CaTiO3－钙钛矿的晶体结构相变、高温流变及其下地幔地球动力学意义

利用高精度的ＭＴＳ材料实验系统，在１４７３－１６７３Ｋ的温度区间和可控热力学环境条件下，对［１００］ｐｃ和［１１０］ｐｃ两个方位的ＣａＴｉＯ₃－钙钛矿单晶体进行了高温蠕变试验，着重观测了晶体结构相变和高温流变的相关性．研究表明，钙钛矿的晶体结构相变对其高温流变性质有明显影响，斜方晶系钙钛矿蠕变强度大大高于四方和立方晶系钙钛矿，后两者具有很强的高温塑性各向异性．在分析相变过程晶体结构变化特征和观测到的高温流变数据基础上，认为钙钛矿晶体结构相变中发生的位错结构的变化是导致其高温流变性变化的主要物理机制．     

钙钛矿的高温塑性：下地幔的流变性

重点介绍了近年来国际地球物理学界在钙钛矿高温塑性研究领域内取得的进展。依据高温高压实验岩石学和矿物物理测试结果，科学家们提出硅酸盐钙钛矿是下地幔最主要的矿物相。采用相似材料理论和高温高压实验技术，在过去几年里对钙钛矿的高温塑性变形进行了深入细致的实验研究，就钙钛矿的高温塑性强度，流变机制和塑性各向异性等进行了探讨，这些开创性的研究特别强调了钙钛矿晶体结构相变和高温塑性的关系，并对应用矿物的晶体结构－－塑性系统相关性外推未知矿物流变强度的可能性作了探讨。作为一个直接的应用，这些研究成果被广泛的用来讨论有关的下地幔地球动力学问题，特别是下地幔的流动强度，流动机制和流动弱化。     

在二维应力状态下土体地震动偏应力的特征

地震作用在土体中产生的动偏应力包括动水平剪应力τｖｈ 和动正应力差［σｐ － σｈ］ ／２ 。本文讨论了这两种动偏应力分量之间相互变化的关系     

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.     

由观测复发间隔得到的经验地震概率

给定断层段在前面一次破裂后某一特定时间Ｔ发生破裂的概率Ｐ，可由观测到的该断层段上复发间隔的样本作经验性的评估。假设的全部条件是特定时间段内破裂的概率与该断层段上所有破裂轮回的概率相同。假定观测到的ｎ个复发间隔中有ｍ个与前面一次地震后段Ｔ内发生破裂的轮回相应。新的一次轮回中破裂也落在最近一次地震后时间段Ｔ内的概率密度由β分布Ｐ（ｐ│ｍ，ｎ）＝｛（ｎ＋１）！／［ｍ！（ｎ－ｍ）！］｝ｐ＾ｍ（１－ｐ）＾ｎ     

俯冲带流变性质的研究

对不同俯冲速度下俯冲带的热结构和相结构进行了耦合计算, 并结合矿物流变的理论和实验结果给出了不同俯冲速度下俯冲带的流变结构. 浅部俯冲带上表层有1034 Pa·s以上的高粘度值, 而俯冲带下表层的粘度则较小, 但也大于1026 Pa·s; 在橄榄石-尖晶石动力学相变界面以下的深度, 俯冲带的粘度有明显的减小, 最低可达约1022 Pa·s; 在700 km附近尖晶石相-钙钛矿相变界面以上的小区域内有一个粘度明显高于周围区域的高粘度区, 其存在是因为尖晶石相-钙钛矿的相变是一个吸热过程; 尖晶石相占1%到99%的等值线之间的距离有从浅部的约1 km到最底部的约5 km的变化, 初步估计这是高粘度的俯冲带深部的一个低粘度区, 其存在将影响俯冲带的动力学性质.     

俯冲带的负浮力及其影响因素

基于Ｍｕｒｎａｇｈａｎ－Ｂｉｒｃｈ状态方程，计算了密度Ｐ、热膨胀系数ａｖ、定压比热ｃｐ等物性参数在上地幔温压条件下的分布，给出了热传导系数ｋ在上地幔温压条件下的分布．采用准动力学计算方案，用有限元方法计算了板块不同俯冲速度、角度、不同厚度及不同俯冲深度情况下俯冲带的温度分布和密度分布．计算了不同俯冲模型下俯冲带的负浮力及其对岩石层板块形成的等效应力，发现负浮力在俯冲过程中是变化的，俯冲速度和板块厚度对其有明显的影响，在稳定俯冲状态，负浮力约为（１．５－２．３）×１０^１３Ｎｍ^－１，等效张应力约为０．２５－０．２９Ｇｐａ．     

上行离子密度分布特性的理论研究

采用动力学方程，求解了定态情况下磁层中上行离子沿磁力线的分布函数，针对不同的Ｋ_ｐ指数，分别对北半球极光带区起源的上行离子Ｏ_＋、Ｈ_＋和Ｈｅ_＋在子午面内沿磁力线的密度分布及其特性进行了研究．结果表明：１．沿磁力线向外，上行离子密度在近地空间呈急剧下降趋势，在远地空间呈缓慢下降趋势；２．重离子或初始能量较小的离子，其密度沿磁力线向外下降较快；３．Ｋ_ｐ指数越大，离子进入磁层的空间范围越大；４．离子的投掷角分布对密度分布的影响甚小；５．离子密度沿磁力线的下降程度随到Ｘ轴距离的增大而呈增大趋势；６．在典型参数条件下，求得上行离子Ｏ_＋在等离子体片边界附近的密度为１０^－３－１０^－２个ｃｍ^－３，这与观测结果相一致．     

带阻尼的一维地震反射波成像

本文是文献［８］的继续，文中提出了地震勘探的带阻尼波动方程反演的新方法，应用此反演算法可以通过微机识别地层的波阻抗，这里把地层分成１００层，地震数据含有１０％的误差，且反演计算时间仅仅是正演计算时间的三倍．文献［１］－［４］的计算方法是基于波动方程的混合问题，而文献［８］所提出的反演算法是基于波动方程的哥西问题，许多例子充分说明后者更有效．     

聊古一井数字化氡观测资料的分析与应用

对聊古一井SD－3A数字化氡观测资料的变化特征进行了分析，与该井原有的模拟观测资料进行了对比研究，并对数字化氡观测资料的应用与分析方法进行了探讨。认为尽管SD－3A自动测氡仪所测为逸出氡与模拟观测溶解氡的物理量有所不同，但两具有基本一致的变化趋势，且数字化氡具有较强的映震能力。     

Elastic and anelastic anomalies in (Ca,Sr)TiO3 perovskites: Analogue behaviour for silicate perovskites

Values of bulk modulus (K), shear modulus (G) and mechanical quality factor (Q) have been determined for polycrystalline samples across the CaTiO₃ (CST0)–SrTiO₃ (CST100) solid solution by resonant ultrasound spectroscopy. Because of similarities with low frequency elastic and anelastic anomalies due to twin wall motion reported in previous studies, a working hypothesis is developed in which dissipation processes are interpreted in terms of twin wall displacements. At high temperatures in CST50 the stability field of the I4/mcm structure is marked by the disappearance of all resonance peaks (superattenuation). This is attributed to anelastic domain wall sliding. At room temperature the I4/mcm phase of CST70 and CST80 has values of G which are lower than those of cubic or orthorhombic phases, and a concomitant drop in Q is interpreted as implying that the domain wall pinning process reported elsewhere to occur below 400–450 K is only partial. A similar drop in G and Q was found in CST95 below the transition at 238 K. The I4/mcm ↔ Pbcm transition in CST70 at 230 K is marked by an abrupt increase in Q, suggesting that mobile twins in crystals with the I4/mcm structure become effectively immobile in antiferroelectric crystals with the Pbcm structure. The I4/mcm ↔ Pnma transition in CST50 is marked by a similarly abrupt increase in Q, consistent with twin walls becoming effectively immobile also in crystals with the Pnma structure. A fall in Q below 800 K in CST0, however, could imply that a degree of twin wall mobility might develop in Pnma crystals if the tetragonal spontaneous strain departs significantly from zero. The remarkable attenuation behaviour of crystals with the I4/mcm structure at the relatively low stress conditions which apply during resonances of a parallelepiped with edge dimensions of 2–4 mm, is consistent with the view that a characteristic signature for tetragonal CaSiO₃ in the Earth's lower mantle should be a marked attenuation of seismic waves.     

Phase transition in CaSiO3 perovskite

A phase transition in pure CaSiO₃ perovskite was investigated at 27 to 72 GPa and 300 to 819 K by in-situ X-ray diffraction experiments in an externally-heated diamond-anvil cell. The results show that CaSiO₃ perovskite takes a tetragonal form at 300 K and undergoes phase transition to a cubic structure above 490–580 K in a pressure range studied here. The transition boundary is strongly temperature-dependent with a slightly positive dT / dP slope of 1.1 (± 1.3) K/GPa. It is known that the transition temperature depends on Al₂O₃ content dissolved in CaSiO₃ perovskite [Kurashina et al., Phys. Earth Planet. Inter. 145 (2004) 67–74]. The phase transition in CaSiO₃(+ 3 wt.% Al₂O₃) perovskite therefore could occur in a cold subducted mid-oceanic ridge basalt (MORB) crust at about 1200 K in the upper- to mid-lower mantle. This phase transition is possibly ferroelastic-type and may cause large seismic anomalies in a wide depth range.     

Experimental evidence of transformation plasticity in silicates: minimum of creep strength in quartz

Mechanical weakening due to solid state transformation of mineral phases has long been proposed to be a significant mechanism for localization of deformation in the Earth’s lithosphere and the mantle transition zone. However, experimental observations confirming such a proposition are lacking. Here we present a novel approach to prove the existence of a minimum in the creep strength of quartz at the α–β transition by observing the deformation of fluid inclusions in a quartz crystal using a hydrothermal diamond-anvil cell. Pressure differences required for permanent deformation of the quartz around fluid inclusions were significantly lower at the phase transition than in either the stability fields of α- or β-quartz. These results indicate that transformation plasticity of silicates can indeed cause a considerable localized reduction in the strength of the Earth’s crust and mantle.     

Experimental study of the elasticity of single crystalline calcite under high pressure (the calcite I—calcite II transition at 14.6 kbar)

The length variations and the ultrasonic wave velocities were measured along principal single crystal calcite axes under high pressure.We have observed that six independent elastic constants undergo drastic changes at the calcite I—calcite II transition point at 14.6 kbar. Furthermore, we have determinated the length and volumetric variations in the phases I and II and during the I–II transition.These results allow us to deduce the following important points: influence of the planar CO₃ groups on the strong linear compressibility anisotropy in phase I; reciprocal relation between the decrease of elastic constants at the transition point and the anomaly of compressibility of the calcite II; compatibility of our data with the monoclinic structure of calcite II.     

On the magnetic phases of natural pyrrhotites

Summary The magnetic phases of natural pyrrhotites were studied by the method of powder patterns, the chemism of the magnetic phases by means of the electron microprobe method, and the crystal phases by etching the polished sections. The method of powder patterns distinguished the ferrimagnetic and antiferromagnetic phases in the transition-type pyrrhotites. The method of absorbed electrons lead to the discovery that both phases have a different chemical composition and that the ferrimagnetic phase has a lower denisty and an average atomic number. The linear and point-by-point analysis disclosed that the ferrimagnetic phase has a lower weight content of Fe and a higher weight content of S than the antiferromagnetic phase. Both the magnetic phases displayed different properties when etched, and therefore they not only have a different chemical composition, but also a different crystal structure. The necessity of studying the image of the absorbed electrons and the image of the etched surface of the polycrystalline samples of pyrrhotites of the transition type separately on individual grains was pointed out.     

Molecular modeling of the 10-Å phase at subduction zone conditions

Molecular dynamics (MD) modeling of the 10-Å phase, Mg₃Si₄O₁₀(OH)₂·xH₂O, with x=2/3, 1.0 and 2.0 shows complex structural changes with pressure, temperature and water content and provides new insight into the structures and stabilization of these phases under subduction zone conditions. The structure(s) of this phase and its role as a reservoir of water in the mantle have been controversial, and these calculations provide specific predictions that can be tested by in situ diffraction studies. At ambient conditions, the computed structures of talc (x=0) and the 10-Å phases with x=2/3 and 1.0 are stable over the 350-ps period of the MD simulations. Under these conditions, the 10-Å phases show phlogopite-like layer stacking in good agreement with previously published structures based on powder X-ray diffraction data for samples quenched from high-pressure and high-temperature experiments. The calculations show that the 10-Å phase with x=2.0 is unstable at ambient conditions. The computed structures at P=5.5 GPa and T=750 K, well within the known stability field of the 10-Å phase, change significantly with water content, reflecting changing H-bonding configurations. For x=2/3, the layer stacking is talc-like, and for x=1.0, it is phlogopite-like. The calculations show that transformation between these two stackings occurs readily, and that the talc-like stacking for the x=2/3 composition is unlikely to be quenchable to ambient conditions. For x=2.0, the layer stacking at P=5.5 GPa and T=750 K is different than any previously proposed structure for a 10-Å phase. In this structure, the neighboring basal oxygens of adjacent magnesium silicate layers are displaced by b/3 (about 3 Å) resulting in the Si atoms of one siloxane sheet being located above the center of the six-member ring across the interlayer. The water molecules are located 1.2 Å above the center of all six-member rings and accept H-bonds from the OH groups located below the rings. The b/3-displaced structure does not readily transform to either the talc-like or phlogopite-like structure, because neither of these stackings can accommodate two water molecules per formula unit. There is likely to be a compositional discontinuity and phase transition between the b/3-displaced phase and the phase with phlogopite-like stacking. The simulations reported here are the first to use the recently developed CLAYFF force field to calculate mineral structures at elevated pressures and temperatures.     

Olivine-spinel Phase Boundary Rheology, Strength and Deep Seismicity

In this paper,we discuss (1) whether a significant change in dominant creep mechanism will occur at 400 km discontinuity in some subducting slabs as a result of olivine-spinel phase transition;(2) could the result influence phase boundary strength and deep seismicity? Through this study,we noticed that a transition of creep mechanism from dislocation to diffusion (or superplasticity) could occur at the olivine-spinel phase boundary where temperature effect on phase transition dominates over pressure,which will result in a weakening phase boundary.Triggered by this phase transition weakening,a deep strong earthquake might be generated in the relatively strengthening region above the phase boundary so that the phase boundary is naturally the ceasing boundary of deep seismicity.Contrasting to this,the transition of the creep mechanism from dislocation glide to dislocation climb may be common at the phase boundary where pressure effect on phase transition dominates over temperature.In this case,olivine-spin     

地幔的蠕变性质

地幔的蠕变特性是地球物理学中一个普遍关心的问题。本文以地球模型和地球热模型为基础,结合理论导出的蠕变方程和岩石蠕变实验的结果,考虑到温度、压力、晶粒直径及相变过程对地幔流变特性的影响,计算了整个地幔的蠕变性质和等效粘滞性。 结果表明,当晶粒直径d≤1毫米时,扩散蠕变是主要的,地幔行为与牛顿流体相似。当d≥10毫米时,幂函数蠕变成为主要的形变机制,地幔呈非牛顿粘滞性。从得出的结果来看,并不存在高粘的下地幔。