BACKSCATTERED 39AR LOSS IN FINE-GRAINED MINERALS: IMPLICATIONS FOR 40AR/39AR GEOCHRONOLOGY OF CLAY

Abstract

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.