Glasses in mantle xenoliths from western Victoria, Australia, and their relevance to mantle processes

Mantle-derived xenoliths from western Victoria, Australia, contain glass in patches and veinlets, and as secondary inclusions within xenolith minerals. Glass patches are commonly associated with primary clinopyroxene, spinel and in some cases relict pargasitic amphibole, phlogopite, apatite or calcitic carbonate. The patches and veins are filled with secondary microphenocrysts of olivine, clinopyroxene and spinel. The siliceous, aluminous, alkali-rich glasses display ranges in major element compositions which are similar to those of glasses in mantle xenoliths worldwide. Textures and compositions of glass and associated phases in the Victorian xenolith suite argue strongly against origins involving equilibrium partial melting of spinel peridotite, or migration of exotic metasomatic melts. Metasomatism of the southeastern Australian lithosphere is expressed by formation of amphibole, phlogopite, and apatite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in lherzolite or harzburgite. Partial or complete melting of this metasomatic assemblage occurred immediately prior to, or during, entrainment of the xenolith in the host magmas, and locally produced disequilibrium liquids, now preserved as glass. Melting in the xenoliths was caused by the thermal and decompressional effects of transport in the host magma, or by thermal and metasomatic effects in the lithosphere associated with adjacent intrusion of magmas. Large inter-xenolithic variation in glass compositions was caused by variations in the nature and proportions of the precursor metasomatic assemblage, by reaction of the melts with primary orthopyroxene in the lherzolites and harzburgites, and by rapid, disequilibrium crystallization from the melts of an assemblage of olivine, clinopyroxene and spinel in all samples.