Deep Hydrothermal Circulation Research Articles

Two Decades of Monitoring Hydrothermal Plumes at the Brothers Submarine Volcano, Kermadec Arc, New Zealand

Abstract Brothers volcano is arguably the most well-studied submarine arc volcano on Earth. Between 1996, when massive sulfides were first recovered by dredging, and 2018, when International Ocean Discovery Program (IODP) Expedition 376 recovered cores from as deep as 453 m below the sea floor at two chemically distinct hydrothermal upflow zones, over 60 conductivity-temperature-depth (CTD) vertical casts and tow-yo operations mapped hydrothermal plumes over and around the edifice by employing hydrothermal tracer-specific sensors. These surveys started in 1999 and were completed during nine separate expeditions at one- to three-year intervals, except for a six-year gap between 2011 and 2017. Hydrothermal plume distributions over this two-decade period show variability in the intensity and vertical rise height of plumes from the four main vent fields (Upper Cone, Lower Cone, NW Caldera, and Upper Caldera, with the latter not discovered until 2017). Upper Cone plumes were more intense than all other sites in 1999, 2002, 2007, and 2009, then significantly diminished from 2011 to 2018. The Lower Cone plume was the most intense in 2004, then the NW Caldera site became the dominant source of hydrothermal particles from 2011 to 2018. Despite the gap of six years between 2011 and 2017, hydrothermal output appears to have increased within the caldera sometime after the 2009 survey while simultaneously decreasing in intensity at the cone sites. This supports other evidence of linkages between the cone and caldera sites in the deep hydrothermal circulation system, and may be related to the predicted deepening of hydrothermal circulation, infiltration of seawater to facilitate “mining” of magmatic brines, and modulation of subseafloor mineralization processes associated with a modeled, pulsed injection of magmatic gasses. The surveys also revealed ways in which the highly variable regional hydrographic environment impacts the flux of hydrothermal products to the surrounding ocean. Plumes from sources located above the caldera rim disperse hydrothermal components without hindrance, but particles and heat from sources within the caldera become trapped and are dispersed episodically by caldera-flushing events. While on site for 18 days in 2018, repeat CTD casts into the deepest part of the caldera, which was isolated from the surrounding ocean, showed a progressive increase in temperature, representing a net heat flux of 79 MW from conductive and advective sources deeper than 1,570 m.

Characterising thermal water circulation in fractured bedrock using a multidisciplinary approach: a case study of St. Gorman\u2019s Well, Ireland

A hydrogeological conceptual model of the source, circulation pathways and temporal variation of a low-enthalpy thermal spring in a fractured limestone setting is derived from a multidisciplinary approach. St. Gorman’s Well is a thermal spring in east-central Ireland with a complex and variable temperature profile (maximum of 21.8 °C). Geophysical data from a three-dimensional(3D)audio-magnetotelluric(AMT) survey are combined with time-lapse hydrogeological data and information from a previously published hydrochemical analysis to investigate the operation of this intriguing hydrothermal system. Hydrochemical analysis and time-lapse measurements suggest that the thermal waters flow within the fractured limestones of the Carboniferous Dublin Basin at all times but display variability in discharge and temperature. The 3D electrical resistivity model of the subsurface revealed two prominent structures: (1) a NW-aligned faulted contact between two limestone lithologies; and (2) a dissolutionally enhanced, N-aligned, fault of probable Cenozoic age. The intersection of these two structures, which has allowed for karstification of the limestone bedrock, has created conduits facilitating the operation of relatively deep hydrothermal circulation (likely estimated depths between 240 and 1,000 m) within the limestone succession of the Dublin Basin. The results of this study support a hypothesis that the maximum temperature and simultaneous increased discharge observed at St. Gorman’s Well each winter is the result of rapid infiltration, heating and recirculation of meteoric waters within a structurally controlled hydrothermal circulation system.

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.

Fate of sulfate in seafloor hydrothermal systems: Insights from in situ observation of the liquid–liquid phase separation in hydrothermal fluids

The ubiquitous hydrothermal circulation within the newly formed oceanic crust plays an important role in the transport of heat, water and chemical constituents in the earth-ocean-atmosphere system. One enigma during this process is whether seawater-derived sulfate participates in the deep hydrothermal circulation. Our results showed that homogeneous sulfate-bearing fluid separated to a sulfate-rich dense liquid phase and a sulfate-poor light liquid phase. The liquid–liquid phase separation (LLPS) temperature increased with rising fluid pressure but decreased with the addition of low dielectric constant (D) components. In situ Raman spectroscopic characterization revealed that the Zn2+-SO42- association increased with the addition of low D components. All these observations confirmed our previous conclusions that strong ion complexation triggered the LLPS. The formation of a sulfate-rich dense liquid phase could postpone the precipitation of sulfate, resulting in the participation of seawater-derived sulfate in the deep hydrothermal circulation. Sulfate would be reduced to S2− and mixed with that leached from the basalt, leading to the elevated δ34S value of the sulfide precipitated on the seafloor.

THM modeling of gravity anomalies related to deep hydrothermal circulation at Soultz-sous-For\xeats (France)

Gravity measurements in the Upper Rhine Graben evidence spatial variations at the regional scale and close to the geothermal sites. They are classically interpreted as linked to the geology. We aim to bring new insights on another potential origin of these gravity changes. Our approach is to quantify gravity anomalies related to the deep hydrothermal circulation. A thermo-hydro-mechanical model is developed at the reservoir scale for the Soultz-sous-Forêts site (Soultz), France. A finite element method is used in 2D and 3D. The size of the representative elementary volume is assumed to be 100 m with no regional fault in the reservoir. Surface gravity profiles and maps associated to the large-scale hydrothermal circulation are computed from the variations of the effective density through the model. Synthetic spatial gravity variations in 2D and 3D models are shown to have an amplitude of 0.02 mGal. They are shown to be mostly linked to the convective system. Their wavelength is about 7.5 km, consistent with the width of the hydrothermal convection cells. The anomaly maximum is located at the top of the maximum surface heat flux. However, gravity anomaly observations show much higher amplitude and heterogeneity. Spatial gravity variations linked to the hydrothermal circulation are shown to be smaller than the observed gravity spatial variations, but still measurable with very sensitive instruments (absolute gravimeters).

Fault-controlled deep hydrothermal flow in a back-arc tectonic setting, SE Tyrrhenian Sea

Understanding magmatic systems and deep hydrothermal circulation beneath arc-volcanoes provides insights into deep processes associated with slab-subduction and mantle-wedge partial melting. Here we analyze hydrothermal flow below a structural high (Capo Vaticano Ridge, CVR) located offshore Capo Vaticano (western Calabria) and affected by magmatic intrusions generated from above the Ionian subducting-slab. In order to explain observations, we combine geophysical and numerical modelling results. Fluid-flow modelling shows that temperature distribution and geothermal gradient are controlled mainly by hydrothermal circulation, in turn affected by heat source, fault pattern, rock permeability, basement topography and sediment thickness. Two main faults, shaping the structural high and fracturing intensely the continental crust, enable deep hydrothermal circulation and shallow fluid discharge. Distribution of seismicity at depth supports the hypothesis of a slab below Capo Vaticano, deep enough to enable mantle-wedge partial melting above the subduction zone. Melt migration at shallow levels forms the magmatic intrusions inferred by magnetic anomalies and by δ3He enrichment in the discharged fluids at the CVR summit. Our results add new insights on the southern Tyrrhenian Sea arc-related magmatism and on the Calabrian inner-arc tectonic setting dissected by seismogenic faults able to trigger high-destructive earthquakes.

Reservoir Imaging Using Ambient Noise Correlation From a Dense Seismic Network

AbstractIn September 2014, a dense temporary seismic network (EstOF) including 288 single‐component geophones was deployed during 1 month in the Outre‐Forêt region of the Upper Rhine Graben (France), where two deep geothermal projects (Soultz‐sous‐Forêts and Rittershoffen) are currently in operation. We apply ambient seismic noise correlation to estimate the empirical Green's function of the medium between the ~41,200 station pairs in the network. The noise correlation functions obtained are comparable to those from previous studies based on the sparse long‐term networks settled in the area mostly to monitor the induced seismic activity. However, the dense spatial coverage of the EstOF network improves our ability to identify the main phases of the Green's function. Both the fundamental mode and the first overtone of the Rayleigh waves are identified between most station pairs. P waves are also evidenced. We analyze the statistical distribution of the Rayleigh wave group velocity between station pairs as a function of the period (between 0.8 and 5 s), the station pair orientation, the distance over wavelength ratio and the signal‐to‐noise ratio. From these observations, we build a high‐resolution three‐dimensional S wave velocity model of the upper crust (down to 3 km deep) around the regional deep geothermal reservoirs. This model is consistent with local geological structures but also evidences nonlithological variations, particularly at depth in the basement. These variations are interpreted as large‐scale temperature anomalies related to deep hydrothermal circulation.

How temperature-dependent elasticity alters host rock/magmatic reservoir models: A case study on the effects of ice-cap unloading on shallow volcanic systems

In geodynamic numerical models of volcanic systems, the volcanic basement hosting the magmatic reservoir is often assumed to exhibit constant elastic parameters with a sharp transition from the host rocks to the magmatic reservoir. We assess this assumption by deriving an empirical relation between elastic parameters and temperature for Icelandic basalts by conducting a set of triaxial compression experiments between 200 °C and 1000 °C. Results show a significant decrease of Young's modulus from ∼38 GPa to less than 4.7 GPa at around 1000 °C. Based on these laboratory data, we develop a 2D axisymmetric finite-element model including temperature-dependent elastic properties of the volcanic basement.As a case study, we use the Snæfellsjökull volcanic system, Western Iceland to evaluate pressure differences in the volcanic edifice and basement due to glacial unloading of the volcano. First, we calculate the temperature field throughout the model and assign elastic properties accordingly. Then we assess unloading-driven pressure differences in the magma chamber at various depths in models with and without temperature-dependent elastic parameters. With constant elastic parameters and a sharp transition between basement and magma chamber we obtain results comparable to other studies. However, pressure changes due to surface unloading become smaller when using more realistic temperature-dependent elastic properties. We ascribe this subdued effect to a transition zone around the magma chamber, which is still solid rock but with relatively low Young's modulus due to high temperatures. We discuss our findings in the light of volcanic processes in proximity to the magma chamber, such as roof collapse, dyke injection, or deep hydrothermal circulation. Our results aim at quantifying the effects of glacial unloading on magma chamber dynamics and volcanic activity.

Understanding hydrothermal circulation patterns at a low-enthalpy thermal spring using audio-magnetotelluric data: A case study from Ireland

Kilbrook spring is a thermal spring in east-central Ireland. The temperatures in the spring are the highest recorded for any thermal spring in Ireland (maximum of 25°C). The temperature is elevated with respect to average Irish groundwater temperatures (9.5–10.5°C), and represents a geothermal energy potential, which is currently under evaluation. A multi-disciplinary investigation based upon an audio-magnetotelluric (AMT) survey, and hydrochemical analysis including time-lapse temperature and chemistry measurements, has been undertaken with the aims of investigating the provenance of the thermal groundwater and characterising the geological structures facilitating groundwater circulation in the bedrock.The three-dimensional (3-D) electrical resistivity model of the subsurface at Kilbrook spring was obtained by the inversion of AMT impedances and vertical magnetic transfer functions. The model is interpreted alongside high resolution temperature and electrical conductivity measurements, and a previous hydrochemical analysis.The hydrochemical analysis and time-lapse measurements suggest that the thermal waters have a relatively stable temperature and major ion hydrochemistry, and flow within the limestones of the Carboniferous Dublin Basin at all times. The 3-D resistivity model of the subsurface reveals a prominent NNW aligned structure within a highly resistive limestone lithology that is interpreted as a dissolutionally enhanced strike-slip fault, of Cenozoic age. The karstification of this structure, which extends to depths of at least 500m directly beneath the spring, has provided conduits that facilitate the operation of a relatively deep hydrothermal circulation pattern (likely estimated depths between 560 and 1000m) within the limestone succession of the Dublin Basin. The results of this study support the hypothesis that the winter thermal maximum and simultaneous increased discharge at Kilbrook spring is the result of rapid infiltration, heating and re-circulation of meteoric waters within this structurally controlled hydrothermal circulation system.This paper illustrates how AMT may be useful in a multi-disciplinary investigation of an intermediate-depth (100–1000m), low-enthalpy, geothermal target, and shows how the different strands of inquiry from a multi-disciplinary investigation may be woven together to gain a deeper understanding of a complex hydrothermal system.

New Perspectives on Ancient Mars

Mars was most active during its first billion years. The core, mantle, and crust formed within approximately 50 million years of solar system formation. A magnetic dynamo in a convecting fluid core magnetized the crust, and the global field shielded a more massive early atmosphere against solar wind stripping. The Tharsis province became a focus for volcanism, deformation, and outgassing of water and carbon dioxide in quantities possibly sufficient to induce episodes of climate warming. Surficial and near-surface water contributed to regionally extensive erosion, sediment transport, and chemical alteration. Deep hydrothermal circulation accelerated crustal cooling, preserved variations in crustal thickness, and modified patterns of crustal magnetization.

Costa Rica Rift revisited: Constraints on shallow and deep hydrothermal circulation in young oceanic crust

New heat-flux observations made along two seismic reflection profiles on 6 Ma crust of the Costa Rica Rift flank show an inverse correlation between heat-flux and sediment thickness similar to that observed on other sedimented ridges and young ridge flanks. Extrapolation of the seafloor heat-flux values to the top of the igneous crust—justified by comparing seafloor and borehole determinations where observations are colocated—shows the surface of the crust to be of uniform temperature despite large local sediment thickness variations. This is consistent with observations made at DSDP/ODP Holes 504B and 896A where basement temperatures are observed to differ by less than 2 K, also despite contrasting sediment thicknesses which would cause a difference of more than 15 K under conditions of conductive heat transport. Efficient lateral heat exchange via vigorous crustal hydrothermal circulation is required to create the degree of uniformity inferred and observed. Permeability measurements at the two drill sites have shown that this vigorous circulation may be restricted to the uppermost 100 m of the crust. The laterally uniform temperatures in the uppermost igneous crust here and elsewhere are remarkable given the small apparent depth extent of the circulation that so efficiently distributes heat. Modeling suggests that the effective formation-scale permeability must be of the order of 10 −9 m 2, higher by several orders of magnitude than borehole packer and flow determinations, and that fluid fluxes are of the order of many tens of m year −1. Deeper in Hole 504B, permeability previously measured is too low to permit thermally significant flow, and this is consistent with geochemical observations which have shown that deep borehole fluids retain characteristics of drilling fluid years after episodes of drilling. Paradoxically, deep temperature logs are best explained by the presence of advective heat transport, and suggest that thermally significant flow may indeed extend throughout the 1800 m section penetrated at Site 504, and cool the deep crust by many tens of K. Permeability in the deeper extrusives and sheeted dikes estimated via numerical models using the constraint of the thermal structure is >10 −15 m 2. Fractures and other zones of high permeability in both the uppermost and deeper crust must be underrepresented by the borehole. While certainly not as vigorous as the inferred flow in the uppermost crust, the deeper circulation suggested by the deep temperature logs is noteworthy in light of previous conclusions based on permeability and fluid chemistry that the deeper crustal water at this site is stagnant. Model results suggest average fluxes of up to 0.1 m year −1, and an average time for exchange between the deeper and shallow layers of <5000 years.

Deep hydrothermal circulation in a granite intrusion beneath Larderello geothermal area (Italy): constraints from mineralogy, fluid inclusions and stable isotopes

A shallow (∼2 km) granite body has been intersected by the MONT-4 well in new producing areas of the Larderello geothermal field. The granite at 2.2–2.5 km and below 3 km underwent pervasive propylitic alteration that is dominated by the assemblage chlorite+adularia+quartz+illite+calcite+albite±epidote±sphene. Fluid inclusion studies indicate two stages of alteration. The first one is characterised by high-temperature fluids that, at 2218 m, were trapped under boiling conditions at about 340°C and 15 MPa, whereas at 3520 m evidence of boiling is not revealed and the temperature of the fluid was calculated to be about 350°C. The second alteration stage shows a cooling of the system, particularly evident in the shallower portion of the intrusion (∼235°C at 2218 m and ∼315°C at 3520 m). The temperature of homogenisation for subsequent cooler inclusions approximates present-day down-hole temperatures, and may be considered as representative of the fluid actually circulating in the granite in recent times. Calculated δ 18O values of 0±1‰, for fluids in equilibrium at 340°C (2218 m) and 350°C (3520 m) with feldspar, epidote and chlorite, and δD values of −51 to −61‰, for fluids in equilibrium with chlorite, together with directly measured δD values of −33 to −61‰ for fluid inclusions in quartz, indicate derivation from meteoric waters. The original meteoric signature of the fluids has been altered through water–rock interaction, but remains clearly distinguished from a primary magmatic signature, for which no isotopic evidence was found. Active fracturing and the high structural relief of granite emplacement promoted hydrothermal circulation in the granitic body drilled by MONT-4 well, suggesting that such characteristics may be a pre-requisite for such circulation at Larderello.

Crustal structure of the continent-ocean transition off the Côte d'Ivoire-Ghana transform margin: implications for thermal exchanges across the palaeotransform boundary

Crustal structure of the continent–ocean transition off the Côte d'Ivoire–Ghana transform margin is determined by 2‐D modelling of wide‐angle seismic data and gravity data. The resulting models, together with geological data recently collected by drilling on the margin, are compared with available thermomechanical models of transform margins. In particular, we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere. These effects include: (i) ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it, from the continent towards the ocean; and (ii) thermal uplift of the transform margin. In the region of the Côte d'Ivoire–Ghana transform margin, the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent–ocean boundary is approached. The interpreted models lack a transitional domain between continental and oceanic crusts, and abnormally thin oceanic crust is found along the margin. These results exclude large‐scale ductile flow of lower continental crust during the margin formation. The geological data are, moreover, not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre. Our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere. Moderate heating of the Côte d'Ivoire–Ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models. Two main factors may have contributed to reduce thermal exchanges across the continent–ocean boundary. The first factor is that the oceanic edge may have been colder than assumed by thermal models. An abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the Côte d'Ivoire–Ghana transform margin. The oceanic crustal structure is similar to that found along oceanic fracture zones, where oceanic crust is also emplaced near a ridge–transform intersection. In both cases, abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of: (i) a cold edge effect of the adjacent continental lithosphere; (ii) accretion at the end of a ridge segment, away from the main magma supply point along the spreading axis; and (iii) an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere. The second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults, which reduces the lateral thermal conduction in the lithosphere, since water is much less conductive than rock. During the margin formation, the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre. We propose that this thermal buffer hypothesis may be generalized to most transform margins, implying a lower amount of continental heating than assumed by the available thermomechanical models.

Role of a Cl‐bearing flux in the origin of depleted ocean floor magmas

Plagioclase‐hosted melt inclusions from ocean floor lavas are characterized by great diversity in their minor and trace element compositions. The incompatible element contents of the inclusions range from enriched to ultradepleted. Ultradepleted inclusions in lavas sampled from areas such as the Galapagos Platform and the Endeavour Segment of the Juan de Fuca Ridge have Ti/Zr values as high as 3000. Such high Ti/Zr (low Ti and much lower Zr, as little as &gt;2 ppm) suggest a source material that was melted past the point of the exhaustion of clinopyroxene (harzburgite melting). Nevertheless, these same inclusions have (La/Sm)n &gt; 0.25 (although with low La and Sm concentrations), low K2O (0.01–0.04 wt%), and high Cl (up to 1600 ppm). In addition, examination of melt inclusion data from over 30 locations worldwide shows that there is a correlation between the level of enrichment of the host lava and the trend in K2O versus Cl described by the inclusion population. Melt inclusions from enriched lavas are characterized by relatively low Cl contents at a given K2O and no high Cl inclusions. A wide range of Cl contents and uniformly low K2O characterizes inclusions from depleted lavas. Transitional lavas exhibit either intermediate slopes or two separate trends. In contrast, other incompatible elements, such as P, Ti, and the high field strength elements (HFSE), behave coherently. The paradox presented by high Cl and (La/Sm)n together with low HFSE and K2O in melt inclusion populations from depleted lavas, coupled with uniformly high incompatible element contents in enriched lava inclusions, is inconsistent with their derivation by variable degrees of melting. Correlation of Ti/Zr with Cl in the ultradepleted inclusions, plus the absence of high Cl contents in inclusions from enriched lavas supports the contention that this signal is a mantle phenomenon and not the result of alteration at or near the seafloor. The close association of ultradepleted and normal mid‐ocean ridge basalt (NMORB) melt inclusions, sometimes in the same phenocryst, together with the existence of a complete array of melt compositions (i.e., not two distinct populations), demonstrates that the magma types were part of the same magma production episode. This combination of characteristics exhibited by melt inclusions may be produced by the interaction of a harzburgite source with a Cl, light rare earth element–bearing fluid derived from deep hydrothermal circulation. Fluctuation of magma supply may allow the upper mantle to be periodically cooled and altered by hydrothermal fluids, then undergo melting during resurgence of the magma supply. The ultradepleted component produced by that fluxed harzburgite may comprise as much as 5–10% of the array of magmas that make up NMORB magmas. If this is true, it has important implications for the thermal and mass budget and perhaps the rheology of the upper mantle and lower crust.

Submarine hydrothermal fossils confirmed

Researchers from Princeton University (D. Crerrar et al, Econ. Geol., May 1982) have documented, in considerable detail, evidence for the formation of some of the 800 or more manganiferous chert deposits occurring in the central belt of the Fransiscan formation in northwestern California. They confirm the surprisingly old conclusion o f Tiaferro and Hudson (Cal. Div. Mines Bull., 125, 217–276, 1943) that the Fransiscan chert deposits probably represent the fossil remains of submarine hydrothermal vents.The deposits resemble recently discovered hydrothermal mounds near the Galapagos rift, the Gulf of Aden, and the Mid‐Atlantic Ridge. As the Princeton investigators point out, there are important implications of the existence of deep hydrothermal circulation systems at oceanic spreading centers throughout geologic time. They note that the calculated annual flow of hydrothermal fluids in such processes is about 1017 g, which implies that the entire volume of the oceans could circulate completely every 10 million years. With such circulation, the hydrothermal processes along midocean ridges could control the composition of seawater and strongly influence the geochemical flux of elements in the marine environment.