Investigating formation processes of secondary sulfate minerals in the semi-arid climate of the Rio Puerco watershed, New Mexico using sulfur and oxygen isotopes – Implications for the origin of gypsum veins in Gale crater on Mars

Abstract

Secondary sulfate minerals are common throughout the sedimentary deposits of Mount Sharp, located within Gale crater on Mars. However, the source of sulfate (SO₄²⁻) and past climatic conditions during their formation are not well understood. Therefore, we investigated the δ³⁴S, δ¹⁸O, and δ²H of gypsum veins and other Mg- and Ca- sulfates forming as salt crusts and cement within the shallow sediments of the Rio Puerco watershed in central New Mexico. The δ³⁴S values of vein gypsum and acid-soluble SO₄²⁻ (cement) varied over the same range (−33.3 to −12.9 ‰ and −34.6 to −12.1 ‰, respectively), which was similar to the δ³⁴S of bedrock sulfide minerals (−37.4 to −5.9 ‰). This implies that sulfide oxidation is the main source of SO₄²⁻ in the Rio Puerco aqueous system. The measured δ¹⁸O values of SO₄²⁻ (−8.9 to +3.1 ‰) as well as δ¹⁸O and δ²H values of gypsum hydration water (−8.9 to +0.6 ‰, and −112 to −82 ‰, respectively) overlapped with the isotope composition of local meteoric precipitation, suggesting that sulfide oxidation to SO₄²⁻ and gypsum formation have occurred under semi-arid climate conditions. The isotope results suggest the top-down infiltration of meteoric water leads to leaching of SO₄²⁻, Mg⁺, and Ca²⁺ from bedrock sulfide weathering followed by abundant formation of Mg- and Ca-sulfates in surface deposits and gypsum veins with depth. Because of spatial and mineralogical similarities in the secondary Mg- and Ca-sulfate mineral occurrences, we hypothesize that chemical weathering of sulfide minerals could have been the main source of SO₄²⁻ in the aqueous system of Gale crater.