mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge east pacific rise 17 to 19 s naudur cruise 1993 phase separation processes controlled by volcanic and tectonic activity during the naudur cruise in december 1993 23 dives using the french submersible nautile were conducted on the axis of the east pacific rise between 17 and 19 s where the spreading rate is among the fastest in the ocean 14 to 16 cm yr twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered respectively at 17 25 s 18 15 s and 18 26 s the fluids exhibit a very wide range of temperature chemical and gas compositions on the 17 25 s and 18 26 s segments fluids have quite uniform compositions low chlorinities are gas enriched and are low in dissolved metals relative to fluids from the 18 15 s segment which show high chlorinities are less gas enriched and show high metal concentrations chloride and metal depletion associated with gas enrichment is consistent with phase separation whereas ch 4 end members show large variations between sites the 13 ch 4 data are very similar with 13 c values in a narrow range 22 0 to 23 9 versus pee dee belemnite pdb 13 co 2 measured in fluids within the 18 15 s and 18 26 s segments are respectively 7 9 and 5 8 versus pdb similar to 13 c of co 2 trapped in mid oceanic ridge basalts suggesting a magmatic origin the variability in fluid composition is linked to the variability of the accretion process observed on the three segments the uniform venting of low chlorinity fluids in the 17 25 s and 18 26 s segments is connected with volcanic activity which causes boiling with preferential venting of vapor enriched fluids high salinity fluids are emitted on the 18 15 s segment where the ridge is tectonics dominated and subseafloor circulation controlled by faults phase separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area as previously observed on the juan de fuca ridge or in the north fiji basin ridge

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

evolution of a brine saturated layer at the base of a ridge crest hydrothermal system vent fluids with chlorinities significantly greater than seawater are indicative of mixing between hydrothermal seawater and brines that have formed as a result of supercritical phase separation we relate the evolution of a brine saturated layer to magmatic and tectonic processes at the base of a ridge crest hydrothermal system a conductive heat balance indicates that under steady state conditions such a brine saturated layer is typically 1 10 m thick and contains 107 to 108 kg of brine with 6 10 times the chlorinity of seawater simple mixing and fractionation models indicate that the depletion and replenishment of brine saturated layers may occur on a timescale of years to centuries a timescale of years is consistent with temporal changes in chlorinity of fluids at vents in areas of recent known dike injection in these systems brines may be undergoing depletion by mixing with liquid phase seawater vents stably emitting high chlorinity fluids on timescales of decades may represent older systems in which brines are being depleted by diffusion into an overlying liquid phase hydrothermal system

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

phase separation and fluid mixing in subseafloor back arc hydrothermal systems a microthermometric and oxygen isotope study of fluid inclusions in the barite sulfide chimneys of the lau basin fluid inclusions in barite and sulfide in chimneys both active and inactive from three hydrothermal sites of the back arc lau basin were studied with microthermometric and isotopic methods to determine the chemistry and evolution of hydrothermal aqueous fluids strontium isotope compositions of sulfides from the lau basin reflect the presence of anhydrite and barite inclusions the 87sr 86sr ratios of these two sulfate minerals vary from 0 7045 to 0 7078 and are interpreted as the result of mixing between various proportions of the hydrothermal end member and pure seawater the microthermometric study of fluid inclusions reveals that mixing with seawater involved different kinds of aqueous fluid end members a high temperature mg depleted end member of high salinity 5 5 wt eq nacl was found at the vai lili site a uncommon low temperature mg rich end member was also identified at the hine hina site in association with barite deposition at the vai lili site very low salinity fluids were produced in addition to a very saline brine 30 wt nacl that was trapped inside anhydrite precipitated from an active vent at a temperature of 342 c oxygen isotope ratios of water inclusions range from 2 to 4 4 for chalcopyrite barite and sphalerite minerals the 18o enrichment and the high salinities of many fluids from the lau basin are accounted for by the specificity of the back arc setting this non mid ocean ridge setting is characterized by the shallow depth of the hydrothermal systems that allows frequent unmixing of high temperature liquids the abundance of silicic magmas also provided magmatic fluids including brines that mixed with seawater derived aqueous fluids

layered hydrothermal barite sulfide mound field east diamante caldera mariana volcanic arc east diamante is a submarine volcano in the southern mariana arc that is host to a complex caldera 5 10 km elongated ene wsw that is breached along its northern and southwestern sectors a large field of barite sulfide mounds was discovered in june 2009 and revisited in july 2010 with the r v natsushima using the rov hyper dolphin the mound field occurs on the northeast flank of a cluster of resurgent dacite domes in the central caldera near an active black smoker vent field a 40ar 39ar age of 20 000 4000 years was obtained from a dacite sample the mound field is aligned along a series of fractures and extends for more than 180 m east west and 120 m north south individual mounds are typically 1 to 3 m tall and 0 5 to 2 m wide with lengths from about 3 to 8 m the mounds are dominated by barite sphalerite layers with the margins of each layer composed of barite with disseminated sulfides rare inactive spires and chimneys sit atop some mounds and also occur as clusters away from the mounds iron and mn oxides are currently forming small 1 m diam 0 5 m tall knolls on the top surface of some of the barite sulfide mounds and may also drape their flanks both diffusely and focused fluids emanate from the small oxide knolls radiometric ages of the layered barite sulfide mounds and chimneys vary from 3 920 to 3 350 years one layer from an outcrop of 10 to 100 cm thick cu rich layers is notably younger with an age of 2 180 years the fe mn oxides were 5 years old at the time of collection in 2009 most mound chimney and layered outcrop samples are dominated by barite silica and sphalerite other sulfides in decreasing order of abundance are galena chalcopyrite and rare pyrite anglesite cerussite and unidentified pb oxychloride and pb phosphate minerals occur as late stage interstitial phases the samples contain high zn up to 23 wt pb to 16 wt ag to 487 ppm and au to 19 ppm contents some layered outcrop samples are dominated by chalcopyrite resulting in 4 78 wt cu in a bulk sample 28 wt for a single lens with a mean of 0 28 wt for other samples other significant metal enrichments are sb to 1 320 ppm cd to 1 150 ppm and hg to 55 ppm the east diamante mound field has a unique set of characteristics compared to other hydrothermal sites in the mariana arc and elsewhere the geochemical differences may predominantly reflect the distribution of fractures and faults and consequently the rock water ratio temperature of the fluid in the upper parts of the circulation system and extensive and prolonged mixing with seawater the location of mineralization is controlled by fractures following resurgent doming within the caldera mineralization resulted from focused flow along small segments of linear fractures rather than from a point source typical of hydrothermal chimney fields based on the mineral assemblage the maximum fluid temperatures were 260 c near the boiling point for the water depths of the mound field 367 406 m lateral fluid flow within the mounds precipitated interstitial sphalerite silica and pb minerals within a network of barite with disseminated sulfides silica was the final phase to precipitate the current low temperature precipitation of fe and mn oxides and silica may represent rejuvenation of the system

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

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

noble gases in vent water from the juan de fuca ridge abstract hydrothermal vent fluids collected with the dsrv alvin from the southern limb of the juan de fuca ridge are chemically unique having chloride concentrations 2 times ambient seawater the same fluids contain noble gases in relative abundances like 2 c air saturated seawater the expected recharge composition however the absolute noble gas abundances are depleted by 30 relative to seawater the combination of very high chloride and moderately depleted noble gases appears to require the formation of a cl rich gas free brine by phase separation this brine is mixed with recharge seawater at temperatures in excess of 340 c and therefore deep in the hydrothermal system

effects of water depth on pumice formation in submarine domes at sumisu izu bonin arc western pacific domes of the sumisu volcanic complex western pacific having summits at ocean depths of 1100 600 245 and 95 m are mantled with compositionally identical rhyolitic pumice that has similar vesicularity but that varies systematically in distribution size and surface texture suggesting that facies and morphology can serve as useful indicators of eruption depth at depths 500 m the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant meters to tens of meters polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace at 500 m depths the pumice occurs as an apron of blocky giant and smaller rough textured clasts enclosed by quenched margins and pockmarked by coarse cm vesicles no carapace pumice occurs and the summit is composed of craggy dense dome rock these shallower water pumice clasts resemble those spalled from historic submarine dome forming eruptions that buoyed to the sea surface we interpret spalling to result from vent derived weak volatile driven explosions that take place at water depths 500 m our study shows that an increase in hydrostatic pressures over a range of 12 mpa reduces volatile driven explosivity for subaqueous rhyolitic dome forming eruptions but does not affect vesicularity we conclude that meter size highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m and its morphology can be used to distinguish between explosive and effusive origins

petroleum characteristics of the oil in a guaymas basin hydrothermal chimney abstract extensive petroleum shows have been sampled from the seabed of guaymas basin these petroleums were generated at depth by hydrothermal activity from immature sedimentary organic matter and migrated to the sea floor there the heavy ends solodify due to the temperature drop and the volatiles are emitted as plumes thus it is of interest to determine the petroleum characteristics of such oils for comparison with normal reservoir petroleums an oil saturated sample of a chimney alvin dive 1630 emitting water at 200 c was studied in contained 21 7 solvent extractable oil and 19 2 s moisture the oil has an api gravity of 27 6 a specific gravity at 20 c of 0 8891 and contains 0 99 s the astm distillation yields vol are gasoline 3 0 ibp 149 c ibp 300 f intermediates 149 315 c 300 660 f 61 0 gas oil 315 538 c 660 1000 f 21 0 and residue 538 c 1000 f 15 0 gas chromatography and ft infrared spectrometry indicate that this oil is similar to typical reservoir petroleum but with the low ends and aromatics depleted due to water washing mild thermal conditions are indicated by the absence of cracking products

chloride depletions and enrichments in seafloor hydrothermal fluids constraints from experimental basalt alteration studies na k ca cl fluid was reacted with diabase at 400 and 425 c 400 bars and fluidrock mass ratio of 0 5 to assess the relative mobility of dissolved cl fluids from the present experiments reveal relatively large time and temperature dependent decreases in cl temperature increase enhances cl removal whereas reaction progress has the opposite effect these changes are not due to boiling and or phase separation at 400 and 425 c 400 bars cl removal from solution is accompanied by ca and na fixation respectively both experiments produced mixed layer chlorite smectite clinozoisite and albite olivine was not detected by x ray diffraction or petrographic analysis of run products from either experiment however we propose that the time dependent changes in dissolved cl result from olivine replacement by an fe hydroxy chloride phase followed by hydration effects quenched and washed alteration products however did not reveal anomalous cl which suggests that the cl bearing phase is characterized by retrograde solubility this is consistent with the relative magnitude of cl fixation observed for experiments at both temperatures and from results of an isobaric cooling experiment the experiments provide evidence for the non conservative behavior of cl during hydrothermal alteration of basalt the data are particularly important in light of the cl depleted nature of some ridge crest hot spring fluids and suggest temperatures of formation for these fluids of approximately 410 c assuming sub seafloor pressure of 400 bars in addition the retrograde solubility of the cl bearing phase responsible for cl fixation during high temperature basalt alteration may help to explain cl enrichment in hot spring fluids which have conductively cooled below 350 c

the drawbacks of sectioning rocks relative to fabric orientations in the matrix a case study from the robertson river metamorphics northern queensland australia understanding the relationships of inclusion trail geometries in porphyroblasts relative to matrix foliations is vital for unravelling complex deformation and metamorphic histories in highly tectonized terranes and the approach used to thin sectioning rocks is critically important for this two approaches have been used by structural and metamorphic geologists one is based on fabric orientations with sections cut perpendicular to the foliation both parallel p and normal n to the lineation whereas the other uses geographic orientations and a series of vertical thin sections studies using p and n sections reveal a simple history in comparison with studies using multiple vertical thin sections the reason for this is that inclusion trails exiting the porphyroblasts into the strain shadows in p and n sections commonly appear continuous with the matrix foliation whereas multiple vertical thin sections with different strikes reveal that they are actually truncated such truncations or textural unconformities are apparent from microstructures textural relationships compositional variations and fia foliation intersection axis trends a succession of four fia trends from ene wsw e w n s to ne sw in the robertson river metamorphics northern queensland australia suggests that these truncations were formed because of the overprint of successive generations of orthogonal foliations preserved within porphyroblasts by growth during multiple deformation events at least four periods of orogenesis involving multiple phases of porphyroblast growth can be delineated instead of just the one previously suggested from an n and p section approach

geochemical time series of hydrothermal fluids from the slow spreading mid atlantic ridge implications of medium term stability abstract time series investigations of hydrothermal fluids from different geological settings at the slow spreading mid atlantic ridge mar indicate a surprising medium term stability of their chemical composition over up to 20 years this does not only apply to tectonically controlled systems with exposure of mantle rocks where magmatic activity is limited such as the logatchev 1 hydrothermal field at 14 45 n we also found the 407 c hot and extremely metal rich fluids at the 5 s site turtle pits 2986 m water depth to discharge at constant composition indicating supercritical phase separation for at least 4 years which contrasts with rather dynamic volcanically driven hydrothermal systems at the fast spreading east pacific rise epr possibly fluid pathways and pressure temperature conditions are more stable in tectonically controlled settings at mar than at the epr where frequent volcanic activity affects permeability within the hydrothermal systems despite some spatial variability hydrothermal fluids discharging at the shallower ca 810 870 m menez gwen hydrothermal field at 37 50 n seem to represent a stable discharge of phase separated vapor type fluids over 16 years this demonstrates that medium term stability of hydrothermal fluid composition seems to be a general feature at slow spreading ridges as a consequence hydrothermal vent organisms at the mar may live in a more stable chemical environment compared to vent systems at fast spreading ridges considering the fact that slow spreading ridges account for more than half of the submarine ridge system such a constant emanation of hydrothermal fluids makes hydrothermal vent systems on slow spreading ridges a medium term stable source of fe and other micronutrients into the ocean

a time series of vent fluid compositions from 21 n east pacific rise 1979 1981 1985 and the guaymas basin gulf of california 1982 1985 a time series study of four hot spring fields at 21 n on the east pacific rise demonstrates that the solution compositions of mid ocean ridge hydrothermal vents are stable on a decadal time scale the ocean bottom seismometer obs national geographic smoker ngs and south west sw vents show no significant changes in the major element composition of their end member fluids since 1979 two of the vents obs and sw have maintained exit temperatures near 350 c since 1979 the ngs vent has remained close to 270 c since 1981 after cooling from 350 c in 1979 only one vent hanging gardens shows important changes in solution chemistry from 1981 to 1985 in 1985 this vent appears to have cooled 10 c relative to the other vents the depth of the reaction zone as determined by quartz geobarometry has increased by about 1 km at this vent the large fluxes of compositionally stable fluids from these vents cannot be explained by transient conditions and unique circumstances rather the constancy of small interfield variations in solution composition is more consistent with control by rock buffering a 4 year time series study of hydrothermal vents in the guaymas basin also shows no significant changes in the major element composition of the end member fluids interfield differences in solution chemistry have been maintained from 1982 to 1985 this phenomenon is best explained in terms of an equilibrium model

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

origin of fluids and anhydrite precipitation in the sediment hosted grimsey hydrothermal field north of iceland the sediment hosted grimsey hydrothermal field is situated in the tjornes fracture zone tfz which represents the transition from northern iceland to the southern kolbeinsey ridge the tfz is characterized by a ridge jump of 75 km causing widespread extension of the oceanic crust in this area hydrothermal activity occurs in the grimsey field in a 300 m 1000 m large area at a water depth of 400 m active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical for this field clear metal depleted up to 250 c hydrothermal fluids are venting from the active chimneys anhydrite samples collected from the grimsey field average 21 6 wt ca 1475 ppm sr and 3 47 wt mg the average molar sr ca ratio is 3 3 10 3 sulfur isotopes of anhydrite have typical seawater values of 22 0 7 34s indicating a seawater source for so42 strontium isotopic ratios average 0 70662 0 00033 suggesting the precipitation of anhydrite from a hydrothermal fluid seawater mixture the endmember of the venting hydrothermal fluids calculated on a mg zero basis contains 59 8 mol kg sr 13 2 mmol kg ca and a 87sr 86sr ratio of 0 70634 the average sr ca partition coefficient between the hydrothermal fluids and anhydrite of about 0 67 implies precipitation from a non evolved fluid a model for fluid evolution in the grimsey hydrothermal field suggests mixing of upwelling hydrothermal fluids with shallowly circulating seawater before and during mixing seawater is heated to 200 250 c which causes anhydrite precipitation and probably the formation of an anhydrite rich zone beneath the seafloor

hydrothermal precipitates associated with bimodal volcanism in the central bransfield strait antarctica polymetallic sulfide sulfate mineralization enriched in pb ag as sb hg occurs in the bransfield strait a late tertiary quaternary marginal basin close to the antarctic peninsula the mineralization is associated with bimodal volcanism and pelagic and volcaniclastic sediment in rifted continental crust hydrothermal precipitates have been recovered from two shallow 1 050 1 000 m water depth submarine volcanoes hook ridge and three sisters in the central bransfield strait mineralization at hook ridge consists of polymetallic sulfides massive barite and pyrite and marcasite crusts in semilithified pelagic and volcaniclastic sediment native sulfur commonly infills void space and cements the volcaniclastic sediment the polymetallic sulfides are dominated by sphalerite with minor galena enargite tetrahedrite tennantite pyrite chalcopyrite and traces of orpiment cemented by barite and opal a the presence of enargite at hook ridge the abundance of native sulfur and the low fe content of sphalerite indicate a high sulfur activity of the hydrothermal fluids responsible for mineralization the sulfur isotopic composition of hook ridge precipitates documents the complexity of the sulfur sources in this hydrothermal system with variable influence of biological activity and possibly magmatic contributions homogenization temperatures and salinities of fluid inclusions in barite and opal a suggest that boiling may have affected the hydrothermal fluids during their ascent the discovery of massive barite silica precipitates at another shallow marine volcano three sisters volcano attests to the potential for hydrothermal mineralization at other volcanic edifices in the area the characteristics of the mineralization in the bransfield strait with rifting of continental crust the presence of bimodal volcanism including highly evolved felsic volcanic rocks the association with sediments and the pb ag as sb hg enrichment are similar to the setting of massive sulfide deposits in the okinawa trough and distinct from those of sediment dominated hydrothermal systems such as escanaba trough middle valley and guaymas basin the geological setting of the bransfield strait is also broadly similar to that of some of the largest volcanogenic massive sulfide deposits in the ancient record such as the iberian pyrite belt

a hot miocene subaqueous scoria flow deposit in the shimane peninsula sw japan abstract a subaqueous scoria flow deposit occurs in the miocene takashibiyama formation in sw japan faunal assemblages in associated sediments suggest an outer shelf to upper bathyal environment with water depth likely between 200 and 1500 m the deposit 5 7 m thick is similar to subaerial scoria flow deposits containing mainly non to moderately vesiculated hyalopilitic basaltic andesite lapilli and coarse ash the essential basaltic andesite clasts are variably vesiculated even within single clasts and are commonly cracked with curviplanar surfaces whereas the inner parts are cracked in a pattern similar to perlitic cracks the clasts are blocky with curviplanar surfaces if non to poorly vesiculated and are fibrous with irregular shapes and flattened by compaction if more vesiculated at one proximal location fines depleted rootless pipes 5 90 cm wide and 1 3 m long occur and spherical to amoebic vesicles 0 01 0 3 mm in diameter and filled with stilbite quartz or greenish brown clay are common in the fine ash matrix and in siltstone clasts contained in the deposit the vesicles represent open spaces once supported by gas and the rootless pipes are interpreted as gas escape pipes magnetization vectors of essential clasts are dispersed not at random but around a single direction suggesting that the clasts gained thermoremanent magnetism mainly during cooling in the flow or during post emplacement cooling and then rotated to some extent by differential movement of the flow or by gas escape in the deposit the vesicularity shape and texture of essential clasts suggest a phreatomagmatic eruption involving magmatic vesiculation and dynamic interaction of magma and ambient water the perlitic cracks in the inside of the essential clasts and the paleomagnetic vectors of the essential clasts indicate that the flow was initially hot and that the deposit cooled slowly internal depositional features change rapidly over only 500 m rootless pipes and vesicles in the fine ash matrix and siltstone clasts are therefore absent at distal locations indicating that an initially cohesionless debris flow quickly became sufficiently turbulent to mix with ambient seawater

genesis of the wolverine volcanic sediment hosted massive sulfide deposit finlayson lake district yukon canada mineralogical mineral chemical fluid inclusion and sulfur isotope evidence the wolverine deposit is a high grade 6 2 mt at 12 7 zn 1 3 cu 1 6 pb 370 9 g t ag and 1 8 g t au volcanic and sedimentary rock hosted massive sulfide deposit in the finlayson lake area yukon territory the area has the potential to become an important mining camp and currently contains more than 30 million tons mt of massive sulfide ore in five deposits a succession of metamorphosed early mississippian felsic volcanic and sedimentary rocks that are interpreted to have formed on the margin of an ensialic back arc ocean basin between the yukon tanana terrane and the ancestral north american craton hosts the wolverine deposit the massive sulfide ore occurs at a single horizon that marks a significant change in the character of the volcanism from quartz and feldspar phyric rhyolite of the footwall to aphyric rhyolite and chemical sedimentary exhalative rock of the hanging wall the deposit consists of two discrete zn pb ag massive sulfide lenses situated at the same stratigraphic level connected by strata bound semimassive replacement style zn pb ag mineralization copper rich massive sulfide commonly replaces the zn pb ag massive sulfide at the base of the lenses and in the footwall replacement zones beneath the massive mineralization several zones of sulfide stringer veins and chlorite sericite carbonate alteration occur within permeable footwall volcaniclastic rocks alteration minerals preferentially associated with massive sulfide mineralization include ba rich phengitic mica biotite mg rich chlorite and siderite microthermometric measurements of fluid inclusions in the massive sulfide bodies and accompanying stringer zones indicate formation at temperatures of 235 to 353 c primary aqueous liquid rich fluid inclusions in hydrothermal quartz from quartz sulfide stringer veins contain low salinity fluids 2 1 8 5 wt nacl equiv mean 6 0 there is no evidence of fluid boiling in the inclusions and based on the average fluid salinity and temperature of homogenization mineralization took place under a minimum water depth of 1 000 m in situ analyses of 34sv cdt in sulfide minerals from massive sulfide lenses means 0 8 and sulfide stringer veins mean 12 0 display a bimodal distribution lower 34s values characterize sulfides near the top of the lenses versus higher more positive values in underlying veins may reflect different processes of reduction of seawater sulfate sulfur in sulfides from the tops of the lenses was likely derived mainly from bacterial reduction of seawater sulfate whereas sulfur in the stringer veins and at the base of the lenses was mainly from inorganic reduction of seawater sulfate sulfide minerals precipitated both on the sea floor and in the subsea floor from hydrothermal fluids that vented from multiple mounds the presence of black shales and the strongly positive sulfur isotope compositions of the sulfides provide evidence that the ambient bottom water in the basin in which the wolverine deposit formed was likely anoxic this environment may have contributed to the preservation of the sulfide mounds based on its tectonic setting host rock types local geologic setting metal grades age and source s of sulfur the wolverine deposit best exemplifies the volcanic sediment hosted massive sulfide vshms class of syngenetic massive sulfide deposits in the district

analysis of seismic data using horizontal cross sections three dimensional data provide the interpreter with a new dimension for interpretation horizontal cross sections this article suggests formats for the display of the data and techniques for deriving contour maps of horizons strike directions dip angles thicknesses of formations and fault blocks

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

native gold and gold rich sulfide deposits in a submarine basaltic caldera higashi aogashima hydrothermal field izu ogasawara frontal arc japan sulfide deposits with extremely high au concentrations up to 275 ppm avg 102 ppm n 15 high au ag ratios 0 24 n 15 and low cu cu zn ratios 0 03 n 15 were discovered in 2015 in active hydrothermal fields at a water depth of 760 m in a basalt dominated submarine caldera in the izu ogasawara frontal arc japan native gold grains occur in massive sulfide fragments concretions and metalliferous sediments from a sulfide mound 40 m across and 20 m high with up to 30 m high black smoker chimneys tiny native gold grains up to 14 m in diameter are mainly present in sulfide fallouts from chimney orifices and plumes larger native gold grains up to 150 m long occur mostly as discrete particles and or with amorphous silica and sulfides the larger gold grains are interpreted to represent direct precipitation from au bearing hydrothermal fluids circulating in and or beneath the unconsolidated sulfide mound deposits sulfur isotope compositions from a limited number of sulfide separates n 4 range from 4 3 to 5 8 34s similar to the quaternary volcanic rocks of the arc barite separates have values of 22 2 and 23 1 close to modern seawater values and indicate probable seawater sulfate origin the cu zn and pb concentrations in bulk samples of sulfide rich rocks are similar to those of volcanogenic massive sulfides formed in continental crustal environments the gold is interpreted to have formed by low temperature hydrothermal activity perhaps genetically different from systems with documented magmatic contributions or from seafloor hydrothermal systems in other island arc settings its presence suggests that basalt dominated submarine calderas situated on relatively thick continental crust in an intraoceanic arc setting such as the higashi aogashima knoll caldera may be perspective for gold mineralization

fluid inclusions in epidote minerals and fluid development in epidote bearing rocks the widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies apart from high to very high temperature and low to intermediate pressure conditions epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust e g poli and schmidt 1998 yet fluid inclusion studies on epidote minerals are surprisingly scarce even in fluid saturated environments like certain vein type deposits or hydrothermal volcanic vugs and druses for example epidote minerals are not mentioned in the subject index of roedder s 1984 outstanding summary and review of fluid inclusion studies and occurrences which lists more than sixty different host minerals for fluid inclusions nonetheless more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems ore deposits and high pressure to ultra high pressure rocks the aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes which are based on temperature t pressure p molar volume v and composition x data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments the review starts with a chapter on some typical mixed volatile solid fluid equilibria involving epidote minerals which are relevant for the here discussed environments this is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals the next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low grade metamorphic rocks and constraints on the p t x properties of the fluids present in these systems this is followed by a short introduction

formation and evolution of the changbaishan volcanic geothermal system in a convergent plate boundary back arc region constrained by boron isotope and gas data abstract the goal of this study is to analyze fluid geochemistry boron isotope and gas data to reassess the formation and evolution processes of the changbaishan volcanic geothermal system jilin china due to the presence of hot fumaroles with temperatures of higher than 100 c and sinters we believe that phase separation is occurring underground which is also indicated by the characteristics of the b and gas data based on their hydrochemical data and b cl ratios the thermal water samples in this area can be divided into three groups 1 group 1 is steam heated condensate water group 1 1 jj jlq group 1 2 sbdg xrq 2 group 2 is karst water cr3 and lsd and 3 group 3 is residual water cr1 and cr2 the preliminary discrimination of samples using b and gas data implies that group 1 1 around the crater is controlled by a second phase separation process which is induced by the input of hot magmatic material group 1 2 in the basin area is controlled by the transformation of clay minerals and the xrq water sample shows contamination by sewage group 2 represents karst water that has leached metamorphic rock and been directly heated by the input of hot magmatic volatiles and group 3 is the result of one seawater phase separation process the mixing and fluid rock interaction models based on the mass balance of boron concentration and isotope imply that the phase separation and input of magmatic volatiles are more important than fluid rock interactions in the formation of the changbaishan geothermal system and the first phase separation process in the deep carbonate reservoir occurred in a late stage with approximately 75 87 5 of water becoming vapor additionally the rayleigh distillation of b isotope implies that new magma is being generated below this area with the low b concentrations and high boron isotopic values of its gases implying that the mixed component may be derived from the subducted pacific plate e g marine sediment the addition of excess heat due to the continued subduction of the pacific plate has further encouraged the phase separation process and the subduction process has also accelerated the transition of the hydrothermal system into a non hydrothermal system

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

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

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

phase relations and adiabats in boiling seafloor geothermal systems abstract observations of large salinity variations and vent temperatures in the range of 380 400 c suggest that boiling or two phase separation may be occurring in some seafloor geothermal systems consideration of flow rates and the relatively small differences in density between vapors and liquids at the supercritical pressures at depth in these systems suggests that boiling is occurring under closed system conditions salinity and temperature of boiling vents can be used to estimate the pressure temperature point in the subsurface at which liquid seawater first reached the two phase boundary data are reviewed to construct phase diagrams of coexisting brines and vapors in the two phase region at pressures corresponding to those of the seafloor geothermal systems a method is developed for calculating the enthalpy and entropy of the coexisting mixtures and results are used to construct adiabats from the seafloor to the p t two phase boundary results for seafloor vents discharging at 2300 m below sea level indicate that a 385 c vent is composed of a brine 7 nacl equivalent in equilibrium with a vapor 0 1 nacl brine constitutes 45 by weight of the mixture and the fluid first boiled at approximately 1 km below the seafloor at 415 c 330 bar a 400 c vent is primarily vapor 88 wt 0 044 nacl with a small amount of brine 26 nacl and first boiled at 2 9 km below the seafloor at 500 c 520 bar these results show that adiabatic decompression in the two phase region results in dramatic cooling of the fluid mixture when there is a large fraction of vapor

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

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

rare earth elements as indicators of hydrothermal processes within the east scotia subduction zone system the east scotia subduction zone located in the atlantic sector of the southern ocean hosts a number of hydrothermal sites in both back arc and island arc settings high temperature 348 c black smoker vents have been sampled at three locations along segments e2 and e9 of the east scotia back arc spreading ridge as well as white smoker 212 c and diffuse 28 c hydrothermal fluids from within the caldera of the kemp submarine volcano the composition of the endmember fluids mg 0 mmol kg is markedly different with ph ranging from 1 to 3 4 cl from 90 to 536 mm h2s from 6 7 to 200 mm and f from 35 to 1000 m all of the vent sites are basalt to basaltic andesite hosted providing an ideal opportunity for investigating the geochemical controls on rare earth element ree behaviour endmember hydrothermal fluids from e2 and e9 have total ree concentrations ranging from 7 3 to 123 nmol kg and chondrite normalised distribution patterns are either light ree enriched lacn ybcn 12 8 30 0 with a positive europium anomaly eucn eu cn 3 45 59 5 or mid ree enriched lacn ndcn 0 61 with a negative eu anomaly eucn eu cn 0 59 by contrast fluids from the kemp caldera have almost flat ree patterns lacn ybcn 2 1 2 2 eucn eu cn 1 2 2 2 we demonstrate that the ree geochemistry of fluids from the east scotia back arc spreading ridge is variably influenced by ion exchange with host minerals phase separation competitive complexation with ligands and anhydrite deposition whereas fluids from the kemp submarine volcano are also affected by the injection of magmatic volatiles which enhances the solubility of all the rees we also show that the ree patterns of anhydrite deposits from kemp differ from those of the present day fluids potentially providing critical information about the nature of hydrothermal activity in the past where access to hydrothermal fluids is precluded

ore genesis and fluid evolution of the kaladawan south zn pb cu ore field eastern altyn mountains nw china evidence from fluid inclusions h o isotopes and geochronology abstract the newly discovered kaladawan south zn pb cu orefield in eastern altyn mountains is composed of five zn pb cu deposits with the largest one containing 414 4 thousand tonnes kt zn 265 2 kt pb and 149 2 kt cu stratiform lenticular orebodies are hosted by the ordovician volcanic sedimentary sequences subjected to greenschist facies metamorphism two major ore forming stages with significant cross cutting relationship are identified at kaladawan south stage i of the earlier massive banded disseminated barite rich sulfide ores and stage ii of the later polymetallic sulfide quartz veins with economically less importance fluid inclusions fis locked in quartz of stage i massive sulfide ores are homogenized completely at 226 354 c with calculated seawater like salinities of 2 7 4 2 wt nacl equiv these characteristics of ore forming fluids coupled with ore textures of typical vms deposits are interpreted to be originated from the modified seawater by sea floor volcanic hydrothermal activities in compared the stage ii fis yielded lower homogenization temperatures ranging 251 c 357 c with higher salinities of 0 8 14 4 wt nacl equiv laser raman analyses identified large amounts of co 2 ch 4 n 2 in the hydrothermal fluids of stage ii the 18 o h2o values 0 5 5 7 and d h2o values of 68 1 to 32 3 of stage ii quartz are plotted in the range of metamorphic fluids in the d o isotopic diagram which indicating that the co 2 rich and mesothermal fluids are metamorphic derived the phlogopite paragenetic in the stage ii polymetallic vein yielded a 40 ar 39 ar plateau age of 435 4 3 9 ma similar to the age of the metamorphic muscovite 451 0 4 1 ma from the ore hosting schist based on the ore deposit geology mineral assemblages and fluid geochemistry the kaladawan south zn pb cu mineralization is attributed to be an ordovician vms type deposit overprinted by the silurian metamorphic fluids

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

shallow water stratigraphy at the mount chalmers volcanic hosted massive sulfide deposit queensland australia mapping of the permian stratigraphy at the mount chalmers massive sulfide footwall stockwork cu au orebody has revealed shallow water body fossils trace fossils and nonwelded to partly welded pyroclastic units within the fiat lying host sequence the massive sulfide bearing lower chalmers sediment and the upper chalmers sediment are each 20 to 40 m thick and comprise fossiliferous tuffaceous mudstone and wacke they are separated by the north star pumice breccia the breccia is 50 m thick at the mine and comprises watertransformed pyroclastic fiow deposits but 2 km to the south and southwest it is 120 m thick and includes probable nonwelded ignimbrite the upper chalmers sediment is overlain by a dacitic tuff which has a 5 m thick blocky to columnar jointed interval near the base termed the chalmers welded tuff the tuff contains strongly spherulitic pumice lenses within a weakly spherulitic matrix with relict fine banding it extends at least 450 m north and 300 m west from the mine the upper chalmers sediment has yielded an assemblage of brachiopods gastropods bivalves and bryozoans from five localities 1 to 2 km north and south of the mine these body fossils show limited reworking and are typical of a shallow marine shelf setting near or below wave base although seawater depths of 50 to 300 m are possible a range of 100 to 200 m depth is considered most likely the upper and lower chalmers sediments contain trace fossils typical of the cruziana ichnofacies below and above the ore position also suggesting shelf sedimentation below the effective wave base these consistent estimates suggest that seawater depths of 200 to 300 m prevailed during deposition of the massive sulfide contrary to the 1 000 m seawater depth usually regarded as necessary to form vhms ore deposits the shallow depth would have allowed the ascending ore forming fluids to boil prior to reaching the sea floor unless prevented by channel constrictions boiling may account for the strong cu rich footwall stringer zone and elevated fluid inclusion salinities at mount chalmers large and both 1980 the duplication of these features at other deposits suggests that mount chalmers is not an isolated case the relatively shallow water depths have also

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

subseafloor phase equilibria in high temperature hydrothermal fluids of the lucky strike seamount mid atlantic ridge 37 17 n abstract as part of an integrated study conducted at the lucky strike seamount mid atlantic ridge 37 17 n in 2008 gas tight sampling devices were used to collect high temperature 300 c hydrothermal fluids issuing from sulfide structures distributed throughout the vent field located in the summit depression compared with previous observations from 1993 to 1997 the most substantial changes in vent fluid compositions are dramatically increased co2 concentrations 5 up to 133 mmol l and the observation of vent fluids enriched in dissolved chloride relative to seawater combined with an increase in 13 c co 2 values by 4 in 2008 the elevated co2 indicates replenishment of the magmatic heat source and may be indicative of a recent magmatic event the additional supporting fluid chemistry is however similar to that of the previous sampling intervals necessitating a reassessment of the subseafloor controls on vent fluid chemistry at lucky strike in the context of recently obtained geophysical data that provides the depth extent of a steady state magma chamber two phase behavior is indicated by the chloride variability in the vent fluids and comparison with experimental data for the associated chloride dependent partitioning of minor trace elements suggests the possibility of a similar source fluid for all the vent structures while limiting the likelihood of shallow phase separation and subseafloor mixing for the hydrothermal end members a recently calibrated fe mn geothermometer indicates minimum subseafloor equilibration temperatures of 350 385 c however constraints imposed by dissolved si cl in conjunction with geophysical observations are consistent with peak reaction conditions at temperatures of 430 475 c and pressures near the top of the axial magma chamber 410 480 bars where magmatic co2 becomes entrained in the circulating fluids the distance between the magma chamber and the seafloor at lucky strike is substantially greater than at most faster spreading ridges and we propose the resulting increased residence time in the up flow zone leads to the re equilibration of temperature sensitive transition metals at conditions less extreme than those associated with peak reaction agreement between experimental data thermodynamic model calculations and dissolved concentrations of fe cu zn h2 and h2s in the lucky strike fluids reinforce the hypothesis of ph redox equilibria for transition metals at relatively oxidizing conditions and temperatures predicted by the empirical fe mn geothermometer in situ ph measurements of the high temperature fluids exiting the seafloor are also consistent with the model calculations

two cells phase separation in shallow submarine hydrothermal system at milos island greece boron isotopic evidence 1 three types of hydrothermal vent fluids herein referred to as cave submarine brine and seawater like were recovered from a shallow submerged system at milos in the aegean sea greece for detailed chemical and isotopic analyses the cave fluids discharge through rock fissures near sea level and have low ph chlorinity and b concentrations relative to seawater the submarine brine fluids are characterized by high cl and contain 10 times seawater b concentrations a scenario involving a two cells circulation is proposed one occurs at 1 2 km and another at shallower depth the deeper saline reservoir has experienced subcritical phase separation partitioning 0 42 mm b in vapor and 6 8 mm in brine with no detectable isotopic fractionation the reaction temperature in the saline reservoir is 313 c calculated from the na k ca geothermometry the vapors rise directly to form the cave vents whereas the saline fluids transport in different pathways and are influenced by seawater mixing to form the variable submarine brine fluids the seawater like fluids circulate at shallower depths where calculated temperature is 248 c and show slightly diluted b 0 36 0 41 mm and seawater 11b these fluids probably resulted from heating of down flow seawater and may have experienced groundwater discharge and partial mg removal this study represents the first two cells circulation occurring at milos and emphasizes the important role of phase separation in shallow submarine hydrothermal system

chemistry of seafloor hydrothermal systems the past four years saw advances in a number of aspects of hydrothermal chemistry high temperature venting was documented at a number of sites in association with the spectrum of seafloor volcanic activity slow to fast spreading normal ridges seamounts back arc settings sediment hosted and sediment starved sites significant progress was made in the description both theoretical and observational of the geometry of circulation and the nature of water rock reaction hydrothermal plumes received extensive study expanding the range of documented anomalies and demonstrating more clearly the links to hydrothermal sedimentation biological chemical interactions began to be explored in a quantitative way pore fluid advection on sedimented ridge flanks was characterized at a number of sites though the general question of the importance of off axis basalt seawater interaction remains unresolved

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

exploring the ocean for hydrothermal venting new techniques new discoveries new insights abstract enumerating active hydrothermal fields on the seafloor has been a challenge since their discovery almost 40 years ago high temperature hydrothermal fields are readily discoverable primarily by detecting mineral laden plumes but low temperature particle poor vent fields resist discovery decades of exploration for vent fields have covered though often cursorily about one third of the global lengths of both oceanic spreading ridges osrs and volcanic arcs identifying some 630 active vent fields about 80 of these fields are on osrs and the spatial frequency of those fields is currently estimated as 0 5 5 100 km generally increasing with spreading rate over the last decade however a few detailed surveys have added sensors capable of detecting ephemeral chemical tracers oxidation reduction potential in addition to standard sensors that detect quasi conservative optical tracers such as light backscattering this approach has revealed a new view of the distribution of venting fields along fast spreading 55 mm yr osrs studies of four such ridge sections totaling 1470 km length suggest that the present inventory of vent fields may underestimate the true global population of vent fields on fast spreading osrs by a factor of 3 6 this increase implies that ridge axes are unexpectedly leaky reservoirs from which hydrothermal fluids escape at far more sites than presently assumed that the supply of dissolved hydrothermal iron which may be fertilizing the primary production of the southern ocean is higher than now calculated and that present estimates of recoverable sulfide tonnage from ridge axes may be too low along slow spreading ridges which account for 60 of the global osr length and 86 of known sulfide tonnage expansive axial valleys present special exploration challenges that will not be easily overcome