Chemical composition and elemental oxide ratios to AI2O3 of bulk solids at DSDP Leg 64 Holes

Analyses of sediments from Leg 64 sites reveal a diverse and in one case unique geochemistry. Sites are characterized by high heat flow along an active, divergent plate boundary, or rapid accumulation of diatom muds, or both. The geochemical trends of Sites 474-476 at the tip of Baja California reflect changes4n the percentages of sedimentary components - particularly biogenous matter and mineralogy - that support interpretations of sedimentary environments inferred to be present since the commencement of subsidence along this young, passive continental margin. The sediments below dolerite sills in Holes 477, 477A, 478, and 481 show major mineralogic and chemical deviations from "average" hemipelagic sediments. The sills appear to have two functions: (1) they allow hydrothermal circulation and metamorphism in a partially closed system by trapping heat and fluids emanating from below, and (2) they expel heated interstitial fluids at the moment of intrusion and mobilize elements, most likely leading to the formation of metalliferous deposits along the surface traces of normal faults in the basin. The hydrothermal system as a whole appears to be localized and ephemeral, as is indicated by the lack of similar geochemical trends and high heat flow at Sites 478 and 481. Site 479 illustrates sedimentation in an oxygen-minimum zone with anoxic sediments and concomitant geochemical trends, especially for MnO. With few exceptions, geochemical trends are remarkably constant with depth, suggesting that Site 479 can serve as an "internal" standard or average sediment against which the magnitude of hydrothermal alteration at the basinal Sites 477, 478, and 481 can be measured.

Grain-size distribution and Folk's statistics at DSDP Leg 64 Holes

Grain size of 139 unconsolidated sediment samples from seven DSDP sites in the Guaymas Basin and the southeastern tip of the Baja California Peninsula was determined by sieve and pipette techniques. Shepard (1954) classification and Inman (1952) parameters correlation were used for all samples. Sediment texture ranged from sand to silty clay. On the basis of grain-size parameter, the sediments can be divided into three broad groups: (1) very fine sands and coarse silts; (2) medium- to very fine silts; and (3) clays and coarse silts.

Mineral chemistry of basalts from DSDP Holes 65-483 and 65-483B

During DSDP Leg 65, a series of holes was drilled into the oceanic basement across the mouth of the Gulf of California to study the composition of the crust and the nature of its construction at a young spreading center. In Holes 483 and 483B, two of the deepest basement holes drilled on this leg, the basement is characterized by an upper sequence of interlayered massive basalts and sediments underlain by a lower sequence of interlayered pillow and massive basalts. Electron microprobe analyses were performed on pyroxene, plagioclase, olivine, spinel, and glass from 14 representative samples of 10 of the 16 major lithologic units. These analyses along with petrographic results can be used to interpret the detailed crystallization history of the basalts. We believe from the results of this study that the basalts were formed by at least a three-stage cooling process, followed by eruption and formation of quench phases. Our data do not support magma mixing.

Anisotropy and acoustic properties at DSDP Holes 62-463 and 62-465A

Variations of acoustic properties within the sediment column may significantly affect the propagation of acoustic energy in the upper portion of the oceanic crust. Moreover, the acoustic properties of sediments reflect their mineral compositions, fabrics, and degrees of compaction and cementation. Hence, the physical properties of indurated deep-sea sediments are of considerable geophysical and geological interest. Chalks and limestones are particularly important because substantial accumulations of biogenic carbonates are generally present at the base of the deep-sea sediment column, and high-standing features such as Hess Rise are capped by calcareous deposits. This paper constitutes a preliminary report of the compressional-wave velocities and densities of 31 indurated calcareous sediment samples recovered at DSDP Sites 463 and 465, in the Mid-Pacific Mountains and on Hess Rise, respectively. The sample set includes nine pairs of samples in which velocities were measured parallel and perpendicular to bedding to determine the velocity anisotropy of the sediment. This research is part of an ongoing study of the seismic properties of indurated deep-sea carbonates.

Organic facies variations in the Mesozoic South Atlantic DSDP Holes

Visual kerogen and total organic carbon determinations indicate that there are two periods of organic enrichment events in the Mesozoic sediments of the South Atlantic. The first period, from the Late Jurassic through the late Aptian, is recorded in sediments from the Falkland Plateau, the Cape Basin, and the Angola Basin. Apparently, salinity stratification in the restricted basin, coupled with rising sea level, led to bottom water anoxia and organic enrichment. The second event, from the late Albian to the Santonian period, is recorded in sediments from the Angola Basin and the Sao Paulo Plateau. It appears to have been caused by development of an anoxic oxygen minimum zone at midwater depths. Organic matter sedimentation in the Mesozoic South Atlantic is controlled by geologic, climatic, eustatic, and Oceanographic factors.

Migration of C1 to C8 volatile organic compounds in sediments at DSDP Hole 75-530A

The distribution of C1 to C8 hydrocarbons in sediment samples from DSDP Leg 75, Hole 530A, indicates that significant amounts of methane and ethane have migrated from organic-rich to organic-lean shales in close proximity. Most compounds larger than ethane are not migrating out of black shales, where they occur in high concentrations. These results lead to a general model for assessing migration. In addition, three shale types are identified on the basis of organic carbon and pyrolysis products and patterns.

Geochemistry at DSDP Leg 82 Holes

The nine holes (556-564) drilled during DSDP Leg 82 in a region west and southwest of the Azores Platform (Fig. 1) exhibit a wide variety of chemical compositions that indicate a complex petrogenetic history involving crystal fractionation, magma mixing, complex melting, and mantle heterogeneity. The major element chemistry of each hole except Hole 557 is typical of mid-ocean ridge basalts (MORBs), whereas the trace element and rare earth element (REE) abundances and ratios are more variable, and show that both depleted Type I and enriched Type II basalts have been erupted in the region. Hole 556 (30-34 Ma), located near a flow line through the Azores Triple Junction, contains typically depleted basalts, whereas Hole 557 (18 Ma), located near the same flow line but closer to the Azores Platform, is a highly enriched FeTi basalt, indicating that the Azores hot-spot anomaly has existed in its present configuration for at least 18 Ma, but less than 30-34 Ma. Hole 558 (34-37 Ma), located near a flow line through the FAMOUS and Leg 37 sites, includes both Type I and II basalts. Although the differences in Zr/Nb and light REE/heavy REE ratios imply different mantle sources, the (La/Ce)ch (>1) and Nd isotopic ratios are almost the same, suggesting that the complex melting and pervasive, small-scale mantle heterogeneity may account for the variations in trace element and REE ratios observed in Hole 558 (and FAMOUS sites). Farther south, Hole 559 (34-37 Ma), contains enriched Type II basalts, whereas Hole 561 (14-17 Ma), located further east near the same flow line, contains Type I and II basalts. In this case, the (La/Ce)ch and Nd isotopic ratios are different, indicating two distinct mantle sources. Again, the existence along the same flow line of two holes exhibiting such different chemistry suggests that mantle heterogeneity may exist on a more pervasive and transient smaller scale. (Hole 560 was not sampled for this study because the single basalt clast recovered was used for shipboard analysis.) All of the remaining three holes (562, 563, 564), located along a flow line about 100 km south of the Hayes Fracture Zone (33°N), contain only depleted Type I basalts. The contrast in chemical compositions suggests that the Hayes Fracture Zone may act as a "domain" boundary between an area of fairly homogeneous, depleted Type I basalts to the south (Holes 562-564) and a region of complex, highly variable basalts to the north near the Azores hot-spot anomaly (Holes 556-561).

Lead and strontium isotopic composition of some metalliferous and pelagic sediments and basalts at DSDP Leg 70 Holes

In recent years, metalliferous sediments have been discovered overlying newly generated oceanic crust in the East Pacific, North Atlantic, Indian Ocean, Red Sea, Gulf of Aden, and elsewhere (e.g., Boström, 1973; Lalou et al., 1977; Bischoff, 1969; Boström and Fisher, 1971; Cann et al., 1977, respectively). Such material has also been recovered by drilling from sediments lying upon older oceanic crust (Boström et al., 1972, 1976; Horowitz and Cronan, 1976). Hydrothermal circulation of seawater at a spreading ridge results in the leaching of Fe, Mn, and possibly other elements from the basaltic volcanic layer and their transport and discharge into ocean bottom waters, whereupon fine-grained Fe-Mn-rich precipitates form and settle into the ambient sediment (cf. Corliss, 1971; Dasch et al., 1971; Spooner and Fyfe, 1973; Bischoff and Dickson, 1975; Heath and Dymond, 1977; Corliss et al., 1979, Edmond et al., 1979). Mn-rich crusts have also been recovered from active ridges and are inferred to have formed in the vicinity of hydrothermal discharge areas (Scott et al., 1974; Moore and Vogt, 1976; Corliss et al., 1978; Hoffert et al., 1978). The source of the trace elements in the metalliferous deposits is generally not clear. They may be derived from seawater by adsorption onto the precipitates or crusts, or from hydrothermal solutions which have leached them from the basalts. Pb, however, can be used as a geochemical tracer because of the known isotopic compositional differences between oceanic basalts and seawater. Isotopic investigations of Pb in ferruginous sediments from the East Pacific have shown that it has been derived partly or mostly from a basaltic source (Bender et al., 1971; Dasch et al., 1971; Dymond et al., 1973). In the present study, Pb isotopic analyses have been made of a suite of metalliferous sediments (nontronite, Mn-oxide crust, Mn-Fe-oxide mud), pelagic sediments, and basalts from the Galapagos mounds area. The main purposes of the Pb study were to determine the source or sources of Pb in the metalliferous sediments, and whether or not stratigraphic variations exist in the isòtopic composition of Pb in the sediments.

Composition of sediment pore water and isotopes in carbonate and benthic foraminifera at DSDP Holes 68-501, 69-505 and Site 69-504

Two sites on the southern flank of the Costa Rica Rift were drilled on DSDP Legs 68 and 69, one on crust 3.9 m.y. old and the other on crust 5.9 m.y. old. The basement of the younger site is effectively cooled by the circulation of seawater. The basement of the older site has been sealed by sediment, and an interval in the uppermost 560 meters of basement recently reheated to temperatures of 60 to 120°C. Although the thickness of the sediments at the two sites is similar (150-240 m versus 270 m), the much rougher basement topography at the younger Site 505 produces occasional basement outcrops, through which 80 to 90% of the total heat loss apparently occurs by advection of warm seawater. This seawater has been heated only slightly, however; the temperature at the base of the sediments is only 9°C. Changes in its composition due to reaction with the basement basalts are negligible, as indicated by profiles of sediment pore water chemistry. Bacterial sulfate reduction in the sediments produces a decrease in SO4 (and Ca) and an increase in alkalinity (and Sr and NH3) as depth increases to an intermediate level, but at deeper levels these trends reverse, and all of these species plus Mg, K, Na, and chlorinity approach seawater values near basement. Si, however, is higher, and Li may be lower. At the older site, Site 501/504, where heat loss is entirely by conduction, the temperature at the sediment/basement contact is 59°C. Sediment pore water chemistry is heavily affected by reaction with the basaltic basement, as indicated by large decreases in d18O, Mg, alkalinity, Na, and K and an increase in Ca with increasing depth. The size of the changes in d18O, Mg, alkalinity, Ca, Sr, and SO4 varies laterally over 500 meters, indicating lateral gradients in pore water chemistry that are nearly as large as the vertical gradients. The lateral gradients are believed to result from similar lateral gradients in the composition of the basement formation water, which propagate upward through the sediments by diffusion. A model of the d18O profile suggests that the basement at Site 501/504 was sealed off from advection about 1 m.y. ago, so that reaction rates began to dominate the basement pore water chemistry. A limestone-chert diagenetic front began to move upward through the lower sediments less than 200,000 yr. ago.

Geochemistry of carbon at DSDP Hole 61-462

Seven quartered sections of Pliocene to Mesozoic (Cenomanian) cores from the Nauru Basin contain primarily marine organic matter admixed with detectable amounts of terrigenous organic matter. The mixture is immature with respect to organic genesis. Chemical properties of this organic matter are compared with properties of other deep-ocean cores from DSDP sites in the central Pacific.

Geochemistry of DSDP Holes 60-453 and 60-454A

Pliocene and Pleistocene volcanic glass fragments from Mariana Trough sediments at Sites 453 (16 samples) and 454 (4 samples), located near the western edge of the trough and just west of the spreading axis, respectively, have been analyzed for major elements with an electron microprobe. They derive from volcanic activity on the present Mariana active arc. The glasses from Site 453 are all tholeiitic with a wide range of SiO2 contents. Those less than 2 m.y. old have slightly lower TiO2 and higher K2O contents than the older ones. The glasses from Site 454 are all Pleistocene and resemble the younger glasses at Site 453. Major element compositions of the older basaltic glasses at Site 453 are similar to those of the Mariana Trough basalts drilled on Leg 60. Both older and younger suites of glasses differ from the composition of rocks exposed on the active arc, which are assumed to be younger than any of the samples studied (i.e., about 200,000 y.). A third suite is represented by the arc rocks exposed on the volcanic islands. These have a smaller range of SiO2 contents and contain more A12O3 but less K2O, TiO2, and FeO1 (total Fe as FeO) than the sediment glasses studied. Further, a plot of FeO1 against MgO for the arc rocks does not follow the island arc tholeiite trend of the trough sediment glasses. Using the major element compositions of the arc rocks and sediment glasses, we can recognize three phases of volcanic activity, as indicated. The first evidence of the oldest phase of activity occurs 5 Ma, about 4.5 m.y. after the trough started to form. The second commenced about 2 Ma, and the last, including present-day activity, began within the last 200,000 y. Initially the rocks had major element affinities with the tholeiitic Mariana Trough seafloor, but this influence declined as the trough widened.

Mineralogy and pyrolysis at DSDP Hole 62-463

A 15-meter sequence of early Aptian organic-matter-rich sediments, cored at Deep Sea Drilling Project Site 463 (western Mid-Pacific Mountains) has been submitted for detailed mineralogical studies (XRD, SEM) and organiccarbon characterization. Although intense diagenesis has obscured the sedimentary record of depositional conditions, the history has been tentatively reconstructed. Through sustained volcanic activity and alteration processes on the archipelago, large amounts of silica were released into the sea water, resulting in a "bloom" of radiolarians. Hard parts settled in large amounts, yielding a hypersiliceous sediment; amorphous silica was diagenetically transformed into chalcedony, opal-CT and clinoptilolite through dissolution and recrystallization. Oxidization of part of the radiolarian soft parts (1) depleted the sea water in dissolved oxygen, allowing the burial of organic matter, and (2) generated carbon dioxide which led to dissolution of most of the calcareous tests. Moderate depositional depth and a high sedimentation rate are though to have prevailed during this episode. An immature stage of evolution is assigned to the studied organic matter, which is of two origins: autochthonous marine material, and allochthonous humic compounds and plant debris. Rhythmic sedimentation characterizes the distribution of the organic matter; each sequence shows (1) an upward progressive increase in organic-carbon content, and (2) an upward enrichment in marine organic matter.

Geochemistry at DSDP Site 60-456

Basalts in two holes spaced 200 meters apart at DSDP Site 456 in the Mariana Trough both show a downward sequence of nonoxidative and oxidative zones of alteration, each 10 to 15 meters thick, overlying fresh basalts. Basalts in the nonoxidative zone have been extensively chloritized and have vein and vesicle fillings of quartz, opal, chlorite, calcite, and pyrite. Minor sulfides are chalcopyrite and digenite. Basalts in the oxidative zone have abundant smectites and iron hydroxides and are variably enriched in K, Rb, and Ba, unlike the nonoxidative basalts above them. We propose that the oxidative zone was a zone of mixing between high-temperature, reduced hydrothermal fluids moving horizontally beneath impermeable sediments at the top of the pillowed basement lavas and cold, oxygenated seawater in interpillow voids deeper in the basement. Recrystallized vitric tuffs immediately above the basalts containing authigenic quartz and wairakite, as well as occurrence of chlorite, epidote, and chalcopyrite in the basalts, suggest temperatures of alteration in excess of 200°C.