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"Applying crystal clocks to metamorphic rocks: Using Li isotopes in garnet to constrain fluid flow processes during subduction metamorphism"

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 and the chemistry of these fluids. Bulk-rock variations in Li isotopic compositions (d⁷Li) are observed in fluid-related features in subduction-related metamorphic rocks at the centimeter-scale suggesting a short duration to fluid infiltration events—weeks to centuries. These measurements record a time-integrated record, while in situ measurements in metamorphic minerals can record individual events experienced by the rock. In this study, in situ traverses across garnet crystals in subduction-related eclogite and amphibolite from the Franciscan Complex, CA and the Monviso Ophiolite, Italy were analyzed for Li isotopes. Observed variations in d⁷Li occur within crystals over a scale of a few hundred microns, including troughs of low d⁷Li measurements in the mantle regions of some of the garnets from both localities. In both localities, variations in d⁷Li are associated with evidence for fluid-rock reaction suggesting a role for fluids fluxing through the slab. The unusual trough pattern suggests that diffusion played a role in generating these features, but the nature of diffusion differs between the two localities. Garnets from the Franciscan experienced intracrystalline diffusion of Li on the scale of a few hundred microns, whereas garnets from Monviso recorded diffusion of Li on a centimeter scale, within an intergranular fluid surrounding the garnets. Multiple troughs in some of the Monviso garnets indicate that episodicity of fluid flow events can be discerned. Variability in profiles from the Franciscan garnets suggest that some aspect of the chemistry of the fluids may be responsible for intracrystalline diffusion; likely candidates are either pH or Eh of the fluid. 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.