A story of diamonds and the hidden chemistry of Earth’s mantle

When most of us think about diamonds, we picture glittering stones in jewelry cases. But for geologists, diamonds are much more than symbols of luxury, they’re tiny time capsules from Earth’s deep interior. Encased within some diamonds are microscopic minerals that formed hundreds of kilometers beneath our feet. These inclusions record secrets about the mantle, the mysterious layer of rock that makes up most of our planet. A study published in [Science Advances] by Mingdi Gao and Yu Wang of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, along with Stephen Foley (Macquarie University and Australian National University) and Yi-Gang Xu, explores one of Earth’s most fundamental questions: **how does carbon traveling deep underground change the chemistry, and even the stability, of continents themselves?** To understand this work, we need to talk about something that might sound abstract: **redox state.** In simple terms, it’s a measure of how oxidized or reduced a material is, that is, how many electrons are floating around in its chemical reactions. Think of it like cooking: some recipes call for adding oxygen (like when you caramelize sugar), while others need things to stay oxygen-free (like fermenting beer). In Earth’s mantle, redox chemistry decides whether carbon exists as solid diamond, as carbon dioxide gas, or locked away in minerals. These differences, in turn, shape volcanism, the carbon cycle, and even the stability of ancient continents known as **cratons.** The surface of the Earth is constantly recycled into the interior through subduction zones, where oceanic plates dive under continents. Along with water and sediments, these slabs carry carbonates, the same kind of minerals found in seashells and limestone, down into the mantle. When these carbonates sink deeper than about 250 kilometers, they encounter a strange environment where **metallic iron** exists. This iron can strip oxygen away from the incoming carbonates, changing their chemistry dramatically. The result is a patchwork mantle with wildly different redox states. And this is where diamonds come in. Under certain conditions, carbon crystallizes as diamond, often trapping bits of surrounding minerals in the process. Those inclusions are like postcards from the deep mantle, telling scientists about the environment in which they formed. The team focused on diamonds from two very different cratons, **the Amazonia Craton** in Brazil, a vast low-lying region with thick, stable lithosphere, and **the Kaapvaal Craton** in South Africa, famous for diamond mines but geologically more restless, with evidence of past volcanism and pieces of its deep root missing. Diamonds from Amazonia often carry mineral inclusions that indicate **highly reduced conditions** (oxygen-poor), while those from Kaapvaal suggest a much more **oxidized environment**. Why the contrast? To answer this, Gao and colleagues recreated mantle conditions in the lab. Using a massive press called a multi-anvil apparatus, they squeezed mixtures of peridotite (a mantle rock) and carbonatite melts (slab-derived carbonate liquids) to pressures of up to 21 gigapascals, equivalent to depths of 660 kilometers, while heating them to over 1700°C. By varying the redox conditions, they could watch what minerals formed. The results showed that under **reduced conditions**, carbonatite melts were consumed and frozen into diamond plus metallic carbon phases. This matched the inclusions seen in Amazonian diamonds. Instead, under **oxidized conditions**, the melts survived, producing carbon-rich magmas that could weaken the lithosphere. This matched the Kaapvaal diamonds. In non-plume settings like Amazonia, the subducted carbonates mostly froze as diamond, strengthening the craton’s keel, the deep root that makes these ancient continents so stable. That’s why Amazonia remains a flat, intact block today. But in plume-influenced regions like Kaapvaal, hot upwellings tipped the balance toward oxidized conditions. Carbon-rich melts penetrated the lithosphere, weakening it, and in some cases, causing pieces of the craton to peel away, a process called **delamination.** This explains the high plateaus, widespread volcanism, and evidence of lost cratonic root beneath southern Africa. Basically, diamonds acted as scientific messengers: their tiny inclusions record the battle between oxidizing and reducing forces deep underground, forces that decide whether a craton survives for billions of years or gets eroded away. Beyond diamonds, this story shows how the deep Earth breathes, recycles, and reshapes itself. The balance of redox reactions in the mantle determines not only the fate of ancient cratons but also how carbon moves between Earth’s surface and interior. In the long run, that affects volcanism, mountain building, and even the climate. If you want to learn more, read the original article titled "Variable mantle redox states driven by deeply subducted carbon" on [Science Advances] at . [Science Advances]: http://dx.doi.org/10.1126/sciadv.adu4985

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