Silicates oxygen isotopic fractionation

Zheng Y. F. (1993a). Calculation of oxygen isotope fractionation in anhydrous silicate minerals. Geochim. Cosmochim. Acta, 57 1079-1091. 

Isotope fractionations in solids depend on the nature of the bonds between atoms of an element and the nearest atoms in the crystal structure (O Neil 1986). The correlation between bond strength and oxygen isotope fractionation was investigated by Schiitze (1980), who developed an increment method for predicting oxygen isotope fractionations in silicate minerals. Richter and Hoemes (1988) applied this method to the calculation of oxygen isotope fractionations between silicate minerals and... 

Compositional effects in silicates. Compositional effects are also important in the oxygen isotope fractionation behavior of silicate minerals. The effects are complex, however, because of the large number of substitution mechanisms that operate in these minerals. Table 3 summarizes some of the major substitutions and their estimated effects on isotope fractionation (cf Kohn and Valley 1998a). Of the common substitution schemes, the plagioclase substitution has the largest isotopic effect. Experimental data (O Neil and Taylor 1967 Matsuhisa et al. 1979 Clayton et al. 1989) and theoretical calculations (Kieffer 1982) indicate a 1.05 to 1.2%o fractionation between albite and anorthite at 1000 K. This fractionation reflects the higher Si to Al ratio of albite, and the affinity of the Si-0 bond relative to the Al-0 bond for 0 (Taylor and Epstein 1962). In contrast to plagioclase, K Na substitution in the alkali feldspars, which does not affect the Si to Al ratio, has no measurable isotopic effect (Schwarcz 1966 O Neil and Taylor... 

Table 3. Effect of compositional substitutions on oxygen isotope fractionation in silicates.

The generalizations that stem from the above observations are similar to the ones made long ago by Taylor and Epstein (1962). Namely, the dominant compositional variable affecting oxygen isotope fractionations between silicates is the identity of the tetrahedral cation. With the exception of Al, the octahedral and cubic (8-fold) cations are of secondary importance, although, as shown in Table 3, not insignificant in all cases. Monovalent cations have little effect on oxygen isotope fractionations. 

Smyth JR, Clayton RN (1988) Correlation of oxygen isotope fractionations and electrostatic site potentials in silicates. EOS Trans Am Geophys Uition 69 1514... 

Temperature gradients can cause thermal diffusion (Soret effect), which has been measured by Kyser et al. (1998) above the solvus in silicate liquids that are immiscible at lower temperatures. Additional isothermal oxygen diffusion experiments were performed below the solvus in the immiscible liquids and the results from the two kinds of experiments were compared. Although the magnitude and direction of oxygen isotope fractionation was found to be different from that expected, the authors conclude that this process is unlikely to play a significant role in natural processes such as mantle metasomatism. 

Zheng Y-F (1993) Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates. Earth Planet Sci Letters 120 247-263... 

Patel A, Price GD, Mendelssohn MJ (1991) A computer simulation approach to modeling the structure, thermodynamics and oxygen isotope equilibria of silicates. Phys Chem Min 17 690-699 Polyakov VB (1997) Equilibrium fractionation of the iron isotopes estimation from Mossbauer spectroscopy data. Geochim Cosmochim Acta 61 4213-4217... 

The theory developed for perfect gases could be extended to solids, if the partition functions of crystals could be expressed in terms of a set of vibrational frequencies that correspond to its various fundamental modes of vibration (O Neil 1986). By estimating thermodynamic properties from elastic, structural, and spectroscopic data, Kieffer (1982) and subsequently Clayton and Kieffer (1991) calculated oxygen isotope partition function ratios and from these calculations derived a set of fractionation factors for silicate minerals. The calculations have no inherent temperature limitations and can be applied to any phase for which adequate spectroscopic and mechanical data are available. They are, however, limited in accuracy as a consequence of the approximations needed to carry out the calculations and the limited accuracy of the spectroscopic data. 

Muehlenbachs K, Byerly G (1982) 0 enrichment of silicic magmas caused by crystal fractionation at the Galapagos Spreading Center. Contr Miner Petrol 79 76-79 Muehlenbachs K, Qayton RN (1972) Oxygen isotope studies of fresh and weathered submarine basalts. Can J Earth Sci 9 471 79... 

Figure 4.9 Schematic illustration displaying the observed oxygen isotopic composition of silicate and that expected via reaction with ozone in a three-isotope plot. Oxidation was postulated as a two-step fractionation process (Kimura et al. 2007) as shown in the diagram.

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