Volatile resorption expedites eruption onset in large silicic systems

Domain: biology
Published: 2026-03-12
Editorial outcome: major_revision

Abstract
Silicic caldera-forming eruptions are among the most hazardous natural phenomena on Earth, yet their
triggering mechanisms remain poorly understood. While volatile exsolution is widely recognized as a
potential eruption driver in large silicic systems, we find that volatile resorption can, counterintuitively,
promote chamber pressurization faster than volatile exsolution. Using a thermo-mechanical magma
chamber model, we show that resorption is a common process in rapidly recharged systems, driven by
pressure increase and crystal melting. The Aso-4 eruption offers a natural case where volatile resorption
may have occurred, with model results predicting resorption at recharge rates >10-2.4 km3/yr. Through
reducing bulk magma compressibility, resorption amplifies pressurization, driving chamber destabilization
and potentially expediting eruption onset. Here, we propose that volatile resorption is a natural process both
accommodating and promoting rapid chamber pressurization, fundamental to destabilizing large-scale
silicic systems. Detecting its signatures in monitoring signals could provide early warning of imminent
eruption.

Introduction
Triggering silicic caldera-forming eruptions (VEI 7) requires extraordinary conditions, as pressurization
of large-volume reservoirs is inherently difficult. Such extensive systems usually form through prolonged
magma injection into the shallow crust over timescales exceeding 104 yrs, leading to thermal maturation of
the surrounding crust 1. The associated reduction in crustal viscosity slows down internal pressure build-
up, allowing magma chambers to grow to extreme sizes (sometimes 500 km3) without erupting. As
reservoirs grow to larger volumes, recharge-induced pressurization becomes less effective, since achieving
critical overpressure for eruption requires proportionally higher recharge rates 2,3. Consequently, eruption
triggering in large-scale silicic reservoirs is significantly more challenging than in average-sized silicic
systems 4,5.

Various triggering mechanisms have been proposed for eruptions at caldera-forming systems, which are
broadly subdivided into two groups: (1) external processes, such as roof collapse or ice unloading, and (2)
internal processes, like reservoir pressurization through magma recharge or continued volatile exsolution
2,6–8. Especially in water-rich silicic systems, volatile exsolution is often considered a main driver for
internal pressurization inducing eruption 9–15. In the early stages of accumulation, subvolcanic reservoirs
can evolve without an exsolved magmatic volatile phase (MVP). Over time, magma differentiation and
crystallization lead upper-crustal reservoirs to volatile saturation and MVP exsolution 8,16–18. This process
increases overpressure as the low-density MVP causes magma to expand in volume against stiffer crustal
rocks 7,11–15,19. However, for volatile exsolution to act as primary eruption trigger, exsolution must outpace
both volatile loss by passive degassing and viscous relaxation of the crust. This, however, requires rapid
crystallization rates that are difficult to maintain in larger, thermally buffered reservoirs 2. In large silicic
systems, exsolved volatiles may therefore exert a primary control on magma compressibility and chamber
growth, rather than directly triggering eruptions. The high compressibility of the MVP increases the bulk
magma compressibility, dampening pressurization from recharge and leading to less frequent, yet larger,
eruptions 2,3,7,8,12,20,21.

We hypothesize that pre-eruptive MVP resorption, the opposite of volatile exsolution, is a naturally
occurring process in large silicic systems that reduces magma compressibility, thereby modulating a
system’s response to recharge, its overall stability, and the timing of eruptions. Our hypothesis is motivated
by a case of observed pre-eruptive MVP reduction at the Aso caldera in Japan 22. In this system, water
saturation levels prior to the Aso-4 caldera-forming event (>500 km3 DRE, 86.4  1.1 ka, 5-25 vol%
crystallinity; 23–25) were reconstructed through a combination of melt inclusion data and volatile partitioning
between apatite, melt, and a MVP. The study reveals a shift from water-saturated conditions during the
Aso-Y event (~91.4 ka, 0.05 km3), the last known eruption prior to Aso-4, to water-undersaturated
conditions during Aso-4. Passive degassing of the MVP could be ruled out as a cause of the observed
volatile reduction, as this processes is generally not efficient enough to remove all exsolved MVP 26,27.
Apatite would therefore continue to record water-saturated partitioning behavior rather than the observed
undersaturated trends. This suggests that other processes must have contributed to reduction of the MVP.
Instead, the volatile loss is linked to an apparent recharge event that replenished the system with a large
quantity of more mafic magma within the last 5 kyrs before eruption. Here, we perform thermo-mechanical
modelling informed with geochemical data from Keller et al., 22 to test whether volatile resorption could
explain the observed MVP reduction and be a potentially overlooked, naturally occurring process. Using
Aso as a case study, we aim to identify the physical and chemical conditions that promote volatile resorption
and investigate whether it has unrecognized impacts on reservoir pressurization and eruption onset.

Results
Volatile dynamics prior to the Aso-4 caldera eruption
We use a 1D thermo-mechanical box model 2,3,21 to reconstruct volatile saturation levels in the 5 kyrs
between the Aso-Y eruption and the Aso-4 caldera-forming event. This model simulates the thermal and
mechanical evolution of magma chambers (the mobile part of a magma reservoir containing less than 50
vol% crystals) situated in a visco-elastic crust, where the residing magma is experiencing phase transitions
such as crystallization and volatile exsolution. Chambers undergo continuous recharge and are subject to
mass withdrawal via eruption once a critical overpressure (here 20 MPa) is reached. We calibrate the initial
model setup to match pre-Aso-4 conditions by parametrizing a melting curve for rhyodacitic Aso-Y
compositions (see methods section). The initial chamber volume is set to 500 km3 24 at 6 km depth 28,29 and
the maximum model runtime is limited to 5 kyrs, reflecting the time interval between the Aso-Y and Aso-
4 eruptions. The inflowing magma is hotter and contains lower H2O and higher CO2 contents than the
resident magma, to simulate pre-eruptive hybridization of the Aso-4 reservoir with less evolved recharge.
To ultimately test if and under which conditions volatile resorption occurred, we vary initial conditions of
the resident magma, such as crystallinity, H2O and CO2 contents, as well as the temperature, injection rate,
H2O and CO2 contents of the recharging magma (Table 1).

As volatile budgets in apatite and melt inclusions show that the Aso-Y reservoir was water-saturated before
its eruption 22, the presence of an MVP is required at the beginning of each simulation. We find that volatile
saturation at the initial time step of our runs is achieved when a resident magma with 15 vol% crystals
contains at least 4 wt% initial H2O (Fig. 1B). Runs with H2O below 4 wt% do not exsolve an initial MVP
and are therefore excluded from further analysis. In the remaining cases, changes in volatile saturation are
systematically linked to recharge-induced variations in crystal and MVP proportions that enable volatile
exsolution as well as resorption. Based on these observations, we define four distinct regimes that describe
the coupled thermal and volatile saturation conditions in the chamber as a function of recharge rate: Regime
1 – Volatile Exsolution and Cooling, Regime 2 – Volatile Resorption and Cooling, Regime 3 – Volatile
Resorption and Heating, and Regime 4 – Volatile Exsolution and Heating (Fig. 1A). For Aso, recharge rates
10-3.1 km3/yr allow volatile exsolution (second boiling) during reservoir cooling and crystallization
(Regime 1, Fig. 1C). Rates >10-3.1 km3/yr initiate a transition to Regime 2, where the MVP is being resorbed,
while reservoir cooling continues. Once supply rates increase, in our case exceeding ~10-2.7 km3/yr,
recharge-induced heating drives crystal melting and simultaneous volatile resorption (Regime 3). Regime
4 is defined by volatile exsolution accompanied with recharge-induced reservoir heating, which is not
observed in any of our runs. In the Aso simulations, eruption onset is achieved once recharge rates increase
to at least 10-2.4 km3/yr (Fig. 1C). We find that while recharge conditions such as higher temperatures (Fig.
1D, Supplementary Fig. 1B), lower H2O (Fig. 1E, Supplementary Fig. 1C) and lower CO2 (Fig. 1F,
Supplementary Fig. 1D) contents can promote resorption, recharge rate is the main control on the
occurrence of this process (Fig. 1C, Supplementary Fig. 1A).

To achieve the transition from volatile saturation to undersaturation in a magma chamber as observed prior
to Aso-4, the system must be at a sweet spot near saturation when the mass fraction of the initial MVP is
small before the recharge event. For Aso, recharge rates >~10-2.6 km3/yr are required to achieve complete
resorption of the MVP, which takes place at different H2O contents depending on the initial crystallinity:
at 15 vol% magma crystallinity undersaturation is only reached in runs at 4 wt% initial H2O (Fig. 1C), while
at 2 vol% initial crystallinity runs with up to 4.5 wt% initial H2O can undersaturate. These values match
water contents (4.4-4.7 wt%) retrieved from plagioclase-melt hygrometry in Aso-4 magmas 25. This overlap
suggests that pre-Aso-4 conditions were ideally suited for recharge-driven volatile resorption, which
potentially critically influenced the events leading up to the cataclysmic Aso-4 eruption.

What drives volatile resorption during magma recharge?
As we find that recharge is the main driver for volatile resorption, we assess three processes linked to
magma recharge as potential drivers for volatile resorption: (1) recharge-induced pressurization of the
magma chamber, (2) mixing of silicic resident magma with less volatile-rich recharge magma, and (3)
variations in phase proportions through crystal melting. Although these processes are not mutually
exclusive, we aim to identify the hierarchy of processes driving volatile resorption by quantifying their
individual contributions to variations in the MVP volume upon recharge. Therefore, we solve the equation
for water mass conservation in a closed system and calculate the associated changes in the MVP volume
fraction based on hypothetical scenarios (see Supplementary Methods 1 and 2). To evaluate the isolated
effect of magma chamber pressurization resulting from high recharge rates (process 1), overpressure within
the system is systematically varied between 0 to 25 MPa, while holding all remaining parameters fixed. As
water solubility is a function of pressure, increasing overpressure values increases the solubility and
therefore initiates resorption 12,30–33. We find that such pressurization leads to significant resorption of the
MVP of up to 4.3 vol%, corresponding to a relative volume decrease by 87 % assuming an initial MVP
volume fraction of 5 vol% (Fig. 2A). However, as our simulations assume a fixed critical overpressure for
eruption (20 MPa), the resulting pressurization effect, and thus the degree of volatile resorption, is
constrained, defining a maximum extent to which resorption can occur. Under such constant critical
overpressure thresholds, variations in the melt volume fraction (process 3) are suggested to account for
significant variations in the MVP fraction. Here, influx of recharge introduces hotter melts into the
previously evolved system, increasing the overall melt volume fraction, while also heating the chamber
inducing crystal melting. The addition of anhydrous crystal melts and recharge melts dilutes the host
magma’s volatile content and drives it away from saturation. Equilibration of the now undersaturated melt
with the still present MVP occurs through volatile diffusion from the MVP into the undersaturated melt,
thereby reducing the MVP volume. Using the same calculations as above, we isolate the effect of changes
in the crystal fraction by assuming no change in overpressure throughout the calculations. Instead, the
values for crystal fraction are systematically varied from 10 vol% to 5 vol%. These changes in the melt
volume fraction cause a loss of up to 2.4 vol% of the MVP, corresponding to a relative volume reduction
by 47% (initial MVP volume fraction = 5 vol%, Fig. 2C). The effect of mixing between the resident and
recharge magmas (process 2), on the contrary, has a significantly smaller impact on changes in the MVP
volume fraction. To simulate mixing of volatile-rich resident magmas with volatile-poorer (yet volatile-
bearing) recharge magmas, we calculate the effects of increasing water mass and magma chamber mass for
a hypothetical recharge scenario. The water mass to magma chamber volume ratio is calculated for a
chamber volume increase of up to 1 vol%, resulting in a reduction of the MVP of 0.003 vol%. This accounts
for a relative volume decrease of 8% assuming an initial MVP volume of 5 vol% (Fig. 2B). Following these
observations, we conclude that volatile resorption is primarily driven by recharge-induced pressurization
of the magma chamber, increasing water solubility in the melt, followed by variations in phase proportions
induced by increasing chamber temperatures through the addition of hotter recharge.

The implications of volatile resorption on eruption timing
Having established that recharge-induced pressurization and decreasing crystal fraction of the resident
magma can initiate volatile resorption, we assess its impact on magma chamber pressurization and stability.
Therefore, we compare three volatile-resorbing cases with variable initial H2O contents (4, 4.5, and 5 wt%)
with a volatile-exsolving case that has the same initial conditions as the 5 wt% H2O resorbing case but
undergoes slower recharge. Looking specifically at the 5 wt% H2O containing cases, we find that
pressurization rate is substantially higher in the resorbing run and that this case experiences an eruption
after ~2.3 kyrs, while the exsolving run does not undergo eruption within the 5 kyrs simulation time (Fig.
3B). The elevated pressurization rates during resorption are caused not only by higher recharge rates
experienced in volatile resorbing runs, but also by a reduction in the magma compressibility stemming from
the resorption-induced decrease in MVP that usually buffers pressurization build up in silicic systems (Fig. 3A).
In runs transitioning from volatile-saturated to undersaturated conditions, chamber pressurization further
accelerates once the system transitions to undersaturation (Fig. 3C). To fully understand the correlation
between the observed lower magma compressibility and enhanced pressurization in resorbing cases, we
establish a scaling relationship capturing these effects across varying magma chamber conditions (Fig. 4).
The alignment of our data along the 1:1 line confirms that the scaling relationship defined by the combined
effects of mass recharge rate (Ṁin), initial magma density (i), initial chamber volume (V) and mean magma
compressibility (mean) captures the first order controls on chamber pressurization. Cases that undergo a
transition from volatile saturation to undersaturation systematically fall above the 1:1-line, indicating
amplified pressurization. These pressurization dynamics can result from an additional reduction of the bulk
magma compressibility as the MVP volume decreases during a return to volatile-undersaturated conditions
(Fig. 3A, Supplementary Fig. 3). In Aso-4, such accelerated pressurization could have led to earlier eruption
onset of ~70 yrs compared to scenarios retaining the MVP (Fig. 3C, Supplementary Fig. 3).

A generalized model for volatile resorption
In conclusion, we test the viability of volatile resorption as a naturally occurring process through thermo-
mechanical modelling over a broad parameter space (Table 1). Focusing on the Aso-4 eruption as a case
study, we confirm that pre-eruptive volatile resorption and subsequent eruption occurred if the magma
chamber was recharged at rates of at least 10-2.4 km3/yr (Fig. 1C). While such recharge rates exceed typical
long-term silicic magma production rates (~10-3 km3/yr), they are consistent with previous estimates
suggesting that high recharge rates on the order of 10-2 km3/yr (which are long-term averages) are required
to build large silicic magma bodies with volumes over 500 km3 5,34–37. We further find that a return to
volatile undersaturation prior to Aso-4 was possible if the resident magma contained ~4 - 4.5 wt% H2O and
~100 ppm CO2 (Fig. 1B). This is in good agreement with geochemical estimates of the magma water content
in Aso-4 magmas ranging from 4.4 to 4.7 wt% H2O. We conclude that the occurrence of volatile resorption
is inherently linked to high recharge rates, where chamber pressurization along with variations in phase
proportions upon recharge are primary drivers for volatile resorption (Fig. 2). Magma chamber
pressurization is then controlled by the combined effects of recharge rate, initial magma chamber volume
and the bulk compressibility of the resident magma (Fig. 4). We further find that volatile resorption during
strong recharge events generates overpressure in large-scale silicic magma chambers more efficiently than
volatile exsolution. This enhanced pressurization results from both the addition of mass through recharge
and the reduction of the MVP, which usually buffers the build-up of overpressure (Fig. 3). Once a system
transitions from volatile-saturated to undersaturated conditions, an additional drop in magma bulk
compressibility amplifies pressurization and expedites chamber destabilization leading to earlier eruption
(Fig. 3C). Such an early eruption onset has significant implications for hazard assessment and risk
mitigation on a human timescale and needs to be accounted for to improve forecasting of eruptive events
in potentially highly hazardous systems.

If volatile resorption is a ubiquitous process in silicic systems, it would fundamentally transform current
models of large-scale eruption triggering. To evaluate its prevalence, we suggest assessing water saturation
levels via coupled apatite-melt inclusion analyses in systems comparable to Aso. Systems of interest should
experience elevated magma flux prior to eruption, which is often considered an eruption trigger in large
silicic systems 2,6–8, and is documented in cases such as the Kos Plateau Tuff 38, Bishop Tuff 39, and
Bandelier Tuff 40. In such cases, volatile resorption may occur due to increased magma flux without fully
undersaturating the system, while still promoting more efficient overpressure build-up necessary for caldera
eruption onset. To fully resorb the MVP and undergo further amplified pressurization, magma chambers
must additionally sit near volatile saturation. Both the Bishop Tuff (3.2 - 6 wt% H2O, saturation ~5.6 wt%
41–45), and the Kos Plateau Tuff (4.5 - 6.5 wt% H2O, saturation ~ 5.9 wt% 43,46) fulfill this requirement,
making them ideal candidates to test for volatile resorption in the lead up to cataclysmic eruptions. We find
additional evidence for volatile resorption at Campi Flegrei, which exhibits a distinct recharge-driven
increase in magma storage temperatures and a decrease in dissolved H2O prior to caldera-forming events.
Here, the last known eruptions preceding both the Campanian Ignimbrite and the Neapolitan Yellow Tuff
record H2O above saturation (5.5 - 6.5 wt%) that dropped below saturation before the caldera-forming
eruptions (3 - 6 wt%, saturation ~5.5 wt%) 43,47, suggesting potential volatile resorption.

We hypothesize that volatile resorption could be recorded in volcano monitoring through variations in the
mass flux and composition of gas emissions prior to eruption. Pre-eruptive declines in gas emissions have
previously been observed in silicic systems and are usually attributed to the high viscosities of silicic
magmas that hinder the escape of exsolved volatiles or progressive sealing of the conduit (e.g. 48,49). Volatile
resorption, however, could have similar emission-reducing effects that were previously unrecognized.
Similarly, the CO2/SO2 ratio of gases emitted prior to eruption could be impacted by resorption. Since SO2
is more soluble in silicic melts than CO2, we expect an increase in the CO2/SO2 ratio as a response to
beginning volatile resorption during continuous recharge. Such dynamics cannot be captured with our
model, which does not solve for SO2, but are frequently observed prior to eruption, being primarily linked
to fresh recharge entering the system (e.g. 50,51). Ground deformation and seismicity are expected to continue
or even intensify during volatile resorption. Kilbride et al., 52 suggest that ground deformation can be muted
by the presence of a compressible MVP. Likewise, we expect a resorption-induced reduction of the MVP
to cause a decrease in magma compressibility, while resorption of bubbles would lead to a decrease in the
magma chamber volume. Such competing effects could fluctuate or enhance ground deformation prior to
large silicic eruptions, as is periodically observed at Campi Flegrei 53. For example, during the 1982-84
unrest an uplift of ~1.9 m was observed accompanied by a sharp increase of the CO2/H2O ratio, which could
be caused by resorption 54,55.

We propose that volatile resorption is a critical, yet potentially overlooked process that significantly impacts
reservoir pressurization and stability even without transitioning a chamber to undersaturation. While our
framework establishes the fundamental viability of this process, we recognize that significant uncertainties
remain when translating a numerical model to complex natural magmatic systems. Therefore, this work is
intended to provide a foundation for future research. Specifically, the integration of more complex physical
parameters, such a magma chamber geometry or the influence of variations in tensile strength of host rocks
would be essential to capture the complexities of volatile resorption in diverse volcanic settings. Identifying
volcanic systems that underwent volatile resorption in the past and monitoring large-scale silicic volcanoes
for it could help us identify highly recharged systems that undergo amplified pressurization with the
potential of near-future eruptions.

Methods
To test the physical and chemical viability of volatile resorption and simulate the thermodynamic and
mechanical response of magma chambers to it, we apply the numerical framework developed by Scholz et
al.,21. This framework is based on a 1D box model introduced by Degruyter and Huber 2 and further
developed by Townsend et al., 3 to investigate the thermo-mechanical evolution of three-phase silicic
magma chambers in a visco-elastic upper crustal setting. Magma chambers are defined as the mobile portion
of a magmatic reservoir, containing less than 50 vol% crystals, silicate melt and if saturation is reached, a
magmatic volatile phase (MVP). During simulation, the chamber undergoes mass injection via recharge
and mass loss through episodic eruption, while experiencing conductive heat loss to the surrounding wall
rocks. The updated model by Scholz et al.,21 expands upon the initial water-only setup by incorporating
H2O-CO2 fluid mixtures and allowing for the simulation of both mafic and silicic compositions. The
solubility of H2O and CO2 is described using the parametrization from Liu et al.43 for rhyolitic melt under
the assumption that volatile partitioning between the melt and MVP takes place under chemical equilibrium
conditions. To test the kinetic feasibility of volatile resorption, we calculate water diffusivity for Aso-Y
compositions at 900 C and 200 MPa after Ni & Zhang 56. With a water diffusivity of 3.643x10-10 m2/s,
complete MVP resorption would take ~0.1 to 1100 seconds for bubble sizes between 10 and 1000 m in a
500 km3 magma chamber containing 4.5 wt% initial H2O. Throughout the simulation, phase proportions of
melt, crystals and MVP vary with changes in magma chamber pressure, temperature, and volatile solubility
in response to continuous magma recharge and conductive heat loss. We use the equations for the
conservation of total mass, H2O mass, CO2 mass, and enthalpy to track variations in magma chamber mass,
pressure and volume over time. Mass removal from the magma chamber occurs via eruptions, which are
initiated once a critical overpressure (i.e. magma pressure in excess of lithostatic pressure) is reached. We
assume the presence of pre-existing weak zones such as fractures and faults around the magma chamber;
therefore, magma overpressure needs to be high enough to reactivate weak zones and propagate a dike
faster than it freezes. In the literature, such overpressure threshold values vary between 10 - 40 MPa
dependent on host rock properties and magma viscosity. Here, we use an overpressure threshold for fracture
re-activation and eruption onset of 20 MPa. Eruptions remove the required amount of magma mass to
restore the magma chamber pressure to lithostatic.

Calibration of the model to Aso compositions
To get a clear understanding of the magma dynamic processes preceding the Aso-4 event, we calibrated the
model presented by Scholz et al., 21 to Aso-specific compositions. We selected a rhyodacitic melt
composition measured from glass shards of the volatile-saturated Aso-Y eruption 22 that occurred ~5 kyrs
prior to the Aso-4 event at ~91.4 ka (Supplementary Table 1). To calibrate our model, we used a Matlab-
based batch processing tool of the rhyolite-MELTS software 57, generating temperature-crystallinity curves
for Aso-Y as model input. These curves were defined across a broad range of pressures (50 – 400 MPa in
25 MPa intervals), temperatures (700 - 1200 C in 1 C intervals), H2O (0.5 – 5 wt% in 0.25 wt% intervals)
and CO2 contents (0 – 500 ppm in 50 ppm intervals with additional runs at 1 and 10 ppm). MELTS runs
were conducted under isobaric equilibrium crystallization conditions at an oxygen fugacity of NNO +2.65
58. The resulting mass fractions for crystals, melt and fluid were validated for mass-balance and converted
to volume fractions for further processing.

To describe crystallinity as a function of temperature, pressure, and H2O content in our model, we fit the
MELTS outputs following the method described in Scholz et al.,21. They found that temperature-
crystallinity curves of rhyolites are best fitted using error functions, while average basaltic compositions
can be described using linear functions. To fit the rhyodacitic Aso-Y compositions, we therefore apply a
segmented regression that combines linear and error functions with a smooth transition, covering a
crystallinity range from 3 vol% (x = 0.03) to 80 vol% (x = 0.8). We find that low crystallinity segments
of the curve are best described using a linear fit following:
x = 𝐴 × 𝑇+𝐵, (1)
while higher crystallinity segments are best described with an error function:
x = 𝐶 × erfc(𝐷 × (𝑇−𝐸))+𝐹. (2)
x stands for the crystal volume fraction, T for the temperature of the run (C), and A, B, C, D, E, and F are
fitting parameters. Each fitting parameter is calculated individually as a function of H2O content (wt) and
pressure (P in MPa) as described in the following equations. Since the effect of CO2 on parameter
fits is small compared to the impacts of H2O, we neglected it here. T0 describes the transition point between
the two curve fitting equations.

A(wt,𝑃) = [F(wt,𝑃)−B(wt,𝑃)+C(wt,𝑃)×erfc(D(wt,𝑃)×(T0(wt,𝑃)−E(wt,𝑃)))] / T0(wt,𝑃) (3)

B(wt,𝑃) = 3.406−1.298wt+6.78×10−5𝑃+0.3318wt2 +0.0001133wt𝑃−3.387×10−8𝑃2−0.02805wt3
−6.757×10−6wt2 𝑃−2.393×10−8wt𝑃2+1.203×10−11𝑃3 (4)

C(wt,𝑃) = 0.2039−0.03967wt+2.62×10−5𝑃+0.006702wt2 +2.523×10−5wt𝑃−3.251×10−8𝑃2
−0.0006055wt3 −3.335×10−6wt2 𝑃+4.296×10−9wt𝑃2 (5)

D(wt,𝑃) = 0.05491−0.02003wt+5.98×10−5𝑃+0.01834wt2 −1.544×10−5wt𝑃−1.275×10−8𝑃2
−0.002331wt3 +7.214×10−7wt2 𝑃+2.786×10−9wt𝑃2 (6)

E(wt,𝑃) = 856.6−7.875wt−0.09838𝑃+1.17wt2 +0.0006271wt𝑃+3.006×10−5𝑃2−0.0001973wt2 𝑃
+5.233×10−8wt𝑃2−3.4×10−9𝑃3 (7)

F(wt,𝑃) = 0.7868−0.4093wt+0.0002331𝑃+0.1121wt2 −4.308×10−5wt𝑃−2.535×10−8𝑃2
−0.009755wt3 +9.498×10−8wt2 𝑃+5.113×10−9wt𝑃2 (8)

T0(wt,𝑃) = 868.5−3.51wt−0.09669𝑃+0.0008189wt𝑃+2.766×10−5𝑃2
−1.037×10−7wt𝑃2−2.959×10−9𝑃3 (9)

A comparison between crystallinity-temperature curves generated via MELTS and our parametrization
using the presented equations can be found in Supplementary Fig. 2. The deviations between the original
MELTS crystal volume fractions and the calculated fits range from 0 to 10 % for all H2O and CO2 contents
(Supplementary Fig. 2).

Parameter Space
A total of over 12,000 simulations were run to analyze magmatic processes influencing the dynamic
behavior of volatiles and their potential impacts on eruption onset. In a first step, we performed a broad
parameter search covering four orders of magnitude of magma chamber volumes (10 - 1000 km3) at crustal
depth of 6 and 8 km. To simulate variable starting temperatures, we apply initial magma crystallinities of 2
and 15 vol%. We use a fixed and prescribed starting crystallinity instead of a starting temperature since
liquidus temperatures can vary considerably with the total volatile content of the magma (starting
temperatures for the 15 vol% crystals vary between ~855-900°C). Initial H2O contents covered a wide range
between 3 and 5 wt%, while initial CO2 contents were set to 100 and 500 ppm to represent high and low
CO2 endmember scenarios. These volatile contents represent total volatile contents within the system,
which include both volatiles dissolved in the melt and exsolved into the MVP. We additionally tested the
impact of variable recharge conditions on volatile dynamics by varying recharge rates from 10-2 km3/yr to
10-4 km3/yr (corresponding to 7.1 to 713.5 kg/s for a melt density of 2250 kg/m3 calculated for Aso-Y melts)
across 100 logarithmically space steps, adjusting recharge temperatures to 900 C and 1300 C, and setting
recharge volatile contents to 1 and 4 wt% H2O and 100, 500, 1000, and 10,000 ppm CO2. Due to model
constraints, the composition of the recharging magma is identical to the resident magma, however through
variations in temperature and volatile enrichment, we model less evolved recharge conditions, as recorded
before the Aso-4 event 25,59. Simulations were then run until one of the following conditions were achieved:
(1) a crystallinity of 50 vol%, marking rheological locking, (2) a crystallinity of 0.01 vol%, preventing
complete remelting of crystals, or (3) a maximum run time of 5 kyrs, representing the time interval between
the Aso-Y and Aso-4 events. In an additional step following the wide range parameter search, we
constrained magma chamber volumes to 500 km3 24 and chamber depth to 6 km (e.g.28,29) to match Aso
specific conditions. An overview of the full and Aso-specific parameter spaces is provided in Table 1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is an interesting paper that explores the role of pressurisation of a magma reservoir upon recharge. Using an established code (ie an already published one by the same group) the authors explore how the excess fluid/gas that a reservoir may hold prior to recharge may be resorbed in response to the pressure increase. The authors show that under some conditions of magma recharge, that excess fluid may be fully redissolved in the melt, which results in a significant decrease of its bulk compressibility, hence in an increase of the rate of pressure increase, which in turn may bring the reservoir to its failure at a faster rate as well. It is true that the conventional wisdom in this area is to consider that cooling increases excess fluid amount hence magma eruptability. This study shows that this may not always be the case and on that ground it provides an interesting and novel view on this complex problem. This study is a logical follow up of previous efforts led by the same team (Keller et al., 2013 in EPSL) in which the authors proposed the idea of a shift between gas saturated and under-saturated conditions of the Aso reservoir (using apatite volatiles systematic), and suggested that this could affect eruption frequency. In the present contribution they carry out the numerical simulations testing their hypothesis, which they find is a physically tenable one.\nBeing not a numerical modeller I can only trust what the model predicts, but the core code has been already used in a number of published papers, so I assume it has been already benchmarked and it is working properly.\nMy comments are mostly suggestions, clarifications asked for, and some are possibly naive questions given my limited experience in numerical simulations:\n1. The authors investigated a range of volatile contents, either for the resident or the intruded magma. For the later, it is my understanding that it aims at simulating conditions relevant to a mafic recharge (as inferred for Aso by previous work, in particular Kaneko et al., 2007). My question is why the bulk CO2 of that mafic end-member is restricted to 1000 ppm or less? There is evidence that the CO2 content of mafic magmas, including in arcs, can reach several wt% (see the case of Stromboli for instance). It would be interesting to expand the CO2 range to at least 1 wt% bulk content (as in Scholz et al., 2023) and see how this could affect model's outcome.\n2. I also guess or understand that the role of recharge is to provide heat and mass (including H2O and CO2?)to the reservoir but only heat is allowed to propagate upwards (in addition to that lost to the host rock)? In other words the melt fraction trends which are shown in extended data Figure 2 are those which are used to simulate the bulk of the magma reservoir? Or do you take into account also the mafic layer? Similarly, are the H2O and CO2 sourced from the mafic recharge transferred into the overlying silicic melt? I would also suggest the authors to compare their curves with experimental data to assess how accurate is MELTs in calculating melt fraction at a given bulk H2O and CO2 content: for instance the phase equilibria of Costa et al (2004) have been performed on a whole rock composition very similar to that selected in the present work (see also Scaillet and Evans 1999, though this uses a slightly more mafic magma).\n3. From what I understand, the compressibility of host rocks β is kept constant in all simulations. Presumably this property is likely to vary between volcanic areas, depending on the level of intrusion, how fractured are the host rocks etc...The authors fix compressibility to β=1/10-10 Pa (correct?). Quoting Gudmundsson (Volcanotectonics, 2020, page 286) it is said that host rock compressibility can be β=3x10-11 Pa-1. I have no opinion on which value is the most appropriate but a comment of why the value 1/10-10 Pa has been selected would be welcome. I am asking this because using my own rudimentary code which simulates the evolution of internal pressure of a cooling magma reservoir, made me understand that the host rock bulk modulus is quite important in determining how much of a pressure increase arises (and not only the magma compressibility).\n4. One of the great appeal of the model, for a non expert like me, is that it allows to simulate the time evolution of the reservoir, which is a highly desirable property for any model aimed at anticipating eruptions with some confidence. However, the age of Aso-4 event seems to come from Hbl Ar dating (Albert et al., 2019 quoted by Keller et al., 2023), but that phase is described as being xenocrystic or not at equilibrium with its host melt (Ishibashi et al., 2018) which is supported by the lack of Hbl in the run products of hydrothermal experiments of Ushioda et al (2020). I wonder therefore how sure one can be about the time interval between Aso-Y and Aso-4? Is there any other data or way to ascertain the age of Aso-4?\n5. One important result or at least, petrologically speaking, the one that brings question to my mind is that the reservoir may undergo a significant pressure increase during cooling (as in previous work). Is this pressure increase documented (aside from apatite volatile record)? I mean, in an ideal world, one could expect to see phenocrysts hosting melt inclusions showing such a pressure increase, ie an increase of both CO2 and H2O contents as MI get closer to the rim? I know this is difficult task (in particular in phenocryst-poor rocks and given the rather modest changes in pressure) but in my opinion it would be the real smoking gun of the process advocated here. Also, if I understood correctly, volatile resorption comes mostly from an increase in pressure but not from a decrease in crystal fraction? It would help the reader to show a graph with the evolution of these two parameters (gas and crystal fractions) with time during a single but representative simulation that culminates in gas-free conditions close to reservoir rupture. Increasing pressure while cooling and preserving no textural evidence of melting/crystallisation requires quite a subtle combination of parameters. According to petrological constraints (Keller et al., 2023, JVGR), the temperature drop between Aso-A and Aso-4 is about 55°C. Does that fit with the model output?\n6. The model considers the EOS derived by Halbach and Chaterjee on H2O to simulate the PVT of the gas phase. This EOS is for H2O only and it seems that it is used to compute the volume properties of the H2O-CO2 fluid mixture. Am I correct? If so, any idea of how a more real EOS (fluid mixture) would affect the results? More specifically, I dont understand how equilibrium distribution of both H2O and CO2 species between gas and melt can be computed with an EOS for water only?\n7. I think that Bishop Tuff may not be a good example of a system sitting near gas saturation. According to Wallace et al (1995,1999) seminal work, this system records good evidence of being already gas saturated. As for Campi Flegrei, I dont believe we have the firm evidence in hand for such a shift toward gas under-saturation, since temperature estimates of that system are not that well constrained in my opinion.\n8. Pre-eruptive declines in gas emissions also arise from progressive sealing of the volcanic conduit, and not only because of the high viscosity of the magma\n9. I dont think it is correct to say that CO2 solubility is higher in mafic melts relative to more silicic ones (line 186). CO2 solubilities are not that much different in mafic and felsic melts, and in fact, a rhyolite could dissolve even more CO2 if held at the same fCO2 and T than a basalt liquid. What happens in nature is that by the time silicic melts are produced most of the CO2 of the system has been lost.\n10. From what you have done, I think that the best pre-eruptive signal of a pressurising system entering a gas-free regime would be a break in slope of geodetic signals recording ground uplift (following what is shown in your figure 3b(inset)), assuming the time series of data is long enough to capture this change? Yet, the Campi Flegrei example shows that the rapid up and down ground motions are not that easy to interpret in terms of deep seated processes..\nBruno Scaillet.", "reviewer_2": "Review of “Volatile resorption expedites eruption onset in large silicic systems” by Keller, Townsend, Troch, and Huber.\nI have read this manuscript closely and with great interest. The authors use the case study of the caldera-forming Aso-4 eruption to investigate the hypothesis that mafic recharge of large silicic magma chambers can cause resorption of exsolved magmatic volatiles, and that this resorption can then cause increased chamber pressurization rates through the corresponding decrease in magma compressibility, resulting in earlier eruption onset. The authors use thermal mechanical modelling in conjunction with previously published geochemical data regarding the pre-eruptive chemistry, volatile contents and magma chamber conditions of the Aso-4 eruption. Their results indicate that when recharge rates are high volatile resorption can occur, primarily through chamber pressurization (which increases volatile solubility in the melt) and increase of the melt volume fraction by addition of hot recharge melts that also act to melt existing crystals in the resident magma (which together adds relatively volatile-poor melt thus reducing the average volatile content of the melt, approaching a state of volatile undersaturation). The authors suggest that elevated pressurization rates caused by complete volatile resorption could have brought forward the onset of the Aso-4 eruption by ~70 years. The authors propose that detecting precursory signals of chamber pressurization caused by volatile resorption could be critical for early warning of future eruptions.\nOverall I am convinced that volatile resorption may be a common (or, not uncommon) process in large silicic magma chambers and that a consequence of the resorption process can be increased rates of chamber pressurization that may trigger eruption. I agree it is an important topic and worthy of future study. However, I am left uncertain about the extent of the authors’ argument regarding volatile resorption as an eruption trigger, and unconvinced by how useful or meaningful it is that the Aso-4 eruption may have been ‘expedited’ by up to 100 years. This latter point is a significant issue for a paper with the current manuscript title.\nThe paper is generally well-written and logically set out, however I was still left with many questions about the authors’ precise meaning. I have included these in my comments below and I expect that most of them can be addressed by tighter and more explicit text, particularly for figure captions, and perhaps some minor adjustments to the figures. Addressing them will avoid unnecessary confusion for the reader, particularly for the manuscript layout that provides few figures in the main text with detailed methods at the end. The most important revisions will be those addressing to what extent and why the authors are arguing for the importance of volatile resorption in understanding large silicic eruptions. If these can be addressed convincingly I would recommend the manuscript for publication in Nature Communications.\nI have listed my detailed comments below. Line numbers refer to the line numbers of the PDF version provided for review.\nTitle\n‘expedites eruption onset’. Is this the most important finding? The slightly earlier eruption?\nAbstract\nL16 What is the meaning of ‘independent’ here? Previously published?\nL16 and throughout: kyrs is a length of time. An eruption 86 thousand years ago should be 86 ka or 86 kyrs BP, etc\nMain text\nL36-56 this is a nice (clear, concise) explanation of the study context\nL52 are vapor loss by passive degassing/viscous relaxation of the crust included in the magma chamber model used? The model description includes ‘in a visco-elastic crust’ (L76), what about passive degassing?\nL55-56 “The high compressibility of the MVP increases bulk magma compressibility, dampening pressurization from recharge and leading to less frequent, yet larger, eruptions”. Reading this, I wonder if the proposed volatile resorption mechanism should have any expected effect on 1) frequency and 2) size of eruptions? Certainly Aso-4 was caldera-forming even though undersaturated... would it have been even bigger if the initial magma was not undersaturated?\nL61 another example where kyrs should be ka or kyrs BP etc\nL64 water undersaturated. Was CO2 saturated i.e. CO2 bubbles present in the initial magma? Or the argument is the eruption could start without any MVP at all?\nL64 why is passive degassing of the MVP ruled out? If the answer is in the other Keller et al paper please add it here concisely for the reader\nL66 kyrs is used correctly here\nVolatile dynamics prior to the Aso-4 caldera-forming eruption\nSee L52 comment above, does model include passive degassing effects\nL81-82 recharge magma is ‘drier and hotter’. Is it mafic? Is the recharge melt composition included in the magma mixing modelling? And is it accurate to call it ‘drier’ or is ‘less volatile-rich’ better – what about CO2 as well as H2O?\nFigure 1 caption is confusing. Do all three panels show data with the identical initial parameters as in the first sentence of the caption? Are the data in B and C for the same runs, i.e. this figure shows the results of varying resident magma H2O wt% and recharge rate and nothing else? It took me some detailed re-reading to reach this (hopefully correct) understanding. Panels B and C easily look to a quick glance like they are plotting temporal variations in crystallinity & MVP during three runs, with start point in upper right and end point in lower left. Are the symbols showing the final change in crystallinity & MVP at the end of each run? Why are some ‘undersaturated’ diamonds shown within Regime 2 – are these runs where the MVP totally resorbed but then some volatiles exsolved again before the model run ended?\nIn general it is better to have a lengthy caption to prevent unnecessary confusion for readers seeing this diagram for the first time. I am not a fan of ‘show don’t tell’ in figure captions – better to ‘show and tell, then let the reader decide if they agree’.\nRelated, Extended Data Figure 1 was extremely helpful in understanding this manuscript and the meaning of Fig. 1 and it is unfortunate it is not in the main manuscript. Could it not be combined to make the original Fig. 1 a 6 panel figure?\nL93 “we define four distinct regimes that describe the coupled thermal and volatile saturation conditions in the chamber as a function of recharge rate (Fig. 1A)”. There is nothing in Panel A to indicate the influence of recharge rate on these regimes, and the recharge rate is argued to be a crucial factor in the volatile resorption mechanism. Can useful schematic arrow(s) of ‘increasing recharge rate’ be applied to this regime illustration? Or again, it is an argument to combine the Extended Data Figure 1 here.\nL98 Regime 4 is not observed for the Aso data, why not? What conditions would need to be met? Is it possible at other volcanic systems or is it an unnatural scenario?\nL108 most(?) of the model runs that resulted in eruption within the 5 kyr model timeframe were not fully undersaturated (i.e. not diamond symbols). Why is it so important that Aso-4 was back to undersaturated conditions? What difference (eruption size, duration?) between the runs that were undersaturated vs partially resorbed but still with MVP? Is the authors’ argument that magma chambers that enter Regime 3 will have eruptions sooner than those that stay in Regimes 1-2 (or 4?), even if the MVP is not fully resorbed? Or is full MVP resorption considered to cause the most rapid return to eruption?\nWhat drives volatile resorption during magma recharge?\nProcesses 2 and 3 need clarification, between the description in the text here and the caption of Figure 2.\nWhat are the 5 data points shown in Figure 2? Is the change in MVP vol% by these difference processes being output from model runs like those in Figure 1, or just a simple calculation of ‘suppose Process X causes an overall change in parameter Y of amount Z’?\nProcess 2:\nLine 111 “(2) mixing of silicic resident magma with drier and hotter recharge magma”, but also\nLine 128 “The effect of chemical mixing between the resident and recharge magmas (2)”, and also\nFig 2B caption “(B) magma mixing of silicic resident magma with drier recharge magmas ( Solubility in wt% )” (Line 461).\nWhat is being included in the calculation for Process 2, as shown in Fig 2B as ‘delta solubility wt%’?\nChange in solubility value caused by change in SiO2 content of resident magma by mixing of initial rhyodacite with mafic recharge? (Does this study’s model include the chemistry of the magma? If not, why not – if it is shown to be negligible, say so).\nAnd/or the change in solubility value caused by temperature increase due to addition of hotter recharge magma?\nAs for ‘drier’, solubility value of a volatile in a given melt composition at a given P,T condition is fixed. Mixing one dry (volatile-poor) version of a melt into a wet (volatile-rich) version of that melt does not change the solubility value. So it seems ‘drier’ should be deleted from the description of Process 2.\nInstead of ‘delta solubility’, are the authors aiming for a meaning of ‘closer to/further from saturated conditions’? This would more accurately reflect the Process 2 mixing described in Line 129-130 (unless, there is an unspoken assumption about different temperature and composition of the recharge vs resident magmas).\nProcess 3:\nLine 112 “(3) variations in phase proportions of crystals, melt and MVP through influx of less crystalline recharge magma”\nFig 2C caption “(C) changes in crystal volume fraction (  x) induced by the dilution of resident magma with anhydrous recharge and crystal-derived melts.” (Line 462)\nIt seems heat (hotter recharge magma) is missing from these outlines of Process 3, although the effect of heat for melting crystals and liberating anhydrous melt is indeed described in the following text.\nWhy doesn’t Fig 2B have change in gas fraction going from 0 on the x axis, like panels A and C? Why does ‘reduced solubility’ (as indicated by the y axis) still correspond to decreases in gas fraction – lower solubility should mean more of the volatiles in the system should be exsolved in bubbles?\nThe implications of volatile resorption on eruption timing\nFig. 3 explain the notation in the panels in the figure caption.\nLine 468 ‘inset’ typo\nThe three eruptions in Fig 3 all happen on much shorter timescales than the 5 kyrs between Aso-Y and Aso-4 that is(?) being modelled. Are these scenarios realistic?\nLine 147-148 The finding that Aso-4 eruption occurs 70 years earlier when the MVP is totally resorbed rather than partially resorbed (but still in Regime 3) seems to be the big crux of this paper. But with timescales of thousands of years since the last eruption, does this plus/minus 70 years have any practical value? We are not able to make eruption predictions at anything like that level of precision (if at all).\nLine 162-164 “We find that once a system transitions from volatile-saturated to undersaturated conditions, a drop in magma bulk compressibility amplifies pressurization expediting chamber destabilization and earlier eruption”.\nIs the change to undersaturated conditions (no MVP) the critical point for ‘expediting’ eruptions? As said above, an earlier onset of 70 years does not seem such a remarkable finding. What about Regime 3 in general? If a large silicic magma chamber is experiencing high recharge rates, does it experience an earlier eruption if its initial conditions (density, size etc) cause it to evolve through the Regime 3 pathway (even if the MVP is not totally resorbed) compared to if its initial conditions led it to evolve through Regimes 1,2 or 4? This might be of greater impact in understanding which systems could erupt quickest and necessitate closest monitoring of temporal variation of precursory signals.\nLine 167 again I am not sure to what extent the authors are arguing for the importance of resorption as an eruption trigger. Does this trigger require total resorption of the MVP or is only partial resorption enough? Or is their argument that these big systems (Bishop Tuff, KPT, CF) require volatile undersaturation to erupt; their high recharge rates are not enough to trigger eruption unless they also trigger undersaturation?\nL187 what is the reasoning for the expected increase in CO2/SO2 ratio? If CO2 becomes more soluble as the system becomes more mafic (through mixing of mafic recharge), then more CO2 dissolves and CO2/SO2 ratio should instead decrease. Or CO2 solubility is higher in mafic melts thus the mafic recharge is bringing in lots more CO2 to the chamber hence higher CO2/SO2 – but how is this different to the case of mafic recharge that doesn’t trigger resorption?\nL192-193 The link between the volatile resorption mechanism and the observable precursory ground deformation and perhaps seismicity signals that might result seems like a more useful and significant outcome for hazard mitigation than the finding that resorption might cause eruptions to occur a few decades earlier than they otherwise would. It needs a lot more work, probably beyond the scope of this study, to know if this is feasible.\nL194 the reference for the CF deformation is for recent observations. Are the authors implying these deformations could be happening because the CF magma chamber is currently undersaturated? Do the CF CO2/SO2 or other observations over the same timescale support their interpretation?\nL197-198 “identify highly recharged systems that undergo amplified pressurization with the potential of near-future eruptions”\nAgain I am left wondering what this means. Systems with the potential for total MVP resorption? Or partial MVP as well? And the meaning of ‘near-future’. The three eruption scenarios in Fig 3 all take place within ~300 to 2500 years. Is this realistic? And what type and size of eruption will result? See also my comment on L55-56.\nMethods\nL359 do different run conditions have different erupted magma volumes? Is volatile undersaturation exclusively associated with large caldera-forming eruptions?\nL363 Aso-specific compositions. The following calibration for the Aso-Y crystallinity with T, P, H2O, does not include the effect of mixing in mafic magma with low SiO2. Is this change in overall melt chemistry shown to be insignificant for the melting of crystals etc that promotes resorption?\nL423-424 if recharge is mafic it doesn’t make sense to use the density of Aso-Y rhyodacite melt", "reviewer_3": "Summary\nThe authors build upon previous observational results of mineral chemistry from the Aso-Y and Aso-4 eruptions to consider the implications of mafic recharge in large silicic systems on volatile saturation state and the consequences on eruptability. Simulations using an existing numerical model solve for the evolution of the state of the magma chamber, including pressure, temperature, and chemistry and how perturbations to these change the volume fractions of crystals and vapor which have an impact on the magma chamber compressibility. While I think the core argument that mafic recharge could induce some degree of volatile resorption is sound, I have questions about the details and the strength of the authors’ claims of magma chamber-wide undersaturation before the manuscript is ready to be published.\nDistinguishing mechanisms of resorption\nOverall, I like the approach taken to tease apart the multiple possible mechanisms to identify the most important. However, I think there are some issues in the calculation:\n1) the assertion for constant chamber volume (V0-Vi) is, I think, causing a problem here. The simulations with volatile resorption presented in Fig. 1 clearly show a change in chamber volume. For example, the high flux and 4.5 wt% intitial water case; Δε¬g≈-0.03 and Δε¬x≈-0.035, by mass conservation of the melt Δεx≈ Δεm, ρx=2800, ρm=2250 and ρg=350 then (-0.03*350 - 0.035*2800 + 0.035*2250)/(0.05*350+0.15*2800+0.8*2250) then the chamber volume is changing by ~1% which could be important here given the small initial bubble volume. Also, you can see with the joint constraint of Δε¬g+ Δε¬m+ Δε¬x=1 (which is true by definition) that you are forcing the decrease in the gas volume fraction (and therefore absolute gas volume) to occur at the expense of crystal and melt volume, which is probably why you come to the conclusion that the crystal melting (and resultant volume expansion of the crystal+melt assemblage) is the important driving factor.\n2) you’re solving here for only the water mass balance between the bubbles and melt, but in the mixed H2O-CO2 system, the CO2 really matters. If you have 4.5 wt% H2O and 300 ppm CO2 total, the vapor would have 30 vol% CO2 (50 wt%). Which has a direct effect on the vapor volume and you need to consider the resorption of CO2 to get the right vapor volume. But also, CO2 has a strong effect on the water solubility. I ran a quick mass balance calculation (ignoring the input magma for hypothesis 1) for joint water and CO2 starting from:\nMH2O =ρm(1-εg,0-εx,0)meq,H2OV¬0 + ρ¬H2Oχ¬H2OεgV\nMCO2=ρm(1-εg,0-εx,0)meq,CO2V0 + ρ¬CO2(1-χ¬H2O) εgV\nWhere χ¬H2O is the mass fraction of water in the bubble. And from a little rearrangement, assuming no change in the melt and crystal volumes: V=[1- εg,0+ εg(1- εg,0)/(1- εg)]V0. I don’t have your exact numbers, but if I choose a pressurization from 200 MPa to 220 MPa along with a temperature increase from 875 °C to 913 °C (for addition of melt at 1300 °C at a rate of 10-2 km3/yr, although the result is similar in the iso-thermal case) and 15% crystallinity along with ρ¬H2O,0=381 kg/m3, ρ¬H2O,i=389 kg/m3, ρ¬CO2,0=922 kg/m3, ρ¬CO2,i=982 kg/m3 using the ideal gas law for CO2 and the Pitzer and Sterner (1994) equation of state for water and the Liu et al. (2005) mixed solubility model, I find that a starting vesicularity of 0.05 and starting melt composition of 4.55 wt% H2O and 370 ppm CO2 (equilibrium for 4.5 wt% H2O total and 300 ppm CO2) has a mass fraction of water in the bubble of 0.71. If I don’t allow for CO2 mobility, I find that the pressurization and heat increase give me εg=0.03 and Δεg=-0.02 quite close to what you report in Fig. 2A. However, if I solve for the joint H2O-CO2 mass conservation, I find εg=0.044 and Δεg=-0.006, much less resorption than you calculate. In this case, the solubility only increases to 4.70 wt% H2O and 419 ppm CO2 with a mass fraction of water in the vapor of 50% versus the water-only mass balance equation which gives 4.83 wt% H2O and 51% water in the vapor. It seemed from the methods description that the numerical simulations are solving the coupled H2O-CO2 problem, so you need to do the same for the analysis of driving mechanisms.\nLines 110-113 & 120-132: I found the text here a little unclear about the difference between mechanisms 2 & 3. My understanding is that you mean for 2) to be about the change in solubility due to melt chemistry from a combination of mixing and crystal melting, but you also discuss the reduction in total water on a per volume basis by introducing the drier magma (and also from crystal melting?), while 3) is about the decrease in vesicularity to accommodate the volume expansion of crystal melting and crystal-poor recharge (which are reduced by allowing the chamber to expand as your simulations do). But, it took me a few reads through the paragraph and figure 2 and the supplement to understand this so maybe make the distinction a little more apparent. And which of these exactly do you mean by “dilution”?\nI also don’t understand how you calculated mechanism 2. Is this the heating effect on solubility? I would expect heating to matter more than your values suggest and would cause exsolution, not resorption. Is this the change in solubility with changing chemistry (you could lose a couple wt% silica), but then how are you calculating that given that the Liu et al. (2005) solubility law doesn’t account for the melt chemistry. Using my calculation from earlier, but now only heating without pressurization I exsolve either Δεg=0.005 for the mixed H2O-CO2 or Δεg=0.017 for the immobile CO2. Also, in Fig. 2B, they all have a change of Δεg≈-0.029, with little variability in there but not passing through 0 gas change when the solubility change is 0. So where is that coming from if you’re isolating only the component from solubility?\nRegime diagram\nIt’s a bit odd here to have a plot for the scaling with really only one dimensionless parameter and then plot things along the 1-to-1 line or not. From just mass balance:\nMin=∂/∂t(ρV) and defining compressibility, β=1/ρ ∂ρ/∂p, such that ρ=ρ¬0exp( Δp) you can find:\nMin=ρ(β ∂p/∂t V + ∂V/∂t), which suggests the scaling 1~ρβV/(Min ∂p/∂t) + ρ/Min ∂V/∂t. You plot the first term divided between the axes, and suggest that the deviation from the 1-to-1 line is the result of “amplified pressurization” which “can result from reduced bulk magma compressibility as the MVP volume decreases during a return to volatile-undersaturated conditions.”\nCompressibility is in your term so it should be explicitly accounted for (although you have not specified how it was calculated). Instead, it looks to me like you are seeing the influence of the second term which is the change in volume of the chamber with time which should scale in some way with the visco-elastic response of the crust. That is, when you lose the vapor phase, the compressibility of the magma chamber becomes comparable to the compressibility of the host rock (or maybe this is accommodated by deformation at the surface through the crust rheology), you could find the relative contribution of these from the model governing equations.\nCompatibility with Aso-4 observations\nThe motivating petrological study from the first author (https://doi.org/10.1016/j.epsl.2023.118400) suggests a zoned magma chamber and volatile undersaturation only in the late-erupted products. Addtionally, you require very fast recharge rates to push the magma chamber to undersaturated conditions. Taken together, these suggest to me that undersaturation may be only a local and probably transient effect during pulsed mafic recharge; the claim of undersaturated conditions reducing the compressibility of the bulk magma chamber allowing for accelerated pressurization and earlier eruption seems unlikely to me. The fully undersaturated model results require the fastest recharge rates and an initial condition of the magma chamber at 4 wt% H2O which I would argue is in fact incompatible with your observations of 4.4-4.7 wt% H¬2O, which should be better matched by the 4.5 wt% H2O simulations that don’t produce undersaturation. In your magnetite paper (https://doi.org/10.1016/j.jvolgeores.2023.107789) you also calculate the temperatures of the pre-Aso-4 eruptions to be ≈925 °C and early Aso-4 erupted material to be ≈870 °C, so I’m having trouble reconciling your conceptual model of a cool, large reservoir being heated by an increase in the recharge rate over the 5 kyr gap between eruptions. I don’t think there’s a big inconsistency between the data and a zoned reservoir with resorption and recharge taking place in the lower zone (late-erupted material) while the upper zone is cooling and evolving over the repose period, but I would then expect that upper portion to have a co-existing vapor phase and remain compressible. I understand the numerical model assumes a mixed reservoir and so you have to push it really far to get the resorption and undersaturation that maybe would be easy to produce in a zoned chamber. You could argue that it might exist in another system, but the examples you give in Lines 173-174 also show observations that span the water solubility calculation as the Aso-4 samples seem to.\nWhat are the noteworthy results?\nThe authors demonstrate several mechanisms by which mafic recharge could produce volatile resorption, which I agree is potentially an overlooked process. However, the model requires quite high recharge rates (accounting for about 10% of the total volume) and initial conditions very close to saturation (an open system?) before it can produce undersaturated conditions in the entire magma chamber. While I agree that an undersaturated magma chamber would have low compressibility and could change the eruptability, I think this case is probably not so widespread and not apparent in the selected case study.\nWill the work be of significance to the field and related fields? How does it compare to the established literature? If the work is not original, please provide relevant references.\nThis work could point the community to have a better discussion about the saturation state of magmatic reservoirs and how that saturation might vary in space and time. The authors could point more directly in the discussion to the implications this would have for petrologists studying these systems and what they should look at (maybe for length at the expense of the hazard implications which I think are not so strong).\nDoes the work support the conclusions and claims, or is additional evidence needed?\nThe analysis of the mechanisms should be improved and while the model can produce the behavior the authors describe, I think it is not as widespread as they claim and doesn’t match their case study.\nAre there any flaws in the data analysis, interpretation and conclusions? - Do these prohibit publication or require revision?\nErrors in the calculations for Figure 2 (see above), which are only for the discussion and are simple to correct, but may change the conclusions. I expect the calculation is correct in the numerical simulations that provide the data for the other figures, although in that case the authors may not have identified the most important processes.\nIs the methodology sound? Does the work meet the expected standards in your field?\nThe numerical simulations are already established by this authorship team and collaborators. The model is simple enough to enable running many simulations, but has the appropriate complexity to answer the question posed in this text.\nIs there enough detail provided in the methods for the work to be reproduced?\nThere is some detail missing. I wasn’t able to determine how the calculations for Fig. 2 and Fig. 4 were done. While I understand the temperature for each model was set according to the desired crystallinity and water content and so varies, it would be helpful to report the range of initial temperatures explained in the text and add the temperature for each run to the relevant supplementary data tables. Same for the bulk composition (even just SiO2) for the recharge material.\nMinor points:\nLines 99-101: “We find that while higher recharge temperatures and lower recharge volatile contents promote resorption, recharge rate is the main control on this process.” I think you should be clear here that the scaling dependence should be (I think) approximately linear for all of these factors, you’re just varying recharge rate over a much large range, which is reasonable, but just be clear about that in the wording here. Unless you really think the pressurization is the driving mechanism.\nLine 192-194: “we expect a resorption-induced reduction of the MVP to cause a decrease in magma compressibility, which could enhance of fluctuate ground deformation prior to large silicic eruptions.” I don’t think this claim is that simple. True, it would decrease the compressibility, but the resorption process itself actually decreases the magma volume so there’s a competing effect here. You’re right in the case of complete resorption, but anything shy of that you would need to actually look into which factor wins.\nMethod line 416: This is a very limited depth range. I understand you are interested in the very large systems, but for the small magma chambers, it would be interesting to see the effect of a broader range. You don’t really present the effect of depth, but I could imagine that changes in the solubility sensitivity to pressure at different depths could be interesting.\nMethods line 417: Please give the range of temperatures investigated.\nMethod line 419: The listed CO2 contents don’t match the table.\nMethod line 423: You don’t give the composition for the recharge melt. Is it supposed to be a basalt? You give a melt density for the rhyodacite which is the most evolved end-member.\nSupplementary information 1 line 41: 0.09 is too high for your simulations, and also not what you present in Fig. 2C. Maybe 0.009? Also, not sure why these calculations were done on the basis of 10% initial crystallinity if the simulations are for 15%.\nSincerely,\nJanine Birnbaum", "reviewer_3_code": "I did not have access to the code repository."}, "author_response": "We thank reviewer 1 for their constructive and insightful feedback, we really appreciate it and believe it helped us to improve the manuscript!\nBelow we provide detailed answers to the reviewer’s comments:\n1. The authors investigated a range of volatile contents, either for the resident or the intruded magma. For the later, it is my understanding that it aims at simulating conditions relevant to a mafic recharge (as inferred for Aso by previous work, in particular Kaneko et al., 2007). My question is why the bulk CO2 of that mafic end-member is restricted to 1000 ppm or less? There is evidence that the CO2 content of mafic magmas, including in arcs, can reach several wt% (see the case of Stromboli for instance). It would be interesting to expand the CO2 range to at least 1 wt% bulk content (as in Scholz et al., 2023) and see how this could affect model's outcome.\nResponse:\nWe initially limited our simulations to a recharge CO content of 1000 ppm, as CO plays a less important\n2 2\nrole in controlling bulk magma compressibility than H O (see Scholz et al., 2023 -\n2\nhttps://doi.org/10.1029/2023GC011151) and because we don’t necessarily regard the recharge as strictly\nbasaltic but more as intermediate to basaltic-andesitic, where CO contents are expected to be lower\n2\noverall.\nTo fully address the reviewers’ concerns, we ran an additional suite of simulations including 1 wt% CO\n2\ncontent in the recharging magmas. The results are depicted in the updated figure 1F and also in\nsupplementary fig. 1D. We see that higher recharge CO does not impact whether resorption occurs but\n2\nrather influence its efficiency – i.e. at the same change in crystallinity, less volatile resorption takes place\nin runs with higher recharge CO . As such high CO contents increase the mass of the MVP, we don’t\n2 2\nobserve resorption to fully undersaturated conditions in these runs.\n2. I also guess or understand that the role of recharge is to provide heat and mass (including H2O and CO2?)to the reservoir but only heat is allowed to propagate upwards (in addition to that lost to the host rock)? In other words the melt fraction trends which are shown in extended data Figure 2 are those which are used to simulate the bulk of the magma reservoir? Or do you take into account also the mafic layer? Similarly, are the H2O and CO2 sourced from the mafic recharge transferred into the overlying silicic melt? I would also suggest the authors to compare their curves with experimental data to assess how accurate is MELTs in calculating melt fraction at a given bulk H2O and CO2 content: for instance the phase equilibria of Costa et al (2004) have been performed on a whole rock composition very similar to that selected in the present work (see also Scaillet and Evans 1999, though this uses a slightly more mafic magma).\nResponse:\nThis question might address two points: 1) the general model setup and how mixing takes place within this\nsetup, and 2) crystal-melt curves used to parametrize the model.\n1) Generally, our model does not implement the composition of recharge. As a 1D box model, the\nmodel simulates recharge by continuously adding mass, heat and volatiles into the silicic magma\nchamber and assumes immediate and complete mixing between the host and recharge magmas.\nTherefore, we cannot model individual silicic and mafic layers unfortunately. What we can do is\ncontrol the volatile content and the temperature of recharge that is being added into the silicic\nreservoir, where we choose water and CO2 contents typical for intermediate to mafic magmas. As\nthese magmas are entirely mixed with the host magma, volatiles are indeed being transferred. We\nfind though that the volatile content and temperature of the recharging magmas is secondary to the\nrecharge rate – so lower recharging volatile contents can ‘dry out’ the magma more efficiently, i.e.\nmore volatile resorption at similar crystallinity changes, but resorption only occurs under high\nrecharge rates (see Figure 1C, Supplementary Fig. 1).\n2) We added the data from Costa 2004 to supplementary fig. 2 and find that MELTS generally predicts\nlower temperatures during crystallization, but the slopes of the curves are very similar to the ones\nproduced in the Costa experiments, which is more essential to the model to predict crystallinity\nchanges.\n3. From what I understand, the compressibility of host rocks β is kept constant in all simulations. Presumably this property is likely to vary between volcanic areas, depending on the level of intrusion, how fractured are the host rocks etc..The authors fix compressibility to β=1/10-10 Pa (correct?). Quoting Gudmundsson (Volcanotectonics, 2020, page 286) it is said that host rock compressibility can be β=3x10-11 Pa-1. I have no opinion on which value is the most appropriate but a comment of why the value 1/10-10 Pa has been selected would be welcome. I am asking this because using my own rudimentary code which simulates the evolution of internal pressure of a cooling magma reservoir, made me understand that the host rock bulk modulus is quite important in determining how much of a pressure increase arises (and not only the magma compressibility).\nResponse:\nIndeed, host rock bulk moduli can vary between 1010 Pa to 1011 Pa - we use a value of 1010 Pa here. This\nvalue is dependent on crustal composition with some sedimentary rocks (e.g. sandstone) sometimes being\nas low as 5x109 Pa (see Jaeger, Cook, Zimmermann, 2007, page 190), while more mafic rocks (e.g. islandic\ncrust) are often closer to 1011 Pa (see your reference from Gudmundsson, 2020). In addition, this value is\nalso impacted by the presence of fractures in the crust, where fractures usually lower the bulk modulus\neven of more mafic rocks to roughly 1010 Pa.\n4. One of the great appeal of the model, for a non expert like me, is that it allows to simulate the time evolution of the reservoir, which is a highly desirable property for any model aimed at anticipating eruptions with some confidence. However, the age of Aso-4 event seems to come from Hbl Ar dating (Albert et al., 2019 quoted by Keller et al., 2023), but that phase is described as being xenocrystic or not at equilibrium with its host melt (Ishibashi et al., 2018) which is supported by the lack of Hbl in the run products of hydrothermal experiments of Ushioda et al (2020). I wonder therefore how sure one can be about the time interval between Aso-Y and Aso-4? Is there any other data or way to ascertain the age of Aso-4?\nResponse:\nWe are not sure if we agree that amphiboles are in disequilibrium with the Aso-4 melts. In our 2021 study,\nwe evaluated the mineral assemblage of the Aso-4 eruption and found pretty striking evidence that the\namphibole should be in equilibrium with the host Aso-4 melts – e.g. the amphiboles are euhedral and do\nnot show any visible disequilibrium structures or resorption rims, they have relatively high REE contents\nsuggesting crystallization from evolved REE-enriched melts and their Eu-anomaly is progressively\nincreasing as expected during co-crystallization with plagioclase. Therefore, we suggest that the hbl age is\nreliable.\nNevertheless, several alternative age constraints were provided before the K-Ar age from Albert et al.,\n2019. For example, Aoki 2008 used sediment cores from the pacific to estimate the Aso-4 age between 86.8\nand 87.3 ka, while a recent study by Hoshizumi et al., 2022 (in Japanese) used loess chronometry to estimate\nthe Aso-4 age to 88 ka (the Aso-Y age also stems from the Hoshizumi study). So overall all these ages fall\nin similar ranges, making us confident that the time interval between Aso-Y and Aso-4 is reliable.\n5. One important result or at least, petrologically speaking, the one that brings question to my mind is that the reservoir may undergo a significant pressure increase during cooling (as in previous work). Is this pressure increase documented (aside from apatite volatile record)? I mean, in an ideal world, one could expect to see phenocrysts hosting melt inclusions showing such a pressure increase, ie an increase of both CO2 and H2O contents as MI get closer to the rim? I know this is difficult task (in particular in phenocryst-poor rocks and given the rather modest changes in pressure) but in my opinion it would be the real smoking gun of the process advocated here. Also, if I understood correctly, volatile resorption comes mostly from an increase in pressure but not from a decrease in crystal fraction? It would help the reader to show a graph with the evolution of these two parameters (gas and crystal fractions) with time during a single but representative simulation that culminates in gas-free conditions close to reservoir rupture. Increasing pressure while cooling and preserving no textural evidence of melting/crystallisation requires quite a subtle combination of parameters. According to petrological constraints (Keller et al., 2023, JVGR), the temperature drop between Aso-A and Aso-4 is about 55°C. Does that fit with the model output?\nResponse:\nAbsolutely agree, it would be incredible if such changes could be traced in melt inclusions! We don’t have\nsuch data available at this point, but we also hope to stir new data collection with this publication. So\nhopefully such work could be part of new research avenues. Another question is also on what timescales\nresorption occurs and if enough time for MI formation is given. Such questions could potentially be\napproached using diffusion chronometry in the MI. However, these questions unfortunately go beyond the\nfocus of this study for now.\nWe added an additional figure (Supplementary figure 3) showing the progression of T, P, , , V, and a\ng x\nplot of the change in T vs. the time until eruption. What we can see is 1) pressure changes substantially\n(~20 MPa) while temperature might change only a couple of degrees, and 2) resorption can occur over\nvarious timescales up until almost 2500 yrs. However, resorption until full undersaturation usually occurs\nvery fast, mainly controlled by the fact that pretty small volumes of gas are present at simulation start.\nWe further looked more into comparing temperature changes to our Aso case study. Unfortunately, we do\nnot have reliable T constraints from the Aso-Y eruption, so we cannot fully reconstruct the temperature\nincrease between Aso-Y and Aso-4. As you mentioned before between Aso-A (97.7 ka) and Aso-4 a\ntemperature drop of 55°C occurred. However, we assume that the actual drop happened already between\nAso-A and Aso-Y, which we do not model here. However, we know that the mineral assemblage of Aso-Y\ncontained significant amounts of biotite, which according to our MELTs simulations starts crystallizing at\n~870°C at 200 MPa and H O of 5 wt%. So, the temperature increase can be subtle between Aso-Y and Aso-\n2\n4 and in our view is realistic. In addition, we find sieved plagioclase cores and reverse zoning patterns in\nplagioclase and amphibole in the late-erupted material of the Aso-4 event (see Keller et al., 2021), which\nimplies that some reheating likely took place.\n6. The model considers the EOS derived by Halbach and Chaterjee on H2O to simulate the PVT of the gas phase. This\nEOS is for H2O only and it seems that it is used to compute the volume properties of the H2O-CO2 fluid mixture. Am I\ncorrect? If so, any idea of how a more real EOS (fluid mixture) would affect the results? More specifically, I dont understand\nhow equilibrium distribution of both H2O and CO2 species between gas and melt can be computed with an EOS for water\nonly?\nResponse:\nIf we understand this comment correctly, it might be pointing to two individual processes within the code.\nThe EOS from Halbach and Chatterjee is mainly used to calculate the density of the gas phase and how it\nchanges with pressure (i.e. the compressibility) and temperature (i.e. the thermal expansion). In exemplary\nrun batches (n=1800) >95% of the runs with an exsolved MVP have XCO <<0.1 and therefore the gas\n2\nphase is heavily water-dominated and the error propagated through using an EOS including only H O is\n2\nsmall enough to neglect.\nThe partitioning of H O and CO between the MVP and melt is then calculated using the H O-CO solubility\n2 2 2 2\nlaw developed by Liu et al., 2005. Here both components are accounted for, which we agree is necessary\nto accurately determine the behavior of H O and CO in the melt and gas phase.\n2 2\n7. I think that Bishop Tuff may not be a good example of a system sitting near gas saturation. According to\nWallace et al (1995,1999) seminal work, this system records good evidence of being already gas saturated. As for Campi Flegrei, I dont\nbelieve we have the firm evidence in hand for such a shift toward gas under-saturation, since temperature estimates of that\nsystem are not that well constrained in my opinion.\nResponse:\nThank you for pointing out the Wallace references, these are really interesting. They might actually support\nour hypothesis. If we get it correctly, the main message from the Wallace papers is that the Bishop Tuff had\ngradients in exsolved gas from early-erupted material that contained higher quantities of H O dominated\n2\ngas to late-erupted material, which contained significantly less gas, which at the same time was enriched\nin CO (even though still H O dominated). This is actually pretty similar to what we see in the natural Aso-\n2 2\n4 samples, where undersaturation is observed in the late-erupted material. Wallace et al explain the\naccumulation of gas in the early-erupted material of the Bishop Tuff through H O enrichment during silicic\n2\nmagma differentiation and density stratification, while CO enrichment in deeper parts is suggested to\n2\nresult from CO -rich recharge. We think that resorption could have similar signatures – we assume that\n2\nthe effects of recharge (e.g. pressurization, heating, dilution) are locally stronger in deeper parts of the\nreservoir making resorption more efficient in these parts. Such a process could contribute to the\nstratification of the Bishop Tuff. In addition, CO enrichment in the Bishop Tuff would lead to an increase\n2\nin the XCO of the MVP, this is something we observe in our runs.\nare sitting around saturation (not excluding that they already exsolved an MVP but still remain close to\nsaturation so that the MVP volume is small), which is also supported by the H O contents reported in\nusing the Liu et al., 2005 solubility model.\nAs for Campi Flegrei, we used data provided in Forni et al., 2018 to argue for a recharge-driven increase\nin storage temperatures. These are equilibrium temperatures calculated using the cpx-liquid thermometer\nfrom Masotta et al., 2013.\n8. Pre-eruptive declines in gas emissions also arise from progressive sealing of the volcanic conduit, and not only because\nof the high viscosity of the magma\nResponse:\nThat’s a good point; we added this to the manuscript.\n9. I dont think it is correct to say that CO2 solubility is higher in mafic melts relative to more silicic ones (line 186). CO2\nsolubilities are not that much different in mafic and felsic melts, and in fact, a rhyolite could dissolve even more CO2 if held at\nthe same fCO2 and T than a basalt liquid. What happens in nature is that by the time silicic melts are produced most of the\nCO2 of the system has been lost.\nResponse:\nThank you for catching this. We corrected our statement here saying that we expect SO to be more soluble\n2\nin the silicic melt than CO , therefore leading to an increase in the CO2/SO2 ratio with beginning\n2\nresorption.\n10. From what you have done, I think that the best pre-eruptive signal of a pressurising system entering a gas-free regime\nwould be a break in slope of geodetic signals recording ground uplift (following what is shown in your figure 3b(inset)),\nassuming the time series of data is long enough to capture this change? Yet, the Campi Flegrei example shows that the\nrapid up and down ground motions are not that easy to interpret in terms of deep seated processes.\nResponse:\nWe agree that interpreting the signals is very hard. We mainly tried to suggest possible monitoring signals,\nin which MVP resorption might be captured, but more research certainly needs to be done on that. By\nincluding our statements here, we primarily want to encourage readers to consider such research.\nBruno Scaillet.\nReviewer #1 (Remarks on code availability):\nThe code has been used several times by the same team, and has led to several publications which I assume\nhave permitted to check it\n_____________________________________________________________________________________\nReviewer #2 (Remarks to the Author):\nReview of “Volatile resorption expedites eruption onset in large silicic systems” by Keller, Townsend,\nTroch, and Huber.\nI have read this manuscript closely and with great interest. The authors use the case study of the caldera-\nforming Aso-4 eruption to investigate the hypothesis that mafic recharge of large silicic magma chambers\ncan cause resorption of exsolved magmatic volatiles, and that this resorption can then cause increased\nchamber pressurization rates through the corresponding decrease in magma compressibility, resulting in\nearlier eruption onset. The authors use thermal mechanical modelling in conjunction with previously\npublished geochemical data regarding the pre-eruptive chemistry, volatile contents and magma chamber\nconditions of the Aso-4 eruption. Their results indicate that when recharge rates are high volatile resorption\ncan occur, primarily through chamber pressurization (which increases volatile solubility in the melt) and\nincrease of the melt volume fraction by addition of hot recharge melts that also act to melt existing crystals\nin the resident magma (which together adds relatively volatile-poor melt thus reducing the average volatile\ncontent of the melt, approaching a state of volatile undersaturation). The authors suggest that elevated\npressurization rates caused by complete volatile resorption could have brought forward the onset of the\nAso-4 eruption by ~70 years. The authors propose that detecting precursory signals of chamber\npressurization caused by volatile resorption could be critical for early warning of future eruptions.\nOverall I am convinced that volatile resorption may be a common (or, not uncommon) process in large\nsilicic magma chambers and that a consequence of the resorption process can be increased rates of chamber\npressurization that may trigger eruption. I agree it is an important topic and worthy of future study.\nHowever, I am left uncertain about the extent of the authors’ argument regarding volatile resorption as an\neruption trigger, and unconvinced by how useful or meaningful it is that the Aso-4 eruption may have been\n‘expedited’ by up to 100 years. This latter point is a significant issue for a paper with the current manuscript\ntitle.\nThe paper is generally well-written and logically set out, however I was still left with many questions about\nthe authors’ precise meaning. I have included these in my comments below and I expect that most of them\ncan be addressed by tighter and more explicit text, particularly for figure captions, and perhaps some minor\nadjustments to the figures. Addressing them will avoid unnecessary confusion for the reader, particularly\nfor the manuscript layout that provides few figures in the main text with detailed methods at the end. The\nmost important revisions will be those addressing to what extent and why the authors are arguing for the\nimportance of volatile resorption in understanding large silicic eruptions. If these can be addressed\nconvincingly I would recommend the manuscript for publication in Nature Communications.\nResponse:\nWe thank reviewer 2 for their thoughtful and supportive review. We agree that the manuscript was very\nstrongly focused on the early eruption onset hypothesis, and we tried to address this issue by reframing the\nmanuscript slightly to point out that we do not assume that resorption is an eruption trigger itself. We rather\nsee it as a process that occurs simultaneously with high recharge rates (acting as a trigger) and might be\nimportant to achieve critical overpressures and destabilization of large silicic magma chambers that are\nusually very difficult to pressurize.\nWe provide more details on these changes in our detailed replies below and hope that the manuscript is\nmore clear now!\nI have listed my detailed comments below. Line numbers refer to the line numbers of the PDF version\nprovided for review.\nTitle\n‘expedites eruption onset’. Is this the most important finding? The slightly earlier eruption?\nResponse:\nOverall, we think that uncovering MVP resorption as a possible and not even uncommon process in silicic\nsystems is the main message for us. So far, recharge paired with volatile exsolution has been interpreted\nto play a crucial role in triggering silicic eruptions. By introducing volatile resorption (occurring coupled\nto high recharge rates), we argue that the opposite process to volatile exsolution might actually play a\nsignificant and so far, overlooked role in chamber destabilization of often fairly stable silicic reservoirs.\nWe do think the expedited eruption onset is a key finding in our study. When comparing model results for\nresorbing cases to cases that exsolve volatiles, pressurization rates are generally significantly higher,\neven\nif the magma does not fully return to undersaturation (see for example figure 3). This is mainly of course\ndue to the elevated recharge rates, however, resorption also decreases the compressibility additionally\naccelerating pressurization. We think that such expedited pressurization rates play a crucial role in\nreaching the critical overpressure for eruption onset, yet we believe that this is more a supporting process\nto allow silicic systems to pressurize critically than an actual eruption trigger.\nAbstract\nL16 What is the meaning of ‘independent’ here? Previously published?\nResponse:\nYes, we refer to previously published; as we updated the abstract, this section is not included anymore.\nL16 and throughout: kyrs is a length of time. An eruption 86 thousand years ago should be 86 ka or 86 kyrs\nBP, etc\nResponse:\nThank you for pointing this out, we corrected this throughout the manuscript!\nMain text\nL36-56 this is a nice (clear, concise) explanation of the study context\nResponse:\nThank you, we are happy to hear that we could capture the context understandably!\nL52 are vapor loss by passive degassing/viscous relaxation of the crust included in the magma chamber\nmodel used? The model description includes ‘in a visco-elastic crust’ (L76), what about passive degassing?\nResponse:\nNo, passive degassing is not considered here. We also don’t think that passive degassing played a major\nrole in controlling the MVP loss observed in the Aso reservoir. Even if passive degassing takes place, the\nMVP fraction will not go back to zero (Parmigiani et al., 2017 – https://doi.org/10.1002/2017GC006912,\nor Degruyter et al., 2019 – https://doi.org/10.1098/rsta.2018.0017), therefore, apatite would have\ncontinued to capture a water saturated signature from the melt rather than showing an undersaturated\nsignature, as we find for Aso-4.\nL55-56 “The high compressibility of the MVP increases bulk magma compressibility, dampening\npressurization from recharge and leading to less frequent, yet larger, eruptions”. Reading this, I wonder if\nthe proposed volatile resorption mechanism should have any expected effect on 1) frequency and 2) size of\neruptions? Certainly Aso-4 was caldera-forming even though undersaturated... would it have been even\nbigger if the initial magma was not undersaturated?\nResponse:\nThis is a very good point, thank you. Given that Aso-4 was a caldera forming eruption, we don’t think that\nresorption significantly impacted the erupted volume in this case, as it is mainly driven by the collapse of\nthe reservoir roof. We believe that resorption might rather play an important role in destabilizing the\nreservoir through enhanced pressurization so that large-scale silicic systems can be brought out of\nequilibrium and achieve eruption. As these systems are usually thermally mature, reaching the critical\noverpressure for eruption is significantly more difficult than in smaller silicic systems and it is not yet very\nwell understood what can trigger large-scale caldera eruptions (hypotheses range from roof collapse to\nvolatile exsolution, see the introduction). We believe that resorption induced by increasing recharge rates\nprior to eruption might be a missing puzzle piece.\nHowever, we do think that in smaller silicic systems resorption might indeed cause smaller volume\neruptions (see Townsend et al., 2020 - https://doi.org/10.1130/G47045.1). Regarding frequency - as\nresorption goes along with high recharge rates, the eruption frequency is generally high in systems that\nundergo resorption, however, we do believe that reactivation of a reservoir is easier following an eruption\nthat underwent resorption as reservoir cooling during eruption is limited, therefore it remains less viscous\nand can be pressurized for eruption more readily again.\nL61 another example where kyrs should be ka or kyrs BP etc\nResponse:\nThanks, we corrected throughout the manuscript.\nL64 water undersaturated. Was CO2 saturated i.e. CO2 bubbles present in the initial magma? Or the\nargument is the eruption could start without any MVP at all?\nResponse:\nWe argue that the eruption could have been triggered without an MVP. We mainly refer to water-saturation\nhere as apatite is predominantly capturing H O partitioning, while our model incorporates a mixed H2O-\nphase and therefore has a more important effect on overall magma compressibility and magma dynamics\n(see Scholz et al., 2023). Rhyolites are usually considered relatively CO poor due to early-stage degassing\n2\nof CO at deeper levels of the crust.\nadd it here concisely for the reader\nResponse:\nPassive degassing is mainly ruled out because apatite records water undersaturation from the magma. This\nwould not be the case if passive degassing was the primary driver for volatile reduction because, as\nmentioned above, passive degassing rates are generally too slow to diffuse out the entire MVP volume\nfraction (see references above). At the same time, any accompanying crystallization of a mostly anhydrous\nmineral assemblage would further drive up water contents in the melt, and keep the magma saturated. OH\ngroups would continue to partition into the (rare) bubbles and OH partitioning between apatite and the\nmelt would not be impacted, continuing to capture water-saturated signatures in apatite. Therefore we\nconclude that passive degassing alone cannot account for the transition to MVP-undersaturation and an\nadditional process must be at play here.\nWe added a little clarification here in the manuscript.\nL66 kyrs is used correctly here\nResponse:\nThanks!\nVolatile dynamics prior to the Aso-4 caldera-forming eruption\nSee L52 comment above, does model include passive degassing effects\nResponse:\nSee our comments above – the model does not include passive degassing; however, we also think that it did\nnot play a major role in controlling volatile loss due to signatures captured in apatite.\nL81-82 recharge magma is ‘drier and hotter’. Is it mafic? Is the recharge melt composition included in the\nmagma mixing modelling? And is it accurate to call it ‘drier’ or is ‘less volatile-rich’ better – what about\nCO2 as well as H2O?\nResponse:\nThat’s a good point; recharge at Aso is mostly basaltic andesite in composition. We cannot prescribe a\nfixed composition to the recharge in the model, we rather determine temperatures and volatile content to\nmimic more mafic recharge (i.e. hotter and drier recharge usually). We renamed ‘drier’ to ‘less volatile-\nrich’ recharge in the main text following your suggestion.\nFigure 1 caption is confusing. Do all three panels show data with the identical initial parameters as in the\nfirst sentence of the caption? Are the data in B and C for the same runs, i.e. this figure shows the results of\nvarying resident magma H2O wt% and recharge rate and nothing else? It took me some detailed re-reading\nto reach this (hopefully correct) understanding. Panels B and C easily look to a quick glance like they are\nplotting temporal variations in crystallinity & MVP during three runs, with start point in upper right and\nend point in lower left. Are the symbols showing the final change in crystallinity & MVP at the end of each\nrun? Why are some ‘undersaturated’ diamonds shown within Regime 2 – are these runs where the MVP\ntotally resorbed but then some volatiles exsolved again before the model run ended?\nIn general it is better to have a lengthy caption to prevent unnecessary confusion for readers seeing this\ndiagram for the first time. I am not a fan of ‘show don’t tell’ in figure captions – better to ‘show and tell,\nthen let the reader decide if they agree’.\nResponse:\nSorry that the figure caption seemed confusing – in trying to keep everything as compact as possible we\nseem to have lost important information on the way. Yes, your interpretation is absolutely correct, both\npanels B and C show the exact same data just color-coding changes for water content (B) and recharge\nrate (C). We remodeled the figure following your next comment and tried to make the figure caption clearer.\nAs for the diamonds in regime 2 (‘resorption and cooling’) – we expect to see some very few points in this\nregime, where resorption itself is predominantly driven by pressurization of the reservoir but without\nadding enough material to really heat up the whole magma chamber.\nRelated, Extended Data Figure 1 was extremely helpful in understanding this manuscript and the meaning\nof Fig. 1 and it is unfortunate it is not in the main manuscript. Could it not be combined to make the original\nFig. 1 a 6 panel figure?\nResponse:\nThank you for the suggestion, we added a modified version of the extended data fig. 1 here in panels D-F.\nThe main difference between the new panels and the extended data fig. 1 is that here data is filtered to\nrepresent only the cases that contain 4 wt% initial H O and 100 ppm CO in the resident magma, while the\n2 2\nrecharge rate is varied through 100 logarithmically spaced steps between 10-4 to 10-2 km3/yr and not kept\nconstant at the endmember values.\nL93 “we define four distinct regimes that describe the coupled thermal and volatile saturation conditions in\nthe chamber as a function of recharge rate (Fig. 1A)”. There is nothing in Panel A to indicate the influence\nof recharge rate on these regimes, and the recharge rate is argued to be a crucial factor in the volatile\nresorption mechanism. Can useful schematic arrow(s) of ‘increasing recharge rate’ be applied to this regime\nillustration? Or again, it is an argument to combine the Extended Data Figure 1 here.\nResponse:\nThis is another great suggestion. As mentioned above, we added extra panels to the figure. To not overload\npanel A, we refrained from adding additional arrows here but added arrows for increasing recharge rate\nin panels C and D for clarity.\nL98 Regime 4 is not observed for the Aso data, why not? What conditions would need to be met? Is it\npossible at other volcanic systems or is it an unnatural scenario?\nResponse:\nWe have not observed any data falling into regime 4 in all our simulations. Generally, this regime\n(‘Exsolution and Heating’) might also be difficult to achieve as MVP exsolution is naturally driven by\ncrystallization and cooling. We could imagine that replenishment of very volatile-rich and hot recharge at\nhigh rates might drive an increase in the exsolved MVP fraction and simultaneously heating the reservoir,\nbut at the same time a rapid depressurization might be required, which does not seem to be very likely to\noccur in nature.\nL108 most(?) of the model runs that resulted in eruption within the 5 kyr model timeframe were not fully\nundersaturated (i.e. not diamond symbols). Why is it so important that Aso-4 was back to undersaturated\nconditions? What difference (eruption size, duration?) between the runs that were undersaturated vs\npartially resorbed but still with MVP? Is the authors’ argument that magma chambers that enter Regime 3\nwill have eruptions sooner than those that stay in Regimes 1-2 (or 4?), even if the MVP is not fully resorbed?\nOr is full MVP resorption considered to cause the most rapid return to eruption?\nResponse:\nThank you for pointing this out – yes cases that return to full undersaturation would trigger eruptions faster\nthan those that keep some of the MVP, i.e. undersaturation accelerates pressurization additionally. This is\nalso visible in Figure 4, where all cases that return to undersaturation are indeed recording elevated\npressurization rates. But overall resorption still triggers eruptions faster than exsolution due to decreasing\nbulk magma compressibility associated with resorption.\nThe main point why it is important for Aso to have returned to undersaturation – at least in parts of the\nreservoir - is to match the observations we made with apatite data recording undersaturated conditions.\nWe are basically aiming to understand the magma dynamics using the model here that led to the\nobservations made in Aso.\nWhat drives volatile resorption during magma recharge?\nProcesses 2 and 3 need clarification, between the description in the text here and the caption of Figure 2.\nResponse:\nWe added a substantial amount of information to the main text, hopefully the process becomes a bit clearer\nnow.\nWhat are the 5 data points shown in Figure 2? Is the change in MVP vol% by these difference processes\nbeing output from model runs like those in Figure 1, or just a simple calculation of ‘suppose Process X\ncauses an overall change in parameter Y of amount Z’?\nResponse:\nFigure 2 is independent of the model runs. The points represent isolated calculations to determine the\namount each process (pressurization, mixing, and changes in phase proportions) can contribute to\nresorption. A detailed explanation is given in the supplementary information, and additional information\nis added to the manuscript as pointed out above.\nProcess 2:\nLine 111 “(2) mixing of silicic resident magma with drier and hotter recharge magma”, but also\nLine 128 “The effect of chemical mixing between the resident and recharge magmas (2)”, and also\nFig 2B caption “(B) magma mixing of silicic resident magma with drier recharge magmas (Solubility in\nwt% )” (Line 461).\nWhat is being included in the calculation for Process 2, as shown in Fig 2B as ‘delta solubility wt%’?\nChange in solubility value caused by change in SiO2 content of resident magma by mixing of initial\nrhyodacite with mafic recharge? (Does this study’s model include the chemistry of the magma? If not, why\nnot – if it is shown to be negligible, say so).\nAnd/or the change in solubility value caused by temperature increase due to addition of hotter recharge\nmagma?\nAs for ‘drier’, solubility value of a volatile in a given melt composition at a given P,T condition is fixed.\nMixing one dry (volatile-poor) version of a melt into a wet (volatile-rich) version of that melt does not\nchange the solubility value. So it seems ‘drier’ should be deleted from the description of Process 2.\nInstead of ‘delta solubility’, are the authors aiming for a meaning of ‘closer to/further from saturated\nconditions’? This would more accurately reflect the Process 2 mixing described in Line 129-130 (unless,\nthere is an unspoken assumption about different temperature and composition of the recharge vs resident\nmagmas).\nResponse:\nThank you for pointing out some of the issues with process 2. We redid the calculations using a modified\nversion of the equation considering changes in water mass and mass of the magma in the chamber. So, we\nnow specifically test how mixing of a less volatile-enriched magma with a volatile-enriched host magma\nimpacts the water mass in the chamber and the chamber volume throughout recharge. Therefore, we do\nnot present delta_solubility anymore but show how the gas fraction changes with volume in the magma\nchamber as more recharge is added. We hope this is clearer now.\nThe model does not directly account for changes in the SiO content of the magma, but it does account for\n2\nchanges in X . We followed your advice above and renamed the recharge from ‘drier’ to less volatile-\nCO2\nrich.\nProcess 3:\nLine 112 “(3) variations in phase proportions of crystals, melt and MVP through influx of less crystalline\nrecharge magma”\nFig 2C caption “(C) changes in crystal volume fraction (x) induced by the dilution of resident magma\nwith anhydrous recharge and crystal-derived melts.” (Line 462)\nIt seems heat (hotter recharge magma) is missing from these outlines of Process 3, although the effect of\nheat for melting crystals and liberating anhydrous melt is indeed described in the following text.\nResponse:\nWe here assume that recharge does both, add crystal-poor recharge magma itself, which increases the melt\nfraction, i.e. dilutes the host magma, and adds anhydrous melt through crystal melting. However, we only\ncapture an increase in overall melt fraction as it would be difficult to account for the individual processes.\nWhy doesn’t Fig 2B have change in gas fraction going from 0 on the x axis, like panels A and C? Why does\n‘reduced solubility’ (as indicated by the y axis) still correspond to decreases in gas fraction – lower\nsolubility should mean more of the volatiles in the system should be exsolved in bubbles?\nResponse:\nAs mentioned above, we redid the calculations for process 2. With this setup now, the change in gas fraction\nis 0 if there is no change in solubility and volume.\nThe implications of volatile resorption on eruption timing\nFig. 3 explain the notation in the panels in the figure caption.\nResponse:\nWe clarified the annotations in the figure caption.\nLine 468 ‘inset’ typo\nResponse:\nThank you for pointing this out, we corrected the typo.\nThe three eruptions in Fig 3 all happen on much shorter timescales than the 5 kyrs between Aso-Y and Aso-\n4 that is(?) being modelled. Are these scenarios realistic?\nResponse:\nThat’s a great question. Yes, we are modelling the time frame between the Aso-Y and Aso-4 eruption here,\nand yes, we believe that these are realistic scenarios, even though the times over which resorption occurs\nis significantly shorter than the 5 kyrs time frame. What this mainly tells us is that a change in recharge\nrate must have occurred at some point prior to the Aso-4 event, which induced resorption and allowed for\nmore efficient pressurization of the reservoir, ultimately inducing eruption. As we cannot really tell when\nsuch a change in recharge rate occurred, this model gives us an estimate on potential time frames. We tried\nto make this clearer in the manuscript.\nLine 147-148 The finding that Aso-4 eruption occurs 70 years earlier when the MVP is totally resorbed\nrather than partially resorbed (but still in Regime 3) seems to be the big crux of this paper. But with\ntimescales of thousands of years since the last eruption, does this plus/minus 70 years have any practical\nvalue? We are not able to make eruption predictions at anything like that level of precision (if at all).\nResponse:\nIndeed the 70 yrs earlier eruption does not seem to be a lot of time. However, on a human timescale such\ntimes are actually considerable, and it would be great to be able to forecast that a system might be prone\nfor eruption within the next few decades. We think of resorption as an indicator that a system is highly\nrecharged and that pressurization rates are climaxing, making it more prone for eruption. Therefore,\nresorption might be a warning sign for elevated pressurization rates, calling for enhanced monitoring.\nIn general, the bigger point we are trying to make is that resorption is a viable process that can occur in\nnature, which so far has not been considered to the best of our knowledge and is the opposite process of\nvolatile exsolution which is often cited together with recharge as an eruption trigger.\nLine 162-164 “We find that once a system transitions from volatile-saturated to undersaturated conditions,\na drop in magma bulk compressibility amplifies pressurization expediting chamber destabilization and\nearlier eruption”.\nIs the change to undersaturated conditions (no MVP) the critical point for ‘expediting’ eruptions? As said\nabove, an earlier onset of 70 years does not seem such a remarkable finding. What about Regime 3 in\ngeneral? If a large silicic magma chamber is experiencing high recharge rates, does it experience an earlier\neruption if its initial conditions (density, size etc) cause it to evolve through the Regime 3 pathway (even if\nthe MVP is not totally resorbed) compared to if its initial conditions led it to evolve through Regimes 1,2\nor 4? This might be of greater impact in understanding which systems could erupt quickest and necessitate\nclosest monitoring of temporal variation of precursory signals.\nResponse:\nThank you for pointing this out. Definitely, the onset of resorption in general does already expedite\npressurization of the system and with that eruption onset. Therefore, whenever a system goes through\nregime 3, an eruption might occur faster than in regime 1. We tried to make this clearer in the manuscript\ntext.\nLine 167 again I am not sure to what extent the authors are arguing for the importance of resorption as an\neruption trigger. Does this trigger require total resorption of the MVP or is only partial resorption enough?\nOr is their argument that these big systems (Bishop Tuff, KPT, CF) require volatile undersaturation to erupt;\ntheir high recharge rates are not enough to trigger eruption unless they also trigger undersaturation?\nResponse:\nWe don’t necessarily argue that resorption is an eruption trigger. Our idea is more that high recharge rates\nthat are often implied for large-scale silicic systems might not go along with the volatile exsolution process\nthat is often being suggested as eruption trigger. We find that exsolution is not as efficient in triggering\neruptions and that rather resorption occurs as a result of high recharge rates. The resorption induced\nchanges in compressibility then accelerate pressurization of the system and might explain how thermally\nbuffered systems reach their critical overpressure.\nL187 what is the reasoning for the expected increase in CO2/SO2 ratio? If CO2 becomes more soluble as\nthe system becomes more mafic (through mixing of mafic recharge), then more CO2 dissolves and\nCO2/SO2 ratio should instead decrease. Or CO2 solubility is higher in mafic melts thus the mafic recharge\nis bringing in lots more CO2 to the chamber hence higher CO2/SO2 – but how is this different to the case\nof mafic recharge that doesn’t trigger resorption?\nResponse:\nThank you for pointing this out, our reasoning of a CO /SO increase was mixed up here. We believe that\n2 2\nonce resorption occurs SO goes back into the melt, while CO remains in the MVP, therefore increasing\n2 2\nthe CO /SO ratio. This is actually a process that we also see in the XCO , which increases as a result of\n2 2 2\nresorption during our model runs. In the end, this process is not really different from adding CO from a\n2\nmore mafic recharge magma, which could complicate the interpretation of increasing CO /SO ratios at\n2 2\nactive volcanoes. We think however, that isotopic ratios of the MVP might be affected differently by\nresorption, yet, as we are not really specialists in gas monitoring of volcanoes it is hard for us to make\npredictions. It would be really interesting though to see future work on that!\nL192-193 The link between the volatile resorption mechanism and the observable precursory ground\ndeformation and perhaps seismicity signals that might result seems like a more useful and significant\noutcome for hazard mitigation than the finding that resorption might cause eruptions to occur a few decades\nearlier than they otherwise would. It needs a lot more work, probably beyond the scope of this study,\nto know if this is feasible.\nResponse:\nWe agree that such work would be beyond the scope of our manuscript. We primarily mention these outlooks\nhere, as we would love to see more research in this direction in the future from specialists in the respective\nfields. We believe that such research would be important as resorption might actually give us significant\nwarning signs of upcoming eruptions, so understanding its effects on monitoring signals and tracing its\noccurrence would be really interesting.\nL194 the reference for the CF deformation is for recent observations. Are the authors implying these\ndeformations could be happening because the CF magma chamber is currently undersaturated? Do the CF\nCO2/SO2 or other observations over the same timescale support their interpretation?\nResponse:\nThat’s an interesting thought. We believe that resorption is a result of strong recharge events, so we would\nexpect ground deformation to be mainly driven by input of large quantities of magma, which could be\nintensified by volatile resorption. Assuming that an existing MVP buffers uplift, the loss of the MVP would\nhave the contrary effect therefore supporting strong uplift. For CF that would mean that we expect a sharp\nuplift event that is associated with a pulse of new magma input as observed in the 1982-84 uplift (~1.86 m\nin less than 3 yrs, which accounts for a recharge rate on the order of 10-2 km3/yr). Changes in gas\ncomposition are a little more difficult to predict. As mentioned above, it will be hard to distinguish\nresorption from input of CO -rich fluids, as we expect predominantly water to diffuse back into the melt,\n2\nleaving the MVP CO enriched (we see an increase in the mole fraction of CO accompanying resorption).\n2 2\nFollowing Chiodini et al., 2012 (https://doi.org/10.1130/G33251.1) the 1982-84 recharge was preceded by\na decrease in the CO /H O ratio indicating the influx of new magmatic fluids – during the following sharp\n2 2\nuplift period though, the CO /H O ratio significantly increased. The authors interpreted this increase as\n2 2\ncontribution from CO -rich fluids from depth; however, we think that this sharp increase could also be a\n2\nsign of ongoing resorption. We added a short sentence about these observations in the manuscript, however,\nsince we are no experts in understanding gas emission data, we prefer to keep these observations on the\nsuggestive side.\nL197-198 “identify highly recharged systems that undergo amplified pressurization with the potential of\nnear-future eruptions”\nAgain I am left wondering what this means. Systems with the potential for total MVP resorption? Or partial\nMVP as well? And the meaning of ‘near-future’. The three eruption scenarios in Fig 3 all take place within\n~300 to 2500 years. Is this realistic? And what type and size of eruption will result? See also my comment\non L55-56.\nResponse:\nThank you for pointing out these open questions! We believe that we can use resorption as an indicator for\nsystems that undergo high recharge rates. We find that if resorption occurs pressurization rates are\ngenerally higher than if the system is undergoing exsolution, therefore we think that already the onset of\nresorption in general, without hitting undersaturated conditions, is already a sign that the system is\nundergoing enhanced pressurization. If a transition to undersaturation is actually met, it is very likely that\nthe system will probably experience eruption soon, given that recharge will be continued at the established\nhigh rates.\nConcerning the time frame, we believe that the time intervals given for resorption in fig. 3 are realistic. As\nit is usually difficult to assess the risk posed by especially large-scale silicic systems, we think that even\nsuch time frames are helpful, in concentrating monitoring efforts. With over >100 active silicic caldera\nonly in arc settings, knowing which ones have the higher potential for renewed eruption, will be already\nhelpful in our opinion.\nMethods\nL359 do different run conditions have different erupted magma volumes? Is volatile undersaturation\nexclusively associated with large caldera-forming eruptions?\nResponse:\nYes, we do find that different run conditions produce different erupted volumes, this is mainly the effect of\ntime – the longer the system has to accumulate volume, the more mass is erupted during the event. When\nlooking at the new Supplementary Fig. 3, we show 2 different runs, the first one going back to\nundersaturation has a significantly smaller erupted mass (5.5x1012 kg) than the second run (3.1x1013 kg),\nwhich experiences eruption only after 2500 yrs. Therefore, we don’t think that undersaturation is directly\nlinked to caldera formation, we rather believe that resorption is an overlooked process that could explain\nhow we destabilize caldera-forming systems that are otherwise really difficult to pressurize to the point of\nfailure.\nL363 Aso-specific compositions. The following calibration for the Aso-Y crystallinity with T, P, H2O, does\nnot include the effect of mixing in mafic magma with low SiO2. Is this change in overall melt chemistry\nshown to be insignificant for the melting of crystals etc that promotes resorption?\nResponse:\nExactly, the calibration only uses the Aso-Y specific composition and does not take lowering of SiO\n2\ncontents through mixing into account. In other words, the recharging magma has the same assumed\ncomposition as the resident magma, but through setting temperature and volatile conditions of the\nrecharging magma to variable conditions, we can mimic more mafic conditions (e.g. hotter, H O poorer\n2\nand CO richer than the host magma). Though lowering the SiO content of the magma can impact the\n2 2\nequilibrium conditions of crystals already present in the chamber, we believe that temperature is the\nprimary driver for crystal melting in our runs. In addition, a portion of changes in melt fraction also stems\nfrom the addition of crystal-poor recharge magma – if the temperature of the recharging magma is higher\nthan the host, the crystallinity of the recharging magma will be adjusted to these higher temperatures and\ntherefore will be less crystal-rich.\nL423-424 if recharge is mafic it doesn’t make sense to use the density of Aso-Y rhyodacite melt\nResponse:\nThis is a valid point, we used the same density here, as we assume that the composition of the recharging\nmagma is the same as the resident magma for simplicity of the model. The more mafic conditions are then\nsimulated through higher temperatures and variable volatile contents compared to the resident magma.\nWe clarified this in the main text by referring to less volatile-rich magma rather than more mafic magma.\n_____________________________________________________________________________________\nReviewer #3 (Remarks to the Author):\nSummary\nThe authors build upon previous observational results of mineral chemistry from the Aso-Y and Aso-4\neruptions to consider the implications of mafic recharge in large silicic systems on volatile saturation state\nand the consequences on eruptability. Simulations using an existing numerical model solve for the evolution\nof the state of the magma chamber, including pressure, temperature, and chemistry and how perturbations\nto these change the volume fractions of crystals and vapor which have an impact on the magma chamber\ncompressibility. While I think the core argument that mafic recharge could induce some degree of volatile\nresorption is sound, I have questions about the details and the strength of the authors’ claims of magma\nchamber-wide undersaturation before the manuscript is ready to be published.\nResponse:\nWe thank reviewer 3 for their detailed review. We provide answers to each point brought up by the reviewer\nbelow:\nDistinguishing mechanisms of resorption\nOverall, I like the approach taken to tease apart the multiple possible mechanisms to identify the most\nimportant. However, I think there are some issues in the calculation:\n1) the assertion for constant chamber volume (V0-Vi) is, I think, causing a problem here. The simulations\nwith volatile resorption presented in Fig. 1 clearly show a change in chamber volume. For example, the\nhigh flux and 4.5 wt% intitial water case; Δε¬g≈-0.03 and Δε¬x≈-0.035, by mass conservation of the melt\nΔεx≈ Δεm, ρx=2800, ρm=2250 and ρg=350 then (-0.03*350 - 0.035*2800 +\n0.035*2250)/(0.05*350+0.15*2800+0.8*2250) then the chamber volume is changing by ~1% which could\nbe important here given the small initial bubble volume. Also, you can see with the joint constraint of\nΔε¬g+ Δε¬m+ Δε¬x=1 (which is true by definition) that you are forcing the decrease in the gas volume\nfraction (and therefore absolute gas volume) to occur at the expense of crystal and melt volume, which is\nprobably why you come to the conclusion that the crystal melting (and resultant volume expansion of the\ncrystal+melt assemblage) is the important driving factor.\nResponse:\nThank you for looking into our approach so carefully. We agree that the reviewer’s point are generally\nreally good and valid, however, we want to emphasize that we use the process calculations to estimate\nhierarchy of the three processes and their contribution to resorption, rather than accurately reproducing\nthe integrated changes in gas fraction produced in the model. We therefore anticipate closed system\nconditions (i.e. V0=Vi and Mw0 = Mwi) to calculate the isolated effect of changes in pressure and phase\nproportions.\nFollowing your suggestions, we adjusted our calculations of process 2 (mixing) by allowing for open system\nconditions, so we are now accounting for changes in the water mass and volume of the system. We agree\nthat in the mixing case this will play an important role, as recharge is adding water mass and volume to\nthe system, which we previously overlooked. Using this approach, we indeed find that magma mixing can\nresorb up to 8% of the MVP, therefore pressurization and changes in phase proportions, remain the main\nprocess driving volatile resorption. We updated our calculations in the supplementary files and updated\nfigure 2. We additionally caught a sign error in the equations, therefore the numbers presented in fig. 2 are\nslightly different than in the previous version.\n2) you’re solving here for only the water mass balance between the bubbles and melt, but in the mixed\nH2O-CO2 system, the CO2 really matters. If you have 4.5 wt% H2O and 300 ppm CO2 total, the vapor\nwould have 30 vol% CO2 (50 wt%). Which has a direct effect on the vapor volume and you need to consider\nthe resorption of CO2 to get the right vapor volume. But also, CO2 has a strong effect on the water\nsolubility. I ran a quick mass balance calculation (ignoring the input magma for hypothesis 1) for joint\nwater and CO2 starting from:\nMH2O =ρm(1-εg,0-εx,0)meq,H2OV¬0 + ρ¬H2Oχ¬H2OεgV\nMCO2=ρm(1-εg,0-εx,0)meq,CO2V0 + ρ¬CO2(1-χ¬H2O) εgV\nWhere χ¬H2O is the mass fraction of water in the bubble. And from a little rearrangement, assuming no\nchange in the melt and crystal volumes: V=[1- εg,0+ εg(1- εg,0)/(1- εg)]V0. I don’t have your exact\nnumbers, but if I choose a pressurization from 200 MPa to 220 MPa along with a temperature increase from\n875 °C to 913 °C (for addition of melt at 1300 °C at a rate of 10-2 km3/yr, although the result is similar in\nthe iso-thermal case) and 15% crystallinity along with ρ¬H2O,0=381 kg/m3,\nρ¬H2O,i=389 kg/m3, ρ¬CO2,0=922 kg/m3, ρ¬CO2,i=982 kg/m3 using the ideal gas law for CO2 and the Pitzer and Sterner\n(1994) equation of state for water and the Liu et al. (2005) mixed solubility model, I find that a starting\nvesicularity of 0.05 and starting melt composition of 4.55 wt% H2O and 370 ppm CO2 (equilibrium for 4.5\nwt% H2O total and 300 ppm CO2) has a mass fraction of water in the bubble of 0.71. If I don’t allow for\nCO2 mobility, I find that the pressurization and heat increase give me εg=0.03 and Δεg=-0.02 quite close\nto what you report in Fig. 2A. However, if I solve for the joint H2O-CO2 mass conservation, I find εg=0.044\nand Δεg=-0.006, much less resorption than you calculate. In this case, the solubility only increases to 4.70\nwt% H2O and 419 ppm CO2 with a mass fraction of water in the vapor of 50% versus the water-only mass\nbalance equation which gives 4.83 wt% H2O and 51% water in the vapor. It seemed from the methods description that the\nnumerical simulations are solving the coupled H2O-CO2 problem, so you need to do the same for the analysis of driving mechanisms.\nResponse:\nThank you again for putting the effort in to go through our approach in detail, we really appreciate the\neffort. We agree that CO is playing an important role, however, most of our run scenarios have XCO\n2 2\nvalues well below 0.1 leaving the MVP strongly water dominated. As we are trying to approach the\nhierarchy of mechanisms driving resorption, we believe that focusing on water, which is the dominant\nvolatile species and has a larger impact on magma compressibility (see Scholz et al., 2023), is sufficient\nfor these approximations – again we are not attempting to reproduce integrated changes of gas fraction\nproduced by the model.\nAs mentioned above, we redid our calculations of process 2 now accounting for changes in volume and\nmass.\nLines 110-113 & 120-132: I found the text here a little unclear about the difference between mechanisms 2\n& 3. My understanding is that you mean for 2) to be about the change in solubility due to melt chemistry\nfrom a combination of mixing and crystal melting, but you also discuss the reduction in total water on a per\nvolume basis by introducing the drier magma (and also from crystal melting?), while 3) is about the decrease\nin vesicularity to accommodate the volume expansion of crystal melting and crystal-poor recharge (which\nare reduced by allowing the chamber to expand as your simulations do). But, it took me a few reads through\nthe paragraph and figure 2 and the supplement to understand this so maybe make the distinction a little\nmore apparent. And which of these exactly do you mean by “dilution”?\nResponse:\nWe clarified the processes in the manuscript. Process 2 calculates how magma mixing of a drier recharge\nwith a wetter host magma impacts resorption, while process 3 accounts for increasing the melt fraction of\nthe host magma through addition of crystal-poor recharge melt and crystal melting.\nI also don’t understand how you calculated mechanism 2. Is this the heating effect on solubility? I would\nexpect heating to matter more than your values suggest and would cause exsolution, not resorption. Is this\nthe change in solubility with changing chemistry (you could lose a couple wt% silica), but then how are\nyou calculating that given that the Liu et al. (2005) solubility law doesn’t account for the melt chemistry.\nUsing my calculation from earlier, but now only heating without pressurization I exsolve either Δεg=0.005\nfor the mixed H2O-CO2 or Δεg=0.017 for the immobile CO2. Also, in Fig. 2B, they all have a change of\nΔεg≈-0.029, with little variability in there but not passing through 0 gas change when the solubility\nchange is 0. So where is that coming from if you’re isolating only the component from solubility?\nResponse:\nWe redid the calculations for processes 2 as described above accounting for variations in the magma\nvolume and water mass rather than solubility. We hope this clears the reviewers concerns.\nRegime diagram\nIt’s a bit odd here to have a plot for the scaling with really only one dimensionless parameter and then plot\nthings along the 1-to-1 line or not. From just mass balance:\nMin=∂/∂t(ρV) and defining compressibility, β=1/ρ ∂ρ/∂p, such that ρ=ρ¬0exp(ꞵΔp) you can find:\nMin=ρ(β ∂p/∂t V + ∂V/∂t), which suggests the scaling 1~ρβV/(Min ∂p/∂t) + ρ/Min ∂V/∂t. You plot the first\nterm divided between the axes, and suggest that the deviation from the 1-to-1 line is the result of “amplified\npressurization” which “can result from reduced bulk magma compressibility as the MVP volume decreases\nduring a return to volatile-undersaturated conditions.” Compressibility is in your term so it should be\nexplicitly accounted for (although you have not specified how it was calculated). Instead, it looks to me\nlike you are seeing the influence of the second term which is the change in volume of the chamber with\ntime which should scale in some way with the visco-elastic response of the crust. That is, when you lose\nthe vapor phase, the compressibility of the magma chamber becomes comparable to the compressibility of\nthe host rock (or maybe this is accommodated by deformation at the surface through the crust rheology),\nyou could find the relative contribution of these from the model governing equations.\nResponse:\nThe scaling law we used here is a common scaling law used in our field (see e.g. Townsend et al., 2019\n- https://doi.org/10.1029/2018GC008103). We followed the reviewers’ recommendations and tried to\nimplemented a second term, however, this term does not help to bring the data together along the 1-to-1\nline, but rather enhances the differences. We expect that increasing volume rather shifts the points to the\nleft in the diagram space, therefore producing the opposite results. We also already previously checked\nwhether crustal viscosity would have an impact on the data distribution, but we could not find any\ncorrelation.\nWe calculated the compressibility following the approach of Townsend & Huber 2020\n(https://doi.org/10.1130/G47045.1) for the effective compressibility.\nCompatibility with Aso-4 observations\nThe motivating petrological study from the first author (https://doi.org/10.1016/j.epsl.2023.118400)\nsuggests a zoned magma chamber and volatile undersaturation only in the late-erupted products.\nAdditionally, you require very fast recharge rates to push the magma chamber to undersaturated conditions.\nTaken together, these suggest to me that undersaturation may be only a local and probably transient effect\nduring pulsed mafic recharge; the claim of undersaturated conditions reducing the compressibility of the\nbulk magma chamber allowing for accelerated pressurization and earlier eruption seems unlikely to me.\nThe fully undersaturated model results require the fastest recharge rates and an initial condition of the\nmagma chamber at 4 wt% H2O which I would argue is in fact incompatible with your observations of 4.4-\n4.7 wt% H¬2O, which should be better matched by the 4.5 wt% H2O simulations that don’t produce\nundersaturation. In your magnetite paper (https://doi.org/10.1016/j.jvolgeores.2023.107789) you also\ncalculate the temperatures of the pre-Aso-4 eruptions to be ≈925 °C and early Aso-4 erupted material to be\n≈870 °C, so I’m having trouble reconciling your conceptual model of a cool, large reservoir being heated\nby an increase in the recharge rate over the 5 kyr gap between eruptions. I don’t think there’s a big\ninconsistency between the data and a zoned reservoir with resorption and recharge taking place in the lower\nzone (late-erupted material) while the upper zone is cooling and evolving over the repose period, but I\nwould then expect that upper portion to have a co-existing vapor phase and remain compressible. I\nunderstand the numerical model assumes a mixed reservoir and so you have to push it really far to get the\nresorption and undersaturation that maybe would be easy to produce in a zoned chamber. You could argue\nthat it might exist in another system, but the examples you give in Lines 173-174 also show observations\nthat span the water solubility calculation as the Aso-4 samples seem to.\nResponse:\nThank you for pointing out these concerns. We find that the onset of resorption is very much dependent on\nthe initial crystallinity of the resident magma. The observed crystallinities in the whole ignimbrite range\nfrom as low as <5% to up to 50% in the most crystalline samples. We chose a crystallinity of 15% to\nrepresent a good average of the chamber, given that volumetrically most of the ignimbrite represents\ncrystal-poor material. However, if we choose lower crystallinities undersaturation is achieved in magmas\nthat have 4.5 wt% H O. Given the uncertainties of the model and the uncertainties stemming from\n2\nhygrometers themselves (here +/- 0.35 wt%), we in fact argue that the water contents are matching.\nFor our magnetite paper, we are referring to Aso-A as the last eruption before Aso-4 – this is because Aso-\nY was first discovered and described in the Hoshizumi 2022 study, therefore we did not have access to Aso-\nY samples during this study. We included Aso-Y for the first time in the apatite study of 2023. Therefore,\nthe temperature variations you are referring to are between the Aso-A eruption before Aso-Y and the Aso-\n4 event. Unfortunately, we do not have reliable T constraints from the Aso-Y eruption, so we cannot reliably\ncheck the temperature increase between Aso-Y and Aso-4. However, we know that the mineral assemblage\nof Aso-Y contained significant amounts of biotite, which according to our MELTs simulations starts\ncrystallizing at ~870°C at 200 MPa and H O of 5 wt%, implying almost no change in temperature between\nsieved plagioclase cores and reverse zoning patterns in plagioclase and amphibole (see Keller et al., 2021).\nThis implied that some reheating likely took place.\nThe recharge rates we are using here are referenced in literature – see Schöpa and Annen 2013\n(https://doi.org/10.1002/jgrb.50127) or Costa 2008 (https://doi.org/10.1016/S1871-644X(07)00001-0) for\nexamples. Based on such studies, it is widely accepted that such high recharge rates (even exceeding 10-2\nkm3/yr) are required to grow caldera-forming reservoirs and their eruption. We therefore argue that our\ninputs here are reasonable.\nWe agree that modelling the whole magma chamber is not ideal for representing a zoned chamber,\nhowever, we also believe that our results are applicable. As we described in the manuscript, the driving\nfactor of resorption is pressurization induced by recharging of the magma chamber. Pressure increases\nwill be felt throughout a zoned reservoir, as mass is added to the system increasing the internal pressure.\nTherefore, resorption could occur throughout the reservoir. We agree that locally, it might be enhanced by\nadditional effects such as heating and changes in phase proportions, and these local changes might be the\nfactor pushing the system back to undersaturated conditions. If recharge continues under such conditions\ncritical overpressurization can be reached more easily, helping to trigger such large-scale eruptions, that\notherwise remain difficult to destabilize. Overall, we want to make clear that yes, resorption is a process\nthat is initiated by high recharge fluxes, but we believe that the implications of resorption, i.e. lowering the\ncompressibility of the magma – even locally- are important to drive pressurization of the system and reach\nthe critical overpressure faster than in exsolving systems.\nWhat are the noteworthy results?\nThe authors demonstrate several mechanisms by which mafic recharge could produce volatile resorption,\nwhich I agree is potentially an overlooked process. However, the model requires quite high recharge rates\n(accounting for about 10% of the total volume) and initial conditions very close to saturation (an open\nsystem?) before it can produce undersaturated conditions in the entire magma chamber. While I agree that\nan undersaturated magma chamber would have low compressibility and could change the eruptability, I\nthink this case is probably not so widespread and not apparent in the selected case study.\nResponse:\nThank you for sharing these concerns. We appreciate hearing that the reviewer agrees that resorption\nmight be an overlooked processes. As we explained in our comment above, we do think that the recharge\nrates used here are reasonable, as they are based on frequently cited literature and recharge rates of up to\n10-2 km3/yr are high, yet not unrealistic values (looking at the 1982-84 unrest period at Campi Flegrei –\nwe have an estimated addition of 0.025 km3 material accounting for a recharge rate of ~10-2 km3/yr – see\nTroise et al., 2019). In addition, such rates given in the literature are typically long-term averages\nintegrated over long timescales, yet realistically recharge rates vary over time, so very likely there will be\nshorter episodes of high recharge rates exceeding these average values, and other periods with lower\nrecharge rates than the averages. Especially in the case of Aso, high recharge rates could have occurred\non a short timescale, making the chosen recharge rates very realistic.\nWe are not quite sure on what evidence the 10% volume change resulting from such recharge referred to\nby the reviewer is based on. Referring to the new supplementary fig. 3, we see a volume increase of 1% in\nour longest run, which we believe is a realistic scenario.\nIt is correct that the conditions to achieve undersaturation have to be very close to saturation, however, we\nwant to emphasize that resorption itself, without achieving undersaturation, already induces accelerated\npressurization of the system. This is of course tied to the high recharge rates but also the fact, that\ncompressibility starts to decrease.\nOur model results suggest that resorption almost always occurs under high recharge rates (in the 15vol%\ninitial crystallinity case as soon as recharge rates exceed 10-3 km3/yr). Assuming that large caldera systems\nare expected to experience enhanced recharge rates (see references mentioned above and in the\nmanuscript) and considering that recharge rates might be pulsed rather than long-time continuous input,\nwe think that such episodes of high flux pulses are common and that the process can be in fact widespread.\nOf course, this is a hypothesis, and we would love to see more research on this topic to get more evidence-\nbased input from more than one case scenario. This is one of the main purposes of this paper, to get\nattention to this potential process and test the frequency of its occurrence in nature.\nWill the work be of significance to the field and related fields? How does it compare to the established\nliterature? If the work is not original, please provide relevant references.\nThis work could point the community to have a better discussion about the saturation state of magmatic\nreservoirs and how that saturation might vary in space and time. The authors could point more directly in\nthe discussion to the implications this would have for petrologists studying these systems and what they\nshould look at (maybe for length at the expense of the hazard implications which I think are not so strong).\nResponse:\nWe agree, we would love to spark more discussion about saturation levels in magmatic systems prior to\neruption. We already give instructions on how petrologists might look for this process in nature and how\nsystems of interest could be identified in the conclusions and implications section.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nThe analysis of the mechanisms should be improved and while the model can produce the behavior the\nauthors describe, I think it is not as widespread as they claim and doesn’t match their case study.\nResponse:\nWe understand the reviewers concern here, however, in our previous comments we lined out why we think\nthat the reviewers concerns might be misled. The water contents cited here are well within the range of\nerror of the water contents determined in Aso samples (taking model and hygrometer uncertainties into\naccount) and the temperature variations addressed by the reviewer previously referred to the Aso-A instead\nof the Aso-Y eruption, which is not the focus of this study. Therefore, we do believe that our case study is\nmatching.\nWe further agree that very specific conditions are required to achieve a system transition to\nundersaturation, however, we do mention a number of systems that have water contents close to saturation,\nwhich therefore might have been subject to similar processes as Aso. We also would like to bring the study\nof Brookfield et al., 2023 to the reviewer’s attention – here the authors suggest a similar sweet spot\ncondition around solubility to achieve high overpressures in magma chambers. In the end, large-scale\nsilicic eruptions are very rare events, and specific conditions might be required to achieve such rare natural\nevents. Finally, we also want to emphasize again that we believe that resorption is a process that is\ngenerally important, even if undersaturation is not reached – referring to figure 3, we see that resorption\nis generally more efficient in pressurizing a system compared to exsolution, which is of course also tied to\nhigher recharge rates. As such high recharge rates are commonly suggested for large-scale silicic system,\nresorption might also be a common process. As we deliver the groundwork on detecting resorption in\nnatural data and can reproduce it with our model, we want to spark an interest for more research on this\ntopic in the future, more petrological data will deliver the evidence for whether resorption is widespread\nor not.\nAre there any flaws in the data analysis, interpretation and conclusions? - Do these prohibit publication or\nrequire revision?\nErrors in the calculations for Figure 2 (see above), which are only for the discussion and are simple to\ncorrect, but may change the conclusions. I expect the calculation is correct in the numerical simulations that\nprovide the data for the other figures, although in that case the authors may not have identified the most\nimportant processes.\nResponse:\nThank you for pointing out these concerns, we addressed the calculations in detail above and approached\nthe calculation of especially process 2 – magma mixing from a new perspective. With these changes and\nidentifying a previous sign error in our equation, we still find that pressurization followed by changes in\nthe phase proportions are the main contributors to volatile resorption.\nIs the methodology sound? Does the work meet the expected standards in your field?\nThe numerical simulations are already established by this authorship team and collaborators. The model is\nsimple enough to enable running many simulations, but has the appropriate complexity to answer the\nquestion posed in this text.\nResponse:\nThank you.\nIs there enough detail provided in the methods for the work to be reproduced?\nThere is some detail missing. I wasn’t able to determine how the calculations for Fig. 2 and Fig. 4 were\ndone. While I understand the temperature for each model was set according to the desired crystallinity and\nwater content and so varies, it would be helpful to report the range of initial temperatures explained in the\ntext and add the temperature for each run to the relevant supplementary data tables. Same for the bulk\ncomposition (even just SiO2) for the recharge material.\nResponse:\nWe supplied additional information on the calculations for figure 2 in the supplementary materials, we\nhope it is clearer now.\nRegarding initial temperatures – initial temperatures for the 15 vol% case usually vary between ~855 to\n~900 °C. We added this in parentheses in the parameter space section of the methods. We, however, would\nlike to refrain from adding all initial temperatures to the supplementary files, initial temperatures can be\neasily assessed with the code available on Zenodo. In addition, Supplementary fig. 3 was added giving\nexamples of temperature curves during resorption.\nMinor points:\nLines 99-101: “We find that while higher recharge temperatures and lower recharge volatile contents\npromote resorption, recharge rate is the main control on this process.” I think you should be clear here that\nthe scaling dependence should be (I think) approximately linear for all of these factors, you’re just varying\nrecharge rate over a much large range, which is reasonable, but just be clear about that in the wording here.\nUnless you really think the pressurization is the driving mechanism.\nResponse:\nWe do believe that pressurization induced by high recharge rates is indeed the driving factor of resorption\n– this is also what is shown with our calculations of figure 2. In addition, we varied the recharge volatile\ncontents and temperatures over ranges that are comparable to values found in natural systems, therefore\nthe variations we receive should also be on the order of magnitude of the expected outcomes. For us this\nmeans that indeed, the recharge rate has more important controls than the other factors – which yes, is in\nparts due to the naturally larger variety of recharge rates than the other parameters.\nLine 192-194: “we expect a resorption-induced reduction of the MVP to cause a decrease in magma\ncompressibility, which could enhance of fluctuate ground deformation prior to large silicic eruptions.” I\ndon’t think this claim is that simple. True, it would decrease the compressibility, but the resorption process\nitself actually decreases the magma volume so there’s a competing effect here. You’re right in the case of\ncomplete resorption, but anything shy of that you would need to actually look into which factor wins.\nResponse:\nWe agree that this statement is a simplification, we added a sentence here in the manuscript to point out\nthe competing effects of volume decrease and lowered magma compressibility. However, we want to make\nclear that in this section, we are brainstorming potential effects on how resorption could be tracked in\nnature rather than providing definite statements. Our main goal is to spark new research, which could give\nthe answers to our hypothesis mentioned here, therefore, we would like to keep the statement.\nMethod line 416: This is a very limited depth range. I understand you are interested in the very large\nsystems, but for the small magma chambers, it would be interesting to see the effect of a broader range.\nYou don’t really present the effect of depth, but I could imagine that changes in the solubility sensitivity to\npressure at different depths could be interesting.\nResponse:\nAbsolutely, we agree that a wider depths range would be interesting especially for smaller scale systems.\nHowever here we oriented our study on Aso to which these depth ranges refer and generally magma\nchambers for large-scale silicic systems are expected to be at a depth around 200 MPa (see the study of\nHuber et al., 2019 https://doi.org/10.1038/s41561-019-0415-6). Future research on the depth effect would\nbe really interesting to see but is beyond the scope of this study.\nOut of interest, we also ran a quick simulation (using a 10-2 km3/yr recharge rate, 4 wt% initial H2O, 100\nppm CO2) showing that high recharge rates would have similar effects on shallower magma chambers. It\nmainly seems like resorption is more efficient in shallower and smaller chambers, which is probably related\nto higher proportions of exsolved volatiles. Due to the higher initial mass fractions of the MVP, these\nchambers do not necessarily reach undersaturation under the same conditions as deeper chambers but\nmight require less volatile-rich initial chamber conditions or stronger recharge at the same initial magma\nconditions, see figure below.\nMethods line 417: Please give the range of temperatures investigated.\nResponse:\nWe added a range of initial temperatures for the 15 vol% crystals case.\nMethod line 419: The listed CO2 contents don’t match the table.\nResponse:\nThank you for pointing this out, we corrected the typo in the text!\nMethod line 423: You don’t give the composition for the recharge melt. Is it supposed to be a basalt? You\ngive a melt density for the rhyodacite which is the most evolved end-member.\nResponse:\nFor model simplicity we actually assume the recharge magma to be of the same composition as the resident\nmagma, therefore, we use the same density here. We simulate less evolved conditions of the recharging\nmelt through higher temperatures and variable recharge contents of the inflowing magma.\nSupplementary information 1 line 41: 0.09 is too high for your simulations, and also not what you present\nin Fig. 2C. Maybe 0.009? Also, not sure why these calculations were done on the basis of 10% initial\ncrystallinity if the simulations are for 15%.\nResponse:\nWith these calculations we want to determine which of the processes in our 3 presented hypotheses has the\nlargest effect on volatile resorption. We are not aiming to reproduce model outputs, as these would be\nsubject to a large set of thermo-mechanical processes influencing the dynamics of the system, therefore\nisolating the individual effects would not be as easily doable. We use exemplary numbers here that are\nindependent of our models. 0.09 was indeed a typo – we meant a ∆ of 0.01, representing a change in\n𝑥\ncrystallinity of 10%, thank you for spotting this!\nSincerely,\nJanine Birnbaum\nReviewer #3 (Remarks on code availability):\nI did not have access to the code repository.\nResponse:\nSorry for that! We added a link to our zenodo repository, but it might have gotten lost on the way. We will\nmake sure to provide the access to the repository during re-submission.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nI have read the revised version of that paper and found that the authors have correctly adressed the\ncomments raised during the first round of review.\nI would simply recommend the authors to change widespread process by common process in their\nabstract (I would even write ..likely a common process). I don't think we have the data to say that this\npaper would represent a paradigm shift in our perception of the mechanical evolution of magmatic\nreservoirs.\nThe excess sulfur conundrum documented for several explosive silicic eruptions is generally explained by\nthe existence of a separate fluid phase, which, if true, shows that in those cases at least, volatile resorption\ndid not play a crucial mechanical role.\nI understand that to get published in journals of wide audience one has to show that the findings provided\nare of wide/global relevance but here it is mostly the attention to a under-appreciated mechanism which\nmakes the paper appealing and interesting. In my opinion it is now the job of future studies to test that\npossibility before we can claim it is a general one.\nOther than that I have no more comments to do and therefore recommend acceptance of this paper which\nadresses a mechanism which was so far not considered in the modelling reservoir evolution.\nBruno Scaillet\nResponse:\nWe thank reviewer 1 for their final comments. As suggested by the reviewer, we changed “widespread\nprocess” to “common process” in the abstract. We also agree that further studies will be required to test\nhow widespread the resorption process really is. We also agree that resorption will likely not take place\nin all systems and also will not always reach undersaturation, as high recharge rates will be required, but\nwe do see the appearance of resorption quite frequently in our models, which makes us curious to see\nfuture studies on this topic.\nReviewer #2 (Remarks to the Author):\nI thank the authors for their engagement with my comments and questions. I appreciate the detail of their\nresponses in the rebuttal and I am satisfied that they have made sufficient changes to the manuscript text\nand figures to address them. I am now happy to recommend this manuscript for publication, and hope that\nit sparks fruitful further discussions and new avenues of research by its readers.\nResponse:\nWe thank reviewer 2 for their time invested in our manuscript and are glad to hear that we could address\ntheir concerns appropriately.\nReviewer #3 (Remarks to the Author):\nNCOMMS-25-51869A\nVolatile resorption expedites eruption onset in large silicic systems\nKeller, F., Townsend, M., Troch, J., Huber, C.\nMain comments\nResorption drivers calculations:\n1) Has this calculation now been removed from the methods?\nResponse:\nWe are not sure which calculation the reviewer is referring to here. As visible in the track changes\nrecorded in the manuscript, we did not remove any calculations from the methods section. The equation\nfor water mass conservation that elaborates on the drivers of resorption has always been in\nsupplementary methods 1, where it is still available and we expanded on our approach.\n2) The discussion of these is still a bit confused – going back and forth in order between the\ndilution of magma mixing and crystal melting\nResponse:\nWe are not sure what exactly the reviewer is referring to here. We worked quite significantly on\nimproving the discussion part as this was also mentioned by other reviewers, who acknowledged that the\nmanuscript has improved. In our opinion the discussion is clearer now, going from the most important\nprocesses hierarchically to the least important processes influencing resorption.\n3) I think the new discussion is a cleaner in terms of the simplicity of the calculations, which\nis not per se a problem. But, I think there are two overlooked (competing) effects that\nneed to be directly addressed in this discussion: you mix the water contents in process 2,\nbut not the CO2 contents which should have the opposite effect. It’s fine to keep them\nvery simple, treat them separately and show the effect of keeping water constant and\nessentially just adding some CO2 (CO2 flushing?) It seems from your supplementary\ndata that the more CO2 you add, the less resorption you get. Additionally, you do just the\npressure effect, please also do just the temperature effect.\nResponse:\nIn the numerical model, we do account for both CO2 and H2O, it is only in the simplified scaling analysis\nthat we focus on H2O alone. We do this for two reasons: First, our model shows that resorption becomes\nless effective at higher CO2 contents, largely because the total gas mass fraction increases, meaning\nmore gas must be resorbed before the system reaches undersaturation. Second, we do not expect CO2 to\nfundamentally control resorption dynamics in rhyolitic systems, which typically contain very little CO2.\nFor this reason, we focus on water primarily and believe that adding terms for CO2 goes beyond the\nscope of this paper.\nWe further understand that the reviewer would like to see additional equations for temperature in the\nsection explaining drivers for volatile resorption. As solubility is more pressure dependent than\ntemperature dependent (see Liu et al., 2005: e.g. a 100 degree decrease in temperature for a water\ndominated gas phase causes a solubility increase of 0.4wt%), we believe that the main effect of increasing\ntemperatures is melting crystals in the reservoir, which drives changes in the phase proportions as\ncaptured in process 3. Therefore, we would like to refrain from adding additional complexity to our\nequations, which might increase the difficulty for the readership to follow our thought process.\n4) I asked on the first draft and I’m still confused. Why in your analysis do you only account\nfor adding 1% of chamber volume through recharge when your undersaturated\nsimulations add 10%? Eruptions occur for 10-2.4 km3/yr * 5x103 yr = 20 km3 which is 4% of\nthe total chamber volume, and you explore up to 10-2 km3/yr * 5x103 yr = 50 km3 (10% of 500 km3). This will bring the effect closer to what you get for 5% crystal melting (albeit\nthat melt is completely anhydrous rather than 1 wt% water). Or is it that they erupt before\ngetting there?\nResponse:\nIndeed, in none of our simulations is a volume change of 10% achieved, as eruptions occur before such a\nvolume change takes place. In our previous response we highlighted the new supplementary figure 3,\nwhich shows the volume change in one of the longest runs. In this run the max. volume change before\neruption is 5 km3, which reflects 1% of the total volume. We would like to point out that the volume is not\nonly controlled by mass influx but also the decrease in volume through resorption etc. Given these\nobservations, we believe that modelling a max. volume change of 1% is reliable here and represented in\nour data.\nFigure 3:\nFrom this plotting it is not apparent to the reader what difference is from the\nresorption/exsolution and what is from the recharge rate. Can you do this in normalized time\n(time*recharge rate) which would just be added volume so that we can distinguish?\nResponse:\nWe don’t think that this plot would actually change the current representation much. Runs 1 through 3\nhave the same recharge rate, therefore their relation will remain the same as presented in figure 3. The\ninclination of run 4 compared to the previous three might indeed change, but we don’t think this will be of\nsignificant value for the reader. See the attached figure supporting our comment (due to the lower\nrecharge rate, Run 4 is markedly shorter than runs 1-3).\nIn addition, we believe that changing the axis as suggested by the reviewer might actually make the figure\nharder to understand for non-modeler readership (see figure below with extended values on x-axis that\nare less intuitive than time in years). As Nature Communication is targeting a broad audience, we believe\nthat keeping time instead of an equation on the x axis might be also preferable to address a broader\nreadership.\nAdditionally, something looks odd to me in the calculation of average compressibility. Why is the\naverage compressibility for run 3 between the value for runs 1 and 2 when it has uniformly\nhigher vesicularity and lower pressure?\nResponse:\nThis is a good point. We double checked our calculations for the effective compressibility and realized\nthat we calculated compressibility including during the time of eruption for this dataset, which can lead\nto wrong calculations. By calculating compressibility until the timestep right before eruption, we received\nvalues with a larger spread, even though the maximum change in compressibility is still low. We\ncorrected the values in the figure.\nRun 3 can be slightly less compressible than Run 2 even though it contains more water than Run 2. This is\ndue to the dependence of solubility to thermodynamic and compositional parameters such as pressure\nand temperature. The temperature of our runs is calculated based on the initial volatile and crystal\ncontent of the respective runs and can therefore vary between individual runs. This is also the case here,\nRun 3 (867 °C) is slightly higher in temperature than Run 2 (856 °C), which can explain small differences\nin compressibility of 2x10^-11 Pa-1 as seen here.\nI’m skeptical of this extrapolated curve. How was this fit? Run 2 also shows a certain\nacceleration despite remaining volatile saturated. If you adopt the scaled axis you could\ncompare two runs that go from just volatile saturated to just undersaturated for a more direct\ncomparison.\nResponse:\nWith these figures we primarily want to highlight the pressurization efficiency of runs with different\nrecharge rates and water contents. For example, a run that is undergoing low recharge is exsolving,\nwhile pressurizing slower than all resorbing cases. Resorption and associated elevated pressurization on\nthe other hand is connected to different water contents, where we see that lower water contents allow for\nresorption to undersaturation (which is not achieved in wetter run due to higher gas volume fractions)\nand allows for increased pressurization. The extrapolation of curve in Run 1 is secondary but included to\nshow that undersaturation, if achieved, can lead to earlier eruption onset.\nTo extrapolate this curve, we extended the linear part of the curve before undersaturation has been\nachieved. We agree with the reviewer that curve 2 also shows inclination, however, as we are aiming to\ngive examples of endmembers here that are secondary to the main message of the figure, and at the same\ntime try to not overstate the early eruption onset (as also later pointed out by reviewer 3), we would like\nto keep the current extrapolation.\nCompressibility calculation:\nSince it is a critical parameter, how is the compressibility being calculated? It seems to me that\nfrom a volume-averaged-type formulation, the increase in compressibility at low volume fraction\nof exsolved volatiles into undersaturation should have a smooth, rather than stepped, increase in\ncompressibility. Why would this produce a sharp change in the pressurization rate?\nAlso, the difference in compressibility is so small between the exsolving and volatile\nundersaturated runs (4e-11 1/Pa) which for 20 MPa of pressure should only be a volume\ndifference of 0.08%. It seems to me that it’s really just the volume reduction of the gas phase\nitself that’s buffering the pressure, more than the “compressibility”. I understand that in a bulk\nsense these are indistinguishable, but the discussion should be clear that this isn’t about the\ninstantaneous suspension compressibility if that’s the case.\nResponse:\nAs mentioned in our previous responses, we did not calculate compressibility using a volume averaged\napproach but using the calculations from Townsend and Huber 2020\n(https://doi.org/10.1130/G47045.1) that expand the derivative of mass density with respect to pressure.\nWe reference this in both the figure captions of Figure 3 and 4.\nThe advantage of this calculation is that it considers the pressure dependence of volatile solubility and\npressure-driven phase changes, which can capture changes in gas fraction better than a volume-average-\ntype formulation. As the appearance/ disappearance of a gas phase has significant impacts on the overall\nsystem compressibility we do see a sudden jump in compressibility and with that in pressurization.\nScaling argument (Fig. 4):\nI still think it would be better to have a non-dimensional factor given that the pressure term\nshould be combined with the rest. But interesting that including the volume change didn’t help.\nThat suggests to me that you haven’t actually identified the driving process since compressibility\nalone can’t explain your result!\nResponse:\nWe would like to re-emphasize that this is a commonly used scaling law and that adding a non-\ndimensional factor would probably not close the gap between the undersaturated and saturated cases.\nTherefore, we would like to keep the current version of the scaling law.\nWhile we agree with the reviewer that additional processes besides compressibility (as calculated here)\nmay be playing a role in changing the pressurization, we also believe that compressibility does play a\nsignificant role here as is shown in figure 4. We also acknowledge that the model includes many coupled\nand time-varying processes that will never be fully captured by a single scaling law. We have highlighted\nthat a return to undersaturation is clearly important in driving up the pressurization rate compared to\nwhat we would expect in volatile saturated cases, and we invite readers to investigate the full effects of\nthis process further.\nSee attached document for line-by-line comments.\nBest regards,\nJanine Birnbaum\nReviewer #3 (Remarks on code availability):\nGiven that the model is well-established, I didn't run the code, but I can see the files and inputs are\navailable at the listed repository and all the data for the figures is provided in the supplementary material.\nLine-by-line:\nLines 12-13: Noun disagreement between “volatile resorption” and “systems undergoing exsolution”.\nResponse:\nDone.\nLine 61: I would avoid the word “couple” here from a mathematics perspective and instead “we\nperform thermos-mechanical modelling tuned to/with inputs for geochemical data from Keller et\nal.”\nResponse:\nWe changed this to “we perform thermo-mechanical modelling informed by geochemical data…”.\nLine 63: Omit “yet”.\nResponse:\nDone.\nLine 78: Are there melt inclusion volatile contents for this system that could hint at the volatile\ncontents for the recharge? Is this a reasonable H2O/CO2 ratio?\nResponse:\nFrom previous geochemical data, we know that recharge is of intermediate composition at Aso. As no MI\ndata exists for the Aso-4 event itself, we tuned the volatile contents to the best of our knowledge to\nintermediate compositions, which are usually hotter and drier than rhyodacites/ rhyolites. In addition, we\ntested over large, yet realistic ranges of these parameters to capture potential correlations. We find these\nare secondary to the recharge rate, as pointed out in the manuscript.\nLine 86: This threshold for H2O solubility needs to be paired with a CO2 content for the mixed\nsolubility model.\nResponse:\nSee table 1: initial concentrations for CO2 used are 100 and 500 ppm CO2. We do not find significant\nvariations in solubility dependent on this small range of CO2 starting compositions, crystallinity plays the\nmajor role as explained in the manuscript.\nLine 88: recharge-induced variations in pressure, temperature, and volatile content; describe the\ndirect impact on state variables and then the modeled effect on system characterization. You\nhave not established through what mechanisms the resorption occurs.\nResponse:\nIn this section we are introducing our regime diagram, which captures the described variations using\nchanges in crystal and gas fractions. We realized a typo here, we meant gas instead of melt volume\nfractions. In the next section of the manuscript, we explain in detail how pressure, temperature and\nvolatile contents impact resorption mechanisms.\nLine 90: It would be nice to directly state the regimes in the main text, not only in the figure.\nResponse:\nDone.\nLine 92: add “second boiling”.\nResponse:\nDone.\nLine 93: how would you describe this second regime? Just pressurization?\nResponse:\nYes, we believe this is a small fraction of cases where the recharge rates are high enough to pressurize\nthe system without adding sufficient recharge to already heat the system. Such processes might be easier\nto achieve in smaller chambers.\nLine 95-96: This question from the first set of reviews is unanswered. Is this regime\nphysical/possible?\nResponse:\nWe would like to refer the reviewer to our answer in the previous reviews, we answered in detail there\nthat we believe it could be possible under very specific conditions, which might be hard to achieve in\nnature but not impossible.\nLine 97-100: This sentence doesn’t tell me which recharge conditions can promote resorption –\nis it high temperatures, low H2O, low CO2?\nResponse:\nWe added these here.\nLine 107: You’ve just told us that starting crystallinity has an important effect. What is the\ncrystallinity of the Aso-4 system?\nResponse:\nWe would like to refer the reviewer to table 1, we tested both crystallinities of 2 and 15 vol% for Aso to\ncover the range of crystallinities observed.\nLine 114: “variation in phase proportions of crystals, melt and MVP through influx of hotter\nrecharge magma” – just crystal melting.\nResponse:\nDone.\nLine 120: Why calculate up to 25 MPa if you limit overpressure to 20 MPa?\nResponse:\nThis is a fair point. Mainly to have symmetry in the plots and have 5 points for each panel.\nLine 121: “fixed” rather than “at zero”.\nResponse:\nDone.\nLine 127: Why discuss 3 before 2?\nResponse:\nWe explain the processes in hierarchical order of their contribution to resorption from the most important\nto the least important processes. Therefore, the order is process 1 – process 3 – process 2. We believe this\nis more intuitive to the reader and would like to keep it.\nLine 134: Really the change in the melt volume.\nResponse:\nWe are not sure what the reviewer is referring to here. If the reviewer is referring to “The addition of\nanhydrous crystal melts and recharge melts dilutes the host magma’s..”, we agree that the underlying\nmechanism is a change in the melt volume, however, we explain this in the previous sentence.\nLine 149: how can you asses the impact of pressurization in magma chamber pressurization?\nPlease rephrase. Also melting rather than decreasing crystal fraction.\nResponse:\nWe believe we have addressed this point both in the manuscript and in our earlier responses related to\nFigure 3. The individual contributions are explicitly described and clarified in the main manuscript text,\nand we believe the current presentation is the most accurate and coherent way to convey our results. We\nwould therefore prefer to keep the structure and content as it is.\nLine 243: Isn’t the Liu et al. 2005 model for rhyolites? Can you speak to the difference from\nrhyodacite?\nResponse:\nIndeed, Liu et al is calibrated for rhyolite, yet the difference between rhyolite and rhyodacite is negligible\n(rhyodacite being slightly lower in SiO2 and alkalis than rhyolite) for such saturation models. In\naddition, to the best of our knowledge, no well-calibrated and well-established coupled CO2-H2O\nsaturation model is established for specifically rhyodacite compositions, making a rhyolite calibrated\nmodel our best fit.\nLine 267: Your crystallinity curves extend only to 1200 °C, but you have recharge at 1300 °C. Is\nthis magma aphyric?\nResponse:\nYes, under these circumstances the 1300C magma would be considered aphyric, which is a realistic\napproximation given that the crystals crystallizing at these high temperatures are very few, close to 0.\nLien 323: Is this the suspension density or the melt density?\nResponse:\nThis refers to the melt density. We clarified this in the manuscript.\nLine 325: There is again disagreement between the listed CO contents in the text here, what is\n2\nshown in the figures, and what is presented in Table 1.\nResponse:\nWe caught a typo here stating 550 ppm instead of 500 ppm, we corrected this. We only present the lower\nrecharge CO2 and the highest recharge CO2 here for easier visibility, the full range of CO2 are given in\nsupplementary fig. 1.\nLine 373: For reference with very long author lists, please still give the first four authors followed\nby et al.\nResponse:\nWe changed this.\nLine 512: Are the conditions for B different?\nResponse:\nWe assume the reviewer is referring to a typo in this line, which we corrected.\nFigure 2: Here as well why “increasing melt fraction” rather than “melting”. And see main\ncomments above, it would be nice to include in this figure the competing effects of temperature\nand CO2 mixing. Panel B should be specified here that this is the effect of water dilution.\nResponse:\nAs pointed out in the main text of the manuscript the melt fraction is not only increasing due to crystal\nmelting but also due to the addition of recharge magmas, which are lower in crystal fraction than the host\nmagma. The addition of less crystal-rich magmas will also drive the crystal fraction of the host magma\ndown. Therefore, it is a combination of crystal melting and crystal-poor recharge melt addition. We refer\nto the main comments regarding an answer to the second part of this comment.\nSupplementary Figure 1: There’s a typo in the panel c legend “conent” instead of “content”. Also\nyou have the CO2 in panel D all in wt% when you use ppm elsewhere in the text.\nResponse:\nThank you for spotting the typo, we corrected this. The CO2 in the panel D legend was in wt% due to\nspace constraints but we updated this.\nSupplementary methods 1:\nLine 10: Define here again all the variables.\nResponse:\nA definition of all variables is provided in the table at the end of this supplementary methods file.\nLine 27: The notation has switched in the middle of the equation for the last two terms from\nm_eq(P_0) to m_eq,0 and from rho_g(P_0) to rho_g,0.\nResponse:\nThank you for spotting this typo, we corrected this.\nLine 30: This equation uses the second notation from line 27, which is in conflict with the\nnoration on lines 19-26.\nResponse:\nWe corrected this.\nLine 30: The way this equation is written is a little chaotic: rho_m partial m_eq/partial P + rho_m\npartial m_eq/partial P (-epsilon_chi0-epsilon_g0) should be combined to rho_m partial\nm_eq/partial P (1- epsilon_chi0-epsilon_g0) to preserve the structure of the original equation and\ncombine terms. You can also preserve more of the original structure by multiplying by keeping\nsome signs more consistent and factoring:\nΔ𝜀𝑔=_(_𝜌𝑚𝜕𝑚𝑒𝑞𝜕𝑃(_1_−𝜀𝜒,_0_−𝜀𝑔,_0_)_+_𝜕𝜌𝑔𝜕𝑃𝜀𝑔,_0_)_Δ𝑃−Δ𝜀𝜒𝜌𝑚(_𝑚𝑒𝑞,_0_+_𝜕𝑚𝑒𝑞𝜕𝑃Δ𝑃)_−\n(_𝜕𝑀𝑤,_0_𝜕𝑉0_−𝜕𝑀𝑤,_𝑖𝜕𝑉𝑖)_𝜌𝑚𝑚𝑒𝑞,_0_−𝜌𝑔,_0_+_(_𝜌𝑚𝜕𝑚𝑒𝑞𝜕𝑃−𝜕𝜌𝑔𝜕𝑃)_Δ𝑃 _\nResponse:\nWe updated the equation structure as suggested by the reviewer.\nLine 43: In this formulation the recharge density must be the same as the chamber melt density,\nwhich is 2250 kg/m3 not 2400 kg/m3. You’ve restricted the compositions to be the same, the\ndensities should be as well.\nResponse:\nFor our code this is correct. Yet the examples we are bringing on here are unrelated to the code, so using\na higher recharge density is more realistic here and we would like to keep it that way.\nPrevious round of review:\nThere was a question from reviewer 1 about the host rock compressibility, which I was also\ncurious about. The value was given in the response, but not added to the text as far as I could\ntell. Fine to go in the methods or supplement, but it should be there. Better if it can be directly\ncompared to the magma compressibility.\nResponse:\nWe believe adding this value to the methods section is not necessary here. The value for the bulk modulus\nis available in the code provided in the supplements and does not play a crucial role in the model setup.\nHornblende in equilibrium with the melt: I’m not a mineralogist, but I would be surprised to find\nthat hornblende was in equilibrium with a water undersaturated melt.\nResponse:\nThe main question arising from the papers cited by reviewer 1 is whether the hornblende grew in the Aso-\n4 magmas or if it was entrained from an underlying mush. This question is mainly geared to how reliable\nthe ages provided by K/Ar dating are, it seems like the reviewer did not aim to comment on this issue.\nResorption is a process on much shorter timescales and we still believe that the hornblende grew from the\nAso-4 magmas due to the reasons stated in the previous reviews before the magma became\nundersaturated shortly before eruption.\nIn addition, we want to point out that indeed hornblende grows in hydrous magmas, which does not\nimmediately mean that the magma was water saturated. There is no evidence that we are aware of that\nhornblende only grows in water-saturated environments.\nI agree with reviewer 2 that the early eruption onset is not the main (or most robust) claim of the\npaper and an alternative title focusing on that recharge might favor resorption is stronger. I\nundersaturation really the critical point either?\nResponse:\nWe would like to bring our answers to reviewer 2 to the attention of the reviewer. There we explain in\ndetail that we agree with reviewer 2 that undersaturation is not the critical point, and why we would like\nto keep the current title.\nResponse to Referee’s\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nI have had now time to read carefully the last version of the paper, as well as the accompanying rebuttal\nletter.\nIt is my opinion that the authors have done a good job in clearly and carefully addressing/considering the\nmajority of the comments raised during the several rounds of reviews.\nThere are obviuously still open questions but in my opinion they pertain to the scientific debate. Those\nquestions may even promote future work on the subject, in particular additional modelling efforts, which\nwill deal with more complete/complex treatment of the physical process addressed here (for instance what\nwould be the role of the geometry of reservoir, or its depth, which may affect the tensile strength of host\nrocks).\nI therefore recommend acceptance.\nBruno Scaillet\nOur reply:\nWe would like to sincerely thank Reviewer 1 for their continued engagement with our manuscript. We\ntruly appreciate the time they took to review this latest version. We are glad that our efforts to address the\nprevious concerns were helpful and that the reviewer found the revised manuscript to be improved.\nWe absolutely agree that there are still some open questions about the process of volatile resorption\nespecially due to the complexity of magmatic systems and hope that this might inspire future research. To\nhighlight this more carefully in the manuscript, we added a couple of sentences addressing the model\nuncertainty at the end of the manuscript in ll. 244.\nThank you for your time again!\nFranziska Keller and co-authors."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have read the revised version of that paper and found that the authors have correctly adressed the comments raised during the first round of review.\nI would simply recommend the authors to change widespread process by common process in their abstract (I would even write ..likely a common process). I don't think we have the data to say that this paper would represent a paradigm shift in our perception of the mechanical evolution of magmatic reservoirs.\nThe excess sulfur conundrum documented for several explosive silicic eruptions is generally explained by the existence of a separate fluid phase, which, if true, shows that in those cases at least, volatile resorption did not play a crucial mechanical role.\nI understand that to get published in journals of wide audience one has to show that the findings provided are of wide/global relevance but here it is mostly the attention to a under-appreciated mechanism which makes the paper appealing and interesting. In my opinion it is now the job of future studies to test that possibility before we can claim it is a general one.\nOther than that I have no more comments to do and therefore recommend acceptance of this paper which adresses a mechanism which was so far not considered in the modelling reservoir evolution.\nBruno Scaillet", "reviewer_2": "I thank the authors for their engagement with my comments and questions. I appreciate the detail of their responses in the rebuttal and I am satisfied that they have made sufficient changes to the manuscript text and figures to address them. I am now happy to recommend this manuscript for publication, and hope that it sparks fruitful further discussions and new avenues of research by its readers.", "reviewer_3": "NCOMMS-25-51869A\nVolatile resorption expedites eruption onset in large silicic systems\nKeller, F., Townsend, M., Troch, J., Huber, C.\nMain comments\nResorption drivers calculations:\n1) Has this calculation now been removed from the methods?\n2) The discussion of these is still a bit confused – going back and forth in order between the dilution of magma mixing and crystal melting\n3) I think the new discussion is a cleaner in terms of the simplicity of the calculations, which is not per se a problem. But, I think there are two overlooked (competing) effects that need to be directly addressed in this discussion: you mix the water contents in process 2, but not the CO2 contents which should have the opposite effect. It’s fine to keep them very simple, treat them separately and show the effect of keeping water constant and essentially just adding some CO2 (CO2 flushing?) It seems from your supplementary data that the more CO2 you add, the less resorption you get. Additionally, you do just the pressure effect, please also do just the temperature effect.\n4) I asked on the first draft and I’m still confused. Why in your analysis do you only account for adding 1% of chamber volume through recharge when your undersaturated simulations add 10%? Eruptions occur for 10-2.4 km3/yr * 5x103 yr = 20 km3 which is 4% of the total chamber volume, and you explore up to 10-2 km3/yr * 5x103 yr = 50 km3 (10% of 500 km3). This will bring the effect closer to what you get for 5% crystal melting (albeit that melt is completely anhydrous rather than 1 wt% water). Or is it that they erupt before getting there?\nFigure 3:\nFrom this plotting it is not apparent to the reader what difference is from the resorption/exsolution and what is from the recharge rate. Can you do this in normalized time (time*recharge rate) which would just be added volume so that we can distinguish?\nAdditionally, something looks odd to me in the calculation of average compressibility. Why is the average compressibility for run 3 between the value for runs 1 and 2 when it has uniformly higher vesicularity and lower pressure?\nI’m skeptical of this extrapolated curve. How was this fit? Run 2 also shows a certain acceleration despite remaining volatile saturated. If you adopt the scaled axis you could compare two runs that go from just volatile saturated to just undersaturated for a more direct comparison.\nCompressibility calculation:\nSince it is a critical parameter, how is the compressibility being calculated? It seems to me that from a volume-averaged-type formulation, the increase in compressibility at low volume fraction of exsolved volatiles into undersaturation should have a smooth, rather than stepped, increase in compressibility. Why would this produce a sharp change in the pressurization rate?\nAlso, the difference in compressibility is so small between the exsolving and volatile undersaturated runs (4e-11 1/Pa) which for 20 MPa of pressure should only be a volume difference of 0.08%. It seems to me that it’s really just the volume reduction of the gas phase itself that’s buffering the pressure, more than the “compressibility”. I understand that in a bulk sense these are indistinguishable, but the discussion should be clear that this isn’t about the instantaneous suspension compressibility if that’s the case.\nScaling argument (Fig. 4):\nI still think it would be better to have a non-dimensional factor given that the pressure term should be combined with the rest. But interesting that including the volume change didn’t help. That suggests to me that you haven’t actually identified the driving process since compressibility alone can’t explain your result!\nSee attached document for line-by-line comments.\n[Editorial Note: See end of file for attachment]\nBest regards,\nJanine Birnbaum"}, "author_response": "We thank reviewer 1 for their final comments. As suggested by the reviewer, we changed “widespread process” to “common process” in the abstract. We also agree that further studies will be required to test how widespread the resorption process really is. We also agree that resorption will likely not take place in all systems and also will not always reach undersaturation, as high recharge rates will be required, but we do see the appearance of resorption quite frequently in our models, which makes us curious to see future studies on this topic.\nWe thank reviewer 2 for their time invested in our manuscript and are glad to hear that we could address their concerns appropriately."}, {"round": 3, "reviewer_comments": {"reviewer_1": "I have had now time to read carefully the last version of the paper, as well as the accompanying rebuttal letter.\nIt is my opinion that the authors have done a good job in clearly and carefully addressing/considering the majority of the comments raised during the several rounds of reviews.\nThere are obviuously still open questions but in my opinion they pertain to the scientific debate. Those questions may even promote future work on the subject, in particular additional modelling efforts, which will deal with more complete/complex treatment of the physical process addressed here (for instance what would be the role of the geometry of reservoir, or its depth, which may affect the tensile strength of host rocks).\nI therefore recommend acceptance.\nBruno Scaillet"}, "author_response": "We would like to sincerely thank Reviewer 1 for their continued engagement with our manuscript. We truly appreciate the time they took to review this latest version. We are glad that our efforts to address the previous concerns were helpful and that the reviewer found the revised manuscript to be improved.\nWe absolutely agree that there are still some open questions about the process of volatile resorption especially due to the complexity of magmatic systems and hope that this might inspire future research. To highlight this more carefully in the manuscript, we added a couple of sentences addressing the model uncertainty at the end of the manuscript in ll. 244.\nThank you for your time again!\nFranziska Keller and co-authors."}]

Decision letter