Chemical composition of Black Sea bottom sediments

Concentrations of major-, trace- and rare earth elements in recent and Old Black Sea bottom sediments are reported in the paper. Data presented suggest that accumulation of black shale deposits was not constrained to a certain time span but proceeds in certain modern basins and generates sediments with metal contents close to those in their ancients analogues in hydrogen sulfide contaminated environments. If REE are involved in the process, their composition can vary depending on such factors as variations in redox conditions and occurrence of phosphate and barite nodules, which can induce development of either positive or negative Eu anomalies.

Trace and rare elements and factors at DSDP Hole 71-511

The distribution of paragenetic assemblages of trace and rare elements, as revealed by factor analysis (R-mode, Q-mode), the ratios of elements to Zr and the interpretation of these data in the context of the known mineralogy, lithology, and geology of the region, provide the bases for the outline of the geochemical history of sedimentation in the study area that forms the subject of this chapter. Two stages may be discerned. 1. Late-Middle Jurassic-Early Cretaceous (160-106? Ma). The sediments that accumulated in relatively shallow water (shelf) were predominantly clay, with dispersed sapropelic organic matter, plant fragments, pyrite, admixtures of acid-medium volcanic glass, and epigenetic crystals of gypsum. The bottom water layers of the basin are notably stagnant. The sediments are characterized by higher amounts of V, Zn, Cu, Cr, Rb, and Be associated with organic matter. Lower Cretaceous sediments, separated from those of the Upper Jurassic by a hiatus, accumulated in a deepened and enlarging basin. These Lower Cretaceous deposits are chemically similar to those of the Upper Jurassic, but contain diagenetic concentrations of Zn, Ni, and La. 2. Early-middle Albian (Unit 5)-middle Maestrichtian (1067-66.6Ma). The prevailing regime was that of an open ocean basin that tended to expand and deepen. During the second half of the early-middle Albian, the biogenic components Ba, Sr, and CaCO3 accumulated. By the end of this interval, Ti/Zr values had increased. In conjunction data on mineral composition, they testify to an outburst of basaltoid volcanism related to tectonic activity before an erosional hiatus (late Albian-Cenomanian). At the end of the Cenomanian-Turonian, residual deposits of predominantly clay sediments with relatively high amounts of Ti and Zr and associated rare alkalis (Li, Rb) accumulated. Clay sediments deposited during the Coniacian-Santonian were characterized by higher concentrations of Ti, Zr, Li, and Rb, by diagenetic carbonate phases of Ni, Zn, and La, and by sulphides and Fe-oxides with an admixture of Ni and Co. The latter half of the interval saw the deposition of fine basaltoid volcanoclastic material, diagenetically altered by zeolitization and carbonatization and enriched with Se, Pb, Ti, Sr, Ba, Y, and Yb. Sediments with a similar chemistry accumulated in the Campanian-middle Maestrichtian. Strong current activity preceding a global hiatus at the Mesozoic/Cenozoic boundary is reflected in both lower sedimentation rates and the presence of higher residual concentrations of Ti, Zr, Ba, Sr, and other elements studied in this chapter.

Chemical and isotopic compositions of sediments and some minerals from the Derugin Basin, Sea of Okhotsk

The paper reports specific mineralogical and geochemical characteristics of deposits from local depressions of the Derugin Basin. They were formed in an environment with periodic changes from oxic to anoxic conditions and show evidence for presence of hydrogen sulfide in bottom waters. Deposits of this type can be considered as a modern model for ancient ore-bearing black shale associations. Compared with typical metalliferous black shale sequences, which are characterized by high contents of organic matter, the sediments described here are depleted in elements of the organophilic association (Mo, Ni, Cu, Zn, V, and U), but have higher Mn contents.

Geochemistry on water profiles from the Kara Sea

As a part of the Russian-German project "Siberian River-Runoff (SIRRO)" the major element composition of the dissolved load and the major and trace element composition of particulate load and bottom sediment of the Yenisei River and Estuary were analyzed and examined in context of the basin lithology and climate. In addition, the processes controlling the transformation of the river load in the estuarine mixing zone were investigated. The chemical composition of the dissolved and particulate load of the Yenisei fluvial endmember is generally comparable to that of other major world rivers. The dissolved load is chiefly controlled by carbonate weathering and the chemical composition of the river suspended particulate matter (SPM) is similar to that of the North American Shale Composite (NASC), which represents the weathering product of the upper continental crust. The Chemical Index of Alteration (CIA) of the Yenisei SPM amounts to 71, which indicates moderate chemical weathering. With regard to the SPM geochemistry, the Yenisei occupies an intermediate position between the adjacent rivers Khatanga and the Lena. Drastic changes in the composition of the river load are seen in the mixing zone between fresh and salt water. While dissolved Na, Ca, Mg, K, CI, S04, F, Br, Sr and HC03 behave conservatively, dissolved Fe is completely removed from solution at very low salinities. Particulate Mn exhibits a pronounced mid-salinity minimum concomitant with a maximum of dissolved Mn, which is probably related to suboxic conditions in the area of the so-called "marginal filter", where highest turbidities are found. The Mn-minimum in SPM is paralleled by depletions of the elements Ba, Zn, Cd, Ni, Cu and V, which can be associated with manganese particles. The estuarine bottom sediments are composed of mud and sand and the sedimentological parameters of the bottom sediments have to be considered for the interpretation of the bulk geochemical data. The chemical composition of the mud is comparable to the SPM, whereas the sand is relatively enriched in Si/Al, Ba/Al, Zr/Al and Sr/Al ratios and depleted in transition metals.

Geochemistry of middle Eocene-lower Oligocene pelagic sediments of ODP Hole 105-647A

Geochemical analyses of the middle Eocene through lower Oligocene lithologic Unit IIIC (260-518 meters below seafloor [mbsf]) indicate a relatively constant geochemical composition of the detrital fraction throughout this depositional interval at Ocean Drilling Program (ODP) Site 647 in the southern Labrador Sea. The main variability occurs in redox-sensitive elements (e.g., iron, manganese, and phosphorus), which may be related to early diagenetic mobility in anaerobic pore waters during bacterial decomposition of organic matter. Initial preservation of organic matter was mediated by high sedimentation rates (36 m/m.y.). High iron (Fe) and manganese (Mn) contents are associated with carbonate concretions of siderite, manganosiderite, and rhodochrosite. These concretions probably formed in response to elevated pore-water alkalinity and total dissolved carbon dioxide (CO2) concentrations resulting from bacterial sulfate reduction, as indicated by nodule stable-isotope compositions and pore-water geochemistry. These nodules differ from those found in upper Cenozoic hemipelagic sequences in that they are not associated with methanogenesis. Phosphate minerals (carbonate-fluorapatite) precipitated in some intervals, probably as the result of desorption of phosphorus from iron and manganese during reduction. The bulk chemical composition of the sediments differs little from that of North Atlantic Quaternary abyssal red clays, but may contain a minor hydrothermal component. The silicon/ aluminum (Si/Al) ratio, however, is high and variable and probably reflects original variations in biogenic opal, much of which is now altered to smectite and/or opal CT. An increase in the sodium/potassium (Na/K) ratio in the upper Eocene corresponds to the beginning of coarsergrained feldspar flux to the site, possibly marking the onset of more vigorous deep currents. Although the Site 647 cores provide a nearly complete high-resolution, high-latitude Eocene-Oligocene record, the high sedimentation rate and somewhat unusual diagenetic conditions have led to variable alteration of benthic foraminifers and fine-fraction carbonate and have overprinted the original stable-isotope records. Planktonic foraminifers are less altered, but on the whole, there is little chance of sorting out the nature and timing of environmental change on the basis of our stable-isotope analyses.

Lithium sources in pore fluids of Peru slope

High Li concentrations, up to a maximum of 1155 µM are observed in the pore fluids of the Peru convergent margin slope sediments. At Ocean Drilling Program Sites 683 and 685 (ca. 9°S), the Li concentration depth gradients are twice as steep as at Site 682 and 688 (ca. 11°S). Within the sediments, the most important Li sources are from aluminosilicate minerals. Biogenic opal-A contains little Li and thus dilutes the Li concentration of the bulk sediments. The sediment compositions and the thermal regimes are similar at 9° and 11°S, suggesting there is an additional, non-sedimentary source for the observed high Li concentrations in the northern pore fluids. At 9°S, the 87Sr/86Sr ratios reach a maximum value of 0.709958. The observed radiogenic 87Sr/86Sr values in the pore fluids support the suggestion that the additional Li may derive from exchange reactions with underlying continental crust. The high concentrations of Li at 11°S may derive from basalt alteration at moderate to high temperatures, as suggested by the non-radiogenic 87Sr/86Sr ratios in these pore fluids, which reach a minimum value of 0.707218. Based on (1) Li concentrations in the pore fluids in slope sediments from Peru and several other margins, and (2) an approximate estimate of fluid flux from continental margins into the ocean, continental margins provide an estimated 1 to 3 * 10**10 moles Li/yr to the ocean. This source of oceanic Li, which has not been considered previously, is of the same order of magnitude as some estimates of hydrothermal and river Li fluxes and may have important consequences for the oceanic Li isotope budget. The sink is unknown for this newly discovered and possibly large Li source, but it may be more pervasive low-temperature alteration of oceanic basement than previously estimated, or burial of mineral phases, such as authigenic clay minerals, or metal oxyhydroxides which may be Li-rich.

Geochemistry of ODP Leg 125 peridotites

Ocean Drilling Program Leg 125 recovered serpentined harzburgites and dunites from a total of jive sites on the crests and flanks of two serpen finite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzbutrgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cx-number = 0.40-0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine-spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite- magnetite- quartz) - 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamotmt contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover; they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ -0.2 toFMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20 % fractional melting) which has subsequently been modified by interaction with boninitic melt ivithin the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzbutgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + l.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20-25 % fractional melting) in a supra-subduction zone environment The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.

Geochemistry of sediments beneath the Tonga-Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga-Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2-10, (b) the top 1-22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be-Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ~800 °C during the breakdown of amphibole +/- plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10-15% partial melts of depleted mantle after it has been enriched by the addition of 0.2-2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ~800 °C by the time it attains ~ 80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partialmelting of the altered oceanic crust.

Geochemistry on Yangla copper deposit, China

The Yangla copper deposit, situated in the middle section of Jinshajiang tectonic belt between Zhongza-Zhongdian block and Changdu-Simao block, is a representative and giant copper deposit that has been discovered in Jinshajiang-Lancangjiang-Nujiang region in recent years. There are coupled relationship between Yangla granodiorite and copper mineralization in the Yangla copper deposit. Five molybdenite samples yielded a well-constrained 187Re-187Os isochron age of 233.3±3 Ma, the metallogenesis is therefore slightly younger than the crystallization age of the granodiorite. S, Pb isotopic compositions of the Yangla copper deposit indicate that the ore-forming materials were derived from the mixture of upper crust and mantle, also with the magmatic contributions. In the late Early Permian, the Jinshajiang Oceanic plate was subducted to the west, resulting in the formation of a series of gently dipping thrust faults in the Jinshajiang tectonic belt, meanwhile, accompanied magmatic activities. In the early Late Triassic, which was a time of transition from collision-related compression to extension in the Jinshajiang tectonic belt, the thrust faults were tensional; it would have been a favorable environment for forming ore fluids. The ascending magma provided a channel for the ore-forming fluid from the mantle wedge. After the magma arrived at the base of the early-stage Yangla granodiorite, the platy granodiorite at the base of the body would have shielded the late-stage magma from the fluid. The magma would have cooled slowly, and some of the ore-forming fluid in the magma would have entered the gently dipping thrust faults near the Yangla granodiorite, resulting in mineralization.