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.
ASJC Scopus subject areas
- Geochemistry and Petrology