Metal-silicate Partitioning of Carbon to 59 GPa and >5000 K with Implications for Earth's Core Formation
Abstract
Earth's core is 10% less dense than pure iron under the same conditions, implying the presence of light elements in the core, such as Si, S, O, and C. The core abundances of these elements were primarily controlled by partitioning between silicate and metallic liquids during core formation, and have implications for Earth's formation, bulk composition, thermal structure, core dynamics, and magnetic field generation. Previous studies of the metal-silicate partitioning of carbon have revealed siderophile behavior, with log10(D) in the range of 2-4 [ref 1-5] (where the partition coefficient D = wt%C(metal) / wt%C(silicate)); however, the highest pressure (P) and temperature (T) at which data were previously obtained was 8 GPa and 2473 K [ref 5], significantly lower than the average P-T of core formation in the Earth. We performed a suite of C-bearing metal-silicate partitioning experiments, using a laser-heated diamond anvil cell to achieve pressures of 37-59 GPa and temperatures of up to >5000 K. The recovered samples were polished and analyzed for major and minor elements using electron microprobe and for carbon using nanoSIMS. The nanoSIMS data reveal >2 wt% carbon in the silicate melts of all experiments, indicating a greatly increased solubility of carbon in silicate melts at high P-T. At these P-T conditions, we find log10(D) in the range 0-1, in agreement with a previous ab initio calculation [6], indicating only slightly siderophile behavior of carbon. Combining our data with those of previous studies [1-5] shows that D decreases significantly with both P and T, and we have preliminarily parameterized log10(D) as a function of P and T. Using this preliminary parameterization, for a single-stage core formation model at 54 GPa and 3350 K [ref 7], we find D=3. For a mantle carbon concentration of 100 ppm [ref 8], the core should contain 300 ppm C, implying that carbon is not an important light element in Earth's core. We will present more detailed models of core formation and discuss implications for the carbon content and C/S ratio of the Earth.
[1] Chi et al. (2014) GCA. [2] Stanley et al. (2014) GCA. [3] Dasgupta et al. (2013) GCA. [4] Li et al. (2015) EPSL. [5] Li et al. (2016) NatGeo. [6] Zhang and Yin (2012) PNAS. [7] Fischer et al. (2015) GCA. [8] Palme and O'Neill (2003) ToG.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI31A..02F
- Keywords:
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- 1015 Composition of the core;
- GEOCHEMISTRYDE: 1030 Geochemical cycles;
- GEOCHEMISTRYDE: 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 3621 Mantle processes;
- MINERALOGY AND PETROLOGY