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U-Pb dating of zircon in the Bishop Tuff at the millennial scale

J.L. Crowley^*,1, B. Schoene^,1 and S.A. Bowring¹

¹ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Zircon from the Bishop Tuff (eastern California) was dated by the U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) method to evaluate time scales of magmatic evolution and applicability of the method to rocks younger than 1 Ma. The ²⁰⁶Pb/²³⁸U dates from 17 of 19 grains are equivalent with a weighted mean of 767.1 ± 0.9 ka that overlaps with the published ⁴⁰Ar/³⁹Ar sanidine age, requiring that most zircon crystallized immediately before eruption. Dates from two grains that are 6 and 12 k.y. older indicate minimum times that a small portion of the grains resided in the magma. These findings are consistent with chemical zoning patterns in zircon that suggest a relatively simple crystallization history, yet contrast with previous results of ion microprobe dating of zircon that show 50–80 k.y. of magma residence. The weighted mean ²⁰⁷Pb/²³⁵U date is 52 k.y. older than the ²⁰⁶Pb/²³⁸U date, a difference that is attributed to excess ²⁰⁷Pb from initial Pa/U disequilibrium. The results demonstrate that high-precision ID-TIMS geochronology can resolve magma chamber dynamics of <1 Ma silicic eruptions at the millennial scale.

Key Words: zircon • U-Pb geochronology • Bishop Tuff • magma residence • disequilibrium

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