Isua Supracrustal Belt Research Articles

Evidence for subduction at 3.8 Ga: Geochemistry of arc-like metabasalts from the southern edge of the Isua Supracrustal Belt

We present new major and trace element data for rare examples of preserved pillow basalts from a locality of little studied > 3800 Ma rocks at the southwestern edge of the Isua Supracrustal Belt (ISB), West Greenland. The 20 samples have 47.3–59.0 wt.% SiO 2, 4.9–12.2 wt.% MgO, 6.5–11.2 wt.% CaO, 60–340 ppm Ni and 63–1094 ppm Cr, consistent with pre-metamorphic basaltic compositions. Nb/La (n) and Ti/Gd (n) are lower than primitive mantle compositions (0.1–0.6 and 0.3–0.8, respectively), with these ratios interpreted to reflect the composition of their source. Correlations of Nb/La with La/Sm, Ti/Gd and Zr/Sm and between Zr/Nb and Nb/Th are comparable to those of modern subduction related magmas, whereby slab melts enriched the overlying mantle wedge and HFSE were retained by Ti-rich phases. Trends in Ba/La vs. Ba/Nb are comparable to modern day arc related basalts, which reflect a sediment influence and may indicate that, at least early in the evolution of Eoarchean subduction zones, the subducted Eoarchean oceanic crust retained a pelagic sediment cover, with this cover influencing the chemistry of the slab-derived fluid. Three of the ISB pillow lavas show extreme enrichments in LREE (La/Sm ( n) = 1.9–2.6), large depletions in Nb, Ta, Zr and Hf relative to REE that are similar to rare high-K arc basalts from the Clark volcano of the presently active Tongan–Kermadec–New Zealand arc (Gamble, J.A., Christie, R.H.K., Wright, I.C., Wysoczanski, R.J., 1997. Primitive K-rich magmas from Clark volcano, southern Kermadec arc: a paradox in the K–depth relationship. The Canadian Mineralogist, 35, 275–290). The compositions of these highly enriched basalts indicate that the mantle source region of the Isua basalts was occasionally overprinted by small-volume sediment dominated melts rather than hydrous fluids. The compositional affinities of these 3.8 Ga pillow basalts with modern island-arc basalts provide strong evidence for the role of slab fluids and melts in basalt genesis in the Eoarchean, further demonstrating arc-like signatures in the oldest rock sequences. The compositions of Archean mafic rocks including samples from the ISB that are interpreted to have a petrogenesis similar to that of modern island-arc basalts, are compared with Archean ‘non-arc’ rocks (i.e. plume-related komatiites, komatiitic basalts and associated tholeiitic basalts). The two groups are distinguished by differences in La/Sm, Gd/Yb, Nb/La, Ti/Gd, Ba/La, Ba/Nb and Al 2O 3/TiO 2. Thus clear compositional distinctions between different basaltic types, attributed to arc and non-arc origins, are a feature of the rock record for at least the last 3.8 Ga.

Dacitic ocelli in mafic lavas, 3.8–3.7 Ga Isua greenstone belt, West Greenland: Geochemical evidence for partial melting of oceanic crust and magma mixing

Mafic volcanic rocks in the 3.8–3.7 Ga Isua greenstone belt, southern West Greenland, contain randomly distributed 1 to 10-centimeters long white spheroidal structures. In this study, these structures are called ‘ocelli’. In the western part of the belt, ocelli-bearing lavas are enclosed in basaltic to picritic flows (MgO = 9–21 wt.%) with a subduction zone geochemical signature. The ocelli are composed predominantly of polycrystalline Na-plagioclase and quartz, with minor hornblende and biotite, whereas the surrounding amphibolite matrix (basaltic host) is composed mainly of hornblende, Ca-plagioclase, and quartz. The ocelli are devoid of radial or concentric internal structure, and display all stages of coalescence. Contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational. Compositionally, the ocelli are calc-alkaline dacites (SiO 2 = 62.9–72.0 wt.%; MgO = 0.60–3.50 wt.%; Ni = 58–143 ppm; Cr = 250–510 ppm), whereas the surrounding matrix is tholeiitic basalt (SiO 2 = 46.6–50.6 wt.%; MgO = 8.70–12.30 wt.%; Ni = 119–175 ppm; Cr = 330–600 ppm). In terms of major element composition, the Isua ocelli closely resemble plagiogranites in Phanerozoic supra-subduction zone ophiolites. Field and petrographic observations, and geochemical data (SiO 2 = 54.2–60.7 wt.%; MgO = 3.95–7.72 wt.%; Ni = 127–158 ppm; Cr = 500–570 ppm) on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts. On a chondrite-normalized diagram, the basaltic host is characterized by variably depleted LREE patterns (La/Sm cn = 0.30–0.94; Gd/Yb cn = 1.03–1.45), whereas the dacitic ocelli display LREE-enriched patterns (La/Sm cn = 1.30–2.60; Gd/Yb cn = 1.32–2.58). The strongly depleted REE patterns in the basaltic host are attributed to LREE loss during carbonate alteration. Partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host. The geochemical characteristic of the ocelli cannot be explained by post-magmatic alteration, slab melting, fractional crystallization of tholeiitic melts, or liquid immiscibility. We suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust (high-Mg volcanic rocks) that fell into the magma chamber, suggesting magma–crust interaction in the early Earth. Formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early Archean.

Archean of Greenland and Fennoscandia

The North Atlantic craton in southern West Greenland mainly consists of a tectonic collage of Mesoarchean continental crustal terranes, which were amalgamated at c. 2.7 Ga and are currently exposed at mid-crustal amphibolite to granulite facies levels. Tonalitic orthogneisses predominate, intercalated with slightly older tholeiitic to andesitic metavolcanic rocks and associated gabbro-anorthosite intrusive complexes. The North Atlantic craton also contains enclaves of Eoarchean, c. 3.86-3.6 Ga orthogneisses and supracrustal rocks including the Isua greenstone (or supracrustal) belt. This is the oldest known assemblage of rocks deposited at the surface of the Earth, comprising mafic pillow lavas, banded iron formations and metasedimentary schists with local disseminated graphite of possible biogenic origin. Eoarchean rocks have not been found in Kola and Karelia in Fennoscandia where most rocks are 2.9-2.7 Ga tonalitic-trondhjemitic- granodioritic orthogneisses with intercalated coeval greenstone belts and amphibolites. Mesoarchean 3.0-3.2 Ga rocks are found in the eastern and western parts of the Karelian province. Subduction-related rocks like the Iringora supra-subduction type ophiolite and basalt-andesite-dacite-rhyolite series volcanic rocks in many greenstone belts, as well as eclogites are found in the Archean of Fennoscandia. A clear distinction between Greenland and Fennoscandia is the abundance of 2.75-2.65 Ga igneous rocks in Fennoscandia which indicates that these two cratons had a separate evolution during the Neoarchean.

Comment on "A Vestige of Earth's Oldest Ophiolite"

Furnes et al. (Reports, 23 March 2007, p. 1704) reported the identification of an ophiolite sequence within the approximately 3.8-billion-year-old Isua supracrustal belt. However, they did not acknowledge that the belt contains supracrustal rocks and mafic dikes of different ages, nor did they demonstrate that the proposed components of the ophiolite are coeval.

Iron isotope, major and trace element characterization of early Archean supracrustal rocks from SW Greenland: Protolith identification and metamorphic overprint

The iron isotope, trace and major element compositions of Eoarchean supracrustal rocks from southern West Greenland (Isua Supracrustal Belt, the islands of Akilia and Innersuartuut) were analyzed in order to identify protoliths and characterize the imprints of metamorphism and metasomatism. Banded iron formations (BIFs) from the Isua Supracrustal Belt (ISB) have trace element characteristics that are consistent with seawater derivation, including high Y/Ho ratios, positive Eu/Eu ∗ anomalies, positive La/La ∗ anomalies, and concave upward REE patterns. These rocks also have heavy Fe isotopic compositions relative to surrounding igneous rocks (∼+0.4‰/amu). The most likely interpretation is that this signature was inherited from partial oxidation in a marine setting of Fe emanating from a source similar to modern mid-ocean ridge hydrothermal vents (∼−0.15‰/amu). Banded quartz-rich rocks from the island of Akilia with high Fe/Ti ratios share many similarities with bona fide BIFs from Isua (heavy Fe isotopic compositions up to +0.4‰/amu, elevated Y/Ho ratios compared to igneous rocks, sometimes positive Eu/Eu ∗ anomalies) suggesting a chemical sedimentary origin. Iron-poor metacarbonates from the southwestern part of the ISB have light Fe isotopic compositions (∼−0.4‰/amu). This is consistent with derivation of these rocks by fluid flow through surrounding ultramafic rocks and deposition as metasomatic carbonates. Iron-rich metacarbonates from the northwest and northeast parts of the ISB have Fe isotopic compositions (from +0.1 to +0.4‰/amu) and trace element patterns (high Y/Ho ratios, positive Eu/Eu ∗ and La/La ∗ anomalies, and concave upward REE) similar to associated BIFs. The most likely interpretation is that these iron-rich metacarbonates were derived from mobilization of Fe in BIFs by metasomatic fluids.

Seawater-like trace element signatures (REE + Y) of Eoarchaean chemical sedimentary rocks from southern West Greenland, and their corruption during high-grade metamorphism

Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive La/La*SN, Eu/Eu*SN, and YSN anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units in the Isua supracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island), display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of “earliest life” rocks (sample G91/26) from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous assertions (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised, mafic rock (Fedo and Whitehouse 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43–60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures. There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but are beginning to lose the distinct La, Eu and Y positive anomalies and are interpreted as metasomatised chemical sediments. Finally there are some siliceous samples with completely different trace element patterns that are interpreted as rocks of non-sedimentary origin, and include metasomatised mafic rocks. The positive La/La*SN, Eu/Eu*SN and YSN anomalies found in Isua BIFs and other Eoarchaean Greenland samples, such as G91/26 from Akilia, suggests that the processes of carbonate ion complexation controlling the REE − Y patterns were already established in the hydrosphere at the start of the sedimentary record 3,600–3,850 Ma ago. This is in accord with the presence of Eoarchaean siderite-bearing marbles of sedimentary origin, and suggests that CO2 may have been a significant greenhouse gas at that time.

A Vestige of Earth's Oldest Ophiolite

A sheeted-dike complex within the approximately 3.8-billion-year-old Isua supracrustal belt (ISB) in southwest Greenland provides the oldest evidence of oceanic crustal accretion by spreading. The geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid-ocean ridge-like setting, and their oxygen isotopes suggest a hydrothermal ocean-floor-type metamorphism. The pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association: the Paleoarchean Isua ophiolite complex. These sheeted dikes offer evidence for remnants of oceanic crust formed by sea-floor spreading of the earliest intact rocks on Earth.

Recognising early Archaean mantle: a reappraisal

This paper examines 3.8 Ga peridotites from Greenland and Labrador to test claims that these samples are unmodified early Archaean mantle. Geochemical cri- teria were applied in which samples were compared to the mantle array in Mg/Si versus Al/Si (wt%) space, their REE patterns were compared to those of different mantle types and their chromite compositions were compared to mantle chromite compositions as expressed by their cr# and fe#. Geochemical data were used from the previously published works of Friend et al. (2002) and Bennett et al. (2002). Only two samples, from the region south of Isua satisfied all criteria, indicating that the area south of the Isua Greenstone Belt in west Greenland is a suitable place to search for early Archaean mantle. This study also confirms the observation by Friend et al. (2002) that early Archaean mantle from south of Isua is of a different character from Archaean mantle from the subcontinental lithosphere. Calculations presented here show that some mantle frag- ments from south of Isua experienced a lower degree of melt extraction and were probably more oxidising than early Archaean mantle preserved in the subcontinental lithosphere. Elemental concentrations of Os in early Ar- chaean mantle are lower than the new estimate for the primitive upper mantle of Becker et al. (2006). Peridotites from the Isua greenstone belt are not mantle, but have an affinity with the layered intrusions found south of Isua.

Pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Québec

Geochemistry and U-Pb ion microprobe zircon geochronology guided by high-resolution mapping (1:50 scale) was used to define a minimum age of ca. 3750 Ma for supracrustal rocks from the Nuvvuagittuq supracrustal belt (NSB) in the northern Superior Province, Québec (Canada). Mineralogy and geochemistry of critical field relationships preserved at the Porpoise Cove locality describe a supracrustal succession of (mafic) amphibolites and ultramafic rocks, finely banded quartz-magnetite units with intermixed coarse-grained ferruginous quartz-pyroxene rocks and quartz-biotite schists that superficially resemble polymict meta-conglomerates with large (up to 10 cm) deformed polycrystalline quartz and mafic clasts. All units in the mapped outcrop have sharp lithological contacts. Narrow (dominantly trondhjemitic) orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals. The total extent of the supracrustal enclave is ∼ 8 km 2; it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions. The quartz-biotite schists record complex zircon growth at ∼ 3500 and ∼ 2800 Ma, interpreted to reflect metamorphic events. Zircons separated from orthogneisses in the enclave – including one sheet that transects a banded quartz-pyroxene (± magnetite) unit – yield magmatic 207Pb/ 206Pb ages of ca. 3750 Ma. These ages are slightly younger than earlier provisional reports for an NSB orthogneiss from Porpoise Cove. The Nuvvuagittuq supracrustals are the oldest rocks thus far reported for the Minto Block, they overlap in age with the ca. 3.70–3.83 Ga Isua supracrustal belt and Akilia association rocks in West Greenland, and they represent an important new area for exploration of the early Earth.

Microscale heterogeneity of Fe isotopes in >3.71 Ga banded iron formation from the Isua Greenstone Belt, southwest Greenland

We present Fe isotope compositions for magnetite crystals from >3.7 Ga banded iron formation (BIF) from the Isua Greenstone Belt and younger, spatially related pyrite obtained in situ by secondary ion mass spectrometry (SIMS). Across a distance of several millimeters, individual magnetite crystals of BIF show δ 56 Fe values up to 2‰, and within single magnetite-rich layers, there is a >2‰ range in δ 56 Fe. The highest positive δ 56 Fe values are consistent with those predicted by the equilibrium fractionation factor for oxidation of ferrous to ferric Fe in aqueous solution at a likely Early Archean ocean temperature of ∼70 °C, and further suggest that subsequent precipitation of ferric oxides was sufficiently slow that kinetic fractionation effects were minimal. These high values also imply that the supply of oxidant was limited. Heterogeneity in δ 56 Fe values is attributed to diagenetic reactions occurring in limited-volume pore waters, isolated from the bulk ocean (δ 56 Fe = 0‰), with development of isotope reservoir effects. Secondary pyrite in BIF and a conglomerate also show a distinct enrichment in heavy Fe isotopes, interpreted as redistribution and/or fractionation during postformational events.

Early Archaean hydrothermal fluids; a study of inclusions from the ∼ 3.4 Ga Buck Ridge Chert, Barberton Greenstone Belt, South Africa

Fluid inclusions have been studied from cavities and breccias in one of the oldest well-preserved, relatively low-grade metamorphosed sedimentary sequences on Earth. The fluid inclusion-bearing samples are from the uppermost part of the early Archaean (∼ 3.4 Ga) Buck Ridge volcano-sedimentary complex in the Barberton Greenstone Belt (South Africa), which was affected by hydrothermal alteration during and immediately after its deposition. In banded, silicified sandy sediments this alteration is characterised, among other features, by quartz-cemented hydrothermal breccias, layer-bound stockworks of cm-size quartz veins and quartz-filled cavities below stacked pebbles. Quartz grains in these hydrothermal deposits contain numerous fluid inclusions filled with either two-phase aqueous or mixed H 2O–CO 2 fluid. The aqueous inclusions are divided into three different groups, based on their microthermometric characteristics: type H A (relatively high salinity), type M A (intermediate salinity) and type L A (relatively low salinity). Optical and microthermometric characteristics of the fourth type of inclusions, the mixed H 2O–CO 2 fluid (type C) indicate that it was homogeneous at the time of trapping, with only some minor changes occurring after trapping. Type H A and type C fluids appear as immiscible fluids in their host crystal, most probably resulting from unmixing of the hydrothermal fluid. Phase relations in the CO 2–H 2O system show that the unmixing must have occurred at a relatively low pressure of ∼ 100 bars. This conclusion is in line with the geological context of the samples, which indicates a shallow environment of deposition for the sediments, (almost) contemporaneous hydrothermal activity, and a very low metamorphic grade. However, it is contrary to the first impression given by the homogeneous character of type C inclusions, which suggests much higher P– T conditions at the time of trapping. The aqueous component of type C fluids may have progressively mixed with high-salinity aqueous fluids (type H A), resulting in a dilution trend from H A to M A to L A fluids. The hydrothermal fluids from the Buck Ridge Chert in the Barberton Greenstone Belt are strikingly similar to those encountered in present-day systems. They show minor differences with older (ca. 3.8 Ga) fluids preserved in hydrothermally altered rocks from the Isua Greenstone Belt in Greenland.

Source heterogeneity for the major components of ∼ 3.7 Ga Banded Iron Formations (Isua Greenstone Belt, Western Greenland): Tracing the nature of interacting water masses in BIF formation

We report trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the ∼ 3.7–3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce ⁎), presodymium (Pr/Pr ⁎) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 ± 41 Ma, indicating that the uranium (U)–lead (Pb) system remained closed after the formation of this BIF. High 207Pb/ 204Pb relative to 206Pb/ 204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-μ ( 238U/ 204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm–Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids ( εNd (3.7 Ga) ∼ + 3.1), the other being ambient surface seawater which reached its composition by erosion of parts of the protocrustal landmass ( εNd(3.7 Ga) ∼ + 1.6). The validity of two different and periodically interacting water masses (an essentially two-component mixing system) in the deposition of alternating iron- and silica-rich layers is also reflected by systematic trends in germanium (Ge)/silicon (Si) ratios. These suggest that significant amounts of silica were derived from unexposed and/or destroyed mafic Hadean landmass, unlike iron which probably originated from oceanic crust following hydrothermal alteration by deep percolating seawater. Ge/Si distributional patterns in the early Archean Isua BIF are similar to those reported from the Paleoproterozoic Hamersley (Western Australia) BIF, but overall Ge concentrations are about one order of magnitude higher in the Archean BIF. This seems consistent with other lines of evidence that the ambient Archean seawater was enriched with iron relative to Proterozoic and recent seawater.

Mass‐independent fractionation of sulfur isotopes in sulfides from the pre‐3770 Ma Isua Supracrustal Belt, West Greenland

ABSTRACTRedox chemistry of the coupled atmosphere–hydrosphere system has coevolved with the biosphere, from global anoxia in the Archean to an oxygenated Proterozoic surface environment. However, to trace these changes to the very beginning of the rock record presents special challenges. All known Eoarchean (c. 3850–3600 Ma) volcanosedimentary successions (i.e. supracrustal rocks) are restricted to high‐grade gneissic terranes that seldom preserve original sedimentary structures and lack primary organic biomarkers. Although complicated by metamorphic overprinting, sulfur isotopes from Archean supracrustal rocks have the potential to preserve signatures of both atmospheric chemistry and metabolic fractionation from the original sediments. We present a synthesis of multiple sulfur isotope measurements (32S, 33S and 34S) performed on sulfides from amphibolite facies banded iron‐formations (BIFs) and ferruginous garnet‐biotite (metapelitic) schists from the pre‐3770 Ma Isua Supracrustal Belt (ISB) in West Greenland. Because these data come from some of the oldest rocks of interpretable marine sedimentary origin, they provide the opportunity to (i) explore for possible biosignatures of sulfur metabolisms in early life; (ii) assess changes in atmospheric redox chemistry from ∼3.8 Ga; and (iii) lay the groundwork to elucidate sulfur biogeochemical cycles on the early Earth. We find that sulfur isotope results from Isua do not unambiguously indicate microbially induced sulfur isotopic fractionation at that time. A significantly expanded data set of Δ33S analyses for Isua dictates that the atmosphere was devoid of free oxygen at time of deposition and also shows that the effects of post‐depositional metamorphic remobilization and/or dilution can be traced in mass‐independently fractionated sulfur isotopes.

Metamorphic fluid flow in the northeastern part of the 3.8–3.7 Ga Isua Greenstone Belt (SW Greenland): A re-evalution of fluid inclusion evidence for early Archean seafloor-hydrothermal systems

Fluid inclusions in quartz globules and quartz veins of a 3.8–3.7 Ga old, well-preserved pillow lava breccia in the northeastern Isua Greenstone Belt (IGB) were studied using microthermometry, Raman spectrometry and SEM Cathodoluminescence Imaging. Petrographic study of the different quartz segregations showed that they were affected by variable recrystallization which controlled their fluid inclusion content. The oldest unaltered fluid inclusions found are present in vein crystals that survived dynamic and static recrystallization. These crystals contain a cogenetic, immiscible assemblage of CO 2-rich (+H 2O, +graphite) and brine-rich (+CO 2, +halite, +carbonate) inclusions. The gas-rich inclusions have molar volumes between 44.8 and 47.5 cm 3/mol, while the brine inclusions have a salinity of ∼33 eq. wt% NaCl. Modeling equilibrium immiscibility using volumetric and compositional properties of the endmember fluids indicates that fluid unmixing occurred at or near peak-metamorphic conditions of ∼460 °C and ∼4 kbar. Carbonate and graphite were precipitated cogenetically from the physically separated endmember fluids and were trapped in fluid inclusions. In most quartz crystals, however, recrystallization obliterated such early fluid inclusion assemblages and left graphite and carbonate as solid inclusions in recrystallized grains. Intragranular fluid inclusion trails in the recrystallized grains of breccia cementing and crosscutting quartz veins have CO 2-rich assemblages, with distinctly different molar volumes (either between 43.7 and 47.5 cm 3/mol or between 53.5 and 74.1 cm 3/mol), and immiscible, halite-saturated H 2O–CO 2–NaCl(–other salt) inclusions. Later intergranular trails have CH 4–H 2 ( X H 2 up to ∼0.3) inclusions of variable density (ranging from 48.0 to >105.3 cm 3/mol) and metastable H 2O–NaCl(–other salt?) brines (∼28 eq. wt% NaCl). Finally, the youngest fluid inclusion assemblages are found in non-luminescent secondary quartz and contain low-density CH 4 (molar volume > 105.33 cm 3/mol) and low-salinity H 2O–NaCl (0.2–3.7 eq. wt% NaCl). These successive fluid inclusion assemblages record a retrograde P–T evolution close to a geothermal gradient of ∼30 °C/km, but also indicate fluid pressure variations and the introduction of highly reducing fluids at ∼200–300 °C and 0.5–2 kbar. The quartz globules in the pillow fragments only contain sporadic CH 4(+H 2) and brine inclusions, corresponding with the late generations present in the cementing and crosscutting veins. We argue that due to the large extent of static recrystallization in quartz globules in the pillow breccia fragments, only these relatively late fluid inclusions have been preserved, and that they do not represent remnants of an early, seafloor-hydrothermal system as was previously proposed. Modeling the oxidation state of the fluids indicates a rock buffered system at peak-metamorphic conditions, but suggests a change towards fluid–graphite disequilibrium and a log f H 2 / f H 2 O above the Quartz–Fayalite–Magnetite buffer during retrograde evolution. Most likely, this indicates a control on redox conditions and on fluid speciation by ultramafic rocks in the IGB. Finally, this study shows that microscopic solid graphite in recrystallized metamorphic rocks from Isua can be deposited inorganically from a fluid phase, adding to the complexity of processes that formed reduced carbon in the oldest, well-preserved supracrustal rocks on Earth.

Geological constraints on detecting the earliest life on Earth: a perspective from the Early Archaean (older than 3.7 Gyr) of southwest Greenland

At greater than 3.7 Gyr, Earth's oldest known supracrustal rocks, comprised dominantly of mafic igneous with less common sedimentary units including banded iron formation (BIF), are exposed in southwest Greenland. Regionally, they were intruded by younger tonalites, and then both were intensely dynamothermally metamorphosed to granulite facies (the highest pressures and temperatures generally encountered in the Earth's crust during metamorphism) in the Archaean and subsequently at lower grades until about 1500 Myr ago. Claims for the first preserved life on Earth have been based on the occurrence of greater than 3.8 Gyr isotopically light C occurring as graphite inclusions within apatite crystals from a 5 m thick purported BIF on the island of Akilia. Detailed geologic mapping and observations there indicate that the banding, first claimed to be depositional, is clearly deformational in origin. Furthermore, the mineralogy of the supposed BIF, being dominated by pyroxene, amphibole and quartz, is unlike well-known BIF from the Isua Greenstone Belt (IGB), but resembles enclosing mafic and ultramafic igneous rocks modified by metasomatism and repeated metamorphic recrystallization. This scenario parsimoniously links the geology, whole-rock geochemistry, 2.7 Gyr single crystal zircon ages in the unit, an approximately 1500 Myr age for apatites that lack any graphite, non-MIF sulphur isotopes in the unit and an inconclusive Fe isotope signature. Although both putative body fossils and carbon-12 enriched isotopes in graphite described at Isua are better explained by abiotic processes, more fruitful targets for examining the earliest stages in the emergence of life remain within greater than 3.7 Gyr IGB, which preserves BIF and other rocks that unambiguously formed at Earth's surface.

Silicon isotopes in ∼3.8 Ga West Greenland rocks as clues to the Eoarchaean supracrustal Si cycle

We report MC-ICP-MS Si-isotopic measurements ( δ 29Si recalculated as δ 30Si) on micro-subsamples (at 500 μm scale) from several major rock types of the ∼3.8-Ga-old Isua Greenstone Belt (IGB, southern West Greenland) and surrounding Eoarchaean terrains. With a large overall range of variations (− 2.80‰ < δ 30Si < + 0.68‰), they demonstrate strong involvement of surface fluids enriched with dissolved Si. The resistance of Si-isotopes to metamorphic resettings and metasomatic overprints is also established. Metabasaltic pillows and metasediments display similar 30Si-enriched signatures, suggesting that emergent surfaces of the Eoarchaean protocrust were composed of slightly weathered, hydrothermally altered, mafic–ultramafic bodies. Isua magnetite–quartz Banded Iron Formation (BIF) is strongly depleted in 30Si relative to all coeval rocks. This depletion supports Rayleigh-controlled precipitation from seafloor-vented hydrothermal fluids. In contrast, banded quartz–pyroxene rocks (from Akilia Island, some 150 km southwest of the IGB), which some authors have identified as BIF-related, yield quartz with Si-isotopic composition ( δ 30Si = − 0.36‰) similar to metamorphic-derived quartz ( δ 30Si = − 0.50‰). This supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers.

Volcanic resurfacing and the early terrestrial crust: Zircon U–Pb and REE constraints from the Isua Greenstone Belt, southern West Greenland

The oldest known bona fide succession of elastic metasediments Occurs in the Isua Greenstone Belt. SW Greenland and consists of a variety of mica schists and rare metaconglomerates. The metasediments are in direct contact with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for 30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon, Those extracted from mica schists yielded ion probe U/Pb ages between 3.70 and 3,71 Ga. One metaconglomerate sample yielded just a single zircon of 3.74 Ga age. The mica schist hosted zircons have U/Pb ages. Th/U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the adjacent 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies but some metasediments contain substantial contribution from more evolved source lithologies. The paucity of zircon in the mica schists is thus explained by incorporation of material from largely zircon-free volcanic lithologies. The absence of older zircon in the mica schists and the preponderance of mafic source material imply intense, mainly basaltic resurfacing of the early Earth. The implications of this process are discussed, Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking oft Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt Outpouring,;, This process Causes a bias towards eruption of Zr-saturated partial melts at the surface with O-isotope corn posit ion,, potentially different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth or formed in plutons that crystallised at some depth from which it took hundreds of millions of years for them to be exhumed and incorporated into much younger sediments. (C) 2005 Elsevier B.V.All rights reserved.

Rare-Earth Element, Lead, Carbon, and Nitrogen Geochemistry of Apatite-Bearing Metasediments from the ∼3.8 Ga Isua Supracrustal Belt, West Greenland

We performed rare-earth element (REE) geochemistry and U-Pb geochronology on apatites in metasediments from the ∼3.8 Ga Isua supracrustal belt (ISB) and Akilia Island, West Greenland, together with stepwise combustion isotopic investigation of carbon and nitrogen for the apatite-bearing quartz-magnetite BIF of uncontested sedimentary origin from northeastern ISB. Ion microprobe analyses reveal that apatites in psammitic schist from the ISB show a U-Pb isochron of 1.5 ± 0.3 Ga. This age is similar to those of Akilia apatite and the Rb-Sr age of 1.6 Ga for the pegmatitic gneiss in the Isukasia area in literature, suggesting a late (∼1.5 Ga) metamorphic event (≤400°C). Pb isotopic ratios of apatite in the quartz-magnetite BIF are also affected by the late metamorphic event around 1.5 Ga. Chondrite-normalized REE patterns of apatites in the BIF show flat patterns with a significant positive Eu anomaly, suggesting hydrothermal influence; this is consistent with a primary depositional origin. In contrast with the quartz-magnetite BIF, apatites in the psammitic schist from the ISB and those in the Akilia BIF show different REE patterns, which resemble those of apatites from secondary mafic and felsic rocks, respectively. Carbon isotopic ratios for the quartz-magnetite BIF by stepwise combustion suggest that two components of reduced carbon are present. One is released below 1000°C (mainly 200-400°C; lowtemperature carbon = LTC), and the other above 1000°C (high-temperature carbon = HTC). δ13C values of the LTC are about -24‰. The LTC is clearly contaminant incorporated after metamorphism, because such a low-temperature component could not have survived the ≤400°C metamorphic event. On the other hand, δ13C values of the HTC are -30‰ for one aliquot and -19‰ for another. The HTC is probably sequestered within magnetite in the BIF, because the decrepitation temperature of magnetite is ∼1200°C. The HTC could not exist within quartz and apatite (decrepitation temperatures: 400-600°C and 600-800°C, respectively), or along grain boundaries. Because the magnetite is concordant with bedding surfaces, it is plausible that the HTC was incorporated in the magnetite during diagenesis. Thus, HTC is the most important candidate for primary carbon preserved in the BIF. δ13C values of HTC cannot be explained as those of Isua carbonate. On the other hand, that the very low δ13C values (-30‰), negative δ15N values (-3‰), and low C/N elemental ratios (86) for the >1000°C fraction of one aliquot are comparable to those of kerogen in Archean metasediments. Therefore, despite the presence of secondary carbon (i.e., LTC), the BIF is suggested to possibly contain highly 13C-depleted kerogenous material, which is unlikely to have been incorporated after metamorphism. Although carbon isotopic change of the kerogenous material due to metamorphic effects cannot be precisely determined from the present data, this study shows that further analysis of magnetite from the Isua BIF is a key to the search for the early life.

Integrated Pb- and S-isotope investigation of sulphide minerals from the early Archaean of southwest Greenland

We present high-spatial resolution secondary ion mass spectrometry (SIMS) measurements of Pb and S isotopes in sulphides from early Archaean samples at two localities in southwest Greenland. Secondary pyrite from a 3.71 Ga sample of magnetite–quartz banded iron formation in the Isua Greenstone Belt, which has previously yielded unradiogenic Pb consistent with its ancient origin, contains sulphur with a mass independently fractionated (MIF) isotope signature (Δ 33S = + 3.3‰). This reflects the secondary mineralization of remobilized sedimentary S carrying a component modified by photochemical reactions in the early Archaean atmosphere. It further represents one of the most extreme positive excursions so far known from the early Archaean rock record. Sulphides from a quartz–pyroxene rock and an ultramafic boudin from the island of Akilia, in the Godthåbsfjord, have heterogeneous and generally radiogenic Pb isotopic compositions that we interpret to represent partial re-equilibration of Pb between the sulphides and whole rocks during tectonothermal events at 3.6, 2.7 and 1.6 Ga. Both these samples have Δ 33S = 0 (within analytical error) and therefore show no evidence for MIF sulphur. These data are consistent with previous interpretations that the rock cannot be proven to have a sedimentary origin. Our study illustrates that SIMS S-isotope measurements in ancient rocks can be used to elucidate early atmospheric parameters because of the ability to obtain combined S and Pb-isotope data, but caution must be applied when using such data to infer protolith. When information from geological context, petrography and chronology (i.e. by Pb isotopes) is combined and fully evaluated, Δ 33S signatures from sulphides and their geological significance can be interpreted with a higher degree of confidence.

Search for traces of the late heavy bombardment on Earth—Results from high precision chromium isotopes

High precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the ∼3.7 Gyr Isua Supracrustal Belt (ISB) in West Greenland cannot be distinguished from the standard terrestrial 53Cr / 52Cr ratio at our present level of resolution. As a consequence, our search for chemical traces of possible impact-derived meteoritic components (asteroidal and/or cometary material, or accreted cosmic dust) in the Earth's oldest chemical and detrital sediments was negative. Our results, based on the 53Mn– 53Cr short-lived radionuclide system (half-life of 3.7 Myr), cannot confirm the recent findings by [1] [R. Schoenberg, B.S. Kamber, K.D. Collerson, S. Moorbath. Tungsten isotope evidence from approximately 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth. Nature 418 (2002) 403–405.] of tungsten isotope anomalies (based on the 182Hf– 182W short-lived radionuclide system; half-life of 9 Myr) in these sediments, which were interpreted as indicating a component derived from meteorites. Possible reasons for the failure to trace cosmic material in the ISB metasediments are various: 1. The samples studied are not representative; 2: The sedimentation period did not overlap with the period of late heavy bombardment of the Moon; and 3. The potential chromium anomalies, if present, are too small to be traceable by our present levels of detection. Unequivocal evidence of a late heavy bombardment on the early Earth therefore remains elusive and uncertain.