Solidus of carbonated fertile peridotite under fluid-saturated conditions

The solidus for a fertile peridotite composition ("Hawaiian pyrolite") in the presence of a CO₂-H₂O fluid phase has been determined from 10 to 35 kbar. The intersection of the decarbonation reaction (olivine + diopside + CO₂ ←→ orthopyroxene + dolomite) with the pyrolite solidus defines the point Q′, located at 22 kbar and 940 °C. At pressures less than Q′, the solidus passes through a temperature maximum at 14 kbar, 1060 °C. The solidus is coincident with amphibole breakdown at pressures less than 16 kbar. At pressures above Q′, the solidus is defined by the dissolution of crystalline carbonate into a sodic, dolomitic carbonatite melt. The solidus is at a temperature of 925 °C at ∼28 kbar. The solidus temperature above the point Q′ is similar to the solidus determined for Hawaiian pyrolite-H₂O-CO₂ for small contents of H₂O (<0.3 wt%) and CO₂ (<5 wt%), thus indicating that the primary sodic dolomitic carbonatite melt at both solidi has a very low and limited H₂O solubility. The new data clarify the roles of carbonatite melt, carbonated silicate melt, and H₂O-rich fluid in mantle conditions that are relatively oxidized (f_O2 ∼ MW to FMQ). In particular, a carbonatite melt + garnet lherzolite region is intersected by continental shield geothermal gradients, but such geotherms only intersect regions with carbonated silicate melt if perturbed to higher temperatures ("kinked geotherm").