I-Xe systematics in LL chondrites

We report Ar and Xe data from stepwise heating analysis of five neutron-irradiated whole rock LL chondrites (Soko Banja, Alta Ameen, Tuxtuac, Guidder and Olivenza). None of these meteorites yields a well-defined ⁴⁰Ar-³⁹Ar apparent age plateau comprised of more than two release fractions, although in some cases intermediate-temperature apparent ages may provide significant lower limits to the age of initial Ar retention. Possibly excepting Guidder, these samples yield well-defined high-temperature correlations between ¹²⁹Xe ¹³⁰Xe and ¹²⁸Xe ¹³⁰Xe, and thus determinations of ¹²⁹I ¹²⁷I and ¹²⁹Xe ¹³⁰Xe at the time of isotopic closure for Xe. Nominal ¹²⁹I-¹²⁹Xe formation ages range from 2.7 Ma before to 8.2 Ma after Bjurbole. Considered in isolation, the present I-Xe results can be viewed in a coherent chronological framework, although possible isotopic heterogeneity of I cannot be excluded. I-Xe systematics do not correlate with metamorphic grade for the LL chondrites, however, nor with chronologies based on other methods (⁴⁰Ar-³⁹Ar, initial ⁸⁷Sr ⁸⁶Sr, metallographic or fission track retention cooling rates). The LL chondrites are, in this respect, like the other ordinary chondrites (H and L groups). Barring serious unrecognized flaws in essentially all the relevant Chronometrie methodologies, the primary conclusion is that the thermal history of the ordinary chondrites, including the LL chondrites, is very complicated.