Laboratory Experiments with Aqueous Solutions Modelling Magma Chamber Processes II. Cooling and Crystallization along Inclined Planes

Abstract

Experiments have been carried out on the cooling and crystallization of aqueous solutions of Na₂CO₃, along an inclined plane. Compositional stratification is generated in homogeneous solutions both below an overhanging roof and above an inclined floor. Experiments with a ternary system (FeSO₄ — Na₂SO₄ — H₂O) along a 45° slope showed the same general behaviour.In addition, layering was produced in the solid. Experiments with initially stratified solutions of Na₂CO₃ showed that the density gradient suppressed vertical mixing as light fluid was released. The stratified solutions developed thermally driven double-diffusive layers and caused a cusped structure to develop in the crystallization front. Contours of constant Na₂CO₃ content in the solid product were discordant to the positions of the crystallization front. In experiments involving large volumes for which the fluid at any height remains essentially constant in composition, the Na₂CO₃ content of the solid increased outwards away from the plane. In experiments involving smaller volumes of fluid, slow cooling rates or more complete crystallization of the solution, the Na₂CO₃ content of the solid decreased outwards in later stages causing areversal. One experiment involved replenishment of the tank by an 8 cm layer of concentrated Na₂CO₃ solution emplaced at the base of the container. The resident stratified solution was passively lifted and caused the solid to temporarily melt back for a few hours before crystallization recommenced. The replenishment caused the Na₂CO₃ content of the solid to increase on the below slope side at all levels.

Marked discordances between rhythmic, phase and cryptic layering have been recorded in layered intrusions. These features have been interpreted in terms of crystallization along inclined margins from compositionally stratified magma (Wilson and Larsen, 1985) and by a lateral accretion model (Irvine et al., 1983) in which rock layers form from adjacent double-diffusive magma layers. The experiments confirm several features of the lateral accretion model: cusps occur adjacent to double-diffusive layers, compositional contours in the solid do eventually dip inwards to the centre of the tank and light residual liquid tends to move as a boundary layer flow parallel to the margin even along an inclined floor. However, the sense of circulation in the adjacent double-diffusive layers was driven by thermal rather than compositional effects and we failed to produce any solid layering adjacent to and equivalent to the double-diffusive layers. Compositional reversals were generated by replenishment. In the initial stages of experiments decreasing the cooling rate caused an increase in Na₂CO₃ content away from the slope and this may be one cause of the basal reversals in cryptic layering observed at the margins of many layered intrusions.