When a tectonic plate is linked to the Earth’s thin crust, it softens significantly. This is proved by X-ray investigations on a mineral found in high amounts in basaltic crust using DESY’s X-ray source PETRA III. According to a multinational team lead by Hauke Marquardt of the University of Oxford, this weakening may even cause the crust to peel away from the underlying plate, as reported in the scientific journal Nature. The physical parameters of the delaminated crust vary from those of the rest of the mantle, which may explain anomalies in the speed with which seismic waves travel through the mantle.
For the first time, scientists were able to analyze the deformation of the mineral davemaoite in circumstances similar to those found in the Earth’s mantle. “Davemaoite belongs to the broad group of materials known as perovskites, but it is only formed from other minerals at depths of about 550 kilometers and beyond due to increasing pressure and temperature,” explains lead author Julia Immoor of the Bavarian Research Institute of Experimental Geochemistry and Geophysics at the University of Bayreuth. The mineral’s presence had been anticipated for decades, but it wasn’t discovered until 2021 that a natural sample of it was discovered. Davemaoite varies from other perovskites in many ways, including its cubic crystal structure. It can account for around a quarter of the descending basaltic oceanic crust at large enough depths.
The team has now succeeded in artificially synthesizing davemaoite and studying it with X-rays using a special device at DESY’s Extreme Conditions Beamline (P02.2) at PETRA III. To do this, the scientists heated finely crushed wollastonite (CaSiO3) at high pressure to roughly 900 degrees Celsius until davemaoite was created. The mineral was then distorted and inspected using X-rays after being subjected to escalating pressures of up to 57 gigapascals (about 570,000 times atmospheric pressure at sea level). These characteristics are representative of the circumstances experienced at depths of up to 1300 kilometers.
“Our results suggest that davemaoite is remarkably soft beneath Earth’s lower mantle,” says the study’s lead author, Hauke Marquardt. “This discovery fundamentally alters our understanding of the dynamic behavior of subducting slabs in the lower mantle.” The dynamics in these so-called subduction zones, when one tectonic plate dives under another, are heavily influenced by the hardness of the minerals present. Because davemaoite is remarkably soft, it may cause the descending crust to split from the underlying plate, causing the subduction process to occur independently for the crust and the remaining plate.
Scientists have long theorized about such a separation because the divided crust might generate the typical differences in seismic wave velocities seen at various depths. However, until recently, it has been unknown what causes such a delamination. “I am pleased that the experimental setup we have developed here is capable of assisting in the resolution of important questions related to processes occurring deep within our planet,” says Hanns-Peter Liermann of DESY, who is in charge of the Extreme Conditions Beamline at PETRA III and a co-author of the study.
The study included researchers from the Universities of Bayreuth, Oxford, and Utah, as well as the GFZ German Research Centre for Geosciences in Potsdam, the California Institute of Technology, and DESY. The project was partially financed by the Deutsche Forschungsgemeinschaft DFG.