^1,2Thomas S. Kruijer, ¹Thorsten Kleine, ¹Mario Fischer-Gödde, ³Christoph Burkhardt, ²Rainer Wieler
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Strasse 10, 48149 Münster, Germany
²ETH Zürich, Institute of Geochemistry and Petrology, Clausiusstrasse 25, 8092 Zürich, Switzerland
³Origins Laboratory, Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL

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 ε 183W (εiWεiW 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 ε 182W and ε 183W, providing a direct empirical means to correct the ε 182W of any CAI for nucleosynthetic isotope anomalies using their measured ε 183W. After correction for nucleosynthetic W isotope variations, the CAI data define an initial 182Hf/180Hf of (1.018±0.043)×10−4(1.018±0.043)×10−4 and an initial ε 182W of −3.49±0.07−3.49±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 Ma4.8±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.

Reference
Kruijer TS, Thorsten Kleine T, Mario Fischer-Gödde M, Christoph Burkhardt C, Wieler R (2014) Nucleosynthetic W isotope anomalies and the Hf–W chronometry of Ca–Al-rich inclusions. Earth and Planetary Science Letters 403,317–327.
Link to Article [DOI: 10.1016/j.epsl.2014.07.003]

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