We conducted experiments at high pressure (P) and temperature (T) to measure hydrogen solubility in plagioclase (Pl) with a range of compositions (An15 to An94). Experiments were run at 700–850 °C, 0.5 GPa, and fO2 close to either the Ni-NiO (NNO) or iron-wüstite (IW) oxygen buffers. Experiments at 700 °C on An15 (containing 0.03 wt% FeO) reveal no dependence of H solubility on fO2 between IW and NNO, but experiments at 800–850 °C on other compositions (with 0.3–0.5 wt% FeO) demonstrate that H solubility is enhanced by a factor of ∼2–3 at IW compared to NNO, consistent with previous experiments by Yang (2012a) on An58. By analogy with synthetic hydrogen feldspar (HAlSi3O8), we infer that the predominant mechanism for H incorporation in Pl is through bonding to O atoms adjacent to M-site vacancies, and we propose likely O sites for H incorporation based on M–O bond lengths in anhydrous Pl structures. Increased uptake of structurally bound H at low fO2 is explained by the formation of defect associates resulting from the reduction of Fe3+ in tetrahedral sites to Fe2+, allowing additional H to be incorporated in adjacent M-site vacancies. This mechanism counteracts the expected effect of water fugacity on H solubility. We also speculate on possible substitutions of H on tetrahedral vacancies, as well as coupled H-F substitution. Enhanced incorporation of H in Pl at low fO2 may have implications for estimating the water content of the lunar magma ocean. However, mechanisms unrelated to low fO2 are needed to explain high H contents in terrestrial Pl xenocrysts, such as those found in basalts from the Basin and Range.
Bibliographical noteFunding Information:
This work was supported by grants from the National Science Foundation (EAR-1347908 and EAR-1625422), NASA (NNX15AH71G and 80NSSC19K0959), and the Undergraduate Research Opportunities Program at UMN. We thank Yunbin Guan for assistance with SIMS and the following people for providing samples used as starting or reference materials: George Rossman, Christopher Nye, Richard Hervig, and Brigitte Stoll. George Rossman is also gratefully acknowledged for discussions about sample localities and other topics related to nominally anhydrous minerals. The manuscript was improved by helpful reviews from Anne Peslier, Xiaozhi Yang, an anonymous referee, and the associate editor.
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