STOPPING CRYSTAL CLOCKS: THE ROLE OF ELECTRONS IN ARRESTING DIFFUSION OF LITHIUM IN SUBDUCTION ZONE GARNETS

"Crystal clocks" record the duration of a geologic event as a diffusion profile within minerals arrested from reaching equilibrium. This approach, also referred to as geospeedometry, has the potential to reveal timescales of short-lived geologic processes. Interpretation requires a detailed understanding of processes controlling the formation and also preservation of arrested diffusion profiles within minerals. Application to minerals in rocks exhumed from paleo-subduction zones has promise for estimating timescales of the duration of fluid infiltration events within subducting crust, revealing details about processes that ultimately contribute to the generation of arc volcanoes. The classic interpretation of these profiles is interruption due to thermal closure of minerals to diffusion, however in deep crustal settings this interpretation sometimes yields unreasonable timescales. Lithium isotopic measurements of garnets from the Franciscan Complex, CA, a paleo-subduction zone, demonstrate the role of electron activity in facilitating and arresting diffusion profiles in garnet. Profiles across garnets reveal troughs in δ⁷Li ranging to 19‰ lower than cores and rims. In order to create and retain these profiles, diffusive fractionation of Li is required in which charge balance is maintained by electron transfer. Fluids fluxing through the subducting slab provide the source for both Li and electrons. Measurement of δ⁷Li in arrested profiles across garnets provides the potential to constrain the duration of fluid infiltration events during subduction.