oxidative mobilization of cerium and uranium and enhanced release of immobile high field strength elements from igneous rocks in the presence of the biogenic siderophore desferrioxamine b abstract polyvalent trace elements such as the high field strength elements hfse are commonly considered rather immobile during low temperature water rock interaction hence they have become diagnostic tools that are widely applied in geochemical studies we present results of batch leaching experiments focused on the mobilization of certain hfse y zr hf th u and rare earth elements from mafic intermediate and felsic igneous rocks in the presence and absence respectively of the siderophore desferrioxamine b dfob our data show that dfob strongly enhances the mobility of these trace elements during low temperature water rock interaction the presence of dfob produces two distinct features in the rare earths and yttrium rey patterns of leaching solutions regardless of the mineralogical and chemical composition or the texture of the rock type studied bulk rock normalized rey patterns of leaching solutions with dfob show i a very distinct positive ce anomaly and ii depletion of la and other light rey relative to the middle rey with a concave downward pattern between la and sm these features are not observed in experiments with hydrochloric acid acetic acid or deionized water in dfob bearing leaching solutions ce and u are decoupled from and selectively enriched relative to light rey and th respectively due to oxidation to ce iv and u vi oxidation of ce3 and u4 is promoted by the significantly higher stability of the ce iv and u vi dfob complexes as compared to the ce iii and u iv dfob complexes this is similar to the relationship between the ce iv and ce iii pentacarbonate complexes that cause positive ce anomalies in alkaline lakes however while formation of ce iv carbonate complexes is confined to alkaline environments ce iv dfob complexes may produce positive ce anomalies even in mildly acidic and near neutral natural waters siderophore promoted dissolution processes also significantly enhance mobility of other immobile hfse and may not only cause or modify ce anomalies and th u fractionation but also mobilization of and fractionation between zr hf th and redox insensitive rey during weathering pedogenesis diagenesis and incongruent dissolution of particles in seawater and freshwater siderophores may significantly affect the use of hfse as geochemical tools concave downward light rey patterns may be used as a biosignature for water rock interaction in the presence of siderophores enhanced and preferential mobilization of u relative to th in the presence of siderophores may produce th u signals comparable to those indicative of weathering under oxidized conditions which might constrain the use of u concentrations and th u ratios as a paleoredox proxy the enhanced mobilization of zr and especially hf from igneous rocks in the presence of dfob might have implications for the use of the latter as a tracer for the impact of continental weathering on seawater chemistry because siderophore complexes affect the particle reactivity of hf and zr they may prevent effective removal of terrigenous hf and zr during aggregation coagulation of riverine particles in estuaries siderophore promoted solubilization and stabilization might hence be an additional way to transport continental hf and zr to the oceans furthermore siderophore enhanced mobilization may also have implications for the remediation techniques employed to immobilize hfse such as u th and rey at nuclear waste and reprocessing sites and at rey ore processing plants where soils are commonly contaminated with these sometimes radioactive heavy metals

composition of impact melt particles and the effects of post impact alteration in suevitic rocks at the yaxcopoil 1 drill core chicxulub crater mexico petrographical and chemical analysis of melt particles and alteration minerals of the about 100 m thick suevitic sequence at the chicxulub yax 1 drill core was performed the aim of this study is to determine the composition of the impact melt the variation between different types of melt particles and the effects of post impact hydrothermal alteration we demonstrate that the compositional variation between melt particles of the suevitic rocks is the result of both incomplete homogenization of the target lithologies during impact and subsequent post impact hydrothermal alteration most melt particles are andesitic in composition clinopyroxene rich melt particles possess lower sio2 and higher cao contents these are interpreted by mixing of melts from the silicate basement with overlying carbonate rocks multi stage post impact hydrothermal alteration involved significant mass transfer of most major elements and caused further compositional heterogeneity between melt particles following backwash of seawater into the crater palagonitization of glassy melt particles likely caused depletion of sio2 al2o3 cao na2o and enrichment of k2o and feotot during an early alteration stage since glass is very susceptible to fluid rock interaction the state of primary crystallization of the melt particles had a significant influence on the intensity of the postimpact hydrothermal mass transfer and was more pronounced in glassy melt particles than in wellcrystallized particles in contrast to other occurrences of chicxulub impactites the yax 1 suevitic rocks show strong potassium metasomatism with hydrothermal k feldspar formation and whole rock k20 enrichment especially in the lower unit of the suevitic sequence a late stage of hydrothermal alteration is characterized by precipitation of silica analcime and na bearing mg rich smectite among other minerals this indicates a general evolution from a silica undersaturated fluid at relatively high potassium activities at an early stage toward a silica oversaturated fluid at relatively high sodium activities at later stages in the course of fluid rock interaction

flooding of ganymede s bright terrains by low viscosity water ice lavas large regions of the jovian moon ganymede have been resurfaced but the means has been unclear suggestions have ranged from volcanic eruptions of liquid water or solid ice to tectonic deformation but definitive high resolution morphological evidence has been lacking here we report digital elevation models of parts of the surface of ganymede derived from stereo pairs combining data from the voyager and galileo spacecraft which reveal bright smooth terrains that lie at roughly constant elevations 100 to 1 000 metres below the surrounding rougher terrains these topographic data together with new images that show fine scale embayment and burial of older features indicate that the smooth terrains were formed by flooding of shallow structural troughs by low viscosity water ice lavas the oldest and most deformed areas the reticulate terrains in general have the highest relative elevations whereas units of the most common resurfaced type the grooved terrain lie at elevations between those of the smooth and reticulate terrains bright terrain which accounts for some two thirds of the surface probably results from a continuum of processes including crustal rifting shallow flooding and groove formation volcanism plays an integral role in these processes and is consistent with partial melting of ganymede s interior

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

mass anomalies on ganymede abstract radio doppler data generated with nasa s galileo spacecraft during its second encounter with jupiter s moon ganymede are used to infer the locations and magnitudes of mass anomalies on ganymede we construct models for both surface and buried anomalies with only one flyby and no global coverage a solution for mass anomalies cannot be uniquely determined however we are able to constrain acceptable solutions for mass anomalies to four broad regions a near polar region and three that are roughly equatorial if the mass anomalies are constrained to lie at the surface the centers of the regions are located near the coordinates 77 n 333 w 36 n 0 w 33 n 130 w and 7 n 194 w if the mass anomalies are located at the deep ice rock interface 800 km below the surface the regions centers are approximately 65 n 17 w 32 n 30 w 37 n 175 w and 15 n 211 w for both models the regions are up to a few thousand kilometers across the magnitude of mass anomalies on the surface is on the order of 10 17 kg mass anomalies at the ice rock interface are on average no more than an order of magnitude larger 10 18 kg there are two positive and two negative mass anomalies in both the surface and ice rock interface models one of the positive mass anomalies at the surface is associated with galileo regio the other positive surface mass anomaly is located at high northern latitudes with no obvious geological association negative surface mass anomalies lie near uruk sulcus and between perrine regio and barnard regio near sicyan sulcus and phrygia sulcus the locations of the ice rock interface mass anomalies lie approximately radially below the surface anomalies positive mass anomalies at the surface could be associated with the silicate rich ice or accumulated silicate layers of the dark regions negative mass anomalies at the surface could be associated with the relatively clean low lying ice of sulci alternatively ganymede s mass anomalies could be associated with the topography or other mass concentrations at the deep ice rock interface

nitrogen and argon isotopic signatures in graphite from the 3 8 ga old isua supracrustal belt southern west greenland abstract the problems involved with the interpretation of carbon isotopes as indicators for early life in highly metamorphosed early archean rocks have prompted the search for additional chemical and isotopic biomarkers here we report an attempt to identify the origin of carbonaceous matter in the 3 8 ga old isua supracrustal belt in southern west greenland by measuring the concentration and isotopic composition of a trapped nitrogen component stepped combustion pyrolysis mass spectrometry of carbonaceous matter in several rock samples revealed three different reservoirs of trapped nitrogen 1 nitrogen associated with a very small amount of reactive carbonaceous material 2 nitrogen intercalated in graphite correlated with intercalated radiogenic argon 3 nitrogen strongly retained at defects or chemically bound in the graphite structure the 15n of nitrogen associated with reactive carbonaceous matter ca 6 overlaps with that of average phanerozoic sedimentary organic matter and is believed to be part of nonindigenous postmetamorphic biologic material in situ raman spectroscopy confirmed the high degree of crystallinity of the metamorphosed indigenous carbonaceous material and this material is further referred to as graphite graphite interpreted as epigenetic associated with mg mn siderite in metacarbonates contains a very small strongly retained nitrogen component with a low 15n ratio 3 to 1 this range overlaps with values that are typically found in archean kerogens but also those of a metamorphically emplaced inorganic basaltic source geological constraints suggest that this graphite incorporated nitrogen from surrounding metabasaltic rocks graphite interpreted as syngenetic and biogenic found in a turbidite deposit is relatively similar to this mg mn siderite derived graphite based on degree of graphite crystallinity amount of trapped radiogenic argon low nitrogen concentration and 15n signature we conclude that nitrogen concentration and its isotope ratio in graphite cannot be used conclusively as a biomarker in these rocks from the highly metamorphosed isua supracrustal belt

reorientation of icy satellites by impact basins 1 large impact basins are present on many of the icy satellites of the outer solar system assuming that their present day topography is uncompensated such basins can cause significant poleward reorientations for slow rotating satellites this reorientation may have been accompanied by transient large amplitude wobble the largest basins on tethys rhea and titania are predicted to have caused reorientations of roughly 4 7 and 12 respectively resulting in global tectonic stresses up to 0 5 mpa the potential anomalies associated with the basins can be up to one third of those expected for a hydrostatic tidally and rotationally deformed body and may complicate interpretation of the satellite interior structure pluto and charon because of their slow rotation are also likely to have undergone reorientation of 10 20 if they possess impact basins of comparable sizes to those of the saturnian satellites

a 3 800 million year isotopic record of life from carbon in sedimentary rocks an increased ratio of 12c to 13c an indicator of the principal carbon fixing reaction of photosynthesis is found in sedimentary organic matter dating back to almost four thousand million years ago a sign of prolific microbial life not long after the earth s formation partial biological control of the terrestrial carbon cycle must have been established very early and was in full operation when the oldest sediments were formed

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

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

the onset and early evolution of life the tension between co 2 dissolved at relatively high atmospheric pressure in the hadean ocean and h 2 generated as ocean water oxidized ferrous iron during convection in the oceanic crust was resolved by the onset of life we suggest that this chemosynthetic life emerged within hydrothermal mounds produced by alkaline solutions of moderate temperature in the relative safety of the deep ocean fl oor exothermic reaction between hydrothermal h 2 hcoo and ch 3 s with co 2 was catalyzed in inorganic membranes near the mound s surface by mackinawite fes nanocrysts and ready made clusters corresponding to the greigite fe 5 nis 8 structure such clusters were precursors to the active centers e g the c cluster fe 4 nis 5 of a metalloenzyme that today catalyzes acetate synthesis viz the bifunctional dehydrogenase enzyme acs codh the water and some of the acetate h 3 c coo produced in this way were exhaled into the ocean together as fl uid waste glycine h 3 n ch 2 coo and other amino acids as well as tiny quantities of rna generated in the same milieu were trapped within tiny iron sulfi de cavities energy from the acetate reaction augmented by a proton gradient operating through the membrane was spent polymerizing glycine and other amino acids into short peptides upon the phosphorylated mineral surface in turn these peptides sequestered and thereby protected the catalytically and electrochemically active pyrophosphate and iron nickel sulfi de clusters from dissolution or crystallization intervention of rna as a polymerizing agent for amino acids also led to an adventitious though crude process of regulating metabolism a process that was also to provide genetic information to offspring the fl uxes of energy and nutrient available in the hydrothermal mound commensurate with the requirements of life encouraged differentiation of the fi rst microbes into two separate domains at the bifurcation the bacteria were to specialize in acetogenesis and the archaea into methanogenesis representatives of both these domains left the mound by way of the ocean fl oor and crust to colonize the deep biosphere once life had emerged and evolved to the extent of being able to reduce nitrogen for use in peptides and nucleic acids light could have been used directly as an energy source for biosynthesis certain bacteria may have been able to do this where protected from hard uv by a thin coating of chemical sediment produced at a subaerial hot spring operating in an obducted and uplifted portion of the deep biosphere

search for traces of the late heavy bombardment on earth results from high precision chromium isotopes abstract high precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the 3 7 gyr isua supracrustal belt isb in west greenland cannot be distinguished from the standard terrestrial 53 cr 52 cr ratio at our present level of resolution as a consequence our search for chemical traces of possible impact derived meteoritic components asteroidal and or cometary material or accreted cosmic dust in the earth s oldest chemical and detrital sediments was negative our results based on the 53 mn 53 cr short lived radionuclide system half life of 3 7 myr cannot confirm the recent findings by 1 r schoenberg b s kamber k d collerson s moorbath tungsten isotope evidence from approximately 3 8 gyr metamorphosed sediments for early meteorite bombardment of the earth nature 418 2002 403 405 of tungsten isotope anomalies based on the 182 hf 182 w short lived radionuclide system half life of 9 myr in these sediments which were interpreted as indicating a component derived from meteorites possible reasons for the failure to trace cosmic material in the isb metasediments are various 1 the samples studied are not representative 2 the sedimentation period did not overlap with the period of late heavy bombardment of the moon and 3 the potential chromium anomalies if present are too small to be traceable by our present levels of detection unequivocal evidence of a late heavy bombardment on the early earth therefore remains elusive and uncertain

constraints on the location magnitude and dimensions of ganymede s mass anomalies abstract previously radio doppler data generated with nasa s galileo spacecraft during its second encounter with jupiter s moon ganymede were used to infer the locations and magnitudes of mass anomalies on ganymede using point mass models however the point mass solutions do not provide the vertical and horizontal extent of the anomalous mass concentrations here we provide the results of a new study using spherical cap disks to model ganymede s mass anomalies the spherical cap disk models not only provide the locations and magnitudes of the mass anomalies but also their vertical and horizontal dimensions the new models show that three disks a positive mass located at 53 0 n 127 0 w and two negative masses located at 22 0 n 87 0 w and 49 0 n 219 0 w can explain the data the magnitudes of the mass anomalies are on the order of 10 18 kg the diameters of the anomalies are a few thousand kilometers the positive anomaly is about 100 meters thick and both negative anomalies have a thickness of less than a kilometer we use the additional information provided by the disk models to investigate the viability of mass anomalies at ganymede s surface by comparing the diameters of the anomalies to the sizes of regiones and sulci and the anomalies thicknesses to accumulated layers of rock and clean ice on the surface we find that the dimensions of the mass anomalies could be explained by concentrations of rock in the regio and rock free ice in the sulci these results confirm that mass anomalies may reside on or near ganymede s surface and that positive mass anomalies are contained within areas of dark terrain and negative mass anomalies within bright terrain

hydrogen based carbon fixation in the earliest known photosynthetic organisms thin carbonaceous laminations preserved in shallow water facies of the 3416 ma buck reef chert south africa have been interpreted to represent some of the oldest known mats constructed by photosynthetic microbes preservation of these mats within a unit containing facies deposited at water depths ranging from 0 m to 200 m provides an opportunity to explore the electron donors employed in early microbial photosynthesis the presence of siderite feco3 as a primary sediment lack of hematite fe2o3 and lack of cerium anomalies throughout the buck reef chert imply that the entire water column was anoxic despite the presence of photosynthetic organisms authigenic uranium ua u th 3 correlates inversely with siderite abundance suggesting that variations in carbonate rather than oxygen activity controlled uranium mobility the inferred lack of oxygen and ferric minerals and the presence of dissolved fe2 in the water column imply that h2o fe2 and h2s could not have served as primary electron donors for carbon fixation it is most likely that buck reef chert bacteria utilized h2 as the primary reductant for photosynthesis

initial pb of the amitsoq gneiss revisited implications for the timing of early archaean crustal evolution in west greenland reply abstract the rapid change in 207 pb 206 pb ratio during early earth history provides a powerful constraint for temporal resolution of archaean crustal evolution new and published data for amitsoq gneiss feldspars and whole rocks define the earth s least radiogenic silicate pb known until now the most important feature of the data is that independent of the exact slope of the regression in a common pb diagram a very well defined intercept with plausible terrestrial pb evolution lines is obtained between 3 63 and 3 66 ga identical to a combined pb pb regression age of 3654 73 ma mswd 17 6 we conclude that the magmatic precursors of the amitsoq orthogneisses separated from a depleted mantle like source at ca 3 65 ga this contrasts sharply with claims based on ion probe u pb zircon dating that the amitsoq gneisses had a prolonged episodic magmatic evolution extending from ca 3 87 to 3 60 ga pb isotopes offer no evidence that any amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3 85 ga we propose that zircons significantly older than 3 65 ga were inherited from chemically evolved rocks as old as ca 3 85 ga geochemical arguments are presented to support this proposal new feldspar and whole rock pb isotope data are also reported for sheets of quartz dioritic gneiss on akilia island that yield the oldest zircon u pb age clusters in the range 3 84 3 87 ga these sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose c isotope ratios are regarded as biogenic in origin as above pb isotopes in the sheets are consistent with a magmatic age of 3 65 ga which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers additional pb isotope data as well as limited published sm nd and u pb zircon data on the akilia enclaves themselves suggest an age between ca 3 65 and 3 70 ga for their original deposition some 150 200 ma younger than proposed previously thus the question of overlap of earliest life with a bolide impact scenario terminating 3 80 ga as on the moon becomes irrelevant

terrestrial impact melt rocks and glasses abstract the effects of meteorite and comet impact on earth are rock brecciation the formation of shock metamorphic features rock melting and the formation of impact structures i e simple craters complex craters and multi ring basins large events such as the 65 ma chicxulub impact are believed to have had catastrophic environmental effects that profoundly influenced the development of life on earth in this review an attempt is made to summarize some of the voluminous literature on impact melting one important aspect of planetary impact provide some comments on this process and to make suggestions for future research the products of impact melting are glasses impact melt rocks and pseudotachylites our treatise deals mainly with the geological setting petrography and major element chemistry of melt rocks and glasses impact glasses in several petrographic aspects are similar to volcanic glasses but they are associated with shock metamorphosed mineral and rock fragments and in places with siderophile element anomalies suggestive of meteoritic contamination they are found in allogenic breccia deposits within fall back suevite and outside fall out suevite impact craters and as spherules in distal ejecta large events such as the k t boundary chicxulub impact are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common on a microscopic scale however impact glasses are commonly strikingly heterogeneous tektites are glasses ejected from craters over large distances they are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal sedimentary rocks four tektite strewn fields are known three of which can be tied to specific impact craters impact melt rocks form sheets lenses and dike like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks dikes of impact melt rocks also intrude the rocks of the crater floor they commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine to coarse grained chemically they are strikingly homogeneous but as with impact glasses exemptions to this rule do exist large and thick melt bodies such as the sudbury igneous complex sic are differentiated or may represent a combination of impact melt rocks sensu strictu and impact triggered deep crustal melts a concerted multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes especially of melting excavation and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater impact triggered pressure release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu strictu this controversial idea needs to be tested by a re evaluation of existing data and models be they based on field or laboratory research only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping in this review we summarize observations made on impact melt rocks and impact glasses in a number of north american brent haughton manicouagan new quebec sudbury wanapitei all in canada asian popigai russia zhamanshin kazakhstan two south african structures morokweng and vredefort the henbury crater field of australia and one european crater ries germany our tables listing major element chemical compositions of impact glasses and melt rocks however include also data from structures not dealt with in further detail

a review of the origins of subparallel ridges and troughs generalized morphological predictions from terrestrial models the morphologies of ridges and troughs on earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites we consider tension fracturing normal faulting thrust faulting buckle folding strike slip tectonism fissure eruption wall diapirism surface folding of a flow and near surface laccolithic intrusion the landform characteristics and geologic associations predicted of each process are generalized to be independent of planet specific parameters we find that the combinations of landforms derived from each process are unique making each distinguishable in the idealized case morphological parameters most diagnostic of formational process are ridge and trough cross sectional shape termination style and incipient character the characteristics described are useful in constraining the origin of ridge and trough terrains observed in voyager images and will aid in analysis of high resolution images from the galileo and cassini spacecraft

diapir induced reorientation of saturn s moon enceladus the recent discovery of active water venting from the surface of saturn s icy moon enceladus has caused a ripple of excitement as it suggests the possibility of subsurface liquid water geologically enceladus poses problems one the reason why the hotspot of geological activity is centred precisely on the south pole has been tackled by francis nimmo and robert t pappalardo they identify reorientation of enceladus s rotation axis as the likely cause and an upwelling of low density material to the surface as the cause of that reorientation comparison of the distribution of impact craters on the leading and trailing hemispheres will be a good test of the reorientation part of the theory enceladus is a small icy satellite of saturn its south polar region consists of young tectonically deformed terrain and has an anomalously high heat flux1 2 this heat flux is probably due to localized tidal dissipation within either the ice shell3 or the underlying silicate core4 the surface deformation is plausibly due to upwelling of low density material diapirism5 as a result of this tidal heating here we show that the current polar location of the hotspot can be explained by reorientation of the satellite s rotation axis because of the presence of a low density diapir if the diapir is in the ice shell then the shell must be relatively thick and maintain significant rigidity elastic thickness greater than 0 5 km if the diapir is in the silicate core then enceladus cannot possess a global subsurface ocean because the core must be coupled to the overlying ice for reorientation to occur the reorientation generates large 10 mpa tectonic stress patterns6 that are compatible with the observed deformation of the south polar region2 we predict that the distribution of impact craters on the surface will not show the usual leading hemisphere trailing hemisphere asymmetry a low density diapir also yields a potentially observable negative gravity anomaly

high phosphate availability as a possible cause for massive cyanobacterial production of oxygen in the paleoproterozoic atmosphere abstract the deposition of major precambrian phosphorites was restricted to times of global change and atmospheric oxygenation at both ends of the proterozoic phosphorites formed after highly positive carbon isotope excursions in carbonates deposited during the paleoproterozoic lomagundi jatuli event and the neoproterozoic cryogenian and ediacaran periods the correlative step wise rise in atmospheric oxygen over the proterozoic has been linked to changes in the carbon cycle however the postulated relations between carbon isotope events phosphorites and atmospheric oxygenation remain unexplained paleoproterozoic carbonates of the aravalli supergroup india preserve evidence for cyanobacterial blooms in the form of tightly packed stromatolitic columns in the world s oldest significant sedimentary phosphate deposit restricted basins of the lower aravalli group with stromatolitic phosphorites in jhamarkotra udaipur jhabua and sallopat exhibit near zero 13 c carb values and large ranges of 13 c org values between 33 3 and 10 1 indicative of a complex carbon cycle because phosphate accumulates primarily in oxic sediments these eutrophic microbial ecosystems likely developed within the photic zone of the shallow oxygenated marine realm this is consistent with deposition during the time of increasingly more oxidizing conditions after the great oxidation event goe approximately contemporaneous basins without phosphate deposits from ghasiar karouli negadia umra and babarmal exhibit a range of positive 13 c carb excursions some with values up to 11 2 that suggest high rates of organic carbon burial and others with moderately high 13 c carb values around 6 or 3 that suggest smaller carbon cycle perturbations the 15 n values of all these rocks vary between 0 7 and 3 4 and are consistent with the predominance of nitrogen fixation during cyanobacterial blooms in all basin types such low nitrogen isotope values are interpreted to have arisen from the biological response to high phosphate availability we conclude that increased phosphate availability during and after the paleoproterozoic lomagundi jatuli event likely caused cyanobacterial blooms and was a key factor in the oxygenation of earth s atmosphere increasing oxygenation of the shallow ocean seafloor favored the removal of excess phosphate as authigenic apatite thus dampening effects of weathering increases on organic burial and marine 13 c carb after about 2 0 ga

local lunar gravity field analysis over the south pole aitken basin from selene farside tracking data 1 we present a method with which we determined the local lunar gravity field model over the south pole aitken spa basin on the farside of the moon by estimating adjustments to a global lunar gravity field model using selene tracking data our adjustments are expressed in localized functions concentrated over the spa region in a spherical cap with a radius of 45 centered at 191 1 e 53 2 s and the resolution is equivalent to a 150th degree and order spherical harmonics expansion the new solution over spa was used in several applications of geophysical analysis it shows an increased correlation with high resolution lunar topography in the frequency band l 40 70 and admittance values are slightly different and more leveled when compared to other global gravity field models using the same data the adjustments expressed in free air anomalies and differences in bouguer anomalies between the local solution and the a priori global solution correlate with topographic surface features the moho structure beneath the spa basin is slightly modified in our solution most notably at the southern rim of the apollo basin and around the zeeman crater

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

lead isotope and trace element geochemistry of paleoproterozoic metasedimentary rocks in the lead and rochford basins black hills south dakota usa implications for genetic models mineralization ages and sources of leads in the homestake gold deposit abstract mixed silicate carbonate sulfide facies banded iron formations with elevated detrital components were deposited in the northern black hills during middle paleoproterozoic time between 2 012 and 1 974 ga homestake iron formation and equivalents and at 1 887 ga rochford iron formation these iron formations deposited atop shales and basalts in intracontinental rifted basins separated in time by 80 130 myr are the hosts of extensive late paleoproterozoic gold mineralization gold sulfide ore bodies occur within dilated segments of late stage ductile brittle shears that formed after isoclinal folding related to wyoming superior continental collision lead stepwise leaching pbsl data for monazite bearing garnet separated from a sample of homestake iron formation has yielded an isochron age of 1746 10 ma 2 mswd 0 42 which represents a maximum age for both the isoclinal folding and subsequent gold mineralization monazite bearing garnet from a nearby harney peak type granitic pegmatite believed to be coeval with latest stage semibrittle shears and quartz veins that crosscut homestake ore bodies has yielded a pbsl isochron age of 1713 10 ma 2 mswd 4 1 which probably represents a minimum age for the gold event pb isotopes of sulfides from replacement ore and shear ore within the homestake iron formation cannot distinguish between these distinct ore types however both pb isotopes and rey rare earth and yttrium abundances indicate slight disequilibrium between the ore sulfides and the barren iron formation thus revealing an epigenetic sensu stricto component to the gold mineralization the pb isotope characteristics of the ore further reflect an admixed component with low 232th 204pb and variable th u ratios in addition to compositions directly derived from bif typical leads this added component corresponds to pb from the anoxic metasedimentary rocks directly underlying the iron formations and from the nearby pegmatitic granite the apparent pb isotopic match between ore and granite is further consistent with the pbsl data for shear ore type arsenopyrite which define an isochron age of 1719 45 38 ma 2 mswd 0 2 and by a striking overlap of the pb isotopic data fields defined by the homestake galenas and pegmatitic feldspars the precambrian ore leads are therefore best explained as mixtures of magmatic hydrothermal and host rock mobilized leads the lead area also contains significant gold bearing deposits of tertiary age which are hosted in both paleoproterozoic and phanerozoic rocks tertiary mineralization was temporally and spatially related to coeval plutons emplaced during laramide uplift of the black hills and represents the final mineralization event recognized in the homestake mine area like the precambrian galena leads the leads of tertiary galena hosted by middle paleoproterozoic rocks including the homestake iron formation define a trend best explained by mixtures of pb derived from magmatic fluids exsolved from tertiary intrusives and pb leached from the same country rocks that contaminated the granite derived hydrothermal fluids ascending through them during the much earlier shearing and mineralization event inferred at 1730 1715 ma

palaeoenvironmental controls on the uranium distribution in an upper carboniferous black shale gastrioceras listeri marine band and associated strata england the mudrocks associated with the gastrioceras listeri g listeri marine band contain 0 to 200 ppm authigenic uranium several geochemical degree of pyritisation dop c s ratios and framboidal pyrite size distribution and palaeontological indicators oxygen restricted biofacies orb analysis suggest that the highest authigenic uranium concentrations are within marine sediments associated with the most oxygen restricted biofacies orbs 2 and 3 the uranium peaks tend to occur at the transition between biofacies rather than within the middle of more persistently anoxic intervals these horizons may have been deposited when the oxygen minimum zone impinged on the seafloor brief oxygenation of an otherwise anoxic environment was conducive to francolite precipitation which then scavenged dissolved uranium slow sedimentation rates are also important in concentrating uranium and francolite thus enriched u values are only encountered under specific conditions of low but fluctuating oxygen regime and extremely slow sedimentation rates truly euxinic facies lacking any fossils and a uniformly small framboid population do not contain high concentrations of francolite and are not u rich

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

evidence for oxygenic photosynthesis half a billion years before the great oxidation event the early earth was characterized by the absence of oxygen in the ocean atmosphere system in contrast to the well oxygenated conditions that prevail today atmospheric concentrations first rose to appreciable levels during the great oxidation event roughly 2 5 2 3 gyr ago the evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise but it has proved difficult to constrain the timing of this evolutionary innovation1 2 the oxidation of manganese in the water column requires substantial free oxygen concentrations and thus any indication that mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing mn oxides are not commonly preserved in ancient rocks but there is a large fractionation of molybdenum isotopes associated with the sorption of mo onto the mn oxides that would be retained here we report mo isotopes from rocks of the sinqeni formation pongola supergroup south africa these rocks formed no less than 2 95 gyr ago3 in a nearshore setting the mo isotopic signature is consistent with interaction with mn oxides we therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen

grooved terrain on ganymede first results from galileo high resolution imaging abstract high resolution galileo imaging has provided important insight into the origin and evolution of grooved terrain on ganymede the uruk sulcus target site was the first imaged at high resolution and considerations of resolution viewing geometry low image compression and complementary stereo imaging make this region extremely informative contrast variations in these low incidence angle images are extreme and give the visual impression of topographic shading however photometric analysis shows that the scene must owe its character to albedo variations a close correlation of albedo variations to topography is demonstrated by limited stereo coverage allowing extrapolation of the observed brightness and topographic relationships to the rest of the imaged area distinct geological units are apparent across the region and ridges and grooves are ubiquitous within these units the stratigraphically lowest and most heavily cratered units lineated grooved terrain generally show morphologies indicative of horst and graben style normal faulting the stratigraphically highest groove lanes parallel ridged terrain exhibit ridges of roughly triangular cross section suggesting that tilt block style normal faulting has shaped them these extensional tectonic models are supported by crosscutting relationships at the margins of groove lanes thus a change in tectonic style with time is suggested in the uruk sulcus region varying from horst and graben faulting for the oldest grooved terrain units to tilt block normal faulting for the latest units the morphologies and geometries of some stratigraphically high units indicate that a strike slip component of deformation has played an important role in shaping this region of grooved terrain the most recent tectonic episode is interpreted as right lateral transtension with its tectonic pattern of two contemporaneous structural orientations superimposed on older units of grooved terrain there is little direct evidence for cryovolcanic resurfacing in the uruk sulcus region instead tectonism appears to be the dominant geological process that has shaped the terrain a broad wavelength of deformation is indicated corresponding to the voyager observed topography and may be the result of ductile necking of the lithosphere while a finer scale of deformation probably reflects faulting of the brittle near surface the results here form a basis against which other galileo grooved terrain observations can be compared

the paleoproterozoic fossil record implications for the evolution of the biosphere during earth s middle age abstract the paleoproterozoic 2 5 1 6 ga era is a decisive time in earth and life history the paleobiological record microfossils stromatolites biomarkers and isotopes illustrates the biosphere evolution during a time of transitional oceanic and atmosphere chemistries benthic microfossil assemblages are recorded in a variety of oxygenated sulfidic and ferruginous environments representative of the spatial heterogeneities and temporal variations characteristic of this era the microfossil assemblages include iron metabolizing and or iron tolerant prokaryotes sulfur metabolizing prokaryotes cyanobacteria other undetermined prokaryotes and eukaryotes the undetermined microfossils represent a majority of the assemblages and thus raise a challenge to determine the nature and role of microorganisms in these changing environments despite the early evolution of the eukaryotic cellular toolkit early eukaryotic crown group diversification may have been restrained in the paleoproterozoic by ocean chemistry conditions but it increased during the late mesoproterozoic early neoproterozoic despite the continuation of similar conditions through the miscalled boring billion then amplified significantly but perhaps within lower taxonomic levels with the demise of euxinic conditions and increase in ecological complexity the emerging picture is one of a changing and more complex biosphere in which the three domains of life archaea bacteria and eukarya were diversifying in various ecological niches marked by the diversification of identified microfossils stromatolites increasing abundance of preserved biomarkers and appearance of macroscopic problematic fossils or trace fossils

oxygenation of the archean atmosphere new paleosol constraints from eastern india it is widely believed that atmospheric oxygen saturation rose from 5 present atmospheric level pal in the archean to 10 2 pal at the great oxidation event goe at ca 2 4 ga but it is unclear if any earlier oxygenation events occurred here we report u pb zircon data indicating that a pyrophyllite bearing paleosol from keonjhar in the precambrian singhbhum craton of eastern india formed between 3 29 and 3 02 ga making it one of very few known archean paleosols globally field and geochemical evidence suggests that the upper part of the paleosol was eroded prior to unconformable deposition of an overlying sequence of shallow marine siliciclastic sediments a negative cerium anomaly within the currently preserved level of the paleosol indicates that ancient oxidative weathering occurred in the original upper soil profile the presence of redox sensitive detrital uraninite and pyrite together with a complete absence of pyrophyllite in the overlying sediments indicate that the mineralogical and geochemical features of the paleosol were established prior to the unconformable deposition of the sediments and are not related to subsequent diagenetic or hydrothermal effects we suggest that a transient atmospheric oxygenation event occurred at least 600 m y prior to the goe and 60 m y prior to a previously documented archean oxygenation event we propose that several pulsed and short lived oxygenation events are likely to have occurred prior to the goe and that these changes to atmospheric composition arose due to the presence of organisms capable of oxygenic photosynthesis

titan s gravity field and interior structure after cassini since its arrival at saturn in 2004 cassini performed nine flybys devoted to the determination of titan s gravity field and its tidal variations here we present an updated gravity solution based on the final data set collected during the gravity dedicated passes before cassini s plunge into saturn s atmosphere the data set includes an additional flyby t110 march 2015 primarily devoted to imaging titan s north polar lakes carried out with the low gain antenna this flyby was particularly valuable because the closest approach occurred at a high latitude 75 n over an area not previously sampled previously published gravity results iess et al 2012 indicated that titan is subject to large eccentricity tides in response to the time varying perturbing potential exerted by saturn the magnitude of the response quadrupole field expressed in the tidal love number k 2 was used to infer the existence of an internal ocean the new gravity field determination provides an improved estimate of k 2 of about 0 62 accurate to a level of a few percent the value is higher than the simplest models of titan suggest and the interpretation is unclear possibilities include a high density ocean as high as 1300 kg m 3 a partially viscous response of the deeper region or a dynamic contribution to the tidal response the new solution includes higher degree and order harmonic coefficients up to 5 and offers an improved map of gravity anomalies the geoid is poorly correlated with the topography implying strong compensation in addition the updated geoid and its associated uncertainty could be used to refine the gravity altimetry correlation analysis and for improved interpretation of radar altimetric data

size and morphology of the chicxulub impact crater the chicxulub impact in mexico has been linked to the mass extinction of species at the end of the cretaceous period from seismic data collected across the offshore portion of the impact crater the diameter of the transient cavity is determined to be about 100 km this parameter is critical for constraining impact related effects on the cretaceous environment with previous estimates of the cavity diameter spanning an order of magnitude in impact energy the offshore seismic data indicate that the chicxulub crater has a multi ring basin morphology similar to large impact structures observed on other planets such as venus

the antiquity of oxygenic photosynthesis evidence from stromatolites in sulphate deficient archaean lakes the tumbiana formation about 2700 million years old was largely deposited in ephemeral saline lakes as judged by the unusual evaporite paragenesis of carbonate and halite with no sulfate stromatolites of diverse morphology occur in the lacustrine sediments some with palimpsest fabrics after erect filaments these stromatolites were probably accreted by phototropic microbes that from their habitat in shallow isolated basins with negligible sulfate concentrations almost certainly metabolized by ozygenic photosynthesis

delayed euxinia in paleoproterozoic intracontinental seas vital havens for the evolution of eukaryotes abstract increased flux of sulfate to the oceans in the aftermath of the great oxidation event goe 2 4 billion years ago ga caused major changes in seawater chemistry which eventually contributed to the cessation of iron formation deposition 1 8 ga it is generally accepted that this engendered heterogeneous stratified redox conditions with anoxic and sulfidic euxinic conditions in shallow open marine environments and anoxic ferruginous conditions in deeper environments however the redox evolution of intracontinental marine basins following the cessation of iron formation deposition remains poorly understood here we report contrasting paleoredox conditions in two shale units of the lower mcarthur basin northern australia soon after the cessation of iron formation deposition 1 84 ga our data shows that the 1 78 ga mcdermott formation was deposited in a sulfur limited anoxic shallow marine environment whereas the younger 1 73 ga wollogorang formation was deposited in a euxinic shallow marine environment this implies a delay in the development of euxinia in a shallow intracontinental basin following the onset of euxinia in the open marine realm since bioessential metals are sequestered by pyrite deposition under euxinic conditions protracted low sulfidic conditions in 1 78 ga intracontinental shallow environments could have provided vital niches for nitrogen fixing prokaryotes and eukaryotes thus the ability for localized paleoproterozoic intracontinental basins to remain non euxinic after the onset of euxinia in shallow open marine shelves highlights the importance of intracontinental environments to the evolution and diversification of microbial life perhaps throughout the wider proterozoic

hydrothermal metasomatic and tectono metamorphic processes in the isua supracrustal belt west greenland a multi isotopic investigation of their effects on the earth s oldest oceanic crustal sequence abstract despite superimposed metamorphic overprinting and metasomatic alterations primary volcanic features remain preserved in low strain domains of mafic volcanic sequences in the western isua supracrustal belt isb west greenland these basaltic successions represent the hitherto oldest known fragments of oceanic crust on earth early archean metasomatic fluids rich in light rare earth elements lree th u pb ba and alkalies invaded the supracrustal package and distinctively altered the basaltic sequences field relationships source characteristics traced by pb isotopes and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the isb between 3 81 and 3 74 ga ago subsequent early archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks allanite occurs as the most characteristic and critical secondary metasomatic metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration even in domains of low strain because of its high concentration of lree th and u this secondary mineral accounts for much of the disturbances recorded by the sm nd and th u pb isotope systematics of the pillowed metabasalts the supracrustal sequences were tectono metamorphically affected to varying degrees during a late archean 2 6 to 2 8 ga old event also recognized in the adjacent gneiss terranes of the isuakasia area the degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite hosted parent daughter elements to the overall whole rock mass budget of the respective isotope systems although low strained initially only weakly metasomatized pillow basalts remained more or less closed with respect to the u pb and rb sr systems since 3 74 ga the sm nd system appears to have been partially opened on a whole rock scale during the late archean event this diversified behavior of the whole rock isotope systems with respect to late archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late archean recrystallization processes with associated renewed fluid flow in reactivated zones of high strain where primary metasomatic alteration is most prominently developed late archean partial resetting also of the u pb isotope system on a whole rock scale occurred this is consistent with an apparent late archean age of kyanite which initially crystallized during the early archean metamorphism its age is controlled by the u pb systematics of allanite inclusions which have exchanged their isotopic properties during the tectono metamorphic event that overprinted the oceanic crustal sequence at isua more than 1000 myr later these results underline the need for care in the interpretation of whole rock geochemical data from polymetamorphic rocks in general and from the isua oceanic crustal sequences in particular to constrain isotopic models of early earth s evolution likewise this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from isua to infer models for geotectonic settings relevant for their formation

on the resurfacing of ganymede by liquid water volcanism abstract a long popular model for producing ganymede s bright terrain involves flooding of low lying graben with liquid water slush or warm soft ice the model suffers from major problems however including the absence of obvious near surface heat sources the negative buoyancy of liquid water and the lack of a mechanism for confining the flows to graben floors we present new models for cryovolcanic resurfacing to overcome these difficulties tidal heating within an ancient laplace like orbital resonance showman and malhotra 1997 icarus 127 93 showman et al 1997 icarus 129 367 provides a plausible heat source and could allow partial melting to occur as shallow as 5 10 km depth our favored mechanism for delivering this water to the surface invokes the fact that topography such as a global set of graben causes subsurface pressure gradients that can pump water or slush upward onto the floors of topographic lows graben despite the negative buoyancy of the liquid these eruptions can occur only within the topographic lows furthermore as the low areas become full the pressure gradients disappear and the resurfacing ceases this provides an explanation for the observed straight dark bright terrain boundaries water cannot overflow the graben so resurfacing rarely embays craters or other rough topography pure liquid water can be pumped to the surface from only 5 10 km depth but macroscopic bodies of slush ascending within fractures can reach the surface from much greater depths due to the smaller negative buoyancy of slush a challenge for these models is the short predicted gravitational relaxation timescale of topographic features at high heat flows the resurfacing must occur before the graben topography disappears we also evaluate alternate resurfacing mechanisms such as pumping of liquid water to the surface by thermal expansion stresses and buoyant rise of water through a silicate contaminated crust that is denser than liquid water and conclude that they are unlikely to explain ganymede s bright terrain

isua supracrustal belt greenland a vestige of a 3 8 ga suprasubduction zone ophiolite and the implications for archean geology abstract the mafic ultramafic units of the 3 8 ga isua supracrustal belt isb in greenland occur in a two armed arcuate zone eastern and western arms and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics 1 undifferentiated amphibolites ua and 2 garbenschiefer amphibolites ga the ua contains all major lithological units of a typical penrose type complete ophiolite sequence the ga is composed dominantly of volcaniclastic and volcanic rocks commonly found in immature island arcs the available geochemical data from ua and ga show distinct differences between the two units compared with the geochemical evolution of some of the well known phanerozoic ophiolites the pillow lavas and associated dikes of the ua show a compositional range that is similar to typical morb type ligurian ophiolites in the western alps apennines and those displayed by lip type caribbean ophiolites the ga is characterized by island arc tholeiite iat to boninite like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone ssz ophiolites in the mediterranean region our proposed geodynamic model for the isb suggests that the ua was built by primary to differentiated mantle generated melts during seafloor spreading little to moderately affected by subduction processes and that the iat to boninitic like rocks of the ga formed at a later stage by melting from a strongly subduction affected depleted and hydrated mantle our interpretation of the isb is that the ua and ga represent early and late stages respectively in the formation of a ssz ophiolite this implies that phanerozoic type plate tectonic processes such as seafloor spreading and subduction were operating by 3 8 ga in the palaeoarchean

geophysical implications of the long wavelength topography of the saturnian satellites 1 we use limb profiles to quantify the long wavelength topography of the saturnian satellites the degree 2 shapes of mimas enceladus and tethys are not consistent with hydrostatic equilibrium we derive 2 d topographic maps out to spherical harmonic degree 8 there is a good correlation with topography derived from stereo techniques if uncompensated topography at degree 3 and higher is large enough to be detectable during close spacecraft flybys if not properly accounted for this topography may bias estimates of a satellite s degree 2 gravity coefficients which are used to determine the moment of inertia we also derive a one dimensional variance spectrum a measure of how roughness varies with wavelength for each body the short wavelength spectral slope is 2t o 2 5 similar to silicate bodies however unlike the terrestrial planets each satellite spectrum shows a reduction in slope at longer wavelengths if this break in slope is due to a transition from flexural to isostatic support the globally averaged elastic thickness te of each satellite may be derived we obtain te values of 5 km 1 5 5 km 5 km and 5 km for tethys dione rhea and iapetus respectively for europa we obtain te 1 5 km these estimates are generally consistent with estimates made using other techniques for enceladus intermediate wavelengths imply te 0 5 km but the variance spectrum at wavelengths greater than 150 km is probably influenced by long wavelength processes such as convection or shell thickness variations impact cratering may also play a role in determining the variance spectra of some bodies citation nimmo f b g bills and p c thomas 2011 geophysical implications of the long wavelength topography of the saturnian satellites j geophys res 116 e11001 doi 10 1029 2011je003835

how earth s atmosphere evolved to an oxic state a status report the evolution of the earth s atmosphere is essentially the story of atmospheric oxygen virtually every realm of the earth sciences biology geology geochemistry oceanography and atmospheric science is needed to piece together an understanding of the history of oxygen over the past decade new data from these fields has shown that there were two significant increases in atmospheric o2 levels at around 2 4 2 3 and 0 8 0 6 billion years ago respectively throughout earth history oceanic sulfate concentrations appear to have increased in accord with greater o2 levels while levels of methane a strong greenhouse gas may have inversely mirrored o2 both oxic transitions occurred in eras characterized by bsnowball earthq events and significant disturbances in the carbon cycle perhaps associated with increases in o2 and losses of methane to understand what controlled the oxygenation of the atmosphere it is necessary to determine how o2 is consumed on geologic time scales through reaction with reductants released from the earth s crust and mantle there was apparently a long delay between the appearance of oxygenic photosynthesis and oxygenation of the atmosphere and a plausible explanation is that excess reductants scavenged photosynthetic o2 from the early atmosphere however a quantitative understanding of how and why o2 became abundant on our reducing planet is still lacking thus the study of the early atmosphere remains a frontier field with much to be discovered d 2005 elsevier b v all rights reserved

dating the oldest greenstone in india a 3 51 ga precise u pb shrimp zircon age for dacitic lava of the southern iron ore group singhbhum craton abstract this article reports a precise 3506 8 2 3 ma u pb shrimp zircon age for dacitic lava in a well preserved low grade metamorphic and low strained greenstone belt succession of the southern iron ore group singhbhum craton india this age makes the succession the oldest known greenstone belt succession in india and one of the oldest low strain greenstone successions in the world after the 3 51 ga coonterunah group of the pilbara craton western australia and the moderately deformed 3 54 ga theespruit formation of the barberton greenstone belt kaapvaal craton south africa the geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting the succession also contains a major 120 m thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the pilbara and kaapvaal cratons this banded iron formation may well be one of the oldest and most well preserved and together with associated volcanics

early archaean hydrothermal fluids a study of inclusions from the 3 4 ga buck ridge chert barberton greenstone belt south africa abstract fluid inclusions have been studied from cavities and breccias in one of the oldest well preserved relatively low grade metamorphosed sedimentary sequences on earth the fluid inclusion bearing samples are from the uppermost part of the early archaean 3 4 ga buck ridge volcano sedimentary complex in the barberton greenstone belt south africa which was affected by hydrothermal alteration during and immediately after its deposition in banded silicified sandy sediments this alteration is characterised among other features by quartz cemented hydrothermal breccias layer bound stockworks of cm size quartz veins and quartz filled cavities below stacked pebbles quartz grains in these hydrothermal deposits contain numerous fluid inclusions filled with either two phase aqueous or mixed h 2 o co 2 fluid the aqueous inclusions are divided into three different groups based on their microthermometric characteristics type h a relatively high salinity type m a intermediate salinity and type l a relatively low salinity optical and microthermometric characteristics of the fourth type of inclusions the mixed h 2 o co 2 fluid type c indicate that it was homogeneous at the time of trapping with only some minor changes occurring after trapping type h a and type c fluids appear as immiscible fluids in their host crystal most probably resulting from unmixing of the hydrothermal fluid phase relations in the co 2 h 2 o system show that the unmixing must have occurred at a relatively low pressure of 100 bars this conclusion is in line with the geological context of the samples which indicates a shallow environment of deposition for the sediments almost contemporaneous hydrothermal activity and a very low metamorphic grade however it is contrary to the first impression given by the homogeneous character of type c inclusions which suggests much higher p t conditions at the time of trapping the aqueous component of type c fluids may have progressively mixed with high salinity aqueous fluids type h a resulting in a dilution trend from h a to m a to l a fluids the hydrothermal fluids from the buck ridge chert in the barberton greenstone belt are strikingly similar to those encountered in present day systems they show minor differences with older ca 3 8 ga fluids preserved in hydrothermally altered rocks from the isua greenstone belt in greenland

earth s early o2 cycle suppressed by primitive continents a decrease in mafic continental crust coincides with the rise of o2 in the earth s surface environments about 3 billion years ago according to an analysis of sediment chemistry reduced rates of serpentinization of mafic material which produces chemicals that react with o2 could explain the link

evidence in pre 2 2 ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota the loss of fe from some pre 2 2 ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2 2 ga i have examined the behavior of fe in both pre and post 2 2 ga paleosols from depth profiles of fe 3 ti fe 2 ti and fe ti ratios and fe 3 ti vs fe 2 ti plots this new approach reveals a previously unrecognized history of paleosols essentially all paleosols regardless of age retain some characteristics of soils formed under an oxic atmosphere such as increased fe 3 ti ratios from their parental rocks the minimum oxygen pressure p o 2 for the 3 0 2 2 ga atmosphere is calculated to be about 1 5 of the present atmospheric level which is the same as that for the post 1 9 ga atmosphere the loss of fe common in paleosol sections of all ages was not due to a reducing atmosphere but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere this reductive dissolution of ferric hydroxides occurred either 1 after soil formation by hydrothermal fluids or 2 during and or after soil formation by organic acids generated from the decay of terrestrial organic matter terrestrial biomass on the early continents may have been more extensive than previously recognized

the martian subsurface as a potential window into the origin of life few traces of earth s geologic record are preserved from the time of life s emergence over 3 800 million years ago consequently what little we understand about abiogenesis the origin of life on earth is based primarily on laboratory experiments and theory the best geological lens for understanding early earth might actually come from mars a planet with a crust that s overall far more ancient than our own on earth surface sedimentary environments are thought to best preserve evidence of ancient life but this is mostly because our planet has been dominated by high photosynthetic biomass production at the surface for the last 2 500 million years or more by the time oxygenic photosynthesis evolved on earth mars had been a hyperarid frozen desert with a surface bombarded by high energy solar and cosmic radiation for more than a billion years and as a result photosynthetic surface life may never have occurred on mars therefore one must question whether searching for evidence of life in martian surface sediments is the best strategy this perspective explores the possibility that the abundant hydrothermal environments on mars might provide more valuable insights into life s origins ancient hydrothermal deposits formed in the martian subsurface may be the best targets for finding evidence for ancient life on mars and clues about the origin of life on earth

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

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

cellularly preserved microbial fossils from 3 4 ga deposits of south africa a testimony of early appearance of oxygenic life abstract fossil record of earliest earth s life is scant and restricted to simple kerogenous filaments spheres and lenticular microfossils which origin and taxonomic affiliation are still ambiguous here we report clusters of cell like bodies found in massive and weakly laminated black cherts of the 3 4 ga kromberg formation onverwacht group barberton greenstone belt south africa known earlier for benthic microbial mats and microfossils morphological traits and mineralization of the kromberg microfossils match those known from modern and fossil cyanobacteria micro raman and sem eds analyses showed that the cell like bodies fossilized mostly due to early mineralization with al silicates enclosing dispersed carbonaceous kerogenous matter derived from their thermally altered cell remains although in widespread opinion the early archean life is predominantly represented by benthic microbial mats the random non laminated distribution of the studied cyanobacteria like microfossils in the kromberg cherts is suggestive for probably benthic planktonic life cycle of these microbiota the paper also discusses how the morphological similarity of the kromberg fm cell like microfossils to geologically much younger and extant coccoidal cyanobacteria may influence the debate concerning the time of origin of earth s oxic atmosphere

iron and carbon isotope evidence for ecosystem and environmental diversity in the 2 7 to 2 5 ga hamersley province western australia abstract the largest excursion in kerogen 13c and bulk mineral 56fe values yet measured in the ancient rock record occurs in rocks of 2 7 to 2 5 ga age new fe isotope data integrated with previously collected c isotope data on the same samples document the metabolic diversity of microbial communities in the neoarchean hamersley province of the pilbara craton in western australia samples of shales carbonates and mixed carbonate shale lithologies were collected from three drill cores two cores from the depocenter of the province and one from the margin shallow water clastic carbonate rocks deposited in the center of the province tumbiana formation record kerogen 13c values that indicate c cycling by various anaerobic or aerobic methane pathways but the restricted range in 56fe values indicates little or no fe redox cycling deep water sediments deposited contemporaneously in both parts of the hamersley province jeerinah formation record slightly positive 56fe values in the relatively shallower and suboxic margin but strongly negative 56fe values in the deeper euxinic depocenter of the province a pattern consistent with fe cycling via a basin fe shuttle driven by bacterial dissimilatory iron reduction dir kerogen 13c values from these units indicate coupling of microbial fe cycling to aerobic methanotrophy or anaerobic oxidation of methane younger black shales intercalated with iron formation marra mamba iron formation in the depocenter record a shift to near zero 56fe values reflecting an fe budget dominated by hydrothermal and clastic sources however time equivalent fe rich carbonate shale lithologies deposited on the margin of the province carawine dolomite have 56fe values that steadily decrease from near zero to strongly negative values these relatively fe rich carbonates may reflect a carbonate trap of a dir driven fe shuttle similar to the sulfidic trap in the euxinic portion of the jeerinah formation in contrast younger shallow water carbonates deposited by turbidity currents in relatively deep water in the center of the province wittenoom formation have 56fe values that correlate with fe concentrations a pattern that indicates fe cycling by rayleigh fractionation through precipitation of iron oxides from aqueous ferrous iron

petrology and geochemistry of the 2 9ga itilliarsuk banded iron formation and associated supracrustal rocks west greenland source characteristics and depositional environment abstract here we present new field petrographic and geochemical data from the 2 9 ga itilliarsuk banded iron formation bif and associated lithologies within the itilliarsuk supracrustal belt south eastern nussuuaq west greenland the supracrustals represent a volcanic sedimentary sequence which rests unconformably on a basement of tonalite trondhjemite granodiorite ttg lithologies felsic metagreywackes meta semipelites and thinly bedded ferruginous shales were identified intercalated with the itilliarsuk bif other associated rocks include metapelites acidic metavolcanics and metagabbroic sills the supracrustals have experienced amphibolite facies metamorphism which has resulted in complete resetting of the u pb system with an apparent age of 1895 48 ma this tectono metamorphic event corresponds well with the paleoproterozoic rinkian orogeny known from this region the itilliarsuk oxide facies bif has been divided into two segments on the basis of major and trace elements chemistry a shaley bif with a strong clastic component and a more chemically pure bif the shaley bif contains high terrigenous influx as reflected by elevated al 2 o 3 up to 12 wt tio 2 high field strength elements hfse and transition metals the chemically pure bif is characterised by alternating high iron 68 wt and high silica 64 wt bands with low total rare earths and yttrium rey al 2 o 3 tio 2 and hfse contents suggesting a low detrital component the least altered bands of the bif record diagnostic archaean seawater features with post archaean average shale paas normalised positive la and eu anomalies enrichment in heavy rare earth elements hree relative to light rare earth elements lree pr yb paas t dm model ages of 3 23 2 85 ga and initial nd nd i values in the range of 0 43 to 4 05 average of 1 35 in contrast the chemically pure fe rich bands yield t dm model ages of 3 61 3 22 ga and nd i values from 2 87 to 0 09 average of 1 29 the associated supracrustal rocks in the study area have significantly higher positive nd i values the 143 nd 144 nd in the itilliarsuk bif therefore contrasts world bifs by exhibiting radiogenic positive nd i values in shallow seawaters where the rey s were controlled by a local depleted continental crust whereas the negative nd i values found in the iron rich layers suggest that the submarine hydrothermal source was influenced by an enriched mantle possibly an older subcontinental lithospheric segment the felsic metagreywackes are immature first cycle sio 2 al 2 o 3 4 4 la yb 1 chon metasediments with affinities to ttg suites primarily extrusives whereas the meta semipelites and metapelites contain a larger mafic contribution with higher content of fe 2 o 3 mgo cr ni and hrees this suggests that the bif was deposited in a highly unstable basin presumably in a palaeo continental slope or outer shelf environment with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources the t dm model ages and the use of th sc zr and la th sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting

ice covered water volcanism on ganymede eruption of liquid h2o magmas along extensional fractures and graben bounding normal faults may have played a critical role in the development of ganymede s grooved terrain the resurfacing potential of a water magma is dependent on a variety of factors including the areal extent of the source region the rate of discharge the thickness of the flow and the time that it takes the flow to completely freeze to its base in this paper the thermal evolution of such a flow is considered in detail the minimum unfrozen lifetime of a 5 m flow is approximately 12 5 days while a 10 m flow would survive for at least 50 days heating resulting from factional head loss could reasonably extend these lifetimes by 50 or more with a discharge rate of the order of 1 10 km3 d 1 an individual volcanic water flow could flood an area of 104 km2 before freezing as the flow solidifies its volume will increase thus lifting and arching its protective ice cover extensional fractures may then develop in the ice subparallel to the graben walls these fractures could result in grooves directly given a sufficiently thick 1 km flow or they may simply act to concentrate various tectonic forces that could initiate groove producing faults

biogeochemistry deepening the early oxygen debate the timing of the earliest production of oxygen by photosynthesis is hotly debated haematite crystals from pilbara australia may provide evidence for a deep ocean that was at least occasionally oxygenated by photosynthetic microbes 3 46 billion years ago

geophysical implications of the long wavelength topography of rhea 1 we use limb profiles to investigate the long wavelength topography and topographic variance spectrum of rhea one dimensional variance spectra show a break in slope at a wavelength of 300 km a similar effect is seen on the moon and may be a signature of an elastic lithosphere having a thickness te 10 km the implied heat flux is 15 mw m 2 much higher than can be explained by radiogenic heating we use the 1 d spectral behavior to constrain our solution for the long wavelength global topography of rhea the degree 3 topography is large enough if uncompensated to contaminate estimates of the degree 2 gravity using existing flyby data current models of rhea internal structures which rely on these degree 2 estimates may thus be inaccurate illustrating the need to acquire further rhea gravity data

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

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

trace elements at the intersection of marine biological and geochemical evolution life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes these requirements differ between organisms and have evolved over geological time likely guided in some part by environmental conditions until recently most of what was understood regarding trace element concentrations in the precambrian oceans was inferred by extrapolation geochemical modeling and or genomic studies however in the past decade the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new and potentially more direct insights into secular changes in seawater composition and ultimately the evolution of the marine biosphere compiled records of many bioessential trace elements including ni mo p zn co cr se and i provide new insight into how trace element abundance in earth s ancient oceans may have been linked to biological evolution several of these trace elements display redox sensitive behavior while others are redox sensitive but not bioessential e g cr u their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere ocean redox conditions that are linked to biological evolution for example the activity of oxygen producing photosynthetic cyanobacteria in this review we summarize available precambrian trace element proxy data and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life we also examine several biologically relevant and or redox sensitive trace elements that have yet to be fully examined in the sedimentary rock record e g cu cd w and suggest several directions for future studies

very early archean crustal accretion complexes preserved in the north atlantic craton the north atlantic craton contains very early archean supracrustal rocks orthogneisses and massive ultramafic rocks most units of supracrustal rocks are dominated by mafic volcanic rocks layered gabbros and banded iron formations bust some also contain abundant felsic volcanic sedimentary rocks quartzites and marbles some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic sedimentary rocks in these sequences ca 3800 ma and also from older ca 3850 ma sources the presence of the ca 3850 ma detrital zircons suggests that the supracrustal units containing them were deposited on or close to ca 3850 ma sialic crust the massive ultramafic rocks have chemical affinities to upper mantle rocks the voluminous suites of tonalitic gneisses are dominated by 3700 3730 ma bodies that intrude the supracrustal sequences but they also locally contain components with ages between 3820 and 3920 ma the diverse supracrustal units upper mantle rocks and ge 3820 ma components in the gneisses were tectonically interleaved in very early archean convergent plate boundaries giving rise to accretion complexes in the period 3700 3730 ma voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels forming juvenilemore continental crust and leaving behind a refractory lower crustal to upper mantle substrate less

geochemistry of impact glasses from the k t boundary in haiti relation to smectites and a new type of glass we have individually analyzed twelve black and seven yellow glasses from the k t boundary section at beloc haiti plus three smectite mantles around black glasses the main chemical differences between black and yellow glasses are higher s ca mg zn as br sb and au but lower k na and si abundances in the yellow glasses we have found high cao yellow glasses with low s contents 0 08 0 17 wt which may be explained by fusion of carbonate rich sediments microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10 100 m level while yellow glasses show much more variability and different intra sample elemental correlations one of the black glasses has higher sio2 and k2o abundances but lower concentrations of all other major elements this suggests the existence of a third glass type which we have named the high si k variety hsi k glass the glass shows areas consisting of pure sio2 maybe lechatelierite and some schlieren with lower sio2 content the chemical composition indicates that the hsi k glass may have originated from melting of a sedimentary source rock such as shale or argillite severe changes in major and trace element composition occur during alteration of the glass as documented by analyses of adhering smectites the ree patterns in the glass are similar to average upper crust but the smectites exhibit much lower abundances and flat patterns not unlike the patterns observed in claystones at numerous k t boundary sites the suggestion that the chicxulub structure in yucatan may be the source crater for the k t impact glasses is not fully supported by the currently available data impact glasses and tektites are produced from the surface layers of their targets at chicxulub the target stratigraphy comprised mainly carbonates and evaporites although we cannot exclude chicxulub as a source the present data do not provide any firm chemical evidence for a connection we conclude however that the haiti glasses have formed by impact and have later undergone alteration by low temperature hydrothermal processes to produce clays the boundary claystones at k t boundary sites may very well be alteration products of these impact glasses

discovery of mass anomalies on ganymede we present the discovery of mass anomalies on ganymede jupiter s third and largest galilean satellite this discovery is surprising for such a large icy satellite we used the radio doppler data generated with the galileo spacecraft during its second encounter with ganymede on 6 september 1996 to model the mass anomalies two surface mass anomalies one a positive mass at high latitude and the other a negative mass at low latitude can explain the data there are no obvious geological features that can be identified with the anomalies

hematite replacement of iron bearing precursor sediments in the 3 46 b y old marble bar chert pilbara craton australia the history of atmospheric oxygen prior to the great oxidation event 2 45 2 2 ga is not well understood hematite in the marble bar chert from a nasa funded drill hole abdp1 in the pilbara craton australia has been cited as evidence for an oxygenated ocean 3 46 b y ago however isotopic data from the same drill hole have been used to argue for an anoxic ocean it is generally agreed that the hematite is primary representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis here we present new petrographic evidence from the marble bar chert in drill hole abdp1 that shows that hematite in jasper bands formed via mineral replacement reactions the hematite mostly occurs as sub micron sized inclusions within chert so called dusty hematite that are typically arranged into polygonal clusters surrounded by a rim of clear quartz resembling shrinkage structures the lateral transition from laminated chert enclosing minute inclusions of greenalite siderite and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron bearing minerals the presence of hematite rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite this interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries the occurrence of dusty hematite in fractures sedimentary laminae and the outer margins of polygonal clusters containing greenalite is consistent with fluid mediated oxidation of iron rich precursor minerals the presence of syn sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition the abundance of dusty greenalite inclusions which are texturally the earliest components of the laminated chert suggests that the precursor sediment contained an iron rich clay mineral our results show that hematite has replaced ferrous rich minerals after deposition and provide a mechanism to explain the origin of hematite in the marble bar chert which is consistent with the origin of hematite in adjacent basalts a secondary origin for hematite invalidates arguments for an oxygen bearing ocean 3 46 b y ago and provides a viable explanation for the formation of archean jasper bands our findings show that misinterpretations about the origin of hematite in early precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere

the paleoproterozoic snowball earth a climate disaster triggered by the evolution of oxygenic photosynthesis although biomarker trace element and isotopic evidence have been used to claim that oxygenic photosynthesis evolved by 2 8 giga annum before present ga and perhaps as early as 3 7 ga a skeptical examination raises considerable doubt about the presence of oxygen producers at these times geological features suggestive of oxygen such as red beds lateritic paleosols and the return of sedimentary sulfate deposits after a 900 million year hiatus occur shortly before the 2 3 2 2 ga makganyene snowball earth global glaciation the massive deposition of mn which has a high redox potential practically requires the presence of environmental oxygen after the snowball new age constraints from the transvaal supergroup of south africa suggest that all three glaciations in the huronian supergroup of canada predate the snowball event a simple cyanobacterial growth model incorporating the range of c fe and p fluxes expected during a partial glaciation in an anoxic world with high fe oceans indicates that oxygenic photosynthesis could have destroyed a methane greenhouse and triggered a snowball event on timescales as short as 1 million years as the geological evidence requiring oxygen does not appear during the pongola glaciation at 2 9 ga or during the huronian glaciations we argue that oxygenic cyanobacteria evolved and radiated shortly before the makganyene snowball

the least radiogenic terrestrial leads implications for the early archean crustal evolution and hydrothermal metasomatic processes in the isua supracrustal belt west greenland abstract vein type galenas from within metasomatized tonalitic gneiss sheets in a metamorphosed oceanic crustal sequence of the arc outbound isua supracrustal belt isb west greenland reveal the hitherto least radiogenic terrestrial lead isotope compositions these leads deviate from commonly applied average continental crust evolution curves for lead and can be modeled by a single stage growth with starting parameters 238u 204pb 7 70 and 232th 238u 4 37 inferring model ages of 3 81 and 3 76 ga for these galenas the initial constraints are indicative of higher u pb and th u in the source of the hydrothermal fluids and arguably in the magmatic precursors of the gneisses they support contentions regarding a time integrated terrestrial pre history with an elevated th u in the early earth s mantle necessary to reconcile the difference between the observed th u and the time integrated th u recorded by pb isotope ratios of present depleted morb mantle derived rocks in addition they are in line with the requirement of preferential enrichment of u relative to pb in a proto continental crust relative to depleted upper mantle in the early earth s history the age equivalency between the galena generations and abundant tonalitic gneiss sheets and their mutual spatial relationship within the isb favors an interpretation of the least radiogenic terrestrial leads to closely approximate the initial pb of 3 75 ga and pre 3 8 ga old tonalitic protoliths and therefore prompts a close genetic relationship between early archean fluid flow and the magmatic events the inferred 3 81 ga model age of the least radiogenic galena pb particularly agrees with previous u pb zircon geochronological results on tonalite sheets within the supracrustals which constrain a ca 3 79 3 81 ga minimum age for the formation to the earth s oldest oceanic crustal sequence in combination with 3 74 ga pb pb ages of post deformational hydrothermal metasomatic minerals our results imply that early archean metasomatic activity is indistinguishable in time from the earliest metamorphic overprint and from the ca 3 75 ga major pulse of tonalite intrusions in the isua area

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

the distribution of bright and dark material on ganymede in relationship to surface elevation and slopes abstract the unique high resolution digital terrain data set available for ganymede s uruk sulcus area and galileo regio is analyzed by quantitative image analysis techniques to identify relationships between surface brightness elevation and slopes broad and narrow ridges and grooves of the uruk sulcus terrain model are found to be aligned with bright and dark lineament patterns visible in images these prominent lineaments represent areas covered by bright and dark albedo material not effects of photometric shading there is a strong correlation between material locations and elevation bright material is found on topographic highs whereas dark material is located in depressions the relative abundance of the dark material increases with lower elevation in galileo regio we find that bright material is also located on slopes facing north we suggest that the bright material in uruk sulcus and galileo regio was emplaced or exposed during or after formation of the terrain in its present location whereas the dark material was eroded and redistributed down slope effects of net solar illumination may play a role in the peculiar azimuthal distribution of bright and dark material on slopes

dark terrain on ganymede geological mapping and interpretation of galileo regio at high resolution abstract during its first two encounters with ganymede the galileo spacecraft obtained images of a 16 500 km 2 portion of galileo regio a large expanse of dark terrain at high resolution 76 86 m pixel through mapping of the g1 and g2 target sites within galileo regio we are able to characterize geological units based on their morphology and relative albedo we find three generally low albedo units an intermediate albedo plains unit a lower albedo plains unit and the lowest albedo unit which is found on furrow and crater floors we also find high albedo units which include crater rims furrow rims and isolated knobs and massifs other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta several processes are interpreted to have occurred within galileo regio these include tectonic deformation mass wasting sublimation resurfacing by impact ejecta and possibly cryovolcanism and isostatic adjustment we observe that the nw se trending furrows lakhmu fossae in galileo regio are degraded and are crosscut by the younger n s trending furrows zu fossae we also find several other tectonic features which may be minor faults or fractures related to one or other of these systems through mapping and crater size frequency distributions we are able to propose a stratigraphy for the galileo regio target site the oldest features in the area are high albedo knobs and massifs which are interpreted to be remnants of early impact related features and furrow rims these may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems the lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting much of the northeast portion of the target area was subsequently obscured by one of the youngest units ejecta from an impact just to the north we use our mapping of the high resolution images of galileo regio to evaluate three end member models for the formation of dark terrain 1 the crust is dark throughout 2 material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust and 3 dark material is distributed in small quantities throughout the crust and geological processes have acted to concentrate low albedo material on the surface although it is possible that elements of more than one of these models are present within the dark terrain we find that the third model that of a thin veneer of low albedo material best fits observations of galileo regio

u rich archaean sea floor sediments from greenland indications of 3700 ma oxygenic photosynthesis abstract 3700 ma metamorphosed pelagic shale from west greenland contains up to 0 4 wt reduced carbon with 13c values down to 25 6 pdb peedee belemnite the isotopic signature and mode of occurrence suggest that the carbon derived from planktonic organisms the pb isotopic composition shows that the shale had high primary u th this indicates that organic debris produced a local reducing environment which precipitated u transported to the site of sedimentation by oxidized ocean water the existence of highly productive plankton that fractionated c isotopes strongly and set up oxidation contrast in the environment suggests that oxygenic photosynthesis evolved before 3700 ma

uranium and thorium isotopes in marine sediments off northeastern taiwan uranium and thorium isotopes have been determined on seven box cores collected aboard r v ocean researcher i within the lower continental slope area off northeast taiwan during may 1991 the goals are to obtain the downcore u and th concentrations and their isotopic compositions in the area and to evaluate their possible source s transport and deposition u 238 contents of the box cores are fairly uniform at about 1 3 dpm g ranging between 0 9 and 1 6 dpm g u 234 has activities between 1 2 and 2 dpm g with an average of 1 4 dpm g th 232 contents are also quite uniform at about 3 2 dpm g th 230 has values roughly between 1 5 and 2 3 dpm g averaging at 1 9 dpm g th 228 is essentially equal to th 232 in activity except for the very surface sediments where th 228 appears to be slightly higher than th 232 the u and th contents of the shale slate and black schist from taiwan are about 1 9 and 3 2 dpm g respectively with a uth ratio of 0 6 th 232 in the cored sediments has similar activities but u 238 has activities lower by about 0 6 dpm g on the average if these crustal rocks have provided their weathered debris or particles as sediments to the cored area then u 238 must have lost by about this amount or 30 in transit by dissolution into the seawater as th 230 in the cores has an average activity identical to the u 238 activity of the crustal rocks mentioned above secular equilibrium between u 238 u 234 and th 230 must have been established when the rocks were exposed to weathering and erosion and no net th 230 gain or loss occurred in transit thus both th 232 and th 230 have probably been retained by the detrital particles which form the cored sediments the excess th 230 defined as the th 230 in excess over the u 234 in activity is about 0 5 dpm g however this excess may be fictitious because it is probably caused by dissolution of uranium from the terrigenous particles in transit rather than scavenging of th 230 from the water column by these particles as the th 230 production from u 234 decay in the water column can only account for 2 12 of the calculated excess th 230 a reasonable explanation is that all the th 230 observed in the cores has been of detrital origin with little authigenic contribution from the water column the th 230th 232 activity ratios are quite constant at about 0 6 for both the cored sediments and the above mentioned crustal rocks suggesting that thorium has not been leached out in transit to final deposition this is further supported by the fact that identical activity ratio has also been observed in the settling particulates collected by traps deployed in the same area the th 228th 232 activity ratios are close to unity except for the top few centimeters where the ratios are greater than unity due to the th 228 derived from ra 228 decay in the water column under rapid deposition the penetration depth for this th 228 excess is about 4 cm which is consistent with the prediction based on the known sedimentation rates of about 0 3 0 5 cm yr

u th pb ree systematics of organic rich shales from the ca 2 15 ga sengoma argillite formation botswana evidence for oxidative continental weathering during the great oxidation event article i nfo article history accepted 23 october 2008 theredox sensitive geochemicalbehaviorof uraniumpermits the useof th uratiosasa geochemicalproxy for the oxidation state of the atmosphere during deposition due to the effects of post depositional uranium mobilityonth uratiosduringeventsinvolvingoxygenated fluids directmeasurementsofth uratiosareoften misleading even for drill core samples because both of these elements radioactively decay and produce lead isotopes the pb isotope composition may reflect the depositional th u ratio although the th u ratios induced bychangesshortlyafterdeposition maynotbe distinguished fromthe true depositional th uratios in order to effectively evaluate the time integrated th u ratio a values for the initial depositional pb isotope composition must be determined or accepted from the models for the whole earth while the timing for the rise of atmospheric oxygen is reasonably well constrained now its effect on continental weathering and ocean redox state remains poorly constrained and debated the ca 2 15 ga sengoma argillite formation of botswana contains organic rich shales deposited during the great oxidation event the slope of the

primary haematite formation in an oxygenated sea 3 46 billion years ago the timing of the origin of photosynthesis remains controversial the discovery of ancient haematite crystals that formed in a jasper formation in australia which was created in a marine setting suggests that oxygen was being produced at least locally by photosynthesis as early as 3 46 billion years ago

the rise of atmospheric oxygen when did atmospheric oxygen rise to near present concentrations and what led to this rise as kasting explains in his perspective recent results have provided stronger than ever evidence that the switch from an anoxic to an oxic atmosphere occurred around 2300 millions years ago but researchers remain divided over the mechanism catling et al resurrect a previously proposed mechanism involving hydrogen escape to space with a couple of new twists whether their mechanism is viable depends on the oxidation state of the continental and oceanic crust at this crucial time of earth9s evolution

archaean palaeosols and archaean air reply ohmoto et al reply the idea of a methane rich archaean atmosphere has become popular since rye et al assumed in their calculation1 that siderite was absent in pre 2 2 gyr palaeosols we have concluded that the absence of siderite in some archaean palaeosols does not constrain the atmospheric pco2 but the presence of much siderite in sedimentary rocks does2 sleep s recognition3 that siderite occurs in archaean palaeosols substantiates our arguments2 although siderite should be absent in well aerated soils of all geological ages it may form in waterlogged soils where po2 became less than about 10 60 atm owing to the abundant anaerobic production of h2 in fact we have reported this in a 2 6 gyr soil profile at schagen south africa4 abundant ferric rich minerals formed while the soil was exposed to air but ferrous rich carbonate formed while it was apparently submerged under an anoxic pond

evidence in pre 2 2 ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota comment and reply the major points of disagreement between holland and rye and ohmoto concern 1 the behavior of ferric iron 2 the mechanisms of fe loss 3 the identification of real paleosols and 4 the ages of some paleosols

impact related dike breccia lithologies in the icdp drill core yaxcopoil 1 chicxulub impact structure mexico petrographic descriptions of three dike breccia lithologies from drill core yaxcopoil 1 yax 1 are presented they occur within allochthonous units of displaced sedimentary megablocks of the chicxulub impact structure the suevitic dike breccias are the uppermost dike lithology they contain melt rock particles and melt injections into the dike groundmass shock features occur ubiquitously and indicate a strong thermal annealing flow textures suggest a highly energetic emplacement process possibly during the excavation stage as a ground surge related deposit the impact melt rock dikes are present in a strongly brecciated megablock interval as flow textured anastomozing veinlets of impact melt rock that were altered to clay minerals the melt impregnated a dolomitic host rock indicating a low viscosity and thus high initial temperatures brecciation of the impact melt rock dikes occurred while they were still below the glass transition temperature suggesting that dynamic conditions prevailed shortly after the emplacement process major element data indicates that the impact melt rock dikes differ in composition from the homogenized impact melt rock of chicxulub this could point to an emplacement during the late compression or early excavation stages of cratering the clastic polymict dike breccias are coeval with pervasive brittle fracturing of the host rocks they bear clasts including some crystalline basement and possible melt rock particles in a fine grained dolomite matrix with turbulent flow textures fabric and texture indicate a granular flow at ambient pressures such conditions could be envisaged for the excavation phase while the transient cavity grew and fractures opened

geology and mapping of dark terrain on ganymede and implications for grooved terrain formation geological mapping of regional and high resolution galileo images reveals a variety of units and structures within ganymede s dark terrain we have made a detailed study of areas within galileo marius and nicholson regiones in order to investigate the style of tectonic deformation experienced by dark terrain adjacent to swaths of grooved terrain dark terrain appears to become fractured as a precursor to grooved terrain formation in places dark fractured swaths are recognized which have similar characteristics to brighter grooved terrain swaths tectonic deformation may be but is not always focused through preexisting weaknesses caused by impact craters and furrows a prominent groove lane anshar sulcus is inferred to have formed by the process of hanging wall rollover accompanied by a small amount of right lateral horizontal shear offset resulting from ne sw extension

some perspectives on isotope biosignatures for early life abstract the idea of using stable isotope compositions of light elements particularly of carbon as a sign of biological activities biosignatures both present and past dates back to the dawn of stable isotope geochemistry in the 1940s in the wake of the discovery of large variations in 13 c 12 c ratios among various carbon bearing materials including plants and fossil fuels the contentious debate ensued between kalervo rankama and harmon craig in the early 1950s whether the origin of graphitic carbon in ancient rocks biogenic vs abiogenic can be uniquely identified by its isotopic compositions during the last half century great progress has been made in understanding biogeochemical processes in modern and ancient terrestrial environments rapid developments in the last decade of novel analytical techniques and the birth of new geoscience fields such as geomicrobiology and astrobiology has prompted resurgence in the application not only of conventional light stable isotopes but also of those of metal and other intermediate elements however recent debates on the origin of graphitic carbons from early archean rocks from australia and greenland are very much reminiscent of the rankama craig debate half a century ago here an attempt is made to review briefly the history of isotope biosignatures and to critically assess current criteria for early life a number of possible abiotic pathways exist that lead to the synthesis of various organic and reduced carbon compounds alkanes pah amino acids lipids graphitic carbon etc depleted in 13 c due to large kinetic isotope effects up to ca 60 it also seems very likely that the mantle contains indigenous carbons graphite diamond dissolved carbons carbide with low 13 c values 20 to 30 which have previously been considered recycled sedimentary organic carbons or surface contamination this analysis prompts us to reassess our current and future strategies for identifying early and extraterrestrial life using isotope biosignatures

oxidative sulfide dissolution on the early earth recent evidence suggests that the biological production of oxygen in earth s surface oceans may have preceded the initial accumulation of large amounts of oxygen in the atmosphere by 100 million years or more however the potential effects of early oxygen production on surface ocean chemistry have remained little explored and questions persist regarding the locus of oxidation of crustal material i e subaerial and or submarine settings here we revisit the notion of spatially restricted oxygen oases in the archean surface ocean by employing a simple steady state box model of the surface ocean in a coastal upwelling system using pyrite as an example we then explore the possibility that oxygenic photosynthesis in such a system could support the widespread oxidation of crustal sulfide minerals without concomitant accumulation of oxygen in the atmosphere we find that it is possible to establish strong air sea gas exchange disequilibrium with respect to o 2 however in marine settings there is an apparent timescale mismatch between the kinetics of oxidative dissolution and the rate at which sulfide minerals delivered physically to shallow marine sediments will be buried below the zone of oxygen penetration estimated dissolution timescales compare somewhat more favorably with typical timescales of soil development and physical weathering transport in the subaerial realm despite the much lower dissolved oxygen concentrations inferred at gas exchange equilibrium with atmospheric po 2 relative to those attainable in productive regions of the surface ocean however broad scale subaerial weathering of sulfides would likely have required either local o 2 production at the site of weathering or transient increase in atmospheric po 2 above the most plausible levels characteristic of a pervasively reducing atmosphere

a new geochemical approach for constraining a marine redox condition of early archean abstract this paper presents a new approach for determining the paleo redox conditions of the early archean deep sea using u th systematics of seafloor hydrothermally altered basalts in the modern ocean system fresh unaltered basaltic glasses from the seafloor exhibit a tight u th correlation which reflects magmatic processes however basalts that have been hydrothermally altered are noticeably enriched in u due to adsorption and precipitation of dissolved u from oxygenated deep sea water onto the altered rocks irrespective of alteration temperature they exhibit much higher u th up to 34 than the primary igneous ratio of 0 35 in marked contrast hydrothermally altered basalts from the early archean 3 46 ga seafloor have maintained igneous u th 0 25 perfectly in spite of having been subjected to intense alteration this can be explained by a lack of dissolved u in early archean deep sea water due to the prevalence of anoxic deep sea conditions

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

in situ iron isotope analyses of pyrites from 3 5 to 3 2ga sedimentary rocks of the barberton greenstone belt kaapvaal craton abstract the barberton greenstone belt bgb south africa consists of volcano sedimentary and sedimentary successions deposited between 3 5 and 3 2 ga which are subdivided into three groups the onverwacht fig tree and moodies after deposition the barberton greenstone belt underwent relatively low grade tectonothermal events enabling estimation of surface environmental events and biological evolution in the middle archean isotopic studies suggest the activity of methanogen sulfate reducing bacteria and photosynthetic bacteria at 3 4 ga microbial dissimilatory iron reduction dir is also considered one of the earliest expressions of metabolism on earth but the isotopic signal of microbial dir is still uncertain in the archean we performed in situ iron isotope analyses of individual pyrites in sedimentary rocks from the bgb using a femtosecond laser ablation multi collector icp ms technique fs la mc icp ms to find isotopic evidence for the microbial activity we obtained a large variation in 56 fe values ranging from 1 84 to 3 79 most of the 56 fe values of pyrites from the hooggenoeg complex show positive values whereas those from the noisy complex and subsequent sedimentary sequences show a wide variation from negative to positive 56 fe values one of the main differences between these complexes is their depositional depth the hooggenoeg complex was probably deposited in a deep ocean environment whereas the noisy complex was in shallow water the negative 56 fe values of pyrite grains in the noisy complex onverwacht group indicate the occurrence of partial reduction caused by microbial dir in a middle archean shallow sea

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

petrography and geochemistry of target rocks and impactites from the ilyinets crater ukraine the 400 ma old ilyinets impact structure was formed in the precambrian basement of the ukrainian shield and is now mostly covered by quaternary sediments various impact breccias and melts are exposed in its southern section the crater is a complex structure with a central uplift that is surrounded by an annular deposit of breccias and melt rocks in the annulus brecciated basement rocks are overlain by up to 80 m of glass poor suevitic breccia which is overlain and partly intercalated by glass rich suevite with a thickness of up to 130 m impact melt rocks occur within and on top of the suevites in some cases in the form of devitrified bomb shaped impact glass fragments we have studied the petrographic and geochemical characteristics of 31 mostly shocked target rock samples granites gneisses and one amphibolite obtained from drill cores within the structure and impact breccias and melt rock samples from drill cores and surface exposures multiple sets of planar deformation features pdfs are common in quartz potassium feldspar and plagioclase of the shocked target rocks the breccias comprise more or less devitrified impact melt with shocked clasts the impact melt rocks bombs show abundant vesicles and in some cases glass is still present as brownish patches and schlieren all impact breccias including the melt rocks are strongly altered and have significantly elevated k contents and lower na contents than the target rocks the alteration could have occurred in an impact induced hydrothermal system the bomb shaped melt rocks have lower mg and ca contents than other rock types at the crater compared to target rocks only minor enrichments of siderophile element contents e g ni co ir in impact melt rocks were found