an ion microprobe study on trace element composition of clinopyroxenes from blueschist and eclogitized feti gabbros ligurian alps northwestern italy some petrologic considerations abstract detailed microtextural investigations coupled with ion microprobe trace element determinations have been performed on clinopyroxenes from blueschist and eclogitized fe ti gabbros from ligurian alps northwestern italy ion microprobe analyses have also been carried out on garnets from eclogitized fe ti gabbros the aim is to understand the relations between metamorphic reactions and trace element compositions of clinopyroxenes in high pressure low temperature terranes trace and major element analyses on relicts of igneous diopsides indicate similar protoliths for both eclogites and blueschists metamorphic clinopyroxenes have variable trace element compositions which reflect the compositions of original mineral phases in particular the clinopyroxenes replacing the igneous diopside commonly retain geochemical signatures of the precursor mineral such as relatively high sc y and hree contents the trace element variations among high pressure clinopyroxenes in different textural domains are greater for blueschists than for eclogites this reflects the more limited major element equilibration of blueschists relative to eclogites in deformed eclogites the clinopyroxenes lack the geochemical signatures of the original igneous minerals most probably due to the enlargement of reaction domains caused by synmetamorphic deformation trace element partitioning between omphacite and garnet suggests that in both undeformed and deformed eclogites chemical equilibrium is locally approached and therefore the trace element composition of clinopyroxenes is controlled by the coexisting minerals for instance clinopyroxenes in eclogites are strongly hree and lree depleted i e with bell shaped pattern due to the equilibration with garnet and allanite respectively the partition coefficients for rees between omphacite and garnet are markedly lower than those reported in the literature on the basis of crystal chemical considerations we suggest that this is probably due to lower temperature conditions during formation

non newtonian rheology of igneous melts at high stresses and strain rates experimental results for rhyolite andesite basalt and nephelinite the stress strain rate relationships of four silicate melt compositions high silica rhyolite andesite tholeiitic basalt and nephelinite have been studied using the fiber elongation method measurements were conducted in a stress range of 10 400 mpa and a strain rate range of 10 6 to 10 3 s 1 the stress strain rate relationships for all the melts exhibit newtonian behavior at low strain rates but non newtonian nonlinear stress strain rate behavior at higher strain rates with strain rate increasing faster than the applied stress the decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt the decrease in viscosity observed at the high strain rates of the present study ranges from 0 25 to 2 54 log10 pa s the shear relaxation times of these melts have been estimated from the low strain rate newtonian shear viscosity using the maxwell relationship s g non newtonian shear viscosity is observed at strain rates time 1 equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time brittle failure of the fibers occurs 2 log10 units of time above the relaxation time this study illustrates that the occurrence of non newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate high silica rhyolite melts involved in ash flow eruptions are expected to undergo a non newtonian phase of deformation immediately prior to brittle failure

partitioning and budget of li be and b in high pressure metamorphic rocks abstract partitioning and budget of li be and b in high pressure metamorphic rocks from the island of syros greece were studied using secondary ion mass spectrometry inductively coupled plasma optical emission spectrometry and prompt gamma neutron activation analysis partitioning between coexisting mineral phases was found to be rather constant and independent of element concentrations for several mineral pairs apparent partition coefficients vary in a narrow range while concentrations vary by more than an order of magnitude hence it was possible to establish sets of inter mineral partition coefficients for li be and b among 15 different high pressure minerals this data set provides important information on the behaviour of the light elements in different lithologies within subducting slabs from the onset of metamorphism to the eclogite stage it is essential for modelling trace element and isotope fractionation during subduction and dehydration of oceanic crust

the effect of fluid and deformation on zoning and inclusion patterns in poly metamorphic garnets within the bergen arcs of w norway caledonian eclogite facies assemblages t 650 c p 15 kbar have formed from grenvillian granulites t 800 900 c p 10 kbar along shear zones and fluid pathways garnets in the granulites grti pyr56 40 alm45 25gro19 14 are unzoned or display a weak ca 1 wt feo over 1000 m zoning the eclogite facies rocks contain garnets inherited from their granulite facies protoliths these relict garnets have certain areas with compositions identical to the garnets in their nearby granulite but can be crosscut by bands of a more almrich composition grtii pyr31 41alm40 47gro17 21 formed during the eclogite facies event these bands orientated preferentially parallel or perpendicular to the eclogite foliation may contain mineral filled veins or trails of eclogite facies minerals omphacite amphibole white mica kyanite quartz and dolomite steep compositional gradients up to 9 wt feo over 40 m separate the two generations of garnets indicating limited volume diffusion the bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites the re equilibration of granulite facies garnets during the eclogite facies event must therefore be a function of fracture density deformation and fluid availability the results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re equilibration of garnets

differential patterns of benthic foraminiferal extinctions near the paleocene eocene boundary in the north atlantic and the western tethys bathyal benthic foraminifera and stable isotopes from the caravaca and zumaya sections of spain indicate that the mass extinction near the paleocene eocene p e boundary upper planktic foraminiferal zone p6a was rapid coinciding with a negative 13c shift of 2 4 the onset of dark grey shale deposition and increased carbonate dissolution prior to the extinction event diverse benthic foraminiferal assemblages and high 13c values suggest the presence of a stable intermediate deep watermass rich in oxygen and saturated in calcium carbonate in both caravaca and zumaya sections after the extinction event small finely agglutinated haplophragmoides glomospira and thin walled epifaunal taxa nuttallides truempyi dominate in the north atlantic zumaya section this assemblage indicates sluggish circulation bottom waters undersaturated in calcium carbonate and dysaerobic conditions low oxygen conditions prevailed for the succeeding 400 kyr and are characterized by 1 2 fluctuations in benthic 13c in the western tethys caravaca section infaunal taxa buliminids dominate after the extinction event and prevailed through the succeeding 400 to 450 kyr of decreased 13c values indicating low oxygen conditions despite these common features benthic foraminiferal assemblages from these two sections exhibit significant differences during the extinction event at the zumaya section the extinction event occurred over less than 50 kyr and resulted in a sudden reduction in species richness after the 13c shift in contrast at the caravaca section species diversity declined gradually beginning prior to and in concert with the gradually declining 13c values this gradual change occurred over a period of 250 kyr and culminated in a 4 13c shift these differences in 13c values and benthic assemblages between caravaca and zumaya suggest that the p e oceanographic changes that triggered the benthic foraminiferal mass extinction exhibited its first manifestation in the tethys region

viscosity of himalayan leucogranites implications for mechanisms of granitic magma ascent the viscosity of the gangotri himalayan leucogranite has been experimentally determined between 80 o and 1100oc 300 and 800 mpa for meltwater contents of 3 98 and 6 66 wt the melt viscosity is independent of pressure and shows an arrhenian behavior relative to temperature within the range of conditions investigated we present an empirical relation that can be used to determine leucogranite magma viscosities knowing their meltwater content and temperature this relation together with phase equilibria experiments constrain the viscosity of the itimalayan leucogranit s to be around 104 5 pas during their eraplacement these viscosities and the widths of dikes belonging to the feeder system are consistent with the theoretical relationship relating these two parameters and show that the precursor magma of the leucogranite was at near liquidus conditions when emplaced within host rocks with preintmsion temperatures around 350oc calculated terminal asa nt rates for the magma in the dikes are around 1 m s magma cmber assembly time is on ths basis estimated to be less than 100 years for a volume of 150 icm3 in addition the dynamical regime of the magma flow in the dikes was essentially laminar thus allowing preservation of any chemical heterogeneity acquired in the source these results constrain the viscosity of melts f rmed during the first steps of crustal anatexis those involving muscovite breakdown to be alsoround 104 5 pas thus compaction may not be the only mechanism of melt segregation in partially melted crustal rocks in view of the very short timescale inferred for magma ascent and emplacement

a potential method to confirm the previous existence of lawsonite in eclogite the mass imbalance of sr and lrees in multistage epidote ganghe dabie uhp terrane lawsonite is an important hydrous mineral that is stable at low temperature lt and high to ultrahigh pressure hp uhp conditions in subducted slabs the occurrence absence of lawsonite in eclogite is a significant constraint for the construction of the metamorphic tectonic and fluid melt evolution histories of an hp uhp terrane however lawsonite is very rarely preserved in natural eclogites and accurate judgment of its former existence is a significant challenge for petrologists at present whether lawsonite has ever existed in lawsonite absent eclogite is mainly judged by i pseudomorphs after lawsonite and ii phase equilibria modelling in this study major element and trace element distributions in multistage minerals were examined in the ganghe lawsonite absent uhp eclogite in the dabie uhp terrane eastern china this work demonstrates that the whole rock sr and light rare earth elements lrees are mainly dominated by epidote other minerals garnet omphacite quartz kyanite barroisite phengite and accessory minerals play a very limited role in the sr and lrees budgets two stages of epidote which have noticeably different sr and lrees contents were recognized in the eclogite i epidote porphyroblasts ep p core which are suspected to be the pseudomorphic mineral after lawsonite contain significantly high sr 7200 10 300 ppm and lrees 160 1300 ppm for la ii an earlier stage epidote ep in core occurs as inclusions in matrix omphacite or in omphacite inclusions in the suspected pseudomorphic minerals after lawsonite spmal this early epidote has significantly lower sr 990 1890 ppm average 1495 ppm n 17 and lrees contents 60 110 ppm for la average 91 ppm n 17 all of the existing early stage minerals predating the spmal have very low contents of sr and lrees and the total amounts of these elements in the early stage minerals do not balance those in the spmal this indicates that a missing ca al sr and lree rich mineral which was previously in equilibrium with the early stage minerals likely existed in the ganghe eclogite on the basis of the mineral geochemistry and phase equilibria modelling we confirm that the missing mineral cannot be anything but lawsonite this study indicates that examining the mass im balance of sr and lrees between multistage hp uhp epidote can be used as a potential method to confirm the previous existence of lawsonite in lawsonite absent eclogite

effects of cation substitutions in garnet and pyroxene on equilibrium oxygen isotope fractionations high grade metamorphic rocks were used to explore oxygen isotope fractionations between pyroxene and garnet and to investigate the effects on fractionation factors of the cation substitutions fe3 al 1 and ca fe mg 1 recrystallized granulite facies 725 c wollastonite ores from the northern adirondack highlands contain essentially only the minerals clinopyroxene a di hd solid solution garnet a grs adr solid solution wollastonite and exhibit a systematic dependence of measured fractionations on the fe3 content of calcic garnet cpx cagrt 0 14 0 12 0 78 0 20 xadr and wo cagrt 0 15 0 22 0 57 0 33 xadr in eclogites formed at t 650 c measured compositions of ca poor garnet and omphacite combined with experimental data indicate that ca poor fe rich garnet is enriched in 18o compared to both diopside and grossular extrapolating to 1000 k alm di c 0 2 and alm grs c 0 5 orthopyroxene and clinopyroxene from gore mountain new york show a constant fractionation that is independent of rock type as expected if they have the same closure temperature these data imply opx cpx c 0 7 at 1000 k measured fractionations among ca poor garnet orthopyroxene clinopyroxene and hornblende in the gore mountain rocks further indicate an 18o enrichment in ca poor garnet over grs c 0 5 at 1000 k the new measurements are indistinguishable from expected equilibrium values based on experiments for the minerals enstatite diopside grossular wollastonite and feldspar but consistently indicate a significant isotope effect for the simple octahedral cation substitutions fe3 al 1 grs vs adr and ca fe mg 1 ca poor garnet vs grs opx vs cpx neither cation substitution has been directly investigated for its effect on 18o 16o fractionation with experiments in silicates chemical characterization of minerals is required prior to petrological interpretation of oxygen isotope trends

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

fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a quartz vein within ultrahigh pressure eclogite quartz veins in high pressure to ultrahigh pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis this flow can occur in the direction of changing temperature pressure during subduction or exhumation shrimp u pb dating of zircon from a kyanite quartz vein within ultrahigh pressure eclogite in the dabie continental collision orogen yields two age groups at 212 7 and 181 13 ma which are similar to two groups of la icpms age at 210 4 and 180 5 ma for the same sample these ages are significantly younger than zircon u pb ages of 224 2 ma from the host eclogite thus the two age groups from the vein date two episodes of fluid flow involving zircon growth the first due to decompression dehydration during exhumation and the second due to heating dehydration in response to a cryptic thermal event after continental collision laser fluorination o isotope analyses gave similar 18o values for minerals from both vein and eclogite indicating that the vein forming fluid was internally derived synchronous cooling between the vein and eclogite is suggested by almost the same quartz mineral fractionation values with regularly decreasing temperatures that are in concordance with rates of o diffusion in the minerals while the quartz veining was caused by decompression dehydration at 700 650 c in a transition from ultrahigh pressure to high pressure eclogite facies retrogression the postcollisional fluid flow was retriggered by heating dehydration at 500 c without corresponding metamorphism in either case the kyanite quartz vein formed later than the peak ultrahigh pressure metamorphic event at the middle triassic pointing to focused fluid flow during exhumation rather than subduction the growth of metamorphic zircon in the eclogite appears to have depended on fluid availability so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles

metasomatism in a subduction complex constraints from microanalysis of trace elements in minerals from garnet amphibolite from the catalina schist trace element abundances and zoning were measured in minerals from a metasomatized garnet amphibolite block from the catalina schist using both ion and proton microprobes zoisite strongly concentrates light rare earth elements rees sr y and pb amphibole concentrates ni and zn garnet concentrates y and heavy rees and titanite concentrates nb major and trace elements in the garnets are zoned garnets in the core of the block display an overgrowth enriched in mn y and heavy ree on a y and heavy ree poor core zirconium values remain relatively constant the element enrichments in the garnet overgrowth suggest mobility of rees at either a hand sample or regional scale at subduction zone pressure temperature conditions metamorphic fluids may selectively transport heavy rees relative to some high field strength elements in some convergent margin settings the distribution of sr and pb within subduction zones may reflect the dehydration and melting behavior of epidote group minerals

lawsonite composition and zoning as tracers of subduction processes a global review abstract lawsonite is an abundant hydrous mineral in oceanic crust sediments and metasomatic rocks at depths of 45 to 300 km in most subduction zones but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism owing to the significance of lawsonite for water and element cycling in subduction zones occurrences of fresh lawsonite in blueschist and more rarely eclogite provide important opportunities to determine lawsonite composition zoning and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation in this review we use new and published data to document lawsonite composition in eight of the nine known lawsonite eclogite localities in which fresh lawsonite coexists with garnet omphacite in the rock matrix as well as the composition of lawsonite inclusions in six of seven known sites in which lawsonite occurs only as inclusions in garnet in eclogite facies rocks that lack matrix lawsonite as lawsonite blueschist is much more common than lawsonite eclogite we survey the composition of lawsonite in representative localities of blueschist including blueschist associated with eclogite lawsonite bearing epidote bearing and blueschist not associated with eclogite at current exposure levels included in this review are metabasaltic rocks silica and carbonate rich metasedimentary rocks metasomatic rocks and lawsonite rich veins this dataset demonstrates that lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation and specifically of fluid rock interaction with implications for element cycling in subduction zones furthermore most exhumed lawsonite eclogite records slab surface conditions that correspond to the location where the slab mantle interface transitions from decoupled to coupled and therefore provides key insights into the thermal history and dynamics of subduction zones

the effect of water on the viscosity of a haplogranitic melt under p t x conditions relevant to silicic volcanism the viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040 c have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques the micropenetration viscometry has been performed in the viscosity range of 109 pa s to 1012 pa s the samples ranged in water content from 0 4 to 3 5 wt for samples with up to 2 5 wt h2o the water contents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements above this water content a measurable loss of water occurs during the viscometry the viscosity data illustrate an extremely nonlinear decrease in viscosity with added water the viscosity drops drastically with the addition of 0 5 wt of water and then shallows out at water contents of 2 wt an additional viscosity datum point obtained from the analysis of fluid inclusions in a water saturated hpg8 confirms a near invariance of the viscosity with the addition of water between 2 and 6 wt these measurements may be compared directly with the data of hess et al 1995 in press for the effects of excess alkali and alkaline earth oxides on the viscosity of hpg8 also obtained at 1 bar the viscosity of the melts compared on an equivalent molar basis increases in the order h2o li2o na2o k2o rb2o cs2o bao sro cao mgo beo the extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of shaw 1972 for the range of water contents investigated here the speciation of water in the quenched glasses can be used to quantify the dependence of the viscosity on hydroxyl content considering only the hydroxyl groups as active fluidizers in the hydrous melts the nonlinearity of the viscosity decrease and the difference with the effects of the alkali oxides becomes larger consequences for degassing calcalkaline rhyolite are discussed

individualization of textural and reactional microdomains in eclogites from the bergen arcs norway consequences for rb sr and ar ar radiochronometer behavior during polymetamorphism 1 rb sr 40ar 39ar and sm nd isotopic data are reported in caledonian eclogites from the lindas nappe bergen arcs norway in order to investigate processes controlling isotopic equilibrium at mineral scale in polymetamorphic rocks the bergen arcs exposes sveconorwegian 950 930 ma granulites partially overprinted by caledonian eclogite facies metamorphism at 425 ma and amphibolite facies metamorphism at 410 ma geochemical and rb sr data from more than 10 phengite fractions separated from one sample reflect the composition of the microdomain a few hundred microns in size in which phengite crystallized phengite crystallized after garnet or plagioclase by dissolution precipitation processes yield apparent age between 700 and 600 ma at the time of their crystallization these phengites inherited the isotopic composition of their precursor minerals at a microdomain scale phengite from quartz veins which crystallized from elements mobilized by the circulating fluid yield an age closer to the eclogite facies metamorphic age the closed system evolution of the eclogitizing fluid the segregation of textural and reactional microdomains the high sr content of the studied phengite and the short duration of the recrystallization processes 1ma are interpreted as the main factors responsible for the lack of a rb sr isotopic equilibrium at the scale of hand samples such equilibrium is nevertheless reached in quartz veins where the crystallization of minerals implies that the fluid circulation acted as a factor of isotopic homogenization the in situ single mineral ar ar datings revealed that both eclogite and amphibolite facies minerals are characterized by an excess of radiogenic argon ages between 425 and 520 ma the excess of argon has been inherited from the previous granulite and has been only partially evacuated from the system by the circulating fluid the apparent sm nd ages from garnets inherited from the granulite facies metamorphism c 930 ma are in agreement with previous estimates this result confirms that the diffusion of nd in garnet does not occur at temperatures lower than 700 c this study highlights the complexity of radiochronometer behavior during hp metamorphism and demonstrates that coupling different radiochronometers such as ar ar and rb sr does not always guarantee the validity of the geochronological results

rare earth element redistribution during high pressure low temperature metamorphism in ophiolitic fe gabbros liguria northwestern italy implications for light ree mobility in subduction zones to unravel the rare earth element ree redistribution during high pressure low temperature metamorphism we have analyzed by ion microprobe all the minerals from representative fe gabbros from ligurian metaophiolites northwestern italy contrary to what is observed for fresh fe gabbros the clinopyroxene contribution to the whole rock ree inventory of blueschists and eclogitized fe gabbros is minor or negligible in both blueschists and eclogites ree are redistributed among newly formed minerals in blueschists lawsonite is the major ree carrier and concentrates almost all the light ree lree in the rock whereas titanite plays an important role for middle and heavy ree mree hree in eclogites lree and hree mainly reside in accessory allanite and garnet respectively the development of blueschist and eclogite facies mineral assemblages in mafic oceanic crust due to subduction to 35 65 km depths was not accompanied by release of significant amounts of lree to the overlying mantle wedge owing to the stability of lawsonite and epidote at ultra high pressure conditions subduction of mafic rocks is considered an effective mechanism to bring lree to great depth within the mantle

progressive eclogitization under fluid present conditions of pre alpine mafic granulites in the austroalpine mt emilius klippe italian western alps metabasites from the austroalpine mt emilius klippe preserve different stages of the evolution from pre alpine granulite to alpine eclogite facies assemblages the pre alpine protolith includes layered garnet i clinopyroxene i plagioclase hornblende granulites and gabbros the eclogitic paragenesis comprises garnet ii clinopyroxene ii epidote ca amphibole chlorite eclogitic deformation is composite and involves two main episodes d1a d1b d1a produces a layer parallel foliation and incomplete transformation of granulites and is concluded by an episode of brittle deformation leading to the formation of eclogitic veins d1b gives rise to widespread s l tectonites and mylonites the eclogitic veins and surrounding haloes enriched in fluids during veining control the nucleation of d1b shear zones deformation is accomplished through ductile flow of reaction weakened plagioclase and hornblende sites and cataclasis of garnet i and clinopyroxene i both meso and microscale observations indicate that fluids play a primary role during eclogitization in both enhancing deformation and the kinetics of metamorphic reactions fluids influence plastic flow rates in matrix aggregates and promote fracturing of hard minerals the extent of metamorphic re equilibration is closely related to the density of the fluid pathways

chromium and manganese zoning in pelitic garnet and kyanite spiral overprint and oscillatory zoning patterns and the role of growth rate x ray composition maps and quantitative analyses for mn ca and cr have been made for six pelitic and calc pelitic garnet crystals and al fe and cr analyses maps have been made for two kyanite crystals from lower and mid upper amphibolite facies rocks from the grenville province of western labrador using an electron microprobe analyser and a laser ablation icp ms garnet with spiral snowball internal fabrics si has spiral zoning in major elements implying that growth was concentrated in discrete regions of the crystal at any one time spiral zoning cr zoning is parallel to si in low amphibolite facies garnet with both straight and spiral internal fabrics indicating that the garnet overprinted a fabric defined by cr rich mica chlorite epidote and cr poor quartz plagioclase layers during growth overprint zoning and that cr was effectively immobile in contrast in mid upper amphibolite facies garnet porphyroblasts lacking si cr zoning is concentric implying that cr diffusion occurred cr zoning in kyanite porphyroblasts appears superficially similar to oscillatory zoning with up to three or four annuli of cr enrichment and or depletion present in a single grain however the variable width continuity cr concentration and local bifurcation of individual annuli suggest that an origin by overprint zoning may be more likely the results of this study explain previously observed nonsystematic cr zoning in garnet and irregular partitioning of cr between coexisting metamorphic mineral pairs in addition this study points to the important role of crystal growth rate in determining the presence or absence of inclusions and the type of zoning exhibited by both major and trace elements during fast growth inclusions are preferentially incorporated into the growing porphyroblast and slow diffusing elements such as cr are effectively immobile whereas during slow growth inclusions are not generally included in the porphyroblast and cr zoning is concentric

structural development and petrofabrics of eclogite facies shear zones bergen arcs western norway implications for deep crustal deformational processes caledonian eclogite facies shear zones developed from grenvillian garnet granulite facies anorthosites and gabbros in the bergen arcs of western norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation cpo in lower continental crust field relations on the island of holsnoy show that the eclogites formed locally from granulite facies rocks by progressive development of 1 eclogite adjacent to fractures 2 eclogite in discrete shear zones 2 m thick 3 eclogite breccia consisting of 80 well foliated eclogite that wraps around rotated granulite blocks and 4 anastomosing subparallel eclogite facies shear zones 30 100 m thick continuous over distances 1 km within the granulite terrane these shear zones deformed under eclogite facies conditions at an estimated temperature of 670 50 c and a minimum pressure of 1460 mpa which corresponds to depths of 55 km in the continental crust detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite garnet rich and kyanite zoisite rich layers a consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite the cpo of omphacite determined from five axis universal stage measurements shows a strong b axis maximum normal to foliation and a c axis girdle within the foliation plane with weak maxima parallel to the lineation direction these patterns are consistent with deformation of omphacite by slip parallel to 001 and suggest glide along 010 the lineation and cpo data reveal a consistent sense of shear zone movement although the displacement was small localized faulting of high grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth deformation of granulite and partial conversion to eclogite such as is exposed on holsnoy island may be an orogenic scale process in the lowermost crust of collisional orogens

oxygen isotope equilibrium between eclogite minerals and its constraints on mineral sm nd chronometer abstract sm nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the sulu terrane in eastern china the results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and sm nd isotope systems of eclogite minerals the omphacite garnet pairs of oxygen isotope equilibrium at eclogite facies conditions yield meaningful triassic sm nd isochron ages whereas those of oxygen isotope disequilibrium give non triassic ages of geological meaninglessness this can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than or close to those of nd diffusion and thus attainment of isotopic equilibrium in the omphacite garnet o system suggests achievement of nd isotope equilibrium in the same mineral pairs the presence or absence of fluid in the eclogite protoliths is a major rate controlling factor for isotopic equilibration during high grade metamorphism it appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the sm nd mineral chronometer in addition the exact timing of the ultrahigh pressure metamorphism in the dabie sulu terranes is constrained at early triassic rather than late triassic

a ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the middle adula nappe central alps switzerland a ternary solid solution model for omphacite with the end members jadeite naalsi2o6 diopside camgsi2o6 and hedenbergite cafesi2o6 was derived from experimental data from the literature the subregular solution model fitted by linear programming is best suited to omphacites with very little aegirine component in common eclogites applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the adula nappe central alps switzerland results in large stability fields for common eclogite assemblages garnet omphacite quartz h2o kyanite within this field the compositions of garnet and omphacite show very little variation a precise determination of the peak pressure and temperature is not possible the occurrence of amphibole overgrowing the peak pressure assemblage in fresh eclogite suggests retrograde re equilibration still under eclogite facies conditions the computation of isopleths for garnet and pyroxene end members allows the estimation of the pressure and temperature conditions of this re equilibration event 19 21 kbar c 700 c

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

sr bearing zoisite and epidote in ultra high pressure uhp metamorphic rocks from the su lu province eastern china an important sr reservoir under uhp conditions sr bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the su lu ultra high pressure uhp province eastern china the sro content of prograde zoisite and epidote reaches 3 2 wt in crystal cores and generally decreases toward crystal margins retrograde epidote is poorer in sro 0 1 wt preliminary rare earth element ree analyses of epidote give la2o3 up to 2 9 wt ce2o3 5 9 wt and nd2o3 3 0 wt ree contents of zoisite are distinctly lower la 2o3 up to 0 16 wt ce2o3 up to 0 26 wt and nd2o3 up to 0 16 wt than coexisting epidote apatite is always more depleted in sro 0 10 0 59 wt on average than coexisting zoisite and epidote and sr ca partition coefficients for zoisite and epidote and apatite sr ca zo ep ap range from 5 to 20 sro content of k white mica 0 012 0 044 wt is an order of magnitude lower than that of apatite an evaluation of the sro content in zoisite and epidote and their modal abundances in seven samples indicates that 70 of the whole rock sro is contained in these minerals apatite and k white mica are only minor reservoirs for sro in these rocks zoisite and epidote are thus regarded as the most important sr reservoirs at uhp conditions where calcic plagioclase and titanite are unstable

implications of near rim compositional zoning in rutile for geothermometry geospeedometry and trace element equilibration in principle compositional profiling of the near rim region of minerals can provide insight into cooling rates but presumes that loss or gain of material from the crystal rim is not kinetically restricted trace element depth profiles collected for zr hf ta nb and u in amphibolite facies rutile grains of the catalina schist southern california show significant variability within a single rock profiles of the same element among different grains can have significantly different slopes grains with indistinguishable zr profiles show vastly different nb profiles and grains with indistinguishable nb profiles show different zr profiles textural and kinetic idiosyncrasies within the matrix apparently affect the ability of specific crystals to accept or release trace elements and impugn the common assumption that mineral surfaces maintain equilibrium at amphibolite facies conditions a new model that limits the flux of zr from rutile grains helps explain commonly observed compositional profiles and implies that inversion of compositional profiles assuming equilibrium among grain surfaces will invariably overestimate cooling rates few grains may record the low closure temperatures that experimentally determined diffusivities imply rather higher temperatures will be retained depending on the proximity of reactants and products in the matrix silicon diffusion does not control zr reequilibration in rutile and relative diffusion coefficients d s of trace elements in rutile are dzr dhf 10dnb 20dta 40du

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

in situ oxygen isotope major and trace element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from roberts victor kaapvaal craton a subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development however eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin e g garnet 18o values and mineral 87sr 86sr compositions to assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond forming and other possible metasomatic fluids we have conducted a multi technique in situ study of a diamondiferous eclogite xenolith from the roberts victor kimberlite s africa using sims we provide the first texturally controlled in situ measurements of 18o values in eclogitic garnet in close proximity to diamond garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from mg 75 8 79 2 grossular proportions 8 05 10 14 mol and omphacitic pyroxene has jd13 24 and mg 86 6 90 0 rare earth element patterns of minerals across the xenolith including grains close to diamond are typical lree depleted garnets and markedly lree enriched pyroxenes these silicate minerals also record detectable intra and inter grain lree abundance variations clinopyroxenes of the studied xenoliths show hfse and sr abundance variations that are decoupled from lree contents and major element variations mineralogical constraints and bulk rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small volume 0 03 wt infiltration of metasomatic fluid s potentially linked to ancient diamond evolution intra grain major element lree and hfse variations in clinopyroxene resulted from late stage metasomatism oxygen isotope compositions in garnet are decoupled from all major and trace element variations with garnet 18o values being uniform across the xenolith in a wide variety of textural settings garnet 18o values of 6 5 0 2 are higher than the mean 5 19 0 26 of the mantle garnet range 4 8 5 5 modelling of the buffering effect of mantle peridotite on co2 rich and h2o rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified after percolative flow through dominantly peridotitic mantle at great depth is very low as we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet 18o values is via inheritance from a crustal protolith altered at relatively low temperatures these results have broader relevance and support the hypothesis of a low pressure protolith for mantle eclogite xenoliths demonstrating the robust nature of garnet oxygen isotope compositions even in diamond bearing eclogites

cr rich magnesiochloritoid eclogites from the monviso ophiolites western alps italy cr rich magnesiochloritoid in the eclogitized ophiolites of the monviso massif occurs in the least differentiated rocks of the gabbroic sequence troctolites to melatroctolites chloritoid xmg 0 63 0 85 cr 0 55 atoms co exists with omphacite talc and garnet minor syn eclogitic minerals are chromite rutile and sometimes magnesite and cr ti oxides coronitic textures indicative of a static recrystallization characterize the analysed samples layers of variable mineral composition develop among igneous plagioclase olivine clinopyroxene and spinel the minerals in the coronitic layers display sharp compositional zonings the igneous minerals are commonly not preserved their presence in the original assemblage is inferred from the mineralogical composition of the pseudomorphs syn eclogitic volatile components are indicated by the development of oh bearing minerals e g chloritoid talc and carbonates e g magnesite and supported by the presence of coarse grained and fibrous mineral growths the complex pseudomorphic replacements of igneous minerals suggest that these rocks changed their mineralogical composition prior to the eclogite facies recrystallization most likely during ocean floor metamorphism it is suggested that syn eclogitic fluids formed by breakdown reactions of pre eclogitic volatile bearing minerals geothermobarometry indicates that the investigated rocks recrystallized at a depth corresponding to 2 4 gpa and temperatures of 620 50 c the attainment of high pressure conditions is supported by the presence of magnesiochloritoid magnesite and garnet with high pyrope content up to 58 mol p t estimates point to a very low thermal gradient about 9 c km 1 comparable to that deduced in the adjacent dora maira ultra high pressure unit

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

cs rb ba systematics in phengite and amphibole an assessment of fluid mobility at 2 0 gpa in eclogites from trescolmen central alps eclogites from trescolmen that contain abundant hydrous minerals phengite amphibole paragonite zoisite talc apatite show petrographic evidence for fluid infiltration under conditions of 2 0 to 1 8 gpa 650 c large ion lithophile elements lile e g cs rb ba and sr were analysed by in situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid mobile elements at high pressure in situ analysis of carefully chosen metamorphic assemblages circumvents the problem of partial late stage alteration which can severely influence the calculated element budgets of whole rock samples phengite is the dominant host for cs rb and ba in both eclogite and adjacent garnet mica schist samples and incorporates 90 of the budgets of these elements in whole rocks lile contents of phengites in phengite rich rocks are likely to record the cs rb and ba rb ratios of their host rock protoliths the lile patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration whereas those of mica schist are almost identical to average upper continental crust in contrast lile patterns of eclogite samples that lack phengite but do contain amphibole are unlike any plausible protolith but are identical to those of amphibole in phengite bearing samples this observation points to homogenization of the lile in different lithologies which we correlate with petrographic evidence for fluid infiltration because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to cs rb and ba homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies implying at least metre scale fluid mobility the amphibole homogenization can be most easily modelled by a pervasive open system fluid flux through the eclogites possibly facilitated by ductile deformation during the early stages of uplift simple calculations give minimum fluid rock ratios of 0 001 to 0 004 demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across arc variations in trace element geochemistry the low fluid rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities fluid mobile elements such as cs rb and ba are more sensitive indicators of small volume fluid rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable

p t x effects on equilibrium carbonate h 2 o co 2 nacl dihedral angles constraints on carbonate permeability and the role of deformation during fluid infiltration fluid solid solid dihedral angles in the nacl h2o co2 calcite dolomite magnesite system have been determined at pressures ranging from 0 5 to 7 kbar and temperatures from 450 c to 750 c at 1 kbar and 650 c both dolomite and magnesite exhibit a dihedral angle minimum for intermediate h2o co2 fluids similar to that previously determined by the present authors for calcite but the depth of the minimum is smaller being above the critical value of 60 for both dolomite and magnesite for all fluid compositions calcite calcite brine dihedral angles at 650 c have been determined in the pressure range 1 5 kbar angles decrease with increasing salt content of the fluid tending towards a constant value of about 65 for strong brines at pressures above 2 kbar there is a general increase of angle with increasing pressure which is most marked for strong brines a positive correlation of angle with pressure is also observed in calcite h2o co2 fluids the position of the minimum moving towards higher angles and towards h2o rich fluids with increasing pressure the permeability window previously observed by the present authors at 1 kbar and intermediate fluid compositions closes at about 1 5 kbar the results demonstrate that the permeability of carbonates to grain edge fluid flow is only possible at low pressures and for fluids of restricted h2o co2 nacl compositions however geochemical evidence from metamorphic terrains suggests that pervasive infiltration does occur under conditions where impermeability is predicted from examination of published studies of infiltrated carbonates we conclude that deformation plays a critical role in enhancing carbonate permeability possible mechanisms for this include shear enhanced dilatancy micro cracking fluid inclusion drag by deformation controlled grain boundary migration and dynamically maintained transient grain boundary fluid films

crystal chemical controls on rare earth element partitioning between epidote group minerals and melts an experimental and theoretical study we have experimentally determined the partitioning of ree rare earth elements between zoisite and hydrous silicate melt at 1 100 c and 3 gpa all ree behave moderately compatible in zoisite with respect to the melt and all d ree zo melt show a smooth parabolic dependence on ionic radius the partitioning parabola peaks at nd left d nd zo melt 4 9 right and the compatibility slightly decreases towards la left d la zo melt 3 9 right and decreases by half an order of magnitude towards yb left d yb zo melt 1 1 right application of the elastic strain model of blundy and wood 1994 to the available zoisite and allanite ree mineral melt partitioning data and comparison with partitioning pattern calculated from a combination of structural and physical data taken from the literature with the elastic strain model suggest that in zoisite ree prefer the a1 site and that only la and ce are incorporated into the a2 site in significant amounts in contrast in allanite all ree are preferentially incorporated into the large and highly co ordinated a2 site as a result zoisite fractionates the mree effectively from the hree and moderately from the lree while allanite fractionates the lree very effectively from the mree and hree consequently the presence of either zoisite or allanite during slab melting will lead to quite different ree pattern in the produced melt

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

the viscosity of hydrous phonolites and trachytes the 1 atm viscosities of hydrated synthetic iron free phonolite and trachyte melts are reported between 108 4 and 1013 1 pa s for water contents between 0 and 5 wt these show a very strong reduction with increasing water content particularly at low contents empirical formulae are derived for the dependence of viscosity on temperature and water content at magmatic temperatures of about 1100 k and water contents of 5 wt viscosities are about 910 pa s for the phonolite and 1120 pa s for the trachyte comparisons with data on peralkaline rhyolites show that the effects of dissolved water are similar even though the viscosities of the different compositions vary this indicates that the mechanism by which water reduces the viscosities of partially depolymerized aluminosilicate liquids is independent of the alkali alkaline earth element ratio in addition no major change in this pattern is observed whether water dissolves primarily as oh or molecular h2o this suggests that a third water dissolution mechanism may be important in these compositions in addition to si o si bond breaking and dissolution of molecular water

evolution of the ree mineralogy in hp lt metapelites of the sebtide complex rif morocco monazite stability and geochronology abstract ree mineralogy was characterized at the micrometer scale using scanning electron microscopy in the four tectono metamorphic units of the beni mzala window sebtide complex internal rif morocco which sample a hp lt metamorphic gradient from subgreenschist to blueschist eclogite facies these tectonic units are composed of garnet free metapelites and associated synmetamorphic veins from the same homogeneous aluminium rich and calcium poor protolith in that context mineralogical differences between units are assumed to result from contrasted pressure and temperature histories light ree bearing al phosphates of the crandallite group with florencite rich composition and variable goyazite content are the dominant lree minerals in the lowest grade units tizgarine 300 c 3 kbar as peak conditions and boquete de anjera 350 c 10 kbar in the latter unit metamorphic florencite is associated with mree rich monazite in the schist of the highest grade units beni mzala 2 420 450 c 12 kbar and beni mzala 1 550 c 16 kbar associations of allanite rich epidote and synchisite a lree fluorocarbonate are found whereas florencite and monazite are no longer observed at the wall of the quartz kyanite veins retrograde monazite ce with grain size of 20 to 50 m is the only lree bearing mineral th pb and u pb sims data on some of these grains yield ages of 21 3 1 7 ma and 20 9 2 1 ma respectively consistent with previous k ar ages obtained on retrograde clay mica mixtures in rocks from the same locality the identification of a stability field for monazite in high pressure aluminous metapelites with an upper thermal limit below 450 c as well as the derivation of meaningful u pb and th pb sims ages demonstrates the monazite potential for dating hp lt metapelites however since under these conditions monazite growth occurs below its admitted closure temperature these ages must be interpreted on the basis of monazite textural relationships

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

timing of high pressure metamorphism in the yukon tanana terrane canadian cordillera constraints from u pb zircon dating of eclogite from the teslin tectonic zone zircon from eclogite near last peak in the teslin tectonic zone yielded a u pb isotopic age of 269 2 ma 2 the first precise age for such a rock in the yukon tanana terrane of the canadian cordillera both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin and the u pb age therefore constrains the timing of peak high pressure metamorphism in this rock the u pb age demonstrates for the first time that an early permian high pressure metamorphic event occurred in rocks now making up the teslin tectonic zone and possibly elsewhere in the yukon tanana terrane this u pb age provides a new geochronologic pin in the evolution of the yukon tanana terrane prior to its mesozoic accretion to the north american continental margin and combined with recent 40ar 39ar muscovite data indicates that high pressure metamorphism at this time was a relatively short lived event

crustal reworking and hydration insights from element zoning and oxygen isotopes of garnet in high pressure rocks sesia zone western alps subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere a better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial in this study major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the sesia zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin garnet shows a variety of textures requiring dissolution precipitation processes in presence of fluids in polycyclic metasediments garnet preserves a partly resorbed core related to pre alpine high temperature low pressure metamorphism and one or multiple rim generations associated with alpine subduction metamorphism in eclogites garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim in metasediments pre alpine garnet relics show 18o values up to 5 3 higher than the alpine rims while no significant variation is observed among different alpine garnet generations within each sample this suggests that an extensive re equilibration with an externally derived fluid of distinct lower 18o occurred before or in correspondence to the first alpine garnet growth while subsequent influxes of fluid had 18o close to equilibrium the observed shift in garnet 18o is attributed to a possible combination of 1 interaction with sea water derived fluids during pre alpine crustal extension and 2 fluids from dehydration reactions occurring during subduction of previously hydrated rocks such as the serpentinised lithospheric mantle or hydrated portions of the basement

eclogite meso and microfabrics implications for the burial and exhumation history of eclogites in the tauern window eastern alps from p t d paths abstract the eclogite zone of the central southern tauern window comprises eclogites and associated high pressure metasediments that are intercalated between penninic basement units in the foot wall and an imbricate stack in the hanging wall this nappe stack consists of continental basement units cover sequences of a distal continental slope and the main part of the ophiolitic glockner nappe textures and microfabrics of eclogites from the central southern tauern window allow the establishment of the eclogitic and post eclogitic deformation histories from burial by subduction to subsequent exhumation the metamorphic evolution of the eclogites is documented by 1 initial greenschist to blueschist facies 2 eclogite facies at ca 550 630 c and 20 kbar 3 a second blueschist facies overprint ca 450 c 10 15 kbar and 4 exhumation to pressures of ca 6 7 kbar at 500 550 c upper greenschist to lower amphibolite facies an eclogitic penetrative foliation s 1 and s dipping stretching lineation l 1 formed during the final stages of subduction of the eclogite bearing unit the p t evolution of the eclogites during d 1 documents the final increment of the prograde p t path later blueschist facies metamorphism is contemporaneous to top to the n emplacement of the eclogite bearing unit onto continental basement units within a subduction zone we discuss two models of eclogite emplacement most probably emplacement is achieved by a thrust along the base of the eclogite bearing unit alternatively the p t path of the eclogite facies rocks suggests an emplacement model similar to corner flow subsequent to eclogite and nappe emplacement the nappe stack is entirely affected by a penetrative deformation d 2 that resulted in the development of a mylonitic foliation s 2 and an e w oriented stretching lineation l 2 within lower amphibolite to upper greenschist facies this deformation is related to a crustal scale detachment zone that formed during the exhumation of the penninic units

the solubility of sulphur in hydrous rhyolitic melts experiments performed at 2 kbar in the temperature range 800 1000 c with fo2 between nno 2 3 and nno 2 9 where nno is the nickel nickel oxide buffer and varying amounts of sulphur added to hydrous metaluminous rhyolite bulk compositions were used to constrain the solubility of sulphur in rhyolite melts the results show that fs2 exerts a dominant control on the sulphur solubility in hydrous silicate melts and that depending on fo2 a rhyolitic melt can reach sulphur contents close to 1000 ppm at high fs2 at fo2 below nno 1 the addition of iron to a sulphur bearing rhyolite magma produces massive crystallization of pyrrhotite and does not enhance the sulphur solubility of the melt for a given fo2 the melt sulphur content increases with fs2 for fixed fo2 and fs2 temperature exerts a positive control on sulphur solubilities at least for fo2 below nno 1 the mole fraction of dissolved sulphur exhibits essentially linear dependence on fh2s at low fo2 and although the experimental evidence is less clear on fso2 at high fo2 the minimum in sulphur solubility corresponds to the redox range where both fh2s and fso2 are approximately equal a thermodynamic model of sulphur solubility in hydrous rhyolite melts is derived assuming that total dissolved sulphur results from the additive effects of h2s and so2 dissolution reactions the model reproduces well the minimum of sulphur solubility at around nno 1 in addition to the variation of the sulphide to sulphate ratio with fo2 a simple empirical model of sulphur solubility in rhyolitic melts is derived and shows good correspondence between model and observations for high silica rhyolites

minor and trace element geochemistry of volcanic rocks dredged from the galapagos spreading center role of crystal fractionation and mantle heterogeneity a wide range of rock types abyssal tholeiite fe ti rich basalt andesite and rhyodacite were dredged from near 95 w and 85 w on the galapagos spreading center computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization however the p2o5 k2o ratio averages 0 83 at 95 w and 1 66 at 85 w and implies distinct mantle source compositions for the two areas these source regions also have different rare earth element ree abundance patterns with la sm ef 0 67 at 95 w and 0 46 at 85 w the sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95 w the mantle source regions for these two areas are interpreted to be depleted in incompatible and volatile elements although the source region beneath 95 w is less severely depleted in la and k incompatible trace element abundances in 26 samples are used to infer that the range of fe ti rich basalt from 85 w represents 19 to 35 residual liquid following crystal fractionation of a mineral assemblage of plagioclase clinopyroxene and lesser olivine the most highly differentiated samples have also had less than 1 titanomagnetite removed most samples from 85 w can be related to a common parental magma that contained approximately 9 wt feo 1 wt tio2 and had an mg number mg 100 mg mg fe2 of about 65 although the samples from 95 w cannot all be derived from a common parental magma the inferred parental magmas may have been derived by varying degrees of partial melting of a common source the fractionation sequence consists of two parts an initial iron enrichment trend followed by a silica enrichment trend we interpret the trace element data to indicate that the most iron rich lavas represent about 32 residual liquid derived by crystal fractionation of plagioclase clinopyroxene and lesser olivine from a parental magma with an mg number of about 66 the silica enrichment trend results from crystallization of titanomagnetite and some apatite fractionation of pigeonite which is a minor phase in the major element models cannot be distinguished from clinopyroxene fractionation by using trace elements

origin of eclogites from the austroalpine otztal basement tirol austria geochemistry and smnd vs rbsr isotope systematics geochemical and combined sm nd vs rb sr isotope data from the otztal metabasite suite point to production of gabbros and basalts in an extensional regime in late cambrian times the source of these magmas and cumulates had a chemical and isotopic composition similar to morb the time of intrusion is constrained by sm nd mineral isochrons clinopyroxeneplagioclase of relictic gabbro occurrences within the eclogite amphibolite series in the time span between 530 and 521 ma subsequently the basic eclogite precursors suffered three metamorphic stages the imprint of the caledonian event is unclear during the hercynian cycle the basic magmatic rocks were transformed into eclogites at surprisingly high p t conditions of 27 kbar 730 c the time of this subduction event is given by mineral isochrons garnet clinopyroxene zoisite whole rock as 370 340 ma with a most probable time span for the pressure peak between 360 and 350 ma during subsequent exhumation and pressure release the eclogites were partly transformed into amphibolites alpine overprint during a greenschist facies grade metamorphism caused further and partly intense retrograde alterations in the metabasic suite

approaches to equilibrium in the distribution of trace elements among the principal minerals in a high grade metamorphic terrane x ray fluorescence instrumental neutron activation and particle induced x ray emission methods were used to determine the distribution of numerous trace elements among garnet grt ca pyroxene cpx hornblende hbl biotite bt plagioclase pl and k feldspar kf in a high grade metamorphic terrane within the grenville province of the canadian shield results are presented as distribution formulae e g sr kf 1 1 pl 16 hbl 2 2 cpx 1 0 bt 1 2 grt sc hbl 1 1 cpx 1 0 grt 7 8 bt 22 pl 2 6 kf v hbl 1 15 bt 2 07 cpx 6 0 grt 1 4 cao 1 pl kf zn bt 1 6 hbl 1 62 cpx 2 9 grt 10 pl ga bt 1 2 hbl 1 2 pl 2 5 cpx 1 3 grt where numbers are distribution ratios e g ppm sr in hbl ppm sr in cpx 2 2 examples of inter element similarities and differences are a both rb and cs are concentrated in biotite relative to k feldspar but for rb the ratio is 2 3 and for cs it is 16 b the distribution formulae for seven lanthanides are similar except for the position of garnet e g ce hbl 2 7 cpx 2 8 pl 1 1 bt 11 kf 16 grt yb grt 2 8 hbl 2 7 cpx 9 pl 1 0 bt 7 kf and c all of sr eight lanthanides zr v and cr are concentrated in hornblende relative to ca pyroxene by a factor that lies in the narrow range of 2 2 3 1 there is a larger variation departure from the mean in some distribution ratios than in others thus the mean ratios hbl cpx for each of six elements and in parentheses the percentage relative standard deviation are zn 1 62 8 6 v 2 38 12 cr 2 42 18 sr 2 7 28 ba 2 9 36 and ni 1 66 38 we suggest that variation of this kind is the result of differences from place to place in the magnitude of deformation and recrystallization which facilitated the rearrangement of atoms combined with rates of lattice and crystal boundary diffusion that are unique for the various elements thus permitting some trace elements to approach equilibrium more closely than others

the combined effects of water and fluorine on the viscosity of silicic magmas abstract the newtonian viscosity of water plus fluorine bearing silicate melt of haplogranitic composition hpg8 has been determined viscosities of hpg8 melt with addition of 3 11 and 4 25 wt of f and up to 3 wt h2o have been obtained using a micropenetration technique in the interval 109 74 to 1011 84 pa s and temperatures varying from 370 to 700 c at ambient pressure determination of the temperature dependence of viscosity from this and previous studies permits the parameterization of the viscosity of melts containing water and fluorine having similar composition within a 0 3 log units standard error the viscosity of water bearing f rich haplogranitic samples is represented by a modified vogel fulcher tammann vft equation which provides a non arrhenian description of the temperature dependence of the viscosity the results of this study indicate that taken individually or together both h2o and f have a strong and similar effect on the viscosity of sio2 rich compositions this similarity between f2o 1 and h2o greatly simplifies the task of predicting viscosity for volatile rich highly silicic magmas the low viscosities of hydrous fluorine bearing granitic melts favour efficient crystallization fractionaction paths for these liquids controlling degassing paths and consequently the eruptive behaviour numerical simulations of eruptive events normally do not take into account the contribution of fluorine this may introduce a significant error in the description of the fluid dynamic properties of magma and therefore in the accurate prediction of eruptive scenarios as well as in hazard assessment studies fluorine unlike water remains dissolved in the melt at high concentrations and low confining pressures the incorporation of fluorine data and the modelling of fluorine bearing viscosity data are therefore of fundamental importance for simulations of magma dynamics and prediction of eruptive scenarios

sulphur solubility in andesitic to basaltic melts implications for hekla volcano the solubility of sulphur in sulphide saturated h2o bearing basaltic andesitic and basaltic melts from hekla volcano iceland has been determined experimentally at 1 050 c 300 and 200 mpa and redox conditions with oxygen fugacity logfo2 between qfm 1 2 and qfm 1 1 qfm is a quartz fayalite magnetite oxygen buffer in the systems containing various amounts of s and h2o the s content of the h2o rich glasses saturated with pyrrhotite decreases from 2 500 ppm in basalt to 1 500 ppm in basaltic andesite at the investigated conditions furthermore the reduction of water content in the melt at pyrrhotite saturation and fixed t p and redox conditions leads to a decrease in s concentration from 2 500 to 1 400 ppm for basaltic experiments for h2o decrease from 7 8 to 1 4 wt and from 1 500 to 900 ppm for h2o decrease from 6 7 to 1 7 wt for basaltic andesitic experiments our experimental data combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas indicate that the parental basaltic melts of hekla were not saturated with respect to sulphide during magmatic differentiation the s content in the residual melts increased and might have reached sulphide saturation with 2 500 ppm dissolved s with further magma crystallization the s concentration in the melt was controlled by the sulphide saturation of the magma decreasing from 2 500 to 900 ppm s

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

sulfur abundance and its speciation in oxidized alkaline melts abstract the sulfur concentrations and the relative proportions of s2 and s6 were measured by electron microprobe in a series of melt inclusions trapped in phenocrysts from different subduction related and within plate volcanoes the melt inclusions correspond to potassic and shoshonitic primary melts to tholeiitic and hawaiitic primitive melts in the tholeiitic and the transitional basaltic melt inclusions sulfur is mainly present as s2 s 6 s total 0 03 to 0 08 and varies from 0 13 to 0 18 wt the occurrence of immiscible sulfides attests to their saturation in shoshonitic and potassic primary melts sulfur s 0 12 to 0 32 wt is dissolved as both s2 and s6 s 6 s total 0 3 to 0 7 their oxygen fugacity estimated from the s 6 s total ratios ranges from ninio to ninio 1 log unit hawaiitic melts may also dissolve up to 0 3 wt sulfur possibly because of their oxidation state close to ninio as illustrated by samples from mt etna italy variations of sulfur at constant s 6 s total ratio and temperature in both the potassic and shoshonitic primary melts indicate that these melts are undersaturated with respect to a s rich condensed phase in agreement with their relatively high oxidation state it strongly suggests that sulfur behavior in relatively oxidized primary alkaline melts is controlled by the mantle source melting conditions

fluid flow paths and mechanisms of fluid infiltration in carbonates during contact metamorphism the beinn an dubhaich aureole skye a textural study of marbles from the beinn an dubhaich granite contact aureole skye has shown that mass transport by diffusion was probably negligible during the metamorphic event and that the bulk of the carbonates reacted as a consequence of silica metasomatism permitting the use of calcsilicates as a tracer for fluid infiltration pathways fracture controlled infiltration was predominant in undeformed marbles whereas pervasive infiltration occurred during synmetamorphic ductile deformation some calcite marbles contain disseminated unoriented calcsilicate minerals that are associated with neither fractures nor a ductile deformation fabric consistent with an origin via infiltration of fluid along an interconnected grain edge porosity the inference of limited pervasive infiltration of undeformed carbonates is consistent with predictions based on experimentally determined fluid solid dihedral angles

geochemistry and origin of eclogites from the type locality koralpe and saualpe eastern alps austria abstract in the koralpe and saualpe polycrystalline basement which is now a part of the austroalpine nappe system eclogites and metagabbros occur intercalated in gneisses and kyanite bearing schists we report here the chemical as well as sr and o isotopic composition of such eclogites and one metagabbro a low ti kyanite bearing and a high ti kyanite free group can be distinguished which may be related by igneous differentiation the low ti group which comprises eclogites and the metagabbro has chemical characteristics indicating formation of the igneous precursors as cumulates by crystallization of pyroxene and plagioclase from a mafic magma in contrast the protoliths of the high ti eclogites appear to be basaltic liquids with normal to enriched type morb affinities which are likely to have formed at an oceanic ridge their sr and o isotope relationships strongly suggest that they have suffered both a phase of low temperature and a phase of high temperature hydrothermal alteration by reaction with seawater

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

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

the effect of sulphur on the glass transition temperature in anorthite diopside eutectic glasses the effect of sulfur dissolved in anorthite diopside eutectic ad glasses on the glass transition temperature tg has been investigated via differential scanning calorimetric measurements dsc and thermogravimetric analysis tga under moderately reducing to oxidizing conditions in a series of ad glasses we have measured the change in tg as a function of s content present as so4 2 hs is also identified to a lesser extent and h2o content the ad glasses investigated have s contents ranging from 0 to 7519 ppm and h2o contents ranging from 0 to 5 3 wt in agreement with previous studies increasing h2o content induces a strong exponential decrease in tg volatile free ad glass has a tg at 758 13 c and ad glass with 5 18 0 48 wt h2o has a tg at 450 11 c the change in tg as a function of h2o is wellreproduced with a third order polynomial function and has been used to constrain tg at any h2o content the effect of s on tg is almost inexistent or towards a decrease in tg with increasing s content for instance at 2 4 wt h2o the addition of s induces a change in tg from 585 10 c with 0 ppm s to 523 3 c with 2365 138 ppm s a further increase in s up to 7239 90 ppm s does not induce a dramatic change in tg measured at 529 2 c the limited effect of s on the glass transition temperature contrasts with recent spectroscopic measurements suggesting that s dissolution as so4 2 groups provokes an increase in the polymerization degree we propose an alternative view which reconciles the spectroscopic evidence with the tg measurements the dissolution of s as so4 2 does not induce the formation of si o si molecular bonding through consumption of available non bridging oxygens nbo but instead we suggest that si o s molecular bonds are formed which are not detectable by dsc measurements but mimic the increase in glass polymerization therefore spectroscopic measurements must be used with caution in order to extract melt physical properties

kyanite eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks adula nappe central alps in the southern adula nappe central alps two stages of regional metamorphism have affected mafic and pelitic rocks earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite hornblende eclogites was followed by a tertiary overprint which varied from greenschist to high grade amphibolite facies despite a common metamorphic history contrasting equilibration conditions are often recorded by high pressure mafic eclogite and adjacent predominantly lower pressure pelite assemblages this pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading the rates are controlled by a mechanism in which dehydrating metapelites provide the h2o required for simultaneous overprinting of enclosed mafic eclogites by hydration

water and the viscosity of andesite melts the viscosity of a synthetic andesite like melt has been measured between 1010 and 1014 p for water contents in the range 0 3 5 wt the very slow kinetics of water exsolution over this viscosity range allowed the measurements to be made at 1 bar with a high precision after a steep viscosity decrease of 5 orders of magnitude for 1 wt h2o an additional 2 5 wt h2o causes a further viscosity decrease of only 2 orders of magnitude these viscosity decreases are qualitatively similar to those observed previously for more silica rich compositions the new data join smoothly with available high temperature measurements made at high pressures on water bearing andesite melts because the intrinsic effects of pressure are as small for water bearing as for water free samples the depressing effect of water on the viscosity of natural andesite melts can be estimated changes in water speciation as a function of either temperature or pressure do not seem to have marked effects on the viscosity although quantitative applications are not yet possible the configurational entropy theory accounts qualitatively for these features

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

identification of growth mechanisms in metamorphic garnet by high resolution trace element mapping with la icp tofms garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism in addition to major elements zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification however mapping trace elements at low concentrations over large areas and with high resolution has remained a major challenge we present a comprehensive investigation of the te distribution in garnet from three alpine samples that underwent a complex evolution at different metamorphic conditions the te distribution in garnet grains is mapped in 2d in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry la icp tofms to achieve a lateral resolution of 5 m and limits of detection for the heavy rare earth elements ree down to 0 2 g g comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional la icpms spot analysis testifies to the accuracy of the measurements garnet in an amphibolite facies metapelite from campolungo central alps that recorded metamorphism to 600 c preserves y ree trace element zoning that closely matches that of ca in this sample there is no notable diffusive modification for trace elements y ree zoning is dominated by rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet a granulite facies garnet from malenco eastern central alps formed during subsolidus heating followed by peritectic melting reactions up to temperatures of 800 850 c major and trace element zoning are decoupled indicating diffusional resetting of major elements whereas trace elements still largely document the growth history enrichment of trace elements in the garnet mantle may be related to the consumption of biotite v cr and the dissolution of zircon zr and monazite y ree in the melt diffusion of y hree at the core mantle boundary occurred over a length scale of 200 m garnet in an eclogite from the sesia zone western alps p 2 gpa t 600 c displays pronounced fluid related veinlets visible in feo mgo and mno which cross cut the primary growth zoning surprisingly complex y ree and cr zoning is not affected by the veinlets indicating that they did not form by a crack seal mechanism but are rather related to a selective replacement process the trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes and assist in determination of p t conditions garnet dating diffusion modelling as well as documenting fluid induced modifications

oh in zoned amphiboles of eclogite from the western tianshan nw china chemically zoned amphibole porphyroblast grains in an eclogite sample ws24 7 from the western tianshan nw china have been analyzed by electron microprobe emp micro fourier transform infrared micro ftir and micro raman spectroscopy in the oh stretching region the emp data reveal zoned amphibole compositions clustering around two predominant compositions a glaucophane end member bna2cm2 3 m3 2tsi8 oh 2 in the cores whereas the mantle to rim of the samples has an intermediate amphibole composition a0 5bca1 5na0 5cm2 4 5 m0 53 tsi7 5al0 5 oh 2 a na and or k m2 mg and fe2 m3 fe3 and or al between winchite and ferro winchite and katophorite and mg katophorite furthermore we observed complicated ftir and raman spectra with oh stretching absorption bands varying systematically from core to rim the ftir raman spectra of the core amphibole show three lower frequency components at 3 633 3 649 3 651 and 3 660 3 663 cm 1 which can be attributed to a local o 3 h dipole surrounded by m 1 m 3 mg3 m 1 m 3 mg2fe2 and m 1 m 3 fe2 3 respectively an empty a site and tsi8 environments on the other hand bands at higher frequencies 3 672 3 673 3 691 3 697 and 3 708 cm 1 are observable in the rims of the amphiboles and they indicate the presence of an occupied a site the ftir and raman data from the oh stretching region allow us to calculate the site occupancy of the a m 1 m 3 t sites with confidence when combined with epm data by contrast m 2 and m 4 site occupancies are more difficult to evaluate we use these samples to highlight on the opportunities and limitations of ftir oh stretching spectroscopy applied to natural high pressure amphibole phases the much more detailed cation site occupancy of the zoned amphibole from the western tianshan have been obtained by comparing data from micro chemical and ftir and or raman in the oh stretching data we find the following characteristic substitutions si t site mg fe m 1 m 3 site al t site al m 1 m 3 site tschermakite ca m4 site a site na m4 site na k a site richterite and ca m4 site mg fe m 1 m 3 site na m4 site al m 1 m 3 site glaucophane from the configurations observed during metamorphism

beryllium and boron in subduction zone minerals an ion microprobe study secondary ion mass spectrometry was used to measure the distribution of be b rb sr y nb ba and ce among minerals in rocks from the franciscan complex and catalina schist paleosubduction zone terranes the rocks examined represent a wide range of metamorphic pressure temperature conditions 6 15 kb 300 750 c and include metasomatized and migmatitic samples white mica is the primary host of be 2 5 ppm and b 107 ppm in the rocks examined average concentrations amphibole be 0 7 b 15 ppm lawsonite be 0 9 b 37 ppm and biotite be 0 9 b 25 ppm may also contain significant amounts of both be and b omphacite may host be 4 1 ppm in veins 0 7 ppm groundmass and chlorite is a minor host of b 22 ppm minerals in these subduction zone metamorphic rocks that lack measurable be or b include albite apatite calcite clinozoisite epidote garnet ilmenite sphene stilpnomelane plagioclase quartz rutile and zoisite white mica biotite and stilpnomelane are the major mineral hosts of rb 90 300 and 30 ppm respectively and ba 2100 1000 and 4700 ppm epidote and lawsonite are the major host minerals of sr 600 and 300 ppm and ce 12 and 15 ppm garnet epidote lawsonite and sphene host y 100 30 40 and 60 ppm respectively sphene 77 ppm 1 analysis ilmenite and rutile host nb evidence of veining and metasomatism is strongly correlated with be and or b enrichment of white mica omphacite amphibole and lawsonite at the p t conditions examined both be and b appear to be transported in aqueous fluids at depths of 20 50 km in subduction zones the transfer of be b and other elements from the subducted slab into the mantle wedge can be accomplished by aqueous fluids transfer of the elements hosted by white mica could result directly from the breakdown of white mica through pressure dependent dehydration reactions or alternatively the be and b in white mica could be partitioned into infiltrating fluids produced at deeper levels in the slab and subsequently transported into the region of are magma genesis

glass transition temperatures of natural hydrous melts a relationship with shear viscosity and implications for the welding process abstract glass transition temperatures tg have been determined for natural multicomponent melts using differential scanning calorimetry trachytic dacitic phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3 75 wt for each sample tg has been obtained over a range of cooling heating rates using the extrapolated onset and the peak temperatures in heat capacity temperature curves tg of all compositions are strongly reduced by increasing water content particularly for the first 1 wt added base composition also has an effect with the lowest tg occurring in the peralkaline phonolite suite for all samples a clear dependence on the cooling heating rate has been recorded these results have been compared with rheological investigations on the same samples on the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study both dry and hydrous our determinations of tg provide a lower limit for the time temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature furthermore by using the viscosity values predicted at the glass transition we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at tg

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

s cl f degassing pattern of water rich alkali basalt modelling and relationship with eruption styles on mount etna volcano abstract our knowledge of the degassing pattern of sulphur chlorine and fluorine during ascent and eruption of basaltic magmas is still fragmental and mainly limited to water poor basalts here we model and discuss the pressure related degassing behaviour of s cl and f during ascent differentiation and extrusion of h2o co2 rich alkali basalt on mount etna sicily as a function of eruptive styles our modelling is based on published and new melt inclusion data for dissolved volatiles co2 h2o s cl f in quenched explosive products from both central conduit 1989 2001 and lateral dyke 2001 and 2002 eruptions pressures are obtained from the dissolved h2o and co2 concentrations and vapour melt partition coefficients of s cl and f are derived from best fitting of melt inclusion data for each step of magma evolution this allows us to compute the compositional evolution of the gas phase during either open or closed system degassing and to compare it with the measured composition of emitted gases we find that sulphur chlorine and fluorine begin to exsolve at respective pressures of 140 mpa 100 mpa and 10 mpa during etna basalt ascent and are respectively degassed at 95 22 55 and 15 upon eruption pure open system degassing fails to explain gas compositions measured during either lateral dyke or central conduit eruptions instead closed system ascent and eruption of the volatile rich basaltic melt well accounts for the time averaged gas composition measured during 2002 type lateral dyke eruptions s cl molar ratio of 5 1 35 bulk cl loss extensive magma fragmentation during the most energetic fountaining phases enhances cl release 55 and produces a lower s cl ratio of 3 7 as actually measured comparatively slower magma rise in the central conduits of etna favours both sulphide saturation of the melt and greater chlorine release 55 resulting in a distinct s cl evolution path and final ratio in eruptive gas in both eruption types any previous bubble melt separation at depth leads to increased s cl and s f ratios in emitted gas high s cl ratios measured during some discrete eruptive events can thus be explained by transitions from closed deep to open shallow system degassing with differential gas transfer extending down to 2 km depth below the vents this depth coincides with the base of the volcanic pile where structural discontinuities and the high magma vesicularity 60 may favour separate gas flow finally the excess s cl f gas discharge through etna summit craters during non eruptive periods requires a mixed supply from shallow magma degassing in the volcanic conduits and deeper derived so2 rich bubbles from the sub volcano plumbing system our modelling provides a useful reference framework for interpreting the monitored variations of s cl and f in mount etna gas emissions as a function of volcanic activity more broadly the observations made for s cl and f degassing on etna may apply to other basaltic volcanoes with water rich magmas such as in arcs

trace element zoning in garnet from the kwoiek area british columbia disequilibrium partitioning during garnet growth trace element zoning in garnets from two contact metamorphosed rocks from the kwoiek area british columbia hollister 1969a was measured with an ion microprobe zoning profiles have three distinct parts with chemical breaks defined by co variation of major and trace elements important features of the trace element zoning profiles are 1 roughly bell shaped zoning profiles for y and the hrees 2 an abrupt increase in ti at a point midway through each garnet with inflections in the zoning profiles of other elements li na cr v y zr and the hree and 3 irregular cr and v profiles unlike mn zoning the zoning profiles of most other trace elements cannot be easily modeled using simple rayleigh fractionation models ti activity in the two samples is buffered by phase relations with ilmenite garnets from a continuously heated contact metamorphic environment should display continuous ti zoning profiles if equilibrium was maintained and provided the ti buffering assemblage did not change during garnet growth the irregular ti profiles suggest disequilibrium behavior several elements cr v may indicate breakdown of a phase enriched in trace elements during metamorphism the source for the mass excess of these elements is probably the refractory cores of ilmenite grains either differing matrix transport rates of trace lements or interface kinetic controlled segregation could explain the unusual trace element behavior at the element inflection point the preferred explanation involves segregation of elements at the interface of the garnet that were trapped during episodes of rapid garnet growth

fluid immiscibility in volcanic environments volatile components enter the atmosphere oceans and other surface waters through pre syn and post eruptive volcanic processes that involve a variety of fluids aluminosilicate poor fluids include aqueous or carbonic to sulfide sulfate chloride fluoride carbonate and phosphate rich compositions in volcanic environments but other more complex combinations of these constituents may be involved giggenbach 1977 roedder 1984 1992 lowenstern 1995 other rare gases and dissolved constituents e g h2 n2 he ar h3bo3 hg ch4 and other hydrocarbon compounds metals and metalloids are also present but in general they are not sufficiently concentrated to form their own phases or control bulk fluid composition and volcanic processes as a result of their diverse compositions volcanic fluids ranging from vapor to liquid exhibit widely different densities and show strong distinctions in heat capacity dielectric behavior viscosity and other chemical and physical characteristics geiger et al 2006a the density of a saline liquid in the system nacl h2o at 50 mpa and 800 c for example is more than 7 times that of the coexisting aqueous vapor e g 0 2 gm cm3 henley and mcnabb 1978 multiple fluids move mix and or unmix in magma chambers and volcanic conduits in the root zone of fumaroles and in the convective hydrothermal systems that are ubiquitous to volcanic environments hydrothermal systems are typically centered on magmatic intrusions and may include crater lakes at the top of eruptive conduits hydrothermal processes involving two or more fluid phases operate through a variety of geologically relevant shallow crustal pressure temperature composition conditions volcanic fluids occur at supersolidus to subsolidus conditions meaning that the dense aluminosilicate melt that is common to volcanism may or may not be present this chapter addresses the evidence for and occurrence of two or more low density aluminosilicate poor fluids

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

micro scale element migration during eclogitisation in the bergen arcs norway a case study on the role of fluids and deformation abstract understanding chemical element mobility during high pressure metamorphism is paramount to the knowledge of the transformations occurring during the course of eclogite facies metamorphism in particular the role of deformation and fluid circulation appear essential in order to better decipher this role we investigated two eclogite facies samples from the lindas nappe in the bergen arcs norwegian caledonides which are related to a fluid deformation metamorphic event this study based on detailed microtextural investigations coupled with major and trace element analyses either in situ empa and la icp ms or by conventional icp ms nebulisation on mineral fractions illustrates the complexity of chemical reactions accompanying the eclogite and amphibolite facies metamorphism a single eclogite facies mineral species two eclogite facies i e phengite and epidote depicts various chemical features depending on its textural location at thin section scale the correlation with the mineralogical reaction occurring in each textural site demonstrates that the compositional variations are inherited from the precursor minerals thus the composition of eclogite facies minerals is locally controlled by the composition of few hundred micron domains on the contrary alteration phases of the amphibolite facies symplectite and calcic amphibole display clear enrichment in lile pb sr and lree compared to their precursor minerals omphacite and garnet this supports that the transport of elements at hand sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another the contrasting behaviour of element mobility between peak eclogite facies metamorphism and retrogression can be explained in terms of rate processes and fluid deformation activity during eclogitisation deformation allows the formation of textural and mineralogical microdomains contemporaneously the fluid circulation favours the development of heterogeneities by transport of elements in excess towards areas protected from deformation where phengite bearing quartz lenses crystallise this together with the very fast recrystallisation of eclogite facies minerals results in a heterogeneous redistribution of elements at sample scale and in a chemical disequilibrium between different microdomains of hundred micron size during retrogression in the amphibolite facies coeval with long term ductile deformation the fluids act as an efficient vector to redistribute the elements from a given textural site to another and thus allow to reach at hand sample scale a new chemical equilibrium between the different crystallising phases

alpine geodynamic evolution of passive and active continental margin sequences in the tauern window eastern alps austria italy a review the penninic oceanic sequence of the glockner nappe and the foot wall penninic continental margin sequences exposed within the tauern window eastern alps have been investigated in detail field data as well as structural and petrological data have been combined with data from the literature in order to constrain the geodynamic evolution of these units volcanic and sedimentary sequences document the evolution from a stable continent that was formed subsequent to the variscan orogeny to its disintegration associated with subsidence and rifting in the triassic and jurassic the formation of the glockner oceanic basin and its consumption during the upper cretaceous and the paleogene these units are incorporated into a nappe stack that was formed during the collision between a penninic zentralgneis block in the north and a southern austroalpine block the venediger nappe and the storz nappe are characterized by metamorphic jurassic shelf deposits hochstegen group and cretaceous flysch sediments kaserer and murtorl groups the eclogite zone and the rote wand modereck nappe comprise permian to triassic clastic sequences wustkogel quartzite and remnants of platform carbonates seidlwinkl group as well as jurassic volcanoclastic material and rift sediments brennkogel facies covered by cretaceous flyschoid sequences nappe stacking was contemporaneous to and postdated subduction related high pressure eclogite and blueschist facies metamorphism emplacement of the eclogite bearing units of the eclogite zone and the glockner nappe onto penninic continental units zentralgneis block occurred subsequent to eclogite facies metamorphism the eclogite zone a former extended continental margin was subsequently overridden by a pile of basement cover nappes rote wand modereck nappe along a ductile out of sequence thrust low angle normal faults that have developed during the jurassic extensional phase might have been inverted during nappe emplacement