Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Bulk chemistry of Saharan shergottite Dar al Gani 476
J. A. Barrat*, J. Blichert-Toft, R. W. Nesbitt and F. Keller
*Correspondence author's address: Université d'Angers, Faculté des Sciences, 2 bd Lavoisier, 49045 Angers Cedex, France; e-mail address: barrat@univ-angers.fr
Abstract–We report on major and trace element analyses obtained
by, respectively, ICP-AES and ICP-MS of three different aliquots of the
new Saharan shergottite Dar al Gani 476 (DaG476). The new analyses
are in excellent agreement with previous data (Zipfel et al., 2000).
Ba, Sr and U abundances, together with the presence of carbonate, suggest
that the sample has been significantly weathered. Three rare earth
element (REE) patterns (normalized to CI) determined on three different
aliquots of the sample all show similar shapes. The heavy REEs are
flat with a slight depletion at the heavy end and a strong depletion from
Dy to Pr. All of the patterns display an upturn to La which we interpret
as being caused by the introduction of a terrestrial component. Taking
the terrestrial contamination into account, this study demonstrates that
DaG476 is one of the most depleted of the shergottites, and, just like
QUE94201 (Dreibus et al., 1996), displays very low Zr/Hf ratios.
It appears that the Zr/Hf ratios of shergottites are not uniform, and have
been significantly fractionated by martian mantle processes.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The optical properties of the finest fraction of lunar soil: Implications for space weathering
Sarah K. Noble*, Carlé M. Pieters, Lawrence A. Taylor, Richard V. Morris, Carlton C. Allen, David S. McKay and Lindsay P. Keller
*Correspondence author's address: Brown University, Providence, Rhode Island 02912, USA; e-mail address: noble@porter.geo.brown.edu
Abstract–The fine fraction of lunar soils (<45 µm) dominates
the optical properties of the bulk soil. Definite trends can be seen
in optical properties of size separates with decreasing particle size:
diminished spectral contrast and a steeper continuum slope. These
trends are related to space weathering processes and their affects on different
size fractions. The finest fraction (defined here as the <10 µm
fraction) appears to be enriched in weathering products relative to the
larger size fractions, as would be expected for surface correlated processes.
This <10 µm fraction tends to exhibit very little spectral contrast,
often with no distinguishable ferrous iron absorption bands. Additionally,
the finest fractions of highland soils are observed to have very different
spectral properties than the equivalent fraction of mare soils when compared
with larger size fractions. The spectra of the finest fraction of
feldspathic soils flatten at longer wavelengths, whereas those of the finest
fraction of basaltic soils continue to increase in a steep, almost linear
fashion. This compositional distinction is due to differences in
the total amount of nanophase iron that accumulates in space weathering
products. Such ground-truth information derived from the <10 µm
fraction of lunar soils provides valuable insight into optical properties
to be expected in other space weathering environments such as the asteroids
and Mercury.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Petrogenesis of Allan Hills 84001: Constraints from impact-melted feldspathic and silica glasses
James P. Greenwood* and Harry Y. McSween, Jr.
*Correspondence author's address: Department of Earth and Environmental Sciences,Wesleyan University, Middletown, Connecticut 06457, USA; e-mail address: jgreenwood@mail.wesleyan.edu
Abstract–Compositional and textural relationships of shock-melted glasses in the ALH84001 meteorite have been examined by optical microscopy, electron microprobe analysis, and compositional mapping. The feldspathic and silica glasses exhibit features which constrain the relative timing of shock events and carbonate deposition in ALH84001. The feldspathic glasses are stoichiometric and have compositions plausibly described as forming from igneous plagioclase (An27–39Ab58–68Or3–7) or sanidine (Or51Ab46An3), or from a mixture of these phases (mixed-feldspar glasses). These observations argue against prior interpretations of feldspathic glasses as unflowed maskelynite, hydrothermal precipitates or alteration products, or shock melts that have undergone alkali volatilization. Carbonate was deposited around previously formed mixed-feldspar glass clasts, suggesting that carbonate deposition occurred after the shock event that formed the granular bands (crushed zones) in this meteorite. SiO2-rich glasses appear to be silica remobilized during shock, with little addition of other material.
A petrogenetic history of ALH84001 consistent with the observations
of feldspathic and silica glasses is: 1) igneous crystallization and cumulate
formation; 2) a pre-carbonate shock event that formed the granular bands
(crushed zones) and sheared chromites, and melted igneous plagioclase and
sanidine to form mixed-feldspar glasses; 3) carbonate and silica deposition
in the granular bands (veining of plagioclase glasses by SiO2
and deposition of carbonate around mixed-feldspar and plagioclase glass
clasts); 4) a post-carbonate shock event that resulted in invasion of carbonate
by feldspathic melts, shock faulting and decarbonation of carbonate, high-temperature
mobilization of silica melts, and minor dissolution of orthopyroxene by
silica melts.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Submillimeter grain-size distribution of Apollo 11 soil 10084
A. Basu *, S. J. Wentworth, and D. S. McKay
*Correspondence author's address: Department of Geological Sciences, Indiana University, Indiana University, Bloomington, Indiana, 47405, USA; e-mail address: basu@indiana.edu
Abstract–Soil 10084 is the only representative soil sample from
Apollo 11 and arguably one of the purest mare soils in the Apollo collection.
It was wet sieved in 1970 and dry sieved in 1971 with different results.
Therefore, some doubt about its grain size distribution persists.
We consider allocation inhomogeneity, if any, to be a minor cause for the
discrepancy. Rather, the difference in methodology is likely to be
the major cause for different results. We report the results of a
new analysis of an allocation of 0.99 g using the contemporary method of
wet sieving at JSC; this method uses water instead of freon. Our
results show that the mean grain size and sorting of the submillimeter
fraction of soil 10084 are 4.28 theta (= 51 µm) and 2.23 theta (=
213 µm), respectively. A significant proportion (14.2%) of
the soil is in the <10 µm size range, which contrasts to previous
determinations of 6.4% and 9.8%, respectively. The newly determined
grain size distribution is skewed towards the finest grain sizes.
This result is more compatible with the high maturity of this soil than
the results of previous determinations.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The role of Fischer-Tropsch catalysis in solar nebula chemistry
Monika E. Kress* and Alexander G. G. M. Tielens
*Correspondence author's address: Department of Astronomy, University of Washington Seattle, Washington 98195-1580, USA; e-mail address: kress@astro.washington.edu
Abstract–Fischer-Tropsch catalysis, the iron/nickel catalyzed conversion of CO and H2 to hydrocarbons, would have been the only thermally-driven pathway available in the solar nebula to convert CO into other forms of carbon. A major issue in meteoritics is to determine the origin of meteoritic organics: are they mainly formed from CO in the solar nebula via a process such as Fischer-Tropsch, or are they derived from interstellar organics? In order to determine the role that Fischer-Tropsch catalysis may have played in the organic chemical evolution of the solar nebula, we have developed a kinetic model for this process. Our model results agree well with experimental data from several existing laboratory studies. In contrast, empirical rate equations, which have been derived from experimental rate data for a limited temperature (T) and pressure (P) range, are inconsistent with experimental rate data for higher T and lower P.
We have applied our model to pressure and temperature profiles for the
solar nebula, during the epoch in which meteorite parent bodies condensed
and agglomerated. We find that, under nebular conditions, the conversion
rate of CO to CH4 does not simply increase with temperature as the empirically-derived
equations suggest. Instead, our model results show that this process
would have been most efficient in a fairly narrow region that coincides
with the present position of the asteroid belt. Our results support
the hypothesis that Fischer-Tropsch catalysis may have played a role in
solar nebula chemistry by converting CO into less volatile materials that
can be much more readily processed in the nebula and in parent bodies.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Phosphate control on the Th/U variations in ordinary chondrites: Improving solar system abundances
Goreva, J.S.* and D.S. Burnett
*Correspondence author's address: Department of Geological and Planetary Sciences, MS 100-23, California Institute of Technology, Pasadena, California, 91125, USA; e-mail address: julia@gps.caltech.edu
Abstract–Isotope dilution thorium and uranium analyses by inductively-coupled
plasma mass spectrometry of 12 samples of Harleton (L6) show a much larger
scatter than was previously observed in equilibrated ordinary chondrites.
Th/U linearly correlates with 1/U in Harleton and in the total equilibrated
ordinary chondrite data set as well. Such a correlation suggests
a two component mixture and this trend can be quantitatively modeled as
reflecting variations in the mixing ratio between two phosphate phases:
chlorapatite and merrillite. The major effect is due to apatite variations,
which strongly control the whole rock U concentrations. Phosphorous variations
will tend to destroy the Th/U vs. 1/U correlation, and measured P concentrations
on exactly the same samples as U and Th show a factor of 3 range.
It appears that the P variations are compensated by inverse variations
in U (a dilution effect) to preserve the Th/U vs. 1/U correlation.
Because variations in whole rock Th/U are consequences of phosphate sampling,
a weighted average of high accuracy Th/U measurements in equilibrated ordinary
chondrites should converge to a significantly improved average solar system
Th/U. Our best estimate of this ratio is 3.53 with sigmamean
= 0.10.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
From interstellar gas to the Earth-Moon system
A. G. W. Cameron
Author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, 85721 USA; e-mail address: acameron@lpl.arizona.edu
Abstract–This paper reports the current status of my SPH simulations
of the formation of the Moon. Since the Moon has recently been found
to have been formed approximately 50 million years after the solar nebula
itself was formed, I have placed the lunar formation problem in the entire
context of the formation and early evolution of the solar nebula.
This set of processes remains controversial, and I have outlined what I
believe to be the essential physical processes involved. These start
with the formation of short-lived (now extinct) radioactive nuclides in
a massive supernova. Then follows the probable role of the supernova
ejecta in triggering the collapse of a core in a molecular cloud to form
the solar nebula, and the injection of the radioactivities into the collapsing
cloud core. Most of the solar nebula dissipates to form the Sun,
and what remains becomes relatively quiescent. Gas drag acting on
interstellar grains and the dustballs formed from them, due both to vertical
descent to midplane and inward spiralling in midplane, quickly causes growth
of the solid materials to form planetesimals. When these bodies reach
the kilometer size range and beyond, gravitational forces dominate the
accumulation process. The accumulation of the Earth requires of the
order of 108 years. About half-way through that process
the giant impact occurs with the next largest accumulating body near the
protoearth. I have been simulating the giant impact using smoothed
particle hydrodynamics (SPH) with 100,000 particles. The simulations
of three of these runs are depicted in detail with a series of color images.
It is shown that conventional accumulation simulations that assume Keplerian
orbits and that merge bodies upon collision are misleading because they
cannot take account of tidal stripping nor of loss and gain of particles
during the accumulation. In addition, the large rotational flattening
of the protoearth renders the orbital motions nonkeplerian. The simulations
that are shown in detail have been followed for just over a week of real
time, and in that time the largest accumulating clump has reached about
half or more of the mass of the Moon and additional clumps have accumulated
into bodies in the range of 1 to 20 percent of a lunar mass. It is
important to note that although these runs have given very promising results,
the parameter space that could plausibly be associated with the giant impact
is not yet adequately explored.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The condensation origin of zoned metal grains in Queen Alexandra Range 94411: Implications for the formation of the Bencubbin-like chondrites
Mikhail I. Petaev*, Anders Meibom, Alexander N. Krot, John A. Wood and Klaus Keil
*Correspondence author's address: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA; e-mail address: mpetaev@cfa.harvard.edu
Abstract–Thermodynamic analysis of the compositional profiles
across large chemically-zoned Fe,Ni metal grains in the Bencubbin-like
chondrite QUE94411 suggests that these grains formed by non-equilibrium
gas-solid condensation under variable oxidizing conditions, isolation degree,
and Cr depletion factors. The oxidizing conditions must have resulted
from the complete vaporization of nebular regions with enhanced dust/gas
ratios (~10 – 40 × solar). Because the origin of each of the
metal grains studied requires different condensation parameters (dust/gas
ratio, isolation degree, and Cr depletion factor), a high degree of heterogeneity
in the formation region of the Bencubbin-like chondrite metal is required.
To preserve compositional zoning of the metal grains and prevent their
melting and sulfidization, the grains must have been removed from the hot
condensation region into cold regions where the accretion of the Bencubbin-like
asteroidal body took place.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The metallographic cooling rate method revised: Application to iron meteorites and mesosiderites
W. D. Hopfe and J. I. Goldstein
*Correspondence author's address: College of Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA; e-mail address: hopfe@ecs.umass.edu
Abstract–A major revision of the current Saikumar and Goldstein (1988) cooling rate computer model for kamacite growth is presented. This revision incorporates a better fit to the alpha/alpha + gamma phase boundary and to the gamma/alpha + gamma phase boundary particularly below the monotectoid temperature of 400 °C. A reevaluation of the latest diffusivities for the Fe-Ni system as a function of Ni and P content and temperature is made, particularly for kamacite diffusivity below the paramagnetic to ferromagnetic transition.
The revised simulation model is applied to several iron meteorites and
several mesosiderites. For the mesosiderites we obtain a cooling
rate of 0.2 °C/Ma, about 10 times higher than the most recent measured
cooling rates. The cooling rate curves from the current model do not accurately
predict the central nickel content of taenite halfwidths smaller than ~10
µm. This result calls into question the use of conventional kamacite
growth models to explain the microstructure of the mesosiderites.
Kamacite regions in mesosiderites may have formed by the same process as
decomposed duplex plessite in iron meteorites.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The Monahans chondrite and halite: 39Ar-40Ar age, solar gases, cosmic-ray exposure ages, and parent body regolith neutron flux and thickness
Donald D. Bogard*, Daniel H. Garrison and Jozef Masarik
*Correspondence author's address: Planetary Sciences, Mail Code SN2, Johnson Space Center, Houston, Texas 77058, USA; e-mail address: donald.d.bogard@jsc.nasa.gov
Abstract–The Monahans H-chondrite is a regolith breccia containing
light and dark phases and the first reported presence of small grains of
halite. We made detailed noble gas analyses of each of these phases.
The 39Ar-40Ar age of Monahans light is 4.533 ±
0.006 Ma. Monahans dark and halite samples show greater amounts of
diffusive loss of 40Ar and the maximum ages are 4.50 Ga and
4.33 Gyra, respectively. Monahans dark phase contains significant
concentrations of He, Ne and Ar implanted by the solar wind when this material
was extant in a parent body regolith. Monahans light contains no
solar gases. From the cosmogenic 3He, 21Ne,
and 38Ar in Monahans light we calculate a probable cosmic-ray,
space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains
twice as much cosmogenic 21Ne and 38Ar as does the
light and indicates early near-surface exposure of 13–18 Ma in a H-chondrite
regolith. The existence of fragile halite grains in H-chondrites
suggests that this regolith irradiation occurred very early. Large
concentrations of 36Ar in the halite were produced during regolith
exposure by neutron capture on 35Cl, followed by decay to 36Ar.
The thermal neutron fluence seen by the halite was 2–4 × 1014
n/cm2. The thermal neutron flux during regolith exposure
was ~0.4–0.7 n/cm2/s. The Monahans neutron fluence is
more than an order of magnitude less than that acquired during space exposure
of several large meteorites and of lunar soils, but the neutron flux is
lower by a factor of less than or equal to 5. Comparison of the 36Arn/21Necos
ratio in Monahans halite and silicate with the theoretically calculated
ratio as a function of shielding depth in an H-chondrite regolith suggests
that irradiation of Monahans dark occurred under low shielding in a regolith
that may have been relatively shallow. Late addition of halite to the regolith
can be ruled out. However, irradiation of halite and silicate for
different times at different depths in an extensive regolith cannot be
excluded.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Presence of an iron-rich nanophase material in the upper layer of the Cretaceous–Tertiary boundary clay
Thomas J. Wdowiak*, Lawrence P. Armendarez, David G. Agresti, Manson L. Wade, Suzanne Y. Wdowiak, Philippe Claeys and Glenn Izett
*Correspondence author's address: Astro and Solar System Physics Program, Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294-1170, USA; e-mail address: wdowiak@uab.edu
Abstract–We report new geochemical evidence from ten Cretaceous-Tertiary
boundary sites in North America and Europe, indicating the presence of
a material remnant of a large asteroid or comet that struck the Earth at
65.0 Ma. Mössbauer spectroscopic data reveals that a ubiquitous
iron-rich nanophase material exists at the uppermost part of the K-T boundary
layer in the Western Hemisphere and in Europe in marine and continental
fine-grained sedimentary rock. The high surface-to-volume ratio of
nanophase material suggests that it may be the carrier of the iridium abundance
enhancement that marks the K-T boundary. Even more provocative is
the possibility that the discovered nanophase material is, for the most
part, composed of the vaporized impactor after the impact-generated high-temperature
vapor plume rose and cooled above the atmosphere.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Compositional properties of coexisting orthopyroxene and spinel in some Antarctic diogenites: Implications for thermal history
H-P. Liermann and J. Ganguly
*Correspondence author's address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA; e-mail address: ganguly@geo.arizona.edu
Abstract–We analyzed the compositional profiles of coexisting
orthopyroxenes and spinels in six diogenite samples from the Antarctic
meteorite collection and used the data to constrain their thermal histories.
The closure temperatures of Fe2+-Mg exchange between spinel
and orthopyroxene in these samples vary between ~630 and 830 °C.
However, those in other diogenite samples, for which the compositional
data are available in the literature, extend up to ~1125 °C.
This wide range of closure temperatures suggests repeated excavation of
the diogenites from their original sites over a long time interval during
cooling. The orthopyroxene grains were found to be homogeneous in
composition while two of the relatively large spinel grains in the samples
EET87530 and TIL82410 showed compositional zoning near the rim. Modeling
of the spinel zoning in TIL82410 suggests that it developed during cooling
under a regolith or ejecta blanket, possibly at a depth of ~ 80–120 m,
and that the spinel composition was homogeneous at ~900 °C. A
nonlinear cooling model in which the cooling rate is given by eta T(K)2,
with eta = 5.8 × 10–9 K–1 Ma–1,
leads to simulated retrograde zoning profile in spinel which match the
observed profile in TIL82410 very well.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Relationship between cooling rate and cooling age of a mineral: Theory and applications to meteorites
Jibamitra Ganguly* and Massimiliano Tirone
*Correspondence author's address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA; e-mail address: ganguly@geo.arizona.edu
Abstract–We reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system, which permits direct retrieval of cooling rate from cooling age when the diffusion parameters, grain size and initial temperature are known. This formulation is used to show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system. The cooling rates of seven H chondrites of the metamorphic types H4, H5 and H6 were retrieved from the available data on the Pb-Pb model ages of the phosphates and the diffusion kinetic data of Pb in apatite. The results are in excellent agreement with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites. We also addressed the problem of ~ 90 Ma younger Sm-Nd mineral isochron age, defined by orthopyroxene, phosphate and plagioclase, of the Morristown mesosiderite compared to the Pb-Pb age of the Estherville mesosiderite. It is shown that this younger age could have been a consequence of resetting during cooling instead of an `impulsive heating' event, as suggested earlier.
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