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. This systematic partial melting study presents carefully determined compositions of incipient melts of a range of peridotites in the presence of CO₂ + H₂O mixtures at 2.5 to 7 GPa. Four different fertile and depleted peridotites were used: Hawaiian pyrolite, K₂O-enriched pyrolite, MORB pyrolite and depleted lherzolite. To arrive at accurate melt compositions, we introduce the melt tomography method that integrates multiple area scans of melt pockets polished to several depths. Results confirm that incipient and low degree melts progress abruptly (within 25 °C) from carbonatitic towards melilititic-nephelinitic compositions at 2.5 GPa, whereas they progress gradually from carbonate-rich to carbonated silicate (aillikitic) compositions at 4–5 GPa. Melt compositions at near-solidus conditions are mainly controlled by the breakdown of carbonate, and hydrous phases such as pargasite and phlogopite, and become less siliceous and slightly more magnesian with increasing pressure at given melt fractions. Melts exhibit strong increases in SiO₂ (2.75 to 44 wt%) with increasing temperature, whereas TiO₂, Na₂O and K₂O decrease. The generally strongly potassic (K₂O ≤ 6.63 wt%) and sodic (Na₂O ≤ 3.06 wt%) character of the volatile-rich, incipient and low-degree melts indicate that these would act as reactive metasomatic agents that may transport large amounts of energy and induce chemical changes in large volumes of the upper mantle.