seismicity and attenuation in the central vanuatu new hebrides islands a new interpretation of the effect of subduction of the d entrecasteaux fracture zone the spatial distribution of the intermediate depth earthquakes in the central vanuatu new hebrides islands includes a remark able gap in seismic activity located between depths of 100 and 200 km and having a lateral extent of about 150 km no well located earth quakes determined from teleseismic d ata during a 16 year period nor any microearthquakes located by a local seismograph network during a 5 year period have occurred in the gap a zone of attenuation of high frequency shear waves over laps the seismicity gap no contortion of the benioff zone in the region of the gap indicative of major disruption of the subducted plate is apparent however the gap is close to the extrapolated location of the subducted part of the d entrecasteaux fracture zone dfz the dfz is a ridgelike bathymetric feature on the oceanic plate being subducted beneath the island a rc a pogitive thermal anomaly associated with the dfz may cause the gap of intermediate depth seismic ity and produce anomalous absorption of high frequency shear waves alternatively a zone of strong scattering associated with the subducted part of the dfz may also account for the anoma lous attenuation

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

the collision zone between the north d entrecasteaux ridge and the new hebrides island arc 2 structure from multichannel seismic data the d entrecasteaux zone dez collides with the central new hebrides island arc and consists of two subparallel ridges that strike east west stand 1 2 km above the surrounding oceanic plate and subduct obliquely 15 northward beneath the arc rocks dredged from the north ridge as well as reflections evident in multichannel seismic reflection data indicate that this ridge has a volcanic origin crystalline volcanic rocks are common along the lower flank of the ridge but sedimentary probably volcaniclastic rock caps the ridge seismic reflection data collected over the lower arc slope reveal that mass wasting deposits locally make up most of the accretionary wedge these deposits appear to form discrete bodies suggesting that mass wasting occurred episodically large anticlines and thrust faults having large vertical separation are not readily evident where the colliding ridge intersects the arc slope apparently slope rocks have low strength so that mass wasting deposits formed instead of large relief structures mass wasting is thought to occur as the accretionary wedge is uplifted in response to the northward oblique subduction of the north ridge the toe of the north ridge flank marks an abrupt transition in the lithologies that make up the footwall of the interplate decollement footwall lithologies change from ocean basin to volcaniclastic ridge material and this transition probably marks a discontinuity in friction along the decollement or in rock mechanical properties because north of the transition thrust faults deform the accretionary wedge whereas south of the transition steep reverse faults crosscut the wedge and pierce the north flank of the ridge this piercement means that the decollement at least locally lies within the ridge and that ridge material exotic to the new hebrides arc may be incorporated into the accretionary wedge

bromide chloride fractionation in seafloor hydrothermal fluids from 9 10 n east pacific rise hydrothermal fluids collected from 9 10 n epr during 5 cruises between 1991 and 1994 were analyzed for br and cl the chlorinity of these fluids ranges between 32 6 and 860 mmol kg in comparison to local seawater values of 540 mmol kg these very low chlorinity fluids some with temperatures high enough to be within the vapor stability field were collected following a volcanic eruption in 1991 1 although fluids with chlorinities less than 250 mmol kg had been sampled once previously at axial volcano 2 the vent fluid chlorinities at 9 10 n epr were much lower than at axial volcano fluids with cl 250 mmol kg hadbr cl ratios identical to seawater in agreement with previous studies in contrast fluids with cl 250 mmol kg collected from this area hadbr cl ratios as much as 40 less than the seawater value bromide depletions observed in the low chlorinity fluids from 9 10 n epr were probably the result of fractionation of br from cl during subcritical phase separation this fractionation process appeared to be limited to fluids with cl 250 mmol kg and to require open system phase separation to produce vapors with cl 250 mmol kg at pressure and temperature conditions appropriate for 9 10 n epr requires conditions that are close to the critical point where the properties of the vapor and liquid phases are quite similar resulting in lack of fractionation in thebr cl ratio between the two phases

br cl na systematics in illinois basin fluids constraints on fluid origin and evolution the authors present here bromide chloride and sodium data for fluids from reservoirs of ordovician through pennsylvania age in the illinois basic which suggest that remnant marine fluids contribute significantly to fluid cl budgets cl br and nabr ratios for ordovician through devonian formation fluids are relatively uniform and near those for seawater despite greater than a factor of ten range in cl concentration in contrast fluids from mississippian and pennsylvanian reservoirs separated from older reservoirs by the new albany shale group have more variable fluid cl br and na br ratios most of which are significantly greater then those of seawater the 1 1 stoichiometry of cl and na increases for mississippian and pennsylvanian formation fluids is consistent with halite dissolution nevertheless br systematics and mass balance considerations indicate that he overall cl budget of illinois basin formation fluids appears to be more significantly influenced by the contribution from subaerially evaporated seawater than by halite dissolution

origin and chemical evolution of formation waters from silurian devonian strata in the illinois basin usa abstract a suite of formation water samples from silurian devonian reservoirs in the illinois basin has been analyzed for major minor and trace element concentrations and for h o and sr isotopic compositions in order to interpret origin of salinity and geochemical evolution of brine compositions in this evaporite and shale poor cratonic basin although chloride concentrations range from 2000 to 137 000 mg l cl br ratios 291 18 are consistent with those of seawater or seawater evaporated short of halite saturation cl br 292 thus during silurian devonian time subaerially evaporated penesaline brine entered the subsurface where it was chemically modified through brine rock interactions cation br ratios and mineralogy of associated strata indicate that na and k were depleted through interaction with clay minerals ca was enriched and mg depleted by dolomitization and sr was enriched as a result of caco3 recrystallization and dolomitization although significant dilution of the modified brine with meteoric water is supported by d 18o covariance original marine waters have not been completely expelled from silurian devonian strata hydrogen and oxygen isotopes exhibit covariant relations with cation and anion concentrations implying that isotopic exchange between h2o and minerals has not greatly influenced the d 18o trend brine 87 sr 86 sr ratios range from 0 7092 to 0 7108 when these ratios are plotted versus 1 sr a two component mixing trend is suggested although sr concentrations have experienced local diagenetic modification a 87sr enriched fluid may have accompanied petroleum migration from new albany shales into adjacent silurian devonian carbonates where it mixed with remnant evaporated seawater this event probably preceded the influx of meteoric water as gdd and 18o are not correlated with sr isotopic compositions of formation waters

chlorine stable isotope composition of the oceanic crust implications for earth s distribution of chlorine abstract this first investigation of the cl stable isotope composition of seafloor rocks reveals systematic variations in 37 cl 35 cl of the oceanic crust and its mantle source s relative to seawater chlorine isotopic measurements expressed as 37 cl relative to the seawater 37 cl 35 cl ratio of fresh morb glass hydrothermally produced amphibole containing whole rocks and smectite veins are presented all the samples analyzed are isotopically enriched 0 2 7 5 relative to seawater 0 uptake of cl into amphibole during axial hydrothermal alteration fractionates cl relative to seawater the isotopic compositions of morb glasses are between 0 2 and 7 2 this range appears to be partially due to the assimilation of cl from hydrothermally altered wallrock into morb magmas samples which are unaffected by assimilation have 37 cl between 3 and 7 2 overall the data imply that cl is isotopically fractionated between the surface and mantle reservoirs of earth calculations based on the degree of fractionation suggest that approximately 40 of the cl originally in the degassed mantle now resides on earth s surface

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

halide systematics in interstitial waters of ocean drilling sediment cores abstract the distribution of dissolved halides cl br i in interstitial waters of deep drill holes in the ocean is discussed for a number of environments hemi pelagic sediments hydrate containing sediments sediments affected by halide brines and also in hydrothermal environments the importance of the diagenesis of organic c in the production of i and br is discussed with reference to the observations of dissolved so 4 2 and nh 4 in the pore fluids more importantly in the zone of methano genesis below the so 4 2 reduction zone and usually associated with high organic c accumulation rates increases in i and br are of importance and usually are much higher than the increases observed in the so 4 2 reduction zone in pore fluids affected by dissolution of evaporites the br cl is often a strong indicator of the degree of evaporation of seawater demonstrated from results obtained in the mediterranean sea dissolution of halites on the other hand leads to br cl much lower than those of sea water in hydrothermal sediments there is no doubt that i and br contents of the fluids are influenced by hydrothermal alteration of the sediments and their organic c but signals are not clear the present study provides an overview of previous dsdp and odp studies of halide distributions in sedimentary pore waters in addition to other contributions on halide distributions in sedimentary pore waters in this symposium volume

nonconservative behavior of br cl ratios during alteration of volcaniclastic sediments abstract other than halite diagenesis and organic matter degradation cl and br are considered to be conservative in marine pore fluids consequently br cl ratios should remain constant during most diagenetic reactions nonetheless br cl molar ratios decrease to 1 27 10 3 18 less than seawater value in pore fluids from site 833 in the aoba basin of the new hebrides convergent margin despite the lack of halite diagenesis and little organic matter sediment at this site is largely volcanic ash which becomes hydrated with depth as it converts to clay and zeolite minerals these hydration reactions remove sufficient water to increase the concentrations of most solutes including cl and br the resulting concentration gradients drive diffusion but calculations indicate that diffusion does not decrease the br cl ratio some cl may be leached from the ash but insufficient amounts are available to cause the observed decrease in br cl ratio the limited source of cl suggests that proportionately more br than cl is lost from the fluids to the diagenetic solids similar nonconservative behavior of cl and br may occur during fluid circulation at ridge crests and flanks thereby influencing the halide distribution in the crust

unusual composition of pore waters found in the izu bonin fore arc sedimentary basin extensively altered brines with a composition different from that of any previously reported natural water have been recovered by deep coring of sediments in the izu bonin fore arc sedimentary basin samples from successive depths document the chemical evolution of these unusual pore waters from sea water to highly enriched cacl2 brines the origin of these pore waters seems to be related to the special environment of a fore arc sedimentary basin where accumulation rates of volcanogenic sediment are extremely high and the sediments are susceptible to alteration

sources of solutes and heat in low enthalpy mineral waters and their relation to tectonic setting new zealand abstract approximately 90 of the low enthalpy spring systems in new zealand are distributed along the present day convergence of the pacific and australian plates which varies from subduction to oblique plate collision forearc subducted waters rising in the hikurangi accretionary prism consist of pore waters containing labile organic material water of hydration from clays and seawater these subducted waters are traceable in the north island axial ranges and the northern marlborough fault system in the south island albeit modified by temperature mixing with surface and heated meteoric waters and protracted interaction with the rock the signature of subducted marine sediments in the hikurangi trench is retained in the chemical and isotopic compositions of the taupo volcanic zone magmatic arc type waters the concentrations of b ba br ca cl i k li mg na rb and sr solutes found in high concentrations in forearc subducted waters increase from northeast to southwest towards the incipient subduction zone in the puysegur trench southwest of south island most thermal mineral waters in the south island are expelled along the alpine fault zone where plate collision results to rapid uplift of the southern alps elevated geothermal gradients and fault shearing of rocks in the alpine fault zone variations in temperatures 175 30 c and mineral water compositions are affected by elevation hydrological gradient intensity of tectonic activity permeability fluid volume and rate of throughput and rates of uplift as the main trace of the alpine fault zone is approached concentrations of cl and other solutes generally increase suggesting increasing contributions from ascending cl rich metamorphic waters outside the active convergent zone low enthalpy springs discharge diagenetically modified ancient forearc waters na cl sedimentary formation waters and heated seawater na hco 3 cl waters and na hco 3 deep circulating heated meteoric waters the total surface heat flow in the north island low enthalpy mineral waters is more than 10 higher than in the south island with projected subsurface temperatures as high as 250 c this is because of the widespread occurrence of shallow magmatic and mantle sources associated with pliocene to holocene volcanism and back arc continental rifting the sources of heat and solutes in low enthalpy mineral waters outside the taupo volcanic zone and ngawha are decoupled except for rapidly ascending hot pore waters in the hikurangi accretionary prism and low temperature serpentinization ca cl waters in fiordland

halogen concentrations in pore waters and sediments of the nankai trough japan implications for the origin of gas hydrates abstract presented here are halogen concentrations cl br and i in pore waters and sediments from three deep cores in gas hydrate fields of the nankai trough area the three cores were drilled between 1999 and 2004 in different geologic regions of the northeastern nankai trough hydrate zone iodine concentrations in all three cores increase rapidly with depth from seawater concentrations 0 00043 mmol l to values of up to 0 45 mmol l the chemical form of i was identified as i in accordance with the anaerobic conditions in marine sediments below the so 4 reduction depth the increase in i is accompanied by a parallel although lesser increase in br concentrations while cl concentrations are close to seawater values throughout most of the profiles large concentration fluctuations of the three halogens in pore waters were found close to the lower boundary of the hydrate stability zone related to processes of formation and dissociation of hydrates in this zone generally low concentrations of i and br in sediments and the lack of correlation between sediment and pore water profiles speak against derivation of i and br from local sediments and suggest transport of halogen rich fluids into the gas hydrate fields differences in the concentration profiles between the three cores indicate that modes of transportation shifted from an essentially vertical pattern in a sedimentary basin location to more horizontal patterns in accretionary ridge settings because of the close association between organic material and i and the similarity of transport behavior for i and ch 4 the results suggest that the ch 4 in the gas hydrates also was transported by aqueous fluids from older sediments into the present layers

sand suspension and transport during inundation of a dutch barrier island overwash and inundation of barrier islands transport large amounts of sediment landward which could potentially increase the aggradation of these islands in times of sea level rise however not much is known about the detailed processes of sediment suspension and transport during inundation here we analyze field data of suspended sediment water levels waves and currents which were collected during five inundation events on a barrier island in the netherlands we found that depth integrated suspended sand concentrations and cross shore sand transport showed high variability during and between inundation events at our location where 80 of the combined transport from all inundation events was completed before high tide this is primarily caused by variations in cross shore flow velocities which were strongest up to 1 2 m s onshore before high tide however episodically high depth integrated suspended sand concentrations defined as 2kg m2 were observed on infragravity time scales 20 200 s suggesting that the contribution of infragravity waves to the combined bed shear stresses of waves and currents was important high contributions of infragravity waves to the transport coincided with observed bore like wave shapes which might partly be attributed to higher short waves riding and suspending sediment at the position of the crest two transport regimes were thus found to govern the transport during inundation a flow driven regime when flow velocities were high 0 5 m s and the ratio of infragravity wave and current related shields numbers was below 0 11 and an episodic regime when this ratio exceeded 0 11

avo and ava inversion for fractured reservoir characterization seismic wave reflection amplitudes are used to detect fluids and fracture properties in reservoirs this paper studies the characterization of a vertically fractured fluid filled reservoir by analyzing the reflection amplitudes of p waves with varying incident and azimuthal angles the reservoir is modeled as a horizontal transversely isotropic medium embedded in an isotropic background and the linearized p waves reflection coefficient are considered the conditioning of the inverse problem is analyzed and fracture density is found to be the best conditioned parameter using diffraction tomography under the born approximation an inversion procedure is proposed in the transformed k domain to detect fracture density variations within the reservoir seismic data are rearranged in pairs of incident and reflected plane waves enlightening only one spectral component of the fracture density field at a time only the observable spectral components are inverted moreover working in the transformed domain p

scattering angle migration of ocean bottom seismic data in weakly anisotropic media scattering angle migration maps seismic prestack data directly into angle dependent reflectivity at the image point the method automatically accounts for triplicated rayfields and is easily extended to handle anisotropy we specify scattering angle migration integrals for pp and ps ocean bottom seismic obs data in 3d and 2 5d elastic media exhibiting weak contrasts and weak anisotropy the derivation is based on the anisotropic elastic born kirchhoff helmholtz surface scattering integral the true amplitude weights are chosen such that the amplitude versus angle ava response of the angle gather is equal to the born scattering coefficient or alternatively the linearized reflection coefficient we implement scattering angle migration by shooting a fan of rays from the subsurface point to the acquisition surface followed by integrating the phase and amplitude corrected seismic data over the migration dip at the image point while keeping the scattering angle fixed a dense summation over migration dip

bromine and iodine in peru margin sediments and pore fluids implications for fluid origins at the peruvian convergent margin two distinct pore fluid regimes are recognized from differences in their cl concentrations the slope pore fluids are characterized by low cl concentrations but elevated br and i concentrations due to biogenic production the shelf pore fluids exhibit elevated cl and br concentrations due to diffusive mixing with an evaporitic brine in the slope pore fluids the br i and nh4 concentrations are elevated following bacterial decomposition of organic matter but the i concentrations are in excess of those expected based on mass balance calculations using nh4 and br concentrations the slope sediment organic matter which is enriched in iodine from oxidationreduction processes at the oxygenated sediment water interface is responsible for this enrichment the increases in dissolved i and the i enrichments relative to nh4 and br correlate well with sedimentation rates because of differential trapping following regeneration the pore fluid i br ratios suggest that membrane ion fiitration is not a major cause of the decreases in cl concentrations other possible sources for low cl water including meteoric water clathrate dissociation and or mineral dehydration reactions imply that the diluting component of the slope low cl fluids has flowed at least 1 km through the sediment the low bottom water oxygenation in the shelf is responsible for the low if any enrichment of iodine in the shelf sediments fluctuations in bottom water oxygen concentrations in the past however may be responsible for the observed variations in the sediment ibr ratios comparison of na cl and br cl molar ratios in the pore fluids shows that the shelf high cl fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution this dense brine located below the sediment sections drilled appears to have flowed a distance 500 km through the sediment

calibration of br cl fractionation during subcritical phase separation of seawater possible halite at 9 to 10 n east pacific rise abstract experiments were conducted to assess br cl fractionatiodn during phase separation of seawater at 400 c and 250 to 275 bars these conditions are applicable to the 9 10 n epr system where low cl concentrations and low br cl rations in vent fluids have been attributed to phase separation of seawater at conditions below the critical point of seawater 408 c 300 bars the level of br cl fractionation observed in experiments is well below that needed to account for br cl systematics at epr 9 10 n based on our experimental results we propose an alternative model involving dissolution and precipitation of halite to account for the anomalous br cl data at 9 10 n halite can be predicted to form at pressure and temperature conditions prevailing during eruption of magma at mid ocean ridges subsequent changes in physical conditions however would induce halite to dissolve and thus decrease br cl ratios of vent fluids

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

formation waters from mississippian pennsylvanian reservoirs illinois basin usa chemical and isotopic constraints on evolution and migration we have analyzed a suite of seventy four formation water samples from mississippian and pennsylvanian carbonate and siliciclastic strata in the illinois basin for major minor and trace element concentrations and for strontium isotopic composition a subset of these samples was also analyzed for boron isotopic composition data are used to interpret origin of salinity and chemical and sr isotopic evolution of the brines and in comparison with a similar data set from an earlier study of basin formation waters from silurian devonian reservoirs systematics of cl br na show that present mississippian pennsylvanian brine salinity can be explained by a combination of subaerial seawater evaporation short of halite saturation and subsurface dissolution of halite from an evaporite zone in the middle mississippian st louis limestone along with extensive dilution by mixing with meteoric waters additional diagenetic modifications in the subsurface interpreted from cation br ratios include k depletion through interaction with clay minerals ca enrichment and mg depletion by dolomitization and sr enrichment through caco3 recrystallization and dolomitization ste genevieve limestone middle mississippian formation waters show 87sr86sr ratios in the range 0 70782 0 70900 whereas waters from the siliciclastic reservoirs are in the range 0 70900 0 71052 inverse correlations between 87sr86sr and b li and mg concentrations suggest that the brines acquired radiogenic 87sr through interaction with siliciclastic minerals completely unsystematic relations between 87sr86sr and 1 sr are observed sr concentrations in ste genevieve and aux vases middle mississippian waters appear to be buffered by equilibrium with respect to srso4 although there are many similarities in their origin and evolution these formation waters are distinguished from silurian devonian brines in the basin by elevated clbr and nabr ratios and by unsystematic sr isotope relationships thus waters from these two major segments of the illinois basin stratigraphic column form distinct geochemical regimes which are separated by the new albany shale group devonian mississippian regional aquitard geochemical evolution appears to have been influenced significantly by paleozoic and mesozoic hydrologic flow systems in the basin

geological carbon storage storage of the carbon dioxide that is produced by burning fossil fuels is one way to avoid the damaging consequences of climate change a range of observations suggests that geological carbon storage is much less risky than unabated carbon emissions to the atmosphere

reconstructing last glacial maximum bottom water salinities from deep sea sediment pore fluid profiles deep sea sediment pore fluids contain a record of past glaciations in their cl and 18 o the signal of the last glacial maximum lgm ice volume increase remains in the pore fluids as a local peak in each of these species using a one dimensional model to account for the diffusive and advective transport within the sediment column since the lgm the past bottom water salinity and 18 o seawater values can be estimated the model is most sensitive to the shape of the forcing function used to represent bottom water variations through time the effective diffusion coefficient and the scatter in the data assuming steady state compaction the model is relatively insensitive to the initial condition the bulk sedimentation rate and the assumed porosity profile though these last two are measured independently overall uncertainties in the relative cl increase at the lgm are between 0 1 and 0 5 where the mean ocean change is about 3 5

stable cl isotopes in subduction zone pore waters implications for fluid rock reactions and the cycling of chlorine stable cl isotope ratios measured in marine pore waters associated with the barbados and nankai subduction zones extend significantly to 8 the range of 37 cl values reported for natural waters these relatively large negative values together with geologic and chemical evidence from barbados and nankai and recent laboratory data showing that hydrous silicate minerals i e those with structural oh sites are enriched up to 7 5 in 37 cl relative to seawater strongly suggest that the isotopic composition of cl in pore waters from subduction zones reflects diagenetic and metamorphic dehydration and transformation reactions these reactions involve clays and or other hydrous silicate phases at depth in the fluid source regions chlorine therefore cannot be considered geochemically conservative in these systems the uptake of cl by hydrous phases provides a mechanism by which cl can be cycled into the mantle through subduction zones thus stable cl isotopes should help in determining the extent to which cl and companion excess volatiles like h 2 o and co 2 cycle between the crust and mantle

the collision zone between the north d entrecasteaux ridge and the new hebrides island arc 1 sea beam morphology and shallow structure sea beam bathymetric data closely spaced single channel seismic reflection sections and geopotential field data were collected aboard the r v j charcot over the collision zone between the north d entrecasteaux ridge ndr and the new hebrides island arc the ndr trends east at a small angle 14 to the plate convergence direction so that the ridge creeps northward along the trench at about 2 5 cm yr requiring a continual structural adjustment of the accretionary complex in this report we study the shallow structure tectonic erosion and consequent mass wasting of the accretionary complex all of which were produced by the collision of the ndr in a complex deformational environment the accretionary complex in this collision zone can be divided into northern central and southern parts the northern part of the collision zone which lies north of the leading flank of the ridge shows a lobate lower accretionary complex that is structured by east to southeast dipping thrust and reverse faults and n60 w trending strike slip faults a middle accretionary complex that bulges seaward and appears to be rotated 45 e from the regional arc trend and an upper accretionary complex that is shaped by slumps and a canyon network the central part of the collision zone which directly overlies the crest of the ridge forms a broad shallow protrusion that is bounded on its northern and southern sides by steep scarps rocks forming this protrusion have been uplifted possibly by as much as 1500 2500 m and tilted to the north causing northward block sliding along an extensional detachment surface the southern part of the collision zone which lies in the wake of the ridge is deformed by large slumps and normal faults that trend parallel to the ridge axis rocks forming this southern part collapse causing widespread mass wasting these geophysical data indicate that normal trench convergence has apparently produced only a few trench parallel structures confined to the toe of the accretionary complex whereas the along trench motion of the ndr has resulted in well developed structures that extend obliquely across the arc slope and form an asymmetric tectonic pattern the area of the accretionary complex that is disturbed by collision appears to be about twice as wide as the width of the ndr and the time required to heal the disturbance may be about 0 8 m y

the evaporation path of seawater and the coprecipitation of br and k with halite brines and salt were sampled at the morton bahamas solar salt production facility on great inagua island in the bahamas the brines were analyzed by ion chromatography to define more precisely than heretofore the evaporation path of seawater to the end of the halite facies at inagua calcium carbonate begins to precipitate at a brine concentration factor of 1 8 times that of seawater gypsum begins to precipitate at a brine concentration of 3 8 times seawater and halite at a concentration factor of 10 6 three of the most concentrated brines from inagua 40 times seawater were evaporated further in the laboratory magnesium sulfate first precipitated at brine concentrations about 70 times those of seawater and potassium bearing phases began to precipitate for these brines at concentrations greater than 90 times those of seawater the distribution of coefficients of br and k between brines and halite were determined by combining analytical data for the inagua brines with measurements of the br and k content of halites from inagua and of halite which had precipitated from inagua brines during storage the observed average value of dbr is 0 032 in good agreement with some of the previous measurements the measured values of dk are highly variable 0 001 to 0 021 dk for halite precipitated early in the halite facies is in the vicinity of 0 015

mobilization of beryllium in the sedimentary column at convergent margins studies of be distributions in subduction zone sediments will help to understand questions regarding the enrichments of cosmogenic be 10 in arc volcanic rocks analyses of be 10 and be 9 in sediments of ocean drilling program site 808 nankai trough and be 9 in porewaters of site 808 and sites 671 and 672 barbados ridge complex show significant decreases in solid phase be 10 and large increases of porewater be 9 at the location of the decollement zone and below or at potential flow conduits these data imply the potential mobilization of be during pore fluid expulsion upon sediment burial experiments involving reaction between a decollement sediment and a synthetic nacl cacl2 solution at elevated pressure and temperatures were conducted in an attempt to mimic early subduction zone processes the results demonstrate that be is mobilized under elevated pressure and temperature with a strong ph dependence the be mobilization provides an explanation of be 10 enrichment in arc volcanic rocks and supports the argument of the importance of the fluid processes in subduction zones at convergent margins