THE LU ISOTOPIC COMPOSITION of ACHONDRITES: CLOSING the CASE for ACCELERATED DECAY of ¹⁷⁶LU

Studies of Lu-Hf isotope systematics in meteorites have produced apparent "ages" that are older than Pb-Pb ages and older than the estimated age of our solar system. One proposed explanation for this discrepancy is that irradiation by cosmic rays caused excitation of ¹⁷⁶Lu to its short-lived isomer that then underwent rapid decay to ¹⁷⁶Hf. This explanation can account for apparent excesses in ¹⁷⁶Hf that correlate with Lu/Hf ratio. Mass balance requires that samples with measurable excess in ¹⁷⁶Hf should also have measurable deficiencies in ¹⁷⁶Lu on the order of 1‰-3‰. To unambiguously test the accelerated decay hypothesis, we have measured the ¹⁷⁶Lu/¹⁷⁵Lu ratio in terrestrial materials and achondrites to search for evidence of depletion in ¹⁷⁶Lu. To a precision of 0.1‰ terrestrial standards, cumulate and basaltic eucrites and angrites all have the same ¹⁷⁶Lu/¹⁷⁵Lu ratio. Barring a subsequent mass-dependent fractionation event, these results suggest that the apparent excesses in ¹⁷⁶Hf are not caused by accelerated decay of ¹⁷⁶Lu, and so another hypothesis is required to explain apparently old Lu-Hf ages.