Water in Mantle‐derived Xenoliths in the Hannuoba Basalt

Abstract

Abstract: Minerals of various mantle‐derived xenoliths from the Hannuoba basalt in Hebei Province have been studied by means of IR spectroscopy. The results show that all xenoliths from the mantle at depths <75 km contain trace amounts of water (0.45%–11.6×10−2% H2O). The data of about 0.1% H2O contained in primary pyrolite estimated by earlier studies may be on the high side. The water might enter the frameworks of olivine, pyroxene and garnet earlier than it entered those of amphibole and phlogopite. The presence of water in amphibole and phlogopite may be a local phenomenon of water enrichment, which is related to relatively small‐scale magmatic or metasomatic events although they can contain a hundred times more water than pyroxene contains. There is a little more water (1.11%–3.01×10−2% of H2O mostly) in xenoliths from the Hannuoba basalt than in those from mid‐ocean ridge basalt and kimberlites of South Africa (less than 1×10−2% of H2O mostly). This indicates the heterogeneity of water in time and space in the upper mantle. The presence of trace amounts of water in mantle‐derived anhydrous silicate minerals provides basic data for us to understand the distribution of water in the upper mantle and the role of water in the mantle dynamic and geochemical processes.