Chemical studies of H chondrites .8. On contemporary meteoroid streams

Using date and time of fall and petrographic classification as criteria, many equilibrated H chondrites that fell during September and October from 1812 to the present form four significant-clusters, denoted as Cluster 2 through Cluster 5, on day-year plots. Using radiochemical neutron 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 increasing 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 Clusters 2 and 5 suites (that fell in September 1880-1991 and October, 1919-1984, respectively), each of which is represented by 10 H chondrite falls, are not compositionally distinguishable from the suite of random falls. However, the 17-member combined suite of Clusters 3 and 4 chondrites (that fell during September-October, 1812-1992) proves compositionally distinguishable from random falls at moderate to strong significance levels of 0.01-0.001. This 17-member suite is less readily distinguished from random falls than are the previously reported suite of Cluster 1 falls (May 1855-1895), or Antarctic H chondrites with nominal 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 results are obtained in the cases of Clusters 2 and 5. However, three H chondrite suites (Clusters 1, 3, and 4) distinguishable from the random background by one property (time of fall) are also distinguishable by another (contents of volatile trace elements or thermal history). Temporal change of H chondrite sources sampled by Earth are indicated by these data.