Fate and role of metastable pyroxenes in the subduction process

俯冲带是在板块构造和地壳物质向地幔再循环中发挥特别重要作用的地质环境。大多数震级大于5.5的地震和火山活动都对人类文明产生了巨大的影响，它们都沿着构造板块的边界分布，并起源于俯冲带（例如火环）。辉石矿物大量存在于俯冲板片中，它们的转化与俯冲过程的控制有关。更好地了解这些矿物在微观尺度上发生的变化将提高我们预测地震和火山爆发的能力，并可能有助于减轻它们造成的一些损害。在该项目中，该团队将使用金刚石砧室设备，能够在地球内部数百公里深处产生压力和温度条件的微型手持设备，并将对辉石家族的不同成员进行调查。 这些实验的结果将得到量子力学计算的补充，并将纳入地球物理建模软件，以评估对俯冲过程和地幔对流的影响。除了具体的科学目标外，在该项目中，研究人员将通过参与夏威夷电视台的现场采访和参与CIDER计划等活动，重点教育更广泛的科学界和公众了解矿物物理学研究。而大多数地幔辉石在过渡带以下消失（或者溶解成石榴石，转变成致密的高压多晶型物，或者分解成氧化物和致密的硅酸盐），在会聚边缘，冷俯冲板片的条件的特征在于低得多的温度，并且促进矿物（例如辉石和橄榄石）的亚稳态保存超过它们的热力学稳定场，可能导致板浮力和地震触发。辉石（Px）结构具有显著的弹性，如果温度不超过1000°C，可以很容易地保存到高达60 GPa的压力（深度超过1000 km）。Px稳定相图的一般拓扑结构已经很好地建立，然而，直到最近，很少有实验研究冒险进入亚稳压缩制度，最重要的冷俯冲带。在之前的NSF资助下，PI和合作者使用同步加速器单晶微衍射技术，发现了辉石家族中一些有趣的，以前未知的相变，在低于软岩地热的温度下压缩时发生。这些转换不连续地改变辉石的热力学性质作为俯冲板内的深度的函数，并存在于广泛的辉石组合物。本项目继续探索辉石的亚稳相界和亚稳多晶型的地球动力学后果。该团队的目标是探索三个主要假设：（H1）即使由亚稳相变引入的结构修改是微妙的，伴随的密度不连续性很小，但它们对热力学和弹性性质，相边界的PT斜率以及俯冲带内的浮力关系的影响可能是显着的;（H2）缺陷，如亚稳相变引起的位错，对辉石相图有重大影响，并可能留下可淬灭的特征，在挖出的岩石样品中可以识别;（H3）具有五配位硅的明确定义的结晶相在冷板坯内的大多数Px组合物的致密化期间作为中间体存在，并且可显著影响反应性和流变性。

Subduction zones are geological environments that play a particularly important role in plate tectonics and recycling of crustal material into the mantle. A majority of earthquakes with magnitude greater than 5.5, and much of volcanic activity, both of which dramatically affect human civilization, are distributed along the boundaries of tectonic plates, and originate within subduction zones (e.g. the Ring of Fire). Pyroxene minerals are present within the subducting slabs in large quantities and their transformations are relevant for controlling the subduction process. Better understanding of the transformations these minerals undergo on microscopic scale will improve our ability to predict earthquakes and volcanic eruptions, and may help to mitigate some of the damage they cause. Within this project the team will use a diamond anvil cell apparatus, miniature hand-held devices capable of generating pressure and temperature conditions found at hundreds of kilometers of depth within the Earth interior, and will conduct an investigation of transformations that different members of the pyroxene family undergo, while transported into the Earth's mantle within the subducting slabs. The results of these experiments will be complemented by quantum mechanical calculations and will be incorporated into geophysical modeling software to assess the effects on the subduction process and mantle convection. Besides specific scientific goals, within this project the investigators will focus on educating the broader scientific community and the general public about mineral physics research through activities such as participation in live interviews on Hawaiian TV and participation in the CIDER program. While most mantle pyroxenes disappear below the transition zone (either dissolved into garnet, transformed to dense high-pressure polymorphs, or decomposed into oxides and dense silicates), at convergent margins the conditions of the cold subducting slab are characterized by much lower temperatures and promote metastable preservation of minerals such as pyroxenes and olivine beyond their thermodynamic stability fields, with possible consequences for slab buoyancy and earthquake triggering. The pyroxene (Px) structure is remarkably resilient, and if the temperature does not exceed 1000°C, can be easily preserved to pressures as high as 60 GPa (depths well beyond 1000 km). The general topology of the Px stable phase diagram has been well-established, however, until recently, very few experimental studies ventured into the metastable compression regime, most important for cold subduction zones. With previous NSF funding the PI and collaborators used synchrotron single-crystal micro-diffraction technique and discovered a number of intriguing, previously unknown phase transformations in the pyroxene family, occurring on compression at temperatures lower than the pyrolite geotherm. These transformations discontinuously change the thermodynamic properties of pyroxenes as a function of depth within the subducting slab, and are present across a wide range of pyroxene compositions. This current project continues exploration of the metastable phase boundaries and geodynamic consequences of the metastable polymorphism of pyroxenes. The team is aiming to explore three primary hypotheses: (H1) Even though the structural modifications introduced by the metastable phase transitions are subtle, and accompanying density discontinuities small, their consequences on thermodynamic and elastic properties, PT slopes of phase boundaries, as well as buoyancy relations within subduction zone can be significant; (H2) Defects, such as dislocations caused by metastable phase transitions, have significant consequences for pyroxene phase diagrams, and may leave quenchable signatures that will be recognizable in exhumed rock samples; (H3) Well-defined crystalline phases with penta-coordinated silicon are present as intermediates during densification of most Px compositions within cold slabs, and may significantly affect reactivity and rheology.