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Raman spectroscopy on carbonaceous material (RSCM) is a method widely used to infer the temperature of metamorphism in metasediments. Furthermore, anomalies in the Raman signature of carbonaceous particles contained in fault zones have been interpreted as reflecting short-lived heating events due to friction during earthquakes. These applications of RSCM rely all on the assumption that temperature is the main factor controlling the reorganization of carbonaceous material (CM) and its expression by Raman spectroscopy. Nonetheless, few examples of natural fault zones and shear zones recently raised questions about the possible role of strain in such reorganization.In this work, we studied the influence of deformation on the Raman spectra of CM. We carried out experiments of deformation on low-grade shales in the Paterson and Griggs-type rigs, corresponding to low and high pressure conditions, for variable conditions of temperature and strain rates, simulating deformation at the brittle-ductile transition. In parallel, we examined in these experiments the evolution of CM using the intensity ratio (IR), defined as the intensity ratio of the Defect band over the Graphite band of the Raman spectrum.Deformation proceeded as a combination of slip on discrete planes, corresponding to macroscopic stick-slip events, and of ductile shear in few hundreds of µm’s-thick zones. The effect of stick-slip events on RSCM signal is unclear, principally because the deformed layers are too thin to contain CM to be analyzed. In contrast, in zones of distributed strain we observed a systematic and significant increase in IR compared to undeformed domains, reflecting an enhancement in the structural organization of CM. On the basis of additional experiments of static heating we carried out in parallel, we show that the IR increase cannot be connected to local and transient increase in temperature during the experiment, and has therefore to be connected to strain itself.The zones of concentrated ductile strain are characterized by comminution of the clasts of quartz and feldspar initially present, accompanied by the development of a porosity at the submicron-scale. Chemical analysis revealed that along with intense grain-size reduction, the chemical composition of the feldspar was modified. On the basis of these microstructural and chemical evidence, it appears that the elementary processes behind ductile deformation include intense mechanical fracturing and mass transfer along the grain boundaries.As a conclusion, it appears that strain has an undisputable effect to increase IR of . When applied to nature, the relevant temperature and mechanical realm of these experiments is the brittle-ductile transition, for temperature of the order or below 300°C. These conditions are also the ones corresponding to seismic deformation. Therefore we show here that Raman spectra in fault zones reflect not only the temperature history (including coseismic flash-heating), but also, and to a large extent, the strain history of the rock.

The Raman Spectroscopy of Carbonaceous Material (RSCM) allows quantifying the degree of crystallinity of carbonaceous material, which increases upon geological heating. Evolution of crystallinity of CM is now routinely used as a reliable geothermometer. Recently, RSCM approach has been used to evidence frictional heating during seismic events. This new application assumes that CM spectra reflect only the thermal record irrespectively of the potential impact of geological strain on CM crystallinity. The aim of this study is to reconsider this postulate by using analyses of Raman spectra in order to understand the effects of seismic deformation on the CM structure. For this purpose, we analyzed six pseudotachylyte veins from the Shimanto Belt (Japan) and the Kodiak Accretionary Complexes (USA), through high-resolution cross-sections. Samples are composed of foliated tectonic mélanges cut by millimetric shear planes filled by black vitreous material accompanied by injection veins, and present most of features described in the melt-origin rocks. The Raman peak area ratio as well as the intensity ratio show a drastic increase within the pseudotachylyte compared to the host rock. In addition, these parameters show a very sharp evolution in few microns along cross-sections across the PST boundaries, which is at variance with thermal diffusivity models applicable for others intrusive bodies. In order to understand such an evolution of the Raman parameters, 1D thermal diffusion and kinetics of RSCM evolution based on static heating experiments modellings were applied. Diffusion models show that the temperature of the pseudotachylyte materials and the surrounding rocks drops down very quickly after few milliseconds and returns to the background temperature after few seconds. Additionally, the kinetics modelling shows that a very large temperature must be applied for at least 1.0 x 10² ms to increase the intensity ratio. Furthermore, the cooling down of a molten layer is not able to reproduce the sharp evolution in RSCM parameters across the PST boundaries. Therefore, these results are not consistent with observation made on natural samples. It suggests that deformation is the main factor controlling CM crystallinity in fault cores. These results therefore also question the maximum temperature reached in fault zones, possibly much lower than previously estimated.

International audience; Heat from inflowing magma may act to seal permeable networks that assist passive outgassing at volcanic conduit margins and in overlying domes, reducing the efficiency of overpressure dissipation. Here we present a study of the evolution of permeability – measured under magmatic conditions - with increasing temperature in glassy and glass-poor basaltic andesites from Merapi volcano (Indonesia). Whereas the permeability of glass-poor rocks decreases little up to a temperature of 1010°C, glassy specimens experience a pronounced decrease in permeability above the glass transition once the viscosity of the crystal suspension is low enough to relax under external stresses. Changes in temperature alone are thus not enough to significantly modify the permeability of the glass-poor rocks that commonly form Merapi's dome. However, the presence of glass-rich domains in a dome may lead to local sealing of the volcanic plumbing between eruptions, exacerbating localized overpressure development that could contribute to explosivity.

International audience; The presence of melts in the first part of upper mantle is image by many geophysical studies. The nature of these melts is referring by different petrological and experimental studies that show a melt rich in volatiles, principally in CO2. To apprehend the deep carbon cycle, the first necessity is to know the mobility of these melts, like the viscosity. As a function of geodynamical context, the temperature and the pressure change, therefore different carbonated melts are produce. In this study, we investigate different SiO2/CO2/H2O content in melt to determine their influences on the viscosity. To investigate the viscosity, the experiment is making in ESRF in Grenoble. We produce in-situ measurements in Paris Edinburg cell with falling sphere viscometry experiments. Experiments are produce between 1 and 3 GPa and liquid temperature of sample. One result show a viscosity of 0.15 Pa.s for 20wt% of CO2. It is one order of magnitude more viscous than dolomite (Kono et al., 2013) and average 5 times less viscous than basalt (Sakamaki et al., 2013). Primarily results obtain in ESRF show a no linear effect of SiO2/CO2 content. Volatiles have an effect on viscosity and this has implication on melt migration in the mantle, so on the deep carbon cycle.

International audience; Partial melting of continental crust is a strong weakening process controlling its rheological behavior andductile flow of orogens. This strength weakening due to partial melting is commonly constrained experimentallyon synthetic starting material with derived rheological law. Such analog starting materials are preferentiallyused because of their well-constrained composition to test the impact of melt fraction, melt viscosity and meltdistribution upon rheology. In nature, incipient melting appears in particular locations where mineral and watercontents are favorable, leading to stromatic migmatites with foliation-parallel leucosomes. In addition, leucosomesare commonly located in dilatants structural sites like boudin-necks, in pressure shadows, or in fractures withinmore competent layers of migmatites. The compositional layering is an important parameter controlling melt flowand rheological behavior of migmatite but has not been tackled experimentally for natural starting material. In thiscontribution we performed in-situ deformation experiments on natural rock samples in order to test the effect ofinitial gneissic layering on melt distribution, melt flow and rheological response. In-situ deformation experimentsusing a Paterson apparatus were performed on two partially melted natural gneissic rocks, named NOP1 & PX28.NOP1, sampled in the Western Gneiss Region (Norway), is biotite-muscovite bearing gneiss with a week foliationand no gneissic layering. PX28, sampled from the Sioule Valley series (French Massif Central), is a paragneisswith a very well pronounced layering with quartz-feldspar-rich and biotite-muscovite-rich layers. Experimentswere conducted under pure shear condition at axial strain rate varying from 5*10-6 to 10-3 s-1. The main stresscomponent was maintained perpendicular to the main plane of anisotropy. Confining pressure was 3 kbar andtemperature ranges were 750C and 850-900C for NOP1 and PX28, respectively. For the 750C experimentsNOP1 was previously hydrated at room pressure and temperature. According to melt fraction, deformation ofpartially molten gneiss induced different strain patterns. For low melt fraction, at 750C, deformation within theinitially isotropic gneiss NOP1 is localized along large scales shear-zones oriented at about 60 from main stresscomponent 1. In these zones quartz grains are broken and micas are sheared. Melt is present as thin film (20m) at muscovite-quartz grain boundaries and intrudes quartz aggregates as injections parallel to 1. For highermelt fraction, at 850C, deformation is homogeneously distributed.

International audience; Though it is of prime importance in terms of seismic and tsunami risk, the mechanical behavior of the shallow(z

National audience; La compaction, la déformation et la diagenèse sont autant de processusqui modifient la rhéologie des sédiments accrétés au prisme d’accrétionou enfouis en subduction. A l’image de ces modifications rhéologiquesmajeures, les mécanismes de déformation dans la partie superficielle(

International audience; Seismic and magnetotelluric 22 field campaigns carried out across the Himalaya and the Tibetan Plateau show mid-crustal low resistivity and low velocity zones. Interpretation of these anomalous observations, either saline fluids or partial melts, is still largely debated partly because experimental data simulating crustal melting under relevant pressure, temperature and water content conditions have not been provided. We report laboratory measurements constraining the resistivity as a function of temperature and the viscosity at 800°C of natural metapelite during partial melting. Dehydration-melting of muscovite is simulated using a Paterson press at 300 MPa in the temperature range 600- 850°C. Electrical resistivity has been measured from the solid state (800°C) and viscosity has been determined at 800°C. Our results together with recent experimental constrains on seismic properties of partially molten rocks strongly suggest that the electrical and seismic anomalies measured underneath the Himalayan-Tibetan collisional orogen are best explained by partially molten rocks or local accumulation of pure melt bodies. This is also remarkably corroborated by the temperature-depth conditions of crustal partial melting and melt ponding expected from petrological surveys in the Himalayan range. However, our data suggest much higher melt fraction than previously thought and this implies regions in the middle crust having viscosities several orders of magnitude lower than previously assumed. High degree partial melting in the middle crust of the Himalayan-Tibetan orogenic system suggests revision of conceptual models on the development of mountain belts or that geophysical models addressing electrical resistivity at depth must be reevaluated.

International audience; Migmatites in the inner part of collisional orogens are markers of past partial melting during burial and/or exhumation of their crustal root. If melt production has a softening effect as soon as the very first percents of melt are produced, as suggested by experimental data, then the exact timing of partial melting initiation and the complete duration of the melting event need to be estimated. The Western Gneiss Region (WGR), a basement window within the Norwegian Caledonides, presents excellent exposures of migmatites associated with (ultra)-high pressure (UHP) eclogites and amphibolitic gneisses, appropriate for a field study on the relationships between partial melting, (U)HP metamorphism and subsequent retrogression. Chemical analyses of natural leucosomes in the WGR define a trend from trondhjemitic to granitic compositions. Their relative chronology allows correlation between trondhjemitic compositions and early melting stages in one hand, and granitic compositions and late melting stages in the other hand (Labrousse et al., 2011). A broader sampling of leucosomes in the WGR and their systematic chemical analysis allows an expansion of the observed trend in space and time. The natural trondhjemitic to granitic trend of the leucosome compositions is comparable to experimental results obtained in vapor present partial melting experiments in piston-cylinder (Labrousse et al., 2011), implying that water-present partial melting may have occurred at depth in the Caledonides. While bulk compositions of leucosomes relate to the partial melting conditions and the beginning of partial melting episode, their petrography is relevant for the determination of their retrograde equilibration stage and the termination of the partial melting. The petrography of the leucosomes displays a great diversity in terms of mineral assemblages and compositions. However, early and late leucosomes have different mineral compositions, implying different retrograde PT paths. First results of syn-kinematic partial melting experiments on a natural gneiss from the low temperature realm of the WGR in a Paterson apparatus at ISTO Orléans indicate that partial melting does not occur at temperature below 850°C (i.e. maximum temperature estimate for the peak conditions in the WGR) without free water. Higher temperature and/or fluid present deformation experiments show partial melting textures with melts segregated in a vein network parallel to uniaxial stress. Both piston-cylinder and Paterson experiments suggest that the Caledonian syn-kinematic partial melting event in the WGR is due to vapor present melting reactions. Even if such a reaction is known and expected for the late stages of melting at amphibolite facies conditions, its relevance at higher pressures has strong implications on the hydration state of the continental crust at depth. Keywords: migmatite, leucosome, partial melting, Paterson experiments, Western Gneiss Region, (U)HP. Labrousse L., Prouteau G. and Ganzhorn A.-C., 2011, Continental exhumation triggered by partial melting at ultrahigh pressure: Geology, v. 39, n°12, p. 1171-1174, doi: 10.1130/G32316.1.

International audience; Over the past 70-50 Ma the Indo-Asian collision induced 1400 km of north-south crustal shortening creating the Himalaya-Tibetan orogenic system. Establishing the three-dimensional geometry of thermomechanical structures of the lithosphere is a key parameter to understand the evolution of the Himalaya-Tibetan orogen. Magnetotelluric data can image the electrical resistivity under the surface. This parameter is influenced by the presence of an interconnected network of fluids in a solid matrix. Magnetotelluric data under the southern margin of the Tibetan Plateau show widespread low resistivity zones (10 to 3 ohm.m) interpreted as possible partial melting of the middle crust. Existing experimental studies on electrical resistivity of partially molten crustal rocks have been conducted on granulite samples at 0.1 MPa confining pressure under dry conditions. These studies establish the resistivity of the partially molten crustal rocks at a temperature similar to 1100°C, which is well above the expected temperature prevailing in the middle crust of orogenic belts. In order to better reproduce natural conditions and interpret the resistivity values in terms of petrological processes, we have conducted experiments on metapelitic rocks undergoing muscovite dehydration melting. The starting material is a paragneiss presenting alternating muscovite/biotite-bearing layers with quartzo-feldspathic layers and containing on average 2 wt% water. An impedance spectrometer was coupled to an internally heated Paterson-type deformation apparatus in order to measure the in situ resistivity variations in static conditions and during deformation in torsion (simple shear, strain rates from 1x10-5 s-1 to 1x10-4 s-1) at 300 MPa confining pressure and temperatures between 500°C and 850°C. In the temperature range 500 - 850°C, resistivity decreases from 4600 ohm.m to 8 ohm.m, and indicates the onset of fluid-absent partial melting at temperature similar to 700°C. Our results show that the resistivity measured during partial melting at 800 - 850°C under static conditions is very low (10 - 8 ohm.m), as melt percentage reaches 30%. Both temperature and melt fraction ranges are consistent with conditions of melt extraction during migmatization and with temperatures of leucogranitic pluton emplacements. We conclude that the resistivity measured in natural metapelitic rocks, undergoing partial melting under P-T conditions relevant to the formation of migmatites in continent-continent collisional belts, corresponds to the anomaly detected under the Tibetan Plateau (10 ohm.m at 20 - 25 km depth). The migration of these partial melts most likely lead to the formation of leucogranitic plutons in the middle crust (with expected resistivity similar to 3 - 4 ohm.m) of the Greater Himalaya Sequence triggering channel flow and associated ductile extrusion.

International audience; The replenishment by a hot, basic magma is supposed to upset a differentiated magma chamber by magma mixing. The changes in temperature, convection patterns, extent of crystallization, or the vesiculation of the re-heated felsic magma may be all factors that ultimately trigger eruption. However, only little experimental work has been performed so far on mixing/mingling processes, and the existing data base concerns essentially atmospheric pressure. In order to characterize magma mixing, hybridization and related events, we performed experiments juxtaposing two contrasted magma compositions. We have chosen to use a synthetic haplotonalitic composition (SiO2: 68.9%, Al2O3: 19.6%, Na2O: 8.0%, CaO: 3.5%) as the felsic end-member, and natural basalt from Santorini Volcano (Greece). The geometries adopted were (1) a mafic core plug inserted into an outer hollow felsic cylinder and (2) two basaltic disks separated by a felsic one. Then, we simulated a simple shear deformation using a Paterson HP-HT apparatus, at 300 MPa confining pressure, and temperatures from 900 to 1200°C. The temperature range allowed us to get the lowest viscosity contrast at which magma mixing is enhanced. Under these conditions calculated relative viscosities of both felsic and mafic end-members range from Log η = 8.2 to Log η = 1.3 Pa.s, depending on temperature. However, crystal content changes considerably the apparent viscosity. Applied shear strain rates range from γr = 7x10-5 s-1 to γr = 1x10-3 s-1. Finite shear strains (γ) range between 0 (static experiments) and 5.4. SEM images on run products show textures and particular fabrics such as enclave formation, mafic material dislocation, whereas microprobe analyses reveal basalt melting and mixing with the felsic end-member producing andesitic melts. In addition, close to the contact between the two materials, basaltic phenocrysts are replaced by a new generation of crystals, different from plagioclases growing from the haplotonalite only. The comparison with natural textures from e.g. Santorini volcano will constrain the conditions required for magma mixing at this volcano. Experiments under hydrous conditions relevant to the Santorini context are in progress. These experiments will help us to find out the consequences of such a common process partly responsible of explosive eruptions in volcanic arcs.

International audience; The rheological behaviour of synthetic crystal-bearing magmas containing up to 75 vol.% of crystals (0≤ ΦS ≤ 0.75) has been investigated experimentally at a confining pressure of 300 MPa and temperatures between 475 and 600°C at shear rates between 10-4 and 2x10-3 s-1. Starting hydrated crystal-bearing glasses were synthesized from a dry haplogranitic glass (Qz36Ab39Or29) and 2.5 wt.% water mixed with 0 (pure hydrous melt), 16, 34, 54, 65 or 75 vol.% of Al2O3 sieved (45 < Ø < 90 µm) crystals. Shear viscosity measurements were performed in torsion in a Paterson gas-medium apparatus. For each composition, the rheological data were fitted to a power law of the form: (1) where is shear rate, is shear stress , T is temperature, R is the gas constant, and A, n and Q are empirical parameters defined as the preexponential term, the stress exponent and the activation energy, respectively. The rheology is found to be newtonian (n=1) for pure hydrated melt and 16 vol.% of crystals. At higher crystal contents, the magmas behave as pseudoplastic materials, and exhibit shear thinning behaviour with stress exponents of 1.3, 1.9 and 2.8 for 34, 54 and 65 vol.% of crystals, respectively. The activation energy seems to be nearly independent of crystal fraction and has a mean value of 231 kJ.mol-1. The Einstein-Roscoe equation adequately estimates viscosities of the crystal-bearing magmas at low crystal contents (ΦS ~≤ 0.25), but progressively deviates from the measured viscosities with increasing crystal content as the rheological behaviour becomes non-newtonian. A modified power-law formulation incorporating crystal content as a variable allows a fit to the experimental data set over the whole range of conditions.

Structures such as Mineral fabrics and mechanical anisotropies together with the rheological properties are key parameters to constraint the flow history of crystallising magmas. In particular, a rheological transition from an heterogeneous granular dilatant suspension to a solid material is expected at solid fractions 40%

Pr. Anne NEDELEC - Université de Toulouse ExaminatriceDr. Micha BYSTRICKY - ETH Zürich RapporteurPr. Michel FAURE - Université d'Orléans ExaminateurDr. Didier LAPORTE - CR CNRS Clermont-Ferrand RapporteurDr. Michel PICHAVANT - DR CNRS Orléans Directeur de thèseDr. Bruno SCAILLET - CR CNRS Orléans Examinateur; The development of shape-preferred orientation in magmas is often attributed to crystal rotation under anisotropic stresses. This mechanism has been investigated on a theoretical and experimental point of view. Previous experimental studies, however, barely consider the crystallisation mechanism under differential stresses.Experiments performed at high pressure and high temperature (HP-HT) aimed at investigating this process, as well as crystal rotation. Such studies, however, require a device capable of magma deformation at HP-HT. This manuscript presents the tests of two devices specifically developed and the results of some runs performed in a Paterson Press. Most of the discussion is devoted to the experimental artefacts encountered and to the means to solve them.; Le développement des Orientations Préférentielles de Forme (OPF) dans les roches magmatiques est souvent considéré comme résultant de la rotation des cristaux sous l'effet de contraintes anisotropes. Ce mécanisme a été étudié tant du point de vue théorique qu'expérimental. Cependant, les travaux expérimentaux sur le sujet, notamment ceux analogiques ont rarement considéré la cristallisation sous l'effet de contraintes différentielles.Les travaux entrepris à haute température - haute pression (HT-HP) se proposaient d'étudier ce phénomène ainsi que la rotation des cristaux. Une telle étude nécessite de disposer d'un appareil de déformation des magmas fonctionnant à HT-HP. Ce mémoire présente les tests de deux appareils développés dans cette optique ainsi que ceux réalisés en presse Paterson. Une large place est également consacrée aux artefacts expérimentaux rencontrés et aux moyens mis en œuvre pour les résoudre.

Na- and Ca-exchanged Wyoming montmorillonites have been dehydroxylated by heating during 1 hour at 750°C and rehydroxylated by hydrothermal treatment (350°C, 100MPa) in a tuttle type pressure vessel with different staying time. Due to the octahedral cations migration induced by the dehydroxylation, the initial layers which are cv (vacant octahedral sites in cis positions) become tv (vacant octahedral sites in trans positions) after the treatment [1]. This transformation is accompanied by a motion of the interlamellar cations to compensate the local modifications in the distribution of charges [1, 2]. In the dehydroxylated state the cations go down in the hexagonal holes

The development of shape-preferred orientation in magmas is often attributed to crystal rotation under anisotropic stresses. This mechanism has been investigated on a theoretical and experimental point of view. Previous experimental studies, however, barely consider the crystallisation mechanism under differential stresses.Experiments performed at high pressure and high temperature (HP-HT) aimed at investigating this process, as well as crystal rotation. Such studies, however, require a device capable of magma deformation at HP-HT. This manuscript presents the tests of two devices specifically developed and the results of some runs performed in a Paterson Press. Most of the discussion is devoted to the experimental artefacts encountered and to the means to solve them., Le développement des Orientations Préférentielles de Forme (OPF) dans les roches magmatiques est souvent considéré comme résultant de la rotation des cristaux sous l'effet de contraintes anisotropes. Ce mécanisme a été étudié tant du point de vue théorique qu'expérimental. Cependant, les travaux expérimentaux sur le sujet, notamment ceux analogiques ont rarement considéré la cristallisation sous l'effet de contraintes différentielles.Les travaux entrepris à haute température - haute pression (HT-HP) se proposaient d'étudier ce phénomène ainsi que la rotation des cristaux. Une telle étude nécessite de disposer d'un appareil de déformation des magmas fonctionnant à HT-HP. Ce mémoire présente les tests de deux appareils développés dans cette optique ainsi que ceux réalisés en presse Paterson. Une large place est également consacrée aux artefacts expérimentaux rencontrés et aux moyens mis en œuvre pour les résoudre.

The microstructures and the rheological behaviour of synthetic magmatic suspensions in the range of 0% to 80% solid fraction have been investigated in torsion experiments. The liquid phase is a haplogranitic melt (HPG) with 2.5 Wt % H20. The solid phase is composed of a sieved fraction (45Μm

The synthetic clays are used in a lot of studies to analyse clay properties. They permit to have samples without impurities and with a specific chemical composition. In synthetic smectite group, trioctahedral smectites as saponite were generally studied. Because the majority of natural smectites are dioctahedral ones, it can be interesting to product dioctahedral synthetic smectites. This is the objective of this work. The synthesized smectites should be mono-phased with well-known characteristics and chemical composition. In this study we have synthesized beidellites with different tetrahedral charges and different structural Fe3+ content and montmorillonites with different octahedral charges. Synthesis protocol : Gels used for starting materials were prepared according to the usual method from the following compounds : tetraethyl orthosilicate (TEOS), Mg(NO3)2.6H2O, Al(NO3)3.9H2O, Fe(NO3)3.9H2O, HNO3, Na2CO3, NH4OH and ethanol. After dissolution of Al, Mg nitrate and Na carbonate in nitric acid, the TEOS and ethanol were added. A precipitate was obtained after neutralization of the solution; this precipitate was dried and heated to 400°C to obtain Al, Mg, Si, Na oxides gel. The samples were synthesized in an internally heated pressure vessel. Starting gel was sealed with water solution in a gold capsule. The mixture was treated at 350°C under a variable pressure between 200 and 1500 Bars (depending on the sample) during 15 days. Results : First, we present results for synthetic beidellites with different tetrahedral charges. In the reaction products, no additional phases was observed by XRD. Secondly, beidellites with iron content were synthesized with a maximum FeV I/AlV I ratio equal to 1. In this syntheses, a small quantity of iron oxide was present in final product. Infrared analysis permit to assign the principal part of iron to the clay phase. Iron rich beidellites can not be obtained in these conditions. If we increase the FeV I/AlV I ratio above 1, the reacted product is not iron rich beidellite. We obtain a mixture between iron oxides, opal and sometimes smectite phase. Lastly, for montmorillonites synthesized at low pressure, only smectite was identified by XRD but this product is probably a mixture between beidellite and saponite. At more important pressure, the results seem to be different because only one smectite phase was identified by XRD.

Magmatic fabrics are common in volcanic and plutonic rocks. In the latter, these textures are often used as markers of regional deformation patterns. However, the origin of such fabrics is still not well understood. The most commonly accepted hypothesis is that these textures originate by a magmatic flow mechanism. We are currently testing another hypothesis, namely: the possibility that deformation affects crystallisation and thus, fabric patterns. The main difficulty in such experiments comes from an antagonism between the low viscosity of magmas (i.e. fast deformation) at relevant geological temperatures and the slow kinetic of crystallisation. To overcome this problem, we start our deformation experiments with an already partially crystallised magma (crystallinity = 30%), which increases the viscosity by about 1 order of magnitude. The experimental procedure is divided in two steps: synthesis in IHPV at 850°C, 300 MPa, during 10 days, of partially crystallised glass cylinders (starting material: a rhyodacitic glass, 69% SiO2, with 3 wt% bulk H2O), followed by the deformation of the cylinder at the same pressure but at a temperature below that of synthesis (in the range 700-800°C), in order to induce a second crystallisation stage. The effect of deformation is characterised by comparing deformed and undeformed charges produced at the same temperature from the same starting materials. To this end, we have designed a deformation apparatus (Champallier et al., 2001). This first prototype works at high strain rate and does not allow to accurately determine the onset of deformation at HP-HT. The resulting deformed cylinders all displayed complex crossed fabric patterns difficult to interpret. A new deformation apparatus, overcoming the problems of both onset and strain rate, is currently being tested. It will be presented with the results on fabrics and nucleation obtained from deformation/crystallisation experiments. Champallier R, Scaillet B, Pichavant M, Pons J, Peilleron L, Bellenoue D & Lefèvre A, Journal of abstracts of the 26th Assembly of the European Geophysical Society, (2001).

Magmatic fabrics are common in volcanic and plutonic rocks. In the latter, these textures are often used as markers of regional deformation patterns. However, the origin of such fabrics is still not well understood. The most commonly accepted hypothesis is that such textures originate by a magmatic flow mechanism. In order to characterise this mechanism and test the possibility of generation of strong magmatic fabrics, in-situ experiments on the orientation of crystals in an already partially crystallised magma are being conducted. The experimental procedure is divided in two steps: partial crystallisation of glass cylinders in Internally Heated Pressure Vessels (IHPV), followed by the deformation of the cylinders at 0.1MPa. Experiments are conducted on the Eburru rhyolite (Kenya), a massive, bubble- and crystal-free, peralkaline obsidian with 0.2 wt % H2O, from which glass cylinders have been cored. In order to explore a wide range of crystallisation, partially crystallised cylinders are produced at different temperatures between 700°C and 900°C and 150 MPa. Kinetics of crystallisation of peralkaline magmas ensure a fast attainment of chemical equilibrium (100 to 300 hours depending on temperature). Deformation experiments are conduced in a new apparatus designed to deform cylindrical samples (L=10-15mm – Ø=5mm) of crystal-bearing magma. This new apparatus works on the principle of the parallel-plate viscosimeter (coaxial strain). It allows deformation to be performed at different temperatures, pressures, strain rates, shortening ratios, and viscosity to be measured as well. Calibration measurements of the apparatus on viscosity of the standard melt DGG1 and the Eburru rhyolite at 0.1 MPa have demonstrated its precision to be within ±0.2 log units. Deformation experiments are conduced at 0.1 MPa between 600°C and 850°C. Due to the short duration of experiments, neither new crystallisation nor bubble nucleation occurs. The results obtained from deformation at 0.1 MPa will be presented and discussed.