Glass inclusions and melt compositions of the Toba Tuffs, northern Sumatra

Abstract

Glass (melt) inclusions in quartz, plagioclase and K-feldspar phenocrysts in Toba Tuff ignimbrites all exhibit highly evolved, rhyolitic compositions, identical to glass forming the matrix of the rocks. About 4% H2O is present, dissolved in the glass, suggesting a water saturation pressure (\(P_{{\text{H}}_{\text{2}} {\text{O}}}\)) of about 1 kbar. Melt compositions are consistent with phase relations for the condition \(P_{{\text{H}}_{\text{2}} {\text{O}}}\) =Ptotal = 1 kbar.