x ray microanalysis of fluid inclusions and its application to the geochemical modeling of evaporite basins direct access to brines trapped in halite is possible by freezing and breaking halite crystals on a cooling stage inside a scanning electron microscope quantitative analyses of major solutes na mg k ca cl and so4 in natural brines were obtained by means of x ray microanalysis of the frozen fluid inclusions using frozen droplets of brines with known composition as standards precision errors were estimated to be lower than 10 and detection limits vary from below 0 02 mol l for ca k and so4 to 0 55 mol l for na analyses of several inclusions of sizes larger than 20 m were performed for each cross section of halite the composition of primary fluid inclusions in halites from a drill core in the messinian basin of lorca se spain were used to constrain the parameters of a geochemical model of the evolution of the basin a computer program to simulate evaporation paths was developed using thermodynamic equilibrium and mass balance as constraints the halite sequence was interpreted as resulting from the evaporation of initial seawater in a hydrologically open basin with seawater replacing the evaporated water and a small fraction of basinal brine leaking out in the middle of the sampled sequence the basin was closed to the sea and only continental water flowed into the basin reworking part of the halite deposited in the margins in contrast with these results only very poor information would have been obtained from the mineralogical record alone consisting mainly of halite and minor gypsum

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

the ellsworth subglacial highlands inception and retreat of the west antarctic ice sheet antarctic subglacial highlands are where the antarctic ice sheets first developed and the pinning points where retreat phases of the marine based sectors of the ice sheet are impeded due to low ice velocities and limited present day change in the ice sheet interior west antarctic subglacial highlands have been overlooked for detailed study these regions have considerable potential however for establishing the locations from which the west antarctic ice sheet originated and grew and its likely response to warming climates here we characterize the subglacial morphology of the ellsworth subglacial highlands west antarctica from ground based and aerogeophysical radio echo sounding res surveys and the moderate resolution imaging spectroradiometer modis mosaic of antarctica we document well preserved classic landforms associated with restricted dynamic marine proximal alpine glaciation with hanging tributary valleys feeding a significant overdeepened trough the ellsworth trough cut by valley tidewater glaciers fjord mouth threshold bars down ice of two overdeepenings define both the northwest and southeast termini of paleo outlet glaciers which cut and occupied the ellsworth trough satellite imagery reveals numerous other glaciated valleys terminating at the edge of deep former marine basins e g bentley subglacial trench throughout the ellsworth subglacial highlands these geomorphic data can be used to reconstruct the glaciology of the ice masses that formed the proto west antarctic ice sheet the landscape predates the present ice sheet and was formed by a small dynamic ice field s similar to those of the present day antarctic peninsula at times when the marine sections of the west antarctic ice sheet were absent the ellsworth subglacial highlands represent a major seeding center of the paleo west antarctic ice sheet and its margins represent the pinning point at which future retreat of the marine based west antarctic ice sheet would be arrested

geochemistry of the fluids related to epigenetic carbonate hosted zn pb deposits in the maestrat basin eastern spain fluid inclusion and isotope cl c o s sr evidence epigenetic zn pb deposits in the maestrat basin eastern spain are hosted by dolostones that replace aptian limestones the deposits have morphologies ranging from mineralized breccias to stratiform massive replacements sphalerite and galena cementing breccias precipitated as well formed crystals whereas the stratiform replacements are characterized by textures such as colloform and skeletal or dendritic growths two mineralized areas are found valdelinares and mas de la mina cedraman the highest ore grades up to 11 zn and largest deposits ca 30 000 t of sulfides occur as stratiform deposits in the valdelinares area resurreccion and restauracion mines in the valdelinares area microthermometry of fluid inclusions reveals that ore stage minerals precipitated by the mixing of two distinct fluids fluid a a high salinity brine with salinity greater than 26 wt percent nacl equiv and temperatures above120oc and fluid b a low salinity fluid with less than 15 wt nacl equiv and temperatures around 40oc fluid a dominated brines are found in mineralized breccias located in the vicinity of major fracture zones whereas more dilute fluids are typical of massive ores in the mas de la mina area the presence of two fluids is less evident although the ranges in both temperature and salinity also point to mixing na k li cl br systematics of the inclusion fluids indicates that fluids a and b were evaporated seawater that did not significantly interact with k and li rich sediments the most diluted mixtures fluid b are characterized by relatively heavy cl 37 cl 1 11 and 1 19 compared to fluid a dominated brines which have 37 cl values close to seawater 0 the sulfur isotope compositions range from 10 5 to 13 4 per mil for sulfides and between 16 and 17 8 per mil for barite such values are consistent with mixing of a metal rich brine with small amounts of 32 s enriched sulfur and a sulfur rich fluid with a 34 s close to the marine sulfate of late cretaceous to early paleocene age the c and o isotope compositions of ore stage calcite 13 c from 0 6 to 7 8 18 o from 17 2 to 20 9 are also explained by the precipitation from different mixtures of fluids a and b from the geochemical data it is inferred that fluid mixing played a key role in the formation of the zn pb deposits in the maestrat basin ore precipitation occurred in places where brines ascending through faults interacted with a dilute sulfur rich fluid deposits with the highest ore grade and size are related to areas where the most extensive mixing occurred

a newly dated cretaceous hydrothermal event in the iberian ranges eastern spain and its significance within the mesozoic thermal history in the iberian peninsula abstract the iberian peninsula hosts the world class hg mining district of almaden besides pre hercynian ore bodies alpine cycle hg bearing veins are also present in the eastern iberian ranges espadan deposits we present both the first absolute ages 84 4 and 85 3 ma for a post hercynian hg deposit in spain obtained from primary muscovites and a complete compilation of published mesozoic radiometric absolute ages of the iberian peninsula we deduce that there are three main thermal episodes that affected the iberian peninsula which have been revealed after the comparison among the magmatic metamorphic and hydrothermal ages moreover the espadan hydrothermal system was active when both the cretaceous alkalic magmatism took place in southern portugal and in the pyrenees and during the opening of the bay of biscay

steep reverse bed slope at the grounding line of the weddell sea sector in west antarctica the bed of the west antarctic ice sheet is in places more than 1 5 km below sea level1 2 it has been suggested that a positive ice loss feedback may occur when an ice sheet s grounding line retreats across a deepening bed1 2 3 applied to the west antarctic ice sheet this process could potentially raise global sea level4 by more than 3 m hitherto attention has focussed on changes at the siple coast5 6 7 and amundsen sea embayment8 9 10 sectors of west antarctica here we present radio echo sounding information from the ice sheet s third sector the weddell sea embayment that reveals a large subglacial basin immediately upstream of the grounding line the reverse bed slope is steep with about 400 m of decline over 40 km the basin floor is smooth and flat with little small scale topography that would delay retreat indicating that it has been covered with marine sediment5 11 and was previously deglaciated upstream of the basin well defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin evidence so far suggests that the weddell sea sector of the west antarctic ice sheet has been stable but in the light of our data we propose that the region could be near a physical threshold of substantial change

the sources of the glacial ird in the nw iberian continental margin over the last 40 ka abstract in this work the first results of ice rafted detritus ird provenance related to heinrich events in the nw iberian margin are presented and discussed this study is based on multi proxies namely on thorough analyses of sr and nd isotope ratios measured in the detrital fraction of the sediment core kc 024 19 from the galician continental slope ne atlantic aiming to unravel the sources of the terrigenous component of the sampled sediments in most of the core layers the isotope signatures are consistent with an origin ultimately in the nearby variscan continental crust however allochthonous contributions were also deposited namely during the last four heinrich events he the non carbonate detrital fraction of the sediments deposited during the he1 in its upper part he2 and he4 displays nd much lower than usual suggesting that those layers incorporate clasts dropped by icebergs ice rafted detritus ird whose sources must have been located in archaean cratons such as those of northeastern america and greenland the low nd values combined with dolomite peaks support the hypothesis that icebergs fed by the laurentide ice sheet and launched through hudson strait played a major role in the deposition of ird during he1 he2 and he4 the he1 layer seems to record a complex evolution of this event since at its base the coarse grained non carbonate fraction displays relatively high nd values pointing to european icelandic sources for the icebergs in the initial stages of he1 the ird content of the he3 contrasts significantly with the other he layers and the non ird layers because it has an isotope signature characterized by low 87 sr 86 sr ratios and high nd values this suggests that the most exotic components of the he3 layer may have come from east greenland fram strait and or result from a mixture of materials from western european and more juvenile iceland faeroes crustal sources

organized flow from the south pole to the filchner ronne ice shelf an assessment of balance velocities in interior east antarctica using radio echo sounding data 1 ice flow through central antarctica has the potential to transmit accumulation changes from deep interior east antarctica rapidly to the shelf but it is poorly constrained owing to a dearth of ice velocity observations we use parameters derived from airborne radio echo sounding res data to examine the onset areal extent and englacial conditions of an organized flow network tributaries feeding an ice stream draining from the south pole to the filchner ronne ice shelf we classified res flight tracks covering the region according to whether englacial stratigraphy was disrupted i e internal layers diverged significantly from the surface and bed echoes or undisrupted i e internal layers closely parallel surface and basal topography and we calculated subglacial roughness along basal reflectors where satellite measured surface ice flow speeds are available covering 39 of the study region regions of fast and tributary flow correspond with res flight tracks that exhibit more disrupted internal layers and smoother subglacial topography than their counterparts in regions of slow flow this suggests that disrupted internal layering and smooth subglacial topography identified from res profiles can be treated as indicators of past or present enhanced flow tributaries where neither satellite nor ground based ice flow measurements are available we therefore use these res derived parameters to assess the balance flux modeled steady state flow regime between the south pole and filchner ronne ice shelf the res analysis confirms that an organized flow network drains a wide region around the south pole into the filchner ronne ice shelf however the spatial extent of this network as delineated by the res data diverges from that predicted by currently available balance flux models

the basal roughness of pine island glacier west antarctica we assess basal roughness beneath pine island glacier pig west antarctica based on a recent airborne radio echo sounding dataset we identify a clear relationship between faster ice flow and decreased basal roughness in significant parts of pig the central portion and two of its tributaries are particularly smooth but the majority of the tributaries feeding the main trunk are rougher we interpret the presence of a smooth bed as being a consequence of the deposition of marine sediments following disappearance of the west antarctic ice sheet in the pliocene or pleistocene and conversely a lack of marine sedimentation where the bed is rough importantly we also identify a patchy distribution of marine sediments and thus a bed over which the controls on flow vary while there is a notable correspondence between ice velocity and bed roughness we do not assume a direct causal relationship but find that an indirect one is likely where low basal roughness results in low basal resistance to flow a lower driving stress is required to produce the flux required to achieve mass balance this in turn means that the surface in that area will be lower than surrounding areas with a rougher bed and this will tend to draw flow into the area with low bed roughness since our studies shows that bed roughness beneath the tributaries of the trunk varies substantially there is a strong likelihood that these tributaries will differ in the rate at which they transmit current velocity changes on the main trunk into the interior of the glacier basin

quantifying subglacial bed roughness in antarctica implications for ice sheet dynamics and history abstract glaciated landscapes consist of complex assemblages of landforms resulting from ice flow dynamic regimes and ice sheet history superimposed over and in turn modifying preglacial topography lithology and geological structure insights into the formation of glaciated landscapes can in principle be obtained by analysing modern ice sheet beds but terrain analyses beneath modern ice sheets are restricted by the inaccessibility of the bed it is however possible to quantify roughness the vertical variation of the subglacial interface with horizontal distance along two dimensional images of the bed obtained from radio echo sounding res here we collate several case studies from antarctica where roughness calculations have been used as a glaciological tool to infer basal processes and ice sheet history over large 500 km 2 areas we present two examples from west antarctica which demonstrate the utility of bed roughness in determining the presence and extent of subglacial sediments glacial dynamics and former ice sheet size we also present two examples from east antarctica which illustrate how roughness provides knowledge of ice sheet dynamics in the interior and pre quaternary ice sheet histories in modern ice sheet settings characterising bed roughness along res tracks has the twin advantages of being relatively simple to calculate while producing informative subsurface data and is especially powerful at furthering understanding when coupled with knowledge of ice flow from field satellite and modelling investigations the technique also offers significant potential for the comparison of modern and former ice sheet terrains contributing to an improved understanding of the formation and evolution of glaciated landscapes

pbzn mineralization associated with diapirism fluid inclusion and stable isotope h c o evidence for the origin and evolution of the fluids at fedj el adoum tunisia pb zn mineralizations occur in the contact zone to a saliferous diapir and its surrounding cretaceous limestonerich sediments the diapir of triassic formations was initially intruded hot from a depth of 3 7 km during the cretaceous three principal types of mineralization occur bedded vein and massive combined microthermometric and stable isotope h c o studies have characterized three principal thermal events before cool meteoric waters entered the system 1 1 hot 170 250 c saline 22 33 eq wt nacl fluids of probable deep formation water origin with d 43 and 18o 20 precipitated calcite with 13c 8 and celestite in the inner part of the contact zone with the diapir 2 2 hot 100 130 c saline 18 23 eq wt nacl fluids with d 60 and 18o 13 containing co2 n2 and hydrocarbons precipitated sphalerite calcite 13c 9 quartz and minor galena as banded mineralization in the outer part of the contact zone more galena rich vein mineralizations in the contact zone were deposited at similar temperatures from comparable fluids mineralizing solutions are interpreted to be formation waters with the carbonate carbon being derived from organic carbon the colloform textures in the banded mineralization suggest that the nearby sedimentary basin underwent episodic dewatering 3 3 warm 70 90 c saline 16 22 eq wt nacl fluids with d 110 and 18o 10 precipitated calcite with 13c 16 galena rich ore in the massive mineralizations and vein mineralizations in the cenomanian turonian limestones and essentially sulphide free calcite in the banded and contact vein mineralizations a locally derived organic water origin is proposed where both hydrogen and carbon were derived from organic constituents such as petroleum during bacterial sulphate reduction reactions variable d and salinity values suggest mixing processes between the d depleted organic water and a more d rich formation or meteoric water 4 4 low temperature 50 c meteoric waters with d 15 to 25 introduced a second generation of calcite with the carbonate largely coming from the sedimentary carbonates in the massive and turonian vein mineralizations anhydrite was probably converted to gypsum at about this time and before fedj el adoum arrived at its present altitude

evidence of a hydrological connection between the ice divide and ice sheet margin in the aurora subglacial basin east antarctica subglacial hydrology in east antarctica is poorly understood yet may be critical to the manner in which ice flows data from a new regional airborne geophysical survey icecap have transformed our understanding of the topography and glaciology associated with the 287 000 km2 aurora subglacial basin in east antarctica using these data in conjunction with numerical ice sheet modeling we present a suite of analyses that demonstrate the potential of the 1000 km long basin as a route for subglacial water drainage from the ice sheet interior to the ice sheet margin we present results from our analysis of basal topography bed roughness and radar power reflectance and from our modeling of ice sheet flow and basal ice temperatures although no clear cut subglacial lakes are found within the aurora basin itself dozens of lake like reflectors are observed that in conjunction with other results reported here support the hypothesis that the basin acts as a pathway allowing discharge from subglacial lakes near the dome c ice divide to reach the coast via the totten glacier

extent and dynamics of the west antarctic ice sheet on the outer continental shelf of pine island bay during the last glaciation abstract swath bathymetry and topas sub bottom profiler acoustic data reveal the presence of a major cross shelf bathymetric trough containing streamlined subglacial bedforms formed in soft till that extends to the continental shelf edge of outer pine island bay amundsen sea west antarctica the new data indicate that the west antarctic ice sheet wais extended to the shelf edge of pine island bay during the last glaciation and was drained by a grounded fast flowing palaeo ice stream through this outer shelf trough topography and subglacial geology are likely to have exerted a strong control on the development and location of palaeo ice streams in pine island bay topas records show that mega scale glacial lineations are formed in an acoustically transparent sediment layer interpreted to be soft till and are inferred to be the product of subglacial sediment deformation beneath the palaeo ice stream the flat to irregular nature of the basal reflector of the acoustically transparent layer of soft till implies that a combination of groove ploughing of the substrate and subglacial sediment deformation were important processes beneath the palaeo ice stream in the outer trough of pine island bay as well as palaeo ice streams elsewhere in antarctica therefore the formation of the soft till layer and possibly associated subglacial bedforms are likely to be a function of these palaeo ice stream processes in a regional context the wais extended to the continental shelf edge of the bellingshausen amundsen and ross seas and was drained by a number of palaeo ice streams during the last glaciation

formation of mega scale glacial lineations observed beneath a west antarctic ice stream most discharge from large ice sheets takes place through fast flowing ice streams and their speed is strongly modulated by interactions between the ice and the underlying sediments seismic surveys and investigations through boreholes have revealed a spatial association between fast ice flow and saturated deformable sediments nevertheless our knowledge of the morphology of the interface between ice and sediments is still limited resulting in only rudimentary understanding of the basal boundary conditions beneath ice streams and the generation of subglacial bedforms here we present radar data from the bed of a west antarctic ice stream that reveal the presence of mega scale glacial lineations we combine these data with previously published seismic data and show that these lineations develop in areas of dilatant deforming till and are part of a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales we find that the mega scale glacial lineations are indistinguishable from those found on beds of palaeo ice streams providing conclusive evidence for the hypothesis that highly elongate bedforms are a characteristic of fast flow regions in ice sheets

cl br and i in igneous standard rocks abstract radiochemical neutron activation analysis rnaa and isotope dilution mass spectrometry idms are applied to the determination of cl br and i in igneous standard rocks seventeen different standard samples of igneous rocks distributed by the geological survey of japan gsj and by the association national de la recherche technique anrt in france were investigated the cl contents range from 10 to 1000 ppm br from 0 05 to 2 5 ppm and i from 5 to 100 ppb in this study four standard samples for cl eight for br and six for i were newly determined the analytical results between rnaa and idms are in good agreement within one standard deviation the halogen contents are found to be well correlated for cl and br but less clearly for i halogens generally are enriched in rhyolites in the volcanic rock series while they are depleted in plutonic rock series especially in granite

radar derived bed roughness characterization of institute and moller ice streams west antarctica and comparison with siple coast ice streams 1 subglacial bed conditions exert a significant control on ice stream behavior and evolution and can be characterized by determining bed roughness from fft analysis of radar imaged basal reflectors here we assess bed roughness across institute and moller ice streams west antarctica and compare our findings with bed roughness determined across the siple coast ice streams we find that variations in bed roughness are spatially organized and attribute this to the varying efficacy of subglacial erosion and deposition with rougher inland slow flowing regions largely manifesting preglacial topography and smoother downstream fast flowing regions evincing significant postglacial modification to the subglacial landscape the observed similarities between bed roughness characteristics of iis mis and the siple ice streams suggest that iis and mis are largely underlain by wet poorly consolidated sediments and may therefore be vulnerable to the types of dynamical instabilities experienced by the siple ice streams

origin of vein hydrothermal carbonates in triassic limestones of the espadan ranges iberian chain e spain dolomitization and hg bearing veins in the lower triassic limestones of the espadan ranges iberian chain e of spain

macro scale bed roughness of the siple coast ice streams in west antarctica subglacial bed conditions are key to understanding ice stream behaviour and evolution with bed roughness re ecting substrate composition and ow resistance here we present an analysis of bed roughness in the siple coast region of west antarctica from airborne radio echo sounding data the ice streams are associated generally with low bed roughness values which decrease downstream the bed of the slow owing ice stream c 10 000 km2 is also characterized by being smooth at all scales wavelengths ranging from 5 km to in excess of 40 km furthermore the bed is smooth either side of ice stream c this suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology if the ice stream were encouraged to migrate laterally when active there would be little resistance offered by the subglacial morphology other inter ice stream regions are rough however indicating a subglacial topographic in uence on ice stream position bed roughness increases up ow of ice streams which unless the bed is modi ed may limit the inland migration of these systems copyright 2004 john wiley sons ltd

basal roughness of the institute and moller ice streams west antarctica process determination and landscape interpretation we present a detailed analysis of bed roughness beneath institute and moller ice streams west antarctica using radio echo sounding data res acquired in the austral summer of 2010 11 we assess roughness using a two parameter approach and also assess the directionality of roughness relative to present day ice flow our work highlights the wealth of additional information that resides in analyses of bed roughness employing these multiple approaches we show that spatially variable roughness patterns are partly a consequence of the ability of flowing ice not only to smooth the bed but also to redistribute and remove sediments and to do this along flow accordingly we identify some fast flow tributaries underlain by topography that has been streamlined and other tributaries that are underlain by sediments we also identify locations that are currently protected from erosion but where more ancient erosion may once have occurred we conclude that detailed roughness analysis is a useful tool for landscape interpretation and we suggest that the roughness of an ice sheet s bed should be viewed not only as the consequence of ancient marine sedimentation but also as a product of more contemporary erosion and redistribution of sediments as well as bedrock smoothing that is ongoing because of continuing dynamic activity in this way basal roughness has the potential to evolve continually with ice sheet form and flow and should not be viewed simply as a snapshot of either present day or palaeo basal conditions

evidence for late pleistocene ice stream activity in the witch ground basin central north sea from 3d seismic reflection data abstract buried submarine landforms mapped on 3d reflection seismic data sets provide the first glacial geomorphic evidence for glacial occupation of the central north sea by two palaeo ice streams between 58 59 n and 0 1 e streamlined subglacial bedforms mega scale glacial lineations and iceberg plough marks within the top 80 m of the quaternary sequence record the presence and subsequent break up of fast flowing grounded ice sheets in the region during the late pleistocene the lengths of individual mega scale glacial lineations vary from 5 to 20 km and the distance between lineations typically ranges from 100 to 1000 m the lineations incise to a depth of 10 12 m with trough widths of 100 m the most extensive and best preserved set of lineations is attributed to the action of a late weichselian ice stream which either drained the ne sector of the british irish ice sheet or was sourced from the sw within the fennoscandian ice sheet the 30 50 km wide palaeo ice stream is imaged along its flow direction for 90 km trending nw se an older set of less well preserved lineations is interpreted as an earlier weichselian or saalian ice stream and records ice flow in an sw ne orientation cored sedimentary records tied to 3d seismic observations support grounded ice sheet coverage in the central north sea during the last glaciation and indicate that ice flowed over a muddy substrate that is interpreted as a deformation till the identification of a late weichselian ice stream in the witch ground area of the north sea basin provides independent geomorphic evidence in support of ice sheet reconstructions that favour complete ice coverage of the north sea between scotland and norway during the last glacial maximum

new boundary conditions for the west antarctic ice sheet subglacial topography beneath pine island glacier 1 predictions about future changes in the amundsen sea sector of the west antarctic ice sheet wais have been hampered by poorly known subglacial topography extensive airborne survey has allowed us to derive improved subglacial topography for the pine island glacier basin the trunk of this glacier lies in a narrow 250 km long 500 m deep sub glacial trough suggesting a long lived and constrained ice stream two tributaries lie in similar troughs others lie in less defined shallower troughs the lower basin of the glacier is surrounded by bedrock which after deglaciation and isostatic rebound could rise above sea level this feature would impede ice sheet collapse initiated near the grounding line of this glacier and prevent its progress into the deepest portions of wais the inland slope of the bed beneath the trunk of the glacier however confirms potential instability of the lower basin containing sufficient ice to raise global sea by 24 cm

mesozoic extensional tectonics stratigraphy and crustal evolution during the alpine cycle of the eastern iberian basin abstract sequence stratigraphy subsidence analysis and the integration of the basin fill data allow to the identification of four successive evolutionary stages in the basins of the eastern iberian margin during mesozoic extension 1 triassic rift late permian hettangian 2 early and middle jurassic postrift sinemurian oxfordian 3 late jurassic and early cretaceous rift kimmeridgian middle albian and 4 late cretaceous postrift late albian maastrichtian the present day crustal structure of the eastern iberian range and evidence of its evolution are deduced from the analysis of a new gravity map and other geophysical data a regional gravity low along the iberian range is interpreted in terms of crustal thickening beneath the orogen as a result of the collision between the ebro block and the iberian plate during the paleogene the boundary between these blocks coincides with a magnetic lineament that is interpreted as an inherited signature of the thinning that occurred during mesozoic extension a geodynamic scenario for the crustal evolution of the eastern part of the iberian peninsula based on the evaluation of mesozoic extensional tectonics and an analysis of the available geophysical data includes three main successive evolutionary stages 1 1 mesozoic crustal thinning 2 2 paleogene crustal thickening 3 3 neogene crustal thinning based on new geological and geophysical arguments this study offers an alternative and a more complete geodynamic history of the eastern iberian basins related to the evolution of the iberian plate and the central north atlantic

fluid inclusions in pre ore minerals from the carbonate hosted mercury deposits in the espadan ranges eastern spain abstract the fluids associated with pre ore minerals in the carbonate hosted mercury veins from the espadan ranges eastern spain have been studied using microthermometry cryo sem eds raman spectroscopy and crush leach analyses in order to determine their origin and the processes responsible for mineralization three primary fluid inclusion types have been found in quartz and dolomite 1 type s composed of liquid vapour mainly co 2 halite 2 type s v composed of liquid high density co 2 double bubble halite and 3 type v composed of high density co 2 n 2 mixtures type s fluid inclusions are preferentially trapped in betxi type veins characterized by the presence of hydrothermal dolomite whereas type v fluid inclusions are trapped almost exclusively in eslida type veins which were without dolomite from these data two different fluids can be inferred to occur during the period of formation of the veins 1 a complex polysaline brine of nacl kcl cacl 2 mgcl 2 so 4 2 h 2 o composition with a salinity of up to 33 eq wt nacl and a 2 co 2 n 2 rich fluid the brine halogen composition points to its secondary origin caused by the dissolution of underlying marine evaporites it is thought that these deposits were formed during jurassic or cretaceous rifting periods as a consequence of fluid mixing between a high salinity secondary brine and connate waters the generation of a co 2 n 2 rich phase caused the pressure increase of the fluids that favoured the hydraulic fracturing of the enclosing rock dolostone the origin of this volatile rich phase probably occurred due to the oxidation and thermochemical breakdown of the organic matter contained in the host rock this event is recorded by the coetaneous trapping of both aqueous and carbonic fluids in quartz crystals the formation conditions for the hypogene sulphide paragenesis are obscure but the coincidence between the preferential trapping of type v fluid inclusions the absence of hydrothermal dolomite and the major abundance of mercury in eslida type veins is thought to be linked to a mercury fractionation event to the vapour phase during the release of the volatiles

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

geochemistry of the mercur gold deposit utah u s a abstract the mercur gold deposits of west central utah are disseminated carlin type deposits which are hosted by the upper portion of the topliff member of the mississippian great blue limestone strata bound alteration and mineralization are related to sedimentary lithologies over a 60 80 m section of the topliff member hydrothermal alteration at mercur can be separated into replacement and vein alteration assemblages two replacement assemblages are recognized jasperoid facies consists of replacement of massive limestone by silica and minor sericite and kaolinite argillic facies alteration is characterized by decarbonation of silty limestones fluids which formed the argillic facies became progressively more reduced and acidic through reaction with carbonaceous argillaceous pyritiferous limestones three vein assemblages are recognized at mercur 1 pyrite orpiment organics marcasite 2 calcite realgar and 3 halloysite barite calcite considerable temporal and spatial overlap exists among all hydrothermal assemblages varying degrees of au mineralization are associated with most styles of alteration fluid inclusion studies show that jasperoid facies rocks were formed at temperatures of 220 270 c hydrothermal calcite at 150 190 c and barite at 150 300 c salinities throughout the paragenesis remained relatively constant at 5 8 eq wt nacl a geochemical model based on fluid inclusion data microprobe data and mineral solubilities allows constraints to be placed on the chemical characteristics of the mercur ore forming fluids jasperoid facies rocks were formed by fluids at ph 3 6 p co 2 f o 2 34 to 32 the argillic facies vein assemblages were formed at a ph of 4 0 p co 2 of 100 bar log f o 2 of 40 4 to 39 6 and log f h 2 s of 3 1 to 2 7 the available data indicate oreforming solutions are near a au solubility minimum but do not allow an exact determination of the mechanism of au deposition

saddle dolomite as a by product of chemical compaction and thermochemical sulfate reduction petrographic stable and radiogenic isotope trace element and fluid inclusion data from the devonian nisku reef trend in the subsurface of alberta canada suggest that saddle dolomite was a by product of chemical compaction and thermochemical sulfate reduction in subsiding dolomitized rocks furthermore thermochemical sulfate reduction may be representative of other processes that increase the amount of saddle dolomite via an increase in carbonate alkalinity the data also confirm that saddle dolomite is most likely to form at elevated temperatures from hypersaline brines

a millennium of variable ice flow recorded by the ross ice shelf antarctica an enhanced composite advanced very high resolution radiometer avhrr image is used to map flow stripes and rifts across the ross ice shelf antarctica the patterns of these flow related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf the most profound variations are visible in the track of rifts downstream of crary ice rise flow stripe bends to the west of this ice rise and adjacent to steershead ice rise and changes in the northern margin of ice stream b the track of rifts downstream of crary ice rise indicates that the ice rise has existed for at least 700 years the character of this track changes about 350 km downstream indicating a rearrangement of flow patterns about 550 years ago the large bulge in the flow stripes to the west of crary ice rise is shown in detail with bent flow stripes extending for several hundred kilometers along flow this feature formed from the south possibly due to a change in the discharge of ice stream a the avhrr image documents a complex history associated with the shutdown of ice stream c with changes in the margins of ice stream c and the northern margin of ice stream b and the grounding of steershead ice rise with an associated bending and truncation of flow stripes landsat imagery shows a region that appears to be actively extending just downstream of the ice rise as the shelf continues to respond to recent changes in ice stream discharge we present a four stage flow history which accounts for the features preserved in the ice shelf

fluid mixing and recycling during pyrenean thrusting evidence from fluid inclusion halogen ratios abstract syntectonic fluids have been sampled through fluid inclusion microthermometry and crush leach analyses cations and halogens from a 50 km n s transect through the central southern pyrenees the fluid inclusions are contained in syntectonic quartz veins in triassic redbeds cretaceous carbonates and hercynian basement rocks with some calcite and dolomite data from limestones and evaporites in more external parts of the belt the main datasets come from 1 alpine shear zones cutting the neouvielle granodiorite in the hercynian axial zone at the north end of the transect 2 an imbricate zone beneath the alpine gavarnie thrust at the pic de port vieux 3 several localities in the footwall and hangingwall of the gavarnie thrust on the southern margin of the axial zone the inclusion fluids generally decrease in salinity from 27 35 at the northern end of the transect to 7 22 on the southern margin of the axial zone the majority of the inclusions have cl br ratios lower than seawater and are interpreted as relict fluids after seawater evaporation and halite precipitation in the upper trias this interpretation is supported by cl br na systematics which are consistent with a change from halite to halite sylvite precipitation with progressive evaporation fluids in the basement shear zones are interpreted to have essentially the same evaporitic origin as those still contained in sedimentary formations although it is possible that final concentration of brines in the neouvielle massif involved retrograde hydration reactions with removal of water by precipitation of hydrous minerals the fluids are also very similar in salinity and halogen chemistry to those found in veins associated with mesozoic pb zn f deposits which predate the thrusting the lower salinities seen at the southern margin of the axial zone are interpreted to reflect mixing of the brines with a higher level fluid connate or meteoric water circulating within the mesozoic carbonates of the higher thrust sheets at one locality where triassic evaporites are still present high cl br ratios at relatively low salinities are present in inclusions within the underlying triassic redbeds but low cl br ratios at higher salinities are seen lower in the sequence this is consistent with dissolution of halite by a dilute fluid but with limited penetration downwards we suggest that the fluid history of the pyrenees evolved through a series of stages 1 upper triassic evaporite formation with sinking of brines into underlying redbeds and fractured basement rocks 2 circulation of brines with formation of pb zn deposits along faults at some time between the triassic and the upper cretaceous 3 renewed extension with erosion of triassic rocks in many areas and further drawing down of triassic brines into the basement 4 deposition of u cretaceous and palaeocene carbonates containing connate waters of marine origin 5 formation of the pyrenean thrust belt with overpressuring and expulsion of the brines along shear zones and faults 6 creation of topography with a high level circulation system in the mesozoic thrust sheets driven largely by topography at the southern margin of the axial zone there was limited mixing of the deeper overpressured brines with these more dilute hydrostatically pressured fluids an important point is that because of their density hypersaline brines are difficult to expel from the upper crust and may be involved in a succession of alteration and mineralisation events in the same general area over hundreds of millions of years

origin of the atrevida vein catalonian coastal ranges spain mineralogic fluid inclusion and stable isotope study atrevida is a ba f base metal vein located in the catalonian coastal ranges of spain about 100 km southwest of barcelona the vein cuts paleozoic metasediments and granites of the hercynian basement and triassic red beds at the base of an overlying transgressive sequence the atrevida vein can be divided into three parts on the basis of mineralogy the lower level contains sugarlike quartz fluorite minor barite the middle level contains a breccia of banded barite basement rocks cemented by a complex sulfide arsenide assemblage with galena ni sulfide ag sulfosalt and native silver veinlets the upper level contains symmetrical bands of white barite late calcite and marcasite are found in geodes the only alteration observed is quartz sericite chlorite after k feldspar and biotite where the vein cuts granite host rocks fluid inclusions in fluorite homogenize at 110 degrees c and have salinities of 21 wt percent nacl equiv inclusions in sphalerite homogenize at 90 degrees c and have salinities of 22 wt percent nacl equiv eutectic results in both minerals are as low as 59 degrees c indicating that the trapped fluids are complex cacl 2 bearing polysaline brines fluid inclusions in late calcite homogenize at 0 3 degrees c indicating that low salinity waters invaded the vein system during calcite deposition sulfur and oxygen isotope analyses reveal that vein barites have delta 34 s values of 14 to 17 9 per mil cdt and delta 18 o values of 9 5 to 12 2 per mil smow barite also occurs as cement in the triassic red beds with delta 34 s 15 3 per mil and delta 18 o 10 6 per mil both the sulfur and the oxygen isotope compositions are within the ranges reported for lower triassic to lower jurassic age evaporites banded galena from the middle level of the vein has delta 34 s values between 6 3 and 8 9 per mil and rimming anglesite has delta 34 s values of 21 to 30 9 per mil galena from the central breccia has delta 34 s values of 14 9 to 16 1 per mil and coexisting sphalerite is about 10 per mil quartz in the lower level of the vein has delta 18 o in the range 20 7 to 22 7 per mil we infer that sulfate in the vein was derived directly from seawater or from a solution which dissolved evaporites as it descended through the triassic section along the fault zone barite and sulfides were precipitated where the sulfate bearing solution mixed with an ascending ba rich brine the reduced sulfur found in the vein may have been carried by the ascending brine banded galena and may also have been produced by bacterial reduction of sulfate from a surficial source galena in breccia oxidizing nonmarine waters were introduced into the hydrothermal system giving rise to anglesite rims on galena mineralization took place in the late triassic during an extensional tectonic regime characterized by a high geothermal gradient atrevida and similar barite veins found throughout the catalonian coastal ranges formed along basin margin faults near a paleosurface formed on hercynian basement on a broader scale low temperature veins are known to occur throughout western europe and northern africa if atrevida is representative of the group then the formation of these veins may be linked to mesozoic extensional tectonics

paleomagnetic results from the southeastern margin of the ebro basin northeastern spain evidence for a tertiary clockwise rotation abstract triassic rocks underlying the paleogene deposits of the ebro basin are exposed along its se margin about 200 samples of middle triassic and paleogene rocks from this margin have been demagnetized and analysed the middle triassic red beds contain one or two secondary components of magnetization the high unblocking temperature component has a mean direction of d 1 and i 6 the eocene red beds display a very stable magnetization nevertheless some samples have a high temperature component with the present day field direction the eocene mean direction is d 22 and i 51 the paleomagnetic results show a systematic 20 clockwise rotation relative to the data from the pyrenean and cantabrian ranges and the western and sw part of iberia to explain these results two hypotheses are presented one implies the rotation of the ne part of iberia whereas the other refers to a rigid block rotation mechanism along the catalan coastal range

characterization of subglacial landscapes by a two parameter roughness index previous studies using fourier transformation ft methods to analyze subglacial roughness have shown promise for distinguishing between different types of subglacial landscape from raw subglacial elevation data we derive a two parameter ft roughness index whereis based on the ft of elevation as previously considered in isolation andis based on both the ft of elevation and the ft of bed slope profile in this way we take account of both vertical and horizontal irregularities in subglacial surfaces we demonstrate the statistical veracity of using to consider roughness in terms of obstacle amplitudes and spacing and consider the use of in studies of ice dynamics and subglacial geomorphological interpretation we show that can be linked to basal sliding rates on the metre scale and can be used to differentiate further than single parameter roughness indices between different classes of subglacial landscape in particular between erosional and depositional settings

radio echo sounding of pine island glacier west antarctica aperture synthesis processing and analysis of feasibility from space airborne radio echo sounding of the west antarctic ice sheet over pine island glacier was performed in the austral summer of 2004 2005 under the national science foundation s west antarctic ice sheet initiative the british antarctic survey flew its newly developed 150 mhz ice sounding radar over pine island glacier and collected approximately 35 000 km of sounding data the synthetic aperture radar sar technique was applied to process those data in order to enhance radar signatures from the bed as a matter of fact airborne ice sounding radar systems are generally affected by surface clutter returns masking the echoes of internal layers and ice bedrock transition at a large depth focused and unfocused doppler filtering sar techniques were compared and their respective advantages disadvantages were analyzed enhancement of bedrock detection at a large depth 2000 m through sar processing is demonstrated finally a simulation analysis was performed for assessing the feasibility of ice sheet sounding from space it is shown that the gain in bed detection threshold is marginal in the satellite sounding geometry airborne radar antarctica ice sounding satellite remote sensing subglacial topography synthetic aperture radar sar processing west antarctic ice sheet

na cl br systematics of mineralizing brines in mississippi valley type deposits new and published analyses of fluid inclusion leachates from mississippi valley type mvt deposits can be divided into two groups on the basis of na br and cl br ratios mvt leachates from the illinois kentucky and cincinnati arch districts and from cubic galena in the viburnum trend have na br and cl br ratios that extend to values significantly above that of seawater which are characteristic of evaporite dissolution brines mvt leachates from polaris and octahedral galena in the viburnum trend have na br and cl br ratios that plot below seawater and along the compositional trend formed by evaporation solubility volume constraints require that brines formed by seawater evaporation had high dissolved metal contents preliminary correlation of leachate compositions allows delineation of two brine provinces in the midcontinent united states 1 an early high br province found only in southeastern missouri and 2 a later low br province of probable permian age that extended from the cincinnati arch to the tri state district these observations along with na cl br data for modern brines from the illinois basin argue against models for single stage midcontinent mvt brine flow based on recharge from late paleozoic arkoma foreland basins to the south

basal conditions beneath enhanced flow tributaries of slessor glacier east antarctica radio echo sounding data are used to investigate bed roughness beneath the three enhanced flow tributaries of slessor glacier east antarctica slow moving inter tributary areas are found to have rough beds while the bed of the northernmost tributary is relatively smooth a reconstruction of potential subglacial drainage routing indicates that water would be routed down this tributary and investigations of basal topography following isostatic recovery reveal that the bed would have been below sea level in preglacial times so marine sediments may have accumulated here together these factors are further support for the dominance of basal motion in this tributary reported elsewhere conversely although the other two slessor tributaries may have water routed beneath them they would not have been below sea level before the growth of the ice sheet so cannot be underlain by marine sediments they are also found to be rough and within the range of uncertainties it is likely that basal motion does not play a major role in the flow of these tributaries perhaps the most interesting area however is a deep trough where flow rates are currently low but the bed is as smooth as the northern slessor trough it is proposed that although ice deformation currently dominates in this trough basal motion may have occurred in the past when the ice was thicker

the structure of west antarctica from geophysical studies the subglacial topographical and geological configurations of west antarctica and the boundary zone between east and west antarctica are poorly known yet they are crucial to our understanding of the tectonic evolution of the pacific proto atlantic margin of gondwanaland preliminary results are presented of simultaneous airborne radio echo and magnetic sounding of 106 km2 of west antarctica including detailed maps of ice sheet and bedrock surfaces magnetics radio echo and other available geophysical data suggest interpretations of sub glacial geology

paleogeographical significance of the rot anisian triassic facies marines clays muds and marls fm in the iberian ranges eastern spain abstract the marines clays muds and marls formation consists of fine grained sediments recording the transition from distal fluvial facies to shallow marine sediments representing the onset of the first marine transgression of the tethys sea in the iberian basin muds show irregular parallel lamination of possible cryptalgal origin wave ripples linsen bedding mud cracks vertical burrows and halite pseudomorphs and contain anisian palynomorphs the carbonate content increases towards the se the deposition of the marines formation took place in a rift the iberian basin during a period of distensive tectonics and probably during a marine transgression of eustatic origin but the relative importance of the tectonic and eustatic factor is a matter of speculation in the area up to now

megagrooves and streamlined bedrock in nw scotland the role of ice streams in landscape evolution abstract new multibeam bathymetry data onshore high resolution elevation data nextmap and fieldwork in the ullapool area of nw scotland reveal large scale megagrooves and streamlined bedrock forms in a well defined 20 km wide zone the landsystem is typical of a coherent flow corridor within a grounded ice sheet on bedrock dominated terrain we describe the morphology of the large scale features discuss their likely formation and consider the wider implications for ice sheet dynamics based on the strongly convergent bedform distribution the presence of megagrooves and highly elongate bedrock forms we interpret the erosional landscape to be the signature of a fast flowing tributary that once fed the the minch palaeo ice stream a major artery of the last british irish ice sheet the exact genesis of bedrock megagrooves remains uncertain although focused subglacial abrasion is likely to have carved most of the shallow strongly parallel features whilst glacial meltwater may have carved or modified others bedform morphometry is used to discriminate zones reflecting the degree of glacial streamlining elongation ratios 5 1 we interpret these zones to represent the transition from potentially cold based slow ice sheet flow to warm based fast flow based on these results and the presence of ribbed moraines we suggest a bedform continuum model for onset zones of palaeo ice streams on rigid beds rapid spatial bedform evolution is suggested to reflect an increase in subglacial erosive power that may be diagnostic of palaeo ice sheet thermal boundaries i e from cold to warm based and is also consistent with the expected downstream increase in ice velocity within an ice stream onset zone finally this study speculates on the role played by basal meltwater in ice stream initiation and the role of ice streams and their tributaries in landscape evolution

the age and origin of the barite fluorite pbzn veins of the sierra del guadarrama spanish central system spain a radiogenic nd sr and stable isotope study fluorite and barite rich veins sulphides of the sierra del guadarrama spanish central system are usually hosted by hercynian granitoids zoned fluorites from one of these veins have a sm nd age of 145 18 ma this age is similar to the k ar ages obtained in other hydrothermal rocks of the spanish central system 156 152 ma and confirms the existence of an important hydrothermal event during the late jurassic probably related to extensional fracturing accompanying the opening of the north atlantic ocean the 87sr 86sr ratio found in fluorite and barite 0 7154 0 7207 is in most cases lower than that found in the host granitoids 0 7194 and 0 7209 suggesting more than one source of strontium this in turn supports previous models that involve mixing of two fluids during mineralization an ascending hot 300 c and low salinity fluid and a shallow cooler 100 c and more saline brine the ascending fluid probably reacted to a variable extent with the host granitoids and the most likely source for the shallow fluid derived from a marine evaporitic environment with a low 87sr 86sr ratio 0 7069 in the late jurassic the calculated end composition of the fluorites 8 8 to 7 6 as well as the 147sm 144nd variation found in this mineral 0 153 0 405 do not conflict with the mixing model although they can be largely explained by interaction of the ascending fluid with the host granitoids sulphur isotopic compositions of sulphides range between 3 4 and 0 8 and those of barites from 15 5 to 15 8 the near 34s value of the sulphides suggests that their sulphur was derived from the hydrothermal leaching of the granitoids by the ascending fluid the 34s value of barite is consistent with upper jurassic marine sulphate as the source of sulphate in the shallow fluid the absence of equilibrium between reduced and oxidized sulphur species can be easily interpreted as a consequence of the short residence time of the mixed fluid in the open fractures at temperatures between 250 and loo c although no pre cretaceous sediments are found on this part of the iberian hercynian basement the data presented here support the existence of a shallow platform joining central spain with the iberian or the subbetic realm in late jurassic times

reactivity of dolomitizing fluids and mg source evaluation of fault controlled dolomitization at the benicassim outcrop analogue maestrat basin e spain abstract the mechanisms responsible for the formation of huge volumes of dolomitized rocks associated with faults are not well understood we present a case study for high temperature dolomitization of an early cretaceous aptian albian ramp in benicassim maestrat basin e spain in this area seismic scale fault controlled stratabound dolostone bodies extend over several kilometres away from large scale faults this work aims at evaluating different mg sources for dolomitization estimating the reactivity of dolomitizing fluids at variable temperature and quantifying the required versus available fluid volumes to account for the benicassim dolostones field relationships stable 13 c and 18 o isotopes as well as radiogenic 87 sr 86 sr isotopes indicate that dolomitization at benicassim was produced by a high temperature fluid 80 c 13 c and 18 o isotopic compositions for dolomite vary from 0 5 to 2 9 v pdb and from 21 1 to 24 3 v smow respectively a mg source analysis reveals that the most likely dolomitizing fluid was seawater derived brine that interacted with underlying triassic red beds and the paleozoic basement geochemical models suggest that evolved seawater can be considerably more reactive than high salinity brines and the maximum reactivity occurs at about 100 c mass balance calculations indicate that interstitial fluids with high pressure and or high temperature relative to the normal geothermal gradient cannot account for the volume of dolomite at benicassim instead a pervasive fluid circulation mechanism like thermal convection is required to provide a sufficient volume of dolomitizing fluid which most likely occurred during the late cretaceous post rift stage of the maestrat basin this study illustrates the importance of fluid budget quantification to critically evaluate genetic models for dolomitization and other diagenetic processes

kar ages of clay size concentrates from the mineralisation of the pedroches batholith spain and evidence for mesozoic hydrothermal activity associated with the break up of pangaea abstract the k ar ages of 32 clay concentrates extracted from samples of ore gangue and wallrock associated with mineralisation in the pedroches batholith lie in the range 119 285 ma although some of the mineralisation is of permian age more than half of the ages lie between 210 and 230 ma and indicate a hydrothermal event at this triassic time a comparison with age data for mineralisation and certain anorogenic magmatism in other areas of the north atlantic reveals a consistent pattern of a major event at ca 210 230 ma and a minor event at ca 160 ma with little cretaceous or tertiary activity it is proposed that the ca 210 230 ma event was related to an initial rapid fracturing of the crust associated with the break up of pangaea which was related with an increase in the geothermal gradient and penetration of the deep crust by surface waters which returned via both new and rejuvenated fissure systems

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

fluorite deposits at encantada buenavista mexico products of mississippi valley type processes ore geol rev 23 2003 107 124 a reply we very much appreciate the comments of dr spirakis regarding the fluorite deposits of la encan tada buenavista the contribution by gonza lez par tida et al 2003 represents a partial report of thework we are carrying out on the fluorite deposits ofnorthern coahuila state mexico we have subse quently conducted further work on the deposit inorder to complement the information already pub lished addressing controls on the distribution of themineralized bodies and halogen studies of fluid inclu sions we discuss here in light of our new results theproblem of fluorine transport and precipitation at laencantada buenavista new developments in the mine showed that there isa strong structural control on the distribution offluorite bodies the latter are found at the intersectionof low permeability barriers with low angle faults layer parallel slip developed at stratification bound aries with abundant clayey materials precipitation offluorite is preceded by a dramatic increase in perme ability due to the hydraulic fracturing of the host rock enclosing rock fragments included in the newlydeveloped breccias are usually recrystallized andslightly corroded but are also rapidly covered byfluorite the precipitation of which appears to precludethe continuance of carbonate replacement a thincalcite blanket typically a few centimeters to deci meters thick is found at the boundary of the fluoritebody smoothly passing to the slightly recrystallizedenclosing limestone fluorite appears as white tocolorless corrosion free crystals indicative of passivemineral precipitation late cavities are filled up byidiomorphic cubic zoned fluorite crystals with adeep purple color in their outer zones minor idiomor phic calcite scalenoedrons and extremely rare crystalsof barite

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

sedimentary processes at the base of a west antarctic ice stream constraints from textural and compositional properties of subglacial debris samples of sediments from beneath ice stream b at camp upb west antarctica provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub ice stream environment piston coring in holes bored by hot water drilling yielded five 1 3 m long undisturbed subglacial sediment cores we analyzed granulometry composition and particle morphology in these cores the upb cores are composed of a clay rich unsorted diamicton containing rare marine diatoms sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments the presence of reworked diatoms and their state of preservation as well as the relative spatial homogeneity of this diamicton suggest that the upb cores sampled a several meter thick till layer and not in situ glacimarine sediments the till does incorporate material recycled from the subjacent poorly indurated tertiary glacimarine sediments of the ross sea sedimentary basin which extends beneath this part of the west antarctic ice sheet we propose that the lack of significant comminution in the upb till is ultimately due to its setting over these easily erodible clay rich source sediments the resulting fine grained till matrix inhibits glacial comminution because it facilitates buildup of high pore water pressures and hinders interparticle stress concentrations our observations are consistent with the conjecture that subglacial deformation of weak fine grained tills does not produce significant comminution of till debris elson 1988 based on our findings we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting poorly indurated fine grained sediments since such weak till layers create a permissive condition for ice streaming sub glacial geology may have an indirect but strong control over the location extent and basal mechanics of ice streams

the gamburtsev mountains and the origin and early evolution of the antarctic ice sheet the manner in which the antarctic ice sheet first formed at a time of rapid global climate change about 34 million years ago has been the subject of considerable speculation several lines of evidence point to central antarctic mountain ranges as the site of the initial ice sheet but our knowledge of the present ice sheet topography is highly restricted in the very zone where ice sheet initiation is most likely along the subglacial gamburtsev mountain range at the centre of the present ice sheet now a detailed survey and analysis of the morphology of the gamburtsev mountains has been performed based on an ice penetrating radar study conducted over two seasons the data reveal a mountain landscape initially incised by rivers then over deepened by ice movement the topography beneath up to 3 000 metres of ice resembles an exploded version of classic alpine valley topography and seems to have developed more than 34 million years ago when mean summer temperatures were about 3 c the nature of initial glaciation on antarctica about 34 million years ago is a mystery results from an intensive radar survey now show that the gamburtsev mountains at dome a at the centre of the present ice sheet were initially incised by rivers and then subsequently overdeepened by ice movement this is suggestive of topographical development before 34 million years ago when mean summer temperatures were about 3 c ice sheet development in antarctica was a result of significant and rapid global climate change about 34 million years ago1 ice sheet and climate modelling suggest reductions in atmospheric carbon dioxide less than three times the pre industrial level of 280 parts per million by volume that in conjunction with the development of the antarctic circumpolar current led to cooling and glaciation paced by changes in earth s orbit2 based on the present subglacial topography numerical models point to ice sheet genesis on mountain massifs of antarctica including the gamburtsev mountains at dome a the centre of the present ice sheet2 3 our lack of knowledge of the present day topography of the gamburtsev mountains4 means however that the nature of early glaciation and subsequent development of a continental sized ice sheet are uncertain here we present radar information about the base of the ice at dome a revealing classic alpine topography with pre existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 c this landscape is likely to have developed during the initial phases of antarctic glaciation according to antarctic climate history estimated from offshore sediment records the gamburtsev mountains are probably older than 34 million years and were the main centre for ice sheet growth moreover the landscape has most probably been preserved beneath the present ice sheet for around 14 million years

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream