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

lead and bromine enrichment in eclogite facies fluids extreme fractionation during lower crustal hydration hydration reactions associated with eclogite facies metamorphism of granulites in the norwegian caledonides led to the formation of saline brines with exceptional compositions primary omphacite and garnet hosted fluid inclusions from norwegian eclogites formed by hydration of granulites contain a plethora of solid phases a total of 18 different minerals have been identified in multiphase brines and 12 of these occur at the badsholmen locality in sunnfjord fluid inclusions from this locality are characterized by a high br cl mass ratio 0 03 and extreme pb enrichment as demonstrated by the presence of pb bearing daughter minerals including galena a pb cl bearing phase and a pb cl br bearing phase the fluid compositions may reflect enrichment of elements incompatible with the silicate structure during consumption of h 2 o by eclogite forming hydration reactions pb is believed to have been scavenged from the parent granulite by the increasingly saline brine during k feldspar breakdown

structural mineralogical and petrophysical effects on deep crustal rocks of fluid limited polymetamorphism western gneiss region norway the proterozoic banded granulite facies complex of bardsholmen western gneiss region norway t 815 845 c was locally transformed to eclogite t 455 510 c p 12 kbar and amphibolite facies rocks t 460 c during the caledonian continental collision the granulite complex consists of mafic two pyroxene granulite and leucocratic orthopyroxene garnet bearing layers alternating on a scale from 1 cm to 10 m the granulite facies rocks change to eclogite facies rocks over centimetre scale distances along well defined fluid infiltration fronts the mafic granulite was transformed to omphacite garnet rich eclogites and the leucocratic rocks were converted to quartz phengite rich assemblages with minor garnet and local omphacite melange like lithologies consisting of mafic lenses of eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust to eclogite facies during amphibolitization this melange like lithology evolves to a rock where amphibolite lenses and layers are surrounded by granitoid gneiss a lithology typical of the western gneiss region the bardsholmen locality illustrates the profound control exerted by fluids on the timing of metamorphism the structural make up and petrophysical properties such as density and rheology of crustal root zones fluid induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyancy of the subducted crust we suggest that orogens may develop differently depending on the fluid budget

accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

deformation processes in eclogitic rocks evidence for the rheological delamination of the oceanic crust in deeper levels of subduction zones the deformation mechanisms and associated growth processes operating in omphacite during foliation development and veining have been studied in a km scale eclogitic ductile shear zone from the monviso meta ophiolitic complex italian western alps dislocation creep due to multiple slip and or climb is the dominant deformation mechanism whereas 100 and 001 twinning fracturing and intergranular fluid phase diffusional creep are subordinate active slip systems include 110 001 12 110 110 100 001 and 12 010 100 fracturing is interpreted in terms of brittle instabilities arising from strain and pore fluid pressure partitioning between adjacent omphacite domains the transition from plastic to brittle behaviour of rocks is cyclic in nature as the internally generated fluids can be released and trapped repetitively during progressive deformation the significance of the eclogitic shear zone is discussed in terms of a mechanically weak zone that developed during burial or at least prior to the peak of temperature at a minimum depth of 40 km in a subduction zone and which subsequently acted as a zone of detachment during a late stage of the alpine orogeny a rheological model for oceanic crust delamination in deeper levels of subduction zones is presented

halogen contents of eclogite facies fluid inclusions and minerals caledonides western norway primary multiphase brine fluid inclusions in omphacite and garnet from low to medium temperature eclogites have been analysed for cl br i f li and so4 halogen contents and ratios provide information about trapped lower crustal fluids even though the major element na k ca contents of inclusion fluids have been modified by fluid mineral interactions and step daughter crystal formation after trapping halogens in the inclusion fluids were analysed with crush leach techniques cl br and cl i mass ratios of eclogite fluids are in the range 31 395 and 5000 33 000 respectively most fluids have a cl br ratio lower than modern seawater and a cl i ratio one order of magnitude lower than modern seawater fluids with the lowest cl br and highest cl i ratios come from an eclogite that formed by hydration of granulite facies rocks and may indicate that br and i are fractionated into hydrous minerals reconstructions indicate that the inclusion fluids originally contained 500 4000 ppm br 1 14 ppm i and 33 438 ppm li electron microprobe analyses of eclogite facies amphibole biotite phengite and apatite indicate that f and cl fractionate most strongly between phengite f cl mass ratio of 1469i1048 and fluid f cl mass ratio of 0 008 and the least between amphibole and fluid the chemical evolution of cl and br in pore fluids during hydration reactions is in many ways analogous to cl and br in seawater during evaporation the cl br ratio remains constant until the ah2o value is sufficiently lowered for cl to be removed from solution by incorporation into hydrous minerals

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

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

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

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

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

primary fluids in low temperature eclogites evidence from two subduction complexes dominican republic and california usa eclogites occur as isolated blocks in melanges of both the samana peninsula dominican republic and the franciscan complex california usa in some of these eclogites fluid inclusions were found in omphacite and sodic calcic amphibole grains textures show that non planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole in addition planar arrays of secondary fluid inclusions are found along healed cracks homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions these data were compared with petrologic geothermobarometry temperature conditions of 500 700 c were estimated from garnetclinopyroxene geothermometry the jadeite contents of omphacite indicate minimum pressures of 8 11 kbar in this temperature range the p t estimates agree well with calculated isochores for primary fluid inclusions from the samana peninsula and show some overlap for both primary and secondary fluid inclusions from the franciscan complex salinities of 1 2 5 3 wt nacl equiv were estimated for both primary and secondary fluid inclusions from samana and franciscan eclogites these data suggest that low salinity aqueous fluids attended eclogite facies metamorphism and perhaps retrograde metamorphism in both subduction complexes the salinities and densities of fluid inclusions in eclogites from the samana peninsula and the franciscan complex resemble those of counterparts from garnet amphibolites of the catalina schist southern california an external source for such fluids is suggested by their homogeneous populations coupled with their low salinities geologic evidence suggests that the samana and franciscan eclogites may have been derived from a catalina like source terrane the catalina rocks are inferred to have interacted with large volumes of sediment derived fluid during subduction zone metamorphism at similar p but higher t conditions than those determined for samana and franciscan eclogite blocks these results contrast with data for fluid inclusions from eclogites of the monviso area western alps the monviso eclogites yield similar estimates for metamorphic p t to those obtained in this study but contain fluid inclusions of brine and of other saline aqueous fluids all of which are less dense than expected for incorporation at the reported eclogite facies conditions the differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the samana franciscan catalina subduction complexes and those of monviso probably reflect differences between fluid flow regimes during metamorphism

redistribution of high pressure fluids during retrograde metamorphism of eclogite facies rocks voltri massif italian western alps abstract high pressure clinopyroxenes in metasomatic mafic rocks rodingites and metagabbros containing titanoclinohumite and eclogites from the voltri massif ligurian western alps retain primary two phase aqueous inclusions h 2 o nacl kcl in the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside the eclogites display mylonitic textures primary fluid inclusions are hosted in early porphyroclasts of omphacite omphacite i and are absent in later fine grained omphacite omphacite ii that forms the mylonitic foliation omphacite i has been frequently replaced by retrograde symplectites clinopyroxene plagioclase amphibole in such cases the fluid inclusions are either absent or occur in much lower number the textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite i the symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles indicating that breakdown of the omphacite i to symplectites still occurred at relatively high pressure post entrapment changes have caused a large range of densities in the analyzed inclusions although the textural evidence constrains the entrapment of fluid inclusions to high pressure conditions the highest density isochores in the eclogites indicate pressure temperature conditions 8 kbar and 550 c lower than those attained during the high pressure stage such p t estimates approach those of symplectite formation after omphacite i thus the fluid inclusions preserved in omphacite were formed by low pressure re equilibration of a fluid presumably trapped at eclogite facies conditions the inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes the compositional overlap between the analyzed inclusions and those related to the latest greenschist evolutionary stage suggests further redistribution of the high pressure fluids and close system behaviour of these rocks during their overall exhumation path

cl br and i analyses of metamorphic and sedimentary rocks by isotope dilution mass spectrometry isotope dilution mass spectrometry with negative thermal ions was applied to determine cl br and i in rocks which in part contained less than 100 ppb of br and i two sets of samples have been investigated a a series of al rich metapelites of increasing metamorphic grade from the damara orogen namibia and b fresh and hydrothermally altered greywackes from the pb zn deposit bad grund in germany it was found that regional metamorphism of al rich metapelites causes no strong fractionation of the halides the br cl ratios in the metapelites are similar to those of sea water the i cl and i br ratios however are 500 times higher the i depleted in the ocean most probably is associated with organic matter stored in sediments the unaltered greywackes from near the pb zn vein have about the same concentrations of halides as the metamorphic al rich pelites by contrast the hydrothemally altered greywackes contain about twice as much cl and i and about 2 5 times as much br as the unaltered samples the element ratios however are similar for all three categories of rocks thereby indicating that sea water played no role in the hydrothermal system

halogen geochemistry of fluid during amphibolite granulite metamorphism as indicated by apatite and hydrous silicates in basic rocks from the bamble sector south norway abstract the halogen chemistry of apatites biotites amphiboles and titanites from metabasites in the proterozoic bamble sector amphibolite granulite facies transition zone norway has been studied to gather data on the behaviour of halogens during original magmatism and subsequent metamorphism three groups of apatites could be recognized 1 f oh apatites with more than 50 mole f and virtually no cl 2 f oh apatites with up to 15 mole of cl 3 cl rich apatites with up to 6 73 wt cl the latter are the most cl rich metamorphic apatites ever reported the f contents of coexisting biotite do not follow this division but their cl contents do log z sfnc hf z sfnc h 2 o values for the coexisting fluid phase calculated from biotite are slightly lower than those calculated from apatite log z sfnc hcl z sfnc h 2 o values in the coexisting fluid phase calculated from biotite are substantially higher than those calculated from apatite the log z sfnc hf z sfnc h 2 o reflects the composition of the peak metamorphic fluid phase the apatites show three types of patterns of f contents 1 w shaped 2 rimward increasing and 3 rimward decreasing pattern 2 reflects progressive metamorphism pattern 3 is thought to reflect growth starting near peak metamorphism and going on during retrograde dilution of the peak metamorphic fluid by a newly introduced aqueous one some apatites show local resorption effects along cracks no regional trend in calculated log z sfnc hf z sfnc h 2 o values is present as different from other granulite terrains the bamble biotites and amphiboles from granulite facies samples do not have higher halogen contents as those from the amphibolite facies area the samples show that halogen bearing fluids were controlled at a very restricted scale during peak as well as retrograde metamorphism

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

n2 and co2 in deep crustal fluids evidence from the caledonides of norway abstract high pressure metamorphic rocks eclogites and high pressure granulites occur along the entire length of the norwegian caledonides and have formed from a variety of protoliths in some cases the relationship between protoliths high pressure rocks and their later retrogression products have been preserved in situ fluid inclusion data suggest a simple correlation between metamorphic grade and metamorphic fluid composition 1 eclogites and high pressure granulites contain n 2 bearing fluids pure n 2 or mixtures with co 2 or h 2 o with x n 2 5 in some eclogite facies rocks co 2 1bn 2 inclusions are associated with aqueous brine inclusions ca 30 wt nacl the two compositions representing immiscible fluids at peak metamorphic conditions 2 granulite facies protoliths and eclogites reworked in the granulite facies contain pure co 2 or co 2 dominated fluids with less than 2 5 n 2 3 rocks retrograded in the amphibolite facies contain h 2 o 1bnacl fluids immiscibility between brine and anhydrous n 2 1bco 2 fluid and between anhydrous fluid and waterbearing aluminosilicate melt have taken place in some eclogites during high grade metamorphism nitrogen may be incorporated in minerals as nh 4 substituting for k or it may occur as n 2 in the free fluid phase the partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism low a h 2 o and or high f o 2 partitioning nitrogen to the fluid phase a rock interacting with a carbonic fluid at granulite facies pt conditions will be depleted in mineralogically bound nitrogen in cases where the protoliths of high pressure rocks have been through a previous granulite facies event a local source for the nitrogen contained in high pressure fluid is therefore unlikely

processing of crust in the root of the caledonian continental collision zone the role of eclogitization segments of the root to the caledonian collision zone are exposed along the west coast of norway allowing the study of processes and petrophysical conditions at the deepest crustal levels p t conditions prevailing in these crustal root zones correspond to eclogite facies eclogitization is associated with marked changes in petrophysical properties notably a density 10 15 and vp increase which may give a mantle signature to an eclogitized crust the observed ductility enhancement may be caused by transformation plasticity reduction in grain size formation of new mineral assemblages and presence of fluid in the western gneiss region and the bergen arcs eclogites form along shear zones interpreted as fluid pathways with sharp boundaries to their metastable protolith this makes eclogite shear zones potential deep crustal reflectors dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the p t space but rather on the introduction of fluids into the system eclogites are not only passive recorders of p t t paths illustrating very deep subduction of crust 100 km rapid changes in petrophysical properties make fluid induced eclogitization a dynamic process that influences geodynamics stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn eclogitic pseudotachylytes in the metastable protolith deformation including seismicity enhances fluid infiltration and further eclogitization dense and rheologically weak eclogites may delaminate and sink into the mantle in the absence of fluids crustal doubling may occur without metamorphic re equilibration the evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to p and t

metamorphic evolution of the eiksunddal eclogite complex western norway and some tectonic implications the metamorphic evolution of the western gneiss region of norway wgr can be evaluated from examination of eclogites formed as an integral part of the gneiss terrain ie in situ petrologic data from one of the larger country rock eclogites within the northwestern part of wgr have been used to construct a p t t path for this crustal volume textural relations as well as mineral composition and zonation suggest an early period of amphibolite facies conditions followed by movement along a very steep positive p t gradient towards very high pressure p 25 kb and moderate temperature t 600 c orthopyroxene coexisting with garnet at such conditions was poor in al 0 5 wt al2o3 and is preserved in the cores of orthopyroxene grains in garnet websterites the increasing al content of orthopyroxene towards garnet and increasing edenite substitution rimwards in zoned amphiboles suggest a period of decompression accompanied by heating towards the metamorphic maximum at t 750 c p 20kb garnet zoning reflects a general prograde history but the reactions that control the zoning are insensitive to pressure changes jadeitic pyroxenes may have formed in the gneisses at p max but would revert to plagioclase subsequent to the metamorphic maximum giving rise to the observed mineralogy further decompression and cooling continued towards high rank amphibolite facies conditions this p t t path reflects crustal thickening to 80 90 km during caledonian time the uplift path is qualitatively similar to theoretically derived paths in which uplift is assumed to be controlled by erosion however uplift following the metamorphic maximum may have been accelerated by a period of suture progradation

partitioning of fluorine and chlorine between apatite and aqueous fluids at high pressure and temperature implications for the f and cl content of highp t fluids abstract experiments in a piston cylinder apparatus equilibrated natural fluorapatite with aqueous hcl nacl naoh na 2 co 3 and co 2 h 2 o mixtures at 1 2 0 gpa and 950 1050 c post experiment apatite compositions were determined by electron microprobe and fluid compositions were calculated from mass balance this enabled calibration of the fluorine and chlorine contents of apatite coexisting with a variety of fluids at high p and t and provides better constraints on the halogen composition of aqueous fluids which may exist in the deep crust and upper mantle results obtained at 2 0 gpa and 1050 c indicate that the ratio x clap x ohap in apatite x fap x clap and x ohap are the mole fractions of fluor chlor and hydroxyapatite is proportional to the chlorine content of the fluid up to 1 h 2 o co 2 5 h 2 o hcl and 15 h 2 o nacl wt cl in contrast the ratio x fap x ohap was found to vary in proportion to the fluorine content of the fluid only at concentrations of 0 15 h 2 o hcl and 0 4 h 2 o nacl wt f arising from changes in the activities of aqueous hcl and hf the results of this study demonstrate that fluid composition in addition to the absolute abundances of f and cl in the fluid will determine the amounts of these elements incorporated into coexisting apatite specifically relatively low cl and f concentrations in acidic h 2 o hcl fluids are required to obtain high ratios of x clap x ohap and x fap x ohap in apatite whereas much higher abundances of cl and f in basic fluids i e na bearing are required to achieve the same result in addition resulting from dilution of the aqueous phase with co 2 and the subsequent reduction in the activity of h 2 o apatites with high ratios of x clap x ohap and x fap x ohap may be produced by co 2 h 2 o fluids with relatively low levels of cl or f application of these results to apatites from a suite of ultramafic xenoliths from southeastern australia indicates chlorine abundances of 500 3500 ppm in the fluid phase and points to chlorine as a potentially important component of some mantle fluids

metamorphic evolution of norwegian country rock eclogites as deduced from mineral inclusions and compositional zoning in garnets abstract detailed studies of pre early syn and post eclogite mineral associations in some west norwegian country rock eclogites have made it possible to reconstruct their metamorphic evolution pre eclogite stages are represented by various amphibolite facies associations found in the central parts of prograde zoned garnets early and syn eclogite stages are represented by various eclogitic assemblages containing low and high jd omphacites respectively the former located in the intermediate zones and the latter in the outermost rims of prograde zoned garnets and in the matrix post eclogite stages include granulite amphibolite and greenschist facies associations representing replacement phases after higher grade assemblages p t estimates indicate a prograde evolution along an extremely steep p t gradient 7 9 kbar over 100 200 c while the initial post eclogite evolution involved a nearly adiabatic decompression on the order of 10 kbar

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

magnetotelluric interpretation of crustal and mantle structure in the grenville province summary accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust a magnetotelluric mt station bat in the grenville province of the precambrian shield in eastern canada has been in operation since 1975 for time dependency studies of electrical resistivity changes related to earthquakes the mt response of the station displays low skew with small to moderate anisotropy one dimensional inversion of the apparent resistivity and phase reveals two well defined conductors in the crust one at 10 km and the second at the base of the crust the latter has a resistivity less than 50 m these results are substantiated by three additional mt stations located up to 40 km distant data from other new mt stations and from stations previously published in the literature are compared with two dimensional computer model results and with the three dimensional analogue scale model results of dosso et al while additional data for periods less than 100 s would be desirable the results from a number of the mt stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the grenville the inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 m this may coincide with a seismic low velocity zone observed in the mantle under the canadian shield

nitrogen bearing aqueous fluid inclusions in some eclogites from the western gneiss region of the norwegian caledonides minerals in eclogites from different localities in the western gneiss region of the norwegian caledonides age 425 ma contain a variety of fluid inclusions the earliest inclusions recognized are contained in undeformed quartz grains protected by garnet and consist of h2o n2 with x text h text 2 text o geqslant 0 8 the reconstructed p v t x properties of these fluid inclusions are compatible with peak or early retrograde metamorphic conditions matrix minerals quartz garnet apatite plagioclase contain a complex pattern of mostly truly secondary inclusions dominated by co2 and n2 the textural patterns and p v t x properties of these inclusions are incompatible with the high pressures of the eclogite forming metamorphic event but suggest that they were formed during uplift by a combination of remobilization of preexisting inclusions and influx of external fluids the fluid introduced at a late stage was dominated by co2 and did not contain n2 the present data agree with theoretical predictions of eclogite fluids from mineral equilibria and highlight the differences between granulite co2 and eclogite h2o n2 fluid regimes the provenance of the nitrogen in the eclogite fluid inclusions represents an important but unsolved question in the petrology of high pressure metamorphic rocks