1.6 billion years old crater awes geologists
doi:10.1038/nindia.2010.82 Published online 21 June 2010
Geologists have for the first time found evidence of a meteorite impact more than 1600 million years old that possibly produced a very large crater at Chaibasa in Jharkhand state of eastern India.
The crater is from the Precambrian era — from the time earth was born 4600 million years ago until 600 million years ago.
The discovery is considered significant since there have been no reports on any Precambrian impact structure from the Indian shield. The well known Lonar Lake in Maharashtra is said to be the only crater in India created by the impact of a meteorite about 30,000 to 50,000 years ago.
The latest finding of a dramatic event 1.6 billion years ago has attracted the attention of earth scientists around the world.
"This is a fascinating paper that provides evidence for a meteorite-impact origin of a Precambrian crater at Chaibasa," William Bassett Department of Earth and Atmospheric Sciences at Cornell University told Nature India.
The Chaibasa area where the meteorite impacted the Earth is known to be 3163 million years old and covers an area of about 40,000 sq.km., says Gopalakrishnarao Parthasarathy, a senior scientist at the National Geophysical Research Institute (NGRI) in Hyderabad and one of the authors of the paper.
Others involved in the collaborative study are physicists Usha Chandra and Pooja Sharma of the University of Rajasthan in Jaipur and Bojja Sreedhar of the Indian Institute of Chemical Technology in Hyderabad.
The scientists got evidence for the impact by analysing pre-1.6 billion year old iron sediments from the area using Mössbauer spectroscopy — a sensitive technique to find out if the iron is in 'native' elemental form. Since iron in the elemental form rarely occurs on the Earth's surface by terrestrial processes and is mostly found in the form of meteorites, the presence of native iron suggests an impact event during the Precambrian. According to the scientists the native iron they found in the Chaibasa rocks was derived from the meteorite impact.
"Our findings suggest the occurrence of native iron in the Precambrian Chaibasa shales to be the oldest native iron ever found, formed due to the impact more than 1.6 billion years ago," Parthasarathy said. "The iron in the native form was preserved for over 1.6 billion years by sudden burial of the molten iron and fast sedimentation in the region," the scientists reported.
"This naturally occurring elemental iron undergoes a pressure-induced transition, from body-centered-cubic (bcc) phase to hexagonal-close-packed (hcp) phase, at abnormally low pressures. This means the crater was, indeed, formed by meteorite impact," Bassett told Nature India. "The authors' conclusions are both logical and believable," he said.
According to Parthasarathy this observation may be useful in identifying more Precambrian impact sites in India. Further field investigations are needed to precisely estimate the diameter of the Precambrian impact crater.
- Chandra, U. et al. “57Fe Mossbauer spectroscopy and electrical resistivity studies on naturally occurring native iron under high pressures up to 9.1 GPa. Am. Mineral. 95, 870-875 (2010) | Article |