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Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the [...]

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]

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]

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]