HR: 10:20h
AN: V12A-01 INVITED     [Abstracts]
TI: U-series disequilibria in crystals: ages as tracers
AU: * Cooper, K M
EM: kmcooper@u.washington.edu
AF: ESS Dept., U. Washington, Seattle, WA 98195 United States
AB: U-series disequilibria offer a unique perspective on the fates of crystals within magmatic systems. In addition to delimiting the timescales of magmatic processes, crystal ages can be used as a tracer of different crystal populations even in the case where only subtle differences exist between major- and trace-element chemistries of populations. For example, ²²⁶Ra-²³⁰Th ages of crystals in Mt St Helens lavas erupted since 2 ka are in some cases several kyr older than eruption ages which, when combined with significant Ra-Th disequilibria in the whole-rocks, suggests protracted crystal storage and entrainment in subsequent batches of magma passing through the reservoir. More broadly, in many cases ²³⁰Th-²³⁸U and ²²⁶Ra-²³⁰Th ages measured in the same crystals are discordant. This pattern likely indicates progressive and/or episodic crystal growth where the Th-U ages more closely represent average crystallization ages while Ra-Th ages are weighted toward recent growth, suggesting in turn that some significant fraction of the mass of crystals represent xenocrysts or "antecrysts" recycled from earlier generations of magmas within the same system. Conversely, in cases where ages of different parent-daughter pairs are concordant, mineral separates must be dominated by crystal growth within a relatively narrow time interval relative to the half-life of the shortest-lived daughter isotope. The duration of the crystal record within a given magma can be complicated by crystal recycling and obscured by average ages derived from measurement of bulk mineral separates. One way to extract more information about the proportion and ages of older and younger parts of the crystal population(s) is to analyze different size fractions within the same sample; for example, analyses of different sizes of plagioclase from the ongoing eruption at Mt St Helens are in progress. U-series ages and other crystal-scale geochemical information can also be a powerful combination. For example, preservation of major- or trace-element disequilibrium between zones within crystals limits the duration of crystal residence at high temperature; when combined with absolute age information from U-series disequilibria, these data can provide clues about the thermal conditions of crystal storage and thus whether such storage is likely to have occurred in a mostly-liquid or mostly-crystalline part of the magma system.
DE: 1036 Magma chamber processes (3618)
DE: 1040 Radiogenic isotope geochemistry
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005