Transient and periodic brittle deformation of eclogites during intermediate-depth subduction

Brecciated eclogite clasts from the Monviso ophiolite in the Western Alps fossil subduction zone preserve healed fractures within garnet crystals. These fractures are divided into two distinct populations based on chemical and textural criteria. Diffusion modeling on profiles across fractures identifies a characteristic diffusive timescale for each population, with a mean calculated difference between population timescales of ∼2 Myr. We interpret this relative difference in diffusional timescales to represent the maximum period between individual fracturing events during prograde subduction at eclogite facies P–T conditions. The period between the two distinct fracturing events is likely controlled by transient fluid pressure and/or strain rate increases during prograde subduction. An increase in pore fluid pressure may be related to the dehydration of adjacent portions of the down-going plate during prograde subduction, while an increase in strain rate may be related to seismicity at shallower depths in the subducting slab. The absolute duration for diffusion, as constrained by diffusion modeling, implies extremely short timescales near peak P–T conditions. This suggests that the subducting slab potentially experienced multiple periods of seismic activity immediately prior to detachment and subsequent exhumation from intermediate depths. The presence of similar healed fractures in garnets from eclogites is relatively common in the rock record, implying that similar transient and periodic processes may be relatively common during subduction worldwide and through time.