Resetting, errorchrons and the meaning of canonical CAI initial 26Al/27Al values

The difference between the precise MC-ICPMS analyses of bulk calcium–aluminum-rich inclusion (CAI) fragments (e.g., Jacobsen et al., 2008) and supra-canonical values obtained by micro-analytical techniques, e.g., laser ablation MC-ICPMS (Young et al., 2005) and SIMS (Taylor et al., 2005), at face value seems to be problematic and therefore leads many to dismiss claims of the solar system (26Al/27Al)0 greater than ~ 5 × 10−5 as spurious. Here we use mass balance calculations to quantify the importance of open system isotopic exchange during CAI evolution and show that in situ supra-canonical, in-situ canonical, and bulk canonical measurements can all exist for an individual CAI. The calculations describe mechanisms of isotopic exchange that may have occurred early (100's ka) and late (~ 1.5 Ma) in the solar nebula and much later (> 10's Ma) on parent body planetesimals. A range of possible modal mineralogies is modeled in order to populate the compositional space defined by in situ and bulk CAI measurements. In support of these simulated data we describe in situ measurements of 27Al/24Mg, 25Mg/24Mg, and 26Mg/24Mg obtained by LA-MC-ICPMS comprising core-to-rim traverses across three CV3 CAIs. The CAIs exhibit distinctive Mg isotopic zoning profiles and varying abundances of daughter products of the short-lived nuclide 26Al that are consistent with varying amounts of open system isotope exchange.

Research Highlights
► Canonical 26Al/27Al values record exchange with nebular gas at “canonical” time.
► Rare evidence for supra-canonical 26Al/27Al initial values exists.
► Bulk CAI fragments record cessation of thermal processing not initial formation.
► In situ supra-canonical and bulk canonical data can exist for individual CAIs.
► Model explains formation of Mg isotope zoning profiles and 26Al–26Mg chronologies.