Hydrothermal activity in the mid-ocean ridge facilitates the chemical exchange of seawater with new oceanic crusts. This activity mostly occurs on the detachment fault of the asymmetric accretion segment in the slow-ultraslow spreading ridge, which is characterised by limited magma supply. Deep faults can readily extract heat from deeper heat sources. Moreover, the repeated movement of faults activates the permeable fluid channels of the overlying oceanic crust, thus driving long-life hydrothermal circulation. Recent studies have found that the response time of the hydrothermal activity of the intermediate-fast spreading ridges differs from that of the slow-spreading ridge to the glacial cycle, and a unified model is expected to explain it. Also, the response of hydrothermal activity to the glacial cycle must consider the differences between oceanic ridges with different spreading rates and types of hydrothermal systems.Here, based on two sediment cores collected near the Yuhuang hydrothermal field (HF)on ultraslow-spreading Southwest Indian ridge, we obtained high-resolution sediment history records spanning three glacial periods, understood the 160 ka history of hydrothermal, volcanic and tectonic activities in the region and attempted to reveal the response mechanism of hydrothermal activities controlled by detachment faults to the glacial cycle. We discovered that in the Yuhuang HF controlled by detachment faults, hydrothermal activity increased significantly during the glacial period, and more active detachment fault activity appeared at the same time. At the end of the glacial period, both activities are reduced at the same time. We believe that in the slow-ultraslow spreading ridge, the magmatism regulated by sea level changes may regulate the evolution of detachment faults and the hydrothermal circulation, which are recorded in the sediments near the hydrothermal field.We established a response model of Sea level change–Magmatism–Detachment fault activity–Hydrothermal activity and concluded that the magmatism of slow-ultraslow spreading ridges is more sensitive to sea level changes; with the synchronous effect of detachment faults, the hydrothermal activity responds faster to the glacial cycle.

Abstract At the Trans‐Atlantic Geotraverse hydrothermal field, metalliferous sediments cover extinct hydrothermal mounds and the surrounding seafloor. Here, we report the morphological, mineralogical and geochemical processes that deposit these sediments, remobilize their metals, and affect their preservation. We found that the initial sediment metal tenor is controlled by physical transport of hydrothermal material from its source, followed by diagenetic redistribution and potentially diffuse fluid flow after high‐temperature hydrothermal activity has ceased. We distinguished three different environments: (a) proximal metalliferous sediments on top of extinct mounds are mainly derived from oxidative weathering of primary sulfide structures and are predominantly composed of Fe oxyhydroxides with low contents of Cu, Co, and Zn; metal enrichments in specific layers are likely related to upward flow of low‐temperature hydrothermal fluids; (b) medial distant metalliferous sediments found at the base of the mounds, deposited by mass transport, contain cm‐thick layers of unsorted sulfide sands with high base metal contents (e.g., up to 28% Cu); these buried sulfides continue to undergo dissolution, resulting in metal release into porewaters; (c) distal metalliferous sediments, found in depositional basins a few hundreds of meters from the extinct mounds, include fining‐upwards sequences of thin sulfide sand layers with Fe oxyhydroxides and were deposited by recurrent turbiditic flows. Dissolved metals (e.g., Cu2+ and Mn2+) diffuse upwards under reducing conditions and precipitate within the sediment. Hence, when using hydrothermal sediments to construct reliable geochronological records of hydrothermal activity, distance from source, local seafloor morphology, mass‐transport and depositional, and diagenetic modification should all be considered.

AbstractChanges in sea level caused by glacial cycles may influence the magmatism and hydrothermal activity of oceanic ridges. Recent studies showed that the response time of the hydrothermal activity in the intermediate-fast spreading ridges differs from that in the slow-spreading ridges to the glacial cycles, and a unified model is expected to explain it. Here, we report the 160 ka sediment record adjacent to the Yuhuang hydrothermal field on the Southwest Indian Ridge. Hydrothermal and detachment fault activities were found to enhance or weaken during glacial and interglacial periods, respectively. The magmatism of slow/ultraslow spreading ridges is more sensitive to sea level changes; with the synchronous effect of detachment faults, the hydrothermal activity responds faster to the glacial cycles. We established a model of Sea level change–Magmatism–Detachment fault activity–Hydrothermal activity to explain the different responses of the hydrothermal activity of the mid-ocean ridges to the glacial cycles.

The redox state of hydrothermal fluids on mid-ocean ridges, which is indirectly affected by the depth of hydrothermal cir-culation and crustal permeability, plays an important role on the diversity of hydrothermal precipitates and associated ecosys-tems. Primary hydrothermal fluids that circulate along detachment faults are generally reducing as a result of the serpentinization of ultramafic rocks, while significant seawater infiltration may shift the redox state from reducing to oxidiz-ing. However, the depth of penetration of oxidizing fluids into detachment related systems remain unclear, largely because current observations are based primarily on hydrothermal products that precipitated at the seafloor. Here, we report the first observations of oxidizing mineral assemblages in stockwork samples from the Suye hydrothermal field on the ultraslow spreading Southwest Indian Ridge. This field is hosted by mafic lithologies, while the low As, high Ni and Co contents, and high Au/As, Ag/As, and Ni/As ratios in pyrite from the stockwork samples indicate that the fluids reacted with both mafic and ultramafic rocks in the subseafloor. The high d34S values (average of 9.8 parts per thousand) indicate a high proportion (up to 50%) of seawater derived reduced sulfur involved in the stockwork zone formation. The high homogenization temperatures (similar to 320 degrees C) and salinity (similar to 12 wt.% NaCl) of fluid inclusions indicate that the stockwork zone of Suye was formed by a fluid that underwent phase separation deeper in the system that was subsequently diluted by 4-5 times subsurface seawater. The deep penetration of seawater is facilitated by the unique tectonic setting of the Suye hydrothermal field, which occurs between the two stage detachment faults that creates high permeability. Our findings demonstrate that hydrothermal fluid associated with detachment faults could be oxidized below the subsurface stockwork zone, and that deep-rooted detachment faults at ultraslow-spreading ridges can sustain both reducing and oxidizing hydrothermal systems in the same fault system. These results call for a reevaluation of the fate of base metal in ultramafic hosted hydrothermal fields. (c) 2022 Elsevier Ltd. All rights reserved.

Chromite oxidation during serpentinization of host peridotites is a well-documented process. Detailed compositional characterization of chromites and Cr-rich spinels from three geotectonic settings provided the basis for this study, focused on the comparison of their oxidation patterns as a means to evaluate the relative mobility of spinel components during serpentinization-related oxidation in those different contexts, namely: (i) tectonic exposures of serpentinized oceanic upper-mantle in the Azores sector of the Mid-Atlantic Ridge (MAR); (ii) serpentinized peridotites from Cabeço de Vide (CV, Alter do Chão Massif, Portugal); and (iii) serpentinized subcontinental mantle peridotites from the Ronda Massif (Spain). Electron microprobe data show that: (i) irrespective of geotectonic setting and original composition, Cr-spinels follow similar oxidation trends; (ii) early Cr-spinel oxidation, involving significant Mg- and Al-depletion leading to the formation of ferritchromit rims, has been more intense in the CV serpentinites than in the more recent MAR and Ronda serpentinites; (iii) with the possible exception of Zn, trace components (Mn, Ti, V, Ni and Co) seem to be relatively immobile during spinel oxidation, thus becoming slightly enriched in ferritchromit rims; (iv) extreme Cr-spinel oxidation is responsible for incomplete outer rims of magnetite on several grains. Time, fluid pH, and fluid/rock ratios seem to be the main factors controlling the intensity and extension of chromite oxidation.

The rare earth elements (REE), comprising 15 elements of the lanthanum series (La-Lu) together with yttrium (Y) and scandium (Sc), have become of particular interest because of their use, for example, in modern communications, renewable energy generation, and the electrification of transport. However, the security of supply of REE is considered to be at risk due to the limited number of sources, with dependence largely on one supplier that produced approximately 63% of all REE in 2019. As a result, there is a growing need to diversify supply. This has resulted in the drive to seek new resources elsewhere, and particularly on the deep-ocean floor. Here, we give a summary of REE distribution in minerals, versatile applications, and an update of their economic value. We present the most typical onshore methods for the determination of REE and examine methods for their offshore exploration in near real time. The motivation for this comes from recent studies over the past decade that showed ΣREE concentrations as high as 22,000 ppm in ocean-floor sediments in the Pacific Ocean. The ocean-floor sediments are evaluated in terms of their potential as resources of REE, while the likely economic cost and environmental impacts of deep-sea mining these are also considered.

The Menez Gwen hydrothermal field (37º50’N), is located near the top of a young volcano at the Mid Atlantic Ridge. It is characterized by small chimneys, diffuse venting associated with breccia cemented by hydrothermal amorphous silica-barite. This work presents in situ trace elemental and S isotopic compositions of pyrite and chalcopyrite in three types of hydrothermal samples from this field: high-temperature chalcopyrite-pyrite chimney (HT-ch); mid-temperature sphalerite-pyrite-barite chimney (MT-ch) and low-temperature amorphous silica-barite breccia (LT-br). The results show a systematic distribution of trace metals from LT-br to HT-ch. Trace metals of pyrite analysed on HT-ch samples contain higher concentrations of elements derived from high-T fluids (e.g. Se, Co, Ni and Cu), indicating preferential incorporation of these elements in the lattice of pyrite at higher-Ts. However, some pyrite-chalcopyrite aggregate identified within the base of LT-br also show high-T derived elements, suggesting higher temperature conditions below the sub-surface, probably as a result of a decrease in permeability during slab formation. Framboidal and colloform pyrite observed in most of the LT-br clearly display an enrichment of elements derived from lower-T and oxidative conditions (e.g. Mn, Tl, Mo, Cr and V). Sulphide phases have δ 34 S values ranging from -2.5 to 7.8‰

The on-board identification of ore minerals during a cruise is often postponed until long after the cruise is over. During the M127 cruise, 21 cores with deep-seafloor sediments were recovered in the Trans-Atlantic Geotraverse (TAG) field along the Mid Atlantic Ridge (MAR). Sediments were analyzed on-board for physicochemical properties such as lightness (L*), pH and Eh. Selected samples were studied for mineral composition by X-ray powder diffraction (XRD). Based on XRD data, sediment samples were separated into high-, low- and non-carbonated. Removal of carbonates is a common technique in mineralogical studies in which HCl is used as the extraction agent. In the present study, sequential extraction was performed with sodium acetate buffer (pH 5.0) to remove carbonates. The ratio between the highest calcite XRD reflection in the original samples (Iorig) vs its XRD-reflection in samples after their treatment with the buffer (Itreat) was used as a quantitative parameter of calcite removal, as well as to identify minor minerals in carbonated samples (when Iorig/Itreat > 24). It was found that the lightness parameter (L*) showed a positive correlation with calcite XRD reflection in selected TAG samples, and this could be applied to the preliminary on-board determination of extraction steps with acetate buffer (pH 5.0) in carbonated sediment samples. The most abundant minerals detected in carbonated samples were quartz and Al- and Fe-rich clays. Other silicates were also detected (e.g., calcic plagioclase, montmorillonite, nontronite). In non-carbonated samples, Fe oxides and hydroxides (goethite and hematite, respectively) were detected. Pyrite was the dominant hydrothermal mineral and Cu sulfides (chalcopyrite, covellite) and hydrothermal Mn oxides (birnessite and todorokite) were mineral phases identified in few samples, whereas paratacamite was detected in the top 20 cm of the core. The present study demonstrates that portable XRD analysis makes it possible to characterize mineralogy at cored sites, in particular in both low- and high-carbonated samples, before the end of most cruises, thus enabling the quick modification of exploration strategies in light of new information as it becomes available in near-real time.

The Southwest Indian Ridge (SWIR) is an ultraslow-spreading ridge where large hydrothermal fields (HFs) are widely distributed. The HFs differ in geological settings, basement rock compositions and mineral associations, but are commonly associated with massive sulfides and sulfide-rich hydrothermal vents that are rich in mercury (Hg). However, the source of Hg remains not well understood. This is a first report on the concentration and isotopic composition of Hg in sulfides from two large HFs, named Duanqiao and Yuhuang, in the SWIR. Sulfides from Duanqiao and Yuhuang showed elevated Hg concentrations, ranging from 3.5 x 10(2) to 8.1 x 10(3) ng/g and 4.4 x 10(2) to 4.4 x 10(4) ng/g, respectively, which suggest that seafloor mass sulfide deposits can be an important sink of Hg to the deep marine environment. In both HFs, pyrite (4.4 x 10(2)-4.4 x 10(4) ng/g) and sphalerite (8.3 x 10(2)-6.0 x 10(3) ng/g) show higher Hg concentrations than chalcopyrite (3.5 x 10(2) ng/g), suggesting that the replacement of Fe(II) and Zn(II) by Hg(II) is a major form of Hg incorporation in sulfides. Sulfides from Yuhuang show relatively larger delta Hg-202 and Delta Hg-199 ranges of -1.23 to -0.05 parts per thousand and -0.10 to 0.20 parts per thousand, respectively, compared with those from Duanqiao (delta(202) Hg: -0.63 to -0.12 parts per thousand; Delta(199) Hg: 0.02-0.10 parts per thousand), suggesting that the Hg sources are different in the two HFs. The differences in Delta(199) Hg suggest the dominance of magmatic/mantle Hg in the Duanqiao HF, but a mixture of magmatic/mantle and seawater Hg to the Yuhuang HF. We thus propose Hg isotopes as a potential source tracer of Hg in HFs and infer that magmatism may not only serve as a direct Hg source in HFs, but may also drive seawater circulation and cause the precipitation of Hg from hydrothermally circulated seawater. (C) 2020 Elsevier Ltd. All rights reserved.

The cost-efficient methods of analysis, such as rapid short-wave infrared (SWIR) spectral analysis, have been applied for the efficient exploration of critical raw materials (CRM), including mineral components and rare earth elements (REE) from the deep-ocean sediments. Gravity cored sediment samples were collected during an oceanographic mission to the Trans-Atlantic Geotraverse (TAG) hydrothermal field of the Mid-Atlantic Ridge (MAR, 26° N). SWIR reflectance spectra (dependent variable) of samples were mathematically tested against referent geochemical data (independent variable), obtained by conventional analysis (ICP/OES, ICP/MS), after applied full cross-validation multivariate partial least square regression (CVPLSR). Value of parameter-residual predictive deviation (RPD) was used for evaluation of CVPLSR modeling: RPD > 2.5 (satisfactory calibration model for the screening purposes) and RPD > 5.0 (model adequate for the quality control of the studied elements). The CVPLSR modeling provided significant results for the determination of several mineral components: major elements (Fe and Si) had the values of RPD equal to 3.65 and 2.84, respectively, which indicated a viable potential for their routine analysis, whereas RPD for Ca was equal to 5.51, thus assuring its quality control by SWIR analysis, in sediment samples of the studied location. Among the REE, Ce (RPD = 2.55) and Er (RPD = 2.59) yielded the most satisfactory results. The findings highlight the benefit of rapidly obtained empirical SWIR-reflectance data, which can be used for near-real-time exploration of geochemical deposits hosted in deep-ocean sediments.

The Rainbow hydrothermal field (36 degrees 14'N) and the Saldanha seamount (36 degrees 34'N), in the Mid-Atlantic Ridge (MAR), are tectonic exposures of serpentinized upper mantle peridotites, both associated with significant hydrothermal activity. On the basis of detailed mineralogical and geochemical characterization of serpentinites from both sites, several serpentinization-related issues are discussed in the present work. As expected in oceanic environments, most of the sampled rocks are lizardite-chrysotile serpentinites exhibiting a variety of pseudomorphic through non-pseudomorphic textures, such textural evolution probably being related to changing water/rock ratios during this retrograde process. Oxygen isotope temperatures indicate that the serpentinization took place at 300-200 degrees C; on the other hand, isotopic data suggest that replacement of early pseudomorphic lizardite by lizardite chrysotile non-pseudomorphic textures requires that temperatures and/or water/rock ratios are high enough to promote the necessary dissolution-recrystallization processes. Mass-balance calculations for olivine-serpentine and orthopyroxene-serpentine pairs provided a basis for establishing serpentinization reactions likely to have produced the present rocks. Moreover, these calculations also showed that, notwithstanding some noticeable loss of MgO from olivine and of SiO2 from orthopyroxene, serpentinization of both minerals implies volume increases on the order of 26-27%, therefore potentially promoting the overall expansion of the rock. The geochemical and isotopic features of the studied rocks indicate that unmodified seawater was responsible for the serpentinization of the MAR peridotites. However, the mineralogy and REE patterns of some of these serpentinites indicate occasional subsequent interaction of the scrpentinized rocks with seawater at much lower temperatures (seafloor alteration, characterized by carbonate deposition and negative Ce anomalies), or with high-temperature ore-forming hydrothermal fluids (ore-forming alteration, characterized by sulfide precipitation and steep positive Eu anomalies).

•Generic geological model of hydrothermally extinct seafloor massive sulphide.•Sub-surface characterisation by combined drilling and geophysics.•New sulphide resource estimate for slow-spreading mid-ocean ridges.•Holistic approach to assessing seafloor massive sulphide deposits.

The Yuhuang-1 hydrothermal field (HF) was discovered recently on the ultraslow-spreading Southwest Indian Ridge (segment 29). This field comprises two main sulfide deposition areas, including the southwest sulfide area (SWS) and the northeast sulfide area (NES), all of which are about 500 m in diameter. Sulfide-rich samples collected on the seafloor consist of zinc-rich, iron-rich, and silicified samples. The Zn-rich samples contain up to 60% sphalerite and 5% chalcopyrite, while iron-rich samples primarily contain pyrite and marcasite, and silicified sulfide rich samples primarily consist of amorphous silicon (70%), pyrite (15%-20%), and minor sphalerite. Sulfide rich samples generally show mound features with similar mineral assemblages, except that the NES contains pyrrhotite, indicating precipitation under relatively reduced conditions. Bulk geochemistry of the SWS and NES massive sulfide samples display similar Au, Co, Ni, and Si contents and Ni/Co ratios, similar to mafic related HFs, while the Sn content and the Cd/Zn ratio are comparable to ultramafic-related HFs, which probably indicates a multi-stage origin. Sulfides rich samples from the SWS have delta S-34 values ranging from -1.37 parts per thousand to 6.02 parts per thousand with a median value of 0.83 parts per thousand. Currently, the delta S-34 value of -1.37 parts per thousand is the lowest sulfur isotope composition observed in HFs on ultraslow-spreading ridges. Bacteria-derived sulfur, which is calculated to be 10%-25% of the total sulfur component, is likely the cause of the negative sulfur isotope composition. Conversely, the delta S-34 values of the seafloor sulfides rich samples in the NES ranges from 3.75 parts per thousand to 8.73 parts per thousand with a median value of 4.74 parts per thousand, which is distinctly heavier than that of the SWS. This study suggests that the SWS and NES of the Yuhuang-1 HF probably formed during different mineralization stages.

Contact metamorphism related to Variscan and late-Variscan granitic plutons in the Iberian Peninsula is superimposed on medium-grade regional metamorphism, making it often difficult to evaluate per se the thermal effects due to those intrusions and explaining the paucity of scientific literature on the subject. An exhaustive set of geochemical, isotopic and mineralogical data on the contact-zone metasediments hosting the Penamacor–Monsanto granite (Central Iberian Zone, Portugal) provides a significant contribution to the characterization of low- to intermediate-grade contact metamorphism in geological contexts formerly affected by regional metamorphism. The metasediments hosting the Penamacor–Monsanto pluton belong to the extensive detrital sequence of the ante-Ordovician Schist-Greywacke Complex. Bulk geochemistry, oxygen isotope data and crystal-chemistry of key minerals from those contact-zone and neighbouring metasediments have made it possible to infer metamorphic conditions on the contact zone of this granitic intrusion, and to distinguish them from late boron-metasomatism at the exocontact. Mineral parageneses (muscovite + biotite + chlorite ± quartz ± plagioclase ± cordierite, in spotted-schists; biotite + chlorite ± quartz ± plagioclase (± cordierite), in hornfelses) and the composition of these coexisting mineral phases indicate that most of the contact rocks reached the biotite zone (or even the cordierite zone , in some cases), equivalent to upper greenschist – lower amphibolite metamorphic grade. The relatively narrow range of O-isotope temperatures estimated for the crystallization of the marginal granites (550–625 °C) explains the absence of significant effects of thermal flow anisotropy on the contact-zone rocks. Besides, textural, paragenetic, mineralogical, isotopic and geochemical nuances observed in hornfelses and spotted-schists seem mainly related to the local host-rock heterogeneities, rather than to thermal effects. The relatively low temperatures estimated for granitoid emplacement and their restricted isotopic and mineralogical impacts on the metasedimentary host-rocks account for the narrow metamorphic aureole associated with the Penamacor–Monsanto pluton, and suggest this massif may correspond to the outcropping tip of a larger granitic intrusion at depth.