Hydrogen incorporation in plagioclase

Abstract

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 MO bond lengths in anhydrous Pl structures. Increased uptake of structurally bound H at low fO2is 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.