Download or read book Petrological and Geochemical Studies of an Abyssal Periodotite from the Atlantis II Fracture Zone written by Kwok-Lin Lee and published by . This book was released on 1997 with total page 120 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis investigates the petrology and geochemistry of an abyssal peridotite dredged from the Atlantis II Fracture Zone in the southwestern Indian Ocean Ridge. Texturally, this sample is a serpentinized peridotite with a crosscutting coarse-grained clinopyroxenite vein. One of the alteration veinlets contains rutile and ilmenite in association with plagioclase and amphibole. This veinlet is not related to the pyroxenite vein. In tenns of mineralogy, the composition of the major silicate minerals indicates that this plagioclase llierzolite represents the depleted residue after mantle melting, similar to other abyssal peridotites from this region. In addition to the presence of the unique pyroxenite vein, this sample was earlier shown to be a carrier of 'orphan Sr-87'. Unfortunately, I was unable to find such high Sr isotopic ratios in the magnetic separations of different fractions of the sample. The sulfide mineralogy, together with the whole rock chemistry, suggests that sea water alteration occurs mainly as a result of serpentinization at temperatures higher than 200°C. Since the sample is less than l Ma old, and the low temperature weathering occurred only after the sample was exposed at the sea floor, it is possible that the weathering process was restricted to major alteration veins. This suggests that the alteration process is highly fracture controlled and time dependent Trace element data from clinopyroxene grains in the peridotite shows large variations from grain to grain. The (Ce/Yb)n ranges from 0.17 to 0.54 in the pyroxenite vein, and from 0.75 to 2.35 away from the vein. The tendency for LREE enrichment with the increase of distance from the vein suggests the presence of highly reacted melts. An assimilation-fractional crystallization (AFC) model was derived which supports the idea that the source of the clinopyroxenite vein reacts with the depleted peridotite to form a central reaction zone. Some of the highly reacted melt, after melt-rock reaction, migrates out of the reaction zone, and precipitates some late magmatic phases while being trapped in the country rocks. Since the sulfide is a major Os reservoir in the abyssal peridotites, as shown in leaching experiments, and the melt is saturated in sulfur as a consequence of the reaction process, it is possible to model the heterogeneous distribution of the Os isotopic data by mixing the residual peridotite with 0.2 to 0.5 wt% of sulfides precipitated from the melt. This mixing process can explain most of the heterogeneity from 1.034 to 1.148 for 187Os/186Os. The impact on the peridotite from the melt-rock reaction and impregnation of the late melt is obvious. As evident in Hess Deep gabbroic rocks, conductive heat loss in the transform fault fulfils the physical requirement to create and to preserve such geochemical signature.
Download or read book Trace Element Geochemistry of Oceanic Peridotites and Silicate Melt Inclusions written by Kevin Todd Michael Johnson and published by . This book was released on 1990 with total page 259 pages. Available in PDF, EPUB and Kindle. Book excerpt: The mantle melting process is fundamental to basalt genesis and crustal accretion at mid-ocean ridges. It is believed that melts ascend more rapidly than the surrounding mantle, implying a process similar to fractional melting may be occurring, but geochemical evidence for this has been lacking. Furthermore, crustal accretion is thought to be episodic at slow spreading ridges, but sampling programs that can test this temporal variability are virtually nonexistent. This dissertation examines the trace element compositions of abyssal peridotites and discusses how they preserve details of the melting process that are not recognizable in mid-ocean ridge basalts. The results support fractional melting as the dominant melting process in the sub-ridge upper mantle. Evidence is also presented supporting non-steady state mantle melting at the Atlantis II Fracture Zone cutting the very slow spreading Southwest Indian Ridge. Trace element compositions of peridotite clinopyroxenes from fracture zones along the American-Antarctic and Southwest Indian Ridges vary as a function of proximity to hotspots. The results presented in Chapter 2 are consistent with higher degrees of melting and greater incompatible element depletion in the upper mantle near hotspots. All peridotites studied are consistent with being residues of fractional melting and inconsistent with batch melting. Some samples recovered near hotspots appear to have begun melting in the garnet stability field, deeper than samples recovered away from hotspots. Most samples show pronounced negative Zr and Ti anomalies, which increase with increasing incompatible element depletion (increased melting), on extended rare earth (spider) diagrams. The results of Chapter 2 indicated the importance of accurately knowing trace element partition coefficients between clinopyroxene and liquid. It was found that existing partitioning studies report either rare earth elements, Ti, or Zr, but not all elements together. Thus, there is ambiguity about relative partition coefficients for these elements. Accurate knowledge of partitioning is important in understanding the formation of negative Zr and Ti anomalies observed in peridotite clinopyroxenes as well as in constructing realistic melting models for peridotites. To that end, Chapter 3 reports the results of a clinopyroxene/basaltic liquid trace element partitioning study carried out on natural dredged basalts and experimental charges of mid-ocean ridge basalts. It was found that there are small negative anomalies in the partiton coefficients of Zr and Ti relative to adjacently plotted rare earth elements on spider diagrams. Fractional melting implies that small parcels of refractory (e.g., high Mg/[Mg+Fe]), incompatible element depleted melts must exist somewhere in the ascending body of melt. Since mixing, wall rock reaction, and fractional crystallization probably alter the compositions of silicate melts extensively on their way from source to surface, representatives of these refractory fractional melts will rarely be erupted as flows on the seafloor. However, some refractory silicate melt inclusions possess compositional characteristics akin to those expected in fractional melts, i.e. low incompatible element concentrations and fractionated trace element ratios. Chapter 4 is a study of refractory melt inclusions from a variety of tectonic settings. The inclusions were obtained from Dr. A.V. Sobolev of the Vernadsky Institute of Geochemistry, Soviet Academy of Sciences, Moscow. They are not ideally suited for studying mid-ocean ridge processes, as only a few of the inclusions are from this environment, but in general, the inclusions show more refractory, incompatible element depleted compositions than their host lavas. Furthermore, the suite of inclusions in different mineral phases contained in a single N-type mid-ocean ridge basalt show variable trace element characteristics indicating unrelated sources for some inclusions. The results of the study do not strongly endorse the fractional melting hypothesis, but some support is suggested by trace element depletions and fractionations warranting a more thorough study of a suite of inclusions. Finally, the along-ridge major and trace element variability in peridotites observed previously and in Chapter 2 is compared to the variability found in a single fracture zone. The high sampling density at the Atlantis IT Fracture Zone on the Southwest Indian Ridge, coupled with its great distance from a hotspot make it a good subject for a baseline study. It was found that the compositional variability observed in peridotites from the Atlantis II Fracture Zone covers nearly the whole range of compositions found along the AmericanAntarctic and Southwest Indian Ridges in Chapter 2. However, there are systematics to this wide range, suggesting different processes may control the depletions. On the eastern side of the transform, a compositional gradient is observed from the center of the eastern wall to the northern ridge-transform intersection. Peridotites on this side have become gradually more depleted in incompatible elements and modal clinopyroxene over at least the last 10-11 million years. Samples from the western side of the transform are, in general, more depleted than those from the eastern side and show some indication of a compositional gradient as well, although sampling is less dense. Basalts from the western side are clearly different in iron composition and degree of rare earth element fractionation. These differences are consistent with higher pressure, higher degrees of melting producing lavas on the western side. It is believed that the long wavelength chemical variations corresponding to hotspot proximity described in Chapter 2 result from regional thermal conditions in the upper mantle imposed, in large part, by the hotspots. On the other hand, the short wavelength variability on a fracture zone or spreading cell scale may result from episodic mantle upwelling and magma production due to non-steady state accretion at very slow spreading ridges.
Download or read book Subseafloor Biosphere Linked to Hydrothermal Systems written by Jun-ichiro Ishibashi and published by Springer. This book was released on 2015-01-10 with total page 651 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book is the comprehensive volume of the TAIGA (“a great river ” in Japanese) project. Supported by the Japanese government, the project examined the hypothesis that the subseafloor fluid advection system (subseafloor TAIGA) can be categorized into four types, TAIGAs of sulfur, hydrogen, carbon (methane), and iron, according to the most dominant reducing substance, and the chemolithoautotrophic bacteria/archaea that are inextricably associated with respective types of TAIGAs which are strongly affected by their geological background such as surrounding host rocks and tectonic settings. Sub-seafloor ecosystems are sustained by hydrothermal circulation or TAIGA that carry chemical energy to the chemosynthetic microbes living in an extreme environment. The results of the project have been summarized comprehensively in 50 chapters, and this book provides an overall introduction and relevant topics on the mid-ocean ridge system of the Indian Ocean and on the arc-backarc systems of the Southern Mariana Trough and Okinawa Trough.
Download or read book The Isotope Geochemistry of Abyssal Peridotites and Related Rocks written by Jonathan E. Snow and published by . This book was released on 1993 with total page 654 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation studies several aspects of the formation of the Earth's oceanic mantle and crust, using a variety of geologic techniques, principally major elements, radiogenic isotopes and trace elements, but including petrography, mineral chemistry, x-ray diffraction, seafloor geomorphology, and analysis of the tectonics of fracture zones. The first chapter is an introduction to the problems to be addressed in this work. The second chapter examines the composition of basalts erupted near the Atlantis II Fracture Zone on the Southwest Indian Ridge. Trends in major element compositions of those basalts can be related directly to the nearby presence of the fracture zone. The effects of mantle composition and crustal level lateral transport of magma in the rift system can be ruled out by the analysis of isotopes and the geomorphology of the fracture zone floor. This is the best demonstration to date of a transform fault effect on basalt compositions. In trying to quantify putative transform fault effects documented at other fracture zones, no systematic correlation of transform offset age with mantle temperature change can be found, suggesting that mantle composition and lateral transport phenomena play a larger than expected role in the evolution of those areas. The third chapter relates to oceanic mantle rocks as they are altered at or near the Earth's surface. The major elements which make up abyssal peridotites are extensively redistributed by the alteration they have undergone. Mg is shown to be extracted from the peridotites, and a variety of trace elements added. This elemental redistribution is taken as evidence for extensive Mg transport by circulating waters. Since the solubility of Mg-bearing minerals in hydrothermal solutions is quite limited, much lower temperatures and much higher water /rock ratios are required to explain the major element compositions of the peridotites than had previously been assumed. The behavior of the Nd, Sr and Os isotopic systems during seafloor alteration was also studied. The isotope systematics of these rocks strongly support the hypothesis of high water /rock ratios in the formation of serpentinized abyssal peridotites. Nonetheless, Nd and Sr reside in a phase which is resistant to alteration (clinopyroxene) and the concentration of Os is high relative to that of seawater, so that it too appears resistant to alteration. Primary mantle isotopic signatures may be obtained from abyssal peridotites by careful analysis, even of extremely weathered rocks. Radiogenic strontium in excess of what could be introduced by seawater contamination or in situ radiogenic growth in a reasonable period of time was also found. These observations confirm earlier work which had been discredited for many years. The only plausible mechanism for the formation of this "orphan" S7Sr is that it is introduced as part of a sedimentary component which infiltrates the rock during metamorphism and/ or weathering. The 87Sr may be contained by or sorbed onto extremely fine clay particulates, or colloidal suspensions, as opposed to the dissolved ionic Sr which is normally thought of as characterizing the Sr isotopic composition of seawater. The high water/rock ratios required by the bulk isotopic analysis, as well as the pervasive elemental redistribution arguing for extensive near-surface weathering at high water /rock ra.tios strongly support this hypothesis. Given pervasive percolation of water throughout the samples, sufficient radiogenic, sediment-derived strontium may be drawn deep into the crust in the course of its weathering to cause such high B7SrfB6Sr ratios. The fourth chapter deals exclusively with primary mantle isotopic information from abyssal peridotites. This is the first study which has attempted to relate the Os isotopic system in the oceanic mantle to other isotopic systems and to trace elements. It is possible, with some extreme assumptions, to model the range of Os isotopes in the oceanic mantle alone in a standard model of formation of the depleted mantle by extraction of the crust. The additional constraints provided by the study of Nd isotopes in depleted mantle rocks from the oceans show that partial melt extraction and the formation of a depleted reservoir alone are not sufficient to account for the range of both N d and Os isotopes in the Earth's mantle. Possible mechanisms for the decoupling of the Os and Nd isotopic systems include elemental fractionation via the porous flow of basalt through the mantle, mantle metasomatism, recycling of a subducted component in the mantle and core formation. The core extraction model is pursued in some detail. Such core extraction models can account for the distributions and isotopic compositions of compatible and incompatible trace elements in the Earth's mantle, but they are highly non-unique, and thus difficult to test
Download or read book The Mantle and Core written by R.W. Carlson and published by Elsevier. This book was released on 2005-11-22 with total page 609 pages. Available in PDF, EPUB and Kindle. Book excerpt: Though largely inaccessible, the geochemistry of Earth's mantle and core can be examined through a wide variety of approaches. Volume 2 focuses first on "remote" sensing using evidence from cosmochemical, seismic, petrologic and geochemical approaches. Mantle composition is then examined in detail through descriptions of mantle samples brought to Earth's surface through tectonic, volcanic, and volatile-outgassing processes. The volume concludes with examination of processes that modify the composition of the mantle and core including an early magma ocean, partial melting, element partitioning between minerals and melts, and physical mixing caused by plate subduction, mantle convection and mass exchange between mantle and core. Reprinted individual volume from the acclaimed Treatise on Geochemistry, (10 Volume Set, ISBN 0-08-043751-6, published in 2003) Comprehensive and authoritative scope and focus Reviews from renowned scientists across a range of subjects, providing both overviews and new data, supplemented by extensive bibliographies Extensive illustrations and examples from the field
Download or read book Ophiolites in Earth History written by Yildirim Dilek and published by Geological Society of London. This book was released on 2003 with total page 734 pages. Available in PDF, EPUB and Kindle. Book excerpt: The 32 papers in this volume examine the mode and nature of igneous, metamorphic, tectonic, sedimentological, and biological processes associated with the evolution of oceanic crust in different tectonic settings in Earth history as revealed in various ophiolites and ophiolite belts around the world, and the geodynamic significance of these ophiolites in the evolution of different orogenic systems. Divided into six thematic sections, the book presents a wealth of new data and syntheses from mainly Phanerozoic ophiolites around the world.
Download or read book Geoscience Canada written by and published by . This book was released on 2000 with total page 448 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Geology and Geochemistry of Abyssal Plains written by P. P. E. Weaver and published by Blackwell Science. This book was released on 1987 with total page 268 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Annual Report written by Woods Hole Oceanographic Institution and published by . This book was released on 1994 with total page 322 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Magmatism in the Ocean Basins written by Andy D. Saunders and published by . This book was released on 1989 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The papers in this volume derive from a meeting in Leicester in January 1987, and concentrate on magmatic and mantle processes. They include: Asthenospheric control on magmatism in the ocean basins; Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism; Pressure of origin of primary mid-ocean ridge basalts; Genesis of mid-ocean ridge basalts; Origin of hybrid lavas from Agua de Pau volcano, Sao Miguel, Azores; Geochemical features of intraplate oceanic plateau basalts; Characterization of the St Helena magma source; Evaluation of rare gas data in relation to oceanic magmas; and others.
Download or read book Mineralogical Abstracts written by and published by . This book was released on 2002 with total page 344 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Geochemistry International written by and published by . This book was released on 2003 with total page 562 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Mantle Xenoliths written by Peter H. Nixon and published by John Wiley & Sons. This book was released on 1987 with total page 892 pages. Available in PDF, EPUB and Kindle. Book excerpt: The eruption of deep-seated xenoliths in basaltic, alnoitic, kimberlitic, etc volcanoes provides the geologist with an important direct means of examining the fragments of the earth's mantle and lower crust.
Download or read book The Crust written by R.L. Rudnick and published by Elsevier. This book was released on 2005-12-19 with total page 708 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Treatise on Geochemistry is the first work providing a comprehensive, integrated summary of the present state of geochemistry. It deals with all the major subjects in the field, ranging from the chemistry of the solar system to environmental geochemistry. The Treatise on Geochemistry has drawn on the expertise of outstanding scientists throughout the world, creating the reference work in geochemistry for the next decade. Each volume consists of fifteen to twenty-five chapters written by recognized authorities in their fields, and chosen by the Volume Editors in consultation with the Executive Editors. Particular emphasis has been placed on integrating the subject matter of the individual chapters and volumes. Elsevier also offers the Treatise on Geochemistry in electronic format via the online platform ScienceDirect, the most comprehensive database of academic research on the Internet today, enhanced by a suite of sophisticated linking, searching and retrieval tools.
Download or read book Magmatic Processes and Plate Tectonics written by H. M. Prichard and published by American Association of Petroleum Geologists. This book was released on 1993 with total page 544 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Proceedings of the Ocean Drilling Program written by Ocean Drilling Program and published by . This book was released on 1996 with total page 572 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Ofioliti written by and published by . This book was released on 2001 with total page 556 pages. Available in PDF, EPUB and Kindle. Book excerpt:
