hydrothermal activity as recorded by red sea sediments sr nd isotopes and ree signatures abstract three cores from different red sea deeps were analysed for sr nd isotopes and ree non metalliferous biodetrital sediment from the shaban deep was used as reference its ree pattern characteristics are similar to those of normal detrital sediment whereas the sr and nd isotope ratios 0 7092 and e nd 4 7 are consistent with deposition from seawater sediment of the most metal enriched hydrothermal core from the atlantis ii deep shows hree depleted patterns with a large positive eu anomaly typical of high temperature fluids sr and nd isotope compositions are the same throughout the core 0 7075 and e nd 0 9 and very different from those of the shaban deep the third core also from the atlantis ii deep consists of biodetrital sediment at the base and of increasingly metalliferous sediment toward the top the positive eu anomaly becomes larger as 87 sr 86 sr decreases upward reaching a maximum eu eu value at 2 58 and a minimum 87 sr 86 sr at 0 7076 sediment containing fish remains oudin and cocherie 1988 has a high ree 776 ppm its high e nd 4 0 indicating that the fluid interacted with underlying basalt to construct the curve of nd evolution in seawater with time use of the nd isotope composition of sediment must be accompanied by ree determinations in order to select samples without potential hydrothermal fluid interaction as the latter can increase the isotope ratio of nd

formation of large sulfide mineral deposits along fast spreading ridges example from off axial deposits at 12 43 n on the east pacific rise abstract submersible investigations with nautile near the 13 n hydrothermal field on the epr were made in 1992 we selected diving areas based on deep towed side scan sonar imaging close to a pit crater at the top of a young seamount 2 km east of the axial ridge we discovered one of the largest 70 m high and 200 m in diameter sulfide mounds formed at a fast spreading ridge 12 cm yr despite an overlap in composition between axial and off axial deposits the general trend reflects real differences in compositions off axis deposit differs from axial deposits by the scarcity of zinc sulfides and cobalt and selenium enrichment sulfur isotope variation is minimal for the off axis seamounts and suggests that sulfides precipitated from unmodifed end member hydrothermal solutions axial sulfides have a broader range due to reduction of seawater sulfate at mixing fronts between hydrothermal and seawater solutions along the axial graben fault system lead isotopic compositions of sulfides are in two overlapping but distinct fields indicating different pb sources and separate convective systems for the seamount and the axial deposits our observations suggest that the plumbing system is stable and centred on a shallow localised magma chamber under the seamount this configuration is by far more efficient than unstable axial processes to create during several hydrothermal episodes major sulfide deposits near a fast spreading ridge off axis seamounts are localised areas with high magmatic budget able to drive and focus hydrothermal systems at the same place for a long time other known occurrences of sulfides related to seamounts are at less than 11 km from the ridge axis thus we suggest that off axial volcanoes close to the ridge are first order targets to discover active or inactive large deposits along fast to medium spreading ridges

hydrothermal activity at the trans atlantic geotraverse hydrothermal field mid atlantic ridge crest at 26 n the first submersible observations of the only known active submarine hydrothermal field on a slow spreading oceanic ridge are incorporated with results of 10 prior years of investigation to derive an understanding of periodicity duration and varying intensity of hydrothermal activity at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge crest near latitude 26 n hydrothermal activity has persisted at this location for at least 1 106 years based on the distribution of hydrothermal and hydrogenous mineralization with respect to crustal age the hydrothermal activity has been cyclic multistage and episodic prior high temperature hydrothermal venting stages with a periodicity of the order of 1 104 years and duration of the order of 101 years are deduced from the estimated ages of discrete sedimentary layers anomalously enriched in cu fe and zn and correspond with the independently determined periodicity of volcanic eruptive cycles on the mid atlantic ridge the most recent episode of high temperature venting is inferred to have ceased in the recent past based on metal enrichment cu fe zn in the surficial sediment layer low temperature hydrothermal venting stages with a duration of the order of 1 104 years intervene between the short high temperature stages and produce stratiform deposits of layered and earthy manganese oxide iron oxide hydroxide and silicate bivalve like forms with the characteristics of vent clams in various stages of dissolution are identified on bottom photographs the fresh appearance of intact tubules composed of iron hydroxide that acted as vents on relict deposits conductive heat flow anomalies in the sediment column and the record of temperature anomalies and excess 3he in the near bottom water column suggest that the low temperature hydrothermal discharge is intermittent at individual vents on a time scale of years

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

hydrographic structure of brine filled deeps in the red sea new results from the shaban kebrit atlantis ii and discovery deep abstract hydrographic profiles with ctd and water samples from four brine filled deeps in the red sea shaban kebrit atlantis ii and discovery deep were taken during the meteor 31 2 expedition in february march 1995 in the shaban deep area two smaller basins containing high saline brine were found in addition to the already known southern basins corresponding salinities 150 7 cl in the upper brine section and near equal brine levels around 1325 m water depth suggest subbottom connections below the sills separating the northern and southern basins an insignificant decrease in temperature to about 24 1 c has occurred in the upper brine since detection of the shaban deep in 1981 and no changes were found for the brine seawater level no significant changes have been observed in the kebrit deep over the last 23 years in temperature 24 3 c chlorinity 154 0 0 3 cl and brine level 1466 1 m the transition from high saline brine to normal seawater salinity probably extends over a depth of only a few decimetres or less strong changes however were registered for the atlantis ii deep the temperature increase known from numerous earlier investigations continued the gradient of increase was the steepest during the last 2 5 years and a temperature of 71 7 c was reached in the lower brine of the sw basin the structure of the lower transition zone between about 1990 m water depth and high saline brine has also changed significantly now containing two convective layers with nearly constant temperatures 61 and 55 c respectively the neighbouring discovery deep also shows evidence of increased hydrothermal activities while the temperature was nearly constant near 44 7 c until 1977 it has since increased to almost 50 c and the brine level has risen by about 17 m

heat and salt fluxes in the atlantis ii deep red sea abstract the atlantis ii deep is located on the red sea axial rift it is a topographic depression enclosing a volume of about 17 km3 between 1 900 m and 2 200 m depth that contains layered brines of temperatures up to 66 c and salinities up to 270 previous geochemical investigations showed that the hot brines result from discharge of hydrothermal solutions that have exchanged heat and chemical components with the basaltic substratum the last investigation of the deep in 1992 showed that the brines occurred in four well mixed layers with the shallowest at a depth of 2 000 m the temperature and salinity profiles describe a transition zone from 2 000 m to 1 900 m above which red sea water occurs the distribution of temperature and salt in this transition zone appears to be controlled by the topography of the deep the hydrographic structure of the brine column has been documented in the literature for seven time intervals between 1966 and 1992 examination of these data shows that the system changed with time the evolution of the entire package of brines that fill the deep shows the following changes 1 the temperature of the brines increased 2 the salinity of the solutions increased 3 the two brine layers described in 1966 still existed in 1992 but new layers appeared above them 4 for 26 years almost all additional heat and salt supplied in the deep were confined to the depression and were not dispersed into the overlying seawater the fourth point indicates that a heat and salt balance for the deep can be calculated the calculation of the heat that entered into the system was divided into two components the temperature increase of the brines and the heat loss at the wall rock the latter component was negligible the rate of heat input to the deep was constant during the period considered and amounted to 0 54 109 w the salt input was also constant and equalled 250 350 kg s during the period 1966 1992 heat and salt were most probably supplied by a hydrothermal solution with an average range of flow rate temperature and salinity of 670 1000 kg s 195 310 c and 270 370 respectively

thermal conductivity and temperature structure of the reydarfjordur borehole one hundred and ten measurements of thermal conductivity have been made on samples over a 1900 m depth interval in the reydarfjordur borehole comprising largely basaltic flows and intrusives the mean conductivity of both flows and intrusives increases with depth by about 25 reaching a maximum value of about 1 9 w m 1 c 1 this increase results from the growth of secondary minerals which progressively fill voids with depth and in some cases replace phases having lower values of thermal conductivity the conductive heat flow between 650 m and the surface is about 100 mw m 2 below 650 m the conductive heat flow appears to be less than half the near surface value and there is strong evidence that in this depth interval the thermal structure is modified by hydrothermal circulation

intermittent euxinia in the high latitude james ross basin during the latest cretaceous and earliest paleocene abstract seymour island in the james ross basin antarctica contains a continuous succession of latest cretaceous sediments deposited in a shallow marine environment at high latitude making it an ideal place to study environmental changes prior to the k pg mass extinction we measured major and trace elements and conducted petrographic analysis of two sections from the maastrichtian danian lopez de bertodano formation of seymour island several lines of evidence point to intermittently anoxic to euxinic conditions during deposition including the presence of pyrite framboids with a size distribution suggesting syngenetic formation in the water column and enrichments in several trace elements including molybdenum arsenic copper zinc and chromium molybdenum enrichments are clearly associated with enrichments in manganese and authigenic iron suggesting shuttling of redox sensitive trace elements across a chemocline that fluctuated across the sediment water interface comparisons with modern systems suggest relatively high frequency redox variability possibly over approximately annual timescales which may be related to the annual cycle of polar sunlight and associated seasonal changes in primary productivity glauconitic horizons are associated with more reducing conditions including at the k pg boundary though this does not appear to be a uniquely euxinic interval similar degrees of trace element enrichment are seen in other highly glauconitic intervals while euxinia may have contributed to low diversity in the lowermost rotularia units redox conditions do not seem to have been the primary control on the transition to a mollusc dominated fauna in the latest maastrichtian redox conditions show little to no response to the eruption of the deccan traps or maastrichtian climatic changes instead intermittent euxinia appears to have been a characteristic feature of this high latitude environment during the cretaceous paleogene transition

textures of the hellyer volcanic hosted massive sulfide deposit tasmania the aging of a sulfide sediment on the sea floor the relatively undeformed polymetallic massive sulfide ore of the hellyer deposit retains many textures developed during the early crystallization history common textures in the least modified ore away from the main feeder zone include pyrite framboids concentric and radial textured spheres dendritic and fibrous forms breccias mineral banding veins and reniform and crustification textures many are typical of early crystallization within a medium at high degrees of supersaturation some result from open space filling and others indicate disturbances of the sulfide mass possibly due to tectonic activity and volume adjustment the banding could be due to current winnowing most of these textures are found in modern midocean ridge and kuroko massive sulfide deposits but at hellyer the framboids are much more common hexagonal outlines in ore sphalerite indicate derivation of much of the sphalerite by inversion of wurtzite the high content of chalcopyrite in the wurtzite derived sphalerite appears in most cases to have developed by exsolution rather than replacement or coprecipitation although there is local evidence of both processes having operated possibly resulting from early asymmetrical crystal growth comparison of hellyer ore and typical kuroko ores reveals other differences than framboid abundance e g metal contents and ratios mineralogy and sulfur isotope compositions the most significant of which are the relatively high salinities of the hellyer ore fluids these together with evidence of a coeval basin imply sulfide deposition in a brine pool as previously proposed the development of a quenched sulfide mud at the bottom of a basin filled with spent ore fluid with little or no involvement of seawater explains the different mineral content e g lack of fe oxides scarcity of barite in the massive sulfide and the pyrite arsenopyrite assemblage the high metal content lack of chimney structures and differences in sulfur isotope compositions the sulfide mud dominated by fe s phases would be an ideal growth medium for framboidal pyrite the weight of experimental evidence concerning the development of framboidal pyrite points to involvement of metastable precursor fe s phases such as mackinawite and greigite in a weakly oxidized environment oxygen may have been added to the reduced acid brine pool as a result of inward diffusion of oxygen from overlying seawater which is shown to have been oxic by the presence of sessile benthic dendroids in the shales overlying the ore deposit however the level of oxidation was insufficient to form marcasite

seismic gravity and magnetic surveys in the central part of the red sea their interpretation and implications for the structure and evolution of the red sea abstract while there is unanimous agreement that the axial part of the red sea has witnessed seafloor spreading since the early pliocene opinions diverge considerably with regard to the situation in the main trough and shelves new high precision data comprising palaeomagnetic seismic gravity magnetic and bathymetric measurements from the central and northern parts of the red sea have been examined to discriminate between the earlier proposed hypotheses and to formulate an evolutionary model for the red sea the results show that oceanic crust extends for about 80 km at 19 n on each side of the spreading axis symmetrical seafloor spreading magnetic anomalies are recognized out to anomaly 3a 6 m y b p between 18 30 and 19 45 n anomaly 2a between 19 45 and 20 n and near 21 n anomaly 2 near 22 n and only anomaly 1 near 23 30 n although anomaly 3a is the oldest recognizable magnetic anomaly the extent and configuration of the oceanic basement as determined from seismic measurements indicate that spreading has started and continued without a hiatus since 10 12 m y b p prior to the initiation of oceanic accretion an extension of 40 55 km i e 50 70 has taken place in the proterozoic continental lithosphere whereas a separation of 70 160 km has occurred during the seafloor spreading phase several lines of evidence indicate that the shelves and the marginal parts of the main trough are of continental nature

mineralogy and geochemistry of epigenetic features in metalliferous sediment atlantis ii deep red sea finely laminated and undisturbed except in the area of the southwest basin in this part bedding resedimentation and brecciation are common in addition some sediment cores contain vein minerals precipitated in open fissures which crosscut unlithified metalliferous sediments these veins are the conduits by which new hot brine vents into the deep vein mineralogy is dominated by anhydrite talc smectite pyrite sphalerite and chalcopyrite vertical zonation due to cooling and reaction of the incoming brine as it rises through the metalliferous sediment near the southwest basin west basin transition metalliferous sediment has been locally recrystallized to a hematite magnetite pyroxene assemblage probably due to the intrusion of basalt into the metalliferous sediments modified journal abstract

a numerical study of local convection in the benthic ocean induced by episodic hydrothermal discharges a nonhydrostatic primitive equation model is used to investigate motions and water column property distributions accompanying the rise of hot hydrothermal fluids into a rotating water column stratified in temperature and salinity after preliminary model experiments used to fix the level of turbulent mixing the model is applied to a megaplume event the release of a large amount of hydrothermal heat into the benthic ocean over a period of several days events of this kind have produced water masses with anomalous temperature and salinity distributions 20 km in diameter 800 m thick centered vertically 700 1000 m off the seafloor the model allows the study of the initial time development and maturation of the plume from such an event preliminary experiments demonstrate the sensitivity of plume rise to the level of turbulent mixing and to the ratio of turbulence viscosity to turbulence diffusivity i e prandtl number rise to maximum plume height occurs in 4 6 n 1 where n is buoyancy frequency consequences of a megaplume sized release of heat are examined over an initial 30 day period vertical circulation accompanying a hydrothermal event leads to a slight negative salinity anomaly below the plume s temperature anomaly core the plume s heat potential temperature anomaly is shown to be a composite of hydrothermal heat and ambient heat that has been redistributed by the same vertical circulation cell horizontal circulation is dominated by an anticyclonic flow vortex centered just above the temperature anomaly maximum at 800 m above bottom and a cyclonic flow vortex at the seafloor azimuthal flow accounts for the largest fraction of kinetic energy within 4 5 hours 1 7 1 after the start of heat release azimuthal velocities reach maximum values of 70 cm s 1 90 of which is attained within the initial 10 hours even as much as 4 weeks past cessation of heat release maximum azimuthal velocities are of the order of 10 cm s 1 the balance of vertical forces is hydrostatic by 12 hours save for near bottom frictional effects during plume formation the radial force balance is primarily cyclostrophic near axis and geostrophic at a distance

off axis hydrothermal circulation parametric tests of a refined model of processes at deep sea drilling project ocean drilling program site 504 we have developed a refined model of coupled heat and fluid flow to numerically simulate off axis hydrothermal circulation through the upper oceanic crust the new representation includes mesh elements with curved sides and noncentered nodes these curvilinear elements allow improved simulation of topography at the seafloor and within underlying sediment and basaltic layers curvilinear simulations of circulation at deep sea drilling project ocean drilling program site 504 confirm the broad conclusions derived from the rectilinear simulations of fisher et al 1990 but also suggest that permeability within the upper few hundred meters of crust around hole 504b must be concentrated within several narrow zones this interpretation is consistent with core observations and wireline logs from hole 504b within these highly permeable zones absolute permeability may be several orders of magnitude higher than the bulk permeability measured for the upper crust as a whole a series of detailed parametric simulations was conducted to determine the quantitative importance of basal heat flow seafloor bathymetry basement relief and differential sediment thickness in modifying the geometry and intensity of off axis hydrothermal circulation these studies reveal that basement relief and differential sediment thickness are more important than seafloor bathymetry in enhancing off axis convection within the seafloor observed natural correlations between seafloor bathymetry and heat flow may exist because bathymetry is a proxy for the other two important parameters basement relief and differential sediment thickness off axis convection at geochemically significant velocities is possible even with heat input appropriate for some of the oldest oceanic crust a thick sediment layer over older crust may cause enough conductive refraction at the seafloor to mask the variations in seafloor heat flow often associated with off axis hydrothermal convection

measurements of plume flow from a hydrothermal vent field simultaneous velocity temperature and conductivity data were taken in the convective flow emanating from a hydrothermal vent field located at 10 56 n 103 41 w on the east pacific rise the horizontal profiles obtained indicate that the flow field approaches an ideal plume in the temperature and velocity distribution such parameters as total heat flow and maximum plume height can be estimated using either the velocity or the temperature information the results of these independent calculations are in close agreement yielding a total heat flow from this vent site of 3 7 0 8 mw and a maximum height of 150 10 m the nonlinear effects of large temperature variations on heat capacity and volume changes slightly alter the calculations applied to obtain these values

superheating in the red sea the heat mass balance of the atlantis ii deep revisited atlantis ii is the only hydrothermally active of the five central red sea deeps hot new brine being supplied by a geyser spring it is filled with two stratified anoxic brine layers namely the lower conductive layer lcl at the bottom and the upper conductive layer ucl above it the other deeps were filled by brine overspill hydrological and geochemical data on the atlantis ii deep show that between 1965 and 1977 only negligible amounts of lower brine have spilled over towards chain a deep and that in contrast the lower brine component of the ucl has increased significantly the heat mass balance of the atlantis ii deep lower brine between 1966 to 1977 is considered mass balance calculations are based on the published bathymetry of the deep and on the measured rising rate of the interface between the ucl and the lcl including a possible loss of mass out of the system heat balance equations involve 1 the conductive flow of heat at the bottom of the deep 2 the heat lost by conduction through the lower interface 3 the flow of heat due to hot brine advection and 4 the heat lost by lower brine advection out of the lcl the equation of pitzer et al 1984 57 extrapolated above 300 c is first used to calculate the c p s of the hydrothermal brine it predicts moderately increasing c p s with temperature heat mass balance equations solved with this c p model yield a spring temperature of 461 c for the time interval 1966 1977 this temperature is unrealistic as it is 70 c higher than 390 c the boiling temperature of the lower brine at 220 bar besides it is shown that none of the parameters in the calculation other than c p can attain values as to decrease t s below 390 c the modified equation of born predicts a near exponential increase of the hydrothermal brine c p s above 300 c spring temperatures of 342 c or 353 c are calculated with this c p model depending whether the mass lost towards the ucl is taken into account or not these temperatures compare favorably with the mean temperature of 330 c obtained for the trapping of fluid inclusions in epigenetic anhydrites from the discharge zone sediments 21 hence it is concluded that 1 the changes in the mass and temperature of the atlantis ii deep lower brine between 1965 and 1977 cannot be interpreted in terms of a realistic spring temperature unless the hydrothermal fluid has presented very high heat transport properties above 300 c 2 this can be either in the stable region below the boiling point as predicted by the equation of born or in the metastable region above 390 c superheated liquids indeed present heat capacities which increase exponentially with temperature two periods in the recent hydrothermal activity of the atlantis ii deep are distinguished 1 from 1965 to 1976 the deep has received an excess heat supply e g compatible with the influx of superheated liquids the spring flow rate including the mass of lower brine expelled towards the ucl 29 was around 367 kg s 1 2 the hydrothermal activity then decreased from 1976 to 1979 the influx of fluids with abnormal heat transport properties was greatly reduced and the mean spring flow rate probably dropped to 280kg s 1

thermal conductivity and radiogenic heat production of sedimentary and magmatic rocks in the northeast german basin thermal rock properties were determined for the northeast german basin a subbasin of the southern permian basin in europe the new thermal data provide new aspects for the determination of heat flow and for hydrocarbon and geothermal resource evaluation in the basin thermal conductivity was measured on drill core samples using the optical scanning method values for permian and pre permian clastic rocks are variable 2 3 4 8 w m k because of different depositional environments compaction and cementation reflected in lithology permian carboniferous igneous rocks show lowest thermal conductivity in basalts 2 2 w m k and highest 3 1 w m k in granitoids anisotropy of thermal conductivity in all rocks is low formation thermal conductivity was determined by upscaling the values determined for single lithotypes radiogenic heat production was determined on drill cores for the upper paleozoic sediments and igneous rocks for the younger formations without sample control a well log approach was used the heat production of sedimentary rocks is lowest 0 4 w m3 in the permian zechstein salt and anhydrite and highest 2 1 w m3 in the permian rotliegende clastic rocks heat production of the permian carboniferous rhyolites and granitoids is on the order of 2 6 3 8 w m3 and heat production of andesites and basaltoids is on the order of 0 9 1 9 and 0 1 0 7 w m3 respectively the contribution to surface heat flow by these up to 2 km 1 2 mi thick igneous complexes amounts to 7 mw m2 at a maximum on the same order is the total heat budget provided by the supra permian sedimentary succession in the basin

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

geochemical dynamics of the atlantis ii deep red sea silica behavior abstract previous chemical analyses of atlantis ii deep sediments and waters have shown that silica concentrations are abnormally high in comparison with those from other marine hydrothermal sites new physical and chemical data have been obtained during recent cruises to the atlantis ii area seawater free and interstitial brines have been analyzed for chloride and silica the hydrographic measurements show that two additional brine layers have been created since 1977 these new observations allow us to re evaluate the hydrodynamics and the consequences for mass transfer silica concentration in the lower brine pool is 913 18 mol 1 within the upper brine layers silica concentrations are higher than those expected from simple mixing between the lower brine and seawater the chemical mass transfers through the brine interfaces are explained by double diffusion phenomena silica concentrations in the interstitial waters mol 1 are much lower than those in the lower brine the high silica concentration gradients observed at the brine sediment interface suggest that silica precipitates at this boundary layer thermodynamic calculation indicates that the interstitial brines are undersaturated with respect to quartz and amorphous silica these species act probably as major sources for silica whereas authigenic clay minerals could be the major sink

shale basins sulfur deficient ore brines and the formation of exhalative base metal deposits abstract the massive sulfide deposits of the iberian pyrite belt are interbedded with felsic volcanic rocks and shale and underlain by several thousand meters of siliciclastic sedimentary rocks known as the pq group isotope geochemistry and regional geology are both consistent with equilibration of the ore forming fluids with the pq group prior to ore deposition near the former seafloor the average cu zn pb ratio of the pq group rocks ca 26 55 19 is similar to the weighted average of all the massive sulfide orebodies combined ca 25 52 23 the genetic relationship between massive sulfide deposits and a siliciclastic sedimentary metal source is explained here by a thermodynamic model proposing that mildly reducing redox conditions imposed by equilibration with the sedimentary rocks are most critical for the formation of an effective ore forming fluid relatively metal rich but organic poor pyrite bearing shale undergoing dewatering of saline pore fluids is an effective source for the generation of sulfur deficient but relatively iron and base metal rich brines thus we propose that the giant deposits of the iberian pyrite belt owe their existence not to exceptionally metal enriched e g magmatic fluids but to the existence of a fairly ordinary but large metal source in reactive siliciclastic sediments combined with an underlying igneous heat source and a particularly efficient mechanism of sulfide precipitation by mixing with h 2 s rich fluids at or near the seafloor essentially similar mineral equilibria are imposed when saline fluids are buffered by typical continental basement rocks leaching of retrograde minerals and possibly residual salts from their magmatic or metamorphic prehistory is expected to generate similar variably metal rich but relatively sulfide deficient fluids thus the existence of mildly reducing rocks can be the dominant chemical control in the source of fluids generating many volcanogenic irish type or sedex deposits many of which are known to precipitate their metal load in response to biogenic sulfide addition at the ore deposition site

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

structure and evolution of the northern red sea combined analyses of new geophysical data in the northern red sea with data from the gulf of suez and the gulf of aqaba dead sea rift strongly suggest that the development of the red sea took place in three stages 1 a gulf of suez stage in the oligocene accompanied by thinning of the continental lithosphere and the formation of an early red sea gulf of suez graben 2 an early dead sea stage with slow seafloor spreading from 25 to 15 myr accompanied by 62km of shear along the aqaba dead sea transform and 3 a recent dead sea stage with slow seafloor spreading from 4 5 to 0 myr accompanied by 45km of shear along the aqaba dead sea transform

before the ice biogeography of antarctic paleogene molluscan faunas abstract new identifications are listed of antarctic paleogene molluscan faunas lopez de bertodano and sobral formations seymour island paleocene la meseta formation seymour island eocene mcmurdo sound erratics eocene and king george island oligocene early miocene these show that both cosmopolitan and antarctic subantarctic genera increased regularly as percentages of faunas with time 15 of paleocene genera 30 of eocene genera and 59 of oligocene early miocene genera still live in the antarctic subantarctic region the present major antarctic families of larger molluscs have remained the most diverse ones since middle eocene time and the antarctic fauna was well established by then however mytilidae and veneridae have decreased markedly in diversity mactridae and oysters have disappeared from the antarctic and antarctic trochidae volutidae and conoidea are now distinct guilds of more offshore taxa this reflects the present antarctic ice cover shallow water taxa that are no longer able physically to live in antarctica provide an index of antarctic glaciation glaciation was not extensive on the antarctic peninsula by late eocene time and a few genera of shoreface molluscs still inhabited the king george island area during oligocene early miocene time subduction has left only sparse localities around the margins of the pacific and indian australian plates containing shallow water early paleogene macrofossils almost none are known from the central regions of these plates few data points are available so apparent indications of taxa originating in antarctica are not interpretable new names are provided for two junior homonyms gaimardia zinsmeisteri replacing gaimardia flemingi zinsmeister 1984 prosipho stilwelli replacing aeneator huttoni stilwell and zinsmeister 1992

buoyancy driven plumes in rotating stratified cross flows plume dependence on rotation turbulent mixing and cross flow strength a three dimensional time dependent convection model is used to describe circulation and property fields in rotating stratified and moving fluids near a point source of heat the study context and consequently model scales are those of chronically discharging hydrothermal vent fields found at submarine ridge crests hydrothermal plumes having distinctive thermal and chemical anomalies have been observed to rise several hundreds of meters above the deep sea floor before being advected away by background cross flows typically of magnitude 1 4 cm s 1 the model is used to study effects of rotation and indicate differences in plumes with respect to variation of subgrid scale turbulence intensity and cross flow strength counterrotating vorticity couplets in all three coordinate directions develop in the lower plume stem at startup and follow the plume to the level of neutral buoyancy for a nonrotational case 0 patterns resemble those previously found for jets injected into homogenous cross flow ambient fluid entrainment into the convecting column is primarily from the upstream side but deflection of background flow around both sides of the rising column is the root of the relative vorticity z couplet in the lower plume turbulence intensity within the buoyant region of the plume and or globally controls smoothness and temporal variability of distal nonbuoyant plume distributions allowing or preventing oscillations of potential temperature for example at background buoyancy frequency n over the range of turbulent mixing studied rise height of plumes did not change appreciably but breadth of plumes counterintuitively increased for decreasing turbulent mixing strength increasing cross flow strength u0 bends model plumes such that rise height u0 0 4 for the two largest values of cross flow for which r the ratio of maximum vertical velocity to u0 took values of 2 8 and 1 0 plumes showed evidence of bifurcation

chemical and isotopic 87sr 86sr δ18o δd constraints to the formation processes of red sea brines abstract about twenty deeps filled with hot brines and or metalliferous sediments are located along the red sea axis these brines present a well suited framework to study the hydrothermal activity in such a young ocean the present study outlines the results of a geochemical approach combining major trace element and isotopic oxygen hydrogen strontium analyses of brines in six of the deeps to evaluate different processes of brine formation and to compare the evolution of each deep important heterogeneities in temperature salinity hydrographic structure and chemistry are recorded each brine having its own characteristics the intensity of hydrothermal circulation varies among the deeps and ranges from being strong atlantis ii and nereus to weak port soudan and even to negligible valdivia and suakin and it varies along the entire red sea axis these observations do not favour a unique formational model for all of the brines for example the brines of the suakin deep appear to have been formed by an old sea water which dissolved evaporite beds without significant fluid circulation and hydrothermal input while others such as atlantis ii or nereus deeps appear to be dominated by hydrothermal influences a striking feature is the absence of a relationship between the position of the deeps along the axis and their evolutionary maturity

the development of layering fluxes through double diffusive interfaces and location of hydrothermal sources of brines in the atlantis ii deep red sea the brines in the atlantis ii deep of the red sea occur in horizontally uniform well mixed layers with the hottest and saltiest water at the bottom separated from the successively cooler and fresher layers above by very sharp vertical temperature and salinity gradients data acccumulated over 3 decades are used to test the widely accepted hypothesis that all heat and salt for the brine layers are supplied from below and that the layered brine structure is the result of double diffusion using the changes in temperature and salinity in each layer over successive time intervals one can deduce the corresponding fluxes of heat and salt across the interfaces it is found that the required flux of salt cannot be sustained by double diffusion alone an alternative calculation shows that most of the salt in the successively forming upper layers must have been injected directly from the bottom of the deep through one or more vents located above the level of the lowest brine interface for the bottom layer however it is not possible to obtain the observed salinity and temperature changes unless hot saline water is injected directly into that layer and some of the heat and a smaller fraction of the salt are transferred upward through the interface this process will also maintain convection in each of the layers and keep them well mixed as is observed the new interpretation in terms of separate inputs at various levels in the atlantis ii deep is also supported by recent geochemical evidence

a multistage genetic model for the metamorphosed mesoproterozoic swartberg base metal deposit aggeneys gamsberg ore district south africa

seismic surveys in the northern red sea asymmetric crustal structure abstract wide angle reflection and refraction seismic warrs surveys at key sections of the northern red sea and adjacent areas enabled a better definition of the crust and upper mantle structure a fundamental asymmetry of the region was revealed by the results of profiles which cover both flanks of the red sea main trough the coastal plains and the bordering precambrian shields this asymmetry is clearly expressed by the presence of different crustal types on both flanks the western flank shows oceanised crust within only 20 km off the african coast while the whole eastern flank is underlain by a continental type of crust likewise the shape of the continent ocean transitions differs significantly on both flanks the arabian margin extends almost into the centre of the red sea and shows considerable stretching and attenuation of the continental crust while the african margin on the other hand even thickens towards the escarpment and appears very abrupt the influence of the red sea opening as reflected by the decrease of seismic velocities and the increase of poisson s ratio in the upper mantle due to partial melting is strictly limited to the area seaward of the egyptian escarpment the asymmetry in the northern red sea is regarded to be the expression of strike slip processes in the initial stages of red sea evolution

rapid dike emplacement leads to eruptions and hydrothermal plume release during seafloor spreading events the creation of ocean crust by rapid injection of magma at mid ocean ridges can lead to eruptions of lava onto the seafloor and release of event plumes which are huge volumes of anomalously warm water enriched in reduced chemicals that rise up to 1 km above the seafloor here we use seismic data to show that seafloor eruptions and the release of hydrothermal event plumes correspond to diking episodes with high injection velocities and rapid onset of magma emplacement within the rift zone these attributes result from high excess magma pressure at the dike source likely due to a new influx of melt from the mantle these dynamic magmatic conditions can be detected remotely and may predict the likelihood of event plume release during future seafloor spreading events

constraints on origin and evolution of red sea brines from helium and argon isotopes brines from three depressions along the axis of the red sea the atlantis ii the discovery and the kebrit deep were sampled and analyzed for helium and argon isotopes we identified two principally different geochemical fingerprints that reflect the geological setting of the deeps the atlantis ii and the discovery brines originating from locations in the central red sea show 4 he concentrations up to 1 2u10 35 cm 3 stp g 31 and a 3 he 4 he ratio of 1 27u10 35 the morb like 3 he 4 he ratio is typical of an active hydrothermal vent system and clearly indicates a mantle origin of the helium component within the brines 40 ar 36 ar ratios are as high as 305 implying that mantle derived 40 ar excesses of up to 3 of the total argon concentration are present in the brines and transported along with the mantle helium signal the mean 4 he 40 ar excess ratio of 2 1 is in agreement with the theoretical mantle production ratio in the kebrit deep located in the northern red sea we found a helium excess of 5 7u10 37 cm 3 stp g 31 the low 3 he 4 he ratio of 1u10 36 points to a predominantly radiogenic source of the helium excess with only a minor mantle contribution of approximately 9 we propose a new scenario assuming that the kebrit brine accumulates a diffusive helium flux that migrates from deeper sedimentary or crustal horizons in contrast to the atlantis ii and discovery deep the kebrit brine shows no sign of an active hydrothermal input fl 2001 elsevier science b v all rights reserved

hydrothermal fluid migration and brine pool formation in the red sea the atlantis ii deep numerical heat and fluid flow simulations of the atlantis ii deep in the red sea were conducted to investigate the development migration and discharge of hydrothermal fluids into a submarine depression and determine the conditions necessary to form a brine pool high salinity fluids are predicted to form by leaching miocene evaporates migrate and convect within young oceanic crust and discharge onto the seafloor predicted fluid discharge temperatures t max 301 c discharge fluid velocities v max 0 09 m s and salinities s max 21 wt increase over time and reach values comparable to modern seafloor observations established convection patterns and discharge behavior are robust and are not greatly affected by geometry of rock property changes modeling results were used to calculate the minimum conditions for hydrothermal fluids from a developing hydrothermal system to mix with seawater reverse buoyancy and begin to form a brine pool in a submarine depression under conditions encountered on the seafloor t 25 300 c s 5 25 wt fluid mixtures predicted to pond on the seafloor range from late in the mixing process 99 at low temperatures t 26 c to much earlier 36 mixing at higher temperatures t 94 c a model of brine pool evolution is proposed that describes the processes and conditions necessary to initiate brine pool formation and compares formation conditions with accumulated ore material in the atlantis ii deep and other locations

bulk properties of hot smoker plumes the hot buoyant fluid from a submarine vent rises as a plume entraining fluid from the stratified ocean until a height is reached where the driving buoyant force vanishes and shortly thereafter the diluted plume fluid intrudes into the ocean interior at its equilibrium depth while a large positive heat flux issues from the vent the potential temperature anomaly of the horizontally spreading fluid at the equilibrium height depends on the oceanic stratification of both salinity and potential temperature and on the salinity of the source fluid at the vent for example in the atlantic ocean the oceanic stratification causes the spreading plume fluid to be cooler and fresher than the surrounding seawater of the same density while in the pacific the spreading fluid is relatively warm here the well known solution for a turbulent plume rising in a linearly stratified environment is used to obtain simple formulae for 1 the potential temperature anomaly of the horizontally spreading fluid 2 the heat flux associated with this injection of diluted vent water at this density level and 3 the nondimensional solution for the evolution of tracer properties e g salinity and potential temperature in the rising plume

tectonic configuration of the western arabian continental margin southern red sea the young continental margin of the western arabian peninsula is uplifted 3 5 to 4 km and is well exposed rift related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18 n and its intensity increases gradually from east to west extension is negligible near the crest of the arabian escarpment but it reaches a value of 8 to 10 in the western asir a highly dissected mountainous region west of the escarpment there is an abrupt increase in extensional deformation in the foothills and pediment west of the asir about 40 km inland of the shoreline where rocks in the upper plate of a system of low angle normal faults with west dips are extended by 60 to 110 the faults were active 23 to 29 ma ago and the uplift occurred after 25 ma ago tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 ma old are concentrated in the foothills and pediment as well the chemistry of the dikes suggests 1 fractionation at 10 to 20 kbar 2 a rapid rise through the upper mantle and lower crust and 3 differentiation and cooling at 1 atm to 5 kbar structural relations between dikes faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval but that most of the extension preceded the expansion a tectonic reconstruction of pre red sea afro arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide steep moho slopes probably developed during rift formation as indicated by published gravity data two published seismic interpretations and the surface geology

sedimentary hosted polymetallic massive sulfide deposits of the kebrit and shaban deeps red sea massive sulfides recovered from the kebrit deep carbonaceous sedimentary succession represent black smoker fragments novel to any red sea brine pool deposit chimneys which were also observed in situ near the seawater brine interface of the kebrit deep pool are primarily comprised of fe zn and pb bearing phases and are often tar and asphalt impregnated cu sulfides are virtually absent from parageneses contrasting rift related smoker and red sea metalliferous sediment deposits concentration of nickel in discrete bravoite points to a basalt seawater leaching process as a source for most metals the sedimentary package which probably hosts cu mineralization in lower stockworks of the smoker deposit is considered the major source of lead prevention of boiling of hydrothermal fluids passing through a succession of organic rich carbonate and clay horizons prior to discharge is essential for smoker formation shaban deep sedimentary hosted massive sulfides are less frequent with pyrite being the dominant ore mineral sulfur isotope data indicate both high temperature inorganic as well as biogenic sulfate seawater and or evaporite reduction in sulfide forming processes cogenetic sulfates formed from residual bacteriogenically reduced seawater sulfate rather low sulfide sulfate precipitation temperatures of 110 130 c for the kebrit brine pool and 100 c for shaban deep massive sulfides are evident

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

seafloor hydrothermal systems driven by the serpentinization of peridotite 1 heat balance models predict that heat released upon serpentinization of peridotites can result in a wide range of hydrothermal venting temperatures temperatures ranging from a few to a few tens of c are likely temperatures 300 c may be possible in conjunction with significant heat input from below low fluid flow rate 10 kg s a high rate of serpentinization 100 kg s or some combination of these factors the volume of fresh lithosphere accessible to serpentinization may limit the duration of an episode of hydrothermal venting when the serpentinization rate exceeds 10 kg s hydrothermal venting driven by high rates of serpentinization may occur episodically as new reaction surfaces are made available by tectonic activity or crack propagation given the uncertainty of several parameters the current episode of venting at the lost city vent field on the mid atlantic ridge may range from 102 to 104 years

active ore formation at a brine interface in metamorphosed deltaic lacustrine sediments the salton sea geothermal system california the salton sea geothermal system is an area of active hydrothermal metamorphism of pliocene to pleistocene deltaic lacustrine and evaporitic sediments deposited in a modern continental rift zone temperatures up to 365 degrees c hypersaline metalliferous brines with up to 26 wt percent total dissolved solids and active base metal ore formation accompany low pressure greenschist and amphibolite facies metamorphism at depths less than 3 2 km drill cores recovered by the salton sea scientific drilling project shed new light on the nature and origin of ore formation in the eastern portion of the salton sea geothermal system base metal ore mineralization occurs in vertical fractures that comprise the major form of fluid permeability at 1 to 3 km in the modern geothermal system type 1 veins are dominated by carbonate contain pyrrhotite and other sulfides are completely sealed and occur in wall rock showing minimal alteration fluid inclusion data indicate that they were deposited under a thermal gradient much higher than that now found in the eastern part of the system type 2 veins are dominated by silicates and hematite are permeable and occur in wall rocks showing pervasive chloritization and epidotization fluid inclusion data indicate that they are forming now from the geothermal brines under the modern thermal gradient fluid inclusions record an apparent steep progressive salinity increase in the vein systems in the upper 2 km of the system however fluid production data from salton sea geothermal wells indicate that reduced metalliferous hypersaline brine averaging 23 wt percent total dissolved solids is overlain by more oxidized metal poor fluid averaging 5 wt percent total dissolved solids a sharp interface exists between the fluids at 1 to 2 km paralleling the 250 degrees c isothermal surface a model is developed for type 2 vein ore formation involving mixing at the interface between the two fluids magmatic intrusion induces fracturing heating and the expulsion of reduced connate fluids to cause early type 1 vein mineralization following the rising thermal front deep hypersaline brine rises diapirically and mixes with overlying fluids in vertical fractures transecting the ascending interface forming type 2 veins the hypersaline brine rises to a level of density temperature stabilization causing pervasive greenschist metamorphism of the host sediments the diapir then begins to cool and descend shallow oxidized brine is drawn in as the hypersaline brine descends promoting late stage hematite in type 2 veins the eastern salton sea geothermal system presently appears to be in this latter mode of cooling and retrograde brine descent ore formation in the system is caused by the coincidence of transaxial entry of a major river into an active rift zone deposition of metal bearing deltaic sediments to form a closed basin sedimentary environment episodic lacustrine evaporite formation and injection of heat and elements by rift related magmatic intrusions at depth the presence of an upwelling brine diapir in the system may be analogous to conditions immediately prior to the exhalation of metalliferous fluids that formed stratiform fe zn pb cu sulfide and oxide deposits in paleorift zone environments

first evidence for high temperature off axis venting of deep crustal mantle heat the nibelungen hydrothermal field southern mid atlantic ridge during segment scale studies of the southern mid atlantic ridge mar 7 12 s we found evidence in the water column for high temperature hydrothermal activity off axis east of ascension island extensive water column and seafloor work using both standard ctd and deep submergence auv and rov deployments led to the discovery and sampling of the drachenschlund dragon throat black smoker vent at 8 17 87 s 13 30 45 w in 2915 m water depth the vent is flanked by several inactive chimney structures in a field we have named nibelungen the site is located 6 km south of a non transform offset between two adjacent 2nd order ridge segments and 9 km east of the presently active northward propagating a2 ridge segment on a prominent outward facing fault scarp both vent fluid compositions and host rock analyses show this site to be an ultramafic hosted system the first of its kind to be found on the southern mar the thermal output of this single vent based on plume rise height information is estimated to be 60 15 mw this value is high for a single black smoker vent but small for an entire field the tectonic setting and low he content of the vent fluids imply that high temperature off axis venting at drachenschlund is driven not by magmatic processes as at the majority of on axis hydrothermal systems but by residual heat mined from the deeper lithosphere whether this heat is being extracted from high temperature mantle peridotites or deep crustal cumulates formed at the duelling non transfrom offset is unclear in either case the drachenschlund vent provides the first direct observations of how cooling of deeper parts of the lithosphere at least at slow spreading ridges may be occurring

mineralogy parageneses and phase relations of copper iron sulfides in the atlantis ii deep red sea new investigations are carried out on the mineralogy and mineral chemistry of sulfide assemblages obtained in samples from one core in the hydrothermally active southwest basin of the atlantis ii deep red sea the most abundant sulfide phases are the exsolved intermediate solid solution iss and chalcopyrite sphalerite pyrrhotite marcasite mackinawite and presumably wurtzite are also observed two distinct groups of paragenesis were encountered a intermediate solid solution with sphalerite incrustations and intergrowths and b intermediate solid solution barren of sphalerite intergrowths the first group is confined to the upper part of the co zone and the soan zone backer and richter 1973 and the second is present in the entire core 100 3 7 an optically isotropic chalcopyrite is found for the first time as a natural mineral in atlantis ii red sea yet its existence as a novel phase needs x ray confirmation it exhibits a lower reflectivity than normal chalcopyrite and is isotropic chalcopyrite occurs either as a single phase or in association with tetragonal chalcopyrite our investigations indicate that the formation of atlantis ii deposits is a result of complex processes these processes are characterized by compositional changes in the ore bearing fluids and the change in sulfur fugacity especially with depth the presence of exsolved chalcopyrite lamellae in iss indicates slow cooling below 450 c however it is difficult to understand why the cubic chalcopyrite is not converted to the tetragonal form even though the temperature of transformation lies above 450 c 470 500 c cabri 1973 the cu fe ratio changes in the exsolved chalcopyrite lamellae from core to rim of the composite grains the ratio is higher in the rims this suggests that primary inhomogenous iss grains formed from solutions with a continuous increase in the cu fe ratio slow cooling is also required to account for the exsolution of chalcopyrite lamellae in iss the low sulfur content in isotropic chalcopyrite is also suggestive of low fs2 the low s content in the chalcopyrite may be the controlling factor for the sluggish conversion from cubic to tetragonal chalcopyrite mackinawite lamellae show the same orientation in iss and exsolved isotropic chalcopyrite indicating that mackinawite exsolved before the breakdown of iss this strongly suggests that mackinawite is stable above 300 c contrary to experimental results by zoka et al 1973 pyrrhotite was probably formed by the sulfurization of ilvaite the pyrrhotite grains with several complex successive zones show the sequence of the sulfurization episodes

impact of seafloor sediment permability and thickness on off axis hydrothermal circulation juan de fuca ridge eastern flank sediment permeability plays a key role in controlling both the flux of solute and heat through marine sediments and patterns of hydrothermal circulation within the underlying oceanic crust interlayered silt rich and clay rich sediments almost completely bury basement on the eastern flank of the juan de fuca ridge laboratory measurements indicate that silt rich sediment permeability is 1 to 2 orders of magnitude greater than clay rich sediment permeability at similar depths numerical simulations of coupled fluid flow and heat transfer using a simplified model of a sedimented ridge flank with smooth basement topography illustrate how differences in permeability between the two sediment types can influence patterns of hydrothermal circulation and the flux of heat and solute across the seafloor the layered structure of the model domain is inferred from reflection seismic and seafloor heat flow data two distinct patterns of hydrothermal circulation are obtained depending on whether silt rich or clay rich sediments compose the sediment layer above a permeable upper basement aquifer a clay rich sediment column nearly isolates circulation within the basement aquifer resulting in closed convection computed seafloor heat flow profiles resemble heat flow measurements made on the eastern flank of the juan de fuca ridge computed vertical fluid fluxes across the sediment column are small in agreement with fluid fluxes estimated from sediment geochemical profiles when the sediments are silt rich 17 of the flow within a convection cell crosses the sediment column open convection where basement topography is smooth is probably rare because sediment columns overlying oceanic crust are unlikely to be entirely silt each pattern of convection persists over a wide range of sediment thicknesses for a given sediment permeability and basal heat flow a transition from open to closed convection occurs when the effective permeability of the sediment column is only slightly less than that of an entirely silt rich column thus the addition of only a small amount of clay to an otherwise silt rich sediment column may effectively isolate the crustal aquifer from the ocean

fe mn hydr oxide carbonate crusts from the kebrit deep red sea precipitation at the seawater brine redoxcline lithified crusts located within the sediment and recovered from the kebrit deep red sea are composed of fe and mn hydr oxides carbonates and sulfides mineralogical and geochemical features of these crusts suggest that they are metalliferous layers lithified by carbonates and formed in the narrow band where the transition zone between the anoxic brine and deep seawater crossed the seafloor carbonates cementing the fe mn hydr oxides precipitated from normal saline seawater at t 20 30 degrees c precipitation of fe mn hydr oxides was superimposed the carbonate precipitation in the transition zone a pumping mechanism involving cyclic fe and mn hydr oxide precipitation redissolution operated through the transition zone and produced an enrichment of a number of elements in the crusts the microbiota played an important role in the geochemical cycle of the elements through the redoxcline and in the mineralogy of the sediments underlying the redoxcline waters diagenetic processes led to formation of bi and ternary carbonates and disulfides c 2006 elsevier b v all rights reserved

the cu stockwork and massive sulfide ore of the feitais volcanic hosted massive sulfide deposit aljustrel iberian pyrite belt portugal a mineralogical fluid inclusion and isotopic investigation the variscan feitais volcanic hosted massive sulfide deposit in the aljustrel district of the iberian pyrite belt consists of 55 million metric tons of zn pb cu massive sulfide overlying a cu rich stockwork the massive ore is overlain by up to 30 m of feldspar phyric rhyolitic volcaniclastic rock and locally by a jasper and or chert layer up to 15 m thick the massive sulfide orebody consists dominantly of pyrite sphalerite galena chalcopyrite tetrahedrite tennantite arsenopyrite and bournonite together with minor quartz chlorite sericite carbonate and barite the orebody is up to 100 m thick and is underlain by a tabular alteration zone of chloritedominated locally silicified felsic volcanic rock the upper 30 to 60 m of which contains chalcopyrite quartzchlorite sericite carbonate bearing stockwork vein let s that prior to deformation were at a shallow angle to the base of the massive orebody chloritized footwall rocks extend up to 20 m below the cu stockwork zone and are underlain by up to 50 m of quartz sericite pyrite altered rhyolitic rock the stockwork veins also contain pyrite tetrahedrite tennantite sphalerite and arsenopyrite pyrite both in stockwork and massive ore locally displays partly recrystallized framboidal reniform and cellular textures two generations of quartz q1 and q2 and carbonate in the stockwork veins contain primary in growth zones and pseudosecondary fluid inclusions with homogenization temperatures of 270 to 315 c and salinities of 2 2 to 8 1 wt percent nacl equiv the 34s cdt values of massive and stockwork ores range from 15 4 to 4 7 mean 2 8 and 11 2 to 11 9 mean 0 4 per mil respectively the lowest values from colloformtextured pyrite with no evidence of oxidation of sulfide sulfur during mineralization the most negative values indicate an origin by biogenic reduction of seawater sulfate the 13c pdb values for carbonates 7 5 to 13 7 and 9 3 to 14 3 per mil in massive and stockwork ore respectively indicate an origin mostly by oxidation of methane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmatic carbon there are no significant lateral or vertical variations in s isotope values in sulfides or c o isotope values in carbonates either in massive or stockwork ore the 18o smow values for quartz in stockwork and massive sulfide are 11 6 to 13 9 and 16 7 to 17 9 per mil respectively coexisting and texturally contemporaneous carbonate and quartz in stockwork veins are not in isotopic equilibrium indicating that the c o isotope values may have been reset the 18o values of fluid calculated to be in equilibrium with quartz at fluid inclusion homogenization temperatures are 4 2 to 5 2 per mil barite from the hanging wall and massive ore yields 34s values 21 9 27 9 equal to or slightly higher than those of coeval seawater 87sr 86sr ratios 0 708438 0 709063 are slightly more radiogenic than those of coeval seawater 0 7080 0 7085 and much more radiogenic than those of coeval volcanic rocks 0 703304 0 706642 probably representing mixtures between seawater sr and radiogenic sr in fluids sourced in the crustal pile deposition of the massive sulfide on the sea floor is suggested by its stratiform nature the stronger alteration of footwall relative to hanging wall rocks the stockwork system terminating sharply at the base of the massive sulfide the presence of sedimentary like textures in the massive sulfide the absence of replacement fronts and the presence of framboidal and other sea floor depositional textures indicative of fluid quenching the sheetlike form lack of rubble mounds and chimneys scarcity of barite reduced mineral assemblage and metal zoning distinguish feitais from kuroko type deposits it shares most of the characteristics of those iberian pyrite belt deposits for which a brine pool origin has been proposed based on fluid inclusion data

evolutionary patterns in macrurous decapod crustaceans from cretaceous to early cenozoic rocks of the james ross island region antarctica

discrete and diffuse heat transfer atashes vent field axial volcano juan de fuca ridge abstract heat fluxes of discrete and diffuse components of hydrothermal discharge are estimated from submersible measurement in the ashes vent field located in the caldera of axial volcano on the central juan de fuca ridge the discrete component comprises discharge from individual vents and the diffuse component comprises seepage from large areas of the seafloor the investigation centered on the high temperature portion of the field a 100 100 m area of fractured lobate and sheet lava flows encompassing discrete discharge 326 c from seven individual vents and diffuse flow from intervening areas a total of 4 4 2 10 6 w was estimated for the discrete heat flux based on measurements of effluent temperature flow rate and orifice diameter at all the known individual vents by d s v alvin in september 1987 a total of 15 75 10 6 w was calculated with a standard plume model for the diffuse flux based on a grid of temperature measurements at altitudes of 1 m and 20 m above the study area using a 1 m long vertical array of temperature sensors mounted at the front of alvin s instrument platform the study indicates that 1 the diffuse component rises separately from the discrete component of hydrothermal discharge and is laterally advected by a prevailing current below the discrete component and that 2 the diffuse component of convective heat flux is approximately an order of magnitude greater than the discrete component in the high temperature portion of the ashes vent field

formation on the sea floor of the hellyer volcanogenic massive sulfide deposit new fluid inclusion data from the stockwork veins beneath the hellyer orebody a relatively undeformed late cambrian polymetallic volcanogenic massive sulfide deposit allow semiquantitative modeling of the behavior of the ore fluids on the sea floor and the manner of growth of the massive sulfide the earliest cool fluids may have been denser than seawater and ponded in a basin identified by sedimentological and structural studies subsequent vent fluids buoyant with respect to the brine pool and subsequently at higher temperature with respect to seawater mixed with the cooler brine during and after filling the basin to the level of its lowest outlet density inhomogeneities within the brine pool were reduced by double diffusive convection across the brine seawater interface and internal boundaries and mixing by currents within the brine pool estimates of likely temperatures and salinities in the brine pool for the first main mineralization stage avg 200 c 11 wt salinity assuming a steady or quasisteady state had been established and using volume fluxes similar to those observed in modern oceans indicate temperatures 100 c sulfides mostly pyrite sphalerite and galena precipitated from the vent fluids after mixing with cooler basin fluid settled from the gravity current to the basin floor a pattern that would have continued whether or not a true steady state was reached if the early fluids were buoyant in seawater they ponded after mixing forming a hybrid brine pool that would have required longer periods to homogenize at the peak temperature 300 c and volume flux the salinity of the vent fluids fell to 7 5 wt percent but the flow paths would have been maintained unless the brine pool had been drained at this time that part of the massive sulfide body over the vent the cu core grew by dilatational veining together with leaching cavity infilling replacement and recrystallization chalcopyrite was deposited both in the cu core and probably at the same time in a layer over the flanking massive sulfide in the final stage as temperature fell and salinity rose avg 250 c 11 wt salinity the fluid oxidation potential increased so that barite precipitation dominated resulting in a formation of a discontinuous barite rich cap over the massive sulfide the brine pool model explains several features of the hellyer ore that differ from those of the ores of the hokuroko basin namely the large metal content high zn cu ratio low aspect ratio absence of chimney fragments presence of mineral banding and high barite sulfur isotope values features seen in many other massive sulfide orebodies

metal remobilization at a spreading centre studied using lead isotopes the metalliferous sediment deposits of the atlantis ii deep in the red sea are an example of submarine hydrothermal deposits at a spreading centre the brine salinities 270 for the atlantis ii lower brine originate from leaching of the miocene evaporites the mantle is considered as a possible source of metal by analogy with east pacific rise deposits during the hydrotherm cruise may 1985 on rv marion dufresne two cores numbered 683 and 684 were collected from the sw and w basins respectively of the atlantis ii deep fig 1 these two cores reached the basaltic basement and therefore allowed full and representative sampling of the metalliferous sediment series as well as of the underlying basalt our lead isotope results suggest that the metals in the sediments are not derived from a basaltic source we show that lead in the metalliferous sediments was derived both from the recent biogenic terrigenous sediments and another unknown sedimentary source the variations in the lead isotopic composition of atlantis ii deep sediments suggest a discontinuous brine supply

numerical computation of hydrothermal fluid circulation in fractured earth structures summary hydrothermal fluid circulation through porous earth materials is an important physical phenomenon occurring in both submarine and continental environments irregularly interconnected discrete fractures are pervasive in nearly all earth materials providing preferential paths for fluid flow and controlling the circulating fluid patterns most mathematical algorithms addressing hydrothermal convection problems treat rocks as ecewise continuous media the representation of local large changes in permeability requires a high level of discretization for accurate results and a corresponding large number of unknowns the alternative is to incorporate fractures discretely through special adaptation of the numerical code we adopt this approach to solve the coupled time dependent heat and fluid transport differential equations using the finite element method the final algorithm is validated against both an analytical solution and merical solutions from a complementary but less general finite difference scheme case studies of some simplified fractured models indicate that fractures can induce and maintain hydrothermal fluid circulation in media which would otherwise be passive fracture location can control both convection pattern and vigour in a closed system discrete fractures can also significantly change an established convection pattern multiply fractured porous media are comparable with the homogeneously aniso tropic media in the numerical solutions if the effective average horizontal and vertical permeabilities are kept the same

isotope geochemistry tracks the maturation of submarine massive sulfide mounds iberian pyrite belt the massive sulfide deposits of the southern iberian pyrite belt ipb occur as shale hosted exhalative mounds or shallow sub seafloor replacive bodies that formed at the devonian carboniferous boundary in anoxic bottoms the geochemistry of the mineralization and the host shale from some of the major deposits including a detailed 50 m thick section through the migollas orebody shows a marked textural and vertical zonation that is also reflected in the chemical textural and isotopic variations of pyrite and carbonates pyrite evolves from primary framboids and colloform aggregates to late euhedral crystals which accompanies a redistribution of the contained base metals and grain coarsening sims analyses depict an extreme variability of 34s values from 42 3 to 4 4 with a gradual increase that is related to the textural maturation there is also a systematic rise in the 87sr 86sr ratios 0 70846 to 0 71354 13c 12 2 to 5 2 and 18o values 14 1 to 27 8 of the associated carbonates which evolve from early dolomite to late siderite this evolution evidences the maturation of the exhalative mineralization during mixing between two fluids modified seawater enriched in biogenically derived reduced sulfur and upflowing hydrothermal fluids early sulfides precipitated in the seawater mound interface towards the core of the mounds there was an increasing influence of deep fluids that modified the early mineral textures and the isotopic signatures this evolution is similar to that observed in modern submarine hydrothermal systems and confirms the importance of mound refining in the evolution of fossil exhalative mounds and the formation of economic ores

new stratification in the hydrothermal brine system of the atlantis ii deep red sea temperature and salinity records from the redsed cruise september 1992 reveal a new stratification in the hydrothermal brine system of the atlantis ii deep this stratification consists of the preexisting lower and upper convective layers lcl and ucl1 with temperatures of about 66 c and 55 c respectively and two additional upper convective layers ucl2 and ucl3 that formed more recently and have distinct temperature salinity and thickness in accordance with the physical process that leads to the formation of brine layers i e double diffusive convection a flow rate of about 200 l s has been estimated for the hydrothermal discharge

surficial structures of the northern red sea axial valley from 23 n to 28 n time and space evolution of neo oceanic structures abstract during recent french cruises in the northern red sea the surficial structures of the axial valley were studied by means of bathymetry magnetics and single channel reflection seismics new compilation maps have been established and comparison of the various data allow different segments of the northern red sea axial valley to the distinguished where only intra evaporitic deformations 24 25 n large volcanic outcrops mabahiss deep 25 30 26 n or several recent isolated volcanic intrusions 26 27 n characterize different stages of evolution this new information allows reconstruction of the evolution of oceanization cells from a simple intra evaporitic deep to a limited oceanic rift zone such as those observed in the central red sea a major characteristic of this evolution also depicted in other young oceanic rifts is a succession of volcanic and tectonic phases the major event in the northern red sea axial valley is a recent episode of voluminous but well localized magmatism it would represent a new stage in the red sea evolution and thus may have some implications on earlier phases

zn pb cu massive sulfide deposits brine pool types occur in collisional orogens black smoker types occur in backarc and or arc basins volcanic hosted massive sulfide deposits of zn pb cu type were derived either from seawater dominated buoyant fluids that built mounds on the seafloor e g the ores of the hokuroku basin japan or from saline fluids that reversed buoyancy on mixing with seawater and filled basins on the seafloor e g several ores of the iberian pyrite belt and the mount read province in tasmania the hokuroku ores formed above subduction zones during protracted periods of regional extension but both the iberian pyrite belt and the mount read province formed under extensional stress during continent continent iberia or arc continent mount read collision and orogenesis in the iberian pyrite belt oblique convergence resulted in transcurrent faulting and this may also be the case for the mount read province transcurrent faulting may have allowed easy vertical access for the ore related magmas some of which were sourced in the asthenosphere as well as the blueschist and eclogitic facies emplaced in adjacent terrains and possibly also saline fluids exsolved from the magmas

evidence from the northern red sea on the transition from continental to oceanic rifting abstract the northern red sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading the rift can be divided into two distinct regions the marginal areas consist of a series of bathymetric terraces and as indicated by geophysical data are underlain by a series of rotated fault blocks the axial depression which forms the axis of deep water is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow a large gravity minimum and very deformed sediments both regions are divided along strike into segments roughly 60 km long by cross trending transfer zones the presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions the intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas and at deeps located in the axial depression halfway between transfer zones although extension is becoming concentrated in the axial depression geophysical data imply that at least the upper few kilometers of the crust in that region consists of non magnetic continental type material and that this ca apace has been ruptured by large intrusions only at the deeps a wide range of crustal seismic velocities has been observed in the northern red sea including velocities normally associated with oceanic crust the velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the eastern desert of egypt the northern red sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin rifting starts in a series of linked half grabens 50 60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust this style of rifting can accommodate at least the amcunt of extension represented by the gulf of suez 35 km at some greater amount of extension the rift acquires a more symmetric shape probably through the development of antithetically faulted blocks in the hanging wall extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks at the stage represented by the northern red sea extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps the position of the deeps is controlled by the transfer zones established in the earliest stages of rifting with further extension the deeps serve as nucleation points that develop into small discrete cells of seafloor spreading these cells grow and coalesce to form a continuous mid ocean ridge spreading center such as is found in the southern red sea

hydraulic run away a mechanism for thermally regulated surges of ice sheets by using a simple parameterized model of thermomechanically coupled flow in cold ice sheets together with a physically based sliding law which includes a description of basal drainage we show that relationships between ice flux and ice thickness can realistically be multi valued and hence that hydraulically induced surges can occur we term this mechanism hydraulic run away as it relies on the positive feed back between sliding velocity and basal melt production for this feed back to operate it is essential that water pressure increases with water storage this is consistent with various recent ideas concerning drainage under ice sheets be it through a system of canals a distributed film or a subglacial aquifer for confined flows such as valley glaciers e g trapridge glacier or topographically constrained ice streams e g hudson strait in the laurentide ice sheet which are underlain by sufficiently deformable sediment we can expect thermally regulated surges to occur while in a laterally unconfined drainage basin such as that which flows into the ross ice shelf we might expect ice streams to develop

formation of fossil hydrothermal chimneys and mounds from silvermines ireland sulphur isotope analyses reveal that the fossil hydrothermal pyrite mound and tubes at silvermines were produced by the reaction of hydrothermal fe ii with bacteriogenically reduced sulphur species 34s range 18 4 to 42 5 this took place in a brine pool dominated by hyper saline carboniferous seawater this is in contrast to their modern but considerably larger morphological counterparts forming by direct precipitation of hydrothermal sulphides on the east pacific rise at 21 n

brine pool deposition for the zn pb cu massive sulphide deposits of the bathurst mining camp new brunswick canada ii ocean anoxia during mineralisation abstract current models of massive sulphide ore genesis in the bathurst mining camp new brunswick involve settling of sulphide particles from a stagnating low salinity hydrothermal plume spreading laterally in an anoxic ocean layer with minimal sulphate content there is fragmentary evidence of ocean anoxia in the form of local fine lamination in the shales that host some of the deposits but the total organic carbon s fe u th ni co v ni and v cr relationships indicate deposition under oxic or dysoxic conditions vanadium and mn values range from oxic to anoxic and sulphate reducing to non sulphate reducing but mn may be anomalously low due to derivation by erosion of acidic volcanic rocks the somewhat equivocal physical and chemical data combined with the likely disturbing effects of penecontemporaneous volcanism considerably weaken the case for an anoxic bottom layer in a static ocean the presence of barite with ambient seawater 34 s values in brunswick no 12 ore and the abundance of sulphate in modern euxinic basin waters make a sulphate free layer unlikely even if anoxic sulphate bearing low salinity fluids mixing with seawater would lead to growth of barite bearing chimneys and baritic rubble mounds which are not observed a model involving brine pool deposition better explains the major features of the bathurst ores

characteristics of hydrothermal plumes from two vent fields on the juan de fuca ridge northeast pacific ocean deep ctd transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally buoyant plumes emanating from each of two major vent fields on the southern symmetrical segment sss and endeavour segment es of the juan de fuca ridge at both vent fields emissions from point and diffuse hydrothermal sources coalesced into a single 200 m thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0 02 0 05 c and light attenuation anomalies of 0 01 0 08 m 1 10 80 g l above background temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped 10 15 km downcurrent light attenuation trends were non uniform and differed between plumes apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates vent fluids from the sss are metal dominated and mostly precipitate very fine grained hydrous fe oxides that remain suspended in the plume vent fluids from the es are sulfur dominated and precipitate a high proportion of coarser grained fe sulfides that rapidly settle from the plume the integrated flux of each vent field was estimated from measurements of the advective transport of each plume heat flux was 1700 1100 mw from the es and 580 351 mw from the sss particle flux varied from 546 312 g s to 204 116 g s at the es depending on distance from the vent field and was 92 48 g s from the sss

shear controlled evolution of the red sea pull apart model abstract results of seismic and other geophysical investigations suggest that strike slip processes controlled the break up of the arabian plate from africa and initiated the red sea rift early oceanisation was facilitated by nucleation of pull apart basins and massive intrusives the evolution of the red sea has gone through different stages it was a zone of structural weakness already during the pan african orogeny approximately 600 ma a major reactivation however that gradually led to the present day configuration was initiated during the late oligocene with intense magmatic activity and the development of a continental rift wrench faulting played a key role in the early evolution of the red sea as it shaped most of its western flank as a sharp plate boundary and resulted in the generation and rapid oceanisation of linearly arranged pull apart basins spatial distribution of these basins reflects the geometry of the strike slip zone which was controlled by pre existing fault systems like the najd shear system the central african fault zone or the onib hamisana and baraka suture zones strike slip motion along the latter zones of weakness influenced mainly the egyptian and sudanese coastal areas arabia was therefore separated from africa by oceanisation in those regions where pull apart basins developed they were still connected in the in between segments by stretched continental crust with arabia as the moving and africa as the stable plate the eastern red sea flank was formed by pure shear through stretching thinning and diffuse extension as a consequence the eastern and western flanks of the red sea are asymmetrical the acceleration of the movement of arabia in early middle miocene could no longer be accommodated by the opening in the gulf of suez and consequently the dead sea strike slip fault developed approximately 14 ma ago since plate motion was still oblique to the major structural trends the pull apart evolution on the western flank continued and the oceanisation was accentuated while in the north the eastern flank developed en echelon fractures of aqaba orientation seafloor spreading commenced 5 ma ago at parts of the central and southern red sea this final stage is also responsible for the bathymetry with maximum depth values of 3000 m in the seafloor spread areas and a much shallower basin in the north the metalliferous red sea deeps are either associated with this infant mid ocean ridge or with the en echelon fractures in the north

a serpentinite hosted ecosystem the lost city hydrothermal field the serpentinite hosted lost city hydrothermal field is a remarkable submarine ecosystem in which geological chemical and biological processes are intimately interlinked reactions between seawater and upper mantle peridotite produce methane and hydrogen rich fluids with temperatures ranging from 40 to 90 c at ph 9 to 11 and carbonate chimneys 30 to 60 meters tall a low diversity of microorganisms related to methane cycling archaea thrive in the warm porous interiors of the edifices macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the mid atlantic ridge but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems