Collaborative Research: Transformation plasticity as a transient creep mechanism in Earth's crust and mantle

构成地壳和地幔的矿物经历相变。它们的晶体结构会随着温度和压力的变化而发生变化。随着压力的增加，橄榄石-上地幔的主要成分-转化为密度更大的矿物;就像石墨转化为钻石一样。当一种矿物转变成另一种矿物时，就会产生晶体缺陷，称为位错。位错允许晶体塑性变形，即，永久的 这些位错会导致暂时的弱化，从而加速地壳和地幔的流动。相变引起的弱化--“相变塑性”--可能影响驱动板块构造的地幔流。它也可以解释发生在地幔深处的地震。 人们对这些深源地震仍然知之甚少。尽管相变塑性很重要，但在实验室中很少观察到。这是因为它涉及复杂的过程，难以用传统方法量化。在这里，研究人员在地幔中普遍存在的极端压力和温度下进行实验。 他们研究铁橄榄石、富铁橄榄石端元和石英的可塑性，石英是大陆地壳的重要组成部分。他们使用国家同步加速器设施中最先进的高压变形装置。 在那里，强大的X射线可以对矿物进行成像，并分析它们在转变过程中的特性。 这个多学科项目，在地球物理学，材料科学和晶体学的交叉点，旨在扩大对地球内部动力学的理解。它支持早期职业科学家的专业发展。它还为一名研究生和几名本科生提供培训。相变塑性可能在以下方面发挥核心作用：1）地幔解耦和层状对流的发展; 2）地球地幔过渡带中的板状积水; 3）造山作用期间的剪切带成核; 4）地幔柱上涌; 5）深源地震的成核。然而，很少有研究主要造岩矿物的相变塑性。这是因为在高温（1000 K）和高压（1 GPa）下难以量化复杂的瞬态行为。在这里，研究小组探索了石英柯石英和橄榄石尖晶石相变-在流体静力学和非流体静力学条件下-使用位于阿贡国家实验室先进光子源（APS）的变形DIA（D-DIA）装置。这个实验装置是唯一适合于研究在地幔压力和温度在原地的转换相关的现象。应力和相变动力学通过能量色散X射线衍射进行量化。轴向和体积应变测量使用X射线照相术。本构定律描述变形塑性推导，基准实验数据，并外推到地球条件。运行产品的微观结构也进行了详细研究-使用高分辨率电子背散射衍射-询问的物理过程中负责的转换引起的削弱。最终，该团队寻求确定地壳和地幔中变形塑性的重要性。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The minerals that comprise Earth’s crust and mantle undergo phase transformations. Changes in their crystal structure occur in response to changes in temperature and pressure. With increasing pressure, olivine — the main constituent of the upper mantle — transforms into denser minerals; much like graphite transforms into diamond. As one mineral transforms into another, crystal defects, known as dislocations, are produced. Dislocations allow crystals to deform plastically, i.e., permanently. These dislocations can cause temporary weakening that speeds up crust and mantle flows. Transformation-induced weakening — "transformation plasticity" — may impact the mantle flows driving plate tectonics. It may also provide an explanation for the earthquakes occurring at great depths in the mantle. These deep earthquakes are still poorly understood. Despite its importance, transformation plasticity has seldom been observed in the laboratory. This is because it involves complex processes, difficult to quantify by conventional methods. Here, The researchers conduct experiments at the extreme pressures and temperature prevailing in the mantle. They study the plasticity of fayalite, olivine Fe-rich end member, and quartz which is an important constituent of the continental crust. They use a state-of-the-art high-pressure deformation device set up at a national synchrotron facility. There, powerful X-rays allow imaging the minerals and analyzing their properties during their transformations. The multidisciplinary project, at the intersection of geophysics, materials science, and crystallography, aims to expand the understanding of Earth’s interior dynamics. It supports the professional development of an early career scientist. It also provides training for one graduate and several undergraduate students.Transformation plasticity may play a central role in: 1) mantle decoupling and the development of layered convection; 2) slab ponding in Earth’s mantle transition zone; 3) shear zone nucleation during orogenesis; 4) mantle plume upwelling; and 5) the nucleation of deep-focus earthquakes. However, few studies have examined transformation plasticity in major rock-forming minerals. This is because of the difficulty to quantify complex transient behaviors at high temperatures (1000 K) and pressures ( 1 GPa). Here, the team explores the quartz-coesite and olivine-spinel phase transformations — under hydrostatic and non-hydrostatic conditions — using a Deformation-DIA (D-DIA) apparatus located at the Advanced Photon Source (APS) at Argonne National Laboratory. This experimental setup is uniquely suited to examining transformation-related phenomena in situ at mantle pressures and temperatures. Stresses and transformation kinetics are quantified via energy-dispersive X-ray diffraction. Axial and volume strains are measured using X-ray radiography. Constitutive laws describing transformation plasticity are derived, benchmarked against experimental data, and extrapolated to Earth conditions. Run product microstructures are also investigated in detail — using high-resolution electron backscatter diffraction — to interrogate the physical processes responsible for transformation-induced weakening. Ultimately, the team seeks to determine the importance of transformation plasticity in Earth’s crust and mantle.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.