Near-seafloor magnetic signatures unveil serpentinization dynamics at ultramafic-hosted hydrothermal sites

A near-seafloor magnetic and bathymetric survey conducted by the autonomous underwater vehicle AutoSub 6000 over intermediate-temperature, ultramafic-hosted Von Damm Vent Field (Mid-Cayman spreading center, Caribbean Sea) revealed a moderate positive magnetic anomaly, in accordance with the magnetic response of other known ultramafic-hosted hydrothermal vent fields. However, compared with low-temperature ultramafic-hosted hydrothermal activity, the magnetic signature of this intermediate-temperature site indicates a slightly stronger magnetization contrast between the hydrothermal system and its host, but it remains considerably weaker than at high-temperature ultramafic-hosted hydrothermal vent fields. This observation highlights the nonlinear increase of magnetization production with temperature, as iron partitions into weakly magnetic brucite under 200 °C, but magnetite dominates above this temperature, leading to a sudden increase in the magnetic signature of a site. Our study is consistent with recent laboratory experiments and unveils the dynamics of the serpentinization reaction, enabling fine tuning of the magnetic technique for remotely locating hydrothermal systems. In addition to refining our understanding of the magnetic behavior of hydrothermal vent fields, these new results also reveal the orientation of the fluid pathway feeding the hydrothermal site and indicate the nonvertical structure of the complex network of fissures within the host rock and its associated tectonic feature—an oceanic core complex

High-resolution magnetic signature of active hydrothermal systems in the back-arc spreading region of the southern Mariana Trough

International audienceHigh-resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back-arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep-sea manned submersible. A new 3-D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic-anomaly-derived absolute magnetization suggests that all five andesite-hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt-hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low-magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off-axis sites than at on-axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off-axis sites

Detachment tectonics at Mid-Atlantic Ridge 26°N

Spreading processes associated with slow-spreading ridges are a complex interplay of volcanic accretion and tectonic dismemberment of the oceanic crust, resulting in an irregular seafloor morphology made up of blocks created by episodes of intense volcanic activity or tectonic deformation. These blocks undergo highly variable evolution, such as tilts or dissection by renewed tectonic extension, depending on their positions with respect to the spreading axis, core complexes, detachment or transform faults. Here, we use near-seafloor magnetic and bathymetric data and seismic profiles collected over the TAG Segment of the Mid-Atlantic Ridge to constrain the tectonic evolution of these blocks. Our study reveals that the presence and evolution of oceanic core complexes play a key role in triggering block movements. The deep subvertical detachment fault roots on the plate boundary, marked by a thermal anomaly and transient magma bodies. Thermal and magmatic variations control the structure and morphology of the seafloor above the subhorizontal detachment surface, occasionally leading to relocating the detachment

Structural control, evolution, and accumulation rates of massive sulfides in the TAG hydrothermal field

The Trans‐Atlantic Geotraverse (TAG) hydrothermal field on the Mid‐Atlantic Ridge is one of the best‐studied hydrothermal systems to date. However, high‐resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle (AUV) reveal new information about the distribution of active and inactive hydrothermal deposits, and their relation to structural features. The discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow‐spreading ridges. The interpretation of ship‐based and high‐resolution AUV‐based data sets allowed for the determination of the main tectonic stress regimes that have a first‐order control on the location and distribution of past and present hydrothermal activity. The data reveal the importance of cross‐cutting lineament populations and temporal variations in the prevalent stress regime. A dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material (~29 Mt). The accumulation rate of ~1,500 t/yr is comparable to those of other modern seafloor vent fields. However, our observations suggest that the TAG segment is different from many other slow‐spreading ridge segments in its tectonic complexity, which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation. The inactive and weakly active mounds contain almost 10 times the amount of material as the active high‐temperature mound, providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits

Index

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

High-resolution magnetics reveal the deep structure of a volcanic-arc-related basalt-hosted hydrothermal site (Palinuro, Tyrrhenian Sea)

High-resolution magnetic surveys have been acquired over the partially sedimented Palinuro massive sulfide deposits in the Aeolian volcanic arc, Tyrrhenian Sea. Surveys flown close to the seafloor using an autonomous underwater vehicle (AUV) show that the volcanic-arc-related basalt-hosted hydrothermal site is associated with zones of lower magnetization. This observation reflects the alteration of basalt affected by hydrothermal circulation and/or the progressive accumulation of a nonmagnetic deposit made of hydrothermal and volcaniclastic material and/or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids. To discriminate among these inferences, estimate the shape of the nonmagnetic deposit and the characteristics of the underlying altered area—the stockwork—we use high-resolution vector magnetic data acquired by the AUV Abyss (GEOMAR) above a crater-shaped depression hosting a weakly active hydrothermal site. Our study unveils a relatively small nonmagnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones. Thermal demagnetization is unlikely but the stockwork extends beyond the limits of the nonmagnetic deposit, forming lobe-shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones

Magnetic signature of hydrothermal sites in various environments: contribution of high-resolution data

Detailed submarine exploration shows a diversity of hydrothermal sites in terms of geology, minerals, fluids and biology. High0resolution, near0seafloor magnetic surveys have been undertaken on various basalt0hosted or ultramafic0hosted hydrothermal sites using a vector magnetometer mounted on deep0sea submersible Nautile, remotely operated vehicle (ROV) Victor, and autonomous underwater vehicle (AUV) AsterX of Ifremer. These surveys reveal different magnetic signatures, depending the characteristics of the hydrothermal sites. (1) Basalt-hosted sites are characterized by a negative (reduced to the pole) anomaly indicating a reduced magnetization. Two processes are often considered to explain this observation: thermal demagnetization or alteration of the magnetic minerals of the basaltic crust by the high temperature hydrothermal fluid circulation. The study of inactive hydrothermal site Krasnov suggests that the observed magnetic signature survives the cessation of hydrothermal activity and therefore alteration plays the dominant role. (2) High-temperature ultramafic-hosted hydrothermal sites are characterized by either a positive magnetic anomaly, indicating a stronger magnetization, at the larger sites, or a lack of significant magnetic signature at the smaller sites. Some measurements performed on rock samples suggest that the magnetic bearer is magnetite, which remains unaltered within sulfur0impregnated serpentinized peridotites. High-resolution magnetic surveys may therefore play an important role to detect and characterize active and fossil hydrothermal sites, and to evaluate their mining potential as well.L'exploration sous-marine a révélé l’existence de variations en termes de géologie, minéralisations, fluides et biologie des sites hydrothermaux océaniques. Des levés magnétiques de haute résolution ont été réalisés avec un magnétomètre vectoriel fixé à la structure du sous-marin Nautile, du robot télé-opéré Victor ou du drone sous0 marin AsterX d’Ifremer. Ils montrent l’existence de différentes signatures magnétiques, suivant les caractéristiques des sites hydrothermaux. (1) Les sites à substrat basaltique se caractérisent par une anomalie magnétique (au pôle) négative, traduisant un défaut d'aimantation. Deux hypothèses sont avancées pour expliquer cette observation : la désaimantation thermique ou l’altération des minéraux magnétiques du substrat basaltique sous l’action des fluides hydrothermaux. L’étude du site inactif Krasnov suggère que la signature magnétique des sites hydrothermaux basaltiques demeure après la fin de l’activité, l’altération étant donc le phénomène dominant. (2) Les sites haute température à substrat ultramafique se caractérisent par une anomalie magnétique positive pour les sites les plus importants, ou par l'absence de signature particulière pour les sites plus petits. Des mesures effectuées sur échantillons suggèrent que la magnétite est porteuse de l’aimantation et qu’elle demeure inaltérée dans les péridotites serpentinisées imprégnées de sulfures. Les levés magnétiques de haute résolution peuvent ainsi jouer un rôle important pour détecter et caractériser les sites hydrothermaux actifs ou fossiles, et évaluer leur potentiel minier

Signature magnétique des sites hydrothermaux dans différents environnements: contribution des données de haute résolution

Detailed submarine exploration shows a diversity of hydrothermal sites in terms of geology, minerals, fluids and biology. High0resolution, near0seafloor magnetic surveys have been undertaken on various basalt0hosted or ultramafic0hosted hydrothermal sites using a vector magnetometer mounted on deep0sea submersible Nautile, remotely operated vehicle (ROV) Victor, and autonomous underwater vehicle (AUV) AsterX of Ifremer. These surveys reveal different magnetic signatures, depending the characteristics of the hydrothermal sites. (1) Basalt-hosted sites are characterized by a negative (reduced to the pole) anomaly indicating a reduced magnetization. Two processes are often considered to explain this observation: thermal demagnetization or alteration of the magnetic minerals of the basaltic crust by the high temperature hydrothermal fluid circulation. The study of inactive hydrothermal site Krasnov suggests that the observed magnetic signature survives the cessation of hydrothermal activity and therefore alteration plays the dominant role. (2) High-temperature ultramafic-hosted hydrothermal sites are characterized by either a positive magnetic anomaly, indicating a stronger magnetization, at the larger sites, or a lack of significant magnetic signature at the smaller sites. Some measurements performed on rock samples suggest that the magnetic bearer is magnetite, which remains unaltered within sulfur0impregnated serpentinized peridotites. High-resolution magnetic surveys may therefore play an important role to detect and characterize active and fossil hydrothermal sites, and to evaluate their mining potential as well.L'exploration sous-marine a révélé l’existence de variations en termes de géologie, minéralisations, fluides et biologie des sites hydrothermaux océaniques. Des levés magnétiques de haute résolution ont été réalisés avec un magnétomètre vectoriel fixé à la structure du sous-marin Nautile, du robot télé-opéré Victor ou du drone sous0 marin AsterX d’Ifremer. Ils montrent l’existence de différentes signatures magnétiques, suivant les caractéristiques des sites hydrothermaux. (1) Les sites à substrat basaltique se caractérisent par une anomalie magnétique (au pôle) négative, traduisant un défaut d'aimantation. Deux hypothèses sont avancées pour expliquer cette observation : la désaimantation thermique ou l’altération des minéraux magnétiques du substrat basaltique sous l’action des fluides hydrothermaux. L’étude du site inactif Krasnov suggère que la signature magnétique des sites hydrothermaux basaltiques demeure après la fin de l’activité, l’altération étant donc le phénomène dominant. (2) Les sites haute température à substrat ultramafique se caractérisent par une anomalie magnétique positive pour les sites les plus importants, ou par l'absence de signature particulière pour les sites plus petits. Des mesures effectuées sur échantillons suggèrent que la magnétite est porteuse de l’aimantation et qu’elle demeure inaltérée dans les péridotites serpentinisées imprégnées de sulfures. Les levés magnétiques de haute résolution peuvent ainsi jouer un rôle important pour détecter et caractériser les sites hydrothermaux actifs ou fossiles, et évaluer leur potentiel minier