An international team of geologists has found highly oxidized iron, similar to the rust we see on the Earth’s surface, in diamond’s garnet inclusions from the deep (124-342 miles, or 200-550 km, below the surface) mantle. The result surprised scientists around the globe because there is little opportunity for iron to become so highly oxidized deep below our planet’s surface.
Diamonds with garnet inclusions (like the one shown here) can form at depths down to 342 miles below the Earth’s surface. Image credit: Jeff W. Harris, University of Glasgow.
“On Earth’s surface, where oxygen is plentiful, iron will oxidize to rust,” said University of Alberta’s Professor Thomas Stachel.
“In the Earth’s deep mantle, we should find iron in its less oxidized form, known as ferrous iron, or in its metal form.”
“But what we found was the exact opposite — the deeper we go, the more oxidized iron we found.”
This discovery suggests that something oxidized the rocks in which the superdeep diamonds were founds.
Professor Stachel and colleagues suspect that it was molten carbonate, carried to these great depths in sinking slabs of ancient sea floor.
“It’s exciting to find evidence of such profound oxidation taking place deep inside the Earth,” Professor Stachel said.
The study also has implications for understanding the global carbon cycle that involves the transport of surface carbon back into the Earth’s mantle.
“We know lots about the carbon cycle on Earth’s surface, but what about in the mantle? Our study suggests that surface carbon goes down as carbonates to at least 342 miles below the surface,” Professor Stachel said.
“There, these carbonates may melt and react with the surrounding rocks, eventually crystallizing into diamonds.”
“Diamonds can then be taken down even deeper in the mantle.”
The study shows that the carbon cycle extends deep into mantle, possibly all the way down to the core-mantle boundary, with billion year storage times.
The findings appear online this week in the journal Nature Geoscience.
_____
Ekaterina S. Kiseeva et al. Oxidized iron in garnets from the mantle transition zone. Nature Geoscience, published online January 22, 2018; doi: 10.1038/s41561-017-0055-7
