Element oxides in basaltic glass fragments from ODP Hole 203-1243B

Sand-sized basaltic glass fragments were recovered in the liner of Core 203-1243B-19R, the deepest recovery from Hole 1243B. Microprobe analysis of 582 glassy cuttings cluster into five compositionally distinct groups, most of which are unlike the lithologic units described on board ship. Drilling operations intended to sweep cuttings from the caving hole and differences between the cuttings and geochemically distinct lithologic units of the upper part of the basement indicate that the cuttings came mainly, if not entirely, from the lower part of the hole. They give information about the part of Hole 1243B that had poor core recovery. Enriched mid-ocean-ridge basalt (MORB) from the upper part of the hole and transitional MORB from two groups of cuttings from sources low in the hole may be a trace of the Galápagos plume on the Pacific plate or may be a normal consequence of eruptions from two distinct magmas on fast-spreading crust.

Geochemistry and stable isotopes of peridotites of ODP Hole 103-637A

A ridge of strongly serpentinized, plagioclase-bearing peridotite crops out at the boundary between the Atlantic oceanic crust and the Galicia continental margin (western Spain). These peridotites, cored at Hole 637A (ODP Leg 103) have been mylonitized at high-temperature, low-pressure conditions and under large deviatoric stress during their uplift (Girardeau et al., 1988, doi:10.2973/odp.proc.sr.103.135.1988). After this main ductile deformation event, the peridotite underwent a polyphase metamorphic static episode in the presence of water, with the crystallization of Ti- and Cr-rich pargasites at high-temperature (800°-900°C) interaction with a metasomatic fluid or alkaline magma. Introduction of water produced destabilization of the pyroxenes and the subsequent development of hornblendes and tremolite at temperatures decreasing from 750° to 350°C. The main serpentinization of the peridotite occurred at a temperature below 300°C, and possibly around 50°C, as a consequence of the introduction of a large amount of seawater, which is suggested by stable isotope (d18O and SD) data. Finally, calcite derived from seawater precipitated in late-formed fractures or locally pervasively impregnated the peridotite at low temperature (~10°C).

Petrographic description and geochemistry of basalts and altered volcanic rocks from ODP Leg 183 sites

The basalts recovered during Legs 183 and 120 from the southern, central, and northernmost parts of the Kerguelen Plateau (Holes 1136A, 1138A, 1140A, and 747C, respectively), as well as those recovered from the eastern part of the crest of Elan Bank (Hole 1137A), represent derivates from tholeiitic melts. In the northern part of the Kerguelen Plateau (Hole 1140A), basalts may have formed from two sources located at different depths. This is reflected in the presence of both low- and high-titanium basalts. The basalts are variably altered by low-temperature hydrothermal processes (at temperatures up to 120°C), and some are affected by subaerial weathering. The hydrothermal alteration led mainly to the formation of smectites, chlorite minerals, mixed-layer hydromica-smectite and smectite-chlorite minerals, hydromica, serpentine(?), clinoptilolite, heulandite, stilbite, analcime, mordenite, thomsonite, natrolite(?), calcite, quartz, and dickite(?). Alteration of extrusive basalts is mainly related to horizontal fluid flow within permeable contact zones between lava flows. Under a nonoxidizing environment of alteration, the tendency to lose most of elements, including rare earth elements, from basalts dominates. Under on oxidizing environment, basalts accumulate many elements.

Igneous geochemistry of OP Leg 126 samples

Major element, trace element, and radiogenic isotope compositions of samples collected from Ocean Drilling Program Leg 126 in the Izu-Bonin forearc basin are presented. Lavas from the center of the basin (Site 793) are high-MgO, low-Ti, two-pyroxene basaltic andesites, and represent the products of synrift volcanism in the forearc region. These synrift lavas share many of the geochemical and petrographic characteristics of boninites. In terms of their element abundances, ratios, and isotope systematics they are intermediate between low-Ti arc tholeiites from the active arc and boninites of the outer-arc high. These features suggest a systematic geochemical gradation between volcanics related to trench distance and a variably depleted source. A basement high drilled on the western flank of the basin (Site 792) comprises a series of plagioclase-rich two-pyroxene andesites with calc-alkaline affinities. These lavas are similar to calc-alkaline volcanics from Japan, but have lower contents of Ti, Zr, and low-field-strength elements (LFSE). Lavas from Site 793 show inter-element variations between Zr, Ti, Sr, Ni, and Cr that are consistent with those predicted during crystallization and melting processes. In comparison, concentrations of P, Y, LFSE, and the rare-earth elements (REE) are anomalous. These elements have been redistributed within the lava pile, concentrating particularly in sections of massive and pillowed flows. Relative movement of these two-element groupings can be related to the alteration of interstitial basaltic andesite glass to a clay mineral assemblage by a post-eruptive process. Fluid-rock interaction has produced similar effects in the basement lavas of Site 792. In this sequence, andesites and dacites have undergone a volume change related to silica mobility. As a result of this process, some lithologies have the major element characteristics of basaltic andesite and rhyolite, but can be related to andesitic or dacitic precursors by silica removal or addition.

Geochemistry of basalts near the Tamayo Fracture Zone

Recent investigations of the southern Gulf of California (22°N) on Leg 65 of the Deep Sea Drilling Project (DSDP) allow important comparisons with drilled sections of ocean crust formed at different spreading rates. During Leg 65 the Glomar Challenger drilled seven basement holes at sites forming a transect across the ridge axis near the Tamayo Fracture Zone. An additional site was drilled on the fracture zone itself, where a small magnetic "diapir" was located. Together with the material from Site 474 (drilled during Leg 64) the cores recovered at these sites are representative of the upper basaltic and sedimentary crust formed since the initial opening of the Gulf. The pattern of magmatic accretion at the ridge axis is conditioned by the moderate to high rate of spreading (~6 cm/y.) and comparatively high sedimentation rates that now characterize the Gulf of California. In terms of spreading rate, this region is intermediate between the "superfast" East Pacific Rise axis to the south (up to 17 cm/y.) and the slow-spreading Mid-Atlantic Ridge (2-4 cm/y.) both of which have been extensively studied by dredging and drilling.

Geochemistry at DSDP Hole 69-504B

Cores from Deep Sea Drilling Project Holes 501, 504B and 505B have an unusual near-vein zonation in basalts. Megascopically, zonation occurs as differently colored strips and zones whose typical thickness does not exceed 6 to 7 cm. Microscopically, the color of zones depends on variably colored clay minerals which are the products of low-temperature hydrothermal alteration in basalt. These differently colored zones form the so called "oxidative" type of alteration of basalts. Another "background," or, less precisely termed, "non-oxidative," type of alteration in basalts is characterized by large-scale, homogeneous replacement of olivine, and filling of vesicles and cracks by an olive-brown or olive-green clay mineral. The compositions of clay minerals of the "background" type of alteration, as well as the composition of co-existing titanomagnetites, were determined with an electron microprobe. There are sharp maxima in potassium and iron content, and minima in alumina, silica, and magnesia in clay minerals in the colored zones near veins. Coloring of clay and rock-forming minerals by iron hydroxides and a decrease of the amount of titanomagnetite, which apparently was the source of redeposited iron, occur frequently in colored zones. We assume that the large-scale "background" alteration in the basalts occurred under the effect of pore waters slowly penetrating through bottom sediments. Faulting can facilitate access of fresh sea water to basalts; thus above the general homogeneous background arise zones of "oxidative" alteration along fractures in basalts. The main factors controlling these processes are time (age of basalt), grain size, temperature, thickness of sedimentary cover, and heat flow.

Diatoms on the lower surface of Arctic sea ice and hydrochemical parameters in the water-ice layer

Morphology, ecology, range and species composition of diatom algae mass accumulations that are biotypically associated with the lower surface of Arctic sea ice are discussed. Materials were obtained by skindivers in the Central Arctic Basin at drift stations SP-23 in August 1977 and SP-22 in July 1980.

Chemical composition and sulfur-isotope ratios in DSDP Legs 69-and 70

As part of the geochemical-petrological study of basalts recovered from DSDP Hole 504B (Leg 70) on the southern flank of the Costa Rica Rift, we investigated specially the relationships between the distribution and isotopic composition of sulfur of scattered and vein sulfides on the one hand, and the observed pattern and processes of secondary alterations on the other. The following groups of observations are essential: (1) variations in the contents and isotopic composition of sulfur of different forms of sulfides are clearly interrelated and are observed solely in porous horizons established on the basis of detailed geophysical experiments; (2) the enrichment of sulfides in the light sulfur isotope decreases from the upper to the lower horizons, and within horizons in the direction of the less-altered rock; (3) the increase of d34S values of scattered sulfides in individual permeable zones parallels a decrease in the degree of iron oxidation in the contents of crystallization water, and in the concentrations of Mg, K, and Li in the rock.

Major, trace and rare earth elements and minerals at DSDP Legs 69-70 Holes

We obtained major and trace element data on 113 samples from basalts drilled during DSDP Legs 69 and 70 in the Costa Rica Rift area. The majority have major and trace element characteristics typical of ocean-ridge tholeiities. Most of the basalts are relatively MgO rich (MgO > 8 wt.%) and have Mg values (MgO/MgO + 0.85FeO x 100) of about 53, characteristics that clearly indicate that the various magmas underwent only a small amount of crystal fractionation before being erupted onto the seafloor. According to their normative mineralogies, the rocks are olivine tholeiites. A few samples plot close to the diopside-hypersthene join of the projected basalt tetrahedron. Except for basalts from two thin intervals in Hole 504B, which differ significantly from all the other basalts of the hole, practically no chemical downhole variation could be established. In the two exceptional intervals, both TiO2 and P2O5 contents are markedly enriched among the major oxides. The trace elements in these intervals are distinguished by relatively high contents of magmatophile elements and have flat to enriched chondrite-normalized distribution patterns of light rare earth elements (LREE). Most of the rocks outside these intervals are strongly depleted in large-ionlithophile (LIL) elements and LREE. We offer no satisfactory hypothesis for the origin of these basalts at this time. They might have originated within pockets of mantle materials that were more primitive than the LIL-element-depleted magmas that were the source of the other basalts. A significant change with depth in the type of alteration occurs in the 561 meters of basalt cored in Hole 504B. According to the behavior of such alteration-sensitive species as K2O, H2O-, CO2, S, Tl, and the iron oxidation ratio, the alteration is oxidative in the upper part and nonoxidative or even reducing in the lower part. The oxidative alteration may have resulted from low temperature basalt/seawater interaction, whereas hydrothermal solutions may be responsible for the nonoxidative alteration.

d18O, lithologies, mineralogy and determination of temperature and depth of formation of DSDP Site 69-504 siliceous rocks

Seven opal-CT-rich and five quartz-rich porcellanites and cherts from Site 504 have a range in oxygen-isotope values of 24.4 and 29.4 per mil. In opal-CT rocks, d18O becomes larger with sub-bottom depth and with age. Quartz-rich rocks do not show these trends. Boron, in general, increases with decreasing d18O for porcellanites and cherts considered together, supporting the conclusion that boron is incorporated within the quartz crystal structure during precipitation of the SiO2. Silicification of the chalks at Site 504 began 1 m.y. ago - that is, 5 m.y. after sedimentation commenced on the oceanic crust. Temperatures of chert formation determined from oxygen-isotope compositions reflect diagenetic temperatures rather than bottom-water temperatures, and are comparable to temperatures of formation determined by down-hole measurements. Opal-A in the chalks began conversion to opal-CT when a temperature of 50°C was reached in the sediment column. Conversion of opal-CT to quartz started at 55 °C. Silicification occurred over a stratigraphic thickness of about 10 meters when the temperature at the top of the 10 meters reached about 50°C. It took about 250,000 years to complete the silica transformation within each 10-meter interval of sediment at Site 504. Quartz formed over a stratigraphic range of at least 30 meters, at temperatures of about 54 to 60°C. The time and temperatures of silicification of Site 504 rocks are more like those at continental margins than those in deep-sea, open-ocean deposits.

Geochemistry of dikes and lavas from the north wall of the Hess Deep Rift

An investigation of ~1-m.y.-old dikes and lavas from the north wall of the Hess Deep Rift (2°15'N, 101°30'W) collected during Alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust. The study area encompasses 25 km of an east-west flow line, representing ~370,000 years of crustal accretion at the East Pacific Rise. Samples analyzed exhibit depleted incompatible trace element abundances and ratios [(La/Sm)N < 1]. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flow line. Rather, over short (<4 m) and long (~25 km) distances, significant variations are observed in major and trace element concentrations and ratios. Modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions. These findings, combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift-dominated systems, suggest a more realistic model of the magmatic system underlying the East Pacific Rise relative to the commonly assumed twodimensional model. In this model, melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir. Dikes emanating from these distinct reservoirs transport magma along axis, resulting in interleaved dikes and host lavas with different evolutionary histories. This model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin.

Zirkon fission track ages of for middle Pleistocene Rangitawa Tephra, New Zealand

Rangitawa Tephra is an important stratigraphic marker in mid-Pleistocene marine and terrestrial sequences in New Zealand and adjacent ocean basins. Zircon fission track ages (ZFTA) on Rangitawa Tephra from five sites in the southern North Island yield mean site ages in the range 0.34 to 0.40 Ma with a weighted mean of 0.35 + 0.04 Ma (1 sigma). On the basis of glass shard major-element chemistry, ferromagnesian mineralogy, ZFTA and similarity of paleomagnetic dates of proposed tephra correlalives in deep-sea cores, it is concluded that Rangitawa Tephra represents a major eruptive event in the Taupo Volcanic Zone most probably associated with eruption of the Whakamaru-group ignimbrites (0.35 0.39 Ma) or less likely the Paeroa Range Group Ignimbrites (0.36 -0.38 Ma). Pollen analyses from two onshore sites, together with regional loess stratigraphy, show that Rangitawa Tephra was erupted during a glacial period. The ZFTA and previously reported oxygen isotope data from DSDP Site 594 indicate that Rangitawa Tephra was erupted near the end of oxygen isotope stage 10.

Primary mineralogy of ODP Leg 115 basalts

Basement rocks were recovered at four sites on Leg 115 along the Reunion hotspot track in the western Indian Ocean. Plate tectonic reconstructions indicate that the drilled structures formed in three different volcanic environments. Sites 706 and 713 from the eastern side of the Saya de Malha Bank and the northern end of the Chagos Bank, respectively, are on a large volcanic platform analogous to Iceland on the Mid-Atlantic Ridge. Lavas at Site 707 on the northwestern side of the Saya de Malha Bank erupted during the early stages of rifting of the Seychelles from India. Basalts from Site 715 were erupted onto an isolated oceanic island that was distant from ocean ridges and continents much as Reunion Island is today. Many of the rocks were examined in thin section and found to be primarily augite-plagioclase basalts with minor olivine and rare opaque oxides. Site 715 is unusual in that it contains a variety of basalts including olivine-rich and aphyric Fe-Ti basalts. At each of the four sites the rocks were grouped into chemical types (units) on the basis of ship- board bulk-rock analyses and at least one thin section from each chemical unit was analyzed by electron microprobe. The plagioclase and augite chemistry reflects the bulk-rock chemistry and, in general, these minerals were in equilibrium with their host magmas at the time the basalts were quenched. Olivine was rarely preserved, but where it is still present it also appears to have crystallized in equilibrium with the host magma. At three of the drill sites plagioclase phenocrysts or megacrysts that crystallized from a primitive magma are also present. The one site (715) that does not contain these primitive plagioclase phenocrysts is also the site that appears to have been influenced the least by ocean- ridge or Deccan-type magmas. Site 715, furthermore, has a mineralogy that is dominated by olivine as compared with the plagioclase-rich lavas of the other sites.

Composition of ferromanganese crusts from the Atlantic, Pacific and Indian Oceans

A collection of layered ferromanganese ores (27 samples) from the Atlantic and Pacific oceans was studied. Trace element and PGE contents were determined layer-by-layer (up to 10 microlayers) in 13 of these samples. The trace, rare earth, and platinum group element distributions, including their layer-to-layer variations, were compared in hydrogenic and hydrothermal crusts from different regions. It was found that the main PGE variations (by a factor of 10-50) are related to their layer-to-layer variations within a given ore field. The distributions of PGE and trace elements are strongly heterogeneous, which is related, first, to different contents of the elements in the layers of different age in ferromanganese crusts (FMC) and, second, to the observed regional heterogeneity and influence of hydrothermal fluids. Geochemical data indicate that CFC formation was mainly caused by the hydrochemical precipitation of material from seawater. This process was accompanied by diagenetic phenomena, water-rock interaction, and influence of volcanic and hydrothermal sources.