238U–206Pb dating of U-series disequilibrium zircons by secondary ion mass spectrometry

Abstract

Determining the timescales of magma accumulation and storage prior to large eruptions is fundamental to understand the pre-eruptive history of magma reservoirs. Zircon ²³⁸U–²³⁰Th dating has been widely used to recover the timescales of magma reservoirs younger than 400 ka. However, theoretically and practically, the uncertainties for zircons older than ca. 150 ka, i.e., two half-lives of ²³⁰Th, are large because (²³⁸U)/(²³⁰Th) activities start to stabilize. On the other hand, the accumulation of radiogenic ²⁰⁶Pb begins to increase significantly after two half-lives of ²³⁰Th. Herein we theoretically demonstrate a calibration method for SIMS (Secondary Ion Mass Spectrometry) ²³⁸U–²⁰⁶Pb dating within the timescales of U-series disequilibrium and apply it to zircons of the 75 ka Youngest Toba Tuff (YTT) in northern Sumatra, Indonesia. The ²³⁸U–²⁰⁶Pb dates vary from ca. 665 to 75 ka after common Pb and initial ²³⁰Th disequilibrium corrections. This age range agrees with that obtained by previous studies using the ²³⁸U–²³⁰Th method, but the precision of each age is improved by a factor of three or more. ²³⁸U–²⁰⁶Pb and ²³⁸U–²³⁰Th dating methods have advantages and disadvantages for zircons in U-series disequilibrium. The ²³⁸U–²³⁰Th method is more suited to samples younger than 150 ka, but requires a large Th/U fractionation between the zircon and magma, and a high U content. The ²³⁸U–²⁰⁶Pb method is more appropriate for samples older than 150 ka, but requires high U and low common Pb contents. Our approach improves the precision of in situ dating of zircon in U-series disequilibrium, which will contribute to investigations of the magmatic evolution beneath young volcanoes.