Description: The ultramafic-hosted Kairei vent field is located at 25°19′ S, 70°02′ E, towards the Northern end of segment 1 of the Central Indian Ridge (CIR-S1) at a water depth of ~2450 m. This study aims to investigate the distribution of trace elements among sulfide minerals of differing textures and to examine the possible factors controlling the trace element distribution in those minerals using LA-ICP-MS spot and line scan analyses. Our results show that there are distinct systematic differences in trace element distributions throughout the different minerals, as follows: (1) pyrite is divided into three types at Kairei, including early-stage euhedral pyrite (py-I), sub-euhedral pyrite (py-II), and colloform pyrite (py-III). Pyrite is generally enriched with Mo, Au, As, Tl, Mn, and U. Pyrite-I has high contents of Se, Te, Bi, and Ni when compared to the other types; py-II is enriched in Au relative to py-I and py-III, but poor in Ni; py-III is enriched in Mo, Pb, and U but is poor in Se, Te, Bi, and Au relative to py-I and py-II. Variations in the concentrations of Se, Te, and Bi in pyrite are most likely governed by the strong temperature gradient. There is generally a lower concentration of nickel than Co in pyrite, indicating that our samples precipitated at high temperatures, whereas the extreme Co enrichment is likely from a magmatic heat source combined with an influence of serpentinization reactions. (2) Chalcopyrite is characterized by high concentrations of Co, Se, and Te. The abundance of Se and Te in chalcopyrite over the other minerals is interpreted to have been caused by the high solubilities of Se and Te in the chalcopyrite lattice at high temperatures. The concentrations of Sb, As, and Au are relatively low in chalcopyrite from the Kairei vent field. (3) Sphalerite from Zn-rich chimneys is characterized by high concentrations of Sn, Co, Ga, Ge, Ag, Pb, Sb, As, and Cd, but is depleted in Se, Te, Bi, Mo, Au, Ni, Tl, Mn, Ba, V, and U in comparison with the other minerals. The high concentrations of Cd and Co are likely caused by the substitution of Cd2+ and Co2+ for Zn2+ in sphalerite. A high concentration of Pb accompanied by a high Ag concentration in sphalerite indicates that Ag occurs as Pb–Ag sulfosalts. Gold is generally low in sphalerite and strongly correlates with Pb, suggesting its presence in microinclusions of galena. The strong correlation of As with Ge in sphalerite from Kairei suggests that they might precipitate at medium temperatures and under moderately reduced conditions. (4) Bornite–digenite has very low concentrations of most trace elements, except for Co, Se, and Bi. Serpentinization in ultramafic-hosted hydrothermal systems might play an important role in Au enrichment in pyrite with low As contents. Compared to felsic-hosted seafloor massive sulfide deposits, sulfide minerals from ultramafic-hosted deposits show higher concentrations of Se and Te, but lower As, Sb, and Au concentrations, the latter often attributed to the contribution of magmatic volatiles. As with typical ultramafic-hosted seafloor massive sulfide deposits, Se enrichment in chalcopyrite from Kairei indicates that the primary factor that controls the Se enrichment is temperature-controlled mobility in vent fluids.

Description: Three volcanic ash layers were identified in a deep-sea Core IR-GC1 from the north-eastern Indian Ocean, adjacent to western Indonesian arc. They were dominated by glass shards with minor mineral crystals, such as plagioclase, biotite, and hornblende. According to the morphology and major element compositions of the representative glass shards, combined with the 18O-based age, it is suggested that ash Layer A is correlated to the youngest Toba tuff (YTT), Layer B is supposed to be associated with a new eruption of Toba caldera in an age of 98 to 100 ka. Ash Layer C is different the geochemistry characteristics than those of Layer A and Layer B, suggesting that Layer C was not originated from Toba but registered another volcanic eruption event.

Description: Highlights • First present seafloor hydrothermal mineralization processes at both Wocan-1 and Wocan-2 on the slow-spreading Carlsberg Ridge. • The Cu-rich chimneys were formed at slightly lower temperatures than Cu-rich and Fe-rich massive sulfides. • The main Ag-carriers were both late-stage Cu sulfides and Fe sulfides, which deposited under low temperatures and oxidized conditions. • Fluid mixing of hydrothermal fluids with seawater might result in significant redistributions of trace metal elements in sulfides. Abstract The basalt-hosted Wocan hydrothermal field (WHF), located on the NW slope of an axial volcanic ridge in a depth of ∼3000 m at 6°22′N on the slow-spreading Carlsberg Ridge, northwest Indian Ocean, was discovered in 2013 during Chinese DY28th cruise. Preliminary investigations show that the field consists of two hydrothermal sites: Wocan-1, which shows indications for recent high-temperature hydrothermal activity, is located near the peak of the axial volcanic ridge in a water depth of 2970-2990 m, and the inactive Wocan-2 site, located at a water depth of 3100 m, ∼1.7 km to the northwest of Wocan-1. The recovered hydrothermal precipitates can be classified into four groups: (i) Cu-rich chimneys; (ii) Cu-rich massive sulfides; (iii) Fe-rich massive sulfides; and (iv) silicified massive sulfides. We conducted mineral texture and assemblage observation and Laser-ablation ICP-MS analyses of the hydrothermal precipitates to study the mineralization processes. Our results show that there are distinct systematic trace element distributions throughout the different minerals in the four sample groups. In general, chalcopyrite from the group (i) is enriched in Pb, As, Mo, Ga, Ge, V, and Sb, metals that are commonly referred to as medium- to low-temperature elements. In contrast these elements are present in low contents in the chalcopyrite grains from other sample groups. Selenium, a typical high-temperature metal, is enriched in chalcopyrite from groups (ii) and (iv), whereas Ag and Sn are enriched only in some silicified massive sulfides. As with chalcopyrite, pyrite also shows distinct trace element associations in grains with different habitus. The low-temperature association of elements (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typically present in colloform/framboidal pyrite, whereas the high-temperature association (Se, Co, and Bi) is enriched in euhedral pyrite. Sphalerite in the groups (i) and (iii) at Wocan-1 is characterized by high concentrations of Ga, Ge, Pb, Cd, As, and Sb, indicating that sphalerite in these sample groups likely precipitated at intermediate temperatures. Early bornite, which mainly occurs in the central part of the Cu-rich chimney, is typically enriched in Sn and In compared to the other minerals. In contrast, late bornite that likely formed during increasing interaction of hydrothermal fluids with cold, oxygenated seawater has low Sn and In, but significantly higher concentrations of Ag, Au, Mo and U. Digenite, also forming in the exterior parts of the samples during the late stages of hydrothermal fluid venting, is poor in most trace elements, except Ag and U. The notable Ag enrichment in the late-stage mineral assemblages at both Wocan-1 and Wocan-2 may therefore be related to lower temperatures and elevated pH. Our results indicate that Wocan-1 has experienced a cycle of heating with Cu-rich chimney growth and subsequent cooling, followed by late seafloor weathering, while Wocan-2 has seen intermediate- to high-temperature mineralization followed by intense silicification of sulfides. Seafloor weathering processes or mixing of hydrothermal fluids with seawater during the waning stages of hydrothermal fluid flow result in significant redistributions of trace elements in sulfide minerals.

Description: The Kairei hydrothermal field was the first confirmed active submarine hydrothermal system on the Central Indian Ridge. It has been suggested to be related to mafic as well as ultramafic host rocks based on vent fluid composition and the presence of ultramafic rocks in its vicinity. In this study, detailed geochemical and mineralogical analyses have been carried out on the hydrothermal precipitates from the Kairei vent field in order to investigate the possible presence of indications for an ultramafic substrate at this vent site. The studied samples included fragments of sulfide chimneys, massive sulfides and talc-bearing and silicified breccias. Three mineralization stages were identified: (1) a high-temperature stage consisting largely of chalcopyrite, isocubanite, and pyrite; (2) a medium to low temperature stage characterized by the mineral assemblages of sphalerite and pyrite; and (3) a weathering stage characterized by secondary Cu-sulfides (bornite, digenite, covellite and idaite), Fe-oxihydroxides, Opal-A, and Cu-chloride (paratacamite and atacamite). The sulfide geochemistry is characterized by high concentrations of Cu and Zn (Cu + Zn up to 29.3 wt.%, n = 17) and Au (mean 5.28 ppm, n = 17), which is comparable to results from seafloor massive sulfides collected from ultramafic-hosted sites in the Atlantic Ocean, but differs from those of typical mafic-hosted deposits. The high concentrations of Cu and Au at the Kairei hydrothermal field could be an indication for the involvement of ultramafic rocks in the subseafloor. Ultramafic-hosted, Au-rich sulfide deposits may not be restricted to the Atlantic Ocean and may be common along all slow- and intermediate-spreading ridges.

Description: Highlights • The vent fluids discharged from the Lutao hydrothermal field experienced low-degree subcritical phase separation. • The temperature and chemical compositions of the vent fluids were modulated by tides. • The time delay between tides and the response of hydrothermal system was about 3 h. • The typhoon “Fung-wong” cooled the reaction zone and decreased the degree of phase separation. • The hydrothermal system began to recover after the typhoon passed by. Abstract The Lutao hydrothermal field is an intertidal arc-volcanic system located offshore southeast Taiwan, hosting a Zhudanqu (ZDQ) vent and a Huwaichi (HWC) spring with strongly contrasting fluid chemistry. Low Mg, moderately enriched Cl, and H+ with respect to seawater indicate that the ZDQ endmember was derived from the brine phase that was formed during low-degree subcritical phase separation. In contrast, the endmember for the HWC vent fluids is related to the vapor phase. Temperature and pressure of the phase separation were estimated as ~150 °C and ~7 bar, respectively. The water/rock ratio was roughly calculated as about 2. The Lutao hydrothermal system was slightly affected by semi-diurnal tides, by some combination of tidal loading and tidal currents. The time delay between tides and the response of the hydrothermal system was about 3 h. While freshwater was almost absent in the HWC vent fluids at normal conditions, the typhoon “Fung-wong” on Sep 21st, 2014, led to intrusions of freshwater into the vent fluids with a percentage of ~16%. Both the ZDQ and the HWC endmember compositions showed some changes after the typhoon event, suggesting a cooling of the reaction zone. After the typhoon passed by, the hydrothermal system began to recover, evidenced by increasing percentages of the HWC endmember and decreasing freshwater contributions. The flux of the HWC endmember was estimated as 460–560 L h−1 based on these observations. This study, for the first time, reports a shallow-depth tidal-influenced hydrothermal system that was temporarily cooled by a tropical storm.

Description: The distribution of hydrothermal vents and the biogeography of associated faunal communities in the Indian Ocean are still not well studied. This is especially true for Carlsberg Ridge, the northernmost part of the Indian Ocean spreading system. Here we report geological, morphological, biological, and hydrochemical data for the newly discovered Daxi Vent Field (DVF) on the slow-spreading Carlsberg Ridge at 6°48′N. The DVF is a basalt-hosted hydrothermal field situated atop a rifted volcanic ridge, located in a non-transform offset between two second-order ridge segments. There are three hydrothermal sites, i.e. Central mound, NE mound, and South mound. Eight vigorously venting black smokers were observed in the central hydrothermal mound. The largest sulfide chimney “Baochu Pagoda” is ~24 m tall. Another inactive chimney, which is silica-rich is observed in the NE mound. The sulfide chimneys are dominated by sphalerite and pyrrhotite containing high Sn, Co and Ag. The silica-rich chimney contains high SiO2 and Ba contents. Seven species of megafauna were identified, including alvinellid worms, which were collected in the Indian Ocean for the first time. Rimicaris kairei and actinostolid anemones dominate the community in the central areas and on the periphery of the vent field, respectively. The occurrence of DVF is quite unique as it is located on a non-transform offset and it is mafic-hosted. So far only nine hydrothermal fields with the similar geological setting have ever been reported among nearly 700 hydrothermal sites in the World’s Ocean. Graphical abstract The mafic-hosted Daxi Vent Field with high-temperature hydrothermal mineralization was discovered at a non-transform offset along the slow-spreading Carlsberg Ridge.

Description: Highlights • The hydrothermal fluids were sampled from a neovolcanic ridge within a non-transform offset. • Serpentinization has been involved on the pathway of hydrothermal circulation • The fluids are strongly affected by phase separation with extremely high Cl content in brine phase • A hybrid model of hydrothermal circulation controlled by tectonic and magmatic activities simultaneously was proposed. The Daxi Vent Field (DVF) is located on a neovolcanic ridge within a non-transform offset at water depths of ∼3500 m, on the Carlsberg Ridge, northwest Indian Ocean. In 2017, we investigated this site using the submersible Jiaolong and collected two fluid samples from orifices of chimneys named “Buddha's Hands” and “A1”, about 37 m apart. Their in-situ measured temperatures are 273 °C and 272 °C, respectively. The Buddha's Hands fluid is highly Cl-enriched (928 mM), while the A1 fluid is Cl-depleted (303 mM). This indicates that they have undergone phase separation. The segregated phases must have remixed during the ascent because the vapor and brine phases sampled cannot be produced by the same phase separation history without other processes. Olivine-rich and/or ultramafic mantle rocks must have been involved during the hydrothermal circulation as evidenced by high dissolved H2 (7.07 mM) and methane (0.884 mM) concentrations, a depletion in B relative to seawater, high Ca and low K, and large positive Eu anomalies. The Fe content in Buddha's Hands fluid is extremely high (11,900 μM) as a result of phase separation, while the Cu concentrations in both fluids are relatively low due to entrainment of seawater which results in precipitation of Cu-rich sulfides in the subseafloor. The concentrations of Zn, Ag, Ga, Sn, Sb, and Cd in A1 vent fluid are significantly elevated due to generation of acidity and remobilization of these elements as Cu-rich sulfides are deposited. The subseafloor processes and associated geochemistry of hydrothermal fluids at the DVF are distinct from other mid-ocean ridge hydrothermal systems due to the specific geologic setting. Hence a hybrid model of hydrothermal circulation is proposed. This study broadens our understanding of the hydrothermal processes occurring in areas of NTO setting and provides more information on mass fluxes discharging from hydrothermal systems and the formation of sulfide deposits.

Description: The chemical and isotopic characteristics of calcium (Ca) in subduction zones are closely related to the budget of Ca and carbon cycles. Here we investigate the ultra-high Ca concentrations that characterize the hydrothermal fluids discharged from two types of vents, named the Zhudanqu brine vent (ZDQ) and the Huwaichi vapor spring (HWC), in the Lutao hydrothermal system at the north Luzon arc. The Ca concentrations of up to 159 mM and Ca/Cl ratio of up to 0.26 in the ZDQ vent fluids are possibly the highest ever reported for Ca enrichment in global seawater-circulated hydrothermal/geothermal systems. The differences in chemical compositions between the ZDQ and the HWC vent fluids are primary controlled by subcritical phase separation. The brine phase constitutes the ZDQ vent fluids, while the HWC vent fluids represent mixtures of the vapor phase and seawater. Both the vapor and the brine phases exhibit similar δ44/40Ca values (0.72 ± 0.05‰), suggesting no significant Ca isotope fractionation has occurred during phase separation. The hydrothermal endmember before phase separation (the “Lutao endmember”) presents depletions of 213 ± 15 mM of Na, 24.4 ± 0.4 mM of SO42−, and 10.2 mM of K, and enrichment of 130.2 ± 5.5 mM of Ca with respect to the percolated seawater. The total gained Ca is 154.6 ± 5.9 mM with a δ44/40Ca value of 0.67‰ – 0.77‰ (0.72 ± 0.05‰), considering anhydrite precipitation during hydrothermal circulation. The Holocene raised coral reef is unlikely to contribute substantial Ca into the Lutao system. Much of the gained Ca (111.6 ± 7.5 mM) is produced by high-degree albitization of the Lutao host rock, which is promoted by the low water/rock ratio (~ 2), slightly alkaline conditions, and relatively lower temperature of the Lutao system with respect to most mid-ocean ridge hydrothermal systems. Ca derived from this process inherits the Ca isotopes of plagioclase in the Lutao host rocks (δ44/40Ca = 0.82 ± 0.06‰). According to mass and isotopic balances, the recycled marine carbonate is proposed to contribute 43 ± 13.4 mM Ca with a δ44/40Ca value of 0.46−0.63+0.35‰ into the Lutao system. Such isotopically lighter Ca is derived from either pore fluids expulsed from underlying Philippine Sea sediments, or more probably, carbonate-bearing subduction fluids from the subducting South China Sea sediments and slab. The carbonate solubility in the subduction fluids could maintain at 600 mM near the reaction zone. The carbonate-rich fluids were subsequently migrated into the Lutao reaction zone and released an additional 43 ± 13.4 mM Ca via dolomitization. A small amount (~ 9%) addition of carbonate-rich fluids would not significantly change the budgets of Na, Mg, and Cl but could generate substantial Ca enrichment and Ca isotopic variations.