The injection of mafic magma into a hydrous felsic magma chamber is a
potential trigger mechanism for bimodal explosive volcanism. As H2O is
the most abundant volatile component in magmas, the interaction and the
degassing behavior of mildly peralkaline hydrous rhyolitic melt in contact
with hydrous basaltic melt were investigated by decompression experiments.
Preparatory hydration experiments and bimodal magma decompression
experiments, as well as reference experiments, were carried out in an
internally heated argon pressure vessel. Pre-hydrated rhyolite and
basalt cylinders were perfectly contacted together in a precious-metal
capsule, heated to 1348 K at 210 MPa, and thermally equilibrated for 10 min.
The initial sample properties were determined by a bimodal reference
experiment, quenched immediately after equilibration. To simulate the magma
ascent, three bimodal samples and a decompression experiment with two
contacted rhyolite cylinders for testing the experimental setup were
decompressed with 0.17 or 1.7 MPa s-1 to the final pressure of 100 MPa and then quenched.
All decompression experiments resulted in vesiculated samples. The H2O
vesicles observed in the decompressed sample of the monomodal
rhyolite–rhyolite reference experiment are homogeneously distributed
throughout the sample. The former interface between the contacted glass
cylinders is invisible after decompression and quench. This reference
experiment proves that the two-cylinder design does not influence the
degassing behavior of the hydrous melt, e.g., an increased formation of
vesicles at possible nucleation sites at the contact plane of the cylinders.
The undecompressed bimodal rhyolite–basalt sample shows crystal-free
rhyolitic glass, whereas 3 µm sized idiomorphic magnetite crystals
coexist with glass in the basaltic part of the sample. Within the 10 min run
time, a ∼ 300 µm wide hybrid composition zone developed
between the hydrous rhyolitic and basaltic endmembers, caused by
diffusion-induced mixing processes.
Decompression and quenching of the bimodal melts resulted in vesiculated
glass samples. A ∼ 100 µm wide zone of alkali-depleted
rhyolitic glass as part of the ∼ 300–560 µm wide
hybrid zone is covered with an enhanced number of H2O vesicles
compared to the pristine rhyolitic and basaltic glass volumes. We suggest
that this enhanced vesiculated zone forms by a rapid diffusional loss of
alkalis from the mildly peralkaline rhyolitic melt into the basaltic melt of
the sample. The reduced alkali concentration significantly reduces the
H2O solubility of the rhyolitic melt. This process enhances the
H2O supersaturation necessary for vesicle formation during
decompression.
In summary, the new findings imply that convective magma ascent driven by
the injection of hot basaltic magma into a hydrous peralkaline rhyolitic
melt reservoir leads to enhanced H2O vesicle formation near the melt
interface and thus to efficient degassing. This in turn can accelerate
buoyancy-driven magma ascent and mingling and mixing processes that
induce further degassing and potentially trigger explosive volcanic
eruptions.
Introduction
Explosive volcanic eruptions are driven by the formation and growth of
volatile vesicles, mainly H2O and subordinate CO2, in hydrous
silicic melts. The formation of fluid vesicles requires supersaturation of
volatiles dissolved in the silicate melt. Vesicles in hydrous rhyolitic melt
form by nucleation (e.g., Sparks, 1978; Navon and Lyakhovsky, 1998; Gonnermann
and Manga, 2007; Hajimirza et al., 2019; Gardner et al., 2022) or, as
recently suggested for phonolitic Vesuvius melt composition, by spinodal
decomposition (Allabar and Nowak, 2018; Allabar et al., 2020; Sahagian and
Carley, 2020; Gardner et al., 2022). However, vesicle formation and growth
lead to an overpressure that might burst the ceiling of a magma chamber
(e.g., Sparks et al., 1977; Sigurdsson and Sparks, 1981; Miller and Wark,
2008; Spera et al., 2016). The resulting sudden pressure (P) drop enhances
the further formation and growth of vesicles, which significantly reduces magma
density. Accelerated magma ascent and rapid expansion of vesicles lead
finally to magma fragmentation. These fundamental mechanisms hold to small
explosive silicic eruptions, such as the 1875 eruption of Askja in Iceland,
which produced ∼ 0.2 km3 of rhyolitic magma (Sigurdsson
and Sparks, 1981) with a volcanic explosive index (VEI) of 4. They also apply
for catastrophic explosions, e.g., the 74 ka supereruption of Toba on
Sumatra with a VEI of 8, which ejected ∼ 2800 km3 of
rhyolitic magma, equivalent to ∼ 8000 km3 of volcanic ash
(Rose and Chesner, 1987; Self and Blake, 2008).
Under the pressure and temperature conditions of subvolcanic magma chambers,
silicate melts can dissolve several weight percent (wt %) of the main volatile H2O.
Several processes to achieve volatile supersaturation and vesicle formation
are discussed: (1) decompression by magma ascent (e.g., Sparks et al.,
1977), (2) sudden pressure drop by earthquakes rupturing hydrous magma
chamber roof rocks, (3) enrichment of volatiles that exceed the saturation
limit of silicate melt due to partial crystallization (e.g., Bachmann and
Bergantz, 2008; Scaillet and Pichavant, 2003), (4) thermal vesiculation
induced by the latent heat of partial crystallization and frictional
heating (Lavallee et al., 2015), and (5) the injection of hot mafic magma
into differentiated volatile-rich silicic magma accompanied by mechanical
mingling and chemical mixing processes (Miller and Wark, 2008; Spera et al.,
2016; Murphy et al., 1998; Leonard et al., 2002; Druitt et al., 2012;
Perugini et al., 2012; Laumonier et al., 2014; Pichavant et al., 2018), as
well as (6) dispersion of partially crystallized mafic fragments acting as
heterogeneous vesicle nucleation sites in hydrous silicic melts
(Paredes-Marino et al., 2017). While the first points have already been
extensively studied (e.g., Martel et al., 2017), experimental approaches to
the last points are rare. Therefore, this experimental work focuses solely
on the effects of magma degassing triggered by magma injection.
Geological background of explosive volcanic eruptions triggered by magma injection
Bimodal basaltic–rhyolitic volcanism in Iceland was first described by
Bunsen (1851). In recent literature partially crystallized mafic streaks,
glass fragments, blebs, and enclaves are described as common features in
products of explosive felsic volcanic eruptions (e.g., Sparks et al., 1977;
Troll et al., 2004; Miller and Wark, 2008; Arienzo et al., 2010; Druitt et
al., 2012; Paredes-Marino et al., 2017; Jarvis et al., 2021).
Injection of hot basaltic magma into a rhyolitic magma chamber may induce
local temperature differences of up to 500 K that shift the system out of
equilibrium (e.g., Sparks et al., 1977; Snyder, 2000; Miller and Wark, 2008).
Accompanied mechanical magma mingling significantly increases the contact
surface of the two melts (e.g., Perugini et al., 2012; Jarvis et al., 2021).
This enhances the transfer of heat and mixing by chemical diffusion. Two
important effects are triggered by heat transfer at the interface: (1) volatile-bearing hot basaltic magma cools down and may partially
crystallize. The possible exceedance of the volatile saturation limit by
sufficient enrichment in the residual melt can lead to vesicle formation and
growth. (2) Volatile-rich silicic magma can be superheated. This may lead to
the formation and growth of fluid vesicles as H2O solubility decreases
with the temperature at pressures of ≤ 300 MPa (Holtz et al., 1995;
Lavallee et al., 2015). In both scenarios, the vesicle formation and growth
significantly decrease the magma density, which is controlled by the pressure–volume–temperature (PVT) behavior of H2Ofluid. The decrease in magma density may trigger
buoyancy-driven convection and a large-scale overturning of an initially
stratified magma chamber within timescales of days or weeks (Sparks et al.,
1977; Snyder, 2000; Phillips and Woods, 2002). Further vesicle formation and
expansion accelerate magma ascent. The rapid build-up of internal fluid
pressure during the growth of bubbles and the increase in melt viscosity
induced by H2O loss may cause melt fragmentation, followed by explosive
volcanic eruptions.
Experimental investigation of the interaction of felsic and mafic magmas
Yoder (1973) investigated experimentally the physicochemical interaction of
H2O-saturated basaltic and rhyolitic melts. The fusion of mixed
fine-grained rock powders and H2O at 100 MPa and 1473 K resulted in
complete melt miscibility. In contrast, melting of layered basaltic and
rhyolitic rock powders resulted in separated glasses with a narrow diffusion
zone with an intermediate (hybrid) composition of ∼ 300 µm across the interface after 2 h of fusion and quench. This
indicates that silicate melts of different compositions can be maintained in
direct contact at least for a short time. Johnston and Wyllie (1988)
confirmed these results with coupled hydrous basaltic and rhyolitic melt reaction experiments at 1 GPa and 1193 K. They demonstrated that alkali
diffusion is orders of magnitude faster than the diffusion of divalent
cations that is limited by the diffusion of the network formers Si and Al
(Watson, 1982).
Experimental mingling of basaltic and silicic melts was successfully
conducted by the mechanical torsion of two contacted melt disks at 1483 K and
ambient P (Kouchi and Sunagawa, 1985). Laumonier et al. (2015) mingled
hydrous basaltic and tonalitic magmas by torsion in a Patterson apparatus at
300 MPa and ≤ 1293 K. The experimental products show entrainment of
mafic crystals into the felsic melt, mafic–felsic banding, and enclave
formation similar to the textures observed in bimodal volcanic rocks.
Perugini et al. (2012) and Morgavi et al. (2013) used a rotating crucible
and a rotating inner cylinder based on the so-called journal bearing system
(JBS) to mingle granitic and basaltic melts at 1623–1673 K. The observed
chaotic mingling by stretching and folding induces a significant increase in
the interface of the silicate melts within hours and enhances diffusional
exchange between the two melts. Wiesmaier et al. (2015) demonstrated magma
mingling of nominally volatile-free rhyolitic and basaltic melts at ambient
P and 1723 K without external forces. Air bubbles were intentionally
entrapped at the coarsely ground interface of the two glass cylinders
stacked on each other. During the experiment, the initial air bubbles
ascended from the melt interface into the overlying rhyolitic melt. The
bubbles dragged filament-like mafic melt streaks into the rhyolitic melt.
Furthermore, numerical simulations of Montagna et al. (2015) show that the
mingling of volatile-rich shoshonitic and partially degassed phonolitic
magma develops over a short timescale of hours.
Despite these numerous experimental studies on bimodal melts, experiments to
simulate degassing processes driven by magma injection are still lacking.
Therefore, this study aims to find an experimental approach to investigate
H2O degassing processes by simulating the injection of volatile-bearing
basaltic melt into an H2O-saturated differentiated felsic melt. The
main questions that we try to answer in this study are as follows: how do
interdiffusion processes between the rhyolitic and basaltic melt influence
the H2O solubility in the developing intermediate melt composition?
Would this further affect the vesicle nucleation? Which zone of bulk melts
is influenced by the degassing and mixing processes, dependent on time and
decompression rate?
Experimental and analytical methodsStarting material
Rhyolitic and basaltic glasses similar to the bimodal magma compositions of
the Askja volcanic complex (Sparks et al., 1977) were synthesized. A
modification was implemented on the rhyolitic composition to obtain a
simplified and slightly peralkaline haplogranitic composition
(Ab38Or34Qz28, in wt %; Holtz et al., 1992) with a
Na2O excess of 2 wt %. The modification was chosen to obtain a nearly
bubble-free glass and a distinct interface between the intense, dark brown
basaltic glass and the iron-free colorless and transparent rhyolitic glass
to best visualize the expected bubble textures and possible mixing effects.
The basaltic composition was synthesized according to the analyses of
Philpotts and Ague (2009) (Table 1). To receive homogeneous starting
glasses of rhyolitic and basaltic composition, the glass syntheses were
conducted following the experimental protocol of Marxer et al. (2015) and
the improvement described in Allabar and Nowak (2018). This procedure
ensured crystal- and tension-free homogeneous glasses that are nearly free
of air bubbles. Cylinders up to 13 mm in length and 5 mm in diameter were
drilled from the glasses. The sharp edges of each glass cylinder were sanded
and rounded to prevent capsule damage during pressurization. The glass
compositions were confirmed with electron microprobe analysis (EMPA) (Table 1).
Hydration experiments
Rhyolitic and basaltic glass cylinders were used as starting material for
preparatory hydration experiments. Previous studies indicated that hydration
experiments at high P and T (temperature) using glass powder induced the formation of
numerous small vesicles in the sample volume due to excess H2O fluid in
the case of saturated conditions (e.g., Gardner et al., 1999; Iacono-Marziano et
al., 2007) or due to H2O–N2 vesicles in the case of H2O-undersaturated conditions (e.g., Preuss et al., 2016). Annealed and cleaned
Au80Pd20 tubes (outer diameter: 5.4 mm; inner diameter: 5.0 mm), with
lengths depending on the specific glass cylinder lengths, were closed at the
bottom with Au80Pd20 lids while the other side was crimped to a star shape,
to stabilize the shape of the capsules at high P and T (Marxer et al., 2015).
To ensure H2O saturation of the melt during the hydration experiments,
6 wt % to 7 wt % water was welded together with the rhyolite or basalt glass
cylinders into the capsules (Fig. 1). Under hydration conditions of 200 MPa and
1523 K, calculated H2O solubilities are 5.7 wt % H2O in the
mildly peralkaline rhyolitic melt (Allabar et al., 2022) and 4.8 wt %
H2O in the basaltic melt (Berndt et al., 2002). Consequently, the
excess water added to the capsules ensured complete H2O saturation for
both melt compositions. To check for possible leakage, the sample capsules
were reweighed after heating to 383 K and after pressurization in a cold
seal pressure vessel to 100 MPa at room temperature and again after storing
at 393 K at ambient pressure. In the case of constant weight, the capsules were
used for hydration experiments.
The hydration experiments were performed in an internally heated argon
pressure vessel (IHPV) at 1523 K and 200 MPa for 96 h at an intrinsic oxygen
fugacity close to ΔlogQFM =+3.5 (Berndt et al., 2002). After
hydration, the melts were quenched isobarically with a moderate quench rate
of ∼ 16 K s-1 (NQ; Allabar et al., 2020) to
room temperature by switching off the IHPV furnace. This ensured the
production of crack-free glass cylinders that were used for the magma
injection experiments. The capsules were then punctured and heated at 383 K
for at least 24 h to gravimetrically determine the amount of released free
water to calculate the H2O content of the glass (Table 2).
Preparation of hydrated samples
The hydrated glass cylinders were prepared for further decompression
experiments and analysis. The cylinders were unwrapped from the capsule
material. The 500 µm slices were cut from the middle of the samples
perpendicular to the cylinder axis for the preparation of double-sided
polished thin sections to be used for sample characterization. The remaining
sample halves were ground and polished each on one cylinder face. This
procedure enabled an ideal contact surface of two glass cylinders required
for the bimodal decompression experiments.
Bimodal decompression experiments
For each bimodal magma injection (MI) experiment, a hydrated rhyolite
cylinder and a hydrated basalt cylinder were inserted into a new Au80Pd20
capsule, oriented as rhyolite on top of basalt or basalt on top of rhyolite
(Table 2). The polished faces of both glass cylinders were horizontally
aligned to prevent air entrapment between the cylinder faces. Air entrapment
could cause the formation of pre-existing vesicles between the two cylinders
during the melting process (e.g., Wiesmaier et al., 2015), which may affect
the degassing process during subsequent decompression as an experimental
artifact. To test this experimental design, a reference sample was prepared
where two rhyolite cylinders of one hydrated sample were contacted in a
capsule (Fig. 1).
The bimodal experiments were conducted in the IHPV equipped with a rapid
quench device (Berndt et al., 2002). Five experiments were performed:
one decompression reference experiment (rhyolite–rhyolite Rt–Rt:
MI_Rt_1) to validate the new experimental
procedure
one bimodal reference experiment (rhyolite–basalt Rt–B: MI_5)
to determine the initial melt conditions after thermal equilibration and
before decompression
three bimodal decompression experiments (Rt–B: MI_T_3, MI_6 and MI_7) to study
H2O degassing in a bimodal magma system.
All samples were heated isobarically at the initial pressure (Pi) of 210 MPa at 25 K s-1 from room temperature to the run
temperature (Tr) of 1348 K or 1403 K (Table 2). The P and T conditions
ensured H2O undersaturation to prevent H2O loss during the
temperature equilibration of 10 min. Except for experiment MI_5, the hydrous bimodal melt assemblages and the Rt–Rt melt assemblage were
decompressed isothermally, using a high-pressure valve optimized for
continuous decompression (e.g., Nowak et al., 2011; Allabar and Nowak, 2018).
Decompression rates of 0.17 or 1.7 MPa s-1 were applied
until the final pressure (Pf) of 100 MPa was reached. To minimize
partial crystallization of the basaltic melt, all MI samples were quenched
at a medium quench rate (MQ) of ∼ 44 K s-1
(Allabar et al., 2020). Because rhyolitic melt is not as prone to the
quench-crystal formation, the Rt–Rt sample was quenched more slowly at
∼ 16 K s-1 to room temperature by
switching off the furnace. After re-weighing the capsules to determine
possible H2O loss due to leakage, the decompressed samples were cut
parallel to their cylinder axes. One-half of each sample was embedded in
epoxy resin, ground, polished, and sputtered twice with a 5 nm carbon
coating for EMPA and scanning electron microscopy (SEM) analysis, while the
other half was used for the preparation of double-sided polished thin
sections of ∼ 200 µm thickness (d; measured with a
Mitutoyo digital micrometer (± 3 µm); Table 2) for Fourier
transform infrared (FTIR) spectroscopy and optical microscopy.
(a) Glass cylinders of rhyolite (Rt_5_h) and basalt (B_4_h), each rounded at the
edges, are welded separately into an Au80Pd20 tube (outer and inner diameter:
5.4 and 5.0 mm) with 7 wt % or 6 wt % H2O, respectively. Hydration
experiments were conducted in the IHPV at 200 MPa and 1523 K and equilibrated
for 96–168 h. (b) The quenched glass cylinders were unwrapped, cut
perpendicular to the cylinder axis, and ground and polished at one side of
each cylinder face. Contact of a pair of hydrated rhyolite cylinders or a
hydrated rhyolite and a hydrated basalt glass cylinder in a new Au80Pd20
tube. Decompression experiments were conducted in the IHPV at 1348 and 1403 K with decompression from 210 to 100 MPa and with rates of 0.17
or 1.7 MPa s-1.
Glass composition of rhyolite and basalt, measured by EMP, similar
to the compositions of the 1875 Askja bimodal eruption (Sparks et al.,
1977). Averaged magnetite composition of eight crystals measured in the basalt
of MI_T_3. Intermediate melt composition of
MI_T_3, MI_7, and
MI_6 in the region of enhanced vesicle formation in the
hybrid zone. The data of all EMP measurements are given in weight percent (wt %).
a Total Fe concentration is given as FeO. bxex is the difference between mole fractions of alkalis and those of alumina ((Na2O + K2O–Al2O3) / 100) (Allabar et al. 2022). c FeO converted to FeO and Fe2O3 (Carmichael, 1967) gives a total of 100.3 wt % and 100.1 wt % without considering alkali oxides, respectively. d Zone where enhanced vesicle formation is observed. e Total < 100 wt % due to the H2O content of the hydrated glass.
Experimental conditions for hydration and decompression experiments
in the IHPV. H2O solubilities (H2O sol.) at 200 MPa and 1523 K in
rhyolitic melt were calculated according to Allabar et al. (2022) and in
basaltic melt according to Berndt et al. (2002).
PEq is equilibration/hydration pressure; Pi is initial pressure prior to decompression; Pf is final pressure; TEq is equilibration/hydration temperature; Tr is run temperature during experiment; tEq is equilibration/experiment duration; dP/ dt is decompression rate; NQ: 16 K s-1; MQ: 44 K s-1; H2O sol. is H2O solubility at 200 MPa and 1523 K; d is thickness; Rt is rhyolite; B is basalt; Mag is magnetite. The cH2Og max is gravimetrically determined H2O concentration in the melt if all weight loss of the sample during the experiment, and after piercing and drying was H2O → maximum possible H2O concentration in melt. The cH2Og min is gravimetrically determined H2O concentration in the melt if all weight loss of the sample during welding, during the experiment, and after piercing and drying was H2O → minimum possible H2O concentration in melt. The cH2Ot is total H2O concentration in rhyolite glass, determined by FTIR spectroscopy. Hydr. is hydration experiment (7 wt % H2O added into the capsule for H2O excess during hydration); Rt / B is Rt on top of B; B / Rt is B on top of Rt; the use of the hydrated samples is assigned by the superscript numbers. * Increased error due to the significantly lower thickness of the thin section of 0.072 mm.
Analytical methodsElectron microprobe analysis
The bulk compositions of the nominally dry starting glasses were analyzed
with a JEOL JXA 8900 R electron microprobe (EMP). Wavelength-dispersive X-ray
spectroscopy analyses were conducted by using an acceleration voltage of 15 kV, a beam current of 3 nA, and a defocused beam with a diameter of 20 µm to optimize for Na analysis (e.g., Morgan and London, 2005; Stelling et
al., 2008). Beam counting times were 10 s (Na), 16 s (K, Si, Ca, Al, Mg,
Fe), and 30 s (Mn, Ti). To test the homogeneity of the glasses and the
agreement with the desired glass composition, 15 points were measured across
an entire glass cylinder section of each composition. In sample
MI_T_3, one concentration profile was measured
across the hybrid zone from the lower basalt to the overlying rhyolite. The
samples MI_5, MI_6, and MI_7
were analyzed using a JEOL JXA 8230 electron microprobe under the same conditions but with a
beam current of 10 nA. Concentration profiles were measured across the
samples, from the initial basaltic composition to the initial rhyolitic
composition. EMPA data are given in Table S1 in the Supplement.
Scanning electron microscopy
The experimental samples were examined with a Hitachi TM3030plus Tabletop
SEM, an electron beam current of 50 nA, and an acceleration voltage of 15 kV.
Backscattered electron (BSE) images of the samples were acquired at up to
5000× magnification (Fig. S1). The overview images of the entire samples
were generated by adjusting the grey values of individual images and
stitching (Fig. 2). BSE images were used to estimate vesicles in the central
volume with a number density (VND) using ImageJ and CSD corrections (Higgins,
2000) as described in Marxer et al. (2015) and Preuss et al. (2016).
FTIR spectroscopy
The total H2O concentrations (cH2Ot) of the hydrated and
decompressed rhyolite glasses were determined by FTIR spectroscopy. The
samples were analyzed in the near-infrared range (4000–6000 cm-1) with
the Bruker Vertex v80 FTIR spectrometer coupled with the HYPERION 3000 IR
microscope by using a tungsten halogen light source, a CaF2 beam
splitter, a liquid-N2-cooled InSb single-element detector, a 15×
Cassegrain objective, and the OPUS 7 software package. Overview images of the
thin sections were recorded, and the positions of the measuring points were
programmed and subsequently automatically measured. With a knife-edge
aperture set to 50×50µm, the samples were measured with 50 scans per
spectrum in transmission mode at a spectral resolution of 4 cm-1. Air
was measured as a reference. Each measurement location was tested to be free
of fluid vesicles in the analyzed glass volume by focusing through the thin
section. To determine spatial variations in the H2O concentrations
throughout the sample, profiles were measured along the longitudinal and
transverse cylinder axes. The infrared data are given in Table S1 in the
Supplement.
The cH2Ot of the hydrated rhyolite samples was determined by measuring
the peak heights of the absorbances (A) of the combination modes of molecular
H2O (H2Om) at ∼ 5210 cm-1 and hydroxyl groups (OH)
at ∼ 4470 cm-1. A linear-tangential baseline correction (Behrens
et al., 1996) was applied by setting points at ∼ 5400
and ∼ 4700 cm-1, ensuring that the straight
line is tangential to the spectrum between the absorption bands at 5210
and 4470 cm-1. For evaluation, the linear molar absorption
coefficients (εH2Om=1.50 L mol-1 cm-1, εOH=1.59 L mol-1 cm-1) and the density correlation (ρAOQ2 [g L-1] = (2351±2) - (12.3±0.6) ⋅cH2Ot) from Allabar et al. (2022) were used. Because near-infrared
measurements in transmission mode were impossible for basaltic glasses, the
total H2O concentration was determined gravimetrically (cH2Og)
(Table 2).
X-ray diffraction
For the identification of crystals of the partially crystallized basaltic
hydration and MI samples, a Bruker D8 Discover μ-XRD (X-ray
diffraction) with a VÅNTEC 500 2D detector was used together with a
cobalt radiation source (λ=1.79 Å), a HOPG primary
monochromator, and a 500 µm monocapillary optic. All basaltic samples
were measured for 180 s with an acceleration voltage of 30 kV and an
electric current intensity of 30 mA. The angle of incidence of the X-ray
beam was set to 10∘ and the detector to 25∘. The
diffraction patterns of the crystalline phases were evaluated using the
Bruker software DIFFRAC.EVA.
Transmitted light microscopy
Vesicles in transparent sample thin sections were quantified using the Zeiss
Axio Imager M2M microscope and the software “Trackworks”. Each measurement
was performed by focusing through the sample in a predefined area, resulting
in an analyzed sample volume that was used to normalize the number of
vesicles to 1 mm3 of vesicle-free glass. The errors in the
transmitted light microscopy (TLM) analyses were calculated using error
propagation, assuming errors of 5 % for vesicle size and vesicle number,
as well as ± 3 µm for sample thickness.
ResultsHydration samples
All quenched capsules were punctured after 96–168 h of hydration. Since all
samples showed a weight loss after 24 h in the compartment drier (383 K),
water in the punctured capsules evaporated, showing that a free H2O-fluid phase during the entire experiment duration was ensured. The mass of
the evaporated excess water was used to calculate by difference the
dissolved H2O concentration in the melt under the given experimental
conditions (Table 2).
However, during the welding of precious-metal capsules, it may happen that
tiny droplets of molten metal form and stick as metal beads to the outside
of the welded capsule. At some point, these beads may fall off and cause a
weight loss of several milligrams. The associated precious-metal loss cannot
be distinguished from any possible water loss during welding. This affects
the gravimetric determination of the H2O content dissolved in the melt.
To account for this uncertainty, a maximum (cH2Og-max) and minimum
(cH2Og-min) H2O concentration in the melt was calculated under the
respective assumption that the weight loss measured after welding is related
to total capsule material or total water, respectively.
The lowest cH2Og was calculated for the sample Rt_4_h (max: 5.14 wt %; min: 3.92 wt %). FTIR spectroscopic
measurements provide a cH2Ot of 4.35 ± 0.16 wt % H2O. FTIR
measurements of samples Rt_1_h,
Rt_3_h, and Rt_5_h result in cH2Ot of 5.67 ± 0.54 wt %, 5.38 ± 0.19 wt %,
and 5.11 ± 0.18 wt % H2O, respectively. The calculated H2O
solubility of 5.7 wt % under hydration conditions was not reached in samples
Rt_3_h, Rt_4_h,
and Rt_5_h. This could be related to the
longer hydration time of more than 96 h required for the sample cylinder
length of 11 mm compared to 6.5 mm (Preuss et al., 2016; Allabar and Nowak,
2018). Nevertheless, FTIR spectroscopic measurements confirmed that a
homogeneous H2O distribution was ensured in the rhyolite glass
cylinders (Rt_1/3/4/5_h).
FTIR spectroscopic measurements of basaltic samples were not possible
because of the partially crystallized glasses. The samples are opaque to the
near-infrared radiation. Even reducing the section thickness of
B_3_h to ∼ 50 µm does not
allow the infrared radiation to penetrate the sample. However, the
cH2Og calculations of B_2_h with 4.88 wt % (max) and 4.85 wt % (min), as well as sample B_3_h with 5.41 wt % (max) and 4.66 wt % (min), agree
sufficiently well with the expected solubility of 4.8 wt % (Berndt et al.,
2002). After hydration of sample B_4_h, the
capsule had a noticeable weight loss of 9.06 mg, which is attributed to
H2O leaking out during the hydration process. Nevertheless, still some
H2O escaped after puncturing the capsule and during drying in the
compartment drier. A calculated cH2Og (max–min) of 4.00 wt %–3.65 wt %
resulted for this sample. However, the reduced H2O concentration in the
undersaturated basalt samples does not significantly affect the degassing
mechanism of the bimodal experiments, as explained in the following
discussion.
The investigation of the hydrated samples with XRD showed that all rhyolitic
samples were crystal-free, while the basaltic samples contain magnetite and
pyroxenes. This crystal analysis was confirmed by investigating the basaltic
samples with the SEM. The BSE images also confirm that a significant volume
of the basalt samples is partially crystallized. Idiomorphic magnetite
crystals up to 2 µm in size are surrounded by dendritic pyroxene
quench crystals (Fig. S1).
Optical inspection and SEM analysis confirmed that all hydrated samples were
vesicle free. Furthermore, all hydrated rhyolitic and basaltic samples had a
cylindrical shape and were therefore ideally suited for the subsequent
bimodal decompression experiments.
The reference decompression experiment MI_Rt_1
was intended to validate the new experimental procedure of contacting two
hydrous glass cylinders without air entrapment in between. Images of the
embedded sample halves and the thin section show heterogeneously nucleated
fringe vesicles attached to the capsule wall with a vesicle-free drainage
zone (Navon and Lyakhovsky, 1998; Iacono-Marziano et al., 2007).
Homogeneously distributed vesicles formed in the entire center sample volume
(Fig. 2a). The former interface (indicated by a small kink at the capsule
wall) between the two hydrous rhyolite cylinders is no longer visible and
enhanced vesicle formation at the former interface is not observed.
Therefore, the developed experimental design can be used for further magma
injection experiments. The sample contains a VND of 1.8×102 mm-3 with a mean diameter of ∼ 75 µm, determined
by TLM. The cH2Ot was measured as a vertical profile from the sample top
to the sample bottom and results in 4.22 ± 0.14 wt %.
Bimodal reference experiment
The bimodal reference experiment MI_5 shows the initial melt
conditions after heating and thermal equilibration for 10 min at 1348 K and
210 MPa before decompression. Due to the fast quench rate of 44 K s-1, glass tensions in the sample induced cracking and only a small
piece of initially contacted bimodal composition volume was preserved (Fig. 2a). No vesicles formed during the experiment, neither in the rhyolitic nor
in the basaltic part of the sample. The basalt is partially crystallized
with 1–3 µm sized magnetites. Quench crystals attached to the
magnetites as found in hydration samples quenched at 16 K s-1 (Fig. S1) are not observed. FTIR measurements of the upper
rhyolitic glass result in a mean cH2Ot of 5.25 ± 0.18 wt % and
are therefore slightly lower than the initial cH2Ot of Rt_1_h with 5.67 ± 0.54 wt %.
The zone between the rhyolitic and basaltic glass that developed during the
experiment can be optically characterized by its color transition towards
the endmember compositions. While the rhyolitic glass is colorless and
transparent, the contact zone turns transparent brownish to nearly black and
opaque towards the basaltic part (Figs. 2a, 5). The onset of crystal
occurrence corresponds to the black coloration in the optical image. EMP
analysis across the sample, parallel to the cylinder axis, shows the
development of an intermediate melt composition at the former contact zone
of rhyolitic and basaltic melt. The diffusing species are charged ions and
complicated multicomponent interdiffusion processes of oxygen and cations
with different charges, as well as different concentrations, and mobilities
are expected to maintain charge neutrality (e.g., Watson, 1982; Baker, 1992;
Johnston and Wyllie, 1988; Zhang et al., 2010). For a simplified illustration
of the interdiffusion processes, we refer to the concentration profiles of
oxide components. Figure 3 shows concentrations of oxide components normalized
to a range between 0 (lowest concentration) to 1 (highest concentration).
This highlights the similarities and differences in the
concentration–distance profiles for all measured oxides. Na2O, K2O,
and SiO2 concentrations decrease from the rhyolitic glass in the direction
to the basaltic glass. Al2O3, TiO2, FeO, MgO, and CaO
concentrations decrease from the basaltic glass in the direction to the
rhyolitic glass. Na2O and K2O show asymmetric concentration
profiles and a significantly greater length compared to SiO2,
TiO2, FeO, MgO, and CaO. The normalized Al2O3 concentration
is characterized by a strong variation over the entire profile, caused by
the small difference in the initial concentration of the starting
compositions of 1.11 wt % (Table 1). MnO concentrations are close to the
detection limit and are not shown in Fig. 3. For a subsequent assessment of all
samples, the SiO2 concentration profiles of all samples were evaluated
to define the length of the mixed hybrid zone. For the reference sample
MI_5, the SiO2 concentration profile extends over 0.3 mm
(Table 3, Fig. 3).
To examine a cH2Ot concentration profile across the hybrid zone, the
FTIR spectra were baseline-corrected according to Ohlhorst et al. (2001).
Straight lines were fitted through the minima on both sides of the NIR
combination bands related to H2Om and OH to evaluate the peak
heights. As this baseline correction systematically underestimates the OH
absorption (Ohlhorst et al., 2001), standard samples of rhyolitic
composition (AOQ2, from Allabar et al., 2022) were baseline-corrected
according to Behrens et al. (1996) and Ohlhorst et al. (2001) for
comparison. The differences in cH2Ot, calculated with the εH2Om and εOH of both corrections, are within the
error in the cH2Ot of each sample, except for sample Std_2_7 with the highest H2O content of 6.9 wt % (Table S1).
Ohlhorst et al. (2001) developed a parabolic equation (Eq. 1) to predict
the linear molar absorption coefficients (ε) as a function of
the SiO2 content of the glass, valid for compositions from rhyolite to
basalt:
ε=a+b×cSiO22.ε values are given in liters per mole per centimeter (L mol-1 cm-1), a=-0.13, b=0.000257 (given parameters for the baseline
correction used), and cSiO2 is in weight percent (wt %), resulting in rhyolite absorption
coefficients of εH2Om=1.57 and εOH= 1.32 L mol-1 cm-1. For the initial
basaltic composition, εH2Om=0.76 and εOH=0.64 L mol-1 cm-1 were
calculated. The cSiO2 concentration profile of MI_5
matches with the cH2Ot profile, since the inflection point of the
cSiO2 profile coincides with the first measurement in the color
transition from brownish to transparent glass (hereafter referred to as the
“midpoint”). To calculate the cH2Ot of the FTIR-measured profiles
across the hybrid zone, each measurement point was assigned the
corresponding SiO2 concentration from the EMP measured profile. For the
presented profile of MI_5 (Fig. 4), this corresponds to 73.0 wt % SiO2 for the first measurement in the rhyolitic part to 60.8 wt % SiO2 for the midpoint and 55.8 wt % SiO2 for the last
measurement in the basaltic part of the profile (Table S1). Using these
concentrations, respectively, absorption coefficients of εH2Om=1.47/0.97/0.80 and εOH=1.24/0.82/0.67 L mol-1 cm-1 were calculated
with Eq. (1). The same procedure was used for all other measurement points.
The glass density, necessary for the calculation of cH2Ot with the Beer–Lambert law, at the corresponding measurement point was calculated according
to the equations of Allabar et al. (2022, Eq. 2), Yamashita (1997,
composition no. 43gm, Eq. 3), and Ohlhorst et al. (2001, Eq. 4) for
rhyolite, dacite, and basalt, respectively. The equation for the density of
dacite was chosen because the hybrid composition at the midpoint is quite
similar to the dacite of the Unzen volcano given in Chen et al. (1993):
2ρRt=(2351±2)-(12.3±0.6)⋅cH2Ot,3ρD=(2515±6)-(11.8±2.0)⋅cH2Ot,4ρB=(2819±13.5)-(20.8±6.6)⋅cH2Ot,
where ρ is in grams per liter (g L-1) and cH2Ot is in weight percent (wt %). The
dependence of the glass density on the H2O content in the glass
required an iterative calculation. As the composition changes over the total
profile, Eqs. (2)–(4) were only applied for three data points. Equation (3) was
applied to the midpoint. Depending on the length of the hybrid zone
determined by EMPA, the measurement points in the hybrid composition were
iteratively calculated by interpolating the density between the dacite
density of the midpoint and the rhyolite density or the basalt density. This
whole procedure was also applied to the other MI samples to be able to
determine the H2O concentrations across the hybrid zone of the samples.
FTIR measurements with absorbance values greater than 1 were excluded from
evaluation because less than 10 % of the light penetrates the samples in
these measurements, which may result in a nonlinear behavior where the
Beer–Lambert law is no longer valid (Mayerhöfer and Popp, 2019). This is
especially relevant in the zone with increasing basaltic influence. Such
measurements are still displayed in the cH2Ot profile but marked as open
circles (Fig. 4). FTIR measurement points to which no cH2Ot data point
was assigned, as in MI_7 and MI_6, did not
yield analyzable NIR spectra due to crystals in the basalt glass.
The cH2Ot profile in Fig. 4 across the hybrid zone of the undecompressed
sample MI_5 shows an increase towards the basaltic
composition. The profile starts with a mean cH2Ot of 5.36 ± 0.20 wt % in the rhyolitic part, followed by a slight decrease in cH2Ot
towards the transition to the hybrid zone down to 5.08 ± 0.21 wt %
in the middle of the hybrid data points at 0.38 mm. Further on, cH2Ot
significantly increases up to 6.15 ± 0.30 wt %. With this data
point, an H2O concentration is reached that exceeds the initial
H2O content (cH2Oi) of the rhyolite melt by 0.5 wt %. If the last
data points with excessive absorbance (> 1) were included,
cH2Ot up to 6.71 ± 0.37 wt % would be reached, exceeding the
initial concentration by 1 wt %. Besides the cH2Ot profile, the hybrid
zone is also characterized by the SiO2 concentration profile, marked
in grey, scaled on the right y axis. The hybrid zone extends over a length
of 0.3 mm.
The cH2Ot measured by FTIR spectroscopy from rhyolite to hybrid
zone to basaltic composition. The cH2Ot values based on measured
absorptions (A) > 1 are listed in a separate column. Hyb. signifies hybrid.
Microscope, BSE, and reflected light images of MI samples, cut and
polished along the cylinder axis. BSE and reflected light images were
captured from the embedded sample halves, while the microscope images show
the IR slides in transmission or reflection mode. All basaltic glasses
contain magnetite crystals. The hybrid melt zone of the MI experiments
developed between the rhyolitic melt and the basaltic melt. (a) The
reference experiment MI_Rt_1 with two hydrated
rhyolitic glass cylinders was thermally equilibrated at 210 MPa and 1348 K
for 10 min and then decompressed with 0.17 MPa s-1 to 100 MPa. Vesicles are homogeneously distributed with mean diameters of
∼ 75 µm. No enhanced vesicle formation occurred at the
previous contact plane. The undecompressed reference sample
MI_5 was thermally equilibrated at 210 MPa and 1348 K for 10
min and subsequently quenched to room temperature. A narrow hybrid melt zone
formed between the endmember compositions (Table 3). (b) The sample
MI_T_3 was thermally equilibrated at 210 MPa
and 1348 K for 10 min, followed by decompression at 0.17 MPa s-1 to 100 MPa. The upper rhyolitic glass is homogeneously vesiculated
(mean vesicle diameter of ∼ 74 µm). The basaltic glass
volume contains vesicles with different sizes ranging from 56 to 224 µm, with an average of 98 µm. The hybrid zone is decorated by a series
of ∼ 99 µm sized vesicles, elongated along the contact
boundary. (c) The sample MI_7 with basalt on top of rhyolite
was thermally equilibrated at 210 MPa and 1348 K for 10 min and then
decompressed with 0.17 MPa s-1 to 100 MPa. The basalt is
nearly vesicle free, while the rhyolite is highly vesiculated (mean vesicle
diameter of ∼ 69 µm). Towards the contact zone of
rhyolite and basalt, the VND increases, while the vesicle diameter decreases to
∼ 61 µm. (d) The sample MI_6 was
thermally equilibrated at 210 MPa and 1403 K for 10 min and then
decompressed with 1.7 MPa s-1 to 102 MPa. The rhyolite glass
is homogeneously vesiculated with vesicle diameters of ∼ 41 µm. The basalt is nearly vesicle free. The images show an increased
VND in the hybrid zone, while the vesicle diameter decreases to ∼ 16 µm.
EMPA-measured concentration–distance profiles of oxide components
between initial rhyolitic composition (left) and initial basaltic
composition (right) of MI_5, MI_T_3, MI_7, and MI_6. The
concentrations of the individual oxides were normalized to a range from 0 to 1.
CH2Ot–distance profile from rhyolite to basalt through the
hybrid melt zone of bimodal samples MI_5 (undecompressed),
MI_T_3, MI_7, and
MI_6. The cH2Ot in rhyolite is colored in violet,
measurements in the hybrid melt zone are in red, and cH2Ot in basalt is
shown in blue. Open circles illustrate cH2Ot based on NIR measurements
with absorbances > 1 that were excluded from evaluation. Data
points of the concentration–distance profile are visualized in the shaded
transmission microscope images of the samples at the bottom of each plot.
The initial H2O concentrations of the hydrated rhyolite (cH2Oi; determined by NIR) and the hydrated basalt (cH2Og; determined
gravimetrically) are displayed by the dashed lines (see Table 2). The hybrid
zone is also characterized by the SiO2 concentration profile
(cSiO2), marked by grey solid lines, scaled on the right y axis.
Illustrated by blue ellipsoids is the beginning of the enhanced vesiculated
zone of the rhyolite-dominated part of the sample.
Bimodal decompression experiments
In samples MI_T_3 and MI_6, a
slight indentation of the basalt into the overlying rhyolite can be seen in
the central sample area. A gravitationally induced convective movement of
the melts is preserved in sample MI_7. The basalt overlying
the rhyolite sank into the rhyolite melt, squeezing the rhyolitic melt into
the upper part of the capsule. This effect is visually enhanced by cutting
the thin section slightly inclined from the cylinder axis (Fig. 2).
The decompressed MI samples (Fig. 2b, c, d) contain in the rhyolitic part
heterogeneously nucleated fringe vesicles attached to the capsule walls with
a vesicle-free drainage zone (Navon and Lyakhovsky, 1998; Iacono-Marziano et
al., 2007) and a homogeneously vesiculated center volume. Vesicles close to
the drainage zone are mostly deformed or elliptically shaped, whereas
vesicles in the rhyolitic sample center are spheres. The basaltic parts of
the samples MI_7 and MI_6 are not vesiculated.
The sample MI_T_3 (Fig. 2b) contains vesicles
distributed over the whole basaltic part. The vesicles near the capsule wall
have small diameters of ∼ 25 µm and increase in size
towards the center of the sample up to ∼ 250 µm,
whereas the number of vesicles decreases towards the center. Opposite to
the rhyolitic glass part, no vesicles nucleated heterogeneously at the
capsule wall in the basaltic part of the sample.
The VND and the mean vesicle diameter of the sectioned vesicles in
MI_T_3 were determined using the BSE image as
described by Marxer et al. (2015) and Preuss et al. (2016). As the thin
section was polished down to 53 µm thickness, each vesicle in the
sample was cut and therefore not measurable under uncut conditions with TLM.
The VND results in 1.9×102 mm-3 and
∼ 74 µm diameter in the rhyolitic glass and
1.1×102 mm-3 and ∼ 98 µm in the basaltic glass. VNDs and vesicle diameters of
MI_7 and MI_6 were determined with TLM. The
VND in the rhyolitic part of the samples MI_7 and
MI_6 are 2.2 × 102 and
1.2 × 103 mm-3, respectively. The mean diameter of
uncut vesicles in MI_7 is ∼ 69 µm and in MI_6
∼ 41 µm (Table 3).
FTIR spectroscopic analysis of the central rhyolitic parts results in
cH2Ot of 4.07 ± 0.18 wt % for MI_T_3, 4.53 ± 0.17 wt % for MI_7, and 4.65 ± 0.2 wt % for MI_6 (Tables 2 and S1). Measurements in the
basaltic sample parts were not successful.
Minerals detected by X-ray diffraction in the partially crystallized
basaltic glasses are magnetites, supported by EMPA. The crystals have sizes
of 1–5 µm. Small dendritic quench crystals have grown around the
magnetite crystals during quench (Fig. 6). EMP measurements of eight idiomorphic
magnetite crystals in MI_T_3 (Table 1) result
in a mean concentration of 80.13 wt % FeO* (*total Fe), 5.83 wt % MgO,
3.64 wt % TiO2, 2.92 wt % Al2O3, 0.75 wt % SiO2, and 0.52 wt % MnO. Very low concentrations of 0.3 wt % CaO, 0.08 wt % Na2O, and 0.07 wt % K2O were measured. The total sum of
oxides is 94.25 wt %. Calculation of FeO and Fe2O3 from FeO*
according to Carmichael (1967) results in an oxide sum of 100.3 wt % or
100.1 wt % without the alkali oxides. Based on the main oxide components
of the composition, the crystals can be assigned to spinel and are hereafter
referred to as magnetite. The quench crystals attached to the magnetite
crystals are probably pyroxenes due to the dendritic texture and the
depletion of SiO2 in the residual glass composition. These quench
crystals could not be measured with the EMP as they are too small for
analysis due to the fast cooling rate of 44 K s-1 and are
present in concentrations that are too low for XRD analysis.
Hybrid zone
A compositional hybrid zone developed during the experiments between the
rhyolitic and basaltic melts (Figs. 2, 3). All decompressed MI samples
show an increase in VND or vesicle size near the contact zone of the two
compositions. In the upper rhyolitic part of the contact zone of
MI_T_3, the still transparent glass is
indicated by a series of large vesicles with a mean diameter of
∼ 99 µm and a VND of 4.1 × 102 mm-3,
elongated along the contact boundary (Fig. 6). The situation is different
for samples MI_7 and MI_6, where a
∼ 0.2 mm wide vesicle-free zone is observed in
MI_7 and a ∼ 0.3 mm wide vesicle-free zone is
visible in MI_6 at the former contact zone (Fig. 7). In
MI_7 and MI_6 the mean vesicle diameter near
the contact zone decreases down to ∼ 61 and
∼ 16 µm, respectively. The VND in MI_7
increases from 2.2 × 102 mm-3 in the rhyolite volume to
4.3 × 102 mm-3 in the hybrid zone. The VND in
MI_6 increases from 1.2 × 103 mm-3 in the
rhyolite volume to 7.0 × 103 mm-3 in the hybrid zone
(Table 3). Strikingly, the zone with the enhanced VND occurs in all bimodal
decompressed samples in the rhyolite-dominated region of the hybrid zone
(Fig. 4).
A color change from nearly opaquely black to semitransparent brownish to
transparent colorless shows the development of intermediate melt compositions
at the former contact zone of rhyolite and basalt, exemplarily shown for
sample MI_7 in Fig. 5. The width of the hybrid zone related
to cSiO2 from the initial rhyolitic to the initial basaltic composition
is 0.39 mm for MI_T_3, 0.31 mm for
MI_7, and 0.56 mm for MI_6 (Fig. 3, Table 3).
Microscope image of sample MI_7. Alignment of the
image is identical to the capsule during the experiment. The basaltic glass
is partially crystallized by magnetite (arrows). The hybrid zone is
characterized by a color transition from opaquely black (basalt) to
semitransparent brownish to transparent colorless (rhyolite).
(a) Hybrid zone of MI_T_3 with a
sharp boundary of the magnetite crystals (Mag) in the basaltic-dominated
volume towards the rhyolitic-dominated glass. (b) BSE image of dendritic
growth of quench crystals on idiomorphic magnetite (Mag).
BSE images of the hybrid zones of MI_7 (a) and
MI_6 (b). A ∼ 200 µm wide vesicle-free
zone developed in MI_7 and a ∼ 300 µm
wide vesicle-free zone developed in MI_6 as part of the
hybrid zone (marked by the dotted line). In the rhyolitic-dominated part,
vesicles of ∼ 61 µm in MI_7 and vesicles
of ∼ 16 µm in MI_6 formed.
The cH2Ot profiles over the hybrid zone of MI_T_3, MI_7, and MI_6 show an
increase in cH2Ot in the transition from the rhyolitic- to the
basaltic-dominated composition. Sample MI_T_3
has a mean cH2Ot of 4.02 ± 0.20 wt % in the rhyolitic part. The
cH2Ot in the hybrid zone starts at 3.89 ± 0.21 wt % and
increases up to 5.00 ± 0.30 wt %. The last NIR measurements in the
direction of the basaltic composition result in absorptions > 1. An evaluation of the data points with absorbances > 1 provides
cH2Ot up to 7.70 ± 0.39 wt % in the hybrid zone and even a
cH2Ot of 8.48 ± 0.43 wt % in the basaltic-related composition
(Table 3). Therefore, the last five measurements are not reliable and were
discarded from further discussion. The appearance of the enhanced VND starts at
the ∼ 0.37 mm position in the profile, which corresponds to
the onset of the decrease in SiO2 concentration (Fig. 4).
In sample MI_7, the data points in the rhyolitic composition
yield an average cH2Ot of 4.24 ± 0.19 wt %, while the
concentration decreases towards the hybrid zone (Fig. 4). The minimum
cH2Ot of the hybrid zone is 4.03 ± 0.18 wt % and increases up to
4.93 ± 0.27 wt % towards the basaltic part. One data point
representing 5.54 ± 0.31 wt % was discarded from further discussion
due to an absorption > 1. Two further spectra, one in the hybrid
and one in the basaltic composition, were not evaluable because of the
magnetite content in the glass. The enhanced vesiculated zone in the sample
starts at the 0.64 mm position of the profile, which corresponds to the
first third of the SiO2 concentration profile.
The cH2Ot profile of sample MI_6 starts with a mean
cH2Ot of 4.80 ± 0.23 wt % in the rhyolitic part and decreases at
the beginning of the hybrid zone to 4.63 ± 0.22 wt %. An increase of
up to 5.55 ± 0.31 wt % is observed over the hybrid zone towards the
basaltic composition. The last measurement (6.19 ± 0.35 wt %) in the
hybrid zone was excluded because of the NIR absorption measurements
> 1. The zone with the enhanced VND in MI_6 starts at
the 0.64 mm position in the profile, which corresponds to the onset of the
decrease in cSiO2 (Fig. 4).
DiscussionInitial sample material
The pre-hydrated sample material was examined to evaluate the starting
conditions just before decompression. For the evaluation of decompression
experiments, the homogeneity and the initial glass porosity are important
properties for the degassing and nucleation processes, as shown in Preuss et
al. (2016). Using glass cylinders with porosities below the critical value
of 6 % (Preuss et al., 2016) excludes the effect of growing pre-existing
hydration bubbles by the uptake of H2O during decompression
(Iacono-Marziano et al., 2007). Macroscopic and microscopic examinations of
thin sections of hydrated rhyolitic and basaltic glasses yield vesicle-free
glasses. This ensures melts without pre-existing vesicles prior to
decompression.
While the rhyolitic glasses are crystal free, and the hydrated basaltic
glasses quenched at 16 K s-1 contain magnetite and
pyroxene crystals, the basaltic MI samples show only magnetite crystals
after quenching at 44 K s-1. A comparison of the crystal
phases in the hydration samples and the MI experiments provides a
dissolution of pyroxene during the reheating and the 10 min thermal
equilibration of the melt. This suggests that pyroxene crystallized during
quench at the normal quenching rate of ∼ 16 K s-1 and was therefore not present in the melt before and during
decompression. This observation is confirmed by calculations of the phase
relations under experimental conditions using the software “Easymelts
v.1.2.0”. The phase relations of basalt at 210 and 100 MPa, 1550–1050 K, and total H2O content in the system (H2Osys) from 0 wt % to 6 wt % are presented in Fig. 8. Calculated but not shown is the phase
diagram of basalt at 200 MPa, relevant for the preparatory hydration
experiments, since the equilibrium curves of the crystal phases overlap
except for a few Kelvin. Under hydration conditions of 200 MPa and 1523 K,
spinel (spl) is the only crystal phase that coexists with melt,
corresponding to the idiomorphic magnetites observed in all hydrated
basaltic samples (Fig. S1). At H2O concentrations > 5.4 wt %, H2O would be present in the system as a fluid phase, along with
hydrous melt and spinel. This calculated value is above the H2O
solubility of 4.8 wt % given by Berndt et al. (2002), which can be
explained by a slightly different melt composition. Thus, it can be
concluded that the dendritic pyroxenes in the hydration samples crystallized
during the cooling process. This is unavoidable due to the melt composition
and the cooling rate of 16 K s-1 since an intact sample
cylinder is essential for the subsequent contact with a rhyolitic glass
cylinder for the MI experiments.
From the phase diagram in Fig. 8, H2Osys can be confined under
decompression starting conditions of 1348 K and 210 MPa to a range of
2.65 wt %–5.2 wt %, since neither pyroxene nor H2O fluid coexisted with
hydrous basaltic melt and spinel. This accounts for the reference sample
MI_5. For sample MI_T_3 it can
be inferred from Fig. 8 that at least 3.2 wt % H2O was dissolved in
the basaltic melt system (B_3_h), since a
significant number of H2O vesicles formed during decompression to 100 MPa. This is consistent with the equilibrium degassing of basaltic melt
experimentally demonstrated by Le Gall and Pichavant (2016). As in sample
MI_7, only a few vesicles formed near the drainage zone, and
in MI_6 only one vesicle is present in the decompressed
glass, so the range of possible H2O content in the basalt cylinder
(B_4_h) can be assigned less clearly. At least
2.25 wt % H2O was dissolved in the melts, as neither pyroxene nor
feldspar crystallized. Possibly the H2Osys was at a maximum of 3.2 wt % so that a free fluid phase could not develop, as the equilibrium
H2O content of 3.3 wt % at 100 MPa final pressure, corresponding to Berndt et al. (2002), is higher than the actual dissolved H2O content
of the basaltic melt. Hence, the basaltic melts of MI_6 and
MI_7 were not supersaturated with H2O during
decompression, and thus no vesicles formed in the basaltic melt volume.
Phase relations of basalt at 210 and 100 MPa as a function of T
and H2Osys, calculated using the software Easymelts
v.1.2.0. spl+, cpx+, fsp+, and H2O+ are the spinel,
clinopyroxene, feldspar, and H2O in curves. The oxygen fugacity was set
to log fO2= QFM + 3, corresponding to the intrinsic conditions in
the IHPV (Berndt et al., 2002). Further phases occurring at lower
temperatures were excluded.
MI decompression experimentsVesicle formation
The images of the decompressed samples (Fig. 2) show similar textures of the
vesicles. The sample volumes can be divided into four different zones: (1) the peripheral zone containing vesicles at the contact of the capsule wall
with rhyolite, referred to as fringe vesicles (Navon and Lyakhovsky, 1998;
Iacono-Marziano et al., 2007); (2) partially crystallized basaltic glass with
vesicles in the central volume; (3) rhyolite glass with vesicles in the
central volume (Navon and Lyakhovsky, 1998); and (4) the hybrid zone with an
increased VND compared to the rhyolite volume (Table 3), with a smaller or
larger vesicle diameter, depending on the cH2Oi of the pre-hydrated
rhyolitic and basaltic glasses (Table 2).
Based on classical nucleation theory (e.g., Navon et al., 1998),
vesicles nucleate from H2O-supersaturated melt by exceeding the
critical size of molecular clusters and grow into vesicles. Diffusion of
additional H2O molecules into the existing vesicles reduces the energy
of the entire system, which further drives the diffusion process during
decompression (Hurwitz and Navon, 1994). Diffusion-driven H2O transfer
from the H2O-supersaturated melt volumes through the depleted
melt–vesicle interface into the vesicles leads to an increase in vesicle
size. The four vesiculated zones may have been the result of different
nucleation mechanisms.
Numerous fringe vesicles in the rhyolitic melt form heterogeneously at the
interface between the capsule and the melt due to a low wettability of
hydrous rhyolitic melt. This reduces the required energy to form vesicles at
a low degree of supersaturation during decompression (e.g., Mangan and
Sisson, 2000; Iacono-Marziano et al., 2007). Due to the diffusion of H2O
from the melt into the fringe vesicles, homogeneously nucleated vesicles can
no longer form in the H2O-depleted melt zone, which is referred to as
the drainage zone (Mangan and Sisson, 2000). The observation that only a few
vesicles nucleated at the capsule wall in the basaltic part of sample
MI_T_3 may be due to good wettability of
Au80Pd20 capsule material with hydrous basaltic melt. In this case, energy
required to nucleate vesicles is not reduced (e.g., Gualda and Ghiorso, 2007;
Edmonds et al., 2015).
Images of sample MI_T_3 show magnetite
crystals in direct contact with vesicles in the basaltic part of the sample
(Fig. 6). During hydration, magnetite crystals formed in the basaltic melt.
This suggests that, during reheating followed by decompression within the
stability field of magnetite, these crystals served as nucleation sites for
the formation of the H2O vesicles in the melt. The reduction in energy
required for heterogeneous vesicle nucleation on magnetites was observed in
hydrous rhyolitic melt (Hurwitz and Navon, 1994; Mangan and Sisson, 2000,
2005; Gardner and Denis, 2004; Gardner, 2007; Gardner and Ketcham, 2011), as
well as in andesitic and basaltic melts (Edmonds et al., 2015).
Vesicle nucleation in the basaltic part was not observed in the samples
MI_7 and MI_6 due to the initially
H2O-undersaturated basaltic melts (Table 2). Despite the magnetite
nucleation seeds, the supersaturation pressure was not yet reached for the
lower H2O contents during the decompression to final pressure of 100
MPa.
As the rhyolite melts in the experiments of this study were free of
pre-existing crystals or inhomogeneities, homogeneous vesicle nucleation has
occurred in the central volume of the rhyolitic melts. According to
nucleation theory, the VND increases strongly with the decompression rate
dP/ dt (Toramaru, 2006). This is confirmed by the MI samples where the rhyolitic
part of MI_6 at the fastest decompression rate of 1.7 MPa s-1 has the highest VND of 1.2 × 103 cm-3 compared to MI_T_3 and
MI_7 decompressed with 0.17 MPa s-1 and having
similar VNDs of 1.9 × 102 and 2.2 × 102 cm-3, respectively (Table 3).
All decompressed MI samples differ in their vesicle textures and VNDs in the
hybrid zone from the initial rhyolitic central melt volume. It can be
excluded that vesicles, nucleated heterogeneously at magnetite crystals,
have ascended from the basalt into the hybrid zone. This is proven by
experiment MI_7, where the basalt was placed on top of the
rhyolite. Due to the difference in density, the basalt has sunk into the
rhyolite, but the zone with the enhanced VND is clearly contained in the lower
rhyolite-dominated region of the hybrid zone and is therefore positioned below
the partially crystallized basalt.
The position and the size of vesicles in the hybrid zone may depend on the
initial cH2Oi conditions. In the hybrid zone of MI_T_3, the vesicles of ∼ 99 µm are
bigger than in the rhyolitic melt center (Table 3) but positioned in the
vicinity of the previous contact plane of the two cylinders. This can be
attributed to H2O-saturated rhyolitic and basaltic melt conditions
before decompression. This was different for MI_6 and
MI_7. Since the basalt of both samples was
H2O-undersaturated, H2O diffused from the rhyolitic melt via the
developing hybrid melt into the basaltic region of the sample. This H2O
depletion resulted in an extended vesicle-free zone in the hybrid region of
samples MI_6 and MI_7, as shown in Fig. 7.
Compared to MI_T_3, however, a higher ΔP (reduced pressure during decompression)
was required to form vesicles, related to the lower cH2Ot in the melt.
This resulted in a smaller vesicle size observed in the hybrid zone compared
to the rhyolitic melt volumes. The later onset of the vesicle nucleation
process during decompression did not allow the vesicles to grow as much as
in MI_T_3, which explains the reduced vesicle
size in MI_7 of ∼ 61 µm. The vesicle
size of MI_6 with a decompression rate of 1.7 MPa s-1 may not be directly comparable to that of MI_T_3 and MI_7 with decompression rates of 0.17 MPa s-1 due to the higher VND related to the faster
decompression rate in the case of nucleation (Toramaru, 2006).
Concentration profiles of melt components
The melt between rhyolite and basalt developed as an intermediate composition
through multicomponent interdiffusion processes of ions, depending on their
concentrations, charges, specific diffusion coefficients, and the
requirement to maintain charge neutrality over the sample. The alkalis
Na+ and K+ diffuse orders of magnitudes faster than other network-modifying cations like Ca2+, Mg2+, Fe2+, Fe3+, and
Ti4+ and network formers with the lowest mobility like Si4+ and
Al3+ coupled to O2- diffusion (e.g., Watson, 1982; Baker, 1992;
Johnston and Wyllie, 1988; Zhang et al., 2010). In particular, the cations
Na+ and K+ diffused much faster than Si4+ in the direction of
the basalt, resulting in a significant depletion of Na2O and K2O
components in the rhyolite-dominated hybrid area. Counter flux
Ca2+, Mg2+, Ti4+, Fe2+, Fe3+, and
Al3+ diffused towards the rhyolitic composition. The asymmetric
concentration profile of Na2O with a greater length on the rhyolitic
profile side is suggested to be caused by different Na+ diffusion
coefficients in basalt and rhyolite melt, as Na+ has a higher
diffusivity in rhyolitic composition (Zhang et al., 2010). This effect is
enhanced by the higher H2O content in the rhyolite, since the mobility
increases with increasing H2O content in the melt (Watson, 1981). Except
for the alkalis, the other oxide components show concentration profiles with
a symmetric pattern in the hybrid zone (Fig. 3).
The SiO2 concentration profiles were evaluated for their length to
compare the hybrid zones of the different samples. The differences in the
length of the hybrid zones are striking when comparing samples
MI_T_3 and MI_7 with sample
MI_6. The higher temperature of ∼ 50 K during
thermal equilibration and decompression of MI_6 caused an
increase in diffusivities of components (Zhang et al., 2010). Based on
experiments on the self-diffusion of Si4+ in dry dacitic melts, Tinker
and Lesher (2001) show that the diffusion coefficient of Si doubles at 1 GPa
between 1628 and 1683 K, from 1.45 × 10-14± 0.45 to
3.75 ± 1.13 × 10-14 m2 s-1. This corresponds to the T difference of 50 K between MI_T_3 and MI_7 and that of
MI_6. The linear trend of lnD with the reciprocal of T provides
a first estimate of the differences in the Si4+ diffusion coefficients
for the experiments shown here, via extrapolation of the data of Tinker and
Lesher (2001) with the equation of Zhang et al. (2010, Eq. 37 therein).
Despite the longer diffusion time of MI_T_3
and MI_7 (1247 s) compared to MI_6 (665 s)
related to the slower decompression rate, the higher T of MI_6
of 1403 K results in a slightly higher diffusion coefficient of
2.5(+0.9/-0.6) × 10-17 m2 s-1 compared to 5.5(+1.9/-1.4) × 10-18 in
MI_T_3 and MI_7 at 1348 K.
These calculations demonstrate that the 50 K temperature difference did
indeed result in a longer diffusion distance. However, this corresponds only
to the assumption of dry melts. Due to the influence of H2O on the
Si4+ diffusion coefficient (up to a factor of 10; Baker and Bossanyi,
1994) and the multidimensional behavior of the interacting melt components,
further decomposition of the temperature dependence of the diffusion of
Si4+ cannot be presented within the scope of this project.
The <inline-formula><mml:math id="M659" display="inline"><mml:mrow><mml:msub><mml:mi>c</mml:mi><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">t</mml:mi></mml:msub></mml:mrow></mml:msub></mml:mrow></mml:math></inline-formula> profiles
The H2O concentrations measured between the vesicles of the zone with
high VNDs are generally slightly higher than the calculated equilibrium
solubility at final pressure. Due to increasing H2O solubility with
decreasing T at P<300 MPa (Holtz et al., 1995), part of the H2O
fluid diffused from vesicles back into the melt during quench, causing
vesicle shrinkage (McIntosh et al., 2014; Allabar et al., 2020). As a result,
resorption halos formed with increased H2O concentrations in the melt
surrounding the vesicles, recorded in the glasses (McIntosh et al., 2014;
Allabar et al., 2020). Vesicle shrinkage is reinforced by the decrease in
the molar volume of H2O fluid during the isobaric quench (Marxer et
al., 2015; Allabar et al., 2020). The size of the halos around the vesicles
depends on the cooling rate and is therefore comparable in all bimodal
decompression samples. A high VND favors fast H2O resorption during
cooling due to the high number of H2O fluid sources per melt volume.
Due to an increased VND in the hybrid melt zone within a decompressed sample,
it can be assumed that H2O resorption was most efficient in the hybrid
melt zones.
Resorption of H2O from vesicles back to the melt might also be an
explanation for the high cH2Ot in the hybrid area with dacitic
composition. It could be possible that vesicles also nucleated in the
basaltic-dominated hybrid zone during decompression. Assuming similar
temperature dependence of H2O solubility in dacite, already formed
vesicles in the basaltic-dominated hybrid region might have dissolved again
during cooling due to resorption of H2O from vesicles into the melt.
Therefore, hybrid melt that resorbed H2O indicates a much higher
cH2Ot in the glass than in the rhyolite-dominated vesiculated hybrid
regions (Fig. 4). Based on the EMPA data, the hybrid melt evolved to a
dacitic composition, comparable to that of the Unzen dacite (Chen et al.,
1993). Sato et al. (1999) showed experimentally that 5.7 wt % H2O is
soluble at 98 MPa and 1303 K, which is quite similar to our final
experimental conditions of 100 MPa and 1348 K. Therefore, at Pf the
solubility of H2O in dacitic melt is much higher than in rhyolitic or
basaltic melt, with 3.8 wt % and 3.3 wt %, respectively (Allabar et al.,
2022; Berndt et al., 2002). This may explain the increase of ∼ 1 wt % H2O in each hybrid cH2Ot profile.
However, the evaluation of cH2Ot is prone to errors. A crucial point is
the use of Eq. (1) of Ohlhorst et al. (2001) for the determination of the
absorption coefficients. The composition and the structure of the glasses
are decisive factors for the application of the calibration. Liu et al. (2004) found that Eq. (1) given by Ohlhorst et al. (2001) is only applicable
to the rhyolitic, dacitic, andesitic, and basaltic compositions that were
also used for calibration due to the Fe2+/ Fe3+ ratio on the
calibration. In our case, the bulk compositions of the individual
measurement points differ from those used by Ohlhorst et al. (2001). The
bimodal compositions are in the compositional range between the rhyolite and
dacite of Ohlhorst et al. (2001). Nevertheless, due to the continuous change
in composition, the method of Ohlhorst (2001) is appropriate for calculating
approximations of the actual H2O content. Gaussian background
correction was also tested to evaluate the spectra, as described in Ohlhorst
et al. (2001), Yamashita et al. (1997), and in Liu et al. (2004). However,
the required punctual overlap of the spectrum with the Gaussian curve
between ∼ 4500 and ∼ 5200 cm-1 is
not possible for all NIR spectra collected along the hybrid zone. Due to the
changing bulk composition, the slope of the spectra changes (Supplement Fig. S2). For the rhyolitic composition, it is not possible to tangent the
minimum between 4500 and 5200 cm-1. Similarly, in the dacite
zone, the minimum between 4500 and 5200 cm-1 cannot be
reached. Only for spectra with significant basaltic influence is it
possible to tangent to the minimum and thus to perform the Gaussian
background correction as proposed by Ohlhorst et al. (2001) and Yamashita et
al. (1997). Since the same background correction must be applied for the use
of absorption coefficients like for the determination of absorption
coefficients, they cannot be used if the Gaussian correction cannot be
applied on the spectra. Thus, the background correction according to
Ohlhorst et al. (2001) with the straight lines under the bands of 4500
and 5200 cm-1 is useful to determine the absorptions of OH
and H2Om, even if the Fe2+/ Fe3+ ratio of the bimodal
decompressed samples does not exactly match the calibration.
Another problem is that the error in the absorption coefficients calculated
according to Ohlhorst et al. (2001) increases with cH2Ot (Table S1).
The slight decrease in cH2Ot between the rhyolite and the hybrid region
is probably caused by the density interpolation between the two
compositions, as the density changes more than the SiO2-dependent
absorption coefficients along the rhyolite-dominated region. However, since
there is no cSiO2-dependent density formula for hydrous rhyolitic to
basaltic glasses, the interpolation of density between two known compositions is
the simplest way to evaluate cH2Ot trends.
For a first check of the validity of the cH2Ot profiles calculated by
the NIR absorption spectra, an uncalibrated “by-difference” method was
performed on the EMPA bulk concentration profiles (Devine et al., 1995). The
difference in the sum of the oxides to 100 wt % was considered H2O
content. Although this is a semiquantitative method for determining the
H2O content in the glasses, the data show a systematic increase in
cH2Ot in the hybrid zone towards the basaltic compositions (Table S1).
Data of the mean, minimum, and maximum cH2Ot of each sample in rhyolite,
hybrid composition, and basalt data correlate remarkably well with the
H2O contents determined from the absorption spectra. The maximum
cH2Ot in dacitic composition, measured by FTIR spectroscopy and EMPA,
differs by 3 % (MI_5), 34 % (MI_T_3), and 6 % each for MI_7 and
MI_6.
Despite the errors in determining H2O contents with both methods, the
increase in the H2O contents across the hybrid zone towards basalt may
be possible. However, H2O contents up to ∼ 6 wt % in
the zone with dacitic composition are surprising and need further
investigation. An improvement for the determination of H2O contents
with NIR spectroscopy would be the synthesis of hydrous glass standards with
intermediate compositions in several mixing steps between rhyolite and
basalt that match bulk compositions of the hybrid zone. However, this
procedure is very time-consuming and considerably beyond the scope of this
paper. The H2O contents in the hybrid range shown here can therefore be
regarded as a first approximation of the actual values.
Alkali depletion as the crucial factor for enhanced vesicle
nucleation in the hybrid zone
The enhanced VND occurs in all decompressed bimodal samples in the
rhyolite-dominated region of the hybrid zones, as presented in Figs. 2 and
4. As a new approach to vesicle nucleation investigated for the first time,
alkali diffusion and the accompanying alkali depletion in the rhyolite-dominated region of the evolving hybrid zone between rhyolite and injected basalt are now discussed. We suggest that the enhanced vesiculated zone
forms due to rapid diffusive loss of alkalis, from the mildly peralkaline
rhyolitic part into the basaltic part of the sample (Fig. 3). The decreased
Na2O concentration significantly reduces the H2O solubility of the
rhyolite and promotes H2O supersaturation in the alkali-depleted
rhyolitic melt near the contact zone during decompression. Dingwell et al. (1997) showed that H2O solubility depends on alkali excess (xex= (Na2O + K2O–Al2O3) / 100), which is the difference
between mole fractions of alkalis and those of alumina. The effect of the
peralkalinity on the H2O solubility in silicate melts was recently
confirmed and refined by Allabar et al. (2022). They found a linear
increase in the H2O solubility with increasing Na2O excess or, in
general, for alkali excess. Based on the provided calculation tool by
Allabar et al. (2022), the alkali excess was calculated for the melt
composition of the hybrid zone in the bimodal samples with enhanced VND. The
glass composition of each sample in the region of enhanced vesicle formation
is presented in Table 1. While the initial xex content in mildly
peralkaline rhyolite was 0.03, xex decreased during the decompression
experiments by more than 90 % close to the H2O solubility minimum at
xex=0. This reduced the H2O solubility of the melt by
∼ 0.4 wt % so that a lower ΔP was required to induce
vesicle formation in the contact region of the developing hybrid melt
compared to the pristine rhyolite. Further decompression allowed these
vesicles to grow, whereas in the pristine rhyolitic melt the vesicles formed
and grew at a higher ΔP (Fig. 9), resulting in VNDs about half the size
of those in the hybrid melt zones (Table 3). From this, it can be concluded
that besides P and T, also the alkali concentration is a decisive factor for
the H2O solubility and thus for the vesicle nucleation in silicate
melts.
Schematic illustration of the development at the contact surface of
the melts during magma injection. (a) (t0): intrusion of a partially
crystallized basaltic melt into a peralkaline rhyolitic magma chamber. The
rhyolitic melt is saturated in H2O. (b) (t1): interdiffusion processes start. A hybrid melt forms at the contact area. The rhyolitic
melt becomes rapidly depleted in alkalis (Na+ and K+ diffuse into
basalt), while cations such as Mg2+, Ca2+, Fe2+, Fe3+,
and Ti4+ diffuse slowly from the basalt into the rhyolite melt, leading
to an incipient dissolution of magnetite crystals (Mag) in the basalt.
H2O solubility decreases in the alkali-depleted zone. (c) (t2):
during magma-ascent-induced decompression vesicles form in the
rhyolite-dominated region of the hybrid melt because the solubility of
H2O is significantly reduced due to alkali depletion. The dissolution
of magnetite in the basaltic-dominated melt is progressing. (d) (t3):
vesicles in the hybrid zone grow and accelerate buoyancy-driven magma ascent
and mingling and mixing processes that induce further vesicle nucleation.
Implications and examples for natural bimodal volcanism
The injection of a mafic melt into a hydrous rhyolitic magma chamber cannot
be directly observed in nature at the time of its occurrence. However,
experiments can give insight into such processes. This experimental study is
based on the basic melt compositions of the Askja volcano in Iceland (Sparks et
al., 1977), considering the composition of basalt enclaves and a slightly
simplified rhyolite composition. To relate these results to other natural
alkaline volcanic systems, further natural bimodal compositions were
analyzed for their alkali content using the method of Allabar et al. (2022)
to calculate xex, as well as the classical molar alkali oxides
(Na2O + K2O) to alumina oxide (Al2O3) ratio. Nine
further natural bimodal systems of rhyolitic and basaltic to andesitic
compositions were analyzed – De Rosa et al. (2002) (Salina Island: rhyolite
and andesite); Smithies et al. (2015) (Mount Palgrave: rhyolite (187 077,
195 670) and basalt (194 662, 195 640)); Sparks et al. (1977) (Askja: rhyolite
and basalt); Saito et al. (2002) (Satsuma-Iwojima: rhyolite (S-1–S-4) and
basalt to basalt–andesite (SM-1b, SM-2, SM-3)); Leat et al. (1986) (Parys
Mountain: rhyolite (P5) and basalt (P2)); Lacasse et al. (2007) (Katla Caldera:
rhyolite (KAT02-18) and basalt (KAT02-17)); Pritchard et al. (2013)
(Yellowstone: rhyolite (GR-1-R) and basalt (GR-1-B)); Jahn et al. (2009)
(eastern Central Asian Orogenic Belt: basalt and rhyolite); and Ngounouno et
al. (2000) (Kapski plateau: rhyolite and basalt) (Table S1) – all resulting in
higher xex and alkali / alumina ratios of the rhyolitic composition
compared to the more mafic compositions (xex ratio Rt / B: 0–0.8;
alkali / alumina ratio Rt / B: 1.3–3.6). Although, according to the model of
Allabar et al. (2022), the peralkalinity of the melts is not ensured, as
xex<0, we suspect from the distinct ratio of alkalis in
rhyolite and basalt in each natural example that, in the case of injection
of mafic melt, alkalis will diffuse from the felsic toward the mafic melt.
It has been shown that the depletion of alkalis in the rhyolitic region of
the contact zone reduces the solubility of H2O. Moreover, this concept
is not limited to peralkaline rhyolitic compositions only. Other bimodal
volcanic systems with compositions of rhyodacite, dacite, and andesite – e.g., Gertisser et al. (2009) (Santorini: rhyodacite and basalt); Di Muro et
al. (2008) (Mount Pinatubo: dacite and andesite); and Sigmundsson et al. (2010)
(Eyjafjallajökull; trachyandesite and basalt) (Table S1) – show a ratio of xex, as well as the alkali to alumina ratio, on the felsic magma composition side (xex ratio Rt / B: 0.4–0.8; alkali / alumina ratio Rt / B: 1.3–1.7). It can therefore be assumed that in these cases, the
alkalis also diffuse from the felsic melt into the mafic melt and thus lead
to rapid depletion in the felsic part of the contact zone. The depleted
melts become supersaturated in H2O because the H2O solubility
decreases strongly with the decreasing alkali concentration of the melt
(Dingwell et al., 1997; Allabar et al., 2022). Vesicles form in the hybrid
zone at an early stage during the injection of mafic melt, leading to an
increased contrast in density and viscosity compared to the pristine
rhyolitic magma. The density contrast and the viscosity contrast between
rhyolitic and basaltic melt are crucial for magma mingling and therefore
enlarge the contact zone and thus the chemical magma mixing processes
(Huppert et al., 1982). The resulting vesicle formation in the contact zone
can accelerate buoyancy-driven magma ascent and mingling as shown by
Wiesmaier et al. (2015). Extensive magma mingling could increase the
possibility of mafic crystals being entrained into the felsic melt (e.g.,
Laumonier et al., 2015; Paredes-Marino et al., 2017), which would then act
as nucleation sites and trigger further heterogeneous vesicle nucleation
processes. Therefore, it can be generally considered that injection of a
mafic melt into a felsic magma chamber can trigger explosive volcanic
eruptions. The experimental results presented here and their compatibility
with natural bimodal volcanic systems demonstrate the importance of alkali
diffusion for H2O solubility and the resulting H2O supersaturation
of melts in the contact region. Consideration of this aspect could open new
perspectives on degassing behavior in such complicated volcanic systems.
Data availability
All data derived from this research are presented in the enclosed tables, figures, and the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/ejm-35-613-2023-supplement.
Author contributions
PLM: sample preparation, experiments, analysis and evaluation,
visualization, writing of the original draft; AA: supervision, validation,
software; MN: conceptualization, supervision.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Probing the Earth: magma and fluids, a tribute to the career of Michel Pichavant”. It is a result of the Magma & Fluids workshop, Orléans, France, 4–6 July 2022.
Acknowledgements
We acknowledge the support of the Open Access Publishing Fund of the
University of Tübingen. We thank Monika Rusiecka and one anonymous
reviewer for their very helpful comments on the manuscript. We thank the DFG
for financial support for the acquisition of the electron microprobe (grant:
INST 37/1026-1 FUGG) and Thomas Wenzel and Sebastian Staude for their support during
the sample measurements. We thank Barbara Maier and Annette Flicker for technical
support and maintenance of the IHPV and the FTIR spectrometer, as well as Simone Schafflick for the high-quality sample preparation.
Financial support
This open-access publication was funded by the University of Tübingen.
Review statement
This paper was edited by Francois Holtz and reviewed by Monika Rusiecka and one anonymous referee.
ReferencesAllabar, A. and Nowak, M.: Message in a bottle: Spontaneous phase separation of
hydrous Vesuvius melt even at low decompression rates, Earth Planet. Sc. Lett.,
501, 192–201, 10.1016/j.epsl.2018.08.047, 2018.Allabar, A., Dobson, K. J., Bauer, C. C., and Nowak, M.: Vesicle shrinkage in hydrous
phonolitic melt during cooling, Contrib. Mineral. Petr. 175, 21,
10.1007/s00410-020-1658-3, 2020.Allabar, A., Petri, P. L., Eul, D., and Nowak, M.: An empirical H2O solubility model for
peralkaline rhyolitic melts, Contrib. Mineral. Petr., 177, 52,
10.1007/s00410-022-01915-8, 2022.Arienzo, I., Moretti, R., Civetta, L., Orsi, G., and Papale, P.: The feeding system of
Agnano–Monte Spina eruption (Campi Flegrei, Italy): Dragging the past into
present activity and future scenarios, Chem. Geol., 270, 135–147,
10.1016/j.chemgeo.2009.11.012, 2010.Bachmann, O. and Bergantz, G.: The Magma Reservoirs That Feed Supereruptions,
Elements, 4, 17–21, 10.2113/GSELEMENTS.4.1.17, 2008.Baker, D. R.: Estimation of diffusion coefficients during interdiffusion of
geologic melts: Application of transition state theory, Chem. Geol., 98,
11–21, 10.1016/0009-2541(92)90089-N, 1992.Baker, D. R. and Bossanyi, H.: The combined effect of F and H2O on interdiffusion
between peralkaline dacitic and rhyolitic melts, Contrib. Mineral. Petr., 117,
203–214, 10.2138/rmg.2010.72.8, 1994.Behrens, H., Romano, C., Nowak, M., Holtz, F., and Dingwell, D. B.: Near-infrared
spectroscopic determination of water species in glasses of the system
MAlSi3O8 (M = Li, Na, K): an interlaboratory study, Chem. Geol., 128,
41–63, 10.1016/0009-2541(95)00162-X, 1996.Berndt, J., Liebske, C., Holtz, F., Freise, M., Nowak, M., Ziegenbein, D., Hurkuck, W.,
and Koepke, J.: A combined rapid-quench and H2-membrane setup for internally
heated pressure vessels: Description and application for water solubility in
basaltic melts, Am. Mineral., 87, 1717–1726,
10.2138/am-2002-11-1222, 2002.Bunse, R. W.: Ueber die Processe der vulkanischen Gesteinsbildung Islands, Annu.
Rev. Phys. Chem., 83, 197–272, 10.1002/andp.18511590602,
1851.Carmichael, I. S. E. and Nicholls, J.: Iron-Titanium Oxides and Oxygen Fugacities in
Volcanic Rocks, J. Geophys. Res., 72, 4665–4687,
10.1029/JZ072i018p04665, 1967.Chen, C. H., DePaolo, D. J., Nakada, S., and Shieh, Y. N.: Relationship between eruption
volume and neodymium isotopic composition at Unzen volcano, Nature, 362,
831–834, 10.1016/j.jvolgeores.2008.03.042, 1993.De Rosa, R., Donato, P., and Ventura, G.: Fractal analysis of mingled/mixed magmas: an
example from the Upper Pollara eruption (Salina Island, southern Tyrrhenian
Sea, Italy), Lithos, 65, 299–311,
10.1016/S0024-4937(02)00197-4, 2002.Devine, J. D., Gardener, J. E., Brack, H. P., Layne, G. D., and Rutherford, M. J.: Comparison of
microanalytical methods for estimating H2O contents of silicic volcanic
glasses, Am. Mineral., 80, 319–328, 10.2138/am-1995-3-413, 1995.Di Muro, A., Pallister, J., Villemant, B., Newhall, C., Semet, M., Martinez, M., and Mariet,
C.: Pre-1991 sulfur transfer between mafic injections and dacite magma in the
Mt. Pinatubo reservoir, J. Volcanol. Geoth. Res., 175, 517–540,
10.1016/j.jvolgeores.2008.02.025, 2008.Dingwell, D. B., Holtz, F., and Behrens, H.: The solubility of H2O in peralkaline and peraluminous
granitic melts, Am. Mineral., 82, 434–437, 10.2138/AM-1997-3-421, 1997.Druitt, T. H., Costa, F., Deloule, E., Dungan, M., and Scaillet, B.: Decadal to monthly
timescales of magma transfer and reservoir growth at a caldera volcano,
Nature, 482, 77–97, 10.1038/nature10706, 2012.Edmonds, M., Brett, A., Herd, R. A., Humphreys, M. C. S., and Woods, A.: Magnetite-bubble
aggregated at mixing interfaces in andesite magma bodies, in: The Role of
Volatiles in the Genesis, Evolution and Eruption of Arc Magmas, Geol. Soc. Sp.,
410, 95, 10.1144/SP410.7, 2015.Gardner, J. E., Hilton, M., and Carroll, M. R.: Experimental constraints on degassing of
magma: isothermal bubble growth during continuous decompression from high
pressure, Earth Planet. Sc. Lett., 168, 201–218,
10.1016/S0012-821X(99)00051-5, 1999.Gardner, J. E. and Denis, M.-H.: Heterogeneous bubble nucleation on Fe-Ti oxide
crystals in high-silicia rhyolitic melt, Geochim. Cosmochim. Ac., 68,
3587–3597, 10.1016/j.gca.2004.02.021, 2004.Gardner, J. E.: Heterogeneous bubble nucleation in highly viscous silicate melts
during instantaneous decompression from high pressure, Chem. Geol., 236,
1–12, 10.1016/j.chemgeo.2006.08.006, 2007.Gardner, J. E. and Ketcham, R. A.: Bubble nucleation in rhyolite and dacite melts:
temperature dependence of surface tension, Contrib. Mineral. Petr., 162,
929–943, 10.1007/s00410-011-0632-5, 2011.Gardner, J. E., Wadsworth, F. B., Carley, T. L., Llewellin, E. W., Kusumaatmaja, H., and Sahagian,
D.: Bubble Formation in Magma, Annu. Rev. Earth Pl. Sc., 51, 131–154,
10.1146/annurev-earth-031621-080308, 2022.Gertisser, R., Preece, K., and Keller, J.: The Plinian Lower Pumice 2 eruption,
Santorini, Greece: Magma evolution and volatile behavior, J. Volcanol. Geoth.
Res., 186, 387–406, 10.1016/j.jvolgeores.2009.07.015,
2009.Gonnermann, H. M. and Manga, M.: The Fluid Mechanics Inside a Volcano, Annu. Rev.
Fluid Mech., 39, 321–356,
10.1146/annurev.fluid.39.050905.110207, 2007.Gualda, G. A. R. and Ghiorso, M. S.: Magnetite scavenging and the buoyancy of bubbles
in magmas, Part 2: Energetics of crystal-bubbles attachment in magmas,
Contrib. Mineral. Petr., 154, 479–490,
10.1007/s00410-007-0206-8, 2007.Hajimirza, S., Gonnermann, H. M., Gardner, J. E., and Giachetti, T.: Predicting homogeneous
bubble nucleation in rhyolite, J. Geophys. Res.-Sol. Ea., 124,
2395–2416, 10.1029/2018JB015891, 2019.Higgins, M. D.: Measurement of crystal size distributions, Am. Mineral., 85,
1105–1116, 10.2138/am-2000-8-901, 2000.Holtz, F., Behrens, H., Dingwell, D. B., and Taylor, R. P.: Water solubility in
aluminosilicate melts of haplogranite composition at 2 kbar, Chem. Geol., 96,
289–302, 10.1016/0009-2541(92)90060-I, 1992.Holtz, F., Behrens, H., Dingwell, D. B., and Johannes, W.: H2O Solubility in Haplogranitic
Melts – Compositional, Pressure, and Temperature-Dependence, Am. Mineral., 80,
94–108, 10.2138/am-1995-1-210, 1995.Huppert, H. E., Sparks, R. S. J., and Turner, J. S.: Effects of volatiles on mixing in
calc-alkaline magma systems, Nature, 297, 554–557,
10.1038/297554a0, 1982.Hurwitz, S. and Navon, O.: Bubble Nucleation in Rhyolitic Melts – Experiments
at High-Pressure, Temperature, and Water-Content, Earth Planet. Sc. Lett., 122,
151–164, 10.1016/0012-821X(94)90001-9, 1994.Iacono-Marziano, G., Schmidt, B. C., and Dolfi, D.: Equilibrium and disequilibrium
degassing of a phonolitic melt (Vesuvius AD 79 “white pumice”) simulated by
decompression experiments, J. Volcanol. Geoth. Res., 161, 151–164,
10.1016/j.jvolgeores.2006.12.001, 2007.Jahn, B. M., Litvinovsky, B. A., Zanvilevich, A. N., and Reichow, M.: Peralkaline granitoid
magmatism in the Mongolian–Transbaikalian Belt: Evolution, petrogenesis and
tectonic significance, Lithos, 113, 521–539,
10.1016/j.lithos.2009.06.015, 2009.Jarvis, P. A., Pistone, M., Secretan, A., Blundy, J. D., Cashman, K. V., Mader, H. M.,
and Baumgartner, L. P.: Crystal and Volatile Controls on the Mixing and Mingling of
Magmas, Geoph. Monog. Ser., 264, 125–150,
10.1002/9781119564485.ch6, 2021.Johnston, A. D. and Wyllie, P. J.: Interaction of granitic and basic magmas:
experimental observations on contamination processes at 10 kbar with H2O,
Contrib. Mineral. Petr., 98, 352–362, 10.1007/BF00375185,
1988.Kouchi, A. and Sunagawa, I.: A Model for mixing basaltic and dacitic magmas as
deduced from experimental data, Contrib. Mineral. Petr., 89, 17–23,
10.1007/BF01177586, 1985.Lacasse, C., Sigurdsson, H., Carey, S. N., Johannesson, H., Thomas, L. E., and Rogers, N. W.:
Bimodal volcanism at the Katla subglacial caldera, Iceland: insight into the
geochemistry and petrogenesis of rhyolitic magmas, Bull. Volcanol., 69,
373–399, 10.1007/s00445-006-0082-5, 2007.Laumonier, M., Scaillet, B., Pichavant, M., Champallier, R., Andujar, J., and Arbaret, L.:
On the conditions of magma mixing and its bearing on andesite production in
the crust, Nat. Commun., 5, 5607, 10.1038/ncomms6607, 2014.Laumonier, M., Scaillet, B., Arbaret, L., Andujar, J., and Champallier, R.: Experimental
mixing of hydrous magmas, Chem. Geol., 418, 158–170,
10.1016/j.chemgeo.2015.10.031, 2015.Lavallée, Y., Dingwell, D. B., Johnson, J. B., Cimarelli, C., Hornby, A.J., Kendrick, J. E., von
Aulock, F. W., Kennedy, B. M., Andrews, B. J., Wadsworth, F. B., Rhodes, E., and Chigna, G.: Thermal
vesiculation during volcanic eruptions, Nature, 528, 544–547, 10.1038/nature16153, 2015.Leat, P. T., Jackson, S. E., Thorpe, R. S., and Stillman, C. J.: Geochemistry of bimodal
basalt-subalkaline/peralkaline rhyolite provinces within the Southern
British Caledonides, J. Geol. Soc. Lond., 143, 259–273,
10.1144/gsjgs.143.2.0259, 1986.Le Gall, N. and Pichavant, M.: Homogeneous bubble nucleation in H2O- and
H2O-CO2-bearing basaltic melts: Results of high temperature decompression
experiments, J. Volcanol. Geoth. Res., 327, 604–621,
10.1016/j.jvolgeores.2016.10.004, 2016.Leonard, G. S., Cole, J. W., Nairn, I. A., and Self, S.: Basalt triggering of the c. AD 1305
Kaharoa rhyolite eruption, Tarawera Volcanic Complex, New Zealand, J.
Volcanol. Geoth. Res., 115, 461–486,
10.1016/S0377-0273(01)00326-2, 2002.Liu, Y., Behrens, H., and Zhang, Y.: The speciation of dissolved H2O in dacitic melt,
Am. Mineral., 89, 277–284, 10.2138/am-2004-2-304, 2004.Mangan, M. and Sisson, T.: Delayed, disequilibrium degassing in rhyolite magma:
decompression experiments and implications for explosive volcanism, Earth
Planet. Sc. Lett., 183, 441–455,
10.1016/S0012-821X(00)00299-5, 2000.Mangan, M. and Sisson, T.: Evolution of melt-vapor surface tension in silicic
volcanic systems: Experiments with hydrous melts, J. Geophys. Res., 110, B01202,
10.1029/2004JB003215, 2005.Martel, C., Brooker, R. A., Andújar, J., Pichavant, M., Scaillet, B., and Blundy, J. D.:
Experimental Simulations of Magma Storage and Ascent, in: Volcanic Unrest, edited by: Gottsmann, J.,
Neuberg, J., and Scheu, B., Adv. Volcanol., 101–110,
10.1007/11157_2017_20, 2017.Marxer, H., Bellucci, P., and Nowak, M.: Degassing of H2O in a phonolitic melt: A
closer look at decompression experiments, J. Volcanol. Geoth. Res, 297,
109–124, 10.1016/j.jvolgeores.2014.11.017, 2015.Mayerhöfer, T. G. and Popp, J.: Beer's Law – Why Absorbance Depends (Almost)
Linearly on Concentration, Chemphyschem, 20, 511–515,
10.1002/cphc.201900103, 2019.McIntosh, I. M., Llewellin, E. W., Humphreys, M. C. S., Nichols, A. R. L., Burgisser, A.,
Schipper, C. I., and Larsen, J. F.: Distribution of dissolved water in magmatic glass records growth and
resorption of bubbles, Earth Planet. Sc. Lett., 401, 1–11, 10.1016/j.epsl.2014.05.037,
2014.Miller, C. F. and Wark, D. A.: Supervolcanoes and their explosive supereruptions,
Elements, 4, 11–15, 10.2113/GSELEMENTS.4.1.11, 2008.Montagna, C. P., Papale, P., and Longo, A.: Timescales of mingling in shallow magmatic
reservoirs, Geol. Soc. Lond. Spec. Publ., 422, 131–140,
10.1144/SP422.6, 2015.Morgan, G. and London, D.: Effect of current density on the electron microprobe
analysis of alkali aluminosilicate glasses, Am. Mineral., 90, 1131–1138,
10.2138/am.2005.1769, 2005.Morgavi, D., Perugini, D., De Campos, C. P., Ertel-Ingrisch, W., and Dingwell, D. B.: Time
evolution of chemical exchanges during mixing of rhyolitic and basaltic
melts, Contrib. Mineral. Petr., 166, 615–638,
10.1007/s00410-013-0894-1, 2013.Murphy, M. D., Sparks, R. S. J., Barclay, J., Carroll, M. R., Lejeune, A.-M., Brewer, T. S.,
Macdonald, R., Black, S., and Young, S.: The role of magma mixing in triggering the
current eruption at the Soufriere Hills Volcano, Montserrat, West Indies,
Geophys. Res. Lett., 25, 3433–3436, 10.1029/98GL00713,
1998.Navon, O. and Lyakhovsky, V.: Vesiculation processes in silicic magmas, Geol. Soc.
Spec. Publ., 145, 27–50, 10.1144/GSL.SP.1996.145.01.03,
1998.Navon, O., Chekhmir, A., and Lyakhovsky, V.: Bubble growth in highly viscous melts:
theory, experiments, and autoexplosivity of dome lavas, Earth Planet. Sc. Lett.,
160, 763–776, 10.1016/S0012-821X(98)00126-5, 1998.Ngounounoa, I., Déruelle, B., and Demaiffe, D.: Petrology of the bimodal Cenozoic
volcanism of the Kapsiki plateau (northernmost Cameroon, Central Africa), J.
Volcanol. Geoth. Res., 102, 21–44,
10.1016/S0377-0273(00)00180-3, 2000.Nowak, M., Cichy, S. B., Botcharnikov, R. E., Walker, N., and Hurkuck, W.: A new type of high
pressure low-flow metering valve for continuous decompression: First 86
experimental results on degassing of rhyodacitic melts, Am. Mineral., 96,
1373–1380, 10.2138/am.2011.3786, 2011.Ohlhorst, S., Behrens, H., and Holtz, F.: Compositional dependence of molar
absorptivities of near-infrared OH- and H2O bands in rhyolitic to basaltic
glasses, Chem. Geol., 174, 5–20,
10.1016/S0009-2541(00)00303-X, 2001.Paredes-Marino, J., Dobson, K. J., Ortenzi, G., Kueppers, U., Morgavi, D., Petrelli, M.,
Hess, K.-U., Laeger, K., Porreca, M., Pimentel, A., and Perugini, D.: Enhancement of
eruption explosivity by heterogeneous bubble nucleation triggered by magma
mingling, Sci. Rep.-UK, 7, 16897, 10.1038/s41598-017-17098-3, 2017.Perugini, D. and Poli, G.: The mixing of magmas in plutonic and volcanic
environments: analogies and differences, Lithos, 153, 261–277,
10.1016/j.lithos.2012.02.002, 2012.Perugini, D., De Campos, C. P., Ertel-Ingrisch, W., and Dingwell, D. B.: The space and time
complexity of chaotic mixing of silicate melts: Implications for igneous
petrology, Lithos, 155, 326–340,
10.1016/j.lithos.2012.09.010, 2012.Phillips, J. C. and Woods, A. W.: Suppression of large-scale magma mixing by
melt-volatile separation, Earth Planet. Sc. Lett., 204, 47–60,
10.1016/S0012-821X(02)00978-0, 2002.Philpotts, A. R. and Ague, J. J.: Principles of Igneous and Metamorphic Petrology,
Cambridge University Press, 700 pp., 10.1017/9781108631419,
2009.Pichavant, M., Poussineau, S., Lesne, P., Solaro, C., and Bourdier, J. L.: Experimental
Parametrization of Magma Mixing: Application to the AD 1530 Eruption of La
Soufrière, Guadeloupe (Lesser Antilles), J. Petrol., 2, 257–282,
10.1093/petrology/egy030, 2018.Preuss, O., Marxer, H., Ulmer, S., Wolf, J., and Nowak, M.: Degassing of hydrous trachytic
Campi Flegrei and phonolitic Vesuvius melts: Experimental limitations and
chances to study homogeneous bubble nucleation, Am. Mineral., 101, 859–875,
10.2138/am-2016-5480, 2016.Pritchard, C. J., Larson, P. B., Spell, T. L., and Tarbert, K. D.: Eruption-triggered mixing of
extra-caldera basalt and rhyolite complexes along the East
Gallatin–Washburn fault zone, Yellowstone National Park, WY, USA, Lithos,
175/176, 163–177, 10.1016/j.lithos.2013.04.022, 2013.Rose, W. I. and Chesner, C. A. : Dispersal of ash in the great Toba eruption, 7 KA,
Geology, 15, 913–917,
10.1130/0091-7613(1987)15<913:DOAITG>2.0.CO;2,
1987.Sahagian, D. and Carley, T. L.: Explosive Volcanic Eruptions and Spinodal
Decomposition: A different Approach to Deciphering the Tiny Bubble Paraox,
Geochem. Geophy. Geosy., 21, 6, 10.1029/2019GC008898, 2020.Saito, G., Stimac, J. A., Kawanabe, Y., and Goff, F.: Mafic-felsic magma interaction at
Satsuma-Iwojima volcano, Japan: Evidence from mafic inclusions in rhyolites,
Earth Planet. Space, 54, 303–325, 10.1186/BF03353030,
2002.Sato, H., Nakada, S., Fujii, T., Nakamura, M., and Suzuki-Kamata, K.: Groundmass pargasite
in the 1991–1995 dacite of Unzen volcano: phase stability experiments and
volcanological implications, J. Volcanol. Geoth. Res., 89, 197–212,
10.1016/S0377-0273(98)00132-2, 1999.Scaillet, B. and Pichavant, M.: Experimental constraints on volatile abundances
in arc magmas and their implications for degassing processes, in:
Volcanic degassing, edited by: Oppenheimer, C., Pyle, D., and Barclay, J., Geol.
Soc. Lond. Spec. Publ., 213, 23–52,
10.1144/GSL.SP.2003.213.01.03, 2003.Self, S. and Blake, S.: Consequences of explosive supereruptions, Elements, 4,
41–46, 10.2113/GSELEMENTS.4.1.41, 2008.Sigmundsson, F., Hreinsdóttir, S., Hooper, A., Árnadóttir, T., Pedersen,
R., Roberts, M. J., Óskarsson, N., Auriac, A., Decriem, J., Einarsson, P., Geirsson,
H., Hensch, M., Ófeigsson, B. G., Sturkell, E., Sveinbjörnsson, H., and Feigl, K. L.:
Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption,
Nature, 468, 426–430, 10.1038/nature09558, 2010.Sigurdsson, H. and Sparks, R. S. J.: Petrology of rhyolitic and mixed magma ejecta
from the 1875 eruption of Askja, Iceland, J. Petrol., 22, 41–84,
10.1093/petrology/22.1.41, 1981.Smithies, R. H., Howard, H. M., Kirkland, C. L., Korhonen, F. J., Medlin, C. C., Maier, W. D.,
Quentin de Gromard, R., and Wingate, M. T. D.: Piggy-back Supervolcanoes – Long-Lived,
Voluminous, Juvenile Rhyolite Volcanism in Mesoproterozoic Central
Australia, J. Petrol., 56,
735–763, 10.1093/petrology/egv015,
2015.Snyder, D.: Thermal effects of the intrusion of basaltic magma into a more
silicic magma chamber and implications for eruption triggering, Earth Planet. Sc. Lett., 175,
257–273, 10.1016/S0012-821X(99)00301-5, 2000.Sparks, R. S. J.: The dynamics of bubble formation and growth in magmas: a review
and analysis, J. Volcanol. Geoth. Res., 3, 1–37,
10.1016/0377-0273(78)90002-1, 1978.Sparks, R. S. J., Sigurdsson, H., and Wilson, L.: Magma mixing: a mechanism for triggering
acid explosive eruptions, Nature, 267, 315–318,
10.1038/267315a0, 1977.Spera, F. J., Schmidt, J. S., Bohrson, W. A., and Brown, G. A.: Dynamics and thermodynamics of
magma mixing: Insights from a simple exploratory model, Am. Mineral., 101,
627–643, 10.2138/am-2016-5305, 2016.Stelling, J., Botcharnikov, R. E., Beermann, O., and Nowak, M.: Solubility of H2O- and
chlorine -bearing fluids in basaltic melt of Mount Etna at
T= 1050–1250 ∘C and P= 200 MPa, Chem. Geol., 256, 102–110,
10.1016/j.chemgeo.2008.04.009, 2008.
Tinker, D. and Lesher, C. E.: Self diffusion of Si and O in dacitic liquid at high
pressures, Am. Mineral., 86, 1–13, 10.2138/am-2001-0101,
2001.Toramaru, A.: BND (bubble number density) decompression rate meter for
explosive volcanic eruptions, J. Volcanol. Geoth. Res., 154, 303–316,
10.1016/j.jvolgeores.2006.03.027, 2006.Troll, V. R., Donaldson, C. H., and Emeleus, C. H.: Pre-eruptive magma mixing in ash-flow
deposits of the Tertiary Rum Igneous Centre, Scotland, Contrib. Mineral. Petr.,
147, 722–739, 10.1007/s00410-004-0584-0, 2004.Watson, B.: Basalt Contamination by Continental Crust: Some Experiments and
Models, Contrib. Mineral. Petr., 80, 73–87,
10.1007/BF00376736, 1982.Wiesmaier, S., Morgavi, D., Renggli, C. J., Perugini, D., De Campos, C. P., Hess, K.-U., Ertel-Ingrisch, W., Lavallée, Y., and Dingwell, D. B.: Magma mixing enhanced by bubble segregation, Solid Earth, 6, 1007–1023, 10.5194/se-6-1007-2015, 2015.Yamashita, S., Kitamura, T., and Kusakabe, M.: Infrared spectroscopy of hydrous
glasses of arc magma compositions, Geochem. J., 31, 169–174,
10.2343/geochemj.31.169, 1997.
Yoder, H. S.: Contemporaneous basaltic and rhyolitic magmas, Am. Mineral., 58,
153–171, 1973.Zhang, Y., Ni, H., and Chen, Y: Diffusion Data in Silicate Melts, Rev. Mineral. Geochem.,
72, 311–408, 10.2138/rmg.2010.72.8, 2010.