A TEM look at microlite and nanolite growth in erupting basalts.

Microlite crystals, 1-30 microns across, are common in the products of explosive basaltic eruptions. They represent precious tools to investigate magma evolution immediately before or during eruptions, and their presence affects eruption dynamics by changing the physicochemical properties of magma. Nanolite crystals, 0.03-1 microns across, are less studied than microlites but increasingly recognized as key modifiers of magma rheology and eruption dynamics. Growing microlites (and crystals in general) pushes incompatible elements into the surrounding melt, forming a so-called compositional boundary layer, or CBL. Here we focus on micro- to nano-scale features of CBLs around microlites that are either intact or broken during magma fragmentation. Study cases include lapilli of mafic composition from Etna and Stromboli (Italy), Xitle (Mexico), and Cumbre Vieja (Spain) volcanoes. Samples were investigated by using Transmission Electron Microscopy for high-resolution imaging, EDS analysis and mapping, SAED, and 4D-STEM. In the CBL around plagioclase microlites and inside glass-filled fractures within, we found evidence of liquid immiscibility, with droplets of a denser phase dispersed in a lighter phase. The denser phase is enriched mostly in Fe and variably in Ti, Mg, Ca, while the lighter phase is depleted in the above elements. The size of the droplets of the denser phase decreases away from the CBL. At the microlite surface, the denser phase often crystallizes into Fe-oxides (magnetite) nanolites, mostly a few tens of nm in size, and, occasionally, into clinopyroxene (augite) nanolites. Incipient crystallization of the denser liquid droplets into nanolites suggest that CBL development and consequent liquid immiscibility are key steps leading to local nanolite enrichment of basaltic melts, which ultimately can affect the bulk viscosity of the magmatic suspension and its rheological response to deformation during fragmentation.