ALBERT

Description: Back‐arc basins open in response to subduction processes, which cause extension in the upper plate, usually along trench‐parallel spreading axes. However, global seismic databases reveal that the majority of seismic events in the Lau Basin occur along transcurrent (strike‐slip) rather than extensional faults. To better characterize active deformation in this region we compared Centroid Moment Tensors (CMTs), calculated for large (Mw 〉5), shallow (〈30 km) seismic events to the orientations of seafloor lineaments mapped throughout the Lau Basin. Ship‐based multibeam was combined with vertical gravity gradient data to provide comprehensive coverage to create the lineament map. By comparing the possible focal planes of the CMTs to the orientations of the lineaments, the most likely fault plane solutions were selected, thus classifying the faults and establishing the nature of the highly variable stress regimes in the basin. We resolved the strike, dip and dip direction of 308 faults, and classified 258 additional structures by fault type. The analysis highlights a stress regime that is dominated by a combination of left‐lateral and right‐lateral strike‐slip faults, large‐scale transcurrent motion along rigid crustal‐scale fault zones, and non‐rigid diffuse deformation along pre‐existing seafloor structures, with extension mainly limited to the tips of propagating rifts and spreading centers. By resolving many of the uncertain motions on the mapped lineaments of the Lau Basin, the CMT analysis addresses a number of questions concerning basin‐scale stress regimes and microplate development, complementing GPS measurements and providing a more complete picture of the complexities of back‐arc basin development.

Description: Hydrothermal fluids in ultramafic‐hosted hydrothermal systems associated with oceanic detachment faults can be more oxidizing compared to mafic‐hosted vent sites. These fluids form a mineral assemblage of pyrite, magnetite and hematite. At 13°30′N on the Mid‐Atlantic Ridge, chlorite‐quartz breccias recovered from an exposed fault scarp contain pyrite, with abundant magnetite and hematite, indicating that the redox of the fluids was variable. In primary micron‐scale zonations in pyrite, Ni, Co, and Se have a decoupled relationship, recording fluctuations in the chemical composition and temperature of hydrothermal fluid as the grains grew. Secondary zonations that erase and overprint primary zonations are limited to the grain margin and permeable regions within the grain core. Secondary zonations formed via two processes: (a) grain dissolution followed by overgrowth, and (b) remobilization of metals during oxidizing fluid flow events. In both instances, Ni and Co have been mobilized and concentrated, and are not lost to the hydrothermal fluid. Superimposed on these features is evidence of grain scale deformation related to periods of fault movement along the detachment surface. Sulfur isotope ratios (δ 34 S) in pyrite systematically decrease from the grain margin to the grain core, indicating that increased amounts of sulfur were derived from thermochemical sulfate reduction of seawater. Thus, pyrite records the evolution of fluid flow and deformation events during exhumation along the detachment surface from ∼1 to 2 km below the seafloor at the base of the lava pile, with temporal fluctuations in fluid redox identified as an important process in controlling Ni and Co enrichment in pyrite. Plain Language Summary Detachment faults are long lived faults that can expose ultramafic rocks at the seafloor. We aim to investigate the links between hydrothermal activity and detachment fault formation. To do this we use pyrite as a tape recorder for past fluid flow events. Across individual mineral grains, distinct zonations in metal content and sulfur isotope ratios show that the incursion of seawater occurred periodically during pyrite growth, increasing during fault movement events that lead to changes in the temperature and pH of the fluids in the fault zone. These changes concentrated metals toward the center of individual mineral grains. Zonations were then overprinted by later deformation‐related events, providing evidence that the samples formed at deeper crustal levels below the seafloor and were progressively exhumed at the seafloor over time. Key Points Microtextural, geochemical, and isotopic variations in subseafloor pyrite record the history of sample exhumation along a detachment fault Nickel and Co are remobilized and concentrated in pyrite across individual mineral grains in response to fluctuating fluid redox conditions Evidence of pyrite deformation and alteration mineralogy of samples indicates sample exhumation from a depth of 1–2 km

Description: A GEOMAR (Kiel, Germany) research team has developed a passive electric field acquisition system for Autonomous Underwater Vehicles (AUVs) to optimize seafloor massive sulfides exploration. This sensor was made of two perpendicular and horizontal pairs of electrodes, and was successfully tested over active basalt-hosted hydrothermal site TAG (26°N, Mid-Atlantic Ridge) and several inactive sites in its vicinity. The resulting data underline the efficiency of combining deep-sea electric and magnetic measurements for searching for active and inactive hydrothermal vent fields. With these datasets, it becomes possible to determine the geological nature of the targets and to constrain the characteristics of fluid circulation at depth without involving costly and invasive underwater tools such as Remotely Operated Vehicles or even manned submersibles to collect samples. Data analysis also revealed that AUV attitude variations induce distortions of the electric signal. These distortions start prevailing for dives at altitudes higher than 90 m above the seafloor, as the distance between the AUV becomes too important to guarantee that the signal produced by the geological target still dominates. To improve the acquisition system and reduce the overall noise, we discuss solutions that limit the impact of such attitude variations. These solutions consist of minor adjustments, such as masts at AUVs stern to tow damping electrodes arrays. In such configurations, we believe that deep-sea passive electric measurements combined with high-resolution magnetic measurements can become a highly efficient seafloor exploration tool, including for sulfide deposits associated with inactive hydrothermal systems.

Description: Mapping and sampling four sections of the slow-spreading Reykjanes Ridge provide insight into how tectonic and volcanic activity varies with distance from the Iceland plume. The studied areas are characterized by significant variations in water depth, lava chemistry, crustal thickness, thermal structure, and ridge morphology. For each study area, fault pattern and dimension, tectonic strain, seamount morphology, and density are inferred from 15 m-resolution bathymetry. These observations are combined with geochemical analysis from glass samples and sediment thickness estimations along Remotely Operated Vehicle-dive videos. They reveal that (a) tectonic and volcanic activity along the Reykjanes Ridge, do not systematically vary with distance from the plume center. (b) The tectonic geometry appears directly related to the deepening of the brittle/ductile transition and the maximum change in tectonic strain related to the rapid change in crustal thickness and the transition between axial-high and axial valley (∼59.5°N). (c) Across-axis variations in the fault density and sediment thickness provide similar widths for the neo-volcanic zone except in regions of increased seamount emplacement. (d) The variations in seamount density (especially strong for flat-topped seamounts) are not related to the distance from the plume but appear to be correlated with the interaction between the V-shape ridges (VSR) flanking the ridge and the ridge axis. These observations are more compatible with the buoyant upwelling melting instability hypothesis for VSR formation and suggest that buoyant melting instabilities create many small magma batches which by-pass the normal subaxial magmatic plumbing system, erupting over a wider-than-normal area. Key Points The distance from the plume center is not the only factor controlling tectonic and volcanic activity along the Reykjanes Ridge Fault dimensions are primarily controlled by the variation of crustal thermal structure with distance from the hotspot Flat-topped seamount abundances peak where a V-shaped ridge intersects the axis, consistent with a buoyant upwelling melting instability

Description: Seafloor massive sulfide deposits form in remote environments, and the assessment of deposit size and composition through drilling is technically challenging and expensive. To aid the evaluation of the resource potential of seafloor massive sulfide deposits, three-dimensional inverse modelling of geophysical potential field data (magnetic and gravity) collected near the seafloor can be carried out to further enhance geologic models interpolated from sparse drilling. Here, we present inverse modelling results of magnetic and gravity data collected from the active mound at the Trans-Atlantic Geotraverse hydrothermal vent field, located at 26o08'N on the Mid-Atlantic Ridge, using autonomous underwater vehicle (AUV) and submersible surveying. Both minimum-structure and surface geometry inverse modelling methods were utilized. Through deposit-scale magnetic modelling, the outer extent of a chloritized alteration zone within the basalt host rock below the mound was resolved, providing an indication of the angle of the rising hydrothermal fluid and the depth and volume of seawater/hydrothermal mixing zone. The thickness of the massive sulfide mound was determined by modelling the gravity data, enabling the tonnage of the mound to be estimated at 2.17 +/- 0.44 Mt through this geophysics-based, non-invasive approach.

Description: Accessible seafloor minerals located near mid‐ocean ridges are noticed to mitigate the projected metal demands of the net‐zero energy transition, promoting growing interest in quantifying the global distributions of seafloor massive sulfides (SMS). Mineral potentials are commonly estimated using geophysical and geological data that lastly rely on additional confirmation studies using sparsely available, locally limited, seafloor imagery, grab samples, and coring data. This raises the challenge of linking in situ confirmation data to geophysical data acquired at disparate spatial scales to obtain quantitative mineral predictions. Although multivariate data sets for marine mineral research are incessantly acquired, robust, integrative data analysis requires cumbersome workflows and experienced interpreters. We introduce an automated two‐step machine learning approach that integrates the mound detection through image segmentation with geophysical data. SMS predictors are subsequently clustered into distinct classes to infer marine mineral potentials that help guide future exploration. The automated workflow employs a U‐Net convolutional neural network to identify mound structures in bathymetry data and distinguishes different mound classes through the classification of mound architectures and magnetic signatures. Finally, controlled source electromagnetic data are utilized together with in situ sampling data to reassess predictions of potential SMS volumes. Our study focuses on the Trans‐Atlantic Geotraverse area, which is among the most explored SMS areas worldwide and includes 15 known SMS sites. The automated workflow classifies 14 of the 15 known mounds as exploration targets of either high or medium priority. This reduces the exploration area to less than 7% of the original survey area from 49 to 3.1 km 2 . Key Points A two‐step machine learning workflow identifies mound structures in bathymetry data and classifies their origins based on auxiliary data Significant increase in potential seafloor massive sulfides (SMS) edifices detected within the trans‐Atlantic geo‐traverse hydrothermal field distributed within latitudinal bands SMS mineral potential is likely lower than previously assumed due to heterogeneously distributed mineralization within mounds