Oldest dated rocks

A sample of gneiss from the site of the Earth's oldest dated rocks (the Acasta River area of Canada). This sample has been dated at 4.03 billion years old.

The oldest dated rocks on Earth, as an aggregate of minerals that have not been subsequently broken down by erosion or melted, are more than 4 billion years old, formed during the Hadean Eon of Earth's geological history. Such rocks are exposed on the Earth's surface in very few places. (Some meteorites, such as the ALH84001 Mars meteorite found in the Allan Hills of Antarctica, are older but they were not formed on Earth).

Some of the oldest surface rock can be found in the Canadian Shield, Australia, Africa and in a few other old regions 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.^[1] 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;^[2] the true age of these samples is still under debate, and they may actually be closer to 3.8 billion years old.^[3] 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.

Oldest rocks by category

Oldest terrestrial material

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.^[4] The 4.404 ± 0.008 Ga zircon is a slight outlier, with the oldest consistently-dated zircon falling closer to 4.35 Ga.^[5] 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. Recent developments in atom-probe tomography have led to a further constraint on the age of the oldest continental zircon, with the most recent age quoted as 4.374 Ga ± 0.006.^[6]

Earth's oldest rock formation

Fragment of Acasta Gneiss

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 of 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 the Baltic Shield are 3.5 Ga old.^[7]

Oldest rock on Earth

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.^[1]

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 model age for extraction from the mantle of 4.28 billion years.^{[8][9][10][11]} 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.^[3]

2008 microprobe research

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.^[12]

Non-terrestrial rocks

The Genesis Rock, (Lunar sample 15415) obtained from the Moon by astronauts during Apollo 15 mission, has been dated at 4.08 billion years [http://curator.jsc.nasa.gov/lunar/lsc/15415.pdf]. During Apollo 16, older rocks, including Lunar sample 67215, dated at 4.46 billion years, were brought back.^[13] Lunar sample 67215 is the oldest known rock on Earth, even though it originated on the Moon.

See also

• Age of the Earth
• History of the Earth

References

Bibliography

• Zircons are Forever
• "Western Australia's Jack Hills". NASA Earth Observatory newsroom. Retrieved 2006-04-28. 
• Bowring, S.A., and Williams, I.S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, v. 134, 3-16.
• Stern, R.A., Bleeker, W., 1998. Age of the world's oldest rocks refined using Canada's SHRIMP. the Acasta gneiss complex, Northwest Territories, Canada. Geoscience Canada, v. 25, p. 27-31
• Yu A., Lee C-D and Halliday, A.N..Lutetium-Hafnium and Uranium-Lead Systematics of Early-Middle Archean Single Zircon Grains, Ninth Annual Goldschmidt Conference. 2

External links

• Very old Australian zircons with a story to tell
• On the Acasta Gneiss
• Abstract and full text of the results from O'Neil's research, published by Science