# Probing the hidden magmatic evolution of El Misti volcano (Peru) with the Pb isotope composition of fumaroles

Ivan Vlastélic, Fredy Apaza, Pablo Masias, Marco Rivera, Jean-Luc Piro, Abdelmouhcine Gannoun

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To cite this version:

Ivan Vlastélic, Fredy Apaza, Pablo Masias, Marco Rivera, Jean-Luc Piro, et al.. Probing the hidden magmatic evolution of El Misti volcano (Peru) with the Pb isotope composition of fumaroles. Bulletin of Volcanology, 2022, 84 (2), pp.17. 10.1007/s00445-021-01521-9. hal-03538390

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1 Probing the hidden magmatic evolution of El Misti volcano

2 (Peru) with the Pb isotope composition of fumaroles

3

4

5

6 Ivan Vlastelic(1*), Fredy Apaza(2), Pablo Masias(2), Marco Rivera(3), Jean-Luc Piro(1),

7 Abdelmouhcine Gannoun(1)

8

9 ¹ Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000
10 Clermont-Ferrand, France

11

12 ² Instituto Geológico Minero y Metalúrgico (INGEMMET), Arequipa, Peru

13

14 ³ Instituto Geofísico del Perú, Observatorio Vulcanológico del Sur, Mz B, Lt 19, Urb. La Marina,
15 Cayma, Arequipa, Peru

16

17

18

19

20 * Corresponding author

21

22 email address: ivan.vlastelic@uca.fr

23

24 Phone : + 33 4 73 34 67 10

25

26 ORCID ID : 0000-0002-6701-4832

27
61 **Introduction**

62
63 El Misti stratovolcano threatens the city of Arequipa, the second most densely populated city
64 of Peru (>1.1 million inhabitants, INEI, 2018) that is continuously growing at its foot (Fig. 1).
65
66 Over the last 50 000 years the volcano has shown a cyclic activity, with Plinian eruptions every
67
68 2000 to 4000 years and smaller to moderate explosive eruptions every 500 to 1500 years, on
69 average (Thouret et al., 2001). The lavas produced since 0.12Ma are andesitic to dacitic in
70 composition, with rare rhyolites and no marked evolution through time (Rivera et al., 2017).
71
72 This relative homogeneity contrasts with the mineral evidence for regular recharge and
73 mixing, which led Ruprecht and Wörner (2007) to suggest that magma recharge events are
74 more frequent in comparison to the eruption frequencies, and that El Misti reservoir evolves
75 most of the time cryptically. Since the last major eruption (A.D. 1440–1470), El Misti crater
76 has showed a persistent but fluctuating degassing activity (Thouret et al., 2001; Moussallam
77 et al., 2017). Over the last decades, the temperature of gases has varied between 125°C and
78 310°C (Hantke and Parodi 1966; Birnie and Hall, 1974; Thouret et al. 2001, and observations
79 from Observatorio Vulcanológico del Ingemmet (OVI) and Instituto Geofísico del Perú (IGP)).
80
81 The gases sampled in 2015 showed a dominant magmatic signature with little evidence for
82 interaction with rocks and hydrothermal fluids (Moussallam et al, 2017), which contrasts with
83 the dominance in 1967 of low-temperature minerals formed by gas-rock interactions (Birnie
84 and Hall, 1974). Moussallam et al. (2017) acknowledged that the current composition of the
85 magmatic source is unknown, and may be similar to either the rhyolitic or andesitic magmas
86 that interacted during the c. 2030 - 2000 years B.P. eruption (Thouret et al., 2001; Cobeñas et
87 al., 2012; Harpel et al., 2011). In the absence of direct measurement, the estimation of the
88 current state and evolution of the El Misti magma reservoir is challenging but important.
89

90
91 This study aims to reconstruct the composition of the El Misti magmatic reservoir from the Pb
92 isotopic composition of gas emitted in the summit crater. Lead isotopes are well suited for
93 this purpose because (1) lead is a volatile metal enriched in El Misti gases and fumarolic
94 deposits (Birnie and Hall, 1974), (2) the heavy Pb isotopes (208Pb, 207Pb, 206Pb and 204Pb) do not
95 fractionate measurably during magma degassing, due to their very small relative mass
96 differences (Vlastelic et al., 2013), so that the Pb isotopic composition of gas reliably records
97 that of the degassing magma source, (3) the Pb isotopic composition of magma is tightly
98 related to the degree of magma differentiation through the Assimilation - Fractional
99
93 Crystallization (AFC) process that dominates the compositional evolution of El Misti magmas
94 (Mamani et al., 2008; Kiebala, 2008; Rivera et al., 2017). Lead isotopes are especially sensitive
95 to AFC as mantle melts and crustal basement display highly contrasted Pb contents and
96 isotopic compositions (Tilton and Barreiro, 1980; Barreiro and Clark, 1984). We have applied
97 this approach to gas condensates and fumaroles deposits sampled in the bottom of El Misti
98 crater in October 2018.

99

100 Geological setting

101

102 El Misti volcano (5822 m above sea level) is located to the north of the Andean Central
103 Volcanic Zone (CVZ) at 16°16' S and 71°22' W (Fig. 1a). It sits on the western flank of the
104 Western Cordillera, about 240 km east of the Peru-Chile trench. A detailed description of El
105 Misti's evolution is given in Thouret et al. (2001) and Rivera et al. (2017). Briefly, the composite
106 edifice comprises an older main stratovolcano (Misti 1, ca. 833-112 ka), on which grew three
107 younger strato-cones (Misti 2, 3 and 4). Large volumes (4 – 6.5 km³) of dacitic pyroclastic
108 density currents (PDC) were emplaced between c. 50 000 and 40 000 and again between c. 13
109 700 and 11 300 years B.P., the last of which resulted in the formation of today's summit crater.
110 The most recent Plinian eruption (c. 2030 – 2000 years B.P.) produced 1.4 km³ of pumice-fall
111 deposits that spread ≥25 km from the vent and 0.01 km³ of PDC deposits (Harpel et al., 2011).
112 The mingling texture of rhyolitic and less evolved andesitic components in the pumices were
113 ascribed to the injection of an andesitic magma from 7-12 km depth into a shallower rhyolitic
114 reservoir (Tepley et al., 2013). This major event is most likely responsible for the formation of
115 today's 900 m-wide summit crater. The last major eruption (AD 1440-1470, VEI 2) produced
116 andesitic ash-fallout and led to the formation of a lava dome inside the present-day inner
117 crater. Since then, the El Misti crater has shown a persistent degassing activity, with periods
118 of increased fumarolic emissions and possible phreatic events in 1667, 1784-1787 and 1949
119 (Bullard, 1962; Chavéz Chavéz, 1992; Thouret et al., 2001). Less than 12 km separate El Misti
120 crater from the nearest Arequipa suburbs (Fig. 1b).

121

122 Samples and methods

123
We carried out field work in the El Misti crater in October 2018, at the end of the dry season. After a survey of the lava dome, gas condensates and sublimates were collected at the hottest fumarole vent (Fig. 1c) where a temperature of 260°C was measured using a Fluke 62 Max infrared thermometer. Gas condensates were collected in a savillex teflon elbow emplaced in the vent. Gas sublimates were subsequently sampled 30 cm underground at the lower surface of a lava block that obstructed the vent. Sub-surface sublimates were also collected at another vent with a temperature of 150°C (Fig. 1c). Samples were chemically treated and analysed at Laboratoire Magmas et Volcans (LMV) in France. The sublimates were investigated by Scanning Electron Microscopy (SEM, JSM 5910LV) operating in Backscattered Electron (BSE) imaging mode. Sublimate mineralogy was determined from qualitative analyses of major elements using Energy Dispersive Spectrometry (EDS). All chemical treatment of the samples was done in LMV's clean room, within hoods equipped with HEPA filters, and using savillex teflon vials and highly pure acids. One millilitre of the gas condensate was gently evaporated at 60°C and re-dissolved in 1 ml of 0.5M HNO₃ - 0.05M HF. Representative chips of the sublimates (0.2 to 100 mg in size) were fully dissolved in 6N HCl at 80°C. The condensate and sublimate solutions were aliquoted for the analysis of trace element abundance and Pb isotope composition.

Prior to ICPMS analysis, the sublimate aliquots were evaporated and re-dissolved in 0.5M HNO₃ - 0.05M HF, to reach dilution factors of 10⁵ (solution/sublimate mass ratio). All volumes were precisely weighted with a precision better than 10⁻⁴g. The trace element abundances were determined by quadrupole ICPMS (Agilent 7500), in plasma robust mode (1550 W), using the reaction cell (He mode) to reduce interferences on masses ranging from 45 (Sc) to 77 (Se). The signal was calibrated externally (every 4 samples) with a synthetic standard (Inorganic Venture). The acid media was also measured every 4 samples to correct for background signal. Repeated analysis of the analysed solutions yielded 2σ errors of < 5% for Ti, V, Cr, Mn, Co, Ni, Cu, Rb, Sr, Zr, Nb, Sn, Ba, Pb and < 8% for As, Cd, In, Sb, Tl and Bi.

Lead was separated using Eichrom Sr specific resin following the method of Deniel and Pin (2001). Procedural blanks were < 0.03 ng for Pb, which is insignificant compared to the amounts of Pb extracted from samples. Lead isotope compositions were measured on a Thermo Fisher Scientific Neptune Plus MC-ICPMS. Purified Pb was dissolved in 0.05M HNO₃ in order to obtain 150 ppb Pb solutions. The sample solutions were introduced in the mass spectrometer using a 100 µL/min nebulizer coupled to a quartz cyclonic spray chamber,
yielding a total Pb signal between 18 and 20 volts. The Pb instrumental mass fractionation was
corrected during measurement by doping the Pb solutions with the Thallium (Tl) NBS997
standard, applying a Pb/Tl ratio of 5. The NBS981 Pb standard was measured between every
two samples to correct Pb ratios for instrumental drift and the slight difference in mass
fractionation between Pb and Tl. Data were normalized to the NBS981 standard using the
values of Todt et al. (1996). Repeated analysis of the NBS981 standard every two samples
yielded intra-session 2σ reproducibility better than 100, 120 and 140 ppm for ²⁰⁶Pb/²⁰⁴Pb,
²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, respectively.

164

165 Results

166

167 The chemical and isotopic composition of the 2018 El Misti condensates and sublimates are
168 reported in Table 1. Images of the sublimates are shown in Fig. 2, and trace element contents
169 are plotted in Fig. 3. The 260°C gas condensate contains 177 ppb Pb, 37-38 ppb of Tl and Zn,
170 and 9-12 ppb of Bi, Cu, and Sb, with the latter concentrations being at the low range of volcanic
gases (Zelenski et al., 2021). The condensate also contains significant amounts of refractory
171 elements (244 ppb Ba, 45-46 ppb of Sr and Ti). The 260°C gas sublimates occur as a foam of
172 anhydrous alum (NaAl(SO₄)₂), several millimetres in thickness, and coated in many places with
173 galena (PbS) (Fig. 2). The five representative chips analysed for bulk composition contain 2 -
174 10 wt. % Pb, 0.4 – 1 wt. % Ti, 4100 - 9500 ppm Tl, 5400 - 9400 ppm Zn, 2200 - 4600 ppm Bi,
175 2100 - 3600 ppm Cd, and 600 - 3200 ppm Sn. The 150°C gas sublimates occur as thin patches
176 of anglesite (PbSO₄) and cotunnite (PbCl₂), with minor anhydrite (CaSO₄), coating the lower
177 surface of a lava block (Fig. 2). The light yellow cotunnite and shiny black anglesite are barely
178 visible without the use of optical aids. The bulk analysis of four separates yielded 16.8 - 44.5
179 wt.% Pb, 142 - 1349 ppm Tl, 399 - 1689 ppm Zn, 81 - 114 ppm Bi, and 539 - 2257 ppm Cd. The
180 ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of the 260°C gas condensate are 17.724 ± 0.003,
181 15.592 ± 0.004, and 38.628 ± 0.005, respectively. The 260°C and 150°C gas sublimates display
182 significantly less radiogenic, but very homogeneous Pb isotope composition, with average
183 ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 17.676 ± 0.002, 15.584 ± 0.004, and 38.598
184 ± 0.014, respectively. These ratios are within the ranges of previously analysed El Misti rocks
185 (Mamani et al., 2008).
186
The composition of the gas condensates and sublimates collected in 2018 show marked differences when compared to those collected in July 1967 by Birnie and Hall (1974) (Fig. 3) while the temperature of the fumaroles was only 100-125°C: (1) The Zn concentration of the 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218

Origin of the Pb isotopic composition of fumaroles samples

The Pb isotopic composition of fumarolic samples potentially records several processes, such entrainment of rock particulates, mixing between magmatic and hydrothermal gases, and variations of the magmatic source composition. The contribution of rock particulates to the Pb content of gas samples is assessed using the enrichment factor of Pb (Eq. 1):

$$EF_{\text{Pb}} = (\text{Pb/X})_{\text{gas samples}}/(\text{Pb/X})_{\text{lava}}$$

where X is a refractory rock-derived element (Symonds et al., 1987). Using titanium as rock-derived element, and the average composition of the lavas from “Misti 4” (12.3 ppm Pb and 5050 ppm Ti; Rivera et al., 2017), EF<sub>Pb</sub> is 1.6 x 10<sup>3</sup> in the condensate, 0.8 – 9.4 x 10<sup>3</sup> in the 260°C sublimates, and 0.8 – 4.4 x 10<sup>5</sup> in the 150°C sublimates. The lead fraction unsupported by silicate input (1 - 1/EF<sub>Pb</sub>) is 99.94 % in the condensate, >99.87 % in the 260°C sublimates and >99.999 % in the 150°C sublimates (Fig. 4). Repeating this calculation with other rock-derived elements, such as Zr, Nb or Y, and “Misti 4” end-member rock compositions, yields in all cases more than 99.7 % Pb unsupported by silicate input, confirming that the contribution of rock particulates is extremely small. This tiny contribution does not account for the small isotopic variations in gas condensates and sublimates, which do not correlate with EF<sub>Pb</sub> (Fig. 4). The dry composition (H<sub>2</sub>O/SO<sub>2</sub> molar ratio of 32 ± 3) of El Misti gases also only requires
219 a minimal contribution of the hydrothermal system (Moussallam et al., 2017). The
220 contribution of low temperature hydrothermal vapours does not seem to affect the Pb isotope
221 signature of the sublimates, which does not vary with vent temperature. The remaining and
222 most likely possibility is that Pb isotope ratios of the fumarolic samples reflect the composition
223 of magmatic gases.
224

225

Assimilation-Fractional Crystallization-Degassing model

226

227 The Pb isotopes of magmatic gases can be linked to the degree of crystallization of their
228 magmatic source through an Assimilation-Fractional Crystallization-Degassing (AFCD) model.
229 This model takes advantage of the properties of heavy Pb isotopes which do not fractionate
230 during magma degassing, allowing us to reconstruct the Pb isotope composition of magmas
231 from gas composition. Then, we infer the degree of magma crystallization from the Pb isotope
232 composition of magmas using an Assimilation-Fractional Crystallization model. The two steps
233 and related assumptions of the AFCD model are described below, and solutions for one and
234 two magma reservoirs are provided.
235

236

Degassing

237

238 Lead is a moderately volatile element in magmatic systems, with a coefficient of emanation
239 (i.e., fraction of Pb outgassed) in the range of 1-2% (Gauthier et al., 2000). Lead volatilizes
240 dominantly as chloride and condenses mostly below 500°C (Symonds et al., 1987). The Pb
241 isotope compositions of gas is related to that of the degassing magma by the coefficient of
242 isotopic fractionation α (Eq. 2) :

243

$$
244 \quad (^{206}\text{Pb}/^{204}\text{Pb})_{\text{gas}} = \alpha_{64} (^{206}\text{Pb}/^{204}\text{Pb})_{\text{magma}}
$$

245

246 in the case of the ²⁰⁶Pb/²⁰⁴Pb ratio. The theoretical kinetic isotope fractionation (αₖin =
247 (m₂₀₄/m₂₀₆)⁰.⁵ = 0.995) represents an extreme value of α₆₄, which is closer to unity in natural
248 systems due to back condensation reactions at the gas - magma interface (Richter, 2004).
249 Practically, isotopic fractionation of Pb during magma degassing is smaller than the precision
250 of Pb isotope measurements (Vlastelic et al., 2013), and is not significant compared to the
variations occurring in lavas. In such situations, α must be very close to unity for all Pb isotope ratios, and the Pb isotope compositions of the fumarole samples must reliably reflect that of the degassing magma. In the case of simultaneous degassing of andesite and rhyolite magma reservoirs, the composition of the gas mixture is obtained by mass balance (Eq. 3):

$$ (^{206}\text{Pb}/^{204}\text{Pb})_{\text{gas}} = \Phi_{\text{andesite}} \times (^{206}\text{Pb}/^{204}\text{Pb})_{\text{andesite}} + \Phi_{\text{rhyolite}} \times (^{206}\text{Pb}/^{204}\text{Pb})_{\text{rhyolite}} $$

$$ \text{with } \Phi_{\text{andesite}} + \Phi_{\text{rhyolite}} = 1 $$

where Φ is the fraction of the total Pb gas flux, which depends on the size and degassing rate of each reservoir.

## Assimilation-Fractional Crystallization

Mantle derived magmas ascending through continental arcs tend to melt and assimilate crustal rocks with low melting temperature. Because wall-rock assimilation consumes heat, while crystallization releases heat, the two processes are potentially coupled. However, the ratio of assimilation to crystallization rate (referred to as r) largely varies in magmatic systems, generally between 0.2 and 2.7. These variations depend on several parameters including the initial temperature and melting point of the country rock, and the thermal potential of the parental magma (Thompson et al., 2002). For instance, r is typically in the range of 1.0-1.5 for an initial magma temperature of 1350-1150°C, compared to 0.6-0.2 for a temperature of 1150-900°C. The efficiency of assimilation is predicted to decrease (1) during differentiation due to the decreasing liquidus temperature, and (2) with decreasing depth as the temperature of host rocks decreases following the geothermal gradient. Both trends suggest that assimilation takes places most pronouncedly during earlier, deeper magmatic stages.

A major outcome of AFC is that the isotopic variations of continental arc lavas often correlate with the degree of differentiation (De Paolo, 1981). The isotopic record of AFC is particularly clear along the CVZ where Pb isotope variations are controlled by the assimilation of Pb-rich rocks from the locally thick continental basement (e.g., Tilton and Barreiro, 1980; Wörner et al., 1992; Mamani et al. 2008; Kiebala, 2008; Sainlot et al., 2020a). At El Misti, the Assimilation-Fractional Crystallization process translates in a decrease of $^{206}\text{Pb}/^{204}\text{Pb}$, $^{207}\text{Pb}/^{204}\text{Pb}$ and
$^{208}$Pb/$^{204}$Pb with increasing SiO$_2$, due to assimilation of the proterozoic Charcani gneiss (Arequipa Massif) of the Western Cordillera basement with unradiogenic Pb ratios (16.792$^{<}_{\sim}$$^{206}$Pb/$^{204}$Pb$<\sim$ 17.088) (Tilton and Barreiro, 1980; Barreiro and Clark, 1984; Mamani et al., 2008; Kiebala, 2008). Tilton and Barreiro (1980) pointed out that the granulitic gneiss must have a lower crustal origin, but is probably assimilated by El Misti lavas at shallower depth.
We hypothesize that the frequent passages of magma in the lower crust favour the disaggregation of crustal rocks into small xenoliths (Thompson et al., 2002). Entrained xenoliths could partially dissolve during magma ascent in the upper crust due to superheating above the liquidus (Brearley and Scarfe, 1986). How and where El Misti magmas are contaminated remains uncertain, but contamination must be related to magma crystallization, which dominantly occurs between 7 and 12 km depth (Tepley et al., 2013).
The AFC relation might have substantially varied during the growth of El Misti volcano due to slight variations in magma path and thermal history, in particular following major collapses, and heterogeneity of the basement. For this reason, we use the AFC relation defined over the most recent Misti 4 stage ($<$11 ky) (Fig. 5), assuming that the composition of mantle derived magmas and assimilated rocks, as well as the r value, do not vary at short time scales. This relation is defined based on four samples analysed for Pb isotopes by Mamani et al. (2008), including 6 ka old scoria from pyroclastic density current deposits (MIS-00-20), two pumices from fall deposits (quebrada San Lazaro) and one pumice (MIS-02-10a) from the 2030-2000 years B.P. eruption. The SiO$_2$ contents of these samples, between 58.7 and 63.2 wt.%, encompass the range of variations of El Misti rocks with the exception of rhyolites. The samples show a Pb isotope range (17.3965$^{<}_{\sim}$$^{206}$Pb/$^{204}$Pb$<\sim$17.8442) that brackets the composition of the fumarole samples (17.6740$^{<}_{\sim}$$^{206}$Pb/$^{204}$Pb$<\sim$17.7243), and are thus appropriate for calibrating the AFC relationship. In addition, these samples do not contain large minerals, with potential Pb isotope disequilibria relative to the matrix, so it can reasonably be assumed that their bulk composition reflects that of the degassing melt. The black ash fall sample (MIS-02-06), with a proposed age of 11 ky (Kiebala, 2008) is not included because (1) this sample pertains to Misti 3 (Rivera et al., 2017), and (2) the juvenile origin of the ashes is uncertain. For the purpose of modelling, we preferentially use the $^{206}$Pb/$^{204}$Pb and $^{208}$Pb/$^{206}$Pb ratios, which show well-defined AFC trajectories owing to large variations in lavas and precise measurements. The $^{207}$Pb/$^{204}$Pb and $^{208}$Pb/$^{204}$Pb ratios, with smaller variations and
larger errors are not used. The relationships between SiO₂ content and Pb isotope ratios in lavas from Misti 4 stage are best fitted (R²> 0.988) by the second-degree polynomial functions:

314
315
316
317 SiO₂ (wt%)<sub>magma</sub> = -26.015 x (206Pb/204Pb)<sub>magma</sub>² + 906.56 x (206Pb/204Pb)<sub>magma</sub> -7834.7 (Eq. 4)

318
319 SiO₂ (wt%)<sub>magma</sub> = -3163.95 x (208Pb/206Pb)<sub>magma</sub>² + 13949.16 x (208Pb/206Pb)<sub>magma</sub> -15311.5 (Eq. 5)

321
322 These relations defined over a SiO₂ range of 58.7-63.2 wt.% probably do not hold for rhyolites
323 for two reasons: (1) if the final crystallisation of rhyolites occurs mostly within the volcanic
324 edifice (~3 km depth), as suggested by Tepley et al. (2013), it is expected that rhyolites
325 assimilate older Misti rocks and not the Charcani gneiss (Arequipa Massif). In keeping with this
326 idea, Barreiro and Clark (1984) noted that the most contaminated El Misti rhyolite has a
327 ²⁰⁶Pb/²⁰⁴Pb ratio of 17.38, barely lower than the dacitic pumice sample MIS-02-10a (17.3965).
328 (2) Crustal assimilation markedly decreases during late stage crystallisation (Thompson et al.,
329 2002). Thus, the Pb isotope composition of gases is probably less sensitive to late stage
330 crystallisation of magmas at shallow depth, which may constitute the main limitation of the
331 model.

332
333 *Reconstructed mean composition of degassing magmas*

334
335 The model provides, as a general solution, the mean composition of degassing magmas. The
336 mean composition does not constrain the composition and distribution of heterogeneities (i.e.
337 multiple magma reservoirs) within the system. The mean composition of degassing magmas
338 can be reconstructed by combining degassing (Eg. 2) and AFC equations (Eqs. 4 and 5). From
339 Eqs. 2 and 4, we estimate the SiO₂ content of degassing magmas at 60.81 wt.% for the gas
340 condensate and at 61.50 wt% for the average of the gas sublimates. Using Eq. 5 based on the
341 ²⁰⁸Pb/²⁰⁶Pb ratio instead of Eq. 4 gives slightly higher SiO₂ concentrations of 61.19 wt.% and
342 61.81 wt%, respectively. In both cases, the gas sublimates record a more evolved composition
343 (+0.62 to +0.69 wt.% higher SiO₂) compared to the gas condensate. The 100 ppm analytical
344 uncertainty on the Pb isotope ratios of the fumaroles samples (Table 1) translates into a
345 relative uncertainty of less than 0.03 wt.% on the estimated SiO₂ values. The absolute error
346

on the SiO₂ estimates still remains controlled by the precision of the SiO₂ concentration data used in Eq. 4-5, which does not exceed 0.3 wt.%. In Fig. 6, the estimated SiO₂ concentrations of the degassing magmas are compared to the long-term evolution of the SiO₂ content of El Misti lavas. The results of the model can be directly used to trace the evolution through time of a single large magma reservoir (Fig. 6a).

347

348

349

350

351

352

## Degassing of two reservoirs of known composition

353

354

In the model of Tepley et al. (2013), an additional smaller rhyolitic magma reservoir transiently forms in the shallow crust (ca. 3km) every 2000 to 4000 years. It is not known whether such a rhyolite reservoir has existed since the last 2030-2000 years B.P. Plinian eruption. If it exists, one can envision that both deep andesite and shallow rhyolite magmas contribute to the gases escaping from El Misti crater. In this case, specific model solutions can be obtained considering that the Pb flux from each reservoir ($Φ$) might also contribute to the Pb isotope composition of the gas mixture at the surface according to Eq. 3, in addition to the contrasted compositions of the magma sources. In an end-member situation, the compositions of the magma reservoirs are constant and the composition of the gas is entirely controlled by varying $Φ$ (Fig. 6b). The model then allows the relative contributions of two reservoirs of known composition to be quantified. The $^{206}$Pb/$^{204}$Pb ratio of the andesite reservoir can be assumed to be the same as the least evolved scoria MIS-00-20 (17.8442). In the absence of published Pb data on rhyolites, the Pb composition of the rhyolite reservoir can be assumed to be the same as the dacitic pumice MIS-02-10a ($^{206}$Pb/$^{204}$Pb =17.3965) because, as argued above, late-stage differentiation of magmas within the edifice probably does not markedly modify Pb isotope composition. With these compositions, solving Eq. 3 yields $Φ_{andesite}=73\%$ and $Φ_{rhyolite}=27\%$ for the gas condensate, and $Φ_{andesite}=62\%$ and $Φ_{rhyolite}=38\%$ for the gas sublimates. In this model, the contribution of gaseous Pb from the rhyolite reservoir is about 10% higher in gas sublimates than in gas condensates.

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374

## Hidden evolution of the El Misti magma reservoir

375

376

### Evolution since the last eruption (AD 1440–1470)

377
The modelled composition (60.81-61.81 wt.% SiO₂) of magmas currently degassing at El Misti volcano is similar to the average composition of El Misti magmas (61.0 ± 3 wt.% SiO₂, n= 116, Rivera et al., 2017). The model results preclude highly evolved magmas, such as those sampled during the last Plinian eruption with >63 wt.% SiO₂ (Thouret et al., 2001; Tepley et al., 2013), being the dominant source of the 2018 gases. Yet, the modelled 2018 composition is slightly more evolved than that of the most recent lavas, in particular those forming the conduit plug with a SiO₂ content generally below 58.9 wt.% (Rivera et al., 2017) (Fig. 6). There are two possible interpretations of this shift depending on the interpretation chosen for the structure of the El Misti plumbing system. In the hypothesis of a single, large magma reservoir undergoing cycles of recharge and differentiation (Ruprecht and Wörner, 2007), the SiO₂ increase inferred here from Pb isotopes indicates that magma differentiation has been on average more important than magma recharge over the last few centuries (Fig. 6a). On the other hand, in the two-reservoir model of Tepley et al. (2013), one can assume that the andesite and rhyolite magmas preserve more or less constant compositions at the secular scale (Fig. 6b). The evolved composition of currently degassing magmas (relative to the lavas from the last eruption) can be explained by the contribution of gases from the shallow rhyolitic magmas to the main gas flux.

## Current evolution

The small Pb isotope difference between gas sublimates and gas condensate cannot reflect the simultaneous input of different gases at the same vent. Instead, this difference can be related to the fact that the condensate provides an instantaneous composition (at the time of sampling in October 2018) whereas the solid deposit records a composition averaged over the time of formation of the deposit. This time must be less than 48 years because the Na-Al sulfates and the Pb-rich minerals were not already formed in July 1967 (Birnie and Hall, 1974). The difference between the instantaneous (60.81-61.19 wt.% SiO₂) and time-integrated (61.50-61.81 wt.% SiO₂) SiO₂ estimates thus records the degassing of less evolved magma through time. A straightforward implication from the single magma reservoir hypothesis is that the magmatic reservoir is currently undergoing recharge by hot mafic magmas (Fig. 6a). In the two-reservoir model of Tepley et al. (2013), the current evolution of gas composition
409 could instead reflect the decreasing contribution of gases from the shallow rhyolitic reservoir,
410 possibility due to its progressive solidification (Fig. 6b).

411 The recharge hypothesis is independently supported by the mineralogy of the gas sublimates.
412 The Pb minerals (PbS, PbSO₄ and PbCl₂) occurring in the El Misti 2018 fumaroles deposits are
413 typically formed by the condensation of high-temperature magmatic gas (Symonds et al.,
414 1987; Taran et al., 2000; Africano et al., 2002). These gases can be similar to those sampled in
415 2015, with an equilibrium temperature of 532 °C and fO₂ equivalent to ΔQFM = +2.8
416 (Moussallam et al., 2017). In these conditions, galena forms first and is replaced by cotunnite
417 and anglesite as the temperature decreases and fO₂ increases. At El Misti, the occurrence of
418 PbSO₄ and PbCl₂ below 300°C requires little or no mixing of magmatic gas with air (Africano et
419 al., 2002). The occurrence of galena at the 260°C vent and cotunnite-anglesite at the 150°C
420 vent is thus consistent with closed-system condensation of the same high-temperature
421 parental gas that probably feeds the whole fumarolic field. The 2018 mineral assemblage
422 contrasts with the 1947 low temperature mineral suite (sulfur, anhydrite, gypsum and
423 ralstonite) that results from extensive gas rock interaction (Birnie and Hall, 1974). Part of this
424 difference might reflect sampling bias because Birnie and Hall (1974) did not sample
425 underground deposits whereas we did not sample sulfur and other minerals from low
temperature vents. However, the absence of high-temperature minerals in all five samples
426 collected in 1967 suggests overall colder degassing conditions in agreement with the lower
427 measured temperature (100-125°C). Fumarole deposits grow very slowly, on the order of
428 µm/day (Johnson and Burnett, 1993; Sainlot et al., 2020b) and average out short-term
429 fluctuations of degassing conditions. The difference in mineral assemblage between 1967 and
430 2018 is a robust observation that indicates a recent increase in gas temperature. The
431 important increase of gases temperature (>100°C) cannot reflect a comparable increase of
432 magma temperature but might reflect both a shallower and slightly hotter magma source, and
433 overall conditions severely limiting gas scrubbing, H₂SO₄ condensation and gas-rock
434 interaction. In summary, both sublimate mineralogy and Pb isotope composition of gas
435 support the idea of a recent hidden recharge of the shallow reservoir with hotter mafic
436 magmas. However, the inferred magma recharge and related temperature increase have not
437 been detected since the onset of thermal monitoring from space of the El Misti lava dome in
438 2000 (Moussallam et al., 2017), suggesting the recharge occurred before 2000. The recharge
439
perhaps could have been related to the last high fumarolic activity observed from Arequipa in 1984–1985 (Thouret et al., 2001).

442

443 *Implications for hazards and volcanic monitoring*

444

The activity of El Misti volcano is monitored from the height of the gas plume, edifice deformation, thermal emission, volcano-tectonic earthquakes, and the composition of geothermal waters. Collectively, the monitored parameters point to rather stable activity of the volcano at the decadal time scale. Moussallam et al. (2017) emphasized the need to also monitor major species of gas (H₂O, H₂, SO₂, H₂S), which allow for an estimation of gas-melt equilibrium temperature and oxygen fugacity. Here we suggest that the Pb isotopic composition of fumarolic minerals is less sensitive to short term change of degassing conditions and represents perhaps the most direct probe of recent magma composition. Monitoring the Pb isotope composition of El Misti gases would provide a window on the hidden cycles of magma recharge and differentiation, and would allow us to detect any departure from the steady-state regime that could potentially lead to an eruption.

456

457 **Conclusions**

458

We developed a new method to probe the hidden magmatic evolution of quiescent Andean volcanoes from the Pb isotope composition of gas emissions. The method is based on the properties of Pb isotopes that vary systematically during Assimilation - Fractional Crystallization (AFC) of magmas but do not fractionate during degassing. The method is applied to El Misti volcano that threatens the nearby city of Arequipa, the second most densely populated city of Peru.

Using this approach, we show that magmas currently degassing at El Misti volcano are on average more evolved than the centuries old lavas forming the conduit plug. This difference may reflect the secular differentiation of a single, large magma reservoir, or, alternatively, the contribution of a more evolved magma reservoir (possibly rhyolitic) to volcanic emanations. Conversely, the slight difference between the instantaneous composition recorded in gas condensate and the time-integrated composition recorded in solid deposits points to the degassing of less evolved magmas over the last decades. This trend indicates that El Misti

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471
472 magma reservoir may be currently undergoing recharge by hotter more mafic magmas,
473 consistent with the lack of Pb-rich sublimates and the predominance of low temperature
474 minerals in fumarole deposits collected half a century ago. This work suggests that the Pb
475 isotope composition of gas emissions is a promising tool for probing the hidden magmatic
476 evolution of quiescent volcanoes where AFC is the dominant process.
477 **Figure captions**

478

479 **Fig. 1 Location maps.** (a) Map of South America showing the location of El Misti Volcano in the Central Volcanic Zone (CVZ). Northern (NVZ) and southern (SVZ) volcanic zones are indicated.

480
481 (b) Satellite map of the El Misti area showing the location of Arequipa city. White line shows
482 the approximate boundary of Arequipa suburbs in 2021. Source: Google, Images©2021
483 TerraMetrics. (3) Photo of El Misti lava plug inside the crater taken in October 2018. The
484 locations and temperatures of the sampled fumaroles are indicated by arrows. The 120 m
485 wide lava dome is located at the bottom of the 200 m deep inner crater.

486

487 **Fig. 2 Images of the sublimates.** (a-c) 150°C sublimates: (a) Binocular loupe photo of a grain covered with PbCl₂ and PbSO₄. (b) Scanning Electron Microscopy (SEM) image of the same grain. (c) SEM image of a grain with CaSO₄.

489 grain. (c) SEM image of a grain with CaSO₄. (d-f) 260°C sublimates: (d) Macroscopic photo of
490 NaAl(SO₄)₂ deposit. (e) and (f): SEM images showing PbS coating NaAl(SO₄)₂ foam. All SEM
491 images were acquired at LMV using a JEOL JSM 5910LV microscope operating in Backscattered
492 Electron (BSE) imaging mode.

493

494 **Fig. 3 Trace element concentrations of El Misti fumarolic minerals and gas condensates.** Data
495 are shown for samples collected in 2018 (this work) and 1967 (Birnie and Hall, 1974). Trace
496 elements are sorted from left to right according to their decreasing abundances in the 260°C
497 condensate. Average concentrations are plotted for each sample type.

498

499 **Fig. 4 ²⁰⁶Pb/²⁰⁴Pb plotted against the Pb fraction unsupported by silicate input.** The silicate
500 unsupported Pb fraction (i.e. the fraction of Pb originating from the gas) is equal to 1 - 1/EF<sub>Pb</sub>,
501 where EF<sub>Pb</sub> is the Pb enrichment factor (see text). Here, EF<sub>Pb</sub> is calculated relative to titanium,
502 assuming that all titanium in gas samples originates from rocks. The absence of correlation in
503 this diagram argues against a control of rock particulates on the small Pb isotope variations.
504

505

505 **Fig. 5 Plot of SiO₂ versus ²⁰⁶Pb/²⁰⁴Pb (a) and ²⁰⁸Pb/²⁰⁶Pb (b) for El Misti lavas younger than 11**
506 ky (stage 4). This relation reflects the assimilation of unradiogenic Pb from the proterozoic
507 Charcani gneiss (Arequipa Massif) during fractional crystallization of El Misti magmas. Samples
508 include 6 ka old scoria bombs (MIS-00-20), two pumices from fall (quebrada San Lazaro) and
one dacitic pumice (MIS-02-10a) from the 2000 years B.P. eruption. Data are from Mamani et al. (2008) (Pb isotopes) and Rivera et al. (2017) (SiO₂ content). Errors bars are for analytical uncertainties. Best fit curves are shown. The single Pb data (²⁰⁶Pb/²⁰⁴Pb of 17.38) reported for El Misti rhyolites (Barreiro and Clark, 1984) suggests that rhyolites are not more contaminated than dacites (vertical arrows), consistent with the final differentiation of rhyolites within El Misti edifice (Tepley et al., 2013).

**Fig. 6** SiO₂ content of El Misti lavas and tephra plotted versus time. Data are from Thouret et al. (2001), Rivera et al. (2017) and Tepley et al. (2013). The star and shaded rectangle show the instantaneous and time-integrated SiO₂ contents of degassing magmas inferred from 2018 gas condensates and sublimates, respectively. (A) Single reservoir model (Ruprecht and Wörner 2007). In this model, arrows indicate the inferred evolution of the magma reservoir through time. Arrow 1 links the composition of the last erupted lavas (the conduit plug) with the inferred composition of the currently degassing magma, pointing to a secular differentiation trend. Arrow 2 links the time-integrated and instantaneous modelled SiO₂ contents. This trend suggests that the El Misti magma reservoir is currently undergoing recharge by hot mafic magmas, in agreement with the evolution of fumaroles minerals during the last half a century. (B) Two reservoir model (Tepley et al., 2013). In this model, emanations from El Misti crater result from mixing gases escaping from the andesite and rhyolite reservoirs (horizontal dashed arrows). In an end-member case, both reservoirs have more or less constant compositions through time (vertical arrows), and the composition of gases reflects varying contributions of the two magma reservoirs. In this model, the contribution of the rhyolite reservoir to the Pb gas flux is estimated at 38% over the last decades, and 27% currently (see text).
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<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /><figcaption>260°C sublimates</figcaption></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
2018 (this work)

<figure><img src="image_12.png" /></figure>

260°C sublimates
(PbS + NaAl(SO₄)₂)

<figure><img src="image_13.png" /></figure>

150°C sublimates
(PbCl₂ + PbSO₄ + CaSO₄)

<figure><img src="image_14.png" /></figure>

260°C condensates

1967 (Birnie and Hall, 1974)

<figure><img src="image_15.png" /></figure>

100-125°C Anhydrite

<figure><img src="image_16.png" /></figure>

100-125°C Gypsum

<figure><img src="image_17.png" /></figure>

100-125°C Ralstonite

<figure><img src="image_18.png" /></figure>

100-125°C condensates

Mass concentration

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /></figure>
AFC Misti 4

<figure><img src="image_21.png" /></figure>
Single reservoir

<figure><img src="image_22.png" /></figure>

Two reservoirs

<figure><img src="image_23.png" /></figure>
<table><caption>Table 1: Chemical and isotopic composition of 2018 El Misti condensates and sublimates</caption><thead><tr><th rowspan="3">Sample type<br>Temperature<br>Mineralogy</th><th rowspan="3">Gas condensate<br>260°C</th><th colspan="5">Gas sublimates<br>260°C<br>PbS + NaAl(SO4)2</th><th colspan="5">Gas sublimates<br>150°C<br>PbCl2 + PbSO4 + CaSO4</th></tr><tr><th>#a</th><th>#b</th><th>#c</th><th>#d</th><th>#e</th><th>#1</th><th>#3</th><th>#4</th><th>#5</th></tr><tr><th>ppb</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th><th>wt%</th></tr></thead><tbody><tr><td>Pb</td><td>177</td><td>3.80</td><td>4.90</td><td>2.01</td><td>2.82</td><td>9.86</td><td>16.8</td><td>36.3</td><td>32.4</td><td>44.5</td></tr><tr><td>Ti</td><td>45.0</td><td>6593</td><td>5734</td><td>10338</td><td>10649</td><td>4228</td><td>822</td><td>410</td><td>522</td><td>405</td></tr><tr><td>V</td><td>2.04</td><td>96</td><td>90</td><td>105</td><td>104</td><td>64</td><td>73</td><td>49</td><td>50</td><td>24</td></tr><tr><td>Cr</td><td>2.27</td><td>33</td><td>30</td><td>39</td><td>38</td><td>22</td><td>40</td><td>15</td><td>19</td><td>7</td></tr><tr><td>Mn</td><td>1.52</td><td>589</td><td>559</td><td>454</td><td>465</td><td>578</td><td>344</td><td>646</td><td>500</td><td>112</td></tr><tr><td>Cu</td><td>11.4</td><td>51</td><td>44</td><td>26</td><td>25</td><td>18</td><td>5</td><td>12</td><td>10</td><td>10</td></tr><tr><td>Zn</td><td>37.0</td><td>5393</td><td>6094</td><td>6842</td><td>7008</td><td>9363</td><td>1689</td><td>645</td><td>589</td><td>399</td></tr><tr><td>Rb</td><td>0.25</td><td>63</td><td>66</td><td>42</td><td>42</td><td>36</td><td>38</td><td>43</td><td>78</td><td>12</td></tr><tr><td>Sr</td><td>45.7</td><td>36</td><td>37</td><td>10</td><td>9</td><td>234</td><td>10</td><td>108</td><td>51</td><td>84</td></tr><tr><td>Zr</td><td>7.65</td><td>233</td><td>195</td><td>605</td><td>601</td><td>305</td><td>134</td><td>121</td><td>96</td><td>46</td></tr><tr><td>Nb</td><td>0.11</td><td>19</td><td>17</td><td>6</td><td>6</td><td>4</td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Cd</td><td>3.35</td><td>2141</td><td>2277</td><td>2158</td><td>2158</td><td>3306</td><td>2257</td><td>1304</td><td>1016</td><td>539</td></tr><tr><td>In</td><td>0.11</td><td>45</td><td>55</td><td>70</td><td>74</td><td>173</td><td>77</td><td>29</td><td>23</td><td>13</td></tr><tr><td>Sn</td><td>3.87</td><td>896</td><td>814</td><td>637</td><td>606</td><td>3160</td><td>138</td><td>51</td><td>51</td><td>35</td></tr><tr><td>Sb</td><td>8.74</td><td>9</td><td>9</td><td>2</td><td>2</td><td>3</td><td>0.2</td><td>0.6</td><td>0.4</td><td>0.6</td></tr><tr><td>Ba</td><td>244</td><td>34</td><td>33</td><td>6</td><td>6</td><td>18</td><td>39</td><td>26</td><td>411</td><td>260</td></tr><tr><td>Tl</td><td>38.3</td><td>5283</td><td>9479</td><td>7731</td><td>7868</td><td>4097</td><td>1349</td><td>234</td><td>142</td><td>514</td></tr><tr><td>Bi</td><td>11.7</td><td>2314</td><td>2210</td><td>3182</td><td>3441</td><td>4617</td><td>81</td><td>115</td><td>82</td><td>113</td></tr><tr><td>206Pb/204Pb<br>duplicate</td><td>17.7253<br>17.7234 r</td><td>17.6743<br>17.6738 d</td><td>17.6751</td><td>17.6764</td><td>17.6776</td><td>17.6778<br>17.6770 d</td><td>17.6764</td><td>17.6762</td><td>17.6753</td><td>17.6765</td></tr><tr><td>207Pb/204Pb<br>duplicate</td><td>15.5935<br>15.5910 r</td><td>15.5843<br>15.5838 d</td><td>15.5851</td><td>15.5853</td><td>15.5866</td><td>15.5866<br>15.5858 d</td><td>15.5824</td><td>15.5820</td><td>15.5818</td><td>15.5823</td></tr><tr><td>208Pb/204Pb<br>duplicate</td><td>38.6298<br>38.6264 r</td><td>38.5999<br>38.5985 d</td><td>38.6033</td><td>38.6048</td><td>38.6073</td><td>38.6076<br>38.6049 d</td><td>38.5929</td><td>38.5906</td><td>38.5898</td><td>38.5923</td></tr></tbody></table>

Pb isotope ratios are relative to the NBS 981 values of Todt et al. (1996)

r: duplicate run

d: duplicate chemistry