partition coefficients for calcic plagioclase implications for archean anorthosites by the use of a multiple aliquot technique combined with statistical calculations to overcome effects of alteration and contamination we have determined partition coefficients d s for rare earth elements rees between natural high ca plagioclase megacrysts and their basaltic matrices some of the basalts that we investigated are shown experimentally to crystallize only high ca plagioclase over approximately a 20 30 c temperature interval below the liquidus the ree d s agree well with experimentally determined values and predict equilibrium melts for archean anorthosites that agree well with coeval basaltic flows and dikes furthermore the d s appear to be valid for both tholeiitic and alkali basalts over a range of mg numbers 34 58 and ree concentrations la from 3 50 ppm similarly calculated d s for co sc cr rb sr cs ba hf ta and th also predict equilibrium melts for anorthosites that agree reasonably well with coeval basaltic flows and dikes the apparent values for co sc and ta seem good to better than 50 whereas the standard deviations for rb and th are equal to their means making the d s of questionable value the others are useful probably to within a factor of 2 experimental studies of the d for total fe as feot indicate a value of about 04 at an oxygen fugacity slightly below that of the qfm buffer a by product of the feot experiments was the experimental d s for mgo tio2 and k2o which could be compared with the d s from natural assemblages from the d s that were determined we suggest that plagioclase megacrysts in the bad vermilion lake anorthosite were in equilibrium with melts having the composition of tholeiitic basalts at an oxygen fugacity slightly below the qfm buffer the predicted melts have an feot content of about 13 k2o of about 15 tio2 of about 1 3 a chondrite normalized ree pattern that is nearly flat at 10 to 20 chondrites with a slight depletion of lrees other incompatible trace elements at 10 to 20 chondrites and cr and co at 0 01 to 0 1 chondrites this range of compositions is essentially identical to the tholeiitic flows and dikes in archean greenstone belts we therefore suggest that the moderately fe rich tholeiites that are hosts to plagioclase megacrysts in greenstone belts form the parental melts for megacrysts which make up the bad vermilion lake anorthositic complex and probably other similar archean anorthositic complexes

a deep subaqueous fan depositional model for the palaeoarchaean 3 46 ga marble bar cherts warrawoona group western australia the 3 46 ga marble bar chert member of the east pilbara craton western australia is one of the earliest and best preserved sedimentary successions on earth here we interpret the finely laminated thin bedded cherts mixed conglomeratic beds chert breccia beds and chert folded beds of the marble bar chert member as the product of low density turbidity currents high density turbidity currents mass transport complexes and slumps respectively integrated into a channel levee depositional model the marble bar chert member constitutes the oldest documented deep sea fan on earth with thin bedded cherts breccia beds and slumps composing the outer levee facies tracts and scours and conglomeratic beds representing the channel systems

on the true antiquity of eoarchean chemofossils assessing the claim for earth s oldest biogenic graphite in the saglek block of labrador abstract a recent claim to have found traces of earth s earliest life 3 95 ga utilising isotopically light carbon in graphite bearing metapelites from the saglek block of northern labrador canada is re evaluated applying rigorous geological and geochronological criteria the establishment of these criteria in previous evaluations of early life claims from southern west greenland and northern canada is reviewed in order to provide a backdrop to discussion of the saglek claim in particular we emphasise the importance of the scale of lithological continuity in determining the veracity of such claims which are considerably easier to demonstrate from large relatively less tectonised supracrustal remnants like the isua greenstone belt than they are from smaller isolated enclaves of the kind found on akilia or the highly tectonised and imbricated unit that is found in the saglek block unambiguous field relationships between ca 3 9 ga tonalitic gneiss and the graphite bearing metasediments have not been demonstrated in the literature that the saglek claim relies upon and earlier u pb hf isotopic studies on zircon from metasediments at one of the localities used in the claim indicate a mesoarchean to neoarchean time of deposition we conclude that irrespective of the validity of the carbon isotopic evidence field relationships and geochronological evidence fail to demonstrate an age of 3 95 ga for the potential traces of life

inter and intra group compositional variations in apollo 15 pyroclastic green glass an electron and ion microprobe study abstract representative samples of the five compositionally distinct groups a e of apollo 15 pyroclastic green glass were systematically analyzed for major and trace element abundances by electron and ion microprobe techniques these primary magma compositions define a high co trend groups a and d and a low co trend groups b and c on sr zr ba and nd versus co variation diagrams group e glasses are significantly enriched in sr relative to the glass groups of the two trends pearce element ratio analyses and sr ca ba ca systematics indicate that olivine orthopyroxene and clinopyroxene controlled green glass compositions simple crystal liquid igneous fractionation processes involving such an assemblage however cannot explain the observed inter and intra group trace element variation trends the magmas represented by the high and low co trends are interpreted to have been erupted from two distinct but compositionally variable source regions a source mixing model involving differentiated cumulate trapped residual liquid and extremely fe rich components can successfully account for the compositional ranges of the two sources required to produce the observed chemical variations upon partial melting group e glasses appear to represent a magma erupted from a source with a more limited chemical variability the apollo 15 green glasses represent multiple eruptive events from three chemically distinct but compositionally variable source regions assuming that this mixing model is correct

mineralogical records of early planetary processes on the howardite eucrite diogenite parent body with reference to vesta abstract mineralogical information recovered from the howardite eucrite diogenite hed meteorites was employed to reconstruct the history of the parent body and relate it to 4 vesta these interpreted crustal evolution processes were then compared to the expected geological features on the surface of a likely proto planet 4 vesta the original crustal materials of the hed parent body were preserved as mineral grains and lithic clasts but in many eucrites fe mg ratios in pyroxenes were homogenized by diffusion after crystallization the crystallization trend of the protocrust has been deciphered by 1 examining monomict and crystalline samples and using their mineralogical and chemical information to formulate a sequence of crystallization and cooling trends and by 2 reconstructing the original crust prior to cratering events from lithic clasts and mineral fragments in polymict breccias such as howardites and polymict eucrites mineral components are identical both in the individual hed and in polymict breccias and no remnants of primitive materials were preserved in the polymict breccias a layered crust model reconstructed from such breccias consists of an upper crust with extrusive lava like eucrites that have been brecciated and metamorphosed diogenite mantle and cumulate eucrites of varying thickness between them this model can be used to explain the surface geological features of vesta observed from the hubble space telescope a large crater with diogenitic orthopyroxene at the crater floor is consistent with the deepest diogenitic layer of the layered crust model and an underlying olivine layer is expected from early crystallized olivine in the crystal fractionation model the old terrain of eucritic surface materials of vesta can be howardites polymict eucrites or regolith like eucrites produced from eucrites extruded and impacted on the surface partial melting models of eucrites seem to be favored by the rare earth element ree chemistry and experimental studies unfortunately partial melting models have not demonstrated how the hed parent body is converted to a layered crust without producing any metamorphosed primitive material in the layered crust the origin of cumulate eucrites with systematic variation of textures and chemistries of pyroxene can be explained by the layered crust model with excavation and mixing of trapped liquid discovery of basaltic materials with na rich plagioclase and augite in iron meteorites which are the products of partial melting suggests that eucrites may be unique to a body that underwent large scale differentiation and metamorphism

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

depositional setting and paleogeographic implications of earth s oldest supracrustal rocks the 3 7 ga isua greenstone belt west greenland abstract new structural and stratigraphic mapping in the isua greenstone belt west greenland has revealed that the exposed lithologic succession is quite different to that depicted in earlier studies the key to a better understanding of the stratigraphy has been the recognition of intense strain and metasomatic effects combined with ductile fault structures that segment the belt into a number of tectonic slices in some of these slices deformation and metasomatism are somewhat lower than in surrounding slices though still significant which permit the recognition of primary depositional features that may be used to compare with lithologies from elsewhere in the belt the belt is dominated by amphibolite that in a number of places show well defined pillows pillow breccias and basaltic debris flows also occur within this package strongly recrystallized ultramafic bodies that occur in the belt are interpreted as intrusions or komatiitic flows the most common sedimentary rock type is chert banded iron formation these lithologies have been strongly affected by brittle and ductile deformation in combination with coarse recrystallization siliciclastic detrital rocks such as conglomerate and sandstone are much less common in the belt and where present have intrabasinally derived sources highly deformed quartzo feldspathic schist crops out in a number of places in the belt and most likely has multiple origins previously these schists have been considered to be felsic volcaniclastic rocks though recent mapping and geochemical and isotopic studies support the hypothesis that at least some of these schists are highy deformed and carbonated tonalitic gneiss sheets or replaced pillow lava successions carbonate rocks in the belt are now considered to be mostly or entirely replacement in origin where metasomatizing fluids have particularly utilized amphibolite chert contacts additionally some carbonate may represent the products of early sea floor alteration we envision a temporarily emergent below wave base sea floor origin for the succession based on the assemblage of lithologies

detrital rutile ages can deduce the tectonic setting of sedimentary basins abstract sedimentary sequences contain a detailed record of the evolution of the earth s surface detrital mineral grains provide for example evidence of source redox conditions insights into the age geochemistry and petrology of basement sources and can also be used to elucidate tectonic environments detrital zircon has been used as a means of analysing the tectonic setting of host sedimentary successions but with potentially ambiguous results it is important to find additional ways to discriminate depositional settings particularly in precambrian sequences where other proxies are either not available or have been lost in this contribution we provide a new way to discriminate between different sedimentary tectonic environments using the mineral rutile we present a large compilation of detrital rutile data to show that the u pb age distribution is sensitive to the tectonic setting of the basin in which the host sediments were deposited we then apply this new approach to two case studies where the depositional setting and age are well constrained siliciclastic units of nw scotland which were deposited in the neoproterozoic foreland of the grenville orogen and on the cambrian passive margin of the iapetus ocean and the brazilian sabara basin located within the palaeoproterozoic foreland of the minas orogen we compare the detrital rutile and zircon age distributions of these successions showing that in some cases they are different and that rutile is most sensitive to the youngest metamorphic events affecting the sources reinforcing the applicability of this tool by testing this new method on polyphasally deformed successions sabara basin we show that detrital rutile can still inform the tectonic setting even at medium grades of metamorphism

stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt southern west greenland the highly deformed c 3800 ma isua supracrustal belt is a fragment of a more extensive early archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic amftsoq gneisses in the period 3800 3600 ma the supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 ma in the late archaean and locally at c 1800 ma layered sequences of rock of sedimentary and probable volcanic origin form over 50 of the belt bodies of high mg al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of amitsoq gneisses the layered rocks which are 1 km thick are divided into two sequences that are in faulted contact with each other the way up of these sequences has been determined from facing directions of locally preserved graded layering in felsic metasediments at several localities the overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments clastic sedimentation took place against a background of chemical sedimentation shown by interlayers of banded iron formation metachert and calc silicate rocks throughout the sequences the felsic rocks locally preserve graded bedding and possible conglomerate structures indicating deposition from turbidite flows and possibly as debris flows nodules in the felsic rocks contain structures interpreted as fiamme there is an irregular enrichment in k2o na2o in many of the felsic rocks at constant sio2 and al2o3 content interpreted as owing to alteration of original andesitic to dacitic volcanic rocks banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking possibly under shallow water conditions lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source

archaean flow top alteration zones formed initially in a low temperature sulphate rich environment the 3 500 3 400 myr onverwacht group in the southern part of the barberton greenstone belt south africa includes up to 12 km of komatiitic and tholeiitic lavas containing minor sedimentary and felsic volcanic units1 komatiitic lavas showing spinifex textures chrome spinels and high ni and cr contents are common throughout the sequence2 the rocks have been subject to polyphase archaean deformation and low grade metasomatism2 4 at several positions within the onverwacht group fig 1 the tops of komatiitic lavas underlying chert beds are marked regionally by 1 50 m thick zones of silicification carbonatization and k2o enrichment fig 2 because these zones have been variously interpreted to represent the felsic tops of mafic to felsic volcanic cycles weathering horizons and shear zones their origin is germane to any interpretation of the stratigraphy structure and evolution of the barberton belt here we present evidence that they formed inlarge part by low temperature near surface alteration of komatiitic lavas during intervals of volcanic quiescence they do not mark fault zones and cannot be used to subdivide the greenstone belt sequence into a series of fault bounded structural slices

reassessing the evidence for the earliest traces of life the isotopic composition of graphite is commonly used as a biomarker in the oldest 3 5 gyr ago highly metamorphosed terrestrial rocks earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3 8 gyr old isua supracrustal belt isb in southern west greenland have suggested the presence of a vast microbial ecosystem in the early archean this interpretation however has to be approached with extreme care here we show that graphite occurs abundantly in secondary carbonate veins in the isb that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks metasomatism during these reactions graphite forms from the disproportionation of fe ii bearing carbonates at high temperature these metasomatic rocks which clearly lack biological relevance were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life the new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed early archaean rock record

microscale heterogeneity of fe isotopes in 3 71 ga banded iron formation from the isua greenstone belt southwest greenland we present fe isotope compositions for magnetite crystals from 3 7 ga banded iron formation bif from the isua greenstone belt and younger spatially related pyrite obtained in situ by secondary ion mass spectrometry sims across a distance of several millimeters individual magnetite crystals of bif show 56 fe values up to 2 and within single magnetite rich layers there is a 2 range in 56 fe the highest positive 56 fe values are consistent with those predicted by the equilibrium fractionation factor for oxidation of ferrous to ferric fe in aqueous solution at a likely early archean ocean temperature of 70 c and further suggest that subsequent precipitation of ferric oxides was sufficiently slow that kinetic fractionation effects were minimal these high values also imply that the supply of oxidant was limited heterogeneity in 56 fe values is attributed to diagenetic reactions occurring in limited volume pore waters isolated from the bulk ocean 56 fe 0 with development of isotope reservoir effects secondary pyrite in bif and a conglomerate also show a distinct enrichment in heavy fe isotopes interpreted as redistribution and or fractionation during postformational events

early life signatures in sulfur and carbon isotopes from isua barberton wabigoon steep rock and belingwe greenstone belts 3 8 to 2 7 ga

cristobalite inclusions in the tatahouine achondrite implications for shock conditions the mineralogy of the tatahouine diogenite was investigated by optical microscopy raman micro spectrometry and scanning and transmission electron microscopies inclusions of alpha cristobalite in orthopyroxenes locally in symplectic association with chromites or associated with metal have been characterized for the first time in a diogenite mosaicism of the orthopyroxenes indicates shock effects in the meteorite the shock history of the meteorite must be consistent with the presence of vein like structures containing inclusions of well crystallized cristobalite it low pressure high temperature phase several possible mechanisms to account for these observations are discussed the simplest one consistent with all observations is that a shock event would have occurred in it hot orthopyroxenite either before extensive cooling of the asteroid or in materials heated by previous impacts and maintained hot under an ejecta blanket

deformation features and critical field relationships of early archaean rocks akilia southwest greenland abstract lithologically varied early archaean rocks on the island of akilia godthabsfjord southwest greenland include a quartz pyroxene unit that has been proposed as a bif carbon isotope data from graphite particles in apatites within this unit have further been proposed to represent evidence for earth s earliest life we present field observations which demonstrate that all lithologies on sw akilia have been intensely deformed during both early and late archaean high grade metamorphism in our view this deformation precludes unambiguous demonstration of purported sedimentary features in the quartz pyroxene rock a unit which we interpret as the result of metasomatic metamorphic and tectonic reworking of a mafic ultramafic protolith such an origin is completely incompatible with the quartz pyroxene rock hosting any signs of early biogenic activity we also reappraise contact relationships between amitsoq gneisses and mafic ultramafic rocks and can find no convincing evidence for the existence of intrusive relationships that have been central to geochronological interpretations on akilia

stratigraphy and sedimentology of the paleoproterozoic thluicho lake group southwestern rae province canada alluvial basin development in the hinterland of the taltson orogen abstract the thluicho lake group is a middle paleoproterozoic succession of greenschist facies continental clastic rocks that was deposited on variably mylonitized crystalline basement rocks in the southwestern rae province of the canadian shield near the end of 2 02 1 92 ga thelon taltson orogenesis two informal formations are proposed for this fining upward succession the powder lake formation comprises predominantly coarse grained rocks including conglomerate gulo lake member pebbly sandstone camel lake member and sandstone wellington lake member whereas the overlying camsell portage formation consists of sandstone to siltstone falls member rhythmic sandstone and argillite waterloo lake member and argillite slate island member the two informal formations represent two principal facies associations a lower conglomerate sandstone facies association representing an alluvial fan to distal braided plain that underwent episodic flash flooding events and an upper sandstone siltstone muds

setting and origin for problematic rocks from the 3 7 ga isua greenstone belt southern west greenland earth s oldest coarse clastic sediments abstract whether or not coarse detrital sedimentary rocks occur within the 3 7 ga isua greenstone belt igb southern west greenland has been debated for some time repeated regional metamorphic deformational and metasomatic events have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environments of deposition rocks here interpreted as meta conglomerate crop out in a fault bounded structural domain that is lower in strain relative to adjacent domains the meta conglomerate has a strike length of 1 km and is 10 m thick bed thickness ranges from 10 cm to more than 1 m and beds may be either framework or matrix supported a poorly sorted and variably rounded polymict assemblage of framework clasts consisting of meta chert bif and a variety of mafic volcanic rock fragments are set into a matrix of biotite quartz garnet schist clast compositions indicate reworking of adjacent stratigraphic units a lack of structural similarity in framework clasts range of grain sizes range of rounding and polymict composition demand a primary sedimentary origin for the deposit inferred depositional processes include traction and debris flow which would be consistent with subaerial or shallow subaqueous environments although the limited extent of the meta conglomerate warrants interpretational caution

the genesis of diogenites and hed parent body petrogenesis abstract samples of clasts and whole rocks from eighteen of the twenty four known diogenites orthopyroxenite breccia meteorites have been studied by combined inaa empa in order to elucidate their petrogenetic history the incompatible elements a12o3 and tio2 are positively correlated in diogenite orthopyroxenes these elements determined by empa are also positively correlated with incompatible trace elements such as yb determined by inaa on bulk samples these correlations indicate that most diogenites may form a single igneous fractionation sequence the incompatible minor and trace elements are decoupled from the major elements for essentially constant mg tio2 varies by about 4 times and yb by about 8 times hence there is no strong evidence for igneous fractionation preserved in the major element composition of orthopyroxenes geothermometry calculations using two pyroxene and spinelorthopyroxene thermometers show that these phases in diogenites maintained equilibrium to temperatures in the range 650 880 c far below likely magmatic temperatures in some diogenites orthopyroxenes of differing composition are in breccia contact therefore the low equilibration temperatures were achieved in the diogenites prior to brecciation decoupling of major and trace elements was most likely caused by slow subsolidus cooling in the diogenite pluton which allowed ca fe and mg to reach diffusive equilibration incompatible trace elements likely have much lower diffusivities in orthopyroxene and therefore may have largely preserved their magmatic distributions the extensive variation in incompatible trace elements indicates that either the diogenites were formed through an extensive crystallization interval or that partition coefficients greatly increased during crystallization if the former were true one would expect to find substantial amounts of plagioclase in the diogenites if they are comagmatic with eucrites this is contrary to observations a model in which diogenites crystallized from one or more melts of approximately orthopyroxenitic composition can satisfy both the mineralogie and trace element constraints

reconstruction of a 3700 ma transgressive marine environment from isua greenland sedimentology stratigraphy and geochemical signatures abstract the rare preservation of a stratigraphy in the northwest of the eoarchean isua supracrustal belt greenland demonstrates the world s earliest recognised marine transgression stratigraphically lowest is the 3710 ma solvang volcanic formation of picrites basalts and basaltic andesites with arc like geochemical signatures close to its erosional top this formation s volcanic rocks show increase in k2o with depletion of na2o depletion in sr with no alteration in eu abundance and 18ovsmow increase to 16 interpreted as subaerial weathering at 3700 ma the sedimentary snowpatch formation was deposited unconformably on the solvang volcanic formation locally the unconformity is marked by the laterally discontinuous conglomerate member dominated by rounded quartz clasts but also with clasts of basalt and andesite up to 10 cm across layered chert and ultramafic rocks the conglomerate member carries sparse detrital zircons the youngest of which are 3710 ma the conglomerates are succeeded by the sandstone and dolostone member of layered quartz and dolomite clastic sandstones that locally preserve relict shallow marine cross bedding with scoured layers tempestite breccia beds and very rare stromatolite horizons the sandstone and dolostone member is succeeded by the 3695 ma bif member the snowpatch formation quartz dolomite sandstones and interlayered stromatolites have seawater like ree y trace element patterns variably modified by a negligible to significant influx of a terrigenous component trace element signatures such as ti v ratios and normalising ree y data to a 3710 ma weathered volcanic rock rather than post archean average shale suggest this component was derived from a juvenile arc source resembling the basement solvang volcanic formation the detailed field and geochemical evidence such as positive correlation between fe mg and pr yb in the snowpatch formation dolostones and bif banded iron formation indicates that the dolomitic lithologies developed in a shallow water setting with clastic sedimentation during a marine transgression over a weathered 3710 ma volcanic arc basement followed by bif deposition in deeper water thus by 3700 ma shallow water exposed settings had already been established which were the site of stromatolite development

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

experimental study of a kiglapait marginal rock and implications for trace element partitioning in layered intrusions abstract experiments on a chilled marginal rock from the kiglapait intrusion labrador have been performed in order to constrain the partitioning behaviour of major and trace elements during formation of kiglapait lower zone lz cumulates at 0 1 mpa and f o 2 close to fmq the liquidus is bracketed between 1225 and 1212 c and the only liquidus phases are plagioclase an 73 74 and olivine fo 82 85 in contrast lz troctolites contain fo 67 5 olivine and an 63 4 plagioclase plagioclase melt partition coefficients d s for li ti rb sr y zr ba la and eu determined by ion microprobe analysis of run products are similar to those for other basaltic systems but differ systematically from apparent d s calculated from bulk analyses of lz plagioclases and modelled liquid compositions obtained by summation the discrepancies in d s and phase compositions are inconsistent with diffusion controlled boundary layer growth crystallisation of trapped intercumulus liquid is quantitatively plausible however a the volume of liquid required to reconcile experimental and model d s 20 1 is larger than that estimated from observed proportions of intercumulus phases 11 2 and b this mechanism alone cannot reduce x an in lz plagioclases due to very slow al si interdiffusion the discrepancy between experiment and model is best resolved if the parental melt to lz cumulates was more evolved higher fe mg na ca ree and alkalis lower sr than the summed bulk composition in accord with a recent revision of the kiglapait fe mg based on olivine compositions

parental magmas of the nain plutonic suite anorthosites and mafic cumulates a trace element modelling approach equilibrium melt trace element contents are calculated from proterozoic nain plutonic suite nps mafic and anorthositic cumulates and from plagioclase and orthopyroxene megacrysts assumed trapped melt fractions tmf 20 generally eliminate all minor phases in most mafic cumulate rocks reducing them to mixtures of feldspar pyroxene and olivine which would represent the high temperature cumulus assemblage in anorthosites tmf 15 generally reduce the mode to a feldspar only assemblage all model melts have trace element profiles enriched in highly incompatible elements relative to normal mid ocean ridge basalt nmorb commonly with negative nb and th anomalies most mafic cumulates yield similar profiles with constant incompatible element ratios and can be linked through fractional crystallization high k la subtypes probably represent crust contaminated facies mafic cumulates are inferred to belong to a tholeiitic differentiation series variably contaminated by upper and lower crustal components and probably related to coeval tholeiitic basaltic dyke swarms and lavas in labrador model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates indicating that mafic cumulates and anorthosites did not crystallize from the same melts orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts precluding an origin from parental melts jotunites have lower k rb ba y yb and higher la ce than model residues from fractionation of anorthositic model melts suggesting they are not cosanguineous with them but provide reasonable fits to evolved mafic cumulate model melts incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites with steep y yb lu segments that suggest residual garnet in the source inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust the similarity of the highly incompatible trace elements and lile between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites the reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase and remobilization of these hybrid plagioclase rich mushes then produced the anorthosite massifs

petrology and geochemistry of the unbrecciated achondrite northwest africa 1240 nwa 1240 an hed parent body impact melt nwa 1240 is an unusual eucrite recently recovered in morocco as a single stone of 98 g it is an unbrecciated greenish brown rock nearly devoid of fusion crust it displays porphyritic texture consisting of skeletal hollow low ca pyroxene phenocrysts set in a variolitic fan spherulitic mesostasis of fine elongate pyroxene and plagioclase crystals minor phases are skeletal chromite iron silica troilite ilmenite and minute amounts of phosphate and fayalite pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a eucrite from en 76 0 wo 1 9 fs 22 1 to compositions nearly devoid of mg unusual ferrosilite and fe augite symplectites and possibly pyroxferroite plagioclase crystals contain significant amounts of fe and mg which are possibly controlled by the ca mg fe2 si3o8 plagioclase component

subsolidus ree partitioning between pyroxene and plagioclase in cumulate eucrites an ion microprobe investigation abstract in an attempt to elucidate the relationship between the cumulate eucrites and the noncumulate main series eucrites we have examined the trace element systematics of plagioclase grains that coexist with inverted pigeonites of the cumulate eucrites analyses were done by secondary ion mass spectrometry sims plagioclase compositions are uniform in trace element abundances within grains from each cumulate eucrite but are variable between the four analyzed moama moore county serra de mage and binda shapes of ci normalized trace element abundance patterns of the plagioclase grains of the cumulates are typical of plagioclase showing a lree enriched pattern with positive eu anomalies reflecting the site preference of eu 2 in the feldspar structure using trace element concentrations of the pyroxenes orthopyroxene pigeonite augite pun and papike 1995 and plagioclase this study we calculate the subsolidus partition coefficients ds for the rees and y between plagioclase and pyroxenes in the examined cumulate eucrites trace element systematics suggest that igneous trace element signatures have been altered by subsolidus exchange for both the plagioclase and pyroxene calculated hypothetical parental melt patterns determined from pyroxene and plagioclase are different and can differ by a factor of 4 5 for individual rees

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

detrital zircon sedimentary provenance ages for the eoarchaean isua supracrustal belt southern west greenland juxtaposition of an imbricated ca 3700 ma juvenile arc against an older complex with 3920 3760 ma components abstract the amphibolite facies eoarchaean isua supracrustal belt northern part of the nuuk region southern west greenland is dominated by strongly deformed metabasalts with chert banded iron formation felsic volcanic and volcano sedimentary rocks and minor gabbro and sedimentary carbonates it comprises a suture zone between a northern terrane formed at ca 3700 ma and a southern one formed at ca 3800 ma at the junction between these two terranes is a strongly tectonised thin unit of metachert bif and carbonate bearing rocks with minor detrital components named the dividing sedimentary unit away from the belt the northern terrane is dominated by ca 3700 ma tonalites and the southern one by ca 3800 ma tonalites the isua supracrustal belt chemical metasedimentary rocks give low yields of small commonly 50 m long prismatic oscillatory zoned zircons with less common somewhat rounded zircons of volcanic and or detrital origin the most siliceous of the felsic schists give a larger yield of volcanic or detrital zircons the ages of these zircons are distinct in different parts of the belt in the northern terrane most have ages of 3710 3690 ma with a few slightly older 3750 ma detrital zircons suggests that these volcanic assemblages evolved remote from 3800 ma crust in contrast the up to 3940 ma detrital zircons from the dividing sedimentary unit and the southern fuchsite quartzite shows that the southern ca 3800 ma terrane although dominated by arc related rocks might contain older pre 3800 ma crust we propose that the dividing sedimentary unit was deposited on top of already deformed southern terrane rocks and then acted as a decollement when the northern terrane ca 3700 ma arc complex collided with the southern terrane by ca 3660 ma from previous age constraints thus the detrital zircon age data is compatible with proposals from other evidence for juxtaposition of ca 3700 and 3800 ma terranes in the isua area some thin siliceous metasedimentary rocks along post assembly mylonite zones in the isua supracrustal belt yield some

the akilia controversy field structural and geochronological evidence questions interpretations of 3 8 ga life in sw greenland abstract field relations structure and geochronology demonstrate that controversial rocks on akilia sw greenland cannot host evidence of earth9s oldest life mafic ultramafic gneiss that contains a layer of quartz amphibole pyroxene gneiss a few metres thick with purported biogenic graphite is not cross cut by the protoliths of 3 82 3 65 ga tonalitic gneiss the contact between these gneisses is tectonic so their relative ages are unknown multiple episodes of intrusion and deformation in the tonalitic gneiss preceded the earliest deformation fabrics and structures seen in the mafic ultramafic gneiss although previously interpreted as a volcano sedimentary stratigraphy the mafic ultramafic gneiss displays no evidence of such an origin and could be partly or wholly derived from igneous intrusions existing geochronology indicates that the mafic ultramafic and qtz am px gneisses have ages c 2 68 ga during late archaean high grade events that complicate any protolith interpretation u pb dating of apatite from a variety of rocks on akilia shows that this mineral crystallized or possibly recrystallized at c 1 75 ga and it thus provides no indication of an early archaean age for any associated graphite regardless of whether or not the latter is biogenic supplementary material ion microprobe u pb analytical methods and isotope data tables are available at http www geolsoc org uk sup18333

earth s oldest preserved unconformity prospect of a beginning in the tectono sedimentary continental cycle abstract earth s oldest preserved conglomerates and basaltic pillow lavas at isua greenland provide robust field evidence for deep and shallow water environments on our planet within the first billion years of its formation the conglomerates represent the first paleoarchaean archive of sub aerial erosion and shallow water sedimentation these ca 3700 million year old sedimentary rocks now metamorphosed at amphibolite grade comprise units of rounded quartz pebbles set in a sandy to muddy matrix that unconformably overlie an ophiolite sequence the pillow lavas of the ophiolite are variolitic and essentially vesicle free indicative of formation in a deep water environment locally an unconformity separates the conglomerates from the deformed ophiolite related rocks elsewhere the contact between these units is tectonic such field relationship between deep and shallow water environments resemble those preserved in younger orogenic belts where obducted oceanic crust has been tectonically emplaced across terrestrial platforms or subaerial parts of forearc and backarc basins

metamorphic diogenite grosvenor mountains 95555 mineral chemistry of orthopyroxene and spinel and comparisons to the diogenite suite abstract grosvenor mountains gro 95555 is a relatively newly discovered and unique metamorphic diogenite it does not show the usual brecciated appearance of other diogenites or wide compositional variability of orthopyroxene or spinel electron microprobe analysis empa and secondary ion mass spectrometer sims analysis of orthopyroxene and empa of spinel show limited compositional variability compositions for orthopyroxene for fe fe mg atomic al zr y and yb fall in the middle of the compositional ranges of the diogenite suite apparently gro 95555 formed at sufficient depth or location to escape brecciation by meteorite bombardment and in a thermal regime that led to high grade metamorphism resulting in homogeneous mineral compositions the believed introduction howardite eucrite and diogenite hed lithologies are to have originated on asteroid 4 vesta during an extensive melting event at 4 6 ga consolmagno and drake

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

distinguishing biological from hydrothermal signatures via sulphur and carbon isotopes in archaean mineralizations at 3 8 and 2 7 ga

rare earth element partition coefficients from enstatite melt synthesis experiments enstatite en80fs19wo01 was synthesized from a hypersthene normative basaltic melt doped at the same time with la ce nd sm eu dy er yb and lu the rare earth element concentrations were measured in both the basaltic glass by empa and the enstatite by sims rare earth element concentrations in the glass were determined by electron microprobe analysis with uncertainties less than two percent relative rare earth element concentrations in enstatite were determined by secondary ion mass spectrometry with uncertainties less than five percent relative the resulting rare earth element partition signature for enstatite is similar to previous calculated and composite low ca pigeonite signatures but is better defined and differs in several details first the enstatite lree signature is steeper than for pigeonite second the signature has a negative eu anomaly which relates to oxygen fugacity and third the dependence of the partition coefficients on alumina content is linked to crystal structural constraints

petrology and geochemistry of the elephant moraine a79002 diogenite a genomict breccia containing a magnesian harzburgite component the elephant moraine a79002 eeta79002 diogenite is a fragmental breccia with a subtle lightdark structure it is composed of orthopyroxene with minor olivine chromite and ubiquitous inhomogeneously distributed approximately 5 500 m sized troilite and metal grains these latter are present in the matrix and as inclusions in and as symplectic intergrowths with orthopyroxene and olivine trace amounts of silica and diopside are also present most orthopyroxene compositions typical orthopyroxenes are in the narrow range wo2 1 2 7en74 1 75 6fs22 2 23 8 like those of most diogenites a few magnesian orthopyroxenes are present with compositions of wo1 7 2 5en77 5 80 2 fs18 2 20 3 these are among the most magnesian orthopyroxenes known from diogenites a few ferroan orthopyroxenes have compositions of wo2 1 2 9en71 7 73 7fs24 2 25 5 differences in al2o3 tio2 and cr2o3 between the different orthopyroxene groups are inconsistent with a simple igneous fractionation relationship between them olivine compositions are fo75 0 76 9 the olivines could be in equilibrium with the magnesian orthopyroxenes but not with the typical or ferroan orthopyroxenes that form the bulk of eeta79002 metal grains exhibit a range of ni and co contents and ni co ratios their compositions indicate that they are primary igneous metal metal and troilite grains are more prevalent in the dark samples the trace incompatible lithophile element contents of 16 samples are remarkably uniform their yb concentrations are all within their 2s analytical uncertainties of the mean the uniformity and low content of light rare earth elements in eeta79002 indicate that negligible amounts of a trapped liquid component or foreign material mixed in the breccia could be present the siderophile and chalcophile element data show that the light dark structure is due to the distribution of metal and troilite grains dark samples contain higher ni co and se compared to light samples meteorite eeta79002 appears to contain material from three or more related plutons a magnesian harzburgite and two orthopyroxenites and is a genomict breccia

the age and pb loss behaviour of zircons from the isua supracrustal belt as determined by ion microprobe ion microprobe uthpb analyses of zircons from a conglomeratic metavolcanic unit in the isua supracrustal belt of west greenland show that their magmatic age is3807 1ma and failed to detect any older zircon cores or xenocrysts low u areas within zircons at a 30 m scale lost only small amounts of radiogenic pb and only in the recent past whereas high u areas within grains lost much greater proportions of their pb both recently and as early as the archaean some areas in the zircons have concordant upb and thpb ages the ion probe results agree with earlier determinations for the isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses

the bouma sequence and the turbidite mind set abstract conventionally the bouma sequence bouma a h 1962 sedimentology of some flysch deposits a graphic approach to facies interpretation elsevier amsterdam 168 pp composed of ta tb tc td and te divisions is interpreted to be the product of a turbidity current however recent core and outcrop studies show that the complete and partial bouma sequences can also be interpreted to be deposits formed by processes other than turbidity currents such as sandy debris flows and bottom current reworking many published examples of turbidites most of them hydrocarbon bearing sands in the north sea the norwegian sea offshore nigeria offshore gabon gulf of mexico and the ouachita mountains are being reinterpreted by the present author as dominantly deposits of sandy debris flows and bottom current reworking with only a minor percentage of true turbidites i e deposits of turbidity currents with fluidal or newtonian rheology in which sediment is suspended by fluid turbulence this reinterpretation is based on detailed description of 21 000 ft 6402 m of conventional cores and 1200 ft 365 m of outcrop sections the predominance of interpreted turbidites in these areas by other workers can be attributed to the following 1 loose applications of turbidity current concepts without regard for fluid rheology flow state and sediment support mechanism that result in a category of turbidity currents that includes debris flows and bottom currents 2 field description of deep water sands using the bouma sequence an interpretive model that invariably leads to a model driven turbidite interpretation 3 the prevailing turbidite mind set that subconsciously forces one to routinely interpret most deep water sands as some kind of turbidites 4 the use of our inability to interpret transport mechanism from the depositional record as an excuse for assuming deep water sands as deposits of turbidity currents 5 the flawed concept of high density turbidity currents that allows room for interpreting debris flow deposits as turbidites 6 the flawed comparison of subaerial river currents fluid gravity flows dominated by bed load transport with subaqueous turbidity currents sediment gravity flows dominated by suspended load transport that results in misinterpreting ungraded or parallel stratified deep sea deposits as turbidites and 7 the attraction to use obsolete submarine fan models with channels and lobes that require a turbidite interpretation although the turbidite paradigm is alive and well for now the turbidites themselves are becoming an endangered facies

graphite and carbonates in the 3 8 ga old isua supracrustal belt southern west greenland we present a systematic study of abundance isotopic composition and petrographic associations of graphite in rocks from the ca 3 8 ga isua supracrustal belt isb in southern west greenland most of the graphite in the isb occurs in carbonate rich metasomatic rocks metacarbonates while sedimentary units including banded iron formations bifs and metacherts have exceedingly low graphite concentrations regardless of isotopic composition of graphite in metacarbonate rocks their secondary origin disqualifies them from providing evidence for traces of life stemming from 3 8 ga recognition of the secondary origin of isua metacarbonates thus calls for reevaluation of earlier interpretations that suggested the occurrence of 3 8 ga biogenic graphite in these rocks thermal decomposition of siderite 6feco3 2fe3o4 5co2 c is the process seemingly responsible for the graphite formation the cation composition fe mg mn and ca of the carbonate minerals carbon isotope ratios of carbonates and associated graphite and petrographic assemblages of a suite of metacarbonates support the conclusion that multiple pulses of metasomatism affected the isb causing the deposition of fe bearing carbonates and subsequent partial disproportionation to graphite and magnetite equilibrium isotope fractionation between carbonate and graphite in the rocks indicates peak metamorphic temperatures between 500 and 600 c in agreement with other estimates of metamorphic temperature for the isb

determination of parental magmas of hed cumulates the effects of interstitial melts an evaluation of trapped melts effects during crystallization and subsolidus equilibration of cumulates is necessary to constrain the composition of their parental magmas in this paper a simple mass balance approach is described it allowes to estimate trace element abundances in these parental melts from phase compositions it is used to discuss the genesis of cumulate eucrites and diogenites the ree behavior is in full agreement with the view that cumulate eucrites formed from melts similar to noncumulate eucrites trapped melt fractions ranging from 10 wt for moama to 30 wt for moore county were involved the origin of diogenites is more complex the assumption that eucrites and diogenites shared the same parental melts cannot satisfactorily explain the diversity of incompatible trace element ratios e g dy yb observed in diogenitic orthopyroxenes even if interstitial melt effects are taken into account moreover some diogenites unambiguously crystallized from magmas displaying significant hree heavy rare earth elements enrichments more likely diogenites formed from distinct batches of parental magmas as previously proposed by mittlefehldt 1994 fowler et al 1995 and shearer 1997

vesta as the howardite eucrite and diogenite parent body implications for the size of a core and for large scale differentiation if vesta is the parent body of the howardite eucrite and diogenite hed meteorites then geo chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of vesta to 1 determine the size of a possible metal core in vesta and 2 model the igneous differentiation and internal structure of vesta the density of vesta and petrologic models for hed meteorites together suggest that the amount of metal in the parent body is 25 mass with a best estimate of 5 assuming no porosity for a porosity of up to 5 in the silicate fraction of the asteroid the permissible metal content is 30 these results suggest that any metal core in the hed parent body and vesta is not unusually large a variety of geochemical and other data for hed meteorites are consistent with the idea that they originated in a magma ocean it appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites excepting such eucrites as bouvante and statinem formed by subsequent crystallization of the residual melts modelling results suggest that the hed parent body is enriched in rare earth elements by a factor of 2 5 3 5 relative to ci chondrites and that it has approximately chondritic mg si and al sc ratios stokes settling calculations for a vesta wide nonturbulent magma ocean suggest that early crystallizing magnesian olivine orthopyroxene and pigeonite would have settled relatively quickly permitting fractional crystallization to occur but that later crystallizing phases would have settled or floated an order of magnitude more slowly allowing instead a closer approach to equilibrium crystallization for the more evolved eucritic melts this would have inhibited the formation of a plagioclase flotation crust on vesta plausible models for the interior of vesta which are consistent with the data for hed meteorites and vesta include a metal core 130 km radius an olivine rich mantle 65 220 km thick a lower crustal unit 12 43 km thick composed of pyroxenite from which diogenites were derived and an upper crustal unit 23 42 km thick from which eucrites originated the present shape of vesta with 60 km difference in the maximum and minimum radius suggests that all of the crustal materials and possibly some of the underlying olivine from the mantle could have been locally excavated or exposed by impact cratering

ion microprobe investigation of exsolved pyroxenes in cumulate encrites determination of selected trace element partition coefficients we have examined the partitioning of selected minor and trace elements in exsolved pyroxenes from cumulate eucrites by secondary ion mass spectrometry sims solid solid partition coefficients d s for al cr ti seven rees sr y and zr have been determined between low ca and high ca pyroxenes from moore county binda serra de mage and moama equilibrium temperatures of the co existing pyroxenes were calculated to constrain the relevant temperatures for the partition coefficients trace element abundance patterns are similar for pyroxenes from the four cumulate eucrites and thus indicate similar distribution behaviors of the trace elements during exsolution there is a significant hree enrichment over lrees in the low ca pyroxenes and abundances of the rees in augite are greater than those found in the low ca pyroxene hosts this ree behavior reflects the pyroxene crystal structures the calculated solid solid partition coefficients are found to vary systematically as expected from pyroxene crystal chemical arguments calculated subsolidus equilibration temperatures are within the expected range produced by pyroxene solvus temperature composition relations and consistent with petrographic observations of the cumulates the trace element patterns and abundances indicate that three of the four cumulates examined binda moama serra de mage were formed from the same melt or similar melts crystallizing under similar conditions in their parent body moore county has much higher trace element concentrations and this suggests that it either formed from a more evolved melt than the other three cumulates and crystallized later in the fractionation sequence or alternatively it was derived from a different melt system

ree patterns of eucrites and their genetic implications abstract rare earth element ree abundances in eucrites were precisely determined on juvinas pasamonte cachari stannem and antarctic eucrites y 74450 63 d g y 75015 20 e f y 75011 73 y 790007 61 f y 75011 84 d 1 and y 790007 61 e 4 the last two samples are eucrite clasts from polymict eucrites the other antarctic eucrite samples are matrix from polymict eucrites like the stannern eucrite all of the antarctic eucrites have relatively high ree abundances ree abundances in two clast samples are higher than those in the stannern and matrix samples of the antarctic eucrites when ree abundances in the antarctic eucrites and the stannem eucrite are normalized by the ree abundance suite of the juvinas or pasamonte eucrites highly linear ree patterns are obtained for the juvinas or pasamonte normalized ree patterns of the antarctic and stannern eucrites the inclination of the linear ree patterns increases with increasing ree abundances these observations suggest that the eucrites with larger ree abundances such as the stannern and antarctic eucrites were produced as residual liquid in a fractional crystallization process from the main group of eucrites e g the juvinas eucrite on the other hand the moore county cumulate eucrite is considered to be a solid phase formed in this process

a magma ocean on vesta core formation and petrogenesis of eucrites and diogenites abstract available evidence strongly suggests that the hed howardite eucrite diogenite meteorites are samples of asteroid 4 vesta abundances of the moderately siderophile elements ni co mo w and p in the hed mantle indicate that the parent body may have been completely molten during its early history during cooling of a chondritic composition magma ocean equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0 80 when the convective system freezes and melts segregate from crystals by gravitational forces the extruded liquids are similar in composition to main group and stannern trend eucrites and the last pyroxenes to precipitate out of this ocean before convective lockup span the compositional range of the diogenites subsequent fractional crystallization of a main group eucrite liquid which has been isolated as a body of magma produces the nuevo laredo trend and the cumulate eucrites the predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites thus eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with vesta

controversial pb pb and sm nd isotope results in the early archean isua west greenland oxide iron formation preservation of primary signatures versus secondary disturbances pb stepwise leaching pbsl determinations on two magnetite enriched fractions of a bif sample from the northeastern part of the isua supracrustal belt west greenland yield an isochron of 3691 22 ma mswd 0 4 in combination with previously published geochronological constraints for a minimum deposition age of 3 71 ga for volcanic sequences nutman et al 1997 comprising the studied bifs and supported by microtextural observations this demonstrates an early archean amphibolite facies metamorphic event in the supracrustal pbsl data on magnetite from slightly discordant veins within the same iron stone together with bulk pb isotope values of paragenetically late pyrite yield a 3 63 0 07 ga mswd 8 6 errorchron with significantly different pb isotopic compositions from those obtained from the main magnetite layers and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later the fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3 6 ga granitic into 3 7 ga tonalitic amitsoq gneisses enclosing the belt tremolite rich layers in the bif are characterized by sm nd ratios close to chondritic end 3800 of 1 7 and 2 1 within error of published results from adjacent basic units in the supracrustal suite in contrast the magnetite dominated layers yield unrealistically high end 3800 of 14 8 and 14 4 indicative of sm nd ratios resembling ree fractionated continental sources these high end 3800 together with radiogenic sr leached from the magnetite enriched separates is ascribed to secondary hydroxyapatite which predominantly forms as crystal overgrowths in the magnetite rich bands the timing of the hydrothermal event during which apatite was deposited within the bif remains uncertain but a tchur model age of 1 85 ga from the apatite dominated hcl leachate may point to a close genetic relationship with local proterozoic metamorphism and granite formation by crustal remelting in the isukasia area step leaching of magnetite from a similar silicate oxide facies bif from the western part of the isua supracrustal belt yield a pb pb isochron age of 2 84 0 05 ga mswd 1 43 the agreement between the pbsl age and previously published field and isotopic evidence for a major late archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2 75 2 83 ga granodioritic ikkattoq gneisses west and southwest of the western limb of the isua supracrustal belt we argue that the pbsl isochron ages date the peak of amphibolite facies metamorphic events which respectively affected the eastern and western sections of the isua supracrustal belt during the early and late archean our results give additional support to the suggestion that the scatter on published end t values from the isua supracrustal belt and adjoining gneisses can be assigned to post formational hydrothermal processes and underline the need for care in the interpretation of sm nd bulk data from polymetamorphic rocks to constrain isotopic models of early earth s evolution

young ages of two diogenites and their genetic implications the achondrite meteorites known as eucrites and diogenites are thought to be igneous rocks derived from the crust and mantle respectively of a common parent planet or asteroid1 2 several eucrites have been dated at 4 5 4 6 gyr by the rb sr method4 6 and a rb sr age of 4 45 0 18 gyr 1 42x 10 11yr 1 has been reported3 for three diogenites from these data one cannot judge whether diogenites and eucrites are the same age here we present precise rb sr data for two diogenites tatahouine and johnstown that yield an age of 4 394 0 011 gyr significantly younger than our age of 4 52 0 15 gyr for the juvinas eucrite the low initial 87sr 86sr ratio of these diogenites and their rare earth element abundance patterns are inconsistent with the interpretation that the young ages reflect secondary metamorphism we suggest that our data raise doubts about a genetically close relationship between eucrites and these diogenites and that there exist groups of diogenites that are genetically distinct from one another our results also suggest that igneous activity in the assumed parent body continued for about one hundred million years after its formation 4 52 gyr ago the parent body may thus have had a complicated evolutionary history

the parent magmas of the cumulate eucrites a mass balance approach the cumulate eucrite meteorites are gabbros that are related to the eucrite basalt meteorites the eucrite basalts are relatively primitive nearly flat ree patterns with la approx 8 30 x ci but the parent magmas of the cumulate eucrites have been inferred as extremely evolved la to greater than 100 x ci this inference has been based on mineral magma partitioning and on mass balance considering the cumulate eucrites as adcumulates of plagioclase pigeonite only both approaches have been criticized as inappropriate here mass balance including magma equilibrium pigeonite equilibrium plagiociase is used to test a simple model for the cumulate eucrites that they formed from known eucritic magma types that they consisted only of magma crystals in chemical equilibrium with the magma and that they were closed to chemical exchange after the accumulation of crystals this model is tested for major and rare earth elements ree the cumulate eucrites serra de mage and moore county are consistent in both ree and major elements with formation by this simple model from a eucrite magma with a composition similar to the nuevo laredo meteorite serra de mage as 14 magma 47 5 pigeonite and 38 5 plagioclase moore county as 35 magma 37 5 pigeonite and 27 5 plagioclase these results are insensitive to the choice of mineral magma partition coefficients results for the moama cumulate eucrite are strongly dependent on choice of partition coefficients for one reasonable choice moama s composition can be modeled as 4 nuevo laredo magma 60 pigeonite and 36 plagioclase selection of parent magma composition relies heavily on major elements the ree cannot uniquely indicate a parent magma among the eucrite basalts the major element composition of y 791195 can be fit adequately as a simple cumulate from any basaltic eucrite composition however y 791195 has lree abundances and la lu too low to be accommodated within the model using any basaltic eucrite composition and any reasonable partition coefficients postcumulus loss of incompatible elements seems possible it is intriguing that serra de mage moore county and moama are consistent with the same parental magma could they be from the same igneous body on the eucrite parent asteroid 4 vesta

metamorphic history suggested by garnet growth chronologies in the isua greenstone belt west greenland abstract compositional maps for garnets from the five tectonic domains of the isua greenstone belt in west greenland are used to identify episodes of garnet growth during the metamorphism of the greenstone belt three distinct episodes of garnet growth are recorded in domains iii v two in domain ii and one in domain i a wide variety of garnet textures are observed including growth zoning post growth homogenisation replacement rim resorption and oscillatory zoning there is no simple correlation of garnet growth events among domains indicating that at least some events are related to localised metasomatic processes in the greenstone belt however the relative growth chronology combined with the available geochronology indicates that the observed metamorphic processes span both the early and late archaean

recognizable primary volcanic and sedimentary features in a low strain domain of the highly deformed oldest known 3 7 3 8 gyr greenstone belt isua west greenland a low strain domain has been identified in the metamorphosed mostly highly deformed volcanic and sedimentary rocks of the early archaean isua supracrustal belt this domain contains well preserved volcanic and sedimentary features including basaltic pillow lavas pillow breccia heterogeneous volcanic breccia amygdules in metabasalt and polymict conglomerate dominated by recrystallized chert and volcanic clasts the low strain domain is bounded by highly deformed rocks mostly derived from basalt chert and banded iron formation these discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and furthermore strengthen the characterization of the isua supracrustals as a typical greenstone belt

mass independent isotope effects in archean 2 5 to 3 8 ga sedimentary sulfides determined by ion microprobe analysis we report sulfur isotope anomalies with 33 s the deviation from a mass dependent fractionation line for the three isotope system 34 s 32 s vs 33 s 32 s ranging up to 2 within individual archean sedimentary sulfides from a variety of localities our measurements which are made in situ by multicollector secondary ion mass spectrometry unequivocally corroborate prior bulk measurements of mass independent fractionations mif in sulfur and provide additional evidence for an anoxic atmosphere on the earth before 2 ga this technique also offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record the presence of mif sulfur in sulfides from a 3 8 ga fe rich quartzite from akilia island west greenland is consistent with a marine sedimentary origin for this rock copyright 2003 elsevier science ltd

aubrites and diogenites trace element clues to their origin we have analyzed by rnaa 25 aubrite and 9 diogenite samples for 13 to 29 siderophile volatile and lithophile trace elements both meteorite classes show a typically igneous siderophile element pattern with ir os re ge more depleted than au ni pd sb but aubrites tend to have about 10 higher abundances 10 3 10 4 cl for the first 4 and 10 2 10 3 cl for the last 4 siderophiles apparently reflecting smaller metal silicate distribution coefficients at lowerf o2 or less complete segregation of metal se is surprisingly abundant in aubrites up to 0 4 cl but te is less so sete 5 cl apparently due to its stronger siderophile character other volatiles ag zn in cd bi t1 show depletions intermediate between lunar dunite and the earth s mantle of 7 aubrites analyzed for ree ce nd eu tb yb lu 6 are depleted in ree 0 08 0 5 cl and 5 show negative eu anomalies the exceptions are bishopville and mt egerton silicate this supports an igneous origin as already noted by boynton and schmitt 1972 no samples of the complementary basaltic and feldspathic rocks have been found thus far but one of our samples of khor temiki dark is a candidate for the basalt it is 5 7 enriched in ree and only slightly less so in rb cs and u though shocked and enriched in siderophiles to 0 05 cl it apparently represents a new meteorite class three diogenites analyzed for ree show very diverse patterns from strongly depleted in light ree for tatahouine ce 0 01 cl to flat for garland 2 5 cl the data confirm the trends found by fukuokaet al 1977 as well as their interpretations factor analysis shows several parallel groupings for aubrites and diogenites siderophiles re ir os pd ge chalcophiles se te volatiles ag in tl and incompatibles u ree and cs or rb but there are some differences for elements such as ni sb cd bi au and zn most of which behave more sensibly in aubrites than in diogenites several element pairs that differ greatly in volatility cs u ge ir correlate closely in aubrites in approximately cl chondrite proportions these correlations and other lines of evidence suggest strongly that aubrites originated by igneous processes in their parent body not by direct nebular condensation the source material may have resembled el chondrites in oxidation state and depletion of refractories metal and volatiles

geneses and evolutions of iron bearing minerals in banded iron formations of 3760 to ca 2200 million year old constraints from electron microscopic x ray diffraction and mossbauer spectroscopic investigations abstract minerals preserved in banded iron formations bifs are diagenetic and or metamorphic rather than primary knowing their geneses and evolutions are essential for understanding the interactions between the atmosphere ocean and biosphere in the first two billion years of life history on earth here we use high resolution electron microscopy x ray diffraction and mossbauer spectroscopy to investigate the mineralogy of six bifs aged from 3760 ma to ca 2200 ma with a variety of genetic conditions the results reveal that each major iron mineral magnetite hematite carbonate and silicate has a range of structural and morphological features that represent different geneses and evolutionary histories euhedral magnetite crystals with homogeneous internal structures and compositions 3 5 nm hematite particles aggregated in iron rich bands and submicrometer sized euhedral hematite in chert matrix of fe si transitional bands are identified to have transformed from precursors directly precipitated from seawaters these minerals retain some primary environmental information when bifs are deposited the other mineral phases display features of alteration due to multi redox reactions hydrothermal or supergene processes such as crosscutting mineral replacement or overgrowth deformation fracture and porous structures that have overprinted the primary signatures such minerals include 1 massive magnetite with fragmentation structures 2 partly to completely oxidized magnetite crystals 3 acicular fibrous hematite in fractures or iron and silica layering 4 microplaty hematite 5 hematite oxidized from other fe ii bearing minerals e g magnetite and stilpnomelane 6 carbonates with heterogeneous chemical compositions 7 carbonates replacing other minerals e g siderite thin film coating the euhedral magnetite crystals 8 irregular carbonate cementing other minerals e g magnetite 9 preferentially orientated fibrous spindle like silicates cutting iron and silica rich bands and 10 massive silicate pieces this study provides detailed mineralogical and petrologic characterizations of bif minerals that can serve as a reference for future in situ geochemical measurements by recent developed techniques

the briksdalsbre event a winter precipitation induced decadal scale glacial advance in southern norway in the ad 1990s and its implications in the ad 1990s maritime glaciers in scandinavia started to advance as a response to positive net mass balance in the preceding years invoking annual advance rates in the order of 50 60 m and a total frontal advance of 285 m at briksdalsbreen in less than a decade records from six south norwegian glaciers with continuous annual front measurements are used to evaluate the magnitude duration climatic causes and frontal time lags involved in this mass balance perturbation and the following frontal response of glaciers in southern norway a climate index based on meteorological data from bergen unequivocally demonstrates that the main cause for the large glacier advances in scandinavia in the 1990s was high winter precipitation linked to positive nao index in the winters of 1988 1989 1989 1990 1992 1993 1994 1995 1997 1998 and 1999 2000 less positive or negative glacier mass balance years were 1990 1991 1993 1994 1995 1997 and 1998 1999 between 1996 1997 and 2009 briksdalsbreen retreated

survey and evaluation of eucrite bulk compositions the purpose of this survey is to establish reference bulk elemental abundances for the eucrites and thereby provide the basis to test core formation models as well as partial melting fractional crystallization and magma ocean theories for the eucrite parent body in order to evaluate bulk elemental abundances for the eucrites 296 peer reviewed articles monographs theses or books and 143 abstracts dating from 1938 to 1997 were surveyed of the 101 eucrites having at least one set of elemental abundance analyses reported in the literature 20 were selected for in depth examination the selection criteria of our sample were based on the total number of analyses available for a given eucrite and the total number of elements for which data exist the mean bulk elemental abundance 1 standard deviation and the percent deviation were calculated for each element in a given eucrite in order to evaluate the quality of the mean abundances the elements were then grouped according to availability of data and percent deviations possible reasons for the different deviations in the different groups are briefly discussed from the major element abundances the normative cipw composition the molar compositions of pyroxene olivine and plagioclase and the bulk densities were calculated and compared to petrographic observations the calculated norms for the noncumulates agree well with the observations while the norms for the cumulates do not possible reasons for this are discussed unfortunately analyses of many elements are poorly represented in the literature and many bulk analyses suffer from unacceptable levels of uncertainty therefore future work requires bulk elemental analyses for some of the more poorly characterized elements in eucrites especially those of key elements used for planetary modeling

in situ iron isotope analysis of pyrites in 3 7ga sedimentary protoliths from the isua supracrustal belt southern west greenland abstract the timing of the emergence of life remains one of the principal unresolved questions in the earth sciences putative relicts of microorganisms in the eoarchean ca 3 6 3 85 ga high grade metamorphic terranes do not preserve morphological evidence for early life but some relicts can be identified by their geochemical signatures created by metabolic processes among the oldest rocks of sedimentary origin ca 3 8 ga occur in the isua supracrustal belt isb southern west greenland these have undergone metamorphism up to the amphibolite facies despite intense metamorphism the carbon isotope compositions of graphite clots from the isua metasedimentary rocks suggest biological carbon fixation and provide the oldest evidence for this biological process microbial dissimilatory iron reduction dir is considered to be an early form of metabolism the microbial dir produced fe 2 aq with a lower 56 fe value from a precursor fe 3 bearing iron mineral however 56 fe values lower than 1 are not found in sedimentary rocks prior to about 2 9 ga here we report in situ iron isotope analysis of pyrites in sedimentary rocks from the isb using a near infrared femtosecond laser ablation multicollector icp ms nir fs la mc icp ms a large variation of 56 fe values from 2 41 to 2 35 was documented from 190 points within pyrite grains from 11 rock specimens including those interpreted to be banded iron formations bifs chert amphibole rich chert quartz rich clastic sedimentary rocks mafic clastic sedimentary rocks carbonate rocks and conglomerates we found that the distribution of 56 fe values depends on the lithology whereas there is no correlation between their 56 fe values and the metamorphic grade the 56 fe values of pyrites in bifs range from 0 25 to 2 35 indicating partial oxidation in the deep ocean especially the high 56 fe values up to 2 35 suggest that the bif was formed through interaction of ferruginous seawater with a highly alkaline hydrothermal fluid under anoxic conditions pyrite grains in a conglomerate carbonate rocks mafic clastic sedimentary rocks and amphibole rich cherts show negative 56 fe values around 1 5 down to 2 41 pointing to microbial dir in the eoarchean shallow sea in addition the relatively low 56 fe values of pyrites in the shallow water sediments suggest anoxic anoxygenic photoautotrophic iron oxidation in the photic zone

sulfur and oxygen isotope constraints on the sedimentary history of apparent conglomerates from the nuvvuagittuq greenstone belt nunavik quebec abstract mafic igneous rocks of the nuvvuagittuq greenstone belt ngb crystallized before 3 8 ga and possibly as early as 4 3 ga potentially making the belt the oldest known supracrustal sequence on earth however detrital zircons from a rare quartz biotite schist in the ngb yield significantly younger ages of 3 77 ga or less these appear to be inconsistent with the ages of the mafic igneous rocks as the quartz biotite schist has been interpreted as a metaconglomerate formed by the dismantling of preexisting lithologies in order to assess this genetic interpretation we performed a sulfur and oxygen isotope study of the quartz biotite schist sulfide grains found in quartz clasts and the matrix show significant mass independent fractionation of sulfur isotopes 0 2 33s 1 0 mean 33s 0 5 0 1 secondary sulfides from crosscutting veins do not show mass independent fractionation of sulfur isotopes 0 1 33s 0 3 mean 33s 0 1 0 1 oxygen isotope compositions of quartz from clasts matrix and a fine grained lens are highly enriched in 18o 16 9 18o 26 7 non zero 33s values indicate a surficial origin for sulfur probably the eoarchean atmosphere while high 18o values suggest a low temperature 65 18 c origin for the quartz likely as chemical precipitation of a chert precursor from si saturated seawater therefore the coupled s and o isotope measurements show that primary isotopic signatures characteristic of surficial environments survived the protracted metamorphic history of the ngb and suggest that the quartz biotite schist contains material that originated as chemical metasediments the near mono mineralic compositions of the clasts quartz and their shared 18o enrichment suggest that they had a common protolith which was deposited prior to the formation of the schist and subsequently reworked whether the quartz biotite schist represents a metaconglomerate or a structural melange it preserves remnants of some of the oldest chemical sediments on earth

magnesium oxide iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites according to a currently popular model for petrogenesis on the howardite eucrite and diogenite hed parent asteroid the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite dominated plutons this model can be tested for consistency with mass balance for mgo and feo assuming the overall diogenite diogenite eucrite ratio d of the parent asteroid is at least comparable to the average d for the eucrite diogenite dominated howardite regolith breccias average mg mgo mgo feo is much lower for eucrites especially noncumulate eucrites than for diogenites unless the diogenite parent magmas eventually produced a large proportion of low mg residual basalt and gabbro rbg the implied initial magma s mg is vastly higher than that of any noncumulate eucrite starting from a source previously depleted by putative primary eucrite genesis melt mg can be estimated as a function of the exchange reaction kd and degree of melting using several very conservative assumptions e g assuming that the total mgo feo concentration is nearly the same in the nascent melt as in the residual solids the degree of melting required to yield a melt with mg high enough to satisfy mass balance without implying an rbg component that accounts for 50 of all eucrites is an implausibly high 60 80 wt the separate orthopyroxenitic plutons sop model also seems inconsistent with the uniform density of melts across the diogenite eucrite compositional spectrum 2 77 2 82 g cm3 which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas this difficulty cannot be reduced by simply assuming that later formed magmas were systematically both more plutonic and more mgo rich than earlier ones because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg than noncumulate eucrites conceivably the bulk mg of the asteroid s silicate system was increased between primary melt eucrite genesis and sop diogenite genesis by graphite fueled reduction of feo however the graphite oxidation process generates a huge proportion of gas which would have enhanced the buoyancy of the nascent diogenite parent magmas thus exacerbating the difficulty of achieving the implied high degrees of partial melting to avoid these difficulties but still form most eucrites as rapidly cooled extrusives i propose the nerd noncumulate eucrites as extruded residua of diogenites model in this model the diogenites form as early cumulates from a large magma system probably a global magma ocean that yields a large proportion of eucritic melt as residuum this residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions dikes and sills of rapidly cooled noncumulate eucrites slight 5 10 porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone the common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone the nerd model s greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the hed asteroid

147sm 143nd and 176lu 176hf in eucrites and the differentiation of the hed parent body the 147sm 143nd and 176lu 176hf systematics of 21 whole rock eucrites including five cumulates have been investigated by mc icp ms a statistically significant sm nd isochron was obtained on 18 samples with an age of 4464 75 ma mswd 1 26 and an intercept of 0 50680 0 00010 this age clearly is controlled by the cumulate eucrites the 21 basaltic and cumulate eucrites together do not form a statistically significant lu hf isochron basaltic eucrites however form an errorchron with an age of 4604 39 ma mswd 4 52 which becomes an acceptable isochron if the error on the lu hf ratio is doubled with respect to the laboratory estimate the initial 176hf 177hf is 0 27966 0 00002 three of the cumulate eucrites regressed together further yield a statistically significant age of 4470 22 ma indistinguishable from their sm nd age we therefore conclude that cumulate eucrites are 100 myr younger than basaltic eucrites and basaltic eucrites are close in age to the solar system there is no evidence in the present data to support a decay constant of 176lu significantly different from 1 93 10 11 yr 1 sguigna can j phys 60 1982 361 364 the broad range of variation and strong correlation of lu hf and sm nd ratios require that eucrites are partial melts and not residual magmas the relative lu hf and sm nd fractionation is controlled by the plagioclase to mafic mineral ratio in the residue the steep correlation between these two ratios is explained by the enhancement of plagioclase saturation in the low gravity field of the eucrite parent body basaltic eucrites probably formed as large degree melts of a chondritic source the strong lu hf and sm hf fractionation observed in cumulates clearly reflects the presence of residual ilmenite during melting our preferred interpretation is that cumulates were impregnated by small degree melts of ilmenite bearing gabbros since pressure on the eucrite parent body never reaches the garnet stability field this observation questions the ubiquitous presence of residual garnet in the mantle sources of magmas formed on larger planets such as the moon mars and possibly earth

microfossil like objects from the archaean of greenland a cautionary note recent reports1 4 have described yeast like microfossils isuasphaera isua pflug2 in 3 800 myr old5 metaquartzites from the isua supracrustal belt of south west greenland a biogenic interpretation of these objects is inconsistent with the tectonic history of the isua region6 8 with the petrology of the metaquartzites and with the morphology of the microstructures themselves the putative microfossils are indistinguishable from limonite stained fluid inclusions microstructures which are demonstrably inorganic and post depositional in origin as such we contend here that these objects should not be regarded as evidence of early archaean life forms

ensialic origin for the ngezi group belingwe greenstone belt zimbabwe volcanic rocks of the ngezi group in the belingwe greenstone belt have been interpreted as continental basalts extruded onto or oceanic crust obducted onto a succession of stable shelf sedimentary rocks that nonconformably overlie continental basement the volcanic succession locally contains quartzose sandstones and the stratigraphy is unlike ocean crust primary volcanic and sedimentary structures indicate a shallow subaqueous to subaerial origin all structural features are compatible with later refolding of an autochthonous successon there is no structural manifestation of obduction these criteria refute an oceanic setting for the volcanic rocks rather they are consistent with an ensialic origin for the ngezi group

protoliths of the 3 8 3 7 ga isua greenstone belt west greenland abstract the isua greenstone belt fig 1 contains the oldest known relatively well preserved metavolcanic and metasedimentary rocks on earth the rocks are all deformed and many were substantially altered by metasomatism but both the deformation and metasomatism were heterogeneous transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated likewise different kinds and different episodes of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths new geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents by this means the primary nature of the schists that make up most of the isua greenstone belt was reinterpreted and a new map that better portrays the primary nature of the rocks has been produced the previously mapped stratigraphy was found to be of little value in understanding the geology stratigraphic units were defined by different and diverse criteria such as current composition structure metamorphic texture and inferred protoliths much of this stratigraphy represents a misinterpretation of the primary nature of the rocks the new work indicates that most of the isua greenstone belt consists of fault bounded rock packages mainly derived from basaltic and high mg basaltic pillow lava and pillow lava breccia chert bif and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks a previously mapped extensive unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

mineral chemistry and the petrogenesis of eucrites ii cumulate eucrites abstract we carried out an ion microprobe study of rees and other selected trace and minor element concentrations in orthopyroxene augite lamellae and plagioclase of two cumulate eucrites moama and moore county in order to reevaluate the relationships between cumulate and noncumulate eucrites and among cumulate eucrites pyroxene and plagioclase in these two meteorites are more homogeneous in major minor and trace elements than in noncumulate eucrites and trace and minor element concentrations in moore county are higher than in moama although subsolidus equilibration erased and original igneous zoning it did not affect the ree patterns of these minerals both pigeonite and plagioclase of moama and moore county have higher lree hree ratios than those phases in noncumulate eucrites this indicates that cumulate eucrites derived from melts with fractionated ree patterns and thus are not related to noncumulate eucrites in addition the inconsistency between degrees of fractional crystallization deduced from major and trace elements and the differences of lree hree ratios recorded in pigeonite and plagioclase also suggest that moama and moore county did not form in the same fractional crystallization sequence this study confirms earlier observations and conclusions of ma and schmitt 1979 and poses problems for prevalent models

presidential address presented 2000 august 28 chicago illinois usa the eucrite vesta story many lines of evidence indicate that meteorites are derived from the asteroid belt but in general identifying any meteorite class with a particular asteroid has been problematical one exception is asteroid 4 vesta where a strong case can be made that it is the ultimate source of the howardite eucrite diogenite hed family of basaltic achondrites visible and near infrared reflectance spectra first suggested a connection between vesta and the basaltic achondrites experimental petrology demonstrated that the eucrites the relatively unaltered and unmixed basaltic achondrites were the product of approximately a 10 melt studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal scale magma ocean mass balance considerations point to a parent body that had its surface excavated but remains intact modern telescopic spectroscopy has identified kilometer scale vestoids between vesta and the 3 1 orbit orbit resonance with jupiter dynamical simulations of impact into vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations hubble space telescope images show a 460 km diameter impact basin at the south pole of vesta it seems that nature has provided multiple free sample return missions to a unique asteroid major challenges are to establish the geologic context of the hed meteorites on the surface of vesta and to connect the remaining meteorites to specific asteroids

biological fe oxidation controlled deposition of banded iron formation in the ca 3770 ma isua supracrustal belt west greenland the redox balance of the archean atmosphere ocean system is among the most significant uncertainties in our understanding of the earliest history of earth s surface zone most workers agree that oxygen did not constitute a significant proportion of the atmosphere until after ca 2 45 ga after the great oxidation event but there is less agreement on when o2 production began and how this may have been consumed by reduced species such as fe ii in the oceans the fe redox cycle through time has been traced using banded iron formations bifs and fe isotopes are increasingly used to constrain the conditions of earth s paleoenvironments including the pathways of formation of bifs iron isotope analyses of bifs from the 3 7 to 3 8 ga isua supracrustal belt isb obtained by micro sampling of magnetite rich layers and conventional analysis as well as by in situ femtosecond laser ablation fs la icp ms indicate a consistently narrow range of non zero 56fe values analysis of magnetite by fs la icp ms allows for precise and accurate micron scale analyses without the problems of orientation effects that are associated with secondary ion mass spectrometry sims analyses magnetite 56fe values range from 0 4 to 1 1 among different bands but within individual layers magnetite grains are mostly homogeneous although these bifs have been metamorphosed to amphibolite facies the metamorphism can neither explain the range in fe isotope compositions across bands nor that between hand samples the isotopic compositions therefore reflect primary low temperature sedimentary values the positive 56fe values measured from the isb magnetites are best explained by deposition of fe iii oxides produced by partial oxidation of fe ii rich ocean water a dispersion reaction model which accounts for rates of hydrothermal fe ii aq input rates of oxidation and rates of fe oh 3 settling suggests exceptionally low o2 contents 0 001 of modern o2 contents in the photic zone such low levels suggest an anoxygenic pathway is more likely and the data can be well modeled by anoxygenic photosynthetic fe ii oxidation comparison of the fe isotope data from the isua bifs with those from the 2 5 ga bifs from the hamersley and transvaal basins australia and south africa respectively suggests a striking difference in fe sources and pathways the 2 5 ga magnetite facies bifs of australia and south africa have 56fe values that range from 1 2 to 1 2 over small scales and are on average close to 0 which is significantly lower than those reported here from the isua bifs the wide range in fe isotope compositions for the hamersley and transvaal bifs in concert with c and o isotope data have been interpreted to reflect bacterial dissimilatory fe iii reduction dir the absence of low 56fe values in the isua bifs as well as the lack of fine scale isotopic heterogeneity may indicate formation prior to widespread dir

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events

micro scale silicon isotope heterogeneity observed in hydrothermal quartz precipitates from the 3 7 ga isua greenstone belt sw greenland pillow basalt and chert form integral lithologies comprising many archean greenstone belt packages to investigate details of these lithologies in the 3 7 ga isua greenstone belt sw greenland we measured silicon isotope compositions of quartz crystals by secondary ion mass spectrometry from a quartz cemented quartz amygdaloidal basaltic pillow breccia recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions the recrystallized chert chert clasts and quartz cement have overlapping 30si values while the 30si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age despite amphibolite facies metamorphism we suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting consistent with the pillow breccia origin on the basis of the present data we conclude that the geological context of each sample must be carefully evaluated when interpreting 30si values of quartz

integrated pb and s isotope investigation of sulphide minerals from the early archaean of southwest greenland we present high spatial resolution secondary ion mass spectrometry sims measurements of pb and s isotopes in sulphides from early archaean samples at two localities in southwest greenland secondary pyrite from a 3 71 ga sample of magnetite quartz banded iron formation in the isua greenstone belt which has previously yielded unradiogenic pb consistent with its ancient origin contains sulphur with a mass independently fractionated mif isotope signature delta 33 s 3 3 parts per thousand this reflects the secondary mineralization of remobilized sedimentary s carrying a component modified by photochemical reactions in the early archaean atmosphere it further represents one of the most extreme positive excursions so far known from the early archaean rock record sulphides from a quartz pyroxene rock and an ultramafic boudin from the island of akilia in the godth a over circle bsfjord have heterogeneous and generally radiogenic pb isotopic compositions that we interpret to represent partial re equilibration of pb between the sulphides and whole rocks during tectonothermal events at 3 6 2 7 and 1 6 ga both these samples have delta 33 s 0 within analytical error and therefore show no evidence for mif sulphur these data are consistent with previous interpretations that the rock cannot be proven to have a sedimentary origin our study illustrates that sims s isotope measurements in ancient rocks can be used to elucidate early atmospheric parameters because of the ability to obtain combined s and pb isotope data but caution must be applied when using such data to infer protolith when information from geological context petrography and chronology i e by pb isotopes is combined and fully evaluated delta 33 s signatures from sulphides and their geological significance can be interpreted with a higher degree of confidence c 2005 elsevier b v all rights reserved