Geoscientist Online

Rock of (extreme) ages

Geoscientist 19.2 February 2009

A short article by O’Neil et al.¹ reports Neodymium-142 data for rocks from a newly discovered greenstone belt on the northeast shore of Hudson’s Bay, Quebec - the Nuvvvuagittuq belt.. The 146Sm-142 Nd isochron has an age of 4280 (+53,-81) Myrs, well back into the Hadean. Previously the oldest reported zircon-based dates for a rock, as opposed to a mineral, had come from the Acasta Gneiss in the Slave Province, Canada². However, this was controversial: the dating came from a mafic band in a gneiss, the more silicic parts of which gave younger dates. Scientists at Oxford, in particular, favoured rocks from Greenland, of less than 4.0 billion years, which were undoubtedly whole-rock ages. There have been several mineral dates on zircons, even on parts of zircon crystals, from the Jack Hills, Mt Narryer, Western Australia, ranging from 4.10-4.27 billion years 2 (and slightly more since). The present date is also controversial, for it is in a faux-amphibolite (a cummingtonite-amphibolite) surrounded by a 3.66 billion year old tonalite. The faux-amphobolite contains no zircon, on which this remarkable age date can be checked. The doubts are thus the same as with the Acasta Gneiss.

The article in Science is preceded by a very lucid analysis of what this new find really means³, by Richard Kerr, who writes: “Either this rock is a 2km sliver of protocrust, resembling today’s iron-rich ocean crust or it was derived from such a crust”. Either way, it seems to be a sample of incompatible-element enriched material, formed shortly after Earth formation 4.567 billion years ago. It may be the oldest preserved crustal section recorded on Earth. Leaving aside the rather fussy arguments over whether the Acasta Gneiss is the oldest whole rock sampled or not, and arguments which must now arise about the significance of this new find, these important discoveries from the Slave Province, Quebec (and the equally important finds in the Jack Hills, Western Australia) show that the Earth developed a crust remarkably soon after its formation, a span of less than 300 million years – a fact that must be regarded as quite surprising.

References cited

1. O’Neil, J., Carlson, R.W., Francis, D., Stevenson, R.K. 2008. Neodymium-142 evidence for Hadean mafic crust. Science 321; 1828-1831.
2. McCall, G.J.H. 2000. Age and early evolution of the Earth and Solar System. In: Hancock, P.L., Skinner, B.J. 2000. The Oxford Companion to the Earth; 8-11.
3. Kerr, R.A. 2008. Geologists find vestige of Early Earth –Maybe World’s oldest rock. Science 321; 1755.