Farley, K. A. and Hurowitz, J. A. and Asimow, Paul D. and Jacobson, N. S. and Cartwright, J. A. (2013) A double-spike method for K–Ar measurement: A technique for high precision in situ dating on Mars and other planetary surfaces. Geochimica et Cosmochimica Acta, 110 . pp. 1-12. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20130516-095248373
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A new method for K–Ar dating using a double isotope dilution technique is proposed and demonstrated. The method is designed to eliminate known difficulties facing in situ dating on planetary surfaces, especially instrument complexity and power availability. It may also have applicability in some terrestrial dating applications. Key to the method is the use of a solid tracer spike enriched in both ^(39)Ar and ^(41)K. When mixed with lithium borate flux in a Knudsen effusion cell, this tracer spike and a sample to be dated can be successfully fused and degassed of Ar at <1000 °C. The evolved ^(40)Ar^∗/^(39)Ar ratio can be measured to high precision using noble gas mass spectrometry. After argon measurement the sample melt is heated to a slightly higher temperature (∼1030 °C) to volatilize potassium, and the evolved ^(39)K/(41)K ratio measured by Knudsen effusion mass spectrometry. Combined with the known composition of the tracer spike, these two ratios define the K–Ar age using a single sample aliquot and without the need for extreme temperature or a mass determination. In principle the method can be implemented using a single mass spectrometer. Experiments indicate that quantitative extraction of argon from a basalt sample occurs at a sufficiently low temperature that potassium loss in this step is unimportant. Similarly, potassium isotope ratios measured in the Knudsen apparatus indicate good sample-spike equilibration and acceptably small isotopic fractionation. When applied to a flood basalt from the Viluy Traps, Siberia, a K–Ar age of 351 ± 19 Ma was obtained, a result within 1% of the independently known age. For practical reasons this measurement was made on two separate mass spectrometers, but a scheme for combining the measurements in a single analytical instrument is described. Because both parent and daughter are determined by isotope dilution, the precision on K–Ar ages obtained by the double isotope dilution method should routinely approach that of a pair of isotope ratio determinations, likely better than ±5%.
|Additional Information:||© 2013 Elsevier Ltd. Received 4 December 2012; accepted in revised form 5 February 2013; available online 16 February 2013. We thank Paul Renne for suggesting and providing the Viluy Traps basalt sample and Tim Becker for facilitating the irradiation of our spike glass. We thank Leah Morgan, Pete Burnard, and two anonymous reviewers for helpful suggestions. This work could not have occurred without the generous and patient support of the Keck Institute for Space Studies. This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Agency (JAH). Associate editor: Pete Burnard|
|Official Citation:||K.A. Farley, J.A. Hurowitz, P.D. Asimow, N.S. Jacobson, J.A. Cartwright, A double-spike method for K–Ar measurement: A technique for high precision in situ dating on Mars and other planetary surfaces, Geochimica et Cosmochimica Acta, Volume 110, 1 June 2013, Pages 1-12, ISSN 0016-7037, 10.1016/j.gca.2013.02.010. (http://www.sciencedirect.com/science/article/pii/S0016703713000951)|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||16 May 2013 18:14|
|Last Modified:||23 Jul 2013 18:01|
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