INTER-MINERAL IRON ISOTOPE FRACTIONATION IN UHP ROCKS: DISEQUILIBRIUM AS A SIGNAL OF HIGH PRESSURE METASOMATISM (Invited Presentation)

Studies measuring inter-mineral iron isotope fractionation in eclogites have reported clear trends for clinopyroxene–garnet pairs, with clinopyroxene always heavier than, or within error of, coexisting garnet (e.g., Williams et al. 2009; Williams and Bizimis 2014). In the case of pyroxene (Fe²⁺ and Fe³⁺ in 6-fold coordination) vs. garnet (Fe²⁺ in 8-fold coordination and Fe³⁺ in 6-fold coordination), we expect pyroxene to concentrate the heavy Fe isotopes preferentially if the two minerals are in equilibrium. This is confirmed by predictions made using the ionic model presented by Young et al. (2009) and modified by Macris et al. (2015) for iron isotopes, incorporating the effect of varying the oxidation state of iron. The degree of Fe isotope fractionation is highly sensitive to the relative amounts of Fe²⁺ and Fe³⁺ in the minerals. According to this model, the equilibrium Fe isotope fractionation between omphacite (Omp) and garnet (Grt) at temperatures associated with UHP metamorphism (~600 °C) will vary with Omp Fe³⁺/∑Fe from ~0.4‰ at the lower end of measured Omp ferric iron content, to ~0.73‰ at the upper end. We measured whole rock and inter-mineral Fe fractionation from a traverse across an eclogitic boudin from the Tso Morari ultrahigh pressure (UHP) terrane. The fractionation measured between Omp and Grt varies with distance from the edge of boudin, with the sample near the center plotting within the range of equilibrium values determined by the ionic model (0.45‰), and the sample closer to the eclogite-gneiss contact plotting outside of the range of equilibrium values (0.08‰). We interpret this change in Δ⁵⁶Fe_Omp-Grt with proximity to the lithologic contact as reflecting an open system process that has perturbed the δ⁵⁶Fe value of Omp, but not Grt, resulting in a disequilibrium fractionation. We hypothesize that the perturbing agent was a low δ⁵⁶Fe slab-derived, high-pressure metasomatic fluid (e.g., Debret et al., 2016), causing spuriously small Omp-Grt fractionation in the sample closer to the eclogite-gneiss contact.

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