Ultraslow Spreading Southwest Indian Ridge Research Articles

3D self-potential tomography of seafloor massive sulfide deposits using an autonomous underwater vehicle

Seafloor massive sulfide (SMS) deposits contain abundant metals, such as Cu, Zn, Au, and Ag, which are of growing economic interest. Currently, the exploration of SMS deposits has entered into a new stage of resource assessment, so we need a geophysical method to rapidly obtain the 3D structure of these SMS deposits. We report here a self-potential (SP) survey performed with an autonomous underwater vehicle (AUV) at the Yuhuang hydrothermal field on the ultraslow-spreading Southwest Indian Ridge with a water depth ranging from 1300 m to 2200 m. The electric-field sensors are attached to the tail of the AUV to record the electric field approximately 50 m above the seafloor. The electric field strength is observed to reach an amplitude of 0.6 mV/m at this known SMS deposit. SP tomography is used to obtain the 3D structure of the ore body, and benchmark numerical tests are first performed on synthetic cases to optimize the inversion algorithm parameters to estimate the ore location. The 3D SP tomography reveals a south-sloping ore body, which is 100 m long in the north–south direction and nearly 200 m long in the east–west direction. The vertical extent of the ore body is approximately 100 m. These results suggest that an AUV-based SP survey and tomography are potentially useful exploration methods to characterize the geometry of SMS deposits. This method may play an important role in resource assessment in concert with other geophysical methods, such as magnetic and induced-polarization surveys.

Oxidizing fluids associated with detachment hosted hydrothermal systems: Example from the Suye hydrothermal field on the ultraslow-spreading Southwest Indian Ridge

The redox state of hydrothermal fluids on mid-ocean ridges, which is indirectly affected by the depth of hydrothermal circulation and crustal permeability, plays an important role on the diversity of hydrothermal precipitates and associated ecosystems. 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 oxidizing. 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 δ34S values (average of 9.8‰) indicate a high proportion (up to 50%) of seawater derived reduced sulfur involved in the stockwork zone formation. The high homogenization temperatures (∼320 °C) and salinity (∼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.

Genesis of the distal axis East Longjing-2 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge: Evidences from fluid inclusions

Tectonic-related hydrothermal fields constitute about half of the hydrothermal activities on slow-ultraslow spreading ridges, and may form sulfides large in size and rich in precious metals. Ore-forming fluids are critical for element migration progressions, thus are key parameter to reveal their genesis and sulfide mineralization processes. However, previous studies of this kind of hydrothermal activities primarily focused on vent fluids which have transient effects or based on surface samples, while the ore-forming fluid evolution and related sub-seafloor mineralization process are still poorly constrained. In this study, we analyzed the characteristics of fluid inclusions in the stockwork mineralization of the East Longjing-2 hydrothermal field (ELHF-2) on the Southwest Indian Ridge. The results showed that the fluids evolved gradually from medium-high temperatures (260–315 °C, mean of 284 °C) and low salinities (0.70–3.70 wt.% NaCl eq.) in the disseminated pyrite mineralization stage (Stage I) to medium temperatures (239–261 °C, mean of 249 °C) and low salinities (1.05–3.85 wt.% NaCl eq.) in the chalcopyrite–pyrite–quartz veinlet mineralization stage (Stage II). Laser Raman specular analysis indicated the fluid inclusions were mainly composed of H2O. The low salinity (lowest 0.7 wt.% NaCl eq.) and various vapor volume (0.05–0.5) observed in these fluid inclusions suggested the ore-forming fluids were probably mixtures of the low-salinity vapor phase formed by phase separation with large amounts of seawater. The precipitation of the ore-forming elements was likely induced by temperature decreasing. We propose that hydrothermal circulation in ELHF-2 was a result of coupled high-permeability detachment fault and shallow gabbro intrusion. Our results suggest that high temperature hydrothermal activities could be developed on distal axis area on ultraslow spreading ridges.

Metagenomic analysis reveals wide distribution of phototrophic bacteria in hydrothermal vents on the ultraslow-spreading Southwest Indian Ridge.

The online version contains supplementary material available at 10.1007/s42995-021-00121-y.

780 Thousand Years of Upper‐Crustal Construction at a Melt‐Rich Segment of the Ultraslow Spreading Southwest Indian Ridge 50°28′E

AbstractMelt supply at the ultraslow‐spreading Southwest Indian Ridge (SWIR) has been shown to vary from nearly amagmatic, leading to ultramafic seafloor, to magmatically robust, producing fully volcanic seafloor. The center of the SWIR 50°28′E segment represents a magmatically robust endmember. High‐resolution bathymetry and backscatter, near‐bottom magnetic data, and visual observations were acquired to infer spatiotemporal variations in upper‐crustal construction over the past 780 kyr. Tectonic strain inferred from observed faults represents <8% of the total plate divergence during this period. Mapped seafloor and lava morphologies show a prevalence of smooth seafloor and smooth hummocky seafloor (64% mapped area), corresponding to a dominance of high‐effusion‐rate, sheet‐lobate lavas, over hummocky seafloor (29%) that mostly comprises low‐effusion‐rate pillow lavas. This prevalence is most pronounced within a ∼5 km‐long domain at the segment center, while hummocky seafloor prevails to its east and west, indicating a substantial along‐axis decrease in average eruption rate. Across‐axis changes of seafloor morphologies and faulting pattern indicate two successive cycles of upper‐crustal construction. These cycles last ∼300 kyr, interpreted as waxing and waning magmatic phases. During waxing phases, shallow axial melt lenses (AMLs) form beneath the segment center, feeding narrow diking systems that build high‐eruption‐rate smooth domal volcanos. During waning phases, lower frequencies of melt replenishment may cause the AML to become deeper and ultimately disappear. This configuration triggers diking and hummocky‐dominated eruptions over a wider axial domain, with more faults. We propose that modes of upper‐crustal construction at slow and ultraslow mid‐ocean ridges are influenced by melt supply more than spreading rate.

Trace-element compositions of sulfides from inactive Tianzuo hydrothermal field, Southwest Indian Ridge: Implications for ultramafic rocks hosting mineralization

The recently explored inactive Tianzuo hydrothermal field, in the amagmatic segment of the ultraslow-spreading Southwest Indian Ridge (SWIR), is closely associated with detachment faults. In this site, sulfide minerals are hosted by serpentine-bearing ultramafic rocks and include high-temperature (isocubanite, sphalerite, and minor pyrrhotite) and low-temperature (pyrite I, marcasite, pyrite II, and covellite) phases. In this study, trace-element concentrations of isocubanite and pyrite II were used to elucidate mineralization processes in ultramafic rocks hosting sulfides. Results show that isocubanite is enriched in metals such as Cu, Co, Sn, Te, Zn, Se, Pb, Bi, Cd, Ag, In, and Mn, and pyrite II is enriched in Mo and Tl. The marked enrichment in Te, Cu, Co, and In in isocubanite (compared with Se, Zn, Ni, and Sn, respectively) is most likely due to the contribution of magmatic fluids from gabbroic intrusions beneath the hydrothermal field. The intrusion of gabbroic magmas would have enhanced serpentinization reactions and provided a relatively oxidizing environment through the dissolution of anhydrite precipitated previously in the reaction zone, within high temperature and low pH conditions. This might have facilitated the extraction of metals by initial hydrothermal fluids, leading to the general enrichment of most metals in isocubanite. Metals in pyrite II have compositions similar to those of isocubanite, except for strong depletion in magmatically derived Te, Cu, Co, and In. This means that serpentinization processes had a dominating role in pyrite II precipitation as well. The enrichment of pyrite II in Mo and Tl is also indicative of seawater contribution in its composition. The study concludes that serpentinization reactions contribute effectively both to high- and low-temperature sulfide mineralization at Tianzuo hydrothermal field, with gabbroic intrusions further promoting high-temperature sulfide mineralization, providing additional metals, fluids and heat. In contrast, low-temperature sulfide mineralization occurred during the cooling of gabbroic intrusions, with decreasing rates of serpentinization reactions and a significant influence of seawater.

Seafloor Hydrothermal Activity around a Large Non-Transform Discontinuity along Ultraslow-Spreading Southwest Indian Ridge (48.1–48.7° E)

Non-transform discontinuity (NTD) is one category of tectonic units along slow- and ultraslow-spreading ridges. Some NTD-related hydrothermal fields that may reflect different driving mechanisms have been documented along slow-spreading ridges, but the discrete survey strategy makes it hard to evaluate the incidence of hydrothermal activity. On ultraslow-spreading ridges, fewer NTD-related hydrothermal activities were reported. Factors contributing to the occurrence of hydrothermal activities at NTDs and whether they could be potential targets for hydrothermal exploration are poorly known. Combining turbidity and oxidation reduction potential (ORP) sensors with a near-bottom camera, Chinese Dayang cruises from 2014 to 2018 have conducted systematic towed surveys for hydrothermal activity around a large NTD along the ultraslow-spreading Southwest Indian Ridge (SWIR, 48.1–48.7° E). Five new potential hydrothermal anomaly sites (2 inferred and 3 suspected) of high or low temperature and the previously inferred Sudi hydrothermal field occurred in diverse morphotectonic settings along a 78 km long ridge axis. The calculated vent frequency (Fs, sites/100 km) was ~7.7 over the entire study area, higher than the modified value (Fs ≈ 6.5) between 48 and 52° E of SWIR. Even only for the 54 km long large NTD, three hydrothermal anomaly sites yielded an Fs of ~5.6, which is higher than that of most ridge sections and is comparable to some fast-spreading ridges with high-resolution surveys. This indicates that NTDs along ultraslow-spreading ridges could be promising areas in fertilizing hydrothermal activities. Moreover, the deeply penetrating faults on the rift valley walls and strain-focused areas may contribute to the formation of NTD-related hydrothermal circulations. We suggest that NTDs along ultraslow-spreading ridges may be potential targets for further exploration of hydrothermal activities and seafloor sulfide deposits.

Formation of Fe-Mn coatings on foraminifera from the ultraslow-spreading southwest Indian Ridge: Implications for hydrothermal and diagenetic overprints in sediments

Detailed fine-scale morphological, mineralogical, and geochemical analyses were conducted on the Si-(Fe) hydroxide formations, planktonic foraminifera (PF) and the Fe-Mn coatings within a typical sediment sample collected from near the newly-discovered active hydrothermal vent field at the ultraslow-spreading Southwest Indian Ridge (SWIR). The analyses were conducted with a combination of different techniques, including Laser Scanning Confocal Microscope (LSCM), Laser Raman, high-resolution XRD, SEM-EDS, C-O isotope measurement, EMPA, and in-situ LA-ICP-MS. The mixed-type hydrothermal-hydrogenetic Si-(Fe) hydroxide formations (commonly occur as cavity-fillings inside the PF chambers) contain a considerable amount of organic matter, and are mainly composed of nontronite, goethite, tridymite, and opal-A with the bulk compositions of 8.13–40.91 wt% SiO2, 6.85–54.50 wt% FeO, and 0.43–23.99 wt% Al2O3. Such high Al content can be mainly attributed to the precipitation and accumulation of hydrothermal-derived Al-rich materials, and the Si-Fe-Al assemblage reveals that it is mainly sourced from plagioclase dissolution/alteration during subseafloor fluid-rock interaction. Meanwhile, the relative enrichments of high field strength elements (HFSEs), heavy rare earth elements (HREE), and Cr-Ni-Sc also imply that these Si-(Fe) hydroxide formations have another source from pyroxene alteration. In addition, their distinct microtextures (filamentous, orbicular, or beaded-like) are indicative of bacterial-related precipitation. The hydrogenetic Fe-Mn coatings have a layered texture and exhibit no apparent fluorescence, and are mainly composed of vernadite and goethite with the bulk compositions of 23.98–30.06 wt% FeO and 8.55– 21.52 wt% MnO. Their average Fe/Mn ratios, growth rates, and Zn-As contents are slightly higher than those of the open-ocean SWIR Fe-Mn deposits, which suggests an additional Fe input from submarine hydrothermal activity. Moreover, the altered PF shell is mainly composed of calcite and have three color types (white/yellowish/yellow). The contents of Si, Al, Fe, Cr, Ni, Cu, Zn, Ba, HFSEs, and REE increase as the yellowish color deepens, which indicates the diagenetic contamination from basalt alteration. Consequently, a new diagenetic Fe-carbonate phase (ankerite) was formed in the inner layer of the altered PF shells, and close correlations were also found between their C-O isotope ratios. Both the δ13C and δ18O values tend to drop from white- to yellow-type PF shells, accompanied with the decreasing contents of Ca, Na, Mg, and Sr. All these parameters and signatures would help to unravel the hydrothermal influence and diagenetic overprinting, which could cause confusion when using foraminifera shell geochemistry (e.g., Sr/Ca and Mg/Ca ratios) as a marine tracer in paleoclimate reconstruction.

Role of compaction in melt extraction and accumulation at a slow spreading center: Microstructures of olivine gabbros from the Atlantis Bank (IODP Hole U1473A, SWIR)

The exposure of gabbroic sequences at Oceanic Core Complexes (OCC) along ultraslow- to slow-spreading ridges permits the study of the processes forming the lower oceanic crust. On top of the Atlantis Bank OCC along the ultraslow-spreading Southwest Indian Ridge, IODP Expedition 360 drilled Hole U1473A, mainly composed of primitive olivine gabbros interspersed with more evolved Ti-Fe oxide-bearing gabbros and minor felsic veins. These rocks record a complex history of protracted magmatism during continuous uplift and deformation of the gabbroic sequence. Extensive crystal-plastic deformation is dominantly recorded in the shallower sections of the drillhole, whereas the deeper sections better preserve primary magmatic features. We focus on microstructures, including intra-crystalline deformation of rock-forming minerals, and plagioclase crystallographic preferred orientations of olivine gabbros lacking evidence for exhumation-related crystal plastic deformation, to gain insights on the relationship between compaction, melt migration and melt accumulation during the early magmatic history of this section of lower oceanic crust. Olivine gabbros are characterized by ubiquitous grain-size variations, from coarse- to fine-grained intervals. Minerals in coarse-grained intervals show intra-crystalline deformation, while fine-grained crystals lack internal strain. Bent coarse-grained plagioclase associated with weak magmatic foliation and lack of lineation suggest that the coarse-grained intervals were deformed under weak compaction. On the other hand, crystallographic preferred orientations of undeformed fine-grained plagioclase show weak lineations, likely indicative of non-coaxial strain. We thereby infer that the coarse-grained intervals underwent ongoing weak compaction from the stage of olivine + plagioclase ± clinopyroxene crystal mush to the melt-poor stage, and that this process likely aided melt extraction and accumulation in discrete melt-rich zones where crystals orientated in the direction of magmatic flow. Crystallization of melts in the melt-rich zones ultimately formed the fine-grained intervals at different depths in Hole U1473A. This indicates that processes of compaction can lead to local chemical and grain-size heterogeneities in a lower crustal section, while had a minor role in the melt movement at larger scales (e.g., the whole crystal mush) within the oceanic crust.

Distal axis sulfide mineralization on the ultraslow-spreading Southwest Indian Ridge: an LA-ICP-MS study of pyrite from the East Longjing-2 hydrothermal field

Distal axis sulfide mineralization on the ultraslow-spreading Southwest Indian Ridge: an LA-ICP-MS study of pyrite from the East Longjing-2 hydrothermal field

Resource estimation of the sulfide-rich deposits of the Yuhuang-1 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge

Abstract Seafloor massive sulfide (SMS) deposits are important deep-sea mineral resources expected to occur predominantly on slow- and ultraslow-spreading mid-ocean ridges. Resource estimates are already available for some of the largest SMS deposits on slow-spreading ridges but not on ultraslow-spreading ridges. Based on geological mapping and sampling, this study investigates the distribution and content of sulfide-rich deposits in the Yuhuang-1 hydrothermal field (YHF), located on the ultraslow-spreading Southwest Indian Ridge. The sulfide-rich deposits in the YHF are composed of two areas ~500 m apart: the southwest sulfide area (SWS) and the northeast sulfide area (NES). We calculated the volume of sulfide-rich moundsin the YHF and arrived at a total accumulation of ~10.6 × 106 tons, including at least ~7.5 × 105 tons of copper and zinc and ~18 tons of gold. Furthermore, considering the coverage of layered hydrothermal sediment mixed with sulfide-rich breccias, which may have underlying massive sulfide deposits, the maximum total mass was estimated at ~45.1 × 106 tons. This suggests that the YHF is one of the largest SMS deposits worldwide and confirm that ultraslow-spreading ridges have the greatest potential to form large-scale SMS deposits.

Formation and origin of Fe–Si oxyhydroxide deposits at the ultra-slow spreading Southwest Indian Ridge

Low-temperature hydrothermal systems are generally dominated by Fe–Si oxyhydroxide deposits. However, the formation process and mechanism of modern hydrothermal Fe–Si oxyhydroxides at ultra-slow spreading centers remain poorly understood. This study focused on six Fe–Si oxyhydroxide deposits collected from different sites at a typical ultra-slow spreading center, the Southwest Indian Ridge (SWIR). The mineralogical and geochemical evidence showed significant characteristics of a low-temperature hydrothermal origin. Sr and Nd isotope compositions of Fe–Si oxyhydroxide deposits at the SWIR probably reflected a combined signature of the hydrothermal fluids and seawater. Pb in the Fe–Si oxyhydroxides exhibited a close association with the substrate rocks and seawater. The Mössbauer spectra and iron speciation data further provided insights into iron-bearing phases in all deposits. Two different types of biomineralized forms were also discovered in these deposits by scanning electron microscopy (SEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) and nanoscale secondary ion mass spectrometry (nanoSIMS) revealed that distinct biogenic structures were mainly composed of Fe, Si, and O, together with some trace elements. Based on these findings, we propose that microbial activity plays a significant role in the formation of Fe–Si oxyhydroxides at the ultra-slow spreading SWIR.

Geochemistry of Basalts from Southwest Indian Ridge 64° E: Implications for the Mantle Heterogeneity East of the Melville Transform

As one of the regional, magmatic, robust, axial ridge segments along the ultraslow-spreading Southwest Indian Ridge (SWIR), the magmatic process and mantle composition of the axial high relief at 64° E is still unclear. Here, we present major and trace elements and Sr-Nd-Pb isotope data of mid-ocean ridge basalts (MORBs) from 64° E. The basalts show higher contents of Al2O3, SiO2, and Na2O and lower contents of TiO2, CaO, and FeO for a given MgO content, and depletion in heavy rare-earth elements (HREE), enrichment in large-ion lithophile elements, and lower 87Sr/86Sr, 143Nd/144Nd and higher radiogenic Pb isotopes than the depleted MORB mantle (DMM). The high Zr/Nb (24–43) and low Ba/Nb (3.8–7.0) ratios are consistent with typical, normal MORB (N-MORB). Extensive plagioclase fractional crystallization during magma evolution was indicated, while fractionation of olivine and clinopyroxene is not significant, which is consistent with petrographic observations. Incompatible trace elements and isotopic characteristics show that the basaltic melt was formed by the lower partial melting degree of spinel lherzolite than that of segment #27 (i.e., Duanqiao Seamount, 50.5° E), Joseph Mayes Mountain (11.5° E), etc. The samples with a DMM end-member are unevenly mixed with the lower continental crust (LCC)- and the enriched mantle end-member (EM2)-like components, genetically related to the Gondwana breakup and contaminated by upper and lower continental crust (or continental mantle) components.

Mineralogy and Geochemistry of Deep-Sea Sediments from the Ultraslow-Spreading Southwest Indian Ridge: Implications for Hydrothermal Input and Igneous Host Rock

Detailed mineralogical and geochemical characteristics of typical surface sediments and hydrothermal deposits collected from the ultraslow-spreading Southwest Indian Ridge (SWIR) were studied by high-resolution XRD, SEM-EDS, XRF, and ICP-MS. The SWIR marine samples can be generally classified into two main categories: surface sediment (biogenic, volcanic) and hydrothermal-derived deposit; moreover, the surface sediment can be further classified into metalliferous and non-metalliferous based on the metalliferous sediment index (MSI). The chemical composition of biogenic sediment (mainly biogenic calcite) was characterized by elevated contents of Ca, Ba, Rb, Sr, Th, and light rare earth elements (LREE), while volcanic sediment (mainly volcanogenic debris) was relatively enriched in Mn, Mg, Al, Si, Ni, Cr, and high field strength elements (HFSEs). By contrast, the hydrothermal-derived deposit (mainly pyrite-marcasite, chalcopyrite-isocubanite, and low-temperature cherts) contained significantly higher contents of Fe, Cu, Zn, Pb, Mn, Co, Mo, Ag, and U. In addition, the metalliferous surface sediment contained a higher content of Cu, Mn, Fe, Co, Mo, Ba, and As. Compared with their different host (source) rock, the basalt-hosted marine sediments contained higher contents of Ti–Al–Zr–Sc–Hf and/or Mo–Ba–Ag; In contrast, the peridotite-hosted marine sediments were typically characterized by elevated concentrations of Mg–Cu–Ni–Cr and/or Co–Sn–Au. The differences in element enrichment and mineral composition between these sediment types were closely related to their sedimentary environments (e.g., near/far away from the vent sites) and inherited from their host (source) rock. Together with combinations of certain characteristic elements (such as Al–Fe–Mn and Si–Al–Mg), relict hydrothermal products, and diagnostic mineral tracers (e.g., nontronite, SiO2(bio), olivine, serpentine, talc, sepiolite, pyroxene, zeolite, etc.), it would be more effective to differentiate the host rock of deep-sea sediments and to detect a possible hydrothermal input.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

The crust and mantle both in ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are critical to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogues for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it with the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dykes and sills cutting both mantle and lower crust. The gabbro units are thin ( c . 350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable with the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analogue of ultraslow-spreading ocean lithosphere. Supplementary material: Single-spot analysis and average compositions for mineral major element compositions (wt.%) of the Jiding gabbro section, Xigaze ophiolite are available at https://doi.org/10.17632/dtzpmwkscp.1

GIS-based Mineral Prospectivity Mapping of Seafloor Massive Sulfide on Ultraslow-spreading Ridges: A Case Study of Southwest Indian Ridge 48.7°–50.5° E

With the depletion of mineral resources on land, seafloor massive sulfide deposits have the potential to become as important for exploration, development and mining as those on land. However, it is difficult to investigate the ocean environment where seafloor massive sulfide deposits are located. Thus, improving prospecting efficiency by reducing the exploration search space through mineral prospectivity mapping (MPM) is desirable. MPM has been used in the exploration for seafloor deposits on regional scales, e.g., the Mid-Atlantic Ridge and Arctic Ridge. However, studies of MPM on ultraslow-spreading ridges on segment scales to aid exploration for seafloor massive sulfide have not been carried out to date. Here, data of water depth, geology and hydrothermal plume anomalies were analyzed and the weights-of-evidence method was used to study the metallogenic regularity and to predict the potential area for seafloor massive sulfide exploration in 48.7°–50.5° E segments on the ultraslow spreading Southwest Indian Ridge. Based on spatial analysis, 11 predictive maps were selected to establish a mineral potential model. Weight values indicate that the location of seafloor massive sulfide deposits is correlated mainly with mode-E faults and oceanic crust thickness in the study area, which correspond with documented ore-controlling factors on other studied ultraslow-spreading ridges. In addition, the detachment fault and ridge axis, which reflect the deep hydrothermal circulation channel and magmatic activities, also play an important role. Based on the posterior probability values, 3 level A, 2 level B and 2 level C areas were identified as targets for further study. The MPM results were helpful for narrowing the search space and have implications for investigating and evaluating seafloor massive sulfide resources in the study area and on other ultraslow-spreading ridges.

Sulfur isotopic compositions of sulfides along the Southwest Indian Ridge: implications for mineralization in ultramafic rocks

The recently explored Tianzuo hydrothermal field in serpentinized ultramafic rocks of the amagmatic segment of the ultraslow-spreading Southwest Indian Ridge displays high-temperature sulfide mineralization (isocubanite, sphalerite, and minor pyrrhotite) and low-temperature (pyrite and covellite) phases. Pyrite can be subdivided into pyrite-I and -II, with the former generally having a pseudomorphic texture after pyrrhotite and the latter typically growing around isocubanite, sphalerite, and pyrite-I or occurring as individual grains in quartz veinlets. The sulfide minerals have the greatest range of δ34S values (− 23.8 to 14.1‰), found so far among modern sediment-starved ridges, with distinct δ34S values for low- and high-temperature mineral phases. The high δ34S values of isocubanite (9.6 to 12.2‰) and sphalerite (9.1 to 14.1‰) suggest that sulfate, which precipitated from seawater during an early low-temperature phase of hydrothermal circulation, was the main sulfur source for these sulfides. Pyrite-II has the lowest and most variable δ34S values (− 23.8 to − 3.6‰), suggesting microbial sulfate reduction. Pyrite-I has variable and generally positive δ34S values (− 0.1 to 12.0‰), with sulfur being inherited from pyrrhotite from the original thermochemical reduction of sulfate, mixed with volcanogenic sulfur. Intermittent magmatism represented by gabbroic intrusions, and high permeability caused by well-developed fractures associated with detachment faults, contributed to the formation of sulfides in the Tianzuo hydrothermal field. These factors possibly control sulfide mineralization in amagmatic segments of ultraslow-spreading ridges.

Enhanced hydrothermal activity on an ultraslow-spreading supersegment with a seismically detected melting anomaly

Seafloor hydrothermal venting fields occur on all ocean spreading ridges (OSRs) regardless of spreading rates. However, the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow-spreading OSRs are poorly known. Chinese Dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system, with an array of turbidity sensors and a near-bottom camera, along the ultraslow-spreading Southwest Indian Ridge. Here we report the discovery of multiple hydrothermal plumes overlying segments 28, 29, and 30 between the Indomed and Gallieni fracture zones. From these data, and earlier explorations in segments 25–27, we identify nine active venting sites. The spatial density (Fs, sites/100 km) of active sites along the 394 km of ridge axis in our study area is thus 2.8, nearly 3× higher than predicted by the global trend of Fs for ultraslow OSRs in the InterRidge database. Previous studies concluded that an enhanced magma supply to the central Indomed–Gallieni supersegment 11–8 Ma is now limited to segment 27. Our results indicate that although hydrothermal activity may be most concentrated in segment 27, the discoveries of active venting in segments 25–30 implies the presence of additional magma bodies across a broad extent of the Indomed–Gallieni supersegment.

Fracture-mediated deep seawater flow and mantle hydration on oceanic transform faults

Fluid-rock interaction on oceanic transform faults (OTFs) is important for both the deformation behavior of the lithosphere and volatile cycling in the Earth. Rocks deformed and exhumed at OTFs preserve information about the depth extent of fluid percolation and the nature of fluid-rock interactions within these fault zones. In this study, we focus on five dredges from the Shaka and Prince Edward OTFs on the ultraslow spreading Southwest Indian Ridge that recovered significant volumes of deformed mantle rocks. Samples are predominantly mylonites that have been deformed to high strains in the fault zone, but also contain several generations of fractures. Based on the mineral assemblages in fractures and shear bands combined with thermobarometry analysis, we identified three distinct temperature ranges of fluid-mantle interactions associated with deformation. At low temperature (LT), this leads to crystallization of serpentine (± talc ± amphibole ± chlorite) at <500–550°C. At medium temperature (MT), chlorite and amphibole crystallized at ∼500–750°C. At high temperature (HT), amphibole (± second generation peridotitic minerals) crystallized. The composition of minerals in HT fractures and shear bands indicates that fracturing and fluid flow occur up to temperatures of at least 850–875°C. Combining these results with modeled geotherms for both faults suggests that seawater percolation extended to depths of 20–25 km and that serpentinization extended to ∼11–13 km. The evolution of fault zone structure induced by deep fluid-rock interaction and progressive formation of LT, MT and HT mylonites on OTFs results in weakening and strain localization within the oceanic lithosphere, and suggests that the global transform system may represent a large reservoir of volatiles in the Earth's lithosphere.

Plate-driven micro-hotspots and the evolution of the Dragon Flag melting anomaly, Southwest Indian Ridge

Micro hotspots represent excess volcanism at the ultraslow spreading Southwest Indian Ridge (SWIR) unrelated to mantle hotspots, but to focused melt flow in the mantle, wide volcano spacing, and/or increased mantle fertility. Individual micro hotspot can reflect variations in regional mantle fertility, simultaneously affecting 100's of km of ridge, or extreme melt focusing to isolated segments in regions with unusually thick lithosphere. The Dragon Flag melting anomaly, erupting isotopically moderately depleted low-K tholeiite, is the best example of the former: an enormous ridge-centered volcano at 50.5°E. It is far from Crozet, and lies at the apex of a V-shaped trace consisting of ∼1 km anomalously elevated seafloor that extends ∼400 km to the northwest and southeast, indicating a sudden onset of excess volcanism at ∼11-8 Ma. This trend is opposite to that predicted by the hotspot framework, with inconsistent geochemistry. Narrowgate at 14.7°E is one of many micro hotspots that represent isolated large volcanic centers bounded by long amagmatic ridge sections. It also lies at the apex of an eastward V-shaped bathymetric trend, but erupts large volumes of alkali basalt.Both micro hotspot varieties are an order of magnitude smaller than Wilson's classic hotspots, and are not fixed in the hotspot referenced frame. They can grow and remain stationary for extended periods, or migrate freely with respect to each other; driven by the evolving plate tectonic stress field. Dragon Flag, represents a plate reorganization during a prolong period of enhanced melt supply. Narrowgate, on the other hand represents excess volcanism due to extreme deep melt focusing of low degree melt from an unusually wide region in the mantle to an isolated volcanic segment where thick lithosphere caps melting at great depth. To constrain the origins of the micro hotspots six examples are evaluated in terms of their major and isotopic composition, prior plate history, depth and extent of melting and tectonic context. This includes new major, trace element, and heavy isotope data for the Dragon Flag Supersegment, and unpublished data for the Joseph Mayes and Narrowgate segments.