An Unassuming Mineral is Actually Sucking Iron Out of the Earth's Crust

Saturday, 19 May 2018 - 3:50PM
Earth
Saturday, 19 May 2018 - 3:50PM
An Unassuming Mineral is Actually Sucking Iron Out of the Earth's Crust
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Flickr/Graeme Churchard
There's a lot of reasons why Earth doesn't look like Mars right now (and most of them involve water), and one thing keeping Earth from looking bright red is iron.

More specifically, it's a lack of iron keeping our planet from looking as red as Mars (and yes, the blue oceans help with that too). Earth definitely has iron within its crust, and it's possible for rocks to "rust" over long periods of time and turn red, but our pale blue dot doesn't have nearly as much as the Red Planet does. And scientists have known for a long time that something in Earth's crust is naturally stealing that iron away.

For a long time, it was believed that the mineral magnetite was responsible for absorbing iron in the Earth's continents, but a new study published in Science Advances rejected that claim and pointed a finger at a different target: garnet.



The researchers are expecting to meet with some skepticism, since magnetite appears much more frequently in samples than garnet, and it's difficult to pinpoint one mineral over the other as being responsible for absorbing large amounts of iron. Since these processes happen deep beneath the surfaces of active volcanoes, it's not an easy process to witness in person.

A problem with magnetite is that even though it shows up near volcanic activity, it doesn't appear frequently in the spots where iron depletion occurs, but garnet does. A specific time of garnet called almadine is laden with iron, and forms in hot, high-pressure environments where the continental crust is especially thick - precisely the conditions where the iron-stealing process would take place.

According to study co-author Ming Tang, who said the following in a statement:

Opening quote
"The accepted wisdom is that magnetite pulls iron from the melt before the melt rises and gets erupted out at continental arcs. Iron depletion is most pronounced at continental arcs, where the crust is thick, and much less so in island arcs, where the crust is thin. However, there is no obvious explanation for why the extent of magnetite involvement would correlate with thickness of the crust."
Closing quote


Much of the study is based on rocks which have found their way to the surface, called xenoliths, which were found to be rich in garnet. It may not be quite as direct as watching the process at its source, but it's as effective as any other means we currently have of examining geological processes deep underground.

So for the time being, magnetite may be innocent. It's deposits of garnet that makes sure our planet isn't as red as Mars.


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