Dhofar 225 and Dhofar 735; relationship to CM2 chondrites and metamorphosed carbonaceous chondrites, Belgica-7904 and Yamato-86720


Ivanova, M.A.; Lorenz, C.A.; Nazarov, M.A.; Brandstaetter, F.; Franchi, I.A.; Moroz, L.V.; Clayton, R.N.; Bychkov, A.Y.

Meteoritics & Planetary Science 45(7): 1108-1123

2010


Dhofar (Dho) 225 and Dho 735 are carbonaceous chondrites found in a hot desert and having affinities to Belgica-like Antarctic chondrites (Belgica [B-] 7904 and Yamato [Y-] 86720). Texturally they resemble CM2 chondrites, but differ in mineralogy, bulk chemistry and oxygen isotopic compositions. The texture and main mineralogy of Dho 225 and Dho 735 are similar to the CM2 chondrites, but unlike CM2 chondrites they do not contain any (P, Cr)-sulfides, nor tochilinite 6Fe(0.9) S*5(Fe,Mg)(OH)2 . H2 O-contents of Dho 225 and Dho 735 (1.76 and 1.06 wt%) are lower than those of CM2 chondrites (2-18 wt%), but similar to those in the metamorphosed carbonaceous chondrites of the Belgica-like group. Bulk compositions of Dho 225 and Dho 735, as well as their matrices, have low Fe and S and low Fe/Si ratios relative to CM2 chondrites. X-ray powder diffraction patterns of the Dho 225 and Dho 735 matrices showed similarities to laboratory-heated Murchison CM2 chondrite and the transformation of serpentine to olivine. Dho 225 and 735's oxygen isotopic compositions are in the high18 O range on the oxygen diagram, close to the Belgica-like meteorites. This differs from the oxygen isotopic compositions of typical CM2 chondrites. Experimental results showed that the oxygen isotopic compositions of Dho 225 and Dhofar 725, could not be derived from those of typical CM2 chondrites via dehydration caused by thermal metamorphism. Dho 225 and Dho 735 may represent a group of chondrites whose primary material was different from typical CM2 chondrites and the Belgica-like meteorites, but they formed in an oxygen reservoir similar to that of the Belgica-like meteorites.

The
Meteoritical
Society
Meteoritics
&
Planetary
Science
45,
Nr
7,
1108-1123
(2010)
doi:
10.1111/j.1945-5100.2010.01064.x
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
chondrites
and
metamorphosed
carbonaceous
chondrites,
Belgica-7904
and
Yamato-86720
Marina
A.
IVANOVA
1*
,
Cyrill
A.
LORENZ
1
,
Mikhail
A.
NAZAROV
1
,
Franz
BRANDSTAETTER
2
,
Ian
A.
FRANCHI
3
,
Lyuba
V.
MOROZ
4
'
5
,
Robert
N.
CLAYTON
6
,
and
Andrew
Yu.
BYCHKOV
7
I
Vernadsky
Institute
of
Russian
Academy
of
Sciences,
Moscow,
Russia
2
Natural
History
Museum,
A-1014,
Vienna,
Austria
3
The
Open
University,
Milton
Keynes,
MK7
6AA,
UK
4
Institute
of
Planetology,
University
of
Milnster,
Wilhelm-Klemm
Str.
10,
D-48149,
Milnster,
Germany
5
Institute
of
Planetary
Research,
German
Aerospace
Center,
Rutherford
Str.
2,
D-12489,
Berlin,
Germany
6
Enrico
Fermi
Institute,
University
of
Chicago, Chicago,
Illinois,
USA
7
M.
V.
Lomonosov
Moscow
State
University,
Moscow
119992,
Russia
*
Corresponding
author.
E-mail:
(Received
20
August
2008;
revision
accepted
30
April
2010)
Abstract—Dhofar
(Dho)
225
and
Dho
735
are
carbonaceous
chondrites
found
in
a
hot
desert
and
having
affinities
to
Belgica-like
Antarctic
chondrites
(Belgica
[B-]
7904
and
Yamato
[Y-]
86720).
Texturally
they
resemble
CM2
chondrites,
but
differ
in
mineralogy,
bulk
chemistry
and
oxygen
isotopic
compositions.
The
texture
and
main
mineralogy
of
Dho
225
and
Dho
735
are
similar
to
the
CM2
chondrites,
but
unlike
CM2
chondrites
they
do
not
contain
any
(P,
Cr)-sulfides,
nor
tochilinite
6Fe
0
.
9
S*5(Fe,Mg)(OH)
2
.
H
2
0-contents
of
Dho
225
and
Dho
735
(1.76
and
1.06
wt%)
are
lower
than
those
of
CM2
chondrites
(2-18
wt%),
but
similar
to
those
in
the
metamorphosed
carbonaceous
chondrites
of
the
Belgica-like
group.
Bulk
compositions
of
Dho
225
and
Dho
735,
as
well
as
their
matrices,
have
low
Fe
and
S
and
low
Fe/Si
ratios
relative
to
CM2
chondrites.
X-ray
powder
diffraction
patterns
of
the
Dho
225
and
Dho
735
matrices
showed
similarities
to
laboratory-heated
Murchison
CM2
chondrite
and
the
transformation
of
serpentine
to
olivine.
Dho
225
and
735's
oxygen
isotopic
compositions
are
in
the
high
18
0
range
on
the
oxygen
diagram,
close
to
the
Belgica-like
meteorites.
This
differs
from
the
oxygen
isotopic
compositions
of
typical
CM2
chondrites.
Experimental
results
showed
that
the
oxygen
isotopic
compositions
of
Dho
225
and
Dhofar
725,
could
not
be
derived
from
those
of
typical
CM2
chondrites
via
dehydration
caused
by
thermal
metamorphism.
Dho
225
and
Dho
735
may
represent
a
group
of
chondrites
whose
primary
material
was
different
from
typical
CM2
chondrites
and
the
Belgica-like
meteorites,
but
they
formed
in
an
oxygen
reservoir
similar
to
that
of
the
Belgica-like
meteorites.
INTRODUCTION
The
discovery
of
three
unusual
Antarctic
carbonaceous
chondrites—Belgica
(B-)
7904,
Yamato
(Y-)
82162
and
Y-86720,
named
the
Belgica
group
meteorites,
led
to
a
series
of
studies
of
their
thermal
history
(Akai
1988,
1990;
Tomeoka
et
al.
1989a,
1989b;
Tomeoka
1990;
Bischoff
and
Metzler
1991;
Ikeda
1992;
Kojima
et
al.
1994;
Lipschutz
et
al.
1999).
It
was
shown
that
the
mineralogic
and
petrologic
properties
of
these
meteorites
suggest
that
they
could
be
classified
as
thermally
metamorphosed
CI
(Y-82162)
or
CM
chondrites
(B-7904
and
Y-86720),
and
that
their
oxygen
isotopic
compositions
differ
from
those
of
typical
CI
and
CM.
Several
more
metamorphosed
C
chondrites,
which
appear
to
have
been
naturally
heated
to
various
temperatures
from
400
to
over
700
°C
on
their
parent
asteroids,
have
recently
been
recognized
(Tonui
et
al.
2002;
Zolensky
et
al.
2005).
They
include:
Y-793321,
WIS
91600,
EET
90043,
A
881655,
PCA
©
The
Meteoritical
Society,
2010.
1108
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
and
metamorphosed
carbonaceous
chondrites
1109
91008,
Y-96029,
Y-86789.
However,
oxygen
isotopic
compositions
for
the
most
of
these
meteorites
have
not
been
determined,
and
they
do
not
belong
to
the
Belgica-
like
meteorites.
Based
on
petrographic
studies,
Krot
et
al.
(1997)
proposed
that
some
asteroids
experienced
at
least
two
cycles
of
aqueous
alteration,
with
a
metamorphic
heating
event
in
between.
Some
heating
events
might
have
included
both
hydration
and
dehydration.
Besides
aqueous
alteration
and
thermal
metamorphism,
postaccretional
processes
may
also
have
included
shock
metamorphism,
oxidation,
sulfidization
and
volatilization
(Lipschutz
et
al.
1999;
Tonui
et
al.
2002).
Thus
so-called
"primitive"
asteroids
may
actually
have
had
a
very
complex
history,
and
the
rare
metamorphosed
carbonaceous
chondrites
give
clues
for
understanding
of
the
possible
alteration
processes
that
may
have
occurred
on
the
"primitive"
asteroids.
Ultraviolet,
visible
and
near
infrared
reflectance
spectra
of
these
meteorites
resemble
those
of
C-,
G-,
B-,
and
F-
asteroids
(Hiroi
et
al.
1993),
which
suggests
that
the
surfaces
of
these
asteroids
may
contain
metamorphosed
carbonaceous
chondrite-like
material.
Before
2002,
all
metamorphosed
carbonaceous
chondrites
were
found
in
Antarctica,
and
none
were
observed
falls.
Two
meteorites,
Dhofar
(Dho)
225
and
Dho
735,
were
recently
discovered
in
the
desert
of
Oman
(Ivanova
et
al.
2004, 2005,
2006).
They
have
similarities
to
the
Belgica-like
chondrites.
In
this
paper
we
report
results
of
mineralogical,
petrological,
chemical,
and
oxygen
isotopic
studies
of
Dho
225
and
Dho
735,
compare
them
with
CM2
chondrites
and
with
the
thermally
metamorphosed
CM
Belgica-like
chondrites—B-7904
and
Y-86720.
We
also
discuss
Dho
225
and
735's
accretional
and
postaccretional
history.
The
official
classification
of
Dho
225
is
CM
anomalous
(Russell
et
al.
2002);
Dho
735
is
classified
as
CM2
(Russell
et
al.
2003).
To
understand
potential
genetic
links
between
typical
and
metamorphosed
CM
chondrites
and
to
study
changes
in
their
oxygen
isotopic
compositions
and
mineralogy,
we
report
results
of
experimental
heating
of
two
CM2
chondrites,
Murchison
and
Mighei,
and
studies
of
their
mineralogical
properties
and
oxygen
isotopic
compositions.
SAMPLES
AND
ANALYTICAL
PROCEDURES
Two
polished
sections
of
Dho
225
(approximately
2
cm
2
total
area)
and
a
polished
section
of
Dho
735
(approximately
1
cm
2
total
area)
were
studied
by
optical
microscopy,
backscattered
electron
(BSE)
imaging,
X-
ray
elemental
mapping,
and
electron
probe
microanalysis
at
the
Vernadsky
Institute
of
the
Russian
Academy
of
Sciences
in
Moscow
and
the
Museum
of
Natural
History
in
Vienna.
Mineral
chemical
compositions
were
determined
with
a
CAMECA
SX100
at
the
Natural
History
Museum
in
Vienna.
Chemical
analyses
were
performed
at
15
kV
accelerating
voltage
for
silicates,
and
20
kV
for
metal
and
sulfides,
20
nA
beam
current,
and
40
s
counting
time.
Well-characterized
silicates,
oxides,
metals
and
sulfides
were
used
as
standards.
Chemical
compositions
of
the
meteorites'
matrices
were
measured
by
acquiring
approximately
7
x
7
gm
broad
beam
analyses.
Matrix
corrections
were
applied
using
the
PAP
software
routine—a
general
model
for
calculating
X-ray
intensities.
Bulk
Chemistry
Traditional
wet
chemical
analyses
were
carried
out
at
the
Vernadsky
Institute
of
the
Russian
Academy
of
Sciences
according
to
the
procedure
described
by
Dyakonova
and
Kharitonova
(1966).
A
1
g
sample
was
crushed
and
powdered
in
an
agate
mortar
to
obtain
a
representative
average
sample
for
bulk-rock
major
and
trace
element
determinations.
100
mg
of
the
sample
we
analyzed
for
Al,
Ti,
Ca,
Mg,
Cr,
Mn,
K,
Na,
Ni,
Co,
Fe,
using
inductively
coupled
plasma
mass-spectrometry
(ICP).
300
mg
samples
of
Dho
225
and
Dho
735
samples
were
used
to
measure
Si,
Ti,
Al,
Cr,
Fe,
Mn,
Mg,
Ca,
Na,
K,
Ni,
Co,
P,
and
S
using
X-ray
fluorescence
(XRF)
analysis.
H
2
O,
C,
and
S
were
measured
with
a
CHN-analyzer
at
a
temperature
(T)
of
1000
°C.
Before
determination
of
structural
H
2
O
the
sample
was
heated
up
to
110
°C
until
a
constant
weight
was
reached,
to
exclude
terrestrial
water.
We
also
determined
water
content
by
the
Penfield
method
(Peck
1964;
Jarosewich
1990)
and
the
results
of
both
methods
were
very
similar.
Oxygen
Isotopic
Composition
Separate
chips
of
Dho
225
and
Dho
735,
weighing
approximately
20
mg,
were
measured
to
determine
bulk
oxygen-isotopic
compositions
at
the
University
of
Chicago,
following
the
procedure
of
Clayton
and
Mayeda
(1984).
Heating
Experiment
A
heating
experiment
on
the
Murchison
and
Mighei
CM2
chondrites
was
conducted
at
the
Vernadsky
Institute
Laboratory
of
Meteoritics.
Chips
of
Murchison
(400
mg)
and
Mighei
(360
mg)
were
powdered
and
homogenized,
and
each
was
split
into
three
aliquots:
67.8,
68.7,
and
66.3
mg
(Murchison)
and
55.2,
58.9
and
65.0
mg
(Mighei).
The
aliquots
were
placed
into
molybdenum
crucibles
and
heated
under
vacuum
.
.
1
.
.
_
."
."
.
1
41
.
J
64
"O
rr
4*
.
Zoned
olivine
.
of
GEOKHI
COMPO
5.1AV
:.•22.0
WEI
8.5mri
a
Koiated
i
4
1:
'
100
pm
1110
M.
A.
Ivanova
et
al.
(approximately
6
x
10
-6
atm).
The
Murchison
aliquots
were
heated:
the
first
aliquot
to
450°,
the
second
to
600°,
and
the
third
to
930
°C.
The
Mighei
aliquots
were
heated:
the
first
aliquot
to
400°,
the
second
to
600°,
and
the
third
to
800
°C.
Each
aliquot
was
heated
for
on
hour
at
its
prescribed
temperatures.
The
highest
temperatures
(800-930
°C)
correspond
to
the
temperatures
at
which
olivine
and
enstatite
form
by
complete
dehydration
of
serpentine
and
saponite
(Akai
1990).
The
samples
were
weighed
before
and
after
heating.
Oxygen
isotopic
compositions
of
all
heated
and
unheated
samples
of
Murchison
and
Mighei
were
determined
by
laser
fluorination
using
the
method
of
Miller
et
al.
(1999)
at
the
Open
University
(UK).
X-Ray
Electron
Diffraction
X-ray
powder
diffraction
patterns
of
the
meteorites'
matrices
were
taken
with
a
universal
X-ray
diffractometer
(DRON-UM1)
that
measures
the
X-ray
intensity
distribution
for
oriented
and
powder
samples
using
CoK
o
,
radiation
with
Fe
filter
(V
=
35
kV,
A
=
25
mA).
Measurements
were
made
automatically
with
step
0.02°
at
speed
of
4°/min.
X-ray
spectra
were
processed
by
the
internal
software
XRAY
and
the
Pcpdf
2003
database
for
the
analysis
of
diffraction
powder
patterns.
RESULTS
Petrography
and
Mineral
Chemistry
Dho
225
Dho
225
consists
of
irregularly
shaped
olivine
aggregates,
chondrules,
CAIs,
isolated
minerals
and
mineral
fragments
embedded
in
a
fine-grained
matrix
(Ivanova
et
al.
2005,
2006)
(Fig.
1a).
Matrix
abundance
of
Dho
225
is
70
vol%;
chondrules
constitute
24
vol%
and
CAIs
constitute
2
vol%.
Porous
and
other
friable
material,
probably
excavated
from
the
section
during
polishing,
constituted
approximately
4
vol%.
Some
coarse-grained
objects
are
surrounded
by
haloes
of
dark,
fine-grained
material.
Rounded
inclusions
with
opaque
minerals
also
occur.
The
objects/matrix
ratio
is
approximately
0.3.
The
chondrules
consist
of
olivine,
rare
orthopyroxene,
chromites
and
sulfides.
Mesostasis
in
chondrules
is
completely
altered.
The
mean
chondrule
diameter
is
around
0.3
mm.
The
irregularly-shaped
olivine
aggregates
are
larger
than
the
chondrules,
up
to
0.6
mm
in
size.
They
consist
of
olivine
grains,
sometimes
with
minor
chromite
and
sulfides
embedded
in
mesostasis.
Rare
CAIs,
up
to
220
gm
in
size,
consisting
of
zoned
spinel,
perovskite,
diopside
and
altered
silicate
mesostasis
are
also
found.
The
matrix,
Fig.
1.
Backscattered
electron
image
of
the
texture
of
Dhofar
225.
Coarse-grained
objects
are
surrounded
by
a
dark,
fine-
grained
matrix
halo.
Isolated
grains
of
olivine
and
pyroxene
occur
in
the
matrix
(a).
Several
chondrules
contain
zoned
olivine
(b).
consisting
of
silicates,
sulfides,
phosphides
and
phosphates,
contains
metal,
chromite,
and
eskolaite.
Several
veins
filled
with
carbonates
crosscut
the
matrix.
Isolated
olivine
and
pyroxene
grains
in
the
matrix
vary
in
size,
from
5
to
200
gm.
Olivine
is
the
main
mineral
phase
in
many
of
Dho
225's
objects
(Table
1,
Fig.
2a),
constituting
93
vol%
of
chondrules
and
isolated
grains
in
the
matrix.
Homogeneous
Fo-rich
olivine
occurs
in
aggregates,
CAIs,
chondrules
of
type
I
and
isolated
grains.
FeO-
rich
olivine
is
present
in
chondrules
of
type
II
and
also
in
isolated
grains.
MnO,
Cr
2
O
4
are
positively
correlated
with
FeO
(Fig.
3a),
and
CaO,
A1
2
0
3
are
slightly
negatively
correlated
with
FeO
in
chondrules
of
type
I
(Fig.
3c).
CaO
contents
reach
as
high
as
1
wt%
and
Al
2
O
3
contents
reach
0.5
wt%
in
the
most
forsteritic
Table
1.
Chemical
compositions
of
olivine,
pyroxene,
and
plagioclase
from
chondrules
of
Dhofar
(Dho)
225
and
Dho
735
(wt%).
SiO
2
TiO
2
A1
2
0
3
Cr
2
O
3
FeO
MnO
MgO
CaO
Na
2
O
K
2
0
Total
Fa
Fs'
Wo
b
1
40.7-43.7
0.01-0.12
0.01-0.53
0.06-0.67
0.42-8.98
0.01-0.56
50.4-57.2
0.06-0.86
<0.03
<0.03
0.41-9.09
[63]
42.2
0.04
0.11
0.36
1.52
0.11
55.8
0.33
100.5
1.51
2
33.2-42.0
<0.03
0.01-0.73 0.19-0.73
14.3-49.1
0.01-0.58
16.7-45.0
0.06-0.77
<0.03
<0.03
15.3-62.3
[6]
36.9
0.09
0.34
28.8
0.29
33.4
0.30
100.2
32.9
3
54.5-57.5
0.12-0.39
1.05-1.79
0.71-1.18
3.31-9.47
0.20-0.42
31.5-35.9
1.61-1.91
<0.03
<0.03
5.03-14.4
3.02-3.53
[6]
55.6
0.23
1.43
0.95
5.65
0.32
34.1
1.80
100.1
8.70
3.34
4
50.4-53.4
0.88-1.15
3.10-3.90
0.62-2.11
1.14-3.11
0.11-1.34
19.0-21.6
17.4-19.5
0.05-0.13
<0.03
1.77-5.25
37.7-39.1
[5]
51.8
1.02
3.42
1.13
2.09
0.52
20.3
18.7
0.06
99.4
3.41
38.5
5
43.9-55.1
0.03-3.94
0.68-11.7
0.03-0.09
0.64-1.07
0.02-0.06
12.3-18.2
24.6-25.3
0.02-0.41
<0.03
0.99-1.82
49.5-57.9
[6]
51.2
1.23
4.86
0.05
0.93
0.04 16.7
24.9
0.09
100.0
1.50
51.3
6
40.0-44.2
n.d.
n.d.
0.06-0.80
0.24-3.40
0.01-0.18
54.3-57.7
0.13-0.81
n.d.
n.d.
0.23-3.36
[67]
47.7
0.27
1.09
0.05
56.9
0.36
100.4
1.06
7
36.9-41.9
n.d.
n.d.
0.31-0.69
4.18-20.4
0.13-0.37
40.9-54.3
0.14-0.35
n.d.
n.d.
4.16-21.8
[12]
39.9
0.45
11.1
0.3
48.5
0.22
100.6
11.7
8
52.2-54.5
0.12-0.39
1.44-3.49
0.95-1.63
4.16-15.5
0.16-0.41
25.9-34.8
0.21-1.90
0.03-0.13
<0.03
6.56-25.2
0.43-3.53
[5]
53.9
0.23
2.24
1.26
7.97
0.31
32.1
1.24
0.06
99.3
12.8
2.3
9
48.5-49.5
2.79-3.35
6.76-8.59
0.79-1.29
0.48-0.90
0.04-0.07
15.9-17.3
21.9-23.3
<0.03
<0.03
0.88-1.60
47.3-50.8
[10]
48.8
3.08
7.76
0.98
0.67
0.06
16.7
22.3
100.4
1.23
48.5
10
39.5-43.7
1.08-1.90
17.7-25.3
0.09-0.17
0.16-0.24
<0.02
9.60-12.1
24.5-24.6
<0.03
<0.03
0.33-0.49
59.2-64.5
[4]
41.6
1.49
21.5
0.13
0.2
10.8
24.5
100.2
0.41
61.8
11
43.5-44.1
n.d.
34.1-34.9
n.d.
0.20-0.77
n.d.
0.66-1.73
18.8-19.9
0.25-0.55
<0.03
94.9-97.8
2.19-5.06
[6]
43.7
34.6
0.34
0.91
19.4
0.41
99.36
96.3
3.69
'An.
b
Ab
for
plagioclase.
1-5:
Dho
225;
6-11:
Dho
735;
Fa,
Fs,
Wo,
An,
Ab
(mol%);
1,
6-magnesian
olivine;
2,
7-ferroan
olivine;
3,
8-orthopyroxene;
4-augite;
5,
9-diopside;
10-Al-diopside;
11-plagioclase.
[
]-number
of
analyses
for
average
value.
b
so
70
60
>,
50
c.)
5
40
D
30
LL
20
10
0
Freq
u
ency
(
%)
70
60
50
40
30
20
10
0
1112
M.
A.
Ivanova
et
al.
a
Dhofar
225
0
5
10
15
20
25
30
35
40
45
50
55
60
65
Fa
(mol.%)
Dhofar
735
0
5
10
15
20
25
Fa
(mol.%)
Fig.
2.
Histograms
of
the
Fa
values
of
olivine
in
chondrules
of
Dhofar
225
(a)
and
Dhofar
735
(b).
compositions.
Olivines
in
several
type
II
chondrules
are
strongly
zoned
(Fig.
1b),
from
Fa
ll
to
Fa
36
and
show
correlations
between
Cr
2
O
3
,
MnO
and
FeO
similar
to
those
of
chondrules
of
type
I.
Cr
2
O
3
content
is
0.34
±
0.09
wt%
on
average.
The
range
of
olivine
compositions
observed
among
isolated
grains
is
similar
to
that
observed
in
chondrules.
Low-Ca
pyroxenes
(Table
1)
are
very
rare
in
this
meteorite-
4
vol%
in
chondrules
and
isolated
grains—and
the
olivine/pyroxene
ratio
is
high.
Rare
augite
occurs
in
chondrule-like
objects,
and
Al-diopsides
(up
to
11.7
wt%,
A1
2
0
3
,
3.9
wt%
TiO
2
)
are
found
in
CAIs
and
their
rims.
Al-diopsides
are
low
in
MnO
(0.05
wt%)
compared
to
augites
(0.11
wt%).
Accessory
mineral
assemblages
are
various
in
Dho
225
(Tables
2
and
3).
Spinel
and
perovskite
occur
in
CAIs.
Chromite
was
found
mostly
in
olivine
chondrules
of
type
II,
it
is
Al-rich
and
contains
17.3
wt%
of
A1
2
0
3
(Table
2).
Single
small
grains
of
eskolaite
are
present
in
the
matrix.
FeNi-metal
blebs
within
Fo-rich
olivines
and
forsterite
fragments
are
poorer
in
Ni
(5.5
wt%)
and
Co
(0.24
wt%)
than
FeNi-metal
in
sulfide-metal
aggregates
and
in
metal
grains
in
the
matrix,
which
are
taenite
and
tetrataenite
(up
to
66.2
wt%
Ni,
2.1
wt%
Co)
(Table
3).
Sulfides
are
pentlandite,
Ni-pyrrhotite
and
troilite.
Opaque
inclusions
in
the
matrix
contain
schreibersite,
eskolaite,
and
a
rare
phase
of
a
Cr-
barringerite
(Fe
1
.
3
Cr
0
.
7
)
2
P
composition
with
low
Ni.
No
tochilinite
6Fe
0
.
9
S*5(Fe,Mg)(OH)
2
or
(Cr,P)-sulfides
were
found.
A
rare
Ca,
Fe,
S-bearing
phase
(CFS)
is
found
as
irregular,
rounded
10-20
gm
inclusions
in
the
matrix,
embedded
in
fine-grained
matrix
mantles
(Fig.
4).
The
CFS
inclusions
consist
mainly
of
small
grains,
yellowish
gray
and
showing
bireflectance
in
reflected
light.
Small
grains
of
pyrrhotite
are
often
found
at
the
edges
of
the
inclusions.
The
inclusions
also
contain
thin
veins
of
a
fayalite-like
phase
and
small
blebs
of
Fe
hydroxide
and
FeNi
metal
grains.
The
CFS
has
unusual
and
constant
composition
(Table
3).
As
distinguished
in
BSE
images,
Dho
225
has
three
types
of
fine-grained
matrix
material:
light,
dark,
and
the
matrix
halo
around
objects.
Chemical
compositions
of
matrix
silicates
correspond
to
serpentine (Table
4,
Fig.
7),
but
compositions
differ
from
one
matrix
type
to
the
other
in
Fe,
Ni,
S,
P,
and
Cr
contents.
The
average
composition
of
the
matrix
shows
depletion
in
Fe
(Fig.
5)
and
S
compare
to
that
of
CM2
chondrites.
Dark
and
light
matrices
contain
small
(3-5
µm)
grains
of
olivine,
troilite,
taenite,
and
tetrataenite
similar
to
matrix
of
CM2
chondrites.
Altered
mesostasis
in
chondrules
resembles
the
dark
matrix
silicates
in
composition
(Table
4),
but
is
enriched
in
Ca.
Dho
735
Dho
735
(Ivanova
et
al.
2002a,
Ivanova
et
al.
2005)
is
highly
fractured
(Fig.
6).
The
meteorite
contains
irregularly-shaped
olivine
aggregates,
chondrules
(approximately
50
vol%)
of
100-800
gm
in
sizes,
CAIs
(<1
vol%),
and
isolated
mineral
grains
embedded
in
a
fine-grained
matrix.
The
mean
diameter
of
chondrules
is
0.35
mm.
The
irregular
olivine
aggregates
and
chondrules
are
mantled
with
a
dark,
fine-grained
matrix.
The
objects/matrix
ratio
in
Dho
735
is
0.67.
CAIs
are
spinel-rich
with
altered
silicate
material.
Olivine
is
the
main
phase
of
the
meteorite
(Table
1,
Fig. 2b),
constituting
92
vol%
in
chondrules
and
isolated
grains.
It
varies
in
composition
from
Fa0
.
3
to
Fa3
.
4
in
chondrules
of
type
I,
and
from
Fa4.3
to
Fa21.8
in
chondrules
of
type
II.
There
are
positive
correlations
between
MnO,
Cr
2
O
3
and
FeO
(Fig.
3b),
and
slightly
negative
correlations
between
CaO,
A1
2
0
3
and
FeO
(Fig.
3d).
Average
CaO
content
in
olivine
is
approximately
0.8
wt%,
and
Cr
2
O
3
content
is
0.40
±
0.10
wt%.
Pyroxene
is
present
in
Dho
735
as
low-Ca
pyroxene,
diopsides
and
Al-diopsides
(Table
1),
constituting
2
vol%
in
chondrules
and
isolated
grains.
Low-Ca pyroxenes
are
extremely
rare,
and
only
two
small
grains
of
orthopyroxene
were
found
in
one
object.
Rare
diopside
occurs
in
chondrule-like
objects,
and
0.6
0
I
1
3
0
2
3
4
5
FeO.
2.5
C
0
0
_,;;
0.5
1;
0
0.4
O
D
e
Eh
g
0.3
a
0
m
0
0
mi:ro
El
rA
0.2
0
E
b
0.1
et
*
Dip
o
-
4-
1.1
oaks
0.9
d
0.8
-
0
0.7
-
O
a
0
0.6
-
00
0,5
-
0
••
1
:
00
0.4
-
0
"
0
4
0D
0.3
0
f
l
it
P
CID
El
fl
ir
0.2
-
Pm
C.
t
••
-
k
*
0.1
. .
\
_
#
•4••
0
".
g
*
*
0
0.5
l
1.5
Fe0.
tly
t'•
a
C
0
Ca0
A1203
a
O
••
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
and
metamorphosed
carbonaceous
chondrites
1113
0.8
-
a
MED
0.9
o
Cr203
0.7
-
0.8
MnO
Cr203
Mn
O,
Cr
2
0 j,
(w
t%)
4.6
4.5
-
:
61
00
0
cp1
,
171
4.4
0E1
2
ni
0.7
ID
0
4.3
.
0
D
0
El
O
0
0
D
0.2
-
0
rn
0
• • • . 4
CP
0,..
-
it
0,1
-
Ntl
Cl•
$
*04
,-
**
#
"No
.
.400'
4
a
sit.•
r.
0.5
1
1.5
2
FeO,
(wa%)
0.9
AL203
0
CaO
0.8
0
0.7
0
o
u
0
C
0.6
o
CI
0
13
u
0.5
D
0
.7t
as
E
i!k
0.3
*a
n
171
0
0.
e .
,....
01
;
0
Fii111,•
12
C
41
8
,
0
4.•
11
0.1-
• t .
.
Cf
l•
0
••
:1•
1
40•4•
0
0_5
1
1_5
Fe0,
(wit%)
Fig.
3.
Compositions
of
olivine
in
chondrules
of
type
I.
Correlations
between
MnO,
Cr
2
0
3
and
FeO
in
Dhofar
(Dho)
225
(a)
and
Dho
735
(b),
between
Al
2
O
3
,
CaO
and
FeO
in
Dho
225
(c),
and
Dho
735
(d).
Table
2.
Representative
analyses
of
oxides
from
Dhofar
225
and
Dhofar
735
(wt%).
SiO
2
TiO
2
A1203
Cr
2
O
3
FeO
MnO
MgO
CaO
V
2
0
3
Total
Dhofar
225
Chromite
0.38
0.86
17.3
46.5
25.2
0.28
7.88
<0.05
0.67
99.07
Spinel
<0.05
0.13
70.8
0.21
0.69
<0.05
29.2
<0.05
0.28
100.3
Perovskite
<0.05
56.8
0.42
<0.05
0.68
<0.05
0.05
40.8
0.50
99.40
Escolaite
1.21
<0.05
0.13
94.1
2.21
<0.05
0.90
0.08 0.88
99.50
Dhofar
735
Ilmenite
0.58
55.6
0.21
0.05
31.9
0.49
10.4
0.15
99.38
Rutile
1.14
86.4
1.92
0.05
7.14
0.13
2.48
0.21
99.47
Chromite
0.18
1.35
7.86
55.7
26.5
0.29
6.64
0.11
0.96
99.59
Al-chromite
0.07
0.42
32.4
35.6
17.7
0.23
13.0
0.07
0.44
99.93
Spinel
0.17
0.32
70.2
0.69
0.82
<0.05
27.8
<0.05
0.27
100.3
Al-diopsides
were
found
in
CAIs
and
their
rims.
were
found
only
in
several
barred
olivine
(BO)
Al-diopsides
are
low
in
MnO
(0.05
wt%)
in
comparison
chondrules
in
mesostasis
between
bars
of
olivine.
The
with
augites
(0.11
wt%).
Plagioclase
crystals
(Table
1)
mesostasis
in
most
chondrules
was
altered.
.•:r
.•.)•
z.'•
I-
.
N
Y
C
1.
4
11.
-cFS
Fel
metal
'
-
-
GEOKHI
COMP°
5
OkV
X2
000
tprrt
WD
6.6mm
1114
M.
A.
Ivanova
et
al.
Table
3.
Representative
analyses
of
opaque
minerals
from
Dhofar
225
and
Dhofar
735
(in
wt%).
P
Fe
Ni
S
Si
Co
Cr
Ca
Mg
Total
Dhofar
225
Kamacite
0.13
92.8
4.84
0.02
0.84
0.24
0.22
99.10
Tetrataenite
<0.05
30.5
66.2
0.22
0.04
2.06
0.05
99.10
Barringerite
21.7
57.5
3.71
0.03
0.48
0.45
15.8
<0.05
0.07
99.81
Pentlandite
<0.05
39.8
25.0
32.8
0.10
1.26
<0.05 <0.05
99.04
Pyrrhotite
<0.05
60.6
0.08
40.0
0.08
<0.05
0.06
0.15
101.0
Troilite
63.2
0.12
35.7
0.04
<0.05 <0.05
0.04
99.20
Shreibersite
15.2
31.2
52.8
<0.05
0.06
0.60
0.43
0.22
0.19
100.8
FCS-phase
<0.05
42.8
0.07
20.1
0.05
<0.05 <0.05
21.4
84.40
Dhofar
735
Kamacite
<0.05
94.9
3.85
<0.05 <0.05
0.35
<0.05
99.10
Tetrataenite
<0.05
36.1
60.4
0.08
0.07
2.51
<0.05
99.20
Pentlandite
<0.05
36.1
28.27
33.1
<0.05
1.73
<0.05
99.21
Pyrrhotite
<0.05
62.5
0.30
36.3
<0.05 <0.05
0.06
99.20
Troilite
62.9
0.12
35.9
<0.05 <0.05 <0.05
98.92
Shreibersite
15.5
39.0
43.8
<0.05
0.06
0.68
0.23
99.27
Fig.
4.
Backscattered
electron
image
of
assemblage
of
Ca,
Fe,
S-bearing
phase
(CFS),
pyrhottite
(Pyr),
Fe
hydroxides
and
FeNi
metal
grains
in
the
matrix
of
Dhofar
225.
Dho
735's
oxide
phases
include
Mg-ilmenite,
rutile,
and
chromite
(Table
2).
Sulfides
are
very
abundant
and
include
troilite,
pentlandite,
and
pyrrhotite
(Table
3).
FeNi
metal
grains
include
kamacite,
and
tetrataenite.
Kamacite
mostly
occurs
within
olivine
grains,
and
tetrataenite
is
in
the
matrix.
Schreibersite
is
rare
and
occurs
in
the
matrix.
Ca-carbonates,
dolomite
and
calcite
are
also
abundant
in
Dho
735,
associated
with
matrix
silicates
and
sulfides,
and
occur
often
in
veins.
No
tochilinite
or
(Cr,
P)
sulfides
were
found
in
this
meteorite.
The
fine-grained
silicates
in
the
matrix,
as
well
as
those
of
Dho
225,
vary
in
chemical
composition
(Table
4,
Fig.
7)
and
are
located
along
the
serpentine
line
on
a
Mg-AI+
Si-Fe
diagram.
In
BSE
images
the
light
matrix
distinguishes
from
the
dark
matrix,
the
light
matrix
is
more
depleted
in
Fe
than
the
dark
matrix
and
the
matrix
around
chondrules.
In
average
chemical
composition
Dho
735's
matrix
shows
depletion
in
Fe
(Fig.
5)
and
S
in
comparison
with
matrices
of
CM2
chondrites,
and
as
well
as
the
matrix
of
Dho
225,
it
contains
small
grains
of
troilite,
and
tetrataenite.
In
comparison
with
the
fine-grained
silicates
in
the
matrix,
fine-grained
silicate
material
inside
chondrules
is
enriched
in
Ca
(Table
4).
X-Ray
Diffraction
We
analyzed
the
matrices
of
Dho
225
and
Dho
735
by
X-ray
diffraction,
and
compared
them
with
the
matrices
of
Murchison
CM2
chondrite,
typical
and
heated
to
900
°C
in
the
laboratory.
Figure
8
presents
the
diffraction
powder
patterns.
Observed
reflections
correspond
to
forsterite
for
both
Dho
225
and
Dho
735.
A
wide
diffusive
region
(F'),
in
20-30°
on
the
pattern,
corresponds
to
olivine
with
disordered
structure.
Similar
structures
were
found
during
thermal
transformation
of
serpentine
to
olivine
and
were
recognized
under
600-
650
°C
(Brindley
and
Zussman
1957).
Therefore,
a
region
(F')
means
a
transformational
region
from
serpentine
to
olivine.
The
matrix
of
Murchison
shows
indices
of
serpentine
(antigorite)
reflections,
unlike
the
matrix
of
heated
sample
of
Murchison,
which
does
not
show
serpentine
reflections.
Bulk
Chemical
Compositions
of
Dho
225
and
Dho
735
The
bulk
chemical
composition
of
Dho
225
and
Dho
735
are
given
in
Table
5
and
Fig.
9.
It
shows
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
and
metamorphosed
carbonaceous
chondrites
1115
Table
4.
Average
analyses
of
altered
silicates
from
Dhofar
225
and
Dhofar
735
(in
wt%).
SiO
2
TiO
2
A
1
2
0
3
Cr
2
0
3
FeO
MnO
MgO
Dhofar
225
(10)
Dark
matrix
31.6
±
2.3
0.07
±
0.03
1.83
±
0.23
0.83
±
0.08
18.3
±
2.1
0.18
±
0.06
29.3
±
1.4
(11)
Mantle
around
29.5
±
2.8
0.15
±
0.06
1.96
±
0.10
0.67
±
0.11
22.0
±
1.3
0.16
±
0.08
24.2
±
2.1
chondrule
(16)
Light
matrix
34.8
±
1.9
0.13
±
0.05
2.81
±
0.13
0.59
±
0.12
33.1
±
3.3
0.32
±
0.10
19.9
±
1.8
(7)
Inside
chondrule
33.8
±
1.7
0.12
±
0.03
2.89
±
0.11
0.60
±
0.09
19.5
±
1.1
0.25
±
0.04
17.5
±
2.0
Dhofar
735
(8)
Inside
chondrule
31.2
±
1.4
0.29
±
0.12
2.15
±
0.12
0.20
±
0.08
7.59
±
1.42
0.15
±
0.06
32.0
±
2.1
(12)
Light
matrix
32.8
±
1.2
0.08
±
0.03
2.18
±
0.07
0.32
±
0.10
28.7
±
2.8
0.28
±
0.11
20.0
±
3.0
(15)
Dark
matrix
35.0
±
2.6
0.12
±
0.06
2.71
±
0.20
0.58
±
0.12
13.1
±
2.3
0.23
±
0.09
35.0
±
2.2
(14)
Mantle
around
27.3
±
2.0
0.18
±
0.10
2.80
±
0.19
0.22
±
0.09
21.2
±
2.0
0.19
±
0.10
24.1
±
1.8
chondrule
Ca0
Na
2
0
IC
2
0
Ni0
P
Cl
Dhofar
225
(10)
Dark
matrix
0.83
±
0.10
0.30
±
0.07
0.15
±
0.05
(11)
Mantle
around
0.68
±
0.14
0.40
±
0.10
0.14
±
0.06
chondrule
(16)
Light
matrix
1.62
±
0.24
0.46
±
0.12
0.20
±
0.04
(7)
Inside
chondrule
5.44
±
1.03
0.79
±
0.07
0.15
±
0.03
Dhofar
735
(8)
Inside
chondrule
8.93
±
1.23
0.15
±
0.04
0.12
±
0.05
(12)
Light
matrix
2.39
±
0.12
0.17
±
0.03
0.10
±
0.03
(15)
Dark
matrix
2.29
±
0.10
0.20
±
0.03
0.15
±
0.04
(14)
Mantle
around
4.08
±
0.81
0.21
±
0.04
0.15
±
0.03
chondrule
1.70
±
0.38
2.23
±
0.58
0.06
±
0.03
3.28
±
1.07
3.25
±
0.44
0.78
±
0.43
1.60
±
0.39
2.34
±
0.60
0.46
±
0.18
1.10
±
0.23
0.68
±
0.28
0.10
±
0.04
0.19
±
0.05
1.01
±
0.10
0.12
±
0.05
1.25
±
0.24
0.23
±
0.06
0.09
±
0.05
0.47
±
0.25
0.30
±
0.09 0.09
±
0.04
2.63
±
1.11
3.35
±
0.26
0.15
±
0.04
0.13
±
0.03
0.32
±
0.06
0.50
±
0.12
0.52
±
0.10
(n)
=
number
of
spots
(5
x
5
gm)
for
analyses.
0
CM
Dhofar
225
B-7904
like
0
Dhofar
735
0
Chundrule
.
90
10
Si
10
-
kulated
groin,-.;
90
M
g
111q
lim
Fe
10
90
Fig.
5.
Chemical
composition
of
matrices
of
Dhofar
225
and
Dhofar
735,
metamorphosed
Belgica-like
chondrites
and
CM
chondrites
(wt%).
Data
for
Belgica-like
chondrites
from
Ikeda
1992;
data
for
CM
chondrites
from
McSween
and
Richardson
1977
and
Zolensky
et
al.
1993.
several
differences
from
CM2
chondrites,
and
from
the
Belgica-like
meteorites.
Dho
225's
and
Dho
735's
structural
H
2
0-contents
of
1.76
wt%
and
1.06
wt%,
respectively,
are
low
in
comparison
with
water
contents
of
CM2
chondrites
(2-13
wt%)
(Jarosewich
1990),
but
Fig.
6.
Backscattered
electron
image
of
texture
of
Dhofar
735.
The
meteorite
contains
chondrule-like
objects,
100-800
gm
in
size,
isolated
grains
of
olivine,
and
refractory
inclusions
embedded
in
a
fine-grained
matrix.
The
rounded
objects
are
mantled
with
a
dark,
fine-grained
matrix
halo.
similar
to
those
in
the
Belgica-like
meteorites
(Ikeda
1992).
The
Fe/Si
ratios
of
the
meteorites
(1.36
for
Dho
225,
and
1.37
for
Dho
735)
differ
from
that
of
CM2
t
Dho
225
CMs
O
B-7904
CI
A
Y-86720
Dho
735
a
6
1116
M.
A.
Ivanova
et
al.
Si+AI
Dhofar
225
0
100
Dhofar
735
20
80
- -
ASmectite
solid
solution
40
Serpa
ne
solid
solution
80
CM
phyllosilicates
20
100
0
0
20
40
60
80
100
Fe
Mg
Fig.
7.
Chemical
composition
of
altered
silicates
in
the
matrix
of
Dhofar
225
and
Dhofar
735.
Chemical
compositions
of
matrix
phyllosilicates
of
CM2
chondrites
and
Orgueil
(McSween
and
Richardson
1977;
Zolensky
et
al.
1993).
2
F
FF
F
10
50
60
70
20
30
40
Degree
20
Fig.
8.
Powder
diffractometer
traces
for
Dhofar
225
(1)
and
Dhofar
735
(2),
in
comparison
with
Murchison
heated
to
900
°C
(3)
and
Murchison
unheated
(4).
F-forsterite
reflections;
F'-diffusive
region
corresponding
to
disordered
olivine;
S-serpentone
(antigorite)
reflection.
chondrites
(Fe/Si
=
1.47-2.24;
McSween
and
Richardson
1977).
Dho
225
and
735's
Fe/Si
ratios
also
differ
from
that
of
the
Belgica-like
chondrites-
B-7904
(Fe/Si
=
1.65),
and
Y-86720
(Fe/Si
=
1.61)
(Ikeda
1992).
Dho
735
is
depleted
in
S
in
comparison
with
CM2
chondrites,
and
it
is
more
depleted
in
Cr,
Mn,
and
Na
than
typical
CM2
chondrites
as
it
is
shown
in
Table
5
and
Fig.
9.
10
0.1
Al
Ti
Ca
Mg
Ni
Fe
Si
Cr
Mn
P
K
Na
S
decreasing
condensation
temperatures
Fig.
9.
Bulk
chemical
compositions
of
Dhofar
225
(Dho
225)
and
Dhofar
735
(Dho
735),
CM2
chondrites
(McSween
and
Richardson
1977),
B-7904,
and
Y-86720
(Y-86720)
(Ikeda
1992).
Table
5.
Major
element
chemical
compositions
of
Dhofar
225,
Dhofar
735,
and
Belgica-7904
(wt%).
Dhofar
225
Dhofar
735
Belgica-7904a
Si
15.60
15.25
14.72
Ti
0.09
0.08
0.10
Al
1.53
1.44
1.75
Cr
0.39
0.25
0.34
Fe
21.18
21.00
24.23
Mn
0.20
0.16
0.19
Mg
14.19
14.10
14.30
Ca
1.63
1.70
1.59
Na
0.58
0.38
0.49
K
0.08
0.07
0.03
P
0.15
0.14
0.16
Ni
1.38
1.32
0.86
Co
0.070
0.086
0.028
0.61
0.695
0.972
S
H
2
O
C
2.10
3.238
3.950
1.76
1.06
2.60
a
Data
from
Ikeda
1992.
Al,
Ti,
Ca,
Mg,
Cr,
Mn,
K,
Na,
Ni,
Co,
Fe
were
analyzed
using
ICP
method;
Si-using
XRF
analysis;
H
2
0,
C,
and
S
were
measured
with
a
CHN
analyzer.
Oxygen
Isotopic
Compositions
of
Dho
225
and
Dhofar
735
Oxygen
isotopic
compositions
of
Dho
225
and
Dho
735
are
anomalous
in
comparison
with
those
of
typical
CM2
chondrites
(Table
6,
Fig.
10).
Bulk
oxygen
isotopic
composition
for
Dho
225:
8
17
0
=
9.22,
8
18
0
=
21.8,
A
17
0
=
-2.14
(%
0
),
and
for
Dho
735:
8
17
0
=
10.7,
8180
=
21.6,
A
17
0
=
-0.58
(X)0).
They
40
Orgueil
phyllosilicates
60
n
Abun
dances
/Si
an
d
CI
Murchison
heated
o
Murchison
Mighei
heated
Mighei
450°C
600°C
600°C
930°C
800°C
400°C
A
B-7904-like
chondrites
,C.Dhofar
225
CMs
Dhofar
735
TF
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
and
metamorphosed
carbonaceous
chondrites
1117
Table
6.
Oxygen
isotopic
compositions
of
Dhofar
225,
Dhofar
735
and
the
unheated
and
laboratory-heated
Murchison
and
Mighei
samples
(L).
Meteorite
Sample
.5'
7
0
61
80
Arlo
Dhofar
225
9.22
21.8
-2.14
Dhofar
735
10.7
21.6
-0.58
Murchison
Unheated
0.09
5.92
-2.99
450
°C
-0.18
5.56
-3.07
450
°C
0.21
6.07
-2.94
630
°C
0.78
7.40
-3.07
630
°C
0.99
7.74
-3.03
930
°C
1.84
9.69
-3.20
930
°C
1.78
9.53
-3.17
Mighei
Unheated
0.70
6.27
-2.56
400
°C
1.10
7.27
-2.68
600
°C
1.52
8.32
-2.81
800
°C
2.19
9.64
-2.82
800
°C
2.14
9.34
-2.72
resemble
those
of
metamorphosed
carbonaceous
chondrites-B-7904,
Y-82162
and
Y-86720
and
locate
in
the
right
end
of
the
three-isotope
oxygen
diagram
showing
enrichment
of
oxygen
isotopic
composition
in
18
0
and
17
0
relative
to
values
for
usual
CM2
chondrites.
Oxygen
Isotopic
Compositions
of
Laboratory-Heated
Murchison
and
Mighei
CM2
Chondrites
For
both
Mighei
and
Murchison,
the
heated
samples
become
more
18
0
rich
with
increasing
temperature
(Table
6,
Figs.
11
a
and
11b).
Values
of
8
18
0
vary
from
5.92
to
9.69°/
00
in
the
range
of
CM2
chondrites.
The
value
of
A'
7
0
varies
from
approximately
-2.99-3.20°/
00
to
less
negative
values
with
increasing
temperature,
away
from
the
less
negative
the
Belgica-like
chondrites
and
towards
the
level
of
partially
dehydrated
CMs
(A-881334,
Y-793321,
Y-82098,
Y-86695)
(Clayton
and
Mayeda
1999).
The
heated
Murchison
samples
display
similar
isotopic
effects
as
those
described
by
Mayeda
and
Clayton
(1998)
for
a
heated
serpentine
sample.
They
showed
results
of
their
hydration
experiments,
done
under
vacuum,
with
continuous
trapping
of
all
volatiles.
Most
CM2
chondrites
fall
on
a
mixing
line
with
a
slope
of
and
display
8
18
0
of
over
8°/
00
(Clayton
and
Mayeda
1999).
The
unheated
samples
of
Mighei
and
Murchison
plot
at
the
low
18
0
end
of
this
trend
are
poorer
in
18
0
than
all
the
heated
samples
within
the
range
of
previously
reported
values,
while
the
value
for
Mighei is
similar
to
previous
results
(Clayton
et
al.
1997;
Clayton
and
Mayeda
1999).
Given
the
mineralogy
and
isotopic
heterogeneity
within
CM2s,
considerable
variation
in
whole-rock
measurements
is
to
be
expected.
15
10
0
-5
5
10
15
20
25
6180
(o/
00
)
Fig.
10.
Bulk
oxygen
isotopic
compositions
of
Dhofar
225
and
735.
Data
for
oxygen
isotopic
compositions
of
CM
chondrites
and
Belgica-like
chondrites
from
Clayton
and
Mayeda
(1999).
Murchison
&
Murchison
heated
Mighei
&
Mighei
heated
Belgica-like
Dho
735
V
Dho
225
JI
10
15
20
8
18
0
(im)
5
6
7
8
9
10
6
18
0
mo)
Fig.
11.
Oxygen
isotopic
compositions
(black
triangles)
of
Dhofar
225,
Dhofar
735,
Belgica-7904,
and
Yamato-86720,
and
experimentally
heated
Murchison
and
Mighei
in
the
range
for
typical
CM2
chondrites
(in
oval)
(a),
large
scale
for
experimental
points
(b).
a)
15
10
0
5
7.
0
0
b)
3
2
8
1
0
-1
4
1118
M.
A.
Ivanova
et
al.
However,
these
affects
are
relatively
small
compared
to
the
variations
observed
in
the
heating
experiments,
which
were
all
performed
on
aliquots
of
homogenized
samples
(Fig.
11b).
DISCUSSION
Petrography,
Mineralogy,
and
Bulk
Chemistry
of
Dho
225
and
Dho
735
Dho
225
and
Dho
735
are
texturally
similar
to
the
typical
CM2
chondrites.
Both
meteorites
have
the
same
set
of
major
components—matrix,
olivine
aggregates
and
chondrules,
isolated
grains,
and
CAIs.
The
abundance
of
matrices
in
both
meteorites
is
similar
to
that
of
CM2
chondrites,
but
the
objects/matrix
ratio
in
Dho
735
is
higher
than
in
Dho
225.
Olivine
aggregates,
chondrules
and
CAIs
in
both
meteorites
have
sizes
that
are
typical
for
those
of
CM2
chondrites.
We
observed
mainly
spinel-rich
CAIs
of
type
B;
however,
chondrules
and
CAIs
have
altered
silicate
material.
Mesostasis
in
chondrules
was
altered
to
material
that
is
similar
in
composition
to
matrix
silicate
material
known
to
have
experienced
aqueous
alteration
after
accretion
into
a
parent
asteroid
like
the
chondrules
in
CM2
chondrites
(Zolensky
et
al.
1993;
Brearley
and
Jones
1998).
Olivine
varies
in
composition
in
both
meteorites
in
ranges
similar
to
the
olivine
compositions
of
CM2
chondrites.
Unlike
Dho
735,
Dho
225
contains
extremely
zoned
olivine
grains.
Ferroan
olivine
in
typical
CM2
chondrites
is
uniformly
rich
in
Cr
2
O
3
,
with
mean
compositions
in
individual
chondrites
ranging
from
0.28
±
0.10
to
0.38
±
0.10
wt%
Cr
2
O
3
,
independent
of
the
degree
of
aqueous
alteration
(Grossman
et
al.
2005).
Dho
225
and
Dho
735
compositions
are
similar
to
those
in
CO3.0
chondrites,
and
lower
than
those
in
type
3.0
ordinary
chondrites
(Brearley
and
Jones
1998).
Cr
2
O
3
content
in
olivines
of
Dho
225
and
Dho
735
is
0.34
±
0.09
and
0.40
±
0.10
wt%,
respectively.
Using
a
similar
scale
to
that
for
ordinary
chondrites,
Dho
225
and
Dho
735
are
not
distinguished
from
usual
CM2
chondrites
and
would
be
lower
than
type
3.0.
This
is
unlike
B-7904,
which
has
mean
Cr
2
O
3
of
0.19
±
0.08
wt%
in
olivine
and
would
be
classified
as
type
3.1.
Dho
225
and
Dho
735
are
distinguishable
from
Belgica-like
meteorites
in
the
chemical
compositions
of
their
FeNi-metal
grains.
The
Dho
225
and
735
grains
are
not
enriched
in
Cr
and
P,
unlike
the
FeNi-
metal
grains
described
in
the
Belgica-like
meteorites
(Bischoff
and
Metzler
1991),
which
are
enriched
in
these
elements.
The
lack
of
tochilinite,
the
main
constituent
of
CM2
chondrite
matrices,
is
an
important
characteristic
of
Dho
225
and
Dho
735
as
well
as
of
Belgica-like
meteorites
and
could
be
explained
by
thermal
metamorphism
that
led
to
decomposition
of
this
mineral.
Fine-grained
objects,
consisting
of
troilite
intergrown
with
matrix
silicates
and
Mg,
Fe-oxides,
are
widespread
in
Dho
225
and
Dho
735.
We
proposed
that
they
probably
could
form
by
decomposition
of
tochilinite.
As
shown
by
Fuchs
et
al.
(1973)
in
heating
experiments
on
the
Murchison
CM2
chondrite,
tochilinite
has
low
thermal
stability
and
decomposes
at
245
°C
to
troilite
and
Mg,
Fe-oxides,
before
the
decomposition
of
coexisting
serpentines.
Tomeoka
and
Buseck
(1985)
also
confirmed
that
tochilinite
transforms
to
troilite
by
exposure
to
the
electron
beam.
On
the
other
hand,
it
is
quite
possible
that
no
tochilinite
was
present
to
begin
with
in
the
material
of
Dho
225
and
Dho
735,
and
in
this
case
assembleges
of
troilite
intergrown
with
matrix
silicates
and
Mg,
Fe-oxides
were
not
products
of
decomposition
of
tochlinite
and
had
their
own
history
of
origin.
Another
notable
difference
between
Dho
225,
Dho
735
and
typical
CM2
chondrites
is
a
lack
of
(Cr,
P)-rich
sulfides
such
as
described
by
Nazarov
et
al.
(2001).
This
is
probably
also
due
to
thermal
metamorphism,
and
P-
rich
sulfides
are
also
thermally
unstable
and
can
easily
be
converted
to
Ca-phosphate-troilite-pentlandite-
chromite
assemblages,
which
are
very
common
in
chondrites
(Murrell
and
Burnett
1983;
Rubin
and
Grossman
1985).
Unlike
other
Belgica-like
chondrites
and
CM2
chondrites,
Dho
225
contains
aggregates
of
an
unusual
CFS
phase,
pyrrhotite
and
iron
hydroxides
that
may
be
oxidation
products
of
a
primary
sulfide
phase.
It
has
not
been
previously
described
and
was
not
found
in
the
Belgica-like
chondrites.
EDS
spectra
and
the
low
analytical
total
suggest
oxygen
is
present.
IR
microspectroscopy
(Moroz
et
al.
2006)
did
not
show
any
water
in
this
mineral.
The
best-fit
chemical
formula
corresponding
to
stoichiometry
is
close
to
(Ca4.66
Fe
2+
0
.
34
)5Fe
3+
6
S
5
0
9
.
At
the
present
time
we
cannot
say
with
certainty
whether
the
CFS
phase
is
a
product
of
terrestrial
weathering,
or
not.
We
hypothesize
that
this
phase
may
have
formed
via
oxidation
of
primary
sulfides.
The
hypothetical
oxidation
could
have
occurred
either
on
the
parent
body,
or
on
Earth.
A
notable
argument
in
favor
of
this
phase
being
the
product
of
terrestrial
weathering
is
indeed
the
presence
of
Fe
3+
in
the
proposed formula
of
the
oxysulfide
(CFS).
This
question
would
need
further
study,
since
during
deep
oxidation
on
the
parent
body,
oxidation
of
Fe
to
the
3
+
valence
state
might
also
be
possible.
The
matrices
of
both
meteorites
show
depletion
in
Fe
and
S
in
comparison
with
CM2
chondrite
matrices
(McSween
and
Richardson
1977;
Zolensky
et
al.
1993).
This
depletion
is
similar
to
that
of
the
Belgica-like
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
and
metamorphosed
carbonaceous
chondrites
1119
chondrites
(Fig.
6).
The
matrix
of
Dho
225
is
more
depleted
in
Fe
and
enriched
in
Mg
than
that
of
Dho
735.
The
matrices
of
Dho
225
and
Dho
735
contain
small
grains
of
olivine,
troilite,
taenite,
and
tetrataenite.
The
presence
of
taenite
and
tetrataenite
is
unusual
for
CM2
chondrites
and
occur
very
rare
in
their
matrices.
The
widely
distributed,
fine-grained
troilite
found
in
Dho
225
and
Dho
735
probably
segregated
from
the
matrix
during
metamorphism
and
resulted
in
the
depletion
of
Fe
and
S
in
the
matrices
of
Dho
225
and
Dho
735.
However,
the
depletion
of
Fe
and
S
could
be
a
specific
feature
of
these
chondrites
as
well
as
the
Belgica-like
chondrites.
On
the
other
hand,
depletion
of
S
in
Dho
225
and
Dho
735
could
be
due
to
removal
of
sulfides
during
terrestrial
weathering
rather
than
metamorphism.
Despite
the
depletion
of
Fe
and
S,
matrices
of
Dho
225
and
735
do
not
demonstrate
significant
depletion
of
Ni
in
comparison
with
CM2
matrices.
Ni
usually
exists
in
sulfides
and
tochilinite
in
CM2
chondrites
and
reacts
with
S
during
alteration.
In
CM2
chondrites,
matrices
consist
mainly
of
Fe-rich
serpentines
and
a
sulfide-rich
phase,
probably
Ni-bearing
tochilinite.
Relative
proportions
of
these
phases
vary
in
a
broad
range
within
each
meteorite
and
between
meteorites.
Thus,
S
should
be
correlated
with
Ni,
and
both
are
inversely
correlated
with
Si
and
Mg.
However,
our
microprobe
analyses
did
not
reveal
any
correlation
between
Ni
and
S,
suggesting
that
Ni
exists
in
different
phases
rather
than
in
sulfides,
hence,
tiny
FeNi
metal
grains
possibly
formed
during
thermal
metamorphism.
The
low
H
2
O
contents
of
Dho
225
and
Dho
735
could
indicate
either
that
these
meteorites
experienced
dehydration,
or
that
their
matrices
did
not
initially
contain
H
2
O.
The
low
Fe/Si
ratios
in
the
bulk
compositions
of
Dho
225
and
Dho
735
are
consistent
with
a
depletion
of
their
matrices
in
Fe.
Dho
225
and
Dho
735
are
depleted
in
S
in
comparison
with
CM2
chondrites,
and
Dho
735
is
more
depleted
in
Cr,
Mn,
Na
than
CM2
chondrites.
These
chemical
characteristics
indicate
that
the
primary
material
of
Dho
225
and
Dho
735
was
different
from
that
of
CM2
chondrites,
and
of
the
Belgica-like
meteorites,
B-7904
and
Y-86720.
X-Ray
Powder
Data
and
Infrared
Spectral
Characteristics
of
Dho
225
and
Dho
735
Matrices
X-ray
powder
diffraction
studies
of
Dho
225
and
735's
matrices
showed
that
both
contain
forsterite
with
disordered
structure
(Fig.
8)
that
could
form from
serpentine
during
thermal
events.
Comparison
of
these
results
with
X-ray
powder
diffractions
of
Murchison
matrix
(original
and
laboratory-heated
to
900
°C)
also
confirmed
that
Dho
225
and
735's
matrices
did
not
contain
phyllosilicate
reflections,
but
did
show
reflections
of
olivine
with
disordered
structure,
diffusion
region
F',
which
corresponds
to
serpentine
heated
to
600-650
°C
(Fig.
8).
Akai
(1988)
investigated
matrix
phyllo
silicates
of
the
Antarctic
metamorphosed
chondrites
B-7904
and
Y-
793321
by
high
resolution
electron
microscopy
(HREM).
Akai's
HREM
micrographs
were
interpreted
as
representing
an
"intermediate
phase"
in
the
transformation
from
serpentine
to
olivine.
In
B-7904,
phyllosilicates
appeared
to
have
been
almost
completely
transformed
to
olivine,
making
it
difficult
to
decide
whether
the
minerals
were
originally
phylliosilicates.
Our
X-ray
powder
diffraction
results
for
the
Dhofar
matrices
found
a
similar
"intermediate
phase"—diffusion
region
F'—indicating
incomplete
transformation
of
phyllosilicates
to
olivines.
The
laboratory-heated
sample
of
Murchison
matrix
showed
reflections
of
forsterite
lacking
diffusion
region
F'.
This
indicates
that
in
Murchison,
heating
to
900
°C
in
the
laboratory
brought
about
a
complete
transformation
of
serpentine
to
olivine.
Synchrotron-based
infrared
micro
spectro
scopy
(SIRM)
of
the
Dho
225
and
735
matrices
(Moroz
et
al.
2006)
showed
that
they
are
different
from
those
of
typical
CM2
chondrites
spectrally
and
resemble
the
matrices
of
CO3
chondrites,
whose
major
matrix
constituents
are
fine-grained
Fe-rich
olivines
(Fig.
12).
A
convex
curvature
of
the
Dho
225,
735,
and
Kainsaz
(CO3)
matrix
spectra
near
10
gm
is
consistent
with
phyllosilicates
(possibly
dehydrated)
and/or
amorphous
mesostasis
as
additional
minor
silicate
constituents.
Spectra
of
normal
CM2
chondrites
are
characterized
by
smooth
reflectance
peak
(Reststrahlen
band)
near
10
gm
caused
by
Si-O
vibrations
in
hydrated
silicates,
while
the
matrix
spectra
of
the
Dho
225
and
Dho
735
are
dominated
by
sharper
reflectance
maxima
at
10.3,
11.4,
and
12
gm
due
to
Si-O
stretches
in
fine-grained
Fe-rich
olivines.
However,
due
to
higher
average
content
of
fayalitic
component,
the
olivine
features
in
the
spectra
of
CO3/CV3
matrices
are
shifted
to
longer
wavelengths
compared
to
the
Dho
225
and
735
matrix
spectra.
The
meteorites
contain
approximately
1
wt%
of
water
in
their
bulk
compositions
(Table
5).
The
shapes
of
the
Dho
225
and
735
IR
spectra
near
9.5-10
gm
(where
strong
Si-O
vibrations
in
phyllosilicate
structures
occur)
suggest
that
low
amounts
of
phyllosilicates
may
be
present
in
Dho
225
and
Dho
735
(Moroz
et
al.
2006),
Moreover,
phyllosilicates
may
also
occur
in
chondrule
mesostasis
of
the
meteorites
that
were
not
investigated
by
SIRM
in
detail.
In
addition,
the
near-
AVERAGE
MATRIX
Dhofar
225
Dhofar
735
Kainsaz
CO3
Average "normal"
CM2
9
10
11
12
13
14
15 16
17
18
1120
M.
A.
Ivanova
et
al.
Re
flec
tance
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Wavelength
(microns)
Fig.
12.
Average
matrix
IR
reflectance
spectra
of
Dhofar
(Dho)
225,
Dho
735,
"normal"
CM2
chondrites,
and
CO3
chondrite
Kainsaz
in
the
spectral
range
dominated
by
silicate
Si-O
stretching
signatures.
Each
spectrum
is
offset
from
a
previous
one
for
clarity.
The
matrix
spectrum
of
"normal"
CM2
chondrites
is
an
average
of
the
Mighei,
Murrey,
and
Cold
Bokkeveld
matrices.
Spectra
of
normal
CM2
chondrites
are
characterized
by
smooth
reflectance
peak
(Reststrahlen
band)
near
10
gm
caused
by
Si-O
vibrations
in
hydrated
silicates,
while
the
matrix
spectra
of
Dho
225
and
735
are
dominated
by
sharper
reflectance
maxima
at
10.3,
11.4
and
12
gm
due
to
Si-O
stretches
in
fine-grained
Fe-rich
olivines.
The
Dho
225
and
735
matrices
spectrally
resemble
matrices
of
CO3,
also
dominated
by
Fe-rich
fine-grained
olivines.
However,
due
to
higher
average
content
of
fayalitic
component,
the
olivine
features
in
the
spectra
of
CO3/CV3
matrices
are
shifted
to
longer
wavelengths
compared
to
the
Dho
225
and
735
matrix
spectra.
infrared
reflectance
spectra
of
Dho
225
and
735
bulk
powders
show
spectral
similarities
to
the
Antarctic
Belgica-like
carbonaceous
chondrites
that
were
metamorphosed
(Moroz
et
al.
2006).
Oxygen
Isotope
Changes
During
Experimental
Heating
of
CM2
Chondrites
Our
results
showed
that
the
oxygen
isotopic
compositions
of
Dho
225
and
735
are
anomalous
for
CM2
chondrites
(Table
6,
Fig.
10)
and
close
to
those
of
B-7904
and
Y-86720
(Clayton
and
Mayeda
1999).
B-7904
and
Y-86720
have
chemical
and
petrographic
characteristics
which
would
appear
to
have
classified
them
as
CM2
before
metamorphism.
They
are
also
similar
in
oxygen
isotopic
composition
to
the
Y-82162
metamorphosed
carbonaceous chondrite
which
has
petrological
characteristics
of
CI
chondrites
before
metamorphism
(Ikeda
1992).
We
conducted
experimental
heating
of
two
CM2
chondrites,
Murchison
and
Mighei,
to
study
changes
in
their
oxygen
isotopic
compositions
and
explore
possible
genetic
relationships
between
Dho
225
and
735
and
usual
CM2
chondrites.
Results
of
the
experiment
showed
that
for
both
Mighei
and
Murchison,
the
heated
samples
became
more
18
0
rich
with
increasing
temperature
due
to
mass-fractionation
effects
(Table
6,
Figs.
lla
and
11b).
Mayeda
and
Clayton
(1998)
show
graphically
the
results
of
their
dehydration
experiments,
done
under
vacuum
with
continuous
trapping
of
all
volatiles.
These
results
differ
from
our
experiment
in
that
Mayeda
and
Clayton
noted
smaller
isotope
shifts
at
similar
temperatures.
The
experimental
conditions
in
the
Mayeda
and
Clayton
(1998)
experiment
were
different
from
ours:
samples
in
their
experiment
were
heated
in
a
Pyrex
glass
tube
by
an
external
furnace;
liberated
water
was
immediately
frozen
into
a
nearby
trap
at
liquid
nitrogen
temperatures.
Mayeda
and
Clayton
(1998)
could
not
have
done
experiments
at
800-900
°C
in
glass
under
vacuum.
In
any
case,
neither
our
experiments
nor
the
Mayeda
and
Clayton
(1998)
experiment
produced
heavy-isotope
enrichments
as
large
as
those
seen
in
the
Belgica-group
chondrites
and
in
Dho
225
and
735.
Changes
in
oxygen
isotopic
composition
during
our
experiment
could
have
resulted
from
dehydration
of
phyllosilicates,
tochilinite
and
decomposition
of
carbonates.
Mineralogical
study
of
polished
sections
of
heated
Murchison
and
Mighei
showed
that
carbonates
survived
heating
to
500
°C
and
700
°C
in
both
meteorites,
as
well
as
in
all
the
Belgica-like
chondrites
(Bischoff
and
Metzler
1991).
According
to
Ikornikova
and
Sheptunov
(1973),
carbonates
decompose
between
400
and
700
°C.
Tochilinite
should
decompose
at
245
°C
(Tomeoka
and
Buseck
1985),
but
we
observed
it
in
the
500
°C
heated
samples
of
both
meteorites
and
traces
of
it
in
the
700
°C
heated
samples
of
Mighei.
Probably
the
rates
of
the
decomposition
reactions
of
tochilinite
and
carbonates
were
not
enough
to
form
final
products
during
our
experiments.
Usually
carbonates
and
tochilinite
are
accessory
phases
in
CM2
chondrites,
and
phyllosilicate
is
the
main
component
of
their
matrices.
Therefore
we
propose
that
the
oxygen
isotopic
system
in
our
experiment
was
more
affected
by
dehydration
of
phyllosilicates
than
by
decomposition
of
carbonates
and
tochilinite,
although
the
carbonate
decomposition
is
clearly
complex.
Furthermore,
in
the
laboratory
dehydration
experiments,
0
17
0
was
unchanged,
whereas
the
Belgica-like
meteorites
all
have
a
less
negative
0
17
0
than
typical
CM2
chondrites,
as
seen
in
Fig.
11a.
The
shift
in
8
18
0
of
the
heated
samples
is
small
and
falls
within
the
CM2
chondrite
field
(Figs.
11
a
and
11b).
Therefore,
we
cannot
explain
the
anomalous
oxygen
isotopic
compositions
of
Dho
225
and
735
and
the
Belgica-like
group
as
a
result
of
thermal
metamorphism
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
and
metamorphosed
carbonaceous
chondrites
1121
of
CM2
chondrites.
The
scale
of
fractionation
in
our
experiments
was
too
small
to
provide
oxygen
isotopes
compositions
equivalent
to
the
Belgica-like
meteorites.
Relationship
of
Dho
225
and
Dho
735
to
the
Belgica-
Group
Chondrites
and
CM2
Chondrites
The
Belgica-like
metamorphosed
carbonaceous
chondrites
have
been
classified
differently
by
many
authors
(i.e.,
Prinz
et
al.
1989;
Poul
and
Lipschutz
1989;
Tomeoka
1990;
Bischoff
and
Metzler
1991;
Kimura
and
Ikeda
1992;
Ikeda
1992;
Grossman
and
Brearley
2005),
but
all
classification
schemes
were
either
complicated
or
are
not
appropriate
for
all
members
of
the
group.
Clayton
and
Mayeda
(1999)
summarized
the
main
characteristics
of
the
Belgica-like
metamorphosed
carbonaceous
chondrites:
(1)
Similar
oxygen
isotopic
compositions
distinct
from
the
CI
and
CM2
main
groups.
(2)
Low
H
2
O
content in
bulk
chemical
compositions
(up
to
2
wt%).
(3)
Infrared
spectral
characteristics
indicating
dehydration
(Hiroi
et
al.
1993).
Further,
Akai
(1988)
demonstrated
incomplete
transformation
of
serpentine-like
phyllosillicates
in
the
matrix
of
Antarctic
metamorphosed
CM
chondrites.
Dho
225
and
735
completely
correspond
to
the
features
listed
by
Clayton
and
Mayeda
(1999)
and
their
incomplete
phyllosilicate
transformation,
as
it
was
identified
by
Akai,
has
also
affinities
to
the
Belgica-like
chondrites.
However,
Dho
225
and
735
texturally
and
mineralogically
belong
to
the
typical
CM2
chondrites
but
their
matricies
were
dehydrated
and
their
bulk
compositions
were
different
from
those
of
typical
CM2s.
It
is
still
difficult
to
determine
the
petrologic
types
of
these
meteorites,
because
their
temperature-time
history
was
probably
different
from
that
of
ordinary
chondrites.
Thus,
based
on
our
discussion
above,
we
belive
that
Dho
225
and
Dho
735
are
not
simply
metamorphosed
CM
chondrites
and
may
represent
a
group
of
chondrites
whose
primary
material
was
different
from
typical
CM2
chondrites
and
the
Belgica-like
meteorites,
but
they
formed
in
a
oxygen
reservoir
similar
to
that
of
the
Belgica-like
chondrites.
CONCLUSIONS
Dho
225
and
735
are
chondrites
from
a
hot
desert
with
similarities
to
the
CM2
chondrites
and
the
Belgica-
like
Antarctic
metamorphosed
carbonaceous
chondrites
(B-7904,
and
Y-86720).
Dho
225
and
735
have
textural
characteristics
similar
to
those
of
typical
CM2
chondrites,
but
their
mineralogy,
bulk
chemistry
and
oxygen
isotopic
compositions
differ
from
those
of
typical
CM2's.
The
main
mineralogical
differences
between
Dho
225
and
735
and
typical
CM2
chondrites
are
the
lack
of
P,
Cr—sulfides
and
tochilinite,
and
a
low
amount
of
hydrated
minerals
in
their
matrices.
The
matrices
of
Dho
225
and
Dho
735
are
depleted
in
Fe
and
S
in
comparison
with
those
of
CM2
chondrites.
X-
ray
powder
diffraction
confirmed
incomplete
replacement
of
phyllosilicates
by
fine-grained
olivine,
as
previously
described
in
the
matrices
of
Belgica-like
meteorites
(Akai
1988,
1990).
This
indicates
the
possibility
of
dehydration
of
previously
existing
phyllosilicates.
The
bulk
chemistry
of
Dho
225
and
735
differs
from
usual
CM2
chondrites
and
from
the
Belgica-like
meteorites,
which
indicates
their
different
primary
genetic
histories.
Dho
225
and
735's
H
2
0-contents
are
low
(<2
wt%
of
H
2
O)
in
comparison
with
CM2
chondrites,
but
are
similar
to
those
of
the
Belgica-like
meteorites.
However,
the
Fe/Si
ratios
of
these
meteorites
are
lower
than
those
of
CM2
chondrites
and
the
Belgica-like
meteorites.
The
oxygen
isotopic
compositions
of
Dho
225
and
735
are
in
the
range
of
those
of
the
Belgica-like
meteorites.
The
oxygen
isotopic
compositions
of
Mighei
and
Murchison
changed
after
laboratory
heating
due
to
a
process
of
dehydration
which
led
to
enrichment
of
their
oxygen
compositions
in
18
0
and
17
0.
Experimental
results
showed
that
the
oxygen
isotopic
composition
of
the
Belgica-like
chondrites
and
Dho
225
and
725
could
not
be
derived
from
that
of
typical
CM2
chondrites
via
dehydration
caused
by
thermal
metamorphism.
Dho
225
and
735,
could
represent
a
separate
group
of
chondrites
whose
primary
material
was
different
from
typical
CM2
chondrites
and
formed
in
a
different
oxygen
reservoir.
Acknowledgments—We
thank
L.
D.
Barsukova,
I.
A.
Roshchina
and
S.
Galuzinskaya
for
their
assistance
in
determination
of
bulk
chemical
compositions
of
Dho
225
and
Dho
735.
We
also
thank
Theodoras
Ntaflos
for
his
assistance
in
microprobe
analyses,
as
well
as
Martin
Schmidt,
Ulrich
Schade,
and
Alexander
Firsov
for
their
assistance
in
SIRM
analyses.
We
thank
Michael
Weisberg,
Michael
Zolensky,
anonymous
author,
Jon
Friedrich,
Alan
Rubin,
and
associate
editor
Cyrena
Goodrich
for
their
fruitful
reviews
and
discussion,
which
led
to
improvement
of
the
paper.
This
work
was
supported
by
grant
RFBR-BSTS
(projects
N14/04
and
03-05-20008),
Austrian
Academy
of
Sciences
(FWF,
Austria)
and
by
PPARC,
UK.
This
work
was
also
supported
by
Program
no.
18
of
the
Russian
Academy
of
Sciences.
Editorial
Handling—Dr.
Cyrena
Goodrich
1122
M.
A.
Ivanova
et
al.
REFERENCES
Akai
J.
1988.
Incompletely
transformed
serpentine-type
phyllosilicates
in
the
matrix
of
Antarctic
CM
chondrites.
Geochimica
et
Cosmochimica
Acta
52:1593-1599.
Akai
J.
1990.
Mineralogical
evidence
of
heating
events
in
Antarctic
carbonaceous
chondrites,
Yamato-86720
and
Yamato-82162.
Proceedings
of
the
NIPR
Symposium
on
Antarctic
Meteorites
3:55-68.
Bischoff
A.
and
Metzler
K.
1991.
Mineralogy
and
petrography
of
the
anomalous
carbonaceous
chondrites
Yamato-86720,
Yamato-82162,
and
Belgica-7904.
Proceedings
of
the
NIPR
Symposium
on
Antarctic
Meteorites
4:226-246.
Brearley
A.
J.
and
Jones
R.
H.
1998.
Chondritic
meteorites.
In
Planetary
materials,
edited
by
Papike
J.
J.
Washington,
D.C.:
Mineralogical
Society
of
America.
pp.
398.
Brindley
G.
W.
and
Zussman
J.
1957.
A
structural
study
of
the
thermal
transformation
of
serpentine
minerals
to
forsterite.
American
Mineralogist
42:461-475.
Clayton
R.
N.
and
Mayeda
T.
K.
1984.
The
oxygen
isotope
record
in
Murchison
and
other
carbonaceous
chondrites.
Earth
and
Planetary
Science
Letters
67:151-166.
Clayton
R.
N.
and
Mayeda
T.
K.
1999.
Oxygen
isotope
studies
of
carbonaceous
chondrites.
Geochimica
et
Cosmochimica
Acta
63:2089-2104.
Clayton
R.
N.,
Mayeda
T.
K.,
Hiroi
T.,
Zolensky
M.,
and
Lipschutz
M.
E.
1997.
Oxygen
isotopes
in
laboratory-
heated
CI
and
CM
chondrites
(abstract).
Meteoritics
&
Planetary
Science
32:A30.
Dyakonova
M.
I.
and
Kharitonova
V.
Ya.
1966.
About
methods
of
chemical
analysis
of
stony
and
iron
meteorites.
Meteoritika
27:89-96.
In
Russian.
Fuchs
L.
H.,
Olsen
E.,
and
Jensen
K.
J.
1973.
Mineralogy,
mineral-chemistry,
and
composition
of
the
Murchison
(C2)
meteorite.
Smithsonian
Contributions
to
the
Earth
Sciences
10:1-39.
Grossman
J.
N.
and
Brearley
A.
J.
2005.
The
onset
of
metamorphism
in
ordinary
and
carbonaceous
chondrites.
Meteoritics
&
Planetary
Science
40:87-122.
Grossman
J.
N.,
Zolensky
M.
E.,
and
Tonui
E.
K.
2005.
What
are
the
petrologic
types
of
thermally
metamorphosed
CM
chondrites?
(abstract
#5169).
68th
Annual
Meteoritical
Society
Meeting.
CD-ROM.
Hiroi
T.,
Pieters
C.
M.,
Zolensky
M.
E.,
and
Lipschutz
M.
E.
1993.
Evidence
of
thermal
metamorphism
on
C,
G,
B,
and
F
asteroids.
Science
261:1016-1018.
Ikeda
Y.
1992.
An
overview
of
the
research
consortium,
"Antarctic
carbonaceous
chondrites
with
affinities,
Yamato-86720, Yamato-82162,
and
Belgica-7904."
Proceedings
of
the
NIPR
Symposium
on
Antarctic
Meteorites
5:49-73.
Ikornikova
A.
and
Sheptunov
D.
1973.
Crystallization
processes
under
hydrothermal
condition,
edited
by
Lobachev
A.
N.
New
York:
Consultants
Bureau,
113-123.
Ivanova
M.
A.,
Nazarov
M.
A.,
Taylor
L.
A.,
Clayton
R.
N.,
Maeyda
T.
K.,
Brandstaetter
F.,
and
Kurat
G..
2002a.
Dhofar
225
vs.
CM
clan:
Metamorphosed
or
a
new
type
of
carbonaceous
chondrite?
(abstract
#1437).
33rd
Lunar
and
Planetary
Science
Conference.
CD-ROM.
Ivanova
M.
A.,
Moroz
L.
V.,
Schmidt
M.,
Schade
U.,
Brandstaetter
F.,
Nazarov
M.
A.,
and
Kurat
G.
2004.
Are
the
MCCs
Dhofar
225
and
Dhofar
735
of
CM3-type?
(abstract
#5113).
Meteoritics
&
Planetary
Science
39
(Supplement).
Ivanova
M.
A.,
Nazarov
M.
A.,
Brandstaetter
F.,
Moroz
L.
V.,
Ntaflos
Th.,
and
Kurat
G..
2005.
Mineralogical
differences
between
metamorphosed
and
non-
metamorphosed
carbonaceous
chondrites
(abstract
#1054).
36th
Lunar
and
Planetary
Science
Conference.
CD-ROM.
Ivanova
M.
A.,
Lorenz
C.
A.,
Greenwood
R.
C.,
Franchi
I.
A.,
Nazarov
M.
A.,
Morris
A.
A.,
Baker
L.,
and
Brandstaetter
F.
2006.
Experimental
study
of
laboratory-
heated
CM2
chondrites
Mighei
and
Murchison
(abstract
#1086).
37th
Lunar
and
Planetary
Science
Conference.
CD-ROM.
Jarosewich
E.
1990.
Chemical
analyses
of
meteorites:
A
compilation
of
stony
and
iron
meteorite
analyses.
Meteoritics
25:323-337.
Kimura
M.
and
Ikeda
Y.
1992.
Mineralogy
and
petrology
of
an
unusual
Belgica-7904
carbonaceous
chondrite:
Genetic
relationships
among
the
components.
Proceedings
of
the
NIPR
Symposium
on
Antarctic
Meteorites
5:74-
119.
Kojima
T.,
Tomeoka
K.,
and
Takeda
H.
1994.
Unusual
dark
clasts
in
the
Vigarano
CV3
carbonaceous
chondrite:
Record
of
parent
body
processes.
Meteoritics
28:649-658.
Krot
A.
N.,
Zolensky
M.
E.,
Wasson
J.
T.,
Scott
E.
R.
D.,
Keil
K.,
and
Ohsumi
K.
1997.
Carbide-magnetite-bearing
type-3
ordinary
chondrites.
Geochimica
et
Cosmochimica
Acta
61:219-237.
Lipschutz
M.
E.,
Zolensky
M.
E.,
and
Bell
M.
S.
1999.
New
petrographic
and
trace
element
data
on
thermally
metamorphosed
carbonaceous
chondrites.
Antarctic
Meteorite
Research
12:57-80.
Mayeda
T.
K.
and
Clayton
R.
N.
1998.
Oxygen
isotope
effects
in
serpentine
dehydration
(abstract
#1405).
29th
Lunar
and
Planetary
Science
Conference.
CD-ROM.
McSween
Jr.
H.
Y.
and
Richardson
S.
M.
1977.
The
composition
of
carbonaceous
chondrite
matrix.
Geochimica
et
Cosmochimica
Acta
41:1145-1161.
Miller
M.
F.,
Franchi
I.
A.,
and
Pillinger
C.
T.
1999.
High
precision
measurements
of
the
oxygen
isotope
mass-
depended
fractionation
line
for
the
Earth-Moon
system
(abstract
#1729).
30th
Lunar
and
Planetary
Science
Conference.
CD-ROM.
Moroz
L.
V.,
Schmidt
M.,
Schade
U.,
Hiroi
T.,
and
Ivanova
M.
A.
2006.
Synchrotron-based
infrared
microspectroscopy
as
a
useful
tool
to
study
hydration
states
of
meteorite
constituents.
Meteoritics
&
Planetary
Science
41:1219-
1230.7.
Murrell
M.
T.
and
Burnett
D.
S.
1983.
The
behavior
of
actinides,
phosphorus,
and
rare
earth
elements
during
chondrite
metamorphism.
Geochimica
et
Cosmochimica
Acta
47:1999-2014.
Nazarov
M.
A.,
Kurat
G.,
and
Brandstaetter
F.
2001.
Phosphorian
sulfides
from
the
ALH
84029,
ALH
85013,
EET
96029,
and
Y
82042
CM
carbonaceous
chondrites
(abstract
#1769).
32nd
Lunar
and
Planetary
Science
Conference.
CD-ROM.
Peck
L.
C.
1964.
Systematic
analysis
of
silicates.
U.
S.
Geological
Survey
Bulletin
1170.89
p.
Poul
R.
L.
and
Lipschutz
M.
E.
1989.
A
modest
proposal
for
carbonaceous
chondrite
classification
on
light
of
the
Antarctic
samples
(abstract).
Meteoritics
24:313-
314.
Prinz
M.,
Weisberg
M.
K.,
Han
R.,
and
Zolensky
M.
E.
1989.
Type
I
and
II
chondrules
in
the
B-7904
carbonaceous
chondrite
(abstract).
Meteoritics
24:317-318.
Dhofar
225
and
Dhofar
735:
Relationship
to
CM2
and
metamorphosed
carbonaceous
chondrites
1123
Rubin
A.
E.
and
Grossman
J.
N.
1985.
Phosphate-sulfide
assemblages
and
Al/Ca
ratios
in
type-3
chondrites.
Meteoritics
20:479-489.
Russell
S.
S.,
Zipfel
J.,
Grossman
J.
N.,
and
Grady
M. M.
2002.
The
Meteoritical
Bulletin
86.
Meteoritics
&
Planetary
Science
37:A157
A184.
Russell
S.
S.,
Zipfel
J.,
Folco
L.,
Jones
R.,
Ggrady
M.
M.,
McCoy
T.,
and
Grossman
J.
2003.
The
Meteoritical
Bulletin
87.
Meteoritics
&
Planetary
Science
38:A189—
A248.
Tomeoka
K.
1990.
Mineralogy
and
petrology
of
Belgica-7904:
A
new
kind
of
carbonaceous
chondrite
from
Antarctica.
Proceedings
of
the
NIPR
Symposium
on
Antarctic
Meteorites
3:40-54.
Tomeoka
K.
and
Buseck
P.
R.
1985.
Indicators
of
aqueous
alteration
in
CM
carbonaceous
chondrites:
Microtextures
of
a
layered
mineral
containing
Fe,
S,
0
and
Ni.
Geochimica
et
Cosmochimica
Acta
49:2149-2163.
Tomeoka
K.,
Kojima
H.,
and
Yanai
K.
1989a.
Yamato-
86720:
A
CM
carbonaceous
chondrite
having
experiences
extensive
aqueous
alteration
and
thermal
metamorphism.
Proceedings
of
the
NIPR
Symposium
on
Antarctic
Meteorites
2:55-74.
Tomeoka
K.,
Kojima
H.,
and
Yanai
K.
1989b.
Yamato-
82162:
A
new
kind
of
CI
carbonaceous
chondrite
found
in
Antarctica.
Proceedings
of
the
NIPR
Symposium
on
Antarctic
Meteorites
2:36-54.
Tonui
E.,
Zolensky
M.,
and
Lipschutz
M.
2002.
Petrography,
mineralogy
and
trace
element
chemistry
of
Yamato-86029,
Yamato-793321
and
Lewis
Cliff
85332:
Aqueous
alteration
and
heating
events.
Antarctic
Meteorite
Research
15:38-58.
Zolensky
M.
E.,
Barret
R.,
and
Browning
L.
1993.
Mineralogy
and
composition
of
matrix
and
chondrule
rims
in
carbonaceous
chondrites.
Geochimica
et
Cosmochimica
Acta
57:3123-3148.
Zolensky
M.
E.,
Abell
P.
A.,
and
Tonui
E.
K.
2005.
Metamorphosed
CM
and
CI
carbonaceous
chondrites
could
be
form
the
breakup
of
the
same
Earth-crossing
asteroid
(abstract
#2084).
36th
Lunar
and
Planetary
Science
Conference.
CD-ROM.