The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1middle dot0 to 3middle dot2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes

Abstract

Fluid-absent melting experiments on a zoisite- and phengite-bearing eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and rutile) were performed to constrain the melting relations of these hydrous phases in natural assemblages, as well as the melt and mineral compositions produced by their breakdown. From 1·0 to 3·2 GPa the solidus slopes positively from 1·5 GPa at 850°C to 2·7 GPa at 1025°C, but bends back at higher pressures to 975°C at 3·2 GPa. The melt fraction is always low and the melt compositions always felsic and become increasingly so with increasing pressure. The normative Ab–An–Or compositions of the initial melts vary from tonalites at 1·0 GPa to tonalite–trondhjemites at 1·5 GPa, adamellites at 2·1 and 2·7 GPa, and to true granites at 3·2 GPa. At pressures <∼2·5 GPa zoisite and phengite break down more or less simultaneously. At 3·2 GPa and 1000°C zoisite is unreacted whereas phengite is absent, so that the first formed melt at these conditions is granitic. Our experiments show that if sufficiently high temperatures (of the order of 1000°C) are attained, zoisite- and phengite-bearing eclogites can produce small fractions of silicic melts of a wide range of compositions. These melts are rich in water and, probably, in Sr and other incompatible elements, so that they can act as metasomatic agents in the mantle wedge.