Host: Abstracts of Annual Meeting of the Geochemical Society of Japan
Pages 62-
The interaction of hydrous fluids and melts with dry rocks of the lithospheric mantle has pronounced effects on its viscoelastic and chemical properties due to the formation of compositionally distinct secondary phases. In addition, melt percolation and the associated metasomatic alteration of mantle rocks may also facilitate alteration of the pre–existing rock texture and olivine crystallographic preferred orientation (CPO) and thus seismic properties. Here we explore the relationship between mantle metasomatism, deformation and seismic anisotropy using subduction–related mantle xenoliths from the Penghu Islands, western Taiwan. The investigated xenoliths have equilibrated at upper lithospheric mantle conditions (879°C to 1127 °C) and show distinct variations in their chemical composition, texture and olivine CPO allowing for the classification of two distinct groups. Group 1 xenoliths show a porphyroclastic texture, olivine grains are mostly characterized by [100]–axial pattern symmetries and are chemically depleted. In contrast, chemically enriched Group 2 xenoliths are characterized by a dominantly fine–grained equigranular texture and olivine frequently displays [010]–axial pattern symmetries. The different chemical compositions are indicative of cryptic and modal to stealth metasomatic alteration of Group 1 and Group 2 xenoliths, respectively. Furthermore, the observed olivine [100]–axial pattern of Group 1 xenoliths reflects deformation by dislocation creep under high temperature, low pressure and dry conditions, whereas olivine [010]–axial patterns of Group 2 xenoliths imply activation of olivine [001] glide planes along preferentially wetted (010) grain boundaries. This correlation indicates that the variation in olivine CPO symmetry from [100]– to [010]–axial pattern in Penghu xenoliths results from deformation and intracrystalline recovery by subgrain rotation during metasomatic alteration induced by melt percolation. The microstructural observations and olivine CPO combined with petrological and geochemical data suggest that Group 1 xenoliths preserve microstructural and chemical characteristics of an old, probably Proterozoic lithosphere, while Group 2 xenoliths record localized Miocene deformation associated with wall–rock heating and metasomatism related to melt circulation. Furthermore, the observed transition of olivine CPO from [100]–axial pattern to [010]–axial pattern by deformation in the presence of variable melt fractions and associated metasomatic alteration can be inferred to have a significant effect on physical properties of mantle rocks.
