Silicate Liquid Immiscibility in Layered Intrusions

Abstract

More and more evidence for the development of silicate liquid immiscibility during cooling of magmas in layered intrusions have been presented. Here, we review some theoretical principles with a focus on the separation of two silicate melts, i.e. silica-rich vs. iron-rich. We discuss the role of melt structure and present phase equilibria relevant to stable and metastable immiscibility. The understanding of immiscibility in magmas has strongly benefited from recent progress in experimental approaches. Kinetics studies evidence the importance of nucleation barriers in producing unmixing, coarsening and potential separation of equilibrium melts. Improvement of analytical tools has also enabled detailed study of major and trace element partitioning. The study of immiscible emulsion in volcanic rocks also brings important information on the evolution of plutonic systems and on the potential formation of compositional gap along liquid lines of descent. We then present the most recent evidence for immiscibility in some major layered intrusions, i.e. the Skaergaard, Sept Iles, intrusions of the Emeishan province, and the Bushveld complex. Paired melts are identified as contrasted melt inclusions trapped in apatite and their segregation can be responsible for the formation of Fe–Ti–P-rich rocks. We finally discuss more broadly the potential effect of immiscibility in interstitial melt and the implications on the evolution of the crystal mush.