Geochemistry at DSDP Legs 56-57 Holes

About 200 volcanic ash layers were recovered during DSDP Leg 57. The volcanic glass in some of these layers was investigated petrographically and chemically in this study. Volcanic glass is mainly rhyolitic and/or rhyodacitic in chemical composition, and its refractive index ranges from 1.496 to 1.529. Some volcanic ash layers consist of multiple grains of different chemical compositions. All the volcanic glass belongs to the tholeiitic and the calc-alkalic volcanic rock series, in SiO2-(Na2O + K2O) diagram and FeO*/MgO-SiO2 diagram. We correlated successfully three volcanic ash layers from the standpoint of chemical composition and biostratigraphy. Hydration of volcanic glass from Leg 57 is less intense than in other DSDP cores.

Strontium- and lead-isotope composition of some basalts at DSDP Hole 69-504B

Seventeen whole-rock samples, generally taken at 25- to 50-meter intervals from 5 to 560 meters sub-basement in Deep Sea Drilling Project Hole 504B, were analyzed for 87Sr/86Sr ratios, and rubidium and strontium concentrations. Ten of these samples also were analyzed for Pb-isotope composition. Strontium-isotope ratios for eight samples from the upper 260 meters of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the interval 330 to 560 meters, five samples have a restricted range of 0.70259 to 0.70279, with a mean of 0.70266, almost identical to the average value of fresh mid-ocean-ridge basalts. In the interval 260 to 330 meters, approximately intermediate strontium- isotope ratios are found. The higher 87Sr/86Sr ratios in the upper part of the hole can be interpreted in terms of strontium-isotope alteration during basalt-sea-water interaction. Relative to average fresh mid-ocean ridge basalts, the upper 260 meters of basalts are enriched by an average of about 9% in sea-water strontium 87Sr/86Sr = 0.7091). This Sr presumably is located in the smectites, which, as the main secondary minerals throughout the hole, replace olivine and matrix glass and locally fill vesicles (analyzed samples contained no veins). The strontium-isotope data strongly suggest that the integrated flux of sea water through the upper part of the Hole 504B crust has been greater than through the lower part. This is also suggested by (1) the common occurrence of Feoxide- hydroxide minerals as alteration products above 270 meters, but their near absence below 320 meters, (2) the presence of vein calcite above 320 meters, but its near absence below this level, and (3) the occurrence of vein pyrite only below a depth of 270 meters. Sea-water circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below 230 meters sub-basement. Although sufficient sea water was present within the lower part of the hole to produce smectitic alteration products, the overall water /rock ratio was low enough to prevent significant modification of strontium-isotope ratios. Lead-isotope ratios of Hole 504B basalts form approximately linear arrays in plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb. The arrays are similar to those reported for basalts from other mid-ocean ridges. There is no trend in Hole 504B lead-isotope ratios with vertical position in the basement. The arrays indicate that the lead-isotope composition of the upper mantle from which the Hole 504B basaltic melts were derived was inhomogeneous.

Velocity-density systematics for basalts at DSPD Leg 54 Holes

Precise velocity and density measurements at atmospheric and elevated pressures have been obtained on basalts drilled in 13 holes during Leg 54. The vp-sigma data show systematics which are controlled by the degree of crystallinity (or conversely, glassiness), microstructure, and original mineralogy and chemistry. Extensively fractionated basalts with marked iron enrichment produce anomalously low measured velocities at densities above 2.90 g/cm**3. Also, the effective in situ pressure acting on Leg 54 basalts is less than hydrostatic, and perhaps close to zero. At zero effective pressure, the measured velocities average 2.5 km/s higher than East Pacific Rise upper crustal velocities determined by seismic refraction. This implies that the in situ velocities are undoubtedly a result of the highly fragmented nature of East Pacific Rise crust.

Magnetic remanence acquisition in DSDP Leg 73 Holes

The major magnetic mineral in the turbidites and slumped sediments recovered at Leg 73 drill sites was near to magnetite in composition and in the form of small multidomain particles. There was no variation in magnetic mineralogy with the lithology. The variations in the intensities and directions of the natural remanent magnetization could be explained in terms of postdepositional grain rotations within the wet sediment. In the sands realignment was partial, whereas in some of the slumps the entire remanent magnetization was reset. Fine-particle magnetite was also the main magnetic constituent of the red clays. A significant proportion of a higher-coercivity mineral was also present. The magnetic characteristics of the red clays are explained as a combination of concentration and grain rotation effects. The implications to the assessment of the reliability of paleomagnetic data are discussed. Note: Conversion factors are as follows: 1 Am**2/kg = 1 emu/g, and 80 A/m about 1 Oe.

Campanian-Maestrichtian benthic foraminifera of DSDP Hole 78-543A

Foraminiferal assemblages in sediments from Hole 543A suggest that toward the end of the Cretaceous there was an oscillating carbonate compensation depth (CCD) in the western Central Atlantic. Changing assemblages of siliceous agglutinated and calcareous foraminifers reflect the changing depositional environment, from a ridge crest environment during Campanian time to a deep abyssal environment during Maestrichtian time.

Elemental and stable isotopic composition of some metalliferous and pelagic sediments at DSDP Leg 70 Holes

Nontronite, the main metalliferous phase of the Galapagos mounds, occurs at subsurface depths of about 2 to 20 meters; Mn-oxide material is limited to the upper 2 meters of the mounds. The nontronite forms intervals of up to a few meters' thickness, consisting essentially of 100% nontronite granules, which alternate with intervals of normal pelagic sediment. Electron microprobe analyses of nontronite granules from different core samples indicate that: (1) there is little difference in major element composition between nontronites from varying locations within the mounds, with adjacent granules from a given sample having very similar compositions; (2) individual granules show little internal variation in composition. This indicates that the granules are composed of a single mineral of essentially constant composition, consistent with relatively uniform conditions of Eh and composition during nontronite formation. Mn-oxide crusts have very low Fe contents, a feature characteristic of rapidly deposited Mn-oxide crusts formed under hydrothermal influences. The rare-earth element (REE) abundances of the nontronites are generally extremely low, totalling less than several ppm. Two samples have the negatively Ce anomaly typical of authigenic precipitates formed relatively rapidly from seawater. A Mn-oxide crust sample has low REE contents, typical of Mn-oxide crusts formed under hydrothermal influences, but no negative Ce anomaly. A sample of unusual Mn-Fe-oxide mud has relatively high REE concentrations and a seawater-type pattern; both of these features are also found for metalliferous sediments from the East Pacific Rise. The oxygen and hydrogen isotopic composition of the nontronites define a restricted field within a d18O-dD plot. In manganiferous sediments, d18O and dD appear to decrease with increase in the Mn-oxide content of the sediment. From the d18O values of the nontronites, formation temperatures in the range of about 20-30°C have been estimated. By comparison, temperatures of up to 11.5 °C at a 9-meter depth have been directly measured within the mounds (Corliss et al., 1979), and heat-flow data suggest present basement/sediment interface temperatures of 15-25°C. In a plot of Fe + Mn vs. d18O, the Mn-oxide crust and Mn-Fe-ooze plot near the tie-lines for authigenic Mn nodules and silicate phases, implying that they have formed in isotopic equilibrium with seawater at or close to bottom-water temperatures.

Geochemistry at DSDP Leg 60

Eocene to Pleistocene volcanogenic sediments from the Mariana Trough and the Mariana arc-trench system have been studied by X-ray diffraction, X-ray fluorescence, and atomic absorption, and with a scanning electron microscope with an X-ray-energy-dispersive attachment. The mineralogical composition of the volcaniclastic sediments (tuffs) is the same as that of the other associated sediments (mudstones). Diagenetic alterations are significant and seem to result from two processes. The first (low-temperature alteration) develops with age and depth; it consists of the genesis of pure smectite, coupled with zeolites (phillipsite, clinoptilolite). The second is limited to sediments immediately overlying basalts and to the altered basalts themselves. It consists of the massive development of palygorskite, and seems to be linked with hydrothermal activity in the igneous basement.

Mass-accumulation rates for the non-authigenic, inorganic, crystalline, eolian component of sediments recovered at DSDP Sites 62-464 to 62-466

The mass-accumulation rate (MAR) of the non-authigenic, inorganic, crystalline component of deep-sea sediments from the Pacific aseismic rises apparently reflects influx of eolian sediment. The eolian sediment usually is dominated by volcanic material, except during glacial times. Sediments from Hess Rise provide a discontinuous record of eolian MARs. During Albian to Cenomanian time, the influx of volcanic material was fairly high (0.35-0.6 g/cm**2/10**3 yr), recording the latest stages of the Albian volcanism that formed Hess Rise. From the Campanian through the Paleocene, influx of eolian sediment was low, averaging 0.03 g/cm**2/10**3 yr. None of the four Hess Rise drill sites show evidence of the Late Cretaceous volcanic episode recorded at many sites now in the equatorial to subtropical Pacific. Pliocene to Pleistocene samples record a peak in volcanic influx about 4 to 5 m.y. ago, which has been well documented elsewhere. The several-fold increase in eolian accumulation rates elsewhere which are correlated with the onset of severe northernhemisphere glaciation 2.5 m.y. ago is not obvious in the Hess Rise data.

Metamorphic processes in sediments at DSDP Leg 65 Holes

Sites 482, 483, and 485 were drilled during Leg 65 on young oceanic crust south of the Tamayo Transform Fault. The Leg 65 drilling program was part of a multinational effort to study the East Pacific Rise (EPR) and complements sea bottom surveys conducted both in this area (Lewis, 1979; Cyamex Scientific Team and Pastouret, 1981) and farther south at 21 °N (Larson, 1971; Normark, 1976; Cyamex Scientific Team; Rise Project Group, 1980). These studies, which included deep-tow, Angus and submersible surveys, were recently complemented by sea-beam surveys conducted by the Jean Charcot on the Tamayo Fracture Zone and farther south along the EPR. They have led to a better understanding of the magmatic, tectonic, and sedimentary processes occurring on the East Pacific Rise between the Tamayo and Rivera fracture zones. The purpose of this chapter is to describe the metamorphic processes affecting Pliocene through Quaternary sediments found in contact, or inter layered, with basaltic units drilled during Leg 65 at the mouth of the Gulf of California.

Geochemistry at DSDP Leg 59

Deep Sea Drilling Project Legs 59 and 60 drilled 15 sites along an east-west transect at 18°N from the West Philippine Basin to the Mariana Trench (Fig. 1) in order to study the nature and genesis of the back-arc, marginal basins and the remnant and active arcs of the region. Leg 59 drilled at five sites at the western end of the traverse: Site 447 in the West Philippine Basin; Site 448 on the Palau-Kyushu Ridge; Sites 449 and 450 in the Parece Vela Basin; and Site 451 on the West Mariana Ridge. Penetration into basaltic basement of these sites was 183.5 meters at 447 (8 basalt flows); 623 meters at 448 (46 basalt flows, sills, and dikes and volcaniclastic units); 40.5 meters at 449 (2 basalt flows); 7 meters at 450 (1 basalt intrusion); and 4 meters of basalt breccia at 451 overlain by 861 meters of volcaniclastic sedimentary rocks.

Nannoliths and paleobiogeographic census from DSDP Site 56-435

Calcareous nannofossils were encountered at only one of the sites (435) drilled during DSDP Leg 56. Cores from Hole 435A yield fairly diverse early and late Pliocene assemblages. The section shows considerable reworking, however. Three to five biostratigraphic datum events provide a reasonable biochronology. The datums range from about 3.3 Ma in Core 11 to about 1.8 Ma in Core 3. Paleobiogeographic data indicate relatively stable and warm climatic conditions in this area in the early Pliocene, becoming more unstable in the late Pliocene when the cosmopolitan species become dominant.

Zeolite and silica diagenesis and sandstone petrography at DSDP Sites 57-438 and 57-439

We detected authigenic clinoptilolites in two core samples of tuffaceous, siliceous mudstone in the lower Miocene section of Hole 439. They occur as prismatic and tabular crystals as long as 0.03 mm in various voids of dissolved glass shards, radiolarian shells, calcareous foraminifers, and calcareous algae. They are high in alkalies, especially Na, and in silica varieties. There is a slight difference in composition among them. The Si : (Al+ Fe3+) ratio is highest (4.65) in radiolarian voids, intermediate (4.34) in dissolved glass voids, and lowest (4.26) in voids of calcareous organisms. This difference corresponds to the association of authigenic silica minerals revealed by the scanning electron microscope: There are abundant opal-CT lepispheres in radiolarian voids, low cristobalite and some lepispheres in dissolved glass voids, and a lack of silica minerals in the voids of calcareous organisms. Although it contains some silica from biogenic opal and alkalies from trapped sea water, clinoptilolite derives principally from dissolved glass. Although they are scattered in core samples of Quaternary through lower Miocene diatomaceous and siliceous deposits, acidic glass fragments react with interstitial water to form clinoptilolite only at a sub-bottom depth of 935 meters at approximately 25°C. Analcimes occur in sand-sized clasts of altered acidic vitric tuff in the uppermost Oligocene sandstones. The analcimic tuff clasts were probably reworked from the Upper Cretaceous terrain adjacent to Site 439. Low cristobalite and opal-CT are found in tuffaceous, siliceous mudstone of the middle and lower Miocene sections at Sites 438 and 439. Low cristobalite derives from acidic volcanic glass and opal-CT from biogenic silica. Both siliceous organic remains and acidic glass fragments occur in sediments from the Quaternary through lower Miocene sections. However, the shallowest occurrence is at 700 meters subbottom in Hole 438A, where temperature is estimated to be 21°C. The d(101) spacing of opal-CT varies from 4.09 to 4.11 Å and that of low cristobalite from 4.04 to 4.06 Å. Some opal-CT lepispheres are precipitated onto clinoptilolites in the voids of radiolarian shells at a sub-bottom depth of 950 meters in Hole 439. Sandstone interlaminated with Upper Cretaceous shale is chlorite- calcite cemented and feldspathic. Sandstones in the uppermost Oligocene section are lithic graywacke and consist of large amounts of lithic clasts grouped into older sedimentary and weakly metamorphosed rocks, younger sedimentary rocks, and acidic volcanic rocks. The acidic volcanic clasts probably originated from the volcanic high, which supplied the basal conglomerate with dacite gravels. The older sedimentary and weakly metamorphosed rocks and green rock correspond to the lithologies of the lower Mesozoic to upper Paleozoic Sorachi Group, including the chert, limestone, and slate in south-central Hokkaido. However, the angular shape and coarseness of the clasts and the abundance of carbonate rock fragments indicate a nearby provenance, which is probably the southern offshore extension of the Sorachi Group. The younger sedimentary rocks, including mudstone, carbonaceous shale, and analcime-bearing tuff, correspond to the lithologies of the Upper Cretaceous strata in south-central Hokkaido. Their clasts were reworked from the southern offshore extension of the strata. Because of the discontinuity of the zeolite zoning due to burial diagenesis, an overburden several kilometers thick must have been denuded before the deposition of sediments in the early Oligocene.

40Ar-39Ar geochronological studies on rocks drilled at DSDP Holes 61-462 and 61-462A

40Ar-39Ar step-heating dating was applied to a basalt from Hole 462 and to basalt and dolerite samples from Hole 462A. Only a basalt sample at Hole 462A yielded a reasonable isochron age, 110 ± 3 million years. The radiometric age is consistent with the fossil record (Cenomanian) in the sediments, into which the basalt sill intruded. However, the age is much less than that of the oceanic basement as deduced from the magnetic anomaly (M-26).