The world's lifelong life in the face of destructive tectonic activity is an important geological background for the emergence of life on our planet. This stability depends on the underlying mantle attached to the landmasses. New research by a group of geoscientists from the Carnegie, Gemological Institute of America and the University of Alberta show that diamonds can be used to reveal how a floating part of the mantle under some of the continents became thick enough to provide long-term stability.
"We have found a way to use traces of sulfur from ancient volcanoes that made their way into the mantle and eventually into diamonds to provide evidence of a certain process of continental construction," explains Karen Smit of the Gemological Institute of America, leading author on the group's papers, this week shown in Science . "Our technology shows that the geological activity that formed the West African continent was due to the tectonic movement of the sea crust singing into the mantle."
Diamonds can be loved by jewelry collectors, but they are truly a geologist's best friend. Because they are deep within the earth, small mineral grains are trapped inside a diamond, which is often considered undesirable in the pearl trade, can reveal details of the conditions under which it was formed.
"That way, diamonds work as mineralogical emissaries from the depths of the Earth," Carnegie co-author Steve Shirey explained.
"Solving this mystery is the key to understanding how the continents existed in their present incarnations and how they survive on an active planet," Shirey explained. "Because this is the only tectonically active, rocky planet that we know, the geology understands how our continents are formed is a crucial part of discerning what makes the Earth habitable."
Some scientists believe that jackets are formed by a process called subduction, through which sea plates descend from the surface of the earth to its depth when a tectonic plate slides under another. Others think keeles are created by a vertical process where plants of hot magma rise from much deeper in the soil.
A geochemical tool that can detect if the source of a mantle body's makeup originates from surface plates or swelling of deeper mantle materials was needed to help resolve this debate. Fortunately, mantle keels have the ideal conditions for diamond formation. This means that researchers can reveal the origin of a sheath body by studying interventions from diamonds formed in it.
The research group's analysis of sulfur-rich minerals, called sulphides, in diamonds mined in Sierra Leone, indicates that the region experienced two subduction events during its history.
They could make this determination because the chemistry of the sulfide mineral grains was only seen in soil samples for more than 2.5 billion years ago – before oxygen became so abundant in our planet's atmosphere. This means that sulfur in these mineral integrations must once exist on the surface of the earth and then be drawn down into the mantle by subduction.
The team's comparison with Botswana diamonds showed similar evidence of keel formation by subduction. However, comparison with diamonds mined from northern Canada does not show the same sulfur chemistry, which means that the sheath hook in this region originates in some way that did not contain surface material.
The group's results indicate that thickening and stabilization of the mantle body during the West African continent occurred when this part of the mantle was pressed by collision with the sinking seabed material. This method of keel thickening and continental stabilization is not responsible for forming the keel during some of northern Canada. The sulphide minerals in Canadian diamonds do not tell the researchers how this keel was formed, just how it did.
"Our work shows that sulfide integrations in diamonds are a powerful tool for investigating the continent's construction processes," Smit concluded. 19659003] This work was supported by GIA, the University of Alberta, NSF and Carnegie. It is a contribution to the Deep Carbon Observatory.