ALBERT

Description: During the cruises 3 and 15 of R.V. "Meteor", representatives of 7 families of benthic abyssal gastropods were collected. The prevalent group was the Turridae; the Naticidae are represented by 2 species, the remaining families by a single species each. A survey of the frequency and distribution of the species within the study area is given. A systematic part deals with the individual species in detail, including notes on taxonomy, material and distribution. Concerning the composition of the abyssal gastropod fauna of the present material, Neogastropoda are 4 times as many as Archaeogastropoda and 6 times as many as Mesogastropoda and Opisthobranchia. 89 % of all specimens are Neogastropoda. Usually deep-sea organisms have a non-pelagic development, but many of the Iberian deep-sea Turrids have a planktotrophic protoconch, that means a long pelagic larval period. Among the 19 species, 2 are cosmopolitan, 9 show a transatlantic distribution and 8 are confined to East-Atlantic basins.

Description: IPOD Leg 49 recovered basalts from 9 holes at 7 sites along 3 transects across the Mid-Atlantic Ridge: 63°N (Reykjanes), 45°N and 36°N (FAMOUS area). This has provided further information on the nature of mantle heterogeneity in the North Atlantic by enabling studies to be made of the variation of basalt composition with depth and with time near critical areas (Iceland and the Azores) where deep mantle plumes are thought to exist. Over 150 samples have been analysed for up to 40 major and trace elements and the results used to place constraints on the petrogenesis of the erupted basalts and hence on the geochemical nature of their source regions. It is apparent that few of the recovered basalts have the geochemical characteristics of typical "depleted" midocean ridge basalts (MORB). An unusually wide range of basalt compositions may be erupted at a single site: the range of rare earth patterns within the short section cored at Site 413, for instance, encompasses the total variation of REE patterns previously reported from the FAMOUS area. Nevertheless it is possible to account for most of the compositional variation at a single site by partial melting processes (including dynamic melting) and fractional crystallization. Partial melting mechanisms seem to be the dominant processes relating basalt compositions, particularly at 36°N and 45°N, suggesting that long-lived sub-axial magma chambers may not be a consistent feature of the slow-spreading Mid-Atlantic Ridge. Comparisons of basalts erupted at the same ridge segment for periods of the order of 35 m.y. (now lying along the same mantle flow line) do show some significant inter-site differences in Rb/Sr, Ce/Yb, 87Sr/86Sr, etc., which cannot be accounted for by fractionation mechanisms and which must reflect heterogeneities in the mantle source. However when hygromagmatophile (HYG) trace element levels and ratios are considered, it is the constancy or consistency of these HYG ratios which is the more remarkable, implying that the mantle source feeding a particular ridge segment was uniform with respect to these elements for periods of the order of 35 m.y. and probably since the opening of the Atlantic. Yet these HYG element ratios at 63°N are very different from those at 45°N and 36°N and significantly different from the values at 22°N and in "MORB". The observed variations are difficult to reconcile with current concepts of mantle plumes and binary mixing models. The mantle is certainly heterogeneous, but there is not simply an "enriched" and a "depleted" source, but rather a range of sources heterogeneous on different scales for different elements - to an extent and volume depending on previous depletion/enrichment events. HYG element ratios offer the best method of defining compositionally different mantle segments since they are little modified by the fractionation processes associated with basalt generation.

Description: Calcareous nannoplankton biostratigraphy has been worked out in the eastern Mediterranean utilizing deep-sea sediments recovered from DSDP Leg 42A Sites 375 and 376. These two drill sites were located approximately 55 km west of Cyprus on the Florence Rise. Sediments, ranging in age from early Miocene (Helicosphaera ampliaperta Zone) through Holocene, contain sufficient age-diagnostic species to recognize essentially all of the lowlatitude nannoplankton zones described by Bukry, although regional, secondary marker species are needed to define some zonal boundaries. Reworked Cretaceous and Paleogene nannoplankton occur throughout the stratigraphic interval studied, but not in quantities large enough to mask indigenous species. Sedimentation rates at Sites 375 and 376 were highest in the late Miocene and late Pleistocene. Open-marine, warm-water species of discoasters are present in significant numbers throughout the Miocene and Pliocene. Earliest Pliocene assemblages contain numerous specimens of ceratoliths. Nannoplankton in post-Messinian sediments at the drill sites and the Zanclean stratotype at Capo Rossello, Sicily, indicate that the base of the Amaurolithus tricorniculatus Zone (base of Triquetrorhabdulus rugosus Subzone) corresponds with the Miocene-Pliocene boundary.

Description: This chapter was previously intended to trace volcanic episodes through the Neogene and Pleistocene geological history recorded in the sedimentary sections drilled on the Emperor seamounts. Drilling disturbance, poor core recovery, and incomplete stratigraphic sections recovered from the seamounts have frustrated that plan, however. Moreover, the Leg 55 sedimentologists found in their smear-slide studies that transported island-arc tephra is scarce in the sediments, if present at all. So we have restricted our objective to description of the volcaniclastic admixture in sediments, as determined by mineralogical and geochemical data. We studied geochemistry of bulk samples (see Murdmaa et al., 1980), coarse-fraction mineralogy, and additional smear slides. The results obtained, however, do not tell much more about the volcaniclastic matter than did shipboard core descriptions.

Description: Petrographic analysis of Quaternary terrigenous sand layers in eastern Mediterranean cores reveals distinct mineralogical differences between the Egyptian Shelf–Nile Cone region and the southern part of the Mediterranean Ridge. A compositionally and texturally immature suite in Ridge cores, mixed with a Nile-derived assemblage, identifies a fresh non-recycled mineral component derived from proximal igneous and metamorphic surface or near-surface exposures, probably in the south–central Ridge area rather than from distal African sources. The presence of such basement terrains would be consistent with a compressive thrust-belt origin for this part of the Mediterranean Ridge.

Description: Regional variations in abundance, morphology, and chemical composition of Fe-Mn nodules have a zonal character. Due to circumcontinental zonality of terrigenous sedimentation the main mass of the nodules occurs in the pelagic part of the ocean, in areas of minimal sedimentation rates. In spatial variations in morphology and chemical composition of the nodules the latitudinal zonality is very clear and associated with latitudinal changes in facial conditions of sedimentation. Elevated contents of Mn, Ni, and Cu and of Mn/Fe ratio occur in nodules from the radiolarian belt. Changes of chemical composition of the nodules with depth (vertical zonality of mineralization) are confirmed. Local variations in abundance, morphology and chemical composition of the nodules are caused by ruggedness of relief and depth variations, variations in sedimentation rate, age of ore formation, intensity of diagenetic redistribution of metals.

Description: Manganese nodules made of radiating rods of well crystallized birnessite were sampled at 8 degree 481.2'N, 103 degree 53.8W, 1875 m below sea level by a dredge that also collected hyaloclastite and basaltic talus. The nodule field is on the floor of a caldera within a young tholeiitic seamount and was discovered and photographed during a deep-two survey. It is interpreted as a brecciated hydrothermal deposit, crystallized from an amorphous manganese oxide precipitate that formed when seawater-based hydrothermal fluids mixed with oxidized seawater. The nodules and surrounding igneous rocks have subsequently been encrusted with hydrogenous ferromanganese oxides.

Description: The successful drilling on Legs 51, 52, and 53 created a rare opportunity to investigate the long-term effects of seawater-rock interactions on the mineralogy and chemistry of basalts erupted on the sea floor. The purpose of this paper is to describe the weathering of the basalts in terms of the changes in their mineralogy and chemistry, and to compare the weathering observed in Hole 418A with that in the upper parts of Hole 417A.

Description: Two distinct sedimentary facies produced by sea-floor hydrothermal activity were cored during Deep Sea Drilling Project Leg 54. The first is equivalent to the typical basal iron- and manganese-rich, clayey mud recovered at many DSDP sites. It was sampled as a dispersed component throughout the cores taken in small sediment ponds in several sites within 120 km of the East Pacific Rise at 9°N. We infer that this component was originally deposited as iron hydroxides dispersed from high-temperature vents over the axial magma chamber of the East Pacific Rise. In the sediments, the iron hydroxides have reacted diagenetically with siliceous microfossil tests and detrital clays to form K- and Fe-rich clays with variable SiO2/Fe2O3 and Fe2O3/Al2O3, ratios.

Description: The cores and dredges described at this site were taken on the ANTIPROD II cruise from 21 February to 9 April 1980 by the Muséum National d'Histoire Naturelle from the R/V Marion Dufresne. A total of 10 cores were recovered and are available at MNHN for sampling and study.

Description: Deep sea photographs in the GH77-1 cruise were obtained by two methods, namely a one-shot 16 mm camera attached to the freefall (boomerang) type photo grab and an independent 35 mm deep sea camera.

Description: Phosphatized bioclastic limestones of Early Tertiary age have been dredged from the crest of a seamount in the equatorial Atlantic near 9°N, 21°W. The samples were recovered from water depths of 700 to 970 m but clearly were deposited in a high-energy reefal environment, thus indicating significant Tertiary subsidence. The vertical movement is beleived to reflect reactivation of an old structural line, the Guinée Fracture Zone, which may have had important effects on the transfer of bottom waters of polar origin between the Sierra Leone and Gambia basins. Examination of foraminiferal limestone coating the shallow water material suggests that deep-water conditions were established on the seamount by Middle Miocene time. All phosphatized samples are covered with a layer of ferromanganese oxides which, from textural and faunal evidence, is Palaeocene or Eocene in age and of shallow water origin. Pervasive phosphatization of the bioclastic limestones appears to have been accomplished before the end of the Eocene while the crest of the seamount was near sea level, because Upper Eocene limestone of shallow water facies is unaffected by such radical diagenesis. Phosphatization probably was associated with vigorous upwelling near the seamount and has been taken to an unusually advanced stage, resulting in a phosphoritesof high economic grade. P2O5 contents are in the range of 37 to 41%. The purity of the phosphorites may be related to the intensity of upwelling during the Early Tertiary, but other factors such as the paucity of terrigenous detritus, frequent reworking of the sedimentary capping of the seamount, and a high primary porosity of the limestones may have been important in their information. The Early Tertiary period of phosphorite deposition on the seamount and in northwest Africa ended before the close of Eocene time in both areas, while shallow water conditions prevailed. It is argued that the cessation of phosphorite formation reflects major changes in the surface circulation of the Atlantic during the Eocene.

Description: The text studies the deep-sea red clays in the East-Central Pacific ocean (Tahiti-Touamotou Archipelago), their authigenic formation, transport and diagenetic character in particular through their composition in REE.

Description: The rate of accumulation of a ferromanganese coating on a fragment of pillow basalt was estimated using a variety of techniques. Unsupported 230 Th activity decrease in the oxide layer, K/A dating of the basalt, fission tracks dating of the glassy layer around the basalt, thickness of the palagonitization rind, and integrated 230 Th activity give ages from approximately 3 x 10-6 years to 5 x 10-3 years. Data suggest that the ferromanganese material formed rapidly (33 mm/10-6 years) and by hydrothermal or volcanic processes.

Description: The volcanic deposits which surround Vulcano Island may be classified in four phases : a detritic phase, fed from the Island itself and from Sicily; a siliceous and carbonateous organic phase; a neogenesis mineral phase; a precipitation phase. The formation of clay and the mineralization of iron and manganese are the subject of particular study. A number of indications suggest the littoral neogenesis of kaolinite around hydrothermal issues, and the formation of smectite at greater depths. The distribution of iron mineralization in the Baie du Levant follows the physicochemical variations of the environment. Melnikovite, pyrite, marcassite and pyrrhotin are dispersed in aureoles around swallow emission points (fumaroles). Close to the hydrothermal spring, iron precipitates are found in the form of amorphous hydroxydes, while geothite and hematite characterize the deeper area as a whole. Manganese mineralizations are exclusively represented by hydroxides and oxides, locally (about - 250 m) organized as incrustations on. lapilli glasses (micronodules). These would appear to be related to the hydrothermal spring.

Description: New data for the direct measurement of the isotopic composition of neodymium in Atlantic Ocean seawater are compared with previous measurements of Pacific Ocean seawater and ferromanganese sediments from major ocean basins. Data for Atlantic seawater are in excellent agreement with Nd isotopic measurements made on Atlantic ferromanganese sediments and are distinctly different from the observed compositions of Pacific samples. These results clearly demonstrate the existence of distinctive differences in the isotopic composition of Nd in the waters of the major ocean basins and are characteristic of the ocean basin sampled. The average eNd(0) values for the major oceans as determined by data from seawater and ferromanganese sediments are as follows: Atlantic Ocean, eNd(0) = -12 ± 2; Indian Ocean, eNd(0) = -8 ± 2; Pacific Ocean, eNd(0) = -3 ± 2. These values are considerably less than eNd(0) value sources with oceanic mantle affinities indicating that the REE in the oceans are dominated by continental sources. The difference in the absolute abundance of 143Nd between the Pacific and Atlantic Oceans corresponds to ab. 10**6 atoms 143Nd per gram of seawater. The correspondence between the 143Nd/144Nd in seawater and in the associated sediments suggests the possible application of this approach to paleo-oceanography. Distinctive differences in eNd(0) values are observed in the Atlantic Ocean between deep-ocean water associated with North Atlantic Deep Water and near-surface water. This suggests that North Atlantic Deep Water may be relatively well mixed with respect to Nd isotopic composition whereas near-surface water may be quite heterogeneous, reflecting different sources for surface waters relative to deep water. This suggests that it may be possible to distinguish the sources of water masses within an ocean basin on the basis of Nd isotopic composition. The Nd isotopic variations in seawater are used to relate the residence time of Nd and mixing rates between the oceans.

Description: The growth rate of a manganese nodule from the South Atlantic Ocean (Rio Grande Ridge) has been determined using a 10Be profile as measured with a Tandem Van de Graaff accelerator. This rate is 4.5 mm /My. A correlation may exist between the 10Be flux and the Mn flux to nodules.

Description: The suite of gabbroic rocks collected by the DSRV ALVIN in 1976 and 1977 from the walls of the Mid-Cayman Rise spreading center were studied in detail to provide the best available data on plutonic rocks sampled directly from the ocean floor. The rock types studied include variably deformed and altered gabbros, orthopyroxene gabbros, olivine gabbros and troctolites, and a few amphibolites. Mineral chemical analyses suggest that the various rock types are representative of a fractionation trend from magnesian troctolites through olivine and clinopyroxene gabbros to iron-enriched orthopyroxene gabbros. Within many individual samples, the primary mineral phases are apparently chemically homogeneous despite sometimes considerable alteration, which suggests reequilibration after original crystallization. Variation in mineral-chemistry across the suite is larger than previously reported for ocean-floor gabbros; this may be due to the larger population of this study. The primary or secondary nature of plagioclase and amphibole must be distinguished in discussions of the igneous processes involved in the genesis of these rocks. In this suite, textural evidence is often inconclusive, and although sodic chemistry is assumed to indicate a secondary origin for plagioclase, no chemical indicator was found to apply to amphiboles. Many different deformation textures are observed in the samples, indicating considerable variation in the conditions of deformation within this slowly accreting ridge environment. Ductile features range from mechanical twins and bent crystals (low strain) to complete recrystallization with a well-developed foliation (high strain). Textures suggestive of cataclasis include zones .1-10 mm wide containing very fine, irregular grains; kinked and cracked grains, usually very undulose; and crosscutting fractures. These textures may occur separately, or next to or overprinting each other. Of the variables controlling the formation of deformation features, temperature and water pressure are most easily estimated. Minimum temperature during deformation is suggested by minimum temperature of formation of mineral assemblages unaffected by this deformation. This temperature is greater than 550°C (epidote-amphibolite facies) for the majority of features observed, although sampling may have introduced a bias away from lower temperature features. Observations suggest enhanced recrystallization where the primary mineralogy has been hydrated to a greater extent. Theoretically, confining pressure for the gabbroic rocks in this suite may have varied from about .3-.9kb; fluid pressure should have been within these limits. Based on mineralogical evidence, cataclastic and ductile textures developed at both high and moderate temperatures. This suggests that strain rates and/or pressure vary considerably within regions where rocks are hot as well as where rocks are cooler, and that after high-temperature deformation some rocks cooled quickly enough to prevent significant recrystallization while others cooled more slowly. These interpretations indicate that the Cayman plutonics were raised to their present position by motions which varied from place to place and time to time, and suggests that the structural history of plutonic.rocks formed at slowly accreting plate boundaries is highly variable and complex.

Description: Ranging in height from gentle hills of less than a meter to steep-sided giants of more than 20 m, the mounds of the Galapagos Rift are spectacular hydrothermal features. Their internal temperatures have been measured at up to 13°C above the bottom water temperature, and total heat flow (conducted plus convected) can be several hundred to several thousand times the normal oceanic values. Fluids, when they discharge from the mound, do so at a very slow rate and at temperatures probably quite near the bottom water temperature. The mounds are principally composed of iron silicates intermixed and incrusted with lesser amounts of manganese oxides. They are generally found in rows, in a uniformly sedimented area above faults or fractures in the crustal rocks which permit fluids to escape from a deep hydrothermal aquifer. The sediment blanket in some way alters the chemistry of the ascending thermal fluids and leads to the development of mounds. The mounds field, covering an area of at least 200 km2 and consisting of thousands of individual mounds, is probably less than 300,000 years old; and many of the mounds may be only a few tens of thousands of years old or less.

Description: Leg 43 was planned to examine a wide variety of geological problems at numerous sites in the deep western North Atlantic basin. A variety of objectives were to be addressed on Leg 43,among these: 1) to investigate the sedimentary development of the North American basin, 2) to recover sediments containing late Mesozoic and Cenozoic fossils for biostratigraphic, paleobiogeographic, and paleoceanographic studies 3) to recover samples of basaltic basement from the smooth crest of the basement ridge associated with the northern part of the New England seamounts, 4) To examine the age and nature of volcanism on Bermuda and along the New England seamount chain, in order to test ideas on volcanic* origin such as the "hotspot" hypothesis, and to examine volcaniclastic lithofacies and assess the role of volcanic sources for certain North Atlantic sediments.

Description: This paper summarizes the petrography and major element chemistry of basalt collected by R/V Atlantis II at 23 °N, and compare these data with published data for 22 °N and with the preliminary shipboard data for DSDP Sites 395 and 396, in order to define the regional petrologic setting of the Leg 45 basalts.

Description: A combined mineralogical, petrological, and geochemical study of altered basaltic rock samples of the upper oceanic crust from Hole 396B IPOD Leg 46. The hole was drilled on the west flank of the Mid-Atlantic Ridge, in anomaly 5 at about 23°N. The site is located in a sediment pond at a water depth of 4450 meters; 205 meters of pillow lavas intercalated with indurated ooze and one exceptionally thick lava unit (sill or flow?) were drilled.

Description: Leg 45 of the Deep Sea Drilling Project was the inaugural cruise of the International Phase of Ocean Drilling. Its primary scientific objective was to develop an understanding of the composition, structure, and origin of oceanic crust by drilling as deeply into it as technology would permit. The feasibility of drilling deep into the oceanic crust was demonstrated during Leg 37 when one hole was drilled 586 meters into basaltic volcanic rocks near the crest of the Mid-Atlantic Ridge at about 37 °N. The areas of principal interest for Leg 45 were survey areas AT-5, situated near 23 °N, 46 °W and AT-6 near 23 °N, 43°W. The two survey areas are on opposite sides of the Mid-Atlantic Ridge, south of the Kane fracture zone. During Leg 45, Holes 395 and 395A were drilled in survey area AT-5 and Hole 396, was drilled in survey area AT-6.

Description: Unlike many cruises of the Deep Sea Drilling Project, Leg 48 was scientifically planned to drill a number of sites to systematically address the problems of passive margin evolution. Site 400 was drilled at the foot of the Meriadzek Escarpment of North Biscay in 4399 meters depth. The site was located in a half-graben forming part of a succession of tilted and rotated fault blocks near the continent/ocean boundary. Site 401 was situated on the planated edge of a tilted fault-block underlying the southern edge of the Meriadzek Terrace on the north Biscay margin. Site 402 was located on the upper slope of the northern continental margin of the Bay of Biscay. The main objectives were to establish the presence or absence of shallow water Upper Cretaceous beds, and to penetrate pre-Aptian synrift sediments and the upslope equivalent of the deep water Albian-Aptian carbonaceous mudstones penetrated at Hole 400A.

Description: Leg 49 of the International Phase of Ocean Drilling (IPOD) was designed with a wide spectrum of problems in mind. Topics investigated were (1) variation with time of mantle sources of spreading-center magmas, (2) aging of the volcanic layer, (3) degree of hydrothermal alteration and metamorphism of the volcanic layer, (4) lateral and vertical variation of the petrochemistry, magnetic properties, and structure of the volcanic layer, (5) north-south variability of spreading-center magmas through the North Atlantic, (6) initiation of glaciation during the late Pliocene, and (7) the influence of bottom currents originating at the Iceland-Faeroes Ridge on sediments of the ridge flank province of the western North Atlantic.

Description: The JOIDES Paleoenvironment Panel had noted that the Neogene section had only been spot-cored on previous legs in the Western Atlantic (Legs 1, 2, 4, 11, and 43). The Panel accordingly requested that the complete section be recovered on Legs 51 through 53 in order to examine the transition from siliceous, Pacific-type Eocene sedimentation to non-siliceous sedimentation resulting from the gradual emergence of the Central American isthmus. It was also detremined to examine in detail the thick Cretaceous-Tertiary section overlying the basement.

Description: Thirty selected samples of pelagic and hydrothermal sediments retrieved on DSDP Leg 54 were chemically analyzed for major and minor oxide concentrations. Additionally, 11 samples of lithified carbonate sediments were petrologically studied. The pelagic sediments, which are described as foraminiferal biomicrite, were found to be generally higher in Fe and Zn, similar in Co content, and lower in Cu content, than the average Pacific pelagic sediments. Mineralogically, these samples are composed principally of calcite with minor amounts of quartz and clays. Hydrothermal sediments from Site 424 are divisible into three classes: (1) silica-rich, iron-poor smectites; (2) silica-poor, iron-rich mixtures of smectites and oxides; and (3) silica-poor, iron-rich materials, comprising mainly amorphous manganese oxides. Thus, two chemically distinct hydrothermal phases are recognizable: Class 1, iron-rich smectites, and Class 3, Mn-rich oxides and oxyhydroxides. Dolomite and pyrite were identified in X-ray diffraction studies of samples from Site 427.

Description: A near-bottom survey of a 24-km length of the East Pacific Rise (EPR) crest near the Leg 54 drill sites has established that the axial ridge is a 12- to 15-km-wide lava plateau, bounded by steep 300-meter-high slopes that in places are large outward-facing fault scarps. The plateau is bisected asymmetrically by a 1- to 2-km-wide crestal rift zone, with summit grabens, pillow walls, and axial peaks, which is the locus of dike injection and fissure eruption. About 900 sets of bottom photos of this rift zone and adjacent parts of the plateau show that the upper oceanic crust is composed of several different types of pillow and sheet lava. Sheet lava is more abundant at this rise crest than on slow-spreading ridges or on some other fastspreading rises. Beyond 2 km from the axis, most of the plateau has a patchy veneer of sediment, and its surface is increasingly broken by extensional faults and fissures. At the plateau's margins, secondary volcanism builds subcircular peaks and partly buries the fault scarps formed on the plateau and at its boundaries. Another deep-tow survey of a patch of young abyssal hills 20 to 30 km east of the spreading axis mapped a highly lineated terrain of inactive horsts and grabens. They were created by extension on inward- and outwardfacing normal faults, in a zone 12 to 20 km from the axis. Sediments sampled on the rise crest and flanks are mixtures of calcareous ooze and metalliferous precipitates, and they have been redistributed by southerly currents with average velocities of 9 cm/s.

Description: 35 km either side of the Pacific-Cocos spreading axis at 846'N, on crust 0.6 x 10SUP-6 years old, a pair of small seamounts rises 1.4 km above the faulted terrain of young abyssal hills. A high-resolution traverse made with the sonars and cameras of a deeply towed vehicle shows that the seamounts are inactive volcanoes, with concave side slopes that steepen upwards to 30, and then abruptly flatten at 2-4 km wide summits. Ledges of pillow basalt outcrop on the lower slopes, and form some talus slopes, but the superficial rock of the upper slopes appear to be mainly hyaloclastite. A transponder-navigated deep-tow survey at the summit area of the eastern seamount mapped an asymmetric caldera containing 3 pit craters, which have vertical walls and flat floors. Dredging across the caldera floor recovered hyaloclastite, angular fragments of primitive tholeiite, and manganese nodules. The paired seamounts probably had a common origin at a 'hot spot' on the spreading axis; continued to grow after they were split by sea-floor spreading; and became extinct and suffered summit collapse after they spread too far from their magma source.

Description: A submersible dive on a turbidite-covered spreading axis in Guaymas Basin photographed and sampled extensive terraces and ledges of talc. The rock contains siliceous microfossils, smectite, and euhedral pyrrhotite as well as rather pure iron-rich talc. Sulfur and oxygen isotopes indicate precipitation around a hydrothermal vent, at about 280°C.

Description: Leg 55 was conceived as part of the decade-long experiment to test the kinematic hot-spot hypothesis and several of its more imporant corollaries for the origin of the Hawaiian-Emperor chain. Also of particular importance was the question of whether the Hawaiian hot spot has remained fixed in the mantle. The specific primary objectives of Leg 55, were to determine (1) whether the known increase in the age of the volcanoes on the Hawaiian chain with distance from Kilauea continues northward along the Emperor Seamounts; (2) whether the lavas of the Emperor volcanoes are of the same chemical composition and were erupted in the same sequence as those of Hawaiian volcanoes; (3) the latitude of formation of Suiko Seamount as a test of hot-spot fixity; and (4) whether the Emperor Seamounts were once islands and, if so, to determine their post-volcanic and subsidence history.

Description: Aragonite mineralization was observed in serpentinized peridotites from the Romanche and Vema Fracture Zones in the Atlantic and the Owen Fracture Zone in the Indian Ocean, either in veins or as radial aggregates in cavities within the serpentinites. Evidence of incipient dissolution of the aragonite crystals was observed in one case. The aragonites tend to have lower Mg content (〈 0.03%) and higher Sr content (〉 0.95%) relative to other marine aragonites. Their 18O16O, 13C12C and 87Sr86Sr isotopic ratios suggest the aragonite was deposited at ocean floor temperatures from solutions derived from sea water circulating in fissures and fractures within the ultramafic rocks. The 18O16O ratios of the serpentines indicate serpentinization occurred at higher temperatures, probably deeper in the crust. Low-T reactions between circulating seawater and Mg-silicates (primarily serpentine and pyroxenes) caused high pH and enrichment of Mg and Ca in the solution, conditions favoring carbonate precipitation. Aragonite was formed rather than calcite presumably because the high Mg2+ concentration in the solution inhibited calcite precipitation. The high Sr content of the aragonites is probably related, at least in part, to their low temperature of formation. Opaque mineral grains containing over 8% NiO and over 40% MnO were observed concentrated along the margins of some of the aragonite veins, suggesting that Ni is one of the elements mobilized during reactions between ultramafic rocks and circulating seawater.

Description: The JOIDES Active Margin Panel for Leg 58 decided that the drilling would occur in the north Philippine Sea, namely, the Shikoku Basin, the Daito Ridge and Basin province. The Shikoku Basin sites were selected to solve several problems concerning back-arc basins formed by ocean-floor spreading. The main objectives were: (1) to determine the age of the oldest sediment, so as to calibrate magnetic-anomaly ages and to provide a test for various spreading models suggested for the basin; (2) to investigate the mineralogy, petrology, and chemistry of basalt samples recovered by drilling, and to compare there basalts with those of midocean ridges, so as to understand the nature and source of magmatic materials in these basins; (3) to determine the distribution of sediment types in time, and to relate that distribution to the tectonic history of the basin.

Description: The Japan Trench area was identified a favorable place for studying convergent margins during the initial deliberations of the IPOD Active Margins Panel. The Leg 56-57 DSDP drill sampling along the Japan Trench transect has indicated that material tectonically accreted during the present convergent episode is limited to a surprisingly small zone. The limits seem well established by the seaward extent of crust with continental thickness and lithologies and by the age and thickness of a terrigenous slope apron covering the presumed but probably unsampled accreted oceanic material.

Description: 40Ar/39Ar incremental heating experiments on three samples dredged from Jingu Seamount indicate that Jingu is 55.4 ± 0.9 m.y. old — older than the Hawaiian-Emperor bend and younger than the two dated Emperor Seamounts to the north. Major-oxide chemistry and petrography show that the samples are similar to hawaiites and mugearites from the Hawaiian Islands. By analogy with Hawaiian alkalic volcanic rocks, groundmass plagioclase compositions (An40-47) indicate that the three Jingu samples are probably mugearites. These results suggest that Jingu is a Hawaiian-type volcano and that the Emperor volcanoes become progressively older from south to north, as predicted by the hot-spot hypothesis.

Description: Forty basaltic rocks collected by submersible during the Cyamex expedition (1978) on the East PacifIc Rise at 21°N, a moderately fast spreading segment (6 cm/year opening rate) of the mid-ocean ridge, consist of angular pillow fragments and glass buds, sheet-flow slabs and samples of columnar pillars standing in collapsed fossillava pools. Most of the rocks are from the crestal are a of the Rise. The collection shows a striking petrographic homogeneity wh en compared with the range of basalts found on other segments of midocean ridges: olivine-phyric, or highly plagioclase-phyric rocks, so common in the slowspreading Famous are a in the Atlantic, are absent. All samples are typical lowpotassium oceanic tholeiites with a limited fractionation trend. Pillow-lavas, thin and thick sheet-flows cannot be distinguished by their major element compositions, as in the Galapagos rift which has the same spreading rate as the EPR at 21°N. Further, ferrobasalts have been described from the Galapagos rift, but do not appear in the Cyamex rocks. In the Cyamex area, olivine and plagioclase are the main silicate phases, and clinopyroxene is absent. In the pillows and sheet-flow samples, four generations of olivine and plagioclase crystals are distinguished. Samples from the fossillava pools are aphyric. The corresponding magma batches are presumed to have migrated rapidly through the magma chamber, and to have been extruded in large volumes, possibly during episodes ofhigh instantaneous opening rate. Fe-Ni and Fe-Cu-rich sulphide phases are common in an lava types as massive globules scatterred through the glass, or as microglobules decorating the walls of empty vesicles. Palagonite and Fe-Mn oxide thicknesses across the strike of the Rise indicate relative ages compatible with successive extrusions at the Rise axis.

Description: In Costa Rica the manganese deposits are an integral part of the Nicoya ophiolite complex. They contain both primary mineralization and, due to weathering, small residual concentrations. The most common manganese minerals in surface outcrops are psilomelane and pyrolusite. Most of them exist in jasper bodies often in a form of breccia in contact with underlaying basalt. In some instances stratiform manganese-oxide crusts and nodule deposits cap jasper bodies and stratabound deposits being overlain by radiolarites.

Description: One of the main objectives of the International Phase of Ocean Drilling (IPOD) of the Deep Sea Drilling Project was to determine the geological, geochemical, and geophysical character of ocean crust formed at mid-ocean spreading ridges. A series of drill holes along the Mid-Atlantic Ridge (MAR), from near Iceland south to approximately 22°N latitude, was planned to compare the crust being generated near an anomalous ridge (Iceland) with that originating near a normal slow-spreading ridge (22°N). At the same time, a series of holes in near zero-age crust at 22 °N was planned along a "flow line" (a transect parallel to local fracture zones) trending west into increasingly older crust, eventually extending the sampling to crust of apparent Jurassic date off the eastern United States coast. This age-transgressive transect was designed to test the evolution of crust formed at one point in the spreading ridge. The intersection of these two series of holes was designated Atlantic Site 5 and was positioned on the upper western flank of the Mid Atlantic Ridge near 22 °N latitude, as close as possible to the spreading ridge crest. The R/V Kana Keoki conducted a detailed 22-day survey of Atlantic Site 5 during Cruise 74-01-01, Leg 7 prior to the DSDP Leg45 was conducted.

Description: In surface sediment at the three DOMES sites in the equatorial North Pacific, the percentage of reworked fauna and the thickness of the acoustically transparent uppermost layer increases from east to west (Sites C to B to A) and the percentage of volcanogenic products decreases. On the basis of lithology and acoustic signature (3.5 kHz) this sediment can be correlated with surface sediment recovered in Deep Sea Drilling Project cores from this same area. It has been assigned to the radiolarian ooze unit of the Clipperton Ocean Formation. The underlying Marquesas Oceanic Formation, well documented in DSDP cores, was not observed in DOMES cores but was identified in the acoustic records as an acoustically transparent to weakly reflective unit. In the study area, the Clipperton Formation is commonly deeply eroded and unconformable with the sea-floor surface, but the Marquesas Formation is present everywhere and has a fairly uniform thickness.

Description: Recent study of many box cores and deep sea photographs from DOMES Sites A, B, and C throws new light on the mode or occurrence of manganese nodules in the Horn Region o f the Pacific. Most bos cores from which the supernatant water has been carefully removed show nodules resting on a relatively firm mud surface, and deep imbedding is rare except for very large nodules; yet many bottom photographs from sites nearby show nodules well-imbedded in what appears to be soft muddy sediment. Observations of nodules still covered by sea water in box cores and experiments designed to preserve the natural relationships between nodules and sediment may explain this paradox. We conclude that at least the upper part of the original sediment, which we call the sediment-water interface "boundary layer", collapses when de-watered to form a wet mud only a fraction the thickness of the material on the sea bed. The relatively firm mud upon which nodules rest in many box cores is thought to underlie the boundary layer at a fairly abrupt contact, but the boundary layer proper probably grades upward, becoming progressively more water-rich, to a diffuse contact with sea water. Small nodules tend to be suspended in the boundary layer and grow largely or totally surrounded by it. As nodules grow, they may reach a size which permits anchoring in the firmer mud below by growth o f oxide dendrites downward, with the result that morphology becomes more asymmetrical as overall growth proceeds. Nodules grow by adding the crystalline manganese oxides todorokite and birnessite where they are in contact with sediment or sediment rich water and iron-rich amorphous oxides where they are in contact with the sea water above. Nodules which show alternating Mn-rich and Fe-rich internal layers may owe this pattern to long-continued sedentary growth during which periods of exposure to sea water alternated with periods during which the upper nodule surface was covered by the boundary layer. Similar nodule structures may also result from periodic overturn of nodules, and many nodules probably experience both kinds of events. The long-standing problem of concentration of manganese nodules at the present deep sea sediment-water interface may be partly explained by the unusual properties of the boundary layer. Although it appears to be both fragile and mobile, it also seems to have a cohesiveness or strength which permits it to hold nodules in suspension in spite of the relatively high density or most nodules. The presence o f organic material (mucoprotein) may contribute to the special properties of the boundary layer. There is photographic evidence that the boundary layer may hide small nodules from view in camera surveys, but the layer is so mobile that even bottom organisms can remove it. The use o f some kind of simple "cleaning" device during camera surveys may produce photographs which give a more accurate record o f nodule distribution than is presently possible.

Description: Methods of estimating manganese nodule grade and concentration were investigated using bottom photographs from a well-explored east-central Pacific manganese nodule deposit.

Description: Determination of radium, thorium, uranium isotopes, and 210Pb in sediments and sediment pore waters from North Equatorial Pacific deep-sea clay-silicous oozes shows that the radium and uranium isotopes are mobile in the pore water. The concentration-depth profiles of radium can be understood in terms of a diagenetic model which takes into account mixing of sediment particles by bioturbation, molecular diffusion in the pore water, adsorption onto particle surfaces, as well as radioactive production and decay. The 234U/238U activity ratios in several samples are higher than the seawater value, indicating some enrichment of 234U in the pore water. However, the absolute concentrations of 238U and 234U are 25% lower than those in seawater, suggesting that the sediments form a sink for uranium isotopes. 210Pb is present in the pore water at concentrations approx. 20% that of 226Ra. The origin of 210Pb in the pore water is uncertain and could be due either to its in situ mobilization in the sediments or subsequent production in the laboratory from the decay of 222Rn. 230Th is present in measurable concentrations in the pore waters, but its distribution does not show any systematic trend with depth or other parameters. The most likely source of 230/Th appears to be minute amounts of sediment particles collected in the pore waters during the squeezing operation.

Description: Cores, submarine photography and dredges described in this report were taken during the R/V Knorr Cruise K79-05 in 1979. Cores and dredges were recovered for 43 stations.