Carbon in Earth's mantle determines if continents remain stable or shift

Earth’s surface tells a complex story, shaped not only by visible forces like tectonics and erosion but also by hidden mantle processes deep beneath our feet. One of the most important yet elusive of these is the movement and transformation of carbon in the planet’s mantle.

A recent study led by researchers at the Guangzhou Institute of Geochemistry (GIG-CAS), shines new light on how subducted carbonates reshape the mantle’s chemistry and structure.

This work reveals how carbon, under extreme pressure and temperature, controls the formation of sublithospheric diamonds and influences the long-term fate of ancient continental cores called cratons.

While carbon’s surface roles – like atmospheric CO₂ levels or ocean chemistry – are widely studied, its deep Earth behavior remains less understood. The new research provides compelling evidence that carbonates carried into the mantle by subducted oceanic plates do not simply vanish.

Instead, they alter the redox state of surrounding rocks, producing contrasting geological outcomes based on the mantle’s thermal regime.

Mantle temperature and subducted carbon

The experiments replicated conditions found at depths between 250 and 660 kilometers using carbonatite melts and peridotite rocks containing metallic iron. These setups recreated environments in which subducted carbon-rich melts encounter different mantle settings.

In cooler, nonplume conditions, carbonatite melts gradually reduce. This process leads to the crystallization of immobile diamonds and iron-rich metallic phases, which then attach to the base of cratons. Over time, these structures add stability to the lithosphere.

In contrast, under hotter, plume-affected conditions, the melts remain mobile and act as oxidizing agents. These oxidized melts infiltrate the mantle, dissolving elements like silicon, magnesium, and iron.

The resulting chemical transformation weakens the lithosphere and allows the ascent of silico-carbonatite melts. These in turn oxidize the surrounding mantle, promote metasomatism, and can drive large-scale geological processes like lithosphere delamination, surface uplift, and volcanism.

“The redox state of the deep mantle is a critical factor controlling how volatiles, such as carbon, cycle between Earth’s surface and its interior,” said Professor YU Wang, lead author of the study.

“Our experiments show that the fate of subducted carbon is heavily influenced by mantle temperature and redox conditions, shaping continent evolution over geological time.”

Mantle carbon effects on continents

To validate their findings, researchers compared experimental mineral products to natural inclusions from diamonds found in the Amazonia (Juína) and Kaapvaal cratons.

These two regions provide a natural laboratory for understanding deep Earth processes due to their contrasting geological features and preserved sublithospheric diamonds.

Diamonds from Juína formed in a predominantly reduced mantle, unaffected by mantle plumes. These diamonds contain inclusions like ferropericlase with magnesium numbers (Mg#) ranging from 44 to 78 and low nickel content.

This matches the experimental results where carbonatite melts froze entirely into diamond and iron-carbon alloys under reducing conditions. These materials, once accreted to the base of the cratonic keel, created a stable and long-lived lithosphere.

In contrast, diamonds from Kaapvaal, such as those from the Jagersfontein and Monastery mines, show inclusions that reflect formation in a fully oxidized plume environment. These diamonds contain majorite garnets with high MgO and FeO but lower calcium and sodium contents.

The study demonstrated that under oxidized conditions, melts penetrate and metasomatize the cratonic keel, causing eventual weakening and destabilization.

Minerals reveal deep carbon conditions

Majorite and ferropericlase inclusions serve as redox fingerprints. The team’s experiments revealed distinct chemical signatures in these minerals, based on the redox conditions under which they formed.

In reduced environments, majorites showed higher sodium and lower calcium contents with increasing pressure. This reflects the progressive incorporation of sodium into the garnet structure as carbon was locked away as diamond.

In the Kaapvaal Craton, majorite inclusions follow different trends. Here, compositions indicate strong interaction between oxidized melts and the mantle, consistent with the presence of mantle plumes. These melts retain carbon in mobile forms and lead to the redistribution of iron and other elements throughout the lithosphere.

Ferropericlase compositions also differ sharply. In the Amazonia Craton, low-Mg# ferropericlases appear to form at lower pressures under reduced conditions.

Some are associated with native iron or cohenite, providing further evidence of their deep and reduced origin. In plume regions, ferropericlase occurs only under very high pressures and as a minor phase, showing the dominance of silicate minerals in oxidized settings.

Cool mantle protects, hot mantle changes

In regions where mantle temperatures remain moderate, subducted carbonatite melts gradually freeze as they ascend. Initially, the melt reacts with metallic iron, forming oxidized zones.

However, as it continues to move, the redox capacity of the surrounding mantle overwhelms the melt, which then freezes into diamond. This prevents widespread oxidation and results in the buildup of reduced, carbon-rich materials beneath the craton.

Plume regions behave very differently. Here, temperatures rise significantly, and the melt does not freeze immediately. Instead, it reacts with the mantle, leading to reactive melting and the generation of oxidized, silico-carbonatite melts.

These melts migrate upward and oxidize the lithosphere, making it chemically fertile and gravitationally unstable. This instability often leads to lithosphere delamination and volcanism.

The schematic on page 7 of the study vividly contrasts these two scenarios. In the Amazonia Craton, melt is consumed and locked in place, preserving the craton. In Kaapvaal, melt infiltration reshapes the lithosphere, triggering uplift and widespread Mesozoic magmatism.

Diamonds hold clues to Earth’s past

The geological histories of Amazonia and Kaapvaal cratons reinforce the experimental findings. The Amazonia Craton remains a low-elevation, intact landmass. Its Juína diamonds formed 450 to 610 million years ago and were retained in the keel for over 300 million years.

These diamonds show signs of having been stored deep in the lithosphere, untouched by plume activity. They represent a preserved record of early mantle processes and carbon sequestration.

In contrast, the Kaapvaal Craton has high plateaus and evidence of lithosphere removal. Diamonds here likely formed during the Mesozoic when the region sat near the edge of the Africa Large Low Shear Velocity Province (LLSVP).

Carbonatite melts released during slab subduction interacted with mantle plumes, leading to metasomatism, delamination, and the eruption of kimberlites. These events contributed to the breakup of Gondwana and significantly altered the landscape.

Deep mantle carbon shapes continents

This study offers powerful evidence that subducted carbon does more than disappear into the depths – it determines whether continents remain stable or are reshaped by internal forces.

The mantle’s temperature and redox state control whether carbon forms diamonds or fuels geological upheaval. By linking lab-based experiments with real-world diamond inclusions, the researchers build a clearer picture of how Earth’s deep interior operates across geological time.

Understanding these deep processes is not just academic. It sheds light on continental evolution, the dynamics of mantle convection, and the movement of carbon through Earth’s interior.

As the study shows, carbon may be invisible underground, but its effects stretch from the deepest mantle to the highest continental peaks.

The study is published in the journal Science Advances.

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