a simple magma driven thermal balance model for the formation of volcanogenic massive sulphides abstract a simple model is used to examine the balance of heat water flow and temperature in a long lived single pass hydrothermal system heated by magmatic heat the geometry of the system is based on mid ocean ridge observations cold water percolates downward to the top of a shallow magma chamber is heated by conduction through a thin boundary layer and discharges through a permeable fault zone modelled as a bundle of narrow rough pipes as heat input is increased the model shows a catastrophic temperature transition at an exit water temperature of between 340 and 410 c depending on seafloor pressure which in nature would probably be represented by surging flow and sudden changes in the specific volume of the circulating fluids to generate a 3 million tonne sulphide deposit with 70 efficiency of deposition from water at 350 c within 4000 years would require a heat flux of 110 w m 2 from the magma chamber for a mass flow rate of 140 kg s 1 if the plan area of the reacting part of the system is 2 km 2 the total heat input would be 3 10 19 j under these conditions the thickness of the solid boundary layer between liquid magma and circulating water will be about 10 m and the integrated reactive water rock ratio over the lifetime of the system will be about 30 decreasing the water temperature to 250 c would require a heat flux of only 60 w m 2 but the time to form a 3 million tonne deposit is increased to 33 000 years and the overall heat requirement is more than quadrupled

deformation pattern in a massive ponded lava flow at odp iodp site 1256 a core and log approach 1 a complete in situ section of upper oceanic crust from extrusive lavas through dikes into gabbros has been recently drilled for the first time in 15 ma old crust created at the superfast spreading east pacific rise epr the upper igneous basement cored at ocean drilling program odp integrated ocean drilling program iodp holes 1256c and 1256d consists of sheet flows and massive flows including a ponded lava flow unit lava pond interpreted as a thick lava flow delivered off axis and accumulated in a topographic depression fine scale structural analysis shows that the lava pond is much less fractured than the deeper lavas probably as the effect of structure spacing that is larger than in the underlying lava flows in hole 1256d structures are mainly subhorizontal in the lava pond and become steeper downhole toward the sheeted dike complex this reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth inducing mostly subvertical eruptive fractures and fracture network and the distance of the site from active rifting at the epr contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns as obtained by shipboard and downhole logging data electrical resistivity compressional velocity and bulk density are generally higher whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole the occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and consequently its permeability regime which in turn controls the geometry and distribution of hydrothermal circulation and basalt alteration

the rootzone of an ancient hydrothermal system exposed in the troodos ophiolite cyprus abstract a well exposed oceanic hydrothermal rootzone in the troodos ophiolite cyprus is composed of the basal sheeted dikes the upper 50 100 m of the gabbroic sequence and a contact aureole interpreted as a fossil conductive boundary layer the basal sheeted dikes and upper gabbros were pervasively altered at high temperatures 400 775 c and cut by a series of broadly distributed mineralized faults that are oriented parallel to the strike of the sheeted dike complex fluid inclusion and mineralogical data for fault breccias show that circulating fluids were 350 400 c fairly oxidizing and relatively h2s rich with a range in salinity 1 5 7 8 wt nacl equivalent field and petrological evidence suggest that the rootzone migrated from the basal sheeted dikes into the upper gabbros and back again perhaps for several cycles these migrations and p t oscillations in the conductive boundary layer were likely caused by fluctuations in thermal conditions imposed by the magmatic system at a spreadin

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

constraints on the energy and chemical balances of the modern tag and ancient cyprus seafloor sulfide deposits the size chemical composition energy flux and fluid composition of the tag hydrothermal sulfide deposit at the mid atlantic ridge and the size chemical composition and reaction zone characteristics of the skouriotissa volcanogenic massive sulfide deposit of the troodos ophiolite in cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit we conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years with episodes of activity as short as a few tens of years separated by thousands of years of inactivity about 2 1019 j of energy supplied at high temperature is required to form a deposit the size of the tag mound metals up to 4 times the mass of any element present in the sulfide deposit are leached out of relatively small reaction zones most likely at the base of the sheeted dikes chemical balance can be struck for all elements except sulfur with a reaction zone 1 2 km3 in volume from which a small proportion of iron and large proportions of copper zinc and manganese are removed a sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate as suggested by stable isotope analyses we argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section furthermore we speculate that activity of the tag hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in hawaii and iceland

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

relict hydrothermal zones in the tag hydrothermal field mid atlantic ridge 26 n 45 w two relict hydrothermal zones were delineated between water depths of 3400 and 3500 m at the lower part of the east wall of the rift valley of the mid atlantic ridge in the tag hydrothermal field using a deep towed side scan sonar tow and a camera temperature tow along the northern 3 km of the wall and a submersible transect named the north and mir relict zones they are located about 4 km and 2 km northeast respectively of the known active high temperature sulfide mound between water depths of 3625 and 3670 m on the rift valley floor near the base of the east wall the north zone extends about 2 km along the northern end of the lower east wall the zone includes two moundlike features up to 30 m high by 200 m in diameter imaged by side scan sonar within a 2 km long line of discontinuous hydrothermal deposits comprising inactive toppled and standing chimneys layered material and patchy dark stains on sediment photographed by the camera temperature tow several other moundlike features were imaged with the side scan sonar outside of the photographic coverage the mir relict hydrothermal zone 2 km south of the north zone named after the mir submersible used to investigate it consists of three subzones 1 a 200 m wide area of diverse types of hydrothermal materials exposed by normal faulting at its western margin 2 a 400 m wide by 700 m long central area of discrete groups of toppled and standing inactive sulfide chimneys up to 25 m high on a substrate of red metalliferous sediment and carbonate lutite spires sampled on the highest chimneys are composed of coarse grained recrystallized sulfides dominated by pyrite and chalcopyrite which contain the first primary free gold grains 2 3 m diameter found at a hydrothermal site on a mid ocean ridge and 3 a 150 m wide hummocky area of layered hydrothermal material with the appearance of low temperature precipitates and carbonate lutite with patchy dark stains at its eastern margin the active sulfide mound the north zone and the mir zone are each located on the fractured western margins of gentle dome shaped areas of pillow flows typically 500 m in diameter interpreted as summits of volcanic centers that may have supplied heat to drive adjacent hydrothermal activity the distribution and size of the active and inactive hydrothermal zones of the tag field the chronology and the characteristics of relict samples recovered indicate a history of at least 100 103 years of high temperature hydrothermal episodes with multiple overprinting stages of mineralization accompanied by alteration

a view of the lower crustal component of hydrothermal systems at the mid atlantic ridge an extensive suite of hydrothermally altered gabbros was recovered by alvin and dredging from the mid atlantic ridge in the vicinity of the eastern ridge transform intersection rti of the kane fracture zone mark where tectonic extension has provided a window into the lower crustal component of hydrothermal cells four alteration types are distinguished on the basis of metamorphic assemblage mineral composition and deformation textures a conceptual model is presented that places temperature fluid rock ratio relationships in conjunction with styles of deformation into a spatial and temporal framework exsolution of late stage magmatic fluids in the vicinity of the magma hydrothermal interface at temperatures 700 c marks the onset of fluid rock interaction in the mark gabbros kelley et al this issue with subsequent alteration involving seawater derived fluids being strongly influenced by deformation mechanisms in zones of ductile shear hydration was initiated at temperatures between 550 and 700 c and moderate fluid rock ratios elsewhere in the lower crust the onset of seawater penetration occurred at temperatures of 400 550 c and was facilitated by brittle fracturing early vein networks indicate very low initial fluid rock ratios and are interpreted as being related to the solidification of plutons later fractures indicate higher water rock ratios and may be related to the downward propagation of shallow fault systems throughout the crust fluid rock interaction ceased at temperatures 180 300 c subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the rti massif produced moderate fluid rock ratios within localized zones hydrothermal alteration in the crustal section exposed at mark was initiated at lower temperatures and water rock interaction proceeded to lower temperatures than many other plutonic suites these differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust

development of a brine dominated hydrothermal system at temperatures of 400 500 c in the upper level plutonic sequence troodos ophiolite cyprus abstract microthermometric analyses of quartz hosted fluid inclusions in fresh and epidotized plagiogranites from the troodos ophiolite cyprus indicate that brine rich fluids 46 56 wt nacl equivalent at temperatures 400 500 c were pervasive throughout many of the plagiogranite bodies high salinity fe bearing inclusions along healed microfractures homogenize by halite dissolution at temperatures of 400 500 c liquid dominated low salinity 2 7 wt nacl secondary fluid inclusions are ubiquitous in all plagiogranite and gabbro samples studied homogenization of the low salinity inclusions occurs at uncorrected temperatures of 200 400 c the quartz hosted high salinity fluids represent either phase separation of hydrothermal seawater or an exsolved magmatic aqueous phase at uncorrected temperatures of 400 500 c migration and segregation of brine and vapor phases into fractures near the margins of the crystallizing plagiogranite bodies resulted in preferential entrapment of the fe rich brines in the deep seated high temperature portions of the hydrothermal system subsequent fracturing of the upper plutonic rocks at temperatures of 200 400 c allowed penetration by seawater which pervasively altered the plagiogranites hydration reactions involving formation of secondary minerals and or mixing with phase separated fluids resulted in fluid salinities two times that of seawater

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

growth of an intermittent ice sheet in iceland during the late pliocene and early pleistocene abstract deep sea paleoclimatic records show that the quaternary climate around iceland was and probably still is very sensitive to rapid shifts in north atlantic oceanic circulation studies of several key sections in iceland indicate that similar oscillations are reflected in the late pliocene and early pleistocene terrestrial stratigraphy correlations between six rock sequences in western northern eastern and southern iceland show a fairly distinct trend during the pliocene pleistocene transition indicating the progressive growth of an ice sheet from southeast toward the north and west the correlations are based on k ar dates and paleomagnetic studies a total of 11 glacial horizons are recorded in a stratigraphic column from eastern iceland extending back 6 5 myr in western iceland 7 glacial horizons are preserved in a rock section dated from 7 0 to 1 8 myr and in northern iceland 14 glacial horizons are identified in a section that extends back to 9 0 myr well over 20 glacial horizons have been identified in the stratigraphic column in iceland full scale glacial interglacial cyclicity with regional ice cover is indicated at approximately 2 6 myr a further amplification leading to islandwide glaciations is identified at 2 2 2 1 myr these results show that changes in the ice cover in iceland correlate with the deep sea oxygen isotope records from benthic and planktonic foraminifera as well as ird studies from the north atlantic

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

marine debris flows late precambrian tillites of the avalonian cadomian orogenic belt abstract correlative late precambrian strata of the avalonian cadomian orogenic cycle deposited after 670 and before 550 ma are now widely dispersed around the margins of the north atlantic these contain several prominent diamictite horizons long interpreted as tillites within thick turbidite successions associated with volcanic horizons the granville formation of the armorican massif in normandy france contains many pebby mud diamicite beds up to 20 m thick that have been interpreted as glaciogenic regionally the granville formation occurs within the lowest part of the upper brioverian supergroup which consists of several kilometres of turbidites and volcanogenic rocks at granville diamictites can be traced along striked into deformed and intermixed graded conglomerates and mudstones diamictites are identified as debris flows generated on a submarine slope by the emplacement of gravel turbidites on a mud substrate and their mixing during downslope slumping a glacial influence cannot be identified in north america the conception group of the avalon peninsula newfoundland canada contains nearly 4 km of turbidites with a conformable sequence of diamictites the gaskiers formation near its base massive and crudely stratified diamicite beds the diamictite successions of the granville and gaskiers formations provide significant data regarding facies types associations and the geometry of resedimented diamictite sequences in deep marine basins along active plate margins similar deposits at the same tectonic stratigraphic horizon occur in new england and africa this paper emphasizes the difficulty of identifying a direct glacial influence on diamictite sedimentation in such settings and the need for a sedimentological reappraisal of other supposed late precambrian tillites preserved in orogenic belts this is a necessary first step prior to modelling late precambrian paleoclimates

numerical heat and fluid flow modeling of the panorama volcanic hosted massive sulfide district western australia exceptional exposure of the archean pilbara block in western australia reveals a cross section through an archean massive sulfide hosting volcanic succession with underlying subvolcanic intrusion in the panorama district a numerical model based on available detailed geologic information has been constructed to simulate heat and fluid flow in the panorama district the modeling provides insight into the evolution of the hydrothermal system and evaluates key geologic parameters and their influence on fluid flow hydrothermal circulation and the genesis of massive sulfide orebodies the model simulates important aspects of the panorama massive sulfide district such as temperature distribution relative alteration zonation and the size and distribution of orebodies predicted temperatures ranging from 150oc at the top of the volcanic pile to 400oc at the andesite diorite interface are comparable to temperature estimates based on previously published oxygen isotope mapping modeled fluid discharge temperatures are highest for the sulphur springs deposit 300o 400oc and lower for the kangaroo caves and other deposits 250o 350oc the most favorable conditions to reproduce the orebodies and their related alteration zonation occur at anisotropic rock permeabilities comparable to the upper oceanic crust 10 15 10 14 m2 and higher fault permeabilities 10 14 10 13 m2 with a specific fault arrangement similar to that mapped in the field the 4 6 million metric tons mt sulphur springs orebody is predicted to form in less than 5 000 yr assuming a hydrothermal fluid with seawater salinity 10 ppm base metal concentration and a low deposition efficiency 10 other deposits form above the faults under similar conditions a large range of base metal concentrations in the fluids can account for the known orebodies but high temperatures 250oc and high flow velocities 10 7 m s are necessary to produce the observed alteration patterns and distribution of ore deposits results indicate that the establishment of a significant hydrothermal system capable of forming economic massive sulfide deposits is favored in fresh volcanic rock packages that have not been affected by earlier compaction or alteration under these conditions economic massive sulfide orebodies 5 mt of 10 zn cu may form in a few thousand years although the overall lifespan of the hydrothermal system may be between 30 000 and 200 000 yr depending on the variations in rock and fault permeability with time

anatectic migmatites from the roof of an ocean ridge magma chamber a well preserved contact aureole at the base of the sheeted dyke is likely to be centred at or very close to the sheeted complex in the troodos ophiolite cyprus records the partial melting dyke gabbro boundary this region of the crust is a of the roof of a mid ocean ridge magma chamber hydrothermally dynamic environment that is subjected to rapid changes altered dykes within the lowermost 10 30 m of the sheeted dyke in rheology and thermal structure it is here that melt is complex were recrystallized to pyroxene and hornblende hornfels episodically extracted to build the upper crust e g deand closest to the underlying gabbros 30 at water undersaturated conditions and temperatures if not all of the lower crust e g phipps morgan chen 875 c geological considerations indicate that the duration of 1993 quick denlinger 1993 boudier et al 1996 melting events was probably on the time scale of eruptive cycles i e and magmatic heat is rapidly transferred across a thin years to centuries this resulted in disequilibrium partial melting 100 m conductive boundary layer e g lowell and highly imperfect melt segregation within the contact aureole burnell 1991 and advectively transported out of the we hypothesize that partial melting is a common process within crust by hydrothermal fluids e g cann et al 1985 the roof zones of axial magma chambers at fast spreading ridges geophysical surveys along the east pacific rise epr and that a component of geophysically imaged axial magma chambers provide indirect evidence that the depth and internal may be partially melted upper crust rather than partially crystallized properties e g melt volumes proportion of melt and magma field relations suggest that where the highest degrees of crystals of amcs are not static and change on the time melting occurred a mixture of anhydrous hornfels and hydrous scale of eruptive cycles years to centuries wilcock leucocratic melt disaggregrated and was assimilated into the magma delaney 1996 singh et al 1998 chamber it is well known from ophiolite studies that the roofs of amcs can be marked by polyphase magmatism and intense interactions between hydrothermal fluids and rocks particularly within the uppermost few hundred

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

supercritical two phase separation of hydrothermal fluids in the troodos ophiolite the hot 350 c black smoker springs of the ocean floor are the vents of hydrothermal systems in which cold seawater descends below the ocean floor is heated reacts with the host rock and is transformed into metal bearing brine the salinity of these brines varies between vents from 0 7 to 2 0 times seawater salinity1 4 this variation has been ascribed to crustal hydration5 crystallization of chloride bearing minerals6 or two phase separation of seawater into components of differing salinity4 7 12 here we present fluid inclusion evidence for two phase separation in the ancient troodos sulphide forming systems of cyprus and discuss with the nacl h2o system as a model how this process could explain observed salinity variations in hydrothermal fluids

late miocene valley confined subglacial volcanism in northern alexander island antarctic peninsula isolated late miocene volcanogenic sequences in northern alexander island antarctic peninsula form an unusual cogenetic association of volcaniclastic sandy gravelly lithofacies including tillites and volcanic lava hyalocalstite lithofacies using simple lithofacies analysis and theoretical considerations of hydrodynamic effects of subglacial eruptions valley confined volcanic activity beneth thin wet based ice is suggested the alexander island successions are complete enough to be regarded as model sequences for this uncommonly recorded type of eruptive depositional activity the sedimentary lithofacies represent resedimented tuffs and meltout or flow tills which were probably deposited in subglacial ice tunnels eroded or enlarged by volcanically heated meltwater the volcanic lithofacies formed by the interaction of hot magma with the ice tunnel walls generating abundant meltwater and water saturatedsediments resulting in the formation of heterogeneous masses of lava and hyaloclastite there is no obvious sequence organisation in the sedimentary sections this is probably due to a complex interplay of eruption related and environmental hydrodynamic factors affecting the relative proportions of water and entrained sediment

a fracture loop thermal balance model of black smoker circulation abstract this time dependent model examines the thermal balance between hot rocks and hydrothermal circulation during the formation of volcanogenic ore deposits in order to establish whether the major heat supply to black smoker systems comes from the rock or from an underlying magma chamber flow is modelled as an open thermosyphon in a single fault which is parallel and very close to the mid ocean ridge axis this configuration is based on seafloor and ophiolite evidence of black smoker systems and their structural location the fault is represented in the model by a series of pipes carrying turbulent flow which is driven by the buoyancy difference between upflow and downflow hydraulic resistance to flow is predominantly in the discharge zone the fault is allowed to propagate downwards the horizontal conductive heat flux from the rock is computed analytically with the water flow treated digitally so that the heat exchange at the rock water interface at every position in the loop can be calculated for each time step the model thus provides the exit water temperature and mass flow rate throughout the lifespan of a hot spring enabling the resulting ore mass to be estimated results show that this system cannot obtain sufficient heat from a thin layer of rock such as the 1 2 km thick layer of lavas and dykes overlying an axial magma chamber to sustain long lived ore forming black smokers if the circulation penetrates through a greater volume of hot rock to the base of a recently solidified magma chamber ore masses of a few million tonnes can be formed if conditions are particularly favourable we conclude that systems producing larger ore masses must tap a magmatic heat supply

deformation processes at fast to ultra fast oceanic spreading axes mechanical approach abstract the morphology of fast to ultra fast oceanic spreading ridges such as the east pacific rise epr is characterized by an axial dome 5 10 km wide culminating at 300 500 m above the surrounding seafloor this dome is bounded by lateral grabens that develop systematically 2 to 6 km apart from the spreading axis a large summit trough 200 m to 2 km wide locally notches the axial high only where the dome is inflated indicative of a time average robust magma supply this summit trough is thought to represent an elongated axial summit caldera asc created as a result of the subsidence of the top of the axial magma chamber amc such subsidence is likely caused by a temporary decrease in melt supply into the shallow magma reservoir suffering continuous regional extension analog experiments using small scale modeling have been performed in order to better constrain the tectonic evolution of the axial region the experimental apparatus includes an elongated balloon filled with water as an analog of the magma reservoir set in a central groove in a table it is capped with a silicone layer representing hot rocks below the brittle ductile transition and is covered by a sand layer representing the brittle crust the experiments integrate withdrawal of the balloon and extension at the boundary of the model by the mean of two mobile walls three experimental setups allowed us to study independently the mechanical parameters controlling the axial tectonic evolution extension without withdrawal withdrawal without extension withdrawal and synchronous extension we show that the morphology of the epr axis can be considered as the result of both horizontal and vertical movements two symmetrical lateral grabens develop on both sides of a non deformed axial dome when single extension is applied to a model with a thin silicone layer normal faults of the lateral grabens are rooted on two divergent velocity zones dvzs located on the edges of the groove this situation is regarded as an analog of the natural case where the top of the amc acts as a stress free boundary that fails to transmit the extensional stresses to the upper brittle layer an important deflation of the balloon without extension results in the creation of a central collapse trough limited by reverse faults during synchronous extension and withdrawal the initiation of the lateral grabens is favored by a balloon deflation even if such deflation is unable to generate a superficial collapse this last case is considered as representative of the evolution of epr segments showing little variations in melt supply into the amc higher deflation rates under continuous extension correspond to epr segments undergoing strong variations in melt supply in such experiments the lateral grabens are created together with a central collapse trough developing in a way similar to that of experiments involving only balloon deflation finally we show that dvzs located at the brittle ductile boundary are the key mechanical elements which may explain the structural evolution of the axial region of fast to ultra fast spreading ridges the distance from axis and the width of the dvzs directly control the location and the distribution of the lateral grabens

hydrothermal iron rich deposits from the teahitia mehitia and macdonald hot spot areas southwest pacific abstract during cruise so 47 of r v sonne iron oxyhydroxide crusts of restricted extent were recovered from teahitia and moua pihaa seamounts in the teahitia mehetia hot spot area and from macdonald seamount in the macdonald hot spot area the crusts show complex layering and incorporate volcanic ash layers they consist of goethite feldspar and pyroxene and display low contents of transition elements apart from fe it is believed that the crusts were deposited from relatively low temperature hydrothermal fluids these low temperatures preclude the formation of sulphide deposits the type of hydrothermal mineral recovered may be a function of the thermal regime of the system and the elevation which controls the pressure of the venting fluids the shallower depths of these seamounts compared to the axes of mid ocean ridges may therefore be responsible for the style of mineralization initially magmatic vapours migrating upwards through the hot spot volcano are thought to be absorbed into seawater at comparatively shallow levels to deposit sulphide minerals within the volcanic pile the fe oxyhydroxide deposits are the residual low temperature deposits formed when the spent hydrothermal fluid reached the crest of the seamount these deposits therefore differ fundamentally from those found at the crest of mid ocean ridges where sulphide mounds are deposited directly upon interaction of the boiling hydrothermal fluid with seawater the discovery of hydrothermal iron oxyhydroxide crusts in these two hot spot areas is only the second such finding after loihi seamount southeast of hawaii

fracture analysis of a volcanogenic massive sulfide related hydrothermal cracking zone upper bell river complex matagami quebec application of permeability tensor theory located in the matagami mining district of the abitibi greenstone belt quebec the bell river complex is a 5 km thick tholeiitic layered gabbro and or anorthosite body 2724 6 2 5 1 9 mau pb which likely acted as the heat source that drove hydrothermal convection and volcanogenic massive sulfide vms hydrothermal mineralization in the area abundant fractures and veins crosscutting the western lobe of the bell river complex formed over a range of temperatures from 250 to 700 c the 250 to 400 c assemblage quartz epidote sericite chlorite plagioclase is the most widespread and occurs as orthogonal anastomosing and random vein sets vein densities average between 15 and 25 veins per m2 locally reaching as high as 40 to 60 veins per m2 these veins typically 1 to 3 mm wide are interpreted to represent thermal cracking associated with hydrothermal fluid mineralization in the district furthermore they crosscut earlier higher temperature pyroxene plagioclase 600 c and magnetite rich 300 600 c veins 1 10 mm wide densities commonly 0 5 veins per m2 detailed field measurements of quartz epidote vein geometries coupled with permeability tensor theory have produced a first order approximation of the maximum model permeability structure veins unfilled of the hydrothermal cracking zone representative district wide values indicate maximum model bulk permeabilities of 10 10 to 10 8 m2 for the hydrothermal cracking zone similar to permeabilities calculated for the fractured sheeted dike complexes of the semail and troodos ophiolites however a high flow zone located within the central parts of the hydrothermal cracking zone is characterized by a maximum model permeability of 10 7 m2 locally 1 m2 the high flow zone reaches maximum model permeability values as high as 10 6 to 10 5 m2 where two or more veins occur with apertures in excess of 2 cm mapping has shown that the hydrothermal cracking zone is confined to an 350 m thick interval located within a strongly layered gabbro and or anorthosite horizon of the layered zone upper bell river complex the base of this interval is 1 000 m below the top of the bell river complex furthermore in and around the town of matagami the dominant orientation of quartz epidote veins parallels the orientation of the layering 110 15 this parallelism is further reflected in the orientation of the calculated quartz epidote vein permeability tensors 94 30 1 n 71 the parallelism suggests that heterogeneities associated with the layer contacts provided planes of low tensile strength along which the hydrothermal cracks preferentially developed such an interpretation further explains why the hydrothermal cracking zone appears to be restricted to the strongly layered zone there are many more low tensile strength planes zones of quartz epidote veining approximately orthogonal to layering are also present the orthogonal veins are usually less continuous and commonly truncated but not crosscut by layer parallel veins the orthogonal subset of veins is interpreted to represent short pathways that allowed fluids to travel between adjacent layer parallel veins however locally longer layering orthogonal quartz epidote veins showing offsets of 10 to 30 cm normal to the layering of the bell river complex may represent initial conduits that allowed fluids to travel into overlying stratigraphy and through to the paleosea floor although volumetrically small in comparison with the permeability structure of the bulk sea floor the significantly higher permeability of the hydrothermal cracking zone indicates its importance in controlling the flow paths and fluxes of fluids deep within the hydrothermal system

thermoelasticity and the formation of black smokers darcy s law flow in a permeable medium consisting of uniform parallel evenly spaced fractures is used to elucidate how thermoelastic effects may modify the permeability and flow in fracture controlled hydrothermal systems some simple permeability models are then used to investigate whether black smoker venting can result from focussing of low velocity porous flow into fractures at shallow depths 100 m the models indicate that a thermoelastic processes may be important in controlling the temporal evolution of hydrothermal upflow zones b permeability structure not just the bulk value of the permeability may be critical for the formation of black smokers c a small zone extending to a depth of 100 m containing a few fractures a factor of 2 or more wider than average may be sufficient to focus upflow into discrete vents provided thermoelastic and chemical effects seal parts of the upper crust

east pacific rise at 21 n the volcanic tectonic and hydrothermal processes of the central axis abstract photographs obtained by the angus survey system at 21 n reveal many similarities to the geological processes delineated at other spreading centers and in particular those observed in the galapagos rift at 86 w the region of recent volcanism is restricted to a narrow zone zone 1 approximately 1 km wide this suggests that the width of the magma chamber feeding these flows is also narrow at the top variations in sediment cover are used to subdivide the flows within zone 1 into three sheet flow pillow flow pairs the youngest pair appears to have erupted form a linear fissure 8 km long running parallel to the valley axis this fissure is part of a larger en echelon pattern of eruptive fissures active hydrothermal vents associated with the youngest flows are situated directly above the apparent eruptive fissure the high 350 c temperature for some of the fluids exiting from the vents suggests a highly restricted circulation system involving at times little to no mixing with cold seawater the lava terrain in the remainder of zone 1 and bordering regions is characterized by extensive fracturing further suggesting a narrow upper dimension to the magma chamber and also that subsurface hydrothermal deposition of massive sulfides may lead to a reduction in the mixing processes

penetration of water into hot rock boundaries of magma at grimsvotn the grimsvotn geothermal area is located within one of the most active caldera volcanoes in iceland in the interior of the ice cap vatnajokull at an altitude of 1 400 m fig 1 its setting oilers unique conditions for calorimetric measurement of the heat release from the subglacial geothermal system the melting of ice due to the geothermal activity creates a depression in the surface of the ice cap ice and water are diverted towards the depression from a 300 km2 drainage basin the meltwater accumulates in a 30 km2 subglacial lake periodic outbursts of water jokulhlaups drain the lake subglacially down to the skeidararsandur plain we propose here that penetration of water into hot rock is the primary reason for the intense heat release 5 000 mw thermal of the subglacial grimsvotn geothermal area injection of water into boundary rocks of magma should be considered as a method of heat exploitation

constraints on ocean volume change since the archean water may be added to earth s surface by the bombardment of large numbers of small comets increasing free water on earth s surface deepening the oceans continental freeboard is tied to the ocean surface because of the balance between erosion mountain building addition of water will also affect continental thickness area i calculate the effect of increasing ocean volume on the balance between continental oceanic area one model also includes crustal volume growth significant oceanic depths are known to exist during the archean because sulphide deposits requiring deep oceans occur in greenstone belts these observations prohibit the constant addition of large quantities of water comparable to the present day ocean volume throughout earth history but smaller quantities could be added

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

direct observation of a fossil high temperature fault hosted hydrothermal upflow zone in crust formed at the east pacific rise fault zones in the ocean crust are commonly hypothesized to act as high permeability conduits that focus fl uid fl ow in oceanic hydrothermal systems however there has been little direct study of faults in crust formed at fast spreading ridges here we describe the geology and geochemistry of an 40 m wide fault zone within the uppermost sheeted dike complex exposed at pito deep northeastern easter microplate titanium in quartz thermometry gives temperatures of 392 33 c for quartz precipitation indicating that this fault zone focused upwelling fl uids at temperatures similar to those of black smoker vent fl uids correlated enrichment in 87 sr 86 sr and mgo in fault breccias along with 87 sr 86 sr ratios higher than in average vent fl uids provide evidence for mixing between high temperature upwelling fl uids and a seawater like fl uid within the fault zone large high temperature fl uid fl uxes are required to maintain high temperatures during mixing if this fault zone is representative of upfl ow zones beneath hydrothermal vents on the east pacifi c rise then it is possible that vent fl uids evolve thermally and chemically during their ascent and may not record the precise conditions at the base of the hydrothermal system

plagioclase and epidote buffering of cation ratios in mid ocean ridge hydrothermal fluids experimental results in and near the supercritical region abstract experiments have been performed with na ca k cl fluids of seawater chlorinity and diabase basalt and plagioclase bearing mineral mixtures at 350 425 c and 250 400 bars to help constrain hydrothermal alteration processes at mid ocean ridges dissolved ca na and ph for all experiments responded systematically to differences in dissolved sio2 concentrations and the compositions of plagioclase reactants diabase alteration at low fluid rock mass ratios 0 5 to 1 produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a ph consistent with plagioclase alteration to epidote trace element data from vent fluids specifically b and sr together with major element chemistry ca na sio2 ph provides evidence that the reaction zone for black smoker fluids at mid ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote nearly fresh plagioclase and clinopyroxene and partially to completely hydrated olivine and orthopyroxene fluids reacting with this rock may be undersaturated with respect to quartz so pressure estimates based on the quartz geobarometer should be regarded as minimums using equilibrium between plagioclase the dominant reactant and epidote the dominant reaction product in experiments we estimate that temperatures in reaction zones are in excess of 375 c for most vent systems these temperatures are higher than measured vent temperatures suggesting that hotspring fluids commonly loose heat during ascent to the seafloor

evidence for early glaciation of southeastern beringia between the klondike plateau and yukon tanana highlands of yukon and alaska respectively current maps explain glaciated alpine locales and periglacial areas in terms of localized pliocene pleisto

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

contaminant transport in a single fracture periodic boundary and flow conditions the effects of time periodic inlet contaminant concentration and groundwater flow velocity on contaminant transport in a single planar fracture imbedded in a relatively impermeable rock matrix are investigated the calculations show that the contaminant propagates as a damped traveling wave into the matrix and along the fracture if longitudinal dispersion is neglected penetration of the wave into the matrix and fracture depends upon the input frequency as under uniform flow conditions a daily oscillation penetrates into the matrix and fracture approximately 1 cm and 1 m respectively an annual oscillation penetrates approximately 10 cm and tens of meters for the matrix and fracture respectively these results depend upon physical properties of the rock matrix and fracture and upon the flow velocity because longitudinal dispersion acts to further damp the amplitude of the traveling waves these results give maximum penetration distances for the waves if the flow velocity also has a small time periodic component the downstream contaminant oscillations are virtually unchanged unless the inlet condition and flow have the same frequency in this case a dc transport of contaminant transport occurs that may have an amplitude as large as one half of the magnitude of the velocity oscillation these results indicate that in the absence of resonance negligible error occurs in treating contaminant transport using mean inlet and flow conditions rather than more realistic time varying ones observations of oscillations however can be used to provide useful information on rock and fracture transport properties such as d and d

the sound field near hydrothermal vents on axial seamount juan de fuca ridge high quality acoustic noise measurements were obtained by two hydrophones located 3 m and 40 m from an active hydrothermal vent on axial seamount juan de fuca ridge in an effort to determine the feasibility of monitoring hydrothermal vent activity through flow noise generation most of the measured noise field could be attributed to ambient ocean noise sources of microseisms distant shipping and weather punctuated by local ships and biological sources long period low velocity water rock interface waves were detected with high amplitudes which rapidly decayed with distance from the seafloor detection of vent signals was hampered by unexpected spatial nonstationarity due to the shadowing effects of the caldera wall no continuous vent signals were deemed significant based on a criterion of 90 probability of detection and 5 probability of false alarm however a small signal near 40 hz with a power level of 10 4pa2 hz was noticed on two records taken within 3 m of the inferno black smoker hie frequency of this signal is consistent with predictions and the power level suggests the occurrence of jet noise amplification due to convected density inhomogeneities

the upper cenozoic white river tillites of southern alaska subaerial slope and fan delta deposits in a strike slip setting the white river tillites crop out over 8 km 2 either side of the totschunda fault in the northern foothills of the wrangell mountains in southern alaska and record glacially influenced slope and fan delta deposition in a high relief basin close to an active volcano between 10 and 2 7 ma two measured sections 1 200 and 450 m thick show diamictite and conglomerates interbedded with augite andesite lavas facies range from massive diamictites deposited subaerially as debris flows to crudely stratified sandstones and conglomerates formed by sheet flow processes on alluvial fans and well stratified and normally graded sandstones and conglomerates that accumulated subaqueously as turbidites large scale southward dipping foresets as much as 17 m high identify a fan delta that prograded into a temporary lake the white river succession is identified as a valley infill complex consisting of subaerial slope and lacustrine fan delta deposits there are no in situ primary glacial deposits present in the succession the glacial component of sedimentation was limited to the supply of coarse grained debris identified by the presence of glacially striated and faceted clasts the white river deposits may be typical of glacial sequences preserved as isolated local basin fills in high relief areas such as occur in convergent or strike slip plate margin settings they provide only a limited and fragmentary record of paleoenvironmental changes compared to coeval glacially influenced marine rocks preserved in the yakataga formation 5 km thick along the gulf of alaska coast

strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems new and published strontium isotope analyses from the troodos ophiolite constrain fluid solid exchange processes and the magnitude and circulation paths of the hydrothermal fluids the87sr 86sr profile reflects alteration in the recharge zone of an evolving hydrothermal system fluid rock strontium isotope exchange in the upper 1 5 km of extrusive lavas was kinetically limited and seawater derived fluids emitted from the base of this zone were buffered to87sr 86sr ratios between 0 7047 and 0 7059 in contrast over the next 1 km depth interval of sheeted dykes and the uppermost plutonics 87sr 86sr values cluster about0 7054 7 2 and fluid flow is inferred to have been pervasive with near equilibrium fluid rock exchange quartz chlorite and epidosite zones the probable pathways of the concentrated high temperature upwelling fluids have identical87sr 86sr ratios to adjacent diabase dykes on troodos a time integrated fluid flux in excess of2 9 107 kg m 2 is required to transport the strontium isotope composition of 0 7054 set in the kinetically controlled exchange zone through the 1 km of sheeted dykes and into the zones of concentrated upwelling the uniformity of the87sr 86sr ratios in the diabase sheeted dykes and high temperature epidosite and quartz chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation the inferred minimum time integrated fluid flux of2 9 107 kg m 2 substantially exceeds that of 5 106 kg m 2 inferred from thermal models of high temperature circulation but is comparable with estimates of the hydrothermal flux from oceanic budgets of3he mg and87sr the high flux estimate for troodos is consistent with the ophiolite venting fluids with87sr 86sr elevated significantly above rock values which contrasts with the near morb87sr 86sr ratios of fluids from active high temperature vents at mid ocean ridges and the87sr 86sr profile in dsdp hole 504b this requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the troodos ophiolite differed from that in normal mid ocean ridges and oceanic strontium budgets are balanced by another source the lack of very elevated87sr 86sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

crustal structure of the trans atlantic geotraverse tag segment mid atlantic ridge 26 10 n implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges new seismic refraction data reveal that hydrothermal circulation at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 10 n is not driven by energy extracted from shallow or mid crustal magmatic intrusions our results show that the tag hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km and we find no evidence for low seismic velocities associated with mid crustal magma chambers our tomographic images support the hypothesis of tivey et al 2003 that the tag field is located on the hanging wall of a detachment fault and constrain the complex dome shaped subsurface geometry of the fault system modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation which may enhance footwall cooling however heat extracted from the footwall is insufficient for sustaining long term high temperature hydrothermal circulation at tag these constraints indicate that the primary heat source for the tag hydrothermal system must be a deep magma reservoir at or below the base of the crust

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

archean synvolcanic intrusions and volcanogenic massive sulfide at the genex mine kamiskotia area timmins ontario u pb geochronology and detailed field mapping has resulted in a new subdivision of the kamiskotia volcanic complex into lower and upper kamiskotia strata with a 3 5 m y volcanic hiatus between the two the hiatus is interpreted as an unconformity in common with newly recognized unconformities throughout the greenstone belt the genex and other volcanogenic massive sulfide vms deposits in the area occur within the upper kamiskotia strata volcanic facies mapping and reconstruction indicate that the genex vms deposit formed within a volcanic graben where synvolcanic mafic and intermediate sills and dikes with peperitic and locally pillowed contacts define a proximal volcanic vent area sulfide mineralization at genex occurs in three zones the first hosted in pillow breccia and hyaloclastite the second occurring at the contact between felsic tuff and an intermediate intrusion and the third hosted within the intermediate intrusion the mineralization represents subsea floor replacement sulfides localized within zones of higher primary permeability although the synvolcanic mafic intrusions are not directly related to the genex vms mineralization they are indicative of a high heat flow thermal regime the synvolcanic faults that controlled the emplacement of synvolcanic dikes also provided conduits for the hydrothermal fluids responsible for the mineralization and alteration at genex this high level mafic dike and sill complex was previously correlated with the upper zone of the kamiskotia gabbroic complex but is now interpreted to be younger and correlative with the upper kamiskotia volcanic complex new geochronological data also indicate that the kamiskotia gabbroic complex is older than the vms hosting upper kamiskotia strata and was emplaced into the lower kamiskotia volcanic complex this negates the possibility of a genetic relationship between the genex and other kamiskotia vms deposits within the kamiskotia gabbroic complex the small size of the synvolcanic intrusions indicates that they were also not the heat source for the genex hydrothermal alteration system however their spatial coincidence suggests that this part of the kamiskotia volcanic complex was the focus of long lived intrusive activity and high heat flow that defined a thermal corridor encompassing the vms deposits in the kamiskotia volcanic complex

mass transfer during sub seafloor alteration of the upper troodos crust cyprus five major alteration zones in the extrusive series and the sheeted dike complex of the troodos ophiolite are each characterized by a distinct elemental changes compared to the original composition and b secondary mineralogy the upper ca 300 m of the extrusive crust the highly oxidatedcold seawater alteration zone csa is strongly enriched in k2o and depleted in na2o it is followed downwards by alow temperature alteration zone 170 c which is most widespread in the troodos extrusives and where na2o and k2o are enriched the latter less strongly than in the csa zone three types ofhigh temperature alteration zones 440 c hta i iii are found in the sheeted dike complex all are marked by thorough leaching of k2o while the behavior of na2o e g unchanged in type iii and cao depleted in type i enriched in types ii iii is variable mass budgets of elemental changes are quantified by calibration of whole rock analyses via systematic stable element variations of fresh glasses found throughout the extrusive section the troodos extrusive crust and upper sheeted dike complex are a major sink for mgo k2o and na2o and a source for cao the overall scale of fluxes drastically exceeds estimates based on fresh basalt compositions from present ocean crust

the critical point and two phase boundary of seawater 200 500 c abstract the two phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200 500 c the pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel the critical point was determined to be in the range of 403 406 c 285 302 bar and was located by the inflection in the two phase boundary and by the relative volume of fluid and vapor as a function of temperature the two phase boundary of 3 2 nacl solution was found to coincide exactly with that of seawater over the range tested in the present study the boundary for both is described by a single seventh order polynomial equation the two phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust thus the boundary puts a limit of about 390 c for seawater circulating near the seafloor at active ocean ridges 2 5 km water depth and about 465 c at the top of a magma chamber occurring at 2 km below the seafloor

reconciling temperatures of metamorphism fluid fluxes and heat transport in the upper crust at intermediate to fast spreading mid ocean ridges 1 current models of the mineralogical and chemical modification of the crust within mid ocean ridge hydrothermal systems at intermediate to fast spreading ridges are difficult to reconcile with the temperature distribution predicted by the required fluid fluxes in particular the sharp increase in metamorphic grade passing down from the lavas into the sheeted dike complex places important constraints on the flow of fluid through the crust this distribution of metamorphic temperatures has typically been discussed in terms of a step in permeability in the downwelling limb of a single pass convection cell simple modeling of this system shows that downwelling fluid cools the crust too efficiently for high temperature metamorphism of the upper dikes to occur in the downflow zone in fact temperatures throughout most of the sheeted dike complex are too high three alternative models are considered fluid rock reaction during the cooling of each dike from magmatic temperatures is difficult to reconcile with cooling rate estimates for dikes reaction rates required to grow metamorphic minerals and the relatively constant composition of many hydrothermal vents metamorphism during reheating of the crust after fluid flow slows or stops is difficult to reconcile with the required fluid fluxes during metamorphism based on the sr isotopic composition of the sheeted dike complex and cannot produce high enough temperatures in the upper dikes and low temperatures in the lower lavas in contrast fluid rock reaction in the sheeted dike complex as hot fluids migrate upward after being heated by the underlying magma source appears to be consistent with the data in this model downwelling seawater cools the recharge zone and the fluid only becomes hot enough to significantly alter the crust once it approaches the conductive boundary layer overlying the magma chamber metamorphism of the sheeted dike complex then largely occurs in the discharge zone

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

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

paleomagnetic evidence for multiple late cenozoic glaciations in the tintina trench west central yukon canadathis article is a companion paper to duk rodkin et al also in this issue the tintina trench in west central yukon has preserved an extensive record of late cenozoic preglacial glacial and interglacial deposits these deposits comprise multiple sequences of tills outwash loesses and paleosols the sediments that were laid down directly by ice tills are of both local montane and regional cordilleran provenance the tintina trench area was impacted repeatedly by montane ice from the southern ogilvie mountains to the northwest 2500 m above sea level asl and also repeatedly along its southern extent by cordilleran ice from the selwyn mountains to the east 2759 m asl the latter forming the continental divide in this region we report here the magnetostratigraphy of three sections rock creek 64 13 n 139 07 w west fifteenmile river 64 29 n 139 55 w and east fifteenmile river 64 23 n 139 48 w the majority of the units identified at these sections record late pliocene to mid pleistocene glaciations although relatively thin surficial sequences of late mid

a model of hydrothermal circulation in fault zones at mid ocean ridge crests summary a model of hydrothermal circulation close to mid ocean ridge crests has been developed the flow is modelled as a transient open loop thermosyphon restricted to a single fracture parallel to the spreading centre circulation is to a depth of 500 1000m and heat exchange occurs at the fault walls conduction is the sole heat transfer mechanism within the rock the aim is to model the conditions under which deposition of ore deposits such as those in the cyprus ophiolites could occur hot springs with the characteristics of the black smokers at 21 n on the east pacific rise are obtained from the model but only when the vents are at the unrealistically large spacing of lokm apart and then water at over 350 c is vented for a maximum of only 175 yr the model demonstrates that heat transfer through the rock by conduction alone is too slow to maintain the power output of a group of black smokers the skin depth of the temperature field limits the total heat available to the system but even without this constraint insufficient high grade heat would be available from the volume of rock within the likely field of influence of a vent it would take about 2500yr to accumulate a 3 million ton ore deposit from 1 vent or 250yr from a group of 10 vents the rate of heat extraction and the duration of the vents must both be at least an order of magnitude greater than those obtained from this model thus requiring a much greater source of heat than in the rocks alone the only source likely to be great enough is the magma chamber itself

syngenetic origin for the sediment hosted disseminated gold deposits in nw sichuan china ore fabric evidence abstract sediment hosted disseminated gold deposits in nw sichuan china have many features in common with the well known carlin type deposits in the western united states they are hosted by middle upper triassic turbidites composed of 1300 4300 m of rhythmically interbedded slightly metamorphosed calcareous sandstone siltstone and slate the ore bodies are typically layer or lens like in shape and generally extend parallel to the stratification of the host sedimentary rocks with a strike length of tens to several hundreds of meters the immediate host rocks consist mainly of calcareous slate and siltstone characterized by high contents of organic matter and diagenetic pyrite the main primary ore minerals associated with gold mineralization include pyrite arsenopyrite realgar and stibnite gangue minerals comprise mostly quartz calcite and dolomite gold is extremely fine grained usually less than 1 m and cannot be seen with an electron microscope two types of ore mineralization have been recognized in the deposits the stratiform ores are composed of rhythmical interbeds of sulfides e g pyrite arsenopyrite realgar stibnite interpreted to be authigenic and detrital quartz quartzite sericite and graphite of allogenic origin they were folded and deformed concordantly with host rocks and grade both vertically and laterally into normal country rocks another type of ore forms a network of numerous gold bearing veins and veinlets of quartz calcite sulfides of millimeter centimeter decimeter and even meter scale in width the network ore randomly fills fissures microfissures and cleavages but still is stratabound in character detailed studies on ore fabrics show abundant evidence for synsedimentary origins although subsequent diagenesis metamorphism tectonic deformation and epigenetic hydrothermal activity have significantly remolded the primary fabrics primary fabrics are shown either by rhythmical interbeds of different mineral components parallel to the bedding or by the change of grain size of the same minerals such as pyrite realgar and stibnite the layer inhomogeneity of the stratiform ore is clarified by parallel overprints of later schistosity planes resulting in distinct grain orientation and elongation aggregate polarization and undulating extinction of ore minerals especially of mechanically and chemically extremely mobile ones such as realgar and stibnite it is proposed that the stratiform ores in these chinese deposits were most probably formed concurrently with their host middle upper triassic turbidites in submarine hot spring environments while the network mineralization was formed as a result of complicated processes such as diagenesis weak metamorphism tectonic deformation and epigenetic hydrothermal activity responsible for the remobilization or reworking of the pre existing stratiform ores geochemical data also support this genetic model

constructional features of troodos type oceanic crust relationships between dike density alteration magnetization and ore body distribution and their implications for in situ oceanic crust quantitative relationships are demonstrated between dike density and hydrothermal alteration magnetization location of volcanic massive sulfide vms ore bodies and a number of other properties in a 40 by 15 km segment of the extrusive series of the troodos ophiolite low temperature alteration extends from the sediment extrusive interface to close to the 25 dike density surface while the onset of greenstone type alteration is closely associated with the 50 dike density surface these associations are explained in terms of the regional scale variations in the dike density contour surfaces following the form of the top of the gabbroic layer of the ophiolite an upper magnetic zone characterized by stable high remanence terminates just below the 25 dike density surface a deeper magnetic zone characterized by high induced magnetization straddles the top of the sheeted complex volcanic massive sulfide bodies are concentrated in a narrow depth range centered about 0 1 km above the 25 dike density surface a model for the postridge crest evolution of ore bodies accounts for this optimum depth range for preservation physical property changes with dike density and the depth distribution of vms ore bodies and prospects for the ophiolite as a whole support this model similar relationships appear to be present in the samail macquarie island newfoundland and chilean ophiolites and icelandic crust as a result of relationships found in the troodos ophiolite an alternative is proposed to the current lithologic profile for ocean drilling program hole 504b

the relationship between flow and permeability field in seafloor hydrothermal systems the permeability of oceanic crust is spatially variable and probably anisotropic as well using realistic permeability fields for young oceanic crust we have performed numerical simulations of finite amplitude steady and unsteady convective fluid flow in layered and or anisotropic porous media heated from below to investigate particular patterns of fluid flow and temperature in mid ocean ridge hydrothermal systems on the flanks of mid ocean ridges permeability measurements in deep sea boreholes suggest that only the top few hundred meters of oceanic crust is permeable given this permeability structure and assuming some minimum permeability and layer thickness our models predict that convection occurs in the form of numerous cells with aspect ratios of order unity within this permeable layer such convection results in fluid flux and diagenetic reactions within the permeable layer with a negligible effect on heat flow at the seafloor in agreement with field observations estimates of oceanic crust permeability based on hydrothermal veins in ophiolites and measurements in deep sea boreholes suggest that pillow basalts lava flows are much more permeable than underlying sheeted dikes this is particularly true at the ridge crest where voids have not yet collapsed and filled given this permeability structure our study suggest that a small percentage of the fluid entering the ridge crest circulates through the sheeted dikes before exiting the system at high temperatures in very focused discharge zones in our models these discharge zones narrow considerably at the interface between the sheeted dikes and the pillow basalts lava flows much of the fluid entering the system however never circulates below the pillow basalts lava flows and exits the seafloor at low temperatures in general discharge zones are more focused than recharge zones these results are consistent with observations of narrow and focused discharge zones in ophiolites and localized high temperature venting amid widespread low temperature flow on the ridge crest the spacing of upflow zones at the surface is strongly controlled by convective flow in the bottom permeable layer in our models near field effects dominate over far field effects in systems with lateral variations in permeability and no large scale flow develops between widely spaced areas of contrasting permeability the time to steady state and evolution of convective flow in porous media vary considerably with initial conditions our simulations suggest that the time to steady state is relatively long and that it is possible that hydrothermal convection at the ridge axis never reaches a steady state flow pattern in the sense that the variations in the system boundary conditions such as basal heat flux may occur on a time scale less than the response time of the hydrothermal system it is possible however that these systems may be quasi steady for significant periods of time

hydrothermal circulation and the dike gabbro transition in the detachment mode of slow seafloor spreading one of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast spreading ocean crust this boundary marks the brittle ductile transition at the ridge crest and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi steady state melt lens in contrast at slow spreading ridges the crustal structure appears chaotic with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor recent work suggests that as much as 50 of the atlantic ocean crust formed by a detachment mode of seafloor spreading including the formation of oceanic core complexes capped by long lived convex upward detachment faults these detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall thus explaining the link between faulting and black smoker systems we suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike gabbro transition in the detachment mode of spreading inevitably leading to exposure of gabbros on the seafloor through continued faulting this concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates

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

sedimentology and mechanism of a lacustrine syn rift fan delta system a case study of the paleogene gaobei slope belt bohai bay basin china abstract fan delta systems have long been studied as a kind of good hydrocarbon reservoirs although many models have been raised according to their local conditions there still exists some controversies regarding the formations of the ones developed in continental syn rift settings taking paleogene es32 3 of gaobei slope belt as an example this paper conducted a comprehensive geological study and a forward depositional modeling the results show that the sediment within the study area came mainly from the northeastern baigezhuang uplift and the northwestern xinanzhuang uplift over a short distance the sedimentary system is characterized by subaerial debris flow subaerial braided channels subaqueous distributary channels front sand bars and front sand sheets furthermore the study area is a balance filled basin that experienced a 3rd order normal regression followed by a transgression during the development of the study interval the results also show that a stable subsidence rate with small magnitude climate changes would be a plausible explain for the entire sequence of the fan delta system in the study area the tectonic subsidence was indeed the dominant driving force to a continental syn rift fan delta system although the climate sediment supply or lake level might bring some lower order e g 4th and 5th order superimpositions this paper proposed a likely model for fan deltas developed in continental syn rift settings which is expected to gain some insights into the forming mechanism and provide some supports for the reservoir characterization of similar and correlative sedimentary systems

black smokers fuelled by freezing magma if ocean floor hot spring activity is to continue long enough to produce a large massive sulphide deposit heat must be stripped by the circulating water from a store within the ocean crust we argue here that this heat store can only be a magma chamber and that production of a large sulphide deposit requires very rapid crystallization of large volumes of magma 1 km3 per 100 yr for several hundred years the effects of such rapid solidification should be apparent in the gabbroic components of ophiolite complexes as well as in plutons in other environments where large high temperature massive sulphides are formed

ice streams of the late wisconsin cordilleran ice sheet in western north america abstract the late wisconsin cordilleran ice sheet cis of western north america is thought to have reached its maximum extent 2 5 106 km2 as late at c 14 5 ka most 80 of the ice sheet s bed consists of high mountains but its core zone sited on plateaux of the intermontane belt of british columbia and coterminous parts of the usa shows broad swaths of subglacially streamlined rock and sediment broad scale mapping from new digital imagery data identifies three subglacial bed types 1 hard beds of variably streamlined bedrock 2 drumlinized soft beds of deformation till reworked from antecedent sediment and 3 mixed beds of variably streamlined bedrock protruding through drumlinized sediment drumlins on soft beds appear to be erosional features cut into till and antecedent sediments and identify the catchment areas of paleo ice streams expressed downglacier as flow sets of megascale glacial lineations msgls grooved and cloned drumlins appear to record the transition from drumlins to msgls the location of paleo ice streams reflects topographic funneling of ice from plateau surfaces through outlet valleys and a soft bed that sustained fast flow rock cut msgls are also present locally on the floors of outlet valleys cis disintegrated in

reply to m church and j m ryder s discussion of sedimentology and clast fabrics of subaerial debris flow facies in glacially influenced alluvial fan

response of two phase fluids to fracture configurations within submarine hydrothermal systems submarine hydrothermal systems at mid ocean ridges evolve within a region of brittle deformation overlying a region of ductile deformation the interplay between brittle and ductile modes of deformation and the geometry of fracture systems within the brittle zone can exert a significant influence on fluid flow especially under conditions leading to aqueous two phase separation within the brittle region fractures vary between large conduits directly open to the seafloor and small veins and microcracks with constricted or indirect connections to the seafloor in the ductile region fluids may be trapped under near lithostatic pressure while fluids within throughgoing veins in the brittle region will experience hydrostatic pressure in a narrow transition zone between these two extremes a steep pore fluid pressure gradient must exist in the brittle region itself fluid pressures will vary between lithostatic and hydrostatic as fluid occupies interstices varying from isolated pores through tortuous microcracks to channels directly connected to the main upflow conduits a hydrostatic gradient modified by the decreased density of hot fluid and the effects of the flow itself should be applicable in major conduits open to the seafloor fluctuating pressure conditions in the transition zone may induce phase changes in a fluid remaining at constant depth as a result of the large volume increase that accompanies two phase separation a two phase fluid must either accelerate upward or exert an increased pressure on its surroundings possibly leading to enhanced fracturing the fracture configuration will also affect the behavior of the mixed fluid after phase separation a two phase fluid experiencing rapid turbulent flow in a large open conduit is more likely to remain a well mixed suspension of vapor and liquid whereas a two phase mixture in a constricted branching series of channels moving more slowly in a laminar flow regime is more likely to undergo physical isolation of the phases leading to venting of a vapor phase and retention of a saline brine phase at depth we propose a possible configuration for the fracture pattern within a submarine hydrothermal system consisting of a feeder zone comprised of anastomozing microcracks and veinlets progressively increasing in diameter and decreasing in abundance feeding into a main upflow stockwork zone in which flow directly to the surface takes place in relatively large unconstricted conduits fluid movement within a feeder zone may be modified by capillary flow tortuous channelways disconnected fractures blocking of cracks by mineral deposition and opening of new fractures

evolution of an active sea floor massive sulphide deposit hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to 400 c before venting at spectacular black smokers these hydrothermal systems exert a strong influence on ocean chemistry1 yet their structure longevity and magnitude remain largely unresolved2 the active transatlantic geotraverse tag deposit at 26 n on the mid atlantic ridge is one of the largest oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black smoker fields here we report ages of sulphides and anhydrites from the recently drilled3 tag substrate structures determined from 234u 230th systematics analysed by thermal ionization mass spectrometry the new precise ages combined with existing data4 5 show that the oldest material 11 000 to 37 000 years old forms a layer across the centre of the deposit with younger material 2 300 7 800 years old both above and below this stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid6 the age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2 000 years

processes of brine generation and circulation in the oceanic crust fluid inclusion evidence from the troodos ophiolite cyprus detailed temporal thermal and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the troodos ophiolite cyprus provide the first documentation that generation of high temperature brines may be common at depth in the oceanic crust anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events the earliest fracturing event at temperatures 450 600 c resulted in entrapment of brine rich aqueous fluids with salinities of 36 61 wt nacl equivalent homogenization of the brine inclusions by haute dissolution the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz hosted high salinity inclusions cooling of the fluids during pulses of fluid migration associated with episodic fracturing events resulted in entrapment of the brines in the deep seated high temperature portion of the hydrothermal system in localized areas the high temperature brines nacl kcl cacl2 caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites arrays of low temperature low salinity fluid inclusions which in some samples crosscut fractures dominated by brine inclusions indicate downward propagation of a cracking front subsequent to collapse of the high temperature magmatic system resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures 200 400 c hydration reactions under greenschist facies conditions or limited mixing with brine rich fluids may have resulted in salinity variations from 70 below to 200 above seawater concentrations temperatures and compositions of the low salinity inclusions are similar to those found in stockwork systems beneath troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid ocean ridge spreading centers the low temperature fracture networks may represent an extensive deep seated feeder system which coalesced to form zones of concentrated hydrothermal upflow