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Imperfections such as the inclusions (dark flecks) in this diamond reveal that tectonic slabs can carry water deep into Earth’s mantle.

Evan Smith/© 2021 GIA

Giant diamonds may hold the key to superdeep earthquakes

Earthquakes shouldn’t occur more than 300 kilometers below Earth’s surface, according to most geophysical models. Yet they commonly do—a phenomenon that has mystified seismologists for decades. Now, researchers suggest water carried by tectonic plates shoved beneath continents could be triggering these deep temblors. The find may also explain another marvel: why a huge number of fist-size diamonds form at this depth.

Earthquakes typically occur when the two sides of a fault, or the opposite sides of a tectonic plate boundary, scrape past each other. But far beneath our planet’s surface, the pressures are too high for such slippage, and rocks are typically so hot they ooze and flow rather than break. That has led geophysicists to come up with alternate explanations for deep seismic activity, which can be very strong but largely too far away for us to feel.  

One idea is that some minerals, under the extreme heat and pressure deep within our planet, can suddenly lose volume, with the runaway collapse over large distances causing strong quakes. A second notion is that once a quake gets going—because of the sudden collapse of minerals or another cause—rocks near the tip of the rupture heat up even further and weaken, fueling the quake. A third cause might be water released from rocks deep below Earth’s surface, which could weaken other rocks nearby, allowing them to fracture more easily. Researchers have largely dismissed that explanation, however, because it wasn’t clear where such water would come from.

Steven Shirey, a geochemist at the Carnegie Institution for Science, had a hunch: diamonds. The precious gems can accumulate layers as they grow, gathering imperfections—such as flecks of surrounding rocks—as they get bigger. Those so-called inclusions can also contain pockets of mineral-rich water.

To see whether the idea could work, Shirey and his team took a closer look at how water might make its way down deep. The answer, they believe, is that it rides down within tectonic slabs as they get shoved beneath continents. There are three sources of water, they postulate. One was the water was locked in the minerals that formed as molten rock hardened at midocean ridges. Another was the wet sediments that accumulated on those slabs as they moved across the ocean floor. And the third was ocean water that infiltrated the slabs as they bent and fractured.

Then, the scientists used computer simulations—and the results of previous lab studies by their team and others—to study how minerals in those slabs would behave as they moved deeper and deeper. In general, as depth within Earth increases, so do temperature and pressure. Although slabs can start out relatively cool at Earth’s surface, they warm up as they sink. And because they’re many kilometers thick, it often takes millions of years for the slabs to heat throughout.

Regardless of depth, Shirey and his team found that once rocks in the slabs reached temperatures above 580°C, they were less able to hold water. As that water flooded out of the slab, it weakened the surrounding rocks and triggered quakes, Shirey and his colleagues report in AGU Advances. This water, typically chock-full of dissolved minerals, would also be available to fuel diamond formation.

“The temperature tells the story,” says Douglas Wiens, a seismologist at Washington University in St. Louis who was not involved in the new study. If the tectonic slab starts out hot, as it would if the rocks are relatively young, he says, the plate will dehydrate at depths between 100 and 250 kilometers and thus won’t carry water far enough down to generate deep quakes. But if rocks in the sinking slab are old and relatively cool, water will stay locked inside the sinking slab for a longer time, persisting there until it is released at depths of 300 to 500 kilometers or more.

Further work in both the lab and the field will be needed to fully understand the relationships between water released from sinking slabs and deep earthquakes, Wiens says. In the meantime, he says, it’s clear that diamonds that form at those depths, imperfections and all, will be critical to teasing out the details of the story.