Call for Development of New Mineral Standards for 40Ar/39Ar Dating

Abstract:

Age determination via the ⁴⁰Ar/³⁹Ar dating method relies on the intercomparison of measured ⁴⁰Ar^*/³⁹Ar_K ratios of geological unknowns with those of co-irradiated mineral standards. Good analytical procedure dictates that these ratios (and the evolution of the Ar ion beams underpinning them) be as similar as practical for the greatest accuracy.

Unfortunately, throughout several intervals of the geological time scale this ‘best practice’ cannot be achieved with existing widely used standards. Only two internationally utilized sanidine standards are available for the middle to late Cenozoic: the Alder Creek Rhyolite sanidine (ACs), at ~1.2 Ma (Turrin et al., 1994; Nomade et al., 2005), and the Fish Canyon Tuff sanidine (FCs) at ~28.2 Ma (e.g., Kuiper et al., 2008; Renne et al, 2011). The situation is even worse throughout much of the rest of the Phanerozoic, as the next oldest standard in common use is the Hb3gr hornblende standard with an age of ~1.1 Ga (Turner, 1971; Jourdan et al., 2006).

We propose, as a community effort, the development a set of standards covering the entire target range of high-precision ⁴⁰Ar/³⁹Ar dating, i.e. the Phanerozoic. Their ages would be stepped in a regular fashion with no more than approximately a factor of 3 between standards, such that in the worse case the ⁴⁰Ar*/³⁹Ar ratios of standards and unknown need differ by no more than a factor of two. While somewhat arbitrary, an approximately 3 X age progression allows the entire time scale to be covered by a manageable number of standards. Anchoring the progression in the widely used ACs, FCs, and Hb3gr (in bold, below) yields the following set of suggested standard ages: 0.4, 1.2, 3.3, 9.4, 28.2, 95, 320, and 1100 Ma.

A suitable standard should be highly reproducible in age at the grain-to-grain and sub-grain levels, and highly radiogenic. The mineral should be abundant and easily separated from the host rock. These criteria may be most easily achieved by focusing on sanidine phenocrysts in unaltered volcanic deposits, though not exclusively so. Examples include the Zabriskie Wash Tuff of Death Valley at 3.3 Ma; several ~9-10 Ma volcanic units of the Bodie Hills and Snake River Plain, and the ~316 Ma Fireclay Tonstein of the central Appalachian basin. We welcome suggestions and discussions regarding other rock units that might be tapped for this endeavor.