Platinum metals and microstructure in magnetic deep sea cosmic spherules

Abstract

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.

Rare earth element abundances in stony spherules from deep-sea sediments.

“…1(d)) strongly supports its extraterrestrial origin. Platinum-group metal nuggets were first discovered in deep-sea iron spherules (Brownlee et al, 1984b) and also iden tified in three stony spherules (Bonte et al, 1987;Maurette et al, 1987). On the basis of semi-quan titative analyses using a SEM-EDS, characteris tic X-rays for Ru, Rh, Os, Ir, and Pt were resolv ed.…”

Internal structure of type I deep-sea spherules by X-ray computed microtomography

“…Based on this and other observations (BontÈ et al 1987;Brownlee et al 1984), it seems likely that nuggets must occur widely in type I spherules that have undergone complete oxidation of Fe. The elements more resistant to oxidation, which persist as metal, may have moved toward the front of the spherule by virtue of their high density.…”

“…Non-spherical, PGE-rich objects have been seen in a few type-I spherules. BontÈ et al (1987) observed irregularly shaped, "disequilibrium" nuggets in magnetite and remarked specifically on their "unexpected geometrical habit." Their relevance here is doubtful, however, as they occur in magnetite interior to the spherule, rather than at the edge.…”

“…Brownlee et al (1984) and BontÈ et al (1987) describe nuggets as spherical based on their circular appearance in polished sections; spherical shapes imply a melt. Our SEM image of the nugget in KK1-98-6 deviates from circularity by about 10%.…”

“…The interior oxides vary in composition and texture and, in polished section, often appear to be interlaced with cracks, which may or may not be polishing artifacts. Some type I spherules also contain micrometer-size nuggets enriched in Ir and other Pt-group elements (PGE) (Brownlee et al 1984;BontÈ et al 1987).…”