The chemical and thermal history of the Earth enabled the reactions required to create the building blocks of life. Our nitrogen-rich atmosphere, for example, is critical for chemical reactions that have fostered the evolution of life.
The Earth is a dynamic planet, and elements critical to the formation of life (like nitrogen and carbon) are recycled through many natural processes. One of these processes is the movement of the Earth’s continents, which recycles bits of the crust and some of the chemicals it contains. Though we have been aware that the Earth’s plates have been moving for quite some time, it has been much more difficult to establish the specifics of when and how plate motion initiated.
Now, new data from some very old diamonds suggests that plate tectonics was functioning more than three billion years ago.
A diamond is forever
Scientists have studied the shifting of the continents extensively, and we now understand that the Earth’s outer shell (lithosphere) is divided into rigid plates that are able to glide over the rocky inner layer (mantle) surrounding the Earth’s core. In locations where one plate is riding over a second (called a subduction zone), material from the surface of the planet is transferred to the Earth’s mantle.
Certain chemicals, like nitrogen, are transferred to the interior during this subduction, and they can readily react with other chemicals in redox reactions, which change the chemical composition of the rocks in that area of the mantle.
If we can identify when these chemical transformations occurred, we can determine more about the history of plate tectonics. Unfortunately, dating this type of subduction can be very challenging. Not only is the mantle itself inaccessible, but the chances of finding any rock remnants from this deep in the interior are extremely low.
Low, but not zero. Though these mantle rocks are difficult to find, scientists have successfully unearthed a few specimens. Sometimes these rocks contain diamonds, which can be particularly useful. Diamonds derived from the mantle can provide vital information on the composition and redox conditions of this inaccessible layer. If the diamonds contain mineral inclusions, elemental and isotopic analysis of these inclusions can provide insight into the complex mantle environment.
In a new investigation, the authors analyzed three diamonds from the Witwatersrand Supergroup of the Kaapvaal craton in South Africa. Based on the atomic arrangement of nitrogen, the scientists were able to determine how long the diamonds resided in the mantle and the temperature of the environment. This data indicated that the diamonds were located in the mantle for somewhere between ~200 and 400 million years.
They also knew that the diamonds were formed 2.9 to 3.1 billion years before the mantle rock they were in reached the surface. Combining this temporal information, they concluded that the diamonds were formed in the upper mantle 3.1 to 3.5 billion years ago.
Breaking the diamond down to the atomic level
Analysis also indicated the diamonds had a higher nitrogen content and different isotope composition compared with the average modern mantle and most diamonds worldwide. Diamonds exhibit variability in their nitrogen isotope composition. Diamonds that have less of the uncommon nitrogen isotopes (15N) are typically from primordial mantle reserve; those high in 15N isotopes are often from mantle reservoir that contains recycled components of the crust.
The authors think that contamination of the mantle by nitrogen-rich sediments during the Archaean Eon (from 4,000 to 2,500 million years ago) could have led to the increased nitrogen content. The carbon isotopic content suggests that oxidized fluid or melt was reduced to form the diamonds. The scientists think that this material entered the mantle at a subduction zone.
Given the age, composition, and source of these Archaean diamonds, the authors conclude that modern-style plate tectonics operated as early as 3.5 billion years ago.
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