chemistry of hydrothermal fluids from the 17 s active site on the north fiji basin ridge sw pacific abstract in june 1989 a diving cruise of the french submersible nautile was made along the north fiji basin spreading ridge the objective was the geological and geochemical study of the active spreading axis and of the hydrothermal processes discovered during the kaiyo cruise in 1988 active anhydrite chimneys associated with underlaying sulfide deposits were studied in the central spreading ridge the transparent venting fluid showed significant salinity depletion the extrapolated concentrations of all the chemical species in the pure end member showed a signature similar to that of the cl depleted hydrothermal fluid from the ashes vent site in the juan de fuca ridge these features suggest that the present hydrothermal end member resulted from subcritical phase separation which generated a low salinity vapor phase followed by condensation and mixing with normal seawater in the last part of the plumbing system of the vents

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

fluctuation of chemical compositions of the phase separated hydrothermal fluid from the north fiji basin ridge ishibashi j grimaud d nojiri y auzende j m and urabe t 1994 fluctuation of chemical compositions of the phase separated hydrothermal fluid from the north fiji basin ridge in j m auzende and t urabe editors north fiji basin starmer french japanese program mar geol 116 215 226 on the central ridge of the north fiji basin southwest pacific hydrothermal activities were located in 1989 it was the second discovery of the hydrothermal fluid undergo phase separation during a dive mission of slrinkai 6500 in 1991 we revisited the hydrothermal area and collected fluid samples from the same anhydrite chimney fluid composition of the end member shows no significant changes during this two years other fluid ventings 150 m apart show similar chemistry suggesting that the phase separation and segregation processes occur throughout the hydrothermal area small fluctuation in chemical composition of the fluids from two sites is attributed to subseafloor fluid rock interaction after the phase segregation

consequences of phase separation on the distribution of hydrothermal fluids at ashes vent field axial volcano juan de fuca ridge recent studies of hydrothermal fluid chemistry at the axial seamount hydrothermal emissions study ashes vent field of axial volcano indicate separation of liquid and vapor phases during the migration of fluids to the seafloor the brine phase liquid emanates from discrete sulfide chimneys while fluid which has been through vapor phase separation flows diffusely from fissures in the surrounding basalt one possible mechanism to explain this spatial segregation of the fluid phases is based on the relative permeability terms in the equations of two phase flow a qualitative model for the distribution of fluids observed at ashes is postulated in which brine phase fluids are confined within flow conduits by a surrounding relative permeability barrier while vapor phase fluids flow diffusely into the surrounding host rock

helium and carbon geochemistry of hydrothermal fluids from the north fiji basin spreading ridge southwest pacific the north fiji basin is one of the active marginal basins in the southwestern pacific hydrothermal fluid emanations were located at two sites on the central ridge of the basin high temperature fluids 230 290 c venting from anhydrite chimneys at a 17 s site have end member compositions of 11 0 14 5 mmol kg of co2 30 4 43 5 mol kg of ch4 and 2 3 4 5 10 5 cm3stp g of he taking phase separation processes into account the gas abundances are not enriched compared with the mid oceanic ridge hydrothermal systems isotopic compositions of co2 13c 6 2 to 5 7 pdb ch4 13c 20 to 18 pdb and helium rra 9 0 10 0 are comparable to the mid oceanic ridge signature together with basalt helium data the helium isotopic signature may be attributed to the incorporation of a hotspot like primitive component low temperature shimmering fluids the highest measured temperature was 5 2 c associated with biological communities at a site at 18 50 s show slight chemical anomalies in some species in sio2 mn li ph and ch4 and helium isotope ratios distinct from ambient seawater evaluated helium isotopic compositions prior to dilution fall between rra 8 4 and 8 9 supporting the suggestion of high temperature hydrothermal activity at this site although this was not observed by dive expeditions the gas geochemistry of these two different types of fluids show several similar characteristics to the mid oceanic ridge hydrothermal systems this result is in accordance with previous petrological studies which demonstrated a dominant n morb source signature and a co existing oib source influence of the north fiji basin magmatism

ultrametamorphism within the amphibolite granulite transition zone upper aldan river siberia abstract the sequence of metamorphic events and granite formation in the nimnyr block of the aldan shield is described metamorphic assemblages polymigmatites and granite varieties developed in the upper aldan river are the results of two tectonothermal events corresponding to the second so called aldan cycle 3 3 3 1 ga and the fourth or ungra cycle 2 ga occurring during the five cycle history of the aldan shield for the second cycle a prograde sequence from the amphibolite facies through a transition zone to the biotite garnet cordierite orthoclase subfacies and then the garnet cordierite hypersthene orthoclase subfacies of the granulite facies is established temperatures and pressures are in the range of 680 850 c and 5 7 kbar biotite garnet cordierite geothermobarometer data metamorphism of the ungra cycle is also inhomogeneous and similar to that of the second cycle enderbite and tonalite gneisses charnockite and granite gneisses 1 stromatic and stockwork migmatites with leucosomes 2 and 3 respectively anatectic and removed charnockites and granites were formed during the aldan cycle migmatites with leucosomes 4 and removed charnockites 5 are related to the ungra cycle the mineral assemblages of granitoids and migmatite leucosomes vary systematically with the metamorphic assemblages comparison of the simultaneous leucosomes of the different metamorphic zones suggests a decrease in si and increase in ti ca mg fe from amphibolite to granulite facies the biotite compositions and lattice ordering of k feldspar also change regularly three stages are recognized in the sequence of granite formation related to the second cycle 1 replacement of initial rocks by granitoids 2 leucosome formation in stromatic and stockwork migmatites and 3 mobilization of granite materials these stages are also recognized in the fourth cycle we consider the predominant processes of granite formation metasomatism anatexis together with replacement and rheomorphism densities of co2 inclusions suggest that a decompressional process forms all the more or less displaced granites whereas conditions generating granite and charnockite gneisses were closer to those of metamorphism during the formation of the metamorphic zone pressure decreased by at least 3 kbar corresponding to an isothermal uplift up to 10 11 km

fluid inclusions in high grade anatectic metamorphites fluid inclusions have been studied by microthermometry in metapelites and metabasites which have undergone partial melting in the transition zone from amphibolite to granulite facies mobilizates contain far more abundant and larger fluid inclusions than non mobilized or restite layers and these inclusions occur as several generations of well defined and chemically contrasted fluids carbonic co 2 ch 4 and hydrocarbon aqueous h 2 o nacl and n 2 a garnet bearing mobilizate in amphibolite contains 4 generations of relatively pure fluids most of them trapped after peak metamorphic conditions in the following p t domains co 2 and n 2 p 2 8 kbar t 550 850 c ch 4 p 1 2 kbar t 400 550 c h 2 o p t 120 400 c

archean crustal evolution of the aldan shield siberia geochemical and isotopic constraints abstract the relatively unknown aldan shield in eastern siberia is made up of archean granite greenstone and high grade gneiss terrains as commonly observed in many precambrian shield areas new results of geochemical and isotopic study on two terrains of contrasting metamorphic grades olekma granite greenstone and west aldan granulite gneiss terrains are present with the following principal conclusions being reached 1 the crustal evolution of the aldan shield started as early as 3 5 ga as suggested by the tdm model ages of a few high grade gneisses and a retrograded granulite enclave but the most important crust forming and tectonothermal event is firmly established at ca 3 0 ga 2 new shrimp and pb evaporation analyses on zircons have confirmed the presence of crustal rocks older than 3 ga however the zircon u pb systems appear to have been disturbed by later thermal effects and no components of 3 5 ga have been identified 3 komatiitic and basaltic amphibolites form important supracrustal sequences in the olekma terrain both group i and ii komatiites occur and garnet or majorite fractionation in mantle melting processes is likely to have taken place during the genesis of group ii komatiites overall depleted mantle sources were involved in the geneses of basic and ultrabasic magmas as evidenced from lree depletion and nd isotopic compositions nd t 2 crustal contamination was not significant as suggested by the low la nb ratios 1 in komatiites and metabasalts 4 as common to most archean tonalite trondhjemite granodiorite ttg rocks the aldan granitic gneisses also show highly fractionated rare earth element ree patterns with heavy ree depletions suggesting that separation of garnet and amphibole has occurred during melting of their mafic sources 5 although the olekma and west aldan terrains have undergone different metamorphic evolution there is no substantial difference in the time of major crust forming events at 3 0 ga and a strong proterozoic thermal disturbance at 2 ga furthermore from the geochemical comparison of the two terrains it is concluded that the nature of mantle sources for basic ultrabasic magmas and the process of ttg magma generation are similar in both terrains and that the granulite facies rocks cannot be considered as the restites of intracrustal melting

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

temporal and compositional differences between subsolidus and anatectic migmatite leucosomes from the quetico metasedimentary belt canada migmatites in the quetico metasedi mentary belt contain two types of leucosome 1 layer parallel leucosomes that grew during deformation and prograde metamorphism these are enriched in si02 sr and eu but depleted in ti02 fe203 mgo cs rb ree sc th zr and hf relative to the quetico metasediments 2 discordant leucosomes that formed after the regional folding events when metamorphic temperatures were at their peak these are enriched in rb ba sr and eu but display a wide range of lree th zr and hf contents relative to the quetico metasediments layer parallel leucosomes formed by a sub solidus process termed tectonic segregation this stress induced mass transfer pfocess began when the quetico sediments were deformed during burial and continued whilst the rocks were both stressed and heterogeneous sub solidus leucosome compositions are consistent with the mobilization of quartz and feldspar from the host rocks by pressure solution the discordant leucosomes formed by partial melt ing of the quetico metasediments possibly dur ing uplift of the belt the range of composition displayed by the anatectic leucosomes arises from crystal fractionation during leucosome emplacement some anatectic leucosomes pre serve primary melt compositions and have smooth ree patterns but those with negative eu anomalies represent fractionated melts and others with positive eu anomalies represent accumulations of feldspar plus trapped melt

chemical composition of hydrothermal fluids in the central and southern mariana trough backarc basin abstract we present a geochemical data set from hydrothermal fluids collected from the central and southern mariana trough during dive programs conducted in the 1990s the fluid samples were collected from two hydrothermal fields 1 the alice springs field 18 13 n 144 42 e water depth 3600 m on the axial ridge crest of the central marana trough and 2 the forecast field 13 24 n 143 55 e water depth 1470 m on peak b seamount one of a cluster of small volcanic seamounts which represents a unique tectonic setting in the southern mariana trough gas geochemistry of the alice springs fluid showed he and c isotope ratios in the range of morb whereas that of the forecast fluid showed a higher co 2 3 he ratio and 13 c enriched isotope ratio both of which are commonly recognized in hydrothermal fluids from intraoceanic arc volcanoes the major and minor element composition of the alice springs fluid is characterized by enrichment in mobile alkali elements such as k rb and cs this geochemical signature is in accordance with the well known characteristics of basaltic lava collected from the same area which show substantial enrichment in incompatible elements as interpreted in previous petrological studies this signature is considered to be an arc geochemical signal reflecting contribution from the subduction component the b concentration of the alice springs fluid is also high and is another arc geochemical signal the forecast fluid by contrast did not show enrichment in the mobile alkali elements although its b concentration is high the forecast fluid is characterized instead by enrichment in ca and sr coupled with depletion in na and li as discussed in previous studies this signature is attributed to hydrothermal reactions at rather low temperature due to the shallow water depth we recognize two different arc geochemical signals in these two hydrothermal fluids mobile alkali elements in the alice spring fluid and gaseous species in the forecast fluid this difference could reflect different behaviors of these species during magmatic processes in the mantle wedge beneath the mariana trough backarc spreading center

formation of hydrothermal vents in the north fiji basin sulfur and lead isotope constraints abstract hydrothermal sulfides were recovered from the 16 50 s triple junction area in the north fiji basin at a water depth of ca 1900 m the chimney samples can be divided into three groups based on their major metal contents 1 type 1 fe cu rich 2 type 2 fe cu rich with minor zn and 3 type 3 zn rich chimneys type 1 chimneys are mainly composed of chalcopyrite and pyrite and are enriched in elements commonly precipitated under high temperature conditions 300 c such as cu co mo and se type 3 chimneys consist dominantly of sphalerite and marcasite with traces of pyrite and chalcopyrite and are enriched in elements commonly associated with relatively low temperatures 150 to 250 c such as zn cd pb as and ga type 2 chimneys have a mineralogy similar to that of type 1 chimneys although their trace metal contents range between those of type 1 and type 3 samples type 2 chimneys appear to be the products of low temperature replacement which might be related to formation of type 3 chimneys of type 1 chimneys the trace element composition of basaltic rocks indicates that magma generation in the triple junction area was influenced by two different sources n morb and e morb sulfur and lead isotope patterns of the hydrothermal chimneys show distinct differences between the type 1 and type 3 chimneys the type 1 sulfides 34 s 2 8 1 2 206 pb 204 pb 18 082 to 18 132 207 pb 204 pb 15 440 to 15 481 208 pb 204 pb 37 764 to 37 916 are depleted in 34 s and have lower pb isotope ratios compared to types 2 and 3 chimneys 34 s 4 1 0 9 206 pb 204 pb 18 122 to 18 193 207 pb 204 pb 15 475 to 15 554 208 pb 204 pb 37 882 to 38 150 the higher 34 s values for types 2 and 3 sulfides than type 1 sulfides can be explained by mixing of hydrothermal fluids with ambient seawater the more radiogenic pb isotope composition of types 2 and 3 chimneys compared to type 1 chimneys suggest that the formation of types 2 and 3 chimneys were possibly related to e morb volcanism that shows higher pb isotope composition than n morb rocks however ferromanganese crust samples have radiogenic pb isotope compositions that are identical to those of e morb therefore input of hydrogenous pb is also a possible source for radiogenic pb isotopes in the types 2 and 3 chimneys

magnesium isotopic composition of submarine vent fluids from arc and back arc hydrothermal systems in the western pacific abstract seafloor hydrothermal systems are important sinks for mg as their concentration in high temperature hydrothermal vent fluids decreases to near zero through the formation of secondary minerals however the problem of mass balance still persists regarding the global sink for mg via sub seafloor high temperature hydrothermal reactions in this study we measured isotopic compositions of mg in high temperature hydrothermal vent fluids from 5 arc and back arc systems in the western pacific this was done to better understand mg behavior during hydrothermal circulation and to quantify the oceanic mg cycle the samples with significantly low mg concentrations showed 26mg depletion due to mg fractionation and mg fixation during hydrothermal circulation addition of mg to the permeable sub seafloor during venting is the probable reason for the isotopic variation of mg the vent fluids return flux with lighter mg isotopic compositions contain much smaller amounts of mg than seawater this confirms that the high temperature hydrothermal sinks have a less significant effect on the oceanic mg isotopic composition based on the simple steady state equations and low temperature hydrothermal sinks of 26mg 0 25 to 0 00 of 26mg 7 26 of the riverine mg input is presumed to be removed through the high temperature sink this estimate is lower than that proposed by mottl and wheat 1994 which was 10 40 thus other mg sinks such as the low temperature hydrothermal sink and or the dolomite sink need to be considered

submarine venting of phase separated hydrothermal fluids at axial volcano juan de fuca ridge since the discovery of high temperature venting on the east pacific rise in 19791 it has been expected2 3 because of the physical properties of sea water at pressures and temperatures encountered during submarine hydrothermal circulation4 5 that phase separated fluids would discharge from ridgecrest vents although this notion is supported by the reported large deviations in vent fluid chlorinity relative to that of sea water 40 200 6 7 by observations of venting at p t conditions clearly within the two phase region 220 bar and 420 c 8 and by fluid inclusion data9 10 unequivocal identification11 of phase separated venting fluids has remained elusive here we report observations of chloride and metal depleted gas enriched fluids from a shallow vent field on the juan de fuca ridge which confirm the expectation that phase separated effluents are delivered to the deep ocean from some sea floor venting systems

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

on the origin of co2 rich fluid inclusions in migmatites nearly pure co2 fluid inclusions are abundant in migmatites although h2o rich fluids are predicted from the phase equilibria processes which may play a role in this observation include 1 the effects of decompression on melt 2 generation of a co2 bearing volatile phase by the reaction graphite quartz biotite plagioclase melt orthopyroxene co2 rich vapour 3 selective leakage of h2o from co2 h2o inclusions when the pressure in the inclusion exceeds the confining pressure during decompression and 4 enrichment of grain boundary vapour in co2 by subsolidus retrograde hydration reactions

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

secular variation in mexico during the last 40 000 years paleomagnetic data are available for mexico from various archives covering irregularly the last 40 000 years data obtained from lake sediments are affected by ambiguous conversion of age to depth and the paleomagnetic data themselves are not of high quality as samples were only demagnetized at one field intensity these lake sediment data are therefore considered as less reliable for construction of a secular variation curve although they may give some qualitative information for the last 25 000 years archeological samples provide data for the period 100 1200 a d their 14 c ages are some times not well constrained with unrecoverable analytical and statistical procedures and often ages were only inferred from the stratigraphic position and the paleomagnetic record the paleomagnetic record of these materials is of high quality with very low internal dispersion although no magnetic cleaning techniques have been applied variations of inclination and declination with time are hampered by large age uncertainties and thus of limited value for the construction of secular variation curves paleomagnetic data from an u th dated stalagmite provide reasonable directions for most of the last 1200 years being of lower quality in those parts with very low remanence intensity the u th age determinations were interpreted in terms of a constant growth rate which might be only valid as a first order approximation recent volcanic rocks from the trans mexican volcanic belt provide 19 paleomagnetic directions of reasonable to high quality and often also absolute paleointensities ages have been determined from 14 c analysis of charcoal or paleosols found below the lavas and extend over the last 40 000 years therefore volcanic rocks provide the most adequate archive for the construction of a secular variation curve but much more volcanic structures need to be dated to obtain a semi continuous secular variation curve comparison of the data from different archives is possible mainly for the last 2500 years and despite the aforementioned problems similar variations of declination and inclination are observed this holds as well when comparing them to data from holocene volcanic rocks from western north america 2002 elsevier science b v all rights reserved

arrested charnockite formation in southern india and sri lanka arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high grade terrains of southern india and sri lanka two major kinds of orthopyroxene producing reactions are recognized breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic granitic gray gneiss produced the regional orthopyroxene isograd manifest in charnockitic mottling and veining of mixed facies exposures as at kabbal karnataka and in the kurunegala district of the sri lanka central highlands chemical and modal analyses of carefully chosen immediately adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of cao mgo and feo and gains of sio2 and na2o rb and y are depleted in the charnockite another kind of charnockitization is found in paragneisses throughout the southern high grade area and involves the reaction of biotite and quartz garnet to produce orthopyroxene and k feldspar although charnockite formation along shears and other deformation zones at such localities as ponmudi kerala is highly reminiscent of kabbal close pair analyses are not as suggestive of open system behavior this type of charnockite formation is found in granulite facies areas where no prograde amphibole bearing gneisses exist and connotes a higher grade reaction than that of the orthopyroxene isograd metamorphic conditions of both kabbaltype and ponmudi type localities were 700 800 c and 5 6 kbar lower p h2o in the ponmudi type metamorphism was probably the definitive factor

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

recent tectonic magmatic and hydrothermal activity on the east pacific rise between 17 s and 19 s submersible observations the objective of the naudur cruise december 1993 of the submersible nautile was to study the interaction among magmatic tectonic and hydrothermal processes at a very fast spreading mid ocean ridge axis twenty three dives were completed both along and across the axis in four areas located between 17 10 and 18 45 s on the east pacific rise rock sulfides water and biological samples have been collected along each of the segments two main types of segments have been distinguished characterized either by the predominance of present day volcanic activity or by predominant tectonic activity linked to both types of activity 69 hydrothermal sites have been discovered and sampled they comprise four types interpreted as successive evolutionary stages the first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna the second in areas dominated by recent volcanic activity have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys the third stage is represented by more mature hydrothermal vents and deposits along the faults bounding the eastern side of the axial graben in tectonic dominated areas the associated fauna is well developed the fourth stage corresponds to the reactivation of volcanic activity with lava flows young black smokers and diffuse venting associated with the faults bounding the axial graben fluids collected range from 200 to 340 c and show a wide variability in chemical and gas composition within each of the explored areas evidence of recent volcanic activity has been observed

compared geochemical signatures and the evolution of menez gwen 37 50 n and lucky strike 37 17 n hydrothermal fluids south of the azores triple junction on the mid atlantic ridge during the diva 1 cruise in may 1994 a series of 19 dives was conducted using the french submersible nautile at the topographic highs of three volcanic segments centered at 37 17 n 37 50 n and 38 20 n respectively south of the azores triple junction atj on the mid atlantic ridge mar hydrothermal fluids were collected at the lucky strike ls hydrothermal field discovered at 37 17 n in 1993 and at a new hydrothermal site called menez gwen mgw discovered at 37 50 n during this cruise both systems are relatively shallow compared to other mar systems with seafloor depths of 1700 and 850 m respectively characteristics which make them unique among the already known sites on the mar the characteristics of the ls fluids are the same as in 1993 and include temperatures ranging from 170 c to 324 c variable chlorinities lower than seawater low hydrogen sulfide 3 0 mmol kg high ba concentrations up to 80 mol kg low metal concentrations and high gas contents and distinct chemical end members indicative of significant geographic control of the venting system in contrast the very clear mgw fluids at 37 50 n show a rather uniform exit temperature 285 c and chemical compositions with chlorinities 360 380 mmol kg lower than at ls and corresponding lower concentrations of cations h2s 1 8 mmol kg metals and silica 8 11 5 mmol kg due to the lower temperature t and pressure p of the system in the two systems fluid chemistry is strongly affected by phase separation at the low pressure conditions of these sites phase relations in the nacl h2o system dictate production of an extremely low salinity the higher salinities observed in fluids thus indicate that mixing of extremely low salinity vapor with single phase hydrothermal seawater is the dominant process controlling cl concentrations depletions and enrichments of elements in solutions are also explained by the involvement of fluid rock reactions the enrichment of ba k cs and rb in the fluids is linked to the enriched character of these elements in basalts relatively high ph and ca low li sr and metals are related to the highly altered basaltic substrate in the reaction zone low sr ca and high ca na ratios are explained by albitization within the crust the increase of degassing when approaching the azores may be related to the carbon enriched basaltic crust near the azores hot spot isotopic ratios show that co2 ch4 and helium gases have a magmatic signature compared to other deeper sites on the mar all fluids collected at ls and mgw are gas enriched except for h2s and helium whose both concentration and isotopic ratio compare well with other hydrothermal fluids the high ch4 concentration associated with unsaturated hydrocarbons and the high ch4 3he ratios also suggest a contribution of ch4 generated by serpentinization of ultramafic rocks by fischer tropsch catalysis of co2 reduction

composition of hydrothermal fluids and mineralogy of associated chimney material on the east scotia ridge back arc spreading centre abstract the east scotia ridge is an active back arc spreading centre located to the west of the south sandwich island arc in the southern ocean initial exploration of the ridge by deep tow surveys provided the first evidence for hydrothermal activity in a back arc setting outside of the western pacific and we returned in 2010 with a remotely operated vehicle to precisely locate and sample hydrothermal sites along ridge segments e2 and e9 here we report the chemical and isotopic composition of high and low temperature vent fluids and the mineralogy of associated high temperature chimney material for two sites at e2 dog s head and sepia and four sites at e9 black white ivory tower pagoda and launch pad the chemistry of the fluids is highly variable between the ridge segments fluid temperatures were 350 c at all vent sites except black white which was significantly hotter 383 c end member chloride concentrations in e2 fluids 532 536 mm were close to background seawater 540 mm whereas cl in e9 fluids was much lower 98 220 mm indicating that these fluids are affected by phase separation concentrations of the alkali elements na li k and cs and the alkaline earth elements ca sr and ba co vary with cl due to charge balance constraints similarly concentrations of mn and zn are highest in the high cl fluids but by contrast fe cl ratios are higher in e9 fluids 3 8 8 1 10 3 than they are in e2 fluids 1 5 2 4 10 3 and fluids with lowest cl have highest cu although both ridge segments are magmatically inflated there is no compelling evidence for input of magmatic gases to the vent fluids fluid d values range from 0 2 to 1 5 ph values 3 02 3 42 are not especially low and f concentrations 34 6 54 4 m are lower than bottom seawater 62 8 m the uppermost sections of conjugate chimney material from e2 and from ivory tower and pagoda at e9 typically exhibit inner zones of massive chalcopyrite enclosed within an outer zone of disseminated sulphide principally sphalerite and pyrite in an anhydrite matrix by contrast the innermost part of the chimneys that currently vent fluids with lowest cl black white and launch pad is dominated by anhydrite by defining and assessing the controls on the chemical composition of these vent fluids and associated mineralisation this study provides new information for evaluating the significance of hydrothermal processes at back arc basins for ocean chemistry and the formation of seafloor mineral deposits