[S-IT16] Deep Earth Sciences

[SIT16-P16] Chemical species of carbon in a deep magma ocean

*Haruki Takezawa¹, Kei Hirose¹, Hirotoyo Nakashima², Masao Takeyama² (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology)

Keywords:high pressure, DAC, magma ocean

Chemical species of carbon in a deep magma ocean under high pressure and temperature (P-T) is not understood yet. It is a key to understanding a chemical reaction regarding the metal-silicate partitioning of carbon and the occurrence of carbon in the mantle after the solidification of a magma ocean. We performed a series of melting experiments on carbon-bearing basaltic materials at about 30-50 GPa and 3500-4000 K using a laser-heated diamond-anvil cell (DAC), which corresponds to conditions at the base of a magma ocean where core-forming metals segregated from silicate. In order to vary the oxygen fugacity of the experiments, we added carbon to the sample as calcium carbonate or graphite with/without iron metal. We also employed a water-bearing basaltic glass as starting material in some runs. The chemical species of carbon dissolved in partial melts was examined on recovered samples based on the soft X-ray emission spectroscopy and the Raman spectroscopy.
The results demonstrate that carbon is dissolved in the silicate melts mainly as elemental carbon under these high P-Tconditions regardless of the oxygen fugacity in the present experiments. It is true for the experiments using the water-bearing sample. The shape of the carbon-related peak in the soft X-ray emission spectra is similar to that of graphite and certainly different from that of diamond, which possibly implies that the local structure of carbon in silicate glasses changes upon decompression from ~3050 GPa. The Raman spectra also showed the intense peaks that are assigned to disordered graphite along with a weak peak that corresponds to carbonate. These results suggest that the incorporation of carbon from silicate into metal in a deep magma ocean is not a redox reaction and thus depends little on the oxygen fugacity.