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Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Formation of orange hibonite, as inferred from some Allende inclusions
S. B. Simon*, A. M. Davis and L. Grossman
*Correspondence author's address: Department of the Geophysical
Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois
60637, USA; e-mail address: sbs8@midway.uchicago.edu
Abstract–We studied three fluffy Type A refractory inclusions
from Allende that contain orange hibonite. The melilite in the present
samples is very Al-rich, averaging Åk6, Åk14,
and Åk12 in the three samples studied. Hibonite in two
inclusions, unlike that in Murchison, has low rare earth element abundances
of <10 × CI; in the other inclusion, the hibonite, melilite and
perovskite have Group II-like patterns. The hibonite and melilite in all
three inclusions studied have excess 26Mg consistent with (26Al/27Al)I
= 5 × 10–5. Much of the hibonite and some of the
spinel in these inclusions is corroded. These phases are found enclosed
in melilite, but based on bulk compositions and phase equilibria, hibonite
should not be an early-crystallizing phase in these inclusions. We conclude
that the hibonite and probably some of the spinel is relict. Reversely
zoned melilite, rounded spinel and isotopically heavy Mg in the inclusions
probably reflect reheating events that involved melting and evaporation.
Alteration of the gehlenitic melilite gave rise to some rare phases, including
corundum and nearly pure CaTs pyroxene.
Studies have shown that blue hibonite contains Ti3+ while
orange hibonite does not (Ihinger and Stolper, 1986; Beckett et al.,
1988). Orange hibonite formed either under oxidizing conditions (such as
at oxygen fugacities at least seven orders of magnitude greater than that
of a solar gas at 1700 K), or under conditions reducing enough (e.g.,
solar) that it contained Ti3+, which was later oxidized in
situ. Although V and Ce oxides are volatile at the temperature and
range of oxygen fugacities at which orange hibonite is known to be stable,
we find that: a) the hibonite is V-rich (~1 wt % V2O3);
and b) there are no negative Ce anomalies in Allende hibonite. This indicates
that the hibonite did not form by condensation under oxidizing conditions.
In addition, there are slight excesses of Ti + Si cations relative to Mg
+ Fe cations (up to 0.1 of 0.8 cations per 19 oxygen anions), probably
reflecting the original presence of Ti3+. The results of this
study strongly support the suggestion (Ihinger and Stolper, 1986) that
Allende hibonite originally formed under reducing conditions and was later
oxidized. Oxygen fugacities within ~2–3 orders of magnitude of that of
a solar gas are implied; otherwise, strong Ce and V depletions would be
observed.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Ultraviolet irradiation of naphthalene in H2O ice: Implications for meteorites and biogenesis
Max P. Bernstein, Jason P. Dworkin, Scott A. Sandford* and Louis J. Allamandola
*Correspondence author's address: NASA-Ames Research Center, Mail Stop 245-6, Moffett Field, California 94035, USA; e-mail address: ssandford@mail.arc.nasa.gov
Abstract–The polycyclic aromatic hydrocarbon (PAH) naphthalene
was exposed to ultraviolet radiation in H2O ice under astrophysical
conditions, and the products were analyzed using infrared spectroscopy
and high performance liquid chromatography. As we found in our earlier
studies on the photoprocessing of coronene in H2O ice, aromatic
alcohols and ketones (quinones) were formed. The regiochemistry of
the reactions is described and leads to specific predictions of the relative
abundances of various oxidized naphthalenes that should exist in meteorites
if interstellar ice photochemistry influenced their aromatic inventory.
Since oxidized PAHs are present in carbon-rich meteorites and interplanetary
dust particles (IDPs), and ubiquitous in and fundamental to biochemistry,
the delivery of such extraterrestrial molecules to the early Earth may
have played a role in the origin and evolution of life.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Formation of resurge gullies at impacts at sea: The Lockne Crater, Sweden
Ilka von Dalwigk* and Jens Ormö
*Correspondence author's address: Department of Geology and Geochemistry, Stockholm University, S-10691 Stockholm, Sweden; e-mail address: ilka@geo.su.se
Abstract–The Lockne crater in Sweden is a marine-target crater, formed in a shelf sea, approximately 460 Ma ago. The crater structure consists of an inner crater surrounded by an outer, inclined surface that extends to almost 12 km from the center.
Marine craters differ in several respects from craters formed on land. One special feature is the formation of resurge gullies excavated by the erosional force of the resurging sea water after the impact. The formation of these gullies strongly depends on the ratio crater-rim height to water depth, as well as on the size of the impact structure. Such gullies are known from very few marine-target craters. At the Lockne impact site, four gullies are identified, each of which cuts radially through the rim of the outer crater.
The rapid collapse of that part of the crater cavity, which formed in
the seawater, resulted in forceful flooding of the crater. The resurging
seawater not only contained fallback-ejecta; on its way towards the cavity
on the sea-bottom it incorporated fractured lithologies from the sea-bottom
as well. This entrained material disintegrated during transport and constitutes
today the dominantly monomict lower part of the resurge sequence. The resurge
flood was highly turbulent, highly erosive, and developed to a probable
hyperconcentrated flow or a possible water flood. The erosion in the gullies
proceeded as headward erosion down to the transition zone between the brecciated
and the less disintegrated crystalline basement.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
A plausible cause of the late heavy bombardment
A. Morbidelli*, J-M. Petit, B. Gladman and J. Chambers
*Correspondence author's address: Observatoire de la Cote d'Azur, Nice, France; e-mail address: morby@obs-nice.fr
Abstract–We show that at the end of the main accretional period
of the terrestrial planets, a few percent of the initial planetesimal population
in the 1–2 AU zone is left on highly-inclined orbits in the inner
solar system. The final depletion of this leftover population would
cause an extended bombardment of all of the terrestrial planets, slowly
decaying with a time scale of order 60 Ma. Because of the large impact
velocities dictated by the high inclinations, these projectiles would produce
craters much larger than those formed by asteroids of equal size on typical
current Near-Earth asteroid orbits: on the Moon, basins could have
been formed by bodies as small as 20 km in diameter, and 10 km craters
could be produced by 400 m impactors. To account for the observed
lunar crater record, the initial population of highly-inclined leftovers
would need to be a few times that presently in the main asteroid belt,
at all sizes, in agreement with the simulations of the primordial sculpting
of both these populations. If a terminal lunar cataclysm (a spike
in the crater record ~3.9 Ga ago) really occurred on the Moon, it was not
caused by the highly-inclined leftover population, because of the monotonic
decay of the latter.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
J. E. Chambers* and G. W. Wetherill
*Correspondence author's address: NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035, USA; e-mail address: john@mycenae.arc.nasa.gov
Abstract–The main asteroid belt has lost >99.9% of its solid
mass since the time at which the planets were forming, according to models
for the protoplanetary nebula. Here we show that the primordial asteroid
belt could have been cleared efficiently if much of the original mass accreted
to form planet-sized bodies, which were capable of perturbing one another
into unstable orbits. We provide results from 25 N-body integrations of
up to 200 planets in the asteroid belt,with individual masses in the range
0.017–0.33 Earth masses. In the simulations, these bodies undergo
repeated close encounters which scatter one another into unstable resonances
with the giant planets, leading to collision with the Sun or ejection from
the Solar System. In response, the giant planets' orbits migrate
radially and become more circular. This reduces the size of the main-belt
resonances and the clearing rate, although clearing continues. If ~3 Earth
masses of material was removed from the belt this way, Jupiter and Saturn
would initially have had orbital eccentricities almost twice their current
values. Such orbits would have made Jupiter and Saturn 10–100 times
more effective at clearing material from the belt than they are on their
current orbits. The time required to remove 90% of the initial mass
from the belt depends sensitively on the giant planets' orbits, and weakly
on the masses of the asteroidal planets. 18/25 of the simulations
end with no planets left in the belt, and the clearing takes up to several
hundred million years. Typically, the last one or two asteroidal
planets are removed by interactions with planets in the terrestrial region.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
A new metal-rich chondrite grouplet
Michael K. Weisberg*, Martin Prinz, Robert N. Clayton, Toshiko K. Mayeda, Naoji Sugiura, Shigeo Zashu and Mitsuru Ebihara
*Correspondence author's address: Department of Physical Sciences, Kingsborough College (CUNY), Brooklyn, New York 11235, USA; e-mail address: mweisberg@kbcc.cuny.edu
Abstract–A new grouplet of primitive, metal-rich chondrites, here called the CB (bencubbinite) chondrites, has been recognized. It includes Bencubbin, Weatherford, HH 237 and QUE 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH chondrites, all of which are members of the more inclusive CR clan. CB chondrites have much greater metal/silicate ratios than any other chondrite group, widely increasing the range of metal/silicate fractionation recorded in solar nebular processes. They also have the greatest moderately volatile lithophile element depletions of any chondritic materials. Metal has compositional trends and zoning patterns that suggest a primitive condensation origin, in contrast with metal from other chondrite groups. CB chondrites, as well as other CR clan chondrites, have much heavier nitrogen (higher 15N/14N) than that in other chondrite groups. The primitive characteristics of the CB chondrites suggests that they contain one of the best records of early nebular processes.
Another chondrite, GRO 95551, is petrographically similar to the CB
chondrites, but its mineral and oxygen and nitrogen isotope compositions
indicate that it formed from a different nebular reservoir.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Iron isotopes in chondrules: Implications for the role of evaporation during chondrule formation
C. M. O'D. Alexander* and J. Wang
*Correspondence author's address: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd. N.W., Washington D.C. 20015, USA; e-mail address: alexande@dtm.ciw.edu
Abstract–We have measured the delta 57Fe of olivines
in nine Chainpur chondrules. All are within error of normal (typically
2 sigma = 1–2‰). Most of the chondules could not have lost more than
~20% of their FeO by Rayleigh evaporation and none can have lost more than
~61%. Yet, the range of Fo contents in these chondrules is Fo78-Fo99.9.
The isotopic compositions of the chondrules clearly demonstrate that, for
instance, Type I chondrules cannot form from Type II chondrules by evaporation
of FeO under Rayleigh conditions. The isotopic compositions also
place constraints on the minimum cooling rates these chondrules could have
experienced. These cooling rates must also be equal to or slower
than those required to produce the chondrule textures. Assuming flash
heating and evaporation rates like those measured in vacuum, the minimum
cooling rates necessary to prevent detectable Fe isotopic fractionation
via Rayleigh evaporation approach those needed to produce barred and porphyritic
textures. The presence of hydrogen in the Nebula, non-linear cooling
and other effects will all tend to increase the cooling rates required
to prevent delta 57Fe > 1–2‰, perhaps by as much as 1–2 orders
of magnitude. The two most likely ways that the cooling rates required
to prevent delta 57Fe > 1–2‰ can be kept below those needed
to produce barred and porphyritic textures are: (1) the pH2 in
the Nebula was low enough to keep evaporation rates close to those in vacuum,
or (2) back reaction of chondrules with Fe in the gas suppressed isotopic
fractionation.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Petrology and geochemistry of Patuxent Range 91501, a clast-poor impact-melt from the L chondrite parent body, and Lewis Cliff 88663, an L7 chondrite
David W. Mittlefehldt* and Marilyn M. Lindstrom
*Correspondence author's address: Mail Code SN2, NASA Johnson Space Center, Houston, Texas 77058, USA; e-mail address: david.w.mittlefehldt1@jsc.nasa.gov
Abstract–We have performed petrologic and geochemical studies of Patuxent Range 91501 and Lewis Cliff 88663. PAT 91501, originally classified as an L7 chondrite, is rather a unique, near total impact-melt from the L chondrite parent body. Lewis Cliff 88663 was originally classified as an "achondrite (?)," but we find that it is a very weakly shocked L7 chondrite.
PAT 91501 is an unshocked, homogeneous, igneous-textured ultramafic rock composed of euhedral to subhedral olivine, low-Ca pyroxene, augite and chrome-rich spinels with interstitial albitic plagioclase and minor silica-alumina-alkali-rich glass. Only ~10% relict chondritic material is present. Olivine grains are homogeneous (Fa25.2–26.8). Low-Ca pyroxene (Wo1.9–7.2En71.9–78.2Fs19.9–20.9) and augite (Wo29.8–39.0En49.2–55.3Fs11.8–14.9) display a strong linear TiO2-Al2O3 correlations resulting from igneous fractionation. Plagioclase is variable in composition; Or3.0–7.7Ab79.8–84.1An8.2–17.2. Chrome-rich spinels are variable in composition and zoned from Cr-rich cores to Ti-Al-rich rims. Some have evolved compositions with up to 7.9 wt% TiO2. PAT 91501 bulk silicate has an L chondrite lithophile element composition except for depletions in Zn and Br. Siderophile and chalcophile elements are highly depleted due to sequestration in cm-size metal-troilite nodules.
The minerals in LEW 88663 are more uniform in composition than those
in PAT 91501. Olivine grains have low CaO and Cr2O3
contents similar to those in L5-6 chondrites. Pyroxenes have high
TiO2 contents with only a diffuse TiO2-Al2O3
correlations. Low-Ca pyroxenes are less calcic (Wo1.6–3.1En76.5–77.0Fs20.4–21.4),
while augites (Wo39.5–45.6En46.8–51.1Fs7.6–9.4)
and plagioclases (Or2.6–5.7Ab74.1–83.1An11.2–23.3)
are more calcic. Spinels are homogeneous and compositionally similar
to those in L6 chondrites. LEW 88663 has an L chondrite bulk composition
for lithophile elements, and only slight depletions in siderophile and
chalcophile elements that are plausibly due to weathering and/or sample
heterogeneity.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Exposure history of separated phases from the Kapoeta meteorite
Marc W. Caffee* and Kunihiko Nishiizumi
*Correspondence author's address: Geosciences & Environmental Technology Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA; e-mail address: caffee1@llnl.gov
Abstract–The cosmogenic radionuclides, 10Be, 26Al,
36Cl, and 53Mn were measured in selected clasts and
matrix samples from the howardite Kapoeta. Previous measurements
of cosmogenic 21Ne indicate higher cosmic ray exposure ages
for bulk samples than for some separated clasts or mineral separates.
A possible interpretation for this difference in apparent exposure ages
is a complex recent exposure history for Kapoeta. In this scenario
some constituents are exposed to cosmic rays in a 2 pi geometry as part
of a larger body immediately preceding it's 4 pi exposure in a smaller
body. To test this scenario we measured cosmogenic radionuclides
in several clasts from Kapoeta. These measurements are consistent
with a simple single-stage 4 pi exposure history during which the entire
inventory of cosmogenic radionuclides was produced. Taken together,
these data are most consistent with a single stage 4 pi exposure lasting
~3 Myr. This scenario is nevertheless consistent with models in which
the exposure of some constituents of Kapoeta to energetic particles occurred
at an earlier time, as is indicated by 21Ne measurements.
However, from our data we conclude that insubstantial quantities of cosmogenic
radionuclides were inherited from this earlier irradiation; this earlier
exposure to energetic particles must have pre-dated the recent exposure
by at least ~10 Myr to allow for the decay of the long half-life cosmogenic
radionuclides.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Dhofar 081: A new lunar highland meteorite
A. Greshake*, R. T. Schmitt, D. Stöffler, M. Pätsch and L. Schultz
*Correspondence author's address: Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany; e-mail address: ansgar.greshake@rz.hu-berlin.de
Abstract–The lunar meteorite Dhofar 081, found as a single fragment of 174 g in the Dhofar region of Oman, is a shocked feldspathic fragmental highland breccia dominated by anorthosite-rich lithic and mineral clasts embedded into a fine-grained mostly shock melted clastic matrix. Major mineral phases in the bulk rock are Ca-rich plagioclase (An96.5–99.5), pyroxene (Fs21.9–46.2Wo3.0–41.4), and olivine (Fa29.3–47.8); accessory phases include Fe-Ni metal, ilmenite, and Ti-Cr-rich spinel. Dhofar 081 contains subordinate crystalline fragments of large anorthosites, intersertal impact melt rocks, microporphyritic impact melt breccias, dark fine-grained impact melt breccias, large cataclastic feldspars, and irregularly shaped brown glass clasts. Mafic components are rare and no genuine regolith components were found in the sections studied. Minerals in Dhofar 081 show homogeneously distributed shock features: intergranular recrystallization, strong fracturing and mosaicism in feldspar as well as a high density of mostly irregular fractures in pyroxene and olivine. Localized impact melting caused by one or several impacts led to a strong lithification. Based on these effects an equilibration shock pressure of about 15–20 GPa is estimated for the strongest shock event in Dhofar 081. Devitrification of the "glassy" material in the rock indicates thermal annealing after shock melting suggesting that the 15–20 GPa shock event predated the ejection event. According to the concentrations of implanted solar noble gases Dhofar 081 represents a polymict clastic breccia deposit with possibly a minor regolith component. A similar noble gas record of Dhofar 081 and MAC 88104/05 suggests the possibility of a source crater pairing of both meteorites. As indicated by noble gas measurements pairing of Dhofar 081 with the other lunar meteorites found in Oman, Dhofar 025 and Dhofar 026, is unlikely.
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