Iodine-xenon studies of petrographically and chemically characterized Chainpur chondrules

We have performed petrographie, instrumental neutron activation analysis (INAA) and noble gas studies on samples of 18 chondrules and matrix from the Chainpur (LL3) meteorite to study variations in R₀, the ratio of ¹²⁹I to ¹²⁹I at the time of xenon isotopic closure. R₀ varies by more than a factor of 10 among the chondrules, corresponding to a span of more than 50 Ma in apparent I-Xe ages, including the latest apparent I-Xe ages ever observed for a chondritic sample. Variations are not closely related to any pétrographie properties, although low values of R₀ (late apparent ages) may be associated with high sulfide abundances and/or non-porphyritic textures. Similarly, R₀ is not closely related to any chemical components, but does seem to correlate with abundance of refractory lithophile elements. Also, R₀is correlated with ²⁴⁴Pu ²³⁸U and anticorrelated with trapped ¹²⁹Xe ¹³²Xe, as might be expected if the variations in iodine isotopic composition are dominated by decay of ¹²⁹I. We have not found a completely satisfying explanation of the variations in R₀. Models involving gas-dust mixing or nebular heterogeneity cannot satisfactorily explain the Chainpur data. However, there are also difficulties with explanations attributing the variations to differences in formation age, metamorphic age, or time of aqueous alteration. We believe the most plausible explanation is that the variations represent times of low-grade shock events.