Geochemical evidence of an extraterrestrial component in impact melt breccia from the Paleoproterozoic Dhala impact structure, India

Abstract

The Paleoproterozoic Dhala structure with an estimated diameter of ~11 km is a confirmed complex impact structure located in the central Indian state of Madhya Pradesh in predominantly granitic basement (2.65 Ga), in the northwestern part of the Archean Bundelkhand craton. The target lithology is granitic in composition but includes a variety of meta‐supracrustal rock types. The impactites and target rocks are overlain by ~1.7 Ga sediments of the Dhala Group and the Vindhyan Supergroup. The area was cored in more than 70 locations and the subsurface lithology shows pseudotachylitic breccia, impact melt breccia, suevite, lithic breccias, and postimpact sediments. Despite extensive erosion, the Dhala structure is well preserved and displays nearly all the diagnostic microscopic shock metamorphic features. This study is aimed at identifying the presence of an impactor component in impact melt rock by analyzing the siderophile element concentrations and rhenium‐osmium isotopic compositions of four samples of impactites (three melt breccias and one lithic breccia) and two samples of target rock (a biotite granite and a mafic intrusive rock). The impact melt breccias are of granitic composition. In some samples, the siderophile elements and HREE enrichment observed are comparable to the target rock abundances. The Cr versus Ir concentrations indicate the probable admixture of approximately 0.3 wt.% of an extraterrestrial component to the impact melt breccia. The Re and Os abundances and the 187Os/188Os ratio of 0.133 of one melt breccia specimen confirm the presence of an extraterrestrial component, although the impactor type characterization still remains inconclusive.

Documentation of shock features in impactites from the Dhala impact structure, India

“…This is a silica‐oversaturated composition similar to the dominant target lithology of granitoids around Dhala (Pati et al. 2008a, , ).…”

“…; French ; Pati ; Pati et al. 2008a, ; French and Koeberl ). A wide range of microscopic shock deformation features, including PDFs, PFs, FFs, ballen quartz, diaplectic glass, and granular zircon, besides unshocked grains of quartz or zircon, have now been reported from Dhala.…”

“…) has yielded diagnostic evidence of shock metamorphism, including the most recent discovery of shatter cones which are reported in this paper (Pati et al. 2008a, , ). This is remarkable, because a large part of the Dhala structure is actually overlain by syn‐ to postimpact sediments, yet impactites with mesoscopic and microscopic shock features and monomict breccia ring are well preserved, despite the overprinting of multiple tectono‐metamorphic and hydrothermal alteration events since its formation (Pati et al.…”

“…The presence of anomalously high iridium (Ir) contents in impact melt breccia (IMB), in comparison with the target rocks, and a very low osmium isotopic ratio indicated the presence of an extraterrestrial impactor component in impact melt breccia (Pati et al. ). However, a number of studies have since continued to advocate an endogenous origin for this structure (Bhattacharya et al.…”

“…The stratigraphy of the Dhala area is based on logging of the Mohar Cauldron Borehole (MCB‐10; 25°17′52.9″N and 78°8′28.9″E; Pati et al. , ) (Fig. ).…”