origin and formation of metabreccia in the parkin offset dike sudbury impact structure canada the 1 85 ga sudbury impact structure is considered a remnant of a peak ring or multi ring basin with an estimated original diameter of 150 to 200 km the offset dikes are radial and concentric dike

the sudbury structure as a terrestrial mare basin this paper reviews the origin development and present status of the hypothesis that the sudbury structure of ontario canada is a deformed precambrian impact crater analogous to the circular lunar maria the petrologic similarity of the maria and terrestrial lopoliths such as the bushveld duluth and sudbury igneous complexes was suggested in 1961 assuming these to be extrusive features roofed by their own silicic differentiates this led to the proposal by r s dietz in 1962 that the sudbury structure was a tectonically deformed impact basin occupied by an extrusive lopolith and analogous to the small lunar maria he then searched for and found shatter cones at sudbury discovery of shock metamorphic features in the onaping formation by b m french in 1967 showed that this formation was a fallback breccia detailed studies since the 1970s have further strengthened the impact theory isotopic and impact modeling investigations have shown that the sudbury igneous complex may be largely impact melt rather than internally generated magma the shape and size of the original impact crater are not well known the absence of compressional deformation of the north range supports an originally elliptical shape which has been shown to be similar to those of impact craters on the moon and mars however field mapping and reflection profiling suggest that much of the present ellipticity may have been produced by northwest directed ductile thrusting concentrated on the south range and interior of the complex the size of the original structure is also uncertain recent investigations indicate that the present igneous complex is only the central part of a much larger and perhaps multiring structure the analogy between the sudbury structure and lunar maria though petrologically wrong is valid to the extent that both classes of structure were initially formed by impact however the size and shape of the original crater and the degree to which the sudbury intrusive complex and its ore deposits are impact generated are still open to debate

application of organic facies studies to sedimentary basin analysis a case study from the yitong graben china abstract an organic facies is defined in this paper as a mappable stratigraphic unit containing organic matter om of a particular genetic type in certain abundance several concepts concerning organic facies are proposed and their significance in basin analysis is discussed in detail a combined sedimentological and organic geochemical investigation has been carried out in the yitong graben and four organic facies and one organic parafacies have been recognized distinct organic basin fill sequences obfs and organic facies association ofa were developed in different sub units of this graben which indicates that the yitong graben had undergone two tectonic subsidence cycles such tectonic subsidence patterns could not have been found out without detailed organic facies analysis confirming that organic facies especially obfs and ofa study is a very useful tool not only for prediction and three dimensional mapping of petroleum source rocks but also for sedimentary basin analysis

vredefort sudbury chicxulub three of a kind vredefort sudbury and chicxulub are the largest known terrestrial impact structures all have been cited as multi ring basins the available data indicate that all have some form of multiple ring attributes most commonly structural features chicxulub however is the only example with morphological ring features there are also commonalities in the structural and lithological features of vredefort and sudbury and it is possible to construct a generalized compilation of the character of 200 300 km diameter impact basins on earth it is not clear however that any of these structures had the original morphological characteristics of large lunar multi ring basins additional data and synthesis are required to fully characterize these structures in order to realize their potential to constrain large scale cratering processes if this is not sufficient incentive for further studies the environmental effects of chicxulub aside the vredefort sudbury and chicxulub impact events are also the reason for

isotope geochemistry of fluid inclusions in permian halite with implications for the isotopic history of ocean water and the origin of saline formation waters abstract d and 18o values have been determined for fluid inclusions in 45 samples of permian halite the inclusions are enriched in 18o relative to the meteoric water line but are depleted in d relative to ocean water inclusions with the more positive values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a d 18o diagram inclusions with more negative values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines the isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that permian sea water was isotopically similar to modern sea water connate evaporite brines can have negative values because of the probable hooked isotope trajectory of evaporating sea water and or synsedimentary mixing of evaporite brines with meteoric waters subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested

detection of a meteoritic component in ivory coast tektites with rhenium osmium isotopes measurement of rhenium re and osmium os concentrations and os isotopic compositions in ivory coast tektites natural glasses with upper crustal compositions that are ejected great distances during meteorite impact and rocks from the inferred source crater lake bosumtwi ghana show that these tektites incorporate about 0 6 percent of a meteoritic component analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals the re os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons i the isotopic compositions of the target rocks and the meteoritic impactor are significantly different ii closed system mixing of target rocks and meteorites is linear on re os isochron diagrams which thus permits identification of the loss of re or os iii osmium isotopic compositions are not likely to be altered during meteorite impact even if re and os are lost

floor subsidence and rebound of large venus craters the topography and geology of large craters on venus reveal no evidence for floor rebound or relaxation rather the floors have subsided depressions of the floor centers relative to the margins are evident in topography and floor faulting is interpreted as contractional failure of the likely processes responsible only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas assuming volcanism occurred within a few tens of millions of years after the impact thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere further the predicted stresses are generally consistent with observed floor fracturing we constrain the impact heat deposited in the lithosphere to be less than roughly 5 1023 j for diameters of 100 km the absence of perceptible floor subsidence at craters this size on the moon and icy satellites is readily explained by the scaling dependence of impact energy not only on transient crater diameter but also on gravity and target density the unfractured melt sheets of three large young bright floored craters imply sufficient lithospheric rigidity to support the crater cavities an elastic flexural rebound model restricts the elastic plate thickness to at least 10 15 km for the three craters corresponding to maximum geotherms of 20 30 k km 1 structural evidence for early rebound in older dark floored craters may have been buried by subsequent volcanism

glacial recharge and paleohydrologic flow systems in the illinois basin evidence from chemistry of ordovician carbonate galena formation waters the illinois basin provides an opportune setting for elucidating the roles of remnant evaporite brines and meteoric waters in the evolution of formation waters in an intracratonic sedimentary basin formation waters from carbonate reservoirs in the upper ordovician galena group have been analyzed geochemically to study the origin of their salinity their chemical and isotopic evolution and their relationship to paleohydrologic flow systems chloride bromide ratios and ci br na br relations indicate that initial brine salinity resulted from subaerial evaporation of seawater rather than from halite dissolution subsequent subsurface dilution of the brines by meteoric waters is disclosed by d 18 o covariance however the remnant evaporite brine has not been completely expelled from these ordovician strata galena formation waters have 87 sr 86 sr ratios that range from 0 708 17 a value nearly equal to that of coeval seawater to 0 710 43 this is the greatest range of sr isotopic ratios found in waters from any stratigraphic unit in the basin two fluid mixing events are revealed in plots of 87 sr 86 sr vs 1 sr introduction of 87 sr enriched fluids from a siliciclastic source probably overlying maquoketa shale and a later event that only affected reservoir waters in the western shelf of the basin general covariance between sr and h o isotopes suggests that the later event is related to meteoric water recharge the point of intersection of the d 18 o trend with the meteoric water line implies that this mixing event involved pleistocene glacial meltwater that recharged galena reservoirs near outcrops along the western margin of the basin ordovician galena formation waters are geochemically distinct from those in both silurian devonian and mississippian pennsylvanian strata a distinction that has evidently been maintained by the overlying maquoketa regional aquitard

geochemistry of the dundonald komatiite basalt suite and genesis of dundeal ni deposit abitibi subprovince canada the genesis of konatiites basalts and associated ninineralization in the dundonald township area on tario is critical to understanding the metallogenic evolution of the kidd munro assemblage one of the most primitive volcanic assemblages in the world the 2 5 kin thick stratigraphic succession has a basal calc alkalic basalt dacite rhyolite sequence 2716 8 2 1 ma u pb zircon cut by the dundonald peridotite gabbro sill 27077 ma u pb zircon overlain by komatiitic and basaltic flows containing magmatic ni sulfide mineral ization which are in turn overlain by chemically distinctive low ti basalt flows the dundonald komatiites correlate with komatiites in the footwall of the giant kidd creek volcanic associated massive sulfide deposit 40 km to the west but are slightly older than the komatiites of munro township which are 40 km to the east the dundonald komatiites comprise a munro type al undepleted komatiite suite with liquid compositions having a120 ffio o 18 to 22 and mgo up to 30 3 vet percent anhydrous n 24 most samples in the ko matiite suite have slightly depleted light bee contents with lan smn 0 6 to 1 1 and lan ybn 0 4 to 0 9 n 21 and negligible ti ti and zr zr anomalies their major and trace element geochemistry is consistent with a derivation by sigfificant partial melting of a chemically primitive mantle with little or no influence of majoritc garnet flows that are host to the dundeal and alexo ni deposits have distinctively high thpm nbpm ratios 1 7 n 3 suggesting crustal contamination and peperite textures that are consistent with contami nation from nearby graphitic argillite the chemically distinctive lmv ti basalts have 49 to 53 percent sios 4 5 to 9 5 percent mgo 18 8 to 21 5 percent a1203 0 30 to 0 37 percent tios moderately depleted light ree con tents with lan smn 0 9 to 1 2 and la n ybn 0 4 to 0 6 high ni cr and ba contents 570 6 800 390 630 90 320 ppm respectively n 6 their high ai203 contents place them in the calc alkalic field on a jensen cation plot although their trace element ratios zr y la yb are more consistent with a tholeiitic affinity they are compositionally identical to and they correlate with low ti basalts in the kidd creek volcanic complex their geochemistry is consistent with a derivation by partial melting of a refractory harzburgitic mantle that has undergone a previous melt extraction possibly influenced by a large ion lithophile enriched hydrous phase the empire flow is the thickest and most primitive komatiite flow and is host to the dundeal ni deposit re source of 0 4 mt at 2 0 ni the empire flow contains an elongate dunite peridotite basal unit interpreted as the core of a flow that thermally eroded into footwall heterolithic breccias and pillowed andesites the main dundeal ni horizon overlies part of the basal dunite peridotite unit within the flow channel and extends over the cutbank wall the dundeal deposit and the nearby dundonald south ni deposit have lo v total sulfide con tents and high ni s ratios 0 9 and 1 0 respectively in comparison to most komatiite hosted magmatic sulfide deposits the high ni tenor in sulfide is believed to be due to assimilation of reducing carbonaceous sediment which causes greater ni partitioning into the magmatic sulfide phase

br cl na systematics in illinois basin fluids constraints on fluid origin and evolution the authors present here bromide chloride and sodium data for fluids from reservoirs of ordovician through pennsylvania age in the illinois basic which suggest that remnant marine fluids contribute significantly to fluid cl budgets cl br and nabr ratios for ordovician through devonian formation fluids are relatively uniform and near those for seawater despite greater than a factor of ten range in cl concentration in contrast fluids from mississippian and pennsylvanian reservoirs separated from older reservoirs by the new albany shale group have more variable fluid cl br and na br ratios most of which are significantly greater then those of seawater the 1 1 stoichiometry of cl and na increases for mississippian and pennsylvanian formation fluids is consistent with halite dissolution nevertheless br systematics and mass balance considerations indicate that he overall cl budget of illinois basin formation fluids appears to be more significantly influenced by the contribution from subaerially evaporated seawater than by halite dissolution

origin and chemical evolution of formation waters from silurian devonian strata in the illinois basin usa abstract a suite of formation water samples from silurian devonian reservoirs in the illinois basin has been analyzed for major minor and trace element concentrations and for h o and sr isotopic compositions in order to interpret origin of salinity and geochemical evolution of brine compositions in this evaporite and shale poor cratonic basin although chloride concentrations range from 2000 to 137 000 mg l cl br ratios 291 18 are consistent with those of seawater or seawater evaporated short of halite saturation cl br 292 thus during silurian devonian time subaerially evaporated penesaline brine entered the subsurface where it was chemically modified through brine rock interactions cation br ratios and mineralogy of associated strata indicate that na and k were depleted through interaction with clay minerals ca was enriched and mg depleted by dolomitization and sr was enriched as a result of caco3 recrystallization and dolomitization although significant dilution of the modified brine with meteoric water is supported by d 18o covariance original marine waters have not been completely expelled from silurian devonian strata hydrogen and oxygen isotopes exhibit covariant relations with cation and anion concentrations implying that isotopic exchange between h2o and minerals has not greatly influenced the d 18o trend brine 87 sr 86 sr ratios range from 0 7092 to 0 7108 when these ratios are plotted versus 1 sr a two component mixing trend is suggested although sr concentrations have experienced local diagenetic modification a 87sr enriched fluid may have accompanied petroleum migration from new albany shales into adjacent silurian devonian carbonates where it mixed with remnant evaporated seawater this event probably preceded the influx of meteoric water as gdd and 18o are not correlated with sr isotopic compositions of formation waters

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

re os dating of two cu ni sulfide deposits in northern xinjiang nw china and its geological significance abstract mafic ultramafic intrusions in northern xinjiang are located along major fault zones some of these intrusions host cu ni sulfide deposits and amongst these are two economically important cu ni deposits kalatongke also called karatungk and huangshan east huangshandong sulfides from these deposits were selected for a re os geochronological study in order to understand the tectonic and metallogenic environment of the mafic ultramafic intrusions and the associated cu ni sulfide deposits in northern xinjiang re os dating of cu ni sulfide ores in the nos 1 and 2 intrusions of the kalatongke ore district and huangshan east deposit gave isochron ages of 282 5 4 8 290 2 6 9 and 284 14 ma respectively with initial 187 os 188 os ratios of 0 2563 0 0073 0 2721 0 0053 and 0 241 0 092 the os values of the kalatongke and huangshan east deposits range from 100 6 to 124 2 and from 25 6 to 235 9 these results indicate that the cu ni sulfide mineralization in both deposits formed during the early permian 280 290 ma integrating our field observations with published literature and the re os data presented in this work we suggest that the mafic ultramafic rocks investigated were emplaced in a post collisional extensional setting and the cu ni sulfide ores were segregated from crustally contaminated magmas

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

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

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

highly siderophile elements re os ir ru rh pd au in impact melts from three european impact craters saaksjarvi mien and dellen clues to the nature of the impacting bodies twenty two large 10 g impact melt samples from three scandinavian craters i e saaksjarvi finland and mien and dellen sweden were analyzed for highly siderophile elements hse platinum group elements rh and au by the nickel sulfide technique in combination with neutron activation the ten impact melt samples from saaksjarvi are enriched in ir and other highly siderophile elements ir 2 48 0 73 ng g relative to average upper crust concentrations 0 03 0 02 ng g ir the twelve dellen and mien samples are marginally enriched in ir 0 48 0 23 ng g for dellen and 0 37 0 23 ng g for mien the amount of meteoritic component corresponds to 0 5 of a nominal ci component for saaksjarvi and about 0 1 for mien and dellen the saaksjarvi pattern is fractionated relative to ci chondrites normalized abundances increase from the refractory to the more volatile siderophile elements ir ni the trend is qualitatively similar to magmatic iron meteorites and corresponds to 0 4 of a nominal magmatic iron meteorite component tamarugal iiiab for mien and dellen no projectile assignment can be made all samples from the three impact craters have low osir ratios compared to chondritic ratios either the projectile had low osir ratios or os was lost during the impact as volatile oso4 based on the results of this study and on a compilation of literature data average upper crustal abundances of the highly siderophile elements in the target area baltic shield are estimated as 0 03 ng g for ir and os 1 1 ng g for ru 0 38 ng g for rh 2 0 ng g for pd 8 g g for co and 37 g g for cr these data are representative of the upper crust they allow for the first time reliable estimates of crustal abundances for rh and ru upper crustal abundances also show that ir and os are the most favourable elements for the identification of meteoritic signatures fractionated osir ratios in magmatic iron meteorites or potential losses of os during the impact leaves ir as the best indicator element for meteoritic contamination with ir extraterrestrial components as low as 2 104 ci may be identified

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 u pb hf and re os isotopic analyses of the xiangshan ni cu co deposit in eastern tianshan xinjiang central asia orogenic belt constraints on the timing and genesis of the mineralization abstract the timing and genesis of the major ni cu co sulfide deposit in the xiangshan intrusion have been studied based on newly obtained in situ u pb hf and re os isotopic analyses the sims u pb zircon ages of the gabbro hosting the ni cu co sulfide deposit indicate that the xiangshan intrusion was emplaced at 279 6 1 1 ma 95 confidence level mswd 1 30 n 15 on the basis of combined geological and geochronological evidence we suggest that the xiangshan and other adjacent ni cu deposits were formed in the same period sulphides have low common os concentrations and high re os ratios similar to sulphide ores from the duluth sally malay and voisey bay complexes the re os isotopic data from the disseminated and massive ores from the xiangshan intrusion do not form a single isochron as they have different initial os ratios the hf and os isotopic data suggest that the xiangshan intrusion and associated ni cu co mineralization were derived from crustally contaminated mantle melts the geochemical data show a tholeiitic affinity and a strong suprasubduction zone signature with negative nb sr and ti anomalies similar to n morb and e morb we suggest that the mafic ultramafic rocks and associated ni cu mineralization of the eastern tianshan orogen formed in an alaska type subduction zone arc setting some diagnostic features of ridge trench interaction are present in the chinese east tianshan orogen e g granites adakites high mg andesites near trench magmatism alaskan type mafic ultramafic complexes high temperature metamorphic belts that prograde rapidly from low grade belts and orogenic gold deposits the above distinctive rock groups are probably related to the same thermal event ridge subduction as in the cenozoic orogen of alaska we suggest that ridge subduction is the most plausible mechanism to provide the necessary heat ridge subduction provides an important promising model for understanding many aspects of the evolution of the chinese eastern tianshan orogen and by inference throughout much of central asia

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

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

the basal onaping intrusion in the north range roof rocks of the sudbury igneous complex the 1 85 ga sudbury impact structure is one of the largest impact structures on earth igneous bodies the so called basal onaping intrusion occur at the contact between the sudbury igneous complex sic and the overlying onaping formation and occupy 50 of this contact zone the basal onaping intrusion is presently considered part of the onaping formation which is a complex series of breccias here we present petrological and geochemical data from two drill cores and field data from the north range of the sudbury structure which suggests that the basal onaping intrusion is not part of the onaping formation our observations indicate that the basal onaping intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the basal onaping intrusion planar deformation features within quartz clasts of the basal onaping intrusion are indicators for shock metamorphism and together with the melt matrix point to the basal onaping intrusion as being an impact melt rock by definition importantly the contact between granophyre of the sic and basal onaping intrusion is transitional and we suggest that the basal onaping intrusion is what remains of the roof rocks of the sic and thus is a unit of the sic and not the onaping formation we suggest henceforth that this unit be referred to as the upper contact unit of the sic

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

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

the sudbury structure ontario canada a tectonically deformed multi ring impact basin the occurrence of shock metamorphic features substantiates an impact origin for the 1 85 ga old sudbury structure but this has not been universally accepted recent improvements in knowledge of large scale impact processes combined with new petrographic geochemical geophysical lithoprobe and structural data allow the sudbury structure to be interpreted as a multi ring impact structure the structure consists of the following lithologies sudbury breccia dike breccias occurring up to 80 km from the sudbury igneous complex sic footwall rocks and footwall breccia brecciated shocked crater floor materials in part thermally metamorphosed by the overlying sic sublayer and offset dikes main mass of the sic and basal member of the onaping formation of geochemically heterogeneous coherent impact melt complex ranging from inclusion rich basal unit through a dominantly inclusion free to a capping inclusion rich impact melt rock grey member of of melt rich impact breccia suevite green member of of thin layer of fall back ejecta black member of of reworked and redeposited breccia material onwatin and chelmsford formations post impact sediments observational and analytical data support an integrated step by step impact model for the genesis of these units analysis of the present spatial distribution of various impact related lithologies and shock metamorphic effects result in an estimated original rim to rim diameter of the final crater of 200 or even 280 km for the sudbury structure prior to tectonic thrusting and deformation during the penokean orogeny

an isotopic study of the ni cu pge rich wellgreen intrusion of the wrangellia terrane evidence for hydrothermal mobilization of rhenium and osmium abstract re os sm nd rb sr and oxygen isotope systematics of the wellgreen intrusion in the wrangellia terrane were investigated in an effort to deduce the origin of this mafic ultramafic sill and its attendant ni cu pge deposit radiogenic initial os ratios 1 06 1 82 and sr ratios 0 7044 0 7062 and heavy 18 o 7 3 7 9 suggest alteration of the intrusion by hydrothermal fluids that carried radiogenic 187 os and 87 sr from the surrounding country rocks the great majority 99 of the os however and by inference the other pges derived from a mantle derived magma that suffered little or no interaction with the crust prior to crystallization initial nd isotope ratios are not as variable nd t ranges from 2 02 4 49 and suggest that the rocks were derived from a light rare earth element depleted mantle source the nd results together with os data from relatively undisturbed wehrlites are compatible with either a plume richards et al 1991 or island arc samson et al 1990 model for wrangellia this study contributes to a growing body of evidence that documents the ready mobilization of os in hydrothermal fluids the potential effects of this mobility must be carefully evaluated prior to invoking crustal assimilation to explain variable and radiogenic os initial ratios in layered intrusions

origin of late triassic mafic ultramafic intrusions in the hongqiling ni cu sulfide deposit northeast china evidence from trace element and sr nd isotope geochemistry there are many late triassic mafic ultramafic intrusions in the hongqiling magmatic ni cu sulfide deposit northeast china research on magma evolution leading to formation of these mafic ultramafi

strontium boron oxygen and hydrogen isotope geochemistry of brines from basal strata of the gulf coast sedimentary basin usa abstract significant spatial heterogeneities exist in the stable isotopic composition of saline formation waters from reservoirs of the smackover formation upper jurassic we focused on the southwest arkansas shelf a structurally simple portion of one of the interior basins of the northern gulf coast sedimentary basin here faulting and facies changes juxtapose dominantly oolitic carbonate strata against basal evaporites red beds and siliciclastics as well as metamorphosed basement rocks brines from this area have exceptionally high br and alkali element concentrations and have spatially heterogeneous hydrogen sulfide concentrations strontium boron oxygen and hydrogen isotope compositions exhibit coherent relations with other aspects of brine geochemistry sr isotope compositions range from those expected for carbonates and evaporites deposited from jurassic seawater 0 7071 to radiogenic ratios as high as 0 7107 generally most radiogenic sr isotope values are associated with h 2 s rich waters which also have elevated alkali element li b k rb concentrations these alkali element rich waters are associated with portions of the south arkansas fault system which reach basement boron isotope compositions are similarly heterogeneous ranging from values of 26 to 50 brines with highest b contents are most depleted in 11 b consistent with boron input from brines generated from high temperature siliciclastic diagenetic reactions normalizing b contents to br in the brines reveals a reasonable mixing trend between a dead sea type composition and texas gulf coast type shale sand reservoir waters oxygen and hydrogen isotope data exhibit regional variations which are controlled by meteoric water invasion along the northern limb of the southwest arkansas fault which has surface expression although oxygen isotope compositions are often near equilibrium with respect to reservoir carbonate it is more difficult to ascribe trends in d values to local water rock interaction the stable isotope trends can be explained either in terms of an evolved marine evaporite brine or a water evolved via water rock interaction and mixing with meteoric water in the vicinity of the southwest arkansas fault spatial heterogeneity in isotopic composition of formation waters from this relatively restricted study area suggests that assumptions of a homogeneous sr reservoir in dating regional mineralization events require careful assessment in ancient systems

solute and isotopic geochemistry and ground water flow in the central wasatch range utah ground water flow systems in the rugged central wasatch range utah were investigated by solute and isotopic methods six types of ground water systems were identified on the basis of rock type and structure the systems are broadly grouped into four categories 1 granitic systems 2 non granitic systems a unconsolidated alluvial systems b consolidated sedimentary rock systems and c fault controlled systems 3 thermal systems 4 mine drainage systems these ground water systems have distinctive solute and isotopic chemistries based on an analysis of 3h and 14c data all of the ground waters have a component of post 1952 recharge water and most of the ground waters are composed almost entirely of post 1952 recharge water a few of the ground water systems contain some water which is hundreds to perhaps thousands of years old the 18o and 2h data plot on a local meteoric water line and none of the data exhibit a positive 18o shift the absence of a positive 18o shift suggests maximum aquifer temperatures are approximately 100 c granitic terrains are dominated by fracture controlled local ground water flow systems which respond rapidly to recharge events the granitic ground water systems have estimated maximum circulation depths of about 160 m below land surface all the alluvial most of the consolidated sedimentary bedrock and some of the fault controlled ground water systems are shallow circulating local flow systems with short travel times ground water circulation of most of the local flow systems is within 200 m of land surface and is bedding plane controlled some of the consolidated bedrock and many of the fault controlled ground water systems are of the intermediate type and have circulation depths to 500 m four thermal ground water systems have been identified the thermal systems are heated by the geothermal gradient and have not circulated deeper than 2 2 5 km the thermal systems are of the intermediate and regional type the solute chemistry of most of the non granitic ground waters may be attributed to the dissolution of carbonate minerals and minor amounts of gypsum contributions of external co2 g to some of the thermal systems from either metamorphic or silicate hydrolysis processes is suspected

geochemistry of metal rich brines from central mississippi salt dome basin u s a abstract oil field brines are the most favored ore forming solutions for the sediment hosted mississippi valley type ore deposits detailed inorganic and organic chemical and isotope analyses of water and gas samples from six oil fields in central mississippi one of the very few areas with high metal brines were conducted to study the inorganic and organic complexes responsible for the high concentrations of these metals the samples were obtained from production zones consisting of sandstone and limestone that range in depth from 1900 to 4000 m 70 120 c and in age from late cretaceous to late jurassic results show that the waters are dominantly bittern brines related to the louann salt the brines have extremely high salinities that range from 160 000 to 320 000 mg l total dissolved solids and are na ca cl type waters with very high concentrations of ca up to 48 000 mg l and other alkaline earth metals but with low concentrations of aliphatic acid anions the concentrations of metals in many water samples are very high reaching values of 70 mg l for pb 245 mg l for zn 465 mg l for fe and 210 mg l for mn the samples with high metal contents have extremely low concentrations 2 s samples obtained from the smackover formation limestone have low metal contents that are more typical of oil field waters but have very high concentrations up to 85 mg l of h 2 s computations with the geochemical code solmineq 87 give the following results 1 both pb and zn are present predominantly as aqueous chloride complexes mainly as pbcl 4 2 and zncl 4 2 respectively 2 the concentrations of metals complexed with short chained aliphatic acid anions and reduced s species are minor 3 organic acid anions are important in controlling the concentrations of metals because they affect the ph and buffer capacity of the waters at subsurface conditions and 4 galena and sphalerite solubilities control the concentrations of pb and zn in these waters

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

re os geochronology on sulfides from the tudun cu ni sulfide deposit eastern tianshan and its geological significance the tudun deposit is a medium sized cu ni sulfide deposit located at the westernmost edge of the huangshan jing erquan belt in the northern part of eastern tianshan nw china sulfide separates including pentlandite pyrrhotite and chalcopyrite from the tudun deposit contain re common os and 187os ranging from 40 46 to 201 2 0 8048 to 6 246 and 0 1709 to 0 9977 ppb respectively they have very low 187os 188os ratios of 1 224 2 352 the sulfides yield a re os isochron age of 270 0 7 5 ma mswd 1 3 consistent within uncertainty with the shrimp zircon u pb age for the tudun mafic intrusion gabbro of 280 0 3 0 ma the calculated initial 187os 188os ratio is 0 533 0 022 and os values range from 283 to 307 with a mean of 297 indicating significant crustal contamination of the parent melt prior to sulfide saturation the tudun deposit shares the same age and re os isotopic compositions with other orthomagmatic cu ni sulfide deposits in huangshan jing erquan belt suggesting that they have formed in early permian

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

re os systematics and geochemistry of cobaltite coass in the idaho cobalt belt belt purcell basin usa evidence for middle mesoproterozoic sediment hosted co cu sulfide mineralization with grenvillian and cretaceous remobilization we report the first study of the re os systematics of cobaltite coass using disseminated grains and massive sulfides from samples of two breccia type and two stratabound deposits in the co cu au idaho cobalt belt icb lemhi subbasin to the belt purcell basin idaho usa using a 185re 190os spike solution magnetic and non magnetic fractions of cobaltite mineral separates give reproducible re os analytical data for aliquot sizes of 150 to 200 mg cobaltite from the icb has highly radiogenic 187os 188os ratios 17 45 and high 187re 188os ratios 600 1800 but low re and total os contents ca 0 4 4 ppb and 14 64 ppt respectively containing 30 to 74 radiogenic 187os cobaltite from the icb is amenable to re os age determination using the isochron regression approach re os data for disseminated cobaltite mineralization in a quartz tourmaline breccia from the haynes stellite deposit yield a model 1 isochron age of 1349 76 ma 2 n 4 mean squared weighted deviation mswd 2 1 initial 187os 188os ratio 4 7 2 2 this middle mesoproterozoic age is preserved despite a possible metamorphic overprint or a pulse of metamorphic hydrothermal remobilization of pre existing cobaltite that formed along fold cleavages during the ca 1190 1006 ma grenvillian orogeny this phase of remobilization is tentatively identified by a model 3 isochron age of 1132 240 ma 2 n 7 mswd 9 3 initial 187os 188os ratio of 9 0 2 9 for cobaltite in the quartz tourmaline breccia from the idaho zone in the blackbird mine all mesoproterozoic cobaltite mineralization in the district was affected by greenschist to lower amphibolite facies garnet zone metamorphism during the late jurassic to late cretaceous cordilleran orogeny however the fine to coarse grained massive cobaltite mineralization from the shear zone hosted chicago zone blackbird mine is the only studied deposit that has severely disturbed re os systematics with evidence for a linear trend of mixing with metamorphic fluids the new re os ages and extremely high initial 187os 188os ratios of cobaltite reported here favor a magmatic hydrothermal genetic model for a multi stage ree y co cu au mineralization occurring at ca 1370 to 1349 ma and related to the emplacement of the big deer creek granite pluton at ca 1377 ma in our model deposition of paragenetically early xenotime and gadolinite was followed by an influx of mesoproterozoic evaporitic brines and magmatic hydrothermal fluids containing metals and reduced sulfur derived from mafic and oceanic island arc archean to paleoproterozoic rocks in the laurentian basement cobaltite mineralization occurred upon cooling of these fluids at an inferred temperature of 300 c or below

igneous rocks formed by hypervelocity impact abstract igneous rocks are the primary building blocks of planetary crusts most igneous rocks originate via decompression melting and or wet melting of protolith lithologies within planetary interiors and their classification and compositional petrographic and textural characteristics are well studied as our exploration of the solar system continues so too does the inventory of intrusive and extrusive igneous rocks settings and processes the results of planetary exploration have also clearly demonstrated that impact cratering is a ubiquitous geological process that has affected and will continue to affect all planetary objects with a solid surface whether that be rock or ice it is now recognized that the production of igneous rocks is a fundamental outcome of hypervelocity impact the goal of this review is to provide an up to date synthesis of our knowledge and understanding of igneous rocks formed by hypervelocity impact following a brief overview of the basics of the impact process we describe how and why melts are generated during impact events and how impact melting differs from endogenic igneous processes while the process may differ we show that the products of hypervelocity impact can share close similarities with volcanic and shallow intrusive igneous rocks of endogenic origin such impact melt rocks as they are termed can display lobate margins and cooling cracks columnar joints and at the hand specimen and microscopic scale such rocks can display mineral textures that are typical of volcanic rocks such as quench crystallites ophitic porphyritic as well as features such as vesicles flow textures and so on historically these similarities led to the misidentification of some igneous rocks now known to be impact melt rocks as being of endogenic origin this raises the question as to how to distinguish between an impact versus an endogenic origin for igneous like rocks on other planetary bodies where fieldwork and sample analysis may not be possible and all that may be available is remote sensing data while the interpretation of some impact melt rocks may be relatively straightforward e g for clast rich varieties and those with clear projectile contamination we conclude that distinguishing between impact and endogenic igneous rocks is a non trivial task that ultimately may require sample investigation and analysis to be conducted caution is therefore urged in the interpretation of igneous rocks on planetary surfaces

accretion rate of extraterrestrial particles determined from osmium isotope systematics of pacific pelagic clay and manganese nodules abstract pelagic clay and mn nodules from domes sites in the north pacific and a varved glacial lake deposit from connecticut were analysed for os concentration and isotopic composition by isotope dilution secondary ion mass spectrometry after treatment by nis fusion or oxalic acid leaching bulk pelagic clay from domes site c has 187 os 186 os 6 5 and os 0 14 ng g oxalic acid leaches of this same sediment and of mn nodules from domes sites a and c have more radiogenic 187 os 186 os ratios which average 8 3 bulk glacial lake hitchcock sediment has 187 os 186 os 12 5 and os 0 06 ng g the total os flux to north pacific pelagic clay is 7 to 10 ng os cm2 106 y lake hitchcock sediment provides an integrated value for the local crustal 187 os 186 os ratio the oxalic acid leaches are assumed to attack hydrogenous phases selectively a simple model in which the only sources of os to the ocean are continental crust with the isotopic composition of lake hitchcock 187 os 186 os 12 5 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of osmium to the sediment of 4 9 ngos cm2 106 y of which 20 is hydrogenous a model in which the sources of os to the ocean are continental crust with an 187os 186os ratio of 30 from the model of palmer and turekian 1986 oceanic mantle or crust with 187 os 186 os 1 04 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of os to the sediment of 5 7 ngos cm2 106 y of which none is hydrogenous these extraterrestrial os fluxes correspond to maximum c 1 chondrite accretion rates of between 4 9 104 and 5 6 104 tonnes y

compositional variations within and between five sudbury ore deposits recent work on the platinum group element pge content in ni sulfide ores has shown that those associated with komatiite magmas generally have low pt pd ru ir os ratios 13 the levack west and little stobie 1 and 2 deposits conform with this generalization having ratios of 18 6 19 2 and 13 2 respectively hoffman et al 1979 pt and pd are present in roughly equal proportions in these deposits ranging between 1 000 and 3 000 ppm in massive sulfides new data from the strathcona deposit indicate values of close to 400 ppm for both pt and pd approximately one third of the levels at the nearby levack west deposit both strathcona and levack west are strongly zoned with respect to platinum group elements au and base metals cu ni zn pt pd and au increase away from the irruptive hanging wall into the footwall and co rh ru ir and os increase in the reverse direction at strathcona this zoning results in a decrease of the pt pd ru ir os ratio from 132 in the footwall to 2 2 in the hanging wall ore relevant phase equilibria data indicate that cu ni zn pt and pd are likely to become enriched in the residual liquid of a fractionating sulfide melt whereas co rh and ru will concentrate in the early crystallizing monosulfide solid solution the correspondence between the experimental data and natural observations is strong evidence that the zoning is the consequence of the fractionation of a sulfide melt with the residual liquid being expelled progressively into the footwall average data obtained for the falconbridge depost resemble those of the hanging wall at strathcona both in the low pt pd ru ir os ratio of 2 3 and in a low cu ni ratio suggesting that the ore sampled represents an early crystallizing fraction and that a late crystallizing pt rich fraction equivalent to the footwall ore at strathcona and amounting to perhaps twice the present volume of ore was not included in the sampling and may well not be within the present orebody the deposit is located largely within a vertical fault zone it is possible that the sulfide melt was introduced up the fault crystallizing as it moved upward and that the pt rich portion crystallized farthest up the fault and has been removed by erosion however sampling over a 1 600 m vertical interval of the ore zone has failed to reveal any progressive variation in the pt pd ru ir os ratio within the fault zone an alternative and preferred hypothesis is that the deposit was originally zoned as at strathcona with the pt rich portion forming away from the irruptive within the footwall this portion may have been faulted off leaving the early crystallizing portion behind to become remobilized into the fault zone if true the present location of the pt rich portion is intriguing in addition to having lower concentrations of platinum group elements than levack west the strathcona deposit also has lower concentrations of ni and cu these compositional differences can be modeled in terms of a larger amount of sulfide equilibrating with a given amount of the host silicate magma at the former deposit than at the latter sequences of similar magmatic events coupled with a proposed fractionation of olivine can account for many of the compositional variations between different sudbury deposits

re os isotope geochemistry of tertiary picritic and basaltic magmatism of east greenland constraints on plume lithosphere interactions and the genesis of the platinova reef skaergaard intrusion abstract re os abundance and isotopic studies on a small number of samples from the lowermost part of the east greenland lava pile and the nearby skaergaard intrusion show that picrite and ankaramite lavas tend to have high os abundances and low re os ratios with most rocks having near chondritic values for initial 187 os 188 os one olivine basaltic sample with very high 187 os 188 os has most likely been affected by upper crustal contamination there is no clear evidence for recycled crustal material in the ancestral iceland plume source or for a significant degree of interaction of the picrites with ancient subcontinental lithospheric mantle of the north atlantic craton the platinova reef the precious metal enriched zone in the skaergaard intrusion has very high au and pd contents but very low pt and os contents compared to platiniferous horizons in other layered intrusions these characteristics are almost certainly due to the very late stage in the evolution of the skaergaard magma chamber at which the platinova reef formed the platinova reef yields a re os tchur model age that is younger than the age of the intrusion which was most likely produced by some form of post depositional disturbance to the re os isotopic system the high pge contents of the east greenland volcanic rocks indicate that they were formed from magmas that had the potential to form major accumulations of ni cu pge sulphide deposits had similar magmas undergone significant interaction with continental crust elsewhere in east greenland they may have formed massive magmatic sulphide deposits thus the potential for the discovery of large magmatic sulphide deposits of the noril sk talnakh type in east greenland must be considered excellent

kalkkop crater cape province south africa confirmation of impact origin using osmium isotope systematics the kalkkop structure a circular depression with a diameter of 640 m is situated in the eastern cape province south africa in 1992 a bore hole was drilled in the center of the crater to a depth of 151 8 m to assess a possible impact origin of this structure the abundances and isotopic ratios of osmium and rhenium were measured in breccias and in sandstones and shales derived from the basement the basement rocks show rhenium and osmium abundances and isotopic compositions of osmium and neodymium 187os188os 0 61 to 0 92 and nd 6 to 7 that are typical for continental crust one of the breccia samples shows a significantly elevated osmium content and a much lower 187os188os ratio of about 0 215 187os188os ratio 1 79 this ratio is much closer to meteoritic compositions than to crust indicating the presence of about 0 05 of an extraterrestrial component in the breccia this is the first time that osmium isotope systematics have been used to confirm the impact origin of a crater structure and demonstrates their sensitivity for impact studies

highly siderophile and strongly chalcophile elements in magmatic ore deposits an ore deposit by definition must be economically viable that is to say it must contain sufficient material at high enough grade to make it possible to mine and process it at a profit bates and jackson 1987 this requires the elements to be collected and concentrated by some phase and for them to be deposited close to the surface of the earth at the oxygen fugacities found in the crust native fe is not normally stable and thus the highly siderophile elements defined as ru rh pd re os ir pt and au cannot behave as siderophile elements except in rare cases such as on disko island klock et al 1986 where the magma is sufficiently reduced for native fe to be present however if mafic magmas become saturated in a base metal sulfide liquid the highly siderophile elements behave as highly chalcophile elements table 1 thus these elements are generally found in association with base metal sulfide minerals which crystallized from a magmatic sulfide liquid namely pyrrhotite pentlandite chalcopyrite cubanite pyrite an exception to this is au although au is strongly chalcophile and is produced as a by product from many platinum group element pge deposits table 2 most primary au deposits consist of native au groves et al 1998 these will not be discussed in this chapter there are many pge deposits i e accumulations of pge minerals and base metal sulfides containing pge bates and jackson 1987 around the world but most of these do not constitute pge ore deposits because they are either too small or their grade is too low or other political or infrastructure factors prevent the economic exploitation of the deposit bates and jackson 1987 for the purpose of this work we have defined pge ore deposits as those which have significant production 2 of the annual world

geochemistry of hco 3 na thermal water from the gudian slope insights into fluid origin formation mechanism and circulation in the yitong basin northeast china abstract the goal of this paper is to systematically analyze the geochemical and isotopic characteristics 18o d t 13chco3 and doc of a hco3 na thermal water to determine its origin formation mechanism and circulation pattern in the gudian slope yitong basin northeast china schoeller diagrams and isotope data indicate that this thermal water is not connected to groundwater or river water and that its recharge source is immature water with low 18o and d values that originates from the changbaishan area this result was also confirmed by its doc 13c value approximately 24 which indicates that the c originates from c3 plants the analysis of na k mg diagrams indicates that the thermal waters except for one sample sjxk are fully equilibrated this observation combined with the results of the analysis for 18o d t and cation geothermometry data for the thermal water indicates that no water underwent mixing with cold water before arriving at the surface instead high temperature immature water flowed up from a deep reservoir and mixed with old formation water approximately 35 4 in the eocene reservoir these data indicate the occurrence of an interesting phenomenon in which the thermal water samples of the gd area exhibit a regression line with a negative slope and similar 18o values potentially due to the fractionation of co2 gas and a buffer of high hco3 concentrations rather than their short circulation times the high so4 cl b cl f cl and hco3 cl ratios and 13chco3 values ranging from 0 84 to 1 24 of these samples imply that magmatic volatiles have been dissolved in the thermal water however we cannot ignore the contributions of 13c from the leaching of limestone with a13c value of approximately 3 4 which is further supported by the br cl and hco3 cl ratios thus we analyzed the geological background and data collected from well chang 27 and deduced that a limestone stratum exists below the study area magmatic volatiles flow up through the lithospheric faults along the northwestern and southeastern edges of the basin encounter changbaishan area recharge water in the limestone reservoir form hot water with high concentrations of hco3 and then recharged into the eocene sandstone reservoir long term mineral hydrolysis and transformation reactions in the reservoir cause the concentrations of na to increase and lead to the formation of hco3 na water

reos dating of molybdenites from ore deposits in japan implication for the closure temperature of the reos system for molybdenite and the cooling history of molybdenum ore deposits abstract rhenium and osmium abundances were determined on molybdenites from eighteen ore deposits in japan fifteen vein type a skarn type a greisen type and a pegmatite type deposit re os ages obtained for molybdenites from northeastern central and southwestern japan are 100 130 ma 50 75 ma and ca 10 ma respectively reflecting the ages of regional igneous activities in individual areas for vein type ore deposits re os ages for molybdenites agree with rb sr whole rock ages of host granitoids while they are 3 12 ma older than k ar mineral ages of the host rocks on the other hand re os ages of molybdenites from skarn pegmatite and greisen type ore deposits agree with k ar ages of the host rocks the comparison of re os ages for molybdenites with rb sr and k ar ages of host rocks in vein type deposits suggests that re os closure temperature for vein molybdenite is close to whole rock rb sr closure temperature for host granitic rocks i e roughly estimated to be around 500 c and higher than k ar closure temperature for host granodiorites i e 230 370 c if the thermal history of vein molybdenites is essentially equivalent to that of host rocks one possible explanation for age discrepancies of 3 12 ma between re os ages for molybdenites and k ar ages for the wall rocks observed in vein type molybdenum deposits is that later thermal events after formation of molybdenum ores may reset k ar ages hydrothermal alteration might have occurred in such a temperature condition as to disturb k ar mineral systems in wall rocks and made them younger but not the re os system in molybdenite the other explanation might be that the age discrepancies correspond to a cooling interval from ca 500 to 300 c of host granitoid on the other hand the agreement of re os ages of molybdenites and k ar ages of host rocks in skarn pegmatite and greisen type deposits in this study indicates no later disturbance for k ar system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits

source processes of the thermal waters from the phlegraean fields naples italy by means of the study of selected minor and trace elements distribution abstract the geochemical characteristics of hydrothermal waters from the phlegraean fields p f naples italy were analysed for minor and trace elements selectively mobilised in hydrothermal systems such as b f hg as pb and tl the water samples collected from a shallow aquifer likely to be fed by deeper fluids showed various geochemical features resulting from the mixing of three components 1 surface waters of meteoric origin 2 hot deep waters deriving from water rock interaction and including deep waters of marine origin 3 magmatic fluids rising from the local magma chamber lying a few kilometres below the town of pozzuoli this setting although very complex provides a reliable means of studying the distribution of the investigated trace elements in particular within the phlegraean area high contents of b 0 1 48 mg l f 0 5 8 mg l as 16 6050 g l and hg 0 7 232 g l were observed the levels of thallium in the springs close to solfatara about 7 g l were in line with those normally recorded in hydrothermal areas whereas high levels of this element up to 23 3 g l were identified in other wells of the study area lead 1 3 to 29 1 g l appears to be anomalous with respect to its normal content in groundwater about 1 g l owing to the presence of high density brines at depth which enhance the solubility of pb in volcanic rocks under hydrothermal conditions the distribution of the investigated trace elements in the phlegraean fields thermal area is probably related to the different ascent pathways of the fluids clearly apart from the influence exerted by anomalous thermal conditions each element shows a different behaviour depending on its geochemical affinity with mineral phases and as a consequence of the different enthalpy values which determine ion partitioning in gaseous phases based on geochemical evidences and on the distribution of minor and trace elements the source processes of the investigated hydrothermal waters were defined five main groups were identified 1 acid sulphate waters resulting from mixing of meteoric water with magmatic gases mainly h 2 s 2 high sulphate chloride waters from a deep reservoir located in the major upflow zone 3 waters associated with significant degassing of magmatic co 2 4 waters from a deep geothermal neutral chloride reservoir resulting from heating of marine water modified by water rock interaction processes 5 cold waters from the inner area influenced by low temperature water rock interaction processes

use of two new na li geothermometric relationships for geothermal fluids in volcanic environments abstract this work has made it possible to obtain two new na li geothermometric relationships in addition to the three already known fouillac and michard 1981 kharaka et al 1982 and confirms that the na li geothermometer unlike the na k na k ca k mg and silica geothermometers or the isotope 18o h2o so4 geothermometer also depends on the fluid salinity and the nature of the reservoir rocks reacting with the geothermal water one of the relationships concerns the fluids derived from seawater basalt interaction processes existing in emerged rifts such as those of iceland reykjanes svartsengi and seltjarnarnes geothermal fields and djibouti asal ghoubbet and obock geothermal areas or in numerous oceanic ridges and rises mid atlantic and middle valley ridges east pacific rise etc the best adapted na li relationship for geothermal fluids discharged from emerged rifts between 0 and 365 c is t k 920 log n a l i 1 105 r 2 0 994 n 27 where na and li are the aqueous concentrations of these elements given in mol l the other na li relationship was determined using dilute waters collected from wells located in different high temperature 200 325 c volcanic geothermal areas of iceland krafla namafjall nesjavellir and hveragerdi this relationship can be expressed as follows t k 2002 log na li 1 322 r2 0 967 n 17 these two relationships give estimations of temperature with an uncertainty close to 20 c the second na li relationship was also successfully applied to ht dilute geothermal waters from the east african rift ethiopia kenya some case studies in the literature and thermodynamic considerations suggest that the na li ratios for this type of fluids could be controlled by full equilibrium reactions involving a mineral assemblage constituting at least albite k feldspar quartz and clay minerals such as kaolinite illite or muscovite and li micas unlike the na li ratios no thermometric relationship using li isotopes could be determined for this type of water however it was noticed that 7li values higher than 16 are always associated with low to medium temperature waters

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

re os ages for archean molybdenite and pyrite kuittila kivisuo finland and proterozoic molybdenite kabeliai lithuania testing the chronometer in a metamorphic and metasomatic setting seven 187re 187os ages were determined for molybdenite and pyrite samples from two well dated precambrian intrusions in fennoscandia to examine the sustainability of the re os chronometer in a metamorphic and metasomatic setting using a new 187re decay constant 1 666 10 11y 1 with a much improved uncertainty 0 31 we determined replicate re os ages for molybdenite and pyrite from the kuittila and kivisuo prospects in easternmost finland and for molybdenite from the kabeliai prospect in southernmost lithuania these two localities contain some of the oldest and youngest plutonic activity in fennoscandia and are associated with newly discovered economic au mineralization ilomantsi finland and a cu mo prospect kabeliai lithuania two re os ages for vein hosted kabeliai molybdenite average 1486 5 ma in excellent agreement with a 1505 11 ma u pb zircon age for the hosting kabeliai granite pluton the slightly younger age suggests the introduction of cu mo mineralization by a later phase of the kabeliai magmatic system mean re os ages of 2778 8 ma and 2781 8 ma for kuittila and kivisuo molybdenites respectively are in reasonable agreement with a 2753 5 ma weighted mean u pb zircon age for hosting kuittila tonalite these re os ages agree well with less precise ages of 2789 290 ma for a rb sr whole rock isochron and 2771 75 ma for the average of six sm nd tdm model ages for kuittila tonalite three re os analyses of a single pyrite mineral separate from the same sample of kuittila pluton that yielded a molybdenite separate provide individual model ages of 2710 27 2777 28 and 2830 28 ma re 17 4 12 1 and 8 4 ppb respectively with a mean value of 2770 120 ma in agreement with the kuittila molybdenite age the re and 187os abundances in these three pyrite splits are highly correlated r 0 9994 and provide a 187re 187os isochron age of 2607 47 ma with an intercept of 21 ppt 187os mswd 1 1 it appears that the re os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt 187os intercept reflects the in situ decay of 187re during the 160 to 170 m y interval from 2778 ma time of molybdenite pyrite deposition to 2607 ma time of pyrite resetting when the re os data for molybdenites from the nearby kivisuo prospect are plotted together with the kuittila molybdenite and pyrite data a well constrained five point isochron with an age of 2780 8 ma and a 187os intercept 2 4 3 8 ppt of essentially zero results mswd 1 5 we suggest that the pyrite isochron age records a regional metamorphic and or hydrothermal event possibly the time of au mineralization a proposed re os age of 2607 ma for au mineralization is in good agreement with radiometric ages by other methods that address the timing of archean au mineralization in deposits worldwide so called late au model molybdenite in contrast provides a robust re os chronometer retaining its original formation age of 2780 ma despite subsequent metamorphic disturbances in archean and proterozoic time

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

geothermal systems on the island of java indonesia abstract this paper presents an overview of the geothermal systems on the island of bali indonesia physicochemical data of hot springs and shallow geothermal wells were collected from four geothermal locations penebel batur banjar and banyuwedang the concentrations for the three main anions varied significantly indicating a different geothermal history the values for cl ranged from 0 1 to 1000 mg l for hco3 from 20 to 2200 mg l and for so42 from 0 1 to 500 mg l although the island of bali is underlain by carbonate rocks a carbonate host rock for the geothermal reservoirs could not be confirmed because the ca2 mg2 hco3 molar ratios were approximately 0 4 well below 1 0 and the k mg ratios were approaching those of a calc alkaline rock reservoir the hco3 of the thermal waters correlated with ca2 mg2 sr2 and k indicating water rock interaction in the presence of carbonic acid phase separation was inferred for the bedugul and banjar geothermal systems because of relatively high b cl ratios boron isotopes were determined for selected samples with values ranging from 11b of 1 3 to 22 5 nbs 951 the heavy 11b of 22 5 together with a low b cl ratio indicated seawater input in the banyuwedang geothermal system the hydrogen and oxygen isotopic composition of the thermal water plotted along the global meteoric water line gmwl and close to the mean annual value for precipitation in jakarta indicating a meteoric origin of the geothermal water comparison of the si na k na k ca and na li geothermometers with actual reservoir temperature measurements and physicochemical considerations led to the conclusion that the na li thermometer provided most reliable results for the determination of geothermal reservoir temperatures on bali using this thermometer the following reservoir temperatures were calculated 1 penebel bedugul from 235 to 254 c 2 batur 240 c and 3 banjar 255 c due to seawater input this thermometer could not be applied to the banyuwedang geothermal system there application of a sio2 thermometer indicated a reservoir temperature below 100 c

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

reliable re os age for molybdenite abstract re os ages for four molybdenites were determined by our newly developed method with isotope dilution mass spectrometry special caution was given to achieve osmium isotopic equilibration between sample and spike during chemical treatment re os data for molybdenites from mt mulgine australia matasvaara finland preissac canada and climax usa define ages of 2790 ma 2600 ma 2680 ma and 26 6 ma respectively the reproducibilities of our re os age determination are shown to be within 2 by the replicate analyses for mt mulgine matasvaara and preissac samples the ages obtained are in good agreement with reference ages estimated for wall rocks of molybdenite deposits concerned thus with careful experimental procedure it appears that the re os geochronometer can be used to reliably determine the age of molybdenite

the evolution of the onaping formation at the sudbury impact structure the 1 4 1 6 km thick onaping formation consists of a complex series of breccias and melt bodies lying above the sudbury igneous complex sic at the sudbury impact structure based on the presence of shocked lithic clasts and various glassy phases the onaping has been described as a suevitic breccia with an origin at least in part as fallback material recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases both as clast types and discrete intrusive bodies the nature of the onaping and that of other suevitic breccias overlying impact melt sheets is reviewed the relative thickness internal stratigraphic and lithological character and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the onaping the sudbury structure formed in a foreland basin and water played an essential role in the evolution of the onaping as indicated by a major hydrothermal system generated during its formation taken together observations and interpretations of the onaping suggest a working hypothesis for the origin of the onaping that includes not only impact but also the interaction of sea water with the impact melt resulting in repeated explosive interactions involving proto sic materials and mixing with pre existing lithologies this is complicated by additional brecciation events due to the intrusion of proto sic materials into the evolving and thickening onaping fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two thirds of the onaping

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

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

trace element and isotopic evidence for archean basement in the lonar crater impact breccia deccan volcanic province abstract the lonar impact crater in the deccan traps of the indian peninsula provides unique opportunities to study physical and chemical processes of impact cratering on basaltic targets because terrestrial impact craters on basalts are extremely rare such studies are needed for determining provenance and other parameters of the excavated rocks and the cratering phenomenon that may have implications for similar crater formations in lunar martian and other basaltic targets in the solar system considering some of these objectives we analyzed trace elements and nd sr pb isotopes of impact breccia rocks and target basalts collected from the lonar crater chondrite normalized rare earth element ree patterns in the target basalts and breccia rocks show similar light ree enriched patterns although in detail the impact breccia are more fractionated in la sm compared to the target basalts the target basalts also show much lower concentrations of rb ba th u and pb compared to the breccia and are characterized by rb ba and pb depletions with respect to the primitive mantle normalized th u nb ta and the ree contents the breccia rocks are significantly enriched in rb ba and pb and to a lesser extent in th and u compared to the target rocks the nd sr and pb isotopic compositions of the lonar target basalts can be correlated with those of the poladpur suite one of the mid section volcano stratigraphic units of the deccan traps in contrast to the host basalts the impact breccia rocks show more radiogenic sr less radiogenic nd and higher rb sr and lower sm nd ratios indicating an additional component other than the target basalt that must have been derived from beneath the basaltic target rocks at the impact site the deccan traps in western india are underlain by archean to mid proterozoic cratonic rocks the overall geochemical signatures of the impact breccia rocks specifically the trace element concentrations negative end values radiogenic sr isotopic composition as well as the high 207pb 204pb at low 206pb 204pb indicate that a major component of the lonar impact breccia was derived from melting of archean basement rocks we argue that the archean component in the breccia cannot be from the incorporation of paleosols that are weathering products of the target basalts or from the inter trappean sediments that are most commonly cherts and limestones of mesozoic age similarly the possible role of eolian sediments in causing the archean pb isotopic signature identical to those of the deccan basement in the breccia rocks can be excluded the basement beneath the lonar region is believed to be similar to the dharwar craton of peninsular india based on their similar pb isotopic compositions with the breccia rocks we suggest the archean chitradurga group of rocks of this craton to be present in the basement beneath the deccan lavas of the lonar region the thickness of the basaltic target rocks at the crater site 400 m and the inferred crater depth 350 m 610 m based on depth to diameter ratios in simple planetary craters 0 2 0 33 are consistent with our conclusion regarding melting and incorporation of these ancient basement rocks in the impact breccia of the lonar crater using pi group scaling relations the observed crater diameter and density of the basaltic target rocks we have estimated with reasonable approximation the diameter of the lonar impactor to be either 70 m 86 m or 120 m assuming the bolide to be an iron meteorite stony iron meteorite or an ordinary chondrite respectively

re os geochemistry and geochronology of the ransko gabbro peridotite massif bohemian massif the ransko gabbro peridotite massif in eastern bohemia is a strongly differentiated intrusive complex which hosts low grade ni cu ores mainly developed close to the contact of olivine rich rocks with gabbros in troctolites and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase rich peridotites gabbro troctolite peridotite and ni cu ores from the jezirka ni cu pge deposit considered to be a typical example of the liquid segregation style of mineralization were analyzed for re os concentrations and isotopic ratios seven barren and mineralized samples from the jezirka deposit yielded a re os regression of 341 5 7 9 ma mswd 69 strongly mineralized peridotite with mantle like initial 187os 188os ratio of 0 125 suggests that os as well as other pge present in the ni cu mineralization are predominantly of mantle origin on the other hand barren and low mineralized samples have radiogenic initial 187os 188os ratios of 0 14 0 16 suggesting some import of re and or radiogenic 187os most likely through contamination by continental crust during magma emplacement the re os age of the ransko massif is significantly younger than the previously suggested lower cambrian age but it is similar to and or younger than the age of metamorphism of the adjacent kutna hora crystalline complex and the moldanubian unit therefore it is likely that the emplacement of the ransko massif and its ni cu mineralization was closely connected with the late stage evolution of the kutna hora crystalline complex

geoscience impact a synthesis of studies of the sudbury structure geophysical probing results are synthesized into a three dimensional framework necessary for understanding the genesis of the sudbury structure based primarily on seismic reflection results centred on the sudbury igneous complex remnants of crustal melting from a catastrophic meteorite impact are superimposed on the juxtaposition of mid crustal rocks exhumed during the archean against deformed paleoproterozoic sedimentary rocks sedimentation metamorphism deformation and metasomatic overprints are all part of the post impact history of sudbury and tend to dominate the geophysical response of the structure pre impact deformation although certainly preserved in some aspects of sudbury geology is not clearly expressed in the geophysical data nor are any elements of impact induced deformation geophysical views of the sudbury igneous complex are thus somewhat biased in representing mostly the post impact but pre grenvillian history of the region with the exception of igneous events establishing th

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

seismic images and three dimensional architecture of a proterozoic shear zone in the sudbury structure superior province canada a broad zone of intense ductile to brittle deformation transects the paleo proterozoic sudbury igneous complex and likely formed during the final stages of the penokean orogeny on the southern margin of the superior province herein we present a three dimensional image of this deformation zone derived from four seismic reflection profiles additional information from rock property measurements drill results and published structural mapping suggests that the seismic data define the three dimensional disposition of listric shear zones the seismic image of the deformation is best developed within the onaping formation however the images of the deformation zone envelope are imperfect being most clear where truncating known contacts within the sudbury igneous complex and its contact with huronian host rocks the southern edge of the seismically imaged shear zones does not extend significantly beyond the exposed sudbury igneous complex suggesting limits to the extent of deformation in the sudbury structure

re os sm nd u pb and stepwise lead leaching isotope systematics in shear zone hosted gold mineralization genetic tracing and age constraints of crustal hydrothermal activity abstract a combined re os sm nd u pb and stepwise pb leaching pbsl isotope study of hydrothermal mo w bearing minerals and base metal sulfides from two adjacent shear zone hosted gold deposits ran kimberley in the harare shamva greenstone belt zimbabwe constrain the timing of the mineralizing events to two periods during an initial late archean event 2 60 ga a first molybdenite scheelite bearing paragenesis was deposited in both shear zone systems followed by a local reactivation of the shear systems during an early proterozoic 1 96 ga tectono thermal overprint during which base metal sulfides and most of the gold was re deposited while pbsl has revealed an open system behavior of the u pb systematics in molybdenite and wolframite from the ran mine initial archean re os ages are still preserved implying that this system in these minerals was more resistant to the overprint a similar retentivity could be shown for the sm nd system in scheelite and powellite associated with the above ore minerals re os isotopic data from the proterozoic mineralization in the kimberley mine point to a recent gain of re most pronouncedly affecting fe rich sulfides such as pyrrhotite a significant re loss in powellitic scheelite an alteration phase of molybdenite bearing scheelite coupled with a marked loss of u in w mo ore minerals complements the observation of a major re uptake in fe sulfides during oxidizing conditions in a weathering environment pyrrhotite under these conditions behaves as an efficient re sink lead isotope signatures from pbsl residues of molybdenite powellite and quartz indicate a continental crustal source and or contamination for the mineralizing fluid by interaction of the fluids with older sedimentary material as represented by the direct host country rocks our investigation reveals the potential of the re os isotopic system applied to crustal hydrothermal ore minerals for genetic tracing and dating purposes the simplified chemical separation of re and os from geological material used in this study together with improvements of chemical yields will enable high precision data to be collected rapidly on crustal material with low os concentrations in the future

the south range breccia belt of the sudbury impact structure a possible terrace collapse feature the south range breccia belt srbb is an arcuate 45 km long zone of sudbury breccia in the south range of the 1 85 ga sudbury impact structure the belt varies in thickness between tens of meters to hundreds of meters and is composed of a polymict assemblage of huronian supergroup 2 49 2 20 ga nipissing diabase 2 2 ga and proterozoic granitoid breccia fragments ranging in size from a few millimeters to tens of meters the srbb matrix is composed of a fine grained 100 m assemblage of biotite quartz and ilmenite with trace amounts of plagioclase zircon titanite epidote pyrite chalcopyrite pyrrhotite and occasionally chlorite the srbb hosts the frood stobie vermilion and kirkwood quartz diorite offset dykes the former being associated with one of the largest ni cu pge sulphide deposits in the world optical petrography and whole rock geochemistry concur with previous studies that have suggested that the matrix of the srbb is derived from comminution and at least partial frictional melting of the wall rock huronian supergroup lithologies rare earth element ree data from all sampled lithologies associated with the srbb exhibit crustal signatures when normalized to c1 chondrite values additionally ree data from the quartz diorites disseminated sulphides in sudbury breccia and a sample of an aphanitic biotite hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites quartz gabbro and granophyre units of the sudbury igneous complex sic which suggests that these lithologies are representative of bulk sic melt we suggest that the srbb was formed by high strain rate 1 m s gravity driven seismogenic slip of the inner ring of the sudbury impact structure during postimpact crustal readjustment crater modification stage failure of the hanging wall may have facilitated the injection of bulk sic melt into the srbb along with the ni cu pge sulphides of the frood stobie deposit postimpact penokean 1 9 1 7 ga tectonism particularly northwest directed shearing along the south range shear zone and associated thrust faulting could account for the present subvertical orientation of the srbb and the apparent lack of a connection at depth with the sic

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

clues to the nature of the impacting bodies from platinum group elements rhenium and gold in borehole samples from the clearwater east crater canada and the boltysh impact crater ukraine seven large 10 g impact melt rock samples from boreholes from the boltysh impact crater ukraine and six samples from the east clearwater crater canada were analyzed for os ir ru rh pd re and au by the nickel sulfide technique in combination with neutron activation earlier analyses of clearwater east impact melt rocks have shown that they are strongly enriched in ir os pd and re in this work i confirm earlier findings and demonstrate similarly high enrichments of rh and ru the average os ir ru ir pd ir rh ir and ru rh ratios of the melt rock samples from clearwater east are ci chondritic and yield an average ir content of 25 2 6 5 ng g relative to an average upper crust concentration of 0 03 0 02 ng g ir the amount of meteoritic component corresponds to 4 to 7 of a nominal ci component for clearwater east the impact melt rock samples from a bore hole from boltysh are low in ir with an average of 0 2 0 1 ng g the ci normalized abundances increase from the refractory to the more volatile siderophile elements os ir ru rh pd au ni co because of the low ir anomaly and uncertainties in making corrections correlations are weak for indigenous siderophile elements no clear projectile assignment can be made

a strontium isotopic study of formation waters from the illinois basin u s a abstract the isotopic composition of sr has been measured in 73 formation water samples from paleozoic strata in the illinois basin 87sr 86sr ratios range from 0 7079 to 0 7108 with the exception of four samples the waters are more radiogenic than corresponding paleozoic sea water values the relatively narrow range of slightly elevated 87sr 86sr rations is uniformly distributed in waters throughout the stratigraphic column and in silurian waters across the basin isotopic analyses of core samples from reservoir rocks show an absence of water rock sr isotopic equilibration basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic sr for the waters silurian and devonian water show a two component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from new albany shales entered silurian devonian carbonate rocks and mixed with marine interstitial water regional migration of the waters and associated petroleum within the silurian devonian strata proposed in other studies is consistent with the sr isotopic data under favorable circumstances subsurface waters are capable of retaining a sr isotopic recor of their evolution

geomorphic evidence for the geometry and slip rate of a young low angle thrust fault implications for hazard assessment and fault interaction in complex tectonic environments abstract we present surface evidence and displacement rates for a young active low angle 20 reverse thrust fault in close proximity to major population centers in southern california usa the southern san cayetano fault sscf active faulting along the northern flank of the santa clara river valley displaces young landforms such as late quaternary river terraces and alluvial fans geomorphic strain markers are examined using field mapping high resolution lidar topographic data 10be surface exposure dating and subsurface well data to provide evidence for a young active sscf along the northern flank of the santa clara river valley displacement rates for the sscf are calculated over 103 104 yr timescales with maximum slip rates for the central sscf of 1 9 1 0 0 5 mm yr 1 between 19 7 ka and minimum slip rates of 1 3 0 5 0 3 mm yr 1 since 7 ka uplift rates for the central sscf have not varied significantly over the last 58 ka with a maximum value of 1 7 0 9 0 6 mm yr 1 for the interval 58 19 ka and a minimum value of 1 2 0 3 mm yr 1 since 7 ka the sscf is interpreted as a young active structure with onset of activity at some time after 58 ka the geometry for the sscf presented here with a 20 north dip in the subsurface is the first interpretation of the sscf based on geological field data our new interpretation is significantly different from the previously proposed model derived geometry which dips more steeply at 45 60 and intersects the surface in the middle of the santa clara river valley we suggest that the sscf may rupture in tandem with the main san cayetano fault additionally the sscf could potentially act as a rupture pathway between the ventura and san cayetano faults in large magnitude multi fault earthquakes in southern california however given structural complexities including significant changes in dip and varying holocene displacement rates along strike further work is required to examine the possible mechanism likelihood and frequency of potential through going ruptures between the ventura and san cayetano faults confirmation of the sscf in a previously well studied area such as southern california demonstrates that identification of young faults is critical for accurate seismic hazard assessment we suggest that many young active faults remain undetected in other structurally complex and tectonically active regions globally and that significant seismic hazards can be overlooked

direct laboratory determination of the 187 re half life the long lived naturally occurring radionuclide 187re is important in geochemistry and cosmology as a nucleochronometer until now there have been no direct laboratory measurements which have avoided the difficulties of both low energy counting and dependence on radiometric ages of rocks and meteorites we report here a half life of 4 35 0 13 1010 yr based on the growth of 187os over a 4 yr period into a large source of osmium free rhenium since our result agrees with the best geochemically determined values no significant revision of the present galactic age limits based on the geochemical values is necessary the agreement at the 5 level of the decay rates determined on laboratory and meteorite age timescales places a tight constraint on cosmological models in which fundamental constants are allowed to vary in particular the fine structure constant must have changed by less than one part in 105 over the past 4 5 109 yr

recognition of eroded astroblemes abstract the morphology of lunar craters degrades only slowly by the formation of new impacts and their ejecta it is not the same on earth because of weathering and tectonism generally the typical crater topography has disappeared and the term impact structure or astrobleme is applied on planetary surfaces as on the moon two classes of craters are recognized based on their depth to diameter ratio simple bowl shaped and complex readjusted flat bottomed craters with a sharp transition from simple to complex pike 1977 1980 on earth small simple craters retain some of their typical rimmed pit morphology because they are generally recent many are associated with meteorite fragments large impact structures or astroblemes in addition to their specific circular features are recognized as such only by the presence of shock metamorphism criteria such as dense minerals coesite stishovite diaplectic minerals and glass maskelynite planar deformation features in minerals principaly in quartz and feldspars and shattercones which are commonly used these criteria are abundant in allochthonous breccia particularly in fall back breccia among which there are glass fragments such as tectite or microtectite glass spherules shards droplets rods etc but on the floor of the astroblemes the criteria of shock metamorphism are restricted to the central part which is often covered or eroded using the measurements of nine astroblemes visited a scaling model is presented for two types of craters readjusted along listric slip surfaces a well readjusted model based on data from charlevoix and manicouagan canada and a partly readjusted model based on data from the ries germany both possess a central uplift and a ring graben there is also on the floor of the astrobleme a specific type of breccia mylolisthenite and pseudotachylite associated with the displacement along the autochthonous masses of rocks in very large impact structures the shock metamorphism is able to melt a part of the floor grieve and cintala 1992 the heat resulting from the superheated impact melt destroys a part of the shock metamorphism criteria

sources of osmium to the modern oceans new evidence from the 190pt 186os system high precision os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to 187os 188os it has 186os 188os that is within uncertainty of the chondritic value marine mn nodule compositions thus indicate that the average continental source of os to modern seawater had long term high re os compared to pt os analyses of loess and freshwater mn nodules support existing evidence that average upper continental crust ucc has resolvably suprachondritic 186os 188os as well as radiogenic 187os 188os modeling the composition of seawater as a two component mixture of oceanic cosmic os with chondritic os compositions and continentally derived os demonstrates that insofar as estimates for the composition of average ucc are accurate congruently weathered average ucc cannot be the sole continental source of os to seawater our analysis of four cambrian black shales confirm that organic rich sediments can have 187os 188os ratios that are much higher than average ucc but 186os 188os compositions that are generally between those of chondrites and average ucc preferential weathering of black shales can result in dissolved os discharged to the ocean basins that has a much lower 186os 188os than does average upper crust modeling the available data demonstrates that augmentation of estimated average ucc compositions with less than 0 1 additional black shale and 1 4 additional ultramafic rock can produce a continental end member os isotopic composition that satisfies the requirements imposed by the marine mn nodule data the interplay of these two sources provides a mechanism by which the 187os 188os of seawater can change as sources and weathering conditions change yet seawater 186os 188os varies only minimally

introduction to the special section on the lithoprobe sudbury project integrated geophysical and rock property studies of the sudbury structure have revealed it is highly deformed with substantial ns shortening in the sudbury igneous complex understanding the nature and extent of deformation is essential to determining the original shape of the structure and the results of these studies are sufficient to discount many of the objections to an impact origin

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

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

oxygen and carbon isotope ratios of hydrothermal minerals from yellowstone drill cores abstract oxygen and carbon isotope ratios were measured for hydrothermal minerals silica clay and calcite from fractures and vugs in altered rhyolite located between 28 and 129 m below surface in situ temperatures ranging from 81 to 199 c in yellowstone drill holes the purpose of this study was to investigate the mechanism of formation of these minerals the 18o values of the thirty two analyzed silica samples quartz chalcedony cristobalite and cristobalite range from 7 5 to 2 8 about one third of the silica 7samples have 18o values that are consistent with isotopic equilibrium with present thermal waters most of the other silica samples appear to have precipitated from water enriched in 18o up to 4 7 relative to present thermal water assuming precipitation at present in situ temperatures available data on fluid inclusion homogenization temperatures in hydrothermal quartz indicate that silica precipitation occurred mostly at temperatures above those measured during drilling and imply that 15o enrichments in water during silica precipitation were generally larger than those estimated from present conditions similarly clay minerals celadonite and smectite have 18o values higher by 3 5 to 7 9 than equilibrium values under present conditions in contrast all eight analyzed calcite samples are close to isotopic equilibrium with present thermal waters the frequent incidence of apparent 18o enrichment in thermal water from which the hydrothermal minerals precipitated may indicate that a higher proportion of strongly 18o enriched deep hydrothermal fluid once circulated through shallow portions of the yellowstone system or that a recurring transient 18o enrichment effect occurs at shallow depths and is caused either by sudden decompressional boiling or by isotopic exchange at low water rock ratios in new fractures the mineralogy and apparent 18o enrichments of hydrothermal fracture filling minerals are consistent with deposition during transient boiling or rock water exchange fracturing events

resetting of neodymium isotopes and redistribution of rees during sedimentary processes the early proterozoic chelmsford formation sudbury basin ontario canada abstract the rees abundances and neodymium isotope compositions of sandstones and siltstones from the early proterozoic chelmsford formation reveal that the rees in some samples were redistributed during the early proterozoic by surficial processes and that prior to redistribution the nd isotope system was partially reset the chelmsford formation is a coarse grained turbidite sequence the youngest of three clastic units that fill the sudbury basin in ontario it is constrained to have been derived from nearby late archean superior province rocks by petrography major and trace element chemistry and pb isotopic compositions of whole rocks and feldspar although ree patterns and nd model ages for samples from around the base of the formation reflect their derivation from undisturbed superior province rocks tdm 2800 ma five samples from higher in the stratigraphic section show evidence that up to 60 of their la has been removed and sm nd has increased by up to 60 these samples also have anomalously high nd model ages tdm 3400 to 4400 ma the presence of pronounced positive ce anomalies in the samples with anomalous tdm indicates the influence of surficial processes on the ree composition the data scatter about a vertical array on a plot of f sm nd vs ndt 1800 nd calculated at 1800 ma approximately the depositional age with values of f sm nd between 0 45 and 0 14 mixing of nd from other sources is difficult to reconcile with the petrography major and trace element geochemistry and nd and pb isotopes removal of an lree enriched phase from the sedimentary system provides a viable mechanism to produce the ree and nd isotopic characteristics of the chelmsford formation samples neodymium isotopes require that this phase was partially isotopically equilibrated with the matrix sediment near the time of sedimentation this study shows the dangers of indiscriminate use of nd model ages in provenance studies and demonstrates that the simultaneous examination of sm nd and nd isotopes on f sm nd vs nd or isochron plots is required for the recognition of sedimentary redistribution of the ree

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated

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