Journal of Volcanology and Geothermal Research 141 (2005) 109 – 122
www.elsevier.com/locate/jvolgeores
Segregation vesicles, cylinders, and sheets in vapor-differentiated
pillow lavas: Examples from Tore-Madeira Rise and
Chile Triple Junction
Renaud Merlea,*, Martial Caroff b, Jacques Girardeaua,
Joseph Cottenb, Christèle Guivela
a
UMR 6112, Laboratoire de Planétologie et Géodynamique, UFR des Sciences et Techniques, Université de Nantes, 2 rue de la Houssinière,
44322 Nantes cedex 3, France
b
UMR 6538 bDomaines OcéaniquesQ, Université de Bretagne Occidentale, 6 avenue Le Gorgeu, C.S. 93837, 29238 Brest cedex 3, France
Received 24 February 2004; accepted 1 September 2004
Abstract
We conducted a detailed field and laboratory study of internal segregation structures of two hand-size pillow lavas samples.
They were dredged, respectively, on the Josephine seamount, Tore-Madeira Rise (TMR), and on a small quaternary volcanic
edifice located on the continental edge of the trench close to the Chile Triple Junction (CTJ). Both pillows display a
combination of four types of segregation structures (spherical vesicles, pipe vesicles, vesicle cylinders, and vesicle sheets)
observed so far only within subaerial basalt flows typically 2–10 m thick. In particular, the samples offer a remarkable exposure
of the transition between pipe vesicles and cylinders. We show that the vesicle sheets are not generated by the same mechanism
in both occurrences; they do not seem to be connected to cylinders in the CTJ pillow as they are in the TMR pillow. The two
pillows are geochemically distinct, the TMR being alkaline and the CTJ calc–alkaline. Two types of internal differentiation are
proposed. The first one implies the extraction of the residual liquid from the host lava and transport towards the segregation
structures, whereas the other one results from in situ crystallization within one given structure. In the latter case, glass
composition is highly dependant on the nature of the neighbouring crystallizing minerals. The degree of crystallization required
to produce a crystal framework strong enough for generating the segregation structures seems to be lower in pillows (ca. 25%
crystallization) than in vapor-differentiated basaltic lava flows (35% crystallization).
D 2004 Elsevier B.V. All rights reserved.
Keywords: vapor differentiation; segregation vesicles; vesicle cylinders; vesicle sheets; crystal framework
* Corresponding author. Present address: Laboratoire de Géochronologie, Géosciences Azur, UMR 6526, Parc Valrose, 06108
Nice cedex 02, France. Tel.: +33 4 92 07 65 88; fax: +33 4 92 07 68
16.
E-mail address: [email protected] (R. Merle).
0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2004.09.007
1. Introduction
Four main types of segregation structures have
been recognized in effusive and intrusive magmatic
110
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
bodies: spherical segregation vesicles, (rare) pipe
vesicles, vesicle cylinders, and vesicle sheets. Many
mechanisms have been put forward to explain the
origin of these segregation structures. Spherical
segregation vesicles may derive from shrinkage of
gas during cooling (Smith, 1967) or from escape of
gas to form microvesicle chains (Bideau and Hékinian, 1984). Pipe vesicles are elongated bubbles
occurring only either near the base of basalt flows
or in the interior part of pillow lavas. Vesicle cylinders
result from solidification of low-density diapirs of gas
and differentiate liquid rising through magma bodies
(Goff, 1977, 1996). Segregation sheets have been
considered to result from thermal contraction (Greenough and Dostal, 1992), compaction (Philpotts et al.,
1996), or compositional convection (Helz, 1980; Helz
et al., 1989).
Only gas filter-pressing is able to generate
together the four types of segregation structures in
basalt flows or dykes. This magmatic differentiation
process was described by Anderson et al. (1984) as
migration of residual liquid through a porous and
permeable but rigid network of interlocking crystals
(i.e., crystal framework). Gas filter-pressing is caused
by the build-up of gas pressure due to second boiling
that is relieved either by expulsion of melt out of the
crystallization zone (Sisson and Bacon, 1999) or by
its migration into the previously formed vesicles
(Anderson et al., 1984). This process has been named
vapor differentiation by Goff (1977, 1996), Sanders
(1986), Puffer and Horter (1993), Rogan et al.
(1996), and Caroff et al. (1997, 2000). In some
basaltic flows, the inflation process may have been an
important factor in formation of segregation structures (Thordarson and Self, 1998; Stephenson et al.,
2000). The main characteristic of vapor-differentiation-related segregation structures (except spherical
vesicles) is the high vesicularity of enclosed melt.
Caroff et al. (2000) have shown that the morphology
of the segregation structures is highly dependent on
the thickness of the magmatic host. However,
exceptions are the spherical segregation vesicles
which are ubiquitous in the whole flow, and the pipe
vesicles, which occur only at the base of the flows.
According to the classification of Caroff et al. (2000),
lava flows thicker than 10–15 m are characterized by
the occurrence of vesicle sheets 10–40 cm thick,
having generally a pegmatoid texture (S3 type).
Fig. 1. Location maps of the Tore-Madeira Rise and Chile Triple Junction. (a) Sketch map of north-central Atlantic Ocean. AGTZ: AzoresGibraltar Transform Zone; TMR: Tore-Madeira Rise; Smt: Seamount. (b) Location map of CTJ relative to the Chile Trench and Patagonia
(modified after D’Orazio et al., 2003). 1: Trenches; 2: Oceanic fracture zones; 3: Liquiñe-Ofqui Fault System; 4: Volcanoes.
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
Conversely, basalt flows typically 2–10 m thick are
devoid of such sheets but contain numerous other
structures, such as different types of segregation
vesicles, partly glassy thin upper sheets (S1 type),
10–40 cm thick S2-type sheets in the central part of
the flow, and vesicle cylinders.
The purpose of this paper is to document the
segregation structures within two pillows, respectively
from the Tore-Madeira Rise (TMR) and from the
Chile Triple Junction (CTJ) area. These hand-size
samples, ca. 15 and 30 cm in diameter, respectively,
expose a combination of the four types of segregation
structures (spherical vesicles, pipe vesicles, vesicle
cylinders, and vesicle sheets) up to now observed only
within basalt flows typically of 2- to 10-m thickness.
The only mechanism rapid enough to form these
structures in such small, fastly cooled pillows is vapor
differentiation. Our geochemical study reveals that the
differentiation trends from the host lavas to the
111
segregation structures are different in the two cases
because of different crystallization sequences.
2. Studied samples
The Tore-Madeira Rise (TMR) is a seamount chain
ranging from 418 to 328N and from 128 to 188W, 300
km off the Portuguese coast (Fig. 1a; Laughton et al.,
1975). It was built up at first by a magmato-tectonic
event during the Barremo-Aptian time (119–108 Ma;
Tucholke and Ludwig, 1982; Peirce and Barton, 1991;
Olivet, 1996) followed by several magmatic episodes
during the Cenozoic (Wendt et al., 1976; Olivet, 1996;
Geldmacher et al., 2000; Cornen et al., in preparation). The petrological studies (Merle et al., in
preparation) show that the TMR magmatism is clearly
alkalic (alkali basalts, basanites, and trachytes). The
studied pillow has been dredged from the Josephine
Fig. 2. Pillow lava TMD17b-1 from Tore-Madeira Rise (TMR). (a) Photograph of a vertical cross-section of the pillow lava. The location of the
three samples analysed by ICP-AES is shown. (b) Sketch of the same cross-section. (c) Photograph of another cross-section through the same
pillow lava. The curve shows the distribution of olivine phenocrysts throughout the log outlined by the rectangle. Inset: photomicrograph of an
olivine cluster. (d) Sketch of the same cross-section.
112
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
seamount, located in the middle part of the rise (ToreMadeira mission, 2001). It was an entire 16-cm-large
pillow, grossly triangular (Fig. 2). The host lava, alkali
basaltic in composition, has a red-brown color
characteristic of low-temperature seawater alteration
under oxidizing conditions. It is olivine–phyric (ca. 10
modal %) and has a vesicular partly glassy groundmass, with a diktytaxitic texture in its central part. The
cylinders show the original orientation of the pillow as
illustrated in Fig. 2. From the base to the top of the
pillow, we observe segregation pipe vesicles, which
progressively become the cylinders, then rare thin
sheets (Fig. 2a,b). Spherical segregation vesicles are
found everywhere throughout the sample.
The Chile Triple Junction (CTJ) corresponds to the
area where a segment of the Chile spreading ridge
enters the Chile Trench near the Taitao Fracture Zone
(Fig. 1b; Bourgeois et al., 2000). The studied pillow
was dredged on a small quaternary volcanic edifice
located on the continental edge of the trench close to
CTJ (CTJ28 dredge; CTJ cruise of R/V L’Atalante,
1997; analyses in Guivel et al., 2003). It is a fresh
basal fragment of a pillow of basaltic andesite
composition, with a 2-mm-thick glassy margin (Fig.
3a). The host lava is glomerophyric, with plagioclaserich olivine-bearing clusters (ca. 15 modal %). The
partly glassy groundmass has a highly vesicular and
diktytaxitic texture. The segregation structures appear
grossly similar to those in the TMR pillow (Fig. 3a,b).
3. Segregation structures
3.1. Segregation pipe vesicles
Pipe vesicles occur either near the base of basaltic
flows (Walker, 1987; Stephenson et al., 2000) or in
pillow lavas (Philpotts and Lewis, 1987). In the latter
Fig. 3. Details of segregation structures in the pillow lavas from Tore-Madeira Rise (TMR) and Chile Triple Junction (CTJ). (a) Photograph of a
vertical cross-section of the fragment of the CTJ pillow lava, showing pipe vesicles, vesicle cylinders, and bubble waves. The location of the
three samples analysed by ICP-AES is shown. (b) Photograph of a vertical cross-section of the fragment of the TMD17b-1 pillow lava showing
the transition between pipe vesicles and vesicle cylinders. (c) Photomicrograph of a vesicle cylinder of TMD17b-1. (d) Photomicrograph of a
segregation spherical vesicles of TMD17b-1.
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
case, they have a radial distribution. They never occur
in the chilled margin but only in the interior of the
pillow that contains crystals (Philpotts and Lewis,
1987). Two interpretations have been advanced to
relate these two occurrences. In lava flows, pipe
vesicles have been attributed to gas bubbles buoyantly
ascending through lava at a stage when it has acquired
a yield strength sufficient to prevent closure behind
them (Walker, 1987). Philpotts and Lewis (1987) have
proposed an alternate model to explain the radial
position of pipe vesicles in pillow lavas. They point
out that formation of a long pipe of gas requires
continuous exsolution of gas onto bubbles attached to
the solidification zone. During cooling of the lava,
continued exsolution of gas causes the pipes to grow
normal to the solidification front. Thordarson and Self
(1998), then Caroff et al. (2000), have shown for
continental lava flows that pipe vesicles could be
partly filled by glassy segregation material.
In the TMR and CTJ pillow lavas, pipe vesicles are
clearly segregation structures, partly or entirely filled
by vesicular glass. They occur only at the base of the
pillow, perpendicular to the chilled crust (5 mm above
in the TMR pillow, 1 cm above in the CTJ one; Figs. 2
and 3a,b). Although this latter feature has been
described by Philpotts and Lewis (1987), the fact that
such structures are limited to the base of the pillow
lava is consistent with Walker’s (1987) model.
Consequently, we suggest a two-step formation of
pipe vesicles. Pipe vesicles start to form according to
the model proposed by Philpotts and Lewis (1987)
followed by buoyancy-driven rise according to
Walker’s model (1987). In other words, the zone of
solidification might act as a guide to orientate the
bubbles at the beginning of their ascent.
3.2. Vesicle cylinders
Vesicle cylinders are the structures which are
probably the most characteristic of a vapor differentiation processes. They are vertical tubes, 2–20 cm
in diameter, filled with residual liquid and bubbles
(Goff, 1977). According to the classification of
Caroff et al. (2000), they occur typically within 2to 10-m-thick lava flows, but they were also
observed in the lower half of some thicker inflated
lava flows (Self et al., 1997; Thordarson and Self,
1998; Stephenson et al., 2000). In continuous
113
exposures, cylinders extend from ca. 25 cm above
the flow base to the bottom of the upper chilled crust
(Goff, 1996; Caroff et al., 2000). To form these
vertical structures, Goff (1996) has proposed that
residual liquids generated within the lower solidification zone move into vesicle-rich low-density
areas, through a gas filter-pressing mechanisms, then
migrate towards the top of the flow. Once trapped
beneath the crust, cylinders end either in sill-like
sheets or in vesicular pods, from whose the differentiated liquid invades the chilled crust as thinner
vesicle sheets. In some inflated lava flows, Thordarson and Self (1998) have observed segregation pipe
vesicles which converge to form cylinders. These
features have also been observed in non inflated lava
flows by Goff (1996) and Caroff et al. (2000).
In the pillows described here, we have also
observed the transition between pipe vesicles and
cylinders. In the TMR pillow, the lowest cylinders
resemble greatly elongated, partly filled pipe vesicles,
1–3 mm wide and 2.5 cm long (Figs. 2 and 3b,c).
They end in rounded bulbs located in the middle part
of the pillow lava. Towards the top of the pillow, the
cylinders become wider (4–5 mm in diameter) and full
of segregated melt. The upper extremity of some
cylinders is connected to sill-like sheets or to diffuse
veinlets (Fig. 2a,b). We observe a clear convergence
of the cylinders towards the central area of the TMR
pillow. This noticeable feature cannot be related to the
mechanism responsible of the pipe orientation, normal
to the basal crust, because some cylinders do not
follow the trend initiated down by the pipes (Fig.
2b,c). We propose that solidification of the two lateral
chilled crusts takes place just before the residual melt
migrates towards the top of the pillow. As its
consequence, cylinders are forced to converge
towards the still molten central part of the pillow.
The vesicle cylinders of the CTJ pillow appear to be
relatively disconnected from the pipe zone (Fig. 3a).
In addition, all the CTJ cylinders have a morphology
similar to the lower TMR cylinders (i.e., small
bulbous structures).
3.3. Vesicle sheets
The vesicle sheets described by Goff (1996) and
Caroff et al. (2000) in 2- to 10-m-thick lava flows are
located either within (or just below) the upper chilled
114
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
crust (S1-type) or in the central part of the flow (S2type). They are filled by vesicular, (partly) glassy
material, which has a composition grossly similar to
the cylinder-filling material. After Goff (1996), the
S1-type sheets either spread below the crust or invade
incipient joints and cracks created at the beginning of
the crystallization history.
We have observed vesicle sheets (2–5 mm thick)
and veinlets (b2 mm) in the TMR pillow lava, where
they occur in the external part of its upper half (Fig.
1). They are generally horizontal structures diverging
towards the lateral edges of the pillow through
incipient cracks of the outer solidified zone. The
thickest sheets are very vesicular. At the left border of
the pillow, one of these thick sheets connected to a
degassing chimney (Fig. 2c,d). We suggest that gas
escaped outside from the thick sheet towards the left
side of the pillow.
3.4. Segregation spherical vesicles
Segregation vesicles have been studied by Smith
(1967), Baragar et al. (1977), Bideau et al. (1977),
Shibata et al. (1979), Bideau and Hékinian (1984),
and Bacon (1986). Anderson et al. (1984) have
proposed a gas filter-pressing model to explain melt
migration into bubbles within certain subaerial basalt
flows. Caroff et al. (2000) have proposed a morphological classification of the vapor-differentiationderived segregation vesicles. The most widespread
ones are the V1-type spherical vesicles, which are
relatively ubiquitous throughout the vapor-differentiated lava bodies, except in their basal part where they
are absent. They are b1 cm in diameter and partly
filled with glassy, differentiated material.
The TMR and CTJ pillows contain such V1vesicles, increasing in size and decreasing in abundance upwards (Fig. 3d). The CTJ pillow displays
waves of bubbles such as described, for instance, by
McMillan et al. (1987) in a N70-m-thick basalt flow.
These layers have been interpreted by Manga (1996)
as the result of an hydrodynamic process. In his
model, suspensions of bubbles initially homogeneous
become unstable and form rising waves of bubbles.
The specificity of bubble waves in the CTJ pillow is
that they are formed by segregation vesicles placed
edge to edge. Such bubble layers are mainly observed
in the lower part of the pillow. Upward layers
resemble to the TMR vesicle sheets. However, it
seems that there is no connection between these upper
layers and cylinders.
4. Petrology and geochemistry
4.1. Textural and mineralogical notes
Microprobe analyses were performed with a
Cameca SX50 automated electron microprobe (Microsonde Ouest, Brest). Analytical conditions were 15
kV, 15 nA, counting time 6 s, correction by the ZAF
method. Concentrations of b0.3% are considered
qualitative.
The pillow from the Tore-Madeira Rise has an
olivine–phyric, partly glassy groundmass. Numerous
microlites of plagioclase (An63-70) occur together
with sparser, partly iddingsitized olivine (Fo80-81)
and Fe–Ti oxide microcrystals. Clinopyroxene was
not detected, probably because of the small size of the
crystals and the slight alteration of the groundmass.
Some olivine phenocrysts are assembled as clusters
b3 mm in diameter. Variation of the olivine phenocryst abundance (crystals/cm2) from the base to the top
of the TMR pillow is shown in Fig. 2c. The curve is
serrated with two more pronounced peaks at 3.5 and
10 cm from the base, respectively. The lower peak,
with 28 olivines per cm2, corresponds to the transition
zone between pipe vesicles and cylinders. The upper
peak, with 36 olivines per cm2, coincides with the area
where the vesicle cylinders feed into sheets. Between
these two high values, the curve displays a wide
saddle-like depression reaching six olivines per cm2.
Such features (variation of the vertical olivine
phenocryst distribution and occurrence of clusters)
have been previously described by Caroff et al. (1997)
in a 20-m-thick basaltic lava flow. In this case, olivine
distribution was interpreted to be the result of
interference between the upward motion of the
residual melt plus gas and the downward, densityrelated settling of olivine. Such a model can be
adapted to the TMR pillow lava. In this view, the
upper peak may be interpreted, on one hand, as a
break of the downward motion of olivines from the
top of the pillow by ascending vesicle cylinders, as
suggested by the olivine depletion near the top of the
pillow (Fig. 2c), or/and, on the other hand, as an
Table 1
ICP-AES major and trace element analyses of TMR and CJT host lavas and microprobe glass analyses of segregation structures
Sample
TMD17b-1B TMD17b-1C TMD17b-1G 16
Site
TMR
113
125
129
130
139
TMR
TMR TMR TMR
TMR TMR
TMR TMR CTJ
CTJ
CTJ
CTJ
CTJ
CTJ
CTJ
CTJ
CTJ
CTJ
CTJ
Structure type HL
C
HL
SP
SP
C
SV
SV
S
S
HL
C
HL
CC
SP
SV
SV
S
S
C
C
EBD (Am)
–
–
10
10
10
5
5
5
5
–
–
–
10
5
10
10
10
10
10
10
45.00
3.21
14.55
13.62
0.18
4.74
11.40
3.35
0.82
0.82
–
2.59
100.28
42.25
2.37
15.30
13.74
0.15
8.20
8.60
2.72
0.59
0.44
–
5.66
100.02
53.42
2.60
17.75
9.59
0.23
1.42
6.29
7.00
0.61
0.54
0.00
–
99.45
45.50 53.55
3.58
2.03
10.87 16.05
12.87
7.47
0.21
0.31
6.41
3.48
15.59 10.35
2.63
5.86
0.45
0.55
0.57
0.59
0.00
0.05
–
–
98.68 100.29
6.8
425
242
37.0
340
136
35
41
34.0
210
38.0
24.5
51
31.5
7.2
2.51
7.70
6.60
3.10
2.55
2.40
5.8
415
192
26.0
258
412
56
256
23.0
180
31.0
16.5
37
23.0
5.6
1.95
5.50
4.55
2.00
1.68
1.80
–
wt.%
SiO2
42.00
TiO2
2.47
Al2O3
14.85
Fe2O3*/FeO* 14.25
MnO
0.15
MgO
8.70
CaO
8.20
2.56
Na2O
K2O
0.61
0.38
P2O5
Cr2O3
–
LOI
5.92
Total
100.09
ppm
Rb
7.1
Sr
400
Ba
154
Sc
25.0
V
242
Cr
386
Co
48
Ni
226
Y
22.5
Zr
175
Nb
30.0
La
17.0
Ce
39
Nd
23.0
Sm
5.5
Eu
1.91
Gd
5.45
Dy
4.60
Er
2.10
Yb
1.67
Th
1.70
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
27
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
35
64
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
75
83
46.34 50.24
4.40
4.99
10.62 15.07
14.25 13.83
0.19
0.17
4.02
0.87
13.50
5.54
4.43
7.67
0.58
0.89
0.70
0.80
0.10
0.08
–
–
99.13 100.15
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
60
48.59 54.67
2.92 1.94
13.33 16.56
9.91 7.27
0.09 0.01
5.33 2.45
14.19 8.36
4.05 6.36
0.30 0.85
0.83 0.51
0.10 0.04
–
–
99.64 99.02
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
CTJ28B CTJ28C CTJ28G 160
53.00
0.94
17.80
6.96
0.12
6.38
9.15
3.27
0.65
0.23
–
1.32
99.82
54.20
1.13
15.80
8.15
0.15
6.40
9.00
3.00
0.80
0.28
–
–
98.91
53.00
0.92
17.75
6.92
0.12
6.43
9.20
6.10
0.87
0.24
–
1.26
99.81
21.0
206
118
23.0
146
156
25
90
26.0
157
8.0
12.9
28
15.2
3.5
1.17
4.30
4.45
2.60
2.52
2.70
22.5
175
152
27.5
183
182
27
80
32.5
188
10.0
15.6
35
19.2
4.4
1.38
4.35
5.40
3.10
3.09
3.45
18.6
204
124
23.0
150
157
27
100
26.5
155
8.0
12.8
28
15.0
3.7
1.17
4.20
4.30
2.50
2.50
2.85
53.23 47.03
0.97
1.41
17.44
9.84
6.70 11.17
0.26
0.25
6.05 14.65
9.44 15.90
3.08
0.30
0.74
0.02
0.35
0.18
0.11
0.06
–
–
98.37 100.81
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
153
55.66 50.83 48.05 61.45 62.26 46.38
0.22
0.74
1.29 0.58 0.73
1.87
24.20 14.27
7.74 17.51 20.20
9.28
1.77 11.34 12.20 4.45 2.05 13.36
0.00
0.17
0.30 0.10 0.01
0.25
1.34 12.70 13.81 2.59 0.90 12.18
10.85
6.44 15.32 6.71 6.86 15.87
5.04
3.01
1.14 4.33 4.72
0.49
0.28
0.29
0.09 1.40 0.97
0.07
0.03
0.25
0.41 0.29 0.39
0.37
0.00
0.05
0.09 0.00 0.07
0.20
–
–
–
–
–
–
99.39 100.09 100.44 99.41 99.16 100.32
–
–
–
–
–
–
–
–
–
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R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
68
TMR
HL: host lava; CC: chilled crust; SP: segregation pipe; C: cylinder; S: sheet; SV: spherical vesicle; EBD: electron beam diameter.
Fe2O3*/FeO*: total iron expressed as Fe2O3 for ICP-AES analyses and as FeO for microprobe analyses.
115
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R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
influx of olivines carried away by ascending cylinders, consistent with the saddle-like depression at the
level of the cylinder zone. The lower peak could
correspond to lower limit of this process.
Caroff et al. (1997, 2000) point out that hightemperature iddingsite (HTI, i.e., magmatic alteration
of olivine phenocrysts as the result of f02 increase) is
commonly observed in vapor-differentiated basaltic
lavas. Although altered, the TMR olivines do not
seem to contain HTI. The main secondary minerals
identified are low-temperature iddingsite, zeolites, and
various clay-minerals.
The pillow lava from the Chile Triple Junction
contains phenocrysts of plagioclase (An88–89), plus
subordinate olivine (Fo86–87). The groundmass consists of similar phases (plagioclase An63–86 and
olivine Fo84–86) with glass. The phenocrysts are
arranged in clusters (glomerophyric texture). The CTJ
pillow is perfectly fresh and contains no alteration
minerals, including HTI.
The segregation material in both pillows is mainly
glassy with a few microcrystals of plagioclase,
clinopyroxene, and Fe–Ti oxides.
4.2. Geochemical variations
We present in Table 1 six ICP-AES analyses (two
host lava and one cylinder analyses for each pillow).
Fig. 4. Al2O3 versus MgO diagrams (ICP-AES and microprobe analyses). (a) Host lava and glassy segregations structures in the Tore-Madeira
Rise pillow lava. (b) Host lava and glassy segregation structures in the Chile Triple Junction pillow lava. (c) TMR diagram with an enlarged
scale. The composition of olivine microcrystals in equilibrium with host lava has been calculated with the formula of Roeder and Emslie (1970).
Fields of microprobe analyses of plagioclase and clinopyroxene microlites and olivine phenocrysts are indicated. (d) CTJ diagram with an
enlarged scale. Composition of olivine microcrystals in equilibrium with host lava have been calculated following the same procedure than for
the TMR pillow lava. Fields of microprobe analyses of plagioclase microlites and olivine microcrystals and phenocrysts are indicated.
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
Analytical methods are described by Cotten et al.
(1995). Relative standard deviations are b2% for
major elements and b5% for trace elements. Numerous microprobe analyses of glass have also been
performed in the CTJ chilled crust and in the
segregation structures of both pillows (Table 1).
Loss on ignition (LOI) values show that the TMR
host lava is relatively altered (TMD17b-1B: LOI=5.92
wt.%; TMD17b-1G: LOI=5.66 wt.%), whereas the
cylinders are fresher (TMD17b-1C: LOI=2.59 wt.%).
CIPW normative nepheline percentages range from
0.37 (host lava) to 3.53 wt.% (cylinders), which
correspond to mildly alkali basalt values. The cylinders
have a mean composition clearly more evolved than the
host lava (e.g., MgO=4.74 vs. 8.45 wt.%, respectively).
Both ICP-AES and microprobe analyses are shown in
the Al2O3 versus MgO diagram of Fig. 4a together with
the field of TMR basalts. The two host lava analyses
(TMD17b-1B and-1G) plot within the TMR basalt
field. Two parallel trends are evident, the first one
including analyses of pipe vesicles, cylinders (microprobe and ICP-AES data) and sheets, the other one
including only spherical segregation vesicles. Primi-
117
tive-mantle normalized trace element patterns (normalization values from Sun, 1982, for compatible
transition elements, and from Sun and McDonough,
1989, for incompatible elements) are shown in Fig.
5a,b for the samples TMD17b-1G and-1C. The two
samples are enriched in the most incompatible elements with respect to the moderately incompatible
elements, which is characteristic of alkali basalts.
Cylinders, enriched in all incompatible elements with
respect to the host lava, display a pattern more or less
parallel to the host lava, except for Sr (Fig. 5a). On the
other hand, the cylinders are depleted in compatible
transition elements (Co, Cr, Ni) with respect to the host
lava (Fig. 5b).
The CTJ host lava (CTJ28B: LOI=1.32 wt.%;
CTJ28G: LOI=1.26 wt.%) is very fresh, as well as the
cylinders (LOI of CTJ28C not determined because of
the little quantity of material). The three CTJ
compositions plot within the field of basaltic andesite
in the TAS diagram of Le Bas et al. (1986). They have
calc–alkaline affinities (Guivel et al., 2003). There is
no discrepancy between microprobe and ICP-AES
analyses of the CTJ glassy crust (BV160 and CTJ28B
Fig. 5. Primitive mantle-normalized trace element patterns. Normalization values for incompatible elements from Sun and McDonough (1989)
and from Sun (1982) for compatible elements. (a) Incompatible element patterns of TMD17b-1G, TMD17b-1C, and calculated cylinder values.
(b) Incompatible element patterns of CTJ28B, CTJ28C, and calculated cylinder values. (c) Compatible elements patterns of TMD17b-1G,
TMD17b-1C, and calculated cylinder values. (d) Compatible element patterns of CTJ28B, CTJ28C, and calculated cylinder values.
118
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
in Table 1). The CTJ cylinders also have a mean
composition more evolved than the host lava, but this
is not expressed through the MgO values, which
remain remarkably constant in the three analyses.
TiO2, P2O5, and the incompatible trace elements
values show the extent of differentiation. Thus, it is
possible to estimate the weight percent of residual
liquid from the ratio (Th host lava/Th cylinders), Th
being considered to be the most incompatible element
(see the detailed procedure in Caroff et al., 1993). The
result is 0.80 for the CTJ pillow lava, wihch is close to
that calculated for the TMR sample (0.73). Both ICPAES and microprobe analyses are shown in the Al2O3
versus MgO diagram of Fig. 4b, together with the
field of CTJ28 dredge. The two host lava analyses
(CTJ28B and G) plot within the CTJ28 dredge field.
Like for the TMR pillow, spherical vesicles plot
slightly apart from the other segregation structures
(except one analyse), but in this case, in contrast to the
TMR, they form a trend on the right side of the other
data. The incompatible trace element patterns (shown
for CTJ28B and CTJ28C) are less enriched in the
most incompatible elements than the TMR ones and
display a negative anomaly in Nb (Fig. 5c), a feature
characteristic of calc–alkaline series. The compatible
element patterns display a surprising Cr enrichment in
the most differentiated liquid (CTJ28C), whereas Ni
decreases slightly from the host lava to the cylinders
(Fig. 5d).
5. Geochemical evolution between and within the
segregation structures
Variations of Al2O3 versus MgO (Fig. 4) show
complex differentiation modalities, which can be dealt
in two stages. First, we should consider the transition
from host lava to spherical vesicles through pipes/
cylinders/sheets (Tinter-trendr evolution), then the
differentiation within the trends themselves (Tintratrendr evolution). The fundamental difference between
the two types of differentiation is that the Tinter-trendr
evolution involves extraction and transport of residual
liquids from the host, whereas the Tintra-trendr
evolution corresponds to in situ crystallization within
one given segregation structure. In the latter case, glass
compositions are highly dependant on the nature of the
neighbouring crystallizing mineral phases.
To explain the Tinter-trendr evolutions, it is
necessary to determine the groundmass phase assemblage crystallizing in the whole host lava. In the TMR
pillow lava, it consists mainly of plagioclase and
olivine, as observed in thin sections. However, the
decrease in Cr from host lava to cylinders (Table 1)
Fig. 6. Cartoon illustrating four stages in the formation of a segregation vesicle (modified after Sanders, 1986).
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
implies that clinopyroxene must also crystallize.
Mineral proportions have been estimated by using a
graphic method derived from the diagram of Fig. 4c. If
we fix the position of the crystallizing assemblage
somewhere in the AAV segment (point C), together
with the percentage of clinopyroxene, it is possible to
estimate the proportion of plagioclase and olivine
through simple mass balance. Our results are 15 wt.%
clinopyroxene, 33 wt.% olivine, and 52 wt.% plagioclase. To validate these estimates, a test has been
performed with trace elements. To recalculate the
cylinder composition, we have introduced into the
Rayleigh fractionation equation the following parameters: fraction of residual liquid (Th host lava/Th
cylinders), host lava composition, mineral proportions,
and distribution coefficients from the literature
(incompatible trace elements: Caroff et al., 1997; Co,
Ni, and Cr: Bédard, 1994; Henderson, 1986, except Ni
in olivine estimated from the formula of Hart and
Davies, 1978: D=[124/MgO] 0.9). The results shown
in the diagrams of Fig. 5a,b reproduce satisfactorily
the analysed data, except for Ni and, to a lesser extent,
heavy rare earth elements. A quasi similar mineral
assemblage might explain the mean composition of
spherical vesicles by evolution of the same initial host
lava (Fig. 4c). The mechanism of formation of such
segregation structure is shown in Fig. 6. The cartoons
illustrate how mean compositions of the segregated
liquids are dependant on the nature and the proportion
of minerals crystallizing in the extraction zone. The
position of the spherical vesicle trend relative to that of
the other segregation structures implies a more
important extent of differentiation (Fig. 4a,c).
A similar procedure has been followed for the
CTJ pillow lava. The crystallizing mineral assemblage is probably devoid of clinopyroxene, given the
incompatibility of Cr from host lava to cylinders
(Fig. 5d). It is situated at point C in the segment
linking olivine and plagioclase compositions (Fig.
4d). A simple lever-rule method gives the following
proportion: 88 wt.% plagioclase and 12 wt.%
olivine. The trace element test can be used to
validate these estimates, as shown in the diagrams
of Fig. 5c,d. Note in particular the good superposition of the calculated and measured values for Sr
and Cr, which is consistent with their contrasted
compatibility. The position of the spherical vesicle
trend is not consistent with the crystallization of the
119
same mineral assemblage, involved in melt evolution
from host lava to the other segregation structures.
Crystallization of Fe–Ti oxides is required to explain
such compositions (Fig. 4d).
The Tintra-trendr evolution is controlled by the
mineral assemblages crystallizing within the segregation structures in the vicinity of the analyzed zone.
Indeed, one microprobe pinpoint cannot deliver an
analysis representative of the bulk chemistry of the
segregated glasses. To identify which type of mineral
is responsible for a given evolution, we have plotted
the glass analyses in the AFM (alkali–iron–magne-
Fig. 7. AFM diagrams for glassy segregation structures in pillow
lavas. (a) Tore-Madeira Rise pillow lava. Inset: trends controlled by
different mineral assemblages. 1: Plagioclase+olivine+clinopyroxene; 2: clinopyroxene; 3: Fe–Ti oxides+clinopyroxene; 4: Fe–Ti
oxides+plagioclase. The main trend corresponds to the crystallization of the Fe–Ti oxides and clinopyroxene assemblage. (b) Chile
Triple Junction pillow lava. Inset: trends controlled by different
mineral assemblages. 1: Plagioclase+olivine; 2: plagioclase+Fe–Ti
oxides; 3: Fe–Ti oxides+olivine+clinopyroxene. This latter assemblage corresponds to the prevailing crystallization trend. Symbols as
in Fig. 4.
120
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
sium) diagrams of Fig. 7. In the TMR pillow, the
AFM diagram shows four trends of differentiation
that each corresponds to a specific groundmass
mineral assemblage. The main trend corresponds to
the crytallization of a Fe–Ti oxides–clinopyroxene
assemblage. One of these two phases is always
present in all trends. In the CTJ pillow, the AFM
diagram displays three trends. The dominant one
involves crystallization of Fe–Ti oxides, clinopyroxene, and olivine. The diagrams show the crucial role
of certain mineral phases during this differentiation,
such as Fe–Ti oxides or clinopyroxene, which have
little influence in the transition from host lavas to
cylinders.
6. Formation of crystal frameworks in basaltic
pillows and lava flows
For Anderson et al. (1984), gas filter-pressing
involves the formation of a rigid and permeable
framework of crystals and locks the size and the shape
of the segregation vesicles. We calculated the degree
of crystallization for the melt evolution from host lava
to cylinder in the CTJ and TMR pillows. For the CTJ
pillow, we obtained 20% crystallization and 27% for
the TMR sample. This means that a crystal framework
strong enough to allow the formation of the segregation structures, forms earlier in the calc–alkaline rock
than in the alkaline one. In pillows, the crystal
framework occurs when crystallization reaches ca.
25%. In vapor-differentiated basaltic lava flows,
previous studies have estimated degrees of crystallization of melt from host lavas to segregation
structures: from a alkali basalt host to cylinders, the
degree of crystallization is ca. 57% (Caroff et al.,
2000); from host lava to sheets, it ranges from 35% to
53% (Caroff et al., 1997, 2000); and from host to
spherical vesicles, 36% to 74% (Caroff et al., 2000;
Anderson et al., 1984). Thus, in vapor-differentiated
basaltic lava flows, crystallization should reach at
least 35% to form the crystal framework required for
generating vapor-differentiated segregation structures.
In the studied pillows, the melt in segregation
structures is less evolved than that in lava flows
(Caroff et al., 1997; 2000; Goff, 1996), probably as a
result of a lesser extent of differentiation in pillows,
due to their faster cooling.
7. Conclusions
(1)
The two studied pillows from Tore-Madeira Rise
and Chile Triple Junction exhibit four types of
vapor-differentiation-related segregation structures (spherical and pipe vesicles, vesicle cylinders, and vesicle sheets), previously observed
typically within 2- to 10-m-thick basalt flows.
(2) Formation of the segregation structure obeys
slightly different modalities specific for each
pillow lavas. For instance, vesicle sheets are not
generated by similar mechanisms. In the TMR
pillow lava, vesicle sheets spread through incipient cracks of the outer solidified zone. In the CTJ
pillow lava, lower vesicles sheets appear to be
waves of bubbles caused by the formation of
instabilities within suspensions of bubble initially
homogeneous.
(3) Geochemical variations in the segregation structures are different. The main difference between
the two pillows can be attributed to their magmatic
affinity, which influences liquidus mineral phases.
The variations of Al2O3 versus MgO show two
differentiation modalities. The first one considers
the differentiation of melt from host lava to
spherical vesicles through pipes/cylinders/sheets
(Tinter-trendr evolution), and the second one, the
differentiation within the segregation structures
themselves (Tintra-trendr evolution). Within each
pillow, the segregated glass within spherical
vesicles displays a mean composition slightly
different from that in the other structures, which
denotes different extents and/or modalities of
differentiation. Composition of the segregated
liquid in the spherical vesicles is dependant on the
nature and the proportion of minerals crystallizing
in the extraction zone. The Tinter-trendr evolution
is controlled by the mineral assemblages crystallizing within the segregation structures, in the
vicinity of the analyzed zone.
(4) We have calculated the degree of crystallization
corresponding to the transition between host lava
and cylinders in each pillows. We deduced the
degree of crystallization necessary to form a
crystal framework required for generating the
segregation structures. It is lower in pillows (ca.
25% crystallization) than in vapor-differentiated
basaltic lava flows (at least 35% crystallization).
R. Merle et al. / Journal of Volcanology and Geothermal Research 141 (2005) 109–122
Acknowledgements
The samples were dredged during the ToreMadeira Cruise, conducted by G. Cornen, and the
Chile Triple Junction Cruise, conducted by Jacques
Bourgeois. We thank H. Loyen and E. Boeuf for thin
sections of TMR and technical assistance, A.
Cossard for pictures and J.P. Oldra for thin sections
of CTJ. We also acknowledge Y. Lagabrielle who
provided the CTJ sample, the assistance of M. Bohn
for microprobe studies and F. Jourdan and U.
Sch7rer for helpful comments. Critical reviews by
Drs. A.R. Philpotts and F. Goff helped us in
improving this work. This work has greatly benefited
of the comments of Dr. B.D. Marsh as editor.
Contribution no. 933 of the IUEM, European
Institute for Marine Studies (Brest, France).
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