iron and oxygen isotope fractionation during iron uv photo oxidation implications for early earth and mars abstract banded iron formations bifs contain appreciable amounts of ferric iron fe 3 the mechanism by which ferrous iron fe 2 was oxidized into fe 3 in an atmosphere that was globally anoxic is highly debated of the three scenarios that have been proposed to explain bif formation photo oxidation by uv photons is the only one that does not involve life the other two are oxidation by o 2 produced by photosynthesis and anoxygenic photosynthesis whereby fe 2 is directly used as electron donor in place of water we experimentally investigated iron and oxygen isotope fractionation imparted by iron photo oxidation at a ph of 7 3 the iron isotope fractionation between precipitated fe 3 bearing lepidocrocite and dissolved fe 2 follows a rayleigh distillation with an instantaneous 56 fe 54 fe fractionation factor of 1 2 such enrichment in the heavy isotopes of iron is consistent with the values measured in bifs we also investigated the nature of the mass fractionation law that governs iron isotope fractionation in the photo oxidation experiments i e the slope of the 56 fe 57 fe relationship the experimental run products follow a mass dependent law corresponding to the high t equilibrium limit the fact that a 3 8 gyr old bif sample if g from isua greenland falls on the same fractionation line confirms that iron photo oxidation in the surface layers of the oceans was a viable pathway to bif formation in the archean when the atmosphere was largely transparent to uv photons our experiments allow us to estimate the quantum yield of the photo oxidation process 0 07 iron atom oxidized per photon absorbed this yield is used to model iron oxidation on early mars as the photo oxidation proceeds the aqueous medium becomes more acidic which slows down the reaction by changing the speciation of iron to species that are less efficient at absorbing uv photons iron photo oxidation in centimeter to meter deep water ponds would take months to years to complete oxidation by o 2 in acidic conditions would be slower iron photo oxidation is thus likely responsible for the formation of jarosite hematite deposits on mars provided that shallow standing water bodies could persist for extended periods of time the oxygen isotopic composition of lepidocrocite precipitated from the photo oxidation experiment was measured and it is related to the composition of water by mass dependent fractionation the precipitate fluid 18 o 16 o isotope fractionation of 6 is consistent with previous determinations of oxygen equilibrium fraction factors between iron oxyhydroxides and water

u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

origin of the neoproterozoic fulu iron formation south china insights from iron isotopes and rare earth element patterns abstract in the neoproterozoic era there was widespread deposition of iron formations in close association with global or near glaciations these snowball earth glaciations likely played a key role in iron formation distribution and deposition however the environmental conditions fe sources and formation mechanisms remain debated here we present the rare earth element geochemistry and fe isotope composition of the synglacial iron formation within the neoproterozoic fulu formation south china the fulu iron formation consists of layers of authigenic minerals mainly hematite and detrital components quartz feldspars fe chlorite and minor biotite positive eu anomalies in one of the fulu localities indicate a hydrothermal influence suggesting that fe was mainly sourced from distal hydrothermal systems the bulk rock fe isotope composition of the fulu iron formation shows a large range with 56fe from 0 23 to 1 78 correlation between bulk rock 56fe values and al fe ratios demonstrates that 56fe variability reflects in part varying proportions of authigenic versus detrital components the fe isotope composition of authigenic hematite is calculated by a linear regression and shows 56fe between 0 83 and 2 21 with an average at 1 54 0 50 2 n 41 using a dispersion reaction model the high 56fe values of hematite constrain local dissolved o2 concentrations of the ocean to less than 0 4 nmol l even in the shallow part of the water column this relationship is consistent with highly reducing conditions in the neoproterozoic oceans favored by isolation from the atmosphere by a sea ice we attribute the extremely positive values to partial iron oxidation in waters that were cold relative to modern surface oceans the dominant occurrence of hematite supports an abiotic precipitation pathway given that biological activity would have introduced organic matter to the sediments and led to partial reduction of fe iii oxides and subsequent formation of magnetite and or siderite as is observed in archean and paleoproterozoic iron formations oxic glacial meltwater and or o2 vertical transfer from the atmosphere to the upper ocean linked to ice dynamics is likely to have mediated the abiotic oxidation we propose that vertical transfer of o2 resulted from the deposition of snow that trapped air bubbles at the top of the glacier coupled to a melting of the bottom of the glacier which in combination delivered a limited but continuous amount of o2 to the ocean

copper isotopic signature of the tiegelongnan high sulfidation copper deposit tibet implications for its origin and mineral exploration we report the copper isotopic composition of cu sulfides in the tiegelongnan high sulfidation hs copper deposit tibet and investigate the possible application of cu isotopes to mineral exploration the copper isotopic values of samples from four drill holes display consistent progressive increase with depth to 400 m below surface with an overall variation of 65cu of up to 7 60 4 76 to 2 84 such a large range is very different from that observed in cu porphyry deposits which commonly exhibit a small 65cu range 1 with decreasing values with depth the large 65cu variation is likely of supergene origin and reflects strong leaching under oxidized conditions the systematic increase of 65cu with depth can be explained by the release of isotopically heavy cu at the top and its transport downward which is supported by the coupling of positive 65cu values with high cu grade and the occurrence of both negative and positive 65cu values in two drill holes mass balance consideration on the basis of cu isotopes can indicate mineralized zones which have been eroded or have not been found yet such mineralized targets if buried at depth could be located using cu isotopes

shallow marine ironstones formed by microaerophilic iron oxidizing bacteria in terminal paleoproterozoic abstract the microbial origin of precambrian iron formations is debated due to the lack of direct fossil evidence in order to reveal the genesis of ironstones under low oxygen levels integrative studies of sedimentology petrography mineralogy and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal paleoproterozoic chuanlinggou formation ca 1 65 1 64 ga of north china using microscopy sem eds icp oes icp ms and mc icp ms techniques mineralogical study shows that the fe rich mineral is predominantly hematite that resulted from dehydration of amorphous fe oxyhydroxide during diagenesis petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus basinward increases of the ironstone abundance eu anomalies from 1 39 to 1 56 and 56fe values of the ironstones from 0 5 to 1 0 suggest that the iron was mainly sourced from seafloor hydrothermal fluids and partially oxidized and precipitated in shallow subtidal to intertidal environments the common existence of fe oxide coated sheaths spiral stalks residual extracellular polymer substances eps and other biogenic fabrics indicates that microaerophilic iron oxidizing bacteria feob may have played an important role in precipitating the chuanlinggou ironstones the extremely low oxygen concentrations implied by the proliferation of microaerophilic feob in the shallow waters the weak positive ce anomalies 0 94 1 12 and low mn concentrations in the ironstones are broadly consistent with the previous result of a cr isotope study thus the establishment of a microaerophilic feob genetic model for the widespread chuanlinggou ironstones in north china provides new insight into the origin of precambrian iron formations and the redox evolution of ocean atmosphere systems during the boring billion

u pb geochronology of zircon and polygenetic titanite from the glastonbury complex connecticut usa an integrated sem empa tims and shrimp study abstract u pb ages for zircon and titanite from a granodioritic gneiss in the glastonbury complex connecticut have been determined using both isotope dilution thermal ionization mass spectrometry tims and the sensitive high resolution ion microprobe shrimp zircons occur in three morphologic populations 1 equant to stubby multifaceted colorless 2 prismatic dark brown with numerous cracks and 3 elongate prismatic light tan to colorless cathodoluminescence cl imaging of the three populations shows simple concentric oscillatory zoning the zircon tims age weighted average of 207pb 206pb ages from group 3 grains 450 5 1 6 ma mswd 1 11 and shrimp age composite of 206pb 238 u age data from all three groups 448 2 2 7 ma mswd 1 3 are interpreted to suggest a relatively simple crystallization history titanite from the granodioritic gneiss occurs as both brown and colorless varieties scanning electron microscope backscatter bse images of brown grains show multiple cross cutting oscillatory zones of variable brightness and dark overgrowths colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning electron microprobe analysis empa clearly distinguishes the two populations brown grains contain relatively high concentrations of fe2o3 ce2o3 up to 1 5 wt nb2o5 and zr cerium concentration is positively correlated with total ree y concentration which together can exceed 3 5 wt oscillatory zoning in brown titanite is correlated with variations in ree concentrations in contrast colorless titanite both as discrete grains and overgrowths on brown titanite contains lower concentrations of y ree fe2o3 and zr but somewhat higher al2o3 and nb2o5 uranium concentrations and th u discriminate between brown grains typically 200 400 ppm u all analyses but one have th u between about 0 8 and 2 and colorless grains 10 60 ppm u th u of 0 0 17 in contrast to the zircon u pb age results shrimp u pb data from titanite indicate multiple growth episodes in brown grains oscillatory zoned cores formed at 443 6 ma whereas white in bse cross cutting zones are 425 9 ma colorless grains and overgrowths on brown grains yield an age of 265 8 ma using the total pb method or 265 5 ma using the weighted average of the 206pb 238u ages however empa chemical data identify zoning that suggests that this colorless titanite may preserve three growth events oscillatory zoned portions of brown titanite grains are igneous in origin white cross cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite colorless titanite replaced and grew over the magmatic titanite during the permian alleghanian orogeny these isotopic data indicate that titanite like zircon can contain multiple age components coupling shrimp microanalysis with empa and sem results on dated zones as presented in this study is an efficient and effective technique to extract additional chronologic data to reveal the complexities of igneous crystallization and metamorphic growth

diagenetic regimes in arctic ocean sediments implications for sediment geochemistry and core correlation dark brown sediment layers are a potential stratigraphic tool in quaternary arctic ocean sediments they are rich in mn fe and trace metals scavenged from the water column and were most likely deposited during interglacial intervals in this study we combine sediment and pore water data from sediment cores taken in different parts of the arctic ocean to investigate the influence of early diagenetic processes on sediment geochemistry in most studied cores mn co and mo are released into the pore waters from mn oxide dissolution in deeper 1 5 m sediment layers the relationship between sedimentary mn co and mo contents in excess of the lithogenic background elementxs shows thatcoxs moxs values are a diagnostic tool to distinguish between layers with diagenetic metal addition from the pore waters coxs moxs 10 and unaffected layers coxs moxs from 1 to 10 steady state calculations based on current pore water profiles reveal that in the studied cores the diagenetic addition of these metals from the pore water pool alone is not sufficient to produce the sedimentary metal enrichments however it seems evident that dissolution of mn oxides in the mn reduction zone can permanently alter the primary geochemical signature of the dark brown layers therefore pore water data and coxs moxs values should be considered before core correlation when this correlation is solely based on mn contents and dark sediment color in contrast to the mostly nonlithogenic origin of mn in the dark brown layers sedimentary fe consists of a large lithogenic 80 and a small non lithogenic fraction 20 our pore water data show that diagenetic fe remobilization is not currently occurring in the sediment the dominant fe sources are coastal erosion and river input budget calculations show that fe seems to be trapped in the modern arctic ocean and accumulates in shelf and basin sediments the fe isotopic signal 56fe of the solid phase is positive 0 2 to 0 3 in samples defined as the lithogenic background without significant fe enrichments with increasing non lithogenic fe contents in the sediment 56fe becomes more negative which indicates a shelf to basin export of an isotopically lighter fe fraction we assume that the same transport process is true for mn

the mesoproterozoic beaverhead impact structure and its tectonic setting montana idaho 40ar 39ar and u pb isotopic constraints abstract new 40ar 39ar and uranium lead u pb zircon data from the beaverhead impact structure first identified by extensive shatter coning of proterozoic quartzite and gneiss from the beaverhead mountains near the montana idaho border indicate that the structure formed at or after 900 ma the 40ar 39ar age spectra from fine grained muscovite and biotite from a breccia zone in high grade gneiss show significant argon loss but yield dates for highest temperature steps that cluster between 899 and 908 ma the dated minerals probably formed by recrystallization of impact glass so on both geologic and isotopic grounds the dates probably represent the minimum age of impact u pb data for zircons from the same breccia are strongly discordant and yield an upper intercept apparent age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepac

step leach pb pb dating of inclusion bearing garnet and staurolite with implications for early proterozoic tectonism in the black hills collisional orogen south dakota united states we report 207 pb 206 pb stepwise leaching pbsl ages of 1762 15 ma 1759 8 ma and 1760 7 ma for almandine garnet staurolite and garnet staurolite combined in an early proterozoic metapelite from the black hills collisional orogen south dakota pbsl derived 208 pb 206 pb th u trends reveal that staurolite contains inclusions of both cogenetic monazite ca 1760 7 ma and older detrital zircon 2040 ma whereas coexisting garnet contains only the zircon these results and petrologic data indicate prograde sequential growth of garnet monazite and staurolite during tectonic burial at temperatures between 400 c and 550 c i e well below pb closure temperatures we interpret the 1760 7 ma date as a maximum age for wyoming superior continental collision in the black hills which apparently postdated the 1800 1900 ma trans hudson orogeny in canada but was nearly synchronous with north directed accretion of the central plains orogen in southeastern wyoming

iron isotopes in hot springs along the juan de fuca ridge we present fe isotope data for high temperature hydrothermal fluids sulfides and basalts from the juan de fuca ridge usgs basalt standard bcr 2 and the canyon diablo iron meteorite the fe isotopic compositions of all samples fall within a range of 1 however there is a small but statistically significant isotopic variation between fe dissolved in hydrothermal fluids and fe in canyon diablo specifically the hydrothermal fluids are lighter compared to canyon diablo yielding 56fe 56fe 54fe sample 56fe 54fe canyon diablo 1 1000 values from 0 30 to 0 77 in contrast the bcr 2 basalt and canyon diablo are isotopically indistinguishable measurements of 56fe in juan de fuca ridge basalts and sulfides suggest that fe isotopes are fractionated in hydrothermal systems however the data do not exclude the possibility of small 56fe differences between the rocks from which fluid fe is derived and other igneous fe in which case fractionation in igneous systems is indicated regardless these results are evidence of non biological mass fractionation of fe isotopes in nature further venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light fe to the deep oceans contributing to fe isotopic variations observed in deep sea sediments

rayleigh fractionation of iron isotopes during pedogenesis along a climate sequence of hawaiian basalt abstract we measured iron isotopic composition of surface 10 20 cm and subsurface 50 70 cm basaltic soil horizons from the island of maui along a climate gradient mcg ranging from 2 2 to 4 2 m mean annual precipitation map all soil forming factors except climate were conserved the mcg has a documented decrease in fe with increasing rainfall that is highly correlated with decreasing mean annual eh values we found that increasing map from 2 8 to 4 2 m resulted in a surface plus subsurface average increase of 0 56 0 09 56fe with the subsurface consistently 0 33 0 06 56fe greater than the surface horizons based on loss of fe relative to nb rayleigh fractionation was observed with 103ln lost retained values of 0 37 0 03 and 0 34 0 04 for the surface and subsurface respectively equivalent 103ln lost retained values for the surface and subsurface soils suggests fe loss is driven by similar mechanisms throughout the soil profile our calculated fractionation factor is about 1 3 the magnitude of laboratory determined fractionation factors for fe reduction suggesting other processes organic complexation fe re precipitation modulate the net fe loss along the mcg these results offer field scale confirmation of laboratory experiments on model systems that show anoxic weathering reactions produce materials enriched in heavy fe isotopes

iron isotope fractionation during oceanic crust alteration abstract the purpose of this work is to study the mobility and budget of fe isotopes in the oceanic crust and in particular during low temperature interaction of seawater with oceanic basalt we carried out this investigation using samples from ocean drilling program odp site 801c drilled during leg 129 and leg 185 in jurassic pacific oceanic crust seaward of the mariana trench the site comprises approximately 450 m of sediment overlying a section of 500 m of basalt which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low temperature 10 30 c ocherous si fe hydrothermal deposits fe was chemically separated from 70 selected samples and 57fe 54fe ratios were measured by mc icp ms isoprobe the isotopic ratios were measured relative to an internal standard solution and are reported relative to the international fe standard irmm 14 based on duplicate measurements of natural samples an external precision of 0 2 2 has been obtained the results indicate that the deep sea sediment section has a restricted range of 57fe which is close to the igneous rock value in contrast large variations are observed in the basaltic section with positive 57fe values up to 2 05 for highly altered basalts and negative values down to 2 49 for the associated alteration products and hydrothermal deposits secondary fe minerals such as fe oxyhydroxides or fe bearing clays celadonite and saponite have highly variable 57fe values that have been interpreted as resulting from the partial oxidation of fe2 leached during basalt alteration and precipitated as fe3 rich minerals in contrast altered basalts at site 801c which are depleted in fe up to 80 display an increase in 57fe values relative to fresh values which suggest a preferential leaching of light iron during alteration the apparent fractionation factor between dissolved fe2 and fe remaining in the mineral is from 0 5 to 1 3 and may be consistent with a kinetic isotope fractionation where light fe is stripped from the minerals alternatively the formation of secondary clays minerals such as celadonite during basalt alteration may incorporate preferentially the heavy fe isotopes resulting in the loss of light fe isotopes in the fluids because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions fe redox state and fe isotope fractionation we evaluated the possible effect of this deep biosphere on fe isotope signatures the fe isotope systematics presented in this study suggest that even though iron behavior during seafloor weathering may be mediated by microbes such as iron oxidizers 57fe variations of more than 4 may also be explained by abiotic processes further laboratory experiments are now required to distinguish between various processes of fe isotope fractionation during seafloor weathering

magnesium isotope fractionation by chemical diffusion in natural settings and in laboratory analogues abstract laboratory experiments are used to document isotopic fractionation of magnesium by chemical diffusion in a silicate melt and the results compared to the magnesium isotopic composition across contacts between igneous rocks of different composition in natural settings the natural samples are from transects from felsic to mafic rocks at vinal cove in the vinalhaven intrusive complex maine and from the aztec wash pluton in nevada two laboratory diffusion couples made by juxtaposing melts made from powders of the felsic and mafic compositions sampled at vinal cove were annealed at about 1500 c for 22 5 and 10 h respectively the transport of magnesium in the diffusion couples resulted in easily measured magnesium isotopic fractionations at the interface 26mg 1 5 these isotopic fractionations provide a distinctive isotopic fingerprint that we use to determine whether chemical gradients in natural settings where melts of different composition were juxtaposed were due to chemical diffusion the magnesium isotopic fractionation along one profile at vinal cove is exactly what one would expect based on the fractionations found in the laboratory experiments this is an important result in that it shows that the isotope fractionation by chemical diffusion found in highly controlled laboratory experiments can be found in a natural setting this correspondence implies that chemical diffusion was the dominant process responsible for the transport of magnesium across this particular contact at vinal cove a second vinal cove profile has a very similar gradient in magnesium concentration but with significantly less magnesium isotopic fractionation than expected this suggests that mass transport at this location was only partly by diffusion and that some other mass transport mechanism such as mechanical mixing must have also played a role the magnesium isotopic composition of samples from aztec wash shows no resolvable isotopic fractionation across the contact between the mafic and felsic rocks the different degrees of magnesium isotopic fractionation associated with otherwise similar composition gradients in natural settings show that kinetic isotope fractionations provide a key discriminator for establishing whether or not molecular diffusion was the process responsible for an observed elemental gradient in the one case of a contact at vinal cove where we are confident that the magnesium elemental and isotopic gradients were produced by diffusion we deduced a cooling rate of about 1 5 c per day

opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes iron is a scarce but essential micronutrient in the oceans that limits primary productivity in many regions of the surface ocean the mechanisms and rates of fe supply to the ocean interior are still poorly understood and quantified iron isotope ratios of different fe pools can potentially be used to trace sources and sinks of the global fe biogeochemical cycle if these boundary fluxes have distinct signatures seafloor hydrothermal vents emit metal rich fluids from mid ocean ridges into the deep ocean iron isotope ratios have the potential to be used to trace the input of hydrothermal dissolved iron to the oceans if the local controls on the fractionation of fe isotopes during plume dispersal in the deep ocean are understood in this study we assess the behaviour of fe isotopes in a southern ocean hydrothermal plume using a sampling program of total dissolvable fe tdfe and dissolved fe dfe we demonstrate that 56fe values of dfe 56dfe within the hydrothermal plume change dramatically during early plume dispersal ranging from 2 39 0 05 to 0 13 0 06 2 sd the isotopic composition of tdfe 56tdfe was consistently heavier than dfe values ranging from 0 31 0 03 to 0 78 0 05 consistent with fe oxyhydroxide precipitation as the plume samples age the dfe present in the hydrothermal plume includes stabilised dfe species with potential to be transported to the deep ocean we estimate that stable dfe exported from the plume will have a 56fe of 0 28 0 17 further we show that the proportion of authigenic iron sulfide and iron oxyhydroxide minerals precipitating in the buoyant plume exert opposing controls on the resultant isotope composition of dissolved fe passed into the neutrally buoyant plume we show that such controls yield variable dissolved fe isotope signatures under the authigenic conditions reported from modern vent sites elsewhere and so ought to be considered during iron isotope reconstructions of past hydrothermalism from ocean sediment records

tightening the siberian connection to western laurentia newly available geological and geophysical data tighten the proterozoic connection between the rifted margins of the northern siberian craton and western laurentia and permit a siberia laurentia australia troika with northern australia connected to the southern margin of the siberian craton the continental assembly is linked by a prominent 2 0 1 8 ga collisional belt and the ca 1 3 1 0 ga grenville orogen the reconstruction also aligns a 1 5 1 45 ga dike sill swarm that extends from the wyoming province through the belt purcell basin into the northern siberian craton separation of australia and siberia may have occurred by the early neoproterozoic but separation of siberia and laurentia may not have been completed until the early cambrian continental extension began along the zigzag siberia laurentia rift zone in the early mesoproterozoic rift related igneous and sedimentary assemblages dated at ca 1 5 1 38 and 1 2 to 1 0 ga correlate across the reconstructed siberia laurentia rift zone renewed rifting in the neoproterozoic and vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders correlative archeocyathan reefs endemic olenellid trilobite fauna and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the atdabanian stage black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the botomian sinsk event

homogeneous distribution of fe isotopes in the early solar nebula to examine the iron fe isotopic heterogeneities of ci and ordinary chondrites we have analyzed several large chips approximately 1 g from three ci chondrites and three ordinary chondrites ll5 l5 and h5 the fe isotope compositions of five different samples of orgueil one from ivuna and one from alais ci chondrites are highly homogeneous this new dataset provides a 56fe average of 0 02 0 04 2se n 7 which represents the best available value for the fe isotopic composition of ci chondrites and probably the best estimate of the bulk solar system we conclude that the homogeneity of ci chondrites reflects the initial fe isotopic homogeneity of the well mixed solar nebula in contrast larger up to 0 26 in 56fe isotopic variations have been found between separate approximately 1 g pieces of the same ordinary chondrite sample the fe isotope heterogeneities in ordinary chondrites appear to be controlled by the abundances of chondritic components specifically chondrules whose fe isotope compositions have been fractionated by evaporation and recondensation during multiple heating events

iron and carbon isotope evidence for microbial iron respiration throughout the archean abstract banded iron formations bifs are voluminous chemical sediments that are rich in iron oxide carbonate and silica and whose occurrence is unique to the precambrian their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early earth however many details regarding the role of microbial activity in bif deposition and diagenesis are unresolved laboratory studies have shown that reaction between carbon and iron through microbial iron respiration 2fe2o3 nh2o ch2o 7h 4fe2 hco3 2n 4 h2o chemical energy can impart fractionation to the isotopic compositions of these elements here we report iron 56fe vs irmm 014 and carbon isotopic 13c vs v pdb compositions of magnetite and of iron rich and iron poor carbonates in bifs from the late archean 2 5 ga hamersley basin australia and the early archean 3 8 ga isua supracrustal belt isb greenland the range of 56fe values measured in the hamersley basin including light values in magnetite and heavy values in iron rich carbonates up to 1 2 are incompatible with their precipitation in equilibrium with seawater rather the data together with previously reported light 13c values in iron rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration i e dissimilatory iron reduction dir iron and carbon isotope data of iron rich metacarbonates from the isb are similar to those of late archean bifs the isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on earth

serpentinization of olivine at 300 c and 500 bars an experimental study examining the role of silica on the reaction path and oxidation state of iron abstract the generation of h 2 during serpentinization of ultramafic oceanic crust along earth s mid ocean ridges provides a key energy source for the deep biosphere and has important implications for metal transport and mineral stability in the subseafloor hydrothermal system nonetheless the driving forces and rates of the coupled processes of fe partitioning and h 2 generation during serpentinization are complicated by many factors but especially the activity of sio 2 aq in the serpentinizing fluid to examine the role of this key variable in h 2 generation and fe partitioning between magnetite and fe serpentine during serpentinization san carlos olivine was reacted with a nacl sio 2 aq bearing fluid at 300 c and 500 bars for approximately 90 days using flexible gold cell hydrothermal equipment time series changes in solution chemistry and dissolved h 2 are coupled with magnetic susceptibility mossbauer spectroscopy and fe isotope measurements of reactant olivine and coexisting alteration products to provide a complete description of fe mass transfer and oxidation as a function of reaction progress utilization of talc as reactant not only provided a source of sio 2 aq for serpentinization but effectively prevented brucite formation in keeping with constraints imposed by many natural hydrothermal systems which are inherently open systems moreover this experimental strategy allowed fe derived from olivine dissolution to transfer completely to serpentine and magnetite facilitating reaction path modeling initial changes in solution chemistry indicate buffering of dissolved sio 2 aq owing to the apparently balanced rates of olivine dissolution and serpentine precipitation during this stage of reaction a negligible amount of h 2 is generated suggesting that fe 2 serpentine is stable and the dominant fe bearing product with continued reaction the rate of sio 2 aq production lessens likely due to reduction in surface area of talc such that the continued hydration and hydrolysis of olivine titrates the previously buffered activity of sio 2 aq eventually approaching serpentine brucite coexistence during this secondary stage of reaction the rate of h 2 generation abruptly increases which reflects the formation of magnetite and fe 3 serpentine temporal changes in the isotope composition of dissolved fe throughout these stages likely reflect different mineral alteration and formation processes where initially 1 preferential mass transfer of isotopically light fe into solution occurred during olivine alteration without considerable oxidation as demonstrated by the low h 2 production rate and 2 the preferential removal of isotopically light fe from solution in connection with enhanced magnetite and h 2 formation during the later stages of reaction post experimental analysis of the fe isotopic composition of fe bearing minerals demonstrates statistically negligible enrichment of 56 fe of magnetite relative to olivine and serpentine consistent with observations from natural altered oceanic crust observations from this study demonstrate the utility of using coupled chemical isotopic and analytical approaches to ascertain time series changes in rates of mass transfer and mechanisms of oxidation of fe during serpentinization of olivine ultimately these insights provide important constraints for the chemical and fe isotopic evolution of fluids and minerals in ultramafic hosted hydrothermal systems at mid ocean ridges

mineral composition control on inter mineral iron isotopic fractionation in granitoids abstract this study reports elemental and iron isotopic compositions of feldspar and its coexisting minerals from four dabie i type granitoids to evaluate the factors that control inter mineral fe isotopic fractionation in granitoids the order of heavy iron isotope enrichment is feldspar pyrite magnetite biotite hornblende feldspar has heavier iron isotopic compositions than its co existing magnetite 56 fe plagioclase magnetite 0 376 to 1 084 56 fe alkali feldspar magnetite 0 516 to 0 846 which can be attributed to its high fe 3 fe tot ratio and low coordination number tetrahedrally coordinated of fe 3 56 fe magnetite biotite of coexisting magnetite and biotite ranges from 0 090 to 0 246 based on homogeneous major and iron isotopic compositions of mineral replicates the inter mineral fractionation in this study should reflect equilibrium fractionation the large variations of inter mineral fractionation among feldspar magnetite and biotite cannot be simply explained by temperature variation but strongly depend on mineral compositions the 56 fe plagioclase magnetite and 56 fe alkali feldspar magnetite are positively correlated with albite mode in plagioclase and orthoclase mode in alkali feldspar respectively this could be explained by different fe o bond strength in feldspar due to different fe 3 fe or different crystal parameters the 56 fe magnetite biotite increases with decreasing fe 3 fe biotite and increasing mole na k mg biotite indicating a decrease of factor in low fe 3 fe and high na k mg biotite high silica leucosomes from dabie migmatites with a feldspar accumulation petrogenesis have higher 56 fe values 56 fe 0 42 0 567 than leucosome that represents pristine partial melt 56 fe 0 117 0 016 indicating that accumulation of feldspar could account for high 56 fe values of these rocks high 56 fe values are also predicted for other igneous rocks that are mainly composed of cumulate feldspar crystals e g anorthosites feldspar accumulation however cannot explain high 56 fe values of most high silica granitoids reported in the literature based on their low sr ba contents and negative eu anomalies

zn fe and s isotope fractionation in a large hydrothermal system abstract the genesis of hydrothermal ore deposits is of crucial economic importance this study investigates the extent causes and consequences of zinc and iron isotope fractionation in a large hydrothermal system at the world class navan zn pb orebody ireland large variations in zn fe and s isotope compositions have been measured in microdrilled sphalerite zns at the millimetre scale 66 zn and 56 fe display a well defined positive correlation and both also correlate with 34 s these relationships represent the combined effects of kinetic zn and fe isotope fractionation during sphalerite precipitation and s isotope variation through mixing of hot metal rich hydrothermal fluids and cool bacteriogenic sulfide bearing brines combined with s isotope data 56 fe and 66 zn data on mine concentrates confirm that hydrothermal sulfide is a minor component of the overall deposit signature our data suggest that incoming pulses of metal rich hydrothermal fluid triggered sulfide mineralisation and that rapid precipitation of sphalerite from hydrothermal fluids will lead to strong kinetic fractionation of zn and fe isotopes at very short time and length scales thereby limiting the use of fe and zn isotopes as exploration tools within deposits but revealing the possibility of detecting new deposits from isotopically heavy zn fe geochemical halos

equilibrium high temperature fe isotope fractionation between fayalite and magnetite an experimental calibration abstract the iron isotopic fractionation factor between magnetite and fayalite was measured at high temperature in a piston cylinder apparatus for the first time the three isotope method matsuhisa j goldsmith j r and clayton r n 1978 mechanisms of hydrothermal crystallisation of quartz at 250 c and 15 kbar geochim cosmochim acta 42 173 182 has been used to determine equilibrium fractionation between two minerals directly the experimentally determined temperature dependent iron isotope fractionation between fayalite and magnetite is described by 10 3 ln 57 mag fa 57 fe mag fa 0 30 0 024 10 6 t 2 the equation predicts measurable fractionation at magmatic temperatures our work bears on the magnitude of fe isotope fractionation attending differentiation in magmatic systems and provides a new isotope thermometer for co existing fayalite and magnetite

iron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the luobusa ophiolite tibet we present high precision measurements of iron fe and magnesium mg isotopic compositions of olivine orthopyroxene and chromite separates from harzburgites dunites and chromitites in the mantle section of the luobusa ophiolite southern tibet to investigate the origins of podiform chromitite two harzburgites in the zedong ophiolite southern tibet are also reported for comparison the olivine and orthopyroxene in the luobusa and zedong harzburgites have similar fe and mg isotopic compositions with fe 56 values ranging from 0 to 0 083 in olivine from 0 034 to 0 081 in orthopyroxene and mg 26 values ranging from 0 25 parts per thousand to 0 20 parts per thousand in olivine from 0 29 parts per thousand to 0 26 parts per thousand in orthopyroxene respectively the olivines of two dunites from the luobusa display small fe and mg isotopic variations with fe 56 values of 0 014 parts per thousand and 0 116 parts per thousand and mg 26 values of 0 21 parts per thousand and 0 29 parts per thousand all chromites in the luobusa chromitites have lighter fe isotopic compositions than the coexisting olivines with fe 56 values ranging from 0 247 parts per thousand to 0 043 parts per thousand in chromite and from 0 146 parts per thousand to 0 215 parts per thousand in olivine fe 56 chr ol 0 294 to 0 101 parts per thousand the chromite mg 26 values span a significant range from 0 41 parts per thousand to 0 14 parts per thousand large disequilibrium fe and mg isotope fractionation between chromite and olivine as well as positive correlation of chromite fe 56 values with their mgo contents could be attributed to fe mg exchange between chromite and olivine in the disseminated chromitites the higher modal abundances of olivine than chromite would result in a more extensive fe mg exchange whereas chromite in the massive chromitite where olivine is rare could not be affected by this process

magnesium isotopic composition of igneous rock standards measured by mc icp ms to provide inter lab comparison for high precision mg isotope analysis mg isotope compositions expressed as 26mg relative to dsm 3 for commercially accessible peridotite basalt andesite and granite geo standards have been measured by multi collector inductively coupled mass spectrometry nu plasma using sample standard bracketing method there is a large tolerance of matrix cations during the measurement of mg isotopes as intensity ratios of 23na 24mg and 27al 24mg of about 20 only change the 26mg by less than 0 1 and low 55mn 24mg 0 1 and 58ni 24mg 0 01 do not cause significant mass bias either concentration match between samples and standards within 90 is adequate to obtain accurate isotope analysis which also mitigates the isobaric interference of 12c14n on 26mg organic matrix from chemical purification can cause significant analytical errors when the mass of mg processed is small the long term reproducibility of 26mgdsm 3 for samples with relatively higher mgo content is about 0 11 2sd and granites with lower mgo content is about 0 2 2sd although the standards in this study have wide ranges of major element compositions with sio2 from 40 to 70 wt and mgo from 0 75 to 49 6 wt they exhibit a variation of mg isotopic compositions with 26mg from 0 07 to 0 40 26mg do not correlate with sio2 or mgo contents suggesting homogenous mg isotope compositions in igneous rocks at the level of current precision relative to low temperature samples including sediments and riverine and sea waters our data do not support a non chondritic mg isotope composition of the earth

paleo redox boundaries in fractured granite at the earth s surface fe ii often oxidises and forms insoluble fe iii oxyhydr oxides whose particle size and structure depend on solution composition and temperature during formation and afterwards bacterial processes and exposure to reducing environments reduces them again releasing dissolved iron to the groundwater during such cycling the fe isotopes fractionate to an extent that is expected to depend on temperature in this study we report on the use of fe oxides as paleo redox indicators using their structure morphology and fe composition as a clue for formation conditions in samples taken from 120 m drill cores in granite from se sweden x ray amorphous superparamagnetic nanometre sized fe oxides are confined to fractures of the upper 50 m whereas well crystalline fe oxides with particle sizes typical for soils occur down to 110 m we also identified hematite with a particle size of 100 nm similar to hematite of hydrothermal origin the fe isotope composition of the fine grained fe oxides 1 56fe 1 irmm 14 referenced scatter significantly compared to the distribution previously observed for hydrothermal material 0 26 56fe 0 12 and they are dominantly heavier than fe bearing silicates from fractures 0 56 56fe 0 35 this is consistent with formation by low temperature weathering where fe silicates dissolve fe ii oxidises and fe iii oxides precipitate the x ray amorphous nanometre sized nature of near surface fe oxides suggests recent formation the deeper situated well crystalline fe oxides are more mature and we interpret that they record earlier oxidising events they exist in fractures that are not significantly altered indicating formation during periods of oxidation our results show that oxygenated water may reach depths of 110 m in fractured granite the absence of natural low temperature fe oxides from deeper drill cores suggests that oxygenated waters do not readily penetrate beyond about 100 m and suggests that radioactive waste repositories located at a depth of 500 m should be well protected from oxygenated waters

iron isotope fractionation during crustal anatexis constraints from migmatites from the dabie orogen central china abstract iron fe isotope fractionation could occur during crustal melting which may contribute to the isotopic variations of granites our current knowledge on fe isotope fractionation during crustal melting remains rare thirteen leucosomes 9 melanosomes and 1 amphibolitic schollen in 11 migmatites from the dabie orogen central china were measured to investigate fe isotope fractionation during crustal anatexis the melanosomes and amphibolitic schollen yield 56fe values from 0 018 0 031 to 0 152 0 027 with an average of 0 106 the leucosomes have 56fe values from 0 107 0 035 to 0 512 0 028 variably higher than their coexisting melanosomes by 0 00 0 36 the 56fe of leucosomes and melanosomes as well as apparent isotope fractionation 56fel m between them do not correlate with the abundance of crystalline carbonate loss on ignition and th u indicating the insignificant effect of fluid components high 56fe values of three leucosomes 0 34 giving large 56fel m from 0 22 to 0 36 were likely produced by feldspar accumulation evidenced by their high eu low feot al2o3 and high plagioclase abundance up to 50 vol capture of peritectic amphiboles in another leucosome may explain its low 56fel m 0 after screening out these samples 7 leucosome melanosome pairs yield identical 56fel m averaging 0 093 0 056 2sd n 7 within analytical uncertainties no correlation between 56fel m and mg feot al2o3 or tio2 feot rules out the possibility that the fractionation observed here is produced by fractional crystallization and or sub solidus isotope re equilibrium between leucosomes and melanosomes therefore we suggest these consistent 56fel m should record equilibrium fe isotope fractionation during crustal anatexis producing these migmatites the leucosomes and melanosomes have comparable fe3 fe 0 40 and 56fel m approximates 0 07 at fe 3 fe leucosome fe 3 fe menalosome 1 fe isotope fractionation during crustal anatexis revealed here thus is not dominantly controlled by distribution of fe3 and fe2 but by the difference in coordination number of iron between granitic melts and residual mafic minerals the observation here argues that fe isotope fractionation during crustal partial melting should also contribute to the previously revealed 56fe variation in high silica granitic rocks

experimental quantification of the fractionation of fe isotopes during metal segregation from a silicate melt abstract fractionation of fe isotopes between a silicate melt and metallic alloys has been quantified experimentally at 1500 c the effects of oxygen fugacity and run duration have been investigated to distinguish between kinetic and equilibrium fractionations a new experimental setup is presented in which metallic fe is produced by reduction of oxidized iron from a silicate melt due to a change of redox conditions this metallic fe is physically removed from the silicate and sequestered as a pt fe alloy bulk analyses of the silicate and metallic fractions using multi collector icp ms methods are coupled with in situ analyses using an ion microprobe experimental results indicate that significant isotopic fractionation of fe occurs during metal segregation during the early stages of the experiments we find evidence for kinetic fractionation caused by faster diffusion of 54 fe along concentration gradients in the alloy this process leads to the formation of a metallic phase which is isotopically light compared to the residual silicate metal silicate fractionation up to 2 35 a m u the data are used to quantify the difference in diffusion coefficient of each isotope of iron providing a means to predict fe isotope fractionation during fe diffusion in metallic alloys on the other hand this state of affairs is transient in nature and at superliquidus temperature the systems rapidly reach a state of isotopic equilibrium in which the metal is isotopically heavier than the silicate melt by about 0 2 0 15 a m u these results are consistent with the range of variation observed in natural samples these kinetic and equilibrium data provide an experimental framework to understand the observed variability among igneous and meteoritic materials formed at high temperature particularly for iron and pallasite meteorites

sources and ore forming fluid pathways of carbonate hosted pb zn deposits in southwest china implications of pb zn s cd isotopic compositions the sichuan yunnan guizhou metallogenic province of southwest china contains more than 200 mt of pb zn ore in carbonate hosted pb zn deposits representing 27 of the total pb and zn resources of china sources of metals and pathways of ore forming fluids were elucidated through a study of the pb zn s cd isotopic compositions of sphalerite from the tianbaoshan fusheng maozu jinshachang and daliangzi carbonate hosted pb zn deposits 34s values indicate that s in the deposits is derived mainly from evaporites in cambrian to triassic sedimentary strata sulfates coexisting with sulfides and meso neoproterozoic folded basement 66zn values and pb isotopic ratios indicate that these metals originate mainly from sinian to lower permian sedimentary rocks and proterozoic basement there is a trend towards isotopically heavier cd and zn compositions of sphalerite from maozu to daliangzi jinshachang and tianbaoshan which are all hosted in the upper sinian dengying formation previous studies showed that there would be a trend of enrichment in heavier zn and cd isotopes following the migration of fluids and precipitation of minerals the above observations suggest that the ore forming fluids of these deposits are probably derived from the same hydrothermal fluid system the fluids most likely flowed through maozu first migrating along the xiaojiang and anninghe fault belts and their branch faults to daliangzi jinshachang and tianbaoshan respectively zn and cd isotopes could be useful tools in tracing the pathways of ore forming fluids in this district and heavier zn and cd isotopic compositions could provide a geochemical fingerprint for detecting remote orebodies in large hydrothermal fluid systems

block and shear zone architecture of the minnesota river valley subprovince implications for late archean accretionary tectonics the minnesota river valley subprovince of the superior province is an archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east northeast trending linear geophysical anomalies two of the block bounding zones are verified regional scale shears the geological nature of the third boundary has not been established potential field geophysical models portray the boundary zones as moderately north dipping surfaces or thin slabs similar in strike and dip to the morris fault segment of the great lakes tectonic zone at the north margin of the subprovince the central two blocks of the subprovince morton and montevideo are predominantly high grade quartzofeldspathic gneiss some as old as 3 6 ga and late tectonic granite the northern and southern blocks benson and jeffers respectively are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late tectonic plutons a belt of moderately metamorphosed mafic and ultramafic rocks h

iron isotopes in an archean ocean analogue abstract iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the proterozoic and phanerozoic this increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time we tested these contrasting scenarios through a detailed study of anoxic and ferruginous lac pavin france which can serve as a modern analogue of the archean ocean a depth profile in the water column of lac pavin shows a remarkable increase in dissolved fe concentration 0 1 1200 m and 56fe values 2 14 to 0 31 across the oxic anoxic boundary to the lake bottom the largest fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron leaving the residual fe enriched in light isotopes the analysis of four sediment cores collected along a lateral profile one in the oxic layer one at the redox boundary one in the anoxic zone and one at the bottom of the lake indicates that bulk sediments porewaters and reactive fe mostly have 56fe values near 0 0 0 2 similar to detrital iron in contrast pyrite 56fe values in sub chemocline cores 60 65 and 92 m are highly variable and show significant deviations from the detrital iron isotope composition 56fepyrite between 1 51 and 0 09 average 0 93 importantly the pyrite 56fe values mirror the 56fe of dissolved iron at the redox boundary where near quantitative sulfate and sulfide drawdown occurs suggesting limited iron isotope fractionation during iron sulfide formation this finding has important implications for the archean environment specifically this work suggests that in a ferruginous system most of the fe isotope variability observed in sedimentary pyrites can be tied to water column cycling foremost to the oxidation of dissolved ferrous iron this supports previous suggestions that enhanced iron isotope variability in the archean may record a unique stage in earth s history where partial ferrous iron oxidation in upwelling water masses was a common process probably linked to oxygenic or anoxygenic photosynthesis

routine isotopic analysis of iron by hr mc icpms how precise and how accurate abstract high temperature isotopic variations documented in natural materials span a narrow range and call for precise and accurate isotopic analyses precisions better than 0 03 95 confidence interval for 56 fe have been obtained by high resolution multi collector inductively coupled plasma mass spectrometry hr mc icpms whether these uncertainties encompass all sources of error has not been carefully evaluated the current study examines all parameters that can affect accuracy and precision of fe isotopic measurements by hr mc icpms it is demonstrated that accurate 56 fe measurements can be routinely achieved within the quoted uncertainties

origin of the archean sask craton and its extent within the trans hudson orogen evidence from pb and nd isotopic compositions of basement rocks and post orogenic intrusions u pb zircon ages from the exposed sask craton are 2450 3100 ma from the peter lake domain 2575 2640 ma and from rocks of the trans hudson orogen 1840 1880 ma u pb monazite and zircon ages of post orogenic pegmatites and aplites are 1770 1800 ma common pb and sm nd isotopic compositions of post orogenic intrusions as probes of crust beneath the orogen were compared to sask craton rocks and ca 1850 ma orogenic rocks to infer the origin and subsurface distribution of the sask craton within the internides of the trans hudson orogen results show that post orogenic intrusions within most of the glennie domain and hanson lake block were derived at least in part from archean source materials demonstrating that the sask craton lies beneath paleoproterozoic orogenic rocks present at the surface in contrast common pb and sm nd isotopic compositions from pegmatites and aplites of the la ronge domain are essentially identical with those of the paleoproterozoic orogenic rocks into which they are intruded indicating derivation by partial melting of similar rocks thus if the sask craton extended to the west beneath the la ronge domain it was beneath the zone of melting that produced the post orogenic intrusions making it unlikely that the sask craton is a detached part of the hearne craton many samples from the sask craton have elevated pb 208 pb 204 ratios unlike superior craton or hearne craton rocks suggesting that the sask craton was derived from an exotic source such as the wyoming craton which shares similar elevated pb 208 pb 204 ratios

copper and iron isotope fractionation during weathering and pedogenesis insights from saprolite profiles abstract iron and copper isotopes are useful tools to track redox transformation and biogeochemical cycling in natural environment to study the relationships of stable fe and cu isotopic variations with redox regime and biological processes during weathering and pedogenesis we carried out fe and cu isotope analyses for two sets of basalt weathering profiles south carolina usa and hainan island china which formed under different climatic conditions subtropical vs tropical unaltered parent rocks from both profiles have uniform 56 fe and 65 cu values close to the average of global basalts in the south carolina profile 56 fe values of saprolites vary from 0 01 to 0 92 in the lower reduced part and positively correlate with fe 3 fe r 2 0 90 whereas 65 cu values are almost constant by contrast 56 fe values are less variable and negatively correlate with fe 3 fe r 2 0 88 in the upper oxidized part where large 4 85 65 cu variation is observed with most samples enriched in heavy isotopes in the hainan profile formed by extreme weathering under oxidized condition 56 fe values vary little 0 05 0 14 whereas 65 cu values successively decrease from 0 32 to 0 12 with depth below 3 m and increase from 0 17 to 0 02 with depth above 3 m throughout the whole profile 65 cu positively correlate with cu concentration and negatively correlate with the content of total organic carbon toc overall the contrasting fe isotopic patterns under different redox conditions suggest redox states play the key controls on fe mobility and isotope fractionation the negative correlation between 56 fe and fe 3 fe in the oxidized part of the south carolina profile may reflect addition of isotopically light fe this is demonstrated by leaching experiments which show that fe mineral pools extracted by 0 5 n hcl representing poorly crystalline fe hydr oxides are enriched in light fe isotopes the systematic cu isotopic variation in the hainan profile reflects desorption and downward transport of isotopically heavy cu leaving the organically bound cu enriched in light isotope as supported by the negative correlation of 65 cu with toc r 2 0 88 the contrasting mostly positive vs negative cu isotopic signatures in the upper parts of these two profiles can be attributed to the different climatic conditions e g high rainfall at a tropical climate in hainan favors desorption and the development of organism whereas relatively dry climate in south carolina favors cu re precipitation from soil solutions and adsorption onto fe hydr oxides our results highlight the potential applications of fe and cu isotopes as great tracers of redox condition ancient climate and biological cycling during chemical weathering and pedogenic translocation

copper isotope fractionation during sulfide magma differentiation in the tulaergen magmatic ni cu deposit nw china abstract although it has been recently demonstrated that cu isotope fractionation during mantle melting and basaltic magma differentiation is limited the behavior of cu isotopes during magmatic differentiation involving significant sulfide segregation remains unclear magmatic ni cu deposits which formed via sulfide segregation from basaltic or picritic magmas are appropriate targets to address this issue here we report cu isotope data for sulfides chalcopyrite from the tulaergen ni cu sulfide deposit in xinjiang nw china sulfides including sparsely disseminated hosted by hornblende gabbro moderately disseminated hosted by hornblende olivine websterite densely disseminated hosted by hornblende lherzolite and massive sulfides sandwiched between country rocks and mafic ultramafic rocks were collected from adits at 1050 m 1100 m and 1150 m levels the sparsely and moderately disseminated sulfides on 1150 m and 1050 m levels have a restricted range of 65 cu values from 0 38 to 0 15 whereas disseminated and massive sulfides on 1100 m level have 65 cu values ranging widely from 1 98 to 0 04 and from 1 08 to 0 52 respectively the 65 cu values of disseminated sulfides are negatively correlated with whole rock s and cu concentrations and sulfides formed at later stages have heavier 65 cu values these observations suggest significant cu isotope fractionation during sulfide magma differentiation above 600 c during the formation of the tulaergen magmatic ni cu deposit sulfide segregation and crystallization of olivine and pyroxene caused the increase of fe 3 contents in the residual magmas which would move the redox reaction cu fe 3 fe 2 cu 2 toward larger amounts of cu 2 in the melt the presence of cu 2 in melt allowed redox transformation to happen during sulfide segregation the residual magmas are enriched in heavy cu isotopes due to the removal of 65 cu depleted sulfides and sulfides formed at later stages will have heavier 65 cu values accounting for the large and systematic cu isotopic variation as observed our results therefore demonstrate that cu isotopes could be significantly fractionated during high temperature sulfide magma differentiation up to 2 and cu isotopes may be used to trace the mineralization process of ni cu pge deposits and indicate the movement direction of sulfide enriched magmas

iron isotope fractionation during skarn type alteration implications for metal source in the han xing iron skarn deposit abstract the han xing iron mineralization in the central north china craton is a typical fe skarn deposit associated with altered diorites here we report the fe isotopic compositions of whole rocks and mineral separates from this deposit with a view to evaluate the fe isotope fractionation during the formation of fe skarn deposit and to constrain the metal source the fe isotopes show a large variation both in whole rocks and mineral separates altered diorites show a wide range in 56 fe values 0 07 to 0 21 relative to the fe isotope standard irmm 014 which positively correlate with their tfe 2 o 3 tio 2 ratios fe 2 o 3 and feo calculated as tfe 2 o 3 the positive correlation indicates that heavy fe isotopes were preferentially leached from diorites during the skarn type alteration among the metallic minerals pyrite and pyrrhotite are isotopically heavier 0 12 to 0 48 than the magnetite 0 07 to 0 21 fe isotope fractionation between mineral pairs demonstrates that magnetite did not attain fe isotopic equilibrium with pyrite and pyrrhotite whereas pyrite and pyrrhotite might have attained isotopic equilibrium petrological observations and major element data also suggest that iron was leached from the diorites during the skarn type alteration if the leached iron provides the main fe budget of the han xing fe skarn deposit magnetite in ores would be isotopically heavier than the unaltered diorite however our results are in contrast with the magnetite being isotopically lighter than the unaltered diorite this suggests that the major fe source of the han xing fe skarn deposit is not from the leaching of diorites and might be from magmatic fluid which is isotopically lighter than the silicate melt our data demonstrate that fe isotopes can be used as important tracers in deciphering the metal source of fe skarn deposits

discerning crystal growth from diffusion profiles in zoned olivine by in situ mg fe isotopic analyses mineral zoning is used in diffusion based geospeedometry to determine magmatic timescales progress in this field has been hampered by the challenge to discern mineral zoning produced by diffusion from concentration gradients inherited from crystal growth a zoned olivine phenocryst from kilauea iki lava lake hawaii was selected for this study to evaluate the potential of mg and fe isotopes for distinguishing these two processes microdrilling of the phenocryst 300 m drill holes followed by mc icpms analysis of the powders revealed negatively coupled mg and fe isotopic fractionations 26mg from 0 1 to 0 2 and 56fe from 1 2 to 0 2 from core to rim which can only be explained by mg fe exchange between melt and olivine the data can be explained with ratios of diffusivities of mg and fe isotopes in olivine scaling as d2 d1 m1 m2 with mg 0 16 and fe 0 27 la mc icpms and mc sims fe isotopic measurements are developed and are demonstrated to yield accurate 56fe measurements within precisions of 0 2 1 sd at spatial resolutions of 50 m 56fe and 26mg stay constant with fo in the rim late stage overgrowth whereas in the core original phenocryst 56fe steeply trends toward lighter compositions and 26mg trends toward heavier compositions with higher fo a plot of 56fe vs fo immediately distinguishes growth controlled from diffusion controlled zoning in these two regions the results are consistent with the idea that large isotopic fractionation accompanies chemical diffusion in crystals whereas fractional crystallization induces little or no isotopic fractionation the cooling timescale inferred from the chemical isotope zoning profiles is consistent with the documented cooling history of the lava lake in the absence of geologic context in situ stable isotopic measurements may now be used to interpret the nature of mineral zoning stable isotope measurements by la mc icpms and mc sims can be used as standard petrologic tools to identify samples for diffusion based geospeedometry

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

paleomagnetism of 2 44 ga mafic dykes in russian karelia eastern fennoscandian shield implications for continental reconstructions abstract results of a paleomagnetic study of paleoproterozoic dykes and archean host rocks in the eastern fennoscandian shield are reported here most of the dykes are related to ca 2440 ma layered intrusions and have isotopic ages between 2349 and 2476 ma the dykes form an extensive nw se and e w trending swarm the majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature the high temperature remanence component d which is thought to represent the primary 2 44 ga magnetization of the dykes has a paleomagnetic pole at 20 s 278 8 e a95 6 0 n 11 sites another d like component d with shallower inclination vgp 9 6 n 256 2 e eight samples at one site from baked archean host rock may also represent the primary remanence of the dykes the commonest remanence component a records an extensive svecofennian partial overprint and is interpreted to be ca 1 84 ga old it yields a paleomagnetic pole at 52 6 n 226 8 e a95 2 7 n 23 the fourth component b has a paleopole at 53 9 n 169 6 e a95 6 3 n 16 and is inferred to be ca 1 75 ga on the basis of isotopic dating although ages of 2 1 1 3 and 0 3 ga are also possible baked contact tests for the 2 44 ga dykes are negative as shown by the occurrence of component d in archean host rocks beyond one dyke width it is proposed that the negative baked contact test indicates regional reheating and remagnetization of the archean basement at ca 2 44 ga ago due to the spatially vast occurrence of coeval 2 44 ga mafic dykes layered intrusions volcanics and granites in the area based on this hypothesis it is suggested that component d in the dykes may be primary and acquired at ca 2 44 ga ago the similarity of pole d with poles from coeval dykes and layered intrusions elsewhere in the karelian province and kola peninsula suggest that within the archean domain of the fennoscandian shield no large scale block movements have taken place since 2 44 ga at 2 44 ga the fennoscandian shield was located at the latitude of ca 30 s which is different to that at 1 84 and 1 75 ga when fennoscandia was located at the latitudes of ca 30 n a continental reconstruction based on pole d and the pole from the 2 45 ga matachewan dykes of the superior craton in laurentia places the kola peninsula adjacent to southern greenland reconstructions based on recent geological interpretations and a reconstruction yielding pole d are also discussed

comparing induced polarization responses from airborne inductive and galvanic ground systems tasmania abstractwe have first analyzed the ability of polarizable and superparamagnetic thin sheets in the near surface to fit airborne electromagnetic aem data using data from western tasmania then we analyzed the results of such fitting in the context of geologic mapping and available ground induced polarization ip data small to large ip effects were found to considerably improve the fit to the observed aem data and the overall fitted ip parameters were spatially consistent however the locations of anomalous ip parameters were quite distinct from anomalies in other geophysical data the airborne chargeability highs were adjacent to or surrounded the ground chargeability highs in the five cases analyzed from tasmanian data modeling using the established cole cole physical property values for sulfides predicts that an inductive airborne system is insensitive to many conventional ip targets unless the mineral grain size is substantially less than 1 mm in the cases in which airborne ip responses were ad

iron isotopic compositions of adakitic and non adakitic granitic magmas magma compositional control and subtle residual garnet effect abstract here we present iron fe isotopic compositions of 51 well characterized adakitic and non adakitic igneous rocks from the dabie orogen central china and panama costa rica central america twelve i type non adakitic granitoid samples from the dabie orogen yield 56fe ranging from 0 015 to 0 184 the good correlations between 56fe and indices of magma differentiation e g sio2 feot mg and fe3 fe suggest fe2 rich silicate and oxide minerals dominated fractional crystallization with 56femelt crystal 0 06 may account for the 56fe variation in these samples one a type granite sample from the dabie orogen has 56fe as high as 0 447 likely indicating less magnetite crystallization and an increase in 103ln melt with magma na k ca mg combined with the literature data most high silica sio2 71 wt granitic rocks define a good positive linear correlation between 56fe and na k ca mg 56fe 0 0062 na k ca mg 0 130 r2 0 66 given that fractional crystallization also tends to increase 56fe with na k ca mg this correlation can serve as the maximum estimate of the magma compositional control on fe isotope fractionation low mg adakitic samples lma have 56fe ranging from 0 114 to 0 253 the melt compositional control on lma 56fe could be insignificant due to their limited na k ca mg variation except for one sample that may be affected by late differentiation 14 out of 15 lma have 56fe increasing with dy yb n reflecting a subtle but significant effect of residual garnet proportion this serves as evidence for that source mineralogy may play an important role in fractionating fe isotopes during partial melting dabie and central america high mg adakitic samples have homogeneous fe isotopic compositions with mean 56fe of 0 098 0 038 2sd n 11 and 0 085 0 045 2sd n 11 respectively these samples have undergone melt mantle interaction which may neutralize the possible 56fe variation of their pristine magmas derived by partial melting of eclogitic rocks

40ar39ar dating of glauconites measured 39ar recoil loss from well crystallized specimens abstract nine glauconite samples with relatively high k concentrations and which appear to be well crystallized using normal x ray diffraction techniques have been studied using the 40 ar 39 ar method the glauconite 40 ar 39 ar apparent ages exceed their k ar rb sr and in most cases stratigraphic ages by substantial amounts 40 ar 39 ar release spectra sometimes yield plateaus but these apparent ages have no geological significance the results indicate that 39ar is lost by recoil from mineral grains during neutron irradiation consistent with previously reported observations the amount of 39ar loss was measured by isotope dilution for four samples and varied from 29 to 17 in contrast radiogenic 40ar is quantitatively retained during irradiation the very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of 39ar loss and unsuitable for 40 ar 39 ar dating

iron stable isotopes beyond biosignatures abstract the stable isotope geochemistry of fe has attracted intense interest in the past five years this interest was originally motivated by the possible use of fe isotopes in biosignature applications particularly in sediments from the ancient earth or mars this application is still being developed with particular attention to fractionation mechanisms understanding such mechanisms should also provide new insights into the environmental biogeochemistry of fe at the same time the fe isotope system holds promise for other exciting frontiers including applications in oceanography solid earth geochemistry and biomedicine such applications will be increasingly attractive as fe isotope analysis becomes routine

potassium argon dating of fine grained basalts with massive ar loss application of the 40ar39ar technique to plagioclase and glass from the kirkpatrick basalt antarctica 40ar39ar incremental heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the kirkpatrick basalt from victoria land antarctica these results are used to address the limitations of whole rock analyses of glassy or very fine grained basalts the use of plagioclase for determining ages of such rocks the ar behavior of matrix glass and feldspar components the use of k cl ratios in interpretation of step heating results and the age of the kirkpatrick basalt the precise age of the kirkpatrick basalt has been a long standing problem the wide range of reported k ar dates has been attributed to loss of ar despite massive ar loss from the matrix plagioclase is shown to yield good 40ar39ar plateaus plateaus are observed for increments above 700 c and are defined not only by consistent apparent ages but also appropriately low k ca and high k cl ratios cl relations help identify ar released from small amounts of matrix which can seriously contaminate a plagioclase separate an age of 176 8 1 8 ma defines the time of formation of the youngest lava at mt frustum in north victoria land a glass sample from south victoria land gives the same age indicating that terrestrial basaltic glasses as old as jurassic do not inherently lose ar the step heating profile of glass with very fine crystals shows discordance which is an artifact of ar recoil during irradiation

paleomagnetism and u pb geochronology of the 2 17 ga biscotasing dyke swarm ontario canada evidence for vertical axis crustal rotation across the kapuskasing zone u pb dates on baddeleyite yield ages of 2167 8 2 2 and 2171 6 1 2 ma on two northeast trending dykes west of the kapuskasing zone in ontario canada these ages identify the dykes as belonging to the biscotasing dyke swarm east of the kapuskasing zone which was previously dated at 2166 7 1 4 ma by u pb on baddeleyite and zircon the new dates show that the biscotasing swarm was emplaced over an area of at least 300 000 km2 much larger than hitherto suspected and in a geologically short period of time of about 5 million years a comparison of paleomagnetic data from biscotasing and 2 45 ga matachewan dykes on either side of the kapuskasing zone suggests that the western half of the superior province has rotated about 10 20 counterclockwise relative to the eastern half across the kapuskasing zone this movement may have been accompanied by rifting farther north which ultimately led to the paleoproterozoic embayment underlying hudson bay that gives the superior province its characteristic butter

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

sedimentary iron cycling in the benguela upwelling system off namibia abstract iron fe is an essential micronutrient and its availability controls primary production in large parts of the ocean further the variability of oxygen minimum zones omz exerts a major control on the availability of fe concentrations of which are elevated off namibia therefore to understand the sources and processes involved in the cycling of fe off namibia we determined major and trace element contents fe al si k p organic carbon mn co zr and the isotopic composition of fe 56 fe in seven sediment cores along a transect from the shelf to the abyssal plain at 5 km water depth meteor cruise m76 1 as well as in the local lithogenic background namibian river bed sediments and dust the depletion of mn vs the local lithogenic background llb reflects the reducing character of sediments from the shelf to the lower slope while depletion of mn and enrichment of co indicates the anoxic sulfidic environment on the shelf the enrichment of both elements in abyssal sediments documents highly oxidizing conditions there iron al ratios are elevated 0 6 to 1 6 and therefore equal to or higher than the llb the highest fe enrichment is seen on the shelf core 1 and the shelf break core 2 with a clear decrease towards the abyss the bulk 56 fe values range from 0 30 to 0 05 in the marine sediments with an average of 0 08 which is slightly lighter than the 56 fe of the llb 0 16 to 0 21 further combined enrichments of k and fe suggest the presence of detrital biotite core 1 and authigenic glauconite core 2 both of which are perfectly correlated with zr al ratios reflecting the highly dynamic conditions of the shelf environment 0 300 m water depth the absent large sedimentary fe isotope fractionation in the sediments of this study compared to peru sediments is likely due to a combination of the following i a highly variable omz leading to random re oxidation of any sediment sourced fe which prevents the export of larger amounts of dissolved fe to the interior of the ocean and which also prevents focused enrichments of fe and ii an enhanced dust flux which frequently resupplies terrestrial fe dampening any fe isotope fractionation this study is a snapshot on sedimentary fe cycling in one area from the vast namibian margin compared to the peruvian margin the data suggest little sedimentary fe release transport and redeposition however previous water column data suggest substantial fe release hence for a more subtle investigation of the fe loss and re supply a higher sampling resolution of multiple cores across the shelf and slope will be required

geology and petrology of proterozoic mafic dikes north central minnesota and western ontario proterozoic diabase and gabbro dikes 2120 ma old form a major northwest oriented swarm extending about 300 km from the mesabi iron range in minnesota to the vicinity of kenora ontario the dikes were emplaced into archean crust at about the time that the early proterozoic basin of the lake superior region was opening by rifting and the swarm and the basin may therefore be tectonically related the dikes are overlain unconformably by the animikie group the upper sedimentary sequence in the proterozoic basin in minnesota but may be approximately coeval with mafic volcanic rocks in the pre animikie mille lacs group a two stage tectonic model involving 1 regional right lateral crustal shear in the late archean and 2 hot spot rifting in the early proterozoic is proposed to account for the swarm the dikes are iron rich quartz tholeiites that are differentiated toward dioritic compositions late alteration to hydrous phases including blue green amphibole chlorite and sericite together with lesser

fe isotope fractionation during phase separation in the nacl h2o system an experimental study with implications for seafloor hydrothermal vents abstract phase separation has been proposed as a possible mechanism contributing to the fe isotope composition of hydrothermal fluids at mid ocean ridges the uncertainty results largely from the emphasis on field data that can involve competing processes that obscure cause and effect of any one process to better understand the potential significance of phase separation in the nacl fe h 2 o system on fe isotope fractionation temperature and pressure of a fe bearing nacl fluid in a titanium flow reactor were carefully adjusted to produce vapor liquid halite while the fe isotope composition between coexisting phases was monitored two different p t regions were emphasized 1 424 420 c 35 2 31 5 mpa and 2 464 466 c 29 8 24 7 mpa both regions were chosen to simulate the range of physical conditions that are experienced by hydrothermal fluids at mid ocean ridges mors decompression induced phase separation in both p t regions results in the vapor phase becoming enriched in the heavier isotopes of fe as the fe cl ratio decreases the coexisting nacl rich liquid phase remains essentially constant with respect to fe cl ratio and fe isotope composition coinciding with the lowest vapor chlorinity in the vapor liquid stability field the fe cl ratio of the vapor abruptly increases while the fe isotope fractionation between the vapor and liquid 10 3 ln v l 56 54 reached a maximum value of 0 145 0 048 subsequently fe isotope fractionation decreased upon transition into the vapor halite stability field p t region 2 we infer that the observed fe isotope fractionation between vapor liquid halite is caused by differences in fe speciation among coexisting chloride bearing phases the experimental study confirms for the first time that measurable fe isotope variability can result from phase separation in high temperature hydrothermal systems the species dependent fe isotope fractionation reported here is small relative to predicted mineral mineral and mineral fluid fractionations especially if redox effects are involved as might occur during vent fluid seawater mixing reactions and or magmatic activity associated with seafloor eruptive episodes

nd isotopic evidence for the antiquity of the wyoming province sm nd isotopic data on late archean age metasedimentary rocks from around the wyoming province yield nd values at 2 6 ga from 12 9 to 3 3 which correspond to depleted mantle model ages of 2 7 to 3 8 ga these results indicate that crust of a wide variety of ages was exposed and eroding in late archean time the predominance of middle archean model ages suggests that continental crust was extensive in the wyoming province by this time no resolvable correlation exists between initial nd value and geographic location or depositional environment of the metasedimentary rock samples thus there is no indication that any part of the province was established at a time different from the rest the nd isotopic characteristics of archean rocks from the wyoming province contrast with those of the superior province confirming that although their proterozoic evolution has been parallel the two cratons did not share a common archean history

oxygen and iron isotope constraints on near surface fractionation effects and the composition of lunar mare basalt source regions oxygen and iron isotope analyses of low ti and high ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources an internally consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low ti basalts from the apollo 12 and 15 missions and eight types of high ti basalts from the apollo 11 and 17 missions high precision whole rock d 18 o values referenced to vsmow of low ti and high ti basalts correlate with major element compositions mg tio2 a l 2o3 the observed oxygen isotope variations within low ti and high ti basalts are consistent with crystal fractionation and match the results of mass balance models assuming equilibrium crystallization whole rock d 56 fe values referenced to irmm 014 of high ti and low ti basalts range from 0 134 to 0 217 and 0 038 to 0 104 respectively iron isotope compositions of both low ti and high ti basalts do not correlate with indices of crystal fractionation possibly owing to small mineral melt iron fractionation factors anticipated under lunar reducing conditions the d 18 o and d 56 fe values of low ti and the least differentiated high ti mare basalts are negatively correlated which reflects their different mantle source characteristics e g the presence or absence of ilmenite the average d 56 fe values of low ti basalts 0 073 0 018 n 8 and high ti basalts 0 191 0 020 n 7 may directly record that of their parent mantle sources oxygen isotope compositions of mantle sources of low ti and high ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing the estimated equilibrium mantle olivine d 18 o value and equilibrium oxygen fractionation between olivine and other mineral phases the differences between the calculated whole rock d 18 o values for source regions 5 57 for low ti and 5 30 for high ti mare basalt mantle source regions are solely a function of the assumed source mineralogy the oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values the d 18 o and d 56 fe values of the lunar upper mantle are estimated to be 5 5 0 2 2r and 0 085 0 040 2r respectively the oxygen isotope composition of lunar upper mantle is identical to the current estimate of earth s upper mantle 5 5 0 2 and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle 0 044 0 030

iron isotope fractionation in planetary crusts we present new high precision iron isotope data d 56 fe vs irmm 014 in per mil for four groups of achondrites one lunar meteorite 11 martian meteorites 32 howardite eucrite diogenite meteorites heds and eight angrites angrite meteorites are the only planetary materials other than earth moon system significantly enriched in the heavy isotopes of fe compared to chondrites by an average of 0 12 in d 56 fe while the reason for such fractionation is not completely understood it might be related to isotopic fractionation by volatilization during accretion or more likely magmatic differentiation in the angrite parent body we also report precise data on martian and hed meteorites yielding an average d 56 fe of 0 00 0 01 stannern trend eucrites are isotopically heavier by 0 05 in d 56 fe than other eucrites we show that this difference can be ascribed to the enrichment of heavy iron isotopes in ilmenite during igneous differentiation preferential dissolution of isotopically heavy ilmenite during remelting of eucritic crust could have generated the heavy iron isotope composition of stannern trend eucrites this supports the view that stannern trend eucrites are derived from main group eucrite source magma by assimilation of previously formed asteroidal crust these new results show that iron isotopes are not only fractionated in terrestrial and lunar basalts but also in two other differentiated planetary crusts we suggest that igneous processes might be responsible for the iron isotope variations docu

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

fe isotope variations in the modern and ancient earth and other planetary bodies iron the fourth most abundant element in the earth s crust has four naturally occurring stable isotopes 54fe 5 84 56fe 91 76 57fe 2 12 and 58fe 0 28 and the natural mass dependent isotope variations of fe in the rock record span a range of 4 per mil in 56fe 54fe ratios fig 1 the field of fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how fe is cycled in different environments iron typically occurs as either reduced ferrous fe in oxygen poor environments or as oxidized ferric iron in oxygen rich environments notably only the reduced species is soluble in oxygenated aqueous solutions unless the ph is low in the archean and early proterozoic the earth may have been relatively oxygen poor e g kasting et al 1979 grandstaff 1980 holland 1994 suggesting that there may have been significant quantities of fe 0 9 millimolar dissolved in the oceans as fe ii aq e g ewers 1983 sumner 1997 the extensive iron formations of archean to early proterozoic age may have been deposited from such fe ii rich oceans e g beukes and klein 1992 in the modern oxic oceans however fe contents are exceedingly low 1 nanomolar in the open oceans e g martin and gordon 1988 bruland et al 1991 martin 1992 johnson et al 1997 and it is now recognized that marine productivity is fe limited in parts of the open oceans e g martin and fitzwater 1988 martin et al 1989 1994 the differences in the behavior of fe with redox state and the significant isotope fractionations 1 or more in 56fe 54fe that are associated with redox conditions suggests that fe isotope studies will be extremely useful for tracing

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

archean rocks of the black hills south dakota reworked basement from the southern extension of the trans hudson orogen the little elk and the bear mountain terranes on the margin of the precambrian core of the black hills provide the only known exposures of archean rocks between the archean wyoming and superior provinces the little elk terrane consists of a supracrustal sequence dominated by biotite feldspar gneiss meta arkose or metatuff that is intruded by the slightly metaluminous to peraluminous calc alkaline little elk granite which has a u pb intercept age of 2 549 11 m y a biotite plagioclase schist metagraywacke dominates the bear mountain terrane and is intruded subparallel to foliation by the peraluminous granite at bear mountain an isochron calculated from six severely discordant u pb zircon analyses yields an age of 2 393 230 m y the archean terranes participated in the trans hudson orogenic event suggested by a published rb sr age for the little elk granite 1 850 m y 1 87 1 97 gaigneous activity in the black hills and the similarity in orientation of the proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the trans hudson orogen the metamorphic conditions estimated for the archean rocks may be the result of a thermal event that produced the proterozoic harney peak granite and thermal high northeast of lead south dakota the unexposed precambrian basement is probably dominated by granites similar to the little elk granite and supracrustal rocks consisting of continental margin deposits based on the ree contents of the archean schists and on the heavy mineral assemblages in the uraniferous nature of and the metasedimentary clasts in the lower proterozoic box elder creek formation the ages and the relatively long lived crustal sources for these archean granites and the similarity of the inferred basement characteristics to those of rocks in the wyoming province suggest that the black hills archean rocks may be the easternmost exposures of this province the probable trans hudsonian reworking of the black hills proterozoic rocks and their similarity to those in northern saskatchewan imply that the black hills occupies a tectonic position comparable to that of rocks described from the cree lake zone of saskatchewan

precise determination of the isotopic composition of potassium application to terrestrial rocks and lunar soils we detail a method for the precise and accurate determination of isotopic variations in the 41k39k ratio with a precision of 0 5 2 m purified potassium is chemically extracted from rocks soils minerals or solutions by ion exchange chromatography complete chemical yields 99 8 are achieved in order to avoid laboratory induced isotopic fractionations the purified potassium is converted to a glass by melting with barium borate flux and the resultant bead is mounted for ion probe analysis the sims method utilized by the ion probe produces extremely stable k ion beams with no measurable temporal variability in the isotope ratio the instrumental fractionation is steady at about 4 and is corrected for by measurement of a standard the measurement of gravimetrically prepared isotopic standards indicates that the method is accurate at the stated level of precision and free of egregious errors analysis of terrestrial samples including peridotite basalts granites carbonatite biotite schists and seawater indicate the complete absence of isotopic variations in 41 1k among terrestrial materials at the 0 5 o level application to lunar soils and a regolith breccia confirms previously observed large isotopic fractionation effects garner et al 1975a church et al 1976 some lunar soils e g 14163 are shown to have large sample heterogeneity 7 o while others e g soils and a regolith breccia at several apollo 15 sites station 79 are homogeneous at the level of analytical precision the presence of potassium isotopic effects in bulk soils up to 12 7 in this study with magnitudes comparable to the rayleigh fractionation factor 25 indicates that volatility during micrometeorite impact melting played a large role in lunar regolith formation as much as 15 of the regolith potassium has been lost from the moon through the tenuous lunar atmosphere

the experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt abstract a first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions experiments were performed in an internally heated gas pressure vessel at 500 mpa and temperatures between 840 and 1000 c for 120 168 h three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by mc icp ms i kinetic experiments using 57 fe enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h ii isotope exchange experiments using mixtures of hydrous peralkaline rhyolitic glass powder 4 wt h 2 o and natural pyrrhotites fe 1 x s as starting materials and iii crystallisation experiments in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt consistently showed that pyrrhotite preferentially incorporates light iron no temperature dependence of the fractionation factor was found between 840 and 1000 c within experimental and analytical precision an average fractionation factor of 56 fe 54 fe pyrrhotite melt 0 35 0 04 2se n 13 was determined for this temperature range predictions of fe isotope fractionation between fes and ferric iron dominated silicate minerals are consistent with our experimental results indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt consequently the fractionation factor determined in this study is representative for the specific fe 2 fe ratio of our peralkaline rhyolitic melt of 0 38 0 02 at higher fe 2 fe ratios a smaller fractionation factor is expected further investigation on fe isotope fractionation between other mineral phases and silicate melts is needed but the presented experimental results already suggest that even at high temperatures resolvable variations in the fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems

crustal structure and evolution of the trans hudson orogen results from seismic reflection profiling a 400 km long deep seismic reflection transect across northeastern montana and northern north dakota reveals the crustal scale structural fabric of the early proterozoic trans hudson orogen beneath the williston basin comparison with deep seismic reflection data across the canadian portion of the same orogen 700 km to the north reveals first order similarities in crustal architecture but documents significant along strike variation in orogenic evolution both transects display a broad crustal scale antiform axial to the orogen in the north geologic data suggest that this antiform is cored by an archean microcontinent in the south west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the archean wyoming craton within the wyoming craton the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the early proterozoic trans hudson orogen as interpreted from potential field and drill core data on the basis of subsurface geochronologic data from the crystalline basement and by analogy with the glennie domain within the exposed trans hudson orogen in canada we suggest that the southern antiform is cored by an archean crustal fragment that was caught up in the terminal collision of the wyoming and superior cratons during hudsonian orogeny the eastern side of the trans hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections we interpret the easterly dip of these reflections as evidence that the superior province was thrust westward over the interludes of the orogen during terminal collision although juvenile early proterozoic terranes characterize the exposed segment of the trans hudson orogen in canada limited drill core information within the dakota segment of the orogen shows a predominance of granulitic archean age crust this difference in basement lithologies along strike within the orogen may indicate the following either juvenile crust comparable to that exposed in the northern trans hudson was never present in the south or it was removed by progressive over thickening erosion and or faulting postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north

the earliest lunar magma ocean differentiation recorded in fe isotopes recent high precision isotopic measurements show that the isotopic similarity of earth and moon is unique among all known planetary bodies in our solar system these observations provide fundamental constraints on the origin of earth moon system likely a catastrophic giant impact event however in contrast to the isotopic composition of many elements e g o mg si k ti cr and w the fe isotopic compositions of all lunar samples are significantly different from those of the bulk silicate earth such a global fe isotopic difference between the moon and earth provides an important constraint on the lunar formation such as the amount of fe evaporation as a result of a giant impact origin of the moon here we show through high precision fe isotopic measurements of one of the oldest lunar rocks 4 51 0 10 gyr4 51 0 10 gyr dunite 72 415 compared with fe isotope results of other lunar samples from the apollo program and lunar meteorites that the lunar dunite is enriched in light fe isotopes complementing the heavy fe isotope enrichment in other lunar samples thus the earliest olivine accumulation in the lunar magma ocean may have been enriched in light fe isotopes this new observation allows the fe isotopic composition of the bulk silicate moon to be identical to that of the bulk silicate earth by balancing light fe in the deep moon with heavy fe in the shallow moon rather than the moon having a heavier fe isotope composition than earth as a result of giant impact vaporization

differentiation and magma mixing on kilauea s east rift zone a further look at the eruptions of 1955 and 1960 part ii the 1960 lavas new and detailed petrographic observations mineral compositional data and whole rock vs glass compositional trends document magma mixing in lavas erupted from kilauea s lower east rift zone in 1960 evidence includes the occurrence of heterogeneous phenocryst assemblages including resorbed and reversely zoned minerals in the lavas inferred to be hybrids calculations suggest that this mixing which is shown to have taken place within magma reservoirs recharged at the end of the 1955 eruption involved introduction of four different magmas these magmas originated beneath kilauea s summit and moved into the rift reservoirs beginning 10 days after the eruption began we used microprobe analyses of glass to calculate temperatures of liquids erupted in 1955 and 1960 we then used the calculated proportions of stored and recharge components to estimate the temperature of the recharge components and found those temperatures to be consistent with the temperature of the same magmas as they appeared at kilauea s summit our studies reinforce conclusions reached in previous studies of kilauea s magmatic plumbing we infer that magma enters shallow storage beneath kilauea s summit and also moves laterally into the fluid core of the east rift zone during this process if magmas of distinctive chemistry are present they retain their chemical identity and the amount of cooling is comparable for magma transported either upward or laterally to eruption sites intrusions within a few kilometers of the surface cool and crystallize to produce fractionated magma magma mixing occurs both within bodies of previously fractionated magma and when new magma intersects a preexisting reservoir magma is otherwise prevented from mixing either by wall rock septa or by differing thermal and density characteristics of the successive magma batches

high precision u pb geochronology in the minnesota river valley subprovince and its bearing on the neoarchean to paleoproterozoic evolution of the southern superior province high precision u pb ages have been obtained for high grade gneisses late kinematic to postkinematic granitic plutons and a crosscutting mafic dike of the archean minnesota river valley tectonic subprovince at the southern ramparts of the superior craton of north america the antiquity of the minnesota river valley terranes is confirmed by a high precision u pb zircon age of 3422 2 ma for a tonalitic phase of the morton gneiss voluminous late kinematic monzogranites of the benson ortonville granite and morton sacred heart granite blocks yield identical crystallization ages of 2603 1 ma illustrating the synchrony and rapidity of deep crustal melting and plutonism throughout the minnesota river valley terranes postkinematic 2591 2 ma syenogranites and aplitic dikes in both blocks effectively constrain the final penetrative deformation of the minnesota river valley subprovince monazite growth from 2609 to 2595 ma in granulitic paragneisses of the benson and montevideo blocks is interpreted to record prograde to peak granulite facies metamorphic conditions associated with crustal thickening and magmatism neoarchean metamorphism and plutonism are interpreted to record the timing of collisional accretion and terminal suturing of the mesoarchean continental minnesota river valley terranes to the southern margin of the superior province along the western great lakes tectonic zone subsequent paleoproterozoic rifting of this margin is recorded by voluminous basaltic dike intrusion expressed in the minnesota river valley by major wnw trending tholeiitic diabase dikes dated at 2067 1 ma only slightly younger than the structurally and geochemically similar 2077 4 ma fort frances kenora kabetogama dike swarm of northern minnesota and adjoining canada

tectonic implications of new shrimp and tims u pb geochronology of rocks from the sask craton peter lake domain and hearne margin trans hudson orogen saskatchewan this study reports new sensitive high resolution ion microprobe shrimp and thermal ionization mass spectrometry tims u pb geochronological data for thirteen rocks from the archean to paleoproterozic sask craton the peter lake domain and hearne margin saskatchewan seven samples from the exposed sask craton in both the glennie domain and pelican window record zircon growth between 2425 and 2525 ma and at 1800 ma older precursor material has been identified most notably a refined crystallization age of 3117 ma for a sample of quartzofeldspatic gneiss q gneiss in the pelican window which also experienced ca 2450 ma zircon growth three samples from the peter lake domain and a sample from the linn island inlier of the wollaston domain yield ages of 2575 ma one sample from the peter lake domain is distinctly older yielding an age of 2640 ma but also displays zircon growth at 2575 ma the simplest interpretation of these data is that the peter lake domain is likely a part of the hearne margin

iron isotope fractionation during magmatic differentiation in kilauea iki lava lake magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks the extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements we report analyses of iron isotopes in basalts from kilauea iki lava lake hawaii the iron isotopic compositions 56fe 54fe of late stagemeltveins are 0 2 permil per thousand greater than values for olivine cumulates olivine phenocrysts are up to 1 2 per thousand lighter than those of whole rocks these results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole rock and crystal scales this characteristic of iron relative to the characteristics of magnesium and lithium for which no fractionation has been found may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

paleomagnetism u pb geochronology and geochemistry of marathon dykes superior province and comparison with the fort frances swarm we report the first detailed study of the paleomagnetism u pb geochronology and major element geochemistry of paleoproterozoic north trending marathon dykes north of lake superior the paleomagnetic and geochemical results demonstrate that marathon dykes can be divided into two subsets one of normal magnetic polarity the other of reverse polarity normal and reverse marathon paleomagnetic poles at 43 n 196 e dm 9 dp 7 number of dykes n 16 and 51 n 175 e dm 9 dp 6 n 12 respectively are statistically distinct and may indicate different ages of normal and reverse dyke emplacement a u pb baddeleyite age of ma has been obtained at a normally magnetized marathon paleomagnetic site the reversely magnetized marathon dykes are undated but have a paleopole rather close to that of the reversely magnetized ma fort frances dykes and major element geochemical signatures as portrayed on jensen plots that are identical to those of the fort frances swarm therefore reverse marathon an

zinc and lead isotope signatures of the zhaxikang pbzn deposit south tibet implications for the source of the ore forming metals abstract stable zn isotopes may be applied to trace the source of ore forming metals in various types of pb zn deposits to test this application zn and pb isotope systematics for sulfides and associated basement rocks as well as fe mn carbonates gangue from the zhaxikang pb zn deposit in south tibet have been analyzed the basement in this region includes metamorphosed mafic to felsic rocks dolerite quartz diorite rhyolite porphyry pyroclastics and porphyritic monzogranite these rocks have similar 66 zn values of 0 33 to 0 37 with an average value of 0 36 0 03 2 except for the more evolved porphyritic monzogranite that has a heavier value of 0 49 fe mn carbonates are present as hydrothermal veins and were probably precipitated from magmatic fluids they have an average 66 zn value of 0 27 0 05 which is slightly lighter than the basement rocks possibly representing 66 zn isotopic compositions of the hydrothermal fluids sphalerite and galena have similar zn isotopic compositions with 66 zn ranging from 0 03 to 0 26 and 0 21 to 0 28 respectively considering the zn isotope fractionation factor between sphalerite and fluids of 0 2 at 300 c as reported in literature hydrothermal fluids from which these sulfides precipitated will have 66 zn values of ca 0 39 0 10 which are consistent with the values of basement rocks and the fe mn carbonates this similarity supports a magmatic hydrothermal origin of the zhaxikang pb zn deposit both pb and s isotopes in these sphalerite and galena show large variations and are consistent with being derived from a mixture of basement and sedimentary rocks in various proportions zn isotopic compositions of the sulfides significantly extend the range of regional basement rocks suggesting that sedimentary rocks e g shales are also a significant source of zn however the zn isotopic compositions of sphalerite and galena differ from those of marine carbonates and those of typical sedex type deposits e g kelley et al 2009 confirming a magmatic hydrothermal model combined with regional geological observations and the age constraints of 20 ma zheng et al 2012 2014 the results of our investigation indicate that the zhaxikang pb zn deposit is most likely a magmatic hydrothermal deposit

2480 ma mafic magmatism in the northern black hills south dakota a new link connecting the wyoming and superior cratons the laramide black hills uplift of southwest south dakota exposes a precambrian crystalline core of 2560 2600 ma basement granitoids nonconformably overlain by two paleoproterozoic intracratonic rift successions in the northern black hills a 1 km thick layered sill the blue draw metagabbro that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift basin sedimentation ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207pb 206pb upper intercept age of 2480 6 ma all age errors 2 comparable to two point 207pb 206pb errorchron ages obtained by pb stepwise leaching of the same titanites nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 16 to 2284 20 ma i e differentially reset along u pb concordia and hornblende from an associated metadiorite yields a partially reset date with oldest apparent a

iron isotopic and chemical tracing of basalt alteration and hematite spherule formation in hawaii a prospective study for mars abstract hematite spherules from mauna kea volcano hawaii resemble martian hematite spherules at meridiani planum in morphology mineralogy and geochemistry and they both formed in association with acid sulfate alteration of basalt to gain insights into the processes involved in the formation of these spherules we have measured the fe isotopic compositions of unaltered basalts and tephras as well as a variety of weathering alteration products including altered basalts hematite rich breccia and individual hematite spherules jarosite bearing tephras palagonitic tephras calcined tephras high t dry oxidation and steam vent tephras compared to other alteration processes that induce little fe mobility acid sulfate alteration produces large fe isotopic variations 56 fe values ranging from 0 15 to 0 94 we used rock slabs from a variably altered basaltic rock to constrain the fe isotope fractionation factor during acid sulfate alteration of basaltic rocks we found that the process followed a rayleigh distillation with an instantaneous fractionation factor of 0 24 between the fluid and the residue for 56 fe resulting in a heavy fe isotope enrichment in the alteration residue however the constant mn fe ratios of the slabs suggest that acid sulfate leaching does not significantly fractionate mn fe ratios in basalts which is consistent with the mobilization of iron as fe ii we also measured the fe isotopic compositions and mn fe ratios using nanosims of individual hematite spherules from a spherule rich breccia hwmk745r the spherules show heavy fe isotopic enrichments and low mn fe ratios compared to unaltered samples which is similar to the fractionations encountered in iron formations a rayleigh fractionation model suggests that both fe isotopic compositions and mn fe ratios of the spherules are consistent with 80 fe oxidation and precipitation from an acid sulfate fluid therefore direct fe precipitation from a fluid can readily explain the hematite spherule formation and a more complex scenario involving jarosite precipitation followed by conversion to hematite is not needed combined thermodynamic and kinetic constraints also suggest that precipitation of jarosite would be hindered the study presents a roadmap for studying future returned martian samples in the laboratory

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

northeast trending early proterozoic dykes of southern superior province multiple episodes of emplacement recognized from integrated paleomagnetism and u pb geochronology integrated paleomagnetic and u pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of early proterozoic tholeiitic dykes of northeast trend in the southern superior province previously referred to collectively as preissac dykes it is demonstrated that they are readily separated on the basis of paleomagnetism into subsets referred to as the biscotasing and senneterre swarms in addition a pair of unnamed dykes may be associated with the north and northwest trending matachewan swarm farther west biscotasing dykes have a down west magnetization of single polarity with a corresponding paleopole at 27 8 n 136 7 w dm 12 3 and dp 9 4 senneterre dykes carry an up north or occasionally down south direction with corresponding paleopole at 15 3 s 75 7 w dm 7 0 dp 4 4 the senneterre direction is indistinguishable from the primary n1 remanence direction that dominates the magnetization of nipissing sills of the southern province paleomagnetic fiel

abundance distribution and origin of 60fe in the solar protoplanetary disk abstract meteorites contain relict decay products of short lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed several studies reported a high abundance of 60fe t1 2 2 62 0 04 myr in chondrites 60fe 56fe 6 10 7 suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn sun we measured 58fe 54fe and 60ni 58ni isotope ratios in whole rocks and constituents of differentiated achondrites ureilites aubrites heds and angrites unequilibrated ordinary chondrites semarkona ll3 0 and nwa 5717 ungrouped petrologic type 3 05 metal rich carbonaceous chondrite gujba cba and several other meteorites cv el h ll chondrites iiiab iva ivb iron meteorites we derive from these measurements a much lower initial 60fe 56fe ratio of 11 5 2 6 10 9 and conclude that 60fe was homogeneously distributed among planetary bodies this low ratio is consistent with derivation of 60fe from galactic background 60fe 56fe 2 8 10 7 in the interstellar medium from ray observations and can be reconciled with high 26al 27al 5 10 5 in chondrites if solar material was contaminated through winds by outer layers of one or several massive stars e g a wolf rayet star rich in 26al and poor in 60fe we present the first chronological application of the 60fe 60ni decay system to establish the time of core formation on vesta at 3 7 1 7 2 5 myr after condensation of calcium aluminum rich inclusions cais

redox state of the baogutu reduced porphyry cu deposit in the central asian orogenic belt abstract reduced porphyry cu deposits are scarce and their origins remain enigmatic the late carboniferous baogutu reduced porphyry cu deposit in the western junggar terrane nw china provides an opportunity to address this issue in this study we conducted systematic analyses in terms of petrography mineral chemistry major elements trace elements and fe isotopes to investigate the redox state of its primary magma and further constrain the origin of baogutu copper deposit based on the petrological observation and mineral chemistry two magmatic stages have been recognized during the formation of ore forming diorite the early stage is characterized by the crystallization of magnetite and high mg mg mg fe2 hornblende under oxidized condition nno 2 36 calculated by the equation given by scaillet and evans 1999 the late stage is characterized by the crystallization of ilmenite and low mg biotite under reduced condition nno in this study we first report the fe isotopic compositions of reduced porphyry copper deposit and discuss its application for the origin of ore deposit the 56fe values of less altered diorites and granodiorites 0 083 and 0 054 fall into the field of arc magmas consistent with their major and trace elements results the 56fe values of sulfides chalcopyrite and pyrite show a large span 0 228 to 0 614 and the early formed disseminated sulfides are enriched in light fe isotopes relative to the late formed vein hosted sulfides which indicates that hydrothermal fluid evolves toward heavy fe isotope enrichment the average 56fe of all hydrothermal sulfides chalcopyrite and pyrite is much higher than that of baogutu silicate rocks which demonstrates that the hydrothermal fluid are fe isotopically heavy and it well explains the positive correlations between 56fe and indices of hydrothermal alteration for altered plutonic rocks and wall rocks in summary major elements trace elements and fe isotopes of the baogutu porphyries show the affinity of arc magmas same as the typical oxidized porphyry cu deposits both petrological observations and fe isotope features of the baogutu porphyry cu deposit indicate that the primary magma of baogutu porphyries is oxidized and its reduced features are secondary

the importance of a ni correction with ion counter in the double spike analysis of fe isotope compositions using a 57fe 58fe double spike we present a new method capable of measuring iron isotope ratios of igneous materials to high precision by multicollector inductively coupled plasma mass spectrometry mc icp ms using a 57fe 58fe double spike after sample purification near baseline signal levels of nickel are still present in the sample solution acting as an isobaric interference on 58 amu to correct for the interference the minor 60ni isotope is monitored and used to subtract a proportional 58ni signal from the total 58 amu beam the 60ni signal is difficult to precisely measure on the faraday detector due to johnson noise occurring at similar magnitude this noise dominated signal is subtracted from the total 58 amu beam and its error amplified during the double spike correction placing the 60ni beam on an ion counter produces a more precise measurement resulting in a near threefold improvement in 56fe reproducibility from 0 145 when measured on faraday to 0 052 faraday detectors quantify the 60ni signal poorly and fail to discern the transient 20ne40ar interference visible on the ion counter which is likely responsible for poor reproducibility another consideration is instrumental stability defined herein as drift in peak center mass which affects high resolution analyses analyses experiencing large drift relative to bracketing standards often yield non replicating data based on this we present a quantitative outlier detection method capable of detecting drift affected data after outlier rejection long term precision on individual runs of our secondary standard improves to 0 046 averaging 3 4 analyses further improves precision to 0 019 allowing distinction between ultramafic minerals this article is protected by copyright all rights reserved