The sulfur budget and sulfur isotopic composition of Martian regolith breccia NWA 7533

Abstract

The sulfur isotope budget of Martian regolith breccia (NWA 7533) has been addressed from conventional fluorination bulk rock analyses and ion microprobe in situ analyses. The bulk rock analyses yield 865 AE 50 ppm S in agreement with LA-ICP-MS analyses. These new data support previous estimates of 80% S loss resulting from terrestrial weathering of NWA 7533 pyrite. Pyrite is by far the major S host. Apatite and Fe oxyhydroxides are negligible S carriers, as are the few tiny igneous pyrrhotite-pentlandite sulfide grains included in lithic clasts so far identified. A small nonzero D 33 S (À0.029 AE 0.010&) signal is clearly resolved at the 2r level in the bulk rock analyses, coupled with negative CDT-normalized d 34 S (À2.54 AE 0.10&), and near-zero D 36 S (0.002 AE 0.09&). In situ analyses also yield negative D 33 S values (À0.05 to À0.30&) with only a few positive D 33 S up to +0.38&. The slight discrepancy compared to the bulk rock results is attributed to a possible sampling bias. The occurrence of mass-independent fractionation (MIF) supports a model of NWA 7533 pyrite formation from surface sulfur that experienced photochemical reaction(s). The driving force that recycled crustal S in NWA 7533 lithologies-magmatic intrusions or impact-induced heat-is presently unclear. However, in situ analyses also gave negative d 34 S values (+1 to À5.8&). Such negative values in the hydrothermal setting of NWA 7533 are reflective of hydrothermal sulfides precipitated from H 2 S/HSaqueous fluid produced via open-system thermochemical reduction of sulfates at high temperatures (>300°C).

A coupled model of episodic warming, oxidation and geochemical transitions on early Mars

“…Aqueous solutions of sulfate in open-water systems near the surface can be reduced by dissolved H 2 or other reducing gas species from the atmosphere itself. For sulfates that percolate deeper into the regolith, the transport rate of gas species from the atmosphere will eventually become limited, but TSR can also be driven by ferrous iron from crustal minerals (20). As is clear from Fig.…”

“…As can be seen from Figure 3a, for O 2 fugacities below around 10 (25,43,44), resulting in the repeated dissolution and reduction of early surface sulfate deposits. Interestingly, TSR of surface-derived sulfates has very recently been proposed as an explanation for non-zero S-MIF values in the pyrite of martian meteorite NWA 7533 (20).…”

“…The fraction of sulfate that becomes reduced as a function of depth 𝑓 𝑆𝑟𝑒𝑑 is then calculated by integrating equation (20) to get…”

“…Martian meteorite data suggest that much of Mars' mantle is more reducing than Earth's (17), and hence that its early atmosphere was also at least intermittently reducing (18,19). Some Mars meteorites also preserve non-zero sulfur mass-independent fractionation (MIF) signatures, suggesting sulfur deposition during anoxic atmospheric intervals (20). Today, however, Mars' surface is highly oxidized, with abundant ferric iron lending the planet its reddish appearance and species such as hydrogen peroxide and perchlorates present in the atmosphere and regolith.…”

Assessment of Sulfate Sources under Cold Conditions as a Geochemical Proxy for the Origin of Sulfates in the Circumpolar Dunes on Mars

“…Although without in situ measurements the exact sources and types of sulfur-bearing minerals/phases in the basaltic sand will remain speculative, the best candidates would be sulfide and sulfate minerals and/or sulfur-rich amorphous phases from past magmatic/hydrothermal activities and secondary aqueous alterations that occurred prior to wind erosion and final deposition of basaltic sand in the north polar depression. A variety of secondary Ca-Mg-Fe-K sulfate minerals are abundant in Martian sedimentary rocks (~10-30 wt% SO 3 ; e.g., [53][54][55][56]) with locally elevated sulfide (pyrrothite, pentlandite, chalcopyrite) contents as measured in Gale crater (~0.3-1 wt%; [57]) and Martian meteorites (~0.2-1 wt%; e.g., [58][59][60][61]). As a result, Martian dust appears to be enriched in sulfate minerals globally (up to~8 wt% SO 3 ; e.g., [62][63][64][65]).…”