The Effect of Iron Minerals on the Thermal Decomposition of Calcium and Magnesium Sulfates: Implications for Interpretation of MSL SAM Evolved Gas Analyses

The Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) rover has been essential in understanding volatile-bearing phases in Gale Crater materials. In SAM evolved gas analysis (EGA) samples are heated to 850^oC. Volatiles detected and their evolution temperatures can constrain sample mineralogy, in the context of measurements by other rover instruments and data from SAM-like analyses of reference materials. Samples analyzed to date have evolved significant SO₂ typically as two "peaks" evolving from 500-700 and from 700-860^oC. In many cases, the only S minerals detected with the Chemistry and Mineralogy (CheMin) instrument are Ca sulfates, which thermally decompose at temperatures above those obtainable by SAM. Sulfides or Fe sulfates detected by CheMin in some samples could evolve a portion of the SO₂ observed but are usually not abundant enough to account for all SO₂. This additional SO₂ could be largely derived from x-ray amorphous sample material. Thermal reactions during analysis which lower the temperatures of SO₂ evolution from sulfates expected to decompose above 850^oC (e.g., Ca and Mg sulfates) are another possible contributor. Our previous work has shown that mixing Mg sulfate with Cl phases detected or inferred in Gale samples (chlorides, perchlorate salts) can result in SO₂ evolution temperatures consistent with SAM data. Chlorine phase-Ca sulfate mixtures have not yet shown SO₂ evolution trends fully consistent with SAM data.

Here we discuss SAM-like EGA of mixtures of Mg and Ca sulfates with 1) iron phases and 2) both chlorine and iron phases, to study the effects of Mars analog iron phases on the evolution of SO₂. Iron phases are known to reduce the decomposition temperatures of perchlorates and chlorates. Based on initial analyses, mixtures with iron phases generally evolve less SO₂ in the SAM temperature range than previous mixtures with chlorine phases.

The first drilled sample of the Vera Rubin Ridge (VRR) was recently analyzed by SAM. The VRR exhibits a striking hematite signature in orbital remote sensing data, and MSL observations indicate several potential iron phases in VRR materials. Constraints on the effects of iron phases on sulfur detected by SAM are both relevant to previous analyses and timely for consideration of data from SAM analyses of VRR samples.