A simple method for in situ U-Th-He dating

Vermeesch, Pieter; Sherlock, Sarah C.; Roberts, Nick M. W. and Carter, Andy (2012). A simple method for in situ U-Th-He dating. Geochimica et Cosmochimica Acta, 79 pp. 140–147.

DOI: https://doi.org/10.1016/j.gca.2011.11.042

Abstract

This paper presents a significantly simplified method for in-situ U–Th–He dating removing the need to know any absolute concentrations. This is done by calculating the normalised U, Th, and He concentrations of a conventionally dated calibration standard from its measured Th/U ratio and known U–Th–He age, and scaling these concentrations to the raw U, Th, and He signals of the sample. The Th/U ratio of the standard can be determined from its measured 208Pb/206Pb ratio, removing the need to use NIST glass as a reference material. We introduce an LA-ICP-MS-based method to correct for variable ablation depths between the standard and the unknown, using the strength of the ablated 29Si signal. Finally, we propose a pseudo-depth profile method to assess the effects of compositional zoning on the accuracy of in-situ U–Th–He data. The effectiveness of the proposed method has been demonstrated on three samples of gem-quality Sri Lanka zircon, which yield ages that are in agreement with previously published conventional U–Th–He measurements.

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