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

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

evidence for mantle plumes geophysical hotspots have been attributed to partially molten asthenosphere fertile blobs small scale convection and upwellings driven by core heat most are short lived or too close together to be deeply seated and do not have anomalous heat flow or temperature many are related to tectonic features bourdon et al investigate the dynamics of mantle plumes from uranium series geochemistry and interpret their results as evidence for thermal plumes here we show why alternative mechanisms of upwelling and melting should be considered

climate change in arid central asia since mis 2 revealed from a loess sequence in yili basin xinjiang china abstract k feldspar pirir dating was used to establish a chronological framework for the ks15 05 loess section in the yili basin northern xinjiang province china in arid central asia the reliability of the dating results was verified from the luminescence characteristics and internal checks the chronology was used to calculate mass accumulation rates mars for the section and the paleoclimatic proxies of grain size and magnetic susceptibility were also analyzed the high dust accumulation patterns during the last glacial maximum in yili basin are consistent with those of the chinese loess plateau clp on a millennial scale the bolling allerod b a interstadial is clearly reflected in the ks15 05 section by the variation of the 2 10 m grain size fraction suggesting the influence of the westerlies which dominate the modern atmospheric circulation of the region however the b a interstadial is poorly recorded in grain size records of the clp and this disparity may provide additional information about the nature of the transmission of climate signals from the north atlantic region to asia specifically the zonal transmission of the b a warm signal to the clp may have been obscured by the cooling effect of high latitude cold air to the north of the clp

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

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

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

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

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

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

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

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

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

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

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

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