Trace-element geochemistry and mineralogy of the Cretaceous/Tertiary boundary; Identification of extraterrestrial components

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Abstract
Mineralogical and geochemical analysis of the basal Cretaceous/Tertiary (K/T) boundary marl at Zumaya, Spain, indicates that anomalous siderophile (Pt, Ni) elements are present in specific mineral phases. The boundary marl at Zumaya is enriched in Ir, Pt, Ni, Cr, and As. These elements are concentrated in the following minerals: Pt in platinum grains, Cr in spinels and aluminosilicate spherules, and Ni in spinels, spherules, and Ni-rich grains. Analysis of cross sections of the basal 2 cm of boundary marl at Zumaya show that magnesioferrite spinel is fairly common; one cross section contains 350 spinel grains per square centimeter. Spherules and platinum grains are much rarer than spinel grains. No other noble metals, such as iridium, were found in specific mineral phases. The platinum grains found at Zumaya may be the first direct evidence of a native platinum group element mineral at the K/T boundary. Unlike terrestrial native platinum, these grains contain only trace amounts of iron. The aluminosilicate spherules from Zumaya contain inclusions of magnesioferrite spinels, chromium, and nickel-rich grains. The chemistry and morphology of the platinum grains, spinels, spherules, and Ni-rich grains suggest that they are impact-derived materials similar to chondritic fireball or ablation debris.

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