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

The formation and differentiation of planetary bodies are dated using radioactive decay systems, including the short-lived Sm-146-Nd-142 (T-1/2 = 103 or 68 Ma) and long-lived Sm-147-Nd-143 (T-1/2 = 106 Ga) radiogenic pairs that provide relative and absolute ages, respectively. However, the initial abundance and half-life of the extinct radioactive iso-tope 146Sm are still debated, weakening the interpretation of Sm-146-Nd-142 systematics obtained for early planetary processes. Here, we apply the short-lived Al-26-Mg-26, Sm-146-Nd-142, and long-lived Sm-147-Sm-143 chronometers to the oldest known andesitic meteorite, Erg Chech 002 (EC 002), to constrain the Solar System initial abundance of Sm-146. The Al-26-Mg-26 mineral isochron of EC 002 provides a tightly constrained initial delta Mg-26* of -0.009 +/- 0.005 parts per thousand and (Al-26/Al-27)(0) of (8.89 +/- 0.09) x 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-Nd-142 mineral isochron returns an initial Sm-146/Sm-144 ratio of 0.00830 +/- 0.00032. By com-bining the Al-Mg crystallization age and initial Sm-146/Sm-144 ratio of EC 002 with val-ues for refractory inclusions, achondrites, and lunar samples, the best-fit half-life for Sm-146 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 Sm-146/Sm-144 abundance of EC 002 translates into an initial Solar Sys-tem Sm-146/Sm-144 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 Sm-146-Nd-142 clock.