reduced reused and recycled detrital zircons define a maximum age for the eoarchean ca 3750 3780 ma nuvvuagittuq supracrustal belt quebec canada abstract a key discovery from the hadean pre 3850 ma detrital zircon record has been that the dichotomy of granitic and basaltic crust was established within about 160 myr of earth s formation harrison 2009 understanding the origin and fate of this primordial crust would greatly add to what we know about the geodynamics of the hadean earth insights emerge from 147 146sm 143 142nd isotope data reported from different eoarchean terranes worldwide including the nuvvuagittuq supracrustal belt nsb in northern quebec some ca poor cummingtonite rich amphibolites and granitoid gneisses of the nsb preserve lower 142nd 144nd than bulk silicate earth bse these also show positive correlations against 147sm 144nd that were used by o neil et al 2008 o neil et al 2012 to assign a ca 4400 ma age alternatively the compositions were inherited during the formation of the nsb at around 3800 ma roth et al 2013 guitreau et al 2013 to resolve this discrepancy ion microprobe u pb ages are reported for detrital zircons from nsb meta sediments from within the same supracrustal successions that preserve low 142nd 144nd the youngest detrital zircon cores of igneous derivation define a maximum age for the nsb of ca 3780 ma this age is about 600 myr younger than that obtained from 142nd 144nd vs 147sm 143nd regressions thus just like the variable 142nd 144nd ratios reported for other eoarchean terranes non bse 142nd 144nd values of the nsb were inherited from an older component

evidence for a widespread mafic cover sequence and its implications for continental growth in the northeastern superior province abstract archean greenstone belts of the northeastern superior province nesp were emplaced over a 160 ma period 2 88 2 72 ga spanning a major episode of crustal reworking in which early tonalite trondhjemite plutonism evolved to dominant granite granodiorite and pyroxene bearing felsic plutonism the numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites 1 mg tholeiites that have chemical compositions typical of many archean basalts with 4 10 wt mgo 9 15 wt fe2o3t 0 4 1 2 wt tio2 and la sm ratios between 1 and 3 2 fe tholeiites that have similar mgo contents and la sm ratios but markedly higher fe2o3t 11 20 wt tio2 1 0 2 6 wt and gd yb ratios that may reflect derivation from a distinct garnet bearing mantle source 3 light rare earth element lree enriched rocks that have the chemical characteristics typical of calc alkaline mafic magmas with higher sio2 al2o3 and la sm ratios but lower fe2o3t and tio2 than the tholeiitic suites at a given mgo content the presence of unconformities within single greenstone belts numerous inheritance ages in volcanic rocks replicating those of older volcanic plutonic assemblages and the recognition of widespread geographically separate yet coeval 2 78 ga extrusive assemblages containing the distinctive fe tholeiites do not support a plate tectonic model for a large portion of the nesp the correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence nd isotopic evidence for the presence of evolved crust in some syn volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust the amalgamation of two isotopically distinct terranes at ca 2 76 2 74 ga created a proto cratonic mass that consisted of a composite tonalite trondhjemite greenstone crust underplating by mafic mantle melts and widespread insulation after 2 75 ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene bearing felsic rocks and granodiorite to granite partial melts the isotopically enriched character of mantle derived magmas after 2 75 ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting furthermore a decrease of nb th ratios in the lree enriched mafic rocks younger than 2 75 ga reflects a change in the nature of the crustal contaminant from tonalite trondhjemite prior to 2 75 ga to granite granodiorite afterwards

protocrustal evolution of the nuvvuagittuq supracrustal belt as determined by high precision zircon lu hf and u pb isotope data abstract the nuvvuagittuq supracrustal belt nsb in northern quebec canada represents one of the oldest known crustal fragments preserved in the earth s crust its age has however been disputed and different authors present crustal formation ages varying from ca 3 8 ga to 4 4 ga e g o neil et al 2012 guitreau et al 2013 here we report new high precision u pb geochronological and coupled lu hf isotope data from zircons that reveal the age of the nsb and provide new constraints on the source rocks to this piece of early crust two rocks have been analysed a felsic schist interpreted to represent a volcanite and a mylonitic tonalite that is intrusive into the nsb the felsic schist was emplaced at 3771 5 3 ma dating the formation of the nsb the lu hf model age indicates that the parental melt to the felsic schist was extracted from the mantle at this age or within a few tens of million years but no more than 300 m y before its extrusion this mantle extraction age provides an absolute maximum age for the nsb but the most probable age of its protocrust is ca 3 8 ga the mylonitic tonalite was emplaced at 3667 3 1 ma its lu hf isotopic composition reveals the presence of an older crustal component than that of the nsb requiring that its precursor melt formed from a hadean source that was older than the nsb based on recent models for archean ttg formation we propose that this source represents re melted hadean hydrated crust that must have been translated below the nsb at ca 3667 ma

petrology and geochemistry of mafic rocks in the acasta gneiss complex implications for the oldest mafic rocks and their origin abstract the acasta gneiss complex located in the western part of the slave province canada is widely recognized as the oldest eoarchean terrane in addition to felsic gneisses with the ages of 3 6 4 0 ga minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses despite serving as potential sources of geochemical information on the hadean mantle the mafic rocks have received less attention in previous studies thus we conducted a comprehensive geological petrological and geochemical investigation on the acasta mafic rocks to constrain their petrogenesis and geodynamic setting the mafic rocks comprise massive to weakly foliated amphibolite garnet amphibolite and hornblendite with variable abundances of hornblende plagioclase chlorite and quartz and subordinate clinopyroxene garnet and cummingtonite they commonly underwent high grade metamorphic recrystallization under amphibolite to upper amphibolite facies conditions the observed variations in mineral assemblages abundances and compositions reflect large differences in whole rock compositions likely caused by crustal anatexis during the eoarchean thermal events responsible for the generation of the surrounding felsic gneisses infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between al 2 o 3 and mgo contents and variable enrichments in the incompatible elements despite the widespread influence of anatexis on the geochemistry of acasta mafic rocks we identified the precursor compositions of the least modified amphibolites as basaltic magmas they are characterized by intermediate al 2 o 3 and mgo contents on the observed array and by near chondritic patterns for incompatible trace elements except for slightly negative nb and ta anomalies we considered two scenarios to explain the origin of eoarchean basaltic rocks with nb ta anomalies 1 generation of nb ta deficient basaltic magma in a suprasubduction setting analogous to modern arcs derived magmas and 2 generation of nb ta deficient basaltic magma from the melting of a nb ta deficient primitive mantle possible if the core contains significant proportions of the earth s nb and ta budget although the operation of plate tectonics and the presence of subduction zones at the end of hadean may be an attractive explanation for the observed nb ta depletions the chondritic relative proportions of other immobile trace elements for acasta mafic rocks leave open the possibility of their formation from an nb ta deficient primitive mantle

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

recognition of 3850 ma water lain sediments in west greenland and their significance for the early archaean earth a layered body of amphibolite banded iron formation bif and ultramafic rocks from the island of akilia southern west greenland is cut by a quartz dioritic sheet from which shrimp zircon 206pb 207pb weighted mean ages of 3865 11 ma and 3840 8 ma 2 sigma can be calculated by different approaches three other methods of assessing the zircon data yield ages of 3830 ma the bifs are interpreted as water lain sediments which with a minimum age of approximately 3850 ma are the oldest sediments yet documented these rocks provide proof that by approximately 3850 ma 1 there was a hydrosphere supporting the chemical sedimentation of bif and that not all water was stored in hydrous minerals and 2 that conditions satisfying the stability of liquid water imply surface temperatures were similar to present carbon isotope data of graphitic microdomains in apatite from the akilia island bif are consistent with a bio organic origin mojzsis et al 1996 extending the record of life on earth to 3850 ma life and surface water by approximately 3850 ma provide constraints on either the energetics or termination of the late meteoritic bombardment event suggested from the lunar cratering record on earth

crustal evolution of the western minto block northern superior province canada abstract geochemical and nd isotope data are presented for suites of meso to late archean 2 7 2 9 ga granitoid rocks from the western minto block of the northeast canadian superior province the samples provide coverage of the inukjuak and tikkerutuk domains and include biotite amphibole and or orthopyroxene bearing tonalites granodiorites and granites from tonalite trondhjemite granodiorite ttg and monzodiorite suites both suites are characterized by strong light rare earth element ree and large ion lithophile lil element enrichment variably negative to positive eu anomalies and moderate heavy ree depletion high la yb and sr y ratios with correspondingly low yb and y concentrations in both suites likely reflect parental melts that left residual garnet correlations between eu anomalies and fractionation indices suggest that compositional variations in both suites reflect plagioclase accumulation in tonalite samples and plagioclase and fe ti oxide fractionation among granitic samples the nd isotope compositions range from nd values of 2 to 10 the largest range found among granitoids of the minto block and indicate the presence of a crustal contaminant of ca 3 8 ga in age there is a decrease in nd values from east to west across the studied region and a measurable decrease across tikkerutuk and inukjuak domain boundary this suggests increasing addition of an older crustal component in the inukjuak domain possibly as a result of recycling of early archean crust from the nearby ca 3 8 ga nuvvuagittuq sequence

microbial mats as bioreactors populations processes and products microbial mats are dynamic and complex ecosystems exhibiting spatial and temporal heterogeneity the physical chemical environment is typified by steep gradients and distinct microenvironments these microenvironments support a great diversity of species with a wide range of metabolic processes these processes often result in coupled reactions and biogeochemical cycles and produce important end products such as trace gases and mineral precipitates the latter can impact the composition and character of the sediment imparting a biosignature these biosignatures can be preserved in the rock record and are useful in the interpretation of fossil record on earth and possibly as an indication of life on other planetary bodies the modern marine stromatolites of the exuma cays bahamas provide an ideal system for studying the populations processes and products in a microbial ecosystem using a multidisciplinary approach in order to acquire redox energy microbial populations need to carry out metabolic reactions at rates faster than the equivalent chemical abiotic reactions as such microbes can be viewed as bioreactors that preferably oxidize carbon to co2 to maximize the energy yield the study of the microbial role in carbonate sedimentation and lithification in these stromatolites provides a picture of microbial mats as bioreactors producing a biosignature

early crustal evolution of the superior craton a u pb hf and o isotope study of zircon from the assean lake complex and a comparison to early crust in other cratons abstract we report u pb hf o isotopic measurements of zircon extracted from meta igneous rocks exposed in the assean lake crustal complex northwestern superior craton canada with crystallization ages of ca 3 22 ga zircon from the assean lake crustal complex are among the oldest reported in the superior craton the hf and o isotope compositions of zircon indicate that these meta igneous rocks derive from a mantle component with a relatively primitive hf isotope composition similar to chondrites that was uncontaminated by elevated 18o material ehf initial 0 and 18ovsmow 5 5 these features are shared by the large majority of the zircon extracted from the oldest meta igneous rocks exposed in archean terranes positive ehf deviations in archean zircons are typically observed in magmas that post date the oldest magmatic activity in a given terrain this temporal variability in magma source regions may reflect a sequence of events comprising i felsic magmatism derived from undepleted mantle components followed by ii re melting of the same mantle component producing the depleted hf signatures in zircon and then iii accretion of the proto craton and reworking of the whole system enabling stabilization

geochemical and textural investigations of the eoarchean ukaliq supracrustals northern quebec canada abstract structural geochronological and geochemical studies of pre 3 75 ga rocks of volcano sedimentary protoliths in the inukjuak domain in the superior province in quebec have been mostly focused on the nuvvuagittuq supracrustal belt nsb the porpoise cove outcrops at the southwestern limit of the nsb are the de facto type locality for the supracrustals of the inukjuak complex yet it remains unclear whether the nsb rocks are geochemically distinct from or are in fact common to a host of other supracrustal enclaves locked in the dominantly gneissic inukjuak domain here we report detailed textural and geochemical studies for a suite of rocks from the ukaliq supracrustal belt usb located approximately 3 km northeast of the nsb we find that the usb and nsb have a similar protracted metamorphic history both experienced amphibolite grade metamorphism and contain granitoid gneiss sheets quartz magnetite rocks banded iron formation s l and quartz biotite schists within amphibolitized rocks of basaltic affinity with local retrogressions if the various inukjuak supracrustal belts were once a part of a larger coherent now dismembered terrane they should show similar emplacement ages and source chemistry new zircon u pb geochronology from five gneissic units and two quartz biotite metasedimentary schists reveal the oldest emplacement ages across all units of each individual rock type to be 3 68 0 07 ga granitoid gneisses and 3 65 0 06 ga quartz biotite schists these new ages are similar to those documented as likely minimum emplacement ages of the nsb determined by u pb geochronology zircons from the quartz biotite schist were also analyzed by ion microprobe for their u pb geochronology and were found to yield statistically identical albeit more precise ages than those obtained by la icp ms possible detrital zircons from the usb quartz biotite schists were analyzed by ion microprobe for their coupled 30sinbs28 and 18ovsmow values with respective values between 0 75 and 0 07 30sinbs28 and 5 61 and 6 59 18ovsmow the 18ovsmow values which are on average above mantle derived zircon indicate contribution of altered non mantle derived material into the source of the rocks that weathered to form the quartz biotite schists zircon mineral inclusions quartz feldspar apatite biotite muscovite and other unrecognized fe al si rich phases along with the major and trace element contents for the rocks were analyzed to substantiate this interpretation together with 30sinbs28 18ovsmow our results suggest that lithologies like authigenic silica and serpentinized rocks re melted to form the parent melts that gave rise to zircons found in the usb quartz biotite schists additional lu hf studies reported here on the same zircons also show similarities with nsb zircons the ehf values showed a positive correlation with the measured u pb age from 22 7 0 8 to 1 9 1 1 the lu hf system also reveals that the usb extracted at ca 3 8 ga carries isotopic signatures of an older hadean reservoir having been formed from an eoarchean mafic melt that incorporated them taken together this supports a co genetic origin for the nsb and the usb

young poorly crystalline graphite in the 3 8 gyr old nuvvuagittuq banded iron formation carbonaceous minerals in 3 8 billion year old rocks from west greenland have been used as evidence for early life on earth geochemical analyses of similar carbon minerals in 3 75 to 4 2 billion year old rocks from canada show that they can be derived from subsequent alteration more than a billion years after the initial formation

occurrence and geochronology of the eoarchean 3 9 ga iqaluk gneiss in the saglek block northern labrador canada evidence for the oldest supracrustal rocks in the world abstract understanding earth s early evolution requires decoding the cryptic geological record that is preserved in a few extremely rare terrains on earth the saglek hebron block in the north atlantic craton is one of the oldest terrains in the world to better understand the eoarchean history of the saglek hebron block we undertook a comprehensive geological and geochronological investigation of this terrain the saglek hebron locality contains orthogneisses and supracrustal rocks which are classified into two groups of the eoarchean and mesoarchean suites based on field associations defined by intrusion of the mesoarchean mafic saglek dykes the eoarchean suites contain the nulliak supracrustal rocks and uivak gneisses in this paper we report cathodoluminescence cl observations and u pb ages of zircons from the eoarchean orthogneisses to constrain the evolution of the saglek hebron block we made detailed sketch maps 1 20 of critical outcrops to establish relative chronological relationships between lithologies at least seven generations of mafic and felsic units were recognized in an outcrop of the st john s harbour south area the first and second generations are mafic supracrustal rocks whereas the third to seventh generations are tonalitic gneisses which are intruded into the supracrustal units we study cl observation geochemistry and u pb dating of zircons from the oldest and youngest tonalitic gneisses the cl observation of internal structures of the zircons showed they typically comprise three domains of core mantle and rim the cores have clear oscillatory zoning whereas the mantles lack oscillatory zoning and exhibit dark cl emission and structureless cl images the obvious correlation between the ages and chemical compositions of u contents and th u ratios indicates that the cores with low u contents and high th u ratios preserve the protolith age the cores with clear oscillatory zoning low u contents and high th u ratios of zircons from the oldest suite plot on a concordia line with ages ranging from 3953 to 3797 ma the oldest and average ages of the six oldest spots on the concordia line are 3953 54 and 3920 49 ma respectively on the other hand the average age of the four old spots of zircons from a younger generation of orthogneiss is 3869 63 ma the geological and geochronological relationships between the third and seventh generation orthogneisses are consistent with each other confirming an eoarchean age of ca 3 9 ga for the oldest suite of the uivak gneisses we name this oldest suite the iqaluk gneiss as the protolith of the iqaluk gneiss was clearly intruded into the nulliak supracrustal rocks the nulliak supracrustals must have a minimum age of ca 3 9 ga indicating that they are the oldest supracrustal rocks on earth but further studies should be necessary to obtain more precise age of the iqaluk gneiss and nulliak supracrustal rocks

establishment of a 3 83 ga magmatic age for the akilia tonalite southern west greenland a recurring challenge to ion microprobe u pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains this has proved particularly problematic in studying the polyphase metamorphic rocks that dominate early archean gneissic terranes for example differing interpretations exist for the origin of complex zircon populations investigated by u pb ion microprobe zircon geochronology from a granitoid orthogneiss body previously used to establish a minimum age for a supracrustal enclave on akilia island in southern west greenland we describe a method whereby the geochemistry of the akilia orthogneiss coupled with a u th pb vs age depth profile in a zircon from the same rock permits direct assessment of zircon inheritance results reveal evidence for three phases of concentric zircon growth at 3 83 ga v3 6 ga and 2 7 2 5 ga zircon growth at both v3 6 ga and 2 7 2 5 ga is consistent with precipitation from a metamorphic fluid depth profile u th pb data demonstrate that only the s 3 8 ga zircon core could have crystallized from the host rock we conclude that the magmatic age of this rock is 3 83 7 0 01 ga not v3 65 ga as has been previously proposed the u th pb zircon depth profile technique has wider applications to resolving other geochronological debates in highgrade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution re precipitation features 2002 elsevier science b v all rights reserved

nature of assean lake ancient crust manitoba a combined shrimp id tims u pb geochronology and sm nd isotope study abstract the assean lake crustal complex alcc located at the northwest periphery of the superior province in northern manitoba canada hosts an ancient assembly of rocks that record a complex and prolonged crustal history spanning more than two billion years from 3 9 to 1 8 ga a supracrustal assemblage consisting of amphibolite grade greywacke quartz arenite arkose mafic to intermediate volcanics and to a lesser extent iron formation dominates the alcc and likely formed in a shallow marine arc back arc setting the supracrustal rocks are intruded by amphibolite grade calc alkaline tonalite to granite orthogneiss nd model ages for felsic para and orthogneisses range from 3 5 to 4 2 ga and suggest that paleoarchean basement may be preserved at assean lake shrimp u pb zircon ages and id tims conventional u pb zircon and monazite ages show that felsic magmatism occurred at 3 1 3 2 ga whereas a record extending back to at least 3 9 ga is preserved in detritus in paragneiss that may have a depositional age of 3 2 ga orthogneiss was metamorphosed at 2 68 ga whereas local migmatisation of paragneiss occurred at 2 61 ga monazite growth events at 2 6 2 45 and 1 8 ga might represent episodes of accretion of archean crustal segments including the alcc and possibly the sask craton trans hudson basement along the northwest superior province margin at present matching segments of the exotic alcc are unknown but remnants may exist in parts of the sask craton

ion microprobe u pb age determinations on zircon from the late archean granulite facies transition zone of southern india abstract the southern dharwar craton of karnataka and tamil nadu states in south india exposes a depth profile of late archean crust amphibolite facies felsic to intermediate peninsular gneiss of protolith ages 3 4 2 9 ga with subordinate infolded supracrustal relics and the large closepet granite body give way southward to granulite facies orthopyroxene bearing gneisses over a transition zone about 40 km wide with mineralogic pressure indicators increasing from about 4 kbar corresponding to about 15 km paleodepth to about 7 kbar corresponding to near 25 km paleodepth previous whole rock and zircon geochronology have established that amphibolite facies metamorphism migmatization and granite intrusion took place in the latest archean at 2 53 2 51 ga however some workers have considered the foliated granulites as products of an earlier metamorphism distinct from the terminal archean events this work presents whole rock geochemical studies coupled with ion microprobe 207pb 206pb ages and th u

metamorphic zircon trace elements and neoarchean metamorphism in the ca 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract the nuvvuagittuq supracrustal belt nsb in northwestern quebec ranks as one of the oldest granitoid gneiss complexes thus far discovered emplacement ages for intrusive trondhjemitic dikes that transect amphibolites and quartz magnetite schists at the porpoise cove outcrops of the nsb cluster at 3 75 ga as with all other pre 3 7 ga terranes thus far documented the nsb has been metamorphosed and multiply deformed in a combined approach to explore the thermal history of the nsb conventional garnet biotite and plagioclase hornblende geothermometry was coupled with zircon u th pb depth profiles ree partitioning and ti in zircon thermometry to show that neoarchean metamorphism in the belt reached upper amphibolite facies conditions 640 c this metamorphism resulted in some cases in thick zircon overgrowths on older cores and likely corresponds to the amalgamation of terranes in the northeast superior province and the initiation of widespread igneous activity in the vicinity of the nsb an earlier resolvable metamorphic episode from a zircon depth profile recorded at 3 62 ga probably corresponds to the intrusion of the granitoids which envelope the nsb

potassic high silica hadean crust understanding hadean 4 ga earth requires knowledge of its crust the composition of the crust and volatiles migrating through it directly influence the makeup of the atmosphere the composition of seawater and nutrient availability despite its importance there is little known and less agreed upon regarding the nature of the hadean crust by analyzing the 87sr 86sr ratio of apatite inclusions in archean zircons from nuvvuagittuq canada we show that its protolith had formed a high 1 rb sr ratio reservoir by at least 4 2 ga this result implies that the early crust had a broad range of igneous rocks extending from mafic to highly silicic compositions

giant granulite terranes of northeastern superior province the ashuanipi complex and minto block the ashuanipi complex and minto block of the superior province are large regions that have been classified as high grade gneiss terranes on the basis of the presence of orthopyroxene bearing units like the granite greenstone and metasedimentary subprovinces of the southern superior province the two terranes consist predominantly of intrusive rocks but are distinguished by their primary magmatic orthopyroxene both high grade and gneiss are misnomers because granulite facies gneisses are only sparingly present and the regions consist dominantly of massive unmetamorphosed plutonic rock the ashuanipi complex consists of a deformed metamorphosed package of metasedimentary rocks and primitive early tonalite cut by widespread orthopyroxene garnet granodiorite diatexite as well as plutons of tonalite granite and syenite based on its lithological and chronological similarity and on strike position the complex appears to be the continuation of metasedimentary subprovinces such as the quetico its evolution involved deposition of immature greywacke in an accretionary prism early arc tonalitic magmatism and deformation followed by widespread intracrustal magmatism in the period 2700 2670 ma both metamorphic and igneous rocks record equilibration under granulite facies conditions 700 835 c 0 35 0 65 gpa a 0 3 and indicate exposure levels of 20 km the minto block at the latitude of leaf river consists of several north northwest trending domains of similar scale and diversity to the east trending subprovinces of the southern superior province the central goudalie domain is dominantly amphibolite facies tonalitic rocks including some with ages 3 ga with small belts of volcanic and sedimentary origin lake minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite clinopyroxene orthopyroxene granodiorite orthopyroxene biotite diatexite and granite the hornblende granodiorite suite constitutes most of the utsalik and tikkerutuk domains and is thought to represent continental arc magmatism on the basis of their distinct aeromagnetic and lithological character two additional domains are evident north of the leaf river area the inukjuak domain in the west and the douglas harbour domain in the east the northerly grain of the minto block appears to have been established in situ with respect to the easterly belts of the southern superior province i e no large scale block rotation during the same interval of time 3 0 2 7 ga modification of the tectonic framework for the superior province is required to explain minto arc magmatism in the interval 2730 2690 ma ago a continental magmatic arc built the berens river and bienville subprovinces and minto block on the southern and eastern edges respectively of a northern protocratonic foundation in the same period primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime

thermal conductivity of hydrate bearing sediments 1 a thorough understanding of the thermal conductivity of hydrate bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone the coexistence of multiple phases gas hydrate liquid and gas pore fill and solid sediment grains and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate bearing sediments previous studies have been unable to capture the full parameter space covered by variations in grain size specific surface degree of saturation nature of pore filling material and effective stress for hydrate bearing samples here we report on systematic measurements of the thermal conductivity of air dry water and tetrohydrofuran thf saturated and thf hydrate saturated sand and clay samples at vertical effective stress of 0 05 to 1 mpa corresponding to depths as great as 100 m below seafloor results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size effective stress porosity and fluid versus hydrate filled pore spaces the thermal conductivity of thf hydrate bearing soils increases upon hydrate formation although the thermal conductivities of thf solution and thf hydrate are almost the same several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment increased mean effective stress due to hydrate formation under zero lateral strain conditions and decreased interface thermal impedance as grain liquid interfaces are transformed into grain hydrate interfaces

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

the geochemistry of archaean shales derived from a mafic volcanic sequence belingwe greenstone belt zimbabwe provenance source area unroofing and submarine versus subaerial weathering shales of the 2 7 ga zeederbergs formation belingwe greenstone belt zimbabwe form thin 0 2 2 m horizons intercalated with submarine lava plain basalts shales of the overlying cheshire formation a foreland basin sedimentary sequence form 1 100 m thick units intercalated with shallow water carbonates and deep water resedimented basalt pebble conglomerates zeederbergs shale is characterized by high contents of mgo and transition metals and low concentrations of k2o and lile as compared to average phanerozoic shale indicative of an ultramafic to mafic source terrain cheshire shales have similar major and trace element contents but mgo and transition metals are less enriched and the lile are less depleted zeederbergs shales have smoothly fractionated ree patterns lan ybn 2 84 4 45 and no significant eu anomaly eu eu 0 93 0 96 ree patterns are identical to those of the surrounding basaltic rocks indicating local derivation from submarine reworking cheshire shales have rather flat ree patterns lan ybn 0 69 2 19 and a small negative eu anomaly average eu eu 0 85 indicative of a mafic provenance with minor contributions of felsic detritus a systematic change in ree patterns and concentrations of transition metals and hfse upwards in the sedimentary succession indicates erosion of progressively more lree depleted basalts and ultramafic volcanic rocks followed by unroofing of granitoid crust weathering indices confirm the submarine nature of zeederbergs shale whereas cheshire shale was derived from a source terrain subjected to intense chemical weathering

metasomatic origin of quartz pyroxene rock akilia greenland and implications for earth s earliest life a quartz pyroxene rock interpreted as a banded iron formation bif from the island of akilia southwest greenland contains 13 c depleted graphite that has been claimed as evidence for the oldest 3850 million years ago life on earth field relationships on akilia document multiple intense deformation events that have resulted in parallel transposition of early archean rocks and significant boudinage the tails of which commonly form the banding in the quartz pyroxene rock geochemical data possess distinct characteristics consistent with an ultramafic igneous not bif protolith for this lithology and the adjacent schists later metasomatic silica and iron introduction have merely resulted in a rock that superficially resembles a bif an ultramafic igneous origin invalidates claims that the carbon isotopic composition of graphite inclusions represents evidence for life at the time of crystallization

crustal growth through successive arc magmatism reconnaissance u pb shrimp data from the northeastern superior province canada abstract north of the la grande subprovince in northern quebec the superior province is dominated by plutonic rocks of the bienville subprovince and minto block which have been further subdivided into domains on the basis of reconnaissance geological mapping and geochronology and projected along strike using aeromagnetic anomalies to test the validity of the extrapolations u pb ages were obtained through shrimp analyses of zircon from 18 small archival samples selected from a suite of 2000 specimens collected during a helicopter reconnaissance survey conducted in the 1960s the results reveal a history of successive arc magmatism between 3 07 and 2 70 ga followed by metamorphism emplacement of crustally derived granites 2 702 2 685 ga and late hydrothermal events ca 2 65 ga in the northeastern minto block the douglas harbour domain previously extrapolated from dated localities to the northeast is substantiated through an age of 2 87 ga the older tonalitic crust is punctured by 2 734 2 725 ga granodioritic plutons of the leaf river suite that locally carry 3 01 ga inherited zircon the northern utsalik domain is made up of pyroxene and hornblende bearing leaf river plutons with consistent 2 725 2 723 ga ages and local inheritance of 2 82 and 2 77 ga however the southern extension of utsalik domain contains pyroxene bearing granodiorites both older 2 762 ga and younger 2 692 ga than the leaf river suite a tonalitic rock from the southern goudalie domain with an age of 2 833 ga supports a linkage to 2 84 2 83 ga units of the qalluviartuuq domain along strike to the north tikkerutuk domain represents a north trending 50 km wide calc alkaline magmatic arc with ages between 2 71 and 2 70 ga that extends southward into the bienville subprovince on the basis of aeromagnetic anomalies and geochronology zircon inheritance ages indicate that the tikkerutuk arc was built upon antecedent arcs of 2 84 2 77 and 2 725 ga the inukjuak domain in the west consists mainly of granite dated in one locality at 2 72 ga

applications of x ray computed tomography in the geosciences x ray computed tomography ct is a technique that allows non destructive imaging and quantification of internal features of objects it was originally developed as a medical imaging technique but it is now also becoming widely used for the study of materials in engineering and the geosciences x ray ct reveals differences in density and atomic composition and can therefore be used for the study of porosity the relative distribution of contrasting solid phases and the penetration of injected solutions as a non destructive technique it is ideally suited for monitoring of processes such as the movement of solutions and the behaviour of materials under compression because large numbers of parallel two dimensional cross sections can be obtained three dimensional representations of selected features can be created in this book various applications of x ray ct in the geosciences are illustrated by papers covering a wide range of disciplines including petrology soil science petroleum geology geomechanics and sedimentology

pre 3750 ma supracrustal rocks from the nuvvuagittuq supracrustal belt northern quebec geochemistry and u pb ion microprobe zircon geochronology guided by high resolution mapping 1 50 scale was used to define a minimum age of ca 3750 ma for supracrustal rocks from the nuvvuagittuq supracrustal belt nsb in the northern superior province quebec canada mineralogy and geochemistry of critical field relationships preserved at the porpoise cove locality describe a supracrustal succession of mafic amphibolites and ultramafic rocks finely banded quartz magnetite units with intermixed coarse grained ferruginous quartz pyroxene rocks and quartz biotite schists that superficially resemble polymict metaconglomerates with large up to 10 cm deformed polycrystalline quartz and mafic clasts all units in the mapped outcrop have sharp lithological contacts narrow dominantly trondhjemitic orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals the total extent of the supracrustal enclave is 8k m 2 it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions the quartz biotite schists record complex zircon growth at 3500 and 2800 ma interpreted to reflect metamorphic events zircons separated from orthogneisses in the enclave including one sheet that transects a banded quartz pyroxene magnetite unit yield magmatic 207 pb 206 pb ages of ca 3750 ma these ages are slightly younger than earlier provisional reports for an nsb orthogneiss from porpoise cove the nuvvuagittuq supracrustals are the oldest rocks thus far reported for the minto block they overlap in age with the ca 3 70 3 83 ga isua supracrustal belt and akilia association rocks in west greenland and they represent an important new area for exploration of the early earth

geology age and origin of supracrustal rocks at akilia west greenland new mapping geochronology and geochemistry of the supracrustal enclave and associated orthogneisses at akilia west greenland support a 3825 6 ma age for the sequence and a chemical sedimentary origin for the controversial fe rich quartz pyroxene gneisses lithologies of the enclave comprise laterally continuous mappable units of mafic amphibolite ultramafic rocks and two fe rich quartz pyroxene units a minor anthophyllite garnet rock with chemical characteristics suggesting a sedimentary protolith was identified the earliest foliation parallels lithologic contacts this foliation was isoclinally folded about a steep hinge surface and then tightly refolded about a steep ns hinge plane the latest folding produced limbs exhibiting contrasting strains the full deformation history is shared by all lithologies of the enclave two previously unidentified orthogneiss sheets preserve deformed magmatic crosscutting relations with amphibolite and ultramafic units in the low strain limb the supracrustal anthophyllite garnet rock experienced zircon growth at 3600 and 2700 ma in common with amphibolites these ages are interpreted to reflect metamorphism at granulite and amphibolite facies respectively consistent with independent regional evidence the crosscutting orthogneiss sheets preserve a complex history of zircon growth but only earliest cores indicate th u exchange equilibrium with the bulk rock at magmatic conditions weighted mean 207 pb 206 pb ages of cores of 3730 7 ma and 3825 6 ma date the crystallization ages of the sheets indicating that the akilia supracrustal rocks were deposited 3819 ma quartz pyroxene rocks on akilia have been interpreted as metamorphosed chemical sediments that may preserve carbon isotope evidence for life at the time of their formation an alternative proposal is an origin by metasomatic alteration our results show that these rocks experienced a history of deformation and zircon growth identical to other lithologies of the supracrustal body metamorphic zircons give a minimum 207 pb 206 pb age of 3589 13 ma regardless of origin the two models of formation of quartz pyroxene rocks were evaluated using field petrologic and geochemical tests the distribution of the units the absence of field or petrologic indicators of metasomatism and geochemistry oxygen and nontraditional stable isotopes immobile elements and ree together support a sedimentary but not a metasomatic origin because their protolith was sedimentary the quartz pyroxene rocks are a primary part of the supracrustal sequence and have the same minimum age despite strong deformation and polymetamorphism the akilia supracrustal enclave contains information about terrestrial surficial processes at 3819 ma

deep earth recycling in the hadean and constraints on surface tectonics the hadean mantle was efficiently heated from high internal heat production high rates of impact bombardment and primordial heat from accretion as a result a strong case is made for extremely high internal temperatures low internal viscosities and extremely vigorous mantle convection previous studies of mixing in such high rayleigh number convective environments indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100 myr however 142nd and 182w isotope studies indicate that heterogeneous mantle domains survived without mixing for over 2 gyr at odds with expected mixing rates similarly concentrations of platinum group elements in archean komatiites purportedly due to the later veneer of meteoritic addition on the earth only achieve current levels at 2 7 ga indicating a time lag of almost 1 to 2 gyr in mixing this material thoroughly into the mantle previous studies have sought to explain slow archean mantle mixing via mantle layering due to endothermic phase changes or anomalously viscous blobs of material with limited efficacy here we pursue another explanation for inefficient mantle mixing in the hadean tectonic regime a number of lines of evidence suggest that resurfacing in the archean was episodic and extending these models to hadean times implies the hadean was characterized by long periods of tectonic quiescence we explore mixing times in 3d spherical cap models of mantle convection which incorporate vertically stratified and temperature dependent viscosities we show that mixing in stagnant lid regimes is at the extreme over an order of magnitude less efficient than mobile lid mixing and for plausible rayleigh numbers and internal heat production the lag in hadean convective recycling can be explained the attractiveness of this model is that it not only explains the long lived 142nd and 182w anomalies but also posits an explanation for the delay between accretion of the late veneer between 4 5 to 3 8 ga on a stagnant surface and its fully mixed signature apparent in elevated pges in 2 7 ga komatiites it also provides an explanation for the 400 myrs of immobility of the mafic protolith from which the jack hill zircons were sourced and retards early heat loss from the mantle providing a solution to the archean thermal catastrophe of parameterized earth evolution models

a yilgarn seed to the pilbara craton australia evidence from inherited zircons knowledge of the age and compositional architecture of archean cratonic lithosphere is critical for models of geodynamics and continental growth on early earth but can be difficult to unravel from

chemical sedimentary protoliths in the 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract analyses of chemical sedimentary precipitates such as banded iron formation bif provide a direct means to explore the nature and composition of the early hydrosphere the recently discovered 3750 myr old nuvvuagittuq supracrustal belt nsb in the northeast superior province quebec canada hosts a suite of iron oxide rich pyroxene and amphibole units that are interpreted to be the metamorphosed equivalents of fe oxide facies bif and a collection of bif like ca fe mg silicate rocks the nsb rocks provide a rare glimpse of trace metal availability in eoarchean ca 3800 ma seawater as they may be contemporaneous with the relatively well studied isua supracrustal belt of southern west greenland their comparison provides an opportunity to enhance our basic understanding of the eoarchean oceans at a global scale work since the initial discovery of the nsb in 2001 has established the basic lithological geochemical and petrographic characteristics of these bif and bif like rocks here we review the current state of knowledge of nsb rocks of probable chemical sedimentary origin including aspects of their geology likely origin and age we conclude by examining the implications of results thus far for our understanding of early seawater compositions and for the emergence of life in the context of early metallo enzyme evolution

geology of the eoarchean 3 95 ga nulliak supracrustal rocks in the saglek block northern labrador canada the oldest geological evidence for plate tectonics the earth is a unique planet which has been highly evolved diversified and complicated through geologic time and underwent many key events including giant impact magma ocean core formation large scale mantle differentiation and late heavy bombardment especially in its dawn but our knowledge of early earth is limited due to the lack of the hadean supracrustal rocks the supracrustal rocks with the eoarchean ages provide key evidence for the earth s early evolution but few supracrustal rocks have been comprehensively investigated therefore we mapped in seven areas of the saglek block northern labrador where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses early studies suggested that some of them have the mesoarchean ages because of the lack of the mesoarchean saglek dyke but we found the saglek dykes in the areas to recognize the eoarchean nulliak supracrustal rocks and uivak gneiss in all the areas recent reassessment of u pb dating and cathodoluminescence observation of zircons from the oldest suites of the uivak gneiss showed that the uivak gneiss has the eoarchean age 3 95 ga and forms the iqaluk uivak gneiss series because our geological survey clearly showed that the iqaluk uivak gneisses were intruded into the nulliak supracrustal belts the nulliak supracrustal rocks are the oldest supracrustal rock in the world the supracrustal belts consist of piles of fault bounded blocks which are composed of the ultramafic rocks mafic rocks and sedimentary rocks in ascending order similar to modern ocean plate stratigraphy ops in addition small scale duplex structures are found over the areas the presence of duplex structure and ops indicates that the 3 95 ga nulliak supracrustal belts originate from an accretionary complex the presence of the accretionary complex ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early earth

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

the role of microbialitic facies in the micro and nano pore system of dolomitized carbonate platforms upper triassic southern italy abstract the pore system of two dolomitized triassic carbonate platforms named lower and upper unit and cropping out in southern italy has been investigated through mercury porosimetry and electron microscopy despite their variability all facies show micro to nano pores systems composed of intra and inter crystal pore types this is due to the prevalence of microbialites in the inner platform margin settings together with an homogenous diagenetic history porosity and permeability are generally very low 60 the porosity derived from nanopores is significant in all facies and particularly in the shallow water environments a positive correlation between nanopores volume and porosity is present in the whole upper unit and in the shallow water facies of the lower unit moreover different sub environments of the platforms show separated amount of nanopores and porosity on the contrary permeability does not seem to be influenced by the nanopores distribution regardless a wide spectrum of fabric and morphologies microbialitic facies of both units show a confined range of porosity average 1 0 standard deviation 0 6 and permeability average 0 4 standard deviation 0 2 most probably because composed of syn sedimentary microbial dolomite which presents a high resistance to compaction during the burial history finally despite the complete dolomitization porosity and permeability ratio varies in function of the main depositional environments and less of the precursor sedimentary facies suggesting a control of the final pore size distribution mainly linked with the presence absence of the microbial primary dolomitic facies instead of the native fabrics of the other sedimentary facies

uranium in iron formations and the rise of atmospheric oxygen abstract the concept of the great oxidation event goe during which atmospheric oxygen rose precipitously and perhaps to near modern levels around 2 4 2 1 billion years ago ga has become entrenched in our views on secular atmospheric evolution multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the archean proterozoic boundary however it remains unclear precisely when this transition occurred due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures here we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium u concentrations in iron formations if just as if are important archives of the redox state of seawater concentrations of redox sensitive u in if are faithful proxies for oxidative continental weathering and associated delivery of dissolved u to seawater our dataset suggests that there was an increase in u redox cycling and transport at ca 2 47 ga just before the permanent loss of mass independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the goe further there is significant temporal variability in the if u record that we propose reflects dynamic precambrian redox conditions we provide additional support for earlier suggestions that the goe was a protracted event marked by vacillating oxygen levels

lithifying microbial mats in lagoa vermelha brazil modern precambrian relics abstract bacterial populations microbial and mineral forming processes and their products were analyzed in lagoa vermelha brazil the microbial mat and underlying sediment were studied as a unique system to define the boundary conditions responsible for high mg calcite and dolomite formation in the uppermost layers of the microbial mat oxygenic photosynthesis and aerobic respiration resulted in calcite precipitation whereas in the underlying anoxic layers of the mat sulfide oxidation and sulfate reduction induced formation of a range of carbonate minerals with increasing mg concentrations the chemical mineralogical and biological conditions presently found in lagoa vermelha may have been more common in the precambrian the microorganisms performing the metabolic processes related to carbonate mineral formation within lagoa vermelha s hypersaline microbial mat may have already been present in the precambrian thus microbial carbonate as a biomineral could be a record of metabolism throughout geological time

application of probe permeametry to the prediction of two phase flow performance in laminated sandstones lower brent group north sea abstract the rannoch formation lower brent group north sea is an important laminated oil bearing reservoir in the northern north sea recent work has shown that at the estimated field frontal advance rates for the rannoch formation capillary effects in laminated sandstones might significantly affect the two phase oil and water flow characteristics previous published reservoir simulation models of the rannoch have not accounted for this newly acquired probe permeameter measurements have been used to map the fine scale permeability structure and develop geologically reasonable effective properties permeability and capillary pressure data were combined for geologically meaningful groups of laminae to define dynamic pseudo properties absolute permeability relative permeability and capillary pressure by numerical simulation a geological model for packages of lamination was then used to combine these lamina groups at a scale of the grid block the flow performance of this geologically reasonable grid block containing the laminated structure is compared with that of simple models based on conventional procedure i e the arithmetic average in a horizontal direction or harmonic average vertical permeability and rock curves at this scale the formation is highly anisotropic using the more detailed model with quite different flow characteristics in the horizontal and vertical directions the simple homogeneous models show more isotropic recovery characteristics and give significantly higher recoveries for vertical displacements from these findings it is suggested that previous models of the rannoch formation should be modified to include the capillary effects associated with lamination the procedure outlined here is referred to as the geopseudo approach the approach is widely applicable to laminated reservoirs

eoarchean to neoarchean evolution of the nuvvuagittuq supracrustal belt new insights from u pb zircon geochronology the nuvvuagittuq supracrustal belt in northern quebec is a rare 9 km2 fragment of the earth9s early crust the belt contains a metamorphosed volcano sedimentary sequence that is at least eoarchean in age including amphibolites of the ujaraaluk unit that may have protoliths as ancient as 4400 million years old ma o9neil and others 2012 upper amphibolite facies metamorphism and high strain deformation have obscured many primary field relationships and disturbed whole rock isotope systematics leading to debate over the interpretation of previous geochronological data we report new shrimp u pb isotopic analyses of zircons from key meta igneous and newly identified metasedimentary units of the belt the analyzed samples fall into four categories a felsic gneisses including sheets interlayered with the supracrustal assemblage and a tonalitic orthogneiss at the margin of the belt b a newly identified metasedimentary unit c fuchsite bearing quartz rich layers d mafic gneisses including a garnet biotite amphibolite from the ujaraaluk unit and a meta gabbro the felsic lithologies place constraints on the minimum age of the supracrustal assemblage oscillatory zoned zircons separated from a felsic orthogneiss sheet form a discordant array with an upper intercept age of 3774 32 ma and the tonalitic orthogneiss yielded oscillatory zoned zircons with an upper intercept age of 3781 11 ma a metasedimentary unit dominated by quartz albite and clinozoisite has been identified within a lower strain domain interfolded with amphibolites of the ujaraaluk unit a sample of this unit yielded zircons with highly variable cl structures and near concordant ages of between 3390 and 3780 ma zircons from a fuchsite bearing quartz layer have oscillatory zonation similar to the orthogneiss grains and define an upper intercept age of 3794 16 ma petrologic observations and geochemical data suggest that this lithology is a metasomatically altered felsic orthogneiss band rather than a meta sedimentary rock belonging to the supracrustal package electron microscopy of polished thin sections was used to locate zircons within 6 samples of the ujaraaluk unit amphibolites and two samples of meta gabbro large 500 m zircons separated from a garnet biotite amphibolite together with grains from a meta gabbro are sector zoned and yield ages of 2700 to 2640 ma recording the timing of pervasive high temperature metamorphism in the region the new zircon u pb data add important new detail to the geological history of the nuvvuagittuq belt the volcano sedimentary sequence and mafic ultramafic intrusive rocks were formed prior to 3800 million years ago subsequent geological evolution included extensive and protracted early archean ttg magmatism and crustal re melting paleo to mesoarchean uplift and sedimentation and neoarchean high grade metamorphism and regional magmatism during consolidation of the superior craton

representation of multiscale heterogeneity via multiscale pore networks 1 developing a better understanding of single multiphase flow through reservoir rocks largely relies on characterizing and modeling the pore system for simple homogeneous rock materials a complete description of the real pore structure can be obtained from the pore network extracted from a rock image at a single resolution and then an accurate prediction of fluid flow properties can be achieved by using network model however for complex rocks e g carbonates heterogeneous sandstones deformed rocks a comprehensive description of the real pore structure may involve several decades of length scales e g from submicron to centimeters which cannot be captured by a single resolution image due to the restriction of image size and resolution hence the reconstruction of a single 3 d multiple scale model of a porous medium is an important step in quantitatively characterizing such heterogeneous rocks and predicting their multiphase flow properties in this paper we present a novel methodology for the numerical construction of the multiscale pore structure of a complex rock from a number of ct images models of a carbonate sample at several length scales the success of this reconstruction relies heavily on image segmentation pore network extraction and stochastic network generation which are provided by our existing software system referred to as pore analysis tools pat specifically the statistical description of pore networks of 3 d rock images at multiple resolutions makes it possible for us to a construct an arbitrary sized network which is equivalent in a specified domain and b integrate multiple networks of different sizes into a single network incorporating all scales using multiscale networks of carbonate rocks generated in this manner two phase network modeling results are presented to show how the resulting flow properties are dependent on inclusion of information from multiple scales these outcomes reinforce the importance of capturing both geometry and topology in the hierarchical pore structure for such complex pore systems the example presented reveals that isolated large scale e g macro pores are mainly connected by small scale e g micro pores which in turn determines the combined effective petrophysical properties capillary pressure absolute and relative permeability it is also demonstrated that multi three scale networks reveal the effects of the interacting multiscale pore systems e g micropores macropores and vugs on bulk flow properties in terms of two phase flow properties

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

isotopic characteristics of subduction fluids in an intra oceanic setting izu bonin arc japan new radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active izu bonin arc the oligocene izu arc and their associated rift basins as with many intra oceanic island arcs the pliocene recent izu bonin frontal arc lavas are highly depleted in zr nb and the rare earth elements relative to typical mid ocean ridge basalt morb indicating that the mantle wedge source has undergone a previous episode of melting ratios between these elements such as nb zr and la sm as well as 143nd 144nd do not vary significantly along the length of the frontal arc these parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge however ba zr ba rb and 87sr 86sr increase along the frontal arc to the north this leads us to propose that a variable enrichment in ba and radiogenic sr is superimposed on the mantle wedge sr nd and pb nd isotope variation indicate that both sr and pb become more radiogenic after fluid addition however pb isotope ratios do not correlate with increases in pb concentration or ratios such as ba zr and nb pb in other words the pb isotopic composition of the arc lavas appears to be independent of the amount of pb introduced by subduction fluids into the mantle source this buffering of pb isotopes along the frontal arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities despite this we are able to determine that the mantle wedge has isotopic characteristics similar to indian ocean morb and that the subduction fluid solute is primarily derived from subducted oceanic basalt with a 2 contribution from subducted sediment lavas in the oligocene izu arc and fore arc basin were derived from a mantle wedge of similar composition to the active arc despite levels of pb enrichment comparable to those of the modern arc the pb isotopes of the oligocene volcanics indicate a lower sediment input into the melting region

vestiges of life in the oldest greenland rocks a review of early archean geology in the godthabsfjord region and reappraisal of field evidence for 3850 ma life on akilia abstract the godthabsfjord region of west greenland contains the most extensive best exposed and most intensely studied early archean rocks on earth a geological record has been described of numerous magmatic events between 3 9 and 3 6 ga and evidence of life at 3 85 ga and 3 8 3 7 ga has been proposed from two widely separated localities some of these claims have recently been questioned and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the isua greenstone belt are being reinvestigated and substantially reinterpreted the first part of this article reviews the evolution of geological research and interpretations outlining the techniques by which the geological history has been determined and the ensuing controversies the second part re examines crucial field evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of akilia

inherited 142nd anomalies in eoarchean protoliths abstract geological records of the earliest history of the earth are rare rocks older than 3700 ma comprise only a few percent of continental surfaces evidence is mounting however that vestiges of primordial planetary differentiation continued to influence the compositions of the oldest rocks during the hadean and into the archean here we report new whole rock 147 146sm 143 142nd data for the ancient nuvvuagittuq supracrustal belt nsb in quebec canada and confirm the 142nd deficits reported by o neil et al 2008 we show that the assigned o neil et al 2008 and recently revised kinoshita et al 2012 142nd age of 4362 54 35 ma claimed for nsb amphibolites is at odds with the younger 147sm 143nd record this discrepancy can be reconciled by partial nd isotope equilibration of rocks with hadean model ages of up to 4500 ma during magmatic and metamorphic perturbations associated with the emplacement of the nsb at ca 3750 ma cates and mojzsis 2009 our model further predicts a whole rock 147sm 143nd age of 3800 ma for other nsb lithologies in agreement with u pb zircon chronology cates and mojzsis 2007 hence 146sm 142nd systematics for the eoarchean nsb rocks represent inheritance of a hadean signature that was stored either in pre existing crust or in early enriched mantle sources the decoupled 147 146sm 143 142nd systematics of the nsb is similar but complementary to the hadean mantle isochron preserved in eoarchean rocks from west greenland bennett et al 2007 rizo et al 2011

upscaling of elastic properties in carbonates a modeling approach based on a multiscale geophysical data set linking ultrasonic measurements made on samples with sonic logs and seismic subsurface data is a key challenge for the understanding of carbonate reservoirs to deal with this problem we investigate the elastic properties of dry lacustrine carbonates at one study site we perform a seismic refraction survey 100 hz as well as sonic 54 khz and ultrasonic 250 khz measurements directly on outcrop and ultrasonic measurements on samples 500 khz by comparing the median of each data set we show that the p wave velocity decreases from laboratory to seismic scale nevertheless the median of the sonic measurements acquired on outcrop surfaces seems to fit with the seismic data meaning that sonic acquisition may be representative of seismic scale to explain the variations due to upscaling we relate the concept of representative elementary volume with the wavelength of each scale of study indeed with upscaling the wavelength varies from millimetric to pluri metric this change of scale allows us to conclude that the behavior of p wave velocity is due to different geological features matrix porosity cracks and fractures related to the different wavelengths used based on effective medium theory we quantify the pore aspect ratio at sample scale and the crack fracture density at outcrop and seismic scales using a multiscale representative elementary volume concept results show that the matrix porosity that controls the ultrasonic p wave velocities is progressively lost with upscaling implying that crack and fracture porosity impacts sonic and seismic p wave velocities a result of paramount importance for seismic interpretation based on deterministic approaches

highly siderophile element abundances in eoarchean komatiite and basalt protoliths plume derived mg rich volcanic rocks komatiites high mg basalts and their metamorphic equivalents can record secular changes in the highly siderophile element hse abundances of mantle sources an apparent secular time dependent enrichment trend in hse abundances from paleoarchean to paleoproterozoic mantle derived rocks could represent the protracted homogenization of a late veneer chondritic contaminant into the pre late veneer komatiite source to search for a possible time dependence of a late accretion signature in the eoarchean mantle we report new data from rare 3700 myr old mafic and ultramafic schists locked in supracrustal belts from the inukjuak domain quebec canada and the akilia association west greenland our analysis shows that some of these experienced hse mobility and or include a cumulate component touboul et al in chem geol 383 63 75 2014 whereas several of the oldest samples show some of the most depleted hse abundances measured for rocks of this composition we consider these new data for the oldest documented rocks of komatiite protolith in light of the late veneer hypothesis

the composition of earth s oldest iron formations the nuvvuagittuq supracrustal belt quebec canada abstract the composition of iron formations in the 3 75 ga yr old nuvvuagittuq supracrustal belt in northern quebec provides a proxy for seawater composition of the eoarchean and perhaps hadean oceans as well as constraints on the types of nutrients available to earth s earliest life forms integrated petrologic and geochemical relationships mapped between mineral phases in thin section and whole rock chemistry provide a framework for interpreting bulk and micro scale variations in these chemical sedimentary precipitates results show that there are two distinct chemical sedimentary units in the nuvvuagittuq belt i a banded iron formation bif consisting of alternating micro bands of magnetite ca mg fe silicates and quartz and ii a more silicate rich fe poor unit the banded silicate formation bsf of alternating micro bands of quartz and ca mg fe silicates precursor bif and bsf deposits were likely layered amorphous silica and ferric oxyhydroxides fine grained carbonate oozes and or ca mg fe rich silicate gels deposited in a marine setting low al2o3 tio2 and hfse concentrations show that they are relatively detritus free with distinctively seawater like ree y profiles and consistently positive eu anomalies these features suggest that the rocks preserved their seawater like compositions despite metamorphic overprinting the most significant trace elements in the sediments are ni and zn experimentally derived partitioning coefficients show that ni was enriched in eoarchean seawater as compared to today up to 300 nm while zn was fairly similar up to 20 nm compositional resemblances between the nuvvuagittuq sediments and those documented in the ca 3 8 ga isua supracrustals west greenland provide a plausible case that global ocean processes in terms of trace metal abundances had reached steady state by the eoarchean