Benoit Dubacq

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Sorbonne University | UPMC · Institut des sciences de la Terre de Paris (ISTeP) - UMR 7193

We present a semi-empirical thermodynamic model with uncertainties that encompasses the full range of compositions in H2O–CO2–NaCl mixtures in the range of 10–380 °C and 1–3500 bars. For binary H2O–CO2 mixtures, the activity–composition model is built from solubility experiments. The parameters describing interactions between H2O and CO2 are indepe...

Plagioclase feldspars are key components of terrestrial planets and deriving their phase diagram and activity–composition relations has been the subject of many studies. Three main approaches have been used for thermodynamic modelling of the behaviour of plagioclase feldspars, and all three disagree in terms of the magnitude and relative importance...

Understanding how halogens are distributed among usual hydrous silicates in the lithosphere is important to constrain their deep geochemical cycle and fluid-rock interactions in subduction zones. This article presents first-principles modelling of halogen (F−, Cl−, Br−) incorporation in hydrous silicates including mica, chlorite, serpentine, amphib...

Understanding and quantifying the partitioning of elements at low concentrations is important for petrology, as minor and trace elements are used as tracers of many geological processes. The lattice strain model has consequently been found to be very useful, as it links mineral/melt partition coefficients to the elastic properties of the mineral an...

Subduction zones are first-order features of plate tectonics on Earth, yet the mechanisms by which subduction initiates remain enigmatic and controversial. Here, we reappraise the timing of metamorphism of the rock units first detached from the leading edge of the downgoing slab during initiation of the Neotethys subduction, now preserved in the me...

Biotite, white mica, and chlorite record petrological processes and pressure-temperature conditions. Unfortunately, their crystal chemistry is complicated and thermodynamic modelling of their phase relations is hindered by this complexity. This article reviews the main atomic substitutions in metapelitic biotite, white mica, and chlorite and their...

Lawsonite is important as a carrier of H2O in subduction zones and as a petrological tracer. The trace-element content of lawsonite in mafic rocks has been used as a record of fluid–rock interactions but has received less attention in metamorphosed oceanic sediments. This study documents the major, and trace-element composition, together with ⁸⁷Sr/...

Very-low-grade mineral veins investigated in the impure limestone massif of the Aravis (Haute-Savoie, France) yielded a rich mineral assemblage typified by dolomite, calcite, quartz, illitic mica, fluorite and three types of chlorite. The vein network extends over more than 5 km and was probably emplaced around peak burial of the limestone (∼7 km d...

The Lyngen Magmatic Complex (LMC) is the lowest unit of the Lyngsfjellet Nappe (Upper Allochthon, North Norwegian Caledonides). The fabrics of the LMC rocks range from undeformed to mylonitic. The undeformed rock is a gabbro-norite formed primarily by anorthite-rich (93%) plagioclase, enstatite, and augite. Two deformation events are distinguished...

This study explores the relative importance of fluid availability, temperature and mineral assemblage variations on the development of macro and microstructures in greenschist-facies shear zones of two granitoids of the Axial Zone (central Pyrenees) emplaced at the end of Variscan orogeny, at similar structural levels. The investigated shear zones...

Please designate the presenter/contributor author(s)?: Sarah Figowy Abstract Content: First-principles modelling quantifies the distribution of halogens in hydrous silicates representative of subduction zones-including mica, chlorite, serpentine, amphibole, epidote and carpholite. Estimation of the energetic cost of the substitution of OH groups by...

Different microstructures and quartz recrystallization mechanisms can be observed in shear zones of granites that formed in similar greenschist-facies conditions. It is generally assumed that temperature plays a major role on quartz rheology and recrystallization. However, at low-grade conditions, fluid percolation also controls strain accommodatio...

The Queyras-Monviso traverse of the Schistes Lustrés complex, is a stack of underplated nappes of oceanic rocks subducted from blueschist- to eclogite-facies conditions. This study reports on salinities and gas contents of primary fluid inclusions trapped in high pressure veins from metasediments and metamafic rocks (445 fluid inclusions in 22 samp...

Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is frequently used for pressure–temperature–time constraints of deformation events, although it is often highly heterogeneous, even in...

Understanding the geochemical behaviour of trace and minor elements in mineral assemblages is of primary importance to study small- and large-scale geological processes. Partition coefficients are frequently used to model the chemical evolution of minerals and fluids during melting and in metamorphic rocks of all grades. However, kinetic effects ha...

This study investigates the reactions allowing crystallization of large amounts of lawsonite (CaAl2Si2O7(OH)2•[H2O]) found in calcschists metamorphosed in a subduction zone setting. Previous studies of lawsonite-forming reactions in metasediments have highlighted its importance for the large-scale budget of CO2, as the calcium required to form laws...

This contribution investigates mechanisms controlling subduction development and stabilization over time (coined as 'slabitization'), from a nascent slab to a mature slab viscously coupled to mantle convection, from grain scale to plate tectonics scale. Frozen-in, deep and warm portions of the subduction plate interface with both sides still preser...

Metamorphic soles are m to ~500 m thick tectonic slices welded beneath most large- scale ophiolites (usually ~20 km thick). They typically show a steep inverted metamorphic structure where the pressure and temperature (T) conditions of crystallization increase upward, from the base of the sole (500 ± 100°C at 0.5 ± 0.2 GPa) to the contact with the...

Pre‐kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of t...

This study sheds light on the deformation mechanisms of subducted mafic rocks metamorphosed at amphibolite and granulite facies conditions and on their importance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab and...

Chlorite is a ubiquitous product of metamorphism, alteration of magmatic rocks and hydrothermal processes owing to its large stability field and wide compositional range. Its composition is governed by several substitutions and has been used as a geothermometer, on the basis of empirical, semi-empirical, and thermodynamic models. As in some other p...

The metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé peninsula, Quebec, Canada) is a sliver of Ordovician oceanic crust accreted to the base of an incomplete ophiolitic sequence, along a suture zone throughout the north-eastern Appalachians linked to the Taconian orogeny. A detailed mineralogical study of the rocks in the metamorphic se...

This study sheds light on the deformation mechanisms of subducted mafic rocks metamorphosed at amphibolite and granulite facies conditions, and on their importance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab an...

The basal mantle of the Semail ophiolite directly overlying the metamorphic sole has been affected by a late and relatively low temperature ductile deformation event, ascribed to the deformation of the mantle just above the plate interface during subduction infancy (which ultimately led to ophiolite obduction). We show that this deformation results...

Metamorphic soles are tectonic slices welded beneath most large-scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intra-oceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate. This s...

The dissolution of CO2 into formation brines and the subsequent reactions of the CO2-charged brines with reservoir minerals are two key processes likely to increase the security of geological carbon-dioxide storage. These processes will be dependent on the permeability structure and mineral compositions of the reservoirs, but there is limited obser...

Despite decades of petrological and geochemical studies, the nature and setting of obducted ophiolites remain controversial: the influence of supra-subduction zone environments on pre-existing oceanic lithosphere is yet to assess, and the processes leading to subduction/obduction initiation are still poorly constrained. Our study documents successi...

L’étude du métamorphisme d’une roche est intimement liée à celle de la migration des espèces chimiques dans la roche. Aux températures inférieures à 500°C, la migration des espèces chimiques est dominée par les processus impliquant les fluides, même en l’absence de mouvement de ces fluides, la diffusion permettant le déplacement des ions dissous. L...

Rocks are commonly polycrystalline systems presenting multi-scale chemical and structural heterogeneities inherited from crystallization processes or successive metamorphic events. This work illustrates how spatially resolved analytical techniques coupled with thermodynamic approaches allow rock compositional variations to be related to large-scale...

XMapTools is a MATLAB©-based graphical user interface program for electron microprobe X-ray image processing, which can be used to estimate the pressure–temperature conditions of crystallization of minerals in metamorphic rocks. This program (available online at http://www.xmaptools.com) provides a method to standardize raw electron microprobe data...

[1] Mobilization of contaminants by CO2-charged brines is one concern relating to injection of CO2 as part of carbon capture and storage projects. This study monitors the mobility of trace metals in an exhumed CO2-charged aquifer near the town of Green River, Utah (USA), where CO2-charged brines have bleached red sandstones, and concentrated trace...

A thermodynamic model for phase equilibria in the H2O-CO2-NaCl system

Obduction, whereby fragments of dense, oceanic lithosphere (ophiolites) are presumably 'thrust' on top of light continental ones, remains a poorly understood geodynamic process, in particular with respect to 1) obduction initiation and 2) effective ophiolite emplacement. Most of our knowledge on obduction initiation comes from the amphibolite to gr...

Obduction corresponds to one of plate tectonics oddities, whereby fragments of dense, oceanic lithosphere (ophiolites) are presumably 'thrust' on top of light continental ones. Though reported from most convergent belts, the emplacement of ophiolites is still poorly understood. The thin HT metamorphic soles (i.e., 800°C - 1 GPa, on average) frequen...

Ductile deformation is partly accommodated by mineral recrystallization. Mineral growth is controlled by both thermodynamics and reaction kinetics, where fluid availability and deformation also play a key role, in particular at temperatures below about 400°C. In these temperature conditions, phyllosilicates are ubiquitous and reactive, and often re...

CO2 sequestration is regarded as an important strategy for reducing anthropogenic CO2 emissions. Both the nature and rate of fluid–mineral reactions in CO2–water–rock systems are crucial, yet poorly constrained, parameters in understanding the fate of CO2 injected in geological formations. This study models reactions and reaction rates in an exhume...

Modelling the progress of geochemical processes in CO2 storage sites is frustrated by uncertainties in the rates of CO2 flow and dissolution, and in the rates and controlling mechanisms of fluid-mineral reactions that stabilise the CO2 in geological reservoirs. Dissolution of CO2 must be controlled by the complexities of 2-phase flow of CO2 and for...

Obduction corresponds to one of plate tectonics oddities, whereby fragments of dense, oceanic lithosphere (ophiolites) are presumably 'thrust' on top of light continental ones. Though reported from most convergent belts, the emplacement of ophiolites is still poorly understood. The thin HT metamorphic soles (i.e. 800 • C-1 GPa, on average) frequent...

Secure storage of anthropogenic carbon dioxide (CO2) in geological reservoirs requires predicting gas–water–rock interactions over millennial timescales. Noble gases and carbon isotope measurements can be used to shed light on the nature of competing dissolution—precipitation processes over different timescales, from the fast dissolution of gaseous...

a b s t r a c t During chemical weathering, magnesium (Mg) is released by the dissolution of both carbonate and silicate sources. The degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs, or cycling by a series of processes associated with weathering is poorly constrained. In...

Red sandstones near Green River, Utah (United States), have been bleached by diagenetic fluids. Field relationships, modeling, fluid inclusion and isotopic data suggest that the causal fluid was a CO2-charged brine, distinguishing this site from hydrocarbon-related bleaching elsewhere on the Colorado Plateau. Mineralogical and chemical profiles fro...

Reactions between CO2 injected into geological formations and aquifer minerals may lead to permanently storage of the CO2 as carbonate minerals, or cause leakage via corrosion of caprock and well seals [1]. Reactive transport models aimed at predicting the long term fate of injected CO2 suffer from a poor knowledge of kinetic reaction rate paramete...

Dissolution of CO2 in brines and reactions of the acid brines ultimately dissolving silicate minerals and precipitating carbonate minerals are the prime long-term mechanisms for stabilising the light supercritical CO2 in geological carbon storage. However the rates of dissolution are very uncertain as they are likely to depend on the heterogeneity...

Pressure–temperature (P–T) paths as complete as possible and with a precision on the km-scale or less are needed to further improve the knowledge of deformation, re-equilibration processes and element ⁄ fluid transfer, in particular along subduction zones. This contribution attempts to (i) critically evaluate the precision and continuity with which...

The timing of onset of modern-style plate tectonics is debated. The apparent lack of blueschist metamorphism--a key indicator of modern plate subduction--in rocks aged more than about 1 billion years calls into question the existence of plate tectonics during the Archaean and Palaeoproterozoic eras. Instead, plate tectonics and subduction could hav...

Predicting phase relations and reactions involving phyllosilicates is of critical importance when modeling geological reservoirs and making thermobarometric estimates at temperatures below 350 C. However, it is difficult to perform experimental studies of phase relations of phyllosilicates because of very slow reaction rates. Simple methods of esti...

Volatiles play important roles in the dynamics of subduction systems from the shallowest levels of the plate interface to the greatest depths of arc melting source region. Near the trench, the release of water during the breakdown of smectite into illite is often proposed to be responsible for the elevated pore pressure associated with low frequenc...

The northeastern continental margin of Oman in the Saih Hatat region is characterized by high-pressure (HP) chloritoid- or carpholite-bearing metasediments and highly deformed mafic eclogites and blueschists in a series of tectonic units bounded by high-strain ductile shear zones. New data on the upper cover units of this HP nappe stack indicate th...

The arguments of Nieto et al. (2010) in favor of the incorporation of H3O+ rather than H2O in interlayer positions of illite are disputable. Stoichiometric arguments suggest that the excess water in the Silver Hill illite is in the form of H2O. Moreover, recent thermodynamic models assuming the incorporation of interlayer H2O in illite provide reas...

The northeastern continental margin of Oman in the Saih Hatat region is characterised by high-pressure (HP) chloritoid- or carpholite-bearing meta-sediments and highly deformed mafic eclogites and blueschists in a series of tectonic units bounded by high-strain ductile shear zones. New data on the upper, carpholite-bearing cover units of this HP na...

Significant progress in metamorphic modelling has opened new opportunities to decode the P-T records of low-temperature metamorphic rocks (Yamato et al, 2007 and refs. therein) that comprise large tracts of exposed Archean and Paleoproterozoic terrains. We have seized these opportunities and report on the first discovery of blueschist metamorphic c...

We propose a solid-solution model for dioctahedral aluminous phyllosilicates accounting for the main compositional variations, including hydration, observed in natural smectites, interlayered illite/smectite, illites, and phengites from diagenetic to high-grade metamorphic conditions. The suggested formalism involves dehydrated micas and hydrated p...

We propose a thermodynamic approach to model the stepwise dehydration with increasing temperature or decreasing H2O activity of K, Na, Ca and Mg-smectite. The approach relies on the relative stability of the different solid-solutions that describe the hydration of di- or trioctahedral-smectites containing 0, 1, 2 or 3 interlayer water layers. The i...

The variation of smectite volume during dehydration with increasing temperature or decreasing aH2O is a step function. Discontinuous variations of volume at discrete temperature or aH2O resulting from the loss of water layers alternate with a continuous dehydration at almost constant volume and number of water layers. The discontinuous character of...