Geological setting

The VDVF is located on the western flanks of the ultraslow spreading Mid-Cayman Spreading Centre, the deepest spreading centre on Earth (Fig. 1a). It is situated at a depth of 2,280 mbsl, 13 km to the west of the rift axis6,7 on the upper slopes of Mt Dent, an oceanic core complex (OCC) formed by detachment faulting (Fig. 1b). At this location, the basement age is estimated to be between 1 and 2 Ma (based on distance from spreading centre and spreading rate) and comprises meta-gabbro, dolerite dykes and serpentinized peridotites that are partially covered by calcareous pelagic sediment7. The hydrothermally active area of the VDVF comprises three overlapping conical-shaped talc mounds, up to 75 m high and 150 m in diameter, aligned north-northwest–south-southeast (NNW-SSE) (Fig. 1c). The summit of the largest mound (Main Cone), which is the northern mound at the active site, hosts a 3-m tall, 1-m diameter chimney (The Spire, Fig. 1d). Hydrothermal fluid venting (215 °C) from The Spire has low concentrations of particles and a pH of 5.8 (determined at standard temperature and pressure (STP)). A 1-m diameter orifice (Main Hole) located at the base of The Spire vents fluids of up to 91 °C. Elsewhere across the VDVF, smaller orifices vent fluids of up to 138 °C (for example, Hotter than Hole, Chimlets 1 and Chimlets 2—Fig. 1c). Talc rubble at the base of the Main Cone onlaps the surrounding calcareous sediment—a relationship reversed at the base of the southern mound, indicating an increase in the age of the mounds towards the south (Fig. 1c). A further series of hydrothermally inactive, conical-shaped talc mounds is located 700 m to the south and east of the three active mounds. These include a 90-m high, sediment-covered cone (Mystic Mountain), which has twice the volume of the Main Cone. Samples from these extinct mounds are similar in composition, mineralogy and texture to the active mounds. With an estimated sedimentation rate of between 2 and 5 cm ka−1 (ref. 8), a thickness of 1 m or more of pelagic sediment covering most of Mystic Mountain indicates that its construction by hydrothermal activity ceased at least 20,000 years ago.

Figure 1: Location and bathymetry of the VDVF. (a) map of the Caribbean showing the location of the Mid-Cayman Rise and the black rectangle represents the area of b; the lightly shaded area is the area occupied of the Cayman Trough; OFZ, Oriente Fracture Zone; SIFZ, Swan Island Fracture Zone. (b) Bathymetry and interpretative geology of the Mid-Cayman Rise showing regional tectonic structures. (c) Bathymetry of the active VDVF showing the location of hydrothermal activity across the vent field. Contours are at 20-m intervals. (d) Photomosaic of The Spire at the top of the main VDVF cone obtained from high-definition video. Full size image

Petrology

Compared with hydrothermally active seafloor deposits elsewhere, the mounds and chimneys of the VDVF are highly unusual, constituting 85–90% talc by volume with up to 10% microcrystalline silica and 5% disseminated sulphides (Supplementary Fig. 1). In hand-specimen, the hydrothermally active chimneys show millimetric layers of laminated and botryoidal talc (Fig. 2a). These layers are parallel and have an internal colloform structure (Fig. 2a). Under scanning electron microscope (SEM), broken surfaces reveal dendritic networks and botryoidal masses of talc, indicative of growth into open void spaces (Fig. 2b,c). Concentric bands of sulphide up to 50-μm thick are present as internal growth bands within the talc masses (Fig. 2d) and microcrystalline silica infills pore spaces (Fig. 2e). Talus forming the flanks of the mounds has a similar mineralogy to the venting chimneys, except that the botryoidal texture and open pore spaces are largely replaced by massive fine-grained talc, and the associated sulphides are mostly oxidized. Contrary to initial reports6, talc is the dominant mineralogy at the VDVF; only two of 50 samples recovered contain trace amounts of anhydrite and gypsum. Together with its botryoidal form, zoning, and layering. The talc deposits indicate precipitation from a hydrothermal fluid. The petrology of the hydrothermally active chimneys indicates a paragenetic sequence comprising initial talc growth into open space with co-precipitation of minor sulphide, followed by infilling of the pore space by later-stage microcrystalline silica. Loss of sulphides and a reduction in base-metal content in the flank talus indicate dissolution following exposure to cold and oxygenated seawater.

Figure 2: Hand-specimen and SEM images of VDVF samples. (a) Collapsed chimney wall showing bands of massive talc. (b) and (c) SEM images of dendritic and botryoidal talc networks. (d) SEM image of a cross section through botryoidal talc with bands of sulphide. (e) SEM image showing microcrystalline silica precipitated into a talc framework with disseminated sulphides. Legend: tlc=talc, su=sulphide, si=microcrystalline silica. Full size image

Mineralogy

Bulk X-ray diffraction analyses (Supplementary Fig. 1) and microscopy of the VDVF chimney and mound material confirm the dominant presence of talc, with microcrystalline silica and sulphide constituting up to 15%. Chalcopyrite is the dominant sulphide (70%), indicative of episodically higher vent temperatures9, with the remainder being 20% pyrite, 5% sphalerite and 5% galena. The presence of sulphides at the VDVF, and a lack of sulphate in the end-member fluids, confirms a reducing environment for the precipitation of the chimney and mound talc deposits10.

Other studies of talc, recovered from seafloor environments elsewhere, have revealed significant amounts of intracrystalline layers of smectite clays reflecting the influence of sediment alteration and precipitation from Mg-rich pore waters11. X-ray diffraction analyses of unorientated air-dried mounts of talc separates from the VDVF chimneys and talus deposits show peaks at ∼9.6 Å (for the 001 plane) and ∼4.7 Å (Supplementary Fig. 2) confirming talc (Mg 3 Si 4 O 10 (OH) 2 ) as the dominant phase. Shifts in peaks of the 001 plane from 9.604–9.627 Å for the air-dried mounts to 9.339–9.525 Å for the glycolated mounts show the presence of up to 10% of a smectite clay that is interlayered within the crystalline talc structure. The presence of peaks at 1.529 and 1.532 Å (for the 060 plane) indicate a tri-octahedral structure for the smectite clay interlayers (Supplementary Fig. 2). The dominance of talc (90%), with up to 10% clay interlayers, contrasts with previously reported, sediment-hosted seafloor talc deposits11.

Deposit geochemistry

The high proportions of talc and microcrystalline silica are reflected in the whole-rock geochemistry (Table 1) by the high concentrations of SiO 2 (52–66 wt.%) and MgO, (25–34 wt.%). Average base-metal concentrations in the hydrothermal chimneys are 1,463 p.p.m. Cu, 239 p.p.m. Zn and 112 p.p.m. Pb. In contrast, concentrations in samples of mound talus are an order of magnitude lower with averages of 241 p.p.m. Cu, 48 p.p.m. Zn and 15 p.p.m. Pb, consistent with the observed oxidation and loss by dissolution of sulphides in the talus samples.

Table 1 Whole-rock geochemistry. Full size table

Rare earth element (REE) patterns for talc, separated from hydrothermally active chimneys and mound talus samples, have shallow U-shaped profiles dominated by a large and positive europium anomaly (Fig. 3). Enrichment in the light RREs (LREEs) is indicated by La (N) /Sm (N) (where N=chondrite-normalized values) ratios ranging between 0.9 and 10.4, with an average of 3.3 for chimneys and 3.1 for mound talus (Table 2). The REE profiles also have slight enrichment in heavy RRE (HREE) with average Dy (N) /Yb (N) ratios of 0.8 for chimney material and 0.9 for the mound talus. The magnitude of the positive Eu anomaly, defined as Eu/Eu* (where Eu/Eu*=Eu (N) /√(Sm (N) × Gd (N) ), ranges between 6 and 228, with an average of 99 for the chimneys and 58 for the mounds (Fig. 3 and Table 2). The VDVF chimney and talus talc have 87Sr/86Sr ratios of between 0.706313 and 0.709168, respectively, which are similar to, but slightly less than, that of modern-day seawater (0.7092)12. This contrasts with the surrounding meta-gabbros that have 87Sr/86Sr ratios of between 0.702902 and 0.703657 (Tables 2 and 3).

Figure 3: REE plots. Chondrite-normalized REE patterns showing the range and average concentrations of the VDVF mound (n=15) and chimney (n=9) materials. The patterns are characterized by light and heavy REE enrichment and large positive Eu/Eu* anomalies. Also shown for comparison are samples from the St Paul Fracture Zone, Conrad Fracture Zone, Escanaba Trough, (n=5), Sivas Basin (n=2), TAG anhydrite (n=24), Rainbow hydrothermal vent fluids (n=2) and seawater2,17,54,55,56. Full size image

Table 2 REE geochemistry. Full size table

Table 3 Fluid data. Full size table

The U-shaped REE patterns and positive Eu anomalies in the chimney and mound material indicate talc precipitation from VDVF vent fluid that has some similarities to high-temperature ‘black smoker’ vent fluid REE chemistry13. Reducing conditions in the VDVF vent fluid, indicated by the presence of sulphides, enhance Eu mobility by the formation of divalent Eu chloride complexes, especially during the dissolution of plagioclase, resulting in large, positive Eu anomalies in fluids and precipitates14. Furthermore, the VDVF talc 87Sr/86Sr ratios indicate precipitation following mixing of the high-temperature VDVF vent fluid with a high proportion of seawater (of at least 10:1). Chlorinity in the end-member VDVF fluid of 667 mmol kg−1 is significantly elevated in comparison with seawater (546 mmol kg−1), enhancing the complexation of LREEs in relation to mid-RREs (MREEs) and HREEs in hydrothermal solutions at high temperatures and pressures, resulting in elevated LREE talc /MREE talc ratios15. In contrast, moderate HREE enrichment is largely a crystallographic effect of the talc mineralogy, where HREEs substitute in the octahedral Mg site as a result of the ionic radii being of a more similar size compared with the LREEs16.

Other examples of seafloor talc deposits, reported from the St Paul and Conrad fracture zones, are inferred to have precipitated from the interaction of hydrothermal fluid with either seawater or a mafic protolith17. These talc samples have positive Eu anomalies and flat HREE profiles (Fig. 3), closely resembling those for the VDVF chimney and mound talus17. Seafloor talc deposits from elsewhere lack a positive Eu anomaly and have flat HREE profiles, consistent with formation as alteration products of an ultramafic protolith or sediment (Fig. 3)17,18. In contrast, the positive europium anomaly for the VDVF talc is consistent with primary precipitation from hydrothermal fluids. The radiogenic 87Sr/86Sr ratios for the talc further indicate that a significant component of seawater is mixed with the vent fluid during talc precipitation.

We conclude from the petrographic and geochemical evidence that the VDVF talc is a primary precipitate from a hydrothermal fluid mixed with seawater. To date, no other seafloor, talc-dominated, active hydrothermal vent field has been reported, making the discovery of the VDVF a new and unique class of hydrothermal system.

Vent fluid chemistry

To explore whether the VDVF is currently precipitating talc, we sampled and analysed the composition of the vent fluids escaping from three different chimneys with a maximum measured temperature range of 108–215 °C (Table 3). It should be noted that the vent temperatures and fluid chemistry samples are decoupled, which precludes the possibility of extrapolating to an end-member temperature using fluid chemistry. Instead, we use the highest measured temperature of 226 °C (ref. 19) and assume that this approaches the end-member temperature. We find that our vent fluid samples lie on a mixing line between seawater and zero Mg (Fig. 4). This is consistent with an end-member vent fluid from which Mg has been quantitatively removed in the subsurface20. When extrapolated to zero Mg, the VDVF end-member fluid has a moderate pH of 5.8 (at STP; Table 3) and a dissolved Si concentration of 7.5 mmol kg−1 (ref. 21). End-member concentrations of K (17.5 mmol kg−1) and Li (241 μmol kg−1) are much higher than those generated by phase separation of seawater alone (Fig. 4c and Table 3), indicating significant exchange with subsurface host rocks20. Strontium isotopes for the VDVF fluid samples also lie on a mixing line against Mg/Sr with seawater indicating an end-member vent fluid 87Sr/86Sr of 0.702908, close to the basement rock values of 0.702902–0.703657 (Fig. 4d and Tables 2 and 3). Base-metal concentrations in the fluid samples range from 6.6 to 604 μmol kg−1 for Fe, 4.6–14.2 μmol kg−1 for Mn and 0.4–460.0 μmol kg−1 for Cu, but do not show conservative mixing with seawater and hence end-members cannot be derived (Table 3). Variation in base-metal concentrations and ratios between hydrothermal vents across the VDVF suggests subsurface processes of precipitation and/or zone refining within the talc mounds. Owing to highly variable concentrations, reliable end-member vent fluid Fe and Mn concentrations could not be determined. However, the range of concentrations of Fe and Mn are 10–1,000 times lower than those reported for the Rainbow ‘black smoker’ end-member vent fluid2. In contrast, a chlorinity of 667 mmol kg−1 for the end-member fluid is 22% higher than ambient seawater (546 mmol kg−1), indicating a process of brine concentration.

Figure 4: VDVF fluid plots. (a,b) The top diagrams show Cl, SO 4 (crosses) and Ca (squares) with best fit lines extrapolated to zero Mg concentration. (c) Plots of Cl versus Li and K. The solid line and crosses indicate the linear trend of Li towards seawater (SW); the dashed line and squares indicate the linear trend of K towards SW. The arrows indicate the expected increase in Li, K and Cl concentration resulting from phase separation of seawater alone. (d) Ratios of 87Sr/86Sr plotted against Mg concentration in fluid samples and extrapolated to zero Mg/Sr to determine the VDVF end-member vent fluid ratio. Full size image

The low metal concentrations as well as the near-neutral pH of the fluids venting at VDVF are consistent with temperatures in the water–rock reaction zone, which are significantly cooler than the ∼500 °C calculated for ‘black smoker’ vents22. It is also known that elevated hydrogen sulphide concentrations in ‘black smoker’ hydrothermal vents are related to magmatic input23. Hence, we suggest the low H 2 S concentrations at the VDVF are indicative of minimal magmatic contribution, consistent with the lower temperature of the end-member vent fluid and the ridge-flank setting of the vent field.

Increases in chlorinity for hydrothermal fluids interacting with an ultramafic basement have been suggested to occur in a number of ways: rock alteration, phase separation and brine and halite addition2,24. Temperatures in excess of 360 °C are required to phase-separate seawater at 200 bar (that is, at the depth of the VDVF) and even higher for deep subsurface reactions25. This is considerably hotter than the maximum temperature recorded at the VDVF21 and would require substantial subsurface cooling by seawater circulating deep within the talc mounds for which there is no evidence. The dissolution of residual halite, or the mixing with residual brine formed during an earlier and higher-temperature period of hydrothermal circulation and phase separation deep in the crust, could increase the end-member chlorinity26,27; however, the VDVF and surrounding area lacks any mineralogical evidence for a ‘black smoker’-like phase of venting. The process of serpentinization also has the potential to increase fluid chlorinity by the removal of water from the fluid2,21. Using the equation for the serpentinization of pure forsterite to serpentine and brucite (equation (1)), we calculate that the increase in chlorinity for the VDVF end-member fluid could result from serpentinization at a ratio of 944 g of forsterite to 1 kg seawater. The presence of any mineralogical Cl in serpentine makes this estimate a minimum.

We have calculated a mass flux of end-member hydrothermal fluid at VDVF of ∼500 kg s−1 (see below) for which the increase in chlorinity requires brine expulsion following serpentinization of pure forsterite at a rate of at least 470 kg s−1. Alternatively, the entrainment of an early-formed brine or halite phase is consistent with a cooling hydrothermal system, while serpentinization is compatible with the tectonic setting of the VDVF on slowly exhumed lower-crust and upper-mantle rocks. Both processes are ultimately constrained by the availability of residual brine, halite or fresh peridotite.

Fluid pH

A measured pH of 5.8 (at STP) for the VDVF end-member fluid is significantly higher than the observed range (pH 2.5–3.5) for sulphide-rich, high-temperature ‘black smoker’ vent fluids. It is also lower than the alkaline, low-temperature fluids vented at the ultramafic-hosted Lost City Vent Field3,20. Hydrothermal fluids with intermediate pH (5–6) can result from the interaction of seawater with mafic and/or ultramafic lithologies at temperatures <300 °C (refs 28, 29). Under these conditions, pH is controlled by the balance between Mg removal from seawater and silicate hydrolysis (equation (2)). At low water/rock ratios (<10), the rate of magnesium consumption is relatively low, and silicate hydrolysis acts as a pH buffer (equation (3)). At high water/rock ratios (>50), H+ is produced at a rate greater than it is used up in hydrolysis reactions, resulting in lower pH28.

With its intermediate pH, 215 °C temperature and setting on lower-crustal and upper-mantle rocks, the fluids venting at the VDVF are consistent with moderate-temperature interaction within a gabbro/peridotite basement (equation (3)).

Talc precipitation

Thermodynamic modelling using the Geochemist’s Workbench and SUPCRT92 (refs 30, 31, 32), under the ambient pressure and temperature conditions of the VDVF, predicts instantaneous precipitation of talc and silica as the primary phases on mixing the 215 °C VDVF end-member vent fluid with cold seawater (Fig. 5a). Both phases remain supersaturated throughout the mixing regime until seawater makes up ∼90%, and the fluid reaches ∼25 °C (Fig. 5b). Below this temperature, talc remains undersaturated that, together with the kinetics of the reaction, may explain why talc was not visibly precipitating in the hydrothermal plume. Oscillatory zoning of talc and silica within active chimneys indicates a fluid composition at the VDVF that fluctuates around the intersection of the talc–silica saturation limits in the H 2 O-HCl-(Al 2 O 3 )-MgO-SiO 2 system. A fluid composition around the eutectic between talc and amorphous silica would be influenced by slight changes in Mg or Si activity, brought about either by precipitation of one or the other phase, or by fluctuating seawater proportions within the mound. Silica is saturated in the end-member fluid, therefore would precipitate when no seawater is available to provide Mg for talc precipitation. Such dynamic conditions, which are well documented at other sites33,34,35, could lead to alternating layers of mineral phases. Bands of chalcopyrite also indicate episodically higher temperatures. The prediction that talc precipitates directly as a result of mixing between the VDVF vent fluid and seawater is further supported by the range of 87Sr/86Sr ratios for the talc (0.706313–0.709168), which lie between the value of modern seawater (0.7092) and the VDVF end-member hydrothermal fluid (0.702908; Tables 2 and 3 and Fig. 4).

Figure 5: Thermodynamic phase diagram. (a) Stability boundaries of talc, quartz and brucite at 215 and 90 °C, calculated in Geochemist’s Workbench (GWB) at the ambient pressure conditions of the seafloor at the VDVF. The percentages indicate the proportion of seawater mixing with the end-member VDVF fluid. (b) Saturation indices for talc, quartz and brucite during modelled mixing of VDVF end-member fluid with seawater from 0.01 to 99%. This indicates that talc becomes saturated on mixing end-member VDVF vent fluid with seawater, and remains stable throughout the mixing regime. Brucite, which is the dominant magnesium phase at the higher pH Lost City vent field, never reaches stability at the VDVF. Full size image

The dissolved silica concentrations in the VDVF end-member fluids are similar to those reported from ‘black smoker’ vents20, where talc is also theoretically stable on mixing with cold seawater. However, the large quantity of sulphides precipitated at these vent sites results in talc and other silicates only occurring as accessory minerals (for example, Middle Valley)36. At the VDVF, the low metal content of the fluid results in only accessory amounts of metal sulphides, allowing talc and silica to become the dominant phases. By comparison, the higher pH of the Lost City vent fluids results in calcium carbonate and brucite precipitation instead of talc3.

In summary, the geochemistry of the solid and fluid phases and thermodynamic modelling are consistent with the observed texture of the talc, its growth into pore spaces, the presence of actively venting talc chimneys and the precipitation of significant volumes of talc-forming large conical mounds on the seafloor. We suggest that similar conditions prevailed during the formation of the other, now hydrothermally extinct, talc mounds (for example, Mystic Mountain) located ∼700 m to the east of the currently active site. The current VDVF represents the latest stage in talc precipitation and hydrothermal circulation in the mafic and ultramafic basements beneath the Mt Dent OCC over tens of thousands of years.