Effects of cation substitutions in garnet and pyroxene on equilibrium oxygen isotope fractionations

High-grade metamorphic rocks were used to explore oxygen isotope fractionations between pyroxene and garnet, and to investigate the effects on fractionation factors of the cation substitutions Fe³⁺ Al_-1 and Ca(Fe,Mg)_-1. Recrystallized, granulite facies (725 °C) wollastonite ores from the northern Adirondack highlands contain essentially only the minerals clinopyroxene (a Di-Hd solid solution)+garnet (a Grs-Adr solid solution) ±wollastonite, and exhibit a systematic dependence of measured fractionations on the Fe³⁺ content of calcic garnet: Δ(Cpx-CaGrt)=(0.14±0.12)+(0.78±0.20)X_Adr and Δ(Wo-CaGrt)=(0.15±0.22)+(0.57±0.33)X_Adr. In ecologites formed at Tκ650 °C, measured compositions of Ca-poor garnet and omphacite combined with experimental data indicate that Ca-poor, Fe-rich garnet is enriched in ¹⁸O compared to both diopside and grossular: extrapolating to 1000 K, A(Alm-Di)A c. 0.2± and Δ(Alm-Grs)Ac.0.5‰. Orthopyroxene and clinopyroxene from gore Mountain, New York, show a constant fractionation that is independent of rock type, as expected if they have the same closure temperature. These data imply Δ(Opx-Cpx)~c.0 .7‰ at 1000 K. Measured fractionations among Ca-poor garnet, orthopyroxene, clinopyroxene and hornblende in the Gore Mountain rocks further indicate an ¹⁸O enrichment in Ca-poor garnet over Grs (~c.0.5‰ at 1000 K). The new measurements are indistinguishable from expected equilibrium values based on experiments for the minerals enstatite, diopside, grossular, wollastonite and feldspar, but consistently indicate a significant isotope effect for the simple octahedral cation substitutions Fe³⁺ Al_-1 (Grs vs. Adr) and Ca(Fe,Mg)_-1 (Ca-poor garnet vs. Grs; Opx vs. Cpx). Neither cation substitution has been directly investigated for its effect on ¹⁸O/¹⁶O fractionation with experiments in silicates. Chemical characterization of minerals is required prior to petrological interpretation of oxygen isotope trends.