submarine metamorphism of gabbros from the mid cayman rise an oxygen isotopic study abstract oxygen isotopic compositions of minerals in 22 samples of submarine gabbros were determined the gabbros were collected using the submersible alvin from the 700 m vertical section of the rift valley wall of the mid cayman spreading center our study indicates that in the mid cayman rise seawater barely reached the bottom of the plutonic layer abundant seawater penetration water rock mass ratio 1 was limited to the upper part of the plutonic layer from the observed oxygen isotopic compositions of coexisting minerals and from the experimental and empirical determinations of equilibrium fractionation of oxygen isotopes for mineral water and mineral mineral pairs we show the following 1 pyroxene and olivine did not exchange oxygen with seawater 2 plagioclase is in isotopic disequilibrium with pyroxene 3 the rate of oxygen exchange in plagioclase was not slowed by the absence of cation exchange 4 plagioclase and amphibole have exchanged oxygen with seawater or isotopically modified seawater 18o 3 and 5 amphibole has exchanged or acquired during formation hydrogen from seawater at 380 c t 600 c the decrease in extent of isotopic exchange of plagioclase and the decrease in amphibole abundance with depth indicate that seawater flux decreased rapidly with depth water rock mass ratio falling from 1 7 to 0 2 over a 300 m interval

the sedimentary expression of oceanic anoxic event 1b in the north atlantic abstract the aptian albian oceanic anoxic event 1b contains the record of severalperturbations in the global carbon cycle and multiple black shale levels particularly in the western tethys the local lithological expression of anoceanic anoxic event depends on palaeogeographical and depositional settingas well as on regional palaeoclimate marine sediments at a particular locationmay therefore be more or less organic rich or not at all and they may consistof different lithologies in most studies however much of the lithologicalvariability associated with oceanic anoxic events is left unaccounted for and thus the exact processes leading to the enrichment of organic matter in thesemarine sediments and their subsequent preservation in the geological recordare unknown this study focuses on the local sedimentary processes behindthe deposition of organic rich sediments at deep sea drilling project site 545and ocean drilling program sites 1049 and 1276 in the north atlantic duringoceanic anoxic event 1b although speci cally dealing with the sedimentsdeposited during this particular event at these localities it is expected that thesame processes were responsible for determining the exact sedimentaryproducts at localities in similar settings as well as during other similar eventsin the mesozoic here it is shown that the deposition of organic richsediments during oceanic anoxic event 1b was a consequence of theenhanced productivity favoured by upwelling and by riverine nutrientinput or aeolian fertilization of the euphotic zone depending ongeographical location slope instability processes resulted in the transfer ofpart of these organic rich sediments from the shelf to deep sea depocentres asmud laden organic rich turbidites especially in the northern north atlantic the so called black shales are much more varied than their name implies the end product of sedimentation during an oceanic anoxic event at aparticular location is commonly the result of several equi nal processes actingon a local scale rather than the direct result of basinal or even globalmechanisms keywords aptian albian kilian event north atlantic oae 1b oceandrilling program oceanic anoxic event paquier event

petrology of the mafic rocks of the xigaze ophiolite tibet the xigaze ophiolite yarlung zangbo suture zone southern tibet china shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros by dolerites intrusive throughout the whole ophiolite sequence and by the injection of dolerites in already serpentinized peridotites the abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge all the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea water circulation for the spreading centre in which they have been formed in good agreement with a slow spreading ridge origin

response of laterally loaded rectangular and circular piles in soils with properties varying with depth continuum based analyses for laterally loaded piles with rectangular and circular cross sections are presented using solutions that can be obtained quickly without requiring any elaborate inputs for the geometry and numerical mesh the analysis is developed by solving the differential equations governing the displacements of the pile soil system derived using the variational principles of mechanics parametric studies are performed to investigate the influence of the pile cross sectional shape soil layering pile slenderness ratio and pile soil modulus ratio on the response of laterally loaded piles in heterogeneous soil in which the soil shear modulus varies continuously or discretely with depth the results show that piles with the same second moment of inertia have similar lateral load response the lateral responses of piles in two layer systems were mainly affected by the thickness and stiffness of the top soil layer soil layering also influences the lateral response of piles in three layer soil deposits consisting of two thin layers overlying the third layer algebraic equations for estimating the pile head deflection and maximum bending moment are proposed that can be readily used in design a user friendly spreadsheet program is developed as a tool to perform calculations of pile response using the analysis numerical examples demonstrating the use of the analysis are provided

the limiting pressure on a circular pile loaded laterally in cohesive soil in the analysis of the undrained loading of laterally loaded piles an important quantity is the ultimate lateral resistance at depth to purely horizontal movement if the soil is modelled as a perfectly plastic cohesive material then the calculation of this quantity reduces to a plane strain problem in plasticity theory in which the load is calculated on a long cylinder which moves laterally through an infinite medium an exact calculation of the load on such a cylinder is presented if this load is non dimensionalized with respect to the soil strength and the diameter of the pile it is found that the load factor varies between for a perfectly smooth pile and for a perfectly rough pile this result is discussed in the context of previous calculations for the lateral load capacity of piles and is compared with approximate calculations using cavity expansion theory and a wedge failure near the soil surface la resistance laterale limite en profondeur au seul mouvement horizontal represente un parametre im

a stable isotope study of serpentinization and metamorphism in the highland border suite scotland u k abstract d h and 18 o 16 o ratios have been measured for whole rock samples and mineral separates from the mafic and ultramatic rocks of the cambro ordovician highland border suite the h and o isotopic compositions of these rocks record individual stages in a relatively complex 500 myr old hydrothermal metamorphic history lizardite serpentinites d 105 18 o 6 2 record a premetamorphic history and indicate that parent harzburgites dunites and pyroxenites were serpentinized through low temperature interaction with meteoric waters during cooling the other rocks of the highland border suite record subsequent interaction with metamorphic fluids amphibolite facies hornblende schists were produced through thrust related dynamothermal metamorphism of spilitic pillow lavas during dehydration d enriched fluids were driven off from the spilites thus leaving the hornblende schists to equilibrate with a relatively d depleted internal fluid reservoir d 45 the expelled d enriched fluids may have mixed with more typical dalradian metamorphic waters which then exchanged with the remaining mafic rocks and lizardite serpentinites during greenschist facies regional metamorphism to produce antigorite serpentinites d 62 18 o 8 and greenschist metaspilites d 57 18 o 7 3 with similar h and o isotopic compositions serpentinites which have been only partially metamorphosed show intermediate h isotopic compositions between that of metamorphic antigorite d 62 and non metamorphic lizardite d 105 end members

the xigaze ophiolite tibet a peculiar oceanic lithosphere the xigaze ophiolite which outcrops along the yarlung zangbo river southern tibet locally displays a complete ophiolitic sequence from marine sedimentary cover over basaltic volcanics to the north to fresh cr diopside rich harzburgites to the south in contrast with other ophiolites the mafic part of the sequence is particularly thin it is almost devoid of cumulate gabbros consisting of a diabase sill complex covered by lava flows or pillow lavas the ultramafic unit is poor in residual harzburgites and dunites and consists dominantly of fresh cr diopside rich harzburgites the origin of this peculiar ophiolite of oceanic crust is discussed

mineral composition of sedimentary formations in the north atlantic ocean sedimentary formations established in the western north atlantic basin byjansa et al 1979 and their coeval counterparts from the eastern north atlantic were studied for their mineral composition using x ray powder diffractometry more than thousand samples from different legs of the deep sea drilling project were analysed semiquantitatively the results are presented as diagrams of about thirty five different mineral phases versus depth stratigraphy in the single boreholes sites 105 106 386 387 391 417 and 418 were studied from the western north atlantic and sites 367 369 370 415 and 416 from the eastern north atlantic off w africa specific mineral spectra were found from most of the formations which can thus be additionally characterized the change of mineral composition along the formation boundaries is in most cases sharp but along some of the boundaries it is transitional local variations of mineral spectra within the same formation can mainly be referred to distance from source areas the sedimentary section studied from the western north atlantic basin ranges in age from oxfordian to quaternary it consists of argillaceous limestones cat gap formation limestones blake bahama formation claystones and shales hatteras formation zeolitic claystones plantagenet formation nanno marls crescent peaks member siliceous oozes clays and cherts bermuda rise formation and hemipelagic muds blake ridge formation with locally redeposited shallow water carbonates great abaco member a corresponding section in the eastern north atlantic is only developed at site 367 where similar mineral assemblages could be found at sites 369 370 415 and 416 the sediment sequence is dominated by turbiditic influx of terrigenous components alternating with pelagic sediments this can clearly be depicted from the mineral suites but it is not possible to establish sedimentary formations which can be paralleled to those within the western north atlantic seismically mappable reflectors which were calibrated at the sediment sequence of the drillsites are in most cases related to lithological changes and mineral composition these acoustic horizons are very often coincident with formation boundaries besides different mineral composition changes in grain size and diagenesis but also erosional unconformities are responsible for the impedance contrasts leading to the reflectors as already pointed out bytucholke 1979

a comparison of gcm simulated cretaceous greenhouse and icehouse climates implications for the sedimentary record abstract two general circulation model simulations of mid cretaceous climate are presented one representing a greenhouse and the other representing an icehouse world through a comparison of greenhouse and icehouse palaeoclimates differences in processes which are intrinsically linked with climate such as continental weathering and snow and ice accumulation may be observed a number of conspicuous and discrete climatic differences can be seen between the two simulations which may be translated into a number of identifiable signatures in the sedimentary record the climate at mid high latitudes in the icehouse simulation was more arid than the greenhouse simulation and could result in deposition of smectite dominated clay suites in contrast the climate of low latitudes of the icehouse simulation resulted in a moister climate than the greenhouse simulation which could affect the weathering regimes and result in deposition of kaolin dominated clay suites thus cool climate episodes do not necessarily result in a global increase in aridity the icehouse simulation predicts a small permanent icecap at high latitudes which could account for the origin of dropstones in marine sediments furthermore a sea level fall as a consequence of an accumulation of ice at high latitudes may also increase continentality and surface albedo which may also contribute to the causes of the drier climate at high latitudes hurricanes are likely to be more frequent in greenhouse than icehouse climates mid latitude depressions also important for generating storms are likely to be both intense and frequent during icehouse and greenhouse periods of the middle cretaceous

inferred oxygen isotope profile of archaean oceanic crust onverwacht group south africa whole rock oxygen isotope data from a suite of mafic and ultramafic samples from the onverwacht group the basal unit of the archaean greenstone succession of the barberton mountain land south africa show a range of values from 18o 3 to 18o 14 a range which coincides with those of phanerozoic ophiolites and oceanic crust when the samples are arranged in an inferred ophiolitic pseudostratigraphy from basal serpentinized ultramafic cumulates through altered mafic extrusives they have an oxygen isotope distribution profile which is indistinguishable from that of phanerozoic ophiolites the distinctive isotopic profiles and secondary mineral assemblages in phanerozoic ophiolites are caused by hydrothermal interaction between seawater 18o 0 and oceanic crust 18o 5 8 the existence of a similar isotopic and metamorphic profile in the onverwacht group argues strongly that these rocks were hydrothermally altered by an archaean ocean whose isotopic composition was indistinguishable from modern sea water

oxygen isotope evidence for submarine hydrothermal alteration of the del puerto ophiolite california abstract the oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the del puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the great valley sequence indicate that their alteration occurred in a submarine hydrothermal system whole rock 18o compositions decrease progressively down section with increasing metamorphic grade 22 4 smow to 13 8 for zeolite bearing volcaniclastic sedimentary rocks overlying the ophiolite 19 6 to 11 6 for pumpellyite bearing metavolcanic rocks in the upper part of the ophiolite s volcanic member 12 3 to 8 1 for epidote bearing metavolcanic rocks in the lower part of the volcanic member 8 5 to 5 7 for greenschist facies rocks from the ophiolite s plutonic member 7 6 to 5 8 for amphibolite facies or unmetamorphosed rocks from the plutonic member modelling of fluid rock interaction in the del puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock 18o can be best explained by isotopic exchange with discharging18o shifted seawater at fluid rock mass ratios near 2 and temperatures below 500 c 18o depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism submarine weathering and later burial metamorphism at the base of the great valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions the pervasive zeolite and prehnite pumpellyite facies mineral assemblages found in the del puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center

mineralogy of the upper albian sediments from the kirchrode i borehole with special emphasis on the clay mineralogy abstract the mineralogy of a 245 m thick section of upper albian marly clay stones to clayey marlstones from the kirchrode i borehole lower saxony basin of nw germany has been studied by x ray diffractometry infrared spectrometry chemical analysis and microscopy the investigations were carried out on bulk samples and grain size fractions the mineralogy of the sediments is characterised by biogenous carbonates mainly calcite by terrigenous detrital components and diagenetic minerals often enriched in concretions the calcite content fluctuates considerably between 20 and 50 most probably because of changes in the marine palaeoenvironment illite montmorillonite and kaolinite are the predominant clay minerals but in changing portions chlorite and alkali feldspars were detected only in traces mixed layer minerals such as illite smectites i s are also present in higher amounts in the lower part and less in the upper part of the core reworked upper carboniferous mudstones and siltstones are interpreted to have delivered part of the well crystallised illite kaolinite chlorite and quartz montmorillonites and mixed layer minerals together with kaolinite and illite may be inherited from weakly consolidated jurassic sediments aptian and albian claystones and marlstones and from palaeosols the increase in the proportion of montmorillonite among the clay minerals in the upper part of the upper albian may be due to redistribution of volcanic sediments after either terrestrial or submarine alteration of volcanic ash volcanic activity is known in this area e g the middle to upper albian flammenmergel at the southern margin of the lower saxony basin and around the north atlantic throughout aptian albian time but in the kirchrode i core ash fall was identified as a source for montmorillonite in only one layer namely at 87 25 m where high zr and nb concentrations were also found the mineralogical composition of the concretionary material is generally a mixture of biogenous calcite siliciclastic debris quartz and clay minerals and microcrystalline diagenetically formed fe mn carbonates rhodochrosite siderite and carbonate fluorapatite francolite rhodochrosite is more abundant in the lower part mn siderite in the upper part of the core concretions with fluorapatite francolite occur sporadically

oxygen and carbon isotope evidence for seawater hydrothermal alteration of the macquarie island ophiolite abstract rocks of the miocene macquarie island ophiolite south of new zealand have oxygen and carbon isotopic compositions comparable to those of seafloor rocks basalt glass and weathered basalts have 18o values at 5 8 6 0 and 7 9 9 5 respectively similar to drilled seafloor rocks including samples from the leg 29 dsdp holes near macquarie island compared to the basalt glass the greenschist to amphibolite facies metaintrusives are depleted in18o 18o 3 2 5 9 similar to dredged seafloor samples whereas the metabasalts are enriched 18o 7 1 9 7 although the gabbros are only slightly altered in thin section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4 5 km there is an approximate balance between18o depletion and enrichment in the exposed ophiolite section the carbon isotopic composition of calcite in the weathered basalts 13c 1 0 2 0 is similar to those of drilled basalts but the metamorphosed rocks have low 13c values 14 6 to 0 9 these data are compatible with two seawater circulation regimes in the upper regime basalts were weathered by cold seawater in a circulation system with high water rock ratios 1 0 based on calcite compositions weathering temperatures were less than 20 c and the carbon was derived from a predominantly inorganic marine source as previously suggested for the samail ophiolite it is postulated that the lower hydrothermal regime consisted of two coupled parts at the deeper levels seawater circulating at low water rock ratios 0 2 0 3 and high temperatures 300 600 c gave rise to18o depleted gabbro and sheeted dikes via open system exchange reactions during reaction the seawater underwent a shift in oxygen isotopic composition 18o 1 0 5 0 and subsequently caused18o enrichment of the overlying metabasalts in the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water rock ratios 1 0 10 0 and temperatures in the range 200 300 c the relatively low water rock ratios in the hydrothermal regime are supported by the low 13c values of calcite interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts

monotonic laterally loaded pile testing in a stiff glacial clay till at cowden this paper describes the results obtained from a field testing campaign on laterally loaded monopiles conducted at cowden uk where the soil consists principally of a heavily overconsolidated glac

an integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems chemical and isotopic changes accompanying seawater basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs incorporating provision for addition and subtraction of a wide range of reactant and product minerals as well as cation and oxygen and hydrogen isotopic exchange equilibria the models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100 to 350 c with an overall water rock ratio of about 0 5 being constrained by an assumed 18 o h 2o at 350 c of 2 0 per mil h craig personal communication 1984 this is a realistic model because 1 the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust 2 the water rock ratio is high 50 at low temperatures and low 0 5 at high temperatures and 3 it allows for back reaction of earlier formed minerals during the course of reaction progress the results closely match the major element chemistry von damm et al 1985 and isotopic compositions craig et al 1980 of the hydrothermal solutions presently emanating from vents at 21 n on the east pacific rise the calculated solution chemistry for example correctly predicts complete loss of mg and so 4 and substantial increases in si and fe however discrepancies exist in the predicted ph 5 5 versus 3 5 measured and state of saturation of the solution with respect to greenschist facies minerals the calculated d h 2o is 2 6 per mil in excellent agreement with analytical determinations the calculated chemical mineralogic and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges humphris and thompson 1978 stakes and o neil 1982 as well as with data from ophiolites gregory and taylor 1981 predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite epidote chlorite tremolite greenschist assemblage at 350 c the models demand that the major portion of the water rock interaction occur at temperatures of 300 350 c interaction at temperatures below approximately 250 c results in negative 18 o h 2o shifts contrary to the observed positive 18 o values of the fluids exiting at midocean ridge vents hydrogen isotope fractionation curves by suzuoki and epstein 1976 lambert and epstein 1980 and liu and epstein 1984 among others are compatible with the model and require d h 2o to increase at all temperatures as a result of seawater basalt interaction

interpretation of t bar penetrometer tests at shallow embedment and in very soft soils the cylindrical t bar penetrometer was developed for profiling the undrained strength of soft soils in the centrifuge and is now a widely used offshore site investigation tool the conventional interpretation of the t bar test is to convert the measured penetration resistance to soil strength using a single bearing factor associated with steady flow of soil around the bar this paper describes a new analysis for the interpretation of t bar penetrometer tests at shallow embedment and in soft soils which is an increasingly significant consideration in the design of seabed infrastructure including pipelines the analysis captures two mechanisms that are usually neglected i soil buoyancy and ii the reduced bearing factor arising from the shallow failure mechanism mobilized prior to the full flow of soil around the bar the framework derives from theoretical considerations and is calibrated using large deformation finite element analyses the depth at which the steady deep penetration condition is reach

cenozoic evolution of continental humidity and paleoenvironment deduced from the kaolinite content of oceanic sediments abstract the distribution of detrital kaolinite in cenozoic sediments of the atlantic and west pacific oceans has been used to reconstruct the variations of the continental humidity and associated paleoenvironmental factors five stages have been recognized 1 1 during the late oligocene chattian c 30 m y p21 and np24 increased kaolinite content at every site marked an increase of global humidity 2 2 during the early miocene aquitanian c 20 m y n5 n6 and nn2 nn3 increasing continental aridity is deduced from low kaolinite contents in the atlantic ocean and philippine sea 3 3 during the middle miocene langhian c 14 m y n10 n12 and nn5 nn6 increased kaolinite contents at most of the sites marked a further increase of global humidity which was especially important at low latitudes in africa 4 4 during the late miocene tortonian c 10 m y n16 and nn10 nn11 kaolinite content and inferred continental humidity increased toward high latitudes but decreased toward low latitudes especially off tropical africa where it marked the development of arid conditions 5 5 during the late pliocene piacenzian c 3 m y nn15 nn16 kaolinite content and humidity increased again toward high latitudes while these conditions decreased again toward low latitudes emphasizing the latitudinal climatic zonation periods of increasing humidity show good correlation with the development of polar ice locally volcanism tectonic activity oceanic circulation and migration of lithospheric obliterated the climatic significance of clay mineral associations which therefore express complementary aspects of the continental and marine paleoenvironments

the concentration of co_ 2 in submarine tholeiitic basalts the concentrations of co 2 and h 2 o in glass separates from submarine tholeiitic basalts have been determined the presence of vesicles in samples precludes accurate determination of the co 2 concentration in glass unless steps are taken to analyze only uncontaminated vesicle free glass special care has been exercised during sample preparation to avoid these problems a new analytical technique for micro determination of h 2 o and co 2 has been applied to five mid ocean ridge and two kilauea submarine east rift zone basalts quenched at pressures as great as 0 5 kb the uncertainty of quenching pressure due to flow of lava subsequent to surface quenching global sea level variations and local deformation of the sea floor are discussed non tectonic uncertainties of quenching pressure have been estimated the concentration of co 2 in glass separates from submarine tholeiitic basalts increases linearly with quenching pressure the apparent solubility of co 2 in tholeiitic basalts

lead isotopic study of the xigaze ophiolite tibet the problem of the relationship between magmatites gabbros dolerites lavas and tectonites harzburgites abstract the pb isotopic study of the xigaze ophiolite tibet shows that the tectonic harzburgites and magmatic gabbros dolerites lavas units bear different isotopic signatures the harzburgitic rocks show much higher207pb 204pb values than the samples from the magmatic unit analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes u pb analysis allowed determination of the crystallization age of the magmatites at 120 10 m y this age is compatible with paleontological age determinations and thus demonstrates that u pb behaved more or less as a closed system since formation of the ophiolite if an exchange with seawater has occurred it must necessarily have happened shortly after crystallization of the xigaze oceanic crust dolerite dykes crossing the harzburgites show similar pb isotopic composition as the magmatites indicating that they were emplaced when the ultramafic rocks were already brittle however the time difference of the emplacement was probably not larger than 280 m y this makes it evident that two sources with different isotopic signatures one similar to normal morb the other with much higher207pb 204pb ratios were active in the immediate neighborhood one possible environment which can account for this association is a propagating ridge

multistage hydrothermal alteration of gabbroic rocks from the failed mathematician ridge oxygen isotopic mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed mathematician ridge the mathematician ridge though initially a fast spreading ridge presently has a deep central rift valley more typical of slow spreading rifts the inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large offset fracture zones undeformed mathematician ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post crystallization hydrothermal alteration metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole low calcium plagioclase and ilmenite oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700 c water rock ratios of less than one and produced modified seawater fluids with isotopic compositions of 3 5 6 0 the amphibolites are cross cut by multiple generation quartz epidote veins forming a network replacement of the amphibole grade mineralogies by greenschist grade mineralogies fluid oxygen isotopic compositions of 2 4 and a two phase separation in the fluid captured in conjugate vapor fluid inclusions are all associated with the formation of this network fluid inclusion data for the quartz indicate initial temperatures of over 500 c these samples may represent rocks from the 1 2 km deep plumbing system of black smoker type venting

structure and petrology of peridotites clues to their geodynamic environment the first part of this paper is a review on the main mineralogical petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth s surface these criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history the unifying concept is that of adiabatic ascent rate related to rifting and spreading rates rates lower than 0 5 cm yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km this inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue higher rates probably not exceeding 1 cm yr correspond to oceanic rifts with the main melt extraction completed around 15 km generating a thin oceanic crust and plagioclase lherzolites as residue finally rates greater than 1 2 cm yr correspond to oceanic ridges with melt being extracted at moho depth thus generating a 6 km thick crust and leaving depleted harzburgites as residue thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin this conclusion must however be tempered by the fact that the spreading rate envisioned here as the principal controlling parameter can change with space and time in a given environment mantle diapirism rifting seafloor spreading crust generation

comparative evolution of the senegal and eastern central atlantic basins from mineralogical and geochemical investigations cretaceous to palaeogene sedimentary series in two long continously recovered boreholes from the senegal coastal basin and adjacent cape verde deep sea basin have been compared by mineralogical geochemical electron microscope and microprobe investigations the transition between the diagenetic influences related to the depth of burial and the palaeoenvironmental influences identified in the thick coastal basin successions is complicated by the tectonic instability of the margin during periods of high subsidence rates and crustal thinning as the deep sea basin sediments are only slightly affected by diagenetic changes the comparison between the deep sea and coastal series allows the relative effects of lithostatic pressure tectonics and other palaeoenvironmental factors to be evaluated the disappearance of synsedimentary signatures tectonics climate depositional conditions etc expressed by clay mineralogy occurs below 4 5 km there are no significant diagenetic changes in sediments at less than 2 km depth by comparing identical time slices in coastal and deep sea basins it is possible to recognize from the clay stratigraphic record the main african detrital sources the hot and often semi arid character of the continental climate during the cretaceous the existence of minor but extensive stages of tectonic activity barremian late aptian late albian early cenomanian the temporary existence of semi closed basins on the shelf late cretaceous early palaeogene and the complexity of transport and sedimentation relationships between proximal and distal environments

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

structure of the xigaze ophiolite yarlung zangbo suture zone southern tibet china genetic implications three massifs in the xigaze area southern tibet china have been preserved from the intense deformation observed everywhere else in the yarlung zangbo suture zone these massifs have similar lithologies and display a remarkable homogeneity in the attitude of most magmatic and tectonic structures such as lava flow planes dolerite dikes and sills and foliation planes in peridotites this indicates that these ophiolites are only slightly dismembered inasmuch as they locally display complete sequences from pillow lavas at the top to fresh cr diopside bearing harzburgites at the base gabbroic rocks are rare and are present as screens between dolerite intrusives or in small sized pockets at the limit between mafic and ultramafic rocks one of the most important feature of this ophiolite is the occurrence of dolerite sills and dikes throughout the whole sequence indicating that this ophiolite results from to two successive magmatic events with 1 creation of the crustal part of the oceanic lithosphere with deposition of minor cumulates in small magma chambers over mantle residual rocks and 2 intrusion of dolerite sills and dikes throughout the whole sequence

effects of pile shape and pile end condition on the lateral response of displacement piles in soft clay a series of lateral tests conducted in a centrifuge on displacement piles in kaolin is used to examine the effects of pile shape pile end condition and clay overconsolidation on the lateral load t

mesozoic carbonate platforms and banks of the eastern north american margin abstract the jurassic lower cretaceous carbonate platforms and banks form a discontinuous belt extending over distance of 6000 km from the grand banks up to the bahamas six types of carbonate buildups are recognized and document the variability of depositional paleo oceanographic and tectonic processes on the eastern north american margin the composition of the carbonates closely resembles the recent deposits of the western great bahama bank since oolitic shoals were present near to the shelf edge and skeletal peloid wackestones and biomicrites were deposited in the inner part of the platform coralstromatoporoid and sponge bioherms were only rare constituents of the carbonate banks the thickness of carbonate buildups progressively increases southward along the margin attaining a thickness of more than 5 km on the bahamas the carbonate platforms also become younger southwards which is thought to reflect the northward movement of the north american plate of less than 1 5 cm yr the carbonate platforms were seeded over the continental basement following the taphrogenic period of plate tectonics building of carbonate ramps which characterized the early jurassic has began during the transitional period between continental rifting and early drift the second stage in construction of the carbonate platforms and offshore banks proceeded mainly after separation of the continental plates

mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks mineralogical and isotopic variations observed in altered glassy and crystalline rocksfrom the east pacific rise and the mid atlantic ridge provide information about the temperatures of alteration and seawater rock ratios for various hydrothermal regimes within the oceanic crust a systematic increase in alteration temperature is evident for the glassy rocksin the sequence 1 nontronite and celadonite vesicle fillings 35 c 2 saponite rich pillow breccias 130 170 c 3 calcite rich greenstone breccias and epidote rich greenstone 200 350 c theseresults include the highest temperatures thus far reported for saponite formation the seawater dominated hydrothermal alteration process that formed the saponite rich pillow breccias is characterized by high water rock ratios 0 1 low to moderate temperatures a seawater origin of most of the carbon in vein calcites 13 c 0 and the predominance of fe rich saponite and calcite as secondary phases greenstones chlorite quartz epidote and greenstone breccias chlorite quartz albite calcite are altered in a rock dominated system with lower water rock ratios 50 1 to 13 c 4 the crystalline rocks diabase gabbro and metagabbro are affected to varying degrees by pervasive high temperature seawater interactions that commence soon after solidification producing varying proportions of fine grained secondary minerals including talc smectite chlorite vermiculite actinolite and sodicplagioclase hydrothermal solutions derived from alteration of the crystalline rocks are of the appropriate temperature and isotopic composition to alter the overlying glassy rocks to the observed mineralogies as well as being the source of metal rich deposits associated with the oceanic spreading centers

amphibolitized sheared gabbros from ophiolites as indicators of the evolution of the oceanic crust bay of islands newfoundland abstract in the bay of islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering the resulting shear bands are narrow and composed either of mylonitic amphibole bearing gabbros or of highly foliated amphibolites the amphibole composition and nature and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450 c this shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle a second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge occurred at lower temperatures ranging from 500 to 300 c the surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450 c all metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

hydrothermal vent waters at 13 n on the east pacific rise isotopic composition and gas concentration abstract gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13 n on the east pacific rise in the most mg depleted samples 5 10 3 moles kg the gas concentrations are 3 4 5 10 5 cm 3 stp kg helium 0 62 1 24 cm 3 stp kg ch 4 10 80 16 71 10 3 moles kg co 2 the samples contain large quantities 95 126 cm 3 kg of h 2 and some carbon monoxide 0 26 0 36 cm 3 kg which result from reaction with the titanium sampling bottles 13 c in methane and co 2 16 6 to 19 5 and 4 1 to 5 5 respectively indicate temperatures between 475 and 550 c whereas 13 c co is compatible with formation by reduction of co 2 on ti at 350 c close to the sampling temperature 3 he 4 he are very homogeneous at 7 5 0 1 r a 3 he 4 he 1 0 10 5 and very similar to already published data as well as ch 4 3 he ratios between 1 4 and 2 1 10 6 18 o and d in water show enrichments from 0 39 to 0 69 and from 0 62 to 1 49 respectively these values correspond to w r ratios of 0 4 7 the distinct 18 o enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations the 3 he enthalpy relationship is discussed in terms of both hydrothermal heat flux and 3 he mantle flux

strontium oxygen and hydrogen isotope geochemistry of hydrothermally altered and weathered rocks in dsdp hole 504b costa rica rift dsdp hole 504b was drilled into 6 ma crust about 200 km south of the costa rica rift galapagos spreading center penetrating 1 35 km into a section that can be divided into four zones zone i oxic submarine weathering zone ii anoxic alteration zones iii and iv hydrothermal alteration to greenschist facies in zone iii there is intense veining of pillow basalts zone iv consists of altered sheeted dikes isotopic geochemical signatures in relation to the alteration zones are recorded in hole 504b as follows zone depth m average87sr 86sr average 18o o average d o i 275 550 0 7032 7 3 63 ii 550 890 0 7029 6 5 45 iii 890 1050 0 7035 5 6 31 iv 1050 1350 0 7032 5 5 36 full size table table options view in workspace download as csv alteration temperatures are as low as 10 c in zones i and ii based on oxygen isotope fractionation strontium isotopic data indicate that a circulation of seawater is much more restricted in zone ii than in zone i fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 20 c in zones iii and iv which is consistent with secondary mineral assemblages the strontium oxygen and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in zones iii and iv are estimated to be 0 7037 2 and 3 respectively strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water rock ratios in weight of about 1 6 this value is close to that of the end member hydrothermal fluids issuing at 21 n epr basement rocks are not completely hydrothermally altered about 32 of the greenstones in zones iii and iv have escaped alteration thus 1 g of fresh basalt including the 32 unaltered portion are required in order to make 1 g of end member solution from fresh seawater in water rock reactions

petrology and texture of the ultramafic rocks of the xigaze ophiolite tibet constraints for mantle structure beneath slow spreading ridges abstract the xigaze ophiolite southern tibet exhibits an unusual ophiolite sequence characterized by 1 1 the limited thickness less than 3 km of its crustal unit which is mostly composed of dolerite intrusives the gabbros being very scarce 2 2 the scarcity of harzburgites and dunites the ultramafic rocks consisting mostly of cr diopside bearing harzburgites and iherzolites although there are some textural and chemical variations from the basal iherzolites to the upper harzburgite and dunites no sharp discontinuity exists between the peridotite series of the xigaze ophiolite which all belong to the same peridotite suite the peridotites have been equilibrated at relatively low temperatures and pressures 1155 c 0 5 0 6 gpa for the cr diopside bearing harzburgites and 1255 c 1 0 1 1 gpa for the stratigraphically 2 km deeper iherzolites indicative of a low thermal gradient at the xigaze paleo ridge accordingly the peridotites have locally been deformed under low stress 0 2 gpa conditions and at decreasing temperatures from up to 1000 c to 900 800 c as evidenced by the activation of the 010 100 ok1 100 and 001 100 olivine glide systems the equilibrium temperatures and pressures estimated for the peridotites the inferred shape of the melting zone beneath the xigaze paleo ridge the low temperature low stress conditions inferred for the deformation of the peridotites and the pattern of the mantle structure determined on the basis of structural and microstructural data are in good agreement with a slow spreading ridge origin for the xigaze ophiolite

origin of the xigaze ophiolite yarlung zangbo suture zone southern tibet abstract the xigaze ophiolite tibet displays unusual lithological petrological textural and structural characteristics there are no large masses of cumulate gabbros but dolerite intrusives throughout the whole ophiolite sequence some of which were intruded into already serpentinized peridotites only minor residual harzburgites and dunites in dominantly iherzolitic peridotites equilibrated at low temperatures and pressures and relatively low temperature deformation structures in the uppermost peridotites these features suggest a very low heat flow at the spreading center where the xigaze ophiolite was formed in good agreement with a discontinuous and slowly accreting spreading center origin however this ophiolite does not represent a typical mid oceanic ridge ophiolite rather it was formed in a small basin located at the southern margin of eurasia hence within a preexisting oceanic lithosphere the opening of the nearly n s xigaze paleo ridge resulted from the w e drift of africa relative to eurasia from 180 to 110 ma the n s emplacement onto the continent of the xigaze ophiolite formed 120 to 110 ma ago can be correlated to changes in direction of motion of the african and indian plates a primary intra oceanic thrusting event probably occurring at 110 or 85 ma and the final obduction near 50 ma during the india eurasia collision

structure and metamorphism of a tectonically thickened continental crust the yalu tsangpo suture zone tibet three tectonic units the active palaeomargin the palaeo oceanic domain and the passive palaeomargin are now juxtaposed and constitute the yalu tsangpo suture zone between india and eurasia in southern tibet we examine the deformation metamorphic and uplift histories of each of these units our observations are accounted for in terms of the processes whereby the crust was thickened to about 70 km we finally discuss the implications for ancient continental collision orogens

rate effects on penetrometer resistance in kaolin this paper describes the results from a centrifuge investigation into the rate dependence of t bar and ball penetrometer resistance in kaolin four t bar diameters and two ball diameters were installed in kaolin with overconsolidation ratios of 1 2 and 5 at penetration velocities varying over five orders of magnitude the penetration resistances are compared with the properties of kaolin as measured in element tests to assist in the development of a framework that describes this resistance over the full velocity range consistent trends emerge when the soil viscosity is assumed to affect the penetration resistance in both the partially drained and undrained conditions enabling the dependence of the t bar and ball resistances on diameter velocity and soil characteristics to be quantified

paleoenvironmental history of the north atlantic region from mineralogical and geochemical data abstract principles and applications of inorganic mineralogy and geochemistry in reconstructing past environments in marine basins are summarized by means of two examples from north atlantic deep sea drilling sites the first approach is chronological and permits by comparing the hatteras and cape verde basins since the oxfordian to identify the effects of continental and marine influences land climate and morphological evolution alternation of tectonically active and stable periods ocean deepening and widening stages and latitudinal migration of lithospheric plates the second approach more recently developed concerns the geographic distribution of paleoenvironments during albian time particularly the role of the bermuda area as a submarine barrier separating the western and eastern detrital input and the contrast between the stable american margin that favored pedologic processes on land and the unstable african margin characterized by strong erosion resedimentation and volcanic processes

hydrothermal activity in a peculiar oceanic ridge oxygen and hydrogen isotope evidence in the xigaze ophiolite tibet china the xigaze ophiolite locally displays a 7 km thick continuous sequence which probably represents an oceanic crust formed at a slow spreading ridge it consists from top to bottom of 1 1 pillow lavas with 18o between 9 and 16 and d around 90 vs smow 2 2 rare trondjhemites 9 7 18o 13 4 and d around 75 3 3 dolerite sills and dikes with 18o between 7 0 and 9 5 and d between 77 and 40 4 4 isotropic gabbros occurring as screens of the sills and dikes and rare layered gabbros these gabbros have 18o between 4 8 and 11 0 and d around 65 and bear high 18o albitized plagioclases 18o 10 which coexist with clinopyroxenes of mantle values 18o 5 5 or having values being slightly depleted down to 3 0 5 5 ultramafic rocks are either fresh or serpentinized they have 18o mainly around mantle values and d s are between 250 and 100 for the mafic part of the ophiolite the isotopic compositions correspond to hydrothermal interaction with seawater derived fluids under zeolite to greenschist facies but the trondjhemites were additionally affected by fluids poorer in d in particular the gabbros reacted with seawater derived fluids under amphibolite facies conditions as seen from their secondary mineralogy and their 18o depleted pyroxenes and subsequently were pervasively reacted under greenschist zeolite facies conditions during which the high 18o albitized plagioclase formed this occurrence of amphibolite facies conditions followed by greenschist zeolite facies conditions can be interpreted either to be a result of slow cooling of the oceanic crust at the ridge or to result from a fundamentally lower temperature hydrothermal interaction if compared with other ophiolites in contrast the d s of the serpentine 100 from the ultramafics are attributed to fluids of meteoric origin at high elevation nevertheless the particular location of the serpentinized harzburgites just beneath the gabbros and the high d dolerite sills and dikes 60 d 40 enclosed in these serpentinized harzburgites suggests an oceanic origin and therefore the possibility of hydrogen isotope exchange between serpentine and meteoric fluids after the obduction the xigaze ophiolite sequence displays very little 18o depleted rocks relative to mantle oxygen composition they cannot balance the amount of 18o enriched rocks consequently the result of the hydrothermal interaction between the xigaze ophiolite sequence and the seawater is an 18o depletion of the seawater however the contribution of xigaze type ridges to the buffering of 18o in cretaceous seawater was probably small

oxygen and hydrogen isotopes in the volcano sedimentary complex of huelva iberian pyrite belt example of water circulation through a volcano sedimentary sequence abstract the minerals of basic and acidic rocks from the volcano sedimentary sequence in the huelva area spain iberian pyrite belt display an extended 18o enrichment quartz 18o values from quartz keratophyres vary from 10 5 to 17 0 and feldspar 18o values from 14 4 to 16 0 for the spilite or spilitized dolerites 18o values vary from 9 9 to 13 4 for feldspar from 6 4 to 9 8 for chlorite from 3 7 to 4 3 for ilmenite and from 13 6 to 14 0 for quartz but pyroxene exhibits magmatic values from 5 3 to 6 1 with an exception at 7 5 the chlorite d values vary from 34 to 43 this is attributed to hydrothermal alteration with seawater enriched in 18o by circulation through sediments the temperatures of interaction determined from isotopic fractionations between minerals range from 400 to 520 c calculated d and 18o values for water in equilibrium with the minerals at isotopic temperatures range from 16 to 5 and from 8 3 to 12 8 respectively a model of circulation of seawater through a pile of sedimentary rocks and then through basaltic rocks is proposed to explain the high18o compositions of the rocks from the huelva district water rock mass ratios calculated from this model range between 0 3 and 0 7 for the determined range of temperatures