HIGH PRESSURE, NEAR-LIQUIDUS PHASE RELATIONS IN Fe-C-S SYSTEMS AND IMPLICATIONS FOR COMPOSITION, STRUCTURE, AND PROCESS OF FORMATION OF METALLIC CORES IN PLANETARY

Introduction: The Earth’s outer core is known to be ~10% less dense than pure metallic Fe-Ni liquid [1, 2] and thus is thought to contain ~10% light elements [e.g., 1] with proposed candidates including sulfur (S), carbon (C), oxygen (O), hydrogen (H), and silicon (Si). The presence of minor, lighter alloying elements is also argued for metallic cores of other planetary bodies including inner terrestrials planets such as Mars and Mercury [e.g., 3,4] and for bodies beyond the asteroid belt including Jupiter’s moons Io, Ganymede, and Europa [5,6]. While the light element composition in cores of various planetary bodies remains uknown, it is likely that more than one light alloying element is present [7,8]. But, experimental data on the effect of light elements on melting relations of iron in multicomponent systems and the mutual solubility of various light elements in molten Fe has been limited. Previous experiments in multi-component systems have explored the phase relations in Fe-S-Si, Fe(±Ni)-S-O, and Fe(±Ni)-C-S. To add to the discussion on the mutual compatibility of carbon and sulfur in the metallic core of planetary bodies and to investigate the crystallization behavior of inner core in a multi component system we have performed new high pressure experiments in the Fe-C-S systems. Experiments and Analysis: We investigated the near-liquidus phase relations in Fe-C-S ternary at 2-6 GPa and 1050-2000 °C. Experiments were performed in a piston cylinder and a multi-anvil device using MgO (Fe-5wt.%C-5wt.%S and Fe-5wt.%C-15wt.%S) and graphite (Fe-13wt.%S, Fe-5wt.%S, Fe-wt.1.4%S) capsules. Run products were imaged and analyzed for Fe, S, C, and O using an electron probe micro analyzer on Al-coated samples. Results: The phase assemblage for Fe-5wt.%C5wt.%S evolve from a completely molten system at high temperatures to Fe-metal+Fe-carbide+melt at low temperatures (Fig. 1) via Fe-carbide+melt at intermediate temperatures. The liquidus increases from 11001150 °C at 2 GPa to 1375-1450 °C at 6 GPa, as the sole liquidus carbide phase changes from Fe3C to Fe7C3. With cooling, Fe-metal appears between 1100 and 1050 °C at 2 GPa and between 1200 and 1100 °C at 6 GPa. At 6 GPa, between 1200 and 1300 °C, Fe3C breaks down to produce a more carbon-rich carbide, Fe7C3 and melt. Over the pressure range of investigation, the liquidus temperature for the bulk composition Fe-5wt.%C-15wt.%S is ca. 50-100 °C lower than that of the bulk composition with 5 wt.% sulfur; it is 1100±50 °C at 2 GPa and between 1250 and 1350 °C at 6 GPa. At 2-5 GPa, above the liquidus, for this composition, we observe the presence of two quenched melt phases (Fig. 1A), one carbide-rich and sulfurpoor and the other sulfide-rich and carbon-poor. Texturally the melt phases appear to be immiscible, with blobs of carbide-rich quenched melt in a matte of sulfide-rich melt (Fig. 1B). No immiscibility texture is observed for superliquidus conditions at 6 GPa, and a single, sulfur and carbon bearing quenched melt is observed.