Metal microspherules in breccias of the Onaping Formation, Sudbury impact structure, Ontario, Canada

Abstract

AbstractMetallic microspheres have been found in rocks from the Onaping Formation of the Sudbury impact structure, Canada. Microspherules are common in contact breccias, the lowest part of the Dowling Member, and rare microspherules have been found in the upper sequences of the Dowling Member. Separate microspherules are dispersed in the breccia matrix and do not form clusters. The sizes of the microspheres range from 5 to 30 μm; most commonly, they are 8–15 μm in size. The microspherules have a regular spherical shape, and in some cases show concentric zonal structures. The microspherules consist mostly of the refractory elements Cr, Co, Fe, Mo, W, and Ti, with a predominant Ni content of 40–75 wt%. The formation of the Sudbury metal microspherules by condensation in a high‐temperature plume is suggested by their spherical shape, concentric‐zoned structure, uniform composition, and distribution in fallback breccias of the crater‐fill Onaping Formation. The content of the most refractory W in the composition of the microspheres indicates early condensation. A decrease in the content of W and an increase in the content of Ni in the microspheres of the upper layers relative to the content of these elements in the earliest microspheres of the contact layers indicate that they could have formed by fractional condensation during the expansion and cooling of the impact vapor plume. As source material, a combination of target rocks with high nickel content with a chondritic impactor is suggested.