dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

sulfidation of major rock types of the oceanic lithosphere an experimental study at 250 c and 400 bars abstract circulation of hydrothermal fluids within the oceanic crust is a major factor in mass and heat transfer between hydrosphere and lithosphere many occurrences of massive sulfide have been reported along the mid ocean ridges showing effective mass transfer of sulfur and metals in these systems besides precipitation of sulfide at the seafloor replacive sulfide formation in the sub seafloor has been documented however different lithologies are expected to have different affinities to sulfidization to investigate the role of rock composition on sulfidization efficiency we have conducted batch experiments at 250 c and 400 bars that placed an h2s rich hydrothermal fluid into contact with four different rock types dunite olivine separates troctolite basalt and serpentinite these rocks were exposed to fluids containing 20 mmol kg total dissolved sulfide for durations between 1 and 10 weeks in situ fluid sampling allowed us to determine the evolution of the concentrations of ionic solutes and dissolved gases solid products were analyzed after termination of the experiments using xrd sem tg and vsm geochemical modeling was employed to compare observed reaction with equilibrium thermodynamic predictions sulfide in the form of euhedral pyrite crystals could be observed only in the basalt experiment in all other runs prevailing h2s activities were too low for sulfide formation despite high total sulfide concentrations instead the troctolite and peridotite experiments showed serpentinization of olivine and magnetite formation plagioclase in troctolite experienced desilicification resulting in andradite formation the stability field of sulfide in the h2 h2s activity plane is diminished due to the prevalence of andradite in hydrothermally reacted troctolite and sulfidization in the troctolite experiment was hence inhibited the serpentinite experiment showed very little reaction and no sulfide formation our results show that olivine rich seafloor lithologies are not very prone to sulfidation basaltic crust appears to have a larger potential to form sulfide deposits in some experiments pyrrhotite did not form although h2 h2s activities plot in the pyrrhotite stability field of the fe o s phase diagram we suggest that the higher oxidation state in basalt allows for the precipitation of pyrite which is not stable under the more reducing conditions prevailing where water interacts with olivine rich lithologies

sulfur in serpentinized oceanic peridotites serpentinization processes and microbial sulfate reduction the mineralogy contents and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes serpentinization of 1 ma peridotites at hess deep occurred at high temperatures 200 400 c and low water rock ratios oxidation of ferrous iron to magnetite maintained low o2 and produced a reduced low sulfur assemblage including nife alloy small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled producing low 34s sulfide 1 5 to 23 7 in contrast serpentinization of iberian margin peridotites occurred at low temperatures 20 200 c and high water rock ratios complete serpentinization and consumption of ferrous iron allowed evolution to higher o2 microbial reduction of seawater sulfate resulted in addition of low 34s sulfide 15 to 43 and formation of higher sulfur assemblages that include valleriite and pyrite the high so4 total s ratio of hess deep serpentinites 0 89 results in an increase of total sulfur and high 34s of total sulfur mean 8 in contrast iberian margin serpentinites gained large amounts of 34s poor sulfide mean total s 3800 ppm and the high sulfide total s ratio 0 61 results in a net decrease in 34s of total sulfur mean 5 thus serpentinization is a net sink for seawater sulfur but the amount fixed and its isotopic composition vary significantly serpentinization may result in uptake of 0 4 14 1012 g s yr 1 from the oceans comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output

global tidal residual mean circulation does it affect a climate ogcm a global description of the ocean tidal residual mean circulation trmc is provided and its effect on a global ocean climate model is evaluated the trmc is computed from a global barotropic tidal model based on pure hydrodynamical equations and forced by the major tidal constituents it thus includes both stokes drift and non linear tidal dynamics the trmc map exhibits remarkable basin scale structures in the indonesian archipelago australian region and on the malvinas shelf regions of largest transport are characterized by sites of high tidal amplitudes abrupt topography and western boundaries yet transport magnitudes remain at a few tenths of a sverdrup this trmc is introduced as an external forcing in an ocean climate model at equilibrium the thermohaline structure and the poleward heat transport of the ocean are almost insensitive to the trmc it is concluded that the trmc is negligible for climate purposes in an ocean climate model

sulfur cycling at the mid atlantic ridge a multiple sulfur isotope approach abstract the role of sulfur in two hydrothermal vent systems the logatchev hydrothermal field at 14 45 n 44 58 w and several different vent sites along the southern mid atlantic ridge smar between 4 48 s and 9 33 s and between 12 22 w and 13 12 w is examined by utilizing multiple sulfur isotope and sulfur concentration data isotope compositions for sulfide minerals and vent h 2 s from different smar sites range from 1 5 to 8 9 in 34 s and from 0 001 to 0 051 in 33 s these data indicate mixing of mantle sulfur with sulfur from seawater sulfate combined 34 s and 33 s systematics reveal that vent sulfide from smar is characterized by a sulfur contribution from seawater sulfate between 25 and 33 this higher contribution compared with epr sulfide indicates increased seawater sulfate reduction at mar because of a deeper seated magma chamber and longer fluid upflow path length and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid ocean ridges additionally isotope data uncover non equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 c 34 s values between 0 2 to 8 8 for dissolved and precipitated sulfide from logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at smar 34 s values between 24 5 and 6 5 and 33 s values between 0 001 and 0 125 for sulfide bearing sediments and mafic ultramafic host rocks from drill cores taken in the region of logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction basalts and basaltic glass from smar sites with 33 s 0 008 reveal lower 33 s lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials we conclude that the combined use of both 34 s and 33 s provides a more detailed picture of the sulfur cycling in hydrothermal systems at the mid atlantic ridge and uncovers systematic differences to hydrothermal sites at different mid ocean ridge sites multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow furthermore based on 33 s we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when 34 s were identical

redox state of seafloor hydrothermal fluids and its effect on sulfide mineralization abstract the redox conditions f o 2 f s 2 of seafloor hydrothermal fluids were calculated from dissolved h 2 and h 2 s concentrations at conditions of 300 and 350 c 71 409 bars and seawater salinity the results were compared with f o 2 f s 2 conditions estimated from black smoker mineralogy as reported by previous studies and our own investigations for various geological settings including the okinawa trough sediment rich hakurei and sediment poor jade fields the manus back arc basin felsic hosted pacmanus and basalt hosted vienna woods fields and the central indian ridge mafic ultramafic hosted kairei and basalt hosted edmond fields in most cases the calculated fluid f o 2 f s 2 conditions agree well with estimates from primary mineral assemblages and fes contents in sphalerite wurtzite this correlation suggests that the redox states of sulfide mineralization in chimneys typically reflect dissolved h 2 and h 2 s concentrations in vent fluids it should be noted that variable h 2 concentrations in vent fluids 0 01 19 mmol kg dominantly control not only fluid f o 2 but also f s 2 whereas the more uniform h 2 s concentrations 1 20 mmol kg have less of an influence on fluid f s 2 fluid sediment interaction elevates h 2 concentrations to over 1 mmol kg which sufficiently lowers f o 2 f s 2 to the pyrrhotite stability field however low h 2 s concentrations do not necessarily account for low f s 2 calculations for the h 2 poor 0 1 mmol kg and h 2 s rich 10 mmol kg fluids in some felsic hosted arc backarc systems indicate relatively high f o 2 f s 2 states consistent with the bornite pyrite chalcopyrite assemblages these redox conditions result from a combination of fluid mineral equilibria at relatively oxidizing conditions elevated h 2 s concentrations from magmatic degassing redox calculations at 350 c indicate that neither pyrite nor pyrrhotite is stable in the initial hot fluids of the tag and ultramafic hosted mid atlantic ridge systems due to the low h 2 s concentrations with cooling of these fluids either conductively or by interacting with seawater higher f s 2 and f o 2 result and fe sulfides are stable in agreement with the higher f o 2 f s 2 mineral assemblages observed

226ra 230th excess generated in the lower crust implications for magma transport and storage time scales excesses of 226 ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs followed by fast ascent rates of magma from slab to surface up to 1000 m yr however we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle derived 226 ra excesses and so reduce inferred vertical transport rates we developed an incongruent continuous melting model and both the incongruent melting reaction and ingrowth effects contribute to the 226 ra excess in particular we found that dehydration melting of amphibolite can produce modeled 226 ra excesses greater than 300 mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 u excess subducted slab signature this amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements similar to those observed in several arc settings which may distinguish these magmas from those with 226 ra excesses produced by slab dewatering alone

energetics of the m2 barotropic ocean tides an estimate of bottom friction dissipation from a hydrodynamic model abstract the aim of this paper is to present a detailed energy budget of the m2 ocean tides over the world ocean based on the hydrodynamic tidal solution fes94 1 which includes not only the characteristics of the sea surface tidal elevations but also of the tidal currents global and regional budgets are given the global energy input by astronomical forcing deduced from the 2 2 spherical harmonic development of the m2 sea surface elevation solution is 2 35 tw lower by 0 1 tw than for the new altimetric solutions produced recently directly from satellite observations one major feature emerging from the present analysis is that a quarter of this energy input takes place over the south atlantic ocean on the other hand ocean tides work against astronomical forcing over three areas eastern indian ocean southwest of the south american continent and around new zealand this energy is dissipated in fes94 1 by bottom friction over the main continental shelves but the outstanding feature is that 40 of the tidal energy is dissipated over the north atlantic ocean energy is thus redistributed from the areas of energy input to feed the areas of dissipation the pacific ocean the indian ocean and the south atlantic ocean receive more energy in total than they dissipate in contrast with the atlantic ocean and the arctic ocean where a large deficit must be compensated by a net influx of energy from the other basins one third of the total m2 energy input by astronomical forcing over the world ocean flows northwards through the equatorial atlantic detailed energy dissipation estimates are computed over the different areas of energy dissipation and compared to the previous estimates published in the literature

sulfur geochemistry of peridotite hosted hydrothermal systems comparing the ligurian ophiolites with oceanic serpentinites abstract serpentinization processes at slow and ultraslow spreading ridges control the exchange of various elements between seawater and the oceanic lithosphere and play a major role in marine geochemical cycles we use opaque mineral assemblages and sulfur isotope geochemistry to reconstruct variations in fluid fluxes redox conditions and microbial activity prevailing during serpentinization and carbonate precipitation of serpentinites and ophicalcites from an ophiolite sequence in the northern apennines italy we then compare our results with calcite veined serpentinites from the iberian margin formed during the opening of the north atlantic and with serpentinites sampled at the active peridotite hosted lost city hydrothermal field on the atlantis massif near the mid atlantic ridge the sulfide and oxide mineral assemblage of the serpentinites from the northern apennines is dominated by pyrite pentlandite millerite siegenite magnetite and hematite reflecting relatively oxidizing conditions while the sulfur geochemistry reveals a dominance of sulfide sulfur over sulfate sulfur 34 s values of sulfide and sulfate indicate that microbial sulfate reduction leaching and oxidation are the main processes that affected the sulfur isotope signature of these serpentinites the opaque mineralogy in the serpentinites from the northern apennines is similar to that of the iberian margin and the southern wall of the atlantis massif and generally represents the late stages of serpentinization where intense fluid circulation leads to fairly oxidizing conditions and alkaline fluids lead to carbonate precipitation however the mineral assemblages also indicate strong fluctuations in oxygen fugacity likely caused by volume expansion during serpentinization and tectonic activity along a mid ocean ridge additionally local enrichment in 34 s in sulfides suggests that fluids interacted at depth with gabbros and subsequently circulated along shear zones similar to observations along detachment fault surfaces at the atlantis massif our results therefore give further evidence that the ophicalcites and serpentinites from the northern apennines formed in a tectonic setting similar to the atlantis massif we estimate a global annual uptake of seawater sulfate of 5 6 to 12 10 11 g s by serpentinization of ultramafic rocks exposed to seawater on the ocean floor we suggest that the thermal structure of the exposed mantle rocks and in turn the efficiency of microbes to reduce seawater sulfate to sulfide is a major factor controlling the storage of sulfur in serpentinites thus cycling of sulfur in peridotite hosted hydrothermal systems during the jurassic was probably very similar to the processes observed today in systems along the mid atlantic ridge

incipient hydration of mantle lithosphere at ridges a reactive percolation experiment a reactive percolation experiment was conducted by injecting seawater into a permeable olivine aggregate at 190 c and 19 mpa to explore the relationships between hydration reactions and hydrodynamic properties during the onset of serpentinization in the ultramafic lithosphere exposed at mid ocean ridges the experiment was stopped after 23 days when the sample became impermeable the initial flow rate was 0 2 ml h and then was decreased down to 0 06 ml h after 8 days permeability decreased continuously throughout the experiment the analyses of fluid chemistry time series and of the mineralogy and structure of the reacted sample showed olivine dissolution and precipitation of proto serpentine brucite and fe oxides these reactions are controlled by coupled hydrodynamic and chemical processes interacting at different time and spatial scales their first characteristic is the production of silica rich outlet fluids in disequilibrium with the observed reacted mineral assemblages these compositions are interpreted as resulting from a suite of coupled dissolution precipitation reactions controlled at the pore scale by surface kinetic processes that rapidly reaches steady state constant fluid composition independent of fluid flux hence the effective rate of serpentinization was controlled to the first order by the transport of reactants during the experiment mass balance calculations show that the rate of olivine conversion was fast 0 2 0 5 wt day yet only 8 wt of the olivine sample reacted because the permeability drop limited fluid circulation porosity varied little during the experiment compared to permeability changes the decrease of permeability was explained by the structure of the newly formed serpentine favouring the clogging of fluid paths the rate at which permeability decreased was the fastest at low flux conditions this suggests that permeability changes were not controlled simply by the kinetics of the serpentinization reactions but mainly reflected the development of low peclet pore scale zones forming micro environments that control the serpentinization reaction paths these results outline the need to take into account the coupling of hydrodynamic and chemical processes when modelling the effects of reactive transport within the hydrating oceanic lithosphere

serpentinization and heat generation constraints from lost city and rainbow hydrothermal systems abstract the discovery of ultramafic hosted hydrothermal systems at rainbow 36 n mar and lost city a vent site approximately 15 km west of the mar at 30 n provides unique perspectives on chemical and heat generating processes associated with serpentinization at a range of chemical and physical conditions heat balance calculations together with constraints imposed by geochemical modeling indicate that significant changes in temperature are not likely to occur at either vent system as a result of the exothermic nature of olivine hydrolysis at rainbow the relatively high temperatures in subseafloor reaction zones in excess of 400 c which must be linked to magmatic processes inhibit olivine hydrolysis effectively precluding mineralization induced heating effects geochemical modeling of the lost city vent fluids indicates temperatures in excess of those measured 40 75 c the relatively high subseafloor temperatures 200 50 c requires conductive cooling of the fluids on ascent to the seafloor a scenario in keeping with the mineralization of chimney structures actually observed although the intermediate temperatures predicted for subseafloor reaction zones at lost city could be expected to enhance olivine to serpentine conversion dissolved cl k cl and na cl ratios of the lost city vent fluids are virtually unchanged from seawater values and indicate little hydration of olivine which is a necessary condition for exothermic heat generation by serpentinization apparently the fluid rock mass ratio is too high or fluid residence times too low for this to occur to any significant extent thus in spite of the off axis location of the lost city vents and apparent lack of a localized heat source mineralization reactions likely play an insignificant role in accounting for hydrothermal circulation it is more likely that tectonic processes associated with the slow spreading mar permit access of seawater to relatively deep and still hot lithospheric units and or near axis magmatic heat sources before venting additional chemical and physical temperature flow rate data for lost city and similar hydrothermal systems are needed to test key elements of the proposed model

biological mediation in ocean crust alteration how deep is the deep biosphere abstract biological mediation has been suggested as a control of the chemical exchange between the oceanic crust and seawater but very little is known about its distribution within the oceanic crust and the relative importance of biotic and abiotic processes alteration textures in glassy pillow lava margins record the proportions of biotic and abiotic alteration and the fraction of biotic alteration may be determined by point counting methods we used this method at dsdp odp sites 417d and 418a 110 ma crust south of bermuda rise and holes 504b and 896a 5 9 ma costa rica rift biotic alteration dominates glass alteration in the upper 250 m of the oceanic crust 60 85 of the total glass alteration and steadily declines in importance down to 10 20 at 500 m the consistency of data between two crustal sections of very different age and tectonic setting suggest that microbially mediated glass alteration may be largely confined to the upper oceanic crust however both sites studied are sealed by thick sedimentary layers and thus are typical for ocean crust underlying the oceanic basins rather than crust at mid ocean ridges with possibly deep and rapid hydrothermal circulation down hole temperature measurements at the costa rica rift suggest that glass altering microbes are hyperthermophilic and thrive at least up to temperatures of about 90 c microbial activity does occur at higher temperatures up to about 110 c but with reduced apparent abundance

sulfur mineralogy and geochemistry of serpentinites and gabbros of the atlantis massif iodp site u1309 abstract in situ uplifted portions of oceanic crust at the central dome of the atlantis massif mid atlantic ridge 30 n were drilled during expeditions 304 and 305 of the integrated ocean drilling program iodp and a 1 4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites olivine rich troctolites and diverse gabbroic rocks recovered from hole 1309d these data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the atlantis massif which hosts the lost city hydrothermal field 5 km to the south the oceanic crust at the central dome is characterized by ni rich sulfides reflecting reducing conditions and limited seawater circulation during uplift and exhumation seawater interaction in gabbroic dominated domains was limited as indicated by homogeneous mantle like sulfur contents and isotope compositions of gabbroic rocks and olivine rich troctolites local variations from mantle compositions are related to magmatic variability or to interaction with seawater derived fluids channeled along fault zones the concomitant occurrence of mackinawite in olivine rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150 200 c for late circulating fluids along local brittle faults below 700 m depth in contrast the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry the serpentinites in the upper part of the hole are characterized by high total sulfide contents high 34ssulfide values and low 34ssulfate values which reflect a multistage history primarily controlled by seawater gabbro interaction and subsequent serpentinization the basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the lost city hydrothermal system our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes temperature oxygen and sulfur fugacities and microbial activity

226ra 230th excess generated in the lower crust implications for magma transport and storage time scales comment and reply reply we welcome the comments by berlo et al hereafter btbh and thank them for the opportunity to further discuss lower crustal melting processes in arc settings our intention in dufek and cooper 2005 1 was not to explain the u series disequilibria in all arc magmas through a single model

the geological setting of the ultramafic hosted logatchev hydrothermal field 14 45 n mid atlantic ridge and its influence on massive sulfide formation the logatchev hydrothermal field 14 degrees 45 n on the mar is one of a few submarine hydrothermal systems associated with ultramafic rocks it is situated on the eastern inner flank of the rift valley wall 7 km away from the spreading axis and its formation has previously been linked to detachment faulting and core complex formation geological mapping during various rov dives geological sampling and shallow drilling reveal a structural control of hydrothermal activity as well as its location in a debris flow consisting of heterogeneous ultramafic and mafic intrusive rocks the mixed mafic ultramafic host rock lithology is in agreement with published vent fluid and gas chemical data showing characteristics for interaction with mafic as well as with ultramafic rocks massive sulfide formation is more focused than previously thought and likely limited to a thin veneer at the seafloor the logatchev hydrothermal field shows a number of peculiarities that are unusual for most other hydrothermal systems one of these are so called smoking craters seafloor depressions that are several meters wide characterized by an elevated crater rim made up partly of sulfide talus but also of abundant wall rock material at these smoking craters hydrothermal venting occurs directly from holes within the craters and from small cm to dm high cu rich chimneys occurring at the crater rim based on geological mapping and sampling we suggest that these smoking craters are the product of processes related to the regional and local geological setting in an ultramafic hosted off axis location with abundant landslides as well as off axis gabbroic intrusions providing the heat for the hydrothermal convection cell c 2009 elsevier b v all rights reserved

on formulating two dimensional vertically integrated hydrodynamic numerical models with an enhanced representation of bed stress a convolution method is developed for computing the bottom stress in a vertically integrated hydrodynamic numerical model by this means the bottom stress is no longer related to the depth mean current as in a conventional vertically integrated model and an enhanced representation of the bottom stress is possible an enhanced vertically integrated model consequently the enhanced model can reproduce phase lags and differences in direction between bed stress and depth mean current which cannot be taken into account in conventional vertically integrated models a linear or quadratic frictional formulation can be used with the method idealized calculations of wind induced flow in a rectangular basin show that the bed stress computed by the convolution method is in excellent agreement with that derived using a full three dimensional model however significant differences in the bed stress between the three dimensional and conventional two dimensional model are evident also in this paper a method is developed for extracting current profiles from vertically integrated models current profiles extracted from the enhanced two dimensional model for a range of frictional effects are found to be in good agreement with those computed using a full three dimensional model

thermodynamic constraints on mineral carbonation of serpentinized peridotite abstract carbonation of serpentinite appears to be a promising process to mitigate rising concentrations of atmospheric co2 however given that chemical affinities of mineral carbonation reactions in serpentinized peridotite change drastically with temperature protolith and the activities of dissolved constituents this process is incompletely understood in this report numerical models are used to investigate the heterogeneous phase equilibria and the speciation of intergranular fluids during mineral carbonation of serpentinite dunite at 100 200 300 and 400 c the results suggest that temperature dependent activity gradients of both aqueous carbon dioxide aco2 aq and aqueous silica asio2 aq determine the carbonation potential of serpentinite at low to moderate temperatures there is ample thermodynamic driving force for the carbonation of serpentinite as the gradients in asio2 aq and aco2 aq set between talc quartz magnesite and serpentine brucite equilibria are steepest with increasing temperature these activity gradients become smaller because the stability fields of brucite serpentine and talc expand toward higher aco2 aq and asio2 aq while the stability field of magnesite contracts in addition the enhanced solubility of quartz toward higher temperatures shifts the talc quartz magnesite equilibrium to very high aco2 aq field observations of carbonated serpentinite reveal a rather simple sequence of arrested carbonation reactions which is typically fossilized as spatially distinct metasomatic zones a zone of serpentinized peridotite is followed by a zone of serpentine carbonate then talc carbonate and finally quartz carbonate our modeling results not only reproduce these observations they also suggest that complete carbonation is favored at low to moderate temperatures since complete carbonation of serpentinite is achieved at relatively small fluid to rock ratios at temperatures 200 c prolonged fluid influx at low temperatures can result in decarbonation and silicification of the altered rock in contrast at temperatures 200 c decarbonation and silicification reactions are unlikely even at very high fluid to rock ratios since serpentinized peridotite is highly reactive in the presence of co2 aq its widespread exposure at slow and ultraslow spreading mid ocean ridges may represent a significant sink for co2 in the global carbon cycle our modeling results can be used to evaluate fluid rock interactions in active or fossil mineral carbonation systems if the mineralogy is known and the temperature of formation can be constrained independently the asio2 aq aco2aq oxygen fugacity fo2 g and ph can be estimated in turn if the alteration temperature asio2 aq and aco2 aq are known governing phase equilibria can be approximated the latter is a promising tool to monitor the reaction progress during co2 sequestration into serpentinized peridotite

crustal evolution of the mid atlantic ridge near the fifteen twenty fracture zone in the last 5 ma author posting american geophysical union 2003 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geochemistry geophysics geosystems 4 2003 1024 doi 10 1029 2002gc000364

sulfide mineralization in an ultramafic rock hosted seafloor hydrothermal system from serpentinization to the formation of cu zn co rich massive sulfides abstract the rainbow vent field is an ultramafic rock hosted seafloor hydrothermal system located on the mid atlantic ridge issuing high temperature acidic metal rich fluids hydrothermal products include cu zn co rich massive sulfides with characteristics comparable to those found in mafic volcanic hosted massive sulfide deposits petrography mineralogy and geochemistry of nonmineralized and mineralized rocks sampled in the rainbow vent field indicate that serpentinized peridotites host the hydrothermal vent system but serpentinization reactions occurred prior to and independently of the sulfide mineralization event the onset of sulfide mineralization is reflected by extensive textural and chemical transformations in the serpentine group minerals that show clear signs of hydrothermal corrosion element remobilization is a recurrent process in the rainbow vent field rocks and during simple peridotite serpentinization ni and cr present in olivine and pyroxene are incorporated in the pseudomorphic serpentine mesh and bastite respectively ni is later remobilized from pseudomorphic serpentine into the newly formed sulfides as a result of extensive hydrothermal alteration bulk rock geochemistry and correlation coefficients discriminate the different processes serpentinization sulfide mineralization and superficial seafloor low temperature processes related to the circulation of seawater e g carbonatization sulfide oxidation and b and u uptake

fourier representation of quadratic friction if a current is composed of a number of constituents with different frequencies then quadratic friction may be analyzed at the same frequencies the ratios of the constituents of the friction differ from the ratios for the current itself with a classic result being that for unidirectional flow a very weak current constituent experiences proportionately 50 more friction than a strong constituent here exact results for the magnitude of the friction constituents are derived and confirmed numerically the results are applied to the tidal currents in juan de fuca strait and the strait of georgia showing that minor constituents experience proportionately more friction than the main constituent by an amount that varies spatially but is typically less than the classic result of 50 for two dimensional currents it is shown that if there are two current constituents with the same ellipticity and major axis direction the friction coefficients are separable functions of the current constituent ratio and the ellipticity some results are derived for two constituents with different ellipticity and major axis direction for the case of two constituents with rectilinear but misaligned currents each constituent experiences friction inclined at an angle to its current last the effect of a tidal current on the bottom friction experienced by a steady flow is investigated for arbitrary relative magnitudes and directions of the tide and steady flow in particular the inclination of the mean friction to the mean flow is quantified

iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15 n on the mid atlantic ridge aqueous dihydrogen h2 aq is produced in copious amounts when seawater interacts with peridotite and h2o oxidizes ferrous iron in olivine to ferric iron in secondary magnetite and serpentine poorly understood in this process is the partitioning of iron and its oxidation state in serpentine although both impose an important control on dihydrogen production we present results of detailed petrographic mineral chemical magnetic and mosbauer analyses of partially to fully serpentinized peridotites from the ocean drilling program odp leg 209 mid atlantic ridge mar 15 n area these results are used to constrain the fate of iron during serpentinization and are compared with phase equilibria considerations and peridotite seawater reaction path models in samples from hole 1274a mesh rims reveal a distinct in to out zoning from brucite at the interface with primary olivine followed by a zone of serpentine brucite magnetite and finally serpentine magnetite in the outermost mesh rim the compositions of coexisting serpentine mg 95 and brucite mg 80 vary little throughout the core about 30 50 of the iron in serpentine brucite mesh rims is trivalent irrespective of subbasement depth and protolith harzburgite versus dunite model calculations suggest that both partitioning and oxidation state of iron are very sensitive to temperature and water to rock ratio during serpentinization at temperatures above 330 c the dissolution of olivine and coeval formation of serpentine magnetite and dihydrogen depends on the availability of an external silica source at these temperatures the extent of olivine serpentinization is insufficient to produce much hydrogen hence conditions are not reducing enough to form awaruite at t 330 c hydrogen generation is facilitated by the formation of brucite as dissolution of olivine to form serpentine magnetite and brucite requires no addition of silica the model calculations suggest that the iron distribution observed in serpentine and brucite is consistent with formation temperatures ranging from 150 to 250 c and bulk water to rock ratios between 0 1 and 5 these conditions coincide with peak hydrogen fugacities during serpentinization and are conducive to awaruite formation during main stage serpentinization the development of the common brucite rims around olivine is either due to an arrested reaction olivine brucite serpentine brucite or reflects metastable olivine brucite equilibria developing in the strong gradient in silica activity between orthopyroxene talc serpentine and olivine serpentine brucite

discovery of ancient and active hydrothermal systems along the ultra slow spreading southwest indian ridge 10 16 e 1 we report the discovery of active and fossil hydrothermal systems during r v knorr cruise 162 legs vii and ix along a 400 km long segment of the ultra slow southwest indian ridge swir between 10 and 16 e where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date 8 4 mm yr 1 full rate eight of forty one optical temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias four deposits ofsepiolite and silica as well as mn oxide and nontronite cemented breccias the massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep seated normal fault bounding the southern rift valley wall silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low to moderate temperature ultramafic hosted hydrothermal activity the abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates and hence the magmatic heat input along this section of the swir are lower than on any other explored segment of the global mid ocean ridge system this observation suggests that high mantle upwelling and magma supply rates are not required to drive mid ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra slow end of the ridge spreading spectrum the frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation with hydrothermal vent sites being preferentially associated with long lived faults that provide fluid pathways our results suggest the contribution of magma starved ridge segments to the global ocean lithosphere geochemical budget is potentially much larger than previously thought

constraints on deformation conditions and the origin of oceanic detachments the mid atlantic ridge core complex at 15 45 n 1 deformed rocks sampled from a corrugated detachment fault surface near the mid atlantic ridge 15 45 n constrain the conditions of deformation and strain localization samples recovered in situ record deformation restricted to the cold shallow lithosphere greenschist facies with no evidence for significant high temperature deformation either at the fault zone or in the footwall near it high temperature deformation 720 750 c is observed only at two sites and cannot be directly linked to the detachment detachment faulting was coeval with dyke intrusions that cross cut it as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones and by the presence of chill margins in diabase against fault rock basalts are very scarce and restricted to clasts in breccias with no evidence of pillows or extrusive structures gabbros crop out along mass wasted and fault scarps structurally below the detachment footwall rocks show little or no deformation due to strain localization along a narrow shear zone 200 m with fluid flow as required to form talc and amphibole schists after an ultramafic protolith we speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold shallow lithosphere is applicable to other corrugated surfaces along slow and intermediate spreading ridges these observations preclude detachment models rooting in melt rich zones i e atlantis bank southwest indian ridge or recording high temperature deformation we infer that oceanic detachment faults 1 localize strain at t 500 300 c 2 persist during active magmatism and 3 root at shallow rheological boundaries such as a melt rich zone or magma chamber hot detachments or an alteration front cold detachments

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

sulfur in peridotites and gabbros at lost city 30 n mar implications for hydrothermal alteration and microbial activity during serpentinization variations in sulfur mineralogy and chemistry of serpentinized peridotites and gabbros beneath the lost city hydrothermal field at the southern face of the atlantis massif mid atlantic ridge 30n were examined to better understand serpentinization and alteration processes and to study fluid fluxes redox conditions and the influence of microbial activity in this active peridotite hosted hydrothermal system the serpentinized peridotites are characterized by low total sulfur contents and high bulk d 34 s values close to seawater composition low concentrations of 34 s enriched sulfide phases and the predominance of sulfate with seawater like d 34 s values indicate oxidation loss of sulfide minerals and incorporation of seawater sulfate into the serpentinites the predominance of pyrite in both serpentinites and gabbros indicates relatively high fo2 conditions during progressive serpentinization and alteration which likely result from high fluid fluxes during hydrothermal circulation and evolution of the lost city system from temperatures of 250 to 150 c sulfate and sulfide minerals in samples from near the base of hydrothermal carbonate towers at lost city show d 34 s values that reflect the influence of microbial activity our study highlights the variations in sulfur chemistry of serpentinized peridotites in different marine environments and the influence of long lived moderate temperature peridotite hosted hydrothermal system and high seawater fluxes on the global sulfur cycle

fault rotation and core complex formation significant processes in seafloor formation at slow spreading mid ocean ridges mid atlantic ridge 13 15 n 1 the region of the mid atlantic ridge mar between the fifteen twenty and marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension normal faults show large amounts of rotation dome shaped corrugated detachment surfaces core complexes intersect the seafloor at the edge of the inner valley floor and extinct core complexes cover the seafloor off axis we have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments 13 and 15 n segments steep outward facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30 the rotation occurs very close to the ridge axis as much as 20 within 5 km of the volcanic axis and is complete by 1 my producing distinctive linear ridges with roughly symmetrical slopes this morphology is very different from linear abyssal hill faults formed at the 14 n magmatic segment which display a smaller amount of rotation typically 15 we suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults if faults are long lived a dome shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex a single ridge segment can have several active core complexes some less than 25 km apart that are separated by swales we present two models for multiple core complex formation a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales either model can explain the observed morphology

tidal spectroscopy of the english channel with a numerical model abstract the possibility of reproducing the complexity of tides in shallow water areas with a classical finite difference numerical model is examined this hydrodynamic model is two dimensional but incorporates topography nonlinear advection and quadratic bottom friction particular care is taken to prescribe sea surface elevations at the open boundaries a one month simulation of real tides is run with a simplified spectrum restricted to only 24 constituents corresponding to the nine main astronomical tides and their nonlinear significant interactions the results are analyzed by spectral decomposition elevations and vertically integrated currents and compared with observational data from the tide gages and current meters and with other solutions produced in the literature it is found that the dominant m2 constituent greatly influences the damping of the other constituents so that it is necessary to run them together for any correct simulation however the quadratic friction law introduced in

the effect of temperature on metal mobility in subseafloor hydrothermal systems constraints from basalt alteration experiments abstract crystalline basalt was reacted with a na ca k cl fluid of seawater chlorinity at 400 350 and 300 c 400 bar and a fluid rock mass ratio of 1 to assess the effect of temperature on metal mobility in ridge crest hydrothermal systems basalt alteration at 400 c yielded a fluid chemistry characterized by high zn cu fe mn sio 2 h 2 s and h 2 concentrations lowering temperature to 350 and then to 300 c caused dissolved metal concentrations to decrease drastically black smoker vent fluids generally have measured temperatures 355 c the dissolved concentrations of zn cu fe and mn observed in these fluids however could only be achieved or exceeded during the 400 c phase of the experiment indicating that hot spring fluids have lost heat and possibly dissolved fe and base metals on ascent to the seafloor from a high temperature reaction zone the relative rates at which zn cu fe and mn achieve new steady state concentrations in response to temperature reduction during basalt alteration are cu fe zn mn thus dissolved zn and especially mn concentrations are more likely to record accurately temperatures characterizing subseafloor reaction zones while fe and cu which respond rapidly to changes in temperature are likely to precipitate along fluid flow paths especially in near seafloor locations this trend is confirmed by the relative abundance of dissolved metals in hot spring fluids variably influenced by heat loss processes calculations suggest that hot spring fluids may lose heat by conduction through sulfide mounds and chimney walls at flow rates observed for many vent systems since heat loss processes influence the chemistry especially dissolved metals as well as measured temperatures of axial hot springs fluids these effects must be taken into account to model accurately phase relations and fluid mineral equilibria in deeply seated subseafloor reaction zones

experimental simulation of clay precipitation within reservoir sandstones 1 techniques and examples experiments have been performed in which cores of clay free sand and clay forming mineral phases placed beneath the cores have been treated hydrothermally with various solutions using experimental conditions in the range 150 500 degrees c and 0 75 1 0 kbar 75 100 mpa illite illite smectite kaolinite 7 a chlorite and trioctahedral smectite have been precipitated in the sand pore space with morphologies similar to those of authigenic clay minerals in sandstone petroleum reservoirs this study demonstrates that the mechanism rate of clay precipitation and controls on clay morphology and mineral stability can all be determined by experimental studies these results have direct application to the interpretation of clay mineral paragenesis in reservoir sandstones the study demonstrates 1 authigenic clays in sandstones are direct precipitates from solution and do not require a preexisting clay coating 2 grain surface effects such as abrasion and detrital clay coatings do promote nucleation of clays and 3 clay morphology is controlled by precipitation mechanism specifically the formation of metastable clays which are subsequently modified the synthesis of authigenic type illite and illite smectite is by direct precipitation from solution following dissolution of mineral phases such as detrital illite or feldspar both platy and fibrous illite morphotypes are synthesized platy illites are more readily formed the compositions of synthetic illite and illite smectite clays are controlled by temperature and a sub k super a sub h super fluid composition ratio platy illites show subtle changes in morphology with compositional variation illite smectite and illite with low k content forms a crenulated boxwork texture while more k rich illites approaching muscovite composition form larger discrete plates no correlation between illite composition and fibrous formation is recognized kaolinite and chlorites were also precipitated in experiments vermiform kaolinite appears to be a direct precipitate kaolinite morphology however is controlled by precipitation mechanism isolated discrete kaolinite plates are associated with early formation of a mixed layered smectitic phase chlorite synthesis proceeds via the formation of metastable 7 a chlorite and expandable phases which have previously been recognized as possible precursors to 14 a chlorites in reservoir sandstones

a three dimensional model of the m4 tide in the irish sea the importance of open boundary conditions and influence of wind a fully nonlinear three dimensional hydrodynamic model of the irish sea is used to examine the processes determining the spatial variability of the m4 tide in the irish sea the model is based upon a mixed spectral finite difference approach with an expansion of functions in the vertical yielding a continuous current profile and a finite difference grid in the horizontal the model has a sufficiently fine grid to enable the nonlinear advective terms to be resolved in their finite difference form and the spatial variability of the m4 tide to be represented computed m4 elevations and currents determined using a radiation open boundary condition with both m2 mid m4 elevation and current input and a flow dependent viscosity are found to be in satisfactory agreement with observations considering the limited grid resolution of the model and the level of noise in the m4 tide as a percentage of the signal is much higher than for m2 this agreement is good enough to establish the credibility of using the model to examine the processes influencing the m4 tide which are discussed in detail the importance of an m4 current input to the radiation condition and the difficulty of using an elevation specified open boundary condition to reproduce the m4 tide in this region are considered in some detail the nonlinear advective terms are shown to have a limited local influence namely around headlands in generating the m4 tide the influence of wind effects upon m4 tidal currents particularly through the nonlinear term associated with a time dependent eddy viscosity is examined and shown to produce a significant increase in the m4 near surface tidal current and a modification of the current profile in essence the change of the m4 tidal current by meteorological effects will give rise to noise in any observed m4 current due to the random nature of the wind fields

tide induced residual current in a coastal lagoon of the gulf of california abstract the non linear tidal interactions in the narrow shallow coastal lagoon yavaros bay are investigated using a vertically integrated two dimensional numerical model the model sea surface elevations and currents are validated by comparison of computed tidal harmonics against those derived from harmonic analysis of the sea surface and current time series calculations show that tides become mixed mainly semidiurnal in the lagoon mk3 ms4 and m4 generated by advection and non linear continuity are the most energetic shallow water constituents a large half basin clockwise eddy was found whose source of energy is the m2 tidal energy flux the size of the shoal where the eddy occurs is almost the same as the maximum tidal excursion length 3 km

the extent of u series disequilibria produced during partial melting of the lower crust with implications for the formation of the mount st helens dacites the extent to which u series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for u th and ra we show that partial melting of mafic lower crust can only produce small disequilibria between 238u 230th and 226ra crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect consequently u series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent unless the timescales of silicic magma production within the crust are short compared to the half life of 226ra our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust we show that the 226ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and or the involvement of mantle derived basalts less than a few thousand years old application to mount st helens suggests that crystallisation of young mantle derived magma is likely to be the dominant process in the formation of these dacites

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

fe ni co o s phase relations in peridotite seawater interactions serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen h2 aq release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by h2o we have compiled and evaluated thermodynamic data for fe ni co o s phases and computed phase relations in fo2 g fs2 g and ah2 aq ah2s aq diagrams for temperatures between 150 and 4008c at 50 mpa we use the relations and compositions of fe ni co o s phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the mid atlantic ridge in the 15820 0 n fracture zone area ocean drilling program leg 209 petrographic observations suggest a systematic change from awaruite magnetite pentlandite and heazlewoodite magnetite pentlandite assemblages forming in the early stages of serpentinization to millerite pyrite polydymite dominated assemblages in steatized rocks awaruite is observed in all brucite bearing partly serpentinized rocks apparently buffering of silica activities to low values by the presence of brucite facilitates the formation of large amounts of hydrogen which leads to the formation of awaruite associated with the prominent desulfurization of pentlandite sulfide is removed from the rock during the initial stage of serpentinization in contrast steatitization indicates increased silica activities and that highsulfur fugacity sulfides such as polydymite and pyrite vaesite solid solution form as the reducing capacity of the peridotite is exhausted and h2 activities drop under these conditions sulfides will not desulfurize but precipitate and the sulfur content of the rock increases the co evolution of fo2 g fs2 g in the system follows an isopotential of h2s aq indicating that h2s in vent fluids is buffered in contrast h2 in vent fluids is not buffered by fe ni co o s phases which merely monitor the evolution of h2 activities in the fluids in the course of progressive rock alteration the co occurrence of pentlandite awaruite magnetite indicates h2 aq activities in the interacting fluids near the stability limit of water the presence of a hydrogen gas phase would add to the catalyzing capacity of awaruite and would facilitate the abiotic formation of organic compounds

mineralization alteration and hydrothermal metamorphism of the ophiolite hosted turner albright sulfide deposit southwestern oregon the turner albright sulfide deposit part of the josephine ophiolite formed on and below the seafloor during late jurassic volcanism at a back arc spreading center ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition the uppermost massive sulfide formed on the seafloor at hydrothermal vents similar to sulfide depositing hot springs on modern spreading centers the bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization infiltration of hydrothermal fluid into the hyalo clastite altered the rock to quartz fe chlorite and quartz sericite fe chlorite intense alteration resulted in replacement of the protolith by quartz sulfide the fluid responsible for the hydrothermal alteration was evolved seawater with low ph and mg and high fe sulfide 534s values average 4 7 and indicate contribution of sulfur from both basalt and seawater sulfate the average value of sulfide 34s and the difference between sulfide and contemporaneous seawater sulfate 34s values are similar to ophiolite hosted sulfide deposits in cyprus chlorite rich hydrothermally altered basalt has 18o values of 3 0 6 8 due to high temperature 250 350 c reaction with a seawater derived ore fluid quartz in silicified basalt has higher 18o values of 12 3 15 5 probably reflecting lower temperature recrystallization of amorphous silica formed during initial alteration of basalt mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore transporting hydrothermal fluid but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust this subseafloor metamorphism produced a mineral assemblage typical of prehnite pumpellyite facies metamorphism exchange with altered seawater increased the whole rock 18o of the basalts to values of 9 4 11 2

the genesis of silicic arc magmas in shallow crustal cold zones abstract a number of currently popular models for the genesis of evolved arc magmas from basaltic andesite to dacite invoke repeated intrusion partial melting and differentiation at the base of the crust however several observations suggest that this may be the exception rather than the norm 1 geobarometry often indicates shallow pressure 0 1 0 3 gpa evolution 2 incongruent melting of amphibolite at elevated pressures should yield magmas in equilibrium with high pressure phases like garnet but rare earth element patterns almost ubiquiously preclude this 3 compositionally zoned caldera forming eruptions suggest differentiation at near surface depths 4 u series data most commonly indicate differentiation over millennia time scales this requires rapid cooling that in turn is most easily explained by relatively small magma volumes undergoing crystal fractionation within the shallow i e cool crust to further test these ideas we combined published experimental data for liquidus equilibria with appropriate silicic arc magma compositions on projections of the ternary liquidus system nepheline silica olivine recent data for tongan silicic lavas plot either on or close to low pressure 1 atm cotectics for the rocks phenocryst phases suggesting low pressure differentiation using our own and published data from arc volcanoes around the world we find that the majority are consistent with differentiation at shallow depths regardless of total crustal thickness combined with the typical timescales of differentiation we estimate that the volumes of magma stored during differentiation in shallow crustal zones are usually on the order of only a few km 3 there is also a clear role for mixing and recharge that involves magmas that are more deeply sourced and primitive in character typically evolved basalts and basaltic andesites whether the latter differentiated in the lower crust or at the crust mantle boundary has important implications for the constitution and average composition of arc crust at present we can only conclude that evidence for more silicic arc magma generation at these depths is generally lacking

observations of cross channel structure of flow in an energetic tidal channel 1 measurements of velocity and density profiles were made to describe the transverse structure of flow in chacao channel southern chile 41 75 s where typical tidal velocities are 4 m s current profiles were obtained with a 307 2 khz acoustic doppler current profiler adcp over 25 repetitions of a cross channel transect during one semidiurnal tidal cycle the 2 2 km long transect ran northeast southwest across the channel a northern channel 120 m deep and a southern channel 85 m deep were separated by remolinos rock a pinnacle that rises to 20 m depth at 0 7 km from the southern side density measurements to depths of 50 m were obtained with a conductivity temperature and depth ctd recorder at the north and south ends of each transect repetition one ctd profile was also taken in the middle of the northern channel the mean flow exhibited weak vertical structure because of strong vertical mixing the predominant lateral structure consisted of mean outflow toward the ocean in the channels and mean inflow toward gulf of ancud over the pinnacle and the sides of the channel this lateral structure pattern was consistent with the mean flow pattern expected from tidal rectification as robust overtides were generated throughout the transect the contributions to flow divergence and vorticity by the lateral variations of the lateral flow v y and by the lateral shears of the along channel flow u y respectively were both of the order of 10 3 s 1 this caused advective and frictional forces both horizontal and vertical to be dominant in the across channel momentum balance as they were more than twenty times the coriolis acceleration the present work then represents one of the few examples reported where lateral friction proportional to 2v y2 appears relevant to the transverse momentum balance

garnets within geode like serpentinite veins implications for element transport hydrogen production and life supporting environment formation geochemical micro environments within serpentinizing systems can abiotically synthesize hydrocarbons and provide the ingredients required to support life observations of organic matter in microgeode like hydrogarnets found in mid atlantic ridge serpentinites suggest these garnets possibly represent unique nests for the colonization of microbial ecosystems within the oceanic lithosphere however little is known about the mineralogical and geochemical processes that allow such unique environments to form here we present work on outcrop scale vein networks from an ultramafic massif in norway that contain massive amounts of spherulitic garnets andradite which help to constrain such processes vein andradite spherulites are associated with polyhedral serpentine brucite ni fe alloy awaruite and magnetite indicative of low temperature 200 c alteration under low fo2 and low asio2 aq geochemical conditions together with the outcrop and micro scale analysis geochemical reaction path modeling shows that there was limited mass transport and fluid flow over a large scale once opened the veins remained isolated closed system forming non equilibrium microenvironments that allowed upon a threshold supersaturation the rapid crystallization seconds to weeks of spherulitic andradite the presence of polyhedral serpentine spheres indicates that veins were initially filled with a gel like protoserpentine phase in addition massive fe oxidation associated with andradite formation could have generated as much as 600 mmol h2 aq per 100 cm3 vein although no carboneous matter was detected the vein networks fulfill the reported geochemical criteria required to generate abiogenic hydrocarbons and support microbial communities thus systems similar to those investigated here are of prime interest when searching for life supporting environments within the deep subsurface

metamorphism of deep seated rocks from the garrett ultrafast transform east pacific rise near 13 25 s in spite of 10 years of extensive exploration of the east pacific rise the petrology of the fast slipping transforms remains poorly documented in 1980 metagabbros and serpentinites were dredged from the deepest trough 4616 4820 m in depth of the garrett ultrafast transform which intersects the east pacific rise near 13 25 s 16 cm yr total spreading rate previous works on their protolith concluded that the samples consist of serpentinized refractory upper mantle and lower crust gabbroic cumulates in addition several ultramafic rocks showing evidences of magmatic impregnation were believed to represent mantle crust transition zone as observed in ophiolite complexes the present work focuses on the postigneous evolution of the rocks affected by hydrothermal circulation an early stage of amphibolitization occurred at about 700 400 c and probably down to 300 c in the most evolved gabbros ti fe gabbros the amphibole compositional variations suggest evolution of the metasomatic solution and temperature through time from the amphibolite facies down to the lower greenshist facies high chlorine ferruginous hornblendes 3 5 wt cl found in veins of the ferrogabbros suggests that pore fluids in this type of material evolved in situ to a ferruginous brine the decreasing temperature 300 200 c and further fracturing favored more or less pervasive serpentinization of the olivine rich samples ultramafics and olivine gabbros the release of ca in the solution contributed to the alteration of the plagioclase bearing rocks troctolites and gabbros which were invaded by calc silicates hydrogrossular prehnite pumpellyite and epidote a process which is referred to as rodingitization the occurrence of monomineralic veins of al pumpellyite in some metagabbros suggests very low o2 conditions in the lower ocean crust ph2o 2 kbar low o2 also inferred from the composition of the other calc silicates and indicates low permeability and limited fluid circulation during retrograde metamorphism further uplift and emplacement of the rocks on the seafloor of the transform domain were accompanied by low temperature crystallization of zeolite and aragonite high mg and co2 in monomineralic veins and clay minerals in veins and in external alteration rims the garrett s samples are by many respects comparable to typical ocean floor metabasites from various settings the rocks recorded igneous and metamorphic transformations similar to those expected beneath spreading ridges unaffected by transform tectonics the metamorphism is a consequence of the dynamics of spreading propagation of the cracks and penetration of seawater deeper in the cooling crust however the extent of fissuring and the subsequent hydration probably increase drastically with proximity to the garrett transform most of the observed metasomatic reactions have a limited extent and occurred at depth without significant deformation of the rocks rapid uplift of the rocks initiated by vertical components of the transform tectonics was probably induced by serpentinization of the ultramafics buoyancy of serpentinite and intense mass wasting at the surface of seafloor however emplacement by diapirism is not completely ruled out if the dredged samples are relatively undeformed poorly altered rocks carried up by protusions of ductile serpentine which is now too fragile to be sampled by dredging

effect of subseabed salt domes on tidal residual currents in the persian gulf geological studies in the persian gulf pg have revealed the existence of subseabed salt domes with suitable filtering of a high resolution pg seabed topography it is seen that the domes leave their signature in the seabed i e numerous hills and valleys with amplitudes of several tens of meters and radii from a few up to tens of kilometers it was suspected that the shark skin of the pg seabed may affect the tidal residual flow the interaction of tidal dynamics and these obstacles was investigated in a nonlinear hydrodynamic numerical tidal model of the pg the model was first used to characterize flow patterns of residual currents generated by a tidal wave passing over symmetric elongated and tilted obstacles thereafter it was applied to the entire pg the model was forced at its open boundary by the four dominant tidal constituents residing in the pg each tidal constituent was simulated separately results i e tidal residual currents in the pg as depicted by lagrangian trajectories reveal a stationary flow that is very rich in eddies each eddy can be identified with a topographic obstacle this confirms that the tidal residual flow field is strongly influenced by the nonlinear interaction of the tidal wave with the bottom relief which in turn is deformed by salt domes beneath the seabed different areas of maximum residual current velocities are identified for major tidal constituents the pattern of trajectories indicates the presence of two main cyclonic gyres and several adjacent gyres rotating in opposite directions and a strong coastal current in the northern pg

a three dimensional model of the principal tides on the european shelf abstract a three dimensional hydrodynamic model of the continental shelf is used to examine the spatial distribution both elevations and currents of 28 of the major tides in the region tidal constituents in the diurnal and semi diurnal band together with higher harmonics and long period tides are used to illustrate the magnitude and spatial variability of the tides over the shelf comparisons are made between model computed profiles and harmonic analyses of 278 current time series at a range of locations in varying water depths with particular emphasis on locations where there are current measurements at a number of points in the vertical observed and computed tidal elevation amplitudes and phases are also compared at up to 257 shore based and off shore tide gauges these detailed comparisons are used to determine any bias in the model and the accuracy of predicted elevations and currents derived with the model results of these comparisons show that although the diurnal and semi diurnal tidal elevations and currents can be accurately reproduced using a 12 km grid the error in the higher harmonics is significantly larger the fact that the model contains all the non linear terms advection quadratic friction wave drift time varying viscosity necessary to generate these harmonics together with a full set of tidal harmonics in order to have the correct level of friction suggests that potentially it should reproduce these constituents however a rigorous comparison was found to be very difficult because of the rapid variation of the higher harmonics in shallow water which cannot be adequately resolved in the model and possible inaccuracy in the measurements of the small currents associated with the higher harmonics

unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 1 the results of detailed textural mineral chemical and petrophysical studies shed new light on the poorly constrained fluid rock reaction pathways during retrograde serpentinization at mid ocean ridges uniformly depleted harzburgites and dunites from the mid atlantic ridge at 15 n show variable extents of static serpentinization they reveal a simple sequence of reactions serpentinization of olivine and development of a typical mesh texture with serpentine brucite mesh rims followed by replacement of olivine mesh centers by serpentine and brucite the serpentine mesh rims on relic olivine are devoid of magnetite conversely domains in the rock that are completely serpentinized show abundant magnetite we propose that low fluid flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open system conditions and formation of magnetite by the breakdown of ferroan brucite modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks citation bach w h paulick c j garrido b ildefonse w p meurer and s e humphris 2006 unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 geophys res lett 33 l13306

hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the mid atlantic ridge 15 20 n odp leg 209 a sulfur and oxygen isotope study whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15 20 n fracture zone on the mid atlantic ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement three processes are identified 1 high temperature hydrothermal alteration 250 350 c at sites 1268 and 1271 is characterized by 18o depletion 2 6 4 4 elevated sulfide s and high 34s up to 2 wt and 4 4 10 8 fluids were derived from high temperature 350 c reaction of seawater with gabbro at depth these cores contain gabbroic rocks suggesting that associated heat may influence serpentinization 2 low temperature 150 c serpentinization at sites 1272 and 1274 is characterized by elevated 18o up to 8 1 high sulfide s up to 3000 ppm and negative 34s to 32 1 that reflect microbial reduction of seawater sulfate these holes penetrate faults at depth suggesting links between faulting and temperatures of serpentinization 3 late low temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor sulfate at all sites contains a component of oxidized sulfide minerals low 34s of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low 34s sulfide derived from microbial sulfate reduction results show that peridotite alteration may be commonly affected by fluids heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor

some results of heterogeneous data inversions for oceanic tides many different instruments directly or indirectly observe the oceanic tidal movements among them we consider here the coastal tide gauge and deep sea pressure recorder which provide at least the principal spectral constituents of the tidal height the gravimeter which indirectly observes the oceanic tides through the gravitational effect generated by the deformation of the solid earth under the ocean tidal mass load and the satellite altimeter which samples the tides in the time domain and over the nearly whole ocean our present paper deals with an empirical method for the recovery of the oceanic tides by inversion of heterogeneous data sets the aim is to take advantage of the different kinds of information coming from the measurement instruments in order to complete more accurate empirical solutions first tide gauge and gravity loading data are separately and jointly inverted to provide global charts of the major semidiurnal m2 wave the results prove that the tide gauge data have a major contribution in the tidal mapping but also suggest that the gravity loading data distort the information this is certainly due to unmodelled geophysical phenomena and instrumental errors second tide gauge and satellite altimeter data are considered in a separate and a joint inversion computation to provide tidal estimations along a predefined geographical line in the north atlantic the results are accurate enough to make provision for the boundary conditions of an hydrodynamical tidal model as an intrinsic advantage of the inversion method a posteriori standard deviations and error covariances for all the solutions displayed are provided and analyzed

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

review of recent developments in tidal hydrodynamic modeling i spectral models in this paper we present a brief overview with a comprehensive set of references to recently published papers for detail of a range of techniques used in tidal hydrodynamic modeling and recent progress in the field although the paper aims to introduce the topic to someone new to the area the main emphasis is on new developments in near coastal shallow sea tidal modeling and consequently we concentrate on three dimensional modeling which has progressed significantly in recent years the paper gives a brief account of methods used to discretize the hydrodynamic equations in the horizontal namely finite difference finite element and boundary fitted coordinates and through time the representation of tidal current structure in a three dimensional model using a spectral method in the vertical is also described in this method the subgrid scale vertical diffusion of momentum which has a major influence on tidal current profiles is parameterized using a flow dependent eddy viscosity recent work on

reactions between basalt and co2 rich seawater at 250 and 350 c 500bars implications for the co2 sequestration into the modern oceanic crust and the composition of hydrothermal vent fluid in the co2 rich early ocean abstract this study aims to understand how basaltic rocks absorb co 2 in high temperature alteration zones in the subseafloor and to reconstruct hydrothermal alteration processes such as carbonatization of archean greenstones to this end we conducted two laboratory experiments simulating hydrothermal reactions between basalt synthesized under quartz fayalite magnetite oxygen fugacity and co 2 rich nacl fluid ph 6 5 at 25 c at high temperature and pressure as the water rock reactions progressed at 250 c and 350 c 500 bars total carbonic acid concentration co 2 reduced from its initial 400 mmol kg to near 0 and 100 mmol kg respectively meanwhile calcite was formed in the basalt as an alteration mineral this indicates that calcite destabilizes as temperature increases in the h 2 o co 2 basalt system and that crustal basalts can absorb almost all co 2 in the fluid as calcite at least at temperatures and initial co 2 concentrations below 250 c and 400 mmol kg respectively although the second aim was realized in the experiments minerals such as sericite dolomite ankerite and siderite present in archean greenstones were not identified in the alteration products possibly because k mg and fe were lacking in the initial solutions steady state concentrations of sio 2 mg and k in the fluids during water rock reactions were similar to those of high temperature fluids 250 c in modern basalt hosted hydrothermal systems however the final experimental ph in situ was 6 6 and 7 2 at 250 c and 350 c respectively higher than that in modern hydrothermal fluids approximately 5 and higher than the neutral ph 5 5 5 6 at 250 350 c 500 bars the results suggest that the presence of abundant co 2 in the initial fluid induced carbonatization of basalt consequently ph was buffered by precipitation and dissolution of calcite because ph in situ was elevated the dissolved fe and mn concentrations in the fluid were two to three orders of magnitude lower than those of modern hydrothermal fluids in modern oceans high temperature hydrothermal vent fluids are the second largest iron source after riverine input however because alkaline metal poor hydrothermal fluids are generated in co 2 rich systems co 2 rich seafloor hydrothermal systems may have behaved as iron sinks in early oceans

experimental constraints on fluid rock reactions during incipient serpentinization of harzburgite the exposure of mantle peridotite to water at crustal levels leads to a cascade of interconnected dissolution precipitation and reduction oxidation reactions a process referred to as serpentinization these reactions have major implications for microbial life through the provision of hydrogen h 2 to simulate incipient serpentinization under well constrained conditions we reacted centimeter sized pieces of uncrushed harzburgite with chemically modified seawater at 300 oc and 35 mpa for ca 1 5 yr 13 441 h monitored changes in fluid chemistry over time and examined the secondary mineralogy at the termination of the experiment approximately 4 mol of the protolith underwent alteration forming serpentine accessory magnetite chlorite and traces of calcite and heazlewoodite alteration textures bear remarkable similarities to those found in partially serpentinized abyssal peridotites neither brucite nor talc precipitated during the experiment given that the starting material contained 4 times more olivine than orthopyroxene on a molar basis mass balance requires that dissolution of orthopyroxene was significantly faster than dissolution of olivine coupled mass transfer of dissolved si mg and h between olivine and orthopyroxene reaction fronts was driven by steep activity gradients and facilitated the precipitation of serpentine hydrogen was released in significant amounts throughout the entire experiment however the h 2 release rate decreased with time serpentinization consumed water but did not release significant amounts of dissolved species other than h 2 suggesting that incipient hydration reactions involved a volume increase of 40 the reduced access of water to fresh olivine surfaces due to filling of fractures and coating of primary minerals with alteration products led to decreased rates of serpentinization and h 2 release while this concept might seem at odds with completely serpentinized seafloor peridotites reaction driven fracturing offers an intriguing solution to the seemingly self limiting nature of serpentinization indeed the reacted sample revealed several textural features diagnostic of incipient reaction driven fracturing we conclude that fracturing must have far reaching impacts on the rates of serpentinization and h 2 release in peridotite hosted hydrothermal systems

shallow water tides in the northwest european shelf region from topex poseidon altimetry during recent years the accurate measurements from the topex poseidon satellite have improved global ocean tide models considerably however these global ocean tide models are still not accurate enough on the continental shelves for detailed oceanographic studies the need for increasing accuracy in shelf regions calls for inclusion of more than just semidiurnal and diurnal constituents in future global ocean tide models this is because a considerable part of the tidal variability on the shelves is caused by shallow water constituents one example is the m4 constituent which exceeds 50 cm at several locations on the northwest european shelf so far topex poseidon has not been considered for resolving these constituents mainly owing to its coarse ground track resolution reliable empirical estimates of several major shallow water constituents can be obtained from topex poseidon by combining along track and crossover observations estimates of the major shallow water constituents for the northwest european shelf m4 m6 and ms4 are presented in this study and compared with tide gauges and existing hydrodynamic models

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

sulphide mineral evolution and metal mobility during alteration of the oceanic crust insights from odp hole 1256d fluxes of metals during the hydrothermal alteration of the oceanic crust have far reaching effects including buffering of the compositions of the ocean and lithosphere supporting microbial life an

a comparison of the nonlinear frictional characteristics of two dimensional and three dimensional models of a shallow tidal embayment the nonlinear frictional behavior of two dimensional 2 d and three dimensional 3 d models are compared in this study of tides in the bight of abaco the shallow depths and the existence of an extensive set of tidal elevation data five astronomical and two overtide constituents at 25 stations from filloux and snyder 1979 offer an excellent opportunity to compare the effects of different frictional formulations in addition previous modeling efforts in the bight have consistently overpredicted the m6 and generally overdamped the o1 k1 and s2 tides the results indicate that although the 2 d and 3 d models may be calibrated to produce nearly identical responses for the dominant m2 tide there are systematic differences in the responses of the primary overrides these differences are explained using analytical expansions of the friction terms and are shown to be due to differences in the terms that are nonlinear in velocity and in water level the investigation concludes that the overgeneration of m6 and the overdamping of secondary astronomical tides will occur in 3 d models as well as 2 d models although several causes for these problems were considered improvement in these constituents could be achieved only by modifying the standard quadratic friction or flow dependent eddy viscosity relations to reduce the nonlinear frictional effect relative to the linear frictional effect the required modifications suggest the presence of a constant background velocity residual turbulence field or possibly the need for a more advanced frictional closure

a chlorine isotope study of dsdp odp serpentinized ultramafic rocks insights into the serpentinization process abstract eight dsdp odp cores were analyzed for major ion concentrations and 37cl values of water soluble chloride 37clwsc and structurally bound chloride 37clsbc in serpentinized ultramafic rocks this diverse set of cores spans a wide range in age temperature of serpentinization tectonic setting and geographic location of drilled serpentinized oceanic crust three of the cores were sampled at closely spaced intervals to investigate downhole variation in cl concentration and chlorine isotope composition the average total cl content of all 86 samples is 0 26 0 16 wt 0 19 0 10 wt as water soluble cl xwsc and 0 09 0 09 wt as structurally bound cl xsbc structurally bound cl concentration nearly doubles with depth in all cores there is no consistent trend in water soluble cl content among the cores chlorine isotope fractionation between the structurally bound cl site and the water soluble cl site varies from 1 08 to 1 16 averaging to 0 21 samples with negative fractionations may be related to reequilibration of the water soluble chloride with seawater post serpentinite formation six of the cores have positive bulk 37cl values 0 05 to 0 36 the other two cores 173 1068a leg hole and 84 570 have negative bulk 37cl values 1 26 and 0 54 the cores with negative 37cl values also have variable cl so42 ratios in contrast to all other cores the isotopically positive cores 153 920d and 147 895e show no isotopic variation with depth the isotopically negative core 173 1068a decreases by 1 with depth for both the water soluble and structurally bound cl fractions non zero bulk 37cl values indicate cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling cores with positive 37cl values are most likely explained by open system fractionation during hydrothermal alteration with preferential incorporation of 37cl from seawater into the serpentinite and loss of residual light cl back to the ocean fluid rock ratios were probably low as evidenced by the presence of water soluble salts the two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization we believe the low 37cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments the resulting serpentinites inherit the characteristic negative 37cl values of the pore waters chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization