Figure showing a thermal model of a subduction zone with the relatively cold (blue) oceanic plate sinking into the comparatively hot (red) mantle.
A sample thermal model of a subduction zone with the relatively cold (blue) oceanic plate sinking into the comparatively hot (red) mantle. Three regions of earthquakes (grey spheres) visible in the oceanic plate: “intermediate depth” dehydration-related earthquakes occurring between ~70 and ~250 km, a region of reduced seismicity between ~250 and ~350 km, and the region of "deep" seismicity below 350 km that extends to ~700 km. Superdeep diamonds (blue octahedra) are known to crystallize from fluids released in this deep region as the oceanic plate warms by the heat from surrounding mantle. Credit: Illustration by Steven Shirey, Peter van Keken, Lara Wagner, and Michael Walter / Carnegie Institution for Science
Source: AGU Advances

Deep earthquakes represent brittle failure in a generally ductile regime and the mechanism that causes these earthquakes below 300 kilometers is debated. Proposed mechanisms for deep earthquakes include a delayed phase transition of olivine at depth, development of weakness zones associated with spinel transitions or a potential role for fluids. Hydrous minerals and carbonates can be carried down by the subducted slab to depths below 300 kilometers.

Shirey et al. [2021] provide the first convincing arguments that reactions that liberate fluid or melt occur at the focus of deep earthquakes. The evidence is found in diamonds, which are the deepest samples for the Earth’s mantle. Their arguments are based on the mineral assemblage of inclusions in “deep”, sublithospheric diamonds and phase equilibria modeling. Thermodynamic models combined with thermal models of downgoing slab show that these carbon or hydrogen containing mineral phases will release their volatiles at depth and can form some of the largest diamonds found. The inclusions in diamonds record their depth of formation which coincides with the depth where the deep earthquakes occur.  It is proposed that the release of these fluids trigger the deep seismicity.

Citation: Shirey, S., Wagner, L., Walter, M., Pearson, G. & van Keken, P. [2021]. Slab Transport of Fluids to Deep Focus Earthquake Depths – Thermal Modeling Constraints and Evidence from Diamonds. AGU Advances, 2, e2020AV000304.

—Vincent Salters, Editor, AGU Advances

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