Fluid inclusions in quartz are known to modify their shapes and microstructures (textures) during weak plastic deformation. However, such changes have not been experimentally demonstrated and criteria are not available to relate them to paleostress conditions. To address these issues, quartz crystals containing natural CO 2 -H 2 O-NaCl fluid inclusions have been experimentally subjected to compressive deviatoric stresses of 90-250 MPa at 700°C and *600 MPa confining pressure. Strains of up to 1% cause the inclusions to develop irregular shapes and to generate microcracks in crystallographic planes oriented subperpendicular to the major compression axis, r 1 . The uniform alignment of the microcracks imparts a planar fabric to the samples. The microcracks heal and form swarms of tiny satellite inclusions. These new inclusions lose H 2 O by diffusion, thereby triggering plastic deformation of the surrounding quartz via H 2 O-weakening. Consequently, the quartz samples deform plastically only in domains originally rich in inclusions. This study shows that fluid inclusions deformed by deviatoric stresses may indeed record information on paleostress orientations and that they play a key role in facilitating crystal-plastic deformation of quartz.

International audienceThe Lavrion peninsula is located along the western boundary of the Attic-Cycladic metamorphic complex in the internal zone of the Hellenic orogenic belt. The nappe stack is well exposed and made, from top to bottom, of (i) a non-metamorphic upper unit composed of an ophiolitic melange, (ii) a middle unit mainly composed of the Lavrion schists in blueschist facies, (iii) and a basal unit mainly composed of the Kamariza schists affected by pervasive retrogression of the blueschist facies metamorphism in greenschist facies. The middle unit is characterized by a relatively steep-dipping foliation associated with isoclinal folds of weakly organized axial orientation. This foliation is transposed into a shallow-dipping foliation bearing a N-S trending lineation. The degree of transposition increases with structural depth and is particularly marked at the transition from the middle to the basal unit across a low-angle mylonitic to cataclastic detachment. The blueschist facies foliation of the Lavrion schists (middle unit) is underlined by high pressure phengite intergrown with chlorite. The Kamariza schists (basal unit) contains relics of the blueschist mineral paragenesis but is dominated by intermediate pressure phengite also intergrown with chlorite and locally with biotite. Electron probe micro-analyzer chemical mapping combined with inverse thermodynamic modeling (local multi-equilibrium) reveals distinct pressure–temperature conditions of crystallization of phengite and chlorite assemblages as a function of their structural, microstructural and microtextural positions. The middle unit is characterized by two metamorphic conditions grading from high pressure (M1, 9–13 kbar) to lower pressure (M2, 6–9 kbar) at a constant temperature of ca. 315 °C. The basal unit has preserved a first set of HP/LT conditions (M1–2, 8–11 kbar, 300 °C) partially to totally transposed-retrogressed into a lower pressure mineral assemblage (M3, 5–8.5 kbar) associated with a slight but significant increase in temperature (∼350 °C)

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Pipe-like ultramafic bodies, hosting Ni-Cu-PGE sulphide deposits, INTRODUCTIONintrude the Main Gabbro and the roof metasediments of the Ivrea The Ivrea Zone, in NW Italy, is believed to represent an Zone, NW Italy. These bodies were emplaced at 287 ± 3 Ma exposed section of the lower continental crust (Mehnert, and represent the last mantle-derived melts associated Garuti et al., 1980;Percival et al., 1992) whose underplating event that largely drove the crustal evolution of this area evolution has been largely driven by the underplating during the late Carboniferous (>300-290 Ma). The pipes are of mantle-derived basic magmas (the Basic Complex), composed of volatile-rich ultramafic rocks and gabbros with an associated with the uplift of large fragments of mantle alkaline signature simultaneously enriched in both incompatible and peridotites (the Mantle Tectonite massifs of Baldissero, the most compatible elements but depleted in elements of intermediate Balmuccia and Finero), beneath the metamorphic basecompatibility. The isotope composition of these pipe rocks is 290 Ma (Nd) ment of the Southern Alps (the Kinzigitic Formation) >3•7 to −1•9 and 290 Ma (Sr) >0•8-26. In a 290 Ma (Nd) vs (Fig. 1). Although this area has been the object of 290 Ma (Sr) diagram they define a linear array between the unnumerous studies that have significantly contributed to metasomatized and metasomatized peridotites of Finero, but distinctly the understanding of the evolution of the lower conoblique with respect to the trend defined by Balmuccia peridotites. tinental crust [see Percival et al. (1992) and references The 34 S ranges from 0•0 to +0•9‰ and is indicative of a mantle therein], many aspects of the underplating process, espesource. We suggest that the pipes represent infiltration of melts derived cially the timing of emplacement of basic magmas and from a depleted mantle protolith flushed with alkaline metasomatic whether the environment was a mantle plume (e.g. fluids, probably of juvenile mantle origin, which underwent partial Shervais & Mukasa, 1991) or a subduction setting (e.g. melting as a consequence of the depression of the solidus owing to Hartmann & Wedepohl, 1993;Zanetti et al., 1999), are the increased activity of water and other volatiles. The similarity in still unresolved. age, trace-element, and isotopic signatures indicates that the pipes A particularly striking group of intrusions, whose study were probably produced in the course of the same metasomatic event may be relevant for understanding the geodynamic setting that affected the Finero ultramafic body. The overall geochemical and evolution of the Ivrea Zone, consists of pipe-like characteristics of the pipes are more consistent with magmatism related bodies formed of amphibole-rich peridotites and pyto a mantle plume than with a subduction setting.roxenites with minor hornblendites and gabbros, anomalously rich in volatiles and incompatible elements, which were emplaced into either the upper part of the

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Fluid inclusions in quartz are known to modify their densities during shear deformation. Modifications of chemical composition are also suspected. However, such changes have not been experimentally demonstrated, their mechanisms remain unexplained, and no criteria are available to assess whether deformed inclusions preserve information on paleofluid properties. To address these issues, quartz crystals containing natural CO 2-H 2 O-NaCl fluid inclusions have been experimentally subjected to compressive deviatoric stresses of 90-250 MPa at 700°C and *600 MPa confining pressure. The resulting microcracking of the inclusions leads to expansion by up to 20%, producing low fluid densities that bear no relation to physical conditions outside the sample. Nevertheless, the chemical composition of the precursor inclusions is preserved. With time the microcracks heal and form swarms of tiny satellite inclusions with a wide range of densities, the highest reflecting the value of the maximum principle stress, r 1. These new inclusions lose H 2 O via diffusion, thereby passively increasing their salt and gas contents, and triggering plastic deformation of the surrounding quartz via H 2 O-weakening. Using microstructural criteria to identify the characteristic types of modified inclusions, both the pre-deformation fluid composition and syn-deformation maximum stress on the host mineral can be derived from microthermometric analysis and thermodynamic modelling.

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