Why LA‐ICPMS Fission‐Track Age Data Are So Dispersed? A Pseudo‐EDM Solution

Fission track (FT) dating is a well‐established thermochronological technique widely used to reconstruct rocks' thermal histories. Over the past two decades, development of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICPMS) has significantly advanced the application of FT dating by enabling rapid measurements of uranium concentrations and other compositions. Despite the advantages, current LA‐ICPMS FT methods often produce overdispersed age data sets. Consequently, interpretation typically relies on the central age model of Galbraith and Laslett (1993), 10.1016/1359‐0189(93)90185‐c , which calculates a weighted mean and a dispersion parameter based on the log‐normal distribution of individual grain ages. However, a synthesis of published data reveals that LA‐ICPMS methods tend to yield systematically older central ages compared to the pooled mean (area‐weighted) of multiple grains. Furthermore, the central ages of reference materials frequently exceed the known true values derived from other independent geochronometers. We found that the overdispersion and central‐age overestimation primarily results from underestimation of age uncertainties at the single‐grain level, especially for either low‐uranium grains or those with abundant spontaneous tracks, both of which tend to yield relatively older ages. Underestimated age uncertainties for such grains would result in overestimation of their weights, thereby leading to higher central ages. To address this issue, we introduce a practical age calculation method tailored to LA‐ICPMS FT data. Validated by a large multi‐laboratory data set, the new method produces pooled ages nearly identical to the conventional approaches, while significantly reducing intra‐sample dispersion and the offsets of central ages from both the pooled (for unknowns) and reference values (for reference materials).