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A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than [+ or -] 0.1 [member of] on [member of][sup.182]W and [member of][sup.184]W (normalized to [sup.186]W/[sup.183]W). Purification of W was achieved through a two-step, ion-exchange procedure. In most cases, the yield is better than 80%, and purified W solutions are clear of matrix elements and direct isobars of W. The final W solutions were analyzed using a Micromass Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Tests performed on mixtures of terrestrial standards and meteorite samples demonstrate that the method is accurate and that [member of][sup.182]W variations as small as ~0.1 [member of] can be detected. Analyses of three different aliquots of the Gibeon (IVA) iron meteorite obtained over a period of 6 months show identical [member of][sup.182]W values with a weighted mean of 3.38 [+ or -] 0.05, consistent with literature data for 1VA iron meteorites, and indicating that the metal-silicate differentiation event in its parent body was either contemporaneous with or slightly postdated (by up to ~2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure [member of][sup.184]W accurately and precisely (within [+ or -] 0.1 [member of]), which is useful for characterizing cosmogenic and nucleosynthetic effects that may be present in iron meteorites. We also report for the first time measurements of [member of][sup.180]W, albeit with large error bars (< [+ or -] 4 [member of], in most cases).

We measured the Ni isotopic composition of metal from a variety of meteorite groups to search for variations in the [sup.60]Ni abundance from the decay of the short-lived nuclide [sup.60]Fe ([t.sub.1/2] = 1.49 My) and for possible nucleosynthetic effects in the other stable isotopes of Ni. We developed a high-yield Ni separation procedure based on a combination of anion and cation exchange chromatography. Nickel isotopes were measured on a single-focusing, multicollector, inductively coupled mass spectrometer (MC-ICPMS). The external precision on the massbias-corrected [sup.60]Ni/[sup.58]Ni ratio ([+ or -].15 [epsilon]; 2[sigma]) is comparable to similar studies using double-focusing MC-ICPMS. We report the first high-precision data for [sup.64]Ni, the least abundant Ni isotope, obtained via MC-ICPMS. The external precision on the mass-bias-corrected [sup.64]Ni/[sup.58]Ni ratio (+ or - ] 1.5 [epsilon]; 2[sigma]) is better than previous studies using thermal ionization mass spectrometry. No resolvable excesses relative to a terrestrial standard in the mass-bias-corrected [sup.60]Ni/[sup.58]Ni ratio were detected in any meteoritic metal samples. However, resolvable deficits in this ratio were measured in the metal from several unequilibrated chondrites, implying a [sup.60]Fe/[sup.56]Fe ratio of ~ 1 X [10.sup.-6] at the time of Fe/Ni fractionation in chondrific metal. A [sup.60]Fe/[sup.56]Fe ratio of (4.6 [+ or -]3.3) x [10.sup.-7] is inferred at the time of Fe/ Ni fractionation on the parent bodies of magmatie iron meteorites and pallasites. No clearly resolvable non-mass-dependent anomalies were detected in the other stable isotopes of Ni in the samples investigated here, indicating that the Ni isotopic composition in the early solar system was homogeneous (at least at the level of precision reported here) at the time of meteoritic metal formation.

A procedure was developed that allows precise determination of Fe isotopic composition. Purification of Fe was achieved by ion chromatography on AG1-X8 strongly basic anion-exchange resin. No isotopic fractionation is associated with column chemistry within 0.02 [per thousand]/amu at 2[sigma]. The isotopic composition was measured with a Micromass IsoProbe multicollection inductively coupled plasma hexapole mass spectrometer. The Fe isotopic composition of the Orgueil CI1 carbonaceous chondrite, which best approximates the solar composition, is indistinguishible from that of IRMM-014 (-0.005 [+ or -] 0.017 [per thousand]/amu). The IRMM-014 reference material is therefore used for normalization of the isotopic ratios. The protocol for analyzing mass-dependent variations is validated by measuring geostandards (IF-G, DTS-2, BCR-2, AGV-2) and heavily fractionated Fe left after vacuum evaporation of molten wustite (FeO) and solar (MgO-[Al.sub.2][O.sub.3]-Si[O.sub.2]-CaO-FeO in chondritic proportions) compositions. It is shown that the isotopic composition of ICe during evaporation of FeO follows a Rayleigh distillation with a fractionation factor [alpha] equal to [([m.sub.1]/[m.sub.2]).sup.1/2], where [m.sub.1] and [m.sub.2] are the masses of the considered isotopes. This agrees with earlier measurements and theoretical expectations. The isotopic composition of Fe left after vacuum evaporation of solar composition also follows a Rayleigh distillation but with a fractionation factor (1.013 22 [+ or -] 0.000 67 for the [sup.56]Fe/[sup.54]Fe ratio) that is lower than the square root of the masses (1.018 35). The protocol for analyzing mass-independent variations is validated by measuring terrestrial rocks that are not expected to show departure from mass-dependent fractionation. After internal normalization of the [sup.57]Fe/[sup.54]Fe ratio, the isotopic composition of Fe can be measured accurately with precisions of 0.2[epsilon] and 0.5[epsilon] at 2[sigma] for [sup.56]Fe/[sup.54]Fe and [sup.58]Fe/[sup.54]Fe ratios, respectively ([epsilon] refers to relative variations in parts per 10 000). For [sup.58]Fe, this precision is an order of magnitude better than what had been achieved before. The method is applied to rocks that could potentially exhibit mass-independent effects, meteorites and Archaean terrestrial samples. The isotopic composition of a 3.8-Ga-old banded iron formation from Isua (IF-G, Greenland), and quartz--pyroxene rocks from Akilia and Innersuartuut (GR91-26 and SM/GR/171770, Greenland) are normal within uncertainties. Similarly, the Orgueil (CI1), Allende (CV3.2), Eagle Station (ESPAL), Brenham (MGPAL), and Old Woman (IIAB) meteorites do not show any mass-independent effect.

A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than ±0.1 ϵ On ϵ182W and ϵ184W (normalized to 186W/183W) Purification of W was achieved through a two step, ion-exchange procedure. In most cases, the yield is better than 80%, and purified W solutions are clear of matrix elements and direct isobars of W. The final W solutions were analyzed using a Micromass Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Tests performed on mixtures of terrestrial standards and meteorite samples demonstrate that the method is accurate and that ϵ182W variations as small as ∼0.1 ϵ can be detected. Analyses of three different aliquots of the Gibeon (P/A) iron meteorite obtained over a period of 6 months show identical ϵ182W values with a weighted mean of 3.38 ± 0.05, consistent with literature data for NA iron meteorites, and indicating that the metal-silicate differentiation event in its parent body was either contemporaneous with or slightly postdated (by up to ∼2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure ϵ184W accurately and precisely (within ±0.1 ϵ), which is useful for characterizing cosmogenic and nucleosynthetic effects that may be present in iron meteorites. We also report for the first time measurements of ϵ180W, albeit with large error bars (<±4 ϵ, in most cases). [ABSTRACT FROM AUTHOR]