Origin and diagenesis of K/T impact spherules—From Haiti to Wyoming and beyond

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Abstract— Impact spherules in Cretaceous/Tertiary (K/T) boundary clays and claystones consist of two types; each type is confined to its own separate layer of the boundary couplet in the Western Hemisphere. The form and composition of each of the spherule types result from its own unique mode of origin during the K/T event. Type 1 splash‐form spherules occur only in the melt‐ejecta (basal) layer of the K/T couplet. This layer was deposited from a ballistic ejecta curtain composed of melt‐glass droplets transported mostly within the atmosphere. In contrast, Type 2 spherules are accreted, partially crystalline, spheroidal bodies that formed by condensation of vaporized bolide and target‐rock materials in an expanding fireball cloud, from which they settled out of buoyant suspension to form the fireball layer. Dendritic and skeletal Ni‐rich spinel crystals are unique to these Type 2 spherules in the fireball layer.Compositions of relict glasses found in Type 1 K/T spherules from Haiti indicate that they were derived from intermediate silicic target rocks. These melt‐glass droplets were deposited into an aqueous environment at both continental and marine sites. We propose that the surfaces of the hot melt droplets hydrated rapidly in water and that these hydrated glass rims then altered to palagonite. Subsequent alteration of the palagonite rims to smectite, glauconite, chlorite, kaolinite, or goyazite occurred later during various modes of progressive diagenesis, accompanied by dissolution of some of the glass cores in spherules from continental sections and from marine sections that were subsequently raised above sea level. In many of the nonmarine sections in the Western Interior, the glass cores altered to kaolinite instead of dissolving.Directly comparable spherule morphologies (splash forms), textural features of the altered shells, and scalloping and grooving of relict glass cores or secondary casts demonstrate that the Haitian and Wyoming spherules are equivalent altered Type 1 melt‐droplet bodies. The spherules at both locations were deposited in a melt‐ejecta layer as part of the K/T impact event.Previously, two types of relict impact glasses had been identified in the Haitian spherule beds: black glass of andesitic composition and high‐Ca yellow glass with an unusually high S content. Most workers agree that the latter probably formed by impact melting and mixing of surficial carbonate (and minor anhydrite) rocks with the more deeply‐buried crystalline parent rocks of the black glasses. However, some workers have suggested that an intermediate compositional gap exists between the two groups of glasses, implying a different origin than simple mixing of end members during impact. We report glass compositional analyses with values extending throughout this intermediate range, lending support to the impact‐mixing model. Inclusions of CaSO4 found by us in relict yellow glasses further support this model.