NUCLEOSYNTHETIC TUNGSTEN ISOTOPE ANOMALIES IN ACID LEACHATES OF THE MURCHISON CHONDRITE: IMPLICATIONS FOR HAFNIUM-TUNGSTEN CHRONOMETRY

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 ∼2 Myr 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 ∼2 Myr of the beginning of the solar system.