Geochemistry of pelagic and hemipelagic sediments of ODP Leg 126 sites

Chemical analyses were performed on major, minor, and rare-earth elements of pelagic and hemipelagic sediments of the forearc, arc, and backarc sites of the Izu-Bonin Arc, Ocean Drilling Program Leg 126. Analyses of the hemipelagic and pelagic sediments of this area indicate that the chemical composition of this arc is highly affected by the chemical composition of rocks of this arc as a source of sediments. The Oligocene sediments, which are characterized by high MgO contents, reflect the chemical composition of the Paleogene volcanic rocks of the immature arc. Moreover, the late Miocene to Quaternary sediments with low MgO contents are attributed to the composition of the present arc. We also suggest that the sedimentation rates and topography of the sedimentary basin affect the MnO and SiO2 contents of pelagic and hemipelagic sediments.

Heavy metal contents in bottom sediments and Fe-Mn nodules and their states in bottom and interstitial waters from the Clarion-Clipperton province

Ultrafiltration with tagged atoms was used to study physicochemical states (dissolved, colloidal, suspended) of Mn, Co, Ni, Zn, and Ce in bottom and interstitial waters collected in two areas of the Pacific Ocean with Fe-Mn nodules of different size, shape, structure and origin in different abundances. Use of filters with pore diameter of 0.05 ?m allowed to identify colloidal forms of the metals in bottom sediments and interstitial waters. It was demonstrated experimentally that differences in physicochemical situation in the studied areas could result in formation of nodules by different mechanisms, producing characteristic differences that were observed.

Abundance of nodules, chemical and mineral compositions of nodules and sediments from the Central Pacific

The monograph considers facial conditions of ore-formation in the Central Equatorial Pacific, as well as lithostratigraphy and local variability of bottom sediments. Mineral composition of nodules, forms of occurrence of chemical elements in sediments and nodules, composition of interstitial waters, age of nodules, regularities and processes of ore formation in the radiolarian belt of the Pacific Ocean zone are also under consideration.

Geochemistry of sediments from the Tyrrhenian Sea

At Holes 650A and 651 A, set respectively in the Marsili Basin and the Vavilov Basin, Pleistocene sediments (turbiditic inputs interbedded with essentially hemipelagic sediments) may show layers of mudrocks with moderate to strong induration. Except in the two samples from Hole 651 A, it seems that zeolite crystallization does not play a role in the induration phenomenon. This latter appears to result from in situ clay authigenesis. Secondary K-Fe beidellite or Fe-Mg beidellite form diagenetic growths and bridges between sedimented particles. Turbidites are rich in volcaniclastics (glass, pumices and other volcanogenic elements) but the induration phenomenon appears to be associated essentially with the occurrence of basaltic detritus. It is proposed that clay authigenesis results from low temperature alteration of basaltic fragments issued from Vavilov and probably Marsili seamounts in sediments isolated from seawater by overlying deposits.

Rare and trace element analyses from DSDP Holes 31-291 and 31-294 from the Philippine Sea

Metal-rich sediments were found in the West Philippine Basin at DSDP sites 291 (located about 500 km SW of the Philippine Ridge or Central Basin Fault) and 294/295 (located about 580 km NE of the Philippine Ridge). In both cases the metalliferous deposits constitute a layer, probably Eocene in age, resting directly above the basaltic basement at the bottom of the sediment column. The chemistry of the major (including Fe and Mn) and trace elements (including trace metals, rare earth elements, U and Th) suggest a strong similarity of these deposits to metalliferous deposits produced by hydrothermal activity at oceanic spreading centers. Well-crystallized hematite is a major component of the metal-rich deposits at site 294/295. We infer that the Philippine Sea deposits were formed at some spreading center by hydrothermal processes of metallogenesis, similar to processes occurring at oceanic spreading centers. A locus for their formation might have been the Philippine Ridge (Central Basin Fault), probably an extinct spreading center. We conclude that metallogenesis of the type occurring at oceanic spreading centers can take place also in marginal basins. This has implications for the origin of metal deposits found in some ophiolite complexes, such as those in Luzon (Philippines), which may represent fragments of former marginal basins rather than of oceanic lithosphere.