Iron-nickel metallic components bearing silicate-melts and coesite from Ramgarh impact structure, west-central India: Possible identification of the impactor

Abstract

The Ramgarh structure (rim-to-rim diameter ~2.4 km) in the Vindhyan Supergroup of sedimentary rocks (including sandstone, shale and minor limestone) of the Mesoproterozoic age in the west-central India, is India’s third confirmed asteroid impact crater. This eroded structure is roughly rectangular in shape and resembles to the Barringer Crater, USA. The presence of central peak and its current crater diameter/depth ratio of ~12 well corroborate the range (10–20) of terrestrial complex asteroid impact craters. The mm-sized, iron-rich (FeO ~50 wt.% in average), spherule-like particles, recovered from the alluvium inside the Ramgarh structure, have internal morphology similar to those of the accretionary lapilli described in known impact craters. The in-situ LA-ICP-MS analyses also suggested high Co–Ni (up to 13,000 and 2500 ppm, respectively)-rich areas locally within these spherules/lapilli. A few non-in-situ, mm-sized particles, recovered from the rim of the structure show the presence of coesite, one of the diagnostic indicators of shock metamorphism. A few fragments of iron-rich, Ca–Al–silicate glasses recovered from the soil inside the structure and outside of the western crater rim include the presence of dendritic magnetite with occasional inclusions of relict native iron. Our microprobe analyses confirm that these metallic irons contain high proportions of Co (~350–3000 ppm), Ni (~200–4000 ppm) and Cu (~2200–7000 ppm) and possibly could be the relict component of a Cu-rich iron meteorite impactor. The field observation and relative enrichment of compatible and incompatible trace elements in the spherule-like substance (recovered from the alluvium inside the Ramgarh structure) as compared to target rocks suggests that hydrothermal activity played an important role in the evolution of the crater.