elemental mobilities produced by low grade metamorphic events a case study from the proterozoic supracrustals of southern norway abstract within the sveconorwegian province of southern norway the telemark supracrustals represent the only low grade metamorphic assemblages these supracrustals are composed of metabasalts acid metavolcanics and metasediments which have been subjected to three metamorphic events an early burial type greenschist facies regional and thermal metamorphism within the supracrustal piles each of the events is heterogeneously preserved and has caused variable degrees of elemental redistribution burial metamorphism is a localised phenomenon involving high fluid rock ratios which has produced extremely anomalous compositions in contrast the regional metamorphism greenschist facies has completely obliterated the primary textures in the basaltic formations and has also produced a large degree of scatter in the more mobile elements the late thermal metamorphism is associated with the intrusion of late kinematic granites 950 ma within the thermal aureoles the metabasalts contain randomly orientated porphyroblasts of biotite hornblende and opaque phases in these basalts the more immobile elements i e zr nb ti y have been mobilised to varying degrees this conclusion is of major significance since these are the elements which are frequently used to discriminate ancient tectonic environments if these elements are used for this purpose in the present case spurious conclusions regarding the genesis and tectonic setting of the volcanics may be reached from this study it is apparent that low grade metamorphic events can have a profound effect upon whole rock geochemical compositions and thus ancient volcanic suites must be effectively screened prior to their use in tectonic modelling

helium and carbon fluxes in lake nyos cameroon constraint on next gas burst on 21 august 1986 a lethal gas burst issued from lake nyos in cameroon western africa killing at least 1700 people although the consensus is that the victims died of asphyxiation by co2 of magmatic origin the frequency of such catastrophic degassing events is still unknown the co2 flux at the bottom of lake nyos is estimated based on the measurement of3he4he and4he20ne ratios and the chemical composition of gases exsolved from the lake water the calculated co2 flux at the bottom of lake nyos is 4 4 2 3 1013cm3stp year combined with the estimated release in the august 1986 event of 8 2 1014 cm3 stp the co2 flux suggests that the gas burst may happen about once in18 10 years although the significant uncertainty should be taken into account for the frequency resulting from assumptions such as steady state fluxes in the lake and small fractionization of the c he ratio during the degassing event several yearly measurements of hypolimnetic fluxes and seasonal measurements of epilimnetic fluxes are needed to constrain better the recurrence interval in addition the results of a regional3he4he survey of carbonated mineral springs in northwestern cameroon are discussed in the context of the regional geotectonics

high grade metamorphism and partial melting of basic and intermediate rocks rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust however the former lack of appropriate thermodynamic models for silicate melt amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite and granulite facies metamorphism in this work we present phase diagrams calculated in the na2o cao k2o feo mgo al2o3 sio2 h2o tio2 o2 nckfmashto compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2 12 kbar and 600 1050 c using newly parameterized activity composition relationships detailed in a companion paper by green et al in this issue particular attention is given to mid ocean ridge basalt morb and diorite protolith bulk compositions calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase with variable proportions of epidote clinopyroxene garnet biotite muscovite quartz titanite or ilmenite present at different pressures the h2o saturated wet solidus has a negative p t slope and occurs between 620 690 c at mid to lower crustal pressures of 5 10 kbar the lowest t melts generated close to the wet solidus are calculated to have granitic major element oxide compositions melting at higher temperature is attributed primarily to multivariate hydrate breakdown reactions involving biotite and or hornblende partial melt compositions calculated at 800 1050 c for morb show good correlation with analysed compositions of experimental glasses produced via hydrate breakdown melting of natural and synthetic basic protoliths with niggli norms indicating that they would crystallize to trondhjemite or tonalite diorite is shown to be significantly more fertile than morb and is calculated to produce high t melts 800 c of granodioritic composition subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family although the p t conditions at which orthopyroxene stabilizes thus defining the prograde amphibolite granulite transition is strongly dependent on bulk rock oxidation state and water content the petrological effects of open and closed system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case study analysis of a natural archean amphibolite from the lewisian complex northwest scotland

mantle helium flux from the bottom of lake mashu japan abstract most of the 52 water samples from lake mashu hokkaido japan show large anomalies in their3he 4he ratios up to 4 6 times the atmospheric ratio and4he 20ne ratios indicating continuous injection of mantle he due to hot spring activity at the lake bottom the3he 4he ratio of the injected mantle he is estimated at3he 4he 9 43 0 17 10 6 which is close to the highest value reported for volcanic gases from the japanese islands accumulation of mantle he in the near bottom water can be reasonably explained by a diffusion process with a constant he flux from the lake bottom off4he 9 2 107 atoms cm2 s andf3he 8 7 102 atoms cm2 s the heat 3he ratio 1 7 10 7j atom falls in the range of values reported for submarine hydrothermal systems 0 4 26 10 7 j atom whereas the c3he ratio 1 8 1011 is about two orders of magnitude higher than the morb average 2 109 the mantle he injected from the lake bottom has a fairly short mean residence time in the lake water and is estimated at about 2 4 years overturn of the lake water which occurs twice a year probably plays an important role in the rapid transportation of the mantle he from the bottom to the surface

nd and sr isotopic ratios and trace element geochemistry of epidote from the swiss molasse basin as provenance indicators implications for the reconstruction of the exhumation history of the central alps abstract epidote is a common heavy mineral in sandstones its use as a provenance indicator however is limited because it appears in a wide range of lithologies and geological settings in order to discriminate between mantle and crust derived source rocks we measured isotopic and trace element compositions of detrital epidote from sandstones of the swiss molasse basin the compositions of the detrital epidotes were compared to epidotes from penninic metabasites and austroalpine metagranites from the present day exposures and to epidote from ophiolitic and metagranitic pebbles from the molasse basin we found that nd isotopic ratios of epidote allow a good discrimination between different types of source rocks with end values ranging between 6 1 and 8 6 for metabasic lithologies and between 9 7 and 2 8 for metagranitic lithologies sr isotope ratios reveal similar trends but do not allow such a clear discrimination ba u rb nb and th contents of epidote correlate well with the end values yielding the same provenance signal according to isotopic and trace element compositions three types of epidote bearing sandstones can be distinguished one received epidote from lower austroalpine metagranites another from penninic ophiolites and a third type of sandstone received epidote from both sources the provenance of the detrital epidote has important bearing on the exhumation history of the central alps elucidating the transition between the erosion of lower austroalpine felsic lithologies and the onset of erosion of the penninic lower plate according to our data the penninic lower plate became exposed simultaneously in the hinterland of all molasse fans at 21 20 ma at the same time the relief collapsed as reflected by a drastic decrease of sediment accumulation rates in the foreland basin the combination of both events fits well to the geodynamic scenario of large scale lateral extension processes affecting the central alps in post collisional times

isotopic and geochemical evidence for magma mixing in the petrogenesis of the coire uaigneich granophyre isle of skye n w scotland field relations petrographic features major and selected trace element compositions and sr and pb isotope characteristics indicate that the coire uaigneich granophyre cug was formed by the mixing of two magmas one of these a liquid of high 87sr 86sr ratio ca 0 731 formed by the anatexis of late precambrian torridonian sediments contained relict quartz and zircon the other liquid was an acid differentiate of basaltic magma and was enriched in incompatible elements such as zr and y the two magmas mixed in the proportions of approximately two parts anatectic melt to one part acid differentiate hydrothermal metamorphism had no significant effect on the bulk chemistry of the cug

isotopic evidence for the age and origin of pitchstones and felsites isle of eigg nw scotland an 8 point whole rock and mineral rb sr isochron for the sgurr of eigg yields an age of 52 1 1 0 ma 2 this is the youngest accurate date yet reported for igneous activity in the british tertiary province and indicates that sub acid rocks of the small isles represent one of the latest episodes of hebridean magmatism pb isotope compositions of these rocks yield an apparent age of 2660 75 ma 2 interpreted as a mixing line between lewisian torridonian and mantle derived pb and indicating the presence of lewisian basement under eigg pb sr and nd isotopic evidence points to contamination of basaltic differentiates and granulite facies lewisian melts by torridonian sandstone but precludes significant involvement of amphibolite facies lewisian which may have been largely removed by erosion before deposition of the torridonian sediments in the small isles

noble gas isotopic compositions and water gas chemistry of soda springs from the islands of bioko sao tome and annobon along with cameroon volcanic line west africa abstract chemical and isotopic compositions are reported for water and co 2 and noble gases in groundwater and soda springs from bioko principe sao tome and annobon all islands located in the off shore part of the cameroon volcanic line in west africa the soda spring waters are of ca mg hco 3 type with d and 18 o values that range from 20 to 8 and 5 4 to 2 7 respectively indicative of a meteoric origin co 2 is the main gas species in the springs 13 c co 2 values vary from 2 8 to 5 0 overlapping the observed mantle c range 3 to 8 co 2 3 he ratios 3 9 10 9 suggest that most c 90 in the samples is derived from the mantle neon has atmospheric isotopic compositions while ar is slightly enriched in radiogenic 40 ar 3 he 4 he ratios 3 0 to 10 1 10 6 or 2 1 to 7 2 r a where r a is the atmospheric ratio of 1 4 10 6 are much higher than those for typical crustal fluids 10 8 but lower than those expected for fluids derived from high 3 he 4 he hotspots like loihi and iceland this precludes significant contributions of such fluids in the source regions of the gases and by inference in the magmatism of these oceanic islands alternatively approximately 90 of the he in sao tome gases is inferred to be derived from a source similar to the morb source the 3 he 4 he ratio for the bioko gas 6 6 10 6 may be derived from a source with a higher time integrated u th 3 he ratio than the morb source

geochemical constraints on the late stage evolution of basaltic magma as revealed by composite dikes within the kangâmiut dike swarm west greenland abstract the kang miut dike swarm in west greenland contains numerous composite dikes with mafic margins and andesitic centers internal chilled margins show that the andesitic centers intruded into the middle of the mafic dikes major element systematics indicate that the fractionation of olivine clinopyroxene plagioclase and fe ti oxides drove the evolution of the kang miut parental magma during its transition from mafic to andesitic compositions incompatible trace elements show a marked relative decrease in middle and heavy rare earth elements ree between the mafic margins and the andesitic centers the decrease in the ree is not explicable by olivine clinopyroxene plagioclase and fe ti oxide fractionation or by the fractionation of the accessory phases apatite zircon or garnet rb sr and sm nd isotopes from margin and center pairs from these composite dikes are nearly identical indicating that crustal contamination had little to no affect on their evolution trace element modeling utilizing the mixing of evolved kang miut magmas and low degree melts derived from partial melting of garnet lherzolite produce excellent fits with the trace element patterns for the andesitic centers these models suggest that the late stage evolution of the kang miut dikes included input of mantle melts produced during the end stages of rifting

trace and rare earth element geochemistry in tourmaline and cassiterite from the yunlong tin deposit yunnan china implication for migmatitic hydrothermal fluid evolution and ore genesis abstract yunlong tin deposit is a medial sized hydrothermal deposit in western yunnan province of china this deposit is unusual because the tin mineralization occurs exclusively within the migmatite bodies the genesis of the ore deposit has been hotly debated and various models have been proposed including granite related magmatic hydrothermal and migmatitic hydrothermal origins this paper reports a systematic trace and rare earth elements ree geochemical study in tourmaline and cassiterite from the orebodies and associated migmatites from the yunlong tin deposit tourmaline and cassiterite from this deposit show light ree lree enriched patterns with variable eu and ce anomalies consistent with hydrothermal fluids being derived from local high grade metamorphism and migmatization the data suggest that tourmaline and cassiterite were likely precipitated from migmatitic hydrothermal fluids and indicate that the yunlong tin deposit was probably formed during migmatitic hydrothermal process the high concentrations of transition metals e g sc ti v cr mn and zn in tourmalines are also consistent with this model the high concentrations of sn sr and especially sc v and cr in tourmalines can be used to discriminate tourmaline from barren deposits relative to those associated with economic tin mineralization

noble gases in hydrothermal plumes of loihi seamount abstract seawater samples from hydrothermal plumes of loihi seamount show very high 3 he 4 he ratios up to 15 8 10 6 and 4 he 20 ne ratios higher than that of air saturated seawater assw the 3 he 4 he ratio in the source from which the helium is released into the seawater is estimated as 23 1 2 3 10 6 from the correlation plot of 3 he 20 ne vs 4 he 20 ne the 3 he heat ratio at loihi is 5 49 times higher than that at the galapagos rift and at 21 n epr the difference may be attributed to the differences in the structure of the mantle beneath hot spots and the mid ocean ridge the former representing the deeper mantle region and the latter the shallower depleted mantle these observations are consistent with the view that helium in hydrothermal plumes of loihi seamount was derived from a deeper mantle region via a hot spot where more primordial helium is still preserved

geology and chemistry of hydrothermal deposits from active submarine volcano loihi hawaii high resolution bathymetric surveys bottom photography and sample analyses show that loihi seamount at the southernmost extent of the hawaiian hotspot is an active young submarine volcano that is probably the site of an emerging hawaiian island hydrothermal deposits sampled from the active summit rift system were probably formed by precipitation from cooling vent fluids or during cooling and oxidation of high temperature polymetallic sulphide assemblages no exotic benthic fauna were found to be associated with the presently active hydrothermal vents mapped

the strontium and oxygen isotopic record of hydrothermal alteration of syenites from the abu khruq complex egypt hydrothermal convection initiated by emplacement of the gabbro syenite complex of abu khruq into the egyptian basement 89 ma ago systematically altered the trace element and isotopic compositions of the syenites the scale of sr transport in migrating solutions was far larger than the scale of sr isotopic equilibration within rocks as a result sr exchange was heterogeneous in the syenites an effect which can be observed on three different scales within grains of a single mineral species heterogeneities are related to grain boundaries and microfractures through which fluids migrated among minerals within rock samples heterogeneities are related to differences in susceptibility to sr alteration among samples within a single unit heterogeneous alteration is apparently related to differences in permeability close to fracture zones during the early stages of alteration radiogenic sr derived from the country rocks was added to the syenites causing small net changes in concentration 5 ppm ave some rb sr mineral isochrons from single rock samples yield the emplacement age because isotopic equilibration of this added sr sometimes occurred within rock specimens however regressions of the whole rock rb sr data yield apparent ages that are about 10 ma too old later stage alteration involved larger changes in whole rock sr concentration 45 ppm ave but had little further effect on the isotopic relationships because the sr was derived from cogenetic gabbros rather than the country rocks alterations of rb sr and sr isotopic compositions are not well correlated with changes in 18o 16o ratio because mineralogy played an important role in decoupling trace element and oxygen isotopic alteration in general the absence of such correlations for whole rock data is not diagnostic of rocks with unaltered trace element and isotopic compositions mineral scale sr isotopic heterogeneities associated with grain boundaries and microfractures may be the most unambiguous evidence of trace element mobility

eddy shedding from a boundary current around a cape over a sloping bottom abstract the authors discuss laboratory experiments that elucidate the mechanism of formation and westward drift of anticyclonic baroclinic vortices from a buoyant surface current flowing along a lateral boundary and around a cape experiments were carried out with a sloping bottom in order to simulate the topographic effect they showed how a vortex can be generated from the current where it separates and reattaches to the cape and that under some conditions the eddy is able to detach from the cape and drift westward following isobaths two important timescales regulate the flow the time tf that the current takes to generate a vortex and the time td that the vortex takes to drift westward for a distance equal to its radius when these two timescales are either of the same order of magnitude or tf td the vortex was able to form at the cape but it did not detach and drift westward the influence of the depth of the lower layer h0 on the flo

a far field hydrothermal plume from loihi seamount an extensive helium plume in the north central pacific emanates from loihi seamount on the flanks of hawaii the maximum helium signal is found at a depth of about 1100 meters the same depth as the near field plume directly above loihi seamount although this helium plume is strongest near hawaii where the 3he 4he ratio at a depth of about 1100 meters reaches values 28 percent above the atmospheric ratio it can be detected quite clearly at latitude 24 n over 400 kilometers to the north excess 3he is also present on the same isopycnal between 15 n and 20 n at 135 w some 2000 kilometers east of the hawaiian islands

numerical simulation of the filchner overflow the plume of ice shelf water isw flowing into the weddell sea over the filchner sill contributes to the formation of antarctic bottom water the filchner overflow is simulated using a hydrostatic primitive equation three dimensional ocean model with a 0 5 2 sv isw influx above the filchner sill the best fit to mooring temperature observations is found with influxes of 0 5 and 1 sv below a previous estimate of 1 6 0 5 sv based on sparse mooring velocities the plume first moves north over the continental shelf and then turns west along slope of the continental shelf break where it breaks up into subplumes and domes some of which then move downslope other subplumes run into the eastern submarine ridge and propagate along the ridge downslope in a chaotic manner the next western ridge is crossed by the plume through several paths despite a number of discrepancies with observational data the model reproduces many attributes of the flow in particular we argue that the temporal variability shown by the observations can largely be attributed to the unstable structure of the flow where the temperature fluctuations are determined by the motion of the domes past the moorings our sensitivity studies show that while thermobaricity plays a role its effect is small for the flows considered smoothing the ridges out demonstrate that their presence strongly affects the plume shape around the ridges an increase in the bottom drag or viscosity leads to slowing down and hence thickening and widening of the plume

on the baroclinic instability of cold core coupled density fronts on a sloping continental shelf a theory is presented to describe the linear baroclinic instability of coupled density fronts on a sloping continental shelf the new baroclinic model equations used to study the instability process correspond to an intermediate lengthscale dynamical balance specifically the frontal dynamics while geostrophic is not quasigeostrophic because frontal height deflections are not small in comparison with the frontal scale height the evolution of the frontal height is strongly coupled to the geostrophic pressure in the surrounding slope water through the hydrostatic balance which expresses the continuity of the dynamic pressures across the frontal interface the deeper surrounding slope water evolves quasi geostrophically and is coupled to the front by baroclinic vortex tube stretching compression associated with the perturbed density front allowing the release of mean frontal potential energy and the topographic vorticity gradient associated with the sloping bottom it is shown that the baroclinic stability characteristics are principally determined by a so called non dimensional interaction parameter denoted p which physically measures the ratio of the destabilizing baroclinic vortex tube stretching compression to the stabilizing topographic vorticity gradient for a given along front mode wavenumber it is shown that a minimum p is required for instability several other general stability results are presented necessary conditions for instability growth rate and phase speed bounds the existence of a high wavenumber cutoff and a semicircle theorem for the unstable modes the linear stability equations are solved exactly for a parabolic coupled density front and a detailed description of the spatial and temporal characteristics of the instabilities is given for physically realistic parameter values the instabilities are manifested as amplifying topographic rossby waves in the slope water and on the density front the unstable perturbations take the form of amplifying anticyclones which have maximum amplitude on the offshore side

integral constraints on bottom and surface isolated eddies abstract general integral relationships are derived for joint eddies in a three layer ocean model as considered by nof the model is composed of two eddies in two superposed layers the third layer being motionless two cases are examined first when the two active layers are near the bottom bottom eddies and second when they are located near the surface of the ocean surface eddies for bottom eddies the effect is due to the presence of a slope as a bottom topography for surface eddies the effect comes in through the variation with latitude of the cariolis frequency the novelty of the present work consists in the derivation of mathematically exact integral properties from which the translation speed of the system is deduced contrary to nof no scaling assumptions are made and therefore the result applies to a wide range of physical situations in particular barotropic isolated vortices and bottom eddies in a two layer ocean are within the range of application of the present results since the

rare earth element mobility in the roffna gneiss switzerland the roffna gneiss a deformed hercynian granite porphyry within the penninic nappes of eastern switzerland underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies this resulted in the selective mobilization of major and trace elements over distances of 10 s to 100 s of meters and the resetting of the rb sr whole rock isotopic systems some 100 my ago the component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric the mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline albite and biotite and the formation of phengite and quartz the fluids introduced mg and h2o promoting the development of phengite and removed the na being released by the breakdown of albite the fluids were most probably derived from the surrounding triassic carbonates and quartzites these relatively high fo2 and carbonate rich fluids also introduced rare earth elements ree into the gneiss the gneiss was progressively enriched in eu up to 60 y up to 40 and yb up to 100 these enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite rich sample while the ree were mobile uranium and thorium were essentially immobile the formation of xenotime was suggested to explain the observed heavy ree enrichment when large differences in the ree contents were found for replicate analyses using hf and then lithium metaborate for dissolution these differences arose because xenotime like monazite can be difficult if not impossible to dissolve in hydrofluoric acid due to the possibility of incomplete sample dissolution we now recommend fusion with lithium metaborate for all ree lu hf or sm nd studies

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

influence of topography on the propagation of isolated eddies abstract an analytical and numerical study of isolated coherent vortices and topography is presented the motivation for this work comes from many observations of vortices influenced in trajectory propagation and decay by encounters with midocean ridges seamounts and bottom slopes in particular analytical predictions relevant to vortex propagation and evolution are compared with numerical results for lenses on bottom slopes and mixed barotropic baroclinic eddies over a variety of topographies the latter case includes examination of short term and long term behavior analytical theories are found to work well for the bottom lenses and short term behavior is captured well by a simple theory that emphasizes barotropic dynamics for mixed vortices the exception for the latter case occurs for counterrotating eddies i e eddies with opposing upper and lower layer swirl for which the evolution is dominated by vortex instability long term evolution has no comparable theory and the various possibili

eddy formation in the denmark strait overflow on the basis of observations along the continental slope of east greenland a primitive equation open boundary model is used to study the formation of eddies in the denmark strait overflow the eddies are produced by vortex tube stretching initiated by cross slope components of the velocity field a sequence of cyclones and anticyclones is formed the cyclones are located along the deeper part of the slope and the anticyclones are along the shallower slope the eddies are almost barotropic with horizontal and vertical velocities of 30 cm s 1 and 0 1 cm s 1 respectively the cyclones migrate along the isobaths with a slight downhill component and intensify on their path until they become influenced by the flat deep basin their translation speed is typically 20 30 cm s 1 and the local change in relative vorticity is nearly entirely determined by vortex tube stretching and vorticity advection the overflow water flows mainly between the cyclonic and anticyclonic eddy chains the overflow is strongly accelerated by the eddies the existence of anticyclonic eddies along the shallower part of the east greenland continental slope predicted by the numerical model has been confirmed by observations

metamorphic gradients in burial metamorphosed vesicular lavas comparison of basalt and spilite in cretaceous basic flows from central chile partial spilitization of a 9 km thick pile of flood basalts with highly vesicular flow tops gave rise to patterns of secondary mineralogy at different scales a a local pattern of mineralogical variation from the almost unaltered bottom towards the altered top of each flow and b an overall pattern comparing flow tops throughout the pile with changes in mineralogical composition within a sequence of metamorphic zones and facies the local patterns mimic the trend of the overall pattern but are of opposite direction and telescoped thus a gradual ordering and andepletion of the secondary albite and increases in the fe al ratio of epidote and pumpellyite upwards within individual flows are comparable in range to corresponding overall changes downwards throughout several kilometres the mineralogical changes within the flows diminish in range towards the more altered deeper part of the pile

preconditioning and formation of maud rise polynyas in a high resolution earth system model abstractopen ocean polynyas oops in the southern ocean are ice free areas within the winter ice pack that are associated with deep convection potentially contributing to the formation of antarct

87sr 86sr in kimberlitic carbonates by ion microprobe hydrothermal alteration crustal contamination and relation to carbonatite carbonates in a 30 cm wide zoned kimberlite dyke from the de beers mine kimberley s africa were studied by cathodoluminescence and electron microprobe techniques and their 87sr 86sr ratios were measured using an aei im20 ion microprobe primary carbonates including calcite dendrites rhombohedral calcites in segregation vesicles and mosaic dolomite have high sr 0 69 1 35 wt sro and ba 0 24 0 44 bao and 87sr 86sr ratios in the range 0 7046 to 0 7056 secondary sparry calcite in amygdales and veins is characterised by low ba 0 05 bao and 87sr 86sr near 0 72 rhombohedral calcite 0 5 cm from a contact with 2 900 my old biotite gneiss has minor element chemistry like that of primary carbonate but an elevated 87sr 86sr ratio of 0 7103 possibly indicating crustal contamination in a boundary layer of the kimberlite magma amygdale like segregations of carbonate and or serpentine originated as gas cavities and were not formed by liquid immiscibility they are now filled either by secondary calcite or by minerals precipitated from residual kimberlite liquid however dendritic calcite and primary dolomite and calcite with high sr ba and low 87sr 86sr demonstrate shared chemical characteristics between these carbonates and carbonatite the primary kimberlite magma had initial 87sr 86sr close to 0 7046

the effects of solidification on sill propagation dynamics and morphology sills are an integral part of the formation and development of larger plutons and magma reservoirs thus sills are essential for both the transport and the storage of magma in the earth s crust however although cooling and solidification are central to magmatism their effects on sills have been so far poorly studied here the effects of solidification on sill propagation dynamics and morphology are studied by means of analogue laboratory experiments hot fluid vegetable oil magma analogue that solidifies during its propagation is injected as a sill in a colder layered gelatine solid elastic host rock analogue the injection flux and temperature are maintained constant during an experiment and systematically varied between each experiment in order to vary and quantify the amount of solidification between each experiments the oil is injected directly at the interface between the two gelatine layers when solidification effects are small high injection temperatures and fluxes the propagation is continuous and the sill has a regular and smooth surface inversely when solidification effects are important low injection temperatures and fluxes sill propagation is discontinuous and occurs by steps of surface area creation interspersed with periods of momentary arrest the morphology of these sills displays folds ropy structures on their surface and lobes with imprints of the leading fronts that correspond to each step of area creation these experiments show that for a given constant injected volume as solidification effects increase the area of the sills decreases their thickness increases and the number of propagation steps increases these results have various geological and geophysical implications the morphology of sills such as lobate structures interpretation of 3d seismic studies in sedimentary basin and ropy flow structures field observations can be related to solidification during emplacement moreover a non continuous morphology as observed in the field does not necessarily involve multiple injections but could instead reflect a continuous yet complex morphology induced by solidification effects during emplacement also a discontinuous sill propagation induced by solidification effects should be associated with bursts of seismic activity finally our study shows that once a sill has initiated the dimensionless flux influences the sill thermal state and in turn its propagation and final extent and thickness in restricting the lateral extent of sills magma cooling and solidification are likely to impact directly the size of plutons constructed by amalgamated sills

behaviour of rare earth elements during hydrothermal alteration at the buena esperanza copper silver deposit northern chile abstract a study of the ree behaviour in alteration zones of the buena esperanza subvolcanic cu ag deposit located in the coastal range of northern chile reveals that the elements are mobile in the rocks during alteration the ore forming process developed in three different stages of alteration mineralization the hydrothermal alteration of the early and middle stages affected basaltic fragments of a breccia pipe la and ce were leached from the rock fragments located at the bottom and at the middle part of the breccia pipe and partly redeposited at the top during alteration sm eu and tb were released selectively and the heavy ree behaved as relatively immobile elements sericite is the most important alteration mineral for fixing the ree during this stage the incoming fluids had low ree contents the late stage of hydrothermal alteration happened simultaneously with the intrusion of a gabbroic volcanic neck affecting mainly these subvolcanic rocks ree patterns of samples from the top and bottom of the alteration zone exhibit roughly parallel trends but are systematically enriched in ree in comparison to unaltered samples this enrichment seems to be more intensive at the top of the altered area coinciding with the enrichment of rb k and cu in these rocks epidote plays the most important role for fixing this hydrothermal input of ree during the latest stage of alteration

experiments on turbulent convection over a rotating continental shelf slope a series of southern hemisphere experiments have been performed to study turbulent convection on a continental shelf slope placed in a large rotating tank filled with fresh water dense salt water was uniformly released at the fluid surface above the shelf the resulting negatively buoyant bottom flow travelled over the shelf and then downslope the non dimensional reduced gravity of the dense downslope flow was found to scale as for a fixed slope angle where b 0 the buoyancy flux at the surface g the reduced gravity of the bottom flow h the water depth above the shelf and w the width of the dense water source under rotation a bottom ekman layer with superimposed roll waves propagated down the slope and towards the left sidewall when looking down the slope as the modified natural rossby number p decreased where p b 0 f 3 h 2 1 2 w h and f is the coriolis parameter the appearance of the bottom layer had four different forms laminar flow the continuous formation of waves the periodic release of wave groups and the periodic generation of eddies vortices generated on the surface were cyclonic suggesting but not proving that eddies in the dense bottom layer as originally formed were anticyclonic with a canyon cut from the middle of the shelf to the bottom of the slope g values measured in dense flows to the left of the canyon were significantly reduced the canyon channelled a large amount of dense fluid with a buoyancy considerably larger than that of dense flows on the slope however the flow regime criteria remained basically unchanged with eddies and downslope ekman layer being able to partially cross the canyon

ensialic spreading subsidence in the mesozoic and palaeogene andes of central chile an oceanic plate subducting eastwards below a continental margin and resulting in crustal shortening can be inferred to have been active in central chile during the palaeozoic however the geological record for the mesozoic and palaeogene shows evidence of a persistent tensional regime and absence of crustal shortening although lateral asymmetry inherent to a subducting model is shown by this record a bilateral symmetry is also well substantiated a coupled action of plate subduction and spreading subsidence the latter taking place within the margin of a continental plate is suggested to explain these features

eddy formation by dense flows on slopes in a rotating fluid properties of the flow generated by a continuous source of dense fluid on a slope in a rotating system are investigated with a variety of laboratory experiments the dense fluid may initially flow down the slope but it turns under the influence of rotation to flow along the slope and initial geostrophic adjustment gives it an anticyclonic velocity profile some of the dense fluid drains downslope in a viscous ekman layer which may become unstable to growing waves provided that the viscous draining is not too strong cyclonic vortices form periodically in the upper layer and the dense flow breaks up into a series of domes three processes may contribute to the formation of these eddies first initial downslope flow of the dense current may stretch columns of ambient fluid by the taylor column process which we term capture secondly the initial geostrophic adjustment implies lower layer collapse which may stretch the fluid column and thirdly viscous drainage will progressively stretch and spin up a captured water column overall this last process may be the most significant but viscous drainage has contradictory effects in that it progressively removes dense lower layer fluid which terminates the process when the layer thickness approaches that of the ekman layer the eddies produced propagate along the slope owing to the combined effects of buoyancy coriolis balance and beta gyres this removes fluid from the vicinity of the source and causes the cycle to repeat the vorticity of the upper layer cyclones increases linearly with l d where l is the rossby deformation radius the bottom slope and d the total depth reaching approximately 2 f in the experiments presented here the frequency at which the eddy dome structures are produced also increases with while the speed at which the structures propagate along the slope is reduced by viscous effects the flow of dense fluid on slopes is a very important part of the global ocean circulation system and the implications of the laboratory experiments for oceanographic flows are discussed

chemical changes associated with deformation of granites under greenschist facies conditions the example of the zawidow granodiorite se lusatian granodiorite complex poland abstract small scale subvertical retrograde shear zones are developed in the zawidow granodiorite located in the eastern part of the lusatian massif southwest poland heterogeneous deformation under conditions of greenschist facies metamorphism has produced mylonite zones that show a broad range of microstructural development three fabric types in the granodiorite deformation have been distinguished a weakly deformed protolith b moderately deformed in lens shaped domains c strongly foliated in fabric types b and c whole rock analyses show a decrease in cao mno cr sr pb whereas na2o and sio2 increase slightly overall al2o3 tio2 and zr remain essentially constant this pattern of element variation is consistent with the observed feldspar breakdown and increase of sericite and also with the transformation of biotite into chlorite mass balance calculations based on major and trace element geochemistry indicate constant volume or less than 10 volume loss from this study it is apparent that a low grade metamorphic deformation event does not have to have a profound influence on whole rock geochemical composition

rare earth and trace element mobility in mid crustal shear zones insights from the mont blanc massif western alps the behaviour of rare earth elements ree during fluid rock interaction in mid crustal shear zones has received little attention despite their potential for mass balance calculation and isotopic tracing during deformation in this study several cases of large ree mobility during alpine fluid driven shear zone development in the pre alpine granitic basement of the mont blanc massif are considered on a regional scale the undeformed granite compositions range within 5 wt sio2 70 5 75 3 wt and magmatic chemical variations are of the order of 10 20 ascribed to minor effects of crystal fractionation major and trace element mobility observed in shear zones largely exceeds these initial variations shear zones developed a range of mineral assemblages as a result of shearing at mid crustal depths at not vert similar0 5 gpa 400 c five main shear zone assemblages involve muscovite chlorite epidote actinolite and calcite respectively as major phases in most cases selective enrichments of light or heavy ree and y ta hf are observed ree mobility is unrelated to deformation style cataclastic mylonitic the intensity of strain and to the shear zone s major metamorphic mineral assemblages instead the changes in ree concentrations are ascribed to the alteration of pre existing magmatic ree bearing minerals during deformation related fluid rock interaction and to the syntectonic precipitation of metamorphic ree bearing minerals mainly monazite bastnasite aeschynite and tombarthite minor proportions 2 of these accessory phases with grain sizes mostly 20 m account for enrichments of up to 5 1 compared to the initial granite whole rock ree budget the stability of the ree phases appears to be largely dependent on the altering fluid composition ree mobility is ascribed to changes in ph and to the availability of co32 po42 and so42 ligands in the fluid such processes are likely to influence the mobility of ree y hf and ta in shear zones

helium accumulation in groundwater iii limits on helium transfer across the mantle crust boundary beneath australia and the magnitude of mantle degassing abstract the groundwaters of the great artesian basin australia have been previously shown to be accumulating in situ production helium for groundwaters ages 100 kyr 1 2 new helium isotope measurements show that the observed in situ production helium 3he 4he 1 6 10 8 is isotopically distinct from the crustal degassing helium flux 3he 4he 6 6 10 8 furthermore the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio 3he 4he 3 5 10 8 and the production ratio in a variety of continental rock types this suggests that the upper limit on volatile transport across the mantle crust boundary beneath the relatively stable and complacent australian continent can be characterized by a conductive diffusive helium heat flux ratio of 2 6 1064he atoms mw 1 s 1 which is two orders of magnitude less than the intrusive volcanic ratio of 2 9 1084he atoms mw 1 s 1 measured at the galapagos 16 these results constrain the transcrustal mantle degassing fluxes of4he and40ar to be much less than the mid ocean ridge degassing fluxes which are much less than the degassing of4he and40ar from continental crust thus the degassing of the earth s interior is dominated by magmatic processes but the dominant fluxes of4he and40ar to the atmosphere must come from the continental crust

miocene fossil hydrothermal system associated with a volcanic complex in the andes of central chile abstract cenozoic deposits in the andes of central chile have been affected by very low grade burial metamorphism at about 33 s in the cuesta de chacabuco area approximately 53 km north of santiago two oligocene and miocene volcanic units form a ca 1300 m thick rock pile the miocene unit corresponds to a volcanic complex composed of two eroded stratovolcanoes secondary mineral assemblages in both units were studied petrographically and using x ray diffraction and electron microprobe analyses most of the igneous minerals are wholly or partially preserved and the ubiquitous secondary minerals are zeolites and mafic phyllosilicates the alteration pattern observed is characterized by a lateral zonation in secondary mineralogy related to a lateral increase in temperature but not to stratigraphic depth the following three zones were established mainly based on the distribution of zeolites zone i comprises heulandite thomsonite mesolite stilbite and tri smectite zone ii contains laumontite yugawaralite prehnite epidote and chlorite and zone iii comprises wairakite epidote chlorite diopside biotite and titanite for each zone the following temperature ranges were estimated zone i 100 180 c zone ii 180 270 c and zone iii 245 310 c the alteration episode was characterized by a high p fluid p total ratio ca 1 0 although slightly variable a high geothermal gradient of ca 160 c km 1 and fluid pressures below 500 bars although temperature was the main control on the mineral zonation several interrelated parameters mainly fluid composition porosity and permeability were also important hot near neutral to slightly alkaline ph alkali chloride hydrothermal fluids with very low dissolved co 2 contents deposited the secondary minerals the alteration pattern is the result of depositing fluids in outflow regions from a hydrothermal system developed inside a volcanic complex during the miocene the hydrothermal system has been eroded to a maximum depth of 1 7 km

the laboratory albitization of mid ocean ridge basalt complete alteration of fresh mid ocean ridge basalt to albite actinolite smectite took place in the presence of quartz and 3 4m nacl at 350 c and 400 bars significant na metasomatism occurred in exchange for ca fe mn in contrast alteration of the basalt quartz mixture with a synthetically prepared natural brine and basalt with 3 4m nacl or natural brine did not produce significant albitization based on these and previous experiments that reacted basalt with a variety of fluids it is concluded that excess silica a high na ca ratio and an absence of strong acidity dissolved mg in the altering fluid are important for the albitization of basalt

contact zone interaction of metabasites with metapelites amphibolite facies mineral assemblages chemical profiles and their origin the puolankajarvi formation finland abstract the amphibolite facies iron rich metapelites garnet staurolite mica schists of the early proterozoic puolankajarvi formation are intercalated with metamorphosed basic rocks which vary from iron rich amphibolites to magnesium rich layered sills and serpentinites garnet cummingtonite hornblende assemblages are abundant at the contacts of the metapelites with amphibolites while garnet chlorite garnet gedrite chlorite garnet gedrite cummingtonite chlorite and chlorite cummingtonite hornblende assemblages occur at their contacts with the layered magnesium rich sill the bulk composition profiles of these contacts normalised with respect to chromium and or aluminium show conspicuous enrichment in fe s ga and co and depletion in si k ba and rb some elements e g mg mn ni p zr and hf are depleted or enriched in only small amounts while others e g ca na sr and the rare earth elements may show both enrichment and depletion in the course of a profile although early infiltration during the hydration of metabasites and tectonic reworking may have produced some interaction of the basites with the pelites the major metasomatic event is considered to have bile elements the composition of the metapelites for example approached that of magnesium rich ultramafites through potassium depletion while the ultramafites became markedly enriched in iron to conserve the volume silica was depleted from the contact which underwent iron enrichment

origin of hydrogen nitrogen gas seeps oman six gas samples were collected from five thermal springs in the semail nappe ophiolite and the calcareous calcite and dolomite hajar formation northern oman the3he 4he 4he 20ne 40ar 36ar and38ar 36ar ratios chemical compositions h2 n2 co2 ch4 o2 ar and he and stable isotope compositions dh2 dh2o 13cco2 13cch4 and 15nn2 are reported samples from the ophiolite region are significantly anoxic with major constituents of h2 ch4 and n2 while those from calcite and dolomite regions are ordinary gas seeps consisting of n2 co2 and or o2 the former h2 rich gas is characterized by relatively high3he 4he ratio 0 4 0 8 ratm with low he content 300 ppm slight radiogenic40ar 36ar ratio high n2 ar ratio 77 97 and low 15nn2 value 0 observed dh2 value of 536 in h2 rich gas is distinguished from the literature value of 699 in the ophiolite region giving discrepant isotope formation temperatures

metamorphic overprinting of smnd isotopic systems in volcanic rocks the telemark supergroup southern norway abstract the telemark supergroup of southern norway consists of a series of metamorphosed mesoproterozoic volcanics and sediments the volcanic pile records three metamorphic events a pervasive regional greenschist facies event ca 1050 ma starmer 1993 a more localised early pre regional metamorphism burial type event and a late thermal metamorphism related to granite intrusion ca 900 ma priem et al 1973 in the present study major and trace element data are employed to illustrate the influence of these metamorphic events on apparent initial nd isotopic compositions in metamorphosed volcanics in the case of the basaltic volcanics burial and thermal metamorphism caused substantial fractionation of the ree and alteration of nd isotopic compositions by interaction with open system fluids such that spurious ages might result from age calculations using the nd isotopic data during the regional metamorphism element mobility was limited to the lile rb sr k and ba the ree were not fractionated and the original nd isotopic signatures were preserved the regional metamorphic effects on the acid volcanics are related to the degree of matrix recrystallisation since this matrix contains a large fraction of the ree where groundmass recrystallisation was limited the original nd isotopic signatures were retained but where the groundmass has been extensively recrystallised the ree have been fractionated resulting in a sm nd isochron age menuge 1985 ca 300 ma younger than the u pb zircon age dahlgren et al 1990 the results of this study emphasise the importance of distinguishing the nature of metamorphic fluid rock reactions the potential for ree mobility during low grade metamorphism and the need for careful selection and geochemical screening of samples for nd isotopic studies in metamorphosed volcanic terrains

relative elemental mobility during hydrothermal alteration of a basic sill isle of skye n w scotland in sconser quarry isle of skye a thin tertiary basic sill is cut by vertical veins which formed fluid conduits in a major meteoric hydrothermal system in order to study the relative mobility of different elements during hydrothermal metamorphism sill material adjacent to a large 6 mm wide vein was cut into slices and subjected to geochemical and isotopic analysis

numerical simulations of the baroclinic dynamics of density driven coupled fronts and eddies on a sloping bottom numerical simulations of the baroclinic dynamics of density driven coupled fronts and eddies are described the simulations are based on a two layer intermediate length scale model which filters out barotropic instability and focuses on the subinertial baroclinic evolution of density driven flows within the context of allowing finite amplitude height variations in the lower layer the baroclinic destabilization of a bottom trapped coupled front on a sloping bottom is described in the overlying fluid the instability takes the form of amplifying topographic rossby waves in the coupled front the perturbations to the downslope incropping are preferentially amplified compared to those on the upslope incropping the perturbations to the downslope incropping develop into downslope propagating plumes which eventually evolve into relatively coherent along slope propagating domes we discuss the propagation characteristics of these domes we also simulate the evolution of density driven eddies or domes the first eddy simulation we describe is for an initial eddy configuration which satisfies a zero topographic rossby wave condition in the upper layer we show that these traveling solutions remain remarkably coherent over a period of about 40 eddy circulation times or about 250 days for typical continental slope values for a sufficiently large initial eddy height upper layer fluid parcels can be transported in the along slope direction by the baroclinic eddy we also simulate the evolution of an initial eddy configuration which does not satisfy a zero topographic rossby wave condition in the upper layer a relatively intense cyclonic circulation develops in the overlying fluid over the traveling dome as does a topographic rossby wave tail however even these solutions remain surprisingly coherent over many eddy circulation times

joint vortices eastward propagating eddies and migratory taylor columns abstract the behavior of an isolated pair of vortices consisting of two eddies situated on top of each other in a three layer ocean is examined analytically the amplitudes of both eddies are high and consequently the two eddies behave as one unit and migrate together in the ocean for this reason it is proposed to call the system joint vortices the eddies are of equal or opposite sign each vortex is situated in a different layer so that there are two active layers and one passive layer attention is focused on the behavior of joint vortices on a sloping bottom in the deep ocean and on a plane in the upper ocean that is we consider deep joint eddies situated on an inclined floor in the lowest two layers of a three layer ocean and upper joint eddies in the upper two layers special attention is given to the cases where one of the vortices is a lens like eddy approximate solutions for slope or induced drifts in the east west direction are obtained it is found that because of the high amplitud

evaluation of the effects of alteration and leaching on sm nd and lu hf systematics in submarine mafic rocks abstract nd and hf isotope systematics of oceanic basaltic rocks are often assumed to be largely immune to the effects of hydrothermal alteration we have tested this assumption by comparing nd and hf isotope data for acid leached cretaceous oceanic basalts from gorgona and dsdp leg 15 with unleached data on the same rocks hf isotope values and lu hf ratios are relatively unaffected by leaching but 143nd 144nd values of leached samples are significantly higher than those of unleached fractions of the same sample in most cases furthermore the sm nd ratios of the majority of leached samples are 10 40 greater than those of unleached samples x ray diffraction studies indicate that selective removal of secondary minerals such as smectite during the acid leaching process is responsible for the fractionation of sm nd ratios these results have implications for interpretation of the hf nd isotope systematics of ancient submarine rocks older than 50 ma as 1 the age corrected 143nd 144nd ratio may not be representative of the primary magmatic signature and 2 the uncertainty of the age corrected end value may exceed the assumed analytical precision

hydrothermal helium plumes in the pacific ocean hydrothermal activity along the global mid ocean ridge system and at active seamounts introduces a 3 he rich signal into the deep ocean basins which can be used to trace patterns of ocean circulation and mixing this hydrothermal helium signal is especially strong in the pacific ocean where the spreading rate of the ridges is the greatest in several areas of the pacific the hydrothermal activity is of sufficient strength to produce intense helium plumes which clearly define the regional circulation among these are 1 a pair of helium plumes at 2500 m depth which extend westward from the east pacific rise at 10 n and 15 s into the interior of the pacific basin 2 a helium plume at 2000 m depth extending southwest from sources on the juan de fuca ridge in the northeast pacific and 3 a plume at 1100 m depth emanating from loihi seamount at 20 n which extends eastward from hawaii to the coast of mexico the flow field implied by the helium distribution appears to be in reasonable agreement with reid s 1997 steric height analysis for the pacific

evidence for limited ree leaching from the roffna gneiss switzerland a discussion of the paper by vocke et al 1987 cmp95 145 154

crustal helium in deep pacific waters we have analyzed deep helium isotope data from world ocean circulation experiment section p17 along 135 w in the pacific ocean combining these data with neon data we have derived the isotopic ratio rt of the nonatmospheric part of helium terrigenic helium the calculated rt values are significantly lower than helium isotope ratios from mid ocean ridge mor sources rmor 8ra ra 3he 4he air which are generally presumed to dominate the isotope characteristics of helium of nonatmospheric origin for the depths most influenced by the southern east pacific rise epr helium plume the isotopic ratio of terrigenic helium is 7 4 ra however both the northern epr plume and the juan de fuca ridge jdfr plume show significantly lower rt values of 6 8 ra we find minimum rt values of 5 6 ra at about 20 n to 30 n from 2000 m depth to the ocean floor we conclude that in addition to the epr and jdfr helium sources having mor isotope characteristics there is a flux of radiogenic 4he out of deep sea sediments and the oceanic crust which causes a significant deviation of rt toward values lower than rmor using our calculated terrigenic 4he concentrations and isotope ratios rt and the helium isotope ratios of the two end members mor helium and crustal helium we derive concentrations and ocean bottom fluxes of crustal 4he 4hecrust maximum concentrations of 4hecrust up to 30 pmol kg 1 are found at about 20 n to 30 n between 2000 m and 4000 m along the p17 section the mean crustal 4he flux is obtained as 1 0 0 4 109 atoms m 2earth s 1

diverse basalt types from loihi seamount hawaii loihi seamount is the southeasternmost active volcano in the hawaiian emperor volcanic chain the seamount is considered representative of the early phase of hawaiian volcanism because of its youth small size and location near the melting anomaly seventeen dredge stations recovered transitional basalt alkalic basalt and basanite in addition to the expected tholeiitic basalt four flows of alkalic basalt contain common small dunite xenoliths the recovered samples have thin palagonite rinds and almost no manganese on the glassy surfaces we estimate that the lavas are less than about 4 000 yr old and many are less than 1 000 yr old loihi seamount is apparently in a transitional growth phase between the early eruption of alkalic lavas and the commonly observed subaerial tholeiitic eruptive phase previously thought to dominate hawaiian volcanism from inception until the postcaldera collapse alkalic stage

geochemistry and tectonic implications of two ordovician bimodal igneous complexes southern french massif central two bimodal meta igneous complexes from the southern massif central provide important tectonic constraints on the initial stage of the palaeozoic orogen the metabasites may be subdivided into two groups according to the distribution of the large ion lithophile lil and high field strength hfs elements n type morbs with low lil hfs ratios and subduction type basalts with high lil hfs ratios these associations suggest a complex tectonic setting such as an extensional zone or a back arc basin above a subduction zone the great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust the association of terrigeneous sediments with n type morbs subduction type basalts and high silica peraluminous rhyolites argue against an intra oceanic setting and suggest that the area was one of ensialic crustal tension during middle ordovician the two complexes are identified as remnants of ensialic back arc basins rather than relicts of major oceanic sutures such marginal basins could have been located in a mainly continental domain such as the armorican microplate

pumpellyite bearing rocks in central sweden and extent of host rock alteration as a control of pumpellyite composition metamorphic assemblages and mineral compositions in basic to intermediate volcanic rocks of two precambrian units the sub jotnian and jotnian in the dala region central sweden reveal a history comprising two metamorphic episodes the sub jotnian rocks were metamorphosed during a first episode the metamorphic grade ranges from prehnite pumpellyite facies in a displaced block with sub jotnian lava occurring in the border zone of a major impact structure to pumpellyite actinolite facies and to greenschist facies in the lower part of the unit the unconformably overlying jotnian rocks were metamorphosed to prehnite pumpellyite facies during a subsequent episode which also left its imprint in the sub jotnian basement two other jotnian sequences in central sweden are in the same facies

dynamics of radiating cold domes on a sloping bottom numerical simulations of benthic gravity driven currents along continental shelves suggest they exhibit considerable time and spatial variability and tend to organize themselves into large scale bottom intensied cold domes or eddies attempts to derive simple relations governing the evolution of the spatial moments of the mass equation for baroclinic eddies have failed because it is not clear how to express the form or wave drag stresses associated with the excited topographic rossby wave eld in the surrounding fluid in terms of the eddy moments we develop a simple model for the leading order time evolution of a cold dome conguration which initially nearly satises the mory stern isolation constraint as the topographic rossby wave eld in the surrounding fluid interacts with the cold dome higher azimuthal modes are excited within the cold dome which develop into spiral like lamentary structures on the eddy boundary the trajectory followed by the position of the maximum height of the cold dome corresponds to sub inertial along and cross slope oscillations superimposed on a mean along slope drift well described by the nof velocity nevertheless the theory suggests that there are no oscillations at least to second order in the horizontal spatial moments of the eddy height that is the centre of mass of the eddy moves steadily in the along and down slope directions i e southwestward relative to the topographic plane the theoretical analysis is in good agreement with a nonlinear numerical simulation which we present

topographic effects on the path and evolution of loop current eddies 1 eddy topography et interactions are important in determining the path and evolution of oceanic eddies including loop current eddies lce in the gulf of mexico gom we use the hybrid coordinate ocean model and satellite altimetry data to investigate the et interactions and the impact on lce pathway evolution in the gom satellite altimetry reveals that lces translate dominantly westward in the central gom and strongly collide and reflect against topography near the continental slope in the northern and western gom the result is the frequent generation of an anticyclone cyclone ac pair in conjunction with the lces in the absence of lateral or surface boundary forcing but including realistic topography simulations initialized with idealized eddies at various locations in the gom reveal the following results southward eddy reflection from the northern slope occurs when a cyclone drastically strengthens east of the anticyclone because of the et collision the prevailing westward propagation in the central gom occurs because the cyclone is very dispersive toward nearby topographic features causing a reduced southward component of drift and or moves to the south forming a meridional ac pair causing an enhanced westward component et collision is strongest over the northwestern slope north of 24 n because of the eddy colliding relatively normal to the steep slope and the eddy typically tracks anticyclonic pathways during the collision reflection process along the western slope a strong et collision produces a southeastward reflection and the accompanying cyclone to the northeast strongly enhances the reflection near the southern gom slope eddy pathways tend to propagate alongslope with an onshore offshore oscillatory trajectory because of the competition between topographic and planetary effects in the southwestern gom a bimodal pathway occurs i e northward migration with an anticyclonic route for relatively stronger eddies and southward dissipative propagation along the shelf edge for weaker ones

a geostrophic vortex over a slope abstract nonlinear quasigeostrophic f plane vortices in two layers over a topographic slope are considered scaling arguments suggest two parameters that dictate the effective strength of the slope the first indicates the likelihood of dispersion at depth and the second relates to baroclinic stability if the deep flow is only weakly dispersive weak slopes an initially barotropic vortex can translate barotropically across the isobaths provided the vortex scale exceeds the deformation scale over stronger slopes the vortex separates into topographic waves and a stationary surface trapped vortex an initially surface trapped vortex larger than deformation scale becomes unstable over a weak slope as it does over a flat bottom however a strong slope can stabilize the vortex to small perturbations despite the large vortex scale the effective slope parameters depend not only on topographic grade but on vortex strength and size and on the ambient stratification parameters obtained with represen

archean mafic metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 1 mobility of the major elements reply in a suite of archean mafic pillows from the rouyn noranda region of quebec s abitibi greenstone belt including both tholeiitic and calc alkaline varieties spanning the prehnite pumpellyite to upper greenschist metamorphic facies three types of alteration can be defined i chlorite epidote actinolite ii chlorite epidote and iii chlorite sericite the number of mineral phases decreases as a result of progressive hydration from type i to type iii alteration albitization resulting from substitution of in calcic plagioclase is highly variable in type i alteration but in types ii and iii the plagioclase is totally albitized and in some cases silicified chloritization is closely linked to increasing hydration and ca leaching with mgo and feo substituting for cao in ferromagnesian minerals calcium was mobilized and carried by solutions as evidenced by the variable concentration of epidote at the margins of pillows this calcium leaching generated an excess of al2o3 with respect to the combine

pumpellyites in two low grade metamorphic terranes north of newcastle nsw australia microprobe analyses of pumpellyites from rocks of variable chemistry formed under similar metamorphic conditions in two palaeozoic low grade metamorphic terranes show that they have an extreme range in composition feo 0 9 22 96 and that fe2 mg2 and fe3 al3 are the dominant substitutions a less extreme variation in composition of pumpellyites has been noted in samples taken from a metamorphosed differentiated metadolerite on an a1 fe mg diagram these pumpellyites extend through the fields of high pressure to low pressure terranes indicating that pumpellyite compositions should be used with caution when determining metamorphic conditions

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

coherent baroclinic eddies on a sloping bottom a coherent and stable baroclinic eddy in a rotating fluid was produced on a sloping bottom by releasing a dome of salt water into the ambient fresh water a strong cyclonic vortex is produced above the heavy dome the entire eddy system moves north westward with the up slope direction designated north as a taylor column the eddy system displays long lifetimes but it is shown that a theory of isolated systems cannot account for the experimental observations instead it is demonstrated that the vortex flow above the lens is along the lines of constant depth producing a net pressure force on the lens which approximately balances the buoyancy force when ekman friction is also included it accounts for the northward motion of the dome

major and minor element mobilization with implications for nicufe sulphide remobilization during retrograde metasomatism at the vammala mine southwest finland abstract granitic melt derived from migmatitic host rocks intruded a d 3 related fracture system in a 1 89 ga ni cu fe sulphide bearing peridotite sheet near vammala southwest finland zones of wall rock alteration mainly comprising amphibole phlogopite chlorite developed in the peridotite adjacent to the leucocratic dykes the minerals in the alteration zones and in the gneissic host rocks together with the production and intrusion of granitic melt are consistent with metamorphic conditions of at least 600 c and 4 5 kbar later hydration under condtions of lower temperature and pressure resulted in partial serpentinization of the peridotite and dykes this later transition precluded direct investigation of the earlier transition that developed the medium to high grade alteration zones in the peridotite at the interface with the leucocratic dykes because serpentinization was largely accommodated by volume increase rather than mass loss of mgo and sio 2 the relative concentrations of most components in the serpentinized peridotite should mirror those in the peridotite based on this contention two methods are used to document the composition volume behaviour of the transition from now serpentinized peridotite to alteration zones apparent transition and hence infer the mass balance trends of the igneous peridotite to alteration zone transition real transition the methods collectively suggest that in the alteration assemblages k 2 o rb 2 o p 2 o 5 tio 2 and al 2 o 3 form a group of relatively enriched components and that al 2 o 3 was particularly mobile an estimated volume increase of 10 10 accompanied serpentinization and a calculated volume decrease of 10 15 accompanied the apparent transition thus a volume increase of real transition for best fit volume factors gresens equation demonstrates that the apparent transition involved mass gains in sio 2 al 2 o 3 k 2 o p 2 o 5 and tio 2 and mass losses in feo mgo cao ni s and volatiles loss on ignition loi these amounting to net losses of 32 5 kg of volatiles and 10 8 kg of other components per 1000 kg of serpentinized peridotite the real transition involved similar relative gains and losses except for sio 2 gain slightly less mgo loss slightly more and loi loss negligible it respectively records inward transfer of components from a feldspar rich source to the peridotite and outward transfer to that source this is consistent with metasomatic exchange between the peridotite and the probably supra solidus granitic intrusions during the real exchange ni cu fe sulphides were retained and redistributed internally remobilized in the outer alteration zones but transferred into the dyke from the zone abutting it externally remobilized the same genetic interpretations probably apply to similar dyke and alteration systems in other ni cu fe sulphide deposits hosted in ultramafic rocks in southwest finland

nd isotopes demonstrate the role of contamination in the formation of coexisting quartz and nepheline syenites at the abu khruq complex egypt the petrogenesis of abu khruq an 89 ma alkaline ring complex of eastern egypt which is composed of alkali gabbros and both silica over and undersaturated syenites has been investigated major and trace element relationships and nd and sr isotope data are consistent with formation of the gabbros from an alkaline mafic magma that experienced extensive fractionation and all syenites from a felsic derivative of this melt the parental magma had an 87sr 86sr of 0 7030 and an 143nd 144nd of 0 512750 nd 4 4 indicating derivation from a depeleted mantle source the initial 143nd 144nd ratios are 0 512721 to 0 512748 for the gabbros 0 512739 to 0 512750 for the alkali syenites and trachytes 0 512717 to 0 512755 for the nepheline syenites and 0 512706 to 0 512732 for the quartz syenites in contrast analyzed precambrian granites from eastern egypt have generally lower 143nd 144nd ratios ranging from 0 51247 to 0 51261 or nd 0 8 to 1 7 for 89ma their nd model ages range from 775 to 935 ma and suggest there was no significant input of pre pan african crust in their formation among abu khruq rocks 143nd 144nd ratios indicate that the quartz syenites formed by open system crustal contamination processes whereas the nepheline syenites experienced little or no contamination modeling shows that contamination occurred at various stages affecting both mafic and more evolved compositions with input of about 20 crustal nd for the most contaminated samples the degree of contamination is related to the silica saturation of the quartz syenites simplified modeling of magma evolution within petrogeny s residua system demonstrates the ability of afc processes to cause a critically undersaturated magma to evolve across the feldspar join and produce oversaturated rocks the oversaturated syenites at abu khruq were produced in this manner whereas the nepheline syenites formed by fractionation without similarly large degrees of contamination the results have broad implications for the formation of subvolcanic complexes in continental settings beyond the important production of silica oversaturated compositions from crustal interaction they underscore the importance of crustal interactions in the formation of the various lithologies such interactions occur at various stages in the evolution of the magmas and as such are not strictly coupled with fractional crystallization while previous study of abu khruq has demonstrated extensive hydrothermal alteration of o and sr isotopes the present work shows that the nd isotope ratios were not significantly affected and thus reflect magmatic signatures this feature combined with relatively small corrections for initial ratios emphasizes the utility of nd isotopes for petrogenetic studies

geochemistry of metabasites from the nevado filabride complex betic cordilleras spain relics of a dismembered ophiolitic sequence abstract major and trace element distribution including ree data in the metabasites of the nevado filabride complex betic cordilleras spain suggests that the whole mafic ultramafic association represents a dismembered ophiolitic sequence all the analyzed samples have been affected by sea floor alteration yet only the fine grained rocks have suffered some significant mobilization of the lree life the other western mediterranean ophiolites this sequence is related to the opening of the liguro piemontese basin during mesozoic times however the metabasites from betic cordilleras differ from the ophiolitic basalts by higher values of the more incompatible over less incompatible element ratios the entire variation of these ratios in the liguro piemontese basin is related to a dynamic melting process coupled with a diapiric uprise of the mantle source from the stability field of garnet peridotites possibly up to the spinel plagioclase facies transition in this process the ophiolites of the betic cordilleras would record the deepest stages and the lowest degrees of melting this feature is related to the peculiar location of these ophiolites at the western end of the liguro piemontese basin

a string function for describing the propagation of baroclinic anomalies in the ocean the authors derive a string function that describes the propagation of large scale potentially large amplitude baroclinic energy anomalies in a two layer ocean with variable topography and rotation parameter the generality of the two layer results allows results for the 1 layer 1 5 layer inverted 1 5 layer lens and dome models to be produced as limiting cases the string function is a scalar field that acts as a streamfunction for the propagation velocity in the linear case the string function is simply c2o f where co is the background baroclinic shallow water wave speed and typically describes propagation poleward on the eastern boundaries westward with some topographic steering over the middle ocean and equatorward on the western boundaries in the more general nonlinear case the string function is locally distorted by the anomaly in the fully nonlinear examples of a lens or dome there is no rest or background string function the string function is generated entirely by the disturbance and propagation is due to asymmetric distribution of the anomalous mass over the string function contours it is shown that conventional beta topographic propagation results e g beta drift of eddies the nof speed of cold domes can be obtained as limiting cases of the string function the string function provides however more general propagation velocities that are also usually simpler to derive the first baroclinic mode string function for the global oceans is calculated from hydrographic data the westward propagation speeds in the ocean basins as derived from the meridional gradient of the string function are typically two to five times faster than those expected from standard theory and agree well with the propagation speeds observed for long baroclinic rossby waves in the topex poseidon data

temperature distribution in the crust and mantle in an attempt to understand the temperature distribution in the earth experimental constraints on the geotherm in the crust and mantle are considered the basic form of the geotherm is interpreted on the basis of two dominant mechanisms by which heat is transported in the earth 1 conduction through the rock and 2 advection by thermal flow data reveal that 1 the temperature distributions through continental lithosphere and through oceanic lithosphere more than 60 million years old are practically indistinguishable 2 crustal uplift is instrumental in modifying continental geotherms and 3 the average temperature through the archean crust and mantle was similar to that at present it is noted that current limitations in understanding the constitution of the lower mantle can lead to significant uncertainties in the thermal response time of the planetary interior

the meridional flow of source driven abyssal currents in a stratified basin with topography part ii numerical simulation abstract a numerical simulation is described for source driven abyssal currents in a 3660 km 3660 km stratified northern hemisphere basin with zonally varying topography the model is the two layer quasigeostrophic equations describing the overlying ocean coupled to the finite amplitude planetary geostrophic equations describing the abyssal layer on a midlatitude plane the source region is a fixed 75 km 150 km area located in the northwestern sector of the basin with a steady downward volume transport of about 5 6 sv sv 106 m3 s 1 corresponding to an average downwelling velocity of about 0 05 cm s 1 the other parameter values are characteristic of the north atlantic ocean it takes about 3 2 yr for the abyssal water mass to reach the southern boundary and about 25 yr for a statistical state to develop time averaged and instantaneous fields at a late time are described the time averaged fields show an equatorward flowing abyssal current with distinct up and downslope groundings with decr

magmatic 87sr 86sr relicts in hydrothermally altered quartz diorites brabant massif belgium and the role of epidote as a sr filter the porphyritic quartz diorites of the caledonian brabant massif have been totally altered ca rb sr zr ce y measurements and sr nd isotopic analyses were performed on the quenast plug and the lessines sill in an attempt to study the relative mobility of sr and evaluate the extent direction and magnitude of the 87sr 86sr alterations sr electron microprobe analyses of epidote were also carried out to assess its role in the sr distribution

injection of mantle type helium into lake van turkey the clue for quantifying deep water renewal helium neon and tritium concentrations have been measured to study long term vertical mixing and deep water renewal in lake van eastern turkey the largest soda lake on earth helium excesses were found in the water column of lake van although neon concentrations were close to air saturation the excesses of both isotopes are strictly correlated and increase with depth in the bottom water 4he supersaturation is about 20 and the corresponding 3he concentration is 2 5 times the air saturation value the mean excess 3he 4he ratio of 1 1 2 10 5 is slightly higher than the morb ratio found in lake nemrut a neighbouring volcanic crater lake in which a large input of mantle helium was detected mantle helium accounts for the majority of the helium excesses in lake van although part of the 3he excess is attributed to the presence of tritium a one dimensional lake model was used which describes tritium input vertical mixing and gas exchange to reconstruct the evolution of the helium isotopes and the tritium as a conclusion the model based on the measured 3he and 4he profiles shows that 1 the vertical exchange of deep water occurs within 1 2 yr 2 the flux of mantle helium into the lake averaged over the total cross section of lake van is 0 23 0 35 cm3 stp m 2 yr 1 2 3 1011 atoms m 2 s 1 the present estimate of the global mantle helium flux averaged over the total surface area of the earth is 3 9 109 atoms m 2 s 1 the flux into lake van would account for at least 0 04 0 06 of this

geochemistry of hydrothermal deposits from loihi submarine volcano hawaii abstract dredging across the northeast rim of the summit crater of loihi seamount recovered several morphologically similar but chemically and mineralogically distinct hydrothermal deposits encrusting the surface of fresh pillow lava talus the multicolored deposits suggest a precipitation sequence that may be controlled by an oxidation reduction gradient in which smectite ranging in composition from fe montmorillonite to nontronite has precipitated along with iron oxide under slightly reducing conditions this deposition was apparently followed by amorphous iron oxide and silica precipitation possibly under more oxic conditions oxygen isotope geothermometry indicates formation temperatures in the range of 31 57 c for the loihi smectites trace element enrichments appear to be positively correlated with the isotopic formation temperature of the smectite suggesting either increased trace element solubility within the higher temperature vent fluids or increased smectite and iron oxide scavenging with increased precipitation rates the trace element abundances further suggest the presence of polymetallic sulfides that either are directly associated with the smectites as amorphous phases or occur beneath these deposits in the volcanic pile of the seamount

a major helium 3 source at 15 s on the east pacific rise an extensive plume of water enriched with helium 3 has been discovered in the deep pacific ocean at latitude 15 s on the east pacific rise in the core of the plume at a depth of 2500 meters over the ridge crest the helium 3 helium 4 ratio is 50 percent higher than the ratio in atmospheric helium indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems the helium 3 plume is completely absent east of the rise but it can be traced over 2000 kilometers to the west above a newly observed physical feature a density discontinuity here caled the ridge crest front the injected plume provides a unique deep sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west the striking intensity and lateral extent of this helium 3 anomaly compared to observations at known oceanic hydrohrmal sites suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 s on the east pacific rise

methane anomalies in seawater above the loihi submarine summit area hawaii hydrothermal activity above loihi submarine volcano was characterized by water column distributions of methane ph and helium 3 it was found that the southern loihi summit is almost covered with hydrothermal plumes which have anomalously high concentrations of methane maximum 569 10 6cm3kg 1 accompanied by high concentrations of helium 3 and low ph values minimum 7 18 the plumes consist of two layers a shallow plume about 200 m above the summit and a deep plume about 100 m above the summit probably derived from different hydrothermal vents the shallow and deep plumes showed different ch43he and ch4ph ratios with the same 3heph ratio which implies that methane concentrations differ between the hydrothermal end members for the two plumes the variation of methane between the end members is suggested to result from inter vent inhomogeneity of bacterial activities that consume or produce methane within the vents comparison of the ch43he ratios of the two plumes with the previous data for loihi and other submarine hydrothermal areas confirms that the loihi hotspot has one to two orders of magnitude smaller ch43he value than those of the east pacific rise and the galapagos spreading centers

archaean metasedimentary rocks from the northwestern pontiac subprovince of the canadian shield chemical characterization weathering and modelling of the source areas abstract the northwestern pontiac subprovince is dominated by a monotonous greywacke sequence metamorphosed and deformed during the late archaean kenoran orogeny limited chemical weathering of the source rocks is suggested by 1 low values of the chemical index of alteration 2 no preferential leaching of ca sr and ba 3 well defined correlations among mgo k2o and al2o3 and 4 small variations of cr ni the source was dominantly felsic as indicated by high th sc average 0 39 la co 1 06 zr cr 0 7 zr y 11 2 and sio2 mgo 19 4 the composition of these metasedimentary rocks can be modelled by mixing of three end member components 1 tonalites trondhjemites 65 in order to explain the sio2 al2o3 th u zr sr and ba abundances na k ratios strong depletion in hree and hfse relative to lree and lile low th y and k2o and fractionated la yb 19 4 2 komatiites 5 in order to explain the high cr 220 ppm and ni 66 ppm contents and 3 tholeiitic basalts 30 accounting for the observed tio2 0 62 wt v 144 ppm and sc 17 ppm concentrations the source region may be considered as a deeply eroded greenstone belt intruded by abundant tonalite trondhjemite plutons possibly now located to the south and east in the grenville province or possibly to the north underlying the dominantly volcanic abitibi subprovince

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions