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High Precision Iron Isotope Measurements of Terrestrial and Lunar Materials

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High Precision Iron Isotope Measurements of Terrestrial and Lunar Materials Geochimica et Cosmochimica Acta, Vol. 63, No. 11/12, pp. 1653–1660, 1999 Copyright © 1999 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/99 $20.00 ϩ .00 PII S0016-7037(99)00089-7 High precision iron isotope measurements of terrestrial and lunar materials BRIAN L. BEARD* and CLARK M. JOHNSON Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA (Received August 5, 1998; accepted in revised form February 11, 1999) Abstract—We present the analytical methods that have been developed for the first high-precision Fe isotope analyses that clearly identify naturally-occurring, mass-dependent isotope fractionation. A double-spike approach is used, which allows rigorous correction of instrumental mass fractionation. Based on 21 analyses of an ultra pure Fe standard, the external precision (1-SD) for measuring the isotopic composition of Fe is Ϯ0.14 ‰/mass; for demonstrated reproducibility on samples, this precision exceeds by at least an order of magnitude that of previous attempts to empirically control instrumentally-produced mass fractionation (Dixon et al., 1993). Using the double-spike method, 15 terrestrial igneous rocks that range in composition from peridotite to rhyolite, 5 high-Ti lunar basalts, 5 Fe-Mn nodules, and a banded iron formation have been analyzed for their iron isotopic composition. The terrestrial and lunar igneous rocks have the same isotopic compositions as the ultra pure Fe standard, providing a reference Fe isotope composition for the Earth and Moon. In contrast, Fe-Mn nodules and a sample of a banded iron formation have iron isotope compositions that vary over a relatively wide range, from ␦56Fe ϭϩ0.9 to Ϫ1.2 ‰; this range is 15 times the analytical errors of our technique. These natural isotopic fractionations are interpreted to reflect biological (“vital”) effects, and illustrate the great potential Fe isotope studies have for studying modern and ancient biological processes. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION Precise isotopic analysis of the transition metals is analyti- cally challenging, because the most widely available appropri- Mass-dependent fractionation of light stable isotopes, such as ate instrumentation is thermal ionization mass spectrometry C, O, N, and S, are a well studied phenomenon (e.g., Hoefs, (TIMS), which produces significant mass fractionation during 1987). Isotopic fractionation of the light stable isotopes are isotopic analysis. In this contribution, we use a double-spike controlled by temperature, phase transitions (e.g., liquid to approach to remove instrumentally-produced mass fraction- vapor), inter-mineral equilibrium fractionation, and biological ation; these methods have produced the most precise measure- activity (e.g., Friedman and O’Neil, 1977; Schidlowski et al., ments of the isotopic composition of Fe that preserve mass- 1983). Determining the relative contributions of isotopic frac- dependent fractionation, and demonstrates that Fe isotopes vary tionation for the light stable isotopes in terms of inorganic (e.g., in nature as a result of mass-dependent fractionation. temperature or phase effects) and organic factors (e.g., “vital” effects) is difficult because both can be significant due to the 2. PREVIOUS IRON ISOTOPE STUDIES large relative mass differences between the isotopes. In contrast, the isotopic composition of the first transition Estimates for the atomic abundances of the stable isotopes of metals may be useful in evaluating kinetically-controlled, bio- Fe (54Fe 5.84%, 56Fe 91.76%, 57Fe 2.12%, 58Fe 0.28%) have logically-induced isotope fractionation, because it is anticipated been made by a number of workers (Valley and Anderson, that their small relative mass differences, as compared to the 1947; Vo¨lkening and Papanastassiou, 1989; Taylor et al., 1992; well-studied light stable isotopes, will produce minimal or Dixon et al., 1993). The two biggest challenges in making unmeasureable inorganic (equilibrium) isotope fractionation. precise Fe isotope ratio measurements that retain naturally- Iron is a critically important element in biological processes, produced isotope variations are the low ionization efficiency of and has a relative mass difference that is likely to be sufficient iron and the necessity to correct for instrumental mass fraction- to record biologically-produced isotopic fractionation, and yet ation. is expected to be minimally fractionated by inorganic (equilib- Corrections for instrumentally-produced mass fractionation rium) processes. Iron availability is one of the most important that preserve natural, mass dependent fractionation can be controlling factors for organic productivity in the oceans (e.g., approached in one of two ways- a double-spike method, which Martin and Fitzwater, 1988). Moreover, iron biominerals are allows for rigorous calculation of instrumental mass fraction- ubiquitous in nature (Lowenstam, 1981), and are produced by ation (e.g., Compston and Oversby, 1969; Eugster et al., 1969; iron-reducing, iron oxidizing, and magnetotactic bacteria as and Gale, 1970), or an empirical adjustment, based on compar- well as higher life forms such as Chiton. Indeed, iron biomin- ison with isotopic analysis of standards. The empirical ap- erals have been inferred to exist in ancient rocks from Mars proach assumes that standards and samples fractionate to the (McKay et al., 1996). same degree during isotopic analysis, requiring carefully con- trolled analysis conditions. Such approaches are commonly used for Pb isotope work. Certified standard material of known *Author to whom correspondence should be addressed (beardb@ Fe isotope composition (determined using gravimetrically pre- geology.wisc.edu). pared standards) has recently become available (Taylor et al., 1653 1654 B. L. Beard and C. M. Johnson Table 1. Collector configuration used for static multicollector analysis of Fe isotope ratios.* Low 2 Low 1 Axial High 1 High 2 High 3 High 4 Background 51.5 52.5 53.5 54.5 55.5 56.5 Scan 1 52Cr 54Fe 56Fe Scan 2 56Fe 57Fe 58Fe 60Ni * The Micromass Sector 54 collector block is designed for a working range in mass spread of 9.5%. To simultaneously analyze all four Fe isotopes and continuously monitor for Cr and Ni isobars would require a relative mass spread of 14.4%. Therefore, 2 multi-collection scans are used. A beam growth correction using the ion intensity measured for 56Fe in scans 1 and 2 is used for measurement of the 54Fe/57Fe and 54Fe/58Fe ratios. 1993). Using these certified reference materials, Dixon et al. as Fe is eluted from the anion exchange resin using 1M HCl. (1993) report Fe isotope ratio precisions of Ϯ0.3‰/mass for Iron isotope fractionation during anion-exchange chromato- standard Fe material. However, it is important to stress that the graphic separation is not believed to be significant. The isotope precision and accuracy of Fe isotope ratios determined on composition of the J-M Fe processed using anion exchange unknown samples may be very difficult to evaluate because chromatography is the same as that measured for J-M Fe taken each filament load in a TIMS analysis is different, and, in the directly from the stock solution. Moreover, the isotope com- case of Fe, important and variable matrix and interference position of J-M Fe eluted from a small part of the tail of the Fe effects are present. The Fe isotope data reported by Dixon et al. peak during elution of Fe by 1M HCl (last 5% of the total Fe (1992) are significantly more variable than those of standards, removed from the anion exchange column) is the same as that and contain internal inconsistencies among the isotope ratios, for J-M Fe from the stock solution. Separation of Fe is essen- suggesting that their practical precision is more on the order of tially a one stage process because of the very high affinity of Ϯ1 to 3‰/mass difference, using their empirical approach. ferric iron for resin at high molarity HCl. Moreover, short We consider rigorous correction for instrumental mass bias columns are used (resin height is approximately 2 cm), to be a prerequisite for evaluating if naturally occurring mass- making it unlikely that significant isotope fractionation dependent isotope fractionation occurs. This concept is well could occur. illustrated by the definitive Ca isotope work of Russell et al. Total blanks for our procedure average 40 ng of Fe, produc- (1978), which used a double-spike approach. Prior to the Ca ing a sample to blank ratio of ϳ2,500 to ϳ75,000, making the isotope investigation of Russell et al. (1978), natural, mass- blank contribution entirely negligible. The bulk of the total dependent Ca isotope variations were estimated to be large procedural blanks are from the HF and HNO3 that are used in (12‰/mass based on an empirical method for instrumental the dissolution of silicates. Procedural blanks for the ion- mass-fractionation correction; e.g., Miller et al., 1966). How- exchange chemistry average 5 nanograms. All reagents used ever, Russell et al. (1978) showed that naturally-occurring, were prepared in a class-100 clean lab. Hydrofluoric, HNO3, mass-dependent variations in Ca isotopes is much smaller (total and HCl were prepared by sub-boiling distillation of analytical range of 0.6‰/mass), and that the previously reported mass- reagent-grade acid. Water was purified by reverse osmosis and dependent variations were entirely a result of imprecise correc- then ion-exchange chromatography using a Nanopure water- tions for instrumental mass fractionation. We suspect that vari- purification system. The silica gel and H3PO4 used for loading ations in instrumental mass fractionation correction from filaments are ultra pure reagents (Seastar chemicals). Iron sample to sample is likely to be more problematic for Fe blanks for water and HCl average 0.1 and 0.3 ng, respectively. isotope measurements, as compared to Ca, because of the low Iron blanks for HF and HNO3 are 9 and 1.6 ng, respectively. ionization efficiency of Fe. Loading blanks average 0.3 ng.
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