Chemical composition of ferromanganese manifestations from the Sea of Okhotsk

New data on microstructures and mineral and chemical compositions of ferromanganese crusts sampled from the western slope of the Kuril Island Arc in the Sea of Okhotsk during cruises of R/V Vulkanolog are discussed. The study of the crusts using analytical electron microscopy methods revealed that their manganese phase is represented by vernadite, Fe-vernadite, todorokite, asbolane, and asbolane-buserite, while iron phase consists of hematite, hydrohematite, ferroxyhite, and magnetite. Lithic mineral assemblage includes apatite, quartz, epidote, and montmorillonite. According to chemical analysis most of the crusts contain significant part of volcanogenic and hydrothermal material. It is evident from elevated values of Mn/Fe and (Mn+Fe)/Ti ratios, low concentrations of some trace elements, and positive Eu anomaly.

U and Th concentrations and isotope ratios of samples from ODP Leg 166

Constraining the history of seawater (234U/238U) is important because this ratio is used to assess the validity of U/Th ages, and because it provides information about the past rate of physical weathering on the continents. This study makes use of U-rich slope sediments from the Bahamas in an attempt to reconstruct seawater (234U/238U) for the last 800 kyr. For the last 360 kyr, U/Th dating of these sediments provides ages and initial (234U/238U) values. Sixty-seven samples, largely from marine highstands, have initial (234U/238U) which scatter somewhat about the modern seawater value (~1.145) but neither this scatter nor the average value increases with age of sample. These data contrast with published coral data and suggest that seawater (234U/238U) has remained within 15? of the modern value for the last 360 kyr. This confirms the rejection of coral U/Th ages where the initial (234U/238U) is significantly different from modern seawater. Data from older highstands, dated with delta18O stratigraphy or by the presence of the Brunhes/Matuyama (B/M) reversal at 780 kyr, allow seawater (234U/238U) to be assessed prior to the range of the 230Th chronometer. Unfortunately, diagenetic scatter in the data between the B/M reversal and 360 kyr is rather large, probably relating to low U concentrations for these samples. But there is no indication of a trend in seawater (234U/238U) with age. High U samples from close to the B/M reversal show less diagenetic scatter and an initial (234U/238U) that averages 1.102. This lower value can be explained by lower seawater (234U/238U) at the time of the B/M reversal, or by progressive loss of 234U from the sediment by alpha-recoil. A simple box model is presented to illustrate the response of seawater (234U/238U) to variations in riverine input, such as might be caused by changes in continental weathering. Comparison of the Bahamas (234U/238U) data with model results indicates that riverine (234U/238U) has not varied by more than 65? for any 100 kyr period during the last 360 kyr. It also indicates that the ratio of physical to chemical weathering on the continents has not been higher than at present for any extended period during the last 800 kyr.

Geochemistry and isotopic ratios of basalts obtained during R/V Akademik Nikolaj Strakhov cruise ANS25

Based on the investigation of samples recovered during Cruise 25 of the R/V ''Akademik Nikolai Strakhov'', the character of magmatism was determined in the flank parts of the rift zone at the 74°05'N and 73°50'N region, where the direction of the rift valley changes from the north-northwest in the Knipovich Ridge to the northeast-trending structures of the Mohns Ridge. It was shown that the tholeiitic magmas of this region shows all the geochemical characteristics of TOR-2, which is typical of the Mohns Ridge and most oceanic rift zones worldwide, and differ from the basalts of the Knipovich Ridge, which are assigned to a shallower type of tholeiitic magmatism (Na-TOR). The persistent depletion of the magmas in terms of lithophile element contents and radiogenic isotope ratios of Sr, Nd, and Pb reflects the conditions of their formation during the ascent of the depleted oceanic mantle, which has occurred without significant complications since the early stages of the formation of the Mohns Ridge.

Geochemistry of sediments from the mid-oceanic Kolbeinsey Ridge

In order to assess recent submarine volcanic contributions to the sediments from the active Kolbeinsey Ridge, surface samples were analyzed chemically. The contribution of major and trace elements studied differ within the study area. A statistical analysis of the geochemical variables using factor analysis and cluster method allows to distinguish possible sample groups. Cluster method identifies three distinct sediment groups located in different areas of sedimentation. Group 1 is characterized by highest contents of Fe2O3, V, Co, Ni, Cu and Zn demonstrating the input of volcaniclastic material. Group 2 comprises high values of CaCO3, CaO and Sr representing biogenic carbonate. Group 3 is characterized by the elements K, Rb, Cs, La and Pb indicating the terrigenous component. The absolute percentage of the volcanic, biogenic and terrigenous components in the bulk sediments was calculated by using a normative sediment method. The highest volcanic component (> 60% on a carbonate free basis) is found on the ridge crest. The biogenic component is highest (10-30%) in the eastern part of the Spar Fracture Zone influenced by the East Iceland Current. Samples from the western and southeastern region of the study area contain more than 90% of terrigenous component which appears to be mainly controlled by input of ice-rafted debris.

Marine osmium isotope record across the Cretaceous-Tertiary boundary

A composite late Maastrichtian (65.5 to 68.5 Ma) marine osmium (Os) isotope record, based on samples from the Southern Ocean (ODP Site 690), the Tropical Pacific Ocean (DSDP Site 577), the South Atlantic (DSDP Site 525) and the paleo-Tethys Ocean demonstrates that subaerially exposed pelagic carbonates can record seawater Os isotope variations with a fidelity comparable to sediments recovered from the seafloor. New results provide robust evidence of a 20% decline in seawater 187Os/188Os over a period of about 200 kyr early in magnetochron C29r well below the Cretaceous-Paleogene Boundary (KPB), confirming previously reported low-resolution data from the South Atlantic Ocean. New results also confirm a second more rapid decline in 187Os/188Os associated with the KPB that is accompanied by a significant increase in Os concentrations. Complementary platinum (Pt) and iridium (Ir) concentration data indicate that the length scale of diagenetic remobilization of platinum group elements from the KPB is less than 1 m and does not obscure the pre-KPB decline in 187Os/188Os. Increases in bulk sediment Ir concentrations and decreases in bulk carbonate content that coincide with the Os isotope shift suggest that carbonate burial flux may have been lower during the initial decline in 187Os/188Os. We speculate that diminished carbonate burial rate may have been the result of ocean acidification caused by Deccan volcanism.

Geochemistry and mineralogy of interstitial waters and clays in sediments of ODP Leg 180 sites

Oxygen and strontium isotopes and Rb and Ba were determined in interstitial water (IW) collected from Sites 1109, 1115, and 1118 drilled on the Woodlark Rise during Ocean Drilling Program Leg 180. The trace element and mineralogical composition of the clay fraction of sediments isolated from the squeeze cakes corresponding to IW samples from Site 1109 was also determined.

Geochemistry of volcanivlastic sediments of ODP Sites 134-832 and 134-833

Volcaniclastic sediments of North Aoba Basin (Vanuatu) recovered during Ocean Drilling Program (ODP) Leg 134 show a mineralogical and chemical overprint of low grade hydrothermal alteration superimposed on the primary magmatic source compositions. The purpose of this study was to identify authigenic mineral phases incorporated in the volcaniclastic sediments, to distinguish authigenic chemical and mineralogical signals from the original volcaniclastic mineralogical and chemical compositions, and to determine the mechanism of authigenic minerals formation. Mineralogical, micro-chemical and bulk chemical analyses were utilized to identify and characterize authigenic phases and determine the original unaltered ash compositions. 117 volcaniclastic sediment samples from North Aoba Basin Sites 832 and 833 were analyzed. Primary volcaniclastic materials accumulated in North Aoba Basin can be divided into three types. The older basin-filling sequences show three different magmatic trends: high K, calc-alkaline, and low K series. The most recent accumulations are rhyodacitic composition and can be attributed to Santa Maria or Aoba volcanic emissions. Original depositional porosity of volcaniclastic sediments is an important factor in influencing distribution of authigenic phases. Finer-grained units are less altered and retain a bulk mineralogical and chemical composition close to the original pyroclastic rock composition. Coarser grained units (microbreccia and sandstones) are the major hosts of authigenic minerals. At both sites, authigenic minerals (including zeolites, clay minerals, Mg-carbonates, and quartz) exhibit complex zonation with depth that crosses original ash depositional boundaries and stratigraphic limits. The zeolite minerals phillipsite and analcime are ubiquitous throughout the altered intervals. At Site 832, the first zeolite minerals (phillipsite) occur in Pleistocene deposits as shallow as 146 meters below seafloor (mbsf). At Site 833 the first zeolite minerals (analcime) occur in Pleistocene deposits as shallow as 224 mbsf. The assemblage phillipsite + analcime + chabazite appears at 635 mbsf (Site 832) and at 376 mbsf (Site 833). Phillipsite + analcime + chabazite + thomsonite + heulandite are observed between 443 and 732 mbsf at Site 833. Thomsonite is no longer observed below 732 mbsf at Site 833. Heulandite is present to the base of the sections cored. The zeolite assemblages are associated with authigenic clay minerals (nontronite and saponite), calcite, and quartz. Chlorite is noticeable at Site 832 as deep as 851 mbsf. Zeolite zones are present but are less well defined at Site 832. Dolomite and rare magnesite are present below 940 m at Site 832. The coarse-grained authigenic mineral host intervals exhibit geochemical signatures that can be attributed to low grade hydrothermal alteration. The altered intervals show evidence of K2O, CaO, and rare earth elements mobilization. When compared to fine-grained, unaltered units, and to Santa Maria Island volcanics rocks, the altered zones are relatively depleted in rare earth elements, with light rare earth elements-heavy rare earth elements fractionation. Drilling at Site 833 penetrated a sill complex below 840 m. No sill was encountered at Site 832. Complex zonation of zeolite facies, authigenic smectites, carbonates and quartz, and associated geochemical signatures are present at both sites. The mineralogical and chemical alteration overprint is most pronounced in the deeper sections at Site 832. Based on mineralogical and chemical evidence at two locations less than 50 km apart, there is vertical and lateral variation in alteration of the volcaniclastic sediments of North Aoba Basin. The alteration observed may be activated by sill intrusion and associated expulsion of heated fluids into intervals of greater porosity. Such spatial variation in alteration could be attributed to the evolution of the basin axis associated with subduction processes along the New Hebrides Trench.

Geochemistry of continental slope and deep sea basin sediments off Uruguay

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement (<6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1 N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe2+. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.

Geochemistry of ODP Site 125-786 volcanic rocks

Eocene-Oligocene volcanic rocks drilled at Site 786 in the Izu-Bonin forearc cover a wide range of compositions from primitive boninites to highly evolved rhyolites. K-Ar dating reveals at least two distinct episodes of magmatism; one at 41 Ma and a later one at 35 Ma. The early episode produced low-Ca boninites and bronzite andesites that form an oceanic basement of pillow lavas and composite intrusive sheets, overlain by flows and intrusive sheets of intermediate-Ca boninites and bronzite-andesites and a fractionated series of andesites, dacites, and rhyolites. The later episode produced high-Ca boninites and intermediate-Ca boninites, exclusively as intrusive sheets.

Age and chemical composition of magmatic rocks from the Belkovsky Island, New Siberian Islands

The study was inspired by information on Paleozoic andesites, dacites, and diabases on the Belkovsky Island in the 1974 geological survey reports used to reconstruct tectonic evolution of the continental block comprising the New Siberian Islands and the bordering shelf. We did not find felsic volcanics or Middle Paleozoic intrusions in the studied area of the island. Igneous rocks are mafic subvolcanic intrusions including dikes, randomly shaped bodies, explosion breccias, and peperites. They belong to the tholeiitic series and are similar to Siberian traps in petrography and trace-element compositions, with high LREE and LILE and prominent Nb negative anomalies. The island arc affinity is due to continental crust contamination of mantle magma and its long evolution in chambers at different depths. K-Ar biotite age (252+/-5 Ma) of magmatism indicates that it was coeval to the main stage of trap magmatism in the Siberian craton at the Permian-Triassic boundary. The terrane including the New Siberian Islands occurred on the periphery of the Siberian trap province where magmatism acted in rifting environment. Magma intruded into semiliquid wet sediments at shallow depths shortly after their deposition. Therefore, the exposed Paleozoic section in Belkovsky Island may include Permian or possibly Lower Triassic sediments of younger ages than it was believed earlier.