AN EXPERIMENTAL INVESTIGATION INTO FLUID TRANSFER MECHANISMS IN THE MANTLE LITHOSPHERE

Static annealing and crack-cook experiments were performed on wet and dry synthetic San Carlos olivine aggregates and a core of Balsam Gap dunite to investigate fluid infiltration mechanisms for low-permeability mantle rocks. Wet samples were fabricated by adding 30 to 40 microliters of deionized water to cold-pressed olivine powders before hot-pressing; samples hot-pressed without added water are considered to be dry (< 30 H/10⁶ Si). Talc jackets were fitted around wet and dry olivine aggregate and dunite samples. One dry olivine sample was cored out to insert an inner talc cylinder. All experiments were annealed at 1200°C and 300 MPa confining pressure for three hours. Prior to annealing, crack-cook experiments (wet olivine and dunite) were subjected to a 300-500 MPa axial differential stress for 5-10 minutes at 600 °C and 125 MPa confining pressure. Whether initially wet or dry, samples contain abundant intragranular and intergranular fluid inclusions (FIs) after annealing, and samples with higher available water content display more abundant FIs. All olivine aggregate samples annealed with talc contain FI-rich regions that contain a higher abundance of intergranular FIs. Compared to the starting material, grain size was reduced in all samples and is smaller in FI-rich regions. In samples examined after the annealing experiments, secondary FIs along healed fractures are only observed in the initially dry olivine sample with a talc jacket, whereas in samples after crack-cook experiments secondary FIs are only observed in the core of dunite. Fluids are thought to have been transferred from dehydrating talc into the sample along grain boundaries in olivine aggregate samples, whereas the dominant fluid infiltration mechanism in dunite was along fractures. FIs appear to inhibit grain growth and grain boundary migration, leading to smaller grain sizes than would be expected for these experimental conditions. Grain boundary migration was limited during annealing owing to early formed intragranular FIs having been swept to grain boundaries and/or excess fluids trapped at the grain boundary (i.e. Zenner pinning). Results have important implications for experiments conducted with excess water, as well as for grain-size sensitive deformation mechanisms where excess fluid is available in the upper oceanic lithosphere.