CHEMICAL STUDIES OF H CHONDRITES .5. TEMPORAL VARIATIONS OF SOURCES

We report Cl-36 (301-kyr half-life) data obtained by accelerator mass spectrometry allowing nominal terrestrial ages to be determined for 39 Antarctic H4-6 chondrites for which contents of volatile trace elemen ts are known. The compositional difference between these Antarctic met eorites and 58 non-Antarctic falls increases with terrestrial age and, using multivariate statistical techniques, becomes highly significant for Antarctic samples with ages >50 kyr. The compositional difference is inconsistent with trivial causes such as weathering and seems to r eflect differences in thermal histories of parent sources. Temporal so urce variations for the H chondrite flux on Earth thus exist not only on a short-term, 40 years, basis (Dodd et al., 1993) but also on a lon g-term, >50 kyr, basis.