direct dating of mid crustal shear zones with synkinematic allanite new in situ u th pb geochronological approaches applied to the mont blanc massif dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes this objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ u th pb geochronology of allanite a greenschist facies shear zone within the mont blanc massif is documented allanite is synkinematic and belongs to the mylonitic assemblage la icp ms u th pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common pb component the use of measured pb isotope compositions of associated minerals feldspars and chlorite is critical for accurate common pb correction and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage a mean 208pb 232th age of 29 44 0 95 ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions this age reflects the timing of the mont blanc underthrusting below the penninic front and highlights the potential of directly dating deformation with allanite

lead isotopic disequilibrium between sulfide and plagioclase in the bushveld complex and the chemical evolution of large layered intrusions abstract the pb isotopic compositions of coexisting plagioclase and sulfide from the bushveld complex were determined by laser ablation multi collector icpms la mc icpms the samples are of the upper critical zone in the northeast corner of the complex and were collected from drill core and underground mine exposures all the rocks are fresh and exhibit no evidence for alteration weathering or disruption of the pb isotope systematics subsequent to the initial cooling of the intrusion furthermore individual plagioclase and sulfide crystals do not contain enough u to warrant correction for radiogenic in growth for these reasons the measured pb isotope ratios approximate the initial ones for plagioclase 207pb 206pb ranges from 0 98 to 1 02 and 208pb 206pb from 2 26 to 2 35 low 207pb 206pb and 208pb 206pb ratios characterize grain boundaries and partially annealed microcracks some of which contain minute fragments of sulfide and other phases and this accounts for most if not all the heterogeneity exhibited by individual samples real compositional differences exist however in plagioclase from different lithologic layers for example plagioclase 207pb 206pb values vary from 1 004 in norite beneath the merensky pyroxenite to 1 009 in the mineralized pyroxenite and 0 997 in overlying norite in most samples in which sulfide and plagioclase coexist the sulfide 207pb 206pb ratio is lower and 208pb 206pb ratio higher than the corresponding ones in plagioclase for example in a mineralized merensky reef sample average sulfide 207pb 206pb and 208pb 206pb ratios are 0 993 and 2 313 respectively while those in plagioclase are 1 000 and 2 292 in one sample the sulfide is extremely heterogeneous with 207pb 206pb and 208pb 206pb ratios as low as 0 84 and 2 12 in this particular sample the compositions must represent an isolated occurrence of addition of a young pb component the array of sulfide and plagioclase compositions requires multiple sources of pb at the time of crystallization or soon thereafter the disequilibrium between plagioclase and sulfide implies that some of the pb originated from the isotopically distinct country rocks and was introduced at temperatures at which the composition of sulfide but not plagioclase could be modified thus bushveld sulfide and to some extent plagioclase do not reliably record the initial pb isotopic composition s of the parent magma s

new tree ring dates for recent eruptions of mount st helens abstract distinctive patterns of growth rings in increment cores from old growth douglas fir pseudotsuga menziesii stands identify a d 1800 as a more precise date for the eruption of tephra layer t by mount st helens washington layer t was previously inferred to date to about a d 1802 growth patterns also establish a d 1480 as the date of eruption of the earlier layer wn previously estimated as dating to about a d 1500 the timing of radial tree growth places a small limitation on the seasonal resolution of these new tree ring dates

frost rings in trees as records of major volcanic eruptions new data about climatically effective volcanic eruptions during the past several thousand years may be contained in frost damage zones in the annual rings of trees there is good agreement in the timing of frost events and recent eruptions and the damage can be plausibly linked to climatic effects of stratospheric aerosol veils on hemispheric and global scales the cataclysmic proto historic eruption of santorini thera in the aegean is tentatively dated to 1628 26 bc from frost ring evidence

a special issue devoted to the mineral deposits of the sudbury basin the magmatic ni cu platinum group element pge deposits in the sudbury area were discovered in 1883 giblin 1984 as ni has grown from a chemical curiosity through early use in armor plating to its present day pervasive role in modern industry resources have continued to be discovered around the periphery of the sudbury igneous complex and even richer ore zones have been identified in the footwall rocks making sudbury one of the world s largest and most productive mining camps the premining resource production current resource of the sudbury mining camp has been estimated to be 1 648 million metric tons at 1 2 percent ni and 1 0 percent cu naldrett and lightfoot 1993 as of 2000 the camp had produced 9 69 million metric tons of ni 9 59 million metric tons of cu 69 6 thousand metric tons of co 116 metric tons of au 319 metric tons of pt 335 metric tons of pd 37 6 metric tons of rh 23 3 metric tons of ru 11 5 metric tons of ir 3 710 metric tons of ag 3 002 metric tons of se and 256 metric tons of te from 781 million metric tons of ore ontario ministry of northern development and mines information and marketing services section 2002 sudbury remains one of the world s largest producers of ni with an annual production of 114 thousand metric tons plus 137 thousand metric tons of cu 1 8 metric tons of au and 11 4 metric tons of total platinum group minerals ontario ministry of northern development and mines information and marketing services section 2000 because of its great economic significance the sudbury basin has been studied intensively by local mining companies federal and provincial survey geologists and numerous university researchers landmark papers include the post war synthesis by stevenson 1963 the first recognition of the impact origin of the structure by dietz

the nature age and petrogenesis of the cartier batholith northern flank of the sudbury structure ontario canada abstract the cartier batholith exposed about 50 km northwest of sudbury ontario canada is a late archean k rich granitic batholith along with the levack gneiss complex it forms the footwall of the north range of the sudbury igneous complex the cartier batholith dominantly monzogranitic to granodioritic is relatively homogeneous unfoliated and contains k feldspar megacrysts it shows several distinctive chemical features including high concentrations of zr th u and lree and low concentrations of nb ti and hree several discriminants suggest a post orogenic tectonic environment for the batholith this inference is consistent with a u pb age of 2642 1 ma obtained on zircon only slightly younger than the inferred timing of high grade metamorphism of the levack gneiss complex the cartier batholith could have formed as a result of approximately 33 partial melting of the levack gneiss complex

geochemical relationships in the sudbury igneous complex origin of the main mass and offset dikes petrological models relating the different rock types constituting the 1 85 ga sudbury igneous complex are constrained with extensive new geochemical data we show that the main mass felsic norite transition zone quartz gabbro and granophyre have similar ratios of the highly incompatible trace elements e g la sm 4 5 7 la nb 2 8 4 2 th zr 0 04 0 05 and that these variations are consistent with the crystallization and differentiation of magma types largely 80 derived from the upper crust with a smaller contribution from a mantle source although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact we find no principal reason why this model should be rejected however we propose that a small contribution of mantle derived picritic magma is required to explain the abundant ni cu and platinum group elements pge in the sudbury deposits as well as the compositions of the ultramafic inclusions mgo 12 36 wt fo sub 68 87 olivines with 450 3 700 ppm ni and abundant chrome rich spinel and the magnesian composition of the mafic norite 8 14 wt mgo and the sublayer 6 12 wt mgo we believe that the main mass of the sudbury complex achieved its present composition through incorporation of up to 20 percent mantle derived picritic magma emplaced along crustal fractures produced by the impact event these picritic magmas entered the melt sheet as a dense plume vigorously mixing with it and due to the marked compositional shift the mixed magma formed magmatic sulfides which sank through the magma column depleting the melt in ni cu and pge since both the felsic norite and granophyre have indistinguishable ratios of the incompatible trace elements we see no requirement to derive these units of rock by the crystallization of magmas derived from different sources rather the compositional difference between the felsic norite and granophyre is attributed to the in situ differentiation of the magma we show that the main mass has many compositional traits similar to those of most of the offset dike quartz diorites e g the parkin offset dike la sm 6 3 la nb 4 5 th zr 0 05 and of embayment related leucocratic norites from the whistle mine la sm 6 2 la nb 5 0 th zr 0 02 these rocks have compositions intermediate between the felsic norite and the granophyre and therefore crystallized from the same magma type arguably the unmineralized quartz diorites provide the best possible estimate of the original magma from which the sudbury complex crystallized in detail there are subtle variations in composition within and between offset dikes with the largest difference being between the north and south range offsets the north range offset dikes cut archean granitoids and gneisses and have elevated sr la yb la sm and gd yb and low tio 2 whereas the south range dikes cut early proterozoic sediments mafic volcanics and intrusions and have low sr la yb gd yb la sm and high tio 2 these differences may be caused by the assimilation of different country rocks during emplacement of the dike a strongly mineralized offset dike at the creighton mine has geochemical variations that are different when compared to the main mass and in the case of creighton are more similar to the local mineralized sublayer these data suggest that mineralized and barren quartz diorites have different geochemical compositions and that these traits may be of value in mineral exploration

evolution of palaeoproterozoic mafic intrusions located within the thermal aureole of the sudbury igneous complex canada isotopic geochronological and geochemical evidence abstract impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth age dating of impact events is critical in correlating cause and effects for these catastrophic processes the falconbridge and drury township twp intrusions were emplaced at the contact between neoarchaean basement and palaeoproterozoic volcanosedimentary rocks and also lie at the southeast and southwest edges of the sudbury igneous complex sic within its thermal contact aureole the falconbridge twp intrusion is dated at 2441 3 ma by u pb zircon with evidence of archaean inheritance from its host granitoids granitoids from the southernmost abitibi subprovince are dated here between 2670 11 ma for an undeformed algoman granite and 2696 18 ma for a foliated granitoid consistent with existing data from the abitibi greenstone belt and from the wawa subprovince major and trace element geochemical evidence common pb isotopic compositions and end2440 values between 0 and 1 are all consistent with a palaeoproterozoic origin for the falconbridge twp intrusion and support inclusion in the east bull lake type suite of leucogabbroic plutons and sills in contrast the drury twp intrusion gives a u pb zircon age of 1859 13 ma coincident with the date of sic emplacement while the major and trace element compositions are comparable to the falconbridge data the drury displays significant heterogeneity in end2440 with values ranging from 3 7 to 0 1 and contains more radiogenic pb isotopic compositions field geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the sic itself and indicates that the drury too is a palaeoproterozoic intrusion this requires that apparently unshocked undeformed magmatic looking zircon has been grown or reset in a postmagmatic setting this has significant implications for the identification of mantle derived magmas and crustal remelts associated with large impact craters a resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen complexing is proposed inducing rapid postshock lead loss and subsequent annealing

shallow impact isotopic insights into crustal contributions to the sudbury impact melt sheet abstract the largest known terrestrial impact melt sheet occurs within the 1850 ma sudbury structure ontario in order to evaluate the relative contributions of different target lithologies to the melt sheet we have investigated the pb isotope compositions of feldspar separates from early formed quartz diorite magmas within offset dykes from around the impact structure the samples define a linear array on plots of age corrected 206 pb 204 pb versus 207 pb 204 pb samples from offset dykes hosted by the huronian supergroup south range have a range of 206 pb 204 pb 1850 from 15 424 to 17 255 and 207 pb 204 pb 1850 from 15 390 to 15 801 whilst those hosted by archean gneisses of the superior province north range cluster around 206 pb 204 pb 1850 14 8 and 206 pb 204 pb 1850 15 1 these values can be approximated by binary mixing between the two major groups of target lithologies a mix of 60 70 of superior province gneisses with 30 40 of huronian metasedimentary material closely matches the pb isotope compositions of north range offset dyke samples whereas in the south range the required huronian component is up to ca 80 these mixing proportions are consistent with sr nd and os isotope and trace element constraints a third minor component either locally exposed paleoproterozoic mafic rocks or the lower crust is also required however the isotopic trace element and ni cu platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component a major contribution of huronian supracrustal material which had a pre impact thickness of up to 12 km is required to explain the chemical characteristics of the impact melts which also have a strong upper crustal affinity e g eu sm 0 22 rb sr 0 2 0 35 as such a shallower level of melting is apparent than that predicted by many previous impact models for the sudbury event this can be accommodated by considering approach trajectories for the impactor oblique to the earth s surface in addition the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage and may not have been completely homogenised during crater formation our findings have significant implications for the nature of the sudbury impact event the evolution of the melt sheet and the crustal sources of metals contained in sudbury s world class ni cu pge sulphide ores

sudden probably coseismic submergence of holocene trees and grass in coastal washington state growth position plant fossils in coastal washington state imply a suddenness of holocene submergence that is better explained coseismic lowering of the land than be decade or century long rise of the sea these fossils include western red cedar and sitka spruce whose death probably resulted from estuarine submergence close to 300 years ago rings in eroded bark free trunks of the red cedar show that growth remained normal within decades of death rings in buried bark bearing stumps of the spruce further show normal growth continuing until the year of death other growth position fossils implying sudden submergence include the stems and leaves of salt marsh grass entombed in tide flat mud close to 300 years ago and roughly 1 700 and 3 100 years ago the preservation of these stems and leaves shows that submergence and initial burial outpaced decomposition which appears to take just a few years in modern salt marshes in some places the stems and leaves close to 300 year old are surrounded by sand left by an extraordinary landward directed surge probably a tsunami from a great thrust earthquake on the cascadia subduction zone

evolution of the sublayer of the sudbury igneous complex geochemical sm nd isotopic and petrologic evidence abstract the mineralized sublayer at the base of the sudbury igneous complex sic consists of two variants the noritic contact sublayer and radial and concentric quartz dioritic offset dykes both are characterized by the presence of significant quantities of ni cu pge sulphides and by a prominent population of recrystallized diabasic textured and melanocratic to ultramafic fragments the two variants of the sublayer contain compositionally distinct inclusion populations and inclusion bearing matrices contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments the compositional variation in sic rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt s the basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase and is geochemically and isotopically identified with huronian basalts which comprise the south range footwall with ce yb n ratios of around 2 5 and e nd 1850 between 2 to 5 the melanocratic inclusions in the sublayer are typically coarse grained and undeformed with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet which has ce yb n ratios of around 10 and e nd 1850 around 9 calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet it is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact this basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling offset dykes were emplaced and the magmas effectively removed from the system subsequently continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer no extracrustal e g mantle component is envisioned in this model to explain the silicate compositional distributions in the sic and mafic crustal rocks in the target zone are implicated as the metal source for the sic deposits

in situ pb isotope analysis of pyrite by laser ablation multi collector and quadrupole icpms pb isotope ratios measured in the mineral pyrite provide a valuable petrogenetic tool with widespread applicability in order to interpret complex structural and mineralogical textures however a method of in situ analysis is essential while laser ablation icpms is ideally suited to this task the low melting point of sulfide the highly variable and often high pb contents and the potential presence of relatively radiogenic inclusions introduce a number of analytical problems unique to pyrite pb isotope analysis here we address these issues using results obtained on two very different analytical systems based around multi collector and quadrupole icpms instruments respectively we suggest that controlled ablation of pyrite is only achieved at low laser fluence and that under these conditions standardisation using silicate reference materials is inappropriate and natural pyrite standards are to be preferred the inherent variability in pb and sometimes u concentrations in pyrite requires careful selection of detector systems for optimal analysis and in this regard both quadrupole and multi collector icpms instruments can play important and complimentary roles multi collector instruments provide higher precision analyses but detector configurations can prohibit simultaneous measurement of u th and pb furthermore the micrometer scale variability in pb concentrations can cause problems for both faraday cup and ion counting detection systems in contrast quadrupole icpms systems allow simultaneous measurement of u th and pb and have more flexible detection systems with many orders of magnitude dynamic range but are unable to produce high precision data results are presented for two different analytical systems and demonstrate a very strong dependence of data quality upon signal size in addition they allow some estimation of the limiting precision obtainable by these methods finally a geological example is provided from the giant sukhoi log sedimentary au deposit of russia

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

osmium isotope evidence for a crustal origin of platinum group elements in the sudbury nickel ore ontario canada abstract sulphide ores from the international nickel company s inco creighton mine sudbury were analysed for osmium isotope ratios by icp ms rhenium and osmium abundances were determined by isotope dilution using a mixed spike in solid solution in a nickel sulphide matrix calculated initial 187 os 188 os ratios at 1 85 ga the emplacement age of the sudbury complex cluster around 0 60 the occurrence of less radiogenic compositions is attributed to post emplacement open system behaviour of the re os system the creighton results strongly overlap the initial ratios of published osmium data from two other sudbury mines suggesting a narrow range of isotope ratio in the original ore within the range of estimated osmium isotope compositions in the country rock at 1 85 ga using published and new data therefore the sulphide ores of the sudbury complex can have an entirely crustal source without a mantle derived contribution this conclusion is consistent with published neodymium isotope data for the complex and supports the meteorite impact hypothesis since this is the most effective means of fusing the large quantity of crustal rocks necessary to generate the complex the extraction of platinum group elements pge from this silicate melt by a nickel sulphide liquid was probably analogous to the laboratory fire assay procedure

isotopic heterogeneity in the sudbury impact melt sheet abstract a unique terrestrial large impact melt sheet is preserved in the 1850 ma sudbury structure ontario we have undertaken a pb isotope investigation of the southern limb of the melt sheet termed the south range main mass the model initial pb isotope ratios 207 pb 204 pb m vary stratigraphically through the predominantly quartz monzogabbroic lower unit varying from 15 40 to 15 45 at the base to ca 15 35 at the top of the sequence lateral variations of similar range occur in basal lower unit samples over scales of less than 5 km the range of these variations is similar to those of locally exposed upper crustal target rocks and it is evident that the melt sheet has efficiently preserved inherited variability during the violent phases of crater formation superheated impact melts are expected to be well mixed mechanically therefore significant post impact melting of target rocks fallback material and entrained clasts is required to explain such heterogeneity the sudbury structure hosts world class ni cu pge sulphide ore deposits systematic variation in 207 pb 204 pb m occurs throughout sulphide ores within the creighton embayment from massive 15 42 15 45 to interstitial ca 15 40 15 41 and disseminated ca 15 39 sulphide linking the pb isotope composition of these ores to the immediately overlying lower unit stratigraphy a protracted sulphide segregation history is apparent massive sulphides segregated early prior to or during initial silicate crystallisation although the total time involved in sulphide accumulation spanned much of the crystallisation of the lower unit it is also shown that lateral variations in ni depletion throughout the main mass correlate with pb isotopes those segments with the strongest chalcophile element depletion signatures reflecting the accumulation of significant basal sulphides have high initial pb isotope values consistent with early sulphide segregation the characterisation of pb isotopic heterogeneity has therefore provided insights into the evolution and scales of mixing of the melt sheet with the identified chemical variability between melt cells having a significant influence on ore forming processes

the sudbury structure controversial or misunderstood the origins of the sudbury structure and associated igneous complex have been controversial most models call for a major impact event followed by impact induced igneous activity although totally igneous models are still being proposed much of the controversy is due in our opinion to a misunderstanding of the size of the original sudbury structure by analogy with other terrestrial impact structures the spatial distribution of shock features and huronian cover rocks at the sudbury structure suggest that the transient cavity was 100 km in diameter which places the original final structural rim diameter in the range of 150 200 km theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in 104 km3 of impact melt more than sufficient to produce a melt body the size of the igneous complex present volume 4 8 103 km3 for the igneous complex to be an impact melt sheet it must have a composition similar to that of the target rocks evidence for this has been presented previously for sr and nd isotopic data which suggest a crustal origin here we also present new evidence from least squares mixing models that the average composition of the igneous complex corresponds to a mix of archean granite greenstone terrain with possibly a small component of huronian cover rocks this is a geologically reasonable mix based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size the igneous complex is differentiated which is not a characteristic of previously studied terrestrial impact melt sheets this can be ascribed however to its great thickness and slower cooling that large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted if this working hypothesis is accepted namely that both the sudbury structure and the igneous complex are impact in origin then previous hybrid impact igneous hypotheses can be discarded and the sudbury structure can be studied specifically for the constraints it provides to large scale cratering and the formation of basin sized multiring impact structures

in situ pb pb isotopic dating of sulfides from hydrothermal deposits a case study of the lala fe cu deposit sw china precisely dating different generations of mineralization is important for understanding the origin and tectonic environment of ore formation in this study we have dated two generations of sulfides from the lala fe cu deposit sw china using in situ pb pb isotopic data obtained by the la mc icpms technique the lala deposit is composed of fe cu ore bodies hosted in sedimentary volcanic rocks of the paleoproterozoic hekou group it has a paragenetic sequence of pre ore banded pyrite stage i and na alteration stage ii followed by early fe stage iii and late cu stage iv mineralization the sulfides in the deposit have dominantly formed in two stages including pyrite from stratiform pyrite bands in the host rocks i e stage i and epigenetic pyrite chalcopyrite in fe cu ore i e stage iv the sulfides of both stages have highly radiogenic pb isotopic ratios which form linear trends in the 207pb 204pb versus 206pb 204pb diagram the regression trend for stage i pyrite grains defines an errorchron age of 1669 110 ma mswd 32 undistinguishable from zircon la icpms u pb ages of volcanic rocks in the hekou group i e 1700 ma the regressions for stage iv pyrite and chalcopyrite yield younger errorchron ages of 1088 88 ma mswd 93 and 1053 220 ma mswd 15 respectively taking pyrite and chalcopyrite together an errorchron age of 1053 75 ma mswd 108 is obtained these errorchron ages agree well with the molybdenite re os age of the fe cu ore 1086 8 ma despite the large error and mswd values that are interpreted to be related to the small variation of initial ratios or post ore open system behavior the good agreement with the precise zircon u pb and molybdenite re os ages indicates that the pb pb errorchron ages for stage i and iv sulfides are geologically meaningful therefore this study shows that the in situ pb isotopic analyses can yield geologically meaningful pb pb ages for radiogenic pb rich sulfide minerals we also propose that in situ pb pb isotopic dating may be more promising for precambrian deposits containing sulfides with ingrown pb particularly for those involving multiple mineralization hydrothermal events but without relevant minerals for precise re os or u pb dating

two high precision u pb zircon ages for the sudbury nickel irruptive two independent u pb zircon ages were measured using norite samples from the north and south ranges of the sudbury nickel irruptive ages of and ma were obtained with a probability of fit within experimental error of 51 and 57 respectively previous determinations by the rb sr whole rock method gave ages between about 1700 and 2000 ma with errors of the order of 100 ma the difference in precision is inherent in the two methods zircons from the norite are highly radiogenic and almost concordant whereas rb sr whole rock ages are determined by very small increments in radiogenic 87sr the two zircon ages have completely different discordia so that all simple models for lead loss by continuous diffusion can be excluded

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

testing for pb isotopic differences between minerals in the kiglapait layered intrusion by la icp ms abstract mineral scale pb isotopic variations in cumulates of the kiglapait intrusion the largest troctolitic intrusion of the proterozoic nain plutonic suite in labrador canada are evaluated directly in thin section by single spot la icp ms analysis of minerals with low pb concentrations e g plagioclase 0 31 3 3 ppm clinopyroxene 0 06 1 1 ppm at high spatial resolution

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

allanite u th pb geochronology by laser ablation icpms abstract the rare earth element ree rich epidote group mineral allanite has great potential as a geochronometer for a wide range of geological processes its utilization has been hampered by several analytical challenges often high common pb contents 208pbcomon 208pbtotal from using a 213 nm nd yag laser coupled with an agilent 7500cs quadrupole icpms analytical protocols focused upon minimizing the causes of matrix sensitivity in the laser ablation icpms process this has primarily been achieved via dynamic raster ablation which greatly reduces time dependent laser induced elemental fractionation accordingly a non matrix matched external standardization approach is adopted utilizing the zircon standards plesovice 91500 and gj1 accurate common pb corrections are critical to allanite geochronology and here we advocate an approach based upon the measured intensity of 204pb as it minimizes assumptions and allows for simple and robust error propagation for each of the allanite reference materials mean common pb corrected 208pb 232th ages and tera wasserburg regression ages are within uncertainty of reference values three of the samples siss bona and avc have significantly higher 206pb 238u ages which reflects incorporation of 230th during crystallization and hence 206pb excess such isotopic disequilibrium together with the high th u ratios of allanites up to 1000 makes the th pb dating system preferable particularly for relatively young allanites i e phanerozoic that accurate age information has been generated from allanites of wide ranging composition suggests that matrix effects are not significant in our analyses on the scale of uncertainties generated 0 5 to 1 5 2 on mean 208pb 232th ages the final propagated uncertainty in ages is a function of the common pb content uncertainty in the measurement of 202hg 204pb and uncertainty in the isotopic composition of the common pb inherited into the mineral our results show that accurate u th pb geochronological information at geologically useful levels of precision can be determined from allanite with relatively simple analytical and data reduction protocols and without the requirement for matrix matched standardization

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

search for solar periodicities in miocene tree ring widths we have analysed ring width records of two miocene coniferous trees with an absolute age of 15 20 million years such data may reflect solar periodicities from the prehistoric past evidence for a main period of 12 5 years has been found this is near the present basic solar activity period of 11 years other periodic components appear at 220 150 and 53 years respectively which may also be related to solar variations

application of gauss s method to magnetic anomalies of dipping dikes quite a few computerized interpretation schemes on the inversion of a magnetic profile across a dike have been reported recently mcgrath and hood 1970 described an essentially trial and error method in which each of the dike parameters is varied in turn by an arbitrary amount until a set of best fit parameters is found rao et al 1973 described a method of differential correction to improve the initially assumed dike parameters similar works may include those by moo 1965 hall 1968 bosum 1968 and others

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

simultaneous measurement of sulfur and lead isotopes in sulfides using nanosecond laser ablation coupled with two multi collector inductively coupled plasma mass spectrometers abstract we herein report the coupling of a nanosecond laser ablation system with a large scale multi collector inductively coupled plasma mass spectrometer nu1700 mc icpms np 1700 and a conventional nu plasma ii mc icpms np ii for the simultaneous laser ablation and determination of in situ s and pb isotopic compositions of sulfide minerals we found that the required aerosol distribution between the two spectrometers depended on the pb content of the sample for example for a sulfide containing 100 3000 ppm pb the aerosol was distributed between the np 1700 and the np ii spectrometers in a 1 1 ratio while for lead contents 3000 and

resolving volcanic activity of 20ma ago with relative accuracy of 1 yr from tree rings of petrified woods tree rings of petrified woods were analyzed to resolve a volcanic activity of 20 ma ago with a relative accuracy of 1 yr samples are those in the volcanic breccia of the early miocene formation of volcaniclastic rocks and tuffaceous lake sediments in central japan field observations indicate that these petrified woods are the fossils of the trees that were carried away from their growth sites with volcanic breccia ejected upon explosive eruption and included in it using a dendrochronological procedure we measured the relative years of the last rings of the petrified woods the relative years of the last rings are concentrated in 17 yrs where the ring width patterns show an anomalous behavior similar to one observed in firs growing in the area devastated by mt st helens we suggest that the last formed tree ring of a petrified wood is a record of volcanic eruption and that the activity of volcano continued over a period of 17 yrs in which eruption occurred at least five times high resolution regarding the activity period demonstrates uniqueness of tree ring study to investigate volcanism in the geologic past

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

re os isotope systematics of ni cu sulfide ores sudbury igneous complex ontario evidence for a major crustal component sudbury igneous complex sublayer ores from the levack west falconbridge and strathcona mines were analyzed for their re and os concentrations and os isotopic compositions the re os isotope systematics of three ores from the different mines give isochron ages of1 840 60 ma 1770 60 ma and 1780 110 ma suggesting that the re os system became closed at the time of or soon after the 1850 1 ma crystallization age of the complex the os isotopic compositions of different portions of the complex at the time of crystallization varied considerably with initial187os 186os1850 ranging from 4 64 at levack west to 7 55 at strathcona these heterogeneities require that the os and probably also the other platinum group elements contained in the ores were derived from at least two sources in addition the high initial187os 186os ratios indicate that the os was derived predominantly from ancient crust previous studies have suggested that the complex either crystallized from a mixture of mantle derived basaltic melt and ancient continental crust or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact results of modelling suggest that if a contemporaneous mantle derived basaltic melt was involved in the origin of the sic it likely contributed 50 of the os to all three ores the large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact

paleoclimatic control on the composition of the paleoproterozoic serpent formation huronian supergroup canada a greenhouse to icehouse transition unlike other feldspathic arenites in the 12 km thick huronian supergroup strata of the 250 350 m thick serpent formation contain abundant plagioclase deposited in a distal alluvial setting the serpent formation consists of arenites with minor shale and siltstone the plagioclase abundance could reflect unroofing of a unique source relative to other huronian units or could be due to less intense chemical weathering major trace and rare earth element ree geochemistry of the serpent formation are used to evaluate these two possibilities serpent formation source rocks form part of the archean superior province which mainly consists of tonalitic granitic and supracrustal assemblages and their metamorphosed equivalents in al2o3cao na2ok2o acnk space shales define a linear array which indicates that the unweathered source contained plagioclase and k feldspar in the ratio of 5 1 this particular array indicates that serpent fluvial systems tapped a variably weathered carapace developed on the superior province rather than purely fresh bedrock a modelled mixture of 80 tonalite and 20 granite group rocks reproduces ree and trace element characteristics of the serpent formation sandstones and shales average eu eu 0 87 lan smn 4 4 gdnybn 1 8 and th sc 1 65 mass balance considerations indicate that less than 5 supracrustal group rocks can be added at the expense of the granite group a unique source for serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition plagioclase k feldspar ratios in serpent sandstones range from 2 1 to 1 2 which achieves mass balance among source mudstone and sandstone compositions when plotted in acnk space achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles these findings indicate that plagioclase preservation in the serpent formation is related to less intense compared with other huronian arenites paleoweathering conditions probably a harbinger of the widespread continental glaciation recorded in the overlying gowganda formation

tree ring evidence for climatically effective volcanic eruptions abstract ringwidth variations from temperature sensitive upper timberline sites in the sierra nevada show a marked correspondence to the decadal pattern of volcanic sulfate aerosols recorded in a greenland ice core acidity profile and a significant negative growth response to individual explosive volcanic events the appearance of single events in the mid latitude tree ring record in connection with ice core evidence from the arctic and historical records from the mediterranean indicates that the majority of these events represent climatically effective volcanic eruptions producing temperature decreases on the order of 1 c for up to 2 yr after the initial eruption clusters of climatically effective volcanic events may serve as a trigger to glaciation and are consistently associated with lowered ringwidths and late holocene glacier advance in the sierra nevada the tree ring record strongly suggests forcing of solar radiation receipt and temperatures by increased volcanic aerosols especially during the recess peak advances and matthes little ice age advances from 1400 to 1850 a d intervals with an absence of significant volcanic aerosol production or historically documented eruptive activity correspond to intervals of significantly increased indexed ringwidth values minimal numbers of severe annual negative ringwidth anomalies and an absence of glacial deposits in the southern sierra nevada

u pb geochronology of the sublayer environment sudbury igneous complex ontario cu ni sulfide deposits of the sudbury igneous complex are hosted by the inclusion rich sublayer which occurs discontinuously between the footwall and main mass of the sudbury igneous complex by brecciated footwall in the vicinity of the sudbury igneous complex and by offset dikes protruding for tens of kilometers into the country rocks the genesis of mafic ultramafic inclusions in the sublayer and the relationships between the sublayer main mass and offset dikes have long been at the center of debate on sudbury geology this paper reports u pb isotope data on zircon and baddeleyite from samples representing norite matrix melanorite and olivine melanorite pods a metapyroxenite inclusion and diabase from the sublayer at the whistle embayment these samples yield ages ranging from 1848 1 or 1 8 to 1849 1 or 1 1 ma similar to a u pb age of 1849 8 or 2 0 ma obtained for zircons in quartz diorite from the copper cliff offset and indistinguishable from the previously published average age of 1850 or 1 ma for the main mass of the sudbury igneous complex none of the samples yield evidence for inherited zircon components the isotopic data prove that the exotic mafic ultramafic inclusions and pods in the sublayer are not xenoliths scavenged from older layered complexes but require the existence of primitive moderately mg rich magmas that crystallized in the early stages of development of the sudbury igneous complex such mg rich cumulates are unlikely to have crystallized from melted average upper crust

tree ring evidence for a two year interval between recent prehistoric explosive eruptions of mount st helens distinctive annual ring patterns in trees in a forest stand 10 km east of mount st helens indicate that the volcano9s voluminous layer we tephra was erupted after the a d 1481 growth season late in 1481 or early during 1482 combined with earlier research that established a d 1480 as the date of eruption of the underlying similar layer wn tephra the new tree ring data provide evidence that these two major explosive tephra eruptions about 500 yr ago were separated by about 2 yr during that eruptive sequence a voluminous explosive eruption was therefore followed by more than 1 yr of minor explosive activity before another voluminous eruption occurred this record suggests that mount st helens might produce a second large explosive eruption during its current period of activity examinations of the tephra deposits that correlate with narrowring patterns in trees at mount st helens also demonstrate that distinctive ring patterns can form in trees across a large range of tephra layer thicknesses the examinations also suggest that tephra coarseness interacts with tephra layer thickness in determining the effects that tephra fallout will have upon forest trees

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

early proterozoic tectonics and metallogeny of the lake huron region of the canadian shield abstract deposition of the huronian supergroup dominantly fluvial clastic sediments with local basal volcanic accumulations began at 2 49 ga in a broad trough that extended far south of the present lake huron area atop a large archean continent that was assembled at 2 7 ga and broken up at 2 15 ga after intrusion of widespread gabbro sheets and dykes including nipissing diabase the wyoming province which contains sequences similar in age and stratigraphy to the distinctive huronian supergroup may be a fragment of this continent the superior province is a much larger fragment tectonic controls of deposition of the huronian and its paleoplacer uranium deposits and of the pre 2 2 ga deformation of these strata were probably unrelated to processes that controlled deposition of the animikie and marquette range sequences in the lake superior region at 2 0 ga the huronian animikie and marquette range sequences were deformed during the 1 85 ga penokean orogeny which occurred when an assembly of island arc terranes collided with the superior block the sudbury structure including the whitewater group and its contained sedex deposits and the sudbury igneous complex and its ni cu pge ores were also formed at 1 85 ga the elliot lake pyritic paleoplacer uranium ores myriad lenses of radioactive pyritic gravels were deposited by early huronian fluvial systems other endogenic huronian mineral deposits include disseminated ni cu sulphides in gabbros cu in red beds high in the huronian succession and silica in quartz arenites intrusions of nipissing gabbro near sudbury contain minor ni cu mineralization these intrusions also caused recycling of metals from archean and huronian rocks to form concentrations of cu au ag fe and w in veins and other epigenetic deposits most important of these are the unconformity related ag veins of the cobalt area which had particularly complex still undeciphered histories that probably included pre huronian weathering metallogenic patterns in the early proterozoic rocks were controlled by a complex interplay of factors including the distribution of mineral deposits in the archean basement tectonically controlled variations in huronian deposition and deformation and oxyatmoversion the transition from anoxic to oxygenic atmospheric conditions that occurred during deposition of the huronian supergroup

pb isotopic zoning of k feldspar megacrysts determined by laser ablation multi collector icp ms insights into granite petrogenesis this study investigates pb isotopic zoning in magmatic k feldspar megacrysts from the monte capanne pluton elba italy using laser ablation multi collector icpms the studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open system processes and better characterize the components involved in the genesis of complex magma systems earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal derived magmas the investigated k feldspar megacrysts exhibit strong zoning in 207pb 206pb and 208pb 206pb correlated with lead elemental variations we interpret these variations as reflecting growth zoning as opposed to secondary diffusive exchange despite a great variety of zoning patterns we were able to correlate different events of megacryst growth reflecting crystallization in a dynamic magma system our two step model includes 1 growth of a granitic magma chamber by addition of low 208pb 206pb magma to a high 208pb 206pb magma contaminated with crustal material i e the megacryst cores and 2 recharge by mantle derived magma i e the megacryst rims we interpret the thorogenic nature of the megacryst rims to reflect the mantle derived component involved in the mixing process taking account of other data from the tuscan magmatic province the mantle source is inferred to have been metasomatized by continental material during subduction tims sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled

application of tree ring analysis to paleoseismology two case studies knowledge of earthquake probabilities is essential for planning earthquake hazard mitigation and valid estimation of future probabilities requires precise information on past earthquake occurrences previous studies documented effects of earthquakes on trees and how these effects are recorded by tree rings tree ring analysis can be combined with other disciplines to date and delineate earthquake induced disturbance two case studies show the impact of the 1964 alaska earthquake on shoreline trees and how a previously unknown southern san andreas fault earthquake was recorded in tree rings

geology and geochemistry of huronian rhyolites and low ti continental tholeiites from the thessalon region central ontario bimodal volcanism associated with early phases of huronian rifting in central ontario dated about 2450 ma produced low ti tholeiitic basalts and two varieties of crustally derived calc alkaline rhyolite early tholeiites are characteristically highly evolved have mg values from 30 to 50 and display pronounced enrichment in large ion lithophile elements lile and light rare earth element lree in comparison with modern oceanic basalts fractionated heavy rare earth element hree patterns and low ti zr p nb ba and k abundances ti zr ratios rise progressively in early basalts and associated basaltic andesite fractionates from about 35 in early flows to 55 in central units late basalts also carry enriched lile and lree but in contrast to early types have average mg values greater than 50 and lower rare earth element ree abundances with flat hree patterns they also display negative ba nb and p anomalies on chondrite normalized distribution diagrams but lack low k zr and ti contents

multiple metamorphic stages within an eclogite facies terrane sesia zone western alps revealed by th u pb petrochronology convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel studies of high pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to midcrustal levels but the details of such dynamics are controversial to understand the dynamics of a subduction channel we report the results of a petrochronological study from the central sesia zone a key element of the internalwestern alps this comprises two polymetamorphic basement complexes eclogitic micaschist complex and gneiss minuti complex and a thin dismembered cover sequence scalaro unit associated with pre alpine metagabbros and metasediments bonze unit structurally controlled samples from three of these units eclogitic micaschist complex and scalaro bonze units yield unequivocal petrological and geochronological evidence of two distinct high pressure stages ages u th pb of growth zones in accessory allanite and zircon combined with inclusion and textural relationships can be tied to the multi stage evolution of single samples two independent tectono metamorphic slices showing a coherent metamorphic evolution during a given time interval have been recognized the fondo slice which includes scalaro and bonze rocks and the druer slice belonging to the eclogitic micaschist complex the new data indicate separate stages of deformation at eclogite facies conditions for each recognized independent kilometer sized tectono metamorphic slice between 85 and 60 ma with evidence of intermittent decompression p 0 5 gpa within only the fondo slice the evolution path of the druer slice indicates a different p t time evolution with prolonged eclogite facies metamorphism between 85 and 75ma our approach combining structural petrological and geochronological techniques yields field based constraints on the duration and rates of dynamics within a subduction channel

geology mineralization and emplacement of the foy offset dike sudbury impact structure the foy offset dike is the longest of the known radial dikes associated with the 1 85 ga sudbury igneous complex the foy dike extends for at least 30 km from the complex into the north range archean and proterozoic footwall rocks of the sudbury impact structure the dike ranges in width from 400 m where it connects to the sudbury igneous complex to 50 m at its most distal known exposure we divide the dike into a proximal segment and a distal segment based on where it intersects the concentric hess offset dike three main dike lithologies are distinguished all referred to as varieties of quartz diorite inclusion rich pyroxene quartz diorite occurs in the embayment adjacent to the base of the main mass of the sudbury igneous complex this constitutes the sublayer northward the sublayer gives way to amphibole biotite inclusion bearing quartz diorite and amphibole biotite inclusion poor quartz diorite the dike is mineralized and deposits in the proximal segment have been mined mineralization is associated with the inclusion rich dike phase at the nickel offset mine the dike contains inclusion bearing massive ni cu sulfides that also host platinum group minerals the orebodies occurred as strike parallel sheets within the central parts of the dike in association with inclusion bearing quartz diorite the geochemistry of the dike lithologies indicates that both the inclusion poor and inclusion bearing quartz diorite varieties are close in composition to the postulated bulk composition of the sudbury igneous complex this implies that the dike was emplaced prior to any significant fractionation of the main melt body and hence very early in the evolution of the sudbury complex we envisage dike emplacement rapidly following rebound and central uplift formation as part of the impact process early dike emplacement implies that the ore was also introduced early in the evolution of the melt sheet when the inclusion poor quartz diorite phases were still unconsolidated uplift would have caused extension in the target rocks and so dilated the footwall fractures and faults that then became hosts primarily via gravitationally driven intrusion to the primitive polymict impact melt breccias and sulfide ores the chilled margin compositions of the foy inclusion poor quartz diorite are considered good indicators of the original bulk composition of the sudbury igneous complex this idea is supported by comparison with calculated estimations of the bulk composition a shift in geochemical trends a grain size increase and a decrease in inclusion content occurs 22 km along the length of the dike which we believe corresponds to a fault system that downfaults to the north given that the dense ore would have preferentially settled toward the base of the foy dike fracture system postimpact downfaulting of the distal segment of the dike may have displaced any mineralized zones present to deeper levels relative to the proximal segment

magnetic anomalies around the north anatolian fault the nature of the observed magnetic anomalies surrounding the san andreas fault saf is of interest to researchers studying fault kinematics and earthquake dynamics in this work we will present and interpret data collected around the north anatolian fault naf by honkura et al 1985 which exhibit anomalies similar to those observed along the saf magnetic and surface geologic data from five profiles straddling the north anatolian fault were used in conjunction with a two dimensional magnetic inversion in an attempt to identify the source of the magnetic anomalies as a result of this study we conclude that all long wavelength anomalies 500m could be attributed to the changes in the relative magnetic susceptibility across geologic units thus they are due to lateral variations in the near surface geology and are not due to the concentration of a particular material in the fault zone however there was one exception a short wavelength anomaly 150m was observed on profile ij based on the results of the inversion it appears that the anomaly may be due to a small sliver of material located in the upper part of the fault zone if that interpretation is correct the short wavelength anomaly is clearly of different nature than the other anomalies

in situ pb isotope analysis of fe ni cu sulphides by laser ablation multi collector icpms new insights into ore formation in the sudbury impact melt sheet abstract laser ablation la multi collector inductively coupled plasma mass spectrometry mc icpms is ideally suited to in situ determination of isotope ratios in sulphide minerals using samples of magmatic sulphide ore from the sudbury impact structure we test la mc icpms analytical protocols that aim to meet a range of analytical challenges in the analysis of pb isotopes these include potential matrix sensitive isotopic fractionation interferences on pb isotopes low melting points of many sulphide minerals the availability of standards magmatic sulphides of wide ranging mineralogy pyrrhotite pentlandite chalcopyrite pyrite and sphalerite were analysed for pb isotopic composition using the silicate glass nist srm 610 as an external standard to correct for instrumental mass fractionation despite matrix sensitive melting and re deposition around ablation pits several lines of evidence indicate that all analyses are accurate within typical analytical uncertainties of 0 003 2 2 and that the defined approach is insensitive to compositional diversity in sample matrix a laser ablation and dissolution based measurements of sulphide powders are in agreement b analyses from each sample define isochron ages within uncertainty of the known crystallization age 1850 ma c the results of sulphide measurements by laser ablation are consistent with age corrected feldspar analyses from the same samples the results have important implications for ore formation in sudbury the pb isotope data regressions are consistent with age corrected feldspar analyses from each respective sample which together with time integrated th u ratios that match whole rock values 3 1 4 0 and 6 1 for the worthington copper cliff and parkin offset dykes respectively indicate chemical equilibrium between the silicate and sulphide systems during ore formation the sulphides within each respective sample have indistinguishable model initial pb isotope ratios 207 pb 204 pb m irrespective of mineralogy or texture indicating a common origin for ores within each of three different offset dykes furthermore variations between offset dykes e g 207 pb 204 pb m 15 514 0 012 15 399 0 009 and 15 275 0 003 show that the ores have differing crustal sources on previously unrecognized scales mass balance considerations particularly for mgo ni and cu indicate that the spatial distribution of mafic target rocks played a significant role in controlling the mineralization potential in different parts of the melt sheet

hf isotope evidence for effective impact melt homogenisation at the sudbury impact crater ontario canada abstract we report on the first zircon hafnium oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets the differentiated melt sheet at the 1 85 ga originally ca 200 km in diameter sudbury impact crater ontario canada yields a tight range of uniform zircon hf isotope compositions e hf 1850 of ca 9 to 12 this is consistent with its well established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised we propose that the heterogeneity in other isotopic systems such as pb in early emplaced impact melt at sudbury is associated with volatility related depletion during the impact cratering process this depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post impact assimilation of country rock whereas the systems of more refractory elements preserve initial homogeneities zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks however they display a marked offset approximately one third up the melt sheet stratigraphy that is interpreted to be a result of post impact assimilation of 18 o enirched rocks into the base of the cooling impact melt given that impact cratering was a more dominant process in the early history of the inner solar system than it is today and the possibility that impact melt sheets were sources of ex situ hadean zircon grains these findings may have significance for the interpretation of the early zircon hf record we speculate that apparent e hf time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust we also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself

mercury and in situ sulfur isotopes as constraints on the metal and sulfur sources for the world s largest sb deposit at xikuangshan southern china it has been well established that metal and sulfur sources in mineral deposits can be very difficult to identify especially for ore deposits hosted in sedimentary rocks using the world s largest sb deposit at xikuangshan in southern china as a case study this study combined hg isotopes and in situ sulfur isotope measurements to constrain the sources of sb and sulfur a variation of 1 1 in 202hg 0 04 to 1 15 was observed in stibnite ore samples suggesting that mass dependent fractionation of hg isotopes occurred during the formation of the deposit significant mass independent fractionation of hg isotopes with 199hg ranging from 0 03 to 0 17 was also observed in the ore samples suggesting that hg transported by the ore fluids was inherited from proterozoic basement metamorphic rocks as these rocks show similar 199hg signatures 0 03 to 0 07 in situ sulfur isotope measurements yielded 34s values that cluster in the range of 6 8 to 10 2 providing evidence that sulfur contained in ore fluids may also have been dominantly derived from underlying proterozoic basement metamorphic rocks 34s 5 6 to 11 5 using the new results from hg and s isotopes we proposed that deep circulated meteoric water mobilized sb hg and s from the proterozoic metamorphic basement ascended along deep faults and subsequently deposited sb at favorable structural zones as a result of boiling of the hydrothermal fluids generating the world class xikuangshan sb deposit this study also highlights the combined use of hg s isotopes as a novel method to provide new and additional insights into the source regions of ore materials for sedimentary hosted sb deposits

origin of the sudbury complex by meteoritic impact neodymium isotopic evidence samarium neodymium isotopic data on whole rocks and minerals of the sudbury complex in canada gave an igneous crystallization age of 1840 21 x 10 6 years the initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust falling on the crustal trend of neodymium isotopic evolution as defined by shales the rare earth element concentration patterns of sudbury rocks are also similar to upper crustal averages these data suggest that the sudbury complex formed from melts generated in the upper crust and are consistent with a meteoritic impact

precise and accurate in situ pb pb dating of apatite monazite and sphene by laser ablation multiple collector icp ms to evaluate in situ pb dating by laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms we analysed apatite sphene and monazite from paleoproterozoic metamorphic rocks from west greenland pb isotope ratios were also determined in the national institute of standards and technology nist 610 glass standard and were corrected for mass fractionation by reference to the measured thallium isotope ratio the nist 610 glass was used to monitor pb isotope mass fractionation in the low tl pb accessory minerals replicate analyses of the glass 1 to 2 min yielded ratios with an external reproducibility comparable to conventional analyses of standard reference material 981 by thermal ionisation mass spectrometry tims mineral grains were generally analysed with a 100 m laser beam although some monazite crystals were analysed at smaller spot sizes 10 and 25 m the common pb isotope ratios required for age calculations were either measured on coexisting plagioclase by la mc icp ms or could be ignored as individual crystals exhibit sufficient pb isotopic heterogeneity to perform isochron calculations on replicate analyses of single crystals mean mineral ages with the 204pb ion beam measured in the multiplier were as follows apatite 1715 23 m y sphene 1789 11 m y and monazite 1783 to 1888 m y with relative uncertainties on individual monazite ages of 0 2 but highly reproducible age determinations on single monazite crystals 1 isochron ages calculated from several mineral analyses without assumption of common pb also yield precise age determinations apatite and monazite pb ages determined by in situ pb isotope analysis are identical to those determined by conventional tims analysis of bulk mineral separates and the analytical uncertainties of these short laser analyses with no prior mechanical or chemical separation are comparable to those obtained by tims detailed examination of the sphene in situ age data does however show a small discrepancy between the la mc icp ms and tims ages 1 younger high resolution mass scans of the sphene during ablation clearly showed several small and as yet unidentified isobaric interferences that overlap with the 207pb peak at the resolution conditions for measurement of isotope ratios these might account for the age discrepancy between the la mc icp ms and tims sphene ages la mc icp ms is a rapid accurate and precise method for in situ determination of pb isotope ratios that can be used for geochronological studies in a manner similar to an ion microprobe albeit currently at a somewhat degraded spatial resolution further modifications to the la mc icp ms system such as improved sensitivity ion transmission and la methodology may lead to this type of instrument becoming the method of choice for many types of in situ pb isotope dating

isotopic evidence for distinct crustal sources of north and south range ores sudbury igneous complex lead isotope ratios were measured on fe cu ni sulphide ores and associated silicate phases from nine mines on the north and south ranges of the sudbury igneous complex sic these data are compared with the lead isotope signatures of country rocks and shock melted sudbury breccias around the complex both ore and feldspar data from the south range fall along a 1 85 ga reference line the age of the igneous complex and indicate a magmatic origin for lead in the ore data from the north range are also consistent with a magmatic origin but are displaced to lower 206pb 204pb and 207pb 204pb ratios these two suites fall close to the lead isotope composition of country rocks adjacent to the south and north ranges respectively furthermore there is a close correspondence between lead signatures of the sic and sudbury breccia on each side of the complex the evidence suggests an origin for the sic by shock melting of different crustal units on the two sides of the complex this is consistent with the impact of a meteorite on the southward thickening apron of huronian strata which overlay the margin of the superior craton at 1 85 ga

mafic intrusions in the footwall of the sudbury igneous complex origin of the sudbury impact melt sheet and its associated ore deposits abstract the majority of the mafic intrusions and volcanic rocks in the footwall of the sudbury igneous complex sic which is believed to have formed from a melt sheet produced by the impact of a very large bolide belong to one of two large mafic igneous suites viz the 2450 ma east bull lake ebl suite of intrusions or the 2200 ma nipissing diabase suite of intrusions this has been established by u pb la icp ms dating of zircons from the newly recognized frood intrusion which hosts the frood and stobie ore bodies and two of the sudbury gabbro intrusions and comparison of the geochemistry of these intrusions with that of the ebl intrusive suite and the nipissing diabase suite the age of the frood intrusion 2421 32 ma falls within error of other ebl type intrusions whereas the ages of the nairn 2203 11 ma and totten 2168 11 ma intrusions fall within error of published ages for the nipissing diabase moreover the primitive mantle normalized patterns of the petrogenetic trace elements on extended spidergrams of the nairn and totten sudbury gabbros are similar to those of the nipissing diabase as are those of the frood intrusion to other members of the ebl intrusive suite the ni cu pge sulfide deposits at sudbury are associated with the lower contact of the sic where a discontinuous inclusion rich norite contains much of the mineralization at the contact and an inclusion bearing quartz diorite is directly associated with the mineralization of the radial and concentric offset dykes the sublayer rocks and the overlying unit of melanocratic norites provide strong evidence for a relationship between the ore deposits and both inclusions and melts of mafic ultramafic target rocks the most common pre impact mafic ultramafic target rocks are intrusions of the ebl and nipissing diabase suites although there are also archean aged mafic ultramafic enclaves in the gneisses beneath the north range of the sic the totten orebody in the worthington offset and the kelly lake ore body in the copper cliff offset are directly associated with footwall rocks comprising sudbury gabbro and nipissing gabbro respectively whereas the frood stobie offset and the associated frood and stobie ore deposits are sandwiched in a breccia belt containing large rafts of amphibole megacrystic gabbro belonging to the extensively broken up frood intrusion olivine bearing mafic and ultramafic rocks some of which exhibit shock metamorphic features but others do not are common inclusion types and occur in diverse sublayer environments the unshocked melanorite and olivine melanorite inclusions share common geochemical characteristicsde indicating that they were derived from a similar source their geochemistry is also similar to that of the frood intrusion as well as the east bull lake intrusion this indicates that they may have been sourced from east bull lake type intrusions that were incorporated into the melt sheet at the time of melt generation mass balance calculations indicate that the felsic country rocks which account for 77 of the total volume of the sic and contain 18 ppm ni and 21 ppm cu together with the sulfide poor mafic country rocks which account for 23 of the sic and contain 133 ppm ni and 92 ppm cu have too little ni and cu to account for the ni and cu contents of the quartz diorite the composition of which provides an indication of that of the initial melt sheet calculations using the r factor equation of campbell and naldrett 1979 indicate that the magmas that formed the norites and the ore deposits had 137 ppm ni and 121 ppm cu mass balance calculations have been used to determine if the country rocks have sufficient ni and cu in them to generate the ore deposits the weighted median ni content of the sulfide poor

pre emplacement re os ages from ultramafic inclusions in the sublayer of the sudbury igneous complex ontario abstract re os isotope analyses have been performed on a suite of 14 melanorite inclusions from the sublayer of the sudbury igneous complex sic at whistle mine 30 km nne of sudbury ontario canada six s poor olivine melanorites have relatively unradiogenic present day 187 os 188 os ratios of 0 16 1 2 and define an imprecise age of 1 97 ga their 187 os 188 os 1 85 ratios of 0 107 0 237 are compatible with derivation either from the mantle at the time of sic formation 1 85 ga ago or from pre existing crustal rocks with low re os ratios and unradiogenic os isotope compositions in contrast the relatively s rich inclusions define much older ages of 2 46 ga melanorites and 2 67 ga olivine melanorites while 187 os 188 os 1 85 ratios of 0 41 1 80 unequivocally demonstrate that their os was largely derived from pre existing crustal lithologies the os isotope compositions and s abundances of the inclusions are consistent with mixing at 1 85 ga between an unradiogenic silicate fraction defined by the s poor suite and a radiogenic sulphide rich component with re os ratios similar to those of pre existing crustal lithologies attempts to evaluate the source of the ni cu s and pge in the sic through mass balance calculations are not definitive because of the difficulties in estimating accurately both present day and pre sic crustal abundances however crude estimates suggest that the overall levels of ni and cu in the pre impact crust were probably sufficient to account for the budget of these elements in the present day sic but do not rule out the possibility of significant late archaean early proterozoic sulphide mineralisation in the pre existing sic target rocks

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

petrogenetic significance of chromian spinels from the sudbury ignecus complex ontario canada chromian spinels occur in mafic ultramafic inclusions in the sublayer of the sudbury igneous complex sic as well as in mafic ultramafic rocks in the immediate footwall of the sublayer the host rocks are pyroxenite and melanorite with minor dunite harzburgite and melatroctolite as common accessory phases in these rocks the chromian spinels display euhedral or subhedral forms and are included in olivine and orthopyroxene chromian spinel grains generally have ilmenite lamellae and contain abundant inclusions zircon olivine diopside plagioclase biotite and sulfide all the chromian spinels have similar trace element abundances and are rich in tio2 0 5 15 wt they have constant cr 100cr cr al 55 70 and exhibit a continuum in composition that traverses the normal fields of spinels in a al fe3 2ti cr triangular diagram this continuum extends to that of the composition of chromian magnetite in the host norite matrix to the mafic ultramafic inclusions this continuum in compositio

heterogeneity of s isotope compositions recorded in the sudbury igneous complex canada significance to formation of ni cu sulfide ores and the host rocks sulfur isotope ratios of sulfide ores and their host rocks and the overlying main mass sequence of norites quartz gabbros and granophyres from the sudbury igneous complex have been analyzed to establish the extent of isotopic heterogeneity of the complex and the impact melt sheet from which it was produced noritic rocks located above the sublayer in the lower portion of the main mass of the sudbury igneous complex are characterized by uniform sulfur isotope ratios with 34s values of 2 59 0 48 rocks from the granophyric portion of the main mass are characterized by elevated and variable 34s values from 4 4 to 14 1 the more 34s enriched s in the granophyre was produced by secondary sulfide precipitation related to hydrothermal alteration systems developed in the upper altered granophyric part of the main mass as it cooled footwall and contact style mineralization from the sublayer show variable 34s values from 0 2 to 4 3 south range mineralization is characterized by low ratios from 0 2 to 2 5 north and northeast range mineralization exhibits locally elevated but variable 34s values for example mineralization from whistle mine varies from 3 1 to 4 3 whereas mineralization from victor varies between 2 4 and 2 9 there is typically only a small difference in s isotope ratio of contact and proximal footwall mineralization country rocks in the area of the sudbury igneous complex are also characterized by a wide range in 34s values and the sulfur isotope homogeneity of the noritic rocks can be explained by mixing of sulfur from several potential sources however the variability present between deposits at the lower contact of the complex requires that country rocks of locally variable sulfur isotope composition interacted with magma at the base of the cooling melt sheet there is no evidence that deposit size correlates with 34s value variations between deposits may involve local s isotope exchange reactions between the country rock and the melt sheet as the melt digested the footwall this study illustrates the application of s isotope data in constraining the scale of convective mixing and contamination in the largest known terrestrial melt sheet

signatures of multiple mineralization processes in the archean orogenic gold deposit of the pampalo mine hattu schist belt eastern finland the hattu schist belt is an emerging gold producing ore district in the western part of the archean karelian province of the fennoscandian shield the belt consists of 2 76 to 2 70 ga tonalite granodiorite and leucogranite intruded into a mafic felsic epiclastic volcanic sequence of an only slightly older age complex and successive folding shearing and hydrothermal processes affected these rocks prior to the lower amphibolites facies peak metamorphism 550 50 c 3 5 kbar at ca 2 70 ga orogenic gold deposits are hosted by the highly strained zones that developed during the archean deformation of the belt however previous k ar and rb sr geochronological studies indicated that a second tectonothermal overprint affected the hattu schist belt between 1 7 and 1 8 ga during the svecofennian orogeny results of our field mineralogical textural and pb isotope studies suggest that ore deposition at pampalo was initiated by hydrothermal processes at the time of the emplacement of feldspar porphyry and tonalite intrusions and dikes at around 2 72 ga albitization and quartz tourmaline biotite muscovite veining characterize this hydrothermal activity the major stage of gold ore deposition can be confined to the subsequent development of high strain zones in an intermediate felsic tuffaceous unit mafic schist characterized by biotite carbonate pyrite alteration ar ar studies revealed the complete resetting of the ar ar system in muscovite and biotite due to the svecofennian orogeny with closure of the isotopic system at 1 81 ga results of pb isotope studies of hydrothermal k feldspar galena and altaite by laser ablation inductively coupled plasma mass spectrometry indicate that k feldspar alteration and remobilization of metals also took place during the svecofennian reactivation of high strain zones the petrographic manifestation of this process is the replacement of albite by hydrothermal k feldspar fluid inclusion data from hydrothermal k feldspar suggest that carbonic aqueous low salinity fluids interacted with the archean ore at 350 to 400 c and 1 8 to 2 4 kbar during the svecofennian overprint results of mass transfer calculations indicate that potassium gain correlates with increase of gold concentrations in the mafic schist and feldspar porphyry units therefore the svecofennian overprint locally also enhanced the grade of gold mineralization mostly along competency differences of the mafic schist and feldspar porphyry blocks dikes later percolation of relatively low temperature 300 c saline basin fluids in some fractures of the crystalline basement also left their pb isotope and fluid inclusion signatures on the mineralization while further modifying the composition of the ore occurrences of hydrothermal proto ores as well as fluid flow events along structures reactivated by overprinting orogenic processes are not exceptional features in archean orogenic gold provinces this study shows that the combination of in situ pb isotope studies of u poor alteration minerals and pb rich ore minerals together with evaluation of relationships between gold enrichment and hydrothermal alteration and fluid inclusion studies can be very useful in determining the significance and conditions of overprinting processes as well as their potential implications for genetic and exploration models

comparative geochemistry of pleistocene and paleoproterozoic huronian glaciogenic laminated deposits relevance to crustal and atmospheric composition in the last 2 3 ga abstract comparison of the major and trace element geochemistry of glaciogenic laminated deposits of pleistocene and paleoproterozoic age from an area north of lake huron ontario has revealed differences and similarities that may provide insight into the erosional history of part of the archean superior province of the canadian shield during the last 2 ga and some aspects of the evolution of the earth s atmosphere both the major element and trace element compositions of the ancient and recent glacial deposits are very similar the paleoproterozoic mudstones are however enriched in fe and u characteristics that may reflect the oxygen content of the archean early paleoproterozoic atmosphere hydrosphere system strong ca depletion is exhibited by the paleoproterozoic glaciogenic argillites but is absent from the pleistocene samples the proterozoic argillites have slightly higher values for a chemical index of alteration than those of pleistocene muds these characteristics may reflect the acid natur

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources

common lead corrected laser ablation icp ms u pb systematics and geochronology of titanite u pb geochronology using laser ablation icp ms is a fairly recently introduced technique and thus far its development has centred on zircon dating using titanite as an example the application of laser ablation icp ms to u pb dating of common pb bearing accessory phases is presented a simple analytical set up comprising a 213 nm nd yag laser coupled to a quadrupole based icp ms instrument is used it is demonstrated that accurate and precise pb u ratios 2 3 at 2 for 206pb 238u can be obtained on small volumes of analysed material using the zircon geostandard 91500 as an external calibration standard a robust common pb correction necessitating the measurement of the stable 204pb isotope is detailed the isobaric 204hg background interference in icp ms is lowered to a level where it does not dominate the total 204 amu signal using in line gold traps on the carrier gas lines the remaining 204hg is stripped from the 204pb signal mathematically by applying a natural hg isotope ratio common pb correction using this method is demonstrated to work well in analyses with elevated u pb signal intensity where this correction fails an alternative method is required and a novel approach using three dimensional u pb concordia is adopted this approach is shown to be extremely powerful in assessing age precision and common pb composition correction the technique is applied to the well characterised fish canyon tuff demonstrating that accurate 7 8 at 2 compared to reference age and precise 2 4 at 2 on 206pb 238u ratios u pb age data can be obtained from titanite even as young as oligocene

compositional variations of olivine and sulfur isotopes in the noril sk and talnakh intrusions siberia implications for ore forming processes in dynamic magma conduits olivine from the noril sk i main and northwestern talnakh ore bearing intrusions in the noril sk region occurs as inclusions discrete grains and poikilitic crystals the original compositions of olivine inclusions have been well preserved by their host oikocrysts from reaction with interstitial silicate and sulfide liquids in contrast the compositions of discrete and poikilitic olivine crystals have been variably modified by reequilibration with trapped silicate and sulfide liquids there are abrupt changes in the contents of nickel in olivine inclusions 500 1 500 ppm between different rock units in all three intrusions which indicates that the different rock units in these intrusions are not related to each other via in situ differentiation of a common parental magma but rather are consistent with separate pulses of magma with variable degrees of differentiation possibly involving olivine crystallization and assimilation of siliceous crustal material in a staging chamber the 34s values of the samples with high sulfur contents 800 ppm sulfur from all three ore bearing intrusions vary between 8 and 14 per mil two sulfur poor samples 600 ppm sulfur from the northwestern talnakh intrusion yield 34s values of 1 5 and 3 6 per mil these values are similar to the values of the sulfur poor rocks from the ore barren intrusions in the region and illustrate the fact that high sulfur contents couple with increasing 34s values such a relationship is consistent with the addition of crustally derived 34s enriched sulfur to the magma of the ore bearing intrusions in contrast to other models of sulfide segregation in closed systems we propose that sulfide segregation occurred in an open system and the magma involved was undersaturated in sulfur and undepleted in chalcophile elements we envision that the 34s enriched sulfur was introduced via circulating fluids that transported sulfur from evaporite in the underlying sedimentary sequence to the magma in a dynamic magma conduit the immiscible sulfide liquids that segregated during sulfur contamination settled down in the wider parts of the conduit as the magma continued to travel successive pulses of new magma entered the conduit reacted with the early sulfide and displaced much of the early magma in the conduit the ore bearing intrusions are thought to have crystallized within the wider parts of the conduits and their voluminous sulfide poor peripheral sills represent some if not all of the magma that flowed out of the conduits

dendrochronology and middle miocene petrified oak modern counterparts and interpretation abstract this study reports the first successful statistical crossdating among many ring width time series from petrified wood thus providing a replicable continuous annual resolution window into tree growth and environmental influences during the middle miocene the petrified samples of the genus quercus originated at the stinking water sw locality in oregon a miocene aged exposure associated with the columbia river basalts 40ar 39ar dating on pillow basalt from the locality yielded a weighted plateau age of 13 79 0 09 ma placing the death of the trees at the end of the langhian stage of the middle miocene 15 97 0 05 to 13 65 0 05 ma during the middle miocene climate transition mmct ring width time series from 26 radii 17 different trees show significant intercorrelation a modified coexistence approach was applied to determine the likely climate range when the sw trees were growing the modified approach included regression of site mean ring width time series statistic values on estimated soil moisture for the site locations using site mean data from 126 modern quercus sites from across the united states identification of highly significant linearities indicated strong relationships between ring width intercorrelation and soil moisture and ring width variability and soil moisture comparison of individual modern site mean statistical values with values calculated for the sw locality suggests a mesic growing environment for the sw quercus with moderate temperatures geographic placement of modern quercus sites with site mean statistics similar to the sw values indicates a modern analogue in the eastern united states modern distributions of mesic species in the genera present at the sw locality suggest similarities with the central and southern appalachian mountains and the ozark ouachita mountains indicating a mean annual temperature range of 10 c to 15 c and a mean annual precipitation range of 750 mm to 1200 mm when the sw quercus were growing

three growth in the mesozoic and early tertiary and the reconstruction of palaeoclimates abstract evidence from the distribution and characteristics of fossil wood in the mesozoic and early tertiary indicates that a much warmer global climate prevailed in those times there appears to have been a broad zone of largely non seasonal climate stretching from about 32 n to 32 s palaeolatitudes in addition to this low latitude zone forest growth extended into very high palaeolatitudes where trees cannot grow at the present day a number of theories have been proposed to account for the palaeoclimate responsible for this distribution of forests most notable have been those involving changes in the amount of carbon dioxide in the atmosphere in the positions of the continents or in the obliquity of the earth s axis of rotation evidence from fossil forests indicates that a combination of the effects of an increased quantity of carbon dioxide in the atmosphere and the palaeopositions of the continents in the mesozoic and early tertiary appears at the moment to be the simplest explanation for the climate of those geological times without the need to invoke axial movement

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

response to the scientific comment by dickin on isotopic heterogeneity in the sudbury impact melt sheet epsl 289 2010 347 356

rare earth element characteristics and origin of the sudbury igneous complex ontario canada abstract systematic geochemical studies have been carried out on the sudbury igneous complex sic and its country rocks the two main rock types of the sic norite and granophyre show contrasting chemical features the norite contains 54 58 wt sio2 whereas the granophyre has a sio2 content of 65 71 wt all rocks are characterized by high absolute rare earth element contents ree 63 314 ppm the granophyre having more than twice the ree contents of the norite and by lree enriched chondrite normalized distribution patterns huronian volcanic rocks and huronian plutonic granites have the highest ree contents and hree enrichment among the country rocks archean rocks are characterized by very steep ree patterns and positive eu anomalies nipissing diabase contains the lowest ree contents and has a flat ree pattern within the sic the ree values are more or less constant through the norite but the abundances of some elements such as ce and yb decrease slightly upward with stratigraphy ree contents increase sharply in passing from norite to granophyre and the eu eu ratio decreases from around unity to 0 66 the contrasting ree contents of the norite and granophyre combined with their distinctive major and trace element contents are considered to indicate that they resulted from two separate batches of magma probably emplaced at about the same time the previous models in which the sic was considered to originate from a single magma either contaminated mantle magma or crustal melt are not adequate a new hypothesis is developed based on the chemical features observed in this study the sic is considered to have comprised two independent intrusions the norite was derived from a heavily contaminated mantle magma 30 60 the main contaminant being archean granulite facies rocks of the lower crust the granophyre represents a crustal melt 75 archean rocks and 25 huronian granite induced by meteorite impact the new hypothesis better accounts for the large proportion of granophyre in relation to norite and for the ni cu sulfide mineralization associated with the sic it is also favored by the rb sr isotopic data

190pt 186os and 187re 187os systematics of the sudbury igneous complex ontario abstract we measured by negative thermal ionization mass spectrometry ntims re os and 186 os 188 os and 187 os 188 os in 26 samples of 18 ni cu sulfide ores from the falconbridge mccreedy west and strathcona mines at sudbury ontario at mccreedy west and falconbridge the isochron re os ages are 1835 70 ma and 1827 340 ma and the initial 187 os 188 os ratios 0 514 0 019 and 0 550 0 024 respectively the ages agree with the canonical value of 1850 1 ma for the sudbury igneous complex sic for hangingwall and deep zone ores at strathcona the age of 1780 7 ma may reflect resetting by dyke activity the high initial 187 os 188 os of 0 934 0 005 in these ores is distinct from those at mccreedy west and falconbridge strathcona deep copper zone ores have highly radiogenic os giving a mean model age of 1883 54 ma that is similar to ages at mccreedy west and falconbridge but distinct from other strathcona sulfides initial 186 os 188 os in two strathcona ores with low 190 pt 188 os average 0 119 826 0 000 009 n 3 and 0 119 827 0 000 004 n 3 respectively with a grand mean of 0 119 827 0 000 003 this ratio may be slightly lower than the chondritic value at that time similar ores at falconbridge and mccreedy west show more scatter averaging 0 119 855 0 000 008 n 6 and 0 119 867 0 000 020 n 3 respectively these values are substantially suprachondritic the re os isotope systematics of sudbury ores are clearly of crustal origin and may be derived from a binary mixture of superior province and huronian metasedimentary rocks with strathcona falconbridge and mccreedy west ores containing respectively 55 16 and 12 of os from superior sediments the suprachondritic 186 os 188 os at mccreedy west and falconbridge may be due to admixture of archean or paleozoic mafic rocks with 190 pt 188 os 0 1 no trace of the asteroid that produced the sudbury structure has been reported at the whistle mine s poor olivine melanorite inclusions with high ni and os and low 187 os 188 os may contain the signature of a magmatically fractionated asteroidal core contributing 1 to 2 5 metal the s poor melanorite ni and os data are equally well explained by admixture of 40 mantle peridotite however