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.

Chemistry and mineralogy of ODP Leg 124 sediment samples

From X-ray mineralogical studies and chemical analyses of the whole rocks and the fine fractions (<2 µm) of ten to fifteen samples at each site of ODP Leg 124, two major sources were identified in the sedimentary components of the Celebes and Sulu basins: (1) a terrestrial and continental contribution; (2) a volcanic influx that gives way to well-defined volcanic units or to a dilute contamination, consisting of coarse-grained minerals (Plagioclase, pyroxene, olivine, spinel) or a smectitic-rich fraction produced by the alteration of volcanic glasses and ashes. The continental signature increases the amount of quartz in the rocks and the phyllitic association is complex: micas, kaolinite, disordered interstratified clay-minerals. The chemical compositions of the bulk rocks and the fractions <2 µm are more potassic and aluminum-rich. The volcanic imprint depends on the grain-size and chemical properties of the components. Ca/Na contents highly variable compared to the K content of the bulk composition are due to the presence of coarse-grained volcanic Plagioclase. The fractions <2 µm are more magnesian than in the continental regime. The diagenesis is revealed by the crystallization of zeolites, the fixation of magnesium into the smectites that depletes the pore fluids in this element. Smectitization of the disordered interstratified clay minerals enriches the alkalinity of the pore fluids. Some deep formations of the Sulu Basin are affected by a thermal event, but no thermal event was recognized in the Celebes Basin.

Chemical composition of ultrabasites and gabbro from rift zones of the Indian Ocean

Results of petrographic studies of ultrabasites and gabbro from rift zones of the Indian Ocean are discussed using materials of Cruise 36 of R/V Vityaz. Rocks sampled from two sites 2700 km apart are close to each other in their composition. Petrographically ultrabasic rocks are divided into four subgroups: I - dunite; II - harzburgite, serpentinite; III - plagioclase lherzolite; and IV - metamorphically altered rocks. Petrographic description and chemical composition of basic rock varieties are presented as well as description of rock-forming minerals and their optical properties. Formation of pyroxene and plagioclase is shown to be related to autometasomatosis. Formation of ultrabasite in rift zones is related to complicated processes.

Mineral composition of mid-ocean-ridge basalts from ODP Leg 187 sites

The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°-46°S, 126°-135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a "maximum olivine fractionation" model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth).

Chemical composition of basalts from DSDP Hole 46-396B

Samples of crystalline basalt from Site 396 B are all more or less altered, usually in strongly zoned patterns. Evidence has been found for several related or independent alteration stages, including (1) minor localized deuteric (amphibole and mixed clay minerals in miarolitic voids); (2) minor widespread nonoxidizing (pyrite on walls of vugs and cracks); (3) localized diffusion-controlled rug filling ("glauconite" in black halos); (4) pervasive low level oxidizing (transformation of titanomagnetite to cation-deficient titanomaghemite); (5) localized diffusion-controlled strongly oxidizing (breakdown of olivine and titanomaghemite in brown zones). Plagioclase and pyroxene are essentially unaltered. Detailed analyses of gray and brown zones in pillow basalts show that low temperature oxidation has proceeded in a step-wise fashion, with the relative stabilities of the igneous minerals controlling the steps. Secondary minerals that crystallized from pore fluids on to the walls of vugs may or may not be related to local alteration of primary phases. During the most intense stage of alteration, brown oxidation zones grew into basalt fragments behind diffusion controlled fronts. The specific reactions and products of this stage differ among the lithologic units at the site. A model is proposed whereby efficient seawater circulation through the pillow units maintains the pH and the concentrations of Mg2+ and SiO2 dissolved at low levels in pore fluids, so that olivine is replaced by hydrous ferric oxides, and Mg and SiO2 are removed from the system. In the massive basalt unit, circulation is somewhat less effective and Mg and SiO2 are retained in smectites. Deposition of authigenic minerals in the sequence saponite/Fe-Mn oxides/phillipsite/calcite in vugs and cracks may reflect the gradual closing of the systems and probably signals the end of localized oxidation in parts of the core. Mineral compositions indicate that most of these deposits formed from seawater at very low temperature.

Chemistry of primary and secondary phases in intraplate basalts and volcaniclastic sediments at DSDP Leg 89 Holes

Electron microprobe data are presented for clinopyroxenes, plagioclases, palagonites, smectites, celadonites, and zeolites in Hole 462A sheet-flow basalts and Site 585 volcaniclastic sediments. Glomerocrystic clinopyroxenes in Hole 462A are predominantly Ti-poor augites with minor fractionation to ferroaugites in rim portions. Quenched plumose clinopyroxenes show considerable variation from Ca-rich to Ca-poor augites, although all are characterized by being Tirich and Cr-poor relative to the glomerocrysts. Two differentiated series of Site 585 pyroxene compositions, calcic augite and diopside-salite, demonstrate the coexistence, in the vitric and lithic clasts, of tholeiitic and alkali basalt types, respectively. Plagioclase compositions in all samples are mainly labradorites, although some zoned Hole 462A glomerocrysts range from An73 to An20 and are characterized by high Mg and Fe contents in the more calcic varieties. The K content of the plagioclases is highest in the more sodic crystals, although the overall higher orthoclase component of Site 585 plagioclases reflects the generally higher bulk-rock K content. The compositions of both secondary smectites and celadonites are similar irrespective of the alteration location (glass, matrix, vesicles, etc.), although brown smectites replacing interstitial glass have marginally higher total Fe contents than pale green and yellow smectites. Analyzed zeolites are mainly phillipsites with variable alkali content, and, together with associated celadonite, represent late-stage alteration repositories for K under mildly oxidizing conditions. The compositions of both early and late secondary minerals are typical of those formed by the submarine alteration of basaltic rocks at low temperatures.

Composition and age of bottom sediments from the Southern Ocean

Geomorphology, geology, stratigraphy, lithology and geochemistry of bottom sediments in the South Ocean are under consideration. Regularities of distribution of iron-manganese nodules, features of occurrence of ore components in the nodules, nodule abundance in bottom sediments have been studied.

Ages, lithologie descriptions and mineral analyses from different DSDP holes in the Indian Ocean

The present work is based on mineralogical studies of sand and silt layers from a number of Deep Sea Drilling Project sites in the Indian Ocean belonging to different physiographic provinces of different ages. The minerals can be grouped into two major associations: a hornblende-opaque association with varying amounts of pyroxene, garnet, epidote, zircon, etc. and a biotite-chlorite-muscovite assemblage. The dominance of unstable minerals indicates a first generation, though evidence of reworking is reflected in the zircon and tourmaline grains at some sites. A large variety of minerals at some sites indicates a complex source. The mineral composition is nearly homogeneous at different sites for the entire length of the core, indicating that they have been derived from the same source during the deposition of that interval. However, the provenance changed by tectonic activity, the effect of which has been reflected in the mineralogy of some sites. An attempt was made to describe the mineralogic characteristics and their tectonic interpretations in the Pliocene and Miocene periods in the Ganges and Indus fan sites and also in the Wharton and Mozambique basin sites. Similar attempts could not be made for other ages in other physiographic provinces as the numbers of samples were too few. Within the limited scope, some idea about the mineralogical character of different basins and different physiographic provinces can be obtained from the present study. Mineralogical evidence also suggests very long transport of sediments in the deep sea. Regional variation of mineralogy has resulted due to source, sea-floor configuration, selective removal, reworking by different agencies and the processes operating in the ocean. There is no relation between a particular age and a set mineral assemblage for the Cenozoic sediments of the Indian Ocean.

Mineralogy and textures of gabbros from ODP Hole 118-735B

Abundant iron-titanium (Fe-Ti) oxide gabbro, olivine gabbro, and troctolite were drilled at Hole 735B adjacent to the Atlantis II Fracture Zone of the Southwest Indian Ridge during Leg 118. The Fe-Ti oxide gabbro occurs as intrusive bodies into olivine gabbro with very sharp intrusive contacts. The size of the intrusive bodies varies from a millimeter to a few tens of meters. Mineralogical parameters, such as anorthite content of plagioclase and Mg/(Mg+Fe) ratios of mafic minerals exhibit bimodal distributions corresponding to olivine and Fe-Ti oxide gabbros, respectively. When the two major gabbro types are looked at separately, several downhole mineralogical cycles are recognized. The Fe-Ti oxide gabbros exhibit two such cycles with plagioclase becoming more sodic and mafic minerals becoming more iron-rich downward in the drill core. The olivine gabbros and troctolites, however, exhibit two cycles showing an upward increase in sodium in plagioclase and iron in mafic minerals. The mineralogical variations of these gabbros and the intrusive contact relationships probably resulted from downward intrusion of evolved magma into underlying solid or almost solidified olivine gabbros and troctolite. The dense evolved melt at the top of the cumulus pile probably formed from the crystallization of olivine gabbro cumulates followed by extreme fractional crystallization of residual melt in an isolated, ephemeral magma chamber. The interlayered occurrence of evolved and primitive gabbros from Hole 735B represents a typical section of lower ocean crust formed at a very slow spreading ridge.

Chemical composition of ultrabasite and gabbro from the rift zones of the Arabian-Indian and Western Indian ridges

Results of petrographic studies of ultrabasite and gabbro from the rift zones of the Indian Ocean ridges are discussed using materials of R/V Vityaz Cruise 36. Rocks sampled from two sites 2700 km apart are close to each other in their composition. Petrographically ultrabasic rocks are divided into four subgroups: I - dunite; II - harzburgite, serpentinite; III - plagioclase lherzolite; and IV - metamorphically altered rocks. Petrographic description and chemical composition of basic rock varieties are presented as well as description of rock-forming minerals and their optical properties. Formation of pyroxene and plagioclase is shown to be related to autometasomatosis, which concludes the magmatic phase proper in rock mass formation accompanied by activity of residual intragranular liquid. Formation of ultrabasite in the rift zones is related to complicated processes.

Geochemistry and ages of rock samples from Kirwanveggen, Antarctica

The basement of southern Kirwanveggen (western Dronning Maud Land) is formed by a SSW-dipping section consisting of (from SW to NE): migmatic gneisses; granitoid; low-grade/prograde meta-pelites, meta-psammites and meta-basalts (= "Polaris Formation"); ortho-gneiss; quartzite mylonite; Polaris Formation; quartzite mylonite; meta-turbidites. These units are (partly) separated by at least four SSW-dipping, NE to N directed major thrusts. Most probably, this thrust system is of Pan-African age. Towards north, the section is followed by the molasse-like Urfjell Group, deposited later than approx. 550 Ma and earlier than 450 Ma. Similarities with the Pan-African of the Shackleton Range (thrusting, molasse) led to the assumption, that the East/West Gondwana suture runs from the Shackleton Range towards Sor Rondane (eastern Dronning Maud Land) passing southern Kirwanveggen at its south-east.