^1,2Zachary F.M. Burton,^2,3Janice L. Bishop,⁴Peter A.J. Englert,⁵Anna Szynkiewicz,⁶Christian Koeberl,⁴Przemyslaw Dera,⁴Warren McKenzie,⁷Everett K. Gibson
American Mineralogist 108, 1017-1031 Link to Article [http://www.minsocam.org/msa/ammin/toc/2023/Abstracts/AM108P1017.pdf]
¹Department of Earth and Planetary Sciences, Stanford University, Stanford, California 94305, U.S.A.
²Carl Sagan Center, The SETI Institute, Mountain View, California 94043, U.S.A.
³NASA Ames Research Center, Moffett Field, California 94035, U.S.A.
⁴Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawaii 96822, U.S.A.
⁵Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, U.S.A.
⁶Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria 7
NASA Johnson Space Center, Houston, Texas 77058, U.S.A
Copyright: The Mineralogical Society of America

Understanding past and present aqueous activity on Mars is critical to constraining martian aqueous
geochemistry and habitability, and to searching for life on Mars. Assemblages of minerals observed
at or near the martian surface include phyllosilicates, sulfates, iron oxides/hydroxides, and chlorides,
all of which are indicative of a complex history of aqueous activity and alteration in the martian past.
Furthermore, features observed on parts of the martian surface suggest present-day activity of subsurface
brines and at least transient liquid water. Terrestrial analogs for younger and colder (Hesperian–Amazonian) martian geologic and climatic conditions are available in the McMurdo Dry Valleys (MDV)
of Antarctica and provide opportunities for improved understanding of more recent aqueous activity
on Mars. Here, we study the VXE-6 intermittent brine pond site from Wright Valley in the MDV
region and use coordinated spectroscopy, X-ray diffraction, and elemental analyses to characterize
the mineralogy and chemistry of surface sediments that have evolved in response to aqueous activity
at this site. We find that brine pond activity results in mineral assemblages akin to aqueous alteration
products associated with younger sites on Mars. In particular, surficial chlorides, a transition layer
of poorly crystalline aluminosilicates and iron oxides/hydroxides, and a deeper gypsum-rich interval
within the upper 10 cm of sediment are closely related at this Antarctic brine pond site. Activity of the
Antarctic brine pond and associated mineral formation presents a process analog for chemical alteration on the martian surface during episodes of transient liquid water activity during the late Hesperian
and/or more recently. Our results provide a relevant example of how aqueous activity in a cold and
dry Mars-like climate may explain the co-occurrence of chlorides, clays, iron oxides/hydroxides, and
sulfates observed on Mars.

^1,2Proteek Chowdhury,²Maryjo Brounce,³Jeremy W. Boyce,³Francis M. McCubbin
Journal of Geophysicaöl Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2022JE007634]
¹Department of Earth, Environmental, and Planetary Sciences, Rice University, Houston, TX
²Department of Earth and Planetary Sciences, University of California, Riverside,CA
³Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX
Published by arrangement with John Wiley & Sons

Apatite can incorporate sulfur in its reduced form (S2-) when apatite equilibrates with a silicate melt at reducing conditions. Incorporation of sulfate (S6+) has been observed in terrestrial apatite under oxidizing conditions. Thus, it has been suggested that the proportions of S6+/S2- in apatite may record the oxygen fugacity (fO2) during the formation and/or equilibration of apatite grains with a silicate melt in a wide variety of igneous and metamorphic rocks, including from Earth, Mars, the Moon, and in materials from the asteroid belt. Martian rocks, which record fO2 values intermediate between those recorded by rocks from the Moon and Earth, may have apatite that contains only S2- or mixtures of S6+ and S2-. Here, we present new measurements of the oxidation state of sulfur in apatite grains in the basaltic shergottite, Shergotty, which exhibits spectral features consistent with the presence of sulfide (S2-) structurally bound in apatite, and no evidence for the presence of sulfite (S4+) or sulfate (S6+). The presence of sulfide-only apatite in Shergotty is consistent with other mineralogical records of fO2 in this meteorite, which are calculated from other late-stage crystallizing phases like Fe-Ti oxides as well as from early crystallizing phases like clinopyroxene (DEuCpx/melt) of ΔIW+1.9 to ΔIW+3.5. At these fO2 values, S is present in silicate melts as only S2-, and this suggests that the oxidation state of sulfur records and preserves the fO2 during the igneous crystallization of apatite reinforcing the idea that sulfur in apatite can be used as an igneous oxybarometer.