ANHYDROUS PARTIAL MELTING OF A FERTILE AND DEPLETED PERIDOTITE FROM 2 TO 30 KB AND APPLICATION TO BASALT PETROGENESIS

Abstract

Reversal experiments have been performed to check the method of Jaques and Green (1980) to determine equilibrium partial melts from peridotite compositions. Reversals of the Jaques and Green (1980) calculated equilibrium partial melt (CEPM) compositions have been carried out in two ways: (1) By running CEPM compositions at original P and T conditions and testing for multiple saturation in residual phases of the original experiments. (2) By sandwich/mixed experiments using the CEPM composition plus peridotite (either Hawaiian pyrolite or Tinaquillo lherzolite). The glass (liquid) compositions from the first series of experiments show that the CEPM compositions of Jaques and Green (1980) are too olivine-rich. The glass (liquid) compositions from the second series of experiments define new olivine+orthopyroxene?clinopyroxene cotectics in a molecular normative tetrahedron. The new cotectics plot towards the Qz apex of the tetrahedron, away from the cotectics defined by the CEPM compositions of Jaques and Green (1980). Partial melt compositions have also been determined at 20 and 30 kb, using both the sandwich technique and the approach by modal analysis and mass balance. The results of the experimental study are used to evaluate the petrogenesis of mid-ocean ridge basalts, Hawaiian tholentes and primary magmas in intraoceanic convergent margin settings.