(Table, page 233) Comparative analysis of manganese nodules

A preliminary report on the manganese nodule field southwest of Western Australia published in this Journal recently (Frakes, Exon and Granath, 1977) quoted chemical analyses which were carried out on air-dried material. Significantly higher metal values have been recorded in some later analyses done on nodules dried at 105°C. Tests have shown that the ground, air-dried material retains considerable moisture, which accounts for the higher metal values of the later analyses. The average water content (after drying at 105°C) has been determined at 16 percent. The relevant chemical data now available on this material are summarised in the accompanying table: in this table metal values (by atomic absorption spectrophotometry) have been recalculated assuming a moisture content of 16 percent.

Data on multidisciplinary research in the Tajura Rift (Gulf of Aden) during Cruise 7 of R/V Akademik Mstislav Keldysh in winter 1983-1984

In the collective monograph results of geological and geophysical studies in the Tadjura Rift carried out by conventional outboard instruments and from deep/sea manned submersibles "Pisces" in winter 1983-1984 are reported. Main features of rift tectonics, geology, petrology, and geochemistry of basalts from the rift are under consideration. An emphasis is made on lithology, stratigraphy, and geochemistry of bottom sediments. Roles of terrigenous, edafogenic, biogenic, and hydrothermal components in formation of bottom sediments from the rift zone are shown.

Chemical and mineral compositions of basalts and volcanic glasses from DSDP Sites 65-483 and 65-485

Major and rare earth element (REE) data for basalts from Holes 483, 483B, and 485A of DSDP Leg 65, East Pacific Rise, mouth of the Gulf of California, support a simple fractional crystallization model for the genesis of rocks from this suite. The petrography and mineral chemistry (presented in detail elsewhere) provide no evidence for magma mixing, but rather a simple multistage cooling process. Based on its lowest TiO2 content (0.88%), FeO*/MgO ratio (0.95 with total Fe as FeO), and Mg# (100 Mg/Mg + Fe" = 70), sample 483-17-2-(78-83) has been selected as the most primitive primary magma of the samples analyzed. This is supported by the REE data which show this sample has the lowest total REE content, a La/Sm_cn (chondrite-normalized) = 0.36, and Eu/Sm_cn = 1.05. Because other samples analyzed have higher SiO2, lower Mg#, and a negative Eu anomaly (Eu/Sm_cn as low as 0.89), they are most likely derivative magmas. Wright-Doherty and trace element modelling support fractional crystallization of 14.1% plagioclase (An88), 6.7% olivine (Fo86), and 4.7% clinopyroxene (Wo41En49Fs10) from 483-17-2-(78-83) to form the least differentiated sample with Mg# = 63. The La/Sm_cn of this derivative magma is almost identical to the parent magma (0.35 to 0.36), but the other samples have higher La/Sm_cn (0.45 to 0.51), more total REE, and lower Mg# (60 to 56). Both Wright-Doherty and trace element modelling indicate that the primary magma chosen cannot produce these more evolved samples. For the major elements, the TiO2 and P2O5 are too low in the calculated versus the observed (1.38 to 1.90; 0.11 to 0.17, respectively, for example). Rayleigh fractionation calculates a lower La/Sm_cn and requires about 60% crystal removal versus 40% for the Wright-Doherty. These more evolved samples must be derived from a parent magma different from the one selected here and, unfortunately, not sampled in this study. A magma formed by a smaller degree of partial melting with slightly more residual clinopyroxene left in the mantle than for sample 483-17-2-(78-83) is required.

Rare earth and other chemical element contents and partitions in bottom sediments, micronodules, and nodules from the bioproductive zone of the Pacific Ocean

Concentrations and compositions of rare earth elements (REE) in three micronodule fractions (50-250, 250-500, and >500 ?m), coexisting macronodules, and host sediments were studied. Samples were collected at three sites (Guatemala Basin, Peru Basin, and northern equatorial Pacific) located in elevated bioproductivity zones of surface waters. Influence of micronodule size is dominant for REE compositions and subordinate for REE concentrations. For example, Ce concentration inversely correlates with micronodule size and drops to the lowest value in macronodules and host sediments. Decrease of Ce concentration is generally accompanied by Mn/Fe increase in micro- and macronodules. Hence, the role of diagenetic source of material directly correlates with micronodule sizes. Contribution of the diagenetic source is maximal for macronodules. REE composition distinctions for micronodules and macronodules can be attributed to variations of hydrogenic iron oxyhydroxides and diagenetic (hydrothermal) iron hydroxophosphates that are the major REE carriers in ferromanganese ore deposits. Relationship and general trend in chemistry of coexisting macronodules suggest that they can represent products of the initial stage of nodule formation.

Composition of Cretaceous basalts from DSDP Hole 16-163

Basalt underlying early Campanian chalk at Deep Sea Drilling Project (DSDP) Site 163 is divided into seven extrusive cooling units bounded by glassy margins. The margins have dips of 15° to 70°, suggestive of pillow flows rather than tabular flows. The margins are fresh sideromelane (glass) grading inward to opaque and reddish-brown globules containing microcrystalline material with radial, undulose extinction. Relative to adjacent sideromelane, the reddish-brown globules are enriched in sodium and calcium, whereas the opaque globules are depleted in these elements and enriched in iron and magnesium. It appears that basalt just inside the pillow margins has differentiated in place into globules of two distinct compositions. This globule zone grades inward to less rapidly cooled pyroxene varioles and intergrowths of plagioclase and opaque minerals. In the center of the thicker cooling units, the texture is diabasic. Alteration and calcite vein abundance are greatest at pillow margins and decrease inward; the interior of the thickest cooling unit is only slightly altered, and calcite veins are absent. Chemical analysis of whole rock by atomic absorption spectrophotometry, and of sideromelane by electron microprobe, indicates that the rock is a slightly weathered tholeiite. The atomic absorption analyses, except the one nearest the top of the basalt, are relatively uniform and similar to the sideromelane microprobe analyses, including those near the top of the basalt. This suggests that deep penetration is not necessary to get through the severely altered layer at the basalt surface, and that within this altered layer, analyses of sideromelane may be more representative of crustal composition than analyses of whole rock.

Geochemical composition and atomic formulas of saponites, celadonites and clays from ODP Leg 168 sites

A drilling transect across the sedimented eastern flank of the Juan de Fuca Ridge, conducted during Leg 168 of the Ocean Drilling Program, resulted in the recovery of samples of volcanic basement rocks (pillow basalts, massive basalts, and volcanic glass breccias) that exhibit the effects of low-temperature hydrothermal alteration. Secondary clays are ubiquitous, with Mg-rich and Fe-rich saponite and celadonitic clays commonly accounting for several percent, and up to 10%-20% by volume. Present-day temperatures of the basement sites vary from 15° to 64°C, with the coolest site being about 0.8 Ma, and the warmest site being about 3.5 Ma. Whereas clays are abundant at sites that have been heated to present temperatures of 23°C and higher, the youngest site at 15°C has only a small trace of secondary clay alteration. Alteration increases as temperatures increase and as the volcanic basement ages. The chemical compositions of secondary clays were determined by electron microprobe, and additional trace element data were determined by both conventional nebulization inductively coupled plasma-mass spectroscopy (ICP-MS) and laser-ablation ICP-MS. Trioctahedral saponite and pyrite are characteristic of the interior of altered rock pieces, forming under conditions of low-oxygen fugacity. Dioctahedral celadonite-like clays along with iron oxyhydroxide and Mg-saponite are characteristic of oxidized haloes surrounding the nonoxidized rock interiors. Chemical compositions of the clays are very similar to those determined from other deep-sea basalts altered at low temperature. The variable Mg:Fe of saponite appears to be a systematic function both of the Mg:Fe of the host rock and the oxidation state during water-rock interaction.

Sedimentation rates and geochemistry of the Atlantic and Indic Ocean of Late Miocene - Early Pliocene

Studies of the late Miocene-early Pliocene biogenic bloom typically have focused on high-productivity areas in the Indian and Pacific Oceans in order to achieve high resolution samples. Thus there is a paucity of information concerning whether the Atlantic Ocean, in general or low-productivity regions in all three basins experienced this bloom. This study measured the phosphorus mass accumulation rate (PMAR). in five cores from low-productivity regions of the Atlantic and Indian Oceans. All cores exhibit a peak in productivity 4-5.5 Ma, coincident with the Indo-Pacific bloom. This suggests that nutrients were not shifted away from low-productivity regions nor out of the Atlantic Ocean. Instead, it appears that the bloom was caused by an overall increase in nutrient flux into the world oceans. Four of the cores record the bloom's PMAR peak as bimodal, indicating a pulsed increase in phosphorus to the oceans. This suggests that there may have been multiple causes of the biogenic bloom.

Wet-bulk densities, mean atomic weights, effective porosities, grain densities, and compressional-wave velocities at DSDP Leg 58 Holes

The effects of water saturation and open pore space on the seismic velocities of crystalline rocks are extremely important when comparing laboratory data to in situ geophysical observations (e.g., Dortman and Magid, 1969; Nur and Simmons, 1969; Christensen and Salisbury, 1975). The existence of fractured rocks, flow breccias and drained pillows in oceanic crustal layer 2a, for instance, may appreciably reduce seismic velocities in that layer (Hyndman, 1976). Laboratory data assessing the influence of porosity and water saturation on seismic velocities of oceanic crustal rocks would certainly aid interpretation of marine geophysical data. Igneous rocks recovered during Leg 58 of the Deep Sea Drilling Project, in the Shikoku Basin and Daito Basin in the North Philippine Sea, are extremely vesicular, as evidenced by shipboard measurements of porosities, which range from 0 to 30 per cent (see reports on Sites 442, 443, 444, and 446, this volume). Samples with this range of porosities afford an excellent opportunity to examine the influence of porosity and water saturation on seismic velocities of oceanic basalts. This paper presents compressional-wave velocities to confining pressures of 1.5 kbars for water-saturated and air-dried basalt samples from the North Philippine Sea. Samples used in this study are from sites 442, 443 and 444 in the Shikoku Basin and Site 446 in the Daito Basin. Excellent negative correlation between porosity and compressional-wave velocity demonstrates that waterfilled pore space can significantly reduce compressionalwave velocities in porous basalts. Velocities measured in air-dried samples indicate that the velocity difference between dry samples and saturated samples is small for porosities exceeding 10 per cent, and very large for lower porosities.