Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v13a2105r&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V13A-2105
Other
0399 General Or Miscellaneous, 1040 Radiogenic Isotope Geochemistry, 1041 Stable Isotope Geochemistry (0454, 4870), 1115 Radioisotope Geochronology, 1199 General Or Miscellaneous
Scientific paper
K-Ar and 40Ar/39Ar geochronologists use the isotopic composition of atmospheric argon to correct for non-radiogenic 40Ar and to determine instrumental mass discrimination, among other purposes. The currently accepted atmospheric composition is based on measurements of Nier (1950), as filtered by Steiger and Jager (1977). A redetermination of the 40Ar: 38Ar: 36Ar of atmospheric argon by Lee et al. (2006), with 40Ar/36Ar = 298.56 ±0.31 (1 σ here and throughout) compared with Nier's (1950) value of 296.0 ±0.7, offers a more precise and likely more accurate reference standard that should be adopted by the K-Ar and 40Ar/39Ar communities. Adopting the revised atmospheric composition has little impact on most age calculations as changes resulting from the magnified atmospheric correction are exactly balanced by those from the adjusted mass discrimination correction, in cases where air aliquots are used to determine mass discrimination. More significant are the effects of modified discrimination on reactor-produced isotopes, though in most realistic cases the resulting age difference is a few thousand years or less. The Lee et al. (2006) data provide a more sensitive benchmark for evaluating both natural and experimental fractionation of argon isotopes. Non-atmospheric, isotopically light initial argon turns out to be widely manifest in volcanic ejecta in which 40Ar/36Ar vs. 38Ar/36Ar correlations appear to follow a kinetic mass fractionation trend. This phenomenon has been recognized for decades (e.g., Krummenacher, 1970; Matsumoto et al., 1989; Ozawa et al, 2006) but has somehow escaped notice of the broader community. New data presented here suggest that the most extreme fractionation occurs in silicic glasses (obsidians), in which we have measured initial 40Ar/36Ar (isochron intercept) and 38Ar/36Ar (unirradiated aliquots) as low as 271.38 ±3.00 and 0.18117 ±0.00055, respectively. We suggest that the data of Lee et al. (2006) be adopted as a new standard for geochronology, and that these data be implemented in efforts to better characterize instrumental mass discrimination and to understand argon isotope fractionation in nature. Krummenacher, D., 1970, Earth Planet. Sci. Lett. 8: 109- 117; Lee, J.-Y., et al., 2006, Geochim. Cosmochim. Acta 70: 4507-4512; Matsumoto, A., et al., 1989, Mass Spectroscopy 37: 353-363; Nier, A.O., 1950, Phys. Rev. 77: 789-793; Ozawa, A., et al., , 2006, Chem. Geol. 226: 66-72; Steiger, R.H., and Jäger, E., 1977, Earth Planet. Sci. Lett. 36: 359-362.
Cassata William S.
Morgan Eric Lease
Renne Paul R.
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