Intercalibration and age of the Alder Creek sanidine 40Ar/39Ar standard

Abstract

The accuracy of 40 Ar/ 39 Ar geochronology relies in large part on precise and accurate calibration of the ages and K--Ar isotopic compositions of standards. A widely used standard for Quaternary samples, the ~1.2 Ma Alder Creek sanidine (ACs), has published ages spanning a range of ~2%. New measurements of ACs co--irradiated with the Fish Canyon sanidine (FCs) standard and sanidines from astronomically dated Miocene tuffs in Crete and Morocco yield results that enable both (i) a direct calibration of ACs relative to FCs, and (ii) stepwise calibrations between these two standards employing the Miocene intermediaries. Results are summarized by the parameter R FCs ACs , defined as the ratio of ( 40 Ar*/ 39 ArK) of ACs to FCs, which embodies the fundamental age relationship between these standards that is independent of systematic variables such as decay constants or absolute ages of standards. Our new measurements, executed using three mass spectrometers and various irradiation and analytical protocols, yield a weighted mean R FCs ACs = 0.041702 ± 0.000014 (σ).This result can be combined with previously published determinations of R values for ACs relative to the Miocene tuffs and to FCs to yield a recommended interlaboratory value of R FCs ACs = 0.041707 ± 0.000011. The weighted--mean age of ACs using this interlaboratory value, based on astronomically--calibrated ages of FCs and the Miocene intermediary sanidines, is tACs = 1.1848 ± 0.0006 Ma (± 0.05%).Applying this result to the most precise published 40 Ar/ 39 Ar data for the Matuyama--Brunhes geomagnetic polarity reversal yields tMBB = 780.1 ± 0.8 ka. In addition, these new data for ACs support previous conclusions that U--Pb zircon ages from the Alder Creek rhyolite incorporate 13 ka of pre--eruptive residence time.

Recordings of Fast Paleomagnetic Reversals in a 1.2 Ma Greigite‐Rich Sediment Archive From Lake Ohrid, Balkans

“…Red stars with error bars: Volcanic ages with uncertainties. (1) Sagnotti et al (2014), (2) Niespolo et al (2017), (3) Simon et al (2017), (4) Macrì et al (2018), (5) Nowaczyk, Haltia, et al (2013), (6) using the cores recovered from six parallel holes. The upper 448 m are composed of muddy sediments, which vary in composition on glacialinterglacial timescales Wagner et al, 2014).…”

The New CLOCIT Irradiation Facility for 40Ar/39Ar Geochronology: Characterisation, Comparison with CLICIT and Implications for High‐Precision Geochronology

“…We individually analysed five to ten grains of ACs and FCs per well, heated in one or two steps and calculated an inverse-variance weighted mean (in the following 'weighted mean') J-value for the respective well. All three Ar analysis lines at BGC (NEXUS, MAP1 and Noblesse) were involved, using measurement routines described in Niespolo et al (2017). Typical uncertainties of J-values of individual wells werẽ 0.15-0.4%.…”

“…We determined a planar fit through the 4-8 wells of given J-value and calculated the deviation of each well from the fitted plane; the calculation considers uncertainty in J-value and predicts a J-value and respective uncertainty for any position. For J-value calculations we used ages of 28.201 Ma (Kuiper et al 2008) and 1.1848 Ma (Niespolo et al 2017) for FCs and ACs, respectively. Other ages are in use for these reference materials but the values used are irrelevant for the present purposes so long as they are internally consistent as was demonstrated by Niespolo et al (2017).…”

“…For J-value calculations we used ages of 28.201 Ma (Kuiper et al 2008) and 1.1848 Ma (Niespolo et al 2017) for FCs and ACs, respectively. Other ages are in use for these reference materials but the values used are irrelevant for the present purposes so long as they are internally consistent as was demonstrated by Niespolo et al (2017).…”

Millennial‐Scale Instability in the Geomagnetic Field Prior to the Matuyama‐Brunhes Reversal

“…Before determining the length of the instability, an age for the final onset of the Bruhnes chron is required. There is significant debate over the age of the boundary with arguments for a 780-784 ka age (Cande & Kent, 1995;Mark et al, 2017;Niespolo et al, 2017;Sagnotti et al, 2014) or a ca. 772 ka age (Channell et al, 2010;Okada et al, 2017;Suganuma et al, 2010Suganuma et al, , 2015.…”