Joint 20 th AIRAPT – 43 th EHPRG, June 27 – July 1, Karlsruhe/Germany 2005 Immiscible two-liquid regions in the Fe-FeO-FeS system at high pressure Kyusei Tsuno *, Eiji Ohtani, and Hidenori Terasaki Institute of Mineralogy, Petrology, and Economic Geology, Sendai, Japan tsuno@ganko.tohoku.ac.jp Summary: Melting relation in the Fe-FeO-FeS system, especially the immiscible two-liquid region has been investigated by using Kawai-type multi-anvil apparatus at 15 GPa, 1800- 2200 o C. The products recovered from 15 GPa and 2200 o C show textures that are totally molten. The immiscible two liquids are observed in the starting compositions with 0-2.6 wt.% sulfur, whereas the miscible liquid is obtained in the composition of 7.5 wt.% sulfur. Sulfur is likely to narrow the immiscible two-liquid region in the Fe-FeO-FeS system at high pressure. 1. Introduction Seismic observation and high-pressure experiments of iron indicate that the Earth’s core is less dense than pure Fe at high-pressure conditions (e. g., Anderson and Ahrens, 1994; Anderson and Isaak, 2002), suggesting that the core contains about 10% of the light elements. The candidates of the light elements are S, Si, O, H, and C and more than two of these are likely to be contained in the core (Hillgren et al., 2000; Poirier, 1994). It is important to investigate what kind and how much amounts of the light elements are contained in the core in order to constrain the evolution and dynamics of the Earth. Sulfur is one of the major light elements in the core because it is abundant in CI chondrites (e. g. Anders and Grevesse, 1989). The amount of sulfur in the Earth’s core is estimated to be about 2 wt.% (e. g., McDonough and Sun, 1995) based on the geochemical constraints. Although sulfur is likely to be contained in the Earth’s core, but it maybe doesn’t solely explain the density deficit of the Earth’s outer core of about 10%. Oxygen may be also one of the major light elements because it is the second most abundant element in bulk Earth. However, oxygen also does not solely explain the density deficit because the eutectic composition contains about 2 wt.% oxygen and a large immiscible two-liquid region exists at 16 GPa in the Fe-FeO system (Kato and Ringwood, 1989; Ringwood and Hibberson, 1990). The experimental study of the Fe-FeO-FeS system was conducted by Urakawa et al., (1987), indicating that large immiscible two liquid region exists up to 15 GPa. Recently Helffrich and Kaneshima (2004) reported that the immiscible two liquids regions still exist at 135 GPa and 3500-4300 K using thermodynamic calculation. The purpose of this study is to re-investigate the liquid immiscible region in the Fe-FeO-FeS system at high pressures. 2. Experimental methods High-pressure experiments were performed using MA-8 Kawai-type multi-anvil apparatus (1000ton-Press) at Tohoku University in the conditions of 15 GPa, and 1800-2200 o C. . The cell assembly consists of ZrO 2 pressure medium, LaCrO 3 heater, and an Al 2 O 3 capsule. WC- anvils with the truncated edge length of 3.5mm were used with pyrophyllite gaskets. Temperature was monitored with a W 97 Re 3 -W 75 Re 25 thermocouple whose junction is placed directly above the Al 2 O 3 capsule. Pressure calibration has been described elsewhere (Litasov and Ohtani, 2002). Starting materials were mixtures of powdered Fe (99.9% pure, Wako product), FeS (99.9% pure, Rare Metallic product), and FeO that was synthesized from Fe 2 O 3 hematite in a gas furnace with controlling oxygen fugacity at 1100 o C for 24 h. The lattice parameters of FeO