Accounting for long alpha-particle stopping distances in (U-Th-Sm)/He geochronology : 3D modeling of diffusion, zoning, implantation and abrasion
Résumé
In apatite (U-Th)/He thermochronology the helium distribution in a crystal is a function of the simultaneous processes of radiogenic production, thermally activated volume diffusion and the ejection of He caused by long alpha stopping distances. These processes are further complicated by zonation of U, Th and Sm within the grain and implantation of 4He from neighboring U-Th-Sm bearing minerals. We use a refined version of the 3D Monte Carlo diffusion code of Gautheron and Tassan-Got (2010) to simulate the interplay between ejection and diffusion with or without zonation, ejection and abrasion for a suite of thermal histories. We examine the phenomenon of over-correction produced by the alpha ejection correction parameter (FT or FZAC for homogeneous or heterogeneous eU repartition) by comparing the raw (measured) and FT- or FZAC-corrected ages for a number of scenarios to the ejection-free age (AEF), which we define as the age that would be obtained if alpha ejection had not occurred, or equivalently if the stopping distance was zero. We show that the use of FT- or FZAC-corrected ages generally reproduces the ejection-free age to within typical (U-Th)/He uncertainties (± 8%), even for zoned apatites. We then quantify the effect of alpha implantation on (U-Th)/He ages, showing that implantation from a single external source with modest relative U or Th enrichment can generate as much as 50% excess He. For more extreme cases where an apatite is surrounded by multiple external sources the measured age can be > 300% of that determined from an isolated crystal. While abrasion of the outer 20-25 microns can significantly reduce the age dispersion for rapidly cooled samples, slowly cooled samples can still retain 10-30% excess He. The removal of the rim of the crystal reduces the thermal information from very low temperatures (< 40 °C), and introduces additional technical complications and biases, and should therefore be used with caution. Overall we demonstrate that although zonation and implantation may not be routinely determined, we now have the 3D modeling capability to fully investigate and constrain the causes of age dispersion within a sample, leading to significant improvement in our ability to interpret (U-Th)/He data.