Establishing the Critical Parameters for Dehydration Embrittlement in Subduction Zones

建立俯冲带脱水脆化的关键参数

• 批准号: 1015264
• 负责人: Harry Green
• 金额: $ 40万
• 依托单位: University of California-Riverside
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-08-01 至 2013-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1015264&HistoricalAwards=false
• 关键词: Establishing; Critical; Parameters; Dehydration; Embrittlement

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.

无辅助脆性剪切破坏和摩擦滑动仅限于地球不到 50 公里的深度，因为压力增加对摩擦滑动有强烈的抑制作用，并且温度随深度增加而导致流应力减少。然而，地震发生的深度接近 700 公里，而发生这种破坏的物理原理仍然不完全清楚。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层水分布的重要信息。现在的实验和地震证据非常有力，表明中深度地震（~50-350公里）是由俯冲海洋岩石圈中含水相的脱水脆化引发的。还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）相从地表输送到俯冲板片内的深处。 在这个项目中，PI 将检查支持前者的证据并对后者提出质疑，并建议回答与两者相关的三个关键问题。 这些问题是： (i) 实现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实际上是如何发生的？也就是说，即使叶蛇纹石的分解导致负体积变化（致密化），通过产生少量含水流体而导致剪切不稳定性的物理机制是什么？ (iii) 地幔过渡带的地震是否与脱水脆化一致，如果不一致，是否表明俯冲板片在 410 km 以下基本上是干燥的？ PI 建议通过光学显微镜、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）对运行产品进行高压实验和分析来回答这些问题。 该提案解决了俯冲岩石圈地质学和地球物理学的基础问题，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表地球科学产生影响。此外，它还可能通过阐述相变引起的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先进分析设备的实践使用。