Development ICP-MS isotope dilution preconcentration techniques for determination of platinum group elements in volcanic rocks

Abstract

Understanding PGE geochemistry in low abundance (i.e. sub ppb level) geological samples has been hampered by the absence of an accurate, low blank, preconcentration technique. Ni- S fire assay, the traditional preconcentration method, has been critically assessed and combined with isotope dilution to enable reproducible analysis of standards in the 1-l0ppb concentration range. In addition, a new anion exchange chromatography, low blank, isotope dilution method has been developed that allows analysis of Re, Os, Ir, Ru, Ft and Pd, at pg levels, from the same sample aliquot by ICP-MS. This method enables PGE abundances and Os isotopic ratios to be determined on the same sample dissolution, permitting geochronological studies. Samples are digested in Carius tubes and total procedural blanks are routinely less than l0pg/g for all elements, except Pt (25pg/g). Reproducibility is sufficient at the lOppt level to confidently identify inter-element PGE fractionations. Using this anion exchange preconcentration technique, PGEs have been characterised in a suite of plume-generated picrites from West Greenland, and a suite of subduction-related lavas, including fractionation series from Grenada (Lesser Antilles arc) and from Izu-Bonin. PGE concentrations are higher within the picrites of West Greenland, than in the picrites of Grenada, or boninites of Izu-Bonin. This is attributed to higher degrees of melting and less fractionation during West Greenland magma genesis. There is similarity in the PGE-pattems between all environments suggesting that relative PGE behaviour is not greatly altered during mantle melting in a subduction zone vs a plume. Subtle differences occur in inter-PGE ratios between the different environments and within the fractionation suites. Os and Ir do not behave in an analogous manner during fractionation indicating "compatibility" with different crystallising phases. The PGEs are associated with olivine fractionation in all of the primitive rock types, but, may also be compatible with other phases (e.g. magnetite/chromite/amphibole) during high-level fractionation in arc magma chambers. The PGE signatures in the evolved arc rocks are controlled by multi-stage fractionation. PGE concentrations decrease as fractionation progresses, except for Re and Ru. PGE compatibility during arc-lava fractionation decreases in the order Os>Ir>Pt>Pd>Ru>Re. There is not a strong sulfide control on PGE fractionation in any of the rock suites analysed, probably because the primary melts were generally S- undersaturated. This places important constraints on the nature of the mantle in the different tectonic environments. Os-isotopic studies indicate the presence of a radiogenic component in both the West Greenland picrite plume source region (recycled crust or outer core material) and the Grenada picrite source region (slab-derived fluids).