Comment on “High-pressure melting experiments on garnet clinopyroxenite and the alkalic to tholeiitic transition in ocean-island basalts” by Keshav et al. [Earth Planet. Sci. Lett. 223 (2004) 365–379] (original) (raw)
Related papers
Earth and Planetary Science Letters, 2008
Uranium series disequilibria in ocean island basalts (OIB) provide evidence for the presence of garnet in their source region. It has been suggested that enriched OIB signatures derive from mantle lithologies other than peridotite, such as eclogite or pyroxenite, and, in particular, that silica-poor garnet pyroxenite is the source lithology for alkali basalts. To test the ability of such a source to produce the U-Th disequilibria observed in alkali OIB, we determined experimentally clinopyroxene-melt and garnetmelt partition coefficients for a suite of trace elements, including U and Th, at 2.5 GPa and 1420-1450°C. The starting composition for the experiments was a 21% partial melt of a silica-poor garnet pyroxenite. Experimentally determined clinopyroxene-melt partition coefficients range from 0.0083 ± 0.0006 to 0.020 ± 0.002 for Th and from 0.0094 ± 0.0006 to 0.024 ± 0.002 for U, and garnet-melt partition coefficients are 0.0032 ± 0.0004 for Th and 0.013 ± 0.002 for U. Comparison of our experimental results with partition coefficients from previous experimental studies shows that the relative compatibilities of U and Th in both garnet and clinopyroxene are different for different mineral compositions, leading to varying degrees of U/Th fractionation with changing lithology. For a given melting rate and extent of partial melting, mafic lithologies tend to produce larger 230 Th excesses than peridotite. However, this effect is minimized by the greater overall extents of melting experienced by eclogites and pyroxenites relative to peridotite. Results from chromatographic, batch, and fractional melting calculations with binary mixing between partial melts of pyroxenite and peridotite, carried out using our new partitioning data for the pyroxenite component and taking into account variable productivities and different solidus depths for the two lithologies, suggest that OIB are not the product of progressive melting of a source containing a fixed quantity of garnet pyroxenite. Melting a peridotite with enriched signatures, and mixing those melts with melts of a depleted, "normal" peridotite, is an alternative explanation for the trends seen in Hawaiian, Azores and Samoan lavas.
Chemical characteristics of island-arc basalts: Implications for mantle sources
Chemical Geology, 1980
Perfit, M.R., Gust, D.A., Bence, A.E., Arculus, R.J. and Taylor, S.R., 1980. Chemical characteristics of island-arc basalts: implications for mantle sources. In: R.W. Le Maitre and A. Cundari (Guest-Editors), Chemical Characterization of Tectonic Provinces. Chem. Geol., 30: 227--256.
Earth and Planetary Science Letters, 2007
Variations of high-field strength element (HFSE) ratios in terrestrial reservoirs, in particular Zr/Hf and Nb/Ta, are critical for understanding crust-mantle differentiation. Growing experimental and observational evidence shows that these ratios are fractionated during magmatic processes, despite their very similar geochemical characteristics. Here we present new high-precision Nb, Ta, Zr, Hf and Lu measurements for a variety of ocean island basalts determined by isotope dilution MC-ICPMS together with Hf isotope compositions in order to constrain OIB source characteristics and HFSE fractionation during mantle melting and crystal fractionation. Observed variations in Zr/Hf are larger than expected from fractional crystallisation alone. Partial melting of garnet and/or spinel peridotite assemblages can produce the observed range in Zr/Hf and Nb/Ta ratios, but require the presence of grossular-rich garnet, i.e. of recycled eclogite or garnet pyroxenite in the source of OIBs. This is consistent with Lu/Hf ratios that are lower in OIBs than expected from partial melting of pure garnet peridotite sources. Nb/Ta ratios in terrestrial reservoirs can be used to place constraints on crust-mantle differentiation and mantle evolution since the Archean. The average Nb/Ta in the OIB source region (15.9 ± 0.6 (1σ)) is identical to values observed in many MORB suites, but higher than the ratio of the bulk silicate Earth (∼ 14) and the estimate for the continental crust (∼ 12-13). Despite the inferred presence of recycled eclogite in OIB sources, which had previously been postulated to be a potential reservoir with superchondritic Nb/Ta ratios, their Nb/Ta ratios are invariably subchondritic and therefore provide no evidence for the existence of a silicate reservoir with superchondritic Nb/Ta in the Earth's mantle, and also exclude significant contributions from core material with superchondritic Nb/Ta ratios. The complementary Nb/Ta ratios in the Earth's crust and mantle with respect to bulk silicate Earth can be explained by partial melting of amphibolite bearing slabs with bulk D Nb/Ta N 1 during crust-mantle differentiation. As melting of subducted amphibolites was probably most intense during the Archean, major portions of the continental crust may have formed early in Earth's history. Such a model is consistent with Nb/Ta ratios in Archean rocks and with 142 Nd and 176 Hf/ 177 Hf evidence for early Earth differentiation.
Alkali basalts from the Kerguelen islands have entrained many mantle peridotites (harzburgites and dunites) in addition to various other ultramafic and mafic xenoliths. The harzburgites and the dunites were equilibrated in the spinel peridotite stability field (T = 850-1150°C). They attest to the existence of a metasomatized refractory upper mantle beneath the southeastern province of the Kerguelen islands. To date no fertile mantle lherzolite has been found in this area.
Journal of Asian Earth Sciences, 2011
Picrite basalt tuffs and lavas from the Miocene basalt plateau of Vitim (Trans Baikal, Russia) contain abundant megacrysts and varied pyroxenite and mantle lherzolite xenoliths (spinel facies and upper part of the garnet-facies) and crustal cumulates. Black pyroxenites and megacrysts show decreasing temperatures from 1350 to 900°C, and range from high-T dark green websterites and clinopyroxenites, to low-T black megacrystalline garnet clinopyroxenites and phlogopite-ilmenite-bearing varieties. Garnet-bearing Cr-diopside veins and zoned veins with mica and rare amphiboles cross-cut peridotite xenoliths. Veins consisting of almost pure amphibole are more common in spinel lherzolite xenoliths. P-T calculations for pyroxenites yield pressure intervals at 3.3-2.3, 2.2-2.0, 1.9-1.5 and 1.3-1.0 GPa, probably corresponding to the locations of dense magmatic vein networks in mantle.
The metasomatic alternative for ocean island basalt chemical heterogeneity
Earth and Planetary Science Letters, 2005
Subduction of oceanic lithosphere is thought to be responsible for producing heterogeneity in Earth's mantle; the heterogeneity is most clearly preserved in the compositions of ocean island basalts (OIB). The variation of trace element and isotopic ratios in OIB is commonly explained by recycling of ancient oceanic crust associated with terrigenous or pelagic sediment. However a variety of chemical and physical arguments seem to indicate that subducted oceanic crust is not the source of OIB. In particular, experimental petrologic studies indicate that the most plausible source for OIB is the partial melting of peridotite in the presence of CO 2 or silica-deficient pyroxenites. Alternative hypotheses for the source of OIB are subducted oceanic basal lithosphere enriched by metasomatic liquids and delaminated metasomatised continental lithosphere. Lithospheric metasomatism is common to both hypotheses and could produce silica-deficient pyroxenite; however, the exact chemical and physical nature of the process and how it leads to chemical variations in recycled lherzolite that produce the various OIB isotopic end-member bsignaturesQ is unclear. Chemical variations observed in the Cantal basalt (France) are interpreted as the result of a lithospheric metasomatic mechanism and provide important new constraints on the nature of this metasomatic process. The Cantal basalts, similar to OIB in composition, show unusual variations in Nb / Th, Nb / U, La / Nb and Ce / Pb ratios from the first (13-9 Ma) to the last (9-3 Ma) emitted basalt. The basalts are homogeneous with respect to their Sr, Nd, and Pb isotopic composition, ruling out variable sediment contamination of their mantle sources. We postulate that these trace element variations result from an evolution of metasomatic vein compositions present in a vein plus enclosing lithospheric mantle source.
Mantle refertilization by melts of crustal-derived garnet pyroxenite
HAL (Le Centre pour la Communication Scientifique Directe), 2015
Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at $ 1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al 2 O 3 , CaO, Sm/Yb and FeO n at a given SiO 2. Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt-peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustalderived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite-peridotite interaction, such as secondary spinel lherzolite in Ronda, may nucleate magmatic channels that remain chemically isolated from the ambient mantle and act as preferential pathways for melts with the signature of recycled crust.
Earth and Planetary Science Letters, 2009
... The pyroxenite data come from compilation of Hirschmann and Stolper (1996) completed with analyses from (Becker, 1996), (Bodinier et al., 2008), (Dessai et al ... Kumar et al., 1996), (Kuno and Aoki, 1970), (Lee et al., 2006), (Liu et al., 2005), (Melcher et al., 2002), (Porreca et al ...
Petrogenesis of rhyolite-trachyte-basalt composite ignimbrite P1, Gran Canada, Canary Islands
Journal of Geophysical Research, 1995
The 14 Ma caldera-forming composite ignimbrite PI on Gran Canaria (Canary Islands) represents the first voluminous eruption of highly differentiated magmas on top of the basaltic Miocene shield volcano. Compositional zonation of the ignimbrite is the result of vertically changing proportions of four component magmas, which were intensely mixed during eruption: (I) Crystal-poor to highly phyric rhyolite (-10 km 3), (2) sodic trachyandesite through mafic to evolved trachyte (-6 km 3), (3) Nafoor trachyandesite (<I km 3), and (4) basalt zoned from 5.2 to 4.3 wt% MgO (-26 km). PI basalt is composed of two compositionally zoned magma batches, B2 basalt and B3 basalt. B3 basalt is derived from a mantle source depleted in incompatible trace elements compared to the shield basalt source. Basaltic magmas were stored in a reservoir probably underplating the crust, in which zoned B2 basaltic magma formed by mixing of "enriched" (shield) and "depleted" (B3) mafic melts and subsequent crystal fractionation. Evolved magmas formed in a shallow crustal chamber, whereas intermediate magmas formed at both levels. Abundant pyroxenitic to gabbroid cumulates in PI support crystal fractionation as the major differentiation process. On the basis of major and trace element modeling, we infer two contemporaneous fractional crystallization series: series I from "enriched" shield basalt through Na-poor trachyandesite to rhyolite, and series II from "depleted" PI basalt through sodic trachyandesite to trachyte. Series II rocks were significantly modified by selective contamination involving feldspar (Na, K, Ba, Eu, Sr), zircon (Zr) and apatite (P, Y, rare earth elements) components; apatite contamination also affected series I Na-poor trachyandesite. Substantial sodium introduction into sodic trachyandesite is the main reason for the different major element evolution of the two series, whereas their different parentage is mainly reflected in the high field strength trace elements. Selective element contamination involved not only rapidly but also slowly diffusing elements as well as different saturation conditions. Contamination processes thus variably involved differential diffusion, partial dissolution of minerals, partial melt migration, and trace mineral incorporation. Magma mixing between trachyte and rhyolite during their simultaneous crystallization in the PI magma chamber is documented by mutual mineral inclusions but had little effect on the compositional evolution of both magmas. Fe-Ti oxide thermometry yields magmatic temperatures of around 850°C for crystal-poor through crystal-rich rhyolite,-815°C for trachyte and-850°-900°C for the trachyandesitic magmas. High 1160°C for the basalt magma suggest its intrusion into the PI magma chamber only shortly before eruption. The lower temperature for trachyte compared to rhyolite and the strong crustal contamination of trachyte and sodic trachyandcsite support their residence along the walls of the vertically and laterally zoned PI magma chamber. The complex magmatic evolution of PI reflects the transient state of Gran Canaria's mantle source composition and magma plumbing system during the change from basaltic to silicic volcanism. Our results for PI characterize processes operating during this important transition, which also occurs on other volcanic ocean islands. Introduction and Geologic Setting During the major, Miocene magmatic cycle [Schmincke, t976, 1982, t990] on the volcanic ocean island of Gran Can aria (28'00'N, l5°35'W) in the Canarian archipelago (east central Atlantic), a major change from basaltic to silicic