A clearer insight into the hidden crystals of the Earth

A view of light passed through a cross-section of a 200-micron peridotite sample showing the three main minerals – olivine (clear-green), orthopyroxene (gray-green) and garnet (pink). Credit: dr. Emma Tomlinson, Trinity College Dublin.

Geologists developed a new theory of the state of the Earth billions of years ago after examining very old rocks formed in the Earth’s mantle below the continents.

Assistant Professor Emma Tomlinson of Trinity College Dublin and Queensland University of Technology professor Balz Kamber have just published their research in a leading international journal, Nature Communications.

Seven continents on Earth today are built around a stable interior called the craton, and geologists believe that the stabilization of the craton some 2.5 – 3 billion years ago was crucial for the emergence of terrestrial masses on Earth.

Little is known about how the cratons and supporting keels of the mantle formed, but important traces can be found in the xenoliths of peridotites, which are mantle specimens that are brought to the Earth’s surface by volcanic eruptions.

Dr. Tomlinson of Trinity’s Natural School said:

“Many of the mantle rocks below the old continents contain a surprising amount of silicon dioxide – much more than is found in the younger parts of the mantle.”

“There is currently no scientific consensus on the reason for this.”

New research, which deals with global data for peridotitis in the mantle, provides a new explanation for this observation.

The research used a new thermodynamic model to calculate that unusual mineralogy developed when very hot molten rock – greater than 1700 ° C – interacted with older parts of the mantle, causing the growth of minerals rich in silica.

“Over more than a billion years ago, from 3.8 to 2.5 billion years ago, volcanoes also erupted very unusual lava of very low viscosity – lava that was very thin, very hot and often contained variable levels of silicon dioxide,” he added. is dr. Tomlinson.

“Our modeling suggests that the unusual lavas were in fact molten rocks that acted with the mantle at great depths and that interaction resulted in varying levels of silica.”

Professor Kamber, QUT, said:

“The Earth stopped creating the Earth with deep silicon rocks and low-viscosity volcanic rocks about 2.5 billion years ago. This time is the boundary between the Archean and Proterozoic eons – one of the most significant breaks in the Earth’s geological time span. “

What caused this boundary remains unknown, but research offers a new perspective.

Professor Kamber added:

“It may have been due to a change in the way the cloak flowed. Once the mantle began to rotate slowly all the way to the core (2,900 km), very high temperatures of the Archean eon were no longer possible. “

Reference: February 17, 2021, Nature Communications.
DOI: 10.1038 / s41467-021-21343-9

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