The Effect of Trace Amounts of H2O on the Rates and Mechanisms of Olivine and Enstatite Phase Transformations in Earth's Mantle Transition Zone

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”通过板块构造和俯冲作用，地球将海洋地壳和岩石圈循环到地幔中。下沉的海洋地壳携带着水，当水被释放到上覆的地幔楔中时，会导致融化，从而导致弧火山作用。如果俯冲洋板块的地幔成分也被水化，那么一些俯冲的水很可能被带到地幔深处。高压实验表明，来自上地幔，特别是过渡带（410至660公里深）的矿物能够储存相当于许多海洋的水。然而，我们还不知道有多少水实际储存在地幔中或通过俯冲循环。与俯冲有关的深地震和危险地震（深震源地震）发生在距离400公里至700公里的地方，传统的地震机制不可能发生。对于深震源地震的起源，最被广泛接受的假设是，它们是由矿物橄榄石引发的，橄榄石被俯冲到过渡带并超出了稳定带。这一假说要求在俯冲板块相对较冷的内部反应速率非常慢。本研究的目的是确定微量的水如何在高压下提高橄榄石和顽辉石的转化速率。这些数据将为转换断层模型提供检验，并约束转换断层与深震源地震兼容的最大水含量。这项研究将更好地了解深层地震和潜在的深层地球水文循环。前人对橄榄石-沃德斯莱石和橄榄石-环伍德石相变的研究表明，H2O能显著提高相变速率。初步实验研究还表明，在1100℃时，290 ppm的D2O使环伍德石的生长速率提高了两个数量级，并使无水橄榄石的活化焓从392 kJ/mol左右降低到274 kJ/mol。根据他们的动力学数据和俯冲带的热动力学模型，290 ppm H2O足以排除亚稳橄榄石的存在作为深震源地震的机制。另外的结果表明，在橄榄石中，30 ppm的水同样快速的生长速率和低的活化焓。建议在活塞缸装置中使用水化的圣卡洛斯橄榄石，在30 ppm H2O条件下继续研究橄榄石-环橄榄石的转化。这些样品将在18、19、20和21 GPa和700、900和1100°C下进行部分转化，使用多砧装置，以确定环伍德石生长在30 ppm H2O下的压力依赖性。在与橄榄石相同的条件下，地幔中第二丰富的矿物顽辉石的水化速率和机制也将被研究。该团队还将使用扫描和透射电子显微复制来表征样品中的转化机制。该结果将用于俯冲带的热动力学模型，以确定与变形断裂引起的深震源地震相适应的橄榄石和辉长岩的最大H2O含量。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." Through plate tectonics and subduction, Earth recycles oceanic crust and lithosphere into its mantle. The down going oceanic crust carries H2O, which, when released into the overlying mantle wedge, causes melting that results in arc volcanism. If the mantle component of the subducting oceanic plate is also hydrated, then some of the subducted H2O is likely to be carried deep into Earth's mantle. High-pressure experiments have shown that minerals from the upper mantle and especially the transition zone (410 to 660 km depth) are capable of storing many oceans worth of H2O. However, we do not yet know how much H2O is actually stored in Earth's mantle or recycled through by subduction. Deep and dangerous earthquakes associated with subduction (deep-focus earthquakes) occur from 400 km up to 700 km where conventional earthquake mechanisms are not possible. The mostly widely accepted hypothesis for the origin of deep-focus earthquakes is that they are triggered by the mineral olivine, which is subducted into the transition zone and beyond where it is stable. This hypothesis requires very slow reaction rates in the relatively cool interiors of subducting plates. The purpose of this research is to determine how trace amounts of H2O enhance olivine and enstatite transformation rates at high-pressure. These data will provide a test of the transformational faulting model and constrain the maximum H2O content compatible with deep focus earthquakes by transformational faulting. This research will provide a better understanding of deep earthquakes and the potential deep-Earth hydrologic cycle. Previous studies on the olivine-wadsleyite and the olivine-ringwoodite phase transformations have shown that H2O greatly increases transformation rates. Preliminary experimental studies have also shown that 290 ppm D2O in olivine increases ringwoodite growth rates by two orders of magnitude at 1100°C and reduces the activation enthalpy from around 392 kJ/mol for anhydrous olivine to 274 kJ/mol. Based on their kinetic data and thermo-kinetic models of subduction zones, 290 ppm H2O is enough to eliminate the survival of metastable olivine as a mechanism for deep focus earthquakes. Additional results for 30 ppm H2O in olivine indicate similarly fast growth rates and low activation enthalpy. It is proposed to continue these investigation of the olivine-ringwoodite transformation with 30 ppm H2O, using San Carlos olivine hydrated in the piston-cylinder apparatus. These samples will be partially transformed at 18, 19, 20ad 21 GPa and 700, 900 and 1100 °C, using a multi-anvil apparatus, to determine the pressure dependence of ringwoodite growth for 30 ppm H2O. Transformation rates and mechanisms will also be investigated for enstatite, the second most abundant mineral in the mantle, hydrated under the same conditions as our olivine. The team will also use scanning and transmission electron microcopy to characterize transformation mechanisms in their samples. The results will be used in thermo-kinetic models of subduction zones to determine the maximum H2O contents of olivine and enstatite that are compatible with deep focus earthquakes by transformational faulting.