EXPERIMENTAL-DETERMINATION OF COMPOSITION OF MELT FORMED BY EQUILIBRIUM PARTIAL MELTING OF PERIDOTITE AT HIGH-PRESSURES USING AGGREGATES OF DIAMOND GRAINS

The composition of melt formed by equilibrium partial melting of peridotite at high pressures using a new experimental method is described. A thin layer of diamond powder or a small loosely sintered diamond chip is sandwiched between finely ground peridotite and heated to partially melt the host peridotite at high pressures. Melt formed by partial melting of peridotite segregates and migrates into the pore space between diamond grains. The melt equilibrates with the host peridotite only after the pore space is completely filled with melt. Microprobe analysis of segregated quenched melt trapped between diamond grains can be made without problems involving quenching or overgrowth of coexisting crystals. Melt which fills the pore space of the diamond grains is equilibrated with residual solids within 24 hours at 1400-degrees-C in the synthetic system Mg2SiO4-NaAlSiO4-SiO2 at 10, 15 and 20 kbar. This method can be applied to determine both compatible and incompatible elements in melt formed by partial melting of peridotite or any other rocks at high pressures. Using this method the compositions of melts formed by partial melting of a spinel lherzolite were determined at pressures between 10 and 20 kbar.