The 2014 eruption at Piton de la Fournaise (PdF), La Réunion, which
occurred after 41 months of quiescence, began with surprisingly little
precursory activity and was one of the smallest so far observed at PdF in
terms of duration (less than 2 days) and volume (less than 0.4
A detailed characterization and understanding of eruptive dynamics and of processes driving and modulating volcano unrest are crucial in monitoring active volcanoes and fundamental for forecasting volcanic eruptions (Sparks, 2003). Many studies suggest that eruptive phenomena are strongly dependent on the physico-chemical properties of ascending magma in the conduit (e.g. temperature, viscosity, porosity, and permeability) (e.g. Sparks, 1978; Rust and Cashman, 2011; Gonnermann and Manga, 2013; Polacci et al., 2014). Integrating petrographic, chemical, and textural data can thus provide critical information to constrain both the pre-eruptive storage conditions and the processes related to magma ascent, degassing, and cooling (e.g. reference in Table 1 in Gurioli et al., 2015). This multidisciplinary approach is of even greater importance in the monitoring of volcanoes that emit relatively uniform magma compositions over time, like basaltic volcanoes (e.g. Di Muro et al., 2014; Gurioli et al., 2015; Coppola et al., 2017). As a result, monitoring of textures and petrochemical properties of lava fragments and pyroclasts is now routinely carried out on a daily basis at active volcanoes such as Kīlauea, Etna, and Stromboli (e.g. Taddeucci et al., 2002; Thornber et al., 2003; Polacci et al., 2006; Swanson et al., 2009; Colò et al., 2010; Houghton et al., 2011, 2013, 2016; Carey et al., 2012, 2013; Lautze et al., 2012; Andronico et al., 2013a, b, 2014; Corsaro and Miraglia, 2014; Di Muro et al., 2014; Gurioli et al., 2014; Eychenne et al., 2015; Leduc et al., 2015; Kahl et al., 2015). In the past, time series of petrographic and geochemical data have been measured for Piton de la Fournaise (PdF) basalts and particularly for effusive products. The aim of these datasets was to constrain the spatial and temporal evolution of magma for one of the most active basaltic volcanoes in the world (e.g. Albarède et al., 1997; Vlastélic et al., 2005, 2007, 2009; Boivin and Bachèlery, 2009; Peltier et al., 2009; Schiano et al., 2012; Lénat et al., 2012; Di Muro et al., 2014, 2015; Vlastèlic and Pietruszka, 2016). However, this type of approach has seldom been coupled with detailed textural studies at PdF and instead has mostly focused on crystal textures and crystal size distribution (Welsch et al., 2009, 2013; Di Muro et al., 2014, 2015). Moreover, only sporadic data exist on the textures of pyroclasts ejected by the eruptions at PdF (Villemant et al., 2009; Famin et al., 2009; Welsch et al., 2009, 2013; Michon et al., 2013; Vlastélic et al., 2013; Di Muro et al., 2015; Morandi et al., 2016; Ort et al., 2016).
Within this paper, we present a multidisciplinary textural, chemical, and
petrological approach to quantify and understand the short-lived 2014 PdF
eruption. This approach combines detailed study of the pyroclastic deposit
(grain size and componentry) with bulk texture analysis (density,
vesicularity, connectivity, permeability, morphology, vesicle distribution,
and crystal content) and a petrochemical study (bulk rock, glass, minerals,
melt inclusions) of the same clasts. This integrated approach has now been
formalized within the French National Observation Service for Volcanology
(SNOV), as routine observational systems Dynamics of Volcanoes
(DynVolc,
In spite of being the first of a series of eruptions, the June 2014 event
was preceded by only weak inflation and by a rapid increase in the number of
shallow (< 2 km below volcano summit) volcano tectonic earthquakes
that happened only 11 days before the eruption (Peltier et al., 2016). The
eruptive event was dominantly effusive, lasted only 20 h, and emitted a
very small volume of magma (ca. 0.4
This eruption occurred just outside the southern border of the summit Dolomieu caldera, at the top of the central cone of PdF (Fig. 1). This is a high-risk sector because of the high number of tourists. Identification of precursors of this kind of activity represents an important challenge for monitoring systems (Bachélery et al., 2016).
Therefore this eruption represents an ideal context for applying our
multidisciplinary approach, with the aim of addressing the following key
questions:
Why was such a small volume of magma erupted instead of remaining endogenic? What caused the rapid trigger and the sudden end to this small-volume
eruption? Which was the source of the eruption (shallow versus deep and single versus
multiple small magma batches)? What was the ascent and degassing history of the magma? What was the time and space evolution of the eruptive event?
Furthermore, this eruption provides an exceptional opportunity to study
processes leading to the transition from mild Hawaiian (< 20 m high
fountains, following the nomenclature proposed by Stovall et al., 2011) to
Strombolian activity (< 10 m high explosions), whose products are
little modified by post-fragmentation processes because of the very low
intensity of the activity.
The 20 June 2014 summit eruption represents the first eruption at PdF after
41 months of quiescence. The last eruption had been on 9 December 2010, with
a shallow (above sea level) intrusion on 2 February 2011 (Roult et al.,
2012). From 2011, the deformation at PdF was constant with two distinct
types of behaviour: (i) a summit contraction of a few centimetres every year
(Fig. 1d) and (ii) a preferential displacement of the east flank at a rate
of 1–3 cm per year (Brenguier et al., 2012; Staudacher and Peltier,
2015). The background microseismicity was very low (less than five shallow
events per day below volcano summit) and low-temperature summit intracaldera
fumaroles emitted very little sulfur (H
The 2014 summit eruption started during the night of 20–21 June, at
21:35 GMT (00:35 local time) and ended on 21 June at 17:09 GMT (21:09 local time),
after less than 20 h of dominantly effusive activity. The volcano
reawakening was preceded, in March and April 2014, by deep (15–20 km below
sea level) eccentric seismicity and an increase in soil CO
Photo collection from the 2014 eruption at the MV, highlighted
with a white cross (see location in Fig. 1). From
We reconstructed the chronology of the events by combining a distribution map of the fissures, pyroclastic deposits, and lava flows (Fig. 1) with a review of available images and videos extracted from the observatory database, the local newspapers, and websites (Fig. 2). The 2014 eruption occurred at the summit and on the SE slopes of the Dolomieu caldera (Fig. 1a, b, and c) and evolved quickly and continuously over 20 h. The full set of fractures opened during a short period of time (minutes) and emitted short (< 1.7 km long) lava flows (Figs. 1 and 2c and d). Feeding vents were scattered along a 0.6 km long fissure set (Fig. 1a) and produced very weak (low) Hawaiian to Strombolian activity (Fig. 2).
Fissures opened from west to east, initially subparallel to the southern
border of Dolomieu caldera and then propagated at lower altitude (Fig. 1).
The summit part of the fractures (ca. 2500 m a.s.l., western fracture, WF in
Fig. 1) emitted only small volumes of lava and pyroclasts. This part of the
fracture set was active only during the first few hours of the eruption, at
night. The eastern part of the fractures (upper fracture, UF in Fig. 1)
descended to lower altitude (between 2400 and 2300 m a.s.l., middle fracture,
Fig. 1) along the SE flank of the summit cone and emitted most of the
erupted volume. As often observed in PdF eruptions, the activity
progressively focused on a narrow portion of the fractures at low altitude
and finally on a single vent located at the lower tip of the fracture system
(main vent, at 2336 m a.s.l., MV in Figs. 1, 2). The first in situ observations
in the morning of 21 June (ca. 04:00 GMT) showed that weak Strombolian
activity (Fig. 2a and b) was focused on a narrow segment of the lower
fractures and that aa lavas had already attained the elevation of 1983 m a.s.l.
(0.2 km before maximum runout, Fig. 2c). A small, weak gas plume was
also blowing northwards. A single sample of partially molten lava was
collected from the still active lava front and partially water quenched
(REU140621-1; Table S1 in the Supplement, Fig. 2d). During most of 21 June, the activity
consisted of lava effusion in three parallel lava streams (Fig. 2c) merging
in a single lava flow (Fig. 2e) and weak Strombolian explosions at
several closely spaced spots along the lower part of the feeding fracture.
At 13:00 (GMT), only weak explosions were observed within a single small
spatter cone (Fig. 2f and g). Most of the lava field was formed of open
channel aa lavas. The total volume of lava was estimated by the MIROVA service
(
Textural features of June 2014 pyroclasts and lava. Clast shows a photo
of the different types of juvenile pyroclasts and lava channel. The photo of
the lava channel is from Laurent Perrier. WF: western fracture (smooth
fluidal scoria); MV: main vent (fluidal scoria, less smooth than the ones
at the WF). Thin section is the thin section imaged with a desktop scanner.
Microscope is the picture taken with an optical microscope using natural
light.
SEM (25X) is the image captured using scanning electron microscopy (SEM) in
BSE mode at 25x magnification: black is vesicles, white is glass, grey is
crystals. VSD is the vesicle size distribution histograms, where the diameter,
in millimetres, is plotted versus the volume percentage.
Apart from the sample from the front of the still active lava flow (Fig. 2d), all other samples were collected in two phases: 3 days (pyroclasts on
24 June, Fig. 3a and Table S1) and 11 days after the eruption (lavas on
2 July, Table S1), and 3 months later (pyroclasts from the MV, Fig. 1, on
18 November and Table S1). The 24 June samples were collected from the main
fractures (WF and UF, Fig. 1a), the MV, and the active lava flow (Fig. 1 and
Table S1). We collected 25 scoriaceous bombs and lapilli (REU140624-9a-1 to
REU140624-9a and REU140624-9b-6 to REU140624-9b-25, in Table S3) from the discontinuous deposit (Fig. 3d) emplaced at the WF site
(Fig. 1a), active only at the beginning of the eruptive event. Because of
the short duration of the activity at the WF, the scoria fragments on the
ground were scarce (Fig. 3d). The strategy was to collect a sample that was
formed by the largest available number of clasts that was representative of
this discrete deposit (REU140624-9 in Table S1). From the UF (Fig. 1a) only
one big scoria was collected (REU140624-13, Table S1) that broke in five
parts, allowing us to measure its vesiculated core and the dense quenched
external part (REU140624-13-a to REU140624-13-e, in Table S3). In contrast,
the sustained and slightly more energetic activity at the lower tip of the
fractures, at the MV site, built a small spatter cone (Fig. 2) and
accumulated a continuous, small-volume deposit (Fig. 3a) of inversely graded
scoria fallout (Fig. 3b and c). This deposit is 10 cm thick at 2 m from
the vent and covers an area of about
Proportion of each type of clast measured from the base to the top
of the 10 cm thick deposit emplaced during the eruption, at the MV site. The
deposit is dominated by Hawaiian-like lapilli fragments at the base (golden
pumice and fluidal scoria) and Strombolian-like bombs and lapilli at the top
(spiny scoria):
Density, connectivity, and permeability data of June 2014 pyroclast
and lava fragments:
We performed grain size analyses on the two bulk samples collected from the
MV, following the procedure of Jordan et al. (2016) (Table S2). The samples
were dried in the oven at 90
Following the nomenclature of White and Houghton (2006) the componentry
analysis is the subdivision of the sample into three broad components: (i) juvenile,
(ii) non-juvenile particles, and (iii) composite clasts. The
juvenile components are vesicular or dense fragments, as well as crystals,
that represent the primary magma involved in the eruption; non-juvenile
material includes accessory and accidental fragments, as well as crystals
that predate the eruption from which they are deposited. Finally, the
composite clasts are mechanical mixtures of juvenile and non-juvenile
(and/or recycled juvenile) clasts. In these mild basaltic explosions, the
non-juvenile component is very scarce, so we focused on the juvenile
component that is characterized by three groups of scoria: (i) spiny opaque,
(ii) spiny glassy, and (iii) fluidal, along with golden pumice (Fig. 4). The
componentry quantification was performed for each grain size fraction
between
In the following, we will use the crystal nomenclature of Welsch et al. (2009), with the strictly descriptive terms of macrocrysts (> 3 mm in diameter) mesocrysts (from 0.3 to 3 mm in diameter), and microcrysts (< 0.3 mm in diameter). Regarding the June 2014 products, these ranges of size may however change in comparison to the December 2005 products studied by Welsch et al. (2009).
For each sample site (WF, UF, and MV; Fig. 1a), we selected all the available
particles within the 8–32 mm fraction for density–porosity, connectivity, and
permeability measurements (Table S3). This is the smallest granulometric
fraction assumed to be still representative of the larger size class in
terms of density (Houghton and Wilson, 1989; Gurioli et al., 2015) and has
been used in previous textural studies (e.g. Shea et al., 2010). In
addition, this size range is ideal for vesicle connectivity measurements
(e.g. Formenti and Druitt, 2003; Giachetti et al., 2010; Shea et al., 2012;
Colombier et al., 2017a, b). Density of juvenile particles was measured using
the water-immersion technique of Houghton and Wilson (1989), which is based
on Archimedes' principle. A mean value for the vesicle-free rock density was
determined by powdering clasts of varying bulk densities, measuring the
volumes of known masses using an AccuPyc II 1340 helium pycnometer, and then
averaging. The same pycnometer was also used to measure vesicle
interconnectivity for each clast using the method of Formenti and
Druitt (2003) and Colombier et al. (2017a). Permeability measurements were
performed on five clasts: two golden pumices, one fluidal, one spiny glassy,
and one opaque scoria, all collected from the MV (Table S3). Following
Colombier et al. (2017a), the clasts were cut into rectangular prisms to
enable precise calculation of the cross-sectional area, which is required to
calculate permeability. These prisms were then embedded in a viscous resin,
which was left to harden for 24 h. The sample surface had been previously
coated with a more viscous resin and then wrapped with Parafilm to avoid
intrusion of the less viscous resin inside the pores. The coated samples
were placed with a sample holder connected to a permeameter built at
Laboratoire Magmas et Volcans (LMV, France) following Takeuchi et al. (2008). The measurements were performed at atmospheric pressure (i.e.
without confining pressure) and the samples were measured at a range of gas
flow rates and upstream air pressures to create a curve that could be fitted
using a modified version of Darcy's law (the Forchheimer equation) to solve
for viscous (
Vesicle size distribution was performed following the method of Shea et al. (2010) and Leduc et al. (2015), while the total crystallinity, the percentages for both crystal phases (plagioclase and clinopyroxene), and size populations (meso- and microcrysts) were calculated using the raw data from the FOAMS program (Shea et al., 2010), the CSD corrections program of Higgins (2000), and the CSDslice database (Morgan and Jerram, 2006) to have the percentage of crystals in 3-D with the corrected assumption for shape. We performed these analyses on eight clasts picked up from each component and density distributions (stars in Fig. 6a and b). The choice of the clasts was made mostly on the typologies, rather than on each density distribution, in order to avoid the analysis of clasts with transitional characteristics. For example, two golden pumice fragments were selected from the largest clasts that were less dense and did not break, even if the values in vesicularity were similar. A larger number of fluidal fragments were chosen (even if the density distribution was unimodal) because this typology of clasts was the most abundant and was emitted all along the active fracture; thus we did our best in order to study products representative of the WF, the UF, and the MV activities. Only one spiny glassy and one spiny opaque were selected because they were emitted only at the MV. A full description of the textural measurements all performed at LMV and the raw data of these measurements are available in the DynVolc database (2017).
Ni-Cr concentration plot.
For the determination of the bulk chemistry (Table S4 and Fig. 7) of the
different pyroclasts we selected the largest pyroclasts of golden pumice and
the largest fluidal, spiny glassy and spiny opaque scoriae (Table S4). We
also analysed two fragments of lava, from the beginning and the end of the
eruption (Table S4). Samples were crushed into coarse chips using a steel
jaw crusher and powdered with an agate mortar. Major and trace element
compositions were analysed using powder (whole-rock composition). In
addition, for a subset of pyroclasts, glass chips (2–5 mm in size) were
hand-picked under a binocular microscope and analysed separately for trace
elements. For major element analysis, powdered samples were mixed with
LiBO2, placed in a graphite crucible, and melted in an induction oven at
1050
Spot analyses of matrix glass and crystal composition (Table S5) were
carried out using a Cameca SX100 electron microprobe (LMV), with a 15 kV
acceleration voltage of a 4 nA beam current with a 15 kV
acceleration voltage and a beam of 5
FeO
Melt inclusions (MIs; Table S6, Figs. 8b and 9) were characterized in the olivine mesocrysts from the three groups of scoriae (fluidal, spiny glassy and spiny opaque) but not in the pumice group because crystals were too rare and small to be studied for MIs.
Olivine crystals were handpicked under a binocular microscope from the 100–250
and 250–600
The pyroclastic deposits at the WF and UF sites (Fig. 1a) are formed by scattered homogeneous smooth fluidal (Fig. 3d) bombs and lapilli scoria. The average dimension of the fragments is around 4 cm (maximum axis) with bombs up to 10 cm and scoria lapilli up to 2 cm in size (Fig. 3e).
At the MV, the reversely graded deposit (Fig. 3b) is made up of lapilli and bombs, with only minor coarse ash (Fig. 3c). The lower 5 cm at the base is very well sorted and characterized by a perfect Gaussian distribution with a mode at 4 mm (Fig. 3c). In contrast, the grain size distribution of the upper 5 cm is asymmetrical with a main mode coarser than 22 cm and a second mode at 8 mm (Fig. 3c). This upper deposit is negatively skewed due to the abundance of coarse clasts. The dataset shows a similarity between the grain size distributions of the basal tephra ejected from the 2014 MV and the ones for the lava fountaining of the 2010 summit event (Fig. 3f and Hibert et al., 2015). Conversely, the top of the 2014 fall differs from fountain deposits, being coarser and polymodal, and it is ascribed to dominantly Strombolian activity (Fig. 3f).
In terms of componentry of the deposits, four types of clasts were
distinguished (Fig. 4): (i) golden pumice, (ii) smooth or rough fluidal
scoriae, (iii) spiny glassy scoria, (iv) spiny opaque scoria. The pumices
are vesicular, low-density fragments, characterized by a golden to light
brown colour, sometimes with a shiny outer surface (Fig. 4a). They are
usually rounded in shape. Golden clasts studied for textures contain a few
microcrysts of plagioclase (up to 0.1 mm in diameter), clinopyroxene up to
0.05–0.06 mm in diameter and small olivine up to 0.03 mm in diameter (Fig. 4),
together with large areas of clean, light brown glass. The fluidal
scoria fragments have dark, smooth or rough shiny surfaces (Fig. 4b). They
can be more or less elongated in shape and have spindle as well as flattened
shapes. The fluidal fragments are characterized by rare mesocrysts of
plagioclase and clinopyroxene and microcrysts of plagioclase, clinopyroxene
and olivine (Fig. 4b). The spiny glassy fragments are dark, spiny scoria
that range in shape from subrounded to angular (Fig. 4c). These fragments
contain abundant glassy areas, while the spiny opaque fragments lack a
glassy, iridescent surface. Both groups of spiny clasts are characterized by
the presence of dark and light brown glass. The spiny opaque fragments are
the densest fragments and have the largest number of crystals. They contain,
as the most abundant phase, relatively large meso- and microcrysts of
plagioclase, up to 3 mm long, together with meso- and microcrysts of
clinopyroxene and olivine (Fig. 4c and d). In the dark portions of their
matrix, tiny fibrous microcrysts of olivine
The componentry results are reported in Fig. 5 only for the MV deposits because the deposits from the WF and UF are characterized exclusively by fluidal clasts (Fig. 3). At the base of the MV deposit, the coarse fraction of the deposit is rich in golden and fluidal components that represent more than 60–70 vol. % (Fig. 5a and b). The proportion of the two groups is similar. In contrast, in the upper coarsely grained fall deposit, the clasts bigger than 8 mm are dominated by the spiny scoria fragments, while the fraction of clasts smaller than 8 mm show a dramatic increase in the golden and fluidal fragments, with the fluidal ones always more abundant than the golden ones (Fig. 5a and b). Abundant low-density golden, coarse lapilli pumice and bombs have been found scattered laterally up to 30 m from the main axis and were not found in the proximal deposit. On the basis of the high amount of pumice in the lower part of the deposit, we correlate the large, low-density clasts with the base of the proximal deposit, and consequently we interpret them as material emitted at the beginning of the June 2014 eruptive event.
Density analyses performed on 200 coarse lapilli reveal a large variation in
density values from 390 to 1700 kg m
In all these samples, the increase in vesicularity correlates with an increase in the number of small (0.1 mm), medium (0.5–1 mm) and large (up to 4 mm) vesicles. In the fluidal clasts, these vesicles have a regular rounded or elliptical shape and are scattered throughout the sample. The low-density pumices are often characterized by the presence of a single, large central vesicle (10–15 mm) with the little vesicles and a few medium vesicles distributed all around it (Fig. 4). The spiny glass texture is characterized by a lower number of small vesicles than in the pumice and by the presence of mostly medium-sized vesicles, while the spiny opaque has more irregular shaped and very large (up to 10 mm) vesicles with a small- and a medium-sized vesicle population. In the spiny glass samples, the glass is more or less brown, with the dark brown portions being the ones with the lowest vesicle content and the highest microcryst content. The opaque samples have a central, very dark glass portion, with low vesicle content, and a more vesicular glassy portion at the outer edges (Fig. 4). The two fragments of lava are poorly vesiculated (Fig. 6a) and characterized by large, irregular vesicles (up to 5 mm in diameter). Clusters of small vesicles (up to 0.1 mm) are scattered between the large ones.
The vesicle size distribution (VSD in Fig. 4) histograms are characterized
by a decrease in percentage of vesicles from the golden pumice to the lava as well
as an increase in coalescence and or expansion signatures in the spiny
fragments, marked by the increase in the large-vesicle population (Fig. 4c and d).
This trend is also marked by the decrease in number of vesicles
per unit of volume (
The connectivity data (Fig. 6c) also indicate that the fluidal and golden
clasts have a larger number of isolated vesicles (up to 40 vol. %) with
respect to the spiny products. The fluidal clasts from the WF are the most
homogeneous with an average percentage of isolated vesicles around 30 vol. %. In contrast, both the pumice and the fluidal fragments from the MV,
characterized by higher values of porosity (> 75%), have a
wide range in percentage of isolated vesicles (between 20 and a few vol. %). The fragments of the bomb collected at the UF are consistent with a
vesiculated core characterized by scarce isolated vesicles and the quenched
rind that has 30 vol. % of isolated vesicles. Finally, the spiny fragments
have the lowest content of isolated vesicles (0–5 vol. %). Despite the
presence of these isolated vesicles, all the samples shear high values of
permeability, with the Darcian (viscous,
Major and trace element concentrations of whole-rock and hand-picked glass
samples are reported in Table S4. Whole-rock major element composition is
very uniform (e.g. 6.5
A closer inspection of Ni-Cr variability in June 2014 whole-rock samples (Fig. 7b) reveals that scoria from the WF (140624-9b-6, Table S4) and early erupted lavas (1406-21-1, Table S4) have the lowest amount of olivine (< 0.9 %) whereas scoria from the UF (140624-13a) and late erupted lavas (140324-12) have a slightly higher amount of olivine (> 1.2 %). This is consistent with the general trends observed at PdF of olivine increase from the start to end of an eruption (Peltier et al., 2009).
The so-called “olivine control trend” in Ni-Cr space cannot be explained
by either addition of pure olivine, which contains less than 500 ppm Cr
(Welsch et al., 2009; Salaün et al., 2010; Di Muro et al., 2015), or by
the addition of olivine plus pyroxene (which would require ca. 50 %
pyroxene with 970 ppm Ni and 4800 ppm Cr; see Fig. 7 caption). Instead,
addition of olivine hosting ca. 1 % Cr spinel (with 25 wt % Cr)
accounts for data and observations and is consistent with crystallization
of olivine and Cr spinel in cotectic proportions (Roeder et al., 2006). The
fact that some samples (golden pumice) plot off the main well-defined
array can be explained by either addition of more or less evolved olivine
crystals (within the range of Fo
The glass chemistry of the four clast types allows us to correlate porosity and oxide contents and shows an increase in MgO from the spiny opaque to fluidal and golden fragments (Fig. 8a). Consistent with petrological and textural observations, the spiny opaque is the most heterogeneous type of clast in terms of glass composition (Fig. 8). The glassy portion at the edge of the clast is similar to the spiny glass, while the interior, characterized by dark areas rich in tiny fibrous microcrysts, shows scattered glass compositions with very low MgO content as well as a decrease in CaO (Fig. 8). We attribute the significant variation in glass composition within the different components to variable degrees of micro-crystallization as the bulk chemistry of all clasts is very similar and globally homogeneous.
MI analyses must be corrected for post-entrapment host crystallization at
the MI–crystal interface. We used a Kd
Host olivines span a large compositional range from Fo
MIs in olivines from June 2014 can best be compared with those of other
recent small-volume and short-lived eruptions that emitted basalts with low
phenocryst contents, like those in March 2007 (0.6
Two populations of low- and high-Ca MIs are also found in the
November 2009 olivines. Low-Ca melt inclusions from the November 2009 and
June 2014 eruptions indicate a single trend of chemical evolution (Fig. 8),
consistent with bulk rock compositions. June 2014 products have lower MgO
and CaO contents than those from November 2009. Significant scattering in
K
All 2014 scoriae (spiny, fluidal, golden) contain the same paragenesis of olivine, clinopyroxene and plagioclase. The composition of minerals found in golden, fluidal and spiny scoriae is indistinguishable.
In olivines, average MgO content decreases from macrocrysts (Fo
Plagioclase–melt equilibrium and melt composition in pyroclastic rocks and
water-quenched lavas were used to estimate both temperature and water
content dissolved within the melt (Fig. 10b and Table S5). Temperature
estimates are based on the (dry) equation of Helz and Thornber (1987)
recalibrated by Putirka (2008). Dissolved water content was calculated from
the plagioclase hygrometer of Lange et al. (2009) at 50 MPa. This pressure
corresponds to the average CO
In order to determine pre-eruptive conditions, calculations were performed
only on paired plagioclase rims and matrix glasses in equilibrium, using the
plagioclase–melt equilibrium constant of Putirka (2008) calibrated for
melts
whose temperature exceeds 1050
The activity fed by the uppermost WF and UF (Fig. 1) was very short-lived, as shown by the presence of only scattered bombs and coarse lapilli (Fig. 3d and e). The homogeneity of these clasts, their coarsely grained nature and the fluidal smooth texture are in agreement with very short-lived fire fountaining and magma jets. Glassy outer surfaces of clasts have been interpreted as a late-stage product of fusion by hot gases streaming past the ejecta within the jet or fountain (Thordarson et al., 1996; Stovall et al., 2011). However, the occurrence of this process is not supported by the homogeneous glass composition in our fluidal clasts. Therefore, we interpret these features here just as rapid quenching and not re-melting. Vlastélic et al. (2011) documented the mobility of alkalis and other elements on PdF clasts that experienced long exposures to acid gases. In the 2014 eruption pyroclasts, the mobility of elements was prevented by the short duration of the events.
At lower altitude and close to the MV (Fig. 1), the 5 cm layer at the base of the fall deposit is finely grained (Fig. 3b and c) and rich in fluidal and golden fragments (Fig. 5), with a perfect Gaussian grain size curve (Fig. 5), and similar to that reported from the weak 2010 fountaining event (Fig. 3f and Hibert et al., 2015). Therefore, we interpret this deposit as being due to weak Hawaiian-like fountaining (sustained, but short-lived) activity. We want to remark here that this activity happened during the night and was not observed. The top of the same deposit is coarsely grained (Fig. 3b and c), bimodal, has a lower content in coarse ash (Table S2) and is rich in spiny opaque and spiny glass fragments (Fig. 5). The reverse grain size likely records the transition from early continuous fountaining to late discrete Strombolian activity (observed and recorded on the 21 of June 2014, Fig. 2). This transition in activity is typical of many eruptions at PdF (Hibert et al., 2015). The reverse grading of the whole deposit (Fig. 3b and c) is thus not correlated with an increase in energy of the event but with two different eruptive dynamics and fragmentation processes. The decrease in coarse ash, which correlates with the decrease in energy of the event, highlights the most efficient fragmentation process within the Hawaiian fountaining with respect to the slow gas ascent and explosion of the Strombolian activity. These conclusions are consistent with (i) the continuous and progressive decrease in intensity of real-time seismic amplitude measurement recorded by the OVPF seismic network (unpublished data) and (ii) satellite-derived time-averaged lava discharge rates, which suggest continuous decay of magma output rate after an initial short-lived intense phase (Coppola et al., 2017).
The first micro-textural analysis of Hawaiian ejecta was performed by Cashman
and Mangan (1994) and Mangan and Cashman (1996) on pyroclasts from 1984 to
1986 Pu`u `Ō`ō fountainings. The authors defined two clast types: (1) scoria
consisting of closed-cell foam of
The data that we found in our study of the typical activity of PdF agree only partially with all these interpretations. The reason is that we sampled and measured products of very weak Hawaiian to Strombolian activities. If we plot the approximate durations and masses of these events on the Houghton et al. (2016) diagram, the 2014 activity of PdF falls into the two fields for transient and fountaining activity, but at the base of the diagram. We here show for the first time that short-lived and weak fountaining can preserve pyroclast textures that record magma ascent and fragmentation conditions before the explosions and also provide some information about the pre-eruptive storage conditions. The occurrence of time-variable ascent conditions is also reflected in the time evolution of eruptive dynamics, with the golden and fluidal scoriae emitted from the low Hawaiian fountaining episodes and the spiny fragments from the Strombolian-like explosions.
Thus, as described in Sect. 5.1, longitudinal variation in eruptive style along the
fracture system produces a spatial variability in the proportions of the
four typologies of clasts. The uppermost fractures (WF and UF, Fig. 1a) are
characterized solely by fluidal fragments (Fig. 4b); they lack both the
spiny and the golden components. In addition, these fluidal clasts are the
ones showing the smoothest surfaces (indicative of rapid quenching in a very
hot environment), low porosity values (between 50 and 77 vol. %, Fig. 6b),
the highest content in isolated vesicles (
The four typologies of clasts, golden pumice, fluidal scoria and the spiny
fragments (Fig. 4) were only found at the MV. The relative
proportions of these four typologies of clasts correlate with the eruptive
dynamics. The golden lapilli and fluidal clasts were in fact dominant in the
more energetic Hawaiian activity at the beginning of the eruption (during
the night between 20 and 21 of June 2014). In contrast, the spiny
fragments were dominant during the Strombolian activity, coinciding with the
decrease in mass discharge rate (early in the morning of the 21;
Fig. 2 and Coppola et al., 2017). The golden and fluidal fragments from the MV
show the highest porosity (86 %, Fig. 6a), variable proportions of
isolated vesicles (Fig. 6c) and high, but variable,
Among spiny fragments, the opaque ones are the densest; they lack a uniform glassy surface, and they are characterized by (i) very high microcryst content, (ii) strong coalescence signature (Fig. 4d), (iii) heterogeneous glass chemistry and (iv) mingling with hotter magma at the clast edges (Fig. 8a). All these features reveal the composite nature of these clasts. We interpret the spiny opaque as spiny glass fragments recycled inside the eruptive vent during the explosions, with the densest portion of the magma prone to falling back in the vent or fracture (Fig. 2b).
Syn-eruptive degassing is favoured by bubble connectivity and/or permeability (Fig. 6c and d) in the ascending magma, enhanced by syn-eruptive crystallization in the conduit (especially microcrysts of plagioclase, Fig. 10a), even for magmas at low vesicularity. However, our dataset also supports the occurrence of magma stratification in the reservoir. Textural and petrological data demonstrate that the initial activity emitted a small volume of melt (represented by golden fragments and a large part of the fluidal fragments) with very scarce crystals. This crystal-poor melt was followed in time by the main volume of magma that contains a larger number of mesocrysts (spiny clasts and lava). Lava flows represent the main volume emitted in the 2014 eruption. Mesocrysts are absent in the golden, scarce in the fluidal, and more abundant in the spiny (Fig. 4b, c and d) and lava (Fig. 4e) fragments and consist in plagioclase and clinopyroxene and minor olivine. Their composition indicates that they formed in the reservoir, as shown by their different composition in respect to the microcryst counterparts that formed during melt degassing in the conduit (Fig. 10a). Most importantly, a large number of microcrysts in lava formed in the reservoir as well as during magma cooling (Fig. 10a). Thus, we have a range of crystallization conditions. The fact that the lighter plagioclase are not concentrated in the upper and early erupted portion of the reservoir can be due either to the fact that often they are locked in clusters with the clinopyroxene or that this melt was expelled from the crystal-rich portion of the reservoir (see Fig. 10b). Water exsolution from the melt can result from its extensive crystallization, which induces an increase in dissolved volatile content, up to saturation (second boiling) and can drive melt–crystal separation.
In conclusion, the crystals in the 2014 fragments do reflect the shallow reservoir conditions and the ascent degassing processes.
Volumetric ratio of vesicles to melt (
To prove that the 2014 vesiculation of the clasts has not been modified by
post-fragmentation expansion processes, following Stovall et al. (2011), we
use a plot of vesicle-to-melt ratio (
According to previous works (listed above), the golden pumice of PdF should
be derived from the central part of the fountains, but they do not show the
strong post-expansion signatures reported by other samples collected from
more energetic Hawaiian fountainings (Fig. 11). It is interesting to note
that the fluidal fragments at the MV are less smooth (Fig. 4), more
vesiculated and have a lower content of isolated vesicles than the fluidal
scoriae from the uppermost fractures (Fig. 6). Therefore fluidal fragments at
the 2014 MV could indeed represent clasts that have been partly modified
during their residence in the external part of the fountains, while the
golden samples could come from the central part (Stovall et al., 2011, 2012).
However, the slight differences in crystallinity and glass chemistry
between the fluidal and golden fragments support the idea that each of these
fragments has an imprint from the pre-fragmentation setting. In contrast,
the spiny fragments from the MV and the fluidal fragments from the fractures
show low
Our vesicle connectivity results are in full agreement with the recent review of Colombier et al. (2017b). According to these authors, connectivity values can be used as a useful tool to discriminate between the basaltic scoriae from Hawaiian (fire fountaining) and Strombolian activity. The broad range in connectivity for pumice and scoriae from fire fountaining is interpreted simply as being due to variations in the time available before quenching due to differences in location and residence time inside the fountain. The fluidal fragments from the WF are the richest in isolated vesicles because they are transported by very short-lived hot lava jets. In contrast, the higher connectivity observed in scoriae from Strombolian activity is probably related to their higher average crystallinity and more extensive degassing prior to the eruption (Colombier et al., 2017b). The spiny surface of these Strombolian fragments is due to the fact that these weak explosions emit only a small solid mass fraction and the partially quenched dense clasts land quickly after a short cooling path through the surrounding atmosphere (e.g. Bombrun et al., 2015).
All the clasts, from golden to spiny, are very permeable, independent on their vesicularity, crystal content and/or of the presence of isolated vesicles. This is in agreement with our interpretation that magma degasses during its ascent in the conduit and that promotes microlite nucleation (see the sodic plagioclase, Fig. 10a) before magma fragmentation (see also Di Muro et al., 2015, with the Pele's hair and tear samples for the three 2008 eruptions). Moreover, we always find that some of the spiny clasts (especially the opaque ones) are slightly less permeable than the golden and fluidal ones, but not with a low permeability as we would expect from their low vesicularity.
In conclusion, we can state that (i) the crystals lower the percolation threshold and stabilize permeable pathways and (ii) this is true for the syn-eruptive sodic plagioclase that favour an efficient degassing in the relatively crystal-rich magma because of their low wet angles that favour degassing over nucleation (Shea, 2017) and their aspect ratio (e.g. Spina et al., 2016). (iii) Therefore permeability develops during vesiculation through bubble coalescence, which allows efficient volatile transport through connected pathways and relieves over-pressure (Lindoo et al., 2017). Pervasive crystal networks also deform bubbles and therefore enhance outgassing (Oppenheimer et al., 2015). Based on Saar et al. (2001) crystals should start to affect the behaviour of the exsolved volatile phase when they approach 20 vol. % (Lindoo et al., 2017). In our dataset, apart from the golden and part of the fluidal fragments, all the other clasts do have microcrysts > 20 %. Our data completely support slow decompression rate allowing more time for degassing-induced crystallization, which lowers the vesicularity threshold at which bubbles start to connect.
Rapid re-annealing of pore throats between connected bubbles can happen due to short melt relaxation times (Lindoo et al., 2016). This phenomenology could explain the high number of isolated vesicles in the fountaining samples. However, vesicle distributions of the golden and fluidal fragments are almost perfect Gaussian curves, so it seems that if the relaxation process does occur then it just merged perfectly with the expected vesicle distribution. In contrast, coalescence and/or expansion (as we observe in the spiny fragments) do not fit the curves (Fig. 4). In addition, we should expect that in crystal-poor fragments, due to melt relaxing and pathway closure, the clasts became almost impermeable after quenching, as revealed by some petrological experiments performed on crystal-poor basaltic magma (Lindoo et al., 2016). In contrast, in high crystalline magmas, the presence of micro-crystals increases viscosity, thus preserving the coalesced textures (see Moitra et al., 2013). The isolated vesicle-rich fragments of the 2014 PdF eruption are highly permeable and are characterized by variable ranges of porosity and numbers of vesicles (Figs. 4 and 6d) that seem more related to the pre-eruptive conditions than to the post-relaxation of low-viscosity melts. In the 2014 crystal-poor samples, the permeability increases rapidly once the percolation threshold has been reached, and efficient degassing prevents bubble volumes from expanding past the percolation threshold (Rust and Cashman, 2011).
In conclusion, the vesicles in the 2014 fragments do also partly reflect the shallow reservoir conditions and mostly the ascent degassing processes.
According to Peltier et al. (2016), the June 2014 eruption emitted magma
from a shallow pressurized source located only 1.4–1.7 km below the volcano
summit. Coppola et al. (2017) suggest that the 2014 event was fed by a
single shallow and small-volume magma pocket stored in the uppermost part of
the PdF central plumbing system. All 2014 clasts show homogeneous and
evolved bulk compositions, irrespective of their textural features. June
2014 products are among the most evolved products erupted since at least
1998 and are moderately evolved with respect to those emitted in 2010, just
before the 2010–2014 quiescence. Bulk rock and MI data suggest
that the 2014 evolved magma can be produced by crystal fractionation during
the long-lasting (4.6 yr) storage and cooling of the magma injected and
partly erupted in November 2009. The different types of scoria and pumice
emitted in 2014 show significant variations in glass composition (Fig. 8b)
due to variable degrees of micro-crystallization. In theory, microcrysts can
reflect late-stage (during magma ascent and post-fragmentation)
crystallization. In this case, their variable amount within, for instance,
the glassy and opaque parts of the spiny scoria might reflect slower ascent
velocity or longer residence time in the system (e.g. Hammer et al., 1999;
Stovall et al., 2012; Gurioli et al., 2014) also in agreement with the
vesicle signature. However, the four typologies of clasts also differ in
terms of mesocryst content (from rare to 5 vol. % for the golden and
fluidal and 14–23 vol. % for the glassy spiny and spiny opaque). Equilibrium plagioclase–melt pairs record an almost constant
and moderate dissolved water content, intermediate between that expected for
melts sitting in the main shallow reservoir (located close to sea level) and
the degassed matrix of lavas. Dissolved water contents are thus consistent
with pre-eruptive magma water degassing during its storage at a shallow level,
as suggested by geophysical data, and suggest that the plagioclase
mesocrysts and some of the microcrysts in the spiny scoria and in the lava
grew during magma storage (Fig. 10a). Melt composition records a potential
pre-eruptive thermal gradient of
Tait et al. (1989) suggest that magma evolution can lead to oversaturation of volatile species within a shallow reservoir and trigger a volcanic eruption. At PdF, the golden and the fluidal clasts might represent the portion of magma located at the top of the shallow reservoir and enriched in bubbles of water-rich fluids, released by the cooler, more crystallized and more degassed “spiny lava” magma (Fig. 10b). The small volume of magma, its constant bulk composition and the very small inflation recorded prior to the eruption (Fig. 1d) could be consistent with an internal source of over-pressure related to volatile exsolution. Larger inflation rates over a broader area are expected when shallow reservoir pressurization is related to a new magma input from a deeper source. Slight baseline extensions both on distal and proximal sites suggest that magma transfer towards shallower crustal levels started shortly before (11 days) the final magma eruption. Geochemical data do not support the occurrence of a new magma input in the degassed and cooled 2014 reservoir. We can thus speculate that stress field change related to progressive deep magma transfer has promoted volatile exsolution, melt–crystal separation and melt expansion in the shallow reservoir. Textural heterogeneity of the 2014 products partly reflects a pre-eruptive physical gradient recorded by the variability in crystal and bubble contents in the shallow reservoir feeding this eruption. The golden and fluidal fragments are the bubble-richer and hotter portion of the melt. The spiny fragments are the degassed and cooler portion of the reservoir, whose progressive tapping led to a decrease in explosive intensity (from fountaining to Strombolian activity). Our results are also consistent with processes of mechanical reservoirs and/or dike stratification, as observed by Menand and Phillips (2007). As explained earlier, magma ascent promoted syn-eruptive degassing-induced crystallization. The spiny opaque clasts can be considered as recycled material that fell back into the system. Accumulation of olivine crystals out of equilibrium with the host magma produces minor variations in mesocryst contents as observed within the same type of clasts sampled at different times and locations during the eruption, with the scoriae from the WF and early erupted lava being the ones with the lowest amount of olivine (Table S4 and Fig. 7b). Again, this temporal variation supports an increase in large heavy crystals within the most degassed magma emitted toward the end of activity, further suggesting that it corresponds to the lower part of the reservoir.
Our dataset permits us to propose that the 2014 eruption was fed by a physically zoned magma reservoir. The low-density crystal-poor, bubble-rich magma located in the upper part of the storage system ascended first rapidly and fed the early, more energetic phase, the Hawaiian fountaining. This low-density magma is not more evolved than the spiny one (same bulk compositions) and it is not necessarily richer in dissolved volatile amounts; it is just poorer in crystal and richer in bubbles. Second boiling, possibly triggered a few days before the eruption by stress field change, is responsible for the extraction of bubble-rich melt from a crystal-rich network. This last one will represent the main volume of the erupted lava. Fast ascent of the foam hinders its crystallization and preserves a high number of vesicles, high vesicularity and it is only a little modified by post-fragmentation expansion. Decrease in initial over-pressure translates into a progressive decrease in magma ascent rate and output rate (e.g. Coppola et al., 2017, and references therein). Nucleation of microcrysts is enhanced in melt ascending with lower speed and is mostly related to syn-eruptive degassing (for the spiny fragments). The larger volume (dense lava) corresponds to crystallized and less vesiculated magma, which experiences a slow ascent in the dike and even further micro-crystallization during its subaerial emplacement.
Melt inclusion results allow us to confirm the involvement of a single and only slightly heterogeneous magma source in 2014, related to cooling and fractional crystallization of an older magma batch (November 2009). Interestingly, this latter short-lived summit eruption was also characterized by the same large textural range of pyroclastic products found in 2014 in spite of its more mafic composition.
This suggests that bubble accumulation and source pressurization is highly dependent on the shallow storage depth, which facilitates rapid water exsolution (Di Muro et al., 2016), and it is not necessarily the outcome of slow magma cooling and differentiation (Tait et al., 1989).
Schematic model of the evolution of the PdF volcanic system from the new deep magmatic input of November 2009 up to the June 2014 eruption. See explanation in the text.
In this paper we show that textural and petrochemical study of the eruptive products can be used to characterize the on-going activity at PdF and to constrain both the trigger and the evolution of short-lived and small-volume eruptions. This approach is extremely valuable in (i) understanding processes that lead to an eruption that was preceded by short-lived and elusive precursors and (ii) in reconstructing the time evolution of eruptive dynamics in an eruption with poor direct observations.
Following the sketch in Fig. 12, we infer that residual magma from the 2009 eruption ponding at shallow levels experienced long-lasting cooling and crystallization (Fig. 12a). Between 2010 and 2014 the volcano progressively deflated (Fig. 12b), possibly because of magma degassing and cooling, facilitated by the shallow depth of the reservoir. During this phase mesocrysts and some microcrysts formed (Figs. 4e and 10a).
The occurrence of deep (> 10 km b.s.l.) lateral magma transfer since
March–April 2014 has been inferred by Boudoire et al. (2017) on the basis
of deep (mantle level) seismic swarms and increase in soil CO
Without this deep magma transfer we believe that the small reservoir activated in 2014 would have cooled down completely to form an intrusion (as suggested by the pervasive crystallization of the lava, one of the densest emitted from 2014 to 2017; Harris et al., 2017). The 2014 event represented instead the first of a long series of eruptions, whose magmas became progressively less evolved in time (Coppola et al., 2017). In this scenario the trigger mechanisms of 2014 activity are both internal and external in the sense that the small shallow reservoir hosting cooled magma permitted the creation of the conditions favourable to a second boiling (Fig. 12c, and Tait et al., 1989). The second boiling was likely triggered by an almost undetectable stress field change and was favoured by the shallow storage pressure of the magma (Fig. 12c) that promoted fast water exsolution and rapid magma response to external triggers. The second boiling possibly contributed to the inflation registered 11 days before the eruption at 1.4–1.7 km (Fig. 12c) caused by both magma expansion and transfer of hot fluids to the hydrothermal system (Lénat et al., 2011).
Our data permit the exclusion of (i) new magma input and/or fluid inputs (CO
We conclude that the over-pressure, caused by the second boiling, triggered the eruption. The occurrence of a hydrous, almost pure melt at shallow depth permitted its fast vesiculation upon ascent towards the surface. In turn, fast ascent of the foam (Fig. 12d) hindered its crystallization and preserved a high number of vesicles. Decrease in initial over-pressure translated into a progressive decrease in magma ascent rate and output rate (e.g. Coppola et al., 2017, and references therein) and a temporal transition from Hawaiian activity to Strombolian activity (Fig. 12d). Nucleation of microcrysts was enhanced in melt ascending with lower speed and in turn this syn-eruptive crystallization favoured bubble connectivity and permeability in the ascending magma, even for magma at low vesicularity. The largest volume (dense lava) corresponds to highly crystallized and degassed magma already in the reservoir that experienced a slower ascent in the dike and even further micro-crystallization during its subaerial emplacement.
The texture of the products allowed us to follow the dynamic evolution of the system in space, from smooth fluidal scoriae emitted from a rapid jet of lava at the fractures, to a more stable activity at the MV, and in time. At the MV, in fact, we observed the transition from the golden and fluidal fragments emitted from Hawaiian fountaining, at the peak of the intensity of the eruption, to the spiny fragments, emitted from a declining Strombolian activity at the end of the eruption.
Therefore we here show for the first time that short-lived and weak Hawaiian fountaining and Strombolian events can preserve pyroclast textures that can be considered a valid approximation to shallow reservoir conditions and ascent degassing processes before the explosions and correlate to the eruptive dynamics as well.
To conclude, these results highlight the importance of petrological monitoring, which can provide complementary information regarding the ongoing volcanic activity to other geophysical and geochemical monitoring tools commonly used on volcanoes.
The textural raw data and glass and bulk chemistry data are
available at the DynVolc database (2017)
The authors declare that they have no conflict of interest.
We thank the OVPF team and T. Lecocq for monitoring and fieldwork. F. van Wyk de Vries provided an English revision for a previous version of the paper. We thank the STRAP project funded by the Agence Nationale de la Recherche (ANR-14-CE03-0004-04). This research was financed by the French Government Laboratory of Excellence initiative no. ANR-10-LABX-0006, the Région Auvergne and the European Regional Development Fund. This is Laboratory of Excellence Clervolc contribution number 288. Edited by: Michael Heap Reviewed by: Amanda Lindoo, Madison Myers, and one anonymous referee