RESEARCH ARTICLE
Heterogeneous vesiculation of 2011 El Hierro xeno-pumice
revealed by X-ray computed microtomography
S. E. Berg1 & V. R. Troll1,2 & F. M. Deegan1 & S. Burchardt1 & M. Krumbholz1,3 &
L. Mancini4 & M. Polacci5 & J. C. Carracedo2 & V. Soler6 & F. Arzilli4,5 & F. Brun4,7
Received: 19 June 2016 /Accepted: 8 October 2016 /Published online: 14 November 2016
# The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract During the first week of the 2011 El Hierro subma-
rine eruption, abundant light-coloured pumiceous, high-silica
volcanic bombs coated in dark basanite were found floating
on the sea. The composition of the light-coloured frothy ma-
terial (‘xeno-pumice’) is akin to that of sedimentary rocks
from the region, but the textures resemble felsic magmatic
pumice, leaving their exact mode of formation unclear. To
help decipher their origin, we investigated representative El
Hierro xeno-pumice samples using X-ray computed
microtomography for their internal vesicle shapes, volumes,
and bulk porosity, as well as for the spatial arrangement and
size distributions of vesicles in three dimensions (3D). We
find a wide range of vesicle morphologies, which are espe-
cially variable around small fragments of rock contained in the
xeno-pumice samples. Notably, these rock fragments are al-
most exclusively of sedimentary origin, and we therefore in-
terpret them as relicts an the original sedimentary ocean crust
protolith(s). The irregular vesiculation textures observed prob-
ably resulted from pulsatory release of volatiles from multiple
sources during xeno-pumice formation, most likely by succes-
sive release of pore water and mineral water during incremen-
tal heating and decompression of the sedimentary protoliths.
Keywords ElHierro .Xeno-pumice .X-CTimaging .Vesicle
morphologies . Vesicle size distribution . Heterogeneous
vesiculation . Sedimentary ocean crust
Introduction
The El Hierro 2011–2012 eruption
El Hierro is the westernmost and the youngest of the seven
Canary Islands (1.2Ma, Guillou et al. 1996), which are widely
attributed to an underlying mantle plume (e.g. Carracedo et al.
2001; Geldmacher et al. 2005; Zaczek et al. 2015). No historic
volcanic activity was known on El Hierro before a submarine
eruption commenced on October 10, 2011, off the southern
Editorial responsibility: J.E. Gardner
Electronic supplementary material The online version of this article
(doi:10.1007/s00445-016-1080-x) contains supplementary material,
which is available to authorized users.
* S. E. Berg
e.sylviaberg@gmail.com
1 Department of Earth Sciences, Centre for Experimental Mineralogy,
Petrology and Geochemistry (CEMPEG), Uppsala University,
Villavägen 16, 752 36 Uppsala, Sweden
2 GEOVOL, Universidad de Las Palmas de Gran Canaria, Las Palmas
de Gran Canaria, Spain
3 Geoscience Centre, Georg-August-University of Göttingen,
Goldschmidtstrasse 1-3, 37077 Göttingen, Germany
4 Elettra-Sincrotrone Trieste, S.C.p.A., S. S. 14-km 163,5 in AREA
Science Park, 34149 Basovizza (Trieste), Italy
5 School of Earth and Environmental Sciences, University of
Manchester, Williamson Building, Oxford Road, Manchester M13
9PL, UK
6 Estación Vulcanológia de Canarias, CSIC, Avda. Astr. Fco. Sánchez
3, 38206 La Laguna, Tenerife, Spain
7 Department of Engineering and Architecture, University of Trieste,
Via A. Valerio, 10, 34127 Trieste, Italy
Bull Volcanol (2016) 78: 85
DOI 10.1007/s00445-016-1080-x
coast. Seismic unrest began several months before the erup-
tion and was associatedwith vertical and lateral magmamove-
ment. Volcanic activity continued for several months but
remained submarine for the entire eruption period until activ-
ity finally ceased in March 2012 (e.g. Carracedo et al. 2012a,
b, 2015; Gonzales et al. 2013). Surface expressions of the
eruption included green discolouration of seawater, together
with intense bubbling and degassing, and the presence of two
distinct types of floating lava bombs during separate stages of
the eruption. During the first week of the eruption, abundant
light-coloured pumiceous bombs, enclosed by a basanite coat-
ing were floating on the sea (‘xeno-pumice’), whereas entirely
basanitic ‘lava balloons’, usually with hollow interiors, oc-
curred throughout the entire eruption (Figs. 1 and 2, e.g.
Kueppers et al. 2012; Meletlidis et al. 2012; Perez-Torrado
et al. 2012; Troll et al. 2012).
El Hierro xeno-pumice and its composition
The 2011 to 2012 El Hierro submarine eruption emitted light-
coloured, high-silica, pumiceous bombs (xeno-pumice)
during the first week of eruptive activity. The xeno-pumice
phenomenon was the source of considerable controversy at
the time, and the origin of these peculiar samples is still de-
bated (see Carracedo et al. 2012a, 2015; Meletlidis et al. 2012;
Perez-Torrado et al. 2012; Troll et al. 2012, 2015; Schmincke
and Sumita 2013; Sigmarsson et al. 2013; Del Moro et al.
2015; Rodriguez-Losada et al. 2015; Zaczek et al. 2015).
The debate focuses on whether the white and essentially
crystal-free xeno-pumice represent either (1) high-silica mag-
ma or altered magmatic rocks that vesiculated during contact
with hot basanite magma of the El Hierro eruption (e.g.
Meletlidis et al. 2012; Sigmarsson et al. 2013; Del Moro
et al. 2015), or (2) recycled sedimentary rocks that degassed
during heating and transport while entrained in the ascending
basanite magma (e.g. Troll et al. 2012; Schmincke and Sumita
2013; Rodriguez-Losada et al. 2015; Zaczek et al. 2015).
The xeno-pumice samples that erupted offshore El Hierro
in 2011 exhibit intense mingling and mixing between the
enclosing basanite carapace and the white to grey pumiceous
cores. The cores contain partly intact sedimentary relicts but
lack distinctly igneous fragments or igneous mineralogy
Fig. 1 a Topographic map of the Canary Islands off the coast of
northwestern Africa (courtesy of Wikimedia Commons). El Hierro is
the youngest and westernmost island in the archipelago. b True colour
RapidEye satellite image of El Hierro island, captured by the Moderate
Resolution Imaging Spectroradiometer (MODIS) aboard the Terra
satellite on December 21, 2011. The stain on the ocean caused by the
submarine eruption can be seen SSE of El Hierro (image courtesy of
NASA GSFC). c Close-up of the discolouration of seawater and d the
floating lava balloons and frothy xenoliths that emerged at the surface
during the eruption (Carracedo et al. 2012a, b)
85 Page 2 of 13 Bull Volcanol (2016) 78: 85
(Fig. 2, Meletlidis et al. 2012; Troll et al. 2012; Zaczek
et al. 2015). X-ray diffraction (XRD) analyses of the light-
coloured cores of xeno-pumice revealed a mineralogy similar
to that of sedimentary rock assemblages in the region, com-
prising quartz, clays, jasper, carbonate and a range of contact
metamorphic minerals, while igneous minerals were notably
absent in the XRD determinations (Troll et al. 2012;
Rodriguez-Losada et al. 2015). Geochemical analyses of
xeno-pumice (e.g. Meletlidis et al. 2012; Troll et al.
2012; Martí et al. 2013; Sigmarsson et al. 2013;
Carracedo et al. 2015; Del Moro et al. 2015; Rodriguez-
Losada et al. 2015) show that they have high silica con-
tents (SiO2 = 68–71 wt.%) and trace element compositions
that differ significantly from known El Hierro magmatic
rocks, including an unusual enrichment in uranium in
many xeno-pumice samples (cf. Carracedo et al. 2015;
Rodriguez-Losada et al. 2015).
In addition, 87Sr/86Sr ratios are elevated in xeno-pumice
(0.706426 to 0.710134, Del Moro et al. 2015) and overlap
with regional sedimentary rocks, while 143Nd/144Nd ratios in
xeno-pumice (0.512905 to 0.512978, Del Moro et al. 2015)
are consistent with magmatic values from the Canary Islands.
However, Nd concentrations of xeno-pumice are considerably
lower than those of El Hierro basanite rocks, despite the fact
that regular magmatic fractionation from basanite to trachyte
and eventually rhyolite would be expected to cause Nd enrich-
ment in evolved residual melts. El Hierro xeno-pumice sam-
ples show Nd concentrations of ∼20–60 ppm, while the 2011
El Hierro basanites have Nd ∼53–70 ppm. Rhyolites from
other Canary Islands have Nd concentrations of ∼70–
125 ppm, which exceed the Nd concentration in El Hierro
basanite and xeno-pumice (e.g. Troll and Schmincke 2002;
Troll et al. 2012). Indeed, El Hierro xeno-pumice samples fall
on a mixing line between El Hierro basanite and local
siliciclastic sediments (Nd = 1 to 25 ppm, e.g. von Rad and
Einsele 1980), which implies that a low Nd sedimentary melt
would be highly susceptible to uptake of Nd from a surround-
ing basanite magma. The 143Nd/144Nd ratios of El Hierro
xeno-pumice were hence conceivably compromised while in
contact with their host basanite. Thus, the Sr isotopes require a
Fig. 2 a–iA selection of xeno-pumice hand specimens that were erupted
during the early days of the 2011 El Hierro eruption. Xeno-pumice show
a variety of vesicle sizes and shapes, mingling textures between white
pumiceous cores and basanite lava rims, highly vesiculated core regions,
interconnected vesicle networks, relict sedimentary fragments, as well as
intense degassing textures around xenocrystals and crustal restite
fragments
Bull Volcanol (2016) 78: 85 Page 3 of 13 85
high sedimentary input, while Nd isotopes are probably a less
robust tracer due to the low concentrations of Nd in the sedi-
mentary protoliths and the potential for overprint from the
basanite host. Notably, oxygen isotope ratios of El Hierro
xeno-pumice also differ markedly from mantle values and are
similar to values found in S-type felsic magmas and sedimentary
rocks and thus support the Sr isotope evidence for a crustal origin
(e.g. Savin and Epstein 1970; Troll et al. 2012; Rodriguez-
Losada et al. 2015). Since oxygen is a major element in magma
and in siliciclastic sedimentary rocks (ca. 50 wt.%), while rare
earth elements like Nd are present in trace quantities only, it is
more difficult to significantly change the oxygen isotope ratio by
minor contamination processes. Oxygen isotopes in tandemwith
the mineralogical and Sr isotope evidence above thus support a
sedimentary rock origin for El Hierro xeno-pumice.
Xeno-pumice textures and vesicularity
The 2011 El Hierro xeno-pumice samples are highly porous,
which allowed them to float on water like magmatic pumice
(e.g. Carracedo et al. 2012a). Upon contact with seawater, El
Hierro xeno-pumice solidified and preserved frozen snapshots
of their degassing history. Because vesicle size, shape and
number density in xeno-pumice are ultimately a function of
(i) the volatile content of the source material and (ii) the phys-
ical conditions the material was exposed to, the vesicle tex-
tures observed in the individual samples can probably offer
insight into evolutionary processes during xeno-pumice for-
mation and vesiculation (cf. Song et al. 2001; Polacci et al.
2008, 2009). From thin section analyses Meletlidis et al.
(2012) and Sigmarsson et al. (2013) found that the xeno-
pumice cores and the basanitic crust are unimodally vesiculat-
ed, which they suggest to be a result of a single vesicle nucle-
ation event in an aphyric magma. These authors argue that
crystal-poor, compositionally homogeneous felsic magma
(or hydrothermally altered aphyric igneous rock) frothed up
in a single pulse when brought in contact with mafic basanite
magma (Meletlidis et al. 2012; Sigmarsson et al. 2013). In
contrast, a study of more than 300 samples by Troll et al.
(2012), Carracedo et al. (2015) and Zaczek et al. (2015) show
a heterogeneous spatial distribution of vesicle sizes within
each individual specimen, with particularly intense vesicula-
tion around sedimentary relicts (Fig. 2). These relicts were
found to contain fossils of Cretaceous to Pliocene age
(Zaczek et al. 2015), which document a wide temporal range
of sedimentation and imply a range of sedimentary protolith
compositions to be present beneath El Hierro. Different sedi-
mentary facies would likely display a variety of melting and
vesiculation points amongst samples when heated, but also
within individual samples due to variable pore water contents
and proportions of anhydrous and hydrous phases present in a
single portion of a sedimentary protolith (cf. Robertson and
Stillman 1979a, b; Gluyas and Cade 1997; Steiner et al. 1998).
Indeed, if the 2011 El Hierro xeno-pumice material is derived
from sedimentary rock, they would be expected to display an
irregular vesicle size distribution that varies spatially within
and amongst samples. Up to now, our understanding of the
vesiculation behaviour of El Hierro xeno-pumice remains lim-
ited because existing studies are exclusively based on 2D
analyses as described above. In this paper, we provide com-
puted X-ray microtomography (micro-XCT) data to resolve
the three-dimensional (3D) spectrum of vesicularities present
in El Hierro xeno-pumice and shed new light on its origin.
Methods
X-ray microtomography
From a sample suite of >300 xeno-pumice specimens collect-
ed during the early days of the El Hierro eruption in 2011
(Troll et al. 2012), we selected six specimens that broadly
reflect the diversity seen in the overall sample suite. Xeno-
pumice samples were cut into 10 × 10 × 10 mm cubes and
were analysed by micro-XCT at the SYRMEP beamline
(Tromba et al. 2010) and at the TomoLab station
(Zandomeneghi et al. 2010) of the Elettra synchrotron light
laboratory in Basovizza (Trieste, Italy), in November/
December 2011 and September 2012. Three out of the six
samples were scanned using a monochromatic, nearly-
parallel synchrotron X-ray beam in phase-contrast mode
(edge-detection regime, Cloetens et al. 1996). The experimen-
tal parameters employed for the micro-XCT scan acquisitions
are reported in Supplementary Table 1. A 12-bit, water-cooled
CCD camera with an effective pixel size of 9 × 9 μm and
12 mm × 18 mm field of view was used as detector. The
collected radiographs were reconstructed into two-
dimensional (2D) axial slices with an isotropic voxel size
(3D pixel) of 9.0 μm, using the custom-developed software
Syrmep_tomo_project 4.0 (Montanari 2003) and selecting the
filtered back-projection algorithm for slice reconstruction (e.g.
Kak and Slaney 1988).
The remaining three specimens were analysed using a
microfocus X-ray source at the TomoLab station of Elettra
(see Supplementary Table 1; Zandomeneghi et al. 2010).
The TomoLab instrument provides a polychromatic X-ray
beam with a cone beam geometry. The experimental parame-
ters employed for the TomoLab station are reported in
Supplementary Table 1. A 12-bit, water-cooled CCD camera
with an effective pixel size of 12.5 × 12.5 μm and a maximum
field of view of 50 mm × 33 mm was used as detector.
Depending on the instrument settings, the isotropic voxel size
used for the XCT scans ranged from 5.0 to 7.1 μm. The anal-
yses were performed under phase-contrast conditions, but
with a lower contrast compared to the synchrotron radiation
analyses. Reconstruction of 2D slices from the X-ray
85 Page 4 of 13 Bull Volcanol (2016) 78: 85
projections was performed by the commercial software
COBRA (Exxim, USA), which is based on the FDK algorithm
(Feldkamp et al. 1984). For further methodological details of
the experimental setup and of the reconstruction procedure,
see Ketcham and Carlson (2001) and Polacci et al. (2006,
2010).
Vesicle analyses of El Hierro xeno-pumice
For all samples, the reconstructed 2D images were stacked,
converted to 8-bit raw format and cropped using the freeware
ImageJ (Abramoff et al. 2004). Volume renderings were ob-
tained with the commercial software VGStudio MAX 2.0
(Volume Graphics). The rendered volumes were processed
(3D filtering and segmentation) and analysedmorphologically
(sample porosity and skeletonization) using the Pore3D soft-
ware library developed at Elettra (Brun et al. 2010;
Zandomeneghi et al. 2010).
For each of the scanned samples, we made qualitative tex-
tural observations by examining the complete 2D image se-
quence (stack) and the 3D representations that were recon-
structed using the whole data range (see Supplementary
Table 2). Further processing of the micro-XCT digital dataset
to extract vesicle volumes for quantitative image analyses in
3D involves the extraction of a volume of interest (VOI) with
dimensions suitable for the available computing resources but
preserving sample representativeness. Based on textural ob-
servations, VOIs were selected for each sample to visualise
the main vesicle textures. To ensure that the analysed VOIs are
representative elementary volumes (REV), we tested the con-
vergence of porosity data frommultiple VOIs of variable sizes
within each sample (see Supplementary Table 2). Potential
cutting effects from sample preparation and artefacts of the
cone-beam reconstruction were avoided by selecting VOIs
in the central parts of the imaged volumes.
To extract objects or phases for morphological analyses,
the next step is image segmentation, which produces binary
images optimised for the phases of interest. Vesicles were
segmented in the 3D domain (by Pore3D) using manual
greyscale thresholding based on the greyscale histogram of
the selected VOI and visual inspection of the slices in different
directions, which allowed high sensitivity to the presence of
noise and artefacts and proved favourable for our purpose
compared to automatic thresholding algorithms. The threshold
values were carefully selected iteratively for each individual
sample. To avoid biasing from texturally complicated micro-
XCT images, pre- and post-segmentation smoothening filters
were required on occasion to ease or refine the segmentation
procedure (see Supplementary Table 2, cf. Polacci et al. 2006;
Brun 2012). The 3D anisotropic diffusion filter was applied by
the Pore3D software to smooth the greyscale input
images prior to segmentation, while preserving object edges
(cf. Perona and Malik 1990). When required, the binary
images were treated further by removal of image noise from
isolated clusters of connected object voxels, i.e. ‘voxel
islands’ that are below a specified threshold value, which
was individually determined for each sample (cf. Soille
2004). The procedure described above is summarised in
Supplementary Fig. 1.
To quantify and graphically represent the degree of pore
interconnection in El Hierro xeno-pumice, we adopted the
gradient vector flow (GVF) skeletonization algorithm in
Pore3D (cf. Brun and Dreossi 2010). For our samples, we
favoured the GVF algorithm over other methods because it
produces a thin and centred curve-skeleton even in the pres-
ence of noisy objects (i.e. voxel islands), which can occur in
densely vesiculated samples. The skeleton is a graph of nodes
and branches that represents an idealised spine running along
the medial axis of a connected pore space (Lindquist and Lee
1996). Each single vesicle within the connected pore volume
represents a skeleton node, and the branches represent gas
pathways between them. Skeleton analysis allows us to quan-
tify the number and the dimensions of nodes and branches,
which determine the pore coordination number (i.e. the num-
ber of near neighbours to a central pore) and thus serves as
characterisation of pore space interconnectivity. Pore thick-
ness is measured via the concept of the maximal inscribed
sphere (cf. Hildebrand and Rüegsegger 1997; Baker et al.
2012). This concept is based on the inflation of a sphere
centred at a skeleton node, where the inflation continues until
the sphere ‘touches’ the walls of the pore. Hence, the diameter
of the maximally inflated sphere gives an idealised but repre-
sentative minimum estimation of the pore size and thus vesicle
size distributions prior to pore interconnection. We used this
approach to analyse vesicle size distributions (VSDs).
Results
Xeno-pumice samples show total porosities from 63 to 75
vol.%, which were calculated from the segmented volume of
voxels attributed to the ‘intra-vesicle’ phase (pore space) rel-
at ive to the total volume in the considered VOI
(Supplementary Table 2). The image descriptor labelling of
connected components in the Pore3D software is a measure of
vesicle interconnectivity and shows that the xeno-pumice ves-
icles are dominantly interconnected and form a coherent vol-
ume. While the resolution limits of the method may have
prevented recognition of very thin bubble walls, a small frac-
tion of vesicles appear to be clearly isolated and the closed
porosities within the analysed VOIs equal about 0.01 vol.%,
which equates to on average one isolated vesicle per 2 mm3 in
our specimens (see Supplementary Fig. 2 and Supplementary
Table 2). Petrographic observations, however, reveal vesicle
walls of all sizes, and it is therefore likely that the volume of
isolated vesicles is somewhat underrepresented as a result of
Bull Volcanol (2016) 78: 85 Page 5 of 13 85
the spatial resolution of the method. Textural observations of
xeno-pumice in hand specimen and 3D reconstructions reveal
that the isolated vesicles represent an estimated 1–2 % of the
total volume, which may be more representative overall than
the low values derived from Pore3D (see Supplementary
Fig. 2).
Individual vesicles in the analysed xeno-pumice samples
are spherical to elongated and occasionally tortuous in shape,
and range up to 1.4 mm in diameter (Fig. 3). From our 3D
observations, we distinguish three general vesicle diameter
groups (V): Vsmall <100 μm, Vmedium 100–300 μm, and
Vlarge >300 μm. The number density (n) of vesicles in each
size group varies broadly following nlarge < nmedium < nsmall,
but the abundance of vesicles within each size group varies
between samples. For instance, Vsmall is larger and Vlarge is
smaller in EH-XP-1 and EH-XP-13 than in the other three
samples. We also find that high porosity samples are dominat-
ed by sub-spherical vesicles throughout all size classes (see
samples EH-XP-2, EH-XP-10 and EH-XP-10LA; Fig. 3),
while tortuously shaped small-sized vesicles become more
frequent in the less porous samples (porosity <65 %, see sam-
ples EH-XP-1 and EH-XP-13; Fig. 3). Spherical to sub-
spherical vesicles are dominant, however, especially within
the larger size fractions (Vmedium and Vlarge), while the
smallest vesicles are more distorted in places (see Vsmall in
EH-XP-1 and EH-XP-13; Fig. 3a, c). Moreover, vesicles co-
exist with relict mineral fragments in most of the samples and
the fragments are then typically observed to display a corona
of particularly large sub-spherical vesicles (Fig. 3b, g, h, cf.
Carracedo et al. 2015; Zaczek et al. 2015).
Coalescence and near-coalescence of vesicle pairs are ob-
served in all of the samples, which is represented by sub-
spherical to elongated and polylobate vesicle shapes. We note
that neighbouring vesicle pairs, which are separated by a thin
glass film only, commonly show either flattened or convex/
concave vesicle walls (Fig. 3c, f, h). Moreover, we observe
wrinkled as well as ruptured vesicle walls, and plateau borders
(i.e. thin glass channels created at the vertex point of three
Fig. 3 a–i Three-dimensional micro-XCT renderings of El Hierro xeno-
pumice. All xeno-pumice reconstructions show cut surfaces that represent
an interior part of xeno-pumice. Scale bar is 500 μm throughout. The
solid phase is shown in grey (a–f), blue (g), yellow (h) or white (i). The
outer lava rim is included in a (see bottom). Note the vesicle coronas
around relict crustal fragments in e.g. b and c, as well as large vesicle
channels in e and f. In i, the vesicle space (gas phase) is presented as a
semi-transparent grey overlay on top of the solid phase (white) and is
shown with vesicle skeletons (red lines) that also visualise the connected
gas pathways. Note the intense vesicle networking in h and i
85 Page 6 of 13 Bull Volcanol (2016) 78: 85
neighbouring vesicles during close packing, cf. Mangan and
Cashman 1996, Fig. 3c, h). Indeed, the dominant fraction of
vesicles in El Hierro xeno-pumice has preserved sub-spherical
shapes but frequently shows punctured vesicle walls, consis-
tent with petrographic observations (Fig. 3d–f, i, e.g. Troll
et al. 2012; Carracedo et al. 2015). The computed vesicle
skeletons run through these narrow apertures (Fig. 3i), which
appear to function as gas pathways into the neighbouring ves-
icles and thus give rise to a continuous network of open po-
rosity and as such the development of rock permeability. The
number of skeleton branches that converge at the centre of
each vesicle, hence approximating the number of gas channels
that flow into or from each vesicle, is represented by the skel-
eton coordination number. The calculated mean coordination
numbers of the vesicle skeleton in each sample range from 3.4
to 5.3 (see Fig. 3i and Supplementary Table 2).
Xeno-pumice VSDs were analysed using maximal
inscribed sphere measurements that were produced from the
skeleton analyses (Supplementary Table 2). For optimal com-
parability between different samples and to minimise artefacts
caused by binning, the data were plotted using probability
density functions (PDFs, Fig. 4). All samples show right
skewed vesicle size distributions, with (main) modes between
0.06 and 0.08 mm. The tails exhibit irregularities, i.e. in-
creases in vesicle number over a given diameter range, that
dominantly occur at bubble diameters between 0.1 and
0.2 mm, and which are interpreted to reflect multiple minor
modes within the bubble size distributions (Fig. 4). Because
the diameters at which we observed the main modes are about
one order of magnitude above the detectability limit of struc-
tures in the 3D images, and because of the automated sam-
pling procedure applied, we note that our data are unlikely to
reflect resolution or sampling artefacts (cf. Degruyter et al.
2010; Giachetti et al. 2011).
Discussion
The appearance of floating xeno-pumice rocks during the ini-
tial phase of the El Hierro eruption led to confusion amongst
the authorities concerning their petrological significance (cf.
Carracedo et al. 2012a, 2015; Perez-Torrado et al. 2012).
Xeno-pumice fragments were viewed as possibly representing
high-silica magma, which might have indicated an imminent
explosive eruption (Meletlidis et al. 2012; Sigmarsson et al.
2013). However, the eruption remained purely effusive and
relatively minor, and the initial concerns regarding explosive
eruptive potential seemed unsubstantiated, at least in hindsight
(Carracedo et al. 2015). The uncertainties associated with
xeno-pumice during the 2011 eruption therefore highlight
Fig. 4 a–f Vesicle size distributions (VSDs) of the analysed El Hierro
xeno-pumice specimens plotted using probability density functions,
where the Y-axis shows probability density and the X-axis shows vesicle
diameter (mm). Vesicles in all samples have their mode at about 0.07 mm
(main peak (MP)), and the VSD curves are skewed to the right, with
bumpy tails throughout (irregularities in tail (IRT)). We interpret the
VSD trends to result from a major degassing pulse superimposed by
smaller degassing pulses. Low density data at >0.5 mm vesicle diameter
are not presented
Bull Volcanol (2016) 78: 85 Page 7 of 13 85
the need to better understand the xeno-pumice phenomenon in
general and to increase the knowledge-base on potential erup-
tive products for future Canary eruptions. In this context, our
3D-textural observations on El Hierro xeno-pumice samples
reveal snapshots of their vesiculation histories that were fro-
zen in on contact with seawater, and which will likely help us
to unravel their ultimate origin.
Vesicle textures in El Hierro xeno-pumice
All El Hierro xeno-pumice samples investigated here show
high-porosity and textural evidence of vesicle interaction
and coalescence (Fig. 3). The initial stages of vesicle interac-
tion are documented by small vesicles that frequently sur-
round the larger ones as a result of the internal pressure dif-
ferential between the differently sized vesicles, which causes a
diffusive exchange of volatiles (Ostwald ripening, cf.
Cashman and Mangan 1994). More advanced stages of vesi-
cle coalescence and subsequent quenching are recorded by
remnant plateau borders and incomplete vesicle wall retrac-
tion following a rupture event (cf. Cashman and Mangan
1994; Liu and Zhang 2000). Intense vesicle interaction prior
to quenching is evidenced by the observed dominance of
spherical vesicle shapes amongst larger vesicles in particular,
which is promoted by coalescence under a continuous supply
of volatiles (cf. Liu and Zhang 2000; Masotta et al. 2014).
In addition, labelling analyses in Pore3D, which distin-
guishes vesicle volumes that are connected from isolated ones,
confirm that the majority of the vesicles in our samples are
connected into a single volume, where gas could move freely
within the system (Fig. 3, Supplementary Table 2). Vesicle
coalescence and interaction essentially contribute to the devel-
opment of permeable vesicle networks that define the chan-
nels for the migration of volatiles from xeno-pumice into the
host basanite magma (Fig. 3h, i, cf. Saar and Manga 1999;
Polacci et al. 2008; Okumura et al. 2009). In addition to the
connected vesicle volumes, varying amounts of coexisting
isolated vesicles are present that do not contribute to the per-
meability of the rock (Fig. 3, Supplementary Table 2). These
isolated vesicle volumes that generally have preserved spher-
ical vesicle shapes, likely testify to a late stage of magma
degassing at the vent before eruption, followed by rapid
cooling that halted the vesiculation process (cf. Saar and
Manga 1999). We therefore infer that xeno-pumice vesicular-
ities capture the instant just prior to chilling against seawater.
Xeno-pumice vesicle size distributions (VSDs)
VSDs in rocks reflect the processes that gave rise to vesicle
size variations, e.g. single or multiple nucleation events, de-
gree of coalescence, and continuous or discontinuous growth
(e.g. Klug and Cashman 1994; Klug et al. 2002; Shea et al.
2010). In the simplest sense, each mode in a VSD diagram
represents a distinct vesicle population that evolves from a
temporally constrained nucleation event, while later coales-
cence tends to positively skew the data towards larger vesicle
diameters and may occasionally form a separate mode (e.g.
Cashman and Mangan 1994; Blower et al. 2002; Shea et al.
2010; Toramaru 2014). Multiple modes in VSD diagrams can
thus be produced by either secondary nucleation events or
vesicle coalescence, or by a combination of both (e.g. Bai
et al. 2008; Polacci et al. 2009). These two processes are
generally distinguished by the principle that secondary nucle-
ation would produce a higher number density of small bub-
bles, whereas coalescence would produce a higher number
density of larger bubbles (Toramaru 2014). We observe a high
number density of small matrix bubbles (<100 μm) in the El
Hierro xeno-pumice and a progressively lower number densi-
ty for large vesicles. This suggests the involvement of several
superimposed vesicle generations or pulses rather than succes-
sive growth and coalescence of a single vesicle population (cf.
Meletlidis et al. 2012; Sigmarsson et al. 2013). This notion is
furthermore supported by recurring irregularities on the VSD
curves (Fig. 4), which likely represent smaller degassing
pulses (cf. Blythe et al. 2015). These pulses are best explained
by diverse volatile sources in the protolith, which in the case
of a siliciclastic sedimentary protolith might represent e.g.
pore water and mineral water bound in a range of hydrous
mineral phases (e.g. micas or clays).
Notably, the frequently observed vesicle concentration and
growth at the margins of relict crystals and rock fragments in
our samples shows that crystal breakdown indeed contributed
a significant fraction of volatiles to the xeno-pumice texture
(Fig. 3b, g, h, see also Carracedo et al. 2015). Many of the
relict crystals are quartz grains, as well as occasional
phyllosilicates, gypsum, jasper, barite or halite (Troll et al.
2012; Rodriguez-Losada et al. 2015), consistent with H2O,
CO2, CH4 and SO2 liberation from hydrous phases, evaporate
minerals, calcareous and organic components, and with car-
bon oxides and H2O from degassing quartz grains (cf. Vasiloi
et al. 1985; Kendrick et al. 2006; Aarnes et al. 2011).
Moreover, the vesicle concentration textures we recorded
around relict sedimentary fragments and crystals are virtually
identical to textures that have been derived from experiments
where quartz was heated to magmatic temperatures. In these
experimental studies, quartz degassing created a halo of ves-
icles that attached to the crystal’s solid boundaries (Vasiloi
et al. 1985; Kendrick et al. 2006; Cluzel et al. 2008), matching
our observations on El Hierro xeno-pumice (Figs. 2c and 3c).
Testing for xeno-pumice origin
A xenolith that undergoes vesiculation controlled by its pore
and mineral volatile content, which can be fairly high in shal-
low sedimentary materials (Gluyas and Cade 1997), is expect-
ed to show pulsed degassing (e.g. Blythe et al. 2015). On the
85 Page 8 of 13 Bull Volcanol (2016) 78: 85
other hand, an isolated pocket of crystal-free and composition-
ally homogeneous felsic magma heated by a basanite magma
is expected to degas continuously from a single nucleation
event that is then further sustained by decompression (see
also Introduction section, e.g. Sigmarsson et al. 2013). The
latter scenario would produce a continuous vesicle size distri-
bution with a smooth unimodal VSD curve closely symmet-
rical around the mean (e.g. Shea et al. 2010). The former
scenario, in turn, would lead to a more irregular VSD curve
caused by variable volatile contributions from different com-
ponents in the protolith (cf. Blower et al. 2002; Shea et al.
2010; Toramaru 2014).
Our key observations of (i) highly variable vesicle mor-
phologies (Fig. 3), (ii) concentration of large vesicles around
relict crystals and sedimentary fragments in xeno-pumice
(Fig. 3b, c), and the (iii) irregular, right-skewed VSD curves
(Fig. 4) support the presence of multiple vesicle generations
that formed during separate degassing pulses. These textural
observations emphasise a rapid, but heterogeneous inflation
process, with degassing predicted to be more intense around
disintegrating residual volatile-rich rock fragments (Carracedo
et al. 2015; Zaczek et al. 2015). Consequently, the irregular
vesicle size distribution and the spatial heterogeneity of vesi-
cles within the samples are more consistent with progressive
release of volatiles from the various components of a sedimen-
tary protolith rather than from a crystal-free, single-phase
magma. The various vesiculation pulses from sedimentary
protoliths might then result from the release of various gas
mixtures derived from (1) pore fluids, (2) mineral water (e.g.
from clays), carbon oxides and methane (e.g. from carbonates
and organics), (3) SO2 (e.g. from sulphates and other evapo-
rates) and (4) water and carbon oxides from nominally anhy-
drous phases (e.g. quartz). In this respect, pore water would
likely constitute the first and strongest vesiculation pulse,
followed by less intense pulses from the various mineral
constituents.
Model for vesiculation of sedimentary derived El Hierro
xeno-pumice
Assuming a water saturated silicic sedimentary protolith for
the El Hierro high-silica xeno-pumice samples (e.g. > 4 wt.%
H2O, cf. Zaczek et al. 2015), pore water devolatilisation will
commence on heating. Once H2O-saturated melting of xeno-
pumice sets in (at ∼850 °C), mineral degassing will create
further vesicle generations (Fig. 5a). Some volatiles would
remain structurally bound in restite minerals until the final
phase of mineral devolatilisation is initiated (e.g. at ambient
basanite temperature, 1150 °C) or when a last gasp of mineral
bound volatiles is released on final decompression (Fig. 5a, b).
Mineral devolatilisation is thus expected to generate small
pulses of vesicle nucleation from the various types of mineral
constituents present, which is consistent with the VSD curves
of the El Hierro xeno-pumice (Fig. 4).
Although degassing of sporadically occurring restite min-
erals would only contribute a few percent of vesicles to the
system, and notably in a spatially random fashion, this late
generation of vesicles would tend to grow rapidly due to the
additional local supply of volatiles from the minerals, which
would promote coalescence with neighbouring vesicles and
consequent vesicle expansion (see Figs. 3c and 5b). This pro-
cess would result in distinctly large vesicles around
dehydrating minerals and fragments (Vlarge), which co-exist
with smaller matrix vesicles (Vsmall, see Fig. 3b, c).
Decompression during xeno-pumice ascent in the conduit
would then cause further vesicle expansion, coalescence, wall
rupture and vesicle networking. We note, however, that small
vesicles in El Hierro xeno-pumice are more often tortuously
shaped (Vsmall) and may reflect viscous deformation during
conduit flow or rapid growth of large vesicles in their vicinity.
Large vesicles, on the other hand, appear to be less affected by
shearing and viscous deformation probably because of the
continuous supply of volatiles into these larger vesicles and
the increasing stiffness of the surrounding melt films that
would become strongly volatile depleted and therefore have
higher viscosity (Schmincke and Swanson 1967; Spark 1978;
Hammer et al. 1999; Fig. 3e).
Pulsed xeno-pumice vesiculation (e.g. Fig. 5b) seems to
have caused the xeno-pumice protolith to rapidly increase in
porosity, thus promoting volatile percolation through the rock
and ultimately into the host magma. Gas loss from xeno-
pumice was then associated with at least partial vesicle con-
traction and deformation, which reduced vesicularity while
permeability probably remained at a similar level (Fig. 5b,
Rust and Cashman 2004; Cashman and Sparks 2013).
Interaction of magma with pre-island sedimentary rocks
Xeno-pumice occurred exclusively during the first week of
the El Hierro 2011/2012 eruption, probably as a result of mag-
ma migrating laterally through the crust at shallow sub-island
levels during the two weeks of shallow unrest prior to the
actual eruption (e.g. López et al. 2012; González et al. 2013;
Carracedo et al. 2015). The assortment of xeno-pumice tex-
tures likely reflects the compositional variability of the
Cretaceous to Pliocene pre-island siliciclastic and subsidiary
calcareous sediments, as well as variable interaction time prior
to eruption (cf. Robertson and Stillman 1979a, b; Gee et al.
2001; González et al. 2013; Zaczek et al. 2015). Xeno-pumice
vesiculation was then further promoted by depressurisation
and the associated decrease in volatile solubility during buoy-
ant rise in the ascending magma (Fig. 5). Shearing along the
conduit lining could have caused vesicle distortion, e.g. via
pressure shadows (cf. Kennedy et al. 2005), while the domi-
nantly (sub-)spherical shape of the largest vesicles likely
Bull Volcanol (2016) 78: 85 Page 9 of 13 85
argues for continuous bubble growth and inflation during final
ascent, i.e. until chilling occurred in seawater. Simultaneously,
the high vesicle density and varying size range promoted ves-
icle coalescence, which, in turn, caused selective growth of
large vesicles at the expense of the total vesicle number (e.g.
Masotta et al. 2014). Rapid quenching in seawater upon erup-
tion then halted xeno-pumice vesiculation and a fraction of
gas was trapped as isolated vesicles, freezing in the ongoing
degassing process before it reached completion (Figs. 3 and
5c, and Supplementary Fig. 2).
Fig. 5 a Phase diagram for predicted melting behaviour of 2011 El
Hierro xeno-pumice calculated by Perple_X (modified after Zaczek
et al. 2015). The yellow stars denote successive degassing steps. Pore
water likely formed the main volatile source during initial H2O saturated
melting (1). Subsequently, mineral water was liberated from dehydration
melting of hydrous mineral phases (2). Finally, decompression during
ascent in the conduit triggered further vesiculation pulses from the melt
and from restite materials (3). b Sequential (pulsed) degassing of xeno-
pumice is schematically summarised in a vesicularity vs. permeability
hysteresis (cf. Cashman and Sparks 2013). Steps i–v represent key pro-
cesses of progressive xeno-pumice evolution during degassing, which is
illustrated by c snapshots of the proposed degassing sequence as recorded
in SEM images and 3D tomographs of El Hierro xeno-pumice
85 Page 10 of 13 Bull Volcanol (2016) 78: 85
Explosive potential of sedimentary xeno-pumice
Eruptive products similar to the El Hierro xeno-pumice are
known from historic and Holocene volcanic activity in adja-
cent Canary Islands, but these may not have been fully appre-
ciated at the time of the eruption at El Hierro in 2011 (see
Carracedo et al. 2015). For example, sedimentary silica-rich
xenoliths with occasional interlayered clay and carbonates, as
well as fossil-bearing limestones and shales have been report-
ed from Gran Canaria, La Palma, Lanzarote, Fuerteventura,
and from the submarine volcanic edifice of Hijo de Tenerife
(e.g. Rothe and Schmincke 1968; Arãna and Ibarrola 1973;
Hoernle 1998; Steiner et al. 1998; Klügel et al. 1999;
Schmincke and Graf 2000; Hansteen and Toll 2003;
Aparicio et al. 2006, 2010). Furthermore, frothy sedimentary
xenoliths often tend to occur in association with explosive
eruptive events. For instance, xeno-pumice fragments from
the 1730–1736 A.D. eruption on Lanzarote are more frequent
in the explosive lapilli beds than in the associated lavas
(Carracedo et al. 2015). This co-occurrence suggests that a
connection between xeno-pumice and locally intensified
explosivity may exist, which stresses the need to investigate
xeno-pumice in yet more detail.
For instance, sedimentary rocks maymaintain porosities up
to 20 vol.% even at several kilometer depth, which provide
pore space to host several wt.% of fluids like e.g. H2O, CO2
and hydrocarbons (Gluyas and Cade 1997). Consequently, the
vesiculation of sedimentary xenoliths has the potential to re-
lease additional volatiles into a magmatic system (cf.
Carracedo et al. 2015). At these shallow levels in the crust,
the released volatiles would contribute directly to the gas out-
put of the eruption, as the volatiles would not likely be
reabsorbed by the melt (cf. Holloway and Blank 1994). This
realisation could help explain the phenomenon of
phreatomagmatic eruptions in otherwise arid areas of the ar-
chipelago (e.g. Clarke et al. 2009), as extra volatiles would be
progressively liberated into the magma during conduit ascent,
perhaps analogous to an effervescent tablet in water. Hence,
xeno-pumice may have the potential to temporarily increase
the intensity of an eruption by shifting eruptive behaviour
towards a more phreatomagmatic character.
Summary
Based on our observations of (i) highly variable vesicle mor-
phologies, (ii) concentration of large vesicles at crystal bound-
aries and sedimentary rock fragments, and (iii) irregularities in
right-skewed VSD tails, we argue that the vesiculation history
of El Hierro xeno-pumice is best explained by pulsed
degassing from a heterogeneous protolith. We conclude that
vesicles in xeno-pumice did not form during a single vesicu-
lation event, but instead during several vesiculation pulses.
Although the observed VSD and vesicle shapes do not by
themselves rule out a magmatic origin of xeno-pumice, when
considered in conjunction with the other available evidence
(e.g. sedimentary mineralogy, low trace element concentra-
tions and elevated oxygen isotopes), our data point to a sedi-
mentary origin of El Hierro xeno-pumice. El Hierro xeno-
pumice samples are probably a product of magma-sediment
interaction during magma migration through the pre-island
oceanic crust prior to eruption. Volatiles contained in sedimen-
tary xenoliths were released on heating to form highly vesic-
ular xeno-pumice, which might have increased the magma’s
volatile load and temporarily intensified the character of the
eruption. These aspects should be considered during future
Canary eruptions where xeno-pumice may be amongst the
erupted products.
Acknowledgments We thank S. Wiesmaier, A. Klügel, M.-A.
Longpré, U. Kueppers, B. Dahrén, L. S. Blythe, T. Hansteen, C. Freda,
D. Budd, E. M. Jolis, E. Jonsson, F. C. Meade and K. Zaczek for stimu-
lating discussions on El Hierro xeno-pumice. Extended gratitude also
goes to D. Dreossi and N. Sodini for their help during XCT experiments
at Elettra. We are grateful to W. Degruyter and an anonymous reviewer
for constructive feedback and to J. Gardner for editorial handling of our
manuscript. Funding from the European Science Foundation (ESF)
through the MeMoVolc research network (Measuring and Modelling of
Volcano Eruption Dynamics, www.memovolc.fr), the Spanish program
MEC-CGL2011-25494, the Royal Swedish Academy of Sciences
(KVA), the Swedish Research Council (VR) and the Centre of Natural
Disaster Science (CNDS) at Uppsala University is gratefully
acknowledged.
Open Access This article is distributed under the terms of the Creative
Commons At t r ibut ion 4 .0 In te rna t ional License (h t tp : / /
creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a link
to the Creative Commons license, and indicate if changes were made.
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