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The formation of nuggets of highly siderophile elements in quenched silicate melts at high temperatures: Before or during the silicate quench?

Abstract : The Highly Siderophile Elements (HSE) are powerful tracers of planetary differentiation. Despite the importance of their partitioning between silicate and metal for the understanding of planetary core formation, especially for the Earth and Mars, there is still a huge discrepancy between conclusions based on different high temperature (HT) experimental studies. These disagreements may be due to the presence of HSE micro and nanonuggets in HT experiments. The formation of these nuggets is still interpreted in different ways. One hypothesis is that these HSE nuggets formed during the quench of the silicate melt, while another hypothesis supposes that these nuggets formed before the quench and represented artefacts of HT experiments. The goal of this work is to clarify whether the presence of HSE nuggets in silicate melts is linked to a quench effect or not. Understanding the formation of these HSE nuggets represents thus a necessary step towards the resolution of the Earth's core formation scenarios. We performed new HT experiments (1275–2000 • C) at different oxygen fugacities (fO 2), between ambient air up to ∼5 log units below the Iron-Wüstite buffer [IW-5], for two different silicate compositions (synthetic martian and terrestrial basalts) mixed with a metallic mixture of Pt–Au– Pd–Ru. Our 1275–1600 • C experiments were contained in either olivine, diopside or graphite crucible; experiments at 2000 • C were performed using a levitation method, so no capsule was necessary. Our samples contained quenched silicate melts, minerals (olivine, pyroxene, spinel depending on the run), a two-phase metallic bead and nano and micro-nuggets of HSE. Our samples underwent fine textural, structural and analytical characterizations. The distribution of the nuggets was not homogeneous throughout the quenched silicate melt. HSE nuggets were present within crystals. Dendritic textures from the quenched silicate melt formed around HSE nuggets, which could be crystallized, showing that the nuggets acted as nucleation sites during the quench. Thus they predated the quench. Finally, these nuggets also had strong heterogeneities suggesting at least a two-stage formation process under reducing conditions. Consequently, our observations clearly show that these HSE nuggets formed before the quench in the silicate melt. Our results agreed with previous studies, which concluded that HSE abundances in the Earth's mantle require the late accretion of chondritic material subsequent to core formation. However, the effects of metallic Si, O, H, or the effect of pressure on the HSE partitioning are still not fully understood. Further work to constrain these effects is to be encouraged to understand the Earth's core formation.
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Submitted on : Tuesday, June 28, 2016 - 8:12:09 AM
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Valérie Malavergne, E Charon, J Jones, P Cordier, K Righter, et al.. The formation of nuggets of highly siderophile elements in quenched silicate melts at high temperatures: Before or during the silicate quench?. Earth and Planetary Science Letters, Elsevier, 2016, 434, pp.197-207. ⟨10.1016/j.epsl.2015.11.037⟩. ⟨cea-01338138⟩



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