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

Highlights

• Potential Paleozoic (Carboniferous) monitor standard for ⁴⁰Ar/³⁹Ar dating

• Zircon selection based on morphology effectively screens out xenocrystic component.

• ²⁰⁶Pb/²³⁸U zircon date of 314.629 ± 0.039 Ma (±0.35 Ma: 2σ total uncertainty)

• Reproducible individual ⁴⁰Ar/³⁹Ar sanidine dates are within 1% of mean age.

• ⁴⁰Ar/³⁹Ar sanidine date: 315.36 ± 0.10 Ma (±1.10 Ma: 2σ total, FCT: 28.201 Ma)

Abstract

Radioisotopic geochronology applied to the high-resolution calibration of Earth history requires a set of synthetic 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.

Introduction

As geochronometers become increasingly more precise we are presented with the issue of age inaccuracy stemming from bias between chronometers and laboratories. ⁴⁰Ar/³⁹Ar in sanidine and U-Pb in zircon are widely applied radioisotopic chronometers for the Geologic Time Scale (Ogg et al., 2016; Schmitz, 2012). Recent advances in U-Pb geochronology, such as the EARTHTIME community-driven production and distribution of calibrated U-Pb isotopic tracers for ID-TIMS analyses (Condon et al., 2015; McLean et al., 2015) have highlighted the need for reciprocal improvements in ⁴⁰Ar/³⁹Ar geochronology, cross-calibration between the two chronometers, and their seamless integration into the Geologic Time Scale.

In order to achieve the EARTHTIME initiative's goal of accurate and precise sequencing of geologic events at the 0.1% level, the ⁴⁰Ar/³⁹Ar and U-Pb communities require a set of samples that permit inter-laboratory and inter-technique comparisons. Natural zircon and sanidine reference materials play a critical role for these comparisons, but are currently limited in number. Plešovice (Sláma et al., 2008), R33, and Temora 2 (Black et al., 2004) are commonly used for zircon, whereas Fish Canyon, Alder Creek, and Taylor Creek are the most frequently used sanidine standards (Renne et al., 1998); we know of none that are available from the same volcanic eruption for both ⁴⁰Ar/³⁹Ar and U-Pb chronometers without evident complexity in one of the chronometers. There is currently no recognized Paleozoic sanidine monitor standard for ⁴⁰Ar/³⁹Ar geochronology let alone a monitor standard older than ~30 Ma, and there is a significant need for samples that contain both sanidines and zircons for inter-laboratory and -method refinements. The sanidine- and zircon-bearing Carboniferous Fire Clay tonstein, a voluminous ash bed from the Appalachian Basin, provides potential natural Paleozoic monitor for ⁴⁰Ar/³⁹Ar geochronology and a reference material for inter-method comparison between ⁴⁰Ar/³⁹Ar and U-Pb. Here we report the results of 263 single-grain sanidine ⁴⁰Ar/³⁹Ar analyses from eight irradiation positions and 41 single-zircon U-Pb CA-ID-TIMS analyses from samples of the Fire Clay tonstein, introducing the Fire Clay sanidine as a natural reference material for ⁴⁰Ar-³⁹Ar geochronology with the advantage of cross-calibration with the U-Pb system.

⁴⁰Ar/³⁹Ar geochronology relies on the assumption of a known standard with a precisely and accurately determined absolute age to which unknowns are referenced. Sanidine from the Oligocene Fish Canyon tuff is the most commonly applied standard, despite reported ages that range from 28.393 (Ganerød et al., 2011) to 27.79 (Cebula et al., 1986) and evidence for complexity in the eruptive system for the Fish Canyon tuff (e.g., Lipman and Bachmann, 2015, and references therein). In the ⁴⁰Ar/³⁹Ar system, radioactive ⁴⁰K undergoes a branched decay to ⁴⁰Ar and ⁴⁰Ca, and significant uncertainties are associated with their decay constants (Min et al., 2000; Renne et al., 2011, Renne et al., 2010). U-Pb geochronology by in situ (microbeam) techniques also requires chemically and isotopically well-characterized reference materials including natural mineral standards. In contrast, U-Pb dates by the isotope dilution method of U-bearing minerals such as zircon are calibrated against ‘tracer’ solutions (usually ²⁰⁵Pb ± ²⁰²Pb and ²³⁵U ± ²³³U) of precisely calibrated isotopic abundances. These tracer solutions allow for U and Pb isotopic ratios in the samples to be accurately determined, and these ratios can be traced back to SI units through weights and measures (Condon et al., 2015; McLean et al., 2015). Additional advantages of the U-Pb system include the precisely and accurately determined ²³⁸U half-life (Jaffey et al., 1971) and recent improvements in the ratio of the ²³⁵U and ²³⁸U decay constants (Mattinson, 2010). Recent efforts made by the EARTHTIME U-Pb community have involved development of standard ‘age solutions’ of known composition, made available to the community, re-evaluation of the isotopic composition of natural uranium (Hiess et al., 2012) and the re-determination of the isotopic composition of some commonly used uranium reference materials which underpin the calibration experiments and results (Condon et al., 2010). With increased precision and accuracy of U-Pb zircon dates, subtle complexities in the U-Pb systematics of magmatic systems become significant and need to be addressed. Improvements in the precision and accuracy of ⁴⁰Ar/³⁹Ar dating will lead to improved ability to scrutinize potential geological complexity in both systems.

The Pennsylvanian Fire Clay tonstein is a kaolinized volcanic air-fall ash bed that covers a minimum of 37,000 km² in eastern Kentucky, West Virginia, and parts of Tennessee and Virginia (Fig. 1; Lyons et al., 1992). The geographic range of this eruptive ash has made it a critical lithostratigraphic marker in the Appalachian Basin. The tonstein is commonly found as a 4 to 20 cm thick layer near the base of the Fire Clay coal bed, although thicknesses up to 39 cm have been reported (Huddle and Englund, 1966; Stevens, 1979; Chesnut, 1985).

Because of its significance as a stratigraphic marker bed, the Fire Clay tonstein has been particularly well-mapped and studied (e.g., Huddle and Englund, 1966; Kunk and Rice, 1994; Lyons et al., 2006, Lyons et al., 1992; Rice et al., 1994; Stevens, 1979; Wanless, 1946). It is comprised of well-crystallized kaolinite with 3–5% accessory minerals (Rice et al., 1994). The tonstein is described as dark brownish-grey in color, beds exhibit conchoidal fracture and show a waxy luster (Kunk and Rice, 1994). Thin-section and single-mineral studies indicate a volcanic origin for the tonstein and previous workers have found no evidence for authigenic or detrital phenocrysts (Kunk and Rice, 1994). ⁴⁰Ar/³⁹Ar geochronology on large, multi-crystal, step-heated samples of sanidine from across the available exposures of the tonstein has shown remarkable reproducibility (Kunk and Rice, 1994), supporting the hypothesized volcanic origin for these phenocrysts. The eruptive source of the Fire Clay ash has been suggested to be the North Carolina piedmont (Rice et al., 1994; Sinha and Zietz, 1982) or the present-day Gulf of Mexico (Lyons et al., 1992).

Previous ⁴⁰Ar/³⁹Ar geochronology on multi-grain sanidine separates from the Fire Clay tonstein gave ages from 312.1 ± 1 Ma (Lyons et al., 1992: four separate step heating experiments, including two of Hess et al., 1988); to 310.9 ± 0.8 Ma (seven step heated sanidine samples of ~100 mg each in Kunk and Rice, 1994). The data from Kunk and Rice (1994) has a range of reported plateaus of 310.25 ± 0.55 Ma to 311.38 ± 0.55 Ma for samples collected over a 300 km distance, showing remarkable internal consistency. Note that if Kunk and Rice (1994) ages are converted from Fish Canyon monitor age of 27.79 Ma to an improved age of 28.201 Ma and from ⁴⁰K decay constants of Steiger and Jäger (1977) to those of Min et al. (2000), this age range translates to 314.83 Ma to 315.98 Ma (converted by ArAr software of Mercer and Hodges, 2016).

Early U-Pb geochronology from the Fire Clay tonstein involved analysis of multi-grain (103–145 mg) fractions of untreated zircon by the ID-TIMS method, each based on a different tonstein sample from West Virginia, Virginia, Tennessee and Kentucky. These analyses produced highly discordant U-Pb results due to the widespread presence of xenocrystic components; the four samples defined a roughly linear array with concordia intercepts at 344 ± 35 Ma and 1224 ± 150 Ma (MSWD = 15). The results did not allow any meaningful comparison between U-Pb and the ⁴⁰Ar/³⁹Ar chronometers at the time (Rice et al., 1994). Lyons et al. (2006) reported five single-zircon U-Pb analyses by the ID-TIMS method on air-abraded zircons from a sample of the Fire Clay tonstein from Pike Co., Kentucky. Four of these analyses were highly discordant due to zircon inheritance. The single, youngest ²⁰⁶Pb/²³⁸U date of 314.6 ± 0.9 Ma (2σ) was interpreted as the age of deposition of the Fire Clay ash (Lyons et al., 2006). The proximity of the latter U-Pb date to the published ⁴⁰Ar/³⁹Ar geochronology suggested that the Fire Clay tonstein held promise as a potential Paleozoic reference material for both systems.