【作者】 张艳飞；

【导师】 金振民； 王雁宾；

【作者基本信息】 中国地质大学 ， 构造地质学， 2013， 博士

【摘要】 方辉橄榄岩是俯冲大洋岩石圈的重要组成部分,在板块的俯冲和地幔对流过程中占据重要地位,通过俯冲作用进入地幔深部的方辉橄榄岩不但会改造原生地幔岩的化学成分、矿物组成和热状态,还与中深源地震的成因、地幔深部水循环等一系列地球动力学问题密切相关,具有重要的地球动力学意义。全球层析成像研究结果表明,地幔深部(从上地幔至下地幔底部)存在广泛分布的高速异常,由于这些高速异常多与俯冲带密切相关,因此通常被认为是滞留在地幔中的俯冲大洋板块。然而,目前国际上还没有对真正意义上的方辉橄榄岩在高温高压下的相变行为开展实验研究,这在一定程度上限制了我们对方辉橄榄岩在地幔深部条件下的相变、演化和物理性质以及相关地球动力学问题的认识。为了探索俯冲方辉橄榄岩在地幔转换带条件下的矿物学特征及其物理性质和赋存状态,我们以天然方辉橄榄岩为实验起始材料,在高温高压相变、超声波速度测量和矿物物理计算等方面开展了一些初步的工作,主要包含以下三个方面的研究内容：方辉橄榄岩高温高压相变实验研究、方辉橄榄岩窒温高压超声波速度测量实验研究和中国东部地幔转换带滞留板块的速度和密度特征。(1)对方辉橄榄岩的高温高压相变实验研究初步确定了方辉橄榄岩在地幔转换带条件下的矿物组成,在此基础上我们初步建立了俯冲方辉橄榄岩的密度和速度剖面。1400℃时：在约14-16GPa条件下,方辉橄榄岩的主要矿物组成为瓦兹利石、石榴石和高压单斜辉石,其中石榴石的含量随着压力的升高而增加,高压单斜辉石的含量随着压力的升高而而减少；至约19GPa时,高压单斜辉石消失,石榴石的含量达到最多：随着压力的继续升高(-20GPa),瓦兹利石相变为林伍德石,部分石榴石相变为斯石英+林伍德石,这导致林伍德石含量增加和石榴石含量的减少；在22GPa时,斯石英与林伍德石反应生成秋本石,造成林伍德石含量的减少和石榴石含量的增加；随着压力的升高,秋本石的含量逐渐减少,在约24GPa时,剩余的秋本石相变为钙钛矿(严格讲为具钙钛矿结构的硅酸盐,本文简称“钙钛矿”),林伍德石也分解为富镁钙钛矿和铁方镁石,部分石榴石也逐渐相变为钙钛矿。1200℃时：在18-20GPa条件下,方辉橄榄岩主要由林伍德石、斯石英和石榴石组成,且斯石英和石榴石密切共生；当压力升高至22GPa时,部分斯石英和林伍德石反应生成秋本石,但仍有部分斯石英和石榴石共生；在约24GPa时,秋本石消失,林伍德石开始分解为钙钛矿和铁方镁石在高温高压相变实验的基础上,我们利用三阶高温Birch-Murnaghan状态方程以及前大对相关矿物热弹性参数的结果计算了方辉橄榄岩沿典型俯冲带地温曲线的密度剖面。结果表明,在约420-650km深度范围内,俯冲方辉橄榄岩的密度比正常地幔岩的密度要高约0.1g/cm3；在约650-680km深度范围内,方辉橄榄岩的密度约比周围地幔低0.2g/cm3,这意味着密度是大洋板块深俯冲的重要驱动力；在地幔转换带底部,由于方辉橄榄岩温度较低,后尖晶石相变的压力偏高,导致方辉橄榄岩的密度低于周围地幔岩,这为俯冲板块在转换带中的滞留提供了浮力作用；方辉橄榄岩在转换带底部所受浮力随着滞留板块厚度的增加而减小,当方辉橄榄岩层的厚度达到一定程度时(>-70-80km),浮力逐渐小于重力,此时方辉橄榄岩将在自身重力的拖动下进入下地幔。我们还利用Voigt-Reuss-Hill (V-R-H)公式计算了方辉橄榄岩沿典型俯冲带地温曲线的速度剖面。结果表明,在地幔转换带中俯冲方辉橄榄岩的速度比止常地幔岩高。在转换带上部,方辉橄榄岩P波速度约比Pyrolite高5-6%，S波速度比Pyrolite高6-8%；在转换带下部,方辉橄榄岩P波速度比地震学模型高约3-4%,S波速度比地震学模型高约4-5%；这意味着俯冲方辉橄榄岩是地幔深部高速异常的重要来源。结果还表明,方辉橄榄岩相变难以在转换带下部形成地震波速度不连续面,这意味着转换带’下部与俯冲板块相关的复杂地震波不连续面(比如中国国东北地区地幔转换带)可能并非是方辉橄榄岩相变造成的。(2)在二维俯冲板块热动力学模型的基础上,将方辉橄榄岩高温高压相变实验结果和矿物物理参数相结合,建立了中国东部地幔转换带滞留板块的矿物物理模型。1)滞留板块的矿物学模型表明,在水平层状模型中(模型Ⅰ),滞留板块的主要矿物组成为林伍德石、斯石英、石榴石。波浪形滞留板块模型(模型Ⅱ)相对复杂一些,在转换带上部主要有瓦兹利石、石榴石和高压单斜辉石组成,在转换带底部主要由伍德石、斯石英、石i榴石和部分秋本石组成。2)密度模型结果表明,在模型Ⅰ中,玄武质洋壳的密度比周围地幔高约5-8%,方辉橄榄岩由于具有较低的温度,因此其密度比正常地幔高约1-3%,而下部二辉橄榄岩层的密度仅比正常地幔高约0-1%。对于模型Ⅱ,在转换带中上部，滞留板块的密度比正常地幔高约4-6%,这主要是由于方辉橄榄岩中含有有较多的瓦兹利石，在转换带底部,由于方辉橄榄岩具有较低的温度以及较高的林伍德石含量,因此其密度比正常地幔高约0-2%。3)速度模型结果表明,在模型Ⅰ中,玄武质洋壳的P-和S-波速度比正常地幔岩低约2-3%,中部方辉橄榄岩的P-和S-波速度分别比正常地幔岩高约3-5%和4.5-7.5%,这主要是因为方辉橄榄岩的温度相对较低,且含有较多的林伍德石以及部分斯石英。在模型Ⅱ中,方辉橄榄岩部分P-和S-波速度分别比正常地幔高约2-3.5%和3-5%,二辉橄榄岩的速度和密度和周围地幔比较接近。为了把根据矿物物理模型计算的结果和地震波层析结果相对比,我们分别对模型Ⅰ和模型Ⅱ中的P波速度异常数据进行高斯滤波,以消除高频信号的影响。结果表明,模型Ⅰ中玄武质洋壳产生的低速异常消失,方辉橄榄岩层产生的高速异常约为1.5-3.5%,比滤波之前降低了约2倍,但从整体上看,滤波之后的结果依然明显高于地震波层析成像的结果,因此作者认为模型Ⅰ难以代表中国东部地幔转换带滞留板块的真实赋存形态。仡模型Ⅱ中,滤波之后由波浪形滞留板块产生的比较尖锐的速度异常特征消失了,P波异常在整体上也呈近似水平的形态,最大异常值约为1-2%(俯冲板块刚进入转换带的部分除外)：与模型相比,模型Ⅱ的P波异常幅度大大减小,与中国东部地震波层析成像的最新研究结果十分接近。从整体上看,模型Ⅱ比模型Ⅰ更接近地震波层析成像的结果,这意味着中国东部地幔转换带的滞留板块很可能也经历了一定程度的弯曲变形。(3)为了进一步约束中国东部地幔转换带高速异常的成因,我们计算了俯冲pyrolite模型的地震波速度特征。计算结果表明,水平层状模型(模型Ⅲ)引起的P波速度异常约为+1-3%,弯曲波浪形滞留板块模型(模型Ⅳ)引起的P波速度异常约为+0.5-1.5%,这意味着地幔岩中单独的温度异常也可以引起与滞留大洋板块类似的速度异常,因此仅从地震波速度异常方面难以区分中国东部地幔转换带高速异常的成因(温度异常还是化学成分异常?)。为了厘清中国东部转换带内高速异常的成因,我们计算了不同滞留板块模型中Vp/Vs的分布特征,并探讨了利用Vp/Vs是识别地幔中不同起源速度异常的可行性。计算结果表明,滞留Pyrolite板块模型(模型Ⅲ和Ⅳ)形成的Vp/Vs异常低于最近地震波层析成像的研究结果；弯曲波浪形滞留大洋岩石圈模型(模型Ⅱ)所引起的Vp/Vs异常约比正常地幔低1-2%左右,与地震学研究结果比较一致。这意味着中国东部地幔转换带的高速异常应该代表滞留的太平洋俯冲板块,而非单纯的温度异常现象。同时,这一结果再次表明,波浪形的滞留板块模型(模型Ⅱ)比水平层状滞留板块模型(模型Ⅰ)更能代表中国东部地幔转换带滞留板块的真实赋存状态。(4)我们在室温高压条件下对具有“林伍德石+斯石英+石榴石”组分的方辉橄榄岩进行了超声波速度测量实验。结果表明,方辉橄榄岩的零压P和S波速度分别约为VP0=9.8km/s和Vso=5.7km/s,波速对压力的导数分别约为aVp/(?)P=6.8×10-2kms-1GPa-1,(?)Vs/(?)P=2.4×10-2.kms-1GPa-1;方辉橄榄岩的零压体积模量K0和剪切模量G0分别约为194GPa和123GPa,体积模量和剪切模量对压力的导数分别约为(?)K/(?)P=4.9和(?)G/(?)P=1.5。与具有MORB成分的石榴石岩相比,方辉橄榄岩的P和S波速度分别要高8%和10%左右；与pyrol ite地幔岩相比,方辉橄榄岩的P和s波速度分别要高5-7.5%和5.5-9.5%左右,这意味着转换带滞留方辉橄榄岩和周围pyrolite地幔岩之间的岩性界面能够引起较大的速度突变。作者认为在利用相变成因解释与滞留板块相关的速度不连续面时,地幔岩-方辉橄榄岩这一岩性界面对地幔转换带速度结构的影响也不能被忽视,这很可能是形成俯冲带区域复杂速度不连续面的重要原因之一。更多还原

【Abstract】 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.更多还原