Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
K asteroids and CO3/CV3 chondrites
T. H. Burbine*, R. P. Binzel, S. J. Bus and B. E. Clark
*Correspondence author's address: Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington D.C. 20560-0119, USA; e-mail address: burbine.tom@nmnh.si.edu
Abstract–Reflectance spectra from 0.44 to 1.65 µm were
obtained for three K asteroids. These objects all have spectra consistent
with olivine-dominated assemblages whose absorption bands have been suppressed
by opaques. The two observed Eos family members (221 Eos and 653
Berenike) are spectral analogs to the CO3 chondrite Warrenton. The
other observed object (599 Luisa) is a spectral analog for CV3 chondrite
Mokoia. These asteroids are all located near meteorite-supplying
resonances with the Eos family cut by the 9:4 resonance and Luisa is found
near the 5:2 resonance. However, K asteroids have been identified
throughout the main belt so it is difficult to rule out other possible
parent bodies for the CO3 and CV3 chondrites.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Capture of particles in hypervelocity impacts in aerogel
M. J. Burchell*, J. A. Creighton, M. J. Cole, J. Mann and A. T. Kearsley
*Correspondence author's address: School of Physical Sciences, University of Kent at Canterbury, Kent CT2 7NR, United Kingdom; e-mail address: M.J.Burchell@ukc.ac.uk
Abstract–The capture in aerogel of 106 micron diameter glass
beads is investigated for impact speeds of 1 to 7.5 km s–1.
Three different aerogel densities were used, 60, 96 and 180 kg m–3.
It was found that the length of the penetration track in the aerogel increases
with speed until a maximum is reached. Above the maximum speed the
track length decreases. This behaviour is similar to that which has
previously been observed for particles impacting polystyrene foams and
porous alumina. Whilst track length was not found to be an unambiguous
indicator of impact speed, the excavated track volume was found to be a
suitable indictor of speed. Further, it was possible to estimate
the original particle size by measurements of the track volume and entrance
hole size. In addition sub-100 micron diameter particles composed
of various minerals were fired into aerogel and the characterisation of
the particles in situ by use of a Raman spectrometer was evaluated.
This was found to work well, giving vibrational spectra essentially similar
to those of the bulk minerals, thus providing a mineralogical rather than
an elemental signature for the captured particles.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The effects of space weathering on Apollo 17 mare soils: Petrographic and chemical characterization
Lawrence A. Taylor*, Carlé Pieters, Lindsay P. Keller, Richard V. Morris, David S. McKay, Allan Patchen and Susan Wentworth
*Correspondence author's address: Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee 37996, USA; e-mail address: lataylor@utk.edu
Abstract–The Lunar Soil Characterization Consortium, a group of lunar-sample and remote-sensing scientists, has undertaken the extensive task of characterization of the finest fractions of lunar soils, with respect to their mineralogical and chemical makeup. These compositional data form the basis for integration and modeling with the reflectance spectra of these same soil fractions. This endeavor is aimed at deciphering the effects of space weathering of soils on airless bodies with quantification of the links between remotely sensed reflectance spectra and composition. A beneficial byproduct is an understanding of the complexities involved in the formation of lunar soil.
Several significant findings have been documented in the study of the
<45 µm size fractions of selected Apollo 17 mare soils.
As grain size decreases, the abundance of agglutinitic glass increases,
as does the plagioclase, whereas the other minerals decrease. The
composition of the agglutinitic glass is relatively constant for all size
fractions, being more feldspathic that any of the bulk compositions; notably,
TiO2 is substantially depleted in the agglutinitic glass.
However, as grain size decreases, the bulk composition of each size fraction
continuously changes, becoming more Al-rich and Fe-poor, and approaches
the composition of the agglutinitic glasses. Between the smallest
grain sizes (10–20 and <10 µm), the IS/FeO values (amount
of total iron present as nanophase Fe0) increase by greater
than 100% (>2X), whereas the abundance of agglutinitic glass increases
by only 10–15%. This is evidence for a large contribution from surface-correlated
nanophase Fe0 to the IS/FeO values, particularly
in the <10 µm size fraction. The surface nanophase Fe0
is present largely as vapor-deposited patinas on the surfaces of almost
every particle of the mature soils, and to a lesser degree for the immature
soils (Keller et al, 1999b). It is reasoned that the vapor-deposited
patinas may have far greater effects upon reflectance spectra of mare soils
than the agglutinitic Fe0.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Calcium-aluminum-rich inclusions in enstatite chondrites (II): Oxygen isotopes
Timothy J. Fagan*, Kevin D. McKeegan, Alexander N. Krot and Klaus Keil
*Correspondence author's address: Hawai'i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawaii 96822, USA; e-mail address: fagan@higp.hawaii.edu
Abstract–In situ ion microprobe analyses of spinel in
refractory Ca-Al-rich inclusions (CAIs) from type 3 EH chondrites yield
16O-rich compositions (delta 18O and delta 17O
~ –40‰). Spinel and feldspar in a CAI from an EL3 chondrite have
significantly heavier isotopic compositions (delta 18O and delta
17O ~ –5‰). A regression through the data results in a
line with slope 1.0 on a three-isotope plot, similar to isotopic results
from unaltered minerals in CAIs from carbonaceous chondrites. The
existence of CAIs with 16O-rich and 16O-poor compositions
in carbonaceous as well as enstatite chondrites indicates that CAIs formed
in at least two temporally or spatially distinct oxygen reservoirs.
General similarities in oxygen isotopic compositions of CAIs from enstatite,
carbonaceous, and ordinary chondrites indicate a common nebular mechanism
or locale for the production of most CAIs.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Cosmic-ray exposure history of two Frontier Mountain H-chondrite showers from spallation and neutron-capture products
K. C. Welten*, K. Nishiizumi, J. Masarik, M. W. Caffee, A. J. T. Jull, S. E. Klandrud and R. Wieler
*Correspondence author's address: Space Sciences Laboratory, University of California, Berkeley, California 94720-7450; USA; e-mail address: kcwelten@uclink4.berkeley.edu
Abstract–We measured the concentrations of 10Be, 26Al,
36Cl, 41Ca and 14C in the metal and/or
stone fractions of 27 Antarctic chondrites from Frontier Mountain, including
two large H-chondrite showers. To estimate the pre-atmospheric size
of the two showers, we determined the contribution of neutron-capture produced
36Cl (half-life = 3.01 × 105 yr) and 41Ca
(1.04 × 105 yr) in the stone fraction. The measured
activities of neutron-capture 36Cl and 41Ca, as well
as spallation produced 10Be and 26Al, were compared
with Monte Carlo-based model calculations. The largest shower, FRO
90174, includes eight fragments with an average terrestrial age of (100
± 30) × 103 yr; the neutron-capture saturation
activities extend to 27 dpm/kg stone for 36Cl and 19 dpm/kg
stone for 41Ca. The concentrations of spallation produced
10Be, 26Al and 36Cl constrain the radius
(R) to 80-100 cm, while the neutron-capture 41Ca activities indicate that
the samples originated from the outer 25 cm. With a pre-atmospheric
radius of 80–100 cm, FRO 90174 is among the largest of the Antarctic stony
meteorites. The large pre-atmospheric size supports our hypothesis
that at least 50 of the ~150 classified H5/H6-chondrites from the Frontier
Mountain stranding area belong to this single fall; this hypothesis does
not entirely account for the high H/L ratio at Frontier Mountain.
The smaller shower, FRO 90001, includes four fragments with an average
terrestrial age of (40 ± 10) × 103 yr; they contain
small contributions of neutron-capture 36Cl, but no excess of
41Ca. FRO 90001 experienced a complex exposure history
with high shielding conditions in the first stage (150<R<300 cm)
and much lower shielding in the second stage (R<30 cm), the latter starting
~1.0 million years (Myr) ago. Based on the measured 10Be/21Ne
and 26Al/21Ne ratios, the cosmic-ray exposure ages
of the two showers are 7.2 ± 0.5 Myr for FRO 90174 and 8 ±
1 Myr for FRO 90001. These ages coincide with the well-established
H-chondrite peak and corroborate the observation that the exposure age
distribution of FRO H-chondrites is similar to that of non-Antarctic falls.
In addition, we found that corrections for neutron-capture 36Ar
(from decay of 36Cl) result in concordant 21Ne and
38Ar exposure ages.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Modeling aqueous alteration of CM carbonaceous chondrites
Nina D. Rosenberg*, Lauren Browning and William L. Bourcier
*Correspondence author's address: Earth and Environmental Sciences Directorate, Lawrence Livermore National Laboratory, Livermore California 94551, USA; e-mail address: rosenberg4@llnl.gov
Abstract–Results from an inorganic geochemical modeling study
support a scenario in which low-temperature aqueous alteration of an anhydrous
CM asteroidal parent body and melt water from H2O and CO2
ices produces the altered assemblage observed in CM carbonaceous chondrites
(chrysotile, greenalite, tochilinite, cronstedtite and minor calcite and
magnetite). We consider a range of possible precursor mineral assemblages,
varying with respect to the Fe-oxidation state of the initial anhydrous
phases. The aqueous solutions produced by this alteration are generally
strongly basic and reducing and a large quantity of H2, and
possible CH4, gas can be released during aqueous alteration.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Exploration of quantitative kinetic models for the evaporation of silicate melts in vacuum and in hydrogen
C. M. O'D. Alexander
*Author's address: Department Terrestrial Magnetism, The Carnegie Institution of Washington, 5241 Broad Branch Road, Washington D.C. 20015, USA; e-mail addess: alexander@dtm.ciw.edu
Abstract–Two basic approaches (pure component reference (PCR) and equilibrium reference (EQR)) to modeling silicate melt evaporation are explored. The PCR model calculates the maximum possible evaporation rates of the pure oxides from their equilibrium vapor pressures and rescales these rates according to the activities of the oxides in the silicate melts and the melt densities. The EQR model calculates the maximum possible evaporation rates based on the equilibrium vapor pressures of the melts. Differences between the calculated and experimentally determined elemental evaporation rates are accounted for with evaporation (aevap) coefficients that are only dependent on temperature. Two versions of the PCR model, Cases 1 and 2, are explored to try to resolve apparently contradictory conclusions about the composition of the evaporating species based on Mg and Si isotope fractionation during evaporation (species are not in thermodynamic equilibrium abundances) and direct measurements of gas species in Langmuir experiments (species are in roughly equilibrium abundances). The Case 2 and EQR models cannot explain the observed isotope fractionations unless evaporation occurred under non-Rayleigh conditions, either because there was significant recondensation during the experiments or because diffusion was playing a limiting role.
Whether or not the role of diffusion is included, the PCR and EQR models are able to reproduce the elemental results of evaporation experiments of "chondritic" melts from temperatures of 1700 to 2000 °C, and up to mass losses of about 95%. However, the models underestimate absolute evaporation rates in very Ca- and Al-rich melts. This may reflect errors in the model used to estimate oxide activities. The EQR model can only reproduce the observed evaporation behavior of Na if, unlike the other oxides, its aevap coefficient is close to unity.
Based on available diffusion data, diffusion is not slow enough in "chondritic"
or forsteritic melts to explain the isotopic fractionations of Mg and O
in the evaporation experiments, but it may play a role in limiting Si isotope
fractionation. Provided recondensation was not a significant factor
in the experiments, at present PCR Case 1 appears to be the best model
if both the Langmuir and the isotopic fractionation experiments are to
be explained.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
A petrologic and trace element study of Dar al Gani 476 and Dar al Gani 489: Twin meteorites with affinities to basaltic and lherzolitic shergottites
M. Wadhwa*, R. C. F. Lentz, H. Y. McSween, Jr. and G. Crozaz
*Correspondence author's address: Department of Geology, The Field Museum, 1400 South Lake Shore Drive, Chicago, Illinois 60605, USA; e-mail address: mwadhwa@fmnh.org
Abstract–We present the results of a combined mineralogic-petrologic
and ion microprobe study of two Martian meteorites recently recovered in
the Lybian Sahara, Dar al Gani 476 (DaG 476) and Dar al Gani 489 (DaG 489).
Having resided in a hot desert environment for an extended time, DaG 476
and DaG 489 were subjected to terrestrial weathering that significantly
altered their chemical composition. In particular, analyses of some
of the silicates show light rare earth element (LREE)-enrichment resulting
from terrestrial alteration. In situ measurement of trace
element abundances in minerals allows us to identify areas unaffected by
this contamination and, thereby, to infer the petrogenesis of these meteorites.
No significant compositional differences between DaG 476 and DaG 489 were
found, supporting the hypothesis that they belong to the same fall.
These meteorites have characteristics in common with both basaltic and
lherzolitic shergottites, possibly suggesting spatial and petrogenetic
associations of these two types of lithologies on Mars. However,
the compositions of Fe-Ti oxides and the size of Eu anomalies in the earliest-formed
pyroxenes indicate that the two Saharan meteorites probably experienced
more reducing crystallization conditions than other shergottites (with
the exception of Queen Alexandra Range (QUE) 94201). As is the case
for other shergottites, trace element microdistributions in minerals of
the DaG Martian meteorites indicate that closed-system crystal fractionation
from a LREE-depleted parent magma dominated their crystallization history.
Furthermore, rare earth element abundances in the orthopyroxene megacrysts
are consistent with their origin as xenocrysts rather than phenocrysts.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Thermal history of the Ibitira noncumulate eucrite as inferred from pyroxene exsolution lamella: Evidence for reheating and rapid cooling
M. Miyamoto*, T. Mikouchi and K. Kaneda
*Correspondence author's address: Mineralogical Institute, Graduate School of Science,University of Tokyo, Hongo, Tokyo 113-0033, Japan; e-mail address: miyamoto@min.s.u-tokyo.ac.jp
Abstract–Ibitira is a strongly recrystallized and unbrecciated
noncumulate eucrite. We measured Ca compositional profiles of Ibitira
pyroxene by electron microprobe and computed the cooling rate and burial
depth from pyroxene exsolution profiles to gain information on early thermal
history of Ibitira. Pyroxene begins to exsolve at 1082 °C and
cools down to 550 °C at a rate of 0.02 °C/yr, forming an augite
lamella about 7.0 µm in width. A notable characteristic of
the Ca profile of augite lamellae in Ibitira pyroxene is a gradient near
the interface between augite and low-Ca pyroxene (pigeonite). This
profile suggests that after thermal metamorphism Ibitira pyroxene experienced
a sudden temperature rise to above solidus temperature of pyroxene (~1082
°C), and subsequent rapid cooling. The 39Ar-40Ar
age of 4.485 Ga for Ibitira, which is the oldest 39Ar-40Ar
ages for noncumulate eucrites, may date this reheating event.
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