HR: 09:30h
AN: V31G-07    [Abstracts]
TI: Magma Chamber Dynamics From High-Precision U-Pb Geochronology of Micro-Sampled Chemical Domains
AU: * Crowley, J L
EM: jimcrowley@boisestate.edu
AF: MIT, EAPS, Cambridge, MA 02139, United States
AU: * Crowley, J L
EM: jimcrowley@boisestate.edu
AF: Boise State University, Dept. of Geosciences, Boise, ID 83725, United States
AU: Bowring, S A
EM: sbowring@mit.edu
AF: MIT, EAPS, Cambridge, MA 02139, United States
AB: Dating of zircon by the U-Pb ID-TIMS method at precision of +/- 0.1% (per single date, internal error) often highlights subtle intra- and intergrain variability that in some cases complicates determination of simple igneous crystallization or eruption ages. The complexity is largely due to extended periods of zircon growth in magmatic systems. We present high-precision U-Pb dates from three magmatic systems: the Bishop Tuff, the Fish Canyon Tuff, and the Half Dome granodiorite. Timescales of magmatic evolution were obtained by dating of micro- sampled chemical domains identified by CL and BSE images of internal zoning and U, Th, and Hf concentrations. Contrasting zircon growth histories allow for the recognition of three end-members. (1) Rapid crystallization and eruption. Dates from 17 of 19 zircon grains from the Bishop Tuff are equivalent at the millennial scale and overlap with the 40Ar/39Ar sanidine age, requiring that most zircon crystallized immediately before eruption. (2) Zoned grains reflecting episodic growth. Rims on zircon from the Fish Canyon Tuff are up to 300 kyr younger than cores and 1% older than the 40Ar/39Ar sanidine age. This bias is identical to the typical ca. 1% bias between U-Pb and 40Ar/39Ar ages, suggesting that the rims formed immediately before eruption. (3) Protracted growth in large episodically replenished magmatic systems. Rims on zircon from the Half Dome granodiorite are up to 1.5 myr younger than cores and several million years older than the youngest titanite, indicating a long period of mineral growth below the closure temperature for Pb diffusion in titanite. Mineral growth in a handsample of granodiorite spans nearly the entire duration of emplacement of the Tuolumne Intrusive Suite, suggesting that growth occurred in crystal mushes in large magma chambers rather than small bodies that were emplaced incrementally. Because the results from Half Dome contrast with data from well-known silicic eruptive centers, we suggest that many plutons reflect protracted post-eruptive evolution as non-eruptive crystal mushes and pre- eruptive residence for large silicic magma chambers is 300 kyr or less. High-spatial resolution ID-TIMS dating of zircon from silicic systems offers the advantage of determining both eruption ages and magmatic histories.
DE: 1036 Magma chamber processes (3618)
DE: 1115 Radioisotope geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting