Physical properties and elastic constants of ODP Holes 137-504B and 140-504B upper crustal rocks

Seismic velocities have been measured at confining pressures of 100 MPa and 600 MPa for sheeted dike samples recovered during Ocean Drilling Program Legs 137 and 140. The compressional- and shear-wave velocities show an increase with depth at Hole 504B, which is in sharp contrast to the atmospheric pressure velocity measurements performed as part of the shipboard analyses. Rocks exposed to different types of alteration and fracture patterns show distinct changes in their physical properties. The seismic reflectors observed on the vertical seismic profile (VSP) experiment performed during Leg 111 may have been caused by low velocity zones resulting from alteration. The amount of fracturing and hydrothermal alteration in several zones also may have contributed to the acoustic impedance contrast necessary to produce the E5 reflector. Poisson's ratios calculated from laboratory velocity measurements show several low values at depths ranging from 1600 mbsf to 2000 mbsf, which tends to follow similar trends obtained from previous oceanic refraction experiments. A comparison of physical properties between samples recovered from Hole 504B and ophiolite studies in the Bay of Islands and Oman shows a good correlation with the Bay of Islands but significant differences from the measurements performed in the Oman complex.

Composition of basalts from the Romanche Fracture Zone

New magnetometric, petrological, and geochemical data on basalts from the central Romanche Fracture Zone allow to classify these rocks into two groups. Igneous rocks from the active part of the fracture zone that have undergone transtension are referred to alkaline rocks. According to some indications, they are younger that oceanic tholeiites of the southern fault-line ridge, which were affected by elevated pressure in the past. These data indicate with a high probability that the Romanche Fracture Zone belongs to a rare group of magmatically active demarcation transform lines that separate large oceanic domains different in structural and geochemical features.

Chemistry of minerals of ODP Hole 103-637A peridotites

The Galicia margin lies northwest of the Iberian Peninsula and is a passive ocean margin with thin sedimentary cover. Altered peridotite was recovered from ODP Site 637, on the north-trending ridge at the western edge of the margin, near the oceanic/continental crust boundary. The altered ultramafics were originally clinopyroxene-rich upper mantle harzburgites and are now extensively serpentinized (>85%) and cut by very late-stage carbonate veins. Despite pervasive late, low-temperature alteration, evidence of early, high-temperature alteration remains. Alteration is apparent as (1) amphibole rims on clinopyroxene (>800°C), (2) hornblende + tremolite (450° to 800°C), (3) breakdown of hornblende to form tremolite + chlorite (<450°C), (4) zoned Cr-spinels, (5) hydration of orthopyroxene and olivine to serpentine, (6) serpentine veins, (7) replacement of pyroxene and olivine by calcite, and (8) calcite veins and vugs. Both the relict igneous and the high-temperature alteration minerals (amphiboles) show evidence of brittle deformation. Subsequent low-temperature alteration veins and minerals are deformed only in faulted and brecciated zones. This textural evidence suggests that the low-temperature alteration occurred after emplacement of the ultramafics at the surface. Serpentine fills tension fractures in orthopyroxene, and both serpentine and calcite fill tension cracks in olivine. The high-temperature alterations in these samples are similar to those found in oceanic fracture zone and ophiolite ultramafics. This widespread occurrence of high-temperature alteration suggests that hot fluids were pervasive in these ultramafic blocks. Localization of high-temperature alteration close to large carbonate veins suggests channelization of the late, low-temperature fluids. Earlier hydrations (e.g., high-temperature alterations and serpentinization) were pervasive.

Thickness and duration of biostratigraphic zones, assemblages, ages and sedimentation rates at DSDP Sites 76-534 and 44-391

In this Initial Report of the Deep Sea Drilling Project, detailed studies of Sites 533 (gas hydrates) on the Blake Outer Ridge and 534 (oldest ocean history) in the Blake-Bahama Basin have provided answers to many geological and geophysical questions posed over the decade that deep drilling has been undertaken in this part of the western North Atlantic. The history of drilling and a historical review of key scientific accomplishments have been presented in the Introduction (Gradstein and Sheridan, this volume). In this final chapter we review highlights of new geological, geophysical and paleoceanographic interpretations presented in this volume, and offer a critical review of this information. We conclude with a listing of some outstanding problems and recommendations for future research, including data collection.

Geochemistry of serpentinized peridotites from ODP Holes 207-1272A and 207-1274A

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (Fifteen-Twenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44-2.8 µg/g) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 µg/g), depleted in Li (most values below 1 µg/g) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4-65.0 µg/g), H2O and Cl contents and to lower Li contents (0.07-3.37 µg/g) of peridotites, implying that-contrary to alteration of oceanic crust-B is fractionated from Li and that the B and Li inventory should depend essentially on rock-water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17-307 kg in a column of 1 m * 1 m * thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25-904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.

Geochemical composition of gabbroic rocks from ODP Hole 179-1105A, southwest Indian Ridge

Oxide-free olivine gabbro and gabbro, and oxide olivine gabbro and gabbro make up the bulk of the gabbroic suite recovered from Ocean Drilling Program (ODP) Leg 179 Hole 1105A, which lies 1.2 km away from Hole 735B on the eastern transverse ridge of the Atlantis II Fracture Zone, Southwest Indian Ridge. The rocks recovered during Leg 179 show striking similarities to rocks recovered from the uppermost 500 m of Hole 735B during ODP Leg 118. The rocks of the Atlantis platform were likely unroofed as part of the footwall block of a large detachment fault on the inside corner of the intersection of the Southwest Indian Ridge and the Atlantis II Transform at ~11.5 Ma. We analyzed the lithologic, geochemical, and structural stratigraphy of the section. Downhole lithologic variation allowed division of the core into 141 lithologic intervals and 4 main units subdivided on the basis of predominance of oxide gabbroic vs. oxide-free gabbroic rocks. Detailed analyses of whole-rock chemistry, mineral chemistry, microstructure, and modes of 147 samples are presented and clearly show that the gabbroic rocks are of cumulate origin. These studies also indicate that geochemistry results correlate well with downhole magnetic susceptibility and Formation MicroScanner (FMS) resistivity measurements and images. FMS images show rocks with a well-layered structure and significant numbers of mappable layer contacts or compositional contrasts. Downhole cryptic mineral and whole-rock chemical variations depict both "normal" and inverse fine-scale variations on a scale of 10 m to <2 m with significant compositional variation over a short distance within the 143-m section sampled. A Mg# shift in whole-rock or Fo contents of olivine of as much as 20-30 units over a few meters of section is not atypical of the extreme variation in downhole plots. The products of the earliest stages of basaltic differentiation are not represented by any cumulates, as the maximum Fo content was Fo78. Similarly, the extent of fractionation represented by the gabbroic rocks and scarce granophyres in the section is much greater than that represented in the Atlantis II basalts. The abundance of oxide gabbros is similar to that in Hole 735B, Unit IV, which is tentatively correlated as a similar unit or facies with the oxide gabbroic units of Hole 1105A. Oxide phases are generally present in the most fractionated gabbroic rocks and lacking in more primitive gabbroic rocks, and there is a definite progression of oxide abundance as, for example, the Mg# of clinopyroxene falls below 73-75. Coprecipitation of oxide at such early Mg#s cannot be modeled by perfect fractional crystallization. In situ boundary layer fractionation may offer a more plausible explanation for the complex juxtaposition of oxide- and nonoxide-bearing more primitive gabbroic rocks. The geochemical signal may, in part, be disrupted by the presence of mylonitic shear zones, which strike east-west and dip both to the south and north, but predominantly to the south away from the northern rift valley where they formed. Downhole deformation textures indicate increasing average strain and crystal-plastic deformation in units that contain oxides. Oxide-rich zones may represent zones of rheologic weakness in the cumulate section along which mylonitic and foliated gabbroic shear zones nucleate in the solid state at high temperature, or the oxide may be a symptom of former melt-rich zones and hypersolidus flow, as predicted during study of Hole 735B.

Chemical composition and accumulation rates of chemical elements in metalliferous sediments from axial zones of the East Pacific Rise and the Mid-Atlantic Ridge

In the monograph metalliferous sediments of the East Pacific Rise near 21°S are under consideration. Distribution trends of chemical, mineral and grain size compositions of metalliferous sediments accumulated near the axis of this ultrafast spreading segment of the EPR are shown. On the basis of lithological and geochemical investigations spatial and temporal variations of hydrothermal activity are estimated. Migration rates of hydrothermal fields along the spreading axis are calculated. The model of cyclic hydrothermal process is suggested as a result of tectono-magmatic development of the spreding centre.