Fluids in the lithosphere, 1. Experimentally-determined wetting characteristics of CO 2[sbnd]H 2O fluids and their implications for fluid transport, host-rock physical properties, and fluid inclusion formation

Abstract

The equilibrium distribution of CO 2 H 2O fluids in synthetic rock samples (principally dunite and quartzite) has been characterized by measurements of the dihedral wetting angle, θ, resulting from 5-day annealing periods at 950–1150°C and 1 GPa. For fluids in equilibrium with polycrystalline quartz, θ varies systematically from ∼ 57° for pure H 2O to ∼ 90° at X CO 2 ∼ 0.9 . Similarly, for San Carlos olivine, θ varies from ∼ 65° for pure H 2O to ∼ 90° at X CO 2 ∼ 0.9 . The addition of solutes (NaCl, KCl, CaF 2, Na 2CO 3) to H 2O causes a major decrease in θ in the quartz/fluid system (to values as low as 40°), but has no effect on fluid wetting in dunite. Reconnaissance experiments on other mono- and polymineralic aggregates indicate universally high wetting angles ( θ ≫ 60°) in upper mantle assemblages and for CO 2 in felsic compositions. For diopside + H 2O, θ ∼ 80°, with large variation due to crystalline anisotropy. In no case does θ approach 0°, the condition necessary for fluid to be present along all grain boundaries. Because a value of θ greater than 60° precludes the existence of an interconnected fluid phase in a rock, our results have important implications not only for fluid transport but also for the physical properties of the bulk fluid/rock system. Any static fluid present in the upper mantle must exist as isolated pores located primarily at grain corners, and transport can occur only by hydrofracture. In the continental crust, aqueous fluids (especially saline ones) are likely to form an interconnected network along grain edges, thus contributing to high electrical conductivity and allowing the possibility of fluid transport by porous flow or surface energy-driven infiltration.