THE LU ISOTOPIC COMPOSITION OF ACHONDRITES: CLOSING THE CASE FOR ACCELERATED DECAY OF (LU)-L-176

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