MICROTEKTITES AND MICROKRYSTITES: DEFINITIONS AND MODES OF FORMATION

There are two kinds of distal impact spherules based on petrography: 1) microtektites, which consist of glass without any primary crystallites/microlites, and 2) microkrystites, which contain microlites/crystallites. Cenozoic microtektites (e.g., Australasian) all appear to have been formed directly as ejected melt droplets consisting almost entirely of melted target rocks with little or no contribution from the projectile. The best examples of microkrystites are the Late Eocene cpx spherules which often contain Ni-rich spinel crystals. They appear to have been formed as condensate droplets from an impact plume consisting of vaporized projectile and target rock. However, the formation of primary microlites/crystallites in melt droplets (whether they form from ejected melt droplets or from vapor condensate droplets) probably depends primarily on the composition of the melt and not on their mode of formation. Most melt droplets produced as condensate droplets from a plume consisting largely of vaporized projectile will generally be mafic in composition and are likely to partially crystallize on cooling and, thus, form microkrystites. Whereas, melt droplets produced by melting and ejection of target rock will usually be more silicic and are likely to produce glass spherules without primary microlites/crystallites (i.e., microtektites). However, if the impactor were an achondrite, condensate droplets from the plume may not be as mafic and microlites/crystallites may not form during solidification resulting in the formation of microtektites. Likewise, if the target rock is mafic, then the ejected melt droplets may undergo partial crystallization to form microkrystites. Thus, four types of distal impact spherules are possible: 1) microtektites formed from ejected melt droplets, 2) microtektites formed from condensate droplets, 3) microkrystites formed from condensate droplets, and 4) microkrystites formed from ejected melt droplets. It should be possible to distinguish the four types based on shape and petrography. Unlike spherules formed from condensate droplets, spherules formed from ejected melt droplets often have rotational shapes (e.g., dumbbells, teardrops) and often contain vesicles, lechatelierite particles, and, rarely, relict mineral grains.