Implications of the MESSENGER Discovery of High Sulfur Abundance on the Surface of Mercury

Details Implications of the MESSENGER Discovery of High Sulfur Abundance on the Surface of Mercury Implications of the MESSENGER Discovery of High Sulfur Abundance on the Surface of Mercury

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41a1584z&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P41A-1584

Physics

[3672] Mineralogy And Petrology / Planetary Mineralogy And Petrology, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6235] Planetary Sciences: Solar System Objects / Mercury

Scientific paper

The unusually high S content detected in Mercury's surface materials with the MESSENGER X-ray Spectrometer (XRS) constrains surface mineralogy, petrology, and the redox state of magmas and rocks. This discovery along with the low FeO content in surface silicates indicates a low oxygen fugacity (fO2) in corresponding melts and the occurrence of S in sulfides, which could be abundant in surface rocks. The detected high S content could reflect anomalously high (up to 8-10 wt%) solubility of sulfide S in extremely reduced magmas. The high bulk S/Fe ratio also suggests the presence of S in sulfides of Mg, Ca, Mn, and Cr, which occur in enstatite chondrites. Although the presence of some troilite (FeS) is possible, niningerite, (Mg, Fe, Mn)S, could be the most abundant sulfide. Niningerite could be partially responsible for Mercury's low surface albedo, its unusual reflectance spectrum at visible and near-infrared wavelengths, and the relatively high neutron absorption, because Mn is a strong neutron absorber. The presence of abundant niningerite would also imply a lower Mg/Si ratio in silicates than in bulk surface materials. It follows that Mg-rich mafic lavas could be present instead of, or in addition to, ultramafic lavas (komatiites). The occurrence of Mg-silicates (enstatite and forsterite) in Mercury's regolith as inferred from mid-infrared spectroscopy, together with the postulated presence of niningerite, helps characterize fO2 and fS2 in corresponding melts. If fS2 is controlled by the Fe-metal-Fe-sulfide equilibrium, the silicate-sulfide equilibria set fO2 values. For temperature less than 1700 K the evaluated values are less than 5.5 log fO2 units below the iron-wüstite buffer (IW-5.5). Lower temperatures and analogous considerations for Ca and Mn silicate-sulfide equilibria lead to lower fO2 values. For Fe-metal-saturated melts at 1700 K the fO2 value is IW-5.5 and corresponds to ~0.1 mol % FeO, which could be considered as an upper limit in magmas and igneous rocks. At 1500 K those values are IW-6.3 and ~0.05 mol % FeO. Therefore, the detected high S abundance is consistent with the low FeO content in surface silicates. In contrast to the behavior of S, the low fO2 implies retention of C in the mantle during magmatism because of low solubility of C in reduced melts. The high S content and low fO2 suggest that Mercury accreted from extremely reduced anhydrous solids, which could be similar to enstatite chondrites.

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