Bina, C. R., Phase relations in slabs and plumes: Velocity, buoyancy, seismicity, and convection, Abstracts of the Workshop on the Effect of Plumes and Convection on Surface Tectonics, Earthquake Research Institute, Tokyo University, 1996.
Subduction zone thermal structures yield a variety of stable phase assemblages. In the simple forsterite-90 olivine system, the usual alpha -> alpha+beta -> beta -> beta+gamma -> gamma transition series is uplifted and broadened within the slab interior. It is superseded by an alpha -> alpha+gamma -> beta+gamma -> gamma series within the coldest core of slab, where unusually Fe-rich gamma is stable within the resulting alpha+gamma field. The deeper gamma -> pv+mw reaction is depressed within slab, and all of these phase proportions and compositions vary nonlinearly with depth. In consequence, a region of fast velocity overlies a region of slow velocity, and a region of negative buoyancy overlies a region of positive buoyancy. Static force balance reveals that a maximum in down-dip compressive stress occurs over the depth range of maximum deep seismicity, so that observed patterns of deep seismicity are consistent with slab stress fields independent of particular failure mechanisms.
Such slab behavior may be further complicated by metastable phases. In the case of olivine kinetics, a cold metastable alpha wedge protrudes into the alpha+gamma and beta+gamma stability fields; the presence of metastable alpha is followed by abrupt transition to gamma with no intervening beta field. This metastable wedge yields a local slow velocity anomaly and a local positive buoyancy anomaly, and the attendant lateral buoyancy contrasts yield large shear stress gradients along the boundaries of the wedge. These stresses are consistent with double-planed seismic zones exhibiting opposing polarities, independent of particular failure mechanisms.
The alpha -> alpha+beta -> beta -> beta+gamma -> gamma transition series in mantle olivine deflects downwards within plume thermal models while the deeper gamma -> pv+mw reaction deflects upwards, so that a region of positive buoyancy overlies a region of negative buoyancy in the plume core. In some regions of such a thermal model, the marginal stability of ferromagnesian silicate perovskite (pv) relative to constituent oxides may result in its disproportionation to a mixed-oxide assemblage of magnesiowustite (mw) and stishovite (st). While some thermodynamic parameterizations of st predict such pv breakdown below 1690 km in the mantle (depressed to ~2115 km in the plume core), evidence for a global 1690 km discontinuity is not observed. More recent st data indicate that pv breakdown to mw and st should occur only over a ~565-950 km depth interval and only in cold mantle at the edge of the plume thermal model. Such localized perovskite breakdown would not only generate negative buoyancy anomalies; it would both complicate local seismic structure over this interval and amplify fast velocity anomalies commonly attributed solely to thermal variations.
Such buoyancy anomalies affect convective flow patterns in the mantle. For endmember forsterite-100 olivine, a change in phase relations and Clapeyron slope at moderate temperatures inhibits slab descent but not plume ascent. For forsterite-90 olivine, however, this change in slope occurs at higher temperatures, inhibiting both slab descent and plume ascent. On the other hand, for the pyroxene (px) and garnet (gt) components (i.e., pyrolite minus forsterite-90) a change in phase relations (involving silicate ilmenite) and in slope occurs at lower temperatures, again inhibiting slab descent but not plume ascent. Real mantle behavior must involve a complex interaction of such effects, in addition to other transition zone reactions such as the stabilization of rhombohedral FeO and the exsolution of calcium silicate perovskite.
Copyright © 1996 Craig R. Bina