Magnesium and (54)Cr isotope compositions of carbonaceous chondrite chondrules – Insights into early disk processes

We report on the petrology, magnesium isotopes and mass-independent (54)Cr/(52)Cr compositions (μ(54)Cr) 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 (27)Al/(24)Mg ratios (0.04–0.27) and display little or no evidence for secondary alteration processes. The CV and CR chondrules show variable (25)Mg/(24)Mg and (26)Mg/(24)Mg 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 (26)Mg (μ(26)Mg*) relative to the solar value defined by CI chondrites, which we interpret as reflecting formation from precursor material with a reduced initial abundance of (26)Al compared to the canonical (26)Al/(27)Al of ~5 × 10(−5). Model initial (26)Al/(27)Al 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 μ(54)Cr 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 μ(54)Cr 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 μ(54)Cr signatures. The observed μ(54)Cr variability in chondrules from CV and CR chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir. The coupled μ(26)Mg* and μ(54)Cr systematics of CR chondrules establishes that these objects formed from a thermally unprocessed and (26)Al-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, (26)Al-bearing precursor material akin to that of inner Solar System solids, asteroids and planets.