Geochemistry of impact glasses from the K/T boundary in Haiti: Relation to smectites and a new type of glass

We have individually analyzed twelve black and seven yellow glasses from the K/T boundary section at Beloc, Haiti, plus three smectite mantles around black glasses. The main chemical differences between black and yellow glasses are higher S, Ca, Mg, Zn, As, Br, Sb, and Au, but lower K, Na, and Si abundances in the yellow glasses. We have found high-CaO yellow glasses with low S contents (0.08, 0.17 wt% ), which may be explained by fusion of carbonate-rich sediments. Microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10-100 pm level, while yellow glasses show much more variability and different intra-sample elemental correlations. One of the black glasses has higher SiO, and KrO abundances, but lower concentrations of all other major elements. This suggests the existence of a third glass type, which we have named the high Si-K variety (HSi,K glass). The glass shows areas consisting of pure SiOr (maybe lechatelierite) and some schlieren with lower SiOr content. The chemical composition indicates that the HSi,K glass may have originated from melting of a sedimentary source rock, such as shale or argillite. Severe changes in major and trace element composition occur during alteration of the glass, as documented by analyses of adhering smectites. The REE patterns in the glass arc similar to average upper crust, but the smectites exhibit much lower abundances and flat patterns, not unlike the patterns observed in claystones at numerous K/T boundary sites. The suggestion that the Chicxulub structure in Yucatan may be the source crater for the K/T impact glasses is not fully supported by the currently available data. Impact glasses and tektites are produced from the surface layers of their targets. At Chicxulub, the target stratigraphy comprised mainly carbonates and evaporites. Although we cannot exclude Chicxulub as a source, the present data do not provide any firm chemical evidence for a connection. We conclude, however, that the Haiti glasses have formed by impact and have later undergone alteration by low-temperature hydrothermal processes to produce clays. The boundary claystones at K/T boundary sites may very well be alteration products of these impact glasses. INTRODUCITON THE CRETACEOUS-TERTIARY (K/T) boundary is marked by signs of a worldwide catastrophe. Ever since ALVAREZ et al. ( 1980) identified an enrichment of Ir and other siderophile elements in rocks marking the K/T boundary and interpmted it as the mark of a giant asteroid (or comet) impact, researchers have tried to understand the complexities of the K/T boundary event. The impact theory received a critical boost by the discovery of shocked minerals which have so far been found only in association with impact craters (BoHOR et al., 1984, 1987). One of the problems of the K/T impact theory was, and still is, the lack of an adequate large crater that is close to the maximum abundance of shocked grains in K/T boundary sections, which was found to occur in sections in Northern America ( BOHOR and IZETT, 1986; IZETT, 1987a; MCHONE et al., 1989; BOHOR, 1990). In this respect, the recent discovery of impact glasses from a K/T section in Haiti ( I~ETT et al., 1990; I~ETT, 199 la; SIGURDSSON et al., 1991a; KRING and BOYNTON, 1991) is of great importance. The K/T boundary layer at Beloc, Haiti (Fig. 1). is much thicker than at other known sites and contains large (up to l-2 mm; MAURRASSE and SEN, 199 1) shocked quartz grains as well as altered spherules interpreted to be of impact origin ( HILDEBRAND and B~YNTON, 1990b). The presence of relict glasses within some of the altered spherules from Haiti was reported by IZETT et al. ( 1990), SIGURDWN et al. ( 199 1 a), IZETT ( 199 la), and KRING and BOYNTON ( 199 1). Microspherules have been reported from a variety of K/T boundary sites around the world (e.g., MONTANARI et al., 1983; BOHOR, 1984; BROOKS et al., 1985; NASLUND et al., 1986; IZETT, 1987b) and have in most cases been interpreted as altered impact spherules. The glass spherules from Haiti provide valuable clues as to the target material of the K/T impact(s). Of great importance also is the finding of glasses within smectite shells, which provides direct data on the alteration of the impact products. Isotopic ( DEPAOLO et al., 1983) and chemical arguments, especially the low abundances of the rare earth elements (REEs) in K/T boundary claystones, have been used to infer an oceanic impact site (e.g., HILDEBRAND and BOYNTON, 1988a,b). Here we attempt to ( 1) provide confirmation of the chemical composition of the Haiti glasses reported elsewhere and present high-precision data for major and trace elements in individual glass particles; (2) describe, for the first time, the presence of a third type of impact glass at the Haiti K/T boundary layer; and ( 3) describe the chemical relationship of the glasses with their smectite alteration products and relate that alteration to the nature of the K/T boundary layers at other sites.