Age of the Lonar Impact Crater, India: First Results from Fission Track Dating

The Lonar crater (1.8 km diameter) is a well-preserved simple impact crater in basaltic target rocks (Deccan Traps). Its age is not well defined, with previous age determinations ranging from ca 10 to 50 ka, the latter one being a thermoluminescence age that is commonly quoted as the age of the structure. In an attempt to refine the crater age, we began a fission track investigation of impact glass from Lonar. The first results from this study indicate an age of about 15 ka, but with a fairly high error. Further fission track work to refine this age is in progress. Introduction. The Lonar crater [1-3] is a relatively small (rim to rim diameter 1830 m), almost perfectly circular simple (bowl-shaped) impact crater centered at 19°58’N and 76°31’E. The depression is excavated from the basalt flows of the Deccan Traps in the Buldana District of Maharashtra, India. Whereas the age of the target area is sufficiently well known (the Deccan Traps erupted around 65 million years ago, around the time of the Cretaceous-Tertiary boundary), the age of the impact event itself is – so far – not well constrained, despite of the fact that the impact feature was already known to Gilbert [4] in 1896. Several dates have been reported in the literature, but they are either not well documented, or do not agree well with each other. For example, Fredriksson et al. [3] noted that preliminary fission track results indicate an age of less than 50,000 years. Radiocarbon ages of post-impact lake sediments gave (unpublished, see [13-15]) radiocarbon ages of ca. 15 – 30 ka, which were thought to be lower age limits due to contamination with young carbon. Sengupta and Bhandari [16] quoted a thermoluminescence age of 62 ka, and Sengupta et al. [15], based on more or less the same dataset, arrived at an age of 52 ± 8 ka. Fission Track Dating for Impact Craters. Fission track dating has been successfully applied during the last thirty years to tektites and impact glasses (e.g. 5-8), mid-ocean ridge. [9-10], and lunar(11) glasses of basaltic composition. Due to our experience with the handling of basaltic glasses in fission track dating we attempt here a fission track reconnaissance in order to find out if Lonar crater glass is datable at all with the fission track method. Methodology. Fission track analyses were performed on three impact glasses (LO-1, LO-2, LO-3 ) collected in 1990 from the Lonar crater. The three discs (saw cuts about 0.8 mm thick) of these glasses were mounted in epoxy, in one section and polished. In order to determine the optimal etching characteristics for the basaltic Lonar crater glasses, the polished section was irradiated with Cf-252 fission fragments. Fission track development was found optimal by etching the polished section for 45 seconds in an aqueous acid solution (2 vol. 40% HF + 1 vol. 96% H 2SO 4 + 1 vol. 65% HNO 3 + 6 vol. H 2 O)at 23°C. After repolishing and reetching the mounted sample, the glass sections were scanned for fossil fission tracks in reflected light under an optical microscope (eyepiece: 10x; objective: 50x). This procedure was repeated two times. For details on the technique and analytical data treatment, see [11, 12].