Alteration of zircon and garnet due to low-pressure shock at the Kärdla crater, Estonia

Immersion analysis with the polarizing microscope and SEM studies revealed fracturing in zircon and garnet grains from impact breccias of the Kardla crater. The zircon grains showed impact-related features, including widening of pre-existing fractures in the internal zones, some fresh fractures, loss of inclusions, decrease in transparency, increase in turbidity, and the intensely cracked outermost coat in zoned zircons. The garnet grains were intensely fractured only in the uppermost subcrater basement, especially in allochthonous suevitic breccias. Zircon and garnet grains from different parts of the crater can serve as useful indicators of low shock pressure (< 8 GPa).