Mineralogy and geochemistry at DSDP Leg 80 Holes

Cenozoic sediments recovered from Sites 548, 549, and 550 were the objects of mineralogical (bulk sample and <2 - µm fraction) and geochemical (HCl extract) studies. Thin sections of rock pebbles embedded in sediments (upper levels at Site 548, particularly) were examined on a polarizing microscope. This study outlines the vertical and lateral variation and evolution of the sedimentation. In the Paleocene and lower Eocene, the clay fraction is abundant and smectite is practically the sole existing clay mineral. High Mn, Al, Fe, Mg, and K contents were measured in HCl extracts. Through the middle Eocene, carbonates become more abundant - highly dominant at Site 548. Metal contents in HCl extracts are very low. The clay fraction, although dominated at all sites by smectites, becomes richer in illite and poorly crystallized chlorite. At the middle/upper Miocene boundary, a significant decrease in the smectite/(illite + chlorite) ratio occurs at all sites, and this decrease continues into the middle Pliocene. This decrease is marked by an abrupt increase of quartz at Site 548. At the two other sites, carbonates remain highly predominant; HCl extracts reflect the relative abundance of the clay and carbonate fractions. After a brief recurrence of smectite in a high-metal-content interval, illite and chlorite become the dominant clay minerals in the upper Pliocene and the Pleistocene, where numerous variations in mineralogical composition occur in the clay fraction (Sites 548 and 549) or in non-clay components (Site 548). Several pebbles of various nature and origin, encountered in different levels of this interval at Site 548, appear to have an ice-rafting origin. This study points out three main breaks in the general evolution of the sedimentation: the first, corresponding to the lower/middle Eocene boundary, is marked by the increase of carbonates and associated elements; the second, corresponding to the middle/upper Miocene boundary, is marked by a major decrease of the smectite/(illite + chlorite) ratio at all sites and by a massive appearance of quartz at Site 548; and the third, which occurred toward the late Pliocene, is marked by the dominance of primary clay minerals and the arrival of ice-rafted pebbles. Our interpretation of results considers paleohydrological and paleoclimatic phenomena. It is suggested that the major middle/late Miocene break was associated with an increase of the deep bottom-water circulation between the Norwegian Sea and the North Atlantic Ocean, and/or a climatic evolution: humidification and cooling of climate. The changes toward the late Pliocene appear to have been the first effects of the glaciations at the end of Cenozoic.

Chemical and mineral compositions of bottom sediments from the East Pacific Rise near 13°N

During underwater photography and sampling of the rift valley bottom in the axial part of the East Pacific Rise, where water transparency is reduced due to hydrothermal input, ore manifestations have been found. The bottom is covered by them as by a jacket on both sides from the EPR axial zone. However, exposed pillow-lavas and clumpy blocks in rift ledges are covered by a thin metal-bearing film. It is supposed that sedimentation results mainly from hydrothermal input of dissolved chemical elements in seawater, their transformation on the geochemical barrier, and subsequent deposition as particulates. Contents of ore components in metalliferous sediments have been measured by atomic-absorption and X-ray radiometry methods. Sediment age has been determined as Middle Pleistocene - Holocene. Maximal hydrothermal activity was at the beginning of Early Holocene, about 10 Ka. A smoker has been found on the western slope of the rift valley.

Geochemical history at DSDP Leg 56 Holes

A geological model of subduction postulated by Karig, Ingle, et al. (1975) and Karig and Sharman (1975) proposes that the sedimentary prism at the foot of the landward wall is being actively built as sediment is scraped off the subducting oceanic and plastered onto the base of the wedge, forming an accretionary wedge containing overthrust sedimentary layers or intense sedimentary folding. Because overlying layers must continually be uplifted and compressed to accommodate new matter at the base, the accreting wedge will provide a geochemical record of this process at or near the Japan Trench. Several recent papers have discussed the metalliferous sediments on the active oceanic ridges. The geochemistry of such sediments is now reasonably well known: generally these deposits are considered products of volcanic processes (Boström and Peterson, 1969; Böstrom et al., 1969; Horowitz, 1970, 1974; Cronan et al., 1972; Cronan and Garrett, 1973). The geochemistry of subduction zone sediments, however, is less well known, and the need for studies of these sediments is particularly urgent if such sediments provide a record of the effects of subduction of oceanic plates under continental crust. Because the Japan Trench contains welldeveloped subduction zone deposits, Leg 56 sampling was of utmost importance to the discovery of how they originate.

Geochemistry of interstitial waters of Lau Backarc Basin sediment samples

Interstitial water samples from Sites 834 through 839, drilled during Ocean Drilling Program Leg 135 in the backarc Lau basin (Southwestern Pacific), have been analyzed for major elements, manganese, copper, strontium, barium, vanadium, and 87Sr/86Sr isotopic composition values. The concentration-depth profiles of the major chemical components show almost straight concentration gradients at all sites, and seem to reflect slight alteration of volcanic material. However, in the lower part of the sedimentary cover, where volcanogenic material is abundant and where diagenetic minerals occur, systematic decreases in calcium, strontium, manganese, copper, and vanadium concentrations are observed. A downwelling flow of bottom seawater, which affected the diagenetic chemical signature of the interstitial water, is probably responsible for the recorded chemical features. This hypothesis is supported by strontium isotope data obtained from interstitial water samples at Site 835. It is also in accordance with data from heat flow and physical properties.

Geochemistry of Loihi Seamount hydrothermal deposits

High-resolution bathymetric surveys, bottom photography and sample analyses show that Loihi Seamount at the southernmost extent of the Hawaiian ëhotspotí is an active, young submarine volcano that is probably the site of an emerging Hawaiian island. Hydrothermal deposits sampled from the active summit rift system were probably formed by precipitation from cooling vent fluids or during cooling and oxidation of high-temperature polymetallic sulphide assemblages. No exotic benthic fauna were found to be associated with the presently active hydrothermal vents mapped.

Pore-water chemistry at DSDP Leg 70 Holes

Geological and geophysical data collected during Deep Sea Drilling Project (DSDP) Leg 70 indicate that hydrothermal solutions are upwelling through the sediments of the mounds hydrothermal field (Sites 506, 507, and 509) and downwelling in the low heat-flow zone to the south (Site 508). Pore-water data are compatible with these conclusions. Pore waters at mounds sites are enriched in Ca and depleted in Mg relative to both seawater and Site 508 pore waters. These anomalies are believed to reflect prior reaction of the interstitial waters with basement rocks. The mounds solutions are also enriched in iron, which is probably hydrothermal and en route to forming nontronite. Concentrations of Si and NH3 in mounds pore water increase upcore as a result of the addition of dissolving biogenic debris to ascending hydrothermal solutions. Some low heat-flow pore-water samples (Site 508) are enriched in Ca and depleted in Mg. These anomalies likely reflect the presence of pockets of hydrothermal solutions in areas otherwise dominated by downwelling bottom water.

Interstitial water and sediment chemistry at DSDP Sites 87-582 and 87-584

Two trenches off Japan were explored during DSDP Leg 87. One is the Nankai Trough and the other is the Japan Trench; Site 582 is located on the floor of the former and Site 584 is situated on the deep-sea terrace of the latter. Cores from Site 582 and 584 consist mainly of hemipelagic sediments and diatomaceous silts and mudstone, respectively. In this report we analyze the chemistry of the interstitial water and sediments, as well as the sediment mineralogy. Sulfate reduction is accompanied by the production of secondary pyrite, which is rich in the sediment at both sites. Dissolved Ca concentration is relatively low and changes only slightly at both sites, probably because of the formation of carbonate with high alkalinity. Concentrations of dissolved Mg decrease with depth at Site 584. The dissolved Mg depletion probably results from the formation of Mg-rich carbonate and/or ion exchange and reaction between interstitial water and clay minerals. Higher Si/Al values are due to biogenic opal in the sediments and roughly correlate with higher values of interstitial water SiO2. Increases in dissolved Li concentrations may be related to its release from clay minerals, to advection that results from dewatering, and/or to fluid transport.

Chemical composition of phillipsites from Core AKO26-35

Behavior of rare earth elements (REE) was examined in oceanic phillipsites collected from four horizons of eupelagic clay in the Southwest Basin of the Pacific Ocean. REE concentrations were determined in >50 ?m size fraction phillipsite samples by the ICP-MS method. Composition of separate phillipsite aggregates was studied by electron microprobe and secondary ion mass-spectrometry. Rare earth elements in phillipsite samples are related to admixture of ferrocalcium hydroxophosphates. Analysis of separate phillipsite aggregates reveals low (<0.1-18.1 ppm) REE(III) concentrations. Ce concentration varies between 2.7 and 140 ppm. The correlation analysis shows that REE(III) present in admixture of iron oxyhydroxides in separate phillipsite aggregates. Based on the REE(III) concentration in iron oxyhydroxides we can identify two generations of phillipsite aggregates. Massive rounded aggregates (phillipsite I) are depleted in REE, while pseudorhombic (phillipsite II) aggregates are enriched in REE and marked by a positive Ce anomaly. Oceanic phillipsites do not accumulate REE or inherit the REE signature of volcaniclastic material and oceanic deep water. Hence, REE distribution in phillipsites does not depend on sedimentation rate and composition of host sediments.

Chemistry of interstitial waters and sediments at DSDP Leg 64 Holes

Studies of interstitial waters obtained from DSDP Leg 64 drill sites in the Gulf of California have revealed information both on early diagenetic processes in the sediments resulting from the breakdown of organic matter and on hydrothermal interactions between sediments and hot doleritic sill intrusions into the sediments. In all the sites drilled sulfate reduction occurred as a result of rapid sediment accumulation rates and of relatively high organic carbon contents; in most sites methane production occurred after sulfate depletion. Associated with this methane production are high values of alkalinity and high concentrations of dissolved ammonia, which causes ion exchange processes with the solid phases leading to intermediate maxima in Mg++, K+, Rb+, and Sr++(?). Though this phenomenon is common in Leg 64 drill sites, these concentration reversals had been noticed previously only in Site 262 (Timor Trough) and Site 440 (Japan Trench). Penetrating, hot dolerite sills have led to substantial hydrothermal alteration in sediments at sites drilled in the Guaymas Basin. Site 477 is an active hydrothermal system in which the pore-water chemistry typically shows depletions in sulfate and magnesium and large increases in lithium, potassium, rubidium, calcium, strontium, and chloride. Strontium isotope data also indicate large contributions of volcanic matter and basalt to the pore-water strontium concentrations. At Sites 478 and 481 dolerite sill intrusions have cooled to ambient temperatures but interstitial water concentrations of Li+, Rb+, Sr++ , and Cl- show the gradual decay of a hydrothermal signal that must have been similar to the interstitial water chemistry at Site 477 at the time of sill intrusion. Studies of oxygen isotopes of the interstitial waters at Site 481 indicate positive values of d18O (SMOW) as a result of high-temperature alteration reactions occurring in the sills and the surrounding sediments. A minimum in dissolved chloride at about 100-125 meters sub-bottom at Sites 478, 481, and particularly Site 479 records a possible paleosalinity signal, associated with an event that substantially lowered salinities in the inner parts of the Gulf of California during Quaternary time.