22 research outputs found
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 âdryâ, 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 OHâ 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 spĂ©cifiques de lâĂ©volution gĂ©odynamique des nappes tectoniques sont enregistrĂ©s par les assemblages de minĂ©raux dans les roches mĂ©tamorphiques, pour lesquelles les conditions de pression (P) et tempĂ©rature (T) peuvent ĂȘtre dĂ©duites par des mĂ©thode thermo-baromĂ©triques. Conventionnellement, la P maximale reflĂšte la profondeur dâenfouissement maximale atteinte par une unitĂ© tectonique durant le mĂ©tamorphisme rĂ©gional. La P est convertie en profondeur en utilisant la formule de la pression lithostatique, qui Ă©tablit que la relation entre la P et la profondeur est linĂ©raire. Toutefois, les estimations de pic de P dans des unitĂ©s tectoniques cohĂ©rentes sont souvent trĂšs hĂ©tĂ©rogĂšnes.
Dans la nappe du Mont Rose, dans les Alpes de lâOuest, les estimations du pic de P rĂ©pertoriĂ©es dans la littĂ©rature varient entre 1.2 et 2.7 GPa. Par consĂ©quent, la profondeur dâenfouissement maximale est situĂ©e soit dans la croĂ»te, soit Ă des profondeurs mantelliques, ce qui implique des modĂšles tectoniques considĂ©rablement diffĂ©rents (par exemple, le chenal de subduction ou le prisme orogĂ©nique). Cette large fourchette de P peut ĂȘtre expliquĂ©e par diffĂ©rents scĂ©narii : i) le pic de P Ă©tait homogĂšne, mais la fourchette de P est un artĂ©fact, rĂ©sultant de lâutilisation de diffĂ©rents outils baromĂ©triques; ii) le pic de P Ă©tait homogĂšne, mais certaines rĂ©actions progrades nâont pas eu lieu, Ă cause de conditions cinĂ©tiques lentes; ou iii) le pic de P Ă©tait hĂ©tĂ©rogĂšne, dĂ» au fait que la distribution des contraintes est hĂ©tĂ©rogĂšne entre des lithologies ayant diffĂ©rentes propriĂ©tĂ©s rhĂ©ologiques.
Le but de cette thĂšse est dâexplorer les trois hypothĂšses mentionnĂ©es ci-dessus dans le but de comprendre la signification des diffĂ©rences de pression rĂ©pertoriĂ©es dans les unitĂ©s tectoniques. Pour cela, le centre dâintĂ©rĂȘt de la thĂšse se concentre sur une localitĂ© dans le haut val dâAyas,
Ă Aoste, oĂč la plus grande diffĂ©rence de pic de P est observĂ©e entre un schiste blanc et son encaissant de mĂ©tagranite. Les schistes blancs rĂ©sultent de lâaltĂ©ration mĂ©tasomatique localisĂ©e dâun granite. Les deux roches sont donc cohĂ©rentes et Ă©taient juxtaposĂ©es tout au long de leur enfouissement durant lâorogenĂšse Alpine. Etant des roches mĂ©tasomatiques, les schistes blancs ont Ă©voluĂ© dans des conditions de saturation en eau durant le mĂ©tamorphisme de haute pression (HP), alors que les granites, qualifiĂ©s de âsecsâ, auraient Ă©voluĂ© en condition de sous-saturation en eau, ce qui auraient potentiellement entravĂ© la progression des rĂ©actions progrades. Lâapproche de cette Ă©tude est dâĂ©valuer lâimpact des conditions de saturation en eau sur lâenregistrement mĂ©tamorphique de HP dans les schistes blancs et les mĂ©tagranites, indĂ©pendamment, et aussi spatialement le long dâun profile partant dâun affleurement de schistes blancs jusquâau mĂ©tagranite.
LâËage relatif de lâĂ©vĂ©nement mĂ©tasomatique Ă lâorigine de la chimie des schistes blancs est Ă©tabli en combinant des observations de terrain et pĂ©trographiques avec des donnĂ©es gĂ©ochimiques ainsi que des donnĂ©es dâisotopes radiogĂ©niques. Lâinteraction entre les
Ă©vĂ©nements fluides et les rĂ©actions mĂ©tamorphiques est discutĂ©e sur la base de donnĂ©es in-situ dâisotopes de lâoxygĂšne sur mica blanc dans les schistes blancs. Pour cela, un nouvel ensemble de matĂ©riaux de rĂ©fĂ©rence a Ă©tĂ© dĂ©veloppĂ© pour lâanalyse des isotopes de lâoxygĂšne dans les micas blancs par spectromĂ©trie de masse Ă ions secondaires (SIMS). Les conditions de saturation en eau des mĂ©tagranites sont Ă©tablies sur la base dâobservations pĂ©trographiques des microstructures de produits de rĂ©action dans les mĂ©tagranites de HP les mieux prĂ©servĂ©s. Le baromĂštre de la silice dans la phengite est utilisĂ©, en combinaison avec une estimation de lâactivitĂ© de H2O dans la phengite. Dans ce but, un ensemble de nouveaux matĂ©riaux de rĂ©fĂ©rence a Ă©tĂ© calibrĂ© pour lâanalyse in-situ de H2O dans les micas blancs par SIMS et la relation entre activitĂ© de H2O et P a Ă©tĂ© Ă©valuĂ©e thermodynamiquement.
Les Ă©tudes de terrain, pĂ©trographiques et gĂ©ochimiques ont rĂ©vĂ©lĂ© une altĂ©ration prĂ©-Alpine localisĂ©e et pĂ©nĂ©trante des granites par des fluides tardi-magmatiques hydrothermaux, le long de structure en forme de tube. Les donnĂ©es dâisotopes de lâoxygĂšne in-situ dans les micas blancs des schistes blancs suggĂšrent que le mĂ©tamorphisme Alpin ultĂ©rieur a Ă©tĂ© fait dans des conditions de systĂšme fermĂ©. Les calculs thermodynamiques sur schiste blanc indiquent un pic de P minimum de 2.2 GPa. La jadĂ©ite nâa jamais Ă©tĂ© observĂ©e dans les mĂ©tagranites de HP les mieux prĂ©servĂ©s, toutefois les pseudomorphes du plagioclase observĂ©s consistent en un assemblage de zoisite, albite ± mica blanc finement cristallisĂ©s. La macle du plagioclase ignĂ© est prĂ©servĂ©e, ce qui atteste que lâalbite nâest pas un produit de dĂ©stabilisation de la jadĂ©ite. La baromĂ©trie de silice dans la phengite dans les mĂ©tagranites indique un pic de P maximal de 1.4 GPa, cohĂ©rent avec lâabsence de jadĂ©ite. Les donnĂ©es in-situ de H2O dans les micas blancs ont rĂ©vĂ©lĂ© une fraction de OHâ Ă©levĂ©e dans le site hydroxyle, suggĂ©rant une activitĂ© de H2O Ă©levĂ©e dans les mĂ©tagranites et les schistes blancs, en dĂ©saccord ce qui avait Ă©tĂ© prĂ©cĂ©demment suggĂ©rĂ© dans la litĂ©rature.
Les rĂ©sultats indiquent une diffĂ©rence de pic de P de 0.8 GPa entre le mĂ©tagranite et les schistes blancs, qui nâest pas due Ă des conditions de cinĂ©tique lente ou Ă une rĂ©trogression complĂšte. Un modĂšle alternatif est prĂ©sentĂ©, qui explique les variations de P avec des conditions de contraintes hĂ©tĂ©rogĂšnes. Un modĂšle mĂ©canique simple, basĂ© sur une solution analytique, est utilisĂ© et permet de reproduire les variations de P observĂ©es.
Cette Ă©tude documente la premiĂšre preuve de terrain que des variations de P de plusieurs kilobars peuvent ĂȘtre gĂ©nĂ©rĂ©es Ă lâĂ©chelle de lâaffleurement, entre des roches ayant des propriĂ©tĂ©s mĂ©caniques diffĂ©rentes dans une certaine configuration de systĂšme dâinclusion et hĂŽte. En consĂ©quence, le paradigme de la pression lithostatique nâest pas applicable dans ce cas et la profondeur dâenfouissement atteinte par la nappe du Mont Rose aux conditions de pic du mĂ©tamorphisme est restreinte Ă des profondeurs crustales. Un modĂšle tectonique impliquant une subduction profonde des unitĂ©s crustales nâest donc pas nĂ©cessaire. Le modĂšle de prisme orogĂ©nique conviendrait mieux pour expliquer lâenregistrement mĂ©tamorphique ainsi que les reconstructions structurales.
Les rĂ©sultats prĂ©sentĂ©s dans cette thĂšse montrent que la P peut ĂȘtre hĂ©tĂ©rogĂšne dans des systĂšmes dâinclusion hĂŽte. Cela est en dĂ©saccord avec le paradigme de la P lithostatique et pourrait avoir un impact consĂ©quent sur la vision de lâhistoire gĂ©odynamique des Alpes et dâautres systĂšmes orogĂ©niques
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 1.9âŻGPa, for a temperature of 500 to 520âŻÂ°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 1.4âŻGPa for the temperature range of 500 to 520âŻÂ°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 VĂ©raz, 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 chloritoidâ+âphengiteâ+âtalcâ+âquartz 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 (mĂ©lange) 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 VĂ©raz 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 mĂ©lange 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 2.1â±â0.2 GPa and 560â±â20 °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 pressureâtemperature history of the whiteschist, we compare measured Mg zoning in chloritoid with Mg zoning predicted by fractional crystallization pseudo-section modelling for several hypothetical pressureâtemperature 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 ÎŽ18O values between 1â14 â° reflects varying protoliths and degrees of fluid-mineral interaction at variable temperatures (180â450 °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-ÎŽ18O values, and (2) meteoric fluids with ÎŽ18O values of â7 â° during a post-intrusive, late hydrothermal stage.
Metasedimentary rocks from the upper reservoir contain distinct white mica populations occurring in close proximity (ÎŒm-scale), including detrital grains (ÎŽ18O = 12â14 â°; high Na, low Mg), partially altered white mica (ÎŽ18O = 8â9 â°) and late hydrothermal white mica (1â6 â°; low Na, mid Mg). The late hydrothermal white mica has similar ÎŽ18O values to other secondary minerals and is in equilibrium with meteoric-dominated fluids with a ÎŽ18O of â6 to 0.5 â°, which circulated in the late hydrothermal stage. Downhole towards the lower reservoir, white mica from two contact metamorphic micaschist samples shows either (1) homogeneous ÎŽ18O values of ca. 9 â° likely due to recrystallization in the contact metamorphic hydrothermal stage (T ca. 600 °C), or (2) a large spread in ÎŽ18O from 2 to 12 â° 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 ÎŽ18O 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 ÎŽ18O of 10 ± 0.6 â°, whereas older granite samples from Radicondoli have magmatic to hydrothermal white micas that vary in ÎŽ18O from 4â10 â°. A pronounced intra-grain ÎŽ18O variability of up to 6 â° 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 ÎŽ18O values of 2.6 â° down from 10 â° 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 Όm-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 Ύ18O 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 Earthâs interior. Here, we show that radial cracks around SiO2 inclusions in ultrahigh-pressure garnets are caused by ultrafast decompression. Decompression rates of at least 8âGPa/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 âŒ2850 kg.mâ3 to âŒ3300 kg.mâ3) 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