Li Isotope Zoning in Garnet from Franciscan Eclogite and Amphibolite: The Role of Subduction-Related Fluids

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Fluids released within subduction zones affect many fundamental Earth processes, but very little is understood about processes of fluid transport within subduction zones and the chemistry of the fluids. Centimeter-scale variations in bulk-rock Li isotope compositions in subduction-related metamorphic rocks are observed associated with fluid-related features. The small spatial scales of these variations suggest very short duration fluid infiltration events – on the order of months to centuries. However, bulk-rock measurements record a time-integrated record, while in situ measurements in metamorphic porphyroblasts such as garnet have the potential to record individual events experienced by the rock. In this study, traverses across garnet in Franciscan eclogite and amphibolite were analyzed for Li isotopes in situ using SIMS to determine the potential for inferring the duration of metamorphic fluid flow and for deciphering fluid chemistry. The Li concentrations of these garnet crystals are very low, ∼5 µg/g. The measured range of δ7Li within some Franciscan garnets is 19‰, which falls well outside the uncertainty of the measurement (7Li occur within crystals over a scale of a few hundred microns, with crystal core-to-rim traverses exhibiting troughs of very low δ7Li measurements in the mantle region of the garnet crystals, surrounded by higher values in the garnet crystal rims and cores. Garnet crystal rims have lower δ18O values, which have previously been interpreted as a product of the infiltration of serpentinite-derived fluids. These results suggest a role for fluids fluxing through the slab to create the observed variability, and the unusual trough pattern suggests a role for intracrystalline diffusion. The low δ7Li troughs are observed only in garnet from one of two Franciscan localities. Nevertheless, garnets are similar in composition from the two localities. This contrast suggests that some aspect of the chemistry of the fluids may be responsible for the differences between the two localities; likely candidates are either pH or Eh of the fluid.