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Bubble development in explosive silicic eruptions: insights from pyroclast vesicularity textures from Raoul volcano (Kermadec arc)

Abstract

Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ∼65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation.

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Acknowledgements

We thank the New Zealand National Institute of Water and Atmospheric Research and the Masters and crew of the R.V. Tangaroa on the NZAPLUME III (2004) and TAN07/06 (2007) voyages for their logistical support. The New Zealand Department of Conservation gave permission for the fieldwork, and we acknowledge the Raoul Conservancy staff in 2004 and 2007 for their hospitality and field support on Raoul Island. Max Borella, Darren Gravley and Mike Rosenberg helped with field studies in 2007, Christopher Davy and David Helliwell helped with image processing. John Harper is thanked for mathematical assistance and Lucia Gurioli, Ben Kennedy, and John Townend are gratefully thanked for helpful discussions. Support from the Royal Society of New Zealand Marsden Fund (VUW0613) to CJNW and ICW, and from the AXA Research Fund to KVC, is gratefully acknowledged. We thank the reviewers T. Giachetti and A. Burgisser, and the editor J. Gardner, for their thorough and helpful comments which significantly improved this manuscript.

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Correspondence to Melissa D. Rotella.

Additional information

Editorial responsibility: J.E. Gardner

Electronic supplementary material

Appendix A—Pyroclast imaging and textural measurements

Appendix A—Pyroclast imaging and textural measurements

Each thin section was imaged with a flatbed scanner at 1,200 or 3,200-dpi resolution using transmitted light to characterize the population of largest bubbles. A series of nested magnification backscattered electron (BSE) images were taken on either a JEOL JXA-8320 electron microprobe (Victoria University of Wellington) or a JEOL-5900LV scanning electron microscope (University of Hawaii) at 1,280 × 960 pixel resolution until the smallest bubbles were >5 pixels in diameter, corresponding to an uncertainty in bubble size of <5 % for one incorrect pixel (Shea et al. 2010). At ×500 magnification, 5 pixels corresponds to 1 μm, which is the approximate size of the smallest bubbles present in all clasts. All measurements in FOAMS used a bubble diameter cut-off size of 5 pixels, therefore ensuring all bubble sizes were included in the subsequent analyses. This resolution also ensured that the thinnest bubble walls (∼1 μm), that correspond to the “critical film thickness” of Klug and Cashman (1996) were imaged. Binary images were created from the BSE images using Adobe Photoshop using similar methods to those outlined in Shea et al. (2010). Bubble area data were obtained from the binary 2D images and converted to 3D bubble volume data using the stereological conversion methods of Sahagian and Proussevitch (1998). Bubble number volumes (N v) were calculated for binned bubble diameters using FOAMS (Shea et al. 2010), and vesicle volume fractions and BND values were calculated following the methods of Sahagian and Proussevitch (1998). BND values were corrected for vesicularity and phenocryst content (>30 μm crystals) to avoid underestimating the nucleation densities of highly expanded clasts (after Klug et al. 2002) and to allow direct comparison to clasts of differing density. BND values in this study can be directly compared to other studies in which all bubble sizes were included in the analyses. For comparison with studies in which a 20-pixel (4 μm at ×500) diameter cut-off was used (e.g., Adams et al. 2006; Carey et al. 2009; Costantini et al. 2010; Houghton et al. 2003, 2010) values using the same cut-off are also reported in Table 1.

The stereological conversion method of Sahagian and Proussevitch (1998) defines the probability distributions for intersecting a polydisperse system of particles in order to convert 2D bubble area to 3D volumes. This technique consists of calculating number volumes (NV) in given geometric size classes per unit volume by successive iterations of the NV of larger objects, assuming a spherical geometry. One shortcoming of the Sahagian and Proussevitch (1998) method of stereological conversion, which is used in the FOAMS program (Shea et al. 2010), is the inability to account for elongated or irregular shapes since the cross-section probabilities for elongate shapes cannot be expressed analytically. This method of obtaining BNDs will therefore result in some degree of bias due to the inherent (and unavoidable) assumption of spherical bubble shapes (Sahagian and Proussevitch 1998; Shea et al. 2010). Additionally, the use of a single shape factor for all bubbles can produce artefacts when there is a range of bubble shapes (Castro et al. 2003), and the assumption of a continuous distribution of bubble sizes generates artefacts for distributions that are polymodal.

In recent years, the method of attaining BSDs through 3D Computed Tomography (CT) scans has gained in popularity (see Baker et al. 2012b for review). This method avoids the assumption of spherical bubbles (inherent in the 2D to 3D conversion method), and has proven effective for studies of crystal size distributions (e.g., Gualda and Rivers 2006; Pamukcu and Gualda 2010), permeability studies (e.g., Okumura et al. 2008, 2009, 2010, 2013; Polacci et al. 2008; Degruyter et al. 2010a, b) or bubble studies of basaltic scoria (Song et al. 2001; Polacci et al. 2006, 2009, 2012). A trade-off between imaging area and resolution has proven problematic, however, for high-vesicularity silicic pumices for which the bubble walls are thin (∼1 μm) and, if broken, require manual rectification of imagery (Giachetti et al. 2011). Poor resolution of thin bubble walls at low imaging resolutions causes over-estimation of both overall vesicularity and bubble connectivity resulting in a reduction of BND values obtained. Therefore, given the current limitations to 3D tomography, the traditional 2D to 3D stereological approach employed in this study provided the most robust approach for these samples.

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Rotella, M.D., Wilson, C.J.N., Barker, S.J. et al. Bubble development in explosive silicic eruptions: insights from pyroclast vesicularity textures from Raoul volcano (Kermadec arc). Bull Volcanol 76, 826 (2014). https://doi.org/10.1007/s00445-014-0826-6

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Keywords

  • Bubble size distribution
  • Vesiculation
  • Explosive volcanism
  • Fragmentation
  • Pumice