Solvent extraction/anion exchange separation and determination of PGEs (Os, Ir, Pt, Pd, Ru) and Re-Os isotopes in geological samples by isotope dilution ICP-MS

A method is described that allows the separation of Re, Os, Pt, Pd, Ir and Ru for determination by isotope dilution via inductively coupled mass spect rometry (ICP-MS) at pg levels, from the same sample aliquot. Samples are di gested in Carius tubes, enabling effective attack of PGEs in a wide range o f geological materials, including pure chromite separates. Os is isolated b y solvent extraction so that it may be precisely analysed for isotopic rati os by ICP-MS or negative-ion thermal mass spectrometry. When Os is analysed by ICP-MS, sample memory is minimised by using a direct injection nebulise r. Following Os extraction, Re, Pt, Pd, Ir and Ru are separated from their sample matrix by anion exchange chromatography. The method thus enables PGE abundances and Os isotopic ratios to be determined on the same sample diss olution, thereby avoiding problems of sample inhomogeneity when comparing d ata. Furthermore, analysis of Re fur the same sample dissolution makes geoc hronological studies possible. Total procedural blanks are routinely less t han 10 pg/g for all elements except Pt (15 to 25 pg/g). Blanks of less than 1 pg/g can regularly be obtained for Ir and Os. We estimate procedural det ection limits (3*STD deviation) of c. 3 pg/g for Os and Ir, 5 pg/g for Re, Ru and Pd, and 15 pg/g for Pt. Such detection limits permit analysis of mos t bulk rock geological samples. The method is evaluated by replicate dissol ution of a komatiite reference material, WITS-1 [Tredoux, M., McDonald, I., 1996. Komatiite WITS-1, low concentration noble metal standard for the ana lysis of non-mineralised samples. Geostand. Newsl. 20, 267-276] together wi th unknowns. For WITS-1, concentrations of Os, Ir and Ru reproduce well, wi thin a factor of 3 of that expected based on sampling theory. Potential het erogeneity for Pt, Pd and Re is identified at the 1 g sampling level used, that may be related to alteration. Replicate analyses of picritic basalts i n the 1 to 10 ppb range show average 2 sigma reproducibilities of better th an 5% for Re, Os, Ir, Pt and Pd, with Ru being better than 10%. This compar es very favorably with other analytical techniques. We present data for and esites with Lr and Os at the sub 10 ppt level. As expected, reproducibility decreases as sample concentrations decrease but are sufficient at the 10 p pt level to confidently identify inter-element PGE fractionations. The anal ytical method works well for a wide variety of rocks types. (C) 2000 Elsevi er Science B.V. All rights reserved.