Beyond Equilibrium: Kinetic Thresholds and Rheological Feedbacks Create a Potentially Complex 410 in Slab Regions

The seismic expression of Earth's 410 km discontinuity varies across tectonic settings, from sharp, high-amplitude interfaces to broad transitions—patterns that cannot be explained by equilibrium thermodynamics without invoking large-scale thermal or compositional heterogeneities. Laboratory experiments show the olivine ⇔ wadsleyite phase transition responsible for the 410 is rate-limited, yet previous numerical studies have not directly evaluated the sensitivity of 410 structure to kinetic and rheological factors. Here we investigate these relationships by coupling a grain-scale, interface-controlled olivine ⇔ wadsleyite growth model to compressible simulations of mantle plumes and subducting slabs. We vary kinetic parameters across seven orders of magnitude and quantify the resulting 410 displacements and widths. Our results reveal an asymmetry between hot and cold environments. In plumes, high temperatures produce sharp 410s (2–3 km wide) regardless of kinetics. In slabs, kinetics exert first-order control on 410 structure through three regimes: (1) quasi-equilibrium conditions producing narrow, uplifted 410s and continuous slab descent; (2) intermediate reaction rates generating broader, deeper 410s with metastable olivine wedges resisting downward slab motion; and (3) ultra-sluggish reaction rates causing slab stagnation with re-sharpened, deeply displaced 410s (< 100 km). Rheological contrasts modulate these kinetic effects by controlling slab geometry and residence time in the phase transition zone. These findings demonstrate that reaction rates strongly influence 410 structure in subduction zones, establishing the 410 as a potential seismological constraint on upper mantle kinetic processes, particularly in cold environments where disequilibrium effects are amplified.