Sulfur cycle in the Valles Caldera volcanic complex, New Mexico – Letter 1: Sulfate sources in aqueous system, and implications for S isotope record in Gale Crater on Mars

Abstract

Initial in situ sulfur (S) isotope measurements of the Martian bedrock in Gale Crater have revealed an unexpectedly wide range of δ34S values (−47 to +28‰). Generally, it is unclear what processes could have contributed to these large isotope fractionations. Therefore, we studied S sources and aqueous SO42− cycling in the Valles Caldera volcanic complex, New Mexico to better understand S isotope fractionations related to S degassing, hydrothermal activity, and low-temperature processes in aqueous environment. Overall, our study demonstrates that volcanic systems show large spatial heterogeneity in δ34S. Magmatic S sources are obvious in steam-dominated H2S degassing and precipitation of secondary minerals from hydrothermal fluids with low δ34S values of +0.9 ± 3‰. Locally, however, hydrothermal processes have resulted in more negative δ34S values in sulfide minerals (−18 to −4‰) and more positive δ34S values in sulfate minerals (−1 to +3‰). Major aqueous SO42− sources are oxidation of H2S from modern hydrothermal gas emission, and oxidation and dissolution of sulfide and sulfate minerals present in the hydrothermally altered bedrock and crater-lake sediments. The δ34S of aqueous SO42− in surface water and groundwater varies widely (−8 to +5‰) and is similar to major S endmembers that undergo oxidation and/or dissolution by active hydrological system. Minor SO42− contributions with more positive δ34S values (+9 to +14‰) come from deeply circulating geothermal fluids and negligible amounts from atmospheric deposition (+5 to +7‰ in snow). Elevated SO42− contents are mainly associated with modern and past H2S emissions and oxidations near the surface. On regional scale, however, most of the intracaldera bedrock is S-depleted, thus the SO42− contents are usually low in the surface aquatic system and younger sedimentary lake deposits formed at times of negligible near surface hydrothermal activity. In general, magmatic-hydrothermal processes apparently cause the largest δ34S variation in S-bearing minerals on volcanic terrains. Therefore, we infer that the measured wide range of δ34S values in the Gale sediments by the Curiosity rover on Mars can be explained by S isotope composition of magmatic-hydrothermal sulfide and sulfate minerals that were present in the initial igneous/volcanic rocks prior to crater formation. Later aqueous processes involved oxidation and dissolution of S minerals initially present in these rocks and led to subsequent formation of diagenetic fluids and alteration products enriched in SO42− with relatively large δ34S variation. Additionally, physical erosion, transport and deposition of detrital hydrothermal S minerals from igneous/volcanic rocks might be in part responsible for the measured wide range of δ34S in Gale Crater. These unique S isotope results, measured in situ on another planet for the first time, imply the importance of magmatic-hydrothermal fluids in S transport on early Mars and their subsequent alteration in low-temperature aqueous environments.