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Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Microdistributions and petrogenetic implications of rare earth elements in polymict ureilites
Yunbin Guan* and Ghislaine Crozaz
*Correspondence author's address: Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287-1404, USA; e-mail address: yunbin.guan@asu.edu
Abstract–Polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites, including plagioclase, enstatite, feldspathic melt clasts and dark inclusions. This paper investigates the microdistributions and petrogenetic implications of rare earth elements (REEs) in three polymict ureilites (Elephant Moraine (EET) 83309, EET 87720 and North Haig), focusing particularly on the mineral and lithic clasts not found in monomict ureilites.
As in monomict ureilites, olivine and pyroxene are the major heavy (H) REE carriers in polymict ureilites. They have light (L) REE-depleted patterns with little variation in REE abundances, despite large differences in major element compositions.
The textural and REE characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body. Simple REE modeling shows that the most common melt clasts in polymict ureilites can be produced by 20-30% partial melting of chondritic material, leaving behind a ureilitic residue. The plagioclase clasts, as well as some of the high-Ca pyroxene grains, probably represent plagioclase-pyroxene rock types on the ureilite parent body. However, the variety of REE patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event. Multiple processes, probably including shock melting and different sources, are required to account for all the REE characteristics observed in lithic and mineral clasts.
The C-rich matrix in polymict ureilites is LREE-enriched, like that in monomict ureilites. The occurrence of Ce anomalies in C-rich matrix, dark inclusions and the presence of the hydration product, iddingsite, imply significant terrestrial weathering.
A search for 26Mg excesses, from the radioactive decay of
26Al, in the polymict ureilite EET 83309 was negative.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Gold Basin Meteorite Strewn Field, Mojave Desert, Northwestern Arizona: Relict of a Small Late Pleistocene Impact Event
David A. Kring*, A.J. Timothy Jull, Lanny R. McHargue, P.A. Bland, Dolores H. Hill, and F.J. Berry
*Correspondence author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721 USA; e-mail address: kring@lpl.arizona.edu
Abstract–Over 4450 meteorite specimens with a total mass of 168,760
g have been found in the Gold Basin (L4) strewn field over an area of 225
km2. The meteorite is a breccia, composed only of fragments
of L-chondrite materials. The parent meteoroid had a kinetic energy equivalent
to ~5 to 50 ktons when it hit the top of the atmosphere. Cosmogenic nuclide
studies indicate the meteorite has a terrestrial age of 15,000 +
600 years, corresponding to the Late Pinedale portion of the Wisconsin
Glaciation. Conditions in the Gold Basin, which is now part of the Mojave
Desert, were wetter and cooler at the time of the fall. Mössbauer
analyses indicate the sample is 30 to 35% oxidized. This is less than that
in meteorites with similar ages found in eastern New Mexico, but comparable
to that found in meteorites from the Sahara and the Nullarbor Region. Oxidation
is likely to have occurred soon after the fall, when exposure to precipitation
was at its maximum. Four other new meteorites were also found in the Gold
Basin strewn field.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Korra Korrabes: A new, large H3 chondrite breccia from Namibia
L. D. Ashwal
Author's address: Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park, 2006, South Africa; e-mail address: LDA@na.rau.ac.za
Abstract–A new, large, ordinary chondrite has been recovered
from near the strewn field of Gibeon iron meteorites in Namibia, and is
designated Korra Korrabes, after the farm property on which the specimens
were found in 1996-2000. A total of ~140 kg of related specimens were recovered,
including a large stone of 22 kg, and hundreds of smaller objects between
2 g and several kg. Cut surfaces indicate that Korra Korrabes is a breccia,
containing 10-20% of light grey-brown clasts up to 3 cm across in a uniform,
darker grey-brown host that contains abundant round chondrules, and irregular
grains of Fe-Ni metal and troilite up to 1 cm across. The vast majority
of the stone is unshocked, although some clasts show mild shock features
(stage S2), and one chondrule fragment is moderately shocked (stage S3).
Weathering grade varies between W1 and W2. Microprobe analyses indicate
variable compositions of olivine (Fa13.8-27.2, n = 152, PMD
= 7.82%) and low-Ca pyroxene (multiply twinned clinobronzite, Fs8.4-16.9,
n = 68). There is excellent preservation of magmatic textures and mineralogy
within many chondrules, including normally zoned olivine (Fa13.8-18.9)
and low-Ca pyroxene (Fs0.2-20.9) phenocrysts, and abundant glass,
some of whose compositions are unusually alkaline (Na2O + K2O
= 13.6-16.3 wt.%) and Ca-deficient (CaO = 0-0.75 wt.%), seemingly out of
magmatic equilibrium with associated clinoenstatite or high-Al calcic clinopyroxene
crystals. Textural and mineralogical features indicate that Korra Korrabes
is an H3 chondrite breccia, which represents the largest and least equilibrated
stony meteorite yet recovered from Namibia; it is now one of the four largest
unequilibrated ordinary chondrites worldwide.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Mineralogy of fine-grained material in the Krymka (LL3.1) chondrite
V. P. Semenenko, A. Bischoff*, I. Weber, C. Perron and A. L. Girich
*Correspondence author's address: Institut für Planetologie, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany; e-mail address: bischoa@nwz.uni-muenster.de
Abstract–Two dark lithic fragments and matrix of the Krymka LL3.1 chondrite were mineralogically and chemically studied in detail. These objects are characterised by the following chemical and mineralogical characteristics, which distinguish them from the host chondrite Krymka: 1) Bulk chemical analyses revealed low totals (systematically lower than 94 wt%) due to high porosity; 2) Enrichment in FeO and depletion in S, MgO and SiO2 due to a high abundance of Fe-rich silicates and low sulfide abundance; 3) Fine-grained, almost chondrule-free texture with predominance of a porous, cryptocrystalline groundmass and fine grains; 4) Occurrence of a small amount of once-molten material (microchondrules) enclosed in fine-grained materials; 5) Occurrence of accretionary features, especially unique accretionary spherules; 6) High abundance of small Ca,Al-rich inclusions in one of the fine-grained fragments. It is suggested that the abundance of CAIs in this fragment is one of the highest ever found in an ordinary chondrite.
Accretionary, fine-grained spherules within one of the fragments bear
fundamental information about the initial stages of accretion as well as
on the evolution of the clast, its incorporation, and history within the
bulk rock of Krymka. The differences in porosity, bulk composition, and
mineralogy of cores and rims of the fine-grained spherulitic objects allow
to speculate on the following processes: 1) Low velocity accretion of tiny
silicate grains onto the surface of coarse metal or silicate grains in
a dusty region of the nebula is the beginning of the formation of accretionary,
porous (fluffy) silicate spherules. 2) Within a dusty environment with
decreasing silicate/(metal+sulfide) ratio the porous spherules collected
abundant metal and sulfide particles together with silicate dust, which
formed an accretionary rim. Variations of the silicate/(sulfide+metal)-ratio
in the dusty nebular environment result in the formation of multi-layered
rims on the surface of the silicate-rich spherules. 3) Soft accretion and
lithification of rimmed, fluffy spherules, fine-grained, silicate-rich
dust, metal-sulfide particles, Ca,Al-rich inclusions, silicate-rich microchondrules,
and coarse silicate grains and fragments followed. 4) After low temperature
processing of the primary, accretionary rock collisional fragmentation
occurred, the fragments were subsequently coated by fine-grained material,
which was highly oxidized and depleted in sulfides. 5) In a final stage
this accretionary "dusty" rock was incorporated as a fragment within the
Krymka host.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Ion microprobe uranium-lead dating of zircons from the Lappajärvi impact crater, western Finland
Irmeli MÄNTTÄRI* and Marjatta KOIVISTO
*Correspondence author's address: Geological Survey of Finland, P.O.Box 96, FIN-02151 Espoo, Finland; e-mail address: irmeli.manttari@gsf.fi
Abstract–The lake Lappajärvi impact crater lies in Paleoproterozoic
Svecofennian metasedimentary rocks, on the western side of the Central
Finland granitoid complex (~1.9 Ga). Two conflicting ages have been reported
for the meteorite impact: an age of 77.3 + 0.4 Ma on the basis of
Ar-Ar whole-rock data from impact melt samples and a paleomagnetic age
of 195 Ma. During studies on impact crater indicator minerals at Lappajärvi,
zircons with an atypical appearance were found in suevite boulders. These
zircons seemed to have been affected by impact shock metamorphism and it
was considered that they would be good candidates for ion microprobe U-Pb
dating, allowing a new and independent age estimate for the impact event
at Lappajärvi. Four spot analyses on two black-coated zircons plotted
close to the upper intercept end of the concordia curve giving an approximate
age of 1.8 Ga for the source rock. Seventeen analyses were done on three
dull zircon grains showing patchy impact-related partial recrystallization.
Most of these data fell fairly well on a single discordia line with intercept
ages of 73.3 + 5.3 Ma and 1854 + 51 Ma. However, five of
the data spots near the lower intercept end fell on the younger side of
the line. This was interpreted to indicate post-impact loss of lead. Importantly,
the new ion microprobe U-Pb age of 73.3 + 5.3 Ma is in a very good
agreement with the previously reported Ar-Ar age.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Assessment of the interstellar processes leading to deuterium enrichment in meteoritic organics
Scott A. Sandford, Max P. Bernstein, and Jason P. Dworkin
*Correspondence author's address: NASA-Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035, USA; e-mail address: ssandford@mail.arc.nasa.gov
Abstract–The presence of isotopic anomalies is the most unequivocal
demonstration that meteoritic material contains circumstellar or interstellar
components. In the case of organic compounds in meteorites and interplanetary
dust particles (IDPs), the most useful isotopic tracer has been deuterium
(D). We discuss four processes that are expected to lead to D enrichment
in interstellar materials and describe how their unique characteristics
can be used to assess their relative importance for the organics in meteorites.
These enrichment processes are low temperature gas phase ion-molecule reactions,
low temperature gas-grain reactions, gas phase unimolecular photodissociation,
and ultraviolet photolysis in D-enriched ice mantles. Each of these processes
is expected to be associated with distinct regiochemical signatures (D
placement on the product molecules, correlation with specific chemical
functionalities, etc.), especially in the molecular population of polycyclic
aromatic hydrocarbons (PAHs). We describe these differences and discuss
how they may be used to delineate the various interstellar processes that
may have contributed to meteoritic D enrichments. We also briefly discuss
how these processes may affect the isotopic distributions in C, O, and
N in the same compounds.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The antiquity indicator 40Ar/36Ar for lunar surface samples calibrated by 235U - 136Xe dating
Otto Eugster, Dario Terribilini, Ernst Polnau and Jan Kramers
*Correspondence author's address: Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland; e-mail address: eugster@phim.unibe.ch
Abstract–Several solar gas rich lunar soils and breccias have
trapped 40Ar/36Ar ratios >10, although solar Ar is
expected to yield a ratio of <0.01. Radiogenic 40Ar produced
in the lunar crust from 40K decay was outgassed into the lunar
atmosphere, ionized, accelerated in the electromagnetic field of the solar
wind, and reimplanted into lunar surface material. The 40Ar
loss rate depends on the decreasing abundance of 40K. In order
to calibrate the time dependence of the 40Ar/36Ar
ratio in lunar surface material, the period of reimplantation of lunar
atmospheric ions and of solar wind Ar was determined using the 235U-136Xe
dating method that relies on secondary cosmic-ray neutron induced fission
of 235U. We identified the trapped, fissiogenic, and cosmogenic
noble gases in lunar breccia 14307 and lunar soils 70001-8, 70181, 74261,
and 75081. Uranium and Th concentrations were determined in the 74261 soil
for which we obtain the 235U-136Xe time of implantation
of 3.25+0.38-0.60 Ga ago. On the basis of several
cosmogenic noble gas signatures we calculate the duration of this near
surface exposure of 393 + 45 Ma and an average shielding depth below
the lunar surface of 73 + 7 g/cm2. A second, recent exposure
to solar and cosmic-ray particles occurred after this soil was excavated
from Shorty crater 17.2 + 1.4 Ma ago. Using a compilation of all
lunar data with reliable trapped Ar isotopic ratios and pre-exposure times
we infer a calibration curve of implantation times, based on the trapped
40Ar/36Ar ratio. A possible trend for the increase
with time of the solar 3He/4He and 20Ne/22Ne
ratios of about 12%/Ga and about 2%/Ga, respectively, is also discussed.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
MUSES-C target asteroid 1998 SF36: A reddened ordinary chondrite
Richard P. Binzel, Andrew S. Rivkin, Schelte J. Bus, Jessica M. Sunshine, and Thomas H. Burbine
*Correspondence author's address: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 USA; e-mail address: rpb@mit.edu
Abstract–Near-Earth asteroid 1998 SF36 is a planned target for
the Japanese MUSES-C sample return mission. High signal-to-noise and relatively
high resolution (50Å) visible and near-infrared spectroscopic measurements
obtained during this asteroid's favorable 2001 apparition reveal it to
have a red-sloped S(IV)-type spectrum with strong 1- and 2-um absorption
bands analogous to those measured for ordinary chondrite meteorites. This
red slope, which is the primary spectral difference between 1998 SF36 and
ordinary chondrite meteorites, is well modeled by the spectrum of 0.05%
nanophase iron (npFe0) proposed as a weathering mechanism by
Pieters et al. (2000). Asteroid 1998 SF36 appears to have a surface
composition corresponding to that of ordinary chondrite meteorites and
is most similar in spectral characteristics and modeled olivine / pyroxene
content to the LL chondrite class.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The formation of rims on Ca-Al-rich Inclusions: Step I - Flash Heating
David Wark and William V. Boynton
*Correspondence author's address: Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton 3168, Australia; e-mail address: davidwark@ozemail.com.au
Abstract–Wark-Lovering rims of six Ca-Al-Inclusions (CAIs) representing
the main CAI Types and Groups in Allende, Efremovka and Vigarano were microsurgically
separated and analysed by Neutron Activation. All the rims have similar
~4x enrichments, relative to the interiors, of highly refractory lithophiles
and siderophiles. The NAA results are confirmed by ion microprobe and SEM
analyses of rim perovskites and rim metal grains. Less refractory Eu, Yb,
V, Sr, Ca & Ni are less enriched in the rims. The refractory element
patterns in the rims parallel the patterns in the outer parts of the CAIs.
In particular, the rims on Type B1 CAIs have the igneously fractionated
REE pattern of the melilite mantle below the rim and not the REE pattern
of the bulk CAI, proving that the refractory elements in the rims were
derived from the outer mantle and were not condensates onto the CAIs. The
refractory elements were enriched in an Al2O3-rich
residue <50 um thick after the most volatile ~80% of the outermost 200
um of each CAI had been volatilized, including much Mg, Si and Ca. Some
volatilization occurred below the rim, and created refractory partial melts
that crystallized hibonite and gehlenitic melilite. The required 'Flash
Heating' probably exceeded 2000 oC, but for only a few seconds,
in order to melt only the outer CAI and to unselectively volatilize slow-diffusing
O isotopes which show no mass fractionation in the rim. The volatilization
did, however, produce 'heavy' mass-fractionated Mg in rims. In some CAIs
this was later obscured when 'normal' Mg diffused in from accreted olivine
grains at relatively high temperature (not the lower temperature meteorite
metamorphism) and created the ~50 um set of monomineralic rim layers of
pyroxene, melilite and spinel.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Early Solar System Events and Time Scales
G. W. Lugmair and A. Shukolyukov
*Correspondence author's address: Max-Planck-Institute for Chemistry, Cosmochemistry, PO 3060, 55020 Mainz, Germany; e-mail address: lugmair@mpch-mginz.mpg.de
Abstract–Some recent information on the Mn-Cr and Al-Mg systems
is reviewed. This information is used to derive constraints on the timing
of processes and events, which took place in the early solar system. Using
reasonable assumptions, a time-line is constructed where the estimated
age of the solar system is ~ 4571 Ma. This age is taken to mark the time
when most Ca-Al rich meteorite inclusions (CAIs) were starting to form,
a process that may have lasted for several 105 years. Almost
contemporaneously small planetesimals have accreted, that served to store
these CAIs for later dispersal among larger planetesimals. By the time
large numbers of planetesimals of several 10s of km in size had formed,
the interior of these objects started to melt through the decay of 26Al.
Collisional disruption of these planetesimals allowed gases, dust, and
melt to escape into the surrounding space. The fine droplets of melt reacted
with gas and dust to form chondrules, which, after rapid cooling, were
partially re-accreted onto the residual rubble pile. This process of primary
chondrule formation, in most cases involving several generations of planetesimals,
most plausibly lasted only for about 2 Ma. Towards the end of this period
and during the following 3 to 4 Ma planetary objects of several 100 km
in size were formed. They still stored enough energy to continue melting
from the inside to finally differentiate into chemically stratified layers,
with basaltic volcanism occurring within a few million years.
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