HIGH-PRECISION CHRONOLOGIES OF MAGMA CHAMBER DYNAMICS: ID-TIMS U-PB DATING OF SMALL CHEMICAL DOMAINS

Dating of U-bearing accessory minerals by the ID-TIMS U-Pb method at precision of +/- 0.1% or better (per single date, internal error) 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 mineral 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 in zircon and titanite identified by CL and BSE images of internal zoning and U, Th, and Hf concentrations.

Contrasting mineral 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 protracted period of mineral growth. The mineral growth history in one handsample of granodiorite spans nearly the entire duration 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 (1) many plutons reflect protracted post-eruptive evolution as non-eruptive crystal mushes and (2) pre-eruptive residence for large silicic magma chambers is 300 kyr or less.