the p t d development at the basement cover boundary in the north eastern tauern window eastern alps alpine continental collision at the basement cover boundary of the north eastern tauern window eastern alps the following alpine p t d development has been reconstructed on the basis of macro and micro structures as well as preferred crystallographic orientations mineral parageneses and compositions during increasing p t conditions in the greenschist facies a first period of deformation produced imbrication of the basement gneisses and cover sediments and then monoclinal folds up to the kilometre scale tectonic transport was continuously top to the ene a second period of deformation began at about peak p t conditions of 9 kbar and c 540 560 c in the south and about 7 9 kbar and 490 500 c in the north this continued locally to lower temperature during the second period transport was continuously top to the se crystallographic orientations of white mica and plagioclase give particularly useful information on the kinematic framework in addition data on the ductile behaviour of dolomite and plagioclase can be inferred at c 7 9 kbar dolomite recrystallization starts at 450 480 c and the beginning of plagioclase recrystallization coincides with the oligoclase boundary in general the alpine geodynamic history of the basement cover boundary may be related to continental collision processes between a northerly plate european or brianconnais and a southerly adriatic one the first deformation period possibly reflects subduction of the gneiss sediment boundary toward the wsw to a depth of 31 32 km the second period may be a result of obduction toward the nw followed by late stage uplift most of the basement domes of the eastern tauern window appear as a result of the final stage of the first deformation formed prior to the peak of metamorphism possibly partly influenced by the final collision between the northern and the southern continents

chronology and dynamics of a large silicic magmatic system central taupo volcanic zone new zealand the central taupo volcanic zone in new zealand is a region of intense quaternary silicic volcanism accompanying rapid extension of continental crust at least 34 caldera forming ignimbrite eruptions have produced a complex sequence of relatively short lived nested and or overlapping volcanic centers over 1 6 m y silicic volcanism at taupo is similar to the yellowstone system in size longevity thermal flux and magma output rate however taupo contrasts with yellowstone in the exceptionally high frequency but small size of caldera forming eruptions this contrast reflects the thin rifted nature of the crust which precludes the development of long term magmatic cycles at taupo 11 refs 4 figs 1 tab

a simple model of interannual displacements of the gulf stream interannual variations in the latitude of the north wall of the gulf stream 1966 1999 can be hindcast by a simple model forced solely by monthly values of the north atlantic oscillation nao index the model is a modified version of the one dimensional model developed by d behringer et al and utilizes the observed tendency for the centers of the azores high and the iceland low to move north or south as the nao strengthens or weakens only one parameter the active upper layer thickness is changed from the original values although the model incorporates the thermal feedback on the wind stress used by behringer et al the results are almost unchanged without it in generating the gulf stream position the model averages the nao index over several months and introduces a time lag of at least a year these delays are associated with the time taken for the lateral advection of heat the model results are also compared with estimates of the latitude of the gulf stream made by a gangopadhyay et al 1977 1988 and by t m joyce et al 1955 1997 and with the series of gulf stream transports 1954 1997 constructed by r g curry and m s mccartney the model reproduces the observed weakness of the seasonal variations in comparison to interannual variations the nao data are used to hindcast the annual latitude of the gulf stream for the period 1825 1999 the results show that since 1970 the gulf stream may have been more consistently farther south than during any period of the past 170 years

chemical and tectonic controls on the formation of sparry magnesite deposits the deposits of the northern greywacke zone austria the sparry magnesite deposits of the northern greywacke zone are situated at the base of thrust sheets model calculations and natural examples suggest that an inverse temperature gradient can only be established at the base of a thrust sheet if thrusting is so rapid as to be geologically unrealistic independently of this thrusting induces a fluid flow from the lower units to the front of the moving thrust sheet stability data of magnesite and dolomite in contact with fluids with different ca ca mg ratios suggest that this tectonically induced fluid flow produce magnesite from preexisting dolomite by metasomatism

in situ la mc icp ms and id tims u pb geochronology of cassiterite in the giant furong tin deposit hunan province south china new constraints on the timing of tin polymetallic mineralization abstract the furong deposit located in southern hunan province china is a newly discovered giant tin deposit spatially associated with the a type qitianling granite the petrogenetic link between the giant furong tin deposit and the qitianling granite batholith remains controversial because of the lack of precise dating for the tin polymetallic mineralization here we report for the first time in situ u pb data on cassiterite obtained by la mc icp ms and the results are compared to id tims data the in situ la mc icp ms analyses of the cassiterites provide a reliable age constraint for tin polymetallic mineralization in the furong deposit yielding an isochron age of 159 9 1 9 ma at 95 confidence level mswd 18 which is indistinguishable from the id tims 206 pb 238 u weighted mean age of 158 2 0 4 ma the in situ u pb ages corroborate the ar ar dates published for the furong tin deposit which indicate that the mineralization is coeval with the emplacement of the qitianling granite batholith these results provide further evidence of a close temporal link between the pluton s emplacement and tin mineralization in the furong area the data obtained clearly demonstrate that in situ la mc icp ms u pb dating of cassiterite is a robust geochronometer for direct dating of tin polymetallic mineralization

upb zircon and rutile chronology of archean greenstone formation and gold mineralization in the val d or region quebec abstract u pb geochronology provides an absolute time framework for the evolution of the sigma gold deposit and surrounding rocks at val d or southern abitibi subprovince the bourlamaque batholith the largest pluton in the area gives a 2699 8 1 0 ma u pb zircon age this pluton cuts the val d or formation which hosts the mineralization a u pb zircon age of 2704 9 1 1 ma on a felsic volcanic rock the colombiere rhyolite 13 km east of the mine dates that formation the gold bearing quartz vein system at sigma is hosted by andesites and two generations of porphyry intrusions all metamorphosed to the greenschist facies the oldest porphyry porphyritic diorite shows the same deformation as the volcanic rocks and has a 2703 7 2 5 ma u pb zircon age the porphyritic diorite and volcanic rocks are cut by feldspar porphyry dykes which post date regional folding and have a 2694 0 2 2 ma u pb zircon age regional greenschist metamorphism has been dated directly with a u pb date of 2684 7 ma on rutile in the colombiere rhyolite the mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals hydrothermal rutile from an alteration halo around the veins in andesite has a 2599 9 ma u pb age textural evidence clearly indicates that the wall rock alteration and vein filling are contemporaneous and hence the vein system and gold mineralization appear to have developed at least 80 m y after the formation and metamorphism of host greenstones

impact of the atlantic meridional mode on gulf stream north wall position abstractthe path of the gulf stream as it leaves the continental shelf near cape hatteras is marked by a sharp gradient in ocean temperature known as the north wall previous work in the literature

on the drivers of decadal variability of the gulf stream north wall abstractthe gulf stream is bounded to the north by a strong temperature front known as the north wall the north wall is subject to variability on a wide range of temporal and spatial scales on int

cassiterite u pb and muscovite40ar 39ar age constraints on the timing of mineralization in the xuebaoding sn w be deposit western china abstract the xuebaoding deposit in the songpan garze orogenic belt western china spatially associated with the xuebaoding granitic complex is a unique pegmatite type sn w be deposit within the giant songpan garze polymetallic belt a lack of precise geochronological constraints on the timing of mineralization for this deposit has precluded definitive statements relating to a genetic link between ore formation and granitoid intrusion up to now to address the temporal relationship between the emplacement of granite and the spatially associated sn w be polymetallic mineralization in the xuebaoding deposit systematic cassiterite u pb and muscovite ar ar dating were undertaken u pb data of cassiterite analyzed by in situ la mc icp ms provided an isochron age of 193 6 6 ma at 95 confidence level mswd 4 the 40 ar 39 ar dating on the muscovite intergrown with the cassiterite sample yielded a plateau age of 194 50 1 02 ma mswd 1 26 and an inverse isochron age of 194 53 1 05 ma mswd 1 34 both the ar ar dates overlap the u pb date within uncertainties placing the timing of tin mineralization at 193 194 ma this is consistent with field geological relationships that show veins cutting marble and granite and with previous sm nd dating within experimental error for scheelite and indicates that sn w polymetallic mineralization at the xuebaoding deposit occurred during the early jurassic combined with previous published trace element and ree data from the xuebaoding granites and other ore minerals as well as the timing of granite emplacement the temporal and genetic associations between granite intrusion events and sn w mineralization of this deposit have been discussed in this paper the results suggest that the pankou granite is genetically related to the polymetallic mineralization in xuebaoding sn w be deposit and further demonstrate that the latest phase of granitic intrusion is commonly favorable for sn and or w mineralization although the pukou granite also exhibits close relationships with the ore formation in its spatial distribution and element characteristics the possibility as a parent for this deposit is excluded duo to the time span from its emplacement to mineralization event that exceed the estimated life of modern geothermal systems

how does the deep western boundary current cross the gulf stream abstract the manner in which the deep western boundary current dwbc crosses the gulf stream is investigated using data from a hydrographic survey conducted in 1990 absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level three density layers are considered in detail two mid depth layers which together make up the shallowest water mass component of the dwbc 500 1200 m and a deep layer consisting of the norwegian greenland overflow water 2500 3500 m the shallowest layer does not make it through the crossover and is completely entrained by the gulf stream however the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover in the intermediate layer which is denser than the gulf stream coming off the shelf part of the dwbc recirculates to the northeast while the onshoremost portion continues equatorward in the deep layer only a small amount of recirculation occurs the lateral

the dominant influence of the icelandic low on the position of the gulf stream northwall 1 the location where the gulf stream separates from the american coast and turns eastward is called its northwall the interannual fluctuations of the northwall are significantly correlated with the north atlantic oscillation with a lag of two years when the azores high and the icelandic low pressures are taken as independent variables the latter dominates the relationship with the northwall and the influence of the azores high pressure is insignificant this is consistent with the hypothesis that the major oceanic control of the northwall is in the southward flow of labrador sea water into the slope sea the alternative mechanism that the interaction of westward propagating rossby waves with the american coast is responsible for northwall fluctuations is considered less likely because its initiation requires perturbations of the eastward winds in the mid atlantic region and they are very likely dependent on the azores high the analysis suggests that the time lag between perturbations of the icelandic low and the northwall varies between one and three years

southeast greenland winter precipitation strongly linked to the icelandic low position abstractgreenland s largest precipitation flux occurs in its southeast se region during the winter controlled primarily by easterly winds and frequent cyclogenesis in the north atlantic several

on the predominant nonlinear response of the extratropical atmosphere to meridional shifts of the gulf stream abstractthe north atlantic atmospheric circulation response to the meridional shifts of the gulf stream gs path is examined using a large ensemble of high resolution hemispheric scale weather research and forecasting model simulations the model is forced with a broad range of wintertime sea surface temperature sst anomalies derived from a lag regression on a gs index the primary result of the model experiments supported in part by an independent analysis of a reanalysis dataset is that the large scale quasi steady north atlantic circulation response is remarkably nonlinear about the sign and amplitude of the sst anomaly chosen over a wide range of gs shift scenarios the nonlinear response prevails over the weak linear response and resembles the negative north atlantic oscillation nao the leading intrinsic mode of variability in the model and the observations further analysis of the associated dynamics reveals that the nonlinear responses are accompanied by the shift of the north atlantic eddy

trace and rare earth elements in cassiterite sources of components for the tin deposits of the mole granite australia trace and rare earth elements have been determined for cassiterite from deposits associated with the mole granite and hosted by granite metasediments and metavolcanics the ree of cassiterite is controlled by the ree of the the ore fluid and the rocks through which this fluid circulated the ree distribution factor and lree hree value of cassiterite is strongly influenced by the associated mineral assemblage the fluid chemistry and the crystal chemical characteristics of the host mineral cassiterite from deposits hosted by granite have trace and rare earth element characteristics similar to those determined for the mole granite cassiterite from deposits hosted by metasediments or acid volcanics have most trace and rare earth element characteristics similar to those of the enclosing rocks and some characteristics similar to the mole granite the ore fluid had chemical components derived from the parental granite and components acquired by passage through the metamorphosed aureole

sm nd dating of hydrothermal carbonate formation an example from the breitenau magnesite deposit styria austria abstract the geochemical evolution and chronology of the breitenau sparry magnesite deposit has been investigated by means of major and trace element abundances stable c o isotope and sm nd isotope analysis chemically separated magnesite fractions leachates show consistent light ree depleted rey patterns marine carbonate normalized and nearly constant normalized values for the middle and heavy ree without significant ce eu or y anomalies non carbonate residues composed of dark matrix material are high in al 2 o 3 and show variably light to middle ree enriched rey patterns similar to shales leachates of limestone from nearby country rocks have normalized rey patterns similar to marine carbonates only ce and y are slightly fractionated based on their trace element chemistry and normalized rey patterns leachates of dolomite from the margin of the deposit closely resemble either limestone type a or magnesite type b with one exception 18 o vpdb for both dolomite types is within the range of the respective magnesite values 18 o vpdb c 14 2 to 15 7 leachates of dolomite from druses within the deposit type c show low rey abundances with normalized patterns similar to those of magnesite sm nd isotope data for magnesite leachates yield a well defined isochron age of 222 5 9 8 ma mswd 3 1 isochron calculations for leachate s and residue s from individual magnesite samples internal isochrons yield ages between 236 16 ma and 193 5 8 6 ma with a low degree of scatter mswd 18 o vpdb c 15 2 37 9 5 4 ma 18 o vpdb c 12 5 and 3 1 7 6 8 ma 18 o vpdb c 11 type c dolomite data indicate that druses were formed by open system fluid flow throughout the deposit

lagrangian observations of the deep western boundary current in the north atlantic ocean part i large scale pathways and spreading rates twenty six rafos floats were deployed in the deep western boundary current dwbc of the north atlantic ocean between the grand banks and cape hatteras in 1994 95 and tracked acoustically for up to two years half of the floats were launched in the upper chlorofluorocarbon cfc maximum associated with upper labrador sea water 800 m and the other half near the deep cfc maximum that identifies the overflow water from the nordic seas 3000 m the float observations reveal the large scale pathways of these recently ventilated water masses in the subtropics the shallow float tracks show directly that upper labrador sea water is diverted away from the western boundary and into the interior at the location where the dwbc encounters the gulf stream near 368n the crossover region consistent with previous hydrographic studies east of the crossover region only one upper labrador sea water float out of seven 15 permanently crossed to the south side of the stream in two years caught in a cold core ring formation event the other shallow floats recirculated north of the gulf stream apparently confined by the mean potential vorticity gradient aligned with the stream the deep floats closely followed the topography to the crossover region then revealed a bifurcation in fluid parcel pathways one branch continues equatorward along the western boundary and the other turns first eastward along the gulf stream path then southward the deep float pathways including the bifurcation in the crossover region can be explained in terms of the deep potential vorticity distribution comparison of the float results with results from recent modeling studies suggests that the deep flow is strongly influenced by both the depth of the main pycnocline and bottom depth the effective spreading rates of upper labrador sea water and overflow water estimated directly from the float data southward at 0 6 6 0 2 cm s21 and 1 4 6 0 4 cm s21 respectively agree well with tracer derived spreading rates mean velocities in the dwbc equatorward at 2 4 cm s21 upper labrador sea water and 4 5 cm s21 overflow water are consistent with other in situ measurements one deep float drifted almost 4000 km along the western boundary in two years revealing a fast track for the spreading of overflow water in the dwbc these observations emphasize the importance of the crossover region in the spreading and mixing of recently ventilated water masses addressed in part ii of this study

loop current variability as trigger of coherent gulf stream transport anomalies abstractsatellite observations and output from a high resolution ocean model are used to investigate how the loop current in the gulf of mexico affects the gulf stream transport through the florida

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

the icelandic low as a predictor of the gulf stream north wall position abstractthe gulf stream s north wall east of cape hatteras marks the abrupt change in velocity and water properties between the slope sea to the north and the gulf stream itself an index of the north wall position constructed by taylor and stephens called gulf stream north wall gsnw is analyzed in terms of interannual changes in the icelandic low il pressure anomaly and longitudinal displacement sea surface temperature sst composites suggest that when il pressure is anomalously low there are lower temperatures in the labrador sea and south of the grand banks two years later warm sst anomalies are seen over the northern recirculation gyre and a northward shift in the gsnw occurs similar changes in ssts occur during winters in which the il is anomalously west resulting in a northward displacement of the gsnw 3 years later although time lags of 2 and 3 years between the il and the gsnw are used in the calculations it is shown that lags with respect to each atmospheric variable are statistica

meridional gulf stream shifts can influence wintertime variability in the north atlantic storm track and greenland blocking author posting american geophysical union 2019 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geophysical research letters 46 3 2019 1702 1708 doi 10 1029 2018gl081087

gulf stream shifts following enso events over the past three decades the annual mean latitude of the gulf stream off the coast of the united states has been forecastable from the intensity of the north atlantic oscillation nao 1 the predictions accounting for more than half the variance here we show that much of the unexplained variance can be accounted for by the southern oscillation in the pacific the gulf stream being displaced northwards following el nino southern oscillation enso events this provides a link between events in the equatorial pacific and the circulation and weather conditions of the north atlantic

age of mineralization by basinal fluids at the el soldado manto type copper deposit chile 40ar 39ar geochronology of k feldspar abstract manto type copper deposits of the southern central andes are strata bound epigenetic mineral concentrations hosted by bimodal volcanic and volcaniclastic rock suites they are economically important but since they are not obviously related to intrusions their genesis has been controversial to some extent because it has proven difficult to date the ore forming event and distinguish it from the age of low grade regional metamorphism el soldado the largest manto type copper deposit in central chile is hosted by marine volcanic and subvolcanic intrusive rocks of the lower cretaceous extensional arc to discriminate between regional metamorphism and mineralization we dated 11 samples of k feldspar by the stepwise heating 40 ar 39 ar method hydrothermally precipitated k feldspar associated with copper sulfides precipitated at temperatures of 300 c and range in age from 100 5 1 5 to 106 1 1 1 ma mean 103 1 6 ma n 8 syn mineralization k feldspar gave an age of 102 5 2 0 which is interpreted to be the age of mineralization k feldspar unrelated to ore yields older ages between 109 4 1 1 and 112 2 ma mean 110 3 1 4 ma n 3 is variably reset by the younger ca 103 ma mineralization episode the above ages are interpreted to represent the age of alkali redistribution and crystallization due to burial metamorphism of the host sequence fission track dating of apatite in the host rocks yields an age of ca 90 ma indicating fast cooling of the system to less than ca 100 c after mineralization time temperature modeling of fission track length data are compatible with initial fast exhumation followed by slow exhumation and cooling to surface temperatures high levels of atmospheric ar were detected in all k feldspar samples and are tentatively attributed to connate meteoric fluids incorporated into the basinal metamorphic mineralizing fluids

time scales of assembly and thermal history of a composite felsic pluton constraints from the emerald lake area northern canadian cordillera yukon knowledge of the time scales of emplacement and thermal history during assembly of composite felsic plutons in the shallow crust are critical to deciphering the processes of crustal growth and magma chamber development detailed petrological and chemical study of the mid cretaceous composite emerald lake pluton from the northern canadian cordillera yukon territory coupled with u pb and 40 ar 39 ar geochronology indicates that this pluton was intruded as a series of magmatic pulses intrusion of these pulses produced a strong petrological zonation from augite syenite hornblende quartz syenite and monzonite to biotite granite our data further indicate that multiple phases were emplaced and cooled to below the mineral closure temperatures over a time scale on the order of the resolution of the 40 ar 39 ar technique v1 myr and that emplacement occurred at 94 3 ma simple thermal modelling and heat conduction calculations were used to further constrain the temporal relationships within the intrusion these calculations are consistent with the geochronology and show that emplacement and cooling were complete in less than 100 kyr and probably 70 5 kyr these results demonstrate that production transport and emplacement of the different phases of the emerald lake pluton occurred essentially simultaneously and that these processes must also have been closely related in time and space by analogy these results provide insights into the assembly and petrogenesis of other complex intrusions and ultimately lead to an understanding of the processes involved in crustal development 2002 elsevier science b v all rights reserved

cassiterite potential for direct dating of mineral deposits and a precise age for the bushveld complex granites u pb and pb isotopic measurements of cassiterite from the 200 ma tin deposits of belitung indonesia and 2100 ma zaaiplaats deposit in south africa demonstrate that cassiterite can be used to directly date mineral deposits rather than associated minerals such as zircon and rutile a precise u pb age of 2099 3 ma has been determined for cassiterites hosted in granites from the bushveld complex on conventional 207 pb 204 pb 206 pb 204 pb plots the data for the indonesian deposits give an age apparently older than that obtained from u pb plots perhaps indicative of inheritance of pb from an older source

precise molybdenite re os and mica ar ar dating of the mesozoic yaogangxian tungsten deposit central nanling district south china the yaogangxian deposit in the central nanling region south china consists of vein type ore bodies hosted in cambrian to jurassic strata and mesozoic granitic intrusions wolframite and molybdenite are the dominant ore minerals intergrown with gangue minerals of quartz feldspar phlogopite and muscovite we have carried out molybdenite re os and phlogopite and muscovite 40ar 39ar dating to better understand the timing and genesis of mineralization re os dating of eight molybdenite samples yielded model ages ranging from 152 0 3 5 to 161 1 4 5 ma with an average of 156 0 ma the re os analyses give a well defined 187re 187os isochron with an age of 154 9 2 6 ma mswd 2 4 hydrothermal phlogopite and muscovite display extremely flat 40ar 39ar age spectra phlogopite yields a 40ar 39ar plateau age of 153 0 1 1 ma whereas muscovite yields a plateau age of 155 1 1 1 ma both 40ar 39ar ages are in good agreement with the re os ages placing the timing of tungsten mineralization at about 154 ma this age is consistent with the field relationships our new data when combined with published geochronological results from other major deposits in this region suggest that large scale w sn mineralization occurred throughout the central nanling region in the late jurassic

slow variations in mean path of the gulf stream east of cape hatteras the gulf stream between cape hatteras and the new england seamounts varies in position over a wide range of time scales we present evidence that interannual path changes may be governed by a time varying outflow of waters from the labrador shelf region when the shelf and slope waters south of new england turn cold and fresh the stream frequently assumes a more southerly track and conversely when the waters are warm and saline these changes in path and surface water properties most likely result from large variations in shelf transport and thus greater or less dilution of the warm saline gulf stream waters that continually leak from the current into the slope sea these observations point to a thermohaline rather than winddriven mechanism for governing the path of the gulf stream after it leaves the coast

gulf stream excursions and sectional detachments generate the decadal pulses in the atlantic multidecadal oscillation abstractdecadal pulses within the lower frequency atlantic multidecadal oscillation amo are a prominent but underappreciated amo feature representing decadal variability of the subpolar gyre e g the great salinity anomaly of the 1970s and wielding notable influence on the hydroclimate of the african and american continents here clues are sought into their origin in the spatiotemporal development of the gulf stream s gs meridional excursions and sectional detachments apparent in the 1954 2012 record of ocean surface and subsurface salinity and temperature observations the gs excursions are tracked via meridional displacement of the 15 c isotherm at 200 m depth the gs index whereas the amo s decadal pulses are targeted through the amo tendency which implicitly highlights the shorter time scales of the amo index the gs s northward shift is shown to be preceded by the positive phase of the low frequency north atlantic oscillation lf nao and followed by a positive amo tendency by 1 25 and 2 5 yea

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

petrographic ree fluid inclusion and stable isotope study of magnesite from the upper triassic burano evaporites secchia valley northern apennines contributions from sedimentary hydrothermal and metasomatic sources sparry and microcrystalline magnesite are minor constituents of the upper triassic burano evaporite formation of the northern apennines in italy petrography and geochemistry of magnesite suggest three modes of formation 1 evaporitic precipitation of stratified microcrystalline magnesite layers associated with sulfate and carbonate rocks most ree are below icp ms detection limits 18o is 20 2 smow and 13c is 2 6 pdb 2 hydrothermal infill of fe rich 9 78 wt feo lenticular sparry magnesite this type of magnesite is characterized by very low lree concentrations whereas hrees are relatively high the fluid inclusion composition is nacl mgcl2 h2o salinity is 30 wt nacl equiv and total homogenization temperatures range from 204 309 c 18o is 17 5 and 13c is 1 3 the partial or total replacement of dolostones by lenticular sparry magnesite lrees are lower in magnesite compared with the partly replaced dolostones magnesite yields 18o and 13c compositions of 17 3 to 23 6 and 0 5 to 1 4 respectively whereas the partly replaced dolostones yield 18o and 13c values of 25 0 to 26 2 and 1 3 to 1 9 respectively complete replacement of dolostones produced massive lenticular sparry magnesite rock containing ooids and axe head anhydrite relicts lrees are depleted compared to unaffected dolostones 18o and 13c compositions range from 16 4 to 18 4 and 0 4 to 0 9 respectively these data and the association between fracture filling and replacive magnesite suggests a metasomatic system induced by hydrothermal circulation of hot and saline mg rich fluids these processes probably occurred in the oligocene miocene when the burano formation acted as main detachment horizon for the tuscan nappe during the greenschist facies metamorphism of the apuane complex thrusting over the apuane zone produced large scale fluid flow focused at the tuscan nappe front sources of mg rich fluids were metamorphic reactions in the apuane complex and dissolution of mg salts at the thrust front considering a maximum tectonic burial depth of 10 km as inferred from the geometry of the chain the pressure corrected temperature of magnesite precipitation 380 to 400 c and the calculated fluid composition 18o 13 3 1 2 are in the range of the published apuane metamorphic temperatures 300 450 c and fluid compositions 18o 7 16 the results of this study support the hydrothermal metasomatic model for the formation of sparry magnesite deposits at the expense of dolostone units involved in thrusting and low grade metamorphism as proposed for the northern graywacke zone alps and the eugui deposit western pyrenees