Testing a potential mantle geohygrometer; the effect of dissolved water on the intracrystalline partitioning of Al in orthopyroxene

The presence of water in the Earth's mantle has wide ranging implications and a detailed picture of the spatial and temporal distribution of water in the mantle is needed to be able to understand fundamental global-scale processes. However the interpretation of measured water concentrations in nominally anhydrous minerals (NAMs) from xenoliths or magmatic phenocrysts is complicated by fast diffusion of H and hence the probability of hydration or dehydration during ascent. Rauch and Keppler [1]M. Rauch, H. Keppler, Water solubility in orthopyroxene, Contrib. Mineral. Petrol. 143 (2002) 525-536 have proposed that the partitioning of Al between octahedral and tetrahedral sites in orthopyroxene has the potential to be used as a more reliable mantle geohygrometer than direct measurements of water concentrations in xenolith minerals. In the present study, we have tested the theoretical basis for this geohygrometer by experimentally producing aluminous orthopyroxene samples and measuring the intracrystalline partitioning of Al using very high-field 27Al magic angle spinning nuclear magnetic resonance. In both dry and hydrous orthopyroxene, aluminium is shown to be incorporated by a Tschermak's substitution (one tetrahedral and one octahedral Al), thus the incorporation mechanism of Al in orthopyroxene is not a valid basis for a geohygrometer. However, the effect of OH on the local environment of Al has been observed, and quantification of all the different types of Al in the hydrous samples suggests that OH is incorporated by protonation of O21 and O22 sites. The results suggest that NMR studies on nuclei other than 1H offer new possibilities for studying the interaction of water with mantle minerals.