γ-ray irradiation in the early Solar System and the conundrum of the 176Lu decay constant

When recent geological calibrations of the Lu-176 decay constant are used, the Lu-176-Hf-176 ages of chondrites are consistently 4% too old (similar to 4.75 Ga). Here, we suggest that this discrepancy reflects the photoexcitation of the long-lived Lu-176 ground state to the short-lived isomeric state (T-1/2 = 3.7 h) by gamma-rays irradiating early condensates. Irradiation may have been of solar origin and taking place at the inner edge of the nebular disk. Alternatively, the source of gamma-rays could have been one or more supernova(e) exploding in the vicinity of the solar nebula. Such photoexcitation has been experimentally observed, but requires gamma-ray photons that have energies in excess of 838 keV. At this stage, we cannot assess whether the Hf isotope composition of the Bulk Silicate Earth differs from that of chondrites, eucrites, and the 4.56 Ga old Martian meteorite ALH84001, and therefore, whether the precursor material for these different planetary bodies received comparable fluences of gamma-rays. (c) 2005 Elsevier Inc. All rights reserved.