CLASSES IN MANTLE XENOLITHS FROM WESTERN VICTORIA, AUSTRALIA, AND THEIR RELEVANCE TO MANTLE PROCESSES

Mantle-derived xenoliths from western Victoria, Australia, contain gla ss in patches and veinlets, and as secondary inclusions within xenolit h minerals. Glass patches are commonly associated with primary clinopy roxene, spinel and in some cases relict pargasitic amphibole, phlogopi te, apatite or calcitic carbonate. The patches and veins are filled wi th secondary microphenocrysts of olivine, clinopyroxene and spinel. Th e siliceous, aluminous, alkali-rich glasses display ranges in major el ement compositions which are similar to those of glasses in mantle xen oliths worldwide. Textures and compositions of glass and associated ph ases in the Victorian xenolith suite argue strongly against origins in volving equilibrium partial melting of spinel peridotite, or migration of exotic metasomatic melts. Metasomatism of the southeastern Austral ian lithosphere is expressed by formation of amphibole, phlogopite, an d apatite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in Iherzolite or harzburgite. Partial or complete melt ing of this metasomatic assemblage occurred immediately prior to, or d uring, entrainment of the xenolith in the host magmas, and locally pro duced disequilibrium liquids, now preserved as glass. Melting in the x enoliths was caused by the thermal and decompressional effects of tran sport 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, b y reaction of the melts with primary orthopyroxene in the Iherzolites and harzburgites, and by rapid, disequilibrium crystallization from th e melts of an assemblage of olivine, clinopyroxene and spinel in ail s amples.