The application of INAA and beta autoradiography in an investigation of the speciation of iridium in silicates in the presence of carbon phases

Beta autoradiography and progressive acid leaching have been used to investigate the distribution of Ir in silicates in the presence of carbon phases as well as without these phases. Determinations of Ir by neutron activation analysis gave results for the Ir content of basaltic glasses after experiments at 1 bar of 82 ng x g-1 (NNO buffer), 30 ng x g-1 (CCO equilibrium) and 37 ng x g-1 (IW buffer). Iridium solubility in basaltic melt in the presence of graphite and (CO, CO2) fluid increases from 1.6 μg x g-1 at 0.5 kbar to 6.2 μg x g-1 at 2 kbar. These data give Ir partition coefficients between solid metal and liquid silicate in the presence of carbon phases. Discrete (Ir, Fe) rich phases were discovered in the basaltic melt at 3 kbar and 1250 deg C preventing the determination of solubility data. These results provide clear evidence that the iridium solubility in a basaltic melt in the presence of carbon is higher than the solubility of iridium in melts without carbon. (author)