Constraints on Mantle Water from Peridotite Pyroxenes

Abstract:

The occurrence of volatiles in the Earth’s mantle influences properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, secondary ion mass spectrometry was used to measure H₂O in olivine, orthopyroxene and clinopyroxene in xenolith, orogenic, and abyssal peridotites. The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H₂O concentrations, in agreement with mineral diffusion data. In all samples, olivine diffusively lost water during emplacement, as indicated by disequilibrium between olivine and co-existing pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high temperature water contents, based on partitioning that agrees with published values from experiments and xenoliths. However, preservation of water in pyroxenes suggests that existing experimental diffusion data overestimates the diffusion coefficient for water in pyroxene.

Among samples in this study, clinopyroxenes have 60-670 ppm H₂O and orthopyroxenes have 10-300 ppm. Using the experimentally determined value of 0.11 for the olivine/orthopyroxene partition coefficient, olivine is calculated to have contained 8-34 ppm H₂O. The highest olivine water concentration translates to an effective viscosity of 6×10¹⁹ Pa s at 1250°C and ~15 km depth, compared to a dry effective viscosity of 2.5×10²¹ Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for re-equilibration.