Chromium isotopes identify the extraterrestrial component in impactites from Dhala impact structure, India

Abstract

The Dhala structure in north-central India is a confirmed complex impact structure of Paleoproterozoic age. The presence of an extraterrestrial component in impactites from the Dhala structure was recognized by geochemical analyses of highly siderophile elements and Os isotopic compositions; however, the impactor type has remained unidentified. This study uses Cr isotope systematics to identify the type of projectile involved in the formation of the Dhala structure. Unlike the composition of siderophile elements (e.g., Ni, Cr, Co, and platinum group elements) and their inter-element ratios that may get compromised due to the extreme energy generated during an impact, Cr isotopes retain the distinct composition of the impactor. The distinct ε54Cr value of −0.31 ± 0.09 for a Dhala impact melt breccia sample (D6-57) indicates inheritance from an impactor originating within the non-carbonaceous reservoir, that is, the inner Solar System. Based on the Ni/Cr ratio, Os abundance, and Cr isotopic composition of the samples, the impactor is constrained to be of ureilite type. Binary mixing calculations also indicate contamination of the target rock by 0.1–0.3 wt% of material from a ureilite-like impactor. Together with the previously identified impactors that formed El'gygytgyn, Zhamanshin, and Lonar impact structures, the Cr isotopic compositions of the Dhala impactites argue for a much more diverse source of the objects that collided with the Earth over its geological history than has been supposed previously.