New insights on petrography and geochemistry of impactites from the Lonar crater, India

Abstract

The Lonar impact crater, India, is one of the few known terrestrial impact craters excavated in continental basaltic target rocks (Deccan Traps, ~65 Ma). The impactites reported from the crater to date mainly include centimeter‐ to decimeter‐sized impact‐melt bombs, and aerodynamically shaped millimeter‐ and submillimeter‐sized impact spherules. They occur in situ within the ejecta around the crater rim and show schlieren structure. In contrast, non–in situ glassy objects, loosely strewn around the crater lake and in the ejecta around the crater rim do not show any schlieren structure. These non–in situ fragments appear to be similar to ancient bricks from the Daityasudan temple in the Lonar village. Synthesis of existing and new major and trace element data on the Lonar impact spherules show that (1) the target Lonar basalts incorporated into the spherules had undergone minimal preimpact alteration. Also, the paleosol layer as preserved between the top‐most target basalt flow and the ejecta blanket, even after the impact, was not a source component for the Lonar impactites, (2) the Archean basement below the Deccan traps were unlikely to have contributed material to the impactite parental melts, and (3) the impactor asteroid components (Cr, Co, Ni) were concentrated only within the submillimeter‐sized spherules. Two component mixing calculations using major oxides and Cr, Co, and Ni suggest that the Lonar impactor was a EH‐type chondrite with the submillimeter‐sized spherules containing ~6 wt% impactor components.

Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure, India

“…(), whereas estimates from Ray et al. () and Das Gupta et al. () are based on submillimeter‐sized spherules, which show a clear enrichment in moderately siderophile elements, thus likely a higher impactor contribution despite possible fractionation.…”

“…In their studies, Ray et al. () and Das Gupta et al. () show that millimeter‐sized spherules have Ni and Cr abundances similar to those of target and impact rocks, whereas submillimeter‐sized spherules can be 2.5–12.6 times richer in these elements.…”

“…; Ray et al. ). While they all agree on the chondritic nature of this component, the type of chondrite involved in this impact event remains unclear.…”

“…Ray et al. () suggested that the major oxides and moderately siderophile element composition of the impact spherules were most compatible with the admixture of 6 wt% of an EH‐type chondrite. This hypothesis is based on the evaluation of the lowest error sum of squares (SS 2 ) of calculated mixtures involving various types of chondrites, and using major oxide, Ni, Co, and Cr abundances.…”

“…However, its impact origin was later confirmed with the discovery of shock metamorphic features in breccias found in drill core samples (Fredriksson et al 1973;Fudali et al 1980), despite the lack at that time of geochemical evidence for a meteoritic component (Osae et al 2005;Son and Koeberl 2007) and of projectile fragments (which were expected to be present because the crater was thought to be very young, similar to Meteor Crater in Arizona, before the current age determination by Jourdan et al [2011] revealed it is much older). Only very recently did different research groups provide geochemical evidence for the presence of extra-terrestrial material in Lonar impact rocks (Misra et al 2009;Schulz et al 2016;Das Gupta et al 2017;Ray et al 2017). While they all agree on the chondritic nature of this component, the type of chondrite involved in this impact event remains unclear.…”