On November 30, 2009, a paper titled “Confirmation of Xiuyan meteorite crater” and authored by Dr. Chen Ming’s group in GIGCAS was published in Vol. 54, No. 22 of Chinese Science Bulletin. In this paper, Chen Ming and his colleagues proved the first meteorite crater in China, i.e., Xiuyan meteorite crater, after three years of research. The discovery of the unique geological structural trace proves to be the first of its kind in China territory.
Situated in the north of Liaodong Peninsula, Xiuyan meteorite crater is located in Gulong Village, Suzigou Town, Xiuyan Man Nationality Autonomous County, Liaoning Province, China, and is about 16 km away from Xiuyan county town to the northeast. Here it sits among a circle consisting of hilly land with low elevations and is close to the beautiful Shaozi River. Even though it appears to be pentagonal in planar outline, the crater takes the shape of a huge bowl with a diameter of 1,800m and a depth of about 150m. The crater body is comprised of metamorphic rocks of Early Proterozoic age 2 billion years ago, while the crater edge and crater bottom are generally covered by topsoil, residual slope sediments and Quaternary lacustrine sediments. Shrubbery grows up along the inner and outer slopes of the crater edge, while crops flourish on the fertile land at the crater bottom. When you climb up the circular edge of the meteorite crater and look down, you can see that the bowl-shaped crater exhibits very spectacular scenery and astonishing outlook.
The high speed impact on the ground surface or the surfaces of other celestial bodies by huge meteorites would result in huge pits or similar circular geological structures, which are called meteorite craters. A great number of such spectacular natural landscapes, i.e., meteorite craters, have been observed on the surfaces of moon, Mars and Mercury. Ever since the 1st meteorite crater occurring on the earth’s surface (the Barringer meteorite crater in USA) was confirmed in the 1960s, more than 170 meteorite impact structures, with diameters varying from dozens of meters to nearly 300km, have been discovered on earth up to now. Meteorite crater is significant scientifically for study of earth formation and evolution, paleoclimate, paleoenvironment and paleobiological changes, as well as petrogenesis and mineralization, some meteorite craters have since become natural science museums and tourist attractions.
The meteorite crater research in China starts in the 1980s, and a series of possible meteorite craters have since been reported. However, due to lack of key evidence for meteorite impact, no breakthrough has been made in such kind of study. As an important research organization engaged in study of shock metamorphism in China, GIGCAS has achieved a series of internationally high profile research achievements in the research field of shock metamorphism of meteorite and meteorite crater. With financial support secured from Chinese Academy of Sciences and Natural Science Foundation of China, Dr. Chen Ming and his colleagues started investigation and research of Xiuyan meteorite crater in early 2007, while Xiuyan People’s Government granted support and coordinated services to this research project. In the first year of their research, Dr. Chen Ming found traces showing physical and chemical changes in rocks occurring at the crater surface as resulted from shock wave influences, and the traces constitute key scientific data promising continuing research and guaranteeing forthcoming technical routes.
After repeated study and corroboration, the research group designed and laid out the drill sites inside the Xiuyan meteorite crater in the 1st half of 2009, and invited No. 404 Geological Brigade of Liaoning Provincial Bureau of Metallurgical and Geological Survey to undertake the drilling task, which lasted for more than 5 months. After the geological drilling, the geological structure at the deep part of the crater was revealed, and a series of rock samples were acquired from the deep part. After careful analysis of the core samples secured through the drilling program, a series of evidences were discovered demonstrating deformation, crushing, partial melting of rocks, and shock metamorphism of rocks and minerals in the target area as resulted from intense impact by meteorite.
There are many circular craters or ring structures occurring on the earth’s surface. However, to determine if a circular crater or ring structure is associated with meteorite impact, the key lies on data showing rocks in the crater is subject to the influence of shock wave accompanied with meteorite impact. As under the influence of such shock wave, shock metamorphism will occur in target rocks. The international academic community has proposed three diagnostic indicators for determining shock metamorphism in a typical meteorite crater, i.e., diaplectic shock lamellae in minerals, diaplectic glass of minerals, high pressure polymorphic transformation of diaplectic minerals. Except being resulted from shock wave due to supersonic impact by extraterrestrial celestial body, no other natural process or geological influence can cause such type of mineral physical changes in geological bodies in nature. Therefore, if any of the three indicators can be discovered for a circular geological structure on earth surface, then the structure is certainly associated with meteorite impact event.
The paper published by Chen Ming and his colleagues on Chinese Science Bulletin reports the basic geological and topographic features of the Xiuyan meteorite crater, and details the discovery of diaplectic lamellae and other shock effects in minerals, hence confirming the origin of meteorite impact for the crater.
According to latest news, Chen Ming and his colleagues discovered successively evidences such as “diaplectic glass in minerals” and “high pressure polymorph of diaplectic mineral”, in addition to diaplectic lamellae in minerals that have already been reported. Therefore, three shock metamorphism indicators diagnostic of a meteorite crater were all discovered for Xiuyan meteorite crater. Through radio isotope analysis of crater materials, it can be primarily determined that the meteorite impact event occurred 50,000 years ago. After the meteorite crater is generated, a small lake was developed inside the crater, where 100m thick lacustrine sediments were deposited in a period of over 10,000 years. About 39,000 years ago, the lake disappeared due to breach at the crater edge, and the crater has since evolved into a bowl-shaped pit in topography.
(Contributed by The Department of Extreme Condition Geology and Geochemistry of GIGCAS on January 22, 2010 and edited by the Administrative Office of GIGCAS)