An experimental investigation on fluid transfer mechanisms in ultramafic rocks

Static annealing and crack-cook experiments were performed on wet and dry fabricated San Carlos olivine aggregates and a core of Balsam Gap dunite to investigate fluid infiltration mechanisms for low-permeability mantle rocks. Deionized water (30–40 μL) was added to cold-pressed olivine powders before hot-pressing to make wet samples; samples hot-pressed without added water are dry (<30 H/10⁶ Si). Talc was added to all samples to provide fluid during the experiments. All samples were annealed at 1200 °C and 300 MPa confining pressure for 3 h, but crack-cook experiments were subjected to a 300–500 MPa axial differential stress at 600 °C and 125 MPa confining pressure before annealing. All samples contain abundant fluid inclusions (FIs) after annealing. Olivine aggregate samples show FI-rich and FI-depleted regions. Grain size was reduced in all samples, particularly in FI-rich regions. Crosscutting FIs associated with fractures are only widespread in the dunite. Fluids were primarily transferred from talc along grain boundaries in olivine aggregate samples and along fractures in dunite. FIs inhibit grain growth and grain-boundary migration, leading to smaller than expected grain sizes at the experimental conditions. Results have important implications where excess fluid may affect grain-size sensitive deformation mechanisms in the upper oceanic lithosphere.