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

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Abstract
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)XAdr and Δ(Wo–CaGrt)=(0.15±0.22)+(0.57±0.33)XAdr. In eclogites formed at T ≤650 °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, Δ(Alm–Di)≈c. 0.2 and Δ(Alm–Grs)≈c. 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.