TY - JOUR

T1 - Sulfur solubility in reduced mafic silicate melts

T2 - Implications for the speciation and distribution of sulfur on Mercury

AU - Namur, Olivier

AU - Charlier, Bernard

AU - Holtz, Francois

AU - Cartier, Camille

AU - McCammon, Catherine

N1 - Funding Information: ON acknowledges support from the von Humboldt Foundation and from a Marie Curie Intra-European Fellowship ( SULFURONMERCURY – 327046 ). ON also acknowledges support from the DFG Core Facility for High-Pressure Research from the German Science Foundation ( KE 501/10-1 ) for the high-pressure experiments (BGI). BC was supported by the von Humboldt Foundation , a BELSPO Grant, and the BRAIN-be program (BR/143/A2/COME-IN). A.M. Welsch is thanked for her help with Raman spectroscopy and D. Lattard for sharing her expertise with evacuated silica tubes. We appreciate comments from C. Sotin (editor), F. Gaillard and an anonymous reviewer that significantly improved the quality of the paper. Publisher Copyright: © 2016 Elsevier B.V. Copyright: Copyright 2018 Elsevier B.V., All rights reserved.

PY - 2016/8/15

Y1 - 2016/8/15

N2 - Chemical data from the MESSENGER spacecraft revealed that surface rocks on Mercury are unusually enriched in sulfur compared to samples from other terrestrial planets. In order to understand the speciation and distribution of sulfur on Mercury, we performed high temperature (1200–1750 °C), low- to high-pressure (1 bar to 4 GPa) experiments on compositions representative of Mercurian lavas and on the silicate composition of an enstatite chondrite. We equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions (IW-1.5 to IW-9.4; IW = iron-wüstite oxygen fugacity buffer). Under these oxygen fugacity conditions, sulfur dissolves in the silicate melt as S2− and forms complexes with Fe2+, Mg2+ and Ca2+. The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from <1 wt.% S at IW-2 to >10 wt.% S at IW-8) and with increasing temperature. Metallic melts have a low sulfur content which decreases from 3 wt.% at IW-2 to 0 wt.% at IW-9. We developed an empirical parameterization to predict SCSS in Mercurian magmas as a function of oxygen fugacity (fO2), temperature, pressure and silicate melt composition. SCSS being not strictly a redox reaction, our expression is fully valid for magmatic systems containing a metal phase. Using physical constraints of the Mercurian mantle and magmas as well as our experimental results, we suggest that basalts on Mercury were free of sulfide globules when they erupted. The high sulfur contents revealed by MESSENGER result from the high sulfur solubility in silicate melt at reducing conditions. We make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and/or that the mantle contains a minor metal phase and combine our parameterization of SCSS with chemical data from MESSENGER to constrain the oxygen fugacity of Mercury's interior to IW-5.4±0.4. We also calculate that the mantle of Mercury contains 7–11 wt.% S and that the metallic core of the planet has little sulfur (<1.5 wt.% S). The external part of the Mercurian core is likely to be made up of a thin (<90 km) FeS layer.

AB - Chemical data from the MESSENGER spacecraft revealed that surface rocks on Mercury are unusually enriched in sulfur compared to samples from other terrestrial planets. In order to understand the speciation and distribution of sulfur on Mercury, we performed high temperature (1200–1750 °C), low- to high-pressure (1 bar to 4 GPa) experiments on compositions representative of Mercurian lavas and on the silicate composition of an enstatite chondrite. We equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions (IW-1.5 to IW-9.4; IW = iron-wüstite oxygen fugacity buffer). Under these oxygen fugacity conditions, sulfur dissolves in the silicate melt as S2− and forms complexes with Fe2+, Mg2+ and Ca2+. The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from <1 wt.% S at IW-2 to >10 wt.% S at IW-8) and with increasing temperature. Metallic melts have a low sulfur content which decreases from 3 wt.% at IW-2 to 0 wt.% at IW-9. We developed an empirical parameterization to predict SCSS in Mercurian magmas as a function of oxygen fugacity (fO2), temperature, pressure and silicate melt composition. SCSS being not strictly a redox reaction, our expression is fully valid for magmatic systems containing a metal phase. Using physical constraints of the Mercurian mantle and magmas as well as our experimental results, we suggest that basalts on Mercury were free of sulfide globules when they erupted. The high sulfur contents revealed by MESSENGER result from the high sulfur solubility in silicate melt at reducing conditions. We make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and/or that the mantle contains a minor metal phase and combine our parameterization of SCSS with chemical data from MESSENGER to constrain the oxygen fugacity of Mercury's interior to IW-5.4±0.4. We also calculate that the mantle of Mercury contains 7–11 wt.% S and that the metallic core of the planet has little sulfur (<1.5 wt.% S). The external part of the Mercurian core is likely to be made up of a thin (<90 km) FeS layer.

KW - core

KW - mantle

KW - MESSENGER

KW - oxygen fugacity

KW - sulfide saturation

UR - http://www.scopus.com/inward/record.url?scp=84988358007&partnerID=8YFLogxK

U2 - 10.1016/j.epsl.2016.05.024

DO - 10.1016/j.epsl.2016.05.024

M3 - Article

AN - SCOPUS:84988358007

VL - 448

SP - 102

EP - 114

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

ER -