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利用组成矿物的物理性质来模拟地震速度。通过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，可以在600公里深度的横向区域分辨100开尔文的温度梯度或几个百分点的化学成分（如Al）变化。弹性特性定义了地震波的传播速度，相平衡定义了给定深度下的稳定矿物，这是模拟地震速度的关键因素。然而，40多年来，地震波与相变的相互作用一直被忽视。在速度模型中，通常假定确切的矿物学是不变的。如果地震纵波的周期与相变速率相当，并且纵波作为一种压缩力波，通过相变驱动少量矿物，则基于固体的行为，纵波速度将降低。我们的初步实验表明，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相容仍然存在，特别是在过渡带。深度在200至1000公里之间的大部分区域包含大量高压和低压相共存。此外，经过减容相变的热平衡材料的有效体积模量明显低于单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减小，因为P波取样松弛且模量较低。因此，为了确定两相区域内P波的软化或衰减量，比较相变达到平衡所需的时间量和采样周期变得至关重要。本课题拟开展一项实验研究项目，旨在确定在200 ？在地球上1000千米。利用同步加速器和多砧装置，我们开发了在地幔P-T和地震频率下测量应力-应变-时间关系的能力。我们的先导实验包括橄榄石-尖晶石相变过程中的动力学、衰减和色散，这意味着相变将显著降低地震测量到的P速度。我们将关注作为松弛过程的体积变化相变。为了确定对地震速度和衰减的影响，将在该模型的背景下评估Mg-Fe交换控制的橄榄石-瓦德利石-环伍德-钙钛矿转变和Al/Si扩散控制的辉石石-石榴石-钙钛矿转变。本文的目标包括：(1)在地震频率时间尺度上，建立一阶相变过程中矿物的弹性和非弹性特性支持的工作模型；这可以推断出地震波的应力振幅。(2)测量地幔P-T和地震频率相变过程中矿物的弹性和非弹性性质。

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.