Geochemical and isotopic study of the Lonar and Dhala impact craters and jarosites from Kutch India as analogues to understand planetary surface processes

Abstract

Impact cratering and aqueous alteration are two important processes that have modified the Earth s surface over time and better understanding of these processes on Earth allow us to understand the surface evolution of other planetary bodies in the inner solar system. This thesis focuses on the study of terrestrial analogues and can be divided into two parts: the first part deals with the study of two terrestrial impact craters, namely Lonar and Dhala from India, and the second part deals with the formation of a unique aqueous alteration product called jarosite. The 1.8 Km diameter Lonar impact crater, hosted on the Deccan basalts formed ~0.5 Ma ago and is one of the best-preserved terrestrial impact craters on basalt. The impactor is rarely preserved at the cratering site because the impact process results in large-scale melting and vaporization. However, the products of impact cratering preserve information related to the cratering process. Chemical and isotopic compositions of the sub-mm sized impact spherules, collected from surface sediments near the south-eastern rim of the Lonar crater, were used to understand the cratering process at Lonar. High Ir and Cr concentrations in some of these spherules suggest 1-8 wt% contribution of a chondritic impactor at Lonar. Enrichment of the light rare earth elements (LREE) and large ion lithophile elements (LILE) in some other spherules and impact breccia samples, supported by Nd and Sr isotopic compositions of some of these samples, suggest up to 15 wt% contribution of the sub-basaltic granitic basement, which melted upon impact. Concentrations of relatively volatile elements like Cu and Zn in the Lonar spherules show evidence of volatile loss during impact, as well as re-condensation from a volatile-rich part of the vapor plume. The chemical heterogeneity observed in the Lonar spherules was further investigated by studying the morphology of spherules and non-spherical impact glasses (NSIG) using X-ray Micro-computed tomography. ..