At first glance, this microscope image is indistinguishable from a modern work of art, but there is much more to it than just a pretty sight. This thin section of rock is a sample of the early Earth’s mantle and comes from the Mid-Atlantic Ridge, a seam in the Earth’s crust that once stretched across the entire Atlantic. At this mid-ocean ridge, the ocean plates move apart and hot molten rock from the Earth’s mantle penetrates to the surface.
At two locations on the Mid-Atlantic Ridge, one in the far north and one far south, a team led by Suzanne Birner from Berea College in Kentucky took and analyzed samples of ancient mantle rock from the sea floor. The rock samples turned out to be about 2.5 billion years old and were thus formed in the Archean, a geological era that occurred between 2.5 and 4 billion years ago.
Under cross-polarized light, a colorful mosaic is created, as can be seen here. It is created by the various minerals and crystal forms contained in the sample. This helps to determine the mineral composition of rock samples. Environmental influences such as heat and pressure that have affected the rock can also be recognized by characteristic transformations in the crystal structure.
Birner and her colleagues were particularly astonished by the chemical composition of the rock: the sample had an oxidation level that was around 10,000 times lower than younger parts of the Earth’s mantle. To put it simply, oxidation is the release of electrons during a chemical reaction. When metals oxidize, metal oxides such as rust, which is nothing other than iron oxide, are formed. The researchers then analyzed this level of oxidation. They found that the mantle rock they examined had maintained its astonishingly low level of oxidation over billions of years, despite its great age.
This result contradicts assumptions that dynamic, changing processes in the Earth’s mantle must have significantly altered all old mantle material long ago. Original, unaltered samples from the Archean are therefore unlikely to have been preserved. Nevertheless, this seems to be the case, as the samples from Birner and her colleagues suggest.
One possible explanation is simpler than expected: 2.5 billion years ago, the Earth’s mantle was between 200 and 300 degrees hotter than it is today. The greater heat caused some components of the Earth’s mantle to melt much more than is possible in the Earth’s mantle today. This heat – and not a fundamentally different composition of the early Earth’s mantle – prevented greater oxidation, as the researchers explain.
At the same time, this great initial heat protected the rock from further geothermal changes later on – the cooling mantle rock could no longer significantly change its structure. As a result, it still exists exactly as it was formed in the Archean. “This opens up the possibility of considering such samples as authentic time capsules of the Archean,” says co-author Elizabeth Cottrell of the Smithsonian National Museum of Natural History in Washington DC.