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. The results suggest that the smallest spherules (< 200 microns diameter) are more likely to preserve the impactor signature while the NSIG’s, are more likely to preserve the signatures of the sub-basaltic granitic basement. As part of this study, a second impact crater with contrasting features compared to the Lonar crater was studied. The 11 Km diameter Dhala impact crater is hosted on the Bundelkhand granite. The heavily eroded nature of the crater suggests that it formed a long-time ago and its age estimates range from 2.4 – 2.2 Ga, making it a rarely preserved example of a large crater formed in the Proterozoic. It is a complex crater with a central elevated area (CEA), which is the only well-preserved feature of this crater. The Nd isotopic compositions of the sedimentary rock samples from the CEA suggest that they belong to the Lower Vindhyan Supergroup. The Ni/Cr and Ni/Co ratios, measured in the melted and brecciated granite samples, suggest a chondritic impactor at Dhala. In addition, these rocks show evidences of alteration by a potassium-rich fluid. Strontium and Ca isotopic compositions of the melt breccia have been used to place chronological constraints on the alteration event by a K-rich fluid, which most-likely took place ~1.8 Ga ago. If this alteration event is related to the hydrothermal alteration that immediately follows an impact cratering event, it is possible that the formation age (2.4-2.2 Ga) for the Dhala impact crater has been over-estimated. The second part of this thesis investigates the origin of the mineral jarosite at Kutch, India. This mineral, widely reported on the surface of Mars, is an endmember of the hydroxy-sulfate mineral group, forms at low pH and high Eh conditions, and under specific water:rock ratios. The jarosites at Kutch are considered as a martian analog because of their spatial proximity with the Deccan basalt. Geochemical compositions of the hydroxy-sulfates from Kutch, collected from multiple stratigraphic sections, show clear signatures of precipitation from a sulfate-rich acidic fluid. Combined Nd and S isotopic compositions suggest that the low pH of the fluid is derived from weathering of pyrite while combined Sr and S isotopic compositions suggest that the source of the fluid, from which the hydroxy-sulfates precipitated at Kutch, is seawater. The 87Sr/86Sr and δ34S compositions of seawater over time have been used to constrain the age of the hydroxy-sulfates at Kutch to less than 20 Ma, which is much younger than the formation of the ~ 65 Ma old Deccan basalt. The inferred time of formation of the Kutch jarosites coincides with the timing of widespread aridity in other parts of the world.