Michael J Heap
email: heap@unistra.fr

Valentin R Troll

Chris Harris

H Albert Gilg

Roberto Moretti

Marina Rosas- Carbajal

Jean-Christophe Komorowski

Patrick Baud

M Rosas-Carbajal

Whole-rock oxygen isotope ratios as a proxy for the strength and stiffness of hydrothermally altered volcanic rocks

Keywords: La Soufrière de Guadeloupe, Merapi, porosity, uniaxial compressive strength, tensile strength, Young

Hydrothermal alteration is considered to increase the likelihood of dome or flank collapse by compromising stability. Understanding how such alteration influences rock properties, and providing independent metrics for alteration that can be used to estimate these parameters, is therefore important to better assess volcanic hazards and mitigate risk. We explore the possibility of using whole-rock δ 18 O and δD values and water contents, metrics that can potentially track alteration, to estimate the strength (compressive and tensile) and Young's modulus (i.e. "stiffness") of altered (acid-sulfate) volcanic rocks from La Soufrière de Guadeloupe (Eastern Caribbean). The δ 18 O values range from 5.8 to 13.2‰, δD values from -151 to -44‰, and water content from 0.3 to 5.1 wt%. We find that there is a good correlation between δ 18 O values and laboratory-measured strength and Young's modulus, but that these parameters do not vary systematically with δD or water content (likely due to their pre-treatment at 200 °C). Empirical linear relationships that allow strength and Young's modulus to be estimated using δ 18 O values are provided using our new data and published data for Merapi volcano (Indonesia). Our study highlights that δ 18 O values can be used to estimate the strength and Young's modulus of volcanic rocks, and could therefore be used to provide parameters for volcano stability modelling. One advantage of this technique is that δ 18 O only requires a small amount of material, and can therefore provide rock property estimates in scenarios where material is limited, such as borehole cuttings or when sampling large blocks is impracticable.

Introduction

Geophysical techniques have revealed large subsurface hydrothermal systems within active volcanoes worldwide [START_REF] Aizawa | Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics[END_REF][START_REF] Finizola | Adventive hydrothermal circulation on Stromboli volcano (Aeolian Islands, Italy) revealed by geophysical and geochemical approaches: implications for general fluid flow models on volcanoes[END_REF][START_REF] Troll | Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Byrdina | Geophysical image of the hydrothermal system of Merapi volcano[END_REF][START_REF] Byrdina | Structure of the acid hydrothermal system of Papandayan volcano, Indonesia, investigated by geophysical methods[END_REF][START_REF] Ghorbani | Complex conductivity of volcanic rocks and the geophysical mapping of alteration in volcanoes[END_REF][START_REF] Finn | Three-dimensional geophysical mapping of shallow water saturated altered rocks at Mount Baker, Washington: Implications for slope stability[END_REF][START_REF] Ahmed | 3D electrical conductivity tomography of volcanoes[END_REF][START_REF] Tseng | Anatomy of active volcanic edifice at the Kusatsu-Shirane volcano, Japan, by magnetotellurics: hydrothermal implications for volcanic unrests[END_REF][START_REF] Kereszturi | Synthesis of hydrothermal alteration, rock mechanics and geophysical mapping to constrain failure and debris avalanche hazards at Mt. Ruapehu (New Zealand)[END_REF][START_REF] Finn | Geophysical imaging of the Yellowstone hydrothermal plumbing system[END_REF]. The hydrothermal fluids circulating within these systems can physically and chemically alter the rocks [START_REF] Browne | Hydrothermal alteration in active geothermal fields[END_REF], which is thought to increase the likelihood of potentially devastating dome or partial edifice flank collapse by compromising their stability [START_REF] Day | Hydrothermal pore fluid pressure and the stability of porous, permeable volcanoes[END_REF][START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF][START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF][START_REF] Voight | The 1997 Boxing Day sector collapse and debris avalanches, Soufrière Hills volcano, Montserrat, WI. The Eruption of Soufriere Hills Volcano[END_REF][START_REF] Reid | Massive collapse of volcano edifices triggered by hydrothermal pressurization[END_REF][START_REF] Cecchi | Flank spreading and collapse of weak-cored volcanoes[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF][START_REF] Ball | The hydrothermal alteration of cooling lava domes[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Ball | Combining multiphase groundwater flow and slope stability models to assess stratovolcano flank collapse in the Cascade Range[END_REF][START_REF] Mordensky | Influence of alteration on the mechanical behaviour and failure mode of andesite: implications for shallow seismicity and volcano monitoring[END_REF]Heap et al., 2021a, b;[START_REF] Harnett | Mechanical and topographic factors influencing lava dome growth and collapse[END_REF][START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF][START_REF] Mordensky | Hydrothermally induced edifice destabilisation: The mechanical behaviour of rock mass surrounding a shallow intrusion in andesitic lavas, Pinnacle Ridge, Ruapehu[END_REF]. Dome or edifice collapse can suddenly decompress pressurised hydrothermal systems or gas-rich magma present at shallow depth hence triggering the formation of devastating highly mobile high-energy pyroclastic density currents (blasts) exemplified by the classic 1980 eruption of Mount St. Helens (USA) as well as other eruptions of varying magnitudes [START_REF] Lipman | The 1980 eruptions of Mount St[END_REF][START_REF] Voight | Catastrophic rockslide avalanche of May 18[END_REF][START_REF] Hoblitt | Origin and stratigraphy of the deposit produced by the May 18 directed blast[END_REF][START_REF] Sparks | Generation of a debris avalanche and violent pyroclastic density current on 26 December (Boxing Day) 1997 at Soufriere Hills Volcano, Montserrat[END_REF][START_REF] Voight | The 1997 Boxing Day sector collapse and debris avalanches, Soufrière Hills volcano, Montserrat, WI. The Eruption of Soufriere Hills Volcano[END_REF][START_REF] Boudon | Martinique[END_REF][START_REF] Belousov | Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits[END_REF]Lube et al., 2012;[START_REF] Komorowski | Paroxysmal dome explosion during the Merapi 2010 eruption: processes and facies relationships of associated high-energy pyroclastic density currents[END_REF]. As a result, the monitoring of hydrothermal alteration at active volcanoes, and understanding how alteration influences rock physical and mechanical properties, are important to better assess volcanic hazards and mitigate risk.

Previous experimental studies have shown that hydrothermal alteration can decrease [START_REF] Del Potro | The decrease in the shear strength of volcanic materials with argillic hydrothermal alteration, insights from the summit region of Teide stratovolcano, Tenerife[END_REF][START_REF] Frolova | Effects of hydrothermal alterations on physical and mechanical properties of rocks in the Kuril-Kamchatka island arc[END_REF][START_REF] Wyering | Mechanical and physical properties of hydrothermally altered rocks, Taupo Volcanic Zone, New Zealand[END_REF][START_REF] Mayer | Hydrothermal alteration of surficial rocks at Solfatara (Campi Flegrei): Petrophysical properties and implications for phreatic eruption processes[END_REF][START_REF] Farquharson | Acid-induced dissolution of andesite: evolution of permeability and strength[END_REF]Heap et al., 2020aHeap et al., , 2021a;;[START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF]Heap et al., 2022a) or increase [START_REF] Marmoni | Gravitational slope-deformation of a resurgent caldera: New insights from the mechanical behaviour of Mt. Nuovo tuffs (Ischia Island, Italy)[END_REF]Heap et al., 2020aHeap et al., , b, 2021b) ) the strength and Young's modulus of volcanic rocks. Whether alteration decreases or increases strength or Young's modulus is thought to depend on (1) whether the alteration increases or decreases the porosity of the rock, a factor known to exert a first-order control on the strength and Young's modulus of volcanic rocks and (2) whether the alteration minerals are weaker/softer or stronger/stiffer than the primary mineral assemblage (Heap et al., 2020a;[START_REF] Heap | The mechanical behaviour and failure modes of volcanic rocks: a review[END_REF][START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF]. [START_REF] Heap | Hydrothermal alteration can result in pore pressurization and volcano instability[END_REF] recently suggested that both porosity-increasing and porosity-decreasing alteration could jeopardise volcano stability. These authors argued that porosity-increasing alteration affects volcano stability by reducing rock strength and stiffness, whereas porosity-decreasing alteration reduces permeability and increases pore fluid pressure [START_REF] Heap | Hydrothermal alteration can result in pore pressurization and volcano instability[END_REF].

A step change in volcano hazard monitoring is possible if rock physical and mechanical properties can be accurately estimated using an independent metric that quantifies alteration. Recently, Heap et al. (2021aHeap et al. ( , 2022a) ) showed that the strength (compressive and tensile) and Young's modulus of variably altered rocks from La Soufrière de Guadeloupe (Eastern Caribbean) can be estimated using the amount of alteration minerals in wt%, quantified using X-ray powder diffraction (XRPD) data. Using Mt. Ruapehu (New Zealand) as a case study, [START_REF] Schaefer | Determining physical and mechanical volcanic rock properties via reflectance spectroscopy[END_REF] showed that it is possible to estimate the physical and mechanical properties of volcanic rocks using rapid, non-invasive reflectance spectroscopy measurements. More recently, [START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF] found that the compressive strength of variably altered rocks from Merapi volcano (Indonesia) can be estimated using whole-rock δ 18 O values.

Whole-rock δ 18 O values can serve as an indicator for alteration because hydrothermal processes often entail an exchange of oxygen isotopes between solid igneous mineral phases and circulating fluids, or the formation of new minerals such as clay minerals or sulfates (Taylor, 1974;[START_REF] Hansteen | Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas[END_REF][START_REF] Donoghue | Low-temperature hydrothermal alteration of intra-caldera tuffs, Miocene Tejeda caldera, Gran Canaria, Canary Islands[END_REF][START_REF] Berg | Exceptionally high whole-rock δ18O values in intra-caldera rhyolites from Northeast Iceland[END_REF]. We highlight that magmatic variations in δ 18 O values are typically small and pristine igneous rocks usually have δ 18 O ratios between 5 and 8‰ (Taylor 1974;[START_REF] Bindeman | Oxygen isotopes in mantle and crustal magmas as revealed by single crystal analysis[END_REF][START_REF] Deegan | Sunda arc mantle source δ 18 O value revealed by intracrystal isotope analysis[END_REF]. Indeed, variations due to closed-system fractional crystallisation are usually <1‰ [START_REF] Bindeman | Oxygen isotopes in mantle and crustal magmas as revealed by single crystal analysis[END_REF]. Whole-rock δ 18 O values generally increase in volcanic rocks as a function of relatively low temperature hydrothermal alteration (≤ 250 °C), whereas higher temperatures (≥ 400 °C) or isotopically very light meteoric waters usually tend to decrease the δ 18 O of the mineral assemblage (Taylor, 1974;[START_REF] Rose | Oxygen isotope evidence for hydrothermal alteration within a Quaternary stratovolcano, Lassen Volcanic National Park, California[END_REF][START_REF] Donoghue | Hydrothermal alteration of the Miocene Tejeda Intrusive Complex, Gran Canaria, Canary Islands: insights from petrography, mineralogy and O-and H-isotope geochemistry[END_REF][START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF]. Here, we further explore the relationship between whole-rock δ 18 O values, as well as δD values and water contents, and the strength (compressive and tensile) and Young's modulus of hydrothermally altered volcanic rocks. The goal of this contribution is to test whether isotopic compositions and water contents of hydrothermally altered (acid-sulfate) volcanic rocks can be used to estimate their physical and mechanical properties.

Materials and Methods

A suite of 19 variably altered rocks from La Soufrière de Guadeloupe, an active andesitic stratovolcano located on the French island of Guadeloupe in the Eastern Caribbean [START_REF] Komorowski | Volcanic Atlas of the Lesser Antilles[END_REF][START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF], were used for this study (sampling locations are shown in Fig. 1).

Volcanic unrest at La Soufrière de Guadeloupe has increased since its reawakening in 1992. This unrest is manifest as the expansion of the hot outgassing area at the top of the current lava dome (which formed in CE 1530), the appearance of steam-dominated fumaroles and acid chloride-sulfate springs, an increase in summit and flank displacement rates, an increase in the heat output from the dome, an abundance of shallow seismicity, and, in April 2018, the largest felt tectonic earthquake since the last eruption in 1976-1977 [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF][START_REF] Villemant | The memory of volcanic waters: shallow magma degassing revealed by halogen monitoring in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles)[END_REF][START_REF] Villemant | Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids[END_REF][START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF][START_REF] Jessop | A multidecadal view of the heat and mass budget of a volcano in unrest: La Soufrière de Guadeloupe (French West Indies)[END_REF]Heap et al., 2021a;[START_REF] Moune | Gas monitoring of volcanic-hydrothermal plumes in a tropical environment: the case of La Soufriere de Guadeloupe unrest volcano (Lesser Antilles)[END_REF]. The link between the stability of the volcano and a combination of hydrothermal alteration resulting from hydrothermal circulation and chemical weathering due to the tropical environment has been underscored in several contributions [START_REF] Komorowski | Volcanic Atlas of the Lesser Antilles[END_REF][START_REF] Friant | Potential flank-collapse of Soufriere Volcano, Guadeloupe, lesser Antilles? Numerical simulation and hazards[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Peruzzetto | Modeling of partial dome collapse of La Soufrière of Guadeloupe volcano: implications for hazard assessment and monitoring[END_REF]Heap et al., 2021a;[START_REF] Moretti | The Basse-Terre Island of Guadeloupe (Eastern Caribbean, France) and Its Volcanic-Hydrothermal Geodiversity: A Case Study of Challenges, Perspectives, and New Paradigms for Resilience and Sustainability on Volcanic Islands[END_REF][START_REF] Metcalfe | Magmatic Processes at La Soufrière de Guadeloupe: Insights From Crystal Studies and Diffusion Timescales for Eruption Onset[END_REF][START_REF] Moune | Gas monitoring of volcanic-hydrothermal plumes in a tropical environment: the case of La Soufriere de Guadeloupe unrest volcano (Lesser Antilles)[END_REF]. As a result, La Soufrière de Guadeloupe represents an ideal natural laboratory to study the influence of hydrothermal alteration on rock physical and mechanical properties and volcano stability. Indeed, La Soufrière of Guadeloupe is characterised by an exceptional recurrence of partial edifice collapse with at least nine flank collapses that have occurred in the last 9150 years, the last of which occurred during the 1530 CE eruption [START_REF] Komorowski | Volcanic Atlas of the Lesser Antilles[END_REF][START_REF] Boudon | Volcano flank instability in the Lesser Antilles Arc: Diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Komorowski | A new Holocene eruptive chronology for la Soufrière de Guadeloupe volcano: implications for credible scenario definition as well as hazard and impact modelling, Colima, Mexico: Cities on Volcanoes[END_REF][START_REF] Legendre | Reconstruction fine de l'histoire éruptive et scenarii éruptifs à la soufrière de Guadeloupe: vers un modèle intégré de fonctionnement du volcan[END_REF][START_REF] Peruzzetto | Modeling of partial dome collapse of La Soufrière of Guadeloupe volcano: implications for hazard assessment and monitoring[END_REF]. Evidence from the geological record indicates that the frequency of partial edifice collapse in the last 9150 years has increased compared to older eruptive episodes of the Grande Découverte volcanic complex, although the collapse volume has decreased. Finally, there is a high probability that edifice collapse will trigger laterally-directed explosions (between two and five of the eight edifice collapses of the last 8500 years generated laterally-directed blasts) [START_REF] Komorowski | Volcanic Atlas of the Lesser Antilles[END_REF][START_REF] Boudon | Volcano flank instability in the Lesser Antilles Arc: Diversity of scale, processes, and temporal recurrence[END_REF][START_REF] Komorowski | A new Holocene eruptive chronology for la Soufrière de Guadeloupe volcano: implications for credible scenario definition as well as hazard and impact modelling, Colima, Mexico: Cities on Volcanoes[END_REF][START_REF] Legendre | Reconstruction fine de l'histoire éruptive et scenarii éruptifs à la soufrière de Guadeloupe: vers un modèle intégré de fonctionnement du volcan[END_REF][START_REF] Peruzzetto | Modeling of partial dome collapse of La Soufrière of Guadeloupe volcano: implications for hazard assessment and monitoring[END_REF].

All of the 19 blocks analysed in this study are sourced from either coherent lava blocks or coherent lavas (i.e. we did not collect blocks of, for example, breccia). Of the 19 blocks collected, nine were collected from a collapse scar to the northeast of dome summit (blocks H2A, H2B, H3, H4A, H5A, H6, H25, H29, and H30). Five blocks were collected from the dome summit: four blocks were collected from the lava spines that protrude the dome (two blocks from Cratère Sud Central, H19 and H20, and two blocks from an adjacent site, H21 and H22), and one block was collected from the wall of the Lacroix Supérieur outgassing fracture on the lava dome (H18).

A block was collected to the southwest of dome summit from a collapse scar into a highly fractured lava that forms the core of a paleo-collapse mega-block of the former volcanic edifice (WP1285). Blocks were also collected from the West wall of the fault "Faille 30 août" on the lava dome (H14 and H15), and from a thick lava adjacent to the Galion waterfall not associated with the CE 1530 dome (H32). The final block, a volcanic non-juvenile bomb from the dome that was ejected during the 1976-1977 explosive eruption [START_REF] Komorowski | Volcanic Atlas of the Lesser Antilles[END_REF], was taken from the roof of a small disused thermal bathhouse to the South of the dome summit (WP1317). These rocks, previously described by Heap et al. (2021aHeap et al. ( , 2022a, b), b), are andesites characterised by a porphyritic texture comprising magmatic phenocrysts of dominantly plagioclase and pyroxene (orthopyroxene and clinopyroxene) within a microcrystalline groundmass. The mineral assemblage present in each block was identified by a combination of optical microscopy, Raman spectroscopy, and X-ray powder diffraction (XRPD), and quantitative phase analysis was performed using the XRPD data and Rietveld program BGMN [START_REF] Bergmann | BGMN-a new fundamental parameters based Rietveld program for laboratory X-ray sources, its use in quantitative analysis and structure investigations[END_REF] (for more details see Heap et al., 2021aHeap et al., , 2022a, b), b). The XRPD data show that all of the rocks contain variable quantities of secondary (alteration) minerals: kaolinite, alunite or natro-alunite, silica polymorphs (quartz, cristobalite, tridymite, and opal-A), hematite, pyrite, gypsum, and talc (Heap et al. 2021a(Heap et al. , 2022a, b, b; Table 1). The predominant hydrous alteration phases are kaolinite, natro-alunite, and opal-A in these materials (Table 1) suggesting fluid-rock interaction with acidic sulfate-chloride-rich fluids at relatively low temperatures (< 150-200 °C) [START_REF] Inoue | Formation of clay minerals in hydrothermal environments[END_REF][START_REF] Zimbelman | Origin of secondary sulfate minerals on active andesitic stratovolcanoes[END_REF][START_REF] Scher | Fumarolic activity, acid-sulfate alteration, and high sulfidation epithermal precious metal mineralization in the crater of Kawah Ijen Volcano, Java, Indonesia[END_REF][START_REF] Fulignati | Clay minerals in hydrothermal systems[END_REF]Heap et al., 2021a). Indeed, theses rocks do not contain smectite, which is not stable in highly acidic (low pH) environments. High-temperature acidic alteration minerals, such a pyrophyllite, dickite, diaspore, zunyite, or topaz, are notably absent, and talc was observed in only one sample (H22) in relatively low amounts (Table 1). The alteration intensity of the 19 blocks from La Soufrière de Guadeloupe has been quantified in previous contributions by the weight percentage (wt%) of secondary (i.e. alteration) minerals (Heap et al., 2021a(Heap et al., , 2022a, b) , b) and these data are available in Table 2.

In this contribution, we relate new whole-rock oxygen and hydrogen isotope ratio data, and H2O concentrations, with previously published data for the physical and mechanical properties of rocks from La Soufrière de Guadeloupe. The uniaxial compressive strength (UCS), indirect tensile strength (ITS), and Young's modulus values for multiple cylindrical samples prepared from the blocks collected from La Soufrière de Guadeloupe were published in previous contributions by Heap et al. (2021aHeap et al. ( , 2022a)). UCS was measured on ovendry cylindrical samples in a uniaxial load frame using a constant axial strain rate of 10 -5 s -1 (Heap et al., 2021a).

Young's modulus, an elastic constant that describes the "stiffness" of a material, was then calculated from the pseudo-linear elastic portion of the resultant uniaxial stress-strain curves (Heap et al., 2021a). ITS was measured on oven-dry discs, deformed diametrically in compression using a uniaxial load frame and a constant displacement rate of 0.025 mm. s -1 (Heap et al., 2022a). UCS (and therefore Young's modulus) and ITS were measured at ambient laboratory pressure and temperature. These studies concluded that hydrothermal alteration, associated with mineral dissolution, weak secondary minerals (such as clays), and an increase in microstructural heterogeneity, resulted in a reduction in compressive strength (Heap et al., 2021a), tensile strength (Heap et al., 2022a), and Young's modulus (Heap et al., 2021a).

Offcuts from the 19 blocks were crushed and powdered by hand, to a grain size of << 1 mm, using a ceramic pestle and mortar. The whole-rock oxygen isotope ratios of these samples were measured at the University of Cape Town (South Africa) using a Thermo DeltaXP mass spectrometer (data unique to this study). Aliquots of ~10 mg of the whole rock powders were dried overnight at 50 °C and then under vacuum in nickel reaction vessels, then reacted with 30 kPa of CIF3 for ~4 hours to extract the oxygen from silicates [START_REF] Borthwick | A note regarding CIF3 as an alternative to BrF5 for oxygen isotope analysis[END_REF].

The extracted oxygen was then converted to CO2 by passing it over a high-temperature platinised carbon rod. For full analytical details see [START_REF] Vennemann | The rate and temperature of reaction of CIF3 with silicate minerals, and their relevance to oxygen isotope analysis[END_REF] and [START_REF] Harris | Oxygen isotope composition of garnet in the Peninsula Granite, Cape Granite Suite, South Africa: constraints on melting and emplacement mechanisms[END_REF]. Unknowns were run with duplicates of the internal quartz standard (MQ), which was used to calibrate the raw data to the SMOW (Standard Mean Ocean Water) scale, using a δ 18 O value of 10.1 for MQ (calibrated against NBS-28). The results are reported in standard δ-notation, where δ = (Rsample/Rstandard -1) × 1000, Rsample is 18 O/ 16 O in the sample, and Rstandard is 18 O/ 16 O relative to SMOW [START_REF] Gonfiantini | Standards for stable isotope measurements in natural compounds[END_REF]. The analytical reproducibility (assumed here to be similar to the error) is estimated as ± 0.2‰ (2 sigma), based on long-term repeated analysis of MQ.

Hydrogen isotopes and water contents of the same powdered separates were determined using the method of [START_REF] Vennemann | A simple and inexpensive method of hydrogen isotope and water analyses of minerals and rocks based on zinc reagent[END_REF]. Samples were first melted in quartz glass tubes using a propane torch. The raw data were normalised to SMOW, and corrected for compression, using the water standards RMW (δD = -131.4‰) and CTMP2010 (δD = -7.4‰) with the in-house Serina kaolinite standard analysed with the unknowns (Serina bulk kaolinite, δD = -57‰, H2O = 12.4 wt%; [START_REF] Harris | Oxygen and hydrogen isotope composition of kaolinite deposits, Cape Peninsula, South Africa; low-temperature, meteoric origin[END_REF]. The measured δD values of the standard gave an average of -59.5 ± 3.6‰ (2σ, n = 3) after corrections to the raw data. All data were adjusted to the accepted Serina kaolinite value. Water abundances (as H2O + ) were measured from the voltage on the mass 2 collector of the mass spectrometer. This was calibrated against measured volumes of standard waters analysed alongside the unknowns. The measured water content of Serina kaolinite gave 12.4 wt% (± 0.5 wt% 2σ, n = 3) (pure kaolinite has an expected water content of 13.95 wt%).

We compare our new data with those for Merapi volcano, collected using the same equipment and techniques described above. For the samples from Merapi volcano, the compressive and tensile strength data were published in [START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF] and Heap et al. (2022a), respectively, the alteration data (the wt% of secondary minerals, mostly sulfates) were published in [START_REF] Heap | Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour[END_REF]Heap et al. ( , 2022a)), and the δ 18 O values were published in [START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF]. We have calculated the Young's modulus for the experiments presented in [START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF] using the pseudo-linear elastic portion of the uniaxial stress-strain curves (these data were not presented in [START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF].

Results

We find that δ 18 O values vary from 5.8 to 13.2‰, that δD values vary from -151 to -44‰, and that water contents range from 0.3 to 5.1 wt% for our samples from La Soufrière de Guadeloupe (Table 2). The δ 18 O values, δD values, and water content data are plotted in Fig. 2 as a function of secondary alteration mineral content in wt% (Table 2), a metric used in previous studies of these materials (Heap et al., 2021a(Heap et al., , 2022a, b), b). Additionally, we labelled samples with a high content of sulfates (≥ 2 wt% mostly natro-alunite, rarely alunite; Table 1) and opal-A (≥ 10 wt%; Table 1).

Although there is scatter in the data, the δ 18 O values positively correlate with the wt% of secondary minerals (Fig. 2a). We note that the two samples rich in opal-A (H20 and H21) plot slightly above the trend delineated by the other data (Fig. 2a). In contrast, no simple correlation is observed for the δD values or the water contents as a function of increasing wt% of secondary minerals (Figs. 2b and2c, respectively). We highlight that samples with δD values much higher or lower than the other samples are typically samples with a high sulfate content (Figs. 2b). To some extent, this observation also applies to the water content data (Fig. 2c). Fig. 3 shows the uniaxial compressive strength, tensile strength, and Young's modulus for samples from La Soufrière de Guadeloupe as a function of δ 18 O value, δD value, and water content. All these data are provided in an Excel® spreadsheet that accompanies this contribution as Supplementary Information. The data of Fig. 3 show that strength (uniaxial and tensile) and Young's modulus decrease as a function of increasing δ 18 O (Figs. 3a, 3d, and 3g), but it is more difficult to discern singular trends in the δD and water content data (Figs. 3b,3c,3e,3f,3h,and 3i).

Discussion

Our data show that the δ 18 O values of variably altered rocks from La Soufrière de Guadeloupe increase as a function of increasing alteration (Fig. 2a), as expected for low temperature hydrothermal alteration (≤ 250 °C) (Taylor, 1974;[START_REF] Rose | Oxygen isotope evidence for hydrothermal alteration within a Quaternary stratovolcano, Lassen Volcanic National Park, California[END_REF][START_REF] Donoghue | Hydrothermal alteration of the Miocene Tejeda Intrusive Complex, Gran Canaria, Canary Islands: insights from petrography, mineralogy and O-and H-isotope geochemistry[END_REF][START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF]. The dominance of kaolinite, very fine-grained natro-alunite, and amorphous silica, and the absence of smectite (Table 1), suggests alteration at temperatures below 150-200 °C in a steam-heated acid sulfate alteration environment [START_REF] Zimbelman | Origin of secondary sulfate minerals on active andesitic stratovolcanoes[END_REF][START_REF] Rye | A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems[END_REF]. The least altered samples have stable oxygen isotope compositions (5.8 to 7.3‰) similar to unaltered andesites from La Soufrière de Guadeloupe and other volcanoes in the Eastern Caribbean [START_REF] Davidson | Mechanisms of contamination in Lesser Antilles island arc magmas from radiogenic and oxygen isotope relationships[END_REF][START_REF] Davidson | Oxygen isotope constraints on the petrogenesis of volcanic arc magmas from Martinique, Lesser Antilles[END_REF][START_REF] Van Soest | Resolving sediment subduction and crustal contamination in the Lesser Antilles Island Arc: a combined He-O-Sr isotope approach[END_REF]. The highest values of about 12 ± 1‰, in turn, are recorded for the opal-dominated altered rocks. This is consistent with the larger oxygen isotope fractionation between amorphous silica and water [START_REF] Kita | Oxygen isotope fractionation between amorphous silica and water at 34-93 °C[END_REF] compared to alunite-water [START_REF] Stoffregen | Experimental studies of alunite: I. 18O-16O and DH fractionation factors between alunite and water at 250-450 C[END_REF] and kaolinitewater [START_REF] Sheppard | Stable isotope geochemistry of clay minerals:"The story of sloppy, sticky, lumpy and tough" Cairns-Smith[END_REF]. The observed positive correlation between δ 18 O values and percentage of alteration minerals in our analysed suite can thus be interpreted as a mixing line between primary igneous isotope signatures and secondary alteration phases. The scatter in the data can be explained by (1) the diversity of alteration phases with their distinct oxygen isotope fractionation factors with water, (2) potential temperature variations, (3) variable isotope compositions of the involved fluids, and (4) the location and the type of processes involved in the alteration (e.g., alteration within the host-rock or deep-seated regions of the dome by low temperature steam circulation of hydrothermal fluids in the roots of surficial fumaroles versus surficial alteration within a steaming and cooling dome following emplacement).

In contrast to the δ 18 O values, no simple correlation is observable for the δD values or water contents as a function of increasing wt% of secondary minerals (Figs. 2b and2c, respectively). This result is surprising due to the very minor amount of primary hydrogen in these rocks, and the fact that the dominant hydrous alteration phases (alunite group minerals, kaolinite) have similar nominal water contents. Based on the spurious data for samples containing sulfates and/or opal-A (Figs. 2b and2c), we suspect that the pre-treatment procedure (heating to 200 °C in vacuum) probably led to significant water loss in the amorphous silica phase [START_REF] Brandriss | Oxygen isotope fractionation between diatomaceous silica and water[END_REF]Martin and Gailliou, 2018) and that future studies should seek alternative methods to measure similar samples. Indeed, if we consider only the samples that contain minor or no sulfates and opal-A (the black symbols in Fig. 2), we observe positive correlations between the δD values and water contents as a function of increasing wt% of secondary minerals (Figs. 2b and 2c,respectively). This may suggest that our applied extraction technique for hydrogen isotope analysis of (Na-)alunite-rich samples has released sulphur-bearing volatiles that reacted with the dehydroxylation water leading to unreliable water contents and, most probably, hydrogen isotope data (see Rye et al., 1992). Further, for stable hydrogen isotopes, no simple correlation with the percentage of secondary alteration should be expected, as no igneous hydrous minerals were observed even in the least altered samples and the hydrogen isotope composition of the altered rocks will depend on the mineral type, temperature, and water isotope composition rather than simply on the wt% of secondary phases. We further note that the hydrogen isotope compositions of the steam-heated acid chloride-sulfate waters will be influenced by recurrent evaporation and condensation processes and thus may be quite variable [START_REF] Berg | Exceptionally high whole-rock δ18O values in intra-caldera rhyolites from Northeast Iceland[END_REF], including positive d 18 O-dD pairs, as also observed for other volcanoes in the Eastern Caribbean [START_REF] Chiodini | Fluid geochemistry of Montserrat Island, West Indies[END_REF][START_REF] Joseph | Gas and water geochemistry of geothermal systems in Dominica, Lesser Antilles island arc[END_REF]. Furthermore, the D/H fractionation factors between some of the dominant hydrous phases and water are either less well constrained (e.g., alunite; [START_REF] Stoffregen | Experimental studies of alunite: I. 18O-16O and DH fractionation factors between alunite and water at 250-450 C[END_REF] or even unknown (e.g., amorphous silica).

Although there are some outliers, Figs. 3a,3d, and 3g show that the uniaxial compressive strength, tensile strength, and Young's modulus of variably altered rocks from La Soufrière de Guadeloupe and Merapi volcano decrease as a function of increasing δ 18 O value. Therefore, strength and Young's modulus are decreasing as a function of increasing alteration, as discussed in Heap et al. (2021a) and [START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF]. However, we find little to no correlation between strength and Young's modulus and the δD value (Figs. 3b,3e,and 3h) or water content (Figs. 3c,3f,and 3i), likely due to the pre-treatment of the samples at 200 °C. As noted above, the δD values are not related to the wt% of secondary alteration phases, and thus alteration intensity, and the measured water contents depend on the alteration mineral type. Therefore, we will focus here on using δ 18 O as a proxy for strength and Young's modulus. Although we focus here on δ 18 O only, we do not rule out the possibility of using δD and/or water contents to predict rock properties in the future, as long as alternate laboratory methods are used (those that avoid pre-treating the samples at 200 °C, but can remove extraneous absorbed water).

Figs. 4a, 4b, and4c show uniaxial compressive strength, tensile strength, and Young's modulus, respectively, as a function of δ 18 O. The outliers in these data, shown as grey circles in Fig. 4, include samples from blocks H14, H29, and H32. Blocks H14 and H29 contained mesoscale fractures, which could explain their low strength and Young's modulus (Heap et al., 2021a(Heap et al., , 2022a)). Block H32, which is much stronger than the other blocks (Fig. 4), was collected from an older (K-Ar age 0.079 ± 0.003 Ma; [START_REF] Carlut | Absolute paleointensities recorded during the Brunhes chron at La Guadeloupe Island[END_REF], lowporosity, and very thick lava adjacent to the Galion waterfall (Fig. 1). Therefore, the alteration history of this block differs from the other blocks collected on, within, or adjacent to, the present-day dome that was emplaced during the 1530 CE eruption nearly 500 years ago (Fig. 1). Excluding these outliers, we provide best-fit linear functions to the entire dataset (La Soufrière de Guadeloupe and Merapi volcano) in Fig. 4. Uniaxial compressive strength, 𝜎 ! , indirect tensile strength, 𝜎 " , and Young's modulus, 𝐸, can therefore be estimated using the following empirical equations (where strength is in MPa, Young's modulus in GPa, and δ 18 O in ‰):

𝜎 ! = -13.8(𝛿 #$ O) + 204.7 (1) 𝜎 " = -0.70(𝛿 #$ O) + 13.1 (2) 𝐸 = -3.42(𝛿 #$ O) + 54.0 (3)
Estimates of the strength (compressive and tensile) and Young's modulus of volcanic rocks are required in a range of laboratory-and large-scale modelling designed to understand, respectively, the mechanical behaviour of volcanic rocks and the stability of volcanic flanks and domes [START_REF] Watters | Rock mass strength assessment and significance to edifice stability, Mount Rainier and Mount Hood, Cascade Range volcanoes[END_REF]Okubu, 2004;[START_REF] Apuani | Stability of a collapsing volcano (Stromboli, Italy): Limit equilibrium analysis and numerical modelling[END_REF][START_REF] Moon | Geotechnical characterisation of stratocone crater wall sequences, White Island Volcano, New Zealand[END_REF]del Potro and Hürlimann, 2008;Heap et al., 2021a, b;[START_REF] Harnett | Mechanical and topographic factors influencing lava dome growth and collapse[END_REF][START_REF] Wallace | Material Properties and Triggering Mechanisms of an Andesitic Lava Dome Collapse at Shiveluch Volcano, Kamchatka, Russia, Revealed Using the Finite Element Method[END_REF]. We show here that rock physical and mechanical property estimates for volcano stability modelling, important assess volcanic hazards and mitigate risk, can be provided using Eqs. ( 1), (2), and (3) by measuring the δ 18 O values of representative samples collected from the flank or dome of a volcano. Composite δ 18 O-value crosssections of volcanoes [START_REF] Rose | Oxygen isotope evidence for hydrothermal alteration within a Quaternary stratovolcano, Lassen Volcanic National Park, California[END_REF] and δ 18 O values determined from borehole drill cuttings [START_REF] Hattori | Oxygen isotope ratios of the Icelandic crust[END_REF] could also be used to prepare strength and Young's modulus maps and profiles, respectively.

Unlike laboratory experiments, which require reasonably large blocks from which to prepare the experimental samples, the approach to estimate strength and Young's modulus outlined herein, i.e. Eqs. (1), (2), and (3), requires only < 1 g of material (as representative as possible; e.g., using a < 1 g aliquot of a larger mass of well-mixed powdered material, if available) and is thus ideally suited to provide rock physical and mechanical property estimates when only limited material is available, such as drill cuttings form boreholes or when sampling large blocks is impracticable.

The three-dimensional structures and rock property distributions inside volcanic systems are usually inferred using targeted geophysical methods. Electrical methods, such as magnetotellurics and electrical resistivity tomography, have shown to be particularly useful to infer rock alteration distributions in hydrothermal systems because of the influence of secondary minerals in the bulk electrical conductivity of rocks [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Byrdina | Structure of the acid hydrothermal system of Papandayan volcano, Indonesia, investigated by geophysical methods[END_REF][START_REF] Finn | Geophysical imaging of the Yellowstone hydrothermal plumbing system[END_REF]. Petrophysical relations that link these properties are, however, far from obvious. The difficulty in establishing simple relations between electrical conductivity and the percentage of alteration minerals seems to arise also from the dependence on the type of alteration (i.e. mineral composition), temperature, and fluid composition [START_REF] Lévy | The role of smectites in the electrical conductivity of active hydrothermal systems: electrical properties of core samples from Krafla volcano, Iceland[END_REF][START_REF] Ghorbani | Complex conductivity of volcanic rocks and the geophysical mapping of alteration in volcanoes[END_REF], similar to what we suggest may happen for whole-rock oxygen isotope. Thus, it may be interesting to compare the whole-rock oxygen isotope and the electrical conductivity dependence for different degrees of altered volcanic rocks. Having both properties measured for representative volcano samples could significantly improve the upscaling of the physical properties measured in the laboratory to the field-scale three-dimensional models obtained by geophysical methods.

Although we focus here on volcanological applications, our approach could be used to provide rock physical and mechanical property estimates for borehole stability assessments and stimulation strategies in volcanic geothermal reservoirs, which typically comprise altered rocks [START_REF] Siratovich | Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand[END_REF][START_REF] Cant | Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand[END_REF][START_REF] Lévy | The role of smectites in the electrical conductivity of active hydrothermal systems: electrical properties of core samples from Krafla volcano, Iceland[END_REF]. Whole-rock δ 18 O values are also often used in mineral exploration [START_REF] Green | Whole-rock oxygen isotope distribution in the Fukazawa-Kosaka area, Hokuroku District, Japan, and its potential application to mineral exploration[END_REF][START_REF] Criss | Oxygen isotope, aeromagnetic, and gravity anomalies associated with hydrothermally altered zones in the Yankee Fork mining district, Custer County, Idaho[END_REF][START_REF] Cathles | Oxygen isotope alteration in the Noranda mining district, Abitibi greenstone belt, Quebec[END_REF][START_REF] Paradis | Oxygen isotope zonation and alteration in the northern Noranda District, Quebec; evidence for hydrothermal fluid flow[END_REF][START_REF] Lentz | Petrology, geochemistry, and oxygen isotope interpretation of felsic volcanic and related rocks hosting the Brunswick 6 and 12 massive sulfide deposits (Brunswick Belt), Bathurst mining camp[END_REF], and could therefore also be used to provide rock physical and mechanical property estimates for rock drillability estimates and underground excavation stability assessments during mineral exploration and extraction in volcanic terrains.

One drawback of our approach, however, is that our analysis (Fig. 4) has been performed on rocks that show a decrease in strength and Young's modulus as a result of hydrothermal alteration (Heap et al., 2021a(Heap et al., , 2022a;;[START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF]. However, hydrothermal alteration associated with pore-and crack-filling precipitation can increase strength and Young's modulus [START_REF] Marmoni | Gravitational slope-deformation of a resurgent caldera: New insights from the mechanical behaviour of Mt. Nuovo tuffs (Ischia Island, Italy)[END_REF][START_REF] Heap | Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: the Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand)[END_REF][START_REF] Heap | Hydrothermal alteration can result in pore pressurization and volcano instability[END_REF]. As a result, care should be taken when using Eqs. (1), (2), and (3) to ensure that the alteration observed in a particular setting does not appear to strengthen or harden the rock. Care should also be taken to ensure that the alteration is indeed low temperature (≤ 250 °C) hydrothermal alteration, and that the rocks have not undergone alteration at higher temperatures, which will decrease whole-rock δ 18 O values. Therefore, a detailed study of the mineral assemblage is required in order to use Eqs. ( 1), (2), and (3) confidently. To improve our approach to estimate rock physical and mechanical properties using δ 18 O values, more data are required for a wider variety of hydrothermally altered rocks, including those with different alteration mineral assemblages (e.g., those containing smectite). We further note that the analysis presented here provides laboratory-scale values of strength and Young's modulus, and so these values likely require upscaling before they are used in large-scale volcano models. Strength can be upscaled using, for example, the Hoek-Brown failure criterion [START_REF] Hoek | Hoek-Brown failure criterion-2002 edition[END_REF] and Young's modulus can be upscaled using the Hoek-Diederichs equation [START_REF] Hoek | Empirical estimation of rock mass modulus[END_REF]Heap et al., 2020a).

Concluding remarks

Rock physical and mechanical properties are required for large-scale models designed to assess the stability of a lava dome or volcanic flank [START_REF] Watters | Rock mass strength assessment and significance to edifice stability, Mount Rainier and Mount Hood, Cascade Range volcanoes[END_REF]Okubu, 2004;[START_REF] Apuani | Stability of a collapsing volcano (Stromboli, Italy): Limit equilibrium analysis and numerical modelling[END_REF][START_REF] Moon | Geotechnical characterisation of stratocone crater wall sequences, White Island Volcano, New Zealand[END_REF]del Potro and Hürlimann, 2008;Heap et al., 2021a, b;[START_REF] Harnett | Mechanical and topographic factors influencing lava dome growth and collapse[END_REF][START_REF] Wallace | Material Properties and Triggering Mechanisms of an Andesitic Lava Dome Collapse at Shiveluch Volcano, Kamchatka, Russia, Revealed Using the Finite Element Method[END_REF]. The ubiquity of hydrothermal alteration at active volcanoes [START_REF] Aizawa | Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self-potential and magnetotellurics[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Byrdina | Geophysical image of the hydrothermal system of Merapi volcano[END_REF][START_REF] Byrdina | Structure of the acid hydrothermal system of Papandayan volcano, Indonesia, investigated by geophysical methods[END_REF][START_REF] Finn | Three-dimensional geophysical mapping of shallow water saturated altered rocks at Mount Baker, Washington: Implications for slope stability[END_REF][START_REF] Tseng | Anatomy of active volcanic edifice at the Kusatsu-Shirane volcano, Japan, by magnetotellurics: hydrothermal implications for volcanic unrests[END_REF][START_REF] Kereszturi | Synthesis of hydrothermal alteration, rock mechanics and geophysical mapping to constrain failure and debris avalanche hazards at Mt. Ruapehu (New Zealand)[END_REF][START_REF] Finn | Geophysical imaging of the Yellowstone hydrothermal plumbing system[END_REF], and evidence suggesting that alteration compromises volcano stability (van Wyk de [START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF][START_REF] Voight | The 1997 Boxing Day sector collapse and debris avalanches, Soufrière Hills volcano, Montserrat, WI. The Eruption of Soufriere Hills Volcano[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF], underscores the need for not only understanding the influence of alteration on rock physical and mechanical properties, but also for well constrained properties for altered volcanic rocks, data that are currently rare. In certain scenarios, such as when rock blocks large enough for laboratory experiments cannot be acquired (e.g., drill cuttings from boreholes, when the material is too friable or delicate to prepare samples for laboratory experiments, or when it is impracticable to sample and export a sufficient number of large blocks), or when laboratory equipment is not available, an independent measure of alteration that can be used to estimate the required rock physical and mechanical properties would be extremely useful for routine volcano stability modelling. Such a method could also be used to estimate pre-failure rock properties from semi-consolidated friable material found in debris avalanches. Here we show that whole-rock δ 18 O values, a method that requires a very small amount of representative material, can be used to estimate the strength (compressive and tensile) and Young's modulus of low-temperature (≤ 150-200 °C) hydrothermally (acid-chloride-sulfate) altered dome rocks.

Based on the promise of the approach documented herein, we recommend that future studies further explore the relationship between whole-rock δ 18 O values and the physical and mechanical properties of altered rocks with different alteration assemblies (such as those that contain smectites). Indirect tensile strength as a function of δ 18 O value (d), δD value (e), and water content (f). Young's modulus as a function of δ 18 O value (g), δD value (h), and water content (i). The strength (compressive and tensile) and Young's modulus data are taken from Heap et al. (2021aHeap et al. ( , 2022a)). Uniaxial compressive strength, tensile strength, and Young's modulus were measured on multiple core samples prepared from each block, which is characterised by a single δ 18 O value, δD value, and water content. Experimental errors are within the size of the symbols. All these data are provided in an Excel® spreadsheet that accompanies this contribution as Supplementary Information. Heap et al., 2021aHeap et al., , 2022a, b), b), whole-rock δ 18 O 450 values, whole-rock δD values, and water content (H2O+) for rocks from La Soufrière de Guadeloupe (Eastern 451 Caribbean) and Merapi volcano (Indonesia; data from [START_REF] Darmawan | Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones[END_REF][START_REF] Heap | Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour[END_REF]Heap et al., , 2022a)). The

Fig. 1 .

 1 Fig. 1. Google Earth image (Google Maxar Technologies CNES / Airbus) of La Soufrière de Guadeloupe (Eastern Caribbean) showing the sampling locations for the 19 rock blocks collected for this study (see text for details). Whole-rock δ 18 O values, measured in this study (Table 2) are given next to sampling location. Inset shows a map of Guadeloupe in which the location of La Soufrière de Guadeloupe is indicated by a red triangle.

Fig. 2 .

 2 Fig. 2. Whole-rock δ 18 O value (a), whole-rock δD value (b), and water content (c) as a function of the wt% of secondary minerals for rocks from La Soufrière de Guadeloupe (Eastern Caribbean; sulfate-rich samples (≥ 2 wt%) are blue, opal-rich samples (≥ 10 wt%) are red, samples that are both sulfate-and opal-rich are blue and red, and all other samples are black) (alteration data from Heap et al., 2021a, 2022a, b). Photographs of 20 mm-diameter cylindrical samples cored from selected blocks (those that preserve different degrees of alteration) are provided in panel (a). Experimental errors are within the size of the symbols. All these data are provided in an Excel® spreadsheet that accompanies this contribution as Supplementary Information.

Fig. 3 .

 3 Fig. 3. Uniaxial compressive strength as a function of δ 18 O value (a), δD value (b), and water content (c) for rocks from La Soufrière de Guadeloupe (Eastern Caribbean; sulfate-rich samples (≥ 2 wt%) are blue, opal-rich samples (≥ 10 wt%) are red, samples that are both sulfate-and opal-rich are blue and red, and all other samples are black).

Fig. 4 .

 4 Fig. 4. Uniaxial compressive strength (a), indirect tensile strength (b), and Young's modulus (c) as a function of whole-rock δ 18 O values for rocks from La Soufrière de Guadeloupe (Eastern Caribbean; black circles; strength and Young's modulus data from Heap et al., 2021a, 2022a) and Merapi volcano (Indonesia; green squares; data from Heap et al., 2019, 2022a; Darmawan et al., 2022). Data outliers are shown in grey (see text for details). Black lines -best-fit linear functions, Eqs. (1), (2), and (3) for the La Soufrière de Guadeloupe and Merapi volcano data (excluding the outliers shown in grey). The coefficient of determination for the data of panels (a), (b), and (c) are 0.52, 0.36, and 0.52, respectively. Uniaxial compressive strength, tensile strength, and Young's modulus were measured on multiple core samples prepared from each block, which is characterised by a single δ 18 O value.Experimental errors are within the size of the symbols. All these data are provided in an Excel® spreadsheet that accompanies this contribution as Supplementary Information.

  452 sampling locations for the blocks from La Soufrière de Guadeloupe are shown in Fig. 1. NA -not analysed 453 454

Table 2 .

 2 The weight percentage of secondary minerals (data from

	WP131
	7

Tridymite*

Acknowledgements

This work was supported by the TelluS Program of INSU-CNRS ("Assessing the role of hydrothermal alteration on volcanic hazards") and ANR grant MYGALE ("Modelling the phYsical and chemical Gradients of hydrothermal ALteration for warning systems of flank collapse at Explosive volcanoes"; ANR-21-CE49-0010). M. Heap also acknowledges support from the Institut Universitaire de France (IUF). V.R. Troll acknowledges support from the Swedish Research Council (Vetenskapsrådet; grant number 2020-03789). We thank the IPGP for general funding for the Observatoires Volcanologiques et Sismologiques (OVS), INSU-CNRS for the funding provided to the Service National d'Observation en Volcanologie (SNOV), and the Ministère pour la Transition

Ecologique (MTE) for financial support for the monitoring of the unstable flank of La Soufrière de Guadeloupe.

We are grateful to the Parc National de Guadeloupe for allowing us to carry out geological fieldwork on La Soufrière. We thank Tomaso Esposti Ongaro and Lucille Carbillet for help in the field. This study contributes to the IdEx Université Paris Cité ANR-18-IDEX-0001. We thank Peter Schaaf and Ben Ellis for comments that helped improve this manuscript, and Ulrich Kueppers for handling our manuscript.

Statements and Declarations

The authors declare no financial or non-financial interests that are directly or indirectly related to this work.

Supplementary Information

We provide an Excel® spreadsheet that contains the data collected for, and analysed in, this contribution. were published in Heap et al. (2022b). All other data were published in Heap et al. (2021a). The relative errors in the quantification are in the order of 5-10%.

Author contributions

Michael