geochemistry u pb geochronology and sm nd data from the paleoproterozoic western finland supersuite a key component in the coupled bothnian oroclines abstract the western finland wf supersuite characterized by supracrustal rocks and intruding plutonic rocks forms the core component of the coupled bothnian oroclines in the central part of the svecofennian orogen we present here geochemical isotope data detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the wf supersuite in the pirkanmaa pb and pohjanmaa pob belt representing the southernmost limb of the southern and the hinge zone of the northern bothnian orocline respectively the tampere belt tb in the north and hame belt hb in the south bound the pb basaltic to picritic volcanic rocks in the wf supersuite with a proposed eruption age at ca 1 91 ga 1 92 1 90 ga show locally pillow structures and are typically associated with black shales cherts carbonate bearing rocks based on geochemistry and sm nd data a variable morb wpb type origin is evident and an evolution scenario from earlier wpb type rifting to more morb affinity ocean sea opening is proposed detrital zircon age distributions from sedimentary rocks with 1 91 ga maximum depositional age in the tb pb and hb have been compared and typically the age distributions of

mobility and fractionation of rare earth elements in argillaceous sediments implications for dating diagenesis and low grade metamorphism abstract we report sm nd and rb sr data for the fine fractions of lower paleozoic argillaceous rocks from wales uk and new york usa spanning the range of low grade metamorphic conditions from the diagenetic zone zeolite facies to the epizone greenschist facies in all cases leaching of the fine fractions results in a high 147 sm 144 nd 0 09 0 29 acid soluble component and a complementary low 147 sm 144 nd 0 05 0 14 residual component the observed fractionation is an ancient feature related to diagenesis burial and metamorphism the magnitude of sm nd fractionation between leachates and residues as well as the resulting sm nd ages vary as a function of grain size and metamorphic grade uncleaved welsh mudrocks of the diagenetic zone yield sm nd leachate residue ages of 453 484 ma in agreement with their llanviian to caradocian biostratigraphic ages whereas higher grade rocks of the anchizone and epizone yield sm nd ages as young as 413 ma these ages are transitional between the time of deposition and the time of regional deformation related to the acadian orogeny at 390 ma distinct convex upward rare earth element ree patterns of the leachates suggest that the precipitation of early diagenetic apatite controls the trace element budget of the rock forcing a depletion of middle rees on the subsequently formed diagenetic phyllosilicates the amount of organic matter present and the extent of later prograde reactions are probable modifiers of this fractionation process ordovician and devonian clastic rocks associated with the trenton and onondaga limestones of new york yield single sample and multi sample sm nd isochron ages that agree well with their biostratigraphic ages of 454 ma and 390 ma respectively the ree fractionation observed in shale leachates of the ordovician utica formation is related to ca mg of the bulk rock and hence to the composition of the diagenetic carbonate cement in all cases the sm nd system remained closed subsequent to the peak of diagenesis or metamorphism including the north american rocks that show no evidence of being isotopically reset during widespread remagnetization of the subjacent limestone units in the late paleozoic

behavior of rare earth elements in a paleoweathering profile on granodiorite in the front range colorado usa abstract a paleoweathering profile on the boulder granodiorite in northern colorado provides an opportunity to trace the behavior of rees from parent rock through a weathering profile into unconformably overlying permian sediments with progressive upward weathering of the granodiorite na2o cao sio2 ta hf co th cr sc cr th zr hf la sc zr y and la th decrease al2o3 and fe2o3t increase and tio2 mgo k2o p2o5 rb zr sc cr co hf nb ta y th u ree ti nb and zr nb increase to maximum values and then either level off or decrease lree enrichment is less in the weathering profile than in the parent granodiorite and although the parent does not have an eu anomaly or only a slight positive anomaly all samples from the weathering profile and overlying sediments have significant negative eu anomalies this observation is especially important in that it shows conclusively that a negative eu anomaly can be produced during chemical weathering of granitoids we suggest these eu anomalies are due to relative enrichment of the other rees and partial loss of eu during the breakdown of plagioclase the boulder weathering profile also has a very minor negative ce anomaly that is within error of a ce anomaly in the parent in the unweathered parent 50 of the ree are contained in sphene and in the case of la also in allanite from 10 20 of the ree are contained in apatite and biotite hornblende and from 7 10 of the hrees are in zircon with exception of eu for which feldspars contribute about 8 negligible amounts of rees occur in the feldspars in weathered samples 75 of the rees are contained in clay minerals the crossover between sphene and clay control of rees occurs over a distance of 1 m near the contact with fresh rock except for their small negative eu anomalies the clay minerals have ree patterns very similar to those of the parent rock isocon plots suggest apparent enrichments of many elements in the boulder weathering profile result from losses of na ca and si during plagioclase weathering in addition variable amounts of sr eu ta nb p and ba were lost during weathering although th u zr y th sc zr hf lu hf and ti zr may have been transferred relatively unchanged from granodiorite parent to the bulk weathering profile most other element ratios and ree distributions were significantly changed during weathering this observation implies that caution needs to be exercised when using ree patterns and element ratios to trace sediment provenance the fact that most element ratios and ree distributions also differ between fountain sediments and the bulk weathering profile may be related to one or a combination of four factors listed in order of probable decreasing importance contribution of other sources to the fountain sediments sorting of minerals during sediment deposition remobilization of elements during diagenesis and leaching of elements by water flow through the upper meter of the weathering profile

u pb and sm nd isotopic constraints on the evolution of the paleoproterozoic perapohja belt northern finland ten localities representing metasedimentary and igneous rocks from the northern part of the paleoproterozoic perapohja belt fennoscandian shield were sampled for zircon u pb dating by la mc icp ms and sm nd analysis by tims the results revealed two phases of granitic magmatism that took place at 1 99 ga and 1 79 1 77 ga the former age 1989 6 ma was given by the kierovaara granite pluton representing the first example of felsic plutonism of this age in northern finland though a type felsic volcanic rocks of the same age have previously been recognized the strongly negative initial nd values obtained for granitoids ranging from 3 9 to 11 4 are compatible with the previous sm nd data indicating the presence of archean basement in northern finland at the time of granite emplacement a garnet whole rock sm nd age of 1752 14 ma was measured for a mica gneiss constraining the time of metamorphic cooling down to the closure temperature of garnet detrital zircon grains from quartzite samples belonging to the jatulian palokivalo formation and a stratigraphically unassigned lithodemic unit the mellajoki suite yielded solely archean zircon populations in contrast our sample from the martimo formation representing kalevian sedimentation revealed a 66 paleoproterozoic detrital zircon component with the youngest zircon grains showing an age of ca 1 91 ga the new age data allow us to constrain the depositional time of this sedimentary unit between ca 1 91 and 1 88 ga the latter being the time of the synorogenic plutonism our geochronological data coupled with recent data from other karelian belts suggest that the depositional time period for rocks assigned to the kaleva spans ca 150 ma and the simple division of these rocks into two non coeval units the lower and upper kaleva has become problematic

precambrian ophiolites and related rocks developments in precambrian geology 13 t m kusky editor pp 800 elsevier 2004 isbn 0 444 50923 2 price 198 the purpose of this collection of papers is to address the still controversial question as to the continuity of modern plate tectonic processes back through earth s early geologic history by cataloguing worldwide precambrian ophiolite sequences for the economic geologist and explorationist this volume provides an opportunity to acquaint themselves with geological and geochemical characteristics of precambrian ophiolite assemblages and of phanerozoic analogues this in turn will allow them to recognize this distinctive type of sea floor assemblage in the field and from geochemical study as well as the implications of its presence within certain precambrian volcano sedimentary terranes the editors organized the series of 22 papers into four sections proterozoic ophiolites and related rocks archean ophiolites and related rocks models for the evolution of oceanic crust with time and analogies to precambrian ophiolites the editor s introduction to these sections is somewhat lengthy and repetitious but at the same time it is a good summary of the different ophiolite settings and their characteristics and also serves as a volume synopsis

diagenesis of sandstones in the back arc basins of the western pacific ocean sandstones occur in back arc basins of the western pacific at dsdp sites 299 sea of japan 297 northern shikoku basin 445 and 446 daito ridge and basin province 453 mariana trough 286 new hebrides basin and 285 south fiji basin these sandstones are dominantly volcaniclastic arenites derived from andesitic island arcs the degree of sandstone diagenesis is dependent on original composition burial rate heat flow history of the basin and timing of sandstone deposition with respect to rifting processes and associated high heat flow sandstones containing a larger proportion of volcaniclastic components showed more diagenetic effects than sandstones containing a significant volume of other rock fragments and mineral components sandstones deposited during early stages of rifting sites 445 446 with a slow burial rate and high crustal heat flow showed the greatest degree of downhole diagenetic change these diagenetic changes include early pore space reduction and rim cementation by clay minerals followed later by calcite and subsequent pore fill cementation by clinoptilolite heulandite analcite and later calcite replacement of recognizable volcanic rock fragments by chert calcite and zeolites was observed in the deepest part of the hole sandstones deposited after rifting under conditions of associated lower heat flow showed considerably less diagenetic changes particularly if burial was rapid the high heat flow associated with earliest rifting associated fluid circulation driven by thermal convection and slow burial rate controlled the diagenetic history of these sandstones thus timing of sandstone deposition with rifting stage and associated burial rates were key factors in controlling sandstone diagenesis in back arc basins

spatial and temporal variation in the isotopic composition of mercury in the south river va abstract historic point source mercury hg inputs from industrial processes on the south river waynesboro virginia ended many decades ago but sediment and surface water hg concentrations remain elevated relative to the regional background to understand hg sources mobility and fate in the south river we analyzed total hg concentrations and hg stable isotope compositions of streambed sediments bank soils suspended particles filtered surface waters and channel margin hyporheic zone pore waters hg isotopes allow for the identification of three distinct hg end member inputs to the south river consisting of a regional background source and two isotopically distinct contaminant sources hydrologic conditions are demonstrated to have an influence on within channel hg isotope fractionation and hg partitioning with no observed isotopic discrimination between suspended particulate hg and filtered surface water hg during elevated flow conditions channel margin hyporheic zone porewaters had significantly higher 202hg values than surface waters 202hg 0 52 0 44 and 202hg 0 89 0 20 respectively mean 1sd a subset of porewaters exhibited mass independent fractionation signatures 199hg 0 33 0 06 200hg 0 19 0 03 mean 1sd that are suggestive of a precipitation derived origin for the dissolved hg pool sediments from a floodplain profile were analyzed to explore the temporal variation in hg isotopic composition within the south river indicating brief excursions up to 202hg 0 61 from the average composition observed in modern samples 202hg 0 52 0 09 by improving understanding of the spatial temporal and hydrologic conditions that contribute to variations in hg isotopic composition this study provides insights into the processes that control hg isotopic end member sources hg loading to the channel during elevated flows and hg fate in the south river

tracing anthropogenic hg and pb input using stable hg and pb isotope ratios in sediments of the central portuguese margin abstract three short marine sediment cores from the cascais submarine canyon csc cores 252 32 and 252 35 and the estremadura spur core 252 16 on the central portuguese margin were analysed for hg pb al and mn concentrations and both pb and hg stable isotope compositions in order to reconstruct trends in hg and pb sources and accumulation sediment ages were determined based on 210pb measurements the studied cores reveal increasing hg and pb concentration independent of grain size variations since the middle of the 19th century towards the present concomitantly a decreasing trend of 206pb 207pb to less radiogenic values towards the surface was found indicating increasing anthropogenic inputs the lowest values of 202hg were generally observed in the older sediments characterized by low hg and pb concentrations suggesting a low input of anthropogenic metals odd mass number 199hg and 201hg hg isotopes in sediment samples from core 252 16 were characterized by positive mass independent fractionation mif while recent sediments from cores 252 32 and 252 35 did not reveal significant mif probably reflecting both the proximity to the source of anthropogenic hg contamination tagus estuary and the importance of the csc as a particle carrier the multi tracer approach based on both stable hg and pb isotopic signatures confirms anthropogenic hg and pb enrichment in recent marine sediments and also allows us to distinguish between areas dominated by detrital e g csc versus hemipelagic estremadura spur sedimentation

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

rare earth element redistribution and its effects on the neodymium isotope system in the austin glen member of the normanskill formation new york usa abstract three groups of ree patterns are observed for the sandstones and shales of the middle ordovician austin glen member of the normanskill formation one sandstone sample is enriched in middle and light rare earth elements the majority of samples are light rare earth element lree enriched with negative eu anomalies and flat heavy rare earth elements hrees whereas a third group appears to have lost lrees and middle rare earth element mrees homogeneous neodymium isotopic compositions for all samples at about the time of deposition nd 470 ma 8 2 1 1 2sd indicate that the source of the austin glen member was well mixed and that the neodymium had a long term enriched history the differences in the shape and abundances of the ree patterns combined with the neodymium isotope characteristics of the samples lead us to the conclusion that samarium and neodymium were fractionated and that the neodymium isotopic system might have been disturbed at about the time of deposition 470 ma most samples appear to have been affected by this process the two most altered shales lost about half of their neodymium and their sm nd ratios are higher than the sm nd ratios of average shales these two shales give clearly erroneous mantle extraction ages but the average tdm of the provenance can still be deduced 1700 1800 ma from the least disturbed sandstones with normal upper crustal sm nd ratios accordingly this study demonstrates that the rees may be transported and neodymium isotopes may be reequilibrated under certain sedimentary conditions e g diagenesis however in spite of evidence for ree redistribution provenance information tdm original ree patterns may still be inferred

early proterozoic deposition and deformation at the karelian craton margin in southeastern finland abstract the margin of the karelian craton underwent complex though not chaotic deformation during the svecokarelian orogeny the resultant structural sequence can be subdivided into an early phase that involved translation of thrust nappes and recumbent folds verging towards the craton and a younger phase having more zonal and upright structures these latter include nw trending ductile shear zones associated with widespread pluton emplacement and later n trending faults and shear zones having substantially vertical displacements these features are interpreted in terms of a relative increase in vertical crustal stresses resulting from gravitational and thermal disequilibrium in the overthrust archaean basement despite this structural complexity two distinct depositional provinces are recognized in north karelia separated by a major thrust zone the more easterly hoytiainen province is a narrow essentially intracratonic feature containing locally derived archaean and proterozoic detritus an early phase of chemogenic and pelitic deposition with sporadic tholeiitic volcanism was followed by an influx of coarse clastic detritus largely deposited from turbidity currents within a prograding submarine fan the province apparently records deposition in a transtensional en echelon system of basins initiated near the craton margin at about 2 1 ga in contrast the savo province is largely allochthonous and includes the distinctive outokumpu assemblage commonly interpreted as a remnant of lower proterozoic oceanic crust provenance for the bulk of the overlying metasediments remains problematical although local evidence for deposition on archaean basement exists if an accretionary margin context is considered several potential niches for savo province deposition can be found that appear to satisfy constraints imposed by both isotopic data relating to provenance and the sequence and geometry of svecokarelian deformation

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

archaean proterozoic transition geochemistry provenance and tectonic setting of metasedimentary rocks in central fennoscandian shield finland the central part of the fennoscandian shield in finland is composed of the palaeoproterozoic svecofennian domain and the archaean karelian craton with a palaeoproterozoic allochthonous and autochthonous cover a cryptic suture separating these areas and another tentative suture dividing the svecofennian into central and southern parts have been proposed the chemical composition of sedimentary rocks n 300 within the study area including the effects of palaeoweathering hydraulic sorting depositional environment and post depositional processes have been studied in order to delineate sediment source components the main proposed source components for the archaean sedimentary rocks are weathered 3 0 3 2 ga greenstone granite ttg and local 2 7 ga sources autochthonous 2 2 1 9 ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol 2 2 2 35 ga sedimentary rocks derived from the palaeosol and mafic dykes and plateau volcanics mainly 2 2 2 1 ga although in places locally derived non weathered archaean sources dominated archaean crust and 2 0 1 92 low k bimodal rocks from a primitive island arc are the proposed source for the allochthonous western kaleva cover rocks these formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone the central svecofennian sedimentary rocks can be divided into local arc derived rocks 1 89 ga and older 1 91 ga rocks from a mixture of western kaleva sources and a 1 91 2 0 ga mature island arc active continental margin source rifting followed by increased subsidence during initial collision in the ne and subsequent arc reversal caused rapid erosion from the mountain belt exposing diverse source compositions as seen in the large variation of th sc 2 0 5 and deposition into an oblique hinterland basin further developing into a subduction related foredeep mature greywackes from the southern svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline affinity complexes and archaean rocks less mature rocks also occur and had sources dominated either by island arc active continental margin rocks or local picritic rocks in the sedimentary record the archaean proterozoic transition up to 2 1 ga was dominated by input of mainly mafic plateau type volcanic contribution to the archaean detritus palaeoproterozoic sediments having a crustal component 2 1 ga show higher th sc th cr and lower sm nd and eu eu relative to the archaean rocks but locally low th cr ratios complicate the situation ba depletion relative to k rb and th is a characteristic feature of the sedimentary rocks of the central fennoscandian shield indicating high amounts of ba lost from the clastic record during 2 3 1 9 ga and further recycled back to the mantle forming a subduction component and an enriched mantle component ba depletion seems to have been especially characteristic of chemical weathering during 2 35 2 2 ga under co2 rich and low o2 atmosphere whether this strong ba depletion is characteristic of the archaean proterozoic transition and quiet supercontinent stages in general remains to be determined

palaeoproterozoic accretionary processes in fennoscandia abstract accretionary processes contributed to major continental growth in fennoscandia during the palaeoproterozoic mainly from 2 1 to 1 8 ga the composite svecofennian orogen covers c 1 10 6 km 2 and comprises the lapland savo fennia svecobaltic and nordic orogens it is a collage of 2 1 2 0 ga microcontinents and 2 02 1 82 ga island arcs attached to the archaean karelian craton between 1 92 and 1 79 ga andean type vertical magmatic additions especially at c 1 89 and c 1 8 ga were also important in the continental growth the palaeoproterozoic crust is the end product of accretionary growth continental collision and orogenic collapse preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening the pirkanmaa belt represents a composite accretionary prism and other preserved palaeosubduction zones are identified in the gulf of bothnia and the baltic sea areas in the southern segment of the lapland savo orogen collision between the archaean continent lower plate and the palaeoproterozoic arc microcontinent assembly upper plate produced a special type of lateral crustal growth the archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision

diagenetic modification of sm nd model ages in tertiary sandstones and shales texas gulf coast sm nd model ages of sandstones and shales from the oligocene frio formation of south texas and the paleocene eocene wilcox group of south central texas increase with increasing depth of burial depleted mantle model ages tdm of frio shales increase from ca 800 ma at 1 km of burial to ca 1000 ma at 4km in frio sandstones tdm increases from ca 750 ma at 1 km to over 1200 ma at 4 km of burial in wilcox shales tdm increases from ca 1300 ma to ca 1500 ma over a burial depth of 1 5 to 4 5 km tdm of wilcox sandstones increases from ca 1450 ma at 2 5 km of burial to as much as ca 2000 ma at 4 0 km the sampling scheme employed minimizes but does not eliminate the possibility that these trends in tdm are due to detrital variability close correlation of these trends with diagenetic changes in the rock and evidence of rare earth element mobility preserved in the diagenetic products suggests that diagenesis has modified tdm diagenetic modification of tdm in frio sandstone and wilcox shale is due primarily to loss of radiogenic nd whereas increased 147sm 144nd produces elevated tdm in deeper wilcox sandstones the tdm of frio shales does not change appreciably with depth owing to the opposite effects on tdm of decreased 147sm 144nd and decreased nd 0 loss of radiogenic nd is due primarily to reactions involving unstable volcanic detritus which is more abundant in the frio than in the wilcox decreased 147sm 144nd in authigenic illite and consequent increased 147sm 144nd in pore fluids results in elevated tdm in wilcox and frio sandstones containing late diagenetic cement and in decreased 147sm 144nd in deeper frio shale the similar tdm trends observed in both units which have different detrital characteristics suggest that modification of sm nd model ages in siliciclastic rocks may be commonplace during burial diagenesis

full scale crustal interpretation of kokkola kymi kokky seismic profile fennoscandian shield the kokkola kymi deep seismic sounding profile crosses the fennoscandian shield in northwest southeast nw se direction from bothnian belt to wiborg rapakivi batholith through central finland granitoid complex cfgc the 490 km refraction seismic line is perpendicular to the orogenic strike in central finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013 the campaign resulted in 63 usable seismic record sections the average perpendicular distance between these and the profile was 14 km tomographic velocity models were computed with jive3d program the velocity fields of the tomographic models were used as starting points in the ray tracing modelling based on collected seismic sections a layer cake model was prepared with the ray tracing package seis83 along the profile upper crust has an average thickness of 22 km average and p wave velocities vp of 5 9 6 2 km s near the surface increasing downward to 6 25 6 40 km s the thickness of middle crust is 14 km below cfgc 20 km in se and 25 km in nw but vp ranges from 6 6 to 6 9 km s in all parts lower crust has vp values of 7 35 7 4 km s and lithospheric mantle 8 2 8 25 km s moho depth is 54 km in nw part 63 km in the middle and 43 km in sw yet a 55 km long section in the middle does not reveal an obvious moho reflection s wave velocities vary from 3 4 km s near the surface to 4 85 km s in upper mantle consistently with p wave velocity variations results confirm the previously assumed high velocity lower crust and depression of moho in central finland

petrology and geochemistry of metabasalts from the 1 95 ga jormua ophiolite northeastern finland the jormua ophiolite exposes a unique fragment of red seatype oceanic crust formed in a setting related to continental break up 1950 ma ago two distinct types of basalt are represented the early dykes and the main basalt suite early dykes have fractionated h ree patterns oib like trace element patterns low 2j nb 6 and ejwfyss ga 0 6 indicative of their derivation from an enriched source the remaining dykes and all lavas belong to the second e morb like main basalt suite which is characterized by high mg number and cr contents flat ree patterns zr b 6 17 chondritic th ta and only moderately depleted isotopic signatures zjufi gs a 4 1 9j most main suite samples cannot be related solely by fractional crystallization to a common parental magma rather they represent distinct melt fractions that underwent variable amounts of chromite olivine plagioclase fractionation during ascent a significant part of the compositional diversity of the main basalt suite can be explained by mixing a depleted source with a relatively uniform proportion of an enriched component similar to that represented by the oib like early dykes it is probable that during the latest stages of continental rifting the oib type melts metasomatized the upper part of the depleted asthenospheric mantle which became the source of the main basalt suite soon after the old continental lithosphere was ruptured the complete absence of any evidence for a subduction related component in the basalts implies that jormua is not a back arc ophiolite

tracing historical trends of hg in the mississippi river using hg concentrations and hg isotopic compositions in a lake sediment core lake whittington mississippi usa abstract concentrations and isotopic compositions of mercury hg in a sediment core collected from lake whittington an oxbow lake on the lower mississippi river were used to evaluate historical sources of hg in the mississippi river basin sediment hg concentrations in the lake whittington core have a large 10 15 y peak centered on the 1960s with a maximum enrichment factor relative to hg in the core of 4 8 in 1966 the hg concentration profile indicates a different hg source history than seen in most historical reconstructions of hg loading the timing of the peak is consistent with large releases of hg from oak ridge national laboratory ornl primarily in the late 1950s and 1960s mercury was used in a lithium isotope separation process by ornl and an estimated 128 mg megagrams of hg was discharged to a local stream that flows into the tennessee river and eventually the mississippi river mass balance analyses of hg concentrations and isotopic compositions in the lake whittington core fit a binary mixing model with a hg rich upstream source contributing about 70 of the hg to lake whittington at the height of the hg peak in 1966 this upstream hg source is isotopically similar to hg isotope compositions of stream sediment collected downstream near ornl it is estimated that about one half of the hg released from the ornl potentially reached the lower mississippi river basin in the 1960s suggesting considerable downstream transport of hg it is also possible that upstream urban and industrial sources contributed some proportion of hg to lake whittington in the 1960s and 1970s

detrital zircon u pb dating hf isotopes and whole rock geochemistry from the songshan group in the dengfeng complex constraints on the tectonic evolution of the trans north china orogen abstract the dengfeng taihua and zhongtiao complexes are located in the southernmost segment of the trans north china orogen along which the western and eastern blocks amalgamated to form the north china craton the dengfeng complex consists of the neoarchean granitoids and supracrustal rocks of which the latter are subdivided into the dengfeng and songshan groups the songshan group consists of basal conglomerates quartzites schists with minor dolomites and phyllites geochemistry of the songshan schists and quartzites are indicative of felsic source rocks and display continental island arc and passive continental margin signatures respectively u pb ages of detrital zircons from the songshan group yielded three age populations of 3700 3100 ma 3000 2550 ma and 2550 1950 ma the dominant 3000 2550 ma detrital zircons were most likely sourced from the ttg gneisses of the dengfeng and taihua complexes the subordinate 2550 1950 ma detrital zircons were probably derived from the paleoproterozoic granitoid plutons and meta rhyolites in the dengfeng and zhongtiao complexes and minor amounts of 3700 3100 ma detrital zircons may have been derived from the paleoarchean and mesoarchean crust of the eastern block the youngest detrital zircon age peaks in the luohandong and wuzhiling formations constrain their maximum depositional ages at 2350 ma and 1960 ma respectively in combination with the 1775 ma undeformed granitoids that cut the songshan group the depositional age of the songshan group can be constrained in the period between 2350 and 1780 ma considering the lithostratigraphic feature provenance and depositional age the songshan group is interpreted as having deposited in a retroarc foreland basin which is consistent with the model that the collision between the eastern and western blocks to form the trans north china orogen occurred at 1 85 ga after long lived subduction

abiotic and biotic controls on methane formation down to 2 5 km depth within the precambrian fennoscandian shield abstract despite a geological history characterised by high temperature and pressure processes and organic carbon deprived crystalline bedrock large amounts of hydrocarbons are found in deep groundwaters within precambrian continental shields in many sites methane comprises more that 80 of the dissolved gas phase reaching concentrations of tens of mmol l 1 in this study we used isotopic methods to study the carbon isotope systematics and sources of crustal methane within the fennoscandian shield the main study sites were the outokumpu deep drill hole and the pyhasalmi mine in finland both of which allow groundwater sampling down to 2 5 km depth and have been previously studied for their groundwater chemistry and microbiology we show that the differences in the amount and isotopic composition of methane are related to the availability of carbon sources as well as processes behind the incorporation of hydrogen and carbon via abiotic and biotic pathways into hydrocarbon molecules supported by previously reported occurrences and isotopic data of deep groundwater methane in lithologically different locations in finland and sweden we show that methane formation is controlled by microbial methanogenesis and abiotic reactions as well as lithology with the metasedimentary environments being the most favourable for methane occurrence rather than a thermogenic relic crustal methane within the fennoscandian shield is more likely the result of low temperature formation from ancient organic compounds or their inorganic intermediates such as graphite such crustal gases are characterised by the lack of major amounts of c 2 hydrocarbons and 13 c rich methane further microbiological and isotopic geochemical evidence suggest that microbial methane is more common at depths shallower than 1 5 km

a petrographic approach for evaluating trace element mobility in a black shale abstract at the time of deposition black shales record geochemical information related to the provenance and conditions of deposition for that rock these initial geochemical conditions are susceptible to alteration and overprinting by the aggressive processes of black shale diagenesis in this study we evaluate a sequence of ordovician black shales from the southern welsh basin for the presence of such a diagenetic overprint through use of cathodoluminescence backscattered electron imaging reflected light microscopy fission tracks and microprobe elemental mapping we can discern textural relationships with in the welsh black shales the relationships observed constrain a paragenetic sequence and overall diagenetic history including the early diagenetic formation of pyrite apatite rare earth element ree rich and carbonate and later formation of diagenetic monazite ree u rich and late stage carbonate the chemical reactions necessary to produce this mineral sequence require postdepositional redistribution of ree including changing cerium anomalies ce ce and uranium as a result ree u and their associated isotopic systems sm nd and u pb can reflect a diagenetic overprint rather than the source area and depositional conditions the paragenetic sequence described in this study records several postdepositional changes in the whole rock mineralogy among these changes are the formation of ree rich apatite and ree and u rich monazite the formation of these phases requires that ree and u are being redistributed on at least a mineral scale and that the initial depositional and provenance information recorded by the whole rock has been disturbed therefore the approach used in this study to carefully characterize the paragenetic sequence from the llandeilo caradoc black shales should be considered necessary in order to fully evaluate the usefulness of trace element proxies susceptible to diagenetic redistribution

archean proterozoic collision boundary in central fennoscandia revisited abstract archean proterozoic ap collision boundary in central fennoscandia is a complex collage of archean basement rocks its paleoproterozoic supracrustal cover rocks and svecofennian 1 93 1 78 ga supracrustal and plutonic rocks the apparent steep nature and complexity of the ap boundary has led to contradictory tectonic models varying from arc continent collision via back arc closure to accretion of arc terranes along a strike slip shear zone we present new geochronology data from detrital zircons combined with whole rock geochemical and sm nd isotopic data on paleoproterozoic sedimentary and volcanic rocks across the boundary to revise its nature the 2 06 ga siilinjarvi volcanic complex and associated turbidites with an inferred deposition age at 2 1 2 06 ga are part of the cover sequence the siilinjarvi volcanic rocks derive from both plume oib and sub continental lithospheric mantle sclm magma sources and are often contaminated by the archean crust the sahara suite sedimentary cover rocks are derived from local 1 95 1 92 ga bimodal volcano plutonic source which source also explains the common age population found in younger sedimentary rocks the nalantojarvi suite psammites with maximum depositional ages of 1 97 1 93 ga are continental slope to deep water sediments deposited at the continental margin contemporaneous mafic magmatism seen as lavas and dykes show a change from sclm dominated present east to asthenospheric depleted mantle dm dominated present west melts the interpretation of the nalantojarvi suite as cover rocks of the lower plate indicates that the archean crust continues at least 50 km more to the west than thought earlier parautochthonous sedimentary rocks from salahmi eastern kansanneva and loutemaki with maximum depositional ages at 1 92 1 91 ga are inferred as collision related foredeep deposits from local sources both the lampaanjarvi suite and pyhajarvi suite sedimentary rocks have deposited at 1 89 1 88 ga or later whereas the lampaanjarvi suite rocks are dominated by a local source and the pyhajarvi suite rocks derive from a mixture of older sediments and 1 89 1 88 ga granitoids at least two tectono metamorphic stages separated with a major unconformity can be differentiated in the study area the early deformation and metamorphic stage in the nalantojarvi suite has occurred before the intrusion of the late to post collisional pyroxene granitoids at 1 88 ga the pyhajarvi suite sedimentary rocks have been deposited after the intrusion of these granitoids between 1880 and 1875 ma and then subsequently deformed and metamorphosed at 1875 ma we propose a tectonic model where the continental breakup occurred at 2 1 2 06 ga followed by a new spreading within the pre existing passive margin at 1 98 1 93 ga during initial continent arc collision the subducted slab steepened strongly at high angle subduction ceased rapidly and finally detached and shallow slab breakoff occurred at 1 92 1 91 ga followed subsequently by ophiolite obduction and a formation of a foreland fold and thrust belt mantle crust decoupling of the archean lithosphere occurred and a crocodile jaw formed where the arc rocks are partly buried beneath the bounding archean continent similar features have been recorded in other ap collision zones which could indicate that the buoyant nature of the archean lithosphere combined with denser paleoproterozoic lithosphere may be a general cause for decoupling of archean crust and mantle during arc continent collision in paleoproterozoic

palaeoproterozoic metasedimentary rocks of the ji an group and their significance for the tectonic evolution of the northern segment of the jiao liao ji belt north china craton in this paper we present new petrological and whole rock geochemical data for the palaeoproterozoic metasedimentary rocks in the upper part of the ji an group within the jiao liao ji belt china as well as zircon u pb age dates and in situ lu hf isotope data the new data improve our understanding of the original nature of the metasedimentary rocks further providing insights into their tectonic setting and the evolutionary history of the northern segment of the jiao liao ji belt the zircons can be divided into two groups viz one of magmatic origin and the other of metamorphic origin zircon u pb dating gave mean or statistical peak ages for the magmatic zircons at 2035 2082 2178 2343 2421 2451 2545 2643 2814 and 2923 3446 ma and mean peak ages for the metamorphic zircons at 1855 and 1912 ma which indicate a maximum depositional age of 2 03 ga and two stage metamorphic events at c 1 91 and 1 85 ga for the metasedimentary rocks geochemical data show that 1 the protoliths of these rocks were mainly sandstones greywackes and claystones together with some shales 2 the main sources of the sedimentary material were palaeoproterozoic granites and acid volcanic rocks with minor contributions from archaean granitic rocks and 3 the sediments were deposited in an active continental margin setting moreover along the northeastern margin of the eastern block of the north china craton there is evidence of ancient crustal materials as old as 3 76 ga and multiple crustal growth events at 3 23 3 05 2 80 2 65 2 54 2 45 and 2 28 2 08 ga

mercury isotopic composition of hydrothermal systems in the yellowstone plateau volcanic field and guaymas basin sea floor rift abstract to characterize mercury hg isotopes and isotopic fractionation in hydrothermal systems we analyzed fluid and precipitate samples from hot springs in the yellowstone plateau volcanic field and vent chimney samples from the guaymas basin sea floor rift these samples provide an initial indication of the variability in hg isotopic composition among marine and continental hydrothermal systems that are controlled predominantly by mantle derived magmas fluid samples from ojo caliente hot spring in yellowstone range in 202hg from 1 02 to 0 58 0 11 2sd and solid precipitate samples from guaymas basin range in 202hg from 0 37 to 0 01 0 14 2sd fluid samples from ojo caliente display mass dependent fractionation mdf of hg from the vent 202hg 0 10 0 11 2sd to the end of the outflow channel 202hg 0 58 0 11 2sd in conjunction with a decrease in hg concentration from 46 6 pg g to 20 0 pg g although a small amount of hg is lost from the fluids due to co precipitation with siliceous sinter we infer that the majority of the observed mdf and hg loss from waters in ojo caliente is due to volatilization of hg0 aq to hg0 g and the preferential loss of hg with a lower 202hg value to the atmosphere a small amount of mass independent fractionation mif was observed in all samples from ojo caliente 199hg 0 13 1 0 06 2sd but no significant mif was measured in the sea floor rift samples from guaymas basin this study demonstrates that several different hydrothermal processes fractionate hg isotopes and that hg isotopes may be used to better understand these processes

paleoproterozoic sediment recycling during multiphase orogenic evolution in fennoscandia the tampere and pirkanmaa belts finland abstract geochronology of detrital zircons and their overgrowths combined with whole rock geochemical and sm nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution this approach is applied to the paleoproterozoic sedimentary rocks of the tampere and pirkanmaa belts southern finland in the center of the composite svecofennian orogen the lower part of the tampere belt succession and bulk of the pirkanmaa belt are characterized by turbidites whereas the upper part of the tampere belt succession is dominated by 1 90 1 89 ga mature arc type volcanic rocks detrital zircon geochronology indicates that the tampere and pirkanmaa belts have a coeval 1 92 1 89 ga depositional and tectonic history ages of pre depositional zircon overgrowths vary from 1 91 ga to 2 0 ga with clusters at 1 92 ga and 1 98 ga within the pirkanmaa belt post depositional zircon overgrowths indicate metamorphic culmination at c 1885 ma in the vammala ni zone and at c 1875 ma in the northern part the lower conglomerates and graywackes in the tampere belt and their equivalents in the pirkanmaa belt are rich both in neoarchean and paleoproterozoic grains the latter ranging in age from 1 9 to 2 1 ga compared to these a sample from the vammala ni zone has an exotic provenance with at least c 1 90 ga 2 04 2 15 ga 2 38 2 48 ga and 2 57 2 63 ga components a sedimentary recycling and tectonic model for the central fennoscandia is proposed in which the paleoproterozoic keitele bergslagen continent was formed during an unnamed orogeny at 1 98 1 97 ga the archean norrbotten microcontinent was attached to the continent at 1 97 1 93 ga upper kaleva turbidites derived from the lapland kola orogen in the north were deposited before 1 92 ga on a passive margin of the archean karelia craton the karelia craton collided with the keitele bergslagen norrbotten continent at c 1 92 ga forming the lapland savo orogen subsequent evolution led to rifting and break up of the latter continent into two microcontinents in the hinterland at 1 92 1 91 ga the rift was developed into a subsiding passive margin of the keitele microcontinent with voluminous turbidite deposition now seen as graywackes in the tampere pirkanmaa and pohjanmaa western finland belts the turbidite material was derived from the rising lapland savo orogen and included recycled upper kaleva recycled sandstones and first cycle 2 03 1 97 ga detritus from keitele 1 93 1 92 ga savo arc material as well as detritus from the archean craton and its cover deposits the collision between karelia and keitele caused a subduction reversal and the onset of tampere arc volcanism at 1 90 ga arc derived materials started to deposit and were mixed with older sedimentary rocks and trench parallel distal turbidites from exotic source were being deposited in the accretionary wedge

delineating ophiolite derived host rocks of massive sulfide cu co zn deposits with 2d high resolution seismic reflection data in outokumpu finland abstractseismic reflection data was applied to a study of the upper crustal structures in the outokumpu mining and exploration area in eastern finland the cu co zn sulfide ore deposits of the outokumpu area are hosted by palaeoproterozoic ophiolite derived altered ultrabasic rocks serpentinite skarn rock and quartz rock and black schist within turbiditic mica schist mining in the outokumpu area has produced a total of 36 mt of ore from three historical and one active mine seismic data comprises 2d vibroseis data surveyed along a network of local roads the seismic sections provide a comprehensive 3d view of the reflective structures acoustic rock properties from downhole logging and synthetic seismograms indicate that the strongly reflective packages shown in the seismic data can be identified as the host rock environments of the deposits reflectors show excellent continuity along the structural grain of the ore belt which allows correlating reflectors with surface geology magnetic map and dri

redistribution of rare earth elements during diagenesis of turbidite hemipelagite mudrock sequences of llandovery age from central wales

long lived lree mobility in the cratonic rift and foredeep to foreland sedimentary cover at the western margin of the karelia province abstract the western margin of the karelia province of the fennoscandian shield consists of archean crust covered by remnants of paleoproterozoic c 2 5 1 9 ga metasedimentary rocks the study area in the eastern part of the margin shows 2 3 2 1 ga cratonic sedimentation followed by 2 1 2 05 ga rift sedimentation and c 1 91 1 92 ga foredeep to foreland sedimentation related to continent arc continent collision extreme lree mobility loss up 95 of la is recorded by many samples from these units monazite bastnasite and in some cases allanite have been the prealteration lree host phases in the lree depleted samples monazite was altered to th silicates with variable loss of the lree the remaining part of which was incorporated in th poor monazite the depositional age of the sedimentary units was studied by dating la mc icpms detrital zircon grains and the age of lree mobility by dating monazite and xenotime and by whole rock sm nd modeling the maximum deposition age of the foredeep to foreland basin is 1 91 1 92 ga based on detrital zircon data and the minimum age of the main foliation in the study area is 1 87 1 86 ga based on the younger age limit of late to post tectonic xenotime and the ages of few probable metamorphic zircons coeval bastnasite with xenotime in a lree enriched sample suggests that this was also one important stage of lree mobility monazite crystallizations have occurred 1 76 1 78 ga ago while the rb sr whole rock data suggest that the closure of this isotope system took place 1732 21 ma ago the sm nd modeling gives ages at 0 4 1 0 ga for lree depletion the sm nd age of 412 27 ma can be considered as the age of the latest major lree depletion event we propose that alkali bearing oxidizing fluids started to form in the cratonic sequences due to diagenetic reactions maybe as early as 2 3 2 1 ga alkalinity of fluids was later increased by interaction with alkaline 2 05 ga source materials before and during the c 1 91 1 92 ga foredeep to foreland basin stage basin inversion and metamorphic crystallization occurred at 1 91 1 87 ga episodes of fluid migration at 1 78 1 73 ga and afterwards were focused along fracture zones and in microfracture networks the 0 4 ga stage is the most prominent of the

isotopic and geochemical constraints on the evolution of the 1 93 1 79 ga svecofennian crust and mantle in finland the central part of the svecofennian domain in finland is characterized by thick lithosphere with crustal thickness up to 60 65 km major collisional events at 1 91 1 90 ga and 1 89 ga and a thrusting event at 1 86 1 84 ga the latter due either to continuation of shortening or to a separate intracrustal collision are considered the major causes of crustal thickening the origin of the svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of archaean crustal component via subduction but the new data and reinterpretation of older data imply that besides juvenile crust the svecofennian domain also contains older palaeoproterozoic crustal components no archaean component is found in the 1 93 1 91 ga gneissic tonalites and related felsic volcanics end t 1 4 which occur in the savo belt sb adjacent to the archaean craton the depleted mantle model ages vary from 1 94 ga to 2 33 ga with most values 2 0 ga the analysed syntectonic post thickening granitoids 1 89 ga in the sb exhibit positive end t values ranging from 2 8 to 3 0 pointing to a juvenile source the hypersthene granitoids are similar in age but show epsilon values around 1 which are only slightly lower than found in em derived mafic rocks the isotopic and geochemical data for the central finland granitoid complex and tampere schist belt indicate the occurrence of evolved thick crust 2 0 ga and associated lithospheric mantle already at 1 91 ga the southernmost part of the southern svecofennian in finland also shows evolved type crust 2 0 ga a tentative model for the development of crust and mantle is presented it includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions two successive collisional events thickened both the crust and the subcontinental lithospheric mantle with the greatest relative amount of thickening occurring in the mobile zones convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere increase in the geotherm and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere mafic magmatism during the post thickening stage 1 89 ga thus varied from dominantly enriched to dominantly depleted mantle melts and was associated with variable crustal contamination the thickening of the lithosphere in the central part of the svecofennian domain was not followed by large extension suggesting that the thickness was already great 1 88 1 80 ga ago this together with the thick high velocity lower crust could have provided the isostatic balance needed to stabilize the thick crust

svecofennian detrital zircon ages implications for the precambrian evolution of the baltic shield metasediments intruded by 1 90 1 87 ga old plutonic rocks form the oldest major proterozoic crustal component in the svecofennian domain of the baltic fennoscandian shield their ndtdm model ages and conventional multigrain zircon u pb ages between 2 4 and 2 1 ga have previously been interpreted either as mixing ages between 1 9 ga old juvenile materials and a minor archaean component or as actual rock and protolith ages to resolve the ensuing controversy 120 individual detrital zircons from svecofennian metasediments in sweden and finland were analysed using the shrimp ion microprobe the oldest materials in this array are a 3 44 ga old zircon from the tampere schist belt in finland and a 3 32 ga old crystal from southeastern sweden about 30 of the analysed crystals are 2 97 2 60 ga old while 65 have ages between 2 12 and 1 88 ga thus there is no evidence of 2 6 2 1 ga old protoliths but the age range of the proterozoic zircons indicates that a major area of 2 1 1 9 ga old crust was in erosional position 1 9 ga ago this implies that the formation of palaeoproterozoic crust in the baltic shield or its one time close neighbourhood must have commenced 100 200 ma earlier than hitherto assumed in conjunction with previously obtained isotopic data the youngest detritus ages of the present study constrain the age of svecofennian sedimentation it can also be concluded that the archaean zircons found in quartzites from southern sweden may have been derived from source areas to the southwest of the central svecofennian marine depositional basin the so called bothnian basin separating southern sweden from the archaean craton in the northeastern part of the shield

structure of the outokumpu ore district and ophiolite hosted cu co zn ni ag au sulfide deposits revealed from 3d modeling and 2d high resolution seismic reflection data abstract the outokumpu district within the north karelia schist belt in eastern finland hosts a number of cu co zn ni ag au sulfide deposits that are associated with palaeoproterozoic ophiolitic metaserpentinites derived from depleted mantle peridotites that were subsequently tectonically interleaved with allochthonous metaturbidites the metaperidotites have been extensively metasomatized to quartz carbonate calc silicate rocks of the outokumpu assemblage the outokumpu area has been affected by a multiple phase tectonic history comprising various phases of folding and shearing followed by several faulting events future exploration has to expand the search into deeper areas and requires knowledge of the subsurface geology in order to unravel the complex structure 3d geologic models of different scales have been built using a variety of information including geological aeromagnetic and gravity maps digital terrain models and mine cross sections as well as data like drill core logs combined with observations from underground mine galleries structural measurements aeromagnetic data and seismic surveys for crustal structures data from seismic surveys lines have been reprocessed for our purpose both deposit scale and regional scale models allow the reconstruction of a sequence of structural events the mined ore has formed during remobilization of a proto ore and is closely related to shear zones thrusts that truncate the outokumpu assemblage later faults dismembered the ore explaining the variable depth of the different ore bodies along the outokumpu ore zone on larger scale at least four km scale thrust sheets separated by major listric shear zones can be identified in the ore belt which are internally further imbricated by subordinate shear zones these thrusts separate a number of lens shape metaperidotite bodies that are probably surrounded by outokumpu assemblage rocks thrust stacking was followed by at least three stages of faulting that divided the ore belt into fault bounded blocks with heterogeneous displacements i faulting along nw dipping faults with unresolved kinematics ii reverse faulting along c 50 60 se dipping faults and iii sw ne to ssw nne striking faults which may have formed at an earlier stage and have been reactivated the specific outokumpu alteration assemblage around metaperidotite bodies combined with shear zones acting as path ways for fluids are the main vectors to mineralization seismic reflection data do not provide a simple tool to directly detect the sites of outokumpu assemblage bodies at depth but they identify strong reflector zones which are characteristic for though not exclusive to the assemblage shear zones can be recognized as curved dislocations in the seismic lines our study shows that 3d modeling when used in combination with surface geology and other geophysical data and good knowledge about the structural evolution clearly improves the interpretation of reflectors and enables the identification of strong reflector packages as outokumpu assemblage that due to absent geological control have first been mapped as unknown reflector it thus enhances the chances for locating potentially economic horizons at depth and to delineate target areas for detailed exploration

characterisation and isotopic evolution of saline waters of the outokumpu deep drill hole finland implications for water origin and deep terrestrial biosphere abstract the isotopic composition of water and dissolved sr as well as other geochemical parameters at the 2516 m deep outokumpu deep drill hole finland were determined the drill hole is hosted by palaeoproterozoic turbiditic metasediments ophiolite derived altered ultramafic rocks and pegmatitic granitoids sodium ca cl and ca na cl rich waters total dissolved solids up to ca 70 g l 1 containing significant amounts of gas mainly ch 4 up to 32 mmol l 1 n 2 up to 10 mmol l 1 h 2 up to 3 1 mmol l 1 and he up to 1 1 mmol l 1 discharge from fracture zones into the drill hole this water is distinct from the shallow fresh groundwater of the area and has an isotopic composition typical of shield brines that have been modified during long term water rock interaction based on water stable isotopes and geochemistry the drill hole water profile can be divided into five water types each discharging from separate fracture systems and affected by the surrounding rocks the 2 h varies from 90 to 56 vsmow and 18 o from 13 5 to 10 4 vsmow plotting clearly above the global and local meteoric water lines on a 2 h vs 18 o diagram the 87 sr 86 sr ratios range between 0 72423 and 0 73668 simple two component mixing between 2 h and 18 o rich end member brine and meteoric water cannot explain the water stable isotopic composition and trends observed instead hydration of silicates by ancient groundwaters recharged under different climatic conditions warmer than at present is the most likely mechanism to have caused the variation of the 2 h and 18 o values water types correlate with changes in microbial communities implying that different ecosystems occur at different depths the different water types and microbial populations have remained isolated from each other and from the surface for long periods of time probably tens of millions of years

multiple causes of earth s earliest global glaciation in the context of earth system evolution the causal factors driving the palaeoproterozoic huronian global glaciations occupy a central position the rise of o2 at 2320 ma which eliminated most of the methane was not apparently a single cause triggering the glaciation at c 2450 ma mantle plume driven continental uplifts led to the emplacement of voluminous continental flood basalts in low latitudes that were subsequently dissected by rifting major topographical features and continental drainage patterns were most likely similar to those in younger continental flood basalt provinces and would have enabled deep weathering and erosion of extensive basalt covered continental areas intense consumption of atmospheric co2 by silicate weathering of fresh basaltic surfaces would have been further amplified by a constant organic carbon burial rate these factors similar to those of younger glacial periods in combination with the diminished ch4 greenhouse led to an onset of global cooling at the million year timescale

new geochronological and sm nd constraints across the pajala shear zone of northern fennoscandia reactivation of a paleoproterozoic suture abstract altogether 1130 u pb analyses on zircons from 25 samples across the pajala shear zone in northern fennoscandia were performed by la icpms methodology to study the origin of the zone the samples were also analyzed for their sm nd isotope systematics and geochemically the new data is tested with existing geological information to elaborate the provenance and maximum depositional ages the stratigraphic position of studied samples and finally to assess the tectonic evolution in the pajala shear zone area chromium enriched metasedimentary rocks are found within and to the east and west of the n s trending pajala shear zone no material originating from proterozoic felsic igneous zircons or mafic sm nd rocks was found in these rocks and thus only neoarchean or very early paleoproterozoic ages of deposition are possible our new results indicate that the bimodal 1 99 1 97 ga supracrustal rocks of the rovaniemi supersuite cover a large area east of the pajala shear zone based on the absence of archean zircons typical of the metasedimentary rocks covering the archean basement it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the rovaniemi supersuite the metasedimentary rocks of the uusivirka supersuite located within the pajala shear zone vary from hornblende bearing metasedimentary rocks metapelite and meta arkose to orthoquartzite their characteristic features include a dominant proterozoic zircon population 65 87 with a high proportion of analyzed grains yielding ages between 1 96 ga and 1 91 ga and maximum depositional ages of 1 92 1 91 ga we record at least three metamorphic events at ca 1 92 1 90 ga ca 1 85 ga and at 1 82 1 78 ga of which the youngest can possibly be separated into two events at 1 83 1 82 ga and 1 79 1 77 ga respectively we propose a tectonic model in which continental breakup occurred at 2 1 2 05 ga followed by the development of a magmatic arc kittila arc at ca 2 0 ga above a subduction zone two archean continental units norrbotten and karelia collided at ca 1 92 1 91 ga the kittila arc was obducted upon the karelia continent as a foreland fold and thrust belt and the metasedimentary rocks of the uusivirka supersuite were emplaced as foreland deposits the proposed tectonic model is based on the absence of 2 44 ga layered intrusions and especially 2 06 ga rocks and 1 99 1 97 ga rocks of the rovaniemi supersuite that are found only east of the pajala shear zone in our model the pajala shear zone originated as a divergent plate boundary and was multiply reactivated after continental collision with both lateral and vertical movements

pyritic event beds and sulfidized fe oxyhydr oxide aggregates in metalliferous black mudstones of the paleoproterozoic talvivaara formation finland the paleoproterozoic 2 0 1 9 ga talvivaara formation of finland was deposited during the shunga event a worldwide episode of enhanced accumulation of organic rich sediments in the aftermath of the lomagundi jatuli carbon isotope excursion sulfidic carbonaceous mudstones in the talvivaara formation contain one of the largest known shale hosted nickel deposits in order to gain new insight into this shungian sedimentary environment sedimentological petrographical and in situ s and fe isotopic microanalyses were carried out on samples representing depositional and early diagenetic conditions the event bedded lithology with tidal signatures in the organic rich mudstones strongly indicates deposition from predominantly river delivered mud on a highly productive coastal area below storm wave base the riverine supply of phosphorus sulfate and iron supported high primary productivity and resulted in strong lateral and vertical chemical gradients in the nearshore waters with a shallow oxic surface layer underlain by euxinic water the stratigraphic upper part of the talvivaara formation contains banded intervals of thin alternating pyrite beds and carbonaceous mudstone beds the pyrite beds were deposited by seaward excursions of the concentrated acidic fe rich river plume subsequent to droughts or dry seasons which led to intense pyrite precipitation upon mixing with euxinic waters s34 and fe56 values of the bedded pyrite median s34 10 3 and fe56 0 79 are consistent with the reaction of dissolved fe ii with h2s from bacterial sulfate reduction organic rich clayey fe monosulfide bearing granules were transported from the muddy estuary and enclosed in fe oxyhydr oxide aggregates that were forming by wave and current reworking in nearshore accumulations of river delivered iron the isotopic composition of these presently pyrrhotitic inclusions median s34 3 3 and fe56 1 6 indicates microbial iron reduction the fe oxyhydr oxide aggregates were transported in muddy debris flows to the distal euxinic seafloor their fe oxyhydr oxide matrix was replaced by pyrite median s34 5 8 and fe56 0 81 at shallow sediment depths with 34s and 56fe enriched porewater wavy crinkly laminae of possible microbial origin developed on the euxinic seafloor during low sedimentation these results indicate episodic deposition at seasonal to multiannual time scales s34 and fe56 values in the studied fe sulfides provide evidence of microbial isotope fractionation processes and syndepositional and early diagenetic origin finding no support for the previously proposed local hydrothermal activity in the talvivaara mudstones

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

geoid anomalies at fracture zones and thermal models for the oceanic lithosphere geoid anomalies derived from the geosat altimeter data set have been used to model the change in the geoid height across four pacific fracture zones after accounting for geoid signals from the flexural response to differential subsidence and thermal bending stresses we find that the estimates of geoid steps across the fracture zones are compatible with existing conductive models for the oceanic lithosphere if flexure is not accounted for the geoid steps will be underestimated by 30 40 departures from the predicted conductive trends may be explained by small scale convection and uncertainties in the crustal ages

noble gas residence times of saline waters within crystalline bedrock outokumpu deep drill hole finland abstract noble gas residence times of saline groundwaters from the 2516 m deep outokumpu deep drill hole located within the precambrian crystalline bedrock of the fennoscandian shield in finland are presented the accumulation of radiogenic 4 he 40 ar and nucleogenic 21 ne noble gas isotopes in situ together with the effects of diffusion are considered fluid samples were collected from depths between 180 and 2480 m below surface allowing us to compare the modelled values with the measured concentrations along a vertical depth profile the results show that while the concentrations in the upper part are likely affected by diffusion there is no indication of diffusive loss at or below 500 m depth furthermore no mantle derived gases were found unequivocally previous studies have shown that distinct vertical variation occurs both in geochemistry and microbial community structuring along the drill hole indicating stagnant waters with no significant exchange of fluids between different fracture systems or with surface waters therefore in situ accumulation is the most plausible model for the determination of noble gas residence times the results show that the saline groundwaters in outokumpu are remarkably old with most of the samples indicating residence times between 20 and 50 ma although being first order approximations the ages of the fluids clearly indicate that their formation must predate more recent events such as quaternary glaciations isolation within the crust since the eocene miocene epochs has also direct implications to the deep biosphere found at outokumpu these ecosystems must have been isolated for a long time and thus very likely rely on energy and carbon sources such as h 2 and co 2 from groundwater and adjacent bedrock rather than from the ground surface

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

new constraints for the source characteristics deposition and age of the 2 1 1 9 ga metasedimentary cover at the western margin of the karelian province abstract the paleoproterozoic cover sequence at the 100 150 km wide western margin of the archean karelian province is dominated by deep water lower and upper kaleva metasediments we present here an interpretation of sm nd isotope and geochemical data on 36 samples tims multi grain u pb zircon analyses on nine samples and ca 100 sims analysis of detrital zircon grains from four upper kaleva and one lower kaleva samples the lower kaleva is characterized by autochthonous parautochthonous lithologically heterogeneous metaturbidites showing common enrichment in quartz all the analysed detrital zircons are of a local neoarchean source but tdm variation up to 2 4 ga combined with geochemical data indicate abundant mixing of paleoproterozoic mafic material presumably from 2 1 ga plateau lavas and dykes in most of the lower kaleva samples the upper kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies and it is characterized by lithological and geochemical isotopic homogeneity geochemical isotopic and detrital zircon data favour material derived from an orogenic domain comprising both archean and proterozoic units followed by effective mixing during the transport the archean zircon grains 25 are mostly neoarchean the paleoproterozoic grains lack zircons at 2 5 2 2 ga and plot dominantly 92 between 1 92 and 2 05 ga the indicated maximum deposition ages vary from 1 95 1 94 ga to 1 92 ga the main source area proposed is the himalaya type lapland kola orogen now in the northeast which experienced mountain building and erosion at 1 95 1 91 ga the western margin of the karelian province shows evidence of rifting and lithosphere thinning from 2 1 to 1 95 ga but it is still under debate whether the craton breakup occurred at 2 06 ga in a volcanic or later at 1 95 ga in a non volcanic margin setting one hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre existing passive margin at 1 97 1 95 ga thus both a volcanic margin at 2 06 ga and a non volcanic margin at ca 1 95 ga could have been operated at the western margin of the karelian province

sm nd isotope evidence of a collisional event in the precambrian of northern sweden abstract the sm nd isotope characteristics of c 1 9 and c 1 8 ga granitoids occurring close to the edge of the archaean craton in northern sweden have been studied strongly negative e nd t values occur farther southwest in the potassic 1 8 ga granitoids than in the calc alkaline 1 9 ga granitoids intrusions of the younger granitoids with distinctly negative e nd t values are surrounded by granitoids of the older group with positive values northeast of the 1 8 ga granitoids discussed here the c 1 9 ga calc alkaline intrusions formed within the archaean continent have the same sm nd isotope characteristics as the 1 8 ga granitoids to the southwest our interpretation is that a juvenile volcanic arc terrane to the south collided with and was thrust onto the archaean continent after the formation of the 1 9 ga granitoids the outcome was that these juvenile rock masses were obducted onto the reworked archaean continent the 1 8 ga granitoids were formed by remobilisation of continental crust partial melting at 1 8 ga resulted in the intrusion of granitoids carrying the sm nd isotopic signature of the archaean continent into juvenile rocks it is probable that the collision discussed here was part of a major accretion and amalgamation to the archaean craton after the formation of the calc alkaline 1 9 ga granitoids but before the formation of the 1 8 ga granitoids

tectonic styles of early proterozoic ore deposition in the fennoscandian shield the early proterozoic metallogenic events of the fennoscandian shield are connected with two major stages of crustal evolution the karelian continental development 2 5 2 1 ga and the svecofennian orogeny 2 0 1 75 ga the karelian continental development was dominated by deposition of terrestrial sediments and tholeiitic volcanism on archaean crust a tensional regime probably associated with a mantle plume and partial melting of the upper mantle culminated in the generation of an ene wsw trending rift zone which was intruded by tholeiitic magma 2 44 ga ago some 15 layered igneous bodies hosting cr pge ni cu and fe v ti deposits are known in the kemi suhanko and koillismaa belts these mafic ultramafic bodies originally formed either several lopoliths or one single layered igneous complex with a minimum length of 260 km nw se and nnw sse trending dolerite dykes intruded the rifted and attenuated archaean crust 2 25 2 0 ga ago preceding ocean opening the coeval ne sw trending dolerite dykes in the northern part of the shield are interpreted as manifestation of an aulacogen the failed arm of a triple armed rift system oceanic crust is preserved in 1 96 ga old ophiolite complexes the cu co zn ni and cr contents of the outokumpu type deposits derive from hydrothermal leaching of oceanic crust and were discharged together with chemogenic cherts and carbonates from ascending hydrothermal fluids on the seafloor above fissure vents in the northern part of the shield rifting and probably ocean opening resulted in the formation of the layered tholeiitic sedimentary complex of the petsenga ni cu district intruded by mineralized mafic ultramafic magma the age of this event is poorly constrained the basin was subsequently closed and continental collision of the kola suture was accompanied by southward obduction of the lapland granulite belt on the archaean crust during the svecofennian orogeny the divergent continental margin was converted into an active margin early subduction of oceanic crust produced oceanic island arcs intruded by 1 93 1 9 ga old tonalites and trondhjemites subsequent subduction of oceanic crust generated 1 9 1 87 ga old continental island arcs associated with massive sulphide cu zn pb ag au deposits the island arcs were accreted to the archaean crust in a concave belt containing the ore districts of kiruna skellefte vihanti pyhasalmi aijala orijarvi and bergslagen bimodal volcanic suites with felsic calc alkaline and mafic tholeiitic volcanites are attributed to rifting of the overriding plate above subduction zones ni cu deposits in synorogenic 1 9 1 87 ga old mafic ultramafic intrusions occur in linear fault controlled belts at the margin of the archaean crust the generation of ni bearing tholeiitic to komatiitic magma is explained by partial melting of the mantle wedge above the early subduction zone magma differentiation and magma upwelling along transcurrent faults on the continental margin sulphur was probably assimilated from the supracrustal wall rocks 1 89 1 87 ga old early svecofennian granitoids are associated with porphyry type cu mo au deposits widespread late svecofennian s type 1 84 1 75 ga granitic magmatism generated minor mo u w sn and ree deposits emphasis is laid on the importance of geotectonic models as a basis for further exploration in the fennoscandian shield

accretion tectonism in the proterozoic svecokarelides of the baltic shield the duality of early middle proterozoic terranes in the baltic shield reflects fundamental differences in history between coeval mobile belts and the manner of their interaction the karelides formed on a continent ocean transcurrent margin in which offsets developed as arcs and back arc basins on stretched continental crust at least one back arc basin was obducted onto the continental margin preserved as an allochthonous terrane in the karelian nappe pile remnant arc fragments form accreted exotic terrane in pohjanmaa the svecofennides developed as a succession of island arcs while the site of subduction moved south and successive extinct arcs were accreted onto the archean craton arc terrane impingement affected deformation style in the karelides which changed from one of major horizontal translation to one dominated by major dextral wrench faults analogies can be drawn with the phanerozoic evolution of the northeast pacific margin

how mantle slabs drive plate tectonics the gravitational pull of subducted slabs is thought to drive the motions of earth s tectonic plates but the coupling between slabs and plates is not well established if a slab is mechanically attached to a subducting plate it can exert a direct pull on the plate alternatively a detached slab may drive a plate by exciting flow in the mantle that exerts a shear traction on the base of the plate from the geologic history of subduction we estimated the relative importance of pull versus suction for the present day plates observed plate motions are best predicted if slabs in the upper mantle are attached to plates and generate slab pull forces that account for about half of the total driving force on plates slabs in the lower mantle are supported by viscous mantle forces and drive plates through slab suction

palaeoproterozoic 2 0 1 95ga pre orogenic supracrustal sequences in the west troms basement complex north norway abstract new u pb data from two supracrustal belts enclosed in archaean crust in the west troms basement complex wtbc of north norway document the presence of volcanic subvolcanic and sedimentary rocks formed in the pre orogenic period of extension and basin formation after 2 0 ga the mjelde skorelvvatn belt of highly deformed supracrustal rocks is dominated by mafic metavolcanic rocks including metagabbro dated by zircon at 1992 2 ma these mafic rocks are temporal correlatives of a number of occurrences in fennoscandia and laurentia such as the pilguyarvi unit in the pechenga structure in the kola peninsula and the purtuniq unit in northern labrador the torsnes belt contains siliciclastic and metavolcanic mafic rocks unconformably overlying neoarchaean crust the indicated maximum deposition age of the torsnes belt is given by the age of 1970 14 ma for the youngest detrital zircon the absence of 1 95 ga and younger detrital zircons which are abundant in svecofennian siliciclastic sequences in fennoscandia suggests that the torsnes belt is a distinct pre orogenic deposit and we interpret it as a marginal continental or intracontinental siliciclastic volcanic basin the new data from the wtbc can be placed in the context of continental divergence and sea floor formation in the period between 2 0 ga and the commencement of orogeny in fennoscandia and laurentia at 1 92 ga

sr and nd isotopic evidence for punctuated clay diagenesis texas gulf coast we report rb sr sm nd and mineralogical data for leached whole rock 1200 m independent of the duration of burial the exchangeable sr in the clay has remained an open system and has undergone isotopic exchange since that time minimum fluid rock ratios of 0 2 to 2 are modeled from sr isotopic data consistent with rock dominated open system diagenesis leachates and residues of the 0 1 zm fraction below the smectite illite transition appear to have achieved nd isotopic equilibrium during clay diagenesis and show no signs of subsequent exchange the clay residues have low 147sm 144nd 0 085 indicating that authigenic illite has a lower sm nd ratio than its bulk rock protolith conversely the coexisting exchangeable ree inventory has elevated sm nd as reflected by leachate 14vsm 144nd as high as 0 188 this sm nd fractionation in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid rock interaction in older rocks

provenance of early proterozoic and archaean metasediments in finland a smnd isotopic study abstract sm nd isotopic analyses of early proterozoic and archaean clastic metasediments from finland are presented they have been used to calculate crustal residence ages and to constrain the sedimentary provenance the mean crustal residence age t dm for the svecofennian metagreywackes in the tampere schist belt is 2 22 ga which is about 0 3 ga in excess of the assumed stratigraphic age this indicates that the bulk of the sediments were eroded from newly formed crust their major source was most likely svecofennian orogenic material mixed with a small amount of archaean derived detritus the precursors of many granitoids in the svecofennian terrain also had similar short crustal residence times the kalevian metasediments which were deposited 2 05 1 9 ga ago on the edge of the archaean craton in eastern and northern finland have a mean t dm of 2 41 ga which is 0 4 0 5 ga more than the inferred stratigraphic age hence their source was on average older than that of the svecofennian metasediments investigated but definitely not purely archaean neither could crust similar to the bulk of the 1 9 ga old svecofennian terrain have been the only source various sources contributed at least locally but the bulk of the sedimentary material probably had a common provenance major sources consistent with existing isotopic data could have been 2 1 ga old jatulian rocks the c 2 0 ga old granulite belt in northern finland and or the svecokarelian early orogenic arc terrain mixed with archaean derived nd the method and data do not allow a distinction to be made between the possible alternatives the t dm model ages for two analysed archaean metasediments are slightly over 3 ga which is about 0 3 ga in excess of their assumed stratigraphic age thus providing no evidence for crust much older than 3 ga in this area

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

paleoproterozoic mafic dyke swarms of the belomorian province eastern fennoscandian shield abstract paleoproterozoic mafic igneous rocks 2450 1970 ma are exposed in the form of layered intrusions dykes and volcanic rocks in the karelian kola and murmansk provinces and in the form of dykes and small intrusions in the belomorian province eastern fennoscandian shield the age and sequence of mafic dyke emplacement during the paleoproterozoic are very similar in these regions further comparisons of geochemical characteristics of mafic dyke swarms in the belomorian province and neighboring cratons show considerable similarities at least three distinct groups of paleoproterozoic mafic dykes exist in the belomorian province of the eastern fennoscandian shield they are i early mg tholeiitic dykes ii ca 2 44 2 36 ga high mg gabbronorites and iii ca 2 12 fe tholeiitic dykes research into mafic dykes of the belomorian province and their comparison with paleoproterozoic dyke swarms of the karelian kola and murmansk provinces has provided evidence for at least two extensive shared magmatic events dated to ca 2 5 2 4 ga and ca 2 1 ga in the paleoproterozoic in the eastern fennoscandian shield confirming that the belomorian province does not represent a paleoproterozoic suture but rather a deep eroded inner part of the lapland kola orogen foreland

sources of mercury to san francisco bay surface sediment as revealed by mercury stable isotopes abstract mercury hg concentrations and isotopic compositions were examined in shallow water surface sediment 0 2 cm from san francisco sf bay to determine the extent to which historic hg mining contributes to current hg contamination in sf bay and to assess the use of hg isotopes to trace sources of contamination in estuaries inter tidal and wetland sediment had total hg hgt concentrations ranging from 161 to 1529 ng g with no simple gradients of spatial variation in contrast inter tidal and wetland sediment displayed a geographic gradient of 202hg values ranging from 0 30 in the southern most part of sf bay draining the new almaden hg district to 0 99 in the northern most part of sf bay near the sacramento san joaquin river delta similar to sf bay inter tidal sediment surface sediment from the alviso slough channel draining into south sf bay had a 202hg value of 0 29 while surface sediment from the cosumnes river and sacramento san joaquin river delta draining into north sf bay had lower average 202hg values of 0 90 and 0 75 respectively this isotopic trend suggests that hg contaminated sediment from the new almaden hg district mixes with hg contaminated sediment from a low 202hg source north of sf bay tailings and thermally decomposed ore calcine from the new idria hg mine in the california coast range had average 202hg values of 0 37 and 0 03 respectively showing that hg calcination fractionates hg isotopes resulting in hg contamination from hg ii mine waste products with higher 202hg values than metallic hg 0 produced from hg mines thus there is evidence for at least two distinct isotopic signals for hg contamination in sf bay hg associated with calcine waste materials at hg mines in the coast range such as new almaden and new idria and hg 0 produced from these mines and used in placer gold mines and or in other industrial processes in the sierra nevada region and sf bay area

rifting of africa and pattern of mantle convection beneath the african plate abstract the mesozoic and cenozoic tectonic setting of the african continent is characterized by an extensional stress regime as is evident from the evolution of the west central and east african rift systems the opening of the indian and atlantic oceans and the development of tethys domain as well as from the results of geophysical investigations the most prominent feature of the bouguer gravity anomaly map of africa is a great belt of negative bouguer anomalies which extends across the entire african continent from ethiopia in the east to benguela in the west in conjunction with the red sea spreading zone this belt appears to encircle a centre in cameroon it probably corresponds to a distinct zone of lithospheric thinning and extension the entire african continent is characterized by higher than normal elevations it is located inside the african residual geoid high earthquake fault plane solutions confirm the extensional character of the present tectonic regime of eastern and southern africa where normal faulting predominates the average orientation of t axes is n s in south africa nw se in zambia mozambique and madagascar e w in kenya and ethiopia and ne sw in the red sea region from north to south the orientation of t axes rotates systematically in a clockwise sense by 130 and describes a radial pattern centred on equatorial west africa this indicates that the african lithosphere has been subject to continent wide radial extension around a hypothetical west african spreading centre referred to as the african spreading centre a asc a which coincides with the geographical centre of the african plate shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements in the case of africa the continent wide regularities in the pattern of lithospheric extension the elevated position of the continent as well as the evolution of the mid oceanic ridge system surrounding africa provide distinct boundary conditions for the pattern of large scale mantle flow beneath the african plate it is proposed that relatively warm and less dense mantle material rises below africa forming a single mega plume or a number of plumes at the base of the african lithosphere the ascending material diverges and flows radially away from the centre of ascent corresponding to the asc a this ascending flow forms part of a very large mantle convection cell which occupies the eurafrican hemisphere the descending branch of this convection cell is located at a distance of 80 90 from the asc a the mid oceanic ridges surrounding the african continent are located between the upwelling and the downwelling branches of this convection cell at a distance of 50 60 from the asc a upper mantle flow below the sea floor spreading axes is uni directional and horizontal and is directed away from the asc a

plate scale potential energy distributions and the fragmentation of ageing plates abstract geoid anomalies associated with mid ocean ridge systems and a number of continental margins imply that on the scale of individual plates the old ocean lithosphere represents a gravitational potential energy sink since lateral variations in potential energy contribute to deviatoric stresses in the lithosphere the changing potential energy distributions in individual plates associated with the growth and ageing of the oceanic lithosphere may be expected to result in changes in the intraplate stress field analytical models for simple plate geometries using lithospheric density models consistent with small positive 6 m geoid anomalies across continental margins show that the growth of oceanic lithosphere over a period of 200 ma may contribute to a decline in the mean plate potential energy u p of about 1 10 12 n m 1 and thus contribute a mean extensional stress difference zz xx in continental lithosphere of up to about 8 mpa averaged over a 125 km thick lithosphere these estimates are sensitive to the assumed mean continental potential energy u c about which there is some uncertainty for higher u c approaching that of the mid ocean ridges u mor the net decline in u p may be as much as 1 7 10 12 n m 1 whereas for significantly lower u c approaching that of old ocean lithosphere plate growth may increase u p transiently by up to 2 7 10 12 n m 1 leading to compression in the continents in the african and antarctic plates the ageing of the ocean lithosphere since the late jurassic is estimated to have contributed to a decline in u p of about 0 6 10 12 and 0 95 10 12 n m 1 respectively contributing a mean stress difference of about 5 mpa and 7 5 mpa in the respective continents whereas the predicted stress changes associated with ageing of the oceanic lithosphere are significantly smaller than most laboratory based estimates of the extensional strength of the lithosphere they may provide an important contribution to the stress fields that eventually lead to the fragmentation of ageing plates

mass independent fractionation of mercury isotopes in the environment 1 the toxicity of mercury s methylated species and its biomagnification in aquatic food chains and global dispersion by the atmosphere are the cause of worldwide health problems recent reports have observed natural mass dependent fractionation in mercury isotopes and recent theoretical work has demonstrated that isotopic separation in mercury is due primarily to nuclear field shifts nuclear volume effect and the magnetic spin effect gives rise to mass independent fractionation mif of odd neutron number isotopes now we present analytical evidence of mass independent isotopic variations in mercury produced by both nuclear volume and magnetic isotope processes even mass number isotopes exhibit a pattern indistinguishable from that produced by mass dependent fractionation with both positive and negative 199hg and 201hg anomalies mif is easier to reliably determine in hg isotopes than mass dependent fractionation alone and thus it provides a potentially important key in constraining models of mercury sources and pathways in the environment

zircon u pb and lu hf isotopic and whole rock geochemical constraints on the protolith and tectonic history of the changhai metamorphic supracrustal sequence in the jiao liao ji belt southeast liaoning province northeast china abstract the changhai metamorphic supracrustal rocks located in the eastern central part of the jiao li ji belt in the north china craton ncc are composed mainly of various garnet mica schists along with minor quartzites and marbles this study presents whole rock major and trace element data zircon u pb dates and hf isotope data for these rocks in order to constrain their protolith age and provenance and discuss the tectonic implications geochemical results indicate that the source rocks were mainly granitoids with a possible minor contribution from clastic sediments with an active continental margin signature detrital zircons have u pb age peaks at approximately 1887 2174 2552 2765 and 3212 ma hf values of 11 1 to 13 0 and three major time windows of average continent crustal model ages t dm c of 2 04 2 33 2 48 2 56 and 2 72 2 93 ga besides these units also contain significant numbers of concordant metamorphic zircons that yielded a peak age of ca 248 ma indicating that the region was modified by an early triassic tectono thermal event these results suggest that sediments of the changhai metamorphic supracrustal rocks were mainly sourced from nearby basement granitoid rocks and to a lesser extent from paleoproterozoic metamorphosed strata such as the north and south liaohe groups furthermore the source rocks of the magmatic zircons analyzed in this study appear to have originated from interaction between old continental crust and juvenile material the youngest concordant zircon age peak at 1879 ma coincides with the timing of formation of regionally widespread granitoids mafic igneous rocks and metamorphism of the south liaohe and ji an groups in the jiao liao ji belt and suggest that the sedimentary protoliths of the changhai metamorphic supracrustal rocks were deposited after this time the results indicate that the archean liaobei jinan complex in the north and the liaonan nangrim complex in the south were already a single continental block by 1887 ma and that the changhai metamorphic supracrustal rocks were deposited at an active continental margin

ni cu pge deposits in finland geology and exploration potential abstract several types of nickel sulphide deposits occur in finland some of them are also important sources of cu co and pge the main deposit types are related to archaean and palaeoproterozoic mafic to ultramafic rocks palaeoproterozoic metamorphosed black shales of the talvivaara type or the ultramafic and metasomatic calc silicate rocks of the ophiolitic outokumpu association several komatiite hosted ni cu pge deposits have been discovered in the archaean and paleoproterozoic greenstone belts in northern and eastern finland but only one has been briefly mined in 1989 many 2 44 ga palaeoproterozoic mafic ultramafic layered intrusions in finland host contact type pge ni cu deposits and significant reef type pge deposits typically the ni resources are of low grade but large tonnage the mafic ultramafic magmatism at ca 2 06 ga produced significant ni cu pge resources in the central lapland greenstone belt including the kevitsa and sakatti deposits the svecofennian domain in central and southern finland contains hundreds of small ca 1 88 ga mafic ultramafic orogenic intrusions many of these host nickel mineralization nickel exploration potential was evaluated in general based on identification of critical factors within a mineral system framework and a quantitative assessment of the potential occurrence of undiscovered nickel resources the mgo content of the parental magma was estimated for 28 deposits 6 komatiite hosted occurrences and 22 svecofennian orogenic intrusions the estimated mgo content of the parental magmas for the selected deposits varies from around 7 to 15 in basaltic to komatiitic basalt systems and up to 30 in more magnesian komatiitic systems the ni cu value varies mostly between 0 7 and 12 correlating in general positively with the mgo content of the magma exceptionally high or low ni cu values in some deposits may result from for example copper loss during post magmatic processes serpentinization and or metamorphism or part of the sulphide ore zoned by the ni cu value still being unknown recent advances in understanding of komatiites in finland indicates high exploration potential for nickel within the paleoproterozoic komatiitic formations which typically are also significantly enriched in cu and pge however the quantitative assessment of undiscovered nickel resources in finland suggests that the contact type ni cu pge deposits hosted by 2 44 ga mafic ultramafic layered intrusions in northern finland have the greatest potential for hosting undiscovered resources among the assessed nickel deposit types these occur within restricted areas resulting in a high deposit density finally considerable nickel resources might exist in undiscovered talvivaara kevitsa or sakatti type deposits these deposit types should be assessed as soon as more geological information becomes available