We’ve discovered how diamonds make their way to the surface and it may tell us where to find them

A representation of the internal structure of the Earth. Credit: USGS

“A diamond is forever.” That iconic slogan, coined for a highly successful advertising campaign in the 1940s, sold the gemstones as a symbol of eternal commitment and unity.

But our new research, conducted by scientists from various countries and published in the journal Nature, suggests that diamonds can also indicate the breakup of Earth’s tectonic plates. In fact, they may provide clues to where we should search for them.

Diamonds, being the hardest naturally occurring minerals, require extreme pressures and temperatures to form. These conditions are only found deep within the Earth. So, how do they make their way to the surface?

Diamonds are transported to the surface by molten rocks called kimberlites. Until now, we did not know what process caused kimberlites to rapidly penetrate the Earth’s crust after spending millions or even billions of years beneath the continents.

Supercontinent cycles

Most geologists agree that the explosive eruptions that release diamonds are linked to the supercontinent cycle, a recurring pattern of landmass formation and fragmentation that has shaped Earth’s history for billions of years. However, the mechanisms underlying this relationship have been the subject of debate.

Two main theories have emerged. One proposes that kimberlite magmas take advantage of “wounds” created when the Earth’s crust is stretched or when tectonic plates split apart. The other theory involves mantle plumes, massive upwellings of molten rock from the Earth’s core-mantle boundary located about 2,900 kilometers beneath the surface.

However, both of these ideas have their limitations. The lithosphere, the main part of the tectonic plate, is incredibly strong and stable, making it difficult for fractures to penetrate and allow magmas to pass through. Moreover, many kimberlites do not exhibit the chemical characteristics expected from rocks derived from mantle plumes. Kimberlite formation is thought to involve a very low degree of mantle rock melting, often less than 1%. Therefore, an alternative mechanism is needed.

In our study, we used statistical analysis, including machine learning, to investigate the connection between continental breakup and kimberlite volcanism. Our global study revealed that most kimberlite eruptions occurred 20 to 30 million years after the tectonic breakup of Earth’s continents. Additionally, our regional study focusing on Africa, South America, and North America, where the majority of kimberlites are found, supported these findings. It also provided a significant clue: kimberlite eruptions tend to gradually move from the edges of continents to the interiors at a consistent rate across all continents.

This leads to the question: what geological process could explain these patterns?

To address this question, we utilized multiple computer models to simulate the complex behavior of continents during stretching, as well as the convective movements within the underlying mantle.

Domino effect

We propose that a domino effect can explain how the breakup of continents ultimately leads to the formation of kimberlite magma. During rifting, a small portion of the continental root, which is composed of thick rock located beneath some continents, is disrupted and sinks into the underlying mantle. This causes a process called edge-driven convection, where colder material sinks and hot mantle upwells.

Our models demonstrate that this convection triggers a series of similar flow patterns that migrate beneath the adjacent continent. As these disruptive flows move along the continental root, they erode a significant amount of rock from the base of the continental plate, up to tens of kilometers thick. Other results from our computer models indicate that this process can bring together the necessary ingredients in the right proportions to induce melting of the mantle rock, resulting in the formation of gas-rich kimberlites. With the help of carbon dioxide and water, the buoyant magma can then rapidly ascend to the surface, carrying precious diamonds.

Finding new diamond deposits

This model does not contradict the spatial association between kimberlites and mantle plumes. In fact, the breakup of tectonic plates may or may not be a result of the warming, thinning, and weakening of the plate caused by plumes.

However, our research clearly demonstrates that the spatial, temporal, and chemical patterns observed in regions rich in kimberlites cannot be fully explained by the presence of plumes alone. The processes triggering the eruptions that bring diamonds to the surface appear to be highly systematic. They initiate at the edges of continents and gradually migrate towards the interior at a relatively constant rate.

This information could be utilized to identify possible locations and timing of past volcanic eruptions associated with this process, offering insights that could aid in the discovery of diamond deposits and other rare elements crucial for the transition to green energy.

If we are to search for new deposits, it is important to consider ongoing efforts by advocacy groups to eliminate conflict diamonds and diamonds from mines with poor working conditions from the global market.

Diamonds may not truly be forever, but our work shows that new diamonds have been created repeatedly over the course of our planet’s history.

More information: Thomas M. Gernon et al, Rift-induced disruption of cratonic keels drives kimberlite volcanism, Nature (2023). DOI: 10.1038/s41586-023-06193-3 Provided by The Conversation

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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We’ve discovered how diamonds make their way to the surface and it may tell us where to find them (2023, July 29) retrieved 29 July 2023 from https://phys.org/news/2023-07-weve-diamonds-surface.html

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