THE LEONARD-AWARD-ADDRESS - PRESENTED 1996 JULY-25, BERLIN, GERMANY -THE ELEMENTAL COMPOSITION OF STONY COSMIC SPHERULES

Five hundred stony cosmic spherules collected from deep-sea sediments, polar ice, and the stratosphere have been analyzed for major and some minor element composition. Typical spherules are products of atmosphe ric melting of millimeter sized and smaller meteoroids. The samples ar e small and modified by atmospheric entry, but they are an important s ource of information on the composition of asteroids. The spherules in this study were all analyzed in an identical Manner, and they provide a sampling of the solar system's asteroids that is both different and less biased than provided by studies of conventional meteorites. Vola tile elements such as Na and S are depleted due to atmospheric heating , while siderophiles are depleted by less understood causes. The refra ctory nonsiderophile elements appear not to have been significantly di sturbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with th e Earth. Typical spherules have CM-like composition that is distinctiv ely different than ordinary chondrites and most other meteorite types. We assume that C-type asteroids are the primary origin of spherules w ith this composition. Type S asteroids should also be an important sou rce of the spherules, and the analysis data provide constraints on the ir composition. A minor fraction of the spherules are melt products of precursor particles that did not have chondritic elemental compositio ns. The most common of these are particles that are dominated by olivi ne. The observed compositions of spherules are inconsistent with the p ossibility that an appreciable fraction of the spherules are simply ch ondrules remelted during atmospheric entry.