fluid and element cycling in subducted serpentinite a trace element study of the erro tobbio high pressure ultramafites western alps nw italy cycling of exogenic components diopside and ti clinohumite are the oceanic serpentinization of peridotites and the influence of such the high pressure minerals acting as repositories for ree and sr an alteration on element cycling during their subduction dewatering and for high field strength elements hfse respectively the are here investigated in a mantle slice erro tobbio peridotite first aqueous fluid equilibrated with such an assemblage is enriched in exposed to oceanic serpentinization and later involved in alpine cl and alkaline elements but strongly depleted in ree and subduction partial dewatering and formation of a high pressure hfse less than chondrite abundances sr is low 0 2 1 6 olivine titanian clinohumite diopside antigorite assemblage chondrites although selectively enriched relative to light ree in the peridotites and in veins previous work indicates that highpressure veins include primary brines representing a residue after crystallization of the vein assemblage and containing recycled oceanic cl and alkalis to reconstruct the main changes during oceanic

lead isotope geochemistry of paleoproterozoic layered intrusions in the eastern baltic shield inferences about magma sources and u th pb fractionation in the crust mantle system abstract u pb data for plagioclase and sulfide are reported from the burakovka and olanga layered mafic ultramafic complexes in karelia these 2 44 2 45 ga complexes differ in the age of their host rocks and in post emplacement metamorphic history acid leaching of optically discrete plagioclase populations revealed three components of pb present in the olanga plagioclases initial radiogenic and alteration related leaching in hcl and hno3 was found to remove most of u and radiogenic pb from plagioclase and to fractionate u from pb the alteration component abundant in the olanga plagioclase is only partially removed by leaching the olanga sulfides also contain a significant radiogenic component plagioclase from the unmetamorphosed burakovka complex yields more reliable initial pb ratios the pb isotopic compositions of plagioclase from the three intrusions of the olanga complex form a single steep linear trend which shows excess scatter due to residual radiogenic and alteration pb calculated single stage 1 of about 7 9 in the kivakka intrusion is close to the archean depleted mantle value but combined pb sr and nd isotopic systematics suggest the presence of an enriched component from either lithospheric or slightly contaminated plume source internal variations in initial pb and sr isotopic ratios and pb isotopic systematics of sulfides are explained by late magmatic fluid exchange with 2 7 ga country rocks the burakovka complex emplaced into the pre 3 1 ga crust shows large correlated variations in 207pb 204pb nd t and 87sr 86sr t in plagioclase initial pb nd and sr isotopic variations in the burakovka complex are interpreted as a result of a multistage contamination process including contamination en route and during emplacement and local assimilation of enclosing gneiss in addition the burakovka mantle source was probably modified with addition of sediment derived pb by a subduction zone fluid time integrated th u 4 3 in mantle sources and crustal contaminants of the olanga and burakovka complexes are uniform and are similar to the archean mantle value this implies that u pb fractionation in the archean paleoproterozoic crust mantle system probably occurred by hydrothermal pb transfer that fractionated pb from u but not th from u

chlorine cycling during subduction of altered oceanic crust eclogitic rocks that have experienced devolatilization with little or no interaction with external fluid sources can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction thus they can be used to evaluate the extent of the recycling of volatile elements such as chlorine we report new oxygen isotope ratios of omphacite and fluid inclusion data determined from eclogitic metagabbros of the rocciavre massif italian alps the data are compared with those obtained for the monviso cyclades and the franciscan complex high pressure rocks in all localities relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts rocciavre monviso and franciscan complex or garnet cyclades salinity estimates of the inclusion fluids range from 32 to 45 wt nacl in rocciavre 17 to 21 wt nacl in monviso and are similar to seawater in other areas omphacite and bulk rock 18o values of rocciavre 5 1 6 8 and monviso 3 0 5 3 metagabbros are markedly lower than those of cyclades 6 8 14 3 and the franciscan 6 7 13 1 metabasites the fluid salinity 18o systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites this suggests that high pressure metamorphism and associated processes did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios inherited from a stage of sea floor hydrothermal alteration under low basaltic layer and high temperature gabbroic layer conditions respectively extrapolating the estimated h2o and cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm cl could be recycled into the mantle during subduction this yields a cl h2o ratio of 3 6 to 7 5 10 2 for the subducted oceanic crust which is similar to e morb on the basis of available cl isotopic data we infer that a large proportion 70 of the cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid ocean ridge basalt 37cl 4 7

stable isotope geochemistry and phase equilibria of coesite bearing whiteschists dora maira massif western alps peak metamorphic temperatures for the coesite pyrope bearing whiteschists from the dora maira massif western alps were determined with oxygen isotope thermometry the 18o smow values of the quartz after coesite 18o 8 1 to 8 6 n 6 phengite 6 2 to 6 4 n 3 kyanite 6 1 n 2 garnet 5 5 to 5 8 n 9 ellenbergerite 6 3 n 1 and rutile 3 3 to 3 6 n 3 reflect isotopic equilibrium temperature estimates based on quartz garnet rutile fractionation are 700 750 c minimum pressures are 31 32 kb based on the pressure sensitive reaction pyrope coesite kyanite enstatite in order to stabilize pyrope and coesite by the temperature sensitive dehydration reaction talc kyanite pyrope coesite h2o the a h2o must be reduced to 0 4 0 75 at 700 750 c the reduced a h2o cannot be due to dilution by co2 as pyrope is not stable at x co2 0 02 t 750 c p 30 kb in the absence of a more exotic fluid diluent e g ch4 or n2 a melt phase is required granite solidus temperatures are 680 c 30 kb at a h2o 1 0 and are calculated to be 70 c higher at a h2o 0 7 consistent with this hypothesis kyanite jadeite quartz bands may represent a relict melt phase peak p t f h2o estimates for the whiteschist are 34 2 kb 700 750 c and 0 4 0 75 the oxygen isotope fractionation between quartz 18o 11 6 and garnet 18o 8 7 in the surrounding orthognesiss is identical to that in the coesitebearing unit suggesting that the two units shared a common final metamorphic history hydrogen isotope measurements were made on primary talc and phengite d smow 27 to 32 on secondary talc and chlorite rite after pyrope d 39 to 44 and on the surrounding biotite d 64 and phengite d 44 gneiss all phases appear to be in nearequilibrium the very high d values for the primary hydrous phases is consistent with an initial oceanicderived connate fluid source the fluid source for the retrograde talc chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization the similar d but dissimilar 18o values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium but not oxygen isotope equilibrium the unusual hydrogen and oxygen isotope compositions of the coesite bearing unit can be explained as the result of metasomatism from slab derived fluids at depth

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

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

diachronous closure of the shuanghu paleo tethys ocean constraints from the late triassic tanggula arc related volcanism in the east qiangtang subterrane central tibet abstract the tectonic evolution of the shuanghu paleo tethys ocean is an important but poorly understood issue in the geology of the tibetan plateau and tethys here we present a new and detailed petrological geochronological major and trace elemental sr nd hf isotopic dataset for the late triassic tanggula volcanic sequence in the east qiangtang subterrane central tibet which together with previously published data would provide critical constraints on the evolution of the shuanghu paleo tethys ocean the volcanic sequence is constituted by two nearly concentric belts namely the outer belt of rhyolites and the inner belt of granite porphyries la icp ms zircon u pb dating indicates that the granite porphyries formed at ca 221 ma later than the rhyolites by ca 15 myr they are characterized by enrichments in large ion lithophile elements liles and light rare earth elements lrees with significant negative anomalies at nb ta and ti but positive anomalies at th u and pb isotopic compositions exhibit enrichment signatures with hf t values ranging from 14 1 to 5 7 and nd t values from 8 84 to 7 06 geochemical characteristics indicate that these volcanic rocks were formed by progressive melting of the subducted sediments accumulated on the shuanghu paleo tethys ocean floor which originated from the weathered paleozoic continental crust in the qiangtang terrane combined with the geological background we further propose that the eastern tanggula changdu segment of the shuanghu paleo tethys ocean was still northward subducting at ca 221 ma the shuanghu paleo tethys ocean has an diachronous closing process where the eastern tanggula changdu segment closed later than the western gangma co shuanghu segment by 20 myr

activation of pacific mantle plumes during the carboniferous evidence from accretionary complexes in southwest japan the identification of ancient activity of large igneous provinces which may be surface manifestations of giant mantle plumes is of primary importance in understanding the evolution of the earth system because such major magmatic redistribution could have profound effects on the global environment the immobile trace element compositions of metamorphosed basaltic rocks called greenstones found in accretionary complexes of southwest japan suggest origins in accreted oceanic lavas at pacific hotspots rather than at mid oceanic ridges furthermore some of these greenstones have geochemical characteristics typical of hotspot lavas in the present superswell region of the south pacific ocean available age data show that the hotspot lavas formed at 350 300 ma greenstones with eruption ages identical to those in southwest japan are widely distributed in the circum pacific region these observations provide evidence for activity of a giant pacific mantle plume during the carboniferous the onset of such giant plume magmatism may have preceded that of the geomagnetic quiet zone

chlorine isotope chemistry of serpentinites from elba italy as an indicator of fluid source and subsequent tectonic history chlorine concentrations and 37cl and d values are reported for serpentinites in different tectonic positions on elba italy serpentinites from high in the nappe sequence contain 0 01 0 1wt cl with 37cl values from 0 9 to 1 4 samples with positive 37cl values occur beneath gabbro and basalt and are interpreted as having been serpentinized by interaction with seawater samples with negative 37cl values are in direct contact with sedimentary rocks these samples were juxtaposed against and or buried by sediments in response to low angle normal faulting on the seafloor and serpentinization probably occurred by subsequent interaction with sedimentary pore fluids most samples from a structurally lower nappe and from areas affected by contact metamorphism lost their original cl during heating low d values for all samples suggest postserpentinization interaction with meteoric water but neither cl nor o isotope ratios were affected by this interaction our results demonstrate that chlorine can be retained in obducted serpentinites up to low greenschist conditions and that the cl isotope composition of such serpentinites preserves a record of seafloor tectonic processes

ultra high pressure metamorphism and orogenesis an alpine perspective article i nfo the pressure temperature time deformation evolution of the high to ultra high pressure units of the western alps has been progressively refined in the last 40 years leading to several paradigm shifts in the understanding of orogenesis this set of information combined with cretaceous tertiary plate kinematic reconstructions and mesozoic palaeogeography indicates that the western alps represent the amalgamation of a cretaceous and an eocene orogen which developed at the expense of the adriatic and european rifted margins respectively in the cretaceous nne directed drift of the adriatic plate parallel to the inherited jurassic structural trends led to the development of a highly oblique subduction boundary in this setting the canavese zone and parts of the sesia zone derived from the hyper extended adriatic margin underwent deformation and metamorphism at conditions ranging from sub greenschist to eclogite facies a lower eocene switch to nnw directed motion perpendicular to the rift related jurassic structural trends culminated in the collision between the proto alps and the brianconnais block at ca 44 ma as a result the brianconnais domain and parts of its hyper extended margin preserved in the u hp piemonte units were accreted to the orogen continued convergence was subsequently accommodated by subduction of the more external valaisan basin and of the thinned european margin the arrival of thick european continental crust at the subduction zone at ca 35 30 ma marked the onset of the final continental collision to a first order the orogen grew through the progressive episodic accretion of units located towards north westerly positions accretion of subducting units at the front of the orogen was coeval with kinematic reworking of tectonic contacts in the hangingwall units locally resulting in renewed deformation metamorphism

stable isotope δ18o δd δ37cl evidence for multiple fluid histories in mid atlantic abyssal peridotites odp leg 209 abstract serpentinized abyssal peridotites sampled by the ocean drilling program leg 209 along the mid atlantic ridge near the 15 20 n fracture zone have been analyzed for oxygen hydrogen and chlorine isotope compositions in order to determine isotopic behavior under a wide range of serpentinization conditions and place constraints on fluid history oxygen and hydrogen thermometry suggests peak serpentinization temperatures of 300 500 c serpentine separates have low d values possibly due to a magmatic fluid component or low temperature exchange during seafloor weathering chlorine geochemistry focused on three holes 1274a and 1272a serpentinized peridotites and 1268a serpentinite locally altered to talc concentrations of both water soluble chloride wsc and structurally bound chloride sbc are significantly lower at hole 1268a compared to holes 1274a and 1272a the 37cl values for wsc and sbc of serpentinites in holes 1274a and 1272a are slightly positive avg wsc 0 20 n 22 and avg sbc 0 35 n 22 representing typical seawater hydration conditions commonly determined for abyssal peridotite the sbc of serpentinites from hole 1268a are also positive avg 0 63 whereas the sbc in talc dominated samples is negative avg 1 22 the wsc of both talc and serpentine dominated samples are also negative avg 0 15 we interpret the chlorine isotope data to preserve a record of multiple fluid events as seawater hydrated the peridotite 37cl was preferentially incorporated into the forming serpentine and water soluble salts yielding similar 37cl values on a regional scale as sampled by holes 1268a 1274a and 1272a the resultant pore fluid was left depleted in 37cl locally hole 1268a this evolved fluid was remobilized possibly due to the initiation of hydrothermal circulation in response to emplacement of a mafic magma body the low 37cl pore fluids attained elevated sio2 and sulfur concentrations due to interaction with the gabbroic intrusion and when ascending through the surrounding serpentinite caused formation of isotopically negative talc this secondary fluid also flushed the preserved serpentinite of its previously formed salts resulting in negative 37clwsc values the 37clsbc values of the serpentinite samples remained unmodified by reaction with the secondary fluid

early permian east ujimqin mafic ultramafic and granitic rocks from the xing an mongolian orogenic belt north china origin chronology and tectonic implications abstract the east ujimqin complex located north of the erenhot hegenshan fault north china is composed of mafic ultramafic and granitic rocks including peridotite gabbro alkali granite and syenite we investigated the tectonic setting age and anorogenic characteristics of the xing an mongolian orogenic belt xmob through field investigation and microscopic and geochemical analyses of samples from the east ujimqin complex and la mc icp ms zircon u pb dating of gabbro and alkali granite petrographic and geochemical studies of the complex indicate that this multiphase plutonic suite developed through a combination of fractional crystallization assimilation processes and magma mixing the mafic ultramafic rocks are alkaline and have within plate geochemical characteristics indicating anorogenic magmatism in an extensional setting and derivation from a mantle source the mafic ultramafic magmas triggered partial melting of the crust and generated the granitic rocks the granitic rocks are alkali and metaluminous and have high fe fe mg characteristics all of which are common features of within plate plutons zircon u pb geochronological dating of two samples of gabbro and alkali granite yielded ages of 280 8 1 5 and 276 4 0 7 ma placing them within the early permian the zircon hf isotopic data give inhomogeneous e hf t values of 8 2 14 7 for gabbroic zircons and extraordinary high e hf t values 8 9 12 5 for the alkali granite in magmatic zircons thus we consider the east ujimqin mafic ultramafic and granitic rocks to have been formed in an extensional tectonic setting caused by asthenospheric upwelling and lithospheric thinning the sources of mafic ultramafic and granitic rocks could be depleted garnet lherzolite mantle and juvenile continental lower crust respectively all the above indicate that an anorogenic magma event may have occurred in part of the xmob during 280 276 ma

chlorine isotope evidence for multicomponent mantle metasomatism in the ivrea zone abstract metasomatism of the finero peridotite ivrea zone italy has previously been ascribed to mantle melts generated in a rift or plume setting and or to subduction fluids we combine cl h and o stable isotope geochemistry with petrologic and major and trace element data to further constrain sources and setting of the metasomatic agents and to assess fluid infiltration mechanisms relatively unmetasomatized samples from the balmuccia peridotite serve as our control population 37cl 0 7 to 0 4 n 13 we analyzed 47 samples from four finero rock types 1 spinel harzburgite with minor amphibole phlogopite 2 harzburgite with several percent phlogopite amphibole 3 amphibole rich segregations and 4 phlogopite segregations 37cl values range from 2 0 to 2 1 and cannot be reconciled with metasomatism by a single homogeneous fluid three components describe most of the sample variability a endmember spinel peridotite as represented by the balmuccia samples b a hydrated clinopyroxenite component with low 37cl and high si al ca and d responsible for formation of type 3 amphibole segregations and c a component with high 37cl d cl cr lile and hfse that formed type 4 phlogopite segregations type 1 and 2 peridotite compositions are consistent with different degrees of mixing between these three components centimeter scale isotopic gradients indicate that much of the fluid infiltration was channelized rather than pervasive the data support metasomatism in a forearc wedge with fluids sourced from at least two subducted components with distinct chemical and isotopic compositions e g 37cl 2 overall our results show that relatively shallow and heterogeneous fluid release events from the downgoing slab can profoundly modify the mantle wedge

late pleistocene to holocene activity at bakening volcano and surrounding monogenetic centers kamchatka volcanic geology and geochemical evolution the different roles of variable mantle sources and intra crustal differentiation processes at bakening volcano kamchatka and contemporaneous basaltic monogenetic centers are studied using major and trace elements and isotopic data three suites of volcanic activity are recognized 1 plateau basalts of lower pleistocene age 2 andesites and dacites of the bakening volcano the new bakening volcano dacitic centers nearby and 3 contemporaneous basaltic cinder cones erupted along subduction zone parallel n s faults age data show that the last eruptions in the bakening area occurred only 600 1200 years ago suggesting the volcano is potentially active major element variations and petrographic observations provides evidence for a fractionation assemblage of olivine clinopyroxene plagioclase magnetite within the basaltic suite the fractionation in the andesites and dacites is dominated by amphibole clinopyroxene orthopyroxene and plagioclase plus minor amounts of magnetite and apatite the youngest cpx opx andesites of bakening main volcano deviate from that trend their source was probably formed by mixing of basaltic magmas into the silicic magma chamber of the bakening volcano overall trace element patterns as well as the sr nd pb isotopic compositions are quite similar in all rocks despite large differences in their chemical composition from basalt to rhyodacite in detail however the andesite dacites of the central bakening volcano show a stronger enrichment in the more incompatible elements and depletion in hree compared to the monogenetic basaltic centers this results in a crossing of the ree pattern for the two suites the decrease in the hrees can be explained by amphibole fractionation a slab component is less likely because it would result in fractionation of the hree from each other which is not observed the higher relative amounts of lile in the dacitic and the large scatter in the basaltic rocks must be the result of a variable source enrichment by slab derived fluids overprinting a variable depleted mantle wedge the plateau basalts are less depleted in hfse and show a more fractionated hree pattern these lavas could either result from a slab component or the addition of an oib type enriched mantle in their source q 2000 elsevier science b v all rights reserved

chlorine isotopic composition in seafloor serpentinites and high pressure metaperidotites insights into oceanic serpentinization and subduction processes bulk rock chlorine content and isotopic composition 37 cl were determined in oceanic serpentinites high pressure metaperidotites and metasediments in order to gain constraints on the global chlorine cycle associated with hydrothermal alteration and subduction of oceanic lithosphere the distribution of insoluble chlorine in oceanic serpentinites was also investigated by electron microprobe the hydrothermally altered ultramafic samples were dredged along the south west indian ridge and the mid atlantic ridge the high pressure metamorphic samples were collected in the western alps metaperidotites in the erro tobbio unit and metasediments in the schistes lustres nappe oceanic serpentinites show relatively large variations of bulk rock cl contents and 37 cl values with mean values of 1105 596 ppm and 0 7 0 4 o respectively n 8 1 serpentines formed after olivine meshes show lower cl content than those formed after orthopyroxene bastites in bastites of two different samples cl is positively correlated with al 2 o 3 and negatively correlated with sio 2 these relationships are interpreted as reflecting preferential cl incorporation into the bastite structure distorted by al substituted for si rather than different alteration conditions between olivine and orthopyroxene minerals high pressure metaperidotites display relatively homogeneous cl contents and 37 cl values with mean values of 467 88 ppm and 1 4 0 1 respectively n 7 1 a macroscopic high pressure olivine bearing vein formed from partial devolatilization of serpentinites at 2 5 gpa and 500 600 c shows a cl content and a 37 cl value of 603 ppm and 1 6 o respectively metasediments n 2 show low whole rock cl contents 15 ppm cl that did not allow cl isotope analyses to be obtained the range of negative 37 cl values observed in oceanic serpentinites is likely to result from water rock interaction with fluids that have negative 37 cl values the homogeneity of 37 cl values from the high pressure olivine bearing vein and the metaperidotite samples implies that progressive loss of cl inherited from oceanic alteration throughout subduction did not significantly fractionate cl isotopes chlorine recycled in subduction zones via metaperidotites should thus show a range of 37 cl values similar to the range found in oceanic serpentinized peridotites

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

stable isotopic and trace element evidence for restricted fluid migration in 2 gpa eclogites mineral stable isotopic and trace element studies in 2 gpa banded eclogites of the tauern window eastern alps record mm to cm scale heterogeneities that reflect compositional variations in the accompanying metamorphic fluids a close correlation between dolomite mode and dolomite 18o is consistent with equilibrium partitioning among coexisting minerals and fluids small variations in dolomite 13c values correspond with 18o variations but an overall decrease in dolomite 13c by c 1 o across a 12 cm sample is a relict feature that pre dates eclogite equilibration garnet omphacite and clinozoisite rims show little systematic mineral mineral partitioning behaviour for ti v cr y sr or zr major elements however are well equilibrated among these same minerals despite the apparent lack of mineral mineral trace element equilibration most of the trace elements vary systematically with water activity calculated in each layer trace element behaviour during the eclogite metamorphism thus appears to have been controlled largely by mineral fluid interactions along grain boundaries shallow structural levels in other subduction complexes c 10 45 km typically exhibit fracture controlled permeability and extensive metasomatism but there is no field or geochemical evidence for extensive fluid advection during high pressure metamorphism in the tauern eclogites because most dewatering and devolatilization during tectonic burial occurs prior to eclogite conditions the volumetric fluid rock ratio in eclogites should generally be low low fluid rock ratios coupled with the possible non wetting nature of the fluids permits the production and preservation of fine scale chemical heterogeneities in deeply subducted eclogites and associated fluids however the eventual breakdown at greater depth of volatile bearing dolomite phengite clinozoisite zoisite or amphibole could lead to renewed fracture controlled fluid release from the subducted rocks to regions appropriate for arc magma generation

slab melting its role in continental crust formation and mantle evolution a possible mechanism for continental crust formation slab melting and subsequent melt mantle interaction was examined by geochemical formulation of partial melting and melt solid reactions the modeling results suggest that such a process can reasonably explain the major and trace element compositions of the andesitic bulk continental crust however the sr nd pb isotopic compositions of melting residues in the subducting slab which may have foundered and been stored in the deep mantle do not match those of any proposed geochemical reservoir this may lead to the conclusion that the slab melting did not play the major role in the continental crust formation during the archean

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

horizontal contraction or horizontal extension heterogeneous late eocene and early oligocene general shearing during blueschist and greenschist facies metamorphism at the pennine austroalpine boundary zone in the western alps mylonitic structures related to two orogenic events are described from the upper and lower contacts of the combin zone and the immediately overlying upper austroalpine dent blanche nappe mont mary klippe and the directly underlying lower austroalpine etirol levaz slice the first event late eocene in age commenced during blueschist facies p t conditions but pre dated the peak of subsequent greenschist facies overprint the second event early oligocene in age took place during retrograde greenschist facies conditions most sense of shear indicators associated with the retrograde mylonites indicate top se shearing but subordinate top nw displacing shear sense indicators have also been mapped mylonitic top se shearing appears to be restricted to the combin zone and its upper and lower contacts within the dent blanche nappe and mont mary klippe and at the base of the etirol levaz slice structures were observed which developed during blueschist greenschist facies conditions and are in conjunction with the p t t history of these rocks inferred to be older associated kinematic data indicate a top nw shear sense comparable blueschist greenschist facies shear sense indicators have not been observed in the combin zone nonetheless the foliation in the combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions this indicates that the combin zone and the immediately over and underlying austroalpine units shared a common tectono metamorphic evolution since the late eocene finite strain data reveal oblate strain fabrics which are thought to result from a true flattening strain geometry flow path modelling reveals a general non coaxial deformation regime and corroborates significant departures from a simple shear deformation in the study area mylonitic top se shearing in the combin zone is attributed to early oligocene backfolding and backthrusting of the mischabel phase temperature time curves suggest slight reheating in the monte rosa nappe underneath and cooling in the dent blanche nappe above the combin zone hence confirming a thrust interpretation for this event the top nw displacing structures are thought to result from late eocene emplacement of the dent blanche nappe and the combin zone onto the middle pennine barrhorn series along the combin fault as related structures initiated during mildly blueschist facies conditions in the dent blanche nappe and the underlying combin zone and both were emplaced together onto the greenschist facial barrhorn series it is concluded that the structures developed as the nappes moved upward relative to the earth s surface thus the combin fault is regarded as a thrust the geometry of this structure indicates that the combin fault is an out of sequence thrust that locally cut down section hence top nw out of sequence thrusting caused local thinning of the metamorphic structural section in association with horizontal shortening out of sequence thrusts cutting down section and back thrusts offer the possibility of explaining the pronounced break in the grade of metamorphism across the combin fault i e the contact between the eclogite facial zermatt saas zone and the overlying lower grade combin zone by contractional deformation

hydrothermal lead transfer from mantle to continental crust the role of metalliferous sediments abstract the amount of lead annually transferred from oceanic crust to metalliferous sediments was estimated in order to test the hypothesis that a non magmatic flux of lead causes the pb surplus in the continental crust a pb surplus has been inferred from global crust mantle lead mass balances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts dsdp odp data on the amount of metalliferous sediments in the pacific ocean and along a south atlantic traverse are used to calculate the mean worldwide thickness of 3 1 m for purely metalliferous sediment componens lead isotope ratios of 39 metalliferous sediments from the pacific define mixing lines between continent derived seawater and mantle derived basaltic lead with the most metal rich sediments usually having the most mantle like pb isotope composition we used this isotope correlation and the pb content of the 39 metalliferous sediments to derive an estimate of 130 70 g g for the concentration of mantle derived lead in the purely metalliferous end member mass balance calculations show that at least 12 8 of the lead annually transferred from upper mantle to oceanic crust at the ocean ridges is leached out by hydrothermal processes and re deposited in marine sediments if all of the metalliferous lead is ultimately transferred to the continental crust during subduction the annual flux of this lead from mantle to continental crust is 2 6 2 0 106 kg assuming this transfer rate to be proportional to the rate of oceanic plate production one can fit the lead transfer to models of plate production rate variations through time integrating over 4 ga hydrothermal lead transfer to the continental crust accounts for a significant portion of the pb surplus in the continental crust it therefore appears to be one of the main reasons for the anomalous behavior of lead in the global crust mantle system

a low viscosity wedge in subduction zones geochemical petrologic and seismological observations indicate that there may be high concentrations of water in the region above a subducting slab the mantle wedge which could decrease the viscosity of the mantle locally by several orders of magnitude using numerical models we demonstrate that a low viscosity wedge has a dramatic influence on the force balance in a subduction zone and leads to an observable signal in the topography gravity and geoid a regional dynamic model of the tonga kermadec subduction zone shows that the viscosity of the wedge is at least a factor of 10 smaller than surrounding mantle lithosphere and asthenosphere consistent with estimates from seismic dissipation and deformation experiments

evidence for forearc seafloor spreading from the betts cove ophiolite newfoundland oceanic crust of boninitic affinity abstract the ordovician betts cove ophiolite of newfoundland has a well developed cumulate sequence in which is rooted a sheeted dyke complex that grades up into pillow lavas dominant chromite olivine orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas dykes and lavas have phenocrysts of olivine high cr al chromite orthopyroxene low tio2 clinopyroxene they have high sio2 and mgo contents and depleted u shaped trace element profiles indistinguishable from those of bonin islands boninites field data imply that cumulates dykes and lavas all are comagmatic while geochemical and mineralogical data indicate that all are of boninitic affinity since boninites are only found in forearcs this implies that the betts cove ophiolitic crust formed in a forearc since the entire oceanic crustal section at betts cove is of boninitic affinity then this implies that true seafloor spreading can initiate in forearc

experimental evidence for the origin of lead enrichment in convergent margin magmas it has been proposed1 5 that the low ce pb ratio of subduction related basalts relative to their oceanic counterparts arises by the preferential transfer of lead to the mantle wedge overlying the subducting slab by non magmatic processes fluxing of the mantle wedge by low ce pb fluids generated by the dehydration of sub ducted oceanic crust is one mechanism favoured for this process see for example ref 5 here we report the results of a series of high pressure experiments which confirm that low ce pb fluids coexist with the dominant mineral phases garnet and clinopyroxene produced during high pressure dehydration of altered basalt our results show that the production of subduction zone magmas from mantle sources fluxed by basalt derived fluid is a mechanism by which relatively lead rich cerium poor mantle derived material is added to the continents the lead enrichment of the earth s continental crust is thus a continuing process occurring at conver gent margins

contrasting behavior of noble metal elements during magmatic differentiation in basalts from the cook islands polynesia concentrations of noble metals ir ru rh pt pd and au in ocean island basalts from the cook islands polynesia were determined by improved fire assay and tellurium coprecipitation techniques with an inductively coupled plasma mass spectrometer isotope major element and trace element compositions of these basalts indicate that the present samples include distinctive himu high high 238u 204pb and normal non himu basalts examination based on ni mg fe partitioning between olivine and liquid suggests an only minor effect of accumulation of phenocrysts in governing the compositional variations of the present samples the fractionation trends obtained show monotonic decrease and increase in noble metal elements with decreasing mgo content in himu and non himu basalts respectively these characteristic trends indicate that himu magmas are differentiated by fractional crystallization and have higher sulfide silicate ratios than non himu basalts

age of the banda sea eastern indonesia the age of the banda sea is critical for palaeogeographical reconstructions of eastern indonesia assumed ages for its formation range from mesozoic to neogene1 10 however geophysical evidence shows that the banda sea is floored by oceanic crust11 12 with a minimum age of eocene4 evidence for a sequence of events involving continental breakup and the initiation of spreading that led to the formation of the banda sea should be found in the microcontinents around the margins of the sea reinterpretation of the geological history of these microcontinents based on new data reported here from western irian jaya9 13 shows that they separated from gondwana during the jurassic and strongly supports the suggestion of bowin et al 4 that the banda sea is trapped mesozoic oceanic crust of indian ocean affinities

chlorine isotope variations along the central american volcanic front and back arc volcanic outputs ash tephra and lava samples of 23 volcanic centers from the volcanic front secondary front and back arc in central america were analyzed for their 37cl values with the goal of using chlorine isotopes as a tracer of fluids sourced from the subducting cocos slab 37cl values range from 2 6 to 3 0 with systematic variations along the length of the front values from the northernmost guatemala and el salvador and southernmost costa rica ends of the front have mantle like signatures in contrast 37cl values are both positive and negative in the center of the front nicaragua implying a sediment and or serpentinite derived component geophysical observations are consistent with extensive hydration of the cocos plate offshore of nicaragua in support of a serpentinite derived fluid source fluids from dehydrating serpentinites may also incorporate cl from overlying sediments resulting in a multiple source chlorine signature

origin and evolution of fluids from mud volcanoes in the barbados accretionary complex a large collection of fluids 54 interstitial fluids and four expelled fluids were sampled at the manon site at the outer edge of the barbados accretionary complex these warm fluids up to 20 c are expelled by sub marine 5000 mbsl mud volcanoes consisting of diapirs unchanneled flow and diatremes channeled chlorine stable isotope ratios of these fluids were measured by irms with a reproducibility of 0 05 1 versus smoc standard mean ocean chloride a large range of 37cl between 5 3 and 0 1 is observed data from each volcanic structure describe a mixing between seawater and a low 37cl fluid the whole set of data is interpreted as the result of a mixing between two deep components and seawater the two deep fluids are chemically distinct e g in ca mg k li sr and br contents and br cl ratio they display low and significantly different 87sr 86sr ratios 0 707790 and 0 707892 respectively and 37cl values 4 51 and 5 24 respectively physicochemical processes such as mineralogical transformation diffusion compaction or ion filtration are known to fractionate chlorine stable isotopes and can produce fluids with negative 37cl values ion filtration due to sediment compaction appears to be the more likely process to explain the negative 37cl values observed at the manon site a model for the generation of these signatures is proposed where a residual negative 37cl fluid reservoir is created at the bottom of the prism or the sediment pile further compaction fracturing and or dewatering of the slab may flush out these fluids and focus them towards the decollement zone mixing between the fluids and ultimately with seawater and water released during gas hydrate destabilizations may explain the data set within the individual cores and between the different structures

chlorine isotope constraints on fluid rock interactions during subduction and exhumation of the zermatt saas ophiolite 1 chlorine isotope compositions of high pressure 2 3 gpa serpentinite rodingite and hydrothermally altered oceanic crust aoc differ significantly from high and ultrahigh pressure 3 2 gpa metasedimentary rocks in the aosta region italy texturally early serpentinites rodingites and aoc have bulk 37cl values indistinguishable from those of modern seafloor analogues 37cl 1 0 to 1 0 in contrast serpentinites and aoc samples that recrystallized during exhumation have low 37cl values 2 7 to 0 5 37cl depletion correlates with progressive changes in bulk chemistry hp uhp metasediments have low 37cl values median 2 5 that differ statistically from modern marine sediments median 0 6 cl in metasedimentary rocks is concentrated in texturally early minerals indicating modification of seafloor compositions early in the subduction history the data constrain fluid sources during both subduction and exhumation related phases of fluid rock interaction 1 marine sediments at the top of the downgoing plate likely interacted with isotopically light pore fluids from the accretionary wedge in the early stages of subduction 2 no pervasive interaction with externally derived fluid occurred during subsequent subduction to the maximum depths of burial 3 localized mixing between serpentinites and fluids released by previously isotopically modified metasediments occurred during exhumation in the subduction channel most samples however preserved protolith signatures during subduction to near arc depths

miocene subduction related magmatism in southern sardinia sr nd and oxygen isotopic evidence for mantle source enrichment abstract subduction beneath southern sardinia in miocene times gave rise to a suite of basalts basaltic andesites high k andesites and rhyolitic ignimbrites the monte arcuentu volcanic complex is a typical product of this activity although the volcanic rocks display some tholeiitic affinities in terms of k 2 o sio 2 relations they resemble calc alkaline lavas of the aeolian and aegean arcs the most mafic members of the suite have moderately high ni 164 ppm and cr 550 ppm contents and high mgo 10 wt and are therefore mantle derived mantle normalised trace element patterns show high k rb ba and th but low nb and ta characteristic of subduction related magmas sr contents in almost all samples are particularly low 165 265 ppm 87 sr 86 sr i values range from 0 7054 to 0 7113 whilst nd i is between 0 9 and 8 8 the sr nd isotopic field overlaps those of the high k rocks of roccamonfina and the roman volcanic province and extends that of the aeolian islands and the aegean arc correlations of radiogenic isotope ratios with sio 2 values in bulk rocks are much stronger than with mgo conventionally analysed 18 o whole rock values range from 6 4 in basalt to 12 4 in a high k andesite however 18 o values for clinopyroxene phenocrysts analysed by a laser fluorination method range from 6 17 to 7 46 good correlations are seen between 18 o cpx values and both mgo and sio 2 differences between 18 o whole rock and 18 o cpx values are probably due to late stage alteration of the groundmass and cast doubt on the validity of petrogenetic interpretations based solely on whole rock oxygen isotope analysis the sr nd o isotopic variation can be modelled either by contamination of mantle derived magmas by hercynian continental crust or by extensive enrichment of the mantle source by subducted sediments in the 18 o cpx vs 87 sr 86 sr i diagram crustal contamination hyperbolae are concave upward because of the low sr contents of the monte arcuentu magmas so both crustal contamination trends and mantle enrichment trends can generate the same isotopic relationships however the strong correlations of radiogenic isotopic ratios with sio 2 and the weak correlations with mgo are most easily explained by extensive 2 10 mantle enrichment by subducted siliceous sediments the correlation between sio 2 and 87 sr 86 sr i for the monte arcuentu magmas is very unusual within subduction related magmatism whereas that between 18 o and 87 sr 86 sr appears to show that the suite forms an end member composition for many island arc and active continental margin volcanic suites

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

equilibrium and disequilibrium trace element partitioning in hydrous eclogites trescolmen central alps elements pb th and u whereas phengite hosts ba and is along despite the widespread presence of hydrous phases in subductionwith clinopyroxene and paragonite an important phase for b related systems experimental d trace element values for many however because of their low modal abundance in eclogite facies hydrous phases are lacking to fill this gap we present a set of rocks phengite and paragonite do not control the b whole rock budget d values where min indicates amphibole zoisite we infer that estimated d values from equilibrium phengite paragonite or apatite derived from equilibrium parageneses assemblages can be used as a good approximation for partition of eclogites from trescolmen central alps switzerland these coefficients under the given pt conditions 650 c and 2 0 data can be combined with experimental data for d gpa deformation induced dynamic recrystallization seems to be to estimate d values for the hydrous phases thus circumventing the driving process necessary for a close approach towards trace experimental problems with the direct determination of such values element equilibrium this process should work efficiently during we analysed li be b sr y zr nb ba ce nd sm pb th subduction close to the slab mantle interface affecting sediments and u in coexisting phases by laser ablation microprobe inductively and low t altered mid ocean ridge basalt but is of less importance coupled plasma mass spectrometry many of the values are extremely inside the slab knowledge of d values from equilibrium assemblages low for example nb ba ce th and u are in the lower ppb determined in this study is useful not only for large scale subductionrange in clinopyroxene attainment of equilibrium was evaluated zone modelling but also as an aid in the selection of equilibrium by textural and major and trace element characteristics nonsubassemblages in imperfectly equilibrated samples so that a equilibrated assemblages are common in most eclogite localities crystallization sequence can be derived including trescolmen and using such samples would lead to the derivation of erroneous values for equilibrium partitioning however four of the 10 studied eclogites from trescolmen having homogeneous

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

fluid flow during slab unbending and dehydration implications for intermediate depth seismicity slab weakening and deep water recycling 1 subducting oceanic plates carry a considerable amount of water from the surface down to mantle depths and contribute significantly to the global water cycle a part of these volatiles stored in the slab is expelled at intermediate depths 70 300 km where dehydration reactions occur however despite the fact that water considerably affects many physical properties of rocks not much is known about the fluid flow path and the interaction with the rocks through which volatiles flow in the slab interior during its dehydration we performed thermomechanical models coupled with a petrological database and with incompressible aqueous fluid flow of a dynamically subducting and dehydrating oceanic plate results show that during slab dehydration unbending stresses drive part of the released fluids into the cold core of the plate toward a level of strong tectonic under pressure and neutral slab normal pressure gradients fluids progressively accumulate and percolate updip along such a layer forming together with the upper hydrated layer near the top of the slab a double hydrated zone dhz where intermediate depth seismicity could be triggered the location and predicted mechanics of the dhz would be consistent with seismological observations regarding double seismic zones dszs found in most subduction zones and suggests that hydrofracturing could be the trigger mechanism for observed intermediate depth seismicity in the light of our results the lower plane of the dsz is more likely to reflect a layer of upward percolating fluid than a level of mantle dehydration in our models a 20 30 km thick dsz forms in relatively old oceanic plates without requiring an extremely deep slab hydration prior to subduction the redistribution of fluids into the slab interior during slab unbending also has important implications for slab weakening and the deep water cycle we estimate that over the whole of earth s history a volume of water equivalent to around one to two oceans can be stored in nominally anhydrous minerals of the oceanic lithosphere and transported to the transition zone by this mechanism suggesting that mantle regassing could have been efficient even without invoking the formation of high pressure hydrous minerals

late archean high mg granitoids sanukitoids in the southern karelian domain eastern finland pb and nd isotopic constraints on crust mantle interactions late archean ca 2 73 ga porphyritic high k and high mg granitoids in the nilsia and lieksa areas eastern finland have geochemical signatures similar to those of high mg sanukitoid series from other parts of the karelian domain of the fennoscandian baltic shield and the archean sanukitoid suites from canada the nilsia and lieksa granitoids have low sio2 contents 62 7 67 0 and high mg numbers 0 45 0 52 they are enriched in k2o 2 40 4 73 wt p2o5 0 25 0 42 wt sr 610 850 ppm ba 1200 2300 ppm cr 40 80 ppm and lree la yb n 19 65 they are characterized by a negative eu anomaly eu eu 0 53 0 81 and show strong depletion in u high th u ratio low u pb ratio high 2 values for the source 9 positive nd 2 7 ga values of 0 3 1 4 and nd depleted mantle model ages 2 75 2 86 ga the high 2 values pointing to a significantly older crustal source apparently contradicts the positive nd 2 7 ga values supporting a mantle origin the geochemical and isotopic data suggest that the nilsia and lieksa granitoids originated at ca 2 73 ga from a mantle wedge source enriched in lile u th and pb by recycling of continental material in subduction related slab dehydration processes shortly up to 200 ma before melting a considerably large amount of crustal lead was contributed from subducting sediments into the overlying mantle wedge consequently the isotopic composition of mantle wedge lead was overprinted by crustal lead isotope signatures it is suggested that crustal recycling through subduction zone processes played an important role for the enrichment of the mantle wedge and generation of mantle derived high granitoids in the archean

determination of trace element partition coefficients between water and minerals by high pressure and high temperature experiments leaching technique a refined approach for the experimental determination of fluid composition at elevated pressure and temperature conditions is developed it effectively determines partition coefficients of trace elements between aqueous fluid and minerals this approach is developed on top of many previously known technical achievements however it differs from the previous ones by the use of calibrated base and acid leaching of quenched samples to strip off the precipitates formed from the fluid during the quench this permits the reconstruction of a fluid composition from an experimental charge that consists of a complete mixture of solid and fluid phases this is a significant difference from the previous analogous studies in which the fluid was separated from the solids all through the experiment mass balance calculations and error propagation assess the merits of the present method applied to a fluid antigorite system

hydrothermal alteration of basalt by seawater under seawater dominated conditions fresh mid ocean ridge basalt glass and diabase have been reacted with seawater at 150 300 c 500 bar and water rock mass ratios of 50 62 and 125 using experimental apparatus which allowed on line sampling of solution to monitor reaction progress these experiments characterize reaction under what we have called seawater dominated conditions of hydrothermal alteration in an experiment at 300 c basalt glass undergoing alteration removed nearly all mg2 from an amount of seawater 50 times its own mass in the process the glass was converted entirely to mixed layer smectite chlorite anhydrite and minor hematite removal of mg from seawater occurred as a mg oh 2 component incorporated into the secondary clay this produced a precipitous drop in solution ph early in the experiment accompanied by a dramatic increase in the concentrations of fe mn and zn in solution as mg removal neared completion and the glass was hydrolyzed ph rose again and heavy metal concentrations dropped at water rock ratios of 62 and 125 and 150 300 c the mineral assemblage produced was similar to that at a water rock ratio of 50 solution chemistry however contrasted with the earlier experiment in that mg concentrations in solution were greater and ph lower this caused significant leaching of heavy metals at 300 c nearly all of the na ca cu zn and co2 and most of the k ba sr and mn were leached from the silicates h2s al si and possibly co were also significantly mobilized whereas v cr and ni were not little or no seawater sulfate was reduced although submarine hot spring solutions sampled to date along mid ocean ridges clearly come from rock dominated hydrothermal systems evidence from ocean floor metabasalts and from heat flow studies indicates that seawater dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures

geochronology geochemistry and hf isotope of late triassic magmatic rocks of qingchengzi district in liaodong peninsula northeast china abstract the initiation timing and mechanism of lithospheric thinning of the north china craton ncc was still controversial late triassic igneous rocks especially mantle derived mafic rocks would provide constrains on early mesozoic lithospheric mantle geodynamics and initiation of lithospheric thinning this paper reports late triassic magmatic rocks including lamprophyre diorite dykes and biotite monzogranite cropped out in qingchengzi district of liaodong peninsula northeastern ncc la icpms zircon u pb dating yield ages of 210 227 ma and 224 ma for lamprophyres and biotite monzogranite respectively lamprophyre is ultrapotassic strongly enriched in ree and liles depleted in hfses and negative hf isotopes which are discriminating signatures of crustal source but distinguishingly high compatible element contents indicate the primary magma originated from mantle source a fertile one lamprophyre derived from partial melting of an enriched lithospheric mantle which was modified by slab derived hydrous fluids melts associated with deep subduction between the yangtze craton and the ncc the diorite displays distinct features with relatively enriched nb ta hree and depleted th u which suggest it derived from a relatively depleted source the depletion was caused by break off of the yangtze slab during deep subduction introducing asthenospheric mantle into the source the biotite monzogranite shows adakitic affinity and originated from partial melting of the thickened lower crust with addition of small proportion of mantle material the recognition of late triassic magmatism implies extensional tectonic settings in liaodong peninsula and suggests initiation of lithospheric thinning of north china craton in eastern segment might begin early in late triassic

a mid palaeozoic ocean continent transition in the mazongshan subduction accretion complex beishan nw china new structural chemical and age data constrain the petrogenesis and tectonic evolution

serpentinites of the zermatt saas ophiolite complex and their texture evolution the zermatt saas serpentinite complex is an integral member of the penninic ophiolites of the central alps and represents the mantle part of the oceanic lithosphere of the tethys metamorphic textures of the serpentinite preserve the complex mineralogical evolution from primary abyssal peridotite through ocean floor hydration subduction related high pressure overprint meso alpine greenschist facies metamorphism and late stage hydrothermal alteration the early ocean floor hydration of the spinel harzburgites is still visible in relic pseudomorphic bastite and locally preserved mesh textures the primary serpentine minerals were completely replaced by antigorite the stable assemblage in subduction related mylonitic serpentinites is antigorite olivine magnetite diopside the mid tertiary greenschist facies overprint is characterized by minor antigorite recrystallization textural and mineral composition data of this study prove that the hydrated mineral assemblages remained stable during high pressure metamorphism of up to 2 5 gpa and 650 c the zermatt saas serpentinites thus provide a well documented example for the lack of dehydration of a mantle fragment during subduction to 75 km depth

chlorine stable isotopic composition of basement fluids of the eastern flank of the juan de fuca ridge odp leg 168 abstract this paper presents chlorine stable isotope compositions 37cl of sediment pore waters collected by squeezing sediment cores from the sediment basement interface along an east west transect through the eastern flank of the juan de fuca ridge odp leg 168 these near basement fluids nbf are generally thought to be representative of low temperature fluids circulating in the off axis basaltic crust the 37cl value of the fluid directly sampled from a flow at the base of site 1026 wstp1026 is also reported nbf display 37cl values between 2 09 and 0 12 relative to the standard mean ocean chloride smoc defined as 0 and small variations in chlorinity 4 these data contrast with the homogeneity of 37cl values associated with highly variable chlorinities observed in high temperature on axis fluids m bonifacie j l charlou n jendrzejewski p agrinier j p donval chlorine isotopic compositions of ridge axis high temperature hydrothermal vent fluids chem geol 221 2005 279 288 the nbf 37cl values show a general decreasing trend with distance from the ridge axis except for two fluids when plotted against 18o values the 37cl of the nbf show two different trends this paper discusses the possible contributions on nbf 37cl values of fluid mixing water rock interactions and transport processes diffusion ion membrane filtration that can occur in the igneous basement however as none of these processes can fully explain the observed 37cl variations the potential effect of the sediment cover is also investigated at site 1026 the interstitial pore fluid displays a 37cl signature significantly lower than that of the fluid discharge sample 1 90 and 0 28 respectively this difference demonstrated here cannot be an artifact of the sampling method rather indicates the influence of the sediment cover on nbf 37cl values the potential contributions of physical processes associated with transport compaction e g diffusion ion membrane filtration adsorption ion exchange on nbf 37cl values are qualitatively discussed here but require additional studies for further insights however this study indicates that near basement fluids nbf are not at least for cl isotopic compositions necessarily as representative of fluids circulating in the basaltic crust as initially thought these results add new constraints on cl geodynamics and show that cl isotopes fractionate during low temperature circulation of fluids in off axis and off margin flow contexts but not to the extent observed for active margins fluids circulating at low temperature in the magmatic and or the sedimentary part of the oceanic crust might have played a major role on the 37cl evolution of seawater over geologic time

chlorine isotope variations across the izu bonin mariana arc chlorine isotope ratios were determined for volcanic gas geothermal well ash and lava samples along the izu bonin mariana volcanic front serpentinite clasts and muds from serpentine seamounts conical south chamorro torishima basalts from the guguan cross chain and sediments from ocean drilling program odp sites 800 801 802 and 1149 there is no systematic variation in 37 cl values along the volcanic front in either gas or ash samples in contrast distinct variations occur across the arc implying variations in the fluid source at different depths within the subduction zone serpentinite clasts and serpentine muds from the seamounts tap a source of 30 km depth and have 37 cl values of structurally bound chloride of 0 4 0 4 n 24 identical to most seafloor serpentinites suggesting a serpentinite chrysotile and or lizardite to antigorite transition fluid source tapping deeper levels of the subduction zone 115 130 km depth volcanic gases and ashes have 37 cl values averaging 1 1 1 0 n 29 precisely overlapping the range measured in sediments from odp cores 1 1 0 7 n 11 and limited altered oceanic crust aoc both sediments and aoc are possible cl sources in the volcanic front the guguan cross chain basalts come from the greatest depths and have an average 37 cl value of 0 2 0 2 n 3 suggesting a second serpentine derived source in this case from antigorite breakdown at 200 km depth