Compositional Convection-Driven Differentiation in the Skaergaard Intrusion: A Reaction- Transport Model

Considerable debate has focused on the role of thermal versus compositional convection and late-stage melt and volatile migration in the differentiation of layered intrusions, including the Skaergaard Intrusion. The result of these coupled processes is a hierarchy of structures from textural re-equilibration, to mm-scale rhythmic layering, to large-scale mobilization and recrystallization involving melt and volatiles. In the Skaergaard Intrusion, there is evidence that the base of the intrusion crystallized from melts strongly enriched in iron, presumably derived from the walls and/or roof. To investigate the scenario that iron-rich melts migrated from or through the crystallizing walls and ponded on the floor, we developed a two-dimensional reaction-transport model having the projected cross-section of the intrusion. Simulations of coupled flow and reaction of melt, heat, and minerals were performed using the RCTMAG code developed by the authors. Processes include conservation of fluid mass, energy, advective and diffusive multicomponent transport, and crystallization/melting. Crystal-melt equilibria and compositions are treated using distribution coefficients based on literature values or derived from lab and/or field data. Permeability and porosity changes are coupled to crystallization and melting, with the resulting volume changes affecting flow. Simulations show that iron-rich melt develops within the sidewall mush and tends to migrate through the mush toward the base. Compositional convection dominates over thermal convection because heat loss through the walls and roof lead to crystallization and melt compositional changes, affecting density more than temperature. Chemical and thermal diffusion within the mush has subtle effects on mineral compositions and modes, primarily because water and alkalis diffuse faster than other components. The propensity for melt to migrate through the mush is clearly aided by the increase in iron and volatiles, counteracting the typical large increase in viscosity that accompanies silica-enrichment.