ice pumps and their rates an ice pump is a heat engine driven by the change of freezing point with pressure which will melt ice at depth in the ocean and deposit it at a shallower location it is self starting calculations of the maximum magnitude of this effect are made which show good agreement with field data available for sea and lake ice the discussion is applied to the general case of a moving pack ice sheet with a well mixed surface layer and to floating ice shelves the rate of melt from an 11 m deep pressure ridge keel due to ice pumping is estimated as 26 cm year and that from the front of the ross ice shelf at mcmurdo sound antarctica is estimated as 5 m year for the level of water movement noted in the authors field observations far from the ice front pumping between shelf areas of different thickness will still occur with tidal motion providing the necessary water exchange but its magnitude is now limited by the ability to remove the potentially stable layer of melt water out of the system it is important to realize that the pumping does not depend on the availability of sensible heat in the water column and its effects are additional to any melting caused by the advection of warmer water to the ice water interface

petrology geochemistry and na metasomatism of triassic jurassic non marine clastic sediments in the newark hartford and deerfield rift basins northeastern usa many sandstones and associated siltstones and shales of the triassic jurassic newark supergroup in the newark hartford and deerfield rift basins are immature feldspathic alluvial and lacustrine closed basin deposits they were derived from felsic igneous and metamorphic continental blocks on the flanks of their depositional basins the provenance was dominantly calc alkaline basement rocks similar to the sierra nevada of california the physical and chemical environments of erosion and deposition were probably similar to those in moist pleistocene holocene alluvial basins and playas in the great basin of california and nevada new petrographic and geochemical data for 113 samples indicate that albite rich sandstones and shales of the stockton lockatong and passaic formations in the newark basin and in the portland and east berlin formations of the hartford basin and analcime rich shales up to 52 normative analcime of the lockatong formation have unusually high na2o contents 4 7 textural evidence indicates both sodium replacement in feldspars and formation of authigenic albite and analcime in these rocks by comparison with modem sediments derived from similar provenance using mixing models it is estimated that up to 4 na2o was diagenetically added to the detrital clastics trace metals co cr cu ni zn abundances were apparently enhanced by deposition from brines in the lockatong formation analcime deposition occurred in shales with al2o3 na2o sio2 0 46 this ratio is found only in sediments to which na has been added from salts brines source rocks and the weathering products derived from them may be considered as a closed system in which sodium was retained partly in solids as feldspar and part went into solution during weathering the dilute solution migrated into the depositional basin where evaporation yielded concentrated brine during semi arid to arid climate in the drier parts of 21 000 yr milankovitch cycles reaction of brine during diagenesis at near surface and or shallow depths 2 km with ca plagioclase k feldspar and layer silicates at estimated temperatures 50 c to 100 c and ph 7 9 yielded thermodynamically stable authigenic albite and analcime in the sodium metasomatised clastic sediments low activity of k precluded development of authigenic k feldspar however k from solution incorporated into muds and silts as smectites were transformed to illite some lockatong beds contain anomalously high p2o5 ce la y th and u in authigenic apatite or monazite suggestive of mobilization of these elements in diagenesis

precise u pb titanite age constraints on the emplacement of the bushveld complex south africa the c 2 06 ga bushveld complex intrudes the transvaal supergroup south africa calcsilicate xenoliths within the mafic phase of the bushveld complex rustenburg layered suite or rls preserve calcsilicate xenoliths with high temperature c 1200 c mineralogies that were later metasomatized by hydrous retrograde c 600 700 c fluids whose timing has been unconstrained new u pb isotope data from newly grown titanite within a completely retrogressed xenolith indicates that retrogression occurred at 2058 9 0 8 ma this suggests that retrogression was due to hydrothermal circulation associated with the cooling rls and not slightly later bushveld related granites the new data place a tight constraint on the minimum age of emplacement of the rls

contrasting low and high ca granites in the archean barberton mountain land southern africa abstract two distinct groups of granites can be recognized among the late plutons of the barberton mountain land discrimination between the two groups is provided by geochemical and mineralogical parameters which are considered to reflect the source material from which the magmas were derived a high ca suite has geochemical parameters consistent with derivation from an igneous source i type and an accessory mineral assemblage comprising zircon apatite allanite and sphene a low ca suite contains distinctly different accessory minerals comprising zircon apatite zirconosilicates ca th phosphates very rare xenotime and early monazite and has a chemical signature consistent with derivation from a metasedimentary precursor s type major and trace element trends on harker diagrams support the existence of two distinct granite suites the i type suite exhibits inverse relationships between sio2 and cao tio2 fe2o3 mgo and p2o5 while al2o3 k2o and na2o remain relatively constant these characteristics are consistent with magma formation from an intermediate parent composition and in equilibrium with a hornblendite restite the s type granites formed by vapour absent melting of siliceous metapelite and exhibit inverse correlations between sio2 and al2o3 na2o and k2o with the other major elements remaining relatively unchanged this interpretation of restite unmixing is not unambiguous since rb sr trends could be viewed as reflecting crystal fractionation and partial melting processes high field strength elements tend to reflect the character of accessory mineral phases and are not amenable to modelling in terms of crystal melt equilibria the recognition of possible s and i type suites among the late granite plutons of the barberton mountain land supports recent models which propose plate tectonic analogues in the region the two granite suites are distributed along two subparallel linear arrays which suggests scenarios invoking subduction or hot spot related magmatism with the i types forming in the time span 2740 2690 ma along a western array the putative s types occurring entirely along an easterly belt were emplaced at 3074 and 2820 ma the presence of s type plutons as old as 3074 ma implies that sediments such as the moodies and fig tree groups or their equivalents had been buried to depths of at least 15 km and that by this time at least in the barberton area granites had evolved beyond the ttg mode that typifies many early archaean shield areas

acceleration and spatial rheology of larsen c ice shelf antarctic peninsula the disintegration of several antarctic peninsula ice shelves has focused attention on the state of the larsen c ice shelf here we use satellite observations to map ice shelf speed from the years 2000 2006 and 2008 and apply inverse modeling to examine the spatial pattern of ice shelf stiffness results show that the northern half of the ice shelf has been accelerating since 2000 speeding up by 15 between 2000 and 2006 alone the distribution of ice stiffness exhibits large spatial variations that we link to tributary glacier flow and fractures our results reveal that ice down flow from promontories is consistently softer with the exception of churchill peninsula where we infer a stabilizing role for marine ice we conclude that although larsen c is not facing imminent collapse it is undergoing significant change in the form of flow acceleration that is spatially related to thinning and fracture copyright 2011 by the american geophysical union

3 07 2 97 ga greenstone belt formation in the northeastern kaapvaal craton implications for the origin of the witwatersrand basin

thirty years of elevation change on antarctic peninsula ice shelves from multimission satellite radar altimetry 1 we use data acquired between 1978 and 2008 by four satellite radar altimeter missions seasat ers 1 ers 2 and envisat to determine multidecadal elevation change rates dhi dt for six major antarctic peninsula ap ice shelves in areas covered by the seasat orbit to 72 16 s regional averaged 30 year trends were negative surface lowering with rates between 0 03 and 0 16 m a 1 surface lowering preceded the start of near continuous radar altimeter operations that began with ers 1 in 1992 the average rate of lowering for the first 14 years of the period was typically smaller than the 30 year average the exception was the southern wilkins ice shelf which experienced negligible lowering between 2000 and 2008 when a series of large calving events began analyses of the continuous ers envisat time series to 81 5 for 1992 2008 reveal a period of strong negative dhi dt on most ice shelves between 1992 and 1995 based on prior studies of regional atmospheric and oceanic conditions we hypothesize that the observed elevation changes on larsen c ice shelf are driven primarily by firn compaction while the western ap ice shelves are responding to changes in both surface mass balance and basal melt rates our time series also show that large changes in dhi dt can occur on interannual time scales reinforcing the importance of long time series altimetry to separate long term trends associated with climate change from interannual to interdecadal natural variability

metallogeny of precious and base metal mineralization in the murchison greenstone belt south africa indications from u pb and pb pb geochronology the 3 09 to 2 97 ga murchison greenstone belt is an important metallotect in the northern kaapvaal craton south africa hosting several precious and base metal deposits central to the metallotect is the antimony line striking ene for over 35 km which hosts a series of structurally controlled sb au deposits to the north of the antimony line hosted within felsic volcanic rocks is the copper zinc line where a series of small ca 2 97 ga cu zn volcanogenic massive sulfide vms type deposits occur new data are provided for the malati pump gold mine located at the eastern end of the antimony line crystallizations of a granodiorite in the malati pump mine and of the baderoukwe granodiorite are dated at 2 964 7 and 2 970 7 ma respectively zircon u pb while pyrite associated with gold mineralization yielded a pb pb age of 2 967 48 ma therefore granodiorite emplacement sulfide mineral deposition and gold mineralization all happened at ca 2 97 ga it is thus suggested that the major styles of orogenic au sb and the cu zn vms mineralization in the murchison greenstone belt are contemporaneous and that the formation of meso to epithermal au sb mineralization at fairly shallow levels was accompanied by submarine extrusion of felsic volcanic rocks to form associated cu zn vms mineralization

channelized bottom melting and stability of floating ice shelves 1 the floating ice shelf in front of petermann glacier in northwest greenland experiences massive bottom melting that removes 80 of its ice before calving into the arctic ocean detailed surveys of the ice shelf reveal the presence of 1 2 km wide 200 400 m deep sub ice shelf channels aligned with the flow direction and spaced by 5 km we attribute their formation to the bottom melting of ice from warm ocean waters underneath drilling at the center of one of channel only 8 m above sea level confirms the presence of ice shelf melt water in the channel these deep incisions in ice shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously

characterization of multiple fluid granite interaction processes in the episyenites of avila bejar central iberian massif spain episyenites are small granite hosted pipe like masses characterized by hydrothermal alteration and the major loss of silica compared with the host granites such bodies are fairly common in the hercynian granites of western europe this study deals with late hercynian amphibole bearing and biotite granodiorites monzogranites from the central iberian massif using petrological geochemical major oxides trace elements and ree and isotopic o h data to constrain the paragenesis of alteration the apparent changes in bulk composition and the sources and conditions of fluids involved in fluid rock interactions episyenitization leads to reduction of quartz content albitized plagioclases and chloritized ferromagnesian phases and the process results in increasingly ordered alkali feldspars there are noticeable variations in whole rock na20 k2o mgo cao rb and sr contents with sio2 in some cases behaving differently depending on whether the episyenite is hosted in amphibole granite or biotite granite the lree y zr and nb have also been modified in those syenites associated with amphibole bearing granites this being attributed to destabilization of accessory minerals and recrystallization of new stable mineral assemblages the process is complex and two generations of chlorite early high fe and late low fe are observed whole rocks and mineral separates were studied for o and h isotopes the data rule out meteoric fluids as the cause of syenitization and the later resiliciftcation of some and suggest that at least two different fluids were involved in generating the syenites the first is interpreted to be a high temperature 300 450 c late magmatic deuteric fluid as recorded by feldspar o isotope data later a different fluid with 18o 1 4 and d 13 isotopically indistinguishable from seawater on a deglaciated earth produced the chlorite secondary quartz and minor epidote such a fluid was only available for a short period since feldspars did not re equilibrate the fault systems occupied by the episyenites provided suitable pathways for fluids of both shallow and deep origins over a long term during the carboniferous

porphyry intrusions and albitites in the bardoc kalgoorlie area western australia and their role in archean epigenetic gold mineralization minor intrusions in the menzies kambalda greenstone belt of the archean eastern goldfields province western australia range from quartz feldspar porphyry to plagioclase hornblende porphyry the porphyries display enrichment of mobile and incompatible elements k to zr and depletion of relatively compatible elements with negative nb p and ti anomalies on mid ocean ridge basalt normalized spidergrams the composition and timing of emplacement of the porphyries are consistent with a genetic relationship with spatially related granitoids porphyries occur in 30 of gold mines in the menzies kambalda belt the association appears to be largely structural since both the intrusions and the mineralizing fluids exploit zones of weaknesses such as lithological contacts and shear zones porphyries have been modified to varying degrees by hydrothermal alteration especially pervasive albitization textural evidence indicates that secondary albite and associated sodic amphibole formed late in the deformatio

uranium deposits associated with na metasomatism from central ukraine a review of some of the major deposits and genetic constraints abstract the paper reviews geological structural mineralogical fluid inclusion geochemical and stable isotope characteristics of u deposits related to na metasomatism in the central ukrainian uranium province cuup the province contains numerous u deposits hosted in gneisses migmatites and granites of the ingul paleoproterozoic megablock and in iron formations along its margin with the fore mid dnieper mesoarchean megablock the richest u deposits are distributed along the nearly e w striking subotin mashorin tectonic zone but they are tightly controlled by n s and more rarely sw ne striking crustal scale shear zones na metasomatism and u mineralisation were controlled by protracted tectonic activity along these shears zones from ductile to brittle regimes of deformation a metasomatic zoning is developed from outermost unaltered lithologies inwards to dequartzified rocks episyenites aegirine albitites na metasomatites andradite epidote ca amphibole calcite rich rocks ca mg metasomatites and ultimately to biotite albitites k metasomatites main u ore deposition predominantly consisting of brannerite and uraninite with lesser amounts of u ferropseudobrookite is associated with the ca mg and k metasomatised zones superimposed on earlier aegirine albitites na metasomatites were formed at temperatures exceeding 500 c possibly triggered by the onset of a plume in the mantle associated with the intrusion of the highly potassic korsun novomyrgorod rapakivi granitic pluton at about 1750 1800 ma as constrained by u pb geochonologic data on the granites and uranium mineralisation these intrusions have been partly reworked tectonically with recrystallisation under lower p t conditions the mineral paragenesis of ca mg and k metasomatites crystallised during the cooling of the system down to 370 340 c ca mg and k metasomatisms resulted in further dequartzification of the rocks and their enrichment mainly in ca mg k u zr fe 2 and hf the hydrothermal fluids were oxidised moderately saline and highly alkaline the high alkalinity of the fluids explains the dissolution of quartz the deposition and stability of albite the high zr hf ratios up to 224 in the metasomatites and the high solubility of u and zr u deposition was controlled by mixing with a reduced fluid which explains the increase of whole rock fe 2 concentration during uranium deposition the last widespread hydrothermal event corresponds to a chlorite epidote paragenesis superimposed on all previous mineral associations at temperatures down to less than 100 c h o and c isotopic ratios in minerals and fluid inclusions extract from albitites suggest a surface formation water origin for the hydrothermal fluids producing na metasomatism and a mixed metamorphic surficial formation water origin for the fluids producing ca mg and k metasomatism and the main u mineralisation

albitization in the antimony line murchison greenstone belt kaapvaal craton a geochemical and geochronological investigation abstract the 3 09 2 97 ga murchison greenstone belt is one of several archean volcano sedimentary belts within the kaapvaal craton in southern africa among the diverse ore deposits found within the belt a set of sb au deposits are located in a major quartz carbonate altered brittle ductile structure known as the antimony line the antimony line is thus clearly related to hydrothermal fluid circulation in this study we focus on albitites that run along the antimony line petrological and geochemical investigations indicate that albitization developed at the expense of a granodioritic protoliths under high fluid rock ratios and also that sb enrichment was concomitant with albitization oxygen isotopes on albitites point to a crustal origin for the hydrothermal fluid responsible for the albitization process geochronology on zircon and hydrothermal monazite identifies a 2 97 2 92 ga magmatic crystallization event disconnected from a ca 2 8 ga hydrothermal alteration and a potentially younger event around 2 0 ga these data highlight a likely magmatic related primary enrichment at 2 97 2 92 ga followed by a secondary metamorphic related mobilization event around 2 8 ga leading to a sb enrichment in the albitites and a last episode of alteration around 2 0 ga

regional scale cretaceous albitization in the pyrenees evidence from in situ u th pb dating of monazite titanite and zircon abstract albitization is a common metasomatic process active in various geodynamic contexts in the northern pyrenees there are several occurrences of albitites but until now only one occurrence has been dated 117 ma ar ar dating this paper presents new u th pb ages for several albitite occurrences throughout the pyrenees to test whether they are contemporaneous and if so to specify the regional extent of the albitization event ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 8 and 98 2 ma respectively the zircon u th pb isotopic system did not record this cretaceous metasomatic event even when grains were selected in metasomatically zr enriched rocks or in hydrothermal structures millimetre sized veins cross cutting granitoids we argue that the total time span of 20 ma recorded by albitites corresponds to a long lived hydrothermal system that was active during the rotation of iberia around europe along the north pyrenean fault because albitization and talc mineralization have the same spatial and temporal distribution in the pyrenees we argue that these two metasomatic phenomena are two independent records of this single regional scale long lived hydrothermal system

role de la deformation du granite dans la genese des episyenites feldspathiques des massifs de lovios geres galice et des zaer maroc central l etude de la deformation du granite dans les zones encaissant les bandes d episyenites feldspathiques de deux massifs hercyniens a permis de degager un modele tectonique rendant compte des caracteristiques structurales de celles ci les bandes d episyenites feldspathiques se forment lors des phases ultimes de la mise en place des intrusions granitiques sous l effet d un champ de contrainte local susceptible d engendrer des surpressions fluides aboutissant a une fracturation de type hydraulique la genese des mineralisations filoniennes intragranitiques a tungstene etain associees spatialement a ces episyenites doit etre rapportee a des evenements tectoniques et a des circulations fluides posterieures

impact of tide topography interactions on basal melting of larsen c ice shelf antarctica 1 basal melting of ice shelves around antarctica contributes to formation of antarctic bottom water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers tides influence ice shelf basal melt rate wb by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf we use a three dimensional ocean model thermodynamically coupled to a nonevolving ice shelf to investigate the relationship between topography tides andwb for larsen c ice shelf lcis in the northwestern weddell sea antarctica using our best estimates of ice shelf thickness and seabed topography we find that the largest modeled lcis melt rates occur in the northeast where our model predicts strong diurnal tidal currents 0 4 m s 1 this distribution is significantly different from models with no tidal forcing which predict largest melt rates along the deep grounding lines we compare several model runs to explore melt rate sensitivity to geometry initial ocean potential temperature 0 thermodynamic parameterizations of heat and freshwater ice ocean exchange and tidal forcing the resulting range of lcis averagedwb is 0 11 0 44 m a 1 the spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by 0 these sensitivities in wbpredictions reinforce a need for high resolution maps of ice draft and sub ice shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

evidence of recent climatic warming on the eastern antarctic peninsula air temperatures at the stations marambio esperanza and matienzo have been analysed to investigate recent climate change on the eastern part of the antarctic peninsula the data are compared with the station orcadas on the south orkney islands the longest record available in antarctica and with faraday on the western coast of the peninsula though the interannual variability is comparatively high and the stations are located in different climatic regimes a pronounced warming trend shows up in all records at marambio a temperature increase of 1 5 c has been observed since the beginning of the record in 1971 this is of similar magnitude as the increase at the station faraday on the west coast with 2 5 c for the longer period since 1945 the steady retreat and collapse of the northern larsen ice shelf coincided with this warming trend of particular importance for the ice shelf mass balance in this region are the summer temperatures which show a statistically significant warming trend at the stations marambio and esperanza the representativity of the summer temperatures of marambio for northern larsen ice shelf is confirmed by intercomparison with the parallel measurements at matienzo which is located on the ice shelf

submarine melting of the 1985 jakobshavn isbrae floating tongue and the triggering of the current retreat 1 photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of jakobshavn isbrae west greenland the thickness of the floating tongue determined from hydrostatic equilibrium tapers from 940 m near the grounding zone to 600 m near the terminus feature tracking on orthophotos shows speeds on the july 1985 ice tongue to be nearly constant 18 5 m d 1 indicating negligible dynamic thinning the thinning of the ice tongue is mostly due to submarine melting with average rates of 228 49 m yr 1 0 62 0 13 m d 1 between the summers of 1984 and 1985 the cause of the high melt rate is the circulation of warm seawater thermal forcing of up to 4 2 c beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone we believe that this buoyancy driven convection is responsible for a deep channel incised into the sole of the floating tongue a dramatic thinning retreat and speedup began in 1998 and continues today the timing of the change is coincident with a 1 1 c warming of deep ocean waters entering the fjord after 1997 assuming a linear relationship between thermal forcing and submarine melt rate average melt rates should have increased by 25 57 m yr 1 sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed

in situ la icp ms u pb titanite dating of na ca metasomatism in orogenic belts the north pyrenean example in the pyrenees in association with the rotation of the iberian plate around europe during the mid cretaceous a na ca metasomatism is recognized as a complementary record of the hydrothermal activity that led to na metasomatism albitization and talc chlorite mineralization it affected metasedimentary rocks as well as hercynian granites in situ laser ablation icp ms u pb analyses of titanite grains formed in albitites during metasomatism date the na ca metasomatism between 110 and 92 ma the temperature of the na ca metasomatism is estimated to be approximately 550 c both the time constraints and temperature estimates suggest that the na ca metasomatism is related to the low p high t north pyrenean metamorphism

the role of meltwater advection in the formulation of conservative boundary conditions at an ice ocean interface abstract upper boundary conditions for numerical models of the ocean are conventionally formulated under the premise that the boundary is a material surface in the presence of an ice cover such an assumption can lead to nonconservative equations for temperature salinity and other tracers the problem arises because conditions at the ice ocean interface differ from those in the water beneath advection of water with interfacial properties into the interior of the ocean therefore constitutes a tracer flux neglect of which induces a drift in concentration that is most rapid for those tracers having the lowest diffusivities if tracers are to be correctly conserved either the kinematic boundary condition must explicitly allow advection across the interface or the flux boundary condition must parameterize the effects of both vertical advection and diffusion in the boundary layer in practice the latter alternative is often implemented although this is rarely done for all tracers

li na metasomatism related to i type granite magmatism a case study of the highly fractionated la pedriza pluton iberian variscan belt abstract this study describes rare albitites associated with highly fractionated i type granitic melts showing a mineral paragenesis that resembles those of high temperature metasomatized rocks fenites associated with alkaline and carbonatitic intrusions the studied albitites occur as lenticular bodies on the meter scale hosted in peraluminous granites and are scattered in a narrow sector next to the peraluminous and highly fractionated i type la pedriza leucogranite lpl pluton in the spanish central system albitites show sharp contacts and preserve textural features of the host granites i e isovolumetric metasomatism these rocks consist of a variably albitized magmatic feldspar framework 70 85 modal and interstitial domains where singular na li fe3 rich amphiboles and pyroxenes crystallized with albite or quartz titanite andradite grossular garnet mg li rich mica magnetite fluorite and apatite are metasomatic accessory minerals mass balance indicates enrichments of na2o feo mgo mno li f be and some hfse high field strength elements such as zn sc and sn and lile large ion lithophile elements depletion u pb data from metasomatic titanite yield 307 10 ma which is coeval with the lpl and within the range of the regional granite emplacement about 309 298 ma the lpl also shows strong compositional affinities f li na sn and zn enrichment with albitites similar sr nd d isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant during metasomatism however the heterogeneity of 18o which shifts toward lower values in the more distant outcrops suggests contribution of meteoric in origin low salinity waters during prograde metasomatism p t fo2 conditions are estimated at 0 2 0 3 gpa 645 700 c and relatively oxidized conditions we propose that na hfse flux rich residual melts of the lpl expelled two immiscible hot liquids an alkaline low density liquid and a high density hydrosilicate liquid hsl the low density fluid dissolved magmatic quartz and provided albitization of the magmatic feldspar framework by a diffusive mechanism quartz dissolution created microchannels for infiltration of the relatively low viscosity hsl forming tubular channels this fluid was enriched in network modifiers li na f be and cations ca al released after albitization and it precipitated in these microchannels to form the new metasomatic paragenesis quartz albite uncommon li rich mafic silicates and accessory minerals

present stability of the larsen c ice shelf antarctic peninsula we modelled the flow of the larsen c and northernmost larsen d ice shelves antarctic peninsula using a model of continuum mechanics of ice flow and applied a fracture criterion to the simulated velocities to investigate the ice shelf s present day stability constraints come from satellite data and geophysical measurements from the 2008 09 austral summer ice shelf thickness was derived from bedmap and icesat data and the density depth relationship was inferred from our in situ seismic reflection data we obtained excellent agreements between modelled and measured ice flow velocities and inferred and observed distributions of rifts and crevasses residual discrepancies between regions of predicted fracture and observed crevasses are concentrated in zones where we assume a significant amount of marine ice and therefore altered mechanical properties in the ice column this emphasizes the importance of these zones and shows that more data are needed to understand their influence on ice shelf stability modelled flow velocities and the corresponding stress distribution indicate that the larsen c ice shelf is stable at the moment however weakening of the elongated marine ice zones could lead to acceleration of the ice shelf due to decoupling from the slower parts in the northern inlets and south of kenyon peninsula leading to a velocity distribution similar to that in the larsen b ice shelf prior to its disintegration

a simple law for ice shelf calving a major problem for ice sheet models is that no physically based law for the calving process has been established comparison across a diverse set of ice shelves demonstrates that iceberg calving increases with the along flow spreading rate of a shelf this relation suggests that frictional buttressing loss which increases spreading also leads to shelf retreat a process known to accelerate ice sheet flow and contribute to sea level rise

a numerical model of the weddell sea large scale circulation and water mass distribution a circumpolar model for regional studies of the wind driven and thermohaline circulation of the southern ocean including the major sub ice shelf areas is described a first series of numerical experiments focusing on the weddell sea reveals a pronounced and persistent double cell structure of the weddell gyre with a maximum transport of 60 sv in agreement with observations experiments with artificial passive tracers point to the shallow shelf areas off the filchner ronne ice shelf as main locations for bottom water production the trajectories of lagrangian floats are used to determine the pathways and timescales of water mass spreading in the model in addition the effect of sub ice shelf forcing on the water mass characteristics is evaluated it is shown that water modified in the sub ice cavities contributes significantly to the deep and bottom water formation along the continental slope and affects the water mass characteristics throughout the weddell sea by increasing the stability of the near surface stratification and preventing deep convection

internal tides and baroclinicity in the southern weddell sea 2 effects of the critical latitude and stratification tidal dynamics affect antarctic bottom water and ice shelf water production in the southern weddell sea and the general circulation and mixing in this region tidal dynamics were found to be extremely sensitive both to the proximity of the continental shelf slope break to the critical latitude crit and to the existence of a shelf break front if the continental slope was not in the vicinity of crit or a shelf break front was absent generation of internal tides was negligible generation of internal tides was greatest when both a shelf break front was present and the continental slope was near and equatorward of crit when a shelf break front was present and the continental slope was near but poleward of crit a strong benthic response occurred this response included development of benthic inertial oscillations at the continental shelf slope break increased benthic shear a thicker benthic boundary layer and increased benthic dissipation a shelf break front with even a slight horizontal density gradient was found to significantly enhance internal tide generation and or the benthic effects

sodium calcium metasomatism chemical temporal and spatial relationships at the yerington nevada porphyry copper deposit tilting associated with extensional tectonics has exposed 1 800 m of continuous palcovertical relief through the jurassic age yerington porphyry copper deposit the rarely observed root zones of a porphyry system are characterized at yerington by sodium calcium metasomatism the principal hydrothermal reactions involve the replacement of primary k feldspar by oligoclase and of primary biotite by actinolite copper has been removed sodic calcic alteration developed at deep intrusive contacts nearly simultaneously with potassic alteration and copper mineralization at shallow intrusive contacts the reactions accompanying potassic alteration are the reverse of those accompanying sodic calcic alteration plagioclase reacted to form k feldspar hornblende reacted to form biotite the potassic and sodic calcic alteration couplet principally accompanied each of two separate intrusive events 1 luhr hill granite stock and associated porphyry dikes and 2 walker river granite porphyry dikes the younger walker river intrusion was emplaced at slightly deeper levels than the older luhr hill complex vertically stacked and partially superimposed alteration and mineralization patterns resulted from differences in depth and in timing of emplacement at lower temperatures late sodic alteration was superimposed on early sodic calcic alteration at deeper levels in the system whereas late quartz sericite pyrite alteration was superimposed on potassic alteration at higher levels sodic alteration is characterized by the replacement of k feldspar by albite and of biotite by chlorite copper deposited during potassic alteration was remobilized the transition to lower temperature alteration is associated only with the walker river intrusion the hydrothermal system associated with the emplacement of the luhr hill complex had not evolved to lower temperature assemblages prior to emplacement of the walker river dikes phase equilibria and geologic reconstruction of depth of emplacement suggest that temperatures during early sodic calcic alteration were limited to the range of 360 degrees to 480 degrees c at pressures between 300 and 800 bars late sodic alteration took place at similar pressures but at temperatures less than 360 degrees c nearly simultaneous development of potassic and sodic calcic alteration resulted from the flow of fluids away from intrusions at shallow levels potassic alteration and from the flow of fluids toward intrusions at deeper levels sodic calcic alteration because k na and na ca activity ratios of a fluid in equilibrium with granitic rocks decrease with decreasing temperature cooling of high temperature fluids can generate potassic alteration and heating of fluids to high temperatures can generate sodic or sodic calcic alteration

retrograde fluid infiltration in the high grade modum complex south norway evidence for age source and ree mobility the high grade metamorphic basement of the modum complex south norway exhibits retrogradation and alteration due to late stage fluid infiltration extensive alteration zones of albite and calcite rich veining occur especiallywithin and around numerous metagabbros the gabbros intruded at 1224 15ma are now partly altered to amphibolites due to the subsequent high grade metamorphism two generations of albite rich rocks have been recognized 1 a fine grained foliated type 2 a coarse grained crosscutting type both types show a typical greenschist facies mineralassemblage albite actinolite chlorite talc the calcite veins dykesrepresent a younger generation of veins than both albite rich types u pb data for spheneof type 1 yielded an age of 1080 3 ma determining a point on the retrogradep t t path of the modum complex increasing albitisation of themetagabbros leads to a decrease in 143nd 144nd oand an increase in 87sr 86 o albite and calcite rich samples show negative nd and positive sr suggesting that fluids which interacted with the metagabbros originated from a crustal reservoir the nd and sr isotopic data show disequilibrium at the microscale as well as at the macroscale negative sm nd model ages of the albite rich rocks demonstrate that rare earth elements rees were mobile and fractionated during albitisation

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

thermohaline circulation and interaction between ice shelf cavities and the adjacent open ocean the circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity which at least provides the potential for melting and freezing processes applying a three dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier we found an important parameter controlling the interaction between these two systems idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction because of the sudden decrease in water column thickness since the water column thickness and the coriolis force determine the characteristics for geostrophic flow separated circulation systems arise for the open ocean and the ice shelf cavity only in areas where constant water column thickness and from the oceanographic point of view constant f h contours can be observed across the barrier an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf this is only the case at lateral sloping sidewalls or at deep depressions which can be found for example in the southern weddell sea in all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barrier

on the coupled response to ice shelf basal melting ice shelf basal melting is tightly coupled to ice shelf morphology ice shelves in turn are coupled to grounded ice via their influence on compressive stress at the grounding line ice shelf buttressing here we examine this interaction using a local parameterization that relates the basal melt rate to the ice shelf thickness gradient this formulation permits a closed form solution for a steady state ice tongue time dependent numerical simulations reveal the spatial and temporal evolution of ice shelf ice stream systems in response to changes in ocean temperature and the influence of morphology dependent melting on grounding line retreat we find that a rapid 1 year re equilibration in upstream regions of ice shelves establishes a spatial pattern of basal melt rates relative to the grounding line that persists over centuries coupling melting to ice shelf shape generally but not always increases grounding line retreat rates relative to a uniform distribution with the same area average melt rate because upstream ice shelf thickness gradients and retreat rates increase nonlinearly with thermal forcing morphology dependent melting is more important to the response of weakly buttressed strongly forced ice streams grounded on beds that slope upwards towards the ocean e g those in the amundsen sea

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

tin bearing sodic episyenites associated with the proterozoic a type agua boa granite pitinga mine amazonian craton brazil this paper reports the first occurrence of tin mineralized episyenites in the amazonian craton the agua boa and madeira plutons in the pitinga region are stanniferous proterozoic a type granites of the rapakivi series featuring metasomatic episyenitization the biotite granite facies of the agua boa pluton is metasomatically altered to sodic episyenite and minor potassic episyenite and micaceous episyenite the sodic episyenites formed by a albitization of k feldspar b vug formation by dissolution of magmatic quartz c vug filling by albite chlorite lithian muscovite cassiterite fluorite k feldspar and d deposition of late quartz cassiterite in remaining cavities in the potassic episyenites vugs produced by quartz dissolution are filled by secondary k feldspar which also replaces magmatic phases micaceous episyenites formed by zinnwaldite replacement of secondary albite in the sodic episyenites in contrast to the sodic episyenites the potassic and micaceous episyenites contain only traces of cassiterite relative to unaltered biotite granite the sodic episyenites are strongly depleted in sio2 k2o lree and sr moderately depleted in hree and enriched in na2o al2o3 rb and sn the potassic episyenites resemble the sodic episyenite in sio2 and al2o3but exhibit higher values of k2o ba y and rb their na2o ree and sn concentrations match those of the biotite granite host metasomatic episyenitization is attributed to 1 a high temperature gradient and fluid rock ratio during subsolidus cooling of the agua boa pluton 2 quartz dissolution in the biotite granite facies by reaction with an high temperature silica undersaturated fluid of probable magmatic origin 3 hydrothermal cavity filling by a low temperature silica saturated fluid of meteoric or mixed magmatic meteoric character significant tin values in metasomatic sodic episyenites near the pitinga mine suggest that exploration of similar rocks in other a type granites may be worthwhile

cretaceous albitization and dequartzification of hercynian peraluminous granite in the salvezines massif french pyrenees abstract the salvezines massif in northern french pyrenees has undergone extensive albitization the massif consists of gneisses and a leucogranite hosted in paleozoic sediments schists and carbonates the leucogranite was emplaced at the end of the hercynian orogeny the granite underwent monazite fractionation during magmatic stages which induced lowering of ree and th contents in the most evolved rocks hydrothermal alteration during late magmatic stages is also identified through the development of a tetrad effect in the ree patterns of the granite and through the fractionation of ratios like zr hf out of the charac charge radius controlled range the granite underwent partial to complete sub solidus albitization progressive metasomatic overprint induced metasomatic replacement of feldspars into albite and dissolution of quartz episyenitization together with new albite formation hydrothermal muscovite with a sheaf like structure sometimes crystallised in albitites inherited muscovites from magmatic stage and newly formed muscovites from albitites have distinct chemical compositions with the latter being much more phengitic the initial peraluminous chemistry of the leucogranite was lost during albitization and composition evolved towards a pure albite one a cnk 1 albitization induced gains in na and al and losses in si and k leaching of ree and u is also identified during albitization as well as the non charac behaviour of some elements 18o values of quartz and muscovite in albitized granites are in the magmatic range about 12 and 9 5 respectively feldspars acquired high 18o values during albitization up to 15 muscovite grains from albitites have higher 18o values than inherited ones and tend towards isotopic equilibrium with albite the albitizing fluids have interacted with high 18o rocks probably local carbonates prior to alter granite muscovites inherited from magmatic stages display very characteristic disturbed 39ar 40ar age spectra with saddle shapes when a muscovite associated with albitization provided a plateau age at 117 5 ma for this alteration event albitization in the salvezines massif just preceded the regional talc chlorite hydrothermal mineralization both events might then represent two outward signs of a huge hydrothermal system at the time of the rotation of the iberian plate around europe the north pyrenean metamorphism is identified by 39ar 40ar analyses at ca or younger than 100 ma

the significance of rb sr ages of biotite and phlogopite for the thermal history of the central and southern marginal zones of the limpopo belt of southern africa and the adjacent portions of the kaapvaal craton abstract generally rb sr biotite and phlogopite ages range from 2050 ma to 1950 ma between the murchison thabazimbi and triangle tuli sabi shear zones of southern africa in the rocks comprising the central and southern marginal zones of the limpopo belt and adjacent portions of the kaapvaal craton in some shear zones however similar ages ocurr as young as 800 ma in contrast within the kaapvaal craton south of the murchison thabazimbi shear zone rb sr biotite ages range between 3150 ma and 2100 ma and show no regular distribution in all cases the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature approximately 320 c the regular pattern of 2050 ma to 1950 ma rb sr biotite ages north of the murchison thabazimbi shear zone suggests that the present erosional surface approximates a paleo isothermal surface this surface was exposed by uplift and erosion which was completed by 1900 ma to 1800 ma ago when the sedimentary and volcanic rocks of the soutpansberg group were deposited the regional uplift was accommodated by motion on the murchison thabazimbi and triangle tuli sabi shear zones and probably was related to tectonism associated with the emplacement of the anorogenic bushveld and phalaborwa complexes other anorogenic granitic magmatism accompanied uplift the depth represented by the paleo isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely it is suggested however that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

influence of sea ice cover and icebergs on circulation and water mass formation in a numerical circulation model of the ross sea antarctica 1 satellite imagery shows that there was substantial variability in the sea ice extent in the ross sea during 2001 2003 much of this variability is thought to be due to several large icebergs that moved through the area during that period the effects of these changes in sea ice on circulation and water mass distributions are investigated with a numerical general circulation model it would be difficult to simulate the highly variable sea ice from 2001 to 2003 with a dynamic sea ice model since much of the variability was due to the floating icebergs here sea ice concentration is specified from satellite observations to examine the effects of changes in sea ice due to iceberg c 19 simulations were performed using either climatological ice concentrations or the observed ice for that period the heat balance around the ross sea polynya rsp shows that the dominant term in the surface heat budget is the net exchange with the atmosphere but advection of oceanic warm water is also important the area average annual basal melt rate beneath the ross ice shelf is reduced by 12 in the observed sea ice simulation the observed sea ice simulation also creates more high salinity shelf water another simulation was performed with observed sea ice and a fixed iceberg representing b 15a there is reduced advection of warm surface water during summer from the rsp into mcmurdo sound due to b 15a but a much stronger reduction is due to the late opening of the rsp in early 2003 because of c 19

the effects of rotation and ice shelf topography on frazil laden ice shelf water plumes a model of the dynamics and thermodynamics of a plume of meltwater at the base of an ice shelf is presented such ice shelf water plumes may become supercooled and deposit marine ice if they rise because of the pressure decrease in the in situ freezing temperature so the model incorporates both melting and freezing at the ice shelf base and a multiple size class model of frazil ice dynamics and deposition the plume is considered in two horizontal dimensions so the influence of coriolis forces is incorporated for the first time it is found that rotation is extremely influential with simulated plumes flowing in near geostrophy because of the low friction at a smooth ice shelf base as a result an ice shelf water plume will only rise and become supercooled and thus deposit marine ice if it is constrained to flow upslope by topography this result agrees with the observed distribution of marine ice under filchner ronne ice shelf antarctica in addition it is found that the model only produces reasonable marine ice formation rates when an accurate ice shelf draft is used implying that the characteristics of real ice shelf water plumes can only be captured using models with both rotation and a realistic topography

summertime water masses off the northern larsen c ice shelf antarctica 1 we report on oceanographic observations made at the northern end of larsen c ice shelf in the western weddell sea it appears that the larsen c continental shelf is flushed not by high salinity shelf water from the southern continental shelf but by modified weddell deep water mwdw flowing across the shelf break mwdw is observed at the ice front having tracked west along the northward facing slopes of depressions that reach to the shelf break ice shelf water observed near the ice front is not however derived from mwdw directly but from mwdw pre conditioned by winter cooling and by salinification from sea ice production if the ice shelf base generally is being melted only by pre conditioned mwdw then contrary to recent suggestions changes in the temperature of the deep weddell sea are unlikely to have a major impact on melt rates at the base of larsen c ice shelf

albite rich silica depleted metasomatic rocks at emuford northeast queensland mineralogical geochemical and fluid inclusion constraints on hydrothermal evolution and tin mineralization late paleozoic fluorine rich granites and their host metasediments in the emuford district of the herberton tin field contain small zones of intense albitization that host cassiterite mineralization sodium metasomatism was accompanied by hydrothermal leaching of granite quartz to form vuggy albite rocks episyenites vugs created by quartz dissolution were filled in by hydrothermal minerals including albite k feldspar and or muscovite and cassiterite late phases are dominated by quartz and fluorite swarms of parallel veinlets crosscut the albitized zones these veinlets were formed by repeated fracturing in response to local fluid over pressures they contain infill mineralogy similar to the vugs and probably formed at least partly contemporaneously with vug infill due to the overprinting at various scales of alteration vug infill and vein formation stages chemical changes associated with episyenite formation are only qualitatively constrained however the mineralogical and textural features of the alteration zones suggest that most major and trace elements had substantial mobility during their formation major chemical changes associated with episyenite formation include leaching of sio 2 k 2 o rb and ba with introduction of na 2 o and minor sr during vug and vein filling stages there was an introduction of sio 2 al 2 o 3 na 2 o k 2 o cao f and sn the erratic behavior of ree is probably related to the growth of fluorite and minor accessory phases zircon monazite xenotime with preferential enrichment of heavy ree in rocks with the highest fluorine concentrations fluid inclusion study indicates that vug and vein filling stages of the hydrothermal evolution involved two types of fluids a high salinity fluid 30 50 equiv wt nacl of probable magmatic origin and homogenization temperatures up to 428 degrees c was associated with vug filling in the granite hosted deposits a lower salinity fluid 5 15 equiv wt nacl containing variable amounts of co 2 and having homogenization temperatures between 160 degrees and 420 degrees c was associated with veinlet formation in the granite hosted deposits and with vein and cavity filling in the metasediment hosted deposit this lower salinity fluid may represent a condensed vapor phase derived from boiling of the magmatic fluid or may be a mixture of magmatic and meteoric components the close association of albitization and quartz dissolution suggests a genetic link between the two processes it is proposed that exchange of na for k in the fluorine rich hydrothermal fluids caused a substantial increase in quartz solubility and led to quartz dissolution in the albitized rocks

structure and metamorphism of the archean murchison belt kaapvaal craton south africa critical examination of five popular prejudices applied to greenstone belts in general and the murchison belt in particular show that 1 the term murchison schist belt is preferred to that of murchison greenstone belt which differs significantly from the barberton greenstone belt 2 the murchison belt is tectonized such that no simple stratigraphy may be erected 3 the murchison belt lacks a synclinal keel 4 heterogeneous oblique to dip slip shear characterizes much of the murchison belt but strike slip deformation occurred in the letaba shear zone along the northeastern margin of the belt and 5 contact metamorphism was a limited phenomenon that was not responsible for the highest grade rocks of the murchison belt the belt comprises a number of domains each with its own characteristic lithologies structural style and metamorphic history bounded by tectonic breaks which have juxtaposed rocks from different crustal levels

persistent iceberg groundings in the western weddell sea antarctica abstract we use the time average of a series of satellite synthetic aperture radar sar images from 2005 to 2007 to identify grounded icebergs in the western weddell sea satellite laser altimetry along repeated ground tracks confirms that regions of high mean backscatter are associated with stationary tabular icebergs the altimeter data also provide iceberg freeboard from which we infer maximum ice draft assuming the lightly grounded limit and a two layer density model consistent with a source at filchner ronne ice shelf the maximum iceberg draft agrees very well with the ocean depth at a point where the bathymetry is well constrained by ship soundings however for an extensive region near 71 s south of larsen c ice shelf the maximum grounding depth for several icebergs reveals the ocean to be locally more than 200 m shallower than in bathymetry products this previously uncharted bank is expected to have a profound effect on the northward flow of the western weddell gyre and the cross slope transport of offshore warm deep water towards the eastern antarctic peninsula ice shelves passive microwave data also show that sea ice concentration is reduced in the vicinity of this group of grounded icebergs indicating an iceberg influence on ocean atmosphere heat and freshwater exchanges

ore forming fluids in archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec canada archean gold bearing quartz veins at the sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries the veins were emplaced during a late compressional tectonic event and they consist primarily of quartz tourmaline carbonates pyrite and scheelite vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions in the veins gold typically occurs in late fractures and in recrystallized quartz microthermometric sem eda leachate analysis and gas chromatographic techniques were used to study fluid inclusions in quartz most fluid inclusions at sigma occur in late fractures and are secondary the fluid inclusions are commonly found in fractures parallel to or less commonly within gold bearing fractures and are genetically related to the gold three types of fluid inclusions are identified 1 high salinity na ca cl aqueous inclusions 25 34 wt salt with small amounts of co 2 some containing halite calcite and nahcolite as daughter minerals 2 co 2 rich inclusions containing small amounts of h 2 o and ch 4 and 3 low salinity h 2 o co 2 inclusions 10 equiv wt nacl with co 2 contents which are variable but commonly in the range of 15 to 30 mole percent co 2 leachate analyses show that in addition to na and ca the aqueous fluids contain minor amounts of k fe mg and li homogenization temperatures of h 2 o co 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees c those of aqueous inclusions range from 60 degrees to 295 degrees c the aqueous and co 2 rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous h 2 o co 2 parent fluid itself occasionally trapped in late fractures the ore bearing fluid is interpreted to be a low salinity 10 equiv wt nacl fluid containing 15 to 30 mole percent co 2 at minimum temperatures in the range of 285 degrees to 395 degrees c the relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of co 2 effervescence from a co 2 saturated aqueous fluid due to pressure fluctuations at the time of entrapment it is also proposed that the onset of gold deposition is related to the onset of fluid unmixing

improving antarctic tide models by assimilation of icesat laser altimetry over ice shelves 1 assimilation of laser altimeter data from the ice cloud and land elevation satellite icesat significantly improves the accuracy of ocean tide models for the ross ice shelf ris for the most energetic tidal harmonic k1 assimilation reduces the root mean square error rmse between the model and a set of 16 independent tide records on and near the ris from 6 0 to 2 8 cm and the combined rmse for the six most energetic tidal harmonics from 7 7 to 5 4 cm when only the six most recent and highest quality tide records are considered the combined rmse is 4 8 cm this value is close to the uncertainty expected from tidal analyses of the short 1 2 month validation records indicating that assessing further improvements in tide model accuracy will require development of a higher quality validation data set

mineralogical and geochemical characteristics of hydrothermal alteration and episyenitization in the konigshain granites northern bohemian massif germany the late hercynian granites of konigshain underwent multistage hydrothermal processes extensive high temperature late magmatic alteration is for example indicated by low zr hf and an ree pattern displaying the tetrad effect intensive post magmatic alteration of the granite occurred along brittle structures at least two main stages of post magmatic hydrothermal alteration are involved the first high temperature stage which is characterized by albitization and or quartz leaching episyenitization resulted from fluid rock interaction with late magmatic fluids that very probably mixed with external low salinity fluids quartz dissolution was triggered by vapour condensation and or the cooling of these fluids below 450 c along brittle structures the high porosity resulting from quartz leaching during stage 1 assisted subsequent circulation of low temperature fluids at stage 2 the latter is characterized by the chloritization and illitization of episyenites almost all major and trace elements were enriched or depleted during one of the main alteration stages however zr hf th and ti were immobile during post magmatic alteration the significant depletion of lree and the enrichment of hree in albitized samples is controlled by the dissolution of monazite and the new formation of hree rich polycrase y or aeschynite y during post magmatic stage 1 negative ce anomalies of episyenites are associated with illitization and suggest oxidizing conditions during stage 2

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

modeling ice shelf cavities in a z coordinate ocean general circulation model 1 processes at the ice shelf ocean interface and in particular in ice shelf cavities around antarctica have an observable effect on the solutions of basin scale to global coupled ice ocean models despite this these processes are not routinely represented in global ocean and climate models it is shown that a new ice shelf cavity model for z coordinate models can reproduce results from an intercomparison project of earlier approaches with vertical or isopycnic coordinates as a proof of concept ice shelves are incorporated in a 100 year global integration of a z coordinate model in this simulation glacial meltwater can be traced as far as north as 15 s the observed effects of processes in the ice shelf cavities agree with previous results from a coordinate model notably the increase in sea ice thickness however melt rates are overestimated probably because the parameterization of basal melting does not suit the low resolution of this configuration

mineralogy and geochemistry of the sodium metasomatism related uranium occurrence of aricheng south guyana the aricheng south uranium occurrence is associated with na metasomatism that affected the granitoids of the kurupung batholith in western guyana the mineral paragenesis indicates that late magmatic albitization was followed by chlorite alteration of biotite a minor amount of uraninite occurs in fractures in the newly formed albite crystals often in company of calcite the main mineralization stage occurred later than albitization and chloritization and is represented by brannerite disseminated in a groundmass of fine grained hydrothermal zircon whole rock geochemistry supports the temporal dissociation of albitization from the main ore stage brannerite zircon and uraninite are often partially altered to secondary brannerite zircon and coffinite respectively stable oxygen chlorite calcite and hydrogen chlorite isotope compositions suggest that a highly evolved meteoric fluid or at least one corresponding to a very high rock fluid ratio 18o of approx 3 4 to 4 and d of approx 80 may have caused the pre ore alteration assemblage the fluids in equilibrium with main ore stage zircon have 18o of approx 6 8 and appear to be of magmatic origin the aricheng occurrence geochemically mineralogically thermally and paragenetically resembles the valhalla u deposit in northern australia despite differences between the deposits host lithologies whereas the lagoa real and espinharas u deposits in brazil have host rock lithology that resembles that of aricheng

surface elevation changes at the front of the ross ice shelf implications for basal melting 1 rapid melting beneath the ross ice shelf ris occurs near the ice front likely in response to a secondary buoyant plume with tidal mixing and is sensitive to seasonal water temperatures in front of the ice shelf the front of the ris is examined using glas icesat laser altimetry data spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front melt rates b increase exponentially as the front is approached from approximately zero at 40 km from the front to an average of 2 8 1 0 m a 1 within the front kilometer melt estimates within the front 60 km are best fit by the relationship b 2 0 exp x 11900 m a 1 where x denotes distance from the front frontal melt totals approximately 16 km3 a 1 in the front 40 km which accounts for between 10 and 40 of current published estimates of total melt beneath the ris spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved frontal melt is modeled as a one dimensional buoyant plume with tidal currents included these model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front

the generation of diurnal period shelf waves by tidal currents abstract continental shelf waves of diurnal period are shown to be generated by tidally induced reynolds stresses within a bottom stokes scale boundary layer the theory is applicable to a uniformly rotating homogeneous ocean of two dimensional depth variability in which alongshore variations occur over scales large compared to the shelf width explicit solutions are derived for the shelf wave velocity components and for the cross shelf sea surface slope in the case of a traveling kelvin wave forcing numerical values are presented for an exponential depth profile h h0 exp 2 x where x is the coordinate normal to the coast results indicate that the amplitude of the shelf wave current can exceed that of the astronomical tidal current and that the alongshore component of the shelf wave current will consistently lead the alongshore component of the tidal current by 180 to 0 over one wavelength in the direction of phase propagation

isotopic peculiarities of an archaean pegmatite union mine mica south africa abstract an isotopic investigation rb sr u pb pb pb and sm nd of the archaean union mine pegmatite northern province south africa is reported the intrusion of this pegmatite devoid of zircon and monazite has been dated at 2912 2 6 ma by pb pb step leaching of magmatic garnet this age is supported by an identical two garnet apatite sm nd isochron of 2917 27 ma a rb sr k feldspar garnet albite isochron of 2023 11 ma corresponds to a later hydro thermal overprint isotope resetting during this overprint was incomplete and revealed that various parent daughter isotope pairs within minerals of the union mine pegmatite behaved in exceedingly different manners under the same physico chemical conditions sr isotope resetting was incomplete and resulted in open system behaviour of the pegmatite k feldspar almost quantitatively lost its radiogenic sr which had accumulated between the time of intrusion and the overprint sr loss from muscovite was variable and incomplete as indicated by a wide range in ages from 2088 74 ma to 2744 13 ma only insignificant portions of this sr were incorporated into adjacent albite as revealed by a micro drilled rb sr profile through a muscovite albite grain boundary the 87 sr 86 sr initial ratio is 0 8301 11 for the proterozoic isochron which is far less radiogenic than the calculated value of 2 24 for a closed system it is therefore concluded that sr was lost from the system and that the rb sr ratio of the whole rock was changed apatite on the other hand seems to have behaved as a closed system and retains a low initial sr ratio of 0 7125 u pb analyses of apatite reveal that this mineral has behaved as an almost closed system for pb diffusion as well but was open for u significant loss during the overprint this has resulted in high reverse discordance of apatite u pb analysis and in a geologically meaningless secondary pb pb isochron the data demonstrate that both preferential loss of daughter sr over parent rb and parent u over daughter pb isotopes can occur in some minerals of one and the same rock this process altered the parent daughter ratios to such an extent that in the case of apatite a secondary pb pb isochron with an erroneous age of 4119 66 ma was established it appears likely that preferential loss of daughter or parent isotopes is the reason for spurious secondary whole rock isochrons reported and reviewed by moorbath and taylor 1986 and moorbath et al 1986 such losses far exceeding predictions based on volume diffusion experiments may occur by means of fast transport diffusion which in part may depend on the micro textures of the host minerals geochemical studies of early archaean rocks require substantial back correction for radiogenic decay and the example of the union mine pegmatite shows that changes of parent daughter isotope ratios may occur long after rock formation however such changes may go unnoticed in high precision geochronology i e garnet dating in this study but will lead to erroneous correction for radiogenic decay and hence to erroneous conclusions regarding the origin of the studied samples

ages of altered granites adjoining the witwatersrand basin with implications for the origin of gold and uranium one of the main problems in understanding the origin of the witwatersrand basin mineralization concerns the source of the prodigious concentrations of gold and uranium in the basin another problem is the age and nature of the rocks in the source area recent data1 2 indicate that the source area was largely 3 200 2 900 myr old becoming progressively younger as the basin evolved granites adjoining the basin are commonly overprinted by pervasive and vein related hydrothermal alteration which resulted in an enrichment of both gold and uranium3 4 here we report uranium lead data demonstrating that these hydrothermally altered granites were intruded sporadically both before and during the 360 myr period5 constraining witwatersrand sediment deposition and that hydrothermal alteration was associated with granitoid intrusion and subsequent cooling thus fertilization of the crust occurred during basin evolution a feature that supports the idea19 that gold and uranium mineralization occurred simultaneously with sediment deposition

numerical treatment of cross shelf open boundaries in a barotropic coastal ocean model abstract using a barotropic coastal ocean model with a straight coastline and uniform cross shelf bottom slope seven different cross shelf open boundary conditions four of which are applied in either implicit or explicit form are compared in three numerical experiments 1 a mound of water is allowed to collapse and radiate waves toward the open boundaries 2 a uniform alongshelf wind stress is applied at zero time over the entire shelf and held constant for the duration of the experiment 3 a uniform cross shelf wind stress is applied at zero time over the entire shelf and held constant for the duration of the experiment the boundary condition which is most transparent to waves consists of a sponge at the outer edge of the model domain with an orlanski radiation condition at the outer edge of the sponge several open boundary conditions perform adequately in the wind stress experiments but the orlanski radiation condition alone without a sponge appears to give the best total performance of thes

diurnal shelf waves in the southern weddell sea abstract strong evidence is presented for the existence of barotropic shelf waves at diurnal frequencies in the shelf break region of the southern weddell sea rotary spectra from long term current meter records on the slope at the shelf break and on the shelf show the same rotational sense as that predicted by a barotropic model phase speeds of the zero mode shelf waves calculated from tidal analysis and rotary coherencies of current meter pairs oriented in an along shelf direction are in the same range as that predicted by barotropic theory except for the o1 constituents the weak stratisfication of the weddell sea in the region of the slope results in nearly barotropic motion at diurnal frequencies the anomalous behavior of the o1 constituent could not be explained

u th pb geochronology of zircon and monazite from syenite and pincinite xenoliths in pliocene alkali basalts of the intra carpathian back arc basin abstract phreatomagmatic eruptions in the lucenec basin in slovakia ejected fragments of igneous rocks of unknown age and ranging in composition from gabbro to syenite and orthopyroxene granodiorite pincinite two kinds of syenites with different fe3 contents composed of anorthoclase and interstitial metaluminous trachyte to alkali rhyolite glass represent the final products of alkali basalt differentiation at depths of 10 and 15 18 km pincinite composed of plagioclase quartz orthopyroxene ilmenite and interstitial peraluminous dacite glass is a quenched supersolidus anatectic melt formed at the depth of 17 20 km by high temperature dehydration melting of biotite bearing gneissic protolith zircon from pincinite and syenite and monazite from syenite were dated using excimer laser ablation icp ms the obtained ages cluster at 5 8 0 1 and 5 4 0 1 ma thus corroborating the genetic link between the two rock types and constraining the residence time of at least 300 100 ka of the parental basalt in the deep crustal magma storage zones the inferred ages overlap the upper miocene pliocene boundary and are considerably younger than spatially associated basaltic lava flows dated using the k ar method consequently the fluviolacustrine poltar formation overlying the maar structure must have sedimented during the pliocene and not during the upper miocene pontian as previously assumed

structures related to the antimony line murchison schist belt kaapvaal craton south africa abstract sporadic epigenetic antimony gold mineralisation along the ene trending antimony line of the murchison schist belt is structurally controlled and spatially related to carbonated rock types with quartz carbonate veins in tension fractures the antimony line is a discontinuous zone of brittle ductile deformation and is locally oblique to lithological layering within a broad ductile shear zone this tectonically early d 1 shear zone has an oblique to dip slip reverse north side up movement sense with downward facing fold structures in the hangingwall later d 2 fold structures deformed the antimony line and its wallrocks into steep structures the antimony line is intruded by granitoids which post date d 1 and are not directly related to mineralisation the source of which may have been metamorphic fluids

a model of tidally dominated ocean processes near ice shelf grounding lines 1 glaciological processes at grounding lines which divide floating ice shelves from grounded ice sheets may strongly influence the dynamics and evolution of inland ice therefore understanding the oceanic forcing on ice shelves in this region is of importance to predictions of cryospheric change and sea level rise as the ocean cavity shallows toward the grounding line tidal mixing becomes proportionately more important until a tidal front forms beyond which the water properties are vertically homogenized the extent of this mixed zone is relevant to several questions because a fully mixed region behaves differently to the stratified ocean offshore in this study a highly simplified one dimensional model is used to examine the size properties and sensitivities of the mixed zone the model suggests that most grounding line mixed zones are small implying that the usual models representing a stratified ocean are generally valid if tidal mixing is also taken into account modeled mixed zones can be significant in near freezing regions with vigorous tides and a shallowly sloping cavity but even these areas are smaller than previously proposed it therefore seems that upwelling of warm water rather than mixing in tidal zones generally maintains ice shelf basal melting near grounding lines where mixed zones are present the model suggests that they insulate the grounding line from offshore ocean waters the model illustrates the origin of ice shelf water plumes and confirms that unlike elsewhere in the cavity melting in the mixed zone increases linearly in response to ocean warming

melting and freezing beneath filchner ronne ice shelf antarctica 1 we use remote sensing data sets to evaluate the spatial distribution of melt beneath the filchner ronne ice shelf fris the net melt rate of 83 4 24 8 gtons yr is 2 5 5 times lower than previous glaciological estimates but is similar to existing oceanographic estimates the spatial distribution however differs significantly from standard conceptual and numerical models in which most melt occurs along the grounding lines our results suggest most grounding line melt is refrozen while the dominant ice shelf water isw source is melting near the ice shelf front probably associated with tidal action this suggests that changes in ice shelf extent can impact isw production rates in the weddell sea

observations of flow and ice ocean interaction beneath the mcmurdo ice shelf antarctica 1 the first oceanographic data from beneath the mcmurdo ice shelf antarctica are presented here with synchronous observations from southern mcmurdo sound multilevel current profiles at three sites over spring tide revealed primarily k1 diurnal tides with net transport from mcmurdo sound into the basin beneath the mcmurdo ice shelf the time series in southern mcmurdo sound captured the 14 day tidal phase shift and showed that transport into the subice shelf cavity was even greater over neap tide with average transport 1 8 sv mcmurdo sound sea ice cover during this period was the heaviest in 25 years of satellite record and was associated with the proximity of two massive icebergs b 15a and c 19 the warmest waters observed beneath the ice shelf were set at the surface freezing temperature probably through recent interaction with this extensive sea ice cover observed ranges of temperature and salinity were narrowed to the extent that neither antarctic surface water nor high salinity shelf water were observed beneath the ice shelf we anticipate that in years with more typical summer sea ice cover surface waters swept beneath the ice shelf will be significantly warmer having even greater potential for basal melt near the ice shelf front our observations also exhibit some of the interactions particular to the subice shelf ocean environment these include occurrence of platelet ice in ice shelf meltwater boundary layer friction effects from the ice cover on the ocean and tidal currents driving enhanced melting near the ice shelf front

stable isotopes c o and fluid inclusion study of quartz carbonate veins from the antimony line murchison greenstone belt in the homogeneously deformed murchison greenstone belt region fluids circulations were focused along a localized deformation brittle ductile zone and the so called antimony line in order to trace the origin of the fluid its composition the temperature of the precipitation along the antimony line we investigate the stable isotope oxygen and carbon and fluid inclusion compositions of the quartz carbonate veins as well as chemical analyses of the carbonates and 40ar 39ar dating on the fuchsite associated with the veins three types of fluid inclusions are recognized for the h2o co2 dominated plus minor ch4 n2 fluid s microthermometric measurements together with the paragenesis of the veins and host rocks indicate that the pressure temperature of the precipitation is 350 to 450 c 200 to 300 mpa which is an unusually high temperature for an antimony deposit the carbonates are fe mg rich and are characterized by flat mree and hree patterns with slight depletions in lree the oxygen isotope compositions of the quartz are rather homogeneous at the scale of the antimony line 18oquartz 10 9 14 3 in detail subtle differences in the 18o values for the quartz and carbonate exist depending on the sampling sites these differences are likely due to minor temperature and or host rock geochemistry variations along the antimony line data point to a metamorphic origin for the fluids responsible for the mineralization which likely escaped from the nearby greenstone lithologies finally we present an overview of the murchison greenstone belt geological evolution by integrating the antimony metallogeny of the antimony line which can be compared to that of an orogenic gold deposit

a model study of circumpolar deep water on the west antarctic peninsula and ross sea continental shelves abstract transport of relatively warm nutrient rich circumpolar deep water cdw onto continental shelves around antarctica has important effects on physical and biological processes however the characteristics of the cdw along the shelf break as well as what happens to it once it has been advected onto the continental shelf differ spatially in the present study high resolution 4 5 km regional models of the ross sea and the west antarctic peninsula coastal ocean are used to compare differences in cdw transport the models compared very well with observations from both regions examining the fluxes not only of heat but also of a simulated dye representing cdw shows that in both cases cdw crosses the shelf break in specific locations primarily determined by the bathymetry but eventually floods much of the shelf the frequency of intrusions in marguerite trough was ca 2 3 per month similar to recent mooring observations a significant correlation between the along shelf break wind stress and the cross shelf break dye flux through marguerite trough was observed suggesting that intrusions are at least partially related to short duration wind events the primary difference between the cdw intrusions on the ross and west antarctic peninsula shelves is that there is more vigorous mixing of the cdw with the surface waters in the ross sea especially in the west where high salinity shelf water is created the models show that the cdw moving across the antarctic peninsula continental shelf towards the base of the ice shelves not only is warmer initially and travels a shorter distance than that advected towards the base of the ross ice shelf but it is also subjected to less vertical mixing with surface waters which conserves the heat available to be advected under the ice shelves this difference in vertical mixing also likely leads to differences in the supply of nutrients from the cdw into the upper water column and thus modulates the impacts on surface biogeochemical processes