Buoyant Migration of Melt with Variable Physical Properties: Effect of Melt Viscosity and Its Dependence on Volatiles

Abstract:

Release of a volatile phase, usually considered to be water, from the downgoing plate and the upward migration of volatile saturated melt toward higher temperatures in the interior of the wedge is one mechanism for generation of melt at a convergent plate boundary (Gill, 1981; Grove et al. 2012). However, other volatiles, including CO₂, may be released. Adding CO₂ reduces the water mole fraction in melt that is stable at a given temperature. Dissolved water has a large effect on melt viscosity; but, CO₂ has practically none, leading to the possibility of significant viscosity variations in melt rising through the mantle wedge. This may have important implications for the heterogeneity of erupted melts as discussed below.

“Saffman-Taylor” instability occurs as a low viscosity fluid flowing in porous media displaces another of higher viscosity. The low viscosity fluid forms fingers that extend progressively into the high viscosity fluid. For miscible fluids (no surface tension effects) in a non-compacting matrix (Chouke, 1982), the development of fingers is controlled by interdiffusion of the fluids.

Numerical experiments to be reported examine viscous fingering in a compacting permeable matrix at conditions appropriate for melt generation in a mantle wedge. Mixing of melts with different CO₂/water by diffusion in silicate melts alone is generally slow; however, fingering reduces the scale of CO₂/water heterogeneity making diffusion more effective. We explore the persistence of a CO₂/water heterogeneity of a given scale rising through the mantle wedge at a rate fast enough to preserve ²³⁰Th disequilibrium. The rise height over which heterogeneity can persist as fingers develop depends on the viscosity, i.e. CO₂/water, variation initially present; viscosity variations on the order of 10% allow km-scale heterogeneity to persist over vertical scales comparable to the height of the wedge.