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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.

© 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

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.

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.

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.

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 (

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 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.

Variations in the bulk-rock compositions of primary high-Mg melts provide important constraints on the thermal and chemical structure of their mantle source regions. Picrites and komatiites of Archean to Recent age exhibit a wide range of FeO and Al2O3 at a given MgO content. Those with FeO* contents (total Fe as FeO)>12.5 wt% have low Al2O3 contents (typically

Abundant dykes in the southern Etendeka region, NW Namibia, mostly contain 8–20% MgO. Almost all can be allocated to previously described Early Cretaceous magma types. Horingbaai-type basalts–picrites occur up to 120 km inland. Some have superficially mid-ocean ridge basalt (MORB)-like compositions: (La/Nb)n 1·0; (Sm/Lu)n 1·5; Nd >8; initial 87Sr/86Sr 0·7032. Others show major- and trace-element, and Sr–Nd–Pb isotopic evidence of contamination during upwelling by up to a few per cent of K-feldspar-rich upper crust. Extremely magnesian olivine macrocrysts (Fo91–93·3) in some Horingbaai picrites indicate that komatiitic (MgO 24%) liquids were associated with this suite, although they were too dense to reach the crustal levels currently exposed. Ferropicrite dykes resemble closely the nearby Etendeka ferropicrite basal lavas, with total iron (as Fe2O3) >MgO; (La/Nb)n 1·0–1·5; (Sm/Lu)n 2·5–7; Nd 2–3, except in samples with geochemical evidence of contamination by Archaean–Proterozoic lower crust. A few low-Ti dykes resemble geochemically the lavas of the main southern Etendeka succession, with (La/Nb)n 2·0–2·5; Nd = 0 to -8; initial 87Sr/86Sr >0·707. Dykes north of the Huab river define a fourth magma type, Nil Desperandum (ND), that may have fed part of the Huab sill complex. The dyke ages are constrained by their field relationships. Ferropicrite and low-Ti dykes are consanguineous with lavas erupted at 133 and 132 Ma, respectively. Both Horingbaai and ND dykes cut 132 Ma Etendeka lavas, and the main swarm of Horingbaai picrites with forsteritic macrocrysts is cut by the 131 Ma Brandberg plutonic complex. Forward and inverse modelling of the genesis of the ferropicrites and the Horingbaai picrite–komatiites gives two temporal ‘windows’ into physicochemical conditions within the head of the impacting Tristan mantle plume. At 133 Ma the southern Etendeka lithosphere was >100 km thick and only incipient melting of predominantly Fe-rich peridotite streaks occurred in the rising plume head (Tp = 1500°C). By 2 my later, pre-Atlantic extension had reduced the lithosphere thickness beneath and adjacent to NW Namibia, and there was intense melting (Tp = 1500–1700°C) of even depleted peridotite in the plume head.

Both scaled laboratory experiments and numerical models of terrestrial mantle plumes produce 'balloon-on-a-string' structures, with a bulbous head followed by a stem-like tail. Discussions have focused on whether their initial upwelling heads are hotter than the tails or cooler, as a result of entrainment of ambient mantle during ascent, and also on whether initial plume upwelling is a newtonian or non-newtonian process. The temperature of the mantle delivered to the base of the lithosphere is a critical parameter in such debates. Dry continental magmas can normally contribute little to this topic because their hottest (ultramafic) examples can be expected to be trapped, owing to their density, beneath the Moho. Here we report a rare case in which olivine (with 93.3% forsterite; Mg2SiO4) phenocrysts, precipitated from an unerupted komatiitic melt (approximately 24% MgO) of the Tristan mantle plume head 132 Myr ago, were carried to upper-crust levels in northwest Namibia by less Mg-rich (9.6-18.5% MgO) magmas. We infer that the hidden melt, generated when the plume impinged on the base of the lithosphere, originated in the mantle with a potential temperature of approximately 1,700 degrees C. This is approximately 400 degrees C above ambient and much hotter than the temperatures previously calculated for steady-state Phanerozoic mantle plumes. Published data show that the same conclusion can be reached for the initial Iceland and Galapagos plumes.

A comparison of high-MgO magmas from both oceanic and continental settings reveals that they exhibit wide variations in their bulk-rock contents of FeO* (9–16 wt% at 15 wt% MgO). The high-FeO* picrites (ferropicrites) range from Archean to recent in age but are relatively rare at the Earth’s surface, typically forming thin isolated flows near the base of thick lava piles in large igneous provinces. They are characterised by high contents of compatible trace elements (e.g. Cr=400–1650 parts per million (ppm) and Ni=250–1050 ppm) and unaltered samples (Parana–Etendeka and Madagascar) have relatively smooth, normalised multi-element patterns that lack significant relative enrichments in strongly incompatible elements (e.g. [Ba/La]n=0.5–1.0) and [La/Nb]n=1.2–1.4). The ferropicrites are distinguished from other picritic rocks (e.g. Deccan, Hawaii, West Greenland) by their relatively low abundances of Al2O3 (∼10 wt%) and heavy rare-earth elements (HREEs, e.g. Lu=

DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK DEPARTMENT OF PHYSICS, THE UNIVERSITY, NEWCASTLE UPON TYNE NE1 7RU, UK DEPARTMENT OF GEOLOGY, McMASTER UNIVERSITY, 1280 MAIN STREET WEST, HAMILTON, ONT., CANADA L8S 4M1 DEPARTMENTO DE GEOQUIMICA E RECURSOS MINERAIS, INSTITUTO DE GEOCIENCIAS, UNIVERSIDADE DE BRASILIA

The role of mantle plumes in the genesis of continental flood-basalts (CFB) remains controversial, primarily due to our limited knowledge of the composition of the subcontinental lithospheric mantle (SCLM). In this study we use the widespread Cretaceous mafic potassic magmatic rocks, emplaced around the margins of the Paranasedimentary basin, to probe large-scale compositional variations of the SCLM beneath southern Brazil and Paraguay. On the basis of Ti contents, together with major-, trace-element and isotopic ratios, these mafic potassic rocks may be subdivided into high-Ti and low-Ti groups. The former have relatively high average TiO2 (4.64), CaO/Al2O3 (1.74) and 143Nd/144Ndi (0.51232), together with low La/Nb (1.1) and 87Sr/86Sri (0.7050). The latter are characterised by much lower average TiO2 (1.77), CaO/Al2O3 (0.72) and 143Nd/144Ndi (0.51182), together with higher La/Nb (2.01) and 87Sr86Sri (0.7068). These high-Ti and low-Ti groups are spatially separate and their distribution correlates with tectonic setting; the low-Ti magmas are associated with cratonic regions, whereas the high-Ti magmas are in Proterozoic mobile belts. The distribution of the subgroups of mafic potassic magmatic rocks correlates closely with the geochemical provinciality of the Early Cretaceous high-Ti and low-Ti Paranaflood-basalts. This is the first reported occurrence of extensive low-Ti mafic potassic magmatism associated both spatially and temporally with the low-Ti region of a major CFB province. Our study further reveals similar relationships between tectonic setting and the geochemical provinciality of mafic potassic magmas and continental flood-basalts across Gondwana. We use the bulk-rock compositions and radiogenic isotopic ratios of both the high-Ti and low-Ti mafic potassic magmatic rocks as end members in models of CFB genesis. Mixing calculations involving Sr and Nd isotopic ratios indicate that the flood-basalts may contain up to 50% of mafic potassic lithosphere-derived melts. Overall, the results of our geochemical modelling agree with geophysical arguments that the convecting asthenosphere is the predominant source of CFB magmas.

S. A. GIBSON*, R. N. THOMPSON, O. H. LEONARDOS, A. P. DICKIN AND J. G. MITCHELL 'DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DHi 3LE, UK "DEPARTMENTO DE GEOQUIMICA E RECURSOS MINERAIS, INSTITUTO DE GEOCIENCIAS, UNIVERSIDADE DE BRASILIA, 70910 BRASILIA DF, BRAZIL 'DEPARTMENT OF GEOLOGY, McMASTER UNIVERSITY, is8o MAIN STREET WEST, HAMILTON, ONTARIO, CANADA L8S 4M1 'DEPARTMENT OF PHYSICS, THE UNIVERSITY, NEWCASTLE UPON TYNE NEi 7RU, UK.

[1] Analyses of basalts from the Galapagos archipelago and adjacent spreading center have shown that the underlying mantle plume is isotopically heterogeneous over length scales of 10s to 100s of km. We show that the convecting mantle is also compositionally heterogeneous on smaller length scales (kms). Our evidence comes from recent small-volume mildly alkaline and tholeiitic basalts on Isla Santiago, central Galapagos. Flows from volcanic vents

The magmatism occurring when a hot convective mantle plume is sited beneath a lithospheric plate may be more complex if the latter is continental, rather than oceanic, because of the characteristic local physical inhomogeneity of continents. Thus, the surface volcanic expression of the plume (hotspot) may be displaced from immediately above its rising stem, if the continent has been previously locally thinned nearby. The magmatism of the British Tertiary Igneous Province and Parana basin, South America, appears to fit this model, and it may also explain why the Miocene Columbia River basalts of the NW United States overlie an Early Tertiary sedimentary basin.

Minette occurs as sparse dykes and sills in the Upper-Miocene Elkhead Mountains igneous province, NW Colorado. At the time of magma emplacement, the region was undergoing crustal extension and probably also heating of the sub-continental lithospheric mantle by the approaching Yellowstone plume. The predominant magmatism of the province comprises lava field remnants and hypabyssal plutons. Elemental variation within the minette suite is explicable in terms of fractional crystallisation which involved phlogopite separation from even the most magnesian (MgO = 10.68%) samples. Wide ranges of incompatible-element abundances and ratios occur in the minettes with MgO > 6.0%. Some of these ratios (e.g. Ti/Zr andLa/Nb) correlate well with 143 Nd/ 144 Nd in this suite. The minettes have a combination of relatively low values of both 87 Sr/ 86 Sr (0.70387–0.70413) and 143 Nd/ 144 Nd (0.51201–0.51227), with 206 Pb/ 204 Pb (17.28–17.47), 207 Pb/ 204 Pb (15.45–15.54) and 208 Pb/ 204 Pb (36.55–37.02). Taken together, these isotopic characteristics fall far outside the range of all oceanic igneous rocks and therefore rule out an exclusively asthenospheric source for the magmas. Genetic models involving crustal contamination of either basaltic (s.l.) or lamproitic liquids do not appear to explain satisfactorily the geochemical features of the minettes. Alternative models invoke either separate subcontinental lithospheric mantle sources for each magma batch or mixing between upwelling basaltic liquids and varying amounts of ultrapotassic lithospheric melts. Both models fit the geochemical data reasonably well but the latter is, in addition, consistent with a recent analysis by D. McKenzie [8] of the physical constraints on strongly potassic magma genesis during continental lithospheric extension and/or heating above a mantle plume. A brief survey of the tectonic settings of minettes and ultrapotassic rocks worldwide shows that a strong case can be made for their association in space and time with heating and/or thinning of sub-continental lithospheric mantle.

The extent of lithological heterogeneity in the Earth's convecting mantle is highly debated. Whilst the presence of pyroxenite in the mantle source regions of Ocean Island Basalts (OIBs) has traditionally been constrained using the minor-element chemistry of olivine phenocrysts, recent studies have shown that the Ni and Mn contents of primitive olivines are influenced by the conditions of mantle melting, as well as magma chamber processes. Nevertheless, constraining the lithological properties of the mantle is important due to its influence on the P-T path followed by solid mantle material during adiabatic ascent, as well as the density of upwelling mantle plumes. We have therefore explored the use of Fe-isotopes as a novel method of tracing lithological heterogeneity in the mantle source regions beneath plume-influenced segments of the global Mid-Ocean Ridge system as well as OIBs. We present new Fe-isotope ( δ 56 Fe) and trace-element data for 26 basaltic glasses from the plume-influenced Galapagos Spreading Centre to investigate the relative roles of pyroxenite and peridotite in the mantle source region of oceanic basalts. Our data reveals significant heterogeneity in the Fe-isotope composition of the Galapagos Spreading Centre basalts (+0.05 to +0.25‰ δ 56 Fe), which correlates with key major- and trace-element parameters (e.g. CaO(8)/Al2O3(8), [La/Sm]n). Application of new models developed to calculate Fe-isotope fractionation during mantle melting, alongside Monte Carlo simulations for melting of a 2-component peridotite mantle, show that this variation cannot be caused by changes in melting processes and/or oxygen fugacity of a peridotitic mantle. Instead, our new δ 56 Fe data is best explained by variations in the proportion of isotopically-heavy pyroxenite-derived melt that contributes to the GSC basalts, and conclusively shows that lithological heterogeneity exists in the Galapagos mantle plume. Our findings have implications for the moderately-heavy δ 56 Fe compositions measured in plume-influenced basalts from the Society Islands, Rochambeau Ridges of the Lau back-arc basin, and the FAMOUS segment of the Mid-Atlantic Ridge, which we suggest may also represent contribution from pyroxenite-derived melts.

The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of magma storage regions located directly above high buoyancy flux mantle plumes (such as Kīlauea, Hawai’i and Fernandina, Galápagos) are relatively well understood. However, far fewer constraints exist on the nature of magma storage beneath ocean island volcanoes that are distal to the main zone of mantle upwelling or above low buoyancy flux plumes, despite these systems representing a substantial proportion of ocean island volcanism globally. To address this, we present a detailed petrological study of Isla Floreana in the Galápagos Archipelago, which lies at the periphery of the upwelling mantle plume and is thus characterized by an extremely low flux of magma into the lithosphere. Detailed in situ major and trace element analyses of crystal phases within exhumed cumulate xenoliths, lavas and scoria deposits indicate that the erupted crystal cargo is dominated by disaggregated crystal-rich material (i.e. mush or wall rock). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveal that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within crystal-rich magma storage regions. In addition, application of three petrological barometers reveals that the Floreana mush zones are located in the upper mantle, at a depth of 23·7 ± 5·1 km. Our barometric results are compared with recent studies of high melt flux volcanoes in the western Galápagos, and other ocean island volcanoes worldwide, and demonstrate that the flux of magma from the underlying mantle source represents a first-order control on the depth and physical characteristics of magma storage.

Funder: Royal Commission for the Exhibition of 1851; Id: http://dx.doi.org/10.13039/501100000700, Funder: University of Cambridge; Id: http://dx.doi.org/10.13039/501100000735, Funder: Geological Society of London; Id: http://dx.doi.org/10.13039/100008066, Geophysical analysis of the Earth's lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core‐mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep‐seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep‐seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in ocean island basalts provides constraints on the structure of the Earth's lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high‐MgO basalts, that the isotopically enriched south‐western region of the Galápagos mantle—assigned to melting of LLSVP material—displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north‐central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.