Post-emplacement melt flow induced by thermal stresses: Implications for differentiation in sills

We present the first steps of a new explanation model for differentiation in sills, using a combination of geochemical data and field observations, numerical modeling and dimensional analysis. Geochemical data from a saucer-shaped dolerite sill intruded into the Karoo basin, South Africa reveal a differentiation process which causes D-shaped profiles. The geometry name is based on the variation in whole-rock Mg-number (Mg# = Mg/(Mg + Fe)) from floor to roof in a sill; the D-shaped geochemical profiles represent sheet-intrusions with the most primitive composition (i.e. high Mg#) in its center, and progressively more evolved composition (i.e. low Mg#) towards the upper and lower margins. The differentiation is reversed compared to the normal differentiation produced by fractional crystallization (C-shaped profiles). C-shaped profiles are believed to be formed by segregation of crystals from the magma. We propose that the opposite, the D-shaped profile, may result from melt segregation from the crystal mush. This is achieved by porous melt-flow through a consolidated crystal network after the main phase of emplacement, and before complete solidification. We show that a significant flow is feasible under natural occurring conditions. An underpressure of magnitude 108 Pa develops at the cooling margins due to volume reduction of the crystallizing porous melt. The resulting pressure gradient is the driving force for the melt-flow towards cooling margins considered in this work. As a result the margins will be enriched in the incompatible elements associated with the melt phase, while the center will be depleted. We show that the amount of flow is primarily a function of viscosity of the melt and permeability of the crystal network, which in turn is a transient phenomenon dependent on a number of parameters. Diagrams have been constructed to evaluate the feasibility of substantial melt extraction in terms of these poorly constrained parameters. Data from the Golden Valley Sill and many other natural occurrences of D- and I-shaped geochemical profiles show a reasonable agreement with our predictions of melt flow potential, and are thus well explained by the presented model. We conclude that in order to fully understand differentiation processes occurring in sheet intrusions, we need to account for post emplacement segregation of melt from crystals, and not only segregation of crystals from melt.