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

The formation and differentiation of planetary bodies are dated using radioactive decay systems, including the short-lived (146)Sm-(142)Nd (T(½) = 103 or 68 Ma) and long-lived (147)Sm-(143)Nd (T(½) = 106 Ga) radiogenic pairs that provide relative and absolute ages, respectively. However, the initial abundance and half-life of the extinct radioactive isotope (146)Sm are still debated, weakening the interpretation of (146)Sm-(142)Nd systematics obtained for early planetary processes. Here, we apply the short-lived (26)Al-(26)Mg, (146)Sm-(142)Nd, and long-lived (147)Sm-(143)Sm chronometers to the oldest known andesitic meteorite, Erg Chech 002 (EC 002), to constrain the Solar System initial abundance of (146)Sm. The (26)Al-(26)Mg mineral isochron of EC 002 provides a tightly constrained initial [Formula: see text] (26)Mg* of −0.009 [Formula: see text] 0.005 ‰ and ((26)Al/(27)Al)(0) of (8.89 [Formula: see text] 0.09) [Formula: see text] 10(−6). This initial abundance of (26)Al is the highest measured so far in an achondrite and corresponds to a crystallization age of 1.80 [Formula: see text] 0.01 Myr after Solar System formation. The (146)Sm-(142)Nd mineral isochron returns an initial (146)Sm/(144)Sm ratio of 0.00830 [Formula: see text] 0.00032. By combining the Al-Mg crystallization age and initial (146)Sm/(144)Sm ratio of EC 002 with values for refractory inclusions, achondrites, and lunar samples, the best-fit half-life for (146)Sm 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 (146)Sm, the (146)Sm/(144)Sm abundance of EC 002 translates into an initial Solar System (146)Sm/(144)Sm ratio of 0.00840 [Formula: see text] 0.00032, which represents the most reliable and precise estimate to date and makes EC 002 an ideal anchor for the (146)Sm-(142)Nd clock.