Mineralogy and glass chemistry of recent volcanic ejecta from Mt Erebus, Ross Island, Antarctica

Abstract

Bombs erupted in 1972 and recent flows of anorthoclase phonolite from Mt Erebus both contain phenocrysts of anorthoclase and microphenocrysts of olivine (Fa₄₆Te₃Fo₅₁), clinopyroxene (Wo₄₈Fs₁₅En₃₇), titanomagnetite (USP₇₀), pyrrhotite, and apatite; the associated glass is peralkaline (agpaitic index = 1.06). The composition of glass inclusions in large anorthoclase (Ab₆₅An₁₆Or₁₉) phenocrysts, which form a lag gravel on the summit cone of Mt Erebus, is generally more evolved than that from the groundmass of recent flows and bombs. Glass inclusions from between the core and rim of anorthoclase phenocrysts represent variations with time and show a small and systematic increase of Al₂O₃ and decrease of FeO_T (total iron as FeO), possibly owing to reaction of the glass with the anorthoclase after entrapment. In contrast, whole rock-groundrnass trends show decreasing Al₂O₃ and increasing FeO_T. The whole rock compositions of recent flows and ejecta are similar to older (up to 1 m.y.) anorthoclase phonolite flows exposed around the base of Mt Erebus.