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SUMMARY:Self-organisation in mafic cumulates: differential migration of im
 miscible silicate liquids in the crystal mush - Marian Holness\, Universit
 y of Cambridge
DTSTART:20250513T110000Z
DTEND:20250513T120000Z
UID:TALK224734@talks.cam.ac.uk
CONTACT:Dr Rachael Rhodes
DESCRIPTION:Self-organisation in plutonic igneous rocks has been suggested
  to form by a variety of mechanisms including oscillatory nucleation and g
 rowth\, competitive particle growth (CPG)\, recrystallisation during compa
 ction\, 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 micros
 tructural signatures. An investigation of the Stillwater inch-scale layeri
 ng and similar layers in the Bushveld intrusion shows that the CPG pattern
 ing mechanism leaves a characteristic microstructural record preserving ev
 idence for slow super- and sub-solidus cooling with a highly interconnecte
 d texturally equilibrated melt phase that enhanced Ostwald ripening. The S
 kaergaard intrusion locally preserves cm-scale micro-rhythmic layering\, s
 uperimposed on single modally-graded layers. The microstructures in the Sk
 aergaard example do not show evidence of CPG. Furthermore\, the energy of 
 all relevant hetero-phase interfaces is less than that of the associated g
 rain 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 sedimentatio
 n precludes patterning by oscillatory nucleation and growth. A new pattern
 ing mechanism is proposed\, whereby immiscible conjugate silicate liquids 
 in the crystal mush self-organise\, due to differences in their wetting pr
 operties in the compositionally-graded mush and the positive feedback due 
 to the fact that the two immiscible conjugates predominantly crystallise t
 he minerals which they preferentially wet. 
LOCATION:Department of Earth Sciences\, Tilley Lecture Theatre
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