Rocks and minerals from the Strakhov Fracture Zone, 4°N Mid-Atlantic Ridge

Geological-geophysical data obtained during Cruises 7, 11, and 12 of R/V Akademic Nikolay Strakhov (1989-1991) within the international project EQUARIDGE in the Strakhov Fracture Zone region (4°N) are presented. The trough of the fracture is interpreted as an open extension joint, a graben produced by stretching along the axis of the Mid-Atlantic Ridge. Bedrock studies showed that typical mid-ocean tholeiitic basalts occur within the narrow (60 nm wide) axial rift zone, whereas igneous rocks not typical for the ocean were found on the eastern and western flank plateaus. This allows to suppose that a reworked relict continental-type basement of pre-Upper Jurassic age possibly exists beneath the flank plateaus, within the segment under discussion. The above data correspond to the hypothesis of E. Bonatti about nonspreading nature of the basement of Mid-Atlantic Ridge within the equatorial segment and the Strakhov Fracture Zone.

Composition and fluxes of sinking particulate material and bottom sediments in the northeast Black Sea in 1998-1999

For the first time deep-sea mooring stations with sediment traps were deployed in the northeast Black Sea. One sediment trap for long-term studies was located at Station 1 (44°15'N, 37°43'E, deployment depth 1800 m, depth 1900 m). The trap collected sinking sedimentary material from January to May 1998. Material collectors were changed every 15 days. Other stations with sediment traps for short-term studies (September-October 1999) were located on the shelf: Station 2 (44°16'N, 38°37'E, deployment depth 45 m, depth 50 m) and on the bottom of the canyon: Station 3 (44°16'N, 38°22'E, deployment depth 1145 m, depth 1150 m), Station 4 (44°11'N, 38°21'E, deployment depths 200, 1550, 1650 m, depth 1670 m). Collected material indicates that vertical particle fluxes are controlled by seasonal changes of in situ production and by dynamics of terrigenous matter input. Higher vertical particle flux of carbonate and biogenic silica was in spring due to bloom of plankton organisms. Maximum of coccolith bloom is in April-May. Bloom of diatoms begins in March. In winter and autumn lithogenic material dominates in total flux. Its amount strongly depends on storms and river run-off. Suspended particle material differs from surface shelf sediments by finer particles (mainly clay fraction) and high content of clay minerals and biogenic silica. This material may form lateral fluxes with higher concentration of particles transported along the bottom of deep-sea canyons from the shelf to the deep basin within the nepheloid layer. In winter such transportation of sedimentary material is more intensive due to active vertical circulation of water masses.

Minerals of bulk and fine-grained (2 µm) samples from DSDP Leg 60

A bulk-sediment and clay-fraction X-ray diffraction study of samples from Deep Sea Drilling Project Leg 60 shows an abundance of the following minerals: plagioclase feldspar, zeolite, smectite, Fe-Mg chlorite, attapulgite, and serpentine. Amorphous compounds are also abundant. The variations in abundance of the different components correspond to episodes of volcanic activity through time. Deposits from periods of great activity are composed of sediments very rich in amorphous matter and in "primary" minerals (e.g., plagioclase feldspars). During relatively quiet periods, clay minerals and zeolites predominate.

Chemical and isotopic compositions of rocks, ores, and hydrothermal minerals from the Rainbow and Logachev-2 hydrothermal fields

The aim of this paper is to analyze and compare mineralogy and geochemistry of copper-zinc sulfide ores from the Logachev-2 and Rainbow hydrothermal fields of the Mid-Atlantic Ridge (MAR) confined to serpentinite protrusions. It was found that Zn(Fe) and Cu, Fe(Zn) sulfides had been deposited in black smokers pipes almost simultaneously from intermittently flowing, nonequilibrium H2S-low solutions of different temperatures. Pb isotope composition confirmed that the deep oceanic crust had been a source of lead. The ores from the Rainbow field are 20-fold higher in Co than ores restricted to basalts and show a high ratio of Co/Ni=46. The ores from the Rainbow field are enriched in 34S isotope (aver. d34S=10 per mil) because of constant flow of cold sea water into the subsurface zone of the hydrothermal system. Ores from the Logachev-2 field are 8 times higher in gold compared to other MAR regions. Sulfide ores from the Rainbow and Logachev-2 fields have no analogues among MAR ore occurrences in terms of enrichment in valuable components (Zn, Cd, Co, and Au).

Chemical and isotopic compositions of rocks and minerals from the Palenyi Island, Por'ya Guba (Bay), White Sea

Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in the Palenyi Island within the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10-11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineral and chemical composition. Closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to values obtained with mineralogical geothermometers (garnet-orthopyroxene and garnet-biotite) and correspond to the high-temperature granulite facies (860-900°C). Identified systematic variations in d18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in d18O cannot be explained by primary isotopic heterogeneity of the protolith. Model calculations of the extent and trend of d18O variations in minerals suggest that fluid-rock interaction at various integral fluid/rock ratios in discrete zones was the only mechanism that could generate the zoning. This demonstrates that focused fluid flux could occur in lower crustal shear zones. Preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.

Geochemistry of lavas from the Lau-Tonga arc and back-arc system

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreading centers, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridge volcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site 834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arc volcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range from andesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-undersaturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites. The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterized by the existence of very magnesian olivines (up to Fo90-92) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclases in Units 2 and 9. The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processes affecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarc magmatism, and (2) enrichment in large-ion-lithophile elements, caused by a subduction contribution. A general trend of increasing depletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec) Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) of elements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jump following prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx of new mantle, as required by the recognition of a two-stage opening of the Lau Basin.

Pebbles and carbonate nodules Legs 56-57 Holes

At all DSDP Leg 56 drilling sites, exotic pebbles occur commonly, throughout the cores. Chips of carbonate nodules occur only at Site 434 on the lower inner trench wall. Both exotic pebbles and carbonate nodule chips sometimes tend to be concentrated at particular levels of cores. Exotic pebbles are generally well rounded and consist of various rock types, such as dacite, andesite, basalt, tuff, gabbro, granodiorite, metaquartzite, biotite hornfels, lithic wacke, mudstone, etc., of which dacite occurs commonly at all the sites. Almost all pebbles at Site 436 and most at Sites 434 and 435 may have been rafted by ice. Some at the latter sites may have been derived by down-slope slumping. Carbonate nodules consist of microcrystalline dolomite, manganoan calcite, and siderite; CaCO3 content ranges from 22 to 65 per cent. They are also generally characterized by a high content of P2O5. The nodules are commonly rich in diatom remains, some of which indicate that the nodules are autochthonous. Some nodules contain abundant glass shards, with a modal refractive index of 1.499, almost identical to shards in the surrounding mud and ooze. These facts suggest that the carbonate nodules may have been formed diagenetically, in situ. This may throw light on problems of the formation of carbonate nodules in ancient "geosynclinal" sediments. It is also very important to point out that these carbonate nodules were formed within sediment deposited well below the CCD.