Comparison of Ca and Ar Diffusion in Phlogopite: Implications for K-Ca and K-Ar Geochronology

Abstract:

Coupled geochronology based upon branched decay of ⁴⁰K-⁴⁰Ar and ⁴⁰K-⁴⁰Ca decay is rarely exploited because ⁴⁰Ca is the major common isotope of calcium and ⁴⁰Ca and ⁴⁰K are difficult to resolve isotopically without resorting to isotope dilution wet chemistry. Recently developed ion microprobe methods based upon measurement of doubly ionized species partially overcome the latter problem and have been applied to high K/Ca micas. The ability to interpret K-Ar and K-Ca results is limited due to uncertainty in the relative diffusion properties of Ca and Ar. To address this problem, we are performing Ar and Ca diffusion experiments and fluid-crystal Ar partitioning experiments with anhydrous F-phlogopite that is stable to 1390°C. As an additional check, we are comparing K-Ca and K-Ar ages from natural mantle phlogopites from a variety of settings to assess the relative retentivity of Ar and Ca. South African xenoliths tend to yield ⁴⁰Ar/³⁹Ar ages that are much older than K-Ca ages from the same phologopites. Possible excess ⁴⁰Ar and high common Ca render the comparisons inconclusive, but this suggests that Ca diffuses more readily than Ar in phlogopite. Our most definitive K-Ca phlogopite results (i.e., least affected by common Ca) come from the Archean Seqi dunite of SW Greenland. The K-Ca ages of Seqi phlogopites is 927 ± 26 Ma (2s). Incremental heating ⁴⁰Ar/³⁹Ar results from the same sample yields a much older result with a terminal age of 3.5 Ga. However, the first 5-10% of ³⁹Ar release are consistent with transient heating at ca. 900 Ma. Considered together, the K-Ca and ⁴⁰Ar/³⁹Ar results from the Seqi dunite locality strongly suggest that Ca diffusion is more rapid than Ar diffusion in phlogopite.