Water plays an important role in geological processes. Providing constraints on what may influence the distribution of aqueous fluids is thus crucial to understanding how water impacts Earth's geodynamics. Here we demonstrate that ductile flow exerts a dynamic control on water-rich fluid circulation in mantle shear zones. Based on amphibole distribution and using dislocation slip-systems as a proxy for syn-tectonic water content in olivine, we highlight fluid accumulation around fine-grained layers dominated by grain-size-sensitive creep. This fluid aggregation correlates with dislocation creep-accommodated strain that localizes in water-rich layers. We also give evidence of cracking induced by fluid pressure where the highest amount of water is expected. These results emphasize long-term fluid pumping attributed to creep cavitation and associated phase nucleation during grain size reduction. Considering the ubiquitous process of grain size reduction during strain localization, our findings shed light on multiple fluid reservoirs in the crust and mantle.

International audienceApatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only U-bearing mineral available to date mafic rocks because primary zircon and/or baddeleyite are not present. In this paper, U-Pb LA-ICP-MS dating of apatite was applied to seven different dike and sill samples of dolerite from the Variscan belt of Brittany (Armorican Massif, western France). These dolerites, which are characterized by a within-plate tholeiite geochemical signature, are organized in several dense swarms across the belt. Their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic-metamorphic settings. Their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40Ar/39Ar or the U-Pb methods on classical minerals like mica, plagioclase, or zircon. Although the closure temperature of apatite is lower than the emplacement temperature of the magma, physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less. Consequently, the U-Pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions. Our results demonstrate that, in all cases, the apatite grains do carry enough radiogenic Pb to be dated by in situ U-Pb analyses and yield a 207Pb-corrected mean age of 363.4 ± 5.8 Ma. These results reveal the existence of a major and short-lived magmatic event in the Variscan belt of Brittany during the Devonian-Carboniferous transition, a feature further highlighted by field evidence. Beyond the geological implications of these results, U-Pb LA-ICP-MS dating of apatite appears to represent an ideal tool to date small size mafic intrusions

Hydrothermal alteration related to Sb-Au mineralization is widespread in the Variscan Armorican Massif, but mineral replacement reactions are not well characterized, in particular the hydrothermal breakdown of ilmenite-titanohematite. Based on petrography, electron probe micro-analyzer and laser ablation-inductively coupled plasma-mass spectrometer analyses, we document mineralogical change at rock-and mineral-scale and the redistribution of Sb and others trace elements during the recrystallization of ilmenitetitanohematite to hydrothermal rutile. Hydrothermal alteration is mainly potassic with associated carbonation. The replacement mechanism is interpreted to be an interface-coupled dissolution-reprecipitation process. Results show that Mn, Zn, Co, Ni, Sn, Mo and U are released during hydrothermal alteration, whereas Sb and W are incorporated in newly-formed hydrothermal rutile from the hydrothermal fluid. Furthermore, the concentration of Sb evolves through time suggesting a change in fluid composition likely related to an enrichment of fluid in Sb during rutile crystallization. Considering that Fe-Ti oxides breakdown during hydrothermal alteration is common within epithermal and mesothermal/orogenic Au-Sb mineralizing systems, results report in this study yield important constraints about metal mobility and exchanges in hydrothermal gold systems.