A Comparison of Excimer Laser Microprobe (U-Th)/He and Conventional Laser-Heating (U-Th)/He Thermochronometry
Abstract
Recent advances in Excimer laser ablation (U-Th)/He thermochronology (ExLA-(U-Th)/He) of monazite have demonstrated the potential of the technique with regard to generating precise cooling age information while dealing with parent element zoning, inclusions and avoiding the alpha ejection correction and the uncertainty therein [1]. ExLA-(U-Th)/He age determinations consist of three independent measurements: 1) Moles of He, 2) Ablated volume, and 3) Parent element (U, Th, Sm) concentrations. After samples are polished, an Excimer laser is used to drill a 10-250 μm diameter hole in the surface, liberating radiogenic helium which is then quantified by isotope dilution. The volume of the hole is then measured by an ADE-Phase Shift MicroXAM vertical scanning interferometer, allowing us to calculate He concentration. U, Th, and Sm concentrations are obtained (in the case of monazite), by electron microprobe analysis (either before or after helium analysis). While the precision of the technique has been adequately demonstrated on monazites as young as ~700 ka [1,2], the question of accuracy has to this point been unanswered because of the lack of a suitable monazite (U-Th)/He standard amenable to conventional (U-Th)/He analysis for comparison. Typical natural monazites are very small and strongly zoned, resulting in large and poorly constrained alpha-recoil corrections, and therefore conventional (U-Th)/He ages with unacceptably large uncertainties. Eighteen ExLA-(U-Th)/He ages on a polished section of the centimeter-sized, gem-quality Brazilian monazite MOM1 yield an error-weighted mean of 447.4 ± 3.4 Ma (0.75% at two standard errors from the mean or 2SE). Ten additional 25 μm spot analyses on a small fragment of a second crystal (MOM3) result a similar weighted mean age, albeit with more scatter, of 454.6 ± 8.9 Ma (2.0% at 2SE). Taken together, the 28 ExLA-(U-Th)/He analyses on the two crystals can be used to calculate one age of 450.4 ± 3.9 Ma (0.86% at 2SE). The weighted mean ExLA-(U-Th)/He age is statistically indistinguishable from the conventional (U-Th)/He age of 449.6 ± 9.8 Ma for MOM3 (the means differ by less than 0.2%), demonstrating the ability of ExLA-(U-Th)/He to reproduce ages generated by the more time-consuming conventional method. [1] Boyce, J.W., Hodges, K.V., Olszewski, W.J., Chatterjee, N., 2005. The importance of material characterization in (U-Th)/He thermochronometry: Diffusivity of He and heterogeneous distribution of U and Th in monazite. General Assembly of the European Geosciences Union, Vienna, Austria. [2] Boyce, J.W., Hodges, K.V., Crowley, J.L., Chatterjee, N., Searle, M., 2005. Laser Microprobe (U-Th)/He thermochronometry of monazite. 15th Annual Goldschmidt Conference, Moscow, ID, USA.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.V41D1484B
- Keywords:
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- 1040 Radiogenic isotope geochemistry;
- 1140 Thermochronology;
- 1194 Instruments and techniques