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Title: Composition of the core from gallium metal–silicate partitioning experiments

We present gallium concentration (normalized to CI chondrites) in the mantle is at the same level as that of lithophile elements with similar volatility, implying that there must be little to no gallium in Earth's core. Metal-silicate partitioning experiments, however, have shown that gallium is a moderately siderophile element and should be therefore depleted in the mantle by core formation. Moreover, gallium concentrations in the mantle (4 ppm) are too high to be only brought by the late veneer; and neither pressure, nor temperature, nor silicate composition has a large enough effect on gallium partitioning to make it lithophile. We therefore systematically investigated the effect of core composition (light element content) on the partitioning of gallium by carrying out metal–silicate partitioning experiments in a piston–cylinder press at 2 GPa between 1673 K and 2073 K. Four light elements (Si, O, S, C) were considered, and their effect was found to be sufficiently strong to make gallium lithophile. The partitioning of gallium was then modeled and parameterized as a function of pressure, temperature, redox and core composition. A continuous core formation model was used to track the evolution of gallium partitioning during core formation, for various magma ocean depths, geotherms, coremore » light element contents, and magma ocean composition (redox) during accretion. The only model for which the final gallium concentration in the silicate Earth matched the observed value is the one involving a light-element rich core equilibrating in a FeO-rich deep magma ocean (>1300 km) with a final pressure of at least 50 GPa. More specifically, the incorporation of S and C in the core provided successful models only for concentrations that lie far beyond their allowable cosmochemical or geophysical limits, whereas realistic O and Si amounts (less than 5 wt.%) in the core provided successful models for magma oceans deeper that 1300 km. In conclusion, these results offer a strong argument for an O- and Si-rich core, formed in a deep terrestrial magma ocean, along with oxidizing conditions.« less
Authors:
 [1] ;  [2] ;  [1] ;  [3]
  1. Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Universite Paris Diderot, CNRS, Paris (France)
  2. Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Universite Paris Diderot, CNRS, Paris (France); Ecole Polytechnique Federale de Lausanne (Switzerland). Earth and Planetary Science Laboratory
  3. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); Institut de Physique du Globe de Paris, Sorbonne Paris Cite, Universite Paris Diderot, CNRS, Paris (France)
Publication Date:
Report Number(s):
LLNL-JRNL-690580
Journal ID: ISSN 0012-821X
Grant/Contract Number:
AC52-07NA27344
Type:
Accepted Manuscript
Journal Name:
Earth and Planetary Science Letters
Additional Journal Information:
Journal Volume: 427; Journal Issue: C; Journal ID: ISSN 0012-821X
Publisher:
Elsevier
Research Org:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; gallium partitioning; light elements; accretion and differentiation of the Earth; deep magma ocean
OSTI Identifier:
1357397