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

1,2Jayanta Kumar Pati, 3Wen Jun Qu, 4,5Christian Koeberl, 6,7,8Wolf Uwe Reimold, 9Munmun Chakarvorty, 6Ralf Thomas Schmitt
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12826]
1Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad, India
2National Center of Experimental Mineralogy and Petrology, University of Allahabad, Allahabad, India
3National Research Center for Geoanalysis, Beijing, Xicheng District, China
4Department of Lithospheric Research, University of Vienna, Vienna, Austria
5Natural History Museum, Vienna, Austria
6Museum für Naturkunde – Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
7Humboldt-Universität zu Berlin, Berlin, Germany
8Geochronology Laboratory, University of Brasilia, Brasilia, Brazil
9Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad, India
Published by arrangement with John Wiley & Sons

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 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.

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