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属性来模拟地震速度。结合现代高分辨率地震层析成像和在地幔条件下测量的最新矿物数据，可以分辨600公里深处横向区域100Kelvin温度梯度或几个百分点的化学成分(如Al)的变化。定义地震波传播速度的弹性属性和定义给定深度的稳定矿物的相平衡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，准确的矿物学通常被认为是不变的。如果地震P波的周期与相变率相当，并且P波作为一种纵力波，通过相变驱动少量矿物，则P波速度将根据固体的性质而降低。我们的初步实验表明，这一过程对地球很重要。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。大多数深度在200至1000公里之间的区域包含大量共存的高压和低压相。此外，经过体积缩减相变的热力学平衡材料的有效体积模量明显低于单个相的有效体积弹性模量。如果P波本身的应力引起相变，那么P波在这些区域的速度将会降低，因为P波具有松弛的和较低的模数。因此，相变达到平衡所需的时间量与采样周期之间的比较对于确定两相带内P波的软化量或衰减量至关重要。这项提议是进行一项实验研究计划，旨在确定相变对地球200？1000公里深度范围内预期P波速度的影响。利用同步加速器和多顶锤装置，我们发展了在地幔P-T和地震频率下测量应力-应变-时间关系的能力。我们的初步实验包括橄榄石-尖晶石相变期间的动力学、衰减和弥散，表明相变将显著降低地震测量的P速度。我们将重点介绍作为驰豫过程的体积变化相变。为了确定对地震速度和衰减的影响，将在该模型的范围内对镁-铁交换控制的橄榄石-水滑石-环木-钙钛矿转变和Al/Si扩散控制的辉石-石榴石-钙钛矿转变进行评估。这一建议的目标包括：(1)建立一个工作模型，该模型由矿物在地震频率时间尺度上的一阶相变过程中的弹性和滞弹性性质所支持；并可外推到地震波的应力幅度。(2)测量矿物在地幔P-T和地震频率相变过程中的弹性和滞弹性性质。

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.