Interpreting and reporting 40Ar/39Ar geochronologic data

Abstract

<jats:p>The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ∼0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity.</jats:p>

Eruptive history and 40Ar/39Ar geochronology of the Milos volcanic field, Greece

“…A reliable plateau age is defined as experiments with at least 3 consecutive steps overlapping at 2-sigma, containing >50% of the 39 ArK, a Mean Square Weighted Deviate (MSWD) value<2.5, and with an 40 Ar/ 36 Ar inverse isochron intercept that does not deviate from atmospheric argon at 2-sigma. All the inverse isochron ages used the same steps as used in the weighted mean ages, and all relevant analytical data for the age calculations following standard practices (Schaen et al, 2020) can be found in in the supplementary material II.…”

Review of geothermochronological and thermobarometric techniques for the construction of cooling and exhumation curves or paths for intrusive igneous rocks

“…To calculate these ages, high-temperature thermochronology is the most suitable approach (cf. Harrison, 1982;Zeitler, 1985;Reiners and Brandon, 2006;Schaen et al, 2020). To clarify, the closure temperatures of the systems will depend on the cooling rate of the rock and on kinetics, geometry, composition and size crystal (cf.…”

“…Furthermore, these thermochronometers have considerably higher closure temperatures than other thermochronometers suitable for calculating initial cooling ages (Harrison, 1982;Zeitler, 1985;Harrison et al, 2009;Reiners et al, 2017, pp. 231-253;Schaen et al, 2020) (Figure 3). In the biotite 40 Ar/ 39 Ar method, the thermometer is considered to be a low-temperature thermometer ( Figure 3) because the closure temperatures range from 330 to 280 °C (cf.…”

“…Thus, the potassium feldspar 40 Ar/ 39 Ar thermochronometer is not recommended for plutonic and metamorphic rocks and is accordingly more often applied to the geochronology of volcanic rocks (cf. Reiners et al, 2017, p. 240;Schaen et al, 2020).…”

Supplemental Material: Longest continuously erupting large igneous province driven by plume-ridge interaction

“…Blanks were monitored every 3 to 4 steps. All parameters and relative abundance values are provided in Table S4 following recommendations of Schaen et al (2020) and have been corrected for blank, mass discrimination and radioactive decay.…”