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

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

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

origin of iron oxide spherules in the banded iron formation of the bababudan group dharwar craton southern india the banded iron formation of the bababudan group western dharwar craton india is composed of millimetric to centimetric alternating quartz and grey to red fe oxide bands major phases are quartz and martite hematized magnetite with minor fe sulfides and ca mg fe carbonates micrometric fe oxide spherules fill cavities in discontinuous micrometric layers of fe oxides that occur in the massive quartz layers and at the interface of massive fe oxide and quartz layers the spherules are composed of micrometric radial plates of hematite intergrown with nanometric magnetite these spherules contain carbonaceous matter cm with nanometric fe particles and have low n contents 900 ppm cm1 the spherule formation is attributed to a low temperature hydrothermal process 150 200 c at around 2 52 ga possibly favored by the presence of cm these hydrothermal fluids dissolved diagenetic interstitial sulfides or carbonates creating cavities which provided space for the spherule precipitation carbonaceous matter of semi anthracite maturity is encapsulated in quartz grains adjacent to the fe oxide spherules cm2 and it is thus concluded that cm1 and cm2 are most likely contemporaneous and of the same origin either incorporated at the time of bif formation or during the hydrothermal event at 2 52 ga from the underlying phyllitised black shales carbonaceous matter cm3 was also found around the fe oxide spherules and the martite grains cm3 has much higher n contents 5000 ppm is of a lower maturity than cm1 and cm2 and is related to weathering which is also indicated by the presence of goethite and kaolinite the 13c of all cms varies from 19 4 to 24 7 similar to values measured in the underlying phyllitised black shales and likely reflect denitrifying microbial activity

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

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

metamorphism of the ca 3800 ma supracrustal rocks at isua west greenland implications for early archaean crustal evolution a detailed mineralogical and petrological study has been carried out on samples from two clastic metasedimentary lithologies from the 3800 ma isua supracrustal belt west greenland semipelitic to pelitic garnet biotite schist contains the limiting akfm assemblage muscovite biotite garnet staurolite quartz plagioclase ilmenite whereas muscovite biotite gneiss derived from felsic volcanogenic graywacke locally contains kyanite quartz plagioclase ca mn rich garnet temperatures calculated from fe mg partitioning between coexisting garnet biotite indicate equilibration for garnet coresat t 550 c and 460 c for garnet rims we interpret the higher t as a minimum estimate for prograde regional metamorphism which we argue occurred before 3600 ma whereas the lower t reflects later retrogression as indicated by the development of chlorite sericite in many samples the presence of kyanite as the stable aluminosilicate polymorph combined with phase assemblage data indicate p 5 kbar during prograde metamorphism and a depthof burial of at least 15 km the isua supracrustals are the oldest comprehensively dated rocks on earth and the metamorphic mineral assemblages reported here constitute the earliest direct record of thermal regimes in archaean crust therefore characterization of the metamorphic history of the isua region places an important constraint on models of early earth history our data and observations indicate that prograde regional metamorphism at isua occurred at conditions which are considered normal for an orogenic system with a metamorphic thermal gradient 35 c km moreover our results contraindicate the universal occurrence of thin archaean crust and excessively steep crustal thermal gradients as proposed by some investigators such conclusion appears at odds with estimates for higher terrestrial heat production during the early archaean but can be resolved by appealing to more rapid convection and generation of oceanic crustas a means of dissipating excess heat from the earth s interior as opposed to conduction through the crust

ion microprobe analysis of graphite from ca 3 8 ga metasediments isua supracrustal belt west greenland relationship between metamorphism and carbon isotopic composition in situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca 3 8 ga isua supracrustal belt west greenland the 13c values of micron scale graphite globules in the metasediments and the carbonate rocks vary from 18 to 2 and from 7 to 3 respectively the maximum 13c value of graphite globules in the metasediment rises from 14 to 5 as the metamorphic grade increases from epidote amphibolite to upper amphibolite facies in a single hand specimen the 13c values of graphite inclusions in garnet are 7 lower on average than those outside garnet similarly graphite armored by quartz apparently shows a few permil lower 13c values than those on grain boundaries between noncarbonate minerals the fact that early crystallized minerals include relatively 13c depleted graphite indicates that the regional metamorphism increased the 13c values of the isua graphite this is consistent with the regional trend of 13c enrichment accompanied by the increase of metamorphic grade the minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 c these observations indicate that isotopic exchange with isotopically heavy carbonate caused 13c enrichment of isua graphite the 13c values of graphite reported here 13c 18 were produced either as a metamorphic modification of organic carbon with initially much lower 13c values or as an abiological reaction such as decomposition of carbonate if the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13c enrichment of isua graphite previously reported large 13c depletion of graphite especially armored by apatite mojzsis et al 1996 was probably premetamorphic in origin this supports the existence of life at isua time ca 3 8 ga

fluid heterogeneity during granulite facies metamorphism in the adirondacks stable isotope evidence the preservation of premetamorphic whole rock oxygen isotope ratios in adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived hot co2 h2o fluids during granulite facies metamorphism this conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations the data suggest that these rocks were not an open system during metamorphism that fluid rock ratios were in many instances between 0 0 and 0 1 that externally derived fluids as well as fluids derived by metamorphic volatilization rose along localized channels and were not pervasive and thus that no single generalization can be applied to metamorphic fluid conditions in the adirondacks

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

raman and ion microscopic imagery of graphitic inclusions in apatite from older than 3830 ma akilia supracrustal rocks west greenland three dimensional molecular structural images of apatite grains and associated minerals embedded in a banded quartz pyroxene magnetite supracrustal rock from akilia southern west greenland were constructed by using raman confocal spectroscopy the rock sample is the same as that from which apatite hosted isotopically light graphitic inclusions were reported by mojzsis and colleagues in 1996 the results were challenged in 2005 by lepland and colleagues who failed to find carbon bearing inclusions in this and other akilia samples here we demonstrate that inclusions of graphite wholly contained within apatite occur in this rock the carbon isotopic composition of one such inclusion its graphitic composition established by raman spectroscopy was measured by secondary ion mass spectrometry to be isotopically light 13 c 29 4 in agreement with earlier analyses our results are thus consistent with the hypothesis that graphite containing apatite grains of the older than 3830 ma akilia metasediments may represent chemical fossils of early life

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

calcite graphite isotope thermometry a test for polymetamorphism in marble tudor gabbro aureole ontario canada graphitization and coarsening of organic material in carbonate bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory pressure independent geothermometer comparison of observed isotopic fractionations between calcite and graphite a13ccal gr with independent petrological thermometers provides the following empirical calibration over the range 400 800 c a13ccal gr 5 81 x lo6 x t k 2 61 this system has its greatest potential in marbles where calcite graphite is a common assemblage and other geothermometers are often unavailable the temperature dependency of this empirical calibration differs from theoretical calibrations reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications both calcite graphite isotopic thermometry and calcite dolomite solvus thermometry are applied to marble adjacent to the tudor gabbro in the grenville province of ontario canada the marble has undergone two metamorphic episodes early contact metamorphism and later regional metamorphism values of a13cca gr decrease regularly from c 8 0 in samples over 2 km from the pluton to values of 3 4 within 200m of the contact these samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer and indicate temperatures of 450 500 c away from the intrusion and 700 750 c near the gabbro this thermal profile around the gabbro is consistent with conductive heat flow models in contrast the distribution of mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c 500 c even at the contact graphite textures are important for interpreting the results of the calcite graphite thermometer coarsening of graphite approaching the tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion in contrast to isotopic exchange during prograde metamorphism which is facilitated by graphitization retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally resistance of the calcite graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems

carbon isotopes as biogeochemical recorders of life over 3 8 ga of earth history evolution of a concept ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes biologically mediated 13c 12c fractionations have come to be recognized as a common corollary of biochemical reactions meanwhile it is firmly established that the universal bias in favour of 12c characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon fixing enzymatic carboxylation reaction in the primary metabolism of co2 fixing autotrophic organisms this preference for 12c has turned out to be one of the most enduring relics of the ordered state of the biological precursor substances that may be preserved in fossil organics over billions of years with the currently known sedimentary record at hand it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3 8 ga of recorded earth history indicating that microbial prokaryotic and archaeoprokaryotic ecosystems had been prolific already on the archean earth while for the time span 3 5 ga the isotopic evidence is unequivocal the information encoded in the preceding record is commonly blurred by a metamorphic overprint this holds particularly for the metasediments of the 3 8 ga old isua supracrustal belt of west greenland which apart from widespread metasomatism have suffered amphibolite grade metamorphism it is known that 13c 12c exchange occurs in organic kerogenous rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available as either fluids or carbonate with isotopic re equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction thermodynamic equilibria predict however that 13c 12c ratios in kerogen and graphite increase during this process hence high t exchange equilibria are always bound to drive 13c values in positive direction the lowermost values encountered being consequently the least exchanged and most pristine with the lowest values of reduced graphitic carbon obtained in early isua studies falling into the range 22 to 28 pdb we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre metamorphic isua suite it was therefore by no means surprising that the results of recent isotope work performed on apatite hosted carbonaceous microdomains in isua banded iron formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions hence the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope discriminating properties of the principal co2 fixing reactions s in biological carbon assimilation suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation as a consequence biological modulation of the geochemical carbon cycle had been established at least 3 8 ga ago having been fully operative by the time of formation of the earth s oldest sediments

apatite in early archean isua supracrustal rocks southern west greenland its origin association with graphite and potential as a biomarker abstract rare earth element ree abundances in individual apatite crystals in banded iron formations bifs metacherts metacarbonates and mafic dykes in the isua supracrustal belt isb have been determined by laser ablation inductively coupled plasma mass spectrometry the results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential proposed in earlier studies for use of the apatite graphite association as a biomarker the chondrite normalized distribution patterns of apatite in metasedimentary bifs and metacherts fall into three groups relatively flat profiles with distinct positive eu anomaly are interpreted as characterizing sedimentary diagenetic apatite that carry the ree signature of the archean ocean secondary apatite in isua metasdiments with either middle ree enriched profiles or with light ree depleted profiles is interpreted to have crystallized from percolating carbonate rich metasomatic fluids or from fluids derived from cross cutting mafic dykes respectively the occurrence together of these different genetic types of apatite with distinct ree signatures within cm scale samples shows the immobility of ree in preexisting apatite during metamorphic episodes apatite crystals in isua rocks of uncontested chemical sedimentary origin bif and metachert samples do not have graphite inclusions or coatings graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks in these rocks graphite is produced by thermal metamorphic reduction of carbonate ion derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor oxidizing to form magnetite in view of the non sedimentary metasomatic origin of isua metacarbonates and the abiogenic source of graphite the apatite graphite assemblage can not be considered as a biomarker and does not provide information on early archean life in the isb

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

magmatic graphite in dolomite carbonatite at pogranichnoe north transbaikalia russia a recently discovered dolomite carbonatite at pogranichnoe north transbaikalia russia dated at 624 3 ma contains xenoliths of calcite bearing dolomite carbonatite with graphite spherulites apatite and aegirine are the other rock forming minerals chemically the carbonatites are ferrocarbonatite and ferruginous calciocarbonatite the graphite forms 1 mm up to 1 5 mm diameter spherulites with raman spectra similar to published spectra of microcrystalline amorphous carbon and disordered graphite with g and d bands at 1 580 1 600 cm 1 and at around 1 350 cm 1 alteration has formed fe bearing calcite to ca bearing siderite compositions not previously reported in nature around the graphite along cracks and fractures mineral and stable isotope geothermometers and melt inclusion measurements for the carbonatite all give temperatures of 700 900 it is concluded that the graphite precipitated from the ferrocarbonatite magma there are three candidates to control the precipitation of graphite a a redox reaction with feii in the magma b potential presence of organics in the magma c seeding of or dissolution in the magma of graphite diamond from the mantle and further work is required to identify the most important mechanism s graphite in carbonatite is rare with no substantial published accounts since the 1960s but graphite at other localities seems also to have precipitated from carbonatite magma the precipitation of reduced carbon from carbonatite provides further evidence that diamond formation in carbonate melts at high mantle pressures is feasible

making magnetite late again evidence for widespread magnetite growth by thermal decomposition of siderite in hamersley banded iron formations abstract banded iron formations bifs are marine chemical sedimentary rocks that were widely deposited before the start of the great oxidation event 2 45 2 32 ga they represent an important archive into the chemistry of the precambrian oceans and atmosphere and the evolution of the marine biosphere it is believed that bifs were derived from ferric oxides hydroxides that were deposited by iron oxidizing bacteria or by reaction with oxygen released by cyanobacteria hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide hydroxide precipitate whereas magnetite is thought to have formed from ferric oxides hydroxides via bacterial iron reduction and oxidation of microbial biomass however studies are equivocal about when and how magnetite formed in bifs we present the results of a detailed petrographic study of magnetite in bifs of the hamersley group western australia we find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction instead most of the magnetite formed by replacement of siderite and other ferrous rich minerals after burial our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern pilbara craton 200 350 c the thermal decomposition of siderite could account for much of the magnetite in the hamersley bifs 3feco3 h2o fe3o4 3co2 h2 the relationship between magnetite and the primary precipitates which are preserved as greenalite nanoparticles and microgranules in chert bands suggests that much of the magnetite formed in siderite rich iron silicate bands that developed by compaction of non silicified iron silicate muds these bands acted as reactive pathways for fluid flow and the formation of magnetite if most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron our results are consistent with models for the deposition of bifs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial

carbon isotope thermometry in marbles of the adirondack mountains new york carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the adirondack mountains new york eighty nine calcite graphite pairs from the amphibolite grade nw adirondacks change systematically in temperature north westwards from 680 to 640 to 670 c over a 30 km distance reflecting transitions from amphibolite facies towards granulite facies to the north west and to the south east temperature contours based on calcite graphite thermometry in the nw adirondacks parallel mineral isograds with the orthopyroxene isograd falling above 675 c and indicate that regional metamorphic temperatures were up to 75 c higher than temperatures inferred from isotherms based on cation and solvus thermometry bohlen et al 1985 fifty five calcite graphite pairs from granulite grade marbles of the central adirondacks give regional metamorphic temperatures of 670 780 c in general agreement with cation and solvus thermometry data for amphibolite and granulite grade marbles show a 12 oo range in 13ccal and 13cgr a strong correlation between carbon isotopic composition and the abundance of graphite cgr crock indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism the trends seen in 13c vs cgr crock and 13ccal vs 13cgr could not have been preserved if significant amounts of co2 rich fluid had pervasively infiltrated the adirondacks at any time the close fit between natural data and calculated trends of 13c vs cgr crock indicates a biogenic origin for adirondack graphites even though low 13c values are not preserved in marble delamination of 17 graphite flakes perpendicular to the c axis reveals isotopic zonation with higher 13c cores these isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite samples located 2km from anorthosite show a decrease of 0 5 0 8 oo in the outer 100 of the grains while samples at distances over 8 km show smaller core to rim decreases of c 0 2 oo correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism core graphite compositions indicate contact metamorphic temperatures were 860 890 c within 1 km of the marcy anorthosite massif if samples with a significant contact metamorphic effect cal gr 3 2 oo are not included then the remaining 38 granulite facies samples define the relation 13c cal gr 3 56 106t 2 k

the influence of salinity on mg ca in planktic foraminifers evidence from cultures core top sediments and complementary δ18o the mg ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions but recent core top sediment observations suggest that salinity may exert a significant secondary control on planktic foraminifers this study compiles new and published laboratory culture experiment data from the planktic foraminifers orbulina universa globigerinoides sacculifer and globigerinoides ruber in which salinity was varied but temperature ph and light were held constant combining new data with results from previous culture studies yields a mg ca sensitivity to salinity of 4 4 2 3 4 7 1 2 and 3 3 1 7 per salinity unit 95 confidence respectively for the three foraminifer species studied here comparison of these sensitivities with core top data suggests that the much larger sensitivity 27 4 per salinity unit derived from atlantic core top sediments in previous studies is not a direct effect of salinity rather we suggest that the dissolution correction often applied to mg ca data can lead to significant overestimation of temperatures we are able to reconcile culture calibrations with core top observations by combining evidence for seasonal occurrence and latitude specific habitat depth preferences with corresponding variations in physico chemical environmental parameters although both mg ca and 18o yield temperature estimates that fall within the bounds of hydrographic observations discrepancies between the two proxies highlight unresolved challenges with the use of paired mg ca and 18o analyses to reconstruct paleo salinity patterns across ocean basins the first step towards resolving these challenges requires a better spatially and seasonally resolved 18osw archive than is currently available nonetheless site specific reconstructions of salinity change through time may be valid

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

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

13c 12c exchange between calcite and graphite a possible thermometer in grenville marbles the fractionation of 13c between calcite and graphite cc gr is consistently small 2 6 4 8 permil in 34 assemblages from upper amphibolite and granulite facies marbles of the grenville province in 25 samples from the adirondack mountains new york it decreases regularly with increasing metamorphic temperature the fractionations are independent of absolute 13c values of calcite 2 9 to 5 0 for t 600 800 c the adirondack data are described by cc gr 0 00748t c 8 68 this good correlation between and t suggests that carbon isotope equilibrium was attained in these high grade marbles and that the theoretical calculations of this fractionation by bottinga are approximately 2 permil too large in this temperature range because of the relatively high temperature sensitivity suggested by these results and by bottinga s calculations and the pressure independence of isotope fractionation cc gr may provide a very good thermometer for high grade marbles comparison of this field calibration for cc gr vs temperature with results from other terranes supports the utility of cc gr for geothermometry and suggests that graphite is much more sluggish to exchange than calcite that exchange between calcite and graphite occurs at temperatures as low as 300 c and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500 600 c because 13c exchange is an unavoidable metamorphic process at temperatures above 300 c high values of 13c gr in moderate to high grade carbonate bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite

methanotrophs regulated atmospheric sulfur isotope anomalies during the mesoarchean tumbiana formation western australia abstract the 2 73 billion year old tumbiana formation of the fortescue group in the pilbara craton western australia preserves one of the best records of the fortescue negative carbon isotope excursion with organic 13 c values as low as 60 the prevailing environmental conditions at this biological excursion are not well understood here we present an integrated carbon 12 c 13 c and multiple sulfur 32 s 33 s 34 s 36 s isotopes study of a pristine diamond drill core through the tumbiana formation collected at meentheena by the pilbara drilling project our results show centimeter to meter scale heterogeneities in sulfur and carbon contents and isotope compositions organic 13 c values vary at the meter scale and display a bimodal distribution with peaks at about 44 and 34 this bimodal distribution is interpreted to reflect variations in the contribution of methanotrophic metabolism to the biomass the 34 s v cdt values of sedimentary pyrite vary from 5 73 to 2 75 sulfur isotopes sulfur contents and s c ratios are variable suggesting a complex local biotic sulfur cycling sedimentary pyrite preserves mass independent fractionation of sulfur mif s with 33 s v cdt ranging between 0 25 and 1 64 average 0 47 corresponding to the characteristic dampened mesoarchean anomalies positive correlation between sulfide mif s and a 13 c depletion of organic matter argues for a causal link between methanotrophs activity and the preservation of mif s anomalies in the rock record

non biogenic graphite in 3 8 ga metamorphic rocks from the isua district greenland abstract a 3 8 ga old metamorphic rock that contains up to 5 5 wt reduced carbon as graphite was found in the isua district greenland this rock exhibits a strong schistosity comprises mostly talc contains high mgo 28 0 wt ni 553 ppm and cr 267 ppm and low al2o3 2 13 wt and bears a high fe 2 o 3 feo ratio 0 94 the original rock was probably a submarine ultramafic tuff the carbon content and the 13c value of the rock vary on a microscopic scale a few mm3 scale from 0 7 to 5 5 wt and from 15 to 12 respectively the second rock a quartzite contains 0 13 wt c as reduced carbon with a 13c value of 11 25 these 13c values fall within the range 27 to 10 reported by previous investigators for graphite from the isua district the 13c values of the isua graphite have been interpreted by previous investigators e g schidlowski and aharon 1992 as metamorphic modification of organic matter with initial 13c values of 30 an important implication of this model has been that the 3 8 ga ocean already supported some biological activity however an examination of the carbon content vs 13c relationship of the graphite bearing rocks from isua has revealed that the 13c value generally increases with increasing carbon contents the rocks with 13c values less than 20 contain extremely small amounts of carbon the mineralogical and elemental characteristics of the rocks of this study a thermodynamic consideration of the temperature and composition of metamorphic fluids and carbon isotope mass calculations suggest that the major component of graphite with 13c values around 12 was formed by an inorganic rather than by a biological process the graphite appeared to have formed at temperatures between 700 and 400 c by one or both of the following two processes 1 reactions between co2 and ch4 co2 ch4 2c 2h2o during cooling of fluids with a co 2 ch 4 molar ratio of 1 and or 2 reactions between the feo component in the ultramafic rock with co2 rich fluids 4 feo co2 2 fe2o3 c the 13c c values for the fluids for both i and 2 were probably between 12 and 5 suggesting that the carbon bearing fluids could have been derived from the mantle

microstructure specific carbon isotopic signatures of organic matter from 3 5 ga cherts of the pilbara craton support a biologic origin the 3 5 ga dresser formation from the north pole dome of the pilbara craton western australia contains some of the oldest evidence for life on earth here we present a detailed study of microstructure specific carbon isotopic composition of organic matter om preserved in dresser formation bedded cherts and hydrothermal chert vein using in situ secondary ion mass spectrometry sims the om in these rocks occurs mainly as clots that together with minor fine om layers and laminae are considered primary textures formed prior to host rock lithification other than rare om rich stylolites no evidence was found for later om migration beyond the micrometer scale average 13c om values in specific microstructural types range between 33 6 and 25 7 no correlation is seen between measured 13c values and h c ratios in the studied om microstructures this lack of correlation and the low metamorphic grade of the rocks studied argue against significant modification of om isotopic composition by later metamorphic alteration it is thus concluded that the range of 13c values found in the samples represents primary om isotopic variability within some individual samples variable 13c om values are correlated with specific microstructural types this observation is not consistent with solely abiotic om formation via fisher tropsch type reactions when compared with associated 13c ankerite values average 13c om values indicate c isotopic fractionation 13c ank om of 25 33 which translates to dissolved co2 om isotopic fractionation 13c co2 om of 20 30 this range of 13c co2 om is consistent with enzymatic c fixation via the calvin cycle utilized by photoautotrophs and the reductive acetyl coa pathway utilized by chemolithoautotrophs photosynthetic om formation is supported by the relatively shallow water depth inferred for the dresser environment and the restricted occurrence of stromatolites to shallow water deposits in this unit whereas chemolithosynthesis is supported by the abundance of om in sub seafloor hydrothermal chert veins the range of 13c om values observed in the samples may therefore represent the remains of different organisms utilizing different c fixation pathways other biologic effects such as the growth rate and density of microbial communities and further heterotrophic overprinting of the autotrophic biomass may have also contributed to the observed range of 13c om values

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

are there ancient soils in the 3 7 ga isua greenstone belt greenland abstract a lens of black schist within 3 7 ga quartzites of greenland may be earth s oldest known alluvial paleosol the suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite but it has a truncated top above dark gray grading down to gray color ptygmatically folded surface cracks filled with silt grains and large sand crystals unusual for sedimentary or metamorphic rocks the paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends and by chemical analysis showing molar weathering ratios and ree distribution like those of soils the schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the isukasia area the protolith can be reconstructed as a saponite clay with a salt rich horizon of kieserite like other acid sulfate paleosols of the early earth models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 510 ppm o2 and 820 201 ppm co2 and humid cool temperate paleoclimate the profile has organic 13cpdb consistently of 24 2 to 27 4 and modest raleigh distillation near the top similar consistent values and trends are produced by decay of organic matter in living soils but biotic carbon isotopic composition of sediments is erratic from bed to bed and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle thus life in this very ancient soil is not precluded by our analyses but ultrastructural and geochemical testing of carbon particles would further test this hypothesis

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

ancient graphite in the eoarchean quartz pyroxene rocks from akilia in southern west greenland i petrographic and spectroscopic characterization because all known eoarchean 3 65 ga volcano sedimentary terranes are locked in granitoid gneiss complexes that have experienced high degrees of metamorphism and deformation the origin and mode of preservation of carbonaceous material in the oldest metasedimentary rocks remain a subject of vigorous debate to determine the biogenicity of carbon in graphite in such rocks carbonaceous material must be demonstrably indigenous and its composition should be consistent with thermally altered biogenic carbon as well as inconsistent with abiogenic carbon here we report the petrological and spectroscopic characteristics of carbonaceous material typically associated with individual apatite grains but also with various other minerals including calcite in a 3 83 ga granulite facies ferruginous quartz pyroxene unit qp rock from the island of akilia in southern west greenland in thin sections of the fine grained parts of akilia qp rock sample g91 26 mapped apatites were found to be associated with graphite in about 20 of the occurrences raman spectra of this carbonaceous material had strong g band and small d band absorptions indicative of crystalline graphite three apatite associated graphites were found to contain curled graphite structures identified by an anomalously intense second order d band or 2d band raman mode these structures are similar to graphite whiskers or cones documented to form at high temperatures raman spectra of apatite associated graphite were consistent with formation at temperatures calculated to be between 635 and 830 c which are consistent with granulite facies metamorphic conditions three graphite targets extracted by focused ion beam fib methods contained thin graphite coatings on apatite grains rather than inclusions sensu stricto as inferred from transmitted light microscopy and raman spectroscopy tem analyses of graphite in these fib sections showed a 0 0 0 2 interplanar spacing between 3 41 and 3 64 a for apatite associated graphite which is larger than the spacing of pure graphite 3 35 a and may be caused by the presence of non carbon heteroatoms in interlayer sites samples analyzed by synchrotron based scanning transmission x ray microscopy stxm also confirmed the presence of crystalline graphite but abundances of n and o heteroatoms were below detection limit for this method graphite in the akilia qp rock was also found to occur in complex polyphase mineral assemblages of hornblende calcite sulfides magnetite that point to high temperature precipitation from carbon bearing fluids these complex mineral assemblages may represent another generation of graphitization that could have occurred during the amphibolite facies metamorphic event at 2 7 ga several observations point to graphitization from high temperature fluid deposition for some of the akilia graphite and our results do not exclude a biogenic source of carbon in graphite associated with apatite but ambiguities remain for the origin of this carbon

exceptional preservation of fossil plant spores in high pressure metamorphic rocks it is commonly believed that high grade metamorphism erases all traces of fossils and more generally of biogenicity in rocks to address this issue we report in this study x ray spectromicroscopy raman microspectroscopy and electron microscopy observations at the nm scale of fossil lycophytes megaspores found in triassic metasedimentary rocks from the french alps these rocks have undergone high pressure metamorphic conditions not vert similar 14 kbar not vert similar 360 c during the alpine orogeny corresponding to a burial of not vert similar 35 km during subduction despite such metamorphism the fossil megaspores are remarkably well preserved texturally an unusual mineralogical zonation consisting of magnetite ankerite and calcite is superposed on the structures of the spore walls in parallel chemical heterogeneities of the organic matter om composing the spore walls were detected we discuss different interpretations for our observations in particular a scenario in which these features are remnants of the original structural heterogeneities in the spores and provide insights on the fossilization processes under high grade metamorphic conditions whatever the processes leading to such heterogeneities our observations oppose the usual assumption that high grade metamorphism erases all indications of biogenicity in rocks and open new avenues for investigation at the nm scale of the evolution of organic matter during diagenesis and metamorphism

assessing the ability of the 14c projection age method to constrain the circulation of the past in a 3 d ocean model radiocarbon differences between benthic and planktonic foraminifera b p ages and radiocarbon projection ages are both used to determine changes of the past ocean circulation rate a global 3 d ocean circulation model with a constant modern ocean circulation is used to study which method is less influenced by atmospheric 14c variations three factors cause uncertainties first the long equilibration time of the ocean after atmospheric 14c changes second different mixing processes in the ocean which cause an ocean response of smaller amplitude than the atmospheric forcing and third the unknown source region and corresponding initial surface 14c reservoir age of subsurface waters the model suggests that b p ages and projection ages have lower uncertainties the closer they are to deepwater formation zones in the north atlantic the b p age method is less influenced by atmospheric 14c variations than the projection age method projections ages vary less in the pacific as long as atmospheric 14c decreases linearly a more irregular atmospheric 14c evolution leads to age variations of similar magnitude with both methods on the basis of the model experiment we suggest a potential improvement of the projection age method

the role of ultramafic rocks in regulating the concentrations of volatile and non volatile components during deep crustal metamorphism the common preservation of h2o and co2 bearing phases in high grade ultramafic rocks indicates that they represent reservoirs of volatile components in the deep crust the upper thermal stabilities of volatile bearing mg silicates and magnesite exceed those of volatile bearing phases in most other crustal lithologies the solubility of many rock forming components e g co2 al2o3 cao sio2 is considerably lower in fluids coexisting with ultramafic rocks than in adjacent quartzo feldspathic units metamorphism of crustal sections that include ultramafic rocks is accompanied by metasomatic reactions that regulate the chemistry of both volatile and non volatile components in deep crustal metamorphic fluids during amphibolite facies metamorphism ultramafic rocks can absorb and store volatile and non volatile components introduced from the country rock under conditions of low activities or fugacities an example is the early fixation of co2 h2o and aluminium by reaction of dunite to form chlorite magnesite and progressively more siliceous mg silicates along an early low xco2 reaction path one dimensional continuum models of fluid transport and reaction indicate that infiltration of fluids derived from quarzo feldspathic country rocks can cause enrichment of aluminium in ultramafic rocks up to the level of wt internal reaction between the metasomatic minerals within ultramafic units release fluids during prograde metamorphism fluids will be released in two pulses one with intermediate xco2 and aluminium activity at the level of aluminosilicate or corundum saturation and a later pulse of pure co2