[S-MP26] Deformed rocks, Metamorphic rocks and Tectonics

[SMP26-01] Deformation coupled with thermal event in the lithosphere-asthenosphere boundary zone: microstructural evidence from crystal interface migration in peridotite xenoliths

*Kazuhito Ozawa¹, Yuto Sato², Nasrrddine Youbi³, Moulay Ahmed Boumehdi³ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Guangzhou Institute of geochemistry, CAS, 3.Geology Department, Cadi Ayyad University)

Keywords:lithosphere-asthenosphere boundary, peridotite xenoliths, crystal interface migration, deformation, spinel lamellae

Deformation and thermal perturbation and their coupling in the lithosphere and asthenosphere boundary (LAB) zone are imperative for better understanding evolution of the lithosphere since the zone governs not only plate motion but also magma generation in the asthenosphere. We found microstructural evidence for crystal interface migration related to deformation taking place with an episodic thermal event in mantle peridotite xenoliths from two distinct localities. One from back-arc (Iichinomegata, Japan) and the other from continental regions (Middle Atlas, Morocco), in both of which lithosphere thinning events have been documented. The crystal interface migration is observed between pyroxenes and olivine in addition to grain boundary migration of pyroxenes, which are identified by the occurrence of spinel blebs in minerals in contact with neighboring pyroxenes with spinel lamellae (see Figure). The spinel blebs show similar morphological features and crystallographic orientation to those of spinel lamellae, and the occurrence show consistent orientation dependence in each sample relative to the foliation. Grain boundary migration of olivine is demonstrated by the presence of contrasting densities and structures of dislocations in the two contacting olivine grains on both sides of the grain boundary. It is thus argued that olivine-olivine grain boundary migration is driven by the difference in strain energy of neighboring grains (Toriumi, 1982). The pyroxene-olivine interface migration is distinct from grain boundary migration in the same mineral species such as olivine. The driving force of the pyroxene-olivine interface cannot be attributed solely to strain energy contrast because there is a large contrast in the strength of the two different mineral species. Moreover, mass conservation to modify the chemical composition of the interface zone should be taken into consideration. We propose that the driving force is a coupling of stress and thermal perturbation, which induced strain of pyroxene and olivine and chemical reaction including partial melting. This has an important implication for the dynamics of the LAB. Episodic deformation of the LAB zone is coupled with heating controlled by asthenosphere dynamics such as a hot upwelling accompanying magma generation, which brought mantle xenoliths to the surface.