EFFECTS OF DEFORMATION ON FLUID DISTRIBUTION IN AGGREGATES

The distribution of fluid in aggregates has important consequences for deformation mechanisms, rheology and bulk transport. Experimental studies reviewed here show that differential stress may affect the wetting angle or cause fluid to coalesce in aligned pockets, and finite strain resulting in reduced grain size and/or LPOs may increase fluid interconnectedness.

Calculations suggest that low differential stresses are sufficient to change the wetting angle. High P and T experiments on fine-grained Ab and Or aggregates show that the aqueous fluid distribution changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation at stresses <25 MPa; the isolated pore distribution is rapidly regained during hydrostatic annealing after deformation. The deformation-enhanced fluid distribution results in very low strength, and increases bulk transport rate through the aggregate by >x10. However, a change in fluid distribution is not observed in fine-grained quartz or An aggregates or granite deformed at similar conditions. Deformation appears to enhance the formation of ultra-thin amorphous films along olivine grain boundaries, but these do not decrease strength.

Fluids tend to form aligned pockets above critical levels of differential stress, grain size and fluid fraction (Daines & Kohlstedt, 1997, JGR, 102, 10257). For Pfl=Pconf these pockets lie at 30 deg to compression whereas for Pfl=mean stress they are parallel to compression. Axial compression and shear experiments on olivine aggregates with basaltic melt develop pockets at ~20 deg; in samples held hydrostatically following deformation, the melt re-equilibrates into shorter and randomly oriented pockets, or along particular crystal planes aligned by dislocation creep. Similar results have been found for partial melt in granitic aggregates and for aqueous fluid in quartz aggregates.

There are contradictory results on whether fluids move onto actively migrating grain boundaries. Aqueous fluid spreads onto bischoffite grain boundaries during dynamic recrystallization, and pulls back to isolated pores when deformation ceases. High P and T experiments on synthetic peridotite with partial melt found evidence for similar behavior, but lower P experiments on olivine aggregates with basaltic melt did not.