Formation timescales of CV chondrites from component specific Hf-W systematics

Carbonaceous chondrites are an important meteorite group that closely resembles the bulk composition of the solar system. We report the first elemental and isotope dataset for Hf-W in carbonaceous chondrites that includes chondrules, matrix, magnetic fractions as well as bulk compositions. Our study focuses on the three CV3 chondrites, Allende, Vigarano and Bali. Compared to bulk chondrites, matrix splits have low Hf/W ratios and ε ¹⁸²W compositions, whereas chondrule splits are characterized by high, but more variable, Hf/W ratios and ε ¹⁸²W compositions. Thus, Hf/W ratios behave complementary between chondrules and matrix in the analysed CV chondrites, supporting the view that both components formed from the same parental reservoir. Strong nucleosynthetic effects were observed in most of the analysed CV3 components, especially in matrices and chondrule splits that were found to have large ε ¹⁸³W anomalies of several ε-units. All separates define a rough correlation between initial ¹⁸²W/ ¹⁸⁴W and ¹⁸³W/ ¹⁸⁴W ratios, in agreement with theoretical model trends based on calculations for stellar nucleosynthesis. Our results, therefore, indicate a heterogeneous distribution of s- and r-process W isotopes among the different CV3 chondrite components, arguing for selective thermal processing of early solar system matter during chondrule formation. After correcting for nucleosynthetic anomalies, chondrules and matrix splits of reduced (Vigarano) as well as oxidised (Allende) CV3 chondrites define a linear correlation in ε ¹⁸²W vs. ¹⁸⁰Hf/ ¹⁸⁴W space, which is interpreted as an isochron, covering an age interval within the first ~2.6 Ma after solar system formation. As peak metamorphic temperatures for CV3 chondrites were well below the ¹⁸²Hf- ¹⁸²W closure temperature, the resulting isochron within its error most likely defines a common formation interval for all components. The calculated age interval is for the first time based on a combined chondrule-matrix isochron, a marked difference compared to previous studies where only chondrules were analysed. Notably, our formation age interval covers previously reported chondrule formation ages determined using ²⁶Al and Pb-Pb chronometry, illustrating that chondrule and matrix formation started contemporaneously with CAI formation and lasted over a time interval of about 2-3 Ma. Our results also corroborate previous models from ordinary chondrites, in that chondrite parent bodies were not the first planetesimals to have formed in the early solar system.