New Bulk Sulfur Measurements of Martian Meteorites - Implications for Sulfur Cycle on Mars

Magmatic degassing was likely critical in giving rise to a thick atmosphere of ancient Mars and SO₂ and H₂S could have been key greenhouse gases. How much S was released depends on (1) the S content of the mantle-derived magma and (2) the magmatic sulfur budget of the basaltic crust. While the former is estimated by S content of basaltic melts at sulfide saturation (SCSS) [1,2], assuming mantle-derived magma is sulfide saturated, it is unclear how much S gets trapped during crystallization of basalts in the crust versus how much is released to the atmosphere. S content of the martian crust can be estimated from martian meteorites, yet bulk S concentration data of martian meteorites is limited [3]. Further, most martian meteorites contain cumulus minerals and some have experienced secondary alteration (weathering/ impact effects), which could either deplete or enrich S in these samples. To better constrain the degassing of S from the martian interior, we measured bulk S contents of 7 martian meteorites via high mass-resolution solution ICP-MS [4]. Basaltic shergottites Los Angeles, Zagami and NWA 856 have S contents of 2865×224, 1954×91 and 1584×10 ppm, respectively while clinopyroxenites Nakhla and NWA 998 give values of 690×60 and 253×42 ppm S. Olivine-phyric shergottites NWA 1068 and Tissint have intermediate S contents of 1280×48 and 2120×68 ppm. The meteorites have lower S contents than the predicted SCSS of ~3500-4500 ppm [2] along liquid line of descent for a liquid similar to Yamato 980459 at 1 GPa, estimated using alphaMELTS. Taking into account the possible proportion of inter-cumulus liquid (f= 6-70 wt.%) in the analyzed meteorites estimated by previous studies, the degassed S could be as low as ~300-1900 ppm (estimated by the difference between the SCSS×f and the S in the meteorites). However, nakhlite Nakhla and basaltic shergottites NWA 856 and Zagami show higher S than the calculated SCSS×f. In these two meteorites, sulfides occur as mesostasis minerals [5,6], so the excess S is not likely to come from cumulus mineral assemblages. It is either that the liquid proportion (f) was underestimated or some S was introduced by secondary alteration. The latter case is supported by Δ³³S anomalies in Nakhla [7] and shock melt pockets in Zagami [8] and NWA 856 [9]. Multiplying the degassing magma S contents obtained above, which could be applicable for the Amazonian, with estimated eruption volume of Tharsis province could give the total mass of degassed S as low as 2.4×10¹⁷ kg. Because magmatism may have been more active in the early Mars, this magmatic S flux is likely a lower bound for the late Noachian Mars and thus could still be a key contributor to the ancient greenhouse. [1] Righter et al. (2009) EPSL, 288, 235-243; [2] Ding and Dasgupta (2013) GCA (submitted); [3] Lodders (1998) MAPS 33, A183-A190; [4] Erdman et al. (2013) Geostand. Geoanal. Res. (accepted); [5] Lorand et al. (2005) MAPS 40, 1257-1272; [6] Chevrier et al. (2011) MAPS 46, 769-784; [7] Greenwood et al. (2000) EPSL 184, 23-35; [8] McCoy et al. (1999) GCA 63, 1249-1262; [9] Jambon et al. (2002) MAPS 37, 1147-1164.