Please contact the correspondence author for reprints of all published articles
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Low-temperature phase decomposition in Fe-Ni metal of the Portales Valley meteorite
Birgit Sepp*, Addi Bischoff and Dirk Bosbach
*Correspondence author's address: Institut für Materialphysik,Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany; e-mail address: bis@uni-muenster.de
Abstract–The Portales Valley meteorite provides an opportunity
to investigate and compare the microstructure in Fe(Ni metal of the metallic
particles in the chondritic portion and in the metal veins. The low-temperature
phase decomposition of Fe(Ni metal was investigated using scanning electron
microscopy, transmission electron microscopy, and atomic force microscopy.
The microstructure is formed as the Portales Valley meteorite cooled from
high temperatures and includes the outer taenite rim, the cloudy zone,
clear taenite, and martensite. Martensite in turn decomposes into
a fine admixture of fcc rods in a bcc matrix. The width of the island
phase of the cloudy zone in the metal particles of the chondritic portion
and the metal veins can be used to estimate a low-temperature cooling rate.
The microstructural evidence indicates that the chondritic portions and
the metal veins in the Portales Valley meteorite cooled together as a mixture
with a cooling rate of roughly 6.5 K/Ma.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Abundance and isotopic composition of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite
Teruyuki Maruoka*, Jun-ichi Matsuda and Gero Kurat
*Correspondence author's address: Institute of Geochemistry, University of Vienna, A-1090 Vienna, Austria; e-mail address: teruyuki.maruoka@univie.ac.at
Abstract–Abundances and isotopic compositions of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite were measured. The abundance ratios of spallogenic components in metal reveal a 3He deficiency which is due to the diffusive loss of parent isotopes, i.e., tritium (Tilles, 1963; Schultz, 1967). The diffusive loss likely has been induced by thermal heating by the Sun during cosmic ray exposure (~160 Ma; Lavielle et al., 1999). Thermal process such as impact-induced partial loss may have affected the isotopic composition of spallogenic Ne. The 129Xe/131Xe ratio of cosmogenic components in the metal indicates an enhanced production of epi-thermal neutrons.
The abundance ratios of spallogenic components in the graphite reveal
that it contained small amounts of metal and silicates. The isotopic
composition of heavy noble gases in graphite itself was obtained from graphite
treated with HF/HCl. The isotopic composition of the etched graphite
shows that it contains two types of primordial Xe, i.e., Q-Xe and El Taco
Xe. The isotopic heterogeneity preserved in the Bohumilitz graphite
indicates that the Bohumilitz graphite did not experience any high-temperature
event and, consequently, must have been emplaced into the metal at subsolidus
temperatures. This situation is incompatible with an igneous model as well
as the impact melting models for the IAB-IIICD iron meteorites as proposed
by Choi et al. (1995) and Wasson et al. (1980).
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Production rates of cosmogenic 3He, 21Ne, and 22Ne in ordinary chondrites and the lunar surface
J. Masarik, K. Nishiizumi* and R. C. Reedy
*Correspondence author's address: Space Sciences Laboratory, University of California, Berkeley, California 94720-7450, USA; e-mail address: kuni@ssl.berkeley.edu
Abstract–The production of 3He, 21Ne, and
22Ne in meteoroids of various sizes and in the lunar surface
was investigated. The LAHET Code System, a purely physical model
for calculating cosmic-ray particle fluxes, was used to simulate cosmic-ray-particle
interactions with extraterrestrial matter. We discuss the depth and
size dependence of the shielding parameter 22Ne/21Ne,
which is used for reconstruction of pre-atmospheric sizes, depth, and exposure
histories. The 22Ne/21Ne ratio decreases with
increasing depth or pre-atmospheric size but then increases with depth
in very large objects. This increase with depth in the 22Ne/21Ne
ratio means that this ratio is a poor indicator of shielding in some large
objects. The dependence of 3He/21Ne as function
of 22Ne/21Ne was also calculated, and differences
between the calculations and the Bern line are discussed.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Forsterite-rich accretionary rims around Ca,Al-rich inclusions from the reduced CV3 chondrite Efremovka
Alexander N. Krot*, Alexander A. Ulyanov, Anders Meibom 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–It was suggested that multilayered accretionary rims composed of ferrous olivine, andradite, wollastonite, salite-hedenbergitic pyroxenes, nepheline, and Ni-rich sulfides around Allende Ca, Al-rich inclusions (CAIs) are aggregates of gas-solid condensates which reflect significant fluctuations in physico-chemical conditions in the slowly cooling solar nebula and grain/gas separation processes. In order to test this model, we studied the mineralogy of accretionary rims around one type A CAI (E104) and one type B CAI (E48) from the reduced CV3 chondrite Efremovka, which is less altered than Allende. In contrast to the Allende accretionary rims, those in Efremovka consist of coarse-grained (20–40 µm), anhedral forsterite (Fa1–8), Fe,Ni-metal nodules, amoeboid olivine aggregates (AOAs) and fine-grained CAIs composed of Al-diopside, anorthite, and spinel, ±forsterite. Although the fine-grained CAIs, AOAs and host CAIs are virtually unaltered, a hibonite-spinel-perovskite CAI in the E48 accretionary rim experienced extensive alteration, which resulted in the formation of Fe-rich, Zn-bearing spinel, and a Ca, Al, Si-hydrous mineral. Forsterites in the accretionary rims typically show an aggregational nature and consist of small olivine grains with numerous pores and tiny inclusions of Al-rich minerals. No evidence for the replacement of forsterite by enstatite was found; no chondrule fragments were identified in the accretionary rims.
We infer that accretionary rims in Efremovka are more primitive than
those in Allende and formed by aggregation of high-temperature condensates
around host CAIs in the CAI-forming regions. The rimmed CAIs were
removed from these regions prior to condensation of enstatite and alkalies.
The absence of andradite, wollastonite, and hedenbergite from the Efremovka
rims may indicate that these rims sampled different nebular regions than
the Allende rims. Alternatively, the Ca, Fe-rich silicates rimmimg
Allende CAIs may have resulted from late-stage metasomatic alteration,
under oxidizing conditions, of original Efremovka-like accretionary rims.
The observed differences in O-isotope composition between forsterite and
Ca, Fe-rich minerals in the Allende accretionary rims (Hiyagon, 1998) suggest
that the oxidizing fluid had an 16O-poor oxygen isotopic composition.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Petrography, geochemistry, and 40Ar-39Ar ages of impact melt rocks and breccias from the Ames Impact Structure, Oklahoma: The Nicor Chestnut 18-4 drill core
Christian Koeberl*, Wolf Uwe Reimold and Simon P. Kelley
*Correspondence author's address: Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; e-mail address: christian.koeberl@univie.ac.at
Abstract–The 15-km-diameter Ames structure in northwestern Oklahoma
is located 2.75 km below surface in Cambro-Ordovician Arbuckle dolomite,
which is overlain by Middle Ordovician Oil Creek Formation shale.
The feature is marked by two concentric ring structures, with the inner
ring of about 5 km diameter probably representing the collapsed remnant
of a structural uplift composed of brecciated Precambrian granite and Arbuckle
dolomite. Wells from both the crater rim and the central uplift are
oil- and gas-producing, making Ames one of the economically important impact
structures.
Petrographic, geochemical, and age data were obtained on samples
from the Nicor Chestnut 18-4 drill core, off the NW flank of the central
uplift. These samples represent the largest and best examples of
impact melt breccia obtained so far from the Ames structure. They
contain carbonate rocks which, therefore, are derived from the target sequence.
The chemical composition of the impact melt breccias is similar to that
of target granite, with variable carbonate admixture. Some impact
melt rocks are enriched in siderophile elements indicating the possible
presence of a meteoritic component. Based on stratigraphic arguments,
the age of the crater was estimated at 470 Ma. Previous 40Ar-39Ar
dating attempts of impact melt breccias from the Dorothy 1-19 core yielded
plateau ages of about 285 Ma, which is in conflict with the stratigraphic
age. The new 40Ar-39Ar age data obtained on
the melt breccias from the Nicor Chestnut core by UV laser spot analysis,
resulted in a range of ages with maxima around 300 Ma. These data
could reflect processes related either the regional Nemaha Uplift or resetting
due to hot brines active on a midcontinent-wide scale, perhaps in related
to the Alleghenian and Ouachita orogenies. The age data indicate
an extended burial phase associated with thermal overprint during Late
Pennsylvanian-Permian.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Nebular thermal evolution and the properties of primitive planetary materials
Patrick Cassen
Author's address: NASA Ames Research Center, 245-3, Moffett Field, CA 94035-1000, USA; e-mail address: pcassen@mail.arc.nasa.gov
Abstract–Models of the solar nebula are constructed to investigate
the hypothesis that surviving planetary objects began to form as the nebula
cooled from an early, hot epoch. The imprint of such an epoch might
be retained in the spatial distribution of planetary material, the systematic
deviations of its elemental composition from that of the Sun, chemical
indicators of primordial oxidation state, and variations in oxygen and
other isotopic compositions. Our method of investigation is to calculate
the time-dependent, two-dimensional temperature distributions within model
nebulas of prescribed dynamical evolution, and to deduce the consequences
of the calculated thermal histories for coagulated solid material.
The models are defined by parameters which characterize nebular initial
states (mass and angular momentum), mass accretion histories, and coagulation
rates and efficiencies. It is demonstrated that coagulation during
the cooling of the nebula from a hot state is expected to produce systematic
heterogeneities which affect the chemical and isotopic compositions of
planetary material. The radial thermal gradient at the midplane results
in delayed coagulation of the more volatile elements. Vertical thermal
gradients isolate the most refractory material and concentrate evaporated
heavy elements in the gas phase. It is concluded that these effects
could be responsible for the distribution of terrestrial planet masses,
the systematic depletion patterns of the moderately volatile elements in
chondritic meteorites and the Earth, the range of oxygen isotopic compositions
exhibited by CAIs and other refractory inclusions, and some geochemical
evidence for a moderately enhanced oxidation state. However, nebular
fractionations on a global scale are unlikely to account for the more oxidizing
conditions inferred for some CAIs and chondritic silicates, which require
dust enhancements greater than a few hundred. This conclusion, along
with the well-established evidence from studies of chondrules and CAIs
for thermal excursions of short duration, make it likely that local environments,
unrelated to nebular thermal evolution, were also important.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Remote sensing and geological studies of the Hadley-Apennine region of the Moon
David T. Blewett* and B. Ray Hawke
*Correspondence author's address: Planetary Geosciences, Hawaii Institute of Geophysics & Planetology, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822, USA; e-mail address: dave@higp.hawaii.edu
Abstract–We have used data from the Clementine and Lunar Prospector
spacecraft in conjunction with reflectance spectra collected with Earth-based
telescopes to study the geology of the Hadley-Apennine portion of the lunar
Imbrium basin. The Apennine Mountains and the Imbrium backslope are
composed of Imbrium basin ejecta with a noritic or anorthositic norite
composition. We find that the two major facies of Imbrium ejecta,
the Apenninus material and the Alpes Formation, differ in iron and titanium
content. "Pure" anorthosite has tentatively been identified in the
ejecta of the crater Conon, based on low iron content. A difference
in Th and REE abundance between the northeast Apennine Mountains (lower)
and the southwest Apennines (higher) is noted. Pyroclastic deposits
are common in the region and are dominated by mare basalt material, probably
plug rock ejected in vulcanian eruptions. The Apennine Bench Formation,
which is likely to be a deposit of non-mare volcanic material, has an Fe,
Ti and Th composition consistent with that of Apollo 15 KREEP basalt samples
thought to be fragments of the Bench. Aristillus crater is a Th and
REE hot spot, and the stratigraphy of the impact target site has been reconstructed
from knowledge of the composition of the crater interior and exterior deposits.
We infer that the target consisted of highland basement, KREEP plutonics
and volcanics, and both high- and low-Ti mare basalt.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Impact-induced hydrothermal activity within the Haughton impact structure, arctic Canada: generation of a transient, warm, wet oasis
Gordon R. Osinski* and John G. Spray
*Correspondence author's address: Planetary and Space Science Centre, Department of Geology, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick E3B 5A3, Canada; e-mail address: f532b@unb.ca
Abstract–Field studies and analytical scanning electron microscopy
indicate that a hydrothermal system was created by the interaction of water
with hot, impact-generated rocks following formation of the 24 km-diameter,
23 Ma Haughton impact structure. Hydrothermal alteration is recognized
in two settings: within polymict impact breccias overlying the central
portion of the structure, and within localized pipes in impact-generated
concentric fault systems. The intra-breccia alteration comprises
three varieties of cavity and fracture filling: (a) sulfide with
carbonate, (b) sulfate, and (c) carbonate. These are accompanied
by subordinate celestite, barite, fluorite, quartz and marcasite.
Selenite is also developed, particularly in the lower levels of the impact
breccia sheet. The fault-related hydrothermal alteration occurs in
1–7 m diameter subvertical pipes that are exposed for lengths of up 20
m. The pipes are defined by a monomict quartz-carbonate breccia showing
pronounced Fe-hydroxide alteration. Associated sulfides include marcasite,
pyrite and chalcopyrite. We propose three distinct stages in the
evolution of the hydrothermal system: (1) Early Stage (>200 °C),
with the precipitation of quartz (vapour phase dominated); (2) Main Stage
(200–100 °C), with the development of a two phase (vapour plus liquid)
zone, leading to calcite, celestite, barite, marcasite and fluorite precipitation,
and (3) Late Stage (<100 °C), with selenite and fibroferrite development
through liquid phase-dominanted precipitation. We estimate that it
took several tens of thousands of years to cool below 50 °C following
impact. During this time, Haughton supported a 14 km diameter crater
lake and subsurface water system, providing a warmer, wetter niche relative
to the surrounding terrain. The results also reveal how understanding
the internal structure of impact craters is necessary in order to determine
their plumbing and cooling systems.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka, Leoville and Vigarano: Products of nebular condensation, accretion and annealing
Mutsumi Komatsu, Alexander N. Krot*, Mikhail I. Petaev, Alexander A. Ulyanov, Klaus Keil and Masamichi Miyamoto
*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–Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Leoville and Vigarano are irregularly-shaped objects, up to 5 mm in size, composed of forsteritic olivine (Fa<10) and a refractory, Ca,Al-rich component. The AOAs are depleted in moderately volatile elements (Mn, Cr, Na, K), Fe,Ni-metal and sulfides and contain no low-Ca pyroxene. The refractory component consists of fine-grained Ca,Al-rich inclusions (CAIs) composed of Al-diopside, anorthite (An100), and magnesium-rich spinel (~1 wt% FeO) or fine-grained intergrowths of these minerals; secondary nepheline and sodalite are very minor. This indicates that AOAs from the reduced CV chondrites are more pristine than those from the oxidized CV chondrites Allende and Mokoia. Although AOAs from the reduced CV chondrites show evidence for high temperature nebular annealing (e.g., forsterite grain boundaries form 120° triple junctions) and possibly a minor degree of melting of Al-diopside-anorthite materials, none of the AOAs studied appear to have experienced extensive (>50%) melting. We infer that AOAs are aggregates of high temperature nebular condensates, which formed in CAI-forming regions, and that they were absent from chondrule-forming regions at the time of chondrule formation. The absence of low-Ca pyroxene and depletion in moderately volatile elements (Mn, Cr, Na, K) suggest that AOAs were either removed from CAI-forming regions prior to condensation of these elements and low-Ca pyroxene or gas-solid condensation of low-Ca-pyroxene was kinetically inhibited.
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