Experimental Investigation On The Strength Of The Material Near The660Km Discontinuity At High Pressures And Temperatures And Its Implications For Geodynamics

The660km seismic discontinuity corresponds, at least at lower temperatures, to the transition of the spinel phase of (Mg,Fe)2Si04, ringwoodite, to (Mg,Fe)SiO3, perovskite (pv)+(Mg,Fe)O, magnesiowustite (mw). The rheological properties of material above and below this discontinuity as well as the chemical/structural nature of the boundary itself play an important role for the understanding of deeply subducted slabs, the termination of deep earthquakes, mantle convection, post-glacial rebound, and other geodynamic processes. Although the rheology of olivine, the dominant upper-mantle mineral, has been extensively studied, knowledge about the rheological properties of the material near the660km discontinuity, especially pv+mw is limited because of technical reasons. As a consequence, the deformation behavior of deep subduction slab at the660km seismic discontinuity and its role in the deeper levels of subduction zones and mantle remain poorly understood. To address this important problem, we have performed preliminary studies on a similar realistic analogue system (disproportionation of Co2TiO4spinel into CoTiO3ilmenite+CoO) with the material near the660km discontinuity under high temperature and high pressure, including an experimental investigation on the rheology of the material near the660km discontinuity, preliminary EBSD study on the deformation microstructure of the material near the660km discontinuity and their significances for the mechanism of deep-focus earthquakes in North-Eastern area of China.We have conducted experiments at strain-rates of10-1～10-5/s, pressures of2-4GPa, and temperatures of1273-1523K to determine and compare the flow strength of spinel with its dissociation product in detail through high pressure and temperature experiments in the5GPa piston-cylinder (Griggs) deformation apparatus in which the deformation can be well controlled, with accurate stress measurement with a realistic analogue system:disproportionation of Co2TiO4spinel into CoTiO3ilmenite+CoO. Our results show that (i) Such decomposition reactions yield typical symplectic microstructures--"wormy" intergrowths of daughter phases-in which the domain size viewed in2D can appear much smaller than the actual size of individual intergrowths of the two phases in3D.(ii) The symplectic high-pressure reaction product (CoTi03+CoO) is stronger than the Co2TiO4spinel parent about25%rather than much weaker as predicted, that is, symplectites are structurally strong, not structurally weak because each is a complexly intergrown2-phase bicrystal, not weak nanocrystalline aggregates. Application to Earth shows that subducting lithosphere entering the lower mantle is not weaker and can be expected to be significantly more viscous than the mantle transition zone as has been shown for the lower mantle in general by geophysical measurements. With our results, the down-dip compressive stress states of subducting slabs can be understood well, as observed in the slabs of western Pacific Ocean (including the north-eastern area of China). In addition, there is no indication of a shearing instability during incipient breakdown of the spinel phase under stress in this system. Thus, the most likely explanation of termination of deep earthquakes at the base of the mantle transition zone is that either metastable olivine (now having been observed in4subduction zones) has been exhausted or that, if carried into the lower mantle, reacts aseismically to form symplectites.In the past decade the electron backscatter diffraction (EBSD) technique has become into a available apparatus for the microstructure and crystallographic analysis of materials equipped on the scanning electron microscope. This method was widely used for measurement of texture and microstructure of minerals because of its convenience, high quality positionand, high orientation resolution, and so on. Recently, it have been applied to phase identification, studying deformation mechanisms, constraining dislocation slip systems, studying metamorphic processes, constraining metamorphic processes and numerical computations of petrophysical properties. Our preliminary results from electron backscatter diffraction (EBSD) study on the deformed or undeformed microstructure of symplectites of CoTiO3+CoO and CO2TiO4spinel phase show that:(a) From the scatter diagrams of orientation map of symplectites, it can been seen that all plane{111} of CoO//{0001} of CoTiO3,{110} of CoO//{10-10} of CoTiO3and{100} of CoO//{2-1-14} of CoTiO3, although in the second (right) symplectite show two groups orientation of CoO, suggesting that CoO and CoTiO3are intergrowths in3D and also follow a special direction.(b) During deformation of the symplectites, dynamic recrystallization and grain growth occurs, however, there is only slight diminution of grain size and no evidence for structural weakening of the symplectite due to'structural superplastic' flow dominated by grain-boundary sliding.(c) There is no pronounce crystallographic preferred orientation (CPO) in deformed Co2TiO4and undeformed or deformed CoO of symplectites. All the misorientation angle distributions correspond to theoretically random distribution curves. The nearly random CPO may be attributed to the high symmetry of crystallographic structure (cubic) or inadequate deformation, and can not be used to imply active slip systems.(d) In contrast, a relative stronger crystallographic preferred orientation be developed in deformed CoTiO3of symplectites, the{110} plane tends to form a girdle in the plane subparallel to the lineation with a relatively weak point maximum subparallel foliation, and{100} plane nearly show the same pattern as{110}. The {001} plane poles normal to lineation and tend to parallel Z direction (σ1compression direction), all suggest dislocation glide on the plane of{001} maybe play a dominant role for the plasticity of CoTiO3.The results from the preliminary study of deep-focus earthquakes in Northeast area of China show that (i) The deep-focus earthquakes region of North-Eastern China is a part of Western Pacific Ocean subduction zones, and is the result of driving action from Western Pacific Ocean subducting slab. The down-dip compressive stress states is showed from the the focal mechanisms of the deep earthquakes in North-Eastern China. It is consensus with the greater viscosity of the lower mantle mentioned above,(ii) Most deep-focus earthquakes in North-Eastern China occur within metastable olivine wedge, so their occurrence should be related to phase transformation. The most likely explanation of termination of deep earthquakes at about660km is that either metastable olivine has been exhausted or that, ringwoodite breakdown yields such aseismically symplectites.It's worth noting that there seems no relation between the chapter6and the title of this paper, however, learning how to operate MA apparatus and how to process the sample for MA is one of main prospective objects during my time as a graduated student, besides, it is important to understand the composition of starting materials, temperature and pressure implications of Ca-Eskola component in clinopyroxene, such as, the explanation of omphacite and diopside from many eclogites and garnet peridotites display tiny, crystallographically oriented, rod-shaped or lamellar inclusions of quartz. So as one of important results during my time as a graduated student, the " Ca-Eskola component in clinopyroxene:experimental studies at high pressures and high temperatures in multianvil apparatus" is shown at the chapter6.A series of anhydrous experiments using powdered glass of omphacite composition (wt.%):Na2O=1.78, CaO=10.37, MgO=5.44, Al2O3=23.12, SiO2=59.29were conducted in a Walker-style multianvil apparatus in the P-T range6-12GPa and900-1200℃to study activity of the Ca-Eskola (Ca-Esk) component. All clinopyroxenes synthesized in equilibrium with garnet, kyanite and SiO2(coesite or stishovite) are nonstoichiometric, with the sum of cations<4, calculated for6oxygens. We systematically have measured the values of the Ca-Esk component of clinopyroxenes as a function of pressure and temperature. The results yield that the highest value of the Ca-Esk component (38-32mol.%) is reached at6GPa, then it slightly decreases up to36-30mol.%at8GPa, and significantly decreases at10-12GPa from26to16mol.%when stishovite becomes stable in the assemblage and the pyroxene progressively dissolves into garnet. Comparison of our data with those available in the literature (which all together cover a wider range of P and T within pyroxenes stability field) show that the Ca-Esk activity strongly depends on bulk chemistry at all ranges of pressure and temperature. However, the sensitivity of the Ca-Esk component to pressure and temperature is not a simple negative or positive correlation, it is characterized by complex relationships which are strongly affected by bulk composition. In general, integrated analyses of published experiments suggest that the Ca-Esk component rises rapidly to its highest values (～36-38mol.%) from1to4-6GPa, then it slightly decreases between8to10GPa followed by remarkable decrease at highest pressure ranges of12to15GPa culminating in replacement of pyroxene by garnet. We conclude that the SiO2exsolution lamellae observed in clinopyroxenes from many ultrahigh pressure terranes can be explained by decompressions of the "Si-rich" nonstoichiometric clinopyroxene, however the depth from which such rocks are uplifted should not exceed200-250km, corresponding to pressure～8-10GPa.