ORIGIN OF FE-NI METAL SPHERULES IN IMPACT MELT AT THE MONTURAQUI IMPACT CRATER, CHILE

A distinctive feature of small impact craters (< 1km diameter) is the presence of Fe-Ni spherules in the impact glass that show significant size-related compositional variations (smaller spherules have higher Ni and Co relative to the larger, Fe-rich spherules). A variety of mechanisms to explain these variations have been proposed (vaporization condensation, selective volatilization/oxidation of Fe, selective shock melting of sulfide, diffusion), and we are evaluating these through a detailed textural and compositional study of metallic spherules found in impact glass at Monturaqui crater, Chile. 3D X-ray tomography data and thin section observations indicate a wide range of spherule shapes and textures that are related to size (range from <1µm to >1mm). The smaller spherules are generally spherical while larger spherules (>100 µm) have more irregular shapes. Both can contain an interstitial sulfide phase. Electron microprobe analyses show the large compositional range of individual spherules (Ni 9 – 68%) and the interstitial sulfide (Ni 1.5 – 47%). Alteration effects such as hollowed out cores, secondary barite, and oxidation of the larger spherules are present. Zonation occurs in some of the larger, oxidized spherules in the form of Ni-rich rims. The rims are not continuous along each spherule and their thickness varies (>100µm to <5µm). Individual spherules, separated by electron pulse disintegration and hand picking, will be analyzed by ICP-MS, along with impactor fragments, to obtain a wider range of elemental data (including PGE, Ga, Ge, and Mo). These data will be used to assess the roles of impact processes and alteration in controlling the compositional variation.