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Melting experiments on Fe-Si-S alloys to core pressures; silicon in the core?

Shigehiko Tateno, Kei Hirose, Ryosuke Sinmyo, Guillaume Morard, Naohisa Hirao and Yasuo Ohishi
Melting experiments on Fe-Si-S alloys to core pressures; silicon in the core?
American Mineralogist (May 2018) 103 (5): 742-748


Melting and subsolidus experiments were carried out on Fe-Si-S alloys (2.2-2.7 wt% Si + 2.0-2.1 wt% S) up to 146 GPa in a laser-heated diamond-anvil cell (DAC). The melting and subsolidus phase relations were examined on the basis of in situ synchrotron X-ray diffraction measurements and ex situ textural and chemical characterizations of recovered samples. The subsolidus phase assemblage changed from Fe-rich hexagonal closed-packed (hcp) phase + Fe (sub 3) S into a single phase of hcp Fe-Si-S alloy above 80 GPa at approximately 2500 K. The melting curve was obtained on the basis of the appearance of diffuse X-ray scattering and/or melting texture found in the cross section of a recovered sample. Microprobe analyses of quenched molten samples showed that liquid Fe-Si-S coexisted with Fe-alloy solid, which is depleted in sulfur but enriched in silicon compared to the liquid. This result indicates that the liquid evolves toward a Si-poor and S-rich composition upon crystallization. Our data further suggest that the ternary eutectic liquid composition is Si-deficient and close to the tie line between the eutectic points in the Fe-Si and Fe-S binary systems at each pressure. The composition of Fe-Si-S liquid that accounts for the outer core density is outside the liquidus field of solid Fe at the inner core boundary (ICB) pressure. Accordingly, the solid alloy crystallizing from such an outer core liquid must be more enriched in silicon/sulfur than the coexisting liquid and thus cannot form the denser inner core required from seismic observations. Furthermore, neither liquid Fe-Si-C nor Fe-Si-O can crystallize a dense solid at the ICB. These results reinforce the conclusion that silicon is not an important light element in the core.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 103
Serial Issue: 5
Title: Melting experiments on Fe-Si-S alloys to core pressures; silicon in the core?
Affiliation: Tokyo Institute of Technology, Earth-Life Science Institute, Tokyo, Japan
Pages: 742-748
Published: 201805
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 55
Accession Number: 2018-052696
Categories: General geophysicsSolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: University of Tokyo, JPN, JapanMuseum National d'Histoire Naturelle, FRA, FranceJapan Synchrotron Radiation Research Institute, JPN, Japan
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2018
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