Diamonds that formed deep in the Earth could help seismologists answer a decades-old question: Do fluids play a role in generating earthquakes at depths where high pressure should prevent brittle rupture? to occur ?
The formation of fluid-assisted faulting in subducted slabs 300 to 700 kilometers deep, in the transition zone between the upper and lower mantle, is a process that could explain deep earthquakes. But good evidence of water or other fluids associated with these sample-based slabs was rare until recently, according to Steven Shirey of the Carnegie Institution for Science.
Now studies of the diamonds that formed in the transition zone provide evidence of fluids transported by subducted slabs. With new modeling of subduction zones, these diamonds clearly show that fluids can‘not to be ignored in the history of deep earthquake generation, Shirey told the Seismological Society of America‘s Annual meeting.
Not all deep earthquakes have to involve water, but modeling by Shirey and colleagues suggests that some subduction slabs stay cool enough to hold and transport water to the bottom of the transition zone. . Deep earthquakes, occurring between 500 and 700 kilometers below the Earth‘s surface, appear to occur only in those slabs that can transport water or can transport carbonate deep enough to initiate melting, they found.
At that depth, water or carbonate fluids could trigger the earthquakes, or earthquakes could trigger the release of fluids — or both could happen, Shirey said.
“I think so‘It’s up to the seismological community at this point to try to figure out why fluids would matter,” he said. “We know the fluids are out there, we know they‘moving, and diamond petrology tells you that, because these diamonds always form in regions of the mantle where fluids are moving.
The internal growth pattern of these super-deep “sublithospheric” diamonds (those that form in the mantle hundreds of miles below the lithosphere) indicate that they were formed from fluids moving through the host rock, noted Shirey. “Diamonds tell you that there is a fissure or vein relationship with the host’s mantle. You must have fluids moving through the host rock in veins or fissures and balancing with it.
These diamonds also have distinctive chemistry and inclusions that are telltale signs of their origin from the subducted oceanic plate, he noted. For example, many sublithospheric diamonds are distinguished by isotopically light carbon — deficient in the heavy carbon isotope (13C)—which is associated with organic matter and is much more abundant in the oceanic plate than in the surrounding mantle.
Some sublithospheric diamonds also contain metallic inclusions enriched in heavy iron isotope (56Fe) and other diamonds are enriched with the light element boron. Both of these features are associated with the serpentinized peridotite of the mantle. Serpentinization occurs in rocks that incorporate seawater in subduction zones during seabed weathering and metamorphism.
The inclusions in the diamond indicate that the fluid-bearing rock is penetrating the mantle, and modeling by the researchers shows how it could be transported by cold subduction slabs. As seismologists refine their estimates of the locations of deep earthquakes, those locations may be better suited to the position of these slabs to further test these patterns, Shirey said.
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