A high-order numerical study of reactive dissolution in an upwelling heterogeneous mantle—I. Channelization, channel lithology and channel geometry

SUMMARY High-porosity channels are important pathways for melt migration in the mantle. To better understand the lithology and geometry of high-porosity melt channels, we conduct high-order accurate numerical simulations of reactive dissolution along a solubility gradient in an upwelling and viscously deformable porous column. In contrast to earlier studies, we assume the dissolution reaction to be at equilibrium, consider a finite soluble mineral abundance, and employ a high-order accurate numerical scheme. Using sustained perturbations in porosity and soluble mineral abundance at the inflow boundary, we explore the structure of steady-state high-porosity melt channels and their associated lithologies over a range of parameters, including soluble mineral abundance, solubility gradient, amplitude and lateral variation in inflow melt flux, melt fraction and upwelling rate. In general, high-porosity dunite channels are transient and shallow parts of pathways for melt migration in the mantle. The lower parts of a high-porosity channel are orthopyroxene-bearing dunite, harzburgite and possibly lherzolite. A wide orthopyroxene-free dunite channel may contain two or three high-porosity melt channels. The depth of dunite channel initiation depends on the solubility gradient, soluble mineral abundance, inflow melt flux, melt suction rate and upwelling rate. The amplitude and length scale of lateral variation in porosity and orthopyroxene abundance at the base of the upwelling column are important in determining the size and dimension of dunite channels, the strength of melt focusing and the melt suction rate, the presence of compacting boundary layer, as well as the number of high-porosity melt branches within a dunite channel. The spatial relations among the high-porosity melt channels, dunite and harzburgite channels documented in our numerical simulations may shed new light on a number of field, petrological and geochemical observations related to melt migration in the mantle.