It was a bit over four years ago when Science published a paper claiming that a set of weird Canadian rocks at Nuvvuagittuq, along the shore of Hudson Bay, included amphibolite that is 4.28 billion years oldby far the oldest rocks ever found on Earth. Since that time no one has claimed to find anything older, but there seems to be ongoing discussion of whether the Nuvvuagittuq ("NOO-voo-AG-it-tuck") rocks are really as old as that.
Jonathan O'Neil was the grad student at McGill University whose lucky choice for a PhD project made him the claimant to the world's oldest rock. Now at the prestigious Carnegie Institute, he has continued to visit Nuvvuagittuq and definitely has a place at the table among the handful of people leading the discussion of the earliest Earth. After the 2008 Science paper, O'Neil published an article in the Journal of Petrology in which he merely referred to the "possibility that the dominant lithology of the [Nuvvuagittuq] belt formed at ~4.28 Ga." The problem appears to be the novel dating method he used for the Science paper which, truth be told, was its real breakthrough. That is based on the extinct isotope samarium-146, which for a brief period in Earth's infancy decayed to neodymium-142, leaving an imbalance in the neodymium isotope inventory. The method is ingenious, but so far it has only been applied to Nuvvuagittuq.
The rocks there don't have any zircon crystals, which are the gold standard for uranium-series dating. Among the rocks that are associated with the amphibolite are some that do have zirconsbut those only testify to 3.8 Ga ages. Therefore we still can't definitively say whether the amphibolite formed at 4.28 Ga, or whether its ingredientsan older rock that melted to make its parentdid instead. Meanwhile, O'Neil told an audience at last year's GSA annual meeting that new data from Nuvvuagittuq shows an even older neodymium age of 4.332 Ga.
Another early-Earth researcher, Steve Mojzsis of the University of Colorado, makes an argument that the neodymium data is merely a "signature" that should be interpreted as recycling of older material. A paper on this theme is still in preparation, but he will be delivering a lecture on the subject on April 12. Thus the Nuvvuagittuq amphibolite, rather than actually being 4.3 billion years old, is instead a younger rock (just 3.78 Ga) remelted from rocks that old, which may have been the original crust of the baby Earth.
In 1785, James Hutton famously declared that in the rock record "we find no vestige of a beginning." Today we seem to have one, but its meaning is not yet clear.
Field dog Shake sits on the Nuvvuagittuq amphibolite -- Photo courtesy Don Francis, McGill Univ.