800 resultados para Minerals.


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This paper provides a brief, descriptive, sedimentological background for the chapters on hydraulic piston core Site 480 in this symposium, and supplements data given in the site chapter for Sites 479-480 (this volume, Pt. 1). Sediments are composed primarily of planktonic diatoms, with minor numbers of silicoflagellates, radiolarians, and varying amounts of both benthic and planktonic foraminifers, along with a large terrigenous component of olive brown, silty clay. The section contains meter-thick intervals of finely laminated facies alternating with nonlaminated zones. A few paleoenvironmental events are documented within the generally uniform sequence by sporadic occurrences of thin turbidites, phosphatic concretions, fish debris concentrations, an ash layer, and a thin layer of diagenetic dolomite. The distribution of nonlaminated and laminated zones is attributed to fluctuations of bottom-water oxygen content caused by variations in circulation, fertility, and productivity. Homogeneous sections are interpreted as coinciding with cooler climatic periods, whereas laminated sections seem to correspond to upwelling conditions during drier periods.

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A collection of dredge samples from the Hunter Fracture Zone includes holocrystalline massive and cumulose basic and ultrabasic rocks and volcanites of the ophiolite complex: from basalts to rhyolites. The ultrabasic rocks are largely serpentinized harzburgites and lherzolites; their relict mineralogy is typical of peridotite considered to be the refractory residue of partial melting of the mantle. Cumulate textured ultramafic rocks probably are related to the cumulate gabbro and granodiorite rather than to the residual mantle material. The gabbroic rocks are dominantly cumulate textured Pl-Opx-Cpx±Ol gabbronorite and Pl-Cpx±Ol gabbros; the mineral features of these rocks are the result of their crystallization at moderate pressure (in a moderate level magma chamber). The massive Pl-Cpx±Ol gabbros are less common. Green and brown-green Ca-amphibole has partially or totally replaced the clinopyroxene in many samples. There is an overlap in mineral chemistry between the cumulate rocks and the Opx-Cpx-Pl volcanic rocks and boninites. It is interpreted as an indication that the cumulate rocks were co-genetic with Opx-Cpx-Pl volcanic rocks and that they both constitute remnants of an island arc volcanic-plutonic series. The petrologic evidence indicates that ophiolite gabbroic rocks were derived from an island-arc rather than from a mid-ocean ridge.

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The paper presents characteristics of the Nd and Sr isotopic systems of ultrabasic rocks, gabbroids, plagiogranites, and their minerals as well as data on helium and hydrocarbons in fluid inclusions of the same samples. Materials presented in this publication were obtained by studying samples dredged from the MAR crest zone at 5°-6°N (U/Pb zircon dating, geochemical and petrological-mineralogical studies). It was demonstrated that variations in the isotopic composition of He entrapped in rocks and minerals were controlled by variable degrees of mixing of juvenile He, which is typical of basaltic glass for MAR (DM source), and atmospheric He. Increase in the atmospheric He fraction in plutonic rocks and, to a lesser degree, in their minerals reflects involvement of seawater or hydrated material of the oceanic crust in magmatic and postmagmatic processes. This conclusion finds further support in positive correlation between the fraction of mantle He (R ratio) and 87Sr/86Sr ratio. High-temperature hydration of ultrabasic rocks (amphibolization) was associated with increase in the fraction of mantle He, while their low-temperature hydration (serpentinization) was accompanied by drastic decrease in this fraction and significant increase in 87Sr/86Sr ratio. Insignificant variations in 143Nd/144Nd (close to 0.5130) and 87Sr/86Sr (0.7035) in most of gabbroids and plagiogranites as well as the fraction of mantle He in these rocks, amphibolites, and their ore minerals indicate that the melts were derived from the depleted mantle. Similar e-Nd values of gabbroids, plagiogranites, and fresh harzburgites (6.77-8.39) suggest that these rocks were genetically related to a single mantle source. e-Nd value of serpentinized lherzolites (2.62) likely reflects relations of these relatively weakly depleted mantle residues to another source. Aforementioned characteristics of the rocks generally reflect various degrees of mixing of depleted mantle components with crustal components (seawater) during metamorphic and hydrothermal processes that accompanied formation of the oceanic crust.

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The basement cored at Site 1201 (west Philippine Basin) during Ocean Drilling Program Leg 195 consists of a 91-m-thick sequence of basalts, mostly pillow lavas and perhaps one sheet lava flow, with a few intercalations of hyaloclastite and interpillow sedimentary material. Hydrothermal alteration pervasively affected the basalt sequence, giving rise to a variety of secondary minerals such as K-Fe-Mg-clay minerals, oxyhydroxides and clay minerals mixtures, natrolite group zeolites, analcite, alkali feldspar, and carbonate. The primary minerals of pillow and sheet basalts that survived the intense hydrothermal alteration were investigated by electron microprobe with the aim of characterizing their chemical composition and variability. The primary minerals are mostly plagioclase, ranging in composition from bytownite through labradorite to andesine, chromian-magnesian-diopside, and spinels, both Ti magnetite (partially maghemitized) and chromian spinel. Overall, the chemical features of the primary minerals of Site 1201 basalts correspond to the primitive character of the bulk rocks, suggesting that the parent magma of these basalts was a mafic tholeiitic magma that most likely only suffered limited fractional crystallization and crystallized at high temperatures (slightly below 1200°C) and under increasing fO2 conditions. The major element composition of clinopyroxene suggests a backarc affinity of the mantle source of Site 1201 basement.

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The interaction of seawater with basalts in DSDP Hole 501 and the upper part of Hole 504B (Costa Rica Rift) produced oxidative alteration and a zonation of clay minerals along cracks. From rock edges to interiors in many cracks the following succession occurs, based on microscopic observations and microprobe analysis: iron hydroxides (red), "protoceladonite" (green), iddingsite (orange), and saponite (yellow). Clay minerals replace olivines and fill vesicles and cracks. Other secondary minerals are phillipsite, aragonite, and unidentified carbonates. Some glass is transformed to Mg-rich palagonite. Bulk rock chemistry is related to the composition of the secondary minerals. The zonation can be interpreted as a succession of postburial nonoxidative and oxidative diagenesis similar to that described in the Leg 34 basalts.

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Two basement holes were drilled during Ocean Drilling Program (ODP) Leg 206. Hole 1256C penetrates 88.5 m into basement and Hole 1256D, ~30 m to the south, penetrates 502 m into basement (Wilson, Teagle, Acton, et al., 2003, doi:10.2973/odp.proc.ir.206.2003). Recovered cores consist of basalts exhibiting the effects of low-temperature alteration by seawater. As part of a larger study of alteration effects, a study of the secondary mineralogy was undertaken. This data report presents the major and some minor element compositions of secondary minerals. Analyses focus on the major secondary phases, phyllosilicates, and less abundant feldspars, but also include limited analyses of carbonates and apatite. Different occurrences of secondary minerals are included (e.g., veins and vesicles replacing olivine and plagioclase) as well as variations with depth.

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The northwestern Cascadia Basin of western North America accumulated high-sedimentation-rate sequences during the Pleistocene sea-level low-stands. The continental shelf was largely exposed at that time, and rivers and estuaries delivered large sediment fluxes directly to the deep ocean. The IODP EXP1301 core, which was taken from the middle portion of the Cascadia Basin, is well preserved and exhibits the deeper and - more distal sedimentary facies. The lithology in this location is composed of two units, 1) hemipelagic mud with a thin sand layer and 2) thick, coarsening upward silt-sand turbidites with a small proportion of granules at the top. We will focus on the detailed sand-grain proportions in order to understand the origin of these sediments. We determined the modal proportions of the heavy minerals, and the chemical composition of olivine and orthopyroxene in fourteen samples. These are characterized by an abundance of amphibole, pyroxenes and epidote, and the presence of minerals derived from peridotite. There is no drastic change in the modal and mineral compositions of the sands and silts between the turbidite and hemipelagic sequences. There were two probable drainage systems on the continent, the Frazer and Columbia rivers, which shed turbidites into the Cascadia Basin after 1.6 Ma, especially at 0.46-0.76 Ma. Based on a comparison of the modal and mineral compositions, the Northern Cascadia Basin has been supplied with sediments, mainly from the Frazer River, through the Straits of Juan de Fuca, by Pleistocene to Holocene turbidites.

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A variety of secondary minerals, formed in response to different oxidation and hydration states, are found in vugs and on fracture surfaces of the basalt cores from DSDP Leg 54. The minerals are smectite (blue to grey), high-magnesium calcite, manganoan calcite, aragonite, iron oxides, phillipsite, todorokite, marcasite, and hydrobiotite. The relationship of the mineral assemblages to four depositional modes of the basalts are delineated. A definite sequence and genetic link exists between mineral type and host rock which is dependent upon the origin and subsequent cooling history of the basalt.