Fluid Assisted Recrystallization in Upper Mantle Peridotite Xenoliths from Kimberlites

The development of recrystallization microstructures has beenstudied in some ‘hot deformed’ peridotite xenolithsfrom the Thaba Putsoa Kimberlite pipe in S. Africa. The xenolithswere deformed to high strains by dislocation creep in the uppermantle and then annealed as they were uplifted by the kimberlitefluid. Static recrystallization occurs during annealing producingeuhedral shaped ‘tablet’ grains. Tablet grain boundariesare sub-parallel to crystal growth habits in olivine and orthopyroxene.This microstructure is characteristic of recrystallization byfluid-assisted grain boundary migration, where a thin fluidfilm is present along the boundary. There is microstructural evidence for a complex fluid infiltrationhistory involving an early Fe-Ti rich metasomatic silicate fluidand later kimberlite fluids. Minor partial melting of clinopyroxenecan also be inferred, which, is consistent with infiltrationof a kimberlite-derived C-H-O rich fluid into the xenoliths.Any of these fluids could have been present along the tabletgrain boundaries during static recrystallization. The occurrenceof tablet grains in ‘cold deformed’ xenoliths, whichhave a simple infiitration history, suggests that a C-H-O richfluid derived from kimberlite is the most probable boundaryfluid in both the hot and cold deformed xenoliths. The occurrence of dynamically stable semi-continuous grain boundaryfluid films during re crystallization indicates that mechanismsof fluid segregation and transport in the upper mantle are likelyto be dependent upon the type of deformation and recrystallizationmechanisms operating. In addition the destabilization of thestatic fluid distribution by grain boundary migration and deformationwill also influence the rheology of the upper mantle where fluidsare present. ^*Present address: Mineralogy Research Centre, Research School of Chemistry, Australian National University Canberra ACT 2601, Australia.