A New Correction for He Loss Applied to (U-Th)/He Dating of Grains with Complex Shapes and Polymineralic Aggregates

Applying the (U-Th)/He chronometer to small domains with complex shapes is challenging because of He recoil loss. The development of date correction factors proves challenging when considering either a single grain with complex geometry or polymineralic crystal aggregates composed of many small U-Th-bearing grains with additional impurities, such as iron-oxide aggregates. We introduce a new geometrical metric for the recoil loss correction factor—the two-point correlation distribution S₂(r)—that is independent of dimensionality (i.e., two-dimensional [2-D] or three-dimensional [3-D] images), grain shape, and existence or absence of internal porosity. S₂(r) measures the probability of finding two points within the target phase (U-Th-He host, such as Fe oxides here) separated by a distance r in the physical sample (single grain or aggregate). It provides a metric that is independent of grain or aggregate size. We use a combination of forward recoil loss simulations in 2-D and 3-D domains (both complex single grains and aggregates) to show a one-to-one correspondence between a recoil loss factor and S₂(r = He stopping distance). We illustrate the workflow to retrieve the loss factors for different isotopes and correct date determination in a complex 3-D jasperoid sample imaged by X-ray computed microtomography. Our jasperoid sample contains appreciable amounts of iron oxide with quartz impurities and is representative of a sample for which completely monomineralic separates cannot be obtained. The computation of loss factors based on S₂(r) is extremely efficient, and the codes, including a simple user interface, are made available at https://repository.library.brown.edu/studio/item/bdr:957056/.