40ar 39ar and oxygen isotope studies of polymetamorphism from tinos island cycladic blueschist belt greece petrological oxygen isotope and 40ar 39ar studies were used to constrain the tertiary metamorphic evolution of the lower tectonic unit of the cyclades on tinos polyphase high pressure metamorphism reached pressures in excess of 15 kbar based on measurements of the si content in potassic white mica temperatures of 450 500 c at the thermal peak of high pressure metamorphism were estimated from critical metamorphic assemblages the validity of which is confirmed by a quartz magnetite oxygen isotope temperature of 470 c some 40ar 39ar spectra of white mica give plateau ages of 44 40 ma that are considered to represent dynamic recrystallization under peak or slightly post peak high pressure metamorphic conditions early stages in the prograde high pressure evolution may be documented by older apparent ages in the high temperature steps of some spectra eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation oxygen isotope thermometry of four quartz magnetite pairs from greenschist samples gives temperatures of 440 470 c which cannot be distinguished from those deduced for the high pressure event the exhumation and overprint is documented by decreasing ages of 32 28 ma in some greenschists and late stage blueschist rocks and ages of 30 20 ma in the lower temperature steps of the ar release patterns of blueschist micas almost flat parts of ar ar release spectra of some greenschist micas gave ages of 23 21 ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a 18o enriched fluid rather the compositions indicate that fluid to rock ratios were very low the isotopic compositions being primarily controlled by those of the protolith rocks we assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid a quartz magnetite sample from a contact metamorphic skarn taken near the miocene monzogranite of tinos gave an oxygen isotope temperature of 555 c and calculated water composition of 9 1 the value of 18o obtained from this water is consistent with a primary magmatic fluid but is lower than that of fluids associated with the greenschist overprint which indicates that the latter event cannot be directly related to the monozogranite intrusion

petrologic and stable isotope constraints on the source and evolution of skarn forming fluids at pine creek california a detailed geochemical analysis of the well exposed multistage pine creek california tungsten deposit has constrained the roles of magmatic metamorphic and meteoric fluids in the development of the zoned scheelite ore and calc silicate skarn metamorphic conditions of p 2 kb t 600 degrees c and x sub co 2 0 25 are recorded in the metasedimentary rocks of the septum metamorphism yielded calcite diopside graphite and calcite dolomite periclase brucite forsterite assemblages in the variably dolomitic siliceous carbonate rocks of the pine creek septurn following intrusion of the morgan creek quartz monzonite metasomatism developed the zoned endoskarn exoskarn at the contact the massive garnet andradite sub 20 50 pyroxene hedenbergite sub 20 80 or quartz skarn assemblage carries most of the scheelite calculated t x sub co 2 equilibria corrected for solid solution effects restrict skarn formation to x sub co 2 0 2 at t 500 degrees or 50 degrees c calculated f sub o 2 f sub f 2 f sub s 2 diagrams indicate f sub o 2 values near quartz fayalite magnetite and f sub s 2 values well below pyrrhotite pyrite during skarn formation slightly falling temperatures were accompanied by increased f sub s 2 and hydration of the skarn assemblages stable isotope data for calcites show a mixing trend from unaltered marble delta 18 o 24ppm delta 13 c 3 5 degrees to values in equilibrium with magmatic fluids delta 18 o 8ppm delta 13 c 6ppm biotite quartz thermometry for the quartz monzonite yields temperatures of 410 degrees to 460 degrees c well below the solidus and suggesting later reequilibration pyroxene quartz analyses suggest that these minerals equilibrated at temperatures of 460 degrees to 560 degrees c the delta 18 o values measured for garnets suggest disequilibrium for the garnet quartz and garnet pyroxene pairs d h results for epidote and biotite suggest that meteoric water influx became more important with time sulfur isotope data are consistent with a reduced magmatic source

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

geochemistry and petrogenesis of late miocene granitoids cyclades southern aegean nature of source components abstract neogene igneous activity in the aegean region comprised both extrusive volcanism and intrusive granitoid plutonism granitoid plutonism in the cyclades was established in the mid miocene period and lasted until late miocene 18 to 9 ma the timing of granitoid intrusion was associated with the initiation of extensional tectonics in the aegean these intrusions form a broad belt about 200 km long running from the west lavrium and serifos to the eastern aegean both s type and i type granitoids are present the former generally being emplaced earlier than the i types 15 to 8 3 ma major and trace element variations reveal that three end member components are involved in the granitoids but the proportions of these vary in the different plutons initial isotopic compositions of all the granitoids are typical of crust derived magmas from heterogeneous metasedimentary sources i type 87 sr 86 sr 0 7091 0 712 e nd 6 4 to 10 4 s type 0 710 0715 e nd 7 5 to 10 1 three end member sources have been identified 1 one end member appears to be a metasedimentary biotite gneiss greywacke type such as that forming the metamorphic core complexes naxos and paros this is a dominant but not the only component in the s types 2a major trace element and sr nd isotopic composition correlations of the s type granitoids with basement gneiss require an extra source of sr and ca having lower initial 87 sr 86 sr indicating a more depleted metasedimentary source at depth due to possible interaction of metamorphic fluids with marbles and amphibolites and infiltration through the gneissic units at mid crustal levels 2b a possible second end member could be the marble component as indicated by the buffered values of the initial sr isotopic ratios major element variation of the mafic microgranular quartz diorites and tonalites enclaves are compatible with dehydration melting of a mafic source similar to the amphibolites island arc tholeiites at medium pressure 8 kb conditions 3 amphibolite is another end member which may contribute mostly to the source of the younger intrusions along the western flank of the arc

modelling an infiltration driven geochemical front modelling of geochemical alteration profiles e g in 18o or 87sr 86sr resulting from fluid infiltration has provided important constraints on the volumes of fluid that move through the crust during metamorphism these models consider that alteration results from a combination of advection with diffusion and or kinetically limited solid fluid exchange models incorporating advection and one of the two broadening mechanisms have analytical solutions whilst the three parameter solution does not given a particular alteration profile it is therefore important from a computational point of view to determine whether use of the more comples three parameter model places tighter limits on the characteristics of the fluid infiltration event in this paper we address this problem through modelling of a measured oxygen isotope alteration profile from naxos greece for each end member model we calculate a set of best fit parameters emphasising which features of the data control the fit of each of the models we then consider numerical solutions which allow all three processes to operate and conduct a grid search in parameter space to map out the structure of the variance between the model and data while it is not possible to distinguish a single combination of the three processes that produces a strong global minimum in variance we can distinguish a family of solutions which are equally valid and we use this minimum variance family to place bounds on the time scales fluid velocity and porosity characteristic of the fluid alteration event

fluid record of rock exhumation across the brittle ductile transition during formation of a metamorphic core complex naxos island cyclades greece fluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the naxos metamorphic core complex attic cycladic massif greece were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile brittle transition three sets of quartz veins v m2 v bd v b are distinguished the v m2 and v bd are totally or respectively partially transposed into the foliation of the leucogneiss they formed by hydrofracturing alternating with ductile deformation accommodated by crystal plastic deformation the v b is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition fluids trapped during crystalplastic deformation comprise two very distinct fluid types namely a co2 rich fluid and a high salinity brine that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively they were initially trapped at approximate to 625 degrees c and 400mpa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end members with later trapping conditions of approximate to 350 degrees c and 140mpa in contrast brittle microcracks contain aqueous fluids trapped at 250 degrees c and 80mpa all veins display a similar 13c pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history in contrast the d signature is marked by a drastic difference between i v m2 and v bd veins that are dominated by carbonic aqueous carbonic and high salinity fluids of metamorphic and magmatic origin characterized by d between 56 parts per thousand and 66 parts per thousand and ii v b veins that are dominated by aqueous fluids of meteoric origin characterized by d between 40 parts per thousand and 46 parts per thousand the retrograde pt pathway implies that the brittle ductile transition separates two structurally chemically and thermally distinct fluid reservoirs namely i the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 degrees c km1 at lithostatic pressure and ii the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 degrees c km1 at hydrostatic pressure

structure and kinematics of upper cenozoic extensional detachment on naxos and paros cyclades islands greece this paper presents a study of the ductile and brittle deformation on naxos and paros islands cyclades greece previous maps and studies of the two islands have shown that a major low angle fault zone separates surface rocks above the contact from an initially deep seated unit below showing a metamorphic evolution from high to low pressures structural analysis as well as available stratigraphical metamorphic and geochronological data taken together demonstrate that this fault zone is a major normal sense detachment zone dipping to the north rapid denudation of footwall rocks subsequent to high temperature metamorphism at an estimated rate of 1 8 9 5 mm yr attests for tectonic unroofing during regional scale top to the north ductile shearing the change from ductile to brittle behavior of the footwall rocks together with a progressive localization of high strain intensity deformations just below the hangingwall is explained by the progressive cooling of the uprisen footwall of the detachment mio pliocene clastic sediments in the hangingwall represent the infilling of half grabens opened in between major normal faults that are synthetic to the underlying ductile shear zone these sediments are as old as aquitanian 25 ma or younger than the earliest recognized evidence of ductile extension in the footwall this provides a minimum age for the onset of extension in the cyclades which appears significantly older than maximum ages reported up to now 13 5 ma structural data strongly suggest that the detachment fault was initially rather low dipping 35 an evolutionary model is proposed in which migmatite domes in the footwall correspond to the uprise of the lower ductile crust between two separating upper crustal blocks during a process of asymmetric boudinage of the crust this detachment model applies to a previously thickened continental lithosphere which then suffers thermal relaxation and weakening allowing extensional deformation to reach a climax during and subsequent to high temperature metamorphism in the cyclades crustal scale extension started after early cenozoic thrusting while the crust was still thick or less likely before late underthrusting below the present surface

comparison of azimuthal seismic anisotropy from surface waves and finite strain from global mantle circulation models summary we present global models of strain accumulation in mantle flow to compare the predicted finite strain ellipsoid fse orientations with observed seismic anisotropy the geographic focus is on oceanic and young continental regions where we expect our models to agree best with azimuthal anisotropy from surface waves finite strain derived models and alignment with the largest fse axes lead to better model fits than the hypothesis of alignment of fast propagation orientation with absolute plate motions our modelling approach is simplified in that we are using a linear viscosity for flow and assume a simple relationship between strain and anisotropy however results are encouraging and suggest that similar models can be used to assess the validity of assumptions inherent in the modelling of mantle convection and lithospheric deformation our results substantiate the hypothesis that seismic anisotropy can be used as an indicator for mantle flow circulation derived models can contribute to the establishment of a quantitative functional relationship between the two

chemical composition stratigraphy and depositional environments of the black river group middle ordovician southwestern ohio the chemical composition and stratigraphy of the black river group in southwestern ohio were studied chemical analyses were done on two cores of the black river from adams and brown counties ohio these studies show that substantial reserves of high carbonate rock 96 to 98 caco3 plus mgco3 are present in the black river at depths of less than 800 ft in proximity to cincinnati and the ohio river stratigraphic studies show that the black river group has eight marker beds in its middle and upper portions and three distinct lithologic units in its lower portion these marker beds and units are present throughout southwestern ohio the black river group correlates well with the high bridge group of kentucky depositional environments of the black river are similar to those of the high bridge and to present day tidal flats in the bahamas

strontium and oxygen isotope decoupling in the hercynian trois seigneurs massif pyrenees evidence for fluid circulation in a brittle regime systematic shifts of oxygen isotopic compositions in the higher grade parts of the high temperature low pressure hercynian metamorphic sequence exposed in the trois seigneurs massif have previously been explained as a result of an influx of surface derived water during the prograde part of the metamorphic cycle it has been suggested that this caused a regional lowering of 87sr 86sr in the metamorphic sequence mapping of strontium isotopic compositions across a 15 m meta carbonate horizon in the higher grade pelite psammite sequence shows that strontium isotopic compositions were homogenised over length scales of metres or less during the hercynian metamorphism which brought the carbonate and pelite psammite to oxygen isotopic equilibrium with a common fluid comparison of model pre hercynian 87sr 86sr profiles across the carbonate based on a depositional diagenetic age of 450 ma and initial 87sr 86sr ratio of 0 7086 given by 10 m length scale averaging with the post hercynian 87sr 86sr profile calculated from analysed 87sr 86sr and rb sr compositions implies strontium isotopic diffusion distances of ca 0 4 m in the carbonate and ca 7 m in the pelite psammite the limited sr isotopic diffusion distance of 0 4 0 7 m within the carbonate is compatible with pervasive oxygen isotopic exchange over distances restricted to 4 15 m if fluid strontium concentrations were between 4 and 50 ppm the strontium isotopic transport distances are not compatible with pervasive oxygen isotopic alteration over the observed 5 km regional scale either the flow was perfectly layer parallel or more probably the regional scale alteration of oxygen took place by fluid circulation in the brittle regime early in or prior to the hercynian metamorphic event flow along cracks with incomplete diffusive exchange between fluid and wall rock would allow greater decoupling of oxygen and strontium isotopic transport than pervasive advective transport with local fluid solid equilibrium

strontium carbon and oxygen isotope compositions of marbles from the cycladic blueschist belt greece the cycladic blueschist belt greece is mostly submerged below sea level and regional correlations are difficult to establish marbles are widespread within the belt and locally used as marker horizons to subdivide monotonous schist sequences however owing to the lack of distinctive petrographic characteristics the marbles have not been used for island to island correlations this study aims to investigate the potential of sr c and o isotope compositions of marbles as a tool for unravelling the litho and or tectonostratigraphic relationships across the cycladic islands and as a proxy for the time of sediment formation for this purpose we have studied metamorphic carbonate rocks from the islands of tinos andros syros sifnos and naxos identical 87sr 86sr values for certain marble horizons occurring on tinos andros and sifnos are interpreted to document coeval regional carbonate precipitation the 87sr 86sr values of the apparently least altered samples intersect the seawater curve multiple times within the most likely time interval of original carbonate precipitation 240 ma as indicated by previously published ion probe u pb zircon data and thus an unequivocal age assignment is not possible very broad temporal correlations are possible but more subtle distinctions are not feasible on andros the overlapping sr isotope values of marbles representing the lowest and highest parts of the metamorphic succession are in accordance with a model suggesting isoclinal folding or thrusting of a single horizon or very fast sedimentation in contrast distinct 87sr 86sr values for samples from tinos representing different levels of the metamorphic succession suggest that these rocks represent a temporal succession and not the tectonic repetition of a single horizon based on sr o and c isotope characteristics alone the time equivalence of marbles occurring on different islands could not be documented unambiguously however by using various combinations of these parameters some occurrences can be discriminated from the overall sample population the new data further accentuate the general potential of coupled sr c and o isotope characteristics for identification of archaeological provenance and complement existing datasets for aegean marbles

the ordovician sebree trough an oceanic passage to the midcontinent united states the sebree trough is a relatively narrow shale filled sedimentary feature extending for several hundred kilometers across the middle and late ordovician carbonate platform of the midcontinent united states the dark graptolitic shales within the trough stand in contrast to the coeval bryozoan brachiopod echinoderm rich limestones on the flanking platforms we infer from regional stratal patterns thickness and facies trends and temporal relations established by biostratigraphy and k bentonite stratigraphy that the sebree trough initially began to develop during late turinian to early chatfieldian time mohawkian series as a linear bathymetric depression situated over the failed late precambrian early cambrian reelfoot rift rising sea level and positioning of a subtropical convergence zone along the southern margin of laurentia caused the rift depression to descend into cool oxygen poor phosphate rich oceanic waters that entered the southern reaches of the rift from the iapetus ocean the trough apparently formed in a system of epicontinental estuarine circulation marked by a density stratified water column trough formation was accompanied by cessation of carbonate sedimentation deposition of graptolitic shales development of hardground omission surfaces substrate erosion and local phosphogenesis the carbonate platforms on either side of the trough are dominated by bryozoan brachiopod echinoderm grainstones and packstones that were deposited in zones of mixing where cool nutrient rich waters encountered warmer shelf waters concurrently lime mudstone and wackestone were deposited shoreward northern illinois wisconsin iowa minnesota michigan in warmer more tropical shallow seas coeval upward growth of the flanking carbonate platforms sustained and enhanced development of the trough shale facies five widespread diachronous late mohawkian and cincinnatian omission surfaces are present in the carbonate facies of the midcontinent these surfaces include sub deicke k bentonite ds1 top of black river limestone ds2 base and top of the guttenberg limestone member of the decorah formation ds3 and ds4 and top of the trenton limestone ds5 some of the surfaces correspond to previously described depositional sequence boundaries all five surfaces which embody deepening phases on top of highstand systems tracts converge in the sebree trough indicating that the trough was a long lived feature and was the source of eutrophic waters that episodically spread across the adjacent platforms terminating carbonate production late turinian and early chatfieldian incipient drowning episodes were followed by a final drowning event that began in the sebree trough during the late chatfieldian climacograptus spiniferus zone and reached southernmost minnesota and other regions far within the platform interior by richmondian time amorphognathus ordovicicus zone

evolution of boudins under progressively decreasing pore pressure a case study of pegmatites enclosed in marble deforming at high grade metamorphic conditions naxos greece during ductile deformation of marbles under high grade metamorphic conditions on naxos greece pegmatites enclosed in the marbles were deformed in a brittle fashion forming blocky boudins with quartz crystallized in the interboudin zones we studied the three dimensional geometry of the boudins in the field and in two large samples the first sample was serially sectioned to observe the structural and microstructural evolution in the second sample we mapped the surface morphology of the deformed pegmatite in profile the boudins can be classified as symmetric torn type boudins evolving towards asymmetric boudins with a later domino boudin component in three dimensions however the morphology of the boudinaged pegma tite is a simple set of normal faults with mode i fractures in the fault tips and rotation of the fault blocks to accommodate the extension in the marble deformation history was constrained by petrology and microstructures in combi nation with simple order of magnitude calculations of both the cooling and pore fluid pressure evolution the dynamically recrystallized coarse grained calcite of the marble provides clear evidence that after the pegmatite intruded the marble and solidified it was deformed at peak conditions of m2b metamorphism 670 c and 0 6 gpa such pegmatitic melts contain 10 percent h2o which is released during crystalliza tion we infer that after crystallization of the pegmatite the pore fluid pressure in the pegmatite remained close to lithostatic due to the very low permeability of the surrounding marble and the pegmatite was deformed at very low effective stress which led to brittle deformation of feldspar and mode i fracturing of the pegmatite forming torn type boudins with time the pore fluid pressure slowly decreased increasing the effective stress ongoing n s extension thus resulted in slip along the quartz filled interboudin zones and in block rotation producing domino boudins

oxygen isotope thermometry of quartz al 2 sio 5 veins in high grade metamorphic rocks on naxos island greece diffusion models predict that peak metamorphic temperatures are best recorded by the oxygen isotope fractionation between minerals in a bi mineralic rock in which a refractory accessory mineral with slow oxygen diffusion rate is modally minor to a mineral with a faster diffusion rate this premise is demonstrated for high grade metamorphism on the island of naxos greece where quartz kyanite oxygen isotope thermometry from veins in high grade metamorphic pelites gives temperatures of 635 690 c these temperatures are in excellent agreement with independent thermometry for the regional m2 peak metamorphic conditions and show that the vein minerals isotopically equilibrated at the peak of metamorphism quartz sillimanite fractionations in the same veins give similar temperatures 680 35 c and suggest that the veins grew near to the kyanite sillimanite boundary corresponding to pressures of 6 5 to 7 5 kbar for temperatures of 635 685 c by contrast quartz kyanite and quartz biotite pairs in the host rocks yield lower temperature estimates than the veins 590 600 and 350 550 c respectively these lower apparent temperatures are also predicted from calculations of diffusional resetting in the polyphase host rock system the data demonstrate that bimineralic vein assemblages can be used as accurate thermometers in high temperature rocks whereas retrograde exchange remains a major problem in many polymineralic rocks

the stable isotopic evolution of a metamorphic complex naxos greece a detailed study of the stable isotope composition of pelites amphibolites and marbles from the m2 metamorphic complex on naxos is presented the low grade rocks of this metamorphic complex have isotopic compositions typical of metasedimentary rocks whilst those at high grades have values typical of igneous rocks the isotopic compositions of rocks at intermediate grades vary smoothly between these two end members this variation is shown to result from the superimposition of at least two fluid flow events on a pre existing isotopic gradient the contrast between the observed seemingly simple mixing trend and the complexity of the isotopic history of these rocks has important implications for quantitative studies of fluidrock interactions

evidence for pre regional metamorphic fluid infiltration of the lower calcsilicate unit reynolds range group central australia grandite garnet rich calcsilicate rocks from the lower calcsilicate unit of the regionally metamorphosed reynolds range group central australia crop out along a strike parallel section in which a transition zone from m22 amphibolite to granulite facies rocks is exposed across this transition the grandite rich layers do not show systematic changes in mineral assemblages compositions and modes or stable isotope compositions these layers are deformed by f22 folds that are associated with the peak of regional low pressure high temperature metamorphism therefore the grandite rich layers appear to pre date regional metamorphism and to have acted as closed chemical systems during prograde m22 metamorphism mineral assemblages in the grandite rich layers are consistent with their formation through the infiltration of oxidized water rich fluids xco2 0 1 0 3 log fo2 16 to 14 the stable isotope values of calcite 13c 4 2 to 0 8 0 pdb 18o 10 5 14 0 0 v smow and bulk silicate fractions 18o 6 1 to 10 8 of the grandite rich layers are most consistent with the infiltrating fluid being from a magmatic source it is most likely that fluid infiltration occurred during the pre m22 contact metamorphism m21 that affected much of the reynolds range group the preservation of these assemblages is probably due to their high variance and little pervasive fluid rock interaction having occurred during m22 the clinopyroxene and feldspar rich calcsilicate rocks that host the grandite rich layers contain poikiloblastic grandite garnet that formed during prograde m22 metamorphism thin marbles that locally occur with the grandite rich layers contain a third garnet generation that is post or late m22 this grossular rich garnet occurs in coronas around calcite plagioclase clinopyroxene wollastonite and scapolite these coronas are consistent with cooling and or compression however because the marble assemblages are themselves overprinted by m21 grandite rich layers the development of coronal garnet does not reflect a continuous p t t path rather it more probably reflects the partial re equilibration of m21 contact metamorphic assemblages to post m22 conditions

diffusional isotopic exchange across an interlayered marble schist sequence with an application to tinos cyclades greece marble schist contacts are frequently characterized by fine scale interlayering which prevents the application of semi infinite layer width assumptions to advection diffusion models of isotope profiles using a finite layer width boundary condition we show that the minimum width w of a layer required in order that diffusional exchange at both boundaries will not mutually interfere is given by the relation w k dt 1 2 where d is the effective diffusion coefficient t is the duration of the diffusion and k is a constant determined by the initial compositional step and the noise i e natural homogeneity and or analytical accuracy when this condition is not satisfied a solution of the diffusion equation for a multilayer transition zone containing several schist and marble layers of different finite widths is applicable application of this refinement of the diffusion model to an oxygen isotope profile from tinos cyclades greece gives 10 4 dt 10 2 m2 this dt range and a dt value of 10 1 m2 previously determined for sifnos indicates that rocks undergoing the oligocene miocene greenschist facies overprint of eocene high pressure metamorphic rocks in the cyclades were characterized by extremely low porosities permeability perpendicular to marble schist contacts was sufficiently low to prevent advection into the marbles correspondingly it is suggested that the layer parallel schistosity permitted infiltration of the fluids required for the hydration reactions of the metamorphic overprint

fluid evolution and transport during metamorphism evidence from the llano uplift texas a field petrologic and stable isotopic investigation of the marbles and calc silicates of the 1 15 b y valley spring gneiss documents the dilution of internally evolved co2 rich fluids by externally derived aqueous fluids introduced along channelways reaction textures within calcsilicates record the evolution through time of initially co2 rich fluids toward increasingly more aqueous compositions assemblage zonations within calc silicates require equilibration within local gradients of the mole fraction of co2 in the fluid and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies localized depletions in 13c and 18o corroborate petrologic evidence for channelized infiltration isotopic compositions reflect both devolatilization and the introduction of low 18o fluids estimated minimum oxygen equivalent fluid to rock ratios are near unity both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc silicate formation the calc silicate assemblages which equilibrated with fluids of the lowest mole fraction of co2 record isotopic exchange equilibrium with fluids of 18o typical of those derived from normal granites as do the granitic aplites and pegmatites which transect most calcsilicate occurrences thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism

isotopic and petrological evidence for the infiltration of water rich fluids during the miocene m2 metamorphism on naxos greece abstract a detailed carbon and oxygen isotopic study in conjunction with petrological work has been carried out across calcite dolomite marble layers in the high grade metamorphic sequence on the island of naxos greece the purpose of this study is to examine fluid flow during the miocene barrovian metamorphic event individual marble bands show two dominant styles of isotopic alteration from core values of 22 to 29 in 18o and 1 to 3 in 13c firstly contacts between the marble bands and surrounding pelitic rocks have altered isotopic ratios across a metre wide boundary layer the isotopic values of the marble along the contacts drop to 15 to 17 in 18o and 1 to 5 in 13c secondly there is a drop in the isotopic composition of the marbles along vein networks associated with the development of calc silicates the isotopic compositions in these veins drop to 14 to 16 in 18o and 3 to 4 in 13c cross cutting relationships observed between these two alterations allow the relative timing of infiltration to be determined the development of boundary layers is shown to result from the infiltration of fluids with isotopic signatures of 12 to 16 in 18o and 3 to 12 in 13c and an xco2 less than 0 3 these fluids are derived from dehydration of graphite bearing pelites during the prograde metamorphism these conclusions are at odds with those of previous workers who suggested that the prograde metamorphism was a result of the pervasive influx of mantle derived fluids with xco2 greater than 0 5 the alteration associated with vein networks results from peak to post peak infiltration of fluids which drive decarbonation reactions at least some of these fluids have an xco2 less than 0 05 but this is obscured by the production of co2 during the decarbonation reactions these fluids were most probably produced from the crystallising partial melts formed during the prograde metamorphism these infiltrating fluids are thought to have 18 o 10 and 13c of 5 to 7

textural and isotopic development of marble assemblages during the barrovian style m2 metamorphic event naxos greece a detailed petrological analysis of the marble assemblages observed within the m2 metamorphic complex on naxos is presented two distinct periods of mineral growth are documented the first is associated with prograde m2 metamorphism and the second with retrograde m2 metamorphism occurring during ductile extensional thinning of the complex the textural and miner alogical characteristics and the carbon and oxygen isotope compositions of each generation are described and the p t xco2 conditions at which these two mineral generations were stable and the compositions of the fluids present during metamorphism are characterised whereas the low variance and stable isotope compositions of prograde siliceous dolomite assemblages are consistent with internally buffered fluid evolution the retrograde mineral generation is shown to have grown as a result of the infiltration of a water rich fluid phase that transported silica al2o3 na2o and feo into the host rocks this observation together with the stable isotope compositions of the retrograde calcite and the fact that occurrences of veins of this type are limited to marbles in the highest grade areas t 600 c of the metamorphic complex suggests that the fluids responsible for vein formation were generated during the crystallisation of melts as the metamorphic complex cooled from peak temperatures the existence of this second generation of minerals has significant implications for previous studies of heat transport by fluid flow on naxos because many of the unusually low 18o compositions of pelites at high grades may be ascribable to the effects of interaction with retrograde m2 fluids rather than with prograde fluids

high integrated fluid rock ratios during metamorphism at naxos evidence from carbon isotopes of calcite in schists and fluid inclusions calcite in schists of the metamorphic complex at naxos is depleted both in 13c and in 18o with respect to massive marbles this effect is attributed to isotope exchange with circulating co2 rich fluids which had an x text co text 2 0 5 according to fluid inclusions the carbon isotopic composition of the calcites is close to equilibrium with fluid inclusion co2 at metamorphic temperatures mass balance calculations assuming initial 13c values of 0 for calcite and 5 for the fluid give integrated fluid rock volume ratios between 0 1 and 2 0 such high fluid rock ratios are supported by observations on the distribution of co2 h2o ratios of fluid inclusions carbon isotopic compositions of fluid inclusion co2 and oxygen isotope systematics of silicates

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

field measurement of high temperature bulk reaction rates i theory and technique knowledge of metamorphic reaction rates is crucial to accurately interpret rock and mineral chemistry the local equilibrium assumption used in geochronology geothermobarometry and material flux estimates requires that local reaction rates among system phases are fast relative to local rates of p t x change natural metamorphic reaction rates are essentially unknown and difficulties and disagreements exist regarding extrapolations of existing laboratory data to natural conditions growing recognition of natural effects that could be attributed to slower reaction rates justifies the need for a field based quantification of reaction rates to assess the accuracy of lab based predictions we describe in detail the theory and methodology of a technique for extracting bulk reaction rates directly from isotopic data derived from natural samples baxter and depaolo 2000 reaction rates measured using this technique may be judiciously applied to isotopic exchange or net reaction kinetics in other natural systems the technique requires collection of whole rock and garnet 87sr 86sr data along a sampling traverse normal to a lithologic contact where there was prior to metamorphism a sharp isotopic discontinuity garnet data provide information on syn metamorphic conditions and the time interval for the exchange process forward modeling of the reactive transport process using numerical methods and the equations for diffusive reactive transport allow determination of the reaction rate and bulk sr diffusivity that provides the best fit to the data measurement of reaction rates is best constrained if an isotopic step is preserved at the contact the size of which is directly proportional to the bulk reaction rate this contribution is intended to serve as a template for future use and development of this technique to acquire natural reaction rate data from metamorphic systems

the late alpine evolution of an extensional shear zone naxos greece the mid miocene structural evolution of the metamorphic complex on naxos involved deformation in a kilometre scale low angle ductile shear zone the structures described are consistent with exhumation of the metamorphic complex during crustal extension after and possibly during localized high temperature barrovian style metamorphism non coaxial fabrics developed throughout the structural history indicate that upper levels were transported to the north during ductile extension this shear sense is opposite that inferred previously for the development cordilleran style metamophic core complexes in the cvclades ductile deformation was accompanied during its later stages by e w directed compression

the role of the grain boundary at chemical and isotopic fronts in marble during contact metamorphism carbon and oxygen isotopic profiles around a low pressure metasomatic wollastonite reaction front in a marble of the hida metamorphic terrain central japan display typical metamorphic fluid enhanced isotopic zonations isotopic profiles obtained from detailed microscale analyses perpendicular to the chemical reaction front in calcite marble show that diffusion enhanced isotopic exchange may control these profiles carbon and oxygen isotopic behaviour in grain boundaries is remarkably different oxygen isotopic troughs 18o depleted rims around the calcite grain boundaries are widely observed in this contact aureole demonstrating that diffusion of oxygen in calcite grain boundary dominates over lattice diffusion in calcite in contrast no difference is observed in carbon isotopic profiles obtained from grain cores and rims there is thus no specific role of the grain boundary for diffusion of carbonic species in the metamorphic fluid during transportation carbon chemical species such as co2 and co3 ions in metamorphic fluid migrate mainly through lattice diffusion the carbon and oxygen isotope profiles may be modelled by diffusion into a semi infinite medium empirically lattice diffusion of oxygen isotopes is almost six times faster than that of carbon isotopes and oxygen grain boundary diffusion is ten times faster than oxygen lattice diffusion oxygen isotopic results around the wollastonite vein indicate that migration of the metamorphic fluid into calcite marble was small and was parallel to the aquifer from the stability of wollastonite and the attainment of oxygen isotopic equilibrium we suggest that diffusion of oxygen occurred through an aqueous fluid phase the timescale of formation of the oxygen isotopic profile around the wollastonite vein is calculated to be about 0 76 106 years using the experimentally determined diffusion constant

correlation of middle ordovician k bentonites based on chemical fingerprinting six separate k bentonite beds in middle ordovician rocks of southwestern ohio and northern kentucky have been analyzed for three major and 22 trace elements and the data used as a basis for stratigraphie correlation samples from eight cores distributed over a distance of 140 km were analyzed for dy eu mn na k sm la as ga lu sb fe yb cr tb se rb co ce ta th hf cs zn and zr by instrumental neutron activation analysis and x ray fluorescence and the results subjected to stepwise discriminant analysis within layer and between layer variations in composition were assessed and a ranking of elements established on the basis of their ability to characterize individual layers eighteen elements are sufficient to establish 100 classification in a group matrix of the six beds with the principle assignment contributions coming from th ta rb cr fe ga yb and k to test the stratigraphie usefulness of chemical fingerprinting over more extended distances seven samples of k bentonites e

buoyant decompression melting a possible mechanism for intraplate volcanism 1 volcanic activity away from plate boundaries occurs in a variety of settings linear age progressive volcanic chains have been explained as the manifestation of fixed hot spots possibly generated by plumes originating deep in the mantle however important examples of oceanic intraplate volcanism cannot be explained in this way including short lived chains and chains violating the predicted age distance behavior furthermore a significant fraction of volcanism does not occur in linear chains and observations suggest control by lithosphere structure in many cases in this study melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism numerical models of this buoyant decompression melting in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small random perturbation for a range of model parameters a predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process buoyant melting could occur in a number of geologic settings since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled spontaneous buoyant decompression melting may be possible only in regions where a large scale mantle upwelling can counteract conductive cooling keeping the mantle at its solidus temperature over some depth range where the mantle is at or sufficiently near its solidus buoyant decompression may spontaneously generate convective storms of melting this may explain the abundance of volcanism associated with the south pacific superswell where mantle is slightly cooler than its melting temperature buoyant melting may not occur spontaneously but may be triggered by some initial upwelling this may provide a physical explanation for volcanism that is controlled by lithosphere structure

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

the correlation of reaction and isotope fronts and the mechanism of metamorphic fluid flow infiltration of a metabasite sill from islay scotland by an h2o co2 fluid caused 1 modification of delta 18 o and 2 carbonation at the sill margins maps of delta 18 o and reaction progress were constructed from a 20 x 47 7 metre sample grid across

sublithospheric small scale convection and its implications for the residual topography at old ocean basins and the plate model 1 the seafloor topography and heat flux differ significantly from the predictions of the half space cooling hsc model at old ocean basins understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large scale mantle dynamics a widely used model that explains significant fraction of the deviations is the plate model but the dynamical feasibility of the plate model has never been demonstrated in this study we investigated the effects of sublithospheric small scale convection ssc and of internal heating on seafloor heat flux and topography and mantle thermal structure and we examined the dynamic feasibility of the plate model by formulating high resolution two dimensional numerical models of mantle convection with strongly temperature and depth dependent rheology we found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere especially when significant internal heating is present this trapped heat may exist in the middle mantle when the ssc is absent and it may also be redistributed by the ssc to shallow depths to reheat the lithosphere and to homogenize mantle temperature when internal heating accounts for 60 of the total heat output the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via ssc while maintaining uniform mantle temperature we suggest that the trapped heat and the ssc are responsible for the residual heat flux and topography at old ocean basins relative to the hsc model predictions our models also show that for the plate model to be dynamically viable both the ssc and significant internal heating 60 are necessary this is because only the ssc in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere which is the basic assumption of the plate model with the viscosity structure and internal heating rate for the present day mantle we think that the plate model is dynamically viable

carbon isotope constraints on volatile mixing and melt transport in granulite facies migmatites abstract migmatites at brattstrand bluffs east antarctica comprise granulite facies mineral assemblages produced by high temperature metamorphism of pelitic sediments these rocks preserve an advanced stage of partial melting with recrystallized melt accounting for 25 of outcrop area many of the melt veins contain cordierite which traps volatile molecules within open channels in its mineral framework and can be used to monitor the syn metamorphic volatile contents of the melts we have analysed the isotopic composition of cordierite channel co2 by step heating mass spectrometry channel co2 in cordierite from brattstrand bluffs has not been significantly affected by retrograde isotope exchange as it preserves high temperature carbon fractionations with coexisting graphite cordierite samples from the same locality have similar isotope compositions 0 6 reflecting efficient mixing of partial melt and dissolved volatiles on a metre scale however samples from different localities have distinct 13c values 10 to 20 and indicate that there was little mixing of melt or volatiles on a scale of 100 m despite the high degree of partial melting the 10 variation between localities reflects mixing of variable proportions of isotopically light biogenic carbon with carbon from a heavier reservoir although this mixing can be interpreted in terms of carbon influx from an external source co2 was not a driving force for granulite metamorphism in this case as granulite assemblages occur at all localities irrespective of isotope systematics co2 played a complex but passive role during metamorphism and melting at brattstrand bluffs where the dissolution of h2o into partial melt bodies was the dominant dehydration mechanism

shallow level metamorphic fluids in a high uplift rat metamorphic belt alpine schist new zealand fluids some of which are co2 rich up to 40 mol co2 and some of which are highly saline up to 18 wt nacl equivalent are trapped as fluid inclusions in quartz calcite metallic minerals veins which cross cut the pumpellyite actinolite to amphibolite facies rocks of the alpine schist fluids were commonly trapped as immiscible liquid vapour mixes in quartz and calcite showing open space growth textures fluid entrapment occurred at fluid pressures near 500 bars possibly as low as 150 bars at temperatures ranging from 260 to 330 c saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low density co2 rich fluid calcite deposited by these fluids has 13c ranging from 8 4 to 11 5 and 18o from 4 to 13 isotopic data fluid compositions and mode of occurrence suggest that the fluids are derived from high grade metamorphic rocks fluid interaction with wall rock has caused biotite crystallization and or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks fluid penetration through the rock is almost pervasive in many areas where permeability probably related to alpine fault activity has focussed fluids on a regional scale into fractured rocks the fluid flow process is made possible by high uplift rates in excess of 10 mm year bringing hot rocks near to the surface

87sr 86sr chemostratigraphy of neoproterozoic dalradian limestones of scotland and ireland constraints on depositional ages and time scales new calcite 87 sr 86 sr data for 47 limestones from the metamorphosed and deformed neoproterozoic cambrian dalradian supergroup of scotland and ireland are used to identify secular trends in seawater 87 sr 86 sr through the dalradian succession and to constrain its depositional age dalradian limestones commonly have sr 1000 ppm indicating primary aragonite and marine diagenesis low mn mn sr 18 o and trace element data indicate that many 87 sr 86 sr ratios are unaltered since diagenesis despite greenschist to amphibolite facies metamorphism consistent with the documented behaviour of sr and o during metamorphic fluid rock interaction thus the 87 sr 86 sr data are interpreted largely to reflect 87 sr 86 sr of coeval seawater currently available data show that neoproterozoic seawater 87 sr 86 sr rose from c 0 7052 at 850 900 ma to c 0 7085 or higher in the latest neoproterozoic temporal changes at c 800 ma and c 600 ma bracket the range in 87 sr 86 sr values of calcite in grampian appin and lowest argyll group c 0 7064 0 7072 and middle and uppermost argyll group c 0 7082 0 7095 limestones consistent with a rise in seawater 87 sr 86 sr around 600 ma 87 sr 86 sr data are consistent with the sedimentary affinity of the islay subgroup with the underlying appin group and with a possible time interval between deposition of islay and easdale subgroup rocks they indicate that the dalradian as a whole is younger than c 800 ma

influence of rheology on realignment of mantle convective structure with plate motion after a plate reorganization 1 small scale convection ssc rolls below the oceanic lithosphere have the tendency to align with the large scale shearing direction and thus with the plate motion direction relative to the deep mantle understanding the timescales of and processes responsible for realignment would contribute significantly to our understanding of the unresolved phenomena in the pacific such as gravity lineations small scale seismic velocity variations and intraplate volcanism that cannot be explained by hot spots in this study we examine the evolution of those convection rolls when this relative plate motion direction is suddenly changed as suggested by the kink in the hawaii emperor seamount chain using three dimensional numerical flow models we investigate the realignment of ssc rolls after a change in plate motion direction from the nature of the ssc it is expected that rheological parameters dominate the characteristics of this realignment our results show that this is indeed the case we find that 1 using constraints from onset timing of ssc realignment of rolls can occur as fast as within 20 ma but might also take much longer dependent on the rheology 2 the realignment period is strongly correlated to the sum of large scale shear stress induced by plate motion and small scale shear stress from the ssc itself 3 in a mantle deforming by dislocation creep realignment occurs faster than by diffusion creep because dislocation creep ssc is more vigorous and 4 activation energy has little influence on the realignment time possible evidence for the realignment period might come from precise age determination of intraplate volcanism or azimuthal seismic anisotropy

the nature and distribution of fluids during amphibolite facies metamorphism naxos greece on the basis of fluid inclusion evidence pervasive influx of deep seated co2 rich fluids has been invoked to account for mid to upper amphibolite facies m2b metamorphism on the island of naxos cyclades greece in this paper mineral devolatilization and melt equilibria are used to constrain the composition of both syn and post peak m2b fluids in the deepest exposed levels of the metamorphic complex the results indicate that peak m2b fluids were spatially and compositionally heterogeneous throughout the high grade core of the complex whereas post peak m2b fluids were generally water rich the observed heterogeneities in syn m2b fluid composition are inconsistent with pervasive co2 flushing models invoked by previous workers on the basis of fluid inclusion evidence it is likely that few co2 rich fluid inclusions on naxos preserve fluids trapped under peak metamorphic conditions it is suggested that many of these inclusions have behaved as chemically open systems during the intense deformation that accompanied the uplift of the metamorphic complex a similar process may explain the occurrence of some co2 rich fluid inclusions in granulite facies rocks

are stable isotopic trends in amphibolite to granulite facies transitions metamorphic or diagenetic an answer for the arendal area bamble sector southeastern norway from mid proterozoic carbon bearing rocks the isotopic 3 i3 c and 3 18 0 compositions of primary marbles and secondary veins calcites and primary fahlbands and metapelites and secondary veins graphites sampled across the am phibolite to granulite facies transition in the bamble sector southeast ern norway have been investigated the primary calcites display a regional trend in a 3 i3c 3i80 plot which is the reverse of the expected trend the granulite grade primary calcites essentially preserved most of their initial sedimentary compositions whereas the amphibolite grade calcites have increasingly lower 3 i3 c values the 3 18 0 composi tions of amphibolite grade primary calcites and of some vein tlpes appear to be buffered by the o reservoir of the country rock the 3 1 c of graphite of different types is specific for a certain lithology but not spatially related to the metamorphic zoning reaction progress calculations and rayleigh fractionation model ling prove that prograde metamorphic decarbonation reactions alone cannot be held responsible for the isotopic trend in the marbles the reversed isotopic trend in the marbles themselves combined with the lack of any trend in the primary graphites is not compatible with pervasive flushing with a homogeneous mantle derived co2 rich fluid at granulite facies metamorphism the marble trend and the constant 3 i3 c composition of j rimary graphite throughout the metamorphic zoning are best explamed by low grade processes that did affect the stable isotopic compositions of the marbles but left the graphites unaffected the processes may even occur under diagenetic conditions later amphibolite granulite facies metamorphism preserved the trend despite the production of calcsilicates subsequent retrograde hydro thermal action produced graphite quartz veins and several types of calcite veins without resetting stable isotope systematics on a regional scale

relations among net transfer reaction progress 18 o 13 c depletion and fluid infiltration in a clinohumite bearing marble a siliceous dolomitic marble xenolith within a mid crustal jurassic diorite exhibits mineralogical and stable isotopic evidence for infiltration of water rich fluid adjacent to endoskarn which bounds the xenolith forsterite has been replaced by clinohumite as a result of a devolatilization reaction driven by addition of aqueous fluid and extraction of heat isotopic compositions of calcite also record infiltration of aqueous fluid concentrated near contacts with endoskarn marble calcite 18osmow values range from 19 5 at the center of the xenolith to 12 6 adjacent to endoskarn calcite 13cpdb decreases sympathetically from 3 0 to 1 4 the calculated equilibrium composition of coexisting c o h f fluid and a quantitative characterization of the whole rock reaction which produced clinohumite and consumed graphite are used to place inequality contraints on the composition of the infiltrated fluid continnum mechanical transport models based on the resulting fluid compositions suggest that a small time integrated darey flux of 44 cm3 cm2 coupled with diffusive transport in the fluid was sufficient to produce both the isotopic shifts and the net transfer reactions evidenced in the xenolith the calculations demonstrate the importance of graphite as an indicator of time integrated flux the maximum possible ch4 content of the infiltrated fluid is sufficiently high to impart a 2 0 uncertainty in the 13c of the fluid the isotopic composition of the fluid is consistent with a magmatic origin when this uncertainty is taken into account

new observations on mid plate volcanism and the tectonic history of the pacific plate tahiti to easter microplate abstract we describe the geology and tectonics of a continuous swathe of seafloor between tahiti and the western edge of the easter microplate imaged by gloria and sea beam on two separate cruise transits in 1987 and 1988 the data reveal that mid plate volcanism is common in this region even on deep seafloor hundreds of kilometres from major lines of seamounts and islands this supports the idea of a thin weak lithosphere over the pacific superswell and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km the mid plate volcanism occurs in two distinct forms over most of our traverse it appears as fields of relatively young and acoustically strongly backscattering lava flows often accompanied by groups of numerous small circular volcanoes east of 122 w about chron 5a however we observed a distinct form major sharp crested constructional volcanic ridges many tens of kilometres long individually trending ene but lying en echelon along an e w regional trend these ridges appear morphologically identical to the cross grain ridges seen elsewhere in the pacific we attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness however although these ridges are parallel to fracture zone trends seen farther west they are morphologically very different from any known fracture zone moreover individual ridges are somewhat oblique to the tectonic spreading fabric around them and so do not seem to follow actual fracture zone traces the whole line of en echelon ridges lies along part of the predicted trace of fracture zone 2 of okal and cazenave 15 and is probably its morphological expression however nowhere did we see a convincing conventional fracture zone trace in or following the predicted position or orientation we suggest instead that magma from an independent source has used lines of weakness along minor fracture zones to produce these en echelon features the austral fracture zone is the only major fracture zone crossed in our transit and here is characterised by four fossil transform strands its marked position on the aapg and gebco maps is found to be in error finally we found that the expected change from nnw to nne trending spreading fabric at chron 6c did not occur in a clear cut way as predicted by earlier tectonic histories of the pacific instead the post chron 6c fabric oscillates in a confused way between nne and nnw suggesting to us that this area has been characterised by an unstable plate boundary probably associated with a succession of propagating rifts or microplates from chron 6c to the present

on the causes of 18 o depletion and 18 o 16 o homogenization during regional metamorphism the east humboldt range core complex nevada previous stable isotope studies at lizzies basin revealed that metasedimentary rocks are 18o depleted relative to protolith values particularly in the lower parts of the section lower zone where the rocks are also isotopically homogeneous on a scale of hundreds of meters quartz 18o 9 0 to 9 6 per mil in contrast metasedimentary rocks at higher levels at lizzies basin upper zone are less 18o depleted and more heterogeneous in 18o in order to understand more fully the isotopic evolution of this terrane a series of detailed meter scale traverses across various metamorphic and igneous lithologies were completed at lizzies basin and at the structurally higher angel lake locality traverses in the lizzies basin lower zone and in the lower parts of angel lake angel lake lower sequence across various silicate lithologies including abundant granitoids reveal similar degrees of homogeneity although the average 18o values are higher at angel lake in contrast traverses which include substantial thicknesses of marble and calc silicate gneiss and very little granitoid have more heterogeneous quartz 18o values 11 9 to 13 4 per mil and also have a higher average 18o 12 9 per mil than observed elsewhere the scale of 18o 16o homogeneity in quartz observed at lizzies basin and angel lake meters to hundreds of meters requires fluid mediated isotope exchange which accompanied tertiary metamorphism there is a correlation between the degree of 18o depletion in metasedimentary rocks 18o 16o homogenization between lithologies and the proportion of granitoids leucogranites in particular within any part of the section and a corresponding anticor relation with the proportion of marble this points to a causal relationship whereby the leucogranites as well as the tertiary hornblende diorite and biotite monzogranite acted as both a relatively low 18o reservoir and a source of fluids to enhance exchange while the marbles hindered isotope deplction and homogenization by acting as relatively high 18o reservoirs and impermeable layers material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments single pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations fluidmediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations the generalized model for the isotopic evolution of the east humboldt range core complex provides and excellent opportunity to establish the main causes and controlling factors of 18o depletion and 18o 16o homogenization during regional metamorphism

the lavrion pb zn fe cu ag detachment related district attica greece structural control on hydrothermal flow and element transfer deposition abstract the impact of lithological heterogeneities on deformation fluid flow and ore deposition is discussed based on the example of the lavrion low angle detachment partly accommodating gravitational collapse of the hellenides orogenic belt in greece the lavrion peninsula is characterised by a multiphase pb zn fe cu ag ore system with a probable pre concentration before subduction followed by progressive remobilisation and deposition coeval with the development of a low angle ductile to brittle shear zone the mylonitic marble below the detachment shear zone is composed of white layers of pure marble alternating with blue layers containing impurities sio2 al2o3 carbonaceous material ductile mylonitic deformation is more pervasive in the less competent impure blue marble we propose that localised deformation in the impure marble is associated with fluid circulation and dolomitisation which in turn causes an increase in competence of these layers mineralised cataclastic zones crosscutting the mylonitic fabric are preferentially localised in the more competent dolomitic layers oxygen and carbon isotopic signatures of marble invaded by carbonate replacement deposits during ductile to ductile brittle deformation are consistent with decarbonation coeval with the invasion of magmatic fluids mineralised cataclastic zones reflecting brittle deformation evolve from low 13c to low 18o signatures interpreted as local interaction with carbonaceous material that trends toward the contribution of a surface derived fluid these features indicate that the lavrion area records a complex deposition history influenced by the evolution of fluid reservoirs induced by the thermal and mechanical evolution of the marble nappe stack ore remobilisation and deposition associated with the activity of the low angle detachment is i firstly related to the intrusion of the plaka granodiorite leading to porphyry type and carbonate replacement mineralisation during ductile brittle deformation and ii then marked by progressive penetration of surface derived fluids guided by strain localisation in the more competent levels leading to epithermal mineralisation associated with brittle deformation

crystal chemistry of a mg vesuvianite and implications of phase equilibria in the system cao mgo al2o3 sio2 h2o co2 chemical analysis including h f feo fe o of a mg vesuvianite from george town calif usa yields a formula r 9 mgl bbfe 0 a 10 7si1 81 06 oh 8 4 0 4 in good agreement on a cation basis with the analysis reported by pabst 1936 x ray and electron diffraction reveal sharp reflections violat ing the space group p4 nnc as consistent with domains having space groups p4 n and p4nc refinement of the average crystal structure in space group p4 nnc is consistent with occupancy of the a site with ai of the half occupied b site by 0 8 mg and 0 2 fe of the half occupied c site by ca of the ca i 2 3 sites by ca and the oh and o 10 sites by oh and 0 we infer an idealized formula for mg vesuvianite to be cai9mg mga1 ai4si 8069 oh 9 which is related to fe3 vesuvianite by the substi tutions mg oh fe3 0 in the b and o 10 sites and fe a1 in the alfe site thermodynamic calculations using this formula for mg vesuvianite are consistent with the phase equilibria of hochella liou keskinen kim 1982 but inconsistent with those of olesch 1978 further work is needed in determining the composition and entropy of synthetic us natural vesuvianite before quantitative phase equilibria can be dependably generated a qualitative analysis of reactions in the system cao mgo

the clumped isotope geochemistry of exhumed marbles from naxos greece exhumation and accompanying retrograde metamorphism alter the compositions and textures of metamorphic rocks through deformation mineral mineral reactions water rock reactions and diffusion controlled intra and inter mineral atomic mobility here we demonstrate that these processes are recorded in the clumped and single isotope 13 c and 18 o compositions of marbles which can be used to constrain retrograde metamorphic histories we collected 27 calcite and dolomite marbles along a transect from the rim to the center of the metamorphic core complex of naxos greece and analyzed their carbonate single and clumped isotope compositions the majority of 47 values of whole rock samples are consistent with exhumation controlled cooling of the metamorphic complex however the data also reveal that water rock interaction deformation driven recrystallization and thermal shock associated with hydrothermal alteration may considerably impact the overall distribution of 47 values we analyzed specific carbonate fabrics influenced by deformation and fluid rock reaction to study how these processes register in the carbonate clumped isotope system 47 values of domains drilled from a calcite marble show a bimodal distribution low 47 values correspond to an apparent temperature of 260 c and are common in static fabrics high 47 values correspond to an apparent temperature of 200 c and are common in dynamically recrystallized fabrics we suggest that the low 47 values reflect diffusion controlled isotopic reordering during cooling whereas high 47 values reflect isotopic reordering driven by dynamic recrystallization we further studied the mechanism by which dynamic recrystallization may alter 47 values by controlled heating experiments results show no significant difference between laboratory reactions rates in the static and dynamic fabrics consistent with a mineral extrinsic mechanism in which slip along crystal planes was associated with atomic scale isotopic reordering in the calcite lattice an intrinsic mechanism enhanced isotopic reordering rate in deformed minerals is contraindicated by these experiments we suggest that 47 values of dynamically recrystallized fabrics that form below the diffusion controlled blocking temperature for calcite constrain the temperature of deformation we find that 47 based temperatures of static fabrics from naxos marbles are 60 80 c higher than commonly observed in slowly cooled metamorphic rocks and would suggest cooling rates of 10 5 c myr 1 a similar thermal history is inferred for dolomite marbles from the core vicinity which preserve apparent temperatures up to 200 c higher than a typical blocking temperature 300 c this finding could be explained by a hydrothermal event driving a brief thermal pulse and locally resetting 47 values rapid cooling of the core complex region is consistent with a compilation of published cooling ages and a new apatite u th he age associating the thermal event with the emplacement of a granodiorite pluton at 12 ma

alpine deformation on naxos greece a detailed structural study of naxos attic cycladic massif greece reveals two major deformation events the first one is associated with large scale thrusting and high pressure low temperature metamorphism during an early alpine subduction episode the second event occurred during continental extension and the associated development of localized thermal domes where lower crustal rocks were brought into contact with upper crustal units along a major shallow dipping shear zone we agree with a model of naxos as a cordilleran type metamorphic core complex however our observations show that the sense of shear was upper plate moving north during the second event calling for a reinterpretation of existing tectonic models of the cyclades

migration of reaction and isotopic fronts in infiltration zones assessments of fluid flux in metamorphic terrains abstract criteria are presented for recognising fluid infiltration driven reactions in metamorphic rocks and quantifying the associated fractionations of oxygen and carbon isotopic compositions in medium or high grade metamorphic rocks with local equilibrium between fluid and solid phases reactions or isotopic changes driven by infiltrating fluid are expected to migrate as relatively sharp fronts with limited diffusive broadening fluid leaving such reaction fronts is compositionally buffered by the reacting phases in suitably reactive rocks a series of reaction fronts will develop each driven by infiltration of fluid buffered by the upstream reaction knowledge of the spatial arrangement of such fronts allows calculation of the time integrated fluid fluxes total fluid volume per unit area distinctive stable isotope fractionations are predicted adjacent to infiltration driven reactions with the distribution and magnitude of the fractionation effects dependent upon the relative velocities of the reaction and isotopic fronts the various possibilities are illustrated by reference to a hypothetical infiltration driven decarbonation reaction the calculated isotopic fractionations do not in general resemble those predicted by the open or closed system water rock ratio calculations

linear ridge groups evidence for tensional cracking in the pacific plate a new class of oceanic bathymetric feature discovered in 1987 consists of en echelon groups of linear ridges there are now at least three such sets of ridges known and their remarkable similarity supports the notion that they result from a widespread tectonic process taking place in the interior of plates in two of these major linear ridge groups the crossgrain and puka puka ridges numerous morphologic features indicate that they originated as tension cracks the form of the individual volcanic structures that make up the ridges correlates with the degree of tension that formed them and the similar orientation of all ridges in a group indicates that the stress that formed them is relatively widespread geographically and temporally the ridges show a characteristic sequence of development beginning with a swath of small volcanoes followed by larger domical volcanoes where the ridge will eventually develop this paper examines proposed mechanisms of formation and concludes that no single source of tension in the lithosphere could plausibly have caused both the crossgrain and puka puka ridges the similarity of the orientations of all the groups of en echelon linear ridges on the pacific plate suggests that the individual ridges form normal to a least compressive stress direction that is geographically variable in the crust parallel ridges constrained to a narrow band then result in an en echelon arrangement though some closely spaced synchronously formed sets of two or three ridge segments may have influenced each other s form

thermal evolution of the oceanic lithosphere an alternative view abstract the most common model used for representing the evolution with age of the oceanic lithosphere is the plate model where the temperature is set at a fixed depth called the base of the plate this base of the plate has no physical meaning but this model provides a mathematical substitute for a system where small scale convection occurs through instabilities growing at the base of the cooling lithosphere and becomes effective only below old ocean another possible view is that convection provides heat at the base of the lithosphere whatever the age of the overlying plate this last process can be modeled by a constant heat flow applied on the bottom lithospheric isotherm chablis model a good fit to the observables bathymetry and geoid as function of age and old age heat flow can be obtained both for plate and chablis models in spite of an experimentally determined thermal expansion coefficient much larger than assumed in previous plate models these models have important consequences for several geodynamic processes the plate at an age of 100 ma is only 80 km thick for both models melting above a hot spot can then occur in the garnet spinel transition field without much plate thinning in the plate model the subsidence is stopped at an age of about 80 ma while according to the chablis model several hundred meters of subsidence are expected after 100 ma thus the two models predict quite a different long term pattern of subsidence in the sedimentary basins finally in the chablis model the global cooling of the mantle coming from cold material eroded by secondary convection at the base of the plates is considerably larger than in plate models it amounts to 40 the remaining 60 being due to the subduction process

fluid rock interaction and thermal evolution during thrusting of an alpine metamorphic complex tinos island greece this study examines the fluid rock interaction and thermal evolution along a thrust that juxtaposes calcite rich marbles of high p t metamorphic unit of the attic cycladic massif greece on top of a lower grade dolomite marble unit the tertiary thrust represents a major phase of tectonic movement related to the decompression of the alpine orogen in the hellenides the stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions the depletion is most pronounced in calcite but is almost entirely missing in coexisting dolomite the isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water rich h2o co2 ch4 fluid x c x co 2 x ch 4 0 05 at water rock ratios on the order of 0 1 to 0 5 by weight the fluid induced calcite recrystallization is viewed as an important rheological control during thrusting the temperature evolution of the footwall hanging wall and mylonitic tectonic contact was determined by calcite dolomite solvus thermometry histograms of calcite dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate conversely the hanging wall marble unit was cooled during the thrusting the calcite dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting

organic geochemistry and oil source correlations paleozoic of ohio routine analytical methods and new kerogen kerogen pyrolyzate techniques used for detailed petroleum geochemical studies permit the correlation of oils to their respective source rocks in ohio in the paleozoic section four good to excellent source units were identified the mississippian sunbury shale the devonian ohio shale and olentangy shale and the shales of the ordovician point pleasant formation these marine shales are dominantly oil prone with maturation levels that span the immature to peak oil generation range no source rocks examined exceeded an ro of 1 0 1 1 or equivalent the reservoired oils in ohio are mostly supermature based on normal alkanes pristane n c17 to phytane n c18 ratios low asphaltene and sulfur contents and high paraffin indices three basic oil groups were identified using oil characterization procedures group 1 consists of cambrian ordovician and some silurian oils group 2 consists of some silurian devonian one mississippian and some pennsylvanian oils and group 3 consists of oil from the mississippian berea sandstone oil source rock correlation techniques permitted the assignment of probable source rocks to each oil group group 1 oil was probably derived from the point pleasant shales group 2 oil was derived from the ohio shale and olentangy shale and group 3 oil came from the sunbury shale because the supermature oils in ohio occur in reservoirs with maturities generally under 0 8 ro the oils would have had to migrate from deep within the appalachian basin migration could have followed fracture zones unconformities or extensive and permeable units such as the silurian clinton sands

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

the formation of metasomatic reaction veins in dolomitic marble roof pendants in the bergell intrusion province sondrio northern italy at its eastern margin the berge11 intrusion contains large numbers of well exposed dolomitic marble roof pendants and inclusions symmetrically zoned reaction veins occurring in these dolomites are related to a regional fracture system which was possibly formed during the cooling and uplift of the intrusion the vein thicknesses range from a few millimeters to a maximum of 0 6 m mineral assemblages found in individual zones of the veins include calcite olivine calcite i diopside calcite amphibole calcite antigorite calcite talc biotite may be present in all zones the zone sequences found in different veins show a large variation the most abundant vein type consists of a single but internally banded amphibole calcite zone the internal zonation is caused by discontinuous changes in the amphibole composition ironand aluminium rich amphiboles are found close to the vein center whereas pure tremolite occurs at the metasomatic front these discontinuous composition profiles across amphibolite calcite zones lead to the conclusion that the transport process for many of the veins was infiltration metasomatism comparing modal abundances of vein minerals with model vein rocks it is suggested that mgo was conserved during vein formation and the veins were formed essentially by si0 metasornatism the composition of the vein minerals suggest however that iron and aluminium were transported as well from phase relations among the different zone assemblages it may be concluded that vein formation occurred over a pressure temperature interval cooling uplift interval but that the formation of an individual vein was an isothermal isobaric process occurring in the presence of gradients in a s ioaq and small gradients in x002 in the vein forming fluid geometric relations suggest that the veins may have grown by both deposition of material along an opening central fracture and reaction of a si0 2 rich aqueous fluid with the adjacent dolomite

oxygen and carbon isotopic constraints on the development of eclogites holsnoy norway abstract eclogite formation on the island of holsnoy required the addition of water to anhydrous granulite facies protoliths in order to assess this process oxygen and carbon isotope ratios of whole rock powders and mineral separates from eclogites and granulites have been measured whole rock oxygen isotope ratios range from 7 3 to 6 0 smow in granulites average 6 38 and 7 2 to 6 1 in eclogites average 6 55 field relations permit identification of the granulite protolith of eclogites oxygen isotope measurements show shifts of up to 0 5 between some eclogites compared to their corresponding granulite protoliths indicating open system and locally heterogeneous fluid behavior mineral pair fractionations in the eclogites show disequilibrium are incompatible with slow cooling and diffusive exchange between phases and suggest that open system fluid movement continued after eclogite facies metamorphism carbonate is also present in some of the eclogites as a primary mineral dolomite and as part of a retrograde assemblage calcite textural evidence suggests that carbonate formation occurred during and after eclogite formation however all measured carbonate is out of isotopic equilibrium with eclogite facies minerals due to the influx of retrograde fluids massive calcite marble pods containing amphibolite facies cale silicate minerals have average 18 o of 9 5 0 6 while calcite in retrograded eclogites has 18 o 17 7 2 7 the 13 c 4 0 8 is indistinguishable between these two groups both whole rock and carbonate stable isotope data are interpreted as indicating a continued history of fluid infiltration during and after peak eclogite facies metamorphism the most probable source of fluids are from dewatered sediments tectonically juxtaposed during the caledonian orogeny

equilibrium dihedral angles in the system h2o co2 nacl calcite and implications for fluid flow during metamorphism fluid calcite calcite dihedral angles have been measured for fluids in the system h2o co2 nacl between 1 and 2 kbar and 550 750 c it is found that the calcite calcite h2o dihedral angle decreases steadily with addition of nacl from a value of about 80 pure water to 44 60 wt nacl the co2 h2o system displays a well defined minimum at x co 2 0 5 with a dihedral angle of 50 in contrast to those of pure co2 and h2o which are 90 and 80 respectively experiments containing fluids which are immiscible at run conditions showed a bimodal distribution of dihedral angles in the co2 h2o nacl system which can be approximately correlated with the compositions of the two fluid phases such bimodality was only observed for immiscible fluids in the h2o nacl system if the quench rate exceeded about 200 c per min this is probably due to the extremely rapid establishment of the single phase dihedral angle on quenching the fluid phase topology in devolatilising marbles will only be a connected network for very saline brines and fluids with x co 2 close to 0 5 fluids trapped in fluid inclusions in calcite grains in marbles may be predominantly h2o rich or co2 rich and of low salinity all other fluid compositions in the h2o co2 nacl calcite system will occupy isolated pores the largest of which will grow at the expense of the smallest escape of fluid produced during devolatilisation reactions under such conditions will occur by fluid overpressuring and hydrofracture in contrast previous experimental studies of quartz fluid dihedral angles between 950 and 1100 c watson and brenan 1987 predict that quartz dominated lithologies will permit pervasive flow of h2o nacl fluids but not of h2o co2 fluids documented geological examples of differences in permeability and fluid flow mechanism between metamorphic argillites psammites and limestones which support the results of the experimental studies are discussed

oxygen and carbon isotope profiles in metasediments from lizzies basin east humboldt range nevada constraints on mid crustal metamorphic and magmatic volatile fluxes 18o 16o ratios vary systematically in the 1 000 m section of interlayered metasediments and granitoids at lizzies basin the deepest structural level exposed in the east humboldt range metamorphic core complex in the lower 300 m of the section the lower zone 18o is homogeneous and low 6 6 to 8 8 in silicate rocks 8 7 to 12 1 in marbles a detailed oxygen isotope profile across an individual marble layer within the lower zone has a similar range in 18o 9 5 to 11 9 with the highest values preserved in the middle of the layer in the upper 700 m of the section the upper zone metasediments have been less strongly 18o depleted the 18o values are higher and more heterogeneous and in a profile across a marble layer of similar thickness rise steeply from marginal values of 12 to core values of 23 quartz from silicate metasediments throughout the upper zone ranges from 11 to 13 and is thus fairly homogeneous particularly where detailed profiles have been measured adjacent to the 18o rich marble layers covariation of 18o 16o and 13c 12c ratios in marbles suggests that the isotopic composition of these elements has been altered by exchange with infiltrating water rich co2 h2o fluids with mantle like isotopic composition in most cases marble cores preserve protolith 13c values and these vary systematically throughout the section according to structural level including some exceptionally 13c rich values up to 12 this range may reflect stratigraphic variation in 13c of the proterozoic sedimentary protoliths of these rocks the upper lower zone boundary and the contrast in isotope systematics to either side has been previously explained either as an impermeable barrier to fluid flow or as an infiltration front wickham and peters 1990 the second alternative has been tested using a numerical model in which low 18o aqueous fluid flows vertically upward through an alternating sequence of monomineralic quartz 18o 12 and calcite 18o 22 layers in a regime in which the oxygen isotopic composition is controlled by advection diffusion in the fluid and fluid solid exchange solutions to the relevant transport equations indicate that the abrupt change in the average 18o value of the layers at the upper lower zone boundary can be reproduced by the model with a uniform porosity throughout the system but the observed contrast in the shapes of the marble layer profiles in the upper and lower zones cannot be reproduced under these conditions however if the calcite layers have two orders of magnitude lower porosity in comparison with the quartz layers and exchange within them is diffusion dominated as opposed to advection dominated in the quartz the contrasting shapes of the marble profiles in the two zones are reproduced as well as the upper lower zone discontinuity the range of conditions that generates both an infiltration front at the appropriate scale and contrasting marble profiles at the appropriate scale is quite narrow but requires a volatile flux that could be generated by plausible volumes of mantle derived magma crystallizing at depth beneath the area providing support for this mechanism as a viable agent of 18o depletion in the deep level rocks of this terrane

three dimensional numerical simulations of mantle flow beneath mid ocean ridges we perform three dimensional 3 d numerical simulations of an oceanic lithosphere cooling above a convective mantle to investigate mantle flow geometry and its associated lithospheric features a constant horizontal velocity half spreading velocity of 2 or 4 cm yr is applied at the box surface to mimic plate motion in all cases because of the temperature dependent viscosity a rigid conductive lithosphere cools progressively beneath the two plates separated by the ridge after 16 40 ma cold downgoing instabilities develop at the base of the lithosphere small scale convection is then superimposed on the large scale circulation it produces and then interacts with a slowly evolving short wavelength isotherms topography at the base of the lithosphere the influence of the imposed ridge geometry on the mantle flow appears by comparison of simulations with case a a linear ridge perpendicular to the plate motion case b a linear ridge strongly oblique to the plate motion and case c a ridge with transform faults along the spreading center with a mean orientation strongly oblique to the plate motion all simulations reveal complex interaction between the isotherm topography within the base of the lithosphere the small scale flow generated at or just below the base of the lithosphere and the large scale flow dominating in the mantle below the large scale flow appears always controlled by the mean ridge axis direction and thus may be oblique to the imposed plate motion direction it is enhanced by the instabilities development we argue that the large scale flow orientation results from the interaction with the small scale velocity field which flows down the isotherm topography at the base of the lithosphere the latter is either inherited lithospheric cooling transform faults or develops as boundary layer instabilities

mobility of trace elements and oxygen in zircon during metamorphism consequences for geochemical tracing this study addresses the mobility of the rare earth elements ree o u and pb during the formation of metamorphic zircon by overgrowth and coupled dissolution reprecipitation for this purpose the texture and ree 18o and u pb compositions of zircon crystals from a metabasite an anatectic granite and a metapelite sampled on naxos island greece have been characterised the behaviour of o and ree during the formation of the zircon metamorphic domains shows that two end members can be distinguished i closed or partially closed system behaviour for oxygen and trace elements in zircon formed by coupled dissolution reprecipitation metamorphic domains partially preserve the textural and geochemical characteristics of the precursor zircon chemical or isotopic equilibrium between the metamorphic zircon and the matrix is not completely reached for all the elements such zircons have to be used with caution for geochemical tracing when re equilibration is not complete the u pb system is also not completely reset and the metamorphic domains yield old scattered ages ii open system behaviour for oxygen and trace elements in zircon formed by overgrowth these metamorphic zircons do not show an inherited shape and are devoid of relic magmatic zoning their composition reflects growth in an open system at the scale of the zircon grain with the fluid phase and the surrounding minerals because equilibrium between the metamorphic zircon and the matrix was thus favoured zircon crystallisation can be more confidently related to the p t history and fluid circulation events by geochemical tracing

granulite facies metasomatism zoned calc silicate boudins from the rauer group east antarctica calc silicate boudins from the rauer group east antarctica were metamorphosed under granulite facies conditions during late proterozoic ca 1 000 ma m3 metamorphism boudin cores contain low to moderate aco2 assemblages including wollastonite grossularandradite grandite garnet clinopyroxene scapolite plagioclase quartz calcite petrological and stable isotopic evidence suggests that these core assemblages resulted from pre peak m3 infiltration of water rich fluids there is no evidence for a pervasive fluid phase under peak m3 conditions the boudins are separated from the surrounding fe rich pelites and semi pelites by a series of concentric high variance reaction zones developed under peak m3 conditions variations in mineral assemblage mineral composition and whole rock composition across these zones suggest that they formed by diffusional masstransfer controlled principally by a chemical potential gradient in ca across the original calc silicate paragneiss lithological boundary as a consequence of the nearcomplete decarbonation of the calc silicatesbefore the m3 peak development of the diffusion controlled reaction zones did not liberate significant co2 during granulite facies metamorphism similar calcite poor low aco2 calc silicate horizons in other granulite facies terrains are unlikely to have been important local fluid sources during deep crustal metamorphism

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

reheating of old oceanic lithosphere deductions from observations deep wide oceanic basins are the only regions of old seafloor where depth is truly representative of thermal isostasy when the depths of these basins are corrected for the effect of sediment accumulation and variation in crustal thickness the principal non thermal factors have been eliminated we collect the most precise and reliable values of heat flow for the same basins from multi penetration measurements with in situ thermal conductivity or deep sea drilling thermal gradients backed up by surface surveys the 9 data points that result from this selection process have been plotted on a depth versus heat flow graph and compared to published thermal models of lithosphere when considered without regard to age all the points fall at greater depths than predicted by the plate models with constant temperature lower boundaries and remarkably close to boundary layer cooling with parameters determined from the pre 80 ma depth and heat flow history of the ocean floor they are differentiated by their heat flows not much by their depths and the order they plot in along the heat flow axis is random with respect to crustal age modeling of discrete reheating events shows that near boundary layer conditions are re established after about 40 myr but corresponding to a younger than real age the data therefore favor discrete reheating events rather than a continuously hot basal boundary as implicitly assumed by the plate model lithospheric reheating appears to start only on ocean floor 100 ma the data alone cannot discriminate between a few discrete reheating events due to convective peel off at the base of the lithosphere or one or more catastrophic events however the distribution of points from the blake bahama basin is more consistent with distal reheating associated with the bermuda hotspot than local convective peel off

persistence of pre metamorphic c and o isotopic signatures in marbles subject to pan african granulite facies metamorphism and u th mineralization tranomaro southeast madagascar abstract the status of fluid regimes during the pan african granulite facies metamorphism in southern madagascar was examined by means of a mineralogical and stable isotope study of marble lithologies in the granulitic tranomaro area which is made up of metasedimentary rocks metapelitic leptynitic felsic and calc silicate metasediments and intruded by syn granulitic granitic bodies syn granulitic mineralization th u and metasomatic transformations marbles pyroxenites demonstrate that fluid circulation has occurred the tranomaro marbles can be subdivided into two compositional groups either mg poor or mg rich this division reflects pre metamorphic compositional differences rather than different p t x co 2 histories on a regional scale the marbles display a large variation in 18 o from 19 to 6 5 relative to smow but have a more restricted 13 c range from 1 4 to 2 5 relative to pdb the lowest 18 o values are observed in magnesian marbles whereas calcic ones do not have 18 o values lower than 13 5 both initially low and high 18 o marbles may have locally been infiltrated by the syn metamorphic fluid flow which gave rise to pyroxenites and th u ores thorianite 18 o values of carbonate in mg rich marbles evolve from 7 up to 8 5 towards the contact with a metasomatic pyroxenite in a meter scale profile across an infiltration zone the mineralogy as well as the large and small scale isotopic results argue for i a pre metamorphic origin for the isotopic variations probably related to pre granulitic dolomitization ii a lack of pervasive fluid infiltration capable of erasing pre metamorphic isotopic compositional differences iii an absence of mantle derived c bearing fluids in the infiltrated zones for the area as a whole the present data are consistent with the decarbonation of impure siliceous limestones and dolostones coupled with fluid release from syn metamorphic granitic intrusions

relationship between structure morphology and carbon isotopic composition of graphite in marbles implications for calcite graphite carbon isotope thermometry carbon isotope exchange between calcite and graphite is a useful and reliable geothermometer for medium to high grade marbles however in rare instances such as at naxos greece apparent disequilibrium carbon isotope fractionation between calcite and graphite has been previously reported in this study new results are presented on the morphological features x ray diffraction studies raman spectroscopic studies and carbon isotope studies of graphite three morphologically distinct graphite types are identified the first type is fine grained with distinct polygonal crystal aggregates having smooth pinacoid faces the second type consists of large botryoidal aggregates with typical crystal overgrowth features and the third type is normal large platy crystals associated with phlogopite graphite morphological features using an fe sem suggests that the botryoidal aggregates are composed of complexly intergrown and stacked graphite layers comprised of cone helix structures different types of graphite display distinct lc 002 and dg values as well as a sharp first order raman peak at 1580 cm 1 with disordered bands the presence of raman disordered bands in fine grained well crystallized graphite is attributed to edge effects carbon isotope analysis of graphite reveals that marble with fine grained well crystallized graphite show a consistent isotope fractionation with the calcite which perfectly match temperature estimates based on the mineral isograds in contrast coarse botryoidal graphite gives heterogeneous carbon isotope values about a few per mill and show anomalous carbon isotope fractionation between the calcite and graphite the graphite associated with phlogopite also shows variations in the carbon isotope values combining morphologic crystallographic and carbon isotopic data we conclude that the variations in carbon isotopic composition were caused by the overgrowth of graphite on the preexisting grain and that morphological observations and structural characterizations of graphite crystals is a key for predicting isotopic equilibration during metamorphism

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

the geology of the polymetamorphic complex of ios cyclades greece and its significance for the cycladic massif the geological structure of the island of ios cyclades greece is a dome consisting of an augengneiss core and a mantle of garnet mica schists that together form the basement which is overthrust by a marble schist series this series is mainly made up of meta volcanics and metamorphosed sediments presumably mesozoic in age it is a tectonically composed pile of marbles alternating with glaucophane schists actinolite schists and chlorite schists petrological relations and isotope dating indicate the polymetamorphic character of ios two alpine metamorphic phases m1 and m2 and relicts of a high grade metamorphic or magmatic phase mo that only affected the basement are demonstrated radiometric ages obtained for the basement confirm the interpretation that the mo phase is pre alpine the m1 and m2 were dated at 43 ma and 25 ma respectively the p t conditions of metamorphism are estimated as 9 11 kb and 350 to 400 c for the m1 phase and 5 7 kb and 380 420 c for the m2 phase the metamorphic history of the cyclades is discussed and it is suggested that pre alpine basement occurs also on sikinos and naxos

stable isotopic signatures of superposed fluid events in granulite facies marbles of the rauer group east antarctica the role of volatiles in the stabilization of the lower granulite facies crust is contentious opposing models invoke infiltration of co2 rich fluids or generally vapour absent conditions during granulite facies metamorphism stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models in this study data are presented from metre scale forsteritic marble boudins within archaean intermediate to felsic orthogneisses from the rauer group east antarctica forsteritic marble layers and associated calcsilicates preserve a range of 13c and 18o depleted calcite isotope values 13c 9 9 to 3 0 pdb 18o 4 0 to 12 1 smow a coupled trend of 13c and 18o depletion 2 5 respectively from core to rim across one marble layer is inconsistent with pervasive co2 infiltration during granulite facies metamorphism but does indicate localized fluid rock interaction at another locality more pervasive fluid infiltration has resulted in calcite having uniformly low carbonatite like 18o and 13c values a favoured mechanism for the low 18o and 13c values of the marbles is infiltration by fluids that were derived from or equilibrated with a magmatic source it is likely that this fluid rock interaction occurred prior to high grade metamorphism other fluid rock histories are not however ruled out by the available data coupled trends of 13c and 18o depletion are modified to even lower values by the superposed development of small scale metasomatic reaction zones between marbles and internally folded mafic interlayers the timing of development of these layers is uncertain but may be related to archaean high temperature 1000dc granulite facies metamorphism

ramp facies and sequence stratigraphic models in an epeiric sea the upper ordovician mixed carbonate siliciclastic glenwood and platteville formations wisconsin usa