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Giant diamonds may be the key to giant earthquakes | Science

 


Faults such as inclusions (dark spots) in this diamond reveal that plate tectonics can carry water deeper into the Earth’s mantle.

Evan Smith / © 2021 GIA

According to most geophysical models, earthquakes should not occur more than 300 km below the surface of the earth. Yet they usually do—a phenomenon that has puzzled seismologists for decades. Now, researchers suggest that water carried by tectonic plates pushed under the continents could be causing these deep tremors. The discovery may also explain another wonder: why so many fist-sized diamonds form at this depth.

Earthquakes typically occur when the two sides of a fault, or opposite sides of a tectonic plate boundary, intersect each other. But below our planet’s surface, pressures are too high for such a slip, and the rocks are usually so hot that they squish and flow rather than break. This has led geophysicists to devise alternative explanations for deep seismic activity, which can be very strong but too far from being felt.

One idea is that some minerals, under the intense heat and pressure deep within our planet, can suddenly lose volume, with runaway collapse over great distances causing powerful earthquakes. The second idea is that once an earthquake occurs—due to the sudden breakdown of minerals or some other reason—the rocks near the tip of the rupture heat up and weaken, fueling the earthquake. A third reason may be water from rocks deep below the Earth’s surface, which can weaken other nearby rocks, allowing them to fracture more easily. However, the researchers largely rejected this explanation, because it was not clear where this water would come from.

Stephen Sherry, a geochemist at the Carnegie Institution for Science, had a hunch: diamonds. Gemstones can build up layers as they grow, collecting flaws—such as patches of surrounding rock—as they get older. These so-called impurities can also contain pockets of mineral-rich water.

To see if the idea would work, Sherry and his team took a closer look at how water can make its way into the depths. The answer, they think, is that it descends within the tectonic plates as it is pushed under the continents. They assume that there are three sources of water. One was water trapped in minerals that formed when molten rock solidified at the mid-ocean ridges. The other was the wet sediment that accumulated on those plates as they moved across the ocean floor. The third is ocean water that infiltrated the panels as they were bent and cracked.

Next, the scientists used computer simulations — and the results of previous lab studies by their team and others — to study how the minerals in those plates behave as they moved deeper and deeper. In general, as the depth inside the Earth increases, so does the temperature and pressure. Although the plates can start out relatively cold at the Earth’s surface, they warm up as they sink. Because they are several kilometers thick, it often takes millions of years to heat the plates.

Regardless of depth, Sherry and his team found that once the rocks in the slabs reached temperatures above 580 degrees Celsius, they were less able to hold water. Sherry and colleagues at AGU Advances said that the flow of water from the slab weakened the surrounding rock and triggered earthquakes. This water, full of dissolved minerals, would also be available to fuel the diamond formation.

“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 plate starts out hot, as if the rocks were relatively small, he says, the plate would dry up at depths of 100 to 250 kilometers, so it wouldn’t carry water far enough to generate deep earthquakes. But if the rocks in the sinking slab were old and relatively cold, the water would remain trapped inside the sinking slab for much longer, continuing there until it was released at depths of 300 to 500 kilometers or more.

More work will be needed both in the lab and in the field to fully understand the relationships between water released from sinking plates and deep earthquakes, Wiens says. In the meantime, he says, the diamonds that form in those depths, the flaws and all, are obviously going to be crucial to teasing out the details of the story.

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