Gas Transport Model For The Magmatic System At Mount Pinatubo, Philippines: Insights From (~(210)pb)/(~(226)ra)

摘要

Measurements of ~(226)Ra-~(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, ~(222)Rn, follows the gas phase. We present measurements of ~(210)Pb-~(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 ~(210)Pb-~(226)Ra disequilibria due to gas accumulation without complications due to ~(222)Rn loss during degassing. Pyroclastic samples have (~(210)Pb)/(~(226)Ra)o ranging from 1.01 to 1.10; averaging 1.06. A sample of the post-climactic dome has (~(210)Pb)/(~(226)Ra)_0=1.12. Previous uranium-series degassing studies have suggested that ~(210)Pb excesses are created by rapid volatile transport (carrying the intermediate daughter ~(222)Rn) and subsequent volatile accumulation and decay of ~(222)Rn to ~(210)Pb. However, bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of ~(222)Rn large enough to cause measurable disequilibria in the ~(210)Pb-~(226)Ra system. In addition, there is little evidence for magmatic sources large enough to supply a rapid flux of ~(222)Rn. Therefore, we present a model in which ~(210)Pb-~(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 ~(210)Pb excess in localized basaltic melt. Transport of volatiles and ~(210)Pb-rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with ~(210)Pb excess. Through this mechanism, the timescale of gas transport and accumulation is constrained, not by the half-life of ~(222)Rn (3.8 days), but rather by the half-life of ~(210)Pb (22.6 years). Bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with ~(210)Pb excess alleviating the need for gas transport on very short time-scales. Using the rate of decay of ~(210)Pb coupled with published trace element data, we present a quantitative investigation of this new conceptual model and propose that changes in ~(210)Pb values with time may suggest changing conditions of magma supply at volcanoes.