Self-organisation in mafic cumulates: differential migration of immiscible silicate liquids in the crystal mush

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• Marian Holness, University of Cambridge
• Tuesday 13 May 2025, 12:00-13:00
• Department of Earth Sciences, Tilley Lecture Theatre.

If you have a question about this talk, please contact Dr Rachael Rhodes.

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), recrystallisation during compaction, and by a reduction in the total grain boundary energy budget based on the assumption that the energy of boundaries between two grains of the same mineral is less than that between two grains of different minerals. These various mechanisms can be distinguished on the basis of their microstructural signatures. An investigation of the Stillwater inch-scale layering and similar layers in the Bushveld intrusion shows that the CPG patterning mechanism leaves a characteristic microstructural record preserving evidence for slow super- and sub-solidus cooling with a highly interconnected texturally equilibrated melt phase that enhanced Ostwald ripening. The Skaergaard intrusion locally preserves cm-scale micro-rhythmic layering, superimposed on single modally-graded layers. The microstructures in the Skaergaard example do not show evidence of CPG . Furthermore, the energy of all relevant hetero-phase interfaces is less than that of the associated grain boundaries in igneous and metamorphic rocks, there is no compelling evidence for compaction in the Skaergaard intrusion, and the supposition of micro-rhythmic layering on modally graded layers formed by sedimentation precludes patterning by oscillatory nucleation and growth. A new patterning mechanism is proposed, whereby immiscible conjugate silicate liquids in the crystal mush self-organise, due to differences in their wetting properties in the compositionally-graded mush and the positive feedback due to the fact that the two immiscible conjugates predominantly crystallise the minerals which they preferentially wet.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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• Department of Earth Sciences Seminars (downtown)
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