Absence of Archean Basement in the Genesis of Lonar Crater, India

Introduction: The Lonar crater, India [1] is believed to have formed entirely on basaltic target rock (Deccan Trap, 65 Ma) and is therefore comparable to impact craters formed on other planetary surfaces with basaltic crusts. The genesis of Lonar crater has recently become complicated because isotopic studies on some impact-melt looking glasses found on the surface or in talus occurring mostly inside the crater around the lake suggested involvement of the Archean basement present beneath the Deccan Trap in this region [2, 3]. However, field and petrographic observations do not favour this idea [4]. These samples have a similar appearance to the old earthen bricks used by ancient peoples [5] and it is possible that all of these are only ancient artifacts with no relationship to cratering processes. In this study, we therefore, have dated many of these samples by paleomagnetic dating technique to evaluate this possibility. Sample Description: The samples are 10-20 cm in length, rectangular to sub-spherical to irregular in shape, black on freshly broken surfaces, highly vesicular and have relatively low specific gravity with flow structure on their hemispherical surfaces [5]. Under microscope, the samples are translucent, black coloured glass with minute squarish crystal of opaque with partially digested relict pieces of unshocked basalt, plagioclase and clinopyroxene. Flow-like structure typically present in the in-situ impact melts occurring within the ejecta outside the crater [1, 5] are not observed within these glasses. Results and Discussion: We have dated ten specimens, two from each of five artifacts, collected inside the crater around the lake using the paleointensity dating technique after Thellier and Thellier [6]. The sample discs measuring 2.5 by 2.5 cm were used for paleomagnetic studies. The estimated paleointensity values worked out to be within 56.8±3.2 μT. When compared with the secular variation curve [7], these values provided an age of 1150±50 AD for our samples. Thermoluminescence dating of impact-bombs gave an age of the Lonar crater of 52±6 ky [8]. Radiocarbon dating of post-impact lake sediments gave a lower age limit of 15-30 ky for Lonar crater [9]. So the present age definitely suggests that the impact-melt looking samples from inside the crater are much younger in age and definitely represents artifacts of ancient people.