Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p14a..02s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P14A-02
Other
[1027] Geochemistry / Composition Of The Planets, [1060] Geochemistry / Planetary Geochemistry, [6200] Planetary Sciences: Solar System Objects
Scientific paper
The Earth is depleted in moderately volatile elements relative to CI chondrites and thus average solar system material. The timing of this depletion has been a matter of debate. Isotopic constraints from the short-lived Pd-Ag, Mn-Cr and Hf-W decay systems can be used to model the accretion history of the Earth and the timing of moderately volatile element depletion [1]. While the Pd-Ag decay system provides evidence for the accretion of volatile-rich material, other systems like Mn-Cr and Rb-Sr require that the Earth accreted volatile-depleted material [2, 3]. As recently shown [1], the contrasting evidence from these decay systems can be reconciled by heterogeneous accretion, which implies that the composition of the material from which the Earth accreted changed over time. A continuous core formation model was used and the best match was obtained for the Earth mainly accreting volatile-depleted material in the beginning and more volatile-rich material towards the end, while core formation was still ongoing [1]. However, a different study proposed that the bulk of the moderately volatile elements was delivered in a volatile-rich late veneer after core formation ceased [4]. This is not supported by the Pd-Ag data (Ag is a moderately volatile element, while Pd is more refractory). A late veneer of volatile-rich CI material (Pd = 556 ppb and Ag = 197 ppb) after core formation is limited to a maximum of ~0.4 % of the Earth's mass by the Pd concentration of the Earth's mantle today (~3.3 ppb). This amount of CI material does not supply enough Ag to substantially modify the Ag isotope composition of the Earth's mantle. In a scenario where the Earth accretes exceedingly volatile depleted material, its high Pd/Ag ratio would lead to an extreme radiogenic Ag isotope composition of the bulk silicate Earth (BSE), which cannot be counterbalanced by the late veneer to match the observed BSE composition. We also tested the heterogeneous accretion scenario using N-body accretion simulations [5] for the Pd-Ag decay system. Again the best results were obtained when materials with different degrees of volatile depletion (= different Pd/Ag ratios) were accreted. The simulations include early accretion of close-in material and later accretion of material from greater heliocentric distances, which is consistent with a transition from volatile-depleted to volatile-enriched material. Therefore, N-body accretion simulations and the continuous core formation model yield similar results, which demonstrates the robustness of the heterogeneous accretion scenario. [1] Schönbächler et al. (2010), Science 328, 884. [2] Carlson & Lugmair (1988), Earth Planet Sci. Lett. 90, 119. [3] Qin et al. (2009), Geochim. Cosmochim. Acta 74, 1122. [4] Albarède (2009), Nature, 461, 1227.[5] O'Brien et al. (2006), Icarus 184, 36.
Nimmo Francis
Schönbächler Maria
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