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

相变对体积模量和体积衰减的影响：地震频率下的地幔 P-T 实验室研究

• 批准号: 0809397
• 负责人: Li Li
• 金额: $ 21.87万
• 依托单位: SUNY at Stony Brook
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-08-01 至 2011-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0809397&HistoricalAwards=false
• 关键词: Effect; Phase; Transitions; Bulk; Modulus

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.

我们对地球内部的看法依赖于使用组成矿物的物理特性对地震速度进行建模。 通过结合现代高分辨率地震层析成像和在地幔条件下测得的最新矿物数据，可以在600 km深度下横向区域的100个开尔文温度梯度（例如AL）变化。 弹性特性定义了地震波的传播速度，而相位平衡（定义给定深度的稳定矿物）是模拟地震速度的关键要素。但是，地震波与相变的相互作用已被忽略了四十年。 通常认为确切的矿物学在速度模型中是不变的。 如果地震P波的周期与相变速率相当，而P波作为压缩力波，尽管相变的相变，则驱动少量矿物质，则P波速度将根据固体行为降低。 我们的试点实验表明，此过程对地球很重要。 确实，地震模型和矿物模型之间的不兼容持续存在，尤其是在过渡区。 200至1000公里深度之间的大多数区域都包含大量共存的高压和低压相。此外，经过体积减少相变的热力学平衡材料的有效散装模量显着低于单个相。 如果P波本身的应力诱导相变，则随着P波样品样本宽松和较低的模量，这些区域将在这些区域降低P速度。 因此，为了定义两相区域内P波的软化或衰减的量，相位过渡所需的时间与采样周期所需的时间之间的比较变得至关重要。 该建议是为了定义相变的影响在200的深度范围内的预期P波速度的影响？地球1000公里。 使用Synchrotron和多动力设备，我们开发了测量在地幔P-T和地震频率下应力 - 应变时间关系的能力。 我们的试验实验包括橄榄石旋转期期间的动力学，衰减和分散体，这意味着相变会大大降低地震速度的P速度。 我们将专注于变化阶段转换作为放松过程。 MG-FE交换控制的橄榄石 - 葡萄木岩石岩石晶矿和Al/Si扩散控制的辉石 - garnet-perovskite Transition将在该模型的背景下进行评估，以定义对地震速度和衰减的影响。该提案的目标包括（1）建立一个工作模型，该模型在地震频率的时间范围内在一阶相变期间矿物质的弹性和弹性特性支持；并且可以将其推断到地震波的应力振幅。 （2）在地幔P-T和地震频率下相变时矿物质的弹性和弹性特性。