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The Continuing Evolution of Laser Ablation (U-Th)/He Methods: From Dates to Intracrystalline Isotopic Distributions
  • Kip Hodges,
  • Matthijs van Soest,
  • Alyssa McKanna
Kip Hodges
Arizona State University

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Matthijs van Soest
Arizona State University
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Alyssa McKanna
Princeton University
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Since initially developing laser ablation (U-Th)/He procedures for high-spatial-resolution dating of monazite more than a decade ago, our research group has refined the technique to the point that laser ablation dating of apatite, titanite, and zircon is now routine in the Arizona State University (Group 18) laboratories. We are actively exploring applications to additional minerals. Compared to conventional single-crystal (U-Th)/He dating, the laser ablation alternative offers some important advantages. Following appropriate analytical protocols, laser ablation dates require no alpha ejection corrections. In principle, most factors commonly believed to cause high apparent age dispersion in conventional datasets – parent element zoning, alpha particle implantation, and the presence of high-(U+Th) inclusions – can be mitigated using the laser ablation method. Analytical throughput is greatly enhanced compared to the conventional method because sample dissolution is not required for U+Th+Sm analysis. This is especially beneficial for detrital studies; in this presentation, we review examples of Group 18 research involving (U-Th)/He and U/Pb laser ablation double dating of detrital apatite and zircon. The principal limitations to the method are that: 1) relatively large grain sizes (≥ 100 μm) are sometimes required for especially young or low-(U-Th) materials; and 2) analytical uncertainties for these materials can be as much as a factor of two larger for laser ablation dates than for conventional dates due to a combination of the much smaller masses analyzed and uncertainties in the U, Th, and Sm concentrations of available appropriate standards. Frontier applications of this technology advance our understanding of the intracrystalline distribution of radiogenic 4He in accessory minerals. Here we show examples of both two-dimensional mapping of 4He in polished crystal interiors and one-dimensional depth profiling as practiced in the Group 18 laboratories. Zoning in 4He is very common in older crystals, and 4He distribution patterns can be much more complex than what might be expected simply from alpha ejection or grain-scale diffusive loss during cooling. Much of this complexity reflects non-concentric zoning in parent elements and, for older crystals, spatially variable radiation damage that results in spatially variable 4He diffusivity. The potential impacts of such phenomena on thermal and exhumation history modeling argue for a greater reliance on microanalytical procedures in (U-Th)/He thermochronology moving forward.