• Jennings, E. S.
• Zhao, X.
• Blanchard, I.
• Rubie, D. C.
• Franchi, I.
• Petitgirard, S.

Abstract

Carbon is an element of great importance in the Earth, because it is intimately linked to the presence of life at the surface, and, as a light element, it may contribute to the density deficit of the Earth's iron-rich core. Carbon is siderophile at low pressures and temperatures, as shown by a very high metal-silicate partition coefficient (D<SUP>met-sil </SUP>&gt; 100, Dasgupta et al., 2013). Based on this behavior, it should be stored mainly in the Earth's core. Nevertheless, we still observe the existence of carbon at the surface, stored in crustal rocks and associated with the presence of life, and in the mantle, as shown by the exhumation of diamonds. The presence of carbon in the crust and mantle could be the result of the arrival of carbon during late accretion, after the process of core formation ceased, or because of a change in its metal-silicate partitioning behavior at the conditions of core formation (e.g. P &gt;40 GPa and T &gt;3500 K). Previous studies reported metal-silicate partitioning of carbon based on experiments using large volume presses up to 8 GPa and 2200°C (Li et al., 2016). In this study, we have performed the first laser-heated diamond anvil cell (LH-DAC) experiments in order to determine carbon partitioning between liquid metal and silicate at the extreme conditions of Earth's core-mantle differentiation. We performed 6 successful metal-silicate partitioning experiments between 41 and 71 GPa and 3500 and 4100 K. We recovered our samples using the Focused Ion Beam technique and welded a 2-3 μm thick slice of each sample onto a TEM grid. Major elements were analyzed by electron microprobe, whereas the concentrations of carbon in the silicate were analyzed by nanoSIMS. We thus have obtained metal-silicate partitioning results for carbon at PT conditions relevant to planetary core formation, where C remains strongly siderophile in all experiments. The measured <SUP>12</SUP>C/<SUP>13</SUP>C ratio also indicates that carbon contamination is common in such DAC experiments. We have integrated our results into state-of-the-art core formation models (Rubie et al., 2015, 2016) in order to determine carbon concentrations in the core and bulk silicate Earth (BSE) at the end of accretion. Dasgupta et al. (2013), GCA 102, 191-212 Li et al. (2016), Nature Geoscience 9 (10), 781 Rubie et al. (2015), Icarus 248, 89-108 Rubie et al. (2016), Science 353 (6304), 1141-1144...

Topics

• density
• impedance spectroscopy
• surface
• Carbon
• experiment
• focused ion beam
• transmission electron microscopy
• iron