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1. Insights on the deep carbon cycle from the electrical conductivity of carbon-bearing aqueous fluids

   https://doi.org/10.1038/s41598-021-82174-8  

   Manthilake, Geeth; Mookherjee, Mainak; Miyajima, Nobuyoshi (February 2021, Scientific Reports)

   Abstract The dehydration and decarbonation in the subducting slab are intricately related and the knowledge of the physical properties of the resulting C–H–O fluid is crucial to interpret the petrological, geochemical, and geophysical processes associated with subduction zones. In this study, we investigate the C–H–O fluid released during the progressive devolatilization of carbonate-bearing serpentine-polymorph chrysotile, with in situ electrical conductivity measurements at high pressures and temperatures. The C–H–O fluid produced by carbonated chrysotile exhibits high electrical conductivity compared to carbon-free aqueous fluids and can be an excellent indicator of the migration of carbon in subduction zones. The crystallization of diamond and graphite indicates that the oxidized C–H–O fluids are responsible for the recycling of carbon in the wedge mantle. The carbonate and chrysotile bearing assemblages stabilize dolomite during the devolatilization process. This unique dolomite forming mechanism in chrysotile in subduction slabs may facilitate the transport of carbon into the deep mantle. 

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

   https://doi.org/10.1029/2020AV000304  

   Shirey, Steven B.; Wagner, Lara S.; Walter, Michael J.; Pearson, D. Graham; van Keken, Peter E. (May 2021, AGU Advances)

   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 theP/Tconditions 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. 

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3. Uncovering and quantifying the subduction zone sulfur cycle from the slab perspective

   https://doi.org/10.1038/s41467-019-14110-4  

   Li, Ji-Lei; Schwarzenbach, Esther_M; John, Timm; Ague, Jay_J; Huang, Fang; Gao, Jun; Klemd, Reiner; Whitehouse, Martin_J; Wang, Xin-Shui (January 2020, Nature Communications)

   Abstract Sulfur belongs among H₂O, CO₂, and Cl as one of the key volatiles in Earth’s chemical cycles. High oxygen fugacity, sulfur concentration, and δ³⁴S values in volcanic arc rocks have been attributed to significant sulfate addition by slab fluids. However, sulfur speciation, flux, and isotope composition in slab-dehydrated fluids remain unclear. Here, we use high-pressure rocks and enclosed veins to provide direct constraints on subduction zone sulfur recycling for a typical oceanic lithosphere. Textural and thermodynamic evidence indicates the predominance of reduced sulfur species in slab fluids; those derived from metasediments, altered oceanic crust, and serpentinite have δ³⁴S values of approximately −8‰, −1‰, and +8‰, respectively. Mass-balance calculations demonstrate that 6.4% (up to 20% maximum) of total subducted sulfur is released between 30–230 km depth, and the predominant sulfur loss takes place at 70–100 km with a net δ³⁴S composition of −2.5 ± 3‰. We conclude that modest slab-to-wedge sulfur transport occurs, but that slab-derived fluids provide negligible sulfate to oxidize the sub-arc mantle and cannot deliver³⁴S-enriched sulfur to produce the positive δ³⁴S signature in arc settings. Most sulfur has negative δ³⁴S and is subducted into the deep mantle, which could cause a long-term increase in the δ³⁴S of Earth surface reservoirs. 

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4. Assessing the Role of Water in Alaskan Flat‐Slab Subduction

   https://doi.org/10.1029/2021GC009734  

   Petersen, Sarah E.; Hoisch, Thomas D.; Porter, Ryan C. (May 2021, Geochemistry, Geophysics, Geosystems)

   Abstract Low‐angle subduction has been shown to have a profound impact on subduction processes. However, the mechanisms that initiate, drive, and sustain flat‐slab subduction are debated. Within all subduction zone systems, metamorphic dehydration reactions within the down‐going slab have been hypothesized to produce seismicity, and to produce water that fluxes melting of the asthenospheric wedge leading to arc magmatism. In this work, we examine the role hydration plays in influencing slab buoyancy and the geometry of the downgoing oceanic plate. When water is introduced to the oceanic lithosphere, it is incorporated into hydrous phases, which results in lowered rock densities. The net effect of this process is an increase in the buoyancy of the downgoing oceanic lithosphere. To better understand the role of water in low‐angle subduction settings, we model flat‐slab subduction in Alaska, where the thickened oceanic lithosphere of the Yakutat oceanic plateau is subducting beneath the continental lithosphere. In this work, we calculate the thermal conditions and stable mineral assemblages in the slab crust and mantle in order to assess the role that water plays in altering the density of the subducting slab. Our slab density results show that a moderate amount of hydration (1–1.5 wt% H₂O) in the subducting crust and upper lithospheric mantle reduces slab density by 0.5%–0.8% relative to an anhydrous slab, and is sufficient to maintain slab buoyancy to 300–400 km from the trench. These models show that water is a viable factor in influencing the subduction geometry in Alaska, and is likely important globally. 

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5. Receiver Function Derived Structural Constraints on Dynamic Processes Associated with the Young Nazca Lithosphere Subducting beneath Colombia

   Bishop, Brandon; Cho, Sung-Won; Warren, Linda M; Pedraza, Patricia; Prieto, German A; Dionicio, Viviana (January 2024, AGU Fall Meeting 2023)

   Subduction of the very young (<15 Myr old) oceanic lithosphere of the Nazca plate in central to southern Colombia is observationally related to an unusually high and unusually variable amount of intermediate (>50 km) depth seismicity. From 2010 through 2019 89% of central and southern Colombia’s 11,466 intermediate depth events occurred between 3.5°N and 5.5°N, highlighting these unusual characteristics of the young slab. In addition, morphologic complexity and possible tears characterize the Nazca slab in Colombia and complicate mantle flow in the region. Prior SKS-phase shear-wave splitting results indicate sub-slab anisotropy is dominated by plate motion parallel-to-subparallel orientations in the region, suggesting the young slab has entrained a relatively thick portion of the sub-slab mantle. These observations suggest the subduction of young lithosphere has significant effects on both the overlying and underlying asthenosphere in the Colombia subduction zone. Here we use more than 10 years of data to calculate receiver functions for the Red Sismológica Nacional de Colombia’s network of broadband seismometers. These receiver functions allow us to tie these prior observations of the Colombia subduction zone to distinct, structural features of the slab. We find that the region of high seismicity corresponds to a low seismic velocity feature along the top of the subducting plate between 3.5°N and 5.5°N that is not present to the south. Moderately elevated P-wave velocity to S-wave velocity ratios are also observed within the slab in the north. This feature likely represents hydrated slab mantle and/or uneclogitized oceanic crust extending to a deeper depth in the north of the region which may provide fluids to drive slab seismicity. We further find evidence for a thick layer of material along the slab’s lithosphere-asthenosphere boundary characterized by spatially variable anisotropy. This feature likely represents entrained asthenosphere at the base of the plate sheared by both the overlying plate and complex flow related to proposed slab tears just north and south of the study region. These observations highlight how structural observations provide key contextual constraints on short-term (seismogenic) and long-term (anisotropic fabric) dynamic processes in the Colombia subduction zone. Plain-language Summary The Nazca oceanic plate is very young (<15 million years old) where it is pulled or subducted beneath the South America plate in central and southern Colombia. Earthquakes occurring in the subducted Nazca plate at depths greater than 50 km are nearly 9x more common in central Colombia than in southern Colombia. The subducted Nazca plate also has a complex shape in this region and may have been torn both in northern Colombia and to the south near the Colombia-Ecuador border. The slow flow of mantle rock beneath the subducted plate is believed to be affected by this and earlier studies have inferred this flow is mostly in the same direction as the subducting plate's motion. We have used 10+ years of data to calculate receiver functions, which can detect changes in the velocity of seismic waves at the top and bottom of the subducted plate to investigate these features. We found that the Nazca plate is either hydrated or has rocks with lower seismic velocities at its top in the central part of Colombia where earthquakes are common. We also find that a thick layer of mantle rock at the base of the subducted plate has been sheared. 

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