Mysterious iron-nickel-zinc arctic spherules

Abstract

An investigation into the sources of ice-rafted detritus in the central Arctic Ocean using microprobe analyses of detrital Fe oxide minerals discovered unique magnetic spherules in 20 of 144 potential source sample sites from the shelves and coastal areas around this ocean. The spherules occur only in samples from the Queen Elizabeth Islands, Canada. These grains are characterized by a 45-60 µm diameter, pitting throughout, and are occasionally found as multiple joint spherules. They have the optical properties of magnetite and, most remarkably, contain both ZnO and NiO in subequal amounts of up to 25%. The Ni suggests either an anthropogenic or meteoritic source. These spherules were probably ice-rafted into the central Arctic Ocean, where they are found in sediments of Holocene age and back to at least 780 ka, eliminating an anthropogenic source. Because Zn is too volatile to survive entry of a meteor through the earth's atmosphere, these spherules were probably formed during impact of an Fe-Ni meteor in an area of abundant Zn, perhaps the Zn rich Paleozoic carbonates of the Queen Elizabeth Islands. Examination of the ejecta and sediments filling the 22 Ma Haughton Astrobleme impact site on Devon Island, a carbonate terrain, revealed few magnetite spherules. None of these were pitted or contained Ni, but a few percent of ZnO were found in three spherules. Thus, the origin of these magnetite spherules remains unknown. The unique appearance and geochemistry of these spherules are useful in tracing Arctic Ocean ice-rafted detritus to its source.