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

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.