Water pumping in mantle shear zones: From field observations to experimental evidence
Résumé
Water plays an important role in geological processes. Providing constraints on what may influence the distribution of aqueous fluids is thus crucial to understanding how H2O impacts Earth's geodynamics. In a deep-seated environment, viscous shear zones have been identified as sites of massive fluid circulation, with many implications for ores deposits and rock rheology. However, although seismic pumping, fluid permeation and/or creep cavitation have been proposed as important processes, the source mechanism of such a fluid concentration remains unresolved. In this contribution based on both field and experimental data, we demonstrate that viscous flow exerts a dynamic control on H2O-rich fluid circulation in mantle shear zones. Using the distributions of amphibole and olivine dislocation slip-systems, we first highlight H2O accumulation around fine-grained shear zones in the Ronda peridotite massif (Spain). These observations give rise to a long-term and continuous process of fluid pumping during ductile deformation, which strongly suggest creep cavitation as the driving mechanism. Secondly, we used secondary ion mass spectrometry to document the H2O content of fine-grained olivine across an experimental shear zone. The latter developed with grain size reduction during a H2O-saturated shear experiment at 1.2 GPa and 900 °C. Through data interpolation, the olivine matrix reveals high H2O concentrations where shear strain is localized. These concentrations far exceed the predicted amount of H2O that grain boundaries can contain, excluding diffusive fluid permeation as a unique source of water storage. We also show that the H2O content increases per unit of grain boundary across the shear zone, highlighting an excess volume of H2O that depends on strain and/or strain rate. Based on tensile experiments in metals, we propose that a larger pore volume is produced with increasing strain rate due to competition between creep cavitation and "healing" processes, which include phase nucleation. Altogether, our findings therefore support creep cavitation to occur in mantle shear zones, providing a dynamic process for H2O to be infiltrated and stored in the deep lithosphere.