Kinetics and the extent of immiscibility in basaltic liquids

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Interpretation of sub-micron liquid-liquid phase separation in quenched silicate glasses is a longstanding problem of experimental petrology and material science. Nanoscale heterogeneity is usually interpreted as a product of sub-liquidus, metastable immiscibility. However, such heterogeneity may also form by super-liquidus, thermodynamically stable liquid immiscibility when phase separation and droplet growth are hampered by sluggish kinetics. New evidence from natural melt inclusions, previous experimental work and our own recent experiments imply that crystallization paths of common natural basaltic-andesitic liquids closely approach the compositional regions of liquid immiscibility. However, the systems show strong tendency to metastable crystallization, and experimental reproduction of liquid immiscibility is very difficult also because of kinetic barriers. Silicate melts in the vicinity of stable miscibility gaps may form long-lasting colloidal emulsions, which greatly complicate the distinction between stable and metastable unmixing. This research proposal addresses experimental problems that have been encountered by previous studies of immiscibility in aluminosilicate melts of geological importance. The distinction between stable and metastable immiscibility is the central theme of this project. We propose to study the formation and stability of sup er-liquidus silicate emulsions and monitor melt unmixing by a combination of in situ experimental techniques including high temperature centrifugation, viscosity measurements, Brillouin spectroscopy, small angle X-ray (SAXS) and neutron (SANS) scattering. The development of phase separation will be also documented by electron microprobe analyses and the statistical analysis of droplet size distribution in quenched glasses. We are planning to measure liquid-liquid interfacial energies and mineral-liquid wetting angles, which are crucial for the coarsening of emulsions and the mobility of immiscibJe liquids in a crystal mush. Experimentally measured physical properties will be used for quantitative interpretation of natural immiscibility recorded in volcanic glasses, melt inclusions and textures of fully crystallized plutonic rocks. This will be done in collaboration with an international team of igneous petrologists and experts on layered gabbroic intrusions, The combination of experimental studies with detailed documentation of natural igneous rocks has been very productive so far, and we are planning to continue this fruitful collaboration. Special attention will be given to relationships between unmixing and crystallization in slowly cooling magma chambers. Liquid immiscibility, even if it does not develop beyond the nanoscale emulsions, is expected to have profound effects on magma dynamics and differentiation in volcanic and plutonic environments.