Bina, C. R., F. C. Marton, S. Stein, and D. C. Rubie, Phase relations in deep subduction zones: Buoyancy forces and latent heats, Terra Nostra, Abstracts of the Alfred Wegener Conference on Processes and Consequences of Deep Subduction, Verbania, Italy, 99/7, 11-12, 1999.
Mantle phase relations are significantly perturbed by the thermal structure of subduction zones. In mantle olivine of (Mg0.9Fe0.1)2SiO4 composition, for example, high-pressure phase transitions between olivine (alpha), wadsleyite (beta), ringwoodite (gamma), ferromagnesian silicate perovskite (pv), and magnesiowüstite (mw) are affected by the thermal environment of the slab. Low temperatures result in upward deflection of the equilibrium alpha -> alpha + beta -> beta -> beta + gamma -> gamma transitions to shallower depths, with their eventual replacement by the alpha -> alpha + gamma -> beta + gamma -> gamma transition series at very low temperatures. The higher-pressure gamma -> gamma + pv + mw -> pv + mw transition, on the other hand, is deflected downward to greater depths. Furthermore, low-temperature disequilibrium may result in metastable persistence of lower pressure phases, such as alpha, into the nominal stability fields of higher pressure phases.
The consequent lateral juxtaposition of higher- and lower-density phase assemblages results in both negative buoyancy anomalies, in the shallower reaches of the transition zone, and positive buoyancy anomalies, near the base of the transition zone, all superimposed upon the slab's negative thermal buoyancy [e.g., Bina, 1998a]. Any metastably persisting material contributes additional local positive buoyancy anomalies. The pattern of stresses arising from these buoyancy forces is largely consistent with both the depth-distribution of seismicity and the principal stress axes of deep earthquakes [Bina, 1996, 1997; Yoshioka et al., 1997; Okal & Bina, 1998]. This suggests that observed patterns of deep seismicity may primarily reflect the state of stress in the subducting slab and that buoyancy forces may contribute significantly to this stress field.
Such buoyancy anomalies may also affect subduction velocities, to the extent that the descent rate of a slab is a balance between buoyancy forces and viscous drag. Thermal perturbation of equilibrium phase relations should cause slabs to accelerate upon entering the transition zone, due to interaction with uplifted alpha-beta-gamma phase transitions, and to decelerate upon reaching the base of the transition zone, due to interaction with depressed gamma-pv-mw phase transitions, thus providing a potential mechanism for geologically abrupt changes in plate motions [Marton et al., 1999]. Moreover, metastable persistence of lower pressure phases may provide a negative feedback mechanism for regulating lithospheric subduction rates. While a low-density metastable wedge would reduce the magnitude of a slab's net negative buoyancy, the resulting slower descent rate would allow the slab to warm, thus thermally eroding the metastable wedge. Faster and hence colder slabs should be slowed more greatly, thereby narrowing the range of feasible subduction rates [Kirby et al., 1996; Marton et al., 1999; Schmeling et al., 1999]. Indeed, buoyant metastable material in the coldest slabs may be sufficient to inhibit slab penetration into the lower mantle until further thermal equilibration has occurred [Okal & Kirby, 1998; Bina & Kirby, 1999].
Metastable persistence of lower pressure phases in cold slab interiors may also amplify the thermal effects of latent heats of transformation. While latent heats of equilibrium transformations reversibly perturb temperatures in an adiabatically subducting slab, those of metastable transformations yield irreversible isobaric temperature changes. Hence, latent heat release by metastable exothermic transformations can yield local superheating above the background adiabat, and the degree of potential superheating increases with the extent of metastable overstep. Such local temperature increases may help to trigger seismic release of accumulated strain energy through generation of shear instability [Green & Zhou, 1996; Karato, 1997; Bina, 1998b]. Such instability could be instigated, for example, by progressive shear localization in material exhibiting temperature-dependent rheology [Karato, 1997; Regenauer-Lieb & Yuen, 1998].