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\begin{document}

\title{The Continuing Evolution of Laser Ablation (U-Th)/He Methods: From Dates
to Intracrystalline Isotopic Distributions}



\author[1]{Kip Hodges}%
\author[1]{Matthijs van Soest}%
\author[2]{Alyssa McKanna}%
\affil[1]{Arizona State University}%
\affil[2]{Princeton University}%


\vspace{-1em}



  
  \date{May 22, 2024}


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\begin{abstract}
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 \selectlanguage{greek}μ\selectlanguage{english}m) 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.%
\end{abstract}%



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