Melting of phengite-bearing eclogite at pressures of 4 and 9 GPa relevant to deep regions of a subduction zone

Abstract

Melting experiments undertaken with finely ground powder of phengite-bearing eclogite yielded solidus temperatures of about 970°C at 4 GPa and 1250°C at 9 GPa. Additional experiments with a rock powder of psammopelitic composition established a solidus at 9 GPa at a temperature of 1350°C. Initial melts produced from both rocks are rich in potassium. The melts generated from eclogite tend to become richer in Na and Ca with rising temperature due to increasing decomposition of clinopyroxene. At the maximum temperatures of the experiments with eclogite, up to 450°C above the solidus at 4 GPa, this phase is still present in the restite together with abundant garnet. In the temperature interval of 1100-1300°C, when 22-30% of the studied eclogite was melted, the melts are quartz monzonitic in composition. According to the reported experimental results, we suggest that partial melting of oceanic crust is unlikely in a subduction zone. However, ascending melange diapirs, composed of material from the upper portion of a deep-seated subducted oceanic slab, can partially melt in the hot mantle wedge. The thus generated melts further ascend to contribute to lavas of magmatic arc systems.