¹Olivier Namur, ²Bernard Charlier, ¹Francois Holtz, ²Camille Cartier, ³Catherine McCammon
¹Leibniz University of Hannover, Institute of Mineralogy, 30167 Hannover, Germany
²University of Liege, Department of Geology, 4000 Liege, Belgium
³Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany

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−S2− and forms complexes with Fe2+Fe2+, Mg2+Mg2+ and Ca2+Ca2+. The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from 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 (fO2fO2), 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±0.45.4±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 (

Reference
Namur O, Charlier B,Holtz F, Cartier C, McCammon C (2016) Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury. Earth and Planetary Science Letters 448,102–114.
Link to Article [doi:10.1016/j.epsl.2016.05.024]
Copyright Elsevier

¹Derek W.G. Sears
¹Bay Area Environmental Research Institute, NASA Ames Research Center, Space Science and Astrobiology Division (MS 245-3), Mountain View, California 94035, U.S.

Thermoluminescence (TL) properties of 29 CO chondrites from the Miller Range (MIL) and five chondrites from the Dominion Range (DOM) have been measured. MIL has a relatively strong natural TL signal (19.6±14.7 krad), while some of the DOM samples have a very weak natural TL signal ( The CO chondrites: Major Recent Antarctic finds, Their Thermal and Radiation History, and Describing the Metamorphic History of Members of the Class 19.6±14.7 krad), while some of the DOM samples have a very weak natural TL signal (<1 krad) whereas others resemble the MIL meteorites. We argue that MIL and some of the DOM samples had a normal perihelion (∼1.0 AU) and terrestrial age of ∼450-700 ka, while some of the DOM samples have a terrestrial age of ∼100 ka but a perihelion of ∼0.8 AU. The DOM meteorites also show considerable heterogeneity in their induced TL properties, also suggesting that the DOM fragments represent more than one fall. The induced TL data for the MIL samples studied here are consistent with them all being from a single fragmented meteorite. Small (50 mg) chips have TL properties similar to 500 mg chips, so that the smaller chips are representative, although samples taken from original masses less than ∼2 g have low natural TL suggesting that they were heated during atmospheric fall. The properties of CO chondrites are reviewed in terms of their petrologic types. Correlations between TL sensitivity, the most quantitative technique for evaluating metamorphic alteration in CO chondrites, and data for olivine composition and heterogeneity, matrix composition, inert gas content, metal composition (Ni, Co, and Cr in the kamacite), bulk carbon, C and O isotopes, graphite ordering, spectral reflectance at 0.8 μm, and textural characteristics of the amoeboid olivine and Ca-rich inclusions are examined. The petrographic types appear to be largely metamorphic in origin with perhaps a minor role for metasomatism. Contrary to recent proposals it is here argued that petrologic type definitions should (1) be specific enough to be meaningful, but broad enough to be simple in application and robust to new developments, (2) be descriptive and not interpretative, (3) should not oversimplify and obscure important class-to-class differences, and (4) take account of all the available information, while avoiding reliance on any one technique or single observation whose application is based on interpretation. With these considerations in mind the petrographic type definitions for CO chondrites are restated and the petrologic type of 3.2 assigned to both the MIL and DOM CO chondrites.

Reference
Sears WG (2106) The CO chondrites: Major Recent Antarctic finds, Their Thermal and Radiation History, and Describing the Metamorphic History of Members of the Class.
Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.05.033]
Copyright Elsevier