Bina, C. R., Comparative buoyancy anomalies from metastable minerals during subduction, Abstracts of the XIII International Workshop on Modelling of Mantle and Lithosphere Dynamics, Hønefoss, Norway, 39, 2013.
Subducting lithospheric slabs encounter two significant families of high-pressure phase transformations as they enter the mantle transition zone: polymorphism of olivine (i.e., the olivine-wadsleyite-ringwoodite transitions) and dissolution of pyroxene into garnet-majorite solid solution (i.e., the eclogite-garnetite transitions). Sluggish reaction kinetics under cold subduction conditions may lead to metastable persistence of lower-pressure phases within the equilibrium stability fields of higher-pressure phases. Metastable low-pressure phases are less dense than corresponding equilibrium high-pressure phases, contributing to diminution of the negative buoyancy driving subduction or to the acquisition of positive buoyancy impeding descent. Consequences may include: lower seismic wavespeeds in the slab, development of down-dip compressive stress fields, slowing of slab descent rates (potentially accompanied by changes in trench retreat), and wholesale flexure of the slab into a subhorizontal "stagnant" posture in the transition zone.
Such kinetic inhibition of equilibrium transformation (e.g., to wadsleyite) has been observed experimentally in olivines. Moreover, transformations in pyroxenes (both disproportionation to a wadsleyite-plus-stishovite assemblage and dissolution into garnet-majorite) have been found to exhibit even greater kinetic inhibition than in olivines. While metastable persistence of olivine appears to be feasible at temperatures of at most 1000 K, the most recent (2013) reports on low diffusion rates in garnet indicate that pyroxene may persist metastably at temperatures up to 1800 K. While metastable pyroxene may potentially persist in subduction zones over greater volumes and to greater depths than metastable olivine, the relative potential contributions to buoyancy anomalies of the two mineral families are complicated by such factors as: intrinsic density contrasts of the respective transitions, relative volumetric proportions of the minerals, differential thermal expansivities of the phases, and possible transition to other metastable phases. Here we attempt to quantify the relative potential contributions of olivine and pyroxene metastability to subduction buoyancy anomalies.