Please contact the correspondence author for reprints of all published articles
A petrologic study of the IAB iron meteorites: Constraints on the formation of the IAB-Winonaite parent body
G. K. Benedix*, T. J. McCoy3, K. Keil and S. G. Love
*Correspondence author's address: Department of Geological Sciences, Virginia Tech, Blacksburg, Virginia 24061-0420, USA; e-mail address: gretchen.benedix@asu.edu
Abstract–We studied 26 IAB iron meteorites containing silicate-bearing inclusions to better constrain the many diverse hypotheses for the formation of this complex group. These meteorites contain inclusions that fall broadly into 5 types: (1) sulfide-rich, composed primarily of troilite and containing abundant embedded silicates; (2) non-chondritic, silicate-rich, comprised of basaltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate-rich, the most common type, with approximately chondritic mineralogy and most closely resembling the winonaites in composition and texture; (4) rounded, often graphite-rich assemblages that sometimes contain silicates; and (5) phosphate-bearing inclusions with phosphates generally found in contact with the metallic host. Similarities in mineralogy and mineral and oxygen isotopic compositions suggest that IAB irons and winonaites are from the same parent body.
We propose a hypothesis for the origin of IAB irons that combines some
aspects of previous formation models for these meteorites. We suggest
that the precursor parent body was chondritic, although unlike any known
chondrite group. Metamorphism, partial melting, and incomplete differentiation
(i.e., incomplete separation of melt from residue) produced metallic,
sulfide-rich and silicate partial melts (portions of which may have crystallized
prior to the mixing event), as well as metamorphosed chondritic materials
and residues. Catastrophic impact break-up and reassembly of the
debris while near the peak temperature mixed materials from various depths
into the re-accreted parent body. Thus, molten metal from depth was mixed
with near-surface silicate rock, resulting in the formation of silicate-rich
IAB irons and winonaites. Results of smoothed particle hydrodynamic model
calculations support the feasibility of such a mixing mechanism.
Not all of the metal melt bodies were mixed with silicate materials during
this impact/reaccretion event and these are now represented by silicate-free
IAB irons. Ages of silicate inclusions and winonaites of 4.40–4.54
Ga indicate this entire process occurred early in solar system history.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Chondrule formation by the ablation of small planetesimals
M. J. Genge
Author's address: Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, Great Britain; e-mail address: m.genge@nhm.ac.uk
Abstract–Numerous models have been proposed to explain the formation
of chondrules, but none can be reconciled with the highly diverse properties
of these objects. Here the formation of chondrules by the surface
melting and ablation of small planetesimals in nebula shock waves is investigated
using a numerical model. It is shown that bodies between ~1 mm and
500 m in diameter would have produced molten droplets by ablation during
gas drag in nebula shocks stronger than ~2.0 Ma. The properties of
chondrules produced by ablation are estimated by comparison with meteorite
fusion crusts and through consideration of the environment within the bow
shock envelope of ablating planetesimals. It is suggested that most
ablation chondrules will have broadly chondritic compositions with depletions
in siderophile and chalcophile elements and relatively high volatile contents
and textures that are mainly porphyritic. The formation of chondrules
by ablation of planetesimals in shock waves was probably most important
at a late stage in nebula history and occurred at the same time as chondrules
formed by the melting of dust particles. The high abundance of dust
particles relative to larger bodies at all stages of accretion implies
that only a proportion of chondrules may have been formed by ablation and
that genetic groups of chondrules with very different origins may coexist
in meteorites.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Australasian microtektites in the South China Sea and the West Philippine Sea: Implications for age, size and location of the impact crater
Meng-Yang Lee and Kuo-Yen Wei*
*Correspondence author's address: Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei, Taiwan, Republic of China; e-mail address: weiky@ms.cc.ntu.edu.tw
Abstract–Microtektites from two deep-sea cores in the South China
Sea and the West Philippine Sea are identified as belonging to the Australasian
tektite strewnfield, based on the morphology, chronostratigraphic occurrence
and geographical location of these microtektites. The higher concentrations
of microtektites (>1000/cm2) in the marginal seas of the western
Pacific with the peak concentration in the South China Sea support the
hypothesis of a large impact crater in Indochina. These two new occurrences
lead to a more precise dating of the impact event at 793 ka, while the
size of the Australasian source crater on the Indochina Peninsula is estimated
to be 90–116 km.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5–1.5 µm
Peter Hoppe*, Roger Strebel, Peter Eberhardt, Sachiko Amari and Roy S. Lewis
*Correspondence author's address: Max-Planck-Institut für Chemie, Abteilung Kosmochemie, Postfach 3060, D-55020 Mainz, Germany; e-mail address: hoppe@mpch-mainz.mpg.de
Abstract–We report isotopic abundances for C, N, Mg-Al, Si, Ca-Ti,
and Fe in 99 presolar silicon carbide grains of type X (84 grains from
this work and 15 grains from previous studies) from the Murchison CM2 meteorite,
ranging in size from 0.5 to 1.5 µm. Carbon was measured in
41 X grains, N in 37 grains, Mg-Al in 18 grains, Si in 87 grains, Ca-Ti
in 25 grains, and Fe in 8 grains. These X grains have 12C/13C
ratios between 18 and 6,800, 14N/15N ratios from
13 to 200, delta29Si/28Si between –750 and +60‰,
delta30Si/28Si from –770 to –10‰, and 54Fe/56Fe
ratios that are compatible with solar within the analytical uncertainties
of several 10 percent. Many X grains carry large amounts of radiogenic
26Mg (from the radioactive decay of 26Al, half-life
~ 7 × 105 years) and radiogenic 44Ca (from
the radioactive decay of 44Ti, half-life = 60 years).
While all X grains but one have radiogenic 26Mg, only about
20 percent of them have detectable amounts of radiogenic 44Ca.
Initial 26Al/27Al ratios of up to 0.36 and initial
44Ti/48Ti ratios of up to 0.56 can be inferred.
The isotopic data are compared with those expected from the potential stellar
sources of silicon carbide dust. Carbon stars, Wolf-Rayet stars,
and novae are ruled out as stellar sources of the X grains. The isotopic
compositions of C and Fe and abundances of extinct 44Ti are
well explained both by type Ia and type II supernova (SN) models.
The same holds for 26Al/27Al ratios except for the
highest 26Al/27Al ratios of >0.2 in some X grains.
Silicon agrees qualitatively with SN model predictions but the observed
29Si/30Si ratios in the X grains are in most cases
too high, pointing to deficiencies in the current understanding of the
production of Si in SN environments. The measured 14N/15N
ratios are lower than those expected from SN mixing models. This
problem can be overcome in a 15 M¤ type II SN if rotational
mixing and/or preferential trapping of N from 15N-rich regions
in the ejecta is considered. The isotopic characteristics of C, N,
Si and initial 26Al/27Al ratios in small X grains
are remarkably similar to those of large X grains (2–10 µm).
Titanium-44 concentrations are generally much higher in smaller grains,
indicative of the presence of Ti-bearing subgrains that might have served
as condensation nuclei for silicon carbide. The fraction of X grains
among presolar silicon carbide is largely independent of grain size.
This implies similar grain size distributions for silicon carbide from
carbon stars (mainstream grains) and supernovae (X grains), a surprising
conclusion in view of the different conditions for dust formation in these
two types of stellar sources.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Lunar surface exposure models for meteorites Elephant Moraine 96008 and Dar al Gani 262 from the Moon
Otto Eugster*, Ernst Polnau, Emma Salerno and Dario Terribilini
*Correspondence author's address: Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland; e-mail address: eugster@phim.unibe.ch
Abstract–We derived the cosmic-ray and solar particle exposure
history for the two lunar meteorites Elephant Moraine (EET) 96008 and Dar
al Gani (DAG) 262 based on the noble gas isotopic abundances including
the radionuclide 81Kr. For EET96008 we propose a model
for the exposure to cosmic rays and solar particles in three stages on
the Moon: an early stage about 500 Ma ago, lasting less than 9 Ma at a
shallow shielding depth of less than or equal to 20g/cm2, followed
by a stage when the material was buried without exposure until it was exposed
in a recent stage. This recent stage at a shielding depth in a range
of 200–600 g/cm2 lasted for about 26 Ma until ejection.
This model is essentially the same as that previously found for lunar meteorite
EET87521; thus, pairing of the two EET lunar meteorites that were recovered
on the same icefield in Antarctica is confirmed by our data. The
cosmic-ray produced isotopes, the trapped solar and lunar atmospheric noble
gases, as well as the radionuclide 81Kr observed for the DAG262
lunar meteorite are consistent with a one-stage lunar exposure history.
The average burial depth of the DAG material before ejection was within
a range of 50–80g/cm2. The exposure to cosmic rays at
this depth lasted 500–1000 Ma. This long residence time for DAG at
relatively shallow depth explains the high concentrations of implanted
solar noble gases.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Experimental constraints on alkali condensation in chondrule formation
P. Georges*, G. Libourel and E. Deloule
*Correspondence author's address: CRPG-CNRS, BP20, F-54501 Vandoeuvre les Nancy, France; e-mail address: pgeorges@crpg.cnrs-nancy.fr
Abstract–To assess whether the alkali behavior observed in chondrules
of primitive meteorites is due to volatilization from the raw materials
of chondrules during chondrule formation events or due to condensation
processes from the nebular gas, we set up a new experimental device able
to expose silicate melt samples to a controlled alkali partial pressure
at high temperature under fixed oxygen fugacity. Using a mixture of potassium
carbonate (K2CO3) and graphite (C) as the source
of the potassium gas (Kg), we studied the condensation kinetics
of potassium and its solubility in CaO–MgO–Al2O3–SiO2
silicate melts, according to the reaction 2 K (g) + 1/2 O2
(g) = K2O (melt). From these results, we show
that alkali entering in chondrules from the nebular gas is a viable mechanism
to explain the chondrules alkali contents and their delta 41K
isotopic signatures, at time scales relevant to chondrule formation. Finally,
we also suggest that chondrules may have formed in non-canonical nebular
environments and that the flash heating scenario is not a prerequisite
to chondrule formation.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Radial transpression ridges: A new structural feature of complex impact craters
Thomas Kenkmann* and Ilka von Dalwigk
*Correspondence author's address: Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany; e-mail address: thomas.kenkmann@rz.hu-berlin.de
Abstract–The gravity-driven collapse of complex impact craters
induces the mass transfer of large rock volumes. In distal parts
of a crater inward movements dominate, whereas in the centre of an impact
structure outward movements occur if the central uplift collapses.
The particle trajectory field is centro-symmetric, which signifies that
the conditions for plane-strain deformation are not fullfilled. Converging
particle trajectories can be compensated either by a bulk thickening of
inward sliding masses (folding, repetition of rock units along thrust faults,
plastic flow) or by the formation of localised radial transpression ridges
(RTR) at the edges of individual landslides. In these transpression
ridges material is uplifted to accommodate the converging mass flow.
Different modes of uplift are possible including radial folding, lateral
overthrusting and the formation of positive flower structures. A
simple geometric model is used to estimate the amount of transpression
thickening and bulk thickening of inward sliding masses on the base of
volumetric considerations. The existence of RTRs is confirmed by
structural investigations at the Siljan impact structure, Sweden, as well
as at other complex impact craters on Earth.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Bulk density of ordinary chondrite meteorites and implications for asteroidal internal structure
Sarah L. Wilkison* and Mark S. Robinson
*Correspondence author's address: Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208, USA; e-mail address: sarah@earth.nwu.edu
Abstract–The bulk densities of 82 samples of 72 ordinary chondrites
(OCs) were measured to an accuracy of ~1% using a modified Archimedian
method. We found the average bulk density to be 3.44 ± 0.19
g/cm3 for the H group, 3.40 ± 0.15 g/cm3 for
the L group, and 3.29 ± 0.17 g/cm3 for the LL group (±
represent one sigma). Bulk density measurements of eleven pieces of one
fall, Pultusk (H group), were also found to vary considerably (3.31 to
3.63 g/cm3). To investigate controls on bulk density within
the OCs, we compared density with bulk chemical composition (the ratio
of iron metal to total iron, the ratio of total iron to SiO2,
the ratio of FeO to total iron, and MgO). Within each OC group, bulk
chemical composition is nearly invariant while bulk density varies from
~3.0 to 3.8 g/cm3. Slight but systematic differences in
average density between the H, L, LL groups presumably relate to differences
in metallic iron abundance. However, considerable overlap between
OC groups and the wide range of bulk densities within each group suggest
differences in porosity dominantly control variations of density within
the OC subgroups.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Exposure history of the Mocs (L6) chondrite: A study of strewn field samples
T. E. Ferko, L. Schultz, L. Franke, D. D. Bogard, D. H. Garrison, R. Hutchison and M. E. Lipschutz*
*Corresponding author's address: Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA; e-mail address: rnaapuml@purdue.edu
Abstract–We measured cosmogenic radionuclides (10Be,
26Al, 36Cl) and noble gases (He, Ne and Ar) in 10
specimens of the L6 chondrite Mocs to determine the exposure history and
pre-atmospheric relationship among fragments from known locations in the
strewn field. Cosmogenic noble gas contents alone are consistent
with a simple irradiation exposure of 15.2 Ma. However, Mocs has
very low 22Ne/21Ne ratios indicative of deep burial
in a large meteoroid, but radionuclide levels at saturation values typical
for much smaller meteoroids: this paradox suggests a possible complex exposure.
For the latter case, we propose a two-stage exposure history in which Mocs
initially was buried deeply in a large object for 110 Ma followed by exposure
in a 65 cm object for 10.5 Ma. Relative shielding was inferred from
the measured 22Ne/21Ne ratios assuming constant 22Ne/21Ne
production for all samples during the first stage. These shielding
levels, which are supported by estimates based on 36Cl production
by neutron capture, indicate a possible relationship between depth of samples
in the Mocs meteoroid and fall location in the strewn field.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Chicxulub impactites: Geochemical clues to the precursor rocks
Bianca Kettrup*, Alexander Deutsch, Markus Ostermann and Pierre Agrinier
*Correspondence author's address: Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany; e-mail address: kettrub@uni-muenster.de
Abstract–The 65 Ma Chicxulub impact structure, Mexico, with a
diameter of about 180 km is in the focus of geoscientific research because
of it's link to the mass extinction event at the Cretaceous–Tertiary (K/T)
boundary. Chicxulub, now buried beneath thick post-impact sediments
is most probably one of the best preserved terrestrial impact structures
known. Because of it's in-accessibility, only limited samples on
the impact lithologies from a few drill cores are available. We report
major element, and Sr, Nd, O, and C isotope data for Chicxulub impact melt
lithologies and basement clasts in impact breccias of drill cores C-1,
and Y-6, and for melt particles in the Chicxulub ejecta horizon at the
K/T boundary in Beloc, Haiti. The melt lithologies with SiO2
ranging from 58 to about 63 wt% show significant variations in the content
of aluminum, calcium, and the alkalies. In the melt matrix samples,
delta13C of the calcite is about –3‰. The delta18O
values for the siliceous melt matrices of Y-6 samples range from 9.9. to
12.4‰. Melt lithologies and the black Haitian glass have rather uniform
87Sr/86Sr ratios (0.7079 to 0.7094); only one lithic
fragment displays 87Sr/86Sr of 0.7141. The
Sr model ages TSrUR for most lithologies range from
830 to 1833 Ma; unrealistic negative model ages point to an open Rb-Sr
system with loss of Rb in a hydrothermal process. The 143Nd/144Nd
ratios for all samples, except one basement clast with 143Nd/144Nd
of 0.5121, cluster at 0.5123 to 0.5124. In an epsilonNd-epsilonSr-diagram,
impactites plot in a field delimited by epsilonNd of –2 to –6,
and epsilonSr of 55 to 69. This field is not defined by
the basement lithologies described to occur as lithic clasts in impact
breccias and Cretaceous sediments. At least one additional intermediate
to mafic pre-cursor component is required to explain the data.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Oxygen-isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia
Byeon-Gak Choi, Alexander N. Krot and John T. Wasson*
*Correspondence author's address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA; e-mail address: jtwasson@ucla.edu
Abstract–We report in situ measurements of oxygen-isotope
compositions of magnetite and primary and secondary olivine in the highly
unequilibrated oxidized CV chondrites Kaba and Mokoia. In both meteorites
the magnetite and the secondary olivine (fayalite, Fa90–100)
have O-isotope compositions near the terrestrial fractionation (TF) line;
the mean delta 17O (= delta 17O-0.52 delta 18O)
value is approximately –1‰. In contrast, the compositions of nearby
primary (chondrule), low-FeO olivines (Fa1–2) are well below
the TF line; delta 17O values range from –3‰ to –9‰. Krot
et al. (1998) summarized evidence indicating that the secondary
phases in these chondrites formed by aqueous alteration in an asteroidal
setting. The compositions of magnetite and fayalite in Kaba and Mokoia
imply that the oxygen isotopic composition of the oxidant was near or somewhat
above the TF line. In Mokoia the fayalite and magnetite differ in
delta 18O by 20‰, whereas these same materials in Kaba have
virtually identical compositions. The difference between Mokoia magnetite
and fayalite may indicate formation in isotopic equilibrium in a water-rich
environment at low temperatures, ~300 K. In contrast, the similar
compositions of these phases in Kaba may indicate formation of the fayalite
by replacement of preexisting magnetite in dry environment, with the O
coming entirely from the precursor magnetite and silica. The delta
17O of the oxidant incorporated into the CV parent body (as
phyllosilicates or H2O) appears to have been much (7–8‰) lower
than that in that incorporated into the LL parent body (Choi et al.,
1998), suggesting that the O-isotopic composition of the nebular gas was
spatially or temporally variable.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Ferrous silicate spherules with euhedral Iron,nickel-metal grains from CH carbonaceous chondrites: Evidence for supercooling and condensation under oxidizing conditions
A. N. Krot*, A. Meibom, M. I. Petaev, K. Keil, M. E. Zolensky, A. Saito, M. Mukai and K. Ohsumi
*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, Hawai'i 96822, USA; e-mail address: sasha@pgd.hawaii.edu
Abstract–CH carbonaceous chondrites contain a population of ferrous
[Fe/(Fe+Mg) ~ 0.1–0.4] silicate spherules (chondrules), ~15–30 µm
in apparent diameter, composed of cryptocrystalline olivine-pyroxene normative
material, ±SiO2-rich glass and rounded-to-euhedral Fe,Ni-metal
grains. The silicate portions of the spherules are highly depleted
in refractory lithophile elements (CaO, Al2O3, and
TiO2 <0.04 wt%) and enriched in FeO, MnO, Cr2O3
and Na2O relative to the dominant, volatile-poor, magnesian
chondrules from CH chondrites. The Fe/(Fe+Mg) ratio in the silicate
portions of the spherules is positively correlated with Fe concentration
in metal grains, suggesting that this correlation is not due to oxidation/reduction
of iron (FeOsil?Femet) during melting of metal-silicate solid precursors.
Rather, we suggest that this is a condensation signature of the precursors
formed under oxidizing conditions. Each metal grain is compositionally
uniform, but there are significant intergrain compositional variations:
~8–18 wt% Ni, <0.09 wt% Cr and sub-solar Co/Ni ratio. The precursor
materials of these spherules were thus characterized by extreme elemental
fractionations, which have not been observed in chondritic materials before.
Particularly striking is the fractionation of Ni and Co in the rounded-to-euhedral
metal grains, which has resulted in a Co/Ni ratio significantly below solar.
The liquidus temperatures of the euhedral Fe,Ni-metal grains are lower
than those of the coexisting ferrous silicates and we infer that the former
crystallized in supercooled silicate melts. The metal grains are
compositionally metastable; they are not decomposed into taenite and kamacite
suggesting fast postcrystallization cooling at temperatures below 970 K
and lack of subsequent prolonged thermal metamorphism at temperatures above
400–500 K.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Image-intensified video results from the 1998 Leonid shower: I. Atmospheric trajectories and physical structure
M. D. Campbell*, P. G. Brown, A.G. LeBlanc, R. L. Hawkes, J. Jones, S. P. Worden and R. R. Correll
*Correspondence author's address: Department of Physics and Astronomy University of Western Ontario, London, Ontario N6A 3K7, Canada; e-mail address: mdcampbe@julian.uwo.ca
Abstract–Two station electro-optical observations of the 1998
Leonid shower are presented. Precise heights and light curves were
obtained for 79 Leonids which ranged in brightness (at maximum luminosity)
from +0.3 to +6.1 astronomical magnitude. The mean photometric mass
of the data sample was 1.4 × 10–6 kg. The dependence
of astronomical magnitude at peak luminosity on photometric mass and zenith
angle was consistent with earlier studies of faint sporadic meteors.
For example, a Leonid meteoroid with a photometric mass of about 1.0 ×
10–7 kg corresponds to a peak meteor luminosity of about +4.5
astronomical magnitudes. The mean beginning height of
the Leonid meteors in this sample was 112.6 km and the mean ending height
was 95.3 km. The highest beginning height observed was 144.3 km.
There is relatively little dependence of either the first or last heights
on mass, which is indicative of meteoroids which have clustered into constituent
grains prior to the onset of intensive grain ablation. The height
distribution, combined with numerical modelling of the ablation of the
meteoroids, suggests that silicate-like materials are not the principle
component of Leonid meteoroids, and hints at the presence of a more volatile
component. Light curves of many Leonids were examined for evidence
of the physical structure of the associated meteoroids: like the 1997 Leonids,
the narrow, nearly symmetric curves imply that the meteoroids are not solid
objects. The light curves are consistent with a dustball structure.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Correlation between crystallographic structure and infrared spectra of SiO films containing iron or magnesium atoms
N. Suzuki*, S. Kimura, T. Nakada, C. Kaito, Y. Saito and C. Koike
*Correspondence author's address: Department of Nanophysics in Frontier Project, Ritsumeikan University, Kusatsu-shi, Shiga 525-8577, Japan; e-mail address: kaito@se.ritsumei.ac.jp
Abstract–Films condensed from vapors containing SiO, Fe, or Mg
showed an amorphous structure. Infrared (IR) spectra and electron
microscopic characterization have been carried out on these films.
After the heat treatment of these films in air, IR peaks at about 18–23
µm appeared, in addition to peaks attributable to SiO2.
These peaks can be attributed to metallic oxides such as FeO, Fe2O3
and MgO. It can be concluded that Fe- or Mg-bearing silicate minerals
cannot be produced by the rapid cooling of SiO, Fe or Mg vapors.
Although IR spectra of FeO have been discussed in order to match some spectra
obtained with the Infrared Space Observatory (ISO), the identification
of FeO as the impurity would be very important because the IR spectra of
FeO grains are very dependent on the shape and size of the grains.
These impurities can also influence the IR spectral feature of SiO2.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Experimental generation of shock-induced pseudotachylites along lithological interfaces
T. Kenkmann*, U. Hornemann and D. Stöffler
*Correspondence author's address: Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität Berlin, Invalidenstraße 43, D-10115 Berlin, Germany; e-mail address: thomas.kenkmann@rz.hu-berlin.de
Abstract–To understand the mechanism of formation of shock-induced
pseudotachylites, particularly the role, that rock heterogeneities and
interfaces play for their formation, shock recovery experiments were carried
out on samples consisting of two distinct lithologies (dunite, quartzite).
It was possible to generate melt veins of 1–6 µm width along lithological
interfaces at moderate shock pressures (6 to 34 GPa). The magnitudes
of displacement along the interface, strain rate and the kinetic heat production
indicate that friction is an important heat source which largely contributes
to the energy budget of the melt veins. The experimentally produced
veins resemble natural S-type pseudotachylites. The geometry of the
veins depends on the orientation of the interface with respect to the shock
front and includes: strong variations in thickness, formation of melt pockets
and injection veins, sudden changes in vein orientation, and sharp vein
margins. Two types of melt were observed: dense, vesicle-free melt
rock is likely to represent high-pressure melts. Vesicular melts
were also generated during shock compression, but they remained in a molten
state during pressure release and continued shearing. Intermingling
of comminuted olivine and melt suggests that ultracataclasis of olivine
induced by a dynamic tensile failure is a precursor stage to frictional
melting. Shock wave interferences at the lithological interface provide
the necessary stress conditions to start dynamic failure of olivine.
The composition of the frictional melts ranges from olivine-normative to
enstatite-normative and is, thus, largely determined by olivine melting.
The validity of the sequence of friction melting susceptibilities of rock-forming
minerals inferred from tectonically produced pseudotachylites is confirmed
and can now be applied to ultra-high strain rates during shock compression.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Iron-rich aureoles in the CM carbonaceous chondrites, Murray, Murchison and Allan Hills 81002: Evidence for in situ aqueous alteration
Nicolaus P. Hanowski and Adrian J. Brearley*
*Correspondence author's address: Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA; e-mail address: brearley@unm.edu
Abstract–Iron-rich aureoles in CM carbonaceous chondrites are
previously unidentified domains of aqueously altered matrix material whose
FeO content may exceed that of the surrounding matrix by up to more than
15 wt%. We describe the petrography and mineralogy of these objects
in the CM chondrites Murray, Murchison and ALH 81002. The size of
Fe-rich aureoles ranges from a few hundred microns to several millimeters
in diameter and appears to be a function of the degree of alteration of
the host chondrite. The origin of Fe-rich aureoles is related
to the alteration of large metal grains which has resulted in the formation
of characteristic PCP-rich reaction products that are frequently observed
at the centers of the aureoles. This suggests that Fe-rich aureoles
in CM chondrites are the result of the mobilization of Fe from altering
metal grains into the matrix. The fact that Fe-rich aureoles enclose
numerous chondritic components such as chondrules, CAIs and mineral fragments,
as well as their radial symmetric appearance, are strong evidence that
they formed in situ and that significant directional fluid flow
was not involved in the alteration process. This and additional constraints,
such as the distribution of S and other elements, as well as the inferred
alteration conditions, are consistent with in situ parent body alteration.
The observations are, however, entirely incompatible with preaccretionary
alteration models in which the individual CM chondrite components have
experienced diverse alteration histories. The presence of numerous
intact aureoles in the brecciated CM chondrites Murray and Murchison further
suggests that the alteration occurred largely after brecciation affected
these meteorites. Therefore, the progressive aqueous alteration of
CM chondrites may not be necessarily coupled to brecciation as has been
previously proposed.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Source regions and timescales for the delivery of water to Earth
A. Morbidelli*, J. Chambers, J. I. Lunine, J.M. Petit, F. Robert, G.B. Valsecchi and K. E. Cyr
*Correspondence author's address: Observatoire de la Cote d'Azur, Nice, France; e-mail address: morby@obs-nice.fr
Abstract–In the primordial Solar System the most plausible sources
of the water accreted by the Earth were in the outer asteroid belt, in
the giant planet regions and in the Kuiper belt. We investigate the
implications on the origin of Earth's water of dynamical models of primordial
evolution of solar system bodies and check them with respect to chemical
constraints. We find that it is plausible that the Earth accreted
water all along its formation, from the early phases when the solar nebula
was still present to the late stages of gas-free sweepup of scattered planetesimals.
Asteroids and the comets from the Jupiter–Saturn region were the first
water deliverers, when the Earth was less than half its present mass.
The bulk of the water presently on Earth was carried by a few planetary
embryos, originally formed in the outer asteroid belt and accreted by the
Earth at the final stage of its formation. Finally, a late veneer,
accounting for at most 10\% of the present water mass, occurred due to
comets from the Uranus–Neptune region and from the Kuiper belt. The
net result of accretion from these several reservoirs is that the water
on Earth had essentially the D/H ratio typical of the water condensed in
the outer asteroid belt. This is in agreement with the observation
that the D/H ratio in the oceans is very close to the mean value of the
D/H ratio of the water inclusions in carbonaceous chondrites.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
The South African polymict eucrite Macibini
P. C. Buchanan*, D. J. Lindstrom, D. W. Mittlefehldt, C. Koeberl and W. U. Reimold
*Correspondence author's address: Department of Geology, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa; e-mail address: 065pcb@cosmos.wits.ac.za
Abstract–The polymict eucrite Macibini is a fragmental breccia predominantly composed of eucritic materials with minor proportions (maximum 2 vol%) of diogenitic material. Hence, it is intermediate between the Y74159-type polymict eucrites, which contain negligible amounts of magnesian orthopyroxene, and the howardites. The present study provides mineralogical and bulk compositional data for the meteorite breccia and for six clasts. These clasts include both volcanic/igneous rocks and a variety of impact-generated rocks. A broad range of degrees of post-crystallization metamorphism affected these materials before the final aggregation of the breccia.
Clast A is a fragment of unequilibrated eucrite with subophitic texture. The edges of the zoned pyroxenes in this clast are composed of a host of Fe-rich augite containing vermicules (blebs) and lamellae composed of a mixture of Fe-rich olivine and silica. Similar features occur as fragments in lunar breccias and are attributed by some workers to the breakdown of pyroxferroite, an Fe-rich pyroxenoid. However, textures and compositions of these augite-olivine-silica intergrowths in clast A suggest that, in this case, they are the result of decomposition in a series of steps of Fe-rich subcalcic augite.
Among the fragments of impact-generated material in Macibini is clast
2, an earlier-formed clastic breccia that was lithified before being broken
apart and included in the meteorite breccia. Clast 3 is an impact
melt breccia that is composed of rock and mineral fragments in a devitrified
groundmass. Clast C is also an impact melt breccia that has a coarser-grained,
hornfelsic groundmass that resulted from extensive metamorphism after formation.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
A petrographic, chemical, and isotopic study of calcium-aluminum-rich inclusions and aluminum-rich chondrules from the Axtell (CV3) chondrite
G. Srinivasan, G. R. Huss* and G. J. Wasserburg
*Correspondence author's address: Department of Geology and Center for Meteorite Studies, Arizona State University, P. O. Box 871404, Tempe, Arizona 85297-1404, USA; e-mail address: gary.huss@asu.edu
Abstract–Petrographic, compositional, and isotopic characteristics were studied for three Ca-Al inclusions (CAIs) and four plagioclase-bearing chondrules (three of them Al-rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial 26Al/27Al ratios ((26Al/27Al)0) ranging from ~5 × 10–5 to <1.1 × 10–5, and plagioclase-bearing chondrules have (26Al/27Al)0 of ~3 × 10–6 and lower. One type A CAI has the characteristics of a FUN inclusion. The Al-Mg data imply that the plagioclase-bearing chondrules began to form >2 million years after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (<600 °C), probably not enough to disturb the Al-Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the sun via gas drag for several million years before final accretion.
We also examined the compositional relationships among the four plagioclase-bearing
chondrules (two with large anorthite laths and two barred-olivine chondrules)
and between the chondrules and CAIs. Three processes were examined:
1) igneous differentiation, 2) assimilation of a CAI by average nebular
material, and 3) evaporation of volatile elements from average nebular
material. We find no evidence that igneous differentiation played
a role in producing the chondrule compositions, although the barred olivine
compositions can be related by addition or subtraction of olivine.
Methods 2 and 3 could have produced the composition of one chondrule, AXCH-1471,
but neither process explains the other compositions. Our study indicates
that plagioclase-bearing objects originated through a variety of processes.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Laser argon-40-argon-39 age studies of Dar al Gani 262 lunar meteorite
Vera A. Fernandes*, Ray Burgess and Grenville Turner
*Correspondence author's address: Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.; e-mail address: vfernandes@fs1.ge.man.ac.uk
Abstract–The laser 40Ar-39Ar dating technique
has been applied to Dar Al Gani 262 lunar meteorite (DAG 262), a polymict
highland regolith breccia, to determine the crystallisation age and timing
of shock events experienced by this meteorite. Laser stepped heating
analyses of three dominantly feldspathic fragments (DAG-1, DAG-2 and DAG-3)
revealed the presence of trapped argon mostly released at intermediate
and high temperatures with a 40Ar/36Ar value ~2.8.
Trapped argon is most likely released from melt glass present as small
veins within the fragments. The 40Ar-39Ar ages
determined for the three fragments are ~3.0 Ga for DAG-1 and DAG-2 and
2.0 Ga for DAG-3 and probably relate to major impact events. Laser spot
analyses were performed on a feldspathic clast, an impact crystalline melt
basalt (ICMB) and the matrix in a polished section of DAG 262. The
feldspathic and ICMB clasts have low contents of trapped Ar compared with
that in the matrix. The feldspathic clast shows a wide range of ages from
3.0–1.7 Ga similar to those obtained by stepped heating. The younger age
is interpreted as a minimum age for the last major event that assembled
this meteorite. The ICMB shows two age clusters at 3.37 Ga. and 3.07
Ga, where the older age may be that of the impact event that formed the
impact melt. Several cosmic-ray exposure (CRE) ages were obtained
as expected for a polymict regolith breccia. The CRE ages are 106
Ma and 141 Ma for the feldspathic clast and the ICMB, respectively. One
of the feldspathic fragments DAG-2 shows a range between 200–400 Ma.
These CRE ages, which are similar to those determined for returned samples
of the lunar regolith, indicate that the different components of DAG 262
experienced pre-exposure prior to assemblage of the meteorite.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Evidence for low-temperature growth of fayalite and hedenbergite in MacAlpine Hills 88107, an ungrouped carbonaceous chondrite related to the CM-CO clan
pages 000–000
Alexander N. Krot*, Adrian J. Brearley, Michael I. Petaev, Gregory W. Kallemeyn, Derek W. G. Sears, Paul H. Benoit, Ian D. Hutcheon, Michael E. Zolensky 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, Hawai'i 96822, USA; e-mail address: sasha@pgd.hawaii.edu
Abstract–The carbonaceous chondrite MacAlpine Hills (MAC) 88107 has bulk composition and mineralogy that are intermediate between those of CO and CM chondrites. This meteorite experienced minor alteration and a low degree of thermal metamorphism (petrologic type 3.1) and escaped post-accretional brecciation. The alteration resulted in the formation of fayalite (Fa90–100), Al-free hedenbergite (~Fs50Wo50), phyllosilicates (saponite-serpentine intergrowths), magnetite, and Ni-bearing sulfides (pyrrhotite and pentlandite). Fayalite and hedenbergite typically occur as veins, which start at the opaque nodules in the chondrule peripheries, crosscut fine-grained rims and either terminate at the boundaries with the neighboring fine-grained rims or continue as layers between these rims. These observations suggest that fayalite and hedenbergite crystallized after accretion and compaction of the fine-grained rims. Fayalite also overgrows isolated forsteritic (Fa1–5) and fayalitic (Fa20–40) olivine grains without any evidence for Fe-Mg interdiffusion; it also replaces massive magnetite-sulfide grains. The initial 53Mn/55Mn ratio of (1.58 ± 0.26) ×10–6 in the MAC 88107 fayalite corresponds to an age difference between the formation of fayalite and refractory inclusions in Allende of either ~9 or 18 Ma, depending upon the value of the solar system initial abundance of 53Mn used in age calculations.
Formation of secondary fayalite and hedenbergite requires mobilization
and transport of Ca, Si, and Fe either through a high-temperature gaseous
phase (Hua and Buseck, 1995) or low-temperature aqueous solution (Krot
et al., 1998a,b). The high-temperature nebular model for the origin
of fayalite (Hua and Buseck, 1995) fails to explain (a) formation of fayalite-hedenbergite
assemblages after accretion of fine-grained rims that lack any evidence
for high-temperature processing; (b) extreme fractionation of refractory
lithophile elements of similar volatility, Ca and Al, in hedenbergite;
and (c) absence of Fe-Mg interdiffusion along fayalite-forsterite boundaries.
We conclude that fayalite and hedenbergite in MAC 88107 formed during late-stage,
low-temperature (approximately 150–200 °C) aqueous alteration.
The data for MAC 88107 extend the evidence for an early onset of aqueous
activity on chondrite parent bodies and reinforce the conclusion that liquid
water played an important role in the chemical and mineralogical evolution
of the first chondritic planetesimals.
Meteoritics & Planetary Science 35 (2000)
© Meteoritical Society, 2000. Printed in USA.
Aqueous alteration without a pronounced oxygen-isotopic shift: Implications for the asteroidal processing of chondritic materials
P. A. Bland*, M. R. Lee, A. S. Sexton, I. A. Franchi, A. E. T. Fallick, M. F. Miller, J. M. Cadogan, F. J. Berry and C. T. Pillinger
*Correspondence author's address: Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, U.K.; email address: phib@nhm.ac.uk
Abstract–Primitive meteorites exhibit certain features that are consistent with aqueous and thermal alteration on asteroids, but O-isotopic analyses show only a modest heavy-isotope shift, interpreted as indicating modification in the nebula. To understand the isotopic effects of asteroidal alteration, we take the L-group ordinary chondrites weathered in Antarctica as an analogue. The data show that alteration is a two-stage process, with an initial phase producing only a negligible isotopic effect. Although surprising, a possible explanation is found when we consider the alteration of terrestrial silicates. Numerous studies report pervasive development of channels a few to a few tens of nanometer wide in the incipient alteration of silicates. We observe a similar texture. Alteration involves a restructuring of clay minerals along these narrow channels, in which access of water is restricted. The clay shows a topotactic relationship to the primary grain, which suggests either epitaxial growth of the clay using the silicate as a substrate or inheritance of the original O structure by the clay. Our data suggests the latter: with extensive inheritance of structural polymers by the weathering product, the bulk O-isotopic composition is comparatively unaffected. This offers an explanation for the lack of an isotopic effect in the weathering of the L chondrites. If substantial modification of chondritic materials may occur without a pronounced isotopic effect, it also reconciles existing O analyses of CV chondrites with an asteroidal model of aqueous alteration.
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