Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21c1680t&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P21C-1680
Physics
[3954] Mineral Physics / X-Ray, Neutron, And Electron Spectroscopy And Diffraction, [5410] Planetary Sciences: Solid Surface Planets / Composition, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [6296] Planetary Sciences: Solar System Objects / Extra-Solar Planets
Scientific paper
Carbon is one of the major volatile elements and very important in the Earth, primitive meteorites and some achondrites, such as ureilites. The abundance of carbon has been estimated to be 100 times higher than that in the CI chondrite, in some of the stars with exoplanets, such as the circumstellar gas around Beta Pictoris (Roberge et al., 2006). In such a gas, carbon-enriched planets, "carbon-planet", may be formed. Carbon-planet interior is likely to be composed mainly of Carbon-bearing phase, such as carbide, carbonate, graphite and diamond. Therefore, it is important to investigate phase relations of carbon-rich systems under high pressure conditions. In this study, C-enriched Mg-Si-Fe-O system was investigated at high pressure and temperature in order to understand the internal structure of the carbon-planets. Phase relations were studied based on 2 series of experiments; (I) textural observation and chemical analysis of the sample recovered from high pressure and temperature and (II) in situ X-ray diffraction experiments. We used several different mineral assemblages for the starting materials, as shown below: (i) (Mg1.8,Fe0.2)SiO4 + Fe + SiO2 + C, (ii) (Mg1.8,Fe0.2)SiO4 + Fe + Si + C, (iii) MgO + Fe + SiO2 + C, (iv) MgO + Fe + Si + C. Oxygen fugacity (fO2) of the sample varies depending on these assembleges due to different O amounts in the starting materials. Chemical analyses of the recovered samples were performed using an electron microprobe. In situ X-ray diffraction experiments were conducted at 4 and 15 GPa, and up to 1873 K at BL04B1 beamline, SPring-8 synchrotron facility. Different mineral assemblages were observed depending on the redox condition of the sample. The compositions of metallic melts changes from Fe-C compositions in oxidizing conditions to Fe-Si compositions in the reducing conditions. Based on in situ X-ray diffraction experiments at 4 GPa, FeSi and SiC peaks appeared at 1373 K in the most reducing sample (iv), whereas Fe3C appeared in the other samples. Metallic phases in all samples were melted at 1673 K. In the experiments about the starting materials (i), (ii) and (iv) at 15 GPa, Fe3C was formed at about 1073 K and Fe7C3 was formed at 1273 K. In the samples (ii) and (iv), FeSi was formed with Fe7C3 at 1273 K. This indicates that carbon reacts with metallic iron and any carbonates were not observed under the present experimental conditions. Therefore, these results may suggest that carbon exists in the mantle and/or core in some carbon-planets and carbon-bearing phases could be graphite/diamond, SiC and Fe-C alloy or Fe-Si-C alloy depending on the redox conditions at high pressure.
Funakoshi Kenichi
Higo Yutaka
Ito Yukari
Ohtani Eiji
Takahashi Saburo
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