Garnet crystal plasticity in the continental crust, new example from south Madagascar

International audienceGarnet (10 vol.%; pyrope contents 3444 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain-size reduction (from a few cm to similar to 300 mu m). Electron back-scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain-size evolution. The garnet grain-size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm-sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain-size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion-assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (1010 m-3 or lower). The rounded Type III garnet experiences rigid body rotation in fine-grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain-size- and shape-dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 mu m or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The PT conditions of garnet plasticity in the continental crust (=950 degrees C; 11 kbar) have been identified using two-feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain-size decrease in a fine-grained steady-state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone

Allergy Narratives in Italy: ‘Naturalness’ in the Social Construction of Medical Pluralism

Based on an ethnographic study conducted in both biomedical and CAM settings in north Italy, I explore how people and practitioners make sense of allergy and how patients utilize plural healing options. Despite a wide range of medical modalities, people categorize and use medicine according to whether they are ‘natural’ or ‘not-natural’, thus dissolving any potential confusion between diverse therapies. I analyse how the concept of naturalness relates to allergy and medical pluralism. Nature is perceived as opposed to pollution, the first associated with a reassuring and idealized past and the latter to a modernity riddled with uncertainties. Participants associated a diverse set of meanings with nature, permitting them the syncretism of different medical modalities. Medical pluralism in the study area is an uneven platform for discussion and experimentation, the outcome of historical and cultural context and local entanglements of power