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

lateglacial and holocene vegetation dynamics in the aegean region an integrated view based on pollen data from marine and terrestrial archives to elucidate the vegetation dynamics in the aegean region during the last 20 kyr we have studied terrestrial palynomorphs in marine core geotu sl152 mount athos basin northern aegean sea at centennial scale 125 to 300 yr resolution the robust chronology of the core allows us to reliably date signals of short term vegetation change in the aegean region for the pleniglacial interval until 14 6 kyr bp our data document steppe vegetation suggesting dry climatic conditions in the borderlands of the aegean sea subaerially exposed parts of the shelf were probably colonized by pinus dominated vegetation until shelf areas were flooded during lateglacial sea level rise the final rapid decrease of pinus pollen percentages between 14 and 13 kyr bp appears to be connected to meltwater pulse mwp 1a the lateglacial interstadial complex ie meiendorf bolling and allerod is characterized by the spread of oak trees but also by the continuous presence of steppe elements indicating only slightly increas

temporal and spatial variability of hydrothermal manganese and iron at cleft segment juan de fuca ridge a unique data set for hydrothermal mn and fe was collected at cleft segment on the juan de fuca ridge between 1983 and 1991 the data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting manganese heat and iron heat ratios for plumes from the north end of the cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for mn of 0 36 0 17 mol s 1 and for fe of 0 61 0 34 mol s 1 over 6 years of plume measurements at north cleft segment observed episodic hydrothermal discharge accounted for 15 of the total vented mn and 35 of vented fe the chronic fluxes for mn and fe at a second venting center located at the south end of the cleft segment were estimated to be approximately equal to the fluxes at north cleft segment chronic plumes at north cleft segment are mixtures of focused and diffuse discharge that contribute heat mn and fe in variable proportions similar examination of south cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in mn and fe but contributing substantially to the overall heat temporal and spatial variations in the concentrations of mn and fe and in mn heat and fe heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes manganese heat and iron heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes the accurate assessment of segment scale hydrothermal fluxes of mn and fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge

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

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

sr isotopic constraints on hydrothermal alteration of ultramafic rocks in two oceanic fracture zones from the south atlantic ocean ultramafic rocks from the islas orcadas and shaka fracture zones have been progressively hydrated starting at temperatures greater than 700 c and continuing to less than 250 c the hydration is manifest as a sequence of alteration minerals including in order of decreasing temperature amphibole talc talc plus secondary olivine chlorite secondary clinopyroxene prehnite and serpentine early high temperature minerals display lower87sr 86sr ratios and higher k rb and sr than later lower temperature minerals one explanation for the change in87sr 86sr ratios is that early high temperature reactions are the result of hydration by fluids characterized by a relatively strong basaltic geochemical signature and the later lower temperature alteration is the result of hydration by fluids characterized by a relatively weaker basaltic signature and relatively stronger seawater signature

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

geochemical controls on hydrothermal fluids from the kairei and edmond vent fields 23 25 s central indian ridge prior to 2000 no mid ocean ridge hydrothermal systems were identified and sampled in the indian ocean the kairei field central indian ridge was discovered and sampled in 2000 gamo et al 2001 hashimoto et al 2001 this paper reports the chemical composition of hydrothermal fluids collected in 2001 from both the kairei and newly discovered edmond hydrothermal systems data are used to infer subsurface processes and conditions at the two sites as well as to place them within the global context of known hydrothermal sites on the mid ocean ridge system while both sites are located on the intermediate spreading central indian ridge their hydrologic systems have affinities with those observed on slow spreading ridges although the compositions of fluids from the four individual vents sampled at edmond and the three individual vents sampled at kairei vary respectively our interpretation is that a single source fluid at each site is subsequently modified by processes including phase separation subsurface mixing with seawater and deposition and or remobilization of metal sulfide deposits to generate the observed range of compositions both fields are located 6 km from the neovolcanic zone on steps on the east wall of the axial valley with the edmond field 160 km north of kairei and almost 1000 m deeper 3300 versus 2450 m the edmond fluids are extremely hot with maximum measured temperatures of 382 c compared to 365 c for kairei all of the fluids sampled have chlorinities greater than local ambient seawater due to phase separation of seawater at supercritical conditions visual observations suggest both sites have been active for a substantial period of time again similar to observations of slow spreading ridges our chemical data suggest that the substrate underlying kairei is highly altered while that at edmond is not significant albitization appears to be occurring below the edmond field but not at kairei the edmond fluids are by far the hottest brines yet observed venting from a mid ocean ridge hydrothermal site resulting in unusually high concentrations of several transition metals

230th 238u disequilibrium systematics in oceanic tholeiites from 21 n on the east pacific rise significant disequilibrium occurs between 230 th and its parent 238 u in a suite of fresh basalt glasses from the rise project study area at 21 n on the east pacific rise the 230 th 232 th activity ratios observed for eight of nine samples from the crest of the axis at this site are constant within analytical uncertainty with a value of 1 22 this observed homogeneity of 230 th 232 th has two possible interpretations first the measured 230 th 232 th can be considered to indicate a mantle source for the rise basalts with th u of 2 5 this interpretation however conflicts with the proposed correlation between 230 th 232 th and 87 sr 86 sr 1 which predicts that 230 th 232 th should equal 1 33 at the rise site a second possible interpretation is that radioactive decay of 230 th in the basalts themselves or in a magma chamber has decreased 230 th 232 th from 1 33 to the observed values the required time span is 11 000 to 100 000 years however petrologic arguments rule against long residence time in a magma chamber and the spreading rate of this section of the east pacific rise 6 cm yr predicts that the maximum age for axis basalts is 27 000 years both interpretations of the measured 230 th 232 th imply a low th u ratio for the rise basalt source and suggest that the morb source at this location is depleted in th with respect to u relative to primitive mantle or bulk earth in spite of their constant 230 th 232 th the basalts from 21 n have wide ranges of measured th u and thorium and uranium concentrations in apparent conflict with the common assumption that these two elements have very small similar crystal liquid partition coefficients participation of an accessory phase with high th and u concentrations during partial melting or fractional crystallization appears to be required to explain this anomaly major and other trace element compositions provide more information about petrogenetic processes although fractional crystallization can explain the variations in major element composition a more complex process such as continuous melting is required to account for the observed trends in trace element compositions in addition to its use as a tracer the 230 th 238 u disequilibrium technique is capable of putting some constraints on the timing of fractionation events and it may eventually lead to a method for dating very young mid ocean ridge basalts

volcanism and metallogenesis of axial and off axial structures on the east pacific rise near 13 degrees n a fast spreading segment 12 cm yr super 1 of the east pacific rise near 12 degrees 50 n explored in detail by surface ship and manned submersible displays intense hydrothermal activity the rise crest about 1 500 m in width with a regional depth of 2 600 m is occupied at its center by an axial graben 1 3 titania alkali enriched olivine ti k basalts with high tio 2 1 4 and high alkali content na 2 o 3 0 k 2 o 0 3 occur on the southeastern seamount the fractionated tholeiites are believed to have been derived from primitive melts during differentiation which took place within a magma reservoir asymmetrical with respect to the axis of the graben which supplied both axial and off axial lava flows the ti k enriched basalts are believed to have been produced from an alkali enriched magmatic source

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

gabbroic xenoliths and host ferrobasalt from the southern juan de fuca ridge rare isotropic gabbroic xenoliths occur in sheet and lobate flow fragments of nearly aphyric ferrobasalt collected along a 12 km section of the southern juan de fuca ridge xenoliths comprise 1 of the dredge contents and range in size from 1 cm3 glomerocryst to 240 cm3 the xenoliths have ophitic to intersertal texture with 5 50 interstitial glass of ferrobasaltic composition more evolved than the host lava on the basis of texture and mineralogy the xenoliths have been subdivided into three types type i plagioclase olivine glass type ii plagioclase augite glass olivine and type iii plagioclase augite olivine glass pigeonite partially inverted fe ti oxides mineral and glass inclusion compositions suggest a sequence of evolution for the three xenolith types in which type i is the least evolved and type iii is the most evolved application of a graphical pyroxene geothermometer to augite in xenolith types ii and iii yields crystallization temperatures of 1100 1200 c and to host lamellae pairs in inverted pigeonite yields subsolidus equilibrium temperatures of 1100 1150 c coexisting titanomagnetite ilmenite pairs in type iii xenoliths yield temperature estimates of 1000 1070 c and log f02 9 7 to 10 8 we infer that the xenoliths represent the partially crystalline mush boundary zone of a magma chamber based on the abundance of interstitial glass zonation of mineral grains in the most crystalline samples and coherence of chemical trends between interstitial glass glass inclusions and mineral phases the evolved composition of the xenoliths provides evidence for the presence of melts more fractionated than the host ferrobasalt in the magma chamber the erupted ferrobasalt is a hybrid lava formed by mixing these highly evolved melts with more primitive melts

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

chemical evolution of mid ocean ridge hot springs abstract chemical analyses yielding elemental concentrations of major and minor elements of four hot springs on the east pacific rise at 21 n and 10 hot springs from the southern trough of the guaymas basin gulf of california provide a basis for thermodynamic modeling of conductive cooling of the hydrothermal endmembers mixing of the hydrothermal endmember with seawater and reaction of an epr type fluid with sediment of bulk chemical composition corresponding to unaltered sediment from dsdp hole 477 in the guaymas basin results of the calculations indicate that conductive cooling of endmember fluids within closed chimneys and conduits accounts for the solution composition of one vent on the east pacific rise and several vents in guaymas basin mixing of the hydrothermal fluids with seawater yields a prediction of mineral assemblages closely approximating those observed in samples of chimneys from the east pacific rise and drill cores in guaymas basin the hypothesis that guaymas basin hydrothermal fluids result from interaction of an epr type fluid with sediment cover in guaymas basin is supported by calculations which predict an increase in ph to a value similar to guaymas basin fluids an order of magnitude decrease in metal concentrations and an excellent agreement between predicted mineral assemblages as a function of extent of interaction with sediment and observed mineral assemblage distribution with depth

geochemistry of fluids from submarine hot springs at punta de mita nayarit mexico abstract thermal springs with a maximum measured temperature of 89 c discharge hot water and gas from a depth of 11 m 400 m offshore of punta pantoque located in the northern part of bahia de banderas near puerto vallarta mexico the composition of all water samples collected from the sea bottom is close to that of sea water nevertheless it was possible to estimate the thermal endmember composition by extrapolating the sulfate concentration to zero this endmember is similar in chemical composition both to waters of the rio purificacion and la tuna thermal springs located to the south along the pacific coast of the jalisco block and to pore waters from the deep sea drilling cores from some accretionary complexes gas composition as well as isotopic composition of he and carbon from co 2 ch 4 and c 2 h 6 suggests an essentially thermo biogenic origin for the gas and the presence of a high proportion of radiogenic crustal helium isotopic composition of he in the punta de mita gas 0 4 r a is the lowest ever measured in mexican hydrothermal gases these findings do not support the idea that there exists a direct connection between the punta de mita springs and the last volcanic events which occurred in this area at 3 ma rather this hydrothermal activity is related to deep active faulting and the existence of a deep regional aquifer or local aquifers of connate waters underlying the granites of the jalisco block

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

chemical controls on the composition of vent fluids at 13 11 n and 21 n east pacific rise six vent fields sampled at 13 11 n east pacific rise epr in may 1984 exhibit large interfield variations and a much wider range of chemical compositions than previously observed at 21 n measured ph at 25 c are acidic ranging from 3 1 to 3 7 sodium and chloride vary from 40 lower to 30 higher than seawater iron concentrations range from 2 to 10 mmol kg compared with 0 7 2 5 mmol kg at 21 n other sulfide forming metals cu zn cd and pb are generally lower at 11 13 n than at 21 n reliable temperature measurements were obtained at only two of the six vents and both were 350 5 c the vent fields at 21 n epr were resampled in august 1985 thus extending to almost 6 years the time period over which they have been monitored previous expeditions were made in november 1979 and november 1981 campbell et al this issue have shown that the chemistry of the hydrothermal fluids from these fields has been very stable over the period of repeated observation equilibrium calculations for the fluids from the fields at 13 n and 21 n using a greatly improved thermodynamic data base are described in this report they indicate that the chemistry is rock buffered and that the stability of these systems over time is a result of equilibrium control with respect to a greenschist type mineral assemblage at depth calculated high temperature ph of the fluids range from 4 1 to 4 7 with those from 13 11 n at the more acidic end of the range calculated affinities show that the fluids are close to or at saturation with respect to quartz albite muscovite smectite chlorite epidote and pyrrhotite the computations imply that lower temperature vents such as national geographic smoker ngs 273 c may have their silica concentrations controlled by equilibrium with respect to a phase other than quartz a comparison of fluid chemistry between ngs and vent 5 at 11 n suggests that the latter vent may also have a temperature 300 c while the fluid chemistry of a given field is compositionally stable the compositions themselves are unique for a particular field and cover a wide range between fields the processes controlling this variability probably include the depth of reaction i e pressure which is difficult to treat thermodynamically at present in addition increasing depth in the crust may be accompanied by decreasing permeability and hence a probable decrease in the water rock ratio it may be difficult to separate these two effects if the age differences between the fields at present unknown are of the order of decades then their temporal histories may have an important influence on their particular chemistries

minor and trace element geochemistry of volcanic rocks dredged from the galapagos spreading center role of crystal fractionation and mantle heterogeneity a wide range of rock types abyssal tholeiite fe ti rich basalt andesite and rhyodacite were dredged from near 95 w and 85 w on the galapagos spreading center computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization however the p2o5 k2o ratio averages 0 83 at 95 w and 1 66 at 85 w and implies distinct mantle source compositions for the two areas these source regions also have different rare earth element ree abundance patterns with la sm ef 0 67 at 95 w and 0 46 at 85 w the sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95 w the mantle source regions for these two areas are interpreted to be depleted in incompatible and volatile elements although the source region beneath 95 w is less severely depleted in la and k incompatible trace element abundances in 26 samples are used to infer that the range of fe ti rich basalt from 85 w represents 19 to 35 residual liquid following crystal fractionation of a mineral assemblage of plagioclase clinopyroxene and lesser olivine the most highly differentiated samples have also had less than 1 titanomagnetite removed most samples from 85 w can be related to a common parental magma that contained approximately 9 wt feo 1 wt tio2 and had an mg number mg 100 mg mg fe2 of about 65 although the samples from 95 w cannot all be derived from a common parental magma the inferred parental magmas may have been derived by varying degrees of partial melting of a common source the fractionation sequence consists of two parts an initial iron enrichment trend followed by a silica enrichment trend we interpret the trace element data to indicate that the most iron rich lavas represent about 32 residual liquid derived by crystal fractionation of plagioclase clinopyroxene and lesser olivine from a parental magma with an mg number of about 66 the silica enrichment trend results from crystallization of titanomagnetite and some apatite fractionation of pigeonite which is a minor phase in the major element models cannot be distinguished from clinopyroxene fractionation by using trace elements

patterns and processes of alteration in the lavas and dykes of the troodos ophiolite cyprus alteration patterns in the lavas and dykes of the troodos ophiolite cyprus record a complex history of axial hydrothermal alteration crustal aging and subsequent uplift and emplacement of the ophiolite field mapping shows that distribution of five alteration zones each with distinct mineralogical geochemical and hydrologie characteristics is influenced by igneous stratigraphy structure and the nature and thickness of the overlying sediments paragenetic sequences of secondary minerals indicate that alteration conditions changed progressively as the crust cooled and moved off axis along spreading axes low temperatures 50 c were maintained by the rapid flow of seawater in and out of the lavas and only minimal alteration took place in contrast lower water rock ratios and higher temperatures 200 c in the dykes promoted extensive seawater rock interaction although the sharp rise in temperature between the two regimes generally coincides with the lava dyke transition late stage intrusions or hydrothermal upwelling zones locally cause high temperature alteration to extend upward into the lavas as a segment of crust moved off axis temperatures remained low in the lavas and progressively decreased from 250 to 80 c in the dykes high permeability in the uppermost lavas led to the downward migration of an oxidative alteration front whose thickness and spatial distribution was dependent upon the rate and nature of sedimentation and thus the original seafloor morphology although field relations show that alteration has a consistent vertical pattern in troodos the alteration zones are not laterally continuous and the stratigraphie depth of their boundaries varies considerably

the formation of high temperature clay minerals from basalt alteration during hydrothermal discharge on the east pacific rise axis at 21 n abstract aluminous high temperature clay minerals form from alteration of tholeiitic basaltic glass and calcic plagioclase during hydrothermal venting on the crest of the east pacific rise at 21 n the clay alteration assemblages are layered crusts up to 1 mm thick completely replacing glass and calcic plagioclase adjacent to surfaces exposed to hydrothermal fluids the interiors of the affected basalt samples have unaltered appearances and oxygen isotopic compositions just slightly heavier than that of morb the surficial alteration crusts are mixtures of beidellitic smectite aluminous dioctahedral randomly interstratified mixed layer al rich chlorite smectite minor chlorite an x ray amorphous aluminosilicate material and possible minor serpentine amesite a 18 o value of 4 1 0 2 smow is determined for the beidellitic smectite assuming that this smectite equilibrated with hydrothermal fluid having an oxygen isotope value between that of seawater 0 and 350 c hydrothermal fluid from epr 21 n vents 1 6 an equilibration temperature between 290 c and 360 c is calculated for the beidellitic smectite this is substantially higher than any previously reported temperature for an oceanic smectite the mixed layer al rich chlorite smectite has a 18 o value of 3 5 which corresponds to equilibration at 295 360 c the aluminous composition of the alteration assemblage is uncommon for clay minerals produced by submarine hydrothermal basalt alteration we suggest that this assemblage is largely the product of high temperature interaction between basalt glass plagioclase and mg poor acidic hydrothermal fluids with possibly some contribution of mg from bottom seawater and that the aluminous clays either incorporate al 3 remobilized from basalt by lowph hydrothermal fluids or are residual phases remaining after intense alteration of basaltic glass plagioclase

hydrothermal alteration processes at midocean ridges experimental and theoretical constraints from ca and sr exchange reactions and sr isotopic ratios diabase and basalt were reacted with na ca k cl fluids of seawater chlorinity at 375 425 c 375 400 bars and fluid rock mass ratios of 0 5 1 0 to assess the role of basalt chemistry and texture on sr and ca mobility during high temperature hydrothermal alteration an additional experiment utilizing an 84sr spiked fluid was performed to help quantify reaction rates of processes affecting sr mobility the experimental results help to constrain reaction processes responsible for the chemistry of hot spring fluids at midocean ridges alteration of basalt and diabase is characterized by formation of tremolite actinolite smectite chlorite and clinozoisitic epidote smectite chlorite respectively diabase alteration produced dissolved sr ca ratios similar to those observed for ridge crest hot spring fluids whereas alteration of a mostly crystalline basalt produced significantly lower ratios this observation supports the premise that the sr ca ratios observed in vent fluids may be produced during deep seated reaction of the hydrothermal fluids with diabase dikes and or gabbro at relatively low fluid rock ratios and suggests that hydrothermal alteration of primary igneous minerals at relatively high temperatures 400 c leads to formation of a mineral with low dsr ca possibly hydrothermal plagioclase results of the 84sr spiked experiment indicated that only 4 of the sr in basalt was mobilized after 800 hours of reaction despite the fact that b and li were nearly quantitatively leached it is thus suggested that b and li are good indicators of the amount of fresh rock encountered by fluids while sr concentration and isotopic data can be used to estimate the degree of alteration that the sr bearing primary minerals have undergone the degree of alteration of subsurface reservoirs may be estimated by dividing the fluid rock ratio obtained for highly mobile elements by the fluid rock ratio obtained from path integrated sr isotope and concentration data based on this method for example it can be estimated that only 5 of the primary sr bearing minerals encountered by fluids in the subsurface at epr 21 n are actually converted to secondary phases

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

direct observation of the evolution of a seafloor black smoker from vapor to brine a single hydrothermal vent f vent occurring on very young crust at 9 16 8 n east pacific rise was sampled in 1991 and 1994 in 1991 at the measured temperature of 388 c and seafloor pressure of 258 bar the fluids from this vent were on the two phase curve for seawater these fluids were very low in chlorinity and other dissolved species and high in gases compared to seawater and most sampled seafloor hydrothermal vent fluids in 1994 when this vent was next sampled it had cooled to 351 c and was venting fluids 1 5 times seawater chlorinity this is the first reported example of a single seafloor hydrothermal vent evolving from vapor to brine the 1991 and 1994 fluids sampled from this vent are compositionally conjugate pairs to one another these results support the hypothesis that vapor phase fluids vent in the early period following a volcanic eruption and that the liquid phase brines are stored within the oceanic crust and vent at a later time in this case 3 years these results demonstrate that the venting of brines can occur in the same location in fact from the same sulfide edifice where the vapor phase fluids vented previously

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

peculiar features of suiyo seamount hydrothermal fluids izu bonin arc differences from subaerial volcanism abstract this is the first report of the dissolved gas composition of a deep sea venting hydrothermal system originating from arc volcanism the origin of dissolved gases in hydrothermal fluids from the suiyo seamount in the southern izu bonin ogasawara arc is discussed on the basis of chemical and isotopic compositions twelve high temperature up to 310 c fluid samples were collected from three vents of the hydrothermal system using the submersible shinkai 2000 the average 3 he 4 he ratio of the dissolved he is 8 1 r r atm which is within the range of values reported for n morb the co 2 3 he ratio is 12 109 which is within the range of arc volcanic gas values the 13c values of co2 and ch4 are 1 and 8 5 respectively which is quite high compared with other subaerial arc volcanic gases and those from mid ocean ridges these results which do not accord with those from subaerial volcanism are unlikely to result from fractionation or addition of carbon sources during the fluid circulation but are regarded as characteristic of the magma source of suiyo seamount a simple mixing of 97 morb mantle with 3 subducting slab explains well the composition of the helium and carbon in the parent magma

alteration of the oceanic crust implications for geochemical cycles of lithium and boron abstract fresh tholeiitic basalt glass has been reacted with seawater at 150 c water rock mass ratio of 10 and fresh diabase has been reacted with a na k ca cl fluid at 375 c water rock mass ratios of 1 2 and 5 to understand better the role of temperature basalt composition and water rock mass ratio on the direction and magnitude of b and li exchange during basalt alteration at 150 c slight but nevertheless significant amounts of b and li were removed from seawater and incorporated into a dominantly smectite alteration phase at 375 c however b and li were leached from basalt b behaved as a soluble element and attained concentrations in solution limited only by the b concentration in basalt and the water rock mass ratio li however was less mobile for example at water rock mass ratios of 1 2 and 5 the percent of li leached from basalt was 58 70 and 92 respectively this suggests some mineralogic control on li mobility during hydrothermal alteration of basalt especially at low water rock mass ratios in general these results as well as those for b are consistent with the temperature dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids based on the distribution and chemistry of products of seafloor weathering low 150 c and high temperature hydrothermal alteration of basalt and the chemistry of ridge crest hydrothermal fluids it was estimated that alteration of the oceanic crust is a li source for seawater this is not true for b however since the hot spring flux estimated for b is balanced by low temperature basalt alteration these data coupled with b and li flux estimates for other processes e g continental weathering clay mineral adsorption authigenic silicate formation and formation of siliceous skeletal material yield new insight into the b and li geochemical cycles calculations performed here indicate relatively good agreement between the magnitude of b and li sources and sinks the geochemical cycle of b however may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones serpentinites are conspicuously enriched in b and if the b source for these rocks is seawater then an additional b sink exists which must be integrated into the b geochemical cycle however until more data are available in terms of areal extent of serpentinization serpentite chemistry and isotopic composition the importance of b in these rocks with respect to the b geochemical cycle remains speculative at best

sub sea floor hydrothermal alteration in the galapagos spreading center abstract oceanic crust was drilled at hole 504 b on the southern crest of costa rica rift galapagos spreading center this is the first hole that contains hydrothermally altered rocks containing secondary minerals of greenschist facies in d s d p holes drilled at the typical mid oceanic ridges altered rocks under high temperature water rock interaction are recovered below 890 m subbottom depth based on the secondary minerals the distribution of veins and the trace element contents the cores are classified into three zones zone 1 from 274 to 890 m zone 2 from 890 to 1050 m and zone 3 from 1050 to 1350 m in zone 1 the rocks remain fresh or are partly altered to secondary minerals smectite and saponite etc while in zones 2 and 3 laumontite chlorite epidote quartz and actinolite appear veins are more frequent in zone 2 than in zone 3 some samples in zone 2 are enriched in mn and zn part of zones 2 and 3 is considered to be the discharge zone in a hydrothermal convection system based upon the chemical composition of actinolite whole rock composition and sr isotope data the estimated alteration temperature changes sharply between zones 2 and 3 it is lower than 100 c in zone 1 while it is higher than 200 c in zones 2 and 3 this gap corresponds to the sharp change of hydrothermal alteration and other geophysical properties the most plausible mechanism for the formation of this gap is the time lag intensive hydrothermal activity occurred during the formation of oceanic crust in the rift valley another lava flow covered the rocks in zones 2 and 3

finite thickness of shear bands in frictional viscoplasticity and implications for lithosphere dynamics permanent deformations in the lithosphere can occur in the brittle as well as in the ductile domain for this reason the inclusion of viscous creep and frictional plastic deformation is essential for geodynamic models however most currently available models of frictional plasticity are rate independent and therefore do not incorporate an internal length scale which is an indispensible element for imposing a finite width of localized shear zones therefore in computations of localization either analytical or numerical a width of the shear zone is obtained which tends to zero in numerical computations this manifests itself in a severe mesh sensitivity moreover convergence of the global iterative procedure to solve the non linear processes is adversely affected which negatively affects the reliability and the quality of predictions the viscosity which is inherent in deformation processes in the lithosphere can in principle remedy this mesh sensitivity however elasto visco plastic models which are commonly used in geodynamics assume a series arrangement of rheological elements maxwell type approach which does not introduce an internal length scale here we confirm that a different rheological arrangement which put a damper in parallel to the plastic slider kelvin type approach introduces an internal length scale as a result pressure and strain and strain rate profiles across the shear bands converge to finite values upon decreasing the grid spacing we demonstrate that this holds for non associated plasticity with constant frictional properties and with material softening with respect to cohesion finally the introduction of kelvin type viscoplasticity also significantly improves the global convergence of non linear solvers