A supernova-induced irradiation event in the early Solar System – not confirmed by new 176Lu-176Hf data

Yuri Amelin
Research School of Earth Sciences, the Australian National U

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     Last modified: July 27, 2011

Abstract
Radioactive decay of 176Lu to 176Hf with a half-life of 37 Ga is a powerful isotopic tracer for studying evolution of the Earth and other planets. The decay rate of 176Lu is well known from the studies of terrestrial rocks and minerals (Scherer et al. 2001, Söderlund et al. 2004) and meteorites that formed more than 10 million years after the beginning of the Solar System accretion (Amelin 2005). However, the excess of 176Hf correlated with Lu/Hf ratios, observed in older meteorites, was interpreted as an evidence for a gamma-ray (Albarède et al. 2006) or neutrino or cosmic ray (Thrane et al. 2010) irradiation event of enormous magnitude that occurred during or shortly after accretion and caused accelerated decay of 176Lu by converting a fraction of nuclei to the faster decaying nuclear isomer 176mLu. Since sufficiently powerful source of gamma-rays, neutrinos or cosmic rays cannot be found within the Solar System, it was suggested that a supernova event took place in close proximity to the accreting Solar System. Such event would have made a profound impact on composition and structure of the Solar System. Considering the importance of this possibility, and the fact that previously determined meteorite176Lu-176Hf isochrons could have been affected by disturbance of the isotopic systems, we have sought a more reliable confirmation for accelerated decay of 176Lu using the 176Lu-176Hf isotope systematics of angrites – the oldest and best preserved igneous meteorites. A 176Lu-176Hf isochron for five angrites, two of which formed three million years after the beginning of accretion and three are slightly younger, yields the age of 4576 ± 49 Ma using the decay constant of 176Lu of 1.867x10-11 a. This isochron does not show an excess slope and does not confirm the existence of a giant irradiation event in the early Solar System.