Garnet as an archive of fluid flow processes during subduction metamorphism: Evidence from in situ measurement of Li isotopes

Fluids released within subduction zones affect fundamental Earth processes, including seismicity and the generation of arc magmas, the formation of continental crust, and the geochemical evolution of the mantle. However, very little is understood about processes of fluid transport within subduction zones. Bulk-rock variations in Li isotopic compositions (δ⁷Li) are observed in fluid-related features in subduction-related metamorphic rocks at the centimeter-scale suggesting a short duration of fluid infiltration events – weeks to centuries. These measurements capture a time-integrated record, while in situ measurements in metamorphic minerals such as garnet can record individual events experienced by the rock. In our work measuring δ⁷Li in situ in garnet from several exhumed subduction zone metamorphic localities, we have found variations in δ⁷Li occurring within crystals over a scale of a few hundred microns, including troughs of negative values of δ⁷Li. Variations in δ⁷Li are associated with evidence for fluid release and fluid-rock reaction suggesting a role for fluids fluxing through the slab. The negative δ⁷Li excursions suggest that diffusion played a role in the history recorded by these garnets - in some cases garnet experienced intracrystalline diffusion of Li on the scale of a few hundred microns, and in others garnet growth zones incorporated variations of δ⁷Li within the metamorphic fluid surrounding the garnet, caused by diffusion of Li within the intergranular fluid on at least a centimeter scale. Multiple troughs in some of the garnets record the episodicity of fluid flow. Ongoing work is focusing on investigating garnets from a wide range of natural samples to look for patterns in fluid flow episodicity. Additionally, experiments determining the diffusivity of Li within garnet are being performed in order to quantitatively constrain timescales of intracrystalline diffusion.