unusual bulk rock compositions in eclogite facies rocks from syros and tinos cyclades greece implications for u pb zircon geochronology low temperature eclogite facies rocks from syros and tinos cyclades greece include meta ophiolitic blocks of unclear origin meta olistostrome or tectonic melange these blocks occur in a matrix of altered serpentinite and or semipelitic to tuffitic schists some samples are characterized by very high trace element concentrations for example eclogites from tinos contain up to 4950 ppm zr and up to 480 ppm y understanding of the geochemical controls on the petrogenesis of these rocks has important consequences for interpretation of geochronological results the emphasis of this study is on the behaviour of zr under eclogite facies p t conditions due to remarkably high modal amounts of zircon some samples from the meta ophiolite suite are suitable for u pb zircon chronology the geological significance of zircon ages can only be correctly interpreted by assessing whether zircon crystallized from a melt or formed during subsequent hydrothermal or metamorphic processes the results of this study lead us to conclude that infiltration of trace element enriched fluids during eclogite facies metamorphism caused local scale compositional changes textural observations document the presence of a fracture network favouring fluid infiltration under high pressure conditions such a system of microfractures might have provided infiltration paths for trace element enriched fluids during earlier stages of high pressure metamorphism in most cases subsequent recrystallization erased any evidence of the early infiltration channels but inclusion of high pressure phases in zircon provide evidence for this model suggesting metasomatic alteration in a subduction zone environment the original fluid source and the conditions e g high f environment highly saline brines under which commonly immobile elements were mobilized remain unclear it is speculated that eclogitization of closely associated metagabbros may have released zr and other trace elements the results of this contribution suggest that new zircon formed during high pressure metamorphism cretaceous u pb zircon ages are interpreted to date subduction metamorphism and not magmatic crystallization

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

eclogite blueschist relationships as evidenced by mineral equilibria in the high pressure metabasic rocks of sifnos cycladic islands greece

metal mobility by de icing salt from an infiltration trench for highway runoff soil from an infiltration trench for highway runoff was leached in columns alternately with nacl and de ionised water to simulate the runoff of de icing salt into the trench followed by snowmelt or

tectono metamorphic evolution of syros and sifnos islands cyclades greece abstract miocene exhumation of metamorphic rocks in the aegean sea is partly a consequence of post orogenic extension if the post orogenic mechanism of exhumation is rather well understood the earlier syn orogenic eocene exhumation is still largely enigmatic previous authors have argued in terms of extension or compression new structural and petrological data on sifnos and syros islands show that exhumation of high pressure low temperature hp lt rocks involves crustal scale extensional ductile shear zones during the eocene we observe a continuum of top to the ne and e ductile shear from the eocene in the blueschist facies to the miocene in the greenschist facies this deformation is distributed in the eclogites and blueschists whereas it is rather localised along ductile shear zones in the greenschists eclogites which are preserved only at the top of the structural pile are exhumed with a cold retrograde p t path in the lower part of the structural pile we observe a progressive retrogression of eclogites in blueschist then greenschist facies this lower part of pile is subsequently exhumed with p t paths showing a nearly isothermal decompression before cooling p t t deformation data suggest that the cycladic blueschists are progressively exhumed by a continuum of accretion at the base of the orogenic wedge and by a partly non coaxial extensional deformation above distributed during the syn orogenic stage then localised during the post orogenic stage we then compare the mechanism of syn orogenic exhumation of crete and the cyclades and we discuss a simple geodynamic scenario for the aegean domain and the external hellenides which accounts for 1 the southward migration of the hellenic trench and arc during the cenozoic 2 the p t t deformation data for the cycladic blueschists and the phyllite quartzite nappe and 3 the transition from syn orogenic to post orogenic in the cyclades

rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

exhumation of the schistes lustres complex in situ laser probe 40ar 39ar constraints and implications for the western alps the schistes lustres sl suture zone occupies a key position in the alpine chain between the high pressure hp brianconnais domain and the ultrahigh pressure uhp dora maira massif and reached subduction depths ranging from c 40 65 km cottian alps in order to constrain the timing of hp metamorphism and subsequent exhumation several phengite generations were differentiated on the basis of habit texture paragenesis and chemistry as belonging to the first or second exhumation episode respectively d2 or d3 or to earlier stages of the tectono metamorphic evolution ten carefully selected samples showing d2 d3 d2 d3 or earlier mostly peak temperature phengite population s were subjected to laser probe 40ar 39ar analysis the data support the results of the petrostructural study with two distinct age groups crystallization ages for d2 and d3 phengite at 51 45 and 38 35 ma respectively the data also reveal a coherent age cluster at 62 55 ma for peak temperature phengite associated with chloritoid which were preserved in low strain domains the age of the d3 event in the sl complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs exhumation rates of the order of 1 2 mm yr 1 are obtained for the sl complex which are compatible with velocities documented for accretionary wedge settings similarly cooling velocities are only moderate c 5 c myr 1 which is at variance with recent estimates in the nearby uhp massifs

field studies of in situ colloid mobilization in a southeastern coastal plain aquifer the release of colloids to groundwater was investigated in situ in an iron oxyhydroxide rich sandy aquifer groundwater amended with various solutes was injected into and immediately withdrawn from the shallow aquifer turbidity and colloid composition were monitored in the retrieved injectate the response of the aquifer material to the amendments generally mimicked that observed in an earlier study using packed columns containing the sediment demonstrating the viability of the single well method for testing colloid mobilization in situ the decline of turbidity in the retrieved injectates with increasing withdrawal volume was analyzed to determine a reaction order n describing the redeposition of mobilized colloids to the immobile matrix differences in the reaction order for the amendments tested presumably indicated the effectiveness of these amendments to generate repulsive colloid immobile matrix interactions

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

structural analysis of basement tectonites from the aegean metamorphic core complex of ios cyclades greece abstract the island of ios in the cyclades aegean sea greece exposes high pressure metamorphic rocks that were subjected to miocene continental extension subsequent to the alpine collisional orogeny the pre alpine history of the lower plate of this aegean metamorphic core complex involves intrusion of granitoids into metasediments which subsequently underwent hercynian amphibolite facies metamorphism during the alpine orogeny d2 these rocks were overthrust by the cyclades blueschist nappe later the lower plate was tectonically exhumed during oligo miocene continental extension and associated plutonism final exhumation occurred as a result of the operation of a south directed crustal scale shear zone and low angle normal faults this study shows that the ios core complex reflects an intrinsic relationship between early thrusting and later extensional tectonism basement rocks have been exhumed from beneath the nappes that originally overrode them

emplacement of clay into sand by infiltration abstract laboratory experiments and examination of core samples from holocene sand bodies demonstrate that vadose infiltration of muddy water through sand is an effective mechanism by which clay can be emplaced into sand infiltrated clays form coatings on and bridges between sand sized framework grains clay particles are mostly oriented parallel to grain surfaces in coatings and perpendicular to grain surfaces in bridges the amount of clay emplaced as coatings and bridges is determined by complexly interacting variables that include sand grain size clay particle size clay particle shape concentration of suspended sediment and clay composition of these relatively coarse sand sizes and high concentrations of suspended sediment appear to promote the emplacement of the most infiltrated clay in holocene sands the formation of grain coatings and bridges is facies dependent infiltrated clays are most effectively emplaced in environments characterized by high suspended sediment concentrations fluctuating water levels and minimal sediment reworking e g point bar and delta plain in contrast virtually no infiltrated clay is emplaced in environments characterized by low suspended sediment concentration and continuous reworking e g beach and tidal delta

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

40ar 39ar and rb sr geochronology of high pressure metamorphism and exhumation history of the tavsanli zone nw turkey geochronological investigations in high and ultra high pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages and secondly the problem of excess argon affects 40ar 39ar systematics the most commonly used isotopic system the tavsanli zone is a belt of high pressure low temperature hp lt rocks spanning nw turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of hp lt metamorphism raising the question of whether they are geologically significant this study presents new data from the tavsanli zone 40ar 39ar ages are in the range 60 ma to 175 ma whilst rb sr ages are restricted to 79 7 ma to 82 8 ma confirming the presence of excess argon detailed ultra violet laser ablation microprobe uvlamp studies have revealed younger 40ar 39ar ages in the cores of coarser white micas which in conjunction with 40ar 39ar ages from the finest grained lithologies and the rb sr white mica crystallisation ages constrain the post hp lt metamorphism exhumation rates of these rocks petrological and regional constraints suggest that syn subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating this is followed by a phase of syn continent continent collision at a rate of approximately 1 5 mma 1 and exhumation to the surface via thrusting the 40ar 39ar hornblende data from a granodiorite intruding the hp lt rocks constrain the later parts of exhumation path this study highlights the importance of a multi system geochronological approach when attempting to determine the history of hp lt rocks

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

evolution of the blueschist and greenschist facies rocks of sifnos cyclades greece the metamorphism on the island of sifnos is characterized by the eocene development of a coherent highpressure blueschist terrane and an early miocene greenschist facies overprint this study documents the metamorphic evolution of the blueschist assemblages still preserved in the northern parts of the island and their subsequent transformation into greenschists in the central and southern parts the oxygen isotope geothermometry is based on calibrations for quartz pyroxenes and magnetite matthews et al 1983a augmented by revised calibrations for the minerals muscovite qz mu 1 55 106 t 2 epidote qz ep 1 56 1 92 ps 106 t 2 and rutile qz ru 4 54 106 t 2 oxygen isotope analyses of minerals from the blueschist unit of northern sifnos give consistent fractionations which are independent of rock type an average temperature of 455 c was obtained although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred the minerals and whole rocks of the greenschists of central sifnos are systematically enriched in 18o relative to the blueschist assemblages chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of ca2 and co2 the petrologic isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18o co2 enriched aqueous solutions it is reasonable to assume that this is connected with the miocene magmatic activity observed throughout the cyclades the marbles separating the blueschist from the greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation the pressure of the blueschist metamorphism is estimated at 14 2 kbar corresponding to a depth of ca 50 km the structural style and stratigraphy of sifnos are suggestive of the subduction of a continental margin sequence it is clear that the considerable tectonic depression may be associated with continental collision and underthrusting

boron isotope record of peak metamorphic ultrahigh pressure and retrograde fluid rock interaction in white mica lago di cignana western alps this study presents boron b concentration and isotope data for white mica from ultra high pressure uhp subduction related metamorphic rocks from lago di cignana western alps italy these rocks are of specific geological interest because they comprise the most deeply subducted rocks of oceanic origin worldwide boron geochemistry can track fluid rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones the highest b contents up to 345 g g b occur in peak metamorphic phengite from a garnet phengite quartzite the b isotopic composition is variable 11b 10 3 to 3 6 and correlates positively with b concentrations based on similar textures and major element mica composition neither textural differences prograde growth zoning diffusion nor a retrograde overprint can explain this correlation modelling shows that b devolatilization during metamorphism can explain the general trend but fails to account for the wide compositional and isotopic variability in a single well equilibrated sample we therefore argue that this trend represents fluid rock interaction during peak metamorphic conditions this interpretation is supported by fluid rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in 11b and b taking into account the local availability of serpentinites as potential source rocks of the fluids the temperatures reached during peak metamorphism that allow for serpentine dehydration and the high positive 11b values 11b 20 5 modelled for the fluids an influx of serpentinite derived fluid appears likely paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have b contents between 12 and 68 g g and 11b values that cluster around 0 11b 5 0 to 3 5 white mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks the variable b geochemistry can be successfully modelled as fluid rock interaction with low to moderate 3 fluid rock ratios where mica equilibrates with a fluid into which b preferentially partitions causing leaching of b from the rock the metamorphic rocks from lago di cignana show variable retention of b in white mica during subduction related metamorphism and exhumation the variability in the b geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones

new insights from 40ar 39ar laserprobe dating of white mica fabrics from the pelion massif pelagonian zone internal hellenides greece implications for the timing of metamorphic episodes and tectonic events in the aegean region

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

characterization of suspended particles collected in groundwater under natural gradient flow conditions microscale eulerian variations in the flux mineralogical composition and size of suspended particles have been found in a contaminated sandy aquifer under natural gradient flow conditions during an 8 month study period particle variability has been detected along a 16 m saturated section of the aquifer at a scale of centimeters and meters in the vertical and horizontal dimensions respectively the average concentration of particles in groundwater varied between 1 and 40 mg l but high concentrations of up to 5000 mg l were determined in specific 3 cm vertical intervals of the aquifer the particles were primarily composed of caco3 11 57 quartz 7 39 and clays 8 43 most of the particles were within the 140 3000 nm size range with size modes varying from 310 to 660 nm the large amounts of suspended particles are considered to be related to high inputs of dissolved organic carbon into groundwater from sewage effluents which have been used for agricultural irrigation since the early 1960s as a result of organic matter biodegradation in the saturated zone anoxic conditions developed and the pco2 content of groundwater increased dramatically pcaco2 10 1 8 to 10 1 3 atm it is postulated that part of the carbonate cement of the rocks dissolved and detrital caco3 quartz and clay were released as colloidal particles part of the clay particles could have also been transported through the unsaturated zone into groundwater after mobilization from surface layers as a result of the intermittent input of water of different chemical quality in the prevailing anoxic conditions of groundwater at the study site dissolved oxygen concentrations of 1 mg l colloidal stability is enhanced by organic matter coating of particles

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

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

the tectonic significance of a porphyroblastic blueschist facies overprint during alpine orogenesis sifnos aegean sea greece the macargon program has been used to model published 40ar39ar apparent age spectra for white micas from the island of sifnos cyclades greece these micas formed during a period of porphyroblastic mineral growth m2 in the epidote blueschist facies at 460 c and 50 c m y and since m2 is followed by d3 we infer that these are the cooling rates during d3 modelling experiments for micas from the greenschist domain gsd structurally underneath the ebd suggest that the gsd cooled rapidly but at 32 ma one tectonic model that might explain the rapidity of cooling rates inferred for the ebd model supposes that ambient temperatures were at 500 c when m2 took place but thereafter the high pressure rocks were thrust to the north and rapidly cooled as the result of their juxtaposition against cooler shallower levels of the crust

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

fluid rock interactions during ultra high pressure metamorphism dabie shan china oxygen isotope analyses of garnet separates and whole rock samples of gneiss and marble hosted eclogites from the ultra high pressure uhp dabie shan terrain fall in the range 6 76 to 10 76 comparison of the isotopic compositions of the eclogites with those of their host rocks leads to the following conclusions 1 the basaltic protolith of the eclogites had isotopic compositions in the range 2 15 to 4 99 it is proposed that this is the result of contamination of the parent basaltic magma with hydrothermally altered18o depleted source materials 2 18o enriched eclogite compositions resulted from local scale 3 m interaction between this basaltic eclogite protolith and fluids produced by decarbonation of impure marble lithologies during uhp metamorphism 3 18o depleted eclogite and gneiss compositions resulted from regional scale meteoric hydrothermal alteration of the basalts and their host rocks prior to uhp metamorphism epidote separates and whole rocks from the cores and rims of mineralogically zoned eclogite boudins within amphibolite grade biotite gneisses show no distinct isotopic differences this suggests that the preferential recrystallisation of the gneisses and the associated amphibolite grade overprint of the rims of eclogite bodies could not have resulted from regional scale infiltration of significant volumes of fluid during the exhumation of the dabie shan terrain

migration of contaminants in groundwater at a landfill a case study 6 hydrogeochemistry multilevel point samplers located on a longitudinal cross section through the plume of contaminated groundwater in the sand aquifer at the abandoned borden landfill were used to obtain a suite of small volume samples for analysis of major ions and of minor and trace level inorganic constituents calcium is the dominant cation and sulphate and bicarbonate are the dominant anions in the contaminant plume with maximum concentrations of 400 2000 and 1200 mg l 1 respectively beneath the landfill the most highly contaminated water in the aquifer has total dissolved solids of 4000 mg l 1 sulphate and iron are the only inorganic constituents that exceed the recommended limits for drinking water the plume contains above background concentrations of dissolved zinc and manganese no heavy metals or other hazardous inorganic trace elements occur at levels above the maximum limits for drinking water the plume contains levels of total dissolved organic carbon that are above background levels ranging from 30 mg l 1 beneath the landfill to 5 mg l 1 near the front of the plume the landfill which is almost entirely above the water table contains high partial pressures of carbon dioxide and methane the high carbon dioxide levels induce substantial dissolution of calcite which occurs in sand layers in the landfill and in the aquifer immediately below the landfill calcite dissolution is the origin of much of the dissolved inorganic carbon and some of the calcium in the plume most of the sulphate and much of the calcium in the plume appears to be derived from gypsum in construction debris in the landfill groundwater beneath the landfill is saturated with respect to gypsum and slightly supersaturated with respect to calcite some of the dissolved iron in the plume results from the release under reducing conditions of iron from ferric iron coatings on sand grains located in the aquifer beneath the landfill and in the layers in the refuse dissolved iron concentrations appear to be limited by the solubility of siderite dissolved sulphide is present at only very low concentrations and is probably controlled by ferrous sulphide solubility exchange of cations between the aquifer material and the leachate contaminated groundwater is an apparent cause of calcium release and subsequent precipitation of calcite in the plume as much of 80 of the k 20 of the mg2 and 15 of the na in the plume exists on the exchange sites the effects of dispersive and geochemical attenuation were evaluated from the ratios of chloride to other species along a central flow path from the landfill to the front of the plume platinum electrode measurements and electrochemical potentials calculated from equilibrium speciation of analysed constituents varied over a wide range of values and provided poor agreement among methods for the same sample only potentials derived from the iron redox couples and pt electrode measurements exhibited fair agreement for any one sample the rather narrow range of pt electrode potential values along the central flow line through the plume suggest that the electrochemical potentials are buffered apparently as a result of fe oh 3 s ag feco3 s equilibria

reaction pathways during the progressive deformation of a blueschist metabasite the role of chemical disequilibrium and restricted range equilibrium there are discrete masses of un deformed metabasite within the blueschist series of the island of syros greece around the margins of these masses are zonal sequences through rocks showing intracrystalline deformation but without a geometric fabric to rocks with discrete and anastomosing shear zones and finally to penetratively foliated rocks with isolated relics of the original undeformed texture textural relics suggest that this spatial sequence is at least qualitatively also a temporal sequence this progressive shear zone deformation took place concurrently with a glaucophane epidote to eclogite reaction the reaction pathways in the rocks that underwent the shear zone deformation can be compared with those in rocks of a similar composition that suffered a longer deformation history and show no relics of an undeformed parent although the final assemblages are in both cases the same the pathways are different these differences are in part related to reactions promoted by the change from local to bulk equilibrium on the onset of deformation in the rocks they are also related to the crystallization and later breakdown during the sequence of progressive equilibration of a metastable phase in this case an impure glaucophane

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

migration of contaminants in groundwater at a landfill a case study 4 a natural gradient dispersion test abstract a natural gradient tracer test using a chloride solution with an initial injection volume of 0 7 m 3 was performed in the sandy aquifer at the borden site the solution was injected into five well points set 1 m below the water table in an uncontaminated zone situated above the contaminant plume at a location 450 m downflow from the landfill under conditions of natural groundwater flow the tracer slug was monitored for a period of 4 months by withdrawing small volume samples from points in a three dimensional array of bundle type multilevel samplers measurements of hydraulic head were obtained from a network of miniature piezometers soon after injection the tracer slug gradually split into two halves one half moving horizontally at an average velocity of 2 9 10 6 ms 1 and the other horizontally at 8 2 10 7 ms 1 although the split has been attributed to local lateral heterogeneity the nature of the heterogeneity and its influence on the hydraulic head distribution were not clearly distinguishable in the field data obtained before during or after the test the chloride patterns for each of the two halves of the tracer slug evolved into gaussian forms although the patterns demonstrated some irregularity at early time the relatively smooth gaussian forms were unexpected because the aquifer has numerous small scale heterogeneities observed in vertical cores obtained from the tracer zone and because the glaciofluvial origin of the aquifer suggests that heterogeneities are not laterally continuous simulated chloride distributions from a three dimensional analytical solution to the advection dispersion equation were fitted to the field data to obtain best fit estimates of the values of longitudinal transverse horizontal and transverse vertical dispersivity at various travel distances for each of the two halves of the tracer zone this is the first known field test that has permitted the estimation of three principal dispersion coefficients in layered media the longitudinal dispersivity was found to increase from 0 01 m at a distance of 0 75 m from the source to 0 08 m at 11 0 m the transverse horizontal dispersivity increased also to a value of 0 03 m at 11 0 m transverse vertical dispersion was very weak and was accounted for by molecular diffusion the relative lack of vertical dispersion is consistent with the shape of the plume of leachate contamination from the landfill it was concluded that the observed increase in dispersivity along the path of migration is likely caused by heterogeneities information on the dispersion controlling heterogeneities is not yet available as practical field methodologies for their identification and description have not yet been developed until such information is incorporated into mass transport models a realistic solution of the dispersion problem in heterogeneous media will remain elusive

excess argon evolution in hp lt rocks a uvlamp study of phengite and k free minerals nw turkey excess argon is a common problem in high pressure metamorphic rocks here we present 40ar 39ar data from metabasite metachert and metapelite lithologies and a range of minerals the results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high pressure metamorphism samples were derived from blueschist and low temperature eclogite facies rocks from the tavsanli zone of nw turkey samples were analysed using the 40ar 39ar ultra violet laser ablation microprobe uvlamp and yield ages ranging from 72 3 to 154 7 ma detailed intra grain profiling of phengites in situ and analyses of sodic amphibole lawsonite quartz and garnet present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite whole rock xrf data indicates a scattered relationship between bulk k2o content of the rock and the concentration of excess argon in phengite we conclude that excess argon was derived in situ in a relatively closed fluid absent system rather than introduced by fluids

diagenesis of iron in proglacial sand deposits of late and post weichselian age abstract iron diagenesis was studied in quaternary sand deposits of a temperate climate using an integrated approach of sedimentary petrography and groundwater chemistry the iron bearing minerals amphiboles pyroxenes magnetite and ilmenite are all extensively altered quantitatively amphiboles and pyroxenes are by far the most important goethite and lepidocrocite are the most important of the iron oxyhydroxide weathering products and they coat quartz and feldspars as well as amphiboles and pyroxenes however lepidocrocite is absent in the unsaturated zone hematite is only present associated with magnetite and ilmenite total iron contents of the sediments range from 0 3 0 9 percent with 30 80 percent as coatings on quartz and feldspar grains 25 60 percent in the amphibole and pyrox ne fraction and only 2 15 percent in the magnetite and ilmenite fraction therefore amphiboles and pyroxenes are the most important sources of iron for these diagenetic processes the groundwater has a ph between 5 5 and 6 0 and contains 10 30 m fe2 and a low total of dissolved solids subtraction of the contribution of the dissolved solids from atmospheric precipitation indicates higher ph and hco3 sio2 ca2 na m and fe2 concentrations due to mineral reactions in the aquifer stability relationships for iron bearing minerals under the assumption of anoxic conditions show strong undersaturation for iron bearing silicates while undersaturation for magnetite and ilmenite is much less the saturation state for magnetite is strongly dependent on the iron oxide phase formed by incongruent dissolution the surfaces of iron silicates and magnetite are highly etch pitted indicating that mineral dissolution is controlled by surface reactions rather than by diffusion through alteration products the rate of mineral alteration is slow in comparison with groundwater flow thus diagenetic changes occur in the deposit as a whole rather than as a diagenetic front advancing through the deposit the abundance of feooh on all detrital grains suggests that this proglacial deposit could develop into a red bed during further diagenesis this possibility should be considered in the paleoclimatic interpretation of red beds

evidence for excess argon during high pressure metamorphism in the dora maira massif western alps italy using an ultra violet laser ablation microprobe 40 ar 39 ar technique ultra high pressure eclogite amphibolite grade metamorphism of the dora maira massif in the western alps is a well established and intensively studied event however the age of peak metamorphism and early cooling remains controversial the 40ar 39 ar step heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 ma which have been interpreted as the time of early cooling after the high pressure event recent u pb and sm nd results challenge this assertion indicating a much younger age for the event around 45 ma and hence a radically different timing for the tectonic evolution of the western alps in a new approach to the problem samples from the undeformed hercynian metagranite brossasco were studied using an ultra violet laser ablation microprobe technique for 40ar 39 ar dating the new technique allowed selective in situ analysis at a spatial resolution of 50 urn of quartz phengite biotite and k feldspar the results demonstrate the frequent occurrence of excess argon with high 40ar 39 ar ratios 1000 10000 and a strong relationship between apparent ages and metamorphic textures the highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases the best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event then mixed with an atmospheric component during rapid cooling and retrogression producing a wide range of argon concentrations and 40ar 36ar ratios step heating analysis of minerals with this mixture would produce linear arrays on a40ar 36ar versus 40ar 36 ar correlation diagram leading to geologically meaningless plateau ages older than the true closure age in the present case some ages in the range 60 110 ma could be explained by the presence of excess argon incorporated around 40 50 ma ago similar results found in other high pressure terrains in the alps may reconcile the argon geochronometer with other systems such as rb sr u pb or sm nd this study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40ar 36 ar ages in many high pressure terrains

excess argon incorporation in phengite of the mulhacen complex submicroscopic illitization and fluid ingress during late miocene extension in the betic zone south eastern spain abstract 40 ar 39 ar induction furnace and laser step heating of well crystallised post tectonic phengitic mica single grains from gneisses of the mulhacen complex with an early alpine tectonic fabric has resulted in 1 highly scattered integrated ages 2 an abnormally high atmospheric contamination and 3 often anomalously old apparent ages during early 39 ar release that is associated with a high 36 ar air and 37 ar ca contamination this low temperature excess argon 40 ar xs component is probably released from carbonate formed during slight alteration of the mica more than 50 of the samples yielded plateau ages ranging from 15 8 0 4 to 90 1 1 0 ma samples taken only a few metres apart may differ in age by as much as 50 ma a grain that was split over the basal plane yielded plateau ages for each half that differ by 12 the age variation on these different scales is explained by heterogeneous 40 ar xs incorporation during a period with a high transient partial argon pressure in the metamorphic fluid resulting from a late stage reheating event the very swift cooling of 50 100 c ma during exhumation of the mulhacen complex concomitant with late miocene extension may have prevented the equilibration of different 40 ar xs levels in the mica hrtem images of the oldest and youngest phengite specimens show that at least 20 of the lattice is affected by submicroscopic illitisation which is concentrated in several micrometer wide zones and veins that cross cut the basal cleavage these are made up of aggregates of 0 07 0 30 m thick crystallites of three illitic micas types which are chemically and structurally progressively closer to pure illite and occur in different textures the oldest specimen is affected most severely as the veins contain newly formed pseudo illite that does not inherit its crystallographic orientation and chemistry from the host mica in contrast to the youngest sample hrtem aem analyses revealed that phengite and the different illitic micas may be depleted in k the oldest sample is derived from a coarse grained augen gneiss with extensively developed hydraulic cracks which are lacking in the youngest sample a fine grained mylonitic gneiss fluid rock interaction and consequently sub microscopic illitization were therefore more intense in the coarser grained rocks growth of the illitic micas in equilibrium with a high partial 40 ar pressure could account for 40 ar xs incorporation in k vacancies and other lattice imperfections variation in illitisation and associated textural dissimilarities between the oldest and youngest mica permit the different levels of 40 ar xs incorporation that account for the observed age discordance the finding of 40 ar xs plateau ages despite the degassing of intimately intergrown micaceous minerals is interpreted by gas release invoked by in vacuo chemical and structural changes that led to a joint collapse of the lattices of phengite and the illitic micas between 800 c and 1000 c

tectonic juxtaposition of blueschists and greenschists in sifnos island aegean sea implications for the structure of the cycladic blueschist belt abstract in the cyclades kilometre thick high pressure rock sequences displaying blueschist and eclogite facies mineral assemblages overlie a rock sequence which was thoroughly overprinted in the greenschist and amphibolite facies during its exhumation both rock sequences subducted and metamorphosed at high pressures in eocene times have been considered by previous workers to have been exhumed as a coherent rock unit in contrast it is suggested here that the preserved high pressure rock sequences were exhumed more rapidly and prior to the underlying greenschists significant metamorphic structural and geochronological discontinuities exist across the blueschist greenschist contact and field evidence suggests that the high pressure metamorphic rocks in sifnos are tectonically juxtaposed above the greenschists these two rock sequences were juxtaposed by a low angle fault subsequent to the oligocene miocene greenschist facies overprint published geochronological data and petrological criteria are used to show that the high pressure sequence cooled below 350 c when the rocks now immediately underlying it suffered a greenschist facies overprint at temperatures of ca 450 c the section inferred to absorb this temperature difference is now missing and it is suggested that it has been cut out by the low angle fault

40ar 39ar laserprobe direct dating of discrete deformational events a continuous record of early alpine tectonics in the pelagonian zone nw aegean area greece abstract the pelagonian zone of the hellenides which occurs in the northwestern aegean region of the eastern mediterranean contains remnants of a high pressure metamorphic belt which subsequently underwent localized reworking during low temperature deformational events formation of alpine deformational structures in the ossa massif northern pelagonian zone occurred at metamorphic temperatures which failed to reset most pre alpine 40ar 39ar cooling ages in muscovite porphyroclasts the low metamorphic temperatures at alpine times have preserved the argon isotopic signature in white mica bearing deformational structures which allows direct dating of these structures and refinement of the alpine tectonic history in the region after separation of carefully selected and characterized mica generations application of sensitive 40ar 39ar laserprobe dating has resulted in a continuous record of the early alpine tectonic history of the ossa massif from the early cretaceous well into the eocene the early alpine history of the ossa massif records cooling of basement thrust slices which are mostly hercynian granites and metamorphics below ca 350 c by 100 ma while lower tectonostratigraphic levels of the basement had already cooled below these temperatures at around ca 285 ma blueschist facies mylonite fabrics related to a top to ene direction of tectonic transport yield ages as old as 84 5 3 3 ma exhumation of the blueschist facies sequences was initiated at ca 54 ma and involved tectonic activity along blueschist facies and greenschist facies mylonites zones final wsw ward transport of the metamorphic sequence across the structurally lowest and supposed autochthonous series occurred at around 45 ma and resulted in termination of the ductile deformation history in the studied area this study confines the onset of high pressure metamorphism in the pelagonian zone to an interval between 100 ma and 85 ma and has shown that high pressure metamorphism had terminated by ca 54 ma the recognition of an early to middle alpine cycle lasting over 30 million years and involving crustal shortening high pressure metamorphism exhumation and subsequent crustal shortening is a key contribution to a better understanding of the early alpine tectonic history of the aegean region and eastern mediterranean

excess argon in phengite from eclogite evidence from dating of eclogite minerals by smnd rbsr and 40ar39ar methods the existence of excess ar in phengites from coesite bearing ultrahigh pressure eclogites of the dabie mountains and su lu terrain china has been confirmed by dating of minerals using sm nd rb sr and 40ar39ar methods 40ar39ar ages of phengites for two samples studied are much older than the sm nd and rb sr mineral isochron ages sample ql 1 has rb sr age of 219 5 0 5 ma sm nd age of 226 3 4 5 ma and phengite 40ar39ar age of 877 5 15 9 ma the corresponding ages for sample zb 4 are 223 9 0 9 ma rb sr 228 4 6 0 ma sm nd and 943 3 12 4 ma 40ar39ar phengite an age of 225 ma based on the sm nd and rb sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the north china and yangtse blocks and formation of these eclogites we call for greater caution in 40ar39ar dating of phengites from other occurrences of ultrahigh pressure environments

rb sr isotope studies on tinos island cyclades greece additional time constraints for metamorphism extent of infiltration controlled overprinting and deformational activity this study presents new rb sr age data concerning the metamorphic evolution of the attic cycladic crystalline belt which represents a complex polymetamorphic terrane within the alpidic orogenic belt of the hellenides two major groups of tectonic units can be distinguished metamorphism in parts of the upper units is commonly considered as a cretaceous event in contrast the group of lower units experi enced tertiary high pressure metamorphism which was followed by a medium pressure overprint we focus on the island of tinos where a representative spectrum of the rock units found in the cyclades is exposed in three tectonic units the upper unit the intermediate unit and the basal unit the complete range of tectono metamorphic and magmatic events affecting the attic cycladic crystalline belt is documented by numerous petrological and tectonic studies phyllites and phyllonites from the ophiolitic upper unit yielded rb sr apparent ages phengite whole rock between c 92 and 21 ma the older age differs from the cretaceous dates reported for upper unit rocks elsewhere in the cyclades it is suggested that the sequence studied belongs to the jurassic ophiolites of the hellenides rather than to cretaceous occurrences the spread to younger ages is related to non pervasive rejuvenation and resetting of the rb sr system during tectonic juxtaposition of the upper unit over the intermediate unit the youngest age obtained so far for a sample from the upper unit 21 ma is believed to approximate the timing of tectonic juxtaposition which probably occurred during a regional greenschist facies episode producing a pervasive overprint in the struc turally lower tectonic unit the major phyllite meta gabbro serpentinite sequence of the upper unit is inter preted as an emplacement related ductile shear zone which experienced reworking under brittle conditions in the intermediate unit a gradient in rb sr ages from top c 40 ma to the bottom c 22 ma was recog nized which is interpreted to represent greater effects of fluid infiltration and overprinting in the lower parts of this unit possibly controlled by variable intensity of deformation which might be related to tectonic jux taposition onto the basal unit we suggest that synmetamorphic stacking of all three tectonic units took place during an oligocene miocene greenschist event extensional deformation continued after tectonic stacking and after intrusion of the main granite as is indicated by a rb sr whole rock isochron 15 1 0 6 ma for a ductilely deformed garnet bearing leucogranite from the marginal parts of the main undeformed pluton application of the rb sr dating technique provided no unequivocal evidence that previously pub lished eocene k ar and 40 ar 39 ar dates for high pressure phengites from the lower units are significantly contaminated with excess argon

distribution and significance of extraneous argon in uhp eclogite sulu terrain china insight from in situ 40ar 39ar uv laser ablation analysis in situ 40ar 39ar uv laser ablation analyses of garnet omphacite epidote phengite barroisite and symplectite in a sample of uhp eclogite from qinglongshan sulu terrain china reveal extremely heterogeneous argon isotopic ratios results from the 220 240 ma eclogite indicate a significant and variable extraneous argon signature that is restricted to the early formed phengite and epidote and later formed barroisite individual compositionally zoned phengite grains record a large variation in 40ar 39ar dates 300 1100 ma that are positively correlated with celadonite content these data are consistent with retrograde chemical exchange and argon isotopic repartitioning in response to decreasing partial pressures of argon during decompression consistent with the observation of atmospheric argon ratios in garnet omphacite and symplectite these data support an interpretation that the extraneous ar was incorporated early in the crystallization history of the eclogite and not from a late infiltrating fluid the ultimate source of the extraneous argon remains speculative however on the basis of supporting stable oxygen and hydrogen isotope data from this locality which support a closed isotopic system it is inferred to have been inherited from the protolith of the uhp eclogite

retrograde evolution of eclogites evidences from microstructures and 40ar 39ar white mica dates munchberg massif northern bavaria phengites from eclogites and pegmatites 3t 2m1 coarse grained and recrystallized of the munchberg massif weissenstein and oberkotzau have been dated by the 40ar 39ar method 3t micas from the eclogites yielded plateau and isochron ages of 365 7 ma 2m1 micas show disturbed degassing spectra micas from pegmatites show a slight excess ar component with an isochron age of 353 to 351 3 ma an age component of approximately 300 ma was also detected in combination with age values from the literature the cooling history of the munchberg massif from eclogite facies conditions 390 ma to cooling below 350 c 350 ma is documented the age component of 300 ma is attributed to a low grade stage of mineral growth accompanied by a transitional ductile brittle deformation the petrological effects include formation of pumpellyite prehnite facies minerals frequently precipitated in microcraks and cleavage planes of earlier formed minerals this stage has to be seen in conjunction with the intrusions of the fichtelgebirge granite

hydrogeochemistry of the new jersey coastal plain 2 transport and deposition of iron aluminum dissolved organic matter and selected trace elements in stream ground and estuary water the geochemical controls on the transport and deposition of iron organic carbon and certain trace elements have been examined in bogs streams groundwater and estuaries of the new jersey coastal plain surface waters are unusually dilute total dissolved solids 25 ppm and acidic ph 4 5 and contain relatively high concentrations of organic c and fe liquid chromatographic analyses show that from 10 to 70 of total dissolved fe 0 2 m fe oxyhydroxide or oxyhydroxides admixed with insoluble organic material the proportion of organically complexed fe present varies seasonally with the doc content of the water these stream waters are near saturation with respect to amorphous fe oh 3 groundwaters are uniformly undersaturated with respect to amorphous fe oh3 both stream and groundwaters are near saturation with respect to halloysite slightly supersaturated with respect to gibbsite and highly supersaturated with respect to kaolinite bog iron deposits are common in streams and swamps where fe precipitation is catalyzed by iron oxidizing bacteria fe and al precipitate rapidly within both estuaries with ionic strength rather than increased ph exerting the prime control on estuarine deposition estuarine floccules contain very little or no organic matter suggesting that inorganic fe and al oxyhydroxides are the primary precipitates the high contents of dissolved 0 2 m fe and al suggest that both are either organically complexed or in the form of small 0 2 m oxyhydroxide colloids in the estuaries

excess 40ar transport scale and mechanism in high pressure phengites a case study from an eclogitized metabasite of the dora maira nappe western alps application of the bulk separate furnace heating laser step heating and in situ laser agemapping 40ar 39ar techniques reveals that high pressure phengites from an alpine eclogite of the dora maira basement nappe western alps were pervasively contaminated with excess 40ar to yield ages 130 ma spot fusion dating of individual grains with the 40ar 39ar laserprobe reveals concentric radial patterns with core ages as high as 320 ma showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse the single grains and population from this hand sample retain major differences in apparent ages indicating that the extent of the excess 40ar contamination varies from grain to grain attempt to model these differences in terms of grain size dependent fractional loss falls short and demonstrates that strong cm scale argon isotopic gradients locally prevailed during syn kinematic high pressure metamorphism on petrostructural grounds a model of internal isotopic buffering is outlined whereby the excess 40ar is argued to have been internally derived from within the eclogite protolith itself low intergranular ar diffusion rates due to reduced water activity buffered below unity by the phengite assemblage resulted in the local segregation and partitioning of the excess 40ar along grain boundaries during ongoing ductile deformation supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in alpine eclogites and very high pressure metasediments which shows that volatile recycling during high pressure metamorphism may occur on a very limited scale

burial rates during prograde metamorphism of an ultra high pressure terrane an example from lago di cignana western alps italy abstract estimation of burial rates and duration of prograde metamorphism of ultra high pressure uhp rocks t 590 630 c p 2 7 2 9 gpa of the zermatt saas ophiolite from lago di cignana italy may be made through combined lu hf and sm nd garnet geochronology in conjunction with petrologic estimates of the prograde p t path we report a lu hf garnet omphacite whole rock isochron age of 48 8 2 1 ma from the uhp locality at lago di cignana which stands in contrast to the sm nd age of 40 6 2 6 ma amato et al earth planet sci lett 171 1999 425 438 obtained from the same sample and mineral material the sm nd and lu hf ages as well as other ages determined on metamorphic garnet zircon and white mica amato et al earth planet sci lett 171 1999 425 438 mayer et al eur union geosci 10 1999 abstr 809 rubatto et al contrib mineral petrol 132 1998 269 287 dal piaz et al int j earth sci 90 2001 668 684 from lago di cignana and elsewhere in the zermatt saas ophiolite lie within a range of 50 38 ma which we interpret to encompass the duration of prograde metamorphism and possibly the duration of garnet growth the difference in measured sm nd and lu hf ages from cignana can be accounted for by expected core to rim variations in lu hf sm and nd the measured yttrium content in garnet which may be a proxy for lu is highest in garnet core and lowest in the mineral rim generally following a profile that is predicted by rayleigh fractionation preferential enrichment of lu in the core produces a lu hf age that is weighted toward the older garnet core sm nd ages as predicted by rayleigh fractionation of sm and nd during garnet growth however reflect later grown garnet as compared to lu hf ages the difference in sm nd and lu hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as sm nd and lu hf blocking temperatures were not exceeded for a long period of time based on the 12 myr duration of prograde garnet growth estimated in this study burial rates for rocks at lago di cignana were on the order of 0 23 0 47 cm yr these values correlate with continuous shortening rates of 0 4 1 4 cm yr between the european plate and the african adriatic promontory between 50 and 38 ma which is on the order of that calculated for plate velocities from plate reconstructions suggesting that the zermatt saas ophiolite may have remained well coupled to the down going slab to uhp conditions

coaxial flattening at deep levels of orogenic belts evidence from blueschists and eclogites on syros and sifnos cyclades greece this work presents new structural data from a high pressure low temperature hp lt metamorphic terrane exposed on the islands of syros and sifnos cyclades greece the structure and the metamorphism of a relatively coherent hp lt rock section were studied in order to elucidate how strain was accommodated at deep crustal levels during the formation and exhumation of hp lt rocks at least three deformation phases associated with eclogite and blueschist facies conditions p 8 15 kbar t 400 550 degreesc were recognised the earliest deformation fabric s1 preserved as inclusion trails within garnet porphyroblasts is aligned to define a sub vertical schistosity at present orientation which is frequently orthogonal to the flat matrix schistosity s2 and may indicate that deep crustal thickening involved upright folding the currently dominant fabric in the hp rock section s2 is usually moderately dipping and locally contains nw trending glaucophane lineations symmetric pressure shadows and eclogitic boudins the symmetric structures associated with this fabric seem to indicate coaxial vertical thinning although the existence of non coaxial structures out of the study area cannot be excluded glaucophane bearing shear bands s3 with top to nw sense of shearing locally crosscut the earlier structures the latest recognised fabric d4 is scarce and often absent within the hp rocks it is associated with top to ne kinematic criteria that formed at greenschist facies conditions p 4 7 kbar t 400 450 degreesc based on these observations it is suggested that partitioning of strain occurred at different crustal levels and at different times deep crustal deformation was governed by thickening via upright folding followed by coaxial vertical thinning whereas non coaxial shearing occurred when the rocks were already exhumed to relatively shallow crustal levels the earliest fabrics d1 to d3 pertain to alpine orogenesis and possibly to syn orogenic extension whereas the latest correspond to whole crust back are extension c 2002 elsevier science ltd all rights reserved

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

prograde mica 40ar 39ar growth ages recorded in high pressure rocks syros cyclades greece abstract phengites from the eclogite and blueschist facies sequences of the cycladic island of syros greece have been dated by the in situ uv laser ablation 40 ar 39 ar method a massive phengite rich eclogite and an omphacite rich metagabbro were investigated the phengites are euhedral and coarse grained several 100 m strain free and exhibit no evidence for late brittle deformation or recrystallization apparent ages in these samples range from 43 to 50 ma for the phengite rich eclogite and 42 to 52 ma for the omphacitic metagabbro this large spread of ages is visible at all scales within individual grains as well as in domains of several 100 m and across the entire sample ca 2 cm such variations have been traditionally attributed to metamorphic cooling or the incorporation of excess argon however the textural equilibrium between the phengites and other high pressure phases and the subtle compositional variations within the phengites especially the preservation of growth textures alternatively suggest that the observed range in ages may reflect variations of radiogenic argon acquired during phengite formation and subsequent growth thus dating a discrete event on the prograde path this implies that the oldest phengite 40 ar 39 ar ages provide the best estimate of a minimum crystallization age which is in agreement with recently reported u pb and lu hf geochronological data our results are consistent with available stable isotope data and further suggest that under fluid restricted conditions both stable and radiogenic isotopic systems can survive without significant isotopic exchange during subduction and exhumation from eclogite facies p t conditions

fine scale isotopic heterogeneities and fluids in the deep crust a40ar 39ar laser ablation and tem study of muscovites from a granulite eclogite transition zone spectacular exposures on holsnoy island western norway reveal that eclogites formed in situ from adjacent anhydrous granulites as a result of the localized infiltration of fluids stepwise and laser40ar 39ar experiments on muscovite from the eclogites have been used in conjunction with electron microprobe and transmission electron microscopy tem analysis to examine the behavior of argon under high pressure metamorphism and subsequent cooling muscovites with 0 10 0 17na na k yield40ar 39ar plateau and integrated ages in the range 450 465 ma 20 35 myr older than40ar 39ar muscovite plateau ages from adjacent eclogite samples laser40ar 39ar analyses on 75 m spots across two single muscovite grains from these samples are approximately uniform at 444 2 1 9 21 spots and 443 3 1 7 ma 26 spots however incremental heating of high sodic 0 18 0 32 na na k muscovite failed to yield a plateau laser40ar 39ar analyses of 100 areas from two single grains from this sample reveal complex two dimensional patterns with variable apparent ages in the range of 445 625 ma electron microprobe measurements obtained from one of the grains reveal that the paragonite content is also highly variable both parallel and perpendicular to 001 all the tem bright field images obtained from this mica yield information consistent with a single homogeneous phyllosilicate phase yet several of the selected area electron diffraction saed analyses document locally intergrown muscovite and sodic muscovite possibly in an exsolution relationship on a unit cell scale analytical electron microscopy aem analyses obtained over a 5 100 nm scale on the same sample reveal great range and heterogeneity inna k including pure muscovite although no separate paragonite domains were found in contrast muscovite grains with consistent laser spot fusion and plateau ages show little heterogeneity inna k at the microprobe level these data provide evidence of variable fluid mediated incorporation of excess40ar under high pressure eclogite facies conditions at the time of the development of the sodic muscovite compositions the data indicate that fine scale heterogeneities in argon isotopic compositions were generated during eclogite facies equilibration and were preserved as the rocks were cycled back to the surface despite the influx of free fluids in the rocks at high pressure the heterogeneities are best reconciled with local buffering and limited mobility of the fluids at least during the final stages of equilibration

hydrogeochemistry of the new jersey coastal plain abstract the new jersey coastal plain pine barrens is an extensive region where inorganic interactions between the soils and shallow groundwater are at a minimum this study compares the geochemistry of precipitation and surface water to analyze transport mechanisms of major elements and to evaluate hydrogeochemical balances in this relatively unusual system precipitation composition is controlled by proximity to the ocean and seasonal variability of storm movements total dissolved solids in precipitation are high 11 5 ppm but decrease inland na and cl are more abundant in winter precipitation whereas k so 2 4 ca 2 and no 3 exhibit greater concentrations during the summer months na and cl are derived from marine aerosols so 2 4 is a product of so 2 pollution in the atmosphere other ions have significant contributions from continental aerosols dissolved solids in rivers are low 20 ppm compared with average river water and ph is acidic 4 5 both reflect paucity of mineral water reactions in the uppermost aquifer cohansey formation quartz sand and gravel however regional variations in river water geochemistry suggest cross aquifer migration of deep groundwaters into the mullica river basin mass balances confirm that river water composition is controlled principally by precipitation however ca 2 po 3 4 and so 2 4 are depleted in river waters probably as a result of removal by biota or cedar swamp processes mg 2 shows an excess in the rivers suggesting a contribution from deep groundwater the poor chemical buffering in the waters of this region render them highly susceptible to contamination

an aem tem study of nanometer scale mineral associations in an aquifer sand implications for colloid mobilization abstract analytical and transmission electron microscopy aem tem techniques were used to identify mineral juxtapositions at the nanometer scale in the interstitial matrix of a shallow southeastern coastal plain aquifer sand georgetown south carolina usa in doing so we sought to infer particle particle interaction mechanisms holding the matrix intact the aquifer is a fine to medium quartz sand with approximately 12 by weight

uplift and exhumation of high pressure metamorphic terrains the example of the cycladic blueschist belt aegean sea the cycladic blueschists emerged from a depth of more than 50 km during the tertiary these rocks are now covered only by a thin fraction of the synmetamorphic overburden upper plate remnants of the upper plate include low p crystalline rocks and sediments which have been juxtaposed onto the lower plate blueschists in several stages since the early miocene the low angle contacts operated as normal faults which removed the overburden from above the cycladic blueschists detailed investigations on tinos island indicate that the fractured blocks of the upper plate lying above a low angle normal fault did not rotate this shows that the faults initiated as rather gently dipping fractures similar structures were also reported from other blueschist terrains kinematic analysis of such flat lying faults implies that the synmetamorphic overburden should be mostly located several hundred kilometers away from the present exposure of the blueschists considering that the cycladic blueschist belt exposes successively shallower units in the west and that rocks of the upper plate have an anatolian nature we suggest that these blueschists were dragged from beneath turkey we recognize that the blueschist unit has undergone further thrusting in the final stage of the exhumation as have its counterparts elsewhere in the hellenides the emergence of the blueschists in the tertiary was concomitant with continental collision and thrusting in the hellenides the operation of low angle normal faults with large extensional displacements occurred within this context of collision tectonics this extension occurred parallel to the strike of the convergent orogen

structural development of high pressure metamorphic rocks on syros island cyclades greece abstract new data obtained during re mapping of the northern part of syros cyclades greece allow the refinement of existing concepts about the structural evolution of this island a metabasite belt and its metasedimentary envelope near kampos and san michali were mapped at a 1 5000 scale with special emphasis on the structural inventory the hp lt rocks p 1 5 gpa t 500 c at 50 ma exposed are intensely deformed by at least two isoclinal folding events relative age relations between deformation and peak metamorphism indicate that isoclinal folding took place before or during peak metamorphism later deformation stages include miocene non penetrative upright kink folding and crenulation transpressional strike slip faulting and open upright cylindrical folding followed by the development of steep normal faults taking into account recent zircon u pb geochronological constraints lithostratigraphic observations and published data on the neogene structural evolution of the aegean we present a history for the rocks of syros beginning with the formation of the oceanic crust represented by the metabasites to the present

colloid mobilization in two atlantic coastal plain aquifers field studies the geochemical mechanisms leading to the mobilization of colloids in groundwater were investigated in the pine barrens of new jersey and in rural central delaware by sampling pairs of wells screened in oxic and anoxic groundwaters in the same geologic formations samples were carefully taken at very low flow rates 100 ml min 1 to avoid suspending immobilized particles the colloidal matter was characterized by light scattering photometry scanning electron microscopy energy dispersive x ray analysis microelectrophoresis and fe al si and organic carbon analyses the colloids composed primarily of clays were observed at high concentrations up to 60 mg colloids l in the anoxic groundwaters while the oxic groundwaters exhibited 1 mg colloids l colloidal organic carbon was present in all groundwaters but under anoxic conditions one third to one half of the total organic carbon was associated with the inorganic colloids the field evidence indicates that anoxic conditions cause the mobilization of soil colloids by dissolving the ferric oxyhydroxide coatings cementing the clay particles to the aquifer solids the depletion of oxidized iron on the surfaces of immobile particles and the addition of organic carbon coatings on the soil particles and colloids apparently stabilizes the colloidal suspension in the anoxic groundwaters

map view restoration of aegean west anatolian accretion and extension since the eocene 1 the aegean region greece western turkey is one of the best studied continental extensional provinces here we provide the first detailed kinematic restoration of the aegean region since 35 ma the region consists of stacked upper crustal slices nappes that reflect a complex paleogeography these were decoupled from the subducting african adriatic lithospheric slab especially since 25 ma extensional detachments cut the nappe stack and exhumed its metamorphosed portions in metamorphic core complexes we reconstruct up to 400 km of trench perpendicular ne sw extension in two stages from 25 to 15 ma the aegean forearc rotated clockwise relative to the moesian platform around euler poles in northern greece accommodated by extensional detachments in the north and an inferred transfer fault se of the menderes massif the majority of extension occurred after 15 ma up to 290 km by opposite rotations of the western and eastern parts of the region simultaneously the aegean region underwent up to 650 km of post 25 ma trench parallel extension leading to dramatic crustal thinning on crete we restore a detachment configuration with the mid cycladic lineament representing a detachment that accommodated trench parallel extension in the central aegean region finally we demonstrate that the sakarya zone and cretaceous ophiolites of turkey cannot be traced far into the aegean region and are likely bounded by a pre 35 ma n s fault zone this fault became reactivated since 25 ma as an extensional detachment located west of lesbos island the paleogeographic units south of the izmir ankara sava suture however can be correlated from greece to turkey

flat plateau and impossible isochrons apparent 40ar 39ar geochronology in a high pressure terrain until recently most ages determined by the 40ar 39ar technique have been represented by the age spectrum and isochron early in the history of the technique a series of criteria were outlined in order to discriminate between spectra which were considered to reflect disturbed isotopic systems and spectra which were considered to reflect the accurate age of the rock or mineral with the advancement of 40ar 39ar instrumentation and the introduction of laser ablation techniques it is now possible to obtain an age from single grains and on a sub grain scale in this case there is no production of an age spectra or determination of a plateau age however isochrons are still readily produced and used in the discussion of ar isotope ratios and specifically excess argon here new data illustrate the possible drawback first of the age spectrum and plateau age in the absence of an independent geochronological constraint in high pressure terranes and second of the accompanying isochron which we demonstrate can be both incorrect and geologically meaningless

pressure temperature time evolution of the high pressure metamorphic complex of sifnos greece model studies of continental collision tectonics and pressure temperature time p t t predictions quantitatively demonstrate the key role of synsubduction uplift in the preservation of blueschist and eclogite facies metamorphic rocks rheological stratification of the subducting lithosphere allows development of detachment faults at the compositional boundary between the upper and lower crust material of supracrustal continental affinity is subducted to depths in excess of 50 km and then delaminated from the downgoing slab subduction continues while the detached domain is uplifted independently from the newly active hanging walls and footwalls two dimensional thermal calculations indicate that the nearby active subduction provides the cooling mechanism required to preserve blueschists and eclogites whereas the differential motions of the independently uplifted domains can explain the observed heterogeneity in p t t pathways with some domains displaying a greenschist overprint

magnetostratigraphy of the koobi fora formation lake turkana kenya the koobi fora formation a pliocene and pleistocene sequence of sedimentary deposits northeast of lake turkana has yielded numerous fossils and stone artifacts of early hominids stratigraphic correlation of the hominid bearing deposits throughout the turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies we have reinterpreted previously published magnetostratigraphy from the upper part of the koobi fora formation because the original stratigraphy and dating of tuffs have been revised in our reinterpretation we include previously unpublished data from the uppermost part of the formation the upper magnetozones correlate with parts of the brunhes normal polarity chron and matuyama reversed polarity chron about 0 6 0 85 ma and are separated from the magnetozones of the upper part of the matuyama 2 0 1 25 ma by a disconformity the olduvai normal polarity subchron is represented within the matuyama but the lower part of the matuyama 2 4 2 0 ma is missing due to an erosional disconformity we have also determined magnetozones in the lower part of the koobi fora formation which had not been sampled for paleomagnetism during the earlier studies our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of koobi fora tuffs with dated tuffs in the shungura formation of southern ethiopia the tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the koobi fora and shungura formations the lower part of the koobi fora spans the interval from about 4 ma to 2 4 ma within the gilbert reversed polarity and gauss normal polarity chrons rock magnetic studies indicate that detrital magnetite carries most of the stable remanence although hematite contributes to the remanence as indicated by thermal demagnetization the hematite which presumably formed by postdepositional oxidation of original iron oxide grains carries chemical remanent magnetization crm that diminishes the sharpness of polarity zone boundaries the crm accumulated continuously within the uppermost 10 m of sediment below the land surface so that the crm reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70 000 years in sections where the sedimentation rate was approximately 15 cm 1000 years

u pb zircon geochronology of unusual eclogite facies rocks from syros and tinos cyclades greece low temperature eclogite facies rocks from syros and tinos cyclades greece include meta igneous blocks of unclear origin meta olistostrome or tectonic melange with very high trace element concentrations e g zr up to 4850 ppm y up to 475 ppm the unusual geochemistry is considered to be the result of metasomatic alteration in a subduction zone environment locally metasomatic exchange with an ultramafic matrix further enhanced compositional anomalies a concordant u pb zircon age of 78 1 ma recorded for an omphacitite from syros is interpreted as the time of pre eocene high pressure metamorphism in the cyclades zircon dates of 61 and 63 ma for a jadeitite from tinos possibly indicate an additional discrete event deformation these results are a first geochronological indication that high pressure metamorphism in the cyclades commenced signif icantly earlier than indicated by previous white mica chronology which provided ages between c 50 and 40 ma

iron oxide coatings on sand grains from the atlantic coastal plain high resolution transmission electron microscopy characterization nanoscale characterization of so called iron oxide coatings on aquifer sands is vital for understanding the fate and transport of naturally occurring and anthropogenic chemical species these coatings which typically have a strong reddish color are commonly assumed to consist primarily of iron oxides and oxyhydroxides this work shows that the yellowish red to strong brown coatings on sediments from an atlantic coastal plain aquifer in virginia are predominantly a mixture of si and al rich nanophases of variable crystallinity with interspersed smectite and agglomerates of goethite nanoparticles clusters of bacterial cells diatom fossils sponge spicules and other trace minerals are also observed

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for