Since the 1960s when Stephen Moorbath of the University of Oxford determined a date of 3.8 Ga for metamorphic rocks in West Greenland discovered by Vic McGregor of the Geological Survey of Greenland, pushing the age of tangible rocks towards that of the Earth itself has been slow. Indeed, geologists found only one geological terrain that pushed the ‘vestige of a beginning’ significantly back in time beyond the famous Isua rocks: the Acasta Gneiss east of Great Slave Lake in northern Canada, dated at 30 Ma more than 4 Ga. In fact in the 30 years between Moorbath’s Greenland date and that for the Acasta Gneiss, stratigraphers seem to have become resigned to a maximum 3.8 Ga age for rocks, and the start of the Archaean was set at that age. All earlier time, some 750 Ma of it, became known as the Hadean – a hellish time from which nothing had survived. Some geochemists perked up with the discovery, sifted from a much younger sandstone in the late 1980s, by Australians Bill Compston and Bob Pidgeon of 17 zircon grains that formed up to 4.4 Ga ago; but they tell us very little about the early world. What had become the lost cause of seeking pre-4 Ga rocks, has suddenly become revitalised with the discovery of a voluminous suite of rocks that are 200 million years closer to Earth’s origin in the eastern part of Arctic Canada (O’Neil, J. et al. 2008. Neodymium-142 evidence for Hadean mafic crust. Science, v. 321, p. 1828-1831).
The rocks are part of a recently mapped greenstone belt on the east shore of Hudson Bay, which contains a variety of mafic igneous rocks along with metasedimentary banded iron formations and cherts. The most dominant of the mafic rocks has yielded a 146Sm-142Nd isochron age of almost 4.3 Ga, and they are intruded by mafic and ultramafic sills dated at around 4.0 Ga. The older meta-igneous rock’s geochemistry suggests that it formed by partial melting of undepleted mantle rocks to produce magmas similar to those forming at modern convergent plate margins. Its major element variability, reflected in very diverse metamorphic mineral assemblages, suggests it to have originally formed as a mafic pyroclastic rock. It would be hard to prove that the BIFs and cherts are the same age in such a structurally complex belt, but that they are as old as the dated material is a distinct possibility. In that case they push back tangible evidence for surface water a great deal more convincingly than the arcane isotopic evidence derived from the oldest known zircons (see Zircon and the quest for life’s origin in the May 2005 issue of EPN). That such a substantial piece of very old crust has turned up a record age owes a great deal to advances in the Sm-Nd dating technique; the use of 146Sm decay to 142Nd (1/2 life of ~108 years), rather than the more readily addressed 147Sm to 143Nd decay (1/2 life of ~1011 years). This proof of concept may unleash a reappraisal of rocks that seem to be the oldest relative to others in Precambrian shields on every continent. It may eventually become possible to show that, apart from its cataclysmic experience that formed the Moon and probably a global magma ocean shortly after accretion, the Earth was by no means a totally hellish period during the ‘Hadean’.
Banding in BIFs
Banded iron formations, or BIFs, from the late Archaean and early Proterozoic are made of interlayered accumulations of iron oxides (and occasionally sulfides) and chert, and are the world’s most important iron ores. The BIFs of the Hammersley Range in Western Australia produce 26 % of the western world’s iron ore, and are hundreds of metres thick. The banding extends down to the scale of a few micrometres, and in some cases seems to record cyclic events. It has been claimed that, sun-spot, tidal, Milankovich and other nature cycles can be discerned. Few dispute that the iron oxides formed by oxidation of dissolved iron(II) ions through the influence of micro-organisms in shallow seawater. A popular candidate is photosynthetic blue-green bacteria, which produce oxygen; abundant reduced iron dissolved in Archaean seawater would have consumed the oxygen to become insoluble iron (III) oxides, delaying the development of an oxygen-bearing environment util about 2.2 Ga. There are other possibilities, such as anoxygenic photosynthesising bacteria, or photoferrotrophs, that could have achieved the Fe(II) to Fe(III) oxidation directly, without the need for free oxygen.. The puzzle is the on-off mechanism needed to produce the banding itself. That may have been resolved by experimental work under simulated Archaean conditions (Posth, N.R. et al. 2008. Alternating Si and Fe deposition caused by temperature fluctuations in Precambrian oceans. Nature Geoscience, v. 1, p. 703-708). The authors based their experiments on primitive, but living photoferrotrophs in conditions that chemically mimic likely Archaean seawater. They discovered that the critical factor in this form of biogenic precipitation of iron is sea-surface temperature: the microbes reproduce fastest to maximise iron-oxide formation at 20-25ºC. Temperatures above or below this range shut down productivity. However, temperatures above 25ºC favour silica remaining in solution, so the alternation of Fe- and Si-rich bands favours cooler sea temperatures for the latter. As well as providing a means of producing the enigmatic BIF banding, the experiments help resolve the controversy over prevailing sea-surface temperatures in the Archaean, which have been suggested by some to be as high as 85ºC. At least for the late Archaean, ocean temperatures seem to have been much the same as at present.