AGE OF THE FISH CANYON TUFF STANDARD RE-EVALUATED

The challenges outlined through the EARTHITME Initiative, of increasing accuracy in radio-isotopic dating systems not only influenced the U/Pb community, but also had its impact on the ⁴⁰Ar/³⁹Ar community. Building upon a background of ⁴⁰Ar/³⁹Ar inter-calibration experiments (e.g., Renne et a., 1998) a novel approach to calibrating the age of the most widely used standard in ⁴⁰Ar/³⁹Ar dating, the Fish Canyon Tuff sanidine (FCs), was developed by intercalibrating the ⁴⁰Ar/³⁹Ar method with absolute chronologies derived from astronomical tuning methods (Kuiper et al., 2008). This age of 28.201 ± 0.046 Ma appeared to minimize bias between different geochronometers, such as U/Pb and ⁴⁰Ar/³⁹Ar, and was subsequently adopted in the standard Geological Time Scale (GTS2012). However, the debate about the FCs age nevertheless continued with estimates based on model approaches, U/Pb dating and alternative astronomical calibrations ranging from 27.89 to 28.394 Ma.

Here we evaluate these attempts to determine the FCs age and present relevant new data as well. We re-examined the astronomical tuning of cyclic late Miocene sections in the Mediter­ranean that underlie the FCs age of 28.201 Ma in detail and show that it is indeed reliable down to the individual precession cycle. This was already confirmed by single crystal U/Pb zircon ages of the Fish Canyon tuff (Wotzlaw et al., 2012). We also present new U/Pb - ⁴⁰Ar/³⁹Ar age pairs from the Cretaceous of North America, that confirm an FCs age ~28.2 Ma. Over the past decade reproducible FCs calibration experiments, ⁴⁰Ar/³⁹Ar and U-Pb ‘equivalence pairs’ and other inter-calibration experiments have significantly increased the accuracy of the ⁴⁰Ar/³⁹Ar dating system and minimized the discordance between U/Pb and ⁴⁰Ar/³⁹Ar chronologies that existed at the start of the EARTHTIME and were a major motivation for efforts in the first part of the initiative.