Ppt range iridium determination by RNAA and application of that method on a Permian/Triassic boundary section, Bálvány, Bükk Mts., Hungary

摘要

A modified fire assay type analytical sample preparation method has been developed in order to extend iridium analysis of geological samples to the ppt level by neutron activation analysis (NAA). This method development was essential to trace a potential Ir signal at the Permian/Triassic boundary, the largest mass extinction event in the Phanerozoic history of Earth. The new analytical procedure was calibrated by several synthetic and natural standards and then applied for measuring the Ir profile in the continuous marine Permian/Triassic sedimentary sequence Bálvány north, Bükk Mts., Hungary. The analytical problem of applying the method also on practically pure limestones has been overcome, adding extra SiO2 sand, to avoid the crucible being attacked by the fire assay flux. The nugget after the nickel fire-assay pre-concentration was identified by X-ray diffractometry and energy dispersive spectrometry (EDS–SEM) as heazlewoodite (Ni3S2), hosting iridium in the form of pure metal inclusions, mainly of 2–5 μm size (EDS–SEM). Based on these observations, the filter size of the original preparation protocol was decreased to 0.2 μm, in order to get the highest yield for Ir in the analytical preparation process. By applying the new technique, we determined the iridium profile in the Bálvány north section. The whole profile studied, except for the lowest 10 cm of the “boundary shale”, can be characterized of a several hundred ppt—up to one ppb Ir concentration. At the lowest part of the “boundary shale” the Ir content drops below 100 ppt. These values show no sign of any meteorite impact at this border section. The detailed geological interpretation of the high Ir background and the negative course Ir peak at the lithological border needs further studies.