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

地球深处的高压和温度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，一些矿物的高密度形式也在地球表面以微量形式被发现。在实验室的冲击压缩实验中，通过用高速抛射物撞击矿物，矿物可以被迅速驱动到地球内部深处的高压和高温条件下。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洞察，在地球科学中发挥了重要作用。然而，过去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射线将在对矿物的高速撞击实验中使用，以直接检查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化。这些发现将提高我们对地幔中矿物的了解，也将使我们更好地了解陨石撞击事件中受冲击的矿物中发生的晶体结构变化。该项目将支持对一名研究生和两名本科生的培训，他们将接受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。分布式控制系统结合了半英寸口径二级气炮，能够发射速度高达6公里/S的炮弹，并具有高能同步X射线衍射能力。顽辉石、金红石和橄榄石等代表性矿物将通过高速撞击受到冲击压缩，压力高达约60千兆帕斯卡，压力相当于大型天然陨石撞击事件中的压力。用速度干涉法测量冲击样品后表面的速度历史；分析速度历史以确定冲击矿物的应力和密度。在撞击事件期间，还将使用透射式x射线几何结构对矿物样品进行粉末x射线衍射测量，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)。在撞击事件期间，将使用x射线探测器获取四个x射线衍射帧(时间间隔为153.4纳秒的100微微秒持续时间的快照)。由此产生的衍射图将被用来识别高压矿物结构。还将分析受冲击的单晶矿物的衍射图，以了解高压结构织构，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.