Abstract
The Patom Crater was formed around 500 years ago. According to geological survey data, it represents a concentrically zoned debris cone with a diameter of 130–160 m at the base and up to 80 m in the upper ring swell from 10–12 to 35–38 m high. The crater is made up mainly of limestones of the Mariinsk Formation (Pt3) and much rarer blocks of sandstones and schists. The debris cone was formed by one or several explosions. The value of δ18O in the limestones, which composes the debris cone (from 12.7 to 13.8‰), on average, is 6.5‰ lower than that of the unexploded Mariinsk limestone. The strontium and carbon isotope compositions vary in narrow ranges typical of the Upper Riphean carbonate of the Baikal mountain system: δ13C (from 8.4 to 8.8‰) and 87Sr/86Sr (0.707864–0.708777). Shift of the oxygen isotope composition indicates that the limestones interacted with hot waters at temperatures above 100°C and, correspondingly, the Patom Crater was formed as a result of phreatic (steam) explosion either during magma emplacement in the hydrous rocks or in response to faulting and decompression of heated hydrous rocks.
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