A VOLATILE SYSTEM

Though seismicity was ambiguous throughout April and May 1991 and might have been just a tectonic swarm, SO₂ emissions demonstrated that magma was involved, and, probably, rising (Daag, Tubianosa, and others). In retrospect, the increases in SO₂ emission may have been controlled more by depletion (by boiling) of the hydrothermal system than by decreasing gas solubility in rising magma, but the hazards implications were the same. A sudden drop in SO₂ emission, perhaps the result of quench sealing of the cap of the rising column of magma, was an immediate precursor to eruption at Pinatubo and has since been noted at Galeras (Colombia) and Colima (Mexico).

Traditionally, volcanologists have assumed that magmatic volatiles are dissolved in the silicate melt until near-surface exsolution and vesiculation occur. Gerlach and others make a strong argument for volatile saturation, development of a separate, H₂O-dominated vapor phase, and strong partitioning of SO₂ into that vapor phase, all while the dacitic magma was still in its reservoir, >6 km deep. This separation facilitated the release of voluminous SO₂ to the atmosphere and attendant effects on global climate. Because of early volatile separation, use of a common petrologic method in which the volatile content of glass inclusions is compared to that in matrix glass would have greatly underestimated the sulfur abundance in the magma and its climatic impact.

Some debate continues as to whether this separate volatile phase was the sole source of SO₂ to the atmosphere, or whether breakdown of anhydrite during magma ascent was also significant (as suggested by L. Baker, cited in Rutherford and Devine). Isotopic analysis by M.A. McKibben and others (in progress) of atmospheric sulfate from Pinatubo will help to resolve this issue.

A small caldera lake began to form during September 1991 and is now rising faster than sediment from the caldera walls can fill it. The lake's pH dropped from near-neutral in October 1991 to strongly acidic by December 1992 as the lake absorbed acidic fluids from the volcano (Campita and others).