The oxidation state of sulfur in Apollo samples 71035 and 71055

Introduction: The discovery of 100s-1000s ppm sulfur [1-3] was initially puzzling because lunar basalts are thought to form at low oxygen fugacity (fO2) conditions (including at the time of apatite crystallization) where sulfur exists in its reduced form (S2-), a substitution not previously observed in natural apatite. Recent S-XANES measurements of apatites and proximal mesostasis in the lunar basalts 12039 and 10044 have shown that sulfur is indeed present as S2in both the mesostasis glass and apatite when measurements are performed far from cracks or pits in the thin section [4]. This observation is consistent with other mineralogical indications of low fO2 (IW-1), such as the presence of Fe-metal, ulvöspinel, ilmenite, fayalite, and silica. In the same study, in addition to clear spectral evidence for the dominance of S2-, analyses of apatite grains in both samples that were acquired near cracks or pits in the thin section sometimes revealed non-negligible spectral evidence for the presence of S6+(e.g., S6+/ΣS > 0.03; [4]). Because S6+ was not observed in the mesostasis glass near the apatite grains measured, or in any phase far from cracks or pits in the studied thin sections, it was interpreted as either primary S2altered to S6+ in the thin section, or S6+ of secondary origin, deposited in the fractures of the samples [4, 5]. It is unknown whether this alteration is lunar or terrestrial in origin, with implications for lunar petrogenesis if lunar [e.g., 5], or sample handling and curation if terrestrial. To test between lunar and terrestrial origins for observed S6+ in Apollo-era thin sections, we will measure the oxidation state of sulfur in apatite and associated phases in a specially frozen sample from a boulder sampled at Station 1A during the Apollo 17 mission as part of the NASA Apollo Next Generation Sample Analysis (ANGSA) program. Here, we present “control” measurements of samples 71035 and 71055, using Apollo era thin sections of aliquots of these rocks that were processed upon return to Earth and subsequently stored under N2 atmospheres at room temperature. These will be compared to measurements using newly made thin sections of aliquots of the same rocks (71035 and 71055). In addition, 71036, which has been in cold storage (-20°C) since several weeks after the return of the Apollo 17 mission to Earth, has been made available recently through the ANGSA program. With 71036 we can test the effect of storage temperature on potential oxidation of S2to S6+ in the relatively oxidizing and warm conditions of Earth’s surface. Sample descriptions: 71035 and 71055 are both samples of the same boulder encountered at Station 1a of the Apollo 17 EVAs. Sample 71055 has been dated at 3.64 ± 0.09 Ga [6]. Both 71035 and 71055 are vesicular high titanium basalts with a cosmic ray exposure age of 110 ± 7 Ma [7]. They are olivine-, Capyroxeneand plagioclase-bearing ilmenite basalts with minor amounts of ulvöspinel, troilite, Fe metal, SiO2, apatite, and mesostasis glass [8]. In addition to these previously reported phases, detailed examination in the present study also reveals the presence of baddeleyite, tranquillityite, merrillite, and K-feldspar [9] (Fig. 1). Importantly, the presence of ulvöpsinel, troilite, Femetal, and silica in the mesostasis with apatite fixes fO2 during apatite crystallization to ≲ IW.