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 Victoria, Australia, are present as interstitial glasses and secondary glass inclusions. 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 anhydrous xenoliths show continuous chemical trends apparently governed by different processes. Glass patches in the hydrous wehrlite are interpreted as the product of decompressional breakdown of hydrous phases like amphibole and phlogopite. However, abundances of some elements suggest mixing and the involvement of an additional source. After precipitation of secondary phases (e.g., olivine, clinopyroxene and spinel), the brown microlite-free melt migrated and reacted with primary clinopyroxene and in rare cases was trapped as glass inclusions. The observed chemical trend can be explained by crystallisation of secondary phases of the amphibole breakdown melt with addition of an alkali-volatile-rich phase. In the anhydrous lherzolites, petrographic and chemical evidences suggest the existence of two glasses: a silica-rich glass (glass A) and a very silica-rich glass (glass B). The silica-rich glass A (SiO2: 60–65 wt.%) is interpreted as an initial silicic melt, possibly generated at mantle depths, with a continuous chemical trend toward low-silica glasses (SiO2: 52 wt.%). This evolution is possibly governed by increasing melt fractions and dissolution of original apatite. Glass inclusions formed by this melt are rich in CO2 and characterised by a feldspar–diopside–olivine normative composition. Furthermore, in the proximity to orthopyroxene and, due to a later event possibly related to the ascent of the xenolith, 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 Eifel, Germany: evidences for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment. Contrib. Mineral. Petrol., 122, 76–96]. The rare glass inclusions formed by this melt are CO2-free and have a quartz-feldspar normative composition.