CHEMICAL STUDIES OF H-CHONDRITES .8. ON CONTEMPORARY METEOROID STREAMS

Using date and time of fall and petrographic classification as criteri a, many equilibrated H chondrites that fell during September and Octob er from 1812 to the present form four significant-clusters, denoted as Cluster 2 through Cluster 5, on day-year plots. Using radiochemical n eutron activation analysis, we determined 15 trace elements, U, Au, Co , Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by incre asing putative volatility during nebular condensation), in 27 members of these four clusters. We used model-dependent and model-independent multivariate statistical techniques to compare contents of the 10 most volatile elements separately in the four clusters with those of a 33- member suite of random H chondrite falls (from 1773 to 1970). The Clus ters 2 and 5 suites (that fell in September 1880-1991 and October, 191 9-1984, respectively), each of which is represented by 10 H chondrite falls, are not compositionally distinguishable from the suite of rando m falls. However, the 17-member combined suite of Clusters 3 and 4 cho ndrites (that fell during September-October, 1812-1992) proves composi tionally distinguishable from random falls at moderate to strong signi ficance levels of 0.01-0.001. This 17-member suite is less readily dis tinguished from random falls than are the previously reported suite of Cluster 1 falls (May 1855-1895), or Antarctic H chondrites with nomin al terrestrial ages >50 kyr, each of which is highly significant at <0 .001 levels. All suites are genomict and exhibit a range of cosmic ray exposure ages with a plurality having 6-8 Ma ages. Inconclusive resul ts are obtained in the cases of Clusters 2 and 5. However, three H cho ndrite suites (Clusters 1, 3, and 4) distinguishable from the random b ackground by one property (time of fall) are also distinguishable by a nother (contents of volatile trace elements or thermal history). Tempo ral change of H chondrite sources sampled by Earth are indicated by th ese data.