Petrology and geochemistry of oceanic intraplate sheet-flow basalts at DSDP Hole 89-462A

Reentry of Hole 462A during Leg 89 resulted in the penetration of a further 140 m of basalt sheet-flows similar to those found during Leg 61 at the same site. Twelve volcanic units (45 to 56) were recognized, comprising a series of rapidly extruded, interlayered aphyric and poorly clinopyroxene-plagioclase-olivine phyric, nonvesicular basalts. All exhibit variable, mild hydration and oxidation, relative to fresh oceanic basalts, produced under reducing, low-CO2-activity conditions within the zeolite facies. Secondary assemblages are dominated by smectites, zeolites, and pyrite, produced by low-temperature reaction with poorly oxygenated seawater. No systematic mineralogical or chemical changes are observed with depth, although thin quenched units and more massive hypocrystalline units exhibit slightly different alteration parageneses. Chemically, the basalts are olivine- and quartz-normative tholeiites, characterized by low incompatible-element abundances, similar to mildly enriched MORB (approaching T-type), with moderate, chrondite-normalized, large-ionlithophile- element depletion patterns and generally lower or near-chrondritic ratios for many low-distribution-coefficient (KD) element pairs. In general, relative to cyclic MORB chemical variation, they are uniform throughout, although 3 chemical megagroups and 22 subgroups are recognized. It is considered that the megagroups represent separate low-pressure-fractionated systems (olivine + Plagioclase ± clinopyroxene), whereas minor variations within them (subgroups) indicate magma mixing and generation of near-steady-state conditions. Overall, relatively minor fractionation coupled with magma mixing produced a series of compositionally uniform lavas. Parental melts were produced by similar degrees of partial melting, although the source may have varied slightly in LIL-element content.

Lithology and geochemistry of ODP Leg 198 sites, Shatsky Rise

Samples of chert, porcellanite, and chalk/limestone from Cretaceous chert-bearing sections recovered during Leg 198 were studied to elucidate the nature and origin of chert color zonations with depth/age. Sedimentary structures, trace fossils, compactional features, sediment composition, texture, geochemistry, and diagenetic history were compared among lithologies. Trends in major and minor element composition were determined. Whereas geochemical analyses demonstrate systematic elemental differences among the different lithologies, there are less distinct patterns in composition for the colored cherts. The color of the chert appears to be related primarily to the amount of silica and secondarily to the proportion of other components. Red cherts are almost pure silica with only minor impurities. This may allow pigmentation from fine Fe oxides to dominate the color. These red cherts are from places where geophysical logs indicate that chert is the dominant rock type of the section. These red chert intervals cannot be unequivocally distinguished from surrounding chert-bearing lithologies in terms of sedimentary structures.

Composition of sedimentary rocks from the Mountainous Crimea and the Northwestern Caucasus

Detailed data obtained on chemistry of sedimentary rocks from the Mountainous Crimea and the Northwestern Caucasus that were dated at the Cenomanian/Turonian boundary and formed during Oceanic Anoxic Event 2 make it possible to calculate dissolved oxygen concentration in bottom waters of the sedimentation basin. Enrichment factors of trace elements in black shales are revised and an explanation is suggested for genesis of the rocks with regard for unusual climatic changes.

Geochemistry and physics on sediment cores from the Peru Basin

We studied preservation/dissolution cycles and paleoproductivity in eight sediment cores from the Peru Basin south of the highly productive surface waters of the eastern equatorial Pacific. Stratigraphy is based on stable oxygen isotopes and on combined magnetostratigraphy and biostratigraphy. Sediment cores which span the last 8 m.y., were retrieved during cruise 79 with RV SONNE close to the carbonate compensation depth (CCD). In general, sediments show Pacific-type carbonate cycles. We interpret a pronounced carbonate peak between 6 and 7 Ma as the result of a western and northern extension of the highly productive Peru Current. Decreased carbonate contents from the late Miocene to the late Pliocene might be associated with a slow contraction of the latitudinal extent of the high-productivity belt north of the study areas. During the Pliocene, carbonate variations showed 400 kyr cycles indicating the growth and decay of ice sheets, which should have been associated with pulsations of the Antarctic ice cap. An abrupt collapse of the carbonate system occurred at 2.4 Ma. Higher frequency variations of the carbonate record indicate the major increase of the northern hemisphere glaciation. During the Quaternary, carbonate fluxes are high during glacials and low during interglacials. Large amplitude variations with long broad minima and maxima, associated with small migrations of the lysocline and the CCD (< 200 m), are indicative of the preservation/dissolution history in the Peru Basin. During the early Pleistocene, climatic forcing by the 41 kyr obliquity cycle is not observed in the carbonate record. During the last 800 kyr, variability in the carbonate record was dominated by the 100 kyr eccentricity cycle. Fluxes of biogenic material (calcium carbonate, organic carbon, opal, and barium) were greatest during glacials, which imply higher productivity and export production of the Peru Current during cold climatic periods. Dissolution was greatest during interglacials as inferred from the relatively poor preservation of planktonic foraminifera and from the low accumulation rate of carbonate. After the Mid-Brunhes Event (400 ka), we observe a plateaulike shift to enhanced dissolution and to intensified productivity.

Geochemistry and mineralogy of Eocene-Oligocene sediments of IODP Hole 302-M0002A

In 2004, Integrated Ocean Drilling Program Expedition 302 (Arctic Coring Expedition, ACEX) to the Lomonosov Ridge drilled the first Central Arctic Ocean sediment record reaching the uppermost Cretaceous (~430 m composite depth). While the Neogene part of the record is characterized by grayish-yellowish siliciclastic material, the Paleogene part is dominated by biosiliceous black shale-type sediments. The lithological transition between Paleogene and Neogene deposits was initially interpreted as a single sedimentological unconformity (hiatus) of ~26 Ma duration, separating Eocene from Miocene strata. More recently, however, continuous sedimentation on Lomonosov Ridge throughout the Cenozoic was proclaimed, questioning the existence of a hiatus. In this context, we studied the elemental and mineralogical sediment composition around the Paleogene-Neogene transition at high resolution to reconstruct variations in the depositional regime (e.g. wave/current activity, detrital provenance, and bottom water redox conditions). Already below the hiatus, mineralogical and geochemical proxies imply drastic changes in sediment provenance and/or weathering intensity in the hinterland, and point to the existence of another, earlier gap in the sediment record. The sediments directly overlying the hiatus (the Zebra interval) are characterized by pronounced and abrupt compositional changes that suggest repeated erosion and re-deposition of material. Regarding redox conditions, euxinic bottom waters prevailed at the Eocene Lomonosov Ridge, and became even more severe directly before the hiatus. With detrital sedimentation rates decreasing, authigenic trace metals were highly enriched in the sediment. This continuous authigenic trace metal enrichment under persistent euxinia implies that the Arctic trace metal pool was renewed continuously by water mass exchange with the world ocean, so the Eocene Arctic Ocean was not fully restricted. Above the hiatus, extreme positive Ce anomalies are clear signs of a periodically well-oxygenated water column, but redox conditions were highly variable during deposition of the Zebra interval. Significant Mn enrichments only occur above the Zebra interval, documenting the Miocene establishment of stable oxic conditions in the Arctic Ocean. In summary, extreme and abrupt changes in geochemistry and mineralogy across the studied sediment section do not suggest continuous sedimentation at the Lomonosov Ridge around the Eocene-Miocene transition, but imply repeated periods of very low sedimentation rates and/or erosion.

Petrographic description and geochemistry of basalts and altered volcanic rocks from ODP Leg 183 sites

The basalts recovered during Legs 183 and 120 from the southern, central, and northernmost parts of the Kerguelen Plateau (Holes 1136A, 1138A, 1140A, and 747C, respectively), as well as those recovered from the eastern part of the crest of Elan Bank (Hole 1137A), represent derivates from tholeiitic melts. In the northern part of the Kerguelen Plateau (Hole 1140A), basalts may have formed from two sources located at different depths. This is reflected in the presence of both low- and high-titanium basalts. The basalts are variably altered by low-temperature hydrothermal processes (at temperatures up to 120°C), and some are affected by subaerial weathering. The hydrothermal alteration led mainly to the formation of smectites, chlorite minerals, mixed-layer hydromica-smectite and smectite-chlorite minerals, hydromica, serpentine(?), clinoptilolite, heulandite, stilbite, analcime, mordenite, thomsonite, natrolite(?), calcite, quartz, and dickite(?). Alteration of extrusive basalts is mainly related to horizontal fluid flow within permeable contact zones between lava flows. Under a nonoxidizing environment of alteration, the tendency to lose most of elements, including rare earth elements, from basalts dominates. Under on oxidizing environment, basalts accumulate many elements.

Major, trace and rare earth elements and minerals at DSDP Legs 69-70 Holes

We obtained major and trace element data on 113 samples from basalts drilled during DSDP Legs 69 and 70 in the Costa Rica Rift area. The majority have major and trace element characteristics typical of ocean-ridge tholeiities. Most of the basalts are relatively MgO rich (MgO > 8 wt.%) and have Mg values (MgO/MgO + 0.85FeO x 100) of about 53, characteristics that clearly indicate that the various magmas underwent only a small amount of crystal fractionation before being erupted onto the seafloor. According to their normative mineralogies, the rocks are olivine tholeiites. A few samples plot close to the diopside-hypersthene join of the projected basalt tetrahedron. Except for basalts from two thin intervals in Hole 504B, which differ significantly from all the other basalts of the hole, practically no chemical downhole variation could be established. In the two exceptional intervals, both TiO2 and P2O5 contents are markedly enriched among the major oxides. The trace elements in these intervals are distinguished by relatively high contents of magmatophile elements and have flat to enriched chondrite-normalized distribution patterns of light rare earth elements (LREE). Most of the rocks outside these intervals are strongly depleted in large-ionlithophile (LIL) elements and LREE. We offer no satisfactory hypothesis for the origin of these basalts at this time. They might have originated within pockets of mantle materials that were more primitive than the LIL-element-depleted magmas that were the source of the other basalts. A significant change with depth in the type of alteration occurs in the 561 meters of basalt cored in Hole 504B. According to the behavior of such alteration-sensitive species as K2O, H2O-, CO2, S, Tl, and the iron oxidation ratio, the alteration is oxidative in the upper part and nonoxidative or even reducing in the lower part. The oxidative alteration may have resulted from low temperature basalt/seawater interaction, whereas hydrothermal solutions may be responsible for the nonoxidative alteration.

Geochemistry of dikes and lavas from the north wall of the Hess Deep Rift

An investigation of ~1-m.y.-old dikes and lavas from the north wall of the Hess Deep Rift (2°15'N, 101°30'W) collected during Alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust. The study area encompasses 25 km of an east-west flow line, representing ~370,000 years of crustal accretion at the East Pacific Rise. Samples analyzed exhibit depleted incompatible trace element abundances and ratios [(La/Sm)N < 1]. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flow line. Rather, over short (<4 m) and long (~25 km) distances, significant variations are observed in major and trace element concentrations and ratios. Modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions. These findings, combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift-dominated systems, suggest a more realistic model of the magmatic system underlying the East Pacific Rise relative to the commonly assumed twodimensional model. In this model, melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir. Dikes emanating from these distinct reservoirs transport magma along axis, resulting in interleaved dikes and host lavas with different evolutionary histories. This model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin.

Geochemistry of the lower sheeted dike complex of ODP Holes 137-504B and 140-504B

Sixty-three samples representing 379 m of sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program Site 504B have been analyzed for major and selected trace elements by X-ray fluorescence. The samples range from microcrystalline aphyric basalts to moderately phyric (2%-10% phenocrysts) diabase that are typically multiply saturated with plagioclase, olivine, and clinopyroxene, in order of relative abundance. All analyzed samples are classified as Group D compositions with moderate to slightly elevated compatible elements (MgÆ-value = 0.65% ± 0.03%; Al2O3 = 15.5% ± 0.8%; CaO = 13.0% ± 0.3%; Ni = 114 ± 29 ppm), and unusually depleted levels of moderate to highly incompatible elements (Nb < 1 ppm; Zr = 44 ± 7 ppm; Rb < 0.5 ppm; Ba ~ 1 ppm; P2O5 = 0.07% ± 0.02%). These compositions are consistent with a multistage melting of a normal ocean ridge basaltic mantle source followed by extensive fractionation of olivine, plagioclase, and clinopyroxene. Leg 140 aphyric to sparsely phyric (0%-2% phenocrysts) basalts and diabases are compositionally indistinguishable from similarly phyric samples at higher levels in the hole. An examination of the entire crustal section, from the overlying volcanics through the sheeted dikes observed in Leg 140, reveals no significant trends indicating the enrichment or depletion of Costa Rica Rift Zone source magmas over time. Similarly, significant trends toward increased or decreased differentiation cannot be identified, although compositional patterns reflecting variable amounts of phenocryst addition are apparent at various depths. Below ? 1700 mbsf to the bottom of the Leg 140 section, there is a broadly systematic pattern of Zn depletion with depth, the result of high-temperature hydrothermal leaching. This zone of depletion is thought to be a significant source of Zn for the hydrothermal fluids depositing metal sulfides at ridge-crest hydrothermal vents and the sulfide-mineralization zone, located in the transition between pillow lavas and sheeted dikes. Localized zones of intense alteration (60%-95% recrystallization) are present on a centimeter to meter scale in many lithologic units. Within these zones, normally immobile elements Ti, Zr, Y, and rare-earth elements are strongly depleted compared with "fresher" samples centimeters away. The extent of compositional variability of these elements tends to obscure primary igneous trends if the highly altered samples are not identified or removed. At levels up to 40% (or possibly 60%) recrystallization, Ti, Zr, and Y retain their primary signatures. Although the mechanisms are unclear, it is possible that these intense alteration zones are a source of Y and rare-earth elements for the typically rare-earth-element-enriched hydrothermal vent fluids of mid-ocean ridges.

Geochemistry and radionuclide content of sediment and water samples obtained from the Ob and Irtysh river, Russia

This paper reports the results of the investigations of 2006-2007 on the distribution and migration forms of artificial radionuclides and chemical elements in the Ob-Irtysh water system. Three regions were studied. One of them is a local segment of the Ob River upstream from the confluence with the Irtysh River; its investigation allowed us to estimate the general radioecological state of the aquatic environment affected by the activity of the Tomsk 7 plant. The second region is a local segment of the Irtysh River upstream from its confluence with the Ob River, where the influence of emissions from the NPO Mayak could be estimated. The third region is the water area of the Ob River after its confluence with the Irtysh River. It characterizes the real level of radioactive and chemical contamination of the middle reaches of the Ob River. In order to explain horizontal variations in the distribution of radionuclides in the upper layer of bottom sediments collected at various sites, the results of sorption-kinetic experiments with radioactive tracers in the precipitate-solution system were used. The investigation of the migration forms of trace elements and radionuclides occurring in river water was based on the method of tangential-flow membrane filtration. Chemical element contents were determined in 400-ml water samples. A set of Millipore polysulfone membranes with pore sizes of 8, 1.2, 0.45, 0.1, and 0.025 µm was employed. Taking into account the ultralow specific concentrations of radionuclides in the water, they were analyzed in 300-500 litre samples using Millipore polysulfone membranes with pore sizes of 0.45 µm and 15 kDa. This allowed us to estimate the percentages of cesium-137 and plutonium-239, 240 in the suspended particulate fraction, colloids, and dissolved species.

Major-oxides and trace-elements at DSDP Hole 78-543A

Four chemically distinct basalts were cored in 44 m of basement penetration at Deep Sea Drilling Project Site 543, in Upper Cretaceous crust just seaward of the deformation front of the Barbados Ridge and north of the Tiburon Rise. All four types are moderately fractionated abyssal tholeiites. The four types have different magnetic inclinations, all of reversed polarity, suggesting eruption at different times which recorded secular variation of the earth's magnetic field. Extensive replacement of Plagioclase by K-feldspar has occurred at the top of the basalts, giving analyses with K2O contents up to 5 %. The earliest stages of alteration were dominantly oxidative, resulting in fractures lined with celadonite and dioctahedral smectite, and pervasive replacement of olivine and most intersertal glass with iron hydroxides and green clay minerals. Latef, non-oxidative alteration resulted in formation of olive-green clays and pyrite veins in a portion of the rocks. Basalts affected by this alteration actually lost K2O (to abundances lower than in adjacent fresh basalt glasses), and gained MgO (to abundances higher than in the glasses). Finally, fractures and interpillow voids were lined with calcite, sealing in much fresh glass. Oxygen-isotope measurements on the calcite indicate that this occurred at 12 to 25C. Either altering fluids were warm or the basalts had become buried with a considerable thickness of sediments, such that temperatures increased until a conductive thermal gradient was established, when the veining occurred.