Application of Mineral Equilibria to Estimate Fugacities of H2O, H-2, and O-2 in Mantle Xenoliths from the Southwestern USA

Small amounts of H2O, on the order of tens to hundreds of parts per million, can significantly influence the physical properties of mantle rocks. Determining the H2O contents of nominally anhydrous minerals (NAMs) is one relatively common technique that has been applied to estimate mantle H2O contents. However, for many mantle NAMs, the relation between H2O activity and H2O content is not well known. Furthermore, certain mantle minerals may be prone to H2O loss during emplacement on Earth's surface. The goal of this study is to apply mineral equilibria to estimate values of alpha(H2O) in rocks that originated below the Moho. The chemical compositions of olivine + orthopyroxene + clinopyroxene + amphibole + spinel +/- garnet were used to estimate values of temperature (T), pressure (P), alpha(H2O), hydrogen fugacity (f(H2)), and oxygen fugacity (f(O2)) in 11 amphibole-bearing mantle xenoliths from the southwestern U.S.A. Application of amphibole dehydration equilibria yields values of alpha(H2O) ranging from 0.05 to 0.26 for these 11 samples and the compositions of coexisting spinel + olivine + orthopyroxene yield Delta logf(O2) (FMQ) of -1 to + 0.6. For nine of the samples, values of f(H2) were estimated using amphibole dehydrogenation equilibria, and these values of f(H2) ranged from 6 to 91 bars. Values of f(H2) and f(O2) were combined, using the relation 2H(2)O = 2H(2) + O-2, to estimate a second value of alpha(H2O) that ranged from 0.01 to 0.57 for these nine samples. Values of alpha(H2O), estimated using these two methods on the same sample, generally agree to within 0.05. This agreement indicates that the amphibole in these samples has experienced little or no retrograde H-loss and that amphibole equilibria yields robust estimates of alpha(H2O) that, in these xenoliths, are generally <0.3, and are often 0.1 or less.