Hydrous, low-carbon melting of garnet peridotite

The presence of volatile species in the Earthś upper mantle drives the formation of low-degree melts at pressures and temperatures at which volatile-free mantle rocks would be subsolidus. The two most abundant volatile species, given the oxidation state of the Earthś upper mantle, are carbon dioxide and water; each species has a distinct effect on the melting process.We present experimental melting results from 3 GPa and 13758C on hydrous systems with controlled water contents and rigorously minimized carbon contamination that constrain the independent effects of these volatiles. The hydrous melts in these experiments are in equilibrium with garnet peridotite at pressures reasonable for hydrous melting under mid-ocean ridges. Compared with anhydrous experiments or carbon-rich silicate melting, the addition of water produces a melt with increased SiO₂ content relative to MgO and FeO, tantamount to an increase in the stability of olivine at the solidus relative to the other crystalline phases.We also report a substantial and unexpected change in the composition of clinopyroxene in equilibrium with the melt; the clinopyroxene stability field contracts when water is added to the system, producing clinopyroxenes with higher CaO and lower A_l2O₃ than found at the same pressure without water.The contraction of the clinopyroxene field decreases the bulk partition coefficients of TiO₂, Na₂ O, heavy rare earth elements, U, and H₂O, with important implications for hydrous melting of the mantle; for example, initiating hydrous melting deeper in the garnet lherzolite stability field.