cryptic variation within the cumulate sequence of the oman ophiolite magma chamber depth and petrological implications the cumulate sequence of the oman ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic long lived crustal magma chamber that underlay a late cretaceous spreading ridge regional variations in cumulate sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions microprobe data are presented from a 600 m section of modally layered olivine gabbros coherent cryptic cyclic variation is shown with both normal and reversed geochemical gradients present these are explained in terms of fractionation mixing and eruption in an open system magma chamber the height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m the implications of this result are discussed a contrast between the fo an co variation between olivine and plagioclase in the oman cumulate sequence and that observed for morb votcanics is noted this may reflect compositional stratification in spreading ridge magma chambers or signifcant differences between the petrogenesis of the oman ophiolite and morb

thermal models of oceanic crustal accretion linking geophysical geological and petrological observations 1 models of oceanic crustal accretion should be able to reproduce both petrological and geological observations from ophiolites and geophysical observations from active spreading ridges we have developed thermal models of crustal accretion at fast spreading ridges that incorporate petrological variation and allow for a variable vertical distribution of crystallization including petrological variation has a major effect on the distribution of melt within the crust and the consequent interpretation of model results in terms of geophysical observables in particular the narrow melt lens imaged by seismic reflection surveys can be produced by the presence of large scale compositional stratification within the crust such as that observed in the oman ophiolite controlling the vertical distribution of crystallization allows a range of crustal accretion mechanisms to be tested including the many sills and gabbro glacier models both gabbro glacier and many sills models can broadly match the geophysical observations but the preferred model is a hybrid where the uppermost 25 or more of the lower crust forms in a gabbro glacier and the lowermost 25 or more of the lower crust forms by crystallization in situ the success of all models is dependent on the strength and distribution of hydrothermal cooling

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

duality of magmatism in the plutonic sequence of the sumail nappe oman abstract detailed field mapping of the crustal sequence in several areas of the sumail ophiolite nappe mainly in the fizh the salahi and the haylayn blocks and comparisons with other well studied areas have shown that the crustal sequence was constructed by two distinct although nearly contemporaneous magmatic series the first a gabbroic series was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit the high level isotropic gabbro unit and their by products the sheeted dyke complex and the lower extrusives fine grained homogeneous planar laminated gabbros commonly occur at the top of the layered gabbro sequence in some specific areas wadi haymiliyah and wadi rajmi these are strongly differentiated fe ti rich gabbronorites and norites which coincide with large scale discontinuities within the plutonic sequence interpreted as axial discontinuities of the spreading magma chambers the second series an ultramafic wehrlitic series is locally rooted in the upper mantle i e the impregnated dunitic transition zone this has intruded the whole crustal sequence up to the pillow lavas countless discordant and concordant wehrlitic bodies intruded the plutonic sequence making up 20 40 of it in volume we describe the various settings and forms of these intrusions which were largely underestimated in previous studies we also describe their mineralogy and textures and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems independently from each other we infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion at the base of the crust however indications of immiscibility between gabbros and wehrlites are common suggesting that both magmatic series were quite contemporaneous the wehrlites are commonly isotropic and may exhibit an internal layering of the dunite wehrlite clinopyroxenite type sometimes appearing at the upper or lower margins of the intrusions in this case the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros these plagioclase free ultramafic layered cumulates therefore are not interbedded within the gabbro cumulates but actually belong to the wehrlitic intrusive series picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives which are for the main part cogenetic with the wehrlites the time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre we believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust

metamorphism and deformation along the emplacement thrust of the samail ophiolite oman abstract the base of the samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement the classic exposure of this metamorphic sole at wadi tayin consists of a few meters of garnet clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite overlying 150 m of lower amphibolite facies mafic and sedimentary schist the sole is bounded by faults but is internally coherent peak metamorphic temperatures decrease exponentially from 825 c at the top of the sole to 500 c at its base as documented by changes in grain size hornblende composition the disappearance of chlorite and epidote garnet clinopyroxene element partitioning and garnet hornblende partitioning the inverted peak temperature gradient decreases from 4000 c km in the upper 50 m to 100 c km in the lower 150 m contrary to earlier reports greenschist facies tectonites are not present at wadi tayin although the entire sole underwent greenschist facies retrogression driven by the influx of fluids from the overlying peridotite or underlying carbonate calculations based on an amphibolite constitutive relation reveal that although the section represents a lithologic continuum the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column

mixing of siliciclastic and carbonate sediments in shallow shelf environments the inhibiting effect that siliciclastic material has on carbonate secreting organisms has lead to the generalization that sediments composed of mixtures of carbonate and siliciclastic material should rarely form however many modern and ancient shelf deposits contain a spectrum of sediments that are of mixed composition the processes responsible for this mixing can be grouped into four categories 1 punctuated mixing where sporadic storms and other extreme periodic events transfer sediments from one depositional environment to another 2 facies mixing where sediments are mixed along the diffuse boundaries between contrasting facies 3 in situ mixing where the carbonate fraction consists of the autochthonous or parautochthonous death assemblages of calcareous organisms that accumulated on or within siliciclastic substrates and 4 source mixing where admixtures are formed by the uplift and erosion of nearby carbonate source terranes the allochemical constituents of mixed sediments are both coralgal and foram mollusc in composition the foram mollusc assemblage is the most common because of the effects of increased turbidity unstable substrates and the clogging of filter feeding mechanisms associated with a siliciclastic influx

carbonate evaporite siliciclastic transitions in quaternary rift sediments of the northwestern red sea abstract the quaternary and recent coastal environments of the egyptian red sea and gulf of suez are characterized by rapid lateral variations carbonates evaporites and siliciclastic sediments exist in close proximity although the climate is exceptionally dry the numerous wadis draining high hinterland relief deliver important quantities of coarse clastics to coastal environments during the rare periods of flash flooding however because this efficient supply is sporadic coastal waters are contaminated for short catastrophic periods which are followed by a rapid return to clear water carbonate sedimentation the warm arid conditions on the other hand favour both carbonate reef production and evaporite mineral precipitation rapid lateral transitions between continental siliciclastic and marine carbonate facies are favoured by a particular morphology closely related to the active distensional tectonic setting within the red sea rift block faulting which began in the early miocene and continues today has resulted in numerous subparallel grabens and horsts situated between the onshore basement relief and the coast the structural depressions grabens tend to trap much of the episodic siliciclastic input favouring permanent carbonate production in the nearby littoral environments

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

oxygen isotope exchange and closure temperatures in cooling rocks retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model which predicts both the zonation profile of 18o within grains and the bulk 18o value of each mineral in the rock apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the 18o values of each mineral pair within the rock in systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values slow cooling the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the dodson closure temperatures tc for the two phases and the mode of the rock adding a third phase into a system which undergoes slow cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two phase system in some systems oxygen isotope reversals can be developed if cooling is not slow then the mineral 18o values resulting from cooling will be partly dependent upon the initial temperature of the system concerned the model successfully simulates the mineral 18o values that are often observed in granitic rocks application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems

petrology and geochemistry of the lower ocean crust formed at the east pacific rise and exposed at hess deep a synthesis and new results 1 the geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle how these melts mix and crystallize and the role of hydrothermal circulation in this portion of the crust unfortunately lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes at hess deep crust formed at the east pacific rise epr is exposed due to the propagation of the cocos nazca spreading center westward here we review our state of knowledge of the petrology of lower crustal material from hess deep and document new mineral major and trace element compositions amphibole plagioclase thermometry and plagioclase crystal size distributions samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive moderate cr high mg orthopyroxene and very calcic plagioclase because primitive mid ocean ridge basalts morbs are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle these apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction the occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal morb in the axial magma chamber amc but rather that some melts partially crystallize in isolation within the lower crust however evidence that primitive melts fed the amc along with steep fabrics in shallow gabbros from near the base of the dyke complex provides support for models in which crystallization within the amc followed by crystal subsidence is also an important process in lower crustal accretion more evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust amphibole plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high temperatures in the shallow gabbros tave 713 c and show that the temperature of amphibole formation was similar in deeper gabbros tave 722 c this thermometry also suggests that fracture and grain boundary permeability for seawater derived fluids was open over the same temperature interval

rates of hydrothermal cooling of new oceanic upper crust derived from lithium geospeedometry episodic emplacement and cooling of lavas and dikes at mid ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity however the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood we have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling we have calibrated this li geospeedometer using new high temperature experiments to determine both the temperature dependence of the partitioning of li between plagioclase and clinopyroxene and the diffusion coefficient for li in clinopyroxene application of this method to lavas and dikes from odp hole 504b shows that cooling rates vary dramatically with depth in the upper oceanic crust extremely rapid cooling rates 450 c hr 1 in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

a thermomechanical numerical model for crustal accretion of medium to fast spreading mid ocean ridges we propose a new thermomechanical numerical model of mid ocean ridge accretion aimed at investigating asymmetric spreading rates diverse configurations of lens and sill magma injections crystallization and depth and on and off axis patterns of hydrothermal cooling the numerical algorithm iteratively resolves temperature and motion equations until it reaches a stationary solution the motion equation was written in a vorticity stream function formalism with boundary and internal conditions applied to the stream function to impose the style of magma injection unlike in previous models our model does not assume an a priori shape for the temperature field which is initiated by an initial half space cooling according to the left and right spreading rates complex patterns of hydrothermal cooling are simulated by enhanced thermal diffusivity the model succeeds in describing the dynamic and thermal effects of spreading rates the style of magma intrusion and the hydrothermal cooling accurate descriptions of these are essential to study the cooling histories of crustal rocks and geophysical observables

geology and petrology of the plutonic complexes in the wadi fizh area multiple magmatic events and segment structure in the northern oman ophiolite 1 multiple magmatic events are recorded in the gabbroic unit in the fizh area of the northern oman ophiolite gabbroic blocks intruded by sheeted dike complex and upper gabbros of the main crustal sequence show the oldest event gabbronorite sills in the gabbroic blocks are nearly coeval with the host gabbro wehrlitic intrusions wehrlite i mark the third event of magmatism these three magmatic events occurred at the retreating dying ridge axis because all these rocks are intruded by dolerite dike swarm which is generally regarded as a precursor of advancing ridge axis the next stage of magmatism is a main phase of oceanic crust generation in this area wehrlite ii and then gabbronorite dikes intrude the still hot main gabbro unit all of these above rocks have similar signatures with respect to clinopyroxene compositions and covariations between plagioclase and mafic minerals though slight differences are present in the compositional ranges and clinopyroxene compositions of each unit after considerable cooling of the main gabbro unit primitive basalt dikes intrude the main gabbro unit which may correspond to the lasail unit finally the fizh south complex intrudes into considerably cooled crustal sequence being below the brittle plastic transition temperatures the fizh south complex which was regarded as a common wehrlitic intrusion is significantly different from all of the above mentioned rocks with respect to the covariation between plagioclase and associating mafic minerals crystallization order and clinopyroxene compositions the clinopyroxenes are characterized by extremely low ti and na contents comparable with those of the v2 unit alley volcanics suggesting that the fizh south complex correlates with the plutonic facies of the v2 unit during arc stage layered gabbros in the wadi zabin area about 10 km north of the fizh area may be a northern extension of the gabbro blocks of the fizh area because they are intruded by numerous dolerite dikes on the other hand basin like structure of the main gabbroic unit in the northern end of the fizh area may demonstrate a fossilized magma chamber beneath the advancing ridge axis due to the ceasing of magmatic crustal accretion on the basis of these lines of evidence we propose that the fizh area indicates the northward propagation tip of the ridge axis

constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the samail ophiolite abstract oceanic crust represents more than 60 of the earth s surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis the two proposed end member models the gabbro glacier and the sheeted sills predict radically different strain accumulation in the lower crust during accretion in order to determine which of these two hypotheses is most applicable to a well studied lower crustal section we present data on plagioclase lattice preferred orientations lpo in the wadi khafifah section of the samail ophiolite we observe no systematic change in the strength of the plagioclase lpo with height above the crust mantle transition no dominant orientation of the plagioclase a axis lineation and no systematic change in the obliquity of the plagioclase lpo with respect to the modal layering and macroscopic foliation evident in outcrop these observations are most consistent with the sheeted sills hypothesis in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a gabbro glacier our data support the hypothesis of macleod and yaouancq 2000 that the rotation of the outcrop scale layering from sub horizontal in the layered gabbros to sub vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber additionally our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis whereas in slow and ultra slow ridges large shear strains are accommodated by plastic deformation

modes of crustal accretion and their implications for hydrothermal circulation hydrothermal convection at mid ocean ridges links the ocean s long term chemical evolution to solid earth processes forms hydrothermal ore deposits and sustains the unique chemosynthetic vent fauna yet the depth extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes remain poorly constrained here based on coupled models of crustal accretion and hydrothermal circulation we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast spreading ridges we integrate numerical models with observations of melt lens depth thermal structure and melt fraction models matching all these observations always require a deep crustal scale hydrothermal flow component and less than 50 of the lower crust crystallizing in situ

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

near conductive cooling rates in the upper plutonic section of crust formed at the east pacific rise abstract a new geospeedometer based on diffusion modeling of mg in plagioclase is used to determine cooling rates of the upper section of the lower oceanic crust formed at fast spreading mid ocean ridges the investigated natural sample suites include gabbroic rocks formed at three different locations along the fast spreading east pacific rise these samples cover a depth interval of 0 840 m below the sheeted dike gabbro boundary and therefore allow the variation of cooling rate as a function of depth within the upper plutonic sequence to be determined we demonstrate that the cooling rates we obtained are robust reproducible and consistent across different vertical sections at fast spreading ridges and decrease significantly with increasing sample depth covering almost 4 orders of magnitude ranging from 1 c y 1 for the shallowest samples to 0 0003 c y 1 for the deepest samples both the absolute cooling rates and the rate of decrease of cooling rate with depth are consistent with conductive thermal models in contrast the absolute cooling rates determined from the deeper samples 300 m below dgb and the large decrease in cooling rate with depth are inconsistent with thermal models that include substantial cooling by off axis hydrothermal circulation within the upper plutonic section of the crust

coral reefs in coarse terrigenous sedimentary environments upper tortonian granada basin southern spain abstract during the upper tortonian coral reefs developed on fan deltas and braid deltas along the active eastern edge of the granada basin the corals grew in environments characterized by deposition of coarse sands and conglomerates frequency of sedimentation together with sea level rises controlled reef development and determined the architecture and composition of the carbonate deposits in direct contrast to these reefs others flourished at the same time along stable coastal margins of the basin their structure was influenced mainly by periodic rises in sea level the mapping of all these reefs serves as an accurate guide to the upper tortonian geography of the granada basin

paragenesis of magma chamber internal wall discovered in oman ophiolite gabbros a detailed study in wadi farah reveals a singular point in jebel dihm southern oman ophiolite here we investigated some primitive features of accretion at a fast spreading ridge that has exceptionally been exempt from hydrous alteration while crossing the tbl thermal boundary layer the conjunction of two factors has made this possible in the farah area its location at the point of sharp foliation rotation from the upper to the transitional gabbros and a locally twisted tbl these factors may have favoured the intrusion within the tbl of hydrous wedges transverse to the ridge deeper into the tbl near the active ridge the wedges merge and are covered by veneers of totally fresh gabbros bearing glittering acicular clinopyroxene these gabbros are interpreted as relics of the crystallizing internal wall of the magma chamber that have interacted with hydrous fluids this internal tbl has anisotropic thermal properties and acts as a thermal blanket maintaining high magma chamber temperatures

tectonic development of the samail ophiolite high precision u pb zircon geochronology and sm nd isotopic constraints on crustal growth and emplacement 1 new high precision single grain u pb zircon geochronology and whole rock nd isotopic data provide insight into the magmatic and tectonic development of the samail ophiolite the analyzed rocks can be broadly divided into two groups based on their structural position dates and isotopic composition an older group related to on axis magmatism and a younger group of post ridge dikes sills and stocks on axis gabbros tonalites and trondhjemites yielded th corrected 206pb 238u dates from 96 441 0 062 to 95 478 0 056 ma these dates combined with dates from rioux et al 2012 suggest that most of the ophiolite crust formed at an oceanic spreading center in 1 ma the post ridge intrusions come from all depths in the crust the upper mantle and the metamorphic sole post ridge gabbros tonalites and trondhjemites from the crust and mantle yielded th corrected 206pb 238u dates of 95 405 0 062 to 95 077 0 062 ma a small trondhjemitic pod from the metamorphic sole yielded younger th corrected 206pb 238u dates of 94 90 0 38 to 94 69 0 12 ma isotopic data suggest two distinct sources for the post ridge magmas five of the gabbros and tonalites from the crust have end 96 ma 6 90 0 12 to 7 88 0 16 and two trondhjemites from the upper mantle and metamorphic sole have end 96 ma 7 77 0 08 and 7 01 0 16 the negative end t and u pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite 0 25 0 5 ma after formation of the crust the bimodal isotopic composition of post ridge magmas may reflect coeval decompression and or fluid fluxed melting of the mantle and melting dehydration or assimilation of sediment in the down going plate at this time the new data place temporal constraints on mid ocean ridge and supra subduction zone models for ophiolite formation

chemical and thermal constraints on focussed fluid flow in the lower oceanic crust the mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems we have investigated the role of zones of focussed fluid flow in the plutonic complex of the oman ophiolite in the near axis cooling of the oceanic crust lineaments identified on aerial photographs that occur at 1 km spacing show evidence for extensive hydrothermal fluid flow through regions 10 to 50 m wide fluid flow is initiated in these regions at 800 c and continues at least into the lower greenschist facies strontium isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the moho computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are 1x108 kgm2 modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the sr isotopic composition of samples from the focussed fluid flow zones the combination of independent estimates of the fluid and heat fluxes such as these can provide more rigorous constraints on the thermal history than either approach used in isolation our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid ocean ridges this is because fluids flowing through channels become chemically rock buffered at smaller fluid fluxes than those flowing pervasively through a rock mass thus if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought

contaminating the lower crust in the oman ophiolite contamination of magmas via assimilation of country rock is widely recognized as an important process in the thermal and chemical evolution of magmas emplaced into continental crust its importance at mid ocean ridges has however received much less consideration lavas from the fast spreading east pacific rise show evidence for contamination through an enrichment in seawater derived chlorine that is not observed in basalts from slow spreading ridges in this study magmatic amphiboles in plutonic rocks from the oman ophiolite an intermediate to fast spreading ridge analogue have been analyzed to determine the extent of this contamination in the lower oceanic crust these data suggest an enrichment in chlorine throughout much of the lower ocean crust if this enrichment is representative of lower oceanic crust formed at typical intermediate to fast spreading ridges assimilation may be a more important process at mid ocean ridges than previously recognized

the effects of spreading rate the magma budget and the geometry of magma emplacement on the axial heat flux at mid ocean ridges we explore two dimensional steady state flow and thermal models of oceanic spreading center structure these models include the effects of hydrothermal heat transport and crustal accretion for two basic accretion geometries 1 a lens model where all crust below the sheeted dike layer passes through a magma lens at the base of the dike layer before subsiding and flowing to deeper levels and 2 a dike model where crust is emplaced in a vertical slot from the surface to moho and subsequently moves horizontally rigidly away from the axis of accretion the axial heat flux resulting from these two end member accretion geometries is compared and an assessment is made of the feasibility of using various observations to differentiate between these two accretion geometries we find that the steady state axial heat flux is predominantly influenced by three factors the spreading rate the magmatic budget crustal thickness and the efficiency of hydrothermal cooling the total steady state heat flux from the neovolcanic zone 2 km wide increases almost linearly with the spreading rate for both the dike and lens accretion geometries the major difference between the dike and the lens models is nonthermal they predict different accumulated strain distributions within off axis crust crustal flow due to crustal accretion within a crustal height dike leads to little accumulated strain while intense crustal strain results from crustal subsidence and flow below a steady state magma lens ophiolite and marine seismic observations of crustal layering appear to be the strongest observational tests to discriminate between these two accretion geometries they currently favor a lens like model of lower crustal accretion

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

continental break up mechanism lessons from intermediate and fast extension settings continental break up mechanisms vary systematically between slow and fast extension systems slow extension break up has been established from studies of the central atlantic european and adria margins this study focuses on the intermediate and fast cases from gabon and east india and draws from the interpretation of reflection seismic gravimetric and magnetic data interpretation indicates continental break up via continental mantle unroofing in all systems with modifications produced by magmatism in faster extension systems break up of the intermediate extension gabon system involves partial upper continental crustal decoupling from continental mantle whereas in the fast east coast india system decoupled and lower crustal regimes underwent upwarping in soggy zones in the footwalls of major normal faults usually upper crustal break up is affected by pre existing anisotropies which form systems of constraining rails for extending continental crust this modifies the local stress regimes they regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle different regions attain different amounts of upper crustal stretching prior to the break up the break up location is then controlled by the upper crustal energy balance principle of wound linkage by which the minimum physical work is performed for linking upper crustal wounds leading to successful upper crustal break up

dating the growth of oceanic crust at a slow spreading ridge nineteen uranium lead zircon ages of lower crustal gabbros from atlantis bank southwest indian ridge constrain the growth and construction of oceanic crust at this slow spreading midocean ridge approximately 75 of the gabbros accreted within error of the predicted seafloor magnetic age whereas 25 are significantly older these anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or more likely crystallization of older gabbros at depths of 5 to 18 kilometers below the base of crust in the cold axial lithosphere which were uplifted and intruded by shallow level magmas during the creation of atlantis bank

constraining the cooling rate of the lower oceanic crust a new approach applied to the oman ophiolite abstract knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid ocean ridges in this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down temperature diffusive exchange of ca from olivine to clinopyroxene in gabbros from the oman ophiolite modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth changing rapidly in the upper third of the gabbros but more slowly in the lower two thirds there is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust the cooling rate versus depth profile closely matches that of a conductive cooling model or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

cooling of the lower oceanic crust thermal models of mid ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust these models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast spreading ridges the rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge with 50 70 kw of hydrothermal cooling for every 1 m of ridge axis at crustal ages of 0 1 0 4 ma these timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges

paleointensity record from the 2 7 ga stillwater complex montana 1 the record of geomagnetic intensity captured in the 2 7 ga stillwater complex montana usa provides a statistical description of the archean geodynamo we present results of modified thellier paleointensity experiments on 441 core specimens 114 of which pass strict reliability criteria the specimens are from 53 sites spanning most of the banded series rocks in the stillwater complex on the basis of thermochronologic and petrologic evidence we interpret the highest temperature component of remanence to be a late archean thermoremanence though the possibility remains that it is a thermochemical remanence thermal models indicate that the highest temperature magnetization component at each of the sites averages 20 200 ka of geomagnetic secular variation the suite of sites as distributed through the banded series samples a roughly a 1 ma time interval the average of the most reliable paleointensity measurements uncorrected for the effects of anisotropy or cooling rate is 38 2 11 3 t 1 remanence anisotropy cooling rate and the nonlinear relationship between applied field and thermoremanence have a significant effect on paleointensity results a corrected average of 30 6 8 8 t is likely a more appropriate value earth s average dipole moment during the late archean 5 05 1 46 1022 am2 pmag 44 5 was well within the range of estimates from phanerozoic rocks the distribution of site mean paleointensities around the mean is consistent with that expected from slow cooling over timescales expected from thermal models and with secular variation comparable to that of the phanerozoic field

tectonic setting origin and obduction of the oman ophiolite the semail ophiolite in the oman mountains is the world9s largest and best preserved thrust sheet of oceanic crust and upper mantle 10 000 km 2 550 km long 150 km wide it was emplaced onto the arabian continental margin during late cretaceous time the ophiolite originated 96 94 ma at a spreading center above a northeast dipping subduction zone associated with initiation of immature island arc tholeiitic lavas lasail arc at the highest levels of the ophiolite simultaneous underthrusting of triassic and jurassic mid oceanic ridge basalt and alkalic volcanic rocks beneath 12 km of upper mantle depleted harzburgites produced garnet clinopyroxene amphibolites formed at temperatures of 850 c dated as 95 93 ma subduction cannot have been initiated at a mid oceanic ridge otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above in the northern part of the oman mountains in the bani hamid area united arab emirates 870 m of granulite facies rocks enstatite spinel diopside quartzites garnet diopside wollastonite calc silicate marbles clinopyroxene bearing amphibolites were formed at temperatures similar to those of the garnet diopside amphibolites of the oman sole 800 850 c but at slightly higher pressures as much as 9 kbar they are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out of sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction subduction of the arabian continental crust beneath the obducting semail ophiolite to 78 90 km depth has been proven by thermobarometry of the as sifah eclogites to 20 23 kbar in the eastern sector in the united arab emirates the subducted continental crust began to partially melt producing unusual biotite muscovite garnet tourmaline cordierite andalusite bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite we suggest that the entire leading northeast edge of the arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks higher temperatures and pressures in the united arab emirates sector possibly due to a thicker or double thickness ophiolite section led to blueschist amphibolite and granulite facies conditions in the metamorphic sole and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite a model for ophiolite obduction is presented which accounts for all the structural and metamorphic conditions reported from the oman mountains

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

cooling rates and the depth of detachment faulting at oceanic core complexes evidence from zircon pb u and u th he ages oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid ocean ridges but many questions persist about the thermal evolution and depth of faulting we present new pb u and u th he zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the mid atlantic ridge ocean drilling program odp holes 1270d and 1275d near the 15 20 n transform and atlantis massif at 30 n closure temperatures for the pb u 800 c 850 c and u th he 210 c isotopic systems in zircon bracket acquisition of magnetic remanence collectively providing a temperature time history during faulting results indicate cooling to 200 c in 0 3 0 5 myr after zircon crystallization recording time averaged cooling rates of 1000 c 2000 c myr assuming the footwalls were denuded along single continuous faults differences in pb u and u th he zircon ages together with independently determined slip rates allow the distance between the 850 c and 200 c isotherms along the fault plane to be estimated calculated distances are 8 4 4 2 km and 5 0 2 1 km from holes 1275d and 1270d and 8 4 1 4 km at atlantis massif estimating an initial subsurface fault dip of 50 and a depth of 1 5 km to the 200 c isotherm leads to the prediction that the 850 c isotherm lies 5 7 km below seafloor at the time of faulting these depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the tag hydrothermal field and high temperature fault rocks recovered from many oceanic detachment faults

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

thermal effects of gabbro accretion from a deeper second melt lens at the fast spreading east pacific rise motivated by recent observations a ridge model has been developed which includes both a shallow melt lens and a second deeper melt lens above the moho to assess the thermal effect of the gabbro accretion from the lower melt lens at the fast spreading east pacific rise modeling results argue for a limited role of the lower melt lens in constructing the layer 3 gabbros if significant 10 amount of the layer 3 gabbros is emplaced near the moho then the liberated latent heat during crystallization would generate a large molten section of the lower crust due to the inability of cold seawater to penetrate into this hot ductile region a large molten section of the lower crust is not consistent with recent seismic tomographic results of the east pacific rise thus it is likely that the amount of the gabbro being emplaced from the lower melt lens at the epr is significantly less than that suggested by oman ophiolite studies such a lower melt lens is short lived or a large portion of the sill intrusions observed at oman was emplaced at a few kilometers off axis 3 4 km

chemical zoning of olivine in several pallasites chemical zoning profiles of olivines in the esquel imilac and yamato 8451 pallasites were measured by using an electron probe microanalyzer in order to study their cooling histories pallasite olivines are considered to be compositionally uniform but show faint zoning for fe mg cao al2o3 cr2o3 and mno within a few hundred micrometers from the rim the cao al2o3 and cr2o3 contents of olivines in pallasites decrease gradually toward the rim the fe mg zoning and the mno enrichment in olivine rims of pallasite olivines may reflect a redox process on the basis of diffusion calculations to estimate the cooling rate by using chemical zoning of pallasite olivine the formation process of chemical zoning was discussed similar cooling rates obtained for the fe mg cao and mno profiles of an olivine imply that these zoning patterns formed during the same event

retrieving timescales of oceanic crustal evolution at oceanic core complexes insights from diffusion modelling of geochemical profiles in olivine abstract the building of oceanic crust at oceanic core complexes occ has been described as a complex process involving multiple intrusions of magma over a protracted period of time the migration of primitive magmas i e mid ocean ridge basalts morbs can lead to melt rock interactions during reactive porous flow processes through the lithosphere the timescales of these reactive processes and the subsequent cooling of the modified crystal matrix remain unconstrained diffusion modelling has been widely used to retrieve timescales of magmatic processes in this study we use diffusion models to constrain i the minimum timescales of melt rock interactions and ii the cooling rates of the gabbroic sequence forming the oceanic crust at an occ we chose samples of the most primitive olivine rich troctolites from the gabbroic sequence sampled in iodp hole u1309d atlantis massif occ mid atlantic ridge 30 n olivine rich troctolites were interpreted as marking local partial assimilation of mantle intervals into the oceanic crust and thus allowed to understand the dynamics of mantle assimilation and the formation of slow spreading oceanic crust at the atlantis massif occ olivines in olivine rich troctolites represent relicts of pre existing mantle olivine while clinopyroxenes and plagioclases are crystallized during reactive percolation olivine chemical compositions show that olivine rich troctolites inherit chemical heterogeneity from the mantle precursor flat geochemical profiles in olivine indicate complete chemical re equilibration of olivine crystals with the locally modified percolating melt exception is made for most ca profiles that show lower ca contents at the olivine rim compared to the relative crystal core as the result of subsolidus cooling three dimensional 3d diffusion models at magmatic conditions t 1210 1300 c and p 2 kbar reveal that complete chemical re equilibration of 4 mm size mantle derived olivine with percolating morb type melts can be attained within durations of less than 300 yr the ca in olivine geospeedometer reveal that cooling rates from 1200 c to 1050 c are constant downhole and on average 0 004 c yr they are comparable with lower temperature cooling rates 850 c 250 c estimated at the atlantis massif occ the minimum timescales from 3d models point to rather fast re equilibration of olivine the downhole chemical heterogeneity inherited from the precursor mantle coupled with the timescales of diffusive re equilibration suggest that the partial assimilation of the upwelling mantle and its incorporation into the oceanic crust occurred in the time frame of a single melt input our cooling data are consistent with building of the oceanic crust at occs controlled by continuous uplift in turn governed by long lived detachment faults the latter contribute to the rapid cooling of the assimilated mantle intervals and the magma bodies diffusion models of geochemical profiles in olivine from a single crustal section allow to reconstitute the early magmatic processes leading to mantle assimilation and early crystallization of gabbros and the cooling history of the oceanic crust at occ from magmatic conditions to hydrothermalism

slow cooling of the lowermost oceanic crust at the fast spreading east pacific rise the cooling history of the lowermost ocean crust formed at the east pacific rise and exposed at the hess deep rift is constrained using fresh primitive olivine gabbro and troctolite cores recently recovered by integrated ocean drilling program expedition 345 the mg in plagioclase and ca in olivine geospeedometers constrain the initial subsolidus cooling rates down to closure temperatures in the range of 900 750 c these independent geospeedometers yield similar slow cooling rates 0 005 0 0001 c yr 1 with a mean value of 0 0011 c yr 1 making these results robust despite the uncertainties inherent in the methods the slow cooling rates derived using natural samples provide a crucial test of predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling the cooling rates are most consistent with thermal models that are dominated by conductive cooling within the plutonic complex this implies that lower crust formed at fast spreading rates remains hot in the near axis environment indicating limited hydrothermal fluid ingress and reaction

hydrothermal fault zones in the lower oceanic crust an example from wadi gideah samail ophiolite oman abstract hydrothermal circulation is a key process for chemical and isotopic exchange between the solid earth and oceans and for the extraction of heat from newly accreted crust at mid ocean ridges however due to a dearth of samples from intact oceanic crust or continuous sample suites from ophiolites there remain major shortcomings in our understanding of hydrothermal circulation in the oceanic crust especially in the lower plutonic crust in particular it remains unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channelled within discrete zones such as faults here we describe a hydrothermally altered fault zone that crops out in the wadi gideah in the layered gabbro section of the samail ophiolite of oman a one metre thick normal fault comprising deformed chlorite epidote fault rock with disseminated chalcopyrite and pyrite offsets gently dipping layered olivine gabbros the chlorite rich fault rocks surround strongly altered clasts of layered gabbro 50 to 80 cm in size layered gabbros in the hanging wall and the footwall are partially altered and abundantly veined by epidote prehnite laumontite and calcite veins in the wall rocks igneous plagioclase an82 2 is partially altered towards more albitic compositions an75 81 and chlorite tremolite partially replaces plagioclase and clinopyroxene clinopyroxene is moderately overgrown by mg hornblende whole rock mass change calculations show that the chlorite rich fault rocks are enriched in fe mn h2o co2 co cu zn ba and u but have lost significant amounts of si ca na cr ni rb sr cs light rare earth elements lree eu and pb gabbro clasts within the fault zone as well as altered rock from the immediate hanging wall show enrichments in na volatiles sr ba and u and depletions of si ti al fe mn mg ca cu zn rb cs lree and pb chlorite thermometry suggests a formation temperature of 300 350 c for the fault rock and based on the si loss and solubility of silica in hydrothermal fluids the intensity of alteration requires a fluid to rock ratio of up to 900 1 strontium isotope whole rock data of the fault rock yield 87sr 86sr ratios of 0 7043 0 7048 which is considerably more radiogenic than fresh layered gabbro from this locality 87sr 86sr 0 7030 0 7033 and similar to black smoker hydrothermal signatures based on epidote measured from epidote veins in the footwall and elsewhere in the ophiolite 87sr 86sr 0 7043 0 7051 altered gabbro clasts within the fault zone show similar values with 87sr 86sr ratios of 0 7045 0 7050 in contrast the hanging and footwall gabbros display values only slightly more radiogenic than fresh layered gabbro the elevated strontium isotope composition of the fault rock and clasts together with the observed secondary mineral assemblages and calculated mass changes strongly supports the intense interaction with seawater derived up welling hydrothermal fluids active during oceanic spreading assuming that such a fault zone is globally representative of faulting in the lower crust an extrapolation of our results from mass change calculations to elemental fluxes shows a significant contribution to the global hydrothermal budgets of si ti fe mn mg ca h2o cu zn sr and cs

variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge plagioclase crystal size distributions in gabbros from the oman ophiolite 1 analysis of crystal size distributions csd of plagioclase in gabbros from the oman ophiolite indicates that cooling rates in the lower crust beneath a medium to fast spreading oceanic ridge did not vary smoothly with depth cooling rates in the upper half of the gabbro section were approximately 1 5 2 times faster than in the lower half of the gabbro section during igneous crystallization and or high temperature grain growth processes our results are inconsistent with thermal models for oceanic spreading ridges that approximate the effect of hydrothermal convection by using an enhanced thermal conductivity together with a conductive heat transfer mechanism instead the oman csd data are consistent with recent seismic and modeling studies that suggest that hydrothermal convection leads to closely spaced near vertical isotherms in the upper half of the plutonic section within a few kilometers of the ridge axis

cooling rates in the lower crust of the oman ophiolite ca in olivine revisited debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models the thermal profile of the crust specifically the roles of hydrothermal circulation and cooling rate is a key component in distinguishing between these two models results of this study show no systematic variation of cooling rate with depth in the lower crust of the khafifah section in the wadi tayin massif of the oman ophiolite additionally very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust while the conclusions presented here do not rule out the possibility of accretion by gabbro glacier they do contradict the initial tenet of the gabbro glacier model convective heat loss by hydrothermal circulation can penetrate to moho depths and crystallization is not by necessity constrained to the upper crust on the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust

oxygen isotope evidence for short lived high temperature fluid flow in the lower oceanic crust at fast spreading ridges millimeter scale amphibole veins in the lower oceanic crust record fracture controlled fluid flow at high temperatures but the importance of this fluid flow for the thermal and chemical evolution of the lower oceanic crust is unclear in the section of lower oceanic crust recovered at hess deep from odp hole 894g which formed at the fast spreading east pacific rise these veins are randomly distributed with an average spacing of 1 m we unravel the history of fluid flow through one of these veins by combining in situ o isotope analyses of wall rock plagioclase with major element analyses geothermometry and diffusion modeling thermometry indicates vein sealing by amphibole at 720 c over a narrow temperature interval 20 c in situ oisotope analyses by ion microprobe with a precision of b0 5 reveal zoning of o isotopes in plagioclase adjacent to the vein the zoning profiles can be reproduced using a diffusion model if the duration of o isotope exchange was 100 yr a similar interval of fluid rock exchange is suggested by modeling potassium depletion in plagioclase adjacent to the vein if representative of fracture controlled fluid flow in the lower oceanic crust the limited duration of fluid flow and its occurrence over a narrow temperature interval suggest that high temperature fluid flow in this porosity network does not transport significant heat 2007 elsevier b v all rights reserved

guild structure and aggradation pattern of messinian porites patch reefs ecological succession and external environmental control san miguel de salinas basin se spain abstract small and isolated messinian patch reefs showing a typical internal succession and an oligospecific reef building community are described from the western margin of the san miguel de salinas basin se spain these thrived in a mixed carbonate siliciclastic environment and are known from two different stratigraphic levels the reef growth is characterized by a shallowing upward sequence and can be subdivided into four divisions reef debris zone coralline algal zone branching coral zone and reef crest zone this ecological succession of messinian reefs reflects an increasing water turbulence and the accommodation of the reef building organisms to shallower water settings which is conditioned by its own upward growth the start of reef growth is controlled by a reduced influx of terrigenous deposits which derived from fan sediments reworked by waves the reefs were killed by smothering with quartzose calcarenites the low diversity of the reef building community indicates an arrested ecological succession without climax stages controlled by sedimentation rate and siliciclastic supply a salinity increase can be excluded because the reef associated fauna indicates normal marine conditions during reef growth there is no evidence for subaerial exposure of the reefs during the formation of evaporites at the end of the messinian

the betic segment of the lithospheric trans alboran shear zone during the late miocene abstract the betic sinistral transcurrent shear zone betic cordilleras crosses the southeastern part of spain the study of the geodynamics of this area is approached from tectonic sedimentary magmatic and geophysical data large strike slip movements tectono sedimentary processes and magmatic events are seen at the boundary between two different lithospheric slabs they are located in a thinned compartment with a high geothermal gradient the large scale movements vertical and horizontal and the intensity of magmatic activity are the expression of the mechanical behaviour of this hot and thin crust all the geological events from the late miocene to recent are integrated in a crustal scale cross section which shows that the betic sinistral transcurrent shear zone widely affects the lithosphere it appears that subduction zones are not the only geodynamic context in which calc alkaline magmatism occurs

chronometry and speedometry of magmatic processes using chemical diffusion in olivine plagioclase and pyroxenes the magmatic processes that fuel volcanism crustal growth ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales from millions of years to just a few seconds for example the production of new oceanic crust at mid ocean ridges is a near continuous process that can operate in any one ocean basin on timescales of more than 100 m y however the driving force for such processes is the spreading of the ocean plates that happens on a cm yr timescale at the other end of the spectrum explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes in this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure at intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years although each of these processes has a characteristic time averaged timescale on which it operates this is typically the end result of one or more natural processes that operate on much shorter timescales for example mid ocean ridges do not extrude magma continuously onto the ocean floor mineralising fluids do not discharge continuously through the shallow crust and granitic magmas do not dribble continuously into evolving batholithic chambers in some cases it is the long term timescales that are important for example the spreading rate of ocean basins in others it is the short term timescales that are important for example the episodic growth of lava domes at active volcanoes although

time scales of magmatic processes from modeling the zoning patterns of crystals the advent of polarized light microscopy in the middle of the 19th century allowed mineralogists and petrologists interested in igneous rocks to recognize the widespread occurrence of fine scale heterogeneities in the optical properties of minerals e g see young 2003 for details the interpretation of mineral zoning patterns as archives of magmatic processes has been with us for some time e g larsen et al 1938 tomkeieff 1939 the development of the electron microprobe in the 1960 s allowed mineral zoning profiles to be quantitatively analyzed and modeled e g bottinga et al 1966 moore and evans 1967 enhanced textural observations permitted recognition of many kinds of detailed structures during the growth and dissolution of magmatic minerals fig 1 e g anderson 1983 pearce and kolisnik 1990 and these processes were also explored using experimental and numerical models e g albarede and bottinga 1972 lofgren 1972 kirkpatrick et al 1976 loomis 1982 major element zoning patterns are routinely measured and with the arrival of the ion microprobe the identification of heterogeneous distribution of trace elements opened the window for more realistic and sophisticated scenarios and models fig 1 e g kohn et al 1989 blundy and shimizu 1991 singer et al 1995 we now measure abundances of naturally occurring isotopes at the scale of tens of micrometers e g davidson et al 2007a ramos and tepley 2008 and this allows in situ dating of crystals e g cooper and reid 2008 figure 1 a nomarski dic image of a plagioclase crystal from a gabbroic xenolith of volcan san pedro costa et al 2002 the crystal shows a well defined core and a rim separated by an oscillatory zoned interval ion microprobe analysis pits and the trace of the electron microprobe traverse between the pits are evident b major

cooling history of atlantis bank oceanic core complex evidence for hydrothermal activity 2 6 ma off axis 1 we report 26 u th he zircon ages from atlantis bank southwest indian ridge which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly 1200 c ma consistent with existing models for the cooling of oceanic crust u th he zircon ages from samples collected along n s and e w trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust these samples have a mean u th he zircon age 2 6 ma younger than their corresponding igneous crystallization ages and record cooling through 200 c well outside the rift valley similar anomalously young ages are recorded by zircon sphene and apatite fission track data from odp hole 735b we interpret these young ages as recording an off axis thermal heating event associated with localized high temperature 300 c hydrothermal fluid flow resulting from underplated mafic magmas

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b