According to a new study presented at the Seismological Society of America’s Annual Meeting, the presence of diamonds deep in the Earth suggests that fluids play an essential role in generating earthquakes at depths where high pressures should keep brittle failure from happening.
Although fluid-assisted faulting in subducted slabs 300 to 700 kilometers deep – in the transition zone between the upper and lower mantle – may explain deep earthquakes, evidence for this process was scarce until recently. However, the discovery of diamonds formed in this transition zone (“sublithospheric diamonds”) provide clear evidence of the presence of fluids carried by subducting slabs, since these diamonds’ internal growth structure indicate that they are formed from fluids moving through host rock.
“I think it’s up to the seismological community at this point to try to understand why fluids would be important,” said study lead author Steven Shirey, a geochemist of the Carnegie Institution for Science. “We know fluids are down there, we know they’re moving, and the petrology of the diamonds tell you that, because these diamonds always form in regions of the mantle where fluids are moving. Diamonds tell you there is a crack or vein relationship with host mantle. You have to have fluids moving through host rock in veins or cracks and equilibrating with it.”
According to Dr. Shirey and his colleagues, deep earthquakes – occurring 500 to 700 kilometers below the Earth’s surface, appear to happen only in the slabs which can transport water or that can carry carbonate deep enough to trigger melting. At such depths, water or carbonate fluids could be triggering the earthquakes, or the earthquakes could trigger fluid release, or both things could happen.
The distinctive chemistry and inclusions of sublitospheric diamonds are telltale signs of their origin from the subducted oceanic plate. They indicate that fluid-bearing rock does make its way into the mantle, and is carried by cold subducting slabs, ultimately giving rise to deep earthquakes. Further research is needed to map the precise locations where these earthquakes form, by clarifying the position of the slabs.