Silicate glasses in spinel lherzolites from Yemen: origin and chemical composition

Abstract

Anhydrous and amphibole-bearing spinel lherzolites found in alkali basalts from Southern Yemen contain melt-pockets with silicate glasses from which have crystallized euhedral clinopyroxenes, olivines and spinels. These glasses have variable major element compositions related to variations in the composition of the melting phases. In the hydrous lherzolites, the variations in the major element composition of the glasses are directly correlated to the variations in the composition of residual amphiboles in the melt-pockets. The same observations can be made for the composition of clinopyroxene and glass in the anhydrous lherzolites. The trace element compositions of the residual phases and the glasses are consistent in the different samples and all the observations indicate that the glasses originated by in-situ melting of amphibole or clinopyroxene in the lherzolites. If infiltration of a metasomatic melt or fluid caused the melting event, these data are indicative of very low fluid/rock ratios and attainment of conditions close to equilibrium between the fluid and the solid phases. Surprisingly, Cl-rich apatite, when present, was not melted in these samples, which indicates that apatite can be a residual phase during partial melting at mantle conditions. Comparison of the glasses in the Yemen lherzolites with published chemical compositions for mantle glasses shows a dichotomy between glasses produced by (a) in-situ melting of amphibole or clinopyroxene (± phlogopite), either in closed-system or during reaction with a metasomatic fluid at low fluid/rock ratios, and (b) glasses resulting from interaction between the peridotite and a metasomatic melt or fluid at higher fluid/rock ratios.