In situ origin for glass in mantle xenoliths from southeastern Australia: insights from trace element compositions of glasses and metasomatic phases

Siliceous, aluminous and alkali-rich glasses, commonly found in patches and veins in spinel peridotite xenoliths, have been attributed to a number of different origins. These include low-degree primary melts of the mantle, ex otic metasomatic melts influxing into the lithosphere, or breakdown of amph ibole, and other phases during high-temperature transport of the xenoliths to the surface in their host magmas. We present new laser ablation-inductiv ely coupled plasma-mass spectrometry (LA-ICP-MS) analyses of trace element abundances in glasses, and in metasomatically introduced phases (clinopyrox ene, amphibole, phlogopite, apatite) from a suite of spinel wehrlite, Iherz olite and harzburgite xenoliths from southeastern Australia. The majority o f glass compositions are best explained by melting of amphibole (usually co mplete, as amphibole is now absent from most samples) with varying but sign ificant contributions from partial melting of clinopyroxene. However, some glasses require additional components derived from partial or complete moda l melting of phlogopite, or apatite. The data confirm our earlier model, th at the glass present in patches in these samples derives from high-temperat ure, transport-related breakdown of a metasomatic phase assemblage (amphibo le + clinopyroxene + phlogopite + apatite) present in the xenoliths prior t o their entrainment in the host magmas. (C) 1999 Elsevier Science B.V. All rights reserved.