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 $P_{{\text{H}}_{\text{2}} {\text{O}}}$ =P _total = 1 kbar.

The residual rhyolitic melt formed as the result of fractional crystallisation from a more basic, possibly rhyodacitic melt, leading to the development of zoned feldspars. Water saturation in the melt probably arose as a result of this process.

Melt temperatures prior to eruption and quenching were probably less than 800° C. However, hot-stage homogenisation experiments yield entrapment temperatures significantly higher (>900° C). This discrepancy is not clearly understood but indicates care must be taken in the interpretation of such experiments.

Ignimbritic magmas at Toba, from pressure estimates, appear to have been erupted from about 3–4 kms depth and represent the silicic cap to a batholithic body consolidating beneath the Toba caldera.