Zhamanshin astrobleme provides evidence for carbonaceous chondrite and post-impact exchange between ejecta and Earth\u2019s atmosphere

Tomáš Magna,Zdeněk Řanda,Frédéric Moynier,Andreas Pack,Karel Žák,Jiří Mizera,Bérengère Mougel,Stefan Peters,Šárka Jonášová,Roman Skála

doi:10.1038/s41467-017-00192-5

Abstract

Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per cent. Here we use chromium and oxygen isotopes to identify the impactor and impact-plume processes for Zhamanshin astrobleme, Kazakhstan. ε54Cr values up to 1.54 in irghizites, part of the fallback ejecta, represent the 54Cr-rich extremity of the Solar System range and suggest a CI-like chondrite impactor. Δ17O values as low as −0.22‰ in irghizites, however, are incompatible with a CI-like impactor. We suggest that the observed 17O depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange with atmospheric oxygen (Δ17O = −0.47‰) following material ejection. In contrast, combined Δ17O–ε54Cr data for central European tektites (distal ejecta) fall into the terrestrial range and neither impactor fingerprint nor oxygen isotope exchange with the atmosphere are indicated.

Highlights

Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per cent
Large mass-independent fractionation (MIF) of oxygen isotopes has been recognized for different Solar System bodies most likely inherited from MIF processes during early Solar System formation, whereas common terrestrial and other planetary materials (e.g., Mars, Moon, Vesta) have 18O/16O and 17O/16O ratios which are solely fractionated by mass-dependent processes[20, 21]
The basic splash forms from Zhamanshin show Δ17O values (−0.10 to −0.07‰) that overlap with crustal rocks

Summary

Introduction

Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per cent. Tektites are found as variably shaped and sized objects; typically, they are of splash-form shapes with a size of few centimetres They usually are interpreted as being formed from the uppermost layers of the target lithologies, i.e., unconsolidated sediments and soils, partly vaporized and partly melted and ejected with a high velocity from the boundary zone between the impactor and the Earth’s surface by jetting[7]. They are mostly silica-rich and with extremely low contents of volatile components (H2O, C, S, halogens, etc.). Because individual chondrite groups have distinctive 17O– 54Cr isotope fingerprints[3], combined O–Cr isotope measurements may allow identification of impactor material and distinction between impactor- or atmospheric exchange-related variations in Δ17O

Methods

Results

Conclusion