Age and chemical composition of volcanic rocks and minerals from the Vityaz Ridge

This paper reports results of geological studies carried out during two marine expeditions of R/VAkademik M.A. Lavrent'ev (Cruises 37 and 41) in 2005 and 2006 at the underwater Vityaz Ridge. Dredging has yielded various rocks from the basement and sedimentary cover of the ridge within three polygons. On the basis of radioisotope age determinations, petrochemical, and paleontological data all the rocks have been subdivided into the following complexes: volcanic rock of Paleocene, Eocene, Late Oligocene, Middle Miocene, and Pliocene-Pleistocene; volcanogenic-sedimentary rocks of Late Cretaceous - Early Paleocene, Paleogene (undifferentiated), Oligocene - Early Miocene, and Pliocene-Pleistocene. Determinations of age and chemical composition of the rocks have enabled to specify formation conditions of the complexes and to trace geological evolution of the Vityaz Ridge. Presence of young Pliocene-Pleistocene volcanites allows to conclude about the modern tectono-magmatic activity of the central part of the Pacific slope of the Kuril Islands.

Chemical composition of ultrabasites and composing minerals from the Tonga Trench

A petrologic-geochemical study (petrochemistry, contents of siderophile and certain lithophile elements, composition of rock-forming silicates and accessory chrome spinels) of ultrabasic rocks dredged from the arc side in the northern end of the Tonga deep-sea trench has been carried out. The ultrabasites included harzburgites and dunites. Peridotites show clearly manifested material characteristics of ultrabasic relicts strongly depleted in low-temperature basaltic components. It is suggested that they have arose in the high degree of partial melting (about 30%) of a matrix mantle source of the lherzolite type. Great similarity of the rocks studied with ultrabasites of many ophiolites that are widespread in folded belts indicates that young island arcs are among the most likely geodynamic environments of ophiolite generation.

Precious metals and volatiles in rocks of the Mid-Atlantic Ridge near 15°20'N

Distribution patterns of gold, platinum group metals (PGE), and volatile components are studied in the main rock types of the Mid-Atlantic Ridge lithosphere (ultrabasites, gabbro, and basalts) from the transform fracture zone at 15°20'N. It is ascertained that PGE content depends on the reduction degree of fluids, on rock types, and on their formation conditions. It is noted that concentrations of refractory elements (Os, Ir, Rh) decrease, while those of fusible elements (Pt, Pd, Au) increase with depth. The chondrite type of distribution is only noted in the ultrabasite rocks. Increase in water and CO2 contents and in oxidation degree of fluids occurs with transition from basalts to ultrabasites, as well as from normal magmatic systems to fluid-enriched anomalous systems.

Composition of Fe-Mn minerals from island arcs of the West Pacific

This paper presents materials on the chemical and mineralogical composition of Fe-Mn mineralization in island arcs (Kuril, Nampo, Mariana, New Britain, New Hebrides, and Kermadec) in the western part of the Pacific Ocean. The mineralization was proved to be of hydrothermal and/or hydrogenic genesis. The former is produced by hydrothermal Fe and Mn oxi-hydroxides that cement volcanic-terrigenous material in sediments. Some Fe oxyhydroxides can be derived via the halmyrolysis of volcaniclastic material. Crusts of this stage are characterized by fairly low concentrations of trace and rare elements, and their REE composition is inherited from the volcanic-terrigenous material. The minerals of the Mn oxyhydroxides are todorokite and "Ca-birnessite". The Mn/Fe ratio increases away from the discharge sites of the hydrothermal solutions. The hydrogenic Fe-Mn crusts are characterized by high concentrations of trace and minor elements of both the Mn group (Co, Ni, Tl, and Mo) and the Fe group (REE, Y, and Th). The hydrogenic crusts consist of Fe-vernadite and Mn-feroxyhyte. Some of the hydrothermal crusts originally had a hydrothermal genesis. The first data were obtained on crust B30-72-10 from the Macauley Seamount in the Kermadec island arc, which contained anomalously high concentrations of Co (2587 ppm) and other Mn-related trace elements in the absence of hydrogeneous Fe oxyhydroxides.

Geochemistry of altered smectites

In basalts and volcanogenic sediments from the Indian Ocean, the successive stages of submarine alteration of volcanic rocks and glasses give rise to the incorporation or the relative increase of iron in smectite lattices. During the first stage, the Mg-smectites are the most abundant; they are occasionally associated with Al-smectites. Afterwards, they are gradually replaced by iron-rich smectites. The REE distribution follows the same trend as the mineralogical changes. During the f'trst stage of alteration, REE distribution in clay minerals is the same as in the fresh glasses but, when the iron-rich smectites increase, the Ce has a specific behaviour. The Ce shows a positive anomaly in iron-rich smectites formed early in palagonitized glasses, and a negative one in authigenic smectites formed later from solutions in equilibrium with seawater.

Chemical and mineral composition of rocks and sediments from the Amirante Arc

New geological and geophysical data on the Amirante Arc, which locates to the south of the Seychelles Islands, are presented. These data were obtained by Pacific Oceanological Institute during the 33-rd cruise of R/V Professor Bogorov in 1990. The Amirante Arc represents a seamount chain, which has submeridional strike and total length about 400 km. To the west of the Amirante Arc there are a deep sea trench and a back-arc basin, i.e. this area is characterized by structural elements associated with the subduction zone of Western Pacific type. According to our data the Amirante Arc is composed by tholeiites of ocean plateau type. This facts are evidences that the Amirante Arc differs from typical Pacific island arcs. This gives an opportunity to distinguish a special type of oceanic structures, i.e. non-volcanic (amagmatic) ridges. The Amirante Ridge has been probably formed as a result of oceanic crust heaping due to horizontal displacements of its blocks in the process of spreding ridge formation in the Indian Ocean during Cretaceous-Paleogene.

Composition and properties of rocks and sediments from the Cape Verde fracture zone

The book presents results of comprehensive geological and geophysical studies, carried out in the Cape Verde fault zone in the 3-rd cruise of R/V Akademik Nikolaj Strakhov (1986). Detailed characterization of bottom relief, thickness and structure of the sedimentary cover, magnetic field, crust structure, lithology and stratigraphy of sediments, petrography and geochemistry of magmatic rocks. Conclusions about tectonic layering of the crust and upper mantle in the fault zone, and about a concurrent structural section of large mantle inhomogeneities have been done. The book is the first monographic description of a major fault structure of the ocean floor.

Age and isotopic and chemical compositions of volcanic rocks from the Japan and Okhotsk Seas

This paper reports results of petrographic and geochemical studies of Miocene-Pleistocene volcanic rocks that accompanied formation of deep-water basins of the Sea of Japan and Sea of Okhotsk. Geochemical types of these rocks, their geodynamic settings, and their derivation from different magmatic sources were determined. Marginal-sea basaltoids from the Sea of Japan are derivatives of fluid-enriched mantle (EMI), while volcanics from the Kuril basin were generated from mantle enriched in continental crust matter (EMU). In spite of different conditions of their genesis, they have some common geochemical features, in particular, their calc-alkaline signatures. These traces of influence of the sialic crust on magma generation confirm development of the basins of both these seas on the continental basement.

(Table S2, page 46-47) Manganese nodule sampling station data

Legacy product - no abstract available

Chemical composition of dredged rocks from the Hess Deep, Eastern Equatorial Pacific

A joint analysis of data on the anomalous magnetic field, seismicity, and structures of the Hess deep basalts have allowed to specify propagation of the spreading zone and to correct position of the neovolcanic zone. A precise petrogeochemical analysis of various types of basalts composing the uneven-aged oceanic crust of the basin showed that magmatics of the neovolcanic zone are related to the primitive type in contrast to rift boards of differential basalts. A model of the deep structure of the Galapagos rift in the area of the western Hess Deep has been suggested.

Chemical composition of Fe-Mn nodules and crusts from oceans and seas

Fluorine concentrations were determined ionometrically with an error of 0.02% in iron-manganese materials of the ocean. They were: 0.02-0.04% in ocean iron-manganese nodules, with the exception of two specimens (0.08% and 0.20% F); up to 0.02% in iron-manganese nodules of seas; 0.02-1.17% in ore crusts from ocean seamounts; and 0.02% in ore sediments of the Red Sea. Elevated fluorine content of ore crusts is associated with presence of calcium phosphate inclusions in them. Fluorine is not accumulated during iron-manganese nodule mineralization. Its average concentration in the nodules is half that in host deep-sea sediments.

(Table 2) Chemical composition of phosphates from the Namibian and Chilean shelves

This study on phosphorites of different compositions and ages from shelf sediments and seamounts of the Pacific Ocean by means of analytical electron microscopy showed that these phosphorites contain ultra-microscopic inclusions of authigenic minerals and, more rarely, of rare earth element (REE) minerals. In some of phosphorite samples of Pleistocene-Pliocene age from the Namibian shelf both kinds of minerals were found. Uranium minerals were represented by uraninite, coffinite, and ningioite; those of REE - by monazite, xenotime, and bastnesite, which points to their potential accumulation not only as isomorphous admixtures in calcium phosphate but also as independent mineral phases. Coexistence of the minerals noted in shelf phosphorites is caused by repeated changes in redox conditions during formation and then redeposition of phosphate concretions. Presence of uranium minerals in phosphorites from seamounts shows that during an initial step of formation of these phosphorites environment was rather suboxic or reductive than oxic.

Chemical composition of basalts and basaltic glasses from the Sierra Leone Fracture Zone region

Major and trace element (including REE) geochemistry of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5-7°10'N) has been studied. Associations of basalts of various compositions with particular ocean-floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts (Variety 1) that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwestern flank of the MAR crest zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are different for depleted tholeiites (44-55 km, 1320-1370°C) and enriched tholeiites (45-78 km, 1330-1450°C). Mantle plume impact is shown to affect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in melts parental to rift-related basalts occurring near the plume. T- and P-MORBs are also developed near areas where mantle plumes are localized. High-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov Fracture Zone) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. Likely causes of temporal and spatial instability of the mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In sulfide-bearing rift morphostructures (so-called "Ore area'' and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. Functioning of these chambers is a combined effect of singular geodynamic regime and plume activity. In these chambers melts undergo deep differentiation leading to progressively increasing concentration of sulfide phase, eventually to be supplied to the hydrothermal plumbing system.