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

When recent geological calibrations of the ¹⁷⁶Lu decay constant are used, the ¹⁷⁶Lu-¹⁷⁶Hf ages of chondrites are consistently 4% too old (∼4.75 Ga). Here, we suggest that this discrepancy reflects the photoexcitation of the long-lived ¹⁷⁶Lu ground state to the short-lived isomeric state (T_1/2 = 3.7 h) by γ-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 γ-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 γ-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 γ-rays.