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了相变。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--会转化为更致密的矿物；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。位错使晶体发生塑性变形，即永久变形。这些位错可能导致暂时的减弱，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动。它还可能为地幔深处发生的地震提供解释。人们对这些深源地震仍知之甚少。尽管相变塑性很重要，但在实验室中很少观察到它的存在。这是因为它涉及复杂的过程，很难用传统方法来量化。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。他们研究了铁闪石、橄榄石富铁端元和作为大陆地壳重要组成部分的石英的可塑性。他们使用最先进的高压变形装置，安装在国家同步加速器设施中。在那里，强大的X射线可以为矿物成像，并在它们转变过程中分析它们的性质。这个多学科项目是地球物理学、材料科学和结晶学的交汇点，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。转换塑性可能在以下方面发挥核心作用：1)地幔去耦合和层状对流的发展；2)地球地幔过渡带中的板块堆积；3)造山过程中剪切带的成核；4)地幔上涌；5)深源地震的成核。然而，很少有研究考察主要造岩矿物的转化可塑性。这是因为很难量化高温(1000K)和高压(1 Gpa)下的复杂瞬变行为。在这里，该团队使用位于阿贡国家实验室高级光子源(APS)的形变-DIA(D-DIA)设备，探索了石英-柯石英和橄榄石-尖晶石的相变-在流体静力和非流体静力条件下。这种实验装置特别适合于在地幔压力和温度下就地考察与转变相关的现象。用能量色散X射线衍射法对应力和相变动力学进行了量化。轴向应变和体积应变是用X射线照相法测量的。推导了描述相变塑性的本构定律，以实验数据为基准，并根据地球条件进行了外推。RUN产物的微观结构也被详细研究--使用高分辨率的电子背散射衍射--以询问导致相变诱导弱化的物理过程。最终，该团队寻求确定地壳和地幔中变形可塑性的重要性。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.