• TEM studies and the shock history of a "mysterite" inclusion from the Krymka LL chondrite

      Weber, I.; Semenenko, V. P.; Stephan, T.; Jessberger, E. K. (The Meteoritical Society, 2006-01-01)
      The microstructure and composition of the matrix of one carbonaceous inclusion (K1) in the Krymka LL3.1 chondrite were studied using transmission electron microscopy (TEM). K1 has previously shown an enigmatic nature and similarities with volatile-rich, fine-grained, dark inclusions of Krymka called "mysterite."In the present study, four minerals were identified by TEM. Olivine, pyroxene, and pyrrhotite typically have grain sizes of one micrometer; graphite occurs as flakes of a similar size. Olivine shows a moderately high dislocation density most probably caused by shock. Pyroxene shows coexisting ortho- and clinoenstatite lamellae that probably originated from shear stress after a shock event or from the rapid cooling of the protoenstatite stability field. However, we demonstrate that in this case, a shock trigger is more likely. Pyrrhotite in the studied sample occurs as a 4C monoclinic superstructure. The graphite flakes in the fragment are well crystallized, as can be seen by discrete spots in the diffraction pattern. In graphite, the degree of crystallization increases with the metamorphic grade. Based on the microstructure of this mineral we conclude that after a first moderate shock event, the residual temperature between 300 degrees C and 500 degrees C led to thermal metamorphism. A second shock event, possibly at excavation from the parent body, is responsible for the shock features observed in olivine, pyroxene, and graphite.
    • Terrestrial ages, pairing, and concentration mechanism of Antarctic chondrites from Frontier Mountain, Northern Victoria Land

      Welten, K. C.; Nishiizumi, K.; Caffee, M. W.; Hillegonds, D. J.; Johnson, J. A.; Jull, A. J. T.; Wieler, R.; Folco, L. (The Meteoritical Society, 2006-01-01)
      We report concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in the metal phase of 26 ordinary chondrites from Frontier Mountain (FRO), Antarctica, as well as cosmogenic 14C in eight and noble gases in four bulk samples. Thirteen out of 14 selected H chondrites belong to two previously identified pairing groups, FRO 90001 and FRO 90174, with terrestrial ages of ~40 and ~100 kyr, respectively. The FRO 90174 shower is a heterogeneous H3-6 chondrite breccia that probably includes more than 300 individual fragments, explaining the high H/L chondrite ratio (3.8) at Frontier Mountain. The geographic distribution of 19 fragments of this shower constrains ice fluctuations over the past 50-100 kyr to less than 40 m, supporting the stability of the meteorite trap over the last glacial cycle. The second H-chondrite pairing group, FRO 90001, is much smaller and its geographic distribution is mainly controlled by wind-transport. Most L-chondrites are younger than 50 kyr, except for the FRO 93009/01172 pair, which has a terrestrial age of ~500 kyr. These two old L chondrites represent the only surviving members of a large shower with a similar preatmospheric radius (~80 cm) as the FRO 90174 shower. The find locations of these two paired L-chondrite fragments on opposite sides of Frontier Mountain confirm the general glaciological model in which the two ice flows passing both ends of the mountain are derived from the same source area on the plateau. The 50 FRO meteorites analyzed so far represent 21 different falls. The terrestrial ages range from 6 kyr to 500 kyr, supporting the earlier proposed concentration mechanism.
    • The beginning heights and light curves of high-altitude meteors

      Koten, Pavel; Spurný, Pavel; Borovička, Jiří; Evans, Stephen; Elliott, Andrew; Betlem, Hans; Štork, Rostislav; Jobse, Klaas (The Meteoritical Society, 2006-01-01)
      In this paper, we provide an overview of meteors with high beginning height. During the recent Leonid meteor storms, as well as within the regular double station video observations of other meteor showers, we recorded 164 meteors with a beginning height above 130 km. We found that beginning heights between 130 and 150 km are quite usual, especially for the Leonid meteor shower. Conversely, meteors with beginning heights above 160 km are very rare even among Leonids. From the meteor light curves, we are able to distinguish two different processes that govern radiation of the meteors at different altitudes. Light curves vary greatly above 130 km and exhibit sudden changes in meteor brightness. Sputtering from the meteoroid surface is the dominating process during this phase of the meteor luminous trajectory. Around 130 km, the process switches to ablation and the light curves become similar to the light curves of standard meteors. The sputtering model was successfully applied to explain the difference in the beginning heights of high-altitude Leonid and Perseid meteors. We show also that this process in connection with high altitude fragmentation could explain the anomalously high beginning heights of several relatively faint meteors.
    • The Benguerir meteorite: Report and description of a new Moroccan fall

      Chennaoui Aoudjehane, H.; Jambon, A.; Bourot Denise, M.; Rochette, P. (The Meteoritical Society, 2006-01-01)
      A meteorite fall was witnessed on November 22, 2004, at 11:45 A.M. (GMT) near the city of Benguerir, Morocco. This is one of the first falls from Morocco to be scientifically described. The total mass of the fall is estimated to be at least 40 and 80 kg. Three impact locations have been identified, two of them in soft ploughed ground and a third on a granite surface, which was apparently broken by the impact. The weight of complete pieces range between approximately 100 g and 10 kg. We have classified the stone as an LL6 ordinary chondrite, based on mineralogy and petrology, with shock grade S3 and alteration W0. The dark fusion crust is perfectly fresh, and a number of samples were collected shortly after the fall by local residents and authorities before rainfall, which occurred a few days later. We show that the magnetic susceptibility of Benguerir is homogeneously high, indicating a constant metal content despite brecciation, in the high range for LL6. The LL6 classification can also be confirmed magnetically, with a specific signature similar to other measured LL6 falls.
    • The fall and recovery of the Tagish Lake meteorite

      Hildebrand, Alan R.; McCausland, Phil J. A.; Brown, Peter G.; Longstaffe, Fred J.; Russell, Sam D. J.; Tagliaferri, Edward; Wacker, John F.; Mazur, Michael J. (The Meteoritical Society, 2006-01-01)
      The Tagish Lake C2 (ungrouped) carbonaceous chondrite fall of January 18, 2000, delivered 10 kg of one of the most primitive and physically weak meteorites yet studied. In this paper, we report the detailed circumstances of the fall and the recovery of all documented Tagish Lake fragments from a strewnfield at least 16 km long and 3 to 4 km wide. Nearly 1 kg of "pristine" meteorites were collected one week after the fall before new snow covered the strewnfield; the majority of the recovered mass was collected during the spring melt. Ground eyewitnesses and a variety of instrument-recorded observations of the Tagish Lake fireball provide a refined estimate of the fireball trajectory. From its calculated orbit and its similarity to the remotely sensed properties of the D- and P-class asteroids, the Tagish Lake carbonaceous chondrite apparently represents these outer belt asteroids. The cosmogenic nuclide results and modeled production indicate a prefall radius of 2.1-2.4 m (corresponding to 60-90 tons) consistent with the observed fireball energy release. The bulk oxygen-isotope compositions plot just below the terrestrial fractionation line (TFL), following a trend similar to the CM meteorite mixing line. The bulk density of the Tagish Lake material (1.64 +/- 0.02 g/cm^3) is the same, within uncertainty, as the total bulk densities of several C-class and especially D- and P-class asteroids. The high microporosity of Tagish Lake samples (~40%) provides an obvious candidate material for the composition of low bulk density primitive asteroids.
    • The fall of Hoima, an H6 chondrite from Uganda

      Greshake, A.; Reimold, W. U.; Tuhumwire, J. T.; Baguma, Z.; De Villiers, M. E. (The Meteoritical Society, 2006-01-01)
      The Hoima meteorite fell on March 30, 2003, in the Hoima district near Butema, Uganda. According to its mineralogy, texture, and mineral chemical characteristics, Hoima is classified as a brecciated H6 ordinary chondrite of shock stage S2 and weathering grade W0. After the meteorites Maziba, Soroti, Awere, and Mbale, Hoima represents the fifth meteorite recorded from Uganda.
    • The formation of fluidized ejecta on Mars by granular flows

      Wada, Koji; Barnouin-Jha, Olivier S. (The Meteoritical Society, 2006-01-01)
      A simple granular flow model is used to investigate some of the conditions under which ejecta may flow as a granular media. The purpose of this investigation is to provide some bounds as to when either volatiles or an atmosphere are required to explain the fluid-like morphology of many Martian ejecta deposits. We consider the ejecta deposition process from when an ejecta curtain first strikes a target surface via ballistics and possibly flows thereafter. A new finding is that either hard-smooth surfaces or slightly erodible surfaces allow ejecta to flow readily as a granular medium. Neither volatiles nor an atmosphere are required to initiate flow. A low friction coefficient between ejecta grains can also generate flow and would be analogous to adding volatiles to the ejecta. The presence of either a rough or a densely packed erodible surface does not permit easy ejecta flow. High friction coefficients between ejecta grain also prevent flow, while changes in the coefficient of restitution (a measure of how much energy is retained after collisions between particles) plays a minor role in the flow dynamics of ejecta. A hard smooth or a somewhat erodible surface could be generated by past fluvial activity on Mars, which can either indurate a surface, erode and smooth a surface, or generate sedimentary terrains that are fairly easy to erode. No ramparts or layered ejecta morphologies are generated by our model, but this may be because several simplifying assumptions are used in our model and should not be construed as proof that either volatiles or an atmosphere are required to form fluidized ejecta morphologies.
    • The formation of plessite in meteoritic metal

      Goldstein, J. I.; Michael, J. R. (The Meteoritical Society, 2006-01-01)
      Plessite is a mixture of body-centered cubic (bcc) kamacite (alpha), face-centered cubic (fcc) taenite (gamma), and/or ordered FeNi-tetrataenite (gamma-") phases and is observed in the metal of iron, stony-iron, and chondritic meteorites. The formation of plessite was studied by measuring the orientation of the bcc and fcc phases over large regions of plessite using electron backscatter diffraction (EBSD) analysis in five ataxites, the Carlton IAB-IIICD iron, and zoneless plessite metal in the Kernouve H6 chondrite.The EBSD results show that there are a number of different orientations of the bcc kamacite phase in the plessite microstructure. These orientations reflect the reaction path gamma (fcc) --> alpha2 (bcc) in which the alpha2 phase forms during cooling below the martensite start temperature, Ms, on the close-packed planes of the parent fcc phase according to one or more of the established orientation relationships (Kurdjumov-Sachs, Nishiyama-Wasserman, and Greninger-Troiano) for the fcc to bcc transformation.The EBSD results also show that the orientation of the taenite and/or tetrataenite regions at the interfaces of prior alpha-2 (martensite) laths, is the same as that of the single crystal parent taenite phase of the meteorite. Therefore, the parent taenite was retained at the interfaces of martensite laths during cooling after the formation of martensite. The formation of plessite is described by the reaction gamma --> alpha2 + gamma --> alpha + gamma. This reaction is inconsistent with the decomposition of martensite laths to form phase as described by the reaction gamma --> alpha2 --> alpha -+ gamma, which is the classical mechanism proposed by previous investigators. The varying orientations of the fine exsolved taenite and/or tetrataenite within decomposed martensite laths, however, are a response to the decomposition of alpha2 (martensite) laths at low temperature and are formed by the reaction alpha2 --> alpha + gamma.
    • The fracture of water ice Ih: A short overview

      Schulson, Erland M. (The Meteoritical Society, 2006-01-01)
      This paper presents a short overview of the fracture of water ice Ih. Topics include the ductile-to-brittle transition, tensile and compressive strength, compressive failure under multiaxial loading, compressive failure modes, and brittle failure on the geophysical scale (Arctic sea ice cover, Europa's icy crust). Emphasis is placed on the underlying physical mechanisms. Where appropriate, comment is made on the formation of high-latitude impact craters on Mars.
    • The I-Xe chronometer and the early solar system

      Gilmour, J. D.; Pravdivtseva, O. V.; Busfield, A.; Hohenberg, C. M. (The Meteoritical Society, 2006-01-01)
      We review the development of the I-Xe technique and how its data are interpreted, and specify the best current practices. Individual mineral phases or components can yield interpretable trends in initial 129I/127I ratio, whereas whole-rock I-Xe ages are often hard to interpret because of the diversity of host phases, many of which are secondary. Varying standardizations in early work require caution; only samples calibrated against Shallowater enstatite or Bjurböle can contribute reliably to the emerging I-Xe chronology of the early solar system.Although sparse, data for which I-Xe and Mn-Cr can be compared suggest that the two systems are concordant among ordinary chondrite samples. We derive a new age for the closure of the Shallowater enstatite standard of 4563.3 +/- 0.4 Myr from the relationship between the I-Xe and Pb-Pb systems. This yields absolute I-Xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system.Absolute I-Xe dates for aqueous and igneous processes are consistent with other systems. Consideration of the I-Xe host phases in CAIs and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre-existing material. The ranges of chondrule ages deduced from the Al-Mg and I-Xe systems in Semarkona (LL3.0) and Chainpur (LL3.4) are consistent. Chainpur I-Xe data exhibit a greater range of ages than Semarkona, possibly reflecting a greater degree of parent body processing. However individual chondrules show little or no evidence of such processing. Determining the host phase(s) responsible for high temperature correlations may resolve the issue.
    • The Meteoritical Bulletin, No. 90, 2006 September

      Connolly, Harold C.; Zipfel, Jutta; Grossman, Jeffrey N.; Folco, Luigi; Smith, Caroline; Jones, Rhian H.; Righter, Kevin; Zolensky, Michael; Russell, Sara S.; Benedix, Gretchen K.; et al. (The Meteoritical Society, 2006-01-01)
    • The orbit and atmospheric trajectory of the Orgueil meteorite from historical records

      Gounelle, Matthieu; Spurný, Pavel; Bland, Philip A. (The Meteoritical Society, 2006-01-01)
      Using visual observations that were reported 140 years ago in the Comptes Rendus de l'Académie des Sciences de Paris, we have determined the atmospheric trajectory and the orbit of the Orgueil meteorite, which fell May 14, 1864, near Montauban, France. Despite the intrinsic uncertainty of visual observations, we were able to calculate a reasonably precise atmospheric trajectory and a moderately precise orbit for the Orgueil meteoroid. The atmosphere entry point was ~70 km high and the meteoroid terminal point was ~20 km high. The calculated luminous path was ~150 km with an entry angle of 20 degrees. These characteristics are broadly similar to that of other meteorites for which the trajectory is known. Five out of six orbital parameters for the Orgueil orbit are well constrained. In particular, the perihelion lies inside the Earth's orbit (q ~0.87 AU), as is expected for an Earth-crossing meteorite, and the orbital plane is close to the ecliptic (i ~0 degrees). The aphelion distance (Q) depends critically on the pre-atmospheric velocity. From the calculated atmospheric path and the fireball duration, which was reported by seven witnesses, we have estimated the pre-atmospheric velocity to be larger than 17.8 km/sec, which corresponds to an aphelion distance Q larger than 5.2 AU, the semi-major axis of Jupiter orbit. These results suggest that Orgueil has an orbit similar to that of Jupiter-family comets (JFCs), although an Halley-type comet cannot be excluded. This is at odds with other meteorites that have an asteroidal origin, but it is compatible with 140 years of data-gathering that has established the very special nature of Orgueil compared to other meteorites. A cometary origin of the Orgueil meteorite does not contradict cosmochemistry data on CI1 chondrites. If CI1 chondrites originate from comets, it implies that comets are much more processed than previously thought and should contain secondary minerals. The forthcoming return of cometary samples by the Stardust mission will provide a unique opportunity to corroborate (or contradict) our hypothesis.
    • The origin of dark inclusions in Allende: New evidence from lithium isotopes

      Sephton, Mark A.; James, Rachael H.; Zolensky, Michael E. (The Meteoritical Society, 2006-01-01)
      Aqueous and thermal processing of primordial materials occurred prior to and during planet formation in the early solar system. A record of how solid materials were altered at this time is present in the carbonaceous chondrites, which are naturally delivered fragments of primitive asteroids. It has been proposed that some materials, such as the clasts termed "dark inclusions" found in type III chondrites, suggest a sequence of aqueous and thermal events. Lithium isotopes (6Li and 7Li) can reveal the role of liquid water in dark inclusion history. During aqueous alteration, 7Li passes preferentially into solution leaving 6Li behind in the solid phase and, consequently, any relatively extended periods of interaction with 7Li-rich fluids would have left the dark inclusions enriched in the heavier isotope when compared to the meteorite as a whole. Our analyses of lithium isotopes in Allende and its dark inclusions reveal marked isotopic homogeneity and no evidence of greater levels of aqueous alteration in dark inclusion history.
    • The planforms of low-angle impact craters in the northern hemisphere of Mars

      Herrick, Robert R.; Hessen, Katie K. (The Meteoritical Society, 2006-01-01)
      We have surveyed Martian impact craters greater than 5 km in diameter using Viking and thermal emission imaging system (THEMIS) imagery to evaluate how the planform of the rim and ejecta changes with decreasing impact angle. We infer the impact angles at which the changes occur by assuming a sin2Phi dependence for the cumulative fraction of craters forming below angle Phi. At impact angles less than ~40 from horizontal, the ejecta become offset downrange relative to the crater rim. As the impact angle decreases to less than ~20 degrees, the ejecta begin to concentrate in the crossrange direction and a "forbidden zone" that is void of ejecta develops in the uprange direction. At angles less than ~10 degrees, a "butterfly" ejecta pattern is generated by the presence of downrange and uprange forbidden zones, and the rim planform becomes elliptical with the major axis oriented along the projectile's direction of travel. The uprange forbidden zone appears as a "V" curving outward from the rim, but the downrange forbidden zone is a straight-edged wedge. Although fresh Martian craters greater than 5 km in diameter have ramparts indicative of surface ejecta flow, the ejecta planforms and the angles at which they occur are very similar to those for lunar craters and laboratory impacts conducted in a dry vacuum. The planforms are different from those for Venusian craters and experimental impacts in a dense atmosphere. We interpret our results to indicate that Martian ejecta are first emplaced predominantly ballistically and then experience modest surface flow.
    • The prospect of high-precision Pb isotopic dating of meteorites

      Amelin, Yuri (The Meteoritical Society, 2006-01-01)
      The radiogenic 207Pb/206Pb ratio is the only extant nuclide chronometer with sufficient time resolution for studies of the solar nebula accretion and early asteroidal differentiation and metamorphism. Pb isotopic dates can be used to link the dates obtained from extinct nuclide chronometers to the absolute time scale. The factors that control precision and accuracy of Pb isotopic dates of meteorites: instrumental mass fractionation in isotopic analysis, mass spectrometer sensitivity, removal of common Pb, multi-stage evolution of U-Pb systems, disturbances caused by diffusion, alteration, and shock metamorphism, and uncertainties in decay constants and the natural ratio of the U isotopes are reviewed. The precision of Pb isotopic dates of meteorites attained with currently available techniques and methodology is +/- 0.5-1.0 Myr in favorable cases. The accuracy of time interval measurements is approximately the same. The most serious limitation on precision and accuracy of Pb isotopic dates is placed by the presence of common Pb of uncertain and/or variable isotopic composition. Improvement in precision and accuracy of Pb isotopic dates would be possible through combined advancement of techniques of isotopic analysis (most importantly, better control over instrumental mass fractionation) and more effective techniques for the removal of common Pb, together with a better understanding of the effects of thermal metamorphism, shock metamorphism, and aqueous alteration on the U-Pb system in meteorites.
    • The rate of small impacts on Earth

      Bland, Philip A.; Artemieva, Natalya A. (The Meteoritical Society, 2006-01-01)
      Asteroids tens to hundreds of meters in diameter constitute the most immediate impact hazard to human populations, yet the rate at which they arrive at Earth's surface is poorly known. Astronomic observations are still incomplete in this size range; impactors are subjected to disruption in Earth's atmosphere, and unlike the Moon, small craters on Earth are rapidly eroded. In this paper, we first model the atmospheric behavior of iron and stony bodies over the mass range 1-10^12 kg (size range 6 cm-1 km) taking into account deceleration, ablation, and fragmentation. Previous models in meteoritics deal with rather small masses (<10^5-10^6 kg) with the aim of interpreting registered fireballs in atmosphere, or with substantially larger objects without taking into account asteroid disruption to model cratering processes. A few earlier attempts to model terrestrial crater strewn fields did not take into account possible cascade fragmentation. We have performed large numbers of simulations in a wide mass range, using both the earlier "pancake" models and also the separated fragments model to develop a statistical picture of atmosphere-bolide interaction for both iron and stony impactors with initial diameters up to ~1 km. Second, using a compilation of data for the flux at the upper atmosphere, we have derived a cumulative size-frequency distribution (SFD) for upper atmosphere impactors. This curve is a close fit to virtually all of the upper atmosphere data over 16 orders of magnitude. Third, we have applied our model results to scale the upper atmosphere curve to a flux at the Earth's surface, elucidating the impact rate of objects 1 km diameter on Earth. We find that iron meteorites >5 x 10^4 kg (2.5 m) arrive at the Earth's surface approximately once every 50 years. Iron bodies a few meters in diameter (105-106 kg), which form craters 100 m in diameter, will strike the Earth's land area every 500 years. Larger bodies will form craters 0.5 km in diameter every 20,000 years, and craters 1 km in diameter will be formed on the Earth's land area every 50,000 years. Tunguska events (low-level atmospheric disruption of stony bolides >108 kg) may occur every 500 years. Bodies capable of producing hazardous tsunami (~200 m diameter projectiles) should strike the Earth's surface every ~100,000 years. This data also allows us to assess the completeness of the terrestrial crater record for a given area over a given time interval.
    • The record of Miocene impacts in the Argentine Pampas

      Schultz, Peter H.; Zárate, Marcelo; Hames, Willis E.; Harris, R. Scott; Bunch, T. E.; Koeberl, Christian; Renne, Paul; Wittke, James (The Meteoritical Society, 2006-01-01)
      Argentine Pampean sediments represent a nearly continuous record of deposition since the late Miocene (~10 Ma). Previous studies described five localized concentrations of vesicular impact glasses from the Holocene to late Pliocene. Two more occurrences from the late Miocene are reported here: one near Chasicó (CH) with an 40Ar/39Ar age of 9.24 +/- 0.09 Ma, and the other near Bahía Blanca (BB) with an age of 5.28 +/- 0.04 Ma. In contrast with andesitic and dacitic impact glasses from other localities in the Pampas, the CH and BB glasses are more mafic. They also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals. In addition to evidence for extreme heating (>1700 degrees C), shock features are observed (e.g., planar deformation features [PDFs] and diaplectic quartz and feldspar) in impact glasses from both deposits. Geochemical analyses reveal unusually high levels of Ba (~7700 ppm) in some samples, which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth. These two new impact glass occurrences raise to seven the number of late Cenozoic impacts for which there is evidence preserved in the Pampean sediments. This seemingly high number of significant impacts over a 10^6 km^2 area in a time span of 10 Myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption. The Pampean record suggests, therefore, that either atmospheric entry models need to be reconsidered or that the Earth has received an enhanced flux of impactors during portions of the late Cenozoic. Evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous, low-energy deposition such as the Pampas. Additionally, the rare earth element (REE) concentrations of the target sediments and impact melts associated with the Chasicó event resemble the HNa/K australites of similar age. This suggests the possibility that those enigmatic tektites could have originated as high-angle, distal ejecta from an impact in Argentina, thereby accounting for their rarity and notable chemical and physical differences from other Australasian impact glasses.
    • The Sirente crater, Italy: Impact versus mud volcano origins

      Stoppa, Francesco (The Meteoritical Society, 2006-01-01)
      The Sirente crater is a circular structure with a diameter of 80 m. The rim deposit is an inverse-graded, matrix-supported breccia. Sedimentological features of the rim deposit suggest that the crater is not related to an explosion or violent mechanical displacement. The structure and texture of the deposit exhibit a primary sedimentary character. The rim deposits do not contain artifacts and do not show evidence of reworking. A multistage formation is reconstructed for the rim growth and associated deposits. The geometry and sedimentology of the deposits indicate that they were produced by the extrusion and accumulation of mudflow deposits. The dominant ejection mechanism was low mud fountains and the transport medium was water. Petrographic and geochemical evidence does not indicate any physical or cryptic trace of an extraterrestrial body.The most realistic agent that explains the observed effects is a rapid local emission of mud and/or water. Geological processes capable of producing these features include piping sinkholes or, more probably, "caldera"-type mud volcanoes, which may result from underground water-table perturbation and/or decompression of deep CO2/hydrocarbon gas reservoirs due to tectonic deformation or faulting activity during a seismic event. In both cases, the name "crater" for this geological form may be maintained, but there is no compelling evidence for an impact origin.In this paper, the scientific literature on the Sirente crater is reconsidered in the light of new morphological, sedimentological, geochemical, and archaeological data. A new mechanism is proposed involving mud-fountaining.
    • The Villalbeto de la Peña meteorite fall: II. Determination of atmospheric trajectory and orbit

      Trigo-Rodríguez, Josep M.; Borovička, Jiří; Spurný, Pavel; Ortiz, José L.; Docobo, José A.; Castro-Tirado, Alberto J.; Llorca, Jordi (The Meteoritical Society, 2006-01-01)
      The L6 ordinary chondrite Villalbeto de la Peña fall occurred on January 4, 2004, at 16:46: 45 +/- 2 s UTC. The related daylight fireball was witnessed by thousands of people from Spain, Portugal, and southern France, and was also photographed and videotaped from different locations of Len and Palencia provinces in Spain. From accurate astrometric calibrations of these records, we have determined the atmospheric trajectory of the meteoroid. The initial fireball velocity, calculated from measurements of 86 video frames, was 16.9 +/- 0.4 km/s. The slope of the trajectory was 29.0 +/- 0.6 degrees to the horizontal, the recorded velocity during the main fragmentation at a height of 27.9 +/- 0.4 km was 14.2 +/- 0.2 km/s, and the fireball terminal height was 22.2 +/- 0.2 km. The heliocentric orbit of the meteoroid resided in the ecliptic plane (i = 0.0 +/- 0.2 degrees), having a perihelion distance of 0.860 +/- 0.007 AU and a semimajor axis of 2.3 +/- 0.2 AU. Therefore, the meteorite progenitor body came from the Main Belt, like all previous determined meteorite orbits. The Villalbeto de la Peña fireball analysis has provided the ninth known orbit of a meteorite in the solar system.
    • Thermodynamic constraints on fayalite formation on parent bodies of chondrites

      Zolotov, M. Yu.; Mironenko, M. V.; Shock, E. L. (The Meteoritical Society, 2006-01-01)
      Thermochemical equilibria are calculated in the multicomponent gas-solution-rock system in order to evaluate the formation conditions of fayalite, (Fe0.881.0Mg0.120)2SiO4, Fa88100, in unequilibrated chondrites. Effects of temperature, pressure, water/rock ratio, rock composition, and progress of alteration are evaluated. The modeling shows that fayalite can form as a minor secondary and transient phase with and without aqueous solution. Fayalite can form at temperatures below ~350 degrees C, but only in a narrow range of water/rock ratios that designates a transition between aqueous and metamorphic conditions. Pure fayalite forms at lower temperatures, higher water/rock ratios, and elevated pressures that correspond to higher H2/H2O ratios. Lower pressure and water/rock ratios and higher temperatures favor higher Mg content in olivine. In equilibrium assemblages, fayalite usually coexists with troilite, kamacite, magnetite, chromite, Ca-Fe pyroxene, and phyllosilicates. Formation of fayalite can be driven by changes in temperature, pressure, H2/H2O, and water/rock ratios. However, in fayalite-bearing ordinary and CV3 carbonaceous chondrites, the mineral could have formed during the aqueous-to-metamorphic transition. Dissolution of amorphous silicates in matrices and/or silica grains, as well as low activities of Mg solutes, favored aqueous precipitation of fayalite. During subsequent metamorphism, fayalite could have formed through the reduction of magnetite and/or dehydration of ferrous serpentine. Further metamorphism should have caused reductive transformation of fayalite to Ca-Fe pyroxene and secondary metal, which is consistent with observations in metamorphosed chondrites. Although bulk compositions of matrices/chondrites have only a minor effect on fayalite stability, specific alteration paths led to different occurrences, quantities, and compositions of fayalite in chondrites.