Major and trace elements at DSDP Leg 74 Holes

Deep Sea Drilling Project Leg 74 drilled basement on the Walvis Ridge at Sites 525, 527, and 528. These sites are located on the crest and flanks of the segment of the Ridge about 68 to 70 m.y. old in the central province of the Ridge. Each site has a number of distinct subaqueous flows separated by sediment layers. Although variation in geochemistry among units and sites is related in part to alteration or crystal fractionation, some is caused by small-scale compositional variation in the mantle source of the basalts. Leg 74 basalts are similar to other basalts recovered from the Walvis Ridge and the Rio Grande Rise. They show distinct compositional differences to mid-ocean ridge basalts in general, to those recovered from the South Atlantic at this latitude, and to basalts presently erupting in Tristan da Cunha. The composition of the Walvis Ridge basalts does not suggest simple mixtures of present-day MORB and Tristan da Cunha melts. If the Walvis Ridge represents the trace of the Tristan da Cunha hot spot as the plates separated, then the composition of the mantle source has differed at different times in the past, which suggests mantle heterogeneity.

Chemical composition of volcanic glass of some tephra layers in Northeastern Pacific Ocean

Five widespread upper Cenozoic tephra layers that are found within continental sediments of the western United States have been correlated with tephra layers in marine sediments in the Humboldt and Ventura basins of coastal California by similarities in major-and trace-element abundances; four of these layers have also been identified in deep-ocean sediments at DSDP sites 34, 36, 173, and 470 in the northeastern Pacific Ocean. These layers, erupted from vents in the Yellowstone National Park area of Wyoming and Idaho (Y), the Cascade Range of the Pacific Northwest (C), and the Long Valley area, California (L), are the Huckleberry Ridge ash bed (2.0 Ma, Y), Rio Dell ash bed (ca. 1.5 Ma, C), Bishop ash bed (0.74 Ma, L), Lava Creek B ash bed (0.62 Ma, Y), and Loleta ash bed (ca. 0.4 Ma, C). The isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments. For example, the widespread Bishop ash bed is correlated from proximal localities near Bishop in east-central California, where it is interbedded with volcanic and glacial deposits, to lacustrine beds near Tecopa, southeastern California, to deformed on-shore marine strata near Ventura, southwestern California, to deep-ocean sediments at site 470 in the eastern Pacific Ocean west of northern Mexico. The correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control. Relative age variations of as much as 0.5 m.y. exist between marine biostratigraphic datums [for example, highest occurrence level of Discoaster brouweri and Calcidiscus tropicus (= C. macintyrei)], as determined from sedimentation rate curves derived from other age control available at each of several sites. These discrepancies may be due to several factors, among which are (1) diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds, (2) upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western United States and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums, (3) dissolution of microfossils or selective diagenesis of some taxa, (4) lack of precision in isotopic age calibration of these datums, (5) errors in isotopic ages of tephra beds, and (6) large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums. Correlation of tephra layers between on-land marine and deep-ocean deposits indicates that some biostratigraphic datums (diatom and calcareous nannofossil) may be truly time transgressive because at some sites, they are found above and, at other sites, below the same tephra layers.

Composition of rocks, minerals, water, and gases from uplift areas of the Central Atlantic

Original geological, geophysical, lithological, mineralogical data on uplifts of the Central Atlantic are given in the book based on materials of Cruise 1 of the R/V Akademik Nikolaj Strakhov. Geological and geophysical studies include description of the obtained material and analysis of structural and morphological elements of the ocean floor. Results of lithological, petrochemical and geochemical studies were extremely innovative and develop a conceptual model. The latter include studies of petrochemical evolution of tholeiitic alkaline plate volcanism, large-scale hydrothermal transformation of basement rocks - palygorskitization, phosphatization and ferromanganese mineralization. Showing imposition Superposition of hydrogenic alteration on hydrothermally altered rocks and its role in Cenozoic history of sedimentation is shown.

Isotopic and chemical compositions of lower crust rocks and minerals from ODP Hole 176-735B

The composition of gabbroic rocks from the drill core of Hole 735B (ODP Leg 176) at the 11 Ma Atlantis II bank close to the slow spreading Southwest Indian Ridge (SWIR) has been analyzed for major and trace elements and Sr, Nd and Pb isotopic composition. The samples are thought to represent much of the mineralogical and geochemical variation in a vertical 1-km section (500-1500 m below the sea floor) of the lower ocean crust. Primitive troctolitic gabbros, olivine gabbros and gabbros that have Mg#=84-70, Ca#>61 and low Na# (Na/(Na+Al)) (8-17) are intruded by patches or veins of more evolved FeTi-oxide rich gabbroic and dioritic rocks with Mg# to 20, Ca# to 32, Na#=14-23, TiO2<7 wt.% and FeOtotal<18 wt.%. All rocks are acdcumulates, and incompatible element concentrations are low, e.g. Pb=0.1-0.7 ppm and Uand up to 2 ppm Pb and 0.2 ppm U in the evolved. The range of isotopic compositions of the unleached rocks is: 87Sr/86Sr=0.70280-0.70299, average 0.70287+/-0.00005 (1 S.D., N=30 samples) (except one felsic vein with 87Sr/86Sr=0.7045), 143Nd/144Nd=0.51304-0.51314, average 0.51310+/-0.00002 (1 S.D., N=28), 206Pb/204Pb=17.43-18.55, 207Pb/204Pb=15.40-15.61 and 208Pb/204Pb=37.19-38.28. The range of Sr and the almost constant Nd isotopic composition resemble that found in the upper 500 m of Hole 735B, while Pb ranges to more radiogenic compositions. In general, there is a decrease in isotopic variation of Sr and Pb as well as ? (238U/204Pb), U and Pb with depth, with a trend towards relatively unradiogenic compositions. This correlates with a decrease in alteration and frequency of evolved rock-types in the core. Leached samples generally have less radiogenic Pb with values trending towards 206Pb/204Pb=17.35, 207Pb/204Pb=15.35 and 208Pb/204Pb=37.0, while their 87Sr/86Sr ratios deviate less systematically from unleached rocks and reach both higher, 0.70307, and lower values, 0.70276. Separated clinopyroxene has elevated 87Sr/86Sr up to 0.7035, while plagioclase generally has close to whole rock Sr. Leaching reduced 87Sr/86Sr in clinopyroxene and in two (out of nine) cases leached separates and whole rock display isotopic equilibrium. Relatively minor hydrothermal seawater alteration is thought to have increased 87Sr/86Sr in the rocks, while a secondary high temperature percolation of a mantle-derived agent is thought to be the cause for the trend towards radiogenic Pb. This material had intermediate 87Sr/86Sr and may have originated from non-MORB off axis mantle. The main primary igneous isotopic variation of the gabbros is suggested to have been derived from the MORB-mantle and is defined mainly by leached samples from both ODP Leg 176 and Leg 118 and can be explained by two-component mixing of an end-member with composition like Central Indian Ridge basalts and an end-member with composition unlike any MORB. The latter is characterized by very unradiogenic Pb, in particular 207Pb/204Pb, and may have an origin with affinity to old depleted mantle (DM). The isotopic composition of the magmas parental to the FeTi-oxide rich rocks cannot be distinguished from the magmas parental to the primitive gabbros and an intimate relationship is indicated. The small-scale inhomogeneity indicated for the SWIR MORB-mantle at the Atlantis II Fracture Zone was probably inherited by the lower crustal rocks due to small-scale melting and monogenetic magma chambers at this slow spreading ridge.

Geochemistry of selected Ferrar samples from Northern Victoria Land and George V Land during GANOVEX VIII

Newly sampled basaltic andesites and andesites from the tholeiitic Ferrar Supergroup of northern Victoria Land and George V Land, Antarctica, are attributed to the known low-Ti and high-Ti series. Aside from known sparsely distributed high-Ti extrusives, a high-Ti sill was found in the Alamein Range outside the Rennick Graben. Low-Ti lavas, sills and dikes display wide petrographical, mineral and geochemical variations, reflecting extensive in-situ differentiation. High-Ti rocks from Litell Rocks are homogeneous with respect to mineralogy and geochemistry, minor deviations are shown by the sampled sill. Chilled margins of low-Ti sills, dikes and lava flows exhibit nearly constant bulk-rock chemistry (mg# ~60) within the studied area. Compared to chilled margins from Tasmanian sills, the striking uniformity of the pre-emplacement chemistry of Ferrar magmas over large distances supports the magma transport model of Elliot et al. (1999, doi:10.1016/S0012-821X(99)00023-0). In the area investigated, compositional variations within the low-Ti series, caused by in-situ differentiation, increase towards the Wilson-Bowers Terrane boundary, possibly displaying the asymmetrical distribution of outcrops over this area. Absence of Ferrar occurrences east of the Bowers Terrane remains a matter of palaeo-geodynamic discussion. Besides, the secondary mineralogy of extrusives from Litell Rocks and Monument Nunataks exhibits noticeable differences, which indicates an elevated thermal gradient in the vicinity of Litell Rocks compared to Monument Nunataks during the Cretaceous.

Chemical and boron isotopic compositions of volcanics from ODP Sites 135-834 to 135-840

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ion microprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relative volumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eight different islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7 Ma. B/Be is variable (5.8-122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from around 3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18-607). These values are too high to be related simply to a sediment influence on petrogenesis. Together with very high d11B values (-11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. High d11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower d11B values noted in the Kermadec Arc (17 to -4.4) are related to the influence of sediment eroded from New Zealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time. The timing of maximum B/Be at 3 to 4 Ma correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maximum degrees of partial melting at this time. Although thinning of the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry, variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other trace element systematics in the Tonga-Kermadec Arc with no indication of slab melting.

Major, trace and rare earth element concentrations of sediments from ODP Leg 207 sites

The Cretaceous and Paleogene sediments recovered during Ocean Drilling Program Leg 207 can be divided into three broad modes of deposition: synrift clastics (lithologic Unit V), organic matter-rich, laminated black shales (Unit IV), and open-marine chalk and calcareous claystones (Units III-I). The aim of this study is to provide a quantitative geochemical characterization of sediments representing these five lithologic units. For this work we used the residues (squeeze cakes) obtained from pore water sampling. Samples were analyzed for bulk parameters (total inorganic carbon, total organic carbon, and S) and by X-ray fluorescence for major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and selected minor (As, Ba, Co, Cr, Cu, Mo, Ni, Pb, Rb, Sr, U, V, Y, Zn, and Zr) elements. Inductively coupled plasma-mass spectrometry analyses for rare earth elements (REEs) were performed on acid digestions of the squeeze cake samples from Site 1258. The major element composition is governed by the mixture of a terrigenous detrital component of roughly average shale (AS) composition with biogenous carbonate and silica. The composition of the terrigenous detritus is close to AS in Units II-IV. For Unit I, a more weathered terrigenous source is suggested. Carbonate contents reach >60 wt% on average in chalks and calcareous claystones of Units II-IV. The SiO2 contribution in excess of the normal terrigenous-detrital background indicates the presence of biogenous silica, with highest amounts in Units II and III. The contents of coarse-grained material (quartz) are enhanced in Unit V, where Ti and Zr contents are also high. This indicates a high-energy depositional environment. REE patterns are generally similar to AS. A more pronounced negative Ce anomaly in Unit IV may indicate low-oxygen conditions in the water column. The Cretaceous black shales of Unit IV are clearly enriched in redox-sensitive and stable sulfide-forming elements (Mo, V, Zn, and As). High phosphate contents point toward enhanced nutrient supply and high bioproductivity. Ba/Al ratios are rather high throughout Unit IV despite the absence of sulfate in the pore water, indicating elevated primary production. Manganese contents are extremely low for most of the interval studied. Such an Mn depletion is only possible in an environment where Mn was mobilized and transported into an expanded oxygen minimum zone ("open system"). The sulfur contents show a complete sulfidation of the reactive iron of Unit IV and a significant excess of sulfur relative to that of iron, which indicates that part of the sulfur was incorporated into organic matter. We suppose extreme paleoenvironmental conditions during black shale deposition: high bioproductivity like in recent coastal upwelling settings together with severe oxygen depletion if not presence of hydrogen sulfide in the water column.

Rare and trace element analyses from DSDP Holes 31-291 and 31-294 from the Philippine Sea

Metal-rich sediments were found in the West Philippine Basin at DSDP sites 291 (located about 500 km SW of the Philippine Ridge or Central Basin Fault) and 294/295 (located about 580 km NE of the Philippine Ridge). In both cases the metalliferous deposits constitute a layer, probably Eocene in age, resting directly above the basaltic basement at the bottom of the sediment column. The chemistry of the major (including Fe and Mn) and trace elements (including trace metals, rare earth elements, U and Th) suggest a strong similarity of these deposits to metalliferous deposits produced by hydrothermal activity at oceanic spreading centers. Well-crystallized hematite is a major component of the metal-rich deposits at site 294/295. We infer that the Philippine Sea deposits were formed at some spreading center by hydrothermal processes of metallogenesis, similar to processes occurring at oceanic spreading centers. A locus for their formation might have been the Philippine Ridge (Central Basin Fault), probably an extinct spreading center. We conclude that metallogenesis of the type occurring at oceanic spreading centers can take place also in marginal basins. This has implications for the origin of metal deposits found in some ophiolite complexes, such as those in Luzon (Philippines), which may represent fragments of former marginal basins rather than of oceanic lithosphere.

Pb-, Sr-, and Nd-isotopic composition of basaltic rocks from the Lau Basin

New Pb, Sr, and Nd isotope data are presented for 64 samples from the six backarc sites drilled during Leg 135. Systematic changes in Pb and Sr compositions illustrate significant isotopic variations between and within sites as well as provide two key pieces of information. First, a recent influx of asthenosphere with Indian Ocean mantle affinities has occurred and has successfully displaced older "Pacific" asthenosphere from the mantle underlying the backarc region. Second, clear evidence exists for mixing between these two asthenospheric end-members and at least one "arc-like" component. The latter was not the same as most material currently erupting in the Tofua Arc, but it must have had a more radiogenic Pb-isotope signature, perhaps similar to rocks analyzed from the islands of Tafahi, and Niuatoputapu. A comparison between the isotopic variations and the tectonic setting of the drill sites reveals consistent and important information regarding the mantle dynamics beneath the evolving backarc basin. We propose a model in which the source of upwelling magmas changes from Pacific to Indian Ocean asthenosphere with the propagation of seafloor spreading, a model with important implications for the rate of mantle influx into this region. Although the chemistries of backarc magmas have been profoundly influenced by this process, an additional consequence is the advection of Indian Ocean asthenosphere into the sub-arc mantle source. The isotopic compositions of arc rocks from the vicinity have been reevaluated on the basis of the proposed mantle advection model. We suggest that the slab-derived flux of trace elements into the arc wedge has remained relatively uniform with time (i.e., ~40 Ma), so that the change in arc chemistry results from mantle source substitution, rather than from differences in the composition of the downgoing plate.

Geochemical analyses of ODP Leg 135 samples

Statistical analysis of X-ray fluorescence data acquired during Leg 135 indicates that this instrument produces data of comparable precision to good land-based laboratories. We also examined contamination of certain elements caused by crushing during the use of the tungsten carbide apparatus. Although the concentrations of most elements are not altered during crushing, the powders prepared on the ship should not be used in subsequent studies where key elements of the investigation include W, Co, Ta, Pb, and low levels of Nb.