Laser Ablation Depth Profiling of Helium in Accessory Minerals: Imaging
Alpha Ejection Zones and Natural Helium Diffusional Loss Profiles
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.