Brian L. Beard

Application of Fe isotopes to tracing the geochemical and biological cycling of Fe

Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in d 56 Fe values (per mil deviations in 56 Fe/ 54 Fe ratios) in nature from � 2.50xto +1.5x. Re-assessment of the range of d 56 Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to F0.05x, which represents an unparalleled baseline with which to interpret Fe isotope variations in nature. All of the isotopic variability in nature lies in fluids, rocks, and minerals that formed at low temperature. Equilibrium (‘‘abiotic’’) isotopic fractionations at low temperatures may explain the range in d 56 Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) (DFe(III)–Fe(II)=2.75x). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in d 56 Fe values are also consistent with the experimentally measured isotopic fractionations produced by Fereducing bacteria. In this work, we touch on a number of aspects of Fe isotope geochemistry that bear on its application to geochemical problems in general, and biological cycling of metals in particular. We report on new state-of-the-art Fe isotope analytical procedures, which allow precisions of F0.05x( 56 Fe/ 54 Fe) on samples <300 ng in size. In addition, we discuss the implications of experimental work on Fe isotope fractionations during metabolic processing of Fe by bacteria and the need to take a ‘‘mechanistic’’ approach to understanding the pathways in which Fe isotopes may be uniquely fractionated by biology. Additionally, we discuss experimental methods, such as the use of enriched isotope tracers that are necessary to evaluate if experimental isotope exchange reactions are transient kinetic fractionations, equilibrium isotopic exchange reactions, or a combination of both, which can be caused by the complexities of multiple isotope exchange reactions taking place in an experimental system. D 2002 Elsevier Science B.V. All rights reserved.

Isotopic fractionation between Fe(III) and Fe(II) in aqueous solutions

Abstract Large equilibrium isotope fractionation occurs between Fe(III) and Fe(II) in very dilute (≤22 mM Cl − ) aqueous solutions, reflecting significant differences in bonding environments. Separation of Fe(III) and Fe(II) is attained by rapid and complete precipitation of Fe(III) through carbonate addition, followed by separation of supernatant and ferric precipitate; experiments reported here produce an equilibrium Δ Fe(III)–Fe(II) =+2.75±0.15‰ for 56 Fe/ 54 Fe at room temperature (22±2°C). The timescales required for attainment of isotopic equilibrium have been determined by parallel isotope tracer experiments using 57 Fe-enriched iron, which are best fitted by a second-order rate law, with K =0.18±0.03 s −1 . Based on this rate constant, ∼15–20% isotopic exchange is estimated to have occurred during Fe(III)–Fe(II) separation, which contributes Fe(III)–Fe(II) . Under the experimental conditions used in this study, >97% Fe(II) exists as [Fe II (H 2 O) 6 ] 2+ , and >82% Fe(III) exists as [Fe III (H 2 O) 6 ] 3+ and [Fe III (H 2 O) 6− n (OH) n ] 3− n ; assuming these are the dominant species, the measured Fe isotope fractionation is approximately half that predicted by Schauble et al. [Geochim. Cosmochim. Acta 65 (2001) 2487–2497] at 20–25°C. Although this discrepancy may be due in part to the experimentally unknown isotopic effects of chloride interacting with Fe-hexaquo or Fe-hydroxide complexes, or directly bonded to Fe, there still appears to be at this stage a >1‰ difference between prediction and experiment.

Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, western Alps, Italy

Abstract Estimation of burial rates and duration of prograde metamorphism of ultra-high-pressure (UHP) rocks (T=590–630°C, P=2.7–2.9 GPa) of the Zermatt–Saas ophiolite from Lago di Cignana, Italy, may be made through combined Lu–Hf and Sm–Nd garnet geochronology in conjunction with petrologic estimates of the prograde P–T path. We report a Lu–Hf garnet–omphacite–whole-rock isochron age of 48.8±2.1 Ma from the UHP locality at Lago di Cignana, which stands in contrast to the Sm–Nd age of 40.6±2.6 Ma [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438] obtained from the same sample and mineral material. The Sm–Nd and Lu–Hf ages, as well as other ages determined on metamorphic garnet, zircon and white mica [Amato et al., Earth Planet. Sci. Lett. 171 (1999) 425–438; Mayer et al., Eur. Union Geosci. 10 (1999) Abstr. 809; Rubatto et al., Contrib. Mineral. Petrol. 132 (1998) 269–287; Dal Piaz et al., Int. J. Earth Sci. 90 (2001) 668–684] from Lago di Cignana and elsewhere in the Zermatt–Saas ophiolite, lie within a range of ∼50–38 Ma, which we interpret to encompass the duration of prograde metamorphism, and possibly the duration of garnet growth. The difference in measured Sm–Nd and Lu–Hf ages from Cignana can be accounted for by expected core to rim variations in Lu, Hf, Sm, and Nd. The measured yttrium content in garnet, which may be a proxy for Lu, is highest in garnet core and lowest in the mineral rim, generally following a profile that is predicted by Rayleigh fractionation. Preferential enrichment of Lu in the core produces a Lu–Hf age that is weighted toward the older garnet core. Sm–Nd ages, as predicted by Rayleigh fractionation of Sm and Nd during garnet growth, however, reflect later grown garnet as compared to Lu–Hf ages. The difference in Sm–Nd and Lu–Hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as Sm–Nd and Lu–Hf blocking temperatures were not exceeded for a long period of time. Based on the 12 Myr duration of prograde garnet growth estimated in this study, burial rates for rocks at Lago di Cignana were on the order of 0.23–0.47 cm/yr. These values correlate with continuous shortening rates of 0.4–1.4 cm/yr between the European plate and the African–Adriatic promontory between 50 and 38 Ma, which is on the order of that calculated for plate velocities from plate reconstructions, suggesting that the Zermatt–Saas ophiolite may have remained well-coupled to the down-going slab to UHP conditions.

Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans

The Fe isotope composition of Proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 Fe values that is only slightly more variable than the range of Fe isotope com- positions measured in terrestrial igneous rocks, indicating that chemical weathering, sedimentary transport, and diagenesis play only a minor role in producing Fe isotope variations in environ- ments where Fe redox conditions have been controlled by current levels of atmospheric oxygen. In contrast, the Fe isotope composi- tions of hot fluids ( .300 8C) from mid-ocean-ridge (MOR) spread- ing centers define a narrow range that is shifted to lower d 56 Fe values by 0.2‰-0.5‰ as compared to igneous rocks. These new data allow a conceptual model for the Fe isotope composition of the oxic oceans that predicts large ranges in Fe isotope composition under conditions of changing aerosol and MOR Fe fluxes, such as during periods of major worldwide glaciation.

Strontium isotope composition of skeletal material can determine the birth place and geographic mobility of humans and animals.

The Sr isotope composition measured in skeletal elements (e.g., bone, teeth, or antlers) can be used to infer the geographic region that an animal or human inhabited, because different regions tend to have distinct Sr isotope compositions, and natural variations in the relative abundance of Sr isotopes are not changed as Sr is processed through the food chain. Therefore, an organism that ingests Sr from one region can have a Sr isotope composition that is different than that of an organism that ingests Sr from another region. The Sr isotope composition of skeletal elements is a reflection of the concentration-weighted average of dietary Sr that was ingested while that skeletal element was produced. Because different skeletal elements grow and exchange Sr at different stages during the life times of organisms, Sr isotope analysis of different skeletal elements can be used to infer changes in geographic location at different stages in an organisms life. The Sr isotope composition measured in human teeth will reflect the average Sr isotope composition that was ingested as a child, due to the immobile nature of Sr and Ca in teeth after formation, whereas the Sr isotope composition of bone will reflect the average isotopic composition over the last ten years of life, due to continuous biological processing of Sr and Ca in bone. Inferring the average isotopic composition of dietary Sr is best done by analyzing skeletal fragments from control groups, which might be animals that have the same feeding habits as the animal in question, or, in the case of humans, analysis of close family relatives. In cases where it is not possible to construct a Sr isotope database from control groups, it becomes necessary to estimate the isotopic composition of dietary Sr based on geologic principles. We present three case studies from our research that illustrate a range of approaches: (1) results from a criminal case where a deer was illegally harvested and the location of the deer was important to establish, (2) a pilot study of commingled human remains from a burial in Vietnam, associated with the Vietnam Conflict, and (3) a study of 13th and 14th century migration of peo ple from an archeological site in the Southwest United States.

Rapid exhumation of the Zermatt-Saas ophiolite deduced from high-precision SmNd and RbSr geochronology

Sm‐Nd isotope data from garnets in ultrahigh-pressure (coesite-bearing) eclogite from the Western Alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the Zermatt‐Saas ophiolite complex. We report an exceptionally well-constrained Sm‐Nd isochron age of 40:62:6 Ma from the Lago di Cignana eclogite. Difficulties in dating Alpine eclogites using this method were overcome by using an HF acid-leaching procedure on garnet to remove mineral inclusions. The Zermatt‐Saas rocks reached greenschist-facies conditions around 38 2M a, on the basis of Rb‐Sr whole rock‐phengite isochrons. These data, combined with existing estimates of the pressure and temperature conditions of the eclogite, give an estimate for the initial exhumation rate of 10 to 26 km=m.y., depending on the inferred pressure for the recrystallization of phengite, followed by slow exhumation of 0.3 km= m.y. from 34 to 14 Ma. The initial exhumation rates at Lago di Cignana are among the highest yet determined for high-pressure terranes, and are well constrained because the age has been determined directly on the high-pressure assemblage. The rapid exhumation rates presented here place critical constraints on collisional tectonic models.

Mobility of Bell Beaker people revealed by strontium isotope ratios of tooth and bone : a study of southern Bavarian skeletal remains

In order to contribute to the continuing discussion of the mobility of the late neolithic Bell Beaker people, 69 skeletons from southern Bavaria were analyzed for the 87Sr/86Sr isotope ratios in tooth enamel and compact bone. Whereas Sr isotope ratios in the enamel of the first permanent molar match the Sr isotopic composition at the place of early childhood, the respective value in the adult femoral bone matches the Sr isotope ratio characteristic of the place of residence over the last few years prior to death. Significant differences between 87Sr/86Sr in these tissues indicate that 17.5–25% of these individuals changed residence during their lifetime. The overall direction of the migration, according to archaeological finds from the area, was toward the southwest. A relative surplus of migrating females and two cases of evidence for migration in children argue for the movement of small groups; exogamy might explain the higher numbers of immigrating females. With regard to current information on migration rates in prehistory, the southern Bavarian Bell Beaker people were indeed highly mobile, especially since the archaeometric method used in this study is likely to underestimate movement.

High precision iron isotope measurements of terrestrial and lunar materials

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 ±0.14 ‰/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 δ56Fe = +0.9 to −1.2 ‰; 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.

Correction of instrumentally produced mass fractionation during isotopic analysis of fe by thermal ionization mass spectrometry

High-precision (∼0.015%/mass) isotope ratio measurements of Fe may be obtained by using magnetic-sector thermal ionization mass spectrometry (TIMS), where rigorous correction of instrumentally produced mass fractionation can be made. Such corrections are best done by using a double-spike approach, which was first introduced several decades ago. However, previous derivations do not lend themselves to the high-precision isotope analysis that modern TIMS instruments are capable of because of various assumptions of mass fractionation laws or constant atomic weights. Moreover, some of these previous approaches took iterative approaches to the calculation, and none presented detailed error propagations. Here we present a completely general derivation to the double-spike approach that may be used for any appropriate isotope system and is applicable to the mass fractionation laws that are known to occur in TIMS. In addition, we present an assessment of error propagation as a function of algorithm and spike isotope composition. This approach has produced the highest precision Fe isotope ratio measurements yet reported, on the order of ±0.2 to 0.3 per mil for the 54Fe/56Fe ratio, that correct for instrumentally produced mass fractionation and yet retain natural, mass-dependent isotopic variations in samples.

A Younger Age for ALH84001 and Its Geochemical Link to Shergottite Sources in Mars

Less Old Martian Meteorite The oldest Martian meteorite known, ALH84001, was thought to be a remnant of primordial martian crust formed during solidification of an early magma ocean. Using isotope data, Lapen et al. (p. 347) revised the crystallization age of this meteorite from 4.51 billion years to 4.09 billion years ago, meaning that this rock cannot be a fragment of primordial crust that escaped the period of intense bombardment that occurred between 4.25 and 4.10 billion years ago. The revised age also suggests that magmatism was ongoing in Mars for a large part of its history and that ALH84001 was actually formed during the heavy bombardment period, just before the martian core dynamo stopped and the planetary magnetic field was lost. The oldest known martian meteorite is younger than previously thought, precluding it from sampling primeval martian crust. Martian meteorite ALH84001 (ALH) is the oldest known igneous rock from Mars and has been used to constrain its early history. Lutetium-hafnium (Lu-Hf) isotope data for ALH indicate an igneous age of 4.091 ± 0.030 billion years, nearly coeval with an interval of heavy bombardment and cessation of the martian core dynamo and magnetic field. The calculated Lu/Hf and Sm/Nd (samarium/neodymium) ratios of the ALH parental magma source indicate that it must have undergone extensive igneous processing associated with the crystallization of a deep magma ocean. This same mantle source region also produced the shergottite magmas (dated 150 to 570 million years ago), possibly indicating uniform igneous processes in Mars for nearly 4 billion years.