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We present a double-spike technique for precise determination of mass-dependent fractionation of tungsten (W) stable isotopes. Instrumental mass bias effects and isotope fractionation during W separation are corrected for by means of a 180W–183W double-spike added prior to sample dissolution. The separation of W from the matrix is achieved via three-step anion-exchange chromatography. Tungsten stable isotopic compositions were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Results are expressed relative to the NIST SRM 3136 W standard as the permil difference in 186W/184W (δ186W). The external reproducibility of the NIST SRM 3163 W standard is ±0.05‰ with an average composition of −0.01‰ on 186W/184W (2s.d., n = 171). Tungsten stable isotope compositions for USGS standard reference materials are as follows: AGV-2 (andesite; δ186W = 0.18 ± 0.05‰, 2s.d., n = 6), SDC-1 (mica schist; δ186W = 0.36 ± 0.05‰, 2s.d., n = 6), SDO-1 (shale; δ186W = 0.26 ± 0.06‰, 2s.d., n = 6), and manganese nodules NOD-A-1 (δ186W = 0.07 ± 0.05‰, 2s.d., n = 10) and NOD-P-1 (δ186W = 0.31 ± 0.04‰, 2s.d., n = 7). The long-term external reproducibility (2s.d.) for these rock standards is ±0.05‰. [ABSTRACT FROM AUTHOR]

The Ninetyeast Ridge is an ∼5500 km long, north–south-oriented, submarine volcanic ridge in the eastern Indian Ocean that formed from magmatism associated with the deep-seated Kerguelen mantle plume as the Indian plate drifted rapidly northward during the Late Cretaceous. Basalts recovered along the ridge have the characteristic Dupal geochemical signature of Indian Ocean basalts, but debate concerning the nature and number of components in their mantle source persists. New multiple collector inductively coupled plasma mass spectrometry (Pb, Hf) and thermal ionization mass spectrometry (Sr, Nd) isotopic analyses were obtained for tholeiites representative of the ∼180 m of basaltic basement recovered from three drill sites (Site 758, 82 Ma; Site 757, 58 Ma; Site 756, 43 Ma) along the Ninetyeast Ridge during Ocean Drilling Program Leg 121. No systematic isotopic variation is observed along the ridge, which is inconsistent with the hypothesis of an aging mantle plume origin for the ridge. The isotopic compositions are generally intermediate between those of the volcanic products of the Kerguelen and Amsterdam–St. Paul hotspots and define mixing trends between components with relatively enriched and depleted signatures. At least three, possibly four, source components are required to explain the observed isotopic variability along the Ninetyeast Ridge. The unradiogenic signatures of some Ninetyeast Ridge basalts (e.g. 87Sr/86Sr = 0·70381–0·70438) are not related to the source of Indian MORB and indicate the presence of a relatively depleted component in a deep mantle source. A similar source component is also identified in other Indian Ocean island basalts (e.g. Crozet, Réunion) not related to magmatic activity of the Kerguelen hotspot. The Pb–Hf–Sr–Nd isotopic compositions of the Ninetyeast Ridge basalts are consistent with the presence of a mixture of recycled sediments and lower continental crust together with altered oceanic crust in their mantle source, hence supporting a deep origin for the enriched Dupal signature encountered in ocean island basalts. [ABSTRACT FROM AUTHOR]

Abstract: Resin-derived contaminants added to samples during column chemistry are shown to cause matrix effects that lead to inaccuracy in multi-collector inductively coupled plasma mass spectrometry measurement of small natural variations in Cd and Zn isotopic compositions. These matrix effects were evaluated by comparing pure Cd and Zn standards and standards doped with bulk column blank from the anion exchange chromatography procedure. Doped standards exhibit signal enhancements (Cd, Ag, Zn and Cu), instrumental mass bias changes and inaccurate isotopic compositions relative to undoped standards, all of which are attributed to the combined presence of resin-derived organics and inorganics. The matrix effect associated with the inorganic component of the column blanks was evaluated separately by doping standards with metals at the trace levels detected in the column blanks. Mass bias effects introduced by the inorganic column blank matrix are smaller than for the bulk column blank matrix but can still lead to significant changes in ion signal intensity, instrumental mass bias and isotopic ratios. Chemical treatment with refluxed HNO3 or HClO4/HNO3 removes resin-derived organic components resulting in matrix effects similar in magnitude to those associated with the inorganic component of the column blank. Mass bias correction using combined external normalization-SSB does not correct for these matrix effects because the instrumental mass biases experienced by Cd and Zn are decoupled from those of Ag and Cu, respectively. Our results demonstrate that ion exchange chromatography and associated resin-derived contaminants can be a source of error in MC-ICP-MS measurement of heavy stable element isotopic compositions. [Copyright &y& Elsevier]