Geochemical test for branching decay of Lu-176

摘要

Two different groups of values for the Lu-176 decay constant have been determined by recent high-precision experiments. The lambda(176)Lu values of 1.86-1.87 X 10(-11) a(-1) were determined by age comparisons using terrestrial minerals of Proterozoic and late Archean age, whereas values of similar to1.94 X 10(-11) a(-1) were determined in age comparison studies of meteorites.A possible branched decay of Lu-176 could be the cause of this discrepancy. The beta(+) decay of Lu-176 to Yb-176 was detected in the early studies of radioactivity of Lu-176, with reported values of lambdabeta(+)/(lambdabeta(+) + lambdabeta(-)) in the total Lu-176 ranging from less than 0.03 to 0.67. If the beta(+) decay fraction is close to the upper limit of the reported values, it can explain the 4%-6% difference between the apparent lambda(176)Lu values.To get a reliable estimate for the beta(+) decay of Lu-176, we have measured Yb isotopic composition in 2.7 Ga zircons with Lu/Yb-N (chondrite-normalized) ratios of 1.40 and 1.45, in 1.0 Ga xenotime with Lu/Yb-N = 1.23, using Yb from the 28.4 Ma Fish Canyon Tuff (FCT) zircon and titanite as the modern reference value. Multiple analyses yielded the following weighted mean values (+/- 2sigma) for the Yb-176/Yb-174 ratio: 0.4022134 +/- 0.0000017 for the FCT zircon and titanite, 0.4022134 +/- 0.0000019 for the 1.0 Ga xenotime, and 0.4022124 +/- 0.0000033 for the 2.7 Ga zircons. These data yield lambdabeta(+)/(lambdabeta(+) + lambdabeta(-)) = -0.005 +/- -0.015 +/- (2sigma) and establish an upper limit of 0.9% of total decays for the beta(+) decay branch. Branching decay can therefore be eliminated as the cause of the discrepancy in Lu-176 decay constant estimates. We discuss other possible causes of the lambda(176)Lu terrestrial vs. meteorite discrepancy. Copyright (C) 2005 Elsevier Ltd.