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

abyssal serpentinites as gigantic factories of marine salts and oil abstract serpentinites occur worldwide in various geological settings such as in magma starved oceanic basins abyssal serpentinites are long term storage for chlorine and carbon the latter forming dominantly reduced and subordinately oxidized compounds hence serpentinites salts and reduced carbon phases coexist in an abyssal association although huge reservoirs of salts and reduced carbonaceous matter in abyssal serpentinites are thought to be lacking this may be more a reflection of intrinsic limits in research methods than an indication of their absence on the other hand shallow seated hydrocarbon rich mud volcanoes and diapirs consisting of salts hydrocarbons oils and gases and clayey muds including mudded serpentines can provide evidence for such an oceanic association here we search for scientific data in support of the oceanic association and hence we propose a reappraisal of exploitable worldwide salt hydrocarbon reservoirs even gigantic in size abyssal serpentinites are well known source rocks of abiotic gaseous hydrocarbons via fischer tropsch type reaction ftt whereas macromolecular carbonaceous matter mostly bitumen found in serpentinites is postulated to be biogenic this inference is based mainly on three assumptions i ftt in nature does not yield oils ii hydrocarbons derive essentially from the thermal alteration of microbial molecules iii the presence of biomarkers traditionally considered evidence of the biological origin of oils nevertheless some lines of strong evidence disprove these accepted conventions i lab experiments demonstrated that ftt can synthesize significant quantities of liquid hydrocarbons oils under conditions compatible with the deeper and more reduced level of serpentinite hosted hydrothermal systems ii macromolecules generally thought to derive from microbial activity can be abiotically generated as documented in extraterrestrial bodies iii biomarkers are evidence for the proliferation of microbial communities which subsist on serpentinization and ftt products thus they are biological pollutants moreover seawater driven serpentinization consumes water producing huge amounts 11 kg halite m 3 of peridotite of salts that can be stored in the deeper zone of abyssal type hydrothermal systems thus the origin of shallow sited salt deposits even gigantic in size can be related to either the advective upwelling of hot hydrothermal brines due to the dehydration of abyssal serpentinites or to the passive upwelling of buoyant saline geobodies piercement structures are indeed seismically imaged in several localities such as south atlantic the gulf of mexico the serpentinite belt extending from cuba to hispaniola the north sea trench and the zagros orogenic belt similar lines of evidence at the microscopic scale have been found in some hyblean serpentinite xenoliths south eastern sicily being representative of the unmetamorfosed in situ relic of the palaeo tethys ocean upon which sicily and its off shore areas lie in spite of the accepted stereotype of hydrocarbons as fossil fuels and hence finite resources reduced carbon compounds abiotically produced in abyssal serpentinites may be copious protracted over time and thus renewable

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

high salinity fluid inclusions formed from recycled seawater in deeply subducted alpine serpentinite abstract the origin of high pressure brines has been investigated in the erro tobbio peridotite western alps a mantle slice that was exposed to the seafloor of the mesozoic ligurian piedmontese tethys and was later involved in alpine subduction hydrothermal alteration by seawater derived fluids led to replacement of the mantle assemblage by cl bearing serpentine 0 35 wt cl brucite 0 2 wt cl and alkali bearing phyllosilicates 0 2 wt cl 0 2 0 55 wt na2o 1 5 wt k2o relics of these hydrous phases occur in olivine titanian clinohumite antigorite assemblages formed at 2 5 gpa and 550 600 c during partial devolatilization and veining of the hydrothermally altered peridotite the high pressure phases lack chlorine and alkalis and are coeval with fluid inclusions trapped in the syn eclogitic veins the inclusions are salt saturated and contain up to 50 wt cl na k mg and fe high fluid chlorinity was probably achieved during rehydration of relict mantle minerals and deposition of hydrous phases in the veins the data presented suggest that the seafloor hydrothermal signature was inherited by the eclogitic fluid due to partitioning of chlorine and alkalis into the fluid phase the presence of salty brines in eclogitized hydrous peridotites can indicate deep recycling of seawater derived fluids hydrous ultramafic systems can therefore act as large scale carriers of seawater into the mantle

electrical conductivity of clinopyroxene nacl h2o system at high temperatures and pressures implications for high conductivity anomalies in the deep crust and subduction zone

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

experimental evidence for diamond facies metamorphism in the dora maira massif the peak metamorphic conditions of subducted continental crust in the dora maira massif western alps have been revised by combining experimental results in the kcmash system with petrologic information from whiteschists textural observations in whiteschists suggest that the peak metamorphic assemblage garnet phengite kyanite coesite talc originates from the reaction kyanite talc garnet coesite liquid in the experimentally determined petrogenetic grid this reaction occurs above 45 kbar at 730 c at lower pressures talc reacts either to orthopyroxene and coesite or together with phengite to biotite coesite and kyanite the liberated liquid contains probably similar amounts of h2o and dissolved granitic components the composition of the liquid in the whiteschists at peak metamorphic conditions a major unknown in earlier studies was probably very similar to the liquid composition produced in the experiments therefore the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists experimentally determined si isopleths for phengite further constrain peak pressures to 43 kbar for the measured si 3 60 of phengite in the natural whiteschists all these data provide evidence that the whiteschists reached diamond facies conditions the fluid absent equilibrium 4 kyanite 3 celadonite 4 coesite 3 muscovite pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra high pressure uhp metapelites for graphite bearing metapelites and kyanite phengite eclogites forming the country rocks of the whiteschists peak metamorphic pressures of about 44 3 kbar were calculated from this barometer for temperatures of 750 c estimated from garnet phengite thermometry therefore the whole ultra high pressure unit of the dora maira massif most likely experienced peak metamorphic conditions in the diamond stability field while graphite is common in the metapelites diamond has not been found so far the absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism the absence of a free fluid phase in the metapelites and a short residence time in diamond facies conditions resulting in kinetic problems in the conversion of graphite to diamond

superzoned garnets in the coesite bearing brossasco isasca unit dora maira massif western alps and the origin of the whiteschists abstract rare centimeter sized superzoned garnets szgs were discovered in two coesite bearing whiteschists of the brossasco isasca unit biu southern dora maira massif dmm western alps the superzoned garnet consists of a reddish brown almandine core crowded with inclusions of staurolite chloritoid kyanite chlorite and paragonite and of a pinkish pyrope rim with sporadic inclusions of kyanite and magnesian chlorite the core rim contact is relatively sharp and marks the termination of the inclusion rich portion the core composition of the superzoned garnet is almost identical to or slightly richer in mg than that of the rim of porphyroblastic garnet in metapelites from the same unit in the rim of the superzoned garnet mg fe ratio increases abruptly towards the outermost rim whose composition is identical to that of the common pyrope in the whiteschist at the core rim boundary there is no chemical gap chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite the staurolite composition si 2 00 and total r 2 grt cld chl the resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages such as st cld chl vs cld chl ky new and literature data and results of experiments in the kfash system suggest that 1 the superzoned garnet was formed under a single prograde high pressure ultra high pressure hp uhp alpine metamorphism 2 the almandine inclusion rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600 c and less than 16 kbar probably from a former metapelite xenolith included in a variscan granitoid and 3 the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth i e at a stage close to the uhp climax

influence of temperature pressure and fluid salinity on the distribution of chlorine into serpentine minerals abstract serpentinization produces serpentine minerals that have abundant water and fluid mobile elements e g ba cs and cl the dehydration of serpentine minerals produces chlorine rich fluids that may be linked with the genesis of arc magmas however the factors that control the distribution of chlorine into serpentine minerals remain poorly constrained we performed serpentinization experiments at 80 500 c and pressures from vapor saturated pressures to 20 kbar on peridotite orthopyroxene and olivine with

water content and nature of solutes in shallow mantle fluids from fluid inclusions abstract this study discusses new and published data on the composition of fluid inclusions contained in mantle minerals of spinel and garnet peridotite xenoliths in samples from geodynamically distinct settings ethiopian plateau hawaii canary islands and western mediterranean region based on spectroscopic raman and ftir analyses we show that contrary to a commonly held view fluid inclusions either contain relevant amounts of unsuspected h2o or represent a dehydrated composition from multicomponent aqueo carbonic fluids we identify water loss from fluid inclusions through decrepitation stretching and hydrogen diffusion we also show that talc magnesite chlorides and sulfates represent common phases in fluid inclusions talc and magnesite form through reactions of fluids with the surrounding minerals thermodynamic modeling in the mfshc system of observed reactions between fluid inclusions and surrounding mantle minerals provides the basis for predicting water amounts in shallow mantle fluids and suggests x h 2 o in the range of 10 50 mol model hydrous fluids are relatively enriched solutions dominated by si cl and alkalies with significant amounts of ca and s and low levels of mg and fe this study argues that multicomponent hydrous fluids may be widespread in the shallow mantle not only in subduction zones but also in intraplate and extensional settings

chlorine stable isotopes and halogen concentrations in convergent margins with implications for the cl isotopes cycle in the ocean abstract chlorine stable isotopes 37cl and halogen concentrations e g br cl in 168 pore fluids and 23 serpentines and other solids from three subduction zones the nankai trough costa rica and mariana forearc provide critical information on fluid sources flow paths and reaction conditions the 37cl values of pore fluids at the nankai and costa rica subduction zones are significantly more negative minimum 7 8 2 0 3 than seawater value 0 at nankai trough the minimum 37cl value is situated below the decollement and evolves laterally from 7 8 at the most arcward odp site 808 to 7 1 at site 1174 2 km seaward from site 808 and to 5 8 at the reference site 1173 at costa rica along the decollement the minimum 37cl value evolves from 5 5 at the most arcward odp site 1040 1254 to 3 2 at site 1043 1255 1 km seaward and to 0 at the reference site 1039 1253 at both subduction zones the br cl ratios are higher than the seawater value 1 5 10 3 and also show seaward evolutions these pore fluids originate from greater depth arcward at 250 c from hydrous mineral formation that preferentially incorporates 37cl and excludes br in contrast the 37cl values in the pore fluids at the mariana serpentine mud volcanoes are higher than the seawater value 0 3 to 1 8 and the br cl ratios are lower these pore fluid values and the high cl concentrations with positive 37cl values 1 2 to 6 0 in the serpentines support that the upwelling pore fluid originates from dehydration of the subducting slab that releases water enriched in 37cl into the fluid phase the constancy of the ocean 37cl over the past 200 ma suggests that the isotopically fractionated chlorine in serpentinites and the cl exchanged in subduction zones are efficiently recycled back into seawater if the efficiency is

dehydration of subducting serpentinite implications for halogen mobility in subduction zones and the deep halogen cycle we investigated the halogen cl f br and i chemistry of serpentinites that record progressive dehydration during subduction from shallow oceanic environments via increased pressure and temperature conditions to complete breakdown of antigorite the aim is to evaluate the relevance of serpentinites for halogen recycling in subduction zones and for deep mantle recharge of these elements the halogen compositions of the analyzed samples indicate input from seawater and sedimentary sources during initial serpentinization of either subducting lithospheric mantle during slab bending or forearc mantle by uprising slab fluids during the first dehydration stage antigorite brucite olivine h2o fluids with high br cl and i cl ratios are released resulting in residual serpentinites with lower br cl and i cl ratios veins associated with this event and with the final antigorite breakdown antigorite olivine orthopyroxene h2o show higher halogen ratios compared to their adjacent wall rocks and they are similar to those found in arc volcanoes f cl and i cl between ca 0 083 1 5 and ca 0 00038 0 0013 respectively all measured deserpentinization samples show a narrow range in 37cl values between 0 42 and 0 92 overlapping the 37cl values of seafloor serpentinites and confirming that no significant cl isotope fractionation occurs during subduction dehydration of serpentinites our findings document the conservative behavior of halogens during subduction mass balance constraints reveal that serpentinites strongly control the halogen chemistry of deep subduction zone fluids and that descent of rock residues after deserpentinization strongly affects the halogen budget of the mantle

serpentinization of olivine at 300 c and 500 bars an experimental study examining the role of silica on the reaction path and oxidation state of iron abstract the generation of h 2 during serpentinization of ultramafic oceanic crust along earth s mid ocean ridges provides a key energy source for the deep biosphere and has important implications for metal transport and mineral stability in the subseafloor hydrothermal system nonetheless the driving forces and rates of the coupled processes of fe partitioning and h 2 generation during serpentinization are complicated by many factors but especially the activity of sio 2 aq in the serpentinizing fluid to examine the role of this key variable in h 2 generation and fe partitioning between magnetite and fe serpentine during serpentinization san carlos olivine was reacted with a nacl sio 2 aq bearing fluid at 300 c and 500 bars for approximately 90 days using flexible gold cell hydrothermal equipment time series changes in solution chemistry and dissolved h 2 are coupled with magnetic susceptibility mossbauer spectroscopy and fe isotope measurements of reactant olivine and coexisting alteration products to provide a complete description of fe mass transfer and oxidation as a function of reaction progress utilization of talc as reactant not only provided a source of sio 2 aq for serpentinization but effectively prevented brucite formation in keeping with constraints imposed by many natural hydrothermal systems which are inherently open systems moreover this experimental strategy allowed fe derived from olivine dissolution to transfer completely to serpentine and magnetite facilitating reaction path modeling initial changes in solution chemistry indicate buffering of dissolved sio 2 aq owing to the apparently balanced rates of olivine dissolution and serpentine precipitation during this stage of reaction a negligible amount of h 2 is generated suggesting that fe 2 serpentine is stable and the dominant fe bearing product with continued reaction the rate of sio 2 aq production lessens likely due to reduction in surface area of talc such that the continued hydration and hydrolysis of olivine titrates the previously buffered activity of sio 2 aq eventually approaching serpentine brucite coexistence during this secondary stage of reaction the rate of h 2 generation abruptly increases which reflects the formation of magnetite and fe 3 serpentine temporal changes in the isotope composition of dissolved fe throughout these stages likely reflect different mineral alteration and formation processes where initially 1 preferential mass transfer of isotopically light fe into solution occurred during olivine alteration without considerable oxidation as demonstrated by the low h 2 production rate and 2 the preferential removal of isotopically light fe from solution in connection with enhanced magnetite and h 2 formation during the later stages of reaction post experimental analysis of the fe isotopic composition of fe bearing minerals demonstrates statistically negligible enrichment of 56 fe of magnetite relative to olivine and serpentine consistent with observations from natural altered oceanic crust observations from this study demonstrate the utility of using coupled chemical isotopic and analytical approaches to ascertain time series changes in rates of mass transfer and mechanisms of oxidation of fe during serpentinization of olivine ultimately these insights provide important constraints for the chemical and fe isotopic evolution of fluids and minerals in ultramafic hosted hydrothermal systems at mid ocean ridges

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

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

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

the partition of nickel between olivine magma and immiscible sulfide liquid abstract the ni contents of glasses in products of earlier experiments on the ni partition between olivine and immiscible sulfide liquid have been determined allowing the distribution coefficients for ni fe 2 exchange between sulfide and silicate k d 1 and olivine and silicate k d 2 to be calculated ni is relatively enriched in s bearing mg silicate liquids by complexing with dissolved s in consequence k d 1 and k d 2 are systematically dependent on both silicate liquid and sulfide liquid composition the present study re affirms the reliability of the existing ni partition data for coexisting olivine and sulfide and therefore supports the earlier conclusion that an immiscible sulfide liquid cannot be a normal product of upper mantle magmatic processes in s bearing systems k d 2 will be somewhat lower than the value for the corresponding s free system and this may account for the discrepancies between previously reported partition data

strontium isotope zoning in garnet implications for metamorphic matrix equilibration geochronology and phase equilibrium modelling in principle garnet growth rates may be calculated from 87 rb 86 sr and 87 sr 86 sr measurements in garnet subsamples and the surrounding rock matrix because of low rb sr garnet should passively record the matrix decay of 87 rb to 87 sr as a progressive increase in 87 sr 86 sr from core to rim this concept was tested by collecting rb sr data for five garnet grains from four major orogenic belts eastern vermont c 380 ma western new hampshire c 320 ma southern chile c 75 ma and northwestern italy c 35 ma both normal sr isotope zoning increasing 87 sr 86 sr from core to rim and inverse sr zoning decreasing 87 sr 86 sr from core to rim were observed garnet and matrix iso tope data commonly yielded grossly inaccurate model ages incomplete rb and sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v measured zoning patterns and the age disparities initially the reactive matrix is dominated by rapidly equilibrating rb rich mica which imparts high 87 sr 86 sr values in garnet cores progressive participation of slower equilibrating sr rich plagioclase buffers or even reduces 87 sr 86 sr possibly leading to flat or decreas ing 87 sr 86 sr from garnet cores to rims unusually high 87 sr 86 sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages so metamorphic ages growth rates and associated heating and loading rates are likely suspect although rb sr may be the most suscep tible because of the profound disparities between mica and feldspar zircon reactivity might influence the lu hf system by up to a few per cent the sm nd system seems generally immune to these effects pseudosection analysis and conventional garnet geochronology which presume complete matrix equilibration during metamorphism may require modification to account for differences between whole rock v reactive matrix compositions

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

partitioning of moderately siderophile elements among olivine silicate melt and sulfide melt constraints on core formation in the earth and mars abstract this study investigates the effects of variations in the fugacities of oxygen and sulfur on the partitioning of first series transition metals v cr mn fe co ni and cu and w among coexisting sulfide melt silicate melt and olivine experiments were performed at 1 atm pressure 1350 c with the fugacities of oxygen and sulfur controlled by mixing co2 co and so2 gases starting compositions consisted of a cao mgo al2o3 sio2 feo na2o analog for a barred olivine chondrule from an ordinary chondrite and a synthetic komatiite the f o 2 f s 2 conditions ranged from log f o 2 7 9 to 10 6 wi log f s 2 values ranging from 1 0 to 2 5 our experimental results demonstrate that the f o 2 f s 2 dependencies of sulfide melt silicate melt partition coefficients for the first series transition metals are proportional to their valence states the f o 2 f s 2 dependencies for the partitioning of fe co ni and cu are weaker than predicted on the basis of their valence states variations in f o 2 f s 2 conditions have no significant effect on olivine melt partitioning other than those resulting from f o 2 induced changes in the valence state of a given element the strong f o 2 f s 2 dependence for the olivine silicate melt partitioning of v is attributable to a change of valence state from 4 to 3 with decreasing f o 2 our experimentally determined partition coefficients are used to develop models for the segregation of sulfide and metal from the silicate portion of the early earth and the shergottite parent body mars we find that the influence of s is not sufficient to explain the overabundance of siderophile and chalcophile elements that remained in the mantle of the earth following core formation important constraints on core formation in mars are provided by our experimental determination of the partitioning of cu between silicate and sulfide melts when combined with existing estimates for siderophile element abundances in the martian mantle and a mass balance constraint from fe the experiments allow a determination of the mass of the martian core 17 to 22 wt of the planet and its s content 0 4 wt these modeling results indicate that mars is depleted in s and that its core is solid

fluid melt rock interaction in mafic eclogites and coesite bearing metasediments constraints on volatile recycling during subduction the question as to whether the metasomatizing slab agent needed to form island arc magmas is an aqueous fluid or a hydrous melt requires a knowledge of fluid melt rock interaction during high pressure low temperature metamorphism in the western alps typical anhydrous mafic eclogites monviso massif and coesite bearing metasediments dora maira massif have experienced a prograde p t path characteristic of a mature cold subduction zone microstructural petrologic geochemical and fluid inclusion studies show that fluid flow was limited during eclogite and high grade blueschist facies metamorphism most of the fluids were driven off the rocks at pressures lower than 1 and 1 6 gpa respectively with increasing pressure and temperature the remaining fluid phase was released during crystal plastic flow processes or devolatilisation reactions but retain the host rocks to form veins partial melts or dense hydrous silicates tectonic erosion at modern convergent margins delivers large amounts of terrigenous sediments kmash system in subduction zones in contrast to the basalt and peridotite systems the kmash system is characterized by a wide range of pressure sensitive reactions which allow the reconstruction of dehydration hydration depths in the subducted slab independently of the thermal structure of subduction zones terrigenous sediments will begin to melt at pressures 2 gpa the melt phase can absorb tremendeous quantities of water and represent major sinks for devolatilisation fluids dense hydrous silicates will form in any subducted sediments at all depths these silicates can store significant amount of water in their structure in the subducted oceanic crust and upper mantle heterogeneous distribution of fluids inherited from hydrothermal alteration at mid ocean ridges will cause contemporaneous dehydration and hydration reactions on a local scale rather than large scale fluid infiltration in the overlying mantle wedge the situation where a cold competent and fluid rich subducted slab is bounded above by a hot relatively weak and anhydrous material is likely to act against buoyant rise of devolatilisation fluids by forcing the fluids to flow downwards at sub arc depths 50 10 to 80150 km and for any geothermal gradient continuous internal buffering of volatile activities will retain the fluid phase in the slab this is in contrast with shallow sub forearc depths 20 to 50 10 km where tectonic vein arrays localized thrust faults serpentine diapirs mud volcanoes and boninite magmas attest to the release of large volume of fluids into the overriding mantle wedge

the effect of metal composition on fe ni partition behavior between olivine and feni metal feni carbide feni sulfide at elevated pressure abstract metal olivine fe ni exchange distribution coefficients were determined at 1500 c over the pressure range of 1 to 9 gpa for solid and liquid alloy compositions the metal alloy composition was varied with respect to the fe ni ratio and the amount of dissolved carbon and sulfur the fe ni ratio of the metal phase exercises an important control on the abundance of ni in the olivine the ni abundance in the olivine decreases as the fe ni ratio of the coexisting metal increases the presence of carbon up to 3 5 wt and sulfur up to 7 5 wt in solution in the liquid fe ni metal phase has a minor effect on the partitioning of fe and ni between metal and olivine phases no pressure dependence of the fe ni metal olivine exchange behavior in carbon and sulfur free and carbon and sulfur containing systems was found within the investigated pressure range to match the ni abundance in terrestrial mantle olivine assuming an equilibrium metal olivine distribution a sub chondritic fe ni metal ratio that is a factor of 17 to 27 lower than the fe ni ratios in estimated earth core compositions would be required implying higher fe concentrations in the core forming metal phase a simple metal olivine equilibrium distribution does not seem to be feasible to explain the ni abundances in the earth s mantle an equilibrium between metal and olivine does not exercise a control on the problem of ni overabundance in the earth s mantle the experimental results do not contradict the presence of a magma ocean at the time of terrestrial core formation if olivine was present in only minor amounts at the time of metal segregation

stable chlorine isotopes in hydrothermal processes abstract values of 37cl can now be routinely measured with an analytical precision of 0 2 as reported herein on fluid inclusion leachates from the elmwood gordonsville mississippi valley type deposit and formation waters from its host formation basin brines from the gulf coast and palo duro basins and fluid inclusion leachates and biotites from the bingham silver bell and panguna porphyry copper deposits isotopic measurements are not affected by reaction yield in the range 85 100 or by generation of free i2 during sample preparation in the elmwood gordonsville deposit five successive paragenetic stages of ore and gangue sphalerite fluorite calcite 2 barite and calcite 3 each yielded multiple 37cl measurements ranges of 37cl vary in alternating fashion between stages with more negative values sphalerite calcites 2 and 3 and those with more positive values fluorite barite an interface between two brine systems with 37 cl and 0 0 appears to have moved back and forth across the zone of mineralization brines from the gulf coast and palo duro basins are apparently bimodal in 37cl with modes near 0 9 and 0 0 consistent with the variation in 37cl observed in fluid inclusion leachates from elmwood gordonsville biotite in the bingham deposit has 37cl values up to 1 7 and chlorine isotopes that are fractionated with respect to salt rich hydrothermal fluid hydrothermal fluids in all three porphyry copper deposits studied have a 37cl range of 0 1 to 0 4 with the one exception of a 0 5 value at bingham loss of low 37 cl chloride in vapor may balance chloride with a net 37 cl 0 in the porphyry deposits

water soluble chlorides in massive seafloor serpentinites a source of chloride in subduction zones abstract dissolved cation and anion species were measured in the water soluble fraction of massive seafloor serpentinites leached with distilled h 2 o soluble chloride contents ranged from 0 09 to 0 51 wt average 0 26 wt so 4 2 to 0 09 wt mg 2 to 0 16 wt and na to 0 09 wt the concentration of evolved ions was the same in samples equilibrated with water at 25 and 230 c demonstrating that a significant amount of cl present in serpentinites occurs as water soluble salts a nearly equal amount of cl exists in the leached serpentinite likely sited in serpentinite itself the formation of intergranular salts in the serpentinite can be explained by seawater hydrothermal alteration of peridotite hydration reactions and h 2 o incorporation in the serpentine leave the residual fluid with ever increasing salinity and ultimately lead to precipitation of water soluble chlorides chlorine isotope ratios in the water soluble fraction average 1 49 vs standard mean ocean chlorine smoc from a core at the iberian abyssal plain and 0 16 from a much younger core at hess deep the 37 cl value of insoluble cl is 0 33 higher than the soluble fraction the nonzero 37 cl values demonstrate that the cl in the serpentinites is not an artifact of seawater incorporation during drilling the following conclusions are presented 1 serpentinite may be the major conduit for surficial chlorine transfer to mantle depths and an important part of the global chlorine cycle the cl flux in serpentinites into the mantle is larger than all other previously identified fluxes 2 cl is sited in both a water soluble and insoluble component 3 a high salinity fluid plume is evolved during serpentinite dehydration of a subducting slab the high salinity fluid affects metamorphic reactions and melting properties above the slab and can cause intense metasomatism the unusual bulk composition of the whiteschist from dora maira can be explained by channeled passage of a high salinity fluid 4 high salinity fluids reduce the stability field of serpentine causing its dehydration to occur over a broad interval which may partly explain the broad locus of intermediate depth earthquakes

ion probe and fluid inclusion evidence for co seismic fluid infiltration in a crustal detachment we have investigated the geochemical pattern of fluid infiltration in the extensional detachment of tinos island cyclades greece ion microprobe o isotope analyses and fluid inclusion studies have been conducted in strain fringes developing around pyrite blasts in the mylonite of the shear zone micro scale traverses in quartz and calcite fibres show that 18o increases from 17 18 to 20 21 in 1 mm towards the blast drops of 3 in 200 m then rises again in the direction of growth 18o variations are interpreted as transient influxes of exotic fluids into the shear zone between periods of closed system buffering by the host rock fluid inclusions trapped in the fibres show fluctuating salinities 0 4 wt nacl eq and densities that reflect drops of the pore pressure from lithostatic 1 to hydrostatic 0 4 values during fringe growth isotopic and microthermometric data are consistent with models of seismic pumping developed for compressive shear zones we therefore suggest that co seismic pore pressure variations developed suction forces sufficient to drive large scale fluid migration in the tinos detachment as in convergent tectonic settings

mg metasomatism of metagranitoids from the alps genesis and possible tectonic scenarios terra nova 24 423 436 2012 abstract mg metasomatic rocks e g whiteschists leucophyllites derived from post variscan granitoids are common in the alps previously reported field petrological geochemical and fluid inclusion data are combined to trace the genetic processes and the associated tectonic scenarios many common features can be recognised in all of the continental mg metasomatic rocks indicating that the genetic process is likely common in the entire range of the alps this process assumes highly channelised fluids derived from ultramafic rocks previously interacting with seawater that infiltrated the continental crust along strain zones and produced chromatographic fractionation of major and trace elements three tectonic scenarios involving distinct mantle sources are proposed rift related ocean continent transition continental subduction and continent continent collision all these data suggest that the mg metasomatism was diachronous and occurred at different structural levels during the alpine history

the origin of salts in unaltered kimberlites comment on was crustal contamination involved in the formation of the serpentine free udachnaya east kimberlite new insights into parental melts liquidus assemblage and effects of alteration by abersteiner et al 2018 journal of petrology 59 1467 1492

fluid composition and evolution in coesite bearing rocks dora maira massif western alps implications for element recycling during subduction fluid inclusions and f cl concentration of hydrous minerals were analysed in the coesite pyrope quartzite the interlayered jadeite quartzite and their country rock gneiss from the dora maira massif using a combination of microthermometry raman spectrometry synchrotron x ray microfiuorescence and electron microprobe analysis three populations of fluid inclusions were recognized texturally and can be related to distinct metamorphic stages a low salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeite quartzite and chloritized pyrope an earlier secondary population is found in matrix quartz of the jadeite and pyro pe quartzites this population can be related to the early decompression and so to incipient breakdown of garnet into phlogopite bearing assemblages the inclusion fluid is highly saline up to 84 wt equivalent nacl and contains na ca fe cu and zn as major cations in pyrope quartzite additional k was found in these brines which locally coexist with co2 rich inclusions the oldest fluid inclusions are preserved in kyanite grains included in fresh pyrope and in pyrope itself in pyrope all inclusions have decrepitated and contain magnesite an mg phosphate sheet silicate s a chloride and an opaque phase with no fluid preser ved in contrast the kyanite inclusions in pyrope preserve primary h2o co2 low salinity fluid inclusions probably owing to the low compressibility of the kyanite inclusions and host garnet in spite of in situ re equilibration these inclusions can be interpreted as relics of the dehydration fluid that attended pyrope growth these correlations between textural and chemical fluid inclusion data and metamorphic stages are consistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite the preservation of different fluid compositions both in time and space is evidence for local control and possibly origin of the fluids in agreement with isotopic data these results in particular the absence of co2 in the jadeite quartzite are best interpreted in terms of a fluid melt system evolution with increasing metamorphism partitioning of h2o na ca fe and heavy metals into melt jadeite quartzite and mg na k f co2 and p into a residual aqueous fluid can account for depletion in na ca and fe of the pyrope quartzite during the retrograde path a h 2 o rose as melt crystallized generating the two populations of hypersaline and water rich fluids that were highly reactive to pyrope the process of fluid melt interaction envisioned here coupled with models of melt extraction in subduction zones provides an attractive opportunity for the instantaneous 1 ma and selective transport of elements between a downgoing slab and the overlying mantle wedge

prediction of plagioclase melt equilibria in anhydrous silicate melts at 1 atm many models for plagioclase melt equilibria have been proposed over the past 30 years but the focus is increasingly on the effects of water content and pressure however many geological and petrological applications concern low pressure and low water systems such as the differentiation of large terrestrial basaltic magma chambers and lunar and asteroidal magmatism there is therefore a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1 atm with this in mind a database of over 500 experimentally determined plagioclase liquid pairs has been created the selected low pressure anhydrous experiments include both natural and synthetic liquids whose compositions range from basalt to rhyolite four equations are proposed derived from this data the first is based on a thermodynamically inspired formalism explicitly integrating the effect of temperature this equation uses free energies and activities of crystalline anorthite available from the literature for the activity of anorthite in the liquid phase it is found that current models of the activity of individual oxides are insufficient to account for the experimental results we have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition based upon inversion of the experimental data using this expression allows the calculation of plagioclase composition with a relative error less than 10 however in light of the fact that temperature is not necessarily known for many petrological applications an alternative set of t independent equations is also proposed for this entirely empirical approach the database has been divided into three compositional groups treated independently for regression purposes mafic ultramafic alkali rich mafic ultramafic and intermediate felsic this separation into distinct subgroups was found to be necessary to maintain errors below acceptable limits but results across group boundaries were found to be comparable overall 50 of plagioclase compositions are predicted to within 2 of the experimentally derived value and 90 to within 5 representing a significant improvement over existing models

mg metasomatism and formation conditions of mg chlorite muscovite quartzphyllites leucophyllites of the eastern alps w hungary and their relations to alpine whiteschists the lower austroalpine orthogneiss micachist complex of the sopron fertorakos area of w hungary contains mg chlorite muscovite quartzphyllites leuco phyllite and mg chlorite bearing kyanite quartzites whose chemical compositions differ greatly from their surrounding rocks formation of leucophyllites took place in shear zones and was associated with depletion in alkalies and iron and enrichment of magnesium and h2o mg zonation of relict igneous muscovites of leucophyllites and changes in the whole rock chemical compositions suggest mg metasomatism material gains and losses have been assessed using the composition volume relationship approach proceeding from metagranite through transition rocks to leucophyllites mgo h2o feo and alkalies show continuously increasing dispersion in isocon plots with mg enrichment even in sheared gneiss not in contact with leucophyllite the metasomatic processes that formed the mg rich rocks may be similar to those responsible for the formation of high pressure whiteschists in the central and western alps the geochemical characteristics of the dora maira whiteschists italy and their country gneisses are very similar to those of the sopron leucophyllites supporting the theory that mg metasomatism produced the whiteschist chemistry on the basis of oxygen isotope compositions of relict igneous muscovites the precursor granitic rock had a 18o value around 13 proving its crustal anatectic origin the leucophyllites have whole rock oxygen isotope compositions around 8 5 which is in conflict with the theory of an mg rich sedimentary protolith rather the low 18o values reflect fluid rock interaction with a low 18o fluid quartz mineral oxygen isotope fractionations yield a metamorphic temperature of 560 30 c which agrees with earlier estimates from mineral stabilities silicon contents of phengites correspond to a metamorphic pressure of 13 gpa at this temperature indicating eclogite facies metamorphism the fluids in equilibrium with leucophyllites had oxygen isotope compositions around 7 9 similar to those calculated for the ultrahigh pressure dora maira whiteschists 7 6 further supporting the genetic link between the leucophyllites and whiteschists hydrogen isotope compositions of mixed white mica chlorite samples from leucophyllites range from 40 to 35 correlating with chlorite contents the calculated endmember chlorite and white mica have d values of 30 and 40 respectively the similar d values of the white micas in leucophyllites gneisses and metagranites suggest an overall equilibration with respect to h isotopes the calculated d value of the fluid is approximately 0 suggesting a seawater origin this conclusion was also reached for the dora maira whiteschists a possible fluid source that satisfies both metasomatic and isotopic data is dehydration of hydrothermally altered oceanic crust the mafic ultramafic complex of the alpine penninic unit underlying the austroalpine nappes is a likely candidate the subduction and subsequent dehydration of the ophiolite series would supply the mg rich fluids whose migration brought about the metasomatic alteration of the overlying gneiss micaschist complexes

elemental and isotopic chloride geochemistry and fluid flow in the nankai trough 1 the first transect of pore fluid cl profiles across the deformation front of an accretionary prism and underthrust section nankai trough and the first cl isotopic profile in this environment are presented these demonstrate that there is channelized horizontal episodic flow through most of the section arcward of the deformation front and in the lowermost section seaward of the deformation front in the underthrust sediments channels appear to be continuous fluid velocities within the channels based on cl gradients are 13 5cm year the near balance of depth integrated flow and the subduction of pore fluid indicates that flow episodes are frequent 37cl values are lower than seawater throughout the section averaging 6 1 per mil this demonstrates fluid mineral exchange of cl and suggests that subducted solids carry enriched 37cl that is efficiently recycled from deeper levels in the subduction zone back into seawater and may be responsible for the seawater mantle 37cl fractionation

springs and streams of the taro ceno valleys northern apennine italy reaction path modeling of waters interacting with serpentinized ultramafic rocks article i nfo in the area of the taro ceno valleys northern apennine emilia romagna region italy waters of meteoric origin interact with ophiolite rocks of the external ligurides fresh water springs issuing from basalts have a ca hco3 composition whereas freshwater springs from ultramafites vary in composition from ca hco3 or mg hco3 to na oh or na so4 types and in ph values from 7 3 8 8 up to ph 11 respectively in addition the boron content of the alkaline waters is up to 13 mg l which is unusually high for freshwaters in general and ultramafites that have undergone oceanic serpentinisation in particular and gives a boric alkalinity to the waters the springs waters show evidence of recent low temperature continental serpentinisation and the process is modeled by reaction paths using an updated geochemical thermodynamic database consistent with the local primary and secondary serpentinite paragenesis for the model bicarbonate waters evolve to alkaline waters supersaturated in ca mg carbonate based on the assumption that the dissolution of serpentinite results in supersaturation with respect to kaolinite ferrihydrite vermiculite fe 2 mg 2 saponite and poorly crystalline serpentine the alkaline composition and the chloride content of the waters suggest a prolonged interaction with the rocks at depth that led to dissolution of albite and leaching of olivine hosted fluid inclusions a similar evolution is also proposed for the more developed springs issuing from the ultramafic rocks of the voltri group liguria region where solutions are supersaturated in brucite and are in equilibrium with enstatite and or chlorite

large salt accumulations as a consequence of hydrothermal processes associated with wilson cycles a review part 1 towards a new understanding abstract the formation of large salt deposits is observed especially in areas with a geological history of high tectonic activity over the last decade it has become a well established fact that heavy brines form and solid salts precipitate due to the thermodynamic and physico chemical properties of seawater at high temperatures and pressures encountered within hydrothermal systems this article reviews the modern theoretical and experimental research behind these findings and also describes geological settings that most likely cause brine and salt forming hydrothermal processes to occur this analysis has led to the identification of a set of specific conditions properties and processes referred to as conceptual elements that are used to explain the often complex processes of brine behavior that leads to hydrothermal formation of solid salt the main objective of this review is to present hydrothermal conditions known to occur during wilson cycles subduction collision and rifting e g zones of repeated tectonic unrest where brines commonly derived from seawater are concentrated into heavy brines and precipitate solid salts the internal heat of the earth and its interaction with deeply circulating seawater in hydrothermal systems and also the immense recycling of crustal materials including porous oceanic crust and serpentinite hydrated rocks via mantle processes may lead to the formation of salt accumulations it is also acknowledged that such brines and solid salts may often be stored sub surface for long time periods extending from one wilson cycle to another thus on the basis of this analysis it is cautiously suggested that large amounts of salts hidden inside subduction zones may appear on the surface during subsequent rifting and oceanization phases in part 2 of this review the conceptual elements which are described and discussed herein part 1 are applied to selected cases including the andean mountains the east african rift the red sea rift and other locations

the partitioning of fe ni and co between olivine metal and basaltic liquid an experimental and thermodynamic investigation with application to the composition of the lunar core abstract the partitioning of fe ni and co between mg rich olivine lunar basaltic liquid and metal has been measured over a range of temperatures and pressures the results of the olivine metal partitioning may be compared to those predicted from thermodynamic calculation in general the agreement is good although the calculated distribution coefficients are slightly greater then those experimentally determined this discrepancy increases with increasing temperature and pressure but it is not possible to ascertain unambiguously which thermodynamic data might be responsible at lower temperatures inversion of the equilibria to yield cosmothermometers for measuring the temperature of equilibration between olivine and metal gives results for the pallasite meteorites in excellent agreement with independent estimates the results have been applied to apollo 15 green glass presumed to approximate a primitive melt from the lunar mantle to deduce the composition of fe ni co metal in equilibrium with the lunar mantle this composition is approximately by weight 54 60 fe 38 45 ni and 1 co consequently if metal has separated from the lunar mantle under equilibrium conditions to form a small lunar core this core will be nickel rich with a ni fe ratio of 0 7 0 15

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

composition of fluids in the lower crust inferred from metamorphic salt in lower crustal rocks knowledge of the rheological properties of the lower crust and the metamorphic processes that operate there is important for our understanding of orogenic processes and granite genesis the rheological properties critically depend on whether fluids are present in the lower crust1 2 and if present on their composition3 4 5 6 fluid inclusion7 8 9 and phase equilibria4 10 studies of lower crustal granulites have shown that fluids with low water activities due to the presence of dissolved components such as ch4 n2 co co2 and chlorides 11 are present at least episodically in the lower crust here we report the occurrence of a solid salt solution nacl kcl found together with chlorine rich amphibole and biotite in lower crustal granulites a desiccation mechanism explains how salt and chlorine rich minerals formed from an originally water rich fluid through a short lived series of hydration reactions in the granulites during which chlorine was progressively enriched in the fluid consequently it would appear that fluid was present in the lower crust in only small amounts and was not stable over geologically long periods of time leading to the conclusion that the lower crust is devoid of a free fluid phase during most of its history

sw pacific arc and backarc lavas and the role of slab bend serpentinites in the global halogen cycle in order to better understand the global halogen cycle and the behaviour of halogens during subduction we investigated the concentrations of f cl br and i in suites of rare submarine glasses formed during subduction initiation on the hunter ridge n fiji basin and from the northernmost part of the tonga arc additional submarine glasses from mature arcs and backarcs including the tonga arc volcano a north fiji lau manus and woodlark basins were also investigated the aim was to constrain the relative abundances of all four halogens f cl br cl and i cl in subducted components in proto arc arc and backarc lavas for the first time this enables an assessment of slab fluid evolution from incipient arcs to mature backarcs and a comparison of halogens in slab fluids with possible subduction inputs and other mantle outputs the various arc and backarc lavas investigated are strongly enriched in cl br and i relative to nb and weakly enriched in f compared to pr demonstrating metasomatism of incipient sub arc mantle and sub arc backarc mantle by slab components with strikingly similar ranges of f cl 0 11 0 09 br cl 0 0028 0 0008 and i cl 0 000013 0 001 2 standard deviations the f cl ratio which is much higher than seawater and other surface fluids is attributed to enhanced mobility of f in saline fluids in sub arc environments with slab melts ascribed an additional minor role the slab fluid br cl and i cl ratios are strikingly similar to altered oceanic crust and seafloor serpentinites and distinctly different from sediment pore waters sediments and most forearc serpentinites the median br cl and i cl ratios of slab fluids in sub arc and sub backarcs are also indistinguishable from median mid ocean ridge and ocean island basalt values which have i cl ratios lower than estimated for the primitive mantle the data are interpreted to indicate that sedimentary volatile components present in sediments and forearc serpentinites are efficiently returned to the surface through forearc regions in contrast halogens in altered ocean crust and lithospheric serpentinites are recycled through magmatic arcs and into the deeper mantle with the result that the mantle s i cl ratio has decreased over time mass balance considerations suggest that serpentinisation of the lithospheric mantle at the slab bend immediately prior to subduction is more significant than estimated previously and is driven by seawater derived fluids without sedimentary components lithospheric serpentinites are likely to represent a dominant source of halogens and other volatiles including noble gases in the earth s mantle

growth of diamond in hydrous silicate melts this study demonstrates that a hydrous halide bearing silicate melt is a viable medium for diamond growth experiments were conducted in the mgo sio2 h2o c kcl nacl system which was used as a model for harzburgitic mantle in no case did we observe crystals that could be interpreted as spontaneously nucleated but growth of diamond on seed crystals at 1 400 1 600 c and 7 gpa in experiments of 4 h duration was observed the addition of kcl to the system produced crystallization of diamond at temperatures as low as 1 400 c at higher temperatures larger growth features were produced than those that seen in the kcl free system at the same conditions the nacl bearing system is different in these experiments the diamond seed crystals show evidence of possible dissolution and layer growth albeit more subdued growth than in the kcl system therefore nacl may be an inhibitor of diamond growth in a hydrous silicate melt based on these results hydrous silicate melts could play a role in formation of diamond in either deep subduction zones or above slabs imbricated against a lithospheric root in the sub continental lithospheric mantle the water and halide necessary for their formation could be transported into the mantle in hydrous phases such as serpentine in subducting lithospheric slabs dehydration of serpentine at 200 km depth would release hydrous halide bearing fluids into the overlying mantle wedge or lithospheric root triggering melting at conditions similar to those of the formation of natural diamond

geochemistry of garnetiferous ti clinohumite rock and talc kyanite phengite almandine schist from the kokchetav uhp terrane kazakhstan an insight to possible origins of some chemically unusual uhp rocks abstract major element trace element and o isotope analyses were carried out for garnetiferous ti clinohumite rocks and eclogites from the kumdy kol area as well as talc kyanite phengite almandine schists previously named as whiteschists and eclogites from the kulet area of the kokchetav ultrahigh pressure uhp terrane to explore their possible origins both garnetiferous ti clinohumite rocks and talc kyanite phengite almandine schists are possibly genetically related to eclogites as they occur in close association with eclogites enclosed within gneiss metapelite in these two areas most eclogites from the kumdy kol and the kulet areas exhibit chemical characteristics similar to that of ocean floor basalt although a few show arc affinity with slight ta and nb depletions garnetiferous ti clinohumite rocks from the kumdy kol region are characterized by very high mg low si ca na and k contents compared with eclogites they also exhibit lree depleted patterns with negative eu anomalies their o isotope compositions are less than 5 6 on the other hand the rocks show comparable al ni cr co and hree contents as eclogites the mg enrichment of these garnetiferous ti clinohumite rocks is suggested to have originated from their mafic protoliths that had been subjected to mafic rock ultramafic rock fluid interactions talc kyanite phengite almandine schists from the kulet area are typically low in ca and na but high in mg k rb ba cs th u and o isotope compositions 6 to 10 relative to eclogites this peculiar chemical composition can be accounted for if the basaltic protoliths had been subjected to sequential high temperature enrichment of mg and low temperature enrichment of k rb ba cs th u and o isotope compositions seawater basalt interactions one talc kyanite phengite almandine schist sample klw 7 is distinctly high in si and k but low in mg and fe which can be attributed to more severe low temperature hydrothermal alterations however except for mg the enrichment of rb ba cs th u and o isotope compositions in these schist samples could also be enhanced by or otherwise be interpreted as a result of fluid influx sourced from enclosing metapelite during subduction

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

the occurrence of chlorine in serpentine minerals partially serpentinized dunites containing small amounts of chlorine 0 5 from dumont quebec and horoman hokkaido japan and one containing less than 0 05 chlorine from higashi akaishi yama ehime japan have been examined using the electron probe microanalyzer and scanning transmission electron microscope with x ray analytical capabilities chlorine was found together with si mg ca and fe in the serpentine minerals of the dumont and hokkaido dunites but not in the ehime dunite chlorine is found associated only with the most finely crystalline facies of the serpentine grain size less than 10 nm the ehime dunite contained no such fine grained serpentine and was thus effectively chlorine free as are the coarser grained serpentines of the other samples the finegrained chlorine bearing serpentine also has a much higher concentration of fe and can contain smaller amounts of ca ni and mn than the coarse grained variety as well as minute awaruite feni3 grains this fine grained serpentine probably represents an early stage in the transformation of olivine to serpentine with chlorine from hydrothermal solutions assisting the necessary chemical changes the cl increases the reaction rate by lowering the activation barrier to reaction by the introduction of reaction steps

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

a synthesis of warm air advection to the south polar plateau surface temperature and wind chronologies of varying length and 10 m snow temperatures have been used to produce a synthesis of airflow into antarctica this synthesis shows relatively warm air over ellsworth land which appears to enter the south polar plateau through a slightly lower lying trough between the summit of the transantarctic mountains and the polar plateau comparison of recent automatic weather station aws records from mount howe 88 5 s 150 w with four aws surrounding south pole at 89 s show that this upslope inflow is a frequent and persistent phenomenon

salts in southern yakutian kimberlites and the problem of primary alkali kimberlite melts abstract alkali carbonates sodalite gypsum anhydrite halite and sylvite are present in the groundmass and matrix of many kimberlites in the southern part of the yakutian kimberlite province the kimberlites were emplaced through 2 km thick evaporite bearing carbonate sediments saturated with brines in the global context southern yakutian kimberlites are unprecedented in the amount of the crustal carbonate and evaporite material included in the pipes as evidenced by the bulk major element chemistry and isotopic compositions of sr c o cl and s we present geological and hydrogeological data on country rocks and kimberlites of the udachnaya mir and international naya pipes the secondary crustal origin of na k cl and s rich minerals is supported by the following 1 a regional correlation between the geology and hydrogeology of the local country rocks and the kimberlite mineralogy in particular the difference between southern and northern yakutian kimberlites 2 a restriction of halite or gypsum mineralization in the mir and international naya pipes to depths where pipes intersect country rock strata with similar mineralogy 3 the localization of the highest abundances of na k cl s bearing minerals in the udachnaya east kimberlite at a depth interval that correlates across three magmatic phases of kimberlites and coincides with the roof of the halite bearing country rock and an aquifer carrying anomalously na rich brines 4 the presence of evaporite xenoliths and veins of halite gypsum and carbonate cutting through the kimberlite and xenoliths 5 a secondary origin of halite and alkali carbonates as observed in their textural relationships to serpentine and other groundmass minerals 6 the geochemical and isotopic evidence for crustal contamination addition of crustal salts to kimberlite melt began prior to the volcanic fragmentation as a result of preferential melting and assimilation of evaporite xenoliths and may have continued in situ after the pipe emplacement via reactions with external saline fluids the hybrid alkali s and cl rich compositions of residual melts and fluids were trapped in secondary inclusions in olivine the crustal origin of salts in the udachnaya east kimberlite demonstrated here is incompatible with a model of alkali rich primary kimberlite melt it is significantly more ca na b and s rich than the deep seated mantle k and ba rich fluid inclusions in fibrous diamonds our analysis suggests that the udachnaya east kimberlite is not exceptional and uniquely fresh but rather typical of the other crust contaminated southern yakutian kimberlites

cl br ratios and stable chlorine isotope analysis of magmatic hydrothermal fluid inclusions from butte montana and bingham canyon utah a bulk geochemical study has been carried out on fluid inclusion leachates extracted from quartz veins from porphyry cu deposits in butte montana usa and bingham canyon utah usa the leachates mostly represent low salinity magmatic hydrothermal fluid inclusions their halogen ratios br cl of fluid inclusion leachates were determined by ion chromatography and 37cl values of the leachates were measured by continuous flow isotope ratio mass spectrometry br cl ratios from early pre main stage and later main stage veins at butte range from 0 60 to 1 88 10 3 m ratios are similar in pre main stage veins with sericite bearing selvages and main stage samples ranging from 0 81 to 1 08 10 3 and from 0 92 to 1 88 10 3 m respectively clustering below seawater 1 54 10 3 m and overlapping mantle values 1 2 10 3 m two samples associated with early pre main stage potassic alteration yield distinctly lower br cl ratios of 0 60 and 0 64 10 3 m butte 37cl values range from 0 8 to 2 3 with no significant difference between pre main stage and main stage samples br cl ratios for quartz veins from bingham canyon range from 0 18 to 3 68 10 3 m br cl ratios from bingham range above and below previously reported for porphyry copper deposits in contrast to butte 37cl values for bingham are lower ranging from 0 9 to 4 1 in the absence of any processes which can significantly fractionate chlorine isotopes at high temperatures we suggest that the porphyry system at bingham and to a lesser extent at butte have inherited negative chlorine isotopic signatures from the subducting slab generated at low temperatures

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

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

halogens of bushveld complex south africa δ37cl and cl f evidence for hydration melting of the source region in a back arc setting abstract the bushveld complex is one of only a few layered intrusions anomalously enriched in chlorine this study presents the first chlorine stable isotope analyses of bushveld and associated rocks designed to constrain the source of bushveld cl most samples overlap the range of values for pristine morb and altered seafloor samples 37 cl 4 relative to sea water this contrasts with distal metasedimentary floor rocks 37 cl 0 with little evidence of contamination away from the marginal rocks the cl isotope composition and high cl f ratio is therefore a fundamental characteristic of the lower and critical zone magmas and not derived from infiltration from the country rock these results the high cl f ratio and the boninitic arc like character of bushveld magma are consistent with hydration partial melting of a depleted source in a subduction related setting with cl supplied either by the dehydration of a subducted slab with a heavy cl signature or an overridden wet mantle hotspot

geologic implications of seawater circulation through peridotite exposed at slow spreading mid ocean ridges peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow spreading centers hosts hydrothermal circulation of seawater the reaction of seawater with peridotite causes serpentinization which generates a high ph strongly reducing fluid rich in methane and hydrogen and is accompanied by as much as 40 volume expansion complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk like limestone this may be the main mechanism of deposition of ophicarbonates common in ophiolites the degree of induration is a function of the fluid flux through the sediment column calcareous ooze infiltrates faults and fractures and can be deformed following lithification focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite aragonite and brucite such as those in the recently documented lost city vent field 30 n mid atlantic ridge geophysical implications of serpentinization include 1 creation of magnetic anomalies due to growth of magnetite in serpentinite and 2 lowered seismic velocity integrated studies of geologic and geophysical effects of serpentinization may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor

a simplified analysis of parameters controlling dewatering in accretionary prisms abstract one of the many dynamic geologic processes taking place at subduction zones is the compaction driven dewatering of seafloor sediments as they are accreted to the overriding plate the rate of dewatering is equal to the divergence of the sediment matrix velocity field this rate can be estimated analytically if simplifying assumptions are made about the geometry of the prism and motion of the sediments the analytical expression depends only on the sediment accretion velocity thickness of the accreted section prism taper angle and sediment porosity distribution a sensitivity analysis of the solution shows that the fluid production distribution is relatively insensitive to the sediment porosity distribution however the solution is very sensitive to the taper angle of the wedge high angle wedges expel almost all of the incoming water within 20 km while low angle wedges may retain a significant fraction of the incoming water for 50 or more kilometers thickness of the incoming section is also important dewatering of thin accreted sections is more concentrated near the toe of the wedge analyses of transects through the northern barbados makran vancouver and nankai accretionary prisms illustrate a range of dewatering rates and spatial distributions of dewatering

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

new w isotope evidence for rapid terrestrial accretion and very early core formation the short lived hf 182 w 182 isotope system is an ideal clock to trace core formation and accretion processes of planets planetary accretion and metal silicate fractionation chronologies are calculated relative to the chondritic hf 182 w 182 isotope evolution here we report new high precision w isotope data for the carbonaceous chondrite allende that are much less radiogenic than previously reported and are in good agreement with published internal hf w chronometry of enstatite chondrites if the w isotope composition of terrestrial rocks representing the bulk silicate earth is homogeneous and 2 24 epsilon 182w units more radiogenic than that of the bulk earth metal silicate differentiation of the earth occurred very early the new w isotope data constrain the mean time of terrestrial core formation to 34 million years after the start of solar system accretion early terrestrial core formation implies rapid terrestrial accretion thus permitting formation of the moon by giant impact while hf 182 was still alive this could explain why lunar w isotopes are more radiogenic than the terrestrial value copyright c 2002 elsevier science ltd

comment on the papers by large salt accumulations as a consequence of hydrothermal processes associated with wilson cycles a review part 1 and 2 abstract the geological model of large hydrothermal salt accumulations linked to wilson cycles as reviewed by hovland and coworkers un adequately addressed 1 the lack of well exposed submarine volcanogenic hydrothermal feeder zone 2 the sequential precipitation of different hydrothermal evaporite minerals and 3 the absence of hydrothermal alteration mineralogy around the major evaporites these aforementioned points are the main classical features to be linked to the hydrothermal model of evaporites experimentally heating seawater during submarine volcanism firstly results in precipitation of anhydrite followed by halite other evaporite minerals serpentine brucite talc metallic sulfides oxides and related hydrothermal alterations in this regard mg ca cl mg ca so4 mg ca na cl and mg ca tds biplots show that hydrothermal brines of riftogenic and subduction zone settings plot in a separate field than the subduction related andean salars as well as seawater this indicates that the hydrothermal serpentinization of basaltic oceanic crust and mantle peridotites removes magnesium from seawater and incorporates the mg into serpentine brucite and other hydrous mg silicates rather than the hydrothermal brines finally our comment arose from a recent global classification of the most distinguished marine non marine and exhalative hydrothermal evaporites

origin of salt giants in abyssal serpentinite systems worldwide marine salt deposits ranging over the entire geological record are generally considered climate related evaporites derived from the precipitation of salts mainly chlorides and sulfates from saturated solutions driven by solar evaporation of seawater this explanation may be realistic for a salt thickness 100 m being therefore inadequate for thicker 1 km deposits moreover sub seafloor salt deposits in deep marine basins are difficult to reconcile with a surface evaporation model marine geology reports on abyssal serpentinite systems provide an alternative explanation for some salt deposits seawater driven serpentinization consumes water and increases the salinity of the associated aqueous brines brines can be trapped in fractures and cavities in serpentinites and the surrounding country rocks successive thermal dehydration of buried serpentinites can mobilize and accumulate the brines forming highly saline hydrothermal solutions these can migrate upwards and erupt onto the seafloor as saline geysers which may form salt saturated water pools as are currently observed in numerous deeps in the red sea and elsewhere the drainage of deep seated saline brines to seafloor may be a long lasting effective process mainly occurring in areas characterized by strong tectonic stresses and or igneous intrusions alternatively brines could be slowly expelled from fractured serpentinites by buoyancy gradients and hence separated salts brines could intrude vertically into surrounding rocks forming salt diapirs serpentinization is an ubiquitous exothermic long lasting process which can modify large volumes of oceanic lithosphere over geological times therefore buried salt deposits in many areas of the world can be reasonably related to serpentinites

fluid inclusion and mineral isotopic compositions hco in eclogitic rocks as tracers of local fluid migration during high pressure metamorphism eclogite facies metagabbros from the monviso ophiolitic complex italian western alps provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of 40 km we have determined the omphacites 18o and the abundance d and 13c of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90 fluid loss the remaining fluid was trapped as primary water rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone oxygen isotopes of omphacite omp and hydrogen isotopes of water in the fluid inclusions fi analyzed in low strain rocks mylonites and undeformed deformed veins show marked variations 18oomp and dfi values ranging from 3 0 to 5 3 and from 31 to 93 respectively detailed isotopic analysis of several individual vein wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter therefore the eclogitic minerals and fluids filling the veins are concluded to be locally derived our results argue against recent models which suggest large scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism on average the h2o contents and dfi value are within the upper mantle range the carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean 13c value of 24 2 1 2 two carbonaceous components can be recognized condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions 18oomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole rock values from hydrothermally altered oceanic crust it is suggested that the isotopic imprint of the monviso eclogitic minerals and fluids represents the signature of mid ocean ridge hydrothermal alteration and subduction zone eclogitization processes the rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths in addition the range in 18oomp values recorded in the different microstructural domains covers most of the 18o values reported in type a b and c eclogitic xenoliths implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths

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

graphite oxidation in the apollo 17 orange glass magma implications for the generation of a lunar volcanic gas phase abstract an apollo 17 picritic orange glass composition has been used to experimentally investigate the conditions at which graphite would oxidize to form a co rich gas and ultimately produce lunar fire fountain eruptions isothermal decompression experiments run above the a17 orange glass liquidus temperature 1350 c suggest that the initial co rich gas phase produced by graphite oxidation would be generated during magma ascent at a pressure of 40 mpa 8 5 km beneath the lunar surface additional experiments with 2000 ppm s and 1000 ppm cl showed that the presence of these dissolved gas species would not affect the depth of graphite oxidation verifying that the first volcanic gas phase would be generated by the oxidation of graphite a simple ideal chemical mixing model for calculating melt feo activity in a fe metal silicate melt system was tested with a series of 0 1 mpa controlled oxygen fugacity experiments agreement between the model and experiments allows the model to be used to calculate oxygen fugacity in picritic lunar glass compositions such as the a17 orange glass using this model in a reanalysis of chemical equilibria between the natural a17 orange glass melt and the metal spherules fe85ni14co1 trapped within the glass beads indicates a log oxygen fugacity of 11 2 0 7 log units more oxidized than previous estimates at the a17 orange glass liquidus temperature 1350 5 c this fo2 corresponds to a minimum pressure of 41 mpa on the graphite c o surface the fact that the same critical graphite oxidation pressure was determined in decompression experiments and from the fe feo activity model for the natural a17 orange glass metal assemblage strongly supports this pressure 8 5 km depth for volcanic gas formation in lunar basalts generation of a gas by oxidation of c in ascending magma is likely to have been important in getting dense lunar magmas to the surface as well as in generating fire fountain eruptions the vesicles common in many lunar basalts and the ubiquitous fe metal in these rocks are also likely generated by the oxidation of carbon the presence of carbon in the lunar basalts and the recent discovery of ppm levels of water in lunar basalts indicate that at least parts of the lunar interior still contained volatiles at 3 9 bybp

comment to chlorine stable isotopes and halogen concentrations in convergent margins with implications for the cl isotopes cycle in the ocean by wei et al a review of the cl isotope composition of serpentinites and the global chlorine cycle abstract wei et al wei w kastner m spivack a 2008 chlorine stable isotope and halogen concentrations in convergent margins with implications for the cl isotopes cycle in the ocean earth planet sci lett 266 90 104 published chlorine isotope data for pore fluids and serpentinites from three subduction zones and two obducted serpentinites the measured 37cl values of serpentinites ranged from 1 2 to 6 0 vs smoc far higher than all other published data for serpentinites the explanation for these high 37cl values defies known fractionation mechanisms instead the anomalous 37cl values are almost certainly due to analytical artifacts known to exist for the method the authors employed wei et al wei w kastner m spivack a 2008 chlorine stable isotope and halogen concentrations in convergent margins with implications for the cl isotopes cycle in the ocean earth planet sci lett 266 90 104 concluded that if high 37cl serpentinites 6 were subducted into the mantle then a change in the 37cl value of the crustal reservoir would be observed in the evaporite record over the last 200ma because the cl isotope composition of the crustal reservoirs has been constant over this time period as constrained by evaporites they argue that cl in serpentinites is recycled back to the oceans this argument is flawed for two reasons first the 37cl values of serpentinites is close to 0 not 6 as suggested in their paper second the 37cl value of the crustal reservoir would change immeasurably over 200ma even if serpentinites of 6 were subducted into the mantle in fact it has been shown that the crust mantle and chondritic reservoir all have 37cl values close to 0 sharp z d barnes j d brearley a j fischer t p chaussidon m kamenetsky v s 2007 chlorine isotope homogeneity of the mantle crust and carbonaceous chondrites nature 446 1062 1065 no secular variation in the 37cl value of the crust is expected a conclusion that is supported by chlorine isotope data on sediments extending back over 3 5ga sharp z d barnes j d brearley a j fischer t p chaussidon m kamenetsky v s 2007 chlorine isotope homogeneity of the mantle crust and carbonaceous chondrites nature 446 1062 1065