Geochemistry of Impact Ejecta from Lonar

Introduction: The young Lonar crater, Maharashtra, India [1-3], is a bowl-shaped, almost circular, simple impact crater [4] of relatively small diameter ~1.8 km [5]. The uniqueness of this crater is that it is one of two known terrestrial impact craters excavated on basaltic target rock. The target-rock of this crater is the sub-horizontal Deccan basalt flows (~67 M.y.) [6]. All around the circumference of the Lonar Crater, except for a small sector in NE, there is a raised rim surrounded by a continuous wedge-shaped ejecta blanket, which extends outward to a distance of ~400 to 1600 m [7] with a very gentle slope of 2-6 [5]. This ejecta blanket has a thickness of ~8 m to few cm [7] and consists of angular blocks of basalt of variable sizes in a matrix of fine ejecta. The Lonar Crater has been suspected to be a meteorite impact crater for the last 40 years [8], but only recently has information on the geochemistry of different impact products become available. Recently, Osae et al. [9] in their extensive geochemical analyses of major oxides and trace elements, including Ir and Au, showed that chemical fractionation between the Lonar target-rock basalt and impact-melts was minimal. They also found a low-level of meteoritic contamination in the Lonar impactites suggesting a non-chondritic or otherwise Ir-poor impactor for this crater. We report extensive geochemistry of fine Lonar ejecta to better determine (a) the extent of post-cratering erosion experienced by this crater, (b) the possibility of impactinduced hydrothermal alteration, and (c) any further clue on the nature of the impactor. Analytical technique: Around thirty samples of fine ejecta of ≤ 250 μm size were collected from around the crater rim from a depth of ~40-50 cm or more. XRD analyses of samples were carried out at the Central Research Facility, IIT, Kharagpur, India, by a PW 1710 Philip machine with Cu target and Ni filter under a working voltage of 40 kv and 20 milliampere current and a scanning speed of 3 2θ/minute. The major element analyses were done at WIHG, Dehradun, India, in a SIEMENS SRS 3000 XRF machine with an overall accuracy better than ± 6 %. Trace elements were analysed at NGRI, Hyderabad, India, in a Perkin-Elmer SCIEX model ELAN® DRC II ICP-MS following the procedure of [10]. The precision and accuracy of analyses, as observed on JB-2 standard, were better than ± 7 % for all the trace elements. Analytical results: XRD analyses of (7) seven representative samples show that these fine ejecta mostly consist of labradorite and augite with some mica. The ejecta samples, however, show considerable variation in LOI upto 10 wt%.