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

Georg F. Zellmer,Naoya Sakamoto,Nozomi Matsuda,Anja Moebis,Hisayoshi Yurimoto,Shyh-Lung Hwang,Yoshiyuki Iizuka

doi:10.3389/feart.2016.00088

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-micrometre 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 nanometre-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 in future lead to more accurate 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.

Highlights

Minerals may provide critical insights into the rates of magmatic processes occurring during magma evolution and ascent at the onset of volcanic eruptions, with analytical methods ranging from crystal size distribution studies (e.g., Cashman and Marsh, 1988; Marsh, 1988, 1998; Hammer et al, 1999; Piochi et al, 2005; Noguchi et al, 2006, 2008; Clarke et al, 2007; Toramaru et al, 2008; Melnik et al, 2011) to diffusion geospeedometry (e.g., Zellmer et al, 1999, 2003, 2011, 2016; Morgan et al, 2004; Costa and Dungan, 2005; Martin et al, 2008; Druitt et al, 2012; Ruprecht and Plank, 2013)
For subsequent analysis by secondary ion mass spectrometry (SIMS), the carbon coat was removed with ethanol, and samples were coated with a thin film (c. 70 nm) of gold (SC-701MC, Sanyu Electron Co., Ltd.) at the Isotope Imaging Laboratory (IIL) of Hokkaido University
Crystal growth and associated compositional boundary layers (CBLs) development appears to result in a more favorable nucleation environment of magnesian phases in magnesian CBLs, which are less favorable for nucleation of Mg-poor phases that will nucleate away from these CBLs (Figure 1C)

Summary

Introduction

Crystal nucleation and growth are first-order processes in the evolution of silicate melts during cooling or degassing (e.g., Cashman and Marsh, 1988; Marsh, 1988; Fokin et al, 2006; Toramaru et al, 2008; Hammer, 2009; Hammer et al, 2010; Melnik et al, 2011) and determine mineral and melt chemistry and their evolution, which form the foundation of thermobarometric and hygrometric constraints (e.g., Putirka, 2008; Lange et al, 2009; Waters and Lange, 2015). Minerals may provide critical insights into the rates of magmatic processes occurring during magma evolution and ascent at the onset of volcanic eruptions, with analytical methods ranging from crystal size distribution studies (e.g., Cashman and Marsh, 1988; Marsh, 1988, 1998; Hammer et al, 1999; Piochi et al, 2005; Noguchi et al, 2006, 2008; Clarke et al, 2007; Toramaru et al, 2008; Melnik et al, 2011) to diffusion geospeedometry (e.g., Zellmer et al, 1999, 2003, 2011, 2016; Morgan et al, 2004; Costa and Dungan, 2005; Martin et al, 2008; Druitt et al, 2012; Ruprecht and Plank, 2013). The latest small-scale variations are likely acquired at the onset or during eruption of the samples, and understanding such variations may be crucial in the characterization and mitigation of volcanic hazards (Zellmer et al, 2016)

Methods

Results

Conclusion