Japan Geoscience Union Meeting 2015

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT03] Structure and dynamics of Earth and Planetary deep interiors

Tue. May 26, 2015 11:00 AM - 12:45 PM 106 (1F)

Convener:*Takashi Yoshino(Institute for Study of the Earth's Interior, Okayama University), Satoru Tanaka(Department of Deep Earth Structure and Dynamics Research Japan Agency for Marine-Earth Science and Technology), Dapeng Zhao(Department of Geophysics, Tohoku University), Masanori Kameyama(Geodynamics Research Center, Ehime University), John Hernlund(Earth-Life Science Institute, Tokyo Institute of Technology), Chair:Satoru Tanaka(Department of Deep Earth Structure and Dynamics Research), Kenji Kawai(Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, University of Tokyo)

11:30 AM - 11:45 AM

[SIT03-24] Melting experiments in the system Fe-FeS at core pressures

*Yuko MORI1, Shigehiko TATENO2, Kei HIROSE2, Guillaume MORARD3, Yasuo OHISHI4 (1.Dept. of Earth & Planetary Sciences, Tokyo Institute of Technology, 2.Earth-Life Science Institute, Tokyo Institute of Technology, 3.IMPMC, Universite Pierre-et-Marie-Curie, 4.Japan Synchrotron Radiation Research InstituteJ)

Keywords:High pressure and temperature, melting experiments, the Earth's core, light element, core density deficit

Most of the Earth’s core is iron and nickel alloy. However, the density of the core acquired from Preliminary reference Earth model (PREM) is smaller than that of pure iron. The amount of deficit is 6-10 wt % at the outer core and 1-3 wt % at the inner core. To explain this deficit, the core might contain one or more light elements (H, C, N, O, Si, S).
S is depleted in the crust and mantle compared to other volatile elements (Poirier ,1994). Moreover, iron meteorites contain S (Chabot, 2004), thus we focus on S. In order to discuss the composition of the core, the phase diagram is important. The bulk core composition must be Fe-rich side at inner core boundary (ICB) because the outer core incorporates larger amounts of light element than the inner core. Fe-FeS phase diagram is determined by X-ray diffraction (XRD) pattern (e.g. Morard et al. 2008) or chemical analysis of recovered sample (e.g. Kamada et al. 2012). However, no accurate determination of the eutectic composition has been obtained by XRD. On the other hand, constraining by chemical analysis has not been done using sample recovered from core pressure. Thus, we examine Fe-S phase diagram at higher pressure.

High pressure and temperature (P-T) conditions were generated in a laser-heated diamond-anvil cell. We used Fe-7.5, 13.5 wt.% S foil as a starting material. Angle-dispersive in situ X-ray diffraction (XRD) measurements at high P-T were conducted at BL10XU, SPring-8. The textural and chemical characterizations of recovered samples were made with a field-emission-type scanning electron-microprobe (FE-SEM) equipped with energy dispersive x-ray spectrometry (EDS) and with a field-emission-type electron probe microanalyzer (FE-EPMA). Cross sections of samples were carefully examined by combining a focused Ga ion beam (FIB) with FE-SEM.

From quantitative analysis, we observed the trend that the amount of S in melt decreases and that the amount of S in the solid Fe increases with increasing pressure.
Obseving the sample using Fe-7.5 wt % S by SEM, we can see the 3 textures; melt, solid Fe, subsolidus phase. Melt includes more S than solid Fe. The content of S in liquid is between 12.5 and 10.4 wt. % in the range of 38-138 GPa. The eutectic composition is expected to have S than molten iron. Moreover, according to the experiment using S rich starting material, the texture of recovered sample contain melt ,Fe3S, subsolidus phase. The S content in melt is larger than that in eutectic composition.Using both of S-rich and S-poor starting material, we can constrain the eutectic composition.

Assuming that the negative pressure dependence of S content in melt is retained up to ICB pressure, the content of S in liquid Fe at ICB pressure is lower than 10 wt %. The S amount required to explain core density deficit (CDD) is 11.9-13.9 wt.% at outer core and 6-6.6 wt. % at inner core (Sata et al. 2010) . In this condition, the S content of eutectic composition is more than 11.9 wt.%. However, this value is great comparing to the eutectic composition expected from this study. Therefore, S is not the only light element inthe Earth’s core.