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Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Meteor storm evidence against the recent formation of lunar crater Giordano Bruno
Paul Withers*
*Author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA; e-mail address: withers@lpl.arizona.edu
Abstract–It has been suggested that the formation of 22 km diameter lunar crater Giordano Bruno was witnessed in June 1178 AD. To date, this hypothesis has not been well tested. Such an impact on the Earth would be "civilization threatening". Previous studies have shown that the formation of Giordano Bruno would lead to the arrival of 10 million tonnes of ejecta in the Earth's atmosphere in the following week. I calculate that this would cause a week-long meteor storm potentially comparable to the peak of the 1966 Leonids storm. The lack of any known historical records of such a storm is evidence against the recent formation of Giordano Bruno. Other tests of the hypothesis are also discussed, with emphasis on the lack of corroborating evidence for a very recent formation of the crater.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite
Yukio Ikeda* and Martin Prinz
*Correspondence author's address: Ibaraki University, Mito 310, Japan; e-mail address: ikeda@mito.ipc.ibaraki.ac.jp
Abstract–Magmatic inclusions occur in type II ureilite clasts (olivine-orthopyroxene-augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar Al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine-pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al. (2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component-enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields.
DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene-metal and pyroxene-plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene-metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene-plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body.
The felsic clasts may have formed mainly from albite component-rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali-bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Greenland Superbolide Event of December 9, 1997
H. Pendersen, R.E. Spalding, E. Tagliaferri, Z. Ceplecha*, T. Risbo and H. Haack
*Correspondence author's address: Academy of Sciences, Astronomical Institute, Observatory, 25165 Ondrejov, Czech Republic; e-mail address: ceplecha@asu.cas.cz
Abstract–Data on the trajectory and orbit of an extremely bright bolide (superbolide) over Greenland on December 9, 1997, are given, and circumstances of the phenomenon and its observations are described. A surveillance-video-camera and satellite-based records enabled computing the trajectory and orbit independently of visual sightings of casual observers. The superbolide body of about 36 000 kg penetrated the atmosphere with an initial velocity of 30.5 ± 1.7 km s-1. Its orbit was a long-period orbit and seems to be at variance with the low value of ablation coefficient (0.017 kg MJ-1) derived from modeling the atmospheric trajectory. However, such an event has been documented previously. Also the intensity and brevity of the satellite detected light flares are highly unusual. The impact area of the main hypothetical remnant of the body is given. Search for meteorites was performed. No meteorites were recovered. Also analysis of snow samples gave no hint of meteoritic dust.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Presidential Address: The eucrite/Vesta story
Michael J. Drake
*Author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721-0092 USA; e-mail address: drake@lpl.arizona.edu
Abstract–Many lines of evidence indicate that meteorites are derived from the asteroid belt but, in general, identifying any meteorite class with a particular asteroid has been problematical. One exception is asteroid 4 Vesta, where a strong case can be made that it is the ultimate source of the howardite-eucrite-diogenite (HED) family of basaltic achondrites. Visible and near infrared reflectance spectra first suggested a connection between Vesta and the basaltic achondrites. Experimental petrology demonstrated that the eucrites (the relatively unaltered and unmixed basaltic achondrites) were the product of approximately a 10% melt. Studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal-scale magma ocean. Mass balance considerations point to a parent body that had its surface excavated, but remains intact. Modern telescopic spectroscopy has identified kilometer-scale "Vestoids" between Vesta and the 3:1 orbit-orbit resonance with Jupiter. Dynamical simulations of impact into Vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations. Hubble Space Telescope images show a 460 km diameter impact basin at the south pole of Vesta. It seems that Nature has provided multiple free sample return missions to a unique asteroid. Major challenges are to establish the geologic context of the HED meteorites on the surface of Vesta and to connect the remaining meteorites to specific asteroids.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Carbon-silicate aggregates in the CH chondrite Pecora Escarpment 91467: A carrier of heavy N of interstellar origin
N. SUGIURA* AND S. ZASHU
*Correspondence author's address: Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan; e-mail address: sugiura@eps.s.u-tokyo.ac.jp
Abstract–Scanning electron microscopy and SIMS (Secondary Ion Mass Spectrometry) of the unequilibrated CH chondrite Pecora Escarpment (PCA) 91467 revealed 4 carriers of isotopically heavy N; (1) aggregates of carbonaceous material and silicates, (2) iron-nickel metal grains with fine Fe-Cr sulfide inclusions, (3) Si-rich Fe-Ni metal associated with Fe-sulfide and (4) hydrated areas in the matrix. N in carbon-silicate aggregates is isotopically heavy (w15N is as high as 2500‰), whereas the other elements are isotopically normal, suggesting interstellar origin of carbonaceous material in the aggregates. Based on isotopic and textural evidence, we suggest that the carriers (2) and (3) were formed by brief heating in the solar nebula, whereas the carrier (4) was formed in a parent-body asteroid. The carbon-silicate aggregates are likely to be related to interstellar graphite found in Murchison and may also be the source of heavy N in bencubbinites.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites
TAKASHI MIKOUCHI*, MASAMICHI MIYAMOTO AND GORDON A. MCKAY
*Correspondence author's address: Mail Code SN2, Planetary Science Branch, NASA Johnson Space Center, Houston, Texas 77058, USA; e-mail address: mikouchi@eps.s.u-tokyo.ac.jp
Abstract–Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a fine-grained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low-Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En76Fs21Wo3~En58Fs30Wo12) than those in lithology A martian of EETA79001 (Env73Fs22Wo5~En45Fs43Wo12), rather similar to pyroxenes in lherzolitic meteorites (En76Fs21Wo3~En63Fs22Wo15). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo76-58) than EETA79001 olivine (Fo81-53), and is also similar to olivines in lherzolitic martian meteorites (Fo74-65). The orthopyroxene-olivine-chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg-rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03-3 oC/h for Dar al Gani 476 and 0.05-5 oC/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range (QUE) 94201, nakhlites?).
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The solar system abundances of phosphorus and titanium and the nebular volatility of phosphorus
DIETER WOLF* AND HERBERT PALME
*Correspondence author's address: Institut für Mineralogie und Geochemie, Universität zu Köln, Zülpicherstrasse 49b, 50674 Köln, Germany; e-mail address: dieter.wolf@min.uni-koeln.de
Abstract–The bulk chemical composition of Orgueil and 25 other carbonaceous chondrites was determined by x-ray fluorescence analysis. The sample sizes of the analyzed meteorites were in all cases 120 mg. The abundances of P and Ti in Orgueil and Ivuna were precisely determined by the standard addition method. The new P CI abundance is 926 ± 65 ppm. Excluding the low P of Ivuna and one Orgueil sample with unusual chemistry gives a CI P content of 930 ± 23 ppm. A CI abundance of 926 ppm corresponds to a P/Si wt ratio of 8.66 10-3 (atomic ratio 7.85 10-3). For Ti a CI content of 458 ± 18 ppm and a Ti/Si wt ratio of 4.28 10-3 (atomic ratio 2.51 10-3) were found. A Si content of 10.69% was obtained for average CI. The new P CI abundance is 20 to 30% below earlier estimates, while the Ti CI abundance is in agreement with earlier determinations.
From the results of the analyses of bulk carbonaceous chondrites it is concluded: (1) refractory element/Mg ratios increase from CI through CM and C3O to C3V, but ratios among Al, Ca and Ti are constant, except for low Ca/Al ratios in the reduced subgroup of C3V. (2). The Si/Mg ratios are constant in all groups of carbonaceous chondrites. (3) There is a volatility related depletion of Cr and Fe, but the Cr/Fe ratios are constant. (4) The sequence of volatility related depletions of the moderately volatile elements P, Au, As, Mn, and Zn follows condensation temperatures (except for As), if in condensation calculations non-ideal solid solution in the host phase is considered.
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