HR: 1340h
AN: DI33A-1131    [Abstracts]
TI: Volatile Transport and Accumulation Timescales Modeled using 210Pb-226Ra Disequilibrium in Tephras From Mount Pinatubo Volcano, Philippines: Moving From the Timescales of 222Rn to 210Pb
AU: * Kayzar, T M
EM: tkayzar@u.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Box 351310, 4000 15th Ave NE, Seattle, WA 98195, United States
AU: Cooper, K M
EM: kmcooper@geology.ucdavis.edu
AF: University of California at Davis, Dept. of Geology, One Shields Avenue, Davis, CA 95616, United States
AU: Reagan, M K
EM: mark-reagan@uiowa.edu
AF: University of Iowa, Dept. of Geoscience, 121 Trowbridge Hall, Iowa City, IA 52242, United States
AU: Kent, A J
EM: kentad@geo.oregonstate.edu
AF: Oregon State University, Dept. of Geosciences, 104 Wilkinson Hall, Corvallis, OR 97331, United States
AU: Kress, V C
EM: kress@u.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Box 351310, 4000 15th Ave NE, Seattle, WA 98195, United States
AB: We measured ²¹⁰Pb-²²²Rn-²²⁶Ra disequilibria in samples representing a time sequence through the June 15, 1991 cataclysmic eruption tephras of Mount Pinatubo volcano, Philippines. Previous U- series degassing studies have interpreted ²¹⁰Pb excess to indicate rapid volatile transport (differential ²²²Rn motion) and subsequent volatile accumulation. Mount Pinatubo deposits show evidence for volatile saturation and the presence of a pre-eruptive volatile phase. In addition, intensive monitoring at Mount Pinatubo prior to eruption provides gas emissions levels which allow us to decouple gas accumulation from passive degassing. Because the 1991 eruption of Mount Pinatubo was well studied, Mount Pinatubo is a uniquely constrained system with which to test hypotheses for the generation of ²¹⁰Pb-²²⁶Ra disequilibria. Analyzed tephra units have (²¹⁰Pb/²²⁶Ra)₀ values that fall outside of equilibrium given 2σ analytical errors. These samples exhibit ²¹⁰Pb excesses with (²¹⁰Pb/²²⁶Ra)₀ ranging from 1.079 to 1.119. The highest excess occurs in the 1992 dome sample extruded after the climactic eruption. Differential gas motion and gas accumulation are typically called upon to generate ²¹⁰Pb excess; however, we find that physical bubble rise and gas fluxing through a permeable media are an ineffective means of ²²²Rn transport in Pinatubo's dacitic magma reservoir. Calculated Stokes rise velocities show that bubbles in dacite magma rise slower than is needed to allow for a flux of ²²²Rn large enough to generate measurable disequilibria in the ²¹⁰Pb -²²⁶Ra system. Instead, we present a conceptual model in which ²¹⁰Pb -²²⁶Ra disequilibria is established during basaltic under-plating of the magma reservoir. Basaltic magma has a viscosity low enough to allow for differential gas motion and the mobilization of ²²²Rn. ²²²Rn decays to ²¹⁰Pb, imparting a ²¹⁰Pb signal to melt at the bubble walls. The timescale of gas transport and accumulation is then constrained not by the half-life of ²²²Rn (3.8 days) but rather by the half-life of ²¹⁰Pb (22.6 years). Our results suggest that the transport of buoyant melts containing exsolved volatiles is a physically viable way to preserve fractionation signals in the ²¹⁰Pb - ²²⁶Ra system and alleviates the need for gas transport on very short time-scales. Further analyses of major and trace elements, including Li measurements, are currently in progress to provide additional constraints on volatile fluxing.
DE: 1036 Magma chamber processes (3618)
DE: 1040 Radiogenic isotope geochemistry
DE: 8430 Volcanic gases
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting