Scientists have discovered that 4.02 billion-year-old silica-rich felsic rocks of the Acasta River, Canada – the oldest rock formation known on Earth – were probably formed at high temperatures and at a surprisingly shallow depth of the planet. budding crust. The high temperatures required to melt the shallow crust were probably caused by a meteorite bombardment about half a billion years after the planet was formed. This melted the iron-rich crust and formed the granite that we see today. These results are presented for the first time at the Goldschmidt conference in Boston (August 14), after publication in the peer-reviewed journal Nature Geoscience .
The felsian rocks (rocks rich in silica / quartz) found on the Acasta River in Canada are the oldest rocks in the world, although there are older mineral crystals *. Scientists have known for a long time that the Acasta rocks differ from the most rocky rocks we see today, such as the granite that is widely used as a building or decorative material. Now a group of scientists from Australia and China modeled the formation of the oldest Acasta felsische rocks and found that they could only have formed at low pressures and very high temperatures.
Scientists believe that the primitive crust consisted largely of dark silica-poor mafic rocks, so there is a question about how the Acasta river felsische rocks could have formed.
"Our modeling shows that the Acasta River were rocks that were derived from the melting of pre-existing iron-rich basalt blocks, which formed the upper layers of crust on the primitive earth," said team leader Tim Johnson, of Curtin University, Perth.
"We used phase equilibria and trace element modeling to show that the Acasta River rocks were produced by partially melting the original mafic rocks at very low pressures.It would have cost something special to produce the 900 ° C temperatures needed to generate these early sheer rocks at such low pressure, and that probably means a dr aical event, most likely the intense heating caused by meteorite bombardment.
We estimate that rocks within the upper 3 km of the corn cistern would have melted in producing the rocks that we see today, we think these old sheer rocks would have been very common, but the passage of 4 billion years, and the development of plate tectonics means that there is almost nothing left.
We believe that these rocks are the only surviving remnants of a barrage of extraterrestial effects that are the first 600 million years of earth history. "
The Acasta River is part of the Slave Craton formation in northern Canada, north of Yellowknife and the Great Slave Lake, the home of the Tlicho people, which led to the geologists discovered that gave them the name "Idiwhaa", derived from the Tlicho word for the old.
Comment, Dr. Balz Kamber (Trinity College Dublin) said: "The idea to melt felsisch melts seem to have large or gigantic effects plausible given the high energetic nature of these events and the pockmarked old surfaces of other inner planets and moons of the solar system. However, the implicit pressure-temperature regime may also allow the melting of shallow crust under an overheated melting sea. In other words, an indirect consequence of the impact itself. "
* Rocks of Jack Hills in Australia contain zircon crystals of up to 4.4 billion years ago, embedded in younger rocks.
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