Diamonds are some of the hardest and most valuable natural materials on Earth, prized for their beauty and industrial applications. Scientists have established that diamonds form under immense pressure and heat within the Earth, primarily in metamorphic rocks called eclogites. One question researchers are still investigating is how diamonds are destroyed within the layer of Earth below the crust, called the mantle, before they reach the surface.
Some researchers hypothesized that igneous rocks called kimberlites are predominant in destroying diamonds. Kimberlites are dark-colored, porphyritic, intrusive igneous rocks that contain diamonds. Diamonds are stable at high pressure but can be broken down or turned into graphite at high temperatures. Kimberlites are formed from very hot magma, so scientists initially assumed the intense heat of kimberlite magma would destroy some diamonds on their journey to the surface. Kimberlites are also rich in gasses, called volatiles. Volatiles can also react with diamonds and degrade them.
To study diamonds, scientists typically separate them from the rock where they were discovered. Researchers from South Africa, Germany, and Canada adopted a new strategy to examine diamonds without extracting them first. They used this method to determine how diamonds in eclogites from the western Kaapvaal and Zimbabwe cratons in southern Africa formed.
Researchers in the past studied diamonds within their original rocks by breaking them open, possibly damaging the diamonds in the process. However, these scientists used an X-ray-based technique, called computed tomography scanning or CT scanning, to peer into the rocks without cracking them open. Just like doctors use a CT scan to see broken bones within the human body, these geologists used a CT scan to see diamonds within the rocks.
The researchers examined 24 eclogite samples from southern Africa with CT scanning. They stitched the individual CT scans together using a computer program to produce detailed 3-dimensional images of the inside of the rocks before cutting them or extracting the diamonds. They explained these images are like intricate maps showing the diamond shapes and their locations within the rock.
The researchers found that most diamonds within these rocks were either single grains or clusters of crystals. The diamond crystals had an 8-sided octahedral shape covered with distinctive “steps.” They found the step-faced crystal structures were sharp and showed no evidence of destruction as their crystal structures were still intact.
However, they also found a few diamonds were rounded with broken crystal structures and, therefore, were partially destroyed. They found that groups of rounded diamonds in the eclogites commonly formed in lines, consistent with changes triggered by diamond-forming fluids. They explained these fluid-based reactions would have occurred deep within the Earth during the diamond-forming process, not as the diamonds were brought to the surface with kimberlites.
The researchers interpreted the CT images to show that some eclogite diamonds were destroyed by remelting or dissolution through a process called resorption. They explained that changes in the temperature, pressure, or composition of the magma can cause the resorption of diamonds into the magma. They suggested this resorption also occurred before the diamonds reached the surface, unlike the destruction that is hypothesized to occur as they’re transported within kimberlites to the surface.
The researchers concluded that more diamonds could be destroyed by resorption in the mantle than previous scientists recognized. However, they acknowledged that their interpretations are based on a selection of rocks from a limited geographic region. They suggested future researchers employ their new CT scanning method on diamonds from around the globe to determine whether this resorption process is widespread.
