[S-IT17] Mass and energy transport in the crust and mantle: from properties to processes

[SIT17-02] Metasomatic mechanisms within an exhuming eclogite–serpentinite mélange: Petrological inspection of Guatemalan lawsonite eclogites

*Ryo Fukushima¹, Tatsuki Tsujimori¹, Jesse B. Walters^2,3, Dominik C. Hezel², Richard Albert², Axel Gerdes², Horst R. Marschall² (1.Tohoku University, 2.Goethe University Frankfurt am Main, 3.University of Graz)

Keywords:metasomatism, serpentinite mélange, lawsonite eclogite, garnet, omphacite, phengite

Understanding mechanisms of metasomatism at subarc depths is crucial to constrain chemical compositions of slab-derived aqueous fluid based on natural high-pressure (HP) metamorphic rocks. Since eclogitized oceanic crust occasionally undergoes HP metasomatism when exhumed together with portions of serpentinized mantle wedge, it is invaluable to investigate the metasomatic histories of exhumed oceanic eclogite. Here, we examined Guatemalan oceanic eclogites (lawsonite-eclogite facies metabasaltic rocks) with petrological observations and in situ analyses of trace-element concentrations and boron-isotope compositions. The investigated eclogite samples (BCG-15, BCG-12, BCG-03-E) were taken from boulders that occur in a serpentinite matrix. They are characterized by garnet porphyroblasts with distinct rims that were formed during exhumation. Garnet rims in BCG-15 and BCG-12 have multiple annuli with high Mn concentrations (or oscillatory zoning), whereas those in BCG-03-E only have high-Mn edges with dissolution textures. In BCG-15, both the metasomatic replacement of garnet by omphacite and the growth of oscillatory-zoned garnet rims are observed. The selective uptake of diffusive cations such as Na and Li suggests that diffusion-controlled metasomatism occurred in this sample. BCG-12 is characterized by the ubiquitous replacement of omphacite by garnet + phengite. The rock texture, the strong enrichment of fluid-mobile elements in oscillatory-zoned garnet rims, and a disequilibrium boron isotope fractionation between phengite and omphacite are evidence for intensive infiltration metasomatism with K–Al-enriched hydrous fluids passing through the serpentinite into the eclogite boulder. Garnet rim compositions in BCG-03-E suggests the presence of modest infiltration metasomatism with fluids with similar compositions as in BCG-12. We propose that textural variation in metasomatized eclogites from this locality was brought mainly by differences in strength and frequency of the fluid infiltration events, rather than by compositional difference in the fluid.