age of emplacement of the schistes lustres nappe alpine corsica abstract structural studies in the schistes lustres nappe west of bastia corsica demonstrate the existence of a tectonic melange in which km scale blocks and smaller lozenges of basement granite gneiss thick layered marble and dismembered mesozoic ophiolite are enveloped in a matrix of calc schist and blueschist the main s1 foliation is developed in both block and matrix and is concordant with lithologie contacts blueschist facies metamorphism was syn kinematic with the main foliation the s1 in the schistes lustres was refolded about ene wsw trending tight similar and monoclinal fold axes f2 these second folds verge to the southeast and show km scale axial culminations and depressions that are reflected by topography and residual bouguer gravity anomalies parautochthonous hercynian basement tenda corte complex beneath the western edge of the schistes lustres nappe contains a mylonitic foliation which is concordant with the main foliation in the schistes lustres the intensity of deformation in the basement decreases away from this contact and undeformed granites are found 3 km to the west whole rock samples of the deformed basement immediately beneath the schistes lustres yield an rb sr isochron diagram n 4 which has an age of 105 8 ma 1 and initial 87 sr 86 sr ratio of 0 7228 0 0005 1 this result is more precise than our preliminary age and initial ratio estimate of 98 14 and 0 7296 0 0068 respectively cohen et al 1979 it is similar to a recently published mid cretaceous 90 ma 40ar 39ar age from glaucophane mineral separates we interpret this date as the age of a metamorphic overprint related to the emplacement of the schistes lustres nappe and associated ophiolites the formation of the main foliation and blueschist facies metamorphism these results indicate that the mid cretaceous blueschist facies metamorphism documented in the western alps formerly extended farther south of its present terminus the data are consistent with mid cretaceous obduction of tethyan oceanic crust onto the present day eastern continental margin of corsica we postulate that during eocene early oligocene time a polarity flip occurred outboard of the obducted crust and a new southfacing subduction zone developed this change in polarity was responsible for the development of southeast vergent second folds and for the resetting of 40ar 39ar and k ar geochronologic clocks described in the literature

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the solea graben northern troodos ophiolite cyprus hydrothermal mineral zonations and o isotope patterns of the northern troodos complex do not parallel the ophiolite pseudostratigraphy but reflect the convective geometry of an upper cretaceous seawater hydrothermal system large areas of the sheeted intrusive complex sic including the subaxial region of the solea graben are composed of 18o rich subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge other areas of the sic are recrystallized to distinctive epidosite rocks granular high variance assemblages of epidote quartz chlorite that are depleted in 18o al2o3 na2o k2o zr cu and zn and are enriched in cao and sr compared with other mafic volcanic and dike rocks of the solea graben epidosite alteration occurred at temperatures of 310 370 c and involved fluids with 18o values and salinities similar to those of upper cretaceous seawater the epidosite zones are discordant with earlier mineral o isotope zonations and with the axis of spreading in the solea graben suggesting a postspreading off axis origin the seawater hydrothermal system responsible for solea graben massive sulfide deposits was probably driven by hypabyssal intrusions not exposed emplaced in a terminal failed spreading episode the geometries of o isotope surfaces within the solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits depletions in base metals show that epidosite alteration liberated cu and zn to mineralizing fluids within the fossil upflow zone

oxygen isotope evidence for submarine hydrothermal alteration of the del puerto ophiolite california abstract the oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the del puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the great valley sequence indicate that their alteration occurred in a submarine hydrothermal system whole rock 18o compositions decrease progressively down section with increasing metamorphic grade 22 4 smow to 13 8 for zeolite bearing volcaniclastic sedimentary rocks overlying the ophiolite 19 6 to 11 6 for pumpellyite bearing metavolcanic rocks in the upper part of the ophiolite s volcanic member 12 3 to 8 1 for epidote bearing metavolcanic rocks in the lower part of the volcanic member 8 5 to 5 7 for greenschist facies rocks from the ophiolite s plutonic member 7 6 to 5 8 for amphibolite facies or unmetamorphosed rocks from the plutonic member modelling of fluid rock interaction in the del puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock 18o can be best explained by isotopic exchange with discharging18o shifted seawater at fluid rock mass ratios near 2 and temperatures below 500 c 18o depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism submarine weathering and later burial metamorphism at the base of the great valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions the pervasive zeolite and prehnite pumpellyite facies mineral assemblages found in the del puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center

oxygen and carbon isotope evidence for seawater hydrothermal alteration of the macquarie island ophiolite abstract rocks of the miocene macquarie island ophiolite south of new zealand have oxygen and carbon isotopic compositions comparable to those of seafloor rocks basalt glass and weathered basalts have 18o values at 5 8 6 0 and 7 9 9 5 respectively similar to drilled seafloor rocks including samples from the leg 29 dsdp holes near macquarie island compared to the basalt glass the greenschist to amphibolite facies metaintrusives are depleted in18o 18o 3 2 5 9 similar to dredged seafloor samples whereas the metabasalts are enriched 18o 7 1 9 7 although the gabbros are only slightly altered in thin section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4 5 km there is an approximate balance between18o depletion and enrichment in the exposed ophiolite section the carbon isotopic composition of calcite in the weathered basalts 13c 1 0 2 0 is similar to those of drilled basalts but the metamorphosed rocks have low 13c values 14 6 to 0 9 these data are compatible with two seawater circulation regimes in the upper regime basalts were weathered by cold seawater in a circulation system with high water rock ratios 1 0 based on calcite compositions weathering temperatures were less than 20 c and the carbon was derived from a predominantly inorganic marine source as previously suggested for the samail ophiolite it is postulated that the lower hydrothermal regime consisted of two coupled parts at the deeper levels seawater circulating at low water rock ratios 0 2 0 3 and high temperatures 300 600 c gave rise to18o depleted gabbro and sheeted dikes via open system exchange reactions during reaction the seawater underwent a shift in oxygen isotopic composition 18o 1 0 5 0 and subsequently caused18o enrichment of the overlying metabasalts in the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water rock ratios 1 0 10 0 and temperatures in the range 200 300 c the relatively low water rock ratios in the hydrothermal regime are supported by the low 13c values of calcite interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts

high pressure veins in eclogite from new caledonia and their significance for fluid migration in subduction zones abstract this paper describes the petrology and geochemistry of high pressure veins and associated metasomatic selvages that are hosted by eclogite in the pouebo eclogite melange of northern new caledonia isotopic and geochemical evidence indicates that the mafic host rock represents seafloor altered fractionated morb that underwent eclogite facies metamorphism in a subduction zone within the host rock are cm thick garnet quartz phengite veins that are enveloped by garnet poor omphacite rich selvages or bleach zones petrography thermometry oxygen isotope characteristics and mass balance calculations are used to show that the veins largely formed by fluid mediated mass transfer from the bleach zones during prograde metamorphism minerals in the veins are free of inclusions but the vein garnets preserve complex chemical zoning features that are not present in the host rock garnets vein garnets have mn and hree zoning patterns that are indicative of progressive garnet growth during prograde metamorphism whereas mg and ca contents reveal prominent sector zoning and fine scale intergrowth features we propose that the veins formed over a prolonged period during subduction by local circulation of fluid that was sourced from prograde dehydration of minerals in the host rock fluid circulation may have been driven by episodic microcracking sealing around garnet porphyroblasts which led to significant mass transfer and progressive vein growth mass balance calculations and phengite trace element compositions also require the additional of pelite derived components to the veins these components were probably introduced into the veins at conditions close to peak metamorphism via a relatively small external fluid flux this model for vein formation is consistent with previous studies that suggest fluid flow in deeply subducted oceanic crust is highly restricted in many cases the delay of fluid migration after hydrous mineral breakdown may provide an important source of water for arc magmas as free fluid is transported to deeper levels in subduction zones

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

the importance of blueschist eclogite dehydration reactions in subducting oceanic crust the metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure temperature p t paths with a model of metabasalt phase equilibria in steady state subduction zones with high rates of shear heating the upper parts of the subducting oceanic crust progress through the greenschist amphibolite granulite eclogite facies whereas lower parts of the subducting oceanic crust progress through the blueschist eclogite facies in steady state subduction zones with moderate rates of shear heating most of the subducting oceanic crust passes through the blueschist eclogite transition in steady state subduction zones with low rates of shear heating the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km for oceanic crust containing 1 2 wt h 2 o dehydration will not begin until the onset of eclogite or amphibolite facies metamorphism depending on the p t path for many subduction zones the most important dehydration reactions in the subducting oceanic crust occur at the blueschist eclogite facies transition associated with the breakdown of lawsonite or clinozoisite glaucophane and chlorite large amounts of h 2 o released by blueschist eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas

distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry examples from tethyan ophiolites in the western alps large amounts of fluid bound up in the hydrated upper layers of the ocean crust are consumed at convergent margins and released in subduction zones through devolatilization the liberated fluids may play an integral role in subduction zone processes including the generation of arc magmas however exhumed subduction zone rocks often record little evidence of large scale fluid flow especially at deeper levels within the subduction zone basaltic pillows from the high pressure corsican and zermatt saas ophiolites show a range of 18o values that overall reflect seafloor alteration prior to subduction however comparison between the 18o values of the cores and rims of the pillows suggests that the 18o values of the pillow rims at least have been modified during subduction and high pressure metamorphism pillows that have not undergone high pressure metamorphism generally have rims with higher 18o values than their cores whereas the converse is the case in pillows that have undergone high pressure metamorphism this reversal in the core to rim oxygen isotope relationship between unmetamorphosed and metamorphosed pillows is strong evidence for fluid rock interaction occurring during subduction and high pressure metamorphism however the preservation of different 18o values in the cores and rims of individual pillows and within and between different pillows suggests that fluid flow within the subduction zone was strongly channelled resetting of the 18o values in the pillow rims was probably due to fluid hosted diffusion that occurred over relatively short time scales 1 myr

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

geometry and mechanisms of fluid flow at 15 to 45 kilometer depths in an early cretaceous accretionary complex the catalina schist southern california contains variably metamorphosed mafic sedimentary and ultramafic rocks that were accreted at depths of 15 to 45 km during early cretaceous subduction fluid flow in the catalina schist was concentrated along fractures and shear zones as indicated by the textures and abundance of veins and evidence in melange zones for metasomatism and homogenization of stable isotope compositions slab parallel orientations of major melange zones with enhanced permeability may dictate that the majority of fluid released by devolatilization at depths 15 km in subduction zones moves updip toward the seafloor such transport could contribute to fluid budgets in shallower parts of accretionary wedges and facilitate large scale mass and energy transfer in forearc regions

bbe systematics in subduction related metamorphic rocks characterization of the subducted component the mobility of b and be in h2o rich fluids and felsic silicate liquids produced during metamorphism of subducted oceanic slab and sediments has been investigated through analysis of subduction zone metamorphic rocks of the catalina schist california in metasedimentary rocks b be and the range in b be decrease with increasing metamorphic grade mean 72 std dev 41 for lowest grade lawsonite albite fades rocks mean 21 std dev 11 for higher grade greenschist and epidote amphibolite facies equivalents this decrease to more uniformly low b be may be attributed to the preferential removal of b in h2o rich fluids produced by devolatilization reactions over the approximate temperature interval of 350 600 c metamafic rocks do not show pronounced decrease in b be with increasing metamorphic grade however all metamafic samples have b be 30 lower than values for many altered seafloor basalts in amphibolite grade exposures felsic leucosomes and pegmatites reflecting partial melting have low b be similar to their metasedimentary and metamafic hosts which presumably experienced prior reduction in b be during lower temperature devolatilization this evidence for b and be mobility during high p t metamorphism complements studies of b be systematics in arc volcanic rocks in further characterizing mechanisms by which slab derived elements can be added to the source regions of arc lavas before subducted mafic and sedimentary rocks reach wadati benioff zone depths beneath arcs 80 150 km the b be of these rocks is likely to have decreased to 30 thus highly fractionated slab derived hydrous fluids may be necessary to generate the high b be signatures observed in many arcs b be of up to 200 the b be data together with previously presented stable isotope data for the catalina schist demonstrate that subduction zone metamorphic processes are capable of homogenizing presubduction variability in the concentrations of particularly fluid mobile elements in rocks and may through mixing produce fluids which trend toward uniform trace element and isotopic compositions these homogeneous fluids could infiltrate parts of the mantle wedge and contribute to the characteristic trace element and isotopic signatures of arc magma source regions in hotter subduction zones e g involving subduction of young hot oceanic lithosphere silicate melts derived from previously devolatilized sedimentary and mafic rocks may contribute relatively low b be signatures to arc source regions thus significant variations among arcs in the ranges of b be observed in front rank volcanoes e g for the bismarck arc b be 20 190 for the cascades arc b be 5 may be related in part to varying thermal structure which could govern both the b be of hydrous fluids and the relative proportions of hydrous fluid and silicate melt derived from the subducted slab and sediments

stable isotope geochemistry of alpine ophiolites a window to ocean floor hydrothermal alteration and constraints on fluid rock interaction during high pressure metamorphism the subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle however despite its potential importance fluid rock interaction during high pressure metamorphism is relatively poorly understood the stable isotope and major element geochemistry of pennine ophiolite rocks from italy and switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites suggesting that they interacted with little pervasive fluid during high pressure metamorphism cover sediments also have oxygen isotope ratios within the expected range of their protoliths in the rocks that escaped late greenschist facies retrogression different styles of sub ocean floor alteration may be identified using oxygen isotopes petrology and major or trace element geochemistry within the basalts zones that have undergone high and low temperature sub ocean floor alteration as well as relatively unaltered rocks can be distinguished serpentinites have 18o and 2h values that suggest that they were formed by hydration on or below the ocean floor the development of high pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub ocean floor alteration and which contained hydrated fine grained reactive assemblages given that the transformation of blueschist facies metabasic rocks to eclogite facies assemblages involves the breakdown of hydrous minerals e g lawsonite zoisite and glaucophane and will thus liberate considerable volumes of fluids metamorphic fluid flow must have been strongly channelled high pressure quartz calcite omphacite glaucophane titanoclinohumite veins that cut the ophiolite rocks represent one possible channel however stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids fluids were more likely channelled along faults and shear zones that were active during high pressure metamorphism such strong fluid channelling may cause fluids to migrate toward the accretionary wedge especially along the slab mantle interface which is probably a major shear zone this may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting additionally because fluids probably interact with relatively small volumes of rock in the channels they cannot scavenge elements from the subducting slab efficiently

high resolution regional climate model validation and permafrost simulation for the east european russian arctic a 15 year long 16 km horizontal resolution present day climate simulation has been conducted with the hirham4 regional climate model the model has been forced by the european centre for medium range weather forecasts reanalyses at the lateral boundary and over sea by sea surface temperature and sea ice concentrations from the same source the model domain is centered over the european part of arctic russia focusing on the usa river basin near the ural mountains it is demonstrated that with this fine mesh the model simulates the annual course and interannual variability in temperature and precipitation quite realistically furthermore the model captures the timing of permanent snow cover very accurately with less than 1 week discrepancy in both autumn and spring an interpretation of modeled subsurface layer temperatures in terms of mean annual temperature and a frost index reveals that the model captures present day regional permafrost patterns to a high degree of accuracy

the gabbro eclogite transformation an oxygen isotope and petrographic study of west alpine ophiolites both magmatic and eclogitic parageneses are preserved in the gabbros of western alpine ophiolites samples with relic magmatic mineralogies display partial transformation to eclogitic assemblages along cracks and grain boundaries gabbros with eclogitic mineralogies contain zoned pseudomorphs after olivine comprising talc rich cores with kyanite mg chloritoid and omphacite in outer cores and garnet rims the compositional zonation of these olivine pseudomorphs closely parallels that shown by olivines in hydrothermally altered ocean floor gabbros the eclogitic gabbros are hydrous containing paragonite zoisite and other water bearing minerals and it has been suggested that water was introduced during high pressure metamorphism however the similarity of olivine alteration patterns to those of ocean floor gabbros suggests that hydration and local metasomatism leading to the stability of aluminous minerals in olivine sites occurred during hydrothermal alteration prior to subduction oxygen isotope systematics are consistent with this proposal alpine gabbros with magmatic relics have a mean 18o value of 5 7 0 7 similar to that of unaltered oceanic crust whereas eclogitic gabbros have a mean 18o value of 4 8 0 9 this statistically significant difference is consistent with the eclogitic samples having undergone high temperature ocean floor alteration the preservation of magmatic and hydrothermal 18o values in ocean floor gabbros that have been metamorphosed at 2 2 5 gpa 60 75 km implies that the deeper levels of ocean crust have not experienced pervasive fluid flow during subduction or subsequent exhumation magmatic assemblages were preserved despite an overstep of eclogitization reactions by at least 0 6 1 1 gpa implying that equilibrium was not attained in undeformed parts of the system because of slow diffusion in water deficient rock volumes

massive fluid influx beneath the colorado plateau usa related to slab removal and diatreme emplacement evidence from oxygen isotope zoning in eclogite xenoliths

sm nd isotopic evidence on the age of eclogitization in the zermatt saas ophiolite rocks within the zermatt saas ophiolite of the western alps have undergone eclogite facies metamorphism during subduction prior to the alpine collision the metamorphic history of these rocks is well defined with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite subsequent greenschist alteration occurs adjacent to faults veins and metasediments away from such sources of water retrogression is very limited sm nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 18 ma the large uncertainty is due to the presence of very small amounts of nd rich epidote present as inclusions within garnet as the closure temperature of garnet to sm nd is thought to be 600dc resetting due to post high pressure diffusion is thought to be insignificant given the fine grained protolith to the sample analysed and its extensive deformation under eclogite facies conditions incomplete homogenization of pre metamorphic isotopic variations is also considered unlikely to be responsible for the young age a tertiary age of eclogitization means that models of early alpine evolution based on the cessation of high pressure metamorphism in the cretaceous need to be revised

distribution of alteration zones in the upper oceanic crust aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics five zones are recognized in the troodos ophiolite and in situ oceanic crust 1 a sea floor weathering zone swz 2 a low temperature zone ltz 3 a transition zone tz 4 an upper dike zone udz and 5 a mineralized zone mz the mz may be overlain by the swz ltz or tz but in the known examples it is always underlain by the udz the development of the swz is dependent upon the duration of cold seawater circulation which is dependent upon seafloor topography and the rate and nature of sedimentation the boundary between the low and high temperature zones is largely controlled by permeability contrasts lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust this along with the evidence for disequilibrium assemblages implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects

the ophiolite bearing schistes lustres nappe in alpine corsica a model for the emplacement of ophiolites that have suffered hp lt metamorphism

focused fluid flow during subduction oxygen isotope data from high pressure ophiolites of the western alps abstract during subduction hydrothermally altered oceanic crust undergoes extensive dehydration the water released migrates upwards and may escape along the slab mantle interface or continue rising to hydrate the mantle overlying the subducting slab oxygen isotope ratios from eclogitic ophiolites of the western alps correlate closely with lithology samples with little altered basalt as a protolith have 18 o values of 5 1 6 0 material representing low temperature alteration zone rocks have 18 o values of 5 9 6 6 and rocks orginating in high temperature alteration zones have 18 o values of 4 6 5 3 these isotopic values closely approach those of the ocean crust and unmetamorphosed ophiolites these data suggest that the alpine rocks have not experienced significant fluid flow during metamorphism implying that the escape of fluid from subducted ocean crust is highly focused probably along structurally controlled pathways to the top of the slab the fluid may then continue largely along the slab mantle interface although a portion of it rises further to hydrate the overlying mantle

evolutionary model for alpine corsica mechanism for ophiolite emplacement and exhumation of high pressure rocks a tectonic model of alpine corsica is proposed based on geological studies its evolution starts after the jurassic with intraoceanic subduction followed by mid cretaceous subduction of the european continental margin under the oceanic segment of the adriatic plate after subduction of the continental crust to a depth of 150 km slices of crustal material are buoyantly uplifted together with high pressure oceanic rocks ophiolites and schistes lustres high pressure low temperature continental gneiss units overthrust the outer segments of the european crust while producing a normal sense motion along the upper surface of the rising crustal body during the eocene the closure of the remnant ligurian oceanic basin separating the proto corsican belt and adria resulted in a second orogenic phase with the emplacement of unmetamorphosed ophiolitic nappes which overthrust the previously exhumed and eroded hp belt this corsican model suggests an original evolutionary path for orogenic belts when continental collision is preceded by intraoceanic subduction

an o isotope profile through the hp lt corsican ophiolite france and its implications for fluid flow during subduction abstract subduction zones are important sites for fluid generation as the downgoing slab undergoes progressive high pressure metamorphism these fluids are thought to play an important role in the generation of arc magmas but their affect on the subducted slab itself is less clear in this study the stable isotope geochemistry of the corsican ophiolite which is variably affected by subduction related metamorphism was investigated to assess the impact of subduction zone fluid flow on the downgoing slab a total of 170 stable isotope analyses oxygen and carbon showed the following features 1 overall the ophiolite preserves the same oxygen isotope profile with depth as unmetamorphosed ophiolites 2 the variation in oxygen isotope values for different areas does not correlate with metamorphic grade 3 oxygen and carbon isotope values for calcite in the ophiolite are not in equilibrium with the ophiolite but are similar to those of the overlying calc schists schistes lustres and 4 hydrogen isotopes indicate that serpentinitisation of ultramafics occurred during sub seafloor hydrothermal alteration and probably also during tectonic emplacement of the ophiolite these features indicate that large scale fluid flow did not affect the corsican ophiolite during subduction zone metamorphism calcite in the ophiolite was probably precipitated from co2 bearing fluids derived from decompression dehydration reactions affecting the overlying schistes lustres during exhumation of the ophiolite bearing schistes lustres nappes given that ocean crust must undergo at least some dehydration during prograde subduction zone metamorphism the evolved fluids must be either highly channelled or exit the slab relatively quickly so as to not interact with it the results of this study indicate that the important fluid pathways on corsica have yet to be identified but are probably the shear zones and fault systems that bound different slices of the high pressure nappe stack

subduction of water into the mantle history of an alpine peridotite the erro tobbio peridotite western alps is a slice of subcontinental mantle that underwent pre jurassic passive rifting followed by sea floor hydration and cretaceous alpine subduction analysis of its extension and subduction related structures and metamorphism suggest the dip below the adria plate of both the extensional detachments and the subduction plane high pressure boudinage structures of rigid metarodingite within ductile antigorite serpentinite prove that antigorite survived subduction and remained stable at eclogite facies conditions subduction of serpentinized peridotites from ophiolite slabs is here considered the most effective mechanism of bringing water to great depth within the mantle

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains