Phase Relations and Melting of Anhydrous K-bearing Eclogite from 1200 to 1600°C and 3 to 5 GPa

To investigate eclogite melting under mantle conditions, we have performed a series of piston-cylinder experiments using a homogeneous synthetic starting material (GA2) that is representative of altered mid-ocean ridge basalt. Experiments were conducted at pressures of 3·0, 4·0 and 5·0 GPa and over a temperature range of 1200–1600°C. The subsolidus mineralogy of GA2 consists of garnet and clinopyroxene with minor quartz–coesite, rutile and feldspar. Solidus temperatures are located at 1230°C at 3·0 GPa and 1300°C at 5·0 GPa, giving a steep solidus slope of 30–40°C/GPa. Melting intervals are in excess of 200°C and increase with pressure up to 5·0 GPa. At 3·0 GPa feldspar, rutile and quartz are residual phases up to 40°C above the solidus, whereas at higher pressures feldspar and rutile are rapidly melted out above the solidus. Garnet and clinopyroxene are the only residual phases once melt fractions exceed 20% and garnet is the sole liquidus phase over the investigated pressure range. With increasing melt fraction garnet and clinopyroxene become progressively more Mg-rich, whereas coexisting melts vary from K-rich dacites at low degrees of melting to basaltic andesites at high melt fractions. Increasing pressure tends to increase the jadeite and Ca-eskolaite components in clinopyroxene and enhance the modal proportion of garnet at low melt fractions, which effects a marked reduction in the Al2O3 and Na2O content of the melt with pressure. In contrast, the TiO2 and K2O contents of the low-degree melts increase with increasing pressure; thus Na2O and K2O behave in a contrasted manner as a function of pressure. Altered oceanic basalt is an important component of crust returned to the mantle via plate subduction, so GA2 may be representative of one of many different mafic lithologies present in the upper mantle. During upwelling of heterogeneous mantle domains, these mafic rock-types may undergo extensive melting at great depths, because of their low solidus temperatures compared with mantle peridotite. Melt batches may be highly variable in composition depending on the composition and degree of melting of the source, the depth of melting, and the degree of magma mixing. Some of the eclogite-derived melts may also react with and refertilize surrounding peridotite, which itself may partially melt with further upwelling. Such complex magma-genesis conditions may partly explain the wide spectrum of primitive magma compositions found within oceanic basalt suites.