A review of water contents of nominally anhydrous natural minerals in the mantles of Earth, Mars and the Moon

Olivine, pyroxene and garnet are nominally anhydrous but can accommodate tens to hundreds of parts per million (ppm) H2O or “water” in the form of protons incorporated in defects in their mineral structure. This review concerns the amount of water in nominally anhydrous minerals from mantle and mantle-derived rocks: peridotites, eclogites, megacrysts, basalts and kimberlites. Trends between internal and external parameters and water content in olivine, pyroxene, and garnet of mantle rocks allow us to identify what controls their H intake. The water content of pyroxenes and garnets in peridotites appears to depend primarily on mineral chemistry, while that of olivines may more readily reflect water activity and water fugacity conditions in the mantle. In eclogites, both mineral chemistry and metamorphic pressure control the water intake of pyroxene and garnet. The water content of minerals in crystallized melts (basalt and kimberlite phenocrysts, xenocrysts, and megacrysts) is determined by the degree of differentiation, the amount of water in the parent melt, and degassing. Basalt and cumulate minerals from Martian meteorites may be as water-rich as their Earth equivalents. No water has been detected at present in Moon minerals but low amounts in degassed basaltic glasses signify that deep Moon reservoirs may still retain water. The presence of water in mantle minerals, typically amounts of tens to hundreds of ppm, enhances their deformation properties. Water contents of peridotite minerals in the oceanic upper mantle, in and around cratons, and in subduction zones may have tremendous influence on Earth's geodynamics.