METAL-SILICATE FRACTIONATION IN CHONDRITIC METEORITES: EXPERIMENTS UNDER MICROGRAVITY CONDITIONS

Introduction. Unequilibrated chondrites consist of three major components, matrix, chondrules, and metal/sulfide grains. Chondrules and metal/sulfide grains exhibit restricted size distributions within major chondrite groups, a phenomenon referred to as “size sorting” [1], with average sizes ranging enough to serve as a guide to meteorite classification in some cases [2]. The process responsible for the “sorting” of chondrules and metal grains is largely unknown. The only major constraints are the average sizes, and the difference in average sizes of chondrules and grains and the composition of the materials. Possible processes can be divided into two major classes: Processes occurring in space (or the early solar nebula), and processes occurring on asteroidal bodies after accretion. Within “nebular” processes, some attribute restricted size distributions to restrictions in the chondrule and metal formation process, and others attribute them to magnetic separation or velocity-based separation [3]. Postaccretionary processes have attributed sorting to passage through an early “atmosphere”, or to fluidization on degassing asteroidal bodies [4, 5]. The minimum gas velocity to fluidize a bed of particles of given grain diameter (d) is: