PGE AND OS ISOTOPIC ANALYSES OF SINGLE-SAMPLE ALIQUOTS WITH NIS FIRE ASSAY PRECONCENTRATION

We have modified methods currently used to determine Os isotope ratios with NiS fire assay preconcentration to allow measurement of PGE conc entrations and Os isotope ratios on the same sample powder. Samples ar e spiked with an isotopically enriched solution of Pt, Pd, Ir and Os p rior to fusion. After filtration of the insoluble residue resulting fr om dissolution of the NiS bead, the filter paper containing the POE-ri ch concentrate is split. One half is used to measure Os isotopes by NT I-MS. The second half of the filter paper is digested and used for det erminations of Pt, Pd and Ir concentrations by isotope dilution ICP-MS . Because we employ isotope dilution, measured PGE concentrations are unaffected by heterogeneous distribution of PGEs on the filter paper. PGE preconcentration by NiS allows a wide range of sample types to be preconcentrated rapidly. We have investigated a variety of mafic igneo us rocks encompassing MORE to picritic plateau basalts. In these sampl es variation in Os and Ir among the powder splits are well correlated, Consequently Os/Ir ratios display less variability among replicate an alyses than Os and Ir concentrations. We interpret these results as ev idence of a 'nugget effect' for Ir and Os in some igneous samples, Sam ples analyzed thus far span three orders of magnitude in PGE concentra tion, from ppb to ppt ranges. PGE blank associated with this procedure is carried mainly by fusion reagents. Blank levels for Pt and Pd are 15-50 pg per gram sample while Os and Ir blanks are 1 to 4 pg per gram sample, These relatively low blank levels allow PGE measurements to e xtend into the MORE concentration range with only modest blank correct ions. Several types of marine sediment have also been investigated. PG E concentrations can be very high, with Pt and Pd concentrations of 20 to 30 ppb and Os and Ir concentrations exceeding 1 ppb. In contrast t o the igneous rocks, where Os and Ir are well correlated over several orders of magnitude in concentration, sediment samples show evidence o f fractionation of Ir from Os by aqueous processes. Organic-rich marin e sediments typically have Os/Ir > 1 while slowly accumulating pelagic clays have Os/Ir < 1. In addition, Pd is enriched in organic-rich sed iments relative to Pt by as much as an order of magnitude. (C) 1997 El sevier Science B.V.