Diamond formation at the core–mantle boundary
The Earth’s core holds the largest reservoir of carbon. Estimates suggest that about 90 percent of the planet’s carbon is stored deep within the core. Researchers have begun to unravel how carbon-rich materials can form under the extreme conditions found at the boundary between the core and the mantle, several thousand kilometers beneath the surface. Diamonds may originate here as carbon moves through the vast depths of the planetary interior.
Hydrated minerals, minerals that contain water, can descend from the oceanic crust through subduction to the Earth’s interior. This process delivers water to the core–mantle boundary where it can participate in chemical reactions under high pressure and temperature. The interplay of water, iron, and carbon at these depths is key to understanding how diamonds may form far below the surface.
At the core–mantle boundary, temperatures are immense, at least twice the temperature of lava, and conditions are hot enough to drive water out of hydrated minerals. In this environment a reaction can occur that resembles how rust forms on iron, but under extreme deep-Earth conditions carbon can separate from liquid iron to become diamond. The boundary hosts a complex, molten-looking alloy of iron and carbon that interacts with water to yield iron oxides and hydroxides before carbon emerges as diamond.
Researchers collaborating with Byeongkwan Ko from Arizona State University report findings on the core–mantle boundary. In experiments carried out at the Advanced Photon Source in Argonne National Laboratory, they simulated the high pressures and temperatures of these depths by compressing a model alloy of iron, carbon, and water. The goal was to observe how such a system behaves at the actual boundary between Earth’s core and mantle.
how diamond forms in this environment
The scientists observed that the presence of water and metal can drive chemical reactions that produce iron oxides and hydroxides. Under core–mantle boundary conditions, carbon is expelled from the iron–carbon liquid and reorganizes into diamond. The deep setting drives the carbon into a form that is exceptionally stable under extreme pressure and temperature, explaining how carbon-rich phases may accumulate in the mantle over geological timescales.
At depths around three thousand kilometers, the silicate mantle meets the metallic core at temperatures near three thousand eight hundred degrees Celsius. This extreme heat can drive out the water trapped in many mineral structures. The high temperature also allows minerals to melt, enabling new chemical pathways for carbon and other elements to migrate and settle in different layers of the Earth. This observation aligns with experimental data from ASU researchers and supports the idea that the core–mantle boundary hosts dynamic chemical exchange.
Although carbon is typically associated with the core due to its affinity for iron, surprisingly more carbon appears in the mantle than earlier models predicted. The exchange of carbon through dehydration at the boundary can locally influence how light elements distribute within the core. In this framework, the stable form of carbon under these deep conditions is diamond, which forms when carbon escapes the liquid iron and enters the surrounding mantle material.
As noted by the team, the presence of diamond-rich structures at the boundary could be detectable by seismic surveys. Diamond is highly incompressible and maintains a lower density than surrounding materials at these depths, which would cause seismic waves to travel unusually fast through such regions. This seismic signature could reveal the hidden carbon pathways from the core into the mantle.
Lead investigator Ko and his colleagues emphasize that this mechanism could account for a significant portion of the mantle’s carbon load. The team plans to explore how the reaction might alter the distribution of other light elements such as silicon, sulfur, and oxygen within the core and how these changes could affect deep mantle mineralogy and behavior at the boundary. The work suggests ancient subduction processes may have sustained diamond formation at the core–mantle boundary for billions of years.
For readers seeking a formal reference, the study is reported in Geophysical Research Letters with detailed experimental methodology and findings published by Ko and collaborators. The findings illuminate how deep carbon cycling operates and how diamond formation at the core–mantle boundary could be a long-standing feature of Earth’s interior. The broader implication is that carbon transport from the core to the mantle helps explain the unexpectedly high carbon content found in deep mantle rocks. The research also invites future inquiry into how this process may influence the behavior of light elements and seismic properties at the deepest layers of our planet.
In sum, the discovery shows that carbon transfer from the core to the mantle through diamond formation provides a plausible mechanism for the deep Earth carbon reservoir. It opens the door to a more integrated view of the carbon cycle that spans from the planetary core to the mantle, with implications for geology, geophysics, and the interpretation of seismic data across North America and beyond. The ongoing work promises to refine our understanding of planetary interiors and the long, slow processes that shape Earth’s deep carbon store.
Reference work: Ko and collaborators. Geophysical Research Letters 2022. Detailed experimental results and analysis accessible through the journal’s archive. This study underscores the importance of deep carbon pathways and the potential for diamond formation at the core–mantle boundary to influence mantle carbon inventories and seismic velocity structures.