Gas transport model for the magmatic system at Mount Pinatubo, Philippines: Insights from ( 210Pb)/( 226Ra)

Abstract

Measurements of 226Ra– 210Pb disequilibria in eruptive products have the potential to track the accumulation or loss of volatiles in magmatic systems on timescales of decades because the intermediate nuclide, 222Rn, follows the gas phase. We present measurements of 210Pb– 226Ra disequilibria for whole-rock samples representing a time sequence of tephra through the June 15, 1991 cataclysmic eruption of Mount Pinatubo volcano, Philippines. Mount Pinatubo volcano is a dacitic system that did not significantly vent gases at the surface prior to eruption, and we can, therefore, isolate the 210Pb– 226Ra disequilibria due to gas accumulation without complications due to 222Rn loss during degassing. Pyroclastic samples have ( 210Pb)/( 226Ra) 0 ranging from 1.01 to 1.10; averaging 1.06. A sample of the post-climactic dome has ( 210Pb)/( 226Ra) 0 = 1.12. Previous uranium-series degassing studies have suggested that 210Pb excesses are created by rapid volatile transport (carrying the intermediate daughter 222Rn) and subsequent volatile accumulation and decay of 222Rn to 210Pb. However, bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of 222Rn large enough to cause measurable disequilibria in the 210Pb– 226Ra system. In addition, there is little evidence for magmatic sources large enough to supply a rapid flux of 222Rn. Therefore, we present a model in which 210Pb– 226Ra disequilibria is established during basaltic recharge of the Pinatubo reservoir. The relatively low viscosity of basaltic magma allows for differential gas motion and the production of 210Pb excess in localized basaltic melt. Transport of volatiles and 210Pb-rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with 210Pb excess. Through this mechanism, the timescale of gas transport and accumulation is constrained, not by the half-life of 222Rn (3.8 days), but rather by the half-life of 210Pb (22.6 years). Bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with 210Pb excess alleviating the need for gas transport on very short time-scales. Using the rate of decay of 210Pb coupled with published trace element data, we present a quantitative investigation of this new conceptual model and propose that changes in 210Pb values with time may suggest changing conditions of magma supply at volcanoes.