Bina, C. R., Temperature dependence of mineralogical phase relations in the mantle: Some implications for plumes and slabs, Abstracts of Superplume International Workshop, Wako-shi, Japan, 177-180, 1999.
Thermal perturbation of mantle phase relations in plumes and slabs yields variations in densities and seismic velocities. The density anomalies result in buoyancy anomalies, which affect both stress fields and convective motions in the mantle. The seismic velocity anomalies may involve local perturbations to both temperature and phase assemblage, thus potentially complicating their interpretation in terms of simple thermal anomalies. The latent heats associated with equilibrium or disequilibrium phase transitions also serve to perturb local thermal structure. Metastable persistence of gamma in an ascending plume would, upon eventual transformation, yield local superheating above the background adiabat.
Equilibrium mineralogical phase relations in the mantle vary considerably from the low temperatures characteristic of subduction zones, through ``average'' mantle temperatures, to the high temperatures presumably associated with mantle plumes. The sequence of significant phase stability fields with increasing depth, in average mantle, for the olivine (alpha, alpha+beta, beta, beta+gamma, gamma, pv+mw) and pyroxene/garnet (opx+cpx+gt, gt, gt+pv, pv) fractions of a peridotite bulk composition are perturbed by thermal variations. Thus, in a cold slab for example, the exothermic alpha -> beta and beta -> gamma transitions are deflected to shallower depths while the endothermic gamma -> pv+mw transition is deflected to greater depths. In a hot rising plume, on the other hand, the exothermic pv+mw -> gamma reaction is deflected upwards, while the endothermic gamma -> beta and beta -> alpha transformations are deflected downwards. Such variations, however, are not strictly symmetric between high- and low-temperature regimes, because changes in the stable mineral assemblages themselves may accompany thermal deflection of phase relations [Bina and Liu, 1995]. In cold slabs, for example, a stability field for alpha+gamma intervenes between those of alpha and alpha+beta, with possibly a small zone of mw+st stability between the gamma and pv+mw fields [Akaogi et al., 1998], and an ilm stability field intervenes between the gt and pv fields [Vacher et al., 1998]. In hot plumes, on the other hand, the beta+gamma field may grow less significant, and there are no alpha+gamma, mw+st, or ilm stability fields. Additional high-pressure transitions in oxides also may affect phase relations deeper in the lower mantle [Bina and Hemley, 1997; Bina, 1998a]. Furthermore, phases may persist metastably beyond their stability fields.
Thus, spatial variations in thermal structure, such as those between slab and plume regions, result in significant variations in the equilibrium proportions and compositions of mantle minerals, yielding variations in both density and seismic velocities. The density variations yield buoyancy anomalies which contribute to local stress fields in a manner consistent with patterns of deep seismicity [Bina, 1996, 1997; Yoshioka et al., 1997; Okal and Bina, 1998] and which affect convective dynamics by influencing, for example, velocities of subducting slabs [Marton et al., 1998; Schmeling et al., 1998]. Such density variations also may perturb the regional geoid. The seismic velocity variations, on the other hand, may result in detectable waveguides or less resolvable anti-waveguides, depending upon their sign [Koper et al., 1998; Bina, 1998b]. Moreover, such velocity variations may be due to two superposed effects, thus complicating interpretation of velocity anomalies in terms of purely thermal anomalies. Thermal anomalies contribute directly to such velocity variations via the temperature dependence of mineral elastic moduli. However, thermal perturbation of phase relations may make additional contributions (of roughly equal magnitude) to such velocity variations via the introduction of anomalous phase assemblages [Bina, 1998b].
The perturbation of phase relations by spatial variations in thermal structure in the mantle are also closely tied to the latent heats of phase transformation. In addition to the equilibrium temperature variations arising from simple refraction of adiabatic geotherms along phase boundaries, any metastable persistence of phases introduces additional latent-heat effects. In subducting slabs, for example, the low-temperature environment should allow low-pressure phases to persist metastably into high-pressure stability fields, yielding local temperature perturbations upon eventual disequilibrium transformation [Daessler et al., 1996; Karato, 1997; Bina, 1998c]. Delayed transformation of metastable alpha in a slab, for example, would yield local superheating [Bina, 1998c]. In ascending plumes, on the other hand, it seems that rapid ascent of crystallized phases may permit high-pressure phases to persist metastably into low-pressure stability fields, despite locally high temperatures. Inclusions of coexisting enstatite (MgSiO3) and periclase (MgO) within diamonds [Harte and Harris, 1994; Harte et al., 1994], for example, suggest that a stable lower mantle assemblage of mw-MgO and pv-MgSiO3 may have persisted metastably to shallower levels in the mantle, rather than reacting to form stable gamma-Mg2SiO4, before the perovskite reverted to enstatite at lower pressures. Coupled with the negative buoyancy anomaly of such a metastable assemblage, eventual disequilibrium transformation of the metastable pv+mw would also yield local superheating.