U and Th zoning in Cerro de Mercado (Durango, Mexico) fluorapatite: Insights regarding the impact of recoil redistribution of radiogenic 4He on (U–Th)/He thermochronology

Fluorapatite from Cerro de Mercado, Durango, Mexico has been used in several important diffusion studies aimed at calibrating the closure temperature of the (U–Th)/He apatite thermochronometer and has become a de facto standard for (U–Th)/He laboratories. Nevertheless, replicate laser fusion analyses of small crystal fragments of the “Durango fluorapatite” typically yield a range of (U–Th)/He dates that cannot be explained by reported analytical imprecision alone. Laser ablation ICPMS measurements of U and Th of a section cut through a representative crystal of Durango fluorapatite (perpendicular to the c axis) reveal the existence of significant zoning in these elements. Since alpha-stopping distances in apatite are comparable to the scale of this zoning, the ejection of radiogenic 4He from one zone to another may explain some of the reported fragment-to-fragment variation in (U–Th)/He dates. Two-dimensional numerical models of the recoil redistribution of 4He in a crystal with the zoning we observed lead to predicted age variations of several percent for fragments with sizes and aspect ratios typical of those used for laser-fusion of Durango fluorapatite. It is likely that large crystals (or large aliquots of small fragments) of zoned materials may be acceptable as “bulk” interlaboratory standards. However, the current trend is toward the use of smaller and smaller sample sizes, including broken crystals, for (U–Th)/He thermochronology. Our work indicates that heterogeneities in the U and Th concentrations of such samples are a serious concern, so much so that high-precision (U–Th)/He geochronologic studies should include a concerted effort to characterize the extent of U and Th zoning in analyzed crystals.