Magnesium and ⁵⁴Cr isotope compositions of carbonaceous chondrite chondrules - Insights into early disk processes

We report on the petrology, magnesium isotopes and mass-independent ⁵⁴Cr/⁵²Cr compositions (micro⁵⁴Cr) of 42 chondrules from CV (Vigarano and NWA 3118) and CR (NWA 6043, NWA 801 and LAP 02342) chondrites. All sampled chondrules are classified as type IA or type IAB, have low ²⁷Al/²⁴Mg ratios (0.04-0.27) and display little or no evidence for secondary alteration processes. The CV and CR chondrules show variable ²⁵Mg/²⁴Mg and ²⁶Mg/²⁴Mg values corresponding to a range of mass-dependent fractionation of ~500 ppm (parts per million) per atomic mass unit. This mass-dependent Mg isotope fractionation is interpreted as reflecting Mg isotope heterogeneity of the chondrule precursors and not the result of secondary alteration or volatility-controlled processes during chondrule formation. The CV and CR chondrule populations studied here are characterized by systematic deficits in the mass-independent component of ²⁶Mg (micro²⁶Mg*) relative to the solar value defined by CI chondrites, which we interpret as reflecting formation from precursor material with a reduced initial abundance of ²⁶Al compared to the canonical ²⁶Al/²⁷Al of ~5 × 10^-5. Model initial ²⁶Al/²⁷Al values of CV and CR chondrules vary from (1.5 ± 4.0) × 10^-6 to (2.2 ± 0.4) × 10^-5. The CV chondrules display significant micro⁵⁴Cr variability, defining a range of compositions that is comparable to that observed for inner Solar System primitive and differentiated meteorites. In contrast, CR chondrites are characterized by a narrower range of micro⁵⁴Cr values restricted to compositions typically observed for bulk carbonaceous chondrites. Collectively, these observations suggest that the CV chondrules formed from precursors that originated in various regions of the protoplanetary disk and were then transported to the accretion region of the CV parent asteroid whereas CR chondrule predominantly formed from precursor with carbonaceous chondrite-like micro⁵⁴Cr signatures. The observed micro⁵⁴Cr variability in chondrules from CV and CR chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir. The coupled micro²⁶Mg* and micro⁵⁴Cr systematics of CR chondrules establishes that these objects formed from a thermally unprocessed and ²⁶Al-poor source reservoir distinct from most inner Solar System asteroids and planetary bodies, possibly located beyond the orbits of the gas giants. In contrast, a large fraction of the CV chondrules plot on the inner Solar System correlation line, indicating that these objects predominantly formed from thermally-processed, ²⁶Al-bearing precursor material akin to that of inner Solar System solids, asteroids and planets.