Technical note: Improved calculation of volume, <i>F</i> _<i>T</i> correction, and other derived data for polished zircon (U-Th)/He thermochronology

Abstract. Polishing mounted zircon crystals prior to bulk grain (U-Th)/He thermochronology analysis provides opportunities for characterizing and subsampling each grain via in situ methods to obtain additional information relevant for (U-Th)/He date interpretation and the broader geologic questions of interest. However, polishing introduces complications for classifying grain geometry and determining grain volume (V), on which many derived (U-Th)/He data partially depend. Derived data that depend on volume include isotope concentrations, effective uranium (eU; a proxy for radiation damage), and alpha-ejection correction factors (FT), which are used to correct (U-Th)/He dates. These derived data are integral to interpreting (U-Th)/He dates, and, without a way to accurately calculate these values for polished grains, a choice must be made between polishing zircon to provide robust in situ data at the expense of the thermochronologic data or not polishing and limiting in situ data to grain rims or one-dimensional depth profiles. To address this issue, this paper presents a comprehensive protocol for calculating volume and alpha-ejection surface area for polished zircon grain fragments, from which additional data, including eU and FT, are derived. This protocol is implemented after grains have been polished and in situ measurements have been made and can easily be integrated into existing workflows for characterizing and measuring grains for conventional (U-Th)/He analysis. An R script accompanying this paper can be used to perform the required calculations and assign uncertainties during analytical data reduction. Applying the new protocol to a synthetic dataset covering a range of zircon geometries, sizes, and grinding conditions shows that the method is an improvement over existing methods to calculate polished grain FT, which only apply to a small subset of possible grain geometries and grinding conditions. The new protocol also calculates all derived data and uncertainties necessary and recommended for (U-Th)/He data reporting, aside from the (U-Th)/He dates themselves, to facilitate integrations with existing data reporting, date interpretation, and thermal history modeling.