BENEFITS OF USING BOTH LASER MICROPROBE AND BULK INCREMENTAL HEATING 40AR/39AR TECHNIQUES TO CONSTRAIN THE TIMING OF IMPACT EVENTS: AN EXAMPLE FROM THE WEST CLEARWATER LAKE IMPACT STRUCTURE, CANADA

The ⁴⁰Ar/³⁹Ar method is often used to date impact craters, but traditional bulk incremental heating (IH) experiments of impactites can sometimes yield ambiguous results. IH methods require larger aliquots of sample, often complex mixtures of variably reset mono- and polymineralic clasts, as well as glassy to recrystallized melt domains. Such mixtures of materials with differing Ar diffusion kinetics can produce inconsistent IH results. Additional complications arise as extra-terrestrial impactite samples may be the products of multiple impact events. High-spatial-resolution, in-situ ultraviolet laser ablation microprobe (UVLAMP) methods can be used to date specific domains within impactites, providing petrologic context to the complex results yielded by IH methods. UVLAMP methods also require smaller aliquots of sample, which is especially important for small precious samples from future exploration targets.

We demonstrate the benefit of using these complementary methods for impact dating studies. Here, we present IH and UVLAMP ⁴⁰Ar/³⁹Ar results from samples from a crater of known age, the West Clearwater Lake impact crater, Canada. Published IH spectra from impact melts produced two indistinguishable plateaus, interpreted to constrain the impact age to ca. 286 Ma (Schmieder et al., 2015). Our IH experiments of visually pristine impact melts do not yield consistent results. For example, IH spectra from two such domains from a single sample yield distinct plateau dates of ca. 275 Ma and ca. 287 Ma. The context provided by our UVLAMP analyses suggests that dates younger than 286 Ma likely are a result of Ar disturbance during post-impact alteration. UVLAMP dates of embedded clasts range up to as old as ca. 2700 Ma (the approximate metamorphic age of the impact target region) and are likely indicative of incomplete Ar isotopic resetting during impact. While many UVLAMP analyses of impact melt domains are indistinguishable from 286 Ma, there still is considerable variability, likely due to unidentified, entrained fine-grained altered and clast material. Careful characterization of impactites is imperative prior to ⁴⁰Ar/³⁹Ar dating, but our work demonstrates that this may not be sufficient to obtain unambiguous dates. A large number of unaltered samples may be needed to accurately determine impact ages.