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

Robin Andrews 18 May 2018, 18:19

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.

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