A test for systematic errors in Ar-40/Ar-39 geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite

摘要

Important sources of systematic error in Ar-40/Ar-39 dating arise from uncertainties in the K-40 decay constants and K/Ar isotopic data for neutron fluence monitors (standards). The activity data underlying the decay constants used in geochronology since 1977 are more dispersed than acknowledged by previous geochronologically oriented summaries, and compilations of essentially the same data in nuclear physics and chemistry literature since 1973 have consistently produced lower estimates (and larger assigned uncertainties) of the constants for K-40 ---> Ar-40 and K-40 --> Ca-40 decay. Considering also uncertainties in K-40/K, and the questionable existence of a gamma-less electron capture K-40 --> Ar-40 decay direct to ground state, the total K-40 decay constant is known to no better than +/-2% at the 2 sigma level. Ar-40*/K-40 ratios for individual standards are known to better than +/-2% in some cases, but interlaboratory discrepancies of more than 2% in the Ar-40/Ar-39 ages of secondary standards like the Fish Canyon sanidine (FCs) suggest larger uncertainties. The very precisely determined decay constants for U-238 and U-235, and the existence of quantitative internal U/Pb, reliability criteria, offer an alternative basis for evaluation of both the 40K decay constants and the ages of Ar-40/Ar-39 standards. High precision U/Pb (zircon) and Ar-40/Ar-39 (alkali feldspar). data from the 1.1-Ga Palisade Rhyolite provide a highly sensitive basis for comparison. Ten U/Pb analyses on abraded single zircons as well as one analysis of six fragmented and HF leached crystals yield a Pb-207/Pb-206 age of 1097.6 +5.2/-5.4 Ma (95% confidence, including decay constant errors). Ar-40/Ar-39 incremental CO2 laser heating of single alkali feldspar grains yields nine undisturbed age spectra with error-weighted plateau ages (based on conventional decay constants and an age of 28.02 Ma for FCs) from 1086.5 +/- 4.8 Ma (R-FCs(F239) = 52.7740 +/- 0.3062) to 1090.4 +/- 4.3 Ma (R-FCs(F238) = 53.0281 +/- 0.2746) (2 sigma, including analytical errors only), with MSWD = 0.95. The weighted mean R-FCs(F239) value of these plateau steps (52.9011 +/- 0.2324), including irradiation-related errors, is inferred to reflect the eruption-age R-FCs(F239) value of the Palisade Rhyolite alkali feldspar. Reconciliation of the Ar-40/Ar-39 and U/Pb results suggests that either the age of the Ar-40/Ar-39 Standard is older, or the K-40 total decay constant is smaller, than values in current use by geochronologists. Comparison with constraints from an historic eruption indicates a total K-40 decay constant of 5.37 x 10(-10)/yr and an age of 28.05 Ma for FCs. Further applications of this approach will provide more robust solutions and allow estimation of uncertainties. Copyright (C) 1999 Elsevier Science Ltd. [References: 79]