A POSSIBLE STANDARD FOR BOTH U/PB AND AR/AR DATING: THE CARBONIFEROUS FIRE CLAY TONSTEIN

A central goal of the EARTHTIME initiative is to create an accurate and precise sequence of geological events that can be used to evaluate the rates of processes. For this purpose, the Ar-Ar and U-Pb communities are in need of better standards for inter-laboratory and inter-technique comparisons. At present there is no standard that can be used both for inter-technique comparison and for calibrating inter-laboratory biases within each dating method, and there is no Paleozoic sanidine monitor standard. We suggest that the Carboniferous Fire Clay Tonstein is a good prospect as a standard for both dating methods. It is well exposed and large samples sufficient for distribution of minerals to interested labs could be processed.

The Fire Clay tonstein is 5-20 cm thick and is interpreted as an ash layer deposited within the Fire Clay (and equivalent) coal bed(s) of the central Appalachian Basin. Kunk and Rice (1994, GSA Sp. P. 294) reported analytically indistinguishable ⁴⁰Ar/³⁹Ar age spectra for bulk sanidine separates distributed over ~300 km transect. These results prompted our initial study aimed at testing the prospects for establishing a new monitor standard for Ar-Ar dating.

Fire Clay sanidine crystals were co-irradiated with several of the monitor standards for the EARTHIME Ar intercalibration experiment. Forty individual sanidine crystals (250-350 μm) from this tonstein yield a simple population and an age in agreement with the published age spectra, and the range of individual sanidine ages is less than 1%. We will further test this tonstein in additional Ar-labs, with the goal of establishing a new ⁴⁰Ar/³⁹Ar standard.

Zircons (ca. 50-400 μm long) were obtained from three tonstein samples. Preliminary results suggest a weighted mean ²⁰⁶Pb/²³⁸U date about 0.4% older than the Ar-Ar sanidine age, consistent with other comparisons. Although there is some xenocrystic inheritance and a few crystals have evidence for Pb-loss, the potential for comparison between the two dating methods justifies the use of this tonstein as a U-Pb zircon standard.