Deformation history of Pinatubo peridotite xenoliths: constraints from microstructural observation and determination of olivine slip systems

The deformation history of the Pinatubo peridotite xenoliths was estimated on the basis of the microstructural observations and the determination of olivine slip systems. The latter was performed by using three methods: lattice-preferred orientation (LPO), crystallographic analysis of subgrain boundaries, and direct characterization of dislocations. The Pinatubo peridotites are composed of coarse olivine grains containing numerous fluid inclusions and some fine aggregates of orthopyroxene and amphibole grains, which implies intense fluid–rock interaction. The development of euhedral fine recrystallized olivine grains along the healed cracks within the coarse olivine grains suggests that the strain-free grains were nucleated and grew during static recovery. The LPO patterns and the analyses of subgrain boundaries indicate the activation of a [100]{0kl} slip system that developed under high temperature, low pressure, and dry deformation conditions. Although dislocations showing the [100]{0kl} slip system are dominantly observed, the other slip systems which could be formed by the deformation under moderate–high water content and lower-temperature conditions are also developed. The discrepancy between the results of dislocation characterization and the other two methods might have been caused by fulfilling the von Mises criterion or overprinting dislocation microstructures. Either way, the possible deformation history of the Pinatubo peridotites can be explained by the following scenario. The peridotites plastically moved from the back-arc to the fore-arc adjacent region, where CO2-rich saline fluid was trapped, by the corner flow of a mantle wedge. They were then annealed and metasomatized during entrapment of the upwelling magma.