Numerous experimental studies have suggested that the mineral phases of the lower mantle has different chemical composition and crystal structure from those of the upper mantle. Therefore, oxygen isotope fractionation can be expected to exist between them. The increment method (Schütze, 1980; Richter and Hoernes, 1988; Zheng, 1991) has been adopted to calculate oxygen isotope fractionation factors for mantle minerals, particularly for the polymorphic phases of (Mg,Ca,Fe)SiO3 and (Mg,Fe)2SiO4. The calculated fractionations involving diopside, enstatite, forsterite, garnet and perovskite (CaTiO3) are in good agreement with known experimental calibrations (Chiba et al., 1989; Gautason et al., 1993; Rosenbaum et al., 1994; Rosenbaum and Mattey, 1995). Therefore, extension of the methodology to the other mantle minerals is potentially valid.
Uncertainty in the resultant fractionation factors is
estimated to be within ±5% of the factor values. This
implies that the calculated fractionations are in the error of 10 ± 0.5 or 1±0.05. Such an uncertainty is small enough to resolve the small d18O differences found in the mantle minerals (about 0.5 to 1.0). Although translating the uncertainty of ±0.05 into reality is presently beyond our analytical capabilities, such a theoretical perspective provides an insight into the "ideal" behavior of oxygen isotopic partitioning among the mantle minerals. Furthermore, the oxygen isotope indices of every solid phases are rigorously govered by the fundamental crystal structure. Thus the I-18O sizes can be well used to quantify the relative sequence of 18O -enrichment in the mantle minerals.
By comparing the size of the I-18O indices obtained, the sequence of 18O -enrichment in the mantle minerals can
be predicted as follows: pyroxene (Mg,Fe,Ca)2Si2O6
> olivine (Mg,Fe)2SiO4 > spinel (Mg,Fe)2SiO4 > ilmenite FeTiO3 > ilmenite (Mg,Fe,Ca)SiO3 > inverse-spinel MgAl2O4 > perovskite CaTiO3 > perovskite (Mg,Fe,Ca)SiO3 > spinel MgAl2O4 > magnisowuestite (Mg,Fe)O. Apparently, the perovskite-structured silicates in the lower mantle and the spinel-structured silicates in the transition zone would fractionate differently with each other than pyroxenes and olivines in the upper mantle. If there would be complete isotopic equilibration in the mantle, the spinel-structured silicates in the transition zone are predicted to be enriched in **O relative to the perovskite-structured silicates in the lower mantle but depleted in **O relative to the pyroxenes and olivines in the upper mantle. Assuming isotopic equilibrium on a whole earth scale, the chemical structure of the mantle can be described by the following sequence of **O -enrichment: upper mantle > transition zone > lower mantle.
Essentially, the oxygen isotope layering of the mantle would result from the differences in the chemical composition and crystal structure of mineral phases at different mantle depths. The physico-chemical differentiation of
the Earth in its formation stage could be the geochemical mechanism by which the mantle would become layered in mineralogical and oxygen isotope compositions. This becomes plausible if the mantle would consist of discrete reservoirs that were mutually in isotopic equilibrium. It is possible that the upper and lower mantles would be in a complete oxygen isotope communication by convection. These reservoirs would interact on such a scale as to develop oxygen isotope equilibrium between the perovskite-
structured and low d**O lower mantle and the olivine-structured and higher d**O upper mantle. The recycling of isotopically **O-enriched crustal materials into mantle depths by plate subduction would only result in local mixing effects with respect to the isotopic zonation in the mantle on a whole earth scale.
Chiba, H., Chacko, T., Clayton, R.N. & Goldsmith, J.R., Geochim. Cosmochim. Acta 53, 2985-2995 (1989).
Gautason, B., Chacko, T. & Muehlenbachs, K., Abstr. Prog. Joint Annual Meeting GAC and MAC, Edmonton, A34 (1993).
Richter, R. & Hoernes, S., Chem. Erde 48, 1-18 (1988).
Rosenbaum, J.M., Kyser, T.K. & Walker, D., Geochim. Cosmochim. Acta 58, 2653-2660 (1994).
Rosenbaum, J.M. & Mattey, D., Geochim. Cosmochim. Acta 59, 2839-2842 (1995).
Schütze, H., Chem. Erde 39, 321-334 (1980).
Zheng, Y.-F., Geochim. Cosmochim. Acta 55, 2299-2307 (1991).