Microstructural evolution of silicate immiscible liquids in solidifying ferrobasalts

• Main Author: Honour, Victoria Claire
• Format: Thesis
• Language: English
• Published: Apollo - University of Cambridge Repository 2020
• Subjects: Fe-rich droplet; ferrobasalt; silicate liquid immiscibility; experimental petrology; heterogeneous nucleation; homogeneous nucleation; droplet coarsening; North-East England dykes; compositional boundary layers; atom probe tomography; igneous petrology; microstructures; Skaergaard; Skaergaard fieldwork; Laki; Iceland
• Online Access: https://dx.doi.org/10.17863/cam.50738; https://www.repository.cam.ac.uk/handle/1810/303661

Evidence of silicate liquid immiscibility in ferrobasalts is provided by co-existing Fe- and Si-rich melt inclusions and Fe-rich droplets dispersed in the Si-rich glassy mesostasis of rapidly cooled rocks. Crucially, the different physical properties of these unmixed liquids mean that they may migrate and separate within a granular medium, forming chemically distinct accumulations. I combine experiments, geochemistry, image analysis and field observations to better quantify the physical behaviour of emulsions in ferrobasaltic magmas. Quantification of the microstructural evolution of an emulsion in ferrobasaltic experiments shows that the Fe-rich liquid forms homogeneously nucleated droplets dispersed in an immiscible Si-rich liquid, together with droplets heterogeneously nucleated on plagioclase, magnetite, and pyroxene. Heterogeneous nucleation is likely promoted by localised compositional heterogeneities around growing crystals. The equilibrium wetting angle of Fe-rich droplets on both plagioclase and magnetite increases with decreasing temperature. Droplet coarsening occurs by diffusion-controlled growth (including Ostwald ripening), with an insignificant contribution from coalescence. The experimental observations are scaled to infer that in magma bodies < ~10 m in size, gravitationally-driven segregation of immiscible Fe-rich droplets is unlikely to be significant. The same concepts are investigated using natural samples with preserved immiscible textures found in tholeiitic basaltic glass from Hawaii (USA), the Snake River Plain (USA), and the Laki eruption (Iceland). High-resolution imaging, electron probe microanalysis, and atom probe tomography are combined to examine the role played by compositional boundary layers in promoting unmixing around growing crystals at melt-crystal interfaces. The effects of cooling rate on silicate liquid immiscibility microstructure are studied using basaltic dykes from Northeast England, coupled with simple 1D thermal models. The size of Fe-rich droplets within a continuous silicic phase is found to increase with decreasing cooling rate. At the even slower cooling rate of the Skaergaard Intrusion, field, whole rock and petrographic observations of late-stage immiscible segregations show that complete segregation of unmixed liquids on the metre scale is feasible; therefore, timescales of cooling are shown to be a key factor in immiscible liquid separation. : VCH was funded by a Natural Environmental Research Council Doctoral Training Programme studentship (Grant Ref: NE/L002507/1)