An Experimental Study of Water in Nominally Anhydrous Minerals in the Upper Mantle near the Water-saturated Solidus

Abstract

The incorporation of water in olivine and pyroxenes interlayered within fertile lherzolite compositions was explored experimentally near the wet solidus of lherzolite at 2·5 and 4 GPa. The concentrations and activities of water were varied to establish the partitioning of water between nominally anhydrous minerals (NAMs) and the hydrous minerals pargasite and phlogopite. The water content in NAMs was determined by Fourier-transform infrared (FTIR) spectroscopy. The main absorption bands in NAMs from these experiments are very similar to those generally found in natural upper mantle peridotites. Olivine, orthopyroxene and clinopyroxene contain 32–190, 290–320 and 910–980 ppm of water under the studied conditions. The partition coefficients between coexisting clinopyroxene and orthopyroxene (Dcpx/opx) are 2·7 ± 1·1 and 3·5 ± 1·5 at 2·5 and 4 GPa respectively, whereas values for coexisting orthopyroxene and olivine (Dopx/ol) are 6·7 ± 2 and 4·7 ± 1·1, at 2·5 and 4 GPa respectively. The storage capacity in NAMs in a model mantle composition close to the vapour-saturated solidus (water-rich vapour) is ∼190 ppm at both 2·5 and 4 GPa. Pargasite is the most important phase accommodating significant amounts of water in the uppermost mantle. Its breakdown with increasing pressure at 3 GPa at the vapour-saturated solidus (which is at ∼1025°C at 2·5 GPa) results in a sharp drop in the water storage capacity of peridotite from ∼1000 ppm to ∼190 ppm H2O. At pressures >3 GPa, melting in fertile lherzolite begins at the vapour-saturated solidus if the bulk H2O concentration exceeds ∼190 ppm.