Backscattered ³⁹Ar loss in fine-grained minerals: Implications for ⁴⁰Ar/³⁹Ar geochronology of clay

The interpretation of ³⁹Ar recoil loss from clay minerals during application of the ⁴⁰Ar/³⁹Ar analytical technique is controversial. To further explore the mechanism for ³⁹Ar recoil loss, we quantified the effects of backscattering where the recoiled ³⁹Ar atom fails to reenter the coherent clay particle upon crossing an incoherent boundary. A numerical simulation with the computer program, SRIM, was conducted. The results indicate that the recoil range distribution is significantly influenced by the incident angle between the attacking direction and c-axis (perpendicular to the plane of clay surface). ³⁹Ar concentration profiles were generated for two spatial analogues to natural illite/smectite (1) a one-component system composed of the K-rich particles with thicknesses ranging from 5 to 100 nm, and (2) a two-component system composed of alternation of the K-rich and K-poor particles having thicknesses ranging from 5 to 100 nm. The models yield 14-15%, 9-10%, 2-3%, and 1-2% backscattered ³⁹Ar for particle thicknesses of 5 nm, 10 nm, 50 nm, and 100 nm, respectively. The initial ³⁹Ar concentration exerts very little influence on the fraction of backscattered ³⁹Ar. If the particles are separated by incoherent boundaries along which ³⁹Ar atoms freely migrate, then the backscattered ³⁹Ar must contribute to the ³⁹Ar recoil loss. The estimates of ³⁹Ar backscatter loss are generally consistent with published experimental data. Good agreement is obtained for samples having a particle thickness larger than 50 nm. The model underestimates the observed ³⁹Ar recoil loss by a factor of two for samples with a particle thickness less than 50 nm. Our results indicate that ³⁹Ar recoil loss in clays is largely controlled by backscattering at the incoherent boundary.