1996 IRIS Abstract

Bina, C. R., Some geophysical implications of phase relations in the transition zone, Abstracts of the Eighth Annual Workshop of the Incorporated Research Institutions for Seismology, Blaine, Washington, 1996.

The complexities of phase relations in the transition zone may be largely responsible for a variety of geophysical phenomena. Within subduction zones, both for the case of equilibrium phase relations and for that of metastable persistence of olivine at low temperatures, the buoyancy anomalies associated with thermal perturbation of mantle phase relations contribute to the state of stress in the slab in a fashion consistent with observed deep seismicity. The thermal fields associated with mantle plumes may give rise to local velocity anomalies arising from local phase destabilization. In a convecting mantle, the various components of flow will encounter different phase boundaries, and hence different impediments to passage, depending upon both their temperature and compositional structures. Several important transformations occur within the region between the major seismic velocity discontinuities at ~410 and ~660 km depth which may be expected to contribute to seismic velocity features in the ~520-560 km depth range and elsewhere.

Subduction zone thermal structures yield an interesting variety of stable phase assemblages. Even in the simple (Mg_0.9Fe_0.1)₂SiO₄ olivine (Fo₉₀) system, the equilibrium phase relations reveal several important features. The usual alpha -> alpha + beta -> beta -> beta + gamma -> gamma transition series is uplifted and broadened within the slab interior, and it is replaced by a alpha -> alpha + gamma -> beta + gamma -> gamma series within the coldest core of slab. Unusually Fe₂SiO₄-rich gamma is stabilized within the resulting alpha + gamma field. The deeper gamma -> pv + mw reaction is depressed within slab. These phase proportions and compositions vary nonlinearly across phase transitions. In consequence of these perturbations, a region of fast velocity overlies a region of slow velocity and a region of negative buoyancy overlies a region of positive buoyancy within the slab. The down-dip static force balance reveals that a maximum in compressive stress occurs over the depth range of maximum deep seismicity. Thus, observed patterns of deep seismicity are consistent with the stress field, independent of particular failure mechanisms.

Subduction zone thermal structures may also be complicated by metastable phase assemblages. Consideration of (Mg_0.9Fe_0.1)₂SiO₄ olivine (Fo₉₀) phase relations incorporating olivine metastability reveals additional features. A metastable alpha wedge protrudes into the alpha + gamma and beta + gamma stability fields, and the presence of metastable alpha is followed by abrupt transition to gamma with no intervening beta field. The metastable wedge yields a local slow velocity anomaly and a local positive buoyancy anomaly. The attendant lateral buoyancy contrasts yield large shear stress gradients along the boundaries of the wedge. Thus, wedge-associated stresses are consistent with double-planed seismic zones exhibiting opposing polarities, independent of particular failure mechanisms.

Stable phase assemblages in the (Mg_0.9Fe_0.1)₂SiO₄ olivine (Fo₉₀) system for mantle plume thermal models also give rise to intriguing structures. The alpha -> alpha + beta -> beta -> beta + gamma -> gamma mantle transition series deflects downwards within the plume, and 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. Perhaps more interestingly, ferromagnesian silicate perovskite is only marginally stable relative to constituent oxides under certain conditions of pressure and temperature. While some sets of relevant thermodynamic parameters, particularly for stishovite, predict disproportionation of (Mg,Fe)SiO₃ perovskite (pv) to an assemblage of (Mg,Fe)O magnesiowüstite (mw) and SiO₂ stishovite (st) at depths greater than 1690 km in mantle (depressed to ~2115 km in the plume core), the effects of such a global perovskite breakdown at 1690 km are not observed. More recent analysis of stishovite behavior suggests that silicate perovskite breakdown to magnesiowüstite and stishovite 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 serve to amplify fast velocity anomalies commonly attributed solely to thermal variations.

The buoyancy contrasts attending thermal perturbation of phase relations must significantly affect convective flow in the mantle. For endmember Mg₂SiO₄ olivine (Fo), a change in phase relations and hence in Clapeyron slope at moderate temperatures inhibits slab descent but not plume ascent. However, for (Mg_0.9Fe_0.1)₂SiO₄ olivine (Fo₉₀) 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 (e.g., pyrolite minus Fo₉₀) 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 these effects.

The transition zone is a busy place. Between the major seismic velocity discontinuities at ~410 km depth, largely due to the alpha -> beta transition, and at ~660 km depth, largely due to the gamma -> pv + mw and gt -> pv transitions, several other important reactions take place. The beta -> beta + gamma -> gamma transition occurs in this depth range, over a somewhat broader interval than the transitions associated with the major discontinuities. Similarly, the family of px -> gt majorite-forming transitions attain completion in this interval; while they are generally quite broad, the nonlinear variation of phase proportions may render their completion somewhat sharper. The exsolution of CaSiO₃ perovskite from garnet also occurs in this depth interval prior to ~600 km, with recent work suggesting a rapid onset as shallow as 520 km. Furthermore, the stable form of FeO changes from the NaCl structure to a rhombohedral structure at roughly 480 km depth, a fact which may shift the extent of (Mg,Fe)O solid solution and the relative stability of (Mg,Fe)SiO₃ perovskite at depths below this transition. For certain compositions in cold regions, silicate ilmenite may briefly become stable below ~600 km. Several of these reactions may contribute to reported ~520-560 km seismic velocity features. Changes in the ability of mantle phase assemblages to host water may accompany such phase transformations. Finally, localized (Mg,Fe)SiO₃ perovskite destabilization in roughly the ~565-950 km depth interval, in addition to amplifying the velocity anomalies to be expected from thermal perturbations, may contribute to local seismic structure reported in both the ~520-560 km and ~900-940 km depth ranges.