by Mary Caperton Morton Friday, August 3, 2018
Iron is the fourth-most common element in Earth’s crust, but why there isn’t more found near the surface in continental crust has been a long-standing question among geologists. In a new study, scientists implicate an overlooked mineral culprit in the theft of iron from continental crust: garnet. But not everybody is ready to exonerate the long-implicated magnetite.
If iron were more concentrated in Earth’s continental rock, our blue planet would look more like Mars, where highly oxidized, iron-rich rocks cause its trademark red color. But the missing iron isn’t just a matter of color — it also has widespread implications for the chemistry of Earth’s oceans and atmosphere: If more iron were found at the surface, our planet might not be habitable. So what do we have to thank for sequestering this iron in the planet’s interior?
“The traditional view … is that iron is removed from continental crust by the mineral magnetite,” says Ming Tang, a geochemist at Rice University in Houston, Texas, and lead author of the new study, published in Science Advances. This theory suggests that magnetite pulls iron out of crustal melt deep in subduction zones, before the melt rises to the surface to erupt. The trouble with the magnetite theory of iron depletion is that depletion is most pronounced beneath thick continental volcanic arcs, such as the Andes, while there’s much less depletion beneath island arcs, where the crust is thinner. “The magnetite theory has been the dominant view for almost half a century, but it doesn’t explain why iron-depleted magmas are preferentially found in thick arcs. The distribution of magnetite does not correlate with crustal thickness,” Tang says.
However, what does correlate with crustal thickness is the distribution of garnet, particularly almandine, an iron-rich type of garnet that forms under high temperature and high pressure — the kinds of conditions found in deep regions of thick continental crust. “Garnet is a mineral that really likes high pressure. The more pressure, the more that crystallizes from magma and the more iron is depleted,” Tang says. At continental arcs, where the crust can be 80 kilometers thick, the pressure is great enough to produce large quantities of almandine crystals, which are heavy and sink out of melt as it moves toward the surface. “The iron [the garnets] pull out is ferrous iron (Fe2+), which is not highly oxidized. It goes back into the mantle, while the more oxidized iron (Fe3+) remains in the melt that rises to the surface.” This creates the highly oxidized, low-iron magmas commonly found at continental arcs.
Tang and his colleagues found new evidence for the role of garnet in iron depletion of continental crust in a collection of xenoliths — bits of deep rock brought to the surface by rising magma — found in Arizona. These fragments originated at depths of 60 to 80 kilometers and “offer a direct window into the deep roots of the continental arc,” Tang says.
Geochemical study of the garnet-rich xenoliths offered some clues about where iron depletion is taking place in the crust, Tang says. If magnetite were the main culprit, the reaction would be taking place deep in subduction zones, which tend to be highly oxidized. But the xenoliths appear to have formed in less oxidized conditions. “When you bring garnet into the equation, everything falls into place,” Tang says. The garnet theory holds that the oxidation occurs above subduction zones, as magma is rising to the surface.
The team tested their hypothesis on a global scale using the GEOROC database at the Max Planck Institute in Germany, a collection of published geochemical and isotopic analyses of volcanic rocks and mantle xenoliths collected all over the world. The researchers found a clear relationship between iron depletion and garnet fractionation, or crystallization, in continental arc volcanic rocks: “Magmas that fractionate more garnet are more depleted in iron,” Tang says. “The evidence is something that wouldn’t be obvious from looking at just one or two cases. It requires a global database, and those have only recently become available.”
The study is “provocative,” says Marc Hirschmann, an experimental petrologist at the University of Minnesota, who was not involved in the research. Senior author Cin-Ty Lee, also at Rice University, is “known for looking at data very differently and proposing unconventional viewpoints,” Hirschmann says. The study doesn’t eliminate the magnetite theory but rather offers an alternative pathway for iron depletion. It’s possible that both the magnetite and garnet pathways play a role in iron depletion of continental crust, he says — one pathway does not eliminate the other. This isn’t the first time the garnet pathway has been considered — the new paper cites a 1968 study that initially proposed the idea — “but this team is definitely advancing it with new data, new analyses and new arguments.”
Hirschmann expects that this study will stimulate other research teams. “People are going to be resistant to accepting this garnet idea right away,” he says. “I think we’re going to see teams looking for igneous garnet in the field and continuing [to use] modeling and thermodynamic studies to find out if garnet is as major a player in iron depletion.”
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