NEUTRON-ACTIVATION ANALYSIS OF MULTIPLE 10-100 MU-G GLASS SAMPLES FROM SIDEROPHILE ELEMENT PARTITIONING EXPERIMENTS

Experimental studies of the metal/silicate partitioning of Ir have pro duced two sets of results differing by a factor of 10(6) (Jones and Dr ake, 1986; O'Neill et al., 1995). In an attempt to understand this dis crepancy, we have performed instrumental neutron activation analyses o f multiple glass chips from these siderophile element partitioning exp eriments. From seven to ten individual chips (similar to 10-100 mu g e ach) were analyzed from each experimental glass. The major target was Ir, which was detected at concentrations ranging from 690 ppb (ng/g) t o less than 1 ppb. Detection limits are less than 10(-13) grams Ir. Al most all of the run products appear to be homogeneous with respect to lithophile elements, but most vary considerably in their Ir concentrat ions. It appears that most of the experiments analyzed so far contain small amounts of metal in the glass separates. One experiment from Jon es and Drake (1983) appears to be homogeneous, but in fact may not be. More recent experiments are definitely not homogeneous, and require t he existence of a metallic phase distinctly different from the bulk so lid metal or liquid metal/sulfide phases also present in the charges. Iridium contents of the glass in the stirred crucible experiment studi ed (O'Neill et al., 1996) decreased continuously over a period of seve ral months, even as the oxygen fugacity was increasing, suggesting tha t the experiment never achieved equilibrium. Iridium contents in multi ple samples of the glasses varied over factors of 2 to 3x. Further sub division of one low-Lr glass did not result in a reduction in the vari ation of Ir between aliquots. Thus, the variation in Ir concentration appears to remain fairly constant, irrespective of the scale of the sa mpling.