Platinum metals and microstructure in magnetic deep sea cosmic spherules

Instrumental neutron activation analysis was used to determine the concentrations of Ir, Fe, Ni, Co, Cr, and Sc in 16 “iron” and 8 “chondritic” magnetic deep sea spherules with sizes of about 200 μm. With the exception of two iron spheres, high indium concentrations were found in each sphere, thus confirming their extraterrestrial origin. Moreover, the “depth profile” of the residual mass of iridium was monitored in individual spheres by comparing the variations of the intensities of the γ ray lines of iridium, cobalt, and iron during a very gradual sequential polishing. Each sharp but partial discontinuity, observed only in the Ir profiles, reflects the intersection of a polished section with a tiny Ir‐rich nugget. These profiles show that such nuggets are much more common in both the iron and the chondritic spheres than previously thought. We found new types of disequilibrated clusters of micrometer‐sized nuggets of platinum metals. All types of nuggets, which were characterized with a scanning electron microscope and an energy dispersive X ray spectrometer, seem to be associated with magnetite. The compositions of the nuggets, as well as the fractionation patterns for Ir, Fe, Ni, Co, and Sc in the spheres, suggest that most of the Ir‐rich iron spheres originate from iron‐rich bodies.