Titanium isotopes and rare earth patterns in CAIs: Evidence for thermal processing and gas-dust decoupling in the protoplanetary disk

Abstract

Titanium isotopic compositions (mass-dependent fractionation and isotopic anomalies) were measured in 46 calcium-, aluminum-rich inclusions (CAIs) from the Allende CV chondrite. After internal normalization to 49Ti/47Ti, we found that ε50Ti values are somewhat variable among CAIs, and that ε46Ti is highly correlated with ε50Ti, with a best-fit slope of 0.162 ± 0.030 (95% confidence interval). The linear correlation between ε46Ti and ε50Ti extends the same correlation seen among bulk solar objects (slope 0.184 ± 0.007). This observation provides constraints on dynamic mixing of the solar disk and has implications for the nucleosynthetic origin of titanium isotopes, specifically on the possible contributions from various types of supernovae to the solar system. Titanium isotopic mass fractionation, expressed as δ′49Ti, was measured by both sample-standard bracketing and double-spiking. Most CAIs are isotopically unfractionated, within a 95% confidence interval of normal, but a few are significantly fractionated and the range δ′49Ti is from ∼−4 to ∼+4. Rare earth element patterns were measured in 37 of the CAIs. All CAIs with significant titanium mass fractionation effects have group II and related REE patterns, implying kinetically controlled volatility fractionation during the formation of these CAIs.