Oldest rocks, earliest life, heaviest impacts, and the Hadean -- Archaean transitionReferencesOldest rocks, earliest life, heaviest impacts, and theHadean–Archaean transitionStephen Moorbath*Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UKEditorial handling by R. FugeMany geologists have their own favourite part of thevast geological time scale, from which there is plenty tochoose for all types of pure and applied research, and fordetailed speculation. My own favourite is that ancienttime range during which the EarthÕs surface changedfrom something quite unfamiliar and shaped by largelynon-uniformitarian processes, to something increasinglyrecognisable in uniformitarian terms. That has been re-ferred to as the Hadean–Archaean transition, often de-fined by arbitrary time boundaries like 4.0, 3.9, and3.8 Ga, which are not necessarily associated with anyspecified event horizon.The first and last sentences of the chapter on the Ha-dean Earth in Jonathan LunineÕs well-known textbookEarth – Evolution of a Habitable World (1999) read asfollows:‘‘The period from the formation of the Earth, some 4.56billion years ago, to the time when the oldest rocks stillin existence were formed, roughly 3.8–4.0 billion yearsago, is called both the Hadean era and Priscoan eon ofEarth. The term Hadean, referring to the Greek versionof hell, is well chosen, because all evidence that we haveis that the Hadean Earth was very hot and extremelyactive, with widespread volcanism and frequent impactsof debris left over from planetary formation.’’‘‘The Hadean Earth, while vastly different from the pres-ent planet, set the stage for what was to follow. By 3.8–4.0 billion years ago, the growth of continents, the sta-bilisation of liquid water, and the decreasing impactrate, made for an increasingly predictable and benignenvironment. Increasing environmental stability charac-terised the transition from the Hadean era to the Archa-eon eon of Earth.’’Some of this makes good sense and serves as a start-ing point for looking at some major event horizons inthis 3.8–4.0 Ga time range.The first thing to ask is where, and how old, are theoldest rocks on Earth (for review, see Kamber et al.,2001)? It appears that the largest, best exposed, best pre-served terrain of the oldest known early Archaean rocksoccurs in southern West Greenland, with undisputeddates in the range of 3.82–3.65 Ga, some of which wereestablished more than 30 a ago. The nearly 3.8 Ga Isuasupracrustal belt (also known as Isua greenstone belt)comprises a great variety of metamorphosed, variablydeformed volcanic and sedimentary rocks, includingbasalts (with pillow lavas), felsic volcanics, chemical sed-iments (e.g., chert, banded iron-formation), clastic sedi-ments (now garnet-mica schists) and other minor rocktypes (for review and references see Appel et al., 2003).The presence of genuine, primary carbonate sedimentsis still much debated. It is quite certain that most, ifnot all, of these rocks were deposited in water. Therewas clearly no shortage of water on the EarthÕs surfaceat around 3.8 Ga. Maybe planet Mars was still flowingwith surface water then, or it might already have par-tially or completely lost its surface water, some to spaceand some to a vast underground reservoir. We shall cer-tainly find out before long!The Isua supracrustal belt is bounded on its northernside by granitoid gneisses of magmatic origin (‘‘orthog-neisses’’), reliably dated at 3.70–3.65 Ga (for summary,see Crowley, 2003). Geochemical evidence suggests that0883-2927/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.apgeochem.2005.01.001*Tel.: +44 186 527 4584; fax: +44 186 527 2072.E-mail address: [email protected] Geochemistry 20 (2005) 819–824AppliedGeochemistrywww.elsevier.com/locate/apgeochemthese are tonalite–trondhjemite–granodiorite (TTG)gneisses (known as Amıˆtsoq or Itsaq gneisses) character-istic of the protoliths of genuine continental crust, differ-entiated from a geochemically slightly depleted, maficArchaean source region. These gneisses exhibit both tec-tonically conformable and intrusive relationships withthe adjacent Isua supracrustal belt, and both rock unitsare cut by a dense doleritic dyke swarm (the Ameralikdykes), of which the oldest components have been datedat 3.4–3.5 Ga (Nutman et al., 2004). The orthogneissescould well mark the initiation of subduction-related pro-cesses and the onset of uniformitarian continental crustproduction. The Ameralik dyke swarm is the oldestknown continental dyke swarm on Earth.To the south of the Isua supracrustal belt there is aterrain of several hundred km2of well exposed, weaklydeformed granitoid orthogneisses with zircon U–Pbdates as old as 3.82 Ga, which is regarded as their trueage of emplacement (Nutman et al., 1999; Crowley,2003). These orthogneisses are rich in man-to-mountainsized enclaves which include mafic/ultramafic metamor-phosed magmatic rocks, varied amphibolitic gneisses,and chemical sediments, such as banded iron-formation.These enclaves must be at least 3.82-Ga old. Thisextraordinary terrain has not been studied much, andonly a few geologists have visited the area. After somuch intense current interest in, and exploration of,the ancient surfaces of Moon and Mars, here we havea sizeable terrain of easily accessible plus-3.8-Ga rockscrying out for detailed exploration and research, at asmall fraction of the cost of interplanetary research! Inmy view, in these enclaves in 3.82-Ga gneisses, there isprobably a better chance than anywhere else on Earthof actually finding bits and pieces of genuine Hadeancrust and, possibly, physical remnants of the late heavybombardment (LHB), of which more below.Kamber et al. (2003) recently reported that Pb iso-tope characteristics of these 3.82-Ga gneisses are quitedifferent to those of the well-studied, nearby 3.65–3.70-Ga gneisses (see above) and they suggest that these old-er gneisses were derived from a mafic source regionwhich had itself separated from the mantle as earlyas ca. 4.2–4.3 Ga. Kamber et al. (2003) suggested thatthis source region was the Hadean basaltic crust which,in the probable absence of the subduction process, en-cased the early Earth. From these two isotopically con-trasted groups of orthogneisses, a fundamental changein global tectonic regime was proposed between about3.8 and 3.7 Ga, with the protoliths of