High-precision Al-Mg isotopic systematics in USNM 3898 - The benchmark “ALL” for initial 87Sr/86Sr in the earliest Solar System

The Allende CAI USNM3898 is the basis for “ALL”, the lowest measured initial 87Sr/86Sr value in any solar system material including other CAIs (Gray et al., 1973). If the value ALL is correct (debated), then USNM3898 must be 1-2 million years older than other CAIs (Podosek et al., 1991). This would require in turn that it have a much higher initial 26Al/27Al value than other CAIs, on the order of 4 × 10−4. Podosek et al. (1991) showed that this is not the case, with initial 26Al/27Al = (4.5 ± 0.7) × 10−5, but their Mg-isotopic data had large error bars and there was clear isotopic disturbance in the data having the highest 27Al/24Mg. Without the latter data, the slope of their isochron is higher (5.10 ± 1.19) × 10−5) and within (large) error of being supracanonical. We used high-precision SIMS to re-determine the initial 26Al/27Al in this CAI and obtained a value of (5.013 ± 0.099) × 10−5, with an intercept δ26Mg⁎=−0.008±0.048 and MSWD = 1.3. This value is indistinguishable from that measured in many other CAIs and conclusively shows that USNM3898 is not uniquely ancient. We also confirmed evidence of later isotopic disturbance, similar to what Podosek et al. found, indicating a re-melting and evaporation event some 200,000 years after initial CAI solidification.