scapolite a tracer for the initial lead isotopic composition in sulfide deposits with later additions of radiogenic lead the initial lead isotopic composition of metamorphosed and tectonically reworked sulfide deposits is not always preserved as sulfides easily change their lead isotopic composition through incorporation of lead derived from external fluids or redistribution and recrystallization of the deposit sulfide trace lead and in cases even galena lead from such deposits may show exceedingly radiogenic lead isotopic compositions thus the initial lead isotopic composition has to be estimated from other minerals scapolite which is a common phase in alteration haloes associated with epigenetic sulfide deposits in northern sweden has very low uranium contents therefore its trace lead contents could preserve the initial isotopic composition of the ore forming fluids as scapolite is more resistant to recrystallization it is more likely to reflect the original lead isotope signature of the deposit this is illustrated using scapolite and sulfides from the pahtohavare cu au deposit in northern sweden which is hosted by palaeoproterozoic mafic tuffites and graphitic schists and was affected by a mild thermal metamorphism during the caledonian orogeny

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

europa s dynamic icy crust abstract europa s icy crust records active resurfacing by tectonic and thermal processes over tens of millions of years as rapidity demonstrated by a paucity of craters tidal working causes rotational torque surface stress internal heating and orbital evolution which can explain the formation of observed tectonic crack patterns ridges crustal displacement and chaotic terrain by processes involving connections between the surface and the underlying ocean through cracks melt sites and occasional impacts these processes were recent and thus most likely continue today the permeability of the crust allows exchange of materials including oxidants and exogenic organics from the surface and endogenic substances from the ocean such that a habitable biosphere might extend to within a few centimeters of the surface continual changes in environmental conditions in the ice crust such as deactivation of individual cracks after thousands of years due to non synchronous rotation and crustal thawing releasing any trapped organisms could provide drivers for biological adaptation as well as opportunity for evolution

lead mobilization during tectonic reactivation of the western baltic shield abstract lead isotope data from sulfide deposits of the western part of the baltic shield define mixing lines in the 206 pb 204 pb 207 pb 204 pb diagram lead from two types of sulfide deposits have been investigated 1 1 exhalative and volcanogenic deposits that are syngenetic with their host rocks 2 2 vein deposits the syngenetic deposits locally show a very wide range of lead isotopic compositions that reflect a variable addition of highly radiogenic lead while the vein deposits although they have radiogenic lead isotopic compositions exhibit only limited isotopic variations in different provinces of the shield both types of deposits fall on the same lead mixing array the slope of the lead mixing lines varies as a function of the age of basement rocks and the age of the tectonic event which produced the lead mobilization and therefore relates the source rock age with the age of lead mobilization calculated mixing ages fall into several short time periods that correspond either to orogenic events or to major phases of continental rifting the orogenic events are the ca 360 430 ma caledonian ca 900 1100 ma sveconorwegian and the ca 1800 1900 ma svecofennian orogenic cycles the rifting events correspond to the formation of the ca 280 ma oslo rift and the ordovician ca 450 ma graben system in the area of the present gulf of bothnia each mixing age indicates that lead was mobilized probably as a consequence of mild thermal disturbances and that the crust was permeable to lead migration the data show that the geographic distribution of sulfide deposits with highly radiogenic lead isotopic compositions coincides with old graben systems orogenic belts and orogenic forelands on the baltic shield the ages of vein deposits and their geographic distribution demonstrate multiple tectonic reactivation of the interior of the baltic shield in response to orogenic events at its margin

lead isotope study of zn pb ore deposits associated with the basque cantabrian basin and paleozoic basement northern spain a total of forty three galena samples from syngenetic and epigenetic pb zn mineralizations emplaced in the lower cretaceous basque cantabrian basin and paleozoic basement of the cinco villas massif in the western pyrenees have been analyzed for pb isotopic composition galena from sedex mineralizations hosted in carboniferous clastic rocks in the cinco villas massif display an homogeneous lead isotopic signature 206pb 2044pb 18 43 207pb 204pb 15 66 208pb 204pb 38 69 suggesting a single lead reservoir these values are slightly more radiogenic than lead from other european hercynian deposits possibly reflecting the influence of a more evolved upper crustal source underlying paleozoic sediments are proposed as lead source for the cinco villas massif ores analyses from twenty six galena samples from the four strata bound ore districts hosted in mesozoic rocks reveal the existence of two populations regarding their lead isotopic composition galena from the western santander districts e g reocin is characterized by more radiogenic isotope values 206pb 204pb 18 74 207pb 204pb 15 67 208pb 204pb 38 73 than those from the central and eastern districts troya legorreta central and western vizcaya 206pb 204pb 18 59 207pb 204pb 15 66 208pb 204pb 38 73 in all districts the most likely source for these mineralizations was the thick sequence of lower cretaceous clastic sediments the existence of two separate lead isotopic populations could be the result of regional difference in the composition of the basement rocks and the clastic sediments derived of it or different evolution histories in both sub basins isotopic ratios indicate an increase in crustal influence as the age of the ores decreases

source of metals and age constraints for epigenetic gold deposits in the skellefte and pohjanmaa districts central part of the fennoscandian shield the lead isotopic compositions of galena in early proterozoic gold deposits have been determined for three districts in northern sweden and central finland the deposits are hosted by a variety of 1870 1890 ma svecofennian host rocks including the volcanosedimentary succession within the skellefte district island arc in sweden as well as i type tonalites at jorn sweden and pohjanmaa finland the deposits are epigenetic in relation to these svecofennian rocks and are part of a goldbearing metallogenetic belt which can be followed for 600 km parallel to the southwestern margin of the archaean domain in spite of these epigenetic relationships the lead isotopic data indicate that the deposits are not dramatically younger than the 1870 1890 ma svecofennian host rocks probably not exceeding 10 20 million years two principal lead sources were activated when the gold deposits were formed the most significant source is represented by the i type tonalites which constitute a relatively primitive 9 3 and widely distributed source in the entire metallogenic belt in addition the volcanic components in the westernmost part of the skellefte district constitute an extremely primitive 9 0 source which only locally was an important contributor to the epigenetic deposits in this metallogenetic belt the significantly different lead isotopic composition estimated for these sources indicates that the volcanic rocks in the western part of the skellefte district were not comagmatic with the i type tonalites recognized at jorn and central finland

chemical weathering of basalts southwest iceland effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean river ground and peat water from a basaltic catchment area laxa in kjos southwest iceland were sampled and analyzed for major element concentrations to define the effect of glassy versus crystalline basalt the formation of secondary minerals and runoff on fluxes of dissolved elements to the ocean the proportions of the dissolved solids in the water samples derived from precipitation air and rock weathering were estimated the amounts of na mg ca k and s originating from precipitation were calculated from the respective solute cl marine ratios and the estimated aqueous cl originated from precipitation which was calculated using measured b content of the waters and b cl molal marine and rock ratios the rock contribution from weathering ranged from 22 to 46 percent of the total dissolved solids of the waters iron and al showed low mobility compared to na this was primarily due to consumption of these elements by ferrihydroxides allophane imogolite and clay minerals also si and to a lesser degree ca show apparent slightly lower mobility than na these results are in good agreement with these phases being dominant in the soils of the study area wada and others 1992 and the main secondary minerals of basaltic glass in iceland crovisier and others 1992 reaction progress calculations support these findings according to a model interaction of basaltic glass and meteoric water al fe and si are consumed by amorphous fe and al hydroxides and imogolite and allophane at low reaction progress with increasing reaction progress ca fe mg smectite became the primary secondary minerals limiting the mobility of ca fe mg si and al these findings suggest that the weathering in the laxa in the kjos catchment area has evolved into the beginning of the smectite weathering stage and that it is dominated by a relatively low reaction progress and a high water to rock ratio in the present study the effect of runoff and rock crystallinity was quantified for dissolved elemental fluxes temperature lithology other than rock crystallinity and rock age were similar in all of the catchment areas in this study silica ca f s al k mg and b had a very similar dependence on runoff on the other hand na fluxes were found to be less dependent on runoff and fe fluxes were found to be independent of runoff basaltic glass dissolves faster than fully crystallized basaltic rocks gislason and eugster 1987a therefore increased glass content of the primary rocks is thought to increase the leaching and elemental fluxes at constant runoff however because some of the solutes particularly al and fe were highly influenced or even controlled by the formation of secondary minerals this increasing dissolution with increasing glass content was hidden on the other hand basaltic glass was observed to enhance the fluxes of more mobile elements like na si ca f and s by a factor of 2 to 5 at a constant runoff of 200 cm yr constant vegetative cover and 0 to 100 percent glass content of the rocks the k and mg fluxes were found to be independent of rock crystallinity however runoff alone cannot explain the fluxes of these elements and it seems that vegetative cover and seasonal variations in biomass activities influence the k and mg fluxes

estimates of europa s ice shell thickness from elastically supported topography 1 the thickness of europa s solid ice shell is uncertain and has important implications for europa s habitability and thermal history and the design of future spacecraft missions here we obtain an estimate of the ice shell thickness from observations of a plateau sw of cilix impact crater stereo topographic profiles suggest that the plateau is flexurally supported with an effective elastic thickness te of 6 2 5 km for a conductive temperature profile this te value implies a solid ice shell thickness of 15 9 20 km if the shell is convecting this estimate is a lower bound combined with independent estimates we infer a probable shell thickness of 25 km the shell thickness is likely to be uniform over the entire satellite

volcano tectonic interactions during rapid plate boundary evolution in the kyushu region sw japan evolution of the local plate tectonic and volcanic system relationship at kyushu island is defined by major changes in tectonics and volcanic style at ca 15 10 6 and 2 ma plate reconstructions presented here suggest that prior to 15 ma the pacific plate subduction dominated kyushu tectonics from 15 to 6 ma the evolving relative plate motions shifted the triple junction between the pacific plate philippine sea plate and southwest japan northwards so that the philippine sea plate was subducted beneath kyushu we suggest that a lack of subduction related volcanism from 10 to 6 ma is due to shallow subduction of the young shikoku basin lithosphere by 6 5 ma changes in the philippine sea plate motion led to more rapid nearly trench normal subduction of the eocene west philippine basin crust beneath kyushu this model is supported by an increase in arc like geochemistry of lavas since ca 6 5 ma subduction of fluid rich features such as the kyushu palau ridge introduced large volumes of fluids into the kyushu arc system leading to voluminous volcanism across kyushu focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics key issues such as the timing of izu arc collision with central japan and the history of motion of the philippine sea plate are reassessed here resulting in a model that favors izu arc central japan collision at ca 8 6 ma rather than the more widely accepted date of ca 15 ma

fluid inclusion characteristics of auriferous quartz veins in archean and paleoproterozoic greenstone belts of eastern and southern finland fluid inclusions in auriferous quartz veins in five deposits in the archean 2 9 2 7 ga and paleoproterozoic 2 0 1 75 ga greenstone belts of eastern and southwestern finland have been characterized on the basis of their composition reconstructed isochores and decrepitation behavior the dominant fluids associated with gold mineralization are h2o co2 ch4 halite archean and h2o co2 ch4 or h2o ch4 paleoproterozoic in composition phase ratios were generally uniform h2o co2 in the archean but were variable in the paleoproterozoic in the archean au sulfide quartz veins the mineralizing fluids contain less ch4 30 mol and have higher salinities up to 39 wt nacl equiv than the corresponding fluids in the paleoproterozoic au sulfide quartz veins up to 38 mol ch4 and below 13 wt nacl equiv additionally the estimated pressure temperature conditions for gold mineralization differ markedly between the two domains mineralization occurred in a pressure temperature range of 300 to 500 c at 2 to 3 5 kbars in the archean domain and 200 to 300 c at 1 to 2 kbars in the paleoproterozoic domain decrepigrams for the archean and paleoproterozoic auriferous quartz veins show different characteristics and permit classification of the different fluid inclusion populations on a regional scale

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

thermal equilibrium states of europa s ice shell implications for internal ocean thickness and surface heat flow abstract ice shell thickness and ocean depth are calculated for steady state models of tidal dissipation in europa s ice shell using the present day values of the orbital elements the tidal dissipation rate is obtained using a viscoelastic maxwell rheology for the ice the viscosity of which has been varied over a wide range and is found to strongly increase if an inviscid internal ocean is present to determine steady state values the tidal dissipation rate is equated to the heat transfer rate through the ice shell calculated from a parameterized model of convective heat transfer or from a thermal conduction model if the ice layer is found to be stable against convection although high dissipation rates and heat fluxes of up to 300 mwm 2 are in principle possible for europa these values are unrealistic because the states for which they are obtained are thermodynamically unstable equilibrium models have surface heat flows around 20 mwm 2 and ice layer thicknesses around 30 km which is significantly less than the total thickness of the h2o layer these results support models of europa with ice shells a few tens of kilometers thick and around 100 km thick subsurface oceans

dynamical investigation of a thickening ice shell implications for the icy moon europa abstract current astrobiological exploration of the outer solar system is largely driven by understanding planetary ice ocean systems such as those of europa and enceladus the ice shell thickness that affects the dynamics of the ice ocean system is determined by the balance between heat loss via convection and conduction and the heat production via tidal dissipation and radiogenic decay in this study we investigate how ice shells of such ocean moons grow and how convection inside them evolves as the system cools we perform geodynamical calculations to investigate the ice shell thickness and growth rate as the system cools from a warm initial condition we study both diffuse and localized end member modes of tidal dissipation within the ice shell we find that as the ice shell cools and thickens there is a reconfiguration of the convection patterns increasing the size of convection cells that leads to a relatively significant increase in the ice shell growth rate

thickness constraints on the icy shells of the galilean satellites from a comparison of crater shapes a thin outer ice shell on jupiter s large moon europa would imply easy exchange between the surface and any organic or biotic material in its putative subsurface ocean1 2 3 4 the thickness of the outer ice shell is poorly constrained however with model dependent estimates ranging from a few kilometres5 6 to ten or more kilometres7 here i present measurements of depths of impact craters on europa ganymede and callisto that reveal two anomalous transitions in crater shape with diameter the first transition is probably related to temperature dependent ductility of the crust at shallow depths 7 8 km on europa the second transition is attributed to the influence of subsurface oceans on all three satellites3 8 9 which constrains europa s icy shell to be at least 19 km thick the icy lithospheres of ganymede and callisto are equally ice rich but europa s icy shell has a thermal structure about 0 25 0 5 times the thicknesses of ganymede s or callisto s shells depending on epoch the appearances of the craters on europa are inconsistent with thin ice shell models1 and indicate that exchange of oceanic and surface material could be difficult

convection in ice i shells and mantles with self consistent grain size 1 the viscosity of ice i is grain size dependent for temperature and stress conditions appropriate for ice i shells and mantles of large and midsized icy satellites satellite thermal evolution heat flux critical shell thickness for convection brittle ductile transition temperature and potential for surface deformation are therefore grain size dependent using measured grain sizes from terrestrial ice sheets experiencing temperature and strain rate conditions similar to convecting ice shells in icy satellites we develop two end member models of grain size and its evolution in the absence of non water ice impurities grain size in a convecting ice shell of a large icy satellite will evolve over time to an equilibrium value which we determine to be between 30 and 80 mm because of dynamic recrystallization such large grains imply viscous ice so convection in an ice shell thinner than 35 km in a large icy satellite such as ganymede and callisto would be sluggish at best and may cease after a few convective overturns if it starts soluble ions and silicate microparticles may keep grains small however of order 1 5 mm and permit convection in shells as thin as 30 km in large icy satellites the dynamic recrystallization model provides a plausible upper limit on grain size in a geodynamically evolved ice shell whereas grain sizes from dust contaminated terrestrial ice cores provide a plausible lower limit on grain size in an icy satellite on the basis of these constraints grain sizes in evolved ice shells of large water ice satellites in the absence of tidal forcing should be of order 1 mm to 10 cm

interpretation of lead isotope data from the uraniferous cu fe sulfide mineralizations in the proterozoic greenstone belt at kopparasen northern sweden the polymetamorphic early proterozoic basic metavolcanites of the kopparasen greenstone belt northern sweden contain u mineralizations which are confined to cu fe sulfide mineralizations in mylonitized zones within basic metatuffs and graphite bearing mica schists trace lead isotope data from sulfides indicate contamination of the sulfide lead with two different lead components at ca 430 ma one lead component was leached from rocks with a 232th 238u ratio of 3 0 3 2 while the other lead component had evolved in a environment with a lower 232th 238u ratio 0 0 0 05 the source for this latter lead component underwent a u th separation probably in relation to the formation of the u mineralizations if this lead component was leached from the u mineralizations these mineralizations have a 207pb 206pb model age of ca 1 780 ma which is less than the least radiogenic lead model age of the supracrustal belt ca 2 050 ma a possible genetic model for the uranium mineralizations includes the transport of u with an oxidized metamorphic fluid which was channelled into the permeable zones and the local reduction of the fluid by sulfides which caused the precipitation of u

synvolcanic mixing of ore lead and the development of lead isotopic provinces in the skellefte district sweden ore lead isotope ratios have been analysed in galenas and sulphosalts from nineteen massive sulphide deposits hosted by svecofennian 1 9 ga supracrustals in the skellefte ore district northern sweden the ore lead data can be grouped on the basis of their geographical distribution most probably this feature is reflecting a number of lead isotopic provinces which correspond to crustal blocks the obtained ore lead data define linear trends in conventional pb pb plots the ore lead signatures are due to synvolcanic mixing processes as lead was leached from mafic and acid volcanic sources the relationships inferred for initial ratios in source rocks at 1 89 ga require a pre svecofennian crustal history it is suggested that magmatic processes at c 2 0 ga involved recycling of archean sedimentary material into the mantle and the formation of a crystalline felsic basement the metasomatized mantle and the basement melted at the time of svecofennian magmatism 1 89 ga which created mafic and acid magma sources subsequently basalts and rhyolites were extruded onto the sea floor ore was formed as hot solutions penetrated isotopically different levels of the volcanic pile

regional variations in the pb isotopic compositions of ore galena across the archaean proterozoic border in northern sweden abstract galena is a rare component in the palaeoproterozic mineralizations in northernmost sweden in the present work pb isotope data from galena from nineteen deposits tend to vary as a function of geographical location basically the zone dividing radiogenic and non radiogenic lead isotopic compositions is consistent with the position of the inferred proterozoic archaean border in northern sweden radiogenic lead is generally found in galena of the 2 0 2 7 ga old greenstone group in the northeast along the finnish border and in some cu occurrences in the 1 9 ga old porphyry group near malmberget non radiogenic lead is found in the huornaisenvuoma zn pb ore in the greenstone group in epigenetic cu zn pb ores in the porphyry group in southern norrbotten and in similar deposits near the caledonides the lead isotopic compositions of galena in the cu zn pb ores of southern norrbotten are fairly homogeneous with 206 pb 204 pb 15 34 to 15 50 207 pb 204 pb 15 16 to 15 30 similar to those of the sulphide deposits associated with the central finnish batholith and to data from the north central skellefte district the coincidence of data for the areas of southern norrbotten and the north central skellefte district is interpreted such that these terrains formed one crustal unit at the time of ore deposition tentatively this event took place at 1 87 ga or slightly thereafter and was related to the suturing of the skellefte arc to the northerly inferred continent although pb model ages should be treated with caution when applied in proterozoic terrains in sweden the present data suggest an age difference between the deposits and between source rocks at tjamotis and huornaisenvouma 2 0 ga old on one hand and the remaining cu zn pb occurrences 1 9 ga old on the other hand possibly the indicated depositional event at 2 0 ga is an expression of simultaneous large scale crust mantle interactions in accordance with the hypothesis of magma forming processes at that time in the pb pb diagrams data for galena in northern sweden indicate linear relationships the radiogenic lead is the result of leaching processes in phanerozoic caledonian times which released rock lead evolved in the time interval 2 0 1 9 ga to 0 4 ga the data furthermore suggest that radiogenic galena was formed by mobilizing lead from u rich rocks 238 u 204 pb ratios ca 25 32 of the greenstone group which are likely to have a similar age possibly close to 2 0 ga since the deposits with radiogenic lead are distributed over a very large area and still define linear trends it is likely that the 2 0 ga age is typical for the bulk of greenstone rocks in norrbotten from the slopes of straight line relations for the pb isotope data it is inferred that only proterozoic rock lead was mobilized at 0 4 ga and that no significant archaean lead was involved in such processes accordingly lead mobilization was quite local and only involved the upper portions of the crust as the radiogenic lead is found at localities far away from the caledonian front the indicated pb mobilization processes were most probably connected with reactivation of old proterozoic fracture zones

metallogeny of gold in the precambrian of northern europe fennoscandian gold deposits have been successfully explored in a wide range of precambrian geological environments in sweden and finland during the last two decades under explored areas still exist at other sites in the fennoscandian shield particularly in russia and norway and there is a high potential for future new discoveries of economic gold in northern europe careful geological documentation of a number of previously productive gold deposits such as haveri and saattopora finland boliden adelfors and enasen sweden and eidsvoll and bidjovagge norway is now available in the literature kutemajarvi pahtavaaraa and suurikuusikko finland and bjorkdal akerberg harnas and pahtohavare sweden represent a new generation of gold mines and recent research on these enables an improved base for formulating regional and local metallogenetic models younging trends from northeast to southwest characterize both the crust and ore forming regional patterns in the fennoscandian shield orogenic or mesothermal gold deposits are found in a wide range of host rocks and are closely linked to either the paleoproterozoic svecokarelian or the neoproterozoic sveconorwegian orogeny unlike these shear zone related deposits significant amounts of gold were concentrated in volcanogenic massive sulfide vms deposits by magmatic hydrothermal processes in conjunction with formation of paleoproterozoic svecofennian juvenile crust hundreds of gold deposits of a variety of ages and genetic styles are therefore now known in the precambrian of northern europe of which some are economic others subeconomic and still others only of scientific interest one hundred representative deposits in sweden norway finland and russia are discussed in this review literature references are provided for each of them and detailed maps show the location of each deposit or prospect listed data on tonnage and grades for the most recent exploration targets as well as the economically and historically most significant deposits show that they were formed during the entire precambrian yield of gold in the fennoscandian shield is so far confined to proterozoic deposits but the potential to find economically viable gold deposits in the russian parts of the archean greenstone belts should not be underestimated

oceans in the icy galilean satellites of jupiter abstract equilibrium models of heat transfer by heat conduction and thermal convection show that the three satellites of jupiter europa ganymede and callisto may have internal oceans underneath ice shells tens of kilometers to more than a hundred kilometers thick a wide range of rheology and heat transfer parameter values and present day heat production rates have been considered the rheology was cast in terms of a reference viscosity 0 calculated at the melting temperature and the rate of change a of viscosity with inverse homologous temperature the temperature dependence of the thermal conductivity k of ice i has been taken into account by calculating the average conductivity along the temperature profile heating rates are based on a chondritic radiogenic heating rate of 4 5 pw kg 1 but have been varied around this value over a wide range the phase diagrams of h2o ice i and h2o 5 wt nh3 ice have been considered the ice i models are worst case scenarios for the existence of a subsurface liquid water ocean because ice i has the highest possible melting temperature and the highest thermal conductivity of candidate ices and the assumption of equilibrium ignores the contribution to ice shell heating from deep interior cooling in the context of ice i models we find that europa is the satellite most likely to have a subsurface liquid ocean even with radiogenic heating alone the ocean is tens of kilometers thick in the nominal model if tidal heating is invoked the ocean will be much thicker and the ice shell will be a few tens of kilometers thick ganymede and callisto have frozen their oceans in the nominal ice i models but since these models represent the worst case scenario it is conceivable that these satellites also have oceans at the present time the most important factor working against the existence of subsurface oceans is contamination of the outer ice shell by rock rock increases the density and the pressure gradient and shifts the triple point of ice i to shallower depths where the temperature is likely to be lower then the triple point temperature according to present knowledge of ice phase diagrams ammonia produces one of the largest reductions of the melting temperature if we assume a bulk concentration of 5 wt ammonia we find that all the satellites have substantial oceans for a model of europa heated only by radiogenic decay the ice shell will be a few tens of kilometers thinner than in the ice i case the underlying rock mantle will limit the depth of the ocean to 80 100 km for ganymede and callisto the ice i shell on top of the h2o nh3 ocean will be around 60 to 80 km thick and the oceans may be 200 to 350 km deep previous models have suggested that efficient convection in the ice will freeze any existing ocean the present conclusions are different mainly because they are based on a parameterization of convective heat transport in fluids with strongly temperature dependent viscosity rather than a parameterization derived from constant viscosity convection models the present parameterization introduces a conductive stagnant lid at the expense of the thickness of the convecting sublayer if the latter exists at all the stagnant lid causes the temperature in the sublayer to be warmer than in a comparable constant viscosity convecting layer we have further modified the parameterization to account for the strong increase in homologous temperature and therefore decrease in viscosity with depth along an adiabat this modification causes even thicker stagnant lids and further elevated temperatures in the well mixed sublayer it is the stagnant lid and the comparatively large temperature in the sublayer that frustrates ocean freezing

genesis of acid sulfate alteration and au cu ag mineralization at summitville colorado the summitville au cu ag deposit occurs within a porphyritic quartz latite primarily in a zone of vuggy silica alteration characterized by leaching of all major elements except silica and iron vuggy silica is best developed between an elevation of 3 700 and 3 500 m where it occurs in irregular pipes and lenticular pods up to 70 m wide and is enclosed by up to 30 m of intense quartz alunite alteration the vuggy silica zones diminish in size with decreasing elevation and rarely exceed a width of 2 m below an elevation of 3 400 m these deep vuggy silica zones are enclosed by quartz kaolinite alteration with little or no alunite sulfide mineralization occurs primarily within the vuggy silica the sulfide assemblage changes with decreasing elevation from covellite luzonite or native gold to chalcopyrite tennantite sulfides occur lining voids or in microveinlets and are accompanied by minor kaolinitc but not by alunite gold also occurs in a near surface barite goethite jarosite assemblage that crosscuts the vuggy silica consideration of alunite kaolinitc and pyrite stability relations and of aluminum speciation at 250 degrees c indicates that a ph of 2 or below and a log f sub o 2 of 31 or 1 at 250 degrees c were necessary to produce vuggy silica alteration the chalcopyrite tennantite mineralization occurring at depth in the vuggy silica zone is indicative of less oxidizing and less acidic conditions the covellite luzonite or gold assemblage requires conditions intermediate between those necessary for vuggy silica alteration and for deep sulfide mineralization barite gold mineralization is interpreted as a near surface assemblage produced under highly acidic oxidizing conditions these textural and geochemical relationships indicate that sulfide and gold mineralization postdate the formation of vuggy silica alteration in the deposit and are not directly related to the highly acidic solutions necessary for this early acid leaching event the change from extreme acid leaching to copper sulfide and precious metal deposition may represent a transition from a magmatic vapor dominated to a liquid dominated system leading to more reducing and less acidic conditions during sulfide deposition

granites and early proterozoic crustal evolution in sweden evidence from smnd upb and o isotope systematics abstract u pb zircon and whole rock rb sr sm nd and o isotope results are reported for granitoids from three proterozoic provinces in sweden that represent different crustal environments the radiometric ages span the period 1 89 1 65 ga and record events ranging from original igneous crystallisation to late metamorphic resetting these events occurred at different times in the crustal segments the o isotope ratios identify different crustal source materials for the granitoids the sm nd results indicate that the major crustal source components common to most of the granitoids formed shortly before 1 9 ga close in time to the formation of the earliest granitoids contributions from recycled archaean basement are minimal the geographically widespread occurrence of such crustal sources indicates that the period around 2 ga was of great importance for the growth of new continental crust in sweden

anhydrite bearing rocks from the rožna district moldanubian zone czech republic high grade metamorphosed exhalites several types of anhydrite bearing rocks have been found in the amphibolite facies metamorphosed rocks at the north eastern margin of the moldanubian zone anhydrite either forms monomineralic bands up to 40 cm thick or occurs in the form of disseminated grains in surrounding calc silicate gneiss together with feldspar scapolite amphibole pyroxene epidote and pyrite the isotopic composition of sulphur 34s 30 6 to 32 3 and strontium 87sr 86sr 0 70797 to 0 70781 in anhydrite may indicate a marine source of sulphate the isotopic ratio of strontium is in the same range as that of metamorphosed strata bound barite sulphide ores which have been previously described in the same area the 34s values of coexisting pyrite range from 21 4 to 22 5 the 34sanhydrite pyrite corresponding to the metamorphic temperature of 600 to 660 c in contrast to many submarine exhalative deposits the oxygen isotopic compositions of anhydrite 18o 9 3 to 10 2 are lighter than that of barite 18o 10 4 to 13 8 this indicates that the both minerals are not in isotopic equilibrium therefore it is probable that anhydrite and barite from the ro na district were deposited from fluids that contained different proportions of seawater and hydrothermal fluids or from hydrothermal fluids that underwent variable extent of oxygen isotope exchange with seafloor rocks the 13c values in calcite 13c 17 2 to 18 7 from anhydrite bearing rock are lower than those in distant marbles as graphite is absent in anhydrite and calcite bearing rocks impoverishment in the 13c isotope cannot be attributed to the graphite carbonate isotopic exchange during metamorphism it is proposed that low 13c values in carbonates are caused by pre metamorphic oxidation of organic matter in course of hydrothermal processes anhydrite and anhydrite bearing calc silicate gneiss from the north eastern part of the moldanubian zone are interpreted to be the high grade metamorphosed analogue of anhydrite rich exhalites commonly found in submarine exhalative hydrothermal deposits

nd isotope data on 1 9 1 2 ga old basic rocks and metasediments from the bothnian basin central sweden abstract sm nd data are presented for basic rocks and metagreywackes from the early proterozoic bothnian basin in central sweden the basic rocks include synsedimentary metabasalts early orogenic intrusions and anorogenic dolerites covering the age range 1 9 1 2 ga the samples thus span the entire period from the early part of the svecokarelian thermo tectonic episode to the time when this region of the baltic shield had been a stable craton for about 0 4 ga all these samples have slightly positive nd values up to 2 for synorogenic rocks and up to 3 5 for anorogenic dolerites if these values are representative for the contemporaneous mantle they suggest a less depleted character for this mantle than that shown for several other proterozoic shield areas three metagreywacke samples have nd values of 2 4 to 3 1 indicating a significant influx of detrital older crustal material into the bothnian basin

numerical simulations of convection in europa s ice shell implications for surface features 1 europa s icy surface displays numerous small 5 to 30 km diameter pits spots and uplifts that have been suggested to result from convection in the ice shell to test this hypothesis we present numerical simulations of convection in europa s ice shell including temperature dependent viscosity and tidal heating ice shells 15 and 50 km thick are considered consistent with several estimates of the shell thickness on europa the convection produces deep pits consistent with some of the observed features when the lithospheric viscosity is 103 105 times greater than that of the underlying ice but greater viscosity contrasts lead to topography insufficient to explain the observed pits if ductile creep is the only deformation mechanism these results imply that convection cannot produce the observed pits and uplifts because in that case the predicted surface viscosity exceeds that of the warm underlying ice by at least a factor of 1010 however the strength of europa s surface may be low enough for plastic deformation to play a role in the convection opening the possibility that convection could produce some of europa s pits for plausible viscosities 1013 pa s at the melting temperature the pits are 100 300 m deep and 10 20 km in diameter greater or lesser viscosities lead to wider or narrower pits respectively none of our simulations produced isolated uplifts of any diameter however so these probably formed by another mechanism the convection can induce surface stresses 1 bar which exceeds the inferred strength of europa s crust and indicates the likelihood of surface disruption the maximum tidal heat fluxes that can be transported by convection at realistic ice viscosities are 0 05 0 07 w m 2

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

crustal residence ages of clastic sediments orogeny and continental evolution abstract the relationships between additions to the continental crust and periods of orogenesis have been investigated by means of sm 1bnd isotopic analyses of fine grained clastic sediments from the western canadian and baltic shields the crustal residence ages of these sediments indicate that proterozoic crustal additions occurred in both regions but in canada preceded the main magmatic stage of the wopmay orogeny demonstrating that proterozoic orogenic events are not always accompanied by identifiable crustal addition a review of sm 1bnd data for clastic sediments world wide suggests that the major sediment sources have undergone broadly similar evolutionary histories in quite separate continental areas and together yield a mean age estimate for the entire sedimentary mass of 2 0 0 2 ga a minimum crustal growth curve can be derived from these data which requires that 90 or more of the continental crust existed at the end of the proterozoic

strike slip motion and double ridge formation on europa layer thickness of 2 km and shear velocity of 6 10 7 ms 1 such a velocity is appropriate for diurnal 85 hour tidal motion the local increase in temperature may cause 100 m uplift around the shear zone through thermal buoyancy the stresses required to produce velocities of order 10 7 ms 1 are similar to estimates for present day tidal stresses on europa 10 4 10 5 pa brittle layer thicknesses 2 km are unlikely to persist at active shear zones because of the effect of shear heating shear velocities greater than or equal to 10 6 ms 1 will give rise to melting at shallow depths the removal of material by downwards percolation of meltwater may cause surface collapse along the shear zone inward motion leading to compression may also result the combination of thermally or compression induced uplift and melt related collapse may be responsible for the pervasive double ridges seen on europa s surface index terms 5475 planetology solid surface planets tectonics 8149 6218 planetology solar system objects jovian satellites 8120 tectonophysics dynamics of lithosphere and mantle general 8160 tectonophysics rheology general keywords ice shear zone melting viscosity brittle ductile transition

europa tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting europa we show that tidal energy can be preferentially focused in rising plumes for viscosities in agreement with laboratory experiments when the plume cores reach the base of the outer cold brittle layer they spread laterally causing shallow melting disruption and formation of terrain similar to lenticulae and chaos we show that this mechanism can readily explain the major characteristics of chaos and lenticulae even if the average outer solid ice layer thickness is larger than 20 km

ice diapirs on europa implications for liquid water early examinations of galileo images of europa revealed features that look like low topographic domes these small 5 10 km in radius domes have been interpreted as surface manifestations of diapirs as a way to probe the subsurface structure of europa we investigate the possibility that thermally driven ice diapirism created these surface features we use a previously developed analytic model for rising diapirs to estimate the initial radii of the diapirs their initial depth the temperature of the medium through which they rise and their formation timescales we assume that the diapirs originate at the boundary layer between solid ice and whatever underlies it whether it be liquid water or solid rock our results show that the diapirs originate at a depth of no more than a few tens of kilometers since the h2o layer on europa is substantially thicker than this our results support the view that liquid water recently existed beneath the ice on europa at this location at a depth of no more than a few tens of kilometers further the ice must have been warm 250 270 k and was therefore likely to have been convecting

oxidation states and ascent history of the apollo 17 volcanic beads as inferred from metal glass equilibria abstract the composition mode of occurrence and relationships of metal blebs in the apollo 17 74001 2 core have been examined to provide insight into variations in oxidation states during ascent and eruption of the lunar volcanic glasses the 74001 2 core is composed of 99 orange volcanic glasses and their crystallized equivalents black beads with olivine phenocrysts metal blebs and in the upper 8 cm of the core lithic fragments composing the remaining 1 in an analysis of this core twenty nine metal blebs larger than 4 m in size were discovered in thin sections taken along the length of the 68 cm deep core these blebs are classified into three types on the basis of their shape composition and distribution fourteen of the metal blebs were found trapped inside or between olivine phenocrysts type i blebs microprobe analyses showed that these spherical blebs have a homogeneous composition of 85 fe 14 ni and 1 1 co from core to rim six type ii rounded blebs were discovered trapped inside volcanic beads their core compositions are similar to the type i but near the rim these blebs have a sharp decrease in fe and increase in ni content in the lower half of the 74001 2 core where the beads are predominantly black many contain 5 m amoeba shaped type iii metal blebs type iii blebs are composed of fe with trace amounts of ni two angular metal fragments could not be classified into any of the three types we propose that the type i blebs formed initially at 4 0 km depth and 1 3 log units below iw at a pressure of 200 bars and a temperature of 1320 c by the formation of c o gas during graphite oxidation once trapped by olivine phenocrysts the blebs were buffered from further reactions with the melt the type ii blebs remained in contact with the melt during ascent and experienced loss of fe to the surrounding melt phase by oxidation reactions which range from 1 3 to 0 9 log below iw finally type iii blebs formed either by exsolution of fe during crystallization in the fire fountain or by loss of volatiles originally coordinated with fe to the volcanic gas phase under the reduced conditions in the fountain

thickness of a europan ice shell from impact crater simulations several impact craters on jupiter s satellite europa exhibit central peaks on the terrestrial planets central peaks consist of fractured but competent rock uplifted during cratering therefore the observation of central peaks on europa indicates that an ice layer must be sufficiently thick that the impact events did not completely penetrate it we conducted numerical simulations of vapor and melt production during cratering of water ice layers overlying liquid water to estimate the thickness of europa s icy crust because impacts disrupt material well beyond the zone of partial melting our simulations put a lower limit on ice thickness at the locations and times of impact we conclude that the ice must be more than 3 to 4 kilometers thick

microbial transformation of magnetite to hematite abstract a microbial consortium enriched from surface water draining the lac du bonnet granitic batholith in manitoba canada was able to mediate the transformation of 11 of the fe in a sample of magnetite into hematite within three weeks holes up to 100 m in diameter were formed in grains of magnetite in a polished section of granite that had been exposed to a microbial biofilm in the laboratory bacteria were seen by scanning electron microscopy to be attached to magnetite after three days incubation in a flask while a covering of biofilm was nearly complete after six days some hexagonal grains of an iron mineral were visible after incubation for eight weeks dissolution of the magnetite and the presence of hematite in the incubated flasks was confirmed by mossbauer spectra and powder x ray diffraction patterns no hematite was found either in the original sample of magnetite or in duplicate sterile controls

the enasen gold deposit central sweden stable isotope analyses of quartz sulphides and magnetite were conducted to provide information on thermal history and source of hydrothermal fluids in the palaeoproterozoic enasen gold deposit reequilibration and homogenization of oxygen isotopes throughout the rock have apparently not occurred despite the upper amphibolite to granulite facies regional metamorphism that has affected the rocks however oxygen isotope geothermometry on a coexisting quartz magnetite pair gave a minimum temperature for peak metamorphism of around 650 c which agrees with fe mg geothermometry this suggests that grain scale equilibrium is achieved the variation in oxygen isotope ratios 18o 7 3 10 5 on quartz from the metamorphosed acid sulphate alteration zone is suggested to represent a cooling trend in the fossil hydrothermal system with higher 18o values in more superficial parts temperatures of alteration and silicification and isotopic composition of hydrothermal fluids could not be defined from the present data but it was recognized that the data is compatible with a epithermal genesis for the deposit it is suggested that alteration silicification and mineralization at the enasen gold deposit took place in a high sulphidation epithermal environment at temperatures of around 200 250 c and that the hydrothermal fluids consisted of meteoric and magmatic water a tentative reconstruction of the fossil hydrothermal system is presented sulphur isotope ratios of sulphides from the fold bearing quartz sillimanite gneiss gave 34s values close to zero indicating a magmatic source of the sulphur

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

some new lead isotope determinations from the proterozoic sulfide ores of central sweden lead isotope determinations were made on galenas from three strata bound sulfide ores in the early proterozoic svecokarelian bergslagen district of central sweden and four epigenetic deposits in the alvdalen and vansbro districts in the early to middle proterozoic post svecokarelian belt the leads from the strata bound bergslagen deposits show exceptional isotopic homogeneity over large areas their isotopic composition suggests the existence of a pre svecokarelian crust in the district and is consistent with exhalative sedimentary ore formation in an active continental margin environment the vansbro and alvdalen leads display constant compositions within each district but marked divergence between the districts their compositions preclude derivation exclusively from recycled svecokarelian lead and suggest a substantial lead contribution from a mantle like source the difference between model ages and geologic ages for many of the deposits with a small but significant excess of radiogenic lead suggests a significant deviation of the fennoscandian shield from conventional global lead evolution models

habitability of europa s crust the role of tidal tectonic processes jupiter s satellite europa has been identified as one of the most likely sites for life in the solar system the tidal tectonic processes that appear to have governed europa s geology seem to require interaction with an ocean under only a very thin crust providing a variety of evolving environmental niches the mutually dependent relationship between orbital evolution and tidal processes in turn controls europa s rotation heating and stress surface lineaments are correlated with global stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude tidal driving of strike slip faulting indicates that cracks penetrate to a fluid layer which is possible only with a very thin ice crust the characteristic ridge sets that cover tectonic terrain are likely built by tidal pumping of fluid and slush to the surface on a daily basis widespread tectonic dilation creates new surface as material rises from below chaotic terrain has morphology and other characteristics indicative of melt through from below surface colorants correlate with locations such as along large scale ridge systems and around chaotic terrain where ocean water reached the surface this model implies that as a result of tides liquid water regularly bathed crustal cracks and surfaces with heat and whatever nutrients are included in the oceanic chemistry creating a variety of habitable environments the processes were recent and thus most likely continue today longer term evolution of environmental conditions provided the need for adaptation and opportunity for evolution

tectonic processes on europa tidal stresses mechanical response and visible features abstract europa s orbital eccentricity driven by the resonance with io and ganymede results in diurnal tides 3 5 day period and possibly in nonsynchronous rotation both diurnal variation and nonsynchronous rotation can create significant stress fields on europa s surface and both effects may produce cracking patterns and time sequences of apparent tectonic features on europa include lineaments that correlate with both sources of stress if we take into account nonsynchronous rotation after initial crack formation by amounts ranging up to several tens of degrees for example the crosscutting time sequence of features in the cadmus and minos linea region is consistent with a combined diurnal and nonsynchronous tensile stress field as it evolves during tens of degrees of nonsynchronous rotation constraints on the rotation rate from comparing voyager and galileo images show that significant rotation requires 104yr but could be fast enough to have allowed significant rotation since the last global resurfacing even if such resurfacing was as recent as a few million years ago once cracking is initiated diurnal tides work cracks so that they open and close daily although the daily effect is small over 105yr double ridges could plausibly be built along the cracks with sizes and morphologies consistent with observed structures according to a model in which underlying liquid water fills the open cracks partially freezes and is extruded during the daily closing of the cracks thus several lines of observational and theoretical evidence can be integrated if we assume nonsynchronous rotation and the existence of a liquid water layer

onset of convection in the icy galilean satellites influence of rheology 1 ice i exhibits a complex rheology at temperature and pressure conditions appropriate for the interiors of the ice i shells of europa ganymede and callisto we use numerical methods and existing parameterizations of the critical rayleigh number to determine the conditions required to trigger convection in an ice i shell with each of the stress temperature and grain size dependent rheologies measured in laboratory experiments by goldsby and kohlstedt 2001 the critical rayleigh number depends on the ice grain size and the amplitude and wavelength of temperature perturbation issued to an initially conductive ice i shell if the shells have an assumed uniform grain size 0 4 mm deformation during initial plume growth is accommodated by newtonian volume diffusion if the ice grain size is between 0 4 mm and 3 cm deformation during plume growth is accommodated by weakly non newtonian grain boundary sliding where the critical ice shell thickness for convection depends on the amplitude of temperature perturbation to the 0 5 power if the ice grain size exceeds 2 cm convection can not occur in the ice i shells of the galilean satellites regardless of the amplitude or wavelength of temperature perturbation if the grain size in a convecting ice i shell evolves to effective values greater than 2 cm convection will cease if the ice shell has a grain size large enough to permit flow by dislocation creep the ice is too stiff to permit convection even in the thickest possible ice i shell consideration of the complex ice rheology implies that estimates of the grain size in the satellites and knowledge of their initial thermal states are required when judging the convective instability of their ice i shells

ore lead isotope variations in the proterozoic massive pyrite deposit at nasliden skellefte district sweden the ore lead isotope ratios from the nasliden deposit are mantle like in terms of the idealized plumbotectonics model they suggest that the nasliden deposit was formed in the proterozoic equivalent of a phanerozoic primitive island arc the mantle like nature of the lead and its homogeneity indicate that crustal lead does not constitute the major component of the ore lead crustal lead contamination is suggested to be the cause of divergences in ore lead isotopic compositions associated with later veins in the deposit

early svecofennian stratigraphy of southern and central norrland sweden and the possible existence of an archaean basement west of the svecokarelides abstract the stratigraphy of supracrustal sequences of seven different areas in southern and central norrland sweden are compared all the sequences belong to the svecofennian part of the early proterozoic svecokarelian fold belt it is concluded that quartzites and meta arkoses are usually found at high stratigraphic levels they are sparse or absent in the north but are more abundant in southern norrland this may suggest that a granitic archaean or early proterozoic basement was exposed to erosion somewhere southwest of the regions under consideration future research should aim at clarifying the age and extent of this basement and the provenance of the various metasedimentary rocks

hafnium isotope evidence for early proterozoic volcanic arc reworking in the skellefte district northern sweden and implications for the svecofennian orogen abstract the skellefte district is a seemingly juvenile and heavily mineralized crustal domain in northern sweden that formed between 1 90 and 1 87 ga it is commonly interpreted as a volcanic arc deposited on a basement known variously as the bothnian or the knaften barsele group that could be represented by older rocks 1 96 1 94 ga found in the vicinity in order to understand the potential genetic relationship between the arc and the basement hf and pb isotopes in magmatic zircons from key lithologies were measured by solution multi collector inductively coupled plasma mass spectrometry it is shown that both geological groups display similar hf isotope compositions which translate into decreasing hf with time overall the data are compatible with reworking of the knaften barsele arc to produce the skellefte rocks over a short time interval from 1 90 to 1 87 ga in a context of crustal extension with ongoing subduction when the data presented here are integrated with general models of tectonic evolution of the svecofennian orogen they fit a scenario in which the juvenile knaften barsele arc formed between 1 96 and 1 94 ga and became accreted onto the karelian continent located further north at about 1 92 1 91 ga systematic north to south variations in pb nd and hf isotope compositions throughout the svecofennides interpreted as resulting from an increase in archean crust involvement toward the south indicate a genetic link between the proterozoic crustal domains of sweden and finland

thermal orbital evolution of io and europa coupled thermal orbital evolution models of europa and io are presented it is assumed that io europa and ganymede evolve in the laplace resonance and that tidal dissipation of orbital energy is an internal heat source for both io and europa while dissipation in io occurs in the mantle as in the mantle dissipation model of segatz et al 1988 icarus 75 187 two models for europa are considered in the first model dissipation occurs in the silicate mantle while in the second model dissipation occurs in the ice shell in the latter model ice shell melting and variations of the shell thickness above an ocean are explicitly included the rheology of both the ice and the rock is cast in terms of a viscoelastic maxwell rheology with viscosity and shear modulus depending on the average temperature of the dissipating layer heat transfer by convection is calculated using a parameterization for strongly temperature dependent viscosity convection both models are consistent with the present orbital elements of io europa and ganymede it is shown that there may be phases of quasi steady evolution with large or small dissipation rates in comparison with radiogenic heating phases with runaway heating or cooling and oscillatory phases during which the eccentricity and the tidal heating rate will oscillate europa s ice thickness varies between roughly 3 and 70 km dissipation in the silicate layer or 10 and 60 km dissipation in the ice layer suggesting that europa s ocean existed for geological timescales the variation in ice thickness including both convective and purely conductive phases may be reflected in the formation of different geological surface features on europa both models suggest that at present europa s ice thickness is several tens of km thick and is increasing while the eccentricity decreases implying that the satellites evolve out of resonance including lithospheric growth in the models makes it impossible to match the high heat flux constraint for io other heat transfer processes than conduction through the lithosphere must be important for the present io

snowball earth ice thickness on the tropical ocean 1 on the tropical oceans of a neo proterozoic snowball earth snow free ice would have existed in regions of net sublimation photosynthesis could have continued beneath this bare ice if it was sufficiently thin and sufficiently clear the steady state ice thickness is determined by the necessity to balance the upward conduction of heat with three subsurface heating rates the heat flux from the ocean to the ice base the latent heat of freezing to the ice base and the solar energy absorbed within the ice a preliminary study using a broadband model for solar radiation and assuming a large freezing rate had indicated that tropical ice might be only a few meters thick here we show that the vertical throughput of ice by surface sublimation and basal freezing would be too slow to keep the ice thin and that the broadband model had exaggerated the absorption depth of sunlight we use a spectral model for solar absorption computing radiative transfer at 60 wavelengths considering absorption by the ice and scattering by bubbles with the spectral model the computed ice thickness is much greater for a solar flux of 320 w m 2 at the equatorial surface and expected albedo of 0 5 for bare sea ice we find that surface temperatures below 12 c generate ice layers too thick for photosynthesis 100 m if the albedo were as low as 0 4 thick ice would occur only for surface temperatures below 25 c but such low temperatures would be difficult to maintain with such low albedo for surface temperatures warmer than these limits the ice becomes thin 1 m and is unlikely to represent a coherent ice layer however glacial deformation of thick floating ice from nearby oceanic regions may preclude the existence of thin or ice free patches

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

an oxygen isotope and geochemical study of meteoric hydrothermal systems at pilot mountain and selected other localities carolina slate belt several epigenetic mineral deposits in the ca1rolina slate belt are intimately related to meteoric hydrothermal systems of late precambrian and early paleozoic age at pilot mountain low 18 o rocks correlate well with zones of strong silicic alteration and alkali leaching accompanied by high alumina minerals sericite pyrophyllite andalusite or topaz and anomalous concentrations of cu mo sn b and au the alteration occurs within andesitic volcanic and volcaniclastic rocks and is associated with a subvolcanic dacite porphyry stock on the southeastern slope of the mountain tilting and erosion have exposed an oblique section through the original system interpreted to expose shallower rocks to the northwest a 4 km 2 central zone of slight 18 o depletion delta 18 o sub whole rock 4 3 6 1ppm occurs on the broad and resistant silicifled western flank of pilot mountain predominantly within quartz sericite schist and quartz granofels containing pods of high a1 minerals a magmatic source for much of the sulfur and metal is likely and a subordinate magmatic water component in the fluid of the central zone is possible this central zone is surrounded by a 30 km 2 peripheral zone of low 18 o sericite schists chlorite sericite schists and andesitic volcanic rocks delta 18 o sub whole rock 3 8ppm with the lowest values 1 4ppm occurring in intensely sericitized rocks on the eastern flank of pilot mountain near the apex of the dacite porphyry stock rhyolites of the uwharrie formation delta 18 o 3 8 6 3ppm are not as strongly altered as nearby andesites and may postdate the hydrothermal alteration the fluid calculated to be in equilibrium with the lowest 18 o quartz veins and country rocks at 300 degrees or 50 degrees c would have delta 18 o approximately 4 5 or 2 0 per mil whereas analyses of radiating pyrophyllite indicate equilibrium with a fluid having delta d approximately 30 per mil consistent with a slightly 18 o shifted low latitude meteoric water subsequent greenschist metamorphism caused intermineral isotopic reequilibration in several samples and may have modified preexisting alteration assemblages but it did not destroy the large delta 18 o anomaly produced by meteoric hydrothermal activity at pilot mountain reconnaissance studies of other alteration zones in the carolina slate belt have so far disclosed the involvement of meteoric hydrothermal fluids at the snow camp pyrophyllite deposit at the hoover hill and sawyer au mines and probably at the haile and brewer au mines

experimental evidence for non redox transformations between magnetite and hematite under h2 rich hydrothermal conditions abstract transformations of magnetite fe ii fe 2 iii o 4 to hematite fe 2 iii o 3 and vice versa have been thought by many scientists and engineers to require molecular o 2 and or h 2 thus the presence of magnetite and or hematite in rocks has been linked to a specific oxidation environment however the availability of reductants or oxidants in many geologic and industrial environments appears to have been too low to account for the transformations of iron oxides through redox reactions here we report the results of hydrothermal experiments in mildly acidic and h 2 rich aqueous solutions at 150 c which demonstrate that transformations of magnetite to hematite and hematite to magnetite occur rapidly without involving molecular o 2 or h 2 fe 3 o 4 mt 2h aq fe 2 o 3 hm fe aq 2 h 2 o the transformation products are chemically and structurally homogeneous and typically occur as euhedral single crystals much larger than the precursor minerals this suggests that in addition to the expected release of aqueous ferrous species to solution the transformations involve release of aqueous ferric species from the precursor oxides to the solution which reprecipitate without being reduced by h 2 these redox independent transformations may have been responsible for the formation of some iron oxides in natural systems such as high grade hematite ores that developed from banded iron formations bifs hematite rich deposits formed on mars corrosion products in power plants and other industrial systems

a free plate surface and weak oceanic crust produce single sided subduction on earth 1 earth s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection subduction zones on present day earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle while global self consistent numerical models of mantle convection have reproduced some aspects of plate tectonics the assumptions behind these models do not allow for realistic single sided subduction here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates 1 the presence of a free deformable upper surface and 2 the presence of weak hydrated crust atop subducting slabs we show that assuming a free surface rather than the conventional free slip surface allows the dynamical behavior at convergent plate boundaries to change from double sided to single sided a weak crustal layer further improves the behavior towards steady single sided subduction by acting as lubricating layer between the sinking and the overriding plate this is a first order finding of the causes of single sided subduction which by its own produces important features like the arcuate curvature of subduction trenches citation crameri f p j tackley i meilick t v gerya and b j p kaus 2012 a free plate surface and weak oceanic crust produce single sided subduction on earth geophys res lett 39 l03306 doi 10 1029 2011gl050046

metallogeny of gold in the fennoscandian shield gold occurs in a number of different ore types in the fennoscandian shield ranging in age from late archean to late proterozoic until recently the metal was exploited primarily as a byproduct in volcanogenic massive sulphide deposits but during the 1980s more gold mines have been opened than during any other episode in the mining history of northern europe the occurrence of gold in the fennoscandian shield is reviewed in the context of the major tectonostratigraphic units 1 in the karelian province gold is hosted by greenstone belts of the archean basement complex e g at ilomantsi eastern finland greenstone belts of the nordkalott province which are interpreted as part of an early proterozoic cover sequence contain gold deposits associated with copper bidjovagge saattopora and pahtohavare gold is also associated with cobalt in the metasomatically altered early proterozoic cover in north eastern finland meurastuksenaho and juomasuo 2 in the svecofennian domain the major gold deposits were generated during the emplacement of 1 92 1 87 ga old accretional magmatism these deposits occur in the northeastern part of the svecofennian domain close to the archean proterozoic boundary they comprise two major types a the porphyry type and shear zone gold hosted by tonalite at tallberg laivakangas kopsa and osikonmaki b as a component of volcanogenic massive sulphide deposits e g holmtjarn boliden and pyhasalmi other types are c gold bearing quartz alumina alteration zones formed during the 1 92 1 87 ga magmatic period enasen d gold in massive sulphide and iron ore deposits in bergslagen 3 gold associated with 1 84 1 54 ga granites has been reported from several sites in the shield including quartz veins and contact metasomatic deposits in addition shear zone related gold deposits post dating these granites have been identified in southeastern sweden adelfors 4 in the sveconorwegian domain the gold deposits at bleka eidsvoll glava and hamas are associated with shear zones which developed penecontemporaneously with the intrusion of late 1 0 0 9 ga granites

lead isotopic evidence for the origin of 1 8 1 4 ga ores and granitoids in the southeastern part of the fennoscandian shield abstract new data for the lead isotopic composition of galena from ore deposits related to 1 8 1 4 ga granitoids in the southeastern part of the fennoscandian shield are presented and discussed together with earlier published information on granitoids and associated ore deposits two types of sulphide deposits genetically related to the granites can be recognized based on general metallogenetic features and isotopic composition granite hosted deposits including sn bearing greisen veins in rapakivi granites and peri intrusive deposits including contact metasomatic skarn deposits like pitkaranta and gruvasen in addition epigenetic deposits with a spatial but not a genetic link to the granites occur at various sites an important metallogenetic belt related to 1 7 1 5 ga granitoids can be followed in an east west direction across the shield from lake siljan in sweden to salmi at lake ladoga within this belt a major difference in metal derivation can be recognized ore deposits southwest of the raahe ladoga line i e in areas without archaean basement indicate metal derivation from remelted 1 9 ga svecofennian crust whereas ore deposits in areas underlain by archaean rock units indicate metal derivation from highly metamorphosed archaean sources

metal sources in the early proterozoic svecofennian terrain of central sweden pb isotope evidence the lead isotope composition of three cu zn pb sulfide deposits and one au deposit in the early proterozoic svecofennian of central sweden reveals the existence of two distinct lead sources in the region rockliden and enstern contain lead of upper crustal affinity and are isotopically homogeneous lead in these deposits probably originates from archean derived detritus which was well homogenized by sedimentary processes and deposited over large areas as a component in early proterozoic sedimentary and tuffaceous rocks lead isotopic homogeneity within the tjarnberget and enasen deposits is less pronounced and the less radiogenic character of the lead suggests a source of mantle affinity in areas where metasedimentary rocks are absent or contain very little archean derived material the isotopic signature of mantle derived volcanic rocks is reflected in the ore lead this approach is also applicable to other svecofennian deposits together with deposits in west central finland the rockliden deposit constitutes the northern boundary for lead of upper crustal affinity i e archean derived lead in svecofennian sulfide deposits

thermodynamic properties for bunsenite nio magnetite fe 3 o 4 and hematite fe 2 o 3 with comments on selected oxygen buffer reactions

does europa have a subsurface ocean evaluation of the geological evidence it has been proposed that jupiter s satellite europa currently possesses a global subsurface ocean of liquid water galileo gravity data verify that the satellite is differentiated into an outer h2o layer about 100 km thick but cannot determine the current physical state of this layer liquid or solid here we summarize the geological evidence regarding an extant subsurface ocean concentrating on galileo imaging data we describe and assess nine pertinent lines of geological evidence impact morphologies lenticulae cryovolcanic features pull apart bands chaos ridges surface frosts topography and global tectonics an internal ocean would be a simple and comprehensive explanation for a broad range of observations however we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm soft ice with only localized or partial melting two different models of impact flux imply very different surface ages for europa the model favored here indicates an average age of 50 myr searches for evidence of current geological activity on europa such as plumes or surface changes have yielded negative results to date the current existence of a global subsurface ocean while attractive in explaining the observations remains inconclusive future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within europa today

formation of cycloidal features on europa cycloidal patterns are widely distributed on the surface of jupiter s moon europa tensile cracks may have developed such a pattern in response to diurnal variations in tidal stress in europa s outer ice shell when the tensile strength of the ice is reached a crack may occur propagating cracks would move across an ever changing stress field following a curving path to a place and time where the tensile stress was insufficient to continue the propagation a few hours later when the stress at the end of the crack again exceeded the strength propagation would continue in a new direction thus one arcuate segment of the cycloidal chain would be produced during each day on europa for this model to work the tensile strength of europa s ice crust must be less than 40 kilopascals and there must be a thick fluid layer below the ice to allow sufficient tidal amplitude

europa s differentiated internal structure inferences from four galileo encounters radio doppler data from four encounters of the galileo spacecraft with the jovian moon europa have been used to refine models of europa s interior europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice liquid outer shell but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice liquid shell the size of a metallic core is uncertain because of its unknown composition but it could be as large as about 50 percent of europa s radius the thickness of europa s outer shell of water ice liquid must lie in the range of about 80 to 170 kilometers

the depositional evolution of the svecofennian supracrustal sequence in finland and sweden abstract on the basis of six new zircon ages from metavolcanic rocks together with other isotopic geophysical and geological data the chronological and tectono stratigraphic evolution of the early proterozoic svecofennian supracrustal rocks in finland and sweden are discussed these data suggest that a nw se displacement from the bothnian bay to lake ladoga and a sinking of the southwestern block was initiated 1950 2000 ma ago accumulation of pelitic sediments and greywackes on the subsiding archaean basement continued until about 1860 ma in south central and northern sweden shallow submarine and subaerial volcanics were extruded 1880 1890 ma ago the thick sedimentary piles and volcanic rocks were subsequently intruded by plutonic rocks forming large batholiths 1860 1890 ma ago

a lead isotope study of two sulphide deposits and adjacent igneous rocks in south central sweden two sulphide ore deposits svardsjo and ammeberg occurring in the bergslagen ore district in south central sweden have been examined for their pb isotope compositions analyses of the ore lead reveals a great isotopic homogeneity with compositions that nearly coincide with results obtained from other deposits in the area the stacey and kramers growth model for pb in conformable ores yields model ages however that are 100 150 ma too low in comparison with the accepted geological ages isotopic data from whole rocks and mineral separates indicate an open system behaviour with respect to u and pb on the basis of geological geochemical evidence the sulphide ores in bergslagen are considered to have formed by volcanic exhalative processes the ore lead is interpreted as a mixture of newly added mantle material and a minor archean component it is likely that the main part of the ore lead was derived from volcanic source rocks but most probably sediments interbedded in the volcanic succession also played some role

geology geochemistry and origin of high sulfidation cu au mineralization in the nansatsu district japan the nansatsu district of southern kyushu has been the site of calc alkaline volcanism for the last 10 m y shifting eastward with time associated hydrothermal activity followed deposition of the volcanic host rocks by about 0 5 m y and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions resulting in the formation of high sulfidation cu au deposits at kasuga iwato and akeshi the orebodies consist of 95 wt percent sio 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride sulfate waters these are inferred to have formed when magmatic vapors containing hcl and so 2 condensed into meteoric water the residual silica now quartz orebodies are best developed where the host was initially permeable the margins of the quartz bodies are abrupt with narrow 1 2 m halos representing the reaction front of acid waters isochemically dissolving the host rock the halo comprises alunite strongly zoned in na and k with p rich cores dickite and or kaolinite or pyrophyllite grading out into illite and interlayered illite smectite clays and finally propylitic alteration this pattern is characteristic of deposits of this type throughout the world for example at summitville colorado and lepanto philippines mineralization occurred after initial leaching by the vapor condensates with metals transported by a dense magmatic fluid mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies ore grades are restricted to the quartz bodies gold is most closely associated with enargite and pyrite later minerals include covellite and then sulfur the last stage of activity was steam heated with descending waters oxidizing sulfides to goethite and locally remobilizing au into fractures this varies in degree between deposits erosion exposed the orebodies to supergene weathering continuing the sulfide oxidation and au remobilization stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid delta 18 o 7 or 2 per thousand delta d 25 or 5 per thousand similar to nearby active volcanic discharges with local meteoric water in contrast the clays in the marginal halo have isotopic compositions indicating a delta 18 o shift of 6 to 8 per mil from local meteoric water values probably due to water rock interaction and the delta 18 o values of residual silica quartz may also be due to meteoric water domination fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent nacl equiv during late quartz growth though there is evidence in one sample for higher salinity fluid having been present up to 30 wt percent nacl equiv some inclusions contain daughter minerals of halite and sulfur the t h values of over 1 000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from 200 degrees c at akeshi to about 200 degrees c at iwato and 230 degrees c at kasuga the presence of vapor rich inclusions some with t hv similar to t hl indicate the presence at times of a two phase fluid in the center of the ore zones with depths of about 150 to 300 m below the paleowater table the mineralizing fluid was relatively oxidized sulfide sulfate ratio about 3 1 close to pyrite alunite coexistence under these redox conditions a ph of 3 and over a temperature range of 200 degrees to 300 degrees c aucl 2 super complexing may dominate over hau sh 2 at salinities above about 2 wt percent nacl several conditions are conducive for high sulfidation mineralization to occur 1 a crystallizing magma exsolves a fluid with lower pressure conditions favoring metal fractionation from melt to fluid 2 the exsolved fluid separates into vapor and saline liquid phases due to immiscibility with the latter being metal rich 3 the gas rich hcl and so 2 h 2 s vapor ascends to the surface with at least a portion condensed into meteoric water forming an acid fluid which leaches the host rock to create permeable zones for later mixing and 4 the dense metal bearing fluid also ascends into this leached zoned and precipitates cu sulfosalts sulfides and au upon mixing with meteoric water if the saline liquid is not released from its source adjacent to the magma due to lack of fracturing or if there is a strong hydraulic gradient caused by high relief only the vapor related stage may occur this will leave leached barren rock which is characteristic of many eroded volcanic terranes

material transport across europa s ice shell jupiter s moon europa exhibits a deformed icy surface with salt deposits concentrated along the varied geological features the topographic alignment of salt deposits has been speculated to indicate an endogenic sourcing of the material two way transport of salts from a liquid water ocean beneath the ice shell to the surface and vice versa has been speculated we present dynamical models that demonstrate the incorporation of newly frozen ice into convective plumes within the ice shell caused by convection within the ice shell that drives dynamic topography along the ice ocean boundary the new ice that forms at the freezing front can be transported by the rising ice plumes toward the surface until it is blocked by a high viscosity lid at the surface weakening of the lid by tidal or tectonic forces could then lead to the surface detection of ocean trace chemistry captured in the newly formed ice

a model for the temperature dependence of tidal dissipation in convective plumes on icy satellites implications for europa and enceladus abstract to explain the formation of surface features on europa enceladus and other satellites many authors have postulated the spatial localization of tidal heating within convective plumes however the concept that enhanced tidal heating can occur within a convective plume has not been rigorously tested most models of this phenomenon adopt a tidal heating with a temperature dependence derived for an incompressible homogeneous zero dimensional maxwell material but it is unclear whether this formulation is relevant to the heterogeneous situation of a warm plume surrounded by cold ice to determine whether concentrated dissipation can occur in convective plumes we develop a two dimensional model to compute the volumetric dissipation rate for an idealized vertically oriented isolated convective plume obeying a maxwellian viscoelastic compressible rheology we apply the model to the europa and enceladus ice shells and we investigate the consequences for partial melting and resurfacing processes on these bodies we find that the tidal heating is strongly temperature dependent in a convective ice plume and could produce elevated temperatures and local partial melting in the ice shells of europa and enceladus our calculation provides the first quantitative verification of the hypothesis by sotin et al sotin c head j w tobie g 2002 geophys res lett 29 74 1 and others that the tidal dissipation rate is a strong function of temperature inside a convective plume on europa such localized heating could help allow the formation of domes and chaos terrains by convection on enceladus localized tidal heating in a thermal plume could explain the concentrated activity at the south pole and its associated heat transport of 2 7 gw

temperatures on europa from galileo photopolarimeter radiometer nighttime thermal anomalies galileo observations of europa s thermal emission show low latitude diurnal brightness temperatures in the range of 86 to 132 kelvin nighttime temperatures form an unexpected pattern with high temperatures on the bright ejecta blanket of the crater pwyll and an equatorial minimum in temperatures after sunset uncorrelated with surface albedo or geology the nighttime anomalies may be due to regional thermal inertia variations of an unknown origin which are equivalent to a two to threefold variation in thermal conductivity or to endogenic heat fluxes locally reaching 1 watt per square meter endogenic heat flow at this high level although consistent with some geological evidence is theoretically unlikely

implications of new upb zircon chronology to early proterozoic crustal accretion in northern sweden abstract u pb ages ranging from 1 89 to 1 78 ga are reported for zircon populations formed during major magmatic events in the proterozoic development of northern sweden the studied area includes parts of the continental and the marine svecokarelian domains of the baltic shield the ages record crustal accretion and reactivation off the margin of an exposed archaean craton and show that magmatism was broadly coeval throughout the entire north swedish portion of the svecokarelian province irrespective of regions involved a detailed chronostratigraphy is presented for the skellefte district of northern sweden and comparisons are made with the evolution in other parts of the svecokarelian province two main magmatic phases commencing at 1 89 and 1 79 ga ago are recognized the first represents large scale volcanism and plutonism which in the skellefte district was characterized by the introduction of mainly mantle derived magmas or by magmas formed from rocks with short crustal residence time in the vicinity of the archaean craton some distance to the north the crust generating processes included the mobilization of archaean sources and appear to have continued until c 1 85 ga ago a relatively long period of consolidation and denudation separates this magmatic phase from the one at c 1 79 ga ago the generation of these later migmatite associated granites in the svecokarelian province coincided in time and was possibly associated with those processes which gave rise to the initial plutonism in the neighbouring transscandinavian granite porphyry belt