Diamonds are often seen as the epitome of luxury and beauty, but these precious gems hold much more than just aesthetic value. They offer a unique insight into the processes happening deep within the Earth’s mantle, providing scientists with critical information about the behavior of carbon in the Earth’s interior. By studying diamonds, geologists are unlocking the secrets of carbon’s journey through the planet’s inner layers, helping us better understand everything from the carbon cycle to the Earth’s geological history.

What Are Diamonds and How Do They Form?

Diamonds are formed under extreme pressure and temperature conditions that exist deep within the Earth’s mantle. The Earth’s mantle is a layer of hot, dense rock that lies beneath the crust and extends down to about 2,900 kilometers (1,800 miles). In these harsh conditions, carbon atoms are forced into a crystalline structure, forming the iconic diamond we see at the surface.

While diamonds can form in several geological settings, the most famous diamonds are formed in “diamond stability zones,” where the pressure and temperature conditions are just right. These diamonds are often carried to the Earth’s surface through volcanic eruptions, where they are eventually mined. What’s remarkable about diamonds is that they can encapsulate information about the mantle conditions at the time of their formation, including the type of carbon they contain.

Diamonds as Carbon Time Capsules

Diamonds serve as “time capsules” of the Earth’s deep history. Within their crystal lattice structure, diamonds can trap and preserve ancient fluids and gases, including carbon, that existed when the diamond was formed. By studying these inclusions, scientists can learn about the composition of the mantle during different geological periods.

The carbon found in diamonds is typically derived from the Earth’s mantle, but it can have different isotopic signatures depending on the conditions at the time of formation. Isotopes are variants of elements with different numbers of neutrons. For example, the ratio of carbon-12 to carbon-13 isotopes in a diamond can reveal important details about the source of the carbon, whether it came from deeper layers of the mantle or from surface materials that were subducted deep into the Earth. This allows scientists to track the movement of carbon through the mantle over time.

The Geodynamics of Carbon in the Mantle

Understanding how carbon behaves in the mantle is crucial for several reasons, particularly in the context of the global carbon cycle. Carbon plays a central role in the Earth’s geology, atmosphere, and climate system. It is one of the most important greenhouse gases, influencing the planet’s climate, and it is a key element in the formation of life.

Diamonds help geologists understand the pathways carbon takes within the mantle. The Earth’s mantle acts as a vast storage area for carbon, which can be released back into the atmosphere through volcanic eruptions. By studying diamonds, scientists can uncover the processes that control the movement of carbon from the mantle to the surface, and how it cycles through the Earth’s interior. This research is vital for understanding the Earth’s carbon storage and release mechanisms, which have long-term implications for global climate patterns.

Diamonds and the Carbon Cycle

The carbon cycle refers to the continuous movement of carbon through the atmosphere, oceans, soil, and rocks. Carbon is stored in various forms throughout the Earth, including in fossil fuels, carbonate minerals, and, notably, in diamonds deep within the mantle. Understanding how carbon is stored and transported within the mantle can help scientists predict how carbon behaves over geological timescales.

Recent studies of diamond inclusions have shown that carbon can be sequestered in the mantle for millions to billions of years, trapped in deep reservoirs that are not easily accessible. However, carbon is not permanently locked away. It can be released into the atmosphere through volcanic activity, where it can contribute to the greenhouse effect. The study of diamond geodynamics helps researchers better understand these processes, and the timescales involved in carbon sequestration and release.

Carbon and Mantle Dynamics: A Glimpse into Earth’s Past

Diamonds are more than just geological curiosities; they provide a snapshot of the Earth’s ancient history. Carbon isotopes trapped in diamonds can reveal information about the Earth’s geological processes, including mantle convection, plate tectonics, and volcanic activity. By analyzing diamond inclusions, scientists can trace the evolution of the mantle, the movement of tectonic plates, and how these processes have shaped the Earth’s carbon cycle over time.

This information is crucial for understanding not only Earth’s past but also its future. The study of diamonds allows scientists to refine models of how carbon behaves within the Earth, helping to predict the long-term effects of carbon release and storage on the climate. As scientists continue to study diamonds and their unique carbon signatures, they are unlocking a treasure trove of information about the Earth’s deep, hidden systems.

Conclusion

Diamonds, while primarily known for their beauty and rarity, are much more than mere gemstones. They are natural records of the Earth’s deep geological history, holding within them secrets of carbon’s journey through the planet’s mantle. By studying diamonds, scientists are uncovering the intricate dynamics of how carbon moves through the Earth, contributing to a deeper understanding of the planet’s carbon cycle, its climate history, and the forces that drive geological processes.

As research in diamond geodynamics continues to evolve, these precious gems will undoubtedly continue to shed light on the mysteries of the Earth’s inner workings and the role of carbon in shaping our planet’s past, present, and future.

Diamonds have fascinated humans for centuries, celebrated for their beauty and rarity. Yet, beyond their allure as precious gemstones, diamonds hold the key to understanding some of Earth’s most profound geological processes. These sparkling gems are formed deep within the Earth’s mantle, where extreme temperatures and pressures transform carbon into one of the most durable materials on Earth. But what role does carbon play in this process, and how does it shape our understanding of Earth’s internal dynamics?

In this blog, we will delve into the important role carbon plays in the mantle, and how it contributes to the formation of diamonds. By studying diamonds, scientists can gain invaluable insights into the behavior of carbon deep beneath the Earth’s surface and better understand mantle dynamics and the broader geodynamic processes that influence the Earth’s geology.

The Role of Carbon in Earth’s Mantle

The Earth’s mantle, located just beneath the Earth’s crust, stretches over 2,900 kilometers deep. Despite being far from the surface, the mantle is crucial for understanding the behavior of many elements, including carbon. Carbon plays a central role in the mantle’s processes, influencing the Earth’s long-term climate, tectonic activity, and even the formation of precious gems like diamonds.

In the mantle, carbon primarily exists in three forms:

1. Carbonates: Found in minerals like calcite and dolomite, carbonates form when carbon dioxide reacts with silicate minerals. These minerals are common in subduction zones, where oceanic plates are forced down into the mantle.

2. Graphite: At deeper levels of the mantle, carbon exists as graphite, which is stable under high-pressure and temperature conditions. Graphite is commonly found in the upper mantle and is considered a precursor to diamond formation at even greater depths.

3. Diamonds: Diamonds are the most well-known form of carbon in the mantle. They form under extreme conditions of pressure and temperature, at depths of about 140 to 190 kilometers beneath the Earth’s surface.

Each form of carbon plays a role in the geodynamics of the mantle, contributing to the cycling of carbon between the Earth’s surface and its deep interior.

From Carbon to Diamond: The Formation Process

Diamonds form when carbon atoms are subjected to extreme pressure and temperature conditions in the mantle. The carbon atoms bond in a specific arrangement to form a crystal structure known as the “diamond lattice.” This structure makes diamonds incredibly stable, dense, and the hardest known material on Earth.

Key Steps in Diamond Formation:

• High Pressure and Temperature: Diamonds are formed under extreme conditions, where temperatures range from 1,000 to 1,300 degrees Celsius, and pressures reach 45 to 60 kilobars—nearly 60,000 times the pressure we experience at sea level. These conditions cause carbon atoms to bond in a crystal structure that forms diamonds.

• Crystallization of Carbon: At these great depths, carbon is forced into a tightly packed arrangement, creating the crystalline structure that makes diamonds so durable. Over millions of years, carbon accumulates and crystallizes into diamonds, which are later transported to the Earth’s surface by volcanic eruptions.

• Inclusions and Trapping of Mantle Materials: Diamonds can also trap small fragments of surrounding mantle material, including gases, minerals, and even other carbon-based compounds. These inclusions provide clues about the environment in which the diamond formed, helping scientists learn more about the conditions within the mantle.

Diamonds often reach the Earth’s surface through volcanic eruptions, particularly from kimberlite pipes—deep, narrow tubes that connect the mantle to the surface. As these volcanic eruptions occur, diamonds are carried up, where they are eventually found in deposits, providing valuable information for researchers studying mantle processes.

The Importance of Carbon in Earth’s Carbon Cycle

The mantle plays a crucial role in the Earth’s carbon cycle, which governs the movement of carbon between the Earth’s surface and its deep interior. Carbon moves between the mantle and the atmosphere through processes like volcanic eruptions, subduction, and tectonic activity, which ultimately influence Earth’s climate and geology.

Carbon’s Role in Mantle Dynamics:

• Subduction and Recycling of Carbon: As tectonic plates move, carbon-containing materials, such as oceanic crust rich in carbonates, are pushed deep into the mantle in subduction zones. This process contributes to the recycling of carbon from the surface to the mantle.

• Volcanic Outgassing: When volcanoes erupt, they release carbon dioxide (CO₂) into the atmosphere. This outgassing is a crucial part of the carbon cycle, as it helps to balance the carbon that is subducted into the mantle. Over time, the Earth’s atmosphere and surface environment are influenced by the amount of carbon released by volcanic activity.

• Mantle Convection and Carbon Transport: Mantle convection refers to the movement of material within the mantle driven by heat from the Earth’s core. This process transports carbon and other volatile compounds, influencing both the formation of diamonds and the broader carbon cycle. The movement of carbon between the mantle and the surface helps regulate Earth’s climate over geological time scales.

Diamonds as Key to Understanding Carbon’s Behavior

Diamonds not only provide insights into carbon’s role in the mantle, but they also help scientists study other deep Earth processes. Diamonds are often found with inclusions—tiny fragments of mantle materials—trapped inside them as they form. By analyzing these inclusions, researchers can gain valuable insights into the composition and behavior of carbon at great depths.

Key Insights Gained from Diamond Inclusions:

• Mantle Composition: Inclusions in diamonds often contain rare minerals, gases, and isotopic signatures, allowing scientists to study the chemical composition of the mantle at depths that are otherwise inaccessible. This helps to improve our understanding of the materials present in the deep Earth.

• Carbon Isotope Analysis: Diamonds can also be used to analyze the isotopic composition of carbon. By studying the isotopes of carbon trapped in diamonds, researchers can gain insights into the history of carbon cycling in the mantle, and how it affects Earth’s climate over millions of years.

• Geodynamic Processes: The study of diamonds also helps to unravel the processes involved in mantle convection, plate tectonics, and subduction. As diamonds form, they record signals of deep mantle flow and material interactions, providing clues about the Earth’s geodynamic behavior.

Conclusion: Diamonds and Carbon

Diamonds are far more than just beautiful gemstones—they are natural time capsules that provide invaluable insights into the Earth’s deep interior. By studying diamonds and the role carbon plays in their formation, scientists are able to unlock mysteries about the Earth’s mantle, carbon cycling, and geodynamic processes. As we continue to explore the relationship between carbon and diamond formation, we deepen our understanding of the Earth’s geological processes and its long-term climate history.

Carbon’s journey from the surface to the mantle, and ultimately into diamonds, helps to shape our understanding of Earth’s evolution, from its early formation to the dynamic processes that continue to shape the planet today. The study of diamonds and carbon in the mantle is a crucial step in unraveling the complex geology of our planet.

Diamonds are one of the most coveted and enigmatic substances on Earth. They’re known for their exceptional beauty and rarity, but they also hold invaluable secrets about our planet’s inner workings. Formed under extreme pressure and temperature conditions, diamonds are more than just a symbol of luxury—they are nature’s time capsules, offering clues to the deep processes occurring in the Earth’s mantle.

In this blog, we explore how diamonds shape our understanding of mantle geodynamics, shedding light on the role of carbon in the Earth’s interior and the broader processes that govern our planet’s geological activities.

The Formation of Diamonds: A Glimpse into the Deep Earth

Diamonds are formed at depths of approximately 140 to 190 kilometers beneath the Earth’s surface, within the mantle. This region is far beyond the reach of conventional drilling, making diamonds one of the few natural samples of the deep Earth’s composition. They are created under extreme conditions: temperatures of about 1,000 to 1,300 degrees Celsius and pressures around 45 to 60 kilobars. These conditions allow carbon atoms to bond in a crystalline structure that is incredibly stable, leading to the creation of the world’s hardest known material.

The fact that diamonds form in such extreme conditions means they can serve as a window into the processes occurring deep in the mantle, especially the behavior of carbon, a fundamental element that plays a crucial role in the Earth’s geodynamics.

The Role of Carbon in Mantle Geodynamics

Carbon is a key element in the Earth’s mantle and is responsible for the formation of diamonds. But carbon doesn’t only form diamonds. It is a versatile element that can exist in various forms within the mantle, including as carbonates, graphite, and in volatile compounds like methane. By studying diamonds, scientists can learn about the behavior of carbon at great depths, which is crucial to understanding how it influences mantle dynamics.

The Earth’s mantle is largely composed of silicate minerals, but carbon is an important trace element. The behavior of carbon at depth is particularly significant for understanding key processes such as:

• Carbon Cycling: Carbon is involved in long-term processes that cycle between the Earth’s surface and its deep interior, including volcanic eruptions, tectonic plate movements, and the subduction of carbon-rich oceanic crust.
• Mantle Convection: The movement of materials within the mantle, driven by heat, is central to plate tectonics and volcanic activity. The presence of carbon can influence the physical properties of the mantle, affecting its convection patterns.
• Sequestration of Carbon: The process of carbon being trapped deep in the Earth (a process known as carbon sequestration) plays a role in regulating the planet’s climate over geological time scales. Understanding how carbon behaves deep in the Earth provides insight into the Earth’s long-term climate history.

Diamonds as Windows into Deep Mantle Processes

Diamonds not only provide a glimpse into the conditions at the moment of their formation, but they also offer unique insights into the processes that govern mantle geodynamics. This is because diamonds can encapsulate tiny inclusions—microscopic fragments of surrounding mantle material—trapping these samples deep within their structure. These inclusions can contain gases, minerals, and even other elements that were present in the mantle at the time the diamond formed.

By studying these inclusions, scientists can gain insights into the following:

• Mantle Composition: Inclusions in diamonds often contain rare minerals or gases, such as nitrogen or noble gases, which provide information about the composition of the deep mantle. This helps to build a more accurate model of how the mantle is structured and how it behaves under extreme pressure.
• Heat Flow: The study of diamonds can also help scientists understand the heat flow within the Earth’s mantle. Because diamonds are stable only under high-pressure, high-temperature conditions, they can act as markers for understanding temperature variations within the mantle.
• Mantle Processes: Diamonds can record the history of mantle processes, including mantle convection and the movement of tectonic plates. As diamonds form, they may record signals of deep mantle flow or material interactions, helping to reveal the ongoing geodynamic processes beneath the Earth’s surface.

Diamonds and the Deep Earth: Unraveling Mysteries of Earth’s Evolution

The study of diamonds has profound implications for understanding the Earth’s geological history and evolution. By investigating the diamonds’ age, formation conditions, and included materials, scientists can better understand how the mantle has evolved over millions of years. This, in turn, helps us piece together the history of Earth’s formation, tectonic activity, and climate changes.

For example, some of the oldest diamonds—forming around 3 billion years ago—may offer clues about the early Earth’s conditions and how it cooled over time. Additionally, diamonds from different geological environments may provide insights into regional differences in mantle composition and behavior.

Conclusion: The Crucial Link Between Diamonds and Mantle Geodynamics

Diamonds are far more than just precious stones—they are powerful tools that allow us to peer into the heart of the Earth’s mantle. Through their unique structure and the inclusions they contain, diamonds offer an unparalleled look at the behavior of carbon and other elements deep within the Earth. Their study enriches our understanding of mantle geodynamics, shedding light on the complex processes that shape our planet.

By continuing to study these natural wonders, we not only learn about carbon’s role in the Earth’s geological processes but also gain valuable insights into the Earth’s long-term evolution, climate history, and geodynamic activity. So, the next time you admire a diamond, remember—it’s not just a symbol of beauty, but also a key to unlocking the mysteries beneath our feet.

Super-deep diamonds, which form more than 380 km deep in Earth’s mantle, were invaluable tools for deep carbon scientists.

Super-deep diamonds, which form more than 380 km deep in Earth’s mantle, were invaluable tools for deep carbon scientists. Not only do they harbor clues about how they formed and therefore the reactions taking place inside Earth, they also trap small samples of mantle minerals, so-called inclusions, within their carbon crystal structure as they grow. These tiny samples of Earth’s deep interior from the region where the diamond forms were preserved under high pressure within a super-strong, unreactive diamond shell.

Many super-deep diamonds were small, have poor clarity, and were not generally used as gemstones. However, in a paper published in the journal Science, a team of researchers led by Evan Smith of the Gemological Institute of America (GIA) and including Deep Carbon Observatory DMGC (Diamonds and Mantle Geodynamics of Carbon) collaborators Steven Shirey (Carnegie Institution for Science, USA) and Fabrizio Nestola (Università degli Studi di Padova, Italy), suggests giant gemstone diamonds, like the 3106 carat Cullinan, were super-deep diamonds formed under special mantle conditions [1].

Co-author Professor Fabrizio Nestola explains the method of X-ray diffraction at the University of Padova, Italy. This method was used to first identify the presence of cohenite (an iron-nickel carbide) within the metallic inclusions. (credit Chiara Anzolini and Fabrizio Nestola)

When gem diamonds were polished and cut, expert diamond cutters often remove sections of the stones with inclusions. These offcut diamond pieces were not normally made available to scientists, and were usually considered waste, but the team made special efforts to get their hands on some.

“The project started with our collaborators at the GIA who have the opportunity to observe a number of large gem diamonds and have access to some of their offcut pieces,” said Shirey. “Evan Smith, a GIA postdoctoral researcher had a hypothesis that large diamonds could form deep in the mantle from metallic liquid, but we needed to the samples to figure it out.”

When they analyzed the offcuts, the team discovered multi-mineral metallic inclusions containing iron-nickel metal, an iron-carbide mineral known as cohenite, and the iron-sulfide mineral pyrrhotite. There were also traces of fluid methane and hydrogen in the thin space between the mineral phases and the encasing diamond. At the original pressure and temperature deep in Earth’s mantle, the composition of these multi-mineral inclusions suggested to the research team that a much larger mass of molten metallic liquid existed from which pure carbon crystallized to form diamonds. As each diamond grew, small droplets of the metallic liquid got trapped. As the diamonds were brought to Earth’s surface by volcanic eruption, the liquid droplets crystallized to the individual minerals.

“My motivation in this work was to solve this long-standing mystery about how these especially large and alluring diamonds form,” said Smith. “Everything about them suggests they form in a special way and that means they might tell us something new about the behavior of mantle carbon. In this research I was chasing an idea that I published a couple years ago, that the low nitrogen content and large size of these (CLIPPIR) diamonds might be linked to metallic iron in the mantle. I was thrilled when I started finding the first few inclusions. With the expertise of everyone involved we saw the observations unfold into an amazing story from the deep Earth.”

As well as diamonds with only the metal inclusions, the team found additional similar diamonds with silicate mineral inclusions –that coexisted with smaller amounts of metal. This assemblage suggests that all the metal-containing diamonds formed between 360 and 750km deep inside Earth. This was much deeper than most other gem diamonds, which form in the lower part of continental tectonic plates at depths of 150–200 km.

These two observations together show not only that Earth’s largest gemstone diamonds form extremely deep in the mantle, but also in regions of the mantle with metallic iron, the first time these aspects of the largest gem diamonds have been recognized.

“The idea of metallic iron in the silicate mantle at far shallower levels than Earth’s iron core , is something Earth scientists have expected for a while,” said Shirey. “A number of experiments and simulations predicted it, but now we have physical evidence that this is the case.”

Previous experiments and theory suggested for many years that small amounts of metallic iron existed in parts of the deep mantle below about 250 km depth. Though it’s still unclear how much metallic iron was present in the lower mantle, this was a key observation for understanding of Earth and the conditions under which it formed and evolved. Because the metallic liquid at these pressures and temperatures contains carbon and hydrogen it plays a hitherto undetected role in the geochemical cycles of these elements in the deep mantle.

“This result provides a direct link between diamond formation and deep mantle conditions, addressing a key goal of the Deep Carbon Observatory,” said DCO Executive Director Robert Hazen (Carnegie Institution for Science, USA). “The fact that it was made possible by a hugely successful collaboration between Diamonds and Mantle Geodynamics of Carbon group and the Gemological Institute of America was also very exciting, highlighting the importance of academic connections with industry and their important role in providing postdoctoral funding and the key specimens for this research.”

The 15 November 2016 edition of the journal Lithos delves into the role of natural diamonds in deep Earth research.

Titled “The nature of diamonds and their use in Earth’s study,” the 15 November 2016 edition of the journal Lithos delves into the role of natural diamonds in deep Earth research. This special issue was edited by DCO scientists involved the Reservoirs and Fluxes initiative, Diamonds and the Mantle Geodynamics of Carbon (DMGC).

This special issue of Lithos was compiled in recognition of the second International Diamond School, which took place in January 2015 in Bressanone, Italy. The Deep Carbon Observatory and the Gemological Institute of America sponsored the school, which brought together more than 80 graduate students and postdocs from around the world. Many of the students of the school are authors in the special issue.

Fabrizio Nestola (Università degli Studi di Padova, Italy), Matteo Alvaro (Università degli Studi di Pavia, Italy), Graham Pearson (University of Alberta, Canada), and Steven Shirey (Carnegie Institution for Science, USA) edited a selection of 30 original research articles from 134 authors for the issue. The papers cover four main research areas: geochemistry, diamond forming fluids, and diamond origin sources; geothermo-barometry and geochronology of diamonds; super-deep diamonds, carbonado-like diamonds and diamondites; and innovative methods for the investigation of diamonds.

 

CONTENTS

The Third International Diamond School took place at the University of Alberta, with the Deep Carbon Observatory as the main event sponsor (together with De Beers and IsoMass).

The Third International Diamond School took place at the University of Alberta, with the Deep Carbon Observatory as the main event sponsor (together with De Beers and IsoMass). DCO’s Graham Pearson (Reservoirs and Fluxes Scientific Steering Committee member; University of Alberta, Canada), Steve Shirey (Carnegie Institution for Science Department of Terrestrial Magnetism, USA), Thomas Stachel University of Alberta, Canada), Bob Luth (University of Alberta) and Fabrizio Nestola (University of Padua, Italy) were the principal conveners. The event continued in the tradition of having a mixed participation of students, senior academics, and industry. Seventy-five delegates, including 2 BSc students, and 30 Ph.D and Masters students from Canada, USA, Australia, and the UK attended, along with 18 delegates from industry and Government/Provincial Geological Surveys.

Prior to the conference 21 people (a mix of academics and students) attended a 2 day field trip to the Northwest Territories diamond mines and to see Archean geology around Yellowknife. The field trips were made possible by the generosity of Dominion Diamonds and Rio Tinto (Diavik Diamond Mine), as well as the staff of the Northwest Territories Geological Survey.

The scientific program consisted of 34 presentations, including 12 student talks, and 4 student posters that spanned a range of topics from the fundamentals of diamond formation through new mantle thermometry methods to aspects of diamond exploration and deposit evaluation. DCO early career scientists (non-students) who presented or attended included Emilie Thomassot (Nancy, France), Yakov Weiss (Columbia, USA) and Andrea Guiliani (Melbourne, Australia). The science program was supplemented by laboratory facility tours at the University of Alberta.

In a recent letter published in Nature, researchers propose that most carbon goes no deeper than about 300 to 500 kilometers, at which point a carbon barrier limits carbon recycling into the deeper mantle.

Carbon was cycled from Earth’s surface to its depths, emerging through the crust from volcanoes, and descending to the mantle in subducting ocean floor. But how far down was the carbon subducted? In a letter by Andrew Thomson, Michael Walter, Simon Kohn, and Richard Brooker (University of Bristol, UK) published in Nature, the authors propose that most carbon goes no deeper than about 300 to 500 kilometers, at which point a carbon barrier limits carbon recycling into the deeper mantle [1].

Downwelling slabs of mid-ocean ridge basalt (MORB) efficiently dehydrate at sub-arc depths, but may retain a considerable portion of their carbon cargo. Thomson et al made high pressure-temperature melting experiments on materials that replicated carbonated basalt from the IODP 1256D site on the East Pacific Rise. They show that upon reaching transition zone depths carbonatite melts are produced along a deep solidus depression. The melts infiltrate and react with the overlying mantle, causing diamond production, refertilization and associated metosomatism of the surrounding mantle. This melting of recycled crust in the transition zone was an effective barrier to carbon transport into the lower mantle.

The major difference between this work and other melting studies of carbonated MORB above 8GPa was the different phase assemblage resulting from lower and more realistic CO₂ and CaO contents of this study’s bulk composition. The resulting change in phase relations produces a deep solidus depression in carbonated oceanic crust at upper-most transition zone depths. The authors estimate that melting would occur to depths of at least 7 kilometers into the crustal section, and that only the coldest modern-day slabs would survive the solidus depression and carry carbonate beyond the transition zone.

The compositions of superdeep diamond-hosted inclusions provide strong evidence of carbonate melt-peridotite reaction. These diamonds form at transition zone depths, and have isotopic characteristics consistent with subducted carbon. The diamonds confirm that carbon must survive subduction beyond sub-arc dehydration reactions, and may record the process of slab melting in the transition zone.

Dr. Andrew Thomson said, “superdeep diamonds are a unique pristine snapshot of the deepest portions of the Earth’s carbon cycle. They contain a wealth of information that makes them invaluable and unparalleled tools for better understanding the interior of our planet”.

In a study published in Nature, a team of scientists describes an unexpected mechanism for diamond formation relying on ancient, subducted seawater.

Diamonds are crystals of carbon, formed deep in Earth. As diamond crystals grow, they sometimes trap fluids or other mineral crystals, micro-samples of their surrounding environment. In a study published in Nature, a team of scientists, including DCO’s Graham Pearson (University of Alberta, Canada), describes an unexpected mechanism for diamond formation relying on ancient, subducted seawater [1].

The team, lead by Yaakov Weiss (Columbia University, USA), analyzed 11 diamonds from the Ekati mine in the Northwest Territories of Canada. These diamonds, so called fibrous diamonds, are less than a millimeter in diameter. The center of many of the stones was familiar, a gem-like diamond. But surrounding this core the diamond was studded with millions of minute inclusions, giving it a “fuzzy” or fibrous appearance under a microscope.

The inclusions in the 11 diamonds studied provided the authors with new information about how, when, and where in Earth this carbon crystalized. For diamonds to have inclusions like these, they must have formed quickly, trapping surrounding fluids and minerals. Through a series of measurements, some involving a unique laser ablation method developed by Pearson’s research group, Weiss and colleagues showed that many of the inclusions contained fluids rich in chlorine and sodium.  The source of such high levels of these two elements, combined with their isotopic fingerprint, are strongly indicative of ancient seawater that reacted with oceanic crust, that was subducted to depth.

During subduction, water, in the form of salty fluids or “brines” was transferred into the deep mantle beneath the Northwest Territories, as oceanic lithosphere descended beneath the overlying tectonic plate. The reaction of these brines with particular rock types in the mantle root appears to be a critical part of the diamond forming process.

“These results are particularly interesting to the Deep Carbon Observatory because they point to a new mechanism whereby carbon was cycled into, and stored in, deep Earth,” said Pearson. “Before now, it was unclear what the starting compositions were for the unusual fluids that form these diamonds. Diamonds with “salty” inclusions appear to be common beneath the Northwest Territories. Similar fluid compositions in diamonds from other parts of the world indicate that this diamond forming reaction was widespread beneath the deepest continents around the world.”