Impact-induced and diagenetic changes in minerals in the sandy ejecta of the Kärdla crater, NW Estonia

The sand-rich ejecta containing a considerable amount of mineral grains with impactinduced features was studied in five boreholes drilled around the Kärdla impact meteorite structure. Quartz grains with planar deformation features and planar fractures, as well as cracked grains showing lowered crystallinity by X-ray diffractometry, were recorded. The orientations of shock-produced deformation features, with the orientation planes {1013} (dominating), {1011},{1012}, and {2241}, suggest the peak pressure of 10-25 GPa. The ejecta deposits of the annular zone 2-5 km outside the crater rim yield specific quartz grains surrounded by C-rich amorphous material. The carbon isotope composition of the particles reminds of that of the Early Ordovician Dictyonema shale, the uppermost part of the target, but also coincides with the general cosmic (meteorite) carbon isotope composition. The ejected detrital minerals, mostly quartz, are often coated with transparent or semitransparent yellowish and dark pyrite-bearing apatite, which is probably of post-impact origin, preceding or partly simultaneous with diagenetic carbonate cementation. Unlike carbon coatings on quartz, known from the Lappajärvi crater, the apatite coatings have not been described anywhere in impact craters. Carbon-rich coatings and particles, and apatite coatings have not been previously found in the Estonian bedrock.