Petrological Constraints on Melt Generation Beneath the Asal Rift Djibouti using Quaternary basalts


Pinzuti, P.; Humler, E.; Manighetti, I.; Gaudemer, Y.

Geochemistry, Geophysics, Geosystems 14(8): 2932-2953

2013


The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 ky). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na8.0 and Fe8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 4 km to 43 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 7 km to 67 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 40 C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 2.0 cm yr-1, a value close to the mean spreading rate (2.9 0.2 cm yr-1) over the last 620 ky.

Article
Volume
14,
Number
8
22
August
2013
doi:
10.1002/ggge.20187
ISSN:
1525-2027
Geochemistry
Geophysics
Geosystems
Published
by
AGU
and
the
Geochemical
Society
Petrological
constraints
on
melt
generation
beneath
the
Asal
Rift
(Djibouti)
using
quaternary
basalts
Paul
Pinzuti
and
Eric
Humler
Laboratoire
de
Planetologie
et
Geodynamique
de
Nantes,
UMR
6112,
2
rue
de
la
Houssiniere
BP
92208,
FR-44322,
Nantes
Cedex
3,
France
(paulpinzuti@gmail.com
)
Isabelle
Manighetti
Geoazur,
UMR6526,
Valbonne,
France
Yves
Gaudemer
Laboratoire
de
Tectonique
et
Mecanique
de
la
Lithosphere,
UMR
7154,
Institut
de
Physique
du
Globe
de
Paris,
Universite
Paris
VII,
France
[1]
The
temporal
evolution
of
the
mantle
melting
processes
in
the
Asal
Rift
is
evaluated
from
the
chemical composition
of
56
new
lava
flows
sampled
along
10
km
of
the
rift
axis
and
9
km
off-axis
(i.e.,
erupted
within
the
last
620
kyr).
Petrological
and
primary
geochemical
results
show
that
most
of
the
samples
of
the
inner
floor
of
the
Asal
Rift
are
affected
by
plagioclase
accumulation.
Trace
element
ratios
and
major
element
compositions
corrected
for
mineral
accumulation
and
crystallization
show
a
symmetric
pattern
relative
to
the
rift
axis
and
preserved
a
clear
signal
of
mantle
melting
depth
variations.
While
FeO,
Fe
8.0
,
Zr/Y,
and
(Dy/Yb)
N
decrease
from
the
rift
shoulders
to
the
rift
axis,
SiO
2
,
Na/Ti,
Lu/Hf
increase
and
Na
2
O
and
Na
g
.
°
are
constant
across
the
rift.
These
variations
are
qualitatively
consistent
with
shallow
melting
beneath
the
rift
axis
and
deeper
melting
for
off-axis
lava
flows.
Na
g.°
and
Fe
u)
contents
show
that
beneath
the
rift
axis,
melting
paths
are
shallow,
from
81
±
4
to
43
±
5
km.
These
melting
paths
are
consistent
with
adiabatic
melting
in
normal-temperature
fertile
asthenosphere,
beneath
an
extensively
thinned
mantle
lithosphere.
On
the
contrary,
melting
on
the
rift
shoulders
(from
107
±
7
to
67
±
8
km)
occurred
beneath
thicker
lithosphere,
requiring
a
mantle
solidus
temperature
100
±
40°C
hotter.
In
this
geodynamic
environment,
the
calculated
rate
of
lithospheric
thinning
appears
to
be
4.0
±
2.0
cm
yr
-1
,
a
value
close
to
the
mean
spreading
rate
(2.9
±
0.2
cm
yr
-1
)
over
the
last
620
kyr.
Components:
9,154
words,
9
figures,
2
tables.
Keywords:
geochemistry;
petrology;
continental
breakup;
rifting;
Asal
Rift.
Index
Terms:
1700
History
of
Geophysics;
1749
Volcanology, geochemistry,
and
petrology;
1000
Geochemistry;
1065
Major
and
trace
element
geochemistry;
1032
Mid-oceanic
ridge
processes;
3600
Mineralogy
and
Petrology;
3614
Mid-oce-
anic
ridge
processes;
8400
Volcanology;
8416
Mid-oceanic
ridge
processes;
4825
Geochemistry.
Received
22
March
2013;
Revised
22
May
2013;
Accepted
24
May
2013;
Published
22
August
2013.
©
2013.
American
Geophysical
Union.
All
Rights
Reserved.
2932
t
i
co
Geochemistry
.
3
-
Geophysics
r
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
Pinzuti,
P.,
E.
Humler,
I.
Manighetti,
and
Y.
Gaudemer
(2013),
Petrological
constraints
on
melt
generation
beneath
the
Asal
Rift
(Djibouti)
using
quaternary
basalts,
Geochem.
Geophys.
Geosyst,
14,
2932-2953,
doi:10.1002/ggge.20187.
1.
Introduction
[2]
From
continental
rifting
to
seafloor
spreading,
the
continental
crust
is
progressively
modified
by
magmatism.
Although
substantial
progress
has
been
made
on
petrology
and
geochemistry
of
con-
tinental
basalts
[e.g.,
Fram
et
al.,
1998;
DePaolo
and
Daley,
2000;
Wang
et
al.,
2002],
the
tempera-
ture
and
pressure
conditions
of
melting
that
lead
to
magmatism
during
this
process
are
still
poorly
constrained
[e.g.,
Bastow
and
Keir,
2011].
The
reasons
are
that
the
genesis
and
evolution
of
conti-
nental
basalts
are
due
to
many
other
factors,
which
can
be
difficult
to
decipher.
For
example,
crustal
contributions
may
partially
or
completely
mask
the
composition
of
magma
that
is
formed
in
the
mantle
[e.g.,
Glazner
et
al.,
1991;
Glazner
and
Farmer,
1992]
and
even
without
crustal
contami-
nation,
there
is
a
debate
concerning
the
role
of
mantle
plume
[e.g.,
Bradshaw
et
al.,
1993
;
Par-
sons
et
al.,
1994]
and
lithospheric
versus
astheno-
spheric
mantle
in
the
basalt
genesis
[e.g.,
Dungan
et
al.,
1986;
Kempton
et
al,
1991;
DePaolo
and
Daley,
2000].
Moreover,
because
of
the
absence
of
glasses
and
the
rarity
of
aphyric
rock
samples,
it
is
also
necessary
to
use
whole
rock
data.
The
prob-
lem
is
that
the
presence
in
the
whole-rock
of
phe-
nocrysts
that
are
out
of
equilibrium
with
their
host
melt
modifies
the
chemical
composition
of
sam-
ples
and
lead
to
uncertainty
in
the
interpretation
of
the
major
elements.
[3]
The
East
African
Rift
System
is
a
classic
example
of
continental
breakup,
where
the
defor-
mation
of
the
continental
crust
is
located
along
magmatic
segments
[e.g.,
Manighetti
et
al.,
1998;
Manighetti
et
al.,
2001;
Doubre
et
al.,
2007a,
2007b;
Ebinger
et
al.,
2008;
Ferguson
et
al.,
2013].
The
central
Main
Ethiopian
Rift
marks
the
transition
between
rifting
of
thick
continental
crust
in
the
southern
and
central
East
African
Rift
Sys-
tem
and
incipient
seafloor
spreading
in
the
north-
ern
Afar,
into
the
Afar
Depression
[e.g.,
Hayward
and
Ebinger,
1996;
Rooney
et
al.,
2007].
The
broad
range
of
lithologies
for
potential
crustal
assimilants
has
prompted
the
application
of
multi-
ple
petrographic
and
geochemical
(e.g.,
Ce/Pb,
Ti/Yb,
La/Nb,
Sr,
Nd,
Hf,
and
Pb
isotopes)
indicators
of
crustal
contamination.
For
instance,
the
relationships
between
Sr-Nd
isotopic
ratios
and
K/P
and
Ti/Yb
ratios
have
been
used
to
assess
the
role
of
crustal
contamination
in
lavas
from
the
Afar
depression
[Deniel
et
a/.,1994;
Barrat
et
al.,
1990;
Vigier
et
al.,
1999;
Hart
et
al.
1989;
Schil-
ling
et
al.,
1992;
Rooney
et
al.,
2012a].
Most
of
the
analyzed
basalts
have
low
K/P
ratios
(<5)
and
there
are
no
correlations
between
these
ratios
and
the
Sr
and
Nd
isotopic
ratios
suggesting
the
ab-
sence
of
significant
silicic
crustal
contamination.
Furthermore,
the
contribution
of
the
upper
mantle
asthenosphere
(mid-ocean
ridge
basalt
(MORB)-
like
source)
is
best
recognized
in
the
young
lavas
(<4
Ma),
particularly
in
the
Tadjoura
area
and
the
Asal
Rift
(3-0
Ma),
where
Hf-Pb
data
have
affinity
to,
and
overlap
with,
the
East
Sheba
Ridge
(Aden
Ridge)
data
and
has
Indian
Ocean-like
Hf
and
Pb
isotope
signatures
[Rooney
et
al.,
2012a].
However
it
has
been
shown
that
these
lava
flows
exhibit
val-
ues
consistent
with
mixing
between
Afar
plume
and
regional
lithospheric
mantle
[Schilling
et
al.,
1992;
Rooney
et
al.,
2012a,
Rooney
et
al.,
2013].
[4]
These
geochemical
data
suggest
that
the
major
element
composition
of
the
recent
basalts
(<1
Ma)
from
the
Gulf
of
Tadjoura
and
the
Asal
Rift can
be
used
confidently
to
constrain
mantle
melting
processes.
[5]
As
shown
elsewhere
[e.g.,
Klein
and
Lang-
muir,
1987;
Klein
and
Langmuir,
1989;
Langmuir
et
al.,
1992;
Lee
et
al.,
2009],
major
element
data
can
be
quantitatively
linked
to
the
extent
of
melt-
ing
(F
mean
),
the
initial
and
final
pressures
of
melt-
ing
(P
o
and
P
f
),
mantle
temperature
and
crustal
thickness
(h
e
).
For
instance,
increasing
F
mean
has
the
main
effect
of
lowering
Na
2
O
in
the
melt
(Na
2
O
behaves
as
an
incompatible
element
during
mantle
melting).
FeO
varies
largely
as
a
function
of
P
o
with
relatively
small
variations
as
a
function
of
F.
Thus,
Na
and
Fe
contents
of
the
mantle
melts
provide
constraints
on
the
final
depth
of
melting
Pf
(from
Na
2
O,
which
reflects
F
mean
and
therefore
P
o
—P
f
),
on
the
initial
depth
of
melting
(P
o
)
and
thus,
on
the
solidus
temperature
(from
FeO).
The
petrologically
constrained
crustal
thickness
can
2933
a)
Passive
(Fe.,.=
10.0,
Na„„
=1.7,
=12.5°4,
he
=
10.5
km,
1
3
=
12.7
kb)
25.00
%
4.4
kb
18.75
%
10.6
kb
12.50
%
4
16.9
kb
6.25
%
23.1
kb
0%
29.4
kb
Solidus
RMC
I I I
b)
Melting
terminating
at
depth
km
P(kb)
(Fe„
Na„,=
2.2,
F=10
%,
he
=
5
km,
is=
21
kb)
0
0
12.4%
4.4
kb
30
10
10.6
kb
12.4%
17
kb
60
20
6.2
%
.....
10.8
kb
9D
30
0%
Solidus
29.4
kb
RMC
e
r
--
'
Geochemistry
I
"j
-
Geophysics
r
Geosystems
Lf
PINZUTI
ET
AL:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
Figure
1.
Models
for
passive
(standard
model)
and
melting
terminating
at
depth
(modified
from
Langmuir
et
al.
[1992]).
All
melting
regimes
are
drawn
to
scale,
where
1
kbar
is
equal
to
3
km
of
mantle
or
3.3
km
of
crust,
and
is
1.2%
kbar
-
1
.
The
mean
properties
of
the
steady
state
ocean
crust
generated
by
each
melting
regime
are
given,
and
may
be
calculated
from
residual
mantle
column
(RMC).
F
n
.:
with
mean
in
subscript
characters
is
the
mean
extent
of
melting,
P
mean
:
with
mean
in
subscript
characters.
is
the
mean
pressure
of
melting,
h
e
is
crustal
thick-
ness,
and
F
max
is
the
maximum
amount
of
melting
in
the
melting
regime.
(a)
The
passive
model
is
for
equilib-
rium
melting
and
complete
melt
focusing.
(b)
The
melting
regime
beneath
thick
lithosphere
is
compressed
by
a
freezing
front.
FeO
corrected
for
fractional
crystallization
at
8%
MgO
(Fe
8
.
0
)
is
identical
in
both
cases
(10%)
but
because
the
melting
column
is
shorter
in
melting
regime
terminating
at
depth,
the
Na
b0
content
is
larger
(2.2%)
compared
to
the
standard model
(1.7%).
Indeed,
the
petrologically
constrained
crustal
thickness
is
thicker
in
the
standard
case
(10.5
km)
relative
to
the
melting
regime
ceasing
at
depth
(5
km).
be
then
compared
to
geophysical
observations
and
the
mantle
flow
regime
can
be
determined
(standard
model
versus
melting
terminating
at
depth
for
instance).
In
Figure
1,
FeO
corrected
for
fractional
crystallization
at
8%
MgO
(Fe
8.0
)
is
identical
in
both
cases
(10%)
but
because
the
melting
column
is
shorter
in
the
melting
regime
terminating
at
depth,
the
Nag
.
°
content
is
larger
(2.2%)
compared
to
the
standard
model
(1.7%).
Indeed,
the
petrologically
constrained
crustal
thickness
is
thicker
in
the
standard
case
(10.5
km)
relative
to
the
melting
regime
ceasing
at
depth
(5
km).
[6]
The
present
study
focuses
on
the
conditions
of
melting
that
lead
to
magmatism
during
continental
rifting
through
the
example
of
the
Asal
Rift.
Our
study
area
covers
12
km
of
the
rift
(Figure
2),
out
of
9
km
from
the
axis
(that
is
to
an
equivalent
age
of
about
620
kyr).
Spatial
petrological
and
geo-
chemical
studies
of
the
rift,
which
is
easily
acces-
sible
to
field
observations,
give
us
the
opportunity
to
constrain
the
magmatic
evolution
during
the
rifting
process
based
on
56
new
lava
flows
sampled
in
this
area.
The
key
questions
we
try
to
solve
are
as
follows:
How
large
are
mantle
tem-
perature
variations
beneath
the
rift
and
at
what
depth
does
the
mantle
cease
to
melt?
Which
melt-
ing
conditions
are
required
to
fit
both
petrological
and
geophysical
data
using
a
"normal"
mantle
composition?
These
constrains
are
useful
to
illus-
trate
the
mantle
thermal
state
beneath
a
newly
fragmented
continent.
[7]
In
the
following,
we
will
first
use
major
ele-
ment
composition
of
basalts
corrected
for
mineral
accumulation
and
for
crystallization
to
estimate
the
temporal
variations
of
the
initial
and
final
pres-
sures
and
temperatures
of
melting.
Subsequently,
trace
elements
ratios
insensitive
to
phenocrysts
accumulation
and
crystallization
are
used
to
test
the
results
obtained
from
the
major
element
approach.
Finally,
we
propose
a
"petrologically
constrained
lithospheric
thickness"
and
deduce
the
rate
of
litho-
spheric
thinning
in
the
studied
area.
2.
Geological
and
Geophysical
Constraints
[8]
The
Afar
depression
is
a
unique
area
where
the
transition
between
continental
and
oceanic
rifting
is
working
and
observable.
This
area
is
stretched
by
the
separation
of
the
Africa
and
Arabia
plate,
whose
boundaries
correspond
to
the
Aden
and
Red
Sea
Ridge
(Figure
2a).
The
Africa-Arabia
separa-
tion
was
initiated
30
Myr
ago
[e.g.,
Hofmann
et
al.,
1997;
Courtillot
et
al.,
1999]
by
the
activity
of
a
mantle
plume
leading
to
intraplate
volcanism
[e.g.,
Schilling,
1973;
White
and
McKenzie,
1989;
Marty
et
al.,
1993;
Deniel
et
al.,
1994;
Courtillot
et
al.,
1999].
The
Afar
area
is
characterized
by
active
rifting
taking
place
along
several
discon-
nected
rift
segments
propagating
on
land
(Figure
2a)
[e.g.,
Varet
and
Gasse,
1978;
Courtillot,
1980;
Manighetti
et
al.,
1998].
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42.24°
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42.53'
42.57°
87
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11.37°
11.62°
11.35'
11.56°
"
11.33°
11.54°
;5
11.31°
42.45° 42.49°
42.41°
42.26°
3.5
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o
.60.
Geochemistry
Geophysics
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
Stratoide
Series
Basalt
Series
Central
Series
Axial
Series
Limestone
Recent
anism
FE
14.
Figure
2.
(a)
Geological
map
of
the
emerged
part
of
the
Asal
Rift
[Stie/yes,
1980]
combined
to
IGN,
ASTER,
and
SRTM
DEM.
K/Ar
ages
of
basaltic
lava
flows
are
from
Manighetti
et
al.
[1998].
Letters
correspond
to
the
name
of
the
major
border
faults
from
Manighetti
et
al.
[2001].
Inset
shows
regional
tectonic
settings
where
the
arrows
represent
the
rift
segments
and
indicate
the
direction
of
propagation:
MI,
Manda
Inakir;
AG,
Asal-Ghoubbet;
T,
Tadjoura;
EA,
Erta
Ale;
TA,
Tat'Ali;
AL,
Alayta;
MH,
Manda
Hararo;
MH-G,
Manda
Hararo-Goba'ad.
Modified
from
Manighetti
et
al.
[2001].
(b)
Sample
location
across
the
Asal
Rift.
Black
and
white
dots,
respectively,
correspond
to
the
samples
collected
on
the
shoulders
and
on
the
inner
floor
of
the
rift.
[9]
The
Asal
Rift
(Figure
2)
is
the
first
emergent
segment
of
the
Aden
Ridge,
which
propagates
westward
on
land
into
the
Afar
depression
(Figure
2a)
[Manighetti
et
al.,
1998].
With
a
'-40
km
length,
whose
15
km
are
emerged
between
the
Ghoubbet
Bay
and
Lake
Asal,
it
currently
opens
at
1.6
±
0.1
cm
yr
-1
in
the
N40°
±
5°E
direction
(Figure
2)
[Ruegg
and
Kasser,
1987;
Vigny
et
al.,
2007].
The
Asal
Rift
evolution
starts
after
the
injection
of
rhyolitic
domes
through
NW-SE
faults,
around
1
Ma.
This
silicic
volcanism
can
be
related
to
the
emplacement
of
the
Aden
Ridge
propagator
[Lahitte
et
al.,
2003].
Around
852
±
85
kyr,
the
rhyolite
domes
were
covered
by
the
Gulf
basalts
series
(Figure
2a),
which
have
been
inter-
preted
as
deriving
from
fissural
eruptions
associ-
ated
with
incipient
rifting
[Richard,
1979].
These
Gulf
basalts
series
started
to
erupt
853
±
35
kyr
and
ended
with
a
hyaloclastite
eruption
episode
326
±
15
kyr
ago
(Figure
2a).
On
either
side
of
the
Asal
Rift,
the
youngest
basalts
of
theses
series
are
bound
or
offset
by
faults,
which
delimit
the
Cen-
tral
series
(Figure
2a).
This
magmatic
episode
filled
the
rift
between
300
and
50
kyr
from
a
central
volcano,
the
Fieale
[Manighetti
et
al.,
1998].
Whereas
basaltic
flows
older
than
150
±
15
kyr
have
apparently
spread
astride
the
rift
zone,
flows
younger
than
87
±
29
kyr
abut
fault
scarps
and
therefore
tend
to
have
concentrated
near
the
rift
axis
[De
Chabalier
and
Avouac,
1994].
After
90
kyr,
magmatic
activity
decreased
and
border
faults,
which
are
currently
observed,
started
to
grow.
The
geometry
of
the
modern
rift
began
with
the
development
of
normal
faults
on
either
side
of
the
new
inner
floor
[Manighetti
et
al.,
1998;
Pin-
zuti
et
al.,
2010].
Later,
the
axial
series
erupted
from
small
volcanic
edifices
aligned
along
erup-
tive
fissures
in
the
inner
floor.
The
latest
volcanic
event
occurred
at
the
northwestern
tip
of
the
vol-
canic
ridge
in
1978
(Ardoukoba
volcano,
Figures
2a
and
2b).
[io]
Mechanical
stretching
models
[e.g.,
[Dunbar
and
Sawyer,
1989;
Lin
and
Parmentier,
1990]
have
been
previously
proposed
to
explain
the
long-term
evolution
of
the
Asal
Rift.
However,
recent
study
[Pinzuti
et
al.,
2010]
shows
that
this
evolution
could
be
related
to
localized
magma
intrusion
at
depth.
2935
(1
.60
Geochemistry
"i
-
Geophysics
r
I
Geosystems
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
While
the
crustal
thickness
outside
of
the
Asal
Rift
is
about
20-25
km
[Hammond
et
al.
2011],
deep
seismic
sounding
[Ruegg,
1975]
and
others
geo-
physical
approaches
give
crustal
thickness
beneath
the
rift
of
—5
km
[Tarantola
et
al.,
1980;
Lepine
and
Hirn,
1992;
De
Chabalier
and
Avouac,
1994;
Doubre
et
al.,
2007a,
2007b],
the
first
500
m
near
the
rift
axis
corresponding
to
the
basalts
flooded
since
—1
Myr
[Demange
and
Puvilland,
1993].
[ii]
Deep
structure
of
the
Afar
depression
is
con-
strained
by
seismic
refraction
and
seismological
data
[Ruegg,
1975;
Berckhemer
et
al,
1975;
Makris
and
Ginsburg,
1987;
Knox
et
al,
1998;
Nyblade
et
al,
2000;
Rychert
et
al.
2012].
Knox
and
et
al.
[1998]
localized
a
pronounced
negative
S
wave
ve-
locity
gradient
beneath
Afar
Depression
between
60
and
110
km,
which
is
characteristic
of
the
upper
mantle.
From
seismic
refraction
studies,
Ruegg
[1975]
estimates
that
the
lithosphere-astenosphere
boundary
at
the
both
side
of
the
Aden
Ridge
is
located
to
a
depth
of
58
km
and
rises
up
to
45
km
at
100
km
of
ridge
axis
(profiles
01
and
06).
3.
Data
Acquisition
and
Laboratory
Work
3.1.
Chemical
Analyses
[12]
We
sampled
56
new
basaltic
lava
flows
across
the
Asal
Rift
during
a
field
trip
organized
in
2000.
The
major
and
trace
element
compositions
of
each
sample
(Tables
SI1—SI4)
were
measured
from
small
volume
of
rock
("few
cm
3
),
which
were
prelimi-
narily
powdered
using
agate
mortar
and
90°
alcohol.
The
major
element
compositions
were
measured
by
Inductively
Coupled
Plasma
Atomic
Emission
Spectrometer
and
the
trace
elements
were
deter-
mined
by
Inductively
Coupled
Plasma
Mass
Spec-
trometry
at
Centre
de
Recherches
Petrographiques
et
Geochimiques
(CRPG-CNRS
Nancy-France).
Measurement
uncertainties
are:
A1
2
0
3
<
1%,
Si0
2
<
1%,
FeO
<
2%,
MgO
<
2%,
CaO
<
2%,
Na
2
O
<
5%,
TiO
2
<
5%,
P205
<
10%,
K20
<
2%,
MnO
<
5%,
and
<
5%
for
trace
elements.
[13]
To
observe
detailed
mineral
assemblages
from
microscope,
we
make
thin
sections
of
sam-
ples
at
the
Laboratoire
de
Planetologie
and
Geodynamique
de
Nantes
(France).
From
12
of
these
thin
sections,
we
analyzed
the
major
chemi-
cal
composition
of
mineral
phenocrysts
from
Microprobe
(CAMECA
SX
100)
at
the
Institut
de
Physique
du
Globe
de
Paris-CAMPARIS
(France).
Average
major
chemical
composition
of
ground-
mass
of
six
samples
were
also
analyzed
from
Scanning
Electron
Microspcopy
(SEM)
(JEOL
7600F
and
JEOL
5800LV)
at
the
Institut
des
Materiaux
Jean
Rouxel
(Nantes,
France).
All
min-
eralogical
descriptions
are
given
in
the
supporting
information
(see
Figures
SI1—S138
and
Tables
SI5—S18).
1
3.2.
Mineral
Accumulation
Correction
[14]
Because
a
large
part
of
the
Asal
Rift
samples
is
affected
by
mineral
accumulation,
we
corrected
the
whole-rock
composition
of
the
samples
from
modal
abundances
and
chemical
composition
of
phenocryst
phases
using
simple
mass
balance
cal-
culations.
We
used
two
different
procedures
based
on
modal
analysis
and
phase
identification,
both
from
digital
photographs
and
scanned
images
of
thin
section.
These
quantitative
modal
analyses
are
based
on
the calculation
of
the
area
occupied
by
individual
minerals
on
an
image,
groundmass
and
porosity
normalized
to
the
total
thin
section
area.
The
main
assumption
of
these
approaches
is
that
the
mineral
abundance
is
related
to
the
percentage
of
the
area
it
occupies,
assuming
that
there
is
a
direct
relation
with
the
volume
percent
[e.g.,
Petruk,
1989]
.
[15]
The
first
procedure
is
based
on
the
analyses
of
20
thin
sections
using
a
binocular
polarizing
microscope
combined
with
a
CANON
EOS
350D.
For
each
thin
section,
we
realized
a
mozaic
of
pic-
tures
under
plane-polarized
light
(Figures
SI1—S138),
using
Autopanogiga©
software.
In
this
configuration,
the
color
of
olivine
and
clinopyroxene
is
light
brown,
iron
and
titan
oxides
are
black,
but
unfortunately,
pla-
gioclases
and
vesicles
filled
of
resin
appear
white.
Consequently,
to
separate
these
two
phases,
we
also
realized
five
mozaics
for
each
thin
section
in
cross
polarized
from
a
series
of
pictures
taken
every
30°,
from
to
150°.
Using
image2003©
software
[Lau-
neau
and
Robin,
1996],
we
then
applied
to
each
set
of
mozaic
a
Fourier
series
decomposition
to
extract
an
ellipse
from
the
second
optical
component.
The
result
corresponds
to
an
image,
where
each
mineral
phase
has
a
maximum
intensity
and
the
vesicles
always
appear
black.
[16]
From
the
plane-polarized
light
and
Fourier
panoramas,
it
is
then
straightforward
to
select
and
assign
to
each
phase
a
separate
color
combining
them
in
one
over
layer
using
Image2003
and/or
Adobe
Photoshop
CS3©
software.
From
the
final
'Additional
supporting
information
may
be
found
in
the
online
version
of
this
article.
2936
l
o
co
Geochemistry
.
3
'
-
Geophysics
r
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
mineral
maps,
a
first
abundance
of
each
phase
is
estimated
as
the
ratio
of
the
pixel
number
of
the
assigned
color
divided
by
the
size
of
the
picture
in
pixels,
using
Image2003
software.
At
last,
the
final
mineral
abundance
of
each
mineral
phase
(Tables
SI1
and
SI2)
is
calculated
by
normalizing
the
first
estimation
by
the
porosity.
[17]
The
remaining
dataset
was
analyzed
using
scanned
images
of
thin
section
from
Epson
Expression
10,000XL
scanner.
Each
thin
section
was
scanned
as
negative
film,
first
under
naturel
light,
and
second
between
two
polarizing
films.
Mappings
of
the
thin
sections
were
realized
from
this
couple
of
scanned
images.
The
main
differ-
ence
with
the
first
approach
is
that
the
phenocrysts
presented
on
the
polarized
images
do
not
show
maximum
intensity.
Consequently,
to
clearly
sepa-
rate
plagioclases
and
porosity
filled
of
resin,
it
is
sometimes
necessary
to
verify
the
mapping
of
the
thin
section
with
a
binocular
polarizing
micro-
scope.
The
procedure
is
less
convenient
than
the
first
one,
but
is
still
faster
because
several
thin
sec-
tions
can
be
scanned
together.
4.
Age/Distance
Relationship
in
the
Asal
Rift
[18]
Since
this
study
deals
with
temporal
variation
of
basalts
chemistry,
we
need
to
check
precisely
the
relationship
between
geochronological
ages
of
samples
and
their
distances
to
the
active
rift
axis.
In
order
to
test
this
relationship,
we
used
K-Ar
age
(Figure
2a)
published
in
Manighetti
et
al.
[1998].
K-Ar
age
(Figure
2a)
versus
distance
to
the
rift
axis
for
10
flows
from
the
northern
and
southern
shoulders
of
the
Asal
Rift
is
plotted
in
Figure
SI39
(see
supporting
information).
The
distances
from
the
rift
axis
are
calculated
from
the
projection
of
the
sample
location
along
the
N43°E
direction
(Figures
2
and
SI39).
This
direction
is
consistent
with
the
azimuth
opening
direction
of
the
Asal
Rift
and
corresponds
to
the
perpendicular
of
the
rift
axis,
which
is
defined
as
the
line
passing
through
the
caldera
of
the
Fieale
volcano
and
through
the
axial
volcanic
chains
located
in
the
central
part
of
the
inner
floor
(Figure
2a).
[19]
The
linear
correlation
between
K-Ar
ages
and
distance
to
the
axial
rift
[e.g.,
De
Chabalier
and
Avouac,
1994;
Ferguson
et
al.,
2013]
defines
a
mean
spreading
rate
of
2.9
±
0.2
cm
yr
-1
.
How-
ever,
anomalous
young
age
for
one
sample
outside
the
rift
indicate
an
off-axis
volcanism
on
the
north-
ern
shoulder
and
suggest
that
eruptions
occurred
66
kyr
ago
within
a
zone
3
km
wide
(subrift
Dan-
kalelo,
Figures
2a
and
2b)
[Manighetti
et
al.,
1998].
If
this
data
set
is
statistically
significant
for
the
studied
area,
we
can
use
the
distance
of
sam-
ples
to
the
rift
axis
as
a
proxy
for
their
geochrono-
logical
ages.
The
calculated
extension
rate
(2.9
±
0.2
cm
yr
-1
)
is
higher
than
the
current
value
obtained
from
Global
Positioning
System
measurements
(1.6
±
0.1
cm
yr
-1
)
[Ruegg
and
Kasser,
1987;
Vigny
et
al.,
2007]
but
is
close
to
the
long-term
spreading
rate
for
the
studied
area
[De
Chabalier
and
Avouac,
1994].
In
the
follow-
ing,
the
data
are
projected
along
the
line
perpen-
dicular
to
the
rift
axis
(distance
of
samples
from
the
rift
axis
and
calculated
ages
are
related
by
Dis-
tance
=
29
Age,
with
distance
in
km
and
ages
in
kyr
(Table
1).
We
organized
the
samples
in
two
groups,
from
the
rift
axis
distance.
The
first
group
corresponds
to
the
samples
from
the
Gulf
Basalt
and
Central
series
(group
B),
and
the
second
one
includes
the
samples
from
the
axial
series,
which
characterized
the
inner
floor
of
the
Asal
Rift
(group
A).
This
inner
floor
is
delimitated
by
the
Southern
(faults
H,
F,
D)
and
Northern
(faults
al,
a2,
a3)
border
fault
systems
(Figure
2a).
Its
width
ranges
from
—2
km
at
the
Fieale
caldera
to
more
than
5
km
near
the
Lake
Asal
(Figure
2).
For
our
study,
we
used
a
mean
width
of
4100
m,
which
is
the
best
estimate
that
characterized
most
of
the
samples
from
the
axial
series
(Figure
2).
5.
Evidence
for
Plagioclase
Accumulation
5.1.
Petrological
Observations
[2o]
Petrological
analysis
shows
that
most
of
the
Asal
Rift
lava
flows
correspond
to
high
vesicular
basalts.
Most
samples
of
the
rift
shoulders
(-64%)
have
an
aphyric
texture.
The
majority
of
the
lava
flows
of
the
inner
floor
(-75%)
correspond
to
porphyritic
basalts
with
plagioclase
megacrysts
(-8%
to
—54%),
olivine
(-0.1%
to
—6%)
and
clinopyroxene
(-5%)
in
lesser
proportion.
Microp-
robe
analyses
reveal
that
the
majority
of
these
min-
eral
phases
have
a
close
chemical
composition
from
one
sample
to
the
other
one
(Tables
SI5—S17).
[21]
The
plagioclase
megacrysts
correspond
to
Bytownite
(An
\
_76_88
;
Table
SI5)
with
a
size
that
can
reach
1
cm
(Figure
SI40A),
which
is
unusual
in
eruptive
rocks.
A
few
part
of
plagioclase
crys-
tals
are
subeuhedral,
but
the
majority
of
them
2937
Table
1.
Compositional
Parameters,
Calculated
Pressure,
and
Depth
of
Melting
for
the
Basalts
of
the
Asal
Rift
Corrected
for
Mineral
Accumulation
a
Sample
ID
Longitude
(°)
Latitude
(°)
Distance
(m)
Location
Fe
8
.
0
*
(%)
Na8.0
*
(%)
Zr/Y
Na*/Ti*
(Dy/Yb)
N
Lu/Hf
P
(kbar)
P
1
(kbar)
Fmeaa
(%)
c
(
k
m)
D
t
(
k
m)
Dm
(km)
AF01 42.4945
11.5751
1357
A
8.91
2.54
5.159
2.776
1.326
0.120
23.75
12.65
10
3.77
38.88
73.06
AF05
42.5158
11.5527
1614
A
8.88
2.42
4.527
3.543
1.254
0.135
23.59
10.94
11
4.52
34.67
72.35
AF06
42.5142
11.5601
1129
A
8.36
2.60
4.554
3.235
1.243
0.134
20.53
8.80
10
3.87
28.20
63.05
AF07
42.5091
11.5671
933
A
8.80
2.10
4.809
2.745
1.326
0.125
23.37
5.14
12
7.61
15.94
55.51
AF08
42.5024
11.5887
335
A
9.30
2.32
4.892
2.787
1.279
0.123
26.07
13.36
11
4.75
41.73
80.00
AF10
42.5272
11.5536
700
A
9.79
2.51
5.084
2.768
1.291
0.116
28.97
18.27
10
3.67
56.65
89.13
AF12
42.4885
11.5834
1122
A
8.96
2.29
4.870
3.124
1.338
0.121
24.08
10.20
11
5.26
32.12
73.82
AF14
42.4885
11.5834
1122
A
10.16
2.22
4.491
3.248
1.311
0.134
31.15
17.91
12
5.19
54.95
95.81
AF15
42.4982
11.5928
360
A
10.2
2.48
4.427
3.888
1.265
0.133
31.38
20.53
10
3.77
63.83
96.31
AF16
42.4957
11.5927
168
A
9.46
2.21
4.674
2.759
1.358
0.123
27.08
13.29
12
5.43
40.81
83.20
AF17
42.4839
11.6012
8
A
9.97
2.39
4.269
3.708
1.270
0.149
30.02
18.27
11
4.25
56.73
92.20
AF18
42.4839
11.6023
81
A
8.88
2.24
4.379
3.913
1.288
0.135
23.64
8.86
12
5.75
26.55
72.50
AF20
42.4815
11.6046
92
A
9.45
2.30
4.870
2.948
1.287
0.125
26.98
14.13
11
4.86
44.16
82.75
AF23
42.4777
11.6069
0
A
9.49
2.12
4.544
3.553
1.268
0.132
27.26
12.22
12
6.21
37.28
83.76
AF25
42.4764
11.6089
57
A
9.87
2.41
4.342
3.464
1.257
0.149
29.45
17.79
11
4.19
55.17
90.52
AF26
42.4764
11.6089
67
A
8.96
2.32
4.336
3.486
1.270
0.138
24.10
10.54
11
5.08
33.16
73.90
AF30
42.4732
11.6127
142
A
9.12
2.34
4.146
3.645
1.304
0.141
25.06
12.32
11
4.72
38.23
76.97
AF31
42.4718
11.6160
308
A
10.19
2.62
4.689
3.142
1.271
0.132
31.34
21.59
10
3.20
66.97
96.36
AF33
42.4708
11.6166
283
A
9.83
2.46
4.742
2.981
1.266
0.129
29.21
18.02
10
3.93
55.83
89.84
AF34
42.4697
11.6163
177
A
8.96
2.04
4.604
3.377
1.281
0.133
24.24
5.66
13
8.00
17.54
56.60
AF52
42.5157
11.5907
1479
A
10.86
2.39
4.657
2.319
1.400
0.112
35.26
23.55
11
4.24
72.77
108.42
AF60
42.4920
11.5847
758
A
9.23
2.45
4.934
2.700
1.270
0.123
25.66
14.30
11
4.01
44.52
78.85
AF62
42.4828
11.5819
1665
A
9.24
2.02
4.732
2.577
1.349
0.119
26.02
5.79
13
8.80
22.11
79.53
AF63
42.4771
11.5862
1734
A
9.53
2.41
4.706
2.750
1.259
0.126
27.46
15.86
11
4.16
49.48
84.34
AF64
42.4750
11.5890
1660
A
9.18
2.52
4.369
3.140
1.234
0.136
25.39
14.82
10
3.61
46.07
78.08
AF65
42.4733
11.5920
1541
A
9.37
2.26
4.160
3.940
1.263
0.143
26.51
13.11
12
5.17
40.87
81.32
AF66
42.4682
11.5906
2031
A
8.96
2.31
4.444
3.474
1.279
0.136
24.08
10.37
11
5.16
32.65
73.85
AF77
42.4868
11.5858
1051
A
9.44
2.42
4.986
2.664
1.287
0.121
26.88
15.41
11
4.10
47.95
82.62
Mean
A
9.41
±
0.21
2.35
±
0.06
4.62
±
0.10
3.16
±
0.20
1.29
±
0.01
0.13
±
0.00
AF02
42.4941
11.5651
2203
B
10.91
2.22
4.871
2.106
1.440
0.113
35.68
22.41
12
5.21
68.95
109.80
AF03
42.4952
11.5623
2350
B
9.80
1.92
4.784
2.073
1.406
0.114
29.52
10.30
14
8.81
34.22
90.27
AF04
42.4945
11.5391
4295
B
10.27
2.49
5.274
2.377
1.352
0.111
31.83
20.97
10
3.77
65.19
97.70
AF41
42.5518
11.6154
6156
B
10.40
2.31
5.301
2.631
1.416
0.108
32.61
20.25
11
4.64
62.79
100.13
AF42
42.5518
11.6154
6156
B
10.86 2.48
5.291
2.913
1.430
0.107
35.28
24.29
10
3.83
75.36
108.32
AF43
42.5506
11.6167
6173
B
11.27
2.44
5.415
2.768
1.420
0.105
37.69
26.34
11
4.02
81.42
115.86
AF44
42.5407
11.6141
5231
B
12.26
2.56
5.886
2.367
1.445
0.101
43.52
33.06
10
3.52
101.90
133.80
AF46
42.5243
11.6178
4324
B
10.18
2.23
5.710
2.671
1.384
0.111
31.29
18.11
12
5.15
55.60
96.24
AF47
42.5168
11.6250
4359
B
11.34
2.48
5.623
2.644
1.381
0.106
38.11
27.12
10
3.82
83.85
117.13
AF48
42.4940
11.6398
3886
B
10.47
2.32
5.693
2.596
1.413
0.105
33.03
20.74
11
4.60
64.22
101.51
AF49
42.5016
11.6132
2276
B
11.33
2.27
5.580
2.419
1.426
0.106
38.14
25.36
11
4.90
78.33
117.20
AF50
42.5002
11.6195
2687
B
10.06
2.24
5.661
2.667
1.401
0.108
30.56
17.41
12
5.12
53.76
93.89
AF51 42.5045
11.6119
2383
B
10.59
2.04
5.605
2.451
1.530
0.111
34.08
18.12
13
6.87
56.93
104.26
AF53
42.4850
11.5267
6007
B
11.97
2.48
5.528
2.411
1.338
0.110
41.79
30.79
10
3.82
95.06
128.46
AF54
42.4466
11.5574
6333
B
12.22
2.30
5.486
2.243
1.364
0.108
43.61
30.73
11
4.87
94.83
133.84
AF56
42.4376
11.5786
5266
B
11.60
2.34
5.089
2.659
1.425
0.111
39.67
27.49
11
4.52
84.57
122.19
AF57
42.4326
11.5966
4165
B
11.27
2.25
5.398
2.664
1.416
0.110
37.78
24.83
12
5.00
76.76
116.05
N)
AF58
42.4326
11.5966
4165
B
10.99
2.24
5.588
2.688
1.416
0.109
36.12
23.06
12
5.09
71.02
111.14
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o
z
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le
ID
Long
itu
de
(
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La
t
itu
de
(
°)
D
is
tance
(m
)
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t
ion
Fe&o
*
(
%)
Ta
ble
1.
(
con
t
inu
e
d)
t
o
.60.
Geochemistry
"i
'
-
Geophysics
1
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
display
melt
inclusions,
anhedral
outlines
or
cor-
roded
boundaries
(Figures
SI40A-SI40C).
[22]
Olivine
(Fo
60
_
80
;
Table
SI6)
and
clinopyrox-
ene
(Di
40
_
42
En
26
_
30
FS
6
_
9
;
Table
SI7)
are
minor
or
absent
in
most
of
the
basalt
of
the
Asal
Rift,
and
the
mean
size
of
these
minerals
is
smaller
than
those
of
plagioclase
(-0.3
to
—6
mm).
In
the
same
thin
section,
few
of
these
minerals
are
subeuhe-
dral,
but
most
of
them
are
anhedral
(Figure
SI40D),
also
showing
fractures,
and
corrosion
dis-
solution
forms
(Figures
SI4OE
and
SI4OF).
The
three
phases
can
also
presents
mineral
inclusions,
and
grains
attached
together
or
to
the
rims
of
other
mineral
phases
(Figures
SI40A
and
SI40G-SI40J).
[23]
Groundmass
(Figures
SI40K-SI4OL)
consists
of
plagioclase
laths
(An
60
-
72
;
Table
SI8),
olivine
(F053_66;
Table
SI8),
clinopyroxene
microlites
(Di32_38
En18-26
FS8-12;
Table
SI8)
and
remote
opaque
oxides
(Table
8).
As
megacrysts,
microp-
robe
and
SEM
analyses
reveal
that
the
majority
of
the
mineral
phases
have
an
equivalent
chemical
composition
from
a
sample
to
the
other,
but
these
compositions
are
slightly
different
than
those
of
the
large
size
minerals
(Tables
SI5-S17).
[24]
As
shown
previously
for
the
basalts
of
the
Asal
Rift
[Bizouard
et
al.,
1980;
Vigier
et
al.,
1999],
these
petrological
characteristics
suggest
that
the
three
phases
are
issued
from
a
complex
thermal
evolution.
The
homogeneity
of
the
chemi-
cal
composition
of
the
Bytownite
megacrysts
(An76-88;
Table
SI5)
implies
constant
high
tem-
peratures
(>1200°C)
over
a
long
time
period
[Clocchiatti
et
al.,
1978;
Bizouard
et
al.,
1980].
Petrological
observations
and
microprobe
analyses
also
reveal
that
the
megacrysts
are
clearly
out
of
equilibrium
with
the
groundmass
and
show
that
most
of
the
Asal
Rift
basalts
correspond
to
cumu-
late
rocks.
Due
to
the
large
proportion
of
bytown-
ite,
which
is
able
to
float
free
of
a
denser
mafic
melt,
the
mineral
accumulation
credibly
occurs
at
the
roof
of
the magma
chamber
[Bizouard
et
al.,
1980].
5.2.
Major
and
Trace
Element
Variations
Across
the
Asal
Rift
[25]
The
chemical
variations
of
the
major
element
across
the
Asal
Rift
for
all
basalts
younger
than
620
kyr
(that
is
9
km
from
the
rift
axis)
are
reported
in
Figure
SI41
and
Tables
SI1
and
SI2
(see
supporting
information).
A1
2
0
3
and
CaO
show
clear
trends,
increasing
from
the
rift
2939
(1.1
.6
0
Geochemistry
"i
-
Geophysics
r
I
Geosystems
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
shoulders
to
the
rift
axis.
Inverse
trends
are
observed
for
MgO
and
FeO
(Figure
SI41).
[26]
However,
we
note
a
large
chemical
variability
for
samples
within
the
rift
relative
to
those
sampled
on
the
rift
shoulders.
Asal
Rift
basalts
do
not
show
simple
relationships
in
binary
diagrams
(Figure
SI42).
For
MgO
=
6%,
A1
2
0
3
varies
from
—14
to
—22%
and
FeO
between
—6%
and
—15%.
Recent
differentiated
lava
flows
(low
MgO)
composition
are
A1
2
0
3
rich
and
thus
do
not
fit
the
classical
pet-
rological
trend.
Increasing
A1
2
0
3
,
CaO,
and
decreasing
SiO
2
,
FeO,
with
decreasing
MgO
reflect
plagioclase
accumulation.
The
presence
of
phenoc-
rysts
in
whole
rocks
drastically
modifies
the
major
chemical
compositions
of
the
samples.
Trace
ele-
ment
contents
(Tables
SI3
and
SI4)
are
also
sensi-
tive
to
the
presence
of
plagioclase
in
whole
rocks,
as
shown
by
the
positive
Eu
anomaly
of
the
Rare
Earth
Elements
(REE)
patterns
of
the
rift
shoulders
and
the
inner
floor
basalts
(Figure
3).
The
high
CaO
(-14%)
and
A1
2
0
3
(-24%)
contents
suggest
the
presence
of
cumulative
plagioclase.
Eu/Eu*
ratios
of
1-1.3
(Tables
SI1
and
SI2)
confirm
these
observations
and
indicate
that
plagioclase
accumu-
lation
occurred
in
most
of
the
samples.
Figure
SI43
shows
that
the
Sr/Sm
ratios
positively
correlate
with
A1
2
0
3
content.
The
trend
can
be
reproduced
by
the
addition
of
variable
proportions
of
plagio-
clases
(Sr
and
A1
2
0
3
rich
and
Sm
poor)
to
an
aphy-
ric
basalt
with
a
low
Sr/Sm
ratios
and
A1
2
0
3
content
(41
and
14%,
respectively).
[27]
Major
and
trace
element
composition
and
petro-
logical
analysis
reveal
that
most
of
the
basalts
of
the
Asal
Rift
are
affected
by
plagioclase
accumulation
(and
to
a
lesser
extent
by
olivine
and
clinopyroxene),
which
leads
to
uncertainty
in
the
interpretation
of
the
chemical
composition.
In
the
following
section,
we
thus
correct
the
whole-rock
chemical
composi-
tion
of
each
sample
from
the
chemical
composition
and
the
abundance
of
phenocrysts.
5.3.
Results
of
the
Mineral
Accumulation
Correction
[28]
The
chemical
variations
due
to
the
plagioclase
accumulation
are
illustrated
in
Figure
4,
where
the
relationship
between
the
plagioclase
abundance
and
Al
2
O
3
,
CaO,
FeO,
and
Si0
2
contents
is
compared
in
binary
diagrams.
This
figure
shows
a
linear
relation-
ship
between
the
plagioclase
abundance
and
major
chemical
composition
of
the
samples.
The
more
the
proportion
of
the
plagioclase
increases,
the
more
the
chemical
composition
of
the
samples
is
shifted
toward
the
chemical
composition
of
Bytownite.
Thus,
the
increase
of
A1
2
0
3
and
CaO
and
the
decrease
of
FeO
and
Si0
2
contents,
revealed
in
the
binary
diagrams,
are
clearly
explained
by
the
addi-
tion
of
plagioclase
to
magmas.
[29]
From
the
abundance
and
the
chemical
compo-
sition
of
the
mineral
phases,
we
corrected
the
whole
rock
major
element
composition for
phenocryst
accumulation
using
equation
(2)
(see
supporting
in-
formation).
The
major
chemical
composition
of
the
mineral
phases
is
based
on
the
average
of
several
core
mineral
microprobe
analyses
(Tables
SI5—S17)
realized
from
12
samples.
In
the
following,
major
elements
contents
corrected
for
mineral
accumula-
tion
are
denoted
by
asterisks
(Tables
SI9
and
SI10).
Figure
SI44
shows
a
clear
linear
relationship
between
the
corrected
whole
rock
major
element
composition
estimated
from
the
mineral
average
composition
of
individual
thin
section
(Tables
SI9
and
SI10)
and
the
total
average
composition
calcu-
lated
from
the
entire
set
of
thin
section.
This
result
was
expected
due
to
the
relative
homogeneity
of
the
chemical
composition
of
the
phenocrysts
(Tables
SI5—S17).
[3o]
In
this
study,
we
considered
that
the
surface
of
the
thin
section
was
representative
of
the
ana-
lyzed
samples.
This
hypothesis
is
opened
to
criti-
cism,
because
the
size
of
the
rock
chip
could
change
from
a
sample
to
the
other
and
because
the
structure
of
each
rock
sample
could
not
be
homog-
enous.
To
verify
our
approach,
we
compared
the
corrected
whole
rock
major
element
composition
of
six
samples
to
the
average
groundmass
compo-
sition
estimated
from
several
SEM
measurements
realized
across
the
thin
sections.
Because,
the
analyses
from
SEM
are
also
quantitative,
system-
atic
bias
exists
between
major
chemical
composi-
tion
estimated
by
this
method
and
geochemical
analyses,
as
shown
by
the
results
obtained
from
the
aphyric
sample
AF04.
Consequently,
the
aver-
age
chemical
compositions
of
the
groundmass
esti-
mated
from
SEM
were
calibrated
using
the
aphyric
sample
AF04
as
standard.
The
comparison
between
both
approaches
is
presented
in
Figure
SI45.
Even
if
FeO
and
MgO
contents
of
sample
AF31
are
slightly
scattered,
the
data
correlate
with
slopes
close
to
—1.
Despite
the
uncertainties
of
both
methods,
these
results
show
that
our
correc-
tion
of
whole-rock
composition
is
correct.
6.
Fractional
Crystallization
[31]
To
discuss
temporal
variation
in
melt
genera-
tion,
we
need
to
"see
through"
the
effects
of
2940
RIft
shoulders
-
Mort,
a
I I
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0
Inner
floor
9
b)
La
Ce
Pr
Nd
Sm
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REE
Geochemistry
Geophysics
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
10
2
0
N
0
0
(3
10
1
102
0
N
E
0
2101
Figure
3.
REE
patterns
for
the
(a)
shoulders
and
(b)
inner
floor
basalts
of
the
Asal
Rift.
Normalization
to
chondritic
val-
ues
is
from
Sun
and
McDonough
[1989].
Enriched
E-MORB,
Transitional
T-MORB,
and
Normal
N-MORB
patterns
are
from
Langmuir
et
al.
[1992].
magmatic
differentiation
and
to
determine
the
geo-
chemical
characteristics
of
the
parental
magmas.
Accordingly,
the
first
problem
is
to
identify
the
differentiation
processes.
Then,
we
can
consider
whether
differentiation
itself
could
have
generated
the
separate
geographical
groups
A
and
B
in
the
Asal
Rift
or
whether
multiple
parental
magmas
were
actually
required.
[32]
Major
elements
variability
across
the
rift
(Figure
5)
shows
new
tendencies
that
were
not
obvious
using
the
uncorrected
data
set
(Figure
SI41).
While
SiO
2
*
(SiO
2
*
is
Si0
2
corrected
for
phenocrysts
accumulation,
Tables
SI9
and
SI10)
increases
and
FeO*,
TiO
2
*
slightly
decrease
from
the
shoulders
to
the
rift
axis,
MgO*,
A1
2
0
3
*,
CaO*,
Na
2
O*,
and
K
2
0*
contents
do
not
show
clear
trends
across
the
rift.
Samples
also
display
better
defined
trends
on
MgO*
variation
diagrams
(Figure
6).
The
data
show
a
rapid
decrease
in
MgO*
accompanied
by
rapid
increases
in
FeO*,
TiO
2
*,
Na
2
O*,
Si0
2
*,
and
decreases
in
CaO*
and
A1
2
0
3
*
consistent
with
gabbroic
fractionation
involving
initial
removal
of
olivine,
followed
by
extraction
of
three-phase
assemblage
olivine-pla-
gioclase-clinopyroxene.
[33]
In
order
to
test
this
hypothesis,
we
used
the
Liquid-Line
of
Descent
(LLD)
calculation
proce-
dure
of
Weaver
and
Langmuir
[1990].
We
have
selected
the
most
MgO*
rich
sample
as
a
parental
magma
composition
(sample
AF51)
to
quantita-
tively
test
the
trends
plotted
in
Figure
6.
Fractional
crystallization
calculations
were
performed
at
1
and
4
kbar
with
water
content
ranging
from
1000
to
4000
ppm.
The
models
satisfactorily
explain
the
FeO*,
A1
2
0
3
*,
and
CaO*
content
of
the
lavas
suite,
suggesting
that
the
data
from
the
inner
floor
could
derive
from
the
crystallization
at
low
pressure
of
magmas
from
the
rift
shoulders.
However,
Si0
2
*,
TiO
2
*,
Na
2
O*,
and
K
2
0*
contents
of
the
lava
flows
do
not
follow
the
computed
LLDs.
Different
pres-
sures
of
crystallization
(1-4
kbar),
or
variable
water
content
(1000-4000
ppm),
are
not
appropri-
ate
to
explain
the
whole
data
set
from
a
single
pa-
rental
magma
(i.e.,
sample
AF51).
[34]
Because
high
MgO*
lavas
are
missing
from
the
inner
floor
Rift,
we
used
melt
inclusions
trapped
in
Bytownite
phenocrysts
from
the
Ardou-
koba
eruption
as
a
starting
parental
magma
com-
position
for
the
recent
lava
flows
[Clocchiatti
and
Massare,
1985].
These
high MgO
liquid
composi-
tions
(red
dots
in
Figure
6)
are
different
from
those
erupted
on
the
rift
shoulders
(black
dots
with
MgO*
>
8%
in
Figure
6).
Fractional
crystallization
calculations
were
performed
at
4
kbar
with
water
content
ranging
from
1000
to
4000
ppm.
The
com-
puted
LLDs
satisfactorily
explain
the
most
differen-
tiated
lavas
from
the
inner
floor
(samples
AF01—
AF06
and
AF63—AF64
for
instance).
We
note
how-
ever
that
for
a
given
MgO*,
the
modeled
K
2
0*
are
shifted
toward
lower
values.
This
apparent
discrep-
ancy
has
been
previously
noticed
by
Clocchiatti
and
Massare
[1985].
It
could
be
due
to
a
systematic
analytical
bias
on
the
measured
K
content
of
melt
inclusions
using
electron
microprobe.
[35]
To
summarize,
in
addition
to
the
changes
in
melt
composition
produced
during
crystallization
it
is
clear
from
the
data
reported
in
the
Figure
6
that
there
is
another
aspect
of
the
chemical
vari-
ability.
For
example,
the
data
at
7-9%
MgO*
show
variations
in
FeO*
contents
ranging
from
9%
to
12%,
Na
2
O*
contents
from
2%
to
3%,
and
Si0
2
*
contents
from
47%
to
49%.
Because
the
samples
are
corrected
for
phenocryst
accumulation,
the
dif-
ference
in
major
element
compositions
at
a
given
MgO*
content
can
only
reflect
changes
in
magma
generation
processes,
such
as
variation
of
pressure
and
extent
of
melting,
mantle
source
heterogene-
ities
or
contamination
and
assimilation
effects.
2941
il
-
o
.6
0
Geochemistry
"i
-
Geophysics
r
I
Geosystems
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
In
the
following,
we
will
detect,
differentiate
and
constrain
the
contribution
of
these
different
processes.
7.
Crustal
Contamination
and
Source
Heterogeneity
[36]
In
rift
continental
zones,
contamination
and
assimilation
processes
could
be
a
significant
factor
in
the
variations
of
the
chemical
composition.
At
the
scale
of
the
Afar
Depression,
previous
works
[Deniel
et
al.,
1994;
Hart
et
al.,
1989]
show
that
the
younger
differentiated
lavas,
from
the
Dalha
series
(<9
Ma)
up
to
the
youngest
series
(Asal
Rift),
do
not
isotopically
differ
from
associated
basalts,
and
were
probably
derived
from
them
by
fractional
crystallization
without
significant
crustal
contamination.
To
assess
the
role
of
contamination
at
the
scale
of
the
Asal
Rift,
we
used
Sr
and
Nd
isotopic
ratios
and
major
chemical
composition
corrected
for
phenocryst
accumulation
from
previ-
ous
studies
[Schilling
et
al.,
1992;
Deniel
et
al.,
1994;
Vigier
et
al.,
1999].
The
data
only
cover
the
southern
part
of
the
rift
(from
—530
kyr
to
the
present).
Because
the
filling
of
the
Asal
Rift
is
symmetrical
(Figure
2)
[Manighetti
et
al.,
1998
;
De
Chabalier
and
Avouac,
1994],
we
made
the
assumption
that
these
samples
are
also
representa-
tive
of
the
northern
part
of
the
rift.
[37]
If
the
crust
contaminates
the
rising
basaltic
magmas,
the
more
differentiated
rocks
should
show
the
best
evidence
for
contamination
(higher
degrees
of
differentiation
imply
longer
periods
of
residence
in
the
upper
crustal
magma
chambers).
Figure
SI46A
shows
that
87
Sr/
86
Sr
basalt
ratios
are
clustered
around
a
mean
value
of
0.70354
±
3
across
the
Asal
Rift.
These
ratios
are
similar
to
those
of
the
Asal
rhyolitic
domes,
but
lower
than
the
Ali
Adde
rhyolite,
which
provides
isotopic
ratios
of
the
local
continental
crust
[Deniel
et
al,
1994;
Hegner
and
Pallister,
1989].
In
Figures
SI46B
and
SI46C,
the
data
suite
also
do
not
show
any
distinct
trends
between
87
Sr/
86
Sr
and
fractio-
nation
indices
(Si0
2
or
Mg#).
These
results
sug-
gest
that
the
differentiation
of
these
magmas
is
not
affected
by
assimilation-fractional
crystallization
process.
The
Nd-Sr
isotopic
diagram
(Figure
SI46D)
also
confirms
the
absence
of
the
litho-
spheric
component
in
the
genesis
of
the
Asal
Rift
basalt
[Deniel
et
al.,
1994].
The
data
form
a
cluster
and
overlap
the
field
of
MORB
and
Ocean
Island
Basalts.
This
mantle
source
affinity
is
also
revealed
from
our
data
set
and
those
of
Deniel
et
al.
[1994]
from
incompatible
trace
element
ratios
(Figure
SI46E).
A
recent
study
[Rooney
et
al.,
2012a]
suggest
that
Hf-Pb
data
from
the
Asal
Rift
have
affinity
to,
and
overlap
with,
the
East
Sheba
Ridge
(Aden
Ridge)
data
and
has
also
Indian
Ocean-like
Hf
and
Pb
isotope
signatures
[Rooney
et
al.,
2012a].
However
it
has
been
also
shown
that
these
lava
flows
exhibit
values
consist-
ent
with
mixing
between
Afar
plume
and
regional
lithospheric
mantle
[Schilling
et
al.,
1992
;
Rooney
et
al.,
2012a;
Rooney
et
al.,
2013]].
In
the
Afar
depression,
the
isotopic
signature
of
a
mantle
plume
is
most
pronounced
toward
Djibouti
[Schil-
ling
et
al.,
1992;
Rooney
et
al.,
2012a],
consistent
with
maximum
temperature
values
recorded
in
this
area
[Rooney
et
al.,
2012b].
The
lava
flows
from
the
Afar
depression
show
that,
with
decreas-
ing
age,
the
isotopic
properties
of
the
basalts
express
a
more
depleted
composition.
This
is
inter-
preted
simply
as
an
increased
contribution from
the
depleted
upper
mantle
and
a
lessening
of
crustal
assimilation
[Hart
et
al.,
1989].
A
similar
pattern
is
observed
in
Djibouti
where
early
vol-
canic
products
(>10
Ma)
exhibit
substantial
litho-
spheric
contributions,
but
which
become
insignificant
as
rifting
and
lithospheric
thinning
progress,
replaced
by
an
increasing
fraction
of
melt
derived
from
depleted
upper
mantle
and
the
Afar
plume
[Deniel
et
al.,
1994].
Due
to
the
small
studied
area,
it
is
probable
that
the
compositional
heterogeneity
beneath
the
Asal
Rift
is
insignificant
and
thus
do
not
impact
the
geneses
of
Basalts.
[38]
Together,
these
results
show
that
the
major
element
variations
at
7-9%
MgO*,
which
are
observed
in
the
binary
diagrams
(Figure
6),
cannot
be
assigned
to
mantle
source
heterogeneities,
con-
tamination
or
assimilation
effects.
These
important
variations
at
a
given
MgO*
can
be
a
consequence
of
pressure-release
melting
beneath
the
Asal
Rift.
To
isolate
the
effects
of
mantle
temperature,
depth
and
extent
of
melting,
it
is
first
necessary
to
cor-
rect
the
variations
caused
by
fractional
crystallization.
8.
Correction
for
Low
Pressure
Fractionation
[39]
To
correct
the
variations
caused
by
fractional
crystallization
[Klein
and
Langmuir,
1987],
FeO*
and
Na
2
O*
contents
have
been
extrapolated
along
the
LLD
olivine-plagioclase-clinopyroxene
slope
to
8%
MgO*,
using
the
algorithms
of
Klein
and
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.60.
Geochemistry
-
Geophysics
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
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2'
0"
40.
0.
0
I
.
0
.
a
.
I
..
50
-
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m
0
,,
49
-
.8.
0
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3
97.
8
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M
0
6
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2
2
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33
10
31
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47
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I
0
4
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0
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4
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I
O
le
C
'.
4
.
4
!O
10
'
0
AC
2
4
6
8
MgO
(wt
%)
Figure
4.
Illustration
of
the
chemical
variations
due
to
the
plagioclase.
The
relationship
between
the
plagioclase
abundance
and
A1
2
0
3
,
CaO,
FeO,
and
SiO
2
contents
(noncorrected)
is
compared
to
the
chemical
composition
plotted
in
bi-
nary
diagrams.
The
linear
relationship
between
the
plagioclase
abundance
and
major
chemical
composition
of
the
samples
show
that
the
more
the
proportion
of
the
plagioclase
increases,
the
more
the
chemical
composi-
tion
of
the
samples
is
shifted
toward
the
chemical
composition
of
Bytownite.
In
the
binary
diagrams,
the
increase
of
A1
2
0
3
and
CaO
and
the
decrease
of
FeO
and
SiO
2
contents
are
clearly
explained
as
a
dilution
effect
caused
by
addition
of
plagioclase
accumulation.
O
O
0
0
u_
cn
46
10
0
50
100
Plagioclases
(%)
2943
Ages
(ky)
Ages
(ky)
345
0
345
620
345
0
345
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,
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1.5
Geochemistry
"i
-
Geophysics
Geosystems
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
Rift
shoulders
0
Inner
floor
Distance
(km)
Distance
(km
Figure
5.
Major
element
composition
of
the
samples
corrected
for
mineral
accumulation
(*)
across
the
Asal
Rift.
Langmuir
[1987],
Klein
and
Langmuir
[1989],
and
[ao]
Samples
from
the
rift
shoulders
(group
Langmuir
et
al.
[1992]
(Table
Sill),
whose
slopes
B)
are
characterized
by
Na
8
.
0
*
=
2.29
±
0.06%,
are
equivalent
to
the
LLD
mean
slopes
in
Figure
6.
Fe
8.0
*
=
10.75
±
0.35%,
SiO
2
*
=
47.70
±
0.27%,
2944
#1,4
1
12
ffi
_7
7
4cr
-
2.
41C
o'
54
63
BO
4
014
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75
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63
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8.
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ST
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as
63
6.
77
0
ill
--;
Geochemistry
.
3
-
Geophysics
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
53
54
69
5
70
72
3.
73
6
525
it_ot
sb
s.
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75
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10
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3
-1
2.5
2
0
1.5
62
Si
.
-6
4
6
8
M
g
O*
(
wt
%)
4
6
8
10
MgO*
(wt
%)
Rift
shoulders
„-)
Inner
floor
Clocchiatti
and
Massare,
(1985)
LLD:
1
kbar
4
kbar
H2O
1000
to
4000
ppm
10
52
50
48
46
15
14
3.5
5
3
O
z
2.5
2
0.6
°
0.5
'6
0.4
0.3
0.2
Figure
6.
MgO
variation
diagrams
for
the
samples
of
the
Asal
Rift
corrected
for
mineral
accumulation
(asterisks).
LLD
are
cal-
culated
using
the
procedure
of
Weaver
and
Langmuir
[1990].
Fractional
crystallization
calculations
were
per-
formed
at
1
kbar
and
4
kbar
and
water
content
ranging
from
1000
(full
line)
to
4000
ppm
(dashed
line).
Na*/Ti*
=2.54
±
0.12
(large
black
dot
in
Figure
SI47,
Tables
1,
SI9,
and
SI10
in
supporting
infor-
mation).
Samples
from
the
inner
floor
(group
A)
are
more
homogeneous
and
characterized
by
lower
Fe
8.0
*,
higher
Si0
2
*
and
Na*/Ti*
values
(9.41
±
0.21%, 48.95
±
0.35%
and
3.16
±
0.20
respectively;
large
white
dot
in
Figure
SI47
in
supporting
information
and
Table
1).
Equivalent
values
to
those
estimated
from
the
whole
data
set
can
be
obtained
only
using
the
aphyric
samples
(Table
SI1
and
SI2)
of
each
group.
For
the
inner
floor
samples
(Group
A),
Fe
8.0
*
=
9.16
±
0.30%,
Na
8
.
0
*
=
2.47
±
0.07%,
Si0
2
*
=
49.81
±
0.30%,
Na*/Ti*
=
2.86
±
0.17,
and
for
the
2945
5
0
5
Distance
(km)
9
5
Dista
n
ce
(km)
5
9
Ages
(ky)
Ages
(ky)
345
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345
620
345
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0
Geochemistry
"i
-
Geophysics
r
I
Geosystems
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
shoulder
samples
(Group
B),
Fe
8.0
*
=
10.73
±
0.45%,
Na
8
.
0
*
=
2.31
±
0.08%,
SiO
2
*
=
47.67
±
0.26%,
Na*/Ti*
=
2.52
±
0.13.
This
comparison
suggests
that
our
correction
of
whole-rock
composi-
tion
is
correct.
We
also
note
that
similar
results
can
be
calculated
using
the
LLD
calculation
of
Danyushevsky
and
Plechov
[2011]
or
simple
linear
regressions
trough
the
data
(Figures
S148
and
S149).
[41]
Figure
7
shows
variations
of
Fe
8.0
*
and
Na
8.0
*
across
the
Asal
Rift.
While
Na
8.0
*
values
do
not
show
a
clear
trend
across
the
rift
(Na
8.0
*
=
2.35
±
0.06),
Fe
8.0
*
decreases
from
the
rift
shoulders
to
the
rift
axis.
Since
Fe
8.0
*
largely
varies
in
proportion
with
initial
pressure
of
melting,
P
o
[e.g.,
Langmuir
et
al.,
1992,
and
references
therein],
the
data
suggest
that
the
pressure
of
melting
substantially
decreases
during
the
last
620
kyr
beneath
the
Asal
Rift.
[42]
Thus,
in
the
following
section,
we
use
a
model
to
quantify
the
intensive
parameters
of
the
melting
process
(mantle
temperature
and
pressure
of
melting)
and
test
different
mantle
flow
models
(active
flow
and
thick
lithosphere).
Figure
7.
(a)
Fe
u)
*
decreases
from
the
shoulders
to
the
axis
of
the
Asal
Rift,
suggesting
a
decrease
of
initial
melting
pressure
and
thus
a
decrease
of
the
depths
of
melting,
since
620
kyr.
(b)
Na
8
.
0
*
do
not
show
a
clear
trend
across
the
rift
(Na
8.0
=
2.30
±
0.04%),
suggesting
that
the
extent
of
melting
is
rather
constant
since
620
kyr.
(c-f)
Variations
of
Zr/Y,
Na*/Ti*,
(Dy/Yb)
N
,
and
Lu/Hf
rations
across
the
Asal.
These
variations
are
qualitatively
consistent
with
shallow
melting
beneath
the
rift
axis
and
deeper
melting
for
off-axis
lava
flows.
2946
l
iv
.60.
Geochemistry
"i
'
-
Geophysics
1
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
9.
Mantle
Melting
Models
[43]
We
use
the
model
developed
by
Langmuir
et
al.
[1992]
to
quantify
the
pressure
and
degree
of
mantle
melting.
This
model
describes
composi-
tional
changes
(MgO,
FeO,
Na
2
O,
SiO
2
,
TiO
2
,
and
K
2
0)
in
mantle
melts
during
adiabatic
decompres-
sion,
assuming
that
olivine-melt
equilibrium
and
trace
element
behavior
for
Na.
The
model
is
based
on
the
partition
coefficients
(IQ)
for
Mg
and
Fe
in
olivine.
The
MgO
and
FeO
concentrations
in
peri-
dotite
melts
are
imposed
by
olivine
saturation
and
can
be
calculated
from
K
d
expressions
(which
are
themselves
a
function
of
P,
T
and
alkali
content).
Na
2
O
is
calculated
from
its
K
d
in
clinopyroxene
(which
is
also
P
and
T
dependent).
Other
inputs
to
the
model
include
the
initial
composition
of
the
mantle, the
mantle
solidus,
and
the
relationship
between
T
and
F
(%
of
melting)
during
isobaric
and
adiabatic
melting.
[44]
In
our
model,
we
use
a
normal
peridotite
com-
position with
15%
clinopyroxene
(MgO
=
38.14
%,
FeO
=
8.30%,
Na
2
O
=
0.26%,
K
2
0
=
0.007%,
and
TiO
2
=
0.13%).
After
the
mantle
rises
above
its
solidus,
P
o
melt
is
extracted
instantaneously
from
the
residue
up
to
the
depth
where
upwelling
and
melting
stop
(P
f
).
Fractional
melting
paths
generated
by
the
model,
which
result
in
melt
com-
positions
between
batch
and
fractional
melts
[Langmuir
et
al.,
1992],
are
showed
in
Figure
SI47
for
Na
2
O*
and
FeO*
corrected
to
8%
MgO*
con-
tent
(see
supporting
information).
Each
curve
rep-
resents
a
different
pressure
of
intersection
with
the
mantle
solidus
P
o
from
20
to
45
kbar.
P
o
will
increase
with
increasing
mantle
potential
tempera-
ture
[McKenzie
and
Bickle,
1988].
As
the
mantle
ascends
above
the
solidus,
the
total
melt
fraction
F
increases
until
the
pressure
where
the
mantle
ceases
to
ascend
adiabatically
(P
f
)
and
therefore
ceases
to
melt.
The
total
melting
column
length
P
o
—P
f
thus
determines
F.
Increasing
F
has
the
main
effect
of
lowering
Na
2
O
in
the
melt,
as
Na
2
O
behaves
as
an
incompatible
element
that
is
diluted
by
further
increments
of
melting.
FeO
varies
largely
as
a
function
of
P
o
,
with
relatively
small
variations
as
a
function
of
F.
The
increase
in
FeO
with
increasing
pressure
is
largely
due
to
the
effect
of
temperature
on
olivine
IQ
and
the
dominating
effect
of
increasing
temperature
as
pressure
increases
along
the
mantle
solidus
[Langmuir
et
al.,
1992].
Thus
the
Na
and
Fe
contents
of
man-
tle
melts
provide
excellent
constraints
on
the
final
depth
of
melting
(from
Na
2
O
which
reflects
F
and
therefore
P
o
—P
f
)
and
on
the
initial
depth
of
melt-
ing
(from
Fe
8
.
0
).
[45]
However,
trace
element
patterns
(Figure
5)
and
isotopic
compositions
[Rooney
et
al.,
2012a;
Roo-
ney
et
al.,
2013]
show
that
the
Asal
Rift
lava
derived
from
a
fertile
source.
Because
the
fertile
mantle
starts
melting
deeper
and
melts
more,
the
calculated
final
pressures
of
melting
slightly
decrease,
and
the
initial
pressures
and
crust
thick-
nesses
slightly
increase
compared
to
a
normal
man-
tle
composition.
Using
a
fertile
source
with
Na
2
O
=
0.28%
and
K
2
0
=
0.011%,
the
calculated
crustal
thickness
beneath
the
Asal
Rift
is
about
5
km
(Table
2).
This
result
is
more
consistent
with
the
values
estimated
from
geophysical
measure-
ment
(Table
2),
than
the
one
estimated from
a
nor-
mal
mantle
composition
(Na
2
O
=
0.26%
and
K
2
0
=
0.007%,
Table
2).
In
the
following,
we
will
use
a
fertile
mantle
composition
in
the
calculations.
9.1.
Results
of
the
Major
Elements
Forward
Modeling
[46]
The
results
of
the
inversions
are
presented
in
Table
1.
The
initial
and
final
pressures
of
melting
decrease
regularly
from
the
oldest
to
the
most
recent
lava
flows.
The
modeling
results
in
Figure
8
show
that
the
initial
pressures
of
melting
are
rang-
ing
from
20
to
43
kbar
and
the
final
pressures
of
melting
from
5
to
33
kbar.
If
the
melting
ceases
when
the
upwelling
mantle
reaches
the
bottom
of
the
lithosphere,
the
final
pressure
of
melting
can
be
converted
to
lithospheric
thickness
using
the
procedure
of
Klein
and
Langmuir
[1987]
and
Langmuir
et
al.
[1992].
Taking
into
account
the
average
thicknesses
estimated
beneath
the
rift
shoulders
and
the
inner
floor
(Table
2),
the
litho-
spheric
thickness
decreases
from
67
±
8
to
43
±
5
km
in
620
kyr,
corresponding
to
a
lithospheric
thinning
rate
of
about
4.0
±
2.0
cm
yr
-1
,
which
is
consistent
with
the
long-term
spreading
rate
of
the
Asal
Rift
(2.9
±
0.2
cm
yr
-1
,
Figure
SI39).
As
the
average
crustal
thickness
(4.95
±
0.16
km),
these
lithospheric
thicknesses
are
consistent
with
those
estimated
from
geophysical
measurement
in
the
area
(see
section
2).
Beneath
the
inner
floor
(group
A,
white
arrows),
melting
paths
are
shallow
(from
81
±
4
to
43
±
5
km)
and
are
consistent
with
adia-
batic
melting
in
normal
temperature
asthenosphere
(1400°C),
beneath
an
extensively
thinned
mantle
lithosphere (Figure
8).
On
the
contrary,
melting
on
the
rift
shoulders
(107
±
7
to
67
±
8
km)
occurred
beneath
thicker
lithosphere
(group
B,
black
arrows),
requiring
a
mantle
solidus
temperature
100
±
40°C
hotter,
which
corresponds
to
a
rate
of
mantle
cooling
of
about
5
x
10
-4
°C/yr.
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Av
er
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.60
Geochemistry
-
Geophysics
r
I
Geosystems
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
[47]
We
also
note
a
good
relationship
between
the
SiO
2
*
contents
of
lavas
and
their
morphologies.
While
the
massive
and
slightly
fractured
basaltic
flows
of
the
rift
shoulders
have
low
SiO
2
*
con-
es
the
viscous
lava
flows
of
the
inner
floor,
which
present
more
fissures
and
cracks,
display
higher
SiO
2
*
values.
Equivalent
values
to
those
estimated
from
the
whole
data
set
are
obtained
only
using
the
aphyric
samples
of
each
group.
vo
For
the
inner
floor
samples
(Group
A),
P
o
=
25.60
±
1.91
kbar,
Pf
=
13.84
±
1.90
kbar,
F
me
.,11.00
±
0.50%,
D
e
=
4.19
±
0.37
km,
D
1
=
43.12
±
O
5.77
km,
D
m
=
78.65
±
5.87
km,
and
for
the
shoulder
samples
(Group
B),
P
o
=
34.67
±
2.62
kbar,
P f =
21.80
±
3.30
kbar,
F
mean
=
11.00
±
0.50%,
D
e
=
4.95
±
0.59
km,
D
1
=
67.45
±
10.12
km,
D
m
=
106.51
±
8.08
km.
Once
more,
this
comparison
shows
that
our
correction
of
whole-
°
rock
composition
is
correct.
[48]
At
face
value,
FeO*
and
SiO
2
*
contents
vary
in
inverse
proportion
across
the
Asal
Rift
(Figure
4).
This
variation
is
precisely
expected
for
mantle
melts
derived
from
different
melting
depths,
where
da"
higher
pressure
melts
will
have
higher
FeO
but
lower
SiO
2
[Niu
and
Batiza,
1991;
Langmuir
et
al.,
1992;
Hirose
and
Kushiro,
1993].
Using
polyno-
mial
equation
from
Wang
et
al.
[2002],
which
p
the
relationship
between
Si0
2
concentra-
..
tion
of
peridotite
melts
and
melting
pressure
from
laboratory
experiments
[Hirose
and
Kushiro,
1993;
Robinson
and
Wood,
1998;
Baker
and
Stolper,
1994;
Walter,
1998],
we
calculated
melting
pres-
sure
for
the
Asal
Rift
samples
from
the
SiO
2
*
con-
tents.
The
comparison
of
melting
pressures
(P)
6*4
calculated
from
SiO
2
*
and
from
Fe
8
.
0
*
and
Na8.0
*
of
Asal
Rift
basalts
is
presented
in
Figure
SI50.
Even
if
a
low
pressure
offset
exists
between
the
two
methods
(underestimation
of
P
based
on
Si
due
to
the
assumption
of
a
single
P
of
equilibration
of
polybaric
melts,
Wang
et
al.
[2002]),
the
average
melting
pressures
calculated
from
Si0
2
*
content
are
reasonably
well
in
agreement
with
the
ranges
calcu-
lated
from
Fe
8.0
*
end
Na
8.0
*
(Table
1).
As
the
Si0
2
1
method
is
independent
and
involves
minimal
treat-
ment
of
the
data
or
assumptions,
this
results
gives
supplementary
support
to
the
pressures
sensitive
calculated
here.
At
last,
we
also
note
that
the
mean
melting
pressures
(P)
calculated
from
Fe
8.0
*
and
Na
8.0
*
are
similar
to
the
predict
pressures
estimated
from
the
thermobarometers
for
mafic
magmas
of
fa4
Lee
et
al.
[2009]
(Figure
SI51).
:E
[49]
In
the
following,
we
model
sensitive
pressure
gL
1
7
P.
trace
element
partitioning
in
order
to
test
the
major
7)
.
element
constraints
on
melting
history
in
this
area.
2948
kY-
1f219
/
-41\1
1
483.k
326
45
;fr
3.26.15y,"
-
220
ky
is
it
e
.60.
Geochemistry
'
-
Geophysics
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
Ildn&adl
7?
-
10
15
20
25
30
35
40
45
20
40
60
80
100
120
140
1111•01•111111911§1
Garnet
600
[%51Eo
ZZoinoceo
700
°
8
6
4
2
0
2
4
Distance
from
the
Asal
Rift
axis
(km)
;
Basaltic
crust
Lithosphere
Spinel
1400
°
C
1500
°
C
Garnet
1712dtar.ell
30nnOtb
6
8
Figure
8.
Melting
regime
across
the
Asal
Rift.
The
small
double
white
arrows
illustrate
the
crustal
thickness
beneath
the
Asal
Rift
(4.95
±
0.16
km).
The
large
black
and
white
arrows
represent
the
melting
columns
calculated
from
each
samples,
based
on
the
Fe
u)
and
Na
b0
values
(Table
1).
The
bottom
of
the
arrow
marks
the
onset
of
melting
at
the
solidus
and
is
a
function
of
mantle
temperature.
The
depths
of
melting
were
calculated
from
the
procedure
of
Klein
and
Langmuir
[1987]
and
Langmuir
et
al.
[1992].
The
model
outputs
show
that
beneath
the
rift
axis,
melting
paths
are
shallow,
from
81
±
4
to
43
±
5
km.
These
melting
paths
are
consistent
with
adiabatic
melt-
ing
in
normal
temperature
fertile
asthenosphere
(about
1400°C),
beneath
an
extensively
thinned
mantle
litho-
sphere.
On
the
contrary,
melting
on
the
rift
shoulders
(107.7
to
67.8
km)
occurred
beneath
a
thick
mantle
lithosphere
and
required
mantle
solidus
temperature
100
±
40°C
hotter.
The
calculated
rate
of
lithospheric
thinning
is
4.0
±
2.0
cm
yr
-1
.
The
height
of
the
Aden
Ridge
melting
column
is
calculated
from
the
samples
of
Cann
[1970]
and
O'Reilly
et
al.,
[1993].
10.
Constraints
From
the
REE
and
HFSE
on
Melting
Depth
[so]
In
basalts,
REE,
High
Field
Strength
Elements
(HFSE),
Na
and
Ti
are
commonly
used
to
constrain
the
mantle
melting
depths
[Frain
et
al.,
1998;
Putirka,
1999;
Wang
et
al.,
2002].
Heavy
REE
and
HFSE
favor
garnet
structure,
because
their
partition
coefficients
change
during
melting
of
spinel
versus
garnet
peridotites.
For
a
given
melting
column,
melts
in
equilibrium
with
garnet
will
produce
high
Zr/Y,
(Dy/Yb)
N
ratios
and
low
Lu/Hf.
These
ratios
will
be
gradually
reversed
as
the
melting
column
enters
in
the
spinel
filed.
Because
the
KN
a
of
cpx/melt
increases
with
increased
P,
while
clinopyroxene
and
garnet
K
T;
min/melt
remain
constant
or
decrease,
Na/Ti
ranges
in
the
same
way
as
Lu/Hf
ratio
[Putirka,
1999].
Figure
7
shows
the
variations
of
Zr/Y,
(Dy/
Yb)
N
,
Lu/Hf,
and
Na*/Ti*
ratios
across
Asal
Rift.
While
Zr/Y
and
(Dy/Yb)
N
increase
with
Fe
8
.
0
*
from
the
rift
axis
toward
the
rift
shoulders,
Lu/Hf
and
Na*/Ti*
ratio
decrease,
which
is
consistent
with
the
depth
variations
of
pressure
of
melting.
To
test
this
hypothesis
more
quantitatively,
we
use
the
pressures
of
melting
(P
o
and
P
f
)
calculated
from
Fe
8.0
*
and
Na
8.0
*
contents
to
predict
what
Lu/Hf
ratio
of
the
basalt
should
be.
[si]
Our
procedure
is
based
on
Fram
et
al.
[1998]
and
Wang
et
al.
[2002].
We
observe
that
in
Asal
basalts,
Fe
8.0
*
scales
linearly
with
both
P
o
and
Pf
(see
Table
1).
The
relationships
between
Fe
8.0
*
2949
(1
.
1
.6
0
Geochemistry
-
Geophysics
r
I
Geosystems
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
10.1002/ggge.20187
and
P
o
,
and
between
Fe
8.0
*
and
P
f
are
expected
because
Na
8.0
*
are
fairly
constant
across
the
rift
and
thus
the
interval
P
o
—P
f
is
also
constant
(13
±
0.5
kbar).
We
used
these
linear
relationships
to
calculate
Lu/Hf
fractionation
in
the
basalts.
As
for
the
major
element
model,
we
compute
poly-
baric
melting
along
an
adiabat
in
a
single
mantle
melting
column,
using
P
o
and
P
f
as
calculated
from
FeO*
and
Na
2
O*,
and
a
linear
melt
produc-
tivity
of
1.2%
per
kilobar
of
pressure
decrease
above
the
solidus.
We
start
with
a
mantle
mineral
mode
consistent
with
a
fertile
lherzolite
(ol
:
0.55
;
opx:
0.25;
cpx:
0.15;
gt:
0.05).
The
primitive
mantle
starting
composition
and
chondrite
normal-
ization
for
Lu
and
Hf
are
from
Sun
and
McDo-
nough
[1989],
and
partitioning
coefficient
mineral
are
from
Gibson
and
Geist
[2010]
and
McKenzie
and
0
'Nions
[1991]
(Table
SI12).
[52]
Based
on
Robinson
and
Wood
[1998],
Kinzler
and
Grove
[1992],
Falloon
and
Green
[1988],
and
Takahashi
and
Kushiro
[1983],
we
consider
that
garnet
to
spinel
transition
occurs
between
30
and
20
kbar
(Table
SI13).
These
ranges
of
pressure
are
coherent
with
the
experimental
data
realized
by
Klemme
[2004]
and
Klemme
and
O'Neill
[2000].
In
between
each
increment
of
melting,
the
mantle
composition
is
revised
to
take
into
account
deple-
tion
due
to
melting
and
pressure
dependent
phase
transitions.
Mantle
composition
is
monitored
by
modal
proportions
of
phases
(Table
SI14).
The
transitions
from
garnet
to
spinel
occur
over
inter-
vals
rather
than
at
discrete
pressures.
This
approxi-
mates
the
natural
situation
where
the
phase
transitions
are
gradual
due
to
solid
solution
in
all
of
the
phases
[Frain
et
al.,
1998].
[53]
Results
of
the
melting
model
are
show
in
Figure
9.
The
curve
is
constrained
by
the
data,
reproducing
high
Lu/Hf
at
low
Fe
8.0
*
and
low
Lu/
Hf
at
high
Fe
8.0
*.
For
high
Fe
8.0
*
values,
Lu
is
held
in
the
garnet,
so
Lu/Hf
remains
low.
As
melt-
ing
continues,
Fe
8.0
*
decreases,
and
garnet
is
ex-
hausted
or
melting
crosses
into
the
spinel
field,
Lu
is
no
longer
retained
in
the
residue,
and
Lu/Hf
ratios
rise.
The
data
are
well
fit
by
the
curve,
which
shows
that
the
older
lava
flows
were
respec-
tively
generated
in
the
garnet
field,
and
the
major-
ity
of
the
recent
lava
flows
were
produced
in
the
garnet-spinel
transition
zone.
The
model
reprodu-
ces
the
range
of
the
Lu/Hf
ratio
and
also
validates
the
melting
pressures
calculated
from
major
ele-
ment
melting
model.
11.
Conclusions
and
Discussions
[54]
Because
it
has
been
shown
in
the
literature
that
the
recent
basalts
(<1
Ma)
from
the
Asal
Rift
"
o„
C.4
p
46
s9
57
5
7-3
T6
46
a
65
49
8
9
10
11
12
Fe
8
.
0
*
Figure
9.
(a)
Lu/Hf
versus
Feu)
for
or
calculated
mantle
melts
illustrating
the
effects
of
mantle
composition,
and
melt-
ing
systematics
on
melt
compositions.
The
data
are
well
fit
by
the
curve,
which
shows
that
the
majority
of
older
lava
and
the
most
recent
lava
were,
respectively,
generated
in
the
garnet
and
spinel
field.
The
model
reproduces
the
range
of
the
Lu/Hf
ratio
and
also
validates
the
melting
pressures
calculated
from
element
melting
model.
cannot
be
assigned
to
mantle
source
heterogene-
ities,
contamination
or
assimilation
effects
[Hart
et
al.,
1989;
Barrat
et
al.,
1990;
Schilling
et
al.,
1992;
Deniel
et
al.,
1994;
Vigier
et
al.,
1999;
Rooney
et
al.,
2012],
the
chemical
composition
of
quaternary
lavas
can
be
confidently
used
to
con-
strain
the
temporal
evolution
of
the
rifting
proc-
esses
over
the
last
620
kyr.
(1)
The
major
element
composition
of
these
lavas
shows
significant
variations.
These
inferred
differences
in
basalt
chemistry
are
due
princi-
pally
to
variable
proportions
of
minerals
in
the
whole-rocks,
which
leads
to
uncertainty
in
the
interpretation
of
the
major
and
trace
elements.
In
the
porphyric
samples
(that
is
—54%
of
the
data
set),
the
mineral
assemblages
can
be
di-
vided
into
two
groups
according
to
their
morphology
and
chemical
composition.
Mega-
crysts
of
plagioclase
(An
_76_88),
olivine
(Fo
60
_
80
)
and
clinopyroxene
(Di40_42
E/126_30
Fs6_9)
can
be
easily
identified
by
their
important
size
(up
to
—1
cm)
and
their
characteristic
resorption
figures
(rounded
shapes,
corrosion
embayments).
The
groudmass
is
characterized
by
equilibrium
tex-
tures,
euhedral
phenocrysts
of
smaller
size
and
more
evolved
compositions
than
those
of
the
first
group.
The
olivine
composition
ranges
from
Fo
53
to
Fo
66
,
the
plagioclase
laths
lie
between
An
60
and
An
72
and
clinopyroxene
microlites
are
Di32-38
En18-26
FS8-12•
(2)
In
order
to
correct
the
whole
rock
chemistry
for
megacrysts
accumulation,
we
used
a
sim-
ple
mass
balance
calculation.
The
corrected
0.16
0.14
—I
0.12
0
.
1
2950
l
o
co
Geochemistry
.
3
'
-
Geophysics
r
PINZUTI
ET
AL.:
MELT
GENERATION
BENEATH
THE
ASAL
RIFT
Geosystems
10.1002/ggge.20187
compositions
correlate
with
the
mean
ground-
mass
compositions
determined
using
Scanning
Electron
Microscopy.
The
major
elements
cor-
rected
for
mineral
accumulation
and
the
aphyric
samples
do
not
define
a
single
trend
in
binary
diagrams.
The
difference
in
major
element
at
a
given
MgO*
content
can
only
reflect
changes
in
melting
processes.
Trace
element
ratios
(Lu/Hf,
Zr/Y,
and
Dy/Yb
N
)
and
major
element
composi-
tions
corrected
for
mineral
accumulation
and
crystallization
(Na
8.0
*,
Fe
8.0
*,
Si0
2
*,
and
Na*/
Ti*)
show
a
symmetric
pattern
relative
to
the
rift
axis.
The
results
obtained
from
the
major
element
inversion
are
coherent
with
seismic
refraction
and
seismological
data.
For
a
fertile
mantle
composition
(Na
2
O
=
0.28%,
K
2
0
=
0.011%),
upwelling
model
combined
with
variable
ini-
tial
and
final
pressures
of
melting
explain
the
observed
Na
8.0
*-Fe
8.0
*
values.
The
final
pres-
sures
of
melting
(P
f
)
are
thus
interpreted
as
a
"petrologically
constrained
lithospheric
thick-
ness"
where
mantle
melting
stopped.
(4)
The
resulting
model
outputs
show
that
beneath
the
rift
axis,
melting
paths
are
shallow,
from
81
±
4
to
43
±
5
km.
These
melting
paths
are
consistent
with
adiabatic
melting
in
normal-
temperature
fertile
asthenosphere,
beneath
an
extensively
thinned
mantle
lithosphere.
On
the
contrary,
melting
on
the
rift
shoulders
occurred
beneath
a
thick
mantle
lithosphere
and
required
mantle
solidus
temperature
100
±
40°
C
hotter
(melting
paths
from
107
±
7
to
67
±
8
km),
which
corresponds
to
a
rate
of
mantle
cooling
of
about
5
x
10
-4
°C/yr.
Our
data
strengthen
the
recent
results
of
Rooney
et
al.
[2012b],
which
show
that
elevated
mantle
temperatures
are
pervasive
feature
of
the
upper
mantle
beneath
the
East Africa.
They
noted
a
maxi-
mum
temperature
anomaly
of
140°C
above
ambient
mantle
recorded
from
magmas
young-
ers
than
10
Myr
erupted
in
Djibouti.
The
modeled
Lu/Hf
ratios
coincide
with
the
observed
ratios
suggesting
that
the
major
element
melting
model
is
correct
and
that
garnet
is
required
to
explain
the
rift
shoulders
basalts
chemistry.
Finally,
the
calculated
rate
of
litho-
spheric
thinning
estimated
from
these
results
is
about
4.0
±
2.0
cm
yr
-1
,
which
is
consistent
with
the
long-term
spreading
rate
of
the
Asal
Rift.
Acknowledgments
[55]
We
thank
Pays
de
la
Loire
Region (France)
for
funding
this
project.
We
are
grateful
to
the
Arta
Observatory
Staff
and
to
the
CERD
researchers
for
their
constant
scientific
help
and
collaboration.
We
thank
Cecile
Doubre
who
greatly
partici-
pated
to
the
data
acquisition
in
the
Asal
Rift,
Antoine
Bezos
for
fruitful
discussions
and
for
provide
us
a
modified
version
of
the
Liquid-Line
of
Descent
calculation
procedure
of
Weaver
and
Langmuir
[1990],
and
Patrick
Launeau
for
pro-
vide
us
the
image2003
software.
We
also
thank
the
French
Army
in
Djibouti,
which
allowed
us
to
work
in
the
best
condi-
tions.
Tyrone
Rooney
and
an
anonymous
reviewer
provided
detailed
and
helpful
reviews,
which
enhanced
the
manuscript.
Finally,
we
thank
Cin-Ty
Lee
for
careful
editorial
handling.
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RIFT
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