Superplastic mantle minerals

Superplastic mantle minerals. Superplasticity is the ability of some crystalline materials to stretch up to several times their own length when heated. Although the minerals in Earth’s mantle don’t endure such large strains, circumstantial evidence suggests that superplasticity helps them respond to the subduction of continental plates and other tectonic processes. Now, a team led by Takehiko Hiraga of Tokyo University and Hidehiro Yoshida of Japan’s National Institute for Materials Science has found direct evidence that mantle minerals are indeed superplastic. Like other superplastic materials—real or presumed—those in the mantle are polycrystalline aggregates. For their study, Hiraga, Yoshida, and their team sintered nanoscale powders to make two analogues of mantle minerals, both of which consisted mostly of forsterite (Mg₂SiO₄). In the absence of strain, a superplastic material is made up of nanoscale grains of the majority component interspersed with smaller grains of the minority component. When heated under strain, the majority and minority grains both grow by merging with their neighbors. That response ensures that grains continue to abut each other, forestalling failure of the bulk material. As the accompanying figure shows for samples that consisted of 90% forsterite and 10% periclase (MgO), the materials could withstand strains of more than 500%. Moreover, two electronic diagnostics, electron back-scattered diffraction and transmission electron microscopy, revealed that grains in the mantle analogues grew like grains in materials whose superplasticity is established. Having measured the temperatures and strain rates for which mantle analogues become superplastic, the team estimated that superplasticity could help Earth’s mantle accommodate a 200-km slab that takes 60 million years to penetrate 3000 km. ( T. Hiraga, T. Miyazaki, M. Tasaka, H. Yoshida, Nature NATUAS 468, 1091, 2010.)