Melting of carbonate-bearing peridotite as a function of oxygen fugacity: implications for mantle melting beneath mid-ocean ridges

The depth of melting beneath mid-ocean ridges (MORs) controls the melt composition as well as its rheology. Since mantle melting below MORs is the main mechanism of mantle degassing and CO₂ emission into the atmosphere and oceans, there is an increasing interest in understanding the sub-ridge mantle conditions leading to its melting. Here we study the effect of oxygen fugacity on melting of carbonate-bearing peridotite at 3 GPa. Two metal—metal-oxide buffers (RRO and IW) were used to influence the fO₂ of the experimental charge. Using Ir–Fe alloy sliding redox sensors, the fO₂ of the two sets of experiments was measured. The solidus at IW + 4.5 was found to be at 950 °C, while at IW + 2.5 melting initiated at 1150 °C. In both sets of experiments, near-solidus carbonatitic melts evolved to carbon-bearing silicate melts with increasing temperature. This study together with previous studies suggest that increasing fO₂ of a carbonate-bearing peridotite results in lowering of its melting temperature. Extrapolating these solidi to higher pressures results in initiation of melting of a relatively oxidizing mantle at ~ 430 km while melting of a more reduced mantle will initiate at depth of ~ 320 km. Very low velocity anomalies in the sub-ridge mantle at depth may reflect the initiation of melting, triggered by the presence of carbonate in the mantle at 1–2 log units below QFM.