Oxygen isotope diffusion and zoning in diopside: the importance of water fugacity during cooling

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The oxygen isotope ratio of diopside correlates with crystal size in many high grade marbles, permitting the intracrystalline self-diffusion rate of oxygen in diopside to be empirically evaluated. Small (75–300 μm) and large (1.2–1.5 mm) diopside grains were analyzed in bulk for their oxygen isotope ratios by laser extraction. In many samples, δ 18O correlates to size; smaller grains have lower δ 18O. Published ion microprobe analyses of diopside in one sample which displays this correlation are in good agreement with predictions based on diffusion modeling and analyses of whole crystals. Other samples analyzed by laser extraction have little or no measurable correlation of δ 18O to size. Investigation using cold cathode luminescence, reflected differential interference contrast microscopy, electron microprobe analysis, and scanning electron microscopy shows that diopside in some of the samples that do not show δ 18O correlation with size were partially dissolved at grain boundaries during cooling, indicating processes in addition to diffusion have affected δ 18O. Cooling histories were calculated using the Fast Grain Boundary diffusion model (Eiler et al., 1992, 1993), assuming equilibrium at peak metamorphic temperatures (700–800°C), slow cooling of 1.5–4°C/Ma, and experimentally determined diffusion coefficients for oxygen in minerals. Predicted δ 18O for diopsides of different sizes are significantly different using diffusion coefficients determined under anhydrous (Pco 2 = 1–100 bars) vs. hydrothermal (P H 2O = 1 kbar) conditions. Samples that show a δ 18O correlation to size are best predicted using the diffusion data measured under hydrothermal conditions. Measurements and calculations to predict differences in δ 18O between large and small diopside grains lead to the following conclusions. (1) Natural diopsides in this study exhibit variations in oxygen isotope ratios between grains of different size, which are related to the peak temperature, cooling rate, and water fugacity during cooling. Diffusion distances are properly modeled by the size of an entire grain; there is no evidence for subdomains. (2) In slowly cooled high grade metamorphic terrains, water fugacity can be highly variable from rock to rock during cooling. For many rocks, water fugacity is the most important constraint on the degree of oxygen isotope retrograde exchange.