Geochemical test for branching decay of 176Lu

To get a reliable estimate for the β+ decay of 176Lu, we have measured Yb isotopic composition in 2.7 Ga zircons with Lu/YbN (chondrite-normalized) ratios of 1.40 and 1.45, in 1.0 Ga xenotime with Lu/YbN = 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.