Effect of Phase Transitions on Bulk Modulus and Bulk Attenuation: Mantle P-T Laboratory Study at Seismic Frequencies

我们对地球内部的看法依赖于使用组成矿物的物理特性来模拟地震速度。 结合现代高分辨率地震层析成像和最近在地幔条件下测量的矿物数据，可以解决100开尔文温度梯度或百分之几的化学成分（如铝）在600公里深度的横向区域的变化。 弹性性质定义了地震波传播的速度，相平衡定义了给定深度处的稳定矿物，这是模拟地震速度的关键因素。然而，地震波与相变的相互作用已经被忽略了四十多年。 在速度模型中，精确的矿物学常被假定为不变的。 如果地震P波的周期与相变速率相当，并且P波作为压缩力波通过相变驱动少量矿物，则P波速度将基于固体的行为而降低。 我们的试点实验表明，这个过程对地球很重要。 事实上，地震模型和矿物模型之间的不兼容性仍然存在，特别是在过渡区。 深度在200至1000公里之间的大多数区域包含大量共存的高压和低压相。此外，经历体积减小相变的热平衡材料的有效体积模量显著低于各个相的有效体积模量。 如果P波本身的应力引起相变，则P波速度将在这些区域中减小，因为P波采样松弛和较低的模量。 因此，为了确定两相区内P波的软化或衰减量，相变达到平衡所需的时间与采样周期之间的比较变得至关重要。 这个建议是进行一项实验研究计划，目的是确定在200？在地球上1000公里。 利用同步加速器和多砧装置，我们已经发展了测量地幔P-T和地震频率下的应力-应变-时间关系的能力。 我们的试点实验，其中包括动力学，衰减和分散在橄榄石尖晶石相变意味着相变将显着降低P速度测量地震。 我们将集中讨论作为弛豫过程的体积变化相变。 Mg-Fe交换控制的橄榄石-wadsleyite-ringwoodite-钙钛矿转变和Al/Si扩散控制的辉石-石榴石-钙钛矿转变将在该模型的范围内进行评估，以确定对地震速度和衰减的影响。该建议的目标包括：（1）建立一个工作模型，该模型由地震频率时间尺度上的一阶相变期间矿物的弹性和滞弹性特性支持;并且可以外推到地震波的应力振幅。 (2)在地幔P-T和地震频率下测量矿物相变期间的弹性和滞弹性特性。

Our view of the interior of the Earth relies on modeling seismic velocities using physical properties of the constituent minerals. One can resolve 100 Kelvin temperature gradient or a few percent chemical component (such as Al) variation in lateral regions at 600 Km depth by combining modern high resolution seismic tomography and recent mineral data measured at mantle conditions. Elastic properties, which define how fast seismic wave travels, and phase equilibrium, which defines the stable minerals at a given depth, are the key ingredients to simulate seismic velocities. However, the interaction of the seismic wave with phase transitions has been ignored for over four decades. The exact mineralogy is often assumed to be unchanging in the velocity models. If the period of the seismic P wave is comparable to the phase transition rate, and the P wave, as a compressional force wave, drives a small amount of minerals though phase transitions, P wave velocities will be reduced based on behavior of solids. Our pilot experiments suggest this process is important for the Earth. Indeed incompatibilities between seismic models and mineral models persist, particularly in the transition zone. Most regions between 200 and 1000 km depth contain significant amounts of coexisting high- and low-pressure phases. Furthermore, the effective bulk modulus of thermodynamically equilibrated materials undergoing a volume reducing phase transformation is significantly lower than that of the individual phases. If the stress of the P wave itself induces phase transitions, then the P velocity will be reduced in these regions as the P waves sample a relaxed and lower modulus. A comparison between the amount of time required by phase transitions to reach equilibrium and the sampling period thus becomes crucial in order to define the amount of softening or attenuation of P waves within a two-phase zone. This proposal is to conduct an experimental research program aimed at defining the effect of phase transformations on the expected P wave velocity in the depth range of 200 ? 1000 km in the Earth. Using synchrotron and a multi-anvil device, we have developed the capability of measuring stress-strain-time relations at mantle P-T and seismic frequencies. Our pilot experiments which include the kinetics, attenuation, and dispersion during the olivine-spinel phase transition imply that phase transitions will significantly reduce P velocities measured seismically. We will focus on volume changing phase transformations as relaxation processes. Mg-Fe exchange controlled olivine-wadsleyite-ringwoodite-perovskite transition and Al/Si diffusion controlled pyroxene-garnet-perovskite transition are to be evaluated within the context of this model in order to define the effects on seismic velocities and attenuation. The goals of this proposal include (1) Establish a working model that is supported by elastic and anelastic properties of minerals during first-order phase transitions at the time scale of seismic frequencies; and that can be extrapolated to the stress amplitudes of a seismic wave. (2) Measure elastic and anelastic properties of minerals during phase transitions at mantle P-T and seismic frequencies.