Reconstruction Of The Subsurface Structure Of The Marquez Impact Crater In Leon County,Texas, Usa,Based On Well‐Log And Gravity Data

Abstract— In Leon County, Texas, USA, the Marquez Dome, an approximately circular 1.2 km diameter zone of disturbed Cretaceous rocks surrounded by shallow dipping Tertiary sediments, has been interpreted by Gibson and Sharpton (1989) and Sharpton and Gibson (1990) as the surface expression of a buried complex impact crater. New gravity and magnetic anomaly data collected over the Marquez Dome have been combined with well‐log and seismic reflection information to develop a better estimate of the overall geometry of the structure. A three‐dimensional model constructed to a depth of 2000 m from all available information indicates a complex crater 13 km in diameter with an uplift in the center of at least 1120 m. The zone of deformation associated with the cratering event is limited to a depth of <1720 m. No impact breccias were recovered in drilling at two locations, 1.1 and 2 km from the center of the structure, and the central uplift may be the only prominent remnant of this impact into unconsolidated, water‐rich sediments. The magnetic anomaly field shows no correlation with the location and extent of the structure.     

New investigations of the strewn field of Libyan desert glass and its petrography

Twenty-six specimens of Libyan Desert glass were collected from the westernmost area of Libyan Desert glass occurrence during a joint expedition, 30 May to 5 June, 1971, of The University of Libya and The University of Texas. The Oasis circular structure, a suspected astrobleme southwest of the glass site, was also visited. In the glass, included minerals are partially altered through shock and temperature effects; microprobe analysis confirms one of these minerals as zircon and indicates that another is the elbaite variety of tourmaline. Tourmaline of the elbaite variety and zircon were also identified in a thin section of an analyzed sample of high-silica (99.6%) Nubian Sandstone collected in the area of glass occurrence. This silica content is higher than that of any Libyan Desert glass yet examined. In a heavy mineral separation of this sandstone, the minerals cordierite; green, brown, and blue tourmaline; and minor amounts of staurolite, zircon, and kyanite were identified. It is concluded, from the mineral and chemical evidence, that Libyan Desert glass was derived from Nubian Sandstone. Whether there is any connection between the origin of Libyan Desert glass and the origin of the nearby impact structures is undetermined.     

Isotopic and textural discrimination between hypogene,ancient supergene,and modern sulfates at the Questa mine,New Mexico

Chemical and mineralogical changes due to pyrite weathering are of interest with respect to understanding long-term physical stability of mine rock piles at the Questa mine, New Mexico. The ability to discriminate between ancient and modern processes is important for establishing the extent of modern weathering within the piles. Initial inventories of sulfur minerals and representative isotope compositions in rocks from orebodies, the hydrothermal alteration zones associated with orebodies, hydrothermal alteration scars, and mine rock piles were determined. Ore body sulfides have δ³⁴S_CDT of 0 ± 4‰, typical for sulfides formed by magmatic processes in stockwork Mo systems. Pyrite from alteration scars has a wide range of δ³⁴S values from 0.0‰ to −13.6‰. Sulfate from the ore body has markedly positive δ³⁴S (5–10‰) accompanied by positive δ¹⁸OSO₄${\delta }^{18}{\text{O}}_{{\text{SO}}_4}$ values (6–15‰) reflecting equilibrium formation from magmatic fluids. Sulfates from alteration scars have δ³⁴S values over a broad range, similar to alteration scar pyrites, from −10.6‰ to 0‰ and δ¹⁸OSO₄${\delta }^{18}{\text{O}}_{{\text{SO}}_4}$ of 0 ± 3‰. Sulfates with fine grained, delicate, and euhedral mineral habits suggesting recent formation within the mine rock piles, have δ³⁴S values similar to orebody pyrite and alteration scars but more negative δ¹⁸OSO₄${\delta }^{18}{\text{O}}_{{\text{SO}}_4}$ values (−3‰ to −10‰). Sulfates from all three sources occur in these piles, and their stable isotope values have proven useful in differentiating them and their environments of formation (i.e., hypogene, ancient supergene, and recent weathering). Correlating the isotopic compositions with textures allows petrographic assessment for the origins of sulfate minerals in the rock piles, but this must be applied with caution because some sulfate mineral recycling has occurred.     

Isotopic fractionation of Mg(aq), Ca(aq), and Fe(aq) with carbonate minerals

Density-functional electronic structure calculations are used to compute the equilibrium constants for ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca isotope exchange between carbonate minerals and uncomplexed divalent aquo ions. The most reliable calculations at the B3LYP/6-311++G(2d,2p) level predict equilibrium constants K, reported as 10³ln (K) at 25 °C, of −5.3, −1.1, and +1.2 for ²⁶Mg/²⁴Mg exchange between calcite (CaCO₃), magnesite (MgCO₃), and dolomite (Ca_0.5Mg_0.5CO₃), respectively, and Mg²⁺(aq), with positive values indicating enrichment of the heavy isotope in the mineral phase. For ⁴⁴Ca/⁴⁰Ca exchange between calcite and Ca²⁺(aq) at 25 °C, the calculations predict values of +1.5 for Ca²⁺(aq) in 6-fold coordination and +4.1 for Ca²⁺(aq) in 7-fold coordination. We find that the reduced partition function ratios can be reliably computed from systems as small as and embedded in a set of fixed atoms representing the second-shell (and greater) coordination environment. We find that the aqueous cluster representing the aquo ion is much more sensitive to improvements in the basis set than the calculations on the mineral systems, and that fractionation factors should be computed using the best possible basis set for the aquo complex, even if the reduced partition function ratio calculated with the same basis set is not available for the mineral system. The new calculations show that the previous discrepancies between theory and experiment for Fe³⁺-hematite and Fe²⁺-siderite fractionations arise from an insufficiently accurate reduced partition function ratio for the Fe³⁺(aq) and Fe²⁺(aq) species.     

Albedo,internal heat flux,and energy balance of Saturn

Full-disk and high-resolution measurements recorded during the Voyager 1 flyby of Saturn by the radiometer of the infrared instrument, IRIS, indicate a geometric albedo of 0.242 ± 0.012, which is lower than previous estimates. The given error is largely due to uncertainties in systematic corrections; random effects are small. Combining this measurement with the Pioneer-derived phase integral yields a Bond albedo of 0.342 ± 0.030. Infrared spectra recorded at the same time by the Michelson interferometer, along with a model extrapolation to wavenumbers not covered by the instrument, yield an effective temperature of 95.0 ± 0.4°K. As in the case of the radiometer, random instrumental errors are small, and the quoted error in the effective temperature reflects primarily uncertainties in systematic corrections. The rings of Saturn significantly affect both the short- and long-wavelength fluxes. From these measurements the internal heat flux of Saturn is 2.01 ± 0.14 10^?4W cm^?2, and the energy balance, defined as the ratio of total emitted to total absorbed energy, is 1.78 ± 0.09.     

Venus cloud properties: Infrared opacity and mass mixing ratio

By using the Mariner 5 temperature profile and a homogeneous cloud model, and assuming that CO₂ and cloud particles are the only opacity sources, the wavelength dependence of the Venus cloud opacity is infrared from the infrared spectrum of the planet between 450 and 1250 cm^?1. Justification for applying the homogeneous cloud model is found in the fact that numerous polarization and infrared data are mutually consistent within the framework of such a model; on the other hand, dense cloud models are not satisfactory.Volume extinction coefficients varying from 0.5 × 10^?5 to 1.5 × 10^?5 cm^?1, depending on the wavelength, are determined at the tropopause level of 6110 km. By using all available data, a cloud mass mixing ratio of approximately 5 × 10^?6 and a particle concentration of about 900 particles cm^?3 at this level are also inferred. The derived cloud opacity compares favorably with that expected for a haze of droplets of a 75% aqueous solution of sulfuric acid.     

Influences of atmospheric variations on Mars's record of small craters

Planetary atmospheres influence cratering rates at small diameters (∼2-250 m) by filtering impactor populations via ablation, aerobraking and breakup of entering objects. The atmosphere of Mars undergoes rapid and drastic obliquity-driven variations in density, corresponding to pressure variations between zero and several tens of millibars. Here a simulation is used to assess the fate of a large population of impactors interacting with the present and predicted past and future martian atmospheres. We find that even Mars's present atmosphere significantly reduces crater production rates at small diameters (<30 m) and past denser atmospheres would have affected cratering even more strongly, and to considerably larger diameters. These effects are increased if the inner Solar System's small impactor population contains significant numbers of icy, cometary bodies. Evidence of recent atmospheric density variations may be detectable in the martian small cratering record with future planned imaging capabilities. Because of martian atmospheric effects and variations, surface ages derived from counts of craters of less than about 250 m on Mars may be underestimated.