Experimental observation on grain boundaries affected by partial melting and garnet forming phase transition in KLB-1 peridotite

•Microstructures of recovered samples from natural peridotite KLB-1: partial melting at 1.5 GPa, spinel peridotite to garnet peridotite phase transition at 5 GPa.•Static melting is dominated by grain boundary melting for times less than 1 min.•The solid–solid phase transition shows analogous microstructures as static partial melting.•Grain boundary sliding (GBS) could happen in multi-phase rocks and may cause strain weakening at mantle condition.

We report the microstructures of two different groups of recovered samples from natural peridotite KLB-1: one has gone through partial melting at 1.5 GPa, and the other group has gone through a solid phase transition from spinel peridotite to garnet peridotite at 5 GPa. Static melting in the KLB-1 sample is dominated by grain boundary melting for times less than 1 min. The melt evolves to the triple junctions for longer times (t > 30 min). Analogous microstructures were also observed for the solid–solid phase transition studies in which spherical garnet grains form both in triple junctions and along olivine grain boundaries, while thin films (500 nm width) inter granular residues remain between the faceted olivine grains, which could be the relic of pyroxenes. The evolution of such morphologies has implication for the mechanical properties of the material.In addition, partial melting of KLB-1 peridotite occurs fast on a minute’s time scale. The chemistry for constructing the new phase is mostly available in adjacent grains, thus requiring little diffusion. Furthermore, grain boundary diffusion is probably much faster than bulk diffusion thereby facilitating the phase transitions. Thus the melting rate may be faster than the seismic time scale and may be a dominant factor in defining the seismic velocity and attenuation of partially molten regions.