THE EL'GYGYTGYN IMPACT STRUCTURE, RUSSIAN ARCTIC: AN ACHONDRITIC IMPACTOR?

The El’gygytgyn impact structure, 18 km in diameter, is located on the Chukotka Peninsula in Arctic Russia. This 3.6 million year old impact crater is unique among the terrestrial impact craters known so far because it is the only such structure that has formed in acid volcanic rocks. The target rocks are dominated by Late Cretaceous volcanic suites of the Okhotsk-Chukotka Volcanic Belt. El’gygytgyn was recently the subject of an ICDP deep drilling project. Earlier analyses of impactites from El’gygytgyn showed minor enrichments in the siderophile elements, including Ir, which, together with distinct Cr enrichments, were difficult to reconcile with a chondritic impactor; instead, an achondritic projectile was proposed. A large number of samples from the ICDP drill core near the center of the structure were analyzed in Vienna for their major and trace element compositions. Suevitic breccias from the upper part of the fallback sequence in the core show higher Cr and Ni contents compared to felsic volcanic rocks in the lower part of the core and from surface samples. However, it is difficult to unambiguously establish a meteoritic component from trace element data as contamination from (rare) mafic rocks is a possibility. In view of the low siderophile element contents and the lack of clarity obtained from such data, we analyzed the chromium isotopic composition of an impact glass sample from El’gygytgyn, which has a slight enrichment in Ir compared to target rocks. This impact glass sample (G4) was found to have a non-terrestrial value of ε54Cr=−0.72±0.31 (2 std. err., n=3). This value is different from all known carbonaceous chondrite values (ε54Cr = 0.95-1.65); is is, however, nearly identical to reported values for ureilites (−0.77), but is also within analytical error of eucrites (–0.38) and ordinary chondrites (–0.42). Our trace element data, and especially interelement correlations of siderophile elements (e.g., Co, Ni, Cr), and low Ir contents, as well the similarity of the Cr isotopic data to ureilites, indicate that the impacting asteroid might have been an F-type asteroid, similar to the object that collided with Earth in 2008 and was later recovered at Almahata Sitta in Sudan.