ORIGIN AND DIAGENESIS OF K T IMPACT SPHERULES - FROM HAITI TO WYOMINGAND BEYOND/

Impact spherules in Cretaceous/Tertiary (WT) boundary clays and clayst ones consist of two types; each type is confined to its own separate l ayer of the boundary couplet in the Western Hemisphere. The form and c omposition of each of the spherule types result from its own unique mo de of origin during the WT event. Type 1 splash-form spherules occur o nly in the melt-ejecta (basal) layer of the WT couplet. This layer was deposited from a ballistic ejecta curtain composed of melt-glass drop lets transported mostly within the atmosphere. In contrast, Type 2 sph erules are accreted, partially crystalline, spheroidal bodies that for med by condensation of vaporized bolide and target-rock materials in a n expanding fireball cloud, from which they settled out of buoyant sus pension to form the fireball layer. Dendritic and skeletal Ni-rich spi nel crystals are unique to these Type 2 spherules in the fireball laye r. Compositions of relict glasses found in Type 1 WT spherules from Ha iti indicate that they were derived from intermediate silicic target r ocks. These melt-glass droplets were deposited into an aqueous environ ment at both continental and marine sites. We propose that the surface s of the hot melt droplets hydrated rapidly in water and that these hy drated 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 f rom continental sections and from marine sections that were subsequent ly raised above sea level. In many of the nonmarine sections in the We stern Interior, the glass cores altered to kaolinite instead of dissol ving. Directly comparable spherule morphologies (splash forms), textur al features of the altered shells, and scalloping and grooving of reli ct glass cores or secondary casts demonstrate that the Haitian and Wyo ming spherules are equivalent altered Type 1 melt-droplet bodies. The spherules at both locations were deposited in a melt-ejecta layer as p art of the WT impact event. Previously, two types of relict impact gla sses 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 impac t melting and mixing of surficial carbonate (and minor anhydrite) rock s with the more deeply-buried crystalline parent rocks of the black gl asses. Ho vr?ever, some workers have suggested that an intermediate co mpositional gap exists between the two groups of glasses, implying a d ifferent origin than simple mixing of end members during impact. We re port glass compositional analyses with values extending throughout thi s intermediate range, lending support to the impact mixing model. Incl usions of CaSO4 found by us in relict yellow glasses further support t his model.