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

地球深处的高压和高温将地球表面附近发现的矿物转化为更致密的晶体结构。由于过去的陨石撞击事件，在地球表面也发现了微量的高密度形式的某些矿物。通过在实验室冲击压缩实验中用高速弹丸撞击矿物，矿物可以迅速进入地球内部深处发生的高压和高温条件。因此，几十年来，冲击压缩实验通过提供对高压矿物的机械和声学特性的深入了解而在地球科学中发挥了重要作用。然而，过去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比较。在目前的研究中，高能同步加速器X射线将用于矿物的高速撞击实验，以直接研究撞击事件期间矿物结构如何在纳秒到微秒的时间尺度上演变。这些发现将提高我们对地幔中矿物的理解，也将使我们能够更好地了解陨石撞击事件期间受冲击矿物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，他们将在国家同步加速器设施上接受培训，该项目将在高级光子源的动态压缩部门进行。DCS结合了一个半英寸口径的两级气体炮，能够发射速度高达6公里/秒的弹丸，并具有高能同步加速器X射线衍射能力。代表性的矿物，如顽火辉石，金红石和橄榄石将通过高速冲击冲击压缩到高达约60吉帕斯卡的应力，相当于那些发生在大型天然陨石撞击事件的压力。将使用速度干涉测量法测量受冲击样品后表面的速度历史;将分析速度历史以确定受冲击矿物的应力和密度。还将在撞击事件期间对矿物样品进行粉末X射线衍射测量，使用透射X射线几何学，其中入射X射线穿过撞击物（氟化锂或聚碳酸酯）、矿物样品和X射线窗口（氟化锂或聚碳酸酯）。在撞击事件期间，将使用一个X射线探测器获取四个X射线衍射帧（100皮秒的快照，时间间隔为153.4纳秒）。由此产生的衍射图案将用于识别高压矿物结构。还将分析冲击单晶矿物的衍射图案，以了解高压结构纹理，这将提供对结构转变过程中发生的原子运动的洞察。

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.