Half-life and initial Solar System abundance of ¹⁴⁶Sm determined from the oldest andesitic meteorite

The formation and differentiation of planetary bodies are dated using radioactive decay systems, including the short-lived 146Sm-142Nd (T½ = 103 or 68 Ma) and long-lived 147Sm-143Nd (T½ = 106 Ga) radiogenic pairs that provide relative and absolute ages, respectively. However, the initial abundance and half-life of the extinct radioactive isotope 146Sm are still debated, weakening the interpretation of 146Sm-142Nd systematics obtained for early planetary processes. Here, we apply the short-lived 26Al-26Mg, 146Sm-142Nd, and long-lived 147Sm-143Sm chronometers to the oldest known andesitic meteorite, Erg Chech 002 (EC 002), to constrain the Solar System initial abundance of 146Sm. The 26Al-26Mg mineral isochron of EC 002 provides a tightly constrained initial δ26Mg* of −0.009 ± 0.005 ‰ and (26Al/27Al)0 of (8.89 ± 0.09) × 10−6. This initial abundance of 26Al is the highest measured so far in an achondrite and corresponds to a crystallization age of 1.80 ± 0.01 Myr after Solar System formation. The 146Sm-142Nd mineral isochron returns an initial 146Sm/144Sm ratio of 0.00830 ± 0.00032. By combining the Al-Mg crystallization age and initial 146Sm/144Sm ratio of EC 002 with values for refractory inclusions, achondrites, and lunar samples, the best-fit half-life for 146Sm is 102 ± 9 Ma, corresponding to the physically measured value of 103 ± 5 Myr, rather than the latest and lower revised value of 68 ± 7 Ma. Using a half-life of 103 Ma for 146Sm, the 146Sm/144Sm abundance of EC 002 translates into an initial Solar System 146Sm/144Sm ratio of 0.00840 ± 0.00032, which represents the most reliable and precise estimate to date and makes EC 002 an ideal anchor for the 146Sm-142Nd clock.