COSMIC NICKEL-IRON ALLOY SPHERULES FROM PLEISTOCENE SEDIMENTS, ALBERTA, CANADA

A large number of Ni-Fe alloy spherules, containing from 27.63% up to approximately 100% Ni by weight, have been found in Pleistocene sediments in Alberta, Canada. Most spherules are Ni-Fe metals; a few have iron oxide rims, and some have iron-rich dendrites in the main metal phase, The concentrations of Ni and Fe in the alloys and X-ray powder diffraction data reveal the dominant mineral phase to be taenite (gamma-Fe). The spherules appear to be similar in composition, morphology, and size to Ni-Fe cores of cosmic spherules found in deep-sea sediments and polar ice. We suggest that the Ni-Fe alloy spherules represent the ejected Ni-Fe cores of cosmic spherules. A four-stage model, which involves ablation, oxidation, core protrusion, and core ejection, is proposed to explain the formation of such spherules. It is estimated that the accretion rate of cosmic Fe spherules to Earth was approximately 8.9 X 10(6) tonnes/yr in the Pleistocene.