Geochemical test for branching decay of 176Lu

Abstract

Two different groups of values for the 176Lu decay constant have been determined by recent high-precision experiments. The λ 176Lu values of 1.86–1.87 × 10 −11 a −1 were determined by age comparisons using terrestrial minerals of Proterozoic and late Archean age, whereas values of ∼1.94 × 10 −11 a −1 were determined in age comparison studies of meteorites. A possible branched decay of 176Lu could be the cause of this discrepancy. The β + decay of 176Lu to 176Yb was detected in the early studies of radioactivity of 176Lu, with reported values of λβ +/(λβ + + λβ −) in the total 176Lu ranging from less than 0.03 to 0.67. If the β + decay fraction is close to the upper limit of the reported values, it can explain the 4%–6% difference between the apparent λ 176Lu values. To get a reliable estimate for the β + decay of 176Lu, 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 (± 2σ) for the 176Yb/ 174Yb 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 λβ +/(λβ + + λβ −) = −0.005 ± 0.015 (2σ) and establish an upper limit of 0.9% of total decays for the β + decay branch. Branching decay can therefore be eliminated as the cause of the discrepancy in 176Lu decay constant estimates. We discuss other possible causes of the λ 176Lu terrestrial vs. meteorite discrepancy.