Submarine Hydrothermal Solutions Research Articles

Petrology of Precambrian metagranites and heavy mineral chemistry of the Olounou area within the Cameroon mineral belt (Ntem complex, South Cameroon): relationships with the Fe mineralization

The petrology of Achaean biotite- and amphibole-rich metagranites, the geochemistry of Fe occurrences and heavy mineral concentrations in stream sediments from Olounou (Ntem complex) help to constrain their inter-relationships. These metagranites display granoblastic textures outlined by microcline, biotite, amphibole and oxides. Ilmenite (60 to 80%), zircon (15 to 20%) and rutile (1 to 2%) are the mineral fractions in pan concentrates. Two types of Fe mineralization as silicified veins cross-cut the above rocks: hematite ± magnetite quartzites with primary hematite enclosing magnetite relicts contain high average Fe2O3 (69.05 wt%) and TiO2 (0.73 wt%); banded magnetite-rich quartzites with magnetite partially replaced by hematite are characterized by Fe2O3 (52.15 wt%), TiO2 (0.27 wt%), relatively high W (286 ppm) and Ni (108 ppm). Both types have Zn, Cu, Ga contents below 10 ppm, positive Eu anomalies, HREE enrichment over LREEs and low ΣREE (9.76 to 12.07). Iron and SiO2 were derived from weathering of Archean biotite and amphibole metagranites, deposited in existing Post-Archean intracratonic basins under greenschist facies and impacted by submarine hydrothermal solutions. These results are genetically comparable to other Precambrian BIFs and underline the iron potential of Olounou. They equally contribute to iron ore exploration in Cameroon and worldwide.Supplementary material: Table S1 is available at https://doi.org/10.6084/m9.figshare.c.5426547

Origin of the Mesoproterozoic Jingtieshan bedded barite deposit, North Qilian Mountains, NW China: Geochemical and isotope (O, S, Sr) evidence

The bedded Jingtieshan barite deposit, which occurs in the western part of North Qilian, NW China, is intimately associated with banded iron formation (BIF) and is hosted in weakly metamorphosed Mesoproterozoic clastic–carbonate rocks. Although numerous studies have focused on the BIF, the origin of the barite mineralization is poorly understood. The barite orebody occurs as a lenticular/stratiform unit overlying and locally interlayered with the BIF. In this contribution, we present a comprehensive and integrated study of the geochemistry and S–O–Sr‐isotope characteristics of the Jingtieshan barite deposit, thereby constraining its origin and the depositional conditions in the Mesoproterozoic ocean. The ore is composed of barite, minor carbonate minerals, specularite, and other minerals and is characterized by low Sr concentrations (711–1469 ppm), total rare earth element concentrations (7–28 ppm), and La/Ce values, relative enrichment in light rare earth element, positive Gd anomalies, and elevated Lan/Cen, which indicate that mineralizing fluids were derived from seawater and submarine hydrothermal solutions. Barite separates yield δ34S = 19.5–29.1‰ and δ18O = 12.7–23.6‰, which are consistent with a coeval seawater origin for SO42− modified bacterial sulfate reduction. Moreover, the 87Sr/86Sr values suggest that Sr, as well as Ba, was derived from contemporaneous seawater and hydrothermal solutions, with additional radiogenic Sr derived from late‐stage hydrothermal alteration. The redox‐sensitive trace element ratios of V/(V + Ni), V/(V + Cr), and V/Cr, combined with negative Ce anomalies (0.11–0.82), indicate that the barite formed under oxic marine conditions in a redox stratified ocean. Geological, geochemical, and isotope data support a sedimentary–exhalative origin for the Jingtieshan barite deposit, where barite was deposited under oxic conditions in a shallow marine environment when ascending Ba‐bearing submarine hydrothermal solutions encountered sulfate‐rich seawater.

Thermodynamic modeling of hydrothermal plumes forming processes in a submarine discharge zone

The main methods are considered and results of the modeling of the geochemical processes in submarine hydrothermal solutions of mid-ocean ridges discharge zones are analyzed. Initial materials for modeling were received at several sea expeditions, including operations at the Russian-French expedition SERPENTINE on RV «Pourquoi Pas?» (2007). For carrying out a computer thermodynamic modeling, hydro-geochemical and physicochemical models of a zone of hydrothermal discharge are created. Verification of model is carried out on the change of concentration of manganese in a hydrothermal plume. The prevailing forms of manganese migration in a plume are Mn 2+ , MnCl + , MnCl 2 . In a plume’s geochemical structure two zones are allocated: 1) high temperatures (350-100 °C), with the prevalence of the chloride complexes - buoyant plume; 2) low temperatures (100-2 °C), with the domination of a transfer in a form of free bivalent ion - lateral plume. The sulphate complex is observed in insignificant quantities (1,5 %) in a lateral plume, hydroxide - is stable at temperatures 325-125 °C and can be observed only in the buoyant plume. The results of the modeling almost completely correspond to the natural observations. A verification of the thermodynamic model testifies to her working capacity and allows to pass to the following stage of researches - to studying the geochemical dispersion nature of the main ore components in hydrothermal solutions - Fe, Cu, Zn, etc.

Two pulses of mineralization and genesis of the Zhaxikang Sb–Pb–Zn–Ag deposit in southern Tibet: Constraints from Fe–Zn isotopes

Zhaxikang is one large Sb–Pb–Zn–Ag deposit located in the North Himalaya of southern Tibet. To date, the genesis of this deposit still remains controversial. Here, we present new pyrite Fe and sphalerite Zn isotopic data for the first three stages of mineralization, Fe–Zn isotopic data for Mn–Fe carbonate that formed during the first two stages of mineralization, and Zn isotopic data for the slate wall rocks of the Jurassic Ridang Formation to discuss the genesis of the Zhaxikang deposit. The overall δ56Fe and δ66Zn values range from −0.80‰ to 0.43‰ and from −0.03‰ to 0.38‰, respectively. The δ56Fe values of Mn–Fe carbonates are lighter than those of associated pyrite in six mineral pairs, indicating that the iron carbonates are preferentially enriched in light Fe isotopes relative to pyrite. The sphalerite has lighter δ66Zn values than associated Mn–Fe carbonates in three mineral pairs.The δ56Fe values of pyrite that formed during the first three stages of mineralization gradually increase from stage 1 (−0.33‰ to −0.09‰) through stage 2 (−0.30‰ to 0.19‰) to stage 3 (0.16‰–0.43‰). In comparison, the sphalerite that formed during these stages has δ66Zn values that gradually decrease from stage 1 (0.16‰–0.35‰) through stage 2 (0.09‰–0.23‰) to stage 3 (−0.03‰ to 0.22‰). These data, in conjunction with the observations of hand specimens and thin sections, suggest that the deposit was overprinted by a second pulse of mineralization. This overprint would account for these Fe–Zn isotopic variations as well as the kinetic Rayleigh fractionation that occurred during mineralization. The temporally increasing δ56Fe and decreasing δ66Zn values recorded in the deposit are also coincident with an increase in alteration, again supporting the existence of two pulses of mineralization. The δ56Fe values of the first pulse of ore-forming fluid were calculated using theoretical equations, yielding values of −0.54‰ to −0.34‰ that overlap with those of submarine hydrothermal solutions (−1‰ to 0‰). However, the δ56Fe values of the stage 3 pyrite are heavier than those of typical submarine hydrothermal solutions, which suggests that the second pulse of mineralization was probably derived from a magmatic hydrothermal fluid. In addition, the second pulse of ore-forming fluid has brought some Fe and taken away parts of Zn, which results the lighter δ66Zn values of sphalerite and heavier δ56Fe values of pyrite from the second pulse of mineralization. Overall, the Zhaxikang deposit records two pulses of mineralization, and the overprint by the second pulse of mineralization causes the lighter δ66Zn values and heavier δ56Fe values of modified samples.

Geochemistry and genetic model of the ore-bearing sediments of the Parnok ferromanganese deposit, Polar Urals

The Parnok ferromanganese deposit is confined to the black shales of the western slope of the Polar Urals. The deposit area is made up of weakly metamorphosed terrigenous-carbonate rocks formed in a marine basin at a passive continental margin. Ore-bearing sequence is composed of coaliferous clayey-siliceous-calcareous shales comprising beds and lenses of pelitomorphic limestones, and iron and manganese ores. The iron ores practically completely consist of micrograined massive magnetite. The manganese ores are represented by lenticular-bedded rocks consisting of hausmannite, rhodochrosite, and diverse manganese silicates. With respect to relations between indicator elements (Fe, Mn, Al, Ti), the shales are ascribed to pelagic sediments with normal concentrations of Fe and Mn, the limestones correspond to metalliferous sediments, ferruginous sediments are ore-bearing sediments, while manganese rocks occupy an intermediate position. It was found that the concentrations of trace elements typical of submarine hydrothermal solutions (As, Ge, Ni, Pb, Sb, Zn, etc.) in both the ore types are in excess of those in lithogenic component. At the same time, the indicator elements of terrigenous material (Al, Ti, Hf, Nb, Th, Zr, and others) in the ores are several times lower than those in the host shales (background sediments). REE distribution patterns in iron ores show the positive Eu anomaly, while those in manganese ores, the positive Ce anomaly. In general, the chemical composition of the ores indicates their formation in the hydrothermal discharge zone. The peculiar feature of the studied object is the manifestation of hydrothermal vents in sedimentary basin without evident signs of volcanic activity. Hydrothermal solutions were formed in terrigenous-carbonate sequence mainly at the expense of buried sedimentation waters. The hydrothermal system was likely activated by rejuvenation of tectonic and magmatic processes at the basement of sedimentary sequences. Solutions leached iron, manganese, and other elements from sedimentary rocks and transported them to the seafloor. Their discharge occurred in relatively closed marine basin under intermittent anaerobic conditions. Eh-pH variations led to the differentiation of Fe and Mn and accumulation of chemically contrasting ore-bearing sediments.

Determination of the HCl dissociation constant at a temperature of 350°C and 200 bars of pressure by the potentiometric method using a ceramic electrode

A technique has been developed for pH measurement in flow-through cells at temperatures from 175 to 350°C and pressures up to 350 bars in a nonisothermal cell consisting of a ZrO2(Y2O3) ceramic electrode and a Ag/AgCl (2 M KCl, 25°C) flow-through reference electrode. This technique has been applied to determination of the HCl dissociation constant at 350°C and 200 bars; previously, the HCl dissociation has not been studied sufficiently reliably under these conditions. The obtained value pK dis = 2.16 ± 0.03 was used to estimate the acidity of fluids from their chemical composition at the vent of high-temperature submarine hydrothermal solutions.

Vendian-Cambrian carbonate sequences in sedimentary cover of the Tuva-Mongol microcontinent

Lihological-geochemical investigations of Vendian-Cambrian carbonate rocks from sedimentary cover of the Tuva-Mongol microcontinent revealed that the Bokson Group (Oka structural-formational zone), as well as the Gorlyk and Ara-Oshei formations (Il’chir structural-formational zone), characterize a common carbonate platform. Together with the Bokson Group, the combined Gorlyk and Ara-Oshei formations, which correlate with the complete Bokson Group, make up the Vendian-Cambrian sedimentary cover of the Tuva-Mongol microcontinent. Carbonate sequences accumulated in a shallow-water basin with differentiated bottom topography. Vendian-Cambrian carbonate rocks are characterized by constant geochemical composition due to the stability of provenance (basic and subordinate ultrabasic rocks) and the lack of admixture of pyroclastic material and influence of submarine hydrothermal solutions. Lithological and geochemical features of carbonate sequences imply a gradual deepening of basin in the course of sedimentary cover formation.

Mobility of high field strength elements (HFSE) in magmatic-, metamorphic-, and submarine-hydrothermal systems

High field strength elements (HFSE) are normally considered as ‘conservative’ elements in most geological settings. However, field and experimental evidence has suggested that these elements are actually mobile in some cases and can be transported by magmatic-, metamorphic-, and submarine-hydrothermal solutions, in particular in those extreme cases such as highly altered hydrothermal ore deposits and highly evolved products of crystal fractionation such as pegmatites or granitic bodies bordering to metamorphic rocks. In this paper, we combine our own data with those from the literature and focus our discussion on HFSE mobility at crustal fluid systems. We suggest that the mobility of HFSE is controlled by a number of factors, including P– T conditions, pH, and solution chemistry. The HFSE mobility can occur in a wide range of P– T conditions, and their solubility is generally enhanced at high pH conditions. The high F contents in the fluids also largely enhance the mobility of HFSE. The mobility of HFSE and formation of hydrothermal minerals such as zircon and titanite offer us a useful tool to directly date related hydrothermal events by U–Pb methods, which otherwise lack suitable minerals for isotope dating.

OXYGEN ISOTOPE STUDY OF METAMORPHOSED MANGANESE DEPOSITS OF THE NODA-TAMAGAWA MINE, NORTHEAST JAPAN

Manganese ores of the Noda-Tamagawa mine are metamorphosed equivalents of bedded-type sedimentary manganese deposits. The ore lenses are typically zoned with a central pyrochroite-hausmannite zone, an outer tephroite zone, and an outermost rhodonite zone closest to the wall-rock chert. The δ 18 O (VSMOW) values of 65 tephroite, 32 rhodonite, and eight quartz samples from the manganese ores and chert are in the ranges of 9.9 to 20.0, 11.7 to 20.3, and 23.0 to 23.9 per mil, respectively. The overall order of 18 O enrichment corresponds to the order of equilibrium fractionation among these minerals. However, the temperatures calculated from quartz-rhodonite and rhodonite-tephroite pairs from neighboring ore zones are not consistent with those estimated from the mineral assemblages in the wall rocks. The δ 18 O values for MnCO 3 , the most probable candidate of the precursor manganese mineral, were calculated from the oxygen isotope mass balance among manganese silicates, silica, and CO 2 to be 19.2 to 23.4 per mil. If the MnCO 3 initially precipitated from fluids of δ 18 O = 0.0 to 2.5 per mil, the temperatures of precipitation can be calculated to be between 62° and 111°C, consistent with the primary MnCO 3 having been precipitated by low-temperature submarine hydrothermal solutions.

Energetics of Amino Acid Synthesis in Hydrothermal Ecosystems

Thermodynamic calculations showed that the autotrophic synthesis of all 20 protein-forming amino acids was energetically favored in hot (100 degrees C), moderately reduced, submarine hydrothermal solutions relative to the synthesis in cold (18 degrees C), oxidized, surface seawater. The net synthesis reactions of 11 amino acids were exergonic in the hydrothermal solution, but all were endergonic in surface seawater. The synthesis of the requisite amino acids of nine thermophilic and hyperthermophilic proteins in a 100 degreesC hydrothermal solution yielded between 600 and 8000 kilojoules per mole of protein, which is energy that is available to drive the intracellular synthesis of enzymes and other biopolymers in hyperthermophiles thriving in these ecosystems.

Lithium isotope geochemistry of pore waters from ocean drilling program Sites 918 and 919, Irminger Basin

The distribution of Li isotopes in pore waters to a depth of 1157 m below seafloor is presented for ODP Sites 918 and 919 in the Irminger Basin, offshore Greenland. Lithium isotope data are accompanied by strontium isotope ratios to decipher diagenetic reactions in the sediments which are characterized by the pervasive presence of volcanic material, as well as by very high accumulation rates in the upper section. The lowering of the 87Sr/ 86Sr ratio below contemporaneous seawater values indicates several zones of volcanic material alteration. The Li isotope profiles are complex suggesting a variety of exchange reactions with the solid phases. These include cation exchange with NH 4 + and mobilization from sediments at depth, in addition to the alteration of volcanic matter. Lithium isotopes are, therefore, a sensitive indicator of sediment-water interaction. δ 6Li values of pore waters at these two sites vary between −42 and −25 ‰. At shallow depths (<100 mbsf), rapid decreases in the Li concentration, accompanied by a shift to heavier isotopic compositions, indicate uptake of Li into alteration products. A positive anomaly of δ 6Li observed at both sites is coincident with the NH 4 + maximum produced by organic matter decomposition and may be related to ion exchange of Li from the sediments by NH 4 +. In the lower sediment column at Site 918, dissolved Li increases with depth and is characterized by enrichment of 6Li. The Li isotopic compositions of both the waters and the solid phase suggest that the enrichment of Li in deep interstitial waters is a result of release from pelagic sediments. The significance of sediment diagenesis and adsorption as sinks of oceanic Li is evaluated. The maximum diffusive flux into the sediment due to volcanic matter alteration can be no more than 5% of the combined inputs from rivers and submarine hydrothermal solutions. Adsorption on to sediments can only account for 5–10% of the total inputs from rivers and submarine hot springs.

Significance of tourmaline-rich rocks in the North Range Group of the Cuyuna Iron Range, east-central Minnesota

Concentrations of tourmaline in Early Proterozoic metasedimentary rocks of the Cuyuna iron range, east-central Minnesota, provide a basis for redefinition of the evolutionary history of the area. Manganiferous iron ore forms beds within the Early Proterozoic Trommald Formation, between thick-bedded granular iron-formation having shallow-water alepositional attributes and thin-bedded, nongranular iron-formation having deeper water attributes. These manganese-rich units were previously assumed to be sedimentary in origin. However, a reevaluation of drill core and mine samples from the Cuyuna North range has identified strata-bound tourmaline and tourmalinite, which has led to a rethinking of genetic models for the geology of the North range. We interpret the tourmaline-rich rocks of the area to be a product of submarine-hydrothermal solutions flowing along and beneath the sediment-seawater interface. This model for the depositional environment of the tourmaline is supported by previously reported mineral assemblages within the Trommald Formation that comprise aegirine; barium feldspar; manganese silicates, carbonates, and oxides; and Sr-rich barite veins.In many places, tourmaline-rich metasedimentary rocks and tourmalinites are associated locally with strata-bound sulfide deposits. At those localities, the tourmaline-rich strata are thought to be lateral equivalents of exhalative sulfide zones or genetically related subsea-floor replacements. On the basis of the occurrence of the tourmaline-rich rocks and tourmalinites, and on the associated minerals, we suggest that there is a previously unrecognized potential for sediment-hosted sulfide deposits in the Cuyuna North range.

Oxygen isotope study of metamorphosed manganese deposits of the Noda-Tamagawa mine, northeast Japan.

Manganese deposits of the Noda-Tamagawa mine are metamorphosed equivalents of bedded type. The ores are typically zoned with a central pyrochroite-hausmannite, outer tephroite, and the outermost rhodonite ores are closest to the wall rock chert. The δ18O(SMOW) values of 11 tephroite, 15 rhodonite and 5 quartz samples in the manganese ores and chert were determined to be in the ranges of 9.9-16.4‰, 11.3-19.6‰, and 22.3-23.8‰, respectively. The overall order of 18O enrichment corresponds to the order of equilibrium fractionation among these minerals. However, the isotope temperatures calculated from quartz-rhodonite and rhodonite-tephroite pairs from neighboring ore zones did not give temperatures consistent with those estimated from the mineral assemblage in the wall rocks. This discrepancy suggests that the amount of oxygen-bearing fluid, either H2O or CO2, was small during the metamorphism. The δ18O values for MnCO3, the most probable candidate of the primary manganese mineral, were calculated from the oxygen isotope mass balance among manganese silicates, silica mineral, and CO2 to be 17.0-20.0‰. If the MnCO3 initially precipitated from fluids of δ18O=0.0-2.5‰, the temperatures of precipitation can be calculated to be between 75° and 127°C. These temperatures suggest that the primary MnCO3 was precipitated by submarine hydrothermal solutions.

Chemistry of submarine hydrothermal solutions at Guaymas Basin, Gulf of California

Reconnaisance ALVIN dives in the sediment-filled southern trough of the Guaymas Basin found active hot springs with temperatures ranging up to 315°C. High temperature activity is generally restricted to the crests of large mounds that rise out of the flat-lying basin sediments. The chemistry of the hydrothermal waters is distinctly different from that characteristic of sediment-starved, open ocean ridge axes in that the solutions are alkaline, contain ammonium as a major ion and are strongly depleted in the “ore-forming” metals. These compositions are interpreted as the result of reaction of a primary solution, similar in composition to those as 21°N, EPR, with the biogenous sediments overlying the intrusion zone. The pH of this fluid is raised both by the dissolution of carbonate and the addition of ammonium from thermocatalytic cracking of immature planktonic carbon. Metal sulfides are consequently precipitated at depth in the sediment column. The Guaymas Basin is thus the site of active formation of a sediment-hosted massive sulfide mineral deposit; the exiting waters are the “spent” ore-forming fluid. The ammonium data demonstrate that organic carbon (black shale) is, by itself, a sufficient source of alkalinity to induce the precipitation of sulfides from ascending solutions. Since ammonium does not participate directly in these reactions but does form secondary aluminosilicate minerals these latter should constitute a valuable exploration tool in the search for shale hosted deposits.

Chemistry of submarine hydrothermal solutions at 21 °N, East Pacific Rise

The three hydrothermal fields at 21°N latitude, East Pacific Rise, were resampled and an additional one was discovered. Maximum fluid temperatures observed were within a few degrees of 350°C and these waters had concentrations of Mg and sulfate indistinguishable from zero. One field, NGS, which had active 350°C springs in 1979, was inactive when first located in 1981. However, when a chimney was broken open during sampling, water issued at 273°C and continued to flow for at least five days. The chemical composition strongly suggests that these waters cooled conductively from 350°C in the sealed conduit. The major ion data are consistent with the estimates based on extrapolation of the original measurements made on the hot springs from the Galapagos Spreading Center ( Edmond et al., 1979a). The fluids have a pH of 3.5 and the sulfide-forming element concentrations show significant inter-field variations. Fe levels range from 0.8 to 2.4 mmoles/kg; the ratio Fe:Mn varies from 0.9 to 2.9 similar to metalliferous sediments on the ridge flanks, but much higher than observed at Galapagos (where sub-surface precipitation of iron sulfides occurs) indicating that the overwhelming proportion of the mass flux from hydrothermal systems occurs at high temperatures. Zn ranges from 40 to 106 μmoles/kg with Cu being substantially lower. Since the ratio of these elements in tholeiites is about unity, there is strong net preferential mobilization of Zn. Lead ranges from 183 to 359 nmoles/kg. Nickel and Be are highly immobile relative to the other trace elements. The abundance of H 2S is about three times that of the total sulfide-forming cations. These data demonstrate that acid solutions at elevated temperatures can transport substantial amounts of ore-forming elements in the presence of large excesses of sulfide.

Metal accumulation rates in sediments from Mid-Indian Ocean Ridge and Marie Celeste Fracture Zone

THE existence of metalliferous sediments on mid-ocean ridges is well documented1 and the general conclusion is that they form by precipitation from submarine hydrothermal solutions. Pacific and Atlantic Ocean sediments have been studied in detail2–6. We describe here material from the Indian Ocean Ridge, collected during cruise 8/75 of RRS Shackleton, and compare metal accumulation rate data for ridge-crest and -flank sediments with those from other regions. The metalliferous sediments are present on the Mid-Indian Ocean Ridge, particularly near the Triple Junction7,8, and Bostrom and Fisher7 suggest that “there is a complete belt of iron-rich sediment along the entire length of the Indian Ocean–Pacific Ocean active-ridge system”. We have found ferromanganese encrustations on basalts from the median-valley wall and have recognised what are probably sulphides in decorated vesicles of basalts. The metal accumulation rate data argue against a hydrothermal source of metals in the ridge sediments, however, and suggest that metalliferous sediment accumulations in the Indian Ocean may be more local phenomena than previously realised.

Manganese fluxes to the oceans

Application of a simple model describing regional variations in the contents of manganese and associated minor metals in deep-sea sediments suggests that solid manganese phases are being removed from the <0.5 μm fraction of seawater at ∼1–7 · 10 12g yr −1 in excess of the rate of stream-supplied manganese. This flux is consistent with: (1) the relative rates of sediment accumulation in the Atlantic and Pacific Oceans; (2) the contrast between the oceanic residence time of manganese calculated from stream-supply data (14 · 10 3 yr) and from the flux of manganese precipitating in marine sediments or as manganese nodules (0.38–2.4 · 10 3 yr); (3) the surplus mass of manganese revealed by geochemical balance calculations (22.9 · 10 2g). On this basis excess manganese is accumulating in deep-sea sediments at 0.2–2.0 · 10 −6 g cm −2 yr −1. Manganese supplied to the upper layers of marine sediments by diagenesis has been evaluated with the aid of vertical advection—diffusion—reaction models. The calculated diagenetic flux of manganese at the sediment surface in a near-shore environment is in agreement with the known accretion rate of manganese deposits (1.7 · 10 −2 g cm −2 10 −3 yr −1) and the regionally variable flux over the area assessed is consistent with the presence or absence of manganese nodules at or near the water-sediment interface. The diagenetic flux at the surface of deep-sea sediments has been calculated at 0.7 · 10 −4 g cm −2 10 −3 yr −1 when the upper, oxic, zone of the sediment is ∼20 cm thick. A limiting factor on the in situ production flux of dissolved manganese in deep-sea sediments appears to be the availability of reducing agents for manganese dissolution rather than the rate of downward transport of manganese-rich sediment to a reaction boundary where dissolution takes place. Various estimates of the rate of upward-migrating manganese suggest that manganese precipitates in the oxic zone with a rate constant of ∼10 −7 sec −1 with the result that diagenetic processes cannot supply the flux of excess manganese through more than ∼0.25 m of oxic sediment. However, estimates of the flux of manganese to the oceans by submarine volcanic processes (0.79–1.1 · 10 12g yr −1) are similar to the surplus mass of manganese detected by geochemical balance calculations (0.7 · 10 12g yr −1). If submarine hydrothermal solutions provide only 10% of this excess then their computed discharge rate (39 g cm −2 yr −1) and residence time in the upper layer of oceanic crust (130,000 yr) agree well with these parameters for continental thermal springs.

The origin of metal-bearing submarine hydrothermal solutions

Instrumental activation analyses for 16 major and trace elements in a suite of mid-Atlantic ridge basalts reveals that the slowly cooled interior portions of these submarine extrusions are depleted, relative to the quenched flow margins, in several elements that are enriched in pelagic sediments and manganese nodules (Mn, Fe, Co, the rare earth elements, and others). Many of these elements are excluded from the solid phases that crystallize from the melt, and thus are concentrated in residual liquids. Additional elements are mobilized during the deuteric alteration of early-formed olivine and the formation of immiscible sulfide liquids. It is suggested that these components of melt occupy accessible sites (e.g., intergranular boundaries) in the hot solid rock mass, and are mobilized by dissolution as chloride complexes in sea water introduced along contraction cracks that form during cooling and solidification. These solutions may be the metal-bearing ‘hydrothermal exhalations’ or ‘volcanic emanations’ that accompany submarine volcanism, which are often cited as a source of metals into the pelagic environment. Reasonable estimates of the amount of material involved suggest that a significant fraction of the mass of these elements that reside in pelagic sediments could have been supplied by this process. Recently described amorphous iron-manganese-silica material from the east Pacific rise may form by direct precipitation from these hydrothermal solutions following their introduction into sea water.