Benjamin C. Schuraytz

Petrology of Impact-Melt Rocks at the Chicxulub Multiring Basin, Yucatan, Mexico

Compositions and textures of melt rocks from the upper part of the Chicxulub structure are typical of melt rocks at other large terrestrial impact structures. Apart from variably elevated iridium concentrations (less than 1.5 to 13.5 +/- 0.9 ppb) indicating nonuniform dissemination of a meteoritic component, bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform-sediment target lithologies. Modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts. Subtle variations in the compositions of early-formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts. Evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite, K-feldspar, quartz, epidote, chlorite, and other phyllosilicates, as well as siderophile element-enriched sulfides, suggesting the possibility that Chicxulub, like Sudbury, may host important ore deposits.

Evidence for a meteoritic component in impact melt rock from the chicxulub structure

Abstract The Chicxulub structure in Yucatan, Mexico, has recently been recognized as a >200-km-diameter multi-ring impact crater of K-T boundary age. Crystalline impact melt rocks and breccias from within the crater, which have compositions similar to those of normal continental crustal rocks and which show shock metamorphic effects, have been studied for trace element and Re-Os isotope compositions. Re-Os isotope systematics allow the sensitive and selective determination of an extraterrestrial component in impact-derived rocks. A melt rock sample shows elevated indium concentrations, an osmium concentration of 25 ppb and a low 187 Os 188 Os ratio of 0.113, which are incompatible with derivation from the continental crust. Even though the 187 Os 188 Os ratio is slightly lower than the range so far measured in meteorites, a mantle origin seems unlikely for mass balance reasons and because the cratering event is unlikely to have excavated mantle material. The data support the hypothesis of a heterogeneously distributed meteoritic component in the Chicxulub melt rock. A sample of impact glass from the Haitian K-T boundary at Beloc yielded about 0.1 ppb osmium and an 187 Os 188 Os ratio of 0.251, indicating the presence of a small meteoritic component in the impact ejecta as well.

Chemical variation of the large Apollo 15 olivine‐normative mare basalt rock samples

Most chemical analyses of Apollo 15 olivine-normative mare basalts have been conducted on subsamples of 4 g) to obtain greater whole-rock representivity. These subsamples were individually ground and homogenized, and splits were taken for analysis. Furthermore, we used both X-ray fluorescence and neutron activation techniques to analyze for a comprehensive set of elements suitable for petrogenetic interpretation. The analyses show that the samples form a single coherent suite with almost all of the variation corresponding with olivine control (15% range). A few of the coarser rocks are not quite represented even at this sampling size. The analyses show that the rocks are individually distinct and that analyses are not merely of unrepresentative pieces of a single rock, undifferentiated rock unit, or rocks differing only by short-range unmixing of residual fluids. The petrographic features, including the low abundance of olivine and its small size, and the vesicularity of even some of the coarser samples, show that the olivine that controlled the chemical variation is not accumulated in any of the rocks. The Apollo 15 olivine-normative mare basalts were extruded as a series of magmas from a shallow but not locally surficial, olivine-accumulating magma system and formed a sequence of thin flows. A greater understanding of the relationships within and among other mare basalt sequences would be obtained by obtaining comprehensive chemical analyses on splits taken from subsamples of 5 g of all rocks large enough to obtain such subsamples.

Iridium Metal in Chicxulub Impact Melt: Forensic Chemistry on the K-T Smoking Gun

Iridium concentrations in successively smaller subsplits of melt rock and melt breccia from the Chicxulub impact basin were tracked to isolate and identify iridium carrier phases. Iridium-rich particles were isolated from two samples, and a micrometer-scale, silicate-enclosed aggregate of subhedral iridium metal grains was identified in one, confirming earlier reports of iridium at ground zero of the impact at the Cretaceous-Tertiary (K-T) boundary. The aggregate may be either a phase formed after the collision or a relict of the Chicxulub basin-forming meteorite. In either case, its presence indicates that even among the largest impact structures on Earth, meteoritic components may be preserved within the crater.

Martian Surface and Atmosphere Workshop

The NASA-sponsored Martian Surface and Atmosphere Through Time Study Project convened its first major meeting at the University of Colorado in Boulder, September 23–25, 1991. The workshop, co-sponsored by the Lunar and Planetary Institute (LPI) and the Laboratory for Atmospheric and Space Physics at the University of Colorado, brought together an international group of 125 scientists to discuss a variety of issues relevant to the goals of the MSATT Program. The workshop program committee included co-convenors Robert Haberle, MSATT Steering Committee Chairman NASA Ames Research Center) and Bruce Jakosky (University of Colorado), and committee members Amos Banin (NASA Ames Research Center and Hebrew University), Benjamin Schuraytz (LPI), and Kenneth Tanaka (U.S. Geological Survey, Flagstaff, Ariz.). The purpose of the workshop was to begin exploring and defining the relationships between different aspects of Mars science—the evolution of the surface, the atmosphere, upper atmosphere, volatiles, and climate. Specific topics addressed in the 88 contributed abstracts included the current nature of the surface with respect to physical properties and photometric observations and interpretations; the history of geological processes, comprising water and ice-related geomorphology, impact cratering, and volcanism; and the geochemistry and mineralogy of the surface with emphasis on compositional and spectroscopic studies and weathering processes. Also addressed were the present atmosphere, focusing on structure and dynamics, volatile and dust distribution, and the upper atmosphere; long-term volatile evolution based on volatiles in SNC meteorites (certain meteorites thought to have come from Mars) and atmospheric evolution processes; climate history and volatile cycles in relation to early climate and the polar caps, ground ice, and regolith; and future mission concepts.