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Free keywords:
Hf-W chronometry; CAI; 26Al; early solar system;
chronology; nucleosynthetic isotope anomalies; Earth Science
Abstract:
Ca-Al-rich inclusions (CAI) are the oldest dated objects formed in the solar system and are pivotal reference points in early solar system chronology. Knowledge of their initial 182Hf/180Hf and 182W/184W is essential, not only for obtaining precise Hf-W ages relative to the start of the solar system, but also to assess the distribution of short-lived radionuclides in the early solar nebula. However, the interpretation of Hf-W data for CAI is complicated by nucleosynthetic W isotope variations. To explore their extent and nature, and to better quantify the initial Hf and W isotope compositions of the solar system, we obtained Hf-W data for several fine- and coarse-grained CAI from three CV3 chondrites. The fine-grained CAI exhibit large and variable anomalies in ɛ183W (ɛi W equals 0.01% deviation from terrestrial values), extending to much larger anomalies than previously observed in CAI, and reflecting variable abundances of s- and r-process W isotopes. Conversely, the coarse-grained (mostly type B) inclusions show only small (if any) nucleosynthetic W isotope anomalies. The investigated CAI define a precise correlation between initial ɛ182W and ɛ183W, providing a direct empirical means to correct the ɛ182W of any CAI for nucleosynthetic isotope anomalies using their measured ɛ183W. After correction for nucleosynthetic W isotope variations, the CAI data define an initial 182Hf/180Hf of (1.018 ± 0.043) ×10-4 and an initial ɛ182W of - 3.49 ± 0.07. The Hf-W formation intervals of the angrites D'Orbigny and Sahara 99555 relative to this CAI initial is 4.8 ± 0.6 Ma, in good agreement with Al-Mg ages of these two angrites. This renders a grossly heterogeneous distribution of 26Al in the inner solar system unlikely, at least in the region were CAI and angrites formed.
