Experimental determination of carbon solubility in Fe-Ni-S melts

TitleExperimental determination of carbon solubility in Fe-Ni-S melts
Publication TypeJournal Article
Year of Publication2018
AuthorsZhang Z., Hastings P., Von der Handt A., Hirschmann M.M
JournalGeochimica Et Cosmochimica Acta
Volume225
Pagination66-79
Date Published2018/03
Type of ArticleArticle
ISBN Number0016-7037
Accession NumberWOS:000424974700005
Keywordsc system; content; core; Deep carbon; earths deep mantle; Geochemistry & Geophysics; group element systematics; high-pressure; Iron; Mantle redox; Metal-rich sulfide melt; ocean-ridge; sulfide liquid; sulfur; Volatile
Abstract

To investigate the effect of metal/sulfide and Ni/Fe ratio on the C storage capacity of sulfide melts, we determine carbon solubility in Fe-Ni-S melts with various (Fe + Ni)/S and Ni/Fe via graphite-saturated high-pressure experiments from 2-7 GPa and 1200-1600 degrees C. Consistent with previous results, C solubility is high (4-6 wt.%) in metal-rich sulfide melts and diminishes with increasing S content. Melts with near M/S = 1 (X-S > 0.4) have <0.5 wt.% C in equilibrium with graphite. C solubility is diminished modestly with increased Ni/Fe ratio, but the effect is most pronounced for S-poor melts, and becomes negligible in near-monosulfide compositions. Immiscibility between S-rich and C-rich melts is observed in Ni-poor compositions, but above similar to 18 wt.% Ni there is complete miscibility. Because mantle sulfide compositions are expected to have high Ni concentrations, sulfide-carbide immiscibility is unlikely in natural mantle melts. An empirical parameterization of C solubility in Ni-Fe-S melts as a function of S and Ni contents allows estimation of the C storage capacity of sulfide in the mantle. Importantly, as the metal/sulfide (M/S) ratio of the melt increases, C storage increases both because C solubility increases and because the mass fraction of melt is enhanced by addition of metal from surrounding silicates. Under comparatively oxidized conditions where melts are near M/S = 1, as prevails at <250 km depth, bulk C storage is <3 ppm. In the deeper, more reduced mantle where M/S increases, up to 200 ppm C in typical mantle with 200 +/- 100 ppm S can be stored in Fe-Ni-S melts. Thus, metal-rich sulfide melts are the principal host of carbon in the deep upper mantle and below. Residual carbon is present either as diamond or, if conditions are highly reduced and total C concentrations are low, solid alloy. (C) 2018 Elsevier Ltd. All rights reserved.

DOI10.1016/j.gca.2018.01.009
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