Two diamonds formed 700 kilometres below the Earth's surface reveal a life-giving synchronicity between shifting continents and the cycling of phosphorus, a vital building block of DNA and cell membranes.
For decades, scientists have tracked how phosphorus cycles through the planet's shallowest layers - to about 100 kilometres - before returning to the surface via volcanoes. Yet a mystery has always remained: why haven't billions of years of plate tectonics permanently locked phosphorus away in the deepest recesses of the Earth's mantle?
New research shows that under normal conditions, the descending slabs of oceanic plates that drive plate tectonics are simply too hot to allow the deep Earth to swallow phosphorus, keeping it near the surface and allowing life to thrive. The study was led by former University of Alberta PhD student Qiwei Zhang, now a post-doctoral fellow at the Carnegie Institution for Science, alongside researchers Dr. Graham Pearson and Dr. Thomas Stachel of the Faculty of Science.
"If 90 per cent of phosphorus did get subducted back into the lower mantle, we would have a real phosphorus crisis," explains Pearson.
"Instead, all of that phosphorus gets torched back off the sinking oceanic plates and stays in the shallow Earth."
Because the deepest human borehole only reaches 13 kilometres, scientists rely on "super-deep" diamonds - which form at staggering depths between 410 and 700 kilometres - to act as deep-mantle probes and time capsules. Partnering with the mining company De Beers, Zhang analyzed two such diamonds, a 450 million-year-old diamond recovered from Brazil and a diamond from Canada's Northwest Territories that is thought to be 1.7 billion years old.
Initially mistaking the inclusions inside the gems for more common minerals known as olivine or enstatite, Zhang used Raman spectroscopy to identify their true crystal structures.
"What I found was a spectrum the software could not match to anything in its database," he says.
