Statistical chronometry of Meteorites: II. Initial abundances and homogeneity of short-lived radionuclides

Abstract

Astrophysical models of planet formation require accurate radiometric dating of meteoritic components by short-lived (Al–Mg, Mn–Cr, Hf–W) and long-lived (Pb–Pb) chronometers, to develop a timeline of such events in the solar nebula as formation of Ca-rich, Al-rich Inclusions (CAIs), chondrules, planetesimals, etc. CAIs formed mostly around a time (“t=0”) when the short-lived radionuclide 26Al (t1/2=0.72 Myr) was present and presumably homogeneously distributed at a known level we define as (26Al/27Al)SS≡5.23×10−5. The time of formation after t=0 of another object can be found by determining its initial (26Al/27Al)0 ratio and comparing it to (26Al/27Al)SS. Dating of meteoritic objects using the Mn–Cr or Hf–W systems is hindered because the abundances (53Mn/55Mn)SS and (182Hf/180Hf)SS at t=0 are not known precisely. To constrain these quantities, we compile literature Al–Mg, Mn–Cr, Hf–W and Pb–Pb data for 14 achondrites and use novel statistical techniques to minimize the discrepancies between their times of formation across these systems. We find that for (53Mn/55Mn)SS=(8.09±0.65)×10−6, (182Hf/180Hf)SS=(10.42±0.25)×10−5, tSS=4568.36±0.20Myr, and a 53Mn half-life of 3.80±0.23 Myr, these four free parameters make concordant 37 out of 38 formation times recorded by the different systems in 14 achondrites. These parameters also make concordant the ages derived for chondrules from CB/CH achondrites, formed simultaneously in an impact, and are apparently concordant with the I–Xe chronometer as well. Our findings provide very strong support for homogeneity of 26Al, 53Mn, and 182Hf in the solar nebula, and our approach offers a framework for more precise chronometry.