Fate and role of metastable pyroxenes in the subduction process

俯冲带是在板块构造和地壳物质再循环进入地幔中发挥特别重要作用的地质环境。大多数5.5级以上的地震和许多火山活动都极大地影响了人类文明，它们都分布在构造板块的边界上，并起源于俯冲带(如火环)。俯冲板块中大量存在辉石矿物，它们的转变与控制俯冲过程有关。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喷发的能力，并可能有助于减轻它们造成的一些损害。在这个项目中，该团队将使用钻石砧座装置，这是一种微型手持设备，能够在地球内部数百公里深的地方产生压力和温度条件，并将对辉石家族的不同成员在俯冲板块内运输到地幔时经历的变化进行调查。这些实验的结果将得到量子力学计算的补充，并将被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，在这个项目中，研究人员将专注于通过参加夏威夷电视台的现场采访和参与苹果酒节目等活动，教育更广泛的科学界和普通公众关于矿物物理研究的知识。虽然大多数地幔辉石消失在过渡带以下(要么溶解为石榴石，转化为致密的高压多晶型，要么分解为氧化物和致密硅酸盐)，而在会聚边缘，冷俯冲板块的条件具有温度低得多的特点，并促进辉石和橄榄石等矿物的亚稳保存，超出其热力学稳定场，这可能会对板块浮力和地震触发造成影响。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(深度远远超过1000公里)。PX稳定相图的一般拓扑结构已经得到了很好的证实，然而，直到最近，很少有实验研究冒险进入亚稳态压缩区域，最重要的是冷俯冲带。在美国国家科学基金会之前的资助下，Pi和他的合作者使用了同步加速器单晶显微衍射技术，发现了辉石家族中一些有趣的、以前未知的相变，这些相变发生在低于地热的温度下的压缩。这些转变不连续地改变辉石的热力学性质，作为俯冲板内深度的函数，并存在于广泛的辉石成分中。目前的项目继续探索辉石的亚稳相边界和亚稳多晶性的地球动力学后果。该团队的目标是探索三个主要假设：(H1)尽管亚稳相变引入的结构变化是微妙的，伴随的密度不连续很小，但它们对热力学和弹性性质、相界的PT斜率以及俯冲带内的浮力关系的影响可能是显著的；(H2)缺陷，如亚稳相变引起的位错，对辉石相图有重大影响，并可能留下可在出土岩石样品中识别的可熄灭的特征；(H3)在冷板坯中大多数PX成分的致密化过程中，具有五配位硅的明确晶相作为中间相存在，并可能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.