strontium isotope constraints on fluid flow in the upper oceanic crust at the east pacific rise strontium isotopes are useful tracers of fluid rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems we have determined the whole rock sr isotopic compositions of a section of upper oceanic crust that formed at the fast spreading east pacific rise now exposed at hess deep this dataset provides the first detailed comparison for the much studied ocean drilling program odp drill core from site 504b whole rock and mineral sr concentrations indicate that sr exchange between hydrothermal fluids and the oceanic crust is complex being dependent on the mineralogical reactions occurring in particular epidote formation takes up sr from the fluid increasing the 87sr 86sr of the bulk rock calculating the fluid flux required to shift the sr isotopic composition of the hess deep sheeted dike complex using the approach of bickle and teagle 1 m j bickle d a h teagle strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems earth planet sci lett 113 1992 219 237 gives a fluid flux similar to that determined for odp hole 504b this suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust unfortunately uncertainties in the modeling approach do not allow us to determine a fluid flux that is directly comparable to fluxes calculated by other methods

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

copper and zinc isotope systematics of altered oceanic crust at iodp site 1256 in the eastern equatorial pacific this paper presents the first combined cu and zn isotopic study of altered oceanic crust aoc at iodp hole 1256d that penetrates a volcanic section a lava dyke transition zone a sheeted dyke complex and a plutonic complex in the volcanic section all but one rocks have cu and zn isotopic compositions similar to fresh mid ocean ridge basalt morb reflecting restricted seawater circulation and low oxygen fugacity rocks in the transition zone have morb like 65cu and 66zn indicating the dominant influence of basalt derived cu and zn during alteration rocks in the dyke complex have more variable 65cu 0 50 0 90 and 66zn 0 19 0 55 and those in the plutonic complex have 65cu of 0 43 to 0 20 and 66zn of 0 21 to 0 41 the rocks with heavier 66zn and heavier or lighter 65cu relative to morb are characterized by cu zn depletions low li yb 1 0 and low 18o 5 suggesting that hydrothermal extraction during high temperature alteration of oceanic crust can result in significant cu and zn isotope fractionation such large cu and zn isotopic variations are the results of redox transformation of cu as well as cu and zn isotope fractionation between altered basaltic rocks and dissolved cu and zn species in hydrothermal fluids e g cucl3 1 zn hs 42 this work is the first to define the distribution of cu and zn isotopes in an intact oceanic crust with concentration weighted averages of 65cu 0 05 0 03 and 66zn 0 27 0 01 the potential implications of these new observations are discussed

accumulations of melt at the base of young oceanic crust as hot solid rock ascends through the mantle the pressure around it decreases while its temperature decreases approximately adiabatically during the ascent the melting temperature decreases faster than the adiabat temperature and if the initial temperature of the rock is high enough the rock will begin to melt at the depth where the adiabat crosses the melting temperature curve models of upwelling beneath mid ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge axis region suggesting that some of the ascending melt must arrive away from the ridge axis here i report ocean bottom seismograph observations of an uncommon and previously unreported marine seismic phase which provide strong evidence that melt accumulates in sills at the base of young oceanic crust the phase results from the conversion of compressional p waves to shear s waves on interaction with the crust mantle boundary the moho this phase is rare and very difficult to model but when visible it is strikingly prominent and usually appears at unexpectedly great source to receiver ranges the observations imply the presence of bodies of extremely low rigidity presumably melt sills at the moho close to the receiver melt may be fairly mobile at the base of the crust allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle escape of this melt to the sea floor may be responsible for the formation of off axis seamounts

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

depth shifting cores incompletely recovered from the upper oceanic crust iodp hole 1256d seafloor drilling operations especially those in crustal rocks yield incomplete recovery of drilled sections and depths of the recovered core pieces are assigned with some uncertainty here we present a new depth shifting method that is simple and rapid requires little subjective input and is applicable to any core log integration problem where sufficient comparable data have been collected in both the open hole and from the recovered core over the depth range for which both core and log data have been collected an automatic algorithm selected the best new depth for each piece the criteria for determining the best depth were as follows 1 find new depths for as many pieces as possible and 2 minimize the difference between core density and log density in this study depth shifting is applied at integrated ocean drilling program iodp hole 1256d which is our first opportunity to study a section of intact in situ upper ocean crust drilled down to gabbro the new depths significantly improve the agreement between an independent data set and the logging record

characterization of transport and water retention properties of damaged callovo oxfordian claystone in the context of the underground storage of radioactive waste the aim of this experimental study is to characterize the effect of damage on transport and water retention properties of callovo oxfordian cox argillite the originality of the study is to simultaneously investigate the pore size distribution water retention the dry effective and relative gas permeability and the gas breakthrough pressure gbp of damaged cox argillite these different properties are all relevant to characterizing the fluid transport ability of cox argillite results show that the damage has a significant impact on the properties of the cox argillite it induces a decrease in its water retention capacity and gbp and it increases its gas permeability and apparent porosity available to water owing to the creation of micro cracks another objective is to show which of these properties is the most suitable to detect early damage states in cox argillite with a potential use being to identify them in situ gbp appears to be the best detector of damage because of its sensitivity to damage even under high confinement pressures gas permeability could be a good indicator of damage as it increases significantly one or several orders of magnitude after the damage finally the water permeability curve is a poor indicator of cox argillite damage

accretion and oxidation of a superfast spread axial melt lens tims and sims zircon analyses of the iodp hole 1256d gabbros abstract iodp hole 1256d penetrates a complete section of upper oceanic crust spread from the east pacific rise at a full rate of up to 220 mm yr here we present u pb zircon data from hole 1256d gabbroic rocks using isotope dilution thermal ionization mass spectrometry id tims and secondary ion mass spectrometry sims three gabbro samples yielded th corrected weighted mean 206pb 238u sims dates of 15 03 0 14 ma 15 13 0 35 ma and 15 23 0 12 ma an id tims th corrected weighted mean 206pb 238u date of 15 191 0 040 ma from the first of the three samples provides verification and refinement of the timing of magmatism the u pb dates agree with the predicted age of the crust based on marine magnetic anomalies suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis zircons from two of the studied samples have anomalously high th u ratios likely reflecting oxidation of u prior to or during zircon crystallization the elevated o2 of late stage melts in the axial melt lens may be related to progressive crystallization assimilation of hydrothermally altered sheeted dikes and gabbros and or contamination of the melt lens by seawater derived saline brines

the roof of an axial magma chamber a hornfelsic heat exchanger the heat flux from an axial magma chamber into the convecting hydrothermal system at mid ocean ridges is governed by the conductive boundary layer separating them the nature of this critical interface has been modeled and seismically imaged but its characteristics have hitherto rarely been documented in ocean crust here hornfelsic rocks from the sheeted dike gabbro transitions at two tectonic exposures of fast spreading east pacific rise crust at the pito and hess deeps and in the oman and troodos ophiolites are described these rocks record thermal metamorphism to hornblende and pyroxene hornfels at 700 1000 c these temperatures in conjunction with their geological relationships imply that the hornfels are preserved fragments of conductive boundary layers their distribution at the base of sheeted dike complexes in narrow contact aureoles and dikes intruding the uppermost gabbros tracks the vertical migration of axial magma chambers over minimum depth ranges of 200 400 m the heat flux across the hornfelsic aureoles 11 44 mw km is comparable to hydrothermal fluxes released along fast spreading ridges confirming that the heat driving hydrothermal convection is transferred across conductive boundary layers

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

how crack porosity and shape control seismic velocities in the upper oceanic crust modeling downhole logs from holes 504b and 1256d mounting evidence from holes 504b and 1256d suggests that porosity is the principal factor affecting velocities in the upper oceanic crust spheroidal inclusion and asperity compression models based on reprocessed sonic velocity logs and apparent fractional porosities estimated from deep resistivity logs reveal how both porosity and the geometry of the pore space affect seismic velocities in layer 2 models that best match the data indicate the following first there are three populations of cracks in hole 504b most of the transition zone and the dike section are populated by low concentrations of weak cracks while the upper part of the transition zone and the extrusive pile contain a higher concentration of stiffer cracks similarly the deepest part of the dike section in hole 1256d can be modeled by a dilute concentration of thin or weak cracks while the overlying dikes the transition zone and the extrusive pile contain a higher concentration of stiffer cracks second in a striking confirmation that porosity controls velocities throughout layer 2 90 or more of the variance of sonic velocities logged at both sites is explained by porosity in these models while third the effect of pressure on the variation of sonic velocity is essentially negligible fourth while there is no direct correlation between crack populations and igneous lithostratigraphy some changes of crack population do correspond to metamorphic transitions and fifth velocities typical of layer 3 are reached when the porosity falls to low values 0 2 in hole 1256d and 0 6 in hole 504b

apatite in the dike gabbro transition zone of mid ocean ridge evidence for brine assimilation by axial melt lens we present textures and halogen and trace element compositions of apatites in intrusive rocks from the dike gabbro transition zone of a fast spreading mid ocean ridge which was formed at the east pacific rise and recently drilled by iodp hole 1256d these data are used to discuss the properties of parental magmas and seawater derived hydrothermal fluids at the roof of the axial melt lens during the formation of oceanic crust in general zoning of apatites from three different lithologies tonalites diorites and gabbros is common and shows a consistent evolution trend with depletion in cl and rees from core to rim the cores are usually homogenous in composition and interpreted as magmatic origin whereas zones with lower cl and rees are disseminated with heterogeneous concentrations indicating exchanges with hydrothermal fluids the apatite cores in tonalites are rich in both f and cl with x ap f proportion of fluorapatite end member up to 0 5 and x ap cl proportion of chlorapatite end member up to 0 4 in contrast the apatite cores in gabbros have high x ap cl up to 0 85 and very low x ap f s in some parts of the apatite crystals mainly rim can be explained by removal of these components via hydrothermal fluids based on available f cl oh exchange coefficients for apatite melt the very high cl oh and cl f ratios and high cl contents calculated for tonalitic melts cannot be reconciled with a formation of these felsic melts by partial melting of amphibole bearing metabasalts but indicate that an assimilation of high cl brines must have occurred similarly the low f chlorapatites in gabbros also imply an assimilation of high cl brines the source of high cl fluids in the axial magmatic system may result from seawater derived fluids which may form immiscible vapor and brine at high temperatures as a result of hydrothermal boiling

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer insights from the east pacific rise at lat 16 n seismic reflection data from the east pacific rise at lat 16 n which is spreading at the high end of intermediate rates suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate rather magma chamber depths form two distinct populations each associated with a distinct axial morphology and with an abrupt transition occurring within the intermediate spreading rate range our data 1 show that the melt lens at high intermediate spreading ridges lies at a shallow level similar to lens depths at faster spreading ridges and 2 provide further support for the spreading rate invariance of ridges with axial highs noted in other ridge properties the axial morphology of the two ridge segments within the study area differs markedly and a large contrast in magma supply is inferred the ridge segment with greater magma supply is associated with a broader and more continuous melt lens a wider region over which the extrusive crust accumulates and a thicker extrusive layer off axis where supply to the ridge segment appears to be centered however on axis the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

magma distribution across ridge axis discontinuities on the east pacific rise from multichannel seismic images detailed studies of the morphology of the east pacific rise axis have shown that its linearity is disrupted by many small but distinct non transform offsets including overlapping spreading centres oscs and deviations from axial linearity devals which display variable geochemical signals1 9 using multichannel seismic reflection profiling we have mapped the distribution of a bright shallow reflector that detrick et al 10 have associated with an axial magma chamber we have found that it is neither continuous across the 9 03 osc6 nor separated into two parallel bodies2 and that its lateral offset does not conform to that of the topographic offset associated with the 9 17 deval these observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the east pacific rise and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery

drilling to gabbro in intact ocean crust sampling an intact sequence of oceanic crust through lavas dikes and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid ocean ridges but it has been an elusive goal of scientific ocean drilling for decades recent drilling in the eastern pacific ocean in hole 1256d reached gabbro within seismic layer 2 1157 meters into crust formed at a superfast spreading rate the gabbros are the crystallized melt lenses that formed beneath a mid ocean ridge the depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid ocean ridges melt lenses occur at shallower depths at faster spreading rates the gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by hole 1256d

velocity and attenuation in young oceanic crust new downhole log results from dsdp odp iodp holes 504b and 1256d 1 to characterize the acoustic properties of oceanic crust and investigate the nature of the oceanic layer 2 3 seismic boundary we analyze sonic logging waveforms recorded in the two deepest holes ever drilled in intact oceanic crust ocean drilling program odp integrated ocean drilling program iodp hole 1256d and deep sea drilling program dsdp odp hole 504b hole 1256d penetrated 15 ma old crust on a superfast spreading segment of the cocos plate while hole 504b was drilled in a 6 ma old section of an intermediate spreading segment of the nazca plate careful reprocessing of different sets of waveforms allows us to draw depth profiles of the primary parameters controlling wave propagation in the crust compressional and shear velocities vp and vs and attenuation the results underline the importance of proper filtering and processing parameters for the acquisition of acoustic logs in hard rocks while the reprocessed vp logs improve previous data particularly at the bottom of hole 1256d the vs logs represent significant enhancements in quality and in extent of previous profiles in the two holes attenuation results provide the first measure of intrinsic attenuation in these holes the velocity and attenuation logs confirm that hole 504b had reached oceanic layer 3 without recovering gabbros conversely while gabbros were recovered in hole 1256d no layer 3 velocity values were recorded in the interval logged however the reprocessed data suggest that the transition to layer 3 might occur within the deepest 87 m of the hole that were not logged or immediately below accordingly the layer 2 3 seismic boundary does not appear to be related to the top of the plutonic crust despite different ages spreading rates and mineralogy however the two sites display remarkably similar velocity and attenuation profiles through the entire crustal sections logged suggesting that they can be reliably considered representative of the acoustic properties of the nazca cocos spreading system and more generally of undisturbed oceanic crust

oxygen isotope composition of a section of lower oceanic crust odp hole 735b 1 the oxygen and hydrogen isotope compositions of vein minerals and 18o values of 64 whole rocks from the deeper kilometer of the 1 5 km lower crustal section of odp hole 735b in the sw indian ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust vein amphiboles have d values of 35 to 65 indicating formation from seawater fluids and hydrothermal fluids containing a component of low d magmatic fluid crystal plastic shear zones reacted with 18o enriched fluids at high temperatures and have 18o values of 6 0 7 0 indistinguishable from surrounding rocks reaction with seawater dominated fluids over a range of temperatures 350 700 c led to local 18o depleted rocks but much of the section exhibits 18o enrichment 18o up to 7 6 as the result of static alteration of plagioclase over a range of temperatures 250 to 600 c and fluid compositions chloritic veins and associated alteration by 18o enriched fluids occurred at temperatures of 150 250 c and late carbonates and fe oxyhydroxides formed from cold 10 seawater associated with open fault related fractures bulk rock 18o values of the lower 1 km range from 3 7 to 10 2 but are mostly 5 6 7 6 and the section has a mean bulk 18o of 6 5 1 0 in contrast rocks from the shallower 500 m exhibit net 18o depletion 5 1 1 0 reflecting high temperature deformation and seawater hydrothermal alteration stakes 1991 the weighted average 18o for the entire gabbro section of hole 735b is 6 0 exhibiting little or no change compared to unaltered crust although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low temperature alteration and 18o enrichment there is no evidence for a consistent difference in the bulk 18o of the plutonic sections of oceanic crust formed at fast versus slow spreading rates

origin and transport of trace metals deposited in the canyons off lisboa and adjacent slopes portuguese margin in the last century abstract submarine canyons play an important role in the transfer of contaminated sediments from shelf areas to the deeper ocean to evaluate the importance of submarine canyons adjacent to the tagus and sado estuaries portuguese margin as sediment pathway major and trace elements 210 pb radionuclides organic carbon calcium carbonate and grain size were analyzed in fifteen short sediment cores from the cascais and lisboa setubal submarine canyons and neighbor slopes at the estremadura spur and off sines mass accumulation rates mar vary between 0 02 and 0 40 g cm 2 y 1 with the highest values in the upper cascais and lisboa canyons mercury and pb enrichment factors exceed 2 and show a continuously increasing trend since 1950 ad in the upper sections of the cascais canyon and in the lisboa setubal canyon system implying an anthropogenic source mercury and pb anthropogenic inventories also present highest values in the canyon heads which is consistent with its proximity to the tagus and sado river mouths to a lesser extent the estremadura and sines slopes and the deeper sections of the canyons also have slightly increasing hg and pb enrichment factor trends towards the present day for these areas with a major influence of pelagic sedimentation we infer atmospheric transport as a complementary source of contaminants therefore our data suggest that although canyons are preferential conduits to transport sediments to the deep sea when compared to the slopes hg and pb derived from human activities have reached both domains with different intensities down to a depth of 2000 mwd

tracing the chemical evolution of fluids during hydrothermal recharge constraints from anhydrite recovered in odp hole 504b a unique record of the chemical evolution of seawater during hydrothermal recharge into oceanic crust is preserved by anhydrite from the volcanic sequences and sheeted dike complex in odp hole 504b chemical and isotopic analyses 87sr 86sr 18o 34s of anhydrite constrain the changing composition of fluids due to reaction with basalt there is a general trend of decreasing 87sr 86sr of anhydrite corresponding to the minor incorporation of basaltic strontium with depth in the volcanic rocks 87sr 86sr ratios decrease rapidly with depth in the dikes to values identical to host basalt 0 7029 sr ca ratios 0 1 mmol mol suggest that recharge fluids have very low sr concentrations and fluids evolve by first precipitating sr bearing phases before extensive exchange of sr with the host basalt there is a background trend of decreasing sulfate 18o with depth from 12 13 in the lower volcanics to 7 in the lower sheeted dikes recording an increase in recharge fluid temperature from 150 to 250 c and confirming the presence of sulfate in hydrothermal fluids at elevated temperatures from the amount of anhydrite recovered from hole 504b and the amount of seawater sulfur that has been reduced to sulfide a minimum seawater recharge flux can be calculated this value is 4 25 times lower than estimates of high temperature fluid fluxes based on either thermal constraints or global chemical budgets and suggests that there is significant deficit of seawater derived sulfur in the oceanic crust only a minor proportion of the seawater that percolates into the crust near the axis is heated to high temperatures and exits as black smoker type fluids a significant proportion of the axial heat loss must be advected at 200 250 c by sulfate bearing hydrothermal solutions that egress diffusely from the crust these fluids penetrate into the dikes and exchange both heat and chemical tracers without the extensive clogging of porosity by anhydrite precipitation which would halt hydrothermal circulation for any reasonable fluid flux the heating of the major proportion of hydrothermal fluids to only moderate temperatures 250 c reconciles estimates of hydrothermal fluxes derived from thermal models and global geochemical budgets the flux of hydrothermal sulfate would be of a magnitude similar to the riverine input and oxygen isotopic exchange at 200 250 c between dissolved sulfate and recharge fluids during hydrothermal circulation provides a mechanism to continuously buffer seawater sulfate oxygen to the light isotopic composition observed

recharge flux to ocean ridge black smoker systems a geochemical estimate from odp hole 504b we use the sr isotopic composition of rock and anhydrite as a monitor of fluid composition in dsdp odp hole 504b to calculate the recharge flux to the axial high temperature hydrothermal circulation the fluid and rock sr isotope profiles are well fit by a tracer transport mass balance model that approximates fluid solid exchange by linear kinetics the calculated time integrated flux of 1 7 0 2 x 106 kg m 2 is significantly less than the 5 x 106 kg m 2 calculated from thermal models that assume all magma is intruded into a high level magma chamber at the base of the sheeted dykes our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in hole 504b and the intrusion of magma as lenses distributed through the lower oceanic crust it leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balance

discovery of epidosites in a modern oceanic setting the tonga forearc the first suite of oceanic epidosites has been recovered from the tonga forearc epidosites are metal depleted rocks composed of subequal proportions of epidote and quartz that lack any original igneous texture epidosites have been interpreted to form in upflow zones at the base of ore forming oceanic hydrothermal systems that vent as black smokers on the sea floor tongan epidosites metasomatically replaced basaltic and plagiogranite protoliths and formed under similar conditions to epidosites hosted in many ophiolites interpreted to have formed in a suprasubduction zone setting the similarities shared by the tongan and suprasubduction zone ophiolite epidosites along with the absence of epidosites in collections from modern mid ocean ridges suggest that tectonic setting plays a role in their formation

crustal structure beneath the gravity lineations in the gravity lineations intraplate melting petrologic and seismic expedition glimpse study area from seismic refraction data 1 the active source seismic component of the gravity lineations intraplate melting petrologic and seismic expedition glimpse experiment investigated the velocity structure and crustal thickness of the pacific plate along a 450 km line paralleling the southern east pacific rise where prominent intraplate ridges lie within lows of the lineated free air gravity pattern compressional wave traveltime data collected by 10 ocean bottom seismometers indicate that the median crustal thickness is 6 2 km and increases to 8 2 km beneath the sojourn ridge the conspicuous absence of crustal thinning beneath both the sojourn and hotu matua ridge systems is strong support against a boudinage style model for the formation of these volcanic complexes our results suggest the ridge loads are partially supported by both plate flexure and underplating with an additional mantle component inferred from the analysis of gravity data rayleigh wave tomography and teleseismic body wave delays small scale convection could produce the observed crustal thickness pattern but fails to explain the geochemical and formation age trends along the ridges we prefer a model where the channelized flow of low density material in the upper mantle possibly undergoing small scale convection is advected toward the ridge crest lows in the gravity field reflect the density contrast within the asthenosphere and decompression melting amid upwelling limbs of the convective cells sustains the development of the associated ridge systems

structural contribution from the oman ophiolite to processes of crustal accretion at the east pacific rise a comprehensive model for the activity of the elementary accretion segment at fast spreading ridges relies on integration of structural data from the oman ophiolite and geophysical results from the east pacific rise epr around 9 n which are of comparable size and spreading rates the axial melt lens at shallow crustal level provides a link between deval segmentation at the seafloor and a lower melt sill at moho level imaged at the epr as a crustal melt zone cmz and mapped in oman as the moho transition zone mtz both are attached to a mantle upwelling at the epr and to a frozen diapir in oman the physical link between diapiric mantle uprising at the moho and devals segmentation at the seafloor is the melt being injected from the mantle into the lower mtz ponding there and then being released by powerful injections into the upper melt lens the magma chamber covers the diapir at a distance of 5 km from the ridge axis

subaqueous lava flow lobes observed on rov kaiko dives off hawaii remotely operated vehicle rov kaiko dives north of oahu island hawaii and on the lower south rift zone of loihi seamount revealed diverse flow morphologies of submarine lava that correlate with slope and rate of lava delivery steep to moderate 10 slopes are covered with elongate pillows and narrow pahoehoe streams bulbous pillows and smooth pahoehoe lobes occur on flat areas and gentle slopes some gentle slopes are covered by lobate sheet flows that supply pillow flows smooth pahoehoe lobes change upslope into lobate sheets indicating that the sheets form by coalescence and inflation of successively emplaced flow lobes many pahoehoe flows contain hollow tumuli like lobes that have inflated and collapsed thin crusts 4 20 cm and large volumes 0 7 1050 m 3 of such inflated lobes suggest lava supply rates of 0 01 8 m 3 min these calculated supply rates are more than one order of magnitude larger than those for subaerial tumuli in iceland thinner viscoelastic layers of subaqueous lobes at the time of inflation allowed higher excess pressures and expansion rates

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

geometry conditions and timing of off axis hydrothermal metamorphism and ore deposition in the solea graben the recent discovery of fossil spreading axes along the northern margin of the troodos ophiolite1 provides a unique opportunity to compare a fossil seawater hydrothermal system with systems of modern oceanic spreading centres2 6 whereas previous accounts of ophiolite hydrothermal systems7 13 including troodos14 16have elucidated many aspects of sea floor hydrothermal metamorphism earlier workers have not placed ophiolite hydrothermal features within any specific spreading centre structural framework most ophiolites exhibit complex histories of post accretion deformation that mask or obliterate early structures related to seafloor spreading the troodos ophiolite is apparently still in the process of being emplaced 17 and most structural elements at troodos appear to have originated beneath a cretaceous sea floor1 we have discovered an extensive area of intense seawater hydrothermal alteration within the sheeted dyke complex of the solea graben fig 1 the metamorphic petrology and oxygen isotope geochemistry of 100 dyke samples from this graben reveal the size and three dimensional geometry of the hydrothermal system and yield estimates of temperature salinity oxygen isotope composition and recharge discharge geometry for fluids that transported the constituents of massive sulfide orebodies geometric discordance between the trends of the hydrothermally altered area and of earlier seafloor spreading structures compels us to consider a post spreading off axis origin for the orebodies of the solea graben

mobility of au and related elements during the hydrothermal alteration of the oceanic crust implications for the sources of metals in vms deposits volcanogenic massive sulphide vms deposits are commonly enriched in cu zn and pb and can also be variably enriched in au as sb se and te the behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known ocean drilling program odp hole 1256d penetrates a complete in situ section of the upper oceanic crust providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration a representative suite of samples was analysed for au as sb se and te using low detection limit methods and a mass balance of metal mobility has been carried out through comparison with a fresh mid oceanic ridge basalt morb glass database the mass balance shows that au as se sb s cu zn and pb are depleted in the sheeted dyke and plutonic complexes by 46 12 27 5 2 5 0 5 27 6 8 4 0 7 9 6 1 6 7 9 0 5 and 44 6 respectively arsenic and sb are enriched in the volcanic section due to seawater derived fluid circulation calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as vms mineralisation the quantity of au mobilised and the ratio of au to base metals are similar to those of mafic vms and ten times enrichment of au would be needed to form a au rich vms the cu rich affinity of mafic vms deposits could be explained by base metal fractionation both in the upper sheeted dykes and during vms deposit formation

relationship between spreading rate and the seismic structure of mid ocean ridges different parts of the global mid ocean ridge system create oceanic crust at rates that differ by more than a factor of ten the rates of supply of heat to these different systems must also vary substantially and it is this heat supply in the form of hot mantle material rising beneath the ridge axis that determines the nature of the magmatic tectonic and hydrothermal processes that control the production of new oceanic crust here we present evidence suggesting that a systematic relationship exists between the depth to zones of elevated temperature and partial melt beneath mid ocean ridges and their spreading rate recent seismic measurements provide robust determinations at six different locations of the depth to the top of the axial low seismic velocity zone which suggest that as full spreading rate decreases from 150 to 20 mm yr 1 the depth below the sea floor to the top of the lvz increases from 1 to 4 km although this relationship might arise in a number of ways our preferred explanation is that the major faults that extend to greater depths on slower spreading ridges1 allow a vigorous circulation of cooling sea water to reach greater depths blocking the upward migration of large melt bodies

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

hydrothermal circulation anhydrite precipitation and thermal structure at ridge axes the thermal structure of the axial region of ridges spreading at 60 mm yr 1 was studied by comparing observations with numerical thermal models data from deep sea drilling project hole 504b constrain the temperature in the pillows and the upper part of the dike complex seismic reflection data constrain the depth to the magma chamber magnetic anomalies constrain the accumulation rate of pillows the thermal models assumed steady state and included the kinematics hydrothermal flow and the accumulation of pillows basalts in the successful thermal models hydrothermal fluid entered the dike intrusion zone at the ridge axis over a depth range from the base of the pillows to about 0 8 km above the top of the magma chamber the computed temperature at the axis in this depth range was nearly isothermal and suitable for the source regions of black smoker vents the computed temperature grid and assumed flow grid were used to model anhydrite precipitation and sulfate transport and thus more fully examine the implications of the flow pattern anhydrite was precipitated as seawater approached the hot axial zone the anhydrite was the carried outward with the spreading seafloor and redissolved when it reacted with pristine downwelling seawater the net effect was to deposit anhydrite near the base of vigorous hydrothermal circulation where it could not easily be leached by lower temperature off axis circulation and returned to the ocean this is a general feature of flow patterns where cold seawater vigorously enters the axial zone in broad downwellings that are not strongly heated by conduction at shallow depths alternatively the cold recharge is concentrated in the axial region and exchanges heat with nearby upwelling flow in the transition zone beneath the pillows anhydrite is then precipitated at shallow depths where it can later be leached back into the ocean the latter possibility is favored by studies of geochemical mass balances as well as the lack of evidence for a deep anhydrite layer in the oceanic crust

in situ evidence for the nature of the seismic layer 2 3 boundary in oceanic crust the igneous oceanic crust is typically thought of as comprising two layers an upper crust seismic layer 2 characterized by a rapid increase in seismic velocity with depth and a thicker lower crust seismic layer 3 which is distinguished from layer 2 by both a higher p wave velocity 6 69 0 26 km s l and a much smaller vertical velocity gradient 1 8 km continuous hole drilled into the oceanic crust11 13 and find that at this location the seismic layer 2 3 boundary lies within the sheeted dyke complex where it is associated with gradual downhole changes in crustal porosity and alteration not a litho logical transition from sheeted dykes to gabbro

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at odp iodp site 1256 from a cluster analysis of rock magnetic properties we analyze magnetic properties from ocean drilling program odp integrated odp iodp hole 1256d 6 44 1 n 91 56 1 w on the cocos plate in 15 2 ma oceanic crust generated by superfast seafloor spreading the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting fuzzy c means cluster analysis and nonlinear mapping are utilized to study down hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization the coercive force the ratio of the remanent coercive force and coercive force the low field magnetic susceptibility and the curie temperature to evaluate the effects of magmatic and hydrothermal processes on magnetic properties a statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas which differ from two distinct clusters mainly representing the dikes and gabbros extensive alteration can obliterate magnetic property differences between lavas dikes and gabbros the imprint of thermochemical alteration on the iron titanium oxides is only partially related to the porosity of the rocks thus the analysis complements interpretation based on electrofacies analysis all clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and or the presence of intergrowths the upper part of the extrusive sequence the granoblastic dikes and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization

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

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

correlation between volcanic and tectonic segmentation of fast spreading ridges evidence from volcanic structures and lava flow morphology on the east pacific rise at 9 10 n 1 combined analyses of volcanic features in dsl 120 sonar data and argo i images along the ridge crest of the east pacific rise 9 09 54 n reveal a consistent decrease in inferred lava effusion rate toward the ends of third order segments the correlation of tectonic segmentation and volcanic style suggests that third order segmentation corresponds to the volcanic segmentation of the ridge along axis changes in volcanic structures from collapse troughs to basaltic lava domes and lava morphology from sheet to pillow flows coincide with the boundaries of morphologically defined third order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry pillow lava flows cover 25 of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third order segment ends an additional 25 of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough the remaining terrain consists of lobate lava flows we interpret the spatial correlations of morphologic structural seismic and petrologic data as evidence that individual volcanic plumbing systems are organized at 20 km spacing along the ridge axis third order segment scale in agreement with the hypothesis that volcanic and tectonic segmentations are correlated for fast spreading ridges we estimate that the longevity of volcanic segments is 104 105 years 1 3 orders of magnitude longer than fourth order segments 102 103 years this implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable

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

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

geologic structure of the uppermost oceanic crust created at fast to intermediate rate spreading centers abstract geological investigations of major fault scarps tectonic windows and dsdp odp drill holes provide direct views of the uppermost oceanic crust generated at fast to intermediate rate spreading centers these areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing inward dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of outward dip widespread fracturing faulting and hydrothermal metamorphism accompanied magmatic construction these geological relationships can be interpreted in terms of dramatic asymmetrical subaxial subsidence of upper crustal rock units that diminishes across the very narrow few kilometers wide zone of lava accumulation and dike intrusion at the ridge axis this type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

evaluation of ocean crustal sites 1256 and 504 for long term co2 sequestration 1 geologic storage of co2 in ocean basalt reservoirs is a potentially long term solution to offset anthropogenic greenhouse gas emissions prior drilling at deep sea drilling project dsdp ocean drilling program odp integrated ocean drilling program iodp sites 1256 and 504 provides evidence that the extrusive ocean crust has sufficient porosity and permeability to enable storage of large volumes of co2 six intervals are identified as potential reservoirs in the shallow crust at sites 1256 and 504 with new and reprocessed estimates of porosity from electrical resistivity logs ranging from 6 to 14 and permeability from 10 14 to 10 15 m2 calculations using specific reservoir thickness and porosity estimates suggest that even the smallest reservoir could provide storage capacity for decades of global anthropogenic carbon emissions in situ hydrologic experiments and pilot injection studies are needed to confirm high permeability and porosity estimates at sites 1256 and 504 as well as the potential for co2 injection and retention in these basalt reservoirs

metal depleted root zones of the troodos ore forming hydrothermal systems cyprus the cupriferous pyrite deposits of cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient troodos spreading centre here we identify the zones in which this interaction took place the zones occur in the lower part of the sheeted dyke complex and within them 30 50 of the rock is made up of epidosite an epidote quartz rock replacing the dykes as sheets and pipes the epidosites contain abundant fluid inclusions which give trapping temperatures of 350 400 c or even higher and contain water normally near seawater in salinity zones of epidosite are elongate parallel to the strike of the sheeted dykes and are up to 1 km wide the rocks throughout these zones are strongly depleted in cu and zn and the metals removed are sufficient to supply the ore deposits in fact several large ore deposits lie along strike from zones of epidosite all of these features support the identification of the epidosites as the hydrothermal reaction zones the location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore forming systems came from the underlying magma as is also likely for modern black smoker springs

fluid circulation in the depths of accretionary prisms an example of the shimanto belt kyushu japan accretionary prisms constitute ideal targets to study fluid circulation and fluid rock interactions at depths beyond the reach of active margin deep drilling the highest grade rocks from the shimanto belt on kyushu were buried under 3 5 kbars at 300 c toriumi and teruya 1988 they contain abundant quartz veins formed throughout burial and exhumation and variably affected by brittle and ductile deformation cathodoluminescence cl reveals the existence of two distinct types of quartz characterized by a blue and brown color respectively cl blue quartz fills macro veins width 10 m while cl brown quartz is present in micro veins width 1 10 m and ductilely recrystallized domains on the basis of microstructures the fluids associated with the cl blue and cl brown quartz are interpreted as external and local respectively quartz growth rims of alternating cl colors as well as mutually cross cutting veins show that the two fluids cyclically wetted the host rock from fluid inclusions analysis the fluid associated with cl blue quartz has a salinity similar to seawater while the fluid associated with cl brown quartz is less saline in addition cl blue quartz is richer in aluminum than the cl brown one in contrast to the salinity aluminum signature the 18o isotopic signature of both quartz types is similar and buffered by host rock the difference between the preservation of the salinity signature of the fluid and the loss of its 18o signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration the local fluid associated with cl brown quartz reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals the external fluid associated with cl blue quartz is interpreted as seawater or pore water from shallow depth 1 2 km below seafloor sediments we propose that downward percolation of shallow water to depths 10km is a transient process associated with mega earthquakes

continuous mantle melt supply beneath an overlapping spreading center on the east pacific rise tomographic images of upper mantle velocity structure beneath an overlapping spreading center osc on the east pacific rise indicate that this ridge axis discontinuity is underlain by a continuous region of low p wave velocities the anomalous structure can be explained by an approximately 16 kilometer wide region of high temperatures and melt fractions of a few percent by volume our results show that oscs are not necessarily associated with a discontinuity in melt supply and that both osc limbs are supplied with melt from a mantle source located beneath the osc we conclude that tectonic segmentation of the ridge by oscs is not the direct result of magmatic segmentation at mantle depths

sulphide mineral evolution and metal mobility during alteration of the oceanic crust insights from odp hole 1256d fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere supporting microbial life an

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material