Li isotope zoning in garnet from Franciscan eclogite and amphibolite: The role of subduction-related fluids

Abstract:

Very little is understood about processes of fluid transport within subduction zones and the chemistry of the fluids. Bulk-rock variations in Li isotopic compositions are observed in fluid-related features at the centimeter-scale in subduction-related metamorphic rocks. The small scales of these variations suggest very short duration fluid infiltration events – on the order of months to centuries. However, these measurements record a time-integrated record, while in situ measurements in 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 crystals is up to 19‰, well outside the uncertainty of the measurement (<~4‰ 2s). Observed variations in δ⁷Li occur within crystals over a scale of a few hundred microns, with core-to-rim traverses exhibiting troughs of very low δ⁷Li measurements in the mantle region of the garnet crystals, surrounded by higher values in rims and cores. Garnet crystal rims have lower δ¹⁸O values, previously interpreted as a product of the infiltration of serpentinite-derived fluids. These results combined suggest a role for fluids fluxing through the slab to create the observed isotopic variability, and the unusual trough pattern suggests a role for intracrystalline diffusion and associated isotopic fractionation. The low d⁷Li troughs are observed only in garnet from one of two Franciscan localities. Similarities in garnet major-element composition from the two localities 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.