26Al–26Mg and 207Pb–206Pb systematics of Allende CAIs: Canonical solar initial 26Al/27Al ratio reinstated

Abstract

Allende meteorite Ca-Al rich inclusions U-Pb age 26 Al/ 27 Al ratio The precise knowledge of the initial 26 Al/ 27 Al ratio [(26 Al/ 27 Al) 0 ] is crucial if we are to use the very first solid objects formed in our Solar System, calcium-aluminum-rich inclusions (CAIs) as the "time zero" age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multicollector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of 27 Al/ 24 Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three microdrilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined 26 Al-26 Mg fossil isochron with an (26 Al/ 27 Al) 0 of (5.23± 0.13)× 10 − 5. Internal mineral isochrons obtained for three of these CAIs (A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with (26 Al/ 27 Al) 0 =(4.96± 0.25) × 10 − 5 , anchored to our precisely determined absolute 207 Pb-206 Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the "canonical" (26 Al/ 27 Al) 0 of 5 × 10 − 5 for the early Solar System. The uncertainty in (26 Al/ 27 Al) 0 corresponds to a maximum time span of ±20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the 26 Al-26 Mg and U-Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the "supra-canonical" 26 Al/ 27 Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supracanonical 26 Al/ 27 Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS. The radiogenic 208 Pb⁎/ 206 Pb⁎ ratio obtained as a by-product from the Pb-Pb age dating is used to estimate timeintegrated 232 Th/ 238 U ratio (κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.

Solar system genealogy revealed by extinct short-lived radionuclides in meteorites

“…1). CAIs formed in the protoplanetary disk surrounding the protoSun on a timescale of a few 10 4 yr (Jacobsen et al 2008) contains 26 Al from the wind of the generation #2 massive star and 60 Fe from the generation #1 SNe (Fig. 2).…”

“…After decades of measurements within CAIs (the first solids to have formed in our protoplanetary disk), the solar system's initial 26 Al/ 27 Al ratio is well established at 5.3 × 10 −5 (MacPherson et al 1995;Jacobsen et al 2008), though its homogeneity is still subject to debate (Villeneuve et al 2009;Liu et al 2012;Gounelle & Russell 2005). The situation is a bit more complicated for 60 Fe because its record in CAIs is hampered by secondary processes and by nickel nucleosynthetic anomalies (Birck & Lugmair 1988;Quitté et al 2007), whose origin is far from being understood (Steele et al 2011).…”

THE ELUSIVE60Fe IN THE SOLAR NEBULA

“…The exact ages of these samples, however, remain poorly determined. Recently, troilite from Muonionalusta (an IVA iron meteorite) turned up a strikingly old precise absolute Pb-Pb age (4565.3 ± 0.1 Ma; Blichert-Toft et al 2010a), which, after allowing for a correction for cooling of 1-2 Ma, indicates that the IVA parent body separated within 1 Ma of the formation of CAIs (Blichert-Toft et al 2010a;Bouvier et al 2007;Jacobsen et al 2008). In addition, Muonionalusta, selected for this study because of its old and precisely known age, is highly depleted in Cu, which is an important feature because Cook et al (2008) argued that cosmic-ray interactions with Cu unrelated to supernovae have the potential of producing measurable effects on 60 Ni and 61 Ni abundances.…”

INFERRED INITIAL26Al/27Al RATIOS IN PRESOLAR STARDUST GRAINS FROM SUPERNOVAE ARE HIGHER THAN PREVIOUSLY ESTIMATED

“…Al Al 26 27 ratio of 5.2 10 5´- (Jacobsen et al 2008), the absolute age of CAIs obtained from U-Pb dating provides an anchor point for the determination of relative ages based on the Al-Mg system. After the discovery of 26 Al, evidence for the presence of a series of other shortlived isotopes in early SS materials has been found (Meyer & Zinner 2006;Huss et al 2009).…”