An assessment of sanidine from the Fire Clay tonstein as a Carboniferous 40Ar/39Ar monitor standard and for inter-method comparison to U-Pb zircon geochronology

Abstract

Radioisotopic geochronology applied to the high-resolution calibration of Earth history requires a set of syn- thetic and natural reference materials for both 40Ar/39Ar and U-Pb techniques that permit both inter-laboratory and inter-technique comparisons. The sanidine- and zircon-bearing Carboniferous Fire Clay tonstein provides a potential natural Paleozoic reference for these two widely used radioisotopic systems. Here we report results for both radioisotopic systems, examining the suitability of this tonstein as a geochronologic reference. Sanidine crystals from the Fire Clay and co-irradiated monitors from eight irradiation positions were divided into eleven 40Ar/39Ar experiments. Single-grain sanidine 40Ar/39Ar analyses (n = 263) of the simplest 9 experiments have internal 2σ uncertainties at the ± 1 Myr level ( ± 0.3%), with a range of dates between ~315 and ~317 Ma (~1% precision), similar to the observed dispersion in the Fish Canyon sanidine monitor dates. Forty-one U-Pb analyses by the CA-ID-TIMS method on carefully selected single Fire Clay tonstein zircons have produced 206Pb/238U dates with an average 2σ precision of ± 0.23 Myr (0.14%). Our Fire Clay preferred mean 40Ar/39Ar date of 315.36 ± 0.10 Ma ( ± 1.10 Ma: fully propagated 2σ uncertainty, relative to a Fish Canyon age of 28.201 Ma) is consistent with our weighted mean 206Pb/238U zircon date of 314.629 ± 0.039 Ma ( ± 0.35 Ma: fully propagated 2σ uncertainty; n = 27). The good single-crystal reproducibility of the sanidine data and the overall consistency between the two chronometers suggest that the tonstein holds promise as a Paleozoic age reference material