University of Copenhagen: The early Earth may have behaved more like Venus

A new study by postdoc Pedro Waterton and associate professor Kristoffer Szilas from the Department of Geosciences and Natural Resource Management, in collaboration with an international team of researchers, focusses on some of the oldest rocks on Earth at Isua, Greenland. These rocks are more than 3.7 billion years old.

The Isua area contains fragments of peridotite, found together with volcanic and sedimentary rocks, which formed at Earth’s surface. Researchers previously thought that these peridotites represent ancient pieces of Earth’s mantle – the middle layer of Earth, which accounts for 80% of its volume – because the mantle is also made up of peridotite. If so, this would make these rocks among of the oldest known fragments of the mantle. It would also represent some of the oldest evidence of plate tectonics found on Earth, because this process is necessary for pushing fragments of the mantle up to the surface.

A completely different style of tectonics
According to Pedro Waterton there is a debate among geologists regarding when plate tectonics developed and if there was another style of tectonics in the early Earth.

”Venus and Mars for example do not have plate tectonics, and their crusts, mountains and volcanoes formed in an entirely different way than on the modern Earth. Some researchers think that the early Earth did not have plate tectonics and behaved in a way more similar to Venus”.

The researchers examined the concentrations of rare platinum-group metals in the peridotites and found that they could not be fragments of ancient mantle. Instead, these rocks formed due to the crystallization of magma of the same type that formed the volcanic rocks that make of the majority of the area at Isua. Therefore, the researchers think that the peridotite at Isua is formed in magma chamber or channels through which lava erupted onto the surface of the Earth more than 3.7 billion years ago. This makes it more likely that there was a more Venus-like style of tectonics early in Earth’s history, which is also supported by recent studies of Isua done by other research groups.

Hard to examine the early mantle
Kristoffer Szilas explains that the new study has significance for the understanding of the tectonic processes in the early Earth.

”Our study implies that it was only much later – at around 2 billion years ago – that fragments of Earth’s mantle were pushed up to the surface via plate tectonics processes. Our finding also means that the only way we can study early Earth’s mantle is indirectly, by studying volcanic rocks which formed from the mantle at that time”, says Kristoffer Szilas.

The new study No mantle residues in the Isua Supracrustal Belt was just published in the journal Earth and Planetary Science Letters

The study is part of associate professor Kristoffer Szilas’ research project titled ”The oldest rocks on Earth”, which is supported by the Carlsberg Foundation.

What is plate tectonics?
Earth’s outer layer is called the lithosphere and it is broken up into large plates. These are constantly moving against, past, or away from each other, which ensures that Earth’s surface is continuously rejuvenated. This process is termed plate tectonics.

Plate tectonics is fundamental for life and the evolution of Earth and is – as far as we know – unique to our planet. Despite its importance there is no consensus regarding when plate tectonics emerged. The estimates extend from over 4 billion years ago until around 1 billion years ago.

However, there is an agreement that stable lithospheric plates, which are a requirement for plate tectonics, did not exist in the very earliest Earth from 4.5 to 4.0 billion years ago. This is due to significantly hotter conditions in the mantle at that time.

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