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Minute polyphase inclusions in garnet of quartzofeldspathic rocks (saidenbachite) from the Saxonian Erzgebirge, Germany, contain microdiamond or graphite, phlogopite, quartz, paragonite, phengite and other minerals in minor amounts. These inclusions are interpreted to represent an original dense COH + silicate fluid, trapped in crystallizing garnet at depths of >150 km. Inspection of the inclusion population in a single garnet by SEM reveals two characteristic features: (i) The shape of most inclusions indicates healed radial cracks in the host garnet, and, thus, the buildup of a significant differential pressure DP, i.e. a contrast in pressure between the inclusion (P i ) and the host mineral (P e ). The mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of $750 ± 50°C and pressures below 2.5 GPa. (ii) The diverse types of inclusions appear to be randomly distributed in the garnet host. Thus, the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet. Combining observations (i) and (ii), we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times, and thus, of distinct histories of P i , as DP at decrepitation is primarily controlled by inclusion size and shape. Applying a flow law for dislocation creep of garnet, a low strength of garnet at 750 ± 50°C for low geological strain rates is predicted. Thus, differential pressure should have been kept low (i.e. P i % P e ) by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks. To attain the differential pressure (P i >> P e ) required for catastrophic brittle failure of the garnet host, the decompression rate must have been extremely high. As a robust lower bound, a minimum exhumation rate on the order of 100 m year )1 is suggested, which corresponds to ascent rates of magma.

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Experiments comprising deformation at 600°C and annealing at 700-1000°C were performed on natural peridotite to examine the microfabric evolution at conditions corresponding to those prevailing just below the seismogenic zone in suboceanic upper mantle. The found olivine microstructures indicate that deformation occurs by low-temperature plasticity. At low and high annealing temperatures, zones of high strain are replaced by subgrains and recrystallized grains respectively. The microstructures after annealing at 1000°C resemble`coreand-mantle structures´from shear zone peridotites, often interpreted as imprint of steady-state dislocation creep. Our study shows that such structures can form by a sequence of low-temperature plasticity at high stress and subsequent recrystallization at low stress, as corresponding to coseismic deformation and post-seismic creep. In this case, palaeopiezometers are not applicable and a local CPO in aggregates of new grains does not necessarily indicate the activated glide system.