Findings challenge standard theory for continental iron content

Rocks from deep in the Earth’s crust suggest new insight into tectonics and volcanoes. Andrew Masterson reports.

Processes deep beneath active volcanoes reduce the iron content of continental rocks.
Processes deep beneath active volcanoes reduce the iron content of continental rocks.
Ezra Acayan/NurPhoto via Getty Images

The iron content of rocks beneath the ocean is on average a bit more than those on, or beneath, the continents – and that’s probably a good thing.

If the amount of iron in continental rocks was just a fraction higher, our planet would look a lot more like the surface of Mars, on which iron-rich rocks are so common the ground is red.

For at least 40 years, geologists have agreed that the relative iron deficit in continental rocks is the result of a process associated with active volcanoes. The standard hypothesis suggests that many kilometres below the continental crust a mineral called magnetite incorporates iron from its molten surroundings, such that the ejecta that eventually spews during an eruption contains less of it.

The hypothesis is supported by strong circumstantial evidence, in that low-iron geology is most commonly found in the vicinity of the arcs of continental volcanoes – the one which spans the Andes Mountains, for instance.

Continental volcanoes form in areas where the Earth’s crust is thick. Island volcano chains, in contrast, form out of a comparatively thin crust, and create deposits that have higher iron content.

Correlation, however, is not confirmation, and, lacking any instruments capable of monitoring processes that take place in ultra-hot, high-pressure environments many kilometres underground, no geologist has yet been able to fully account for the role of magnetite in iron removal.

Now, however, a team from Rice University in Texas, US, have found evidence to suggest that magnetite may not actually be the iron-stealing culprit. The true thief, suggests a team led by Ming Tang, is another mineral entirely, called garnet.

“The standard view, which even we agreed with and wrote papers agreeing with, is that iron is removed from continental crust by another mineral called magnetite,” says co-author Cin-Ty Lee.

“I think people haven't thought much about garnet, possibly because it doesn't show up very much and magnetite shows up in a lot of samples.”

And, indeed, it might never have even been suspected as the agent of change had not Lee and some students visited Arizona in 2009 to hunt for xenoliths – rock fragments much older than the volcanoes that spewed them out.

“‘Xeno’ meaning foreign and ‘lith’ meaning rock,” he explains. “They are much older than the volcanoes they came from.

“These volcanoes ripped up the rocks from 60 to 80 kilometres deep, and the xenoliths came up as little fragments. It’s difficult to find rocks like this, but when you do, they give you a window, a direct window, into the deep parts of the continental arc, the root.”

Analysis of the samples established that they were formed in a continental arc and contained a great deal of garnet.

Subsequent investigation established that the rocks also contained an unusual form of a chemical element called europium.

The scenario that posits magnetite as the iron-stealing mineral relies on the assumption that that in subduction zones – where continental arcs form – an oceanic tectonic plate recycled back into the Earth’s mantle brings with it a lot of oxygen, which must result in europium that shares three electrons with nearby atoms. This is known as the element’s oxidised state.

The europium found in the garnet xenoliths was less oxidised, with much of it sharing only two electrons, implying that the subduction zone involved carried much less oxygen than assumed – meaning magnetite action was less likely.

This discovery tied in with another anomaly in the magnetite idea. Iron content in rocks in thick continental arcs is less than that in thinner island arcs, but the amount of magnetite in each does not vary sufficiently to account for the difference.

Garnet, however, does. The researchers suggest that in very thick crustal columns typical of continental arcs, heat and pressure are enough to produce iron-rich garnets, which, being heavy, sink, leaving less iron in the material erupted.

To test the idea the scientists examined the world’s largest database of volcanic rocks and xenoliths, held by the Max Planck Institute in Germany. The relationship between iron content, crust thickness, and garnet held up.

“There is a relationship between iron depletion and the garnet fractionation signatures, which means magmas that fractionate more garnet are more depleted in iron,” says Tang.

“This is borne out in the global record, but the evidence is something that wouldn't be obvious from looking at just one or two cases. It's the kind of thing that requires a global database, and those have only recently become available.”

The research is published in the journal Science Advances.

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Andrew Masterson is news editor of Cosmos.
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