Inferring the Effects of Compositional Boundary Layers on Crystal Nucleation, Growth Textures, and Mineral Chemistry in Natural Volcanic Tephras through Submicron-Resolution Imaging

Abstract

Crystal nucleation and growth are first order processes captured in volcanic rocks and record important information about the rates of magmatic processes and chemical evolution of magmas during their ascent and eruption. We have studied glass-rich andesitic tephras from the Central Plateau of the Southern Taupo Volcanic Zone by electron-and ion-microbeam imaging techniques to investigate down to sub-micrometer scale the potential effects of compositional boundary layers (CBLs) of melt around crystals on the nucleation and growth of mineral phases and the chemistry of crystal growth zones. We find that CBLs may influence the types of mineral phases nucleating and growing, and growth textures such as the development of swallowtails. The chemistry of the CBLs also has the capacity to trigger intermittent overgrowths of nanometer-scale bands of different phases in rapidly growing crystals, resulting in what we refer to as cryptic phase zoning. The existence of cryptic phase zoning has implications for the interpretation of microprobe compositional data, and the resulting inferences made on the conditions of magmatic evolution. Identification of cryptic phase zoning may improve thermobarometric estimates and thus geospeedometric constraints. In future, a more quantitative characterization of CBL formation and its effects on crystal nucleation and growth may contribute to a better understanding of melt rheology and magma ascent processes at the onset of explosive volcanic eruptions, and will likely be of benefit to hazard mitigation efforts.

Eruption style and crystal size distributions: Crystallization of groundmass nanolites in the 2011 Shinmoedake eruption

“…This shows that the growth rates of the pyroxene and magnetite nanolites were rapid enough to form diffusive boundary layers in which the iron was depleted. Similar compositional boundary layers without ultrananolites were reported for glass-rich andesitic tephra from Southern Taupo Volcanic Zone (Zellmer et al 2016).…”

Microstructural evolution of silicate immiscible liquids in ferrobasalts

“…Kinetic processes must therefore play a role. We suggest the Fe-rich liquid distribution in the quenched experiments is controlled by the creation of compositional boundary layers (CBL) around the growing grains (Zellmer et al 2016): compositional boundary layers adjacent to plagioclase will be enriched in Fe, Ti, Mg, and P, promoting unmixing and heterogeneous nucleation of Fe-rich droplets. Conversely, any compositional boundary layer developing around growing grains of whitlockite will be depleted in P, thus decreasing the size of the silicate liquid immiscibility field in composition space (Bogaerts and Schmidt 2006;Charlier and Grove 2012;Visser and Van Groos 1979), and inhibiting unmixing in the adjacent liquid.…”

Compositional boundary layers trigger liquid unmixing in a basaltic crystal mush

“…Previous studies suggest that cryptic crystal chemical zonation in plagioclase on the 1 µm scale may result from CBL development 24 . Our data showing the Na-rich rims on the same scale as the surrounding Fe-rich CBLs (Fig.…”

“…The modal percentage of glass increases linearly with increasing depth in the upper crust of the Kīlauea Iki lava lake; in the 1976 drill core the modal glass percentage increases from 14.3% at 42.8 m depth to 44.4% at 45.5 m 23 . In some samples from all three localities, optimising the contrast-brightness in backscatter electron (BSE) images at high magnification reveals that plagioclase grains are surrounded by a continuous CBL 24 , which is enriched relative to the bulk melt with elements incompatible in plagioclase, chiefly Fe, with Mg, P, Mn and Ti. The Fe-rich CBLs are compositionally similar to the Fe-rich immiscible liquid analysed in experimental and natural sample studies 25 (Supplementary Fig.…”

“…1a; Supplementary Table 2), and the relatively Na- and Al-poor composition of the Fe-rich CBL compared with the nanoemulsion Fe-rich liquids (Supplementary Table 4), are consistent with the plagioclase Na-rich rim forming at the same time as the Fe-rich CBL. This may lead to a cyclic process of formation and destruction 24 , whereby the development of Fe-rich CBLs in a continuously evolving system facilitates the growth of Ca-poor plagioclase, which then reverts to relatively Ca-rich growth if the CBL is destroyed by magma flow. The morphology of the plagioclase grains from the Kīlauea Iki lava lake (Fig.…”