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The longevity of terrestrial continents in the face of destructive tectonic activity is an essential geological context for the emergence of life on our planet. This stability depends on the underlying mantle attached to the land masses. A new study by a group of geoscientists from Carnegie, the Gemological Institute of America and the University of Alberta shows that diamonds can be used to reveal how a floating section of the mantle under certain continents has become thick enough to provide long-term stability.
"We have found a way to use traces of sulfur from ancient volcanoes that have penetrated into the mantle and then into diamonds, to prove a particular process of continent-building," explained Karen Smit, from the main author, the Gemological Institute of America. on the group's paper, which appears this week in Science. "Our technique shows that the geological activity that formed the West African continent was due to the tectonic movement of the oceanic crustal plates sinking into the mantle."
Jewelery collectors love diamonds, but they are really the best friends of geologists. Because they come from the interior of the Earth, tiny grains of minerals trapped in a diamond, often considered undesirable in the gem trade, can reveal details of the conditions in which it was formed.
"In this way, diamonds act as mineral emissaries from the depths of the Earth," said Carnegie co-author Steve Shirey.
At about 150 to 200 km below the surface, geological formations called mantle hulls serve as stabilizers for the continental crust. The material that composes them must thicken, stabilize and cool under the continent to form a strong and floating keel, essential to the preservation of the surface landmass against the relentless destructive forces of tectonic activity of the earth. But the way this has been done has been the subject of debate within the scientific community.
"Solving this mystery is essential to understanding how the continents took shape in their current incarnations and how they survive on an active planet," said Shirey. "Since it is the only tectonically active and active rock planet we know, understanding the geology of the formation of our continents is a crucial part of the discernment of what makes the Earth habitable."
Some scientists believe that mantle keels are formed by a process called subduction, whereby oceanic plates sink from the surface of the Earth into its depths when a tectonic plate slips under another. Others think that the pins are created by a vertical process in which plumes of hot magma come out much deeper from the Earth.
A geochemical tool to determine whether the composition of the mantle keel came from surface slabs or from deeper material lifts was needed to resolve this debate. Fortunately, coat keels provide the ideal conditions for diamond formation. This means that scientists can reveal the origin of the keel of the mantle by studying the inclusions of diamonds that are formed there.
The analysis of the research group on sulfur-rich minerals, called sulphides, found in diamonds extracted in Sierra Leone indicates that the region has experienced two subduction events throughout its history.
They were able to make this decision because the chemistry of sulphide mineral grains is only visible in samples taken from the surface of the Earth more than 2.5 billion years ago – before oxygen became so abundant in the atmosphere of our planet. This means that the sulfur in these mineral inclusions had to exist at one time on the surface of the Earth and then be sucked into the mantle by subduction.
The team's comparison with Botswana's diamonds showed similar evidence of keel creation by subduction. But the comparison with diamonds from northern Canada does not show the same sulfur chemistry, which means that the mantle keel in this region has been created so as not to incorporate surface materials.
The group's conclusions suggest that the thickening and stabilization of the keel of the mantle under the West African continent occurred when this section of the mantle was pinched by a collision with the material being sank. This method of thickening the keel and stabilizing the continents is not responsible for the formation of the keel under a part of northern Canada. The sulphide minerals contained in Canadian diamonds do not tell researchers how this keel formed, but only how.
"Our work shows that sulphide inclusions in diamonds are a powerful tool for studying the construction processes of a continent," concluded Smit.
This work was funded by the GIA, the University of Alberta, the NSF and Carnegie. This is a contribution to the Deep Carbon Observatory.
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