Hornblende alteration and fluid inclusions in Kärdla impact crater, Estonia: Evidence for impact-induced hydrothermal activity

Abstract

The well-preserved Kärdla impact crater, on Hiiumaa Island, Estonia, is a 4-km diameter structure formed in a shallow Ordovician sea about 455 Ma ago into a target composed of thin (~150 m) unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites, amphibolites and gneisses. The fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals. The most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift. Alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater. Chloritic mineral associations suggest formation temperatures of 100-300 degrees C, in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150-300 degrees C in allochthonous breccia. The rather low salinity of fluids in Kärdla crater (<13 wt% NaCleq) suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the cater rim walls raised above sea level after the impact, or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement. The well developed hydrothermal system in Kärdla crater shows that the thermal history of the shock heated and uplifted rocks in the central crater area, rather than cooling of impact-melt or suevite sheets, controlled the distribution and intensity of the impact-induced hydrothermal processes.