Fluid inclusions in microstructures of shocked quartz from the Keurusselkä impact site, Central Finland


Corresponding author: E-mail: matti.poutiainen@helsinki.fi


Granitoid rock samples from the assumed center of the Keurusselkä impact site were subjected to a systematic study of fluid-inclusion compositions and densities in various microstructures of the shocked quartz. The results are consistent with the following impact-induced model of formation. After cessation of all major regional tectonic activity and advanced erosional uplift of the Fennoscandian shield, a meteorite impact (approximately 1.1 Ga) caused the formation of planar fractures (PFs) and planar deformation features (PDFs) and the migration of shock-liberated metamorphic fluid (CO2 ± H2O) to the glass in the PDFs. Postimpact annealing of the PDFs led to the formation of CO2 (±H2O) fluid-inclusion decorated PDFs. The scarce fluid-inclusion implosion textures (IPs) suggest a shock pressure of 7.6–10 GPa. The postimpact pressure release and associated heating initiated hydrothermal activity that caused re-opening of some PFs and their partial filling by moderate-salinity/high temperature (>200 °C) H2O (+ chlorite + quartz) and moderate-density CO2. The youngest postimpact endogenic sub- and nonplanar microfractures (MFs) are characterized by low-density CO2 and low-salinity/low-temperature (<200 °C) H2O.