RAPID 40CA/40K DATING OF DETRITAL MUSCOVITE

The ⁴⁰K–⁴⁰Ca decay system is rarely utilized in geochronology despite ~ 90% of ⁴⁰K atoms decaying to ⁴⁰Ca and only 10% to ⁴⁰Ar. The high mass resolving power (MRP) of ~25k to separate ⁴⁰K from ⁴⁰Ca further restricts in situ methods. The advent of ‘double-plus’ ⁴⁰Ca/⁴⁰K dating (i.e., analysis of ³⁹K⁺⁺ and ⁴⁰Ca⁺⁺) allows isotopic analyses to be made at low MRP (~5k) without chemical separation using a CAMECA ims1270 ion microprobe. The ‘double-plus’ advantage results from K⁺⁺ having a 2^nd ionization potential 3x higher than Ca⁺⁺, effectively suppressing ⁴⁰K⁺⁺ on the ⁴⁰Ca⁺⁺ peak with abundant ³⁹K (at constant ⁴⁰K/³⁹K = 0.001167) still easily measurable without significant interferences. Radiogenic ⁴⁰Ca (⁴⁰Ca*) can be corrected for common Ca by measuring ⁴⁴Ca⁺⁺.

We tested the feasibility of ‘double-plus’ K-Ca dating on a suite of detrital muscovites from modern Colorado River sediments. Muscovite is resistant to abrasion and chemical dissolution with crystals large enough to be rapidly hand-picked. A total of ~150 crystals from two sands collected near Moab (Utah) and Baja California, were mounted in epoxy and analyzed by SIMS. Relative sensitivity was calibrated using 2.4 Ga U68 standard muscovite with ~5% reproducibility on individual crystals and per-spot analysis duration of ~3 min. The majority of muscovites from both locations had ⁴⁰Ca* >50%, and age reproducibility within individual detrital grains is mostly within analytical uncertainties (<5 %). In some cases, precise and reproducible K-Ca ages as young as ~200-300 Ma were obtained, but the overall detrital muscovite population is overwhelmingly dominated by ages ~1.4 to 1.7 Ga which comprise >90% of the analyzed crystals. This suggests a strong influx from Paleoproterozoic-Mesoproterozoic cratonal basement sources that is also evident in U-Pb detrital zircon age populations (Kimbrough et al. 2007). U-Pb zircon ages, however, differ from K-Ca muscovite ages in two aspects: (1) the presence of a slightly older (1.9 Ga) population, and (2) more abundant Paleozoic/Mesozoic zircons. These initial results demonstrate the potential for determining the thermo-tectonic evolution of cratonic crust chronicled in detrital muscovite, with the advantages of having much higher spatial resolution and sample throughput compared to ⁴⁰Ar/³⁹Ar.