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Plagioclase, the most abundant phenocryst at Mount Pinatubo, displays varying textures and compositions within the 1991 eruption products. In June 7-14 dome-forming ande­site, plagioclase phenocrysts show prominent rims with higher MgO (0.04-0.06 wt%), Fe 2 O 3  T  (0.6-0.8 wt%), and K 2 O at given An than the interiors. The compositions of the rims are identical to those of microlites, which are abundant in the groundmass glass. White dacitic pumice, the most voluminous product of the June 15 eruption, contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites. The interiors of phenocrysts from the dacitic pumice and the dome-forming andesite are re­markably similar in terms of textures and compositions. They show oscillatory zoning (mostly An 35-60 ), low MgO (&lt;0.02 wt%) and Fe 2 O 3  T  (0.10-0.30 wt%), and similar K 2 O at given An. This similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt. The oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids.

Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products