Nickel-rich magnesiowiistite in Cretaceous/Tertiary boundary spherules crystallized from ultramafic, refractory silicate liquids

Abstract

We have found trace inclusions of Ni-rich magnesiowastite within grains of magnesioferrite spinel recovered from Cretaceous /Tertiary boundary sediments from DSDP Site 596, South Pacific (23°51.20′S, 169°39.27′W) and DSDP Site 577, North Pacific (3°6.51′N, 157°43.40′E). Measured compositions of these inclusions range from (Mg. 85Ni .74Fe ,17) O to (Mg. 74Ni .09Fe. 1.7) O. Coexisting magnesioferrite and magnesiowastite can only crystallize from ultramafic, refractory, Mg-rich liquids with Mg Si > 2 (atom ratio). Such liquid compositions cannot form as a result of fractional crystallization and are unknown to occur as a result of terrestrial igneous processes or meteoroid ablation. We infer that these minerals crystallized from liquid droplets that equilibrated with silicate vapor at high temperatures (probably >2300°C), resulting in fractionation of volatile SiO 2 from more refractory MgO. The most plausible source of this high-temperature vapor is in the fireball of the major impact event that terminated the Cretaceous.