Scientists recently found highly oxidized iron at ~550 km (341.8 mi) beneath the Earth’s surface, according to researchers at the University of Alberta (Edmonton, Alberta, Canada) who helped lead the study.1
The iron is similar to rust seen on the Earth’s surface within garnets found within diamonds. The result surprised geoscientists because there is little opportunity for iron to become so highly oxidized deep below the surface.
“On Earth’s surface, where oxygen is plentiful, iron will oxidize to rust,” says Thomas Stachel, professor in the earth and atmospheric sciences department at the University of Alberta who co-authored the study. “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 the depths at which the diamonds were found. The scientists suspect that it was molten carbonate, carried to those depths in sinking slabs of ancient sea floor.
“It’s exciting to find evidence of such profound oxidation taking place deep inside the Earth,” Stachel says.
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?” Stachel explains. “Our study suggests that surface carbon goes down as carbonates to at least 550 km [341.8 mi] below the surface. 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.
Source: University of Alberta, www.ualberta.ca.
Reference
1 “Scientists Find Oxidized Iron Deep within the Earth’s Interior,” Science News, Jan. 22, 2018, https://www.ualberta.ca/science/science-news/2018/january/oxidized-iron-diamonds-earths-interior (Feb. 14, 2018).