- Email: [email protected]
Early Archaean processes and the Isua Greenstone Belt, West Greenland
Precambrian Research 126 (2003) 173–179
Preface
Early Archaean processes and the Isua Greenstone Belt, West Greenland Peter W.U. Appel a,∗ , Stephen Moorbath b , Jacques L.R. Touret c,1 a
Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark b Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK c Department of Petrology, Vrije Universiteit, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands Accepted 5 February 2003
This Special Issue contains papers based on talks presented at a Workshop of participants of the Isua Multidisciplinary Research Project (IMRP) 1997–2001, held at the Harnack Haus Conference Centre of the Max-Planck-Gesellschaft in Berlin during 17–20 January 2002. The Workshop was held to mark the termination of official funding for the IMRP mainly from official Danish and Greenlandic sources, and to present some of the scientific results obtained in the previous 4 years by an international group of scientists working in loose affiliation. IMRP funding had provided transport and subsistence expenses within Greenland (as well as shipment of rocks from Greenland) enabling researchers to visit the remote Isua region in southern West Greenland in order to carry out geological field work and to collect samples for miscellaneous laboratory-based work. We emphasise that the papers presented here are by no means the first IMRP-related papers so far published. Furthermore, this Preface is not meant to be a comprehensive review, so that the list of appended ∗
Corresponding author. E-mail addresses: [email protected] (P.W.U. Appel), [email protected] (S. Moorbath), [email protected] (J.L.R. Touret). 1 Present address: Mus´ ee de Min´eralogie, Ecole des Mines, 60 Boulevard Saint-Michel, 75006 Paris, France.
references represents only a small selection from the available publications on the early Archaean rocks of West Greenland. Since the first realisation of the scientific importance of the 3.7–3.8 Ga Isua Greenstone Belt (IGB) (Fig. 1) for early Archaean studies in 1971, many research papers have discussed varied aspects of the belt, whilst a geological map of the Isua region was published by the Geological Survey of Greenland in 1986. In the early- to mid-1990s it became evident that some of the older work on the IGB, as well as the published map, were in need of major re-interpretation and revision. In addition, it was time for the application of new research techniques. Highly conflicting geological interpretations of the IGB have been published in recent years. At one extreme, it is implied that intense metamorphism and/or metasomatism and/or deformation (abbreviated here as MMD) have transformed most IGB rocks to such a degree that they now represent only a pale, ghostly relic of their protoliths, enabling only a tenuous identification to be made between rocks and protoliths, except for a few prominent rock-types such as banded iron-formation (BIF) and metabasalts (e.g. Rosing et al., 1996). At the other extreme, some workers claim that preservation of primary depositional features from the protolith is fully adequate to enable detailed stratigraphical and structural mapping of
0301-9268/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-9268(03)00093-7
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P.W.U. Appel et al. / Precambrian Research 126 (2003) 173–179
Fig. 1. Satellite image of central West Greenland with location of Isua Greenstone Belt.
exposed IGB rocks and even to make close comparisons with Phanerozoic oceanic plate stratigraphy (Komiya et al., 1999). The truth may lie somewhere in-between, and varies greatly within the belt. Just where in the spectrum it lies depends on the lithology of a given rock and the intensity of MMD it has undergone. In some cases, protolith features (e.g. pillow lavas) are easily identifiable in the field. In other cases, sophisticated metamorphic and geochemical studies are essential in order to determine which features have survived from the protolith and which have changed during subsequent MMD. Remapping of the IGB by John Myers and colleagues commenced in 1997, and the former
interpretation of coherent stratigraphic units within the IGB (Nutman et al., 1984) has been replaced by a model of fault-bounded rock packages and tectonically juxtaposed slices of complexly deformed and recrystallised volcanic and sedimentary rocks (Myers, 2001; Fedo et al., 2001). In contrast, there are sizeable low-strain domains with structurally well-preserved basaltic pillow lavas (metabasalts), despite ubiquitous recrystallisation and changes in bulk rock chemistry (e.g. Myers, 2001). Before discovery of the pillows in the early 1990s, some workers regarded the major mafic volcanic rock unit as an igneous intrusion. Much rarer are volcanic breccias and pillow breccias (Appel et al., 1998). Some of these breccia fragments
P.W.U. Appel et al. / Precambrian Research 126 (2003) 173–179
contain quartz-filled amygdules in which unstrained quartz contains tiny inclusions filled with entrapped gas and fluids, interpreted as remnants of an early Archaean marine hydrothermal system (Appel et al., 2001). Also in low-strain zones there are rare, spectacular conglomerate horizons with abundant rounded chert pebbles and rare metabasalt pebbles set in a fine-grained, metamorphosed sedimentary matrix (Fedo, 2000). No conglomerates with clasts of granitoid gneiss have ever been found in the IGB. Comparatively overlooked in recent descriptive field work in the IGB are some prominent horizons of massive garnet–mica–plagioclase–quartz (±amphibole, staurolite, tourmaline, etc.) schists, which contrast strongly in the field with adjacent metabasalts. These rocks were probably pelitic in origin (Boak and Dymek, 1982; Hayashi et al., 2000), with mafic volcanogenic sediments as immediate protoliths. Work in progress suggests that these metasediments can yield much information on the geological and geochemical nature of their source region. Some of these rocks contain a tungsten isotope anomaly, resulting from the short-lived (half-life, 9 Ma) decay scheme 182 Hf–182 W (Schoenberg et al., 2002). This indicates derivation of the sediments from a source region which had suffered meteoritic infall prior to erosion and deposition. It has been known for 30 years that IGB rocks are at least 3.7 Ga old (Moorbath et al., 1973). This has not changed much, but debate persists whether deposition occurred closer to 3.7 or 3.8 Ga, or even whether there are two quite separate periods of deposition within this time range (e.g. Nutman et al., 1997a). Measured dates depend on rock type, isotopic method used, and interpretation of field evidence in different sectors of the IGB. There is no convincing evidence for in situ rocks older than ca. 3.8 Ga, but recent Pb-isotope data on IGB metasediments and Pb-ores (Kamber et al., 2001; Kamber et al., 2003) suggest that important geochemical features pertaining to pre-4.0 Ga mantle and crust can be recognised. Such work follows on from previous modelling of the age of the least radiogenic Pb-isotopic compositions so far found on Earth, namely in small Pb mineralisations (galena) in the IGB (Appel et al., 1978; Richards and Appel, 1987; Frei and Rosing, 2001). Other radiogenic isotope data (e.g. Nd, Hf) on IGB rocks must be interpreted with care on account of MMD open-system behaviour
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(e.g. Moorbath et al., 1997), but tentatively suggest that the mantle source region of IGB rocks and adjacent early Archaean granitoid orthogneisses was slightly depleted-to-bordering-on chondritic (Kamber et al., 1998; Villa et al., 2001), thus not supporting a model of gross mantle heterogeneity or of pre-3.8 Ga mantle-to-continental crust differentiation on a grand scale (Bennett et al., 1993). Within the broadly circular outcrop of the IGB, there are granitoid orthogneisses reliably dated at ca. 3.65–3.70 Ga (Crowley et al., 2002). A large region (>1000 km2 ) south of the IGB comprises granitoid orthogneisses with abundant enclaves of varied mafic and ultramafic volcanics, as well as chemical sediments such as BIF. In places deformation is remarkably low, and zircon U–Pb dates of ca. 3.8 Ga have been reported (Nutman et al., 1999). Parts of the region had already been mapped by Brian Chadwick of Exeter University (maps held by the Geological Survey of Denmark and Greenland in Copenhagen). This gneiss terrain to the south of IGB, as well as its immediate contact with the IGB, is currently being mapped by Jim Crowley, John Myers, and others. Clearly one of the main regional aims is to use detailed field and age data to understand the tectonic intercalation of the IGB between two gneiss terrains of different early Archaean ages. Furthermore, the varied enclaves in the gneisses south of Isua must be older than ca. 3.8 Ga, and could even turn out to be older than similar, but in situ, rocks from the IGB, depending on whether the real age(s) of the latter are closer to 3.7 or 3.8 Ga. This region south of Isua clearly remains an important focus for research into the oldest known terrestrial rocks. Possibly biogenic carbon particles have been reported from a sample of BIF from the eastern IGB (Mojzsis et al., 1996), as well as from a metamorphosed pelitic sediment in the western IGB (Rosing, 1999). The problem is to decide whether the supposedly diagnostic carbon isotope fractionation (i.e. low ␦13 C‰) is truly biogenic or produced by an inorganic process. The analogous, much publicised situation on Akilia Island, some 150 km southwest of the IGB (Fig. 1), is particularly problematic with respect to early life because on Akilia the petrological identification of the carbon-bearing rocks (Fedo and Whitehouse, 2002), the exact nature of their field relationship with the adjacent, dated granitoid gneiss (Myers and Crowley, 2000), as well as the
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interpretation of the measured ages themselves (Whitehouse et al., 1999), all pose severe uncertainties for claims suggesting that life already existed by 3.80–3.85 Ga (Mojzsis et al., 1996; Nutman et al., 1997b). Indeed, the latest work on Akilia Island (Fedo and Whitehouse, 2002) demonstrates that the crucial carbon-bearing rocks, formerly identified as BIF (Mojzsis et al., 1996; Nutman et al., 1997), are actually banded quartz-pyroxene rocks of mixed igneous and metasomatic parentage, of no possible biological relevance whatever. Furthermore, the use of carbon isotopes as unambiguous proxies for biogenicity in strongly metamorphosed, metasomatised and deformed rocks (particularly of dubious parentage) has been strongly criticised. Within the context of the IGB, this applies particularly to a claim for biogenic carbon in a sample of BIF (Mojzsis et al., 1996), subsequently shown to be infiltrated by metasomatic carbonate, in which thermal disproportionation to elemental carbon has occurred (Van Zuilen et al., 2002). Most of the papers presented here touch on the issues mentioned above and offer significant advances in our understanding of these topics. There follows a brief statement on each paper: (1) Rollinson uses detailed garnet growth patterns in IGB rocks to characterise the sequence of metamorphic and metasomatic events, and to tie them in with available geochronological data. Adjacent domains exhibit different metamorphic histories, in accordance with the tectonic model of Myers (2001). (2) Polat and Hofmann study the effect of post-magmatic alteration on IGB volcanic rocks, and demonstrate the strong mobility of some trace elements and the relative immobility of others. They show, in particular, how Nb/Ta ratios can be used to characterise the composition of early Archaean mantle. (3) Touret describes early Archaean primary fluid inclusions containing methane and brines in undeformed, annealed quartz-bearing vesicles in an IGB pillow breccia, and suggests a possible correlation with present-day deep-seafloor hydrothermal systems in Mid-Atlantic Ridge environments. (4) Crowley applies zircon U–Pb dating to a part of the extensive gneiss terrain south of the IGB. He
(5)
(6)
(7)
(8)
(9)
confirms that ca. 3.8 Ga tonalitic orthogneisses were emplaced into supracrustal rocks (i.e. volcanics and sediments), and that both magmatic and metamorphic events affected these rocks at ca. 3.65–3.55 Ga and again at ca. 2.65–2.60 Ga. These early tonalites are ca. 100 million years older than the orthogneisses to the north of the IGB. Whitehouse and Fedo return to the contentious domain of Akilia Island in the Godthaabsfjord region and demonstrate even more convincingly than before that the lithology for so long regarded as a BIF containing biogenic carbon is actually a quartz-pyroxene rock resulting from metasomatic, metamorphic and tectonic reworking of a mafic-ultramafic protolith, totally incompatible with biological activity. Furthermore no valid age constraints on these rocks can be made from an adjacent granitoid sheet, because the two rock units are tectonically concordant. Lowry et al. present O-isotope studies on a ca. 3.8 Ga metamorphosed, layered ultramafic body in the orthogneiss terrain south of the IGB. They describe petrogenetic processes and temperatures, and postulate that mantle oxygen isotope composition at 3.8 Ga was already similar to its present-day composition. Kamber et al. have analysed many early Archaean mafic and ultramafic supracrustal rocks, including IGB samples, for immobile elements such as Nb, Ta, Th and others. They conclude that some elemental ratios, in particular Nb/Th, are robust proxies for amount of continental crust extracted through Earth history, and present a new parametrisation of the continental crust volume-versus-age curve. Appel et al. counter repeated claims for the existence of 3.8 Ga spherical microfossils in an IGB chert by demonstrating that the rock is highly deformed, stretched and recrystallised, and that the objects are clearly post-tectonic and probably represent chemical artefacts and/or recent biological contamination. Westall and Folk demonstrate that carbonaceous microstructures and microfossils in IGB chert and BIF do not represent 3.7–3.8 Ga organisms. Recent contamination of the rocks along cracks and microfissures presents a more convincing
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(10)
(11)
(12)
(13)
explanation, and the authors caution against premature interpretation of carbon isotope ratios and other chemical biomarkers in recently contaminated rocks as signifying early Archaean life. Van Zuilen et al. report that carbon (graphite) in the IGB is closely associated with carbonate-rich metasomatic rocks, and that graphite is produced by thermal decomposition of siderite (FeCO3 ). They find that IGB metachert and BIF are virtually free of graphite. The authors call for complete re-evaluation of earlier proposals for the existence of 3.8 Ga biogenic graphite in these rocks. Carbon isotope variability is attributed to metasomatic processes. Strauss reviews knowledge of the early Archaean sulphur cycle, including new data from the IGB, and concludes that in view of the absence of any unequivocal evidence for a biological origin, sulphur-isotope variability must be attributed to magmatic/hydrothermal processes in sedimentary environments of early Archaean age. Whitehouse and Kamber show how essential it is for characterising the magmatic and metamorphic history of complex, polyphase zircon grains to make detailed ion-microprobe and REE studies. Only in this way can the true significance of zircon dates be reliably assessed. They use zircons from the Godthaabsfjord region, where there has been much debate on this issue. In a final paper, not directly connected with the main topic of this Volume, Kramers compares a range of volatile elements in the outer reservoirs of the Earth with their abundances in solar matter and carbonaceous chondrites, in order to assess possible sources for these elements, as well as quantifying mechanisms of gain and loss. He then relates all this to possible scenarios for the earliest terrestrial environment.
In conclusion, the IGB and adjacent granitoid orthogneisses arguably represent the closest possible approach to the time of putative, massive global impact at around 3.9 Ga, after which total restructuring and renewal of the Earth’s surface occurred within a period not exceeding about 100 million years. This might conceivably be the result of impact-related mantle penetration followed by decompression melting and magmatic auto-obliteration of surface features on a
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huge scale, as suggested recently for the end-Permian Siberian traps (Jones et al., 2002). Furthermore, the adjacent granitoid orthogneiss terrains probably represent some of the earliest true continental crust, formed by petrogenetic and tectonic mechanisms not unlike those in later periods of Earth history. In consequence, this unique regional package of in situ 3.8–3.7 Ga rocks deserves much further study, because it represents a period in Earth history when geological processes as we know them became recognisably uniformitarian and when conditions for life became favourable, even though geological evidence in these rocks for its earliest existence still remains frustratingly inconclusive.
Acknowledgements We thank Editors-in-Chief A. Kröner and K. Eriksson for their interest and support in the planning of this Special Issue, and Patricia Massar of Elsevier for supporting its publication. The excellent work of the reviewers was essential in finalising the papers. The Editors are grateful to the following (as well as several anonymous) individuals for reviewing the papers in this Volume: Nick Arndt, Donald E. Clanfield, Wouter Bleeker, Gilles Chazot, Fernando Corfu, Jim Crowley, Hilary Downes, John Hanchar, Simon Hanmer, Claude Herzberg, Paul W.O. Hoskin, Jan M. Huizenga, John F. Kasting, A.M. van den Kerkhof, Roland Maas, Thomas Naegler, Euan Nisbet, Paddy O’Brien, Hugh O’Neill, Hugh Rollinson, Minik Rosing, John Schumacher, Maud Walsh. Generous support for the Isua Multidisciplinary Research Project (IMRP) was provided by the Geological Survey of Denmark and Greenland (GEUS), the Danish Natural Sciences Council, the Commission for Scientific Research in Greenland, and the Bureau of Minerals and Petroleum, Nuuk, Greenland. Additional funding for field work in Greenland was provided by the National Geographic, the Max-Planck Institut für Chemie, Mainz, the Natural History Museum, Toronto, and the European Science Foundation. Valuable cooperation is acknowledged with research groups from the Geological Museum and the Geological Institute in Copenhagen, as well as the Danish Lithosphere Centre. Special thanks are due to Martin Ghisler, Director of GEUS, and to Hans Christian Schoenwandt,
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Director of the Bureau of Minerals and Petroleum in Greenland, for their interest in, and support of, the activities of IMRP.
References Appel, P.W.U., Moorbath, S., Taylor, P.N., 1978. Least radiogenic terrestrial lead from Isua, West Greenland. Nature 272, 524– 526. Appel, P.W.U., Fedo, C.M., Moorbath, S., Myers, J.S., 1998. Recognisable 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. Terra Nova 10, 57–62. Appel, P.W.U., Rollinson, H.R., Touret, J.L.R., 2001. Remnants of an early Archaean (>3.75 Ga) sea-floor, hydrothermal system in the Isua Greenstone Belt. Precambrian Res. 112, 27–49. Bennett, V.C., Nutman, A.P., McCulloch, M.T., 1993. Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth. Earth Planet. Sci. Lett. 119, 299–317. Boak, J.L., Dymek, R.F., 1982. Metamorphism of the ca. 3800 Ma supracrustal rocks at Isua, West Greenland: implications for early Archaean crustal evolution. Earth Planet. Sci. Lett. 59, 155–176. Crowley, J.L., Myers, J.S., Dunning, F.R., 2002. Timing and nature of multiple 3700–3600 Ma tectonic events in intrusive rocks north of the Isua greenstone belt, southern West Greenland. Geol. Soc. Am. Bull. 114, 1311–1325. Fedo, C.M., 2000. Setting and origin for problematic rocks from the >3.7 Ga Isua Greenstone Belt, southern West Greenland: Earth’s oldest coarse clastic sediments. Precambrian Res. 101, 69–78. Fedo, C.M., Whitehouse, M.J., 2002. Metasomatic origin of quartz-pyroxene rock, Akilia, Greenland, and implications for Earth’s earliest life. Science 296, 1448–1452. Fedo, C.M., Myers, J.S., Appel, P.W.U., 2001. Depositional setting and palaeogeographic implications of earth’s oldest supracrustal rocks, the >3.7 Ga Isua Greenstone Belt, West Greenland. Sedimentary Geol. 141/142, 61–77. Frei, R., Rosing, M.T., 2001. The least radiogenic terrestrial leads; implications for the early Archaean crustal evolution and hydrothermal-metasomatic processes in the Isua Supracrustal Belt (West Greenland). Chem. Geol. 181, 47–66. Hayashi, M., Komiya, T., Nakamura, Y., Maruyama, S., 2000. Archaean regional metamorphism of the Isua Supracrustal Belt, southern West Greenland: implications for a driving force for Archaean plate tectonics. Int. Geol. Rev. 42, 1055–1115. Jones, A.P., Price, G.D., Price, N.J., De Carli, P.S., Clegg, R.A., 2002. Impact induced melting and the development of large igneous provinces. Earth Planet. Sci. Lett. 202, 551–561. Kamber, B.S., Moorbath, S., Whitehouse, M.J., 1998. Extreme Nd-isotope heterogeneity in the early Archaean—fact or fiction? Case histories from northern Canada and West Greenland— reply. Chem. Geol. 148, 219–224.
Kamber, B.S., Moorbath, S., Whitehouse, M.J., 2001. The oldest rocks on Earth: time constraints and geological controversies. In: Lewis, C.L., Knell, S.J. (Eds.), The age of the Earth: from 4004 BC to AD 2002. Geological Society, London, Special Publications 190, 177–203. Kamber, B.S., Collerson, K.D., Moorbath, S., Whitehouse, M.J., 2003. Inheritance of early Archaean Pb-isotope variability from long-lived Hadean protocrust. Contrib. Mineral. Petrol. 145, 25–46. Komiya, T., Maruyama, S., Nohda, T., Masuda, M., Hayashi, M., Okamoto, S., 1999. Plate tectonics at 3.8–3.7 Ga: field evidence from the Isua accretionary complex, southern West Greenland. J. Geol. 107, 515–554. Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P., Friend, C.R.L., 1996. Evidence for life on Earth before 3,800 million years ago. Nature 384, 55–59. Moorbath, S., O’Nions, R.K., Pankhurst, R.J., 1973. Early Archaean age for the Isua iron formation, West Greenland. Nature 245, 138–139. Moorbath, S., Whitehouse, M.J., Kamber, B.S., 1997. Extreme Nd-isotope heterogeneity in the early Archaean—fact or fiction? Case histories from northern Canada and West Greenland. Chem. Geol. 135, 213–231. Myers, J.S., 2001. Protoliths of the 3.8–3.7 Ga Isua Greenstone Belt, West Greenland. Precambrian Res. 105, 129–141. Myers, J.S., Crowley, J.L., 2000. Vestiges of life in the oldest Greenland rocks? A review of early Archaean geology in the Godthaabsfjord region, and reappraisal of field evidence for >3850 Ma life on Akilia. Precambrian Res. 103, 101–124. Nutman, A.P., Allaart, J.H., Bridgwater, D., Dimroth, E., Rosing, M., 1984. Stratigraphic and geochemical evidence for the depositional environment of the early Archaean Isua supracrustal belt, southern West Greenland. Precambrian Res. 25, 365–396. Nutman, A.P., Bennett, V.C., Friend, C.R.L., Rosing, M.T., 1997a. ∼3710 and ≥3790 Ma volcanic sequences in the Isua (Greenland) supracrustal belt; structural and Nd isotope implications. Chem. Geol. 141, 271–287. Nutman, A.P., Mojzsis, S.J., Friend, C.R.L., 1997b. Recognition of ≥3850 Ma water-lain sediments in West Greenland and their significance for the early Archaean Earth. Geochim. Cosmochim. Acta 61, 2475–2484. Nutman, A.P., Bennett, V.C., Friend, C.R.L., Norman, M.D., 1999. Meta-igneous (non-gneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation. Contrib. Mineral. Petrol. 137, 364–388. Richards, J.R., Appel, P.W.U., 1987. Age of the “least radiogenic” galenas at Isua, West Greenland. Chem. Geol. 66, 181–191. Rosing, M.T., 1999. 13 C-depleted carbon microparticles in >3700 Ma sea-floor sedimentary rocks from West Greenland. Science 283, 674–676. Rosing, M.T., Rose, N.M., Bridgwater, D., Thomsen, H.S., 1996. Earliest part of Earth’s stratigraphic record: a reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence. Geology 24, 43–46. Schoenberg, R., Kamber, B.S., Collerson, K.D., Moorbath, S., 2002. Tungsten isotope evidence from ∼3.8 Gyr
P.W.U. Appel et al. / Precambrian Research 126 (2003) 173–179 metamorphosed sediments for early meteorite bombardment of the Earth. Nature 418, 403–405. Van Zuilen, M.A., Lepland, A., Arrhenius, G., 2002. Reassessing the evidence for the earliest traces of life. Nature 418, 627–630. Villa, I.M., Kamber, B.S., Nägler, T.F., 2001. Comment on “The Nd and Hf isotopic evolution of the mantle through the Archaean: results from the Isua supracrustals, West Greenland, and from
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the Birimian terranes of West Africa” by Blichert-Toft et al. (1999). Geochim. Cosmochim. Acta 65, 2017–2021. Whitehouse, M.J., Kamber, B.S., Moorbath, S., 1999. Age significance of U–Th–Pb zircon data from early Archaean rocks of West Greenland—a reassessment based on combined ion-microprobe and imaging studies. Chem. Geol. 160, 201– 224.
Preface
Early Archaean processes and the Isua Greenstone Belt, West Greenland Peter W.U. Appel a,∗ , Stephen Moorbath b , Jacques L.R. Touret c,1 a
Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark b Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK c Department of Petrology, Vrije Universiteit, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands Accepted 5 February 2003
This Special Issue contains papers based on talks presented at a Workshop of participants of the Isua Multidisciplinary Research Project (IMRP) 1997–2001, held at the Harnack Haus Conference Centre of the Max-Planck-Gesellschaft in Berlin during 17–20 January 2002. The Workshop was held to mark the termination of official funding for the IMRP mainly from official Danish and Greenlandic sources, and to present some of the scientific results obtained in the previous 4 years by an international group of scientists working in loose affiliation. IMRP funding had provided transport and subsistence expenses within Greenland (as well as shipment of rocks from Greenland) enabling researchers to visit the remote Isua region in southern West Greenland in order to carry out geological field work and to collect samples for miscellaneous laboratory-based work. We emphasise that the papers presented here are by no means the first IMRP-related papers so far published. Furthermore, this Preface is not meant to be a comprehensive review, so that the list of appended ∗
Corresponding author. E-mail addresses: [email protected] (P.W.U. Appel), [email protected] (S. Moorbath), [email protected] (J.L.R. Touret). 1 Present address: Mus´ ee de Min´eralogie, Ecole des Mines, 60 Boulevard Saint-Michel, 75006 Paris, France.
references represents only a small selection from the available publications on the early Archaean rocks of West Greenland. Since the first realisation of the scientific importance of the 3.7–3.8 Ga Isua Greenstone Belt (IGB) (Fig. 1) for early Archaean studies in 1971, many research papers have discussed varied aspects of the belt, whilst a geological map of the Isua region was published by the Geological Survey of Greenland in 1986. In the early- to mid-1990s it became evident that some of the older work on the IGB, as well as the published map, were in need of major re-interpretation and revision. In addition, it was time for the application of new research techniques. Highly conflicting geological interpretations of the IGB have been published in recent years. At one extreme, it is implied that intense metamorphism and/or metasomatism and/or deformation (abbreviated here as MMD) have transformed most IGB rocks to such a degree that they now represent only a pale, ghostly relic of their protoliths, enabling only a tenuous identification to be made between rocks and protoliths, except for a few prominent rock-types such as banded iron-formation (BIF) and metabasalts (e.g. Rosing et al., 1996). At the other extreme, some workers claim that preservation of primary depositional features from the protolith is fully adequate to enable detailed stratigraphical and structural mapping of
0301-9268/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-9268(03)00093-7
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Fig. 1. Satellite image of central West Greenland with location of Isua Greenstone Belt.
exposed IGB rocks and even to make close comparisons with Phanerozoic oceanic plate stratigraphy (Komiya et al., 1999). The truth may lie somewhere in-between, and varies greatly within the belt. Just where in the spectrum it lies depends on the lithology of a given rock and the intensity of MMD it has undergone. In some cases, protolith features (e.g. pillow lavas) are easily identifiable in the field. In other cases, sophisticated metamorphic and geochemical studies are essential in order to determine which features have survived from the protolith and which have changed during subsequent MMD. Remapping of the IGB by John Myers and colleagues commenced in 1997, and the former
interpretation of coherent stratigraphic units within the IGB (Nutman et al., 1984) has been replaced by a model of fault-bounded rock packages and tectonically juxtaposed slices of complexly deformed and recrystallised volcanic and sedimentary rocks (Myers, 2001; Fedo et al., 2001). In contrast, there are sizeable low-strain domains with structurally well-preserved basaltic pillow lavas (metabasalts), despite ubiquitous recrystallisation and changes in bulk rock chemistry (e.g. Myers, 2001). Before discovery of the pillows in the early 1990s, some workers regarded the major mafic volcanic rock unit as an igneous intrusion. Much rarer are volcanic breccias and pillow breccias (Appel et al., 1998). Some of these breccia fragments
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contain quartz-filled amygdules in which unstrained quartz contains tiny inclusions filled with entrapped gas and fluids, interpreted as remnants of an early Archaean marine hydrothermal system (Appel et al., 2001). Also in low-strain zones there are rare, spectacular conglomerate horizons with abundant rounded chert pebbles and rare metabasalt pebbles set in a fine-grained, metamorphosed sedimentary matrix (Fedo, 2000). No conglomerates with clasts of granitoid gneiss have ever been found in the IGB. Comparatively overlooked in recent descriptive field work in the IGB are some prominent horizons of massive garnet–mica–plagioclase–quartz (±amphibole, staurolite, tourmaline, etc.) schists, which contrast strongly in the field with adjacent metabasalts. These rocks were probably pelitic in origin (Boak and Dymek, 1982; Hayashi et al., 2000), with mafic volcanogenic sediments as immediate protoliths. Work in progress suggests that these metasediments can yield much information on the geological and geochemical nature of their source region. Some of these rocks contain a tungsten isotope anomaly, resulting from the short-lived (half-life, 9 Ma) decay scheme 182 Hf–182 W (Schoenberg et al., 2002). This indicates derivation of the sediments from a source region which had suffered meteoritic infall prior to erosion and deposition. It has been known for 30 years that IGB rocks are at least 3.7 Ga old (Moorbath et al., 1973). This has not changed much, but debate persists whether deposition occurred closer to 3.7 or 3.8 Ga, or even whether there are two quite separate periods of deposition within this time range (e.g. Nutman et al., 1997a). Measured dates depend on rock type, isotopic method used, and interpretation of field evidence in different sectors of the IGB. There is no convincing evidence for in situ rocks older than ca. 3.8 Ga, but recent Pb-isotope data on IGB metasediments and Pb-ores (Kamber et al., 2001; Kamber et al., 2003) suggest that important geochemical features pertaining to pre-4.0 Ga mantle and crust can be recognised. Such work follows on from previous modelling of the age of the least radiogenic Pb-isotopic compositions so far found on Earth, namely in small Pb mineralisations (galena) in the IGB (Appel et al., 1978; Richards and Appel, 1987; Frei and Rosing, 2001). Other radiogenic isotope data (e.g. Nd, Hf) on IGB rocks must be interpreted with care on account of MMD open-system behaviour
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(e.g. Moorbath et al., 1997), but tentatively suggest that the mantle source region of IGB rocks and adjacent early Archaean granitoid orthogneisses was slightly depleted-to-bordering-on chondritic (Kamber et al., 1998; Villa et al., 2001), thus not supporting a model of gross mantle heterogeneity or of pre-3.8 Ga mantle-to-continental crust differentiation on a grand scale (Bennett et al., 1993). Within the broadly circular outcrop of the IGB, there are granitoid orthogneisses reliably dated at ca. 3.65–3.70 Ga (Crowley et al., 2002). A large region (>1000 km2 ) south of the IGB comprises granitoid orthogneisses with abundant enclaves of varied mafic and ultramafic volcanics, as well as chemical sediments such as BIF. In places deformation is remarkably low, and zircon U–Pb dates of ca. 3.8 Ga have been reported (Nutman et al., 1999). Parts of the region had already been mapped by Brian Chadwick of Exeter University (maps held by the Geological Survey of Denmark and Greenland in Copenhagen). This gneiss terrain to the south of IGB, as well as its immediate contact with the IGB, is currently being mapped by Jim Crowley, John Myers, and others. Clearly one of the main regional aims is to use detailed field and age data to understand the tectonic intercalation of the IGB between two gneiss terrains of different early Archaean ages. Furthermore, the varied enclaves in the gneisses south of Isua must be older than ca. 3.8 Ga, and could even turn out to be older than similar, but in situ, rocks from the IGB, depending on whether the real age(s) of the latter are closer to 3.7 or 3.8 Ga. This region south of Isua clearly remains an important focus for research into the oldest known terrestrial rocks. Possibly biogenic carbon particles have been reported from a sample of BIF from the eastern IGB (Mojzsis et al., 1996), as well as from a metamorphosed pelitic sediment in the western IGB (Rosing, 1999). The problem is to decide whether the supposedly diagnostic carbon isotope fractionation (i.e. low ␦13 C‰) is truly biogenic or produced by an inorganic process. The analogous, much publicised situation on Akilia Island, some 150 km southwest of the IGB (Fig. 1), is particularly problematic with respect to early life because on Akilia the petrological identification of the carbon-bearing rocks (Fedo and Whitehouse, 2002), the exact nature of their field relationship with the adjacent, dated granitoid gneiss (Myers and Crowley, 2000), as well as the
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interpretation of the measured ages themselves (Whitehouse et al., 1999), all pose severe uncertainties for claims suggesting that life already existed by 3.80–3.85 Ga (Mojzsis et al., 1996; Nutman et al., 1997b). Indeed, the latest work on Akilia Island (Fedo and Whitehouse, 2002) demonstrates that the crucial carbon-bearing rocks, formerly identified as BIF (Mojzsis et al., 1996; Nutman et al., 1997), are actually banded quartz-pyroxene rocks of mixed igneous and metasomatic parentage, of no possible biological relevance whatever. Furthermore, the use of carbon isotopes as unambiguous proxies for biogenicity in strongly metamorphosed, metasomatised and deformed rocks (particularly of dubious parentage) has been strongly criticised. Within the context of the IGB, this applies particularly to a claim for biogenic carbon in a sample of BIF (Mojzsis et al., 1996), subsequently shown to be infiltrated by metasomatic carbonate, in which thermal disproportionation to elemental carbon has occurred (Van Zuilen et al., 2002). Most of the papers presented here touch on the issues mentioned above and offer significant advances in our understanding of these topics. There follows a brief statement on each paper: (1) Rollinson uses detailed garnet growth patterns in IGB rocks to characterise the sequence of metamorphic and metasomatic events, and to tie them in with available geochronological data. Adjacent domains exhibit different metamorphic histories, in accordance with the tectonic model of Myers (2001). (2) Polat and Hofmann study the effect of post-magmatic alteration on IGB volcanic rocks, and demonstrate the strong mobility of some trace elements and the relative immobility of others. They show, in particular, how Nb/Ta ratios can be used to characterise the composition of early Archaean mantle. (3) Touret describes early Archaean primary fluid inclusions containing methane and brines in undeformed, annealed quartz-bearing vesicles in an IGB pillow breccia, and suggests a possible correlation with present-day deep-seafloor hydrothermal systems in Mid-Atlantic Ridge environments. (4) Crowley applies zircon U–Pb dating to a part of the extensive gneiss terrain south of the IGB. He
(5)
(6)
(7)
(8)
(9)
confirms that ca. 3.8 Ga tonalitic orthogneisses were emplaced into supracrustal rocks (i.e. volcanics and sediments), and that both magmatic and metamorphic events affected these rocks at ca. 3.65–3.55 Ga and again at ca. 2.65–2.60 Ga. These early tonalites are ca. 100 million years older than the orthogneisses to the north of the IGB. Whitehouse and Fedo return to the contentious domain of Akilia Island in the Godthaabsfjord region and demonstrate even more convincingly than before that the lithology for so long regarded as a BIF containing biogenic carbon is actually a quartz-pyroxene rock resulting from metasomatic, metamorphic and tectonic reworking of a mafic-ultramafic protolith, totally incompatible with biological activity. Furthermore no valid age constraints on these rocks can be made from an adjacent granitoid sheet, because the two rock units are tectonically concordant. Lowry et al. present O-isotope studies on a ca. 3.8 Ga metamorphosed, layered ultramafic body in the orthogneiss terrain south of the IGB. They describe petrogenetic processes and temperatures, and postulate that mantle oxygen isotope composition at 3.8 Ga was already similar to its present-day composition. Kamber et al. have analysed many early Archaean mafic and ultramafic supracrustal rocks, including IGB samples, for immobile elements such as Nb, Ta, Th and others. They conclude that some elemental ratios, in particular Nb/Th, are robust proxies for amount of continental crust extracted through Earth history, and present a new parametrisation of the continental crust volume-versus-age curve. Appel et al. counter repeated claims for the existence of 3.8 Ga spherical microfossils in an IGB chert by demonstrating that the rock is highly deformed, stretched and recrystallised, and that the objects are clearly post-tectonic and probably represent chemical artefacts and/or recent biological contamination. Westall and Folk demonstrate that carbonaceous microstructures and microfossils in IGB chert and BIF do not represent 3.7–3.8 Ga organisms. Recent contamination of the rocks along cracks and microfissures presents a more convincing
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(10)
(11)
(12)
(13)
explanation, and the authors caution against premature interpretation of carbon isotope ratios and other chemical biomarkers in recently contaminated rocks as signifying early Archaean life. Van Zuilen et al. report that carbon (graphite) in the IGB is closely associated with carbonate-rich metasomatic rocks, and that graphite is produced by thermal decomposition of siderite (FeCO3 ). They find that IGB metachert and BIF are virtually free of graphite. The authors call for complete re-evaluation of earlier proposals for the existence of 3.8 Ga biogenic graphite in these rocks. Carbon isotope variability is attributed to metasomatic processes. Strauss reviews knowledge of the early Archaean sulphur cycle, including new data from the IGB, and concludes that in view of the absence of any unequivocal evidence for a biological origin, sulphur-isotope variability must be attributed to magmatic/hydrothermal processes in sedimentary environments of early Archaean age. Whitehouse and Kamber show how essential it is for characterising the magmatic and metamorphic history of complex, polyphase zircon grains to make detailed ion-microprobe and REE studies. Only in this way can the true significance of zircon dates be reliably assessed. They use zircons from the Godthaabsfjord region, where there has been much debate on this issue. In a final paper, not directly connected with the main topic of this Volume, Kramers compares a range of volatile elements in the outer reservoirs of the Earth with their abundances in solar matter and carbonaceous chondrites, in order to assess possible sources for these elements, as well as quantifying mechanisms of gain and loss. He then relates all this to possible scenarios for the earliest terrestrial environment.
In conclusion, the IGB and adjacent granitoid orthogneisses arguably represent the closest possible approach to the time of putative, massive global impact at around 3.9 Ga, after which total restructuring and renewal of the Earth’s surface occurred within a period not exceeding about 100 million years. This might conceivably be the result of impact-related mantle penetration followed by decompression melting and magmatic auto-obliteration of surface features on a
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huge scale, as suggested recently for the end-Permian Siberian traps (Jones et al., 2002). Furthermore, the adjacent granitoid orthogneiss terrains probably represent some of the earliest true continental crust, formed by petrogenetic and tectonic mechanisms not unlike those in later periods of Earth history. In consequence, this unique regional package of in situ 3.8–3.7 Ga rocks deserves much further study, because it represents a period in Earth history when geological processes as we know them became recognisably uniformitarian and when conditions for life became favourable, even though geological evidence in these rocks for its earliest existence still remains frustratingly inconclusive.
Acknowledgements We thank Editors-in-Chief A. Kröner and K. Eriksson for their interest and support in the planning of this Special Issue, and Patricia Massar of Elsevier for supporting its publication. The excellent work of the reviewers was essential in finalising the papers. The Editors are grateful to the following (as well as several anonymous) individuals for reviewing the papers in this Volume: Nick Arndt, Donald E. Clanfield, Wouter Bleeker, Gilles Chazot, Fernando Corfu, Jim Crowley, Hilary Downes, John Hanchar, Simon Hanmer, Claude Herzberg, Paul W.O. Hoskin, Jan M. Huizenga, John F. Kasting, A.M. van den Kerkhof, Roland Maas, Thomas Naegler, Euan Nisbet, Paddy O’Brien, Hugh O’Neill, Hugh Rollinson, Minik Rosing, John Schumacher, Maud Walsh. Generous support for the Isua Multidisciplinary Research Project (IMRP) was provided by the Geological Survey of Denmark and Greenland (GEUS), the Danish Natural Sciences Council, the Commission for Scientific Research in Greenland, and the Bureau of Minerals and Petroleum, Nuuk, Greenland. Additional funding for field work in Greenland was provided by the National Geographic, the Max-Planck Institut für Chemie, Mainz, the Natural History Museum, Toronto, and the European Science Foundation. Valuable cooperation is acknowledged with research groups from the Geological Museum and the Geological Institute in Copenhagen, as well as the Danish Lithosphere Centre. Special thanks are due to Martin Ghisler, Director of GEUS, and to Hans Christian Schoenwandt,
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Director of the Bureau of Minerals and Petroleum in Greenland, for their interest in, and support of, the activities of IMRP.
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