Oxygen and carbon isotope data from late calcite and zeolite pore-fill cements at DSDP Site 445

Oxygen isotopic composition of zeolite pore-fill cements in andesitic volcaniclastic sandstones recovered from DSDP Site 445 ranges from +30.1 to +17.8? (SMOW) downhole. This change is controlled by large heat flow from the basement which caused early diagenetic emplacement of zeolites during early basin rifting. d18O-values of late calcite cements range from +25.1 to +27.4? (SMOW); their petrographic relation and inferred temperature of formation suggest that calcite cements were formed during late stages of diagenesis. Isotopic composition in these sandstones is in agreement with mineral paragenesis determined microscopically.

(Table 1) Summary of horizon, lithology, and paleomagnetic of ODP Hole 126-791B basaltic mousse samples

A paleomagnetic study was made on the highly vesiculated basaltic tuff breccia (the basaltic mousse) drilled by Ocean Drilling Program Leg 126 from the Izu-Bonin backarc, Sumisu Rift, to estimate the mode of its emplacement. Thirty-four 10-cm**3 minicore samples were collected from almost all the horizons of the basaltic mousse. Stepwise thermal and alternating-field demagnetization experiments show that the natural remanent magnetizations of many samples are mainly composed of a single stable component. Although remanence inclinations are not expected to be disturbed by rotary drilling, the measured inclinations of remanence show a random directional distribution as a whole. The remanence inclinations, however, show directional consistency on a smaller scale. High-density sampling and measurements from a limited interval of drilled cores, and the measurement of small disks cut from a single minicore sample show that there is directional consistency over several centimeters. Strong and stable remanent magnetization, the existence of remanence direction consistency, and the fresh lithology of the samples suggest the thermal origin of remanence. Combining the paleomagnetic results with other geological, petrographical, and paleontological characters, the Hole 791B basaltic mousse can be interpreted as a subaqueous explosion breccia produced by deep-sea pyroclastic fountaining.

Tab. 1: U-Pb analyses of zircons from rock samples in MT. Newton and Cumpston Massif, East Antarctica

We present new U-Pb zircon (SHRIMP) data on rocks from Mt Newton and Cumpston Massif in the southern Prince Charles Mountains. Our data demonstrate that Mt Newton was affected by a newly proposed Palaeoproterozoic "Newton" Orogeny at c. 2100-2200 Ma. Sedimentation, felsic volcanism (c. 2200 Ma), metamorphism and folding, followed by granite intrusion (c. 2100 Ma), suggest development of a trough or aulacogene in the area during the early Palaeoproterozoic. An orthogneiss from Cumpston Massif yielded an age of c. 3180 Ma for granitic protolith emplacement, which is in good agreement with many U-Pb zircon ages from similar rocks in the southern Mawson Escarpment. A syn- to late-tectonic muscovite-bearing pegmatite from Cumpston Massif yielded a c. 2500 Ma date of emplacement, which indicates early Palaeoproterozoic activity in this block, probably in response to a tectono-magmatic episode in the Lambert Terrane bordering the Ruker Terrane in the northeast. The correlation of tectono-magmatic events in both the Ruker and Lambert terranes of the southern Prince Charles Mountains provides evidence for their common evolution during the Proterozoic.

Chemistry of peridotites of ODP Hole 103-637A

A ridge of peridotite was drilled off of the Galicia margin (Hole 637A) during ODP Leg 103. The ridge is located at the approximate boundary between oceanic and continental crust. This setting is of interest because the peridotite may be representative of upwelling upper mantle beneath an incipient ocean basin. The composition of the Galicia margin peridotite is compared with those of other North Atlantic peridotites. Hole 637A ultramafic lithologies include clinopyroxene-rich spinel harzburgite and lherzolite, as well as plagioclase-bearing peridotites. Variations in mineral modal abundances and mineral compositions are observed but are not systematic. The peridotites are broadly similar in composition to other peridotites recovered from ocean basins, but the mineral compositions and abundances suggest that they are less depleted in basaltic components than other North Atlantic peridotites by about 10%. In particular, the peridotites are enriched in the magmaphilic elements Na, Al, and Ti, as compared with other abyssal peridotites. The high abundances of these elements suggest that the Hole 637A peridotites had experienced, at most, very small amounts of partial melting prior to their emplacement. The presence of plagioclase rimming spinel in some samples suggests that the peridotite last equilibrated at about 9 kbar, near the transition between plagioclase- and spinel-peridotite stability fields. Temperatures of equilibration of the peridotite are calculated as 900°-1100°C. The relatively undepleted composition of the peridotite indicates that it was emplaced at a shallow mantle level under a relatively cool thermal regime and cooled below solidus temperatures without having participated in any significant partial melting and basalt production. This is consistent with the emplacement of the peridotite during incipient rifting of the ocean basin, before a true spreading center was established.

(Table 1) Lead, thorium and uranium isotopic concentration and ages of zircons from Yenisey Ridge metapelites

Reconstruction of the geologic history of the Yenisey Ridge, which developed as an accretionary collision orogen on the western margin of the Siberian craton is essential to understanding the evolution of mobile belts surrounding older cratons, as well as to resolving the recently much debated problem of whether Siberia was part of the supercontinent Rodinia. Available paleotectonic models suggest that this supercontinent was assembled at the Middle-Late Riphean boundary (1100-900 Ma) as a result of the Grenville orogeny, the first long-lived mountain building event which occurred in geosynclinal areas during the Neogaea. However, the character of crustal evolution at that stage is still speculative due to the lack of reliable and conclusive isotope data. In many current geodynamic models, a common underlying assumption is that the Yenisey Ridge showed very little endogenic activity for 1 Gyr, from the time of Tarak granite emplacement (1900-1840 Ma) to the Middle Neoproterozoic (~750 Ma). On the basis of this assumption, several recent studies suggested the absence of Grenvillian collisional events within the Yenisey Ridge. The results of the SHRIMP II U-Pb analysis of rift-related plagiogranites of the Nemtikha Complex, Yenisey Ridge (1380-1360 Ma) suggest an increase in magmatic activity in the Mesoproterozoic. Interpretation of these results in terms of a supercontinent cycle may help find evidence for possible occurrence of the Grenville orogeny on the western margin of the Siberian craton. With this in mind, we attempted to reconstruct using recent geochronological constraints the evolution of metapelitic rocks from the Teya polymetamorphic complex (TPMC), which is a good example of superimposed zoning of low and medium-pressure facies series. High precision age determinations from rock complexes formed in different geodynamic settings under different thermodynamic conditions and geothermal gradients were used to distinguish several major metamorphic events and unravel their time relations with tectonic and magmatic activity in the region.

Stable-isotope evidence for the origin of secondary carbonate veins at DSDP Leg 58 Holes

The stable-isotope composition of carbonate minerals is a function of the temperature and isotopic composition of the materials from which they were precipitated or recrystallized. Because carbonates are among the most abundant secondary phases in oceanic volcanic rocks, information derived from their isotopic composition is useful in determining the environment(s) of seafloor alteration. Isotopic analyses of secondary carbonates in basalt recovered from numerous DSDP sites have been reported previously (Anderson and Lawrence, 1976; Brenneke, 1977; Lawrence et al., 1977; Seyfried et al., 1976; among others). These results are consistent with the formation of most secondary carbonates with sea water at low temperatures. The good recovery of basalts during DSDP Leg 58 provided the opportunity to extend the isotopic study of low-temperature alteration and vein formation to the crust of marginal ocean basins. The evidence for complex off-ridge volcanism and intrusive emplacement encountered at Leg 58 sites (Klein et al., 1978) suggested that modes of alteration at these sites might differ from those previously observed and described.

Calcite composition and stable isotope record of ODP Hole 103-637A peridotites

The peridotite recovered from Ocean Drilling Program Hole 637A, Galicia margin, has suffered extensive low-temperature alteration that includes serpentinization, calcite veining, and calcite replacement. This note presents textural and geochemical data on the serpentine and calcite. Such data indicate that the serpentinization, serpentine veining, and calcite veining of the peridotite occurred in several stages late in the history of the peridotite emplacement, probably after the peridotite was emplaced at crustal levels. It is also apparent that some deformational events (evidenced by faulting and brecciation of both serpentine and calcite veins) continued after the main phase of low-temperature alteration. The geochemistry and petrology, structure, and high-temperature alteration of the peridotite are discussed in separate papers in this volume (Evans and Girardeau, 1988, doi:10.2973/odp.proc.sr.103.138.1988; Girardeau et al., 1988, doi:10.2973/odp.proc.sr.103.135.1988; Kimball and Evans, 1988, doi:10.2973/odp.proc.sr.103.140.1988; Agrinier et al., 1988, doi:10.2973/odp.proc.sr.103.136.1988).

Mud, sand, heavy and light minerals composition in lower Cretaceous at DSDP Hole 93-603B

During the drilling of Hole 603B on Deep Sea Drilling Project Leg 93, an unexpected series of sand-, silt-, and claystone turbidites was encountered from Cores 603B-45 through -76 (1224-1512 m sub-bottom depth). Complete and truncated Bouma sequences were observed, some indicating deposition by debris flows. Sand emplacement culminated with the deposition of a 30-m-thick, unconsolidated sand unit (Cores 603B-48 through -45). The purpose of this preliminary study is to determine the nature of the heavy mineral suites of this sediment in order to make tentative correlations with onshore equivalents. The heavy mineralogy of Lower Cretaceous North American mid-Atlantic coastal plain sediment has been extensively studied. This sediment is classified as the Potomac Group, which has a varied heavy mineral suite in its lower part (Patuxent Formation), and a limited suite in its upper part (Patapsco Formation). The results of this study reveal a similar trend in the heavy mineral suites of sediment in Hole 603B. Hauterivian through lower Barremian sediment has a heavy mineral suite that is dominated by zircon, apatite, and garnet, with minor amounts of staurolite and kyanite. Beginning in the mid-Barremian, a new source of sediment becomes dominant, one which supplies an epidote-rich heavy mineral suite. The results of the textural analyses show that average grain size of the light mineral fraction increases upsection, whereas sorting decreases. The epidote-rich source may have delivered sediment with a slightly coarser mean grain size. This sediment may represent a more direct continental input at times of maximum turbidite activity (mid-Barremian) and during deposition of the upper, unconsolidated sand unit.

Geochemistry of Atlantis Massif serpentinites

The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30°N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, O, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150-250 °C and at high integrated W/R ratios that led to a marked boron enrichment (34-91 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in d11B (11-16 per mil) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature-pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270-350 °C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater-gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of d11B values (9-10 per mil), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings.

Middle Eocene-Early Pliocene Subantarctic planktic foraminiferal biostratigraphy of ODP Site 1090 on the Agulhas Ridge, South Atlantic

The analysis of planktic foraminiferal assemblages from Site 1090 (ODP Leg 177), located in the central part of the Subantarctic Zone south of South Africa, provided a geochronology of a 330-m-thick sequence spanning the Middle Eocene to Early Pliocene. A sequence of discrete bioevents enables the calibration of the Antarctic Paleogene (AP) Zonation with lower latitude biozonal schemes for the Middle-Late Eocene interval. In spite of the poor recovery of planktic foraminiferal assemblages, a correlation with the lower latitude standard planktic foraminiferal zonations has been attempted for the whole surveyed interval. Identified bioevents have been tentatively calibrated to the geomagnetic polarity time scale following the biochronology of Berggren et al. (1995). Besides planktic foraminiferal bioevents, the disappearance of the benthic foraminifera Nuttallides truempyi has been used to approximate the Middle/Late Eocene boundary. A hiatus of at least 11.7 Myr occurs between V78 and V71 m composite depth extending from the Early Miocene to the latest Miocene-Early Pliocene. Middle Eocene assemblages exhibit a temperate affinity, while the loss of several planktic foraminiferal species by late Middle to early Late Eocene time reflects cooling. During the Late Eocene-Oligocene intense dissolution caused impoverishment of planktic foraminiferal assemblages possibly following the emplacement of cold, corrosive bottom waters. Two warming peaks are, however, observed: the late Middle Eocene is marked by the invasion of the warmer water Acarinina spinuloinflata and Hantkenina alabamensis at 40.5 Ma, while the middle Late Eocene experienced the immigration of some globigerinathekids including Globigerinatheka luterbacheri and Globigerinatheka cf. semiinvoluta at 34.3 Ma. A more continuous record is observed for the Early Miocene and the Late Miocene-Early Pliocene where planktic foraminiferal assemblages show a distinct affinity with southern mid- to high-latitude faunas.

Geochemistry and lithology of clayey nannofossil ooze turbidites and hemipelagites at ODP Holes 135-834A and 135-835A

The western Lau Basin, between the Central and Eastern Lau Spreading Centers and the Lau Ridge, contains several small, elongate, fault-bounded, partially sediment-filled sub-basins. Sites 834 and 835 were drilled in the oldest part of the Lau Basin in two of these small extensional basins close to the Lau Ridge, formed on late Miocene to early Pliocene oceanic crust. Both sites show a similar sediment sequence that consists of clayey nannofossil oozes and mixed sediments interbedded with epiclastic vitric sands and silts. The vitric sands and silts are largely restricted to the deeper part of the sediment column (early Pliocene-late Pliocene), and the upper part of the sediment column at both sites consists of a distinctive sequence of brown clayey nannofossil ooze, stained by iron and manganese oxyhydroxides (late Pliocene-Holocene). However, the clayey nannofossil ooze sequence at Site 835 is anomalously thick and contains several medium- to very thick beds of matrix-supported, mud-clast conglomerate (interpreted as muddy debris-flow deposits), together with large amounts of redeposited clayey nannofossil ooze and coherent rafted blocks of older hemipelagic material. Redeposited clayey nannofossil oozes can be distinguished from hemipelagic nannofossil oozes using several sedimentological criteria. These include variation in color hue and chroma, presence or absence of bioturbation, presence or absence of scattered foraminifers, grain-size characteristics, variability in calcium carbonate content, presence or absence of pumice clasts, and micropaleontology. Clayey nannofossil ooze turbidites and hemipelagites are also geochemically distinct, with the turbidites being commonly enriched in Mn, Ni, Pb, Zn, Cr, and P. The sediment sequence at Site 835 is dominated by allochthonous sediments, either muddy debris-flow deposits, coherent rafted blocks, or thick clayey nannofossil ooze turbidites. Since 2.9 Ma, only 25% of the 133 m of sediments deposited represents hemipelagic deposition, with an average sedimentation rate of 1.5 cm/k.y.. Allochthonous sediments were the main sediment type deposited during the Brunhes geomagnetic Epoch and make up 80% of the thickness of sediment deposited during this period. Short intervals of mainly hemipelagic deposition occurred from 0.4 to 0.9 Ma, 1.0 to 1.4 Ma, and 1.7 to 2.1 Ma. However, allochthonous sediments were again the dominant sediment type deposited between 2.1 and 2.5 Ma, with a large slide complex emplaced around 2.5 Ma. We conclude that the adjacent high ground, surrounding the basin in which Site 835 was drilled, was affected by marked instability throughout the late Pliocene and Pleistocene. In contrast, sedimentation at Site 834 during this period has been dominated by hemipelagic deposition, with redeposited sediments making up slightly less than 17% of the total thickness of sediment deposited since 2.3 Ma. However, there was a marked increase in frequency and magnitude of redeposited sediments at around 0.2 Ma at Site 834, which broadly corresponds to the onset of a major episode of turbidite and debris-flow emplacement beginning about 0.4 Ma at Site 835. This episode of instability at both sites may be the effect of the approach and passing of the Central Lau propagator at the latitude of Sites 834 and 835 at about 0.5 Ma.

(Table 1) I, Br, B, d11B and Mn concentrations in ODP Site 131-808

This study of the interstitial water concentration-depth distributions of iodide, bromide, boron, d11B, and dissolved organic carbon, as represented by absorbance at 325 nm (yellow substance: YS) and laser-induced fluorescence (LIF), is a follow-up of the extensive shipboard program of interstitial water analysis during ODP Leg 131. Most of the components studied are associated with processes involving the diagenesis of organic matter in these sediments. Three zones of the sediment column are discussed separately because of the different processes involved in causing concentration changes: 1. The upper few hundreds of meters: In this zone, characterized by very high sedimentation rates (>1200 m/m.y.), interstitial waters show very sharp increases in alkalinity, ammonia, iodide, bromide, YS, and LIF, mainly as a result of the diagenesis of organic carbon; 2. Whereas below 200 mbsf concentration gradients all show a decreasing trend, the zone at ~ 365 mbsf is characterized by concentration reversals, mainly due to the recent emplacement of deeper sediments above this depth as a result of thrust-faulting; 3. The décollement zone (945-964 mbsf) is characterized by concentration anomalies in various constituents (bromide, boron, d11B, manganese, LIF). These data are interpreted as resulting from an advective input of fluids along the zone of décollement as recent as ~ 200 ka. Possibly periodic inputs of anomalous fluids still seem to occur along this décollement zone.

Magnetic properties of igneous rocks recovered at ODP Leg 127 sites

Measurements of natural remanent magnetization (NRM), initial susceptibility (K), anisotropy of magnetic susceptibility, frequency dependent susceptibility (Xfd), and viscous remanent magnetization (VRM) are reported from volcanic rocks recovered during ODP Leg 127 in the Japan Sea. The results indicate a significant difference between the basalts drilled in the Yamato Basin (Site 794 and 797) and in the Japan Basin (Site 795). The Koenigsberger ratios (Q) show very low values in the Yamato Basin attesting that the remanence is not dominant over the induced magnetization. This evidence could explain why no magnetic anomaly pattern has been recognized in this basin. Experiments of VRM acquisition and decay show that both the processes are multistage with the acquisition process proceeding more rapidly and deviates more from a log (t) law than the corresponding decay. The sediments interlayered with the basalts in the acoustic basement of the Yamato Basin show processes of remagnetization related to the emplacement of the dikes. Temperatures of heating between 200° and 250°C were estimated from the different unblocking temperatures of the two components of magnetization.

Stable oxygen, hydrogene and chlorine isotopes of mid-Atlantic abyssal peridotites

Serpentinized abyssal peridotites sampled by the Ocean Drilling Program Leg 209 along the mid-Atlantic Ridge near the 15°20'N Fracture Zone have been analyzed for oxygen, hydrogen, and chlorine isotope compositions in order to determine isotopic behavior under a wide range of serpentinization conditions and place constraints on fluid history. Oxygen and hydrogen thermometry suggests peak serpentinization temperatures of 300-500°C. Serpentine separates have low deltaD values possibly due to a magmatic fluid component or low-temperature exchange during seafloor weathering. Chlorine geochemistry focused on three holes: 1274A and 1272A (serpentinized peridotites) and 1268A (serpentinite locally altered to talc). Concentrations of both, water-soluble chloride (WSC) and structurally bound chloride (SBC) are significantly lower at Hole 1268A compared to Holes 1274A and 1272A. The delta37Cl values for WSC and SBC of serpentinites in Holes 1274A and 1272A are slightly positive (avg. WSC = 0.20 per mil, n = 22 and avg. SBC = 0.35 per mil, n = 22), representing typical seawater-hydration conditions commonly determined for abyssal peridotite. The SBC of serpentinites from Hole 1268A are also positive (avg. = 0.63 per mil); whereas, the SBC in talc-dominated samples is negative (avg. = -1.22 per mil). The WSC of both talc- and serpentine-dominated samples are also negative (avg. = -0.15 per mil). We interpret the chlorine isotope data to preserve a record of multiple fluid events. As seawater hydrated the peridotite, 37Cl was preferentially incorporated into the forming serpentine and water-soluble salts, yielding similar delta37Cl values on a regional scale as sampled by Holes 1268A, 1274A and 1272A. The resultant pore fluid was left depleted in 37Cl. Locally (Hole 1268A), this evolved fluid was remobilized possibly due to the initiation of hydrothermal circulation in response to emplacement of a mafic magma body. The low delta37Cl pore fluids attained elevated SiO2 and sulfur concentrations due to interaction with the gabbroic intrusion and, when ascending through the surrounding serpentinite, caused formation of isotopically negative talc. This secondary fluid also flushed the preserved serpentinite of its previously formed salts, resulting in negative delta37Cl WSC values. The delta37Cl SBC values of the serpentinite samples remained unmodified by reaction with the secondary fluid.