We Might Finally Know What's Been Stealing Earth's Continental Iron

Who's stealing all the iron from Earth's continents? D'July/Shutterstock

Iron, an element that helps to define our world, is sinking down away from Earth’s continental crust. Exactly why has remained somewhat enigmatic, and a new paper by Rice University geoscientists presents a bold explanation as to what’s been stealing it.

Controversially, they suggest geologists have been suffering from a case of mistaken identity for nearly half a century.

Billions of years ago, the incandescent, primitive Earth began to be compressed under its own gargantuan mass. This caused a spike in internal temperature – something boosted by the decay of radioactive materials – which pushed the metaphorical mercury above the melting point of iron.

Said element, as you’d expect, began to melt. These dense droplets began to sink slowly through the world and into its heart, where they condensed. Over time, this process sped up to the point where vast amounts of iron clumped together and formed the iron-rich core of the world.

The so-called “iron catastrophe” was a one-off event, but thanks to this dramatic geochemical robbery, iron is heavily depleted in Earth’s surface realm, and vastly more concentrated at depth.

That wasn’t the end, though: Iron depletion (and enrichment) just beneath our feet is still gradually happening in different parts of the world today. The specifics as to why, however, are up for debate.

Writing in Science Advances, the Rice team explain how they zeroed in on continental arcs, parts of the crust where two tectonic plates meet and the denser segment slips under (or subducts) beneath the less dense one.

Volcanoes that form along these arcs tend to contain magma and erupt lava that’s depleted in iron relative to, say, oceanic crust or island arc geologies. The thicker the crust, the greater the iron depletion.

Magnetite: an imposter? Rob Lavinsky/Wikimedia Commons; CC BY-SA 3.0

For more than 40 years, it’s been generally suspected that magnetite was to blame here. This is an oxygen-rich iron ore, one that was thought to be dragging the iron down through the subduction zone before it has a chance to erupt or solidify and form much of the new continental crust.

The team’s analysis of some Arizona-sourced, rare xenoliths – “alien rocks” – dragged up from the deep roots of an ancient continental arc by long-gone volcanism suggested this might be wrong.

Serendipitously, these contained Europium, an element that can exist in a variety of oxidation states. These states indicate what the oxygen content of the environment at the time of formation was, and painstaking measurements revealed that they formed in low-oxygen conditions, something not really compatible with magnetite’s properties.

These xenoliths also happened to contain plenty of garnet. Specifically, they contained almandine, an iron-rich variant that is more commonplace and easily formed at continental arc roots.

Finding that this tell-tale geochemical signature could be found globally, the team reasoned that almandine is formed during the high-temperature, high-pressure, oxygen-poor, water-rich conditions present in the very thick parts of these arcs, before settling out and sinking down away from the crust. Garnet, it seems, is the underworld’s ferrous thief, not magnetite.

Remember, though that this is just a hypothesis, one of several. The researchers readily acknowledge that their idea, despite first appearing half a century ago in a speculative study, will come in for criticism, and their evidence at present is somewhat circumstantial.

Almandine, the true thief? Didier Descouens/Wikimedia Commons; CC BY-SA 4.0

Regardless of how game-changing this ultimately proves to be, it can’t be understated how important this research actually is.

Iron is a planetary kingmaker. Thanks in part to the earlier “catastrophe”, it’s responsible for our planet’s magnetic field. This subsequent continental iron removal process, whether driven by magnetite or garnet, is just as vital too.

"It would be a bit hard to predict what Earth would exactly look like, but without this garnet-based iron removal, Earth might not have the continents as we know them today," lead author Dr Ming Tang, a post-doc at Rice, told IFLScience.

Thanks to the abundance of oxygen-loving iron on the surface world, "Earth would be much more barren, perhaps more like Venus or Mars, and complicated life forms, including us, would not have existed."

Understanding iron's various journeys, then, isn’t just a niche endeavor. It’s as epic a quest as you’re likely to find, one that, indirectly, has affected everyone and everything.

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