Journal of Petrology Advance Access originally published online on August 31, 2005
Journal of Petrology 2005 46(12):2495-2526; doi:10.1093/petrology/egi063
© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
A Melt Inclusion Record of Volatiles, Trace Elements and Li–B Isotope Variations in a Single Magma System from the Plat Pays Volcanic Complex, Dominica, Lesser Antilles
ANDREY A. GURENKO1,2,*,
ROBERT B. TRUMBULL1,
RAINER THOMAS1 and
JAN M. LINDSAY3
1 GEOFORSCHUNGSZENTRUM POTSDAM, SECTION 4.2, TELEGRAFENBERG, 14473 POTSDAM, GERMANY
2 MAX-PLANCK-INSTITUT FÜR CHEMIE, ABTEILUNG GEOCHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY
3 SEISMIC RESEARCH UNIT, UNIVERSITY OF THE WEST INDIES, ST. AUGUSTINE, TRINIDAD
RECEIVED
JANUARY 30, 2004;
ACCEPTED
JUNE 16, 2005
 |
ABSTRACT
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|---|
Glass inclusions in plagioclase and orthopyroxene from dacitic
pumice of the Cabrits Dome, Plat Pays Volcanic Complex in southern
Dominica reveal a complexity of element behavior and Li–B
isotope variations in a single volcanic center that would go
unnoticed in a whole-rock study. Inclusions and matrix glasses
are high-silica rhyolite with compositions consistent with about
50% fractional crystallization of the observed phenocrysts.
Estimated crystallization conditions are 760–880°C,
200 MPa and oxygen fugacity of FMQ + 1 to +2 log units (where
FMQ is the fayalite–magnetite–quartz buffer). Many
inclusion glasses are volatile-rich (up to 6 wt % H
2O and 2900
ppm Cl), but contents range down to 1 wt % H
2O and 2000 ppm
Cl as a result of shallow-level degassing. Sulfur contents are
low throughout, with <350 ppm S. The trace element composition
of inclusion glasses shows enrichment in light rare earth elements
(LREE; (La/Sm)
n = 2·5–6·6) and elevated
Ba, Th and K contents compared with whole rocks and similar
or lower Nb and heavy REE (HREE; (Gd/Yb)
n = 0·5–1·0).
Lithium and boron concentrations and isotope ratios in melt
inclusions are highly variable (20–60 ppm Li with
7Li
= +4 to +15 ± 2
; 60–100 ppm B with
11B = +6 to
+13 ± 2
) and imply trapping of isotopically heterogeneous,
hybrid melts. Multiple sources and processes are required to
explain these features. The mid-ocean ridge basalt (MORB)-like
HREE, Nb and Y signature reflects the parental magma(s) derived
from the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlations
in inclusion glasses indicate coupled enrichment in strongly
fluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements,
which can be explained by fractional crystallization of plagioclase,
orthopyroxene and Fe–Ti oxides. The
7Li and
11B values
are at the high end of known ranges for other island arc magmas.
We attribute the high values to a
11B and
7Li-enriched slab
component derived from sea-floor-altered oceanic crust and possibly
further enriched in heavy isotopes by dehydration fractionation.
The heterogeneity of isotope ratios in the evolved, trapped
melts is attributed to shallow-level assimilation of older volcanic
rocks of the Plat Pays Volcanic Complex.
KEY WORDS: subduction; volcanic arcs; igneous processes; melt inclusions; SIMS; trace elements; lithium and boron isotopes; diffusion
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INTRODUCTION
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One of the greatest geochemical challenges in understanding
arc magmatism is to identify the components involved in, and
to evaluate their separate contributions to, the origin and
evolution of the erupted magmas. This information is essential
for global cycling models and for assessing what materials and
how much of them are fed into the deeper mantle at subduction
zones. The widely accepted conceptual model for generation of
volcanic arc magmas envisions production of a primary magma
by melting of peridotite in the mantle wedge, the melting process
being enhanced by addition of water and other fluxing components
from the subducting slab (Arculus, 1994
; Pearce & Peate,
1995
; Tatsumi & Eggins, 1995
). Identifying the source components
is difficult enough for near-primary magmas, but these are rarely
erupted and one usually has to deal with intermediate compositions
with the added complexity of magma evolution processes including
fractional crystallization, crustal contamination and magma
mixing. As a consequence, the erupted rocks will commonly be
some mixture of crystals and melts that can have partially separate
histories of evolution or even source, before being finally
aggregated in the eruption column. A viable way forward is to
combine conventional whole-rock and mineral studies with analyses
of melt, fluid and solid phases trapped and isolated in host
phenocrysts. Furthermore, melt inclusion analyses offer the
only direct way to determine pre-eruptive concentrations and
isotope ratios of volatile components that are lost to the atmosphere
and undergo isotopic fractionation during shallow degassing
and eruption.
This paper discusses an application of the melt inclusion approach toward understanding the origin and evolution of intermediate and silicic arc magmas from southern Dominica, which is the volcanically most productive island in the Lesser Antilles arc (Wadge, 1984) and one where explosive silicic eruptions have been especially prominent. Our focus is the late Pleistocene Plat Pays Volcanic Complex (PPVC) in southern Dominica, an area that has been well studied geologically (Lindsay et al., 2003) and on which a comprehensive geochemical and isotopic study of whole-rock samples has recently been completed (Lindsay et al., 2005b). For our in-depth study of mineral and melt inclusions we chose fresh pumice from an airfall eruption of the Cabrits dome, one of 12 dacitic domes that formed within the PPVC by magma resurgence after eruption of the 34 ka Grand Bay Ignimbrite. The whole-rock geochemical and radiogenic isotope compositions of the domes and the Grand Bay Ignimbrite are virtually indistinguishable (Lindsay et al., 2005b), so the results of our analysis of the Cabrits dacite are significant for magma genesis in the PPVC as a whole. We determined the full spectrum of major, trace and volatile (S, Cl, H2O) element concentrations by in situ microanalysis using electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS) and confocal laser Raman spectroscopy. Special effort was made to determine the Li and B isotopic ratios in the melt inclusions because of their utility as tracers of the volatile elements and the slab contribution to subduction zone magmatism (e.g. Leeman et al., 2004). To date, there has been only one other study of B isotopes in the Lesser Antilles Arc, from Martinique (Smith et al., 1997), and ours is the first study to determine Li isotope ratios. Combined with the mineral and whole-rock compositions, these data are used to identify the composition and T, P and 
conditions in the magma prior to eruption and to evaluate the source components and physical processes that contributed to magma genesis. The results demonstrate that a diversity of magma compositions were present within a single dacitic system. This diversity is obscured through amalgamation and mixing in the bulk pumice but is preserved in mineral-hosted inclusions.
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GEOLOGICAL SETTING
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Subduction of the North American plate under the Caribbean plate
resulted in the 850 km long chain of islands that forms the
Lesser Antilles volcanic arc. An excellent thorough review of
Cenozoic volcanism of the arc has been given by Macdonald
et al. (2000)
. Dominica is located in the central group of volcanic
islands in the arc (
Fig. 1a) and it is the volcanically most
productive as well as the potentially most hazardous one, with
nine centers classified as potentially active (Lindsay
et al.,
2005
a) and with a record of explosive ignimbrite activity (Lindsay
et al., 2003
). With the exception of uplifted Pleistocene conglomerates
and coral banks on the west coast (Sigurdsson & Carey, 1991
),
the island of Dominica is made up entirely of Cenozoic volcanic
rocks and their erosional products. The oldest record of volcanism
is from deeply eroded basaltic lavas and breccias of Miocene
age. Pliocene volcanism comprises mainly basaltic to basaltic-andesite
stratovolcanoes (Cochrane-Mahaut and Foundland: Briden
et al.,
1979
; Bellon, 1988
). Quaternary volcanic centers are concentrated
in southern Dominica. The rocks are characteristically silicic,
with compositions dominated by silicic andesite and dacite,
although an exception is Morne Anglais (26–28 ka), which
erupted basalts and basaltic andesite very similar in composition
to those of Foundland (Lindsay
et al., 2005
b). Among the Quaternary
volcanics are three large ignimbrites: the Roseau Tuff (
c. 30
ka: Sigurdsson, 1972
; Carey & Sigurdsson, 1980
), the Grand
Savanne Ignimbrite (>22 ka to >40 ka: Sparks
et al., 1980
a,
1980
b), and the Grand Bay Ignimbrite (39 ka: Lindsay
et al.,
2003
). The Grand Bay eruption is thought to have formed the
Soufrière depression in southern Dominica (Lindsay
et al., 2003
), within and around which are the Morne Plat Pays
stratovolcano and some 12 dacitic dome complexes whose ages
range from about 36 ka to 450 years
BP (
Fig. 1b). Lindsay
et al. (2003)
introduced the term Plat Pays Volcanic Complex to
include the Grand Bay ignimbrite and all centers associated
with the Soufrière depression, and a close magmatic affinity
among these units was affirmed by the geochemical and isotopic
study by Lindsay
et al. (2005
b)
. Continuing hot spring activity
and a large swarm of volcanic earthquakes in 1998–2000
demonstrate that the magma system is still active, and indeed
the Soufrière depression is regarded as the most imminent
volcanic hazard on the island (Lindsay
et al., 2003
, 2005
a).
Petrography and bulk composition of the Cabrits Dome volcanic rocks
The Cabrits dome is a typical representative of the group of
12 dacitic domes that formed in and around the Soufrière
depression after eruption of the Grand Bay Ignimbrite (
Fig. 1b).
It is one of the oldest of the post-collapse domes, with
a
13C date of 35·8 ka, compared with 38·8–38·6
ka for the Grand Bay Ignimbrite itself (Lindsay
et al., 2003
).
The main deposits from Cabrits and the other PPVC domes comprise
block and ash flows, lava screens and basal units of pumiceous
airfall and surge deposits. Pumice clasts in the airfall unit
are up to 43 cm in diameter, typically light grey in color with
a reddish oxidized rind. Their crystal content is about 15–20%,
the dominant phenocrysts being plagioclase and orthopyoxene
in a proportion of approximately 2:1; there are also minor to
variable amounts of clinopyroxene or hornblende and Fe–Ti
oxides. The sample D-JL18, which was selected for the melt inclusion
study, contains fresh matrix glass and abundant, naturally quenched
melt inclusions large enough for multiple SIMS analyses (60–150
µm) (
Fig. 2). The host plagioclase (Pl) and orthopyroxene
(Opx) grains are 0·5–3 mm in size; rare microphenocrysts
(<0·3 mm) of the same phases are also present. Equant
crystals of Fe–Ti oxides up to 200 µm across are
present both in the matrix and as inclusions in other minerals.
The glassy matrix commonly shows a discontinuous lamination
and phenocrysts are aligned, probably a result of compaction
and partial welding of the pumice (
Fig. 2a). Orthopyroxene,
plagioclase and titanomagnetite contain abundant primary melt
inclusions, almost all of which are naturally quenched to clear
glass, with or without a shrinkage bubble. A very few inclusions
contain small daughter crystals, and such inclusions were not
used for analysis (
Fig. 3b–d). Inclusions of crystals
within the phenocrysts include apatite and Fe–Ti oxides
in Opx and Pl, Opx inclusions in Pl, and Pl inclusions in Opx
(
Fig. 2b and d).
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Fig. 2. Photomicrographs showing groundmass texture and various types of melt and crystal inclusions in orthopyroxene and plagioclase phenocrysts. (a) Fluidal glassy matrix composing the groundmass of the dacite pumice D-JL18 from the Morne Cabrits dome (Plat Pays Volcanic Complex, Dominica). (b) Two glass inclusions in one orthopyroxene phenocryst containing gas bubble and combined with magnetite crystals; crystalline inclusions of plagioclase, magnetite and numerous needle-like apatite are also present. (c) Series of primary glass inclusions in plagioclase core. (d) Plagioclase-hosted glass inclusion combined with needle-like apatite crystal. Ap, apatite; Mt, magnetite; Pl, plagioclase; Gl incl, glass inclusion.
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Fig. 3. Selected features of geochemical variations and Sr–Nd–Pb isotopic ratios of whole-rock samples from the Plat Pays Volcanic Complex, southern Dominica, from Lindsay et al. (2005b). Data from the Cabrits dome are shown by stars, other units by open triangles. (a) All PPVC units have very similar radiogenic isotope composition, shown here compared with the wide range found in other Lesser Antilles islands (data from GEOROC databank: http://georoc.mpch-mainz.gwdg.de/georoc). (b) The multielement diagram normalized to the composition of sample D-JL18 emphasizes the extreme homogeneity of PPVC rocks and justifies the use of this sample as representative for the PPVC magmas. Other features of the PPVC whole-rock compositions are illustrated in combination with melt inclusion data in the results section.
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The silicic andesites and dacites erupted from the PPVC are
extremely homogeneous in composition, including the Plat Pays
stratovolcano, pumice from the Grand Bay Ignimbrite, and the
post-collapse domes, of which Cabrits is a typical example.
To illustrate this point and to put the composition of the Cabrits
dome into perspective, selected features of the geochemical
and isotopic composition of these rocks are illustrated in
Fig. 3.
For a full description of the PPVC and other southern Dominica
centers the reader is referred to Lindsay
et al. (2005
b)
.
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ANALYTICAL TECHNIQUES
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Electron microprobe analysis
Major elements in minerals and glasses were analyzed using the
CAMECA SX-100 electron microprobe at the GeoForschungsZentrum
Potsdam (GFZ). Accelerating voltage was 15 kV, and the beam
current and size were varied: 20 nA and 1–5 µm for
minerals, and 10 nA and 10–15 µm for glasses. Counting
times were 20 s for Na and 40 s for other major elements. The
CAMECA set of standards (mostly synthetic and natural minerals
and oxides) was used for routine calibration, and the Smithsonian
set of natural glasses and minerals (Jarosewich
et al., 1980
)
was used to monitor the stability of the instrument during analytical
sessions. Typical analytical uncertainties obtained from replicate
measurements (
n = 38) of the USNM 111240/52 VG-2 basaltic glass
were 0·2–0·5% for SiO
2; 0·5–1·4%
for Al
2O
3, MgO and CaO; 0·7–3·2% for TiO
2,
FeO and Na
2O; 5–10% for K
2O and P
2O
5; and
c. 15% for MnO.
Sulfur and chlorine in inclusion glasses were analyzed in the
same block as the major elements, using 60 s counting times
on the peaks and 30 s for background. Under these conditions,
the detection limit was around 250–350 ppm for both S
and Cl. As monitor samples to control precision and accuracy
of S and Cl measurements, we used VG-2 basaltic glass (0·134–0·137
wt % S and 0·029–0·032 wt % Cl; Dixon
et al., 1991
; Thordarson
et al., 1996
) and KE12 pantellerite glass
(0·323 wt % Cl; Mosbah
et al., 1991
). Values obtained
during this study were 0·149 ± 0·012 wt
% S and 0·029 ± 0·008 wt % Cl (
n = 38)
for the VG-2 glass, and 0·324 ± 0·010 wt
% Cl (
n = 10) for the KE12 glass. These agree with the reference
values within the ±2
uncertainty. Sulfur values similar
to our results (0·147 ± 0·003 wt %) were
obtained for VG-2 by N. Metrich (personal communication, 2003).
The total analytical error for S and Cl, including both precision
and accuracy of measurements, was better than 10%, or up to
20% for low concentrations (<500 ppm).
Hydrous Si-rich glasses are susceptible to Na loss under the analytical conditions applied (e.g. Morgan & London, 1996). We tested for this effect using the reference albite (Micro-Analysis Consultants Ltd, Cambridgeshire, UK), basaltic (USNM 111240/52 VG-2) and rhyolitic glasses (USNM 72854 VG-568), as well as two selected Dominica glass inclusions. Glasses were exposed for varying duration under a 15 kV, 12 nA and 10 µm beam of the JEOL JXA-8200 electron microprobe and the results are shown in Fig. 4. Under these conditions, nearly no Na loss was found for VG-2 glass, whereas up to 50% loss of Na from albite occurred after beam exposure times of 40–120 s, and about 25% Na loss occurred in VG-568 glass for 30–60 s beam exposure (Fig. 4a). The Dominica inclusions appear stable under the electron beam during the first 20 s of exposure, and then show similar behavior to the VG-568 rhyolite (Fig. 4b). We conclude that under the conditions applied for glass inclusion analyses, Na loss is less than the estimated analytical error of 3·2% for Na2O.
Secondary ion mass spectrometry
Li, Be and B concentrations, Li and B isotopic composition
Glass inclusions were analyzed for Li, Be and B concentrations
and Li and B isotope ratios using the CAMECA IMS6f instrument
at the GFZ Potsdam, following techniques described by Chaussidon
& Libourel (1993)
, Chaussidon & Jambon (1994)
, Chaussidon
et al. (1997)
and Chaussidon & Robert (1998)
. Element concentrations
and isotope ratios were measured in subsequent analytical sessions.
Standards and unknown samples were sputtered with a nominal
12·5 kV
16O
– primary beam of
2–10 nA current
and
15–20 µm in diameter. The energy slit was centered
and opened at 25–50 V. A 150 µm contrast aperture
and a 750 µm field aperture were used, giving an
60 µm
field of view. Secondary ions were accelerated by 10 kV and
analyzed at a mass resolving power
M/
M 
1650–1750. The
isotopes
6Li
+,
7Li
+,
9Be
+,
10B
+,
11B
+ and
30Si
+ were counted
for 8 s, 2 s, 4 s, 8 s, 4 s and 2 s, respectively. The ion intensities
were corrected for the electron multiplier dead time (14 ns).
We analyzed only high-energy (>80 eV) secondary ions during
measurement of element concentrations, whereas low-energy ions
(>0 eV) were analyzed during
7Li/
6Li and
11B/
10B measurements.
Typical ion intensities observed under these conditions were
200–300 cps for
7Li
+ and
11B
+, 10 cps for
9Be
+ and 3
x 10
4 cps for
30Si
+ during analyses of element concentrations
at high-voltage offset (HVO) of –80 V, and 1
x 10
4 cps
for
6Li
+, 2
x 10
5 cps for
7Li
+, (1–2)
x 10
3 cps for
10B
+,
and (4–8)
x 10
3 cps for
11B
+ at HVO = 0 V. The measurements
were started after a 300 s unrastered preburn to remove the
gold coating and possible surface contamination. Each measurement
included 70–100 cycles. Element concentrations relative
to SiO
2 and isotope ratios were calculated using calibration
curves obtained by linear regressions from a set of six reference
glasses (
Table 1). The observed internal precision and external
reproducibility of Li/Si, Be/Si and B/Si ratio measurements
was better than 5%. Variations of primary beam intensity, between
2 and 10 nA depending on element concentrations in a given reference
material, were found to have no systematic influence on the
relative sensitivity factors [RSF, defined as the ratio of total
ionic intensity for a given element to the ionic intensity of
Si divided by the respective element ratios in atomic concentration
for the material; i.e. (Li
+/Si
+)/(Li/Si)]. The Li and B isotopic
ratios are expressed as
7Li and
11B values relative to NBS-LSVEC
(
7Li/
6Li = 12·0192 ± 0·0002) and NBS 951
(
11B/
10B = 4·04558 ± 0·00033) reference
materials, respectively (Flesh
et al., 1973
; Spivack & Edmond,
1986
). The instrumental mass fractionation (IMF) values (
Table 2)
used to calculate
7Li/
6Li and
11B/
10B ratios were determined
from analyses of the NIST SRM 610 and 612 reference glasses
(
7Li = 31 ± 1
and
11B = –1·1 ± 0·8
for both glasses; Dalpe
et al., 2001
; Kasemann
et al., 2001
;
T. Zack, personal communication, 2004) and the CRPG-CNRS GB4
internal reference glass (
11B = –12·9 ±
1·0
; Chaussidon & Jambon, 1994
; Chaussidon, 1995
).
The applied instrumental settings and analytical conditions
resulted in a typical internal precision (1
) ranging from 0·6
to 1·7
for both
7Li/
6Li and
11B/
10B ratios (
Table 2),
and the average external reproducibility (1
) was ±2
.
H2O and trace elements
The concentrations of H
2O and selected trace elements [rare
earth elements (REE), Nb, Th, Sr, Y, Zr, V and Cr] in glass
inclusions were determined using the CAMECA IMS3f instrument
at the Max Planck Institute for Chemistry (MPI), Mainz. Analytical
conditions for H
2O were similar to those described by Sobolev
(1996)
and Sobolev & Chaussidon (1996)
; that is, 12·5
kV accelerating voltage for O
– primary ion beam, 4·5
kV secondary accelerating voltage, –80 V offset and
M/
M = 300, as no species interfering with
1H mass are expected.
The energy slit was centered and opened to 25 V. A 150 µm
contrast aperture and a 750 µm field aperture were used.
The analyses were performed in three blocks containing six cycles
each of scans over
1H,
30Si and
47Ti masses, each counted for
2 s. Titanium was monitored to detect and if necessary correct
for overlap with the host mineral in the case of small (<40
µm) inclusions. Orthopyroxene and plagioclase hosts were
repeatedly analyzed throughout the analytical session to monitor
the H
2O background level, and analysis was started when H
2O
concentration on orthopyroxene was equivalent to, or lower than,
0·03 wt %. Typical count rates were (3–5)
x 10
4 cps on the
1H peak, (2–4)
x 10
2 cps on the
47Ti peak and
(2–3)
x 10
4 cps on the
30Si peak. The external reproducibility
based on reference glasses with H
2O contents ranging from 0·11
to 8·5 wt % was better than 5% (
Table 3). Estimates of
accuracy obtained by multiple measurements of selected reference
glasses (analyzed as unknowns and not used to assess the calibration
line) are within 5% of the accepted value.
The analysis of trace elements employed similar instrument settings
as for H
2O, except that a larger field aperture was used (1800
µm). Each analysis consisted of five scans, starting from
16O mass (using for magnet adjustment), then over the sequence
30Si,
39K,
44Ca,
47Ti,
51V,
52Cr,
88Sr,
89Y,
90Zr,
93Nb, each
mass for REE from 133 to 180 and finally
232Th. Oxide interferences,
for example, light rare earth element (LREE) oxides interfering
with heavy rare earth elements (HREE), were corrected using
peak deconvolution (e.g. Zinner & Crozaz, 1986
; Fahey
et al., 1987
). Instrument drift was controlled and necessary correction
applied based on daily replicate analyses of KL2-G reference
glass (Jochum
et al., 2000
;
Table 4). Estimated analytical error
was better than 6% for all elements, except Gd, Tb, Lu, Hf and
Th (between 10 and 15%).
Raman spectroscopy
In many samples, H
2O analyses were also made by confocal laser
Raman spectroscopy at GFZ Potsdam, following techniques described
by Thomas (2000)
. The instrument used is a Dilor XY Laser Raman
Triple 800 mm spectrometer fitted with an Olympus optical microscope
(80
x long working distance objective). Raman spectra were collected
with a Peltier-cooled CCD detector. The 488 nm line of a coherent
Ar
+ laser (Innova 70-3), with a power of 320 mW, was used for
sample excitation. The beam diameter was about 2 µm, which
produces an excitation volume of
c. 10 µm
3. To obtain
H
2O concentrations dissolved in the glass, the ratio of the
3550 cm
–1 band intensity to the intensity at 490 cm
–1 was measured for each glass inclusion. Each measurement consisted
of two spectra taken at different spectrometer positions during
five accumulations, each of 50 s acquisition time. Integral
intensities instead of peak heights were used, and two integral
limits of 3100–3750 cm
–1 and 300–700 cm
–1 were applied. The method gives an average accuracy of ±0·25
wt % for H
2O concentrations of a few wt % (Thomas, 2000
). The
results of Raman and SIMS analyses on the same inclusions agree
fairly well, the range of discordance being 0·1–1·6
wt %.
|
RESULTS
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Mineral compositions and conditions of magma crystallization
Orthopyroxene phenocrysts are euhedral to subhedral and their
compositions correspond to hypersthene (Wo
1·5–2·5 En
50–58 Fs
40–48;
Table 5). Titanium and aluminum
contents range from 0·06 to 0·18 wt % TiO
2 and
from 0·4 to 0·8 wt % Al
2O
3, respectively. Both
correlate positively with
mg-number [100
x Mg/(Mg + Fe
tot),
atomic ratio], which varies between 51 and 59 (
Table 5). Plagioclase
forms euhedral and elongated, commonly broken crystals with
a total compositional range from An
47 to An
67. Grains typically
show normal zoning with maximum within-grain variations of 13
mol % An (
Table 5). The plagioclase inclusions in orthopyroxene
are at the calcic end of the total range (An
51–67), and
similarly, orthopyroxene in plagioclase has relatively high
Mg-number (En
55) compared with the total range of En
50–58.
This mutual inclusion of relatively Ca-rich plagioclase and
Mg-rich orthopyroxene suggests fairly early cotectic crystallization
of the two phases. The Fe–Ti oxides comprise coexisting,
commonly intergrown, magnetite and ilmenite. The compositions
of Fe–Ti oxides were determined only on grains included
in Opx to avoid the problem of subsolidus exchange during cooling.
Magnetite [Mt; (Fe
2+,Mg)Fe
2O
4] contains significant amounts
of ulvöspinel [Usp; Fe
2TiO
4], the range being Mt
64–71Usp
24–31,
whereas the spinel [Sp; (Fe
2+,Mg)Al
2O
4] and chromite [Crt; (Fe
2+,Mg)Cr
2O
4]
components are negligible (
Table 6). The coexisting ilmenite
(Ilm; FeTiO
3) contains varying amounts of hematite (Hmt; Fe
2O
3),
with compositions ranging from Ilm
85Hmt
15 to Ilm
88Hmt
12 (
Table 6).
Apatite (Ap) is an accessory mineral that occurs in the
groundmass of the pumice and as inclusions in Opx and Pl phenocrysts.
We analyzed only the apatite inclusions, and found 1·2–1·7
wt % Cl and 1·4–1·7 wt % F, giving Cl/F
ratios ranging from 0·83 to 1·04.
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Table 5: Major elements, volatile abundances, Li, Be, B concentrations and Li and B isotopic composition of glass inclusions; compositions of Opx and Pl host crystals
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Table 6: Representative compositions of magnetite and ilmenite inclusions, host minerals and derived temperature and redox conditions of crystallization*
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Quartz (Qz) is absent from the phenocryst assemblage and from
the groundmass, even though the composition of groundmass glass
is rhyolitic (73–76 wt % SiO
2; see below). One explanation
for the lack of Qz could be eruption and quenching of the magma
at a temperature exceeding the quartz-in curve. Mineral thermometry
from the Cabrits sample yields 760–880°C (see below),
and crystallization experiments with intermediate bulk compositions
have shown Qz growth from residual melts with >70 wt % SiO
2 at temperatures below 840°C (Barclay
et al., 1998
; Devine
et al., 1998
; Murphy
et al., 1998
). Another factor could be
the relatively high H
2O concentrations in the residual melt
(up to 6 wt %; see below), as high H
2O fugacity decreases the
liquidus temperature of Qz in granitic systems (Pichavant, 1987
,
and references therein).
Estimates of the temperature and oxygen fugacity during magma crystallization were made from mineral equilibria among coexisting Pl, Opx and Fe–Ti oxides using the QUILF program of Andersen et al. [(1993); based on calibrations of Andersen & Lindsley (1988) and Frost et al. (1988)]. Because clinopyroxene is not part of the phenocryst assemblage and was not found as mineral-hosted inclusions, we used the QUILF program in a single pyroxene mode to calculate T and conditions using compositional data from orthopyroxene, titanomagnetite and ilmenite. We also tested the option of fixing the activity of SiO2 in the melt, which allows the composition of equilibrium Cpx to be calculated by QUILF. A similar approach was used by Murphy et al. (2000) to constrain conditions of magma crystallization at the Soufrière Hills (Montserrat). Our samples yielded the following temperature and oxygen fugacity estimates, calculated for 1 atm total pressure and the given mineral assemblages (Table 6): T = 796–878 ± 19°C and FMQ = 0·8–1·1 ± 0·3 log units (where FMQ is the fayalite–magnetite–quartz buffer) for Mt + Ilm + Opx, when considering the activity of SiO2; T = 761–792 ± 36°C and FMQ = 1·2–2·0 ± 0·4 log units for Mt + Ilm + Opx with no SiO2 activity constraint; and T = 768–821 ± 54°C and FMQ = 1·2–1·6 ± 0·6 log units for the Mt + Ilm assemblage alone.
Our geothermometry results agree with temperature estimates for the PPVC rocks by Lindsay et al. (2005b). That study reported magnetite–ilmenite geothemometry in the range of 810–840°C and log values from –13·2 to –12·5 (FMQ = +0·8 to +1·6 log units) for several post-caldera domes including Cabrits. Hornblende–plagioclase geothermometry for the few samples having the appropriate assemblage (Cabrits dome, La Vue dome, Morne Plat Pays) yielded temperatures of 800–830°C, in the same range as the magnetite–ilmenite results. Slightly higher temperatures (840–890°C) were obtained from two-pyroxene QUILF geothermometry.
Composition of glass inclusions
Major and trace elements
The compositions of glass inclusions in the Opx and Pl hosts are given in Tables 2 and 3 together with representative analyses of groundmass glass and the whole-rock composition for sample D-JL18. For many of the glass inclusions, analytical totals are high (up to 103 wt %), but taking into account the individual uncertainties of the electron microprobe, SIMS and Raman methods, the totals are acceptably close to 100%. For discussion and plots presented in this section all major element compositions of inclusions and whole rocks were recalculated to 100% on a volatile-free basis. Comparisons with the PPVC whole-rock compositions are made from data reported by Lindsay et al. (2005b).
The Cabrits dome and associated units of the PPVC erupted calc-alkaline, medium-K, silicic andesites to dacites based on their relationships of SiO2 to FeO/MgO, K2O, and K2O + Na2O (Miyashiro, 1974; Tatsumi & Eggins, 1995; Arculus, 2003). The total SiO2 range in whole rocks is 64–68 wt % (volatile-free) and the Cabrits dome samples including D-JL18 are in the silica-rich part of this range (Fig. 5). In contrast to the bulk rock, all glass inclusions have rhyolitic compositions, whether enclosed by plagioclase or orthopyroxene. The independence of inclusion composition from the host mineral is good evidence that there has not been significant effect on melt composition by post-entrapment crystallization of the host (see mineral vectors in Fig. 5). The excellent correspondence between inclusion compositions and the composition of the glassy groundmass (Fig. 5) also suggests that the inclusions are reasonable representatives of the trapped melt compositions. The gap between the melt inclusion or groundmass glasses and the bulk-rock composition is consistent with the idea that the glasses represent residual melts, from crystallization of the major phenocryst phases in the dacite, as described in more detail in the discussion section below.
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Fig. 5. Major element compositions of Opx- and Pl-hosted glass inclusions and comparison with other volcanic rocks from the Plat Pays domes on Dominica (Lindsay et al., 2005b). (a) Total alkalis vs SiO2; (b) CaO vs SiO2; (c) FeO vs SiO2; (d) FeO vs TiO2. All concentrations are in wt % after recalculation on a 100% volatile-free basis. The inclusions show systematically more evolved compositions than those of erupted volcanic rocks and match the composition of the groundmass glass. Arrows show control vectors for Opx, Pl and Mt addition to melt, demonstrating that the whole-rock dacite can result from a mixture of earlier-formed crystals with rhyolitic residue melt and that the inclusion compositions are not significantly affected by host-mineral crystallization.
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Trace element analyses of glass inclusions are given in
Table 7 and selected features of element abundances and ratios are
shown with respect to whole-rock compositions in
Figs 6 and
7. With a few exceptions described separately (Li, H
2O), there
are no systematic differences in minor or trace element concentrations
between inclusions trapped in plagioclase and in orthopyroxene.
To organize the discussion it is useful to distinguish groups
of trace elements according to their intercorrelations and relationship
to bulk-rock compositions, as these features may relate to different
components in the trapped melts or processes of melt evolution
(see discussion). We consider the following points to be significant.
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Fig. 6. Trace element variation diagrams illustrating the compositional range of Opx- and Pl-hosted glass inclusions (circles) and whole-rock samples from the Plat Pays Volcanic Complex (triangles) and Cabrits dome (stars; data from Lindsay et al., 2005b). Compositions are in ppm except where noted.
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Fig. 7. Variation diagrams illustrating the compositional range of volatile and light elements in the Opx- and Pl-hosted glass inclusions from the Cabrits dacite. Compositions are in ppm except where noted.
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(1) The most internally coherent group of trace elements is
the high field strength elements (HFSE), Ti, Nb, Zr and Y and
the middle REE (MREE) to HREE (Sm to Lu). These elements show
strong positive correlations among each other (
R = 0·78–0·99),
exemplified in
Fig. 6 by the Zr–Nb, Zr–Y and Zr–Ti
diagrams. It should be noted that the inclusion glasses plot
off the whole-rock compositional trends in plots with TiO
2 and,
to a lesser extent, with Y. We believe this is due to some accumulation
of Fe–Ti oxides and other minor phases in the whole rocks,
as most other trace and major element variations in the inclusion
glasses follow reasonable extensions of the bulk-rock compositions.
(2) A second and rather diverse group of trace elements shows weaker intercorrelations than the first, and a good correspondence between inclusions and bulk-rock compositional trends. These elements include Th, Ba, Sr, K and B. Examples of good positive correlations are: B and K (R = 0·84), B and Ba (R = 0·84), K and Sr (R = –0·88), and K and Th (R = 0·80). It should be noted that Sr behaves as a compatible element, with lower contents in the glasses than bulk rock, and Ba shows the opposite behavior (Fig. 6). This is consistent with plagioclase-dominated fractionation and the lack of sanidine or biotite in the crystal assemblage.
(3) Neither Li nor Be shows significant correlation with other elements in the inclusion glasses, with all correlations yielding values of R < 0·6. The concentrations of these elements are apparently controlled by other processes. The concentrations of Li in the inclusion glasses vary widely and are significantly lower in plagioclase-hosted inclusions than in those hosted by Opx (see discussion).
(4) The mid-ocean ridge basalt (MORB)-normalized multi-element diagram (Fig. 8) demonstrates the overall similarity in trace element pattern between inclusion glasses and bulk rocks, but at different concentration ranges. The glasses have higher contents of incompatible elements Ba, Th, K and LREE (La, Ce), lower Nb, Sr, P, Eu and Ti, and also lower but much more variable Y and HREE. The depletions in Ti, Eu and Sr of glasses relative to bulk rock may be explained by fractionation of plagioclase, apatite and oxide phases.
(5) Chondrite-normalized REE patterns of glass inclusions and
bulk rocks (
Fig. 8 inset) are similar or even virtually identical
but typically the inclusions have slightly higher LREE (La,
Ce) contents and lower and much more variable MREE and HREE
(Sm to Lu). The ‘V-shape’ patterns are exemplified
by strong negative correlations between (La/Sm)
n = 2·5–6·6
and (Gd/Yb)
n = 0·5–1·0 (
R = –0·89)
or Sm/Yb (
R = –0·80) ratios. Europium contents
are highly variable in the inclusions, and many show significantly
larger negative Eu anomalies than the PPVC whole rocks. It is
interesting that the strongest Eu anomalies are found in inclusions
hosted by orthopyroxene.
Volatile concentrations
The concentrations of H2O in the inclusion glasses determined by Raman spectroscopy and SIMS mostly show good agreement; the relative difference between the two methods ranges from 2 to 16% and in three extreme cases reaches 21–31% (Table 5). We have taken the average value for the two methods as an estimate of true concentrations. The total range of H2O contents is 2–6 wt % and, generally, inclusions in Opx have higher water concentrations than plagioclase-hosted inclusions. With one exception, the former yielded 5–6 wt % H2O whereas all inclusions hosted by plagioclase have <5 wt % H2O. This difference is found in both SIMS and Raman results. Water contents do not correlate with incompatible elements or with other volatile components. Similar high water concentrations of 5·8 ± 0·7 wt % H2O were obtained for Mt. Pelée, Martinique by Martel et al. (1998) and the Soufrière Hills Volcano, Montserrat (4·3 ± 0·5 wt % H2O; Barclay et al., 1998). Chlorine concentrations in glass inclusions have a wide total range from 1700 to 2880 ppm (presented in Fig. 9). These values are similar to those in andesitic glass inclusions from the Izu volcanic arc (Straub & Layne, 2003) and examples of intermediate arc magmas from Guatemala (Sisson & Layne, 1993), St. Vincent (Heath et al., 1998) and Montserrat (Edmonds et al., 2001). Degassing of Cl is probably one reason for the variation in Cabrits melt inclusions, and is supported by the fact that the groundmass Cl content (1900 ppm; see Table 5) is at the low end of the inclusion range. The solubility of Cl in silicate melts is a complex function of pressure, temperature and melt composition (e.g. Metrich & Rutherford, 1992; Webster, 1992; Carroll & Webster, 1994; Webster et al., 1999; Signorelli & Carroll, 2001). Chlorine is less soluble in hydrous, low-alkali, siliceous melts than in basic and intermediate magmas (Webster et al., 1999; Signorelli & Carroll, 2001), and is thus likely to become oversaturated during differentiation.
Because the solubilities of H
2O and Cl in felsic melts are pressure
dependent (Webster, 1997
; Webster
et al., 1999
), their concentrations
in the inclusions can be used to estimate the confining pressure
at the time of trapping. Compared with the experimentally estimated
solubilities from Webster
et al. (1999)
, the inclusion compositions
correspond to a pressure of about 200 MPa, or a depth of
c.
6 km, for Opx and Pl crystallization (
Fig. 9). A similar estimate
of
5–6 km was obtained for the Soufrière Hills,
Montserrat by Barclay
et al. (1998)
based on water solubility
alone, using the model of Moore
et al. (1998)
and H
2O contents
of 4·3 ± 0·5 wt % in rhyolitic melts.
The sulfur contents in the Cabrits melt inclusions are low, less than the c. 350 ppm S detection limit for our electron microprobe configuration in all inclusions. Edmonds et al. (2001) reported very low S contents (average 70 ppm) in rhyolitic melt inclusions and matrix glass from resurgent dome samples at Soufrière Hills, Montserrat, and attributed them to loss of S by magma degassing before entrapment. This could be the case for Cabrits as well, but there is no direct evidence that sulfur concentrations in the magma were originally higher. Recent work on S solubility in rhyolitic melts at 800–1000°C (Clemente et al., 2004) showed that solubility is dominated by the sulfide species (
) and under the conditions of P, , 
, 
and FeO concentration they examined, the melts contained only 800–1000 ppm dissolved sulfur [for FeO <0·5 wt %, NNO of –2 to –1 log units (where NNO is the nickel–nickel oxide buffer), log 
, log 
and –1 > log 
]. We did not perform special studies to estimate fugacity of H2S, SO2 and S2 for the Dominica glass inclusions, but the calculated redox conditions of FMQ + 1 to FMQ + 2 log units and FeO contents of 1·3–2·3 (Tables 5 and 6) suggest that sulfur solubility in the Cabrits residual melt would have been under 1000 ppm, so degassing may not be necessary to explain the low concentrations.
Li and B concentrations and isotopic composition
The glass inclusions contain 20–60 ppm Li and 60–100 ppm B (Table 5, Figs 7 and 10). The concentrations of these elements are generally higher than the range for other island arcs shown in Fig. 10, including the study of Smith et al. (1997) from Martinique (boron only). At least part of this difference is due to the fact that our data represent high-silica residual melts whereas Smith et al. (1997) analyzed whole-rock samples of andesite to dacite composition. The high B and Li concentrations in Cabrits inclusions are similar to those found in rhyolitic groundmass and melt inclusions from caldera systems (Long Valley: Anderson et al., 2000; Schmitt & Simon, 2004, Jemez: Stix & Layne, 1996; La Pacana: Lindsay et al., 2001; Schmitt et al., 2003). The relationships of Li and B variations with other chemical components in the inclusions are not simple. Boron is one of the ‘group 2’ elements described above (K, Ba, Sr, Th), which show consistent positive correlations with one another. Boron also shows fair positive correlation with Cl and negative correlations with the HFSE Zr, Y, Nb and HREE (Fig. 7). Lithium behaves differently from B and shows generally poor correlations with other elements [also noted by Stix & Layne (1996)]. There also seems to be a systematic difference in the Li abundances depending on the nature of the host mineral, with 23–40 ppm for plagioclase-hosted inclusions and up to 58 ppm for inclusions in Opx (Fig. 7).
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Fig. 10. Li and B concentrations and isotopic composition of Opx- and Pl-hosted glass inclusions shown with reference to potential magma source components. It should be noted that inclusions in Pl have systematically higher 7Li values and lower Li concentrations than those in Opx. This difference is ascribed to post-entrapment diffusion and isotopic fractionation (see text). The MORB box and other reference fields are based on data of Chan et al. (1992, 1999), Chaussidon & Jambon (1994), Chaussidon & Marty (1995), Ishikawa & Tera (1997), Smith et al. (1997), Moriguti & Nakamura (1998), Tomascak et al. (2000) and Straub & Layne (2002). Assumed composition of AOC (altered oceanic crust) is 12 ppm Li with 7Li = +11 (Leeman et al., 2004) and 5·2 ± 1·7 ppm B with 11B = +3·4 ± 1·1 (Smith et al., 1995). Assumed composition of LAS (Lesser Antilles Sediment) is 55 ppm Li with 7Li = –0·3 [median values of Bouman et al. (2004)] and 120 ppm B with 11B = –10 [for western Atlantic marine sediments near Martinique, from Smith et al. (1997)].
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The isotopic composition of Li and B in the inclusion glasses
is variable despite the fact that the inclusions come from a
single sample of erupted magma. The values of
7Li (+4 to +14
)
and
11B (+6 to +13
) in the Cabrits inclusions are considerably
higher than MORB values and they overlap at the high end of
the ranges reported from other volcanic arcs (Chaussidon &
Jambon, 1994
; Chaussidon & Marty, 1995
; Ishikawa & Tera,
1997
; Moriguti & Nakamura, 1998
; Chan
et al., 1999
; Tomascak
et al., 2000
; Straub & Layne, 2002
; Elliott
et al., 2004
).
To our knowledge, no previous Li isotope study has been made
in the Lesser Antilles, but the B-isotope study of Martinique
by Smith
et al. (1997)
found much lower values of
11B than our
results (–5
to +0·7
), which were attributed to
input of subducted terrestrial sediments to the magma source
(see discussion). The variations in Li and B isotope ratios
in the Cabrits inclusions do not correlate with each other and
there is only a weak correlation of isotope ratios with the
corresponding element concentrations (
Fig. 10). The Li results
suggest a complex behavior. Li isotope ratios do not correlate
with other compositional variables in the inclusions but there
is a systematic difference depending on the type of host mineral.
The
7Li values in Pl-hosted inclusions are higher than those
in Opx with one exception (
Fig. 10,
Table 5), and duplicate
analyses (e.g. inclusions 38-1, 75-1 and 76-1;
Table 5) confirmed
that this difference is not due to analytical error. Because
the Pl-hosted inclusions also have lower Li and H
2O concentrations,
and taking into account that post-entrapment loss of hydrogen
from inclusions is commonly observed (Sobolev & Danyushevsky,
1994
), we suggest that the Pl-hosted inclusions may have suffered
diffusive loss of Li and H, with the difference in isotope composition
caused by higher diffusion rate for the lighter
6Li isotope
as described by Richter
et al. (2003)
and discussed below. In
contrast to Li, there is no dependence of B concentration or
isotope ratio on the host mineral and the measured
11B values
show weak but significant correlations with Ba, Th, Nb and some
of the REE or their ratios (
Fig. 11).
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Fig. 11. Boron isotopic composition of the Cabrits glass inclusions plotted against trace element concentrations and ratios.
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DISCUSSION
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Melt inclusion–host rock relationships and implications for magma storage and assembly
An interpretation of melt inclusion compositions in terms of
magmatic processes must first consider if the inclusions are
representative of the trapped melts. The melt inclusions analyzed
are large enough (many >50 µm) that concentration gradients
in the melt around the growing crystals are unlikely to have
affected their element contents and concentration ratios (Lu
et al., 1995
). Furthermore, the inclusion glass compositions
are largely independent of the type of host mineral and the
plots of inclusion data compared with orthopyroxene or plagioclase
control lines in major element variation diagrams show that
post-trapping crystallization or exchange with the host mineral
walls was insignificant (
Fig. 5). Therefore we conclude that
the inclusion compositions can be treated as realistic estimates
of the melts present during crystallization and trapping. A
first-order observation that holds for all inclusions analyzed
is their rhyolitic composition, whereas the host rock is a dacite
and there are no known eruptions of rhyolite from the PPVC.
We suggest that the rhyolitic glasses represent residual melts
from the Cabrits magma system, and that the whole-rock composition
results from a mixture of earlier-formed crystals and the melt
phase (see also Naumov
et al., 1996
; Kamenetsky
et al., 2000
;
Schmitt, 2001
). Indications for this are the similarity of melt
inclusions and groundmass glass compositions and the fact that
inclusion data plot along extensions of the whole-rock variation
trends (
Figs 5 and
6). To test the hypothesis we calculated
least-squares mixing models (e.g. Wright & Doherty, 1970
)
using the whole-rock composition and average analyses of orthopyroxene,
plagioclase, titanomagnetite, ilmenite and apatite from the
rock (
Table 8). The results suggest that the gap between whole-rock
composition and inclusion or matrix glasses represents about
50% of phenocryst accumulation. This estimate is about twice
the observed crystal content of the erupted pumice, so there
must have been considerable separation of crystals and melt
before and/or during eruption.
A striking characteristic of the Cabrits dome and other volcanic
units of the PPVC is their very homogeneous compositions at
the scale of whole-rock samples (
Fig. 3). This contrasts with
the wide range of trace element and Li–B isotope composition
of the trapped melts, and the variation in cation ratios of
the host minerals (e.g. plagioclase phenocrysts with An
47 to
An
67). Unfortunately, the systematics of host crystallization
and inclusion trapping in the dacite were not regular enough
to link melt inclusion compositions with particular growth stages.
The implication, nevertheless, is that the erupted magma represents
a mixture of melt fraction(s) and entrained phenocrysts that
represent different local histories of magma differentiation
and segregation processes within the Cabrits system.
The thermobarometry results from Opx, Pl and Fe–Ti oxide equilibria (Table 6) indicate that equilibration of the phenocryst assemblage took place over a considerable range of temperature, about 760–880°C, with oxygen fugacity 1–2 log units above the FMQ buffer. Pressure estimates are much less precise, but the measured H2O and Cl concentrations in inclusion glasses, compared with experimental solubilities (Fig. 9), indicate a pressure of about 200 MPa. This agrees with the 5–6 km depth of swarms of volcanic seismicity that indicate recent magma movement beneath the Soufrière depression (Lindsay et al., 2003).
Source components and controls on B–Li isotope variations
The large number of petrogenetic studies of the Lesser Antilles arc [for a thorough recent review see Macdonald et al. (2000)] reached a consensus view that primary magmas were derived from fluid-enhanced melting of a depleted mantle similar in composition to the MORB source above the subducting slab. Therefore, the major factor affecting the chemical variation of primary magmas in this setting is the composition of the slab-derived components and the relative proportion of slab vs mantle-wedge components in the final magma. The rhyolitic glass inclusions from the Cabrits dacite are far removed from any parental magma composition and can give only indirect information about the magma source. However, Li and B isotopes can be valuable for fingerprinting different slab components because the mantle Li and B concentrations are so low, and the isotope ratios are not much affected by magma differentiation (Tomascak et al., 1999). It is also worth pointing out that despite their high degree of differentiation, HREE and HFSE ratios in the Cabrits inclusions are similar to those from basaltic rocks of Morne Anglais, southern Dominica (e.g. Gd/Yb = 0·7–1·2 for inclusions vs 1·3–1·6 for Anglais basalt; Zr/Nb = 25–34 for inclusions and 29–39 for Anglais basalt; data from Lindsay et al., 2005b). The corresponding ratios in average N-MORB (Gd/Yb = 1·2 and Zr/Nb = 32 from Sun & McDonough, 1989) are not much different, so in terms of these relatively fluid-immobile incompatible elements, the residual melts retained
TOP
ABSTRACT
INTRODUCTION
