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Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Boltysh, Another end-Cretaceous Impact
Simon P. Kelley* and Eugene Gurov
*Correspondence author's address: Department of Earth Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom; e-mail address: S.P.Kelley@open.ac.uk
Abstract–The Chixculub impact occurred at the Cretaceous/Tertiary (K/T) boundary, and although several other Late Cretaceous and Paleogene impact craters have, at times, been linked with the K/T boundary, isotope geochronology has demonstrated that all have significantly different ages. The currently accepted age of the 24 km diameter Boltysh crater, a K-Ar whole rock age, places it in the Coniacian at 88±3 Ma. However, comprehensive Ar-Ar dating of a range of melt samples yields a mean age of 65.17±0.64 Ma, within errors of the K/T boundary. Several of the fresh samples exhibit signs of excess argon but this seems to be concentrated in rapidly crystallized glass rich samples.
The Ar-Ar age confirms an earlier fission track measurement and thus two dating techniques have yielded an age within errors of the K/T boundary for this crater. Crucially, although the ages of Boltysh and Chixculub are within errors, they may not formed synchronously. Craters of 24km diameter occur much more commonly than impacts of Chixculub dimensions, but their proximity does raise the important question of how many impacts there might have been close to the K/T boundary.
Nitrogen in Diamond-free Ureilite ALH78019: Clues to the Origin of Diamond in Ureilites
V. K. Rai, S. V. S. Murty* and U. Ott
*Correspondence author's address: Physical Research Laboratory, Ahmedabad 380009, India; e-mail address: murty@prl.ernet.in
Abstract–Nitrogen and noble gases were measured in a bulk sample and in acid resistant carbon-rich residues of the ureilite ALH78019 which has experienced low shock and is free of diamond. A small amount of amorphous carbon combusting at <=500oC, carries most of the noble gases, while the major carbon phase consisting of large crystals of graphite, combusts at >=800oC, and is almost noble-gas free. Nitrogen on the other hand is present in both amorphous carbon and graphite, with different delta15N signatures of -21%o and +19%o respectively, distinctly different from the very light nitrogen (~ -100%o) of ureilite diamond. Amorphous carbon in ALH78019 behaves similar to phase Q of chondrites with respect to noble gas release pattern, behavior towards oxidizing acids as well as nitrogen isotopic composition. In situ conversion of amorphous carbon or graphite to diamond through shock would require an isotopic fractionation of 8 to 12% for nitrogen favoring the light isotope, an unlikely proposition, posing a severe problem for the widely accepted shock origin of ureilite diamond.
Nannobacterial Alteration of Pyroxenes in Martian Meteorite ALH84001
Robert L. Folk and Lawrence A. Taylor*
*Correspondence author's address: Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996, USA; e-mail address: lataylor@utk.edu
Abstract–In Martian meteorite ALH84001, this SEM study was focused on the ferromagnesian minerals, which are extensively covered with nanometer-size bodies mainly 30-100 nm in diameter. These bodies range from spheres to ovoids to caterpillar shapes and resemble, both in size and shape, nannobacteria that attack weathered rocks on Earth and that can be cultured. Dense colonies alternate with clean, smooth cleavage surfaces, possibly formed later. Statistical study shows that the distribution of presumed nannobacteria is very clustered. In addition to the small bodies, there are a few occurrences of ellipsoidal 200-400 nm objects, that are within the lower size range of "normal" Earthly bacteria. We conclude that the nanobodies so abundant in ALH84001 are indeed nannobacteria, confirming the initial assertion of McKay et al. (1996). However whether these bodies originated on Mars, or are Antarctic contamination remains a valid question.
Spaceborne ultraviolet 251-384 nm spectroscopy of a meteor during the 1997 Leonid shower
Peter Jenniskens*, Ed Tedesco, Jayant Murthy, Christophe O. Laux, and Stephen Price
*Correspondence author's address: SETI Institute, 2035 Landings Drive, Mountain View, CA 94043, USA; e-mail address: pjenniskens@mail.arc.nasa.gov
Abstract–We used the ultraviolet to visible spectrometers onboard the Midcourse Space Experiment (MSX) to obtain the first ultraviolet spectral measurements of a bright meteor during the 1997 Leonid shower. The meteor was most likely a Leonid with a brightness of about -2 magnitude at 100 km altitude. In the region between 251 and 310 nm, the two strongest emission lines are from neutral and ionized magnesium. Ionized Ca lines, indicative of a hot T ~ 10,000 K plasma, are not detected. The Mg and Mg+ line intensity ratio alone does not yield the ionization temperature, which can be determined only by assuming the electron density. A typical air plasma temperature of T = 4,400 K would imply a very high electron density: ne = 2.2 x 1018 m-3, but at chondritic abundances of Fe/Mg and Si/Mg ~ 1. For a more reasonable LTE air plasma electron density, the Mg and Mg+ line ratio implies a less than chondritic Fe/Mg = 0.06 abundance ratio and a cool non-LTE T = 2,830 K ionization temperature for the ablation vapor plasma. The present observations do not permit a choice between these alternatives. The new data provide also the first spectral confirmation of the presence of molecular OH emission in meteor spectra, while the presence of NO is indicated only in the low T scenario.
Ponded Deposits on Asteroid 433 Eros
A. F. Cheng*, N. Izenberg, C. R. Chapman and M. T. Zuber
*Correspondence author's address: Applied Physics Laboratory, The Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723, USA; e-mail address: andrew.cheng@jhuapl.edu
Abstract–In late January 2001 the NEAR Shoemaker spacecraft performed low altitude passes over the surface of 433 Eros.
Coordinated observations of the asteroid surface were obtained at sub-meter resolution by the NEAR Laser
Rangefinder (NLR) and the Multispectral Imager (MSI). This paper presents three independent, coordinated
observations of a 90 m pond adjacent to a granular debris flow, including the highest resolution altimetric
measurements of ponded deposits on Eros. The ponded deposits appear to have been emplaced by fluid-like motion
of dry asteroidal regolith. A simple model of seismic agitation from impacts is developed to account for pond
formation on Eros. The model predicts that ponds should form readily on Eros but not on the Moon, where ponds
are not observed. The model also suggests that the absence of observable ponds in the largest craters of Eros, as well
as on Phobos and Deimos, may be related to regolith depth.
The Dar al Gani Meteorite Field (Libyan Sahara): Geological Setting, Pairing of Meteorites, and Recovery Density
J. Schlüter*, L. Schultz, F. Thiedig, B.O. Al-Mahdi and A.E. Abu Aghreb
*Correspondence author's address: Mineralogical Museum, University of Hamburg, Grindelallee, D-20146 Hamburg, Germany; e-mail address: minmushh@public.uni-hamburg.de
Abstract–1238 Libyan meteorites have been reported up until July 2001. Most were found in two areas called Dar al Gani (DaG) and Hamadah al Hamra (HaH). DaG is located on a plateau of marine carbonate rocks with marly components. 869 meteorites between 6 g and 95 kg totalling 687 kg have been found here but the calculated mean recovery density is comparatively low with one meteorite on 6.5 km2.
DaG is a perfect site for the recognition and preservation of meteorites. The existence of meteorites is the result of a combination of specific geological and geomorphological conditions: there is an bright-colored, old limestone plateau (< 2 Ma), under arid weather conditions over long periods of time, with rapid elimination of surface water if present and low erosion rates. The preservation of meteorites is guaranteed through the absence of quartz-sand on the plateau, strongly reducing wind erosion and a basic environment emerging from the carbonate ground retards rusting of metallic meteorite components. A supposed soil cover during pluvial times has probably protected older meteorites and led to a concentration of meteorites of different periods.
An evaluation of DaG meteorites suggests the existence of at least 26 strewnfields and 26 meteorite pairs reducing the number of falls to at most 534. Shock and weathering grades as a tool for the recognition of pairings turned out to be problematic, as several strewnfields showed paired meteorites which had been classified to different shock and weathering grades.
Martian meteorite dhofar 019: A new shergottite
L.A. Taylor*, M.A. Nazarov, C.K. Shearer, H.Y. McSween, Jr., J. Cahill, C.R. Neal, M.A. Ivanova, L.D. Barsukova, R.C. Lentz, R.N. Clayton, and T.K. Mayeda
*Correspondence author's address: Planetary Geosciences Institute, Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA; e-mail address: lataylor@utk.edu
Abstract–Dhofar 019 is a new Martian meteorite found in the desert of Oman. In its texture, mineralogy, and major- and trace-element chemistry, this meteorite is classified as a basaltic shergottite. Olivine megacrysts are set within a groundmass composed of finer-grained olivine, pyroxene (pigeonite and augite), and maskelynite. Minor phases are chromite-ulvöspinel, ilmenite, silica, K-rich feldspar, merrillite, chlorapatite, and pyrrhotite. Secondary phases of terrestrial origin include calcite, gypsum, celestite, Fe hydroxides, and smectite.
Dhofar 019 is most similar to the Elephant Moraine (EETA) 79001 lithology A and Dar al Gani (DaG) 476/489 shergottites. The main features that distinguish Dhofar 019 from other shergottites are: lack of orthopyroxene; lower Ni contents of olivine; the heaviest oxygen-isotopic bulk composition; and larger compositional ranges for olivine, maskelynite, and spinel, as well as a wide range for pyroxenes. The large compositional ranges of the minerals are indicative of relatively rapid crystallization. Modeling of olivine chemical zonations yield minimum cooling rates of 0.5-0.8°C/hour. Spinel chemistry suggests that crystallization took place under one of the most reduced conditions for Martian meteorites, at an fO2 3 log units below the QFM buffer.
The olivine megacrysts are heterogeneously distributed in the rock. Crystal-size distribution analysis suggests that they constitute a population formed under steady-state conditions of nucleation and growth, although a few grains may be cumulates. The parent melt is thought to have been derived from partial melting of a LREE- and PGE-depleted mantle source. Shergottites, EETA 79001 lithology A, DaG 476/489, and Dhofar 019, although of different ages, comprise a particular type of Martian rocks. Such rocks could have formed from chemically similar source(s) and parent melt(s), with their bulk compositions affected by olivine accumulation.
Queen Alexandra Range 93148: A new type of pyroxene pallasite?
Christine Floss*
*Correspondence author's address: Laboratory for Space Sciences and the Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA; e-mail address: floss@wuphys.wustl.edu
Abstract–Trace elements, including the rare earth elements (REE), were measured in olivine and orthopyroxene from QUE 93148, and in olivine from two main group pallasites, Springwater and Mount Vernon. Although QUE 93148 was originally classified as a lodranite, a variety of data including oxygen isotopic compositions (Goodrich and Righter, 2000), preclude a genetic relationship with the acapulcoites/lodranites. Incompatible trace element (e.g., Ti, Zr) distributions in orthopyroxene do indicate large amounts of melting and are consistent with the ultramafic assemblage observed in this meteorite. Trace element abundances in olivine are consistent with suggestions that QUE 93148 may be related to the main group pallasites (Goodrich and Righter, 2000), although there are some inconsistencies. Its trace element distributions are most like those of the pyroxene pallasites, suggesting that it may have formed in a similar manner. QUE 93148 may represent a new type of pyroxene pallasite with links to the main group pallasites.
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