Cyclic fluid flow in a high-pressure shear zone: Insights from Li isotope zoning in garnet

Abstract:

Models of subduction zone seismicity often invoke cycles of fluid overpressure and escape. Bulk Li speedometry in high-pressure (HP) metamorphic rocks produces short time-integrated durations of fluid flow but cannot be resolved into the individual fluid pulses proposed by seismic models. New methodological developments (e.g. Penniston-Dorland et al., 2020) allow Li isotopes to be measured in situ in zoned minerals potentially resolving individual fluid pulses to test seismic models. Here we present evidence for multiple fluid infiltration events using SIMS measurements of Li isotopes in eclogitic garnet from a shear zone within the Monviso ophiolite. All analyzed garnets show core-rim zoning in δ⁷Li, and strong negative excursions (NEs) in their mantle zones. These NEs cannot be explained by instrumental mass fractionation during analysis or equilibrium fractionation at eclogite facies. Instead, NEs could only be produced by diffusive fractionation of Li isotopes occurring: 1) within the garnet or 2) in the surrounding matrix and passively incorporated during garnet growth. The NEs are not found at consistent distances from garnet rims as would be expected for intra-crystalline diffusion, but rather are associated with asymmetrical oscillatory or annular major element zoning. This suggests that NEs were produced by bulk diffusive fractionation of Li in the matrix, driven by fluid infiltration along the shear zone. In this scenario, these transient NEs were passively incorporated into the garnet during syn-diffusion growth. The presence of two NEs in one garnet mantle require at least two distinct fluid pulses and associated diffusion. Bulk Li diffusion in HP metamorphic rocks occurs on timescales less than hundreds of years suggesting that the NEs in these rocks reflect cycles of rapid fluid flow and garnet growth. The association of NEs with oscillatory and incipient skeletal zoning indicative of disequilibrium growth supports this conclusion. In the future, evidence for cyclic fluid flow from in situ δ⁷Li measurements will be paired with bulk Li diffusion modeling to resolve the timescales of individual fluid pulses from time-integrated durations of fluid flow.