TRIALLING THE USE OF CONTROLLED EXPOSURE EXPERIMENTS FOR OPTICAL SURFACE EXPOSURE DATING ON QUARTZITE QUARRY SURFACES IN WASHINGTON STATE

Optically stimulated luminescence (OSL) depth profiling utilizes an OSL-at-depth signal to extrapolate an exposure age from a rock's surface (for short timescales within 10⁴ years). A commonly used depth profiling model represents luminescence intensity at depth x (L) relative to an unbleached intensity (L_o) in the form L = L_oe^{(-¯σφo¯ e^(-ux))} which follows the double exponential shape of bleached depth profiles based on first order luminescence kinetics from a single electron trap, as well as the exponential optical attenuation (µ) of surface photon fluence rates relative to depth into the surface. Exposure ages (t) are obtained by fitting the forms of bleached profiles, which depend on parameter estimates of µ as well as the variable ¯σφ_o¯ that combines the effective photoeviction cross-section for a trapped charge σ with φ₀, the incident solar photon flux.

Current procedures for obtaining variables ¯σφ_o¯and µ for a rock surface require matching OSL depth profiles from compositionally and morphologically matched rock surfaces with known exposure ages. Derived variables from such proximal sites are used to calculate ages from the age-unknown rock surface. Problems with the procedure can arise in that: (1) similar rocks can produce inconsistent ¯σφ_o¯ and µ variables, (2) the procedure can only be performed where well-dated proximal matches are available, severely limiting the scope of applications, and (3) any uncertainty in the proximal sample exposure ages reduces precision.

A modified technique will be presented to improve the accuracy and applicability of depth profiling methods. This new procedure aims to reliably determine ¯σφ_o¯and µ directly from the rock surface of interest using luminescence saturated samples subjected to controlled light exposures. The technique will be tested on a quartzite quarry bench with an 11 year surface age at Lane Mountain Quarry in eastern Washington, USA. The derived surface ages using this technique will be compared with surface ages produced using the proximal surface method. If serving an improvement over the proximal method, the controlled exposure technique can offer depth profiling applications at sites where either no proximal rock surfaces exist or proximal samples are deemed problematic. Future applications will be considered on glacial erratic quartzites in southwest Alberta.