geochemistry of laterites stability of al goethite al hematite and fe super 3 kaolinite in bauxites and ferricretes an approach to the mechanism of concretion formation

preliminary evaluation of the runoff processes in a remote montane cloud forest basin using mixing model analysis and mean transit time in this study the mean transit time and mixing model analysis methods are combined to unravel the runoff generation process of the san francisco river basin 73 5 km2 situated on the amazonian side of the cordillera real in the southernmost andes of ecuador the montane basin is covered with cloud forest sub paramo pasture and ferns nested sampling was applied for the collection of streamwater samples and discharge measurements in the main tributaries and outlet of the basin and for the collection of soil and rock water samples weekly to biweekly water grab samples were taken at all stations in the period april 2007 november 2008 hydrometric data mean transit time and mixing model analysis allowed preliminary evaluation of the processes controlling the runoff in the san francisco river basin results suggest that flow during dry conditions mainly consists of lateral flow through the c horizon and cracks in the top weathered bedrock layer and that all subcatchments have an important contribution of this deep water to runoff no matter whether pristine or deforested during normal to low precipitation intensities when antecedent soil moisture conditions favour water infiltration vertical flow paths to deeper soil horizons with subsequent lateral subsurface flow contribute most to streamflow under wet conditions in forested catchments streamflow is controlled by near surface lateral flow through the organic horizon exceptionally saturation excess overland flow occurs by absence of the litter layer in pasture streamflow under wet conditions originates from the a horizon and overland flow copyright 2011 john wiley sons ltd

vertical soil water movement in a tropical rainforest catchment in northeast queensland in a tropical rainforest catchment shallow piezometers respond almost instantaneously to rainfall but the dominant ground water recharge mechanisms are not well understood to improve understanding the downward movement of soil water on a runoff plot was traced using tritiated water injected at 0 20 m below the surface which marks the lower boundary of active subsurface storm flow the tritium pulse was translated slowly down the profile apparently dominated by interstitial piston flow on the lines described by zimmermann s theoretical model this recharge mechanism accounted for about 35 per cent of rainfall or 50 per cent of throughfall the pulse s advance may have also been delayed by the upward movement of soil water indicated by the distribution of hydraulic potential under different hydrological conditions the result was an increase in soil water transit time particularly below 1 0 m there was also evidence in the tracer profiles for rapid by pass flow but the volumes concerned could not be quantified in this experiment

constraints on the timing and duration of orogenic events by combined lu hf and sm nd geochronology an example from the grenville orogeny abstract grenville age basement rocks of the virginia blue ridge province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent rodinia although the igneous origins of these rocks have been the subject of much previous work few studies have examined the complex metamorphic history of these rocks during the grenville orogeny to better constrain the timing and duration of metamorphism we determined lu hf and sm nd garnet and u pb zircon rim dates of garnet bearing rocks exposed in the virginia blue ridge multiple periods of mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before 1065 ma and subsequent local episodes occurring at least until 1000 ma zircon overgrowths and monazite dates suggest metamorphic events between 1050 and 960 ma our new lu hf garnet dates range from 1032 3 to 1019 3 ma new sm nd dates determined on the same garnet dissolutions define a younger range from 965 6 to 949 6 ma our zircon dates are consistent with previously measured ages 1090 940 ma and often overlap with the lu hf garnet dates from the same samples there is a large and systematic difference 60 80 m y between the garnet dates from the two isotopic systems although sm nd dates slightly younger than those recorded by lu hf are not uncommon the large discrepancies between the two systems that we see here are unusual the dated garnets are unzoned and yield lu hf dates that coincide with the zircon overgrowth dates sm nd ages overlap with the younger monazite dates we interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems further these data require slow cooling following large scale crustal thickening after the assembly of rodinia during grenville orogenesis these data illustrate that the use of the coupled lu hf and sm nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events

a proterozoic granulite facies terrane on roan mountain western blue ridge belt north carolina tennessee polymetamorphosed gneisses of proterozoic age crop out on roan mountain north carolina tennessee near the western edge of the blue ridge belt these gneisses display grenville age granulite facies assemblages including two pyroxene metabasites and gar net sillimanite orthoclase metapelites all gneisses were intruded by diabase dikes of the late precambrian bakersville gabbro paleozoic amphibolite facies metamorphism locally formed garnet and amphibole in mafic rocks and kyanite in metapelitic rocks two pyroxene geothermometry and mineral stability curves define temperature pressure ranges of 750 847 c and 6 5 8 0 kbar for the granulite facies metamorphism this metamorphism probably occurred within the lower crust at depths of 23 28 km pressure temperature conditions for the major paleozoic metamorphism are not well constrained but are estimated to be 700 740 c and 7 5 9 0 kbar

the colima volcanic complex mexico part ii late quaternary cinder cones since the late pleistocene eleven cinder and lava cones have erupted on the floor of the southern colima graben ne and nw of the large active andesitic volcano colima scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites 9 leucite basanites 3 and minettes 15 the commonest variety of mica lamprophyre these samples represent primitive high temperature magmas with 47 6 50 3 sio2 7 4 15 3 mgo 2 5 4 4 k2o and 2 2 9 9 normative nepheline all members of this basic alkalic suite contain mg olivine fo75 94 chromite augite and late plagioclase and titanomagnetite the petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile bearing phases phlogopite apatite and analcime during late stages of crystallization as these phases increase in abundance presumably reflecting a rise in magmatic volatile content there are corresponding increases in the whole rock concentrations of 16 incompatible elements although these incompatible elements are relatively abundant even in the basanites many are highly concentrated in the minettes ba 4 200 ppm sr 3 100 ppm zr 550 ppm ce 190 ppm hf 18 ppm among the incompatible elements the degrees of enrichment in the minettes relative to the basanites decrease in the order h th ce la nd zr hf u ba sm eu pb p nb sr ti these enrichments may reflect the increasing importance of minor incompatible element rich mantle phases during partial melting the concentrations of alkali metals k na rb and cs do not correlate with these other elemental enrichments the leucite basanties have similar incompatible element contents to many minettes differing from them only in the presence of leucite rather than analcime and ti f rich groundmass phlogopite rather than hydrous phlogopite phenocrysts thus the leucite basanites represent relatively dry equivalents of minettes two of the eleven cinder cones are calc alkaline in nature and do not belong to the basanite minette group the easternmost cone is constructed of high al basalt and the northernmost of basaltic andesite the high al basalt 49 5 sio2 9 3 mgo 221 ppm ni closely approximates a parental magma to the post caldera andesitic suite of v colima 56 5 61 6 sio2 the basaltic andesite is relatively enriched in incompatible elements compared to the high al basalt v colima trend the ne normative basanite minette samples are highly enriched in incompatible elements while the contemporaneous hy qz normative calc alkaline suite encompassing the high al basalt and v colima s andesites is characterized by relatively low abundances of these elements no likely mineral assemblage can relate the alkaline and calc alkaline suites through crystal fractionation they probably represent fundamentally different melting events during the quaternary the main focus of andesitic volcanism in the southern colima graben has migrated southward with time volcan colima marks its present position 5 km south of the pleistocene volcano nevado de colima and another 15 km from the still older volcan cantaro the eruptions of basic alkalic magma probably occurred during the late stages of nevado s life and through the life of v colima they generally migrated from west to east with time towards v cantaro the most recent cone v apaxtepec is the only one east of the andesitic colima cantaro axis the oldest and the two youngest cones produced basanites while minettes dominated at cones of intermediate ages the cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of v colima the southern end of the colima graben can be viewed then as the volcanic analog of many classical post plutonic hypabyssal lamprophyre localities

constraints on lake agassiz discharge through the late glacial champlain sea st lawrence lowlands canada using salinity proxies and an estuarine circulation model during the last deglaciation abrupt freshwater discharge events from proglacial lakes in north america such as glacial lake agassiz are believed to have drained into the north atlantic ocean causing large shifts in climate by weakening the formation of north atlantic deep water and decreasing ocean heat transport to high northern latitudes these discharges were caused by changes in lake drainage outlets but the duration magnitude and routing of discharge events factors which govern the climatic response to freshwater forcing are poorly known abrupt discharges called floods are typically assumed to last months to a year whereas more gradual discharges called routing events occur over centuries here we use estuarine modeling to evaluate freshwater discharge from lake agassiz and other north american proglacial lakes into the north atlantic ocean through the st lawrence estuary around 11 5 ka bp the onset of the preboreal oscillation pbo faunal and isotopic proxy data from the champlain sea a semi isolated marine brackish water body that occupied the st lawrence and champlain valleys from 13 to 9 ka indicate salinity fell about 7 8 range of 4 11 around 11 5 ka model results suggest that minimum 1600 km3 and maximum 9500 km3 estimates of plausible flood volumes determined from lake agassiz paleoshorelines would produce the proxy reconstructed salinity decrease if the floods lasted 1 day to 5 months and 1 month to 2 years respectively in addition champlain sea salinity responds very quickly to the initiation within days and cessation within weeks of flooding events these results support the hypothesis that a glacial lake flood rather than a sustained routing event discharged through the st lawrence estuary during the pbo

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

rayleigh fractionation of heavy rare earths and yttrium during metamorphic garnet growth we present laser ablation microprobe inductively coupled mass spectrometry analyses of y yb er dy and gd in garnet crystals from high grade metamorphic and migmatitic rocks from the sierras pampeanas of argentina these rocks contain large garnet porphyroblasts 4 mm that are compositionally zoned rare earth element concentrations in the rims are typically one order of magnitude lower than those in the cores a notable feature of this zonation is the inversion of yb er and yb dy ratios from 1 in the cores to 1 in the rims we show quantitatively that the spatial distribution of these trace elements in garnet can be most simply and effectively explained as arising from rayleigh fractionation during garnet growth we also analyzed a small garnet crystal 2 mm representative of a garnet population that is associated with migmatitic leucosomes this crystal displays uniformly low concentrations of all trace elements that are virtually identical in both absolute and relative magnitudes to those in the rims of the larger garnet porphyroblasts the small crystals and perhaps part of the porphyroblast rims are likely to be peritectic products of incongruent melting reactions we argue that melts formed from garnet bearing sources may generally be unable to equilibrate with metamorphic garnet cores and may thus be more strongly depleted in heavy rare earth elements and y than modeling based on bulk source rock abundances of these elements would indicate

use of δ18o tracer to identify stream and spring origins of a mountainous catchment a case study from liddar watershed western himalaya india summary water samples from precipitation streams and springs were collected and analyzed for oxygen isotope composition to identify the stream and spring origins in liddar watershed nw himalaya india samples were collected from eight precipitation sites 14 streams and six springs on bimonthly basis from january march 2008 to january 2009 the data generated indicated a strong spatial and temporal variation in oxygen isotope composition an altitude effect of 0 1 to 0 5 100 m change in elevation mean altitude effect 0 19 100 m was discernible in precipitation isotopic composition the precipitation was 18 o depleted in cooler seasons months and at higher elevations the 18 o of precipitation showed a narrow range 2 5 during the periods months of more precipitation and wide range 8 13 during the other months like precipitation the isotopic composition of streams is also controlled by the elevations of their catchments the stream water was 18 o depleted in the mountain areas with their catchments at higher altitudes than at lower elevations the streams tributaries were most 18 o depleted in may and least depleted in september the oxygen isotope composition of springs is similar to that observed in the streams being most 18 o depleted in may and least 18 o depleted in september on the basis of local vertical isotopic gradient of oxygen the mean altitude of the recharge by rainfall infiltration was estimated the widespread melting of snow the high discharges of the streams and the springs and 18 o depleted stream and spring waters indicate the dominant contribution of snow melt in may negligible snow melting precipitation in the form of snow only less stream and spring discharges and less variance indicate the contribution from baseflow in january while as exhausted snow moderate stream and spring discharges and 18 o enriched stream and spring water indicate the contribution from rainfall the best correlation was observed between the 18 o of springs and streams

rate and mechanism in prograde metamorphism for a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature disequilibrium depends on the slowest of three sequential steps a surface detachment of reactant minerals b transport of material to the site of mineral growth and c nucleation and growth of the product mineral we have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism the dissolution and growth steps are linear functions of the driving force g of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself

chemical fingerprints of hydrological compartments and flow paths at la cuenca western amazonia a forested first order catchment in western amazonia was monitored for 2 years to determine the chemical fingerprints of precipitation throughfall overland flow pipe flow soil water groundwater and streamflow we used five tracers hydrogen calcium magnesium potassium and silica to distinguish fast flow paths mainly influenced by the biological subsystem from slow flow paths in the geochemical subsystem the former comprise throughfall overland flow and pipe flow and are characterized by a high potassium silica ratio the latter are represented by soil water and groundwater which have a low potassium silica ratio soil water and groundwater differ with respect to calcium and magnesium the groundwater controlled streamflow chemistry is strongly modified by contributions from fast flow paths during precipitation events the high potassium silica ratio of these flow paths suggests that the storm flow response at la cuenca is dominated by event water

the structural geometry and evolution of foreland thrust systems northern virginia seismic reflection data reveal that the structural geometry of the central appalachians of northern virginia consists of three distinct thrust systems each thrust system is characterized by a unique internal geometry the blue ridge thrust sheet is a composite thrust sheet composed primarily of imbricated precambrian crystalline rocks it over rode cambrian ordovician carbonates and formed a sheared basement cored fault bend fold thrusts within the sheet may be taconic and earliest alleghanian whereas final thrusting and emplacement of the sheet were probably slightly younger but still early alleghanian the north mountain thrust sheet is characterized by imbricated cambrian ordovician carbonates that are deformed into large amplitude mode ii fault bend folds and fault propagation folds rocks of this sheet were transported more than 60 km across a similar section of carbonates the leading edge of the north mountain thrust sheet was deformed into a fold with a mode ii fault bend fold geometry and was juxtaposed against middle paleozoic rocks the middle paleozoic rocks occur in a ramp across which displacement along the north mountain thrust was transferred to a higher detachment more than 60 km of cover rocks displaced during the emplacement of the north mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western valley and ridge province or were backthrust above the sheet the timing for imbrication and emplacement of the north mountain thrust sheet is probably main phase alleghanian the lower carbonate duplex extends from beneath the blue ridge and north mountain thrust sheets westward across the western valley and ridge province the lower carbonate duplex is characterized by imbricated cambrian ordovician carbonates that form low amplitude fault bend folds this thrust system also probably formed during main phase alleghanian deformation

alternative routing of lake agassiz overflow during the younger dryas new dates paleotopography and a re evaluation abstract overflow from glacial lake agassiz has been implicated in affecting late glacial ocean circulation and climate the timing of the diversion of lake agassiz overflow away from its southern routing to the gulf of mexico correlates closely with the younger dryas ca 11 10 14 c ka new models of paleotopography in the eastern outlet region of lake agassiz adjusted for differential isostatic rebound show that overflow to the superior basin would have occurred when the lake formed its herman norcross and tintah beaches just after 10 9 14 c ka if the laurentide ice sheet lis had retreated from the thunder bay ontario region by that time however the absence of coarse flood deposits normally associated with large overflows plus the morphology of possible overflow channels in the thunder bay region suggest that agassiz overflow may not have been routed into the great lakes during the younger dryas as previously believed in addition non carbonate radiocarbon dates in the thunder bay area are all younger than 10 14 c ka suggesting that this area may not have been deglaciated until after the younger dryas thus preventing lake agassiz overflow through this area an alternative route of overflow from lake agassiz during the younger dryas could have been through the northwestern clearwater outlet to the arctic ocean although all research to date shows that this region was covered by ice until 10 14 c ka on the other hand it is possible that older and coarser overflow sediment may now lie buried beneath finer deposits associated with the 10 14 c ka marquette readvance of the lis which ponded waters in valleys between lake agassiz and lake superior or that eastward overflow in this area was not catastrophic

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

long term patterns and short term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed 1 the 326 ha rio icacos watershed in the tropical wet forest of the luquillo mountains northeastern puerto rico is underlain by granodiorite bedrock with weathering rates among the highest in the world we pooled stream chemistry and total suspended sediment tss data sets from three discrete periods 1983 1987 1991 1997 and 2000 2008 during this period three major hurricanes crossed the site hugo in 1989 hortense in 1996 and georges in 1998 stream chemistry reflects sea salt inputs na cl and so4 and high weathering rates of the granodiorite ca mg si and alkalinity during rainfall stream composition shifts toward that of precipitation diluting 90 or more in the largest storms but maintains a biogeochemical watershed signal marked by elevated k and dissolved organic carbon doc concentration doc exhibits an unusual boomerang pattern initially increasing with flow but then decreasing at the highest flows as it becomes depleted and or vigorous overland flow minimizes contact with watershed surfaces tss increased markedly with discharge power function slope 1 54 reflecting the erosive power of large storms in a landslide prone landscape the relations of tss and most solute concentrations with stream discharge were stable through time suggesting minimal long term effects from repeated hurricane disturbance nitrate concentration however increased about threefold in response to hurricanes then returned to baseline over several years following a pseudo first order decay pattern the combined data sets provide insight about important hydrologic pathways a long term perspective to assess response to hurricanes and a framework to evaluate future climate change in tropical ecosystems

comments on the allerod younger dryas holocene sequence in the west central champlain sea eastern ontario a record of glacial oceanographic and climatic changes by u brand and f m g mccarthy

planar dispersion of folds in ductile shear zones and kinematic interpretation of fold hinge girdles abstract as indicated by theory and multiple case studies hinge lines of minor folds in ductile shear zones are dispersed in a plane that is incompletely rotated toward mylonitic foliation this yields a geometric relationship between the fold hinge girdle and mylonitic foliation that can be used to determine sense of shear statistical study of published data from various shear zones and of new data from the blue ridge thrust complex reveals a small to moderate angle between the best fit plane to hinge lines and mean mylonitic foliation this angle is 4 9 in eight of 12 test cases moderate angles 22 34 are associated with nappes kinematic analyses of 12 fold hinge girdles indicate the sense of shear otherwise determined for host shear zones a theoretical model of contemporary fold rotation in simple shear closely simulates the geometric relationships described above for natural folds using this model large shear strains 10 are implied by the 4 9 angle between the fold hinge girdle and mylonitic foliation

the use of stream flow routing for direct channel precipitation with isotopically based hydrograph separations the role of new water in stormflow generation abstract understanding the pathways by which event water contributes to stream stormflow provides insight into stormflow generation mechanisms we analyze the impact of storm size on the relative contribution of event water to stormflow by using natural variations in the oxygen isotopic composition of precipitation and stream water to separate multiple stormflow hydrographs from a single fourth order 1212 ha catchment we extend previous isotope based hydrograph separations by independently accounting for the contribution of event water via direct channel precipitation to the stream hydrograph the direct channel precipitation contribution is determined using a numerical model of stream flow routing though the catchment taking precipitation and digital elevation data as input variables for the range of storm sizes sampled having recurrence intervals ranging from less than a week to 4 months essentially all the event water in stream stormflow can be attributed to direct channel precipitation event water not directly falling on the stream channel indirectly contributes to stormflow by increasing the subsurface discharge of pre event water to the stream neither the hydrograph separation data field observations during the precipitation events nor experimental observations of flow in a large scale natural soil column extracted from the watershed are consistent with macropore flow or groundwater ridging as the primary mechanism responsible for increasing subsurface discharge results from a series of artificial rain experiments using the unsaturated natural soil column are consistent with a preferential kinematic flow model and indicate that the discharge of pre event water to the stream during a storm event may be controlled by kinematic flow processes within the watershed soils

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

the grenville orogen explained applications and limitations of integrating numerical models with geological and geophysical data numerical models offer powerful insights into tectonic processes especially when their validity can be tested against geological and geophysical observations from natural orogenic belts here we explain some of the criteria for success in integrating orogenic models with data using examples from the grenville orogen model designs must be simplified by comparison with nature to illuminate the first order processes that control orogenic evolution which limits the extent to which model results can reproduce geological observations for the western grenville orogen observed variations in geological properties are represented by lower crustal blocks with strength decreasing from the exterior to the interior of the model go series models with this design reproduce the first order crustal architecture of the georgian bay and montreal val d or lithoprobe transects both constant convergence and stop convergence models produce similar geometries but only stop convergence models produce normal sense shear

timing of eclogite facies metamorphism in the southernmost scandinavian caledonides by lu hf and sm nd geochronology the scandinavian caledonides contain several non cogenetic eclogite bearing terranes that were metamorphosed before the main orogenic stage in scandian time 430 395 ma although petrological and geochronological data from these terranes have provided essential information on the geodynamic history of the caledonian orogenic cycle the general picture is still patchy to refine existing geodynamic models we have dated the eclogite occurrence in the jaeren nappe sw norway by lu hf and sm nd geochronology five out of the six studied samples provide a weighted mean lu hf age of 469 9 1 2 ma 2 one sample provided a significantly younger age of 457 9 2 4 ma garnet from the younger sample grew exclusively at eclogite facies conditions in contrast garnet from the other samples comprises prograde cores and peak metamorphic rims age estimates that take lu contributions of each zone into account provide an age of 471 0 0 9 ma for the cores and suggest a ca 455 ma age for the rims which is identical to the bulk garnet age of the younger sample the same pattern is indicated by sm nd ages although these are relatively imprecise and reflect isotopic disturbance during thermal overprinting upon exhumation the data define a new high pressure age population for the scandinavian caledonides which allows more detailed insight into the subduction history that affected the baltoscandian margin before scandian continental collision furthermore this study highlights the potential complexities involved in garnet geochronology and shows the strength of lu hf dating for unraveling the geochronological record of hp rocks

metamorphic fractional crystallization and internal metasomatism by diffusional homogenization of zoned garnets metamorphic fractional crystallization involves fractionation of material into the interior of a crystallizing mineral such as garnet with consequent change in the effective bulk composition in assemblages with thermodynamic variance of 3 or greater the mineral assemblage compositions of coexisting phases and the zoning profile preserved in the garnet are a function of the amount of material that is fractionated homogenization of zoned garnet by intracrystalline diffusion at elevated temperatures operates on a time scale of millions to tens of millions of years depending on the temperature time path and radius of the garnet one effect of homogenization is to drive chemical reactions by a metasomatic mechanism internal metasomatism even in the absence of changes in pressure or temperature this process may affect the appearance or disappearance of minerals and may in some rocks be responsible for the first appearance of kyanite or the disappearance of staurolite in barrovian terranes

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

an investigation of dissolution methods for lu hf and sm nd isotope studies in zircon and garnet bearing whole rock samples 1 the distinct chemical behavior of lu and hf can limit complete sample dissolution and spike sample equilibration here we investigate the effects that four dissolution methods hotplate low temperature microwave high temperature microwave and steel jacketed parr bomb have on the sm nd and lu hf isotope compositions of rocks any of the dissolution methods produce high quality sm nd isotope data and hotplate dissolution of young volcanic rocks produces adequate lu hf data however low temperature microwave and hotplate dissolution methods produce significantly different lu hf and 176hf 177hf ratios as compared to parr bomb dissolution data for old samples or those that contain garnet rutile or zircon a characteristic feature of incomplete spike sample equilibration and sample dissolution for garnet bearing samples is anomalously high 176lu 177hf ratios relative to 176hf 177hf ratios our results show that for lu hf isotope studies of rocks of any significant age the errors in calculated initial hf values may exceed 10 hf units if complete dissolution and spike sample equilibration are not attained for garnet or zircon bearing rocks high temperature and pressure dissolution such as that which can be achieved through parr bomb dissolution is critical to ensure complete dissolution of refractory phases such as garnet or zircon

distinguishing and estimating recharge to karst springs in snow and glacier dominated mountainous basins of the western himalaya india recharge assessment is a challenge in snow and glacier dominated himalayan basins quantification of recharge to karst springs in these complex geological environments is important both for hydrologic understanding and for effective water resource management we used spring hydrographs and environmental tracers isotopes and solutes to distinguish and estimate the sources of spring water and to identify the flow paths of the recharging waters in three mountainous basins of the western himalaya the karst springs are perennial with high discharge amplitudes the results indicate that ambient temperature has a strong influence on the hydrological behavior of the springs although the spring flow is dominantly controlled by the melting of snow and or glaciers rain events produce sharp spikes in spring hydrographs the facies patterns in springs within the bringi basin ca hco3 and the liddar basin ca hco3 and ca mg hco3 suggest flow dominantly through limestone and dolomite higher concentrations of so42 and na in warm springs of the kuthar basin indicate flow through carbonate silicate and other rocks the isotopic composition 18o 2h of precipitation snowpacks glacier melt and karst springs show wide variation both in space and time and are strongly influenced by the basin relief and meteorology the tracer based two and three component mixing models suggest that the snowmelt dominantly contributes to the spring flow 55 96 followed by glacier melt 5 36 and rain 4 34 based on tracer tests with good recovery rates springs are dominantly recharged through point sources rather than by diffuse infiltration changes in the timing form and amount of winter precipitation substantially affect the timing and magnitude of spring discharge during the rest of the year

the significance of biogenic element cycling in ancient tropical soils many lateritic tropical soils on stable cratonic areas have developed over millions to 10 s of millions of years some elements e g alkalies alkaline earths si are extremely depleted by leaching and others a1 fe ti are greatly enriched commonly this enrichment is regarded as merely the accumulation of insoluble constituents but inspection of the soil shows development of pisolites nodules and other features indicating extensive dissolution precipitation of feand al oxides and hydroxides this paper suggests that cycling of elements through vegetation is an important part of soil development in these ancient soils this project was initiated to investigate the processes forming lateritic gold deposits in which a relatively small primary vein or similar au deposit in bedrock mushrooms in the soil to become a significant au enrichment in the laterite zone davy and e1 ansary 1986 lecomte and colin 1989 the site of our investigation is the gold deposit of igarape bahia in the serra carajas mining region para province brazil in this region a very ancient plateau surface furnishes the environment for major fe mn and a1 deposits of the carajas district beisiegel 1982 machamer et al 1991 as well as a lateritic gold deposit at igarape bahia the plateau surface is interpreted to have been developed in cretaceous or early tertiary beisiegel 1982 kotschoubey and lemos 1985 the entire region is covered by dense tropical rain forest at the igarape bahia au deposit a narrow primary zone of au cu mineralization at depth gives rise to a relatively rich au deposit in lateritic soil the deposit appears analogous to other lateritic au accumulations in which the initial primary agbearing gold grains are etched and become smaller and new grains of ag poor gold grow in the laterite lecomte and colin 1989 a preliminary study indicated that considerable au was present in vegetation over the deposit so in a later sampling expedition an extensive group of vegetation samples was collected along with soils and waters andrade et al 1991 bliss 1991 machesky et al 1993 vegetation sampling was concentrated along a traverse across the unmined f30 deposit a satellite au deposit 1 km from the main igarape bahia deposit additional samples were collected at three background sites 1 10 and 30 km from the deposit at the f30 deposit and at the 1 km background site all species of tree and shrub were collected with leaves twigs and bark being collected from most species five specific species a large tree two bamboo species and two bushes were sampled along the traverse and at the background sites samples were washed dried chopped homogenized and analyzed for 40 elements by neutron activation activation laboratories ltd ancaster ontario and xray fluorescence materials characterization lab penn state university

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

age verification of the lake gribben forest bed and the younger dryas advance of the laurentide ice sheet analysis of nine wood samples from the lake gribben forest bed near lake gribben michigan yielded a combined age of 10 025 100 14c years bp which confirms and refines prior age estimates for the bed the stratigraphic position of these samples below a prograding ice contact fan indicates the time that a glacial margin reached the southern edge of the lake superior basin geomorphic tracing and correlation of associated deposits indicate that a contemporaneous margin extended almost 1000 km from duluth minnesota across the lake superior basin to north bay ontario along the southern shore of lake superior ice margin expansion began during and ended at the close of the younger dryas a surging glacier system would not produce a nearly linear moraine system across both a major basin lake superior and a major upland abitibi upland therefore we attribute this advance of the laurentide ice sheet to climatic forcing of the younger dryas event

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology a case study from the lewisian terrain abstract monazite is a valuable chronometer for many geological processes and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes this contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in lewisian granulites which is of general significance for monazite geochronology in high grade metamorphic rocks detailed investigations on two lewisian granulite samples using sem epma and sims have revealed that lewisian monazites frequently show petrographic chemical and age zonation the petrographical zonation which reflects the chemical compositional variation can be characterised as follows on the basis of the bse images 1 concentric zoning 2 patchy zoning and 3 intergrowth like zoning the chondrite normalised ree distribution patterns are remarkably similar between different zones of single monazite grains this rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin some possible mechanisms are proposed for the formation of monazite zonation these include 1 intergrowth of monazite crystals with different composition 2 episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event s which may involve recrystallisation or replacement of the original crystals in short the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism accordingly interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism line scans of th u pb la ce nd sm ca and si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism the very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism the effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase and recrystallisation of the mineral itself thus great care needs to be taken for geological application of the closure temperature concept

sm nd and u pb isotopic evidence of juvenile crust in the adirondack lowlands and implications for the evolution of the adirondack mts sm nd investigations of hyde school gneiss yield t dm ages only slightly older than u pb zircon emplacement ages thus indicating substantial addition of juvenile calcalkaline crust to the adirondack lowlands at ca 1300 ma juvenile crust of similar age has been recognized within the adirondack highlands the elzevir terrane further to the northwest in canada and in the green mts of vermont suggesting the presence of an extensive magmatic arc at that time while the precise nature of the arc remains uncertain an indonesian analogue is favored u pb zircon ages of ca 1150 1160 ma are reported for lowlands samples of syenitic and granitic rocks belonging to the anorthosite mangerite charnockite granite amcg suite that also occurs in the highlands as well as in the canadian sector of the frontenac terrane the regional occurrence of these anorogenic intrusions indicates that the adirondack highlands and frontenac terrane including the adirondack lowlands constituted a single contiguous entity

u pb geochronology of zircon and monazite from mesoproterozoic granitic gneisses of the northern blue ridge virginia and maryland usa abstract mesoproterozoic granitic gneisses comprise most of the basement of the northern blue ridge geologic province in virginia and maryland lithology structure and u pb geochronology have been used to subdivide the gneisses into three groups the oldest rocks group 1 are layered granitic gneiss 1153 6 ma hornblende monzonite gneiss 1149 19 ma porphyroblastic granite gneiss 1144 2 ma coarse grained metagranite about 1140 ma and charnockite 1145 ma these gneisses contain three proterozoic deformational fabrics because of complex u pb systematics due to extensive overgrowths on magmatic cores zircons from hornblende monzonite gneiss were dated using the sensitive high resolution ion microprobe shrimp whereas all other ages are based on conventional u pb geochronology group 2 rocks are leucocratic and biotitic varieties of marshall metagranite dated at 1112 3 ma and 1111 2 ma respectively group 3 rocks are subdivided into two age groups 1 garnetiferous metagranite 1077 4 ma and quartz plagioclase gneiss 1077 4 ma 2 white leucocratic metagranite 1060 2 ma pink leucocratic metagranite 1059 2 biotite granite gneiss 1055 4 ma and megacrystic metagranite 1055 2 ma groups 2 and 3 gneisses contain only the two younger proterozoic deformational fabrics ages of monazite separated from seven samples indicate growth during both igneous and metamorphic thermal events however ages obtained from individual grains may be mixtures of different age components as suggested by backscatter electron bse imaging of complexly zoned grains analyses of unzoned monazite imaged by bse and thought to contain only one age component from porphyroblastic granite gneiss yield ages of 1070 1060 and 1050 ma the range of ages of monazite not reset to a uniform date indicates that the grenville granulite event at about 1035 ma did not exceed about 750 c lack of evidence for 1110 ma growth of monazite in porphyroblastic granite gneiss suggests that the short hill fault might be a grenvillian structure that was reactivated in the paleozoic the timing of proterozoic deformations is constrained by crystallization ages of the gneissic rocks d1 occurred between about 1145 and 1075 ma or possibly between about 1145 and 1128 ma d2 and d3 must be younger than about 1050 ma ages of mesoproterozoic granitic rocks of the northern blue ridge are similar to rocks in other grenville terranes of the eastern usa including the adirondacks and hudson highlands however comparisons with conventional u pb ages of granulite grade rocks from the central and southern appalachians may be specious because these ages may actually be mixtures of ages of cores and overgrowths

groundwater fluctuations and footslope ferricrete soils in the humid tropical zone of southern cameroon reference ecos article 2007 065view record in web of science record created on 2007 11 28 modified on 2016 08 08

behaviour of chemical solutes during a storm in a rainforested headwater catchment the aim of this study is to identify in a small catchment area located within a tropical forest the pedological compartments in which the export of nutrients and chemical erosion of solutes occur during a stormflow event the catchment area displays two types of lateral flow i overland flow at the surface of the soil in the litter and root mat and ii groundwater flow in a macroporous subsurface horizon we interpret the variations of stream water chemistry during a storm flow event using the separation of storm flow hydrograph data between overland and groundwater flow and cl as a chemical parameter characterizing the residence time of water in the soil it appears that k especially was released into the throughfall whereas ca mg and na were clearly released from the litter k disappeared rapidly from soil solution whereas ca and mg were more progressively absorbed by the vegetation the ca and mg contents in groundwater increased with increasing residence time owing to the transpiration of trees the export of h4sio4 in the overland flow was moderate i e 24 of total h4sio4 export in the stream flow as overland flow represented 39 of total runoff the subsurface horizon where active groundwater flow occurs was successively affected by chemical erosion during the storm flow peak and then by neoformation of kaolinite favoured by increasing water residence time copyright 2003 john wiley sons ltd

whole rock pb and sm nd isotopic constraints on the growth of southeastern laurentia during grenvillian orogenesis the conventional view that the basement of the southern and central appalachians represents juvenile mesoproterozoic crust the final stage of growth of laurentia prior to grenville collision has recently been challenged new whole rock pb and sm nd isotopic data are presented from mesoproterozoic basement in the southern and central appalachians and the granite rhyolite province as well as one new u pb zircon age from the granite rhyolite province these data combined with existing data from mesoproterozoic terranes throughout southeastern laurentia further substantiate recent suggestions that the southern and central appalachian basement is exotic with respect to laurentia sm nd isotopic compositions of most rocks from the southern and central appalachian basement are consistent with progressive growth through reworking of the adjacent granite rhyolite province however pb isotopic data including new analyses from important regions not sampled in previous studies do not correspond with pb isotopic compositions of any adjacent crust the most distinct ages and isotopic compositions in the southern and central appalachian basement come from the roan mountain area eastern tennessee western north carolina the data set indicates u pb zircon ages up to 1 8 ga for igneous rocks inherited and detrital zircon ages 2 0 ga sm nd depleted mantle model t dm ages 2 0 ga and the most elevated 207 pb 204 pb observed in southeastern laurentia the combined u pb geochronologic and sm nd and pb isotopic data preclude derivation of southern and central appalachian basement from any nearby crustal material and demonstrate that grenville age crust in southeastern laurentia is exotic and probably was transferred during collision and assembly of rodinia these new data better define the boundary between the exotic southern and central appalachian basement and adjacent laurentian granite rhyolite province

preferential flow and the generation of runoff 1 boundary layer flow theory approaches to soil water flow which are based on the theory of potential flow often overestimate either the volume of flow or the response time at the hillslope scale when their results are compared with observations macropore flow is frequently thought to be the reason for the discrepancy between observations and model performance the expression macropore flow is considered to be too restricting it confines the rapid flow and transport processes to the easily observed channels concealing the more general question as to what degree potential flow theory adequately describes flow in porous media the application of potential flow theory to infiltration leads to the richards equations only when hydrodynamic dispersion of soil water and capillary potential are perfectly correlated in this paper a boundary layer infiltration model is presented which is based on kinematic theory it is also expanded to lateral flow in a sloping soil

impacts of post glacial lake drainage events and revised chronology of the champlain sea episode 13 9 ka abstract lithologic chirp compressed high intensity radar pulse sonar paleomagnetic stable isotopic and micropaleontological analyses of sediment cores from lake champlain new york vermont were used to determine the age of the post glacial champlain sea marine episode the timing of salinity changes and their relationship to freshwater discharge from mid continent glacial lakes calibrated radiocarbon ages on plant material provide an improved post glacial chronology overcoming problems from shell ages caused by carbon reservoir effects up to 1500 yr the final drainage of glacial lake vermont and the inception of marine conditions occurred 13 1 12 8 ka kiloannum calendar years and a sharp decrease in champlain sea salinity from 25 to 7 8 psu practical salinity units occurred approximately 11 4 11 2 ka reduced salinity was most likely caused by rapid freshwater inflow eastward from glacial lake algonquin into the champlain basin the timing of inferred freshwater event coincides with the widespread climatic cooling called the preboreal oscillation

titaniq a titanium in quartz geothermometer titanium is one of many trace elements to substitute for silicon in the mineral quartz here we describe the temperature dependence of that substitution in the form of a new geothermometer to calibrate the titaniq thermometer we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt at temperatures ranging from 600 to 1 000 c at 1 0 gpa the ti contents of quartz in ppm by weight from 13 experiments increase exponentially with reciprocal t as described by text log left x text qtz text ti right 5 69 pm 0 02 frac 3765 pm 24 t k application of this thermometer is straightforward typically requiring analysis of only one phase quartz this can be accomplished either by epma for crystallization temperatures above 600 c or by sims for temperatures down to at least 400 resulting temperature estimates are very precise usually better than 5 c potentially allowing detailed characterization of thermal histories within individual quartz grains although calibrated for quartz crystallized in the presence of rutile the thermometer can also be applied to rutile absent systems if tio2 activity is constrained

contrasting flow pathways within tropical forest slopes of ultisol soils there are very few experimental studies identifying hydrological pathways within rain forest slopes such knowledge is however necessary to understand why forest disturbance affects rainfall riverflow response and nutrient migration this study examines flow pathways within lowland rain forest slopes comprising udults of the ultisol soil order experimentation was conducted on four se asian hillslope units each 5 5 m in plan in the bukit timah catchment singapore island and in the w8s5 catchment sabah borneo island the flow pathways were identified by artificial tracer experiments we evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns the tracer work indicated much faster subsurface flows at bukit timah than w8s5 for the storms studied some explanation of the greater subsurface waterflows at bukit timah in comparison to w8s5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries vertical flow of the tracer through the upper 1 m of soil predominated 90 per cent of percolation in the bukit timah slopes in some contrast a major component approximately 60 per cent of the tracer percolation was directed laterally within the w8s5 slopes the flow vectors calculated using the hydrometric methods did however grossly under estimate the degree of lateral deflection of waterflow generated at w8s5 and to a lesser extent over estimated it at bukit timah in part these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small natural soil pipes present within both catchments in conclusion the study indicates that marked variations in flow vectors exist within the udult great group of se asian soils and hydrometric calculations may be poor predictors of these dominant pathways copyright 2005 john wiley sons ltd

freshwater outbursts to the oceans from glacial lake agassiz and their role in climate change during the last deglaciation abstract lake agassiz was the largest lake in north america during the last deglaciation as the laurentide ice sheet lis retreated large volumes of water stored in this proglacial lake were episodically released into the oceans these waters were variably routed to the gulf of mexico arctic ocean north atlantic ocean and hudson bay during this period the three largest cooling events in the northern hemisphere closely followed 4 of the 5 largest outbursts from lake agassiz 1 the younger dryas which was preceded by a release of 9500 km 3 2 the preboreal oscillation preceded by releases of 9300 km 3 and 5900 km 3 and 3 the 8 2 ka cold event preceded by a 163 000 km 3 outburst these are respectively fluxes of 0 30 sv 0 29 sv 0 19 sv and 5 2 sv if released in 1 year because the influx of freshwater reaching the north atlantic ocean can inhibit thermohaline circulation partly depending on whether the ocean was in a glacial interglacial or transitional mode of circulation we believe that at least these large outbursts from lake agassiz may have provided the triggers for changes in ocean circulation and in turn for widespread climate change

modeling catchment scale mixing in the near stream zone implications for chemical and isotopic hydrograph separation 1 the saturated region immediately surrounding the stream channel has been characterized as a zone of active mixing but the dynamics of this process are not accounted for in conventional hydrograph separations we employ a simple mixing model to explore the possible consequences chemically distinct hillslope and groundwater runoff components are routed through a near stream reservoir representing the channel and surrounding alluvium to produce streamflow for physically plausible near stream zones groundwater contributions estimated from hydrograph separation on streamflow ranged from 45 to 80 percent larger than the true inflow values on the rising limb and 19 to 57 percent larger at peak flow the overestimates resulted from a near stream reservoir induced lag in the hillslope chemical signature relative to increases in hillslope water flux and were consistent for reservoir dynamics ranging from constant volume to linear the results suggest that caution be exercised when interpreting hydrograph separations without knowledge of the hydrology of the near stream zone

the allerod younger dryas holocene sequence in the west central champlain sea eastern ontario a record of glacial oceanographic and climatic changes abstract the aragonite mineralogy and geochemistry of the mollusc faunas preserved at navan and bearbrook ontario serve as proxies of original seawater chemistry the composite section spanning 12 980 10 980 cal yr bp includes the younger dryas yd paleoclimatic oscillation oxygen isotopes demonstrate the onset of cooling with the yd event in addition to the lowering of marine values by the influx of isotopically light glacial meltwater from lake agassiz impact of cooling and dilution is reduced or eliminated with the start of the holocene when water temperatures and salinities for champlain sea cs seawater were 8 16 c and 27 34 ppt respectively overall oxygen isotope values deceased to 3 5 during the yd mainly due to freshening by glacial meltwater carbon isotopes confirm the rise in atmospheric co 2 concentration at the yd holocene transition marine strontium isotope values for the allerod yd earliest holocene range from 0 709151 16 210 cal yr bp to 0 709145 12 980 cal yr bp and 0 709142 10 950 cal yr bp the oceanographic changes recorded for the cs are in agreement with the evolutionary phases of lake agassiz and deglaciation dynamics of the laurentide ice sheet the volume and direction of meltwater discharge from lake agassiz alternated between the gulf of mexico during the allerod via the great lakes through the cs to the north atlantic during the yd and back to the gulf of mexico during the early holocene but with diminished impact

variation of δ18o δd and 3h in karst springs of south kashmir western himalayas india water samples were collected from cold and warm karst springs for stable isotopes 18o and d and 3h from se of kashmir valley western himalayas to distinguish the sources of recharge and infer their recharge areas the spring water samples were most depleted in heavier isotopes in may average 18o 8 87 and d 50 3 and enriched in september average 18o 7 58 and d 48 1 the depleted 18o and 2h of spring waters bear the signatures of winter precipitation while as the enriched 18o and 2h of spring waters bear the signature of summer rainfall d excess and 3h corroborate with the stable isotope results that the spring flow in spring season may and autumn september is dominantly controlled by the melting of winter snowmelt and summer rainfall respectively the results showed that unlike d the 18o value in the karst spring waters decreases in january suggesting 18o shift the spring water samples also fall above the local meteoric water line and global meteoric water line indicating the 18o shift due to interaction of groundwater with the host carbonate rocks during its traverse the mean elevation of the recharge areas of the springs using 18o and d tracers was also estimated copyright 2014 john wiley sons ltd

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

chemical mixing model of streamflow generation at la selva biological station costa rica abstract la selva biological station occupies an area of lowland tropical rainforest in central costa rica sodium and chloride data were used to quantify the mixing proportions of local runoff and geothermal groundwater at several sites throughout the stream channel system in late april at the end of the dry season the fraction of streamflow accounted for by geothermal groundwater varied spatially between 0 and 0 85 indicating a significant contribution to streamflow and to stream solute loads from geothermal groundwater at some sites in general higher inputs of geothermal groundwater were found at lower elevations over half the flow from one basin the salto was due to geothermal groundwater suggesting a minimum annual runoff of about 0 7 m of geothermal groundwater from this basin a plot of na cl vs fraction of geothermal groundwater revealed watershed scale chemical differences between the two major drainage systems the sura and the salto differences that were not apparent from a traditional two solute plot of cl vs na concentration a small 21 mm storm produced relatively little change in mixing proportions as most throughfall was apparently retained in the relatively dry soils

high chloride concentrations in the soil and groundwater under an oak hedge in the west of france an indicator of evapotranspiration and water movement chloride is a major anion in soil water and its concentration rises essentially as a function of evapotranspiration compared to herbaceous vegetation high transpiration rates are measured for isolated trees shelterbelts or hedgerows this article deals with the influence of a tree hedge on the soil and groundwater cl concentrations and the possibility of using cl as an indicator of transpiration and water movements near the tree rows cl concentrations were measured over 1 year at different depths in the unsaturated zone and in the groundwater along a transect intersecting a bottomland oak hedge we observed a strong spatial heterogeneity of cl concentrations with very high values up to 2 g l 1 in the unsaturated zone and 1 2 g l 1 in the upper part of the groundwater this contrasts with the low and homogeneous concentrations 60 70 mg l 1 in the deeper part of the groundwater cl accumulation in the unsaturated zone at the end of the vegetation season allows us to identify the active root zone extension of trees in winter upslope of the tree row downwards leaching partly renews the soil solution in the root zone while the slow water movement under the trees or farther downslope results in cl accumulation and leads to a salinization of the soil and groundwater this salinization is of the same order as experimental conditions produce negative effects on oak seedlings the measurement of cl concentrations in the unsaturated zone under tree rows at the end of the vegetation season would indicate whether certain topographic pedological or climatic conditions are likely to favour a strong salinization of the soil as observed in the present study copyright 2009 john wiley sons ltd

direct dating of paleomagnetic results from precambrian sediments in the amazon craton evidence for grenvillian emplacement of exotic crust in se appalachians of north america abstract we apply a new diagenetic dating technique to determine the age of magnetization for precambrian sedimentary rocks in the sw amazon craton two new paleomagnetic poles are reported from the rocks of the aguapei gp red beds of the fortuna fm p lat 59 8 n p lon 155 9 e a 95 9 5 k 14 18 sites n n 128 115 q 5 and the reverse polarity mudstones of the overlying vale da promissao formation p lat 49 5 n p lon 89 3 e a 95 12 5 k 30 6 sites n n 94 80 q 4 the fortuna fm magnetization is hosted by massive interstitial hematite cement and constitutes a post depositional remanence the age of diagenesis of the red beds is well constrained by the 1149 7 ma u pb age of authigenic xenotime rims on detrital zircons determined by shrimp analysis the magnetite hosted remanence of the vale da promissao fm may be detrital in origin but the age of deposition is poorly constrained the reliable and precisely dated fortuna fm paleomagnetic pole fixes the paleogeographic position of the amazon craton near the se appalachians portion of north america at 1 15 ga these data demonstrate a mobile grenvillian link between these two cratons and support the recent identification of amazon crust in the blue ridge province region of north america

runoff generation in a steep tropical montane cloud forest catchment on permeable volcanic substrate 1 most studies to date in the humid tropics have described a similar pattern of rapid translation of rainfall to runoff via overland flow and shallow subsurface stormflow however study sites have been few overall and one particular system has received very little attention so far tropical montane cloud forests tmcf on volcanic substrate while tmcfs provide critical ecosystem services our understanding of runoff generation processes in these environments is limited here we present a study aimed at identifying the dominant water sources and pathways and mean residence times of soil water and streamflow for a first order tmcf catchment on volcanic substrate in central eastern mexico during a 6 week wetting up cycle in the 2009 wet season total rainfall was 1200 mm and storm event runoff ratios increased progressively from 11 to 54 with the increasing antecedent wetness conditions our isotope and chemical based hydrograph separation analysis showed increases of pre event water contributions to the storm hydrograph from 35 to 99 stable isotope based mean residence times estimates showed that soil water aged only vertically through the soil profile from 5 weeks at 30 cm depth to 6 months at 120 cm depth a preliminary estimate of 3 years was obtained for base flow residence time these findings all suggest that shallow lateral pathways are not the controlling processes in this tropical forest catchment rather the high permeability of soils and substrate lead to vertical rainfall percolation and recharge of deeper layers and rainfall runoff responses appeared to be dominated by groundwater discharge from within the hillslope

hydrograph separation using stable isotopes silica and electrical conductivity an alpine example abstract hydrograph separation of runoff events in two nested alpine subalpine basins in the coast mountains of british columbia was carried out using electrical conductivity specific concentration of silica and the stable isotopes oxygen 18 and deuterium as hydrological tracers the methods predicted consistent high pre storm water contribution for the subalpine site 60 90 but more variable contribution at the alpine basin outlet 25 90 the pre storm water contribution is much larger than had previously been expected precipitation is believed to run off as overland flow due to the steep slopes in combination with the hydrophobic soils until it can enter the subsurface environment the rapid influx of stored water is possibly caused by pressure propagation in the macropore system which could be enhanced by the heavily fractured bedrock and permeable landslide debris acting as efficient hydrological conduits the study suggests that alternative hydrological tracers such as electrical conductivity and silica concentration can be used under certain hydrological and lithological conditions these alternative tracers should however be verified against more conventional tracers before use as the behaviour depends on specific characteristics of each basin at the upper basin outlet both electrical conductivity ec and silica underestimated the pre storm contribution at the lower station silica and ec showed a similar pattern to deuterium and oxygen 18 tracers the calculated pre storm component using ec was however 10 20 lower than the calculated values from the other three tracers the advantage of using these alternative tracers is that hydrograph separation results can a priori be anticipated

monazite allanite phase relations in metapelites abstract calculations of the transition from allanite to monazite bearing assemblages in typical pelitic bulk compositions have been made using thermodynamic data estimated from oxide sums and inferred from natural parageneses calculations in the cfashpce and mnnckfmashpce systems place the allanite to monazite transition in the middle amphibolite facies 525 600 c for a bulk composition similar to shaw s average pelite the temperature of the transition is pressure dependent and strongly dependent on the bulk rock cao content consistent with inferences from natural parageneses the transition is also a function of the bulk al 2 o 3 content although the calculated result is opposite to that inferred from natural samples comparison with published results in the la mg system suggest that the nature of the ree phosphate does not greatly influence the conditions of the transition

flowpaths source water contributions and water residence times in a mexican tropical dry forest catchment summary runoff in forested tropical catchments has been frequently described in the literature as dominated by the rapid translation of rainfall to runoff through surface and shallow subsurface pathways however studies examining runoff generation in tropical catchments with highly permeable soils have received little attention particularly in tropical dry forests we present a study focused on identifying the dominant flowpaths water sources and stream water residence times in a tropical dry forest catchment near the pacific coast of central mexico during the wet season pre event water contributions to stormflow ranged from 72 to 97 with the concentrations of calcium magnesium sodium and potassium closely coupling the geochemistry of baseflow and groundwater from the narrow riparian near stream zone baseflow from the intermittent stream showed a strongly damped isotopic signature and a mean baseflow residence time of 52 110 days was estimated these findings all suggest that instead of the surface and near surface subsurface lateral pathways observed over many tropical catchments runoff is generated through vertical flow processes and the displacement and discharge of stored water from the saturated zone as the wet season progressed contributions from the saturated zone persisted however the stormflow and baseflow geochemistry suggests that the contributing area of the catchment increased our results show that during the early part of the wet season runoff originated primarily from the headwater portion of the catchment as the wet season progressed and catchment wetness increased connectivity among sub basin was improved resulting in runoff contributions from across the entire catchment

hydrologic pathways and stormflow hydrochemistry at south creek northeast queensland abstract earlier investigations at south creek in northeastern queensland established the importance of overland flow as a hydrologic pathway in this tropical rainforest environment since this pathway is fast transmitting presumably new water its importance should be reflected in the stormflow chemistry of south creek the greater the volumentric contribution to the stormflow hydrograph the more similarity between the chemical composition of streamwater and of overland flow is to be expected water samples were taken during two storm events in an ephemeral gully gully a an intermittent gully gully b and at the south creek catchment outlet additional spot checks were made in several poorly defined rills the chemical composition of old water was determined from 45 baseflow samples collected throughout february the two events differed considerably in their magnitudes intensities and antecedent moisture conditions in both events the stormflow chemistry in south creek was characterized by a sharp decrease in ca mg na si cl ec anc alkalinity and total inorganic carbon ph remained nearly constant with discharge whereas k increased sharply as did sulfate in an ill defined manner in event 1 this south creek stormflow pattern was closely matched by the pattern in gully a implying a dominant contribution of new water this match was confirmed by the spot samples from rills gully b behaved like south creek itself but with a dampened new water signal indicating less overland flow generation in its subcatchment in event 2 which occurred five days later the initial new water signal in gully a was rapidly overwhelmed by a different signal which is attributed to rapid drainage from a perched water table this study shows that stormflow in this rainforest catchment consists predominantly of new water which reaches the stream channel via fast pathways where the ephemeral gullies delivering overland flow are incised deeply enough to intersect a perched water table a delayed old water like signal may be transmitted

newly discovered eclogite in the southern appalachian orogen northwestern north carolina abstract recent detailed geologic mapping in the blue ridge belt of northwestern north carolina has revealed the first reported eclogite sensu stricto in the southern appalachian orogen the eclogite occurs at the base of the ashe metamorphic suite within a shear zone that separates the spruce pine thrust sheet from the underlying grenville age basement rocks this contact juxtaposes rocks of apparent oceanic affinities predominantly metapelite metabasite and meta ultramafic rocks with rocks of the ancient north american craton the primary metamorphic assemblage in the eclogite is garnet omphacite quartz rutile hornblende and plagioclase occur only as retrograde amphibolite facies phases in the eclogite pressure temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 13 17 kbar at 625 790 c while the retrograde amphibolite facies assemblage suggests conditions of 8 5 12 kbar and 650 730 c ultramafic blocks also occur within the shear zone containing the eclogite the presence of high pressure metamorphic rocks associated with ultramafic rocks and pelitic schists within a shear zone separating continental rocks from oceanic rocks is strong evidence for an ancient subduction event and indicates that at least part of the ashe metamorphic suite is a subduction related accretionary melange

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

chemical diversity and origin of precambrian charnockitic rocks of the central pedlar massif grenvillian blue ridge terrane virginia abstract middle proterozoic rocks of the pedlar massif are comprised of varied charnockitic and granulitic lithologies representing 1150 1000 ma grenvillian metamorphic and igneous events in the central appalachians of virginia chemically and mineralogically diverse units include metaluminous to peraluminous charnockites enderbites jotunites and granulite gneisses ranging in sio2 from 47 to 79 wt relative to bulk continental crust typical charnockites are enriched by 3 6 in rb k ba lree zr and hf less enriched in ta sr p and hree and depleted in cs th ti and sc although low sio2 members have relatively high ti and p and some high sio2 units are depleted in k rb and ba the pedlar river charnockite suite prcs vesuvius megacrystic charnockite vmc nellysford granulite gneiss ngg and the lady slipper granulite gneiss lsgg exhibit common chemical signatures and trends notably evident in uniform lree enriched patterns with variable eu anomalies which reflect their derivation from a mixed protolith of volcanics and reworked volcaniclastic sediments dated at 1130 1150 ma pedlar river charnockites represent portions of deep seated granulite gneisses that were mobilized by granulite facies dehydration melting and emplaced en masse as plutons into overlying and surrounding gneisses during the grenville episode divergent ree patterns in the prcs are attributed to crystallization of quartz feldspar garnet or hree compatible mafic and accessory phases leading to complementary chemical signatures in mafic and felsic layers depletion of cs throughout the pedlar massif is attributed to its incompatibility with major mineral phases during either protolith evolution or granulite facies dehydration while dispersions in k rb sr ba and eu are due to the relative proportions of the major phases quartz k feldspar plagioclase and biotite mobility of insoluble trace elements zr hf ta and th require partial melting and crystallization of accessory mineral phases whereas sc cr and ree mobility depends on the proportions of pyroxene and garnet several leucocharnockites and other felsic rocks of the prcs show markedly elevated th values ranging from 10 to 83 ppm which complement depleted th in typical prcs units these charnockitic rocks represent low degree partial melts 1 5 from normal prcs or ngg protoliths to produce high silica magmas which separated from the main charnockitic body

40ar 39ar and k ar data bearing on the metamorphic and tectonic history of western new england 40ar 39ar ages of coexisting biotite and hornblende from proterozoic y gneisses of the berkshire and green mountain massifs as well as 40ar 39ar and k ar mineral and whole rock ages from paleozoic metamorphic rocks suggest that the thermal peak for the dominant metamorphic recrystallization in western new england occurred 465 5 m y ago taconian although textural data indicate a complex metamorphic tectonic history for paleozoic rocks no evidence in rocks at least as high as kyanite grade dictates an acadian age for the barrovian metamorphism available 40ar 39ar and k ar data suggest that the low grade metamorphism and cleavage formation in taconic allochthons and the higher grade metamorphism and emplacement of the berkshire massif allochthon are taconian 40ar 39ar age data from a poorly defined terrane beginning near the east margin of the green mountain massif and extending along the eastern one third of the berkshire massif as far south as otis massachusetts suggest that the area has been retrograded during a metamorphism that peaked at least 376 5 m y ago acadian available age and petrologic data from western new england indicate the presence of at least three separate metamorphic structural domains of taconian age 1 a small area of relict high pressure and low temperature metamorphism in northern vermont t 1 domain 2 a broad area in vermont and eastern new york of normal barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient t 2 domain and 3 a rather narrow belt of steep gradient barrovian series metamorphic rocks extending from near the cortlandt complex northeastward through dutchess county new york to the berkshire massif in western massachusetts t 3 domain areas of maximum metamorphic intensity within the t 3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting berkshire massif or from recumbent folding manhattan prong of remobilized north american continental crust in the later stages of the taconic orogeny

age and origin of anorthosites charnockites and granulites in the central virginia blue ridge nd and sr isotopic evidence rb sr isotopic data for anorthosites charnockites ferrodioritic to quartz monzonitic plutons and high grade gneisses of the blue ridge of central virginia show evidence of post emplacement metamorphism but in some cases retain grenville ages the pedlar river charnockite suite yields an isochron age of 1021 36 ma initial 87sr 86sr ratio of 0 7047 6 which agrees with published u pb zircon ages five samples of that unit which contain paleozoic mylonitic fabrics define a regression line of 683 ma interpreted as a mixing line with no age significance samples of the roseland anorthosite complex show excessive scatter on a rb sr evolution diagram probably due to paleozoic 475 m y metamorphism data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 26 ma inital ratio 0 7058 4 which is in the range of the u pb zircon ages of 1000 1100 ma the stage road layered gneiss yields an age of 1147 34 ma initial ratio of 0 7047 5 sm nd data for the pedlar river charnockite suite reflect a pre grenville age of 1489 118 ma nd 6 7 1 2 data for the roseland anorthosite complex and the ferrodioritic to quartz monzonitic plutons yield grenville isochron ages of 1045 44 ma nd 1 0 0 3 and 1027 101 ma nd 1 4 1 0 respectively two roseland anorthosite samples plot far above the isochron demonstrating the effects of post emplacement disturbance of sm nd systematics while mylonitized pedlar river charnockite suite samples show no evidence of sm nd redistribution the disparity of the sm nd age and other isotopic ages for the pedlar river charnockite suite probably reflects a sm nd source age suggesting the presence of an older crust within this portion of the ca 1 ga old basement

fluid mediated partial alteration in monazite the role of coupled dissolution reprecipitation in element redistribution and mass transfer monazite ce lree th u ca p si o4 with complex zoning in th and other elements is commonly observed in metamorphic and igneous rocks the hypothesis that this alteration is a product of fluid mediated element mass transfer has been tested in the piston cylinder press caf2 assembly cylindrical graphite oven at 1 000 mpa and 900 c and in cold seal autoclaves on a hydrothermal line at 500 mpa and 600 c experiments included a relatively homogeneous monazite ce 7 8 wt tho2 from a heavy mineral sand plus a series of alkali bearing fluids including 2n naoh 2n koh and na2si2o5 h2o experiments were conducted using bse imaging emp analysis and both tem and hrtem a subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in th si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior these thsio4 enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks in the na2si2o5 h2o experiments scarce inclusions of britholite formed in the altered monazite the altered monazite is also characterised by strong depletion in pb ca and y thorium and si mobility coupled with the formation of britholite inclusions during partial alteration in the monazite grain is considered to be the product of fluid aided coupled dissolution reprecipitation as opposed to solid state diffusion since other fluids including nacl and kcl brines do not result in the formation of these textures the experimental replication of thsio4 enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced specifically by alkali bearing fluids if true complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event presuming that all the original radiogenic pb has been removed

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

the duration of prograde garnet crystallization in the uhp eclogites at lago di cignana italy abstract the distinct core to rim zonation of different rees in garnet in metamorphic rocks specifically sm relative to lu suggests that sm nd and lu hf isochron ages will record different times along a prograde garnet growth history therefore ree zonations in garnet must be measured in order to correctly interpret the isochron ages in terms of the garnet growth interval which could span several m y new ree profiles garnet crystal size distributions and garnet growth modeling combined with previously published sm nd and lu hf geochronology on a uhp eclogite of the zermatt saas fee zsf ophiolite lago di cignana italy demonstrate that prograde garnet growth of this sample occurred over a 30 to 40 m y interval relative to peak metamorphism at 38 to 40 ma garnet growth is estimated to have begun at 11 to 14 kbar pressure at 70 to 80 ma although such a protracted garnet growth interval is surprising this is supported by plate tectonic reconstructions which suggest that subduction of the liguro piemont ocean occurred through slow and oblique convergence these results demonstrate that ree zonations in garnet coupled to crystal size distributions provide a powerful means for understanding prograde metamorphic paths when combined with sm nd and lu hf geochronology

numerical simulation of unsaturated flow along preferential pathways implications for the use of mass balance calculations for isotope storm hydrograph separation an objective common to many watershed studies is to separate storm hydrographs into two components water that was present in the watershed prior to a storm event soil moisture and groundwater and water which fell on the watershed during the storm to use this approach a number of assumptions must be made including that the composition of water in the soil moisture and groundwater reservoirs are constant and known the objective of this paper is to show that in settings where flow and transport are dominated by preferential pathways for flow steady state mass balance calculations for quantitative hydrograph separation may be in error we present field data from a site where flow and transport are dominated by preferential pathways relict fractures in saprolite of sedimentary rocks which indicate that the 18o content of the water in the unsaturated and shallow saturated zones is not constant over the course of a storm event we use a numerical model to further explore the interactions between the fractures and surrounding matrix both the field data and modeling results indicate that the 18o of the previous storm event s has a strong influence on water in the fractures on the time scale of a storm event only the water in the matrix immediately surrounding the fracture mixes with water in the fracture while the bulk of the matrix is isolated from fracture flow the spatial and temporal heterogeneity of the 18o in the subsurface and the isolation of the most of the matrix water from flow in fractures make the measurement of a singular 18o value for subsurface reservoirs problematic and the assumption of a constant value doubtful since most near surface geologic materials have preferential flow paths we suggest that quantitative hydrograph separation using mass balance techniques is not possible in most situations future field and modeling investigations using the approach outlined here could be designed to explore the important temporal and spatial scales of variability in watersheds and lead to a more quantitative approach to storm hydrograph separation

overland flow generation in two lithologically distinct rainforest catchments streams on uniformly rainforest covered but lithologically very diverse barro colorado island in central panama show remarkable differences in their runoff response to rainfall this lithological diversity is reflected in equally diverse soilscapes and our objective was to test the hypothesis that contrasting runoff responses derive from soilscape features that control the generation of overland flow we determined the soil saturated hydraulic conductivity ks of two neighboring but hydrologically contrasting catchments lutz creek with a flashy and conrad trail with a delayed response to rainfall and quantified the spatial and temporal frequency of overland flow occurrence the median ks values at a depth of 12 5 cm are large enough to rule out hortonian overland flow but a marked decrease in ks in lutz creek catchment at 30 cm suggests the formation of a perched water table and the generation saturation overland flow the decrease in ks in the conrad trail catchment is more gradual and a perched water table is expected to form only at depths below 50 cm in lutz creek overland flow was generated frequently in time and space and regardless of topographic position including near the interfluve with very low thresholds of storm magnitude duration intensity and antecedent wetness whereas in conrad trail overland flow was generated much less frequently and then only locally we conclude that soilscape features and microtopography are important controls of overland flow generation in these catchments our results contribute to the growing evidence that overland flow and forests are not a priori a contradiction in terms

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

tectonic evolution of the grenville orogen in the central appalachians abstract remnants of the grenville orogeny a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent rodinia between ca 1 3 and 1 0 ga are exposed throughout eastern north america for ca 3000 km from labrador to alabama although tectonic models for the development of the grenville have historically been based on observations from the large and well exposed canadian grenville province recent investigations suggest that initial continental collision occurred proximal to the appalachian blue ridge mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during paleozoic orogenesis to better understand the tectonic history of this critical region we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho and paragneiss from the appalachian blue ridge province the resultant dates together with new results from trace element thermobarometry define a prolonged record of high temperature metamorphism coincident with the timing of continent continent collision as recognized in the northern portion of the orogen initial garnet and monazite growth commenced between 1100 and 1065 ma with peak granulite facies conditions 750 900 c 0 55 0 85 gpa occurring from 1050 to 1010 ma burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring 20 30 myr after deposition of the sedimentary protoliths local hydrothermal alteration followed peak metamorphism and occurred at 1000 990 ma with later cooling through 660 c between 980 and 935 ma 3 2 c myr the metamorphic history outlined here together with insights from previous studies is consistent with formation of the blue ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the himalayan tibetan orogen

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

rare earth diffusion kinetics in garnet experimental studies and applications abstract we determined the diffusion coefficient of sm in almandine garnet as function of temperature at 1 bar and fo 2 corresponding to that of wustite iron buffer and to a limited extent that of a few other selected rare earth elements in almandine and pyrope garnets both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields the experimental diffusion profiles were analyzed by secondary ion mass spectrometry and in addition by rutherford back scattering spectroscopy for two samples transmission electron microscopic study of an almandine crystal that was diffusion annealed did not reveal any near surface fast diffusion path using reasonable approximations and theoretical analysis of vacancy diffusion the experimental data were used to develop an expression of rare earth element ree diffusion coefficient in garnet as a function of temperature pressure fo 2 ionic radius and matrix composition calculation of the closure temperature for the sm nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently modeling of the ree evolution in melt and residual garnet suggests that for dry melting condition the ree pattern in the melt should commonly conform closely to that expected for equilibrium melting however for much lower solidus temperatures that would prevail in the presence of a h 2 o co 2 fluid the concentration of light ree in the melt could be significantly lower than that under equilibrium melting condition a reported core and rim differences in the ree content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the ree zoning was a consequence of entrapment in a magma derived from an external source for 32 000 yr before the eruption

u pb monazite geochronology of granitic rocks from maine implications for late paleozoic tectonics in the northern appalachians crystallization ages of monazites from peraluminous granites and granitic pegmatites in southwestern maine constrain the timing of final ductile motion of a major shear zone to approximately 280 ma u pb monazite ages of granite at brunswick 278 pm 2 ma and evolved granitic pegmatites in topsham 275 269 ma provide evidence that synkinematic intrusion was followed by final emplacement at the end of ductile deformation these granitic rocks were evidently generated during the final high temperature event experienced by this portion of the appalachian orogen during this time the host rocks record initial rapid cooling c 16 c m y followed by slower cooling c 4 c m y into the mesozoic the similarity of ages of granite in brunswick and pegmatites in topsham permit the rocks to be related to the same magmatic system the spatially associated sebago batholith the largest exposed pluton in new england has a concordant u pb monazite age of 293 pm 2 ma thus the sebago batholith cannot be

quantification of temporal distribution of recharge in karst systems from spring hydrographs the estimation of the temporal distribution of recharge in karst aquifers is a challenge due to large heterogeneities in geometric and hydraulic parameters of the vadose and phreatic zone this article provides a time continuous approach for the estimation of inflow into the conduit system of a karst aquifer which consists of the sum of direct recharge and flow from the fissured matrix blocks into the conduit system the approach employs the first time derivative of the spring hydrograph and the recession coefficient of the conduit system for the determination of this function the first time derivative of the hydrograph describes the rate of change in spring discharge it reflects the ratio of inflow to and outflow from the conduit system the recession coefficient depends on the hydraulic diffusivity which controls the velocity of the signal transmission through the system as shown in parameter studies with a simplified serial two reservoir model direct recharge into the conduit system clearly dominates the early hydrograph response during recharge events even if the fraction of direct recharge represents just a few percent of total recharge this behaviour is caused by a large contrast in recession coefficients between conduit system and fissured matrix blocks the direct recharge component can therefore be separated from the estimated total inflow into the conduit system estimation of inflow into the conduit system of the gallusquelle spring swabian alb germany after a storm event yields similar results as those obtained from parameter studies the separation of the direct recharge component is in agreement with information from an independent isotope study the methodology has been further applied to a recharge event initiated by snowmelt as a result of daily variations of the air temperature a clearly diurnal cycle of inflow into the conduit system is estimated that is not easily recognizable from the spring hydrograph the applicability of the proposed methodology requires knowledge about the magnitude of the recession coefficient of the conduit system the characteristics of the conduit system become also apparent in the rapid transport of tracers the recession coefficient of the conduit system can for example directly be estimated from the reciprocal of the mean tracer travel time however for this type of analysis only tracer experiments covering the extent of the catchment should be taken into account for the gallusquelle catchment the recession coefficient obtained from the tracer experiment corresponds to that obtained from hydrograph recession analysis the estimated inflow into the conduit system and interpretation of temporal distribution of recharge is therefore based on a plausible and physically based parameter

petrology and u pb geochronology of the robertson river igneous suite blue ridge province virginia evidence for multistage magmatism associated with an early episode of laurentian rifting

investigating the hydrogeological properties of springs in a karstic aquifer in dorfak region guilan province iran karstic aquifers are considered as the main sources of groundwater in the northeast of rudbar iran the present study was conducted to evaluate the hydrogeological properties of karstic springs in this region for this purpose saturation indices si values were calculated using the geochemical phreeqc model for a number of minerals in the groundwater in the karstic aquifer moreover aqqa rockware software packages were used to prepare hydrogeochemical plots for the aquifer using which the sources of the ions in the water were identified the origin of bicarbonate calcium and magnesium ions in water was determined using chloro alkaline indices moreover through plotting a piper diagram for spring water samples it was discovered that water type of all springs is the ca hco3 type confirming the karstic characteristic of springs in the area a durov diagram also suggests that the water composition of the springs is of the bicarbonate type with the dominant ca cation suggesting the calcareous effects of the region on the quality of groundwater and exhibiting a single source for the springs the calculated saturation indices show that most of the water samples are undersaturated with respect to calcite dolomite and co2 the stable isotopes 18o and 2h and deuterium excess values were used to get information about transport pathways in groundwater atmospheric moisture and the degree of interaction between these reservoirs the degree of karstification of the recharge area of the karst aquifer was determined to be 5 5 from an analysis of the hydrograph sefidab spring

implications for late grenvillian rigolet phase construction of rodinia using new u pb data from the mars hill terrane tennessee and north carolina united states new data for zircon external morphology cathodoluminescence zoning and sensitive high resolution ion microprobe shrimp u pb ages from the carvers gap granulite gneiss of the mars hill terrane tennessee and north carolina united states require reevaluation of interpretations of the age and origin of this rock the new results indicate that the zircon is detrital and that the sedimentary protolith of this gneiss and related cloudland gneiss was deposited no earlier than ca 1 02 ga and was metamorphosed at ca 0 98 ga tectonic models that included the gneiss as a piece of 1 8 ga amazonian crust perhaps as part of the hypothetical columbia supercontinent are now untenable the remarkably fast cycle of exhumation erosion deposition and deep burial also is characteristic of other late grenvillian post ottawan mesoproterozoic paragneisses that occur throughout the appalachians these rocks provide new evidence for the duration of the formation of the rodinia supercontinent lasting until at least 0 98 ma

paleoindians and the younger dryas in the new england maritimes region abstract this paper examines environmental and archaeological data for the younger dryas yd 12 900 11 600 calibrated years before present cal bp and early holocene 11 600 10 000 cal bp in the new england maritimes nem to model environmental changes and possible human responses for some other regions of north america researchers argue for negligible environmental changes and human responses while others suggest that ecological changes associated with cold conditions at the yd onset disrupted regional biota causing subsistence stress for paleoindian populations and the end of the clovis cultural adaptation circa 13 200 12 900 cal bp the nem shows abrupt cooling at the yd onset which fostered more open habitats favorable to both long distance migrating and local herds of caribou and may have encouraged early paleoindian colonization and settlement of this unoccupied deglacial region comparison of the paleoindian point sequence with calibrated radiocarbon dates indicates fluted point groups probably occupied the nem during but not after the yd abrupt warming at the yd terminus circa 11 600 cal bp caused a rapid reorganization of the region s vegetation and prey species populations coinciding in the archaeological record with a decline in paleoindian biface fluting technology and altered regional site distributions in the closed forests of the succeeding early holocene nem late paleoindian groups 11 600 10 000 cal bp used unfluted lanceolate points that may signal post yd immigration to the nem

topographic controls on overland flow generation in a forest an ensemble tree approach summary overland flow is an important hydrological pathway in many forests of the humid tropics its generation is subject to topographic controls at differing spatial scales our objective was to identify such controls on the occurrence of overland flow in a lowland tropical rainforest to this end we installed 95 overland flow detectors ofds in four nested subcatchments of the lutzito catchment on barro colorado island panama and monitored the frequency of overland flow occurrence during 18 rainfall events at each ofd location temporal frequency for each such location we derived three non digital terrain attributes and 17 digital ones of which 15 were based on digital elevation models dems of three different resolutions these attributes then served as input into a random forest ensemble tree model to elucidate the importance and partial and joint dependencies of topographic controls for overland flow occurrence lutzito features a high median temporal frequency in overland flow occurrence of 0 421 among ofd locations however spatial temporal frequencies of overland flow occurrence vary strongly among these locations and the subcatchments of lutzito catchment this variability is best explained by 1 microtopography 2 coarse terrain sloping and 3 various measures of distance to channel with the contribution of all other terrain attributes being small microtopographic features such as concentrated flowlines and wash areas produce highest temporal frequencies whereas the occurrence of overland flow drops sharply for flow distances and terrain sloping beyond certain threshold values our study contributes to understanding both the spatial controls on overland flow generation and the limitations of terrain attributes for the spatially explicit prediction of overland flow frequencies