Silicic glasses in hydrous and anhydrous mantle xenoliths from Western Victoria, Australia: at least two different sources

Glasses in a hydrous wehrlite and in anhydrous lherzolites from Western Vic toria, Australia, are present as interstitial glasses and secondary glass i nclusions. Interconnections between each other, generally observed as thin necks, are still preserved. These petrographic characteristics are suitable for establishing a space-time relationship. Glasses in hydrous and anhydro us xenoliths show continuous chemical trends apparently governed by differe nt processes. Glass patches in the hydrous wehrlite are interpreted as the product of decompressional breakdown of hydrous phases like amphibole and p hlogopite. However, abundances of some elements suggest mixing and the invo lvement of an additional source. After precipitation of secondary phases (e .g., olivine, clinopyroxene and spinel), the brown microlite-free melt migr ated and reacted with primary clinopyroxene and in rare cases was trapped a s glass inclusions. The observed chemical trend can be explained by crystal lisation of secondary phases of the amphibole breakdown melt with addition of an alkali-volatile-rich phase. In the anhydrous lherzolites, petrographi c and chemical evidences suggest the existence of two glasses: a silica-ric h glass (glass A) and a very silica-rich glass (glass B). The silica-rich g lass A (SiO2: 60-65 wt.%) is interpreted as an initial silicic melt, possib ly generated at mantle depths, with a continuous chemical trend toward row- silica glasses (SiO2: 52 wt.%). This evolution is possibly governed by incr easing melt fractions and dissolution of original apatite. Glass inclusions formed by this melt are rich in CO2 and characterised by a feldspar-diopsi de-olivine normative composition. Furthermore, in the proximity to orthopyr oxene and, due to a later event possibly related to the ascent of the xenol ith, the silica-rich glass acquired a very silicic composition (glass B) by reaction with orthopyroxene and crystallisation of microlites [Zinngrebe, E., Foley, S.F., 1995. Metasomatism in mantle xenoliths from Gees, West Eif el, Germany: evidences for the genesis of calc-alkaline glasses and metasom atic Ca-enrichment. Contrib. Mineral. Petrol., 122, 76-96]. The rare glass inclusions formed by this melt are CO2-free and have a quartz-feldspar norm ative composition. (C) 1999 Elsevier Science B.V. All rights reserved.