Lead isotope ratios of basalts from DSDP Leg 37 (Table 1)

Analyses of Pb from mid-ocean ridge basalts obtained from DSDP Leg 37 lie on a line of near-zero-age slope. The data seem consistent with an evolutionary model of three stages, the beginning of the second stage being at about 600 m.y. - the model age obtained for the least radiogenic Pb. The beginning of the third stage of evolution at the time of formation of the rocks apparently did not change the average Th/U ratio since measured values are consistent with the ratio deduced from Pb isotope ratios. Ratios of U/Pb are not consistent with Pb isotope ratios, however, thus ruling out a simple two-stage evolutionary model.

Strontium isotope and rare earth element geochemistry of basalts from DSDP Leg 37

87Sr/S6Sr ratios have been determined on eleven whole rock basalt samples from DSDP Leg 37. The 87Sr/S6Sr ratios range from 0.70305 +/- 4 to 0.70451 +/- 4 due to alteration and contamination with seawater Sr. Leaching with 5% HF has only a small effect on the 87Sr/86Sr of the samples. However, treatment with 6M HCl in acid digestion bombs at 130°C removes the contaminant more effectively. Altered plagioclase and olivine are dissolved during this process. The mean 87Sr/86Sr of four HCl-treated samples from hole 332A is 0.70299 and that for five samples from hole 332B is 0.70297. The 87Sr/86Sr ratios of treated samples from holes 333A and 335 are 0.70304 +/- 4 and 0.70316 +/- 4, respectively. These 87Sr/86Sr ratios are within the range observed for other basalts elsewhere along the Mid-Atlantic Ridge in the North Atlantic. REE distribution patterns have been determined for four samples, three from hole 332B and one from hole 335. CeN/YbN ratios range from 0.58 to 1.30 and do not correlate with 87Sr/86Sr ratios. The source regions of these basalts appear to have been variable in REE abundances.

The distribution of sulfur in oceanic crust with respect to oxidative diagenesis (Table 1)

A detailed geochemical-petrological examination of layer 2 basalts recovered during Leg 37 of the DSDP has revealed that the original distribution, form and abundance of igneous sulfide have been profoundly altered during low-grade oxidative diagenesis. The net result appears to have been a rather pervasive remobilization of igneous sulfide to form secondary pyrite accompanied by a bulk loss of sulfur equivalent to about 50-60% of the original igneous value, assuming initial saturation. It is suggested that during infiltration of seawater into the massive crystalline rock, igneous sulfide has experienced pervasive oxidation, under conditions of limited oxidation potential, to form a series of unstable, soluble sulfur species, primarily in the form of SO3[2-] and S2O3[2-]. Spontaneous decomposition of these intermediate compounds through disproportionation has resulted in partial reconstitution of the sulfur as secondary pyrite and the generation of SO4[2-] ion, which, due to its kinetic stability, has been lost from the basalt system and ultimately transferred to the ocean. This model not only satisfies the geochemical and petrological observations but also provides a suitable explanation for the highly variable delta34S values which characterize secondary sulfides in deep ocean floor basalts.

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).

Rubidium and Strontium chemistry of oceanic crust

Times of vein mineral deposition in the ocean crust have been determined both by Rb-Sr isochron ages of vein smectites and by comparison of 87Sr/86Sr ratios of vein calcites with the known variations of seawater 87Sr/86Sr ratio with time. Results from drilling sites 105, 332B and 418A, Atlantic Ocean, which have basement formation ages of 155 m.y., 3.5 m.y., and 110 m.y., respectively, show that vein deposition is essenrially complete within 5-10 m.y. after formation of the basaltic crust. This provids direct evidence that hydrothermal circulation of sea-water through the oceanic crust is an important process for only 5-10 m.y. after crust formation.

Results of biological and geological bottom investigations carried out during Cruise 43 of R/V Akademik Kurchatov in the South Atlantic

The book is devoted to investigations of benthic fauna and geology of the Southern Atlantic Ocean. These works have been carried out in terms of exploring biological structure of the ocean and are of great importance for development of this fundamental problem. They are based on material collected during Cruise 43 of R/V Akademik Kurchatov in 1985-1986 and Cruise 43 of R/V Dmitry Mendeleev in 1989. Problems of quantitative distribution, group composition and trophic structure of benthos in the Southern Scotia Sea, along the east-west Transatlantic section along 31°30'S, and offshore Namibia in the area of the Benguela upwelling are under consideration in the book. Authors present new data on fauna of several groups of deep-sea bottom animals and their zoogeography. Much attention is paid to analysis of morphological structure of the Scotia Sea floor considered in terms of plate tectonics. Bottom sediments along the Transatlantic section and facial variation of sediments in the area of South Shetland Islands and of the continental margin of Namibia are under consideration.

Chemical composition of bottom sediments and rocks from DSDP Holes 3-14 through 64-477

Data on analyses of chemical composition of DSDP samples of bottom sediments and rocks carried out in P.P. Shirshov Institute of Oceanology are reported. Basal sediments and sedimentary rocks prevail in the sample set.

Nitrogen isotopic composition and chemical composition of altered oceanic crust from ODP Legs 129 and 185

Knowledge of the subduction input flux of nitrogen (N) in altered oceanic crust (AOC) is critical in any attempt to mass-balance N across arc-trench systems on a global or individual-margin basis. We have employed sealed-tube, carrier-gas-based methods to examine the N concentrations and isotopic compositions of AOC. Analyses of 53 AOC samples recovered on DSDP/ODP legs from the North and South Pacific, the North Atlantic, and the Antarctic oceans (with larger numbers of samples from Site 801 outboard of the Mariana trench and Site 1149 outboard of the Izu trench), and 14 composites for the AOC sections at Site 801, give N concentrations of 1.3 to 18.2 ppm and d15N_air of -11.6? to +8.3?, indicating significant N enrichment probably during the early stages of hydrothermal alteration of the oceanic basalts. The N-d15N modeling for samples from Sites 801 and 1149 (n=39) shows that the secondary N may come from (1) the sedimentary N in the intercalated sediments and possibly overlying sediments via fluid-sediment/rock interaction, and (2) degassed mantle N2 in seawater via alteration-related abiotic reduction processes. For all Site 801 samples, weak correlation of N and K2O contents indicates that the siting of N in potassic alteration phases strongly depends on N availability and is possibly influenced by highly heterogeneous temperature and redox conditions during hydrothermal alteration. The upper 470-m AOC recovered by ODP Legs 129 and 185 delivers approximately 800 kg/km N annually into the Mariana margin. If the remaining less-altered oceanic crust (assuming 6.5 km, mostly dikes and gabbros) has MORB-like N of 1.5 ppm, the entire oceanic crust transfers 5100 kg/km N annually into that trench. This N input flux is twice as large as the annual N input of 2500 kg/km in seafloor sediments subducting into the same margin, demonstrating that the N input in oceanic crust, and its isotopic consequences, must be considered in any assessment of convergent margin N flux.

Paleointensity of various DSDP sites (Table 3)

The existing database for paleointensity estimates of the ancient geomagnetic field contains more than 1500 data points collected through decades of effort. Despite the huge amount of work put into obtaining these data, there remains a strong bias in the age and global distribution of the data toward very young results from a few locations. Also, few of the data meet strict criteria for reliability and most are of unknown quality. In order to improve the age and spatial distribution of the paleointensity database, we have carried out paleointensity experiments on submarine basaltic glasses from a number of DSDP sites. Of particular interest are the sites that provide paleointensity data spanning the time period 0.3-5 Ma, a time of relatively few high quality published data points. Our new data are concordant with contemporaneous data from the published literature that meet minimum acceptance criteria, and the combined data set yields an average dipole moment of 5.49 +/- 2.36*10**22 Am**2. This average value is comparable to the average paleofield for the period 5-160 Ma (4.2 +/- 2.3*10**22 Am**2) (Juarez et al., 1998, doi:10.1038/29746) and is substantially less than the value of approximately 8*10**22 Am**2 often quoted for the last 5 Myr (e.g. McFadden and McElhinny (1982) J. Geomagn. Geoelectr. 34, 163-189; Goguitchaichvili et al., 1999, doi:10.1016/S0012-821X(99)00010-2).

(Table 2) Chlorine geochemistry and mineralogy of altered oceanic crust samples

Bulk chlorine concentrations and chlorine stable isotope compositions were determined for hydrothermally altered basalt (extrusive lavas and sheeted dikes) and gabbro samples (n = 50) from seven DSDP/ODP/IODP drill sites. These altered oceanic crust (AOC) samples span a range of crustal ages, tectonic settings, alteration type, and crustal depth. Bulk chlorine concentrations range from < 0.01 wt.% to 0.09 wt.%. In general, higher chlorine concentrations coincide with an increase in temperature of alteration and amphibole content. d37Cl values of whole rock AOC samples range from -1.4 to +1.8 per mil. High d37Cl values (>=~0.5 per mil) are associated with areas of higher amphibole content. This observation is consistent with theoretical calculations that estimate amphibole should be enriched in 37Cl compared to co-existing fluid. Negative to near zero d37Cl values are found in areas dominated by clay minerals. Chlorine geochemistry is a rough indicator of metamorphic grade and mineralogy. AOC is a major Cl host in the subducting oceanic lithospheric slab. Here we show that bulk chlorine concentrations are ~3 times higher than previous estimates resulting in a greater contribution of Cl to the mantle.

(Table 1) Basement penetration, recovery, and lithology of crustal DSDP holes in the North Atlantic Ocean

Oceanic crustal drilling by R. V. Glomar Challenger at 15 sites in the North Atlantic has led to a complex picture of the upper half kilometer of the crust. Elements of the picture include the absence of the source for linear magnetic anomalies, marked episodicity of volcanic activity, ubiquitous low temperature alteration and evidence for large scale tectonic disturbance. Comparison sections in the Pacific and much deeper crustal drilling are needed to attack problems arising from the North Atlantic results.

Nd- and Sr- isotope ratios of oceanic basalts

143Nd/144Nd ratios have been determined on 37 samples of oceanic basalt, with a typical precision of +/- 2-3 * 10**-5 (2 sigma). Ocean island and dredged and cored submarine basalts are included for which reliable measurements of 87Sr/86Sr ratios exist in the literature or have been measured as part of this study. A strong negative correlation exists between 143Nd/144Nd and 87Sr/86Sr ratios in basalts from Iceland and the Reykjanes Ridge, but such a clear correlation does not exist for samples from the Hawaiian Islands. However, when other ocean island basalts from the Atlantic are included there is an overall correlation between these two parameters. Increases and decreases in Rb/Sr in oceanic basalt source regions have in general been accompanied by decreases and increases respectively in Sm/Nd ratios. The compatibility of the data with single-stage models is assessed and it is concluded that enrichment and depletion events, which are consistent with transfer of silicate melts, are responsible for the observed variation.