Peter Halbach

Sequential leaching of marine ferromanganese precipitates: Genetic implications

Abstract Sequential leaching experiments were carried out on twenty-one hydrogenetic crust samples from different locations in the central Pacific and the results are compared with four crust and nodule samples of different genetic origin. Fe-vernadite (δ-MnO2) is the most important minor and trace metal-bearing mineral phase and shows high concentrations of Co, Ni, Cd, TI, Ba, Zn, and Cu, which are present in seawater mainly as hydrated and labile complexed cations. Elements forming carbonate and hydroxide complexes and oxyanions in seawater like Pb, Mo, V, and minor proportions of Cu and Zn are bound in the Fe00H fraction. The Ca carbonate phase, and the residual fraction which consists of aluminosilicates and crystalline oxides, show only minor heavy metal associations, e.g., Cu and Zn. Only a small part of the high Ti concentrations in hydrogenetic crusts is of detrital origin; Ti mainly forms a hydrogenetic phase, probably consisting of Ti02 ∗ 2H20 intergrown with the amorphous Fe00H phase. Analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements. The most notable change was observed for Pb which, during phosphatization of crusts, is transferred from the MnFe oxide fraction to the apatite fraction. A colloid-chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed. In the first stage, Mn2+—rich water from the oxygen minimum zone is mixed with oxygen-rich deep-water, and oxidized Mn(IV) and other metals like Fe, Ti, Al, and Si form oxide and hydroxide colloids. These form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species. In stage two, the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations, incorporating the sorbed heavy metals into the mineral phases.

Iron and manganese oxide mineralization in the Pacific

Abstract Iron, manganese, and iron-manganese deposits occur in nearly all geomorphologic and tectonic environments in the ocean basins and form by one or more of four processes: (1) hydrogenetic precipitation from cold ambient seawater, (2) precipitation from hydrothermal fluids, (3) precipitation from sediment pore waters that have been modified from bottom water compositions by diagenetic reactions in the sediment column and (4) replacement of rocks and sediment. Iron and manganese deposits occur in five forms: nodules, crusts, cements, mounds and sediment-hosted stratabound layers. Seafloor oxides show a wide range of compositions from nearly pure iron to nearly pure manganese end members. Fe/Mn ratios vary from about 24 000 (up to 58% elemental Fe) for hydrothermal seamount ironstones to about 0.001 (up to 52% Mn) for hydrothermal stratabound manganese oxides from active volcanic arcs. Hydrogenetic Fe-Mn crusts that occur on most seamounts in the ocean basins have a mean Fe/Mn ratio of 0.7 for open-ocean seamount crusts and 1.2 for continental margin seamount crusts. Fe-Mn nodules of potential economic interest from the Clarion-Clipperton Zone have a mean Fe/Mn ratio of 0.3, whereas the mean ratio for nodules from elsewhere in the Pacific is about 0.7. Crusts are enriched in Co, Ni and Pt and nodules in Cu and Ni, and both have significant concentrations of Pb, Zn, Ba, Mo, V and other elements. In contrast, hydrothermal deposits commonly contain only minor trace metal contents, although there are many exceptions, for example, with Ni contents up to 0.66%, Cr to 1.2%, and Zn to 1.4%. Chondrite-normalized REE patterns generally show a positive Ce anomaly and abundant ΣREEs for hydrogenetic and mixed hydrogenetic-diagenetic deposits, whereas the Ce anomaly is negative for hydrothermal deposits and ΣREE contents are low. However, the Ce anomaly in crusts may vary from strongly positive in East Pacific crusts to slightly negative in West Pacific crusts, which may reflect the redox conditions of seawater. The concentration of elements in hydrogenetic Fe-Mn crusts depends on a wide variety of water column and crust surface characteristics, whereas concentration of elements in hydrothermal oxide deposits depends of the intensity of leaching, rock types leached, and precipitation of sulphides at depth in the hydrothermal system.

Effects of phosphatization on the geochemical and mineralogical composition of marine ferromanganese crusts

Most hydrogenetic ferromanganese crusts in the central Pacific consist of two growth generations: a phosphatized older growth generation that is impregnated with carbonate fluorapatite (CFA), and a younger non-phosphatized growth generation. Geochemical and mineralogical investigations of non-phosphatized and phosphatized crust layers revealed significant differences that are not simply dilution effects by CFA. Certain elements are depleted in old crusts compared to young crusts in the order Si > Fe > Al > Ti ≥ Co > Mn ≥ Pb; others like Ni, Zn, Cu, Y, and REEs (rare earth elements) are mostly enriched. Suboxic conditions in the phosphate-rich water of the oxygen-minimum-zone that infiltrated the crusts caused a partial redissolution of crust phases, resulting in mobilization of associated elements, a partial recrystallization of more stable phases, and a secondary input of nutrient-type elements. The frequent occurrence of todorokite in phosphatized crust layers indicates that Mn mobilized from the vernadite crust phase was partially recrystallized as todorokite, which is more stable under suboxic conditions, incorporating Ni, Cu, and Zn into the crystal lattice, but rejecting Co. Other metals like Pb, Y, and REEs probably form stable phosphate phases in phosphatized crusts. Our investigations emphasize that under the influence of phosphatization, diagenetic remobilization and reorganization in FeMn crusts took place. In contrast to non-phosphatized crusts, the older phosphatized crust generation no longer represents a complete primary crust precipitate. Therefore, young non-phosphatized and old phosphatized crust generations should be considered as two different sample types with respect to geochemical composition, mineralogy, and evolution. We also suggest that most methods for age determination like the 87Sr86Sr isotopic dating method or Co chronometry cannot be used for phosphatized crusts without caution.

Origin of negative Ce anomalies in mixed hydrothermal–hydrogenetic Fe–Mn crusts from the Central Indian Ridge

Abstract Layered Fe–Mn crusts from the off-axis region of the first segment of the Central Indian Ridge north of the Rodrigues Triple Junction were studied geochemically and mineralogically. Vernadite (δ-MnO2) is the main mineral oxide phase. 230Thxs and Co concentrations suggest high growth rates of up to 29 mm/Myr and a maximum age of the basal crust layer of 1 Ma. Whereas most of the major and minor elements show concentrations which are typical of hydrogenetic formation, Co, Pb, Ni and Ti concentrations are strikingly lower. Concentrations and distribution of the strictly trivalent rare-earths and yttrium (REY) are typical of hydrogenetic ferromanganese oxide precipitates, but in marked contrast, the crusts are characterized by negative CeSN (shale normalized) anomalies and (Ce/Pr)SN ratios less than unity. Profiles through the crusts reveal only minor variations of the REY distribution and (Ce/Pr)SN ratios range from 0.45 to 0.68 (compared to ratios of up to 2 for typical hydrogenetic crusts from the Central Indian Basin). The apparent bulk partition coefficients between the crusts and seawater suggest that for the strictly trivalent REY the adsorption–desorption equilibrium has been reached. Positive Ce anomalies in the partition coefficient patterns reveal preferential uptake of Ce, but to a lesser extent than in normal hydrogenetic crusts. A new parameter (excess Ce, Cexs) to quantify the degree of decoupling of Ce from REY(III) is established on the basis of partition coefficients. Cexs/Cebulk ratios suggest that the CIR crusts formed by precipitation of Fe–Mn oxides from a hydrothermal plume and that in hydrothermal plumes and normal seawater the enrichment of Ce results from the same oxidative sorption process. The growth rates, calculated with 230Thxs data as well as with the Co formula, are inversely related to Cexs.

Fluid inclusion and sulfur isotope studies in probable modern analogue Kuroko-type ores from the JADE hydrothermal field (Central Okinawa Trough, Japan)

Abstract Fluid inclusions and sulfur isotopic compositions were studied in sulfides and barites from the JADE active hydrothermal field in the Central Okinawa Trough. The mineral assemblages in the JADE field strongly resemble ancient Kuroko-type deposits. The formation of massive sulfide mineralization is related to alteration of felsic volcanic rocks by deep penetrating heated seawater. Fluid inclusions in sphalerite from stockwork mineralization show homogenization temperatures that range between 270°C and 360°C and strong variations in salinity (2–15 equiv. wt.% NaCl). Varying salinities are attributed to two-phase separation that partially occurred at deeper levels. The resulting brine favored the transportation of other metals besides Pb, Zn, Fe, Cu, and led to a complex tetrahedrite/tennantite and enargite-bearing sulfide mineralization in the stockwork region. Fluid inclusions in enargite, therefore, show salinities more than three times higher than seawater. Even though the δ34S values of sulfide separates from stockwork mineralization suggest a common sulfur source, ore deposition occurred under non-equilibrium isotopic conditions. Barite in stockwork mineralization, smokers, and mounds on the seafloor precipitated via mixing of hydrothermal solution with seawater. Microthermometric data of barite-hosted fluid inclusions point to maximum formation temperatures of between 150°C and 200°C. Partial reduction of seawater sulfate is indicated by locally observed δ34S values in barite heavier than the ambient seawater.

Marine tephra from the Cape Riva eruption (22 ka) of Santorini in the Sea of Marmara

Abstract A discrete tephra layer has been discovered in three marine sediment cores from the Sea of Marmara, eastern Mediterranean. The rhyodacitic glass chemistry and the stratigraphical position suggest a Santorini provenance and, in particular, a correlation with the marine Y-2 tephra that is known from the southern Aegean Sea and eastern Levantine Basin. This tephra represents the distal facies of the Cape Riva eruption of Santorini, which has been dated by 14C on land at 21 950 cal. yr BP. Hitherto, the Y-2 tephra has been detected only in marine sediment cores recovered south to southeast of its volcanic source. The new occurrence in the Sea of Marmara approximately 530 km NNE of the Santorini eruptive centre suggests a more north-easterly dispersal of fallout products of the Cape Riva eruption than previously supposed.

Geochemical processes controlling the relationship between Co, Mn, and Fe in early diagenetic deep-sea nodules

Abstract Early diagenetic manganese nodules from the northeast Pacific nodule belt and from the southeast Pacific (Peru Basin) show primary growth features of dendritic microtextures consisting of alternating laminae of crystalline 10-Amanganate (A1 substance) and amorphous material which is composed of an intimate mixture of ferric hydroxide, silicate, and δ-MnO 2 (A2 substance). The formation of rhythmic sequences of A1 and A2 microlayers is explained by physico-chemical changes in the peneliquid sediment layer and in the microenvironment of the accreting nodule surface: (a) upward diffusion of Mn 2+ in the interstitial water as a result of decay of organic matter and Mn mobilization; (b) oxidation of Mn 2+ and formation of 10-Amanganate in the upper part of the peneliquid sediment layer, leading to pH depression in the microenvironment of the nodule surface which decreases the mobility of silicate, resulting in formation of A2 layers; (c) restoration of pH and renewed precipitation of 10-Amanganate. Based on data of 171 bulk analyses and on electron microprobe investigations, interelement relationships between Co, Mn, and Fe are pointed out. Fe and Co show a significant positive correlation ( r = 0.84 ), while Mn and Co are poorly negatively correlated ( r = −0.21 ), assuming linear regression. The enrichment of Co within the amorphous A2 phase is attributed to specific surface adsorption and subsequent oxidation of Co 2+ to Co 3+ in the strong electric field of Si 4+ . Robust complexes of Co(III) and ≡ FeH 2 SiO 4 − prevent most of the Co from being available for the 10-Amanganate precipitation. Concerning the relation between Mn and Fe and Co respectively, the highest correlation coefficients are obtained using an inverse logarithmic regression. Under early diagenetic conditions, the concentration and precipitation of Mn 2+ in the interstitial water depends on the redox gradient which is controlled by the amount of decomposing organic matter. The Nernst equation describes the relationship as a reciprocal logarithmic function. However, the quantities of colloidal ferric hydroxide and of dissolved silicate are not affected by variations of the redox potential. These different characteristics in the precipitation of Mn and the Fe- and Si-rich colloidal phase may cause the significantly negative logarithmic correlation of Mn versus Fe and Co, respectively.

The uptake of uranium by organic substances in a peat bog environment on a granitic bedrock

Abstract The contents of uranium in the Brocken granite are bound to mineral phases like biotite, apatite, sphene and zircon. Due to the weathering during humid climatic conditions, U is primarily mobilized as UO 2 2+ from the bedrock; thus the U concentration in the Brocken granite has diminished from about 14 ppm to 2–3 ppm in the weathered parts. The surface waters of the investigated area show oxidative and acid conditions. These facts support a mobilisation of U. The bog waters transport U mainly as dissolved uranyl fulvate, but a steady feed of rainwater (rainfall rate: more than 1500 mm/a) prevents a higher U concentration in the aqueous environment. In the peats of the high bog plateau and in the phytogenic sediments of Lake Oderteich there exist average U concentrations of between 3 and 4 ppm. In the sediment samples from the bottom of Lake Oderteich, the U concentration (2.5–5.4 ppm) correlates positively with the portion of sedimentary material extractable in alkaline solution. Thus U is to be associated with the contents of humic and fulvic acids. The sample material was fractionated into humic acids, fulvic acids and insoluble residues. Chemical investigations show that being soluble in water, the isolated organic acids are acid derivates of carbohydrates. Experiments in enrichment of U by these fractions show that the functional acid groups of the organic substances take up equivalent quantities of uranyl ions by ion-exchange mechanisms, depending on the pH value. A content of U of 47.5 wt.% was found as a maximum result in humic acid with about 6 meq. total acidity.

Sulfide-impregnated and pure silica precipitates of hydrothermal origin from the Central Indian Ocean

Abstract This is the first report about silica-rich hydrothermal precipitates which were sampled together with hydrothermal sulfides (chimney fragments) in an extinct vent field in the Central Indian Ocean. There are two kinds of silica-rich rocks: a jasper which is impregnated and replaced to various degrees mainly by sphalerite, and to a lesser extent by barite, pyrite and traces of chalcopyrite, and an opalite which is an almost pure silica-phase without any sulfide or sulfate impregnations, but which is sometimes covered by manganese crusts. No internal concentric zoning indicating typical chimney structures can be recognized in the jasper and/or opalite samples, the textures rather suggest a sedimentary silica and/or iron deposition from diffuse, low-temperature (±60 °C) vent fluids, partly with still visible indications of former bacterial mats and synsedimentary deformation structures; the sphalerite- and barite-impregnations within the jasper, however, are considered to have precipitated from white-smoker-type fluids since they were deposited under intermediate temperatures between 155 and 265 °C, according to fluid inclusion studies. The sulfur isotopic composition ( δ 34 S) of our sulfide samples has mean values of 6.1% for sphalerite and 5.7% for pyrite indicating a mixture of predominantly basaltic sulfur with subordinate amounts of reduced seawater sulfur. The oxygen isotope signals of some pure jasper concentrate samples indicate that the mean formation temperature calculated from these values lies at 63.2 °C. The relationship between the massive pyrite- and chalcopyrite-ores from the extinct chimney structures and the silica-rich precipitates can be explained by different cycles of hydrothermal activity: one high-temperature (above 300 °C) cycle dominated by pyrite and chalcopyrite formation and one later epithermal (below 300 °C) cycle which resulted in sphalerite- and silica-dominated precipitates. Furthermore, zonation and zone-refining processes are part of the evolution of the mineralized field.

Enrichment of Mo in hydrothermal Mn precipitates: possible Mo sources, formation process and phase associations

Hydrothermal Mn crusts were sampled at a water depth of 1970 m from the SO-99 field, which lies within a hydrothermally active area just south of the Triple Junction of the Central Fiji Ridge in the North Fiji Basin (NFB). The crusts are up to 30 mm thick and consist of dense massive Mn oxide layers with submetallic lustre. Individual layers are only a few micron thick, occur as rhythmic sequences and form spheroidal or botryoidal structures when infilling cavities. The crusts consist of pure todorokite ((Mn2+,Ca,Mg)Mn34+O7·H2O) and birnessite (Na4Mn14O27·9H2O) with no other crystalline mineral phases detected. The molybdenum concentrations are between 826 and 1090 μg/g. The Mn/Fe ratio varies between about 100 and >1000. The crusts also contain high Ba concentrations between 1400 and 1930 μg/g and slightly higher Cu and Zn contents than MOR and other back-arc crusts. Leaching experiments demonstrated that Mo is mainly (>90%) associated with the Mn oxide phase. X-ray absorption spectroscopy (XAS) shows that Mo is present predominantly as octahedrally coordinated polymolybdate and that any adsorbed tetrahedral Mo constitutes only a small fraction of the total adsorbed species. Thermodynamic calculations demonstrate that heptamolybdate is the dominant Mo species which is only stable in slightly acid fluids. It is therefore suggested that Mo was retained by the Mn crusts through adsorption from Mo-rich hydrothermal fluids.