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

Abstract

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.

Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation

“…The investigation of light stable isotope systems in BIFs, namely O, C and S isotope ratios, has a long tradition (e.g. Clayton, 1972, 1976;Perry et al, 1973Perry et al, , 1978Goodwin et al, 1976;Baur et al, 1985;Beukes et al, 1990;Kaufman et al, 1990;Mojzsis et al, 1996;Fedo et al, 2006). In recent years, advances in analytical techniques have provided the opportunity to study the stable isotope fractionation of the two major elements in BIFs, Fe and Si.…”

Cell evolution and Earth history: stasis and revolution

“…(c) Antiquity of negibacterial phyla, the oldest prokaryotes Sound evidence for life more than 3.5 Gyr ago is wanting (Fedo et al 2006). Diverse evidence makes it highly probable that life, negibacteria in particular, existed in the Late Archaean, 2.5-2.9 Gyr; no evidence absolutely requires the presence of Unibacteria (Posibacteria, Archaebacteria) or eukaryotes.…”

Metamorphic alteration, mineral paragenesis and geochemical re-equilibration of early Archean quartz–amphibole–pyroxene gneiss from Akilia, Southwest Greenland

“…The retrogression may have occurred in connection with an amphibolite facies metamorphic event at ca. 2.7 Ga (Nutman et al , 1997Whitehouse et al 1999Myers and Crowley 2000;Mojzsis and Harrison 2002a;Fedo et al 2006;Nutman and Friend 2007), but association with earlier thermal and igneous events (e.g. intrusion of dolerite dykes at ca.…”

“…It has been argued that the major and trace element characteristics of this dataset are inconsistent with a metasomatised ultramafic protolith, and rather reflect the sedimentary origin of at least some parts of the quartz-amphibole-pyroxene gneiss Mojzsis and Harrison 2002b;Manning et al 2006). Fedo and Whitehouse (2002a, b, c) and Fedo et al (2006) admit that some of the major and trace element comparisons are not easily reconcilable, but the combined structural, mineralogical, and geochemical data reflect, in their view, the metasomatised ultramafic protolith.…”

“…Fedo and Whitehouse (2002a), on the other hand, have argued on the grounds of structural and geochemical characteristics that the Akilia quartz-amphibole-pyroxene gneiss is an intensively and multiply deformed and metasomatised (silica and carbonate) ultramafic rock. Structural and mineralogic data demonstrate that layering within the quartz-amphibole-pyroxene gneiss is not a sedimentary feature formed by alternating quartz and magnetite precipitation (magnetite was shown to be a minor component in the quartz-amphibole-pyroxene gneiss and several portions of it are nearly magnetite free), but is part of the overall penetrative tectonic layering (Fedo and Whitehouse 2002a;Fedo 2003, 2007;Fedo et al 2006). A metasomatised ultramafic protolith has also been favored on the basis of silicon isotope composition (André et al 2006) and trace element signatures (Bolhar et al 2004).…”