54 research outputs found

    Oxygen isotope heterogeneity of the mantle beneath the Canary Islands : insights from olivine phenocrysts

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    Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 162 (2011): 349-363, doi:10.1007/s00410-010-0600-5.A relatively narrow range of oxygen isotopic ratios (δ18O = 5.05.4‰) is preserved in olivine of mantle xenoliths, mid-ocean ridge (MORB) and most ocean island basalts (OIB). The values in excess of this range are generally attributed either to the presence of a recycled component in the Earth’s mantle or to shallow level contamination processes. A viable way forward to trace source heterogeneity is to find a link between chemical (elemental and isotopic) composition of the earlier crystallized mineral phases (olivine) and the composition of their parental magmas, then using them to reconstruct the composition of source region. The Canary hotspot is one of a few that contains ~1-2 Ga old recycled ocean crust that can be traced to the core-mantle boundary using seismic tomography and whose origin is attributed to the mixing of at least three main isotopically distinct mantle components i.e., HIMU, DMM and EM. This work reports ion microprobe and single crystal laser fluorination oxygen isotope data of 148 olivine grains also analyzed for major and minor elements in the same spot. The olivines are from 20 samples resembling the most primitive shield stage picrite through alkali basalt to basanite series erupted on Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro, Canary Islands, for which shallow level contamination processes were not recognized. A broad range of δ18Oolivine values from 4.6 to 6.1‰ was obtained and explained by stable, long-term oxygen isotope heterogeneity of crystal cumulates present under different volcanoes. These cumulates are thought to have crystallized from mantle derived magmas uncontaminated at crustal depth, representing oxygen isotope heterogeneity of source region. A relationship between Ni×FeO/MgO and δ18Oolivine values found in one basanitic lava erupted on El Hierro, the westernmost island of the Canary Archipelago, was used to estimate oxygen isotope compositions of partial melts presumably originated from peridotite (HIMU-type component inherited its radiogenic isotope composition from ancient, ~12 Ga, recycled ocean crust) and pyroxenite (young, <1 Ga, recycled oceanic crust preserved as eclogite with depleted MORB-type isotopic signature) components of the Canary plume. The model calculations yield 5.2 and 5.9±0.3‰ for peridotite and pyroxenite derived melts, respectively, which appeared to correspond closely to the worldwide HIMU-type OIB and upper limit N-MORB δ18O values. This difference together with the broad range of δ18O variations found in the Canarian olivines cannot be explained by thermodynamic effects of oxygen isotopic fractionation and are believed to represent true variations in the mantle, due to oceanic crust and continental lithosphere recycling.This work was supported by the CNRS “poste rouge” grant to AG, the NSF EAR-CAREER-0844772 grant to IB and the CRPG-CNRS and at its initial stage by the DFG (grant SCHM 250/64) and the Alexander von Humboldt Foundation (Wolfgang Paul Award to A.V. Sobolev who provided access to the electron microprobe at the Max Planck Institute, Mainz, Germany)

    Moonage daydreams of space rock

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    Halogens in chondritic meteorites and terrestrial accretion

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    <p>Volatile element delivery and retention played a fundamental role in Earth’s formation and subsequent chemical differentiation. The heavy halogens (Cl, Br and I) are key tracers of accretionary processes due to their high volatility and incompatibility, but have low abundances in most geological and planetary materials. Noble gas proxy isotopes produced during neutron irradiation provide a high sensitivity tool for the determination of heavy halogen abundance. Here we show that Cl, Br and I abundances in carbonaceous, enstatite, Rumuruti and primitive ordinary chondrites have concentrations ~6, ~9 and between 15-37 times lower, respectively, than previously reported and most commonly accepted estimates1. This is independent of the chondrites’ oxidation state or petrological type. Bromine/Cl and I/Cl in all studied chondrites show a limited range, indistinguishable from bulk silicate Earth (BSE) estimates. Our results demonstrate that BSE depletion of halogens relative to primitive meteorites is now consistent with lithophile elements of similar volatility. The new results for carbonaceous chondrites demonstrate that late accretion, constrained to a maximum of 0.5 ± 0.2 % of Earth’s silicate mass2–5, cannot solely account for present-day terrestrial halogen inventories6,7. It is estimated that 80−90% of heavy halogens are concentrated in Earth’s surface reservoirs7,8 and have not undergone the extreme early loss observed in atmosphere-forming elements9. Therefore, in addition to late accretion of halogens and mantle degassing, which is <50% efficient over Earth history10, efficient extraction of halogen-rich fluids6 from the solid Earth during the earliest stages of Earth formation is also required. The hydrophilic nature of the halogens supports this requirement, and is consistent with a volatile/water rich late-stage terrestrial accretion</p

    Asteroids and andesites

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    Ubiquitous late veneer

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    Iron fog of accretion

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