Chemical and isotopic compositions of sediments from DSDP Hole 21-204

The highly depleted intra-oceanic Tonga-Kermadec island arc forms an endmember of arc systems and a unique location in which to isolate the effects of the slab flux. High precision TIMS uranium, thorium, strontium, neodymium, and lead isotopes, along with complete major and trace element data, have been obtained on an extensive sample set comprising fifty-eight lavas along the arc as well as nineteen samples of the subducting sediments at DSDP site 204 just to the east of the Tonga-Kermadec trench. Ca/Ti and Al/Ti ratios extend from values appropriate to an N-MORB source in the southern Kermadecs to very high ratios in Tonga interpreted to reflect increasing degrees of depletion of the mantle wedge due to backarc basalt extraction. The isotope data emphasize the need for four components in the petrogenesis of the lavas: (1) the mantle wedge; (2) a component with elevated 207Pb/204Pb towards which the Kermadec and southern Tongan lavas extend; (3) a component characterised by high 206Pb/204Pb, Ta/Nd, and low 143Nd/144Nd observed only in the northernmost Tongan islands of Tafahi and Niuatoputapu; (4) a fluid component characterised by strong enrichments of Rb, Ba, U, K, Ph, and Sr, relative to Th, Zr, and the REE and producing large 238U excesses ((230Th/238U) = 0.8-0.5) in the more depleted lavas. The mantle wedge (Component 1) is isotopically similar to the source of the Lau BABB. Component 2 is average pelagic sediment on the downgoing Pacific plate as observed at DSDP sites 595/596 and in the upper sections of the sediment pile at DSDP site 204. Mass balance calculations indicate that less than 0.5% is recycled into the arc lavas; essentially all the subducted sediment is returned to the upper mantle (~0.03 km**3/yr). Exceptionally low concentrations of Ta and Nb relative to Th and the LREE requires that this sediment component is added as a partial melt which was in equilibrium with residual rutile or ilmenite. Component 3 is identified as volcaniclastics from the Louisville Ridge which comprise the lower 44 m of the sediment section intersected at DSDP site 204. These volcaniclastics are spatially restricted to the vicinity of the Louisville Ridge and provide a unique sediment tracer which can be used to show that it takes 4 Myr from the time of subduction to its first appearance in the arc lava signature. Component 4, the fluid contribution to the lava source is inferred to contribute ~1 ppm Rb, 10 ppm Ba, 0.02 ppm U, 600 ppm K, 0.2 ppm Ph, and 30 ppm Sr. It has 87Sr/86Sr = 0.7035 and 206Pb/204Pb = 18.5 and thus it is inferred to have been derived from dehydration of the subducting altered oceanic crust. U-Th isotope disequilibria reflect the time since fluid release from the subducting slab and a reference line through the lowest (230Th/232Th) lavas constrains this to be 30000-50000 yr. The U-Th and Th-Ra isotope systematics are decoupled, and it is suggested that Th-Ra isotope disequilibria record the time since partial melting and thus indicate rapid channelled magma ascent. Olivine gabbro xenoliths from Raoul are interpreted as cumulates to their host lavas with which they form zero age U-Th isochrons indicating that minimal time was spent in magma chambers. The subduction signature is not observed in lavas from the backarc island of Niuafo'ou. These were derived from partial melting of fertile peridotite at 130-160 km depth with melt rates around 0.0002 kg/m**3/yr.

Carbonate, organic carbon, pyrolysis and composition of the extracts at DSDP Holes 76-534A and 44-391

The geochemical studies of Sites 534 and 391 and their comparison allow us to improve the chemical characterization of different geological formations dating from the early Callovian to the Maestrichtian along the continental margin of eastern North America. Three of the formations are favorable for the preservation of organic matter: (1) the unnamed formation (middle Callovian to Oxfordian), (2) the Blake-Bahama Formation (Berriasian to Barremian), and (3) the Hatteras Formation (Aptian to Cenomanian). The organic matter is mainly detrital, except for a few organic-rich layers where a contribution of aquatic material occurs. In these organic-rich layers, the petroleum potential is medium to good. Maturation has not quite reached the beginning of the oil window even for the deepest organic material.

Thickness and duration of biostratigraphic zones, assemblages, ages and sedimentation rates at DSDP Sites 76-534 and 44-391

In this Initial Report of the Deep Sea Drilling Project, detailed studies of Sites 533 (gas hydrates) on the Blake Outer Ridge and 534 (oldest ocean history) in the Blake-Bahama Basin have provided answers to many geological and geophysical questions posed over the decade that deep drilling has been undertaken in this part of the western North Atlantic. The history of drilling and a historical review of key scientific accomplishments have been presented in the Introduction (Gradstein and Sheridan, this volume). In this final chapter we review highlights of new geological, geophysical and paleoceanographic interpretations presented in this volume, and offer a critical review of this information. We conclude with a listing of some outstanding problems and recommendations for future research, including data collection.

Organic geochemical characteristics at DSDP Holes 76-534A and 44-391C

Pyrolysis assay, bitumen analysis, and elemental analysis of kerogen were used to characterize the organic matter of selected core samples from Hole 534A (Leg 76) and Hole 391C (Leg 44) on the Blake-Bahama Plateau. The organic matter throughout the stratigraphic section appears to be principally of a terrestrial origin. The data from several isolated horizons in the Hatteras and Blake-Bahama Formations imply the presence of significant quantities of autochthonous marine organic matter. However, these horizons appear so limited that they cannot be considered potential liquid hydrocarbon source rocks. All the analyzed samples are immature and have not evolved sufficiently to enter into the main stage of hydrocarbon generation. The temporal and spatial restrictions of strata rich in marine organic matter suggest that they do not represent major expansions and contractions of anoxic bottom-water masses, but represent limited occurrences of anoxic conditions.

Composition of volcanic rocks from the Alchan Basin, Northwestern Primorie

Geological, petrochemical, and geochemical data are reported for volcanic rocks of a Cretaceous pull-apart basin in the Tan Lu strike-slip system, Asian continental margin. A comparison of these volcanic rocks with magmatic rocks from typical Cenozoic transform margins in the western North America and rift zones of Korea made it possible to distinguish some indicator features of transform-margin volcanic rocks. Magmatic rocks from strike-slip extension zones bear island-arc, intraplate, and occasionally depleted MORB geochemical signatures. In addition to calc-alkaline rocks there are bimodal volcanic series. The rocks are characterized by high K2O, MgO, and TiO2 contents. They show variable enrichment in LILE relative to HFSE, which is typical of island-arc magmas. At the same time they are rich in compatible transition elements, which is a characteristic of intraplate magmas. Trace element distribution patterns normalized to MORB or primitive mantle usually show a negative Ta-Nb anomaly typical of suprasubduction settings. Their Ta/Nb ratio is lower, whereas Ba/Nb, Ba/La, and La/Yb ratios are higher than those of some MORB and OIB. In terms of trace element systematics, for example, Ta-Th-Hf, Ba/La-(Ba/La)_n, (La/Sm)_n-La/Hf, and others, they fall within the area of mixing of magmas from several sources (island arc, intraplate, and depleted reservoirs). Magmatic rocks of transform settings show a sigmoidal chondrite-normalized REE distribution pattern with a negative slope of LREE, depletion in MREE, and an enriched or flat HREE pattern. Magmas with mixed geochemical characteristics presumably originated in a transform margin setting in local extension zones under influence of mantle diapirs, which caused metasomatism and melting of the lithosphere at different levels, and mixing of melts from different sources in variable proportions.

Geochemical processes and rare earth element analysis in cold-seep pore waters of Hydrate Ridge, northeast Pacific Ocean

The concentrations of rare earth elements (REEs), sulphate, hydrogen sulphide, total alkalinity, calcium, magnesium and phosphate were measured in shallow (<12 cm below seafloor) pore waters from cold-seep sediments on the northern and southern summits of Hydrate Ridge, offshore Oregon. Downward-decreasing sulphate and coevally increasing sulphide concentrations reveal sulphate reductionas dominant early diagenetic process from ~2 cm depth downwards. A strong increase of total dissolved REE concentrations is evident immediately below the sediment-water interface, which can be related to early diagenetic release of REEs into pore water resulting from the remineralization of particulate organic matter. The highest pore water REE concentrations were measured close to the sediment-water interface at ~2 cm depth. Distinct shale normalized REE patterns point to particulate organic matter and iron oxides as main REE sources in the upper ~2-cm depth interval. In general, the pore waters have shalenormalized patterns reflecting heavy REE (HREE) enrichment, which suggests preferential complexation of HREEs with carbonate ions. Below ~2 cm depth, a downward decrease in REE correlates with a decrease in pore water calcium concentrations. At this depth, the anaerobic oxidation of methane (AOM) coupled to sulphate reduction increases carbonate alkalinity through the production of bicarbonate, which results in the precipitation of carbonate minerals. It seems therefore likely that the REEs and calcium are consumed during vast AOM-induced precipitation of carbonate in shallow Hydrate Ridge sediments. The analysis of pore waters from Hydrate Ridge shed new light on early diagenetic processes at cold seeps, corroborating the great potential of REEs to identify geochemical processes and to constrain environmental conditions.

Chemical composition and accumulation rates of chemical elements in metalliferous sediments from axial zones of the East Pacific Rise and the Mid-Atlantic Ridge

In the monograph metalliferous sediments of the East Pacific Rise near 21°S are under consideration. Distribution trends of chemical, mineral and grain size compositions of metalliferous sediments accumulated near the axis of this ultrafast spreading segment of the EPR are shown. On the basis of lithological and geochemical investigations spatial and temporal variations of hydrothermal activity are estimated. Migration rates of hydrothermal fields along the spreading axis are calculated. The model of cyclic hydrothermal process is suggested as a result of tectono-magmatic development of the spreding centre.