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New high-precision trace-element analyses of magmatic olivines point to a pyroxenite-dominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (

Sheared peridotites from the Kaapvaal craton may be broadly divided into two types: (1) high T and refertilized and (2) low T and highly depleted, which equilibrated at conditions lying either above or along the Kaapvaal craton conductive geotherm, respectively. Here, we have studied 14 low-T sheared peridotites from Kimberley entrained by several Late Cretaceous (90 Ma) kimberlites in order to constrain the nature and timing of the deformation. The sample suite comprises nine garnet peridotites (GPs) with various amounts of clinopyroxene ± isolated spinel, three garnet-free phlogopite peridotites (PPs) with minor amounts of spinel, one garnet–spinel peridotite (GSP) and one dunite. The peridotites have intense deformation textures, ranging from porphyroclastic to fluidal mosaic. Olivine and orthopyroxene compositions (Mg# = 91–94) indicate varying degrees of depletion, similar to coarse-grained peridotites from the same localities. Pre-deformation conditions of the GPs are preserved in the cores of large (>100 μm–mm diameter) porphyroclasts and give a range in temperature of 930–1000°C at pressures of 4.0 ± 0.4 GPa. The GSP was equilibrated at 840°C and 3.1 GPa. Projected onto a 40-mW/m2 geothermal gradient, the PP samples yield temperatures of 850–870°C at 3.3–3.4 GPa. Trace element measurements by laser ablation inductively coupled plasma mass spectrometry and electron microprobe indicate that the ‘cold’ sheared peridotites were influenced by several metasomatic events, ranging from ‘old’ pre-deformation metasomatism to interactions shortly before or during deformation. The old pre-deformation metasomatism is recorded in garnet, clinopyroxene and orthopyroxene porphyroclasts and implies interactions with phlogopite–ilmenite–clinopyroxene- or muscovite–amphibole–rutile–ilmenite–diopside-related metasomatic agents, which also led to crystallization of phlogopite in the garnet-free peridotites. A ‘young’ metasomatic event caused an enrichment in Fe, Ti, Ca and Y (+heavy rare earth elements) and is evident in zoned orthopyroxene and clinopyroxene and phlogopite, the crystallization of new clinopyroxene porphyroclasts and compositional heterogeneities in garnet. This young event marks the beginning of extensive kimberlite-related metasomatism in the late Cretaceous beneath Kimberley. The metasomatism caused the deformation (triggered by a kimberlite pulse?), resulting in the recrystallization of fine-grained, mainly olivine, neoblasts (down to

The 1982 and 2015 eruptions are the first at Wolf volcano, Galápagos Archipelago, with eyewitness accounts and satellite imagery. Both eruptions are characterized by a rapid, intense initial phase and multiple eruptive vents leading to the formation of large ‘a‘ā lava fields with scarce pāhoehoe deposits, mostly associated with the waning phases. The 1982 eruption started on 28 August from an intra-caldera vent that produced high lava fountaining, but also occurred from a radial fissure on the SE flank. This eruption lasted for at least 9 days and generated approximately 70E+6 m3 of lava. The 2015 eruption started on 25 May from a circumferential fissure that also produced high lava fountaining and deposited reticulite scoria on the flanks of the volcano. For the first time since monitoring Galápagos eruptions, we observed cryptotephra from the 2015 eruption reaching and depositing in mainland Ecuador, 1400 km away from the source. Lava from the 2015 circumferential vents covered large areas on the SE and E flanks. On 13 June 2015 the eruption switched to an intra-caldera vent that was active until 30 June, which produced lava flows that covered most of the caldera floor. This eruption lasted 36 days and produced ~116E+6 m3 of lava, making it one of the largest eruptions in the Galápagos since the eruption of Sierra Negra in 1979. The combination of ground-based geophysical surveillance, remote sensing, eyewitness accounts, and detailed field work allows us, for the first time, to constrain the eruptive dynamics of this remote volcano with a day-by-day time resolution. In particular, our approach allows quantification of eruption rates, which represents critical information for understanding volcanic systems and for hazard assessment. First order rheological calculations further enable us to constrain the eruption dynamics and emplacement of the lava fields.

Abundant mantle-derived ultramafic xenoliths occur in Cenozoic (7.7–1.5 Ma) mafic alkaline volcanic rocks along the former active margin of West Antarctica, that extends from the northern Antarctic Peninsula to Jones Mountains. The xenoliths are restricted to post-subduction volcanic rocks that were emplaced in fore-arc or back-arc positions relative to the Mesozoic–Cenozoic Antarctic Peninsula volcanic arc. The xenoliths are spinel-bearing, include harzburgites, lherzolites, wehrlites and pyroxenites, and provide the only direct evidence of the composition of the lithospheric mantle underlying most of the margin. The harzburgites may be residues of melt extraction from the upper mantle (in a mid-ocean ridge type setting), that accreted to form oceanic lithosphere, which was then subsequently tectonically emplaced along the active Gondwana margin. An exposed highly depleted dunite–serpentinite upper mantle complex on Gibbs Island, South Shetland Islands, supports this interpretation. In contrast, pyroxenites, wehrlites and lherzolites reflect percolation of mafic alkaline melts through the lithospheric mantle. Volatile and incompatible trace element compositions imply that these interacting melts were related to the post-subduction magmatism which hosts the xenoliths. The scattered distribution of such magmatism and the history of accretion suggest that the dominant composition of sub-Antarctic Peninsula lithospheric mantle is likely to be harzburgitic.

The extent to which Earth’s sub-continental lithospheric mantle modulates the flux of volatile elements from our planet’s deep interior to its atmosphere (via volcanism) is poorly constrained. Here, we focus on “off-craton” sub-continental lithospheric mantle because this long-lived reservoir potentially acts as both a volatile “sink” and “source” during major heating and rifting events. The sub-continental lithospheric mantle is primarily formed of peridotites with subordinate amounts of pyroxenites. While both lithologies are dominated by nominally-volatile-free mantle minerals, some of these phases have been shown to contain non-negligible amounts of H2O (e.g. 100’s of ppmw in clinopyroxene). Data for volatile elements other than Li are, however, limited. We present new, high-precision, in-situ Secondary Ion Mass Spectrometry analyses of H, F, Cl, Li and B in olivine and pyroxenes from well-characterised garnet- and spinel-bearing peridotites and pyroxenites (from southern Patagonia and the Antarctic Peninsula). Our study confirms that clinopyroxene is the main host of H2O and F. The maximum F contents we report (up to 154 ppmw) are higher than those in previous studies and occur in Ti-Cr diopsides in highly-metasomatised peridotites and Ti-Al augites from clinopyroxenite veins. Water contents of clinopyroxenes (up to 615 ppmw) are within the range previously published for continental mantle. Lithium concentrations are low ( Unique to our study is the large variation in major- and trace-element concentrations of the clinopyroxenes, which allows us to place quantitative constraints on how volatiles are stored in the mantle. We demonstrate that: (i) F contents of clinopyroxenes closely correlate with Ti and (ii) D H Cpx - O p x and D F Cpx - O p x is systematic and inversely correlated with temperature. Despite the redistribution of volatiles during sub-solidus re-equilibration, we show that the first order control on the concentration of volatiles in clinopyroxene is the style of metasomatism, i.e. channellised flow versus reactive percolation. The mean bulk volatile contents of peridotites from Pali Aike and the Antarctic Peninsula (H2O = 89 ± 31 ppmw, F = 16 ± 11.2 ppmw and Li = 2 ± 0.7 ppmw) are within the range previously published for continental “off-craton” mantle. The pyroxenites have significantly higher mean bulk concentrations of H2O (260 ± 59 ppmw), F (86 ± 43 ppmw) and Li (1.0 ± 0.35 ppmw). While the greater capacity of mantle pyroxenites to host H2O relative to the associated peridotites has previously been observed in global “off-craton” mantle xenolith suites (e.g. Oahu, Hawaii; eastern China and the Rio Grande Rift, SW USA), here we show for the first time that pyroxenites are also major hosts of F (but not Cl, Li or B). Because of their relatively low solidus temperatures, pyroxenites in “off-craton” settings will be readily re-mobilised during lithospheric extension (and heating). We suggest these pyroxene-rich mantle lithologies may be responsible for the elevated concentrations of H2O and F observed in basalts and volcanic gasses from major continental rift zones and flood basalt provinces, and hence an important consideration in models of global volatile cycles.

Opals are widespread within Miocene volcanic sequences in the North Shoa province of Central Ethiopia. The opal occurs as cavity fillings in a 5 m thick seam of glassy rhyolitic ignimbrite that is sandwiched between basaltic lava flows. The opals occur over a large area (>25 km2). X-ray diffraction analyses show that they are CT-type. The opals contain lower concentrations of trace elements (up to 100 times) than the host rhyolite. Ratios of most trace elements are, however, similar in both the opals and rhyolites. The opals have high δ18O values (28.4–33.8‰) that imply a low temperature of formation, between 20.55 and 25.74 °C. We therefore propose that the opals precipitated from meteoric waters that had percolated through and interacted with the host rhyolite. Field evidence indicates that this weathering and alteration occurred immediately after emplacement of the rhyolites, but prior to the extrusion of the overlying basalt flows.

Despite their relatively low concentration in most oceanic basalts, volatile species (e.g. H2O, CO2 and S) have a disproportionately large influence on a wide range of mantle and magmatic processes. However, constraining the concentration of H2O (and other volatiles) in basaltic magmas is not straightforward as submarine glass analyses are influenced by assimilation of hydrothermal brines, and the melt inclusion record is often reset by post-entrapment processes. Nevertheless, in this study we show that it is possible to reconstruct a detailed history of the volatile content of basaltic magmas through integration of multiple discreet volatile records and careful consideration of secondary processes. We present new analyses of volatiles in olivine-hosted melt inclusions, melt embayments and nominally anhydrous minerals (NAMS, clinopyroxene and orthopyroxene) found in basalts erupted on Floreana Island in the south-eastern Galapagos Archipelago. Our results indicate that the Floreana magmas, which are characterised by the most radiogenic Pb and Sr isotope signatures in the Galapagos Archipelago, contain H2O concentrations between 0.4 and 0.8 wt% (at a melt Mg# of 0.65, where Mg# = Mg/(Mg + Fe) molar). These are marginally greater than the H2O contents of magmas beneath Fernandina in the western Galapagos Archipelago (cf. 0.2–0.7 wt% H2O at Mg# = 0.65). While the volatile content of magmas from the western archipelago follow trends defined by concurrent mixing and crystallisation, NAMs from Floreana reveal the presence of rare, volatile-rich magmas (∼2 wt% H2O) that form as a consequence of reactive porous flow in mush-dominated magmatic systems beneath the south-eastern Galapagos. Furthermore, the Floreana magmas have similar H2O/light Rare Earth Element ratios to basalts from the western Galapagos but contain F/Nd and Cl/K ratios that are ∼2 – 3 times greater, indicating that the mantle source of the Floreana lavas might represent an important halogen reservoir in the Galapagos mantle plume.

The flow of high-temperature and compositionally-enriched material between mantle plumes and nearby spreading centres influences up to 30% of the global mid-ocean ridge system and represents a significant, but currently unconstrained, flux of volatiles out of the mantle. Here we present new analyses of H2O, F, Cl and S in basaltic glass chips from an archetypal region of plume-ridge interaction, the Galapagos Spreading Centre (GSC). Our dataset includes samples from the eastern GSC, on ridge segments that are strongly influenced by the adjacent Galapagos mantle plume, and complements published analyses of volatiles largely from the western GSC. We use forward models of mantle melting to investigate the role of solid and melt-phase transport from a lithologically heterogeneous (peridotite-pyroxenite) mantle in plume-ridge interaction along approximately 1000 km of the GSC. Our results indicate that the observed geochemical and geophysical variations cannot be recreated by models which only involve solid-state transfer of material between the Galapagos mantle plume and the GSC. Instead, we show that the geochemical and geophysical data from the GSC are well-matched by models that incorporate channelised flow of volatile-rich melts formed at high-pressures (>3 GPa) in the Galapagos plume stem to the GSC. In addition, our new models demonstrate that channelised flow of enriched, plume-derived melt can account for up to ~60% of the H2O outgassed from regions of the GSC which are most strongly influenced by the Galapagos mantle plume.

Ocean-island basalts (OIBs) provide a unique insight into the extent of lithological heterogeneity (peridotite vs. pyroxenite) in Earth’s convecting mantle. However, crustal processing of these mantle melts significantly influences minor-element concentrations in olivine phenocrysts, challenging the suitability of this widely used approach to identify lithological variations in their mantle source. Using a numerical model of magma recharge, mixing, and diffusional reequilibration, we show that this type of crustal processing—which is widely observed in OIBs—results in elevated Ni and lower Ca contents of forsterite-rich olivine, causing erroneously high estimates of the proportion of pyroxenite-derived melt. We applied our model of magma recharge and mixing to several OIBs, including the Galapagos Islands, Canary Islands, and La Reunion. In particular, we critically examined olivine compositional variations in basalts from the eastern Galapagos, which display Sr- and Pb-isotope ratios similar to normal mid-ocean-ridge basalts. While previous interpretations (based on olivine chemistry) argued for a significant contribution from pyroxenite-derived melt, our results indicate that the postulated presence of pyroxenite in the eastern Galapagos mantle is an artifact of processing of magmas and their olivine cargo as they transition through the crust, consistent with major-element and isotopic evidence for a dominantly peridotitic source in this region. This new model for magma recharge and mixing may have important implications for our understanding of lithological heterogeneity beneath OIBs globally, and it highlights the importance of considering crustal processes when attempting to interpret olivine compositions with regard to mantle heterogeneity.

The impact of deteriorating climatic conditions on variability in the archaeological record towards the Last Glacial Maximum (LGM) remains uncertain. Partly as a result of poor-quality data, previous studies on Upper Palaeolithic (UP) societies of North-Western Europe prior to the LGM have focused on techno-typological traditions and diversification to outline the diachronic processes through which assemblage composition changed. This study addresses the adaptive trade-offs brought about by the general climatic downturn towards the LGM in North-Western Europe, by investigating the impact of local climate and habitat characteristics on the behavioural variability that characterises Gravettian technological organisation compared to the previous Aurignacian, based on two assemblages from Walou Cave, Belgium. This site is one of the rare well-stratified sites in North-Western Europe with evidence for multiple occupation events accompanied by a fine-grained palaeoenvironmental record. We use a combination of analytical techniques (AMS, LA-ICP-MS and ZooMS) to evaluate questions about hunter-gatherer adaptations. Faunal remains at Walou Cave mirror the faunal diversity documented at numerous other Aurignacian and Gravettian sites in the broader European context, which is similar between both periods. The overall picture presented here, using multiple lines of evidence, is not entirely clear; nonetheless, the results suggest that Gravettian technologies are unlikely to solely be a product of heightened risk in relation to a significant reshuffling of food resources compared to the previous Aurignacian. Future research of the factors structuring assemblage variability prior to the LGM will have to assess whether Aurignacian and Gravettian technologies indeed offer no relative material advantage over one another, a phenomenon called ‘technological equivalence’.

A 2 to 4 °C warming episode, known as the Latest Maastrichtian warming event (LMWE), preceded the Cretaceous–Paleogene boundary (KPB) mass extinction at 66.05 ± 0.08 Ma and has been linked with the onset of voluminous Deccan Traps volcanism. Here, we use direct measurements of melt-inclusion CO(2) concentrations and trace-element proxies for CO(2) to test the hypothesis that early Deccan magmatism triggered this warming interval. We report CO(2) concentrations from NanoSIMS and Raman spectroscopic analyses of melt-inclusion glass and vapor bubbles hosted in magnesian olivines from pre-KPB Deccan primitive basalts. Reconstructed melt-inclusion CO(2) concentrations range up to 0.23 to 1.2 wt% CO(2) for lavas from the Saurashtra Peninsula and the Thakurvadi Formation in the Western Ghats region. Trace-element proxies for CO(2) concentration (Ba and Nb) yield estimates of initial melt concentrations of 0.4 to 1.3 wt% CO(2) prior to degassing. Our data imply carbon saturation and degassing of Deccan magmas initiated at high pressures near the Moho or in the lower crust. Furthermore, we find that the earliest Deccan magmas were more CO(2) rich, which we hypothesize facilitated more efficient flushing and outgassing from intrusive magmas. Based on carbon cycle modeling and estimates of preserved lava volumes for pre-KPB lavas, we find that volcanic CO(2) outgassing alone remains insufficient to account for the magnitude of the observed latest Maastrichtian warming. However, accounting for intrusive outgassing can reconcile early carbon-rich Deccan Traps outgassing with observed changes in climate and atmospheric pCO(2).

A 2 to 4 °C warming episode, known as the Latest Maastrichtian warming event (LMWE), preceded the Cretaceous–Paleogene boundary (KPB) mass extinction at 66.05 ± 0.08 Ma and has been linked with the onset of voluminous Deccan Traps volcanism. Here, we use direct measurements of melt-inclusion CO2 concentrations and trace-element proxies for CO2 to test the hypothesis that early Deccan magmatism triggered this warming interval. We report CO2 concentrations from NanoSIMS and Raman spectroscopic analyses of melt-inclusion glass and vapor bubbles hosted in magnesian olivines from pre-KPB Deccan primitive basalts. Reconstructed melt-inclusion CO2 concentrations range up to 0.23 to 1.2 wt% CO2 for lavas from the Saurashtra Peninsula and the Thakurvadi Formation in the Western Ghats region. Trace-element proxies for CO2 concentration (Ba and Nb) yield estimates of initial melt concentrations of 0.4 to 1.3 wt% CO2 prior to degassing. Our data imply carbon saturation and degassing of Deccan magmas initiated at high pressures near the Moho or in the lower crust. Furthermore, we find that the earliest Deccan magmas were more CO2 rich, which we hypothesize facilitated more efficient flushing and outgassing from intrusive magmas. Based on carbon cycle modeling and estimates of preserved lava volumes for pre-KPB lavas, we find that volcanic CO2 outgassing alone remains insufficient to account for the magnitude of the observed latest Maastrichtian warming. However, accounting for intrusive outgassing can reconcile early carbon-rich Deccan Traps outgassing with observed changes in climate and atmospheric pCO2.

Carbon is central to the formation and environmental impact of large igneous provinces (LIPs). These vast magmatic events occur over geologically short timescales and include voluminous flood basalts, along with silicic and low-volume alkaline magmas. Surface outgassing of CO2 from flood basalts may average up to 3,000 Mt per year during LIP emplacement and is subsidized by fractionating magmas deep in the crust. The large quantities of carbon mobilized in LIPs may be sourced from the convecting mantle, lithospheric mantle and crust. The relative significance of each potential carbon source is poorly known and probably varies between LIPs. Because LIPs draw on mantle reservoirs typically untapped during plate boundary magmatism, they are integral to Earth's long-term carbon cycle.

Long‐distance raw material transfers across Romania prior to the Last Glacial Maximum have previously been inferred from either visual and/or petrographic observations of East Carpathian sites. We investigated the potential to ‘fingerprint’ flint from archaeological sites at Mitoc‐Malu Galben and Bistricioara–Lutărie III in Eastern Romania, using in situ high‐precision analyses of 28 major, minor and trace elements determined by laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) in combination with multivariate statistical analysis. Our results suggest that geochemical analyses have the ability to distinguish between different geographical sources but are unable to positively associate flint artefacts from archaeological contexts to these geochemical groups. The mismatches of signatures between artefacts and geological materials, however, raise new questions and open unforeseen perspectives.

The widespread occurrence of pyrope garnet in Archean lithospheric mantle remains one of the 'holy grails' of mantle petrology. Most garnets found in peridotitic mantle equilibrated with incompatible-trace-element enriched melts or fluids and are the products of metasomatism. Less common are macroscopic intergrowths of pyrope garnet formed by exsolution from orthopyroxene. Spectacular examples of these are preserved in both mantle xenoliths and large, isolated crystals (megacrysts) from the Kaapvaal craton of southern Africa, and provide direct evidence that some garnet inthe sub-continental lithospheric mantle formed initially by isochemical rather than metasomatic processes. The orthopyroxene hosts are enstatites and fully equilibrated with their exsolved phases (low-Cr pyrope garnet ± Cr-diopside). Significantly, P-T estimates of the postexsolution orthopyroxenes plot along an unperturbed conductive Kaapvaal craton geotherm and reveal that they were entrained from a large continuous depth interval (85 to 175 km). They therefore represent snapshots of processes operating throughout almost the entire thickness of the sub-cratonic lithosphericmantle.New rare-earth element (REE) analyses show that the exsolved garnets occupy the full spectrum recorded by garnets in mantle peridotites and also diamond inclusions. A key finding is that a few low-temperature exsolved garnets, derived from depths of ∼90 km, are more depleted in light rare-earth elements (LREEs) than previously observed in any other mantle sample. Importantly, the REE patterns of these strongly LREE-depleted garnets resemble the hypothetical composition proposed for pre-metasomatic garnets that are thought to pre-date major enrichment events in the sub-continental lithospheric mantle, including those associated with diamond formation. The recalculated compositions of pre-exsolution orthopyroxenes have higher Al2O3 and CaO contents than their post-exsolution counterparts and most probably formed as shallow residues of large amounts of adiabatic decompression melting in the spinel-stability field. It is inferred that exsolution of garnet from Kaapvaal orthopyroxenes may have been widespread, and perhaps accompanied cratonization at ∼2.9 to 2.75 Ga. Such a process would considerably increase the density and stability of the continental lithosphere.

© 2019 Elsevier B.V. High-Fo olivine (Fo = Mg/(Mg+Fe) mol%) is an ideal proxy for establishing the compositions of primary melts and their mantle sources. This has been exploited in establishing lithological variations in the mantle source regions of oceanic basalts, including in Iceland. However, previous studies on Icelandic olivine lack spatial and temporal coverage. We present high-precision in-situ major, minor and trace element analyses of Fo-rich olivine from a suite of 53 primitive basalts erupted in the neovolcanic rift and flank zones of Iceland, as well as in older regions of Quaternary and Tertiary crust. Most of these samples have previously been analysed for 3He/4He, which ranges from 6.7 to 47.8 RA, the largest span reported for any oceanic island. By combining trace elemental variability with 3He/4He, we assess the extent of lithological variability in the Icelandic mantle plume. Trace-element ratios that are likely to preserve information about mantle source regions (e.g., Mn/Fe, Ni/(Mg/Fe), Ga/Sc, Zn/Fe and Mn/Zn) suggest a peridotitic mantle source in all rift-related volcanic regions, as well as in the off-rift flank zones of Öræfajökull and Snæfellsnes. However, a signal of a more pyroxenitic mantle lithology is clearly visible in olivine from the South Iceland Volcanic Zone, which represents the southward propagation of the Eastern Rift Zone, while olivine from Tertiary lavas suggests a mixed peridotite-pyroxenite source composition. We are able to identify four components present in the Icelandic mantle: a lithologically heterogeneous plume component with 3He/4He >MORB; a depleted MORB-like peridotite; an isotopically enriched MORB-like peridotite; and a peridotitic component with 3He/4He

Rare earth elements (REEs) generate characteristic absorption features in visible to shortwave infrared (VNIRSWIR) reflectance spectra. Neodymium (Nd) has among the most prominent absorption features of the REEs and thus represents a key pathfinder element for the REEs as a whole. Given that the world's largest REE deposits are associated with carbonatites, we present spectral, petrographic, and geochemical data from a predominantly carbonatitic suite of rocks that we use to assess the feasibility of imaging REE deposits using remote sensing. Samples were selected to cover a wide range of extents and styles of REE mineralization, and encompass calcio-, ferro-and magnesio-carbonatites. REE ores from the Bayan Obo (China) and Mountain Pass (United States) mines, as well as REE-rich alkaline rocks from the Motzfeldt and Ilímaussaq intrusions in Greenland, were also included in the sample suite. The depth and area of Nd absorption features in spectra collected under laboratory conditions correlate positively with the Nd content of whole-rock samples. The wavelength of Nd absorption features is predominantly independent of sample lithology and mineralogy. Correlations are most reliable for the two absorption features centered at ∼744 and ∼802 nm that can be observed in samples containing as little as ∼1,000 ppm Nd. By convolving laboratory spectra to the spectral response functions of a variety of remote sensing instruments we demonstrate that hyperspectral instruments with capabilities equivalent to the operational Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) and planned Environmental Mapping and Analysis Program (EnMAP) systems have the spectral resolutions necessary to detect Nd absorption features, especially in high-grade samples with economically relevant REE accumulations (Nd> 30,000 ppm). Adding synthetic noise to convolved spectra indicates that correlations between Nd absorption area and whole-rock Nd content only remain robust when spectra have signal-to-noise ratios in excess of ∼250:1. Although atmospheric interferences are modest across the wavelength intervals relevant for Nd detection, most REE-rich outcrops are too small to be detectable using satellite-based platforms with>30-m spatial resolutions. However, our results indicate that Nd absorption features should be identifiable in high-quality, airborne, hyperspectral datasets collected at meter-scale spatial resolutions. Future deployment of hyperspec-tral instruments on unmanned aerial vehicles could enable REE grade to be mapped at the centimeter scale across whole deposits. University of Cambridge Higher Education Innovation Funding (HEIF) Natural Environment Research Council (NERC)

Compositions of basalts erupted between the main zone of Galapagos plume upwelling and adjacent Galapagos Spreading Centre (GSC) provide important constraints on dynamic processes involved in transfer of deep-mantle-sourced material to mid-ocean ridges. We examine recent basalts from central and northeast Galapagos including some that have less radiogenic Sr, Nd and Pb isotopic compositions than plume influenced basalts (E-MORB) from the nearby ridge. We show that the location of E-MORB, greatest crustal thickness and elevated topography on the GSC correlates with a confined zone of low-velocity, high-temperature mantle connecting the plume stem and ridge at depths of ~100 km. At this site on the ridge, plume-driven upwelling involving deep melting of partially-dehydrated, recycled ancient oceanic crust, plus plate-limited shallow melting of anhydrous peridotite, generate E-MORB and larger amounts of melt than elsewhere on the GSC. The first order control on plume stem to ridge flow is rheological rather than gravitational, and strongly influenced by flow regimes initiated when the plume was on axis (> 5 Ma). During subsequent northeast ridge migration material upwelling in the plume stem appears to have remained “anchored” to a contact point on the GSC. This deep, confined NE plume stem-to-ridge flow occurs via a network of melt channels, embedded within the normal spreading and advection of plume material beneath the Nazca plate, and coincides with locations of historic volcanism. Our observations require a more dynamically complex model than proposed by most studies, which rely on radial solid-state outflow of heterogeneous plume material to the ridge. This article is protected by copyright. All rights reserved.

The flux of elements into Earth's sub-continental lithospheric mantle is facilitated by the passage of small-fraction melts that either crystallise new phases or react with pre-existing minerals.Metasomatised peridotite records the end product of this exchange but rarely captures the process in the act due to subsolidus re-equilibration. We present the results of a systematic investigation of a metasomatic melt channel preserved in a mantle peridotite from the Late Cretaceous Bultfontein kimberlite (Kaapvaal craton) that shows rare direct evidence of the melt-rock reaction processes. We show that the metasomatic proto-kimberlite melt underwent variable crystallisation of clinopyroxene, sulfides, phlogopite, spinel and zircon together with interaction and diffusive exchange with the surrounding olivine-rich mantle.\ud \ud Element profiles across large olivine porphyroclasts (Fo88) show significant core-to-rim variations in Ni (NiO = 0.18–0.32 wt%) and Cr (Cr = 35–60 ppm), while concentrations of all other elements (e.g. Mg, Fe, Mn, Co, V)are remarkably homogeneous. Electron backscatter diffraction analysis shows that the disequilibrium of Ni and Cr is greatest where the crystal contains large components of the [100] and [010] axes. The disequilibrium is preserved in certain orientations because diffusion of Ni and Cr in olivine is more anisotropic than Fe-Mg and Mn, and slower in the [100] and [010] directions. We present the first observations of Ni and Cr decoupling from other elements in mantle olivine and suggest that this is a consequence of: (i)changing mineral-melt concentration gradients associated with the reactive percolation of a precursory kimberlite melt; and (ii) late-stage sulfide and spinel precipitation.\ud \ud We use the diffusion limited re-equilibration of Ni in olivine to quantify the timing of metasomatism prior to xenolith entrainment by the host kimberlite. Our modelling indicates that reactive percolation occurred on the order of 103–105 years prior to entrainment; this provides an additional line of support for the hypothesis that a period of metasomatism by proto-kimberlite melts precedes the final kimberlite ascent to the surface. The broader implication of our finding of variable rates of minor element diffusion in natural olivine is that it highlights the importance of anisotropy and the impact of changing local concentration gradients during subsolidus re-equilibration.

Continental flood basalts (CFB) are amongst the most voluminous volcanic eruptions in Earth's history. They are rapidly emplaced, and in rare cases the thick lava piles are associated with primitive magmas that have high MgO contents. The compositions of these primitive melts are consistent with a deep-sourced, high-temperature mantle plume origin. Whilst the association of CFBs with impacting mantle plumes is widely accepted, the magnitude of the thermal anomaly is not yet resolved. The development of Al-in-olivine thermometry, however, allows the crystallisation temperature of (near-)liquidus olivine to be determined without knowing the composition of the co-existing melt. This provides both a robust minimum estimate of mantle temperature and a value from which potential temperature (TP) can be back-calculated. This technique has previously confirmed that crystallisation temperatures in CFB settings can be a few hundred degrees greater than those estimated for MORB, and the results hint at a diversity in crystallisation temperatures between different CFB settings. In this study, we re-assess the TP of the mantle source of the Parana-Etendeka CFB province by applying the Al-in-olivine thermometer to olivine–spinel pairs from picrites and ferropicrites. We show that the mean crystallisation temperature of olivine with Fo>90 in the picrites is 1458 °C, with a maximum temperature of 1511 °C. Using the mean value, we calculate a preferred TP of 1623 °C, for an assumed lithospheric thickness of 50 km and magma emplacement pressure of 0.5 GPa. This represents a thermal anomaly of around +300 °C relative to ambient mantle, and confirms that the mantle source of the Parana-Etendeka CFB is the second hottest known from Phanerozoic Large Igneous Provinces, after the Caribbean Large Igneous Province. The ferropicrites record a cooler mean olivine crystallisation temperature of 1296 °C. Given that these low-volume melts derive from deeper and earlier melting of mantle pyroxenite, their temperature is not directly comparable to that of the picrites but they appear to require a somewhat cooler mantle source – perhaps found at the front or edges of a rising plume head.

Accurately dating the formation and modification of Earth’s sub-cratonic mantle still faces many challenges, primarily due to the long and complex history of depletion and subsequent metasomatism of this reservoir. In an attempt to improve this, we carried out the first study on peridotites from the Kaapvaal craton (Finsch Mine) that integrates results from Re-Os, Lu-Hf, Sm-Nd and Sr-isotope systems together with analyses of major-, trace- and platinum-group elements. The Finsch peridotites are well-suited for such a study because certain compositional features reflect they were highly depleted residues of shallow melting (1.5 GPa) at ambient Archean mantle temperatures. Yet, many of them have overabundant orthopyroxene, garnet and clinopyroxene compared to expected modal amounts for residues from partial melting. Finsch peridotites exhibit a wide range of rhenium depletion ages (TRD) from present day to 2.7 Ga, with a prominent mode at 2.5 Ga. This age overlaps well with a Lu-Hf isochron of 2.64 Ga (eHf (t) = +26) which records silico-carbonatitic metasomatism of the refractory residues. This late Archean metasomatism is manifested by positive correlations of Pt/Ir and Pd/Ir with 187Os/188Os ratios and good correlations of modal amounts of silicates, especially garnet, with Os isotope ratios. These correlations suggest that the Highly Siderophile Elements (HSE) and incompatible element reenrichment and modal metasomatism result from one single major metasomatic event at late Archean. Our detailed study of Finsch peridotites highlights the importance of using multiple isotopic systems, to constrain the ages of events defining the evolution of lithospheric mantle. The Re-Os isotope system is very effective in documenting the presence of Archean lithosphere, but only the oldest TRD ages may accurately date or closely approach the age of the last major partial melting event. For a meaningful interpretation of the Re-Os isotope systematics the data must be combined with HSE patterns, trace-element compositions and ideally other isotopic systems, e.g. Lu-Hf. This is highlighted by the widespread evidence in Finsch peridotites of Pt, Pd and Re enrichment through significant Base Metal Sulfide (BMS) addition (mainly in the range of 0.002–0.08 wt%) that systematically shifts the mode of TRD model ages to younger ages.

Here we report the first occurrence of ultra-depleted pyrope garnets with rare earth element (REE) patterns similar to those of hypothetical garnets proposed to have formed in the subcontinental lithospheric mantle prior to those of metasomatic origin. These unique ultra-depleted garnets have not previously been identified in global suites of mantle xenoliths or diamond inclusions. They occur in certain harzburgite members of the xenolith suite from the Lashaine tuff cone, northern Tanzania. The Lashaine ultra-depleted garnets are characterized by low concentrations of CaO (

Active volcanism in Galapagos is far more widespread (> 40,000 km2) than in other hotspot-related archipelagos, such as Hawaii (~ 20,000 km2). Here we employ both geochemical and geophysical models to constrain the causes of this large spatial extent of melt generation and the diverse compositions of erupted basalts. Insight in to the physical as well as the chemical nature of the melting regime beneath Galapagos –– and the cause of the relatively widespread, non-linear age-progressive distribution of volcanism – is provided by incompatible-trace-element ratios of basaltic magmas. Whilst variations in these (and isotopic) ratios of basalts from individual Galapagos volcanoes are limited, considerable differences have been observed in basalts erupted across the archipelago. We have used rare-earth-element inversion modelling for basalts dominated by “plume” and depleted MORB mantle components to constrain the depth to the top of the melt column beneath different Galapagos volcanoes. By converting S-wave data from a recently published tomographic experiment [Villagomez, D.R. et al., 2007. Upper mantle structure beneath the Galapagos Archipelago from surface wave tomography. J. Geophys. Res. 112] to temperature we have been able to map the base of the Galapagos lithosphere, i.e. where the geotherm, with a mantle potential temperature of 1315 °C, intersects the anhydrous peridotite solidus. An excellent correlation exists between the results of our geophysical and geochemical models. These predict that lithospheric thickness varies from ~60 km beneath islands in the south west of the Galapagos Archipelago (e.g. Fernandina and Isabela) to ~ 45 km below those in the northeast (e.g. Genovesa, Marchena, eastern Santiago and northern Santa Cruz). The thinner lithosphere away from the postulated site of the present-day plume axis, combined with the lateral deflection of the plume, is responsible for active volcanism over a relatively large area. Non-uniform variations in lithospheric thickness relative to distance from the Galapagos Spreading Centre are consistent with the complex nature of the oceanic lithosphere beneath this part of the Pacific, perhaps due to ridge jumps.

Miocene rhyolites and basalts from the Ethiopian Large Igneous Province (LIP) crop out in north Shewa on the central Ethiopian plateau. The volcanics were emplaced at the onset of development of the Main Ethiopian Rift and postdate eruption of the Oligocene Ethiopian continental-flood basalt (CFB) province by ~ 15 Ma. Variations in major- and trace-element chemistry indicate that fractionation of the north Shewa parental magmas occurred at ~ 15 km depth; this is consistent with geophysical studies which have predicted the presence of gabbroic intrusions in the mid to upper parts of the underlying ~ 50 km thick crust. High-to-moderate [La/Nb]n (1.1–1.4) and Ce/Pb ratios (17–19.5) for the basalts, together with mantle δ18O values for the rhyolites, suggest that the north Shewa magmas contain only minimal contributions of crustal melts. The formation of the Ethiopian CFB province has been associated with the sub-lithospheric impact of the Afar mantle plume ‘starting-head’ whereas recent volcanic activity in the Main Ethiopian rift has been linked to lithospheric extension and adiabatic decompression melting in the steady-state ‘tail’ of the plume. Rare-earth element (REE) inversion modelling of Oligocene, Miocene and Recent mafic magmas from the Ethiopian LIP has allowed us to determine changes in the amount of partial melting that accompanied the head-to-tail transition of the Afar plume. The results also provide information on the approximate depth of melting and mantle potential temperature and allow us to assess how these have varied both spatially and temporally. The REE inversion results suggest that, since the Oligocene, the amount of adiabatic decompression melting has decreased two-fold, from 13 to 7%, and the potential temperature of the Afar mantle plume has decreased from ~ 1450 to 1350 °C. The ~ 15 Ma north Shewa basalts formed by slightly lower degrees of partial melting (5%) than those in the present-day rift, presumably due to the presence of thicker lithosphere (60 km) prior to the main phase of rifting during which the lithosphere thinned to ~ 50 km at the rift axis.

In 1835 Charles Darwin's geological observations on Isla Santiago (James Island) in the Galápagos Islands led him to important insights as to the process by which different varieties of igneous rock might be produced from the same volcanic vent. His work figured in a tradition of interpretation that began with the work of George Poulett Scrope and would end in the twentieth century with the theory of magmatic differentiation of igneous rocks through the process of crystal fractionation. This article reports on the findings of an expedition to Isla Santiago in July 2007 during which we were able to locate samples of igneous rocks similar to those collected by Darwin. We have used these, together with Darwin's original specimens and transcriptions of his field notes, to examine how his understanding of the separation of the trachytic and basaltic series of magmas developed from his initial field observations through to publication of Volcanic Islands in 1844.

We present major- and trace-element analyses of mineral phases present in a suite of 16 garnet-peridotite xenoliths from the western terrane of the Kaapvaal craton. The xenoliths were entrained by a Group II Finsch kimberlite at 118 Ma, shortly prior to a major metasomatic event that caused widespread enrichment of the Kaapvaal lithospheric mantle. Compositionally homogeneous grains of olivine, orthopyroxene, garnet and clinopyroxene and coarse–equant textures indicate equilibrium relationships between mineral phases in the majority of xenoliths. Pressure and temperature estimates suggest that clinopyroxene-bearing garnet peridotites last equilibrated at 1130–1270°C and 45–59 kbar whereas clinopyroxene-free xenoliths record temperatures of 1000–1070°C and pressures of 34–42 kbar. The Finsch xenoliths plot on a conductive palaeogeotherm with a surface heat flux of ~46 mW m2. Combined Ca and Cr abundances of Finsch pyrope garnets suggest that both lherzolitic and harzburgitic parageneses are present. Samples bearing sub-calcic (harzburgitic) garnets are from the shallowest depths. The lherzolitic garnets are depleted in light rare earth elements (LREE) relative to the middle and heavy REE (MREE and HREE) and have ‘smooth’ chondrite-normalized patterns. In contrast, the sub-calcic garnets are characterized by sinusoidal chondrite-normalized REE patterns that peak at Nd and Lu and exhibit lows at La and Er. The sub-calcic garnets also have lower Zr, Hf, Ti and HREE, and higher LREE and Sr, than lherzolitic garnets. The variations in REE ratios correlate with temperature and pressure and also Cr/Ca ratio. The high Cr content of harzburgitic and some lherzolitic Finsch garnets may have a significant effect on the crystal framework. Substitution of the larger Cr3+ ion for the smaller Al3+ ion increases with decreasing temperature and pressure and distorts the crystal lattice; this permits a greater substitution of Ca by large cations, such as Sr and the LREE, but also limits the replacement of Al by Ti, Zr and Hf. Positive HREE slopes displayed by harzburgitic garnets on chondrite-normalized plots are believed to result from metasomatic enrichment by a melt that had already undergone significant garnet fractionation during ascent through the lithospheric mantle. The low-temperature Finsch peridotites are characterized by much lower orthopyroxene (< 17%) and higher olivine (up to 96%) modal abundances than have been reported from xenolith suites elsewhere in the Kaapvaal craton. Significantly, they resemble residues generated in partial melting experiments. The Finsch harzburgites have very low Al2O3 (0·18 wt %) and CaO (0·38 wt %) and high MgO contents (49·75 wt %) and appear to be highly refractory. They also have high bulk-rock Mg/(Mg + Fe) and high modal olivine contents, and in this respect resemble some of those recently described from NW Canada and Greenland. We suggest that some of the Finsch low-temperature peridotites represent Kaapvaal lithospheric mantle that formed as a residue of adiabatic decompression melting between 4·5 and 1·5 GPa. The inferred mantle potential temperature of 1550°C would have been similar to that of ambient Archaean mantle. Importantly, it appears that the sub-Finsch lithospheric mantle has remained unmodified by the silica enrichment that has been so prevalent elsewhere in the craton. This may reflect the remoteness from the subduction zone that is believed to have been in existence at 2·9 Ga on the eastern margin of the craton.

New rhenium–osmium data for high-MgO (>9 wt.%) basalts from the Galápagos Archipelago reveal a large variation in $^{187}$Os/$^{188}$Os (0.1304 to 0.173), comparable with the range shown by primitive global ocean island basalts (OIBs). Basalts with the least radiogenic $^{187}$Os/$^{188}$Os occur closest to the Galápagos plume stem: those in western Galápagos have low $^{187}$Os/$^{188}$Os, moderate $^{87}$Sr/$^{86}$Sr, $^{143}$Nd/$^{144}$Nd, $^{206}$Pb/$^{204}$Pb and high $^{3}$He/$^{4}$He whereas basalts in the south also have low $^{187}$Os/$^{188}$Os but more radiogenic $^{87}$Sr/$^{86}$Sr, $^{143}$Nd/$^{144}$Nd, $^{206}$Pb/$^{204}$Pb and $^{3}$He/$^{4}$He. Our new Os isotope data are consistent with the previously established spatial zonation of the common global isotopic mantle reservoir “C” and ancient recycled oceanic crust in the mantle plume beneath western and southern parts of Galápagos, respectively. Galápagos basalts with the most radiogenic $^{187}$Os/$^{188}$Os (up to 0.1875) typically have moderate MgO (7–9 wt.%) and low Os (

We have determined mineral-melt partition coefficients (D values) for 20 trace elements in garnet-pyroxenite run products, generated in 3 to 7 GPa, 1,425–1,750°C experiments on a high-Fe mantle melt (97SB68) from the Parana-Etendeka continental-flood-basalt (CFB) province. D values for both garnet (∼Py63Al25Gr12) and clinopyroxene (∼Ca0.2Mg0.6Fe0.2Si2O6) show a large variation with temperature but are less dependent on pressure. At 3 GPa, D cpx/liq values for pyroxenes in garnet-pyroxenite run products are generally lower than those reported from Ca-rich pyroxenes generated in melting experiments on eclogites and basalts (∼Ca0.3–0.5Mg0.3–0.6Fe0.07–0.2Si2O6) but higher than those for Ca-poor pyroxenes from peridotites (∼Ca0.2Mg0.7Fe0.1Si2O6). D grt/liq values for light and heavy rare-earth elements are ≤0.07 and >0.8, respectively, and are similar to those for peridotitic garnets that have comparable grossular but higher pyrope contents (Py70–88All7–20Gr8–14). 97SB68 D LREE grt/liq values are higher and D HREE grt/liq values lower than those for eclogitic garnets which generally have higher grossular contents but lower pyrope contents (Py20–70Al10–50Gr10–55). D values agree with those predicted by lattice strain modelling and suggest that equilibrium was closely approached for all of our experimental runs. Correlations of D values with lattice-strain parameters and major-element contents suggest that the wollastonite component and pyrope:grossular ratio exert major controls on 97SB68 clinopyroxene and garnet partitioning, respectively. These are controlled by the prevailing pressure and temperature conditions for a given bulk-composition. The composition of co-existing melt was found to have a relatively minor effect on 97SB68 D values. The variations in D values displayed by different mantle lithologies are subtle and our study confirms previous investigations which have suggested that the modal proportions of garnet and clinopyroxene are by far the most influential factor in determining incompatible trace-element concentrations in mantle melts. The trace-element partition coefficients we have determined may be used to place high-pressure constraints on garnet-pyroxenite melting models.

We have determined high-precision 40Ar/39Ar ages for alkaline igneous rocks from the western margin of the Early-Cretaceous Parana–Etendeka large igneous province (Paraguay). These show that small-fraction melt generation occurred beneath the region in two phases; at 145 Ma and 127.5 Ma, i.e. before and at the end of the 139–127.5 Ma Parana–Etendeka flood-basalt eruptions. Previously published 40Ar/39Ar ages for alkaline igneous rocks on the proto-Atlantic coastal margins range from 134 to 128 Ma and indicate that small-fraction melt generation in the east of the province was either synchronous or slightly later than the main pulse of tholeiitic volcanism (between 134 and 132 Ma). Our new 40Ar/39Ar phlogopite ages confirm that: (i) the earliest melts associated with the initial impact of the Tristan plume were generated in the west of the Parana–Etendeka large igneous province and (ii) igneous activity was long lived and immediately predates continental break-up. The Early-Cretaceous Paraguayan alkaline magmas are silica-undersaturated, enriched in incompatible-trace elements, have very-low initial eNd values and probably represent melts of phlogopite-bearing, carbonate-metasomatised peridotite in the subcontinental lithospheric mantle. Our simple one-dimensional, conductive-heating models suggest that the early-phase (145 Ma) alkaline magmas were emplaced on the margins of the Rio de La Plata craton at the time of sublithospheric impact of the proto-Tristan plume. The late phase (127.5 Ma) of Paraguayan alkaline magmatism is concentrated in an intra-cratonic rift zone and melt generation appears to have been triggered by lithospheric extension, perhaps facilitated by conductive heating and thermal weakening associated with the upwelling Tristan plume. The location and timing of both alkaline and tholeiitic melt generation in the Parana–Etendeka province appear to have been significantly influenced by the non-uniform composition and thickness of the South American and south-west African lithosphere. The long duration of Parana–Etendeka magmatism (17 Myr) relative to other Phanerozoic large igneous provinces (e.g. Siberia, Karoo, and Deccan) may be an artefact of the limited available high-precision age data for CFB-related alkaline igneous rocks.

The enriched mantle 1 (EM-1) component in ocean–island basalts (OIB, e.g., Kerguelen, Pitcairn and Walvis Ridge) has been attributed to melting in upwelling mantle plumes of either: (i) shallow-recycled delaminated subcontinental-lithospheric-mantle or crust; or (ii) deep-recycled metasomatised lithosphere, oceanic plateau or oceanic crust plus a few percent of pelagic sediment. We present new geochemical data for OIB samples from the central South Atlantic; these include 100 to 30 Ma alkali and tholeiitic basalts from the Walvis Ridge and Rio Grande Rise and

The anhydrous phase relations of an uncontaminated (primitive), ferropicrite lava from the base of the Early Cretaceous Parana´- Etendeka continental flood basalt province have been determined between 1 atm and 7GPa. The sample has high contents of MgO (� 14� 9 wt %), FeO* (14� 9 wt %) and Ni (660ppm). Olivine phenocrysts have maximum Fo contents of 85 and are in equilibrium with the bulk rock, assuming a K Olliquid DFeMg of 0� 32. A comparison of our results with previous experimental studies of high-Mg rocks shows that the high FeO content of the ferropicrite causes an expan- sion of the liquidus crystallization field of garnet and clinopyroxene relative to olivine; orthopyroxene was not observed in any of our experiments. The high FeO content also decreases solidus temper- atures. Phase relations indicate that the ferropicrite melt last equili- brated either at � 2� 2GPa with an olivine-clinopyroxene residue, or at � 5GPa with a garnet-clinopyroxene residue. The low bulk-rock Al2O3 content (9 wt %) and high (Gd/Yb)n ratio (3� 1) are consistent with the presence of residual garnet in the ferropicrite melt source and favour high-pressure melting of a garnet pyroxenite source. The garnet pyroxenite may represent subducted oceanic litho- sphereentrainedbytheupwellingTristanstartingmantleplumehead. During adiabatic decompression, intersection of the garnet pyroxenite solidus at � 5GPa would occur at a mantle potential temperature of � 1550 � C and yield a ferropicrite primary magma. Subsequent melting of the surrounding peridotite at � 4� 5GPa may be restricted by the thickness of the overlying sub-continental lithosphere, such that dilution of the garnet pyroxenite melt component would be signific- antly less than in intra-oceanic plate settings (where the lithosphere is thinner). This model may explain the limited occurrence of ferropicrites at the base of continental flood basalt sequences and their apparent absence in ocean-island basalt successions.

The

The Baikal Rift is a zone of active lithospheric extension adjacent to the Siberian Craton. The 6-16 Myr old Vitim Volcanic Field (VVF) lies approximately 200 km east of the rift axi's and consists of 5000 km3 of melanephelinites, basanites, alkali and tholeiitic basalts, and minor nephelinites. In the volcanic pile, 142 drill core samples were used to study temporal and spatial variations. Variations in major element abundances (e.g. MgO = 3.3-14.6 wt reflect polybaric fractional crystallization of olivine, clinopyroxene and plagioclase. (0.7039-0.7049), (0.5127-0.5129) and (0.2829-0.2830) ratios are similar to those for ocean island basalts and suggest that the magmas have not assimilated significant amounts of continental crust. Variable degrees of partial melting appear to be responsible for differences in Na2O, P2O5, K2O and incompatible trace element abundances in the most primitive (high-MgO) magmas. Fractionated heavy rare earth element (HREE) ratios (e.g. Gd/Lun > 2.5) indicate that the parental magmas of the Vitim lavas were predominantly generated within the garnet stability field. Forward major element and REE inversion models suggest that the tholeiitic and alkali basalts were generated by decompression melting of a fertile peridotite source within the convecting mantle beneath Vitim. Ba/Sr ratios and negative K anomalies in normalized multi-element plots suggest that phlogopite was a residual mantle phase during the genesis of the nephelinites and basanites. Relatively high light REE (LREE) abundances in the silica-undersaturated melts require a metasomatically enriched lithospheric mantle source. Results of forward major element modelling suggest that Melting of phlogopite-bearing pyroxenite veins could explain the major element composition of these melts. In support of this, pyroxenite xenoliths have been found in the VVF. High Cenozoic mantle potential temperatures (∼1450° C) predicted from geochemical modelling suggest the presence of a mantle plume beneath the Baika1, Rift Zone

© 2015 Geological Society of America. New constraints on the timing of the Cretaceous- Paleogene mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the "main-stage" eruptions of the Deccan continental flood basalt province suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was under way well before Chicxulub/Cretaceous-Paleogene time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1-1.0 J/m3 throughout the upper ~200 km of Earth's mantle, sufficient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume "head." It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai Subgroup) lava flows, which together may account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/Cretaceous- Paleogene time, a huge pulse of mantle plume-derived magma passed through the crust with little interaction and erupted to form the most extensive and voluminous lava flows known on Earth. High-precision radioisotopic dating of the main-phase Deccan flood basalt formations may be able either to confirm or reject this hypothesis, which inturn might help to determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed significantly to the Cretaceous-Paleogene extinction.

Although general accounts of carbonatites usually envisage Ca–Mg carbonate melts evolving by fractional crystallisation to Fe-rich residua, there is longstanding concern that ferrocarbonatites may actually be products of hydrothermal rather than magmatic processes. All previously published examples of ankerite- and/or siderite-carbonatites fail to show one or more of the isotopic criteria (all determined on the same sample) thought to be diagnostic of crystallised magmatic carbonate liquids. Ferrocarbonatite dykes cut Archaean-Proterozoic basement at Swartbooisdrif, adjacent to the NW Namibia-Angola border. Their age is uncertain but probably ~1,100 Ma and their associated fenites are rich in sodalite. Where unaffected by subsequent recrystallisation, their petrographic textures resemble those of silicate layered intrusions; ankerite, magnetite and occasionally calcite are cumulus phases, joined by trace amounts of intercumulus pyrochlore. Ankerite is zoned, from Ca(Mg, Fe2+)(CO3)2 cores towards ferroan dolomite rims. Calcite contains ~1.7% SrO, plus abundant, tiny exsolved strontianite grains. Magnetite is close to pure Fe3O4. Pyrochlore has fine-scale euhedral oscillatory zoning and light-REE-enriched rims. ICP-MS analysis of magnetite and pyrochlore from the carbonatite allows calculation of their modal amounts from mass-balance considerations. Sodalite from the fenite is REE poor. Geothermometry, using either the calcite-dolomite solvus or oxygen isotope fractionation between calcite and magnetite, gives temperatures in the range 420–460 °C. Initial Sr, Nd and Pb isotopic ratios of the ferrocarbonatites (87Sr/86Sr=0.7033; eNd=0.2–1.0; 206Pb/204Pb=16.37; 207Pb/204Pb=15.42; 208Pb/204Pb=36.01) are appropriate for an ~1,100-Ma magmatic carbonatite. Likewise, carbonate δ18O=8.0 and δ13C=–7.36 indicate little or no subsequent shift from magmatic values. It appears that dense ankerite and magnetite dominated crystal accumulation from a melt saturated in these phases, plus calcite and pyrochlore, with prior fractionation of a silicate mineral and apatite. The resulting ferrocarbonatite lacks a silicate mineral (excluding fenite xenocrysts) and apatite. It has unusually low (basalt-like) REE abundances and (La/Lu)n, and low concentrations of Ba, Rb, U, Th, Nb, Ta, Zr and Hf. Very high Nb/Ta and low Zr/Hf imply that the evolution of the parental magma involved immiscible separation of a carbonate from a silicate melt. The sodalite-dominated Swartbooisdrif fenites suggest that the parental melt also had a substantial Na content, in contrast with the ferrocarbonatite rock.

During the last decade, alkaline igneous rocks and carbonatites have become of increasing scientific importance. Their significance to our understanding of both small- and large-scale crust and mantle processes has been highlighted by numerous recent detailed, field, petrographic and geochemical studies. These igneous rocks are also of growing economic importance as they are the host of a wide variety of metals (including Nb and the REEs). Because of their relatively small volumes, alkaline igneous …

For lecturers and students of igneous petrology, the end of the last century was marked by a void of appropriate course textbooks. I was therefore delighted to discover that Myron Best and Eric Christiansen had compiled Igneous Petrology. The text is written at the level appropriate for second or third year undergraduate students. It is an up-to-date and comprehensive compilation of all aspects of igneous petrology, ranging from the nucleation and growth of crystals to magma genesis. It is well illustrated with line drawings and numerous photomicrographs and field photographs. The book is divided into 13 chapters based on igneous processes. At the end of each chapter …