Geochemistry of the oceanic crust from ocean drilling samples

This book presents new data on chemical and mineral compositions and on density of altered and fresh igneous rocks from key DSDP and ODP holes drilled on the following main tectonomagmatic structures of the ocean floor: 1. Mid-ocean ridges and abyssal plains and basins (DSDP Legs 37, 61, 63, 64, 65, 69, 70, 83, and 91 and ODP Legs 106, 111, 123, 129, 137, 139, 140, 148, and 169); 2. Seamounts and guyots (DSDP Legs 19, 55, and 62 and ODP Legs 143 and 144); 3. Intraplate rises (DSDP Legs 26, 33, 51, 52, 53, 72, and 74 and ODP Legs 104, 115, 120, 121, and 183); and 4. Marginal seas (DSDP Legs 19, 59, and 60 and ODP Legs 124, 125, 126, 127, 128, and 135). Study results of altered gabbro from the Southwest Indian Ridge (ODP Leg 118) and serpentinized ultramafic rocks from the Galicia margin (ODP Leg 103) are also presented. Samples were collected by the authors from the DSDP/ODP repositories, as well as during some Glomar Challenger and JOIDES Resolution legs. The book also includes descriptions of thin sections, geochemical diagrams, data on secondary mineral assemblages, and recalculated results of chemical analyses with corrections for rock density. Atomic content of each element can be quantified in grams per standard volume (g/1000 cm**3). The suite of results can be used to estimate mass balance, but parts of the data need additional work, which depends on locating fresh analogs of altered rocks studied here. Results of quantitative estimation of element mobility in recovered sections of the upper oceanic crust as a whole are shown for certain cases: Hole 504B (Costa Rica Rift) and Holes 856H, 857C, and 857D (Middle Valley, Juan de Fuca Ridge).

Lithology, biostratigraphy, and geochemistry of the Black Sea sediments, DSDP Leg 42 (pt. 2) data

The monograph presents results of deep-sea drilling in the Black Sea carried out in 1975. Detailed lithological, biostratigraphic and geochemical studies of Miocene-Holocene sediments have been carried out by specialists from institutes of the USSR Academy of Sciences, Moscow State University and other organizations. Drilling results are compared with geophysical data. Geological history of the Black Sea basin is considered as well.

Temperature reconstruction of sediment core SO9-93KL

The Toba volcanic event, one of the largest eruptions during the Quaternary, is documented in marine sediment cores from the northeastern Arabian Sea. On the crest of the Murray Ridge and along the western Indian continental margin, we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5-4 boundary with the chemical composition of the "Youngest Toba Tuff". Time series of the Uk'37-alkenone index, planktic foraminiferal species, magnetic susceptibility, and sediment accumulation rates from this interval show that the Toba event occurred between two warm periods lasting a few millennia. Using Toba as an instantaneous stratigraphic marker for correlation between the marine- and ice-core chronostratigraphies, these two Arabian Sea climatic events correspond to Greenland interstadials 20 and 19, respectively. Our data sets thus depict substantial interstadial/stadial fluctuations in sea-surface temperature and surface-water productivity. We show that variable terrigenous (eolian) sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record. Within the provided stratigraphic resolution of several decades to centennials, none of these proxies shows a particular impact of the Toba eruption. However, our results are additional support that Toba, despite its exceptional magnitude, had only a minor impact on the evolution of low-latitude monsoonal climate on centennial to millennial time scales.

Petrography and geochemistry of organic matter at DSDP Holes 80-549 and 80-550B

Three lower Barremian to middle/upper Cenomanian samples from DSDP Hole 549 and three lower Cenomanian to lower Maestrichtian samples from DSDP Hole 550B were investigated by organic geochemical and organic petrographic methods. The samples came from wells drilled in the area of the Goban Spur in the northeastern Atlantic; they represent gray to greenish gray carbonaceous mud or siltstones from the deeper parts of the Cretaceous sequences penetrated and light-colored chalks from the shallower ones. The total amount of organic carbon is below 1% in all samples; it is especially low in the Cenomanian to Maestrichtian chalks. Terrigenous organic matter predominates; only the Barremian sample shows a moderate number of marine phytoclasts. As indicated by several parameters, the maturity of the organic matter is low, corresponding to about 0.4% vitrinite reflectance.

(Table 2) Chemistry of the organic matter at DSDP Leg 62 Holes

The wide distribution of sapropelic deposits in the sedimentary cover of the oceans, their Cretaceous age, and their possible oil- and gas-generating characteristics allow us to regard these deposits as a regular global stage in the history of oceanic sedimentation. So, Cretaceous sapropelic deposits are a unique object for study. Cretaceous sapropelic deposits of DSDP Sites 463, 465, and 466, as well as similar sediments of the Atlantic and Indian Oceans, are characterized by enrichment in organic matter, which sometimes reaches 33% (Cape Verde Basin, DSDP Sites 367 and 368). The objective of this study is the elucidation of genesis, paleogeographic environment of sedimentation, and oil-generating potential of Cretaceous sapropelic deposits at these sites. Attention is given to petrographic composition and distribution of the organic matter.

Tab.1: Representative bulk-rock and trace element chemistry

High-pressure/low-temperature metabasites occupy a definite geological position within the structure of the Polar Urals and have a very important bearing on the understanding of the early history of the Ural Mountains. Recently obtained geological, petrographic, geochemical and isotope data allow some conclusions on this history. The metabasites of the Khord"yus and Dzela complexes contain relics of a Neoproterozoic (578 ±8 Ma) oceanic crust. This crust formed part of the base of the early Paleozoic (500 Ma) ensimatic island arc and experienced Ca-Al-Si±Na metasomatism and, probably, partial melting with the formation of boninite melts. However, so far no boninite volcanics have been found. The metabasites at the base of the island arc took part in the collision and as a consequence experienced glaucophane schist and greenschist facies metamorphism during the collision and obduction over the passive Baltic margin 350 ±11 Ma ago.

(Table 1) Stable isotope data of chemoherm carbonates at Hydrate Ridge on the Cascadia continental margin

Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly 13C-depleted carbon isotope values of the samples (as low as -48.1 per mill PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The d18O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 per mill PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 per mill PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.

Isotope composition of dolomite, calcite and pore-waters of ODP Leg 210 samples

Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, d13C, d18O and 87Sr/86Sr values. The results are compared with the chemistry, and d13C and 87Sr/86Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre-dates the formation of framboidal pyrite. Most dolomite layers show 87Sr/86Sr-ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the d13C values of the dolomite are not in equilibrium with the d13C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87Sr/86Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (<30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co-occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate-methane interface. It has been hypothesized that microbial 'hot-spots', such as the sulphate-methane interface, may act as focused sites of dolomite precipitation. Varying d13C values from -15 per mil to +15 per mil for the dolomite are consistent with precipitation at a dynamic sulphate-methane interface, where d13C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate-methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon-rich sedimentary sequences can provide a useful window into the palaeo-dynamics of the deep biosphere.

Geochemistry and petrology of ODP Site 136-843 basalts

About 13 m of Cretaceous, tholeiitic basalt, ranging from normal (N-MORB) to transitional (T-MORB) mid-ocean-ridge basalts, was recovered at Ocean Drilling Program Site 843 west of the island of Hawaii. These moderately fractionated, aphyric lavas are probably representative of the oceanic basement on which the Hawaiian Islands were built. Whole-rock samples from parts of the cores exhibiting only slight, low-temperature, seawater alteration were analyzed for major element, trace element, and isotopic composition. The basalts are characterized by enrichment in the high field strength elements relative to N-MORB, by a distinct positive Eu anomaly, and by Ba/Nb and La/Nb ratios that are much lower than those of other crustal or mantle-derived rocks, but their isotope ratios are similar to those of present-day N-MORB from the East Pacific Rise. Hole 843A lavas are isotopically indistinguishable from Hole 843B lavas and are probably derived from the same source at a lower degree of partial melting, as indicated by lower Y/Nb and Zr/Nb ratios and by higher concentrations of light and middle rare earth elements and other incompatible elements relative to Hole 843B lavas. Petrographic and trace-element evidence indicates that the Eu anomaly was the result of neither plagioclase assimilation nor seawater alteration. The Eu anomaly and the enrichments in Ta, Nb, and possibly U and K relative to N-MORB apparently are characteristic of the mantle source. Age-corrected Nd and Sr isotopic ratios indicate that the source for the lavas recovered at ODP Site 843 was similar to the source for Southeast Pacific MORB. An enriched component within the Cretaceous mantle source of these basalts is suggested by their initial 208Pb/204Pb-206Pb/204Pb and epsilon-Nd-206Pb/204Pb ratios. The Sr-Pb isotopic trend of Hawaiian post-shield and post-erosional lavas cannot be explained by assimilation of oceanic crust with the isotopic composition of the Site 843 basalts.

Chemical composition of basalts from ODP holes at the Kerguelen Plateau

Petrographic and geochemical study of basalts in the Kerguelen Plateau basement revealed changes in composition and character of volcanism during development of this tectonovolcanic structure. The Kerguelen Plateau is one of the largest intraplate rises in the World Ocean. It started to form about 120 Ma ago. Age of basalts and overlying sediments shows that plateau formation was in the northwest direction. Basalts of the Kerguelen Plateau basement are products of tholeiitic melts in terms of geochemistry, but differ from mid-ocean ridge basalt (MORB). They are enriched in incompatible trace elements and rare earth elements (REE) relative to MORB, and degree of enrichment varies in basalts from different segments of the plateau. Composition of basalts does not directly depend on their age. Specific features of plateau magmatism are commonly explained in terms of a long-living deep magma plume, which variously interacted with a depleted upper mantle source at different stages of plateau formation. However, taking into account block morphology and deep structure of the plateau, one can suggest that plateau volcanism was initiated by a large fault. As the volcanism prograded to the northwest, depth of fault penetration into the mantle changed. Composition of basalts in the plateau basement was also governed by formation depth of primary melts.

Major and trace elements and minerals at DSDP Hole 79-544A

Fifty m of basement rocks underlying 185 m of Neogene and Mesozoic sediments were drilled seaward of the Mazagan Slope about 100 km west of Casablanca during Leg 79. These rocks are metagranites with mylonitic textures consisting dominantly of quartz, plagioclase, and potassium feldspar. Chemically, they are strongly peraluminous. This along with the absence of hornblende suggest that these rocks are similar to the S-type granites. Petrographic and chemical data suggest the possible existence of a former weathering surface on top of the Mazagan metagranite.