Chemical studies of H chondrites-10: Contents of thermally labile trace elements are unaffected by late heating

Abstract

We have used radiochemical neutron activation analysis (RNAA) to determine 15 trace elements, including 10 moderately to highly volatile ones--Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, In (in increasing volatility order), in 6 H chondrite falls with low-3He contents. These (plus prior RNAA data) provide a compositional database of 92 H4-6 chondrite falls. Three suites of samples can be identified from their noble gas contents: 44 with 'normal' contents and, therefore, 'normal' orbits and cosmic-ray exposure histories; 8 that lost radiogenic gases, presumably by shock late in their histories; and 17 that lost cosmogenic gases by heating during close solar approach. We used the standard multivariate statistical techniques of linear discriminant analysis and logistic regression to compare contents of the 10 moderately and highly volatile trace elements, listed above, in these three suites. We found no significant differences. This contrasts sharply with similar comparisons involving random falls and H4-6 chondrites that landed on Earth at specific time intervals. Apparently, contents of volatile trace elements in H4-6 chondrites were established early in their histories, and they are so retentively sited that loss during later heating episodes did not occur.