The Cretaceous‐Tertiary fireball layer, ejecta layer and coal seam: Platinum‐group element content and mineralogy of size fractions

Abstract

Abstract— Grain size separates of the Cretaceous‐Tertiary (K‐T) fireball layer at marine sites and the entire K‐T boundary interval at nonmarine sites (fireball layer, ejecta layer, coal seam) were analysed for platinum‐group elements (PGE: Ru, Ir, Ft, Pd, Rh) and Au using inductively coupled plasma mass‐spectrometry. X‐ray diffractometry and scanning electron microscopy were performed on each fraction to establish the mineralogy. It was determined that the most abundant minerals in the finest fraction were a smectite‐group mineral at marine K‐T sites and illite‐smectite mixed layer clay at nonmarine K‐T sites. Positive correlation of PGE content and mineralogy indicates that the PGE are presently associated with these clay minerals which were likely formed by alteration of a condensed mafic phase (e.g ., olivine or pyroxene). The correlation of clay mineral abundance with PGE content is imperfect because not all of the clay minerals in the fireball layer are derived by alteration of the original condensed ejecta. In addition, undetected PGE host(s), present in trace amounts, are likely also present.