T87 Non-Traditional Stable Isotope Fractionation at Extreme Conditions: In Honor of Anat Shahar, 2016 MSA Awardee

Monday, 26 September 2016
8:00 AM - 9:45 AM
Room 404

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

Paper number: T87-2
Program: Technical Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

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 Δ56FeOmp-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.

Debret et al., Geology 44, 215 (2016)

Macris et al., Geochim. Cosmochim. Atca 154, 168 (2015)

Williams et al., Earth Planet. Sci. Lett. 283, 156 (2009)

Williams & Bizimis, Earth Planet. Sci. Lett. 404, 396 (2014)

Young et al., Earth Planet. Sci. Lett. 288, 524 (2009)

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-283501

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