Shock Wave Studies of Mineral and Melt Physics at Deep Mantle Pressures

对地球深处的研究对于了解地球的长期演化至关重要，包括我们所居住的地表的宜居性和自然灾害。我们特别感兴趣的是深部内部的融化，以及它对地幔化学演化的影响。然而，以大深度为特征的极端条件，特别是高压和高温，以及在这种深度直接观测的局限性，需要应用专门的实验技术。一系列技术使用大型固定枪支以高速将矿物和金属目标碰撞在一起，从而在短时间内产生非常高的压力，这种方法被称为冲击波研究。我们可以用这些实验进行的测量是独一无二的，但也是对其他实验和理论技术获得的结果的补充。在接下来的三年里，我们将致力于加州理工学院冲击波实验室，研究下地幔条件下矿物和熔体的熔融和热力学性质。最终目标是开发简化地幔成分的相平衡和物理性质的准确模型，这些模型可与地球物理观测数据进行比较，或插入今天可能在地幔深处发生的过程的动力学模型中，例如形成地幔热柱，或可能在地球历史早期发生的过程，如岩浆海洋结晶。通过对部分熔融区的合理温度、成分和相对浮力的限制，我们将限制这种带的存在的动力学和化学后果。同样，液体和固体的液线矿物学和相对浮力的知识有助于确定早期陆地全地幔岩浆海洋可能是如何通过结晶-分异而演化的，从而确定固体地幔演化的初始状态。我们的实验方法将是同时测定透明物质中的冲击波速度、声速和冲击温度，主要是冲击成下地幔相组合的MgO和MgSiO。所有这些量的确定共同提供了对固体和液体的熔化曲线和热状态方程参数的最佳约束。我们将这项工作的重点放在具有地球物理意义的材料上，这些材料由于现有数据和实验能力的限制，这些量仍然最不确定。我们已经通过对多孔氧化镁的声速测量证明，我们能够冲击熔化这种材料，并确定其高度不确定的熔化曲线的位置。我们将继续使用不同的孔隙率和预热的单晶，以获得对熔化曲线的更多限制，并定义未知的氧化镁液体的热物理性质。在我们现有的镁硅系统冲击结果数据库的基础上，我们建议在单晶顽火辉石和预热的硅酸镁液体上增加一系列冲击波速度、声速和冲击温度的测量，以消除这种液体组成的热力学描述中的参数权衡。

Study of the deep interior of the Earth is essential to understanding the long-term evolution of our planet, including the habitability and natural hazards of the surface where we live. We are particularly interested in the melting of the deep interior, and its consequences for the chemical evolution of the mantle. The extreme conditions that characterize great depth, particularly high pressure and temperature, and the limits of direct observations at such depths, however, require the application of specialized experimental techniques. One family of techniques uses large stationary guns to collide mineral and metal targets together at high speed and so generate very high pressures for a short time, a method referred to as shock wave research. The measurements that we can make with these experiments are unique, but also complementary to results obtained by other experimental and theoretical techniques. We will dedicate the Caltech shock wave laboratory over the next three years to the study of melting and the thermodynamic properties of minerals and melts under lower mantle conditions. The ultimate goal is development of accurate models of phase equilibria and physical properties of simplified mantle compositions that can be compared to geophysical observables or inserted into dynamic models of processes that may occur in the deep mantle today, such as formation of mantle plumes, or that may have occurred early in Earth history, such as magma ocean crystallization. With constraints on the plausible temperature, composition, and relative buoyancy of partially molten regions that might constitute the ultra-low velocity zones seismically observed at the base of the mantle, we will place limits on the dynamical and chemical consequences of the existence of such zones. Likewise, knowledge of the liquidus mineralogy and the relative buoyancy of liquids and solids helps to define how an early terrestrial whole-mantle magma ocean might have evolved by crystallization-differentiation and so the initial state of solid mantle evolution.Our experimental approach will be simultaneous determination of shock velocity, sound speed, and shock temperature in transparent materials, principally MgO and MgSiO3 shocked into lower mantle phase assemblages. Determination of all these quantities together provides optimal constraints on melting curves and thermal equation of state parameters of solids and liquids. We focus this effort on materials of geophysical interest where these quantities remain most uncertain due to limitations of existing data and experimental capabilities. We have demonstrated through sound-speed measurements on porous MgO that we are able to shock melt this material and define the location of its highly uncertain melting curve. We will continue this program using different porosities and pre-heated single crystals both to obtain more constraints on the melting curve and to define the unknown thermophysical properties of MgO liquid. To our existing database of shock results in the MgSiO3 system, we propose to add a series of shock velocity, sound speed, and shock temperature measurements on single crystal enstatite and on preheated MgSiO3 liquid in order to remove parameter trade-offs in the thermodynamic description of this liquid composition.