The longevity of the Earth's continents in the face of destructive tectonic activity is a major geological background to the emergence of life on our planet. This stability depends on the main mantle attached to the earth masses. New studies by a group of geologists from Carnegie, the Gemological Institute of America and the University of Alberta show that diamonds can be used to reveal how the floating part of the mantle beneath some of the continents becomes thick enough to provide long-term stability.
"We have found a way to use sulfur traces of ancient volcanoes that have come to the mantle and possibly diamonds to provide evidence of a particular process of building the continent," said Karen Smith of the Gemological Institute of America, the lead author of the document the group that appears this week in science, "Our technique shows that the geological activity forming the West African continent is due to the tectonic movement of the ocean crust plates sinking into the mantle."
Diamonds can be loved by jewelery collectors, but they are really the geologist's best friend. Since they come deep in the ground, small grains caught in diamonds are often considered undesirable in the trade in precious stones may reveal details of the conditions in which they are formed.
"In this way diamonds act as mineralogical emissaries from the depths of the Earth," Carnegie co-author Steve Shirey explained.
About 150 to 200 kilometers, 93 to 124 miles below the surface, geological formations called mantle keels act as stabilizers for the continental crust. The material that contains them must be thickened, stabilized and cooled under the continent to form a strong, lively keel that is essential to maintaining the surface earth mass against the merciless destructive forces of the tectonic activity of The Earth. But how this is achieved is a matter of debate in the scientific community.
"Solving this mystery is key to understanding how the continents existed in their current incarnations and how they survive on an active planet," Shirey explained. "Since this is the only tectonically active, rocky planet we know, the understanding of the geology of the formation of our continents is an important part of recognizing what makes the Earth habitable."
Some scientists believe that the mantiss keels are formed by a process called a subduction whereby ocean plates sink from the surface of the Earth in depth when a tectonic plate slides beneath another. Others think that the keels are created by a vertical process in which hot magma jets rise from deeper into the Earth.
A geochemical tool that can detect whether the source of make-up make-up originates from surface plates or from the deeper material of the mantle to help solve this debate. Fortunately, the mantle keels have ideal conditions for diamond formation. This means scientists can reveal the origin of the mantle keel by studying the diamonds that are formed in it.
The analysis of the research group of sulfur-rich minerals called sulphides in diamonds extracted in Sierra Leone shows that the region has experienced two events in its history.
They have succeeded in doing so because the chemistry of sulfide mineral beads can be seen only in samples from the surface of the Earth more than 2.5 billion years ago – before oxygen becomes so abundant in the atmosphere of our planet. This means that the sulfur in these mineral inclusions must have once existed on the surface of the Earth and then drawn into the mantle by subduction.
The comparison of the Botswana diamond team showed similar evidence of the creation of a keel by subduction. But the comparison with the extracted diamonds from northern Canada does not show the same chemistry of sulfur, which means that the mantissa keel in this region originates in some way that does not include the surface material.
The group's findings indicate that the thickening and stabilization of the mantissa keel across the West African continent occurred when this part of the mantle was pressed by a collision with the sinking material of the ocean floor. This method of thickening the keel and stabilizing the continent is not responsible for forming a keel under part of northern Canada. Sulfid minerals in Canadian diamonds do not tell researchers how this keel is formed, just how not.
"Our work shows that sulphide inclusions in diamonds are a powerful tool for exploring continental building processes," Smith concluded.
This work is backed by the GIA, the University of Alberta, NSF and Carnegie. This is a contribution to the Deep Carbon Observatory.