of 1
Meteoritics
&
Planetary
Science
37
(Supplement),
A9-A156
(2002)
Available online at
http://www.uark.edu/meteor
Abstracts
MICROSTRUCTURAL
CHARACTERIZATION
OF
INCLUSIONS
IN
VIGARANO
MATRIX
OLIVINES.
Neyda
M.
Abreu and
Adrian
J.
Brearley.
Dept.
of
Earth
and
Planetary
Sciences,
University
of
New
Mexico,
Albuquerque,
NM
87131,
USA;
abreu@unm.edu.
Introduction:
The CV
chondrites
are an
important
group
of
carbonaceous chondrites that provide significant insights into a range of
nebular and parental body processes. One key question concerns the
differencesbetweenthe oxidized and reduced sub-groups
ofCV
chondrites.
In this study, we have used
transmission
electron
microscopy
to
characterize the matrix
of
the reduced type CV chondrite Vigarano, as
part of a continued effort to differentiate primary nebular characteristics
from secondary overprints.
Results:
FeO-rich olivine is the dominant phase in both matrix and
rims and has an average composition
of
approximately 56.4 mol% Fa.
TEM studies show that inclusions are commonly present in many olivine
grains in both rims and matrix, although a significant number of grains are
inclusion-free. So far, we have identified five distinct types
of
inclusions
in olivine grains [1,2]: (1) hercynitic spinel, (2) Fe-Ni metal; (3) carbon-
rich; (4) voids, and (5) rare inclusions rich inAI and Ca. Here we focus on
the spinel and Fe-Ni inclusions. The spinels are 5-50 nm in diameter with
rounded shapes and are
heterogeneously
distributed from one grain to
another. Compositionally, they are consistently higher in Fe than the
host olivine grains. Due to their fine-grained nature, we cannot obtain
clean analyses
of
the spinel phase. However, using a compositional
extrapolation
technique
[3], we
determined
an
approximate
spinel
composition
of
FeS.7sMgl.1SCro.33Cao.llAlll.3
0
24,
(Mg/
(Mg+Fe)=0.17).
We also identified Fe-Ni metal grains included in several fine-grained
rim olivine grains. These inclusions are texturally similar to the spinel
inclusions and show sharp interfaces with the host grains. They are
approximately 20 nm in diameter. Their compositions have not been
established clearly, but they appear to be relatively Ni-rich.
Discussion: Several previously unrecognized inclusion phases occur
in Vigarano matrix olivines that may provide useful constraints on the
origin
ofthe
matrix materials. There is considerable heterogeneity in the
abundance and nature
of
these inclusions, which suggests that there are
several different sources for the matrix olivines. The inclusion population
may therefore represent a primitive, nebular, signature. However, unlike
the highly unequilibrated olivines in matrix
ofKaba,
olivines in Vigarano
matrix have a narrow range
of
compositions
that is probably due to
equilibration during mild asteroidal metamorphism. This is supported by
the composition
of
the spinel inclusions within olivine, which are always
more Fe-rich than the host olivine (Mg/(Mg+Fe) = 0.17
sp
cf. 0.56
0
1),
based on a comparison with coexisting hercynitic spinel and olivine in the
metamorphosed CK4 chondrite,
Maralinga
[4]. In Maralinga, the spinel
is also enriched in Fe (Mg/(Mg+ Fe) = 0.43) relative to coexisting olivine
(Mg/(Mg+Fe) = 0.66). Although these values are significantly different
from Vigarano, probably due to a higher metamorphic temperature, the
relative sense
of
Mg-Fe partitioning is the same and is consistent with
solid state elemental partitioning. Wetherefore suggest that the hercynitic
spinels may have originally been condensates of MgAl204 that could
have served as seeds for olivine nucleation during condensation in the
solar nebula.
Acknowledgements:
Supported by NASA grant NAG5-9798 to
AJ.
Brearley (PI).
References:
[1] Abreu, N.M. and Brearley,
AJ.
(2002)
LPSe
XXXIII, 1542. [2]
Brearley,AJ.
and Abreu, N.M.
(2001)M4PS36,A28.
[3] Cliff, G. (1983)
Inst. Phys.
Can!
Ser.
68, 63-66. [4] Keller, L. et al.
(1992)
Meteoritics
27, 87-91.
A9
MARTIAN
ROCKS,
MINERALS,
AND
MANTLES.
Arden Albee,
California
Institute
of
Technology,
1201 E. Calif.
Blvd.,
Pasadena
CA.,91125.
The
variable
nature
of
Mars was first
observed
almost
400
years
ago and
modern
observations
began
almost
40 years ago,
culminating
with the
flotilla
of
spacecraft
now at or
heading
for Mars. We now
know that the
atmosphere,
which
produced
the
visible
variation
of
Mars, has also
covered
it with
a mantle
that makes
difficult
any
detailed
investigation
of
the rocks and
minerals
of
Mars.
This mantle was produced by
disaggregation
of
bedrock, by settling
of
wind-lofted
dust and by
aeolian
drifting
and sorting. The
thickness
varies
greatly-in
some areas simply
softening
the outlines
of
bedrock
and
geomorphic
features and in others totally
obscuring
such evidence.
Nevertheless,
remote
sensing
has
provided
some
insight
into the
petrology
and
mineralogy
of
Mars.
Thermal
emission
spectroscopy
in the low albedo areas has shown
the
presence
of
two
somewhat
different,
but
basically
basaltic,
rock
compositions
whose
distribution
has been
globally
mapped.
Basalt
dominates
the
ancient
southern
highlands
and
andesitic
basalt
the
younger
northern
lowlands.
Elemental
analyses
by
landers
help to
confirm
this
spectroscopic
identification.
Infrared
spectra
have
demonstrated
the
abundant
presence
of
plagioclase,
calcic
pyroxene,
and glass, local abundant
of
olivine and hematite, and the low abundance
or lack
of
quartz, micas,
carbonates,
and
sulfates.
The
absence
of
a
pervasively
weathered
surface,
at least in the
darker
areas is
strongly
indicated.
High-resolution
images
show
an
incredible
abundance
of
depositional
aeolian
landforms
as well as the
pervasive
mantle. The
nature
of
the
surface
soil, sand, and dust
remain
a problem.
The soils
are
commonly
interpreted
as
secondary
weathering
products
of
mafic
igneous
rocks,
possibly
resulting
from
palagonitization
of
basalt.
However,
a high
abundance
of
weathering
minerals
has not been
confirmed
by remote sensing
observations.
The bright and dark albedo
areas are well matched,
respectively,
by a fine and coarse
aggregate
of
pyroxene
and
plagioclase.
Thus the
bright
and the dark
regions-and
much
of
the soil and
dust-may
simply
be
basaltic
material,
broken
up by
impact
processes
or
physical
weathering
and not
involving
extensive
hydrolytic
or
sulfuric
alteration
at all.
©
Meteoritical
Society,
2002.
Printed in USA.