Chemical composition, characteristics and age of glauconites at DSDP Leg 66 Holes

During the analysis of "glaucony" recovered during Leg 66, off Mexico, we reviewed the data on previously studied glaucony layers in active margin areas. We found the depth of Leg 66 glaucony sediments to be significantly greater than conventionally assumed appropriate to their genesis (100-500 m). Accordingly, we hypothesize their occurrence at unusual depth to be due to (1) transport of shallow sediments and redeposition at greater depths, (2) margin subsidence, or (3) genesis at greater depth than is generally assumed. For the area off Mexico, we reject (1). (2) has already been verified in Japan and is possible as an explanation for the present phenomenon without excluding (3), which we investigate in this chapter.

Mineralogy and chemical composition of ODP Sites 129-800 and 129-801

Sites 800 and 801 in the Pigafetta Basin allow the sedimentary history over the oldest remaining Pacific oceanic crust to be established. Six major deposition stages and events are defined by the main lithologic units from both sites. Mineralogical and chemical investigations were run on a large set of samples from these units. The data enable the evolution of the sediments and their depositional environments to be characterized in relation to the paleolatitudinal motion of the sites. The upper part of the basaltic crust at Site 801 displays a complex hydrothermal and alteration evolution expressed particularly by an ochre siliceous deposit comparable to that found in the Cyprus ophiolite. The oldest sedimentary cover at Site 801 was formed during the Callovian-Bathonian (stage 1) with red basal siliceous and metalliferous sediments similar to those found in supraophiolite sequences, and formed near an active ridge axis in an open ocean. Biosiliceous sedimentation prevailed throughout the Oxfordian to Campanian, with rare incursions of calcareous input during the middle Cretaceous (stages 2, 4, and 5). The biosiliceous sedimentation was drastically interrupted during the Aptian-Albian by thick volcaniclastic turbidite deposits (stage 3). The volcanogenic phases are pervasively altered and the successive secondary mineral parageneses (with smectites, celadonite, clinoptilolite, phillipsite, analcime, calcite, and quartz) define a "mineral stratigraphy" within these deposits. From this mineral stratigraphy, a similar lithologic layer is defined at the top of the Site 800 turbidite unit and the bottom of the Site 801 turbidite unit. Then, the two sites appear to have been located at the same distal distance from a volcanic source (hotspot). They crossed this locality, at about 10°S, at different times (latest Aptian for Site 800, middle Albian for Site 801). The Cretaceous siliceous sedimentation stopped during the late Campanian and was followed by deposition of Cenozoic pelagic red clay (stage 6). This deep-sea facies, which formed below the carbonate compensation depth, contains variable zeolite authigenesis in relation to the age of deposition, and records the global middle Cenozoic hiatus events. At the surface, the red clay from this part of the Pacific shows a greater detrital component than its equivalents from the central Pacific deep basins.

Element concentrations and isotopic composition of tholeiite and whole rocks samples from the east Mariana Basin

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40Ar/39Ar ages determined in this study for two Leg 129 basalts average 114.6 +/- 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites (87Sr/86Sr init = 0.70360-0.70374; 143Nd/144Nd init = 0.512769-0.512790; 206Pb/204Pb meas = 18.355-18.386) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 +/- 1.0 Ma 40Ar/39Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Porosity contributions of radiolarians in ODP Site 110-671 and Hole 110-672A sediments

This study quantitatively addresses the significance of porosity within radiolarian tests in the décollement zone at the toe of the northern Barbados accretionary prism. Quantification was accomplished using scanning electron microscope images of core samples taken from Ocean Drilling Program (ODP) Sites 671 and 672, representing the décollement and proto-décollement, respectively. The décollement is localized to a radiolarian claystone, and its depth correlates with a low-density anomaly that has been attributed to high porosity at all relevant ODP drilling sites in the area (Moore, Klaus, et al., 1998, doi:10.2973/odp.proc.ir.171A.1998; Shipley, Ogawa, Blum, et al., 1995, doi:10.2973/odp.proc.ir.153.1995; Mascle, Moore, et al., 1988, doi:10.2973/odp.proc.ir.110.1988). Porosity in the décollement zone is presumably lost between Sites 672 and 671 because of shear enhanced consolidation (Moore et al., 1998, doi:10.1130/0091-7613(1998)026<0811:CPDIAE>2.3.CO;2).

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.

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.

Geochemical analysis of minerals from sediment core CRP-2 in the Ross Sea, Antarctica

Distribution patterns, petrography, whole-rock and mineral chemistry, and shape and fabric data are described for the most representative basement lithologies occurring as clasts (granule to bolder grain-size class) from the 625 m deep CRP-2/2A drillcore. A major change in the distribution pattern of the clast types occurs at c. 310 mbsf., with granitoid-dominated clasts above and mainly dolerite clasts below; moreover, compositional and modal data suggest a further division into seven main detrital assemblages or petrofacies. In spite of this variability, most granitoid pebbles consist of either pink or grey biotite±hornblende monzogranites. Other less common and ubiquitous lithologies include biotite syenogranite, biotite-hornblende granodiorite, tonalite, monzogranitic porphyries (very common below 310 mbsf), microgranite, and subordinately, monzogabbro, Ca-silicate rocks, biotite-clinozoisite schist and biotite orthogneiss (restricted to the pre-Pliocene strata). The ubiquitous occurrence of biotite±hornblende monzogranite pebbles in both the Quaternary-Pliocene and Miocene-Oligocene sections, apparently reflects the dominance of these lithologies in the onshore basement, and particularly in the Cambro-Ordovician Granite Harbour Igneous Complex which forms the most extensive outcrop in southern Victoria Land. The petrographical features of the other CRP-2/2A pebble lithologies are consistent with a supply dominantly from areas of the Transantarctic Mountains facing the CRP-2/2A site, and they thus provide further evidence of a local provenance for the supply of basement clasts to the CRP-2/2A sedimentary strata.

Analysis of the iron oxide assemblages in the ANDRILL AND-1B drill core, Ross Sea, Antarctica

The AND-1B drill core recovered a 13.57 million year Miocene through Pleistocene record from beneath the McMurdo Ice Shelf in Antarctica (77.9°S, 167.1°E). Varying sedimentary facies in the 1285 m core indicate glacial-interglacial cyclicity with the proximity of ice at the site ranging from grounding of ice in 917 m of water to ice free marine conditions. Broader interpretation of climatic conditions of the wider Ross Sea Embayment is deduced from provenance studies. Here we present an analysis of the iron oxide assemblages in the AND-1B core and interpret their variability with respect to wider paleoclimatic conditions. The core is naturally divided into an upper and lower succession by an expanded 170 m thick volcanic interval between 590 and 760 m. Above 590 m the Plio-Pleistocene glacial cycles are diatom rich and below 760 m late Miocene glacial cycles are terrigenous. Electron microscopy and rock magnetic parameters confirm the subdivision with biogenic silica diluting the terrigenous input (fine pseudo-single domain and stable single domain titanomagnetite from the McMurdo Volcanic Group with a variety of textures and compositions) above 590 m. Below 760 m, the Miocene section consists of coarse-grained ilmenite and multidomain magnetite derived from Transantarctic Mountain lithologies. This may reflect ice flow patterns and the absence of McMurdo Volcanic Group volcanic centers or indicate that volcanic centers had not yet grown to a significant size. The combined rock magnetic and electron microscopy signatures of magnetic minerals serve as provenance tracers in both ice proximal and distal sedimentary units, aiding in the study of ice sheet extent and dynamics, and the identification of ice rafted debris sources and dispersal patterns in the Ross Sea sector of Antarctica.

Geochemistry, isotopes, trace elements and fluid inclusion geothermometry at DSDP Hole 83-504B

Stockwork-like metal sulfide mineralizations were found at 910-928 m below seafloor (BSF) in the pillow/dike transition zone of Hole 504B. This is the same interval where most physical properties of the 5.9-m.y.-old crust of the Costa Rica Rift change from those characteristic of Layer 2B to those of Layer 2C. The pillow lavas, breccias, and veins of the stockwork-like zone were studied by transmitted and reflected light microscopy, X-ray diffraction, and electron microprobe analysis. Bulk rock oxygen isotopic analyses as well as isolated mineral oxygen and sulfur isotopic analyses and fluid inclusion measurements were carried out. A complex alteration history was reconstructed that includes three generations of fissures, each followed by precipitation of characteristic hydrothermal mineral parageneses: (1) Minor and local deposition of quartz occurred on fissure walls; adjacent wall rocks were silicified, followed by formation of chlorite and minor pyrite I in the veins, whereas albite, sphene, chlorite and chlorite-expandable clay mixtures, actinolite, and pyrite replaced igneous phases in the host rocks. The hydrothermal fluids responsible for this first stage were probably partially reacted seawater, and their temperatures were at least 200-250° C. (2) Fissures filled during the first stage were reopened and new cracks formed. They were filled with quartz, minor chlorite and chlorite-expandable clay mixtures, traces of epidote, common pyrite, sphalerite, chalcopyrite, and minor galena. During the second stage, hydrothermal fluids were relatively evolved metal- and Si-rich solutions whose temperatures ranged from 230 to 340° C. The fluctuating chemical composition and temperature of the solutions produced a complex depositional sequence of sulfides in the veins: chalcopyrite I, ± Fe-rich sphalerite, chalcopyrite II ("disease"), Fe-poor sphalerite, chalcopyrite III, galena, and pyrite II. (3) During the last stage, zeolites and Mg-poor calcite filled up the remaining spaces and newly formed cracks and replaced the host rock plagioclase. Analcite and stilbite were first to form in veins, possibly at temperatures below 200°C; analcite and earlier quartz were replaced by laumontite at 250°C, whereas calcite formation temperature ranged from 135 to 220°C. The last stage hydrothermal fluids were depleted in Mg and enriched in Ca and 18O compared to seawater and contained a mantle carbon component. This complex alteration history paralleling a complex mineral paragenesis can be interpreted as the result of a relatively long-term evolution of a hydrothermal system with superimposed shorter term fluctuations in solution temperature and composition. Hydrothermal activity probably began close to the axis of the Costa Rica Rift with the overall cooling of the system and multiple fracturing stages due to movement of the crust away from the axis and/or cooling of a magmatic heat source.