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K-Pg impactor was likely carbonaceous chondrite asteroid

From: Ben Creisler

A recent paper that may be of interest:

S. Goderis, R. Tagle, J. Belza, J. Smit, A. Montanari, F. Vanhaecke,
J. Erzinger & Ph. Claeys (2013)
Reevaluation of siderophile element abundances and ratios across the
Cretaceous–Paleogene (K–Pg) boundary: Implications for the nature of
the projectile.
Geochimica et Cosmochimica Acta 120: 417–446

The discovery over 30 years ago at Gubbio (Italy) and Caravaca (Spain)
of an enrichment in the concentrations of iridium (Ir) and the other
platinum group elements (PGE) by up to four orders of magnitude (Irmax
= 0.10–87 ng/g) compared to average continental crustal background
levels remains one of the most important discoveries in the Earth
sciences. Since then, similar anomalies have been detected in more
than 120 Cretaceous–Paleogene (K–Pg) boundary sites worldwide. Highly
elevated Ir and other siderophile element abundances in roughly
chondritic ratios are considered strong indicators for the presence of
a meteoritic contribution in impact-related lithologies (melt rocks,
impact ejecta material, etc.), delivered when an extraterrestrial
object strikes Earth. The presented work adds 113 unpublished PGE
analyses of 38 K–Pg sections worldwide to the existing literature. The
analytical protocol relied on for this purpose consisted of a
combination of a nickel-sulfide fire assay pre-concentration technique
and subsequent trace metal determination via inductively coupled
plasma-mass spectrometry (ICP-MS). Through repeated determination of
key siderophile elements (i.e., Cr, Co, Ni, and PGE), the importance
of sampling, nugget effects, and analytical methodologies applied
becomes more apparent. Even more critically, these analytical effects
are superimposed by the local syn- and post-depositional conditions
that have affected the pristine meteoritic signature of the K–Pg
impactor, including potential fractionation during vaporization and
condensation, dissimilar PGE carrier phases, terrestrial PGE input,
sedimentation rate, reworking, diagenesis, bioturbation, and chemical
diffusion. While chondrite-normalized PGE patterns of individual sites
appear relatively flat (i.e., chondritic), strong variations in
siderophile element content and inter-element ratios exist between
K–Pg locations, inter-laboratory measurements, and replicate analyses,
hampering a precise projectile identification using (highly)
siderophile elements. Only when considering improved databases of
siderophile element concentrations in meteorites, in combination with
linear regression analysis to calculate inter-element ratios from a
large suite of ejecta deposit sites, the nature of the K–Pg projectile
can be resolved. Application of this methodology to an extensive data
set of continental and marine sites, very proximal to distal to the
Chicxulub impact structure, supports a carbonaceous chondritic
impactor (type CM or CO).