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PubMed1997

A unified theory of impact crises and mass extinctions: quantitative tests.

Several quantitative tests of a general hypothesis linking impacts of large asteroids and comets with mass extinctions of life are possible based on astronomical data, impact dynamics, and geological information. The waiting times of large-body impacts on the Earth derived from the flux of Earth-crossing asteroids and comets, and the estimated size of impacts capable of causing, large-scale environmental disasters, predict the impacts of objects > or = 5 km in diameter (> or = 10(7) Mt TNT equivalent) could be sufficient to explain the record of approximately 25 extinction pulses in the last 540 Myr, with the 5 recorded major mass extinctions related to impacts of the largest objects of > or = 10 km in diameter (> or = 10(8) Mt events). Smaller impacts (approximately 10(6) Mt), with significant regional environmental effects, could be responsible for the lesser boundaries in the geologic record. Tests of the "kill curve" relationship for impact-induced extinctions based on new data on extinction intensities, and several well-dated large impact craters, also suggest that major mass extinctions require large impacts, and that a step in the kill curve may exist at impacts that produce craters of approximately 100 km diameter, smaller impacts being capable of only relatively weak extinction pulses. Single impact craters less than approximately 60 km in diameter should not be associated with detectable global extinction pulses (although they may explain stage and zone boundaries marked by lesser faunal turnover), but multiple impacts in that size range may produce significant stepped extinction pulses. Statistical tests of the last occurrences of species at mass-extinction boundaries are generally consistent with predictions for abrupt or stepped extinctions, and several boundaries are known to show "catastrophic" signatures of environmental disasters and biomass crash, impoverished postextinction fauna and flora dominated by stress-tolerant and opportunistic species, and gradual ecological recovery and radiation of new taxa. Isotopic and other geochemical signatures are also generally consistent with the expected after-effects of catastrophic impacts. Seven of the recognized extinction pulses seem to be associated with concurrent (in some cases multiple) stratigraphic impact markers (e.g., layers with high iridium, shocked minerals, microtektites), and/or large, dated impact craters. Other less well-studied crisis intervals show elevated iridium, but well below that of the K/T spike, which might be explained by low-Ir impactors, ejecta blowoff, or sedimentary reworking and dilution of impact signatures. The best explanation for a possible periodic component of approximately 30 Myr in mass extinctions and clusters of impacts is the pulselike modulation of the comet flux associated with the solar system's periodic passage through the plane of the Milky Way Galaxy. The quantitative agreement between paleontologic and astronomical data suggests an important underlying unification of the processes involved.

Biological Evolution↗
PubMed1985

Sulfur in achondritic meteorites.

Total sulfur abundances have been measured for 48 achondrites. For twenty eucrites they ranged from 370 to 3700 micrograms S/g with a median sulfur content of 1180 micrograms S/g. Sulfur abundances for howardites ranged from 1490 to 3240 micrograms S /g and had a median sulfur concentration of 2340 micrograms S/g. Diogenites' sulfur abundances ranged from 130 to 3170 micrograms S/g, with a median value of 1280 micrograms S/g. Four shergottites had a median sulfur content of 1940 micrograms S/g and ranged from 740 to 2540 micrograms S/g. Enstatite achondrites contained the greatest sulfur abundances of any achondrite group. They ranged from 2450 to 8580 micrograms S/g and had a median sulfur content of 6020 micrograms S/g and had a median sulfur content of 6020 micrograms S/g. A single Chassignite had a sulfur concentration of 360 micrograms S/g. The wide variations in sulfur concentrations for the achondrites reflect the small scale heterogeneous nature of these unique extraterrestrial materials due in large part to discrete sulfide mineral grains.

Geological Phenomena↗
PubMed1988

Carbonates and sulfates in CI chondrites: formation by aqueous activity on the parent body.

Compositions and morphologies of dolomites, breunnerites, Ca-carbonates, Ca-sulfates and Mg, Ni, Na-sulfates, and their petrologic interrelations, in four CI chondrites are consistent with their having been formed by aqueous activity on the CI parent body. Radiochronometric data indicate that this activity took place very early in Solar-System history. No evidence for original ("primitive") condensates seems to be present. However, alteration apparently took place without change in bulk meteorite composition.

Calcium Carbonate↗
PubMed1989

The formation of weathering products on the LEW 85320 ordinary chondrite: evidence from carbon and oxygen stable isotope compositions and implications for carbonates in SNC meteorites.

Isotopic analysis of nesquehonite recovered from the surface of the LEW 85320 H5 ordinary chondrite shows that the delta 13C and delta 18O values of the two generations of bicarbonate (Antarctic and Texas) are different: delta 13C = +7.9% and +4.2%; delta 18O = +17.9% and 12.1% respectively. Carbon isotopic compositions are consistent with equilibrium formation from atmospheric carbon dioxide at -2 +/- 4 degrees C (Antarctic) and +16 +/- 4 degrees C (Texas). Oxygen isotopic data imply that the water required for nesquehonite precipitation was derived from atmospheric water vapour or glacial meltwater which had locally exchanged with silicates, either in the meteorite or in underlying bedrock. Although carbonates with similar delta 13C values have been identified in the SNC meteorites EETA 79001 and Nakhla, petrographic and temperature constraints argue against their simply being terrestrial weathering products.

Antarctic Regions↗
PubMed1993

The mid-infrared transmission spectra of Antarctic ureilites.

The mid-infrared (4000-450 cm-1; 2.5-22.2 micrometers) transmission spectra of seven Antarctic ureilites and 10 Antarctic H-5 ordinary chondrites are presented. The ureilite spectra show a number of absorption bands, the strongest of which is a wide, complex feature centered near 1000 cm-1 (10 micrometers) due to Si-O stretching vibrations in silicates. The profiles and positions of the substructure in this feature indicate that Mg-rich olivines and pyroxenes are the main silicates responsible. The relative abundances of these two minerals, as inferred from the spectra, show substantial variation from meteorite to meteorite, but generally indicate olivine is the most abundant (olivine:pyroxene = 60:40 to 95:5). Both the predominance of olivine and the variable olivine-to-pyroxene ratio are consistent with the known composition and heterogeneity of ureilites. The H-5 ordinary chondrites spanned a range of weathering classes and were used to provide a means of addressing the extent to which the ureilite spectra may have been altered by weathering processes. It was found that, while weathering of these meteorites produces some weak bands due to the formation of small amounts of carbonates and hydrates, the profile of the main silicate feature has been little affected by Antarctic exposure in the meteorites studied here. The mid-infrared ureilite spectra provide an additional means of testing potential asteroidal parent bodies for the ureilites. At present, the best candidates include the subset of S-type asteroids having low albedos and weak absorption features in the near infrared.

Antarctic Regions↗
PubMed1994

Fractionated martian atmosphere in the nakhlites?

Considerable evidence points to a martian origin of the SNC meteorites. Noble gas isotopic compositions have been measured in most SNC meteorites. The 129Xe/132Xe vs, 84Kr/132Xe ratios in Chassigny, most shergottites, and lithology C of EETA 79001 define a linear array. This array is thought to be a mixing line between martian mantle and martian atmosphere. One of the SNC meteorites, Nakhla, contains a leachable component that has an elevated 129Xe/132Xe ratio relative to its 84Kr/132Xe ratio when compared to this approximately linear array. The leachable component probably consists in part of iddingsite, an alteration product produced by interaction of olivine with aqueous fluid at temperatures lower than 150 degrees C. The elevated Xe isotopic ratio may represent a distinct reservoir in the martian crust or mantle. More plausibly, it is elementally fractionated martian atmosphere. Formation of sediments fractionates the noble gases in the correct direction. The range of sediment/atmosphere fractionation factors is consistent with the elevated 129Xe/132Xe component in Nakhla being contained in iddingsite, a low temperature weathering product. The crystallization age of Nakhla is 1.3 Ga. Its low-shock state suggests that it was ejected from near the surface of Mars. As liquid water is required for the formation of iddingsite, these observations provide further evidence for the near surface existence of aqueous fluids on Mars more recently than 1.3 Ga.

Atmosphere↗
PubMed1995

Lunar soil evolution processes and Apollo 16 core 60013/60014.

Soils of the 62-cm deep Apollo 16 double drive tube 60013/14 are mature at the top and submature at the bottom. Modal analyses of 5529 grains from the 90-150 micrometers and the 500-1000 micrometers fractions from 12 levels of the core show that, in general, agglutinate abundance increases somewhat monotonically to the top and mimics the Is/FeO profile. There is a general decrease in the modal abundance of monomineralic fragments towards the top, suggesting that agglutinates were formed in part at the expense of monomineralic grains, especially feldspars, which are by far the most abundant mineral in these soils. In detail, the top 27 cm of the core differs from the bottom 21 cm, and the middle 14 cm is intermediate in its properties. In the upper segment, variations in the abundances of feldspars correspond with those of feldspathic fragmental breccias and cataclastic anorthosites; in the bottom segment, a similar but weak correspondence between feldspars and crystalline matrix breccias is observed. Mixing of the comminuted products of these three rock types likely produced the bulk of the core material. Many single feldspars in all size fractions are remarkably fresh, show no damage from shock, and are similar in appearance to the large feldspars in anorthosites and feldspathic fragmental breccias, which we consider to be the primary sources of single feldspars in this core. Major (Na, Al, Si, K, Ca) and minor (Fe, Ba) element analyses of 198 single feldspar grains indicate the presence of only one population of feldspars, which is consistent with our interpretation of feldspar provenance. Classification of 890 monomineralic feldspar, olivine, pyroxene, and glass spherules on the basis of the presence or absence of thin brownish coating--related to reworking at the surface--shows that coated grains are much more abundant in the top segment than in the bottom segment. A comparison with the mixing and maturation model (McKay et al., 1977) of soils in the core 60009/10, some 60 m away from 60013/14, shows that mixtures of an immature, nearly pure plagioclase soil (dominant in 60009/10) and another immature, crystalline breccia-rich soil (dominant in 60013/14) may have matured through in situ reworking to produce the soils under investigation. We conclude that the soils in this core are products of mixing along soil evolution Path 2 of McKay et al. (1974). Superimposed on that soil column is the reworking of the upper part, which has evolved more recently along Path 1. This core thus represents a consanguineous column of the lunar regolith with an upper reworked segment.

Aluminum Silicates↗
PubMed1996

The inventory of interstellar materials available for the formation of the solar system.

Tremendous progress has been made in the field of interstellar dust in recent years through the use of telescopic observations, theoretical studies, laboratory studies of analogs, and the study of actual interstellar samples found in meteorites. It is increasingly clear that the interstellar medium (ISM) contains an enormous diversity of materials created by a wide range of chemical and physical processes. This understanding is a far cry from the picture of interstellar materials held as recently as two decades ago, a picture which incorporated only a few generic types of grains and few molecules. In this paper, I attempt to review some of our current knowledge of the more abundant materials thought to exist in the ISM. The review concentrates on matter in interstellar dense molecular clouds since it is the materials in these environments from which new stars and planetary systems are formed. However, some discussion is reserved for materials in circumstellar environments and in the diffuse ISM. The paper also focuses largely on solid materials as opposed to gases since solids contain a major fraction of the heavier elements in clouds and because solids are most likely to survive incorporation into new planetary systems in identifiable form. The paper concludes with a discussion of some of the implications resulting from the recent growth of our knowledge about interstellar materials and also considers a number of areas in which future work might be expected to yield important results.

Astronomy↗
PubMed1997

Fullerenes, fulleranes and polycyclic aromatic hydrocarbons in the Allende meteorite.

In this paper, we confirm our earlier observations of fullerenes (C60 and C70) in the Allende meteorite (Becker et al., 1994a, 1995). Fullerene C60 was also detected in two separate C-rich (approximately 0.5-1.0%) dark inclusions (Heymann et al., 1987) that were hand picked from the Allende sample. The amounts of C60 detected were approximately 5 and approximately 10 ppb, respectively, which is considerably less than what was detected in the Allende 15/21 sample (approximately 100 ppb; Becker et al., 1994a, 1995). This suggests that fullerenes are heterogeneously distributed in the meteorite. In addition, we present evidence for fulleranes, (C60Hx), detected in separate samples by laser desorption (reflectron) time-of-flight (TOF) mass spectrometry (LDMS). The LDMS spectra for the Allende extracts were remarkably similar to the spectra generated for the synthetic fullerane mixtures. Several fullerane products were synthesized using a Rh catalyst (Becker et al., 1993a) and separated using high-performance liquid chromatography (HPLC). Polycyclic aromatic hydrocarbons (PAHs) were also observed ppm levels) that included benzofluoranthene and corannulene, a cup-shaped molecule that has been proposed as a precursor molecule to the formation of fullerenes in the gas phase (Pope et al., 1993).

Carbon↗
PubMed1997

Tookoonooka, a large buried early Cretaceous impact structure in the Eromanga Basin of southwestern Queensland, Australia.

The 66 km wide Tookoonooka impact structure (27 degrees 07'S, 142 degrees 50'E) was first recognised, from seismic profiles, as a circular structure consisting of a concentric arrangement of anticlines and synclines, which surround a complex central dome, approximately 22 km wide. A gravity low and a central magnetic high characterize the structure. Now buried by up to 900 m of Cretaceous and Tertiary clastic sediments, the Tookoonooka structure was formed approximately 128 Ma ago, during deposition of the paralic Cadna-owie Formation. Thin sections from a centrally located exploration well reveal an impact melt breccia, composed of local Ordovician quartz-mica schist bedrock. Detailed study of planar deformation features (PDFs) in quartz grains from this breccia show 64 lamellae sets in 25 grains. Most of the PDF measurements correspond to xi ¿mean 1122¿ and r/z ¿mean 1011¿/¿mean 0111¿ crystallographic indices, with five other orientations also measured. This distribution of PDFs corresponds to that found in quartz from impact structures in porous sedimentary rock targets, thus confirming an impact origin for Tookoonooka.

Australia↗
PubMed1998

The origin of chondritic macromolecular organic matter: a carbon and nitrogen isotope study.

The N and C abundances and isotopic compositions of acid-insoluble carbonaceous material in thirteen primitive chondrites (five unequilibrated ordinary chondrites, three CM chondrites, three enstatite chondrites, a CI chondrite and a CR chondrite) have been measured by stepped combustion. While the range of C isotopic compositions observed is only delta 13C = 30%, the N isotopes range from delta 15N approximately -40 to 260%. After correction for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3-4 wt% of the insoluble C in all the chondrites studied. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites, suggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in Renazzo and Semarkona. These two meteorites have almost M-shaped N isotope release profiles that can be explained most simply by the super-position of two components, one with a composition between delta 15N = -20 and -40% and a narrow combustion interval, the other having a broader release profile and a composition of delta 15N approximately 260%. Although isotopically more subdued, the CI and the three CM chondrites all appear to show vestiges of this M-shaped profile. How and where the components in the acid-insoluble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most 15N-rich material probably formed in the interstellar medium, and the fraction of organic N in Renazzo in this material ranges from 40 to 70%. The isotopically light component may have formed in the solar system, but the limited range in nanodiamond to total organic C ratios in the chondrite groups is consistent with most of the organic material being, presolar.

Carbon↗
PubMed1998

Exposure of Allan Hills 84001 and other achondrites on the Antarctic ice.

The enrichment of F on Antarctic meteorites is the result of their exposure to the atmosphere, and its measurement allows a subdivision of the terrestrial age into a duration of exposure on the ice and the time a meteorite was enclosed by the ice. In many cases, the periods of surface exposure are only small fractions of the terrestrial ages of meteorites collected in Antarctica. The enrichment of F on the surfaces of Antarctic achondrites was investigated by means of nuclear reaction analysis (NRA): scanning proton beams with an energy of 2.7 and 3.4 MeV were used to induce the reactions 19F(p, alpha gamma)16O and 19F(p,p gamma)19F, respectively. Gamma signals proportional to the F content were measured. The following Antarctic achondrites were investigated: Martian meteorite ALH 84001; diogenite ALHA77256; the eucrites ALHA81011 and ALHA78132; and in addition, the H5 chondrite ALHA79025. For ALH 84001, our data indicate a period of exposure on the ice of <500 years. Thus, this specimen was enclosed in the ice >95% of its terrestrial age of 13 000 years.

Antarctic Regions↗
PubMed1998

Carbonates in fractures of Martian meteorite Allan Hills 84001: petrologic evidence for impact origin.

Carbonates in Martian meteorite Allan Hills 84001 occur as grains on pyroxene grain boundaries, in crushed zones, and as disks, veins, and irregularly shaped grains in healed pyroxene fractures. Some carbonate disks have tapered Mg-rich edges and are accompanied by smaller, thinner and relatively homogeneous, magnesite microdisks. Except for the microdisks, all types of carbonate grains show the same unique chemical zoning pattern on MgCO3-FeCO3-CaCO3 plots. This chemical characteristic and the close spatial association of diverse carbonate types show that all carbonates formed by a similar process. The heterogeneous distribution of carbonates in fractures, tapered shapes of some disks, and the localized occurrence of Mg-rich microdisks appear to be incompatible with growth from an externally derived CO2-rich fluid that changed in composition over time. These features suggest instead that the fractures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual Mg-rich fluid that was squeezed along fractures. Carbonate in pyroxene fractures is most abundant near grains of plagioclase glass that are located on pyroxene grain boundaries and commonly contain major or minor amounts of carbonate. We infer that carbonates in fractures formed from grain boundary carbonates associated with plagiociase that were melted by impact and dispersed into the surrounding fractured pyroxene. Carbonates in fractures, which include those studied by McKay et al. (1996), could not have formed at low temperatures and preserved mineralogical evidence for Martian organisms.

Calcium Carbonate↗
PubMed1998

Bulk and stable isotopic compositions of carbonate minerals in Martian meteorite Allan Hills 84001: no proof of high formation temperature.

Understanding the origin of carbonate minerals in the Martian meteorite Allan Hills (ALH) 84001 is crucial to evaluating the hypothesis that they contain traces of ancient Martian life. Using arguments based on chemical equilibria among carbonates and fluids, an origin at >650 degrees C (inimical to life) has been proposed. However, the bulk and stable isotopic compositions of the carbonate minerals are open to multiple interpretations and so lend no particular support to a high-temperature origin. Other methods (possibly less direct) will have to be used to determine the formation temperature of the carbonates in ALH84001.

Calcium Carbonate↗
PubMed1998

The history of Allan Hills 84001 revised: multiple shock events.

The geologic history of Martian meteorite Allan Hills (ALH) 84001 is more complex than previously recognized, with evidence for four or five crater-forming impacts onto Mars. This history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient Martian life in ALH 84001. Allan Hills 84001 formed originally from basaltic magma. Its first impact event (I1) is inferred from the deformation (D1) that produced the granular-textured bands ("crush zones") that transect the original igneous fabric. Deformation D1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater. An intense thermal metamorphism followed D1 and may be related to it. The next impact (I2) produced fractures, (Fr2) in which carbonate "pancakes" were deposited and produced feldspathic glass from some of the igneous feldspars and silica. After I2, carbonate pancakes and globules were deposited in Fr2 fractures and replaced feldspathic glass and possibly crystalline silicates. Next, feldspars, feldspathic glass, and possibly some carbonates were mobilized and melted in the third impact (I3). Microfaulting, intense fracturing, and shear are also associated with I3. In the fourth impact (I4), the rock was fractured and deformed without significant heating, which permitted remnant magnetization directions to vary across fracture surfaces. Finally, ALH 84001 was ejected from Mars in event I5, which could be identical to I4. This history of multiple impacts is consistent with the photogeology of the Martian highlands and may help resolve some apparent contradictions among recent results on ALH 84001. For example, the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition, whereas the elongate magnetite grains, epitaxial on carbonates, could be ascribed to vapor-phase deposition during I3.

Aluminum Silicates↗
PubMed1998

Epitaxial growth of nanophase magnetite in Martian meteorite Allan Hills 84001: implications for biogenic mineralization.

Crystallographic relationships between magnetite, sulfides, and carbonate rosettes in fracture zones of the Allan Hills (ALH) 84001 Martian meteorite have been studied using analytical electron microscopy. We have focused on those magnetite grains whose growth mechanisms can be rigorously established from their crystallographic properties. Individual magnetite nanocrystals on the surfaces of carbonates are epitaxially intergrown with one another in "stacks" of single-domain crystals. Other magnetite nanocrystals are epitaxially intergrown with the surfaces of the carbonate substrates. The observed magnetite/carbonate (hkl) Miller indices orientation relationships are (1, 1,3)m ¿¿ (1, 1 ,0)c and (1, 1 ,1)m ¿¿ (0,0, 3)c with lattice mismatches of approximately 13% and approximately 11%, respectively. Epitaxy is a common mode of vapor-phase growth of refractory oxides like magnetite, as is the spiral growth about axial screw dislocations previously observed in other magnetite nanocrystals in ALH 84001. Epitaxy rules out intracellular precipitation of these magnetites by (Martian) organisms, provides further evidence of the high-temperature (> 120 degrees C) inorganic origins of magnetite in ALH 84001, and indicates that the carbonates also have been exposed to elevated temperatures.

Carbonates↗