Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products

Abstract

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°jo), Fe20j (0.6-0.8 wt°/o), and K20 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 An35-6o),low MgO «0.02 wtOjo) and Fe20j (0.10-0.30 wt°jo), and similar K20 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. A portion of the felsic magma (-800°C) mixed with a mafic melt (-1000-1100 °C) to become an andesitic magma that extruded to form the June 7-14 dome. All plagioclase phenocrysts in the andesite were derived from the felsic magma. The mixing caused destabilization of phenocrysts, forming sieve textures, dusty zones, and partial resorption. Extrusion of the mixed magma resulted in overgrowths on once-resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass. In the unmixed, remaining portion of the felsic magma, some plagioclase underwent partial resorption but did not develop overgrowths. This lack of overgrowths and the absence of micro lites in the groundmass glass of the June 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption, supporting the linkage between the two.