Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Heterogeneous chemical processes as source of persistent meteor trains
Edmond Murad
Author's address: Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts 01731, USA; e-mail address: ed.murad@hanscom.af.mil
Abstract–Observations of long-lasting persistent trains following
the entry of some meteoroids into the Earth's atmosphere are suggested
to arise in part from the interaction between meteoroid components and
the atmosphere and in the heterogeneous recombination reaction of atmospheric
O atoms with NO. The latter occurs on the surfaces of dust left by
the explosive fragmentation of larger meteoroids. A strong role is
attributed to reactions of troilite (FeS), a meteorite component, with
the atmosphere at elevated temperatures. The suggestions made
in this paper complement previous work that suggested that long-lived emissions
results from a variety of species made in the shock of larger meteoroids.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Refractory Ca, Al-rich inclusions and Al-diopside-rich
chondrules in the metal-rich chondrites Hammadah al Hamra 237 and QUE 94411
Alexander N. Krot*, Kevin D. McKeegan, Sara S. Russell, Anders Meibom, Michael K. Weisberg, Jutta Zipfel, Tatiana V. Krot, Timothy J. Fagan and Klaus Keil
*Correspondence author's address: Hawaii Institute of Geophysics & Planetology, School of Ocean & Earth Science & Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA; e-mail address: sasha@pgd.hawaii.edu
Abstract–The metal-rich chondrites Hammadah al Hamra 237 (HH
237), and QUE 94411 (QUE), paired with QUE 94627, contain relatively rare
(<1 vol%) Ca, Al-rich inclusions (CAIs) and Al-diopside-rich chondrules.
Forty CAIs and CAI fragments and 7 Al-diopside-rich chondrules were identified
in HH 237 and QUE. The CAIs, ~50–400 µm in apparent diameter,
include: (a) 22 (56%) pyroxene-spinel±melilite [+forsterite rim],
(b) 11 (28%) forsterite-bearing, pyroxene-spinel±melilite±anorthite
[+forsterite rim] (c) 2 (5%) grossite-rich [+spinel-melilite-pyroxene rim],
(d) 2 (5%) hibonite-melilite [+spinel-pyroxene±forsterite rim],
(e) 1 (2%) hibonite-bearing, spinel-perovskite [+melilite-pyroxene rim],
(f) 1 (2%) spinel-melilite-pyroxene-anorthite, and (g) 1 (2%) amoeboid
olivine aggregate. Each type of CAI is known to exist in other chondrite
groups, but the high abundance of pyroxene-spinel±melilite CAIs
with igneous textures and surrounded by a forsterite rim are unique features
of HH 237 and QUE. Additionally, oxygen isotopes consistently show
relatively heavy compositions with 
17O
ranging from –6‰ to –10‰ (1 sigma = 1.3‰) for all analyzed CAI minerals
(grossite, hibonite, melilite, pyroxene, spinel). This suggests that
the CAIs formed in a reservoir isotopically distinct from the reservoir(s)
where "normal", 16O-rich (17O
< –20‰) CAIs in most other chondritic meteorites formed.
The Al-diopside-rich chondrules, which have previously been observed
in CH chondrites and the unique carbonaceous chondrite Adelaide, contain
Al-diopside grains enclosing oriented inclusions of forsterite, and interstitial
anorthitic mesostasis and Al-rich, Ca-poor pyroxene, occasionally enclosing
spinel and forsterite. These chondrules are mineralogically similar
to the Al-rich barred-olivine chondrules in HH 237 and QUE, but have lower
Cr concentrations than the latter, indicating that they may have formed
during the same chondrule-forming event, but at slightly different ambient
nebular temperatures. Aluminum-diopside grains from two Al-diopside-rich
chondrules have O-isotopic compositions (17O
~ –7 ± 1.1‰) similar to CAI minerals, suggesting that they formed
from an isotopically similar reservoir. The oxygen isotopic composition
of one Ca, Al-poor cryptocrystalline chondrule in QUE was analyzed and
found to have 17O
~ –3 ± 1.4‰.
The characteristics of the CAIs in HH 237 and QUE are inconsistent with
an impact origin of these metal-rich meteorites. Instead they suggest that
the components in CB chondrites are pristine products of large-scale, high-temperature
processes in the solar nebula and should be considered bona fide
chondrites.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Radar Observations of Asteroid 1998 ML14
Steven J. Ostro*, R. Scott Hudson, Lance A. M. Benner, Michael C. Nolan,
Jon D. Giorgini, Daniel J. Scheeres, Raymond F. Jurgens and Randy Rose
*Correspondence author's address: Jet Propulsion Laboratory,
Mail Code 300-233, California Institute of Technology, Pasadena, California
91109-8099, e-mail address: ostro@reason.jpl.nasa.gov
Abstract–Goldstone and Arecibo delay-Doppler radar imaging of
asteroid 1998 ML14 shortly after its discovery reveals a 1-km-diameter
spheroid with prominent topography on one side and subdued topography on
the other. The object's radar and optical properties are typical
for S-class near-Earth asteroids. The gravitational slopes
of a shape model derived from the images and assumed to have a uniform
density are shallow, exceeding 30° over only 4% of the surface.
If 1998 ML14's density distribution is uniform, then its orbital environment
is similar to a planetary body with a spheroidal gravitational field and
is relatively stable. Integration of a radar-refined orbit
reveals that the 1998 apparition was the asteroid's closest approach to
Earth since at least 1100 and until 2283, when it approaches to within
2.4 lunar distances. Outside of that time interval, orbit uncertainties
based on the present set of observations preclude reliable prediction.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Negative correlation of 129I/127I
and 129Xe/132Xe: Product of closed system evolution
or evidence of a mixed component
J. D. Gilmour*, J. A. Whitby and G. Turner
*Correspondence author's address: Department of Earth Sciences, University of Manchester, Manchester M13 9PL, U.K.; author's email address: jamie.gilmour@man.ac.uk
Abstract–Anti-correlation of initial iodine ratios with trapped
129Xe/132Xe ratios has been interpreted as resulting
from 129I decay to 129Xe in a closed system.
However, many of the 129Xe/132Xe ratios that contribute
to the anti-correlations are lower than 1.04, the value characteristic
of major solar system reservoirs; 129I decay cannot lead to
a decrease in this ratio. We offer an alternative explanation for
the anti-correlations, based on trapped iodine and xenon components similar
to those observed in Nakhla, that does not require the existence of components
with 129Xe/132Xe lower than solar.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Report of the Working Party on the classification
of the lunar igneous rocks
M. J. Le Bas
Author's address: School of Ocean and Earth Sciences, University of Southampton, European Way, Southampton, SO14 3ZH, U.K.; e-mail address: mjlb@soc.soton.ac.uk
Abstract–A report is presented for a possible revised classification
of lunar igneous rocks that still uses the division of Moon rocks into
mare and highland types. It subdivides the mare rocks into basalts
depending on TiO2 content and glasses depending on colour, and
subdivides the highland rocks principally into KREEP basalts and into coarse-grained
igneous rocks comparable to and using terrestrial igneous rock terminology.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
A transmission electron microscope study
of exsolution and coarsening in iron-bearing clinopyroxene from synthetic
analogues of chondrules
Stephan Weinbruch*, Wolfgang F. Müller and Roger H. Hewins
*Correspondence author's address: Institut für Mineralogie, Technische Universität Darmstadt, Schnittspahnstr. 9, 64287 Darmstadt, Germany; e-mail address: dh6d@hrzpub.tu-darmstadt.de
Abstract–The microstructure of Fe-rich clinopyroxene from synthetic analogues of chondrules was studied by transmission electron microscopy. The samples were cooled at various rates from 1455 °C to the quench temperature of 1000 °C. Slow cooling at rates below approximately 50–60 °C/h leads to the development of coherent pigeonite/augite exsolution lamallae on (001). A final wavelength of 19.6 ± 1.1 nm was obtained at a cooling rate of 10 °C/h, and 17.4 ± 2.4 nm at a cooling rate of 50 °C/h. Faster cooling at rates between approximately 50–450 °C/h yields only modulated structures with a wavelength on the order of 17–19 nm for the (001) orientation.
Coherent exsolution lamellae on (001) in clinopyroxene occur in chondrules
of H, L, LL, and CV chondrites, indicating that slow cooling of chondrules
at subsolidus temperatures is a widespread phenomenon. The variation
of the lamellar wavelength observed in natural chondrules corresponds to
a variation of the subsolidus cooling rates between approximately 0.1 and
50 °C/h. The low cooling rates at subsolidus temperatures deduced
from the microstructure of Fe-rich clinopyroxene point to nonlinear cooling,
with cooling rates decreasing with decreasing temperature.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Evidence for shock metamorphism in sandstones
from the Libyan desert glass strewn field
Barbara Kleinmann*, Peter Horn and Falko Langenhorst
*Correspondence author's address: Windeckstrasse 6, D 68163 Mannheim, Germany; e-mail address: Katrin.Behaghel@gmx.de
Abstract–Microscopic analysis of sandstones from the southern
Libyan Desert Glass (LDG) strewn field reveals a sequence of progressive
deformation features, which range from unaffected rock to extreme brecciation.
The quartz grains show a wide range of deformation features: crushing and
fracturing, undulatory extinction, mosaicism, oriented cleavage, partial
isotropization and apparent multiple sets of planar deformation features
(PDF). This preliminary report provides the first description of
these shock indicators, which are typical for hypervelocity impact, in
bedrock material that is assumed to be related to the Libyan Desert Glass.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Chemical and isotopic characteristics of the
Didwana-Rajod (H5) chondrite
B. S. Paliwal, R. R. Mahajan, S. V. S. Murty, A. D. Shukla, P. N. Shukla, N. Bhandari*, R. Natarajan, R. Hutchison, S. Russell and I. A. Franchi
*Correspondence author's address: Physical Research Laboratory, Ahmedabad 380009, India; e-mail address: bhandari@prl.ernet.in
Abstract–The mineralogical and chemical characteristics of the
Didwana-Rajod Meteorite are described. The mean mineral composition
is found to be olivines: (Fo83.2), pyroxenes: (En83.5Wo0.7Fs15.8)
and feldspar is mainly oligoclase. Oxygen isotopic analysis shows 
18O
= +3.8‰ and 17O
= +2.59‰. The nitrogen content of Didwana-Rajod is about 2 ppm with 15N
~ 3.4‰. Based on microscopic, chemical, isotopic and electron probe
microanalysis, the meteorite is classified as H5 chondrite. Cosmogenic
tracks, radionuclides and the isotopic composition of rare gases were also
measured in this meteorite. The track density in olivines varies
in a narrow range with an average value of (6.5 ± 0.5) × 105/cm2
for four spot samples taken at the four corners of the stone. The
cosmic ray exposure age based on neon and argon is 9.8 Ma. 22Na/26Al
~0.94 is lower than the solar-cycle average value of about 1.5 and is consistent
with irradiation of the meteoroid to lower GCR fluxes as expected at the
solar maximum. The track density, rare gas isotopic ratios, 60Co
activity and other radionuclide data are consistent with a preatmospheric
radius of about 15 cm, corresponding to a mass of about 50 kg. The
cosmogenic properties are consistent with a simple exposure history in
the interplanetary space.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Siderophile elements in Earth's upper mantle
and lunar breccias: Data synthesis suggests manifestations of the same
late influx
J. W. Morgan*, R. J. Walker, A. D. Brandon and M. F. Horan
*Correspondence author's address: Department of Earth Resources, Colorado State University, Fort Collins Colorado 80523, USA; e-mail address: jmorgan@cnr.colostate.edu
Abstract–The platinum group elements (PGE; Ru, Rh, Pd, Os, Ir, Pt), Re and Au comprise the highly siderophile elements (HSE). We re-examine selected isotopic and abundance data sets for HSE in upper mantle peridotites to resolve a longstanding dichotomy. Re-Os and Pt-Os isotope systematics, and approximately chondritic proportions of PGE in these rocks suggest the presence in undepleted mantle of a chondrite-like component, which is parsimoniously explained by late influx of large planetisimals after formation of the Earth's core and the Moon. But some suites of xenolithic and orogenic spinel lherzolites, and abyssal peridotites, have a CI-normalized PGE pattern with enhanced Pd that is sometimes termed "non-chondritic". We find that this observation is consistent with other evidence of a late influx of material more closely resembling enstatite, rather than ordinary or carbonaceous, chondrites. Regional variations in HSE patterns may be a consequence of a late influx of very large objects of variable composition.
Studies of many ancient (>3.8 Ga) lunar breccias show regional variations in Au/Ir and suggest that "graininess" existed during the early bombardment of the Earth and Moon. Reliable Pd values are available only for Apollo 17 breccias 73215 and 73255, however. Differences in HSE patterns between the aphanitic and anorthositic lithologies in these breccias show fractionation between a refractory group (Re, Os and Ir) and a normal (Pd, Ni, and Au) group and may reflect the compositions of the impacting bodies. Similar fractionation is apparent between the EH and EL chondrites, whose PGE patterns resemble those of the aphanitic and anorthositic lithologies, respectively.
The striking resemblance of HSE and chalcogen (S, Se) patterns in the Apollo aphanites and high Pd terrestrial peridotites suggest that the "non-chondritic" abundance ratios in the latter may be reflected in the composition of planetisimals striking the Moon in the first 700 Ma of Earth-Moon history. Most notably, high Pd may be part of a general enhancement of HSE more volatile than Fe suggesting that the Au abundance in at least parts of the upper mantle may be 1.5 to 2 times higher than previously estimated.
The early lunar influx may be estimated from observed basin-sized craters. Comparison of relative influx to Earth and Moon suggests that the enrichment of HSE is limited to the upper mantle above 670 km. To infer enrichment of the whole mantle would require several large lunar impacts not yet identified.
 |
 |