High Pressure And High Temperature Phase Transitions In Harzburgite:Implications For The Mantle Transition Structures Beneath Eastern China

Harzburgite is generally accepted as an important part of subducting oceanic plate. In subduction zones, the oceanic plate was subducting into the deep mantle and stagnated in the mantle transition zone (MTZ) or core mantle boundary, e.g., circum-Pacific subduction zone. These subducted or stagnated materials have significant effect on the physical and chemical properties of the pyrolitic mantle due to thermal and compositional heterogeneities, e.g., cause slow (subducted basalt material) or fast anomalies (subducted harzburgite material), intensive hydration or partial melting in the deep mantle, and complex velocity structures in the MTZ. However, high pressure and high temperature (HPHT) experimental studies on harzburgite are still rarely by now, which limited our understanding of the geodynamics of subducting plate and the nature of the stagnant slabs. In order to explore the phase transitions of harzburgite and the physical properties of stagnant slabs in the MTZ, we conducted HPHT phase transition experiments by using a natural harzburgite, then calculated the velocity and density properties in different stagnant slab models and measured the ultrasonic sound velocity of hazrburgite under high pressure and room temperature conditions. The main contents of this thesis include the following three points:HPHT phase transformation in harzburgite. physical properties of the stagnant slabs beneath eastern China, ultrasonic sound velocity measurement of harzburgite under high pressure and room temperature conditions.(1). We determined the mineral constituents of harzburgite in the mantle transition zone through HPHT experiments and provided the velocity and density profiles of subducting plate. Under1400℃:harzburgite is mainly composed of wadsleyite, garnet and some amount of high pressure clinopyroxene at～14-19GPa. the proportion of garnet increased with the increase of pressure and the proportion of clinopyroxene decreased with pressure. Between20and22GPa, some stishovite crystals formed within large garnet grains, in association with an increase in the volume proportion of ringwoodite. When pressure increased to22-23GPa, about10vol%of akimotoite were observed in the run products, accompanied by the disappearance of stishovite. The volume fraction of akimotoite decreased with increasing pressure. At24.2GPa, perovskite and magnesiowilstite were formed together with majoritic garnet. Under1200℃:harzburgite was mainly composed of ringwoodite, stishovite and garnet, and some amount of akimotoite was formed when pressure increased to22GPa. All akimotoite disappeared and ringwoodite decomposed into perovskite+magnesiowilstite at circa24GPa.Density profile of harzburgite was calculated along a chosen subductiing geotherm by using the third-order high-temperature Birch-Murnaghan equation of state. The results indicate that harzburgite is～0.1g/cm3denser than pyrolite between～420and650km, and～0.2g/cm3less dense between650and680km, indicating that buoyancy is an important driving force during the subduction of oceanic plate. For various thicknesses from20to60km, the harzburgite layer is denser in the MTZ until～660km depth owing to combined effects of composition, thermal, and phase relations. When the subducting harzburgite layer has a thickness of greater than～70-80km, the subducitng harzburgite will become denser than the surrounding mantle and the stagnant slab will be driven to the lower mantle.The widely used Voigt-Reuss-Hill (V-R-H) method was used to calculate the velocity profile of harzburgite along a subducting slab geotherm, the results indicate that sound velocities of harzburgite is higher than those of the surrounding mantle. In the upper part of MTZ, sound velocities of harzburgite are about5-6%and6-8%higher than normal pyrolitic mantle for P and S waves respectively. In the bottom of MTZ, sound velocities of harzburgite are about3-4%and4-5%higher than the surrounding mantle for P and S wave velocities respectively; this indicates that subducting harzburgite is an important source for high velocity anomalies in the deep mantle. Our results also indicate that phase transitions in harzburgite can not form seismic velocity discontinues in the lower part of MTZ, this may have some implications for the formation of complex velocity structures beneath eastern China.(2). Mineral physics models of the stagnant slabs beneath eastern China were established based on the phase transition experimental results and2-D thermal dynamic subdcuting slab models. The calculation results indicate that:1) the main minerals in the horizontal slab model (Model Ⅰ) were ringwoodite, stishovite and garnet; the buckled slab model (Model Ⅱ) were mainly composed of ringwoodite, stishovite, garnet and some amount of akimotoite.2) The densities of the upper basaltic crust in Model I were about5-8%higher than the surrounding mantle, harzburgite is about1-3%denser than normal pyrolitci mantle, and the lower Iherzolite is circa0-1%denser than normal mantle. In Model Ⅱ, the stagnant slab is about4-6%denser than surrounding mantle in the middle to upper part of MTZ, and harzburgite is about0-2%higher than the surrounding mantle in the lower part of MTZ.3) In Model I, the upper oceanic crust is about2-3%slower than normal pyrolitic mantle; the middle harzburgite layer is about3-5%and4.5-7.5%faster than the surrounding mantle for P and S wave velocities respectively, due to lower temperature and higher ringwoodite content in the harzburgite layer. In Model Ⅱ, harzburgite is about2-3.5%and3-5%faster than normal pyrolitic mantle for P and S wave velocities respectively, the lower lherzolite layer have very similar P and S wave velocities with the surrounding mantle.Before comparing mineral physics results with seismic tomography results, low-pass spatial Gaussian filters were used to filter the P wave velocity anomaly images in the two models. The results indicate that:In Model I, the low-velocity region caused by the upper basalt layer disappeared due to the reduction in spatial resolution; amplitudes of the P-wave anomalies caused by the harzburgite layer are reduced by a factor of～2for the horizontal slab model to about1.5-3.5%, still significantly higher than seismic tomography results. In Model II, the sharp, sinusoidal velocity anomalies are smeared to a smooth near-horizontal feature. The amplitude of P-wave velocity anomalies is reduced to about1-2%throughout the slab except for the kneel point of the subducting slab. The horizontal extent of the fast anomaly is greatly reduced compared to Model I. Overall, the buckled model (after filtering) better matches the P-wave anomalies observed in seismic tomography images.(3). We calculated the velocity structures in two subducting pyrolite models. The P wave velocity anomaly is about+1-3%in the horizontal slab model (Model Ⅲ), and that in the buckled slab model (Model IV) is about0.5-1.5%1. These indicate that temperature anomalies along can also cause velocity anomalies that observed in seismic tomography studies. In order to determine the origin of the fast velocity anomalies beneath eastern China, we then calculated the Vp/Vs properties in the above four mineral physics models. The calculation results show that the Vp/Vs anomalies in the stagnant pyrolite models (Model Ⅲ and Ⅳ) are lower than recent tomography studies, while the Vp/Vs anomalies in Model Ⅱ are very similar to that observed in tomography studies (～1-2%). These implicate that temperature anomalies along can not account for the velocity anomalies beneath eastern China, and the stagnated buckled oceanic plate may represent the nature of the fast velocity anomalies under eastern China.(4). We conducted ultrasonic sound velocity measurements for harzburgite under high pressure and room temperature conditions. The harzburgite sample was mainly composed of ringwoodite, stishovite and garnet. Our experimental results indicate that the zero pressure P and S wave velocities of the tarzburgite sample are VP0=9.8km/s and VS0=5.7km/s, the pressure derivative of P and S wave velocities are about (?)Vp/(?)P=6.8×10-2kms-1GPa-1and (?)Vs/(?)P=2.4×10-2kms-1GPa-1. The zero pressure bulk and shear modulus are about K0=194GPa and Go=123GPa, the pressure derivative of bulk and shear modulus are about (?)K/(?)P=4.9and (?)G/(?)P=1.5. The sound velocities of harzburgite are about8%and10%higher than that of garnetite with MORB composition, and are about5-7.5%and5.5-9.5%higher than that of pyrolite. These indicate that the velocity jump across the interface between hazrburgite and the surrounding pyrolitic mantle is large enough for seismic detecting. The author believes that the harzburgite-pyrolite interface around the stagnant slabs must contribute to the complex velocity structures in the lower part of MTZ beneath eastern China.