Establishing the Critical Parameters for Dehydration Embrittlement in Subduction Zones

由于压力增加对摩擦滑动的强烈抑制作用和温度随深度增加而引起的流动应力的降低，无辅助脆性剪切破坏和摩擦滑动仅限于小于50 km的深度。然而，地震发生在接近700公里的深度，这种故障可能发生的物理学仍然没有完全了解。这些地震发生在俯冲带;了解它们发生的基本过程可以提供有关地幔对流和地球深部水分布的重要信息。实验和地震证据现在非常有力地表明，中深度地震（~50-350 km）是由俯冲洋岩石圈中含水相的脱水脆化引发的。也有大量的实验证据表明，大量的H2O可以通过含水（和名义上无水）相从表面输送到俯冲板内的很深处。 在本项目中，PI将检查支持前者的证据，并对后者提出质疑，并将提出回答与两者相关的三个关键问题。 这些问题是：（i）使脱水脆化不稳定所需的叶蛇纹石的最小量是多少？(ii)叶蛇纹石的脱水脆化究竟是如何发生的？也就是说，是什么物理机制激活的生成少量的含水流体，导致剪切不稳定性，即使当叶蛇纹石分解导致负体积变化（致密化）？(iii)地幔过渡带的地震是否与脱水脆化一致？如果不是，这是否表明俯冲板块在410公里以下基本上是干燥的？PI建议通过高压实验和通过光学显微镜、扫描电子显微镜（SEM）、透射电子显微镜（TEM）、nanoSIMS和各种光谱技术（例如FTIR、拉曼、EELS等）分析运行产物来回答这些问题。 这一建议涉及俯冲岩石圈地质学和地球物理学的基本问题，因此将对固体地球科学的广泛领域产生影响，包括地震学、地球化学、地幔岩石学和地球动力学。它还将通过其与地球水循环的相关性对地球表面科学产生影响。此外，它可能会通过阐述相变诱导剪切不稳定性的潜在机制对材料科学产生影响。 它将进一步增加受过高等教育的地球科学家的数量，为研究生和本科生提供实际使用复杂分析设备的机会。

Unassisted brittle shear failure and frictional sliding are restricted to depths in Earth of less than ~50 km because of the strong inhibition of frictional sliding by increasing pressure and the reduction of flow stress due to increasing temperature with depth. Nevertheless, earthquakes occur to depths approaching 700 km and the physics by which such failure can occur remains incompletely understood. These earthquakes occur in subduction zones; understanding of the fundamental processes by which they occur can provide important information about mantle convection and about the distribution of water within the deeper portions of our planet.The experimental and seismic evidence is now very strong that intermediate-depth earthquakes (~50-350 km) are initiated by dehydration embrittlement of hydrous phases in the subducting oceanic lithosphere. There is also abundant experimental evidence that large volumes of H2O could be transported from the surface to great depths within subducting slabs by hydrous (and nominally anhydrous) phases. In this project the PI will examine the evidence that supports the former and calls into question the latter and will propose to answer three critical questions that are relevant to both. Those questions are: (i) What is the minimum amount of antigorite serpentine that is necessary to enable the dehydration embrittlement instability? (ii) How does dehydration embrittlement of antigorite really work? That is, what are the physical mechanisms activated by generation of small amounts of hydrous fluid that lead to a shearing instability even when breakdown of antigorite leads to a negative volume change (densification)? (iii) Are earthquakes in the mantle transition zone consistent with dehydration embrittlement and, if not, does that indicate that subducting slabs are essentially dry below 410 km? The PI proposes to answer these questions with high-pressure experiments and analysis of the run products by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), nanoSIMS, and a variety of spectroscopic techniques (e.g. FTIR, Raman, EELS, etc.). This proposal addresses problems that are fundamental to the geology and geophysics of subducting lithosphere and therefore will have impact on a very broad spectrum of the solid-earth sciences, including seismology, geochemistry, mantle petrology and geodynamics. It also will have impact on the surficial Earth sciences through its relevance to Earth's water cycle. In addition, it potentially will have impact on materials science through elaboration of the underlying mechanisms of phase-transformation-induced shearing instabilities. It further will add to the numbers of highly educated Earth Scientists, providing graduate and undergraduate students with hands-on use of sophisticated analytical equipment.