Accounting for long alpha-particle stopping distances in (U-Th-Sm)/He geochronology: Refinement of the baseline case

This contribution proposes a number of methodological refinements for accounting for ejection of energetic particles, most particularly in the case of long alpha-particle stopping distance effects in (U-Th-Sm)/He thermochronometry. Recent stopping- distance estimates for minerals commonly used for thermochronometry are up to 1.5 mu m shorter than previously thought, which directly affects the magnitude of the ejection correction parameter F-T. The revision in stopping distances necessitates re-fitting the empirical polynomials that have been used to calculate F-T corrections for various crystal forms. Reformulation and re-parameterization of these polynomials enables them to account for present and any future change in stopping distances without needing to be recalibrated. The library of shapes described with these polynomials to accommodate habits is expanded for all crystal systems and also ellipsoids, thus covering all minerals currently used and under development for (U-Th-Sm)/He thermochronometry. In addition, the inaccuracy caused by characterizing a set of alpha particles from a decay chain with a single mean stopping distance is examined, and a simple method for compensation is recommended. A new method of using F-T parameters to calculate corrected ages by applying them to parents instead of daughters or age is presented, which maximizes accuracy for old specimens such as meteorites. Altogether, the refinements proposed will affect (U-Th-Sm)/He ages by similar to 1-5%, depending on size and age. Finally, the concept of an F-T-equivalent sphere is introduced as a sphere with the same effective F-T value as a given mineral grain. Testing indicates that the F-T-equivalent sphere is of comparable or superior accuracy to a sphere of the same surface to volume ratio for diffusion modeling. This finding greatly facilitates utilization of (U-Th-Sm)/He data from standard data tables for forward and inverse modeling. (C) 2011 Elsevier Ltd. All rights reserved.