Decrepitated UHP fluid inclusions: about diverse phase assemblages and extreme decompression rates (Erzgebirge, Germany)

Abstract

AbstractMinute 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 ΔP, i.e. a contrast in pressure between the inclusion (Pi) and the host mineral (Pe). 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 Pi, as ΔP 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. Pi≈ Pe) by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks. To attain the differential pressure (Pi >> Pe) 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.