橄榄石成核动力学方程的修正及其对动力学数据的约束

本文从成核动力学理论出发,考虑新相晶核的表面能对其形状的影响,修正了晶面成核情形的成核动力学方程.新的成核率方程还原了传统成核率方程中通常简化为常数的指前系数.结合修正的相变体积分数计算公式,新的成核率方程可以确定出成核过程中的两个物理参量:新相晶粒与反应相晶粒的接触角度及新相物质与反应相物质之间的表面能.分析结果表明...     

加载、加热路径上橄榄石的相变及其对相变动力学参数计算的影响

本文研究了橄榄石原位相变实验中加载、加热路径上的相变及其对确定相变动力学参数的影响.利用文献［1］所给出的退火后先加温后加压,且相对低温条件下实验结果确定出的Ni₂SiO₄橄榄石相变动力学参数,计算了加载、加热路径上所发生的相变对确定成核率、长大率及相变体积分数的影响.结果表明,退火后先加压后加温,且相对高温条件下的实验数据受到加热路径上成核的影响.根据这样的实验数据得到的成核率会明显高于实际温压条件下的成核率.尤其是当多数实验都是高温实验时,根据这些成核率数据所确定的成核率参数会严重偏离其真值,从而严重影响对俯冲带颗粒粒度及俯冲带流变结构的计算.尽管目前有很多关于Mg₂SiO₄橄榄石长大率的实验数据,也有通过对挤碰物理图像的分析对(Mg_0.89Fe_0.11)₂SiO₄橄榄石成核率的估算,但只有文献［2］通过退火后先加压后加温的原位实验得到了Mg₂SiO₄橄榄石的相变成核率,且属于高温实验.根据本文的研究结果,我们认为亟需补充退火后先加温后加压或相对低温的实验数据以得到正确的地幔橄榄石成核率参数.     

对橄榄石相变形核率的估算

地幔主要矿物橄榄石［(Mg089Fe011)2SiO4］的相变与深源地震的解释以及俯冲带和周围地幔的相互作用相关. 其中,形核率和长大率是刻画其相变动力学的两个重要参数. 因为实验技术的原因,目前人们还没有基于实验数据给出橄榄石的形核率. 本文首先通过对挤碰物理图像的分析,对边界形核情形的相变动力学理论中表征挤碰程度的无量纲参数进行了修正. 在此基础上,根据已有的实验资料,首次对橄榄石相变的形核率进行了估计. 通过对形核率数据的拟合,可以得到形核率参数K0和γ1/3. 得到两个形核率参数的变化范围分别为K0=55×1021～87×1027s-1·m-2·K-1,γ1/3=0～020 J·m-2,最佳拟合值为：K0=69×1024s-1·m-2·K-1,γ1/3=016 J·m-2.     

存在外源线电流元的电磁场边界积分方程

根据电磁场理论,应用δ函数及广义极限导出了存在外源线电流元的电场和磁场的亥姆霍兹方程.在此基础上,应用物理近似方法引用矢量格林第二公式导出了相应的电磁场边界积分方程,可供用边界元法计算有关勘查地球物理的正演问题.     

亚稳态橄榄石相变与深源地震研究进展

由于岩石圈俯冲板块内部温度低, 在400km左右的深度橄榄石不发生相变, 低压相的橄榄石便以亚稳态的形式进入更深的深度。 高温高压实验研究表明, 亚稳态橄榄石相变机制以相变形成的超塑性透镜状尖晶石反裂纹破裂为深源地震的触发机制, 能很好的解释深源地震的特征。 亚稳态橄榄石在俯冲带中存在的范围,是制约亚稳态相变机制的重要因素。 然而亚稳态橄榄石在俯冲带存在的范围尚存争议, 不同模型给出结果相差较大, 文中给出一些解决这个问题的建议。     

橄榄石的同系温度T/T_m:对上地幔蠕变与橄榄石组构转变的启示

同系温度为晶体材料的绝对温度(T)与其熔点(T_m)的比值,是对比晶体流变强度的重要参数之一.橄榄石(Mg,Fe)_2Si O_4的熔点随含铁量和含水量的增加而降低,随压力增加而升高,高压下橄榄石的相变可导致熔点随压力发生不连续变化.本文对前人的铁橄榄石(Fe2Si O_4)熔融实验结果进行校正,确定铁橄榄石-γ相Fe2Si O_4-熔体的三联点位于6.4GPa和1793K.然后使用广义混合律建立了常压至6.4GPa,无水橄榄石的固相线和液相线与含铁量和压力的关系.橄榄石T/T_m随深度的变化可用于定性对比具有不同热状态和橄榄石成分的上地幔流变强度,T/T_m=0.5的深度界定了上地幔从半脆性变形到完全塑性变形的转换深度.克拉通岩石圈地幔的橄榄石T/T_m显著低于造山带和伸展盆地,一直到岩石圈-软流圈边界才与周围地幔趋于一致(T/T_m0.66),表明克拉通的岩石圈强度较高.此外,使用T/T_m分析橄榄石的变形实验结果,发现含水量对橄榄石组构的影响与压力密切相关,但是水对橄榄石流变强度的弱化作用及其与橄榄石同系温度的关系尚需进一步研究.低于6.4GPa(200km),T/T_m控制了橄榄石[100]和[001]位错滑移的转变.在上地幔10~-(12)~10~-(15)s~(-1)的应变速率和低应力条件下,橄榄石[100](010)位错滑移系(A型组构)的稳定域为T/T_m0.55~0.60.T/T_m0.55~0.60时,[001]位错滑移更易于发生,低T/T_m使橄榄石的主控位错滑移系转变为[001](100),形成C型组构.这与天然变形的橄榄岩普遍发育橄榄石A型组构,而超高压变质带中经历了深俯冲的橄榄岩发育橄榄石C型组构一致.而[001](010)位错滑移系(B型组构)受T/T_m和应力的共同影响.因此,橄榄石的同系温度建立了将变形实验结果外延到上地幔流变的桥梁.克拉通地区的上地幔地震波各向异性需要一个四层模型来拟合:上层为残留的A型组构,中间为B型组构,岩石圈-软流圈边界为新生的A型或B型组构,以及Lehmann不连续面之下以扩散蠕变为主的软流圈地幔.对橄榄石组构转变机制的认识是通过地震波各向异性追踪上地幔含水量的分布和地幔流动的关键.     

橄榄石矿物弹性研究进展

橄榄石是上地幔含量最丰富的矿物,对其弹性性质的研究对解释地球内部的物质组成和动力学过程至关重要.近年来,对橄榄石矿物弹性的研究,无论是实验测试还是模拟计算都取得了很大的进展.本文首先介绍了研究橄榄石弹性的主要方法,然后从铁、镁不同配分比例下橄榄石的弹性性质,水对橄榄石弹性的影响以及橄榄石各向异性等三个方面,对近年来橄榄石弹性研究所取得的主要进展做了介绍.实验方法方面主要介绍了1)超声共振谱法(Resonant Ultrasound Spectroscopy,RUS);2)X射线衍射法;3)X射线照相法;4)超声干涉法;5)布里渊散射法.     

橄榄石-尖晶石相转变流变学的实验研究

对(Mgx,Ni1-x)2GeO4系统中橄榄石-尖晶石两相混合物的流变性进行了实验研究.利用不同温压条件对混合物进行热压,得到的橄榄石和尖晶石的比例不同.温度为1 573K,试件的化学组成从x=0.95-0.70时,得到的两相材料从100%橄榄石到100%尖晶石.三轴压缩试件为圆柱状,直径8mm,两相混合物的平均颗粒尺寸为2.21-9.43μm.流变学试验在气体介质试验机中进行,围压为300MPa,温度为1 073-1 573K.试验分为常应力和常位移率两种,差应力σ1-σ3为45-320MPa,应变率在2.8×10-6-2.8×10-5s-1之间,将实验测得的混合物力学性质与两端材料的强度进行对比,发现两相多晶介质的强度介于两端材料的强度之间.当橄榄石到尖晶石的相变发生在变形过程中时,尖晶石比率的增加使得多晶材料的总体强度上升.     

人工合成橄榄石单晶方法综述

橄榄石是上地幔的主要造岩矿物,研究橄榄石在高温高压下的物理化学性质是了解地幔对流、板块运动等地球动力学过程的基础.一些研究橄榄石物理化学性质的实验需要用到橄榄石单晶,例如在实验室中进行的变形实验,无论是双轴压缩、三轴压缩、简单剪切,还是扭转变形实验等等.近年来,水对橄榄石蠕变性质的影响被广泛关注,橄榄石的含水测量也需要用到大尺寸的橄榄石单晶.除了实验室需要大晶体,合成晶体也是新材料领域的研究热点和发展前沿,目前国内外人工合成单晶的方法包括从固相、液相、气相以及从熔体中合成晶体.在人工合成橄榄石单晶方面,目前主要使用的方法包括提拉法、浮区法和烧结法.本文综述了从固相、液相、气相以及从熔体中合成晶体的方法,详细介绍了人工合成橄榄石单晶所采用的提拉法、浮区法和烧结法等三种方法的原理与合成过程,讨论了不同方法的优缺点,其中烧结法合成单晶需要的时间短,但是对温度以及实验的氧化还原环境要求较高;浮区法生长大的单晶耗时短、可以得到大尺寸的单晶,但对工艺技术要求高;提拉法可以通过籽晶的取向来制备不同晶体取向的单晶,过程容易控制,但是由于使用坩埚而容易污染,不适用于对于固态下有相变的晶体.通过以上研究,本文提出了以后生长大尺寸橄榄石单晶的可能途径,希望对橄榄石单晶的制备方面工作提供参考.     

亚稳态橄榄石对俯冲带负浮力的影响及其动力学意义

利用有限元方法,计算了不同俯冲速度及热传导系数俯冲岩石层的负浮力.在h-lt;400km和h-gt;740km的深度范围,低温高密俯冲带的负浮力随深度单调增加.因为尖晶石相到后尖晶石相的相变有负的克兰帕龙斜率,俯冲带冷的物质在660km间断面以下不到100km的深度范围内仍以低密度的低压物质相存在.所以在该深度范围,俯冲带受到了周围地幔阻止其插入下地幔的浮力作用.在400km-lt;h-lt;660km深度范围,由于受橄榄石相变的影响,不同计算模型负浮力随深度的变化有明显的不同.对于可能的相变动力学模型,亚稳态橄榄石的存在使负浮力随深度的增加值减小.其作用是不利于俯冲带直接穿透660km间断面,并引起该深度范围俯冲带沿俯冲方向压应力分布的变化.     

Hugoniot equation of state of olivine and its geodynamic implications

Large olivine samples were hot-pressed synthesized for shock wave experiments. The shock wave experiments were carried out at pressure range between 11 and 42 GPa. Shock data on olivine sample yielded a linear relationship between shock wave velocity D and particle velocity u described by D=3.56(?0.13)+2.57(?0.12)u. The shock temperature is determined by an energy relationship which is approximately 790°C at pressure 28 GPa. Due to low temperature and short experimental duration, we suggest that no phase change occurred in our sample below 30 GPa and olivine persisted well beyond its equilibrium boundary in metastable phase. The densities of metastable olivine are in agreement with the results of static compression. At the depth shallower than 410 km, the densities of metastable olivine are higher than those of the PREM model, facilitating cold slab to sink into the mantle transition zone. However, in entire mantle transition zone, the shock densities are lower than those of the PREM model, hampering cold slab to flow across the "660 km" phase boundary.     

Experimental investigation of phase transformations of olivine and enstatite at the lower part of the mantle transition zone: Implications for structure of the 660 km seismic discontinuity

High-pressure polymorphs of olivine and enstatite are major constituent minerals in the mantle transition zone(MTZ).The phase transformations of olivine and enstatite at pressure and temperature conditions corresponding to the lower part of the MTZ are import for understanding the nature of the 660 km seismic discontinuity.In this study,we determine phase transformations of olivine(MgSi2O4) and enstatite(MgSiO3) systematiclly at pressures between 21.3 and 24.4 GPa and at a constant temperature of 1600℃.The most profound discrepancy between olivine and enstatite phase transformation is the occurency of perovskite.In the olivine system,the post-spinel transformation occures at 23.8 GPa,corresponding to a depth of 660 km.In contrast,perovskite appears at 23 GPa(640 km) in the enstatite system.The ~1 GPa gap could explain the uplifting and/or splitting of the 660 km seismic discountinuity under eastern China.     

The nature of the boundary between the upper and the lower mantle and its influence on convection

In the PREM seismic model, the boundary between the upper and the lower mantle is accepted at a depth of 670 km, where seismic velocities and density increase. However, until recently there was an obvious inconsistency in this model. The density increases abruptly, and the velocities, in addition to the jumps, have also the subsequent zones of increased gradient. The discontinuity between the upper and the lower mantle is related to the transition of olivine from the ringwoodite phase into the mixture of perovskite and magnesiowustite. However, in the pyrolyte model, the transition zone of the upper mantle consists not wholly of olivine, but partly of olivine (60%) and partly of garnet (40%). The latest data of the garnet measurement at high pressures show that it also experiences phase transition, being converted into magnesium perovskite with the impurity of calcium perovskite. In contrast to the sharp transition in olivine (within a depth interval of only 5 km), the transition in garnet is spread over the interval of depths of 660–710 km. In the widely used PREM and AK135 models, this additional transition corresponds to the zone of the increased gradient in seismic velocities, while in the density distribution it is included in the sharp transition of ringwoodite. Thus, the mineralogy data indicate the need for correction of the PREM and AK135 seismic models: the density jump at a depth of 660 km should be reduced by approximately a factor of two, and a subjacent layer with the increased density gradient should be added at the depth interval of 660–710 km. The phase transition in olivine hampers the mantle flows, although in garnet it accelerates them. Therefore, with an allowance for the smaller jump in density, the decelerating effect of the subducting plates, caused by the phase transition in olivine, decreases, and, furthermore, the effect of their acceleration, caused by the phase transition in garnet, is added. The decrease in the density jump by almost a factor of two will lead to essential changes in the results of the majority of recent works addressing the assessment of the deceleration of convection at the upper/lower mantle discontinuity on the basis of the PREM model.     

日本海沟俯冲带热结构与深源地震

本文利用有限差分方法,计算了全地幔对流模式和双层地幔对流模式下日本海沟俯冲带热结构、浮力及P波速度异常分布,基于亚稳态橄榄石相变模型推测亚稳态橄榄石的存在范围,同时分析了热传导系数、热膨胀系数和热源对俯冲带热结构的影响,以及俯冲带所受浮力与俯冲带形态的关系.结果表明,双层地幔对流模式下模拟的P波速度异常分布与层析成像结果更为相符,也与深源地震的分布有较好的相关性.板块内部亚稳态橄榄石的存在范围随热传导系数和热膨胀系数的减小而增大,同时忽略相变潜热和剪切生热的影响也会造成模型所预测的亚稳态橄榄石范围偏大.俯冲带所受负浮力在400 km深度附近达到最大值,亚稳态橄榄石的存在使负浮力逐渐减小,甚至在板块内部产生正浮力,不利于俯冲带穿透660 km相变界面.     

Mantle dynamics: The strong control of the spinel-perovskite transition at a depth of 660 km

The strong control that the endothermic phase change from spinel to perovskite and magnesiowüstite at a depth of 660 km has on mantle convection is discussed. The phase transition determines the morphology and length scales of upflow and downflow structures and, through retardation of sinking slabs, can cause an avalanche phenomenon involving rapid flushing of cold upper mantle material down to the base of the lower mantle. The phase change significantly heats plumes that rise from the lower mantle and penetrate into the upper mantle. The exothermic phase change from olivine to spinel at a depth of 400 km in the mantle mitigates the effects of the dynamically and thermally dominant endothermic phase transition.     

Influence of an endothermic phase transition on mass transfer between the upper and the lower mantle

Geochemical data indicate that two major reservoirs 1–2 Ga in age are present in the mantle. The upper mantle, feeding mid-ocean ridges, is depleted in chemical elements carried away into the continental crust. The lower mantle, feeding hotspot plumes, is close in composition to primordial matter. The 660-km depth of an endothermic phase transition in olivine has been considered over the last two decades as a possible boundary between the reservoirs. In this period, many models of mantle convection were constructed that used values of the phase transition parameters, which led to temporal (up to 1 Gyr long) convection layerings and periodic avalanche-induced mantle intermixing events. However, laboratory measurements with new improved instrumentation give other values of the phase transition parameters that require a revision of the majority of the existence of large-scale avalanches in the Earth’s history becomes disputable. The paper is devoted to comprehensive study of the phase transition effect on the structure of mantle flows with different values of phase transition parameters and Rayleigh numbers; in particular, the mass transfer through the phase boundary is calculated for different regimes of steady-state convection.     

Phase transition zone width implications for convection structure

A temperature and pressure increase in the mantle causes phase transitions and related density changes in its material. The transition boundary in the pressure-temperature phase diagram is determined by the curve of phase equilibrium with the slope γ = dp/dT. If the slope is nonzero, a phase transition in hot ascending and cold descending mantle flows occurs at different depths and, therefore, either enhances (γ > 0) or slows down convection (γ < 0). The mantle material has a multicomponent composition. Therefore, phase transitions in the mantle are distributed over an interval of pressures and depths. In this interval, the concentration of one phase smoothly decreases and the concentration of the other increases. The widths of phase transition zones in the Earth’s mantle vary from 3 km for the endothermic transition in olivine at a depth of 660 km to 500 km for the exothermic transition in perovskite, and the high-to-low spin change in the atomic state of iron takes place at a depth of about 1500 km. This work presents results of calculations demonstrating the convection effect of phase transitions as a function of the transition zone width. Transitions of both types with different slopes of the phase curve and different intensities of mantle convection are examined. For the first time, the convection enhancement and an increase in the mass transfer across the phase boundary are quantitatively investigated in the presence of an exothermic phase transition as a function of the slope of the phase curve. The mixing of material under conditions of partially layered convection is examined with the help of markers.     

俯冲带深部应力场的二维弹性有限元数值模拟

基于一些简化的数值模型，根据弹性本构关系，用平面应变有限元方法计算了相变引起的体积变化、板块内部温度差、密度异常及边界力产生的应力场分布情况．数值模拟结果显示，热应力能够解释俯冲带深源地震的应力场方向特征，但解释不了深源地震的深度分布特征；有亚稳态橄榄石存在时密度异常所产生的应力场特征与地震观测结果所显示的应力场特征有所偏离；虽然橄榄石——尖晶石相变体积变化所产生的应力在橄榄石——尖晶石相变过渡区附近有最大值，其数值远远超过温度差和密度异常产生的应力场的最大剪应力数值，但在相变界面附近的区域，主压应力方向垂直于相变界面的方向，与地震观测结果所显示的主压应力的方向不一致．所以，不能用弹性模拟得出的俯冲带温度变化所产生的热应力、俯冲带密度变化所产生的应力、相变体积变化所产生的应力来对深源地震进行简单化的解释．