A CENSUS OF THE METEORITES OF ROOSEVELT COUNTY,NEW MEXICO

In the five years from June, 1967, to June, 1972, a total of 99 meteorites were found in Roosevelt County, New Mexico and in adjoining Curry County. Of this number, 74 were found by one man. The finds include two achondrites, one pallasite, one carbonaceous chondrite, and 95 chondrites. They appear to represent more than 50 separate meteorite falls. The finding of a large number of meteorites in a small area provides data for an estimate of the probable quantity and average size of the meteorite specimens reaching the earth. The problems involved in allocating a total of 17 available local place names among more than 50 meteorite falls are discussed.     

Infrared Spectroscopy of Jupiter's Atmosphere after the A and E Impacts of Comet Shoemaker–Levy 9

Infrared spectra of Jupiter's atmosphere were obtained with the infrared spectrometer (IRS) on the 1.5-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) during the first 2 days of the impacts of the fragments of Comet Shoemaker–Levy 9 (1993e). We monitored 3.51 ± 0.17 μm radiation from the impact areas, undisturbed areas, and auroral regions of Jupiter after the A and E impacts. The strong emission of a portion of the P-branch of the ν₃band of CH₄was detected on the A impact area 4 hr after the impact. H⁺₃emissions are found to be decreased at the A and E impact sites after 4 hr and 10 hr 50 min of the impacts, respectively, compared with undisturbed areas at the same latitude. The temperatures of the southern H⁺₃aurora were normal within the first several hours following the A and E impacts.     

Petrogenesis of Al-rich chondrules: Evidence from bulk compositions and phase equilibria

We measured major, minor, and trace-element compositions for eleven Al-rich chondrules from unequilibrated ordinary chondrites to investigate the relationships between Al-rich chondrules, ferromagnesian chondrules, Ca-, Al-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs). Phase equilibrium considerations show that, for the most part, mineral assemblages in Al-rich chondrules are those expected from melts of the observed compositions. The diversity of mineral assemblages and Al-rich chondrule types arises mainly from the fact that the array of compositions spans both the spinel-saturated anorthite-forsterite reaction curve and a thermal divide defined by where the anorthite-forsterite join crosses the reaction curve. The reaction curve accounts for the two principal varieties of Al-rich chondrule, plagioclase-phyric and olivine-phyric, with or without aluminous spinel. The thermal divide influences the subsequent evolution of each variety. A third variety of Al-rich chondrule contains abundant sodium-rich glass; trace-element fractionation patterns suggest that these glassy Al-rich chondrules could have been derived from the other two by extensive alteration of plagioclase to nepheline followed by remelting. The bulk compositions of Al-rich chondrules (except sodium-rich ones) are intermediate in a volatility sense between ferromagnesian chondrules and type C CAIs. The combined trend of bulk compositions for CAIs, Al-rich chondrules, and ferromagnesian chondrules mirrors, but does not exactly match, the trend predicted from equilibrium condensation at P_T ∼ 10^-3 atm; the observed trend does not match the trend found for evaporation from a liquid of chondritic composition. We thus infer that the bulk compositions of the precursors to CAIs, Al-rich chondrules, were ferromagnesian chondrules were controlled primarily by vapor-solid reactions (condensation or sublimation) in the solar nebula. Some Al-rich chondrules are consistent with an origin by melting of a compound CAI-ferromagnesian chondrule hybrid; others cannot be so explained. Any hybrid model is restricted by the constraint that the CAI precursor consisted dominantly of pyroxene + plagioclase + spinel; melilite cannot have been a significant component. Amoeboid olivine aggregates also have the inferred mineralogical characteristics of Al-rich chondrule precursors—they are mixtures of olivine with plagioclase-spinel-pyroxene-rich CAIs—but the few measured bulk compositions are more olivine-rich than those of Al-rich chondrules.     

Two forsterite-bearing FUN inclusions in the Allende meteorite

We have discovered two FUN inclusions, CG-14 and TE, among a group of five forsterite-rich inclusions in Allende, two of which are described for the first time herein. All five consist of euhedral forsterite and spinel crystals poikilitically enclosed by fassaite. Forsterite and spinel are usually segregated from one another, sometimes into a spinel-rich mantle and a forsterite-rich core. Some inclusions contain vesicles, indicating that they were once molten. The crystallization sequence inferred from textures is: spinel, forsterite, fassaite and, finally, Mg-rich melilite. One concentrically-zoned inclusion contains melilite in its mantle whose composition lies on the opposite side of the liquidus minimum in the melilite binary from that in its core. This suggests that segregation of forsterite from spinel in all of these inclusions could be due to volatilization of MgO and SiO₂ relative to Al₂O₃ and CaO from the outsides of droplets. CG-14 is relatively uniformly enriched in refractory elements relative to Cl chondrites by a factor similar to that for Ca-, Al-rich coarse-grained inclusions except for Ca, Al and Hf which are unusually low. No Ce anomaly such as found in FUN inclusions Cl and HAL is present in CG-14. Whole-rock samples of CG-14 and TE are more strongly mass-fractionated in oxygen relative to “normal” Allende inclusions than the FUN inclusion EK 1-4-1 and less so than Cl. Relative to bulk Allende, both inclusions have strongly massfractionated magnesium and silicon and ²⁵Mg excesses or deficits of ²⁴Mg or ²⁶Mg. CG-14 has a ²⁹Si excess or a deficit of ²⁸Si or ³⁰Si. Volatilization loss cannot be responsible for the magnesium and silicon isotope fractionations, as this would require prohibitively large mass loss from these magnesium-rich inclusions. The remarkable similarity in textures between FUN and non-FUN inclusions implies similar thermal histories, arguing against different rates of evaporative loss of major elements. Sputtering alone may be insufficient to account for the magnitude and direction of oxygen isotope fractionation in FUN inclusions.     

X-ray absorption spectroscopic studies of silicate glasses and minerals

Selected results of x-ray absorption spectroscopy (XAS) studies of amorphous silicates and minerals are presented in order to show their utility in providing short-and, in certain cases, medium-range structural and bonding information for cations and anions. EXAFS and XANES studies of amorphous silicates are reviewed with the objective of illustrating variations in structural environments of the various types of glass-forming cations, including Si, Al, Na, K, Ca, Ti, Fe, Yb, and U. Al is shown to occur in tetrahedral coordination in all aluminosilicate glasses examined, including peraluminous compositions. The weakly bonded Na and Ca cations are shown to occur in sites with observed coordination numbers (ranging from 6 to 7) and distances similar to those predicted by molecular dynamics simulations. Elements like Ti, which form bonds of intermediate strength, may show some order beyond the first coordination shell at low concentrations in silicate glasses. EXAFS studies of Yb and U in silicate glasses at trace to minor concentration levels provide unique structural information about the environments of these cations. K-edges and XANES of transition element sulfides, third-row tetrahedral oxyanions, and oxygen in minerals are interpreted in terms of band theory or molecular orbital theory.     

A corundum-rich inclusion in the Murchison carbonaceous chondrite

A corundum-hibonite inclusion, BB-5, has been found in the Murchison carbonaceous chondrite. This is the first reported occurrence of corundum as a major phase in any refractory inclusion, even though this mineral is predicted by thermodynamic calculations to be the first condensate from a cooling gas of solar composition. Ion microprobe measurements of Mg isotopic compositions yield the unexpected result for such an early condensate that ²⁶Mg excesses are small: δ_N²⁶Mg = 7.0 ± 1.6%. for hibonite and 5.0 ± 4.8%. for corundum, despite very large ${}^{27}\text{Al}{}^{24}\text{Mg}$ ratios, 130 and 2.74 × 10⁴, respectively. Within the errors, δ_N²⁶Mg does not vary over this exceedingly large range of ${}^{27}\text{Al}{}^{24}\text{Mg}$ ratios. The extreme temperature required to melt this inclusion makes a liquid origin unlikely, except possibly by hypervelocity impact involving refractory bodies. If, instead, BB-5 is a direct gas-solid condensate, textural evidence implies that corundum formed first and later reacted to produce hibonite. In this model, BB-5's uniform enrichment in ²⁶Mg must be a characteristic of the reservoir from which it condensed. Because severe difficulties are encountered in making such a reservoir by prior decay of ²⁶Al, nebular heterogeneity in magnesium isotopic composition is a preferred explanation.     

Bio-controlled thermostasis involving the sulfur cycle

The Gaia hypothesis proposed by Lovelock and Margulis presumes the existence of an unspecified biological means of ameliorating climate that has operated since the emergence of life 3500 Myr ago: Recently it was suggested that the mechanism of thermostasis may involve biospheric cycling of atmospheric carbon dioxide. We suggest an alternative hypothesis of biothermostasis operating through the sulfur cycle, rather than the carbon cycle. The mechanism would operate by altering planetary albedo through the selective creation of biospheric organic sulfide gases which go on to metamorphize into submicron particles and introduce cooling. In contrast to the carbon-cycle mechanism, sulfur-based cooling would have the ability to ameliorate climate well into the future, in principle over stellar Main Sequence time intervals. The main feature of interest is that the S cycle represents a particularly favorable thermodynamic pathway, involving three to four orders of magnitude less mass of active material cycled through the biospheric-atmospheric system (in response to a given temperature-imposed stress) than would be the case for a greenhouse gas hypothesis. There is no evidence that the suggested biospheric controlled particle-albedo change mechanism is actually operating, but we speculate that the probability of its rising importance and perhaps eventual dominance will improve when the partial pressure of atmospheric CO₂ drops low enough to impose stress on metabolic processes. The intriguing thing about the process is its extremely high efficiency.     

In situ oxygen,magnesium, and silicon isotopic compositions of the FUN inclusion Vigarano 1623-5

Oxygen, magnesium, and silicon isotopic abundances in Vigarano 1623-5 were studied using secondary ion mass spectrometry to investigate correlations between isotopic and petrologic properties of this unique forsterite-bearing FUN inclusion. Vigarano 1623-5 displays large, correlated mass-dependent fractionation effects, tightly linked to mineralogy within distinct petrologic units of the inclusion. The pyroxene-rich and melilite-rich interior parts of the inclusion display highly mass-fractionated isotopic compositions of oxygen, magnesium, and silicon, consistent with Rayleigh distillation during evaporation of a melt with initial oxygen composition close to a solar composition. However, the chemical composition, enriched in magnesium and silicon, suggests a precursor already fractionated by prior melt evaporation. A discontinuous igneous rim was produced by a flash-melting event followed by isotopic exchange in the rim melilite with planetary-like oxygen, mechanical fragmentation, and reassembly with an accretionary rim of heterogeneous materials. Al-rich minerals in 1623-5 show evidence for having crystallized with live ²⁶Al but at less than the “canonical” level of most CV calcium-aluminum-rich inclusions. However, well-defined ²⁶Al-²⁶Mg isochrons are not found and temporal implications are ambiguous.