Abstract:

Recent geodynamic models and geothermometers suggest that slabs in intermediate to hot subduction zone cross the water-saturated basalt solidus, indicating that hydrous silicate melts are important agents of mass transfer from slab to mantle wedge beneath arcs. Yet the effects of basaltic crust-derived hydrous melt fluxing on mantle wedge melting are poorly known. Here we present the melting phase relations of a depleted peridotite + a MORB-derived hydrous silicate melt at a melt:rock mass ratio of 0.1 and 0.05 (3.5 and 1.7 wt.% H₂O, respectively) to simulate fluid-present partial melting of a depleted peridotite, which has been metasomatized by a hydrous silicate melt derived from subducting basaltic crust. Experiments were performed at 2–3 GPa and 900–1250 °C in a piston cylinder, using Au and Au₇₅Pd₂₅ capsules. Amphibole (7–10 wt.%) is stable up to 1000 °C at 2 and 3 GPa coexisting with an assemblage dominated by olivine and opx and with minor fractions of cpx and garnet at 3 GPa. The apparent fluid-saturated solidus of our bulk composition is located at 1000–1050 °C, coinciding with the exhaustion of amphibole at 2 and 3 GPa. Amphibole is exhausted between 0 and 5 wt.% melting at 2 and 3 GPa and dominates the melting reactions in this melting interval along with opx, generating SiO₂ and Al₂O₃-rich, and FeO*- and MgO-poor primitive andesites under fluid-saturated conditions. The melting reactions during low-degree, fluid-saturated melting are incongruent, consuming opx and producing olivine + SiO₂-rich melts and is observed over a wide range of starting compositions and pressures from this study and others. As extent of melting increases and the free fluid phase is consumed, a spectrum of basaltic andesites to basanites are produced. Comparison of experimental partial melts from this and other hydrous peridotite melting studies with natural primitive arc magmas suggests that melting of peridotites with varying bulk compositions but with 2.5 – 4.2 wt.% H₂O can reproduce the major oxide spread and trends of primitive arc magmas globally. From this comparison, it is clear that differences solely in the pressure of hydrous mantle melting, where the partial melts are fluid-under saturated, can account for the first order trends observed in experimental and natural data, with differences in temperature and composition contributing to the compositional spread within these trends. The ubiquity of andesite genesis over a wide range of pressures and bulk compositions during aqueous fluid-saturated melting suggests that the relative rarity of primitive andesitic melt flux through the crust could be related to the fact that such melts are only produced at the base of the mantle wedge where temperatures are relatively low. As fluid-saturated andesitic melts ascend into the hotter core of the mantle wedge, they are likely consumed by higher-degree, fluid-undersaturated melting generating more common hydrous basaltic melts.

Lara, M. & Dasgupta, R. (2020) Partial Melting of a Depleted Peridotite Metasomatized by a MORB-Derived Hydrous Silicate Melt–Implications for Subduction Zone Magmatism. Geochimica et Cosmochimica Acta 290: doi:10.1016/j.gca.2020.09.001