A NEW METRIC FOR ASSESSING ALPHA EJECTION CORRECTIONS IN MINERAL PHASES WITH COMPLEX TOPOLOGIES: APPLICATION TO THE FE-OXIDE (U-TH)/HE CHRONOMETER

The Fe-oxide (U-Th)/He system represents an emerging geochronometric tool that has seen renewed attention for its ability constrain the dates and rates of fluid-rock interactions, fault-slip events, and exhumation. The technique has been applied in a number of geologic settings and important advances in diffusion kinetics, sample characterization, and sampling protocols have been made in the last half decade. Fe-oxide crystal topology is often complex though, with individual grains showing irregular shapes, or arrangements of Fe-oxide crystallites separated by significant void space or non-U, Th bearing inclusions such as quartz. These complexities in turn greatly complicate alpha-ejection corrections that are needed to produce accurate date information, especially when voids and inclusions are spaced at similar length-scales to the alpha ejection distance, which limits the types of Fe-oxide samples that can be dated with this technique. Traditional methods of alpha-ejection correction as applied to more straight-forward phases such as apatite and zircon are not applicable for Fe-oxide crystal aggregates and we introduce here a new geometrical metric for calculating this correction factor: the two-point correlation distribution, S₂(r). This metric describes the probability of finding two points within the target Fe-oxide phase separated by a distance r in the polycrystalline aggregate. We demonstrate the accuracy of this new correction factor in 2 and 3-D domains with a comparison between recoil loss simulations and S₂(r) calculated on model aggregates, which show a one-to-one correspondence. We also demonstrate a method for constructing 3-D aggregate topologies from X-ray computed microtomography images of natural samples, and then using this imagery to calculate alpha ejection correction factors.