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 a mixed Pt, Pd, Ir and Os spike 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 NTIMS. The second half of the filter paper is digested and used for determinations of Pt, Pd and Ir concentrations by isotope dilution ICPMS. 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 abyssal plagioclase peridotite. Triplicate analyses of a Costa Rican picritic basalt powder show appreciable variations in PGE concentrations among powder splits. The standard deviations of Pd, Pt, Ir and Os, expressed as a percent of mean concentrations are 6%, 3%, 14% and 18% respectively. Variation in Os and Ir among the powder splits are well correlated. Consequently Os/Ir ratios display less variability (Os/Ir=l .01 + 0.05 1 sd). We interpret these results as clear evidence of a "nugget effect" for Ir and Os in this sample. 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 - 30 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, sediments sample 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. In contrast data from igneous rocks show systematic variations reflecting the greater compatibility of Os and Ir relative to Pt and Pd. We suggest that unusual PGE patterns observed in marine sediments may serve as a finger print for assimilation of sedimentary material and aid in interpret Os isotopic variations in complex magmatic systems.