Slab Transport of Fluids to Deep Focus Earthquake Depths—Thermal Modeling Constraints and Evidence From Diamonds

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.

Hydrous silicate melts and the deep mantle H2O cycle

“…However, in the mantle portion of cooler slabs a significant fraction of the H 2 O dissolved in both hydrous and nominally anhydrous phases may survive to greater depths (Iwamori, 2004;van Keken et al, 2011). Stalling and stagnation of slabs in the transition zone may lead to dehydration of serpentinized mantle in cooler slabs providing a mechanism for deep mantle hydration (Iwamori, 2004;Komabayashi and Omori, 2006), a process implicated in deep focus earthquakes and consistent with the petrology of some sublithospheric diamonds from the deep transition zone or shallow lower mantle (Omori et al, 2004;Pearson et al, 2014;Shirey et al, 2021).…”

“…dehydration occurs deeper in the upper mantle. For each mass increment, we add 70% of the slab H 2 O content to the transition zone and 30% to the lower mantle mimicking deeper slab dehydration (Iwamori, 2004;Komabayashi and Omori, 2006;Shirey et al, 2021); model outcomes for the upper mantle and transition zone are relatively insensitive to this choice, whereas the lower mantle water content depends critically on this assumption. We also include H 2 O loss from the upper mantle in this model, connecting the hydrosphere to the deep-H 2 O cycle through subduction ingassing and magmatic outgassing.…”

Mineral Inclusions in Lithospheric Diamonds

“…The viability of a scenario of diamond formation via interaction of slab-derived fluids with oceanic crust is supported by thermal modelling, which indicates that carbon and volatiles in subducting slabs can escape shallow decarbonation and dehydration (Shirey et al 2021). It can be assessed by considering phase relations of oceanic slabs and the stability of diamond vs. other carbon minerals as a function of pressure, temperature and oxygen fugacity.…”

Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor

“…Seawater circulation in faults associated with ridges, transforms, and slab bending (1) before subduction leads to variable but widespread, penetrative serpentinization of lithospheric mantle peridotite (2,3). The serpentinized portion of subducting slabs, especially within cool slab interiors, is recognized as a way to carry surficial materials beyond arcs and recycle them deeply into the convecting mantle (2,(4)(5)(6)(7)(8)(9). However, the signature of the serpentinized peridotitic portion of these recycled slabs has only been inferred indirectly, based on the geochemistry of oceanic basalts, erupted at ocean islands and midocean ridges.…”