⁴⁰Ar/³⁹Ar

From the outset the EARTHTIME community has sought to improve the ability of the entire community through sharing of best practices. A recent (2017) workshop at New Mexico Tech initiated a discussion of best practices.

The ⁴⁰Ar/³⁹Ar method requires a priori knowledge of a mineral standard, or neutron flux monitor, which is co-irradiated with samples of interest. The age of a mineral standard can be determined in several ways, including first-principles measurements, intercalibration with primary standards, astronomical calibrations, and optimizations involving U-Pb – ⁴⁰Ar/³⁹Ar data pairs.

Among the most commonly used mineral standards in ⁴⁰Ar/³⁹Ar geochronology is Fish Canyon sanidine (FCs). FCs has been separated from the Fish Canyon Tuff (FCT), which erupted from the La Garita Caldera in the San Juan Mountains of southern Colorado. Between 1998 and 2008, many geochronologists used an age of 28.02 ± 0.16 Ma for FCs. More recently, most geochronologists use an age of either 28.201 ± 0.023 Ma or 28.294 ± 0.036 Ma.

Given the dwindling supply of high purity FCs, a new sample (FCs-EK) is now available on request to Darren Mark.

The ⁴⁰Ar/³⁹Ar method relies on the branched decay of ⁴⁰K to ⁴⁰Ar and ⁴⁰Ca. Decay constants for these decays are critical parameters to calculating ⁴⁰Ar/³⁹Ar ages.

Commonly used values for decay constants have also changed, along with mineral standard ages, as they have been reassessed. The ‘classic’ values agreed on at the IUGS Subcommission on Geochronology meeting in 1976 were reassessed in 2000 and 2011.

A number of experiments have been carried out to quantify the agreement between ⁴⁰Ar/³⁹Ar laboratories.

2004/5:  This experiment involved analyses of the Fish Canyon, Alder Creek, and Taylor Creek sanidine standards. Initial results were presented at the EARTHTIME IV workshop in 2008, where they demonstrated that inter-laboratory bias was present at a level of ~1%, substantially larger than analytical precision (~0.1%).

2011-current: The NSF-funded APIS (Argon Pipette Intercalibration System) has been lead by Brent Turrin, Sid Hemming, and Al Deino, and discussed here and here. The APIS is a traveling pipette system that has visited laboratories at Rutgers, Lamont, Berkeley Geochronology Center, Arizona State, New Mexico Tech, USGS Menlo Park, and USGS Denver. The system is designed to allow all laboratories to measure the exact same gases, in hopes of identifying the issues involved in interlaboratory biases.

There are several software packages used by laboratories to collect and/or reduce ⁴⁰Ar/³⁹Ar data:

MassSpec, by Al Deino, Berkeley Geochronology Center

ArArCALC, by Anthony Koppers, Oregon State University

PyChron, by Jake Ross, New Mexico Bureau of Geology, with Github and Documentation

ArArRedux, by Pieter Vermeesch, University College London

There are also several packages designed for viewing and/or recalculating ⁴⁰Ar/³⁹Ar data to new standard ages and decay constants. Care should be taken in using these, as standard ages require the use of certain decay constants (ie: the same ones used to calculate the standard age itself):

ArAR, by Cameron Mercer and Kip Hodges, Arizona State University

ArArCALIBRATION, by Anthony Koppers, Oregon State University

A ‘data exchange format’ working group, including software developers and other analysts, formed recently (2017) to enable the integration and exchange of data between the most common software packages.

A publication intended to guide authors, reviewers, and editors of manuscripts including ⁴⁰Ar/³⁹Ar data resulted from the EARTHTIME IV workshop.

2017 working group meeting – to be convened.

Jicha, B.R., Singer, B.S., Sobol, P., 2016. Re-evaluation of the ages of ⁴⁰Ar/³⁹Ar sanidine standards and supereruptions in the western US using a Noblesse multi-collector mass spectrometer. Chemical Geology, v. 431, p. 54-66.

Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne, P.R., and Wijbrans, J.R., 2008. Synchronizing rock clocks of Earth history. Science, v. 320 i. 5875, p. 500-504. Klaudia and co-authors present an astronomically calibrated age for Fish Canyon sanidine.

Koppers, A.A.P., 2002. ArArCALC—software for⁴⁰Ar/³⁹Ar age calculations. Computers & Geosciences v. 28, i. 5, p. 605-619. Anthony describes his ArArCALC software package.

Min, K. W., Mundil, R., Renne, P.R., and Ludwig, K.R., 2000. A test for systematic errors in ⁴⁰Ar/³⁹Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite. Geochimica et Cosmochimica Acta, v. 64, i. 1, p.  73-98. Kyle and co-authors sift through the existing activity data for ⁴⁰K and present new values (with more reasonable, increased uncertainties) for decay constants.

McDougall, I., and Harrison, T.M., 1999. Geochronology and Thermochronology by the ⁴⁰Ar/³⁹Ar Method. Oxford University Press. In what could fairly be called ‘The Argon Bible’, Ian and Mark cover many aspects of argon geochronology. Essential reading for argon geochronologists.

Morgan, L.E., Postma, O., Kuiper, K.F., Mark, D.F., van der Plas, W., Davidson, S., Perkin, M., Villa, I.M., Wijbrans, J.R., 2011. A metrological approach to measuring ⁴⁰Ar* concentrations in K‐Ar and ⁴⁰Ar/³⁹Ar mineral standards. Geochemistry, Geophysics, Geosystems, v. 12, i. 10. Leah, Klaudia, Darren, Igor, Jan, and co-authors present a system for a first-principles, metrological calibration of ⁴⁰Ar concentrations in mineral standards.

Niespolo, E.M., Rutte, D., Deino, A.L., and Renne, P.R., 2017. Intercalibration and age of the Alder Creek sanidine ⁴⁰Ar/³⁹Ar standard. Quaternary Geochronology, v. 39, p. 205-213.

Phillips, D., Matchan, E.L., 2013. Ultra-high precision ⁴⁰Ar/³⁹Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required? Geochimica et Cosmochimica Acta 121, 229-239.

Renne, P. R., et al., 2009. Data reporting norms for ⁴⁰Ar/³⁹Ar geochronology. Quaternary Geochronology, v. 4, i. 5, p. 346-352. Paul, along with a long list of working group co-authors, presents requirements for the publication of ⁴⁰Ar/³⁹Ar data.

Renne, P. R., Cassata, W.S., and Morgan, L.E., 2009. The isotopic composition of atmospheric argon and ⁴⁰Ar/³⁹Ar geochronology: Time for a change? Quaternary Geochronology, v. 4, i.4, p. 288-298. Paul, Bill, and Leah discuss the ramifications of using a newly published value for atmospheric ⁴⁰Ar/³⁶Ar.

Renne, P.R., Mundil, R., Balco, G., Min, K., Ludwig, K.R., 2010. Joint determination of ⁴⁰K decay constants and ⁴⁰Ar*/⁴⁰K for the Fish Canyon sanidine standard, and improved accuracy for ⁴⁰Ar/³⁹Ar geochronology. Geochimica et Cosmochimica Acta, v. 74, i. 18, p. 5349-5367. Paul, Roland, Greg, Kyle, and Ken present a model for determining mineral standard ages and decay constants using available data and ⁴⁰Ar/³⁹Ar – U-Pb data pairs.

Renne, P.R., Balco, G., Ludwig, K.R., Mundil, R., Min, K., 2011. Response to the comment by W.H. Schwarz et al. on “Joint determination of ⁴⁰K decay constants and ⁴⁰Ar*/⁴⁰K for the Fish Canyon sanidine standard, and improved accuracy for ⁴⁰Ar/³⁹Ar geochronology” by PR Renne et al. (2010). Geochimica et Cosmochimica Acta 75, 5097-5100. Paul and others update results from their 2010 paper (above).

Rivera, T.A., Storey, M., Zeeden, C., Hilgen, F.J., Kuiper, K., 2011. A refined astronomically calibrated ⁴⁰Ar/³⁹Ar age for Fish Canyon sanidine. Earth and Planetary Science Letters 311, 420-426.

Simon, J.I., Renne, P.R., Mundil, R., 2008. Implications of pre-eruptive magmatic histories of zircons for U-Pb geochronology of silicic extrusions. Earth and Planetary Science Letters 266, 182-194.

Schmitz, M.D., 2012. Radiogenic isotope geochronology. Geologic Time Scale, 115–126  In this chapter, Mark reviews the developments in radio-isotopic dating methods (esp. U-Pb ID-TIMS) from 2004 to 2012, covering many of the EARTHTIME and related innovations.

Schmitz, M.D. and Kuiper, K.F., 2013. High-precision geochronology.  Elements.  Elements, v. 9, i. 1, p. 25-30.  In this contribution to the ‘One Hundred Years of Geochronology’ issue of Elements, Mark and Klaudia discuss…

Schoene, B., Condon, D.J., Morgan, L., and McLean, N., 2013. Precision and Accuracy in Geochronology.  Elements, v. 9, i. 1, p. 19-24  This short note outlines and discusses issues around the precision and accuracy of geochronologic data and interpretations

Vermeesch, P., 2015. Revised error propagation of ⁴⁰Ar/³⁹Ar data, including covariances. Geochimica et Cosmochimica Acta, v. 171, p. 325-337. Pieter describes his method for including covariances in ⁴⁰Ar/³⁹Ar data propagation.

Wotzlaw, J.-F., Husing, S.K., Hilgen, F.J., Schaltegger, U., 2014. High-precision zircon U-Pb geochronology of astronomically dated volcanic ash beds from the Mediterranean Miocene. Earth and Planetary Science Letters 407, 19-34.

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