PRESSURE VARIATIONS IN THE MONTE ROSA NAPPE, WESTERN ALPS

Specific instances throughout the geodynamic evolution of tectonic nappes are recorded by mineral assemblages in metamorphic rocks, from which pressure (P) and temperature (T) conditions can be assessed through thermobarometry. Conventionally, the maximum P is assumed to reflect the maximum burial depth reached by a coherent tectonic unit during regional metamorphism. P is converted to depth using the lithostatic P formula, which states that P and depth are linearly related. However, peak P estimates in coherent tectonic units are often very heterogeneous. In the Monte Rosa nappe, Western Alps, Alpine peak P estimates reported in the literature vary from 1.2 to 2.7 GPa. Accordingly, maximum burial depths are either within crustal depths or at mantle depths, implying therefore considerably different tectonic models (e.g. subduction channel or orogenic wedge). Such large P ranges can be explained by three scenarios: i) peak P was homogeneous, hence the P range is an artefact due to various geobarometric tools used; ii) peak P was homogeneous, but some prograde reactions were inhibited due to slow kinetics; or iii) peak P was heterogeneous, due to a heterogeneous stress distribution between lithologies of contrasting rheologies. The aim of this thesis is to explore the three above-mentioned hypotheses in order to under- stand the significance of P variations within coherent tectonic units. To do so, the focus of the thesis is set on a specific region of the Monte Rosa nappe, located in the Upper Ayas Val- ley, in Aosta, where the largest P variation is recorded between a metagranite and enclosed whiteschist. Whiteschists result from the localized metasomatic alteration of the granite, hence the metagranite and whiteschist are coherent and were always at the same depth during the Alpine orogeny. Being metasomatic rocks, whiteschists evolve under fluid-saturated condi- tions during high-pressure (HP) metamorphism, whereas the metagranite, qualified as “dry”, would have evolved in water undersaturated conditions, potentially impeding the progression of prograde reactions. The approach of this study is to evaluate the impact of fluid saturation conditions on the HP record in both the whiteschist and metagranite independently; as well as spatially along a profile going from the whiteschist towards the metagranite. The relative age of the metasomatic event at the origin of the whiteschist chemistry is es- tablished by combining field and petrographic observations with whole rock geochemistry as well as radiogenic isotopes. The interplay between fluid events and metamorphic reactions is discussed based on in situ oxygen isotopes measurements in white mica. For this, a new set of white mica reference materials was developed for in situ oxygen isotope measurements by Secondary Ion Mass Spectrometry (SIMS). The water-saturation conditions of metagranites were assessed by petrographic observations of reaction product microstructures in the best- preserved HP metagranites. Silica in phengite barometry was used in combination with water activity (aH2O) estimate in phengite. To this end, a new set of white mica reference materials was calibrated for in situ H2O content measurement by SIMS and P - aH2O relationships were evaluated. Field and geochemical studies of the whiteschist revealed a pre-Alpine localized pervasive alteration of the granite by late magmatic hydrothermal fluids along tube-like structures. A subsequent closed-system Alpine metamorphic evolution is suggested based on oxygen isotopes in white micas from the whiteschist. Thermodynamic calculations on whiteschists resulted in a minimum peak P of 2.2 GPa. Jadeite has never been observed in the best-preserved HP metagranites, however plagioclase pseudomorphs consist of fine-grained assemblages of zoisite, albite ± white mica. Igneous twinning is preserved in these pseudomorphs, attesting that albite is not a retrograde product after jadeite. Silica in phengite barometry indicates a maximum peak P of 1.4 GPa, consistent with the absence of jadeite. In situ H2O content data in phengite revealed high OH− content of the hydroxyl site, pointing towards high aH2O in both metagranites and whiteschists; thus, in contrast with what is proposed in the literature. These results point towards an Alpine peak P difference of 0.8 GPa between the metagranite and whiteschists, that is not due to slow kinetics or total retrogression. An alternative model explaining P variations related to heterogeneous stress conditions is presented. A simple mechanical model, based on an analytical solution is used, that reproduces the observed P variation. This study documents the first field evidence that demonstrates outcrop-scale P variations up to several kilobars can be produced between rocks of contrasting mechanical properties. Consequently, the lithostatic paradigm does not apply in this case and the burial depth reached by the Monte Rosa nappe at Alpine peak conditions is restricted to lower crustal depth. Hence, a tectonic model involving deep subduction of the crustal units is not necessary and a model involving an orogenic wedge would better fit with both the metamorphic record and structural reconstructions. The results of this thesis show that P can be heterogeneous for inclusion - hosts systems. This is not in agreement with the lithostatic paradigm and could have great impacts on the current view of the geodynamic history of the Alps and other orogenic systems. -- Des instants spécifiques de l’évolution géodynamique des nappes tectoniques sont enregistrés par les assemblages de minéraux dans les roches métamorphiques, pour lesquelles les conditions de pression (P) et température (T) peuvent être déduites par des méthode thermo-barométriques. Conventionnellement, la P maximale reflète la profondeur d’enfouissement maximale atteinte par une unité tectonique durant le métamorphisme régional. La P est convertie en profondeur en utilisant la formule de la pression lithostatique, qui établit que la relation entre la P et la profondeur est linéraire. Toutefois, les estimations de pic de P dans des unités tectoniques cohérentes sont souvent très hétérogènes. Dans la nappe du Mont Rose, dans les Alpes de l’Ouest, les estimations du pic de P répertoriées dans la littérature varient entre 1.2 et 2.7 GPa. Par conséquent, la profondeur d’enfouissement maximale est située soit dans la croûte, soit à des profondeurs mantelliques, ce qui implique des modèles tectoniques considérablement différents (par exemple, le chenal de subduction ou le prisme orogénique). Cette large fourchette de P peut être expliquée par différents scénarii : i) le pic de P était homogène, mais la fourchette de P est un artéfact, résultant de l’utilisation de différents outils barométriques; ii) le pic de P était homogène, mais certaines réactions progrades n’ont pas eu lieu, à cause de conditions cinétiques lentes; ou iii) le pic de P était hétérogène, dû au fait que la distribution des contraintes est hétérogène entre des lithologies ayant différentes propriétés rhéologiques. Le but de cette thèse est d’explorer les trois hypothèses mentionnées ci-dessus dans le but de comprendre la signification des différences de pression répertoriées dans les unités tectoniques. Pour cela, le centre d’intérêt de la thèse se concentre sur une localité dans le haut val d’Ayas, à Aoste, où la plus grande différence de pic de P est observée entre un schiste blanc et son encaissant de métagranite. Les schistes blancs résultent de l’altération métasomatique localisée d’un granite. Les deux roches sont donc cohérentes et étaient juxtaposées tout au long de leur enfouissement durant l’orogenèse Alpine. Etant des roches métasomatiques, les schistes blancs ont évolué dans des conditions de saturation en eau durant le métamorphisme de haute pression (HP), alors que les granites, qualifiés de “secs”, auraient évolué en condition de sous-saturation en eau, ce qui auraient potentiellement entravé la progression des réactions progrades. L’approche de cette étude est d’évaluer l’impact des conditions de saturation en eau sur l’enregistrement métamorphique de HP dans les schistes blancs et les métagranites, indépendamment, et aussi spatialement le long d’un profile partant d’un affleurement de schistes blancs jusqu’au métagranite. L’ˆage relatif de l’événement métasomatique à l’origine de la chimie des schistes blancs est établi en combinant des observations de terrain et pétrographiques avec des données géochimiques ainsi que des données d’isotopes radiogéniques. L’interaction entre les événements fluides et les réactions métamorphiques est discutée sur la base de données in-situ d’isotopes de l’oxygène sur mica blanc dans les schistes blancs. Pour cela, un nouvel ensemble de matériaux de référence a été développé pour l’analyse des isotopes de l’oxygène dans les micas blancs par spectrométrie de masse à ions secondaires (SIMS). Les conditions de saturation en eau des métagranites sont établies sur la base d’observations pétrographiques des microstructures de produits de réaction dans les métagranites de HP les mieux préservés. Le baromètre de la silice dans la phengite est utilisé, en combinaison avec une estimation de l’activité de H2O dans la phengite. Dans ce but, un ensemble de nouveaux matériaux de référence a été calibré pour l’analyse in-situ de H2O dans les micas blancs par SIMS et la relation entre activité de H2O et P a été évaluée thermodynamiquement. Les études de terrain, pétrographiques et géochimiques ont révélé une altération pré-Alpine localisée et pénétrante des granites par des fluides tardi-magmatiques hydrothermaux, le long de structure en forme de tube. Les données d’isotopes de l’oxygène in-situ dans les micas blancs des schistes blancs suggèrent que le métamorphisme Alpin ultérieur a été fait dans des conditions de système fermé. Les calculs thermodynamiques sur schiste blanc indiquent un pic de P minimum de 2.2 GPa. La jadéite n’a jamais été observée dans les métagranites de HP les mieux préservés, toutefois les pseudomorphes du plagioclase observés consistent en un assemblage de zoisite, albite ± mica blanc finement cristallisés. La macle du plagioclase igné est préservée, ce qui atteste que l’albite n’est pas un produit de déstabilisation de la jadéite. La barométrie de silice dans la phengite dans les métagranites indique un pic de P maximal de 1.4 GPa, cohérent avec l’absence de jadéite. Les données in-situ de H2O dans les micas blancs ont révélé une fraction de OH− élevée dans le site hydroxyle, suggérant une activité de H2O élevée dans les métagranites et les schistes blancs, en désaccord ce qui avait été précédemment suggéré dans la litérature. Les résultats indiquent une différence de pic de P de 0.8 GPa entre le métagranite et les schistes blancs, qui n’est pas due à des conditions de cinétique lente ou à une rétrogression complète. Un modèle alternatif est présenté, qui explique les variations de P avec des conditions de contraintes hétérogènes. Un modèle mécanique simple, basé sur une solution analytique, est utilisé et permet de reproduire les variations de P observées. Cette étude documente la première preuve de terrain que des variations de P de plusieurs kilobars peuvent être générées à l’échelle de l’affleurement, entre des roches ayant des propriétés mécaniques différentes dans une certaine configuration de système d’inclusion et hôte. En conséquence, le paradigme de la pression lithostatique n’est pas applicable dans ce cas et la profondeur d’enfouissement atteinte par la nappe du Mont Rose aux conditions de pic du métamorphisme est restreinte à des profondeurs crustales. Un modèle tectonique impliquant une subduction profonde des unités crustales n’est donc pas nécessaire. Le modèle de prisme orogénique conviendrait mieux pour expliquer l’enregistrement métamorphique ainsi que les reconstructions structurales. Les résultats présentés dans cette thèse montrent que la P peut être hétérogène dans des systèmes d’inclusion hôte. Cela est en désaccord avec le paradigme de la P lithostatique et pourrait avoir un impact conséquent sur la vision de l’histoire géodynamique des Alpes et d’autres systèmes orogéniques

The role of H2O on metamorphism and deformation at high pressure: A combined petrological and thermo-mechanical study based on the Gran Paradiso Unit, Western Alps

Quantitative phase petrology analysis of two samples showing variable strain record and different peak pressure (P) conditions show that H2O is a key parameter in the development of contrasting high-pressure (HP) peak conditions in adjacent polymetamorphic rocks. A shear zone in a paragneiss from the Gran Paradiso nappe, Western Alps, shows an Alpine foliation and records a peak P of 1.9 GPa, for a temperature of 500 to 520 °C. A few tens of meters away from the shear zone, a paragneiss showing no apparent Alpine age deformation records a peak P of maximum 1.4 GPa for the temperature range of 500 to 520 °C. The H2O content of the latter has potentially been reduced to low contents following the pre-Alpine, Variscan amphibolite facies, and the absence of re-hydration prior to Alpine orogeny could have inhibited the formation of HP mineral assemblages. The validity of this interpretation is questioned here by considering the mechanical effect of H2O undersaturated rocks on deformation and P during deformation. Based on a thermo-mechanical numerical modelling study, we show that heterogeneities in fluid saturation conditions between rocks lead to strength contrasts that are sufficient to trigger a dynamic P in the range of several hundreds of MPa. In particular, the models successfully reproduce the measured peak P between the two paragneiss studied. This model could be applied to other H2O deficient rocks from HP tectonic units to further explore the role of H2O on the rheology and hence assess its potential impact in the preservation of low P bodies in otherwise HP units from continental collision settings

Alpine peak pressure and tectono-metamorphic history of the Monte Rosa nappe: evidence from the cirque du Véraz, upper Ayas valley, Italy

International audienceThe Monte Rosa nappe consists of a wide range of lithologies that record conditions associated with peak Alpine metamorphism. While peak temperature conditions inferred from previous studies largely agree, variable peak pressures have been estimated for the Alpine high-pressure metamorphic event. Small volumes of whiteschist lithologies with the assemblage chloritoid + phengite + talc + quartz record peak pressures up to 0.6 GPa higher compared to associated metapelitic and metagranitic lithologies, which yield a peak pressure of ca. 1.6 GPa. The reason for this pressure difference is disputed, and proposed explanations include tectonic mixing of rocks from different burial depths (mélange) or local deviations of the pressure from the lithostatic value caused by heterogeneous stress conditions between rocks of contrasting mechanical properties.We present results of detailed field mapping, structural analysis and a new geological map for a part of the Monte Rosa nappe exposed at the cirque du Véraz field area (head of the Ayas valley, Italy). Results of the geological mapping and structural analysis shows the structural coherency within the western portions of the Monte Rosa nappe. This structural coherency falsifies the hypothesis of a tectonic mélange as reason for peak pressure variations. Structural analysis indicates two major Alpine deformation events, in agreement with earlier studies: (1) north-directed nappe emplacement, and (2) south-directed backfolding.We also analyze a newly discovered whiteschist body, which is located at the intrusive contact between Monte Rosa metagranite and surrounding metapelites. This location is different to previous whiteschist occurrences, which were entirely embedded within metagranite. Thermodynamic calculations using metamorphic assemblage diagrams resulted in 2.1 ± 0.2 GPa and 560 ± 20 °C for peak Alpine metamorphic conditions. These results agree with metamorphic conditions inferred for previously investigated nearby whiteschist outcrops embedded in metagranite. The new results, hence, confirm the peak pressure differences between whiteschists and the metagranite and metapelite. To better constrain the prograde pressure–temperature history of the whiteschist, we compare measured Mg zoning in chloritoid with Mg zoning predicted by fractional crystallization pseudo-section modelling for several hypothetical pressure–temperature paths. In order to reach a ca. 0.6 GPa higher peak pressure compared to the metapelite and metagranite, our results suggest that the whiteschist likely deviated from the prograde burial path recorded in metapelite and metagranite lithologies. However, the exact conditions at which the whiteschist pressure deviated are still contentious due to the strong temperature dependency of Mg partitioning in whiteschist assemblages. Our pseudo-section results suggest at least that there was no dramatic isothermal pressure increase recorded in the whiteschis

Garnet microstructures suggest ultra-fast decompression of ultrahigh-pressure rocks

International audienceRadial microcracks surrounding retrogressed SiO2 inclusions in UHP garnets are key microstructural observations allowing to constrain the mechanisms of exhumation of ultra-high-pressure (UHP) rocks. The major challenge lies in identifying whether the microstructures formed during their ascent from mantle depths, or as a consequence of transient variations in the tectonic regime. By combining petrographic observations, petrochronological data and numerical thermo-mechanical modelling, we show that radial cracks around SiO2 inclusions in ultrahigh-pressure garnets from Dora Maira are caused by ultrafast decompression during the early stage of exhumation (< 0.5 Ma). Decompression rates higher than 10-14 s-1 are, for the first time, inferred from natural microstructures independently of current petrochronological estimates1. We demonstrate that the SiO2 phase transition generates shear stresses sufficiently large to trigger plastic yielding, resulting in the generation and propagation of radial and bent shear bands, mimicking the fractures observed in UHP garnet. Our results question the traditional interpretation of the exhumation from great depth of ultrahigh-pressure tectonic. Instead, we propose that such ultrafast decompression rates are related to transient changes in the stress state of the buried continental lithosphere, favoring an exhumation mechanism involving nappe stacking

Brittle failure at high-pressure conditions: the key role of reactioninducedvolume changes

International audienceMetamorphic reactions can lead to drastic changes in rocks mechanical properties. Indeed, duringsuch transformations, the nucleation of new phases with different strength, grain size and/ordensity compared to the primary phases is enhanced, and transient processes due to the ongoingreaction are then activated.Eclogitization of lower crustal rocks during continental subduction constitutes an emblematictransformation illustrating these processes. In such tectonic context, it has been shown thateclogitization seems to be closely associated with the occurrence of seismogenic events. However,the mechanisms that trigger brittle failure in such high pressure environments remain highlydebated. Indeed, whether the change in density or the change in rheology can lead toembrittlement is still enigmatic.By using 2D compressible mechanical numerical models we studied the impact of the strongnegative volume change of the eclogitization reaction on the rocks rheological behaviour. We showthat eclogitization-induced density change occurring out of equilibrium can, by itself, generatessufficient shear stress to fail the rocks at high-pressure conditions.Rupture initiation at depth in continental subduction zones could therefore be explained byvolume changes, even without considering the modifications of the rheological properties inducedby the transformation. Our results also indicate that the negative volume change associated withbrittle failure can enhance the propagation of the eclogitization process by a runaway mechanismas long as the reaction is not limited by the lack of reactants.Powered b

Episodic hydrothermal alteration recorded by microscale oxygen isotope analysis of white mica in the Larderello-Travale Geothermal Field, Italy

Microscale oxygen isotope analysis (18O/16O) of minerals can identify distinct events of fluid-rock interaction. This method is, however, still limited to a few major and accessory minerals of which most are anhydrous minerals. We present the systematic study of oxygen isotope distribution in white mica by Secondary Ion Mass Spectrometry (SIMS). Texturally and chemically distinct white mica populations in granitic and contact metamorphic rocks from the Larderello-Travale geothermal field (LTGF), Italy, record stages of magmatic crystallization, metamorphic-hydrothermal replacement and fluid-rock interaction. The large range in intra- and inter-grain white mica δ18O values between 1–14 ‰ reflects varying protoliths and degrees of fluid-mineral interaction at variable temperatures (180–450 °C present-day temperatures; p.d.T.). This variability reflects the large-scale circulation of both (1) magmatic, syn-intrusive to early contact metamorphic hydrothermal fluids with high-δ18O values, and (2) meteoric fluids with δ18O values of –7 ‰ during a post-intrusive, late hydrothermal stage. Metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity (μm-scale), including detrital grains (δ18O = 12–14 ‰; high Na, low Mg), partially altered white mica (δ18O = 8–9 ‰) and late hydrothermal white mica (1–6 ‰; low Na, mid Mg). The late hydrothermal white mica has similar δ18O values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a δ18O of –6 to 0.5 ‰, which circulated in the late hydrothermal stage. Downhole towards the lower reservoir, white mica from two contact metamorphic micaschist samples shows either (1) homogeneous δ18O values of ca. 9 ‰ likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600 °C), or (2) a large spread in δ18O from 2 to 12 ‰ within and across grains of variable texture and chemistry in the host rock and a cross-cutting quartz-white mica vein (ca. 300 °C, present day temperature; hereafter p.d.T.). This contrasting δ18O signature of white mica is also recorded in granite cored at up to 4.6 km depth. The Carboli granite contains white mica with a homogeneous magmatic δ18O of 10 ± 0.6 ‰, whereas older granite samples from Radicondoli have magmatic to hydrothermal white micas that vary in δ18O from 4–10 ‰. A pronounced intra-grain δ18O variability of up to 6 ‰ occurs in white mica domains with higher Fe-Mg-Ti halos around inclusions of chloritized biotite, as a result of interaction with dominantly meteoric fluids that infiltrated to depths of at least 4.6 km. In the Porto Azzurro granite on Elba, Italy, altered white mica has δ18O values of 2.6 ‰ down from 10 ‰ in unaltered grains. The distribution of oxygen isotope ratios in white mica is thus firstly a result of pervasive versus selective fluid alteration (at depth, sample and grain scale). Secondly, the actual preservation of these μm-scale variabilities indicates that volume diffusion is not detectable at microscale at p.d.T at or below 350 °C where most of the heterogeneous white mica is found. Selective, sample- and grain-scale fluid penetration occurs episodically and anisotropically, on micro- and megascale, along faults, fractures and cleavages, producing lower δ18O white mica at various times in zones of higher secondary permeability and active hydrothermal fluid circulation

Decompression of host-inclusion systems in UHP rocks: insightsfrom observations and models

International audiencePolymorphic transformations are key tracers of metamorphic processes, also used to estimate thepressure and temperature conditions reached by a rock. In particular, the quartz-coesite transitionis commonly used to define the lower boundary of the ultrahigh-pressure (UHP) metamorphicfield. The partial preservation of coesite included in garnets from UHP rocks bring considerableinsights into the burial and exhumation mechanisms of the continental crust involved inconvergent zone. Coesite was first described in the Western Alps by Chopin[1], in the Dora-Mariawhiteschist, one of the most emblematic UHP rock worldwide. Although the partial preservation ofcoesite inclusions in garnet has long been attributed to the pressure vessel effect, theinterrelationship and relative timing between fracturing and retrogression is still contentious.Here we study the reaction-deformation relationships of coesite inclusions initially enclosed ingarnet and transforming into quartz during the decompression process. We combine 2Dnumerical thermo-mechanical models constrained by pressure-temperature-time (P-T-t) estimatesfrom the Dora-Maira whiteschist. The model accounts for a compressible visco-elasto-plasticrheology including a pressure-density relationship of silica based on thermodynamic data. Thisallows us to study the effect of reaction-induced volume increase during decompression. Ourresults capture the typical fracture patterns of the host garnet radiating from retrogressed coesiteinclusions and can be used to study the relative role of volume change associated with a change ofP-T conditions on the style of deformation during decompression.The mechanisms of the coesite-quartz transformation and geodynamic implications are presentedand validated against geological data. The effect of fluids on the phase transition and theconditions of access of fluids during the transformation are discussed in the light of the results ofthe thermo-mechanical models.This study demonstrates the high potential of thermo-mechanical modelling in enhancing ourunderstanding of the processes involved in the formation and evolution of metamorphic minerals.[1]Chopin (1984) Contributions to Mineralogy and Petrology 86, 2, 107-11

Garnet microstructures suggest ultra-fast decompression of ultrahigh-pressure rocks

Abstract Plate tectonics is a key driver of many natural phenomena occurring on Earth, such as mountain building, climate evolution and natural disasters. How plate tectonics has evolved through time is still one of the fundamental questions in Earth sciences. Natural microstructures observed in exhumed ultrahigh-pressure rocks formed during continental collision provide crucial insights into tectonic processes in the Earth’s interior. Here, we show that radial cracks around SiO2 inclusions in ultrahigh-pressure garnets are caused by ultrafast decompression. Decompression rates of at least 8 GPa/Myr are inferred independently of current petrochronological estimates by using thermo-mechanical numerical modeling. Our results question the traditional interpretation of fast and significant vertical displacement of ultrahigh-pressure tectonic units during exhumation. Instead, we propose that such substantial decompression rates are related to abrupt changes in the stress state of the lithosphere independently of the spatial displacement

Brittle failure at high-pressure conditions: the key role of reactioninducedvolume changes

International audienceMetamorphic reactions can lead to drastic changes in rocks mechanical properties. Indeed, duringsuch transformations, the nucleation of new phases with different strength, grain size and/ordensity compared to the primary phases is enhanced, and transient processes due to the ongoingreaction are then activated.Eclogitization of lower crustal rocks during continental subduction constitutes an emblematictransformation illustrating these processes. In such tectonic context, it has been shown thateclogitization seems to be closely associated with the occurrence of seismogenic events. However,the mechanisms that trigger brittle failure in such high pressure environments remain highlydebated. Indeed, whether the change in density or the change in rheology can lead toembrittlement is still enigmatic.By using 2D compressible mechanical numerical models we studied the impact of the strongnegative volume change of the eclogitization reaction on the rocks rheological behaviour. We showthat eclogitization-induced density change occurring out of equilibrium can, by itself, generatessufficient shear stress to fail the rocks at high-pressure conditions.Rupture initiation at depth in continental subduction zones could therefore be explained byvolume changes, even without considering the modifications of the rheological properties inducedby the transformation. Our results also indicate that the negative volume change associated withbrittle failure can enhance the propagation of the eclogitization process by a runaway mechanismas long as the reaction is not limited by the lack of reactants.Powered b

Reaction-induced volume change triggers brittle failure at eclogite facies conditions

International audienceMetamorphic reactions can lead to important changes in rock strength and density. Eclogitization constitutes one of the most emblematic transformations in continental subduction zones, where conversion of lower crustal rocks into eclogite facies rocks correlates with the occurrence of seismogenic events. The relationship between eclogitization and seismicity has been highlighted in several studies, but the processes that trigger brittle failure remain highly debated. Indeed, whether the change in density (from ∼2850 kg.m−3 to ∼3300 kg.m−3) or the change in rheology can lead to embrittlement is still enigmatic. Here we show that eclogitization-induced volume change occurring out of equilibrium can, by itself, generate sufficient shear stress to fail the rocks at high-pressure conditions. Intermediate-depth earthquakes in continental subduction zones could therefore be explained by volume changes, even without considering rheological modifications induced by mineral reactions. Our results also indicate that interplay between negative volume change and frictional plastic yielding can enhance the propagation of the eclogitization process by a runaway mechanism as long as the reaction is not limited by the lack of reactants