Water content and nature of solutes in shallow-mantle fluids from fluid inclusions

This study discusses new and published data on the composition of fluid inclusions contained in mantle minerals of spinel and garnet peridotite xenoliths, in samples from geodynamically distinct settings (Ethiopian plateau, Hawaii, Canary Islands, and western Mediterranean region). Based on spectroscopic Raman and FTIR analyses we show that, contrary to a commonly held view, fluid inclusions either contain relevant amounts of unsuspected H2O, or represent a ‘‘dehydrated’’ composition from multicomponent aqueo-carbonic fluids. We identify water loss from fluid inclusions through decrepitation, stretching and hydrogen diffusion. We also show that talc, magnesite, chlorides, and sulfates represent common phases in fluid inclusions. Talc and magnesite form through reactions of fluids with the surrounding minerals. Thermodynamic modeling in the MFSHC system of observed reactions between fluid inclusions and surrounding mantle minerals provides the basis for predicting water amounts in shallow-mantle fluids, and suggests XH2O in the range of 10–50 mol%. Model hydrous fluids are relatively enriched solutions, dominated by Si, Cl, and alkalies, with significant amounts of Ca, and S, and low levels of Mg and Fe. This study argues that multicomponent hydrous fluids may be widespread in the shallow mantle, not only in subduction zones but also in intraplate and extensional settings.