Slab Transport of Fluids to Deep Focus Earthquake Depths—Thermal Modeling Constraints and Evidence From Diamonds. Issue 2 (26th May 2021)

• Record Type: Journal Article
• Authors: Shirey, Steven B.; Wagner, Lara S.; Walter, Michael J.; Pearson, D. Graham; van Keken, Peter E.
• Is Part Of: AGU advances. Volume 2:Issue 2(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-26
• Subjects: deep focus earthquakes -- fluids -- mantle transition zone -- sublithospheric diamonds -- thermal modeling; Earth sciences -- Periodicals; Space sciences -- Periodicals; 550
• DOI: 10.1029/2020AV000304
• Languages: English
• ISSNs: 2576-604X

Abstract

Abstract: The nature and cause of deep earthquakes remain enduring unknowns in the field of seismology. We present new models of thermal structures of subducted slabs traced to mantle transition zone depths that permit a detailed comparison between slab pressure/temperature ( P/T ) paths and hydrated/carbonated mineral phase relations. We find a remarkable correlation between slabs capable of transporting water to transition zone depths in dense hydrous magnesium silicates with slabs that produce seismicity below ∼300‐km depth, primarily between 500 and 700 km. This depth range also coincides with the P/T conditions at which oceanic crustal lithologies in cold slabs are predicted to intersect the carbonate‐bearing basalt solidus to produce carbonatitic melts. Both forms of fluid evolution are well represented by sublithospheric diamonds whose inclusions record the existence of melts, fluids, or supercritical liquids derived from hydrated or carbonate‐bearing slabs at depths (∼300–700 km) generally coincident with deep‐focus earthquakes. We propose that the hydrous and carbonated fluids released from subducted slabs at these depths lead to fluid‐triggered seismicity, fluid migration, diamond precipitation, and inclusion crystallization. Deep focus earthquake hypocenters could track the general region of deep fluid release, migration, and diamond formation in the mantle. The thermal modeling of slabs in the mantle and the correlation between sublithospheric diamonds, deep focus earthquakes, and slabs at depth demonstrate a deep subduction pathway to the mantle transition zone for carbon and volatiles that bypasses shallower decarbonation and dehydration processes. Plain Language Summary: Earthquakes occurring below ∼300 km, especially in the mantle transition zone are some of the strongest events experienced on Earth. Deep earthquakes, whose nature and cause are poorly known, occur with regularity and are a deep and prominent result of plate tectonics. We model the paths of subducting slabs to relate pressure‐temperature conditions to the experimentally determined mineralogies of the slab crust and mantle. We present a synthesis of mantle minerals included in diamonds derived from same depths as the deep earthquakes to show that fluids exist there. We show that decarbonization/melting reactions in the slab crust and dehydration reactions in the slab mantle can provide fluids to the earthquake generation regions, suggesting that fluids cause or are related to deep earthquakes. Key Points: Mineral inclusions in sublithospheric diamonds are evidence for mobile fluids in the mantle transition zone Thermal modeling shows that carbonated crust and hydrated mantle in cold slabs can produce fluids at the depths of deep‐focus earthquakes Fluids should be considered as significant contributors to the genesis of deep earthquakes