CCNet DIGEST, 17 June 1998

    NASA NEWS <>

    James Perry <>

    Vol.33  Iss.3  MAY 1998 



Douglas Isbell
Headquarters, Washington, DC                       June 16, 1998
(Phone:  202/358-1753)

Michael Mewhinney
Ames Research Center, Moffett Field, CA
(Phone:  650/604-3937)

Anne Watzman
Carnegie Mellon University, Pittsburgh, PA
(Phone:  412/268-3830)

RELEASE:  98-105

NASA scientists soon will explore a barren Arctic meteorite impact
crater to attempt to learn more about Mars and its early history, while
testing technologies useful for future robotic and human exploration of
the planet.

From June 22 to July 26, a 20-member science team from NASA and several
other research organizations will explore the Haughton Impact Crater
and its surroundings on Devon Island in the Arctic Circle.

Scientists consider the site a potential Mars analog because many of
its geologic features, such as the crater's ice-rich terrains, its
ancient lake sediments and nearby networks of small valleys, resemble
those reported at the surface of Mars.  The site may shed light in
particular on the early history of Mars, when the planet's climate may
have been wetter and warmer.

"The cold, relatively dry, windy and unvegetated environment at the
Haughton site is milder and wetter than present-day Mars, but it may
give us an idea of what early Mars was like and how some of its surface
features were formed," said Principal Investigator Dr. Pascal Lee of
NASA's Ames Research Center, Moffett Field, CA.

During the expedition, Dr. Omead Amidi and other engineers from
Carnegie Mellon University's Robotics Institute, Pittsburgh, PA, will
conduct field tests of an experimental, robotic helicopter. "The
mission provides a great opportunity to demonstrate the feasibility
and the value of robotic aircraft for mapping and surveying
applications," Amidi said.

Carnegie Mellon's small, 160-pound autonomous helicopter has
vision-based stability and position control, as well as an onboard
navigation computer, laser rangefinder and video system for site
mapping.  More information about the unpiloted helicopter may be found
at the following website:

In addition to the tests with the autonomous helicopter, scientists
also will conduct experiments with a ground-penetrating radar system, a
field spectrometer, drilling equipment and a stereo camera.

The radar system will be deployed in an attempt to map ground-ice and
other subsurface conditions within and outside the crater's 12-mile
(20-kilometer) diameter.  "The ability to find underground ice, both
for human consumption and geologic studies, will be critical in the
exploration of Mars," said Dr. Aaron Zent of Ames, Dr. Lee's post-
doctoral research advisor.

Scientists will use a field spectrometer to determine the site's
reflective qualities and better understand the crater's compositional
evolution.  In another experiment, scientists will use a portable drill
to obtain core samples from ten feet deep in the frozen ground. Core
samples of sediments from a lake that once occupied the crater will
provide information about local climate evolution.  Since the use of
liquid drilling lubricants might be precluded on Mars, none will be
used in this test.

A portable stereo camera system previously used by Carnegie Mellon's
Nomad rover during its unprecedented 133-mile wheeled trek through
Chile's Atacama Desert last summer will provide high-resolution images
of the site, and produce images for a 360 degree photo-realistic
virtual reality project being developed by Ames' Intelligent Mechanisms

Using laptop computer systems and "mobile workstations" developed by
Ames' Intelligent Mobile Technologies Team, scientists will communicate
with other field team members and send live images via a wireless link.
Team members will operate from a base camp on a terrace of the Haughton
River within the crater's perimeter and explore the site with
All-Terrain Vehicles.  Supplies will be brought in by Twin Otter
airplane, while a helicopter will aid exploration of remote sites.

As part of the expedition's educational outreach program, the following
website will be updated regularly with new data and images as

The total cost of the project is $80,000. NASA is partially funding the
project through a National Research Council grant. Additional support
is provided by Ames Research Center; NASA's Johnson Space Center,
Houston, TX; the Geological Survey of Canada; the Polar Continental
Shelf Project of Canada; the Nunavut Research Institute, Canada; the
Robotics Institute of Carnegie Mellon University; NovAtel
Communications, Calgary, Alberta, Canada; and the National Geographic


From James Perry <>

It's all a matter of priorities...

News of the Weird, April 24, 1998:

"In March, the Oregon Lottery Commission awarded a $124,000 contract to
a company to advise it on how best to restore its gambling games to
operating status in case of a catastrophic earthquake or asteroid
collision, with a goal of having video poker back up within two hours
of a disaster.  Several critics suggested there might be more pressing
problems after an earthquake, but the Commission pointed out that
gambling generates $1 million a day for the state."

I suppose they want to be sure the patrons enjoy at least an hour or
two of quality video poker before the tsunami arrives! 

Let's see, if an asteroid detection system costs $300 million, and the
casino generates $1 million a day in revenues for the state, Oregon
could have a nice asteroid detection system in less than a year...

James Perry


    Vol.33  Iss.3  MAY 1998 

A.E. Rubin: Correlated petrologic and geochemical characteristics of
CO3 chondrites. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3,


Many properties of CO3 chondrites have been shown previously to have
resulted from thermal metamorphism; petrologic subtypes 3.0-3.7 have
been assigned to members of the group. Additional properties that
correlate with the metamorphic sequence but seem to have resulted from
hydrothermal alteration include the modal abundance of amoeboid olivine
inclusions (AOI), chondrule size, the types of refractory inclusions
and whole rock O isotopic composition. The percentage of rimmed AOI
increases with petrologic subtype. The rims most likely formed during
hydrothermal alteration. The previously reported correlation between
AOI abundance and chondrite subtype is probably an artifact due to the
difficulty in recognizing small unrimmed AOI in the least metamorphosed
CO3 chondrites. Because large (greater than or equal to 200 mu m size)
porphyritic chondrules have nearly the same mean size in all CO3
chondrites, it seems likely that the correlation between chondrule size
and subtype is due to alteration of the smallest chondrules to the
point of unrecogizability as complete objects in the more metamorphosed
CO3 chondrites. The previously reported decrease in the proportion of
melilite-rich refractory inclusions with increasing petrologic subtype
may have resulted from more extensive hydrothermal alteration in
CO3.4-3.7 chondrites that converted primary melilite into Ca-pyroxene,
andradite and nepheline. Alteration probably caused the preferential
occurrence of O-16-poor oxygen isotopes in the more metamorphosed whole
rock samples. Copyright 1998, Institute for Scientific Information Inc.


N. Sugiura: Ion probe measurements of carbon and nitrogen in iron
meteorites. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3,


Carbon and nitrogen distributions in iron meteorites, their
concentrations in various phases, and their isotopic compositions in
certain phases were measured by secondary ion mass spectrometry (SIMS).
Taenite (and its decomposition products) is the main carrier of C,
except for IAB iron meteorites, where graphite and/or carbide
(cohenite) may be the main carrier. Taenite is also the main carrier of
N in most iron meteorites unless nitrides (carlsbergite CrN or roaldite
(Fe,Ni)(4)N) are present. Carbon and N distributions in taenite are
well correlated unless carbides and/or nitrides are exsolved. There
seem to be three types of C and N distributions within taenite. (1)
These elements are enriched at the center of taenite (convex type). (2)
They are enriched at the edge of taenite (concave type). (3) They are
enriched near but some distance away from the edge of taenite (complex
type). The first case (1) is explained as equilibrium distribution of C
and N in Fe-Ni alloy with M-shape Ni concentration profile. The second
case (2) seems to be best explained as diffusion controlled C and N
distributions. In the third case (3), the interior of taenite has been
transformed to the alpha phase (kamacite or martensite). Carbon and N
were expelled from the alpha phase and enriched near the inner border
of the remaining gamma phase. Such differences in the C and N
distributions in taenite may reflect different cooling rates of iron
meteorites. Nitrogen concentrations in taenite are quite high
approaching 1 wt% in some iron meteorites. Nitride (carlsbergite and
roaldite) is present in meteorites with high N concentrations in
taenite, which suggests that the nitride was formed due to
supersaturation of the metallic phases with N. The same tendency is
generally observed for C (i.e., high C concentrations in taenite
correlate with the presence of carbide and/or graphite). Concentrations
of C and N in kamacite are generally below detection limits. Isotopic
compositions of C and N in taenite can be measured with a precision of
several permil. Isotopic analysis in kamacite in most iron meteorites
is not possible because of the low concentrations. The C isotopic
compositions seem to be somewhat fractionated among various phases,
reflecting closure of C transport at low temperatures. A remarkable
isotopic anomaly was observed for the Mundrabilla (IIICD anomalous)
meteorite. Nitrogen isotopic compositions of taenite measured by SIMS
agree very well with those of the bulk samples measured by conventional
mass spectrometry. Copyright 1998, Institute for Scientific Information

S.B. Simon*), A.M. Davis, L. Grossman, E.K. Zinner: Origin of
hibonite-pyroxene spherules found in carbonaceous chondrites
METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3, pp.411-424

   AVE, CHICAGO, IL, 60637

We have studied both of the known glass-free, hibonite-pyroxene
spherules: MYSM3, from Murray (CM2), and Y17-6, from Yamato 791717
(CO3). They consist of hibonite plates (similar to 2 wt% TiO2tot)
enclosed in Al-rich pyroxene that has such high amounts of CaTs
(CaAl2SiO6) component, up to similar to 80 mol%, that it must have
crystallized metastably. Within the pyroxene, abundances of MgO and
SiO2 are strongly correlated with each other and are anticorrelated
with those of Al2O3, reflecting an anticorrelation between the diopside
and CaTs components of the pyroxene. in contrast with previous results
for Type B fassaite, however, we do not observe an anticorrelation
between MgO and TiO2tot, possibly reflecting different relative
distribution coefficients for Ti3+ and Ti4+ in the aluminous pyroxene
of the spherules from those found for fassaite in Type B inclusions.
Previously described hibonite-silicate spherules have Mg-26 deficits
but the present samples do not. Furthermore, the pyroxene in Y17-6 has
excess Mg-26, while the hibonite it encloses does not, indicating that
the two phases either had different initial Al-26/Al-27 ratios or
different initial Mg-26/Mg-24 ratios. The Ti isotopic compositions of
the present samples are highly unusual: delta(50)Ti = 103.4 +/- 5.2
parts per thousand in MYSM3 and - 61.4 +/- 4.1 parts per thousand in
Y17-6, which are among the largest Ti-50 anomalies reported for any
refractory inclusion. The textures suggest that hibonite crystallized
first; but based on the calculated bulk compositions of both spherules,
it is not the liquidus phase in either sample, which, suggests that the
hibonite in both samples is relict. The presence of ragged hibonite
grains in MYSM3 and rounded hibonite grains in Y17-6 and a lack of
isotopic equilibrium between pyroxene and hibonite support this
conclusion. The spherules crystallized from liquid droplets that
probably formed as a result of the melting of solid precursor grains
that included hibonite. The heating events were too short and/or not
hot enough to melt all the hibonite. The droplets cooled quickly enough
that CaTs-rich pyroxene crystallized instead of anorthite. Based on the
observed differences in isotopic composition, it is unlikely that the
precursors of the present samples formed in the same reservoir as each
other or as the previously described hibonite-silicate spherules,
providing further evidence of the isotopic heterogeneity of the early
solar nebula. Copyright 1998, Institute for Scientific Information Inc.

M.J. Genge & M.M. Grady: Melted micrometeorites from Antarctic ice with
evidence for the separation of immiscible Fe-Ni-S liquids during entry
heating. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3,


We report the discovery of four large (>50 mu m) cosmic spherules (CSs)
and a single scoriaceous micrometeorite (SMM) that contain evidence for
the separation of immiscible Fe-Ni-S liquids during atmospheric entry
heating. The particles contain segregated Fe-rich regions dominated by
either Ni-S-bearing Fe-oxides or iron sulphides and have textural
relations that suggest these separated from the silicate portions of
the particles as metallic liquids. The oxides, which may be hydrous,
are thought to result from alteration of metal and sulphide. The
compositions of the silicate portions of the CSs are equivalent to
spherules without Fe-rich regions, implying that metallic liquids are
exsolved during the heating of most spherules, but completely separate.
The single SMM has a very different composition from other scoriaceous
particles, and the occurrence of an exsolved metallic liquid probably
indicates extreme reduction during entry heating. The pyrolysis of
carbonaceous materials is the most likely explanation for reduction and
suggests that the precursor material of this particle was unusually
C-rich. This SMM might be, therefore, an appropriate candidate for a
large melted anhydrous or smectite interplanetary dust particle (IDP).
The exsolution of immiscible Fe-Ni-S liquids during entry heating will
result in systematic changes in the compositions of the remaining
silicate melt. Copyright 1998, Institute for Scientific Information

Y. Lin*) & M. Kimura: Anorthite-spinel-rich inclusions in the Ningqiang
carbonaceous chondrite: Genetic links with Type A and C inclusions.
METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3, pp.435-446


Ca-Al-rich inclusions (CAIs) were molten once, and the question of what
their precursors consisted of is controversial. In this paper, we
report a new type of anorthite-spinel-rich inclusion (ASI) in the
Ningqiang carbonaceous chondrite. The modal and bulk compositions of
the ASIS are similar to those of Type C inclusions but with nonmolten
textures. The ASIs are proposed to be the precursors of Type C
inclusions. The ASIs show alteration textures, with melilite being
replaced by anorthite and Ca-pyroxene. Furthermore, compositions of the
melilites and the calculated modal and bulk compositions of the
possible precursors of the ASIs are close to those of the spinel-rich
Type A inclusions in the Ningqiang meteorite, which suggests that ASIs
formed by alteration of the latter in the solar nebula. Petrography and
mineral chemistry of one Type C inclusion in Ningqiang are also
reported. Copyright 1998, Institute for Scientific Information Inc.

D. Brandt*), W.U. Reimold, A.J. Franzsen, C. Koeberl, L. Wendorff:
Geophysical profile of the Roter Kamm impact crater, Namibia.
METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3, pp.447-453


New gravity and magnetic data were obtained along ground profiles
across the Roter Kamm impact crater in the southern Namib desert of
Namibia. As the traverses of previous studies did not extend
sufficiently beyond the crater rim, it had not been possible to
adequately determine the regional background values. The gravity
results of this study are similar to those obtained by Fudali in 1973,
in that a negative, near-symmetrical anomaly was obtained over the
crater center. This anomaly conforms to the results expected for a
sediment and impact breccia-filled, simple bowl-shaped crater. The
magnetic results of this study, however, are different to those
previously reported, which is most probably as a result of the longer
profiles used in this new study. A Slight positive magnetic anomaly was
obtained over the crater interior. Short-wavelength, high-amplitude
anomalies observed in the vicinity of the crater rim reflect
magnetization contrasts that are probably related to brecciation and
block rotation. Modelling of the positive magnetic anomaly indicates
the possibility of a small magnetic body or lining at the crater
floor-breccia interface in the interior of the crater. Also presented
is a 10 m contour digital elevation model of the crater and its
environs. Copyright 1998, Institute for Scientific Information Inc.

N. Bhandari*), S.V.S. Murty, K.M. Suthar, A.D. Shukla, G.M. Ballabh,
M.S. Sisodia, V.K. Vaya: The orbit and exposure history of the Piplia
Kalan eucrite. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3,


Cosmogenic radionuclides, particle tracks and rare gases have been
measured in two fragments of the Piplia Kalan eucrite that fell in
Rajasthan, India on 1996 June 20. The cosmic-ray exposure age of the
meteorite is calculated to be 23 Ma, which is similar to ages of some
other eucrites. The track density in feldspars and pyroxenes varies
between 0.2 x 10(6) to similar to 4.5 x 10(6) cm(-2) The mass ablation
of the meteorite, based on the distribution of track density in
near-surface samples of the two fragments, is calculated to be similar
to 75%, which corresponds to an entry velocity of similar to 17 km/s.
The orbital parameters of the eucrite have been computed from the
radiant of the meteor trail and the geocentric velocity. The best
estimates are a = 2.47 AU, e = 0.62 and i = 7.54 degrees, which is
similar to the orbital elements of other meteorites, most of which have
been inferred to originate within 2..6 AU of the Sun. The activity of
the radionuclide Al-26 agrees with the expected production rate;
whereas the shortlived radionuclides Na-22, Mn-54, Sc-46 etc. have
levels that are consistent with the galactic cosmic-ray fluxes that are
expected during the solar minimum period before the time of fall. All
the cosmogenic effects (ie., radio-and stable-nuclides and particle
tracks) are consistent with the meteoroid having had a simple,
one-stage exposure history in interplanetary space. Lower radiogenic
ages of U, Th-He (0.7 Ga) and K-Ar (3.6 Ga) indicate severe losses of
He-4 and Ar-40, as observed in most eucrites. A Pu-Xe age, concordant
with Angra dos Reis, shows that Piplia belongs to the ''old'' eucrite
group. Copyright 1998, Institute for Scientific Information Inc.

N. Sugiura*), K. Kiyota, K. Hashizume: Nitrogen components in primitive
ordinary chondrites. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33,
No.3, pp.463-482


Nitrogen and Ar in more than 20 primitive ordinary chondrites were
studied by a stepped combustion method. Several N carriers that are
characterized by N isotopic composition, N release pattern and trapped
Ar release pattern are recognized in the primitive ordinary chondrites.
Large fractions of anomalous N and associated Ar are removed by acid
treatment in most cases. The N isotopic anomalies cannot be explained
by known presolar grains (with a possible exception of graphite), and
some of the N isotopic anomalies may be due to unknown presolar grains.
There is no specific relationship between the type of N carriers
contained in an ordinary chondrite and the chemical type (H, L, or LL)
of the chondrite. It is likely that as a result of impacts, the
carriers of isotopically anomalous N were mixed in various parent
bodies as rock fragments rather than as individual fine particles. The
presence of distinctive N isotopic anomalies in primitive meteorites
indicates that the primitive solar nebula may have been heterogeneous
either spatially or temporally. Copyright 1998, Institute for
Scientific Information Inc.

E.P. Turtle & E. Pierazzo: Constraints on the size of the Vredefort
impact crater from numerical modeling. METEORITICS & PLANETARY SCIENCE,
1998, Vol.33, No.3, pp.483-490


The Vredefort structure in South Africa was created by a meteorite
impact about two billion years ago. Since that time, the crater has
been deeply eroded; so to estimate its original size, researchers have
had to rely heavily upon comparison to other terrestrial impact
structures. Recent estimates of the original crater diameter range from
160 km to as much as 400 km. In this study, we combined the
capabilities of both hydrocode and finite-element modeling, using the
former to predict where the pressure of an impact-generated shock wave
would have been high enough to form planar deformation features (PDFs)
and shatter cones and the latter to follow the subsequent displacement
of these shock isobars during the collapse of the crater. We
established constraints on the sizes of the projectile and the
transient crater (and, thus, on the size of the final crater) by
comparing the observed locations of PDFs around Vredefort to the
results of our simulations of impacts by projectiles of various sizes.
These simulations indicate that a rocky projectile with a diameter of
similar to 10 km, impacting vertically at a velocity of 20 km/s
generates shock pressures that are consistent with the distribution of
PDFs around Vredefort. These projectile parameters correspond to a
transient crater similar to 80 km in diameter or a final crater similar
to 120-160 km in diameter. Allowing for uncertainties in our modeling
procedures, we consider final craters 120 to 200 km in diameter to be
consistent with the observed locations of PDFs at Vredefort. The shock
pressure contour corresponding to the formation of shatter cones is
almost horizontal near the surface, making the locations of these
features less useful constraints on the crater size. However, they may
provide a constraint on the amount of erosion that has occurred since
the impact. Copyright 1998, Institute for Scientific Information Inc.

A.J. Conway*) & P.A. Bland: A genetic algorithm scheme for pairing
meteorite finds. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3,


A genetic algorithm is employed to perform the pairing of meteorite
fragments based on various characteristics measured from thin sections
using an image analysis program and from analyses routinely carried out
during classification. The genetic algorithm searches for best group
pairings by generating a population of trial pairs, linking them
together to form groups, and evolving the population, so that only
pairs that are members of likely pairing groups survive to the next
generation of the population. In this way, meaningful pairing groups
will emerge from the population, as long as characteristics from within
real pairing groups have variance sufficiently small compared to the
variance between groups. What constitutes ''sufficiently small'' is
discussed and investigated by testing the genetic algorithm method on
artificial data, which shows that, in principle, the method can achieve
a 100% success rate. The method is then tested on real data whose
pairing groups are definitely known. This is achieved by gathering data
from the image processing of several scenes of the same meteorite thin
section, treating each scene as a separate fragment. Using thin
sections from the Reg el Acfer meteorite population, we find that the
genetic algorithm identifies almost all of the main pairing groups,
with about half the groups being found in their entirety; the pairwise
success rate being 76%. Although this methodology requires some
refinement before it could be applied to a population of meteorite
fragments, these preliminary results are encouraging. The potential
benefit of an automated approach lies in the tremendous savings in time
and effort, allowing meaningful and reproducible pairings to be made
from data sets that are prohibitively large for a human being.
Copyright 1998, Institute for Scientific Information Inc.

Y. Lin*) & M. Kimura: Petrographic and mineralogical study of new EH
melt rocks and a new enstatite chondrite grouplet. METEORITICS &
PLANETARY SCIENCE, 1998, Vol.33, No.3, pp.501-511


Enstatite chondrites are classified into EH and EL groups, and some of
them were melted once. In this paper, we report petrography and mineral
chemistry of five new Antarctic enstatite chondrites. Yamato 793225 is
characterized by intermediate properties between the EH and EL groups
and probably represents a new grouplet of enstatite chondrites. The
three paired meteorites (Y-8404, Y-8414 and Y-86004) may have cooled
rapidly near the surface of the parent body as impact EH melt rocks.
Yamato 82189 is also classified as EH melt rock, but it experienced
slow cooling as did Y-793225. Yamato 82189 contains the first
occurrence of phlogopite in enstatite meteorites. Copyright 1998,
Institute for Scientific Information Inc.

C. Koeberl*), W.U. Reimold, S.B. Shirey: The Aouelloul crater,
Mauritania: On the problem of confirming the impact origin of a small
crater. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3, pp.513-517


The impact origin of small craters in sedimentary rocks is often
difficult to confirm because of the lack of characteristic shock
metamorphic features. A case in point is the 3.1 Ma Aouelloul crater
(Mauritania), 390 m in diameter, which is exposed in an area of
Ordovician Oujeft and Zli sandstone. We studied several fractured
sandstone samples from the crater rim for the possible presence of
shock metamorphic effects. In thin section, a large fraction of the
quartz grains show abundant subplanar and planar fractures. Many of the
fractures are healed and are evident only as fluid inclusion trails. A
few grains showed sets of narrow and densely spaced fluid inclusions
trails in one (rarely two) orientations per grain, which could be
possible remnants of planar deformation features (PDFs), although such
an interpretation is not unambiguous. In contrast, an impact origin of
the crater is confirmed by Re-Os isotope studies of the target
sandstone and glass found around the crater rim, which show the
presence of a distinct extraterrestrial component in the glass.
Copyright 1998, Institute for Scientific Information Inc.

K. Tomeoka & T. Kojima: Arcuate band texture in a dark inclusion from
the Vigarano CV3 chondrite: Possible evidence for early sedimentary
processes. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3,


A dark inclusion in the Vigarano CV3 carbonaceous chondrite consists
almost exclusively of small (<5 mu m in diameter) grains of Fe-rich
olivine and is devoid of chondrules, Ca-Al-rich inclusions (CAIs) and
their pseudomorphs. In backscattered electron images, this dark
inclusion shows an unusual texture comprising a network of arcuate
bands. Two or more bands occur roughly parallel, forming a set of
succesive parallel bands, some crosscutting one another. The bands
contain slightly higher amounts of relatively small (<1 mu m) olivine
grains and so are more densely packed than other areas. The olivine
grains in the bands are slightly more Fe-rich than those in other
areas. The bands commonly show gradation on the concave side due to a
decrease in the abundance of the small Fe-rich olivine grains.
Texturally, the arcuate bands closely resemble ''dish structures'' that
are commonly observed in siltstones and sandstones on Earth. Dish
structures are characterized by thin, dark-colored, subhorizontal to
concave-upward laminations that are rich in relatively fine-grained
material. On Earth, dish structures form during compaction and
dewatering of unconsolidated fine-grained sediments; they are one of
the characteristic sedimentary structures formed through fluidization
of fine grains. The dark inclusion in Vigarano, therefore, provides the
first evidence that sedimentary processes due to water migration may
have taken place within planetesimals and further suggests that
fluidization may have played a significant role in the formation of the
carbonaceous chondrites. Copyright 1998, Institute for Scientific
Information Inc.

J.N. Goswami*), S. Sahijpal, K. Kehm, C.M. Hohenberg, T. Swindle,
J.N. Grossman: In situ determination of iodine content and iodine-xenon
systematics in silicates and troilite phases in chondrules from the LL3
chondrite Semarkona. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33,
No.3, pp.527-534


Iodine concentrations in small domains (similar to 10 mu m) of
silicates and troilite (FeS) phases in three chondrules from the
Semarkona (LL3) meteorite were determined by an ion microprobe.
Independent determination of I content in some of these phases was
accomplished by in situ laser probe mass spectrometric analysis of
I-derived Xe-128 in one of these neutron-irradiated chondrules. The ion
microprobe data suggest low I content for olivines (20-45 ppb) and
relatively higher values for pyroxene and glass (mesostasis) (40-160
ppb). The broad similarity in the measured I contents in pyroxenes in a
porphyritic pyroxene chondrule by ion microprobe (42-138 ppb) and by
laser probe (37-76 ppb) demonstrate the feasibility of in situ
determination of I content in silicate phases via ion microprobe. The I
contents in troilite measured by ion microprobe, however, are prone to
uncertainty because of the lack of a sulfide standard. The ion
microprobe data suggest I content of >1 ppm in troilite, if the
calibration from our silicate standard is used. However, the noble gas
data suggest that the I content in troilite is comparable to that in
silicates. We attribute this apparent discrepancy to an enhanced
sputter ion yield of I from sulfides. Iodine-derived Xe-129, excesses
were observed in both pyroxene and troilite within this chondrule. The
I-Xe model ages of these selected phases are consistent with the I-Xe
studies of the bulk chondrule. The individual data points fall on or
near the isochron obtained from the bulk chondrule, although all except
the most radiogenic data point contain evidence of low-temperature
uncorrelated iodine. Copyright 1998, Institute for Scientific
Information Inc.

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