The Dense MORB Fraction Controls the Dynamic Stability and Hydration Potential of the Mantle Transition Zone

Abstract The mantle transition zone (MTZ; ∼410–660 km depth) is a major reservoir of water and mid‐ocean ridge basalt (MORB)‐like materials, yet how composition and hydration regulate its physical stability remains unclear. Using geodynamical modeling, we examine the joint effect of water content and dense MORB fraction on the density, viscosity, and dynamics of the MTZ. We find that the MTZ remains dynamically stable only within a narrow range of MORB‐to‐water ratios; outside this window, buoyancy‐driven instabilities and dehydration occur. Combining this stability constraint with global MORB fraction estimates, we evaluate the theoretical hydration potential of the MTZ, which is strongly laterally heterogeneous and controlled not only by intrinsic mineralogical capacity but also by dynamic stability. Our results align with observed water distributions and suggest that most of the MTZ resides in stable or slowly dehydrating regimes, supporting a sustained deep water cycle and long‐term lithospheric stability.