Stable isotope and geochemical record of ODP Hole 124-769A

Deep marine late Pleistocene sediments from Ocean Drilling Program Sulu Sea Site 769 contain a high-resolution record of paleoceanographic change in this strongly monsoonal climatic setting in the tropical western Pacific. Detailed time series of planktonic foraminifer (G.ruber; white variety) d18O, d13C, and bulk CaCO3 mass accumulation rate (MAR) were generated, spanning the last 750 k.y. Sedimentation rates in this portion of the record average 8.5 cm/k.y., and vary from 4 to 16 cm/k.y. Cross spectral analysis of the d18O and d13C time-series demonstrate that each contains increased variance at the primary orbital periodicities. The d18O record shows strong variability in the precessional-band and closely correlates with the SPECMAP d18O record and other high-resolution records. The dominance of a 23-k.y cycle in the d18O record agrees with other studies of the monsoon system in the Indian Ocean that have documented the importance of precessional insolation as a monsoon-forcing mechanism. In addition, d13C is strongly coherent, with d18O at a period of 41 k.y (obliquity), suggesting a connection between surface water CO2 chemistry in the Sulu Sea and high- latitude climatic change. The d18O and d13C time-series both contain increased spectral variance at a period of 30 k.y. Although the source of 30-k.y. variability is unknown, other studies have documented late Pleistocene Pacific Oceanographic variability with a period of 30 k.y. Major- and trace-metal analyses were performed on a second, less-detailed sample series to independently assess paleoproductivity changes and bottom-water conditions through time. Glacial periods are generally times of increased calcium carbonate and copper accumulation. The positive association between these independent indicators of paleoproductivity suggests an increase in productivity in the basin during most glacial episodes. Changing bottom-water redox conditions were also assessed using the geochemical data. Low concentrations of molybdenum throughout the record demonstrate that bottom waters at this site were never anoxic during the last 750 k.y. The bioturbated character of the sediments agrees with this interpretation.

Chemistry of altered basaltic glass shards and zeolites of volcaniclastic sediments from ODP Hole 157-953C

Trace element concentrations of altered basaltic glass shards (layer silicates) and zeolites in volcaniclastic sediments drilled in the volcanic apron northeast of Gran Canaria during Ocean Drilling Program (ODP) leg 157 document variable element mobilities during low-temperature alteration processes in a marine environment. Clay minerals (saponite, montmorillonite, smectite) replacing volcanic glass particles are enriched in transition metals and rare earth elements (REE). The degree of retention of REE within the alteration products of the basaltic glass is correlated with the field strength of the cations. The high field-strength elements are preferentially retained or enriched in the alteration products by sorption through clay minerals. Most trace elements are enriched in a boundary layer close to the interface mineral-altered glass. This boundary layer has a key function for the physico-chemical conditions of the subsequent alteration process by providing a large reactive surface and by lowering the fluid permeability. The release of most elements is buffered by incorporation into secondary precipitates (sodium-rich zeolites, phillipsite, Fe- and Mn-oxides) as shown by calculated distribution coefficients between altered glasses and authigenic minerals. Chemical fluxes change from an open to a closed system behavior during prograde low-temperature alteration of volcaniclastic sediments with no significant trace metal flux from the sediment to the water column.

Pedogeomorphology, geochronology and paleobotany of soil profiles obtained in the Nagqu area, central Tibet

Vast areas on the Tibetan Plateau are covered by alpine sedge mats consisting of different species of the genus Kobresia. These mats have topsoil horizons rich in rhizogenic organic matter which creates turfs. As the turfs have recently been affected by a complex destruction process, knowledge concerning their soil properties, age and pedogenesis are needed. In the core area of Kobresia pygmaea mats around Nagqu (central Tibetan Plateau, ca. 4500 m a.s.l.), four profiles were subjected to pedological, paleobotanical and geochronological analyses concentrating on soil properties, phytogenic composition and dating of the turf. The turf of both dry K. pygmaea sites and wet Kobresia schoenoides sites is characterised by an enrichment of living (dominant portion) and dead root biomass. In terms of humus forms, K. pygmaea turfs can be classified as Rhizomulls mainly developed from Cambisols. Wet-site K. schoenoides turfs, however, can be classified as Rhizo-Hydromors developed from Histic Gleysols. At the dry sites studied, the turnover of soil organic matter is controlled by a non-permafrost cold thermal regime. Below-ground remains from sedges are the most frequent macroremains in the turf. Only a few pollen types of vascular plants occur, predominantly originating from sedges and grasses. Large amounts of microscopic charcoal (indeterminate) are present. Macroremains and pollen extracted from the turfs predominantly have negative AMS 14C ages, giving evidence of a modern turf genesis. Bulk-soil datings from the lowermost part of the turfs have a Late Holocene age comprising the last ca. 2000 years. The development of K. pygmaea turfs was most probably caused by an anthropo(zoo)-genetically initiated growth of sedge mats replacing former grass-dominated vegetation ('steppe'). Thus the turfs result from the transformation of pre-existing topsoils comprising a secondary penetration and accumulation of roots. K. schoenoides turfs, however, are characterised by a combined process of peat formation and penetration/accumulation of roots probably representing a (quasi) natural wetland vegetation.

Geochemistry of tephra from the Taupo Volcanic Zone

The Taupo Volcanic Zone (TVZ), central North Island, New Zealand, is the most frequently active Quaternary rhyolitic system in the world. Silicic tephras recovered from Ocean Drilling Programme Site 1123 (41°47.16'S, 171°29.94'W; 3290 m water depth) in the southwest Pacific Ocean provide a well-dated record of explosive TVZ volcanism since ~1.65 Ma. We present major, minor and trace element data for 70 Quaternary tephra layers from Site 1123 determined by electron probe microanalysis (1314 analyses) and laser ablation inductively coupled plasma mass spectrometry (654 analyses). Trace element data allow for the discrimination of different tephras with similar major element chemistries and the establishment of isochronous tie-lines between three sediment cores (1123A, 1123B and 1123C) recovered from Site 1123. These tephra tie-lines are used to evaluate the stratigraphy and orbitally tuned stable isotope age model of the Site 1123 composite record. Trace element fingerprinting of tephras identifies ~4.5 m and ~7.9 m thick sections of repeated sediments in 1123A (49.0-53.5 mbsf [metres below seafloor]) and 1123C (48.1-56.0 mbsf), respectively. These previously unrecognised repeated sections have resulted in significant errors in the Site 1123 composite stratigraphy and age model for the interval 1.15-1.38 Ma and can explain the poor correspondence between d18O profiles for Site 1123 and Site 849 (equatorial Pacific) during this interval. The revised composite stratigraphy for Site 1123 shows that the 70 tephra layers, when correlated between cores, correspond to ~37-38 individual eruptive events (tephras), 7 of which can be correlated to onshore TVZ deposits. The frequency of large-volume TVZ-derived silicic eruptions, as recorded by the deposition of tephras at Site 1123, has not been uniform through time. Rather it has been typified by short periods (25-50 ka) of intense activity bracketed by longer periods (100-130 ka) of quiescence. The most active period (at least 1 event per 7 ka) occurred between ~1.53 and 1.66 Ma, corresponding to the first ~130 ka of TVZ rhyolitic magmatism. Since 1.2 Ma, ~80% of tephras preserved at Site 1123 and the more proximal Site 1124 were erupted and deposited during glacial periods. This feature may reflect either enhanced atmospheric transport of volcanic ash to these sites (up to 1000 km from source) during glacial conditions or, more speculatively, that these events are triggered by changes in crustal stress accumulation associated with large amplitude sea-level changes. Only 8 of the ~37-38 Site 1123 tephra units (~20%) can be found in all three cores, and 22 tephra units (~60%) are only present in one of the three cores. Whether a tephra is preserved in all three cores does not have any direct relationship to eruptive volume. Instead it is postulated that tephra preservation at Site 1123 is 'patchy' and influenced by the vigorous nature of their deposition to the deep ocean floor as vertical density currents. At this site, at least 5 cores would need to have been drilled within a proximity of 10's to 100's of metres of each other to yield a >99% chance of recovering all the silicic tephras deposited on the ocean surface above it in the past 1.65 Ma.

Geochemistry and mineralogy of ultramafic rocks from conical seamount

The Mariana arc-trench system, the easternmost of a series of backarc basins and intervening remnant arcs that form the eastern edge of the Philippine Sea Plate, is a well-known example of an intraoceanic convergence zone. Its evolution has been studied by numerous investigators over nearly two decades (e.g., Kang, 1971; Uyeda and Kanamori, 1979; LaTraille and Hussong, 1980; Fryer and Hussong, 1981; Mrosowski et al., 1982; Hussong and Uyeda, 1981; Bloomer and Hawkins, 1983; Karig and Ranken, 1983; McCabe and Uyeda, 1983; Hsui and Youngquist, 1985; Fryer and Fryer, 1987; Johnson and Fryer, 1988; Johnson and Fryer, 1989; Johnson et al., 1991). The Mariana forearc has undergone extensive vertical uplift and subsidence in response to seamount collision, to tensional and rotational fracturing associated with adjustments to plate subduction, and to changes in the configuration of the arc (Hussong and Uyeda, 1981; Fryer et al., 1985). Serpentine seamounts, up to 2500 m high and 30 km in diameter, occur in a broad zone along the outer-arc high (Fryer et al., 1985; Fryer and Fryer, 1987). These seamounts may be horsts of serpentinized ultramafic rocks or may have been formed by the extrusion of serpentine muds. Conical Seamount, one of these serpentine seamounts, is located within this broad zone of forearc seamounts, about 80 km from the trench axis, at about 19°30'N. The seamount is approximately 20 km in diameter and rises 1500 m above the surrounding seafloor. Alvin submersible, R/V Sonne bottom photography, seismic reflection, and SeaMARC II studies indicate that the surface of this seamount is composed of unconsolidated serpentine muds that contain clasts of serpentinized ultramafic and metamorphosed mafic rocks, and authigenic carbonate and silicate minerals (Saboda et al., 1987; Haggerty, 1987; Fryer et al., 1990; Saboda, 1991). During Leg 125, three sites were drilled (two flank sites and one summit site) on Conical Seamount to investigate the origin and evolution of the seamount. Site 778 (19°29.93'N, 146°39.94'E) is located in the midflank region of the southern quadrant of Conical Seamount at a depth of 3913.7 meters below sea level (mbsl) (Fig. 2). This site is located in the center of a major region of serpentine flows (Fryer et al., 1985, 1990). Site 779 (19°30.75'N, 146°41.75'E), about 3.5 km northeast of Site 778, is located approximately in the midflank region of the southeast quadrant of Conical Seamount, at a depth of 3947.2 mbsl. This area is mantled by a pelagic sediment cover, overlying exposures of unconsolidated serpentine muds that contain serpentinized clasts of mafic and ultramafic rocks (Fryer et al., 1985, 1990). Site 780 (19°32.5'N, 146°39.2'E) is located on the western side of Conical Seamount near the summit, at a depth of 3083.4 mbsl. This area is only partly sediment covered and lies near active venting fields where chimney structures are forming (Fryer et al., 1990).

Geochemistry of ODP Hole 183-1137A basalts

The Kerguelen Plateau and Broken Ridge in the southern Indian Ocean together represent one of the most voluminous large igneous provinces (LIPs) ever emplaced on Earth. A scientific objective of Ocean Drilling Program (ODP) Leg 183 was to constrain the post-melting magma evolution of Kerguelen Plateau magmas. In an effort to better understand this evolution, isotopic and trace element analysis of individual plagioclase crystals hosted within two Kerguelen Plateau basalts recovered from Elan Bank were undertaken. Previous whole-rock studies established that the two host basalts investigated in this study are samples of crustally contaminated (lower group) and relatively uncontaminated (upper group) basalt. Plagioclase phenocrysts from the uncontaminated basalt are dominantly normal zoned and exhibit a 87Sr/86SrI range of 0.704845-0.704985, which overlaps uncontaminated group whole-rock values previously reported. Plagioclase crystals from the contaminated basalt are dominantly reverse zoned and exhibit a 87Sr/86SrI range of 0.705510-0.705735, which all lie within contaminated group whole-rock values previously reported. There are no systematic within crystal core to rim variations in 87Sr/86SrI from either group, with the exception that contaminated group crystal rims have overall less radiogenic 87Sr/86SrI than other zones. These observations indicate that crustal assimilation occurred before the formation of Unit 10 plagioclase phenocrysts, which is supported by parent magma trace element abundance data inverted using carefully calculated partition coefficients. Trace element diffusion modeling indicates that the upper group basalt (Unit 4) experienced a more vigorous eruptive flux than the lower group basalt (Unit 10). We suggest that plagioclase phenocrysts in both the upper and lower group basalts originated from the shallowest section of what was likely a complex magma chamber system. We contend that the magmatic system contained regions of extensive plagioclase-dominated crystal mush. Crustal assimilation was not a significant ongoing process in this portion of the Elan Bank magmatic system. Both basalts exhibit compelling evidence for remobilization and partial resorption of crystalline debris (e.g., reverse zoned crystals, glomerocrysts). We suggest Unit 4 and 10 magmas ascended different sections of the Elan Bank magma system, where the Unit 10 magmas ascended a section of the magma system that penetrated a stranded fragment of continental crust.

Geochemistry at DSDP Holes 85-573B and 85-574C

Eocene-Oligocene metalliferous sediments and associated lithologies from the central equatorial Pacific are described in detail. Geochemical analyses of 54 sediment and 2 basalt samples are presented for 34 elements. Detailed stratigraphic and statistical analyses of these data, combined with mineralogic studies, indicate the presence of volcanic glass and seven main mineral phases: biogenic calcite and opal, Fe smectite, goethite, dMnO2, carbonate fluorapatite, and barite. Fe smectite formed by reactions between Fe oxyhydroxides and biogenic opal, causing the dissolution of calcite and the precipitation of barite. Diagenesis was oxic. Sediments have rare earth element distributions similar to those in seawater. The metal content of the sediments is related to competition between the supply rates of hydrothermal and biogenic particles, but has been enhanced by early diagenetic processes. Eocene-Oligocene metalliferous sediments compare closely to those currently being deposited in the Bauer Basin and on the flanks of the East Pacific Rise. There is, however, no evidence that they were deposited in close proximity to an active hydrothermal system.

Geochemistry of the Kimmeridge Clay Formation, Dorset Coast

The Kimmeridge Clay Formation (KCF) and its equivalents worldwide represent one of the most prolonged periods of organic carbon accumulation of the Mesozoic. In this study, we use the molybdenum (Mo) stable isotope system in conjunction with a range of trace metal paleoredox proxies to assess how seawater redox varied both locally and globally during the deposition of the KCF. Facies with lower organic carbon contents (TOC 1-7 wt %) were deposited under mildly reducing (suboxic) conditions, while organic-rich facies (TOC >7 wt %) accumulated under more strongly reducing (anoxic or euxinic) local conditions. Trace metal abundances are closely linked to TOC content, suggesting that the intensity of reducing conditions varied repeatedly during the deposition of the KCF and may have been related to orbitally controlled climate changes. Long-term variations in d98/95Mo are associated with the formation of organic-rich intervals and are related to third-order fluctuations in relative sea level. Differences in the mean d98/95Mo composition of the organic-rich intervals suggest that the global distribution of reducing conditions was more extensive during the deposition of the Pectinatites wheatleyensis and lower Pectinatites hudlestoni zones than during the deposition of the upper Pectinatites hudlestoni and Pectinatites pectinatus zones. The global extent of reducing conditions during the Kimmerigidan was greater than today but was less widespread than during the Toarcian (Early Jurassic) oceanic anoxic event. This study also demonstrates that the Mo isotope system in Jurassic seawater responded to changes in redox conditions in a manner consistent with its behavior in present-day sedimentary environments.

Composition of hydrothermal manifestations in the World Ocean

The monograph summarizes results of studies of hydrothermal fields on the ocean floor, hydrothermal plumes and metalliferous sediments. Hydrothermal ore manifestations formed in different geodynamic settings, with different character of volcanism in different facial conditions of deposition are described. Causes of non-uniformity of hydrothermal system functioning in different parts of the ocean and therefore variability of hydrothermal deposits are under consideration. On the base of found relationships of these irregularities with geodynamics, volcanism and sedimentation a new classification of hydrothermal processes and genetic models of hydrothermal ore formation in the ocean have been created. Regularities of hydrothermal sedimentary material dispersion in bottom waters are discussed.

Geochemistry of rare earth elements in basalts from Walvis Ridge

Selected basalts from a suite of dredged and drilled samples (IPOD sites 525, 527, 528 and 530) from the Walvis Ridge have been analysed to determine their rare earth element (REE) contents in order to investigate the origin and evolution of this major structural feature in the South Atlantic Ocean. All of the samples show a high degree of light rare earth element (LREE) enrichment, quite unlike the flat or depleted patterns normally observed for normal mid-ocean ridge basalts (MORBs). Basalts from Sites 527, 528 and 530 show REE patterns characterised by an arcuate shape and relatively low (Ce/Yb)N ratios (1.46-5.22), and the ratios show a positive linear relationship to Nb content. A different trend is exhibited by the dredged basalts and the basalts from Site 525, and their REE patterns have a fairly constant slope, and higher (Ce/Yb)N ratios (4.31-8.50). These differences are further reflected in the ratios of incompatible trace elements, which also indicate considerable variations within the groups. Mixing hyperbolae for these ratios suggest that simple magma mixing between a 'hot spot' type of magma, similar to present-day volcanics of Tristan da Cunha, and a depleted source, possibly similar to that for magmas being erupted at the Mid-Atlantic Ridge, was an important process in the origin of parts of the Walvis Ridge, as exemplified by Sites 527, 528 and 530. Site 525 and dredged basalts cannot be explained by this mixing process, and their incompatible element ratios suggest either a mantle source of a different composition or some complexity to the mixing process. In addition, the occurrence of different types of basalt at the same location suggests there is vertical zonation within the volcanic pile, with the later erupted basalts becoming more alkaline arid more enriched in incompatible elements. The model proposed for the origin and evolution of the Walvis Ridge involves an initial stage of eruption in which the magma was essentially a mixture of enriched and depleted end-member sources, with the N-MORB component being small. The dredged basalts and Site 525, which represent either later-stage eruptives or those close to the hot spot plume, probably result from mixing of the enriched mantle source with variable amounts and variable low degrees of partial melting of the depleted mantle source. As the volcano leaves the hot spot, these late-stage eruptives continue for some time. The change from tholeiitic to alkalic volcanism is probably related either to evolution in the plumbing system and magma chamber of the individual volcano, or to changes in the depth of origin of the enriched mantle source melt, similar to processes in Hawaiian volcanoes.

Chemical composition and description of a ferromanganese crust, sample 89D, obtained during R/V Capricorne cruise ARCANTE 1

A manganese oxide encrustation (2.5 kg) was dredged, in an island arc setting, downslope of Bertrand bank, a seamount culminating at 70-m depth and located NNE of Grande-Terre, Guadeloupe, and SE of Antigua, West Indies. A thorough texturai analysis indicated a rhythmic precipitation and growth polarity as well as mineralogical ( 10 A tektomanganate) and geochemical (low concentrations of Ni, Cu, Co, Zn, Pb and REE) criteria, point to a submarine hydrothermal origin for most of the sample. The crust was coated with a fine ferromanganese oxide cortex deposited iii a "normal" oceanic environment; it also included micritic fillings, a main pyroclastic zone near the top of the crust, and a Mg-Al sulphate deposit. Planktonic foraminifera coeval with the precipitation of the manganese oxide indicate an age of ca. 3 m. y. (upper Pliocene); i.e., more than 20 m. y. after the cessation of the volcanic activity of the Lesser Antilles outer arc that was responsible for the buildup of the Bertrand seamount. Furthermore, the genesis of the crust is not linked to the activity of the contemporaneous inner arc (Miocene to Present), particularly of its nearmost segment (Basse Terre, Guadeloupe-Montserrat) located about 50 km to the West. The authors suggest that the manganese oxide is the result of convective circulation of sea water through a faulted system occurring in an area of intense seismic activity. The remobilization of chemical elements (Mn, S, etc.) within the seamount volcanic core bas probably affected a substratum that was still hydrothermally altered during the previous volcanic activity of the outer arc. The authors insist on the interest in using texturai analysis for Fe/Mn oxide investigations.

Utility of thallium and geochemical analysis of Mariana arc sediments from ODP Hole 129-801A and 129-801B

We provide the first exploration of thallium (Tl) abundances and stable isotope compositions as potential tracers during arc lava genesis. We present a case study of lavas from the Central Island Province (CIP) of the Mariana arc, supplemented by representative sedimentary and altered oceanic crust (AOC) inputs from ODP Leg 129 Hole 801 outboard of the Mariana trench. Given the large Tl concentration contrast between the mantle and subduction inputs coupled with previously published distinctive Tl isotope signatures of sediment and AOC, the Tl isotope system has great potential to distinguish different inputs to arc lavas. Furthermore, CIP lavas have well-established inter island variability, providing excellent context for the examination of Tl as a new stable isotope tracer. In contrast to previous work (Nielsen et al., 2006b), we do not observe Tl enrichment or light epsilon 205Tl (where epsilon 205Tl is the deviation in parts per 10,000 of a sample 205Tl/203Tl ratio compared to NIST SRM 997 Tl standard) in the Jurassic-aged altered mafic ocean crust subducting outboard of the Marianas (epsilon 205Tl = - 4.4 to 0). The lack of a distinctive epsilon 205Tl signature may be related to secular changes in ocean chemistry. Sediments representative of the major lithologies from ODP Hole Leg 129 801 have 1-2 orders of magnitude of Tl enrichment compared to the CIP lavas, but do not record heavy signatures (epsilon 205Tl = - 3.0 to + 0.4), as previously found in similar sediment types (epsilon 205Tl > + 2.5; Rehkämper et al., 2004). We find a restricted range of epsilon 205Tl = - 1.8 to - 0.4 in CIP lavas, which overlaps with MORB. One lava from Guguan falls outside this range with epsilon 205Tl = + 1.2. Coupled Cs, Tl and Pb systematics of Guguan lavas suggests that this heavy Tl isotope composition may be due to preferential degassing of isotopically light Tl. In general, the low Tl concentrations and limited isotopic range in the CIP lavas is likely due to the unexpectedly narrow range of epsilon 205Tl found in Mariana subduction inputs, coupled with volcaniclastic, rather than pelagic sediment as the dominant source of Tl. Much work remains to better understand the controls on Tl processing through a subduction zone. For example, Tl could be retained in residual phengite, offering the potential exploration of Cs/Tl ratios as a slab thermometer. However, data for Tl partitioning in phengite (and other micas) is required before developing this application further. Establishing a database of Tl concentrations and stable isotopes in subduction zone lavas with different thermal parameters and sedimentary inputs is required for the future use of Tl as a subduction zone tracer.

Description, geochemistry, mineralogy and isotopes of sulfides from ODP Site 193-1189, PACMANUS field

Chemical and isotopic data for rare massive and semimassive sulfide samples cored at Site 1189 (Roman Ruins, PACMANUS) suggest their genetic relationship with sulfide chimneys at the seafloor. Sand collected from the hammer drill after commencement of Hole 1189B indicates that at least the lower section of the cased interval was occupied by material similar to the stockwork zone cored from 31 to ~100 meters below seafloor (mbsf) in this hole, but with increased content of barite, sphalerite, and lead-bearing minerals. Fractional crystallization of ascending hydrothermal fluid involving early precipitation of pyrite may explain vertical mineralogical and chemical zoning within the stockwork conduit and the high base and precious metal contents of Roman Ruins chimneys. A mineralized volcaniclastic unit cored deep in Hole 1189A possibly represents the lateral fringe of the conduit system. Lead isotope ratios in the sulfides differ slightly but significantly from those of fresh lavas from Pual Ridge, implying that at least some of the Pb within the Roman Ruins hydrothermal system derived from a deeper, more radiogenic source than the enclosing altered volcanic rocks.

Chemical composition of basalts from DSDP Hole 46-396B

Samples of crystalline basalt from Site 396 B are all more or less altered, usually in strongly zoned patterns. Evidence has been found for several related or independent alteration stages, including (1) minor localized deuteric (amphibole and mixed clay minerals in miarolitic voids); (2) minor widespread nonoxidizing (pyrite on walls of vugs and cracks); (3) localized diffusion-controlled rug filling ("glauconite" in black halos); (4) pervasive low level oxidizing (transformation of titanomagnetite to cation-deficient titanomaghemite); (5) localized diffusion-controlled strongly oxidizing (breakdown of olivine and titanomaghemite in brown zones). Plagioclase and pyroxene are essentially unaltered. Detailed analyses of gray and brown zones in pillow basalts show that low temperature oxidation has proceeded in a step-wise fashion, with the relative stabilities of the igneous minerals controlling the steps. Secondary minerals that crystallized from pore fluids on to the walls of vugs may or may not be related to local alteration of primary phases. During the most intense stage of alteration, brown oxidation zones grew into basalt fragments behind diffusion controlled fronts. The specific reactions and products of this stage differ among the lithologic units at the site. A model is proposed whereby efficient seawater circulation through the pillow units maintains the pH and the concentrations of Mg2+ and SiO2 dissolved at low levels in pore fluids, so that olivine is replaced by hydrous ferric oxides, and Mg and SiO2 are removed from the system. In the massive basalt unit, circulation is somewhat less effective and Mg and SiO2 are retained in smectites. Deposition of authigenic minerals in the sequence saponite/Fe-Mn oxides/phillipsite/calcite in vugs and cracks may reflect the gradual closing of the systems and probably signals the end of localized oxidation in parts of the core. Mineral compositions indicate that most of these deposits formed from seawater at very low temperature.

Geochemical and isotopic composition of basalts from the axial Mid-Atlantic Ridge, South Atlantic

One of the essential problems of oceanic tectonics is estimation of the influence of plumes of the deep hot mantle on processes in the axial spreading zone. Areas of two giant (St. Helena and Tristan da Cunha) plumes in the Mid-Atlantic Ridge (MAR) rift zone (South Atlantic) are characterized by the effusion of basalts that differ from typical depleted riftogenic tholeiites by anomalously high contents of lithophile components and specific isotopic compositions. Moreover, the rift valley floor with basalt effusion is significantly uplifted above the adjacent sectors of the rift. The formation of the St. Helena Seamount located 400 km east of the MAR axis is related to magmatism that is active to this day. St. Helena Island is a member of the structural ensemble of large volcanic seamounts (Bonaparte, Bagration, and Kutuzov). Like St. Helena Island, each seamount incorporates a series of smaller rises of different morphologies and dimensions. Thus, a system of subparallel series of NE-trending (~45°) rises extend from the seamount ensemble to the African continent. According to the plate tectonics concept, the seamount series represent hotspots related to a deep mantle plume that can be projected onto the present-day St. Helena Island area (St. Helena plume). At the same time, the inferred topographic map based on satellite altimetry data shows that the seamount series also extend along the opposite southwestern direction (~225°) toward the axial MAR and even intersect the latter structure. This fact cannot be explained by the hotspot hypothesis, which suggests stationary positions of plumes relative to the mobile oceanic plate. In the course of Cruise 10 of the R/V Akademik Ioffe (2002), detailed geological and geophysical investigations were carried out at the junction of one structural series with the MAR rift zone located near the Martin Vaz Fracture Zone (Martin Vaz test area, 19°-20° S). The present communication is devoted to the study of lithology, geochemistry, and isotopy of basalts dredged at the test area.