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

微量H2O对地幔过渡带橄榄石和顽辉石相变速率及机制的影响

• 批准号: 0838159
• 负责人: Thomas Sharp
• 金额: $ 29.39万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-07-01 至 2013-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0838159&HistoricalAwards=false
• 关键词: Effect; Trace; Amounts; H2O; Rates

"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.

该奖项是根据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，地球将洋壳和岩石圈回收到地幔中。向下的大洋地壳携带着H2O，当H2O释放到上面的地幔楔形物中时，会导致熔融，从而导致弧形火山活动。如果俯冲大洋板块的地幔成分也是水合的，那么一些被俯冲的H2O很可能被带到地球地幔深处。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大洋的H2O。然而，我们还不知道有多少H2O实际上储存在地球的地幔中，或者通过俯冲循环。与俯冲有关的深部和危险地震(深源地震)发生在400公里至700公里的范围内，常规地震机制是不可能的。对于深源地震的起源，最被广泛接受的假设是，它们是由矿物橄榄石触发的，橄榄石被俯冲到过渡带，并超出其稳定的位置。这一假说要求俯冲板块相对较冷的内部有非常慢的反应速度。这项研究的目的是确定微量H2O如何在高压下提高橄榄石和顽火辉石的转化率。这些数据将检验转换断层模型，并通过转换断层作用约束与深源地震相容的最大H2O含量。这项研究将对深部地震和潜在的深部地球水文循环有更好的了解。前人对橄榄石-瓦兹利石和橄榄石-环木石相变的研究表明，H2O极大地提高了转化率。初步实验研究还表明，橄榄石中290ppm的D2O可使1100℃下环木石的生长速率提高两个数量级，并使活化热从无水橄榄石的392kJ/mol降至274kJ/mol。根据他们的动力学数据和俯冲带的热动力学模型，290ppm的H2O足以消除作为深源地震机制的亚稳橄榄石的存留。橄榄石中30ppm H2O的额外结果也表明了类似的快速生长速度和低活化热。建议使用活塞筒装置中水合的圣卡洛斯橄榄石，以30ppm的H2O继续这些橄榄石-环木岩相变的研究。这些样品将在18、19、20AD 21 GPA和700、900和1100°C下进行部分转化，使用多砧座设备，以确定30ppm H2O下环木石生长的压力依赖关系。顽辉石是地幔中第二丰富的矿物，在与橄榄石相同的条件下水化，其转化速度和机制也将被研究。该团队还将使用扫描和透射电子显微镜来表征他们样品中的转化机制。这一结果将被用于俯冲带的热动力学模型，以确定与转换断裂作用的深源地震相容的橄榄石和顽辉岩的最大H2O含量。