NICKEL-RICH MAGNESIOWUSTITE IN CRETACEOUS TERTIARY BOUNDARY SPHERULESCRYSTALLIZED FROM ULTRAMAFIC, REFRACTORY SILICATE LIQUIDS/

We have found trace inclusions of Ni-rich magnesiowustite within grain s of magnesioferrite spinel recovered from Cretaceous/Tertiary boundar y sediments from DSDP Site 596, South Pacific (23 degrees 51.20'S, 169 degrees 39.27'W) and DSDP Site 577, North Pacific (32 degrees 26.51'N , 157 degrees 43.40'E). Measured compositions of these inclusions rang e from (Mg.85Ni.11Fe0.4)O to (Mg.74Ni.09Fe.17)O. Coexisting magnesiofe rrite and magnesiowustite can only crystallize from ultramafic, refrac tory, Mg-rich liquids with Mg/Si > 2 (atom ratio). Such liquid composi tions cannot form as a result of fractional crystallization and are un known to occur as a result of terrestrial igneous processes or meteoro id ablation. We infer that these minerals crystallized from liquid dro plets that equilibrated with silicate vapor at high temperatures (prob ably >2300 degrees C), resulting in fractionation of volatile SiO2 fro m more refractory MgO. The most plausible source of this high-temperat ure vapor is in the fireball of the major impact event that terminated the Cretaceous.