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Understanding dendritic crystallization is key to obtaining petrological information about rapid crystallization events. Clinopyroxene dendrites from a basaltic rock fulgurite from Nagpur, India, exhibit curved branches with corresponding lattice rotation that exceeds 180° for some branches. This paper combines crystallographic orientation mapping with microstructural observations and compositional information to determine the dendrites' 3-D morphology and their bending mechanism. Dendrites exhibit a network of branches in the (010) plane, following either {001}* (normal to {001} planes, strong lattice curvature) or < 10–1 > (weak lattice curvature). Three or more orders of branches are observed in the (010) plane, alternating between {001}* and < 10–1>. Side branches with weak lattice curvature extend sub-perpendicular to the (010) plane, following either {021}* (sprouting from {001}* branches) or < 12–1 > (from <10–1 > branches) and defining curved 'ribbons' containing their respective central branch. All branches rotate about [010], with a consistent rotation sense regardless of elongation direction in sample or crystal coordinates. Bending must therefore be caused by local asymmetric thermal and compositional fields in the melt, generated by dendritic growth itself, not by sample-scale compositional, thermal or mechanical gradients. The most likely cause of bending is asymmetric distribution of melt supersaturation around branch tips, related to unequal growth rates perpendicular to different facets. Lattice rotation is inferred to occur via preferential incorporation of high densities of [001] (100) edge dislocations of one sign. High inferred dislocation densities imply that the preservation of bent dendrites requires rapid quenching. Higher inferred degree of undercooling (based on microstructural observations) correlates with greater lattice curvature. Bent dendrites can thus potentially be used to deliver information about spatial variations in degree of undercooling and place limits on the history of a sample after dendritic crystallization. Finally, finding lattice rotation exclusively about [010] is a new criterion to identify cryptic dendritic growth stages in euhedral crystals. [ABSTRACT FROM AUTHOR]

Field relations in dissected volcanic terrains and the internal evidence of persistent low-pressure cotectic character in erupted basalts point to the frequent and substantial modification of liquid compositions by some form of partial crystallization within the crust In contrast, the highly compatible trace elements do not display the marked variations and extreme depletions which are predicted to result from perfect fractional crystallization (PFC). Imperfect fractional crystallization, refilling of magma chambers during fractionation and in situ crystallization are important factors which can help to explain this apparent paradox. This paper explores another effect, the integration of residual liquids from differing extents of partial crystallization, which can help to resolve this paradox, even while still permitting perfect fractional crystallization at all points in the magma chamber. Integration of such residual liquids through the thickness of the crystallization zone is explicit, although not implemented, in the model of in situ crystallization proposed by Langmuir ('Nature 340, 199–205, 1989). It may be separated as a process for purposes of mathematical modelling from the basic concept of partial crystallization of small packets of magma with remixing of the residual liquids into the main body of magma. Integration of melts from differing extents of partial crystallization might in principle also be applied to the case of lateral variations in the mass fraction crystallized with position in the magma chamber. Integrated PFC itself can develop residual liquids which differ little from products of equilibrium (batch) crystallization (FTC) at the same average mass fraction of liquid remaining in both incompatible and compatible trace element concentrations. For one specific combination of parameters these integrated liquids are identical in composition at all values of the distribution coefficient to the EPC liquid. At other values of the parameters the integrated liquids may even—a new paradox— have higher relative concentrations of highly compatible elements than the EPC products. Any integration of residual liquids from different mass fractions of PFC rapidly eliminates what have in the past been taken to be the diagnostic differences between PFC and EPC Integration of EPC liquids (towards which the products of imperfect fractional crystallization processes will tend) produces even more pronounced effects, with highly compatible elements less depleted even than in EPC and far less depleted than would be predicted by simple models. When interpreted according to oversimplified models, sequences of residual liquids produced in such processes might appear to be inconsistent with products of a partial crystallization process and to require a process of progressively smaller mass fractions of melting of inhomogeneous and progressively more refractory (higher mg-number) source regions. [ABSTRACT FROM AUTHOR]

The mineralogy of the oceanic basalt source region is examined by testing whether a peridotite mineralogy can yield observed whole-rock and olivine compositions from (1) the Hawaiian Islands, our type example of a mantle plume, and (2) the Siqueiros Transform, which provides primitive samples of normal mid-ocean ridge basalt. New olivine compositional data from phase 2 of the Hawaii Scientific Drilling Project (HSDP2) show that higher Ni-in-olivine at the Hawaiian Islands is due to higher temperatures (T) of melt generation and processing (by c. 300°C) related to the Hawaiian mantle plume. DNi is low at high T, so parental Hawaiian basalts are enriched in NiO. When Hawaiian (picritic) parental magmas are transported to shallow depths, olivine precipitation occurs at lower temperatures, where DNi is high, leading to high Ni-in-olivine. Similarly, variations in Mn and Fe/Mn ratios in olivines are explained by contrasts in the temperatures of magma processing. Using the most mafic rocks to delimit Siqueiros and Hawaiian Co and Ni contents in parental magmas and mantle source compositions also shows that both suites can be derived from natural peridotites, but are inconsistent with partial melting of natural pyroxenites. Whole-rock compositions at Hawaii and Siqueiros are also matched by partial melting experiments conducted on peridotite bulk compositions. Hawaiian whole-rocks have elevated FeO contents compared with Siqueiros, which can be explained if Hawaiian parental magmas are generated from peridotite at 4–5 GPa, in contrast to pressures of slightly greater than 1 GPa for melt generation at Siqueiros; these pressures are consistent with olivine thermometry, as described in an earlier paper. SiO2-enriched Koolau compositions are reproduced if high-Fe Hawaiian parental magmas re-equilibrate at 1–1·5 GPa. Peridotite partial melts from experimental studies also reproduce the CaO and Al2O3 contents of Hawaiian (and Siqueiros) whole-rocks. Hawaiian magmas have TiO2 contents, however, that are enriched compared with melts from natural peridotites and magmas derived from the Siqueiros depleted mantle, and consequently may require an enriched source. TiO2 is not the only element that is enriched relative to melts of natural peridotites. Moderately incompatible elements, such as Ti, Zr, Hf, Y, and Eu, and compatible elements, such as Yb and Lu, are all enriched at the Hawaiian Islands. Such enrichments can be explained by adding 5–10% mid-ocean ridge basalt (crust) to depleted mantle; when the major element composition of such a mixture is recast into mineral components, the result is a fertile peridotite mineralogy. [ABSTRACT FROM AUTHOR]

This paper presents field, geochemical and isotopic (Sr, Nd, Pb) results on basalts from the Antipodes, Campbell and Chatham Islands, New Zealand. New 40Ar/39Ar age determinations along with previous K–Ar dates reveal three major episodes of volcanic activity on Chatham Island (85–82, 41–35, ∼5 Ma). Chatham and Antipodes samples comprise basanite, alkali and transitional basalts that have HIMU-like isotopic (206Pb/204Pb >20·3–20·8, 87Sr/86Sr <0·7033, 143Nd/144Nd >0·5128) and trace element affinities (Ce/Pb 28–36, Nb/U 34–66, Ba/Nb 4–7). The geochemistry of transitional to Q-normative samples from Campbell Island is explained by interaction with continental crust. The volcanism is part of a long-lived (∼100 Myr), low-volume, diffuse alkaline magmatic province that includes deposits on the North and South Islands of New Zealand as well as portions of West Antarctica and SE Australia. All of these continental areas were juxtaposed on the eastern margin of Gondwanaland at >83 Ma. A ubiquitous feature of mafic alkaline rocks from this region is their depletion in K and Pb relative to other highly incompatible elements when normalized to primitive mantle values. The inversion of trace element data indicates enriched mantle sources that contain variable proportions of hydrous minerals. We propose that the mantle sources represent continental lithosphere that host amphibole/phlogopite-rich veins formed by plume- and/or subduction-related metasomatism between 500 and 100 Ma. The strong HIMU signature (206Pb/204Pb >20·5) is considered to be an in-grown feature generated by partial dehydration and loss of hydrophile elements (Pb, Rb, K) relative to more magmaphile elements (Th, U, Sr) during short-term storage at the base of the lithosphere. [ABSTRACT FROM AUTHOR]

This paper presents new geochemical data from Kistufell (64°48′N, 17°13′W), a monogenetic table mountain situated directly above the inferred locus of the Iceland mantle plume. Kistufell is composed of the most primitive olivine tholeiitic glasses found in central Iceland (MgO 10·56 wt %, olivine Fo89·7). The glasses are interpreted as near-primary, high-degree plume melts derived from a heterogeneous mantle source. Mineral, glass and bulk-rock (glass + minerals) chemistry indicates a low average melting pressure (15 kbar), high initial crystallization pressures and temperatures (10–15 kbar and 1270°C), and eruption temperatures (1240°C) that are among the highest observed in Iceland. The glasses have trace element signatures (Lan/Ybn <1, Ban/Zrn 0·55–0·58) indicative of a trace element depleted source, and the Sr–Nd–Pb isotopic ratios (87Sr/86Sr 0·70304–0·70308, 143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb 18·343–18·361) further suggest a long-term trace element depletion relative to primordial mantle. High He isotopic ratios (15·3–16·8 R/Ra) combined with low 207Pb/204Pb (15·42–15·43) suggest that the mantle source of the magma is different from that of North Atlantic mid-ocean ridge basalt. Negative Pb anomalies, and positive Nb and Ta anomalies indicate that the source includes a recycled, subducted oceanic crustal or mantle component. Positive Sr anomalies (Srn/Ndn = 1·39–1·50) further suggest that this recycled source component involves lower oceanic crustal gabbros. The δ18O values (4·2–4·7‰), which are lower than those observed in mantle peridotites but similar to those observed in ophiolites and in situ oceanic gabbros, are consistent with this interpretation. The elevated 3He/4He ratios are primarily attributed to a primitive, relatively undegassed component in the Iceland mantle plume, which dominates the He isotope signature as a result of long-term depletion of U, Th and He in the recycled gabbroic component. [ABSTRACT FROM PUBLISHER]

Basaltic oceanic plateaux are important features in the geological record. Not only do they record ancient mantle plume activity, but they also are believed to be important building blocks in the formation of the continental crust. In this paper we review the salient features of two Cretaceous oceanic plateaux (the Ontong Java and the Caribbean–Colombian): thick sequences of predominantly homogeneous basalt; the occurrence of high-MgO basalt, including komatiites; and an apparent absence of sheeted dyke complexes. In addition, pyroclastic deposits may be scarce. We then explore ways of distinguishing plateaux from basaltic sequences erupted in different tectonomagmatic settings: continental flood basalt provinces; island arcs; back-arc basins; ocean islands and mid-ocean ridges. Using these criteria, potential Archaean and Proterozoic oceanic plateaux are reviewed and identified. Finally, we explore how these remnant oceanic plateaux became incorporated into the continents, by reviewing the proposed accretion mechanisms for the Cretaceous Caribbean–Colombian oceanic plateau, on the basis of evidence from South America and the tonalites of the southern Caribbean island of Aruba. [ABSTRACT FROM PUBLISHER]

Siliceous High Magnesium Basalt (SHMB) magmas have been implied to play critical roles in the formation of platiniferous horizons in layered intrusions such as the Bushveld Complex, South Africa. However, their role in the formation of such complexes and particularly in the generation of the Cu–Ni–Platinum Group Element (PGE) sulphide mineralization has been deduced rather than proven. In this paper, we present an example of Cu–Ni–PGE sulphide mineralization which was unequivocally formed from SHMB magmas. This example is provided by the Noritic Ring Complex of the northern Vestfold Hills of Antarctica. The Noritic Ring Complex is co-magmatic with the early Proterozoic High Magnesium Tholeiite (HMT) dykes which were previously demonstrated to have formed from SHMB magmas. Chilled margins from the HMT have very similar PGE (average values: 16.0 p.p.b. Pd, 15.2 p.p.b. Pt, 0.98 p.p.b. Ru, 0.34 p.p.b. Ir) and other chemical features common to SHMB volcanics and intrusives from Western Australia, as well as to sills from the Bushveld Complex that are believed to have formed from the magmas that formed the Critical Zone of the Complex. The high Pd and Pt contents combined with the low S contents (291 p.p.m.) of the HMT dykes indicate that they were formed from S-undersaturated magmas, a fundamental requirement for the formation of major Cu–Ni–PGE sulphide mineralization. Sub-economic Cu–Ni–PGE sulphide mineralization occurs as disseminated sulphides and sulphide lenses within the last intrusive phase of the Noritic Ring Complex, the Rubbly Norite, which is characterized by the presence of up to 40 modal % orthopyroxene phenocrysts as well as xenoliths of country rock. It is suggested that the SHMB magma which formed the Rubbly Norite lagged behind the rest of the SHMB magma and spent some time in a residence chamber below the Noritic Ring Complex in which it underwent cooling, assimilation of country rock fragments and extensive crystallization of orthopyroxene which accumulated along the floor of the temporary magma chamber. These factors caused S saturation of the SHMB magma and the generation of the Cu–Ni–PGE sulphides which accumulated along with the orthopyroxene phenocrysts; the orthopyroxene- and sulphide-bearing magma was subsequently emplaced into the Noritic Ring Complex magma chamber. Economic concentrations of Cu–Ni–PGE sulphides did not form in the Noritic Ring Complex because the SHMB magmas did not interact with an abundant source of S. [ABSTRACT FROM AUTHOR]

Apatite can incorporate significant amounts of H2O and CO2, potentially recording volatile abundance and behavior during magma evolution. Here we conducted in situ measurements of CO2 and H2O concentrations in apatite, along with elemental compositions, from two contemporaneous alkaline volcanic suites (Tianbao and Tudiling) in the South Qinling belt in central China to better understand the CO2 and H2O contents and behavior in evolving alkaline melts. Apatite from alkali basalts in Tianbao contains variable CO2 contents ranging from <100 ppm to 2.7 wt.% and H2O contents ranging from 0.1 to 0.6 wt.%. Apatite from REE-enriched trachytes, which evolved from alkali basalt, shows significantly lower CO2 contents and a continuous decrease in H2O during magma fractionation. The observations suggest that CO2 loss commenced at the early stages of magma evolution, whereas significant H2O loss occurred during subsequent magma fractionation in tandem with REE-enrichment. In comparison, apatite grains from the Tudiling trachyte, which is genetically linked with carbonatite, contain higher CO2 contents (0.6 to 1.5 wt.%) but lower REE concentrations than the Tianbao trachytes. Apatite in the Tudiling trachyte is inferred to have crystallized from a carbonated alkaline magma prior to significant CO2 loss and the separation of Tudiling carbonatitic melts, where subsequent liquid immiscibility led to REE enrichment into the carbonatitic melts. The volatile characteristics of apatite from the two volcanic suites provide valuable insights into two different evolutionary processes of alkaline/carbonatitic magmas, the behavior of CO2 and H2O, and the enrichment of REE in alkaline systems. [ABSTRACT FROM AUTHOR]

Many lines of evidence from high P – T experiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon. However, constraints on the effects of H2O–CO2 fluids on mantle melting, particularly at mantle wedge P – T conditions, are limited. Here, we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2 [= molar CO2 / (CO2 + H2O)] of 0.04–0.17. Experiments were performed at 2–3 GPa and 1350°C to assess how temperature, peridotite fertility, and XCO2 of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene +7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO*, and MgO and poorer CaO, Al2O3, and alkalis when influenced by H2O–CO2 fluids. At constant P – T and bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2 of slab-fluid. High XCO2 depleted compositions generate ~7 wt.% melt, whereas, at identical P – T , low XCO2 fertile compositions generate ~30 to 40 wt.% melt. Additionally, peridotite fertility and XCO2 have significant effects on peridotite partial melt compositions. At a constant P – T –XCO2, fertile peridotites generate melts richer in CaO and Al2O3 and poorer in SiO2, MgO + FeO, and alkalis. Similar to previous experimental studies, at a constant P – T fertility condition, as XCO2 increases, SiO2 and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2 on melt SiO2 and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2 studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across most P – T –fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2 ≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2 = 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2 limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2 < 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3 signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites with 0 < XCO2 < 0.11 at 2–3 GPa, or from nominally CO2-free hydrous fertile peridotites at P > 3 GPa. [ABSTRACT FROM AUTHOR]

The author comments on the article "Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar Petrogenesis Revisited," by M. J. O'Hara. He discusses the enrichment of europium (Eu) in the highland crust and its role in the lunar evolution. He considers the depletion of volatile elements in the Moon. He describes the role of basalts from the lunar interior.

An introduction is presented in which the editors discuss various reports within the issue on topics including the diversity of conclusions on the importance of shallow-level processes, the crystal contamination of basaltic magmas and the theoretical aspects of ocean-island magmatism.

The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau. [ABSTRACT FROM AUTHOR]

The origin of intraplate volcanic provinces in continental interiors remains equivocal. One of the major gaps is the absence of a clear understanding of thermal state and the role of recycled carbon. Here we present results from detailed temperature estimation, trace element content of olivine and integrated Sr-Nd-O-Mg-Zn isotopic studies on Meso-Cenozoic basalt and basanite from the Tuoyun Basin in the western Central Asian Orogenic Belt. The calculated crystallization temperatures are in the range of 1101°C–1273°C for basalt and 1037°C–1295°C for basanite, consistent with ambient upper mantle-derived melts. Olivine Zn/Fe, Mn/Zn and Fe/Mn ratios fall in the hybrid source region composed of peridotite and pyroxenite for both basalt and basanite, which is also in agreement with their bulk-rock CaO-MgO trends, FeO/MnO ratios (62.8–77.4 for basalt, 57.8–65.1 for basanite) and FCKANTMS values (overall major element compositions; 0.52–0.68 for basalt, 0.43–0.64 for basanite). Trace elemental modeling shows that the basalt was produced by higher degrees of partial melting with more pyroxenite contribution than basanite. Mg and Zn isotopes show lighter Mg (−0.45 to −0.55‰ for basalt, −0.45 to −0.47‰ for basanite) and heavier Zn (0.37–0.41‰ for basalt, 0.38–0.48‰ for basanite) isotopic compositions than primitive mantle, indicating the possible involvement of recycled sedimentary carbonate in mantle source. Conversely, the basalt and basanite lack typical 'carbonatitic fingerprints' such as high CaO/Al2O3 ratios and negative Zr, Hf and Ti anomalies. Furthermore, the Sr-O isotopes display typical mantle affinity without explicit crustal signatures. Such a decoupling is attributed to carbonate species transition and decarbonation reactions between carbonate and surrounding silicate during plate subduction at a depth of >150 km where the following reactions take place: CaMg(CO3)2 (dolomite) = MgSiO3 (magnesite) + CaCO3 (aragonite) and MgCO3 (magnesite) + SiO2 (coesite) → MgSiO3 (enstatite) + CO2. When the recycled carbonates are exhausted, crustal Sr-O is released but the Mg-Zn isotopic anomalies are inherited by the silicate products ('ghost carbonate'). These silicates are expected to be involved in the subsequent mantle upwelling that induces decompression melting to generate intraplate magmas. We therefore propose recycled carbonates in the mantle sources of the Tuoyun lavas that underwent recycling at depths >150 km were incorporated as 'ghost carbonate' with recycled Mg and Zn elements in the partial melting process rather than directly in the carbonate form. [ABSTRACT FROM AUTHOR]

Highly magnesian, olivine-phyric tholeiitic basaltic and picritic lavas with >5 wt% TiO2 from the Prinsen af Wales Bjerge (PAWB) region are chemically distinct from all other Paleogene East Greenland flood basalts and from basalts in the North Atlantic Igneous Province. The ~100-m-thick lava succession rests on the 61 Ma Urbjerget Formation, is intercalated with volcaniclastic sediments, and has 57 Ma 40Ar/39Ar stepwise degassing ages. It is part of the Milne Land Formation, the first of the major flood basalt formations in East Greenland, and the result of plume impingement of the Kangerlussuaq area in East Greenland during the initial stages of continental breakup. The Ti-rich picrites have relatively primitive compositions and contain Mn- and Ni-rich olivine up to Fo88. Intermediate to high 87Sr/86Sri (0.7034–0.7044) and low Pb isotopic compositions reflect 4–11% crustal contamination, whereas the initial εNd (+4 − +5) and 187Os/188Os ratios (0.121–0.129) overlap with recent Icelandic basalts and appear little affected by contamination processes. The mantle source of the Ti-rich picrites contained garnet and was pyroxene-rich and similar to that of later low-Si alkaline basalts. The Ti-rich picrites of the PAWB, similar to other Ti-rich melts of the Kangerlussuaq region, represent analogies of MgO-rich and variably TiO2-enriched melts from pyroxene rich sources of traditionally accepted mantle plumes like Hawaii. [ABSTRACT FROM AUTHOR]

Melting phase relations involving model carbonated basalt with excess silica were studied in experiments over the pressure range of 4–7 GPa in the system CaO-MgO-Al2O3-SiO2-CO2 to determine if there is a sharp decrease in the melting temperatures along the transition from carbon dioxide vapor (vapor) to dolomite. The phase assemblages of clinopyroxene + garnet + coesite + vapor + carbon dioxide-bearing silicate liquid (silicate liquid) and clinopyroxene + garnet + coesite + dolomite + carbonate liquid, exist over 4–5 and 5.8–7 GPa, respectively. These two distinct phase assemblages form the two, vapor + silicate liquid and dolomite + carbonate liquid-bearing divariant surfaces. The dissolved carbon dioxide and the molar calcium number [Ca# 100*(Ca/Ca + Mg)] of the silicate and carbonate liquids are approximately 4–8 wt% and between 50–55 and 35–40 wt% and 69–71, respectively. The compositions of phases vary little, implying minimal topography along the two surfaces, and the temperatures rise linearly along the silicate liquid-bearing divariant surface over 4–5 GPa. Between 5.2 and 5.6 GPa, however, the temperatures decrease precipitously by ~200–250°C and, along with this steep decline, the liquid changes from silicate to carbonate, with the rest of the phase assemblage of clinopyroxene + garnet + coesite + vapor, persisting. Hence, and this is important to emphasize, this liquid, coexisting with vapor, is carbonate in composition in the absence of dolomite. Isobaric invariance, at 5.4 GPa/1250°C, 5.6 GPa/1150°C, and 5.8 GPa/1100°C, consists of the six-phase assemblage of clinopyroxene + garnet + coesite + vapor + dolomite + carbonate liquid. Melting phase relations are thus univariant, and correspond to that of a solidus 'ledge', i.e. with a negative Clapeyron slope, in this part of the composition space. The melting reaction along the ledge is clinopyroxene + vapor = garnet + coesite + dolomite + carbonate liquid, and the ledge separates the two divariant surfaces. The Ca# of the coexisting carbonate liquid and dolomite here are opposite to those of the carbonate liquid and dolomite on the calcite-magnesite join at similar pressures as in this study. This is most likely a consequence of the combined effects of (a) observations from experiments and theory that the fusion curve of calcite starts to diverge from that of magnesite toward lower temperatures at pressures in excess of ~5 GPa, and (b) the pressure, where ultrabasic silicate–carbonate (~2.5–3 GPa) and excess-silica carbonate-basalt (>4 GPa, as inhere) systems undergo carbonation. These, in turn, cause the liquid and dolomite in experiments here to become more calcic and more magnesian than observed in experiments on the calcite-magnesite join. The solidus ledge, here, has a profound effect because the most plausible modern-day model ocean crust subduction zone geotherms in Earth will, in all likelihood, intersect it and cause fusion of dolomite, thereby, in effect, liberating all carbon from what once was a carbonate-basalt mixture. Thereafter, little exists to suggest that there is anything 'deep' to the carbon cycle, through recycling, with most of it likely confined to less than ~200 km in Earth. [ABSTRACT FROM AUTHOR]

It is not well known which chemical differentiation pathways basaltic melts will take when they are iron metal saturated. Thermodynamically, the pathway seems predictable. So long as Fe metal is a stable liquidus phase and relative oxygen fugacity (fO2) is not subject to major fluctuations, the activity of FeO (aFeOmelt) is buffered by the iron–wüstite (IW) equilibrium 2Femetal + O2 → 2FeOmelt. Metallic Fe also stabilizes olivine through the equilibrium 2Femetal + O2 + SiO2 melt → Fe2SiO4 olivine. That equilibrium tends to suppress the enrichment in bulk SiO2 when Fe saturated basaltic melts differentiate. We document the differentiation history of tholeiitic cumulates from the Siberian craton that carry up to 30 modal % metallic Fe. Our study is complemented by differentiation experiments at two redox states, one set in Fe metal capsules at 1.6 log units below IW (IW-1.6) and a second set in graphite capsules at IW + 1.5. Iron saturated differentiation pathways do not show enrichments in FeO nor in bulk SiO2 because olivine remains stable along the entire liquid line of descent. By contrast, melts equilibrated at IW + 1.5, that is, outside metallic Fe saturation, crystallize pigeonite as first silicate and follow a normal (terrestrial) differentiation pathway involving marked SiO2 enrichment. The Fe-saturated path duplicates in detail the liquid line of descent we derive for the cumulates. Iron-saturated experiments have limited applicability to the Earth because there are so few terrestrial basalts saturated with metallic Fe; however, they might apply to the Moon. Many lunar basalts appear to have been saturated with an Fe-Ni phase during their emplacement on the lunar surface, and potentially during generation within the lunar mantle. [ABSTRACT FROM AUTHOR]

Mantle preconditioning may be defined as the extraction of small melt fractions from mantle asthenosphere during its flow to the site of magma generation. Equations may be written for mantle preconditioning, assuming that the mantle comprises enriched plums in a depleted matrix. The equations take into account variations in mass fraction of plums, the relative rate of melting of plums and matrix, the temperature and pressure of melt extraction, the mass fraction of melt extracted, the extent of chemical exchange between plums and matrix, and the efficiency of melt extraction. Monitoring mineralogical changes and variations in partition coefficients along the inferred PTt path of the mantle asthenosphere allows the equations to be correctly applied to the conditions under which melt extraction takes place. Numerical experiments demonstrate the influence of petrogenetic variables on the shape of melt extraction trajectories and provide new criteria for distinguishing between melt extraction and mixing as the cause of regional geochemical gradients. Representative examples of arcback-arc systems (Scotia), continental break-up (Afar) and plumeridge interaction (Azores) indicate that the compositions of the mantle sources of mid-ocean ridge basalts and island arc basalts may be determined, at least in part, by the melt extraction histories of their asthenospheric sources. [ABSTRACT FROM AUTHOR]

This study explores the effects of high-pressure crystallization and immiscible sulfide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to >10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulfide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulfide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulfide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulfide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g. Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of high-pressure crystallization of clinopyroxene and garnet. The calculated pressure (P) and temperature (T) conditions of the clinopyroxene megacrysts are close to those of the lithosphere–asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts. [ABSTRACT FROM AUTHOR]

Systematic variations in the crystal cargo and whole-rock isotopic compositions of mantle-derived basalts in the intraplate Dunedin Volcano (New Zealand) indicate the influence of a complex mantle-to-crust polybaric plumbing system. Basaltic rocks define a compositional spectrum from low-alkali basalts through mid-alkali basalts to high-alkali basalts. High-alkali basalts display clinopyroxene crystals with sector (hourglass) and oscillatory zoning (Mg#61–82) as well as Fe-rich green cores (Mg#43–69), whereas low-alkali basalts are characterized by clinopyroxenes with unzoned overgrowths (Mg#69–83) on resorbed mafic cores (Mg#78–88), coexisting with reversely zoned plagioclase crystals (An43–68 to An60–84 from core to rim). Complex magma dynamics are indicated by distinctive compositional variations in clinopyroxene phenocrysts, with Cr-rich zones (Mg#74–87) indicating continuous recharge by more mafic magmas. Crystallization of olivine, clinopyroxene and titanomagnetite occurred within a polybaric plumbing system extending from upper mantle to mid-crustal depths (485–1059 MPa and 1147–1286°C), whereas crystallization of plagioclase with subordinate clinopyroxene and titanomagnetite proceeded towards shallower crustal levels. The compositions of high-alkali basalts and mid-alkali basalts resemble those of ocean island basalts and are characterized by FOZO-HIMU isotopic signatures (87Sr/86Sri = 0.70277–0.70315, 143Nd/144Ndi = 0.51286–0.51294 and 206Pb/204Pb = 19.348–20.265), whereas low-alkali basalts have lower incompatible element abundances and isotopic compositions trending towards EMII (87Sr/86Sri = 0.70327–70397, 143Nd/144Ndi = 0.51282–0.51286 and 206Pb/204Pb = 19.278–19.793). High- and mid-alkali basalt magmas mostly crystallized in the lower crust, whereas low-alkali basalt magma recorded deeper upper mantle clinopyroxene crystallization before eruption. The variable alkaline character and isotope composition may result from interaction of low-alkaline melts derived from the asthenosphere with melts derived from lithospheric mantle, possibly initiated by asthenospheric melt percolation. The transition to more alkaline compositions was induced by variable degrees of melting of metasomatic lithologies in the lithospheric mantle, leading to eruption of predominantly small-volume, high-alkali magmas at the periphery of the volcano. Moreover, the lithosphere imposed a filtering effect on the alkalinity of these intraplate magmas. As a consequence, the eruption of low-alkali basalts with greater asthenospheric input was concentrated at the centre of the volcano, where the plumbing system was more developed. [ABSTRACT FROM AUTHOR]

The southern part of the eastern branch of the East African Rift is characterized by extensive volcanic activity since the late Miocene. In the Crater Highlands, part of the North Tanzanian Divergence zone, effusive and pyroclastic rocks reflect nephelinitic and basaltic compositions that formed between 4·6 and 0·8 Ma. The former are best represented by the Sadiman volcano (4·6–4·0 Ma) and the latter occur in the giant Ngorongoro crater (2·3–2·0 Ma), the Lemagarut volcano (2·4–2·2 Ma) and as a small volcanic field in the Laetoli area (2·3 Ma), where basaltic rocks known as Ogol lavas were erupted through fissures and several cinder cones. Compositionally, they are alkaline basalts with 46·0–47·9 wt% SiO2, 3·0–4·3 wt% of Na2O + K2O, Mg# of 61 to 55, and high Cr and Ni content (450–975 and 165–222 ppm respectively). Detailed textural and compositional analysis of the major minerals (olivine, clinopyroxene, plagioclase and spinel-group minerals) reveals the heterogeneity of the rocks. The primary mineral assemblage that crystallized from the Ogol magmas comprises macro- and microcrysts of olivine (Fo89·5–84·2), Cr-bearing diopside to augite, magnesiochromite–chromitess, magnetite–ulvöspinelss, andesine–oligoclasess and fluorapatite, with glass of phonolitic composition in the groundmass. All samples contain appreciable proportions of xenocrystic minerals of macro- and microcryst size, with large variations in both concentration and mineral populations between samples. Xenocrysts include olivine with reverse zonation (Fo84·1–72·5), rounded and embayed clinopyroxene cores of variable composition, anhedral Cr-free magnetite–ulvöspinelss and embayed oligoclase. These xenocrysts as well as variations in major and trace element contents, 87Sr/86Sr(i) (0·70377–0·70470) and 143Nd/144Nd(i) (0·51246–0·51261) ratios provide evidence of multi-stage magma mixing and mingling between Ogol and adjacent Lemagarut volcano basaltic melts with only very minor contamination by Precambrian granite–gneisses. Elevated alkalinity of Ogol lavas, which positively correlates with isotope ratios, and the presence of xenocrystic green core clinopyroxene, perovskite, schorlomite and titanite indicate additional mixing and mingling with evolved nephelinitic magmas and/or assimilation of nephelinitic Laetolil tuffs or foidolitic rocks related to the Sadiman volcano. Owing to their heterogeneity, estimates on the crystallization conditions for the Ogol rocks are difficult. Nevertheless, clinopyroxene–liquid thermobarometry indicates crystallization temperatures of around 1150–1220 °C and records upper-crustal depths of 3–12 km (1–4 kbar). Despite the fact that Ogol basalts are hybrid rocks that formed under open-system conditions with well-documented mixing and mingling processes, they seem to be the best examples closest to primary basaltic melts within the Crater Highlands. [ABSTRACT FROM AUTHOR]

We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track. [ABSTRACT FROM AUTHOR]

The rock type most commonly associated with komatiite throughout Earth's history is tholeiitic basalt. Despite this well-known association, the link between komatiite and basalt formation is still debated. Two models have been suggested: that tholeiitic basalts represent the products of extensive fractional crystallization of komatiite, or that basalts are formed by lower degrees of mantle melting than komatiites in the cooler portions of a zoned plume. We present major and trace element data for tholeiitic basalts (∼7·5 wt% MgO) and dunites (46–48 wt% MgO) from the Palaeoproterozoic Winnipegosis Komatiite Belt (WKB), which, along with previous data for WKB komatiites (17–26 wt% MgO), are utilized to explore the potential links between komatiite and basalt via crystallization processes. The dunites are interpreted as olivine + chromite cumulates that were pervasively serpentinized during metamorphism. Their major and immobile trace element relationships indicate that the accumulating olivine was highly magnesian (Mg# = 0·91–0·92), and that small amounts (4–7 wt% on average) of intercumulus melt were trapped during their formation. These high inferred olivine Mg# values suggest that the dunites are derived from crystallization of komatiite. The tholeiitic basalts have undergone greenschist-facies metamorphism and have typical geochemical characteristics for Palaeoproterozoic basalts, with the exception of high FeO contents. Their REE patterns are similar to Winnipegosis komatiites, although absolute concentrations are higher by a factor of ∼2·5. The ability of thermodynamic modelling with MELTS software to reproduce komatiite liquid lines of descent (LLD) is evaluated by comparison with the crystallization sequence and mineral compositions observed for Winnipegosis komatiites. With minor caveats, MELTS is able to successfully reproduce the LLD. This modelling is extended to higher pressures to simulate crystallization of komatiitic melt in an upper crustal magma chamber. All major and rare earth element characteristics of the tholeiitic basalts can be reproduced by ∼60 % crystallization of a Winnipegosis komatiite-like parental melt at pressures of ∼1·5–2·5 kbar at oxygen fugacities between QFM − 1 and QFM + 1, where QFM is the quartz–fayalite–magnetite buffer. Winnipegosis basalts have low Mg# values that are not in equilibrium with mantle peridotite. They therefore cannot represent primary mantle melts derived from cooler mantle than the komatiites, and require fractional crystallization processes in their formation. Furthermore, their trace element characteristics indicate a depth of melting indistinguishable from that of the Winnipegosis komatiites, and derivation from an identical depleted mantle source. All geochemical and geological evidence is therefore consistent with their derivation from a komatiitic melt, and the presence of a large komatiite-derived dunite body in the WKB provides evidence of extensive fractionation of komatiite in the upper crust. The observed uniform basalt compositions are interpreted as the result of a density minimum in the evolving komatiitic melt at temperatures between clinopyroxene and plagioclase saturation, with efficient extraction of melt from a mixture containing ∼60 % crystals. We conclude that the WKB basalts formed by fractional crystallization of a komatiitic parental melt, and suggest that this model may be more broadly applicable to other localities where komatiites and associated basalts show similar geochemical or isotopic characteristics. [ABSTRACT FROM AUTHOR]

The island of Pantelleria, located in the Sicily Channel Rift Zone (Italy), has been the site of violent peralkaline silicic magmatism alternating with minor effusive to low-intensity Strombolian eruptions of basaltic composition. The basaltic rock suites exposed on the island were sampled to investigate the plumbing system dynamics through the study of chemical stratigraphy and temporal records of olivine crystals. Our petrographic and geochemical observations, together with the compositional variability of olivine, suggest different evolutionary histories for basaltic magmas erupted over two major periods divided by the ∼45 ka Green Tuff (GT) eruption. Core-to-rim compositional traverses across olivine crystals document different types of zoning. We recognized olivine zones affected by Fo oscillations at very fine scales in the inner cores, rims and/or in intermediate portions of crystals and used them to reconstruct the residence and passage of crystals through different magmatic environments, with P–T –ƒO2 and compositional characteristics constrained by thermodynamic modeling. The sequence of magmatic environments evidenced by olivine zoning indicate that the pre-GT volcanic period was dominated by injection at shallow crustal levels (∼300–200 MPa) of primitive melts, initially moving from a deep storage zone at the crust-mantle boundary. Supply of this magma significantly decreased after the GT eruption, while the dynamics of magma transfer within the upper portion of the plumbing system were greatly enhanced. The diffusive relaxation of olivine zoning provided the timing of storage and migration of a crystal through different environments. For magmas feeding the ancient (>45 ka) basaltic activity we retrieved transfer histories that are much longer (up to ∼3 years) if compared with those calculated for the post-GT basalts (1–9 months). The compositional and temporal dataset presented in this study supports the idea that the GT eruption and the subsequent collapse of the volcanic edifice could have caused major changes to the internal structural setting of Pantelleria, creating more favorable conditions for the migration of magmas in the upper portions of the plumbing system. [ABSTRACT FROM AUTHOR]

Magmatism in the East African Rift System (EARS) contains a spatial and temporal record of changing contributions from the Afar mantle plume, anciently metasomatized lithosphere, the upper mantle and the continental crust. A full understanding of this record requires characterizing volcanic products both within the rift valley and on its flanks. In this study, three suites of mafic, transitional to alkaline lavas, were collected over a northeast-southwest distance of ∼150 km along the southeastern Ethiopian Plateau, adjacent to the Main Ethiopian Rift. Specifically, late Oligocene to Quaternary mafic lavas were collected from Chiro, Debre Sahil and the Bale Mountains. New major element, trace element, 40Ar/39Ar ages and isotopic results (Sr, Nd, Pb, Hf, Os, He) show spatial and temporal variation in the lavas caused by dynamical changes in the source of volcanism during the evolution of the EARS. The trace element compositions of Oligocene and Miocene Chiro lavas indicate derivation from mildly depleted and nominally anhydrous lithospheric mantle, with variable inputs from the crust. Further south, Miocene Debre Sahil and alkaline Bale Mountains lavas have enriched incompatible trace element ratios (e.g. Ba/Nb = 12–43, La/SmN = 3·1–4·9, Tb/YbN = 1·6–2·4). Additionally, their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf and 206Pb/204Pb values trend toward a radiogenic Pb (HIMU) component. Radiogenic 187Os/188Os in these lavas correlates positively with 206Pb/204Pb and trace element indicators consistent with ancient metasomatic enrichment of their mantle source. In contrast, transitional Miocene Bale Mountains lavas have lower incompatible trace element abundances, less enriched trace element ratios (Ba/Nb ∼7, La/SmN = 2·3–2·5) and less radiogenic isotopic signatures that originate from melting garnet-bearing, anhydrous lithospheric mantle (Tb/YbN = 2·5–2·9). Pliocene and Quaternary Bale Mountains basaltic lavas are chemically and isotopically similar to Main Ethiopian Rift lavas. Trace element and isotopic indicators in both of these suites denote an amphibole-bearing source distinct from that sampled by the older Bale Mountains lavas. Isotopically, Pliocene and Quaternary Bale lavas have notably less radiogenic Sr–Nd–Pb–Hf isotopic ratios. Quaternary Bale Mountains lavas have the strongest mantle plume contribution (3He/4He = 12·1–12·5 RA), while other Bale Mountains, Debre Sahil and Chiro lavas were derived dominantly by melting of lithospheric or upper mantle sources (3He/4He = 5·1–9·1 RA). A multi-stage, regional-scale model of metasomatism and partial melting accounts for the spatial and temporal variations on the southeastern Ethiopian Plateau. Early Debre Sahil and alkaline Bale Mountains mafic lavas are melts derived from Pan-African lithosphere containing amphibole-bearing metasomes, while later transitional Bale basalts are melts of lithosphere containing anhydrous, clinopyroxene-rich veins. These ancient metasomatized domains were eventually removed through preferential melting, potentially during thermal erosion of the lithosphere or lithospheric foundering. Pliocene and Quaternary Bale Mountains lavas erupted after tectonic extension progressed throughout Ethiopia and was accompanied by increased plume influence on the volcanic products. [ABSTRACT FROM AUTHOR]

We present new U-series disequilibria results for 37 basalts collected from the flanks and axial valley of the Endeavour segment of the Juan de Fuca Ridge, making Endeavour one of the most well-characterized spreading ridges along the global ridge system. Detailed studies of Endeavour basalts provide a geological framework for the tectonic, magmatic, and hydrothermal evolution of the ridge, and the geochemical diversity of basalts that result from short spatial- and temporal-scale variations in mantle sources and melting processes. Because the geologic and geochemical context of these basalts is well known, new uranium series disequilibria and model ages place additional constraints on the recent magmatic evolution of the ridge. While Endeavour basalts vary widely and include depleted, normal, transitional, and enriched compositions, relatively little variation is observed in Th/U ratios or238U–230Th disequilibria in near zero-age basalts. The largest variation in both exists in lavas erupted most recently in the axial valley (the 'Graben Trend') with overall higher230Th excesses. Some basalts with lower230Th excesses collected from pillow mounds on the flanks of the ridge (the 'Inflated Trend') have aged significantly since eruption and document the episodic nature of volcanic and tectonic regimes at Endeavour. Uranium-series melting models using two lithologies (peridotite and eclogite) can account for U–Th systematics in near zero-age basalts. These models indicate that mixing of diverse, deeply-generated, enriched melts and homogeneous, shallowly-generated, depleted melts can account for the geochemistry of Endeavour basalts. In addition, decreasing porosity and melting rate of the eclogite component is primarily responsible for chemical and physical changes occurring at the Endeavour segment through time. [ABSTRACT FROM AUTHOR]

As one of the most geochemically unique drill cores recovered within the Yellowstone–Snake River Plain (YSRP) province, the Sugar City geothermal test well was drilled into intra-caldera rhyolite lavas and tuffs erupted during the middle to late Pliocene and the resurgent basaltic volcanism erupted during the Pleistocene. This sequence parallels the two main stages proposed for YSRP hotspot calderas: i.e. the eruption of several large-volume, ash-flow tuff sheets followed by caldera collapse, then cessation of major rhyolitic activity and gradual subsidence accompanied by filling and eventual burial of the caldera by basalt lava flows. We employ stratigraphic relationships, paleomagnetism, and major, trace element, and Sr–Nd isotope geochemistry to develop models for the origin of the basaltic and rhyolitic magmas within a geographical and temporal context. The basalts are characterized by distinct groupings based on depth and geochemistry and reflect the dominant compositions observed on the surface, e.g. Snake River olivine tholeiite (SROT) and evolved type (e.g. Craters of the Moon). We also observe contaminated basalts that interacted with rhyolite/granite. The basaltic magma formed by shallow partial melting in the plume channel carved into the lithosphere. The older rhyolites preserve the classical characteristics of A-type granites and display major element and trace element concentrations typical for Eastern SRP caldera centres and minimal stratigraphic variation. Multiple lines of evidence document extensive magmatic differentiation and coupled basalt–rhyolite interactions. We find that the most plausible origin for the rhyolites is via partial melting of a hybrid source, comprising Archean crustal components and younger juvenile mafic intrusions. Assimilation of hydrothermally altered material is also required for some eruptive units. The rhyolites did not evolve from residual magma left over from the climactic Kilgore eruption (4·0 Ma), but instead represent discrete magma generation events in the course of a few hundred thousand years between 4·0 to 3·8 Ma. Beginning at approximately 3.3 Ma, basalts were able to erupt through the solidified composite pluton that formed below the caldera. The transition from rhyolite to basalt is tied to the declining flux of basaltic magma as North America moved away from the Yellowstone hotspot core. [ABSTRACT FROM AUTHOR]

The sulfur (S) and copper (Cu) contents of primitive mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) are similar, although the latter are thought to be derived from hotter mantle. To reconcile the sulfur and chalcophile element budgets of OIB, we developed a model to describe the behavior of sulfide and Cu during decompression melting of mantle by combining experimental constraints on decompression melting at different excess mantle potential temperatures, T P, between 1400 and 1650°C, and empirical sulfur content at sulfide saturation (SCSS) models, which take into account the effect of Ni and Cu present in the equilibrium sulfide melt. Model calculations at T P = 1450–1650°C were applied to explain the S and Cu inventory of high-Mg# 'reference' OIB. Modeling indicates that partial melts relevant to OIB generation have higher SCSS than those of primitive MORB, because of the positive effect of temperature on SCSS. Therefore, for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle. Calculation of SCSS along melting adiabats at T P = 1450–1550°C, with variable initial S content of peridotite, indicates that sulfide-undersaturated primitive Icelandic basalts with ∼720 ppm S and 74–115 ppm Cu can be generated by 10–25 wt % melting of peridotite containing 100–150 ppm S. The S and Cu budgets of OIB that are thought to represent low-degree melts can be satisfied by (1) peridotite partial melting if a sulfide-saturated partial melt with a Ni content in the sulfide melt ≥25–30 wt % is derived from relatively cold mantle (T P ≤ 1450°C), or (2) if primitive melts parental to OIB are enriched in S (>1500 ppm), or (3) an extremely low (≤1%) degree of melting is applicable. Alternatively, if the Ni content in the equilibrium sulfide in the peridotitic mantle is ≤20–25 wt %, mixing of partial melts, derived from low-degree melting of MORB-eclogite and metapelite, with S-depleted peridotite partial melts may be necessary to reconcile the measured S and Cu contents in the low- F (<10%, where F is melt fraction) basalts from Galapagos spreading center, Lau Basin and Loihi for T P of 1450–1650°C. In this last case, sulfides, equivalent to 50–100 ppm S in the peridotite mantle, can be exhausted by 1–9 wt % partial melting. The total S inventory of the heterogeneous mantle source of these basalts is higher because of the presence of subducted eclogite ± sediments. Our analysis also suggests that compared with peridotite, which is likely to become sulfide-free during partial melting owing to the high SCSS for its partial melts, subducted MORB-eclogite and metapelite probably play important roles in retaining sulfide in the Earth's shallow mantle, owing to low SCSS in their partial melts and high initial sulfide abundances. [ABSTRACT FROM AUTHOR]

A new set of thermodynamic models is presented for calculating phase relations in bulk compositions extending from peridotite to granite, from 0·001 to 70 kbar and from 650°C to peridotite liquidus temperatures, in the system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3–Cr2O3 (KNCFMASHTOCr). The models may be used to calculate phase equilibria in partial melting of a large range of mantle and crustal compositions. They provide a good fit to experimental phase relation topologies and melt compositions across the compositional range of the model. Compared with the preliminary model of Jennings, E. S. & Holland, T. J. B. (2015) (A simple thermodynamic model for melting of peridotite in the system NCFMASOCr. Journal of Petrology 56, 869–892) for peridotite–basalt melting relations, the inclusion of K2O and TiO2 allows for better modelling of small melt fractions in peridotite melting, and in reproducing rutile-bearing eclogite melting at high pressures. An improved order–disorder model for spinel is now incorporated. Above 10 kbar pressure, wet partial melting relations may be significantly affected by the dissolution of silicates in aqueous fluid, so the set of models includes an aqueous low-density silicate-bearing fluid in addition to a high-density H2O-bearing silicate melt. Oxygen fugacity may be readily calculated for the whole range of bulk compositions investigated, and the effect of water content on melt f O2 is assessed. [ABSTRACT FROM AUTHOR]

Alkaline magmatism associated with the West Antarctic rift system in the NW Ross Sea (NWRS) includes a north-south chain of shield volcano complexes extending 260km along the coast of Northern Victoria Land (NVL), numerous small volcanic seamounts located on the continental shelf and hundreds more within an ~35 000km2 area of the oceanic Adare Basin. New 40Ar/39Ar age dating and geochemistry confirm that the seamounts are of Pliocene-Pleistocene age and petrogenetically akin to the mostly middle to late Miocene volcanism on the continent, as well as to a much broader region of diffuse alkaline volcanism that encompasses areas of West Antarctica, Zealandia and eastern Australia. All of these continental regions were contiguous prior to the late-stage breakup of Gondwana at ~100 Ma, suggesting that the magmatism is interrelated, yet the mantle source and cause of melting remain controversial. The NWRS provides a rare opportunity to study cogenetic volcanism across the transition from continent to ocean and consequently offers a unique perspective from which to evaluate mantle processes and the roles of lithospheric and sublithospheric sources for mafic alkaline magmas. Mafic alkaline magmas with >6wt % MgO (alkali basalt, basanite, hawaiite, and tephrite) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in silica-undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, as well as LREE/HREE (heavy REE) and Nb/Y ratios. Radiogenic isotopes also vary, with Nd and Pb isotopic compositions increasing and Sr isotopic compositions decreasing oceanward. These variations cannot be explained by shallow-level crustal contamination or by changes in the degree of mantle partial melting, but are considered to be a function of the thickness and age of the mantle lithosphere. We propose that the isotopic signature of the most silica-undersaturated and incompatible element enriched basalts best represent the composition of the sub-lithospheric magma source with low 87Sr/86Sr (≥0≥7030) and δ18O olivine (≥5·0), and high 143Nd/144Nd (~0·5130) and 206Pb/204Pb (≤20). The isotopic 'endmember' signature of the sub-lithospheric source is derived from recycled subducted materials and was transferred to the lithospheric mantle by small-degree melts (carbonate-rich silicate liquids) to form amphibole-rich metasomes. Later melting of the metasomes produced silica-undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continentward. Ancient and/or more recent (~550-100 Ma) subduction along the Pan-Pacific margin of Gondwana supplied the recycled subduction-related material to the asthenosphere. Melting and carbonate metasomatism were triggered during major episodes of extension beginning in the Late Cretaceous, but alkaline magmatism was very limited in its extent. A significant delay of ~30 to 20 Myr between extension and magmatism was probably controlled by conductive heating and the rate of thermal migration at the base of the lithosphere. Heating was facilitated by regional mantle upwelling, possibly driven by slab detachment and sinking into the lower mantle and/or by edge-driven mantle flow established at the boundary between the thinned lithosphere of the West Antarctic rift and the thick East Antarctic craton. [ABSTRACT FROM AUTHOR]

The microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition. For a given width of intrusion (or depth of lava lake), vertical tabular bodies are coarser grained than horizontal bodies, with an invariant plagioclase shape across the intrusion. When comparing samples from sills and dykes for which the average grain size is the same, the dyke samples contain fewer small grains and fewer large grains than the sill samples. In contrast, the variation of median clinopyroxene-plagioclase-plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss. These patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well-mixed convecting magma, with the final stage (during which dihedral angles form) occurring in a crystal-rich static magma in which heat loss is primarily diffusive. In contrast, crystallization in sills occurs predominantly in marginal solidification fronts, suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow. An exception to this general pattern is provided by members of the Mull Solitary Dykes, which propagated 100-1000 km SE from the Mull Palaeogene Igneous Centre, Scotland, through the shallow crust. These dykes, where sampled >100 km from Mull, have a microstructure indistinguishable from that of a sill of comparable thickness. We suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased. [ABSTRACT FROM AUTHOR]

Neogene back-arc mafic volcanism in the northern Puna Plateau produced a suite of mainly high-Mg, calc-alkaline, basaltic andesites to andesites that form small scoria cones and lava fields. We present the first comprehensive geochemical study of the mafic suite in the northern Puna that complements work on similar rocks from the southern Puna Plateau. The emphasis is on magma genesis and evolution in both areas, and on a combined interpretation of the two regional datasets in the geodynamic context of back-arc magmatism in the central Andes. The results from the northern Puna suite (bulk-rock and mineral compositions, thermobarometry and radiogenic isotope ratios) are consistent with a predominantly asthenospheric source for the mafic magmas, with variable but locally significant contamination by crustal material. Quantifying the crustal and mantle input fails in most cases because the data display contradictory features, such as high compatible element contents (Mg, Ni, Cr) paired with moderate contents of silica and incompatible lithophile elements that defy classical models of magma mixing, fractionation and assimilation. We suggest that magma evolution involved selective assimilation during turbulent flow, probably at more than one level in the crust. Comparison with the southern Puna mafic suite reveals many features in common (high magma temperatures, textural evidence of rapid magma ascent and cooling, assimilation of crust at different depths). However, the volume of erupted magma is greater in the south than in the north and the volcanism in the south is slightly younger. There is much compositional overlap between the two regions, but the southern Puna suite extends to more primitive compositions. These differences suggest a stronger crustal influence in the northern Puna andesites, which we suggest is due to the presence of an extended upper-crustal melt zone associated with the Altiplano-Puna ignimbrite province. Radiogenic Sr and Nd isotope data from both suites define two diverging trends of variation with MgO that can be explained with a crustal component common to both trends, similar to the silicic ignimbrites, and two contrasting mantle components. The more common and regionally more widespread of the two mantle components (also seen in the frontal arc magmas) has 87Sr/86Sr and 143Nd/144Nd values of 0·705 and 0·5126, respectively, which we attribute to an asthenospheric source enriched by subduction erosion. The less common of the two has Sr and Nd initial ratios (0.708 and 0.51235) that we attribute to melting or assimilation of enriched lithosphere. This component has been found only in the northern Puna and it may have an origin in delaminated lithosphere. [ABSTRACT FROM AUTHOR]

Although the axial melt lens (AML) beneath fast-spreading mid-ocean ridges has been detected by seismic reflection for decades, its nature and role in the accretion of lower oceanic crust and the evolution and eruption of mid-ocean ridge basalts (MORB) are still poorly constrained. Plutonic rocks consisting of quartz-bearing gabbros, diorites and tonalites, which might represent the upper part of a fossilized AML, have for the first time been recovered from an intact fast-spreading oceanic crust section by Integrated Ocean Drilling Program (IODP) Hole 1256D. Whole-rock major elements show a wide and continuous compositional range (e.g. Mg# 24-70) and apparent enrichments in Ti and Fe at intermediate MgO contents (4-6 wt %). Trace element characteristics are coherent for the different lithology groups defined by petrography and mineral modes; that is, gabbro, clinopyroxene-rich diorite, amphibole-rich or oxide-rich diorite and tonalite. The gabbros and diorites are consistent with modeled products of MORB fractional crystallization, composed of mixed melt and cumulate in varying ratios. Modeled trace elements (especially with respect to Eu) support a model in which the tonalites originated from low-degree partial melting of the sheeted dikes overlying the AML, rather than extreme fractional crystallization. Enrichments in rare earth elements (REE) in clinopyroxenes from the gabbroic and dioritic intrusive rocks suggest strong assimilation of REE-rich tonalitic components by evolved MORB magmas. Hydrothermal alteration was pervasive during cooling of the plutonic system, which can be traced by petrography, mineral compositions and bulk-rock geochemistry. The upper part of AML, largely composed of lowdensity and high-viscosity felsic magmas, may serve as a barrier to eruptible MORB melts in the lower part of AML. [ABSTRACT FROM AUTHOR]

Crustal metasomatism by subduction is considered as an important mechanism for generating mantle heterogeneity through infiltration of different metasomatic agents into the mantle. As a consequence of the subduction of oceanic crust (including oceanic basaltic rocks and seafloor sediments), both carbonate and silicate metasomatism are expected to occur at the slab-mantle interface in an oceanic subduction channel. This is demonstrated by an integrated study of major and trace elements, stable Mg-isotopes and radiogenic Sr-Nd-Hf isotopes in Cenozoic and Mesozoic alkali basalts from the West Qinling orogen, China. Although the two series of continental basalts show ocean island basalt (OIB)-like trace element distribution patterns and relatively depleted Sr- Nd-Hf isotope compositions, they exhibit differences in other geochemical variables. The Cenozoic basalts have low SiO2, but high CaO and MgO concentrations and high CaO/Al2O3 ratios but low d26Mg values of -0.54 to -0.32‰. They have relatively high abundances of melt-mobile incompatible trace elements such as the light rare earth elements (LREE) and most large ion lithophile elements (LILE), but low abundances of K, Pb, Zr, Hf and Ti, with high (La/Yb)N values but low Ti/Eu and Hf/Sm ratios. In contrast, the Mesozoic basalts have relatively high SiO2, but low CaO and MgO concentrations and low CaO/Al2O3 ratios, but high d26Mg values of-0.35 to -0.21‰. They have relatively low abundances of melt-mobile incompatible trace elements such as the LILE and LREE, but are relatively enriched in Zr, Hf and Ti, with low (La/Yb)N values but high Ti/Eu and Hf/Sm ratios. These observations indicate a significant difference in the composition of the mantle sources between the two periods of alkali basalt magmatism. Whereas the low d26Mg values of the Cenozoic basalts indicate the involvement of sedimentary carbonate in their mantle source, the high d26Mg values of the Mesozoic basalts point to a primary contribution from a silicate component. Metasomatic reaction of depleted mid-ocean ridge basalt (MORB)-source mantle peridotite with carbonate and silicate melts, respectively, at the slab-mantle interface in the Paleotethyan oceanic subduction channel are responsible for the generation of their mantle sources. Both carbonate and silicate melts have been incorporated into the depleted MORB-source mantle wedge producing 'crustal metasomatis'm in an oceanic subduction channel. We suggest that the type of metasomatic agent in the mantle sources is the key to the composition of the resultant alkali basalts. [ABSTRACT FROM AUTHOR]

Plagioclase-bearing ultraphyric basalts, which can have up to 35% accumulative, millimetre-sized plagioclase crystals, were episodically erupted during some stages of building of La Réunion Island volcanoes. Selected rock samples were analysed from the two volcanoes of the island (four samples from Piton des Neiges and two from Piton de la Fournaise). We summarize the results of petrographic and geochemical studies of the whole-rocks and silicate melt inclusions trapped within plagioclase (An84.2-71.7) macrocrysts, which contain aliquots of the parental melts of their hosts. Melt inclusion compositions are used to discuss the origin of La Réunion plagioclasebearing ultraphyric basalts, with special emphasis on the magma storage system that led to their production. Experimentally re-homogenized melt inclusions were analysed by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for their major and trace elements. Together with the textural observations, these results indicate that the host crystals are inherited, although their parental magma compositions are close to that of the host lava. Petrographic lines of evidence suggest that the plagioclase macrocrysts originate from batches of plagioclase-rich mush or fragments of anorthositic gabbro ripped from the magma chamber walls. The parental melts of the plagioclases evolve predominantly by clinopyroxene+plagioclase crystallization. The density contrasts between minerals and liquid allow their efficient segregation. We propose that plagioclases are collected below the magma chamber roof as a flotation cumulate, thereby forming plagioclase-rich mush and anorthositic gabbro, whereas clinopyroxenes sink. The plagioclase-ultraphyric basalts are derived from the remobilization of this mush, or fragments of the anorthositic cumulate, upon the input of a new batch of magma that triggers their eruption. We postulate that the periodic occurrence of these striking basalts during specific periods of the volcano's growth corresponds to decreases in the magma supply, which promoted plagioclase crystallization and its segregation by flotation. Finally, we suggest that the plagioclase-ultraphyric basalts from the Riviè re des Remparts and Riviè re de l'Est valleys, previously regarded as components of the Piton de la Fournaise volcano, are instead products of the declining activity of an older volcano, possibly Les Alizé s volcano. [ABSTRACT FROM AUTHOR]

We present a detailed study of the mineralogical and chemical modifications of peridotite xenoliths during magma transport and cooling at the Earth's surface. The xenoliths are entrained in three small-volume (<1 km3), monogenetic, Miocene-Oligocene, basanite domes in the Moesian Platform, north Bulgaria, arranged along a NNE-directed right-lateral strike-slip fault. The domes show symmetrically decreasing volumes of erupted magma correlated with the size and quantity of the entrained xenoliths, according to their position along the fault. The xenoliths exhibit different degrees of mineralogical and chemical interaction with their host rocks, with the extent of interaction strongly depending on their position in the dome. Xenoliths from the fine-grained brecciated carapaces of the domes show very thin, fine-grained, reaction rims around orthopyroxene and spinel, and thin diffusion zones around olivine, limited mostly to the rims of the xenoliths. Clinopyroxene shows almost no visible reaction and is always strongly depleted in light rare earth elements (LREE) and Sr. Melt pockets in the xenoliths are small, composed mostly of fine-grained olivine and clinopyroxene. Modelling of Fe-Mg zoning in olivine suggests a very short residence time of a few days in the magma during transport and fast cooling at the Earth's surface. In contrast, xenoliths from the interior of the domes, hosted in holocrystalline groundmass, are much more strongly affected by the host basanite magma. Their constituent minerals have wider reaction rims around orthopyroxene, sometimes leading to its entire consumption, and show transformation of spinel into chromite. Fe-Mg diffusion profiles in olivine are up to 400 μm wide and calculations indicate diffusion times up to 200 days, recording protracted cooling in the inner part of the dome. Melt pockets are much larger and coarser-grained, composed of minerals identical to the host groundmass. With few exceptions, clinopyroxene is sieve-textured and shows variable enrichment in LREE and Sr, ranging from several times higher than in the depleted xenoliths to complete equilibration with the host basanite. Strongly veined xenoliths show stronger chemical modifications, facilitated by infiltrated melt, which also progressively increase depending on the position of the xenoliths in the dome. The most enriched xenoliths from the core of the dome exhibit large inter- and intra-grain variations in Sr and LREE. Our study demonstrates that chemical and mineral modifications, although starting at the time of entrainment of the xenoliths at mantle depths, were completed mostly during their residence in the magma at the surface. The reaction phenomena are the result of post-entrainment partial melting, and reactions between xenolith minerals and infiltrated fluids and melts from the host basalts. The large inter- and intra-crystal chemical variations in a single xenolith suggest that reactions strongly depend on the access of fluids and melts (permeability) in different parts of the xenoliths. The results of this study allow us to introduce the term 'host basalt metasomatism' for those mantle xenoliths that have undergone chemical alteration at or near the surface during cooling of the host magma. Comparison with xenoliths stored in large-scale magmatic systems under La Palma (Canary Islands) shows that, although the products of interaction between xenoliths and host rocks are similar, there are considerable differences in the mechanism of entrainment, depth and longevity of the reactions between small-volume and large-scale magmatic systems. [ABSTRACT FROM AUTHOR]

The Rum Eastern Layered Intrusion (ELI) is the product of part of an ~60 Ma open-system magma chamber. The 16 coupled peridotite-troctolite ± gabbro macro-rhythmic units it contains represent crystallization of multiple batches of basaltic and picritic magma. Within the ELI, Unit 10 has been considered the type example of batch fractionation of magma on Rum for more than 50 years, successively producing peridotite, troctolite and olivine gabbro. Detailed field observations and logs of the Unit 10 peridotite cumulate are presented here, together with mineralogical and textural analyses of Cr-spinel seams and their peridotite host-rocks. Numerous harrisite layers are commonly associated with diffuse, laterally discontinuous, platinum-group element (PGE) enriched Cr-spinel seams. Multiple millimetre- to centimetre-thick Cr-spinel seams occur at the bases and tops and within harrisite layers. These relationships are inconsistent with simple batch fractionation of magma. Critically, the harrisite layers also exhibit centimetre- to metre-scale, upward oriented apophyses that point to injection of magma into the overlying cumulate, indicating an intrusive origin for the harrisite. Quantitative textural and chemical analysis suggests that the Cr-spinel seams formed via in situ crystallization within the crystal mush together with intrusive peridotites from an assimilation reaction between the replenishing magma and peridotitic crystal mush. Intrusive magma replenishment in Unit 10 caused significant compositional disequilibrium between the crystallizing phases in response to the postcumulus migration of reactive liquid, resulting in chemical zoning of intercumulus plagioclase crystals. We propose that the Unit 10 peridotite is intrusive and that repeated small-volume magma replenishments are responsible for incremental construction of a large proportion of the peridotite body, similar to recent interpretations of parts of Unit 12 and Unit 14. Moreover, it is suggested that some or all of the injections of magma occurred into the crystal mush, rather than at the magma chamber floor. This new model of intra-mush Cr-spinel formation may have significant economic implications for PGE enrichment in other layered intrusions, such as the peridotite-hosted chromitites of the Stillwater Complex Ultramafic Series (Montana, USA). It is also worth noting that thin platiniferous chromitite seams considered to have formed in situ occur below the Merensky Reef of the Bushveld Complex (South Africa). [ABSTRACT FROM AUTHOR]

To understand fully the role of amphibole in the fractionation of Nb/Ta during arc magma evolution, we conducted experiments with mid-K and high-K basalts to determine amphibole/melt Nb, Ta and Ti partition coefficients (DNb, DTa and DTi) at variable conditions of bulk TiO2, P, T, H2O and fO2. The experimental results show that, at crustal pressures, amphibole is the most important crystalline phase in hydrous basaltic systems. The amphibole/melt Nb, Ta, and Ti partitioning results are 0.16-0.90 for DNb, 0.13-0.68 for DTa, 1.81-10.63 for DTi and 0.76-2.81 for DNb/DTa. Bulk TiO2 and fO2 show no observable effects. T and H2O, in addition to the compositions of amphibole and melt, are the main affecting factors. DNb, DTa, DTi and DNb/DTa increase with decreasing temperature, amphibole Mg# and melt H2O content and increasing melt polymerization. During cooling and crystallization of arc magmas at crustal pressures, amphibole Mg# decreases and melt polymerization increases, leading to significant increase in amphibole/melt DNb, DTi and DNb/DTa. Nb/Ta fractionation in evolved melts will thus be enhanced with crystallization progress. Meanwhile, melt H2O content will increase with the degree of crystallization, which slows down the increase in these D values. Therefore, the trend and extent of Nb/Ta fractionation in the melt by amphibole critically depends on temperature and melt H2O content. Only low temperatures or low H2O contents at high temperatures lead to high D values. For arc magmas with an average H2O of ~3.9wt %, DNb and DNb/DTa are in general >0.40 and >1.20, respectively, which explains why amphibole fractionation results in lower Nb/Ta ratios in evolved arc magmas. The bulk Nb/Ta fractionation trend during arc magma evolution appears to be generally controlled by fractional crystallization of amphibole. Experimental and modeling results suggest that amphibole is a main fractionating phase during arc magma evolution and continental crust formation. [ABSTRACT FROM AUTHOR]

The chemistry of ocean floor basalts (OFB) has been much studied for the insights that the compositions of their parental magmas may give into the thermal structure of the mantle, and its compositional heterogeneity. But as Mike O'Hara pointed out, OFBs are not primary magmas, and their extensive low-pressure differentiation 'must be quantified before chemical features of lavas are ascribed to the nature of the underlying upper mantle'. O'Hara also emphasized that individual magmatic eruptions are products of complex magmatic systems. Volcanoes are not formed by isolated events, but are sites of long-term activity at which eruptions are caused by the arrival of replenishing magma that mixes with earlier magma through repeated cycles. In the intervals between replenishment events, part of the magma crystallizes, concentrating incompatible elements in the remaining magma. If the compositions of OFB are to be used to infer differences in the conditions of melting owing to mantle thermal structure, first it is necessary to disentangle the effects of this low-pressure evolution. It is a remarkable feature of OFB magmatism that these potentially complex replenish- mix-tap-crystallize (RMTX) cycles give rise to a simple pattern of chemical evolution when averaged at the global scale. Here we look at the deviations of 45 minor and trace element concentrations from these global average trends. The global trends are taken as the logarithms of the concentrations of the elements as a function of MgO concentration (log [M] vs [MgO]), and the deviations from these trends are defined as 'variabilities'. An element's variability is expected to result from a combination of source heterogeneity, partial melting in the mantle and melt extraction, and from the local variations in the RMTX process during crustal evolution. There is a strong correlation of an element's variability with its slope in the global average trend, where the slope is a proxy for the element's incompatibility (bulk partitioning) during global average RMTX. This correlation implies that the chemical controls on source heterogeneity and partial melting and melt extraction are similar to those during RMTX. Striking exceptions are the variabilities in plagioclase-hosted Na and Sr, indicating that a substantial part of the variability of Na in OFBs is due to variations in RMTX, rather than reflecting differences in extent of partial melting. If Iceland is excluded, there is no evidence from OFB chemistry to suppose that the distribution of potential temperatures in the sub-ridge mantle has a standard deviation, σ(Tp), larger than about 10°C, and if variability of the mantle source composition is allowed for, σ(Tp) could be even less. A principal component analysis of the variabilities reveals two more-orless equally important principal components, suggesting that two distinct factors have affected incompatible trace element concentrations in global OFB. We suggest that these two factors are source heterogeneity and the RMTX process rather than the extent of partial melting or shape of the melting regime. An unexpected finding is the decoupling of the variabilities of very incompatible elements (VICE) such as Ba, Th and Nb, from those of the highly incompatible elements (HICE), such as La and the other light rare earth elements, Zr, P and Be. [ABSTRACT FROM AUTHOR]

Mid-ocean ridge basalts (MORB) are arguably the most abundant and also the simplest igneous rocks on the Earth. A correct understanding of their petrogenesis thus sets the cornerstone of igneous petrogenesis in general and also forms the foundation for studying mantle dynamics. Because major element compositions determine the mineralogy, phase equilibria and physical properties of rocks and magmas, understanding global MORB major element systematics is of prime importance. The correlated large MORB major element compositional variations are well understood as the result of cooling-dominated crustal-level processes (e.g. fractional crystallization, magma mixing, melt-rock assimilation or reaction, and other aspects of complex open-magma chamber processes), but it remains under debate what messages MORB major elements may carry about mantle sources and processes. To reveal mantle messages, it is logical to correct MORB melts for the effects of crustal-level processes to Mg#≥0.72 to be in equilibrium with mantle olivine of ≥Fo90. Such corrected MORB major element (e.g. Si72, Ti72, Al72, Fe72, Mgc, Ca72 and Na72) compositional variations thus reflect fertile mantle compositional variation, composition-controlled mantle physical property variation (e.g. density and solidus), variation in the extent and pressure of melting, and uncertainties associated with the correction. The correction-related uncertainties can be removed through justified heavy averaging. Because ridge axial depth variation (~0 to ~6000m below sea level) and plate spreading rate variation (<10 to >150mm a-1) are the two largest known physical variables along the global ocean ridge system, possible correlations of MORB major element compositions at Mg#≥0.72 with these two physical variables are expected to reveal intrinsic controls on global MORB petrogenesis and ocean ridge dynamics. Indeed, global MORB major element data averaged with respect to both ridge axial depth intervals and ridge spreading rate intervals show significant first-order correlations. These correlations lead to the conclusion that the ridge axial depth variation and MORB chemistry variation are two different effects of a common cause, induced by fertile mantle compositional variation. The latter determines (1) variation in both composition and mode of mantle mineralogy, (2) variation of mantle density, (3) variation of ridge axial depth, (4) source-inherited MORB compositional variation, (5) density-controlled variation in the maximum extent of mantle upwelling, (6) apparent variation in the extent of melting, and (7) the correlated variation of MORB chemistry with ridge axial depth. These correlations also confirm the recognition that the extent of mantle melting increases with, and is caused by, increasing plate spreading rate. Mantle temperature variation could play a part, but its overstated role in the literature results from a basic error (1) in treating ridge axial depth variation as solely caused by mantle temperature variation by ignoring the intrinsic control of mantle composition, (2) in treating mantle plume-influenced ridges (e.g. Iceland) as normal ridges of plate spreading origin, and (3) in treating seismic low velocity at great depths (>300 km) beneath these mantle plume-influenced ridges as evidence for hot ridge mantle. There is no evidence for large mantle temperature variation beneath ridges away from mantle plumes. The suggested conclusions of this study may continue to be debated, but they are most objective, and are most consistent with petrological, geochemical, geological and geophysical principles and observations. [ABSTRACT FROM AUTHOR]

The pattern of trace element enrichment and variability found in differentiated suites of basalts is a simple observable, which nonetheless records a wealth of information on processes occurring from the mantle to crustal magma chambers. The incompatible element contents of some midocean ridge basalt (MORB) sample suites show progressive enrichment beyond the predictions of simple models of fractional crystallization of a single primary melt. Explanations for this overenrichment have focused on the differentiation processes in crustal magma chambers. Here we consider an additional mechanism and focus instead on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts supplied to magma chambers. A primary observation motivating this strategy is that there is significant chemical diversity in primitive high-MgO basalts, which single liquid parent models cannot match. Models were developed to simulate the compositional effects of concurrent mixing and crystallization (CMC): diverse parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crystallization. Using a simple statistical model to explore the effects of concurrent mixing and crystallization on apparent liquid lines of descent, we show how significant departure from Rayleigh fractionation is possible as a function of the diversity of trace elements in the incoming melts, their primary MgO content, and the relative proportion of enriched to depleted melts. The model was used to make predictions of gradients of trace element enrichment in log[trace element]- MgO space. These predictions were compared with observations from a compilation of global MORB and provide a test of the applicability of CMC to natural systems. We find that by considering the trace element variability of primitive MORB, their MgO contents and degree of enrichment, CMC accurately predicts the pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the relationship between over-enrichment and incompatibility can result from mantle processes: the fact that during mantle melting maximum variability is generated in those elements with the smallest bulk Kd. Magma chamber processes are therefore filtering the signal of mantle-derived chemical diversity to produce trace element over-enrichment during differentiation. Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies as a function of melt supply. There is no correlation between over-enrichment and melt supply in the global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and crystallization, are very rarely detected. This supports a model whereby trace element over-enrichment is an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may operate across all rates of melt supply. [ABSTRACT FROM AUTHOR]

On 1 January 2008 Llaima volcano, a basaltic andesite stratocone in southern Chile, entered a phase of violent Strombolian eruption. Llaima, like many passively degassing systems, has experienced prolonged (decades-long) periods of persistent summit degassing from its open vent. The rapid transition from long-lived passive degassing to violent explosive eruption occurred with limited precursory monitoring signals. This study is motivated by the desire to understand what occurs in these systems when that switch takes place. To this end, we study the products of the 2008 violent Strombolian eruption of Llaima volcano. We present new textural analyses of scoria and geochemical data for five whole-rock samples, troctolite glomerocrysts with and without Cr-spinel, and 182 olivine-hosted melt inclusions from tephra samples. Two populations of scoria ('brown' and 'black') are distinguished by their variable crystallinity and vesicularity, but are geochemically indistinguishable. Black scoria contains abundant microlites with tabular to acicular morphologies and convolute vesicles up to 1⋅75 mm in effective diameter. The brown scoria tends to have fewer, acicular microlites, abundant matrix glass, and round vesicles with a narrower size distribution, constrained to < 0⋅4 mm in diameter. Overall, the textures of the black and brown scoria provide evidence for a textural maturation process in which shallow system magma becomes more crystal rich and probably rheologically stiffer as a result of prolonged passive degassing. The Cr-spinel-bearing and Cr-spinel-free troctolite glomerocrysts have plagioclase and olivine compositions of An65-92 and Fo81, respectively. The Cr# in the Cr-spinel ranges from 26 to 37, consistent with magma originating from the deeper plumbing system. Whole-rock compositions for the tephra average 51 wt % SiO2, 18 wt % Al2O3, and ~6 wt % MgO. The major element compositions of olivine-hosted melt inclusions range from 49 to 56 wt % SiO2 and 3⋅72 to 7⋅76 wt % MgO; there is no distinct compositional difference between olivine-hosted melt inclusions sourced from the different scoria. Melt inclusion volatile contents range from below detection to 2⋅95 wt % H2O and 1973 ppm CO2 (though not in the same melt inclusion). H2O and CO2 concentrations are consistent with open-system degassing and, when compared with differentiation indices (e.g. K2O), indicate coupled degassing and crystallization throughout the system. The majority of melt inclusions define a single liquid line of descent indicative of plagioclase and olivine fractionation. Entrapment pressures range from 8 to 342 MPa and fall into two groups: 8-100 MPa (300 m to ~4 km depth) and >100 MPa (4-14 km depth), revealing that this eruption tapped a deep plumbing system. We propose here that passive degassing at Llaima is maintained by periodic, small-batch magma injections. Consequently, owing to extensive degassing the upper plumbing system magma crystallized and increased in viscosity. Before the 2008 eruption, some volatiles sourced from the repeatedly injected magmas exsolved from the inferred crystal mush and ascending from deeply sourced degassing magmas, and gradually accumulated within the crystal mush and beneath the stiffened conduit magma. Our results support a model in which eruption triggering occurred when magma injection remobilized the mush and, importantly, unlocked the accumulated gases, which ascended rapidly and generated the observed violent Strombolian explosive activity. Our proposed model contrasts with those models for explosive mafic volcanism that require rapid magma ascent under closed-system degassing conditions. Importantly, our proposed mechanism provides a means for systems with dominantly open-system degassing behavior to switch from passive degassing to explosive eruptions. [ABSTRACT FROM AUTHOR]

Trachytes are typically interpreted in terms of extreme fractional crystallization from basaltic magmas. Data from Mauritius suggest otherwise. Here, intrusive, nepheline-bearing trachytes are associated with Older Series basalts (9.0-4.7 Ma), as confirmed by a U-Pb zircon age of 6.8 Ma. Trachyte mineralogy is dominated by low-Ca alkali feldspar with variable Na/(Na + K), lesser high-Na nepheline, aegirine-rich clinopyroxene, titanomagnetite and accessory zircon and apatite. A few samples contain xenocrysts of anorthoclase, Al- and Ti-rich clinopyroxene and kaersutite; these phases show prominent reaction rims that approach typical trachyte mineral compositions. Trachyte major elements cluster at ~63 wt % SiO2 and Na2O + K2O ~12 wt %, but reconstructed pre-alteration compositions suggest that they were originally mainly phonolites, and form a prominent Daly Gap when plotted with the basalts. Incompatible trace elements are enriched in all trachytes, except for Ba, Sr and Eu, which show prominent negative anomalies. Rare earth element patterns have variable abundances, prominent negative Eu anomalies and shapes that differ markedly from those of the basalts. Initial εNd values cluster at + 4.03 ± 0.15 (n = 13), near the lower end of the range for basalts (εNd = +3.70 to + 5.75), but the initial 87Sr/86Sr is highly variable (ISr = 0.70408-0.71034) compared with the relatively constant ISr of 0.70411 ± 19 for the basalts. Fractional crystallization models, using the PELE and MELTS algorithms, starting with a primitive, nepheline-normative Mauritian basalt parent (P = 1 kbar, fO2 = QFM - 3, where QFM is quartz-fayalite-magnetite buffer) fail, because when plagioclase joins olivine in the crystallizing assemblage, successive liquids become depleted in Al2O3, do not produce nepheline, and do not approach phonolitic or trachytic compositions. Similar results are obtained using more alkaline parent liquids including basanite; however, such compositions are not observed anywhere amongst the Older Series basalts. Plutonic xenoliths from Mauritius do not fill the Daly Gap as in some other occurrences (e.g. Hawaii, Pantelleria, Azores). Fractional crystallization was not the operative process that produced Mauritian trachytes. Likewise, liquid immiscibility is excluded because the compositions do not fall at the ends of known miscibility gaps. What remains as plausible is some type of partial melting process, although the source cannot be Precambrian continental crust, as suggested to exist under Mauritius, because such material should not yield nepheline-bearing melts, and would not account for the Sr-Nd isotopic compositions. Small amounts of contamination with such continental crust, however, can account for the Sr-Nd isotopic compositions of the trachytes. Partial melting of basaltic volcanic rocks or extant gabbroic bodies, either from the oceanic crust or from Réunion plume-related magmas, should yield quartz-saturated melts different from the critically undersaturated Mauritian trachytes. A remaining possibility is that the trachytes represent direct, small-degree partial melts of fertile, perhaps metasomatized mantle; we suggest that the xenocryst assemblage present in some samples might represent fragments of this source. A mantle source is supported by the presence of trachytic and phonolitic glasses in many mantle xenoliths, and experimental results show that low-degree alkaline melts can be produced from mantle peridotites even under anhydrous conditions. If some feldspar is left behind as a residual phase, this would account for the negative Ba, Sr and Eu anomalies observed in Mauritian trachytes. These considerations may also apply to other trachyte and phonolite occurrences worldwide. [ABSTRACT FROM AUTHOR]

Intraplate basaltic volcanic rocks ranging in age from Late Cretaceous to Holocene are distributed across southeastern Australia in Victoria and eastern South Australia. They comprise four provinces differentiated on the basis of age and spatial distribution. The youngest of these (<4.6 Ma) is the Newer Volcanic Province (NVP), which incorporates lava flows, scoria cones and maars, distributed across western and central Victoria into South Australia. The oldest eruptive rocks belong to the 95-19 Ma Older Volcanic Province, which comprises basaltic lava flows and shallow intrusions distributed across eastern and central Victoria. When examined within the broader framework of geochemical data available for Cretaceous to Cenozoic intraplate volcanism in southeastern Australia, new major, minor and trace element and Sr, Nd and Pb isotope analyses of volcanic rocks from the NVP suggest that their parental magmas originated from a distinctively different mantle source compared with that of the Older Volcanics. We propose that the magmas represented by the Older Volcanics originated from low degrees of partial melting of a mixed source of Indian mid-ocean ridge basalt (MORB)-source mantle and calcio-carbonatite metasomatized subcontinental lithospheric mantle (SCLM), followed by up to 20% fractional crystallization. The magmas of the youngest (<500 ka) suite of the NVP (the Newer Cones) were generated by up to 13% partial melting of a garnet-rich source, followed by similar degrees of fractional crystallization. We also suggest that the temporally intermediate Euroa Volcanics (~7 Ma) reflect chemical evolution from the source of the Older Volcanics to that of the Newer Cones. Furthermore, energyconstrained recharge, assimilation and fractional crystallization (EC-RAxFC) modelling suggests that the Sr isotope signature of the ~4.6-1 Ma Newer Plains component of the NVP can be explained by up to 5% upper crustal assimilation. On the basis of these results and data from the literature for mantle xenoliths, we propose a geodynamic model involving decompression melting of metasomatized veins at the base of the SCLM generating the Older Volcanics and modifying the ambient asthenosphere of Indian MORB isotope character. This was followed by thermal erosion and entrainment of the resulting depleted SCLM into the modified Indian MORB-source asthenospheric mantle, generating the Newer Cones. Such a model is in agreement with recent geophysical observations in the area suggesting edge-driven convection with shear-driven upwelling as a potential geodynamic model resulting in temporal upwelling in the region. [ABSTRACT FROM AUTHOR]

Major ± trace element and Sr-Nd-Pb-Hf-He isotope data are presented for more than 300 geochemically diverse basalt samples collected by submersible from the Inflated Central Endeavour Segment of the Juan de Fuca Ridge. Seven chemically distinct basalt types are present, from depleted (D-) to enriched mid-ocean ridge basalt (E-MORB). By combining the geochemical data with high-resolution bathymetry and age determinations, the detailed spatial and temporal scale of on-axis mantle-derived basalt heterogeneity is determined. The basalts define binary mixing arrays in all isotope plots that are usual in their correlations, but unusual in the limited range of Sr-Nd-Hf isotope compositions for D- to E-MORB, and greater range in Pb isotopes. The basalts also define two different styles of enrichment of moderately incompatible elements. Geochemical enrichment began when the currently inflated axial ridge formed <105 years ago. One enrichment style (the Inflated Ridge Trend) characterizes basalts erupted across the ~5km wide ridge from >10000 to ~4000 years ago, whereas the other enrichment style (the Graben Trend) characterizes most basalt types erupted within the axial graben after it formed ~2300 years ago. We attribute the Inflated Ridge Trend to a relatively high proportion of pyroxenite (or melt derived therefrom) to enriched peridotite in the mantle during a phase of ridge inflation that lasted at least 6000 years. The Graben Trend reflects the reduced effect of pyroxenite after the axial graben formed. Because at least 14 different samplings of mantle components occurred within <1 km of ridge length and width during a time when <1 km of upwelling occurred, we infer that the scale of mantle heterogeneity far from a plume is < 1 km. The enriched mantle component at Endeavour is young with 206Pb/204Pb ~19·0; Hf and He isotope ratios trend toward HIMU characteristics. These traits are regionally widespread and are shared with the next two ridge segments to the north (West Valley and Explorer). [ABSTRACT FROM AUTHOR]

Mount Merapi is one of Indonesia's and the world's most hazardous volcanoes. Existing constraints on the location and the crystallization conditions of its pre-eruptive magma reservoir based on mineral and glass thermobarometry and geophysical surveys are inconclusive, yet of immediate importance for future hazard mitigation. Here, we use two series of phase equilibrium experiments to quantify the conditions of pre-eruptive magma storage and magma recharge in Merapi's upper-crustal reservoir: (1) to characterize crystallization at total equilibrium conditions; (2) to characterize crystallization at local equilibrium conditions between crystal rims and host melt. We demonstrate that this experimental approach can constrain pre-eruptive crystallization conditions and their variation in crystal-rich, mixed magma systems, such as that of Merapi, for which standard crystallization experiments and mineral thermobarometry fail. In agreement with geophysical estimates, we infer that Mount Merapi's pre-eruptive reservoir partially crystallizes at ≥100-200±75MPa and thus at relatively shallow depths of c. ≥4.5 to ~9 km. Magmas are stored at ≥925-950±25°C with a melt H2O content of ~3-4 wt % and a vapour phase with an XH2O [H2O/ (H2O+CO2)] of ~0.5-0.660.1. Pre-eruptive recharge magmas, in contrast, have a temperature >950 to <1000°C, a higher melt H2O content of ~4-5 wt % and a vapour phase with a higher XH2O of ~0.8±0.1. We hypothesize that these variations in melt H2O content and vapour XH2O between resident and pre-eruptive recharge magmas relate to variable degrees of open-system degassing of magma parcels en route into the reservoir rather than to variations in volatile fluxing of the stored magmas. [ABSTRACT FROM AUTHOR]

Numerous models have been developed to simulate the reaction of magmas to changes of thermodynamic variables, such as pressure, temperature, oxygen fugacity, and water activity. However, the extensive experimental database still lacks information on the distinct effect of small amounts of H2O on olivine+plagioclase+clinopyroxene cotectic crystallization in tholeiitic basalt. We present an experimental study addressing the effects of pressure (at 100, 200, 400, and 700 MPa) and small amounts of H2O on phase relations and liquid lines of descent in three tholeiitic basalts representing different evolutionary stages of the Shatsky Rise oceanic plateau magmatic system (compositions AH6, AH3, and AH5 with 8.6, 8.0, and 6.4 wt % MgO, respectively). Two experimental approaches (dry and low H2O) are designed to maintain contrasting H2O activities during crystallization using (1) graphite-platinum double capsules to perform nearly anhydrous experiments (<0.15 wt % H2O in the melt) and (2) Fe pre-saturated Au20Pd80 capsules to obtain low melt H2O contents ranging from 0.4 to 1.1wt % H2O. Under dry conditions, at lower pressures (⩽400 MPa), the crystallization in the MgO-rich AH6 and intermediate AH3 basalts follows the typical sequence of tholeiitic differentiation with olivine crystallization at the liquidus followed by olivine+plagioclase and olivine+plagioclase+clinopyroxene. Both basalts are close to multiple saturation at pressures between 400 and 700 MPa. At high pressure (700 MPa) the crystallization sequence is reversed, starting with clinopyroxene at the liquidus. Under low-H2O conditions, AH6 and AH3 are very close to multiple saturation, even at the low pressures of 100 and 200 MPa, and the reversed crystallization sequence (clinopyroxene, plagioclase+clinopyroxene, olivine+plagioclase+clinopyroxene) is observed already at 400 MPa. In contrast to the two more MgO-rich basalts, in the most evolved AH5 basalt, clinopyroxene is the liquidus phase at all investigated pressures and under both dry and low-H2O conditions, followed by crystallization of plagioclase+clinopyroxene and olivine+plagioclase+clinopyroxene. The most striking observation in our experiments is that the stability of clinopyroxene increases not only with pressure increase but also in the presence of small amounts of H2O (when compared with dry counterparts at similar pressures). Small amounts of H2O increase the proportion of clinopyroxene in the olivine+plagioclase+clinopyroxene phase assemblage. Our experiments clearly show that the effect of adding 0.4wt % H2O to cotectic melt compositions (e.g. CaO/Al2O3 ratio at a given MgO) is similar to that caused by an increase of pressure from 100 to∼300 MPa. This implies that small amounts of H2O can lead to significant overestimation of cotectic crystallization pressures (by up to 300 MPa) and that H2O contents need to be taken into account in geobarometric models. Our new experiments emphasize the role of low melt H2O contents in stabilizing clinopyroxene and provide some new insights into the problem of the 'pyroxene paradox'. The apparent mantle pressures obtained for some mid-ocean ridge basalts using 'dry' geobarometric approaches can actually represent depths within the lower crust, if small amounts of H2O are present. The application of our experimental data to natural Shatsky Rise basalts implies that the magmas record partial crystallization processes occurring mainly at low pressure (100 MPa), corresponding to depths of ∼3km beneath the former spreading center, although the more primitive lavas show evidence of differentiation in a deeper reservoir at ∼14km depth (400 MPa). [ABSTRACT FROM AUTHOR]

Modelled primary magma compositions of Palaeogene basalts from the North Atlantic Igneous Province (NAIP) require melting at mantle potential temperatures (TP) in the range 1480-1550°C. Modern lavas from Icelandic rift zones require TP∼1500°C and those from the rift flanks TP∼1450°C. Secular cooling of the NAIP thermal anomaly was therefore of the order of ∼50°C over the past 61 Myr. There were systematic variations in TP of 50-100°C from the centre of the thermal anomaly to its margins at any one time, although limits on the stratigraphical distribution of TP determinations do not rule out thermal pulsing on a timescale of millions of years. Variation in extent of melting at similar TP was controlled by local variability in lithospheric thickness. In the west of the NAIP, lithosphere thickness varied from ∼90km at Disko Island to ∼65km at Baffin Island, with similar thickness variations being evident for magmatism in the Faroe Islands, Faroe-Shetland Basin and the British Palaeogene Igneous Province (BPIP). Mean pressure of melting was greater than or equal to the final pressure of melting; the two values converge for melting columns with a melting interval of <1.5 GPa, regardless of TP. The majority of the BPIP magmas were generated in the garnet-spinel transition in the upper mantle. Calculated and observed rare earth element distributions in NAIP lavas are entirely consistent with the melting regimes derived from major element melting models. This allows a calibration of rare earth element fractionation and melting conditions that can be applied to other flood basalt provinces. [ABSTRACT FROM AUTHOR]

The physics and chemistry of ore formation in magmatic sulphide systems are dominantly controlled by the dynamics and kinetics of transported sulphide liquid droplets. We examine the relative importance of chemical dissolution (owing to changes in sulphide concentration at sulphide saturation in the magma) on droplet size, and conclude that the timescales for dissolution (of the order of several days to thousands of years) are much longer than the timescales for dynamic processes (of the order of seconds). We examine dynamic processes that lead to droplet break-up, and delineate the different domains of behaviour that arez encountered over a range of magma flow regimes from stagnant to turbulent, and according to droplet size. Droplets can break up via a variety of mechanisms including deformation and rupture in turbulent flows, and ligament stretching and relaxation in chaotic laminar flows. Droplets larger than a few millimetres in diameter are likely to break up owing purely to the viscous forces acting on them as they settle through stagnant environments. We conclude that droplet break-up, rather than coalescence, is the dominant mechanism for modifying droplet size populations during flow, and that break-up is particularly likely to be prevalent in turbulent komatiite magma and chaotic flow in basaltic magma. The size distributions observed in sulphide blebs and droplets in nature are interpreted as the result of multiple superimposed processes that are active on different portions of the droplet size distribution: growth of sulphide droplets from sulphide-saturated silicate magma and mechanical accumulations of transported assimilated droplets that have undergone break-up during transport. By determining which droplets are stable and which will undergo break-up we show that the presence of large (>2mm) sulphide droplets or blebs in cumulate rocks is an indicator of proximity to a sulphide source or reworked sulphide liquid pool. [ABSTRACT FROM AUTHOR]

To investigate the nature and origin of across-arc geochemical variations over time in mantle wedge derived magmas, we have carried out a geochemical study of basalts in the NE Japan arc spanning an age range from 35Ma to the present. Back-arc basalts erupted at 24-18 Ma, 10-8 Ma, 6-3Ma and 2-5-0Ma have higher concentrations of both high field strength elements (HFSE) and rare earth elements (REE) [particularly light REE (LREE) and middle REE (MREE)], and higher incompatible trace element ratios compared with frontal-arc basalts at any given time. Geochemical modeling of Nb/Yb versus Nb shows that the frontal-arc and back-arc compositional differences are independent of subduction modification and can, in many cases, be explained by different degrees of melting (higher degrees of melting for frontal-arc magmas and lower degrees of melting for back-arc magmas) of a nearly homogeneous depleted mid-ocean ridge basalt (MORB) mantle (DMM)-like source, although there are several exceptions. These include some Pliocene frontal-arc basalts that may originate from a source that is slightly more depleted than DMM, several 35-32Ma and 24-18Ma back-arc basalts derived from a lithospheric mantle source that is enriched in HFSE compared with DMM, and a rare 16-12Ma basalt that was erupted in the back-arc but was produced by a similar degree of melting to frontal-arc basalts erupted at the same time. Variations in ratios of fluid-mobile and -immobile elements and those of melt-mobile and -immobile elements for the 35-0Ma NE Japan basalts indicate that the principal subduction component added to the source mantle prior to generation of these basalt magmas is a sediment-derived melt. Comparison of Sr and Nd isotopic compositions for Pacific Ocean MORB, the NE Japan basalts and subducting sediments suggests that the isotopic compositions of most post-16Ma more depleted back-arc basalts can be explained by the addition of <2% bulk sediment; the most enriched isotope compositions of the subcontinental lithosphere-derived magmas can be accounted for by addition of a maximum 5-7% Japan Trench Sediment (JTS), if the original Sr and Nd compositions of the lithosphere approximated that of DMM. The Sr and Nd isotope composition of the frontal-arc basalts can be accounted for by the addition of 1-5% JTS. A depleted asthenospheric mantle (DMM-like) upwelling model with interaction between asthenospheric mantle-derived magmas and overlying lithospheric mantle can account for the geochemical characteristics of the 35-0Ma NE Japan basalts. The frontal-arc magmas were generally generated by higher degrees of melting of the shallower part of the asthenospheric mantle, whereas the back-arc magmas resulted from lower degrees of melting of the deeper part of asthenospheric mantle. These latter magmas underwent interaction with the lithospheric mantle, resulting in more enriched Sr and Nd isotopic signatures for the pre-18Ma back-arc basalts and post-22Ma frontal-arc basalts, but less interaction, resulting in more depleted Sr and Nd isotopic signatures, for most of the back-arc basalts younger than 16 Ma. [ABSTRACT FROM AUTHOR]