Two diamonds formed at depths of 410–700 km provide evidence that phosphorus, a key component of DNA and cell membranes, remains in the Earth's shallow mantle rather than being subducted into the deep mantle.
The Phosphorus Puzzle
Scientists have long questioned why plate tectonics have not permanently sequestered phosphorus in the deep mantle over billions of years. A study led by former University of Alberta PhD student Qiwei Zhang (now at Carnegie Institution for Science) with researchers Graham Pearson and Thomas Stachel indicates that descending oceanic plates are too hot to allow deep phosphorus drawdown.
Graham Pearson stated that if 90% of phosphorus were subducted into the lower mantle, it would cause a phosphorus crisis; instead, phosphorus is released back into the shallow Earth.
Diamonds as Deep Probes
Because human drilling reaches only 13 km depth, scientists use super-deep diamonds as probes of the mantle. Zhang analyzed two diamonds provided by De Beers: a 450-million-year-old diamond from Brazil and a 1.7-billion-year-old diamond from Canada's Northwest Territories.
Using Raman spectroscopy, Zhang identified the inclusions as minerals distinct from common olivine or enstatite, confirming their deep origin.