The depths, widths, and magnitudes of the 410‐km and 660‐km seismic discontinuities are largely consistent with an isochemical phase change origin, as is the observation that the topography on these discontinuities is negatively correlated and significantly smaller than predicted for chemical changes. While most thermodynamic studies of the relevant phase changes predict greater topography on the 410 than the 660, recent seismic studies suggest greater topography on the 660. The seismic results are consistent with some recent thermochemical studies which suggest that the Clapeyron slopes of the perovskite‐forming reactions exceed in magnitude those of the spinel‐forming reactions; however, we have reexamined the relevant Clapeyron slopes in light of other, more recent, experimental studies as well as the requirements of internal thermodynamic consistency. We conclude that the bulk of the evidence indicates a greater Clapeyron slope magnitude for the 410 than for the 660. Thus the recent seismic results are unexpected. One explanation might be that lateral temperature variations near 660 km depth exceed those near 410, consistent with a model of the 660 as a thermal boundary layer. An alternate interpretation, which requires neither a thermal boundary nor metastable olivine, is that the 410 does possess greater topography but is simply less visible seismically than the 660. This latter idea, and recent short‐period observations of P’410P’ seismic phases in conjunction with an elevated 660, is consistent with thermodynamic modeling of subduction zones illustrating the extreme broadening of the olivine α → β transition in cold slab interiors and, conversely, its sharpening in regions of high temperature.
The sequence of high-pressure phase transitions a-•/•-•ff in olivine is traditionally used as a model for seismic velocity variations in the 200-to 650-km-depth interval in a mantle of peridotitic bulk composition. It has been proposed that the observed seismic velocity increase at 400-km depth is too sharp and of too small a magnitude to be attributable to the a--,/• phase change and that the upper mantle must thus be chemically stratified, with the 400-km discontinuity being due either to a combination of phase changes in a layer of pyroxene/garnet-rich "piclogite" composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer. Using available calorimetric, thermoelastic, and synthesis data (and their associated experimental uncertainties), we have derived internally consistent high-pressure phase relations for the Mg2SiO4-F%SiO 4 join. We find that the divariant transition a•a+/•/•, which is generally regarded as occurring over a broad depth interval for mantle olivine compositions, is, in fact, extremely sharp. The seismic discontinuity corresponding to the a--}a+/•/• transition in (Mg0. oFe0. x)2SiO4 should occur over a depth interval (isothermal)of about 6 km at a depth of approximately 400 km; the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric, thermoelastic, and synthesis data. In addition, we have computed seismic velocity profiles for a model mantle consisting of pure olivine of (Mg0.oFe0. x)2SiO • composition. Comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400-km depth is consistent with a mantle transition zone composed of about 60-70% olivine. We conclude that there is no need to infer the existence of pyroxene/garnet-rich compositions, such as eclogite or "piclogite," in the transition zone, since an upper mantle of homogeneous olivine-rich peridotitic composition is consistent with the available seismic velocity data. strated that the major mineralogic transformation in eclogite at about 400-km depth, pyroxene dissolving into garnet (majoritc), is multivariant and would not produce an appropriate seismic discontinuity (Figure 1). The major purpose of this study has been to determine whether or not a phase transformation in the Mg9.SiO4-Fe9.SiO• system could produce, under isochemical conditions, a sharp seismic discontinuity. We therefore follow Navrotsky and Akaogi [1984] and Weidner [19851 in examining the main 4853 4854 B•• WOOD: OLIVINFrSP]iNEL TRANsrr•oNS Wood, B. J.,' and J. R. Holloway, A thermodynamic model for subsolidus equilibria in the system CaO-MgO-Al•Oa-SiO •, Geochim. Cosmochim. Acta, •8, 159-176, 1984. Wood, B. J., and O. J. Kleppa, Thermochemistry of forsteritefayslite olivine solutions, Geochim. Cosmochim. Acta, •5, 529-534, 1981.
During the subduction of oceanic lithosphere, water is liberated from minerals by progressive dehydration reactions and is thought to be critical to several geologically important processes such as island-arc volcanism, intermediate-depth seismicity and chemical exchange between the subducting lithosphere and mantle. Although dehydration reactions would yield supercritical fluid water in most slabs, we report here that the stable phase of H2O should be solid ice VII in portions of the coldest slabs. The formation of ice VII as a dehydration product would affect the generation, storage, transport and release of water in cold subduction zones and equilibrium conditions of dehydration would shift, potentially affecting the depths of seismogenesis and magmagenesis. Large amounts of pure ice VII might accumulate during subduction and, as a sinking slab warms, eventual melting of the ice would release large amounts of water in a small region over a short period of time, with a significant positive volume change. Moreover, the decreasing availability of fluid water, owing to the accumulation of ice VII and its subsequent reaction products in a cooling planetary interior (for example, in Mars or the future Earth), might eventually lead to a decline in tectonic activity or its complete cessation.
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