ILLITE/MUSCOVITE 40AR/39AR THERMOCHRONOLOGY: A TEST FROM MIDDLE CAMBRIAN PELITES OF THE SOUTHWESTERN US

Despite being one of the earliest applications of K-Ar geochronology, conflicting models still exist for the interpretation of K-Ar and ⁴⁰Ar/³⁹Ar data from greenschist to sub-greenschist pelites. We conducted ⁴⁰Ar/³⁹Ar step-heating experiments on micron to submicron grain sizes of illite- and muscovite- rich Middle Cambrian pelites from the western US than range in metamorphic grade from the early diagenetic zone (zeolite facies) to epizone (greenschist facies). With increasing metamorphic grade, maximum ages reached on ⁴⁰Ar/³⁹Ar spectra decrease, as do total gas ages and retention ages. Complete resetting of the illite/muscovite Ar thermochronometer appears to occur between the high anchizone and epizone, or between roughly 200 and 300 °C. This empirical result is in agreement with experimentally-derived muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.1-1 µm have closure temperatures in the same range. ⁴⁰Ar/³⁹Ar data from illite appear suitable for detrital studies in samples that have not been at least partially reset, and as a thermochronometer for samples that have been completely reset. In the latter case, age variations from different grain sizes may correspond with cooling rate. Our results demonstrate both applications: ⁴⁰Ar/³⁹Ar data from zeolite-facies Middle Cambrian pelites in the western US suggest that they contain detrital muscovite with an apparent age of ≥ 950 Ma. Data from correlative greenschist-facies Middle Cambrian pelites in the footwall of the Bullfrog/Fluorspar Canyon detachment near Death Valley reveal a period of Cretaceous cooling.