From Wikipedia, the free encyclopedia - View original article
|This article needs additional citations for verification. (June 2012)|
The oldest dated rocks on Earth, as an aggregate of minerals that have not been subsequently melted or disaggregated by erosion, are from the Hadean Eon. Such rocks are exposed on the surface in very few places.[a]
Some of the oldest surface rock can be found in the Canadian Shield, Australia, Africa and in other more specific places around the world. The ages of these felsic rocks are generally between 2.5 and 3.8 billion years. The approximate ages have a margin of error of millions of years. In 1999, the oldest known rock on Earth was dated to 4.031 ± 0.003 billion years, and is part of the Acasta Gneiss of the Slave craton in northwestern Canada. Researchers at McGill University found a rock with a very old model age for extraction from the mantle (3.8 to 4.28 billion years ago) in the Nuvvuagittuq greenstone belt on the coast of Hudson Bay, in northern Quebec; the true age of these samples is still under debate,[not in citation given] and they may actually be closer to 3.8 billion years old. Older than these rocks are crystals of the mineral zircon, which can survive the disaggregation of their parent rock and be found and dated in younger rock formations.
The oldest material of terrestrial origin that has been dated is a zircon mineral of 4.404 ± 0.008 Ga enclosed in a metamorphosed sandstone conglomerate in the Jack Hills of the Narryer Gneiss Terrane of Western Australia. The 4.404 ± 0.008 Ga zircon is a slight outlier, with the oldest consistently-dated zircon falling closer to 4.35 Ga. This zircon is part of a population of zircons within the metamorphosed conglomerate, which is believed to have been deposited about 3.060 Ga, which is the age of the youngest detrital zircon in the rock.
The oldest rock formation is, depending on the latest research, either part of the Isua Greenstone Belt, Narryer Gneiss Terrane, Nuvvuagittuq greenstone belt, or the Acasta Gneiss (on the Slave craton). The difficulty in assigning the title to one particular block of gneiss is that the gneisses are all extremely deformed, and the oldest rock may be represented by only one streak of minerals in a mylonite, representing a layer of sediment or an old dike. This may be difficult to find or map; hence the oldest dates yet resolved are as much generated by luck in sampling as by understanding the rocks themselves.
It is thus premature to claim that any of these rocks, or indeed that other formations of Hadean gneisses, is the oldest formations or rocks on Earth; doubtless new analyses will continue to change our conceptions of the structure and nature of these ancient continental fragments.
Nevertheless, the oldest cratons on Earth include the Kaapvaal craton, the Western Gneiss Terrane of the Yilgarn craton (~2.9 - >3.2 Ga), the Pilbara Craton (~3.4 Ga), and portions of the Canadian Shield (~2.4 - >3.6 Ga). Parts of the poorly studied Dharwar craton in India are greater than 3.0 Ga. The oldest dated rocks of Baltic Shield are 3.5 Ga old.
The Acasta Gneiss in the Canadian Shield in the Northwest Territories, Canada is composed of the Archaean igneous and gneissic cores of ancient mountain chains that have been exposed in a glacial peneplain. Analyses of zircons from a felsic orthogneiss with presumed granitic protolith returned an age of 4.031 ± 0.003 Ga.
On September 25, 2008, researchers from McGill University, Carnegie Institution for Science and UQAM announced that a rock formation, the Nuvvuagittuq greenstone belt, exposed on the eastern shore of Hudson Bay in northern Quebec had a Sm-Nd age for extraction from the mantle of 4.28 billion years. However, it is argued that the actual age of formation of this rock, as opposed to the extraction of its magma from the mantle, is likely closer to 3.8 billion years, according to Simon Wilde of the Institute for Geoscience Research in Australia.
The zircons from the Western Australian Jack Hills returned an age of 4.404 billion years, interpreted to be the age of crystallization. These zircons also show another interesting feature; their oxygen isotopic composition has been interpreted to indicate that more than 4.4 billion years ago there was already water on the surface of the Earth. The importance and accuracy of these interpretations is currently the subject of scientific debate. It may be that the oxygen isotopes, and other compositional features (the rare earth elements), record more recent hydrothermal alteration of the zircons rather than the composition of the magma at the time of their original crystallization. In a paper published in the journal Earth and Planetary Science Letters, a team of scientists suggest that rocky continents and liquid water existed at least 4.3 billion years ago and were subjected to heavy weathering by an acrid climate. Using an ion microprobe to analyze isotope ratios of the element lithium in zircons from the Jack Hills in Western Australia, and comparing these chemical fingerprints to lithium compositions in zircons from continental crust and primitive rocks similar to the Earth's mantle, they found evidence that the young planet already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed.
The Genesis Rock, obtained from the Moon by astronauts during Apollo 15 mission, has been dated at 4.46 billion years. This is one of the oldest known rocks on Earth, even though it originated on the Moon. During Apollo 16, older rocks were brought back.