geology and molybdenite re os age of the dahutang granite related veinlets disseminated tungsten ore field in the jiangxin province china this is a brief research report about the recently discovered and currently being explored dahutang tungsten deposit or ore field in northwestern jiangxi south central china the deposit is located south of the middle lower yangtze river valley cu au mo fe porphyry skarn belt yrb the mineralization is genetically associated with cretaceous porphyritic biotite granite and fine grained biotite granite and is mainly hosted within a neoproterozoic biotite granodiorite batholith the dahutang ore field comprises veinlets disseminated 95 of the total reserve breccia 4 and wolframite scheelite quartz vein 1 ore styles the mineralization and alteration are close to the pegmatite shell between the cretaceous porphyritic biotite granite and neoproterozoic biotite granodiorite and the three styles of ore bodies mentioned above are related to zoned hydrothermal alteration that includes greisenization k feldspar alteration silicification carbonatization chloritization and fluoritization arranged in time early to late and space bottom to top five samples of molybdenite from the three types of ores have been collected for re os dating the results show re os model ages ranging from 138 4 ma to 143 8 ma with an isochron age of 139 18 0 97 ma mswd 2 9 the quite low re content in molybdenite falls between 0 5 ppm and 7 8 ppm that is indicative of the upper crustal source this is quite different from molybdenites in the yrb cu au mo fe porphyry skarn deposits that contain between 53 ppm and 1169 ppm re indicating a mantle source the dahutang tungsten system is sub parallel with the yrb porphyry skarn cu au mo fe system both are situated in the north margin of the yangtze craton and have a close spatial temporal relationship this possibly indicates a comparable tectonic setting but different metal sources both systems are related to subduction of the paleo pacific plate beneath the eurasian continent in early cretaceous the cu au mo fe porphyry skarn ores are believed genetically related to granitoids derived from the subducting slab whereas the porphyry w deposits are associated with s type granitoids produced by remelting of the upper crust by heat from upwelling asthenoshere

syntectonic carbonate veins from the barbados accretionary prism odp leg 110 record of palaeohydrology syntectonic carbonate veins were found along the decollement zone between the atlantic and caribbean plates and in two sites 12 and 17 km landward of the deformation front of the barbados accretionary prism odp leg 110 both calcite and lesser amounts of rhodochrosite mnco3 occur in microcrystalline fibrous and blocky mosaic forms in veins deposited along scaly shear zones descriptions of the occurrences textures mineralogy and stable isotope geochemistry of these veins presented herein help trace the hydrological evolution of the barbados accretionary prism which has seen the impact of more deeply derived fluids on elevated sediment temperatures and altered geochemistry of interstitial fluids at one site for example vein forming fluids have been as much as 10 c warmer and 1 more enriched in 18o than expected carbon isotope ratios of veins indicate a mixture of carbon derived from seawater and from oxidized organic matter during sulphate reduction although dissolution of calcareous skeletons may be an additional component the presence of methane is recorded in one vein sample in the accretionary prism indicating that either the plumbing of the prism has changed with time allowing more fluid to leak into the prism in the past or that materials currently 1 km above the decollement zone were once deformed in the decollement using oxygen isotope analyses of rhodochrosite samples from two sites in conjunction with measurements of fluid temperature and 18o values of interstitial fluids it is proposed that the fractionation of oxygen isotopes between rhodochrosite and water 1000 1n 38 at 25 c 10 greater than the fractionation between calcite and water however it appears that substitution of 10 20 mol ca into rhodochrosite reduces the fractionation factor by 2 9 so impure diagenetic rhodochrosite samples from the deep ocean will rarely be this enriched in 18o

experimental study of boron geochemistry implications for fluid processes in subduction zones a comprehensive experimental study utilizing an autoclave hydrothermal apparatus with a 10b isotopic tracer has been conducted to monitor the geochemical behavior of sediment b during early subduction zone processes the partition coefficient of exchangeable b kd was determined over a temperature range of 25 350 c at 800 bars and a water rock ratio of 3 1 5 w w these kd are shown to be a complex function of temperature ph and possibly mineralogy at low temperatures kd is significantly high at 4 in contrast to the value of essentially zero at temperatures higher than 100 c a kd of zero represents no b adsorption implying efficient mobilization of exchangeable b at shallow depths during sediment subduction our experimental results demonstrate high mobilization of bulk b in sediments both exchangeable and lattice bound at elevated temperatures 200 350 c in good agreement with previous observations of b in metasediments indicating progressive depletion during metamorphism in addition this study emphasizes the importance of a possible water rock ratio dependence of b mobilization in other words the degree of sedimentary b mobilization in subduction zones strongly depends on the local thermal structure and porosity distribution in low geothermal gradient areas large amounts of porewater are expelled before significant b mobilization has occurred so that some sedimentary b will survive and get into the deeper parts of the subduction zone our results imply that efficient mobilization of b from the subducted slab must occur and that arc magmatism recycles most of the remaining subducted b back to surface reservoirs a reconsideration of the b budget in subduction zones provides critical information with respect to b sources and sinks in the ocean

interplay between fluid flow and fault fracture mesh generation within underthrust sediments geochemical evidence from the chrystalls beach complex new zealand the chrystalls beach complex in the otago schist on the south island of new zealand is a melange comprising sheared trench fill sediments and fragments of oceanic crust it represents an exhumed analogue for underthrust sediments actively deforming along modern subduction thrust interfaces the melange is cross cut by a fault fracture mesh comprising subvertical extension veins and subhorizontal slickenfibre coated shear surfaces both shear and extension veins have a crack seal microstructure indicating episodic growth shear veins are associated with pressure solution selvages along the shear surface whereas wall rock alteration is not observed adjacent to extension veins electron microprobe analyses of selvage seams indicate dissolution of silica from the immediate surroundings of slickenfibre shear veins and therefore these slickenfibres probably grew by local dissolution precipitation of silica on the contrary no depletion or addition of silica is detected around extension veins indicating these veins grew by precipitation from advecting fluids oxygen isotope ratios measured in vein quartz show that shear and extension veins both precipitated from an aqueous fluid with 7 18o 10 consistent with a fluid derived from low grade metamorphic dehydration reactions fluid pressure therefore probably increased as fluids were introduced to a relatively impermeable melange with increasing metamorphic grade and decreasing porosity fault fracture mesh generation therefore involved localized shear assisted by dissolution precipitation creep and concomitant extension fracturing this led in turn to transient permeability associated with a fluid pressure drop allowing episodic vein growth this process may be analogous to geophysically observed episodic tremor and slow slip which also involves a mixture of deformation styles that put together achieve shear slip along the subduction thrust interface

fluid flow in and around a mud volcano field seaward of the barbados accretionary wedge results from manon cruise a field of mud diapirs and mud volcanoes situated in the barbados trench at 13 50 n and extending along an old oceanic fracture zone mercurus was investigated during the manon cruise using both surface ship and nautile submersible sampling and measurements the entire zone from 13 50 n up to 14 20 n has an anomalously high heat flow which implies that fluids are drained into it from a segment of the accretionary wedge a few hundred kilometers wide two structures interpreted as diatremes atalante and cyclops expell large amounts of water and methane we propose that they were formed from the release of a light fluid when gas hydrates were dissociated in the sediment as the result of the circulation of warm fluid in the area however they expell only a small fraction of the incoming fluid implying that disperse flow is the dominant mode of expulsion in this area the chemoautotrophic communities on the surface of the structures rely mostly on sulfides submersible observations temperature measurements in the sediment and the chemistry of the pore fluid indicate that convection of seawater occurs within the first few meters of sediment through high permeability channels such as cemented carbonate conduits we propose that this convection is driven by the density difference between the pore fluid and seawater but fresh water released by the dissolution of shallow hydrates may also contribute this shallow convection may be a frequent process in cold seep environments

the action of water films at a scales in the earth implications for the nankai subduction system abstract water properties change with confinement within nanofilms trapped between natural charged clay particles we investigated nanofilm characteristics through high stress laboratory compression tests in combination with analyses of expelled pore fluids we utilized sediments obtained from deep drilling of the nankai subduction zone at site c0002 of the integrated ocean drilling program iodp we show that below 1 2 km there should be widespread ultrafiltration of migrating fluids experiments to 100 mpa normal compression collapse pores below a few ion monofilm thicknesses a reduction towards a single condensing dehydrating ion monofilm occurs as stresses rise 100 200 mpa and clay separations are reduced to

hydrothermal alteration of hemi pelagic sediments experimental evaluation of geochemical processes in shallow subduction zones abstract laboratory hydrothermal experiments provide unique information regarding the fate of volatile and or incompatibles e g b li and as during oceanic crust subduction examination of chemical redistribution between the subducted slab mantle wedge arc volcanics and overlying ocean water during subduction is critical to gain further insight into arc volcanism and chemical oceanic budgets for instance efficient mobilization of b at shallow depths may be a key aspect of its oceanic budget and can help to explain the systematics of b 11b and b 10be in arc lavas fluid rock interactions at elevated temperatures and pressures in accretionary prisms were studied using a rocking autoclave hydrothermal apparatus to monitor sediment porewater interaction over the range of 25 to 350 c at 800 bars clay rich hemi pelagic sediments from the decollement zone of ocean drilling program site 808 nankai trough were reacted with nacl cacl2 solutions at water rock ratios of 3 5 to 1 5 w w to mimic alteration processes in the shallow subduction zone fluids were extracted at 25 50 c intervals and were analyzed for major and trace chemical constituents the fluid chemistry changed significantly during the course of these experiments but there was generally only minor modification of the solid phase only b li as br and pb are significantly depleted during the heating cycle dissolved na mg and so4 decreased sharply and nh4 sio2 k b and li increased at t 300 c calcium drops gradually at low temperatures but concentrations rise sharply at t 300 c decomposition of organic matter so42 depletion and mg fixation dominate at low temperatures however albitization of calcic plagioclase leads to marked na depletions and ca enrichments at t 300 c dissolved sio2 remained below saturation with respect to quartz and amorphous silica throughout the entire experiment b and li mobilization with large isotopic fractionations occurred at low temperature exchangeable b 11b 15 is completely leached before reaching 150 c substantial o2 exchange between fluids and the solid phase occurred at t 200 c in the spiked experiment where 18o varies more than 100 in the fluids during retrograde cooling dissolved mg so4 ca si k and sr are released as a result of carbonate or anhydrite dissolution and marked b re adsorption occurred at temperatures below 60 c

table nunatak a key outcrop of upper cretaceous shallow marine strata in the southern larsen basin antarctic peninsula the northern james ross island region of the larsen basin on the eastern back arc margin of the antarctic peninsula magmatic arc includes one of the thickest and most complete upper cretaceous sedimentary successions exposed in the southern hemisphere however the southern part of the basin remains poorly known mainly owing to inaccessibility and lack of exposure table nunatak an isolated 1 km long 400 m wide outcrop at the tip of kenyon peninsula is the only known exposure of upper cretaceous or younger strata in this region the 62 m thick succession exposed there is assigned to the newly defined table nunatak formation it consists mainly of sharp based amalgamated beds of fine grained sandstone up to 2 8 m thick with subordinate intervals of intensely bioturbated mudstone wave ripples are present at some levels and locally developed swaley cross stratification provides evidence for storm generated combined flow deposition however most sandstone beds appear to be internally structureless apart from normal grading and are interpreted as the direct suspension deposits of highly sediment charged storm and or flood related flows the succession represents relatively nearshore deposition probably at the mouth of a river or deltaic distributary channel charcoalified plant debris abundant at the tops of some sandstone beds suggests a periodically wildfire swept hinterland forested largely by coniferous trees dinoflagellate cyst assemblages indicate a late santonian age and suggest correlation with the basal part of the lachman crags member of the santa marta formation marambio group on james ross island palaeocurrents sandstone petrography and the high sediment supply rate proposed for the table nunatak formation suggest a relatively high relief source area to the west with large scale erosion of granitoid plutons and metamorphic rocks possibly related to arc uplift during a mid cretaceous compressional episode the formation is evidence of a major southward extension of the upper cretaceous strata exposed in the northern larsen basin and suggests lateral continuity of shallow marine deposition for at least 500 600 km along the weddell sea margin of the antarctic peninsula in santonian times

fluid accumulation and channeling along the northern barbados ridge decollement thrust a volume of three dimensional seismic reflection data acquired in 1992 imaged the decollement beneath the northern barbados ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault we model the seismic reflection by inversion for seismic impedance the product of velocity and density throughout the 5 x 25 km survey area and thus map physical property variations in 1997 ocean drilling program leg 171a penetrated the protodecollement and decollement at five sites with a logging while drilling lwd tool to log density and other physical properties of the decollement we construct a regional map of density and inferred porosity within the decollement from seismic models calibrated with lwd density data in the sediments out in front of the trench the protodecollement forms in a radiolarian rich miocene mudstone with an anomalously high porosity 70 75 that appears as a pervasive inherent characteristic of this interval seaward of the deformation front in the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70 at the wedge toe to 50 4 km from the wedge toe a second distinct trend also forms along a 10 km long 1 to 2 km wide ne sw zone in which porosity is 70 as high as it is in the protodecollement this zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content we postulate that high fluid content is maintained by continuous recharge flowing into and along this channel this porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out of sequence thrusts above the decollement

syntectonic fluid flow and deformation mechanisms within the frontal thrust of a foreland fold and thrust belt example from the internal jura eastern france abstract structural mapping and microstructural analyses are combined with stable clumped and radiogenic isotopes to unravel deformation mechanisms and syntectonic fluid flow during upward propagation along the frontal thrust fuans thrust of the internal part of the jura fold and thrust belt the fuans thrust cuts through upper jurassic lower cretaceous carbonate deposits branches into two splays and generates a system of fault propagation folds folding was accomodated by layer parallel shortening and layer parallel shear which generated flexural slip along bedding surfaces stylolites and foliated fabric in marly limestone interbeds thrusting was characterized by cycles of mutually overprinting pressure solution and brittle processes such as frictional sliding and fracturing veining this generated foliated tectonites in marls and limestones without significant cataclasis of the host rock pressure solution and micrometer slip along slickenfibers are evidence of aseismic slip in agreement with the ongoing aseismic growth of anticlines in the jura fold and thrust while crackle like vein texture may account for episodic seismic fracturing geochemical data show that fuans thrust acted as a conduit for the downward infiltration of meteoric derived fluids which channeled along foliated tectonites during syntectonic fluid flow calcite mineralizations precipitated at temperatures between 54 c and 10 c showing an increased contribution of organic carbon from soils in colder paleofluids this indicates a progressive opening of the system characterized by a more efficient and extensive meteoric fluids circulation considering the highest recorded temperature of 54 c and assuming the modern geothermal gradient of 30 c km thrusting in the study outcrop initiated at a burial depth of at least 1 4 km

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

fault structure control on fault slip and ground motion during the 1999 rupture of the chelungpu fault taiwan the chelungpu fault taiwan ruptured in a mw 7 6 earthquake on 21 september 1999 producing a 90 km long surface rupture analysis of core from two holes drilled through the fault zone combined with geologic mapping and detailed investigation from three outcrops define the fault geometry and physical properties of the chelungpu fault in its northern and southern regions in the northern region the fault dips 45 60 east parallel to bedding in both the hanging wall and footwall and consists of a narrow 1 20 cm core of dark gray sheared clay gouge the gouge is located at the base of a 30 to 50 m zone of increased fracture density confined asymmetrically to the hanging wall microstructural analysis of the fault gouge in dicates the presence of extremely narrow clay zones 50 300 lm thick that are interpreted as the fault rupture surfaces few shear indicators are observed outside of the fault gouge implying that slip was localized within the gouge zone slip localization along a bed parallel surface resulted in a narrow gouge zone that pro duced less high frequency ground motion and larger displacements average 8 m during the earthquake than in the southern region displacement in the southern region averaged only 2 m but ground shaking consisted of large amounts of high frequency ground motion the fault in the southern region dips 20 30 at the surface and consists of a wide 20 70 m thick zone of sheared foliated shale with numerous gouge zones these data demonstrate a potential correlation between fault structure i e gouge width geometry and earthquake characteristics such as displacement and ground motion i e acceleration

isotopic geochemistry of burial metamorphosed volcanogenic sediments great valley sequence northern california isotopic and mineralogic data from an 8500 m thick section of the great valley sequence northern california indicate that changes in the 18o values of authigenic minerals resulted from the conversion of smectite to a 10 a clay mineral as temperature increased with burial in the jurassic cretaceous outer arc basin the clay mineral assemblage in mudstone is characterized by a proportional increase of the 10 a clay mineral with increasing stratigraphic depth and by a depletion in the 18o value of the mixed layer smectite 10 a clay mineral with descending stratigraphic position from 21 9 to 15 5 smow modeling of the oxygen isotopic data from authigenic phases based on equilibrium fractionation during clay mineral diagenesis indicates that 18o values of calcite in mudstones and of calcite cements in sandstone precipitated along a temperature gradient of about 25 c km during maximum burial to about 6 7 km d values of the mixed layer smectite 10 a clay mineral range between 69 to 44 smow using temperatures calculated from the oxygen isotopic data the deuterium and oxygen isotopic data indicate that the smectite underwent late stage dehydration and probably buffered the composition of formation waters from sea water values to isotopic compositions of 18o 8 smow and d 25 smow the 13c values of calcite from mudstone and sandstone imply that crystallization of authigenic calcite was linked to organic diagenesis during which dissolved hcot 3 was continuously enriched in 13c as temperature increased with burial at the base of the sequence and immediately overlying the ophiolitic basement rocks several hundred meters of strata were altered by more oxygen depleted 18o 4 to 5 hydrothermal fluids emanating from the ophiolitic rocks probably at maximum burial depth

abundance of smectite and the location of a plate boundary fault barbados accretionary prism isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics the frontal decollement zone of the northern barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy fluid flow chemical interactions and sediment physical properties affect strain localization this plate boundary fault is centered at a lithologic contact between miocene claystone and fine grained oligocene turbidites the fault zone is nearly 40 m thick at ocean drilling program site 948 and its upper part passes through smectite rich deposits a sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement there is a consistent increase in percent smectite with distance above the base of the decollement but the top of the decollement is poorly defined by clay mineralogy the intrinsic mechanical weakness of strata with abundant smectite group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the atlantic abyssal plain a second inherited parameter is the local abundance of radiolarians which contribute to higher than normal porosities sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic the principal cause of excess pore pressure seems to be updip fluid advection in theory however decreases in pore fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals the imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions if fresher pore water migrates to the propagating tip of the decollement its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more the location and evolution of the decollement therefore are controlled by a complicated interplay of static factors inherited from the abyssal atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry

constraints on water transport and alteration in the oceanic crust from the isotopic composition of pore water the oxygen isotopic compositions of pore fluids from sediments of older oceanic crust commonly decrease with depth increases in calcium ion concentrations always accompany the 18o 16o decreases these changes are attributed principally to the alteration of basalts or alteration of volcanic ash in the sediments oxygen isotope material balance calculations suggest that 5 20 of the upper kilometer of basalt is altered at the time of subduction of the oceanic crust this corresponds to a flux of 18o into the oceanic crust of 2 3 1015 moles per million years evaluation of the oxygen and hydrogen isotope data from the pore fluids also suggests that the principal process of mass transfer in the sediments of older oceanic crust is diffusion convection of water in the basalts of the oceanic crust appears to continue until subduction

contrasting geothermal regimes of the barbados ridge accretionary complex three e w transects of closely spaced heat flow measurements across the barbados accretionary complex are reported one transect about 100 km in length at 14 20 n and a shorter 15 km line at 14 35 n show that heat flow decreases arcward roughly in proportion to the thickening of the wedge over the actively accreting zone this decrease is due to the combined thermal effects of the underthrusting oceanic lithosphere and thickening of sediments that have been ingested into the wedge except for anomalously high heat flow over a mud volcano on the eastern end of the 100 km transect fluid expulsion appears to have only a secondary effect on the seafloor heat flow in this region an increase of heat flow is observed at the western end of the 100 km transect where high angle arcward vergent thrusting is occurring we attribute this increase primarily to a decrease in the arcward thickening of wedge material and heat generated by friction on the decollement in strong contrast a short line of stations at 15 30 n the deep sea drilling project ocean drilling project dsdp odp drilling transect found anomalously high heat flow a 40 increase over the heat flux from the lithosphere with large variability over the most recently accreted material and also in the adjacent basin the thermal regime in the vicinity of the dsdp odp transect is dominated by a strong flow of pore fluids along the decollement and upward flow along fault planes in the toe of the wedge and adjacent basin sediments heat transport by pore fluids also appears to dominate the subseafloor thermal regime in a zone near 13 50 n the thermal effects of dewatering of the sediments by upward diffusive flow were examined using a simple one dimensional model which showed that the resulting effects are too small to produce detectable heat flow anomalies consequently the flow of fluids must be channelized in subseafloor conduits to produce the observed anomalies in heat flow

heat flow tectonics and fluid circulation at the toe of the barbados ridge accretionary prism detailed patterns of near surface tectonics and heat flow were investigated at two locations at the toe of the barbados ridge accretionary prism at the two locations in the thickly sedimented southern portion of the prism near 12 20 n and in the northern area drilled during ocean drilling program odp leg 110 near 15 32 n relative highs of heat flow were associated with zones of thrusting we attribute them to upward advection of water rising from the decollement along faults through the sediments in the odp area a significant heating plate effect from warm fluids flowing along the decollement and thrusts is required to account for the high regional heat flow this effect implies high rates of fluid flow at the decollement which can only be non steady state or would require massive lateral transport of fluids along strike

geochemical and geothermal evidence for fluid migration in the barbados accretionary prism odp leg 110 measured geochemical and geothermal effects of fluid migration in the northern barbados accretionary prism indicate that 1 fluid flows laterally along low angle active faults 2 the entire length of the leg 110 transect is a fluid discharge zone 3 migration results in heating of sediments across the transect but most strongly at the deformation front and 4 migration results in dilution of pore water cl and sup 18 o enrichment more in the arcward than the seaward side of the prism suggesting that the cl poor sup 18 o rich fluids that intrude the prism are derived from smectite illite reactions occurring arcward these conclusions have strengthened those derived from the initial appraisal of shipboard data modified ideas about where fluids are heated most and where the geochemistry is strongly altered and provided new ideas about the sources of fluids and their relative contributions

diagenesis in tephra rich sediments from the lesser antilles volcanic arc pore fluid constraints we present sediment pore fluid and sediment solid phase results obtained during iodp expedition 340 from seven sites located within the grenada basin of the southern lesser antilles volcanic arc region these sites are generally characterized as being low in organic carbon content and rich in calcium carbonate and volcanogenic material in addition to the typical reactions related to organic matter diagenesis pore fluid chemistry indicates that the diagenetic reactions fall within two broad categories 1 reactions related to chemical exchange with volcanogenic material and 2 reactions related to carbonate dissolution precipitation or recrystallization for locations dominated by reaction with volcanogenic material these sites exhibit increases in dissolved ca with coeval decreases in mg we interpret this behavior as being driven by sediment water exchange reactions from the alteration of volcanic material that is dispersed throughout the sediment package which likely result in formation of mg rich secondary authigenic clays in contrast to this behavior sediment sequences that exhibit decreases in ca mg mn and sr with depth suggest that carbonate precipitation is an active diagenetic process affecting solute distributions the distributions of pore fluid 87sr 86sr reflect these competitive diagenetic reactions between volcanic material and carbonate which are inferred by the major cation distributions from one site where we have solid phase 87sr 86sr site u1396 the carbonate fraction is found to be generally consistent with the contemporaneous seawater isotope values however the 87sr 86sr of the non carbonate fraction ranges from 0 7074 to 0 7052 and these values likely represent a mixture of local arc volcanic sources and trans atlantic eolian sources even at this site where there is clear evidence for diagenesis of volcanogenic material carbonate diagenesis appears to buffer pore fluid 87sr 86sr from the larger changes that might be expected given the high abundance of tephra in these sediments part of this carbonate buffering at this site as well as throughout the region derives from the fact that the sr concentration in the non carbonate fraction is generally low 200 ppm whereas the carbonate fraction has sr concentrations approaching 1000 ppm

transient fluid flow through the toe of the barbados accretionary complex constraints from ocean drilling program leg 110 heat row studies and simple models thirty four sediment and mud line temperatures were collected from six drill holes on ocean drilling program odp leg 110 near the toe of the barbados accretionary complex when combined with thermal conductivity measurements from sediment cores and results from earlier surveys these data delineate the complicated thermal structure on the edge of this convergent margin heat flow values at the seafloor of 92 192 mw m sup 2 are 80 300 higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude heat flow tends to decrease downhole at four sites suggesting the presence of heat sources within the sediments these results are consistent with the flow of warm fluids through the complex along high permeability conduits including thrust faults the major decollement zone and sandy intervals simple calculations suggest that this fluid flow is transient occurring on time scales of tens of thousands of years fluid flow velocities along the decollement zone estimated with a simple thermal model are about 10 sup minus 7 m s 100 times faster than predicted by numerical steady state analyses the estimated maximum sediment permeability within the decollement zone which was basedmore on the above fluid flow velocity is about 10 sup minus 12 m sup 2 also 100 times higher that that calculated numerically high heat flow in the vicinity of 15 degree 30 prime n and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism water discharge zone with dewatering more active here than elsewhere along the accretionary complex less

effect of smectite dehydration on pore water geochemistry in the shallow subduction zone an experimental approach 1 the diagenetic smectite to illite transition is widely accepted to cause ubiquitous pore water freshening at convergent margins however experimental consolidation tests show that smectite also dehydrates partially with increasing effective stress to shed light on this process three hydrothermal consolidation tests on a silty smectite rich claystone were conducted with stresses p up to 70 mpa and at constant temperatures t of 20 c 60 c and 100 c fluids expelled during the tests were analyzed for major and trace elements to evaluate dehydration and fluid rock interaction with increasing pt conditions the results document that fluid freshening starts when the effective stress exceeds 1 3 mpa the smectite interlayer water content decreases from 27 wt to 20 wt during the experiments which is equivalent to an interlayer collapse from 18 5 to 15 4 a the released interlayer h20 accounts for up to 17 of the total fluid volume released from the consolidating sediment major and volatile element geochemistry is influenced by temperature and smectite interlayer collapse the interlayer collapse is characterized by k and ca uptake while is b released from smectite application of experimental results to the barbados accretionary margin shows that stress dependent smectite dehydration is an important fluid source mechanism and accounts for substantial fluid freshening modeled smectite dehydration suggests a decrease of smectite interlayer water content from 27 wt at the surface down to 22 wt at 2 km depth this leads to chlorinity values as low as 460 mm

paleoatmospheric consequences of co2 released during early cenozoic regional metamorphism in the tethyan orogen the eocene was the warmest epoch of the cenozoic with published estimates of eocene atmospheric co2 content ranging from two to six times the current value calculations of co2 consumption by silicate weathering show that co2 fluxes to the atmosphere of ca 1018 myr could account for inferred paleocene eocene atmospheric co2 contents and the resulting greenhouse effect would have contributed to eocene warmth extensive portions of the tethyan orogen underwent regional metamorphism culminating in the eocene prograde metamorphism may have been contemporaneous with the late paleocene global warming we calculated the amount of metamorphic co2 produced at depth in the himalayan orogen with data on the timing of the india asia collison duration of prograde regional metamorphism and proportions and bulk compositions of metamorphic co2 source rocks if co2 was generated at a constant rate over a 10 myr period of prograde metamorphism in the himalayan orogen we estimate that ca 1018 1019 moles myr of metamorphic co2 were produced at depth significant expulsion of metamorphic co2 to the atmosphere may have occurred by focused fluid flow along shear zones such as the extensive main central thrust in the himalayan orogen an additional ca 1018 myr could have been contributed by eocene regional metamorphism in the mediterranean tethys i e from the alps to turkey the extensive metamorphism associated with the india asia collision and the closing of tethys may have contributed to co2 greenhouse warming in the early to mid cenozoic

elevated fluid pressure and fault zone dilation inferred from seismic models of the northern barbados ridge decollement in 1992 a large volume of three dimensional seismic reflection data were acquired in a 5 25 km area across the toe of the barbados accretionary complex that covers the deep sea drilling project leg 78a and ocean drilling program legs 110 and 156 drilling sites these data are used to examine the acoustic character of the decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone seismic models have been constructed across a 6 km region of the decollement where it has been mapped as a moderate to bright polarity reversed reflection the models show that this segment of the decollement reflection is caused by a low velocity interval usually 12 16 m thick the top of the low velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1 8 km s to between 1 7 and 1 65 km s with some localized bright reflections with an even lower velocity of 1 6 km s the base of the low velocity layer is less certain from modeling the base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10 m thick interval comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the decollement fluid pressure is at or near lithostatic furthermore the reflection coefficients of the decollement are sufficiently large that some dilation of the fault zone is required the dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent decollement reflection along a 5 km segment of the decollement it is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength