A comparative study of continental vs. intraoceanic arc mantle melting: Experimentally determined phase relations of hydrous primitive melts

It is widely recognized that H2O and other volatile components play a crucial role in mantle melting in subduction zones in both continental and intraoceanic arc settings. Here we report the results of a study aimed at determining the H2O-undersaturated, near-liquidus phase relations for two primitive arc magma compositions with the goal of determining the P–T–H2O conditions of mantle melt segregation beneath an arc of each type. These samples, JR-28, a basaltic andesite (SiO2 = 53.2 wt.%) from Volcán Jorullo, Mexico, and ID-16, a basalt (SiO2 = 48.9 wt.%) from Okmok Volcano, Aleutian Islands, have compositions consistent with near-primary, mantle-derived origins. H2O-undersaturated (2–7 wt.% added H2O) piston-cylinder experiments have been carried out at upper mantle pressures and temperatures (1.0–2.0 GPa and 1100–1350 °C). The near-liquidus mineralogy of these compositions was mapped in P–T–H2O space to constrain the conditions under which these melts are multiply saturated with a mantle residue (lherzolite or harzburgite). Previous measurements of dissolved H2O in olivine-hosted melt inclusions suggest pre-eruptive H2O-contents for JR-28 of ≥ 5.5 wt.% H2O. Inverse and forced saturation experiments show JR-28 melts to be saturated with a harzburgite assemblage at ~ 1175 °C and ~ 1.2 GPa with 5 wt.% H2O and ~ 1150 °C and ~ 1.4 GPa with 7 wt.% H2O. The experimentally determined phase relations of ID-16 suggest equilibrium with a lherzolite residue with ~ 2 wt.% H2O at 1280 °C and 1.4 GPa. The presence of Ca-rich pyroxene in these latter experiments likely reflects the higher bulk CaO content of ID-16 (~ 10.7 wt.% CaO) relative to JR-28 (~ 8.3 wt.% CaO). The temperatures of melt–mantle equilibration for both samples are higher than those predicted by steady-state geodynamic models at the inferred equilibration depths. Our results thus support the hypothesis that ascent of melts from the hottest region of the mantle wedge perturbs the isotherms upward to create a hot region in the shallow mantle beneath the arc. We propose that final melt–mantle equilibration of primitive, H2O-undersaturated melts occurs in the shallow mantle prior to their ascent into the crust and that heterogeneities in uppermost mantle mineralogy therefore influence the compositions of erupted primitive melts. We suggest that basalt ID-16 last equilibrated with lherzolite, whereas the primitive basaltic andesite JR-28 formed by melting of a depleted source and last equilibrated with a harzburgite residue.