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Much of our knowledge of the North American lithosphere comes from imaging seismic velocities. Additional constraints on the subsurface can be gained by studying seismic attenuation, which has different sensitivity to physical properties. We produce a model of lateral variations in attenuation across the conterminous U.S. by analyzing data recorded by the EarthScope Transportable Array. We divide the study area into 12 overlapping tiles and differential attenuation is measured in each tile independently; and twice for four of the tiles. Measurements are combined into a smooth map using a set of linear inversions. Comparing results for adjacent tiles and for repeated tiles shows that the imaged features are robust. The final map shows generally higher attenuation west of the Rocky Mountain Front than east of it, with significant small length scale variations superimposed on that broad pattern. In general, there is a strong anticorrelation between differential attenuation and shear wave velocities at depths of 80–250 km. However, a given change in velocity may correspond to a large or small change in attenuation, depending on the area; suggesting that different physical mechanisms are operating. In the western and south‐central U.S., as well as the Appalachians, velocity variations are large compared to attenuation changes, while the opposite is true in the north‐central and southeastern U.S. Calculations with the Very Broadband Rheology calculator show that these results are consistent with the main source of heterogeneity being temperature and melt fraction in the former regions and grain size variability in the latter ones. Plain Language Summary: Seismic waves in the mantle propagate at lower speeds when temperatures are higher, rocks have higher water content, and small amounts of melt are present. These conditions also affect how much energy the wave loses as it passes through, which we call seismic attenuation. In this study we produce a map of seismic attenuation for the conterminous United States. We find that, in most places, where seismic velocities are low, attenuation is high, and vice versa. This is what we expect. Interestingly, the size of the change in attenuation that corresponds to a given change in velocity varies by region. In places with thicker lithosphere and without recent tectonic activity attenuation anomalies are large compared to velocity anomalies, the opposite is true in places with thin lithosphere and recent tectonic activity. Considering the results of lab experiments on velocity and attenuation, this suggests that in the regions with thick lithosphere and without recent tectonic activity the main cause of the anomalies is changes in the size of the mineral grains in the mantle, whereas in the regions with thin lithosphere and recent tectonic activity the main cause of the anomalies is changes in temperature and the amount of melt. Key Points: Teleseismic P attenuation is largely anti‐correlated with upper mantle seismic velocitySignificant lateral variations in attenuation are observed in the tectonically quiescent regionsThe ratio of attenuation to velocity anomaly amplitudes may be indicative of mantle conditions [ABSTRACT FROM AUTHOR]

The self‐potential (SP) method is expected to play an important role in the exploration of seafloor massive sulfide (SMS) resources. Since the redox potential changes with depth below the seafloor, SMS deposits are the source of an electrical (source) current density, generating in turn a recordable electrical field. This electrical field can be remotely measured at and above the seafloor. By integrating this electrical field, we obtained a so‐called SP profile or map. The electrical conductivity distribution of SMS deposits is an important ingredient in the inversion of these SP data in order to distinguish between causative primary sources (i.e., associated with the SMS deposits themselves) and secondary current sources associated with conductivity contrasts below the seafloor. Such ingredient is therefore necessary to properly localize SMS deposits and eliminating ghosts associated with high conductivity contrasts. The conductivity structure of the seafloor is however unknown in most SP surveys. This study presents the first joint interpretation of transient electromagnetic and SP surveys. The field data were acquired at the Trans‐Atlantic Geotraverse (TAG) hydrothermal mound. The conductivity structure of TAG mound was obtained by inverting transient electromagnetic data. This electrical conductivity distribution is then used to invert the SP data. The obtained (primary) source current distribution delineates the geometry of the SMS deposits. The deposits are located in the center of the mound as well as at its edges, in a way that is consistent with water column data. Combining the two geophysical methods improves our ability to better decipher the fine geometry of SMS deposits below the seafloor. Plain Language Summary: Seafloor massive sulfide (SMS) deposits contain abundant amounts of Cu, Zn, Au, and Ag, which are of growing economic interest for our civilization. In recent years, the exploration of SMS deposits has entered into a new era with the goal of better assess and characterize these resources. In this perspective, geophysical methods are in high demand to localize SMS deposits and obtain their three‐dimensional geometry. The self‐potential (SP) method provides an efficient non‐intrusive and cost‐effective approach to image SMS deposits. Previous surveys have documented the existence of recordable negative SP anomalies above the seafloor in association with SMS deposits. SP tomography can be used to assess the 3D geometry of SMS deposits. That said, the conductivity pattern below the seafloor is unknown in previous SP surveys while it is an important ingredient in the inversion of the SP data. Active source measurements like the transient electromagnetic (TEM) method can be used to obtain the electrical conductivity distribution below the seafloor. So, we propose to combine SP and TEM data to improve our ability to image SMS deposits. We apply the new methodology to a famous submarine hydrothermal field located in the Atlantic mid‐oceanic ridge Trans‐Atlantic Geotraverse and investigated in the past by various scientific teams. Our study demonstrates that the combination of the two methods has the ability to better image SMS deposits by eliminating source current ghosts associated with conductivity contrasts. Key Points: A joint self‐potential (SP) and transient electromagnetic survey has been performed at Trans‐Atlantic Geotraverse (TAG) hydrothermal field (Mid‐Atlantic Ridge)The transient electromagnetic method provides the electrical conductivity needed for SP tomographyThe inversion of the SP data reveals the distribution of the massive sulfide deposits at TAG [ABSTRACT FROM AUTHOR]

The outcrops of ultramafic rock along mid‐ocean ridges experience variable extents of serpentinization related to tectonic and hydrothermal processes. It involves the water‐assisted alteration of primary ferromagnesian minerals (e.g., olivine and pyroxenes) to produce secondary minerals including serpentine, magnetite and brucite. This hydration process induces profound changes in both the rheological and petrophysical properties of the ocean lithosphere. That said, the evolution of the low‐frequency electrical polarization properties of these rocks along the serpentinization path remains poorly explored. A total of 47 core samples were collected at the seafloor along the Southwest Indian Ridge (SWIR) in order to investigate the process of serpentinization in terms of combined magnetization and polarization signatures. We found a linear relationship connecting chargeability and magnetic susceptibility for both fresh basalts and ultramafic rocks. A unique trend between these two properties is associated with the production of magnetite during serpentinization of peridotites. Magnetite acts indeed as a semi‐conductor responsible for a high chargeability in a low‐frequency (<10 kHertz) electrical field and because of its magnetic properties, it increases the magnetic susceptibility of the ultramafic rocks during serpentinization. In addition, both the chargeability and the magnetic susceptibility change with the grain density of the core samples, which depends on the degree of serpentinization. Plain Language Summary: For the first time, a relationship connecting two geophysical properties of interest (chargeability and magnetic susceptibility) is developed for fresh basalts and serpentinized peridotites. A mineral, magnetite, is responsible for a high chargeability and is the only highly magnetic mineral in serpentinites. Our database is built using core samples from the Southwest Indian Ridge. It illustrates the variation of magnetic and electrical properties along the serpentinization path from fresh peridotites to pure serpentinites. This unique database provides strong constraints on the production of magnetite during the progression of the serpentinization in an ultraslow‐spreading ridge. Furthermore, integrating both induced polarization, magnetic and gravity surveys as complementary geophysical methods has therefore the potential of better characterizing the process of serpentinization at oceanic ridges. Key Points: Serpentinization of peridotites is responsible for a change in magnetite content and densityMagnetite affects both the magnetic susceptibility and the chargeability of peridotitesMagnetic susceptibility and chargeability depend exponentially on the degree of serpentinization [ABSTRACT FROM AUTHOR]

Deep‐sea polymetallic sulfides associated with hydrothermal systems are considered a potential viable resource for base and precious metals. The Southwest Indian Ridge (SWIR, Indian Ocean) hosts active and inactive hydrothermal systems. Inactive hydrothermal fields are more abundant than active fields but are difficult to remotely characterize. We report a deep‐sea self‐potential investigation to locate inactive ore deposits at the Yuhuang hydrothermal field on the ultraslow‐spreading SWIR. A horizontal array of six nonpolarizing AgCl electrodes are attached to a deep‐towed transient electromagnetic system to record the electrical field at a height of 40 m above the seafloor. The observed electrical field strength near a previously recognized sulfide deposits (verified by drilling) reaches an amplitude of 0.5 mV/m. Negative self‐potential anomalies (~ −27 mV) were observed by integrating the measured electrical field. The high electrical conductivities (up to 12 S/m) of sulfide samples measured in the laboratory and the oxidized sulfides recovered from the surface of the deposit suggest that the self‐potential anomalies are due to sulfide mineralization and corrosion of the polymetallic sulfides. Inversion of the self‐potential data reveals a localized body with a thickness of ~65 m, which is interpreted as the Yuhuang ore deposit. Our field data demonstrate that the self‐potential method is a useful exploration method to identify and image seafloor massive sulfide deposits hosted in inactive hydrothermal fields at mid‐ocean ridges. Key Points: Seafloor sulfide deposits exhibit negative self‐potential anomaliesThe self‐potential mechanism results from redox processesExtend of sulfide body at depth is characterized using self‐potential tomography [ABSTRACT FROM AUTHOR]