numerical reconstructions of lgm climate across the eurasian arctic abstract although limits of the eurasian ice sheet at the last glacial maximum lgm are known relatively well from geological evidence the palaeoclimate coexistent with this ice sheet has yet to be identified confidently a simple ice sheet model has been used to determine lgm climate conditions by finding the ice sheet mass balance configuration that permits model results to match with geological data relating to the extent of lgm ice cover these conditions are compared with atmospheric general circulation model agcm calculations of the climate at the lgm which reveal the meteorological conditions associated with the ice sheet mass balance the western margin of scandinavia is characterised by a maritime climate influenced by the nearby warm seasonally sea ice free ocean conditions however further east the warming effect of the north atlantic diminishes and an extreme polar desert climate dominates across the kara sea similar to east antarctica today the lgm climate that we establish is in contrast to many previous reconstructions involving snow laden warm westerly winds originating from the north atlantic affecting the entire eurasian arctic severely cold conditions across the eurasian arctic are compatible with a stable circulation cell over the kara sea that isolates the region climatically and or cold dry westerly winds from the fennoscandian ice sheet that flow across the rest of the eurasian arctic

comment on paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing by hua ning qiu and j r wijbrans abstract in a recent study qiu and wijbrans 2006 qiu h n and wijbrans j r 2006 paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing geochimica et cosmochimica acta 70 2354 2370 analyzed ar isotopes extracted by crushing garnets from the dabie shan bixiling eclogite and claimed to have constrained a pre triassic 450 ma episode of uhp metamorphism from primary fluid inclusions however in the absence of careful sample characterization and stepped heating analyses the reported ages are more easily explained as experimental artifacts related to ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix dabie shan garnets commonly contain mineral impurities such as k rich omphacite and or k feldspar if dabie shan uhp metamorphism is of the generally accepted triassic age 210 240 ma the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous 40ar component with mean 40ar 36ar value of 5000 this value is similar to the composition of extraneous 40ar in other eclogite facies terrane

rapid climatic shifts during isotope stages 2 4 in the polar north atlantic sediment cores from the polar north atlantic provide evidence of six periods of sea ice breakup during isotope stages 4 3 and 2 probably caused by inflow of north atlantic surface water into the polar north atlantic these periods are characterized by having a high number of foraminifera g they last from 2000 to 10000 yr and constitute 50 of the total time span these periods of sea ice breakup correlate temporarily to the heinrich events and the early temperature maximum in the bond cycles of the north atlantic and greenland ice record our hypothesis is that massive iceberg discharges that flooded the north atlantic during each heinrich event probably triggered an oceanographic regime that gave a much more vigorous surface circulation pattern in the polar north atlantic which contributed to the breakup of the sea ice cover open water conditions in the polar north atlantic are inversely related to terrestrial interstadials of coastal norway suggesting ice sheet starvation during the cold periods and ice sheet growth when an open water surface circulation existed in the polar north atlantic our data document synchronous variations on a october 1900 yr time scale between arctic oceanic climate northern hemisphere ice sheet dynamics and ocean atmosphere temperature changes

flux of ice rafted detritus from sea ice in the fram strait abstract a three year particle flux record from the eastern fram strait between greenland and svalbard revealed a rather untypical seasonal flux pattern compared to other particle flux studies from the nordic seas in the eastern fram strait this pattern is characterised by a sudden four to six fold increase of the particle flux in january when no daylight is available to support any biological productivity comparison with sea ice distribution maps led to the conclusion that the sudden increase in the flux is due to ice rafted detritus released from sea ice which originated from the svalbard archipelago and from the northern barents sea detailed grain size analyses of the silt fraction indicated the 10 m fraction of the lithogenic matter to be clearly enriched due to ird input even more important is the observation that lithogenic material 40 m occurs exclusively during the ice rafting event and therefore appears to be a suitable indicator for ird transported on sea ice thus in addition to coarse ird e g 500 m which is mainly derived from icebergs the analysis of fine ird 40 m in deep sea sediments can be used to reconstruct paleo sea ice extensions

structural and gold mineralizing evolution of the world class orogenic mana district burkina faso multiple mineralizing events over 150 million years abstract the mana district located in the northern part of the birimian hounde greenstone belt in western burkina faso is a world class paleoproterozoic orogenic gold district 8 moz including five gold deposits fofina nyafe siou wona kona and yaho these deposits are located in specific lithostratigraphic domains and gold is controlled by various structural features deposit and regional scale mapping intrusion age and geochemistry as well as airborne aeromagnetic and electrical resistivity geophysical data were used to decipher the tectonic evolution of each gold deposit and the district five deformational and four gold mineralizing events were recognized the first deformation event d1md e w oriented shortening affected the metamorphosed volcanic and sedimentary rocks of the lower birimian group this early deformation episode was correlated with the formation of gently n plunging folds f1md and n s striking thrusts faults coeval with emplacement of the pre to synkinematic wona kona and siou plutons dated at 2172 ma under greenschist facies metamorphism the quartz carbonate veins v1md at fofina and siou formed during d1md at eoeburnean time manifesting the first gold event at approximately 2172 ma the following deformation event d2md e w oriented extension is associated with the deposition of the upper birimian group mana basin overlying the lower birimian group the geometry of the mana basin is controlled by the mana and maoula shear zones the tarkwaian type rock formation overlying the upper birimian group controlled by the wona kona and siou shear zones is constrained at the end of d2md or at the beginning of the d3md event with a maximum deposition age at 2113 ma the third deformation event d3md e w to wnw ese transpression affected the entire supracrustal rock such event is correlated with the formation of map scale f3md folds and dextral shear zones during the eburnean orogeny 2113 2090 ma a second gold mineralizing event occurred during d3md and is manifested by quartz carbonate veins v3md and disseminated sulfides at the yaho fofina and nyafe and possibly wona kona deposits the fourth deformation event d4md nnw sse transpression is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric nne striking folds f4md associated with vertical fold axes syn d4md mineralization is characterized mainly by a strong silicification si4md with disseminated pyrite and arsenopyrite along the wona kona shear zone and by tiny quartz carbonate veinlets v4md this event is considered the main gold bearing event in the western margin of the mana district the fifth and last deformation event d5md is brittle in character and was responsible for the formation of e w subvertical crenulation cleavages and reverse faults under overall n s shortening this late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the wona kona and siou deposits this event could be as young as 2022 ma the age obtained from ar ar datation of muscovite schists at the wona kona deposit our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit and regional scale mapping it is interpreted that gold was introduced as early as 2172 ma and possibly as late as 2022 ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the birimian belts

moisture supply for northern ice sheet growth during the last glacial maximum during the last ice age the barents sea ice sheet began to grow 22 kyr ago1 only 8 kyr before it began to disintegrate2 this implies that the ice must have grown very rapidly from the coast to the edge of the continental shelf such rapid growth of a large ice sheet requires significant amounts of moisture3 but the origin of this moisture has been unclear particularly as the climap climate reconstruction suggests4 5 that the greenland iceland norwegian gin seas were perennially ice covered during this period here we present data from deep sea sediment cores from the fram strait which suggest that relatively warm water from the north atlantic ocean was advected into the gin seas in two short term events 27 22 5 and 19 5 14 5 kyr ago we suggest that the resulting seasonally ice free waters were an important regional moisture source for the barents sea ice sheet and that the gin seas played a much more active role in climate during the last glaciation than has previously been supposed

new 40ar 39ar ages from the kalatag district in the eastern tianshan nw china constraints on the timing of cu mineralization and stratigraphy abstract the kalatag region in the eastern tianshan of nw china contains a number of economically important cu deposits among these the south meiling vms deposit is hosted in the late ordovician to early silurian dacitic tuff belonging to the daliugou formation the deposit is composed of two domains a concordant massive sulfide ore domain and a discordant vein type ore domain located in the footwall phyllic alteration zone here we report new 40 ar 39 ar ages on sericite from the phyllic alteration zone to better constrain the timing and genesis of mineralization the sericite displays markedly flat 40 ar 39 ar age spectra yielding a 40 ar 39 ar plateau age of 438 8 3 0 ma and an isochron age of 443 0 11 0 ma the 40 ar 39 ar age is in good agreement with the re os age reported earlier placing the timing of cu zn mineralization at 434 2 3 9 ma the age is also consistent with the field relationships that concordant massive sulfide orebody formed later than discordant vein type orebody another deposit at hongshi is hosted in basaltic andesites of the daliugou formation the orebodies cut the phyllic alteration zone indicating that the cu mineralization formed later than the phyllic alteration hydrothermal sericite from the phyllic alteration zone from this deposit yielded flat 40 ar 39 ar age spectra with 40 ar 39 ar plateau ages of 441 5 3 2 ma and 446 4 4 6 ma and isochron ages of 445 8 5 6 ma and 448 8 7 3 ma respectively this age is obviously older than the re os age which is also in conformity with the field relations that quartz sulfide veins cut the sericitization zone our 40 ar 39 ar ages offer precise constraints on early paleozoic cu mineralization in the eastern tianshan orogen these results indicate that the lower unit basaltic andesites of the daliugou formation formed earlier than 446 ma we thus consider that the daliugou formation formed during middle ordovician to early silurian we also propose several metallogenic events in the kalatag area including 1 an early vms cu zn and hydrothermal vein cu mineralization at 440 430 ma 2 porphyry cu and skarn fe mineralization at 390 380 ma and 3 a late cu au mineralization at ca 300 ma these three metallogenic events were coeval with the distinct magmatic pulses in the kalatag area

episodic gold mineralisation correlated with discrete structural events at ballarat east southeast australia abstract new 40 ar 39 ar geochronological data suggest orogenic gold mineralisation at the ballarat east deposit southeast australia occurred in three main episodes at ca 445 435 ma ca 420 415 ma and ca 380 370 ma the gold mineralisation is localised in muscovite bearing quartz and quartz carbonate veins hosted in the steep faults 70 90 on limbs of tight and isoclinal folds in an ordovician turbidite sequence and within west dipping 45 faults historically known as leather jacket lodes initiation of the 45 faults that are confined to fold culminations begins at ca 445 ma with peak metamorphic conditions at 440 ma the earliest vein sets v 1 were emplaced on limb thrusts at ca 445 435 ma and are characterised by arsenopyrite dominated quartz veins these v 1 veins parallel arsenopyrite rich shale units historically referred to as indicator beds both the steep and 45 faults were reactivated during fold amplification with deposition of the v 2 auriferous veins at ca 420 415 ma a later set of auriferous veins v 3 v 4 with ages of 380 370 ma dominated by pyrite sphalerite galena white mica quartz v 3 or carbonate rich v 4 veins are predominantly associated with reactivation of the 45 west dipping faults this new geochronological data constrains the local kinematic history of the ballarat east deposit and has regional implications the v 1 v 2 vein development appears to be synchronous across the entire western section of the lachlan orogen where previous studies have suggested that initial gold mineralisation was linked to orogenesis at 440 ma as a result of metamorphic devolatilisation reactions in the lower crust in contrast a close spatial and temporal relationship exists between the felsic dykes and the mineralisation recognised in the v 3 v 4 veins the deformation that accompanies v 3 v 4 vein development is attributed to small localised events during east west shortening utilising pre existing fold and fault structures the origin of the fluids producing the v 3 v 4 veins may be metamorphic devolatilisation associated with widespread felsic magmatism that occurred at this time across central victoria

dating of archean auriferous quartz vein deposits in the abitibi greenstone belt canada 40 ar 39 ar evidence for a 70 to 100 ma time gap in between plutonism metamorphism and mineralization discussion

weathering versus circulation controlled changes in radiogenic isotope tracer composition of the labrador sea and north atlantic deep water geological reconstructions and general circulation models suggest that the onset of both northern hemisphere glaciation 2 7 myr ago and convection of labrador sea water lsw were caused by the closure of the panama gateway 4 5 myr ago time series data that have been obtained from studies of ferromanganese crusts from the northwestern atlantic suggest that radiogenic isotopes of intermediate ocean residence time pb and nd can serve as suitable tracers to reconstruct these events however it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering in this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in lsw but can be explained in terms of increases of erosion levels due to the glaciation of greenland and canada furthermore we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic pb and nonradiogenic nd from continental detritus eroding into the labrador sea this can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic pb should provide such a rich supply of radiogenic pb to the oceans an important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources perhaps more importantly the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater

a new approach to the nd residence time in the ocean the role of atmospheric inputs concentrations of rare earth elements ree and nd isotopic ratios were analyzed for seawater filtered suspension and sediment trap samples collected in the tropical atlantic ocean eumeli program eutrophic mesotrophic and oligotrophic sites 20 n 18 21 w this is the first ree nd dataset on solution and different sized particles collected at the same site we present direct evidence of the nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer this exchange is probably one of the main factors that simultaneously constrains the nd concentration and isotopic ratio budget we propose a new approach to estimate the residence time of nd in the ocean nd based on isotopic exchange 200 yr nd 1000 yr the exchange requires a partial dissolution of lithogenic nd we estimate that the fraction of soluble nd proportion in atmospheric dust is of the order of 20 based on the isotopic ratios we suggest that the partial dissolution of atmospheric fallout is probably one of the main ree sources of the ocean

variability of sea surface temperature and sea ice cover in the fram strait over the last two millennia abstract a sediment core located on the west spitzbergen margin in the fram strait 78 54 931 n 6 46 005 e water depth 1497 m was analyzed for its dinocyst content in order to reconstruct hydroclimatic variations of the last 2500 years the relative abundance of dinocyst taxa and principal component analysis show a major transition at about 300 cal years bp it is characterized by the disappearance of thermophilic taxa spiniferites mirabilis hyperacanthus and impagidinium sphaericum and the increase of polar subpolar taxa impagidinium pallidum and pentapharsodinium dalei sea surface temperature sst estimates suggest warmer conditions than present anomaly 2 c averaging at 7 c in summer until 300 cal years bp although cooling pulses are recorded around 1700 1500 1200 and 800 cal years bp the last 300 years were marked by a cooling from 7 6 to 3 5 c and sea ice cover increasing up to 7 months yr the results demonstrate that the fram strait area is sensitive to hydroclimatic variations notably with respect to sea ice and ssts which are linked to the relative strength of northward flow of north atlantic waters to the east and southward outflow of cold and fresh waters from the arctic ocean based on our data the warmest part of our record around 1320 cal years bp is the only interval of the last 2500 years that provides a possible analogue for the modern post ad 2000 interval which is characterized by sea ice free conditions

neocrystallization or cooling 40ar 39ar ages of white micas from low grade mylonites abstract 40 ar 39 ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or alternatively ages of deformation induced neocrystallization to address these interpretations three types of information are needed 1 the effective diffusion length scale controlling volume diffusion of argon in white micas 2 an estimate of the diffusion parameters for volume diffusion of argon and 3 the thermal history completely recrystallized quartzites exhibit smooth grain size distributions for white micas with good exponential or power law fits suggesting that the grains grew under similar conditions the microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history measured grain size distributions provide a maximum limit for the diffusion length scale thermal histories are estimated by 40 ar 39 ar diffusion experiments on k feldspars using the multi diffusion domain method of lovera lovera o m richter f m harrison t m 1989 the 40 ar 39 ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes j geophys res 94 17917 17935 these results are consistent with the expected temperature time path for the thrusting environment and predict maximum temperatures of 250 350 c during the time span recorded by the white micas experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages forward modeling of the passage of the white micas through the closure window for argon diffusion using the thermal histories derived from the k feldspars indicates that the white micas record neocrystallization rather than cooling and supports the idea that the white micas record the timing of when ductile deformation ceased

single crystal 40ar 39ar dating of pyrite no fool s clock single crystals of dispersed trace pyrite from two igneous rocks were dated by using the 40 ar 39 ar laser probe method and the resulting ages were compared to step heating ages of associated amphibole and phlogopite the isochron ages for 0 1 0 3 mm cubes of pyrite agree well with the respective amphibole ages of these igneous samples preliminary step heating analyses of single pyrite crystals yield internal isochrons with indistinguishable although less precise ages both pyrite and amphibole are significantly older than associated phlogopite in one sample which has undergone postcrystallization alteration these results suggest that pyrite can give reliable and precise 40 ar 39 ar ages even in the presence of subsequent alteration given the ubiquity of pyrite in many geologic environments this technique has great promise for application to the dating of ores sediments and hydrocarbon migration all of which have been very difficult to date directly

evidence for diachronous archean lode gold mineralization in the yilgarn craton western australia a shrimp u pb study of intrusive rocks the currently accepted model for the archean lode gold deposits of the yilgarn craton postulates that they represent a coherent group of epigenetic deposits the majority of which formed during a craton scale broadly synchronous hydrothermal event late in the tectonothermal evolution of the granite greenstone terranes at ca 2640 to 2630 ma felsic rocks from the southern eastern goldfields which host or are cut by gold mineralization have shrimp ii u pb zircon ages of 2673 or 3 ma at mount charlotte 2669 or 17 ma at mount percy 2663 or 3 ma at racetrack and 2657 or 8 ma at porphyry all these ages are consistent with gold mineralization at ca 2640 to 2630 ma intermediate to felsic dikes cut typical syn to postmetamorphic lode gold mineralization at the mount mcclure and jundee deposits in the yandal greenstone belt in the north of the kurnalpi terrane the dikes give ages of 2656 or 4 2663 or 4 and 2668 or 10 ma from mount mcclure and 2656 or 7 ma from jundee requiring that mineralization and peak regional metamorphism in the belt occurred prior to ca 2660 ma however both the characteristics of the jundee and mount mcclure deposits and the relative timing of mineralization with respect to the metamorphic and structural history of the belt are similar to that seen for gold deposits elsewhere in the yilgarn craton this implies that mineralization at jundee and mount mcclure was produced prior to 2660 ma by similar processes to those seen elsewhere in the yilgarn at 2640 to 2630 ma peak metamorphism in the western higher metamorphic grade terranes of the yilgarn was not reached until ca 2630 ma some 10 to 30 m y after peak metamorphism in the kalgoorlie terrane and more than 30 m y after metamorphism in the yandal belt in addition almost all of the published robust ages supporting gold mineralization at ca 2640 to 2630 ma are from the west of the craton consideration of the new data from the yandal belt in conjunction with previously published geochronology throws doubt on the hypothesis that lode gold mineralization occurred approximately synchronously across the yilgarn craton rather it suggests that mineralization along with regional metamorphism is earlier by at least 30 m y in the northeastern yilgarn craton

neoarchaean metamorphic evolution of the yilgarn craton a record of subduction accretion extension and lithospheric delamination abstract the greater part of the metamorphic record of the yilgarn craton formed during a protracted middle neoarchaean orogenic cycle spanning from about 2750 to 2620 ma at least six distinct metamorphic events are defined by parageneses in different tectono stratigraphic settings this review characterizes the middle neoarchaean metamorphic response of the entire craton using large databases rock descriptions from 60 000 sites in government databases and 1040 pt determinations from all sources are used to construct metamorphic map patterns for each of these metamorphic events p t paths and timing of each metamorphic event are characterized using petrology and the well constrained pt determinations from this study n 287 combined with metamorphic age determinations from all sources n 114 the spatial patterns and metamorphic conditions of each event provides critical constraints on tectonic settings and changes in tectonics over 130 million years the record of metamorphism in conjunction with stratigraphic kinematic and magmatic constraints shows that the neoarchaean craton evolved from an accretionary crustal growth phase ma and m1 to thermal reworking of the crust during massive influx of granitic intrusions m2 and lithospheric extension after the termination of subduction m3 these events were followed by lithospheric delamination resulting in a craton wide diffusion delayed thermal pulse that coincided with renewed contraction m4 m1 metamorphism at 2748 19 2727 8 and 2706 10 ma experienced high p moderate t hairpin clockwise p t paths during burial of magmatic arc margins during terrane accretion events m2 regional contact type metamorphism at 2671 6 ma tracked low p moderate t clockwise p t paths in back arc settings during voluminous felsic magmatism related to shallow subduction m3 metamorphism at 2656 5 ma experienced low p moderate t anticlockwise p t paths in post volcanic rift basins formed during lithospheric extension m4 metamorphic peaks at 2644 4 and 2629 7 ma experienced low p high t clockwise p t paths during regional scale thermal pulses resulting from lower crust and mantle lithosphere delamination at 2665 2668 ma

application of the no fool s clock to dating the mukodek gold field siberia russia the 40ar 39ar dating method on pyrite encapsulated sericite known as the no fool s clock is promising for bracketing gold ore formation processes using this dating technique we show that gold mineralization at the mukodek gold field siberia russia took place in the late paleozoic at the time of emplacement of the angara vitim granitoid magmas late paleozoic magmatism however was not the source of gold it was the source of heat and fluids that preconcentrated already available gold within alteration zones and hydrothermal veins

40ar 39ar thermochronology of the fossil ll6 chondrite from the morokweng crater south africa studies of meteorites are based mostly on samples that fell to earth in the recent past i e a few million years at most the morokweng ll chondrite meteorite is a particularly interesting specimen as its fall is much older ca 145 ma than most other meteorites and because it is the only macro meteorite clast width intersected in drill core 25 cm found in a melt sheet of a large impact structure when applied to the morokweng meteorite 40ar 39ar thermochronology provides an opportunity to study 1 effects associated with pre impact and post impact processes and 2 collision events within a potentially distinct and as yet unsampled asteroid population a single multi grain aliquot yielded an inverse isochron age of 625 163 ma this suggests a major in space collisional event at this time we have modeled the diffusion of 40ar within the meteorite and plagioclase during and after the 145 ma impact on earth to tentatively explain why pre terrestrial impact 40ar has been preserved within the plagioclase grains the 145 ma terrestrial impact age is recorded in the low retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 15 ma and thus confirms that age information about major terrestrial or extraterrestrial impacts can be recorded in the k rich mineral phases of a meteorite and measured by the 40ar 39ar technique more studies on fossil meteorites need to be carried out to understand if the rough 0 6 ga age proposed here corresponds to major ll chondrite asteroid population destructions or rather to an isolated collision event

remobilisation of the continental lithosphere by a mantle plume major trace element and sr nd and pb isotope evidence from picritic and tholeiitic lavas of the noril sk district siberian trap russia the late permian to early triassic siberian traps have been sampled by drill core core sg 9 and from surface exposure section 1f in the noril sk region of the siberian platform russia combined major trace element and nd sr and pb isotope data on selected samples through the siberia trap offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near noril sk a lower sequence of sub alkalic basalts tholeiites and picritic basalts upwards these are the ivakinsky syverminsky and gudchichinsky formations are overlain by an upper sequence of picritic basalts and tholeiites interbedded with tuffs upwards these are the khakanchansky tuklonsky nadezhdinsky morongovsky mokulaevsky and kharayelakhsky formations the gudchichinsky and tuklonsky formations contain both picritic and tholeiitic lavas the tuklonsky formation tholeiites and picrites have moderate gd yb 1 6 1 8 low tio2 0 45 0 95 wt a significant negative ta and nb anomaly nb la 0 42 0 57 and unradiogenic nd nd chur to 4 6 in contrast both the gudchichinsky formation tholeiites and picrites have high gd yb 2 3 3 1 and tio2 1 2 2 3 wt no significant nb or ta anomaly nb la 0 8 1 1 and radiogenic nd nd chur to 7 3 the low ti and nb la high la sm and unradiogenic nd isotope signatures of the picritic tuklonsky formation lavas and the tholeiitic lavas of the upper sequence are characteristic of magmas strongly influenced by material from the continental lithosphere whereas the high ti and nb la low la sm and radiogenic nd isotope signatures of the lower sequence are more comparable to deeper asthenospheric mantle plume generated lavas similar to oceanic island basalts the lavas overlying the tuklonsky formation have mg numbers of 0 63 to 0 68 and are more evolved than the tuklonsky mg number 0 62 and have more radiogenic nd chur tuklonsky 0 03 to 4 66 mokulaevsky 0 60 to 1 61 but have many of the incompatible trace element features of the tuklonsky sky type magma these lavas show a progressive upwards decline in sio2 55 49 wt la sm 4 6 2 0 and ur sr 67 to 13 which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma this paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust

broad synchroneity of three gold mineralization styles in the kalgoorlie gold field shrimp u pb and 40ar 39ar geochronological evidence there has been a long standing controversy regarding the timing and number of gold mineralization events at kalgoorlie uranium pb dating of zircons and hydrothermal monazite and xenotime as well as ar 40 ar 39 analysis of metasomatic fuchsite and white mica are used to date pre to synore dikes alteration and orebodies in order to resolve this issue the majority of gold mineralization at kalgoorlie including ductile brittle fimiston brittle ductile oroya and brittle mount charlotte style gold are different expressions of a complex mineralizing system that was active at broadly the same time at ca 2 64 ga gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time limier similar pt conditions this giant ore system formed after ea 2 69 ga basic magmatism intrusion of the golden mile dolerite sill at 2680 9 ma and intrusion or calc alkaline feldspar quartz porphyry dikes at 2670 5 ma gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 6 ma and overlapped the waning stages of hornblende and albite bearing porphyry dike emplacement at 2650 6 ma and regional metamorphism subsequent brittle deformation in the kalgoorlie gold held way accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the golden mile between about 2 61 and 2 60 ca this late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks it kalgoorlie and may be related to uplift lift and final cooling of the terrane despite this late event the geometry of the kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization

how accurately can we date the duration of magmatic hydrothermal events in porphyry systems an invited paper determining the absolute duration of magmatic hydrothermal events leading to the formation of porphyry systems i e including porphyry copper skarn and epithermal deposits is one of the key questions in ore geology this is so because the duration of magmatic hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy the problem of determining the absolute duration of magmatic hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo annum ka isotopic dating in contrast is the only possible way to determine the life span of magmatic hydrothermal events in fossil porphyry systems analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating u pb 40ar 39ar re os allow us to date minerals with internal precisions 0 2 2 for a 10 ma old mineral this corresponds to a 20 ka uncertainty which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo annum scale however many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events which extend the overall life of the porphyry systems to a few 0 x and up to 2 ma in some cases internal precisions of the above mentioned dating methods allow us in theory to comfortably discriminate events at the 0 x scale and the combination of u pb 40ar 39ar and reos geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic hydrothermal events in porphyry systems in this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems we show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated we also provide basic guidelines to follow when trying to resolve the duration of magmatic hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable

sources of labrador sea sediments since the last glacial maximum inferred from nd pb isotopes pb isotopes have been measured in the clay size fraction of late glacial and holocene deep sea sediments recovered from two labrador sea piston cores that have been previously analyzed for nd isotopes the newly acquired pb isotopic data allow us to better constrain the different source areas that supplied clay size material during the last deglaciation until 8 6 kyr calendar ages nd pb data can be modeled mainly as a mixture between a precambrian crust and lower paleozoic material originating from the denudation of the pan african orogen the old material originates mainly from the archean lower proterozoic or both terranes of greenland and also probably corresponding terranes of labrador although minor input of other precambrian material is recorded in some detrital carbonate rich deglacial samples from orphan knoll the phanerozoic crustal end member consists of sediment material mainly originating from northwestern europe this source area is found to be the only significant source of young crustal material in early holocene sediments from the greenland rise no significant input from the mid atlantic volcanism is apparent this study puts further constraints on the deep circulation pattern during the last deglaciation it is concluded that at that time european phanerozoic material was carried from the norwegian sea through the wyville thompson ridge into the iceland basin by the north east atlantic deep water no evidence for an overflow is found either south of the iceland iceland scotland ridge or through the denmark strait

sphalerite 40ar 39ar progressive crushing and stepwise heating techniques abstract it is difficult to determine the mineralization ages of sulfide deposits because no suitable minerals can be selected for the traditional isotopic dating methods the 40ar 39ar method rarely applies well to sulfide minerals because of their trace potassium content and strong radioactivity after irradiation this preliminary study investigates the possibility of dating the hydrothermal sphalerites by 40ar 39ar using crushing and heating techniques the 40ar 39ar crushing experiments of two sphalerite samples from the fankou lead zinc deposit yield well defined and concordant isochron ages of 265 8 1 0 1 ma and 269 7 6 2 ma corresponding to initial 40ar 36ar ratios of 290 7 2 4 and 294 6 8 1 respectively the crushed powders of the later sphalerite yields with stepwise heating a flat age spectrum with 40ar 39ar plateau age of 267 2 2 5 ma which define a good isochron with an age of 271 3 5 4 ma and initial 40ar 36ar ratio of 294 0 1 7 this is the first report to distinguish secondary and primary fluid inclusions within sphalerite to obtain their ages using the crushing technique and to obtain good concordant ages both of fluid inclusions by crushing and heating of k containing minerals we expect progressive crushing and stepwise heating techniques will become an effective approach to 40ar 39ar dating of the mineralization ages of hydrothermal sulfide deposits

dating of archean auriferous quartz vein deposits in the abitibi greenstone belt canada 40 ar 39 ar evidence for a 70 to 100 m y time gap between plutonism metamorphism and mineralization the sigma mine at val d9or quebec in the abitibi belt is typical of vein type gold deposits in archcan greenstone belts although the relative timing of gold mineralization with respect to magmatism metamorphism deformation and hydrothermal events in the mine is well documented precise isotopic age constraints have until recently been lacking this paper presents the results of a 40 ar 39 ar dating study of the sigma mine and of the davidson tisdale deposit in the abitibi belt in the sigma mine dates on metamorphic amphibole indicate that the regional metamorphism which predates the vein formation occurred at ca 2 68 ga muscovite from the gold bearing quartz veins yields a plateau date of 2579 or 9 ma 2sigma whereas biotite from the veins shows disturbed saddle shaped age spectra the mild disturbance of the biotite is attributed to later diabase dike emplacement and a 40 ar 39 ar date on a dike indicates that this event is younger than 2 3 ga thermal effects of the grenville orogeny 1 2 1 0 ga may also be present a muscovite separate from a gold bearing quartz vein at the davidson tisdale deposit yields a 2607 or 10 ma 2sigma plateau date although the amphibole and muscovite dates at the sigma mine bracket the gold mineralization event at between ca 2 68 and 2 58 ga a number of tectonothermal histories are possible for this time interval it is concluded based on all evidence that the muscovite plateau dates are the time of vein and gold formation which thus occurred in the sigma mine ca 100 m y and in the davidson tisdale deposit ca 70 m y after syn to post tectonic magmatism and regional metamorphism

young porphyries old zircons new constraints on the timing of deformation and gold mineralisation in the eastern goldfields from shrimp u pb zircon dating at the kanowna belle gold mine western australia abstract the kanowna belle gold mine is a late archaean orogenic lode gold deposit hosted by felsic volcaniclastic and intrusive rocks porphyries of the kalgoorlie terrane western australia rare gold occurs in fragments of veins and alteration that form clasts within the black flag group volcaniclastic rocks at the kanowna belle mine indicating that epithermal gold mineralisation accompanied black flag group volcanism the shrimp u pb zircon age of the volcaniclastic unit is 2668 9 ma and xenocrystic zircons with 2 68 2 70 and 2 71 ga age groupings are common the black flag group rocks are faulted by a d1 thrust and 2670 ma is thus an older limit for regional d1 deformation although shrimp u pb zircon ages of felsic porphyries commonly give the best constraints on the timing of deformation and structurally controlled gold mineralisation the data are complex and dates from single samples can be ambiguous four porphyry samples from the kanowna belle gold mine were analysed backscattered electron and cathodoluminescence imaging show that most magmatic zircon in the porphyries is either high u and metamict or restricted to rims on older xenocrysts that are too narrow to be dated by shrimp some porphyries appear to have been saturated with zircon at source and contain only xenocrystic zircons zircons that are interpreted to be magmatic in a sample of the mineralised kanowna belle porphyry gives a mean age of 2655 6 ma the kanowna belle porphyry is cross cut by regional d2 fabrics and 2655 ma is thus the maximum age for regional d2 deformation this is a maximum age for epigenetic lode gold mineralisation the age of resetting of high u zircon grains 2 63 ga and the age of ore related pb pb galenas 2 63 ga serves as an approximate date for lode gold mineralisation if the complex zircon history of the felsic porphyries at kanowna belle is typical of this suite throughout the eastern goldfields province it is clear that existing single zircon dates from this province require reevaluation backed up by careful backscattered and cathodoluminescence imaging and textural studies

vanadio oxy chromium dravite nav3 cr4mg2 si6o18 bo3 3 oh 3o a new mineral species of the tourmaline supergroup vanadio oxy chromium dravite nav3 cr4mg2 si6o18 bo3 3 oh 3o a new mineral species of the tourmaline supergroup

zirconolite and xenotime u pb age constraints on the emplacement of the golden mile dolerite sill and gold mineralization at the mt charlotte mine eastern goldfields province yilgarn craton western australia in situ shrimp u pb dating of magmatic zirconolite cazrti2o7 in the golden mile dolerite from the mt charlotte gold deposit yilgarn craton australia has yielded the first robust emplacement age 2 680 9 ma for the principle host rock of gold mineralization in the kalgoorlie district in contrast co magmatic zircon gave ages from 2 68 ga to 2 17 ga reflecting isotopic resetting of high u and th crystals in situ shrimp analysis of hydrothermal xenotime ypo4 which co exists with gold in alteration pyrite provided a pb pb isochron age of 2 655 13 ma this date indicates that the youngest deposit in the kalgoorlie district mt charlotte formed at 2 65 ga and provides a new minimum age for the structurally older golden mile deposit our results indicate that gold mineralization at mt charlotte is 50 million years older than indicated by recent 40ar 39ar dating and places new constraints on the timing of late stage regional faulting d4 in the province

revealing mineralization and subsequent hydrothermal events insights from 40ar 39ar isochron and novel gas mixing lines of hydrothermal quartzs by progressive crushing abstract increasingly more data have indicated that the 40ar 39ar crushing technique provides a very useful approach to determine the mineralization ages of hydrothermal ore deposits in this study novel gas mixing lines are proposed based on the data of two quartz samples by 40ar 39ar crushing electron microprobe analyses indicate that the quartz grains contain k rich silicate microlites both quartzs by crushing yield two mixing lines with ages corresponding respectively to those of k feldspar or sericite post mineralization fluids and muscovite samples ore forming fluids this is the first report that both ages of secondary and primary fluid inclusions sfis and pfis are confirmed correspondingly by those of the k rich minerals this study strongly indicates that the 40ar 39ar crushing technique is very useful to determine the ages of geofluid activities the newly proposed gas mixing patterns are shown on the inverse and normal isochron diagrams indicating 1 the mixtures are in different proportions of the pfis and sfis in different crushing steps 2 the gas mixing lines yield pfi ages with low initial 40ar 36ar ratios 3 an intermediate age between pfi and sfi ages cannot be obtained because the progressive crushing technique never release gas mixtures with a constant pfi sfi proportion for each step the gas mixing lines are thus interpreted to yield significant geological ages the mixing patterns will help us better understand the 40ar 39ar dating results and be used for references to other isotopic mixing isochron systems

strontium neodymium and lead isotope variations of authigenic and silicate sediment components from the late cenozoic arctic ocean implications for sediment provenance and the source of trace metals in seawater proven ance changes of silicate sediment deposited during the late cenozoic 5 0 ma on the alpha ridge central arctic ocean are determined from variations in strontium lead and neodymium isotope compositions whereas strontium and lead isotope compositions are relatively invariant from 5 to 1 7 ma end values start to increase at 3 ma subsequently s7sr 86sr and 2 6pb z 4pb ratios progressively increase and end values progressively decrease from 1 7 ma to the present day from these isotope variations three different endmember compositions for sediment source regions are defined the two endmember compositions defined by sediment that was deposited by sea ice from 5 to 1 7 ma are consistent with a significant component being derived from the east siberian shelf the progressive change in isotope compositions from 1 7 to 0 ma correlates with an increase in coarse detritus deposited by icebergs over this time period the isotope data are consistent with a progressive increase in the proportion of sediment that was deposited in the central arctic ocean since 1 7 ma having been derived from the northern canada or queen elizabeth island region changes in the strontium neodymium and lead isotope compositions of arctic seawater are determined by analyzing the oxide fractions of fe mn micronodules strontium isotope compositions of the microno dule oxide fractions are similar to published late cenozoic seawater data and indicate that 97 of the sr is seawater derived but minute contributions of sr from silicate detritus prohibit using the hydrogenous fractions for chronostratigraphic purposes neodymium and lead isotope compositions of the micronodule oxide fractions which reflect those of arctic seawater follow the isotope variations of the silicate components throughout the late cenozoic these relations indicate that river water can not be the primary source of rare earth elements and pb to the dissolved reservoir in arctic seawater and that ice rafted detritus by dissolution or exchange processes is an important source of trace elements to seawater in ice covered oceans copyright 1997 elsevier science ltd

arctic ocean deep sea record of northern eurasian ice sheet history the sediment composition of deep sea cores from the central arctic ocean the fram strait and the yermak plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200 000 years layers with high amounts of coarse terrigenous ice rafted debris ird and often high contents of smectite were deposited during extensive glaciations in northern eurasia when ice sheets reached the northern continental margins of the barents and kara seas and discharged icebergs into the arctic ocean intercalated layers with relatively low ird and smectite contents but abundant planktic foraminifers in the coarse fraction were deposited during periods of atlantic water inflow to the arctic ocean and seasonally open waters leads in a sea ice cover with only few icebergs in the arctic ocean high ird contents in the sediments reflect the presence of ice sheets on the kara and barents seas shelves and the hinterland during the entire oxygen isotope stage 6 ca 190 130 ka in substage 5b ca 90 80 ka at the stage boundary 5 4 around 75 ka and in late stage 4 early stage 3 ca 65 50 ka these results are in excellent correlation with those from recent field work in northern scandinavia european russia siberia and on the shelves relatively low amounts of ird in central arctic ocean sediments from the late weichselian glacial maximum ca 24 18 ka correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the arctic ocean the synchroneity of freshwater events and ird input suggests a common source strongest events were associated with deglaciations of the barents and kara seas after the ice sheets had blocked the outflow of large rivers for several millennia the outflow of freshwater from large ice dammed lakes occurred at ca 130 80 75 and 52 ka freshwater events in the central arctic ocean during the last deglaciation ca 18 ka were relatively small compared to the previous events this indicates that during most of the late weichselian glacial maximum a river outflow from northern siberia to the arctic ocean was possible atlantic water inflow to the arctic ocean and seasonally open waters in the ice leads occurred during the interglacials ofoxygen isotope stage 1 and substage 5e during several interstadials stage 3 substages 5a and 5c and to a lesser degree within stadials and glacials stages 2 4 and 6 with the exception ofthe interglacials these periods were times ofstrong ice growth on the continents as revealed by terrestrial data the coincidence suggests that open waters in the arctic ocean and the nordic seas were an important moisture source in addition to more southerly sources which fostered the growth of ice sheets on northern eurasia

part ii evaluation of 40ar 39ar quartz ages implications for fluid inclusion retentivity and determination of initial 40ar 36ar values in proterozoic samples the argon isotope systematics of vein quartz samples with two different k reservoirs have been evaluated in detail potassium is hosted by ultra high salinity fluid inclusions in quartz samples from the eloise and osborne iron oxide copper gold iocg deposits of the mt isa inlier australia in contrast k is hosted by accidentally trapped mica within lower salinity fluid inclusions of a sample selected from the railway fault 13 km south of the mt isa copper mine australia imprecise apparent ages have been obtained for all of the samples studied and conclusively demonstrate that quartz fluid inclusions are retentive to ar and have not leaked over billions of years iocg samples that host k in fluid inclusions only have k cl values of 1 and the ages obtained represent the maximum ages for mineralization in contrast the railway fault samples that include accidentally trapped mica have k cl values of 1 excess 40 are plus cl hosted by fluid inclusions and radiogenic 40 arr plus k are strongly correlated in these samples and define a plane in 3d 40 ar 36 ar k cl space in this case the plane yields an excess 40 are corrected age of 1030 ma that is 100 s of ma younger than nearby cu mineralization at mt isa the age is interpreted to reflect 40 ar loss from the accidentally trapped mica into the surrounding fluid inclusions and is not related to the samples age of formation the initial 40 ar 36 ar value of fluid inclusions is widely used to provide information on fluid origin for the iocg samples that host k in fluid inclusions only the initial 40 ar 36 ar values are close to the measured values at every temperature of stepped heating experiments for samples that include accidentally trapped mica the correction for post entrapment radiogenic 40 arr production is significant furthermore because 39 ark present in accidentally trapped mica crystals is released at different temperatures to radiogenic 40 arr lost to the surrounding fluid inclusions intra sample 40 ar 36 ar variation cannot be reliably documented the results demonstrate that noble gas analysis is readily applicable to proterozoic or older samples but that if k mineral impurities are present within quartz the abundance of k must be determined before calculation of mean 40 ar 36 ar values that are

neodymium isotopic composition and rare earth element concentrations in the deep and intermediate nordic seas constraints on the iceland scotland overflow water signature neodymium isotopic composition and rare earth element concentrations were measured in seawater samples from eleven stations in the nordic seas these data allow us to study how the iceland scotland overflow water isow acquires its neodymium signature in the modern ocean the waters overflowing the faroe shetland channel are characterized by nd 8 2 0 6 in good agreement with the only other data point published 19 years ago in the greenland and iceland seas the water masses leading to the formation of the isow display lower neodymium isotopic composition with nd around 11 and 9 respectively since no water masses in the nordic seas are characterized by nd 8 the radiogenic signature of the isow likely reflects inputs from the highly radiogenic norwegian basin basaltic margins jan mayen iceland faroe with nd 7 in addition to the neodymium isotopic composition the rare earth element patterns suggest that these inputs occur via the remobilization which includes resuspension and dissolution of sediments deposited on the margins whereas the neodymium isotopic composition behaves conservatively in the oceans in the absence of lithogenic inputs and can be used as a water mass tracer these results emphasize the role of interactions between sediments deposited on margins and seawater in the acquisition of the neodymium isotopic composition of water masses these results should allow a better use of this parameter to trace the present and the past circulation in the north atlantic

fluid sources and the role of abiogenic ch4 in archean gold mineralization constraints from noble gases and halogens abstract the st ives goldfield of western australia hosts high grade gold mineralization in a variety of lithologies within 2 7 ga greenstone belts the low salinity co 2 bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones or from external magmatic sources and independently sourced abiogenic ch 4 has been suggested as an important control on gold mineralization in order to better constrain possible fluid sources we applied a combination of he ne ar kr xe isotope and cl br i analyses to quartz and carbonate veins containing h 2 o co 2 fluid inclusions pyrite magnetite hematite and quartz veins containing ch 4 rich fluid inclusions pyrrhotite pyrite samples containing h 2 o co 2 fluid inclusions have higher br cl and i cl values and less radiogenic noble gas isotope signatures than suggested for ch 4 rich fluid inclusions the samples containing ch 4 fluid inclusions have maximum 40 ar 36 ar of 50 000 20 ne 22 ne of 9 8 and maximum 21 ne 22 ne of 0 56 in contrast h 2 o co 2 fluid inclusions have maximum 40 ar 36 ar of only 20 000 and maximum 21 ne 22 ne of 0 14 the use of 40 ar 39 ar methodology to measure k simultaneously with 40 ar 36 ar enables us to show the highest 40 ar 36 ar ratios are representative of the fluids trapped at 2 65 ga and have not been significantly modified by post entrapment 40 ar r production 40 ar r radiogenic 40 ar furthermore the 40 ar 36 ar ratio is correlated with the 21 ne 22 ne and 136 xe 130 xe ratios demonstrating that other heavy noble gas isotope ratios are also close to their original composition the highly radiogenic noble gas isotope signatures are unlikely to have been generated within the greenstone host rocks because seawater altered volcanic rocks have high abundances of non radiogenic atmospheric noble gas isotopes in contrast the strongly radiogenic noble gas isotope signatures are likely to have been generated in ancient basement rocks underlying the greenstones at 2 65 ga the combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced h 2 o co 2 and ch 4 localized high grade gold mineralization we suggest that the fluids were intimately associated with crustal magmatism in the st ives goldfield and that abiogenic ch 4 could be an important moderator of redox processes in other crustal environments

40ar 39ar dating of mica bearing pyrite from thermally overprinted archean gold deposits previous studies have suggested that pyrite may armor potassium bearing mineral inclusions from alteration induced argon loss thus providing improved 40ar 39ar age results to test this hypothesis matrix muscovite and pyrite crystals hosting muscovite inclusions were selected from two variably overprinted yilgarn gold deposits western australia and analyzed by combined single crystal 40ar 39ar laser probe and in vacuo crushing methods pyrite grains from the ca 2 60 ga mount charlotte gold deposit exhibit evidence of minor argon loss but give concordant high temperature ages averaging 2594 8 ma 2 indistinguishable from previous age estimates matrix mica from the ca 2 63 ga kanowna belle gold deposit yielded a discordant age spectrum with all ages younger than 2 5 ga indicating substantial 40ar loss related to middle proterozoic thermal overprinting crushing and low temperature heating experiments on pyrite crystals from kanowna belle produced anomalously old apparent ages up to 7 0 ga however in contrast to the matrix mica total gas pyrite ages generally approach or overlap the inferred time of gold mineralization this behavior is attributed to diffusion of 40ar to internal muscovite pyrite grain boundaries in response to the middle proterozoic thermal event with limited external loss of argon it is concluded that pyrite acts as a partially closed system for argon but that the 40ar 39ar pyrite dating method has the potential to see through later thermal and or alteration events and resolve controversial aspects of ore deposit geochronology

retreat of the laurentide ice sheet tracked by the isotopic composition of pb in western north atlantic seawater during termination 1 during the last glacial maximum much of north america was covered by the laurentide ice sheet its melting during termination 1 led to systematic changes in proglacial lake formation continental runoff and possibly north atlantic meridional overturning circulation the accompanying change in chemical weathering rates in the interior of north america throughout the deglaciation resulted in a pronounced change in seawater pb isotope composition in the western north atlantic ocean here we present the first high resolution records of seawater pb isotope variations of north atlantic deep water extracted from authigenic fe mn oxyhydroxides in three sediment cores 51ggc 1790 m depth 31ggc 3410 m depth 12jpc 4250 m depth from the blake ridge off florida these data reveal a striking excursion from relatively unradiogenic 206pb 204pb as low as 18 93 towards highly radiogenic pb isotope compositions that was initiated during the bolling allerod interstadial and was most pronounced in both intermediate and deep waters during and after the younger dryas 206pb 204pb as high as 19 38 at 8 8 ka in 4250 m this pattern is interpreted to be a direct function of increased inflow of continent derived radiogenic pb into the western north atlantic supplied through chemical weathering of north american rocks that had been eroded and freshly exposed during the preceding glacial cycle these sediment derived data are complemented by new laser ablation pb isotope data from a ferromanganese crust from the blake plateau at 850 m water depth which show only small glacial interglacial pb isotope variations of the florida current 206pb 204pb between 19 07 and 19 16 the lack of change in the blake plateau record at the same time as the radiogenic excursion in the deeper sediments supports a northern origin of the pulse of radiogenic pb after the younger dryas the deep western north atlantic has experienced a persistent highly radiogenic pb supply that was most pronounced during the first half of the holocene and still lasts until today

40 ar 39 ar and u pb age constraints on the timing of gold mineralization in the kalgoorlie gold field western australia the 40 ar 39 ar ages of three hydrothermal muscovite samples associated with auriferous gold stockwork systems at the mount charlotte deposit in the kalgoorlie gold field have a weighted mean of 2602 or 8 ma 40 ar 39 ar age errors are 2sigma at the nearby golden mile deposit 40 ar 39 ar ages from two muscovite samples associated with shear hosted lode gold mineralization have a weighted mean of 2629 or 9 ma on the balance of available evidence these ages are interpreted to represent the timing of gold mineralization in the two deposits these data indicate that formation of auriferous gold stockwork systems at mount charlotte occurred at least 10 million years after formation of the gigantic golden mile lode system this is consistent with geologic relations between the two different styles of mineralization and suggests that separate hydrothermal episodes were responsible for stockwork and shear hosted gold mineralization a weighted mean 207 pb 206 pb zircon age for a postregional metamorphism biotite porphyry intrusion of 2674 or 6 ma 95 confidence limits provides a minimum estimate for the timing of regional metamorphism of the surrounding greenstone sequence and associated compressive deformation hypotheses for the formation of archcan gold deposits which invoke fluids that are derived from regional metamorphic devolatilization of upper middle crustal greenstone sequences or from intrusion of spatially related porphyries cannot account for the gold mineralization in the kalgoorlie gold field these data indicate that the events occurred 30 to 40 and 60 to 70 million years prior to mineralization at the golden mile and mount charlotte respectively formation of the golden mile appears to have been part of a widespread gold mineralizing event at ca 2630 to 2640 ma supporting models in which crustal scale hydrothermal circulation at this time is used to explain archcan gold mineralization within the yilgarn block however such models must now contain the caveat that discrete younger episodes of mineralization such as that responsible for mount charlotte may also produce significant gold deposits with features similar to typical archcan gold deposits

single grain pyrite rb sr dating of the linglong gold deposit eastern china abstract this study employs the single grain pyrite rb sr technique to date mineralization using the example of the linglong lode gold deposit jiaodong peninsula eastern china the gold deposits in this area are interpreted to have a consistent spatial temporal relationship with widespread late jurassic early cretaceous magmatism in eastern china likely related to lithospheric thinning around 120 ma pyrite of the linglong gold mine contains different types of mineral inclusions such as sericite and feldspar that cause distinguishable variations in rb and sr contents and rb sr ratios four pyrite samples from veins of different mineralization stages give an average isochron age of 120 6 0 9 ma analyses of one quartz and two sericite samples yield isochron ages between 122 ma and 120 ma dating of mineralization is often hampered by disequilibrium of isotopic systems and or the lack of suitable minerals for dating purposes hence the successful attempt of the single grain rb sr technique of pyrite which occurs as a common mineral phase in orebodies and is genetically related to the mineralization has great potential for precise geochronology of hydrothermal mineral deposits

pb isotopes and geochemical monitoring of arctic sedimentary supplies and water mass export through fram strait since the last glacial maximum 1 elemental and pb isotope measurements were performed on leachates and residues from surface sediments and two 50 cm cores mc04 and mc16 collected along a ne sw transect through fram strait geochemical and isotopic properties of residues from surface sediments define three distinct spatial domains within the strait 1 the easternmost edge of the strait 2 the eastern part of the strait off the svalbard margins and 3 the western part of the strait influenced by supplies from svalbard the nordic seas with possible contributions from northwestern siberian margins and sea ice and water outflow from the arctic respectively core mc16 in the third domain beneath the outflowing arctic waters spans the last glacial maximum present interval sediments from this core were leached to obtain detrital residues and exchangeable leachates fractions detrital supplies to core mc16 are believed to originate mainly from melting of the overlying sea ice and thus can be used to document changes in arctic sedimentary sources detrital 206pb 204pb and 208pb 206pb ratios illustrate two mixing trends trends a and b corresponding to the pre and post younger dryas yd intervals respectively these trends represent binary mixtures with a common end member canadian margins and either a siberian trend a or greenland trend b margin end member the yd is marked by an isotopic excursion toward the canadian end member suggesting a very active beaufort gyre possibly triggered by massive drainage of the laurentide ice sheet pb isotope compositions of leachates thought to represent the signature of the overlying water masses define a unique linear trend coincident with trend a this suggests that water masses acquired their signature through exchange with particulate fluxes along the canadian and siberian continental margins

authigenic pb isotopes from the laurentian fan changes in chemical weathering and patterns of north american freshwater runoff during the last deglaciation climate dependent chemical weathering trends have a strong impact on the dissolved pb isotopic composition of continental runoff during glacial terminations so that this tracer can be used to reconstruct the impact of the north american deglaciation on aspects of freshwater runoff and ocean chemistry here we present authigenic fe mn oxyhydroxide derived pb isotope records from lower laurentian fan sediments that trace the local deglacial continental runoff signal through the gulf of st lawrence we use these records to investigate changes in the pb isotopic composition of the north american runoff and their relationship to deglacial processes the new pb isotope records are very similar to those from the subtropical deep nw atlantic locations though the new data are at higher resolution and exhibit much greater amplitudes of change due to their laurentide ice sheet proximal setting unradiogenic compositions 206pb 204pb as low as 18 8 at 18 5 ka during the latest glacial change towards highly radiogenic compositions during the early holocene 206pb 204pb 20 1 at 11 2 ka late holocene 206pb 204pb values return to intermediate values around 206pb 204pb 19 6 these new data support previous inferences that the pb isotopic composition of the deep atlantic is controlled during deglaciation by changes in the pattern and intensity of continental chemical weathering in more detail the most significant change in the entire pb isotope record starts at 12 5 ka and lasts until about 11 ka this observation of strong pb isotopic changes across a climatically cold period in the north atlantic region suggests that at this location pb isotopic compositions recorded in deep marine authigenic fe mn oxyhydroxides not only trace deglacial changes in chemical weathering intensity but also highly depend on major freshwater runoff routes in interior north america our data suggest a gradual opening of the eastward freshwater runoff route in the course of the younger dryas

pre metamorphic arar ages from biotite inclusions in garnet two populations of ar ar ages in biotite inclusions occluded by garnets from an indian polymetamorphic gneiss have been measured using uv laser ablation one population yields a reproducible age of 455 my identical to biotite ages in the adjacent groundmass and is interpreted as slow cooling following high grade metamorphism during the pan african event the second population yields ages in the range 1800 2100 ma and corresponds closely with zircon ages from the same rock these results imply that the ar ar system in a small population of biotite inclusions remained closed during high grade metamorphism 750 c and 5 kb the results of this first experiment accord with results for helium diffusion in garnet and suggest a mechanism for the long term storage of heavy noble gases in the deep crust

nd and pb isotope signatures of the clay size fraction of labrador sea sediments during the holocene implications for the inception of the modern deep circulation pattern holocene on the basis of sediment supply variations for the last 12 kyr three sources have contributed to the sediment mixture the north american shield the pan african and variscan crusts and the mid atlantic ridge clay isotope signatures indicate two mixtures of sediment sources the first mixture 12 2 6 5 ka is composed of material derived from the north american shield and from a young crustal source from 6 5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the midatlantic ridge since the significant decrease in proximal deglacial supplies the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the western boundary undercurrent over the past 8 4 kyr the progressive intensification of the western boundary undercurrent was initially associated mainly with the transport of the northeast atlantic deep water mass until 6 5 ka and with the denmark strait overflow water thereafter the establishment of the modern circulation at 3 ka suggests a reduced influence of the denmark strait overflow water synchronous with the full appearance of the labrador seawater mass our isotopic data set emphasizes several changes in the relative contribution of the two major components of north atlantic deep water throughout the holocene index terms 4267 oceanography general paleoceanography 4558 oceanography physical sediment transport 1040 geochemistry isotopic composition chemistry 9325 information related to geographic region atlantic ocean 9604 information related to geologic time cenozoic keywords clay size fraction sedimentary mixings deep circulation nd and pb isotopes north atlantic labrador sea

40ar 39ar laser probe studies of clinopyroxene inclusions in eclogitic diamonds abstract 40 ar 39 ar laser probe ages are reported for individual clinopyroxene inclusions in thirty two eclogitic diamonds from four localities jwaneng and orapa both in botswana udachnaya siberia and argyle australia ages obtained from cleaved diamonds from jwaneng 244 ma orapa 102 ma and udachnaya 425 ma are similar to those of their host kimberlites however there is considerable variation in age between individual laser extractions obtained from the same inclusion which we suspect is mainly due to partial retention of radiogenic argon that diffused to the inclusion diamond interface while the diamond was in the mantle some of this argon was lost when the diamonds were cleaved as part of the experimental procedure the presence of an ambient mantle 40 ar component trapped in inclusions may be an additional cause of scatter in ages however this cannot be validated with the present data in support of the diffusive loss process the results from an udachnaya inclusion completely enclosed within host diamond with the argon being released by laser drilling yielded an age of 1149 37 ma which is considerably older than the host kimberlite about 340 400 ma likewise the scatter in 40 ar k ratios for eclogitic diamonds from jwaneng and orapa indicates that they too are probably xenocrysts therefore a combination of sample preparation technique and laser drilling can be used to offer a useful extension of the original method enabling both diamond formation from buried inclusions and host rock emplacement ages from cleaved inclusions to be determined argyle clinopyroxenes similarly cleaved to the diamond surface also show a considerable range of inclusion ages 1159 1540 ma only in this case there is a relationship between age and k ca ratio of the inclusions such a relationship is difficult to reconcile in terms of partial loss of 40 ar and instead indicates that there was a protracted period of diamond growth

provenance studies from 40ar 39ar dating of mineral inclusions in diamonds methodological tests on the orapa kimberlite botswana abstract previous studies indicate that clinopyroxene inclusions extracted from their host diamonds give 40ar 39ar ages approaching the time of source kimberlite eruption this behaviour is attributed to diffusion of argon to the diamond inclusion interface region during mantle residence with subsequent loss of this component on cleaving of the diamond to release the inclusion s in this study we investigate the potential of the 40ar 39ar inclusion dating method as a niche provenance tool as diamondiferous kimberlites are uncommon igneous rocks largely restricted to archaean cratons and have limited age ranges provenance studies of detrital diamond occurrences should provide unique information on craton erosional histories palaeo drainage evolution and related basin development as a methodological test 40ar 39ar step heating and in vacuo crushing experiments were conducted on 50 eclogitic clinopyroxene inclusions extracted from diamonds from the 93 ma orapa kimberlite in botswana low temperature steps and crushing experiments produced anomalously old ages up to 2 98 0 10 ga consistent with partial retention of pre eruption inherited argon in sub micron defect sites high temperature fusion steps yielded significantly younger ages although only 35 are within error of the time of orapa kimberlite eruption however 77 of results are within 50 ma and 92 within 100 ma of the orapa eruption age these results mandate that individual fusion ages be treated as maximum estimates of source kimberlite eruption ages with analyses of several 6 inclusions required to date single age detrital populations for multi source diamond deposits only the youngest detrital population or populations separated by 100 ma will be resolvable despite these limitations we conclude that the 40ar 39ar inclusion dating method is still capable of resolving key provenance questions such as the source of detrital diamond deposits in southern africa where controversy exists as to whether the diamonds originated from proximal permo carboniferous glacial deposits or distal cretaceous jurassic kimberlites finally a model is presented to account for the variable retention of pre eruption argon in clinopyroxene inclusions extracted from their host diamonds the proportion of this component retained by inclusions is considered dependent on interface porosity volume partitioning of argon into defect structures and the timing of stress fracture development around inclusions during entrainment to surface