Chromium isotopic composition

Lee T, Tera F (1986) The meteoritic chromium isotopic composition and limits for radioactive Mn in the early solar system. Geochim Cosmochim Acta 50(2) 199-206 Lemly AD (1985) Toxicology of selenium in a freshwater reservoir Implications for environmental hazard evaluation and safety. Ecotoxicol Environ Saf 10 314-338 Lemly AD (1998) Pathology of Selenium Poisoning in Fish. In Environmental Chemistry of Selenium. 

The Mn- Cr system can be studied by TIMS, ICPMS, and SIMS techniques. For TIMS and ICPMS work, bulk samples or mineral separates are dissolved and the solutions are passed through ion-exchange columns to produce clean solutions of manganese and chromium. For minerals with high Mn/Cr ratios SIMS can obtain isotopic data while retaining the petrographic context of the measurements. The chromium isotopic compositions may have to be corrected for small additions of chromium from spallation reactions induced by cosmic rays. This is particularly important in iron-rich meteorites. 

C02-0012. Figure 2-17 gives the isotopic composition of naturaiiy occurring chromium (Cr). Draw a bar graph that shows the mass spectrum of Cr. 

Virag A, Zinner E, Lewis RS, Tang M (1989) Isotopic compositions of H, C, and N in C8 diamonds from the Allende and Murray carbonaceous chondrites. Lunar Planet Sci XX 1158-1159 Volkening J, Papanastassiou DA (1989) Iron isotope anomalies. Astrophys J 347 L43-L46 Volkening J, Papanastassiou DA (1990) Zinc isotope anomalies. Astrophys J 358 L29-L32 Wadhwa M, Zinner EK, Crozaz G (1997) Manganese-chromium systematics in sulfides of unequilibrated enstatite chondrites. Meteorit Planet Sci 32 281-292... 

Beaudoin G, Taylor BE, Rumble III D, Thiemens M (1994) Variations in the sulfur isotope composition of troilite from the Canon Diablo iron meteorite. Geochim Cosmochim Acta 58 4253-4255 Birck JL, Allegre CJ (1988) Manganese-chromium isotope systematics and the development of the early solar system. Nature 331(6157) 579-584... 

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. 

Bulk techniques still have a place in the search for presolar components. Although they cannot identify the presolar grain directly, they can measure anomalous isotopic compositions, which can then be used as a tracer for separation procedures to identify the carrier. There are several isotopically anomalous components whose carriers have not been identified. For example, an anomalous chromium component enriched in 54Cr appears in acid residues of the most primitive chondrites. The carrier is soluble in hydrochloric acid and goes with the colloidal fraction of the residue, which means it is likely to be submicron in size (Podosck el al., 1997). Measurements of molybdenum and ruthenium in bulk primitive meteorites and leachates from primitive chondrites show isotopic anomalies that can be attributed to the -process on the one hand and to the r- and /7-processes on the other. The s-process anomalies in molybdenum and ruthenium correlate with one another, while the r- and /7-process anomalies do not. The amounts of -process molybdenum and ruthenium are consistent with their being carried in presolar silicon carbide, but they are released from bulk samples with treatments that should not dissolve that mineral. Thus, additional carriers of s-, r-, and/ -process elements are suggested (Dauphas et al., 2002). 

Chromium spectral lines are observed in stars. They show that the abundance ratio Cr/Fe has remained constant, even while each has increased in its ratio to H. Although the spectral lines do not require it, this observed Cr abundance is surely dominated by the 52Cr isotope. Itis quite expected thatin the surfaces of some stars and in the nucleosynthesis ejecta of certain stars, Cr isotopes will have nonsolar proportions. However, Cr isotopic compositions have not been measurable by astronomers owing to their low abundance relative to 52Cr and to the small differences in the wavelengths of the radiation emitted by each isotope. 

For most other elements there is no difference between the isotopic composition of carbonaceous chondrites and the Earth. As of early 2000s, only two exceptions, chromium and titanium, are known for these two elements very small differences in the isotopic composition between carbonaceous chondrites and the Earth were found. Bulk carbonaceous chondrites have isotope anomalies in chromium and titanium. Isotopically unusual material may have been mixed to the CC-source after proto-earth material has accumulated to larger objects. 

Figure 2 Dissolved Cr(VI) (A) and Cr(lll) (B) in the eastern North Pacific Ocean, 23°N, 115°W. (Data from Murray JW, Spell B and Paul B (1983) The contrasting geochemistry of manganese and chromium in the eastern tropical Pacific Ocean. In Wong CS et al. (eds) Trace Metals in Seawater, NATO Conference Services 4 Marine Science vol. 9, pp. 643-668. New York Plenium Press.) (C) Dissolved rhenium in the eastern North Pacific Ocean, 9°46 N, 104°1TW. (Data from Woodhouse OB, Ravizza G, Falkner KK, Statham PJ and Peucker-Ehrenbrink B (1999) Osmium in seawater vertical profiles of concentration and isotopic composition in the eastern Pacific Ocean. Earth and Pianetary Science Letters 173 223-233.)...

Herzog GF, Xue S, Hall GS, Nyquist LE, Shih C-Y, Wiesmann H, Brownlee DE (1999) Isotopic and elemental composition of iron, nickel, and chromium, in t>pe I deep-sea spherules Implications for origin and composition of the parent micrometeoroids. Geochim Cosmochim Acta 63 1443-1457 Holland HD (1994) Early Proterozoic adnospheric change. In Early Life on Eardi, Noble Symposium No 84. 

Isotopic trace studies show that the chromium in the two monomeric species originates in the chromium(n). The rate law for the redox reaction is comparatively simple, giving a transition state with the composition [CrC10aCr +]. ... 

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