A LREE-depleted component in the Afar plume; further evidence from Quaternary Djibouti basalts


Daoud, M.A.; Maury, R.C.; Barrat, J.A.; Taylor, R.N.; Le Gall, B.; Guillou, H.; Cotten, J.; Rolet, J.

Lithos 11(4): 3-4

2010


Major, trace element and isotopic (Sr, Nd, Pb) data and unspiked K-Ar ages are presented for Quaternary (0.90-0.95 Ma old) basalts from the Hayyabley volcano, Djibouti. These basalts are LREE-depleted (La(n) /Sm(n) =0.76-0.83), with(87) Sr/(86) Sr ratios ranging from 0.70369 to 0.70376, and rather homogeneous(143) Nd/(144) Nd (epsilon(Nd) =+5.9-+7.3) and Pb isotopic compositions ((206) Pb/(204) Pb=18.47-18.55,(207) Pb/(204) Pb=15.52-15.57,(208) Pb/(204) Pb=38.62-38.77). They are very different from the underlying enriched Tadjoura Gulf basalts, and from the N-MORB erupted from the nascent oceanic ridges of the Red Sea and Gulf of Aden. Their compositions closely resemble those of (1) depleted Quaternary Manda Hararo basalts from the Afar depression in Ethiopia and (2) one Oligocene basalt from the Ethiopian Plateau trap series. Their trace element and Sr, Nd, Pb isotope systematics suggest the involvement of a discrete but minor LREE-depleted component, which is probably an intrinsic part of the Afar plume.

LSEVIER
Lithos
114
(2010)
327-336
Contents
lists
available
at
ScienceDirect
Lithos
journal
homepage:
www.elsevier.com/locate/lithos
A
LREE-depleted
component
in
the
Afar
plume:
Further
evidence
from
Quaternary
Djibouti
basalts
Mohamed
A.
Daoud
a
'
b
,
Rene
C.
Maury
a,*
,
Jean-Alix
Barrat
a
,
Rex
N.
Taylor
C,
Bernard
Le
Gall
a,
Herve
Guillou
d
,
Joseph
Cotten
a
,
Joel
Rolet
a
a
Universite
Europeenne
de
Bretagne,
Universite
de
Brest,
CNRS,
UMR
6538
Domaines
Oceaniques,
Institut
Universitaire
Europeen
de
la
Mer,
Place
N.
Copernic,
29280
Plouzane,
France
b
Centre
d'Etudes
et
de
Recherches
Scientifiques
de
Djibouti,
B.P.
486,
Djibouti
School
of
Ocean
and
Earth
Science,
NOC,
University
of
Southampton,
Southampton
S014
3ZH,
UK
d
UMR
1572
LSCE/CEA-CNRS,
Domaine
du
CNRS,
12
avenue
de
la
Terrasse,
91118
Gif-sur-Yvette,
France
ARTICLE INFO
Article
history:
Received
16
May
2009
Accepted
21
September
2009
Available
online
6
October
2009
Keywords:
Depleted
basalts
Trace
element
geochemistry
Isotopic
compositions
Afar
plume
Djibouti
NE
Africa
ABSTRACT
Major,
trace
element
and
isotopic
(Sr,
Nd,
Pb)
data
and
unspiked
K-Ar
ages
are
presented
for
Quaternary
(0.90-0.95
Ma
old)
basalts
from
the
Hayyabley
volcano,
Djibouti.
These
basalts
are
LREE-depleted
(Lan/Smn
=
0.76-0.83),
with
87
Sr/
86
Sr
ratios
ranging
from
0.70369
to
0.70376,
and
rather
homogeneous
143
Nd/
144
Nd
(6Nd
=
+
5.9-+
7.3)
and
Pb
isotopic
compositions
(
206
Pb/
2°4
Pb
=18.47-18.55,
7pbi204
Pb
=15.52-15.57,
2°8
1
3
b/
204
Pb
=
38.62-38.77).
They
are
very
different
from
the
underlying
enriched
Tadjoura
Gulf
basalts,
and
from
the
N-MORB
erupted
from
the
nascent
oceanic
ridges
of
the
Red
Sea
and
Gulf
of
Aden.
Their
compositions
closely
resemble
those
of
(1)
depleted
Quaternary
Manda
Hararo
basalts
from
the
Afar
depression
in
Ethiopia
and
(2)
one
Oligocene
basalt
from
the
Ethiopian
Plateau
trap
series.
Their
trace
element
and
Sr,
Nd,
Pb
isotope
systematics
suggest
the
involvement
of
a
discrete
but
minor
LREE-depleted
component,
which
is
probably
an
intrinsic
part
of
the
Afar
plume.
©
2009
Elsevier
B.V.
All
rights
reserved.
I.
Introduction
The
study
of
basalts
from
intra-oceanic
islands
and
plateaus
as
well
as
from
traps
and
rifts
has
shown
the
considerable
chemical
heterogeneity
of
plume
materials
(Hart,
1988).
This
heterogeneity
might
indicate
very
complex
plume
structures
and
dynamics
(Lin
and
van
Keken,
2006).
However,
it
may
not
only
result
from
the
initial
chemical
heterogeneity
of
mantle
plumes
at
depth
but
also
from
the
entrainment
of
surrounding
mantle
materials
(Hart
et
al.,
1992;
Furman
et
al.,
2006).
In
addition,
a
lithospheric
component
is
clearly
recognized
in
some
intracontinental
basalts,
e.g.
in
the
Afar
province,
but
its
origin
is
still
debated
(Rogers,
2006).
Some
authors
have
suggested
that
melting
of
the
Afar
lithospheric
mantle
explains
a
significant
proportion
of
the
erupted
lavas
(Hart
et
al.,
1989;
Vidal
et
al.,
1991;
Deniel
et
al.,
1994)
whilst
others
point
out
that
continental
crust
contamination
can
also
contribute
to
the
isotopic
signature
of
these
basalts
(Barrat
et
al.,
1993;
Baker
et
al.,
1996;
Pik
et
al.,
1999).
The
vast
majority
of
plume-related
basalts,
including
the
Afar
ones
(Furman
et
al.,
2006;
Beccaluva
et
al.,
2009)
are
dominated
by
a
component
that
is
chemically
and
isotopically
enriched.
However,
the
occurrence
of
subordinate
components
characterized
by
a
light
rare
earth
element
(LREE)
depletion
has
been
suggested
from
the
study
of
*
Corresponding
author.
Tel.:
+33
298498708;
fax:
+33
298498760.
E-mail
address:
mauly@univ-brestfr
(R.C.
Maury).
0024-4937/$
-
see
front
matter
C
2009
Elsevier
B.V.
All
rights
reserved.
doi:10.1016/j.lithos.2009.09.008
basalts
from
major
mantle
plumes
in:
(1)
Iceland
(Zindler
et
al.,
1979;
Hemond
et
al.,
1993;
Taylor
et
al.,
1997;
Chauvel
and
Hemond,
2000;
Skovgaard
et
al.,
2001;
Fitton
et
al.,
2003;
Thirlwall
et
al.,
2004;
Kokfelt
et
al.,
2006);
(2)
Hawaii
(Chen
and
Frey,
1985;
Yang
et
al.,
2003;
Frey
et
al.,
2005);
(3)
the
Galapagos
(White
et
al.,
1993;
Hoernle
et
al.,
2000;
Blichert-Toft
and
White,
2001;
Saal
et
al.,
2007);
and
(4)
the
Kerguelen
Archipelago
(Doucet
et
al.,
2002).
However,
the
character-
ization
of
this
reservoir
is
difficult
because
its
signature
may
be
overprinted
by
either
the
dominant
enriched
plume
component
or
the
lithospheric
reservoirs.
Therefore,
the
presence
of
an
intrinsic
depleted
component
in
plumes
is
still
an
open
question.
LREE-depleted
basalts
associated
to
the
Afar
mantle
plume
have
long
been
recognized
in
the
Quaternary
Manda
Hararo
volcanic
chain,
Ethiopia
(Treuil
and
joron,
1975;
Joron
et
al.,
1980;
Barrat
et
al.,
2003).
A
single
LREE-depleted
Oligocene
Ethiopian
Plateau
basalt
has
also
been
so
far
analysed
(sample
E88:
Pik
et
al.,
1998,
1999).
The
purpose
of
this
paper
is:
(1)
to
describe
another
newly
discovered
occurrence
of
such
basalts
in
the
SE
part
of
the
Afar
triangle,
i.e.
the
rather
large
Hayyabley
Quaternary
volcano
in
Djibouti
(Fig.
1),
and
(2)
to
discuss
its
bearing
on
the
composition
and
heterogeneity
of
the
Afar
mantle
plume.
2.
Analytical
techniques
Ar
isotopic
compositions
and
K
contents
(Table
1)
were
measured
at
Gif-sur-Yvette
and
IUEM
(Institut
Universitaire
Europeen
de
la
A
A
A
A
A A
A
A
A
A
A
A
A
A
A A A
A
A
A
A A
A
A
A
A A
A
RA"
A
A A
AAA
A
A
A A
A
jibttuti
42.45'E
TADJOURA
GULF
Djibouti
o
5
km
ayyable
vo
feah
11°35'E
I2°N
TADJOURA
GULF
Wadi
Warabor
Djibouti
5
km
71
0
A A
A
A A
A
AAAA
AAAA
A
A A
A
A A
A
AAAA
328
MA
Daoud
et
aL
/
Lithos
114
(2010)
327-336
43°10'E
43°10'E
43°E
11°35NI
Recent
deposits
Upper
Pleistocene
fluvial
deposits
and
reef
limestone
Asal
basalts
Quaternary
volcanoes
Tadjoura
Gulf
Basalts
Mabla
rhyolites-Dalha
and
Somali
basalts
Main
normal
faults
Secondary
normal
faults
A
Signal
Bou't
127m
Summit
zone
Flow
direction
54o
Sampling
location
Fig.
1.
Geological
setting
of
the
Djibouti
Plain.
(a)
Location
of
the
study
area
in
the
Tadjoura
Gulf
context.
(b)
ASTER
satellite
image
showing
the
Hayyabley
volcano
post-dating
the
coastal
fault
belt
related
to
the
Tadjoura
rift.
(c)
Geological
interpretation
of
panel
b.
Mer),
respectively.
The
samples
were
crushed,
sieved
to
0.25-0.125
mm
size
fraction
and
ultrasonically
washed
in
acetic
acid. Potassium
and
argon
were
measured
on
the
microcrystalline
groundmass,
after
removal
of
phenocrysts
using
heavy
liquids
of
appropriate
densities
and
magnetic
separations.
This
process
improves
the
K
yield
as
well
as
the
percentage
of
radiogenic
argon,
and
removes
at
least
some
potential
sources
of
systematic
error
due
to
the
presence
of
excess
'Ar
in
olivine
and
feldspar
phenocrysts
(Laughlin
et
al.,
1994).
Ar
analyses
were
performed
using
the
procedures
detailed
in
Yurtmen
et
al.
(2002)
and
Guillou
et
al.
(2004).
The
unspiked
technique
differs
from
the
conventional
isotope
dilution
method
in
that
argon
extracted
from
the
sample
is
measured
in
sequence
with
purified
aliquots
of
atmospheric
argon
at
the
same
working
gas
pressure
in
the
mass-spectrometer.
This
suppresses
mass
discrimination
effects
between
the
atmospheric
reference
and
the
unknown,
and
allows
quantities
of
radiogenic
as
small
as
0.14%
to
be
detected
on
a
single-run
basis
(Scaillet
and
Guillou,
2004).
Argon
was
extracted
by
radio
frequency
heating
of
2.0-3.0
g
of
sample,
then
transferred
to
an
ultra-high-vacuum
glass
line
and
purified
with
titanium
sponge
and
Zr-Ar
getters.
Isotopic
analyses
were
performed
on
total
'Ai
.
contents
ranging
between
2.4
Sample
id
Split
Weighted
mean
Experiment
#
K
(wt.%)
±
la
DJ54B
Mass
molten
(g)
40Ar*
10
-13
(mol./g)
f
lo
-
40Ar*
10
-13
(mol./g)
f
lo
Age
(Ma)±2a
7040
0.09
±
0.005
2.10972
1.254
1.430±
0.057
7056
2.07428
1.213
1.468
±
0.058
1.449±
0.041
0.93
±
0.06
DJ54F
7039
0.04
±
0.004
2.77642
0.577
0.642
±
0.046
7063
3.01844
0.699
0.745
±
0.040
0.701
±
0.030
1.06
±
0.09
See
text
for
the
analytical
procedures.
and
3.2
x
10
-11
moles
using
a
180°,
6
cm
radius
mass
spectrometer
with
an
accelerating
potential
of
620
V.
The
manometric
calibration
(Charbit
et
al.,
1998)
was
based
on
periodic,
replicate
determinations
of
international
dating
standards
including
LP-6
(Odin
et
al.,
1982)
and
HD-B1
(Fuhrmann
et
al.,
1987).
The
total
40
Ar
content
of
the
sample
can
be
determined
with
a
precision
of
±
0.2%
(2a)
according
to
this
procedure.
Ages
were
calculated
using
the
constants
recommended
by
Steiger
and
Jager
(1977).
Major
element
compositions
of
minerals
and
glasses
were
deter-
mined
using
a
Cameca
SX50
five
spectrometer
automated
electron
microprobe
(Microsonde
Ouest,
Plouzane,
France).
Analytical
condi-
tions
were
15
kV,
10-12
nA
and
a
counting
time
of
6
s.
(see
Defant
et
al.,
1991,
for
further
analytical
details).
Major
and
trace
element
data
on
bulk
rocks
(Table
2)
were
first
obtained
by
Inductively
Coupled
Plasma-Atomic
Emission
Spectrometry
(ICP-AES)
at
IUEM,
Plouzane.
The
samples
were
finely
powdered
in
an
agate
grinder.
International
Table
2
Major
and
trace
element
analyses
of
Hayyabley
basalts
(major
oxides
in
wt.%,
trace
elements
in
ppm).
MA.
Daoud
et
al.
/
Lithos
114
(2010)
327-336
329
Table
1
Unspiked
40
K-
40
Ar
datings
of
Hayyabley
basalts.
DJ54B DJ54B
DJ54C
DJ54D
DJ54F
DJ54G
DJ54H DJ54H
DJ57 DJ57
DJ58 DJ58
DJ59 DJ59
ICP-AES
ICP-MS
ICP-AES
ICP-AES
ICP-AES ICP-AES ICP-AES
ICP-MS
ICP-AES
ICP-MS
ICP-AES
ICP-MS
ICP-AES
ICP-MS
Si02
47.15
47.00
46.8
46.6
46.8
46.5
47.2
46.6
47.6
TiO
2
0.93
0.91
0.95
0.95
0.89
0.89
0.91
0.99
0.92
0.90
0.85
0.88
0.96
0.97
A
1
2
0
3
16.53
16.55
16.40
17.00 16.60 16.50
16.52
16.60
17.05
Fe203
1133
11.50
1133
11.16
11.22
11.29
11.41
10.29
11.07
Mn0
0.17
0.16
0.17
0.17
0.17
0.16 0.16
0.17
0.17
0.16
0.15
0.14
0.16
0.15
Mg0
9.45
8.92
8.75
9.46
9.47
930
9.80
9.18
9.25
Ca0
12.90
13.25
13.50
12.50
12.85
13.10
12.90
13.80
12.90
Na20
1.99
2.00
1.90
1.98
1.98
1.92
1.96
1.96
2.08
K
2
0
0.08
0.07
0.08
0.05
0.09
0.06
0.07
0.1
0.06
P205
0.07
0.07
0.07
0.07
0.07 0.07
0.07
0.07
0.07
WI
0.46
0.6
0.72
0.63
0.49
0.92
0.05
136
-
038
Total
101.06
101.08
100.67
100.51
100.62
100.73
101.07
100.96
100.82
Li
2.77
3.09
2.81
2.67
3.05
Be
024
026
0.25
0.27
0.29
Sc
45
43.9
46 46
44
44 44
47.4
45
45.5
38
39.6
40
41.1
V
285
266
290 290
275
280
275
291
285
279
245
244
270
253
Cr
380
362
340 340 350
359
355
351
405
388
362
352
370
364
Co
51
52
52
50
53
52 52
55
53
53
48
51
51
51
Ni
198
192
175
175
208
209
198
208
210
201
173
179
175
174
Rb
1.05
0.82
0.8
1
0.5
1
0.85
0.32
0.5
0.42
1.15
1.24
0.5
0.45
Sr
154
149
156
157
155
153
155
160
147
144
174
174
170
168
Y
20.5
20.89
21
20.5
19.5
20
20.5
21.08
20
19.99
17.5
18.49
19
19.57
Zr
48
44.91
48
47
43
46
47
44.09
44
42.81
45
46.87
48
47.77
Nb
2.7
228
2.5
2.4
2.5
2.45
23
226
2.4
2.37
2.6
2.59
2.4
2.56
Ba
34
32.19
41
55
40
50
30
29.81
25
24.11
40
40.01
22
20.49
La
2.6
237
2.5
2.4
2.5
2.4
2.5
234
2.7
2.40
2.7
2.53
2.5
2.44
Ce
7.1
634
7.0
6.5
63
6.4
6.7
638
6.5
647
73
6.81
7.0
6.73
Pr
1.00
1.02
1.00
1.06
1.08
Nd
53
539
5.5
53
4.8
5.2
5.4
5.43
5.4
5.34
5.4
5.73
5.8
5.89
Sm
2.0
1.82
2.0
1.7
1.9
1.7
1.8
1.85
1.9
1.82
1.8
1.99
1.9
2.02
Eu
0.8
0.75
0.78
0.78
0.73
0.76
0.77
0.79
0.74
0.76
0.75
0.81
0.82
0.83
Gd
23
2.45
23
2.4
2.5
2.05
2.4
2.52
23
2.43
2.5
2.48
2.5
2.53
Tb
0.47
0.47
0.45
0.46
0.47
Dy
33
326
3.4
33
3.1
3.15 3.25
3.40
3.25
3.21
2.9
3.10
3.25
3.18
Ho
0.73
0.76
0.72
0.67
0.68
Er
2.1
2.19
2.1 2.1
2
2.1
2.15
223
2
2.14
1.8
1.93
1.9
2.02
Yb
2.18
223
2.29
2.2
2.08
2.1
2.15
228
2.1
2.20
1.75
1.84
1.9
1.95
Lu
033
033
0.32
0.27
0.28
Hf
123
123
1.21
1.34
1.34
Ta
0.17
0.18
0.18
0.19
0.19
Pb
026
023
0.26
0.24
0.26
Th
025
022
0.23
0.25
0.22
U
0.06
0.06
0.05
0.07
0.03
ICP-AES
and
ICP-MS
analytical
methods
described
in
the
text.
330
MA
Daoud
et
aL
/
Lithos
114
(2010)
327-336
standards
were
used
for
calibration
tests
(ACE,
BEN,
JB-2,
PM-S
and
WS-E).
Rb
was
measured
by
flame
emission
spectroscopy.
Relative
standard
deviations
are
±
1%
for
Si0
2
,
and
±
2%
for
other
major
elements
except
P
2
0
5
and
Mn0
(absolute
precision
±
0.01%),
and
ca.
5%
for
trace
elements.
The
analytical
techniques
are
described
in
Cotten
et
al.
(1995).
Concentrations
of
additional
trace
elements
were
measured
by
Inductively
Coupled
Plasma
Mass
Spectrometry
(ICP-
MS
)
at
IUEM,
using
a
Thermo
Element
2
spectrometer
following
procedures
adapted
from
Barrat
et
al.
(1996,
2000).
Based
on
standard
measurements
and
sample
duplicates,
trace
element
concentration
reproducibility
is
generally
better
than
5%
(Barrat
et
al.,
2007),
and
are
in
good
agreement
with
the
ICP-AES
results
(Table
2).
Isotopic
compositions
of
Sr
and
Nd
(Table
4)
were
determined
at
IUEM.
Conventional
ion
exchange
techniques
were
used
for
separation
of
Sr,
and
isotope
ratio
measurements
were
carried
out
by
thermal
ionization
mass
spectrometry
using
a
Thermo
Triton
equipped
with
7
collectors.
Isotopic
ratios
were
normalized
for
instrumental
mass
fractionation
relative
to
86
Sr/
88
Sr
=
0.1194.
87
Sr/
86
Sr
of
the
NBS
987
Sr
standard
yielded
0.710213
±
22
(2o;
n
=14)
and
the
sample
Sr
isotopic
compositions
are
reported
relative
to
87
Sr/
86
Sr
=
0.71024.
The
Nd
purification
was
done
according
to
the
procedure
described
in
Dosso
et
al.
(1993).
TRU
Spec
chromatographic
resins
from
Eichrom
were
used
to
separate
the
REE
fraction
from
the
sample
matrix.
Then,
the
separation
and
elution
of
Nd
and
other
REE
were
realized
on
Ln.
Spec
resin.
During
the
course
of
the
study,
analyses
of
the
14
Jolla
standard
were
performed
and
gave
an
average
of
143
Nd/
144
Nd
=
0.511845
±
6
(n
=
15).
All
Nd
data
were
fractionation-corrected
to
..
146
Nd/
144
Nd
=
0.7219
and
further
normalized
to
a
value
of
143Nd/144Nd
=
0.511860
for
the
14
Jolla
standard.
Isotopic
compositions
of
Pb
were
determined
at
the
National
Oceanography
Centre,
Southampton,
using
the
SBL
74
double
spike.
Powdered
samples
were
leached
with
6
M
HCl
at
140
°C
for
1
h
and
then
rinsed
up
to
6
times
with
ultrapure
water
prior
to
dissolution.
Lead
separation
was
then
performed
on
an
anionic
exchange
resin.
High-
resolution
Pb
isotopic
analyses
were
carried
out
on
a
VG
sector
54
multi-
collector
instrument,
using
the
double
spike
technique
with
the
calibrated
Southampton-Brest
207
Pb/
204
Pb
spike
(Ishizuka
et
al.,
2003).
The
true
Pb
isotopic
compositions
were
obtained
from
the
natural
and
mixture
runs
by
iterative
calculation
adopting
a
modified
linear
mass
bias
correction
(Johnson
and
Beard,
1999).
The
reproducibility
of
this
Pb
isotopic
measurement
(external
error:
2a)
by
double
spike
is
<200
ppm
for
all
2
'Pb/
204
Pb
ratios.
Measured
values
for
NBS
SRM-
981
during
the
measurement
period
were
266
Pb/
204
Pb
=
16.9414
±
26,
207pb/204pb
=
15.4997
±
30
and
268
Pb/
204
Pb
=
36.726
±
9
(2o;
n
=
9).
Pb
blanks
measured
using
this
procedure
were
<100
pg,
and
thus
negligible
relative
to
the
amount
of
sample
analysed.
3.
Geological
setting
and
K-Ar
ages
3.1.
Geological
and
tectonic
framework
The
geology
of
the
Republic
of
Djibouti
records
the
effects
of
the
activity
of
the
Afar
mantle
plume
since
30
Ma
(Schilling,
1973;
Barberi
et
al.,
1975;
Barberi
and
Varet,
1977;
Furman
et
al.,
2006).
Plume-
related
basaltic
and
derived
magmas,
variably
enriched
in
incompat-
ible
elements
(e.g.,
Joron
et
al.,
1980;
Deniel
et
al.,
1994)
cover
ca.
90%
of
its
surface,
and
range
in
age
from
at
least
23.6
±
0.5
Ma
to
Present
(Barberi
et
al.,
1975;
Courtillot
et
al.,
1984;
Zumbo
et
al.,
1995).
Since
the
Miocene,
the
most
salient
tectono-magmatic
process
observed
in
the
area
was
the
penetration
of
the
Gulf
of
Aden
(GA)
oceanic
ridge
between
the
Arabia
and
Somalia
plates,
leading
to
the
opening
of
the
Tadjoura
Gulf
(Courtillot
et
al.,
1980;
Manighetti
et
al.,
1997),
at
the
southwestern
edge
of
which
the
emerged
Asal
Rift
shows
spectacular
evidence
for
both
tectonic
and
magmatic
activities
(Stieltjes
et
al.,
1976;
Needham
et
al.,
1976).
Onland,
the
principal
marker
of
the
Pliocene
opening
of
the
Tadjoura
Gulf
(TG)
was
the
emplacement
of
a
<350
m-thick
basaltic
lava
flow
pile,
referred
to
as
the
"initial
basaltic
series
from
the
borders
of
the
Tadjoura
Gulf"
(Fournier
et
al.,
1982;
Gasse
et
al.,
1983),
which
will
be
named
hereafter
the
Tadjoura
Gulf
Basalts
(TGB).
These
very
fluid
subaerial
lava
flows
are
generally
assumed
to
have
been
emitted
from
now
submerged
fissures
in
the
Gulf,
and
emplaced
rather
symmetrically
outwards
on
the
twin
margins
(Fig.
1,
inset)
(Richard,
1979).
Additional
feeder
dykes,
and
associated
neck-like
features,
have
been
identified
onshore,
along
the
northern
flank
in
the
Tadjoura
area.
TGB
range
from
olivine
tholeiites
to
ferrobasalts,
and
in
thin
section
are
subaphyric
to
sparsely
phyric,
with
3-6
modal%
calcic
plagioclase,
and
1-3
modal%
olivine
set
in
a
microlitic
groundmass.
They
display
mild,
but
significant,
enrichments
in
light
rare
earth
elements
(LREE)
and
other
highly
incompatible
elements
(Joron
et
al.,
1980;
Barrat
et
al.,
1990,
1993;
Deniel
et
al.,
1994).
In
the
Djibouti
plain,
the
TGB
are
involved
in
a
coastal
network
of
Gulf-parallel
tilted
fault
blocks,
bounded
by
dominantly
extensional
N-facing
structures,
in
association
with
N140°E
normal
faults
outlined
by
a
swarm
of
small
cinder
cones
(Fig.
1).
To
the
East,
they
are
post-
dated
by
the
Hayyabley
elongated
volcano,
the
long
axis
of
which
also
strikes
NW-SE,
parallel
to
the
regional
fault
scarp
bounding
the
eastern
coastal
plain
further
SE.
3.2.
The
Hayyabley
volcano
The
youngest
volcanic
units
in
the
Djibouti
plain
are
a
set
of
generally
small
(less
than
100
m
high)
ash
and
cinder
strombolian-
type
cones
with
associated
basaltic
flows
(e.g.
the
Nagad
volcano,
Fig.
1),
aligned
along
a
young
NNW-SSE
fracture
network
(Fournier
et
al.,
1982).
They
overlie
the
TGB
and
have
been
dated
to
1.75-
1.70
Ma
(Gasse
et
al.,
1983).
The
largest
of
these
post-TGB
volcanic
centers
is
the
Hayyabley
volcano,
east
of
Djibouti
town
(Fig.
1).
Although
it
was
shown
on
the
1:50000
geological
map
of
Djibouti
(Fournier
et
al.,
1982),
and
further
well-described
and
dated
by
Gasse
et
al.
(1983),
it
was
apparently
never
investigated
despite
the
obviously
unusual
characteristics
of
its
basaltic
lavas.
The
Hayyabley
volcano
in
map-view
is
a
5
x
10
km
elliptic
edifice,
with
a
NNW-SSE
trending
axis.
It
has
a
shield-like
and
rather
flat
morphology,
and
culminates
at
147
m
at
Signal
Bouet.
It
overlies
the
TGB
lava
flows
outcropping
W
and
N
of
Wadi
Ambouli
valley
(Fig.
1),
and
seals
the
EW
to
WNW-ESE
normal
fault
pattern
related
to
the
Tadjoura
rift.
Despite
the
rather
large
aerial
extent
of
its
lavas,
we
estimate
its
volume
to
ca.
0.6-0.8
km
3
only.
Its
eruptive
vents
are
no
longer
identifiable,
possibly
because
of
the
strong
anthropic
imprint
and
constructions
of
the
Djibouti
suburbs:
they
are
thought
to
be
located
in
its
summit
zone,
and
aerial
photograph
data
suggest
radial
emplacement
of
the
lava
flows
away
from
this
summit
(Fournier
et
al.,
1982).
The
total
thickness
of
the
Hayyabley
lava
flow
pile
is
estimated
at
120
m.
The
best
section
is
exposed
in
Wadi
Warabor,
along
the
northern
coast
(Fig.
1).
There,
we
sampled
seven
superimposed
basaltic
lava
flows
(DJ54B
to
DJ54H),
resting
conformably
upon
a
15
m-thick
columnar-jointed
lava
flow
(DJ54A)
belonging
to
the
TGB
sequence.
These
flows
are
vesicle-rich,
and
their
thickness
decreases
upwards
from
ca.
4
m
to
less
than
20
cm.
Only
the
thickest
lava
flows
show
columnar
jointing,
and
the
uppermost
ones
are
highly
vesicular
and
often
scoriaceous
(Gasse
et
al.,
1983).
A
sample
(TF
914)
collected
from
a
possible
eruption
vent
in
the
summit
area
had
been
dated
by
the
K-Ar
unspiked
method
to
0.98
±
0.10
Ma
and
0.83
±
0.08
Ma
(Gasse
et
al.,
1983),
the
youngest
K-Ar
dates
obtained
so
far
in
the
area.
We
have
checked
the
previous
results
by
dating
two
basaltic
flows
from
the
Wadi
Warabor
section
(Fig.
1).
The
results
are
shown
in
Table
1.
The
two
ages
obtained,
0.93
±
0.06
Ma
and
1.06
±
0.09
Ma,
are
mutually
consistent,
and
compatible
as
well
with
those
previously
published
(Gasse
et
al.,
1983).
Indeed,
the
four
results
almost
overlap
t
ectib„
e
%fi
b
OSQ/
ts
Hayyabley
basalts
../
.**
Red
Sea
N-MORB
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Yb
Lu
MA.
Daoud
et
al.
/
Lithos
114
(2010)
327-336
331
100
10
Fig.
2.
Chondrite-normalized
REE
patterns
of
Hayyableh
basalts
compared
to
the
field
of
older
Tadjoura
Gulf
basalts
located
onland
in
Djibouti
(Barrat
et
al.,
1993;
Daoud,
2008).
The
reference
chondrite
is
from
Evensen
et
al.
(1978).
The
pattern
of
a
southern
Red
Sea
N-MORB
(sample
V84,
Barrat
et
al.,
1990)
is
shown
for
comparison.
at
around
0.91-0.97
Ma,
and
are
remarkably
convergent
considering
the
very
low
concentration
of
potassium
in
the
studied
samples
and
the
young
age
range.
4.
Petrologic
and
geochemical
results
4.1.
Petrographic
and
mineralogical
features
The
Hayyabley
basalts
are
rather
homogeneous
from
a
petro-
graphic
point
of
view,
and
also
quite
different
from
the
underlying
TGB.
They
are
moderately
to
highly
vesicular
(10
to
30
modal%
vesicles
in
thin
section).
These
vesicles
are
usually
empty,
or
sometimes
partly
filled
by
calcite,
especially
in
the
summit
part
of
the
volcano.
The
rocks
are
also
sparsely
to
moderately
phyric,
with
5
to
15
modal%
phenocrysts,
the
size
of
which
ranges
from
0.5
to
3
mm.
They
include
olivine
(dominant)
and
calcic
plagioclase
(subordinate),
in
a
roughly
2:1
ratio.
These
phenocrysts
are
set
in
a
holocrystalline
groundmass,
showing
doleritic
or
intersertal
textures.
It
contains,
by
order
of
decreasing
abundance,
plagioclase
laths,
olivine
microcrysts
(the
periphery
of
which
is
often
replaced
by
iddingsite),
calcic
pyroxene
grains
and
titanomagnetite.
Olivine
compositions
range
from
F084_82
for
the
phenocryst
cores
to
F078_54
for
their
rims
and
the
microcrysts,
the
smallest
ones
being
the
most
Fe-rich.
The
plagioclase
phenocryst
cores
are
bytownitic
(An
86
-
77
)
and
contain
negligible
amounts
of
Or
component
(<03%).
The
corresponding
rims
are
less
calcic
(An7o-32)
and
the
small
laths
from
the
groundmass
are
clearly
enriched
in
alkalis
(up
to
An
27
.
15
Ab
70
_
80
Or2_5).
Groundmass
clinopyroxenes
are
augitic
(W
0
45-41
E043_40
FS12-16)
and
their
low
TiO
2
(<1
wt.%)
and
Na
2
O
(<0.3
wt.%)
contents
are
typical
of
tholeiitic
clinopyroxenes.
4.2.
Major
and
trace
elements
on
bulk
rocks
Nine
samples
taken
from
different
flows
from
four
locations
(Fig.
1)
were
analysed,
and
the
results
are
given
in
Table
2.
Their
major
and
trace
element
abundances
are
rather
uniform.
These
lavas
display
high
A1
2
0
3
(16.4-17.05
wt.%)
and
Ca0
(12.5-13.8
wt.%)
abundances,
low
Na
2
O
(1.9-2.1
wt.%)
abundances
and
Fe0*/Mg0
ratios
close
to
1.
Although
not
primitive,
these
lavas
are
amongst
the
least
evolved
basalts
collected
so
far
from
the
Republic
of
Djibouti.
Indeed,
they
exhibit
the
highest
compatible
trace
element
abundances
(e.g.,
Ni,
Co,
Cr)
measured
in
samples
from
this
area
(e.g.,
Joron
et
al.,
1980;
Barrat
et
al.,
1990,
1993;
Deniel
et
al.,
1994).
More
importantly,
their
incompatible
trace
element
abundances
are
low,
and
these
samples
are
characterized
by
light
REE
depletions
(La
n
/Sm
n
=
0.76-0.83),
and
small
but
significant
positive
Eu
anoma-
lies
(
Eu/Eu*
=
1.08-1.12,
Fig.
2).
These
features
unambiguously
distinguish
the
Hayyabley
basalts
from
both
the
TGB
and
the
older
post-TGB
basalts,
which
are
always
LREE-enriched
(Joron
et
al.,
1980;
Barrat
et
al.,
1990,
1993;
Deniel
et
al.,
1994).
The
unusual
features
of
the
Hayyabley
basalts
are
strengthened
by
their
primitive
mantle
normalized
patterns
that
exhibit
large
positive
Ba
(Ba
n
/Rb
n
=
2.9-8.6)
and
Sr
(Sr
n
/Ce
n
=
1.8-2.1)
anomalies
(Fig.
3).
Although
LREE-depleted,
the
Hayyabley
basalts
are
clearly
distinct
from
typical
N-MORB
and
basalts
erupted
by
the
nearby
nascent
oceanic
ridges.
For
example,
basalts
with
N
to
T-MORB
affinities
are
known
from
the
eastern
part
of
the
Tadjoura
Gulf
(Barrat
et
al.,
1990,
1993).
Although
a
positive
Sr
anomaly
has
been
observed
in
a
single
roc
kic
ho
n
dr
ite
10
E
E
10
2
1
Manda
Hararo
Ethiopian
Plateau
E
Gulf
of
Tadjoura
Hayyabley
.•-•
Red
Sea
N-MORB
Rb
Ba
Th
Nb
La
Ce
Sr
NdSm
Hf
Eu
Gd
Dy
Er
Yb
Lu
Rb
Ba
Th
Nb
La
Ce
Sr
NdSm
Hf
Eu
Gd
Dy
Er
Yb
Lu
Fig.
3.
Primitive
mantle-normalized
element
patterns
for
Hayyabley
basalts,
LREE-depleted
Manda
Hararo
basalts
(Barrat
et
al.,
2003),
two
submarine
MORB
from
the
East
of
the
Gulf
of
Tadjoura
(Barrat
et
al.,
1990,1993),
the
southern
Red
Sea
N-MORB
sample
V84
(Barrat
et
al.,
1990),
and
the
LREE-depleted
sample
E88
from
the
Oligocene
Ethiopian
Plateau
(Pik
et
al.,
1999).
The
primitive
mantle
values
are
from
Sun
and
McDonough
(1989).
332
MA
Daoud
et
aL
/
Lithos
114
(2010)
327-336
Table
3
Compositions
of
LREE-depleted
basalts
from
Hayyabley
(average
of
the
samples
analysed
by
ICP-MS),
Manda
Hararo
(average
data
from
Barrat
et
al.,
2003),
Ethiopian
Plateau
(sample
E88,
Pik
et
al.,
1999),
and
of
a
N-MORB
from
Tadjoura
Gulf
(sample
A3D3,
Joron
et
al.,
1980;
Barrat
et
al.,
1993).
Hayyabley
Manda
Hararo
Ethiopian
Plateau
Tadjoura
Gulf
(n=5)
(n=4)
(n
=
1)
(n
=
1)
Si02
47.01
48.50
48.05
48.40
TiO2
0.91
1.04
1.08
0.83
A1203
16.64
15.50
16.05
15.50
Fe203
11.08
11.94
11.63
9.78
Mn0
0.16
0.18
0.17
0.13
Mg0
9.40
8.47
8.67
8.83
Ca0
13.12
1132
10.58
12.90
Na20
1.98
233
238
2.06
K20
0.07
0.08
0.16
0.09
1320
5
0.07
0.08
0.10
0.05
Total
100.93
99.44
98.87 98.57
Sc
43.5
36.7
37.9
267
260
229
Cr
363
64
182
Co
52.4
54.80
47.7
Ni
191 101
152
139
Rb
0.65
0.68
1.0
0.53
Sr
159
177
224
114.5
Y
20.0
23
21
Zr
45
41
61
Nb
2.41
2.43
2.1
Ba
2932
28.01
48
8.68
La
2.42
2.63
2.9
1.68
Ce
6.55
7.41
8.2
4.90
Nd
5.56
6.03
7.2
433
Sm
1.90
1.94
2.40
1.58
Eu
0.79
0.80
1.00
0.63
Gd
2.48
2.73
330
230
Dy
3.23
3.40
3.60
2.96
Er
2.10
2.14
2.00
1.88
Yb
2.10
2.04
1.90
1.77
Lu
031
030
0.29
0.28
Hf
1.27
134
2.0
0.95
Ta
0.18
0.18
0.1
0.14
Th
0.23
0.17
0.19
0.18
U
0.05
0.06
0.06
028
(La/Sm)n
0.80
0.85
0.76
0.67
Eu/Eu*
1.11
1.06
1.09
1.01
Ba/Rb
45.11
41.26
48.00
1638
(Sr/Ce)n
2.05
2.01
230
1.97
Major
oxides
in
wt.%,
trace
elements
in
ppm.
n
denotes
ratios
normalized
to
the
primitive
mantle
composition
from
Sun
and
McDonough
(1989).
LREE-depleted
basalt,
positive
Ba
and
Eu
anomalies
are
missing
(Barrat
et
al.,
1990,
1993
and
unpublished
results).
In
addition,
the
Nb/Y
and
Zr/Y
ratios
(0.11-0.15
and
2.20-2.57,
respectively,
Table
2)
of
Hayyabley
basalts
are
such
that
these
lavas
plot
within
the
field
of
Icelandic
plume
basalts,
and
well
above
the
N-MORB
field,
in
Fitton
et
al.'s
(1997,
2003)
rectangular
plot
(not
shown).
Interestingly,
the
Hayyabley
basalts
are
remarkably
similar
to
the
scarce
LREE-depleted
basalts
which
were
sporadically
emitted
by
the
Manda
Hararo
rift,
Ethiopia
(Barrat
et
al.,
2003).
Indeed,
the
latter
display
incompatible
element
abundances
and
distributions
very
similar
to
those
of
the
Hayyabley
basalts
(Fig.
3).
The
noticeable
differences
are
minor.
The
Manda
Hararo
basalts
are
somewhat
more
evolved
than
the
Hayyabley
basalts
and
have
for
example
lower
Ni
and
Cr
concentrations
(Table
3).
In
addition,
an
Oligocene
basaltic
flow
with
the
same
features
(sample
E88)
was
reported
by
Pik
et
al.
(1999)
from
the
Ethiopian
Plateau.
4.3.
Sr,
Nd,
Pb
isotopic
data
The
isotopic
compositions
of
five
samples
are
given
in
Table
4,
and
are
almost
uniform,
with
the
exception
of
87
Sr/
86
Sr
ratios
which
vary
significantly
in
the
range
0.70369-0.70396
(Table
4).
Although
relatively
fresh,
the
Hayyabley
basalts
display
some
evidence
of
weathering.
One
may
suspect
that
their
87
Sr/
86
Sr
ratios
are
not
pristine,
and
have
been
affected
by
secondary
processes.
Indeed,
the
least
radiogenic
sample
DJ59
displays
a
negative
Loss
On
Ignition
(LOI)
value
(
-
038
wt.%).
Conversely,
the
LOI
value
of
the
most
radiogenic
sample
(DJ54H)
is
much
higher
(0.92
wt.%),
and
in
a
87
Sr/
86
Sr
vs.
LOI
plot
(not
shown),
a
weak
positive
correlation
is
apparent.
In
order
to
check
if
the
Sr
isotopic
compositions
of
the
samples
were
modified
by
alteration,
150
mg
of
sample
DJ54B
was
leached
for
2
h
in
hot
(150
°C)
6N
HC1,
and
rinsed
in
deionized
water
prior
to
dissolution.
Its
87
Sr/
86
Sr
ratio
is
significantly
lower
than
the
value
obtained
on
the
unleached
powder
(Table
4),
a
result
which
suggests
that
the
Sr
isotopic
compositions
have
been
modified
by
secondary
processes.
Similar
observations
were
made
by
Deniel
et
aL
(1994)
on
other
samples
from
Djibouti
Thus,
87
Sr/
86
Sr
obtained
on
unleached
samples
from
this
area
should
be
discussed
only
with
extreme
caution,
even
ratios
obtained
from
apparently
fresh
basalts.
We
believe
that
only
two
87
Sr/
86
Sr
measurements
can
be
safely
used
in
the
discussion:
the
least
radiogenic
one
(DJ59),
and
the
value
obtained
on
the
leached
residue
of
DJ54B.
The
Sr,
Nd,
and
Pb
isotopic
compositions
of
the
Hayyabley
basalts
are
compared
to
those
of
other
volcanics
from
the
Horn
of
Africa
in
Figs.
4
to
6.
In
these
plots,
Hayyabley
basalts
lie
significantly
outside
the
fields
defined
by
the
submarine
basalts
erupted
from
the
nascent
oceanic
ridges
of
the
Red
Sea,
the
Eastern
part
of
the
Tadjoura
Gulf,
and
the
Aden
Gulf.
These
features
indicate
that
these
LREE-depleted
lavas
are
unlike
MORB
(Figs.
5
and
6).
For
example,
they
display
87
Sr/
86
Sr
ratios
more
radiogenic
than
N-MORB,
and
significantly
lower
6Nd
values
(Ito
et
al.,
1987).
In
contrast,
the
6isid
vs.
87
Sr/
86
Sr
plot
(Fig.
4)
shows
that
the
Hayyabley
basalts
and
LREE-depleted
basalts
from
Manda
Hararo
are
isotopically
very
similar.
The
Hayyabley
basalts
display
almost
uniform
Pb
isotopic
compositions
(206Pb/204Pb
=
18.47-18.55,
207
pb/
204,-
no
=
15.52-15.57,
2°8
Pb/
2°4
Pb
=
38.62-38.77)
well
above
the
NHRL
(Hart,
1984,
1988;
see
Table
4).
In
the
Sr-Nd,
Pb-Pb
and
Nd-Pb
plots
(Figs.
4
to
6),
the
Hayyabley
basalts
extend
the
range
of
the
compositions
displayed
by
the
young
(<4
Ma)
Table
4
Sr,
Nd
and
Pb
isotopic
compositions
of
Hayyabley
basalts
(B:
bulk
rock;
R:
residue
after
leaching).
DJ54B
DJ54H
DJ57
DJ58
DJ59
87
Sr/
86
Sr
(B)
0.703909±3
0.703962
±
5
0.703869
±
4
0.703871
±4
0.703693
±
5
87
Sr/
86
Sr
(R)
0.703762
±
9
143
Nd/
144
Nd
(B)
0.512961
±4
0.513001
±
3
0.512965
±4
0.512942
±
3
0.513010
±
4
ENd
+63
+7.1
+6.4
+5.9
+73
206pb
/
204pb
(R)
18.4856±
15
18.4776
±
15
18.5502
±
27
18.4842
±
17
18.4979
±
21
207pb
/
204pb
(R)
15.5478
±
14
15.5421
±
14
15.5662
±
25
15.5407
±
16
15.5292
±
20
2°8
Pb/
2°4
Pb
(R)
38.6917
±
43
38.6658
±
44
38.7692
±
78
38.6217
±
50
38.5842
±
61
47/4
53
4.8
6.4
4.6
33
48/4
71.6
69.9
71.5
64.7
593
See
text
for
the
analytical
procedures.
47/4
and
48/4
denote
the
deviation
(in
%.)
of
207
Pb/
204
Pb
and
208
Pb/
2°4
Pb
ratios
with
respect
to
the
Northern
Hemisphere
Reference
Line
(NHRL:
Hart,
1984,
1988).
o
Djibouti
(<4
Ma)
Hayyabley
DM
0
it•
•••••
Ci•D
+
JP
UP
ES
Erta'Ale
MH
HT2
I-
0
o
#
E88
..
E
Tadjoura
Gulf
.
+Aden
Gulf
0.703
87
Sr/
86
S
r
0.704
E
Tadjoura
Gulf
+Aden
Gulf
0
HT2
DM
o
Djibouti
(<4
Ma)
Hayyabley
10
9
8
ENd
7
6
5
4
19.5
w
a•
18.5
18.0
000
HT2
o
E'A
o
coo
E88
o
-
N
•••
.•
.•
‘‘`
.
E
Tadjoura
Gulf
DM
+AdenGulf
S
Red
Sea
0
S
Red
Sea
.•
HT2
o
Djibouti
(<4
Ma)
Hayyabley
E'A
.•
E
Tadjoura
Gulf
+Aden
Gulf
.•••
\
DM
E88
MA.
Daoud
et
al.
/
Lithos
114
(2010)
327-336
Fig.
4.
Plot
of
ENd
vs.
87
Sr/
86
Sr
for
young
onland
basalts
from
Djibouti
(Deniel
et
al.,
1994,
and
this
study).
Only
the
two
reliable
Sr
isotopic
ratios
of
Hayyabley
basalts
have
been
plotted.
Basalts
older
than
4
Ma
have
been
omitted
because
of
their
possible
contamination
by
continental
crust.
The
fields
of
(1)
basalts
from
the
South
Red
Sea
occurrences,
which
include
oceanic
ridge
segments,
Ramad
seamount
and
Zubair
and
Hanish
islands
(Barrat
et
al.,
1990,
1993;
Volker
et
al.,
1993,
1997),
(2)
submarine
basalts
from
the
East
of
the
Gulf
of
Tadjoura
and
the
Aden
Gulf
(Barrat
et
al.,
1990,
1993;
Schilling
et
al.,
1992),
(3)
Erta
'Ale
volcanics
(Barrat
et
al.,
1998),
(4)
LREE-depleted
basalts
from
Manda
Hararo
(MH,
Barrat
et
al.,
2003),
and
(5)
some
Ethiopian
samples
(E88:
depleted
Oligocene
basalt;
H12:
average
composition
of
high-Ti
basalts,
Pik
et
al.,
1999)
are
shown
for
comparison.
DM
refers
to
the
regional
depleted
mantle
composition
deduced
from
the
study
of
South
Red
Sea
and
Gulf
of
Aden
basalts.
15.6
n
0
15.5
39.5
o_
38.5
38.0
i8
18.5
19
19.5
20
206
Pb/
204
Pb
Fig.
5.
Plot
of
207
Pb/
264
Pb
and
208
Pb/
204
Pb
vs.
206
Pb/
2°4
Pb
for
young
(less
than
4
Ma)
onland
enriched
basalts
from
Djibouti
(Deniel
et
al.,
1994)
and
Hayyabley
depleted
basalts
(this
study).
Other
fields
as
in
Fig.
4.
E'A:
field
of
Erta
'Ale
volcanics
(Barrat
et
al.,
1998).
Most
207
Pb/
264
Pb
data
taken
from
the
regional
literature
(e.g.
on
E88
and
Erta
'Ale)
are
less
precise
than
those
measured
on
Hayyabley
basalts,
and
should
therefore
be
considered
with
caution.
4
5
6
7
8
9
10
E
Nd
Fig.
6.
Plot
of
266
Pb/
264
Pb
vs.
Eryd
for
young
(less
than
4
Ma)
onland
enriched
basalts
from
Djibouti
(Deniel
et
al.,
1994)
and
Hayyabley
depleted
basalts
(this
study).
Other
fields
as
in
Fig.
5.
basalts
from
Djibouti.
They
might
reflect
the
contribution
of
a
distinct
LREE
component
in
their
petrogenesis.
5.
Discussion
Although
Ethiopian
Plateau
basalts
(Pik
et
al.,
1999;
Kieffer
et
al.,
2004;
Meshesha
and
Shinjo,
2007;
Beccaluva
et
al.,
2009),
and
Afar
basalts
(Treuil
and
joron,
1975;
Joron
et
al.,
1980;
Deniel
et
al.,
1994)
are
dominantly
enriched,
previous
studies
(Barrat
et
al.,
1993;
Pik
et
al.,
1999;
Barrat
et
al.,
2003;
Meshesha
and
Shinjo,
2007)
have
demonstrated
that
minor
depleted
components
were
also
involved
in
their
petrogenesis.
The
discovery
of
a
new
occurrence
of
LREE-
depleted
basalts
in
Djibouti,
i.e.
further
east
in
the
Afar
rift
setting,
might
provide
new
constrains
on
their
origin.
Two
main
points
will
be
discussed
below:
(1)
the
origin
of
the
Ba,
Sr
and
Eu
positive
anomalies
observed
in
the
Hayyabley
basalts,
and
(2)
the
occurrence
of
a
specific
LREE-depleted
component
in
the
sources
of
the
Afar
basalts.
5.1.
The
Ba,
Sr
and
Eu
positive
anomalies
in
the
Hayyabley
basalts
The
origin
of
Ba,
Sr
and
Eu
positive
anomalies
in
LREE-depleted
basalts
has
been
previously
investigated
in
the
cases
of
some
Icelandic
basalts
(e.g.,
Kokfelt
et
al.,
2006
and
references
therein)
and
of
the
Manda
Hararo
basalts
(Barrat
et
al.,
2003).
The
compositions
of
LREE-
depleted
basalts
such
as
those
erupted
by
the
Hayyabley
volcano
might
be
related
to
those
of
common
MORB.
The
chief
differences
between
them
could
be
due
to
secondary
processes,
such
as
hot-
desert
weathering,
crystal
accumulation,
or
contamination
by
a
crustal
component.
Alternatively,
they
could
be
derived
from
an
unusual
mantle
source,
located
in
the
lithospheric
or
asthenospheric
mantle
or
in
the
plume
itself.
In
a
hot-desert
environment,
surface processes
are
able
to
generate
positive
Ba
and
Sr
anomalies
in
a
very
short
time.
The
studies
of
meteorites
from
Sahara
have
demonstrated
that
some
of
them,
and
not
only
the
most
weathered
ones,
exhibit
marked
Ba
and
Sr
enrichments
that
are
sensitive
indicators
of
the
development
of
secondary
calcite,
gypsum,
or
barytes
(e.g.,
Barrat
et
al.,
1998,
2003).
Such
processes
would
have
generated
a
range
of
Ba
and
Sr
abundances
from
low
values
typical
of
unweathered
N-MORB
(about
10
ppm
Ba
and
100
ppm
Sr)
to
much
higher
concentrations.
However,
Ba
and
Sr
abundances
in
the
Hayyabley
basalts
are
uniform,
and
strikingly
similar
to
the
concentrations
measured
in
the
distant
Manda
Hararo
basalts.
Furthermore,
the
development
of
secondary
phases
is
unable
334
MA
Daoud
et
aL
/
Lithos
114
(2010)
327-336
to
increase
the
Eu/Eu*
ratio
and
to
generate
positive
Eu
anomalies,
hence
ruling
out
this
first
explanation.
Positive
Ba,
Sr
and
Eu
anomalies
in
basaltic
rocks
are
usually
explained
by
plagioclase
accumulation
or
assimilation.
However
this
process
is
unable
to
produce
Sr
anomalies
as
high
as
those
displayed
by
the
Hayyabley
or
Manda
Hararo
basalts
without
increasing
drastically
the
A1
2
0
3
contents
of
the
resulting
rocks.
The
fact
that
the
A1
2
0
3
abundances
of
the
LREE-depleted
basalts
are
not
anoma-
lously
high
(Table
2)
is
inconsistent
with
the
hypothesis
of
plagioclase
accumulation.
Assimilation
of
plagioclase-rich
gabbros
from
the
oceanic
lithosphere
during
ascent
of
plume-related
magmas
has
been
proposed
in
the
cases
of
offshore
Tadjoura
Gulf
basalts
(Barrat
et
al.,
1993)
and
Galapagos
basalts
(Saal
et
al.,
2007).
However,
reproducing
the
compositions
of
Hayyabley
basalts
through
this
process,
and
especially
their
positive
Ba,
Sr
and
Eu
anomalies,
would
require
rather
high
rates
of
assimilation.
In
addition,
the
Hayyabley
and
Manda
Hararo
basalts
overlie
thinned
continental
crust
which
is
25-26
km
thick
(Dugda
and
Nyblade,
2006),
while
the
depleted
plateau
basalt
sample
E88
(Pik
et
al.,
1999)
is
located
on
normal
(ca.
40
km
thick)
African
crust.
Due
to
the
presence
of
a
substantial
plume-
related
basaltic
cover
in
both
cases,
one
may
expect
the
occurrence
at
crustal
or
even
subcrustal
depths
of
associated
gabbroic
cumulates.
However,
these
gabbros
should
be
LREE-enriched
like
the
vast
majority
of
Afar
basalts.
Therefore,
their
interaction
with
depleted
(N-MORB
type)
melts
is
likely
to
lead
to
variably
LREE-enriched
magmas
with
isotopic
compositions
close
to
those
of
the
flood
basalts.
The
Hayyabley
basalts
have
radiogenic
Sr
isotopic
compositions
and
low
ENd
values
relative
to
Aden
Gulf
or
Red
Sea
MORBs
(Schilling
et
al.,
1992;
Volker
et
al.,
1993;
Hase
et
al.,
2000).
The
assimilation
of
a
continental
component
could
explain
this
shift
from
usual
N-MORB
values,
but
incompatible
trace
element
ratios
give
no
support
to
this
interpretation.
Contamination
of
MORB-like
melts
by
continental
crust
would
produce
significant
changes
in
incompatible
trace
element
ratios.
The
Hayyabley
basalts,
like
the
Manda
Hararo
LREE-
depleted
basalts,
lack
the
negative
Nb
or
Ta
anomalies
observed
in
the
multi-element
plots
of
crust-contaminated
basalts.
Moreover,
they
show
a
limited
range
of
Ce/Pb
ratios
from
24
to
28,
similar
to
values
measured
in
oceanic
basalts
(e.g.,
Sun
and
McDonough,
1989).
Therefore,
there
is
no
indication
for
assimilation
of
significant
amounts
of
material
derived
from
the
continental
crust
in
the
LREE-
depleted
basalts.
In
the
case
of
the
Manda
Hararo
basalts,
this
conclusion
is
strengthened
by
their
5
18
0
values
close
to
5.5%.,
which
are
typical
of
mantle
composition
(Barrat
et
al.,
2003).
Another
possible
explanation
of
the
specific
features
of
Hayyabley
and
Manda
Hararo
basalts
is
that
they
might
result
from
the
interaction
between
ascending
depleted
(N-MORB
type)
melts
and
the
African
subcontinental
lithospheric
mantle.
Once
again,
such
a
mantle
is
expected
to
be
LREE-enriched
(Hart
et
al.,
1989;
Vidal
et
al.,
1991;
Deniel
et
al.,
1994)
and
thus
should
transmit
its
trace
element
and
isotopic
fingerprint
to
LREE-poor
ascending
magmas.
In
addition,
the
remarkably
similar
chemical
features
of
Hayyabley,
Manda
Hararo
and
E88
basalts
suggest
that
they
derive
from
almost
identical
sources
and
petrogenetic
processes.
Their
distinct
locations,
emplacement
ages
(Oligocene
for
E88,
ca.
1
Ma
for
Hayyabley
and
less
than
0.2
Ma
for
Manda
Hararo)
and
underlying
crustal/lithospheric
thickness
(normal
for
E88,
thinned
for
the
two
other
occurrences)
are
hardly
consistent
with
a
similar
petrogenetic
history.
Therefore,
as
previously
pointed
out
for
the
Manda
Hararo
basalts
(Barrat
et
al.,
2003),
the
positive
Sr,
Ba
and
Eu
anomalies
and
the
particular
Sr-Nd-Pb
isotopic
features
of
the
Hayyabley
basalts,
are
more
likely
a
genuine
feature
inherited
from
their
deep
mantle
sources.
The
same
conclusions
have
been
reached
for
depleted
basalts
with
similar
positive
anomalies
from
Iceland.
Chauvel
and
Hemond
(2000),
Skovgaard
et
al.
(2001),
and
Kokfelt
et
al.
(2006)
have
suggested
that
the
sources
of
Icelandic
lavas
contained
an
old
recycled
oceanic
lithosphere
component
and
that
melting
of
the
gabbroic
portion
of
this
lithosphere
led
to
the
formation
of
basalts
that
exhibit
large
positive
Ba,
Sr
and
Eu
anomalies.
At
first
glance,
such
an
explanation
is
attractive
because
if
this
recycled
gabbroic
component
has
been
hydrothermally
altered,
one
may
expect
87
Sr/
86
Sr
ratios
much
more
radiogenic
than
those
of
typical
MORB.
Hence,
the
involvement
of
such
component
could
account
for
the
relatively
high
87
Sr/
86
Sr
ratios
of
the
Manda
Hararo
and
Hayyabley
depleted
basalts.
However,
an
old
LREE-depleted
recycled
gabbroic
component
from
the
oceanic
lithosphere
would
also
be
characterized
by
high
Eryd
values.
On
the
contrary,
the
Manda
Hararo
and
Hayyabley
lavas
display
ENd
values
unexpectedly
low
(E
N
d
=
5-7)
for
depleted
basalts.
Thus,
we
conclude
that,
at
best,
this
model
only
partially
fits
the
observations.
5.2.
The
depleted
components
in
the
sources
of
Djibouti
and
Ethiopian
basalts
Previous
geochemical
studies
have
demonstrated
the
participation
of
a
depleted
component
during
the
genesis
of
the
Horn
of
Africa
basalts.
In
the
case
of
basalts
emitted
by
the
young
oceanic
ridges
from
the
Red
Sea
or
the
Aden
Gulf,
major
involvement
of
MORB-related
sources
has
been
proposed
(e.g.,
Barrat
et
al.,
1990;
Schilling
et
al.,
1992;
Barrat
et
al.,
1993;
Volker
et
al.,
1993
).
These
submarine
basalts
do
not
have
the
unradiogenic
Pb
isotopes
of
the
Carslberg
Ridge
ca.
1600
km
east
of
Hayyabley
volcano
(Hart,
1984)
but
do
extend
away
from
the
Indian
Ocean
MORB
toward
a
more
HIMU
composition.
On
land,
huge
volumes
of
enriched
basalts
were
emplaced
in
Afar
and
Ethiopia.
The
trace
element
and
isotopic
features
of
the
depleted
reservoirs
which
have
been
involved
during
the
genesis
of
the
scarce
LREE-depleted
lavas
are
very
difficult
to
constrain.
Two
distinct
LREE-
depleted
components
have
been
unambiguously
detected.
First,
a
depleted
MORB
mantle
component
is
clearly
involved
in
the
genesis
of
Quaternary
basalts
from
Northern
Afar.
The
Sr-Nd-Pb
isotopic
relationships
displayed
by
the
Erta'Ale
basalts
(Figs.
4
to
6)
point
to
the
participation
of
two
mantle
end-members,
namely
a
HIMU
component
and
a
depleted
mantle
(DM)
component
undis-
tinguishable
from
the
source
of
the
Red
Sea
MORB
(Barrat
et
al.,
1998).
Furthermore,
a
similar
depleted
component
has
been
detected
in
the
sources
of
the
Oligocene
lavas
from
the
Northwestern
Ethiopian
volcanic
province
(Meshesha
and
Shinjo,
2007).
The
entrainment
of
depleted
asthenospheric
mantle
during
plume
ascent
(Furman
et
al.,
2006)
is
a
possible
explanation
for
the
contribution
of
this
component
to
the
sources
of
some
of
the
basalts
erupted
in
Afar
and
Ethiopia,
as
well
as
to
those
of
Kerguelen
basalts
(Doucet
et
al.,
2002).
However,
numerical
models
(Farnetani
et
al.,
2002;
Farnetani
and
Samuel,
2005)
suggest
that
incorporation
of
depleted
upper
mantle
within
ascending
plumes
is
unlikely
to
occur.
In
addition,
the
compositions
of
LREE-depleted
basalts
from
Hayyabley
and
Manda
Hararo
point
to
a
depleted
end-member
chemically
(Fig.
3)
and
isotopically
(Figs.
4
to
6)
distinct
from
an
asthenospheric
MORB-like
component.
A
single
Oligocene
LREE-
depleted
basalt
displaying
chemical
features
similar
to
those
of
the
Quaternary
depleted
ones
has
been
collected
in
Ethiopia
(sample
E88,
Pik
et
al.,
1999).
Although
its
isotopic
composition
is
slightly
different
from
those
of
the
Hayyabley
basalts
(Figs.
4
to
6),
the
occurrence
of
this
sample
indicates
that
a
depleted
component
distinct
from
the
MORB
source
was
involved
in
this
area
at
an
early
stage
of
plume
emplacement.
Therefore,
we
suggest
that
a
depleted
component,
intrinsic
to
the
plume
at
depth,
has
contributed
to
the
sources
of
both
young
and
old
lavas
related
to
the
Afar
plume.
Similar
conclusions
have
been
reached
for
the
Hawaiian
(Frey
et
al.,
2005)
and
Icelandic
(Thirlwall,
1995;
Kerr
et
al.,
1995;
Fitton
et
al.,
1997;
Chauvel
and
Hemond,
2000;
Skovgaard
et
al.,
2001;
Thirlwall
et
al.,
2004;
Kokfelt
et
al.,
2006)
plumes.
However,
the
nature
of
this
component
is
currently
difficult
to
constrain
in
the
Afar
case.
Indeed,
melting
of
the
gabbroic
part
of
an
old
recycled
oceanic
lithosphere
(e.g.,
Kokfelt
et
al.,
MA.
Daoud
et
al.
/
Lithos
114
(2010)
327-336
335
2006)
would
produce
high
ENd
magmas
and
therefore
this
process
does
not
account
for
the
low
6ryd
values
of
Hayyableh
and
Manda
Hararo
basalts.
Alternatively,
LREE
depletion
could
be
due
to
a
previous
melting
event
affecting
the
plume
materials,
as
proposed
by
Thirlwall
et
al.
(2004)
for
their
ID2
(or
RRD2)
depleted
component
of
the
Icelandic
plume.
This
hypothesis
may
account
for
the
Pb
isotopic
differences
between
Hayyabley/Manda
Hararo
basalts
and
the
other
(enriched)
Djibouti
basalts
(Figs.
4
to
6)
but
can
hardly
explain
the
higher
Sr
isotopic
ratios
of
Hayyabley
and
Manda
Hararo
basalts.
Finally,
another
intriguing
problem
is
the
causal
mechanism
for
the
sporadic
eruption
of
small
volumes
of
such
nearly
pure
"depleted"
melts
in
spatially
and
temporally
distinct
locations,
without
any
significant
contamination
by
the
dominant
enriched
materials.
Indeed,
such
features
are
difficult
to
reconcile
with
models
postulating
a
large
concentrically-zoned
Afar
plume
(e.g.,
Beccaluva
et
al.,
2009).
Numerical
simulations
of
the
evolution
of
thermal
and
thermo-
chemical
plumes
(Farnetani
et
al.,
2002;
Farnetani
and
Samuel,
2005;
Farnetani
and
Hofmann,
2009)
suggest
that
small
heterogeneous
mantle
domains
present
in
the
thermal
boundary
layer
feeding
the
plume
are
converted,
during
the
ascent
of
the
latter,
into
long-lived
elongated
and
narrow
filaments
within
the
plume
conduit.
Such
filaments
would
melt
sporadically,
and
then
eventually
communicate
their
specific
geochemical
fingerprint
to
small
volumes
of
basaltic
lavas
(Farnetani
and
Hofmann,
2009).
6.
Conclusions
The
-1
Ma-old
Hayyabley
volcano
(SE
Djibouti)
has
emitted
ca.
0.6-0.8
km
3
of
LREE-depleted
basalts
(La
n
/Sm
n
=
0.76-0.83)
that
display
unusual
chemical
features
(positive
Ba,
Sr
and
Eu
anomalies).
These
lavas
are
chemically
distinct
from
the
N-MORBs
erupted
from
the
nearby
Red
Sea
and
Gulf
of
Aden
oceanic
ridges,
and
instead
closely
resemble
the
LREE-depleted
basalts
from
the
Manda
Hararo
rift
in
Central
Afar
(Barrat
et
al.,
2003).
Another
similar
occurrence,
Oligocene
in
age,
has
been
reported
from
the
trap
series
in
the
Ethiopian
Plateau
by
Pik
et
al.
(1999).
Our
new
results
confirm
the
presence
within
the
Afar
region
of
basalts
derived
from
an
uncommon
depleted
component,
isotopically
distinct
from
the
source
of
the
Red
Sea
MORBs
and
from
the
similarly
depleted
mantle
(DM
in
Figs.
4
to
6)
which
contributes
to
the
genesis
of
Erta'Ale
volcanics
(Barrat
et
al.,
1998).
This
component
is
not
unusual
from
an
isotopic
(Sr,
Nd,
Pb,
0)
point
of
view,
and
is
mainly
recognizable
from
the
specific
trace
element
signature
of
the
corresponding
basalts
(positive
Ba,
Sr,
Eu
anomalies
combined
with
LREE
depletion).
The
origin
of
the
Hayyabley-Manda
Hararo
basalts
fingerprint
could
be
ascribed
to
the
interactions
between
(i)
depleted
(N-MORB
type)
basalts
derived
from
an
asthenospheric
mantle
component
similar
to
the
Erta
'Ale
depleted
end-member
and
(ii)
enriched
lithospheric
materials
which
would
be
responsible
for
the
positive
Ba,
Sr
and
Eu
anomalies.
These
materials
could
be
either
the
African
continental
crust,
flood
basalt-related
gabbroic
cumulates
stored
within
or
below
it,
or
finally
the
subcontinental
lithospheric
mantle.
However,
all
these
materials
are
mostly
LREE-enriched,
and
the
contamination
hypothesis
can hardly
explain
the
clear
LREE,
Rb
and
Th
depletion
and
concomitant
Ba,
Sr
and
Eu
enrichment
of
Hayyabley
basalts
(Figs.
2
and
3)
as
well
as
their
Pb
isotopic
signature
(Figs.
5
and
6).
Moreover,
contamination
in
plume-related
volcanic
series
is
often
described
as
a
variable,
occasional
or
random
process.
Thus,
it
can hardly
account
for
the
very
specific
trace
element
and
isotopic
signature
of
the
Afar
depleted
basalts,
which
were
erupted
in
three
separate
locations,
with
distinct
emplacement
ages
and
underlying
crustal/lithospheric
thickness.
Therefore,
our
preferred
conclusion
is
that
these
depleted
basalts
derive
from
an
intrinsic
(although
volumetrically
minor)
depleted
component
from
the
Afar
plume,
possibly
present
as
elongated
and
narrow
filaments
within
the
plume
conduit.
Sporadic
melting
of
such
filaments
might
account
for
the
restricted
spatial
and
temporal
distribution
of
the
Afar
depleted
basalts.
The
precise
origin
of
this
deep
mantle
component
is
currently
difficult
to
constrain,
given
the
small
number
of
depleted
basalt
samples
and
the
limited
amount
of
corresponding
geochemical
data.
The
most
likely
hypothesis
is
the
contribution
of
recycled
gabbros
from
ancient
oceanic
crust.
Acknowledgements
This
study
has
been
funded
by
the
French
Embassy
in
Djibouti,
and
the
grant
of
the
first
author
(M.A.D.)
provided
by
the
MAWARI
international
program
managed
by
the
CIFEG,
Orleans,
France.
Analytical
expenses
were
funded
by
the
MAWARI
program
and
UMR
6538,
Plouzane.
We
especially
thank
Dr.
Mohamed
Jalludin,
Director
of
the
CERD,
for
his
interest,
scientific
discussions
and
logistic
support,
Ali
Abdillahi
for
his
efficiency
in
organizing
fieldwork,
and
Marcel
Bohn
for
his
help
with
microprobe
analysis.
Careful
reviews
by
Tania
Furman
and
Godfrey
Fitton
led
us
to
improve
significantly
the
organization
and
contents
of
this
manuscript.
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