Super-deep diamonds reveal why phosphorus remains on Earth's surface

Super-deep diamonds reveal why phosphorus remains on Earth's surface

The survival of life on Earth may depend on the searing heat of the planet's interior. New research suggests that the depths of the Earth's mantle act as a thermal barrier, preventing vital phosphorus—a building block of DNA and cell membranes—from being permanently swallowed by the deep Earth.

Under normal conditions, the descending slabs of oceanic plates that drive plate tectonics are too hot to allow the deep Earth to absorb phosphorus. Instead, this element is torched back off the sinking oceanic plates, according to Dr. Graham Pearson of the University of Alberta, and remains in the shallow Earth where life can thrive.

Because the deepest human borehole reaches only 13 kilometres, scientists utilize "super-deep" diamonds as geological probes. These gems form at depths between 410 and 700 kilometres, trapping microscopic mineral inclusions that serve as time capsules from the lower mantle.

The Discovery of Tuite

Qiwei Zhang, a former University of Alberta PhD student and current post-doctoral fellow at the Carnegie Institution for Science, analyzed two such diamonds in partnership with the mining company De Beers. One diamond from Canada's Northwest Territories is thought to be 1.7 billion years old, while another recovered from Brazil is 450 million years old.

Using Raman spectroscopy, Zhang identified inclusions of tuite, a high-pressure transformation of apatite. While apatite is common in the Earth's crust, tuite has previously only been found in highly shocked meteorites. This discovery marks some of the first terrestrial examples of the mineral.

The presence of tuite proves that phosphorus can occasionally reach the lower mantle, but Zhang's modelling indicates this process is incredibly inefficient. Transporting phosphorus 700 kilometres down requires a "cool" subduction zone with temperatures around 1,100 C, which is significantly lower than the typical 1,700 C found at those depths.

The diamonds also contained stishovite, a dense version of quartz. Both stishovite and tuite were trapped at temperatures at least 500 C cooler than the surrounding ambient mantle, confirming a new mechanism for phosphorus cycling.

The Journey of Super-Deep Diamonds

Super-deep, or sublithospheric, diamonds originate from depths of approximately 300–800 km, distinguishing them from standard gemstone diamonds that form at 150–200 km. They make up an estimated 2% of mined diamonds globally, though they account for 90% of the world's largest and most valuable gems, including the Cullinan diamonds in the British Crown Jewels.

Industrial and Scientific Impact

Understanding these processes has direct implications for the diamond industry. Because super-deep diamonds are difficult to locate, Zhang's research focuses on creating new "indicator mineral" methods to help geologists find lucrative deposits. Current tools, such as chromium-pyrope garnets, are blind to super-deep sources.