PGE and Os isotopic analyses of single sample aliquots with NiS fire assay preconcentration

Abstract

We have modified methods currently used to determine Os isotope ratios with NiS fire assay preconcentration to allow measurement of PGE concentrations and Os isotope ratios on the same sample powder. Samples are spiked with an isotopically enriched solution of Pt, Pd, Ir and Os prior to fusion. After filtration of the insoluble residue resulting from dissolution of the NiS bead, the filter paper containing the PGE-rich concentrate is split. One half is used to measure Os isotopes by NTI-MS. The second half of the filter paper is digested and used for determinations 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 igneous rocks encompassing MORB to picritic plateau basalts. In these samples variation in Os and Ir among the powder splits are well correlated. Consequently Os/Ir ratios display less variability among replicate analyses than Os and Ir concentrations. We interpret these results as evidence of a ‘nugget effect’ for Ir and Os in some igneous samples. Samples analyzed thus far span three orders of magnitude in PGE concentration, 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 extend into the MORB concentration range with only modest blank corrections. Several types of marine sediment have also been investigated. PGE 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 to the igneous rocks, where Os and Ir are well correlated over several orders of magnitude in concentration, sediment samples show evidence of fractionation of Ir from Os by aqueous processes. Organic-rich marine sediments typically have Os/Ir > 1 while slowly accumulating pelagic clays have Os/Ir < 1. In addition, Pd is enriched in organic-rich sediments relative to Pt by as much as an order of magnitude.