Geochemical test for branching decay of Lu

Two different groups of values for the ¹⁷⁶Lu decay constant have been determined by recent high-precision experiments. The λ¹⁷⁶Lu 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 λ¹⁷⁶Lu 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 λ¹⁷⁶Lu terrestrial vs. meteorite discrepancy.