Andrew J. Cross, Rellie Goddard, Kathryn M Kumamoto, David L Goldsby, Lars Hansen, Haiyan Chen, Diede Hein, Christopher A. Thom, M Adaire Nehring

Phase transformations are widely invoked as a source of rheological weakening during subduction, continental collision, mantle convection, and various other geodynamic phenomena. However, the likelihood and magnitude of such weakening in nature remains undetermined. Here, experiments performed on a synchrotron beamline reveal dramatic weakening across the polymorphic quartz↔coesite phase transition. Under non-hydrostatic conditions, we observe a transient decrease in effective viscosity of up to two orders of magnitude. Such weakening occurs only when the transformation outpaces deformation. We suggest that this behavior is broadly applicable among silicate minerals and examine the likelihood of slab weakening due to the olivine-spinel phase transformation. Our model suggests that cold, wet slabs are most susceptible to transformational weakening, consistent with geophysical observations of slab stagnation beneath the western Pacific.