Testing the ureilite projectile hypothesis for the El'gygytgyn impact: Determination of siderophile element abundances and Os isotope ratios in ICDP drill core samples and melt rocks

Abstract

Abstract-The geochemical nature of the impactites from International Continental Scientific Drilling Project-El'gygytgyn lake drill core 1C is compared with that of impact melt rock fragments collected near the western rim of the structure and literature data. Concentrations of major and trace elements, with special focus on siderophile metals Cr, Co, Ni, and the platinum group elements, and isotope ratios of osmium (Os), were determined to test the hypothesis of an ureilite impactor at El'gygytgyn. Least squares mixing calculations suggest that the upper volcanic succession of rhyolites, dacites, and andesites were the main contributors to the polymict impact breccias. Additions of 2-13.5 vol% of basaltic inclusions recovered from drill core intervals between 391.6 and 423.0 mblf can almost entirely account for the compositional differences observed for the bottom of a reworked fallout deposit at 318.9 mblf, a polymict impact breccia at 471.4 mblf, and three impact melt rock fragments. However, the measured Os isotope ratios and slightly elevated PGE content (up to 0.262 ng g À1 Ir) of certain impactite samples, for which the CI-normalized logarithmic PGE signature displays a relatively flat (i.e., chondritic) pattern, can only be explained by the incorporation of a small meteoritic contribution. This component is also required to explain the exceptionally high siderophile element contents and corresponding Ni/Cr, Ni/Co, and Cr/Co ratios of impact glass spherules and spherule fragments that were recovered from the reworked fallout deposits and from terrace outcrops of the Enmyvaam River approximately 10 km southeast of the crater center. Mixing calculations support the presence of approximately 0.05 wt% and 0.50-18 wt% of ordinary chondrite (possibly type-LL) in several impactites and in the glassy spherules, respectively. The heterogeneous distribution of the meteoritic component provides clues for emplacement mechanisms of the various impactite units.

Target rocks, impact glasses, and melt rocks from the Lonar crater, India: Highly siderophile element systematics and Sr‐Nd‐Os isotopic signatures

“…), reflecting a difference in the distribution of projectile components between impact spherules and bulk impactites, which was also observed at other impact craters (e.g., Goderis et al. ; Foriel et al. ; Raschke et al.…”

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

“…), modified after Tagle and Hecht () by Goderis et al. (), which indicates most likely mixing of a chondritic impactor with the Dhala impact melt breccia. Given an impactor of chondritic composition, an admixture of approximately 0.3 wt% of an extraterrestrial component is estimated.…”

“…; Goderis et al. , , ; French and Koeberl ; Koeberl et al. )—provided the PGEs contributed from terrestrial sources are negligible.…”

“…Therefore, the 187 Os/ 188 Os ratio can show dramatic change with very minor contribution from a meteoritic component to an impactite (Koeberl and Shirey ; Goderis et al. , , ; Koeberl et al. ), as continental crustal rocks have values of about 1.0–1.4 (Esser and Turekian ; Peucker‐Ehrenbrink and Jahn ) compared to average values of 0.13 for most primitive meteorites (Lee et al.…”

Geochemical studies of impact breccias and country rocks from the El'gygytgyn impact structure, Russia

“…Impact melt occurs in four different settings (1) blocks of impact melt breccia and glass bombs in the lake terraces, (2) tiny (0.5–1.5 mm) glass spherules on the lake terrace and along the Enmyvaam River (Glushkkova and Smirnov ), (3) similar spherules in the reworked suevite section of the ICDP drill core (Goderis et al. ; Wittmann et al. ), and (4) small (altered) melt particles in the drilled suevite section (Pittarello et al.…”