Experimental Constraints on the Origin of the 1991 Pinatubo Dacite

Abstract

Crystallisation and mantle-interaction experiments have been performed on the 1991 Pinatubo dacite to constrain its petrogenesis. In the dacite- H2O system at 1000 MPa, magnetite and either clinopyroxene or amphibole are the liquidus phases. No garnet is observed at this pressure. Dacite-mantle interaction at 920 MPa produces massive orthopyroxene crystallisation, in addition to amphibole ± phologopite. Amphibole crystallising in dacite at 1000 MPa faithfully reproduces aluminium-rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite, suggesting a high pressure stage of dacite crystallisation under high melt H2O contents (> 10 wt%) and low temperatures (< 950C). Altogether, plagioclase, amphibole and melt inclusion compositions suggest that the Pinatubo dacite was water-rich, oxidized and not much hotter than 900C, when emplaced in the shallow reservoir, in which most phenocrysts precipitated until the onset of the 1991 eruptions. The strongly fractionated REE pattern demands garnet somewhere during dacite genesis, which in turn requires high pressure derivation. Partial melting of subducted oceanic crust gives melts unlike the Pinatubo dacite. Interaction of these slab melts with sub-arc peridotite is unable to produce such type of dacite, nor is a direct mantle origin conceivable on the basis of mantle-dacite interaction experimental results. Dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low FeO/MgO, high H2O, Ni and Cr contents typical of Pinatubo-like magmas. On the face of chemical and phase equilibrium constraints, the most plausible origin of the Pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous, oxidised, primitive basalt that crystallises amphibole and garnet upon cooling, as shown by recent phase equilibrium work. Dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally, following injection of H2O-poor mantle-derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts, parental to dacite magmas, that rest at mantle depths. Dacites such as the Pinatubo one may thus witness particular H2O-rich conditions of the sub-arc mantle rather than the melting of a young and hot subducting oceanic plate.