Cr isotope ratios

If remobilization of Cr(III) in an aquifer were to occur, this process may be detectable using Cr isotope ratios. When Cr(VI) reduction is occurring, Cr(III) in the aquifer matrix should be enriched in the lighter isotopes relative to the Cr(I V) in the water. If part of a Cr(VI) plume were to change over time from enrichment in heavier isotopes to emichment in lighter isotopes, this would suggest remobilization of previously reduced Cr has begun. 

Schoenberg et al. (2008) presented the first set of Cr isotope data for rocks and Cr(ll) rich ores. Mantle derived rocks and chromite ores from layered intrusions have a uniform Cr/ Cr isotope ratio very close to the certified Cr standard NIST SRM 979. The Cr isotope composition of hydrothermal lead chromates is substantially heavier (S Cr from 0.6 to 1.0%o) than the rocks from which the chromium was leached. 

Frei et al. [37] hypothesized that a paleoredox proxy based on Cr isotope ratios would likely reflect atmospheric oxygenation, rather than marine oxygenation. When oxygen is abundant in the atmosphere, Cr(lll) in exposed rocks on land is converted during oxidative weathering to Cr(Vl), which is highly soluble in water... 

If the interpretation of Cr isotope ratio variation by Frei et al. [37] is valid, Cr isotope ratios provide an extremely valuable complement to other geochemical proxies for paleoredox conditions, since they reflect atmospheric, rather than marine, oxygenation and can be measured in a rock type that is not suitable for applying other paleoredox proxies, such as Mo or S isotope ratios. Some critical assumptions in Frei et al. s study [37] must be verified, namely (i) that oxidative weathering of Cr(III) from rocks indeed results in an isotopically heavy pool of dissolved Cr and (ii) that reduction and coprecipitation of Cr with Fe oxyhydroxides either does not fractionate Cr isotopes or results in complete removal of Cr from the water column in continental shelf environments where BIF are deposited. Although Cr isotope ratio behavior is not yet fully understood at a fundamental level, several studies have been published that provide important insights into this system. 

In principle, the three isotope method may be widely applied to new isotope systems such as Mg, Ca, Cr, Fe, Zn, Se, and Mo. Unlike isotopic analysis of purified oxygen, however, isotopic analysis of metals that have been separated from complex matrices commonly involves measurement of several isotopic ratios to monitor potential isobars, evaluate the internal consistency of the data through comparison with mass-dependent fractionation relations (e.g., Eqn. 8 above), or use in double-spike corrections for instrumental mass bias (Chapter 4 Albarede and Beard 2004). For experimental data that reflect partial isotopic exchange, their isotopic compositions will not lie along a mass-dependent fractionation line, but will instead lie along a line at high angle to a mass-dependent relation (Fig. 10), which will limit the use of multiple isotopic ratios for isobar corrections, data quality checks, and double-spike corrections. 

Chromium. Much smaller variations are evident in this isotope usually a few e. They are not correlated with Cr variations but with the Mn/Cr elemental ratios. They are also present in the higher metamorphic grades with even larger spreads. Cr is not produced directly in stars but through Mn which then decays to Cr. These arguments favor the interpretation of Cr as solely due to in situ Mn decay (Birck et al. 1999). 

Selenium (masses 74, 76, 77, 78, 80, and 82 Table 1) and chromium (masses 50, 52, 53 54 Table 1) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. 

General information about analytical techniques can be found in another chapter of this volume (Albarede and Beard 2004). Details specibc to Se and/or Cr are covered here. Most of the existing Se and Cr isotope measurements were made by TIMS, but MC-ICP-MS methods are supplanting TIMS methods for Se (Rouxel et al. 2002) and will probably do so for Cr within a few years. With both TIMS and MC-ICP-MS analyses, there is a measurement bias, or discrimination, that must be corrected for (Albarede and Beard 2004). For example, Se is particularly evaporation prone because of its volatility lighter isotopes of Se evaporate from the hlament faster than heavier isotopes, resulting in large changes in measured ratios over time (Johnson et al. 1999). 

When isotopes are fractionated kinetically during chemical reactions, the isotope ratio shift of the reaction products relative to the reactants often depends on reaction mechanisms and rates. This contrasts with isotopic fractionations between phases in isotopic equilibrium, where the isotopic differences are thermodynamic quantities and thus do not depend on reaction mechanisms or rates. In this section, we briefly review the well-developed theory for kinetic isotope effects that appears in the S isotope literature. This background serves as a guide for interpreting and predicting Se and Cr isotope systematics. 

Ellis AS, Johnson,TM, Bullen TD (submitted) Using chromium stable isotope ratios to quantify Cr(VI) reduction lack of sorption effects. Environ Sci Technol submitted 9/2003 Ellis AS, Johnson TM, Bullen TD, Herbel MJ (2003) Stable isotope fractionation of selenium by natural microbial consortia. Chem Geol 195 119-129... 

Chromium is a common anthropogenic contaminant in surface waters, therefore Cr isotope fractionations are of potential interest in tracking Cr + pollution in groundwaters. Ellis et al. (2002, 2004)) and Izbicki et al. (2008) analyzed ground-water samples from contaminated sites and observed an increase in Cr/ Cr ratios up to 6%c during the reduction of chromate. Equilibrium fractionations between Cr(VI) and Cr (III) have been estimated by Schauble et al. (2002), who predicted Cr isotope fractionations >l%c between Cr species with different oxidation states. 

One problem is the availability of reliable reference materials for isotope ratio measurements. Certified isotope standard reference materials are available from NIST (National Institute of Standard Reference Materials, http //www.nist.gov/srd/, formerly NBS - National Bureau of Standards) and IRMM (Institute for Reference Materials and Measurements, http //www.irmm.jrc.de). There are isotope standard reference materials for light elements (H, Li, , C, N, O), Mg (magnesium metal isotopic standard NIST SRM 980), and for moderately heavy elements such as (potassium chloride isotopic standard NIST SRM 985), Cr (chromium metal isotopic standard NIST... 

Major elements show a vertical trend for potassium in the mafic rocks (Fig. 6.16c). These have moderate enrichments in incompatible elements, which are lower than in other Campanian volcanoes ferromagnesian trace element contents are high in the mafic rocks (Ni up to 230 ppm, Cr up to 600 ppm). REE are fractionated, with small or no Eu anomalies (Fig. 6.17a). Incompatible element patterns of mafic rocks are fractionated and contain small negative anomalies of HFSE and positive spikes of Pb (Fig. 6.17b). Sr and Nd isotopic ratios respectively display lower and higher... 

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