JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 11
PAGES 1513–1539
1997
The Origin of Highly Silicic Glass in Mantle
Xenoliths from the Canary Islands
E.-R. NEUMANN∗ AND E. WULFF-PEDERSEN
MINERALOGISK-GEOLOGISK MUSEUM, UNIVERSITY OF OSLO, SARSGT. 1, N-0562 OSLO, NORWAY
RECEIVED APRIL 5, 1997; REVISED TYPESCRIPT ACCEPTED JULY 1, 1997
Spinel harzburgite, lherzolite, dunite and wehrlite mantle xenoliths
from the Canary Islands (La Palma, Hierro, Tenerife and Lanzarote) contain a spectrum of silicate glasses as inclusions in
minerals, along grain boundaries, and in interstitial glass pockets.
These glasses show a range in composition from basaltic (~44 wt
% SiO2), to highly silicic, TiO2–FeO–MgO–CaO–P2O5-poor
types (up to 71 wt % SiO2). Glasses in spinel harzburgites and
lherzolites are generally silica oversaturated, whereas those in spinel
dunites and wehrlites have somewhat lower SiO2 contents and are
generally silica undersaturated. Glasses in xenoliths from La Palma
and Tenerife are rich in K2O compared with those from Hierro and
Lanzarote. Daughter minerals coexisting with highly silicic glass
in polyphase inclusions are similar in composition to the main
phases in the host xenoliths (Fo>90, Cr-diopside, chromite),
whereas those in less silicic glasses are richer in Al2O3, TiO2 and
FeO, and poorer in MgO. The systematic relations found to exist
between glass composition, mineralogy of the host xenolith and
locality (island) cannot reflect random variations in the geochemistry
of ‘exotic’ melts infiltrating the mantle lithosphere, but instead
suggest a cogenetic relationship between the melts and their mantle
host xenoliths. The silicic glasses are interpreted as the products of
reactions at 8–12 kbar between infiltrating alkali basaltic magmas
and peridotitic wall-rocks which, in orthopyroxene-bearing rocktypes, involves formation of silicic melt+olivine at the expense of
orthopyroxene. In xenoliths from La Palma and Tenerife, where
interstitial phlogopite is commonly present, phlogopite has been
partly or totally consumed by the reactions between relatively mafic
melts and peridotite, giving rise to silicic glasses with high K2O
contents and K2O/Na2O ratios. The low K2O concentrations and
K2O/Na2O ratios in glasses in anhydrous xenoliths suites from
Hierro and Lanzarote are believed to result from reactions between
infiltrating melts and anhydrous and/or amphibole-bearing mantle
wall-rocks. The silicic melts appear to have been mobile over
distances exceeding the diameter of a xenolith, that is, at least
20–30 cm.
Highly silicic glass (up to 72 wt % SiO2) is commonly
found as inclusions and as interstitial glass pockets in
upper-mantle peridotites (e.g. Frey & Green, 1974; Francis, 1976, 1987; Jones et al., 1983; Siena et al., 1991;
Schiano et al., 1992, 1994, 1995; Ionov et al., 1994;
Schiano & Clocchiatti, 1994; Neumann et al., 1995;
Zinngrebe & Foley, 1995; Wulff-Pedersen et al., 1996a).
Reported host xenoliths include both anhydrous and
hydrous spinel-bearing harzburgites, lherzolites and dunites from continental, oceanic and island-arc tectonic
settings (e.g. Schiano et al., 1992, 1994, 1995; Schiano
& Clocchiatti, 1994; Ionov et al., 1994; Neumann &
Wulff-Pedersen, 1995).
The origin of silicic glass in mantle xenoliths, and its
role in mantle processes, has been the subject of vigorous
debate. A number of workers (e.g. Frey & Green, 1974;
Francis, 1976) have attributed the formation of silicic
melts to the breakdown of amphibole in response to
decompression during transport of the xenoliths to the
surface, and heating by the host lava. It has also been
proposed that silicic melts may form by partial melting
of mantle xenoliths during short residence times (up to
a few years) in crustal magma chambers during ascent
to the surface in the host magma (Klu¨gel et al., 1996).
Such melts would have no bearing on mantle processes.
Other models imply formation of silicic melts at mantle
depths. These include immiscible separation of a single
melt into coexisting silicic and carbonate melts (Schiano
et al., 1994); small degrees of partial melting of subducted
∗Corresponding author.
Extended Data Set can be found at
http://www.oup.co.uk/jnls/list/petroj
 Oxford University Press 1997
Canary Islands; silicic glass inclusions; mantle xenoliths;
melt–wall-rock reactions
KEY WORDS:
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 38
crust followed by percolation of the melts into the overlying depleted mantle wedge in volcanic arcs (Schiano et
al., 1995); infiltration by migrating metasomatic melt
phases genetically unrelated to the mantle rock in which
they are found (Edgar et al., 1989; Schiano et al., 1992,
1994, 1995; Schiano & Clocchiatti, 1994); reactions between infiltrating basaltic melts and peridotite wall-rock
(Zinngrebe & Foley, 1995; Wulff-Pedersen et al., 1996a);
in
situ
melting
involving
breakdown
of
amphibole±phlogopite (Amundsen, 1987); in situ melting
of clinopyroxene + spinel±amphibole (Francis, 1987;
Chazot et al., 1996); disequilibrium in situ melting involving largely clinopyroxene and spinel owing to reaction
with migrating fluids (Ionov et al., 1994); and partial
melting of peridotite which has previously been metasomatized by carbonatitic melts (Hauri et al., 1993).
Recently, experimental investigations have indicated
that highly silicic melts may form by small degrees of in
situ partial melting in the upper mantle. Baker et al. (1995)
showed that at 10 kbar, near-solidus melts (melt fraction
F=0·02–0·05) are enriched in SiO2 (57 wt % SiO2 at
F=0·02), Al2O3 and Na2O, and depleted in FeOtotal,
MgO and CaO relative to melts formed by higher degrees
of melting, and exhibit a strong increase in SiO2 and
alkalis and decrease in FeO, MgO and CaO with decreasing melt fraction at near-solidus conditions. Similar
results were obtained by Drury & FitzGerald (1996)
with a melt fraction of 0·0002. Draper & Green (1997)
observed that under upper-mantle pressures and temperatures, silicic (56–62 wt % SiO2), aluminous, alkaline
melts, typical of silicic glasses found in mantle xenoliths,
have near-liquidus mineral assemblages and mineral compositions which indicate equilibrium with a harzburgite
residue, both in the presence of a CO2–H2O fluid and
under anhydrous conditions. Draper & Green (1997)
proposed that silicic, aluminous, alkaline melts may form
by low-degree partial melting of peridotite enriched in
alkalis, volatiles, and other low-melting-temperature components.
The aim of this study is to establish the origin of silicic
glasses in unveined upper-mantle xenoliths from the
Canary Islands. Wulff-Pedersen et al. (1996a) suggested
that highly silicic glass in veined xenoliths from La Palma
have formed as the result of reactions between infiltrating
basaltic melts and peridotite wall-rock. An important
question is whether this model has general application
to silicic melts in peridotites in the Canary Islands (and
other localities). Our approach in the present study is to
test if any relationship exists between glass composition
and type of host xenolith, mode of occurrence (or relative
age) of the glass, and/or locality. As a basis for this study
we have chosen ultramafic xenolith suites from four
islands: essentially anhydrous xenoliths from Hierro and
Lanzarote, and hydrous xenoliths from La Palma and
Tenerife. The main rock types are spinel harzburgites
NUMBER 11
NOVEMBER 1997
and dunites; lherzolites and wehrlites are relatively rare,
but have been included where available. This study has
revealed systematic relationships between glass composition and type of host xenolith, mode of occurrence
of the glass, and locality (island), which strongly support
the ‘infiltrating melt–wall-rock reaction model’ and indicate that the compositions of highly silicic melts to a
large extent are controlled by the modal and chemical
composition of the mantle wall-rock. We found no relationship between the compositions of the glasses and
the host lavas.
THE HOST XENOLITHS
The silicic glasses discussed in this paper occur in ultramafic xenoliths from the Canary Islands belonging to
Group I of Frey & Prinz (1978). Group II xenoliths
(wehrlites, clinopyroxenites, dunites with Fo<87 in olivine
and relatively Ti–Fe-rich clinopyroxene and spinel) from
the same localities contain basaltic glasses (<50 wt % SiO2;
Neumann & Wulff-Pedersen, 1995), but these glasses will
not be discussed here. The Group I xenoliths comprise
refractory spinel harzburgites, rare spinel lherzolites,
spinel dunites, and rare spinel wehrlites. All these rocktypes contain Mg-rich olivine (Fo>89), Cr-diopside and
chromite; orthopyroxene is a major phase in harzburgites
and lherzolites, but is generally absent in dunites and
wehrlites. Phlogopite is a common accessory phase in all
types of xenoliths from La Palma and Tenerife, but has
not been observed in xenoliths from Lanzarote, and only
in one Group I xenolith from Hierro. The Group I
xenoliths are interpreted as fragments of the oceanic
lithospheric mantle that have been subjected to alternating episodes of partial melting and metasomatic
enrichment in highly incompatible trace elements (Neumann et al., 1995; Wulff-Pedersen et al., 1996a). Detailed
discussion of the petrography and mineral chemistry,
chemical composition and origin of the xenoliths may
be found in Johnsen (1990), Neumann (1991), Hansteen
et al. (1991), Neumann et al. (1995) and Wulff-Pedersen
et al. (1996a). A summary of these data is given below.
P–T estimates, based on conventional mineral geothermometry and densities of CO2 inclusions, give minimum temperatures of about 900°C, and minimum pressures of origin of 12 kbar for Hierro (Hansteen et al.,
1991; Neumann, 1991), and 6–8 kbar for Lanzarote
( Johnsen, 1990; Neumann et al., 1995).
The discussion is based on 674 analyses of glasses in
50 mantle xenoliths (29 spinel harzburgites, 5 spinel
lherzolites, 14 spinel dunites and 2 spinel wehrlites). The
xenoliths were collected in one locality in each of the
islands La Palma, Hierro and Tenerife, and four localities
in Lanzarote (Fig. 1); each locality contains different types
of xenoliths. All the xenoliths were collected in cinder
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NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Fig. 1. Map of the Canary Islands showing xenolith localities as black crosses.
cones. In Hierro and Tenerife the xenoliths appear to
be concentrated in a single layer. Sample identifications
are of the type XXii-jj, where the letters XX indicate
island (PAT, La Palma; H, Hierro; TF, Tenerife; LA,
Lanzarote), the number ii gives locality in that island,
and the number jj is sample number at that locality
(sample TF14-52 thus means sample 52 from locality 14
in Tenerife).
Spinel harzburgite and lherzolite xenoliths
Harzburgite and lherzolite xenoliths are protogranular or
porphyroclastic [following the nomenclature of Mercier &
Nicolas (1975)]; three generations of crystal growth may
be distinguished. The oldest generation consists of highly
strained porphyroclasts of olivine (up to about 25 mm
long) and orthopyroxene (containing exsolution lamellae
of spinel±clinopyroxene) with highly irregular grain
boundaries. In La Palma xenoliths large rounded spinel
grains are also interpreted as porphyroclasts. Porphyroclasts are commonly very rich in glass and fluid
inclusions (see below). The second generation of
crystal growth is represented by mildly strained to unstrained neoblasts of olivine+Cr-diopside+chromite
phlogopite±orthopyroxene. Neoblasts occur as granular,
equidimensional grains with interlocking grain boundaries (<1·0 mm in diameter), as irregular, interstitial
grains,
or
as
symplectitic
intergrowths
of
clinopyroxene + spinel±olivine. Equidimensional neoblasts with interlocking grain boundaries are often found
along the rims of phenocrysts with very irregular grain
boundaries, and in narrow zones crosscutting porphyroclasts, and appear to grow at the expense of these.
Interstitial grains of clinopyroxene frequently enclose
corroded
grains
of
olivine
and
orthopyroxene, and/or vermicular spinel. Clinopyroxene neoblasts often exhibit spinel exsolution lamellae, and orthopyroxene neoblasts occasionally have exsolution
lamellae of clinopyroxene. The neoblast generation has
been related to in situ heating and metasomatism during
the Canary Islands magmatism (Neumann, 1991; Neumann et al., 1995; Wulff-Pedersen et al., 1996a). In addition to these textures, many spinel harzburgites from
Tenerife exhibit large, irregular, poikilitic, mildly strained
or unstrained orthopyroxene (up to about 6 mm long) and
clinopyroxene grains (up to about 4 mm long), enclosing
smaller grains of rounded to corroded olivine, pyroxene
and spinel; fluid inclusions are rare. Some samples contain
exsolved orthopyroxene porphyroclasts with exsolutionfree domains surrounding olivine or clinopyroxene grains
(up to 1 mm in diameter). Where several such domains
occur close to one another, they resemble the poikilitic
pyroxenes. Poikilitic orthopyroxene and Cr-diopside thus
appear to have formed as the result of extensive recrystallization during the neoblast generation. Another
interesting feature is closely spaced, parallel rows of
minute, platy spinel inclusions that may sometimes be
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JOURNAL OF PETROLOGY
VOLUME 38
followed continuously from orthopyroxene porphyroclasts into olivine neoblasts (Neumann, 1991). Some
samples show similar parallel rows of spinel inclusions
cutting straight through clusters of olivine neoblasts of
different crystallographic orientations (Fig. 2). Interstitial
silicic glass is occasionally present in such areas. Olivine
neoblasts with parallel rows of minute spinel inclusions
are believed to have formed by incongruent melting of
orthopyroxene, and to have inherited the spinel inclusions
from the pre-existing orthopyroxene. Phlogopite (in xenoliths from La Palma and Tenerife) occurs as interstitial
grains, and in polyphase inclusions (glass +
phlogopite ± clinopyroxene ± spinel). Interstitial phlogopite often encloses spinel or small neoblasts of olivine,
and is frequently associated with interstitial glass. In
Hierro phlogopite has only been found in a single polyphase inclusion in olivine (glass + phlogopite + spinel)
in a spinel harzburgite xenolith. Sulphide globules occur
as scattered monomineralic inclusions in minerals, as
parts of polyphase inclusions, and as inclusions in interstitial glass. The third, and youngest, generation of crystal
growth consists of microlites of spinel, olivine, and clinopyroxene in interstitial glass pockets.
Spinel dunite and wehrlite xenoliths
Spinel dunites and wehrlites are porphyroclastic to equigranular. Olivine porphyroclasts are strongly deformed
whereas neoblasts and equigranular rocks are moderately
deformed. Orthopyroxene is rarely present, and the
xenoliths do not contain glass–olivine aggregates which
might be interpreted as the result from incongruent
melting of pre-existing orthopyroxene. Phlogopite is common in dunites from La Palma and Tenerife. Amphibole
(pargasite) has only been observed in Group I dunites
and wehrlites from La Palma where it is a rare accessory
phase in dunites, but present in considerable amounts in
wehrlites. Sulphide globules are more common in dunites
and wehrlites than in harzburgites and lherzolites.
MICROSTRUCTURES OF GLASS
Glass shows different modes of occurrence and relative
age. Glass inclusions in olivine porphyroclasts generally
form trails, indicating a secondary origin. The inclusion
trails sometimes stop at the boundaries of neoblasts,
indicating an age younger than the porphyroclasts but
older than the neoblasts. The inclusions in these trails
range from negative crystal shape (Fig. 3a) to elongate
irregular shapes (Fig. 3b and c), indicating different stages
of healing. The glass is colourless or brownish, and both
types may occur in the same thin-section (e.g. spinel
harzburgites H1-4 and H1-7). Glass inclusions generally
contain fluid bubbles (Fig. 3b) and in some cases also
NUMBER 11
NOVEMBER 1997
daughter minerals (clinopyroxene, spinel, phlogopite; Figs
2d, 4a and b). With the exception of some dunites from
Lanzarote that contain CO2 + N2 (Andersen et al., 1995),
fluid-bearing inclusions in xenoliths from the Canary
Islands have been found to consist of pure CO2 (Hansteen
et al., 1991; Frezzotti et al., 1994; Neumann et al., 1995;
Wulff-Pedersen et al., 1996a). In some samples glass
inclusions or glass-bearing inclusions also contain sulphide globules (e.g. H1-4).
Olivine and clinopyroxene neoblasts frequently exhibit
a central domain rich in small inclusions of colourless
to pale brown glass±fluid±spinel±phlogopite; these
inclusions have negative crystal shape, or they are
rounded to vermicular (Fig. 3a). Their mode of occurrence suggests that they are primary, and represent
melt trapped during growth of the neoblasts. Small,
scattered, vermicular glass±fluid inclusions also occur
in the rims of olivine porphyroclasts.
Orthopyroxene porphyroclasts commonly contain
numerous, scattered inclusions of colourless glass. Small
glass inclusions (a few micrometres in diameter) frequently
exhibit negative crystal shape, whereas larger ones are
irregular and are often found along the rims of euhedral to
subhedral olivine neoblasts located inside porphyroclasts
(Fig. 4a). These inclusions may contain fluid bubbles and/
or daughter minerals of chromite and/or Cr-diopside,
and in some cases sulphide globules. In xenoliths from
La Palma and Tenerife the phase assemblage in glassbearing inclusions in orthopyroxene porphyroclasts may
include phlogopite. The observed transition from exsolved orthopyroxene with silicic glass + olivine inclusions, to large clear poikilitic orthopyroxene in
Tenerife xenoliths, and parallel rows of platy spinel
inclusions cutting several olivine neoblasts, indicates that
the neoblast generation involved formation of silicic
glass + olivine±clinopyroxene at the expense of orthopyroxene.
Spinel porphyroclasts often contain numerous rounded
to vermicular inclusions and ‘tunnels’ consisting of glass±
clinopyroxene ± orthopyroxene ± olivine ± fluid.
These inclusions are commonly concentrated along the
rims, but may be found throughout large grains. Locally
glass-filled ‘tunnels’ continue into interstitial glass. The
host spinels have embayed, highly irregular outlines and
are zoned with higher Cr2O3, FeOtotal and TiO2 in inclusion-rich than in inclusion-free domains (WulffPedersen et al., 1996a).
In addition, many xenoliths contain brownish or colourless interstitial glass. Contacts between olivine grains
often display a thin coating of very small, glass ‘droplets’.
Locally these layers expand into continuous films of glass
along grain boundaries, and larger domains or glass
pockets, and may continue into narrow glass + fluid
veinlets that cut porphyroclasts. These veinlets are clearly
younger than the inclusion trails. However, we wish to
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NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Fig. 2. (a) Closely spaced parallel rows of myriads of minute, platy spinel inclusions (black) cutting several olivine neoblasts of different
crystallographic orientation in spinel dunite PAT2-113 from La Palma (crossed polars). (b) Same in ordinary light. The spinel lamellae show a
sharp termination inside a larger olivine grain. This is believed to mark the boundary of the pre-existing orthopyroxene grain. (c) Clear glass
(gl) (medium grey), skeletal lazurite (laz) (bright), and euhedral clinopyroxene (cpx) in interstitial glass pocket in spinel dunite PAT2-116.
emphasize that the interstitial glass and associated veinlets
are local phenomena inside the xenoliths; they are not
connected with the enclosing basaltic magma (host
magma), or veinlets of host magma penetrating into the
xenoliths. Large pockets of interstitial glass are most
common around the rims of orthopyroxene and spinel
porphyroclasts. In xenoliths from La Palma and Tenerife,
glass is also commonly found along the rims of interstitial
phlogopite, which shows corroded contacts against this
glass. Interstitial glass commonly contains large, rounded,
empty
vesicles,
sometimes
microlites
of
olivine±clinopyroxene±spinel, and occasionally sulphide globules. Sulphide globules are more common in
glass in spinel lherzolites, dunites and wehrlites than in
harzburgites, and more common in hydrous than in
anhydrous xenoliths, and are believed to be related to
metasomatism. In two spinel dunites from La Palma
(PAT2-27 and PAT2-116) lazurite {Na5·7K0·2Ca2·0
[Al5·9Si6·1O24](SO4,S)1·6; E. Wulff-Pedersen & E.-R.
Neumann, unpublished data, 1996} has been found in
highly Na2O-rich, silica-undersaturated, colourless glass
(SiO2 51–54 wt %) present in inclusion trails in olivine
and in interstitial glass pockets (Fig. 2c). In sample PAT116 the silicic glass carries sulphide globules in addition
to lazurite.
Contacts between colourless, silicic glass inclusions and
host minerals are sharp, showing no evidence of reaction.
This is in direct contrast to the contacts between xenolith
minerals and the enclosing alkali basalt and basaltic
veinlets, which are frequently marked by a reaction
zone. Reaction zones may consist of (a) a vermicular
intergrowth of clinopyroxene and spinel, both of which
are considerably more Ti–Al-rich than the clinopyroxene
and spinel inside the xenoliths; (b) an intergrowth of
Ti–Al-rich clinopyroxene and amphibole, (c) formation
of clinopyroxene±olivine±spinel±glass at the expense
of orthopyroxene, or (d) a combination of (a)–(c). Similar
reaction zones are found against basaltic glass in veined
spinel harzburgite and dunite from La Palma (WulffPedersen et al., 1996a).
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NUMBER 11
NOVEMBER 1997
Fig. 3. (a) Secondary trail of glass (g) + fluid (F) inclusions with negative crystal shape in olivine porphyroclast (o) cutting fluid inclusion trails
with very small inclusions (spinel dunite TF14-4). The glass is in the early stages of devitrification. (b) Secondary trail of irregular glass (g) + fluid
(F) inclusions cutting fluid inclusion trails in olivine porphyroclast (o) (spinel harzburgite PAT2-59). (c) Glass (g) in poorly healed fracture through
olivine porphyroclast (o) (spinel dunite PAT2-116). (d) Polyphase inclusion consisting of phlogopite (ph), glass (g) and fluid (F) in spinel dunite
PAT2-49.
ANALYTICAL PROCEDURE
Major element analyses of glasses and minerals were
obtained using an automatic wavelength-dispersive CAMECA Camebax Microbeam electron microprobe fitted
with a LINK energy dispersive system at the Mineralogisk-geologisk museum in Oslo. Glass was analysed
by scanning an area of 5 lm × 5 lm to 20 lm × 20
lm, counting light elements first, and minerals were
analysed by point analyses, using an acceleration voltage
of 15 keV, sample currents of 10 nA for glass, 20 nA for
minerals, and counting times of 10–30 s per element. The
composition of groundmass in host lavas was estimated as
the average of 10–20 scanning analyses (20 lm × 20 lm)
of adjacent areas, avoiding phenocrysts and xenocrysts.
Oxides and natural and synthetic minerals were used as
standards. Matrix corrections were performed by the
PAP procedure in the CAMECA software. Analytical
precision (2r) evaluated by repeat analyses of individual
grains is <1% for oxides in concentrations of [20 wt
%, <2% for oxides in the range 10–20 wt %, <5% for
oxides in the range 2–10 wt %, and <10% for oxides in
the range 0·5–2 wt %. Representative analyses of glass
in different types of mantle xenolith are listed in Table 1,
together with (CIPW) normative quartz, nepheline and
leucite. An Fe3+/Fetotal ratio of 0·5 was arbitrarily chosen
for the norm calculations. As the glasses contain very little
iron, the estimated amounts of quartz and feldspathoids in
the norms are only marginally influenced by the choice
of Fe3+/Fetotal ratio. A reduction in this ratio from 0·5 to
0·0 causes an increase in normative quartz (or reduction
in nepheline + leucite) of <1%. The compositional
ranges of glass inclusions in different types of host xenolith
from each of the islands included in this study are given
in Table 2.
CHEMICAL COMPOSITIONS OF THE
GLASSES
Glasses in mantle xenoliths from the Canary Islands
exhibit a considerable range in composition (e.g. 44–71
wt % SiO2, 0·0–5·8 wt % TiO2, <1–8 wt % MgO,
1518
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Fig. 4. (a) Inclusion consisting of clear glass (g) and fluid (F) (empty) and euhedral olivine daughter mineral or neoblast (o) inside orthopyroxene
porphyroclast (p) (spinel harzburgite PAT2-61). The presence of small, scattered, primary glass (g) and spinel (s) inclusions in the olivine grain
should be noted. (b) Polyphase inclusion consisting of glass (g) + cpx (c) + fluid (F) (empty) in olivine porphyroclast (o) (spinel harzburgite PAT259). The compositions of different phases are given in the tables to the right of each figure; analysis locations are indicated in the figures by
numbers corresponding to the column numbers.
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NUMBER 11
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Table 1: Representative major element analyses and normative quartz (Qz), nepheline+leucite (Ne)
contents of glass inclusions and interstitial glass in mantle xenoliths from the Canary Islands, listed by
island and type of host xenolith
Island:
Rock type:
La Palma
Sp harz
Sp lherz
Sp dunite
Sp dunite Sp dunite
Sp wehr
anhydr
anhydr
anhydr
amph
phl/amph
amph/phl
Sample:
PAT2-41
PAT2-59
PAT2-25
PAT2-42
PAT2-116
PAT2-74
Incl in:
opx por
opx por
ol por
sp por
Other phases:
Interstitial:
ol por
ol por
ol por
ol por
sp+F
lz+F
F
F
pocket
pocket
pocket
SiO2
65·65
67·16
67·48
56·92
49·15
64·87
51·60
51·28
57·44
54·54
TiO2
0·08
0·04
0·30
0·30
0·96
0·15
0·52
0·72
0·29
0·40
0·85
Al2O3
15·77
15·43
14·67
21·93
21·19
22·00
20·23
20·09
21·71
20·44
20·31
FeO
1·51
1·98
1·61
2·17
3·27
0·66
3·27
3·79
2·75
3·03
3·40
MnO
0·07
0·06
0·04
0·08
0·09
0·05
0·07
0·03
0·07
0·08
0·14
MgO
1·52
1·56
1·49
0·40
2·65
0·05
2·20
2·69
1·66
2·15
2·81
CaO
1·56
1·27
1·33
1·76
8·04
0·55
6·40
9·15
4·09
5·51
6·88
Na2O
6·95
6·57
6·61
6·90
6·87
6·80
8·15
5·18
6·74
8·86
7·05
K 2O
4·47
4·63
4·43
4·43
5·13
2·41
4·67
4·43
4·00
3·90
4·10
P 2 O5
0·03
0·10
0·68
0·78
0·09
0·51
0·46
0·53
0·25
0·47
Cl
0·08
0·02
0·12
0·31
0·22
0·56
0·32
0·12
0·49
0·33
0·16
97·69
98·72
98·18
95·88
98·35
98·19
97·94
97·94
99·77
99·49
98·28
8·34
11·74
—
—
15·38
—
—
—
29·46
—
29·60
15·97
Sum
n.d.
52·11
CIPW norm
Qz
Ne
5·50
—
—
—
3·05
<1–15 wt % CaO, <1–12 wt % K2O, 1–2 wt % P2O5
and 0·0–0·8 wt % Cl), and show general trends of
decreasing concentrations and ranges of TiO2, FeOtotal,
MgO, CaO and P2O5 with increasing SiO2 (Tables 1
and 2; Figs 5 and 6). Na2O and K2O exhibit considerable
scatter (K2O/Na2O 0·02–2·0) with the highest concentrations in the most silicic glasses, whereas the Al2O3
contents appear to reach a maximum at 55–60 wt %
SiO2. The compositional ranges of glass inclusions in
spinel harzburgites and lherzolites from Hierro and Lanzarote obtained by us overlap those published by Schiano
et al. (1994), whereas Siena et al. (1991) reported a slightly
higher SiO2 range for glasses in spinel harzburgites from
Lanzarote (Table 2).
In each island the most SiO2-rich, and TiO2–
FeO–MgO–CaO–P2O5–poor glasses are found in spinel
harzburgites and lherzolites (Table 2; Figs 5 and 6), that
is, in orthopyroxene-bearing rock-types. In La Palma all
glasses in harzburgites and lherzolites fall within the
range 62–69 wt % SiO2 (Table 3; Fig. 5). Furthermore,
6·88
—
—
26·24
21·77
most glasses in harzburgites and lherzolites are moderately to strongly silica oversaturated, whereas those in
dunites and wehrlites tend towards silica undersaturation
(Fig. 7). We have found no systematic compositional
differences between glasses in anhydrous and hydrous
xenoliths within the same island, nor between harzburgites and lherzolites. However, Amundsen (1987) reported higher K2O contents and K2O/Na2O ratios in
glasses in phlogopite-bearing than in phlogopite-free
xenoliths from Gran Canaria (Canary Islands). Available
data (Fig. 7) indicate that silicic glass in spinel harzburgite
and lherzolite xenoliths from other locations and tectonic
settings also tends towards silica oversaturation. Unfortunately, very few data on glass in spinel dunites and
wehrlites are as yet available. The glasses also show
inter-island differences with respect to Na2O–CaO–K2O
relations. Most of the glasses in the anhydrous xenolith
suites from Hierro and Lanzarote are poorer in K2O
and richer in CaO and show lower K2O/Na2O ratios
than glasses with similar SiO2 contents in the xenolith
1520
ol por
sp neo
1521
0·00
1·81
2·16
5·69
2·62
0·16
0·59
MnO
MgO
CaO
Na2O
K 2O
P 2O5
Cl
18·50
—
Qz
Ne
CIPW norm
96·08
2·87
FeO
Sum
13·31
Al2O3
—
4·23
96·80
0·40
0·34
4·62
5·33
3·28
1·50
0·11
2·60
17·63
0·15
60·84
—
4·15
97·75
0·19
0·53
2·62
4·11
8·40
3·12
0·05
4·10
16·83
2·66
55·14
60·10
—
11·81
97·12
0·03
0·08
1·46
4·30
5·69
4·73
0·10
3·94
16·19
0·50
51·95
11·00
—
97·64
0·07
0·28
1·10
6·08
11·23
3·60
0·13
5·06
17·55
0·59
—
7·96
99·25
0·06
0·18
0·86
4·90
8·22
3·77
0·12
4·27
17·12
0·33
3·55
—
99·06
0·06
0·21
1·47
6·30
7·46
3·42
0·13
4·65
19·07
0·42
55·87
18·97
—
96·66
0·06
0·40
0·99
5·41
15·12
4·84
0·17
5·66
16·20
1·01
46·80
59·42
0·02
TiO2
66·85
ol por
SiO2
ol+sulph F
ol por
grbd/opx pocket/cpxveinlet/ol
opx+F
Other phases:
Interstitial:
opx por
Incl in:
H1-19
anhydr
H1-4
anhydr
Sp harz
Rock type:
Sample:
Sp harz
Hierro
Island:
Table 1: continued
—
15·67
99·24
0·24
0·15
0·10
4·50
7·75
1·39
0·06
3·13
20·99
0·05
60·88
cpx neo
H1-58
anhydr
Sp harz
—
11·05
97·79
0·11
0·06
0·80
3·69
8·98
3·19
0·06
3·73
19·28
0·61
57·28
ol+F
oxp por
H1-7
anhydr
Sp lherz
ol por
3·45
—
96·57
0·05
1·99
1·71
3·91
10·65
4·21
0·15
7·22
16·06
5·77
44·85
cpx+F
7·52
—
97·40
0·18
2·17
1·62
4·45
11·12
4·48
0·19
7·12
15·85
5·62
44·60
grbd/ol
—
8·23
95·62
0·31
0·10
1·01
1·46
5·63
13·66
0·16
5·32
14·29
0·25
53·43
F
ol por
H1-13
anhydr
Sp dunite
ol por
2·65
—
94·89
0·33
0·13
1·01
5·01
6·44
11·35
0·10
4·31
14·59
0·25
51·37
F
ol por
16·87
—
96·63
0·29
0·09
0·55
4·88
14·91
5·34
0·07
5·38
17·88
0·36
46·88
F
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
opx neo
Incl in:
1522
0·27
Cl
11·28
—
—
Qz
Ne
Lc
CIPW norm
99·62
0·13
—
—
20·85
97·14
0·21
0·12
4·67
3·57
3·48
2·00
0·02
1·79
13·77
—
—
19·25
97·60
0·02
0·52
4·72
3·00
4·80
3·26
0·08
2·31
12·19
1·97
64·73
—
—
3·10
99·43
0·07
0·81
0·20
8·30
4·20
3·76
0·02
2·29
16·52
0·45
62·81
ol neo
sp neo
—
—
5·84
96·79
0·07
0·90
6·37
3·22
3·82
4·35
n.d.
1·79
14·54
2·35
59·38
F
phlog
—
5·51
—
96·59
0·02
1·24
4·88
3·99
8·59
5·18
0·31
3·18
13·68
2·52
53·00
F
—
—
9·79
96·32
0·03
0·43
4·51
4·21
5·29
4·39
0·09
2·66
11·53
1·36
61·82
pocket
—
—
7·24
98·81
0·34
0·25
4·67
4·26
3·68
1·40
n.d.
2·52
20·67
2·00
59·02
sp+F
ol por
TF14-41
—
—
7·55
97·12
0·04
0·74
3·93
3·99
6·04
2·30
0·10
3·06
16·49
2·71
57·72
ol por
—
16·00
—
100·66
n.a.
0·59
3·64
6·26
8·10
3·16
0·21
4·99
19·45
1·95
52·31
F
ol
TF14-11
ol
0·64
4·00
4·37
8·99
3·10
0·18
5·32
19·65
2·15
51·91
F
—
7·78
—
100·31
n.a.
Sp dunite
phlog
0·48
26·45
—
97·77
0·35
0·25
7·91
5·63
3·99
3·38
0·14
2·96
21·61
0·96
50·59
ol por
TF14-50
phlog
Sp dunite
ol por
36·01
27·66
—
98·38
0·05
n.d.
11·80
5·94
2·95
0·31
n.d.
1·94
23·67
0·43
51·29
cpx
sp neo
—
3·38
—
97·16
0·24
0·37
1·17
8·29
2·42
3·00
0·06
3·71
20·64
0·88
56·38
cpx
—
—
3·89
99·30
0·05
0·77
3·70
3·72
7·36
3·18
0·25
5·46
16·58
3·22
55·01
pocket
TF14-46
phlog
—
—
2·04
97·67
0·05
0·93
3·25
3·55
8·28
3·72
0·21
5·72
15·97
3·92
52·07
pocket
Sp wehrlite
NUMBER 11
Sum
5·43
P 2O5
0·70
MgO
K 2O
0·03
MnO
1·39
0·65
FeO
5·44
18·11
Al2O3
Na2O
0·06
ol neo
TF14-42
Sp harz
phlog
VOLUME 38
CaO
67·38
67·41
SiO2
TiO2
0·13
pocket
Interstitial:
Other phases: F
TF14-40
phlog
Sp harz
anhydr
Rock type:
Sample:
Sp harz
Tenerife
Island:
Table 1: continued
JOURNAL OF PETROLOGY
NOVEMBER 1997
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Table 1: continued
Island:
Lanzarote
Rock type:
Sp harz
Sp dunite
anhydr
anhydr
Sample:
LA6-35
LA6-25
Incl in:
ol por
Other phases:
opx por
opx por
ol+F
ol
ol
ol
F
F
F
Interstitial:
grbd/ol
grbd/sp
SiO2
66·97
70·51
63·62
65·32
62·26
56·27
59·79
TiO2
0·04
0·07
0·04
0·21
0·12
0·74
0·07
0·28
Al2O3
16·76
17·59
22·16
18·03
22·19
20·01
22·02
20·76
1·37
2·87
2·75
3·45
2·59
4·43
0·07
0·01
0·04
0·05
0·09
1·16
1·84
2·13
1·94
2·53
FeO
MnO
MgO
1·19
n.d.
0·37
1·28
n.d.
0·87
n.d.
0·65
55·43
CaO
4·90
4·52
5·23
4·54
2·64
6·89
2·86
6·39
Na2O
5·73
3·90
5·44
4·40
5·82
7·09
8·96
7·15
K 2O
1·75
1·58
0·93
0·66
1·73
2·05
1·69
2·15
P 2 O5
0·04
0·03
0·10
0·20
0·22
0·28
0·22
0·34
Cl
0·10
0·11
0·14
0·71
0·11
0·32
0·16
0·26
97·85
100·46
99·68
98·17
99·69
99·27
100·35
99·81
Qz
18·17
30·54
16·33
26·43
13·55
—
Ne
—
—
—
—
Sum
CIPW norm
8·10
—
7·09
—
9·70
Additional analyses of glass and coexisting minerals in polyphase inclusions are given in Table 3 and Fig. 3. Anhydr,
anhydrous; amph, amphibole; phlog, phlogopite; ol, olivine; opx, orthopyroxene; cpx, clinopyroxene; sp, spinel; sulph,
sulphide globule; lz, lazurite; por, porphyroclast; neo, neoblast; grbd, grain boundary; grbd/ol, grain boundary along olivine;
pocket, glass pocket; F, fluid. n.d., not determined; n.a., not analysed.
suites from La Palma and Tenerife, where phlogopite
is a common accessory phase (Figs 5 and 6). In an
Na2O–CaO–K2O diagram, glasses in harzburgite and
lherzolite xenoliths from La Palma and Tenerife define
trends towards the central part of the Na2O–K2O sideline, whereas those from Hierro and Lanzarote fall close
to the Na2O–CaO tie-line (Fig. 8). Glasses in dunites
show less clear differences in Na2O–CaO–K2O relations.
We have also tested compositional differences against
mode of occurrence: (a) glass in secondary inclusion
trails in olivine porphyroclasts (assumed to be the oldest
generation of glasses), (b) irregular glass-bearing inclusions
in orthopyroxene porphyroclasts, and (c) glass in interstitial glass pockets. Clusters of (primary) inclusions in
olivine and clinopyroxene neoblasts (intermediate in age
between type a and type c) are generally too small to
allow analysis; we have therefore not been able to test
those as a separate group. Glasses in spinel harzburgites
and lherzolites from La Palma show no compositional
relation to mode of occurrence. In the other islands there
is a weak tendency for glass found in secondary inclusion
trails in olivine and as inclusions in orthopyroxene to
show a more restricted range in high-SiO2 compositions
and higher average SiO2 than interstitial glasses (Fig. 5).
Dunites show no chemical differences between glass
inclusions in olivine and interstitial glass (Fig. 6).
MINERALS COEXISTING WITH
GLASS IN INCLUSIONS AND GLASS
POCKETS
The compositions of daughter minerals and microlites
are clearly correlated with the coexisting glass. Euhedral
to subhedral daughter minerals in polyphase inclusions
associated with highly silicic glass have compositions
typical of the main phases in the host xenoliths, that
1523
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 11
NOVEMBER 1997
Table 2: Compositional ranges in glass inclusions and interstitial glasses from different types of mantle
xenoliths from the Canary Islands
Island:
La Palma
Rock type:
harz/lherz
harz/lherz
dun
dun
wehr
harz/lherz
dun
anhydr
hydr
anhydr
hydr
hydr
anhydr
anhydr
4
3
1
4
1
4
1
1
incl
incl
incl
incl
incl
incl
incl
incl
59·09±0·35
n (rocks):
Hierro
harz/lherz (1)
SiO2
62·7–67·5
62·3–68·9
52·5–60·5
46·2–56·9
53·4–57·4
44·9–66·9
46·9–53·4
TiO2
0·0–0·5
0·1–0·6
0·2–0·9
0·3–1·0
0·3–1·0
0·0–5·8
0·2–0·4
0·26±0·02
Al2O3
14·7–17·2
15·1–17·3
21·5–23·2
19·2–22·0
20·4–22·5
13·3–21·0
14·6–17·9
18·96±0·13
FeO
1·5–2·5
1·4–2·2
1·6–3·5
2·2–4·6
2·8–3·3
2·0–7·2
4·3–5·4
3·21±0·11
MgO
1·3–1·8
1·0–1·8
0·4–1·6
0·4–2·4
1·5–2·7
1·4–7·8
5·3–11·4
3·36±0·18
CaO
1·2–2·3
0·6–2·2
2·0–8·4
1·7–12·1
4·1–7·5
1·0–11·2
5·6–14·9
8·67±0·19
Na2O
6·6–7·7
6·6–7·4
7·2–8·2
6·5–9·1
5·2–8·7
3·1–7·4
1·5–5·0
4·29±0·02
K2O
4·4–4·8
4·5–4·8
3·1–4·5
2·8–4·7
3·4–4·0
0·1–4·6
0·6–1·0
1·14±0·05
P2O5
0·0–0·2
0·0–0·3
Host (Fo)
89·8–91·0
90·2–90·8
Island:
Tenerife
Rock type:
harz/lherz
harz/lherz
dun
dun
wehr
anhydr
hydr
anhydr
hydr
hydr
4
4
1
1
incl
incl
incl
n (rocks):
0·1–0·2
89·8
0·2–0·8
89·9–90·7
0·1–0·5
0·0–2·0
89·4
90·1–92·5
0·1–0·2
0·12±0·02
91·2
91·0
harz/lherz
dun
harz (2)
dun (2)
anhydr
anhydr
anhydr
anhydr
anhydr
1
2
2
3
1
1
incl
interst
incl
incl
interst
interst
incl
56·5–55·0
Lanzarote
harz/lherz (1)
SiO2
58·5–68·4
61·6–69·2
51·9–52·3
50·0–51·3
52·1–55·0
55·8–70·5
54·6–65·3
62·0–72·6
62·9
TiO2
0·0–2·7
0·0–2·0
2·0–2·2
0·4–2·1
3·0–3·9
0·0–0·3
0·1–0·7
0·1–1·3
1·1
0·0–0·2
Al2O3
16·7–20·1
14·8–18·0
19·4–19·7
20·6–23·7
16·0–16·6
16·8–22·2
18·3–22·2
17·1–19·6
17·4
21·2–21·9
FeO
0·7–3·1
0·6–2·7
5·0–5·3
1·9–3·7
5·3–5·7
1·2–2·0
2·8–3·9
1·1–1·7
2·8
2·2–2·3
MgO
0·7–3·0
0·0–3·3
3·1–3·2
0·3–3·4
3·2–3·7
0·4–3·0
1·2–5·4
0·4–2·4
1·7
1·4–2·1
CaO
1·4–6·0
0·2–4·8
8·1–9·0
2·4–5·8
5·1–8·3
1·5–12·7
2·6–6·9
0·7–4·5
1·5
8·8–7·8
Na2O
3·0–6·2
2·7–6·9
4·4–6·3
4·1–8·3
3·6–4·1
3·7–7·0
4·4–7·7
1·3–5·3
2·4
4·7–6·2
K 2O
3·2–6·9
4·4–9·8
3·6–4·0
1·2–11·8
3·3–3·7
0·2–1·8
0·7–3·6
1·5–1·8
2·9
2·1–2·3
0·0–0·3
0·1–0·3
0·5–0·8
0·5
91·6–91·7
89·7–91·6
P 2 O5
Host (Fo)
0·0–0·7
90·3–90·6
0·0–0·5
91·2
0·6–0·7
87·2
0·0–0·3
89·9
0·7–0·9
89·7
0·1–0·2
91·0
Data published by (1) Schiano et al. (1995) and (2) Siena et al. (1991) are included for comparison. n, number of rock
samples; incl, inclusions in orthopyroxene and olivine; interst, interstitial glass.
is, olivine with Fo>90, and Mg–Cr-rich, Ti–Al-poor
pyroxenes (Table 3, Fig. 4). The same is true for microlites
in glass pockets with highly silicic glass. Less silicic glass,
in contrast, contains minerals which are markedly richer
in TiO2, Al2O3 and FeO, and poorer in MgO than those
in the mantle wall-rock. Minerals of similar compositions
are typically found as phenocrysts in alkali basalts, and
in Group II wehrlites and clinopyroxenites. The latter
are interpreted as mantle cumulates formed from alkali
basaltic Canarian magmas (Hansteen et al., 1991;
Neumann, 1991). Similarly, veined xenoliths from La
Palma show a continuous shift in mineral compositions
from relatively Fe-rich olivine and Ti–Al–Fe-rich clinopyroxene, amphibole and phlogopite in the least silicic
glass, to Mg-rich olivine and Ti–Al-poor, Mg–Cr-rich
clinopyroxene and phlogopite coexisting with the most
silicic glass (Wulff-Pedersen et al., 1996a). Corresponding
relations between the compositions of glass and daughter
minerals–microlites were observed by Zinngrebe & Foley
(1995) in mantle xenoliths from Gees, Germany.
1524
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Fig. 5.
CHEMICAL COMPOSITIONS OF THE
HOST LAVAS
As the xenoliths were collected in cinder cones, their
host lavas were often difficult to analyse. However, we
obtained data on host lava (bulk rock, groundmass, or
glass) for a number of samples. Representative analyses
are presented in Table 4 and plotted in Figs 5–7. Like
Canary Islands basalts in general, the host lavas of the
xenoliths are TiO2 rich, and silica saturated to undersaturated. K2O/Na2O values range from 0·31 to 0·71.
Within each locality we found only minor compositional
1525
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 11
NOVEMBER 1997
Fig. 5. Compositional variations vs SiO2 content in glasses in anhydrous (left column) and hydrous spinel harzburgite and lherzolite xenoliths
(right column) from the Canary Islands. Groundmass and glass in host lavas of the xenoliths are indicated by letters: P, La Palma; H, Hierro;
T, Tenerife; L, Lanzarote. For comparison are also shown trends defined by glasses in the vein system of veined spinel harzburgite from La
Palma, PAT2-4 (grey field; Wulff-Pedersen et al., 1996a), by aphyric lavas in Tenerife (field outlined by dotted line; E.-R. Neumann & E. WulffPedersen, unpublished data, 1996), aphyric lavas in Hierro (field outlined by continuous line; data from Pellicier, 1977, 1979), and mafic MORB
(star). Incl, inclusions in orthopyroxene and olivine. The horizontal dashed line in the K2O–SiO2 figure shows the lower limit of the range
defined by glasses in hydrous xenoliths. (See text for discussion.)
1526
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Fig. 6.
1527
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 11
NOVEMBER 1997
Fig. 6. Compositional variations vs SiO2 content in glass inclusions in olivine and interstitial glass in anhydrous (left column) and hydrous (right
column) Group I spinel dunite xenoliths in the Canary Islands. Symbols as in Fig. 5. (See text for discussion.)
differences between host lava attached to different xenoliths. This means that different types of xenoliths are
hosted by lava of the same composition. Furthermore,
host lavas from different localitites and islands fall on, or
close to, the trends defined by aphyric lavas in Tenerife
(E.-R. Neumann & E. Wulff-Pedersen, unpublished data,
1996) and Hierro (data from Pellicier, 1977, 1979;
Figs 5–7).
DISCUSSION
The origin of the silicic glasses
We noted above that silicate glasses in peridotite xenoliths
from the Canary Islands cover a wide range in compositions from basaltic, to SiO2–Al2O3–Na2O–K2O-rich,
TiO2–FeO–MgO–CaO–P2O5-poor types (Tables 1 and
2; Figs 5 and 6). There is considerable evidence that the
highly silicic melts are in, or close to, equilibrium with
the spinel peridotites in which they are found, whereas
the basaltic melts are not. This is indicated by the sharp
contacts between silicic glass and peridotite minerals, in
contrast to the reaction-type contacts between xenoliths
and basaltic glass seen in both unveined (this study)
and veined xenoliths. Furthermore, the compositions of
daughter minerals and microlites in highly silicic glass
are similar to those of the main phases in the host xenolith
(Mg-rich olivine Cr-diopside, chromite; Fig. 4, Table 3),
whereas those in basaltic glass are not (e.g. Ti–Fe-rich
augite and titanomagnetite; Figs 5 and 6). Another important result of this study is that it reveals systematic
relations between glass composition and type of host
xenolith, mode of occurrence (or relative age) of the glass,
and locality (island). The mineral–melt relations, and
contact relations between glass and peridotite minerals
found for unveined Canary Islands xenoliths closely
1528
1529
3·42
7·77
4·30
2·78
0·41
MgO
CaO
Na2O
K 2O
P 2O5
63·5
91·6
mg-no.
Fo (host)
Sum
S
100·07
2·81
0·04
MnO
Cl
3·11
3·50
FeO
90·3
99·29
0·42
49·16
0·07
0·52
Cr2O3
92·5
67·0
98·59
0·03
0·31
6·76
2·64
0·03
2·32
0·49
16·64
17·70
1·79
61·66
Al2O3
9·39
40·25
1·81
57·82
TiO2
SiO2
glass
glass
ol
incl in sp
interstitial
Phase:
anhydr
anhydr
gl+ol
H1-8
H1-4
Phase ass: gl+ol
Sample:
harzburgite
Rock type: Sp
91·2
98·30
0·20
49·07
0·11
8·47
40·45
ol
91·1
65·5
98·67
0·07
1·88
2·20
4·54
8·23
4·88
0·01
4·58
0·00
14·72
4·85
52·71
glass
86·4
100·98
0·27
46·50
0·17
13·02
41·02
ol
gl+ol+cpx+sp+F
interstitial
anhydr
H1-12
89·9
100·64
0·81
20·39
16·67
0·07
3·33
1·37
5·38
0·05
52·27
cpx
56·2
99·87
0·10
15·09
0·17
20·99
38·21
23·41
1·90
sp
92·5
100·29
0·46
23·68
16·63
0·07
2·41
0·92
2·51
0·05
53·56
cpx por
host rock
58·1
99·20
0·00
15·16
0·17
19·46
33·33
31·08
0·00
sp
91·1
66·5
98·91
0·12
0·10
6·48
5·51
0·23
1·48
0·06
1·33
16·00
0·04
67·56
glass
gl+ol+F
incl in opx
phlog
TF14-14
92·4
99·48
0·05
50·27
0·25
7·41
41·50
ol
90·8
67·7
96·65
0·05
0·10
4·83
3·66
2·69
1·88
0·02
1·60
0·01
16·22
0·27
65·32
glass
91·8
100·04
0·27
0·01
1·28
20·55
16·81
0·03
2·68
1·94
1·48
0·00
54·99
cpx
gl+sp+cpx+F
incl in ol
phlog
TF14-53
59·4
97·89
0·03
14·38
0·17
17·49
49·91
15·49
0·34
0·08
sp
Table 3: Representative compositions of euhedral to subhedral crystals coexisting with highly silicic glass in polyphase inclusions (incl) and interstitial
glass (interstitial), compared with wall-rock phases (host rock)
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
cpx
host rock
18·56
Al2O3
1530
5·44
0·64
0·04
Na2O
K 2O
P 2O5
Cl
53·8
90·5
mg-no.
Fo (host)
91·7
98·23
90·5
99·84
0·01
1·75
19·32
16·83
90·1
48·8
98·95
0·38
0·40
3·58
7·77
10·04
2·41
60·8
94·81
0·00
0·03
0·03
0·11
0·09
16·80
0·06
49·6
99·28
0·00
0·14
13·65
0·12
24·69
37·99
22·11
0·57
0·01
45·9
98·81
0·07
0·17
12·97
0·20
27·25
39·07
18·31
0·75
0·02
sp(rim)
89·0
71·9
100·95
0·00
0·00
4·56
5·46
4·69
4·18
0·00
2·91
19·24
0·92
58·99
glass
88·2
100·89
0·86
21·03
17·94
0·36
4·26
3·39
0·83
52·22
cpx
88·5
99·92
0·70
20·49
16·90
0·16
3·93
1·02
2·25
0·89
53·58
cpx
host rock
Phase ass., phase assemblage. Other abbreviations as in Table 1. Additional data are given in Fig. 2.
98·06
Sum
0·01
0·88
20·54
17·44
0·03
19·32
22·09
35·63
0·33
0·32
sp(core)
incl in ol
gl+cpx+F
90·1
53·6
99·64
0·10
0·13
3·50
6·34
7·41
2·43
0·09
3·75
0·26
21·04
0·12
54·47
glass
89·0
99·98
0·23
48·01
0·15
10·58
41·01
ol
gl+ol+cpx+F
intersititial
anhydr
LA6-12
89·4
65·5
99·08
0·33
0·09
4·19
3·17
2·70
2·72
0·12
2·55
0·04
21·62
0·35
61·20
glass
gl+cpx+F
incl in ol
phlog
amph/
PAT2-74
90·6
100·52
0·01
0·27
0·01
0·88
22·33
16·62
0·07
3·06
0·62
1·36
0·11
55·18
cpx
91·8
99·23
0·55
23·76
15·94
0·05
2·55
0·40
2·72
0·37
52·85
cpx
host rock
NUMBER 11
S
2·08
5·38
CaO
1·47
MgO
0·09
0·02
MnO
4·50
0·00
21·90
0·74
sp
host rock
sp por
phlog
TF14-4
wehrlite
Sp
VOLUME 38
0·11
3·48
3·13
2·81
2·25
FeO
0·44
0·02
Cr2O3
1·36
0·00
47·20
0·09
54·77
62·09
SiO2
TiO2
55·08
glass
glass
Phase:
cpx
gl+ sp+F
incl in ol
anhydr
anhydr
incl in ol
PAT2-34
TF14-5
Sp dunite
Phase ass: gl+cpx+F
Sample:
lherzolite
Rock type: Sp
Table 3: continued
JOURNAL OF PETROLOGY
NOVEMBER 1997
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Fig. 7. Normative quartz and nepheline + leucite (CIPW) plotted against SiO2 for glass inclusions in olivine and orthopyroxene and interstitial
glass in different types of host xenoliths in the Canary Islands (a–d, f ). The figure shows that glasses in spinel harzburgites and lherzolites are
generally silica oversaturated, whereas those in spinel dunites and wehrlites are silica undersaturated. The ‘open-star’ symbol represents average
N-MORB. Groundmass and glass in host lavas of the xenoliths are indicated by letters: P, La Palma; H, Hierro; T, Tenerife; L, Lanzarote.
Trends defined by glasses in the vein system of veined spinel harzburgite PAT2-4 and veined spinel dunite PAT2-62, are shown as grey fields
in (a) and (b), and in (c) and (d), respectively (grey fields; Wulff-Pedersen et al., 1996a). Trends are defined by aphyric lavas from Tenerife (field
outlined by dotted line; E.-R. Neumann & E. Wulff-Pedersen, unpublished data, 1996), and Hierro (field outlined by continuous line; data from
Pellicier, 1977, 1979). (f ) Published data on glass in spinel harzburgite and lherzolite xenoliths from other locations: Yemen—interstitial glass,
glass in glass pockets and veinlets in spinel lherzolites (Chazot et al., 1996); W Eifel—interstitial glass in spinel harzburgites (Zinngrebe & Foley,
1995); Mongolia—glass in glass pockets in spinel lherzolites from Mongolia (Ionov et al., 1994); Oceanic and Continental—glass inclusions in
spinel harzburgites and lherzolites from various oceanic and continental localities, respectively (Schiano et al., 1992, 1994; Schiano & Clocchiatti,
1994); Philippines—glass inclusions in spinel harzburgites from Philippine arc lavas (Schiano et al., 1995). Incl, inclusions in orthopyroxene and
olivine.
resemble those found in the vein systems of veined
xenoliths from La Palma (Wulff-Pedersen et al., 1996a).
The small compositional differences found to exist among
the host lavas cannot account for the chemical contrasts
exhibited by glasses in different types of peridotites and
different localitites (islands). Most striking is the fact that
the host lavas fall on a common K2O–SiO2 trend, whereas
the silicic glasses in hydrous xenoliths are markedly
richer in K2O than glasses with similar SiO2 contents in
anhydrous xenoliths (Figs 5 and 6). Furthermore, whereas
all the host lavas fall close to the TiO2–SiO2 trend defined
by aphyric lavas in Tenerife and Hierro (Figs 5 and 6),
glasses in Hierro xenoliths appear to define both a highTiO2 and a low-TiO2 trend.
The above relations have important implications for
our understanding of the origin of silicic glasses in mantle
xenoliths. They indicate that the compositional diversity
found to exist among glasses in Canary Islands xenoliths
does not reflect random chemical variations among infiltrating melts migrating undisturbed through a column
1531
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 11
NOVEMBER 1997
Fig. 8. Na2O–CaO–K2O relations among glasses in (a) spinel dunite and (b) harzburgite and lherzolite xenoliths from the Canary Islands. Glass
in veined spinel harzburgite (PAT2-4) and dunite (PAT2-62; Wulff-Pedersen et al., 1996a) are shown as grey fields marked ‘vein glass’. Mafic
basaltic lavas (MgO>7 wt %) from the islands of La Palma, Hierro, Tenerife and Lanzarote (data from Fuster et al., 1968a; Pellicier, 1977,
1979; Staudigel, 1981; Hernandez-Pacheco & Valls, 1982; Staudigel et al., 1986; E.-R. Neumann & E. Wulff-Pedersen, unpublished data, 1996)
are shown as grey fields marked ‘basalts’. (c) Published data on series of glass inclusions and glass pockets in spinel harzburgites and lherzolites
from other localities: GC hydr and GC anhydr—glasses in phlogopite-bearing and phlogopite-free spinel harzburgite xenoliths from Gran
Canaria (Amundsen, 1987); Mongolia (Ionov et al., 1994); Gees, West Eifel (Zinngrebe & Foley, 1995), Mt Lessini, Southern Alps, and Cape
Verde (Siena & Coltorti, 1993). (See text for discussion.)
of different mantle rock types (Model 1: Edgar et al.,
1989; Schiano et al., 1992, 1994, 1995; Schiano & Clocchiatti, 1994). Instead, the observed relations imply a
direct association between melts and host xenolith which
may arise through reactions between infiltrating basaltic
melts and peridotite wall-rock (Model 2: Zinngrebe &
Foley, 1995; Wulff-Pedersen et al., 1996a), or through in
situ partial melting (Model 3: Amundsen, 1987; Francis,
1987; Hauri et al., 1993; Ionov et al., 1994; Baker et al.,
1995; Chazot et al., 1996; Draper & Green, 1997).
Model 2 (reactions between infiltrating melts and
mantle wall-rocks) is also supported by a strong decrease
in TiO2/Al2O3 ratios from the least silicic to the most
silicic glasses (exemplified by data on Hierro in Fig. 9).
Experiments at pressures of 10–20 kbar (e.g. Mysen &
Kushiro, 1977; Jaques & Green, 1980; Falloon & Green,
1987) indicate only moderate fractionation of Ti relative
to Al over a wide range of partial melting of spinel
peridotite. Similar results were obtained in near-solidus
melting experiments (2–5% melting of spinel lherzolite)
at 10 kbar (Baker et al., 1995). This implies that a silicate
melt and spinel peridotite in equilibrium should have
similar TiO2/Al2O3 ratios. Spinel harzburgites and lherzolites from La Palma, Hierro and Lanzarote are characterized by TiO2/Al2O3 <0·07. This is within the range
of ‘normal’ oceanic peridotites and primitive tholeiitic
basalts (highest MgO/FeO ratios) collected along ‘normal’ segments of the Mid-Atlantic Ridge (Bryan et al.,
1981; Sigurdsson, 1981; Schilling et al., 1983; Weaver et
al., 1985; Menzies, 1991). Mafic Canarian lavas (MgO
>7 wt %), in contrast, have TiO2/Al2O3 >0·15. Glass in
veined spinel peridotite xenoliths from La Palma shows
a gradual decrease in TiO2/Al2O3 from about 0·2 in
alkali basaltic glass in broad veins, to TiO2/Al2O3 <0·08
in the most SiO2-rich glass in very narrow veinlets penetrating peridotite fragments (Wulff-Pedersen et al., 1996a;
Fig. 9). The variation in glass chemistry in the veined
xenoliths was interpreted by Wulff-Pedersen et al. (1996a)
as the consequence of melt–wall-rock reactions. These
reactions start with infiltration by alkali basaltic melts
(with high TiO2/Al2O3 ratios) out of equilibrium with
the peridotite wall-rock, and their end-products are silicic
1532
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
Table 4: Representative analyses of host lavas of xenoliths from different localities
Island:
La Palma
Hierro
Tenerife
Lanzarote
Sample:
PAT2
PAT2-84
H1-6
H1-32
TF14-15
TF14-39
TF14-62
LA6-6
LA6-19
LA8-9
WR
grmass
WR
glass
grmass
grmass
grmass
grmass
grmass
grmass
SiO2
44·28
49·36
41·68
43·15
43·66
43·24
42·27
45·70
47·10
TiO2
3·64
2·26
5·07
5·44
4·85
4·96
4·82
2·98
3·31
2·46
Al2O3
13·11
20·48
12·59
13·49
14·11
14·14
13·98
16·58
16·88
16·94
FeOtotal
11·80
6·84
14·14
15·09
13·40
13·44
13·90
10·69
9·88
7·52
0·19
0·16
0·19
0·19
0·20
0·22
0·21
0·13
0·17
0·08
MgO
8·26
2·12
9·47
4·16
4·87
4·69
4·81
4·94
3·98
4·97
CaO
11·37
8·72
11·01
9·85
12·02
12·54
12·72
10·05
9·81
13·16
MnO
48·24
Na2O
3·83
6·54
2·13
3·75
3·01
2·66
2·22
5·28
5·11
4·55
K 2O
1·76
2·71
0·81
1·79
0·95
1·75
1·57
2·35
2·19
1·41
P 2 O5
0·89
1·36
1·17
1·62
1·32
1·42
1·34
1·06
1·28
1·26
Cl
0·10
0·08
0·04
0·04
0·04
0·04
0·09
0·06
S
0·09
0·06
0·13
0·20
0·15
0·04
0·04
0·04
98·61
98·56
99·30
98·30
99·76
99·71
100·59
Sum
99·13
100·67
98·26
WR, whole rock; grmass, groundmass; glass, glass in host lava.
Fig. 9. TiO2/Al2O3 vs SiO2 for glass inclusions and interstitial glasses in spinel harzburgite and lherzolite xenoliths from Hierro. Xen, peridotite
xenoliths from Hierro; basalt, basaltic lavas from Hierro with MgO>7 wt % (data from Pellicier, 1977, 1979); vein, field of vein glasses in veined
spinel harzburgite PAT2-4 (data from Wulff-Pedersen et al., 1996a). (See text for discussion.)
melts (with low TiO2/Al2O3 ratios) in, or close to, equilibrium with the wall-rock. The postulate that highly
silicic, FeO–MgO–CaO-poor melts may be in equilibrium with spinel peridotite is supported by the experiments of Baker et al. (1995) and Draper & Green
(1997).
Model 3 (different degrees of in situ partial melting)
cannot account for the large differences in TiO2/Al2O3
ratios (Fig. 9) observed among glasses in Canary Islands
xenoliths. It is also difficult to reconcile the fact that the
least silicic glasses appear not to be in equilibrium with the
host peridotite with partial melting sensu stricto. However,
mixing between melts formed by very small degrees
of in situ partial melting (low TiO2/Al2O3 ratios) and
infiltrating mafic melts with high TiO2/Al2O3 ratios cannot be excluded.
Decompression melting (Model 4: e.g. Frey & Green,
1974; Francis, 1976) has been dismissed as a general
mechanism for the formation of highly silicic melts by
Edgar et al. (1989) and Zinngrebe & Foley (1995). Textural
relations indicate that in the Canary Islands xenoliths
SiO2-rich melts were present in the peridotites before
and during the neoblast generation of crystal growth (see
description of glass inclusions). This was followed by a
period of cooling and exsolution of pyroxenes before
entrainment of the xenoliths in the magma which
1533
JOURNAL OF PETROLOGY
VOLUME 38
transported them to the surface. Decompression melting
has therefore been discarded as a general model for
formation of silicic melts in the Canary Islands xenoliths,
although we cannot exclude the possibility that some
silicic melt has also formed during ascent.
Partial melting during short residence times in shallow
magma chambers during ascent (Model 5: Klu¨gel et al.,
1996) has also been discarded as a relevant model for
the xenoliths of this study. Studies on dissolution rates
indicate that ultramafic xenoliths can survive in a host
lava for only a few days before they are dissolved and
consumed [e.g. Scarfe & Brearley (1987), and references
therein]. Although the Canary Islands xenoliths frequently show narrow reaction rims against the enclosing
basaltic magma, their surfaces are well defined. In contrast to glasses in the veined xenoliths from La Palma
(Wulff-Pedersen et al., 1996a), occasional veinlets of host
magma penetrating xenoliths have maintained basaltic
compositions. Formation of the silicic glass by partial
melting of mantle xenoliths in crustal magma chambers
during ascent to the surface therefore seems highly unlikely.
Also, immiscible separation of mantle melts into a
silicate + carbonate melt pair (Model 6: Schiano et al.,
1994) seems an unlikely mode of formation for the silicic
glasses discussed here. Kogarko et al. (1995) reported
interstitial silicate glass (~64 wt % SiO2, mildly silica
oversaturated) with carbonate and sulphide globules in
wehrlitic alteration zones of Group II composition in
spinel harzburgite xenoliths from the Montana Clara
Island, Canary Islands, and interpreted this as the result
of immiscible separation. The composition of the silicate
glass is within the range of glasses analysed by us. Mixed
silicate glass + carbonate inclusions have also been observed in composite Group II clinopyroxene–spinel dunite xenoliths cut by clinopyroxenite veinlets from Gomera
(Fo83–85; Frezzotti et al., 1994). However, there the silicate
glass associated with carbonate is ultramafic (33–46 wt
% SiO2, 24–38 wt % MgO, 5–18 wt % FeO) and has
very low concentrations of Al2O3 (0·1–2·1 wt %) and
alkalis (<0·1 wt % Na2O and K2O), and has no apparent
relation to the highly silicic, alkali-rich glass which is
the topic of this paper. We have observed carbonate
aggregates in interstitial glass in xenoliths from Tenerife,
but these aggregates appear to be secondary, filling
rounded vesicles. Occurrences which may represent carbonate melt inclusions have not been observed in Group
I xenoliths. Without visible evidence that carbonate melts
were present, we find it hard to believe that liquid
immiscibility has been an important process in the formation of silicic melts in the Group I xenoliths.
We conclude that silicic melts in the upper mantle
under the Canary Islands mainly result from reactions
between infiltrating, alkali basaltic melts ascending from
NUMBER 11
NOVEMBER 1997
deeper parts of the mantle, and spinel peridotite wallrocks (Model 2). The observed tendency for interstitial
glass pockets to contain a higher proportion of relatively
SiO2-poor glasses than the inclusions in olivine and
orthopyroxene porphyroclasts (Figs 5 and 6) suggests that
many interstitial melts have not had time to go through
the extensive reactions which may eventually lead to
equilibrium with the peridotite wall-rock. Glass in secondary inclusion trails in olivine represents interstitial
melts that were trapped in fractures at some earlier stage.
The more restricted range of high SiO2 contents found
among these glasses may be the result of reaction and
crystallization during healing of the fractures in which
the melts were trapped. However, although we regard
reaction processes as most important, this does not preclude the possibility that formation of silicic melts by in
situ melting, and mixing between infiltrating melts and
in situ melts (Model 3, modified) may also have taken
place at some stage of the evolution.
Reaction processes
Silicic glasses in unveined, hydrous peridotite xenoliths
from the Canary Islands define compositional trends
which resemble those exhibited by melts in veined xenoliths from La Palma (Wulff-Pedersen et al., 1996a). It
therefore seems likely that the reactions which caused
the evolution from infiltrated, basaltic melt to highly
silicic melts in the veined xenoliths may be used to gain
insight into the evolution from basaltic to silicic melts in
the unveined xenoliths. Textural relations in the veined
xenoliths combined with petrographic mixing calculations (Wulff-Pedersen et al., 1996a) imply that the
highly silicic melts are the products of a series of reactions
between infiltrating basaltic melts (and melts formed
through the reactions) and peridotite. Reactions between
basaltic melts and peridotite include formation of the
daughter phases cpx±amph±ol + melt at the expense
of primary opx±ol±phlog in harzburgite and lherzolite,
and the daughter phases cpx + sp±amph±phlog±melt
at the expense of primary ol±phlog in spinel dunite
and wehrlite. Daughter minerals formed through these
reactions are TiO2–Al2O3–FeO rich (e.g. augite, kaersutite, Fe-rich olivine). As the reactions progress, the
melts produced become progressively enriched in SiO2
and alkalis, and depleted in TiO2, FeO, MgO and CaO,
and phlogopite takes the place of amphibole in the
daughter mineral assemblages. The daughter minerals
become progressively depleted in TiO2, Al2O3 and FeO,
and enriched in MgO, and in highly silicic glass they
have compositions close to the peridotite wall-rock phases.
Also, the unveined harzburgites and lherzolites provide
considerable petrographic evidence (see description
above) for the formation of olivine and silicic melt at the
1534
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
expense of orthopyroxene (e.g. Figs 2 and 4a). This is
clearly a major source of SiO2 enrichment in the melt.
Under dry conditions, incongruent melting of orthopyroxene is restricted to pressures below about 5 kbar,
but this pressure limit increases to more than 20 kbar in
the presence of an H2O-rich fluid (e.g. Eggler, 1972;
Mysen & Boettcher, 1975a, 1975b). Incongruent melting
of orthopyroxene within the pressure range of origin of
the xenoliths of this study, 8–12 kbar ( Johnsen, 1990;
Hansteen et al., 1991; Neumann et al., 1995), requires
the presence of an H2O–CO2 fluid. Silicic glass and
phlogopite in harzburgite and dunite xenoliths from
La Palma contain 0·21–0·36 and 2·7–3·1 wt % H2O,
respectively (Wulff-Pedersen et al., 1996b). The large
vesicles frequently found in glass pockets (Fig. 2) indicate
that a fluid phase unmixed from the silicic melts during
ascent of the xenoliths, implying that the original fluid
content of the melts was considerably higher than that
indicated by analyses of the degassed glass. The
H2O + CO2 content in the upper mantle may thus
have been high enough to allow incongruent melting of
orthopyroxene to take place at pressures of 8–12 kbar.
The frequent presence of phlogopite in mantle xenoliths
from Tenerife suggests the presence of H2O in addition
to CO2 (seen as fluid inclusions) also in the upper mantle
under Tenerife. The near absence of phlogopite and
amphibole in xenoliths from Hierro and Lanzarote suggests higher CO2/(CO2 + H2O) ratios, or lower fluid
partial pressure in the upper mantle below these islands.
However, also, mantle xenoliths from these islands bear
evidence of incongruent melting of orthopyroxene.
The postulated effects of reactions between infiltrating
basaltic melts and peridotite wall-rock are also supported
by Kelemen’s (1990) modelling. He showed that liquids
saturated in olivine and undersaturated in low-Ca pyroxene, reacting with lherzolite or harzburgite, will dissolve
low-Ca pyroxene and crystallize a smaller mass of olivine.
Kelemen (1990) predicted that this kind of reaction will
lead to silica enrichment in the derivative liquids. We
believe that addition of SiO2 to the melt through reaction
between harzburgite or lherzolite and alkali basaltic
melts and derivative liquids, all undersaturated in lowCa pyroxene, is responsible for the high SiO2 contents
and silica oversaturation in the majority of glasses in
spinel harzburgites and lherzolites, as compared with
glasses in spinel dunites and wehrlites (Figs 5–7), which,
in the Canary Islands, generally do not contain orthopyroxene. Addition of SiO2 to the melts as the result
of breakdown of orthopyroxene accounts for the fact that
the most silicic glass in spinel harzburgites and lherzolites
is significantly richer in SiO2 than any of the mineral
phases in the host xenolith. Furthermore, Kelemen (1990)
predicted that melts saturated in olivine and calcic pyroxene will stay that way during melt–wall-rock reactions.
Formation of clinopyroxene and olivine through melt–
wall-rock reactions is in agreement with the decreasing
concentrations of CaO, FeO and MgO with increasing
SiO2 in the glass (Figs 5 and 6).
In unveined Canary Islands xenoliths high K2O contents and high K2O/Na2O ratios are found in glasses of
xenolith series where phlogopite is a common accessory
interstitial phase (La Palma and Tenerife), whereas low
K2O contents and low K2O/Na2O ratios are typical of
glasses in xenolith series where interstitial phlogopite is
very rare (Hierro and Lanzarote; Tables 1 and 2; Figs 5,
6 and 8). It seems likely therefore that the K2O content
in glass is directly related to the presence or absence of
phlogopite in the mantle wall-rock with which the melt
has reacted. This is in agreement with petrographic
mixing calculations which indicated that the strong increase in K2O with increasing SiO2 defined by the veined
xenoliths required consumption of a K2O-rich phase,
such as phlogopite (Wulff-Pedersen et al., 1996a). Consumption of phlogopite is supported by the corroded
contacts between interstitial phlogopite and glass in unveined xenoliths, which indicate that interstitial phlogopite was already present in the upper mantle under
La Palma and Tenerife before formation of the silicic
melt and was partially consumed during its generation.
The following reactions involving phlogopite are compatible with petrographic observations:
{Na0·3K1·7Mg5·0Fe0·5Al0·5[Al2·2Si5·8O20](OH)4}phlog
→3[(Mg,Fe)2SiO4]ol + [(Na0·3K1·7)O + 2·8SiO2 +
1·35Al2O3 + 2H2O]melt
(1)
{Na0·3K1·7Mg5·0Fe0·5Al0·5[Al2·2Si5·8O20](OH)4}phlog
+ 5·8(CaO)melt→5·8[Ca(Mg,Fe)(Si,Al)2O6]cpx
+ 0·2[(Mg,Fe)Al2O4]sp + [(Na0·3K1·7)O
+ 0·95Al2O3 + 2H2O]melt
(2)
Where phlogopite is present, it will play an important
role in melt–wall-rock reactions, resulting in melts with
high K2O contents, and high K2O/Na2O ratio compared
with the initial infiltrating melt. In anhydrous mantle
domains alkalis will not be added to the melt through
the breakdown of hydrous phases, although some Na2O
may be partitioned into the melt from clinopyroxene.
The main change in Na2O–CaO–K2O relations will
result from removal of CaO from the infiltrating melt
(to form new clinopyroxene), causing the Na2O/CaO
ratio in the melt to increase as reactions progress. The
K2O/Na2O ratio in glasses formed by reactions with
anhydrous peridotite is expected to be similar to, or
slightly lower than, that of the infiltrating melt, and
(Na2O + K2O)/CaO and K2O/Na2O ratios are expected
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JOURNAL OF PETROLOGY
VOLUME 38
to be significantly lower than in melts with similar SiO2
contents in hydrous peridotite. If amphibole is present
in the peridotite wall-rock, breakdown of amphibole will
add sodium and minor amounts of potassium to the melt,
resulting in marked Na2O enrichment and low K2O/
Na2O ratios compared with the infiltrating melt.
The high K2O contents and K2O/Na2O ratios in silicic
glasses in xenoliths from La Palma and Tenerife as
compared with those in xenoliths from Hierro and Lanzarote (Fig. 8) are compatible with reactions involving
phlogopite in the peridotite wall-rocks. It should also be
noted that the K2O/Na2O ratios of these glasses are
higher than those of the host basalts and Canary Islands
basalts in general (Fig. 8). Within the xenolith suites from
La Palma and Tenerife, we see no difference in K2O
content between glasses in xenoliths with, and those
without, interstitial phlogopite. This suggests that phlogopite may originally have been more common in the
upper mantle under La Palma and Tenerife than is
indicated by the modal composition of the xenoliths, but
that it locally has been totally consumed through reactions
and/or partial melting. Glasses in the anhydrous xenolith
suites from Hierro and Lanzarote show K2O/Na2O ratios
similar to, or below, those in Canary Islands basalts. These
ratios are compatible with reactions between anhydrous
mantle wall-rock and infiltrating melts covering a range
in K2O/Na2O ratios, including very low values. However,
if the K2O/Na2O ratios of the Canary Islands basalts are
representative of the infiltrating melts, reactions involving
amphibole-bearing peridotite are likely. Amphibole has
not been reported in mantle xenoliths from Hierro or
Lanzarote (Fuster et al., 1968b; Sagredo Ruiz, 1969;
Johnsen, 1990; Neumann, 1991; Neumann et al., 1995),
but pre-existing amphibole may have been consumed by
reaction processes.
Our data imply that the phase assemblage of the
mantle wall-rocks is not only a factor controlling the
compositions of melts formed by in situ partial melting,
but is also a factor that strongly affects the compositions
of silicic melts which are modified through melt–wallrock reactions. The composition of glass inclusions in a
mantle rock series may thus give important information
about the evolutionary history of the mantle domain.
Silicic glasses in mantle xenoliths from other localities
and tectonic settings also show wide ranges in Na2O–
CaO–K2O relations (Fig. 8c). Some of these peridotite
suites show ‘expected’ relationships between the Na2O–
CaO–K2O relations in silicic glasses and the modal
composition of their host xenoliths, whereas in other
suites such connections appear to be lacking. In Gran
Canaria glasses in phlogopite-bearing xenoliths are relatively enriched in K2O and define a trend parallel to
the K2O–Na2O tie-line, whereas glasses in anhydrous
xenoliths are significantly poorer in K2O and define a
trend parallel to the CaO–Na2O tie-line (Amundsen,
NUMBER 11
NOVEMBER 1997
1987). Amundsen (1987) interpreted these glasses as the
results of in situ partial melting involving breakdown
of phlogopite in the phlogopite-bearing xenoliths, and
breakdown of pre-existing amphibole in the anhydrous
xenoliths. The highest degrees of K2O enrichment are
observed in glasses in xenoliths from Cape Verde [data
from Siena & Coltorti (1993)] and Gees, West Eifel [data
from Zinngrebe & Foley (1995)]. The high K2O contents
of these glasses strongly suggest partial melting of phlogopite (or another K-rich phase) in the mantle wall-rock.
Phlogopite is common in West Eifel xenoliths, both as
interstitial grains and in veins where it is associated with
amphibole (C. Shaw, personal communication, 1997).
Descriptions of xenolith suites from Cape Verde (Siena
& Coltorti, 1989, 1993) indicate no K-bearing minerals
which may account for the high K2O contents in the
silicic glasses. Glasses in xenoliths from Mt Lessini, Southern Alps, show moderate enrichment in K2O, and no
hydrous minerals are reported in their host xenoliths
(Siena & Coltorti, 1989, 1993). Glasses in xenoliths
from Yemen define a trend of strong Na2O enrichment,
interpreted by Chazot et al. (1996) as the result of in
situ partial melting of clinopyroxene and/or amphibole,
which are observed as residual phases. Glasses in anhydrous spinel lherzolite xenoliths from Mongolia [data
from Ionov et al. (1994)] define a kinked trend of Na2O
enrichment with decreasing CaO among the more CaOrich glasses, and marked K2O enrichment with decreasing
CaO among the most CaO-poor ones. These rocks show
a clear negative correlation between modal clinopyroxene
and glass, and Ionov et al. (1994) interpreted the glasses
‘as the result of disequilibrium in situ melting, involving
largely clinopyroxene and spinel, resulting from reaction
with migrating fluids’. Resorption of clinopyroxene explains the increase in Na2O relative to CaO, whereas
the mechanism behind the trend of K2O enrichment
remains an open question.
Origin of the infiltrating melts
Based on the discussion above, we assume that the
least silicic glasses are closest to the compositions of the
infiltrating melts. The mildly to strongly silica-undersaturated nature of these glasses (Table 1, Fig. 7) suggests
that the reaction processes started with infiltration by
alkali basaltic melts. Low-SiO2 glasses are most common
in xenoliths from Hierro and Tenerife. These glasses
form two groups: a low-TiO2 group, and a group with
relatively high TiO2, CaO and P2O5 contents (Figs 5–7).
Low-SiO2 glasses in harzburgites and lherzolites from
Tenerife belong to the high-TiO2 group (Fig. 5), whereas
low-SiO2 glasses in dunites from La Palma appear to
1536
NEUMANN AND WULFF-PEDERSEN
MANTLE XENOLITHS FROM CANARY ISLANDS
belong to the low-TiO2 group (Fig. 6). Neither group
shows affinity to mid-ocean ridge basalt (MORB). The
observed compositional diversity among the low-SiO2
glasses strongly suggests that the melt–wall-rock reactions
involved infiltrating melts of different compositions. Furthermore, although the reaction processes which gave
rise to the silicic glasses must have taken place during
the formation of the Canary Islands, the glasses include
more extreme compositions (TiO2–P2O5 rich and TiO2–
P2O5 poor) than seen among the aphyric basalts from
Hierro and Tenerife, and lavas from the Canary Islands
in general. This suggests that the total compositional
range of melts produced during the Canary Islands
magmatism is greater than that reflected in the exposed
lavas. It is unclear if the compositional diversity seen
among the low-SiO2 glasses is the result of the melting
processes in the source region (different degrees of partial
melting, different pressures, etc.), or if they are imposed
through interaction with different types of wall-rock during ascent through the upper mantle.
Mobility of the silicic melts
Wulff-Pedersen et al. (1995, 1996a, 1996b) showed that
silicic melts in mantle rocks from the Canary Islands are
highly enriched in strongly incompatible as compared
with mildly incompatible elements [e.g. heavy rare earth
elements (HREE), Ti and Zr], and proposed that silicic
melts may be important agents of cryptic metasomatism
in the mantle. The potential of a melt to act as a
metasomatic agent is highly dependent on its mobility.
Silicic glass in Canary Islands xenoliths generally makes
up a very small proportion of the total rock volume
(0·1–4·3% in La Palma; Wulff-Pedersen et al., 1996a).
Although glass in secondary inclusion trails represents
melts which at one time moved into cracks and fractures,
it is clear that the ability of a fluid to move through the
mantle is greatly reduced if the fluid is present in such
small amounts that fluid–mineral cohesive forces are
large relative to the buoyancy of the fluid. Furthermore,
the most silicic glasses consist almost exclusively of SiO2,
Al2O3, Na2O and K2O (Tables 1 and 2). Unless they are
relatively rich in H2O, such melts are expected to be
highly polymerized and viscous. As indicated above, it is
likely that the silicic glass and phlogopite of this study
contained H2O + CO2. There is also evidence in Canary
Islands xenoliths that the silicic melts have moved over
moderate distances. The presence of glass that is clearly
not in equilibrium with the host peridotite in some
unveined xenoliths implies that silicic melts have been
mobile over distances at least corresponding to the dimensions of the xenoliths, that is, up to about 30 cm.
The existence of an open system is supported by the
films of glass and glass inclusions commonly present along
grain boundaries.
CONCLUSIONS
Detailed studies of glasses and associated minerals in
Group I xenoliths (spinel harzburgite, lherzolite, dunite
and wehrlite) from different Canary Islands lead to the
following observations and conclusions:
(1) The glasses cover a considerable compositional range
from TiO2–FeO–MgO–CaO–P2O5-rich basaltic glasses
with ~44 wt % SiO2, to highly silicic, TiO2–
FeO–MgO–CaO–P2O5-poor glass with up to 71 wt %
SiO2.
(2) Several features suggest that highly silicic melts are
in, or close to, equilibrium with the host spinel peridotites,
whereas the basaltic melts are not: daughter minerals
associated with highly silicic glass in polyphase inclusions
are similar in composition to the main phases in the host
xenoliths (Fo>90, Mg–Cr-rich, Ti–Al-poor pyroxenes,
chromite); minerals in less silicic glasses are richer in
Al2O3, TiO2 and FeO, and poorer in MgO; reaction
rims are common between basaltic glass and peridotite
minerals, whereas contacts between highly silicic glass
and peridotite are smooth.
(3) Each island shows a systematic relationship between
glass composition and type of host xenolith. Glasses in
spinel harzburgites and lherzolites are typically silica
oversaturated and extend to higher SiO2 contents than
glasses in dunites and wehrlites (no orthopyroxene), which
are generally silica undersaturated.
(4) The Canary Islands show differences with respect
to Na2O–CaO–K2O relations which are related to the
phase assemblages of their host peridotites. Glasses in
xenoliths from La Palma and Tenerife where interstitial
phlogopite is common have higher K2O contents and
K2O/Na2O ratios than glasses in xenoliths from Hierro
and Lanzarote, where phlogopite is very rare.
(5) The silicic melts are interpreted as the results of
complex series of reactions between infiltrating alkali
basaltic melts and peridotite wall-rocks. The higher SiO2
contents in melts in orthopyroxene-bearing than in orthopyroxene-free rock types result from formation of
SiO2-rich melt + olivine at the expense of orthopyroxene
in the presence of an H2O–CO2 fluid.
(6) The high K2O contents and K2O/Na2O ratios in
glasses from La Palma and Tenerife reflect reactions
involving partial or total consumption of phlogopite produced by earlier metasomatic events. The low K2O
concentration and K2O/Na2O ratios of glasses in the
anhydrous xenolith suites from Hierro and Lanzarote
reflect reactions between infiltrating melts and anhydrous
and/or amphibole-bearing mantle wall-rocks.
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JOURNAL OF PETROLOGY
VOLUME 38
(7) In the upper mantle under the Canary Islands highly
silicic melts have been mobile over a distance at least
equal to the xenolith dimension, that is, 20–30 cm.
ACKNOWLEDGEMENTS
This project was supported by the Commission of the
European Communities, DGXII, Environment Programme, Climatology and Natural Hazards Unit, under
Contract EV5V-CT-9283, and grants from the Norwegian Research Council (NFR) and Nansenfondet and
associated funds. We also gratefully acknowledge the
permission from the Ayuntamiento de Fuencaliente de
La Palma (given to E.-R.N. in 1988) to collect xenolith
samples from the volcanoes of San Antonio and Teneguı´a.
P. Bottazzi, T. H. Green, W. L. Griffin, B. B. Jensen, L.
Ottolini, C. Shaw, R. Vannuzzi and M. Wilson are
gratefully acknowledged for enlightening discussions and
constructive criticism of earlier versions of this paper.
Fieldwork in Tenerife was made pleasant because of the
very comfortable accommodation at the Parador de
Can˜adas del Teide, and the efforts of the staff there.
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