evolution of the sanbagawa and shimanto high pressure belts in sw japan insights from k ar ar ar geochronology abstract the sanbagawa belt sensu lato in japan is one of the world s classical high pressure hp metamorphic belts and has long been considered to be a coherent hp belt of the cretaceous age however recent progress in detrital zircon geochronology together with the accumulation of abundant phengites k ar ar ar ages revealed that the belt is comprised of two petrotectonic belts sanbagawa sensu stricto hp belt and shimanto hp belt these two belts have distinctly different ages of peak metamorphism and different p t conditions of metamorphism the sanbagawa schists sensu stricto were metamorphosed in the conditions of the pumpellyite actinolite facies through the epidote blueschist to epidote amphibolite facies and up to the eclogite facies during a prograde stage in a subduction zone at 120 115 ma phengite k ar geochronology revealed that the subsequent exhumation after the peak metamorphism took placed in a manner that the higher grade rocks exhumed more rapidly in comparison with the lower grade rocks this is supported by a positive correlation between age and apparent metamorphic gradient that have formed during the post metamorphic peak hydration recrystallization moreover the schists have experienced intense ductile deformation and long term exhumation it took longer than 31 m y to reset the phengite k ar system in contrast the shimanto hp schists were metamorphosed in the epidote blueschist greenschist transitional facies to the epidote amphibolite facies and have experienced a short term deformation for less than 13 m y this short deformation formed a negative correlation between k ar age and apparent metamorphic gradient these two contrasting age temperature relationships suggest different exhumation processes between the two hp belts most likely due to a change in subducting oceanic plates in the cretaceous along the paleo japanese convergent margin

downward particle fluxes in the guadiaro submarine canyon depositional system north western alboran sea a river flood dominated system abstract three moorings equipped with sediment traps were deployed in the north western alboran continental margin to study downward particle fluxes in the guadiaro submarine canyon depositional system this area is located close to the strait of gibraltar and is influenced by the upwelling induced by the atlantic jet and by episodic flood events from the guadiaro river sediment traps were installed in the guadiaro canyon in the guadiaro channel and in the adjacent continental slope the overall duration of the deployment was 12 months from november 1997 to october 1998 time series of downward particle fluxes major constituents organic carbon nitrogen biogenic opal calcium carbonate and lithogenics and 210pb were determined near the surface at mid depths and near the bottom total mass fluxes tmf in this area fluctuated more than two orders of magnitude and showed an important seasonal variability with higher fluxes in winter increases in tmf and lithogenics together with decreases in 210pb organic carbon and opal were recorded in all traps coinciding with river floods indicating a direct response of the system to these events and a rapid offshore transport of suspended matter affecting the entire water column the channel site received similar particle fluxes to the western open slope site indicating that this channel did not act as a preferential sediment conduit during the deployment period in the guadiaro canyon tmf were more than one order of magnitude higher 210pb concentration was lower one half and organic matter was more degraded than at the channel site during spring and summer as a consequence of receiving particles resuspended by internal waves and occasionally by trawling activities these particles were mainly retained in the canyon which works as a trap also during spring and summer the opal and organic carbon percentages increased in all traps both in magnitude and variability and peaks seem to be associated with biological blooms

k ar age and chemistry of phengite from the sanbagawa schists in the kanto mountains central japan and their implication for exhumation tectonics abstract epma analyses and k ar age determinations were carried out on phengite in pelitic schist from the sanbagawa metamorphic belt of the kanto mountains central japan phengite from the sanbagawa pelitic schist in the kanto mountains generally occurs as aggregates of fine grained crystals it is extremely fine grained in domains adjacent to relatively rigid garnet and albite porphyroblasts this suggests that deformation induced grain size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists epma analysis shows that phengite is chemically heterogeneous at the thin section scale suggesting that it formed during retrograde metamorphism in restricted equilibrium domains the retrograde chemical reaction was promoted by the ductile deformation k ar ages of phengite get younger from the southern unit 82 ma to the northern unit 58 ma in the kanto mountains the age range is similar to that in central shikoku the older schists occur in the higher metamorphic grade zone in central shikoku and in the lower grade zone in the kanto mountains the thermal structures in central shikoku are inverted so that the highest grade zone occurs in the upper or middle parts of the apparent stratigraphic succession in contrast the kanto mountains have a normal thermal structure the higher grade zone is in the lower part of the apparent stratigraphic succession the different tectonic features in exhumation produced the two contrasting age temperature structure relations at the western side of sanbagawa belt in central shikoku and the eastern end of the sanbagawa belt in the kanto mountains that are 800 km distant from each other namely the western sanbagawa belt in central shikoku underwent longer ductile deformation during the exhumation than the eastern sanbagawa belt in the kanto mountains

the oldest uhp eclogites of the world age of uhp metamorphism nature of protoliths and tectonic implications coesite bearing eclogitic rocks from the pan african belt in northern mali were dated by multichronological methods rb sr sm nd and ar ar rb sr and sm nd isotope analyses on whole rock and mineral separates of an omphacite kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 ma this is interpreted as the time of the eclogitisation which represents the oldest ultrahigh pressure uhp metamorphic event so far recorded in the continental crust the nearly identical rb sr and sm nd ages suggest very rapid exhumation as observed in many phanerozoic uhp rocks 40ar 39ar dating of phengites from two uhp micaschists gave much older ages of 1045 and 760 ma phengite from a quartzite collected from the same site yielded a plateau date of 623 3 ma which is in agreement with the rb sr and sm nd dates the two 40ar 39ar ages older than 625 ma testify once more the recurrent problem of excess ar in ultrahigh pressure phengites the occurrence of the mali uhp eclogites indicates that by the late precambrian the earth has cooled enough to sustain the formation and preservation of deeply subducted uhp metamorphic rocks the new ages of ca 620 ma also constrain the amalgamation in nw gondwana to have occurred in the latest proterozoic but not in the middle cambrian this is supported by several recent chronological data for the southern segment of the major suture zone in the dahomeyide orogenic belt

age k ar phengite temperature structure relations a case study from the ishigaki high pressure schist belt southern ryukyu arc japan the ishigaki high pressure schist belt in the southern ryukyu arc is correlated with the suo high pressure schist belt in southwest japan the former metamorphic sequence is composed mainly of basic and pelitic schists and is subdivided into three zones the lower grade zone a the medium grade zone b and the high grade zone c based on the mineral assemblages of the basic schists the k ar phengite age gives 188 205 ma for zone a 196 206 ma for zone b and 208 220 ma for zone c while the apparent d 002 spacing of carbonaceous materials is 3 590 3 437 a 3 415 3 390 a and 3 387 3 364 a respectively the age d 002 relationships suggest that the ages become older with increasing metamorphic temperature this positive age temperature relationship in the ishigaki area contrasts with a negative relationship in the nishiki area in the suo belt the two areas also display a contrasting thermal structure with the former area having an inverted metamorphic gradient and the latter displaying a normal thermal structure these contrasting age temperature structure relationships in the metamorphic belt could be due to different tectonic styles relating to the exhumation of the metamorphic sequences we suggest that the ages obtained are related directly to the ductile deformation history of the matrix phengite below the closure temperature 500 c during exhumation of the host rocks the duration from the beginning of exhumation to the apparent resetting of the phengite k ar system was different between the two metamorphic sequences and significantly longer in the ishigaki than the nishiki

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

impact of suboxia on sinking particulate organic carbon enhanced carbon flux and preferential degradation of amino acids via denitrification fluxes of particulate organic carbon poc through the oxygen deficient waters in the eastern tropical north pacific were found to be relatively less attenuated with depth than elsewhere in the eastern north pacific the attenuation coefficient b for the flux was found to be 0 40 versus the composite value of 0 86 determined by martin et al 1987 to examine this further sinking poc was collected using sediment traps and allowed to degrade in oxic and suboxic experiments using a kinetic model it was found that degradation proceeded at similar rates roughly 0 8 day 1 under oxic and suboxic conditions but a greater fraction of bulk poc was resistant to degradation in the suboxic experiments 61 vs 23 amino acids accounted for 37 of poc collected at 75m but following degradation the value dropped to 17 and 16 in the oxic and suboxic experiments respectively poc collected from 500m was 10 amino acids the non aa component of poc collected at 75m was not degraded under suboxic conditions while under oxic conditions it was these results suggest that microbes degrading oc under suboxic conditions via denitrification preferentially utilize nitrogen rich amino acids this preferential degradation of amino acids suggests that 9 more nitrogen may be lost via water column denitrification than is accounted for when a more redfieldian stoichiometry for poc is assumed

chemical features of white micas from the piemonte calc schists western alps and implications for k ar ages of metamorphism abstract anomalously large chemical ranges in muscovite paragonite and muscovite celadonite systems are observed in white micas from the piemonte calcschists in the chisone valley area internal western alps the petrographical and chemical observations on white mica strongly suggest that most mica crystals with high na k ratios in the chlorite zone are of detrital origin and were derived from the pre alpine high temperature metamorphic sequence such the caledonian and or variscan submicroscopic muscovite ms paragonite pg composite aggregates occur in the chlorite zone and their epma analyses give an apparent chemical composition range from ms 0 6 pg 0 4 to ms 0 2 pg 0 8 in the rutile zone the paragonite content of the white micas is less than 20 suggesting that the white micas have been homogenized during the alpine metamorphism even if detrital white micas existed metamorphic mica is also very heterogeneous the total range in si content becomes wider with increasing of metamorphic grade 3 22 3 39 pfu for the chlorite zone 3 07 3 45 pfu for the chloritoid zone and 3 06 3 59 pfu for the rutile zone this clearly indicates that the micas have experienced significant retrogressive chemical reactions during cooling and exhumations of the host schists the detrital white mica in the chlorite zone has not reset well in its k ar system during the alpine subduction related metamorphism the wide range of the white mica k ar ages from 115 to 41 ma must be due to a mixture of various amounts of detrital white mica in the separates this feature is also observed in the chloritoid zone though the age variation is not so large as that in the chlorite zone in contrast the mica in the rutile zone which was higher than 450 c has been reset completely during alpine hp metamorphism

evidence for protracted prograde metamorphism followed by rapid exhumation of the zermatt saas fee ophiolite major and trace element zoning in garnet in combination with rb sr sm nd and lu hf geochronology provide evidence for a protracted garnet growth history for the zermatt saas fee zsf ophiolite western alps four new lu hf ages from pfulwe c 52 46 ma and one from chamois c 52 ma are very similar to a previously published lu hf age from lago di cignana overall the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history commensurate with their similar structural position near the top of the zsf samples from the lower part of the zsf at saas fee and st jacques however produced much younger lu hf ages c 41 38 ma neither differences in whole rock geochemistry which might produce distinct garnet growth histories nor rare earth element zoning in garnet can account for the age differences in the two suites this suggests a much later prograde history for the lower part of the zsf supporting the idea that it was subducted diachronously such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions where early oblique subduction which produced long prograde garnet growth changed to more orthogonal subduction which corresponds to shorter prograde garnet growth six new rb sr phengite ages range from c 42 to 39 ma and in combination with previously published rb sr ages constrain the timing of the transition from eclogite to upper greenschist facies p t conditions the proximity of the zsf in the saas fee region to the underlying continental monte rosa unit and the similarity of peak metamorphic ages suggest these two units were linked for part of their tectonic history this in turn indicates that the monte rosa may have been partly responsible for rapid exhumation of the zsf unit

metamorphic evolution of the sesia lanzo zone western alps time constraints from multi system geochronology the sesia lanzo zone is a polymetamorphic unit containing hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the alpine orogeny different parts of the unit record different stages on the p t deformation evolution allowing multi system isotopic studies to unravel the precise timing of the metamorphic history new rb sr white mica and u pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60 70 ma this substantially alters the common view of early to mid cretaceous eclogite facies metamorphism in this unit the new results are more consistent with the established geotectonic framework for the alpine orogeny since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension it is also more concordant with recent studies of other units that demonstrate post cretaceous high pressure metamorphism step heated 40ar 39ar analysis of phengites yields good plateaux giving ages older than the corresponding rb sr age such anomalously high ages indicate the presence of radiogenic argon rich fluids in the grain boundary network under the fluid pressure conditions acting during this high pressure metamorphic event the u pb sphene ages are variable in polymetamorphic rocks and show inheritance of older pb or sphene crystals into the high pressure event two monometamorphic assemblages yield concordant ages at 66 1 ma reflecting crystallisation of the eclogite facies assemblage the gneiss minuti complex gmc lies structurally below the eclogitic micaschists and its pervasive greenschist facies fabric yields tightly clustered rb sr white mica ages at 38 39 ma this greenschist event did not affect the majority of the emc the 40ar 39ar ages of micas formed at this time were very disturbed whereas micas surviving from an earlier higher pressure assemblage had their 40ar 39ar system reset the greenschist event did not strongly affect u pb systematics in hercynian age sphenes suggesting that the gmc did not uniformly suffer an eclogite facies metamorphism during the alpine cycle but was juxtaposed against the emc later in the orogeny this model still requires that the locus of deformation and metamorphism and possibly fluid flux moved outboard with time leaving the sesia lanzo basement as a shear bounded unreactive block within the orogenic wedge

prograde high to ultrahigh pressure metamorphism and exhumation of oceanic sediments at lago di cignana zermatt saas zone western alps pelagic metasediments and morb type metabasalts of the former tethyan oceanic crust at cignana valtournanche italy experienced uhp metamorphism and subsequent exhumation during the early to late tertiary maximum pt conditions attained during uhp metamorphism were 600 630 c 2 7 2 9 gpa which resulted in the formation of coesite glaucophane eclogites in the basaltic layer and of garnet dolomite aragonite lawsonite coesite phengite bearing calc schists and garnet phengite coesite schists with variable amounts of epidote talc dolomite na pyroxene and na amphibole in the overlying metasediments during subduction the rocks followed a prograde hp uhp path which in correspondance with the jurassic age of the tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle plastic transition in the quartz aragonite dominated rocks induced convection in the asthenospheric mantle wedge and or heat consumption by endothermic reactions over a restricted pt segment during subduction after detachment from the downgoing slab some 50 70 ma before present the cignana crustal slice was first exhumed to ca 60 km and concomitantly cooled to ca 550 c tracing back the uhp hp prograde path displaced by 50 80 c to higher temperatures exhumation at this stage is likely to have occurred in the benioff zone while the subduction of cool lithosphere was going on subsequently the rocks were near isothermally exhumed to ca 30 km followed by concomitant decompression and cooling to surface conditions at 500 c 1 gpa during this last stage the uhpm slice arrived at its present tectonic position with respect to the overlying greenschist facies combin zone in contrast to the well preserved hp uhpm record of the coesite glaucophane eclogites the hp uhp assemblages of the metasediments have been largely obliterated during exhumation relics from which the metamorphic evolution of the rocks during prograde hp metamorphism and the uhp stage can be retrieved are restricted to rigid low diffusion minerals like garnet dolomite tourmaline and apatite

pressure effects on the biological degradation of organo mineral aggregates in submarine canyons organo mineral aggregates are more easily remobilized in the benthic boundary layer than in the sediment surface beneath playing a crucial role in the transport of organic material from the continental shelf to the deep ocean floor while being transported to greater depths these aggregates are subjected to increasing hydrostatic pressure which translates into a metabolic inhibition of the bacterial community colonizing them the reduced microbial activity on these aggregates may allow relatively labile organic matter to arrive at the deep ocean floor in submarine canyons the speed of transport might translate into a greater percentage of labile organic matter to fuel deep sea food webs and in a larger amount of carbon being sequestered in the deep ocean the effect of increased pressure on benthic boundary layer aggregates is virtually unknown particularly due to the methodological difficulties inherent to these studies through the use of a new pressure laboratory the a proach adaptive pressurized ocean analysis chamber we studied the effects of pressure in organic matter degradation in organo mineral aggregates from two different canyon systems in the west coast of portugal the nazare canyon and the setubal canyon our results suggest that if the downward transport is fast the hydrostatic pressure can inhibit the microbial community reducing the degradation of organic matter this effect varies among different canyons probably due to different microbial communities and is highly dependent on the rate of down slope transport the stronger the canyon s down slope currents the more effectively the system will act as a sink for labile carbon the a proach can be a valuable instrument for the study of deep sea microbial communities under realistic conditions generating appropriate thermodynamic and hydrodynamic conditions

preservation of old prograde metamorphic u th pb ages in unshielded monazite from the high pressure paragneisses of the variscan ulten zone italy abstract three samples of garnet kyanite paragneiss from the variscan ulten zone northern italy were studied in detail for u th pb monazite dating monazite in these gneisses is abundant shows highly variable grain size and occupies different textural positions within the matrix as inclusion in garnet and kyanite within apatite aggregates monazite shows different deformation features as a function of the textural position enclosed shielded monazite is generally more fractured than matrix unshielded monazite the integration of textural information with deformation features and in situ u th pb analyses by la icp ms indicates that there is no direct correlation between textural site and monazite ages old ages of 351 343 ma determined on portions of large matrix unshielded monazite and on rare domains of monazite shielded by garnet have been related to a prograde stage of the variscan metamorphic evolution of the ulten zone ages of 330 326 ma which are related to the thermal peak are recorded by small matrix monazite external domains of large matrix monazite and by domains of fractured monazite enclosed in garnet and kyanite large old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation re crystallisation partially occurred

continuous metamorphic gradient documented by graphitization and k ar age southeast otago new zealand the chrystalls beach brighton coastal section southwest of dunedin new zealand exposes otago schist including the weakly metamorphosed triassic accretionary melange of the chrystalls beach complex and higher grade rocks of a largely felsic arc derived provenance melange zones contain pelagic sediments some of which are manganiferous and metabasites the grade of metamorphism increases progressively from southwest to northeast four mineral zones are recognized largely on the basis of mineral assemblages in psammitic and semipelitic rocks pumpellyite chlorite pumpellyite actinolite epidote actinolite and biotite d 002 data for carbonaceous material b 0 values for phengite and metamorphic phengite k ar ages are reported for 32 pelitic and semipelitic samples remarkably tight correlations are revealed between d 002 for progressively graphitized carbonaceous material k ar ages that range from 184 6 to 138 7 ma and linear distance along the section both the d 002 values and the k ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes all rocks cropping out in the section have undergone a coherent episode of progressive metamorphism associated with terrane collision and have not been disturbed by any later major displacements a convex trend for phengite b 0 values plotted against advancing graphitization suggests p t conditions during metamorphism as in the high pressure intermediate facies series the age of the metamorphic peak is inferred to be 175 155 ma middle to late jurassic with cooling to the mica closure temperature at 155 135 ma late jurassic to early cretaceous unloading denudation and cooling were accompanied by localized hydrothermal events the oldest covering strata were deposited at ca 100 ma and denudation was completed by ca 75 80 ma middle triassic radiolarian nodules in melange zones are dated as preceding the metamorphic peak by about 65 ma associated turbidites contain suspected late middle to early late triassic tube fossils

excess 40ar free phengite in ultrahigh pressure metamorphic rocks from the lago di cignana area western alps abstract ar ar analyses of phengites and paragonites from the ultrahigh pressure metamorphic rocks zoisite clinozoisite schist garnet phengite schist and piemontite schist in the lago di cignana area western alps were carried out with a laser probe step heating method using single crystals and a spot dating method on thin sections eight phengite and two paragonite crystals give the plateau ages of 37 42 ma with 96 100 of 39 ar released each rock type also contains mica crystals showing discordant age spectra with age fractions 20 35 ma significantly younger than the plateau ages phengite inclusions in garnet give ages of 43 2 1 1 ma and 44 4 1 5 ma which are significantly older than the spot age 36 4 1 4 ma from the matrix phengites and the plateau ages from the step heating analyses inclusion ages 43 and 44 ma are consistent with a zircon shrimp age 44 1 ma in this area these results suggest that the oceanic materials that underwent a simple subduction related uhpm form excess 40 ar free phengite and that the peak metamorphism is ca 44 ma or little older we suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40 ar from the crystals this has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age

effects of fluid rock interaction on 40ar 39ar geochronology in high pressure rocks sesia lanzo zone western alps abstract in situ uv laser spot 40ar 39ar analyses of distinct phengite types in eclogite facies rocks from the sesia lanzo zone western alps italy were combined with sims boron isotope analyses as well as boron b and lithium li concentration data to link geochronological information with constraints on fluid rock interaction in weakly deformed samples apparent 40ar 39ar ages of phengite cores span a range of 20 ma but inverse isochrons define two distinct main high pressure hp phengite core crystallization periods of 88 82 and 77 74 ma respectively the younger cores have on average lower b contents 36 g g than the older ones 43 48 g g suggesting that loss of b and resetting of the ar isotopic system were related phengite cores have variable 11b values 18 to 10 indicating the lack of km scale b homogenization during hp crystallization overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40ar 39ar ages than the respective cores they also show variable effects of heterogeneous excess 40ar incorporation and ar loss one acceptable inverse isochron age of 77 1 1 1 ma for rims surrounding older cores 82 6 0 6 ma overlaps with the second period of core crystallization compared to the phengite cores all rims have lower b and li abundances but similar 11b values 15 to 9 reflecting internal redistribution of b and li and internal fluid buffering of the b isotopic composition during rim growth the combined observation of younger 40ar 39ar ages and boron loss yielding comparable values of both parameters only in cores and rims of different samples is best explained by a selective metasomatic overprint in low permeability samples this overprint caused recrystallization of phengite rims whereas higher permeability in other samples led to complete recrystallization of phengite grains strongly deformed samples from a several km long blueschist facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent 40ar 39ar ages 64 7 68 8 ma yielding an inverse isochron age of 65 0 3 0 ma almost complete b and li removal in mylonitic phengite is due to leaching into a fluid the b isotopic composition is significantly heavier than in phengites from the weakly deformed samples indicating an external control by a high 11b fluid 11b 7 4 we interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone this event also caused partial resetting of the ar isotope system and further b loss in more permeable rocks of the adjacent unit we conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40ar 39ar data in partially metasomatized rocks

40ar loss in experimentally deformed muscovite and biotite with implications for 40ar 39ar geochronology of naturally deformed rocks abstract the effects of deformation on radiogenic argon 40ar retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite cylindrical cores 15 mm in length and 6 25 mm in diameter were drilled from granite collected from the south armorican massif in northwestern france loaded into gold capsules and weld sealed in the presence of excess water the samples were deformed at a pressure of 10 kb and a temperature of 600 c over a period 29 of hours within a solid medium assembly in a griggs type triaxial hydraulic deformation apparatus overall shortening in the experiments was approximately 10 transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking cracking and grain segmentation of the micas infrared ir laser co2 heating of individual 1 5 2 5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well defined 40ar 39ar plateau ages of 311 2 ma 2 identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40ar loss of 0 35 in muscovite and 2 3 40ar loss in biotite intragrain in situ ultraviolet uv laser ablation 40ar 39ar ages 4 10 1 of deformed muscovites range from 309 13 to 264 7 ma consistent with 0 16 40ar loss relative to the undeformed muscovite the in situ uv laser ablation 40ar 39ar ages of deformed biotite vary from 301 to 217 ma consistent with up to 32 40ar loss no spatial correlation is observed between in situ 40ar 39ar age and position within individual grains using available argon diffusion data for muscovite the observed 40ar loss in the experimentally treated muscovite can be utilized to predict average 40ar diffusion dimensions maximum 40ar 39ar ages obtained by uv laser ablation overlap those of the undeformed muscovite indicating argon loss of

exhumation of ultrahigh pressure metamorphic oceanic crust from lago di cignana piemontese zone western alps the structural record in metabasites the metamorphic and deformational history of coesite bearing ultrahigh pressure metamorphic oceanic crust from lago di cignana valtoumanche western alps as recorded along the decompression and cooling path has been resolved to derive constraints on the physical state of the crust during exhumation the following history is proposed eclogites derived from pillow basalts were deformed by dislocation creep of pyroxene at a depth of more than 80 km in the stability field of coesite the deformation was progressively localised into shear bands over the first 40 km of subsequent exhumation the rocks remained essentially unmodified any deformation must have been localised beyond the limits of the present outcrop retrogression first in a closed system started without concomitant deformation at a depth of about 40 km and temperatures of 550 to 500 c subsequently the system size increased fluid infiltration focussed along tensile fractures is documented by the formation of veins vein shapes and orientations indicate high pore fluid pressures and low differential stress after cooling to about 350 450 c at depths of 12 to 18 km localised ductile deformation transformed the retrogressed eclogites into greenschists for that stage the microstructure of the quartz veins indicates deformation by dislocation creep at falling temperatures and increasing differential stress final cooling to below ca 300 c took place at depths of 7 to 12 km as indicated by the density of fluid inclusions later deformation in the brittle field cannot be correlated with the p t path

transport of persistent organic pollutants by organo mineral aggregates omas in the lisboa setubal canyon system abstract in this work we investigate the role of organo mineral aggregates omas in the transport and deposition of persistent organic pollutants in the lisboa setubal canyon system omas are more easily remobilized than the underlying sediment surface and are easily transported in the benthic boundary layer since aquatic sediments act as sinks for hydrophobic organic contaminants omas must be a factor in the lateral transport of organic contaminants submarine canyons further facilitate the flux of these contaminants from source to deep sea our results show that the difference between the behavior of the omas and the underlying superficial sediment is very important in the transport of carbon and persistent organic pollutants to the deep sea the lisboa setubal canyon system is clearly exporting organic pollutants to the deep sea through the transport of omas

preservation of sub microscopic structural relicts in micas from the gran paradiso massif western alps implications for 40ar 39ar geochronology abstract permian igneous biotite and white mica that were re heated at 500 550 c in the middle late eocene were investigated by laser step heating and in situ 40ar 39ar techniques in conjunction with scanning electron microscopy sem electron microprobe emp and transmission electron microscopy tem to assess the influence of short lived metamorphism on k ar systematics large intra and inter grain age variations ranging from the triassic to the late eocene are primarily related to the extent of metamorphic re equilibration brown biotite porphyroclasts from weakly re equilibrated samples are characterized by core to rim zoning in major element composition mineral structure and 40ar 39ar ratios titanium concentration and fe mg fe ratios decrease from crystal cores towards the rims which are compositionally indistinguishable from green biotite aggregates in metamorphic coronas compositional changes are coupled with structural modifications towards the crystal edges where the disordered igneous 1m 2m1 stacks are replaced topotactically by highly ordered metamorphic 1m polytypes apparent ages up to 66 ma were determined in crystal cores by in situ laser probe analyses while ages as young as 45 ma are typical of crystal rims step heating experiments of these zoned biotites yielded a discordant saddle shaped age spectrum with a concordant central segment yielding an error weighted mean age of 44 9 0 3 ma biotite porphyroclasts from a more re equilibrated specimen gave markedly different results with uniform major elements compositional profiles homogeneous argon distribution throughout the crystals and a flat age spectrum at 36 5 0 3 ma tem investigations of these optically homogeneous porphyroclasts revealed that the original igneous structure had been replaced by highly ordered 1m polytypes during alpine metamorphism white mica yielded comparable results with spot ages up to 218 ma in domains characterized by high na na k ratios and low si and ti content representing the best preserved igneous relicts significantly younger apparent ages down to 44 39 ma are typical of domains affected by partial metamorphic re equilibration at high pressure conditions as indicated by increasing si and ti contents or by exhumation related exsolution of amoeboidal quartz these results indicate that different intra grain and inter grain ar concentration patterns arise primarily from the relative amount and distribution of relict igneous and metamorphic crystal chemical domains core to rim age gradients in biotite result from inward directed recrystallization from the crystal edges eventually leading to flat age profiles as metamorphic replacement of pre existing porphyroclasts runs to completion this study calls for caution when estimating thermal histories from apparent age gradients or from age vs grain size correlations unless the presence of pre metamorphic mineral relicts can be ruled out

white mica k ar geochronology of hp uhp units in the lago di cignana area western alps italy tectonic implications for exhumation abstract high pressure and ultra high pressure hp uhp blueschist and eclogite facies metabasaltic and metasedimentary rocks occur in four different tectonic units near lago di cignana western alps we have determined k ar ages for white micas matrix phengite and paragonite from the lago di cignana uhp unit lcu 39 41 ma the lower and upper units of the zermatt saas meta ophiolite lu and uu 37 38 ma and 38 41 ma respectively and the combin unit cu 36 40 ma these k ar ages overlap with single grain ar ar plateau ages 36 42 ma previously determined for phengites from lcu metasediments matrix white micas have been severely deformed during exhumation and their chemistries differ from those of micas included in garnet although individual mica grains in the matrix could have experienced different degrees of deformation which have reset their k ar systems bulk white mica separates provide the average age of all the individual grains in the separate the similarity of ages determined for white micas from the lcu lu uu and cu units regardless of rock type and mineral species suggests that these four units were metamorphosed together as part of a single metamorphic sequence in the piemonte liguria paleosubduction zone and were subsequently exhumed together however present day structural relationship among those units and the limited occurrence of uhp minerals in lcu suggests that the exhumation of lcu was more rapid than that for lu uu and cu the age gaps between the youngest value of white mica k ar ages in each unit and the inferred timing of the metamorphic peak u pb age 44 ma is 5 7 6 and 8 myr for lcu lu uu and cu respectively these intervals are considerably shorter than that determined for the sanbagawa hp metamorphic belt of southwest japan 31 myr the short interval observed for the lago di cignana units that we have studied is consistent with the model of rapid exhumation of the uhp bearing metamorphic domain suggesting the exhumation rate is higher than 18 mm y in the early stage of exhumation from the deepest level ca 120 km to the lower crust ca 30 km

ultrahigh pressure metabasaltic garnets as probes into deep subduction zone chemical cycling 1 we demonstrate an approach to examining the metamorphic history of subducting oceanic crust that can complement records of subduction zone chemical cycling derived from studies of igneous rocks produced at volcanic arcs by merging methods utilizing garnet zoning to establish prograde reaction histories with in situ high resolution trace element geochemistry and application to coesite bearing mafic eclogites representing subduction to depths beneath arcs we are able to directly identify geochemical manifestations of reactions contributing to element mobility in the subducting slab that are only inferred in studies of volcanic arcs or theoretical metamorphic models specifically we identify a prograde metamorphic reaction based solely on the zoning of geochemistry and mineral inclusions within garnet and infer that these features are a record of the breakdown of coexisting clinozoisite titanite and probable liberation of trace element laden fluid from the rock during prograde metamorphism we are then able to assign a specific depth interval for the reaction through calculation of the p t dependence of the reaction for these eclogites and comparison with a published p t trajectory because of the robust preservation of records of petrologic and geochemical processes by garnet this methodology is particularly suited for study of ultrahigh pressure uhp eclogites in which severe retrograde alteration generally related to exhumation commonly obscures prograde history

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

dating of ultramafic rocks from the western alps ophiolites discloses late cretaceous subduction ages in the zermatt saas zone the zermatt saas zone was part of the middle to late jurassic tethyan lithosphere that underwent oceanic metamorphism during mesozoic time and subduction during eocene time hp to uhp metamorphism in upper valtournanche serpentinite metarodingite and eclogite record a dominant s2 foliation that developed under 2 5 0 3 gpa and 600 20 c during alpine subduction serpentinites contain clinopyroxene and rare zircon porphyroclasts clinopyroxene porphyroclasts show fringes within s2 with similar compositions to that of grains defining s2 zircon cores show zoning typical of magmatic growth and thin fringes parallel to the s2 foliation these features indicate crystallization of clinopyroxene and zircon fringes during hp syn d2 metamorphism related to the alpine subduction the u pb zircon dates for cores and fringes reveal crystallization at 165 3 2 ma and 65 5 5 6 ma respectively the middle jurassic dates are in agreement with the known ages for the oceanic accretion of the tethyan lithosphere the late cretaceaous paleocene dates suggest that the zermatt saas zone experienced high pressure to ultra high pressure hp uhp metamorphism at c 16 ma earlier than previously reported this result is in agreement with the evidence that in the western alps the continental sesia lanzo zone reached the subduction climax at least from 70 ma and was exhumed during ongoing oceanic subduction our results are further evidence that the zermatt saas ophiolites diachronically recorded heterogeneous hp uhp metamorphism

effect of physical variables on particle critical erosion shear stress hydrostatic pressure slope and changes in water density abstract in recent years the importance of organo mineral aggregates omas in the transport of organic material from the continental shelf to the deep ocean floor has been recognized attempts to model the transport of these omas rely on measurements of several physical parameters such as the critical erosion shear stress however due to technical constraints such measurements are not done in situ making it difficult to assess the impact of environmental parameters on the measurements made and their accuracy in representing the study site changes in hydrostatic pressure slope angle and water density potentially can affect the critical erosion shear stress of omas using new methodologies we found no measurable effect of hydrostatic pressure variation on the omas resuspension behavior changes in slope angle resulted in the decrease of u cri up to 35 the u cri decreased by 9 with an exchange with denser water and it increased by 6 with an exchange with lighter water these changes were temporary and after a 3 h stabilization the u cri returned to the original value we will discuss the future applications of these results on modeling of particle transport

ultrarapid exhumation of ultrahigh pressure diamond bearing metasedimentary rocks of the kokchetav massif kazakhstan the diamond bearing ultrahigh pressure kokchetav massif recrystallized at eclogite facies conditions deep in the mantle at 180 km depth at 535f3 ma and yet new 40 ar 39 ar ages suggest that it may have been exhumed to crustal depths as indicated by closure of mica to ar loss by f529 ma these data indicate a possible exhumation rate of tens of kilometers per million years i e a rate that is comparable to rates of horizontal plate motion and subduction d 2003 elsevier b v all rights reserved

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

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

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

tertiary remagnetization of paleozoic rocks from the eastern cordillera and sub andean belt of bolivia paleomagnetic samples were collected from 98 sedimentary horizons in eight different devonian to permian sedimentary units at eight localities in the eastern cordillera and the sub andean belt of bolivia for 77 sites thermal demagnetization allowed determination of a characteristic magnetization chrm with site mean 95 confidence limit 95 15 the chrm is carried predominantly or entirely by hematite fold and reversal tests from two of the sampled localities indicate that the characteristic magnetization is synfolding likely acquired during the earliest stages of deformation additionally a modified conglomerate test at one locality and the nearly uniform direction of chrm across the devonian to permian age units clearly reveals the secondary nature of the characteristic magnetization finally the chrm directions are discordant from any expected paleozoic directions paleomagnetic poles calculated from the chrm directions fall near the cenozoic portion of the apparent polar wander path for south america we interpret these observations to indicate widespread chemical remagnetization of these paleozoic strata during but prior to completion of cenozoic andean folding

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

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

muscovite k ar ages of the sanbagawa schists japan and argon depletion during cooling and deformation seventy muscovites from schists in the sanbagawa terrain in central shikoku were dated by the k ar method the muscovite ages are consistently older with increasing metamorphic grade within the same zone the ages are significantly younger in schists which have been more severely deformed these k ar age variations could be due to systematic argon depletion during deformation i e to the dynamic recrystallization of muscovites during ductile deformation that formed a large scale recumbent fold during the uplift and cooling argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low temperature deformation than the higher grade zone the relationships between age and grain size in a pelitic schist suggest that coarse grained muscovites lost more argon than the finegrained ones there was no significant resetting of ages in the vicinity of major strike slip faults such as the median tecotonic line or near thrust faults the combination of geochronological and geological data constrains the cooling rate of the sanbagawa schists to 9 12 c ma in the oligoclase biotite zone in central shikoku japan

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

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

cretaceous isochron ages from k ar and 40ar 39ar dating of eclogitic rocks in the tso morari complex western himalaya india abstract the tso morari complex which is thought to be originally the margin of the indian continent is composed of pelitic gneisses and schists including mafic rock lenses eclogites and basic schists eclogites studied here have the mineral assemblage grt omp ca amp zo phn pg qtz rt they also have coesite pseudomorph in garnet and quartz rods in omphacite suggesting a record of ultrahigh pressure metamorphism they occur only in the cores of meter scale mafic rock lenses intercalated with the pelitic schists small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism the garnet omphacite thermometry and the univariant reaction relations for jadeite formation give 13 21 kbar at 600 c and 16 18 kbar at 750 c for the eclogite formation using the jadeite content of clinopyroxene x jd 0 48 phengites in pelitic schists show variable si al and na k ratios among grains as well as within single grains and give k ar ages of 50 87 ma the pelitic schist with paragonite and phengite yielded k ar ages of 83 5 ma k 4 9 wt for paragonite phengite mixture and 85 3 ma k 7 8 wt for phengite and an isochron age of 91 13 ma from the two dataset the eclogite gives a plateau age of 132 ma in ar ar step heating analyses using single phengite grain and an inverse isochron age of 130 39 ma with an initial 40 ar 36 ar ratio of 434 90 in ar ar spot analyses of phengites and paragonites the cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature 500 c for phengite k ar system the phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic 40 ar from the crystals the argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions

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

recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin abstract recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin were investigated on the basis of water column profiles of suspended particulate matter records of near bottom currents and settling fluxes of particulate matter obtained with benthic landers and analysis of surface sediments the results show that fine grained predominantly lithogenic sediment derived from adjacent shelf areas accumulates in the upper reaches of the canyons sediment deposited further down in the middle and lower reaches of the canyons is essentially similar to the hemipelagic sediment found on the adjacent continental slope indicating that down canyon transport of sediment from the upper to the lower canyon is limited tidal currents measured at various depths in the lisbon setubal canyon appear sufficiently strong to resuspend and transport sediment but net up canyon flow of the bottom water may retain shelf derived lithogenic sediment in the upper canyon sediment gravity flows which in the nearby nazare canyon are an effective mechanism for down canyon sediment transport appear rare in the lisbon setubal canyon and probably also in the cascais canyon turbidity current events recorded in the sedimentary record of the canyons may correspond to seismic events of 1969 and 1755 ad

mono lake excursion recorded in sediment of the santa clara valley california 1 two intervals recording anomalous paleomagnetic inclinations were encountered in the top 40 meters of research drill hole ccoc in the santa clara valley california the younger of these two intervals has an age of 28 090 330 radiocarbon years b p calibrated age 32 8 ka this age is in excellent agreement with the latest estimate for the mono lake excursion at the type locality and confirms that the excursion has been recorded by sediment in the san francisco bay region the age of an anomalous inclination change below the mono lake excursion was not directly determined but estimates of sedimentation rates indicate that the geomagnetic behavior it represents most likely occurred during the mono lake laschamp time interval 45 28 ka if true it may represent one of several recurring fluctuations of magnetic inclination during an interval of a weak geomagnetic dipole behavior noted in other studies in the region

structural evolution of the contact between two penninic nappes zermatt saas zone and combin zone western alps and implications for the exhumation mechanism and palaeogeography the boundary zone between two penninic nappes the eclogite facies to ultrahigh pressure zermatt saas zone in the footwall and the blueschist facies combin zone in the hanging wall has been interpreted previously as a major normal fault reflecting synorogenic crustal extension quartz textures of mylonites from this fault were measured using neutron diffraction together with structural field observations the data allow a refined reconstruction of the kinematic evolution of the pennine nappes the main results are 1 the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast directed normal faulting 2 exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening 3 the sesia dent blanche nappe system originated from a continental fragment cervinia in the alpine tethys ocean and the combin zone ophiolites from the ocean basin southeast of cervinia 4 out of sequence thrusting played a major role in the tectonic evolution of the penninic nappes