Dissolution of orthopyroxene in basanitic magma between 0.4 and 2 GPa: further implications for the origin of Si-rich alkaline glass inclusions in mantle xenoliths

Abstract

A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO2-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures of 1 GPa and below under anhydrous and CO2-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and CO2-bearing experiments is only slightly enriched in SiO2 and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass of similar composition whereas the glass from CO2-bearing experiments is less SiO2 rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene, combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K also causes minor SiO2 enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor.