Self-organisation in gabbroic cumulates: a new patterning mechanism driven by differential migration of immiscible liquids in a crystal mush?

Abstract

Self-organisation in plutonic igneous rocks has been suggested to form by a variety of mechanisms including oscillatory nucleation and growth, competitive particle growth (CPG), and preferential dissolution and re-precipitation during fluid infiltration enhanced by compaction, with driving forces including reduction of the interfacial energy budget by either Ostwald ripening or because the energy of boundaries between two grains of the same mineral is less than that between two grains of different minerals. An investigation of the Stillwater inch-scale layering shows that the CPG patterning mechanism leaves a characteristic microstructural signature preserving evidence for a highly interconnected melt in textural equilibrium, and slow super- and sub-solidus cooling: such a signature is also preserved in chromite-bearing fine-scale layers in the Bushveld intrusion. The cm-scale micro-rhythmic layering of the Skaergaard intrusion, superimposed on single modally-graded layers, does not have these microstructural features. Furthermore, the energy of all relevant inter-phase grain boundaries in the Skaergaard gabbros is less than that of grain boundaries involving only one mineral, viscous compaction was not a significant process in the Skaergaard intrusion, and patterning by oscillatory nucleation and growth is precluded by the fact that the micro-rhythmic layering is superimposed on modally graded layers formed by sedimentation. A new patterning mechanism is proposed, operational only in intrusions in which the interstitial liquid of the crystal mush intersects a binode and splits into two immiscible conjugates. Cm-scale separation of the immiscible conjugate liquids in a compositionally-graded mush, due to both gravity and capillary forces, leads to layering due to differences in their wetting properties. The positive feedback required for pattern formation is due to the two immiscible conjugates predominantly crystallising the minerals which they preferentially wet.