New method could simplify detection of diamond deposits

Geologists from ETH Zurich and the University of Melbourne have established a link between diamond occurrence and the mineral olivine.

In a paper published in the journal Nature Communications, the scientists explain that their method will simplify the detection of diamond deposits. The process relies on the chemical composition of kimberlites, which occur only on very old continental blocks that have remained geologically unchanged for billions of years, predominantly in Canada, South America, central and southern Africa, Australia and Siberia.

According to the study’s lead author, Andrea Giuliani, olivine is a mineral that makes up around half of kimberlite rock and consists of varying proportions of magnesium and iron. The more iron olivine contains, the less magnesium it has and vice versa.

“In rock samples where the olivine was very rich in iron, there were no diamonds or only very few,” Giuliani, who has been studying the formation and occurrence of the gemstones since 2015, said in a media statement. “We started to collect more samples and data, and we always got the same result.”

His investigations ultimately confirmed that olivine’s iron-to-magnesium ratio is directly related to the diamond content of the kimberlite. Giuliani and his team took these findings back to De Beers, who had provided them with the kimberlite samples. The company was interested and provided the scientific study with financial support and asked the researchers not to publish the results for the time being.

A slow, repetitive process
In 2019, Giuliani moved from Melbourne to ETH Zurich and, supported by the Swiss National Science Foundation, began to look for explanations for the connection between olivine’s magnesium and iron content and the presence of diamonds.

With his new colleagues, he examined how the process of metasomatism, which takes place in the earth’s interior, affects diamonds. In metasomatism, hot liquids and melts attack the rock. The minerals present in the rock react with the substances dissolved in the fluids to form other minerals.

The geologists analyzed kimberlite samples that contained olivines with a high iron content—and hence no diamonds. They discovered that olivine becomes richer in iron in the places where melt penetrates the lithospheric mantle and changes the composition of mantle rocks significantly. And it is precisely in this layer, at a depth of around 150 kilometres, that diamonds are present.

The infiltration of the melt that makes olivine richer in iron destroys diamonds. If, on the other hand, no or only a small amount of melt from underlying layers penetrates the lithospheric mantle and thus no metasomatism takes place, the olivine contains more magnesium—and the diamonds are preserved.

“Our study shows that diamonds remain intact only when kimberlites entrain mantle fragments on their way up that haven’t extensively interacted with previous melt,” Giuliani said.

A key point is that kimberlites don’t normally reach the earth’s surface in one go. Rather, they begin to rise as a liquid mass, pick up fragments of the mantle on the way, cool down and then get stuck. In the next wave, more melt swells up from the depths, entrains components of the cooled mantle, rises higher, cools, and gets stuck. This process can happen multiple times.

“It’s a real stop-and-go process of melting, ascent and solidification. And that has a destructive effect on diamonds,” Giuliani noted. If, on the other hand, conditions prevail that allow kimberlites to rise directly to the surface, then this is ideal for the preservation of the gemstones.

De Beers is already using olivine analysis
Olivine analysis is as reliable as previous prospecting methods, which are mainly based on the minerals clinopyroxene and garnet. However, the new method is easier and faster: it takes only a few analyses to get an idea of whether a given kimberlite field has diamonds or not.

“The great thing about this new method is not only that it’s simpler, but also that it finally allows us to understand why the previous methods worked,” Giuliani said. “De Beers is already using this new method.”

Source: DCLA

What Causes Diamonds To Erupt? Scientists Crack the Code

New findings hold the potential to spark future diamond discoveries.

An international team of scientists, led by the University of Southampton, has found that the breakup of tectonic plates is the main driving force behind the generation and eruption of diamond-rich magmas from deep inside the Earth.

This insight could significantly influence the trajectory of the diamond exploration industry, guiding efforts to locations where diamonds are most probable.

Diamonds, which form under great pressures at depth, are hundreds of millions, or even billions, of years old. They are typically found in a type of volcanic rock known as kimberlite. Kimberlites are found in the oldest, thickest, strongest parts of continents – most notably in South Africa, home to the diamond rush of the late 19th century. But how and why they got to Earth’s surface has, until now, remained a mystery.

The new research examined the effects of global tectonic forces on these volcanic eruptions spanning the last billion years. The findings have been published in the journal Nature.

Southampton researchers collaborated with colleagues from the University of Birmingham, the University of Potsdam, the GFZ German Research Centre for Geosciences, Portland State University, Macquarie University, the University of Leeds, the University of Florence, and Queen’s University, Ontario.

Tom Gernon, Professor of Earth Science and Principal Research Fellow at the University of Southampton, and lead author of the study, said: “The pattern of diamond eruptions is cyclical, mimicking the rhythm of the supercontinents, which assemble and break up in a repeated pattern over time. But previously we didn’t know what process causes diamonds to suddenly erupt, having spent millions – or billions – of years stashed away 150 kilometers beneath the Earth’s surface.”

To address this question, the team used statistical analysis, including machine learning, to forensically examine the link between continental breakup and kimberlite volcanism. The results showed the eruptions of most kimberlite volcanoes occurred 20 to 30 million years after the tectonic breakup of Earth’s continents.

Dr. Thea Hincks, Senior Research Fellow at the University of Southampton, said: “Using geospatial analysis, we found that kimberlite eruptions tend to gradually migrate from the continental edges to the interiors over time at rates that are consistent across the continents.”

Geological processes

This discovery prompted the scientists to explore what geological process could drive this pattern. They found that the Earth’s mantle – the convecting layer between the crust and core – is disrupted by rifting (or stretching) of the crust, even thousands of kilometers away.

Dr Stephen Jones, Associate Professor in Earth Systems at the University of Birmingham, and study co-author said: “We found that a domino effect can explain how continental breakup leads to the formation of kimberlite magma. During rifting, a small patch of the continental root is disrupted and sinks into the mantle below, triggering a chain of similar flow patterns beneath the nearby continent.”

Dr. Sascha Brune, Head of the Geodynamic Modelling Section at GFZ Potsdam, and a co-author on the study, ran simulations to investigate how this process unfolds. He said: “While sweeping along the continental root, these disruptive flows remove a substantial amount of rock, tens of kilometers thick, from the base of the continental plate.”

The typical migration rates estimated in models matched what the scientists observed from kimberlite records.

“Remarkably, this process brings together the necessary ingredients in the right amounts to trigger just enough melting to generate kimberlites,” added Dr Gernon.

The team’s research could be used to identify the possible locations and timings of past volcanic eruptions tied to this process, offering valuable insights that could enable the discovery of diamond deposits in the future.

Professor Gernon, who was recently awarded a major philanthropic grant from the WoodNext Foundation to study the factors contributing to global cooling over time, said the study also sheds light on how processes deep within the Earth control those at the surface: “Breakup not only reorganizes the mantle, but may also profoundly impact Earth’s surface environment and climate, so diamonds might be just a part of the story.”

Reference: “Rift-induced disruption of cratonic keels drives kimberlite volcanism” by Thomas M. Gernon, Stephen M. Jones, Sascha Brune, Thea K. Hincks, Martin R. Palmer, John C. Schumacher, Rebecca M. Primiceri, Matthew Field, William L. Griffin, Suzanne Y. O’Reilly, Derek Keir, Christopher J. Spencer, Andrew S. Merdith and Anne Glerum, 26 July 2023, Nature.

Source: DCLA