Composition of bottom sediments and serpentinites from Core PS-846

A core of foraminiferal-coccolithic oozes filling a valley of the transform fault located at 29°40'S on the South Atlantic Ridge contains layers composed of angular fragments of igneous and metamorphic rocks. They include many serpentinites deriving from serpentinized ultrabasic rocks, probably exposed on the lower section of the southern slope of the fault valley. A mineral and chemical description of these serpentinites is given.

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.

Toba Tephra across India

A controversy currently exists regarding the number of Toba eruptive events represented in the tephra occurrences across peninsular India. Some claim the presence of a single bed, the 75,000-yr-old Toba tephra; others argue that dating and archaeological evidence suggest the presence of earlier Toba tephra. Resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples, allowing comparison of the Indian samples to those from the Toba caldera in northern Sumatra, Malaysia, and, importantly, the sedimentary core at ODP Site 758 in the Indian Ocean - a core that contains several of the earlier Toba tephra beds. In addition, two samples of Toba tephra from western India were dated by the fission-track method. The results unequivocally demonstrate that all the presently known Toba tephra occurrences in peninsular India belong to the 75,000 yr B.P. Toba eruption. Hence, this tephra bed can be used as an effective tool in the correlation and dating of late Quaternary sedimentary sequences across India and it can no longer be used in support of a middle Pleistocene age for associated Acheulian artifacts.

Composition of post-Jurassic deposits from the Northwest Pacific and the Galapagos Rift, data of DSDP Legs 62, 65, and 70

The monogragh contains results of mineralogicai and geochemical studies of Mesozoic and Cenozoic deposits from the Pacific Ocean collected during Deep Sea Drilling Project. Special attention is paid on the aspects of geochemical history of post-Jurassic sedimentation in the central part of the Northwest Pacific, detailed characteristics of the main stages of sedimentary evolution are given: Early Cretaceons (protooceanic), Late Cretaceons (transitional) and Cenozoic (oceanic). Results of mineralogical and geochemical studies of hydrothermal deposits from the Galapagos Rift are given as well.

Geochemistry at DSDP Leg 92 Holes

The major-element and most of the trace-element data from the different laboratories that contributed to the study of samples recovered during Leg 82 are presented in the following tables. The different basalt groups, identified on the basis of their chemical properties (major and trace elements), were defined from the data available on board the Glomar Challenger as the cruise progressed (see site chapters, all sites, this volume). Most of the data obtained since the end of the cruise and presented in these tables confirm the classification that was proposed by the shipboard party (see site chapters, all sites, this volume). Nevertheless, special mention should be made about Site 564. The shipboard party proposed a single chemical group at this site but noticed significant variations down the hole, mainly in trace-element data. However, the range of variation was small compared to the precision of the measurements. These variations were confirmed by the onshore studies (see papers in Part IV of this volume, especially Brannon's paper, partly devoted to this topic).

Composition of bottom sediments and rocks from the Philippine Trough

New data on bottom sediments and igneous rocks of the Philippine Trench are under consideration. They show differences in geological structures of the island slope and the ocean slope of the trench. The island slope is comparable to the accretionary prism formations on the Philippines; there processes of gravitational re-deposition of sediments occur. The ocean slope is an edge of the Philippine Plate sinking into the trough, where basalts of the oceanic crust are exposed.

Minerals and geochemistry abundance and occurance, as well as K-Ar determinations for DSDP Site 92-597 basalts

Preliminary studies of hydrothermally altered massive basalts formed at the fast-spreading Mendoza Rise and recovered from DSDP Holes 597B and 597C indicate the presence of three secondary mineral assemblages which formed in the following order: (1) trioctahedral chlorite and talc, (2) goethite and smectite, and (3) calcite and celadonite. The sequential precipitation of these mineral assemblages denotes high water:rock ratios and time-varying conditions of temperature (early >200°C to late <30°C) and state of oxidation (early nonoxidative to late oxidative). A decrease in the relative proportion of oxidative mineral assemblages with depth to 70 m in Site 597 basement indicates a zone of oxidative alteration that became shallower with time as the deeper, more constricted fracture systems were filled by secondary mineralization. In this report we present the first results of the K-Ar dating of celadonite formation age; celadonite formation reflects end-stage hydrothermal alteration in Site 597 basement. Three celadonite dates obtained from Site 597 samples include 13.1 ± 0.3 m.y. from 17 m basement depth (Hole 597B), 19.9 ± 0.4 m.y. from 18 m basement depth (Hole 597C), and 19.3 ± 1.6 m.y. from 60 m basement depth (Hole 597C). The age of host rock crystallization (28.6 m.y.) and the K-Ar dates of celadonite formation establish that hydrothermal alteration in the upper 70 m of Site 597 basement continued for at least 10 m.y. and possibly as long as 16 m.y. after basalt crystallization at the ridge crest. Assuming a half-spreading rate of 55 km/m.y., we calculate that hydrothermal circulation was active in shallow basement at a distance of at least 550 km off ridge crest and possibly as far as 1000 km off ridge crest.

Major- and trace-element chemistry of DSDP Leg 70 Holes basalts and their alteration products

Major and trace element compositions of basalts from the lower part of Hole 504B indicate their cogenetic nature. The cored sequence of interlayered pillow lavas and massive lava flows was produced by eruption of lavas, slightly variable in composition. Plagioclase and olivine crystallization in a shallow magma chamber, followed by small-scale fractionation at higher levels, is responsible for these variations. Except in highly fractured zones within the basement, there are systematic variations in the style and degree of rock alteration with depth. Trace element characteristics of altered rocks and secondary minerals indicate that progressive changes in sea water composition occurred as it reacted with basaltic crust.

Physical properties and geochemistry at DSDP Leg 70 Holes

Ashes occurring both as distinct layers and mixed with pelagic sediments of the hydrothermal mounds lying south of the Galapagos Rift are mainly rhyolitic and basaltic. The ashes, of rhyolitic to intermediate composition, appear to belong to a calc-alkalic series and were probably derived from Plinian eruptions in Ecuador or Colombia. Basaltic ashes are made of nonvesicular sideromelane spalling shards and are of tholeiitic composition. They probably were derived locally from fault scarps. Most rhyolitic and basaltic glass shards studied are fresh except for hydration of the rhyolitic shards. Some shards are severely altered, however. Basaltic ash may be more common in pelagic sediments deposited near accretion zones and may be a source of silica and other elements released during diagenesis

Geochemistry of vein rocks of ODP Hole 118-735B

Rock samples from Hole 735B, Southwest Indian Ridge, were examined to determine the principal vein-related types of alteration that occurred, the nature of fluids that were present, and the temperatures and pressures of these fluids. Samples studied included veined metagabbro, veined mylonitic metagabbro, felsic trondhjemite, and late-stage leucocratic diopside-bearing veins. The methods used were standard petrographic analysis, mineral chemical analysis by electron microprobe, fluid inclusion petrography and analysis by heating/freezing techniques and laser Raman microspectroscopy, and oxygen isotopic analyses of mineral separates. Alteration in lithologic Units I and II (above the level of Core 118-735B-3OR; approximately 140 meters below the seafloor) is dominated by hydration by seawater-derived fluids at high temperature, up to about 700°C, and low water/rock ratio, during and immediately after pervasive ductile deformation. Below Core 118-735B-30R, pervasive deformation is less common, and brittle veining and brecciation are the major alteration styles. Leucocratic centimeter-scale veins, often containing diopside and plagioclase, were produced by interaction of hot (about 500°C) seawater-derived fluid and gabbro. The water/rock ratio was locally high at the veins and breccia zones, but the integrated water/rock ratio for the lower part of the hole is probably low. Accessory hydrous magmatic or deuteric phases formed from magmatic volatiles in some gabbro and in trondhjemite. Most subsequent alteration was affected by fluids that were seawater-derived, based on isotopic and chemical analyses of minerals and analyses of fluid inclusions. Many early-generation fluid inclusions, associated with high-temperature veining, contain appreciable methane as well as saline water. The source of methane is unclear, but it may have formed as seawater was reduced during low water/rock interaction with ultramafic upper mantle or ultramafic and mafic layer 3. Temperatures of alteration were calculated on the basis of coexisting mineral chemistry and isotopic values. Hydrothermal metamorphism commenced at about 720°C and continued to about 550°C. Leucocratic veining took place at about 500°C. Alteration within brecciated horizons was also at about 500° to less than 400°C, and the trondhjemite was altered at about 550° to below 490°C. Pressures calculated from a diopside-bearing vein, based on a combination of fluid inclusion and isotopic analysis, were 90 to 100 MPa. This pressure places the sample, from Core 118-735B-70R in Unit V, at about 2 km below the seafloor.

Geochemistry and ages at DSDP Hole 19-192A

Conventional K-Ar and 40Ar/39Ar age data on altered basalts from DSDP Hole 192A on Meiji Guyot, Emperor Seamount chain, indicate a minimum age for the volcano of 61.9 ± 5.0 m.y. The K-Ar data are consistent with the early Maestrichtian date of the overlying sediments, but do not provide either a positive or negative test of the hypothesis that Meiji is older than Emperor volcanoes to the south. The most prominent alteration affecting the age measurements is potassium metasomatism, particularly of feldspar phenocrysts. The K-Ar apparent ages of feldspar separates from the Meiji basalts show that more than half of the potassium metasomatism occurred within the last 25 m.y. or so, and that if the potassium replacement rate has been constant, then the alteration of the Meiji basalts did not begin for 10 to 20 m.y. after the volcano formed.

Major, trace and rare earth element concentration of ODP Site 210-1277 basalts, Newfoundland margin

Drilling of the distal Newfoundland margin at Ocean Drilling Program Site 1277 recovered part of the transition between exhumed sub-continental mantle lithosphere and normal mid-ocean-ridge basalt (N-MORB) volcanism perhaps related to the initiation of seafloor spreading, which may have occurred near the Aptian/Albian boundary, coincident with the final separation of subcontinental mantle lithosphere. Subcontinental mantle lithosphere was recovered near the crest of a basement high, the Mauzy Ridge. This ridge lies near magnetic Anomaly M1 and is inferred to be of Barremian age. The recovered section is dominated by serpentinized spinel harzburgite, with subordinate dunite and minor gabbroic intrusives, and it includes inferred high-temperature ductile shear zones. The serpentinite is capped by foliated gabbro cataclasite that is interpreted as the product of a major seafloor extensional detachment. The serpentinized harzburgite beneath is highly depleted subcontinental mantle lithosphere that was exhumed to create new seafloor within the ocean-continent transition zone. After inferred removal of overlying brittle crust, the detachment was eroded, producing multiple mass flows that were dominated by clasts of serpentinite and gabbro in a lithoclastic and calcareous matrix. Basaltic lavas were erupted spasmodically, mainly as sheet flows, with subordinate lava breccia, hyaloclastite, and possible pillow lava. The sedimentary-volcanic succession and the exhumed mantle lithosphere experienced later high-angle extensional fracturing and probably faulting. Extensional fissures opened incrementally and were filled with silt-sized carbonate, basalt-derived clastic sediment, and hyaloclastite, forming neptunian dykes and geopetal structures. Chemical analysis of representative basalts for major elements and trace elements were made using a high-precision, high-accuracy X-ray fluorescence method (utilizing increased count times) and by whole-rock inductively coupled plasma-mass spectrometry that yielded additional evidence for rare earth elements. The analyses indicate N-MORB to slightly enriched compositions. The MORB was produced by relatively high degree melting of a fertile mantle source that differed strongly from the cored serpentinized peridotites. The basalts exhibit a distinct negative Nb anomaly on MORB-normalized plots that can be explained by prior extraction of melt from upper mantle that had previously been affected by subduction, possibly during closure of the Iapetus or Rheic oceans. In the proposed interpretation, mantle lithosphere was exhumed to the seafloor and experienced mass wasting to form serpentinite-rich mass flows. The interbedded MORB records the beginning of a transition to "normal" seafloor spreading. This interpretation takes into account drilling results from the Iberia-Galicia margin and the Jurassic Alps-Apennines.

Reflection microscopy and geochemistry at DSDP Leg 55 Holes

Magnetic properties of volcanic rocks are controlled mainly by the physical and chemical state of their constituent ferromagnetic minerals. The most important parameters determining magnetic properties are concentration, composition, grain size, and oxidation state. In sea floor basalts, the main ferromagnetic minerals are titanomagnetites which are either unoxidized or, more commonly, have undergone various degrees of posteruptive low-temperature oxidation to become cationdeficient titanomagnetites, or titanomaghemites. The effects of this low-temperature alteration are seen in the increase of Curie temperature and decrease of saturation magnetization and lattice parameter of ferromagnetic minerals (Readman and O'Reilly, 1972). It is now believed that titanomaghemitization of newly formed mid-ocean ridge crust proceeds with a time constant of about 1 m.y., accompanying drastic decrease of the intensity of the natural remanent magnetization (NRM) (Johnson and Atwater, 1977).