Microstructural evolution of silicate immiscible liquids in ferrobasalts

Abstract

An experimental study of the microstructural evolution of an immiscible basaltic emulsion shows that the Fe-rich liquid forms homogeneously nucleated droplets dispersed in a continuous 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 wetting angle of Fe-rich droplets on both plagioclase and magnetite increases with decreasing temperature. Droplet coarsening occurs by a combination of diffusion-controlled growth and Ostwald ripening, with an insignificant contribution from coalescence. Characteristic microstructures resulting from the interaction of immiscible Fe-rich liquid with crystal grains during crystal growth can potentially be used as an indicator of liquid unmixing in fully crystallised natural samples. In magma bodies < ~ 10 m in size, gravitationally driven segregation of immiscible Fe-rich droplets is unlikely to be significant.

The effect of cooling rate on immiscible silicate liquid microstructure: an example from the Palaeogene dykes of Northeast England

“…Note, this is likely to be a maximum timescale as the glass transition temperature increases with faster cooling rates (Giordano et al , 2005) and the Kīlauea Iki lava lake cooling rate was three orders of magnitude slower than even the central regions of the dykes studied here (Helz et al , 2014). The results of Honour et al (2019 b ) show that 60 h is insufficient time for coarsening of the Fe-rich droplets to be a result of Ostwald ripening.…”

“…The coarseness of Fe-rich droplets in unmixed silicate melts is a function of diffusion-controlled droplet growth, Ostwald ripening and/or coalescence. Diffusion-controlled Fe-rich droplet growth is driven by widening of the silicate liquid immiscibility binodal during cooling or chemical equilibration (Honour et al , 2019 b ). Ostwald ripening is driven by differences in the surface curvature of the Fe-rich droplets as a function of size (Ostwald, 1897; McNaught, 1997).…”

“…Coalescence occurs as droplets collide and is a function of droplet radius, viscosity, density and the interfacial energy (Yao et al , 2005). Recent progress in defining the binodal for silicate liquid immiscibility (Charlier and Grove, 2012) and investigating the microstructure of unmixing magmas with constant cooling rates (Farr et al , 2017; Honour et al , 2019 b ) indicates that liquid composition may also affect droplet size due to an expansion or reduction in the volume of Fe-rich liquid.…”

Settling of Immiscible Droplets: A Theoretical Model for the Missing Link Between Microscopic and Outcrop Observations

“…Journal of Geophysical Research: Solid Earth ZHANG ET AL. (Vona et al, 2011) and interact with the immiscible melt droplets (Honour et al, 2019). Thus, the application of this model is limited to aphyric magmas (i.e., magmas with a small fraction of suspended crystals).…”

“…This process can lead to unmixing of homogeneous silicate melt to form ferrobasaltic and andesitic-to-rhyolitic melts (hereafter referred to as "iron-rich" and "silica-rich" melts, respectively). Microscopic exsolution of immiscible silica-rich (or iron-rich) droplets has been observed in both laboratory experiments and volcanic rocks (Charlier & Grove, 2012;Dixon & Rutherford, 1979;Honour et al, 2019;Hou et al, 2018;Philpotts, 1982). This unmixing process has been proposed to have produced mafic and/or felsic plutonic outcrops, and magnetite-apatite deposits (Charlier et al, 2011;Chen et al, 2010;McBirney, 1975;Tornos et al, 2017;VanTongeren & Mathez, 2012).…”