Osmium, tungsten, and chromium isotopes in sediments and in Ni‐rich spinel at the K‐T boundary: Signature of a chondritic impactor

Abstract

Abstract-It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re-Os, Hf-W, and Mn-Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous-Tertiary (K-T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni-rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite.

Absence of geochemical evidence for an impact event at the Bølling–Allerød/Younger Dryas transition

“…In terms of siderophile element content, these groups do not differ strongly [CI: Ir ϭ 472 ng/g, Ni ϭ 10,863 g/g, Cr ϭ 2,796 g/g; CM: Ir ϭ 605 ng/g, Ni ϭ 12,396 g/g, Cr ϭ 3,059 g/g; CR: Ir ϭ 622 ng/g, Ni ϭ 13,794 g/g, Cr ϭ 3,810 g/g; (38)]. The Chicxulub impact structure was formed by an asteroidal CM carbonaceous chondrite, compositionally similar to comets without an icy component (64,69). It is also worth mentioning that comets have never been analyzed for their PGE-composition, but cometary interplanetary dust particles thought to have originated from comets lack nanodiamonds (70).…”

Projectile Identification in Terrestrial Impact Structures and Ejecta Material

“…Applications of the method to crater impactites and ejecta layers are described in detail by Lugmair (1998), Shukolyukov et al (2000), Koeberl et al (2002Koeberl et al ( , 2007, Kyte et al (2003), Trinquier et al (2006), Quitté et al (2007), and Simonson et al (2009). The conventional methodology relies on the measurement of the relative abundances of radiogenic isotope 53 Cr, the daughter product of the long since extinct radionuclide 53 Mn (half life of 3.7 Myr).…”

“…Tungsten ( 182/184 W) isotope ratios (based on the decay of 182 Hf to 182 W with a half-life of 9 Myr) have been proposed to evaluate the degree of mixing that occurred between meteoritic material and the surrounding terrestrial sediments (Quitté et al, 2007). Similar to Cr isotope systematics, the terrestrial W isotope composition is constant (ε 182 W) = 0 by definition; Lee and Halliday, 1996;Schoenberg et al, 2002), while different classes of meteorites show a high variability in W isotope ratios.…”

“…Normalized 53 Cr/ 52 Cr ratios for meteorites vary between +0.1 and +1.3 ε, except for carbonaceous chondrites, which show values around −0.4 ε. The original normalization procedure made possible the first isotopic recognition of the presence of a carbonaceous chondritic projectile component at the K-Pg boundary layer Quitté et al, 2007). The 'raw' ε( 53 Cr) and ε( 54 Cr) ratios (although less precise than the normalized equivalent) can distinguish between different classes of carbonaceous chondrites and refined the type of impactor at the K-Pg boundary down to a CM2 carbonaceous meteorite (Trinquier et al, 2006).…”

“…Seven of the Phanerozoic structures (>1 km) for which the projectiles are known down to the level of a specific class or type were produced by ordinary chondrites (OC). Only the projectile of the largest known impact in the last 1 billion years, the Chicxulub structure, stands out as a carbonaceous chondrite (CM2-type; Kyte, 1998;Shukolyukov and Lugmair, 1998;Trinquier et al, 2006;Quitté et al, 2007). Stony-iron projectiles prevail in the non-chondritic fraction of the projectiles identified, including the mesosiderite fragments have the lowest 182/184 W ratios (−4.4 to −2.5 ε W ), reflecting the early removal of metal-loving W from silicate into the core of planetesimals, when most 182 Hf was still present.…”