Statistical Chronometry of Meteorites. II. Initial Abundances and Homogeneity of Short-lived RadionuclidesOPEN ACCESS 

Steven J. Desch, Daniel R. Dunlap, Curtis D. Williams, Prajkta Mane, Emilie T. Dunham

Submitted to Geochimica et Cosmochimica Acta


“• We present a new method for combining and averaging data from the Al-Mg, Mn-Cr, Hf-W, and Pb-Pb radiometric dating systems, to: attain greater accuracy and precision in the initial (53Mn/55Mn)SS and (182Hf/180Hf)SS ratios the Pb-Pb age tSS of “t=0” in the Solar System, when (26Al/27Al)SS ≡ 5.23 × 10−5; and better assess concordancy.
• In meteorites and components where it is expected, we find substantial concordancy between the times of formation measured by the different
isotopic systems, provided tSS = 4568.7 ± 0.1 Myr, (53Mn/55Mn)SS = (7.8 ± 0.4) × 10−6, and (182Hf/180Hf)SS = (10.4 ± 0.1) × 10−5, and the 53Mn half-life is ≈ 4.0 Myr; this strongly implies homogeneity of 26Al, 53Mn, and 182Hf in the solar nebula from early times.”

“Astrophysical models of planet formation require accurate radiometric dating of meteoritic components by short-lived (Al-Mg, Mn-Cr, Hf-W) and long-lived (U-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 x 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 13 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 = (7.80 +/- 0.36) x 10^-6, (182Hf/180Hf)SS = (10.41 +/- 0.12) x 10^-5, tSS = 4568.65 +/- 0.10 Myr, and a 53Mn half-life of 3.98 +/- 0.22 Myr, these four free parameters make concordant 18 formation times recorded by the different systems in all six known volcanic achondrites (D’Orbigny, SAH 99555, NWA 1670, Asuka 881394, Ibitira, NWA 7325). These parameters also make concordant the ages derived for chondrules from CB/CH achondrites, formed simultaneously in an impact. The other seven achondrites are not quite concordant, but are plutonic angrites or ‘carbonaceous achondrites’ for which simultaneous closure of the isotopic systems might not be expected. Our findings provide very strong support for homogeneity of 26Al, 53Mn and 182Hf in the solar nebula, and our approach offers a path for more precise chronometry.”