Isotopic fractionation, disequilibrium

Figure 14. Inter-mineral Fe isotope fractionations among olivine and clinopyroxene from spinel peridotite mantle xenoliths. Data are from Zhu et al. (2002) ( ) and Beard and Johnson (2004) ( ). In the study by Beard and Johnson (2004), the difference in the Fe isotope composition between clinopyroxene and olivine is larger as a function of their 5 Fe values, suggesting disequilibrium fractionation.

Sulfate-sulfide isotopic exchange disequilibrium (where sulfate is reduced to sulfide without isotopic exchange but sulfide minerals and H2S isotopically equilibrate) in closed and open isotopic systems result in significantly smaller shifts in isotopic compositions (Figure 1). In addition, the calculated shifts are to higher sulfide compositions, generally consistent with the measured values for seafloor sulfide minerals. Analyses of the natural samples, however, provide no evidence of systematic isotopic differences between pyrite, sphalerite, and chalcopyrite, as predicted by both standard fractionation approaches and these calculated isotopic reaction pathways, possibly due to dynamic variations of mixing and precipitation on a local scale. 

Figure 17. Carbon isotope fractionations as a function of metamorphic grade. (17A) Compilation of measured values of A(Cc-Gr) shows scatter and disequilibrium at greenschist facies and lower temperature conditions. Many amphibohte facies and most granuhte facies samples show a tight clustering of values consistent with isotope equilibration above 600°C. Values in black are from the Adirondacks (from Kitchen and Valley 1995). (17B) Values of 5 C for a Liassic black shale formation (Hoefs and Frey 1976) showing successive approach to equihbiium at maximum T = 500-600°C (from Sharp et al. 1995).

In a simple chemical apparatus, isotopic fractionation occurs by two mechanisms. The first is isotopic disequilibrium, in which the mass difference between the isotopes causes a significant difference between the equilibrium constants or rates of reactions in which the two isotopic species are involved. The magnitude of the isotopic fractionation is greater for traced elements in which there is a large fractional difference between the mass number of the tracer and that of the traced element this effect is considered neghgible for isotopes of aft elements with atomic numbers >10 (Duncan and Cook 1968). 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

Most of the reactions that involve significant fractionation of Se and Cr isotopes appear to be far from chemical or isotopic equihbrium at earth-surface temperatures. Redox disequilibrium is common among dissolved Se species. Dissolved Se(IV) and solid Se(0) are often observed in oxic waters despite their chemical instability (Tokunaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGrath 1998). In one study of shallow groundwater, Se species were found to be out of equilibrium with other redox couples such as Fe(III)/Fe(II) (White and Dubrovsky 1994). The kinetics of abiotic Se(VI) reduction, like those of sulfate, are quite slow. In the laboratory, conversion of Se(VI) to Se(IV) requires one hour of heating to ca. 100°C in a 4 M HCl medium. 

At equilibrium, the activity ratio between any two members of a decay series is 1.00. However, at and near the Earth s surface, disequilibrium of the various nuclides of the uranium series is found to occur. The disequilibrium is especially pronounced in groundwaters (Cherdyntsev, 1971 Osmond and Cowart, 1976). The fractionation of the nuclides can occur as a result of chemical differences between elements, the fractionation of isotopes of a given element may occur because of preferential leaching of one (because of its radiogenic origin), or by the direct action of recoil during radioactive decay (Osmond and Cowart, 1976). 

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