Experimental investigation of the composition of incipient melts in upper mantle peridotites in the presence of CO2 and H2O

The compositions of mantle-derived magmas indicate a substantial variety in the abundances of volatiles in the upper mantle. CO2 and H2O depress the melting point of mantle peridotites considerably, delineating a pressure-temperature region of incipient melting where small degrees of melt exist over a large temperature range (~300 °C) before major melting begins. However, the chemical characterization of these melts in high-pressure experiments is challenging at low melt fractions (melt pockets may occupy volumes of only 10–50 μm3) because of analytical uncertainties related to the ubiquitous formation of metastable phases during quenching.