Geochemical test for branching decay of 176Lu

Details Geochemical test for branching decay of 176Lu Geochemical test for branching decay of 176Lu

Jan 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005gecoa..69..465a&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 69, Issue 2, p. 465-473.

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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/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.

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