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

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

3d seismic interpretation of dissolution pipes in the south china sea genesis by subsurface fluid induced collapse a large suite of remarkable pipe structures has been identified from high resolution 3d seismic data in the dongsha massif northern south china sea the diameters and heights of the collapse structures range from c 100 m to 710 m and c 134 m to 1010 m respectively these pipes vary from cylindrical to a steep conical geometry narrowing upwards they are rooted in a major carbonate succession that was originally deposited as a large carbonate platform system these pipe structures are interpreted as resulting from carbonate diagenesis and dissolution at a deep burial stage i e they are not classical karst collapse structures as suggested in previous studies instead they are analogous to pipe like collapse structures described from the ordovician ellenburger group of west texas the dissolution pipes formed at different times during the neogene with the earliest pipes developing during the early middle miocene 15 0 ma and with at least one peak of pipe formation during the dongsha event began at 5 5 ma this regional event was associated with magma intrusion uplifting and faulting in the study area stable isotope data of reservoir rocks in the area of pipe development suggest that cementation was linked to magmatically driven hydrothermal circulation during the dongsha event small scale pipes are densely clustered above the main oil reservoir and given that petroleum is known to have been generated during the time interval of pipe formation we suggest that corrosive carbonic acid derived from flushing of petroleum reservoirs may have been a contributory factor to the dissolution and collapse process c 2013 elsevier b v all rights reserved

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

imaging through gas filled sediments using marine shear wave data marine multicomponent sea floor data of excellent quality have been acquired over the tommeliten alpha field the most dominating wave modes are interpreted to be conventional compressional pp waves and converted ps waves the most important geophysical problem associated with the tommeliten alpha field is the presence of a gas chimney obscuring the conventional 3 d seismic image of the reservoir zone the converted ps waves effectively undershoot the gas chimney leading to substantially improved images of the reservoir subsequent interpretation indicates the tommeliten alpha structure is a faulted dome

messinian evaporites and fluid flow abstract thick evaporite sequences deposited in saline giant basins are traditionally viewed as impermeable barriers to fluid flow this paradigm has recently been challenged by documented evidence of fluid migration pathways through several km thick series of evaporites such as the late miocene messinian sediments in the mediterranean basin deposited during the messinian salinity crisis msc our paper reviews the occurrence of fluid expulsion events in these evaporites in the depocentres of the basin and analyses their potential as seal bypass systems the messinian salt giant was deposited in peculiar conditions where massive sea level changes occurred in a relatively limited time interval and a thick basin centre evaporitic series was deposited between pre msc and post msc deep water sediments consequently rapid water and sediment loading unloading events contributed to the creation of overpressures up to fracture and possibly lithostatic gradient causing the fluids to be released in explosive events examples of fluid expulsion events are here grouped and classified in relation to the long and short term effects of the messinian salinity crisis as well as the local controls and pre conditioning basin factors while peaks of fluid expulsion activity during the short time frame of the msc can be mainly linked to mediterranean wide events local controls appear to play a major role in post msc fluid mobilisation in these cases evaporite breach is largely dependent on the availability of pre msc undersaturated fluid sources and the capability of the pre msc sediments to be overpressured the analysis here presented can be used as a basis to understand the controls on syn and post depositional movement of fluids in sedimentary basins moreover the analysis of temporal and spatial distribution of fluid expulsion events can help define hydrocarbon migration style and pathways in deep seated petroleum plays

perturbation of polygonal fault propagation by buried pockmarks and the implications for the development of polygonal fault systems abstract evidence is presented that constrains the earliest part of the growth history of a polygonal fault tier using high resolution 3d seismic data from the west african margin we document the interaction between a shallow buried wedge shaped polygonal fault tier and pockmarks located at the base of the fault tier strongly curved patterns of perturbed polygonal faults are mapped in the shallowest portions of a tier overlying a series of pockmark craters these perturbed patterns differ markedly from the background polygonal pattern developed away from areas of pockmark influence perturbation is attributed to local stress effects relating to residual topography of the underlying pockmarks by analysing the thickness variations of mapped layers within the polygonally faulted tier we demonstrate that any residual topography was most likely healed after approximately one third of the total tier thickness was deposited hence the nucleation of the polygonal faults into their perturbed propagation trajectories must have been established within the first 100 150 m of burial from these observations of early nucleation in the basal third of a tier we propose a new model of polygonal fault growth in which individual faults nucleate near the base of a tier at an early stage of burial and then propagate preferentially upwards with basal tips restricted by an underlying mechanically competent unit the base of the tier continued growth then shifts the locus of displacement such that final displacement distributions are more axio symmetrically disposed with respect to the base and top of the tier

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

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

seafloor instabilities and sediment deformation processes the need for integrated multi disciplinary investigations abstract in this paper we present the current practice of investigations of seafloor instabilities and deformation processes based on extensive research conducted over the last years which sets the scene for future research activities in this field the mapping of the continental margins and coastal areas with ever increasing resolution systematically reveals evidence of instabilities and deformation processes both active and palaeo features in order to properly assess the hazards and risks related to these features an integrated and multi disciplinary approach is essential but challenging such an approach consists of combining field data geophysics geology sedimentology geochemistry and geotechnical data with numerical simulations constrained by results from laboratory data as such it is of paramount importance to build a common knowledge base and understanding that unify these disciplines into more complete and conceptual models constrained by all the data we review the status of this integrated approach adapted to palaeo landslides e g storegga ana vesteralen and recent deformations finneidfjord nice gulf of guinea allowing to identify gaps in our knowledge at these sites by reviewing these case studies one can conclude that each case remains highly site specific in which both the regional and local geological tectonic settings have a distinct effect of the type of instability or deformation taking place or that can take place our knowledge on the actual triggers remains poorly constrained and there is even ambiguity for historic landslides e g finneidfjord also our knowledge of the preconditioning factors is incomplete there is a general lack of geotechnical data both in situ and from laboratory and therefore modelling the dynamics e g rheology of the instabilities relies on a number of assumptions rather than facts in addition excess pore pressure and its evolution is one of the key parameters driving instabilities despite this fact in situ excess pore pressure is rarely measured or monitored much work remains to be done to relate and integrate geotechnical data with geophysics e g through inversion and rock physical models in order to obtain additional quantitative information from the sub surface but also with respect to partial saturation free gas hydrate and pore pressure behaviour or lithologies it is of critical importance to be able to identify the different processes which can lead to hazardous situations which include establishing recurrence intervals timing and frequencies through event recognition and age control and magnitudes so that proper mitigation measures can be developed in this perspective the smaller scale instabilities deserve much attention as there are many instances where such features had far reaching consequences for society e g nice finneidfjord in that perspective human interferences e g exploitation drilling blasting loading must be one of the factors that should be taken into consideration

seismic modeling of gas chimneys we propose a simple acoustic model explaining the main features of gas chimneys the main elements of the model consist of gas diffusing from a connected fracture network and into the surrounding shale creating an inhomogeneous gas saturation the gas saturation results in an inhomogeneous fluctuating compressional velocity field that distorts seismic waves we model the fracture network by a randomwalk process constrained by maximum fracture length and angleofthefracturewithrespecttothevertical thegassaturation is computed from a simple analytical solution of the diffusion equation and pressure wave velocities are locally obtained assuming that mixing of shale and gas occurs on a scalemuchsmallerthanseismicwavelengths syntheticseismic sections are then computed using the resulting inhomogeneous velocity model and shown to give rise to similar deterioration in data quality as that found in data from real gas chimneys also synthetic common midpointcmpgathers show the same distorted and attenuated traveltime curves as those obtained from a real data set the model shows clearly thatthefeaturesofgaschimneyschangewithgeologicaltime a model parameter in our approach the deterioration of seismicwavesbeingsmallestjustafterthecreationofthegas chimney it seems likely that at least some of the features of gas chimneys can be explained by a simple elastic model in combinationwithgasdiffusionfromafracturenetwork

gas hydrate distributions in sediments of pockmarks from the nigerian margin results and interpretation from shallow drilling a joint research expedition between the french ifremer and the german marum was conducted in 2011 using the r v pourquoi pas to study gas hydrate distributions in a pockmark field 1141 1199 meters below sea surface at the continental margin of nigeria the sea floor drill rig mebo of marum was used to recover sediments as deep as 56 74 meters below seafloor the presence of gas hydrates in specific core sections was deduced from temperature anomalies recorded during continuous records of infrared thermal scanning and anomalies in pore water chloride concentrations in situ sediment temperature measurements showed elevated geothermal gradients of up to 258 c km in the center of the so called pockmark a which is up to 4 6 times higher than that in the background sediment 72 c km the gas hydrate distribution and thermal regime in the pockmark are largely controlled by the intensity periodicity and direction of fluid flow the joint interaction between fluid flow gas hydrate formation and dissolution and the thermal regime governs pockmark formation and evolution on the nigerian continental margin

evidence for fluid migration following pockmark formation examples from the nile deep sea fan a large number of pockmarks have been identified in the rosetta region of the western nile deep sea fan co located high resolution 2d and chirp datasets have been used to identify seabed and buried pockmarks which are interpreted to span the holocene to pleistocene period the pockmarks range in size and stratigraphic position from large buried pockmarks to small unit seabed pockmarks clusters of small unit pockmarks are stratigraphically positioned above the centre of large buried pleistocene pockmarks and are used to infer 1 rejuvenation of the larger pockmark conduit once burial has occurred and 2 a change in the frequency and magnitude of pockmark formation events

the spatial and temporal distribution of pipe formation offshore namibia a group of nearly 400 pipe structures from the continental slope of northern namibia are analysed for their spatial and temporal distribution the pipes most likely formed as a result of highly focused fluid venting and understanding the factors controlling their distribution in space and time is key to their genesis we analysed their spatio temporal distribution using an arbitrary chronostratigraphic timescale from which it is concluded that the pipes did not form at the same time pipe formation is shown to be intermittent and persistent with 2 29 pipes forming in each of the 20 arbitrary time intervals that are considered to span the neogene period the spatial distribution of these pipes is clustered to dispersed spatial statistics conducted on the distribution of pipe formation timings have shown that two statistically significant groups of pipes exist within the population 1 in the north and west and 2 in the south with the former occurring prior to the latter locally pipe formation is sporadic with clusters and outliers occurring during the same time period a conceptual model is proposed whereby pipe formation in specific locations is the result of localised breaching of the seals for isolated pressure cells which are locally independent yet broadly controlled an inferred basinal fluid source is thought to determine the broader patterns of pipe formation and the focus of this fluid source shifts from north to south with time at a local scale multiple local factors interact producing a sporadic pipe formation distribution through a prolonged period of highly focused fluid migration once formed the pipes continued to focus fluids intermittently leading in some cases to later pockmark formation

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

seal failure assessment of a major gas field via integration of seal properties and leakage phenomena we present a seismic and well based interpretation of a large leakage zone above the scarborough gas field exmouth plateau north west shelf of australia this leakage zone well imaged on three dimensional seismic data extends over a region of 100 km2 39 mi2 encompassing both the crest and flanks of the anticlinal trap and is termed here as distributed crestal leakage the present day gas water contact is 85 m 278 ft and the spillpoint is 110 m 328 ft below the crest implying that the trap is underfilled at present the leakage zone comprises more than 500 pockmarks at the present day seabed with no crosscutting or cannibalization suggesting that they formed in a short interval of time these are underlain by sediment remobilization features and amplitude anomalies consistent with a relatively high flux leakage of gas from the underlying cretaceous deep water sand rich reservoir by analyzing the geometrical relationship between the leakage zone the top seal properties and the gas water contact we conclude that the mode of leakage in this specific setting is not the result of gradual addition of gas charge but is instead consistent with a sudden increase of aquifer overpressure we suggest two alternative models for seal failure in this case study a conservative model consistent with a modest but rapid increase in aquifer overpressure leading to membrane seal failure and a model dominated by high aquifer overpressure leading to leakage through hydraulically dilated faults and fractures

multiproxy late quaternary stratigraphy of the nile deep sea turbidite system towards a chronology of deep sea terrigeneous systems abstract understanding the recent formation of a sedimentary system such as a deep sea turbidite system dsts requires an accurate stratigraphic control on deposits due to the important terrigeneous input which disrupts the sedimentary record dsts is an environment where stratigraphic control is difficult to assess most of the time traditional stratigraphic tools are not accurate enough this has led to a rather limited number of studies concerning stratigraphy in dsts in this study we examine several hemipelagic long piston cores collected from the nile dsts eastern mediterranean in order to understand the recent evolution of the complex sedimentary system in this area the first aim of this study is to show how to obtain a reliable timeframe in dsts indeed we provided a detailed ecostratigraphical scheme based on planktonic foraminiferal distribution oxygen isotope records and lithostratigraphy sapropels and tephra of three cores where the sedimentation is least disturbed we have identified 29 foraminiferal ecozones during the last 250 000 years bp with an approximately 2000 year time resolution the time span of each ecozone was constrained by the oxygen isotope record 14 c ams radiometric data tephrochronology and the sapropel chronology these high resolution ecostratigraphical time subdivisions have been applied in discontinuous mixed hemipelagic turbiditic sequences of a levee record this example shows how to date gravity events formation and time periods of sedimentary accumulations

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

spectrum of gas migration phenomena across multilayered sealing sequences we describe a spectrum of seismic high amplitude anomalies from the great south and canterbury basins offshore new zealand that have many characteristics that make them distinct from previously described hydrocarbon related amplitude anomalies we propose a new classification scheme that is based on the specific vertical stacking of the anomalies and thereby reflects their genetic interrelationships we demonstrate by combining amplitude versus offset and other attribute analyses that the anomalies are the product of gas migration across thick sequences of low permeability sediments and identify specific units where migration is focused through discontinuities e g faults and storage units where the gas spreads laterally based on our observations we argue that fluid flow phenomena in which darcy flow must have occurred can be hosted within low permeability layers that would normally be regarded as high quality sealing sequences the documentation of a wide range of gas related anomalies developed in a relatively uniform lithostratigraphy and in similar basinal contexts allows us to infer a migration sequence based on the morphostructural and geophysical characteristics of the anomalies we suggest that the shape of the composite anomalies is directly controlled by gas flux and by probably quite subtle variations in physical properties of the host sediments the distribution of the anomalies is generally correlated with maximum burial regions of the most prospective source rock intervals that are currently in the gas maturation window this spatial coincidence strongly suggests that the anomalies result from vertical migration of thermogenic gas from these underlying source kitchens

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

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

seismic indicators of focused fluid flow and cross evaporitic seepage in the eastern mediterranean abstract we present for the first time a synthesis of the evidence of focused fluid flow in the eastern mediterranean providing an updated record that includes recent and past occurrences through the last ca 6 my of evolution of the basin we do this by adding the interpretation of a previously unpublished regional 2d seismic dataset to the existing occurrences of focused fluid flow reported in the literature our interpretation shows a high number 141 of focused fluid flow features which span the stratigraphic interval from late miocene to recent of these features 82 are at the seabed and 59 are buried the previous published record is more difficult to quantify but in comparison shows an overwhelming majority of seabed features with only rare examples of buried features the spectrum of the buried and seabed features covers pockmarks pipes mud volcanoes clastic intrusions and collapse structures clustering of the fluid flow features is observed at different times in specific areas including the nile cone and the levant herodotus cyprus and latakia basins with the buried record we are able to highlight the evolution of the leakage points through time focused fluid flow venting has been occurring since the onset of the messinian salinity crisis and the start of basinwide deposition of evaporites we focus in particular on seismic indicators of leakage through evaporites and of sub evaporitic source for fluids and remobilised sediments we also discuss the role of the evaporites as a seal to ascending fluids and in which circumstances they can be breached fluids and associated remobilised sediments are sourced from different intervals from the sub and supra evaporitic section and possibly within the evaporites only a minor proportion of the fluid flow features are certainly sourced from below the messinian evaporites and most of them are located in the nile levant eratosthenes areas the few examples of pathways that are able to cross thick undeformed and well preserved evaporites are typically correlated to overpressure release and hydrofracturing this confirms that the evaporites do act regionally as a very good seal as expected while fluids are able to cross the evaporites only in their most extreme expression i e in near lithostathic overpressure conditions this is confirmed by our observations made in the eastern mediterranean where in the presence of relatively undisturbed evaporites cross evaporite vertical fluid pathways are only observed at the high end of the flux pressure range and involve sediment remobilisation maps combining these different elements can be used to detect areas potentially more prone to breaching

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

stress dependent wave velocities in sedimentary rock cores why and why not abstract rocks exhibit stress dependent wave velocities when their constitutive behaviour shows nonlinear elasticity the main physical mechanism responsible for stress dependency is opening or closure of cracks which is particularly important in brittle rocks crack formation is also associated with breaking of grain contacts we observe by using synthetic sandstones formed under stress a strong stress dependence during unloading simulated coring the reloaded material also shows a strong stress dependency in particular at stress levels below the virgin stress state but also above thus stress dependency as seen in cores taken from great depth is mainly a result of core damage stress independency is associated with linear elasticity such as in well cemented rocks or near the virgin stress conditions as observed here with synthetic sandstones but also occurs in ductile materials where there is a strong component of plasticity we demonstrate stress independence in synthetic sandstone at stress close to the virgin stress state in the elastic regime but also well above in the non elastic deformation regime high porosity high clay content shale was found to rather stress insensitive this is thought to be a result of the ductility of the rock caused mainly by the strong affinity of water to clay mineral surfaces

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

multi disciplinary investigation of fluid seepage on an unstable margin the case of the central nile deep sea fan we report on a multidisciplinary study of cold seeps explored in the central nile deep sea fan of the egyptian margin our approach combines in situ seafloor observation geophysics sedimentological data measurement of bottom water methane anomalies pore water and sediment geochemistry and 230 th u dating of authigenic carbonates two areas were investigated which correspond to different sedimentary provinces the lower slope at 2100 m water depth indicates deformation of sediments by gravitational processes exhibiting slope parallel elongated ridges and seafloor depressions in contrast the middle slope at not vert 1650 m water depth exhibits a series of debris flow deposits not remobilized by post depositional gravity processes significant differences exist between fluid escape structures from the two studied areas at the lower slope methane anomalies were detected in bottom waters above the depressions whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities carbonate u th age dates 8 kyr bp pore water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past in contrast large 1 km2 carbonate paved areas were discovered in the middle slope with u th isotope evidence for ongoing carbonate precipitation during the late holocene since 5 kyr bp at least our results suggest that fluid venting is closely related to sediment deformation in the central nile margin it is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of fossil carbonate ridges whereas pervasive diffuse flow prevails at the unfailed middle slope

1000 m long gas blow out pipes abstract this study presents seismic observation of pipe anomalies from offshore nigeria outcrops of blow out pipes from rhodes greece and geophysical modelling of an acoustic pipe the studies give insight into how pipes form their internal structure the seismic image and geophysical artefacts related to the pipes over one hundred seafloor craters 100 m 700 m wide and up to 30 m deep have been observed on the seafloor offshore nigeria they are underlain by interpreted cones and seismic pipe anomalies that can be traced down to reservoir zones at 1000 m 1300 m below the seafloor the seismic pipe anomalies are 50 m 150 m wide and almost vertical they are interpreted as up scaled pipes found in outcrops on rhodes greece the outcrops show pipe related structures at three levels lowest the reservoir rock contains metre sized cavities which are filled with a mixture of clay derived from the overlying cap rock in the middle several circular to oval structures in plane view of pipes are observed in the lowest part of the cap rock highest 15 m into the clay cap rock strongly sheared country rock forms circular structures with a core of structureless clay based on outcrop observation on rhodes we constructed an acoustic model of a 50 m wide and 1000 m long pipe seismic modelling proves that such pipes would be expressed in seismic data that they are similar to the seismic pipe anomalies offshore nigeria but this study also revealed that prominent intra pipe reflections are artefacts a formation model for the pipes is suggested high fluid overpressure in the reservoir generated hydro fractures from the reservoir to seafloor where a mixture of gas and fluid flowed at high speed to form pipes cones and seafloor craters after hours to weeks of gas and fluid flow through the pipe the pore pressure in the reservoir dropped and the blow out terminated muddy slurry fell back and plugged the cavity in the reservoir and the pipe

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

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

shallow gas and fluid migration in the northern arafura sea offshore northern australia abstract a variety of shallow gas indicators and fluid migration pathways have been interpreted from new sub bottom profiler multibeam bathymetry side scan sonar and echo sounder data together with geochemical analyses of sampled sediments from the northern arafura sea offshore northern australia this study provides new geological data and a seismic stratigraphy of the youngest units in the money shoal basin in an area that has not been closely studied in over 30 years the shallow gas indicators include pockmarks low frequency enhanced reflectors and acoustic blanking these indicators are supported by gas within shallow cores geochemistry indicates that this gas has a microbial origin but deeper fluid movement is also suggested by the presence of interpreted hydrocarbon slicks based on synthetic aperture radar data a region of likely hydrocarbon seepage is indicated by the clear coincidence of shallow gas indicators with an area of increased faulting and a zone of poor quality seismic data recognised on conventional seismic data together with anomalies interpreted on remote sensing data aligned with mapped deep structures

recent depositional patterns of the nile deep sea fan from echo character mapping the nile deep sea fan was surveyed in 1998 using swath multibeam bathymetry backscatter imagery and 3 5 khz and seismic profiling on the basis of this new data set the fan has been divided into three main provinces western central and eastern recent sedimentary patterns as deduced from echo character mapping show that gravity induced sedimentary deposits are predominant and are expressed through either slides debris flows or turbidites turbidity processes are particularly active in the westernmost province through a well developed network of deep sea channels this network controls turbiditic flows that subsequently overflow levees mass wasting processes are extremely efficient all over the nile deep sea fan in the upper central province a broad allochthonous tongue of mass flow deposits generated by slope destabilizations may have led to channel avulsions and migrations in the eastern province a northwest southeast deformed belt resulting from combined deep seated and salt related tectonics contains large proportions of destabilized sediments i e originating from destabilization processes such as mass movements probably settled by recent tectonic activity this article highlights 1 the influence of both thin skinned and thick skinned tectonics on sedimentary distribution and 2 the predominance of gravity induced sediments turbidites slides and mass flows in such submarine fans the distribution of debris flows and sedimentary deposits settled by turbidity currents appears fundamental in predicting reservoir geometry and economic potential

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

focused hydrocarbon migration in shallow sediments of a pockmark cluster in the niger delta off nigeria the niger delta is one of the largest hydrocarbon basin offshore africa and it is well known for the presence of active pockmarks on the seabed during the guineco mebo cruise in 2011 long cores were taken from a pockmark cluster in order to investigate the state of its current activity gas hydrates oil and pore water were sampled for geochemical studies the resulting dataset combined with seismic data reveal that shallow hydrocarbon migration in the upper sedimentary section was focused exclusively within the pockmarks there is a clear tendency for gas migration within the hydrate bearing pockmarks and oil migration within the carbonate rich one this trend is interpreted as a consequence of hydrate dissolution followed by carbonate precipitation in the course of the evolution of these pockmarks we also demonstrate that anaerobic oxidation of methane aom is the main process responsible for the depletion of pore water sulfate with depths of the sulfate methane transition zone smtz ranging between 1 8 and 33 4 m in addition a numerical transport reaction model was used to estimate the age of hydrate layer formation from the present day sulfate profiles the results show that the sampled hydrate layers were formed between 21 and 3750 years before present overall this work shows the importance of fluid flow on the dynamics of pockmarks and the investigated cluster offers new opportunities for future cross site comparison studies our results imply that sudden discharges of gas can create hydrate layers within the upper sedimentary column which can affect the seafloor morphology over few decades this article is protected by copyright all rights reserved

anomalous depressions in the northern yellow sea basin evidences for their evolution processes abstract depressions of both known and unknown origin are observed in continental margin basins and ocean floors worldwide a field of anomalous depressions with dome structures within the centers are documented for the first time using sub bottom seismic profiles multibeam bathymetry data and well data in the northern yellow sea basin they have complex seabed morphologies circular elliptical polygonal linear and crescent shapes with kilometer scale diameters and shallow depths 10 s decimeters to meters focused fluid flow pathways such as gas chimneys faults and pipes and free gas enhanced amplitude anomalies are observed within the shallow strata the depressions are interpreted as immature pockmarks possibly caused by insufficient fluid supply compared to fully developed normal pockmarks they are suspected to still be in the middle stage of pockmark evolution their formation processes are suspected to be controlled by sub seabed plumbing fluid flux and bottom currents this study is important for geologic hazard assessments in the study area and it also contributes to the full understanding of pockmark evolution

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