Fate and role of metastable pyroxenes in the subduction process

俯冲带是一种地质环境，在板块构造和地壳物质向地幔的再循环中起着特别重要的作用。大多数5.5级以上的地震和许多火山活动都沿着构造板块的边界分布，并起源于俯冲带（例如火环），这两者都对人类文明产生了重大影响。辉石矿物大量存在于俯冲板块内，其转化与控制俯冲过程有关。更好地了解这些矿物在微观尺度上的转变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破坏。在这个项目中，研究小组将使用一种金刚石砧细胞装置，一种微型手持设备，能够在地球内部数百公里深处产生压力和温度条件，并将对辉石家族的不同成员在被运送到俯冲板块内的地幔中时所经历的转变进行调查。这些实验的结果将由量子力学计算补充，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，在这个项目中，研究人员将通过参加夏威夷电视台的现场采访和参加CIDER计划等活动，重点教育更广泛的科学界和公众关于矿物物理研究的知识。虽然大多数地幔辉石在过渡带以下消失（要么溶解为石榴石，要么转化为致密的高压多晶岩，要么分解为氧化物和致密硅酸盐），但在辐合边缘，冷俯冲板块的条件以温度低得多为特征，并促进辉石和橄榄石等矿物在其热力学稳定场之外的亚稳保存，这可能导致板块浮力和地震触发。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公里）。Px稳定相图的一般拓扑结构已经建立，然而，直到最近，很少有实验研究冒险进入亚稳态压缩状态，这对冷俯冲带最重要。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，发现了许多有趣的，以前未知的辉石族相变，发生在低于软锰矿地热温度的压缩下。这些转变不连续地改变辉石的热力学性质，作为俯冲板块内深度的函数，并且存在于广泛的辉石成分中。目前的项目继续探索辉石亚稳相边界和亚稳多态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改变是微妙的，并且伴随的密度不连续很小，但它们对热力学和弹性性质、相边界的PT斜率以及俯冲带内浮力关系的影响可能是显著的；（H2）缺陷，如由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，这些特征将在挖掘的岩石样品中被识别；（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界相作为中间体存在，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.