Geochemistry of volcanic rocks of ODP Holes 126-792E and 126-793B

The Izu-Bonin forearc basement volcanic rocks recovered from Holes 792E and 793B show the same phenocrystic assemblage (i.e., plagioclase, two pyroxenes, and Fe-Ti oxides ±olivine), but they differ in the crystallization sequence and their phenocryst chemistry. All the igneous rocks have suffered low-grade hydrothermal alteration caused by interaction with seawater. As a result, only clinopyroxenes, plagioclases, and oxides have preserved their primary igneous compositions. The Neogene olivine-clinopyroxene diabasic intrusion (Unit II) recovered from Hole 793B differs from the basement basaltic andesites because it lacks Cr-spinels and contains abundant titanomagnetites (Usp38.5-46.4) and uncommon FeO-rich (FeO = 29%) spinels. It displays petrological and geochemical similarities to the Izu Arc volcanoes and, thus, can be considered as related to Izu-Bonin Arc magmatic activity. The titanomagnetites (Usp28.5-33) in the calc-alkaline andesitic fragments of the Oligocene volcaniclastic breccia in Hole 793B (Unit VI) represent an early crystallization phase. The Plagioclase phenocrysts enclosed in these rocks show oscillatory zoning and are less Ca-rich (An78.6-67.8) than the plagioclase phenocrysts of the diabase sill and the basement basaltic andesites. Their clinopyroxenes are Fe-rich augites (Fs ? 19.4; FeO = 12%) and thus, differ significantly from the clinopyroxenes of the Hole 793B arc-tholeiitic igneous rocks. The 30-32 Ma porphyritic, two-pyroxene andesites recovered from Hole 792E are very similar to the andesitic clasts of the Neogene breccia recovered in Hole 793B (Unit VI). Both rocks have the same crystallization sequence, and similar chemistry of the Fe-Ti oxides, clinopyroxenes, and plagioclases: that is, Ti-rich (Usp25.5-30.4) magnetites, Fe-rich augites, and intensely oscillatory zoned plagioclases with bytownitic cores (An86-63) and labradorite rims (An73-68). They display a calc-alkaline differentiation trend (Taylor et al., this volume). So, the basement highly porphyritic andesites recovered at Hole 792E, and the Hole 793B andesitic clasts of Unit VI show the same petrological and geochemical characteristics, which are that of calc-alkaline suites. These Oligocene volcanic rocks represent likely the remnants of the Izu-Bonin normal arc magmatic activity, before the forearc rifting and extension. The crystallization sequence in the basaltic andesites recovered from Hole 793B is olivine-orthopyroxene-clinopyroxene-plagioclase-Fe-Ti oxides, indicating a tholeiitic differentiation trend for these volcanic rocks. Type i is an olivine-and Cr-spinel bearing basaltic andesite whereas Type ii is a porphyritic pyroxene-rich basaltic andesite. The porphyritic plagioclase-rich basaltic andesite (Type iii) is similar, in most respects, to Type ii lavas but contains plagioclase phenocrysts. The last, and least common lava is an aphyric to sparsely phyric andesite (Type iv). Cr-spinels, included either in the olivine pseudomorphs of Type i lavas or in the groundmass of Type ii lavas, are Cr-rich and Mg-rich. In contrast, Cr-spinels included in clinopyroxenes and orthopyroxenes (Types i and ii lavas) show lower Cr* and Mg* ratios and higher aluminium contents. Orthopyroxenes from all rock types are Mg-rich enstatites. Clinopyroxenes display endiopsidic to augitic compositions and are TiO2 and Al2O3 depleted. All the crystals exhibit strong zoning patterns, usually normal, although, reverse zoning patterns are not uncommon. The plagioclases show compositions within the range of An90-64. The Fe-Ti oxides of the groundmass are TiO2-poor (Usp16-17). The Hole 793B basaltic andesites show, like the Site 458 bronzites from the Mariana forearc, intermediate features between arc tholeiites and boninites: (1) Cr-spinel in olivine, (2) presence of Mg-rich bronzite, Ca-Mg-rich clinopyroxenes, and Ca-plagioclase phenocrysts, and (3) transitional trace element depletion and epsioln-Nd ratios between arc tholeiites and boninites. Thus, the forearc magmatism of the Izu-Bonin and Mariana arcs, linked to rifting and extension, is represented by a depleted tholeiitic suite that displays boninitic affinities.

Chemical and germanium isotopic compositions of sediments and sedimentary rocks from ODP Sites 129-801 and 185-1149

A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC-ICP-MS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of 74Ge/70Ge ratios are expressed as d74Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2? for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4? in d74Ge, representing 12 times the uncertainty of the measurements and demonstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of 74Ge/70Ge, 73Ge/70Ge and 72Ge/70Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of d74Ge in igneous rocks is only 0.25? without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a d74Ge of 1.3+/-0.2? can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher d74Ge values between 2.0? and 3.0?. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the d74Ge of the seawater to +3.0?.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to seawater. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have d74Ge values ranging from 0.7? to 2.0?. The variable values of the cherts cannot be explained by binary mixing between a biogenic component and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification.

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.

Chemical and mineral compositions of recent and ancient sediments and sedimentary rocks from the Mariana Trench

On the bed and on the ocean slope of the southern latitudinal part of the Mariana Trench ancient sediments, as well as sedimentary and igneous rocks are exposed. In the lower part of the sampled part of the studied section Late Oligocene to Early Miocene chalk-like limestones and marls occur. Upward marly tuffites and tuffs (apparently alternating with carbonate rocks) occur. These rocks are overlain by Early Miocene tuffaceous clays and siliceous-clayey muds. In the upper part of the section there are Pleistocene pelagic clays and ethmodiscus oozes.

Conglomerate and sandstone petrography at DSDP Hole 58-445

Conglomerates and sandstones in lithologic unit V at DSDP Site 445 comprise lithic clasts, detrital minerals, bioclasts, and authigenic minerals. The lithic clasts are dominantly plagioclase-phyric basalt and microdolerite, followed by plagioclase-clinopyroxene-phyric basalt, aphyric basalt, chert, and limestone. A small amount of hornblende schist occurs. Detrital minerals are dominantly plagioclase, augite, titaniferous augite, olivine, green to pale-brown hornblende, and dark-brown hornblende, with subordinate chromian spinel, epidote, ilmenite, and magnetite, and minor amounts of diopside, enstatite, actinolite, and aegirine-augite. Bioclasts are Nummulites boninensis, Asterocyclina sp. cf. A. penuria, and some other larger foraminifers. Correlation of cored and dredged samples indicates that the Daito Ridge is mainly composed of igneous, metamorphic, ultramafic, and sedimentary rocks. The igneous rocks are mafic (probably tholeiitic) and alkalic. The metamorphic rocks are hornblende schist, tremolite schist, and diopside-chlorite schist. The ultramafic rocks are alpinetype peridotites. Mineralogical data suggest that there were two metamorphic events in the Daito Ridge. The older one was intermediate- to high-pressure metamorphism. The younger one was contact metamorphism caused by a Paleocene volcanic event, possibly related to the beginning of spreading of the west Philippine Basin. The ultramafic rocks suffered from the same contact metamorphism. During the Eocene, exposed volcanic and metamorphic rocks on the uplifted Daito Ridge may have supplied pebble clasts to the surrounding coast and shallow sea bottom. The steep slope offshore may have caused frequent slumping and transportation of the pebble clasts and shallow-water benthic organisms into deeper water, forming the conglomerates and sandstones treated here.

Chemical composition of rocks from the Eastern Indian Ocean

The monograph presents results of comprehensive geological and geophysical studies carried out in 1973 and 1976 during Cruises 54 and 58 of R/V "Vityaz" in the Eastern Indian Ocean. On the base of obtained data a description of topography, magnetic and gravity fields, structure of the sedimentary series and deep crustal structure of the East Indian Ridge, Central, West Australian and Cocos Basins, the Sunda Trench has been done. Materials on petrography, petrochemistry and geochemistry of igneous rocks in the region have been summarized. New geological and geophysical information has been compared with with DSDP materials. Tectonics and geological history of the Eastern Indian Ocean are under consideration.