Establishing the Critical Parameters for Dehydration Embrittlement in Subduction Zones

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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.