Origin of excess ¹⁷⁶Hf in meteorites

After considerable controversy regarding the ¹⁷⁶Lu decay constant (λ¹⁷⁶Lu), there is now widespread agreement that (1.867 ± 0.008) × 10^-11 yr^-1 as confirmed by various terrestrial objects and a 4557 Myr meteorite is correct. This leaves the ¹⁷⁶Hf excesses that are correlated with Lu/Hf elemental ratios in meteorites older than ∼ 4.56 Ga meteorites unresolved. We attribute ¹⁷⁶Hf excess in older meteorites to an accelerated decay of ¹⁷⁶Lu caused by excitation of the long-lived ¹⁷⁶Lu ground state to a short-lived ^176mLu isomer. The energy needed to cause this transition is ascribed to a post-crystallization spray of cosmic rays accelerated by nearby supernova(e) that occurred after 4564.5 Ma. The majority of these cosmic rays are estimated to penetrate accreted material down to 10-20m, whereas a small fraction penetrate as deep as 100-200 m, predicting decreased excesses of ¹⁷⁶Hf with depth of burial at the time of the irradiation event.