In situ X-ray observations of the melting relations in the Fe-S-H system under high pressure and high temperature

Sulfur and hydrogen are two of the most plausible light elements in the planetary cores. Particularly the cores of icy satellites, such as Ganymede, are considered to contain a significant amount of those elements based on studies of meteorites. Therefore, it is essential to investigate the properties of iron alloyed with those light elements at high pressure and high temperature, in order to shed light on the composition and structure of the cores. To date, the Fe-FeS system has been extensively investigated at high pressure and temperature. Hydrogen is known to dissolve in interstitial sites of Fe and FeS lattices and strongly depresses the melting temperatures. However, it is still not clear how hydrogen affects the eutectic point (temperature and composition) of the Fe-FeS system. In order to understand the melting relations in the Fe-S-H system, we have performed in situ X-ray diffraction experiments at high pressure and high temperature. The experiments were carried out using the multi-anvil apparatus at the BL04B1 beamline of SPring-8, Japan, up to 10 GPa and 1700 K. Fe-FeS powder mixtures (15.5 wt% S and 30 wt %S) were packed into a NaCl capsule along with LiAlH4. Hydrogen was supplied to the Fe-FeS sample by a thermal decomposition of LiAlH4. The Fe-FeS sample was separated from LiAlH4 using a thin MgO disk to avoid the direct chemical reaction between the sample and LiAlH4. The NaCl capsule is able to seal hydrogen effectively at high pressure and high temperature. The diffraction patterns were collected for a period of 300 s at a temperature interval of 50-100 K. The collected diffraction data show that FeHx and FeSHx were synthesized at high temperature and then the sample was totally molten via a partial melting with increasing temperature. Since the atomic volumes of Fe and FeS expand due to the hydrogen dissolution (hydrogenation), the hydrogen concentrations in FeHx and FeSHx were estimated by comparing the volumes of between Fe and FeHx or between FeS and FeSHx. The solidus temperature was close to the eutectic temperature of pure Fe-FeS system, while the liquidus temperature was much lower than the liquidus of the Fe-FeS system. The results will be discussed in conjunction with the structure of the planetary cores.