Letter: Extraterrestrial spherules in glacial sediment from the Transantarctic Mountains, Antarctica: Structure, mineralogy, and chemical composition

Silicate and glassy spherules of probably extraterrestrial origin have been discovered in glacial till from localities in the Beardmore glacier region, Transantarctic Mountains, Antarctica. These spherules are similar in structure, appearance, and composition to spherules found in deep-sea sediments and in Greenland. Most spherules have a shiny and smooth dark surface, while some show a characteristic brickwork pattern composed of intergrown olivine crystals and magnetite dendrites. Several spherules have been sectioned after trace element analysis by INAA. The interiors of the spherules contain olivine crystals of varying size (predominantly of forsteritic composition) set in a glassy Fe-rich and Mg-poor matrix. The matrix contains very fine-grained dendritic magnetite crystals and rare accessory wustite. SEM and microprobe studies show that most of the spherules have not experienced significant weathering. Trace element analyses for individual particles confirm the extraterrestrial nature of the spherules. They have undifferentiated chondritic rare earth element (REE) patterns and are enriched in Ir and other siderophile elements (with Ir contents between 10 and >1000 ppb). Some spherules have experienced loss of volatile elements and some differentiation of siderophile elements ( e.g., Ni). The Antarctic spherules—which might be micrometeorites that melted during passage through the atmosphere, or ablation spherules—are a significant addition to the collection of extraterrestrial microparticles. Of key importance is their high abundance in the sediment and the fact that they have been concentrated by glacial action.