Partial melting of phlogopite-bearing synthetic spinel- and garnet-lherzolites

Abstract

Liquid appears in equilibrium with phlogopite, forsterite, enstatite, diopside, garnet and vapour (phlogopite-garnet lherzolite) in the system CaOK 2OMgOAl 2O 3SiO 2H 2O at 1080 ± 10°C, 30 kbar. This liquid contains 14% olivine, 19% hypersthene and 12% diopside in the C.I.P.W. norm, and contains 3.6% K 2O by weight. At 15 kilobars liquid appears in equilibrium with phlogopite, forsterite, enstatite, diopside, spinel and vapour (phlogopite-spinel-lherzolite) at 980 ± 10°C. This liquid contains 1.0% K 2O and has 6.5% quartz, 28.5% hypersthene and a mere trace of diopside in the C.I.P.W. norm. Neither of these liquids bears any resemblance to kimberlite fluids. The relatively low K 2O and high MgO of these liquids are unambiguously established by bracketing of the relevant compositions. At much higher pressures than 30 kbar it is conceivable that the initial partial melting product of phlogopite-garnet-lherzolite may become critically undersaturated in silica and rich in K 2O, when it would resemble kimberlite. The extract required to modify the 30-kbar liquid to become the 15-kbar liquid consists essentially of diopside and phlogopite.