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Laser Ablation Depth Profiling of Helium in Accessory Minerals: Imaging Alpha Ejection Zones and Natural Helium Diffusional Loss Profiles
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  • Matthijs van Soest,
  • Michelle Aigner,
  • Kip Hodges,
  • Alexandra Pye
Matthijs van Soest
Arizona State University

Corresponding Author:[email protected]

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Michelle Aigner
Arizona State University
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Kip Hodges
Arizona State University
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Alexandra Pye
Arizona State University
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Abstract

The Ultraviolet Laser Ablation Microprobe (UVLAMP) method of releasing helium from samples is an excellent, but under-utilized, tool in the diverse toolkit of gas extraction approaches available to researchers working with the (U-Th-Sm)/He thermochronology method. So far, most applications have involved some form of Laser Ablation (U-Th-Sm)/He dating (LAHe) or combined LAHe and Laser Ablation U-Th/Pb double dating (LADD) (e.g. 1, 2, 3, 4, 5, 6, 7). Other applications using UVLAMP have focused on 2D-mapping of helium distributions within zircon crystals (8) and stepwise Laser Ablation Depth Profiling (LADP) of induced helium diffusional loss profiles in apatite and zircon (9, 10). Based on the latter examples the stepwise helium LADP method would appear to be an excellent method to study the intricacies associated with a variety of aspects of the (U-Th-Sm)/He dating method and the interpretation and modeling of its results. Given that it creates high resolution helium profiles from the crystal margin to its core without the need to heat the sample to release the gas. Thus, it avoids issues of within-experiment radiation damage annealing, diffusional flattening of helium zonation, and/or the sudden release of helium from fluid and/or melt inclusions that can be associated with approaches using step heating of samples to acquire similar information about the helium distribution within a sample. In this contribution we focus on the results of high spatial resolution helium LADP experiments in a variety of accessory minerals (apatite, zircon, monazite, and titanite). The experiments are intended to a) empirically determine the alpha ejection distance and how those results compare to the distance for each mineral derived from SRIM calculations (11) and b) image natural helium distribution profiles from rim to core in zircons to produce data that are equivalent to those produced by 4He/3He thermochronology (12) experiments, but without the need to proton irradiate the sample. Initial LADP results on Durango apatite yielded an alpha ejection distance that is within error of the theoretical value, while results from several larger (>5 mm) zircon crystals did not yield profiles consistent with the presence of a straightforward alpha ejection zone. The helium depth profile results from the zircons were suggestive of either natural diffusional loss profiles, showing evidence of U-Th zoning, or a combination thereof. 1 Boyce et al. GCA 70, 2006; 2 Vermeesch et al. GCA 79, 2012; 3 Tripathy-Lang et al. JGR-ES 118, 2013; 4 Evans et al. JAAS 30, 2015; 5 Horne et al. GCA 178, 2016; 6 Horne et al. CG 506, 2019; 7 Pickering et al. CG 548, 2020; 8 Danisik et al. Sci Adv 3, 2017; 9 Van Soest et al. GCA 75, 2011; 10 Anderson et al. GCA 274, 2020; 11 Ziegler and Biersack, 1985; 12 Shuster and Farley EPSL 217, 2004.