Nucleosynthetic tungsten isotope anomalies in acid leachates of the Murchison chondrite: Implications for hafnium-tungsten chronometry

Abstract

Progressive dissolution of the Murchison carbonaceous chondrite with acids of increasing strengths reveals large internal W isotope variations that reflect a heterogeneous distribution of s- and r-process W isotopes among the components of primitive chondrites. At least two distinct carriers of nucleosynthetic W isotope anomalies must be present, which were produced in different nucleosynthetic environments. The co-variation of ¹⁸²W/ ¹⁸⁴W and ¹⁸³W/¹⁸⁴W in the leachates follows a linear trend that is consistent with a mixing line between terrestrial W and a presumed s-process-enriched component. The composition of the s-enriched component agrees reasonably well with that predicted by the stellar model of s-process nucleosynthesis. The co-variation of ¹⁸²W/¹⁸⁴W and ¹⁸³W/¹⁸⁴W in the leachates provides a means for correcting the measured ¹⁸²W/¹⁸⁴W and ¹⁸²W/ ¹⁸³W of Ca-Al-rich inclusions (CAI) for nucleosynthetic anomalies using the isotopic variations in ¹⁸³W/¹⁸⁴W. This new correction procedure is different from that used previously, and results in a downward shift of the initial ε¹⁸²W of CAI to -3.51 ± 0.10 (where ε¹⁸²W is the variation in 0.01% of the ¹⁸²W/¹⁸³W ratio relative to Earth's mantle). This revision leads to Hf-W model ages of core formation in iron meteorite parent bodies that are 2Myr younger than previously calculated. The revised Hf-W model ages are consistent with CAI being the oldest solids formed in the solar system, and indicate that core formation in some planetesimals occurred within 2Myr of the beginning of the solar system. © 2012. The American Astronomical Society. All rights reserved..