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Published February 2019 | Supplemental Material
Journal Article Open

The oxidation state of sulfur in lunar apatite


Lunar apatites contain hundreds to thousands of parts per million of sulfur. This is puzzling because lunar basalts are thought to form in low oxygen fugacity (f_(O_2)) conditions where sulfur can only exist in its reduced form (S^(2–)), a substitution not previously observed in natural apatite. We present measurements of the oxidation state of S in lunar apatites and associated mesostasis glass that show that lunar apatites and glass contain dominantly S^(2–), whereas natural apatites from Earth are only known to contain S^(6+). It is likely that many terrestrial and martian igneous rocks contain apatites with mixed sulfur oxidation states. The S^(6+)/S^(2–) ratios of such apatites could be used to quantify the f_(O_2) values at which they crystallized, given information on the portioning of S^(6+) and S^(2–) between apatite and melt and on the S^(6+)/S^(2–) ratios of melts as functions of f_(O_2) and melt composition. Such a well-calibrated oxybarometer based on this the oxidation state of S in apatite would have wide application.

Additional Information

© 2019 Mineralogical Society of America. Manuscript received August 31, 2018. Manuscript accepted November 1, 2018. Manuscript handled by Ian Swainson. We thank A. Lanzirotti and M. Newville for assistance in beamline operations at the Advanced Photon Source, Argonne National Laboratory (APS ANL). We also thank the curatorial staff at NASA Johnson Space Center for allocations of Apollo samples for this study. We thank A. Bell, G. Ustunisik, and an anonymous reviewer for constructive comments. This research was performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), APS ANL. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1634415) and Department of Energy—GeoSciences (DE-FG02-94ER14466). This research used resources of APS, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by ANL under Contract No. DE-AC02-06CH11357. Support for this research was provided by the University of California and by NASA's Planetary Science Research Program.

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