Fluids in the Shallow Mantle of Southeastern Australia: Insights from Phase Equilibria

Small amounts of water (10s to 100s of ppm) can have a profound effect on the properties of mantle peridotites, including viscosities, conductivities, and melting temperatures. Measuring the water content of nominally anhydrous minerals (NAMs) has provided insight into the amounts of water contained within mantle rocks. However, converting from NAM water contents to the activity of H2O is non-trivial. Equilibria involving amphibole can be used to determine values of the activity of H2O (aH2O) at the time of mineral equilibration. This approach yields low values of the activity of H2O (< 0.3) for four peridotite xenoliths from Southeastern Australia. These four xenoliths also record values of oxygen fugacity (fO2) that range from -0.2 to -1.2 (Log units relative to FMQ). All these values of fO2 are inconsistent with the presence of a CH4-rich fluid (too oxidizing), and the lowest value of oxygen fugacity, as recorded by one sample, is inconsistent with the presence of a CO2-rich fluid.