科研之友
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摘要
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 W-182/W-184 and W-183/W-184 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 W-182/W-184 and W-183/W-184 in the leachates provides a means for correcting the measured W-182/W-184 and W-182/W-183 of Ca-Al-rich inclusions (CAI) for nucleosynthetic anomalies using the isotopic variations in W-183/W-184. This new correction procedure is different from that used previously, and results in a downward shift of the initial epsilon W-182 of CAI to -3.51 +/- 0.10 (where epsilon W-182 is the variation in 0.01% of the W-182/W-183 ratio relative to Earth%26apos;s mantle). This revision leads to Hf-W model ages of core formation in iron meteorite parent bodies that are similar to 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 similar to 2 Myr of the beginning of the solar system.
- 出版日期2012-7-1
