halogen bearing minerals in syenites and high grade marbles of dronning maud land antarctica monitors of fluid compositional changes during late magmatic fluid rock interaction processes meta sedimentary rocks including marbles and calcsilicates in central dronning maud land cdml in east antarctica experienced a pan african granulite facies metamorphism with peak metamorphic conditions around 830 20 c at 6 8 0 5 kbar which was accompanied by the post kinematic intrusion of huge amounts of syenitic charnockitic magmas at 4 5 0 7 kbar the marbles and calcsilicates may represent meta evaporites as indicated by the occurrence of metamorphic gypsum anhydrite and cl rich scapolite that formed in the presence of saline fluids with x nacl in the range 0 15 0 27 the marbles and calcsilicates bear biotite tremolite and or hornblende and humite group minerals clinohumite chondrodite and humite which are inferred to have crystallized at about 650 c and 4 5 kbar the syenitic intrusives contain late magmatic biotite and amphibole formed between 750 and 800 c as well as relictic magmatic fayalite orthopyroxene and clinopyroxene two syenite and two calcsilicate samples contain fluorite corona textures in the marbles and calcsilicates suggest very low fluid rock ratios during the formation of the retrograde 650 c assemblages biotite in all but two syenite samples crystallized at log f h 2 o f hf ratios of 2 9 0 4 while in the calcsilicates both biotite and humite group minerals indicate generally higher log f h 2 o f hf values of up to 5 2 a few samples though overlap with the syenite values log f h 2 o f hcl derived from biotite covers the range 0 5 2 6 in all rock types within a single sample the calculated values for both parameters vary typically by 0 1 to 0 8 log units water and halogen acid fugacities calculated from biotite olivine orthopyroxene feldspar quartz equilibria and the above fugacity ratios are 1510 2790 bars for h2o 1 3 5 3 bars for hf and 7 600 bars for hcl the results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids in terms of log f h 2 o f hf derived from the late magmatic stages of the syenites with both earlier crystallized still hotter parts of the syenites and with adjacent country rocks during down temperature fluid flow fluorine is successively removed from the fluid and incorporated into f bearing minerals close to the syenite into metamorphic fluorite in the course of this process log f h 2 o f hf increases significantly chlorine preferably partitions into the fluid and hence log f h 2 o f hcl does not change markedly during fluid rock interaction

arctic sea ice variability in the context of recent atmospheric circulation trends abstract forty years 1958 97 of reanalysis products and corresponding sea ice concentration data are used to document arctic sea ice variability and its association with surface air temperature sat and sea level pressure slp throughout the northern hemisphere extratropics the dominant mode of winter january march sea ice variability exhibits out of phase fluctuations between the western and eastern north atlantic together with a weaker dipole in the north pacific the time series of this mode has a high winter to winter autocorrelation 0 69 and is dominated by decadal scale variations and a longer term trend of diminishing ice cover east of greenland and increasing ice cover west of greenland associated with the dominant pattern of winter sea ice variability are large scale changes in sat and slp that closely resemble the north atlantic oscillation the associated sat and surface sensible and latent heat flux anomalies are largest over the portions of the marginal sea ice zone in which the tr

synmetamorphic veining origin of andalusite bearing veins in the vedrette di ries contact aureole eastern alps italy discordant andalusite biotite quartz bearing veins occur in the contact aureole of the vedrette di ries pluton italian eastern alps never outside the area of contact metamorphic andalusite development andalusite veins are found only within andalusite bearing hornfelses and vein biotite occurs wherever host rock garnet is partially replaced by biotite veins formed during contact metamorphism synchronously with the crystallization of andalusite and biotite within host rocks their pegmatitic structure and their orientation suggest that vein parageneses crystallized within fluid filled cavities that opened by hydraulic fracturing a mechanism of synmetamorphic veining is proposed to explain rock failure and subsequent mineral deposition within veins during hydrofracturing induced by dehydration reactions in response to heating in the aureole fissures were immediately filled with locally derived fluids the lack of large scale flux together with high fluid pressures required by hydrofracturing suggest fluid in the cavities was a virtually stagnant passive medium and that mass transport toward fractures was driven by intergranular diffusion because temperature and pf values within veins are similar to those in the host rock vein assemblages are interpreted as the stable high t side of reactions taking place within pelitic schists at the time when fractures opened once nucleation of product phases occurred chemical components released by dissolution of reactant minerals were driven to precipitation sites by chemical potential gradients since nucleation was favoured at the strained grains of vein walls andalusite and biotite simultaneously grew in vein and host rock the proposed genetic model contrasts with generally adopted metasomatic mechanisms for the genesis of al2sio5 bearing veins in not requiring large fluid rock ratios or a highly aggressive fluid composition the mechanism of synmetamorphic veining may be particularly useful in the interpretation of vein occurrences in medium and deep crustal rocks which have undergone extensive devolatilization

case studies of high wind events in barrow alaska climatological context and development processes the beaufort chukchi cyclones of october 1963 and august 2000 produced the highest winds ever recorded in barrow alaska in both cases winds of 25 m s21 were observed with gusts unofficially reported at 33 m s 21 the october 1963 storm caused significant flooding contaminated drinking water and interrupted power supplies the august 2000 storm caused the wreck of a 6 million dredge and removed roofs from 40 buildings both storms were unusual in that they tracked eastward from the east siberian sea into the chukchi and beaufort seas rather than following a more typical northward track into the arctic ocean this paper addresses through modeling and analysis the development processes of these two storms the october 1963 system was a long lived warm core zonally elongated cyclone that traversed around the arctic basin through the canadian archipelago the august 2000 system was an open wave cyclone that dissipated rapidly into a weak cold core eddy in the alaskan sector of the beaufort sea approximating the contributions to development using terms in a quasigeostrophic omega equation it was found that both storms were characterized by the increasing importance of the convergence of the q vector representing differential vorticity advection and thermal advection in the midtroposphere at the expense of forcing by the turbulent fluxes of heat moisture and momentum in the boundary layer however the influence of surface turbulent fluxes in the early stages of development was important particularly for the august 2000 cyclone which passed over an extensive coastal lead in the east siberian sea this study concludes that the observed retreat in western arctic ice cover is unlikely to be an important contributor to increasing cyclonic activity in the region but that ice retreat north of eurasia could have an impact

3 d crustal structure along the north anatolian fault zone in north central anatolia revealed by local earthquake tomography summary 3 d p wave velocity structure and vp vs variations in the crust along the north anatolian fault zone nafz in north central anatolia were investigated by the inversion of local p and s wave traveltimes to gain a better understanding of the seismological characteristics of the region the 3 d local earthquake tomography inversions included 5444 p and 3200 s wave readings obtained from 168 well located earthquakes between 2006 january and 2008 may dense ray coverage yields good resolution particularly in the central part of the study area the 3 d vp and vp vs tomographic images reveal clear correlations with both the surface geology and significant tectonic units in the region we observed the lower limit of the seismogenic zone for north central anatolia at 15 km depth final earthquake locations display a distributed pattern throughout the study area with most of the earthquakes occurring on the major splays of the nafz rather than its master strand we identify three major high velocity blocks in the mid crust separated by the izmir ankara erzincan suture and interpret these blocks to be continental basement fragments that were accreted onto the margin following the closure of neo tethyan ocean these basement blocks may have in part influenced the rupture propagations of the historical 1939 1942 and 1943 earthquakes in addition large variations in the vp vs ratio in the mid crust were observed and have been correlated with the varying fluid contents of the existing lithologies and related tectonic structures

diagnosis of the record minimum in arctic sea ice area during 1990 and associated snow cover extremes the arctic sea ice cover exhibited its record minimum area during 1990 characterized by extensive ice free conditions during august along the siberian coast these reductions are consistent with warm windy conditions in may and continued warmth in june promoting early melt and reductions in ice concentration followed in august by strong coastal winds forcing a final breakup and retreat of the pack ice the unusually warm arctic conditions in 1990 are part of a larger scale temperature anomaly pattern linking the sea ice anomaly to accompanying record minima in eurasian snow cover

a case study of the anomalous arctic sea ice conditions during 1990 insights from coupled and uncoupled regional climate model simulations 1 the regional coupled atmosphere ice ocean model hirham mom has been developed to simulate the arctic climate the model has been applied for the year 1990 which was characterized by anomalous ice retreat along the eastern arctic coasts many observed features of the anomalous atmospheric circulation and sea ice concentration pattern during spring to early summer are reproduced by the model it was even able to generate the extreme summer sea ice conditions in the eurasian arctic and the associated high surface air temperatures the model shows less success in simulating the large ice retreat in the siberian arctic in late summer this is explained mainly by deviations in the simulated atmospheric circulations results from an ocean ice alone model confirm that the atmospheric forcing dominates the simulated ice retreat on the other hand a sensitivity study with the atmosphere alone model shows the impact of the sea ice conditions on the atmospheric circulation only with the accurate satellite derived sea ice data is the model able to reproduce the anomalous late summer atmospheric pressure patterns

two circulation regimes of the wind driven arctic ocean the major goal of this paper is to demonstrate the existence in the arctic ocean of two regimes of wind forced circulation we simulated the vertically averaged currents sea level heights and ice drift in the arctic ocean from 1946 to 1993 using a two dimensional wind forced barotropic model that includes frictional coupling between the ocean and ice the model has a spatial resolution of 55 5 km and is driven by winds river runoff and an imposed but realistic sea level slope between the pacific and the atlantic oceans there is a good agreement between velocities from observed buoy motions and velocities of modeled ice drift even though the model lacks ocean baroclinicity and ice thermodynamics the results indicate that wind driven motion in the central arctic alternates between anticyclonic and cyclonic circulation with each regime persisting for 5 7 years based upon our analysis of the modeled sea level and ice motion anticyclonic wind driven motion in the central arctic appeared during 1946 1952 1958 1963 1972 1979 and 1984 1988 and cyclonic motion appeared during 1953 1957 1964 1971 1980 1983 and 1989 1993 shifts from one regime to another are forced by changes in the location and intensity of the icelandic low and the siberian high the two regimes may help explain the significant basin scale changes in the arctic s temperature and salinity structure observed recently the great salinity anomaly and the variability of ice conditions in the arctic ocean

halite and sylvite as solid inclusions in high grade metamorphic rocks solid inclusions of halite and sylvite formed during regional and contact metamorphism have been identified by microscopy and by electron microprobe analysis in rocks from campolungo switzerland and cornone di blumone italy the solid inclusions occur in several of the major minerals crystallized during metamorphism and have been observed as idiomorphic crystals and dendrites the compositions measured in 100 analyses from campolungo switzerland and 40 analyses from cornone di blumone italy extend across the two phase region in the system kcl nacl indicating that the salt inclusions are high temperature precipitates in both localities compositionally zoned and unzoned crystals have been found measured compositions on the temperature maximum of the two phase region indicate at least 500 c which can be compared with 500 20 c determined by mercolli 1982 and walther 1983 from the mg content of calcites from campolungo the solid inclusions have been trapped apart from co2 rich and saline h2o rich fluid inclusions which have been described by mercolli 1982 as the earliest preserved fluid inclusions in the rocks the early precipitation of salt minerals at campolungo indicates that fluids were saturated with nacl and kcl at 500 c and pressures of 2 000 bars or higher similar relationships exist between solid and fluid inclusions in the rocks of cornone di blumone which formed at temperatures as high as 800 c and pressures between 0 5 and 1 kilobar ulmer 1983 the entrapment of halite and sylvite as solid inclusions preserves the composition of the minerals which may therefore be useful as geothermometers

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

spatial distribution of trends and seasonally in the hemispheric sea ice covers 1978 1996 we extend earlier analyses of a 8 8 year sea ice data set that described the local seasonal variations and trends in each of the hemispheric sea ice covers to the recently merged 18 2 year sea ice record from four satellite instruments the seasonal cycle characteristics remain essentially the same as for the shorter time series but the local trends are markedly different in some cases reversing sign the sign reversal reflects the lack of a consistent long term trend and could be the result of localized long term oscillations in the hemispheric sea ice covers by combining the separate hemispheric sea ice records into a global one we have shown that there are statistically significant net decreases in the sea ice coverage on a global scale the change in the global sea ice extent is 0 01 0 003 106 km2 per decade the decrease in the areal coverage of the sea ice is only slightly smaller so that the difference in the two the ice free areas within the packs has no statistically significant change

silica mobility and fluid movement during metamorphism of the connemara schists ireland quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of connemara and attest to considerable migration of silica contrary to common assumptions there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower p t down the regional geothermal gradient under amphibolite facies conditions a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated while cooling over the temperature interval from 650 to 500 c the absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through flowing externally derived fluids the oxygen isotopic composition of matrix quartz shows considerable differences between different rock types quartzite pelite and marble with a range of 18osmow from c 11 5 quartzite to 18 5 marble in each rock type vein quartz compositions closely match those of the matrix quartz these results demonstrate the importance of local segregation processes in the formation of veins and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations if at all both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies with only a relatively minor component of through flow of externally derived fluid veins are clearly not the major pathways of metamorphic dewatering it is proposed that abundant veins in the predominantly pelitic ballynakill formation formed during peak metamorphic d3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins while irreversible loss through the rest of the succession was a much less important process in the central mountains of connemara rather pure unreactive quartzites are cut by widely spaced laterally extensive quartz veins that are axial planar to d3 folds these veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile but here too oxygen isotope data indicate considerable segregation of locally derived quartz reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow

an investigation of boundary forcing sensitivities in a regional climate model the sensitivity of a regional climate model to lateral upper and lower dynamical forcing is evaluated for a pan arctic domain the experiments focus on a month long case study of may 1990 but the results are shown to be extendable to january and july 1990 although the control simulation is found to adequately represent major tropospheric circulation features a large negative geopotential height bias of up to 700m is found at the upper levels of the model experiments find these biases are remedied through the replacement of a reflective rigid lid upper boundary condition with a local wave absorbing layer geopotential height biases are reduced to a maximum underestimation of only 50m at the upper boundary substantial improvement is also found in the simulated temperature and zonal wind profiles transient heat and momentum fluxes and stationary eddies as seen through the eliassen palm flux divergence with this wave absorbing upper boundary condition in place additional improvement is found using a gravity wave drag parameterization appropriate for this domain s 100km horizontal resolution additional sensitivity to lateral forcing is examined and the relative role of the upper boundary condition to skillful simulations within the arctic basin domain is discussed

sea ice albedo climate feedback mechanism abstract the sea ice albedo feedback mechanism over the arctic ocean multiyear sea ice is investigated by conducting a series of experiments using several one dimensional models of the coupled sea ice atmosphere system in its simplest form ice albedo feedback is thought to be associated with a decrease in the areal cover of snow and ice and a corresponding increase in the surface temperature further decreasing the areal cover of snow and ice it is shown that the sea ice albedo feedback can operate even in multiyear pack ice without the disappearance of this ice associated with internal processes occurring within the multiyear ice pack e g duration of the snow cover ice thickness ice distribution lead fraction and melt pond characteristics the strength of the ice albedo feedback mechanism is compared for several different thermodynamic sea ice models a new model that includes ice thickness distribution the ebert and curry model the maykut and untersteiner model and the semtner level 3 an

on the record reduction in 1998 western arctic sea ice cover during summer 1998 record reductions in ice cover occurred in the beaufort and chukchi seas open water formed earlier than in prior years and september ice extent in this region was 25 percent less than the previous minimum for 1953 1997 seven percent of the arctic basin that had been perennially ice covered was ice free in 1998 this reduction in western arctic ice extent can be attributed in part to preconditioning by light ice cover in autumn 1997 and to atmospheric circulation patterns during the following winter through autumn that favored southerly and easterly winds such decreases in northerly winds and the associated weakening or displacement of the beaufort gyre is found to be typical of winters that precede years with below normal ice extent in the western arctic

recent changes in arctic sea ice the interplay between ice dynamics and thermodynamics abstract it is well established that periods of high north atlantic oscillation nao index are characterized by a weakening of the surface high pressure and surface anticyclone in the beaufort sea and the intensification of the cyclonic circulation in the eastern arctic ocean the response of arctic sea ice to these atmospheric changes has been studied with a thickness distribution sea ice model coupled to an ocean model during a period of high nao 1989 96 the model shows a substantial reduction of ice advection into the eastern arctic from the canada basin and an increase of ice export through fram strait both of which tend to deplete thick ice in the eastern arctic ocean and enhance it in the western arctic in an uneven dipolar pattern we call the east west arctic anomaly pattern ewaap from the period 1979 88 with a lower nao index to the period 1988 96 with a high nao index the simulated ice volume in the eastern arctic drops by about a quarter while that in the western arctic increases by

fluid regimes during late stages of a continental collision physical chemical and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the central alps switzerland fluid evolution during neo alpine metamorphism during late stages of the continental collision between europe and africa was studied by analyzing fluid inclusions in alpine fissure quartz collected in forty nine localities along a geotraverse through the central alps switzerland the methods employed include microthermometry micro raman spectroscopy k na thermometry and stable isotope analysis early fluid inclusions provide evidence of close to peak metamorphic temperatures of the late tertiary or neo alpine metamorphic event fluid composition evolved along the geotraverse from north to south as follows higher hydrocarbons were dominant in the low and medium grade diagenetic zones methane was the main volatile in the high grade diagenetic and low grade anchizone water dominated in the highgrade anchizone and low grade epizone with co2 10 mol in the high grade epizone and in the mesozone higher hydrocarbons and ch4 were the products of kerogen maturation and cracking of preexisting petroleum large water supplies originated from the dehydration of cooler metasedimentary rocks that were overthrust by crystalline basements of the lepontines aar and gotthard massifs carbon isotope analyses suggest that the co2 component was derived from oxidation of graphitic matter especially in the vicinity of sulfate bearing metasediments and from decarbonation reactions in the aar and gotthard massifs as well as in the helvetic axen nappe and its underlying north helvetic flysch high fluid pressures prevailed and favored nappe transport by contrast in the southern lepontine area very low early fluid pressures were probably related to dry rocks and scarce metasediments and to high geothermal gradients that resulted from intense uplift and erosion between 26 and 18 ma retrograde fluid evolution was recorded by a succession of fluid inclusion populations in each alpine fissure it was controlled by uplift and cooling and characterized by decreasing contents of volatiles and an increase in 18o of host quartz tectonic activity led to episodic pressure drops of at least 0 5 to 2 kbar and promoted fluid unmixing channelized flow and rapid growth of skeletal quartz channelized rather than pervasive fluid migration at temperatures 450 c and under conditions of brittle deformation is documented by episodic increases in salinity and by fluid flushing through the massifs there is stable isotope evidence for involvement of meteoric water only in late crystallizing quartz formation of alpine fissures and fissure minerals was the result of a unique coincidence of late continental collision 450 c fluid expulsion from overthrust metasediments uplift and erosion

nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions abstract in the neves area the pre alpine intrusive protolith of the zentralgneise unit tauern window eastern alps is well preserved in a kilometric scale low strain domain without pervasive alpine deformation it is compositionally heterogeneous consisting predominantly of granodiorites with lesser leucocratic granites and different generations of lamprophyres and aplites the intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes incipient alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities resulting in the development of several different types of shear zones fractures without alteration haloes initially accommodate slip entirely on the fracture itself with increasing deformation a foliation is progressively developed in the adjacent host rock eventually producing a single heterogeneous ductile shear zone with the typical sigmoidal foliation pattern strong layers aplite dykes and bleached alteration haloes developed to either side of precursor fractures localize shear on their boundaries to produce characteristic paired shear zones shearing is more evenly distributed within weak layers lamprophyres and quartz veins with a marked discontinuity in shear strain against the adjacent little deformed granodiorite shear zone development was accompanied by the formation of new fractures and quartz rich veins in the host rock which in turn also localized shear magmatic contacts fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations the kinematics of the overprinting strike slip shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis 1 here oriented ca 345 discontinuities of almost all orientations show shear reactivation even in the case of very low resolved shear stress indicating an overall viscous response of the system without a specific yield stress the geometry and kinematics of the shear zone network suggest that the overall deformation in low strain domains was close to coaxial during deformation along the shear zone network compatibility was maintained by fracturing with the development of new quartz veins and by distributed ductile deformation of the host rock especially within contractional domains of the shear network and at shear zone intersections deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities shear zone development in intact rock is always preceded by fracturing which localizes subsequent shear reactivation

book review the al 2 sio 5 polymorphs derrill m kerrick reviews in mineralogy volume 22 mineralogical society of america 1991 xii 406 p us 20 00 isbn 0 939950 27 8

complicated secondary textures in zircon record evolution of the host granitic rocks studies from western tauern window and otztal stubai crystalline complex eastern alps western austria abstract samples of metamorphosed and deformed granitic rocks were collected from two alpine complexes with well constrained metamorphic history western tauern window and otztal stubai crystalline complex zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques cathodoluminescence cl imaging and raman spectroscopy the aims were to describe and interpret complicated secondary textures and microstructures in zircon based on cross cutting relationships between secondary microstructures reconstruct the sequence of processes affecting zircon crystals link the evolution of zircon with the history of the host rocks the results indicate that zircon in the sampled granitic rocks forms growth twins and multi grain aggregates which are unusual for this mineral moreover various secondary textures have been found in the sampled zircon often cross cutting each other in a single crystal these include distorted oscillatory cl zoning with inner zones forming inward penetrating cl bright embayments which are the evidence of dry recrystallization via annealing lattice recovery cl mosaicism with no preservation of growth zoning but abundant nano and micro scale pores and mineral inclusions which are the evidence of recrystallization by coupled dissolution reprecipitation and or leaching embayed zircon boundaries filled with apatite monazite epidote and mylonitic matrix indicating mineral fluid reactions resulting in zircon dissolution and fragmentation overgrowth cl dark rims which contain nano pores and point to transport and precipitation of dissolved zircon matter we conclude that zircon in our meta granites is sensitive to metamorphism deformation events and was reactive with metamorphic fluids additionally we have found evidence of crystal plastic deformation in the form of low angle boundaries and bent grain tips which is a result of shearing and ductile deformation of the host rock we suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization prograde metamorphism peak of amphibolite facies metamorphism post peak cooling and exhumation formation of ductile shear zones and final cooling to 250 c

mechanisms in the development of anomalous sea ice extent in the western arctic a case study interannual cycles and trends in arctic ice cover are dominated by strong regional variability arctic sea ice extent exhibited a record minimum within the arctic basin during september 1990 dominated by a rapid retreat in the chukchi east siberian and laptev seas simulations using a coupled regional model reproduced the enhanced cyclonic activity and enhanced ice melt which led to this large retreat in sea ice cover sensitivity experiments showed that accurate initial ice conditions are crucial for a realistic simulation of the ice anomaly pointing to the need for adequate spin up in coupled model experiments it was also found that thermodynamic melt can account for the bulk of the total loss in ice mass over the summer season in 1990 but without dynamics the spatial patterns characteristic of the ice reductions are not reproduced well also transport of ice was crucial to initiate the formation of the anomaly even if the formation was delayed until later in the season the ice albedo feedback is important in allowing the continued formation of the ice anomaly throughout the summer interannual variations in atmospheric circulation that yield regional differences in ice thickness provide a preconditioning that significantly affected development of the ice anomaly this suggests that circulation modes play an important role in determining ice severity along the siberian coast

time calibration of a pt path from the western tauern window eastern alps the problem of closure temperatures new hornblende k ar and 39ar 40ar and mica rb sr and k ar ages are used to place specific timemarks on a well constrained pressure temperature path for the late alpine metamorphism in the western tauern window after identification of excess 40ar the closure behavior of ar in hornblende is compared with that of sr and ar in phengite and biotite samples were collected in three locations whose maximum temperatures were 570 c zemmgrund 550 c pfitscher joch and 500 540 c landshuter hutte

retreat of the cold halocline layer in the arctic ocean we present a comparison of arctic ocean hydrographic data sets from the 1990s with a focus on changes in the upper few hundred meters of the eurasian basin the most recent observations discussed here were collected during the spring 1995 scientific ice expedition scicex 95 the second in a series of scientific cruises to the arctic ocean aboard u s navy nuclear submarines although the 1990s have seen an abundance of synoptic cruises to the arctic this was the only one to take place in winter spring conditions other data considered here were collected during the first scicex cruise in summer 1993 scicex 93 and during an icebreaker cruise to the eurasian basin in summer 1991 oden 91 a new russian american winter climatology is also used as a reference these comparisons reveal that the eurasian basin cold halocline layer has retreated during the 1990s to cover significantly less area than in previous years specifically we find a retreat from the amundsen basin back into the makarov basin the latter is the only region with a true cold halocline layer during scicex 95 changes are also seen in other halocline types and in the atlantic water layer heat content and depth since the cold halocline layer insulates the surface layer and thus the overlying sea ice from the heat contained in the atlantic water layer this should have profound effects on the surface energy and mass balance of sea ice in this region using a simple mixing model we calculate maximum ice ocean heat fluxes of 1 3 w m 2 in the eurasian basin where during scicex 95 the surface layer lay in direct contact with the underlying atlantic water layer the overall cause of water mass changes in the 1990s might have been a shift in the atmospheric wind forcing and resulting sea ice motion during the late 1980s which we speculate influenced the location where fresh shelf waters flow into the deeper basins of the arctic ocean finally we discuss two different mechanisms that have been proposed for cold halocline water formation and we propose a compromise that best fits these data

thinning of the arctic sea ice cover comparison of sea ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1 3 m in most of the deep water portion of the arctic ocean from 3 1 m in 1958 1976 to 1 8 m in the 1990s the decrease is greater in the central and eastern arctic than in the beaufort and chukchi seas preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s

modeling the thermodynamics of a sea ice thickness distribution 1 sensitivity to ice thickness resolution a one dimensional ice thickness distribution model is presented to determine the minimum number of ice thicknesses necessary to resolve the area averaged annual cycles of ice thickness and turbulent fluxes the baseline case includes 40 ice thickness categories ice thickness and area meltwater ponds ice salinity and age snow cover and surface albedo evolve independently for each ice category a ridging and ice export parameterization and a coupled one dimensional ocean mixed layer model are also included sensitivity studies indicate that 16 ice thickness categories can accurately resolve the baseline annual cycles of area averaged ice thickness and the summertime turbulent fluxes in this model provided that one third of the thickness categories represent ice thinner than 0 8 m resolving the distribution of ice in this thickness range is important in simulating the area averaged ice characteristics wintertime values of the turbulent fluxes differ from the baseline by up to 1 w m 2 for fewer than 40 ice thickness categories the large difference in turbulent fluxes between open water and ice thicker than 1 6 m makes the area averaged value sensitive to the number of ice thickness categories since this number can affect the categories that are merged the categories that melt completely ice that is ridged and the resolution of the ice thickness distribution this makes an accurate simulation of the baseline wintertime turbulent fluxes difficult model simulations with fewer than 40 categories provide a reasonable estimate of the baseline response to a surface longwave heat flux perturbation of greater than 5 w m 2 the annually area averaged ice thickness is within 10 cm the model response to a heat flux perturbation of less than 5 w m 2 is similar for a wide range of ice thickness categories since the thickness distribution of ice thinner than 1 m is not affected

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

why calcite can be stronger than quartz 1 in the neves area eastern alps calcite forms asymmetric centimeter scale single crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at 550 c under these conditions coarse calcite was clearly stronger than the surrounding polycrystalline dynamically recrystallized quartz matrix experimental results indicate that coarse calcite is less strain rate sensitive than wet quartzite consistent with an inversion in strength on extrapolation to natural strain rates for this to occur wet quartzite must be weak flowing at differential stress of 10 mpa the lack of high temperature twins showing bulging or recrystallization in calcite clasts is consistent with such low stresses during shear zone development under near peak metamorphic conditions the maximum effective viscosity ratio of coarse calcite to quartzite for these conditions is probably not large 10 however numerical modeling shows that ratios of around 2 are sufficient to maintain near rigid calcite clast behavior for power law rheology with stress exponents appropriate to quartz n 3 4 and coarse calcite n 6 the inversion in relative strength reflects the difference in influence of water on the crystal plastic flow of calcite and quartz water has a dramatic effect for quartz but little or no effect for calcite quartz rich rocks under hydrous amphibolite facies conditions in the middle to lower crust are therefore relatively weak in fact weaker than coarse calcite and flow at much lower stresses than dry quartz rich rocks at similar crustal levels

alpine metamorphism and veining in the zentralgneis complex of the sw tauern window a model of fluid rock interactions based on fluid inclusions abstract the granitic to tonalitic gneisses of the zentralgneis complex in the sw tauern window contain numerous discordant quartz biotite plagioclase veins occurring either as isolated or in parallel sets with a preferential orientation perpendicular to the main foliation of host rocks veins show a zonal distribution of minerals with plagioclase and biotite at the walls and quartz in the centre quartz contains abundant fluid inclusions fi mostly occurring along secondary planes all primary and most secondary inclusions three phase h2o co2 inclusions formed from a dominant fluid type of aqueous carbonic composition fi have been characterized by microthermometry laser raman and laser ablation optical emission spectroscopies which have confirmed their similarity also with respect to composition and density fluids display a somewhat unusual composition especially a wide range of rather high salinities 3 7 mol deduced from the final melting temperature of clathrate which covers a wide range from 4 2 to 4 0 c the volatile phase is dominated by co2 3 12 mol with small amounts of n2 the final homogenization temperature of these inclusions is generally 300 c but independently constrained trapping temperatures of 550 600 c yield to pressure estimates of 3 5 7 5 kbar highest pressures are in agreement with the p t conditions reached during the tauern metamorphism peak veining was triggered by hydrofracturing induced by pl bt producing devolatilization and probably decarbonation reactions in the adjacent gneisses and quartz precipitated in response to the significant pressure and or salinity fluctuations shown by fi during each microfracturing healing stage

late magmatic healed fractures in granitoids and their influence on subsequent solid state deformation abstract late magmatic fractures developed during final stages of magma crystallization are recorded in many granitoid plutons by planar trails of magmatic biotite and or leucocratic veins but have not been previously described such biotite rich trails with lengths of a few centimetres to several tens of centimetres widths of a few millimetres and a variable orientation are ubiquitous in meta granitoids of the neves area tauern window eastern alps italy these pre alpine granitoids were metamorphosed to upper amphibolite facies and deformed during the alpine orogeny allowing the effect of planar healed magmatic fractures on the nucleation of subsequent solid state ductile shear zones to be directly assessed numerical models considering power law viscous materials predict that planar arrays of a weaker mineral e g biotite oriented at around 45 to the shortening direction should nucleate and localize viscous deformation however no discernible localization on the natural small scale biotite trails is observed even when they delineate planes that were well oriented for shear reactivation in contrast heterogeneous shear zones nucleated on larger scale 1 10 m long planar compositional or structural boundaries such as joints dykes quartz veins alteration layers surrounding veins and zones of magma mingling outlined by densely packed clusters of basic enclaves irrespective of their orientation relative to the imposed shortening direction clearly some crucial aspect is missing from purely viscous numerical models we propose that this missing aspect is the observed interplay between fracture and flow with new fractures developing and localizing shear at bulk strains too low for discernible localization to occur on the pre existing healed magmatic fractures natural granitoids have a truly elasto plasto viscous rheology even at low differential stress as in the neves example and for high grade metamorphic conditions considered as typical of wet ductile middle crust

formation of saline brines and salt in the lower crust by hydration reactions in partially retrogressed granulites from the lofoten islands norway anorthosites gneisses and mangeritic rocks from the lofoten islands in northern norway contain cl rich amphibole up to 3 5 wt percent cl and biotite up to 1 5 wt percent cl that developed mostly during regional metamorphism at 600 c and 9 kb by replacement of older anhydrous granulite facies mineral assemblages na k cl halide solid solutions are closely associated spatially with amphiboles that have the highest measured cl contents rocks with cl rich amphiboles and biotites always have minerals that exhibit a wide range in oh cl one sample from a shear zone contains virtually cl free amphibole within a few millimeters of cl rich amphibole coexisting with halide crystals textural evidence indicates that amphibole and biotite evolved from cl poor to cl rich compositions within individual samples oh rich cl poor compositions are always much more abundant than cl rich ones cl rich amphiboles and biotites exclusively occur in rocks that contain relict granulite facies minerals pyroxene and or olivine samples in which no granulite facies minerals remain contain amphiboles and biotites without elevated cl contents in one particular case the restricted development of clrich amphiboles and biotites within a shear zone and the absence of these minerals from undeformed lithologic equivalents outside the shear zone demomstrate that the amphibole and biotite developed by reaction between a preexisting granulite facies mineral assemblage and an aqueous fluid rather than by fluid ansent metamorphism or by magmatic processes we infer from this example that cl rich amphibole nad biotite with the same grani size and texture as in the shear zone developed throughout the lofoten islands by fluid rock reaction mineralfluid equilibria indicate that the aqueous fluid was variable in composition ranging from xh2o h2o h2o cl of 0 99 for equilibrium with the most cl poor amphiboles and biotites to 0 4 0 6 for equilibrium with the most cl rich minerals that coexist with salt the most saline fluids correspond to nacl equivalent concentrations of over 25 mol l the range in oh cl of amphiboles and biotite in individual samples and the restricted occurrence of cl rich minerals in rocks with relict pyroxene and or olivine can be explained by a desiccation model in which small amounts of normal low cl crustal fluid infiltrates rocks with granulite facies minerals and is subsequently enriched in cl by preferential consumption of h2o during the resulting hydration reaction saline fluids developed at the extremes of reaction progress and then produced the cl rich amphibole and biotite compositions coexisting with halide crystals the desiccation model further explains variations in the whole rock concentration of cl in rocks from the lofoten islands the absence of cl rich minerals from completely retrogressed granulites that lack olivine or pyroxene the measured frequency distribution of amphibole and biotite compositions in individual samples and the calculated evolution of fluid composition with reaction progress our results have implications for interpreting measurements of elevated electrical conductivity in the lower crust and for models of granulite genesis that appeal to fluids with low h2o activity amer j science 1998 298 705 757