Precambrian Research 183 (2010) 725–737 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres ≥3700 Ma pre-metamorphic dolomite formed by microbial mediation in the Isua supracrustal belt (W. Greenland): Simple evidence for early life? Allen P. Nutman a,b,∗ , Clark R.L. Friend c , Vickie C. Bennett d , David Wright e , Marc D. Norman d a School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia b Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China c 45 Stanway Road, Headington, Oxford OX3 8HU, UK d Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia e Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK article info Article history: Received 19 August 2009 Received in revised form 22 July 2010 Accepted 13 August 2010 Keywords: Isua supracrustal belt Early life Dolomite REE + Y chemistry Chemical sedimentary rocks Pillow lava interstices abstract Chemical (meta)sedimentary rocks in the amphibolite facies ≥3700 Ma Isua supracrustal belt (W. Green- land) are mostly strongly deformed, so there is only a small chance of the survival of features such as stromatolites or microfossils that would be direct proof of a ≥3700 Ma biosphere. Therefore the search for evidence of ≥3700 Ma life in Isua rocks has focused on chemical signatures, particularly C-isotopes. The new approach presented here is based on whole rock chemistry rather than isotopic signatures. Isua chemical sedimentary rocks have Ca–Mg–Fe bulk compositions that coincide with ferroan dolomite – siderite/Fe-oxide mixtures. Most have low Al 2 O 3 , TiO 2 contents (<0.5 and <0.05 wt% respectively) show- ing minimal contamination from terriginous materials. Identical seawater-like REE + Y shale-normalised trace element signatures with La, Ce, Eu and Y positive anomalies are found in magnetite-rich banded iron formation (BIF – such as the geochemical standard IF-G), dolomite-rich rocks and quartz–carbonate–calc- silicate rocks. Additionally from a rare, small area of low deformation in Isua, there are ∼3700 Ma pillow lava interstices consisting of quartz + tremolite + calcite derived from pre-metamorphic dolomite + silica. Thus the dolomite in the chemical sediments and the pillow interstice was part of the pre-metamorphic assemblage, and was deposited from seawater and/or low-temperature groundwater (as shown by the REE + Y chemistry). Therefore, at least some Isua carbonate rocks are sedimentary or diagenetic in origin rather than being formed by metasomatism at 600–500 ◦ C as proposed by Rose et al. (1996. American Journal of Science 296, 1004–1044). Low-temperature dolomite formation in modern sediments (sabkha to deep ocean) and its deposition from low-temperature groundwater within basalts has only been directly observed in the field and repli- cated in laboratory experiments through anaerobic microbial mediation. Therefore, microbial mediation appears to be essential for the formation of low-temperature dolomite. From this, we propose that the evi- dence for the formation of low-temperature pre-metamorphic dolomite in Isua prior to metamorphism provides a new, simple, and relatively direct line of evidence for ≥3700 Ma life. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Widely accepted evidence for Paleoarchean life is found in essentially undeformed and non-metamorphosed 3500–3400 Ma sedimentary rocks of the Pilbara region, Western Australia in the form of stromatolites and probably some microfossils, in associa- tion with stable isotope signatures consistent with a biotic origin (Schopf, 2006; van Kranendonk, 2007 and references therein). In ∗ Corresponding author at: Department of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW, 2515, Australia. Tel.: +61 02 4298 1347. E-mail address: Allen.Nutman@gmail.com (A.P. Nutman). contrast, all pre-3500 Ma (Eoarchaean) rocks have been metamor- phosed to at least amphibolite facies conditions and are mostly strongly deformed (Nutman, 2006), which makes it much harder to find convincing evidence of early life in them. The ≥3700 Ma Isua supracrustal belt in the Itsaq Gneiss Complex of southern West Greenland contains the largest and best-preserved example of ancient terrestrial sedimentary and volcanic rocks in the world (Moorbath et al., 1973; Allaart, 1976; Nutman et al., 1996; Rosing et al., 1996; Nutman and Friend, 2009). Since the realisation of the belt’s great age in the early 1970s, the Isua rocks have remained the focus of the search for evidence of earliest life. Because of the deformation and amphibolite facies metamor- phism of the Isua rocks, the survival of ≥3700 Ma microfossils in them (Pflug and Jaeschke-Boyer, 1979) has been regarded as 0301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2010.08.006