FLUID ASSISTED RECRYSTALLIZATION IN UPPER MANTLE PERIDOTITE XENOLITHS FROM KIMBERLITES

Abstract

The development of recrystallization microstructures has been studied in some ‘hot deformed’ peridotite xenoliths from the Thaba Putsoa Kimberlite pipe in S. Africa. The xenoliths were deformed to high strains by dislocation creep in the upper mantle and then annealed as they were uplifted by the kimberlite fluid. Static recrystallization occurs during annealing producing euhedral shaped ‘tablet’ grains. Tablet grain boundaries are sub-parallel to crystal growth habits in olivine and orthopyroxene. This microstructure is characteristic of recrystallization by fluid-assisted grain boundary migration, where a thin fluid film is present along the boundary. There is microstructural evidence for a complex fluid infiltration history involving an early Fe-Ti rich metasomatic silicate fluid and later kimberlite fluids. Minor partial melting of clinopyroxene can also be inferred, which, is consistent with infiltration of a kimberlite-derived C-H-O rich fluid into the xenoliths. Any of these fluids could have been present along the tablet grain boundaries during static recrystallization. The occurrence of tablet grains in ‘cold deformed’ xenoliths, which have a simple infiitration history, suggests that a C-H-O rich fluid derived from kimberlite is the most probable boundary fluid in both the hot and cold deformed xenoliths. The occurrence of dynamically stable semi-continuous grain boundary fluid films during re crystallization indicates that mechanisms of fluid segregation and transport in the upper mantle are likely to be dependent upon the type of deformation and recrystallization mechanisms operating. In addition the destabilization of the static fluid distribution by grain boundary migration and deformation will also influence the rheology of the upper mantle where fluids are present.