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

Measurements of (super 226) Ra- (super 210) Pb 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, (super 222) Rn, follows the gas phase. We present measurements of (super 210) Pb- (super 226) Ra 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 (super 210) Pb- (super 226) Ra disequilibria due to gas accumulation without complications due to (super 222) Rn loss during degassing. Pyroclastic samples have ( (super 210) Pb)/( (super 226) Ra) (sub 0) ranging from 1.01 to 1.10; averaging 1.06. A sample of the post-climactic dome has ( (super 210) Pb)/( (super 226) Ra) (sub 0) = 1.12. Previous uranium-series degassing studies have suggested that (super 210) Pb excesses are created by rapid volatile transport (carrying the intermediate daughter (super 222) Rn) and subsequent volatile accumulation and decay of (super 222) Rn to (super 210) Pb. However, bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of (super 222) Rn large enough to cause measurable disequilibria in the (super 210) Pb- (super 226) Ra system. In addition, there is little evidence for magmatic sources large enough to supply a rapid flux of (super 222) Rn. Therefore, we present a model in which (super 210) Pb- (super 226) Ra 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 (super 210) Pb excess in localized basaltic melt. Transport of volatiles and (super 210) Pb-rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with (super 210) Pb excess. Through this mechanism, the timescale of gas transport and accumulation is constrained, not by the half-life of (super 222) Rn (3.8 days), but rather by the half-life of (super 210) Pb (22.6 years). Bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with (super 210) Pb excess alleviating the need for gas transport on very short time-scales. Using the rate of decay of (super 210) Pb coupled with published trace element data, we present a quantitative investigation of this new conceptual model and propose that changes in (super 210) Pb values with time may suggest changing conditions of magma supply at volcanoes.