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A nearly balanced carbon budget between subduction input and degassing output has likely controlled the long‐term surface environment and habitability of Earth throughout geological history. However, the ongoing extensive hydration and carbonation of the mantle in trench outer‐rise regions may affect the global carbon budget. In this study, we show that the carbon content of the lithospheric mantle can be inferred from geophysical data and thermodynamic modeling. Based on the seismic velocity anomaly in trench outer‐rise regions, we estimated that the total carbon flux due to mantle carbonation is 7–31 Mt C/year, with possible fluid‐to‐rock mass ratios of 250–1000. These values are similar to the carbon uptake by altered oceanic crust, indicating that mantle carbonation has a significant effect on the subduction carbon budget. Although there are large uncertainties on the estimates of the subduction and degassing carbon fluxes, secular cooling of the mantle leads to the development of outer‐rise faults associated with bending of the oceanic lithosphere and increased mantle carbonation, which may disrupt the self‐regulating system of the global carbon cycle on geological timescales. [ABSTRACT FROM AUTHOR]

The seismic anisotropy observed in Earth's lowermost mantle (D″ layer) is thought to originate from the lattice‐preferred orientation (LPO) of mantle materials induced by plastic deformation. Ferropericlase is the second most abundant mineral in the lower mantle, and its LPO induced by deformation may significantly contribute to the seismic anisotropy in the D″ layer. The slip system(s) of ferropericlase controlling the LPO in the D″ layer is still controversial, owing to technical difficulties in conducting high‐pressure deformation experiments. We conducted shear deformation experiments on MgO under pressures of up to 120 GPa and reported the LPO after the deformation experiments. Our results indicate that {100}<011> is the dominant slip system in the D″ layer. The LPO developed during shear deformation would result in shear wave splitting with VSH > VSV, consistent with seismic observations in the regions expected to accommodate large shear strain by subducted slabs in the D″ layer. Plain Language Summary: Heat from Earth's core induces active thermal convection involving plastic flow in the Earth's solid mantle. Owing to plastic flow, minerals with elastic anisotropy in the mantle are thought to be aligned in certain directions (lattice‐preferred orientation, LPO) and generate the seismic anisotropy observed in the lowermost mantle. In particular, the LPO of ferropericlase, the second most abundant mineral in the lower mantle, has garnered attention as a likely candidate for attributing the seismic anisotropy of the lowermost mantle. To quantitatively interpret the dynamics of mantle convection in the lowermost mantle from seismic observations, it is essential to understand the LPO patterns of ferropericlase during deformation under the lowermost mantle conditions (for instance, extremely high‐pressure conditions at 1.2 million atmospheric pressure). However, owing to technical difficulties in conducting high‐pressure deformation experiments, there have been uncertainties in the deformation‐induced LPO of ferropericlase in the lowermost mantle. We conducted shear deformation experiments of MgO under the lowermost mantle pressure for the first time and demonstrated that the LPO of MgO produced by shear deformation is indeed consistent with the seismic anisotropy observed in parts of the lowermost mantle expected to accommodate large shear deformation by subducted slabs. Key Points: Strain rate controlled shear deformation experiments on MgO were conducted under lowermost mantle pressure (120 GPa) for the first timeLarge strain deformation experiments on MgO show that the {100}<011> slip is dominant in the D″ layerA {100}<011> slip of MgO by shear deformation results in VSH > VSV shear splitting, consistent with seismic observations in the D″ layer [ABSTRACT FROM AUTHOR]

We document an exhumed plate boundary shear zone—a subduction channel—developed at the contact between a fossil Cretaceous‐Eocene accretionary prism (Ligurian Units) and the underlying continent‐derived nappes of the Northern Apennines. The subduction channel, referred to as the Norsi‐Cavo Complex (NCC), is continuously exposed for ∼10 km along strike on the island of Elba. The NCC consists of oceanic sediments and serpentinites, coupled at the base of the prism. In its northernmost exposures the NCC crops out as a serpentinite mélange with blocks of sedimentary rocks. There, evidence of pervasive fluid‐rock interaction is present. We interpret this mélange as the result of material transfer from the upper plate to the subduction channel through tectonic erosion at the base of the prism. The southern part of the NCC preserves a series of tectonic slices of shales and limestones (Palombini shales) coupled with ultramafic rocks through serpentinite shear zones containing lenses of massive serpentinites, mafic rocks, and sediments. Since sedimentary slices preserve the same structures of the overlying Ligurian Units, produced by accretionary prism deformation, we interpret this complex as the result of material transfer from the prism to the subduction channel and its subsequent underplating at the base of the prism. Based on our interpretation, the NCC fossil subduction channel formed during E‐verging deformation over the W‐directed subduction of the Ligurian Ocean during the early development stages of the Apennines. This deformation terminated when the continental margin entered the subduction. Plain Language Summary: Subduction is the process by which the ocean floor disappears, pushed by plate tectonics to sink under a continental margin of another oceanic plate. Subduction produces the most destructive earthquakes on Earth and large submerged chains of oceanic rocks called accretionary prisms. Here we describe a belt of deformed rocks on the island of Elba Italy, which we interpret as the emerged analog of a subduction zone that formed between 145 and 35 million years ago in the Northern Apennines of Italy. Subduction produced a complex of intensely deformed rocks, derived from an extinct ocean, underneath a pile of ocean‐derived material—called the Ligurian Units. This complex, which we call Norsi‐Cavo Complex, witness the crushing of rocks in the subduction zone, which were folded, fragmented, and mixed up in an environment rich in fluids. Deformation structures show that the rocks were transported toward the East, a direction consistent with that of the subduction system. We also propose that during subduction, material was scraped away from the accretionary prism and brought at depth in the subduction zone. At about 35 million years ago, the margin of a continent (Adria) entered subduction and sealed this fossil subduction zone. Key Points: An exhumed plate boundary zone is preserved at the contact between oceanic and continental units in the inner Northern ApenninesSubduction caused the coupling of sediments with upper plate serpentinites, producing tectonic mélanges and duplexesEpisodes of tectonic erosion and underplating are proposed as viable mechanisms to explain the architecture of the subduction channel [ABSTRACT FROM AUTHOR]

Geophysical surveys of the oceanic crust indicate that hydrothermal circulation universally occurs in seismic layer 2, which results in a low seismic velocity and high VP/VS due to the occurrence of cracks. However, the anomalously low VP/VS observed at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VS due to crack development. In this study, we present the first evidence that shows the low VP/VS in the oceanic crust is caused by epidotization due to upward fluid flow in hydrothermal systems. Simultaneous measurements of elastic wave velocity and porosity of epidosites collected by the Oman Drilling Project show that quartz precipitation and spheroidal pores results in low VP/VS, in contrast to diabases that contain thin cracks. The presence of spheroidal pores in epidosites is supported by CT imaging, and is consistent with predictions from the effective medium model. Plain Language Summary: The low velocity and high VP/VS structure in the upper oceanic crust ("seismic layer 2") is generally associated with the development of fluid‐filled thin cracks. However, the anomalously low VP/VS ratios that have been observed in the layer 2/3 transition cannot be explained by the classical thin crack model. We provide the first direct evidence of low VP/VS ratios in epidotized rocks that are associated with the occurrence of spheroidal pores and quartz precipitation. Our theoretical calculations show that both the crack geometry and alteration minerals are required to explain the anomalously low VP/VS ratios in the transition zone. Knowledge of these unique physical properties of epidotized rocks will facilitate the detection of fluid upwelling zones in the oceanic crust, which are possibly linked to the hydrothermal ore resources. Key Points: Epidosites are characterized by low VP/VS due to the presence of spheroidal poresLow VP/VS anomaly in the layer 2/3 transition can be explained by alterations and pore development during epidotizationThe unique characteristics of epidosites may be useful to detect the fluid upwelling zones in the oceanic crust [ABSTRACT FROM AUTHOR]

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic "stratigraphy" of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO2‐rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid‐rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front. Plain Language Summary: This paper reports initial results from study of core from Oman Drilling Project Hole BT1B and the surrounding area. It provides insights into subduction zone processes, including large fluxes of recycled CO2 from subducting sediments into the leading edge of the mantle wedge, and surprisingly low temperature ductile deformation at less than 200°C. Recycling of CO2 via carbon mineralization in the hanging wall of subduction zones may produce an important, lithospheric reservoir in the global carbon cycle. Ductile deformation of serpentinite, and during or after transformation of peridotite to listvenites (mixtures of carbonates and opal or quartz) could explain aseismic subduction atop some subduction zones. Key Points: The area at and around OmanDP Hole BT1B preserves nearly flat‐lying tectonic "stratigraphy" of the basal thrust of the Samail ophioliteRecycling of CO2 from subducting sediment into the mantle wedge produced listvenites (carbonated peridotite) at <200°CSubduction beneath the ophiolite was accommodated, in part, by ductile deformation of serpentinite and listvenite at <200°C [ABSTRACT FROM AUTHOR]

We quantified mineral proportions in listvenite (completely carbonated peridotite) in Hole BT1B drilled across the basal thrust of the Samail ophiolite by the International Continental Scientific Drilling Project Oman Drilling Project using 3D X‐ray Computed Tomography (XCT). We analyzed >250,000 XCT images from a continuous ∼200 m listvenite core. Histograms of the intensity of X‐ray attenuation of each XCT core‐slice image were fitted assuming that the listvenites are composed of magnesite, quartz, and dolomite. The XCT mineral peaks were confirmed by comparison with chemical mapping data obtained using an X‐ray fluorescence (XRF) core scanner. Listvenite matrix is composed almost entirely of magnesite and quartz, consistent with discrete XRD and XRF data. Veins are composed mostly of dolomite. The mean abundance of dolomite in listvenite from BT1B is 11 vol.%, whereas that in core sections within 15 m of the basal thrust is >50 vol.%, suggesting the basal thrust acted as a source of Ca‐ and CO2‐rich fluids. The SiO2:MgO:CaO molar ratio in the entire core from BT1B is 42:52:6, similar to that of onboard XRF data for discrete samples (41:54:5), whereas average Oman peridotites have ratios of (39:60:1), indicating Ca addition perhaps during carbonation. P‐ and S‐wave velocities and density of listvenite are close to those of peridotite and are higher than those of serpentinites. These results suggest that limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O. Listvenites formed in the mantle wedge above subduction zones may be an overlooked reservoir for carbon in the Earth's interior. Plain Language Summary: To understand planetary‐scale fluid circulation from the surface to deep regions of Earth, it is important to constrain how fluids are captured/released by hydration/dehydration and carbonation/decarbonation of minerals and the associated volumes of these fluids. Listvenite, completely carbonated mantle material, occurs in the Samail ophiolite. Continuous ∼200 m drill core samples of listvenite overlying the basal thrust of the Samail ophiolite were obtained by the International Continental Scientific Drilling Project Oman Drilling Project. We evaluated the mineral fractions of listvenite from 3D X‐ray Computed Tomography (XCT) images of these core samples with confirmation using chemical mapping data from an X‐ray fluorescence core scanner. More than 125,000 XCT images were analyzed using this method. We found that dolomite (CaMg(CO3)2) abundance is highest within 15 m of the base of listvenite layer, which is the paleo‐plate boundary. These results suggest that the mantle carbonation involved CO2‐, H2O‐, and Ca‐rich fluids and that the paleo‐plate boundary acted as a pathway for these fluids. Because listvenite contains 36 mol% CO2 and has seismic properties different from serpentinites and similar to volatile‐free peridotites, listvenite in the mantle wedge overlying subduction zone may be an overlooked reservoir for carbon in the Earth's interior. Key Points: A method was developed to determine the downhole mineral fractions of drill cores from continuous X‐ray Computed Tomography (XCT) imagesQuartz‐magnesite‐dolomite fractions were estimated from XCT images of listvenite cores from International Continental Scientific Drilling Project Oman Drilling Project Hole BT1BResults indicate limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O [ABSTRACT FROM AUTHOR]

Seismic velocity structures of the oceanic lithosphere are variable due to the variation of alteration and heterogeneities in porosity. In order to interpret geophysically determined velocity structures, it is necessary to understand how the seismic velocity of the oceanic lithosphere is affected by alteration and porosity. We conducted petrological analysis and elastic wave velocity measurements at 200 MPa on mafic and ultramafic rocks drilled from the Samail ophiolite. The elastic wave velocity calculated from the mineral abundances of the ultramafic rocks was sensitive to serpentinization, whereas that of the mafic rocks was nearly independent of the alteration, suggesting that velocity changes with depth in the oceanic crust are mainly due to porosity reduction. Based on the effective medium theory and pressure dependence of the experimentally determined velocity, the change in the porosity was estimated to be 0.04%–0.42% at 200 MPa, indicating porosity reduction by the increase of lithostatic pressure in the oceanic crust in the fast‐spreading system such as the Pacific plate. We estimated the degree of hydration in the mantle to account for the geophysical observations based on the laboratory experiments. Both the crustal porosity and mantle serpentinization increase towards the trench due to fracture related plate bending, indicating extensive hydration of the oceanic lithosphere close to the trench. From these estimates, water contents in the oceanic lithosphere were calculated to be as high as 3 wt.% in the shallow crust and 6 wt.% in the upper most mantle, which is significant for water transport into Earth's interior. Plain Language Summary: Geophysical observations have found evidence for hydration of the oceanic lithosphere, because the physical properties of rocks are controlled by mineralogy and pore fluids. To understand how the seismic velocity of the oceanic lithosphere is affected by mineral alteration and porosity, we conducted petrological analysis and elastic wave velocity measurements at high confining pressure on mafic and ultramafic rocks drilled from the Samail ophiolite, where the ancient fragments of oceanic lithosphere are emplaced on land. Our results indicate that the seismic velocity calculated from the mineral abundances of the ultramafic rocks was sensitive to serpentinization, whereas that of the mafic rocks was nearly independent of the mineral alteration, suggesting that the change of velocity in the oceanic crust are mainly due to porosity. Based on the laboratory experiments and the effective medium theory, we estimated the water contents in the Pacific plate from the geophysical observations. The water content increases toward trench due to fracture related plate bending, and the mantle may have more than twice the water content of crust, which significantly influences water cycling on Earth. Key Points: The seismic velocity of ultramafic rocks is sensitive to serpentinization, but that of mafic rocks is nearly independent of alterationPorosity variation in the oceanic crust is estimated to be 0.04%–0.42%, based on the pressure dependence of elastic wave velocitiesWater contents in the oceanic lithosphere are calculated to be as high as 3 wt.% in the shallow crust and 6 wt.% in the upper‐most mantle [ABSTRACT FROM AUTHOR]

An accurate understanding of the relationship between the impact conditions and the degree of shock‐induced thermal metamorphism in meteorites allows the impact environment in the early Solar System to be understood. A recent hydrocode has revealed that impact heating is much higher than previously thought. This is because plastic deformation of the shocked rocks causes further heating during decompression, which is termed post‐shock heating. Here we compare impact simulations with laboratory experiments on the impact devolatilization of calcite to investigate whether the post‐shock heating is also significant in natural samples. We calculated the mass of CO2 produced from the calcite, based on thermodynamics. We found that iSALE can reproduce the devolatilization behavior for rocks with the strength of calcite. In contrast, the calculated masses of CO2 at lower rock strengths are systematically smaller than the experimental values. Our results require a reassessment of the interpretation of thermal metamorphism in meteorites. Plain Language Summary: Shock‐induced thermal metamorphism found in meteorites could be used as "the Rossetta stone," which records the ancient impact environment in the Solar System. If we could understand impact heating, then it would be possible to decode the impact history of the Solar System. We show that the temperature increase after impact events is much higher than conventionally thought, if we consider a recently realized heat source, that is post‐shock heating due to shear deformation. Impact simulations were compared with a previous experiment on the impact devolatilization of natural calcite marble to explore the influence of shear deformation on heating. The simulation reproduces the devolatilization behavior of calcite with increasing impact velocity, which demonstrates that heating due to plastic deformation is also significant in natural samples as shown in idealized numerical models. This finding suggests that mutual collisions between planetesimals, which are parent bodies of meteorites, could much easily produce shock‐induced thermal metamorphism in rocky materials than previously expected. Consequently, our Solar system might not have a number of dynamically excited bodies. Key Points: A numerical model was developed to describe impact devolatilization of calcite by taking strength effect on impact heating into accountThe mass of CO2 due to impact devolatilization in the numerical model was compared with that in a previous impact experimentThe numerical model with material strength reproduces CO2 devolatilization behavior with increasing impact velocity [ABSTRACT FROM AUTHOR]

Large earthquakes and slow slip events typically nucleate along plate boundaries near the depth limit of the seismogenic zone, which is also recognized as the brittle‐plastic transition zone (BPT). High Vp/Vs ratios are commonly observed at the BPT in subduction zones, indicating the presence of aqueous fluid in pore spaces. We conducted experiments to investigate the rheology of quartz with different fluid fractions at deformation conditions that cross the BPT. The strengths of quartz aggregates with fluid‐filled porosities of 5–25 vol% are significantly lower than predicted by wet quartzite flow laws, and decrease with increasing fluid fraction. Recovered samples deformed in the ductile regime exhibit S‐C´ mylonitic structures characterized by elongate grains, shear localization and fluid segregation. Variations in strength are explained by a combination of a constitutive law for dislocation creep that includes the geometric effects of fluid fraction, a friction law that includes the effect of fluid fraction through its role on the real area of contact, and an empirical function to describe the smooth brittle‐plastic transition. Our results indicate that the presence of fluid‐filled porosity promotes significant weakening in shear zones, and that variations in fluid fraction (together with temperature) can explain transitions in the spectrum of slip behaviors observed along plate boundaries. Key Points: The presence of fluid‐filled porosity promotes significant weakening in quartz shear zones across the brittle‐plastic transitionThe rheology of quartz shear zones is controlled by pore fluid pressure, fluid fraction, and fluid topologyVariations in fluid fraction can explain transitions in the spectrum of slip behaviors observed along plate boundaries [ABSTRACT FROM AUTHOR]

Permeability profiles in the crust‐mantle sequences of the Samail ophiolite were constructed based on onboard measurements of the electrical resistivity of cores recovered during the Oman Drilling Project. For each sample, we measured dry and brine‐saturated resistivity during the description campaign on the drilling vessel Chikyu. Owing to the conductive brine in the pore space, wet resistivity is systematically lower than dry resistivity. The difference between dry and wet resistivity is attributed to the movement of dissolved ions in brine that occupies the pore space. We applied effective medium theory to calculate the volume fraction of pores that contribute to electrical transport. Using an empirical cubic law between transport porosity and permeability, we constructed permeability profiles for the crust‐mantle transition zone and the serpentinized mantle sections in the Samail ophiolite. The results indicate that (1) the gabbro sequence has a markedly lower permeability than the underlying mantle sequence; (2) serpentinized dunites have higher permeability than serpentinized harzburgites; and (3) discrete sample permeability is correlated with ultrasonic velocity, suggesting that the permeability variations predominately reflect crack density and geometry. Plain Language Summary: Aqueous fluids that circulated beneath the seafloor play an important role in heat transfer, chemical exchange, and microbial activity in the oceanic lithosphere. The Oman Drilling Project was successful in obtaining continuous drill cores through the crust‐mantle sequences in the Samail ophiolite, where the paleo‐oceanic basement was thrust onto the continental crust in the Late Cretaceous. In this study, we constructed profiles of permeability across the crust‐mantle sections using the effective medium theory and resistivity data, which provide insights into fluid circulation in the oceanic lithosphere. The results indicate higher permeability in the uppermost mantle sequence between the crust and the underlying mantle, suggesting that present‐day fluid transfer is predominant at the crust‐mantle boundary. Although the fluid flow and chemical reactions are likely coupled, the high permeability could promote the transformation of crust‐mantle materials and hence the recycling of water into the mantle. Application of this technique to forthcoming deep drilling projects through the Mohorovičić discontinuity (Moho) and into the upper mantle may provide insights into the permeability structure and fluid circulation system in the oceanic lithosphere. Key Points: Brine‐saturated resistivity is systematically lower than dry resistivity, with the differences attributed to the volume fraction of poresPermeability through the crust‐mantle sequences was modeled using the effective medium theory and resistivity dataThe dunite sequence is characterized by higher permeability than the overlying gabbro and underlying harzburgite sequences [ABSTRACT FROM AUTHOR]

An accurate understanding of the relationship between the impact conditions and the degree of shock‐induced thermal metamorphism in meteorites allows the impact environment in the early Solar System to be understood. A recent hydrocode has revealed that impact heating is much higher than previously thought. This is because plastic deformation of the shocked rocks causes further heating during decompression, which is termed post‐shock heating. Here we compare impact simulations with laboratory experiments on the impact devolatilization of calcite to investigate whether the post‐shock heating is also significant in natural samples. We calculated the mass of CO2 produced from the calcite, based on thermodynamics. We found that iSALE can reproduce the devolatilization behavior for rocks with the strength of calcite. In contrast, the calculated masses of CO2 at lower rock strengths are systematically smaller than the experimental values. Our results require a reassessment of the interpretation of thermal metamorphism in meteorites. Plain Language Summary: Shock‐induced thermal metamorphism found in meteorites could be used as "the Rossetta stone," which records the ancient impact environment in the Solar System. If we could understand impact heating, then it would be possible to decode the impact history of the Solar System. We show that the temperature increase after impact events is much higher than conventionally thought, if we consider a recently realized heat source, that is post‐shock heating due to shear deformation. Impact simulations were compared with a previous experiment on the impact devolatilization of natural calcite marble to explore the influence of shear deformation on heating. The simulation reproduces the devolatilization behavior of calcite with increasing impact velocity, which demonstrates that heating due to plastic deformation is also significant in natural samples as shown in idealized numerical models. This finding suggests that mutual collisions between planetesimals, which are parent bodies of meteorites, could much easily produce shock‐induced thermal metamorphism in rocky materials than previously expected. Consequently, our Solar system might not have a number of dynamically excited bodies. Key Points: A numerical model was developed to describe impact devolatilization of calcite by taking strength effect on impact heating into accountThe mass of CO2 due to impact devolatilization in the numerical model was compared with that in a previous impact experimentThe numerical model with material strength reproduces CO2 devolatilization behavior with increasing impact velocity [ABSTRACT FROM AUTHOR]