the evolution of water in the contact metamorphic aureole of the duluth complex northeastern minnesota contact metamorphism of the rove formation by the duluth complex northeastern minnesota resulted from the discontinuous reaction chlorite muscovite quartz cordierite biotite h 2 o and the continuous reaction muscovite biotite quartz cordierite k feldspar h 2 o bulk compositions of hornfelses and mass balance calculations indicate that metamorphism occurred isochemically except for the loss of 0 6 to 2 7 wt h 2 o calculated values of temperature and f h 2 o indicate that during progressive metamorphism h 2 o evolved from the rocks progressively diluted a co 2 rich fluid phase the relations among temperature x h 2 o and progress of the continuous reaction in an average bulk composition suggest that at t 500 c and p 1 500 bars chlorite muscovite quartz in 1 m 3 of rock reacted with at least 0 5 m 3 of a co 2 rich fluid producing a fluid with x h 2 o 0 15 the continuous reaction buffered the composition of this fluid to a value of x h 2 o 1 0 at t 600 c the relation between x h 2 o and the progress of the continuous reaction suggests that the volumetric fluid rock ratio decreased from a value of 0 55 to essentially 0 0 as temperature increased from 500 c to 600 c the study indicates that multivariate devolatilization reactions can be capable buffers of fluid composition the dynamic variables rate of temperature change rate of h 2 o evolution and rate of reaction were determined by a heat conduction model for contact metamorphism the calculated width of the aureole 180 m and temperature distribution within the aureole 500 to 620 c are consistent with observations the calculated reaction rates for the continuous reactions ranging from 1 2 10 4 to 3 10 2 moles m 3 yr are much lower than reported rates for other reactions minimum permeabilities implied by the rate of h 2 o evolution ranging from 10 12 to 5 10 15 cm 3 sec cm 3 rock are 4 to 7 orders of magnitude lower than measured permeabilities of shales these results strongly support the assumption of chemical equilibrium during metamorphism even in contact metamorphic aureoles where the rate of temperature change is great

variations in os isotope ratios of pyrrhotite as a result of water rock and magma rock interaction constraints from virginia formation duluth complex contact zones abstract os isotope ratios in pyrrhotite bearing pelitic rocks of the 1 85 ga virginia formation are variable with perturbations linked to the emplacement of the 1 1 ga duluth complex pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1 1 ga involving a low 187 os 188 os fluid however because rocks with perturbed pyrrhotite os isotope ratios occur 1 km or more from the duluth complex the fluid is unlikely to have been of magmatic origin fluid inclusions in layer parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between 300 and 400 c analyses of fluid inclusions via la icp ms show that the fluids contain up to 1 7 wt na 1 1 wt k 4330 ppm fe 2275 ppm zn and 415 ppm mg the veins also contain pyrite or pyrrhotite plus minor amounts of chalcopyrite bornite pentlandite and sphalerite the re os isotopic ratios of pyrite from the veins indicate that they crystallized from low 187 os 188 os fluids 18 o values of vein quartz range from 7 7 to 9 5 consistent with an origin involving fluid with a relatively low 18 o value between 2 and 5 meteoric water with such a low 18 o value could have interacted with the igneous rocks of the complex and would have acquired os with a low 187 os 188 os ratio strongly serpentinized olivine rich rocks of the complex are commonly characterized by such low 18 o values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the os isotopic system recorded by pyrrhotite in the virginia formation two important observations are that only pyrrhotite bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing os exchange are spatially restricted and not uniformly distributed os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite and where time integrated water rock ratios in the aureole were high enough to provide a supply of os troctolitic and gabbroic rocks of the partridge river intrusion duluth complex are characterized by os isotope ratios that are indicative of variable degrees of crustal contamination os values of 0 543 xenoliths of carbonaceous and sulfidic pelitic rocks of the virginia formation found in the igneous rocks provide evidence that os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle derived magma however residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by 187 os 188 os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole the os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of re os liberation during devolatilization kerogen maturation and pyrite to pyrrhotite conversion processes that contribute to magma contamination followed by os uptake by pyrrhotite during back reaction involving magma and or fluid characterized by a relatively low 187 os 188 os ratio the extreme os uptake recorded by pyrrhotite in the xenoliths as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole is in line with the high os diffusivity in pyrrhotite experimentally determined by brenan et al 2000 our data confirm that os isotope ratios in pyrrhotite bearing rocks may be readily perturbed for this reason caution should be exercised in the interpretation of os isotope ratios in rocks where pyrrhotite may be the primary host of os

selective crustal contamination and decoupling of lithophile and chalcophile element isotopes in sulfide bearing mafic intrusions an example from the jingbulake intrusion xinjiang nw china abstract the jingbulake mafic ultramafic intrusion in the south tianshan orogenic belt xinjiang nw china is a zoned intrusive body composed of gabbro diorite olivine gabbro and wehrlite locally intruded by pyroxenite within a major w e structure both olivine gabbro and wehrlite contain disseminated ni cu sulfides whereas pyroxenite hosts an ore body containing massive net textured and disseminated sulfide ores both silicate rocks and sulfide ores from the jingbulake intrusion have low pd ir ratios 10 63 the silicate rocks contain ni poor olivine generally 187 os 188 os i ratios 0 277 to 0 916 of the silicate rocks are consistent with a major contribution of os from crustal materials on the other hand the net textured sulfide ores from no 8 mineralization zone have high os contents 15 1 to 32 6 ppb low re os 2 0 to 2 3 and low 187 os 188 os i ratios 0 125 to 0 141 indicating a mantle derived origin modeling indicates that 30 crustal contamination is required to explain the radiogenic os isotopic composition if average upper crust is adopted as the crustal contaminant or 5 25 if sulfide rich crustal rock is used as the crustal contaminant however only 5 10 crustal contamination is required to explain the nd and sr isotopes we propose that the decoupling of os from sr nd isotopes in the silicate rocks was due to selective crustal contamination addition of external sulfur from a crustal source may be the key factor triggering sulfide saturation the silicate magma that underwent sulfide extraction before emplacement to shallow depth preserved a crustal os isotope signature continued reaction of sulfide melts with new pulses of mantle derived magma increased the os content and decreased the 187 os 188 os i ratios effectively masking the crustal os contribution in the sulfides

cu isotope variations between conduit and sheet style ni cu pge sulfide mineralization in the midcontinent rift system north america abstract disseminated ni cu pge sulfide mineralization in sheet like intrusions of the duluth complex are characterized by elevated bulk rock cu ni ratios typically in the range of 3 to 8 in order to evaluate whether the addition of crustal cu to parental magmas may have contributed to the high cu ni ratios cu isotope ratios were measured from sulfide minerals in the disseminated mineralization found within the sheet style partridge river and south kawishiwi intrusions of the duluth complex as well as sulfide mineralization in geometrically distinct conduit style deposits in the eagle intrusion and tamarack igneous complex these intrusions were emplaced during early stages of rift evolution 1110 1105 ma from picritic parental magmas with associated mineralization characterized by lower cu ni ratios in addition cu isotope ratios were determined from sulfidic and carbonaceous sedimentary country rocks to the intrusions 65cu values of disseminated sulfide mineralization in intrusions of the duluth complex range from 0 85 to 0 45 whereas values from the eagle intrusion and the tamarack igneous complex range from 0 69 to 1 84 65cu values of the sedimentary rocks range from 0 33 to 1 18 with two samples showing higher values of 2 65 and 3 12 the 65cu values of the duluth complex mineralization is consistent with the assimilation of significant amounts of sedimentary cu only if the 65cu value of the parental mantle derived magma was lower than 0 25 a value less than that proposed for the mantle based on analyses of terrestrial mafic igneous rocks the distinct difference in 65cu values between the sheet style and conduit style mineralization suggests that either mantle sources of the parental magmas in the rift differed or that cu isotope fractionation accompanied magmatic processes i e variable degrees of melting and sulfide retention sulfide liquid fractionation that were responsible for the relative enrichment of cu in the sheet style intrusions of the duluth complex

world class ni cu pge deposits key factors in their genesis magmatic ni cu sulfide deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma and the partitioning of chalcophile elements into these from the silicate melt sulfide saturation of a magma is not enough in itself to produce an ore deposit the appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals and then is concentrated in a restricted locality so that the resulting concentration is of ore grade the deposits of the noril sk region have developed within flat elongate bodies 15 2 0 2 km that intrude argillites evaporites and coal measures adjacent to a major trans crustal fault and immediately below the centre of a 3 5 km thick volcanic basin studies of the overlying basalts have shown that lavas forming a 500 m thick sequence within these have lost 75 of their cu and ni and more than 90 of their pge overlying basalts show a gradual recovery in their chalcophile element concentrations to reach normal values 500 m above the top of the highly depleted zone the ore bearing noril sk type intrusions correlate with those basalts above the depleted zone that contain normal levels of chalcophile elements the high proportion of sulfide 2 10 wt associated with the noril sk type intrusions the high pge content of the ores the extensive metamorphic aureole 100 400 m around the bodies and the heavy sulfur isotopic composition of the ores 8 12 34s are explicable if the ore bearing bodies are exit conduits from high level intrusions along which magma has flowed en route to extrude at surface the first magma to enter these intrusions reacted with much evaporitic sulfur at a low r value and thus gave rise to sulfides with low metal tenors successive flow of magma through the system progressively enriched the sulfides in the conduits losing progressively less of their chalcophile metals and thus accounting for the upward increase in metals in successive lava flows above the highly depleted flows the voisey s bay deposit lies partly within a 30 100 m thick sheet of troctolite interpreted as a feeder for the 1 334 ga voisey s bay intrusion and partly at the base of this intrusion where the feeder adjoins it studies of olivine compositions indicate that an early pulse of magma through the feeder and into the intrusion was ni depleted but that subsequent pulses were much less depleted trace element re os and s and o isotope data and mineralogical studies indicate that the magma pulses interacted with country gneiss probably principally in a deeper level intrusion extracting sio2 na2o k2o and possibly sulfur form the gneiss which accounts for the magma becoming sulfide saturated the jinchuan deposit of north central china occurs within a 6 km long dyke like body of peridotite the compositions of olivine within the dyke the igneous rocks themselves and the ore are all inconsistent with derivation of the body from ultramafic magma as originally supposed and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt magma flow of magma into the intrusion has resulted in olivine and sulfide being retained where the keel was widening out into the intrusion the west australian komatiite related deposits occur in thermal erosional troughs which have developed due to the channelisation of magma flow and the resulting thermal erosion of underlying sediments and basalt by the hot komatiite magma the sediments are sulfide rich and may have contributed substantially to the sulfide of the ores the mineralisation in the duluth complex occurs in troctolitic intrusions along the western margin of the complex as a result of magma interacting with and extracting sulfur from the underlying graphite and sulfide bearing sediments no magma flow channels have been identified so far and the lack of magma flow subsequent to the development of sulfide immiscibility is regarded as the reason why these deposits are not of economic grade when most major ni cu sulfide deposits are compared they prove to have a number of features in common olivine rich magma proximity to a major crustal fault sulfide bearing country rocks chalcophile element depletion in related intrusive or extrusive rocks field and or geochemical evidence of interaction between the magma and the country rocks and the presence of or proximity to a magma conduit the features are thought to explain the three key requirements sulfide immiscibilty adequate mixing between sulfides and magma and localisation of the sulfides discussed and have important implications with respect to exploration

steady state secondary convection beneath lithospheric plates with temperature and pressure dependent viscosity we have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self consistently by the heat transported by convection in this study of the interaction between the lithosphere and upper mantle strongly temperature and pressure dependent rheologies for both newtonian and non newtonian creep mechanisms are employed for a strictly temperature dependent rheology an insignificant amount of heat less than 12 5 mw m2 can be transported convectively for an interior viscosity 0 1021 pas compatible with postglacial rebound on the other hand for similar values of the interior viscosity steady heat fluxes between 20 and 40 mw m2 are produced by introducing pressure dependence into the rheology for the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated once all of the rheological parameters are fixed this finding may have important ramifications for parameterized convection we employ both the single mode mean field approximation and the complete two dimensional equations using finite elements to obtain solutions for the various types of rheologies from evaluating the geophysically relevant observables such as topography free air gravity anomalies surface heat flow and stress fields in the lithosphere we find that the lateral variations of these quantities predicted by a non newtonian rheology are much smaller than those derived from a linear rheology these results suggest that surface variations of geophysical observables are more compatible with a non newtonian rheology in the upper mantle

stable isotope geochemistry of organic matter alteration in animikie basin sediments within the thermal aureole of the duluth ccomplex abstract we have determined total organic carbon toc total carbonate carbon tcc and 13 c values for toc in samples from the early proterozoic animikie basin sediments virginia and biwabik iron formations that have been variably metamorphosed by the duluth complex toc values for the variably metamorphosed virginia formation show no systematic variation with distance from the intrusion toc from 0 06 to 2 54 in the biwabik iron formation samples outside the thermal aureole of the intrusion have toc generally below 1 compared to 1 7 6 2 for samples within the thermal aureole this enrichment may reflect extensive remobilization of organic carbon within the thermal aureole however 13 c org values measured for both the virginia and biwabik iron formations within the thermal aureole are not significantly different from those outside the thermal aureole it thus appears that in the iron formation the thermal shock caused by the intrusion induced migration of labile organic carbon that was isotopically similar to the residual kerogen the relatively low tcc in samples within the thermal aureole of the intrusion appear to indicate decarbonation of the iron formation also occurred

deep convection and brine rejection in the japan sea received 15 october 2002 accepted 13 january 2003 published 19 february 2003 1 direct water mass renewal through convection deeper than 1000 m and the independent process of dense water production through brine rejection during sea ice formation occur at only a limited number of sites globally our late winter observations in 2000 and 2001 show that the japan east sea is a part of both exclusive groups japan sea deep convection apparently occurs every winter but massive renewal of bottom waters through brine rejection had not occurred for many decades prior to the extremely cold winter of 2001 the sites for both renewal mechanisms are south of vladivostok in the path of cold continental air outbreaks index terms 4283 oceanography general water masses 4243 marginal and semienclosed seas 4223 4215 climate and interannual variability 3309 citation talley l d v lobanov v ponomarev a salyuk p tishchenko i zhabin and s riser deep convection and brine rejection in the japan sea geophys res lett 30 4 1159 doi 10 1029 2002gl016451 2003

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

diffusion of osmium in pyrrhotite and pyrite implications for closure of the re os isotopic system abstract in order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite experiments consisted of either 1 isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered os bearing fe sulfide or 2 relaxation of initially high near surface osmium concentrations produced in the latter experiments pyrite only osmium penetration into samples was characterized by depth profiling using rutherford backscattering spectroscopy rbs pyrite or electron microprobe analyses across sectioned run products pyrrhotite results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces with the extent of penetration uncorrelated with run duration images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near surface occurred by solute uptake during step growth and not by volume diffusion prolonged 1000 h relaxation experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for rbs a maximum diffusion coefficient of 2 5 10 23 m2 s at 500 c was calculated experiments involving pyrrhotite over the temperature range of 950 1100 c showed extensive osmium uptake and osmium concentration gradients that conform with fickian diffusion behavior we found that pyrrhotite fe s could be varied by changes in the composition of the starting material and osmium source and over the range of fe s produced in experiments molar fe s 0 83 0 90 we observed no systematic variation in the osmium diffusion coefficient diffusion coefficients measured parallel to the a crystallographic axis were on average 1 4 higher than values measured parallel to c and regression of the c axis data yielded the arrhenius relation d t 1 31 0 16 10 5 m 2 s exp 211 5 14 7 kj mol rt the application of these diffusion data to simple models of diffusive exchange during static or polythermal time temperature histories is used to assess the conditions under which radiogenic osmium will be retained during isothermal annealing calculations indicate that the cores of millimeter sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in 0 5 ma at temperatures exceeding 400 c pyrite undergoing the same process at 500 c requires in excess of 10 ma before crystal cores are affected the relatively short core retention time scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events thus possibly accounting for the scatter that commonly occurs in re os isochrons generated from massive sulfide samples calculated closure temperatures tc for osmium exchange in pyrrhotite yielded values of 300 400 c for grain sizes ranging from 10 to 1000 m these values of tc are similar to those calculated for ar retention in biotite and considerably lower than for sr in apatite and plagioclase for example such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late stage hydrothermal fluids this latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage

homogenization of potential vorticity in planetary gyres the mean circulation of planetary fluids tends to develop uniform potential vorticity q in regions where closed time mean streamlines or closed isolines of mean potential vorticity exist this state is established in statistically steady flows by geostrophic turbulence or by wave induced potential vorticity flux at the outer edge of the closed contours the expelled gradients of q are concentrated beyond this transition lies motionless fluid or a different flow regime in which the planetary gradient of q may be dominant the homogenized regions occur where direct forcing by external stress or heating within the closed isoline is negligible upon the potential density surface under consideration in the stably stratified ocean such regions are found at depths greater than those of direct wind induced stress or penetrative cooling in channel models of the atmosphere we again find constant q when mesoscale eddies cause the dominant potential vorticity flux in the real atmosphere the results here can apply only where internal heating is negligible the derivations given here build upon the prandtl batchelor theorem which applies to non rotating steady two dimensional flow supporting evidence is found in numerical circulation models and oceanic observations

deep convection in the japan east sea a modeling perspective 1 the japan east sea jes is a region of intense air sea interactions recent observations show deep open ocean convection occurring south of vladivostok near the subpolar front to a depth of roughly 1000 m during the winter of 2000 with even more intense convection during the extremely cold winter of 2001 in this region wintertime cold continental air outbreaks combined with strong very cold mountain gap winds give rise to deep convection and intermediate water formation we have reproduced the 2000 event in a three dimensional numerical model and examine its onset and evolution thus shedding some more light on this important process the study demonstrates that the confluence of warm and cold water masses at the subtropical front south of vladivostok is the preferred location of deep convection events in jes as the confluence induced downwelling assists in the deeper penetration of the convective turbulence generated by strong wintertime cooling

stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the duluth complex minnesota abstract thin 3 20 m picritic to noritic sills are found in the proterozoic metasedimentary footwall rocks of the duluth complex minnesota the sills were emplaced prior to the major intrusions that comprise the duluth complex and underwent contact metamorphism along with the country rocks oxygen hydrogen and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals melts and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the midcontinent rift system the pre duluth complex sills exhibit a range in 18o values from 4 9 to 14 8 with values between 6 and 7 generally found in sill interiors high 18o values near sill contacts with high 18o metasedimentary rocks of the pelitic virginia formation or biwabik iron formation coupled with a smooth sigmoidal isotopic profile centered at the contact suggest that oxygen diffusion was an important exchange mechanism the elevated 18o values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma dehydration reactions in the pelitic rocks of the contact aureole liberated high 18o fluids that enhanced subsolidus diffusive exchange advective displacement of the diffusion profiles toward the sill interior is less than 40 cm and suggests that layer parallel flow dominated in the dehydration of the contact aureole elevated but uniform 18o values 9 7 to 10 5 in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity perhaps related to extensive development of microcracks although d values of the pelitic country rocks record a history of dehydration systematic variations of d 64 to 143 and h2o 0 15 to 5 40 wt content are not found in the sills 18o values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium and are consistent with a diffusion dominant exchange process at temperatures near 500 c results of diffusion modelling suggest a duration of isotopic exchange that may have extended from tens of thousands of years to 1 4 ma depending on local controls of porosity and permeability as well as rates of fluid production in the contact aureole localized areas of 18o and d depletion in the sills values as low as 4 9 and 143 respectively denote exchange with meteoric water after interaction with the high 18o metamorphic fluids in the contact aureole although all of the elevated 18o samples in the contact environment may have suffered 18o depletion most exchange with meteoric water appears to be spatially localized and is thought to reflect highly channelized fracture controlled fluid flow sulfur isotopic values of the sills are variable 2 7 to 11 2 and indicative of an evolution involving pre emplacement contamination of basaltic magma and sub solidus exchange with an h2s bearing metamorphic fluid sulfur contents exceed 3 0 wt only within troctolitic to melatroctolitic sills and 34s values of 7 8 to 8 3 are strongly suggestive of pre emplacement contamination by sulfur derived from a proterozoic sedimentary unit high 18o and 34s rocks particularly at sill margins are consistent with either hydrothermal precipitation of fine grained sulfide minerals or isotopic exchange between magmatic sulfides and an h2s bearing metamorphic fluid

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

connections between surface fluxes and the deep circulation in the sea of japan 1 the extent to which seasonably variable surface fluxes and bottom friction can exert a control on the abyssal circulation above localized bottom topography is analyzed in the japan sea region through the use of a three dimensional eddy resolving ocean model our simulations show that the bottom friction exerts a first order control on the magnitude of the eddy driven deep circulation over f h geostrophic contours which is inversely dependent on the bottom drag in accordance with theory seasonal surface flux variability acts as a modifier to this theory depending on the timescale of active forcing relative to bottom friction spin down time possible physical mechanisms for a varying bottom drag coefficient are proposed winter ventilation of isopycnals during deep convection is followed by enhanced drain energy to small scale turbulence and strong energy dissipation in intermediate layers leading to a weakening of deep circulation that contradicts the classic ventilation theory

heterogeneous os isotope compositions in the kalatongke sulfide deposit nw china the role of crustal contamination re os isotope compositions of mantle derived magmas are highly sensitive to crustal contamination because the crust and mantle have very different os isotope compositions crustal contamination may trigger s saturation and thus the formation of magmatic ni cu pge sulfide deposits the 287 ma kalatongke norite intrusion of nw china are hosted in carboniferous tuffaceous rocks and contain both disseminated and massive sulfide mineralization the re os isotope compositions in the intrusion are highly variable norite and massive sulfide ores have os values ranging from 59 to 160 and a re os isochron age of 239 51 ma whereas disseminated sulfide ores have os values from 117 to 198 and a re os isochron age of 349 34 ma the variability of os isotope compositions can be explained as the emplacement of two distinct magma pulses massive sulfide ores and barren norite in the intrusion formed from the same magma pulse whereas the disseminated sulfide ores with more radiogenic os isotopes formed from another magma pulse which underwent different degrees of crustal contamination re os isotopes may not be suitable for dating sulfide bearing intrusions that underwent variable degrees of crustal contamination to form magmatic sulfide deposits

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

origin of pge depleted ni cu sulfide mineralization in the triassic hongqiling no 7 orthopyroxenite intrusion central asian orogenic belt northeastern china the 216 ma hongqiling no 7 intrusion is located in the southern margin of the central asian orogenic belt northeastern china it was the second largest ni producer after jinchuan in china the intrusion crops out in an area of 0 013 km2 on the surface it intruded gneisses and marbles of the paleozoic hulan formation the intrusion is mainly composed of orthopyroxenite 95 vol with minor harzburgite and norite over 90 vol of the intrusion contains disseminated net textured or massive sulfides the bulk sulfide ores recalculated to 100 sulfide have low average pge concentrations 5 6 ppb os 3 1 ppb ir 4 7 ppb ru 2 2 ppb rh 31 ppb pt and 11 ppb pd and high cu pd ratios 3 105 3 107 consistent with the compositions of sulfide liquids segregated from pge depleted magma olivine composition fo 86 mol indicates a fractionated parental magma back calculation shows that the parental magma was possibly derived by olivine fractionation from a primary magma containing 16 wt mgo numerical modeling indicates that pge depletion in the parental magma of the hongqiling no 7 intrusion was due to previous sulfide segregation in the staging chamber whole rock data show light rare earth element ree enrichments negative nb ta anomalies and positive nd t 216 ma values from 3 to 5 which can be explained by mixing of a mantle derived magma with a granitic melt formed by magma underplating in the crust the os t 216 ma values of sulfide ores vary from 50 to 187 the highly variable os t values coupled with similar nd t values indicate preferential assimilation of crustal sulfides during magma rock interaction based on the observations we propose that the parental magma of the hongqiling no 7 intrusion formed by contamination of mantle derived s undersaturated magma by crustally derived granitic melt together with olivine crystallization in a staging chamber the contaminated magma became pge depleted due to sulfide segregation in response to crustal contamination the pge depleted magma became s undersaturated again during ascent due to the negative effect of pressure on the maximum solubility of sulfur in magma upon arrival near the present level of the hongqiling no 7 intrusion a second event of sulfide segregation took place due to addition of crustal sulfides from immediate country rocks the immiscible sulfide liquids and orthopyroxene crystallizing from the magma were retained in a dynamic conduit to form the ore bearing intrusion while much of the magma left the conduit to form some of the nearby sulfide poor intrusions we suggest future exploration in the region to focus on olivine bearing and orthopyroxene dominant intrusions with ages similar to that of the hongqiling no 7 intrusion

the energetic surface circulation patterns of the japan east sea abstract in the period of 1988 2001 226 argos fixed drifters were deployed in the japan east sea jes from these and the historical file of temperature at a depth of 100 m 15 m depth circulation and its variances in relationship to the seasonal mean warm and cold water regions were constructed on 0 5 resolutions the strongest seasonal currents were the east korean warm current ekwc the tsushima current tc the north korean cold current nkcc and local eastwards intensifications along the sub polar frontal patterns the tc and nkcc were observed during summer and drifters passed through the soya strait only in summer the sub polar front occurred along 40 n in the northeastern jes a separate front occurred in winter that was associated with the flow of warm water restricted to the southern jes small eddy energy was found in the cold water regime and large eddy energy in the warm water regime satellite altimeter and drifter tracks were used to mark the wonsan eddy found in the east korean bay a yearly late summer occurrence mechanical energy flux from the mean circulation to mesoscale eddies occurred in ekwc tc and the sub polar front in the eastern jes with eddy kinetic energy doubling time of 15 60 days the yearly mean circulation east of the korean coast is organized into a broad eddy filled east sea current that contains weak anticyclonic gyres in the ulleung and yamato basins it accelerates toward the yamato rise and against the eastern coast of the jes the published numerical model solutions show several historical circulation features but fail to capture many of the new structures described by the drifter data

sulfide saturation and magma emplacement in the formation of the permian huangshandong ni cu sulfide deposit xinjiang northwestern china the eastern tianshan orogenic belt northwestern china hosts 120 mafic ultramafic intrusions and at least six of them host significant ni cu sulfide mineralization of which the permian huangshandong intrusion is the largest the mafic ultramafic rocks of the huangshandong intrusion comprise lherzolite gabbronorite gabbro and diorite individual orebodies are composed of pyrrhotite pentlandite and chalcopyrite as disseminated to net textured or massive to semimassive sulfide mineralization oxide bearing sulfide mineralization contains 1 to 5 magmatic magnetite that has ilmenite and spinel exsolution lamellae silicate rocks and sulfide mineralization have variable whole rock platinum group elements pge 0 38 362 ppb cu 42 97 000 ppm and ni 26 33 000 ppm concentrations the mineralization is broken out into two types termed oxide rich and oxide poor the oxide rich sulfide mineralization has high pge concentrations with low 187re 188os ratios 38 9 151 whereas oxide poor sulfide mineralization has low pge concentrations with high 187re 188os 159 781 both oxide rich and poor sulfide mineralization have highly variable os values 49 189 and 30 278 respectively oxide rich sulfide mineralization contains magnetite with variable amounts of mgo tio2 al2o3 feototal cr v zn and sn indicating that they crystallized from different stages during magma differentiation we propose that the host intrusion and associated sulfide mineralization were derived from high mg basaltic magmas from a mantle source that was previously modified by subducted oceanic slab oxide poor sulfide mineralization of the huangshandong intrusion formed from a magma that underwent 0 03 sulfide removal before emplacement and was thus pge depleted oxide rich sulfide mineralization formed from magma that had much lower amounts of sulfide 0 003 removed in the early stage before entering the huangshandong magma chamber an early sulfide saturation event was likely associated with fractionation of silicate minerals in a deep staging magma chamber the second sulfide saturation event that formed the sulfide mineralization was probably triggered by selective crustal contamination and fractionation in the high level chamber where mixing of magmas and accumulation of immiscible sulfide occurred

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

hydrothermal alteration in the babbitt cu ni deposit duluth complex mineralogy and hydrogen isotope systematics hydrothermal alteration at the babbitt cu ni deposit of the duluth complex is widespread but highly variable in terms of intensity alteration styles range from the filling of microfractures in plagioclase and olivine or minor alteration along grain margins to more intense alteration in the form of macroscopic fracture fillings or massive replacement of primary minerals with or without well developed fractures chlorite occurs as a macroscopic fracture filling within microveinlets in plagioclase and as an alteration of pyroxene biotite and rarely olivine along grain margins or fractures mats of chlorite are intergrown with amphibole epidote calcite and prehnite as part of massive replacement of calcic plagioclase greenalite is found in areas of intense alteration particularly in the vicinity of sulfide mineralization iron rich serpentine occurs along fractures in olivine or as a more massive replacement of olivine in picritic units serpentine composition is related to that of the parent olivine as well as to the degree of serpentinization in addition to its involvement in plagioclase alteration calcic amphibole hornblende actinolite is found as a partial replacement of primocryst and interstitial clinopyroxene ca poor amphibole occurs as an alteration product of orthopyroxene the presence of the assemblage chlorite prehnite actinolite epidote suggests that alteration continued to temperatures less than 400 degrees c the delta d values and h 2 o contents of igneous rocks at babbitt range from 56 to 138 per mil and 0 49 and 4 60 wt percent respectively biotite separates are characterized by delta d values of 67 to 86 per mil and delta 18 o values of 4 2 to 6 1 per mil a range considered normal for biotite in mafic igneous rocks because most of the low temperature reactions between fluids and minerals in the cooling igneous system involved hydration h 2 o content is an approximate indicator of the extent of low temperature alteration rocks with h 2 o contents in excess of approximately 1 5 percent are characterized by delta d values greater than 87 per mil except for a sample of highly serpentinized picrite which has a delta d value of 120 per mil modeling of the isotopic composition of a fluid exsolved from interstitial melt indicates that for samples with minor alteration and h 2 o contents less than approximately 1 wt percent secondary mineral assemblages were probably produced as a d depleted magmatic vapor reacted with primary minerals along cooling fractures and grain margins areas of more intense alteration may have formed from localized accumulations of d enriched vapor released from underlying interstitial melt alternatively intensely altered zones may have formed due to the introduction of relatively d enriched fluid derived from devolatilization of metasedimentary xenoliths and footwall the latter interpretation is consistent with previously reported isotopic ripley and taib 1989 and fluid inclusion pasteris 1989 data both oxygen and hydrogen isotope compositions of one serpentinized picrite suggest that a low 18 o low d fluid of probable meteoric origin has been involved in the serpentinization of olivine rich units an insufficient number of samples have been analyzed to determine if a direct positive correlation exists between platinum group element pge content and degree of alteration in the babbitt deposit however the presence of discrete platinum group minerals in intensely altered zones suggests that most pt and pd remobilization occurred via transport by a d enriched fluid a positive correlation between whole rock delta d values and pd content for samples that contain less than 300 ppb pd suggests that magmatic fluids as well as high delta d fluids derived from metasedimentary country rocks may have been involved in local pt pd redistribution

the role of fluids in the formation of platinum group minerals duluth complex minnesota mineralogic textural and chemical evidence several new occurrences of platinum group minerals pgm including sperrylite ptas 2 taimyrite pd cu 3 sn froodite pdbi 2 michenerite pdbite and moncheite pt bi 2 te 3 are recorded from drill core samples of the basal troctolites of the duluth complex the first documentation of a vanadium chromium hercynite v 2 o 3 ranging from 5 14 5 23 wt in graphite rich samples of the mineralized zone of the duluth complex is also reported the pgm are associated predominantly with serpentinized olivine and secondary magnetite or with altered calcic plagioclase approximately an 75 prehnite actinolite hornblende and chlorite the pd bismutho tellurides are associated with parkerite ni 3 bi 2 s 2 tellurobismuthite bi 2 te 3 bismuthinite bi 2 s 3 hessite ag 2 te native gold and clausthalite pb s se with low temperature sulfides like valleriite 2 fe cu s 3 mg fe oh 2 violarite feni 2 s 4 heazlewoodite ni 3 s 2 and in some instances graphite these associations especially the occurrence of the pgm in altered silicates suggest that the pgm reported in this study were transported and deposited during a late magmatic hydrothermal event which could be related to the introduction of volatiles from the metasedimentary virginia formation and the biwabik iron formation the high cl contents of the serpentine biotite apatite and graphite as well as the high an contents of the plagioclase in contact with the sulfides and pgm further suggest that chloride complexing was a possible mechanism for the transport and deposition of the pgm during the hydrothermal event the occurrence of high vanadium chromium hercynite is also consistent with this interpretation

segregation of magmatic fluids and their potential in the mobilization of platinum group elements in the south kawishiwi intrusion duluth complex minnesota evidence from petrography apatite geochemistry and coexisting fluid and melt inclusions abstract pegmatitic and other felsic rock pockets and dike like intrusions are abundant in the south kawishiwi intrusion of the duluth complex including the basal cu ni pge mineralized units these occurrences are found as pockets pods or as veins and contain abundant accessory apatite and quartz quartz hosts primary fluid inclusions as well as silicate melt inclusions combined microthermometry and raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are co 2 rich 95 mol and contain small amounts of h 2 o 4 5 mol ch 4 0 4 mol and trace n 2 respectively this combined technique also made it possible to measure total homogenization temperatures of the inclusions th tot 225 10 c otherwise not detectable during microthermometry silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt composition of the entrapped melt is granitoid peraluminous and is very poor in mafic components we interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the south kawishiwi intrusion the presence of co 2 in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval the composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the south kawishiwi intrusion and the fluid and melt phases coexisted as a heterogeneous melt fluid system until entrapment of the inclusions coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure 1 7 kbar and thus estimate formation depth 5 8 km for the inclusions chlorine is suggested to behave compatibly in the silicate melt phase in the fluid melt system represented by the inclusions indicated by the high up to 0 3 cl concentrations of the silicate melt and co 2 rich nature of the fluid apatite halogen contents provide further details on the behavior of cl apatite in pegmatitic pockets often has elevated cl concentrations compared to troctolitic rocks suggesting enrichment of cl with progressive crystallization systematic trends of cl loss at some differentiated melt pockets suggest that in some places cl exsolved into a fluid phase and migrated away from its source the segregation of cl from the melt is probably inhibited by the presence of co 2 rich fluids until the last stages of crystallization increasing the potential for the development of late stage saline brines platinum group minerals are often present in microcracks in silicate minerals in late stage differentiated sulfide veinlets and in association with chlorapatite indicating the potential role of cl bearing fluids in the final distribution of pges

sulfur and oxygen isotopic evidence of country rock contamination in the voisey s bay ni cu co deposit labrador canada abstract the emplacement of basaltic magma into sulfide bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation induced changes in magma composition pelitic country rocks of the proterozoic tasiuyak gneiss in the area of the voisey s bay ni cu co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the voisey s bay intrusion sulfur isotopic compositions of the tasiuyak gneiss range from 0 9 to 17 0 values typical for sulfides produced via bacterial sulfate reduction in an open marine environment archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally derived sulfur sulfur isotopic compositions of the sulfide minerals from the voisey s bay deposit show consistent variations both spatially and with rock types disseminated and massive sulfides show a decrease in 34 s to the west with values typically between 0 and 2 in the eastern deeps ovoid and discovery hill zone and between 2 and 4 in the reid brook zone 34 s values of the mushua intrusion to the north and the normal troctolite in the eastern deeps are more positive ranging between 0 5 and 1 8 this range is taken to represent the isotopic composition of primary mantle derived sulfur in the area because the mushua intrusion and normal troctolite show the least geochemical evidence for contamination by country rocks sulfur isotopic data from the reid brook zone are consistent with up to a 50 sulfur contribution from the tasiuyak gneiss correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low sulfide enderbitic and quartzofeldspathic gneisses oxygen isotopic values of gneiss fragments in the basal breccia sequence and feeder breccia suggest that the assimilation process involved a greater proportion of high 18 o contaminant to the west 18 o values of the tasiuyak gneiss 5 9 to 14 0 enderbitic gneiss 6 4 to 8 7 and archean quartzofeldspathic gneiss 9 5 to 9 7 are consistent with an increased proportion of tasiuyak gneiss contaminant to the west isotopic data strongly indicate that sulfur from the tasiuyak gneiss has been involved in ore deposition at the voisey s bay deposit however sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization and that assimilation of both proterozoic and archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic physical mechanisms

platinum group element geochemistry of cu ni mineralization in the basal zone of the babbitt deposit duluth complex minnesota the babbitt cu ni sulfide deposit located in the duluth complex of minnesota contains associated low grade platinum group element pge mineralization interelement correlations show that for both massive and disseminated ore zones rh ru os re and ir are positively correlated to s abundance and suggest that they have behaved as immobile elements however pt pd and au are strongly decoupled from the rh ru os re ir group and show no significant correlation with s cu or ni a pge enriched zone up to 3 000 ppb pt and 1 100 ppb pd is located laterally adjacent to the highest grade cu ni ore the pge enriched zone is characterized by abundant bornite lower sulfide delta 34 s values 1 4 to 8 9ppm than those from other sulfide bearing zones at babbitt and oxide assemblages that include cr bearing hercynite and magnetite rich spinels as well as ilmenite sperrylite ptas 2 is the only discrete pge mineral identified in the babbitt deposit synchrotron x ray fluorescence microprobe analyses indicate that some rocks with a high bulk pge content are characterized by ppm quantities 16 132 of pd in cubanite chalcopyrite maucherite and mackinawire although cl contents of biotite and apatite from mineralized units range from 0 10 to 0 71 and 0 87 to 1 43 wt percent respectively no direct relationship between pge enrichment and cl content of volatile rich phases has been observed geochemical data suggest that both magmatic and hydrothermal processes have been important in the localization of pge at the babbitt deposit the distribution of rh ru os re and ir reflects magmatic concentration processes whereas pt pd and au have been locally mobilized and redistributed during hydrothermal alteration

origin of ultra nickeliferous olivine in the kevitsa ni cu pge mineralized intrusion northern finland the 2 058 4 ma mafic ultramafic kevitsa intrusion is located in the central lapland greenstone belt northern finland it is hosted by a paleoproterozoic volcano sedimentary sequence that contains komatiitic volcanic rocks and sulfide and graphite rich black schists economic ni cu pge sulfide mineralization occurs in the middle part of the ultramafic lower unit of the intrusion two main types of ore are distinguished normal and ni pge ores the normal ore is characterized by 2 to 6 vol disseminated sulfides and average ni and cu grades of 0 3 and 0 42 wt respectively ni cu 1 the ni pge ore has broadly similar sulfide contents but a higher ni grade and lower cu grade as a result the ni cu ratio reaches 15 much higher than in the normal ore the ni pge ores occur as irregular discontinuous lense like bodies in the ultramafic rocks notably the olivines in the ni pge ore contain extremely high ni contents of up to 14 000 ppm which is significantly higher than the ni content of olivine in other mafic ultramafic igneous rocks globally up to 5 000 ppm and in harmony with the associated ni rich sulfide assemblage containing pentlandite millerite and pyrite microprobe mapping of olivine from the ni pge ore suggests relatively low and homogeneous s contents and homogeneous distribution of ni mg fe which is inconsistent with the presence of sulfide inclusions in the olivine grains or diffusion of ni from interstitial sulfides into the olivine grains we therefore conclude that ni substitutes for mg in the olivine lattice the clinopyroxenes from the ni pge ore also have unusually high ni concentrations reaching 1 500 ppm and show a positive correlation with the nickel content of the associated olivine the nicpx niolivine is 0 1 to 0 2 corresponding to high t partitioning of ni between clinopyroxene and olivine k d of 20 can account for the partitioning of nickel between olivine and the sulfide phase consistent with magmatic equilibration these data suggest that the olivine clinopyroxene and sulfides all crystallized from a basaltic magma with an unexceptionally high ni content ranging from 300 to 1 100 ppm the ni pge ores are spatially associated with ultramafic xenoliths olivine in these ultramafic xenoliths have relatively high fo contents up to 90 mol and high ni contents up to 5 200 ppm suggesting that the xenoliths formed from a komatiitic parental magma it is proposed that assimilation by the kevitsa magma of massive or semi massive sulfides associated with komatiitic rocks elevated the ni content of the magma and resulted in the formation of ni pge ores and related extremely ni rich olivines

sulfur and oxygen isotope studies of the interaction between pelitic xenoliths and basaltic magma at the babbitt and serpentine cu ni deposits duluth complex minnesota sulfur and oxygen isotope compositions of two pelitic virginia formation xenoliths and surrounding igneous rocks have been measured in drill core intersects at the babbitt and serpentine cu ni deposits duluth complex the xenolith at the babbitt deposit xenolith 1 shows compositional and textural evidence for partial melting whereas the xenolith at the serpentine deposit xenolith 1 may still be in part connected to the footwall and primary sedimentary mineral assemblages are present in its center the delta 34 s values of the xenoliths range from 6 to 27 per mil a well developed vertical profile of upward increasing delta 34 s values in xenolith 2 is suggestive of initial pyrite formation due to bacterial reduction of sulfate in a sedimentary environment where the rate of sulfate reduction was rapid relative to sulfate supply the extreme variation in delta 34 s values over a vertical interval of less than 20 m illustrates the potential variability of delta 34 s values in the virginia formation the delta 34 s values of magmatic sulfides that formed in response to s assimilation may be equally variable depending upon local sites of contamination and convective homogenization in the magma in situ conservation of sulfur is suggested for xenolith 2 where the fe mg ratio of biotite decreases with increasing bulk rock sulfur content sulfidation reactions involving iron bearing silicates or oxide minerals limited the amount of sulfide liberated by the xenolith assimilation of country rock sulfide occurred either at depth within crustal staging chambers or during magma ascent an oxygen isotope gradient is well developed at the upper contact between gabbronorite norite and xenolith 2 oxygen isotope exchange fronts are poorly developed at the lower contact of xenolith 2 and at both contacts of xenolith 1 at the babbitt deposit the igneous rock at the lower contact is a compositionally distinct troctolite that shows little evidence for country rock contamination intrusion of distinct thin magmatic pulses that cool rapidly after previous devolatilization and possible partial melting of the xenoliths can cause the obliteration of well developed isotopic profiles the lack of concentration gradients that coincide with the oxygen isotope gradient together with results of modeling of mass transfer by diffusive processes suggests that the observed delta 18 o profile was produced during subsolidus cooling the observed profile may be produced by oxygen isotope exchange via fluid assisted grain boundary diffusion within 500 000 yr a duration consistent with the 0 5 to 1 m y time span estimated for emplacement of the major intrusions of the duluth complex

possible sources for ch4 rich fluid inclusions in olivine from troctolitic rocks of the bathtub intrusion duluth complex u s a abstract cumulus olivine from the babbitt deposit in the troctolitic bathtub intrusion duluth complex contains abundant ch4 rich fluid inclusions with common hydrous silicate and carbonate precipitates at their inclusion walls given the active rifting environment in which magma of the duluth complex 1 1 ga has intruded as repeated pulses fluid inclusions in olivine are traceable throughout an entire stratigraphic drill core section with comparable textural features that enable a classification of three generations 1 primary ch4 rich fluid inclusions oriented parallel to growth planes of olivine or arranged in clusters 2 intragranular fluid inclusion planes of ch4 rich fluid inclusions extending towards grain boundaries and 3 transgranular fluid inclusion planes of ch4 rich fluid inclusions closely associated with serpentinization the hydrous minerals and carbonates contained in fluid inclusions of generations 1 and 2 indicate that original constituents of the early homogenous magmatic fluid must have been h2o and co2 upon cooling below 700 c at fixed pfluid 2 kbar and fo2fluid at or below qfm conditions an increase of xch4 at the expense of co2 results in the ch4 rich chemistry of the fluid the formation of late secondary fluid inclusion planes of generation 3 is related to extensive magma country rock interaction these inclusions contain no solids and their composition reflects the reduction of co2 to ch4 during serpentinization in an upper stability limit of 620 c this study emphasizes fluid inclusions as powerful indicators in remodeling chemical changes of the magmatic fluid during crystallization of cumulus phases magma country rock interaction and subsequent alteration

geodynamics of magmatic cu ni pge sulfide deposits new insights from the re os isotope system in this study we reassess crustal contamination and sulfide ore forming processes in some of the largest magmatic ore deposits using published re os isotope data and a modeling methodology that incorporates the r factor defined as the effective mass of silicate magma with which a given mass of sulfide magma has equilibrated in an re os isotope mixing equation we show that there is less disparity between conclusions based on re os isotope data compared to other isotopic systems if the r factor is considered komatiite associated ni sulfide ore systems typically have high os concentrations low re os ratios and near chondritic initial os isotope compositions for magmatic sulfide ores that are interpreted to have experienced relatively low r factors 10 000 for noril sk talnakh and the j m reef stillwater complex in these very dynamic magmatic ore systems crustal contamination processes are more difficult to assess using re os isotopes as the effects of contamination are masked by the r factor process in which sulfide magma equilibrates with extensive amounts of asthenospheric mantle derived magma sulfide protore for these systems may then have been very radiogenic and of crustal origin prior to r factor processes that occurred during transport in feeder conduits and in upper crustal magma chambers this study therefore highlights the need for caution when interpreting the re os isotope geochemistry of sulfide ores from dynamic magmatic systems the results of our reinvestigation of these giant ore deposits suggest that geodynamic processes associated with large magmatic systems including major lithospheric pathways to the surface changes in flow regime coupled magma flow through and magma mixing providing enhanced r factors may be critical to our understanding of the emplacement localization and quality of magmatic sulfide deposits thus the timing and exact mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localization of economic concentrations of magmatic sulfides

secular rotational motions and the mechanical structure of a dynamical viscoelastic earth we give a survey of the analytical methods for computing the linear responses of the rotational axis of a layered viscoelastic earth to surface loading the two classes of eigenspectra involving isostatic relaxation of the mantle and readjustment of the spin axis are emphasized as being essential for a uniformly valid description of polar wander the rotational responses due to the application of an equilibrium phase or a chemically layered boundary condition at the 670 km discontinuity are studied the displacement fields associated with rotational deformation are affected to a much greater extent by the amount of chemical enrichment than are the eigenfunctions associated with isostatic relaxation the present polar motion data from international latitude service i l s can be fit equally well by a pure chemical or an equilibrium phase boundary with a 9 change in density across the interface on the other hand responses from models with a smaller amount of chemical stratification 7 are not compatible with the data the secular variation of the earth s gravitational harmonic j2 is insensitive to the chemical nature of the 670 km discontinuity from the rotational data it is not possible to rule out the existence of a high viscosity region with values of o 1023 p at the bottom 500 km of the mantle although a solution with a relatively uniform viscosity profile o 1022p still can be found inferences of the globally averaged thickness of the elastic lithosphere from rotational data are not affected too much by the particular density stratification used for the lithosphere and upper mantle they lie in the range between 120 and 170 km

petrogenesis of pelitic xenoliths at the babbitt cuni deposit duluth complex minnesota u s a abstract the babbitt deposit consists of disseminated cu fe ni sulfides found within mafic rocks of the duluth complex generally near contacts with underlying metasedimentary rock types host rocks for the deposit include troctolites olivine gabbros gabbronorites norites and occasionally country rock hornfels xenoliths of country rocks are abundant in the deposit and suggest a relationship between sulfide mineralization and country rock contamination country rocks in the babbitt area include those of the middle precambrian biwabik iron formation and both calcareous and non calcareous pelites of the virginia formation xenoliths contain the assemblage cordierite plagioclase biotite orthopyroxene and are thought to have been derived from virginia formation protoliths comparison of protoliths and xenoliths using composition volume element ratio and mass balance techniques suggests that xenoliths have been strongly depleted in volatiles alkalis and si footwall rocks show only a depletion in volatiles neither fluid phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred extensive partial melting of xenoliths with residual enrichment of feo mgo and al2o3 is the most viable transfer process the lack of sio2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible and prior to final emplacement sulfide saturation may have been initiated due to si assimilation with an auxiliary magma chamber however the composition of ores in the babbitt deposit is consistent with saturation being achieved by addition of sediment derived volatile sulfur independent of major element assimilation

deep flow field in the japan east sea as deduced from direct current measurements abstract the mean flows and variability in the abyssal japan east sea are investigated on the basis of direct current observation data the distribution of the mean flow vectors indicates relatively strong cyclonic circulations along the basin periphery and sluggish flows in the interior region strong mean flows of faster than 6 0 cm s 1 are found at the western edge in the yamato and ulleung basins considered to be the western boundary currents in each basin the similarity between the distributions of the mean flow vectors in the upper 400 1000 m and lower layers 1000 m bottom indicates common circulation patterns throughout the water column of the proper water progressive vector diagrams show meandering curves with clockwise and anticlockwise turns due to the passing of mesoscale eddies since a dominant background flow is absent in the basin interior the ratio of eddy kinetic energy to the mean flow kinetic energy reaches more than 50 in the interior region this indicates that the flows observed in the interior region are comparable to those in the basin periphery even though the mean flow is generally weak spectral analysis has revealed that tidal fluctuations are prevailing in the ulleung basin while they are absent in the yamato and japan basins though the inertial oscillation is dominant in all of the basins this indicates that the tidal energy from the east china sea is confined within the ulleung basin suggesting the barrier effect of the yamato rise and the oki spur significant energy peaks are also found in the 2 5 and 5 10 day period bands in the yamato basin the former variation represents the shelf wave activity trapped on the basin edge and the latter is due to eddy migrations associated with the tsushima current

stacking investigations of the dispersion of higher order mantle rayleigh waves and normal modes abstract using a number of stacking techniques including non linear stacking we have systematically investigated both in the travelling wave and normal mode formalisms the dispersion of the third fourth and fifth branches of rayleigh overtones which travel with identical group velocities at similar frequencies and are a prominent part of the horizontal away from the source components of long period seismograms from major earthquakes using data from six major events recorded on the global digital seismological network we have extracted more than 100 normal modes from these three branches in the period range 82 160 s our results are generally in excellent agreement with values computed from available models of the earth and also with previously published values for a well separated part of the third branch

representing topographic stress for large scale ocean models abstract interaction of eddies with seafloor topography can exert enormous systematic forces on the ocean circulation this interaction has been considered previously under idealized circumstances theoretical results are here simplified and extended toward practical application in large scale ocean circulation models among the suggestions is that coarse resolution models can correct a depth independent part of the velocity field toward a velocity given by z flh where z is the vertical unit vector f is coriolis form l is a length scale o 10 km and h is the total depth absence of this tendency may be implicated in a number of systematic defects that appear in present ocean models

re os sm nd and pb isotopic constraints on mantle and crustal contributions to magmatic sulfide mineralization in the duluth complex abstract previous petrologic and stable isotopic studies of sulfide mineralization in the duluth complex have led to the premise that sulfide genesis is strongly linked to the interaction between mantle derived magmas and sedimentary country rocks in a rift zone environment in order to more fully evaluate the nature of this interaction and to gain an insight into the possible importance of externally derived metals in the ore forming process pb sm nd and re os isotopic studies of the babbitt cu ni deposit were initiated rock types examined include low sulfide troctolite that show very little petrologic evidence for contamination disseminated sulfide bearing troctolitic to gabbroic rocks that occur close to the basal contact with country rocks and contain metasedimentary xenoliths and massive sulfide lead isotopic values of whole rocks plagioclase mineral separates and massive sulfides show only subtle differences and are compatible with 3 to 5 contamination of a mantle derived melt with a proterozoic crustal contaminant e nd 1 1 ga values of the troctolite and gabbro samples are chondritic and only the massive sulfides show strong evidence for contamination based on sm nd isotopic values massive sulfides tend to be more sensitive indicators of contamination in the sm nd system because of late stage incorporation of a light rare earth element rich fluid into a ca po 4 apatite component of the immiscible sulfide liquid os 1 1 ga values are also strongly anomalous and range from 500 to 1200 in disseminated sulfide bearing troctolites and massive sulfides these values are also consistent with from 1 to 3 contamination by c rich and strongly radiogenic proterozoic sedimentary rocks elemental and isotopic mass balance calculations suggest that up to 50 of the pb and at least 35 of the os in the sulfide mineralization have been derived from external sources a corollary is that other metals may also be in part derived from external sources which would aid in explaining the compositional diversity of the sulfide mineralization selective assimilation of country rocks by mantle derived magmas may occur in crustal staging chambers during ascent to shallower levels or in situ as a result of devolatilization reactions and partial melting

zircon u pb dating and sulfide re os isotopes of the xiarihamu cu ni sulfide deposit in qinghai province northwestern china the xiarihamu cu ni sulfide deposit is the second largest cu ni deposit in china located in the eastern kunlun orogenic belt however despite previous study the formation and evolution of this d

m m a variable period mantle magnitude for intermediate and deep earthquakes we extend to the case of intermediate and deep earthquakes the mantle magnitude developed for shallow shocks byokal andtalandier 1989 specifically from the measurement of the spectral amplitude of rayleigh waves at a single station we obtain a mantle magnitude mm theoretically related to the seismic moment of the event through m m log 10 m 0 20 the computation ofmminvolves two corrections the distance correction is the same as for shallow shocks for the purpose of computing the frequency dependent source correction we define three depth windows intermediate a 75 to 200 km intermediate b 200 400 km and deep over 400 km in each window the source correctioncs is modeled by a cubic spline of log10t

petrochemical studies of the dunka road cu ni deposit duluth complex minnesota troctolitic sequence divided into three major units oxygen isotope values confirm the chemical studies and indicate that major element assimilation is restricted to within approximately 10 m of country rock inclusions pyrrhotite crystallization followed by separation of a cu rich sulfide melt blending and partial crystallization of chemically similar magma and enrichment of cu in the final residual liquid modified journal abstract

a reassessment of the deep fiji earthquake of 26 may 1932 abstract the 1932 deep fiji earthquake is studied from the point of view of comparing it to the recent large events in the transition zone spectral analysis and body wave modeling suggest a moment of 3 4 10 27 dyn cm within a multiplicative or divisive factor of 1 5 which makes it comparable to but not significantly larger than the great 1994 earthquake 770 km to the north its seismic moment remains well below that of the largest deep shocks 1994 in bolivia or 1970 in colombia relocation efforts show that the earthquake took place on the front edge of the wadati benioff zone in an area where the latter features a complex geometry involving significant warping the mechanism of the earthquake involves down dip compression and can be readily explained in terms of the prevailing large scale stresses in the slabs

on the dynamics of the zapiola anticyclone recent observations obtained in the south atlantic suggest the existence of a strong anticyclonic flow positioned over a major bottom topographic feature known as the zapiola drift here a numerical simulation of the south atlantic is described in which this anticyclone is reproduced and the model is used to diagnose the dynamics maintaining this flow with a mean barotropic transport of 140 sv and bottom velocities of 10 cm s 1 the simulated zapiola anticyclone compares well with in situ observations furthermore the model surface eddy kinetic energy shows a local minimum over the drift in agreement with observations from topex poseidon numerical experiments show that the circulation feature is sensitive to the intensity of the eddy field and to the particular value of the bottom friction both of these tendencies are in agreement with a theoretical explanation of the zapiola anticyclone that has recently appeared elsewhere thus we argue that the anticyclone is maintained by eddy driven potential vorticity fluxes accelerating flow within topographically closed ambient potential vorticity contours as far as we know this is the first south atlantic simulation to reproduce the zapiola anticyclone in a realistic fashion the quantitative success of this experiment is attributed to the use of a topography following sigma coordinate in a spatially well resolved model

re os and o isotopic variations in magnetite from the contact zone of the duluth complex and the biwabik iron formation northeastern minnesota abstract osmium and oxygen isotopic analyses of magnetite from the south kawishiwi intrusion ski of the duluth complex and country rocks of the biwabik iron formation bif were undertaken to evaluate the potential importance of country rock assimilation in the genesis of pge enrichment in the intrusive rocks and to track the response of the re os isotopic system to contact metamorphism of the iron formation oxygen isotopic values of magnetite from contact metamorphosed biwabik iron formation and magnetite rich oxide assemblages from highly contaminated troctolitic rocks range from 7 0 to 13 7 distinctly higher than oxide minerals from less strongly contaminated troctolitic to melatroctolitic rocks of the ski that range from 2 3 to 6 9 187 re 188 os ratios of metamorphosed bif tend to be much lower than values from low grade weakly recrystallized bif only one sample of bif from the contact aureole shows the type of os isotope ratios expected as a result of mixing with a chondritic fluid at 1 1 ga the time of emplacement of the ski both low 18 o and high 18 o magnetite populations from the ski are characterized by 187 re 188 os values less than 7 2 and os isotopic ratios that plot above a 1 878 ga reference isochron for the low grade bif samples although elevated os values are consistent with the premise that the ski was contaminated by crustal material we suggest that the os isotopic compositions reflect a later lowering of re os ratios related to interaction with a fluid potentially that which was involved in the locally extensive serpentinization of the melatroctolitic rocks oxygen and h isotopic values of serpentine and serpentinized melatroctolites indicate that the fluids responsible for hydrothermal alteration t 300 c were low in 18 o and characterized by a large meteoric water component magnetite 18 o values record less than a 15 approach to equilibrium with the fluid involved in the serpentinization process suggesting that perturbations to the os isotopic system under hydrothermal conditions may occur in the absence of significant shifts in magnetite 18 o values

origins of outliers of the huronian supergroup within the sudbury structure abstract the spatial relations between outliers of huronian metasediment and the current outline of the sudbury igneous complex sic have been used previously to constrain the original size of the meteorite impact crater despite uncertainty regarding their structure and origin the outliers are tight to isoclinal gently plunging synclines with northeast southwest trending axes slightly overturned to the northwest major deformation of the huronian supergroup preceded the 1849 ma sudbury meteorite impact event by sufficient time to permit peneplanation of the region impact occurred in a foredeep related to the subduction of the laurentia plate under the wisconson magmatic terranes faults truncate the northwest margins of most of the outliers where examined at several localities the northwest margin bounding faults are invariably occupied by pseudotachylite structural considerations provide evidence for thousands of meters of slip along these surfaces each of which was apparently accomplished in

sensitivity of simulated circulation dynamics to the choice of surface wind forcing in the japan east sea abstract the circulation sensitivity to the choice of wind forcing product is investigated with the nrl layered ocean model nlom configured for the japan east sea monthly climatologies from seven different wind stress data sets and wind stress curl are formed from observed and model derived atmospheric data sets the resulting maps of wind stress curl reveal significantly different spatial patterns and magnitudes even two with largely opposite large scale patterns of wind stress curl these wind sets were used in forcing three sets of simulations 1 8 linear and 1 8 and 1 32 nonlinear in addition seasonally varying straits forcing inflow through tsushima balanced by outflow through tsugaru and soya was included in all the simulations and simulations with straits forcing only were performed the 1 5 layer linear reduced gravity simulations include only the lowest order dynamics mainly munk 1 3 western boundary layers due to both wind and straits forcing and a sverdrup interior the nonlinear simulations add bottom topography multiple internal modes diapycnal mixing and ventilation of layer interfaces at 1 8 resolution only weak barotropic baroclinic instabilities occur but at 1 32 resolution these are much stronger allowing vigorous transfer of energy from the upper ocean to the abyssal layer via baroclinic instability this drives much stronger mean abyssal currents that more strongly steer upper ocean current pathways than at 1 8 i e there is much stronger upper ocean topographical coupling the linear model simulates most of the basic features e g the subpolar gyre with all but the straits forcing only the east korean warm current ekwc and its connection to the subpolar front with all but one wind forcing set but the remaining wind set gives a continuous nearshore branch nb of the tsushima warm current along the coast of honshu in all of the linear simulations with an ekwc the separation latitude from the coast of korea is determined by the latitude 41 n of the tsugaru strait outflow too far north except for one which is also too far north all of the nonlinear simulations include the nb which is mainly due to isopycnal outcropping this creates another inflow to outflow pathway diverting some of the flow from the ekwc and allowing the wind stress curl to compete better with straits forcing in determining the ekwc separation latitude using the 1 8 nonlinear model three of the wind forcing sets yield a realistic separation latitude for the ekwc 37 38 n and two more are marginally successful the two remaining and the simulation with straits forcing only yield ekwc overshoot to 40 n at 1 32 resolution one of the simulations one with forcing that gave marginally realistic separation at 1 8 has a realistic ekwc separation latitude the three wind products that yielded realistic ekwc separation at 1 8 continue to give realistic separation at 1 32 via the wind stress curl but the remainder give realistic separation via upper ocean topographical coupling at 1 32 resolution all but one of the simulations showed a strong yamato rise impact on the pathway of the subpolar front due to steering by abyssal currents at 1 8 resolution only two of the eight showed any such impact one due to abyssal currents driven by vertical mixing in the subpolar gyre in all the other 1 8 simulations the mean abyssal currents were weak but in one case weak abyssal currents were aided by a region of strong negative wind stress curl not seen in the other wind products in addition regions of the ventilation of intermediate and deep water in response to each wind set are identified a series of six 1 32 simulations forced interannually with 6 hourly winds 1979 93 are used to separate the model variability due to wind forcing deterministic and flow instabilities nondeterministic this analysis indicates that flow instabilities are the dominant source of ssh variability over a large majority of the basin especially near 37 38 n where the ekwc separates from the coast and along the pathway of the subpolar front following the north rim of the yamato rise towards the tsugaru strait wind forcing dominates the ssh variability in the coastal wave guide and over the deep basins and troughs

regional dispersion of first order overtone rayleigh waves summary records of the first rayleigh overtone in the 70 110s period range following deep subduction zone events are analysed through standard surface wave techniques to retrieve dispersion characteristics in the oceanic environment our data are in general agreement with published models obtained from regionalization of fundamental rayleigh wave paths a general increase in phase velocity with age is observed with a somewhat smaller effect on group velocities continental and trench area data are processed through a regionalization scheme indicating that the difference between average continental and oceanic velocities both phase and group is about 0 2 km s l at periods of the order of 95 s an investigation of a set of simple models allowing structural heterogeneities between oceans and continents down to variable depth suggests that they probably do not extend past the bottom of the asthenosphere 250 km

gulf stream path and thermocline structure near 74 w and 68 w the synoptic ocean prediction synop experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the gulf stream in the synop inlet array off cape hatteras and in the central array near 68 w modred observations were collected from october 1987 through august 1990 the inlet array measured the surface path and bottom currents where the gulf stream leaves the continental margin to enter the deep water regime small amplitude propagating and growing meanders characterized the variability there the central array measured velocity and temperature as a proxy for density at four levels in the water column as well as the upper and deep level streamfunctions all with mesoscale resolution near 70 w the path envelope exhibited a relative node confined within a 40 km band 55 of the time near 68 w the path envelope was over 3 times as wide due to several elongated steep meander troughs and relatively steep meander crests the crests typically propagated downstream without much growth the troughs often stalled near 68 w steepened and persisted for one to several months two cases evolved into s shaped paths and subsequently formed rings even the time averaged fields showed a small trough in the mean path and thermocline structure whereas meanders of 20 to 60 day periods had similar spectral levels throughout 70 67 w meanders with long periods 85 day accounted for the local minimum in variance at 70 w bottom pressure and velocity observations revealed repeated periods of intense swirl speeds 0 30 m s 1 abyssal eddies the time averaged deep currents exhibited a mean cyclone centered 30 km offshore and downstream of the upper layer mean trough the cross stream slope of the thermocline steepened linearly with path curvature consistent with gradient wind balance structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form

long period love wave overtone data in north america and the pacific ocean new evidence for upper mantle anisotropy abstract two tonga trench earthquakes recorded in north america on wwssn long period instruments have been processed by making use of spatial analysis techniques to retrieve the phase velocities of love wave modes dispersion data up to the third overtone at periods longer than 30 s have been obtained for two broad areas the pacific ocean and north america it is found that the observed love wave phase velocities are not in agreement with the values predicted from the previously published models inferred from rayleigh wave data phase velocity discrepancies as large as 3 4 in the pacific ocean and 2 in north america are observed up to the second overtone preliminary direct computations which have been performed in the framework of transversely isotropic models with the axis of symmetry oriented vertically suggest that the cause for this departure from isotropy probably extends to depths 200 km

pt os isotopic constraints on the age of hydrothermal overprinting on the jinchuan ni cu pge deposit china platinum group element pge mineralization occurs associated with mafic ultramafic rocks in different environments although the pge enrichment is primarily caused by magmatic processes remobilization of pd and pt by hydrothermal fluids has likely been an important mechanism in increasing the precious metal grade in many cases however the timing of pge enrichment by hydrothermal fluid processes is commonly difficult to constrain the jinchuan ultramafic intrusion in northwest china is ranked the world s third largest magmatic ni cu sulfide deposit besides the main ore body consisting of net textured and disseminated sulfides there is hydrothermal mineralization associated with sheared contact zones of the intrusion which shows elevated cu and pt concentrations the unusually high pt is hosted mainly in sperrylite within altered silicates in this study we applied the pt os geochronometer to a cu pt rich ore body yielding an isochron age of 436 23 ma this age is significantly younger than the main ore formation age of ca 825 ma but similar to that of the continental collision event between the qaidam qilian block and alax block of north china this indicates that the intrusion may have been uplifted during the paleozoic orogenic processes from deeper crust resulting in the generation of the cu pt rich hydrothermal ore body our new data provide the first strong age constraints on the hydrothermal pge enrichment showing that the pt os isotope system is potentially a powerful tool for dating hydrothermal overprinting on ni cu pge sulfide deposits

re os isotopic systematics of the voisey s bay ni cu co magmatic ore system labrador canada abstract re and os concentrations and os isotopic compositions have been obtained for massive matrix and disseminated sulphide ores from three environments within the voisey s bay intrusion the ovoid eastern deeps and discovery hill zone in order to assess the role of crustal contamination in the genesis of this large cu ni co deposit these samples have high re concentrations 148 to 288 ppb in 100 sulphide for their common os concentrations 4 8 to 24 ppb in 100 sulphide yielding high re os ratios 12 to 33 these data confirm that the magma parental to the voisey s bay ore system was broadly basaltic in major element chemistry rather than picritic consistent with the low ni cu ratio of the ores 1 5 re os isotopic data exhibit a limited spread in 187 re 188 os 57 to 157 and define an imprecise 1323 135 ma model 3 isochron likely the result of small r factor variations within the ore system the re os isochron age is within error of 1334 ma u pb ages obtained for baddeleyite from the ore bearing troctolites demonstrating that whole rock re os isotopic systematics have remained closed since crystallisation the initial os isotopic composition of the isochron os 1040 200 implies significant magma interactions with radiogenic os that most likely resides in the nain churchill province crust these data are therefore consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components with further addition of base and precious metals as a function of r factor analyses of sulphide separates from the proterozoic tasiuyak churchill and archaean nain gneisses confirm that both units contained significant os 8 6 ppb and 0 38 ppb respectively that was very radiogenic at 1334 ma os 1908 and 5202 respectively yielding crustal residence tchur model ages of 2200 to 2400 ma however these model ages may have been affected by re and or os mobility during the 1 85 ga torngat and 1 1 ga grenville orogeny re os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the nain plutonic suite nps with either tasiuyak 2 contamination or nain 16 contamination gneiss followed by an r factor process 200 to 1000 which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active replenished voisey s bay intrusion magma chamber however the dominance of plagioclase rich high al2o3 magmas in the mafic members of the nps troctolites and anorthosites may indicate that other deeper forms of lithospheric interaction potentially involving mafic lower crust may be crucial to understanding this style of magmatic sulphide ore system

re os isotopic variations in carbonaceous pelites hosting the duluth complex implications for metamorphic and metasomatic processes associated with mafic magma chambers abstract previous mineralogic and isotopic studies of sulfide mineralization in the 1 1 ga duluth complex have suggested that sulfur and possibly metals have been derived from proterozoic sedimentary country rocks the re os isotopic system offers great potential in the evaluation of crustally derived metals in magmatic ore forming processes but an understanding of the mechanisms that could cause isotopic variations in country rock contaminants is a prerequisite a suite of sulfidic and carbonaceous pelitic rocks of the virginia formation country rock to the duluth complex ranging from unmetamorphosed through strongly recrystallized and partially melted xenoliths have been analyzed for their os isotopic composition sulfide mineral separates mainly pyrrhotite kerogen graphite isolates and whole rocks were analyzed whole rocks kerogen and graphite from metamorphosed samples plot between 1 85 ga and 1 1 ga chondritic reference isochrons sulfide separates from all metamorphosed samples plot nearer the chondritic 1 1 ga reference isochron and require exchange with a fluid or melt characterized by a chondritic os isotopic composition potentially derived from the duluth complex devolatilization and partial melting may perturb isotopic systematics when accompanied by the loss of re and os bearing fluids these processes could in part explain the scatter in isotopic values found in kerogen and metamorphosed whole rocks however metasomatic addition of os via interaction with a fluid or melt is required to explain the sulfide isotopic ratios and is consistent with sulfide minerals being more susceptible to os exchange with a fluid than kerogen the relative ease with which pyrrhotite may be reset during fluid interaction as well as the variable degrees of exchange shown by kerogen graphite suggest that a knowledge of the petrologic and isotopic evaluation of country rocks is necessary before a meaningful assessment of the isotopic variations found in igneous rocks or associated ore deposits can be undertaken

re os isotopic composition and pge contents of proterozoic carbonaceous argillites virginia formation northeastern minnesota abstract carbonaceous sulfidic argillites from the proterozoic virginia formation in minnesota have been investigated in terms of platinum group elements pge distribution and re os isotopic signatures kerogen from protoliths and slightly metamorphosed equivalents is characterized by re and os enrichment factors relative to whole rock concentrations that range from 4 to 272 re os isotopic values of kerogen in samples that show no evidence of extensive recrystallization fall near a 1 85 ga reference isochron this age is consistent with that estimated for the virginia formation and suggests that dating of organic rich precambrian sedimentary rocks and the retrieval of data pertinent to the os isotopic evolution of seawater may be possible using the re os isotopic system

use of the mantle magnitude m m for the reassessment of the moment of historical earthquakes the mantle magnitudemm is used on a dataset of more than 180 wavetrains from 44 large shallow historical earthquakes to reassess their moments which in many cases had been previously estimated only on the basis of the earthquake s rupture area we provide 27 new or revised values ofmo based on the spectral amplitudes of surface waves recorded at a number of stations principally uppsala and pasadena among them and most significantly we document a large low frequency component to the source of the 1923 kanto earthquake the low frequency seismic moment is 2 9 1028 dyn cm in accord with geodetic observations on the other hand we revise downwards the seismic moment of the 1906 ecuador event which did not exceed 6 1028 dyn cm

structure of the earth mantle and core the deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years most studies have emphasized quantitative three dimensional mapping of the lateral velocity heterogeneity of the mantle and core these aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes a first generation of global models for the lateral velocity variations in the deep earth has been produced providing tantalizing images of large scale structures suggestive of a non steady state thermal convection system the upper 400 km of the mantle has the strongest lateral velocity variations of up to 10 for shear velocity surface wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations thus the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics the upper mantle models support the contention that continents have deep roots with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km very long wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km as well as throughout the lower mantle these deep seated variations have little correspondence to surface tectonics and efforts to interpret their nature are just beginning the lowermost 200 km of the mantle d region has lateral velocity fluctuations comparable to those in the upper mantle and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the d layer a combined thermal and compositional boundary layer roughly mirroring the lithosphere is a likely explanation for this anomalous zone the core mantle boundary appears to have significant 10 km long wavelength topography presumably sustained by dynamic stresses from deep mantle convection the inner core may have strong lateral heterogeneity or axially symmetric anisotropy suggesting a complex thermal and compositional state significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short period body wave band models for teleseismic p wave attenuation operators have converged with t values of 0 7 1 0 s appropriate at 1 hz and t values of 0 4 0 6 s appropriate at 4 hz regional variations of attenuation are slowly being mapped out as well

investigation of upper mantle structure under young regions of the southeast pacific using long period rayleigh waves abstract long period rayleigh wave group velocity dispersion curves are presented for paths across the east pacific ocean the records are from the ipg site at pamatai tahiti and the ida station at nana for the first time direct path observations of group velocities up to 300 s have been obtained this study shows that for young oceanic regions group velocities are low even at long periods the observations are interpreted in terms of an s wave velocity model by a generalized inversion scheme in the models for young ages the low velocity zone under the asthenosphere lid is well developed with a strong velocity gradient at the bottom of this zone followed at larger depths by a plateau representing a lower velocity zone and a marked gradient at 400 km

shear velocity and shear attenuation models inverted from the world wide and pure path average data of mantle rayleigh waves 0s25 to 0s80 and fundamental spheroidal modes 0s2 to 0s24 summary global and regional structures of the earth s mantle are studied original phase velocities and q 1 of rayleigh waves are converted to equivalent free oscillation periods and q 1 global average upper mantle shear velocity models are derived from the inversions of the world wide average eigenperiods corrected for attenuative dispersion resolution analysis shows that the thickness and velocity of the lithosphere cannot be determined from our data set inverted models have a well developed low velocity zone in the depth range between 70 and 280 km global average q 1 models for the whole mantle are derived from the world wide average q 1 of mantle rayleigh waves and fundamental spheroidal modes the resolution analysis indicates that q 1 at depths shallower than about 100 km cannot be determined by our data set and that the depth resolution for q 1 of the lower mantle is poor all inverted q 1 models are characterized by a well developed high q 1 zone in the depth range between about 100 and 300 km in the upper mantle q has a minimum value of about 90 at depths around 200 km begins to increase at a depth of about 300 km rapidly increases in the depth range from 300 to 400 km and gradually increases at depths between 400 and 800 km q of the lower mantle is about 330 the boundary between the high q 1 upper mantle and low q 1 lower mantle is sharper than has been suggested in the past inverted q 1 models predict qscs 226 to 250 the earth s surface is divided into mountainous oceanic ridge shield and trench and marginal sea regions and a regionalized shear velocity model of the upper mantle is derived the inversions of the pure path eigenperiods without correction for attenuative dispersion show that strong lateral variations in shear velocity exist at depths shallower than 250 km in the upper mantle a low velocity zone for shear waves exists in the upper mantle beneath all the tectonic regions except perhaps the shield region if the sn velocity of the starting model is equal to or higher than 4 7 km s 1 the shield data require a moderate low velocity zone data from the mountainous region require a much lower shear velocity than the shield region at depths shallower than 250 km the oceanic ridge and trench and marginal sea models have a well developed low velocity zone the shear velocity model for the trench and marginal sea region is characterized by a velocity increase at depths between 250 and 400 km this increase in velocity is interpreted as a consequence of the subducted lithospheric slabs beneath many of our trench and marginal sea regions regional differences in rayleigh wave q 1 are found for modes from 0s25 to 0s45 since the resolving power of this data set is limited we could not locate the depths at which regional variations of q 1 exist since the oceanic q 1 for modes from 0s45 to 0s78 appear to be stable the oceanic q 1 model is derived this model has a well developed high q 1 zone q has a minimum value of about 110 at depths between 200 and 300 km and rapidly increases to a high value of about 250 at a depth of about 400 km

re os geochronology of organic rich sediments an evaluation of organic matter analysis methods rhenium and osmium in organic rich sedimentary rocks are dominantly hydrogenous but any nonhydrogenous component will influence the accuracy and precision of the re os date obtained to minimize the influence of any nonhydrogenous re and os we evaluate analysis of isolated organic matter from the whole rock together with whole rock analysis using a cro3 h2so4 digestion medium instead of inverse aqua regia for a black shale unit of the exshaw formation canada this unit previously returned a whole rock re os date of 358 10 ma model 3 geochim cosmochim acta 2002 using inverse aqua regia dissolution organic matter isolated from the whole rock matrix using the hf bf3 technique org geochem 20 1993 249 yields scattered data and a re os date of 449 220 ma model 3 mswd 616 the organic matter analyses show similar 187os 188os values but significantly lower 187re 188os values in comparison to the whole rock analyses we show that the re os systematics of organic matter are altered during chemical isolation and as such we suggest that the hf bf3 method should not be used for re os analysis of organic matter whole rock re os analysis using a cro3 h2so4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method and the re os data identify two distinct initial 187os 188os values for the sample set separate regressions of these data yield precise dates 366 1 9 6 mswd 2 2 and 363 4 5 6 ma mswd 1 6 model 3 which are indistinguishable from the age constraints for this formation 363 4 0 4 ma u pb monazite comparison of the re os dates obtained from aqua regia and cro3 h2so4 methods suggests that the former may contain nonhydrogenous re and os whereas the cro3 h2so4 method dominantly liberates hydrogenous re os from organic matter allowing for better stratigraphic age determinations and evaluation of the os isotope composition of seawater

mineralogic and oxygen isotopic studies of open system magmatie processes in the south kawishiwi intrusion spruce road area duluth complex minnesota the south kawishiwi intrusion located along the western margin of the duluth complex minnesota is one of several composite intrusions that are found in the complex the duluth complex is the principal exposed plutonic portion of the 1 1 ga midcontineni rift system in the spruce road area the south kawishiwi intrusion is divided into seven distinct units that are part of the broader south kawishiwi troctolite series defined by severson tech rep nrri tr 91 13a natural resources research institute university of minnesota duluth 1994 units may be characterized as follows unit i basal accumulation of heterogeneous gabbro troctolite and norite unit ii norite with abundant inverted pigeonite unit iii troctolite and olivine gabbro with local oxide rich layers unit iv melatroctolite troctolite olivine gabbro unit v increased plagioclase abundance in troctolites and leucocratic troctolites unit vi strongly altered troctolite unit vii similar to unit v troctolite and leucocratic troctolite country rocks in the spruce road area are granodiorite to quartz monzonite of the archean giants range batholith sutfide mineralization consisting of 1 5 vol of disseminated pyrrhotite cubanite chalcopyrite andpentlandite occurs in units i ii iii and vi oxygen isotopic analyses indicate that unit ii has experienced extensive crustal contamination 5o values of unit ii range from 69 to 7 l o and are 0 enriched compared with values of 5 1 6 8 o found in other units silica contamination is indicated based not only on 5 0 values but also by the predominance of orthopyroxene in the unit possible high 0 contaminant rocks include the giants range batholith and pelitic rocks of the lower proterozoic virginia formation or biwabik iron formation mass balance computations suggest that units in the spruce road area may be related through varying degrees offractionation of a high al olivine tholeiite magma modeling of trace element concentrations and variations in mineral chemistry suggest that discontinuities within the major units developed by in situ boundaryaayer equilibrium crystallization of solidification zones 20 50 m in thickness followed by recharge of fresh magma upward enrichment of incompatible elements olivine fa content and plagioclase ab content may be effectively explained by this process 5 0 values of uncontaminated rock types are strongly correlative with modal mineralogy and can also be modeled by boundary layer fractionation a parental magma 5 0 value of 6 3 o is calculated for unit vii based on olivine and plagioclase values and is similar to that of several other large layered mafic intrusives