submarine metamorphism of gabbros from the mid cayman rise an oxygen isotopic study abstract oxygen isotopic compositions of minerals in 22 samples of submarine gabbros were determined the gabbros were collected using the submersible alvin from the 700 m vertical section of the rift valley wall of the mid cayman spreading center our study indicates that in the mid cayman rise seawater barely reached the bottom of the plutonic layer abundant seawater penetration water rock mass ratio 1 was limited to the upper part of the plutonic layer from the observed oxygen isotopic compositions of coexisting minerals and from the experimental and empirical determinations of equilibrium fractionation of oxygen isotopes for mineral water and mineral mineral pairs we show the following 1 pyroxene and olivine did not exchange oxygen with seawater 2 plagioclase is in isotopic disequilibrium with pyroxene 3 the rate of oxygen exchange in plagioclase was not slowed by the absence of cation exchange 4 plagioclase and amphibole have exchanged oxygen with seawater or isotopically modified seawater 18o 3 and 5 amphibole has exchanged or acquired during formation hydrogen from seawater at 380 c t 600 c the decrease in extent of isotopic exchange of plagioclase and the decrease in amphibole abundance with depth indicate that seawater flux decreased rapidly with depth water rock mass ratio falling from 1 7 to 0 2 over a 300 m interval

newly discovered eastern dispersal of the youngest toba tuff volcanic glasses with minor mafic mineral fragments such as biotite and hornblende found in deep sea sediments of the south china sea basin scsb have been clearly identified as eruptive products of the youngest toba tuff ytt northern sumatra indonesia the tephra layer occurs between marine oxygen isotopic event 5 1 79 3 ka and event 4 22 64 1 ka with an interpolated age of 74 0 ka which is in good consistence with previous radiometric dating 73 75 ka and ice core dating 71 5 ka of the ytt the tephra consists predominantly of bubble wall shards with minor elongated vesicles of pumice fragments geochemical characteristics of the tephra such as high total alkali content high 87 sr 86 sr ratio and uniformity of their compositions all suggest that the recovered tephra is of the youngest toba tuff this finding supports an extended dispersal of coarse 63 mm glass shards over 1500 km northeast of the toba caldera a direction opposite to what previously conceived while providing a better documentation of the distribution extent of the toba ash this report points to the need to reestimating the eruptive volume of the ytt and re evaluating its environmental impact q 2000 elsevier science b v all rights reserved

discriminating hydrothermal and terrigenous clays in the okinawa trough east china sea evidences from mineralogy and geochemistry abstract the okinawa trough ot in the east asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic hydrothermal activities in this study the clays collected during iodp expedition 331 to the middle ot iheya north knoll were analyzed for mineralogical and geochemical compositions by comparing with the clays from the east china sea shelf and surrounding rivers we examine different clay origins the hydrothermal field in the mid ot is dominated by mg rich chlorite while the recharge zone has clay mineral assemblages similar to the shelf and rivers showing high content of illite subordinate chlorite and kaolinite and scarce smectite compared to the terrigenous clays the hydrothermal clays in the ot have high concentrations of mg mn and zr but low fe na k ca ba sr p sc and ti while the hydrothermal clays in the mid ocean ridge are relatively enriched in fe and v and depleted in al mg zr sc and ti different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays diagnostic of variable clay origins we infer that the ot hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids the remarkably different compositions of hydrothermal clays between the sediment rich back arc basin like ot and the sediment starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid rock sediment reactions under various geologic settings

hydrothermal circulation and metamorphism in crustal gabbroic rocks of the bay of islands ophiolite complex newfoundland canada evidence from mineral and oxygen isotope geochemistry

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

quartz cemented breccias from the mid atlantic ridge samples of a high salinity hydrothermal upflow zone textural mineralogical chemical and fluid inclusion studies on a suite of quartz rich sulfide bearing greenstone breccias indicate that high salinity high fluid flow systems were recently active on the mid atlantic ridge south of the kane fracture zone these systems involved fluids with temperatures close to 300 c and salinities almost 3 times that of seawater rounded pebblelike fragments enclosed within a matrix of euhedral quartz crystals appear to have been cemented by quartz deposition from a moving fluid which attained velocities in excess of 1 m s homogenization temperatures and freezing point depression effects in fluid inclusions from texturally early and late quartz crystals indicate temporal evolution of fluid temperatures and nacl equivalent salinities from 290 c and 10 wt to 200 c and 4 7 wt during alteration and cementation of the breccias highly variable cobalt concentrations in the evolving hydrothermal solutions are indicated by fine scale oscillatory zoning in euhedral pyrite within the breccias a continuous supply of the high salinity fluids may be generated either by supercritical two phase separation at depths well below the level of inclusion entrapment or by retrograde dissolution of hydroxy chloride phases in the deeper portions of the hydrothermal system either model requires the initial temperature of the deeply circulating fluids to have been distinctly higher than 415 c and the fluid to have been cooled by conduction or by mixing with lower temperature seawater prior to entrapment within the inclusions

synthetic fluid inclusions in natural quartz iii determination of phase equilibrium properties in the system h2o nacl to 1000 c and 1500 bars phase equilibria in the system h2o nacl have been determined to 1000 c and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion free brazilian quartz in the presence of the two coexisting immiscible h2o nacl fluids at various temperatures and pressures petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present or mixtures of the two salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single homogeneous fluid phase results of the present study are consistent with previously published data on the h2o nacl system at lower temperatures and pressures and indicate that the two phase field extends well into the p t range of most shallow magmatic hydrothermal activity as a consequence chloride brines exsolved from many epizonal plutons during the process of second boiling should immediately separate into a high salinity liquid phase and a lower salinity vapor phase and produce coexisting halite bearing and vapor rich fluid inclusions this observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions particularly the porphyry copper deposits in which halite bearing and coexisting vapor rich inclusions are commonly associated with the earliest stages of magmatic hydrothermal activity

sulfide composition and microthermometry of fluid inclusions in the leg 111 sheeted dike section of ocean drilling program hole 504b costa rica rift hole 504b of the ocean drilling program is dedicated to the study of crustal structure and hydrothermal processes in 5 9 m y old oceanic basement continuing the work of previous legs hole 504b was extended 212 3 m to a total depth of 1562 3 m below seafloor bsf during leg 111 in 1986 quartz sulfide veins occur at a depth of 1369 1388 m bsf in basalts of the sheeted dike complex the ore minerals are predominantly pyrite less chalcopyrite rare corich cu fe s phases and a thiospinel linnaeite carrollite microprobe analyses yield a high co content in zoned vein pyrites 8 wt as well as in the cu fe s phases 5 wt up to 35 8 wt co was detected in the thiospinel a co ni ratio of 100 distinguishes the vein pyrite from pyrite in the basaltic wall rock and from pyrite formed as an alteration product of olivine co ni 5 the co ni ratios correlate positively with cu and negatively with as co rich nonstoichiometric cu fe s sulfides in chalcopyrite are interpreted as metastable phases which have been quenched at a high temperature and prohibited from exsolution of the stable products chalcopyrite and pyrite fluid inclusions in quartz from the quartz sulfide veins are two phase and vary from liquid to vapor dominated their salinities range from 4 2 to 7 2 wt equivalent nacl and average 5 5 wt pressure 360 bars corrected average filling temperatures vary from 271 to 408 c with a maximum of 486 c this is consistent with calculated quartz formation temperatures for a single quartz separate 4 2 18o using oxygen isotope thermometry the 18o value of the hydrothermal fluid was determined to be 1 7 the temperature data indicate fluid alteration of the sheeted dikes at about 350 to 500 c the maximum homogenization temperatures intersect the liquid vapor two phase boundary above the critical point of seawater thus phase separation could have occurred before or during the formation of the mineralized veins and the alteration of the sheeted dike sequence

do lower crustal rocks record reversals of the earth s magnetic field magnetic petrology of oceanic gabbros from ocean drilling program hole 735b ocean drilling program odp leg 118 at the southwest indian ridge recovered the first vertically oriented continuously cored gabbros from in situ ocean crust magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the earth s magnetic field primary igneous magnetite is common only in evolved fe ti oxide rich gabbros and averages by volume 2 of the rock although substantially higher contents are common secondary magnetite produced by high temperature exsolution and hydrous alteration of olivine and pyroxene is observed in all gabbros studied and is the most important magnetic phase in the majority of recovered gabbros cumulate olivine gabbros because of the high temperatures associated with this alteration it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization or a very high temperature chemical remanent magnetization acquired very near the ridge crest hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant stable remanent magnetization finally previously published drilling results and field studies from odp site 735b indicate that although this crustal section has been tectonically uplifted it formed at a mid ocean ridge spreading center similar to much of the oceanic crust therefore the results of this magnetic study can be extrapolated to include the oceanic gabbro section in general and argue strongly that layer 3 crustal rocks are capable of recording reversals of the earth s magnetic field

two high levels of continental waters in the southern tunisian chotts at about 90 and 150 ka uranium and thorium content and activity ratios were measured on 20 samples 19 molluscs and 1 calcareous concretion from the great chotts in southern tunisia results were studied by the isochron method and by age frequency histograms they suggest that two major flood episodes took place about 150 and 90 ka uranium activity ratios indicate a ground water supply of continental origin in agreement with biological indicators which show large variations in salinity from fresh to marine like conditions the existence of a lacustrine phase radiocarbon dated to 17 40 ka as previously suggested for the northern sahara is highly questionable

the xigaze forearc basin evolution and facies architecture cretaceous tibet abstract the mid cretaceous to eocene flysch deposits of the xigaze forearc basin in southern tibet were investigated in a 120 km segment along the indus yarlung suture zone the basin evolved south of the magmatic arc gangdise belt of the lhasa block on top of trapped oceanic or transitional crust remnants of shelf carbonates are preserved along the northern edge of the basin the mapped segment was shortened by about 65 metamorphism reached low grade conditions along the northern margin of the basin the flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic andesitic to dacitic material shed from the magmatic arc gangdise belt particularly in the western part of the study area plutonic and sedimentary rocks from deep erosion levels and or more distal sources contributed to the basin fill rivers from the lhasa block acted as point sources and fed five major deep sea channel systems turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing the forearc flysch is subdivided into at least three megasequences which begin with wide up to several km incised 10 to 50 m coarse grained channel fills and their associated fan deposits the upper parts of the megasequences contain hemipelagic dark shales and marls deposited above the calcite compensation depth lateral channel migration channel lobe switching but also volcanic pulses generated a predominantly fining upward high frequency cyclicity after continental collision the marine sedimentation in the forearc basin was replaced by fluvial deposits of the eocene oligocene qiuwu formation which is time equivalent to the kailas and indus molasses farther west and rich in coarse gravel derived from the gangdise belt both forearc flysch and qiuwu formation were deformed simultaneously during the miocene we assume that the molasse type qiuwu formation represents the final continental facies of the forearc basin filling

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

an integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems chemical and isotopic changes accompanying seawater basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs incorporating provision for addition and subtraction of a wide range of reactant and product minerals as well as cation and oxygen and hydrogen isotopic exchange equilibria the models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100 to 350 c with an overall water rock ratio of about 0 5 being constrained by an assumed 18 o h 2o at 350 c of 2 0 per mil h craig personal communication 1984 this is a realistic model because 1 the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust 2 the water rock ratio is high 50 at low temperatures and low 0 5 at high temperatures and 3 it allows for back reaction of earlier formed minerals during the course of reaction progress the results closely match the major element chemistry von damm et al 1985 and isotopic compositions craig et al 1980 of the hydrothermal solutions presently emanating from vents at 21 n on the east pacific rise the calculated solution chemistry for example correctly predicts complete loss of mg and so 4 and substantial increases in si and fe however discrepancies exist in the predicted ph 5 5 versus 3 5 measured and state of saturation of the solution with respect to greenschist facies minerals the calculated d h 2o is 2 6 per mil in excellent agreement with analytical determinations the calculated chemical mineralogic and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges humphris and thompson 1978 stakes and o neil 1982 as well as with data from ophiolites gregory and taylor 1981 predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite epidote chlorite tremolite greenschist assemblage at 350 c the models demand that the major portion of the water rock interaction occur at temperatures of 300 350 c interaction at temperatures below approximately 250 c results in negative 18 o h 2o shifts contrary to the observed positive 18 o values of the fluids exiting at midocean ridge vents hydrogen isotope fractionation curves by suzuoki and epstein 1976 lambert and epstein 1980 and liu and epstein 1984 among others are compatible with the model and require d h 2o to increase at all temperatures as a result of seawater basalt interaction

submarine metamorphism of gabbros from the mid cayman rise petrographic and mineralogic constraints on hydrothermal processes at slow spreading ridges the coarse grained submersible collected rocks from the mid cayman rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro petrographic and mineralogic studies showed that 1 some deformation is present in all rocks 2 the distribution of deformation is not uniform within a specimen and 3 alteration has taken place preferentially in the deformed zones the abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth the compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied na k and h2o to and removed ca from the rock in all about 15 of the original rock has been transformed to amphibolite in the 700 m vertical section our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur if deformation continues to take place during the spreading of seafloor alteration may also continue to occur along newly formed fractures and cracks

synthetic fluid inclusions viii vapor saturated halite solubility in part of the system nacl cacl2 h2o with application to fluid inclusions from oceanic hydrothermal systems abstract halite solubility along part of the vapor saturated liquidus in the system nacl cacl 2 h 2 o has been determined using the synthetic fluid inclusion technique data allow the construction of liquidus isotherms for temperatures up to 500 c and bulk compositions containing 60 wt total salt and as much as 25 wt cacl 2 combined with previous data for the binary system nacl h 2 o and for the ternary system nacl cacl 2 h 2 o in the low salinity low temperature region a preliminary ternary phase diagram can be constructed that remains incomplete only in the cacl 2 rich region results are applied to the interpretation of saline fluid inclusions from quartz veins in oceanic metagabbros and can be applied to many other natural inclusions containing aqueous solutions with nacl and cacl 2 the dominant solutes microthermometric measurements at equilibrium of the melting temperature of ice tm ice and of the dissolution temperature of halite tm halite are sufficient to determine the bulk composition of the nacl cacl 2 h 2 o fluid

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

high temperature alteration of oceanic gabbros by seawater hess deep ocean drilling program leg 147 evidence from oxygen isotopes and elemental fluxes two major stages of high temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the hess deep rift valley at the east pacific rise 2 n 101 w the 154 m of plutonic rocks cored at site 894g show mineralogies and textures that suggest that they represent the roof of an approximate 1 myr magma chamber a late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite upper greenschist facies conditions the hydration of the gabbros occurred locally through the development of well crystallized green amphiboles significant chemical fluxes were associated with the fluid flow and precipitation dissolution mechanisms oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the hess deep gabbros at temperatures above 500 c hydration was limited with only minute amphibole lamellae in pyroxenes that trapped cl na and minor k from seawater ca plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns we propose that this high temperature event lowered the 18o of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion although it is unclear whether the 18o depletion of hole 894g gabbros is representative of the entire lower oceanic crust it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans during this stage water rock ratios calculated using sr isotopes are underestimated 0 1 0 5 because of insignificant diffusion of this cation in plagioclases better estimates of water rock ratios are provided by oxygen isotopes 0 2 1 which are important in unraveling the cooling history of the oceanic crust

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

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

criteria for the recognition of salt pan evaporites layered evaporites can accumulate in 1 ephemeral saline pans 2 shallow perennial lagoons or lakes and 3 deep perennial basins criteria for recognizing evaporites deposited in these settings have yet to be explicitly formulated the characteristics of the ephemeral saline pan setting have been determined by examining eight holocene halite dominated pans salt pans and their deposits marine and non marine from the u s mexico egypt and bolivia these salt pans are typified by alternating periods of flooding resulting in a temporary brackish lake evaporative concentration when the lake becomes saline and desiccation which produces a dry pan fed only by groundwater the resulting deposits consist of alternating layers millimetres to decimetres of halite and mud the layers of halite are characterized by 1 vertical and horizontal cavities rounded crystal edges and horizontal truncation surfaces due to dissolution during flooding 2 vertical chevrons and cornets grown syntaxially on the bottom during the saline lake stage 3 halite cements overgrowths and euhedral cavity linings and disruption of layering into metre scale polygons produced during the desiccation stage the muddy interbeds are characterized by displacive growth of halite during the desiccation stage immediately below the surface of the pan the halite layers are matured by repeated episodes of dissolution and diagenetic crystal growth this results in porous crusts with patches of chevron and cornet crystals truncated by dissolution clear diagenetic halite cement and internal sediment these layers of mature halite closely resemble the patchy cloudy and clear textures of ancient halite deposits holocene salt pans are known to cover thousands of square kilometres and cap halite deposits hundreds of metres thick so they are realistic models for ancient evaporites in scale e g permian salado formation of new mexico texas which preserves many primary salt pan features

elemental composition of concentrated brines in subduction zones and the deep continental crust abstract it has been well established that fluids played an important part in determining chemical characteristics in many crustal terranes studies of fluid inclusions in eclogites have established that brines coexisted with the primary mineral assemblages during their metamorphic crystallization these brines are currently multiply saturated in halide salts carbonates oxides and sulfides as a first step in quantitatively bounding the composition of the brines during metamorphism the equilibrium compositions of the brines at room temperature were computed using the aqueous speciation codes eq3 6 the results demonstrate that the brines are high density solutions ca 1 4 g cm 3 that have ionic strengths of approximately 8 mol and are approximately 40 dissolved solids by weight these are predominately na and k rich brines with subordinate ca and mg the approximate na k ca mg molar ratios are 4 2 0 5 0 2 but are sensitive to the assumed hco 3 concentrations charge balance is primarily maintained by the very high cl concentrations these brines bear resemblance to brines analyzed from fluid inclusions in metamorphic rocks reported by roedder roedder e 1972 composition of fluid inclusions us geol surv prof paper 440jj p 164 although these fluids have the potential of acting as significant metasomatic agents in subduction zones and deep crustal environments their impact will be mineralogically discernible only if the fluid release and movement is channelized

application of avhrr to monitoring a climatically sensitive playa case study chott el djerid southern tunisia the importance of monitoring changes in the levels of lakes within endorheic basins using remotely sensed data as a means of assessing changes in regional aridity is noted large salt playas are highlighted as ephemeral lakes that can display extreme sensitivity to changes in regional rainfall patterns and which commonly do not have extensively managed catchments to explore the application of high temporal frequency monitoring of salt playas using remote sensing the chott el djerid a large salt playa situated in southern tunisia was targeted a short time series of 39 advanced very high resolution radiometer avhrr resolution 1 1km at nadir images of the chott el djerid spanning 36 months between 1987 and 1990 were compiled along with climate information from a weather station at tozeur using image histogram manipulation lake areas were extracted from the time series a good level of agreement was observed between recorded rainfall events and the presence of surface water on the playa and for a limited sample of large flood events it was found that there were significant relationships between rainfall evaporation and estimated lake areas r 2 98 5 p50 001 overall these data suggest that contemporary lake formation is largely controlled by temporal changes in effective precipitation within the basin in addition it was found that the coefficient of variation of the time series and a combination of temporal reflectance profiles extracted from it could be used to give a direct indication of which sedimentary surfaces on the playa are affected by large flood events and the extent to which these events may be preserved within the recent sedimentary record at these sites copyright 1999 john wiley sons ltd

misr based passive optical bathymetry from orbit with few cm level of accuracy on the salar de uyuni bolivia abstract we demonstrate that under ideal circumstances passive optical measurements can yield surface water depth estimates with an accuracy of a few centimeters our target area is the salar de uyuni in bolivia it is a large active salt flat or playa which is maintained as an almost perfectly level and highly reflective surface by annual flooding to a mean depth of 30 50 cm we use misr data to estimate spatial and temporal variations in water depth during the waning portion of the 2001 flooding cycle we use a single icesat laser altimetry profile to calibrate our water depth model though the salt surface is probably the smoothest surface of its size on earth with less that 30 cm rms height variations over an area of nearly 104 km2 it is not completely featureless topography there includes a peripheral depression or moat around the edge of the salt and several sets of prominent parallel ridges with 5 km wavelength and 30 cm amplitude the process by which these features form is still not well characterized

evolution and source of ore fluids in the stringer system hellyer vhms deposit tasmania australia evidence from fluid inclusion microthermometry and geochemistry the hellyer deposit is a classic large tonnage high grade mound style volcanic hosted massive sulphide vhms deposit in the cambrian mt read volcanic belt of western tasmania in the footwall directly underlying the deposit there is an extensively altered pipe which cootains a well developed and preserved stringer zone the vein paragenesis at hellyer indicates that premineralization stage i veins consist entirely of quartz and occur throughout the alteration pipe the synmineralization stage 2 veins are the most abundant veins in the stringer zone and consist of three sub stages stage 2a veins of crustiform quartz pyrite and carbonate with minor amounts of chalcopyrite sphalerite and galena stage 2b veins with abundant base metal sulphides minor quartz carbonate and barite gangue and stage 2c veins of coarsely crystalline barite with variable amounts of pyrite sphalerite galena and carbonate stages 3 6 veins are postmineralization veins and are related to the devonian tabberabberan orogeny textural petrographic and microthermometric investigations of fluid inclusions in the hellyer stringer system indicate that type i primary liquid vapour inclusions occur along growth planes of crustiform quartz crystals or within colour banding of zoned sphalerite these inclusions are 10 15 m in size and yielded homnogenisation temperatures of 170 220 c in early 2a veins 165 322 c in main stage 2b veins and 190 256 c io late stage 2c veins these data suggest a waxing and waning thermal history however the average salinity remained between 8 11 naci equiv wt in all stage 2 veins chalcopyrite bearing primary fluid inclusions have been recognised in the base metal rich stage 28 veins no evidence for presence of co2 e g formation of clathrates was recorded by rnicrothermometry however laser raman spectroscopic lrs analysis indicates the presence of co2 1 mole in the stage 2b veins and no detectable co2 in 2a and 2c vein stages semi quantitative sem wds microprobe analyses of fluid inclusion decrepitates indicate that the hellyer ore fluid was enriched in potassium and calcium but depleted in magnesium relative to seawater pixe microanalysis of fluid inclusions in quartz indicates that the stage 2b ore fluids have a significantly higher base metal concentration compared to the stage 2a veins the postmineralization stage 4 veins have a variable but lower base metal content in this study there was no fluid inclusion evidence of boiling cation composition higher salinities relative to seawater and the presence of co2 suggest that recycled seawater alone cannot be the sole source of the ore fluids this interpretation is in agreement with previous isotopic studies in the hellyer stringer system although direct input of bulk ore constituents from a magma chamber cannot be demonstrated from the present fluid inclusion data such a contribution of ore fluids from a magmatic source cannot be ruled out the possible input from the magmatic source may have occurred during the base metal rich stage 2b vein formation characterised by the intensifying temperature of deposition higher base metals and co2 contents

fluid temperature and salinity characteristics of the matagami volcanogenic massive sulfide district quebec fluid inclusions hosted within various lithologic units of the 40 million metric ton mt matagami district abitibi greenstone belt preserve samples of archean volcanogenic massive sulfide vms and postvolcanogenic massive sulfide hydrothermal fluid microthermometric measurements on ore hosted primary two phase liquid vapor inclusions from matagami s south limb deposits indicate that the vms hydrothermal fluid was highly saline 16 2 4 7 wt nacl cacl2 equiv 1 n 230 and of moderate temperature trapping temperature tt 208 32 c 1 n 230 a fluid with these characteristics is capable of transporting 5 10 4 m 30 ppm zn as zncl3 and zncl4 2 chloride complexes however the low temperature of this fluid precluded efficient cu transport 3 ppm which may in part explain the relatively cu poor nature of the matagami deposits calculated densities of this ore fluid as high as 1 10 g cm3 are consistent with a bottom hugging brine model however a subset of the data indicate a fluid less dense than ambient seawater suggesting that buoyant hydrothermal plumes were also present a microthermometrically determined high cacl2 content xnacl 0 55 molar na ca 2 3 1 for the vms ore hosted primary fluid is consistent with an archean hydrothermal fluid more ca rich than modern day seawater quartz epidote veins located in the hydrothermal cracking zone of the bell river complex host primary liquid vapor halite inclusions these inclusions are interpreted to be samples of the deep seated equivalent to the vms ore hosted hydrothermal fluid described above microthermometry indicates that these inclusions trapped a high temperature brine tt 373 44 c 1 n 92 38 2 1 9 wt nacl equiv 1 n 92 we interpret this brine to be a phase separated product of modified model seawater 3 2 wt nacl an exsolved magmatic fluid or a combination thereof deep within the hydrothermal system at 650 to 670 c and a near lithostatic pressure of 90 mpa phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone the occurrence of high temperature brine overlain by lower temperature salinity fluid suggests a two cell convection model consistent with metal mass balance calculations for the south limb the high salinity of the ore hosted fluid inclusions indicates two possibilities 1 a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection 2 archean seawater itself was very saline and of variable salinity with the cooling of the bell river complex lower temperature fluids dominantly of seawater origin circulated deep within the hydrothermal system modified by water rock interaction yet not phase separated these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid vapor fluid inclusions homogenization temperature th 242 17 c 9 1 1 6 wt nacl equiv 1 n 14 hosted within the bell river complex quartz epidote vein material post and or waning stage vms hydrothermal activity is evident from the presence of quartz epidote veins crosscutting wabassee group hanging wall rocks microthermometry on quartz hosted primary liquid vapor fluid inclusions suggests that this activity occurred at relatively low temperatures th 76 177 c n 212 over a wide range of salinity 6 0 32 4 wt nacl cacl2 equiv n 212 and with a high apparent cacl2 content xnacl 0 06 these fluid inclusion data illustrate the importance of subsea floor chemical and physical processes directly related to metal transport and deposition in vms systems in particular phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits

geochemistry of north cleft segment vent fluids temporal changes in chlorinity and their possible relation to recent volcanism hydrothermal vent fluids from the north cleft segment of the juan de fuca ridge between 44 54 and 45 00 n were sampled in 1988 1990 1991 and 1992 in 1988 chloride depleted relative to seawater diffuse fluids with low dissolved metal concentrations relative to other mid ocean ridge mor fluids were sampled over a distance of 10 km along axis in 1990 and 1991 both high temperature 200 c and diffuse vents were consistently metal and chloride enriched the end member compositions of high temperature vents and nearby diffuse vents were very similar there were small but significant correlated changes in diffuse and high temperature composition from 1990 to 1991 the large scale but temporary venting of low chlorinity fluids over the entire north cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride enriched fluids it is possible that a volcanic event along the north cleft segment prior to 1987 for which there is firm geologic evidence initially caused a boiling event which resulted in the preferential venting of vapor enriched fluids through 1988 followed by a transition to brine enriched fluids by 1990 high iron low sodium and low sr ca ratios in the high chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event the absence of chloride depleted fluids from 1990 onward and the systematics of lithium boron and manganese with chloride in the high temperature fluids from north cleft suggest that the evolution toward lower chlorinity at monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater

marine like potash evaporite formation on a continental playa case study from chott el djerid southern tunisia abstract an exceptional flood in january 1990 led to the formation of a large ephemeral lake on the chott el djerid a salt playa in southern tunisia repeated observations made during 1990 show that the ephemeral lake underwent four evolutionary stages 1 initial flooding 2 evaporative concentration of lake waters 3 the movement of concentrated brine pools over the playa surface as a result of wind action and 4 total desiccation of the lake by september 1990 during all four stages the brine chemistry of the lake was monitored water inflow into the chott el djerid basin was found to have a consistent ca so 4 cl rich and hco 3 co 3 poor chemistry reflecting the recycling of homogeneous assemblages of cretaceous mio pliocene and quaternary evaporites within the catchment as the ephemeral lake shrank these waters produced an na mg k cl so 4 brine which was similar to modern sea water mineral saturation data show that during the desiccation of the lake saturation with respect to both gypsum and halite was achieved and that the most concentrated brines were ultimately saturated with respect to potash phases after the desiccation of the lake the main mineral phases found on the chott included gypsum and halite in addition ephemeral deposits of carnallite observed as carnallitite 3kmgcl 3 6h 2 0 nacl were found this assemblage is that which would be expected to form if the waters had undergone salt norm evaporation at 1 bar pressure at 25 c snorm in the evaporation model proposed by jones and bodine 1987 the nature of both the brine chemistry and evaporite mineralogy provides a new and rare example of marine like potash bearing evaporites being formed in a contemporary continental playa

petrology isotope geochemistry and chemical budgets of oceanic gabbros seawater interactions in the equatorial atlantic petrological and chemical variations as well as oxygen and strontium isotopic data are presented for metagabbros from the romanche and vema fracture zones these rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization five major kinds of alteration processes ranging from late magmatic deuteric alteration stage i to low temperature 150 c alteration stage v were identified water rock interactions between 300 and 650 c are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and or greenschist facies amphibole plagioclase epidote titanite chlorite prehnite hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater derived fluids these exchanges lead to decrease of gabbroic 18o toward values as low as 3 9 and larger sr isotopic variations than other oceanic gabbroic rocks 87sr 86sr ratios shift to 0 7029 0 7051 calculation of a chemical budget indicates that metagabbros are hydrated and enriched in fe and probably in mg and cl while si ca and ti are released to the hydrothermal fluids in addition to metamorphic recrystallization and geochemical transformation hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry

petrologic evolution of large failed rifts in the eastern pacific petrology of volcanic and plutonic rocks from the mathematician ridge area and the guadalupe trough

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

multistage hydrothermal alteration of gabbroic rocks from the failed mathematician ridge oxygen isotopic mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed mathematician ridge the mathematician ridge though initially a fast spreading ridge presently has a deep central rift valley more typical of slow spreading rifts the inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large offset fracture zones undeformed mathematician ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post crystallization hydrothermal alteration metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole low calcium plagioclase and ilmenite oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700 c water rock ratios of less than one and produced modified seawater fluids with isotopic compositions of 3 5 6 0 the amphibolites are cross cut by multiple generation quartz epidote veins forming a network replacement of the amphibole grade mineralogies by greenschist grade mineralogies fluid oxygen isotopic compositions of 2 4 and a two phase separation in the fluid captured in conjugate vapor fluid inclusions are all associated with the formation of this network fluid inclusion data for the quartz indicate initial temperatures of over 500 c these samples may represent rocks from the 1 2 km deep plumbing system of black smoker type venting

metamorphism in oceanic layer 3 gorringe bank eastern atlantic gorringe bank is an anomalously high structure of the eastern part of the north atlantic which was known to be composed of mantle derived peridotites layer 4 and gabbros layer 3 during the submersible cruise cyagor ii in 1981 the contact between layer 4 and layer 3 was observed on mount gettysburg and interpreted as tectonic the overlying series of gabbro was extensively sampled on both mounts composing the bank gettysburg and ormonde coarse grained to pegmatoid clinopyroxene gabbros predominate and are associated with differentiated rocks ferrogabbros and diorites cumulate gabbros are missing the gabbroic section sampled is therefore interpreted as the upper part of the plutonic section most samples were strongly recrystallized during two distinct events metamorphism occurred close to the ridge axis from interaction of a seawater derived fluid with still hot gabbros high temperature shear zones favoured fluid circulation but the water rock ratio estimated from the sodium input was very small in undeformed rocks 1 the low w r ratio explains the strong evolution of the fluid phase and therefore some particular compositions of secondary minerals low temperature alteration occurred when the gabbros were tectonically emplaced close to the sea bottom

anomalies in rare earth distributions in seawater gd and tb we have measured profiles of the rare earth elements ree in atlantic and pacific ocean waters the data normalized versus shales exhibit a pronounced anomaly in gd relative to its neighbors eu and tb in the ree series such that the gd concentrations are high by 30 50 closer inspection reveals that the anomaly is made up of both elevated gd and depressed tb concentrations likely associated with solution chemistry shifts in the transition from an exactly half filled 4f electron shell anomalies in gd and tb solution complexation are also indicated by the turner whitfield dickson speciation model the overall trend of heavy ree iii enrichment in seawater and the gdtb anomaly described here tend to support scavenging as an important removal mechanism for the ree from seawater

systematics of and postulated controls on submarine hydrothermal solution chemistry the chemical compositions of seafloor hydrothermal solutions from the galapagos spreading center 21 n and 13 n on the east pacific rise the southern juan de fuca ridge and the guaymas basin gulf of california are examined for systematic variations several elemental ratios elemental products and empirical geothermometers show much consistency among the various areas in spite of large absolute differences in elemental concentrations the results suggest that most of the major cations in these solutions are solubility controlled with respect to a similar phase assemblage at all of the sites examination of the solution chemistry in conjunction with experimental results and seafloor rock alteration data permits identification of phases which may be important in controlling the solution chemistry as potassium appears to be solubility controlled its use to calculate water rock ratios is inappropriate lithium appears to be a better element to use in these calculations a model to explain the observed solution chemistries as a result of three component mixing is discussed all of the solutions are postulated to be either the result of mixing between a seawater hydrothermal solution which has not undergone a phase separation and is hence gas containing a gas rich vapor phase and a gas poor brine phase or any one of these individual components alone the vapor and brine phases are the result of phase separation which may have occurred at some time in the past essentially all of the reported data fall within the field defined by these three components the solutions apparently have had sufficient time after the mixing event s to reequilibrate with the rock this is true for quartz as well as other chemical components the best test of this model is chloride together with gas and isotropic data as the model predicts that solutions with elevated chlorinities will be depleted in gases and enriched in 18 o and low chlorinity solutions will have elevated gas contents and lower 18 o

mesozoic volcanogenic massive sulfide vms deposits in mexico abstract volcanogenic massive sulfide vms deposits are the most conspicuous type of deposits that formed during the mesozoic in mexico many mexican vms deposits display classical kuroko type mineral zonation and structure and some of them as cuale and la minita formed in shallow submarine environments the most prospective time window for the formation of vms deposits in mexico comprises the late jurassic and the early cretaceous vms stopped forming during the progressive continentalization of mexico since its metallotectonic processes dominated by extensive tectonics changed giving way to compression during the late early cretaceous new vms deposits did not form until after the opening of the gulf of california mesozoic vms deposits in mexico occur in submarine volcano sedimentary sequences that deposited essentially in association with back arc basins now found roughly along the boundaries between tectonostratigraphic terranes or within juvenile and slightly evolved arcs at the internal parts of the terranes while few others occur on the epicontinental seafloor and hinterlands of eastern mexico vms deposits are especially abundant in the guerrero composite terrane and are also present in the alisitos and parral terranes however new evidence referred in this paper indicate the western continental edge of the cratonic block of oaxaquia as a promising new prospective region for vms deposits interestingly no vms deposits are found in the northern part of the guerrero composite terrane despite the occurrence of marine volcano sedimentary sequences similar to those in the south such absence can be related to differential extensional unroofing much larger in the southern part of the guerrero composite terrane than in the northern part many vms deposits occur along or close to terrane boundaries especially around the guerrero composite terrane this distribution reflects the association between vms deposits and back arc basins which represented the frontal part of terranes or sub terranes that were ultimately accreted to the oaxaquia cratonic block as a consequence vms deposits usually display strong deformation and thrusting and their mineral and compositional zonation can be found overturned due to such common association between these deposits and terrane boundaries that reactivated during the cenozoic vms deposits are especially susceptible to overprinting by later metallogenic processes unrelated to vms producing environments this susceptibility may be held accountable for the complex mineral associations found in the francisco i madero deposit the inclusion fluids in the cuale la minita el rubi tizapa and campo morado deposits have salinities that range from 2 5 to 20 0 wt nacl equiv and temperatures of homogenization from 110 to 420 c although a particular fluid inclusion assemblage with daughter halite and sylvite from a stockwork at the tizapa deposit ranged salinities from 39 7 to 64 7 wt nacl and from 35 9 to 43 5 wt kcl and temperatures of homogenization from 440 to 550 c the general characteristics listed above may account for mineralizing fluids from magmatic marine and modified marine sources the entrainment of cool and oxidizing seawater within upwelling fluids increased as the paleohydrothermal systems waned the waning stages are normally represented by barite rich mineralizations and in the case of the la minita deposit this notoriously outlasted by mn oxide mineralization that reflects even more oxidizing conditions a similar evolution is deduced in cuale which is the only known mesozoic vms deposit in mexico that formed in a shallow submarine environment together with la minita the shallower the formation of vms deposits is the more quickly mineralizing fluids evolve into highly oxidized end members once the paleohydrothermal systems cooled down the extremely high salinity and hot inclusion fluids in tizapa despite the unavailability of further geochemical data are likely to represent magmatic brines that did not undergo significant mixing with seawater as for the temporal evolution of the studied deposits they generally exhibit 1 an increasing magmatic contribution from early to middle stages for both fluids as mineralizing fluids grew hotter and more saline and sulfur and 2 the waning of hydrothermal activity towards the last stages on mineralization which is characterized by the prevalence of seawater as fluids grew cooler and more diluted and oxidizing thus reflecting quite a typical evolution for most vms deposits elsewhere new 34s values for the same vms deposits reflect multiple sources of sulfur in the mineralizing fluids magmatic sedimentary metasedimentary and marine different degrees of dominance of either source for sulfur in these deposits and in their stages of mineralization have been inferred in a way that also responds to a schematic evolution from magmatic to seawater dominated paleohydrothermal systems

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

geochemistry of brines of the chott el jerid in southern tunesia application of pitzer s equations abstract in the chott el jerid in southern tunisia waters are strongly concentrated by evaporation under an arid climate the diluted waters are of the ca 1bmg 1bso4 type whereas the brines are of the na 1bcl type relatively concentrated in k and mg in concentrated waters ca and silica concentrations remain at a very low level the use of pitzer s equations for calculating the ion activities has permitted accurate saturation tests and a fairly good evaluation of the mineral sequences controlling the geochemistry of the basin

salt ramps wind induced depositional features on tunisian playas abstract semicircular and crescent shaped accumulations of salt crystals developed on salt crusts were measured on three tunisian playas in september 1990 these features have been termed salt ramps their morphology and chemistry suggest that they are formed in the late stages of shallow ephemeral lake desiccation in playa basins they form by salt precipitation from shallow brine lakes that are blown across salt encrusted playa surfaces by the wind moreover they appear to be short lived features and their degradation is related to the flooding of playas with less saline water and possibly rainfall and deflation key words playas salt crusts halite brine salt ramps tunisia introduction salt crusts in playas develop microtopographic features as a result of salt efflorescence desiccation and solution neal 1965 teller et al 1982 last 1984 only remaining smooth when covered by water or when above the capillary fringe eugster and kelts 1983 lowenstein and hardie 1985 describe the different sedimentary and surface features of a typical flooding and desiccation cycle in a playa as surface water evaporates it becomes increasingly saline and eventually begins to precipitate salts precipitation may occur in the brine and or at the water sediment interface halite crystals which precipitate at the lake surface develop into characteristic cup shapes known as hopper crystals teller

chemistry of hydrothermal solutions from the southern juan de fuca ridge fluids from three vent fields on the southern juan de fuca ridge were sampled in september 1984 using the dsrv alvin the fluids are uniquely high in both chloride which ranges up to twice the seawater value and in metal content simple vapor liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids the cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes 1 subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and 2 dissolution of a chloride rich phase combined with a possible small loss of gas during sampling procedures although measured temperatures were all less than 300 sup 0 c quartz geothermometry suggests that the fluids have equilibrated at greater than 340 sup 0 c quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber copyright american geophysical union 1987

mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks mineralogical and isotopic variations observed in altered glassy and crystalline rocksfrom the east pacific rise and the mid atlantic ridge provide information about the temperatures of alteration and seawater rock ratios for various hydrothermal regimes within the oceanic crust a systematic increase in alteration temperature is evident for the glassy rocksin the sequence 1 nontronite and celadonite vesicle fillings 35 c 2 saponite rich pillow breccias 130 170 c 3 calcite rich greenstone breccias and epidote rich greenstone 200 350 c theseresults include the highest temperatures thus far reported for saponite formation the seawater dominated hydrothermal alteration process that formed the saponite rich pillow breccias is characterized by high water rock ratios 0 1 low to moderate temperatures a seawater origin of most of the carbon in vein calcites 13 c 0 and the predominance of fe rich saponite and calcite as secondary phases greenstones chlorite quartz epidote and greenstone breccias chlorite quartz albite calcite are altered in a rock dominated system with lower water rock ratios 50 1 to 13 c 4 the crystalline rocks diabase gabbro and metagabbro are affected to varying degrees by pervasive high temperature seawater interactions that commence soon after solidification producing varying proportions of fine grained secondary minerals including talc smectite chlorite vermiculite actinolite and sodicplagioclase hydrothermal solutions derived from alteration of the crystalline rocks are of the appropriate temperature and isotopic composition to alter the overlying glassy rocks to the observed mineralogies as well as being the source of metal rich deposits associated with the oceanic spreading centers

initiation and collapse of active circulation in a hydrothermal system at the mid atlantic ridge 23 n gabbro and basalt collected from an area south of the kane fracture zone along the mid atlantic ridge have three stages of alteration which record the cooling of a hydrothermal system 1 stage 1 penetration of seawater began between 400 and 550 c altering pyroxene to fibrous green amphibole 2 stage 2 propylitic alteration formed along connected fractures between 250 and 300 c as fracture density increased the fe mg ratio of chlorite increased the final result being an fe chlorite quartz sulfide breccia 3 stage 3 late smectite veinlets formed at low temperatures 200 c after active circulation ceased the study focuses on stage 2 alteration by assuming local equilibrium between alteration minerals and the hydrothermal fluid constraints can be placed on the fluid composition responsible for stage 2 alteration the stage associated with deposition of sulfides the following activities of species in solution were determined for the system feo na2o cao mgo al2o3 sio2 h2o at 350 bars and 250 c log a ca a2 h 8 0 log a na a h 5 0 log a fe a2 h 1 7 log a mg a2 h 6 0 log a sio2 was set at quartz saturation 2 3 at 350 bars and 250 c fluid inclusions record the introduction of a low temperature seawater salinity fluid during formation of the latest quartz veins associated with stage 2 alteration mixing of this and the hydrothermal fluid caused a drop in temperature and increase in oxidation state resulting in increased precipitation of quartz pyrite and chalcopyrite the salinities of fluid inclusions trapped in quartz during stage 2 alteration are as much as 3 times that of seawater concentration of a fluid initially of seawater salinity may be the result of boiling at 350 c and 3000 m depth

metabasalts from the mid atlantic ridge new insights into hydrothermal systems in slow spreading crust an extensive suite of hydrothermally altered rocks were recovered byalvin and dredging along the mark mid atlantic ridge south of the kane fracture zone 23 24 n where detachment faulting has provided a window into the crustal component of hydrothermal systems rocks of basaltic composition are altered to two assemblages with these characteristics i type i albitic plagioclase an02 10 mixed layer smectite chlorite or chlorite actinolite quartz sphene 20 of the clinopyroxene is altered and cu and zn are leached the geochemical signature of these alteration types reflects the relative proportion and composition of secondary minerals and the degree of alteration of primary phases and does not show simple predictive relationships element mobilities indicate that both alteration types formed at low water rock ratios the mark assemblages are typical of the greenschist and transition to the amphibolite facies and represent two distinct albeit overlapping temperature regimes type i 180 to 300 c and type ii 250 to 450 c by analogy with dsdp odp hole 504b and many ophiolites the mark metabasalts were altered within the downwelling limb of a hydrothermal cell and type i and ii samples formed in the upper and lower portions of the sheeted like complex respectively episodic magmatic and hydrothermal events at slow spreading ridges suggest that these observed mineral assemblages represent the cumulative effects of more than one hydrothermal event groundmass and vein assemblages in the mark metabasalts indicate either that alteration conditions did not change during successive hydrothermal events or that these assemblages record only the highest temperature event lack of retrograde reactions or overprinting of lower temperature assemblages e g zeolites suggests that there is a continuum in alteration conditions while crustal segments remain in the ridge axis environment the type ii samples may be representative of thereaction zone where compositions of hydrothermal fluids actively venting at the seafloor today become fixed this prediction necessitates interaction between hydrothermal fluids and intersertal glass and or mafic phases in addition to plagioclase in order to produce the observed range in vented fluid ph

monitoring salt playa dynamics using thematic mapper data various image analysis techniques have been applied to multitemporal landsat thematic mapper data collected over a four year period the results have been used to monitor sediment transport processes in semi arid south central tunisia process domains on salt playas in the study area have been identified and mapped seasonal interseasonal and spatial changes on these playas have been related to sediment salt and moisture regimes surface water and groundwater activity and eolian processes are identified as important sediment transport processes the observations bring into question the traditional groundwater based model of salt playa geochemistry a revised set of integrated geochemical geomorphological models is invoked

rapid high temperature metamorphism of east pacific rise gabbros from hess deep abstract metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid ocean ridges but the temperature range rate and location of subsolidus events are poorly known we combine hornblende plagioclase thermometry statistical analysis and thermal models to estimate precisely the temperature time distance from axis and duration of metamorphism in east pacific rise gabbros from hess deep odp hole 894g metamorphic hornblende and plagioclase which formed during microfracturing and sea water penetration equilibrated at a mean temperature of 716 8 c 90 confidence level comparison of the properties of the observed temperature distribution with those of model events indicates that metamorphism spanned 60 c when combined with thermal models of fast spreading centers this implies that metamorphism was rapid 6000 yr and occurred 1 4 km off axis application of this approach to other gabbros will allow comparison of spatial and temporal characteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting

paleohydrological indicators in playas and salt lakes with examples from canada australia and africa abstract the sedimentary records of playas and salt lakes provide some of the best evidence for hydrological change in a region these records are comprised of siliciclastic sediments and chemically precipitated minerals that typically vary temporally and spatially in some cases the interpretation is straightforward with mineralogical sequences related to changing brine concentrations and hydrology as well as to climate in other cases it is difficult to identify the controlling factors which include climate groundwater geological setting and basin morphology in addition diagenesis commonly affects the sedimentary record models of lake level fluctuations are given for basins in the canadian prairies and southeastern australia the lake manitoba model is based mainly on secondary changes associated with pedogenesis that alters the deeper water sediment during low water stages smaller canadian lakes in more arid regions display a larger variety of diagnostic paleohydrological indicators such as specific carbonate and salt mineralogy nature of bedding grain size and secondary alterations decreasing water levels in shallow lakes tend to show a progression from clastics to carbonates to salts and then back to clastics as groundwater levels fall below the lake floor similar parameters are used in australian lakes the hydrological model for the lake tyrrell playa in victoria whose present ionic composition is similar to seawater shows clastics being deposited when average lake levels are high followed by deposition of primary carbonates gypsum and halite once seasonal drying on the lake floor begins secondary gypsum grows within already deposited sediment and clay pelletization in the capillary fringe of surface sediment leads to basin deflation and the accumulation of adjacent eolian lunettes continued water level decline leads to pedogenesis subsequent lake level rises in all of these models may initiate sedimentation in any part of the hydrological cycle and a complete cycle in the sedimentary record appears to be uncommon

interactions between magma chambers and hydrothermal systems oceanic and ophiolitic constraints a compilation of published exit temperatures and salinities for seafloor hydrothermal fluids along with fluid inclusion data on oceanic and ophiolitic rocks shows that ridge axes hydrothermal systems are generally characterized by maximum temperatures around 300 360 c thus clearly being in the one phase field they overlie a lower system marked by the generation of highly saline brines at high temperatures the brines may have originated either when seawater approached the top of the magma chamber or more probably as exsolution products of the differentiated melts that typify the roof of oceanic magma chambers in ophiolites the brines typically occur within the transition zone between the sheeted dike complex and the plutonic sequence i e on top of magma chambers this transition zone relatively well known from ophiolite studies oman troodos bay of islands josephine is characterized by mutually intrusive relationships between gabbro plagiogranite and dikes and xenoliths of altered diabase within plagiogranite and gabbro in fast to medium spreading ophiolites oman it is the locus of a major downward decrease in the density of hydrothermal veins and alteration the vein system is strongly anisotropic and shows a well marked preferred vertical along strike orientation the underlying cumulates are almost unaltered and exhibit only a diffuse net of amphibole veins while the overlying sheeted dike complex is densely veined and pervasively altered this zone is marked by major gradients in the physical parameters over short distances temperature several degrees celsius per meter pressure several hundred kilopascals transition from brittle to ductile in steady state systems direct interactions between the magma chamber and the hydrothermal system are restricted to this zone characterized by exsolution of magmatic volatiles into the hydrothermal system and the incorporation of hydrothermal fluids into the magma chamber by digestion of altered roof rocks complications to this steady state model resulting from a non steady state magma chamber e g josephine ophiolite are mainly the development of late stage faulting and more pronounced retrogressive alteration

controls of fluid chemistry and complexation on rare earth element contents of anhydrite from the pacmanus subseafloor hydrothermal system manus basin papua new guinea ocean drilling program odp leg 193 successfully drilled four deep holes 126 to 386 m into basement underlying the active dacite hosted pacmanus hydrothermal field in the eastern manus basin anhydrite is abundant in the drill core material filling veins and vesicles cementing breccias and occasionally replacing igneous material we report rare earth element ree contents of anhydrite from a site of diffuse venting site 1188 which show extreme variability in terms of both absolute concentrations e g 0 08 28 3 ppm nd and pattern shape lan smn 0 08 3 78 smn ybn 0 48 23 1 eu eu 0 59 6 1 the range of ree patterns in anhydrite includes enrichments in the middle and heavy rees and variable eu anomalies the patterns differ markedly from those of anhydrite recovered during odp leg 158 from the tag hydrothermal system at the mid atlantic ridge which display uniform lree enriched patterns with positive eu anomalies very similar to tag vent fluid patterns as the system is active the host rock composition is uniform and the anhydrite veins appear to relate to the same hydrothermal stage we can rule out predominant host rock and transport control instead we propose that the variation in ree content reflects waxing and waning input of magmatic volatiles hf so2 and variable complexation of rees in the fluids ree speciation calculations suggest that increased fluoride and possibly sulfate concentrations at pacmanus may affect ree complexation in fluids whereas at tag only chloride and hydroxide complexes play a significant role the majority of the anhydrites do not show positive eu anomalies suggesting that the fluids were more oxidizing than in typical mid ocean ridge hydrothermal systems we use other hydrothermal fluids from the manus basin vienna woods and desmos which bracket the pacmanus fluids in terms of acidity and ligand concentrations to examine the dependence of ree complexation on fluid composition geochemical modeling reveals that under the prevailing conditions at pacmanus ph 3 5 t 250 300 c eu oxidation state and the relative importance of fluoride versus chloride complexing are very sensitive to small variations in oxygen fugacity temperature and ph patterns with extreme mid ree enrichment may reflect speciation effects free ion abundance coupled with crystal chemical control we conclude that the great variability in ree concentrations and pattern shape is likely due to variable fluid composition and ree complexation in the fluids

late quaternary climatic reconstruction for the maghreb north africa abstract the paleoclimatic evolution of the maghreb since 40 000 yr b p is not yet known in great detail for that purpose data from more than 35 sites have been collected although only about 11014c dates and a few prehistoric remains can be used to correlate these data however the sites are well distributed over the maghreb and the convergent results from paleosoils paleolakes alluvian or eolian deposits pollen analysis etc induce one to draw up a certain pattern in climatic evolution for the period between 40 000 and 20 000 yr b p the data are still fragmentary but are concordant in indicating wetter environments generalized as far as the northern sahara the effect of dropping temperatures on the increasing water budget and soil moisture is not yet clear but rainfall could have been more regular and more abundant as a consequence of the shifting of middle latitude cyclone paths towards the south in both summer and winter the environmental indicators show that between about 20 and 18 000 yr b p rainfall progressively became more irregular however proofs of aridity in the south maghreb and the northern sahara is evident only for the period between 12 10 000 yr b p at that time the saharan high pressure belt was reinforced and well established at higher latitudes than at present this dry climate lasted more or less until the thermal optimum after 6000 yr b p there is some evidence that the climate became wetter again but the environmental conditions were rather different from those of before 20 000 yr b p for instance the middle holocene dark humic sois are the result of a pedogenesis different from that of the late pleistocene reddish paleosoils for the last two millennia it is not easy to distinguish between a natural trend towards aridification and desertification through man s intervention

climate surface pore water interactions on a salt crusted playa implications for crust pattern and surface roughness development measured using terrestrial laser scanning sodium accumulating playas also termed sodic or natric playas are typically covered by polygonal crusts with different pattern characteristics but little is known about the short term hours dynamics of these patterns or how pore water may respond to or drive changing salt crust patterning and surface roughness it is important to understand these interactions because playa crust surface pore water and roughness both influence wind erosion and dust emission through controlling erodibility and erosivity here we present the first high resolution 10 3m hours co located measurements of changing moisture and salt crust topography using terrestrial laser scanning tls and infra red imagery for sua pan botswana maximum nocturnal moisture pattern change was found on the crests of ridged surfaces during periods of low temperature and high relative humidity these peaks experienced non elastic expansion overnight of up to 30 mm and up to an average of 1 5 mm night during the 39 day measurement period continuous crusts on the other hand showed little nocturnal change in moisture or elevation the dynamic nature of salt crusts and the complex feedback patterns identified emphasise how processes both above and below the surface may govern the response of playa surfaces to microclimate diurnal cycles

chemical interaction of aqueous solutions with epidote feldspar mineral assemblages in geologic systems ii equilibrium constraints in metamorphic geothermal processes thermodynamic analysis of the system na sub 2 o k sub 2 o cao feo fe sub 2 o sub 3 al sub 2 o sub 3 sio sub 2 h sub 2 o hcl co sub 2 at pressures and temperatures to 5 kb and 600 sup 0 c affords quantitative description and interpretation of phase relations among epidote garnet plagioclase and alkali feldspar solid solutions in hydrothermal systems comparison of computed and observed compositions of these minerals and coexisting fluids suggests that the calculations afford close approximation of equilibrium and mass transfer in metamorphic geothermal processes the standard molal enthalpies of decomposition reactions for plagioclase are of the order of 4 to 50 kcal mole sup 1 which requires the activity of the anorthite component of plagioclase coexisting with epidote solid solutions to increase with increasing temperature at constant pressure in contrast because the standard molal volumes of these reactions are also negative a sub caal sub 2 si sub 2 o sub 8 decreases with increasing pressure at constant temperature either stoichiometric epidote or clinozoisite is compatible with plagioclase quartz calcite and an aqueous phase but only at x sub co sub 2 less than or equal to 0 2 the complex zoning commonly exhibited by both plagioclase and epidote solid solutions in geologic systems can be attributed to minor isothermal isobaric changes in the composition of coexisting aqueous solutions logarithmic activity and fugacitymore diagrams constructed with provision for solid solution permit documentation of such changes and facilitate thermodynamic interpretation of mineral and fluid compositions in metamorphic geothermal systems less

ocean floor hydrothermal metamorphism in the limousin ophiolites western french massif central evidence of a rare preserved variscan oceanic marker the limousin ophiolite is located at the suture zone between two major thrust sheets in the western french massif central this ophiolitic section comprises mantle harzburgite mantle dunite wehrlites troctolites and layered gabbros it has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and or pargasite bearing chloritites with various thermobarometric methods it is possible to show that the different varieties of amphibole have registered low p c 0 2 gpa conditions with temperature ranging from high t late magmatic conditions to greenschist zeolite metamorphic facies the abundance of undeformed metamorphic rocks which is typical of the lower oceanic crust the occurrence of ca al mg metasomatism illustrated by the growth of ca al silicates in veins or replacing the primary magmatic minerals the p t conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from ocean drilling program and worldwide ophiolites are the main arguments for an ocean floor hydrothermal metamorphism in the vicinity of a palaeo ridge among the west european variscan ophiolites the limousin ophiolites constitute an extremely rare occurrence that has not been involved in any hp subduction related or mp orogenic metamorphism as observed in other ophiolite occurrences i e france spain and germany

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

microthermometry and chemical composition of fluid inclusions from the mt chalmers volcanic hosted massive sulfide deposits central queensland australia implications for ore genesis mt chalmers is a mound style volcanic hosted massive sulfide vhms deposit in central queensland australia the ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of early permian age the two ore lenses main lode and west lode consist of cu zn pb massive sulfide underlain by cu rich stringer mineralization textural and petrographic investigations of fluid inclusions indicate that primary type i inclusions up to 25 microns are found in quartz from the stringer mineralized zone and microthermometric studies of these inclusions yielded homogenization temperatures of 160 285 degrees c and salinities of 5 10 5 nacl equiv wt laser raman spectroscopic lrs analysis indicates the presence of co2 0 1 1 mol in the mt chalmers vhms systems semiquantitative sem wds microprobe analyses of fluid inclusion decrepitates indicate that the mt chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater pixe microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids high base metal content and the presence of co2 in the fluid inclusions imply that magmatic input of ore metals copper in particular accompanying seawater leaching of the footwall volcanic pile is a distinct possibility in terms of fluid composition the k ca fe variation of the mt chalmers ore fluid is comparable with those of typical epithermal deposits e g thames new zealand and porphyry copper deposits bingham ut the cu zn fe 10 plot of the mt chalmers ore fluids indicates that there is significant copper in the system comparable to copper enrichment in a porphyry copper system the mt chalmers ore fluids also show similar copper content with the cu rich end member ore fluid composition of the mineralized stage 2b veins of the hellyer vhms deposit whereas the stage 2a veins of hellyer are more enriched in lead and zinc overall the ore fluids have a variable chemistry with a continuum of compositional data from vhms to epithermal porphyry style ore fluids shallow water emplacement 300 m for the vhms mineralization has been postulated for the mt chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies however fluid inclusion studies do not rule out a moderate to deeper submarine environment as there is no fluid inclusion evidence of boiling at mt chalmers boiling of ore fluids would probably have occurred as in most epithermal systems if the ore fluid exhaled onto the seafloor under a shallow environment

gabbroic rocks from the mathematician ridge failed rift plutonic rocks from the ocean crust provide direct evidence on the petrological processes leading to its formation and evolution the nature of such rocks is important for interpreting the origin of on land ophiolites so far oceanic plutonic rocks have been mainly recovered from slow spreading ridges and their associated fractures1 13 and from trenches14 15 here we report the results of petrological studies of a suite of gabbros and related plutonic and volcanic rocks dredged from the mathematician ridge mr which was a fast spreading ridge 43 mm yr 1 until it was abandoned 3 5 myr ago16 textures and mineral chemistry of the rocks together with structural considerations indicate that before abandonment the mr produced fractionated mid ocean ridge basalt morb and fractionated gabbros the gradual decrease of spreading rate during abandonment resulted in morphological change eruption of more primitive morb and possibly tectonic emplacement of the older ferrogabbros alternatively the latter could have been tectonically emplaced after abandonment

a stable isotope study of the hydrothermal alteration of the east taiwan ophiolite abstract the east taiwan ophiolite eto occurs as allochthonous fragments in the lichi melange and has been suggested to have formed at an active ridge ridge transform fault in a marginal basin the brecciated plutonic sequence has experienced a pre brecciation ocean ridge type high temperature metamorphism as well as a post brecciation off axis type low temperature metamorphism whereas the extrusive sequence has been subjected only to an off axis low temperature hydrothermal alteration stable isotopic studies of the eto show a slightly decreasing trend in oxygen isotope compositions from basalt through plagiogranite gabbro to ultramafic rocks the isotopic compositions of the eto basalts indicate low temperature interaction with seawater under variable water rock ratios the amphiboles from the eto gabbro show low 18o i e 3 1 to 3 8 and high d i e 19 to 9 values this is interpreted as having resulted from high temperature interaction with seawater derived hydrothermal fluid the plagioclase in the gabbro is mostly albitic in composition and exhibits very high 18o values i e 12 0 which may have resulted from low temperature reaction with 18o enriched fluid such 18o enriched fluid may have been derived from a high temperature seawater plutonic rock interaction system the antigorite and lizardite chrysotile serpentinites in the eto formed in oceanic environments under pre and post brecciation metamorphic conditions respectively the isotopic compositions of the antigorite are compatible with its formation in an oceanic environment although the 18o values of the lizardites chrysotiles are comparable to those of oceanic serpentines their d values ranging from 91 to 57 are too low to be accounted for by reaction with seawater derived fluid post formation hydrogen isotope exchange between the lizardite chrysotile and local meteoric waters during after on land emplacement of the serpentinites is postulated as a probable explanation calcite veins are ubiquitous in the eto three carbon sources are identified inorganic seawater carbon juvenile carbon and organic carbon in the last case it is shown that some calcite veins may contain a substantial amount of co2 derived from the ch4 oxidation process the organic carbon was most likely derived from organics in the surrounding lichi mudstone therefore the calcite veins with an organic carbon signature are suggested to have formed after the eto emplacement within the lichi melange

marine deposits 35 000 25 000 years old in the chott el djerid southern tunisia the chott el djerid is a large salt covered depression in southern tunisia fig 1 the late nineteenth century idea that the basin had been invaded by the sea1 2 has been superseded by the view that any quaternary flooding of the chott must have resulted from climatic changes as the fossils found in its sediments are not exclusively marine and there is no indication of the fluctuations in sea level or of the crustal movements that would be required to fill and then empty the basin2 3 we report here evidence for a marine connection 35 000 25 000 years ago and show that it implies subsequent uplift by some 80 metres in an area long regarded as either stable2 or prone to subsidence throughout the quaternary4

hydrothermal anorthitization of plagioclase within the magmatic hydrothermal transition at mid ocean ridges examples from deep sheeted dikes hole 504b costa rica rift and a sheeted dike root zone oman ophiolite abstract the unique petrological record from the deep sheeted dikes of odp hole 504b gives insight into the magmatic hydrothermal transition and the hydrothermal reaction zone at mid ocean ridges a high temperature peak hydrothermal event at hole 504b defined as stage 1 corresponds to an early stage in the evolution of an axial reaction zone and perhaps to an initial high temperature phase of black smoker activity the temperatures and or the a ca a na 2 ratios of hydrothermal fluids overlapped and even exceeded those inferred for the reaction zones of many modern black smoker systems the volume of hydrothermally altered rock at this stage of evolution of the reaction zone was much less than 10 stage 1 is manifest in hand specimen by reaction halos adjacent to hornblende veins and by irregular alteration patches there is no epidote associated with stage 1 wallrock reactions involved the replacement of magmatic clinopyroxene by magnesiohornblende and of magmatic plagioclase by hydrothermal plagioclase these secondary minerals were not in equilibrium hydrothermal plagioclase varies in composition from an 54 to an 95 and has low mgo and feo tot it has distinctive bright cathodoluminescence and a porous appearance stage 1 at hole 504b occurred at pressures between 38 and 85 mpa 0 38 and 0 85 kbar and temperatures in excess of about 425 c these p t conditions closely approach the two phase field for seawater stage 1 is overprinted by retrograde alteration that caused further replacement within halos and patches as well as 10 40 alteration mostly to chlorite and albite of all the fresh sheeted dikes that were unaffected during stage 1 stage 1 alteration at hole 504b is unique among oceanic and ophiolitic samples however we report one area from the oman ophiolite that exhibits features similar to stage 1 at hole 504b early halos around hornblende veins contain secondary hornblende and porous hydrothermal calcic plagioclase an 55 an 78 with low mgo and feo tot fluid inclusions in quartz show that some early hydrothermal fluids had 5 6 2 3 wt nacl equivalent at temperatures constrained to be above 320 and below 510 c as well as a saline brine with 31 to 40 wt equivalent nacl and similar temperature constraints although evidence for stage 1 alteration of sheeted dikes is far from ubiquitous it is now identified in an oceanic setting hole 504b and in an ophiolitic example from oman

calcium rich brines and other hydrothermal fluids in fluid inclusions from plutonic rocks oceanographer transform mid atlantic ridge abstract fluid inclusions in metagabbro plagiogranite and epidosite from the oceanographer transform mid atlantic ridge were investigated using microthermometric measurements and laser raman spectroscopy inclusions in quartz reveal the presence of a high salinity average 40 wt nacl cacl 2 high temperature 340 c calcium rich na ca about 1 by weight fluid in metagabbro and plagiogranite at room temperature these fluids bear a halite daughter crystal and may also contain small crystals of calcite titanite and hematite a low density vapor rich fluid bearing a small amount of co 2 is closely associated with the briney fluids in one sample all samples contain a population of high temperature 150 to 400 c aqueous liquid dominated fluid inclusions commonly of secondary origin having seawaterlike salinity 3 to 8 wt nacl equivalent we conclude that the calcium rich brines are produced by a combination of two processes fluid phase separation and segregation of brines from vapor prior to trapping to generate the high salinities and extensive fluid mafic rock interaction at low water rock ratio to promote the calcium enrichment later introduction along microfractures of low salinity modified seawater at high water rock ratio is recorded by the secondary fluid inclusions the chemical complexity and association with co 2 may indicate that there is a magmatic component to the brines in addition to a seawater source differences between the oceanographer transform brines and those reported in previous studies of oceanic and ophiolite samples are attributed to variations in the degree of water rock interaction the host rock lithology and the influence of a magmatic fluid component

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

o and h stable isotope compositions of oceanic crust and upper mantle rocks exposed in the hess deep near the galapagos triple junction abstract a large variety of rocks consisting of basalts dolerites gabbros and ultramafics are exposed on the intra rift ridge in the hess deep representing a section of the oceanic crust formed at the east pacific ridge 1 we conducted a stable isotope study of o and h to assess the nature of the interaction between seawater and the oceanic crust in this region fresh basalt and dolerite samples have magmatic morb values 18o 6 0 105 d 70 and low h2o content 57 and h2o contents 1 3 most gabbros have 18o 5 6 essentially identical to their initial magmatic values their d values are generally higher than 57 suggesting that they have reacted with some seawater derived fluids as is also suggested by their mineralogy in gabbros displaying little or no visual evidence of alteration of their plagioclases and pyroxenes 18oplagioclase pyroxene is small 0 3 and consistent with magmatic values the calcic an 50 plagioclases have magmatic 18o 5 8 albitized prehnitized plagioclases have variable 18o values 4 8 it also appears that the 18o modified gabbros are those in which plagioclases are transformed into albite and or prehnite which are characteristic of greenschist facies conditions the lack of calcic an 50 plagioclases depleted in 18o and hornblende suggests that most of the gabbros did not react extensively under amphibolite facies conditions with seawater derived fluids t 400 c or that they were subsequently altered to greenschist facies gabbros the serpentinized peridotites were depleted in 18o 3 3 4 9 in one sample the 18oserpentine magnetite 5 7 corresponds to a temperature of about 325 50 c these values are compatible with serpentinization by seawater derived fluids at temperatures between 200 and 350 c another sample has a higher 18o value of 10 corresponding to a serpentinization temperature of less than 50 c the 18o mass balance between seawater and the oceanic crust seems to have attained steady state where enrichment in 18o of metabasalts is balanced by the depletion in 18o of metadolerites the contribution of the gabbros is minor the depletion in 18o of the serpentinized peridotites is not taken into account since we believe that the high temperature serpentinization is characteristic of the hess deep and cannot be extrapolated to the normal oceanic crust

deep crustal seawater penetration and circulation at ocean ridges evidence from the oman ophiolite abstract detailed field mapping of the plutonic sequence and sheeted dyke complex in several areas of the sumail ophiolite nappe mainly in the haylayn block wadi haymiliyah area and comparisons with other well studied areas has shown that early hydrothermal circulation affected the whole crust down to the petrological moho five major vein systems have been distinguished in the field and their mineralogy is as follows amphibole zoisite epidote epidote quartz and prehnite vein systems the amphibole veins occur in the lower plutonic sequence and are interpreted as the fossil trace of a cracking front at the interface between a hydrothermal and a magmatic system the plutonic sequence and the zoisite epidote and epidote vein systems were formed in an off axis environment after the formation of the magma chamber in the sheeted dyke complex the epidote and quartz sulphide veins are restricted to the dyke margins and represent early hydrothermal circulation at the top of a magma chamber these two systems formed contemporaneously the quartz sulphide veins marking zones of mixing with colder seawater in the stockwork zone prehnite veins along with minor calcite veinlets are the fossil trace of the latest hydrothermal circulations and occur down to the petrological moho in a diffused system shear zones are rare in the whole ophiolite and are essentially developed in the northern fizh block diffusing from these veins seawater penetrates the crust and induces retrograde metamorphic transformations mineralogical and petrological data show that the entire crust down to the petrological moho has been affected by hydrothermal metamorphism pervasive in the sheeted dyke complex more localized around veins in the underlying plutonic unit the mineralogical parageneses in the sheeted dyke complex are typical of the greenschist facies albite chlorite actinolite and sphene whereas the high level isotropic gabbros are characterized by the association of calcic plagioclase and actinolite actinolitic facies retrogressive in the greenschist facies the almost unaltered underlying units although all samples show traces of metamorphic alteration exhibit mineral parageneses typical of the amphibolite to greenschist facies the main crustal transformations during this metamorphism are hydration a na gain and leaching of ca superimposed ca rich and na mg poor epidotised zones in the sheeted dyke complex are produced by ca rich upflowing discharge fluids high cl contents were found in the amphibole veins suggesting that they formed during the circulation of seawater or of its modified equivalent the hydration of the wadi haymiliyah area plutonic sequence and sheeted dyke complex haylayn block was calculated as requiring a minimum seawater consumption of a column of 260 m fluids trapped in minerals during their growth quartz epidote amphiboles calcite and sphalerite or during a fracturing event quartz and plagioclase consist of aqueous solutions with an average salinity close to that of seawater 3 2 nacl equivalent thermal gradients deduced from fluid inclusion microthermometric studies decrease exponentially with depth and vary between 300 c km in the volcanic unit and 30 c km in the plutonic sequence with temperatures up to 530 c circulation was active until cooling of the crust to a general thermal gradient of 30 c km

fluid evolution during tectonic exhumation of oceanic crust at a slow spreading paleoridge axis evidence from the lizard ophiolite u k zoned quatz crystals within late syntectonic veins from brittle ductile shear zones situated within the lowermost plutonic rocks of the lizard ophiolite contain three groups of fluid inclusions type i are a phase separated primary and pseudosecondary group liquid rich inclusions contain 3 2 8 0wt nacl equivalent with a mode at 5 7wt nacl equiv vapour rich inclusions contain 0 0 6 1wt nacl equiv with a mode at 2 6wt nacl equiv inclusions homogenize either by disappearance of the liquid or vapour phase with a mode at 380 c type ii inclusions contain 2 1 5 7wt nacl equiv with a mode at 4 1 wt nacl equiv inclusions homogenize by disappearance of the vapour phase l v l with a mode at 230 c type iii inclusions are a pseudosecondary and secondary group with textures which are suggestive of underpressure induced re equilibration they contain methane and a low salinity aqueous solution 0 3 2 2wt nacl equiv liquid rich satellite inclusions homogenize at 310 240 c l v l vapour rich inclusions homogenize by disappearance of the liquid phase l v v at 370 320 c by assuming that the range in salinity is exclusively the product of phase separation of a 3 2wt nacl seawater solution and by applying microthermometric data to the nacl h2o system analysis of the data suggests two phase boundary constrained conditions of crystal growth initiated within the super critical field at around 410 c with declining temperature passing into subcritical portions of the system beyond which the fluid entered single phase liquid conditions of crystal growth microthermometric and petrographic data in comparison with present day ocean ridge hydrothermal systems is consistent with a reaction zone situated within an inferred decollement at or near the petrological moho from which fluids were derived from and expelled along intersecting listric faults at temperatures broadly comparable to shallow subsurface levels within present day systems this interpretation suggests that high vent temperatures were maintained as the heat flux waned and that the hydrothermal system migrated downward through the uplifting gabbros culminating in the reaction zone intersecting the upper mantle during the terminal pulses of tectonic exhumation

ridge axis deformation and coeval melt migration within layer 3 gabbros evidence from the lizard complex u k ridge parallel extensional shear zones within the layer 3 gabbros of the lizard ophiolite have recorded varied and numerous chemical and mineralogical changes under a variety of p t conditions these changes accompany tectonic exhumation of lower crustal levels by a listric fault mechanism during amagmatic extension at a slow spreading centre deformation within the mid crustal level of layer three resulted in the migration and impregnation of an evolved iron titanium rich silica poor melt along the shear zones this syntectonic infiltration occurred along the lowermost portions of listric faults which rooted in a partially molten zone beneath the ridge axis residual melt was tapped from this underlying zone and tectonically redistributed about deforming gabbros situated immediately above the partially molten zone the occurrence of sub solidus synkinematic assemblages within the shear zones which were generated at various temperatures and water rock ratios is explained in terms of their simultaneous development on listric fault planes in the presence of a hydrothermal fluid phase the sequential overprinting of pre existing assemblages arises from repeated block rotation during amagmatic episodes of extension within an oceanic slow spreading environment

on the ages of humid holocene and late pleistocene phases in north africa remarks on late quaternary climatic reconstruction for the maghreb north africa by p rognon discussion sur l article de rognon p paru en 1987 dans palaeogeogr palaeoclimatol palaeoecol vol 58 p 11 34

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

oceanic and orogenic fluid rock interaction in 18o 16o enriched metagabbros of an ophiolite tinos cyclades abstract two spatial scales of fluid rock interaction in an ophiolite suite are revealed by oxygen isotope and hydrogen isotope studies of metagabbros on the island of tinos cyclades greece sequentially formed mineral generations in the metagabbros include relict igneous augite hornblende of sub seafloor hydrothermal origin and actinolite and albite formed by regional greenschist facies metamorphism during orogenesis with the exception of augite 18 o 4 4 5 6 the metagabbros are characterized by unusually high 18 o values hornblende 5 8 7 4 actinolite 6 5 10 2 feldspar 14 6 14 9 and whole rocks 7 0 10 5 hornblende d values range from 57 to 66 the high 18 o values and the d range of the hornblendes are compatible with interaction of oceanic gabbro with seawater that had previously been enriched in 18 o 16 o 18 o 6 5 8 by isotopic exchange at moderate to high temperatures the high degree of oceanic alteration in the layered gabbros mass balance calculations of isotopic exchange and field evidence for early oceanic thrusting suggest that seawater could have penetrated deeply into the ocean crust becoming 18 o 16 o enriched through isotopic exchange with gabbros at progressively increasing temperature upward down temperature flow of the high 18 o water would be very effective in elevating the 18 o values of gabbros the regional greenschist metamorphic overprint of the ophiolite possibly the result of continued thrusting and piling up of nappes during obduction is characterized by localized fluid rock exchange actinolite in massive gabbroic layers has 18 o values 6 5 7 2 close to those of the hornblende whereas in deformed meter sized gabbroic blocks the amphiboles have significantly higher values 8 4 10 2 likewise albite in the gabbroic blocks has high 18 o values of ca 15 that are ascribed to meter scale exchange with 18 o rich fluids derived from dehydration reactions in low temperature hydrothermally altered basaltic host rock enclosing the blocks deformation enhanced permeability facilitated fluid infiltration in gabbroic blocks whereas the relatively undeformed and therefore less permeable massive gabbros experienced minor interaction with fluids the orogenic fluid rock interaction thus represents local scale redistribution of hydrous mineral components introduced during seafloor hydrothermal exchange

monitoring playa sedimentation using sequential radar images a series of ers 1 radar images of the chott el djerid playa in tunisia obtained at intervals of 35 days demonstrates the ability to monitor geological change from space the c band radar backscatter is shown from field measurements to be sensitive to changes in surface roughness at millimetric scale due to evaporative growth of halite and gypsum crystals backscatter increases gradually during the dry summer months and decreases rapidly in early winter as the rains begin episodic hydrogeological and sedimentological events including lake development channel flow and aeolian deposition can also be detected in addition to the seasonal pattern multitemporal spaceborne radar offers the prospect of being able to map the dynamics of evaporite sedimentation in continental playas

extinct spreading center in the labrador sea crustal structure from a two dimensional seismic refraction velocity model the labrador sea contains a rare example of an abandoned mid ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates seismic refraction data were collected along a line which transversely crosses the extinct spreading center two dimensional analyses of the refraction data using ray tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center in the extinct spreading center a crustal thickness of approximately 4 km is determined compared with 5 5 km for the flanks substantial lateral variations in p wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration the anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction

hydrothermal activity in a peculiar oceanic ridge oxygen and hydrogen isotope evidence in the xigaze ophiolite tibet china the xigaze ophiolite locally displays a 7 km thick continuous sequence which probably represents an oceanic crust formed at a slow spreading ridge it consists from top to bottom of 1 1 pillow lavas with 18o between 9 and 16 and d around 90 vs smow 2 2 rare trondjhemites 9 7 18o 13 4 and d around 75 3 3 dolerite sills and dikes with 18o between 7 0 and 9 5 and d between 77 and 40 4 4 isotropic gabbros occurring as screens of the sills and dikes and rare layered gabbros these gabbros have 18o between 4 8 and 11 0 and d around 65 and bear high 18o albitized plagioclases 18o 10 which coexist with clinopyroxenes of mantle values 18o 5 5 or having values being slightly depleted down to 3 0 5 5 ultramafic rocks are either fresh or serpentinized they have 18o mainly around mantle values and d s are between 250 and 100 for the mafic part of the ophiolite the isotopic compositions correspond to hydrothermal interaction with seawater derived fluids under zeolite to greenschist facies but the trondjhemites were additionally affected by fluids poorer in d in particular the gabbros reacted with seawater derived fluids under amphibolite facies conditions as seen from their secondary mineralogy and their 18o depleted pyroxenes and subsequently were pervasively reacted under greenschist zeolite facies conditions during which the high 18o albitized plagioclase formed this occurrence of amphibolite facies conditions followed by greenschist zeolite facies conditions can be interpreted either to be a result of slow cooling of the oceanic crust at the ridge or to result from a fundamentally lower temperature hydrothermal interaction if compared with other ophiolites in contrast the d s of the serpentine 100 from the ultramafics are attributed to fluids of meteoric origin at high elevation nevertheless the particular location of the serpentinized harzburgites just beneath the gabbros and the high d dolerite sills and dikes 60 d 40 enclosed in these serpentinized harzburgites suggests an oceanic origin and therefore the possibility of hydrogen isotope exchange between serpentine and meteoric fluids after the obduction the xigaze ophiolite sequence displays very little 18o depleted rocks relative to mantle oxygen composition they cannot balance the amount of 18o enriched rocks consequently the result of the hydrothermal interaction between the xigaze ophiolite sequence and the seawater is an 18o depletion of the seawater however the contribution of xigaze type ridges to the buffering of 18o in cretaceous seawater was probably small