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. Zellmer1* 
Naoya Sakamoto2 Shyh-Lung Hwang3 
Nozomi Matsuda2 Yoshiyuki Iizuka4 Anja Moebis1 Hisayoshi Yurimoto2- 1Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand
- 2Isotope Imaging Laboratory, Department of Natural History Sciences, Hokkaido University, Sapporo, Japan
- 3Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan
- 4Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
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.
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 also 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). Thus, understanding the processes that govern crystal nucleation and growth, as well as crystal morphology, structure, and composition, are key to unlocking volcanic hazards and many other compelling questions. Small scale variations in crystal chemistry have become apparent in the last few decades through advances in microanalytical techniques (Anderson, 1983; Wallace and Bergantz, 2002; Davidson et al., 2007; Jerram and Martin, 2008, and references therein). At present, the most advanced instruments provide submicroscopic resolution (down to nanometer scale) of crystal chemistry and crystal structure (Zellmer et al., 2015, 2016). With increasing magnification, the small-scale complexity of compositional and structural variations has become apparent, elucidating magmatic processes that were previously not resolvable. In natural tephras, the latest small-scale variations are likely acquired at the onset or during eruption of the samples, and understanding such variations may thus be crucial in the characterization and mitigation of volcanic hazards (Zellmer et al., 2016).
In this perspective article, we focus on the narrow compositional boundary layers (CBLs) of melt around growing crystals (Zhang, 2009) and their potential effects on crystal nucleation, growth, and compositional as well as phase zoning, using as examples some tephras from New Zealand's recent Central Plateau eruptions in the Southern Taupo Volcanic Zone (Moebis, 2010; Moebis et al., 2011). CBLs are the interface-melt layers around growing or dissolving crystals with composition different to that of the bulk melt. CBLs develop when crystal growth or dissolution rates exceed the rate of diffusion of the crystal-forming ions within the melt. Their thicknesses therefore depend on growth/dissolution rate of the crystal, diffusion rate of the ions, and on the relative motion of crystal and melt (Levich, 1962). We demonstrate here through semiquantitative imaging techniques that there are complex interdependencies between CBLs and crystal nucleation, growth, and zoning. A detailed quantification of these interdependencies is beyond the scope of this contribution, but we argue that their study is important and may ultimately improve volcanic hazard characterization and mitigation strategies.
Methods
Sampling Details and Sample Preparation
Tephra samples from the Central Plateau were collected by one of us (AM) for tephrochronological work. In the present study, we report data from Mangatawai tephra 407-28 and Papakai tephra 606-30 sourced from Ngauruhoe volcano, with ages of c. 2800 and c. 3980 cal. years B.P., respectively (Moebis, 2010), and from Tufa Trig tephra 108-137 (TF13 of Donoghue et al., 1997) sourced from Ruapehu volcano, with an age of c. 540 cal. years B.P. Volcanic glass shards from individual tephra units were handpicked at Massey University. The particles were embedded in an epoxy plug (EPOTEK 301), then cut and polished with diamond pastes of successively finer grades. The final polish was made at the Institute of Earth Sciences (IES), Academia Sinica, using a vibration polisher (Buhler: Vibromet-2) with 0.3 μm alumina compounds for several hours. For electron microbeam work, the polished specimens were then coated by a layer of carbon (Q150TE, Quorum Technologies Ltd., UK). 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.
Electron Probe Microanalysis
Crystals within tephra shards were analyzed at IES using a JEOL JXA-8900R electron microprobe equipped with four wavelength-dispersive spectrometers. A 2 μm defocused beam was operated for analysis at an acceleration voltage of 15 kV with a beam current of 12 nA. Measured X-ray intensities were ZAF-corrected using the standard calibration of synthetic (s) and natural (n) chemical-known standard minerals with various diffracting crystals, as follows: diopside (s) or wollastonite (s) for Si with TAP crystal, rutile (s) for Ti with PET crystal, corundum (s) for Al (TAP), chromium oxide (s) for Cr (PET), fayalite (s), or hematite (n) for Fe with LiF crystal, tephroite (s) for Mn (PET), periclase (s) for Mg (TAP), wollastonite (s) for Ca (PET), albite (n) for Na (TAP), and adularia (n) for K (PET). Peak and both upper and lower baseline X-rays were counted for 10 s for each element, respectively. Standards run as unknowns yielded major oxide relative standard deviations for Si, Na, and K of less than 1%, and less than 0.5% for other elements. Detection limits, based on 3σ of standard calibration, were less than 600 ppm for all elements.
Electron Microscopy and Isotopography
Backscattered electron (BSE) images were obtained using a field emission scanning electron microscope (FE-SEM; JEOL JSM-7000F) at the Isotope Imaging Laboratory (IIL), Hokkaido University, equipped with a high sensitivity energy dispersive X-ray spectrometer (EDS; Oxford X-Max 150). However, high-magnification work on crystal rims could not be conducted with this technique due to edge effects. Therefore, the IIL isotope microscope system, a Cameca ims-1270 SIMS instrument equipped with a stacked CMOS-type active pixel sensor (SCAPS) for ion imaging, was applied to visualize at high magnification the elemental distribution on the sample surface (“isotopography,” for details see Yurimoto et al., 2003). An O- primary beam of 23 keV was irradiated on the sample surface of approximately 60 × 60 μm2 with beam currents ranging from c. 1.5 to 20 nA. The exit slit was narrowed enough to eliminate the contribution of interference ions to the isotopic images. The positive secondary ion images of 23Na, 24Mg, 27Al, 28Si, 40Ca, and 56Fe on the sample surface were collected in a SCAPS detector, with exposure times ranging between 200 and 750 s. A spatial resolution of about 300 nm was achieved for our samples. Ablation rates of only about 1 μm/h allowed the successful acquisition of images at 5 s per frame of essentially the identical sample surface across all elements. Secondary ion intensity varied with primary beam intensity. Image gray scales were adjusted to balance these variations.
Transmission Electron Microscopy (TEM)
For TEM analyses, several samples of c. 50 nm thickness were prepared from the corona pyroxene overgrowing an olivine crystal of a selected Ruapehu tephra by applying the focused ion beam technique (FIB; SMI-3050). Selective area electron diffraction patterns and images were obtained using a transmission electron microscope (JEOL JEM-3010) operated at 300 kV. The instrument was equipped with an energy dispersive X-ray (EDX) spectrometer (Oxford EDS-6636) featuring an ultrathin window and a Si(Li) detector, capable of detection of elements from boron to uranium. EDX spectra were collected for 200 s. Semi-quantitative analysis was based on the Cliff-Lorimer thin film approximation with experimental k-factors obtained from natural minerals (Loretto, 1994).
Results
Electron probe mineral chemical data are given in Supplementary Table 1. In summary, besides minor oxide phases that have not been analyzed but are present in all studied samples, Mangatawai tephra 407-28 contains crystals of plagioclase (An57−85) and orthopyroxene (En63−72Fs23−27Wo3−5); Papakai tephra 606-30 contains crystals of plagioclase (An45−85) and orthopyroxene (En67−73Fs24−29Wo3−5); and Tufa Trig tephra 108-137 contains crystals of plagioclase (An51−66), orthopyroxene (En63−84Fs12−33Wo3−5), clinopyroxene (En44−55Fs10−17Wo35−43), and olivine (Fo71−75).
The present study focusses on results from semi-quantitative high-resolution imaging. Figures 1A–C show SCAPS elemental maps of a large zoned plagioclase crystal, set in a glassy groundmass with microlites of plagioclase and pyroxene. A complexly zoned large plagioclase crystal experiencing synneusis (Vance, 1969; Dowty, 1980) with a microphenocryst (bottom right) is seen, with zoning particularly evident in the Na-image. Microlites show swallow-tail textures indicating rapid crystal growth. There is striking evidence of distinct CBLs of melt present around all crystals, about 1 micron in width. CBLs are enriched in Mg and depleted in Al and Na around plagioclase crystals, and vice versa around pyroxene microlites (cf. small pyroxene microlite in center right of image). Complex zoning of Na in plagioclase is evident even in the microlites, with a wavelength of similar width to the CBL. The black arrow in c indicates plagioclase nanolites forming just outside the CBL of the large plagioclase crystal, the white arrow points to a magnesian nanolite forming in the CBL. Figure 1D is a BSE image of groundmass microlites and nanolites of another sample. Pyroxene microlites have a Fe-rich rim and are surrounded by Fe-poor CBLs, similar in grayscale as plagioclase microlites. Magnetite nanolites are seen distributed throughout the groundmass, but the largest are surrounding the plagioclase microlites.
Figure 1. (A–C) SCAPS elemental images of part of a complexly zoned plagioclase phenocryst in a glass shard with plagioclase and pyroxene microlites from Mangatawai tephra 407-28. Melt boundary layers around plagioclase, with their width indicated at one point in the image by square brackets, are depleted in Al and Na and enriched in Mg. Several microlites show swallowtail textures (cf. arrows in A). Two small cracks indicated in (A) result in localized imaging artifacts. Plagioclase and pyroxene nanolites discussed in the text are indicated by black and white arrows, respectively, in (C). (D) SEM-image of swallow-tail textured pyroxene and plagioclase microlites, as well as magnetite nanolites, in a glass shard from Papakai tephra 606-30. Melt boundary layers around pyroxenes (px) are depleted in Fe. Plagioclase (plag) microlites nucleate in these boundary layers and grow outwards. Magnetite (mt) nanolites are preferentially found within melt boundary layers around plagioclase. Numbers indicate possible nucleation and growth sequence.
Figure 2 provides an analysis of an olivine crystal with pyroxene corona texture. Panel (a) shows the BSE image of part of this crystal set in a glassy groundmass. SCAPS elemental maps of the corona texture are provided for Mg (panel b) and Ca (panel c). Variations in Ca concentration within the overgrowth are evident, as observed previously in other corona orthopyroxenes (Zellmer et al., 2016), with a very strong enrichment in the rim of the overgrowth, and concomitant depletion in the CBL. Weaker Ca-enrichment zones are evident within the overgrowth. Such calcic lamellae are ubiquitous in overgrowth orthopyroxenes from the southern Taupo Volcanic Zone, and also occur in some orthopyroxene microphenocrysts (cf. Zellmer et al., 2016). Panels (d) and (e) are TEM bright field images of FIB sections, the location of which are indicated in panel (b). The images show clear phase boundaries of nm-thin layers of Ca-rich clinopyroxenes within Ca-poor orthopyroxene overgrowth. Two clinopyroxene domains can be identified by semiquantitative EDX spectrometry: domain A adjacent to the glass, with Al-enrichment (panel f), and domain B, inside the overgrowth, low in Al. Neither domain is in Fe/Mg exchange equilibrium with their orthopyroxene host, the EDX spectrum of which is provided in Supplementary Image 1. In equilibrium, KD(Fe-Mg) should be 1.09 ± 0.14 (Putirka, 2008). Domain A yields a KD(Fe-Mg) of >2.00, while domain B yields a KD(Fe-Mg) of < 0.80. Two-pyroxene thermobarometry therefore cannot be conducted.
Figure 2. Pyroxene corona textures on olivine displaying cryptic phase zoning in a glass shard from Tufa Trig tephra 108-137. (A) Backscattered electron image of part of the grain, indicating the area investigated by SCAPS imaging. (B) SCAPS Mg-image, showing the Mg-rich olivine (bright), the pyroxenes (gray), and the Mg-poor glass (dark). Position of focussed ion beam (FIB) sections for TEM work is indicated. (C) SCAPS Ca-image showing the Ca-poor olivine (dark), the dark-gray low-Ca pyroxene with some bright high-Ca bands, and the glass with variable Ca-content: low in the boundary layer (indicated by square brackets) right next to the outermost high-Ca growth zone, high in between the two pyroxene crystals, and highly variable away from the crystals. (D,E) TEM bright field images of FIB sections showing nm-scale bands of the high Ca-phase at the pyroxene rims (domain A), as well as inside the pyroxene crystal (domain B). Semi-quantitative analytical electron microscopy energy-dispersive X-ray spectra of high-Ca domains A and B are provided in (F,G), respectively, yielding a higher-Ca, low Mg/Fe aluminous pyroxene in domain A and a lower-Ca, high Mg/Fe pyroxene in domain B. See Supplementary Image 1 for the EDX spectrum of the orthopyroxene host crystal, provided for comparison.
Discussion
Crystal nucleation may initially be randomly distributed within the glass, i.e., may be homogeneous (Fokin et al., 2006). However, crystal growth and associated CBL development appears to result in a more favorable nucleation environment of magnesian phases (such as pyroxene) in magnesian CBLs, which are less favorable for nucleation of Mg-poor phases (such as plagioclase) that will nucleate away from these CBLs (Figure 1C). Mg-poor phases will preferentially nucleate in Mg-poor CBLs. For example, preferential plagioclase nucleation would be expected in the CBLs developing around growing orthopyroxene crystals (further discussed below, cf. Figure 1D). Inhomogeneous crystal nucleation has previously been described (e.g., Hammer et al., 2010), and has recently been associated with CBLs that developed during dissolution of natural olivine in SiO2-rich melts (Zellmer et al., 2016).
CBLs will be less depleted in crystal-forming elements at crystal corners, and this may be the reason for the development of swallow-tail textures (Vernon, 2004), which form by more rapid crystal growth out from those corners in response to greater availability of crystal-forming elements within these local environments (Figures 1A–C). Crystal chemical zonation at circa 1 micron wavelength (Figure 1B) may be the response of CBL development at the same length scale, and subsequent CBL destruction, e.g., by movement of the crystal through the melt and concomitant erosion of the CBL, resulting in a regular local variations of ions available to the growing crystals. Such variations are even seen in microlites. Repetitive CBL development and destruction may thus represent an alternative way to form the fine-scale outer zoning in crystal composition observed in many volcanic phenocrysts, as opposed to being due to growth during magmatic rejuvenation and convection in subvolcanic reservoirs (cf. Shelley, 1993). Timescales predicted by modeling diffusion at intracrystalline boundaries of incompatible element enriched zones may thus not always be associated with magmatic intrusion events, for example, but instead might be recording timescales of crystallization during cooling or decompression-induced degassing.
As a result of these complex interactions of crystal nucleation and CBL formation by crystal growth, small scale heterogeneities in the mineral distribution of the groundmass may result. Figure 1D provides an example where orthopyroxene microlite growth has apparently resulted in Ca and Al enrichment of developing CBLs, which served as nucleation environments for plagioclase microlites. These, in turn, generated Fe-rich CBLs that represented favorable environments for nucleation of growth of magnetite nanolites.
The effect of CBLs may not only be restricted to variations in major (and presumably trace) element zonation in phases displaying solid solution, but may extend to triggering phase changes on microscopic to submicroscopic scales: Figure 2 illustrates the submicroscopic phase changes within rapidly growing pyroxene crystals, which is what one may refer to as cryptic phase zoning. These lamellae are suggestive of exsolution textures, with clinopyroxene exsolving from orthopyroxene (e.g., Vernon, 2004). However, if this was the case, one would expect Fe/Mg exchange equilibrium between the host and exsolution zones. Although the EDX chemical analysis is only semiquantitative, given the narrow width of the clinopyroxene bands, the magnitude of the measured disequilibria of both clinopyroxene domains with their orthopyroxene host is likely too great to be attributable to analytical uncertainties. Thus, we prefer a scenario where rapid orthopyroxene growth results in a calcic CBL, which intermittently triggers brief precipitation intervals of clinopyroxene (domain B) instead of orthopyroxene. These intermittent intervals of clinopyroxene precipitation would then rapidly deplete the CBL in Ca, such that the system switches back to orthopyroxene as the preferred phase. Finally, domain A at the edge of the crystal likely represents growth related to quenching. In this scenario, cryptic phase zoning is related to crystal growth and associated chemical variations of the evolving CBL over time.
The presence of cryptic phase zoning in pyroxenes represents a challenge for two-pyroxene thermobarometry as well as pyroxene-melt thermobarometry (e.g., Putirka, 2008). Electron microprobe analysis (EMPA) is unable to resolve such sub-micron features and will yield average compositions with somewhat elevated amounts of calcium. Detailed imaging of submicron phase zonation may in future allow more successful targeting of crystal growth zones (cf. Zellmer et al., 2015) and thus may improve thermobarometric constraints. Temperature uncertainties still represent one of the principle limiting factors in diffusion geospeedometry (Costa and Morgan, 2010; Petrone et al., 2016), and tighter temperature constraints would be a significant advance in this respect.
Nucleation and growth of microlites are processes occurring during the last stage of magma ascent at the onset and during eruption of volcanic tephras (Hammer et al., 1999; Piochi et al., 2005; Noguchi et al., 2006, 2008; Clarke et al., 2007). Understanding these processes down to submicrometer scale is important to better characterize the crystallization of microlites in volcanic conduits, which have been used to estimate the timescales of magma ascent in order to characterize and mitigate the hazards of explosive volcanic eruptions (Zellmer et al., 2016). Our study shows that CBLs in natural magmas may significantly influence crystal nucleation and crystal growth, as well as the resulting crystal morphologies, crystal chemical zonation, and the distribution of small scale heterogeneities within the crystallizing groundmass. We have outlined some of the complex interplays of crystal nucleation, growth, CBL formation, and crystal chemical as well as phase zonation. Additional work will be required to properly quantify these processes, including the characterization of potential variations in the width of CBLs between different samples, and what this might reveal in terms of relative movement of crystals and melt, and thus about potential variations in melt rheology and melt ascent processes. Given the growing importance of the role of igneous petrology in informing volcanic hazards and their mitigation (Saunders et al., 2012), we anticipate significant advances in this field in the near future.
Author Contributions
GFZ designed the project, assisted with analytical work, interpreted the data and wrote the paper. SH undertook TEM analyses. NS and NM undertook SEM and SIMS imaging work. YI undertook EPMA work. NS and HY checked SEM and SIMS data quality. AM collected and prepared the samples. All authors contributed to the discussion of results.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgments
We thank Fred Davis and Bob Stewart for useful discussions and Oliver Jagoutz for editorial handling of this contribution. We are grateful for constructive comments of Fabio Arzilli and Christy B. Till, which improved the script. GFZ acknowledges funding through the Ministry of Business, Innovation and Employment (MBIE, grant MAUX1507), and the National Geographic Society (grant 9577-14).
Supplementary Material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/article/10.3389/feart.2016.00088
Supplementary Table 1. Electron microprobe data of plagioclase, orthopyroxene, clinopyroxene, and olivine crystals from the studied tephras.
Supplementary Image 1. EDX spectrum of the orthopyroxene host of the cryptic phase zonation (see text for details).
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Keywords: crystal nucleation, crystal growth, swallow tail textures, cryptic phase zoning, Ngauruhoe volcano, Ruapehu volcano
Citation: Zellmer GF, Sakamoto N, Hwang S-L, Matsuda N, Iizuka Y, Moebis A and Yurimoto H (2016) Inferring the Effects of Compositional Boundary Layers on Crystal Nucleation, Growth Textures, and Mineral Chemistry in Natural Volcanic Tephras through Submicron-Resolution Imaging. Front. Earth Sci. 4:88. doi: 10.3389/feart.2016.00088
Received: 28 July 2016; Accepted: 15 September 2016;
Published: 28 September 2016.
Edited by:
Oliver Jagoutz, Massachusetts Institute of Technology, USAReviewed by:
Christy B. Till, Arizona State University, USAFabio Arzilli, University of Manchester, UK
Copyright © 2016 Zellmer, Sakamoto, Hwang, Matsuda, Iizuka, Moebis and Yurimoto. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Georg F. Zellmer, g.f.zellmer@massey.ac.nz