APPLICATION OF ISOTOPE-DILUTION FOR PRECISE MEASUREMENT OF ZR AND HF IN LOW-ABUNDANCE SAMPLES AND INTERNATIONAL REFERENCE MATERIALS BY INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY - IMPLICATIONS FOR ZR (HF) REE FRACTIONATIONS IN KOMATIITES

Precise analysis of all REE's and HFSE's in mantle-derived rocks is cr itical to understanding the chemical compositions of different mantle reservoirs and the development of mantle lithosphere. An isotope dilut ion inductively coupled plasma mass spectrometry (ID-ICP-MS) study of Zr and Hf in komatiites and some low-abundance international reference materials (i.e. BIR-1) was undertaken, in an effort to evaluate previ ous data obtained by ICP-MS using external calibration with pure eleme ntal standards. Isotopic spikes enriched in Zr-91 and Hf-179 were empl oyed. The isotope ratios used, Zr-91/Zr-90 and Hf-179/Hf-178, are virt ually free of isobaric interferences (e.g., formation rate of (DyO)-Dy -162-O-16 and (DyO)-Dy-163-O-16 is 0.4-0.6%, translating into uncertai nties in measured Hf-179/Hf-178 Of similar to +/- 0.02%). Mass bias, s howing reduced response for light masses, is insensitive to analyte so lution concentration in the range 0.08-1.8 ppm for Zr, and 0.003-0.051 ppm for Hf. Mass bias for spiked samples was corrected with reference to the natural isotope ratios of Zr and Hf in unspiked counterparts. The results for Zr and Hf in unknowns and standards are in good agreem ent with previous results obtained by ICP-MS using external calibratio n, but with improved precision. Precision for Zr in BCR-1 (184 ppm, 0. 005% RSD) is comparable to that obtained by thermal ionization mass sp ectrometry. The results confirm previously reported positive or zero n ormalized anomalies of Zr and Hf relative to MREE's in Al-undepleted k omatiites, but negative anomalies in Al-depleted komatiites, that have significance for the depths of mantle melting and Archean geodynamics .