In Situ X-ray Diffraction Study of Phase Transitions in Shock-Compressed Minerals

地球内部深处的高压和高温将地球表面附近的矿物转化为密度更大的晶体结构。由于过去的陨石撞击事件，在地球表面也发现了一些高密度形式的微量矿物质。在实验室的冲击压缩实验中，通过高速弹丸撞击矿物，矿物可以迅速被驱动到地球内部深处的高压和高温条件下。因此，冲击压缩实验几十年来在地球科学中发挥了重要作用，提供了对高压矿物的力学和声学特性的深入了解。然而，以往对矿物的冲击压缩实验并没有直接揭示冲击过程中形成的晶体结构，冲击压缩实验的解释依赖于与静态高压实验结果的对比。在本研究中，高能同步x射线将用于对矿物的高速撞击实验，以直接研究在撞击事件中矿物结构在纳秒到微秒时间尺度上的演变。这一发现将提高我们对地幔中矿物质的理解，也将使我们更好地理解陨石撞击事件中受到冲击的矿物质中发生的晶体结构变化。该项目将支持培训一名研究生和两名本科生，他们将接受有关国家同步加速器设施的培训。该项目将在先进光子源的动态压缩部门（DCS）进行。DCS结合了半英寸口径两级气炮，能够发射速度高达6公里/秒的弹丸，具有高能同步加速器x射线衍射能力。具有代表性的矿物，如长辉石、金红石和橄榄石，将通过高速撞击被冲击压缩到高达约60千兆帕斯卡的应力，与大型天然陨石撞击事件中产生的压力相对应。用速度干涉法测量受冲击样品后表面的速度历史；将对速度历史进行分析，以确定受冲击矿物的应力和密度。粉末x射线衍射测量也将在撞击事件中对矿物样品进行，使用透射x射线几何结构，其中入射x射线穿过撞击器（LiF或聚碳酸酯），矿物样品和x射线窗口（LiF或聚碳酸酯）。在撞击事件中，将使用x射线探测器获得四个x射线衍射帧（100皮秒持续时间快照，时间间隔为153.4纳秒）。所得的衍射图将用于识别高压矿物结构。受冲击的单晶矿物的衍射模式也将被分析，以了解高压结构的纹理，这将为结构转变过程中发生的原子运动提供见解。

High pressures and temperatures deep within the Earth transform minerals found near the Earth's surface to denser crystalline structures. High density forms of some minerals are also found in trace amounts at the Earth's surface as a result of past meteorite impact events. By impacting minerals with high-speed projectiles in laboratory shock-compression experiments, minerals can be rapidly driven into the high pressure and high temperature conditions occurring deep within the Earth's interior. As such, shock compression experiments have played an important role in earth science for decades by providing insight into the mechanical and acoustic properties of high-pressure minerals. However, past shock compression experiments on minerals have not directly revealed the crystal structures formed during impact and interpretation of shock compression experiments has relied on comparison with results of static high-pressure experiments. In the present research, high-energy synchrotron x-rays will be used during high-speed impact experiments on minerals to directly examine how mineral structures evolve on nanosecond to microsecond timescales during an impact event. The findings will improve our understanding of minerals in the Earth's mantle and will also allow us to better understand the crystal structure modifications occurring in shocked minerals during meteorite impact events. This project will support the training of a graduate student and two undergraduates who will be trained on National synchrotron facilities.This project will be performed at the Dynamic Compression Sector (DCS) at the Advanced Photon Source. The DCS combines a half-inch bore two-stage gas gun capable of launching projectiles with velocities up to 6 km/s with high-energy synchrotron x-ray diffraction capability. Representative minerals such as enstatite, rutile and olivine will be shock compressed via high-speed impact to stresses up to about 60 gigapascals, pressures corresponding to those occurring in large natural meteorite impact events. The velocity histories at the rear surface of the shocked samples will be measured using velocity interferometry; the velocity histories will be analyzed to determine the stress and density of the shocked minerals. Powder x-ray diffraction measurements will also be made on the mineral samples during the impact event using a transmission x-ray geometry in which the incident x-rays pass through the impactor (LiF or polycarbonate), the mineral sample and an x-ray window (LiF or polycarbonate). An x-ray detector will be used to obtain four x-ray diffraction frames (100 picoseconds duration snapshots separated in time by 153.4 nanoseconds) during the impact event. The resulting diffraction patterns will be used to identify the high-pressure mineral structures. Diffraction patterns for shocked single crystal minerals will also be analyzed to understand the high-pressure structure texture which will provide insight into the atomic motions occurring during the structural transformations.