Source: http://www.earth-time.org/working-groups/40ar-39ar/
Timestamp: 2019-04-18 16:25:42+00:00

Document:
The 40Ar/39Ar working group is an informal collective of people from 40Ar/39Ar laboratories worldwide who are working together to improve the 40Ar/39Ar method. Below we provide a short summary of the topics being discussed and progressed by this working group.
Interested in joining the working group? Email: earthtime4567@gmail.com with ‘Argon Working Group’ in the subject line.
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.
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.
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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