Bulk chemistry and oxygen isotopic compositions of lunar meteorites Dhofar 025 and Dhofar 026


Taylor Lawrence, A.; Nazarov, M.A.; Cohen, B.A.; Warren Paul, H.; Barsukova, L.D.; Clayton, R.N.; Mayeda, T.K.

Lunar and Planetary Science 32

2001


The major- and trace-element composition of highlands meteorites Dh25 and Dh26 show that both are dominated by a FAN component. Incompatible element depletion and low Ti abundances suggest a farside origin. O-isotopes are typical for lunar meteorites.

14
1 1 1
Fig.
1
12
Dhofar
10
ALHA
810
E
8-
-
a
6—
_
47
25
6032**
M
G
262-Q,
069•'
DAG
400
88104
Dhofar
026
Apollo
16
Regolith
[6]
QUE
ill
22
24
26 26
30
32
34
36
A1203
(wt%)
2
Lunar
Highland
*
Meteorites
[4,5,8,]
0
100
E
I I
Apollo
14
4:
1
.
egolith
[9]
I I
Fig.
2
10-
a
a
E
co
1.=
Apollo
16
-
Regolith
[9]
Luna
Highland
*
Meteorites
[4,5,8]
Luna
20
Regolith
[9]
M8810
Dhofar
025
4
4,
,
ALHA
81005*
4
:k?
DAG
262
Dhofar
026
Y86032
*
DAG
400
E8
16
20
24
1
I
I
18
A1203
(wt
2
%)
32
BULK
CHEMISTRY
AND
OXYGEN
ISOTOPIC
COMPOSITIONS
OF
LUNAR
METEORITES
DHOFAR
025
AND
DHOFAR
026.
L.A.
Taylor',
M.A.
Nazarov
2
,
B.A.
Cohen',
P.H.
Warren
3
,
L.D.
Barsukova
2
,
R.N.
Clayton'',
and
T.K.
Mayeda
4
;
'Planetary
Geosciences
Institute,
Univ.
of
Tennessee,
Knoxville,
TN
37996,
USA
(latay-
),
2
Vernadsky
Institute
of
Geochemistry,
Moscow
117975,
Russia;
3
Institute
for
Geophysics
and
Planetary
Sciences,
UCLA,
Los
Angeles,
CA
90024;
"Enrico
Fermi
Institute,
Univ.
of
Chicago,
Chicago
IL
60637.
Introduction:
Dhofar
025
(Dh25)
and
Dhofar
026
(Dh26)
are
new
lunar
meteorites
found
—20
km
apart
in
the
desert
of
Dhofar,
Oman.
These
meteorites
are
very
different
in
texture,
mineral
chemistry,
and
noble
gas
composition
[1-3].
Dh25
is
a
gas-rich
regolith
breccia
containing
abundant
lithic
clasts
and
mineral
fragments
in
a
glassy
matrix
[1].
Dh26
is
a
gas-poor,
crystalline
melt
breccia
consisting
of
rare
mineral
fragments
and
lithic
clasts
in
fine-grained,
crystalline
matrix
[2].
Here
we
report
the
first
data
on
major-
and
trace-element
chemistries
and
oxygen
isotopic
compo-
sitions
of
these
meteorites.
Both
lunar
samples
are
similar
in
composition
to
other
lunar
highland
meteor-
ites.
However,
each
meteorite
is
distinctly
different
within
the
compositional
range
of
lunar
highland
me-
teorites.
Methods:
1-g
samples
were
powdered
in
an
agate
mill
to
obtain
homogeneous
material
for
whole-rock
analyses.
Si,
Ti,
Al,
Cr,
Fe,
Mn,
Mg
and
Ca
were
de-
termined
by
XRF
and
ICP.
Na
and
K
were
measured
by
atomic
absorption.
Trace
elements
were
analyzed
by
INAA.
The
major-
and
trace-elements
are
given
in
Table
1.
Separate
splits
weighing
—10
mg
were
used
to
measure
oxygen
isotopic
compositions
[4].
Results:
Dh25
and
Dh26
have
anorthositic
compo-
sitions,
which
are
typical
for lunar
highland
meteorites
[e.g.,
5,6].
The
very
high
Sr
content
in
Dh25
is
ap-
parently
caused
by
terrestrial
weathering
and
suggests
that
Dh25
is
more
altered
than
Dh26,
which
does
not
show
the
Sr
anomaly.
This
indicates
that
the
primary-
element
relationships
may
also
be
altered
to
various
degrees
by
terrestrial
weathering
in
both
meteorites,
particularly
for
Ca,
K,
Cs
and
possibly
Si.
Compared
to
Dh25,
Dh26
is
richer
in
Al,
Ca,
and
Eu
and
poorer
in
Fe,
Mg,
and
all
other
trace
elements.
This
is
due
to
the
higher
feldspar
content
in
Dh26.
The
lower
Na/Ca
ratio
of
Dh26
further
indicates
that
the
feldspar
is
higher
in
An
content
relative
to
Dh25.
Also,
the
mafic
constituent
of
Dh26
must
be
richer
in
Fe/Mg
than
that
in
Dh25.
Within
the
compositional
range
of
lunar
highland
meteorites,
the
differences
between
Dh25
and
Dh26
are
significant
(Fig.1-3).
Dh26
is
one
of
the
highest
in
Al
and
poorest
in
Mg#
and
Fe.
In
contrast,
Dh25
is
one
of
the
highest
in
Mg#
and
Sc
and
poorest
in
Al.
Dh25
and
Dh26,
as
well
as
other
lunar
highland
meteorites,
are
compositionally
similar
to
lunar
high-
land
rocks.
The
similarity
is
reflected
in
the
range
of
concentrations
of
elements
like
Al
and
Sc
which
are
carried
mainly
by
plagioclase
and
pyroxene,
respec-
tively
(Fig.
1).
Though
similar
to
other
lunar
highland
meteorites,
Dh25
and
Dh26
are
poor
in
Na,
Ti,
and
incompatible
elements
compared
to
the
typical
Apollo
16
highland
regolith
(Fig.
2).
Compared
to
pristine
FAN
and
HMS
rocks,
these meteorites
have
interme-
diate
Ti/Sm
ratios
(Fig.
3).
They
are
similar
in
com-
position
to
noritic
anorthosites
60135
and
67513
[7].
Siderophile
element
contents
of
Dh25
and
Dh26
fall
within
the
range
reported
for
lunar
meteorites
[5,6]
and
indicate
the
presence
of
a
meteoritic
compo-
nent.
Ni/Ir
ratios
are
practically
chondritic
and
show
low
indigenous
contents
of
Ni.
The
indigenous
con-
tent
of
Co
is
calculated
to
be
—8
ppm,
using
the
chon-
dritic
Co/Ir
ratio
to
correct
for
meteoritic
contribution.
This
value
is
typical
for
lunar
highland
meteorites.
The
Au/Ir
ratio
of
Dh25
is
lower
than
that
in
chon-
drites
whereas
Dh26
shows
a
relative
enrichment
in
Au/Ir.
10000T
E
Fig.
3
_
Dunite
-
Ferroan
Anorthosites
[1
1000-2
Dhofar
02611
*M
*
*
Dhofar
025
Mg-rich
suite
[10
KREEP
[1
Luna
Highland
Meteorites
[4,5,0
100
0
4
0.5
0.6
0.7
0.8
0.9
Mg
/
(Mg+Fe)
at.
0
Lunar
meteorites
Dhofar
O
0
0
3.5
3.3
F
2
3.1
O
2.9
2.7
6.0
62
2.5
5
0
5.2
5.4
5.6
5.8
d'o
(14.,
rel.
SMOW)
LUNAR
METEORITES
DHOFAR
025
AND
DHOFAR
026:
L.
A.
Taylor
et
al.
Dh25
has
8
18
0
=
+5.47
and
6
17
0
=
+2.81;
Dh26
has
6
18
0
=
+5.77
and
6
18
0
=
+2.87.
The
oxygen
iso-
topic
compositions
lie
on
the
terrestrial
fractionation
line
and
are
consistent
with
those
reported
for
all
other
lunar
meteorites
[6,8]
(Fig.
4).
Discussion:
Dh25
and
Dh26
are
breccias,
and
therefore,
bulk
compositions
of
the
meteorites
result
mainly
from
mixing
of
primary
lunar
lithologies.
The
chemistry
of
the
lithologies
may
have
been
modified
due
to
impact-induced
fractionation
before
and
during
the
mixing.
It
appears
that
both
these
meteorites
are
dominated
by
a
component
with
a
ferroan
anorthosite
(FAN)
composition.
A
minor
Mg-rich
component
is
also
present
in
Dh25.
The
anorthosite
and
perhaps
the
Mg-rich
components
are
depleted
in
incompatible
elements
and
are
similar
to
the
Luna
20
regolith
(Fig.
2).
It
is
possible,
therefore,
that
,
that
the
source
area
of
these
lunar
highland
meteorites
might
be
located
on
the
far-eastern
limb
of
the
lunar
nearside
or
on
the
farside,
where
FAN
materials
are
thought
to
dominate.
In
addition,
these
meteorites
do
not
show
any
geo-
chemical
evidence
of
KREEP
material
that
is
typical
for
the
highlands
at
least
on
the
central-
and
western-
regions
of
the
lunar
nearside.
The
very-low
Ti
content
in
Dh25
and
Dh26
also
suggests
that
any
mare
basalt
component
is
insignificant
in
these
rocks,
or
might
be
a
VLT
type
if
present
at
all.
Additional
data
on
the
major-
and
trace-element
chemistry
of
the
lunar
meteorites
Dh25
and
Dh26
and
of
another
lunar
meteorite,
Dhofar
081,
are
discussed
in
[12].
References:
[1]Cahill
et
al.
(2001)
this
volume;
[2]Cohen
et
al.
(2001)
this
volume;
[3]Shukolyukov
(2001)
this
volume;
[4]Clayton
and
Mayeda
(1983)
EPSL
62,
1-6;
[5]Palme
et
al.
(1991)
GCA
55,
3105-
3122;
[6]Bishoff
et
al.
(1998)
MAPS
33,
1243-1257;
[7]Korotev
(1997)
MAPS
32,
447-478;
[8]Clayton
and
Mayeda
(1996)
GCA
60,
1999-2017;
[9]Zipfel
et
al.
(1998)
MAPS
33,
A171;
[10]Warren
and
Wasson
(1980)
PLSC
11th,
431-470;
[11
]Taylor
(1982)
Plane-
tary
Science-A
Lunar
Perspective;
LPI.
[12]Warren
(2001)
this
volume.
Table
1:
Whole-rock
chemistry
of
Dh25
and
Dh26.
Dhofar
025
Dhofar
026
Si0
2
wt%
43.86
44.31
TiO
2
wt%
0.302
0.222
A1
2
0
3
wt%
26.65
29.59
Cr
2
O
3
wt.%
0.102
0.08
Fe0
wt%
4.98
4.06
Mn0
wt%
0.076
0.064
Mg0
wt%
6.53
3.92
Ca0
wt.%
16.11
16.99
Na
2
0
wt%
0.282
0.243
1(
2
0
wt.%
0.065
0.082
P205
wt.%
0.08
0.05
H
2
O
wt%
0.27 0.57
Total
wt.%
99.31
100.18
Sc
ppm
10.2
7.9
Cr
ppm
674
465
Co
ppm
16.5
13.6
Ni
ppm
200
170
Sr
ppm
2010
200
Zr
ppm
62
32
Cs
ppm
0.55
0.44
La
ppm
3.6
2.9
Ce
ppm
8.6
6.6
Nd
ppm
5.2
3.8
Sm
ppm
1.5
1.1
Eu
ppm
1.3
1.1
'Fb
ppm
0.35 0.25
Yb
ppm
1.2
0.85
Lu
ppm
0.21
0.15
Hf
ppm
1.3
0.86
Ta
ppm
0.24
Ir
ppb
7.2
6.3
Au
ppb
3
9
Th
ppm
0.8
0.36
U
ppm
0.27