Curtin University researchers suggest Earth’s oldest known evolved rocks, which are four billion years old, were the result of asteroids slamming into the Earth’s crust and causing it to melt.
The research, published in Nature Geoscience today, found that Earth’s oldest evolved, or granitic, rocks, which form part of the Acasta Gneiss Complex in northwest Canada, have compositions that are distinct from those typical of Earth’s ancient continental crust. These differences suggest they formed through a different process.
Lead researcher Dr Tim Johnson, from the School of Earth and Planetary Sciences at Curtin University, said he and fellow researchers used modelling to show that the rocks were produced by partial melting of iron-rich hydrated basaltic rocks at very low pressures, equivalent to the uppermost few kilometres of the crust.
“The melting of these rocks at such shallow levels is most easily explained by meteorite impacts, which would have supplied the energy to attain the extreme temperatures required for melting,” Dr Johnson said.
“Our computer simulations of asteroid impacts show that not only is this scenario physically plausible, but the region of shallow partial melting needed to form these ancient evolved rocks would have been widespread.
“Given the predicted high flux of meteorites about four billion years ago, impact melting may have been the predominant mechanism that generated granitic rocks at that time.”
Research co-author John Curtin Distinguished Professor Phil Bland, also from the School of Earth and Planetary Sciences at Curtin University, said Earth’s Hadean and earliest Archean eons, 4.5 to 3.9 billion years ago, were dominated by a barrage of asteroid impacts that would have caused widespread melting and recycling of the Earth’s surface.
“Consequently, there are almost no rocks preserved from Earth’s formative Hadean eon,” Professor Bland said.
“The only known evolved rocks from the Hadean eon are those in northwest Canada, which have chemical compositions clearly distinct from those that dominate ancient continental crust worldwide, suggesting they were formed in a different way.”
The research paper was co-authored by Dr Nicholas Gardiner, Associate Professor Christopher Kirkland and Early Career Research Fellows Dr Katarina Miljkovic and Dr Christopher Spencer, all from the School of Earth and Planetary Sciences at Curtin University, along with Dr Hugh Smithies of the Geoscience Directorate at the Western Australian Department of Mines, Industry Regulation and Safety.
The report, ‘An impact melt origin for Earth’s oldest evolved rocks’, can be found online here.