Morphological aspects, textural features and chemical composition of spherules from the Colônia impact crater, São Paulo, Brazil

Abstract

A wide variety of spherules and aluminum-silicate accretionary particles are found associated to allochthonous breccia deposits within the Colônia impact crater, SE Brazil. Using morphological, textural and compositional variation parameters, four types of spherules can be identified: (i) iron spherules and (ii) silicate-iron spherules, both dominant, and scarce (iii) titanium-silicate-iron spherules and (iv) copper-nickel-iron spherules. The spherules range in size from 0.1 mm up to 0.5 mm, and exhibit noticeable splash kinematic shapes with variations for spherical, oval, prolate and droplet. Textural patterns include granular massive, polygonal junctions, and several types of dendritic growth. Iron spherules are mainly pure iron oxides (magnetite and/or hematite) and contain low contents in Mg, Na, K, P and also in REE; occasionally, they may contain Si and Mn. Silicate-iron spherules differ for the higher concentrations in Si and in a few times in Ti and Ca. The accretionary particles have nothing in common with the spherules, except for its close spatial and temporal occurrence. In terms of morphology, they do not show symmetric splash-form shapes and some of them are clearly composed of different parts of fine pieces of mineral fragments mechanically aggregated in a partially molten state before the deposition. Chemical composition of accretionary particles reveals abundance of the elements Si, Fe and Al, presence of K and less commonly of Ti, Ca, Na and P. Because these elements are found in great amounts in minerals of the regional country rocks, it is assumed that the formation of both splash-form spherules and accretionary particles is probably related to the melting of metamorphic lithologies of the crystalline basement due to hypervelocity impact event.