PARTIAL MELTING OF DRY PERIDOTITES AT HIGH-PRESSURES - DETERMINATION OF COMPOSITIONS OF MELTS SEGREGATED FROM PERIDOTITE USING AGGREGATES OF DIAMOND

The compositions of melts formed by partial melting of two relatively fertile spinel lherzolites were determined at pressures between 10 and 30 kbar under dry conditions using a layer of diamond aggregates sandwiched between peridotite layers. Partial melts segregate and migrate into the pore space between diamond grains soon after their formation. Overgrowth of minerals at quenching modifies the composition of coexisting melt, but this modification does not extend to the trapped melt in the diamond layer. Microprobe analyses of the trapped melt, therefore, can determine the melt compositions without the quench problem. Melts formed by low degrees of partial melting (approximately 5%) can be analyzed successfully by this method. The effect of the source composition on partial melting was examined for the two different lherzolites. Both partial melts have nearly the same SiO2 and MgO contents at the same pressure and temperature conditions regardless of their different Mg/(Mg + Fe) ratios, suggesting that SiO2 and MgO contents in partial melts depend little on the source composition. In contrast, FeO, Al2O3, CaO and incompatible elements are controlled by the composition of the source peridotite as well as by the degree of partial melting. In the normative projections, the differences in the lherzolite compositions do not significantly shift the isobaric compositional trends of the partial melts.