Isotopic fractionation aqueous equilibria

Constraints on Li isotopic fractionation from equilibrium laboratory experiments in magmatic-hydrothermal systems have been examined in only one instance (Lynton 2003). This study examined Li in a quartz-muscovite-aqueous fluid system at conditions of formation of magmatic hydrothermal porphyry-type deposits (400-500°C, 100 MPa). In the presence of a fluid containing L-SVEC (5T i = 0), quartz showed rapid shift from 5 Li = +27 in the starting material to c. +10 at both run temperatures. Muscovite (initial 5 Li = +9), shifted more sharply at 500°C (to c. +20) than at 400°C (to c. +13). Although the results are difficult to put directly into the context of a natural mineral deposit, they do indicate that over geologically relevant time scales, minerals in magmatic-hydrothermal systems should show appreciable Li isotopic fractionation, and that this may permit the composition and/or temperature of ore... 

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

It is reasonable to expect that isotopic substitution on solvent molecules will affect both equilibrium and rate constants. This is especially true for reactions in aqueous media, many of which are acid or base catalyzed and therefore sensitive to pH or pD. Furthermore H/D aqueous solvent isotope effects often display significant nonlinearity when plotted against isotope fraction of the solvent. The analysis of this effect can yield mechanistic information. The study of aqueous solvent isotope effects is particularly important in enzyme chemistry because enzyme reactions universally occur in aqueous media and are generally pH sensitive. 

Adsorption of Mo to Mn oxyhydroxides produces an isotopic fractionation that appears to follow that of a closed-system equilibrium model as a function of the fraction of Mo adsorbed (Fig. 8). Barling and Anbar (2004) observed that the 5 TVlo values for aqueous Mo (largely the [MoOJ species) were linearly correlated with the fraction (/) of Mo adsorbed (Fig. 8), following the form of Equation (14) above. The 5 Mo-f relations are best explained by a MOaq-Mn oxyhydroxide fractionation of +1.8%o for Mo/ Mo, and this was confirmed through isotopic analysis of three solution-solid pairs (Fig. 8). The data clearly do not lie... 

Skulan JL, Beard BL, Johnson CM (2002) Kinetic and equilibrium Fe isotope fractionation between aqueous Fe(III) and hematite. Geochim Cosmochim Acta 66 2995-3015 Stewart MA, Spivack AJ (2004) The stable-chlorine isotope compositions of natural and anthropogenic materials. Rev Mineral Geochem 55 231-254... 

This review will introduce basic techniques for calculating equilibrium and kinetic stable isotope fractionations in molecules, aqueous complexes, and solid phases, with a particular focus on the thermodynamic approach that has been most commonly applied to studies of equilibrium fractionations of well-studied elements (H, C, N, O, and S) (Urey 1947). Less direct methods for calculating equilibrium fractionations will be discussed briefly, including techniques based on Mossbauer spectroscopy (Polyakov 1997 Polyakov and Mineev 2000). 

The analysis of fractionation law exponents quantifies the impression from the A -5 plots that aqueous Mg is related to primitive mantle and average crustal Mg by kinetic processes while carbonates precipitated from waters approach isotopic equilibrium with aqueous Mg. In any case, the positive A Mg values of carbonates relative to the primitive chondrite/mantle reservoir and crust is a robust feature of the data and requires a component of kinetic Mg isotope fractionation prior to carbonate formation, as illustrated schematically in Figure 3. 

Accordingly, isotopic equilibration for Cr and Se species is expected to be much slower than for the aqueous Fe(III)-Fe(II) couple, which reaches equilibrium within minutes in laboratory experiments (Beard and Johnson 2004). Additionally, Cr(III) and Se(0) are highly insoluble and their residence times in solution are small, which further decreases the likelihood of isotopic equilibration. In the synthesis below, isotopic fractionations are assumed to be kinetically controlled unless otherwise stated. However, definitive assessments of this assumption have not been done, and future studies may find that equilibrium fractionation is attained for some reactions or rmder certain conditions. 

The large Fe isotope fractionations predicted to occur between aqueous ferric and ferrous Fe species (Fig. 3) has been investigated experimentally at two temperatures for hexaquo Fe(III) and Fe(II), as well as the effect of CF substitution (Johnson et al. 2002 Welch et al. 2003). The kinetics of isotopic exchange in these experiments was determined using an enriched Te tracer for the ferric Fe phase. The Fe tracer experiments reported in Johnson et al. (2002) and Welch et al. (2003) indicate that 95% isotopic equilibrium between Fellll) and Fe(II)gq occurs within —60 seconds at room temperature (22°C), or within —5 minutes at 0°C. The relatively slower isotopic exchange rates at lower temperatures are expected. 

Figure 15. Illustration of possible variations in isotopic fractionation between Fe(III),q and ferric oxide/ hydroxide precipitate (Aje(,n),q.Fenicppt) and precipitation rate. Skulan et al. (2002) noted that the kinetic AF (ni)aq-Feiricppt fractionation produced during precipitation of hematite from Fe(III), was linearly related to precipitation rate, which is shown in the dashed curve (precipitation rate plotted on log scale). The most rapid precipitation rate measured by Skulan et al. (2002) is shown in the black circle. The equilibrium Fe(III),-hematite fractionation is near zero at 98°C, and this is plotted (black square) to the left of the break in scale for precipitation rate. Also shown for comparison is the calculated Fe(III),q-ferrihydrite fractionation from the experiments of Bullen et al. (2001) (grey diamond), as discussed in the previous chapter (Chapter lOA Beard and Johnson 2004). The average oxidation-precipitation rates for the APIO experiments of Croal et al. (2004) are also noted, where the overall process is limited by the rate constant ki. As discussed in the text, if the proportion of Fe(III),q is small relative to total aqueous Fe, the rate constant for the precipitation of ferrihydrite from Fe(III), (Ai) will be higher, assuming first-order rate laws, although its value is unknown.

The large isotopic fractionations observed between oxidized and reduced forms of sulfur compounds and aqueous complexes require accurate appraisal of the effective cogeneticity of sulfur minerals utilized as geothermometric couples, and of their equilibrium condition, to avoid erroneous deductions. In fact, besides temperature, the isotopic composition of sulfur minerals is also affected by the bulk isotopic composition of the sulfur in the system (which is controlled... 

Table 4.2 gives current estimates for the relative abundances of the isotopes in the solar system. The isotopic compositions of most elements, especially those that exist as solids, come from measurements of terrestrial materials. Because the Earth has experienced extensive melting and differentiation, it can be considered a homogeneous isotopic reservoir. However, each of the elements can experience both equilibrium and kinetically based isotopic fractionations during igneous, evaporative, and aqueous processes. The range of compositions introduced by such processes is small for most elements and so does not obscure the overall picture. 

The degree of equilibrium isotopic fractionation among phases depends on temperature, so the isotopic compositions of co-existing phases can be used for thermometry. Oxygen is widely used in this way. For example, Clayton and Mayeda (1984) found that the oxygen isotopic compositions of calcite and phyllosilicates from Murchison lie on a mass-dependent fractionation line and differ in 6180 by 22%o. This difference requires a temperature of around 0 °C, which is interpreted to be the temperature of aqueous alteration on the Murchison parent asteroid. Similar measurements for Cl chondrites indicate that aqueous alteration for these meteorites occurred at higher temperature, 50-150 °C (Clayton and Mayeda, 1999). 

Equilibrium isotopic fractions rebect the combined, unidirectional kinetic isotopic fractionations. In considering the one-way buxes as in Equation (12), estimates of the one-way kinetic fracbonations are needed. Knowledge of kinetic effects is obtained from controlled experiment with pure CO2 and water or with salt solutions, but for the chemically complex system that is ocean-water, empirical values are adopted. Eor example, laboratory experiments show that the hydration of aqueous CO2 to bicarbonate involves fractionation of 13%c and the dehydration reaction fractionate by 22%c (O Leary et al., 1992). The difference between these two kinetic fractionations, 9%c, corresponds to the equilibrium fractionation depicted above (Marlier and O Leary, 1984 O Leary et al., 1992). In practice, it was estimated... 

The second step is to establish whether the calcites were deposited in isotopic equilibrium with the waters from which they are derived. During slow degassing, the isotopic fractionation between the aqueous and solid phases is controlled indirectly by cave temperature under equilibrium conditions. However, under an evaporative cave regime or rapid crystallisation, the isotopic fractionation is modulated by kinetic effects which inhibit isotopic equilibrium between the calcite and the drip waters (Hendy, 1971). 

Subareal plants use atmospheric (gaseous) C02 (C02(g)) as their photosynthetic carbon source, which has a mean 813C value of c— 7%o. Subaquatic plants use the dissolved (aqueous) C02 (C02(aq)), which is at one end of the series of equilibria shown in Eqn 3.8. Both these assimilatory processes are accompanied by isotopic fractionations, as discussed in Section 5.8.2. In marine environments there are also C isotopic fractionations associated with the formation of calcium carbonate tests (using bicarbonate) by some organisms that for formation of calcite is different from that for aragonite. The overall fractionation, caicjee co2(aq) s l31 6 and temperature dependent (Fig. 5.55 Mook et al. 1974 Morse Mackenzie 1990), primarily because of the equilibrium between dissolved C02 and bicarbonate... 

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