Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Thermal decomposition pattern and particle size
estimation of iron minerals associated with Cretaceous-Tertiary boundary
at Gubbio
H. C. Verma*, Chandan Upadhyay, Amita Tripathi, R.P Tripathi and N. Bhandari
*Correspondence author's address: Department of Physics, Indian Institute of Technology, Kanpur 208016, India; e-mail address: hcverma@iitk.ac.in
Abstract–Mössbauer studies of the samples from the Cretaceous–Tertiary
Boundary (KTB) layer at Gubbio, Italy show that iron appears mainly in
two phases, magnetically ordered hematite and a paramagnetic silicate phase.
The average particle size of hematite is estimated to be in the range 16–27
nm from the Transmission Electron Micrographs and lack of Morin transition.
The hyperfine magnetic field at the iron nucleus is observed to be somewhat
less than that of bulk hematite, which may be explained by collective magnetic
excitation. Stepwise heating up to 1000 °C shows a decomposition
pattern of the paramagnetic phase, which suggests it to be a tri-octahedral
layer silicate. The iron-bearing phases found in the bulk sedimentary
KTB material are different from those found in the spherules separated
from this material indicating that the REDOX conditions changed rapidly
after the impact, becoming more oxidizing during the period these bulk
phases were formed.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Noble gas compositions of Antarctic micrometeorites
collected at the Dome Fuji Station in 1996 and 1997
Takahito Osawa* and Keisuke Nagao
*Correspondence author's address: Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan; e-mail address: osawa@eqchem.s.u-tokyo.ac.jp
Abstract–The noble gases He, Ne, Ar, Kr, and Xe were measured
in 27 individual Antarctic micrometeorites (AMMs) in the size range 60
to 250 µm that were collected at the Dome Fuji Station. Eleven
of the AMMs were collected in 1996 (F96 series) and sixteen were collected
in 1997 (F97 series). One of the F97 AMMs is a totally melted spherule,
whereas all other particles are irregular in shape. Noble gases were
extracted using a Nd-YAG continuous wave laser with an output power of
2.5–3.5W for about 5 minutes. Most particles released measurable
amounts of noble gases. 3He/4He ratios are
determined for twenty-six AMMs (0.85–9.65 × 10–4).
Solar energetic particles (SEP) are the dominant source of helium in most
AMMs rather than solar wind (SW) and cosmogenic He. Three samples
had higher 3He/4He ratios compared to that of the
solar wind, showing the presence of spallogenic 3He. The
Ne isotopic composition of most AMMs resembled that of SEP as in the case
of helium. Spallogenic 21Ne was detected in three samples,
two of which had extremely long cosmic-ray exposure ages (>100 Ma), calculated
by assuming SCR (solar cosmic rays) + GCR (galactic cosmic rays) production.
These two particles may have come to Earth directly from the Kuiper Belt.
Most AMMs had negligible amounts of cosmogenic 21Ne and exposure
ages of less than 1 Ma. 40Ar/36Ar ratios for
all particles (3.9–289) were lower than that of the terrestrial atmosphere
(296), indicating an extraterrestrial origin of part of the Ar with a very
low 40Ar/36Ar ratio plus some atmospheric contamination.
Indeed, 40Ar/36Ar ratios for the AMMs are higher
than solar wind, SEP, and Q-Ar values, which is explained by the presence
of atmospheric 40Ar. The average 38Ar/36Ar
ratio of twenty-four AMMs (0.194) is slightly higher than the value of
atmospheric or Q-Ar, suggesting the presence of SEP-Ar which has a relatively
high 38Ar/36Ar ratio. According to the elemental
compositions of the heavy noble gases, Dome Fuji AMMs can be classified
into three groups: chondritic (eight particles), air-affected (nine particles),
and solar-affected (eight particles). The eight AMMs classified as
chondritic preserve the heavy noble gas composition of primordial trapped
component due to lack of atmospheric adsorption and solar implantation.
The average of 129Xe/132Xe ratio for the sixteen
AMMs not affected by atmospheric contamination (1.05) corresponds to the
values in matrices of carbonaceous chondrites (~1.04). One AMM, F96DK038,
has high 129Xe/132Xe in excess of this ratio.
Our results imply that most Dome Fuji AMMs originally had chondritic heavy
noble gas compositions, and carbonaceous chondrite-like objects are appropriate
candidate sources for most AMMs.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
433 Eros: Problems with the meteorite
magnetism record in attempting an asteroid match
P. Wasilewski*, M. H. Acuña and G. Kletetschka
*Correspondence author's address: NASA Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, Maryland 20771, USA; e-mail address: u1pjw@lepvax.gsfc.nasa.gov
Abstract–The Magnetometer experiment (MAG) onboard the NEAR-Shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of (2.1 × 10–6 A m2 kg–1) for asteroid 433 Eros. This is in sharp contrast with the estimated magnetization of other S-class asteroids (Gaspra ~2.4 × 10–2 A m2 kg–1, Braille ~2.8 × 10–2 A m2 kg–1) and is below published values for all types of ordinary chondrites. This includes the L/LL types considered to most closely match 433 Eros based on preliminary interpretations of NEAR remote geochemical experiments.
The ordinary chondrite meteorite magnetization intensity data was reviewed
in order to assess the reasonableness of an asteroid - meteorite match
based on magnetic property measurements. NRM (natural remanent magnetization)
intensities for the ordinary chondrite meteorites show at least a 2 order
of magnitude range within each of the H,L,LL groups, all well above the
2.1 × 10–6 A m2 kg–1 level for 433
Eros. The REM values (ratio of the NRM to the SIRM (saturation remanent
magnetization)) range over 3 orders of magnitude for all chondrite groups
indicating no clear relationship between NRM and the amount of magnetic
material. Levels of magnetic noise in chondrite meteorites can be
as much as 70% or more of the NRM. Consequently published values
of the NRM should be considered suspect unless careful evaluation of the
noise sources is done. Goddard studies of per unit mass intensities
in large (> 10,000 gm) and small (down to <1 gm) samples from the same
meteorite demonstrate magnetic intensity decreases as size increases.
This would appear to be explained by demagnetization due to magnetic vector
randomness at unknown scale sizes in the larger samples. This would
then argue for some level of demagnetization of large objects such as an
asteroid. The possibility that 433 Eros is an LL chondrite cannot
be discounted.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Production of krypton and xenon isotopes
in thick stony and iron targets isotropically irradiated with 1600 MeV
protons
E. Gilabert* , B. Lavielle, R. Michel, I. Leya, S. Neumann and U. Herpers
*Correspondence author's address: LCNAB, UMR 5084, C.E.N. Bordeaux-Gradignan, BP 120, 33175 Gradignan, France; e-mail address: gilabert@cenbg.in2p3.fr
Abstract–Two spherical targets made of gabbro with a radius of
25 cm and of steel with a radius of 10 cm were irradiated isotropically
with 1.6 GeV protons at the SATURNE synchrotron at Laboratoire National
Saturne (LNS)/CEN Saclay, in order to simulate the production of nuclides
in meteorites induced by galactic cosmic-ray protons in space. These
experiments supply depth dependent production rate data for a wide range
of radioactive and stable isotopes in up to 28 target elements. In
this paper, we report results for 78Kr, 80-86Kr isotopes
in Rb, Sr, Y and Zr and for 124Xe, 126Xe, 128-132Xe,
134Xe, 136Xe isotopes in Ba and La. Krypton
and xenon concentrations have been measured at different depths in the
spheres by using conventional mass spectrometry. Based on Monte-Carlo
techniques, theoretical production rates are calculated by folding depth-dependent
spectra of primary and secondary protons and secondary neutrons with the
excitation functions of the relevant nuclear reactions. The comparison
of the model calculation results with experimental data in the thick target
experiments performed at LNS and previously at CERN have allowed adjustments
of the poorly known excitation functions of neutron-induced reactions.
Thus, for the two experiments at SATURNE, excellent agreement is obtained
between experimental and calculated production rates for most Kr and Xe
isotopes in all investigated target elements. Only Xe production
in Ba in the gabbro is underestimated by the calculations by about 25%.
This work validates the approach of the thin-target model calculations
of cosmogenic nuclide production rates in the attempt of modeling the interaction
of galactic cosmic-ray protons with stony and iron meteorites in space
as well as with lunar samples.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
The oxygen isotopic composition of water from
Tagish Lake: Its relationship to low temperature phases and to other carbonaceous
chondrites
L. Baker*, I. A. Franchi, I. P. Wright and C. T. Pillinger
*Correspondence author's address: Planetary and Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K.; e-mail address: l.baker@open.ac.uk
Abstract–The fall and recovery of the Tagish Lake meteorite in
British Columbia in January 2000 provided a unique opportunity to study
relatively pristine samples of carbonaceous chondrite material. Measurements
of the oxygen isotopic composition of water extracted under stepped pyrolysis
from a bulk sample of this meteorite have allowed us to make comparisons
with similar data obtained from CI and CM chondrites and so further investigate
any relationships that may exist between these meteorites. The much
lower yield of water bearing a terrestrial signature in Tagish Lake is
indicative of the pristine nature of the meteorite. The relationship
between the isotopic composition of this water and reported isotopic values
for carbonates, bulk matrix and whole rock, have been used to infer the
extent and conditions under which parent body aqueous alteration occurred.
In Tagish Lake the difference in 
17O
isotopic composition between the water and other phases is greater than
that found in either CMs or CIs suggesting that reaction and isotopic exchange
between components was more limited. This in turn suggests that in
the case of Tagish Lake conditions during the processes of aqueous alteration
on the parent body, which ultimately controlled the formation of new minerals,
were distinct from those on both CI and CM parent bodies.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
An experimental and petrographic investigation
of EET 79001 Lithology A: Implications for its petrogenesis and the
partitioning of Cr and V in a martian basalt
Christopher D. K. Herd*, Craig S. Schwandt, John H. Jones and James J. Papike
*Correspondence author's address: Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, Texas 77058, USA; e-mail address: herd@lpi.usra.edu
Abstract–A composition approximating the Lithology A groundmass
of the EET 79001 martian basalt (e.g., McSween and Jarosewich, 1983)
has been used to investigate the petrogenesis of the meteorite and the
behavior of Cr and V at different oxygen fugacities. Crystallization
experiments were carried out over a range of temperatures, and oxygen fugacities
of either IW or IW + 2 (i.e., 1.5 log units below QFM). Comparison
of trace element concentrations (obtained by SIMS analysis) in experimental
silicates with those of natural silicates supports the Fe-Ti oxide-derived
oxygen fugacity of QFM -1.8 ± 0.3 for this basalt (Herd et al.,
2001). Experimental distribution coefficients, in conjunction with
SIMS analyses of rims from the olivine and pyroxene xenocrysts in Lithology
A, as well as analyses of Lithology A groundmass pigeonite cores, are used
to calculate coexisting liquid concentrations of V and Cr. Liquid
compositions derived from pigeonite xenocryst rims and groundmass pigeonite
cores are similar, suggesting that the rims of orthopyroxene xenocrysts
are overgrowths, which have not previously been accounted for when reconstructing
the groundmass composition. This implies that the Eg composition
requires modification. A similar exercise for the ferroan rims on
olivine xenocrysts yields very different liquid compositions, indicating
that these rims are not overgrowths but are part of the xenocryst assemblage.
These results are shown to be consistent with the petrography of Lithology
A xenocrysts.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Aluminum-26 in H4 chondrites: Implications
for its production and its usefulness as a fine-scale chronometer for early-solar-system
events
Ernst Zinner and Christa Göpel
*Correspondence author's address: Laboratory for Space Sciences and the Physics Department, Washington University, St. Louis, Missouri 63130, USA; e-mail address: ekz@howdy.wustl.edu
Abstract–In order to investigate whether or not 26Al
can be used as a fine-scale chronometer for early-solar-system events we
measured, with an ion microprobe, Mg isotopes and Al/Mg ratios in separated
plagioclase, olivine, and pyroxene crystals from the H4 chondrites Ste.
Marguerite, Forest Vale, Beaver Creek and Quenggouk and compared the results
with the canonical 26Al/27Al ratio for Ca,Al-rich
inclusions (CAIs). For Ste. Marguerite (SM) and Forest Vale (FV)
Pb/Pb and Mn-Cr ages have previously been determined (Göpel et
al., 1994; Polnau et al., 2000; Polnau and Lugmair, 2001).
Plagioclase grains from these two meteorites show clear excesses of 26Mg.
The 26Al/27Al ratios inferred from these excesses
and from isotopically normal Mg in pyroxene and olivine are (2.87 ±
0.64) × 10–7 for SM and (1.52 ± 0.52) × 10–7
for FV. The differences between these ratios and the ratio of 5 ×
10–5 in CAIs indicate time differences of 5.4 ± 0.1 Ma
and 6.1 ± 0.2 Ma for SM and FV, respectively. These differences
are in agreement with the absolute Pb/Pb ages for CAIs and SM and FV phosphates
but there are large discrepancies between the U-Pb and Mn-Cr system for
the relative ages for CAIs, SM and FV. For example, Mn-Cr ages of
carbonates from Kaidun are older than the Pb/Pb age of CAIs. However,
even if we require that CAIs are older than these carbonates, the time
difference between this "adjusted" CAI age and the Mn-Cr ages of SM and
FV require that 26Al was widely distributed in the early solar
system at the time of CAI formation and was not mostly present in CAIs,
a feature of the X-wind model proposed by Shu and collaborators (Gounelle
et al., 2001; Shu et al., 2001). From this we conclude
that there was enough 26Al to melt small planetary bodies as
long as they formed within 2 Ma of CAIs, and that 26Al can serve
as a fine-scale chronometer for early solar system events.
Meteoritics & Planetary Science 37 (2002)
© Meteoritical Society, 2002. Printed in USA.
Pre-atmospheric depths and thermal histories
of Canyon Diablo spheroids
I. Leya, R. Wieler, P. Ma, C. Schnabel and G. F. Herzog*
*Correspondence author's address: Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854-8087, USA; e-mail address: herzog@rutchem.rutgers.edu
Abstract–Despite having melted during formation, seven of eight Canyon Diablo spheroids weighing from 0.6 to 13 mg retain cosmic-ray-produced 38Ar (38Arcos) in concentrations [10–10 cm3 STP/g] ranging from 0.35 to 68. The presence of 38Arcos is consistent with pre-atmospheric depths of less than 2.3 meters and most likely rules out an origin for the spheroids deep within the projectile, which had a radius of about 15 m. Low levels of 21Necos indicate gas loss from these spheroids. Relative to most Canyon Diablo meteorites, the spheroids contain lower concentrations of cosmogenic noble gases. The difference partly reflects diffusion losses from the spheroids, especially for 3He and 21Ne, but also suggests deeper locations on average for the precursor material, consistent with independent results from 59Ni.
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