Liquid segregation parameters from amphibolite dehydration melting experiments

Abstract

We have experimentally obtained some of the parameters necessary for understanding the segregation of silicate liquid from amphibolite undergoing dehydration melting at 1 GPa and 750–1000°C. The solidus for this calcic amphibolite (68% Mg-Hb, 32% Pl (An_(90))) is <750°C. Amphibolites begin to melt at relatively high temperatures in the garnet-absent field, but the solidus appears to backbend at ∼1 GPa, coincident with the garnet-in boundary. Hornblende breakdown due to garnet formation releases H_2O and causes melting. Thus, in the garnet-present field (≥1 GPa), the amphibolite dehydration melting solidus may be coincident with the H_2O-saturated solidus. Liquid interconnectivity may be achieved at <900°C and <5 vol % liquid, based on both physical and chemical data from solid rock runs. Mass balance calculations from powdered rock runs suggest that small amounts (∼5–15 vol %) of hydrous (≥4 wt % H_2O), low-viscosity (10^3–10^4 Pa s), heavy rare earth element-depleted, felsic liquid may be segregated during amphibolite dehydration melting at ≥875°C. The rapid breakdown of coarse-grained Hb cores may lead to the formation of transient H_2O-saturated liquids with even lower viscosities (∼10^2 Pa s). Although comprising only a small portion of the melting cycle, these H_2O-rich conditions may enhance the segregation of liquid by reduction of liquid viscosities and by mechanical effects on the restite (e.g., increased deformation and liquid fracturing). During anatexis of coarse-grained (natural) rocks, transient conditions may control the initial stages of liquid segregation.