Highly Undersaturated Rocks in Upper Mantle Conditions

Abstract

OLIVINE nephelinite and the closely related magmas, basanite and olivine melilite nephelinite, are products of widely distributed but small volcanic eruptions. They commonly contain dense, coarse grained xenoliths produced at high pressures in the upper mantle, particularly lherzolites, pyroxenites and garnet pyroxenites. These magmas therefore derive directly from the mantle and cannot have been modified by shallow, crustal fractionation processes9. It is important to know the variation of the mineralogy of olivine nephelinite as a function of pressure and temperature. Exploratory measurements using a piston-cylinder high pressure apparatus to 36 kilobars, and using techniques which have given equilibrium mineral assemblages in basaltic composition1, have shown a variation in the mineralogy of a nepheline-rich basic composition at sub-solidus temperatures over the pressure range of the crust and upper mantle. The composition studied (Table 1) has very low albite and is classified as transitional between olivine nephelinite and nepheline-rich basanite. The results of the experiments are given in Table 2; phases were identified by X-ray powder diffraction and by optical examination. What follows is a discussion of the mineral reactions required by the data of Table 2.