Bull Volcanol (2009) 71:1077–1089
DOI 10.1007/s00445-009-0287-5
RESEARCH ARTICLE
The November 2002 eruption of Piton de la Fournaise,
Réunion: tracking the pre-eruptive thermal evolution
of magma using melt inclusions
Nathalie Vigouroux & A. E. Williams-Jones &
Paul Wallace & Thomas Staudacher
Received: 19 November 2007 / Accepted: 29 April 2009 / Published online: 6 June 2009
# Springer-Verlag 2009
Abstract The November 2002 eruption of Piton de la
Fournaise in the Indian Ocean was typical of the activity of
the volcano from 1999 to 2006 in terms of duration and
volume of magma ejected. The first magma erupted was a
basaltic liquid with a small proportion of olivine phenocrysts
(Fo81) that contain small numbers of melt inclusions. In
subsequent flows, olivine crystals were more abundant and
richer in Mg (Fo83–84). These crystals contain numerous melt
and fluid inclusions, healed fractures, and dislocation
features such as kink bands. The major element composition
of melt inclusions in this later olivine (Fo83–84) is out of
equilibrium with that of its host as a result of extensive postentrapment crystallization and Fe2+ loss by diffusion during
cooling. Melt inclusions in Fo81 olivine are also chemically
out of equilibrium with their hosts but to a lesser degree.
Using olivine–melt geothermometry, we determined that
melt inclusions in Fo81 olivine were trapped at lower
temperature (1,182±1°C) than inclusions in Fo83–84 olivine
(1,199–1,227°C). This methodology was also used to
Editorial responsibility: M. Clynne
A. E. Williams-Jones
Department of Earth and Planetary Sciences, McGill University,
Montreal, QC, Canada
P. Wallace
Department of Geological Sciences, University of Oregon,
Eugene, OR, USA
T. Staudacher
Observatoire Volcanologique du Piton de la Fournaise, IPGP,
Réunion, France
Present address:
N. Vigouroux (*)
Department of Earth Sciences, Simon Fraser University,
Burnaby, BC, Canada V5A 1S6
e-mail: nvigouro@sfu.ca
estimate eruption temperatures. The November 2002 melt
inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before
eruption and quenching. This relatively wide temperature
range may reflect the fact that most of the melt inclusions
were from olivine in lava samples and therefore likely
underwent minor but variable amounts of post-eruptive
crystallization and Fe2+ loss by diffusion due to their
relatively slow cooling on the surface. In contrast, melt
inclusions in tephra samples from past major eruptions
yielded a narrower range of higher eruption temperatures
(1,163–1,181°C). The melt inclusion data presented here and
in earlier publications are consistent with a model of magma
recharge from depth during major eruptions, followed by
storage, cooling, and crystallization at shallow levels prior to
expulsion during events similar in magnitude to the relatively
small November 2002 eruption.
Keywords Piton de la Fournaise .
Volcanic plumbing system . Melt inclusions .
Olivine cumulates . Post-entrapment modifications .
Volatiles . Magma evolution
Introduction
The 530-ka Piton de la Fournaise shield volcano is the
current manifestation of the Réunion hot spot, which
produced the Deccan Traps in India at about 65 Ma
(Courtillot et al. 1986). It is located at the southern end of
the Mascarene Basin, encompassing the islands of Mauritius and Rodrigues. Piton de la Fournaise rests on the south
flank of the older and now extinct Piton des Neiges, which
forms most of present day Réunion Island (Fig. 1), and is
one of the world’s most active volcanoes.
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The anatomy of Piton de la Fournaise is similar to that of
Kilauea in that a central conduit links a shallow magma
chamber with the summit craters, and flank eruptions are
fed by lateral dyke intrusions connected to the central
conduit (Peltier et al. 2007). Fissures from historic flank
eruptions are mostly distributed along an arcuate rift zone
extending from the northeast of the caldera to the southeast,
passing through the central craters (Peltier et al. 2005).
Recent models based on seismicity and deformation of the
volcano prior to eruption suggest that Piton de la Fournaise
has a magma chamber with a volume of ~300×106 m3
located between sea level and 500 m above sea level (a.s.l.;
Sapin et al. 1996; Nercessian et al. 1996; Peltier et al.
2007). A deeper magma chamber, located at 5–6 km below
sea level (b.s.l.), has also been postulated (Battaglia et al.
2005) and olivine crystallization has been shown to occur
as deep as 15 km b.s.l. (Bureau et al. 1998a, b). Finally, a
crystal cumulate zone, evident as a high-density plug on the
basis of seismic data, is likely to be present within the
central cone above sea level (Nercessian et al. 1996; Sapin
et al. 1996) and to extend to the Moho at a depth of about
12.5 km b.s.l. (Gallart et al. 1999).
Information on the depth of crystallization and accumulation of magma within the volcano is crucial to further
understanding the plumbing system of Piton de la Fournaise. Melt inclusions provide an effective tool for this
purpose because they trap small pockets of magma at
different stages during the crystallization of crystals in
response either to undercooling (Faure et al. 2003) or
changes in the chemical environment (e.g., magma mixing)
(Sobolev 2007). Analysis of the chemical composition,
including volatile contents (H2O and CO2), of melt
inclusions, yields information on the depth of crystallization and storage, as well as trends in the chemical evolution
of the magma. At Piton de la Fournaise, melt inclusion
studies of olivine from both prehistoric (~3,000–50 ka) and
historic (1931, 1977, 1998) eruptions have shown that
olivine crystallization and accumulation occurs over a wide
range of depths from the Moho to approximately sea level
(Bureau et al. 1998b, 1999). The samples analyzed in these
studies were from large-volume, mostly olivine-rich magmas erupted from vents located outside the current caldera
and rift system (prehistoric eruptions) and intracaldera vents
active in historical times (1931, 1998).
For this study, we analyzed olivine-hosted melt inclusions from the moderate-volume (8×106 m3) flank eruption
of November 2002, a seismically shallow event (down to
1.5 km b.s.l.) characteristic of the activity at Piton de la
Fournaise from 1999 to 2006. Using the compositions of
these inclusions after correction for post-entrapment modifications, we have characterized the thermal history of the
magma from which the olivine crystallized and in which it
erupted. Results of this study suggest that olivine ejected in
Bull Volcanol (2009) 71:1077–1089
the early phase (first day) of the November 2002 eruption
crystallized from the magma hosting it, which was more
evolved and cooler than the magma erupted during past
events studied by Bureau et al. (1998b). The bulk of the
olivine, which was ejected in increasing proportions during
the later stages of the 2002 eruption (late phase) and is less
evolved, appears to have been entrained in this lower
temperature magma. This caused it to experience a larger
amount of pre-eruptive cooling than olivine of similar
composition ejected during the earlier events studied by
Bureau et al. (1998b). These results are consistent with the
model of crystallization, accumulation, storage, and eruption proposed previously by Bureau et al. (1998b, 1999), in
which they used melt inclusions to reveal the crystallization
and storage of olivine Fo83–85 at a variety of depths ranging
from ~5 km b.s.l. to the near surface, prior to eruption from
the central conduit area of the volcano. They showed that
the volatile contents found in the melt inclusions from a
single eruption could not be reconciled with a plausible
degassing trend and therefore must represent trapping of
individual melt batches with variable degassing histories.
Background geology
The summit area of Piton de la Fournaise volcano is
enclosed on three of its sides by the 4,500-year-old Enclos
Fouqué, the remnant of the youngest of three calderas,
whereas its east flank is open to the Indian Ocean. The
summit rises to 2,631 m a.s.l. and is crowned by two
intersecting craters, Bory and Dolomieu, the latter one
being the larger and representing the current center of
activity (Fig. 1).
Most of the historic eruptions have been from vents
located inside the walls of the current caldera, at various
elevations along the rift zone (summit to 1,000 m a.s.l.) but
three of the last four large-volume (>50×106 m3) deepseated eruptions (1977, 1986, 1998) have had at least one
vent located outside of the caldera rim along an extension
of the intracaldera rift zone. Prior to the collapse of the
current caldera (Enclos Fouqué), the southeast and the
northeast rift zones were active along the margins of the old
caldera rim (Plaine des Sables collapse; Fig. 1) and a few
vents/eruptive centers were preserved after the sector
collapse that created the current caldera (e.g., Piton
Manapany; Bureau et al. 1998b).
The Piton de la Fournaise volcano is still in its shield
building stage, erupting lavas that range from aphyric
basalts to oceanites, which are olivine-rich basalts equivalent to the Hawaiian picrites (Upton and Wadsworth 1966).
Most of the olivine erupted at Piton de la Fournaise ranges
in composition from Fo81 to Fo87 (Albarède et al. 1997;
Bureau et al. 1998a, b, 1999). However, basalts containing
Fo83–84 olivine have been the dominant lava type during the
Bull Volcanol (2009) 71:1077–1089
1079
Fig. 1 Digital elevation model
of the summit area of Piton de la
Fournaise. Inset shows location
on Reunion Island. The two
summit craters (Bory and
Dolomieu) are within the
youngest caldera (L’Enclos
Fouqué). The two older collapse
features are also labeled (Rivière
des Remparts and Plaine des
Sables). The cones of the 1998
eruption (Hudson, Krafft, and
Kapor), the November 2002
cone (Guanyin), and the extent
of the 2002 lava flows are
shown. The 2002 lava flowed
down the steep slope of the
Grandes Pentes before crossing
the Grand Brûlé and entering the
ocean. Also shown is the
location of the prehistorically
active Piton de Caille cone, and
arrows point toward the
locations of two other
prehistoric cones referred to in
this study. The map is modified
from Longpré et al. (2007)
last ~400 ka. These lavas contain 6 to 8 wt.% MgO and
show little chemical variability, which has led them to be
referred to as “steady-state basalts” (Albarède et al. 1997).
The restricted compositional variability of the erupted lavas
is probably caused by the buffering effect of cumulate crystal
mush zones, which also serve to filter out more Mg-rich
olivine that crystallized at higher pressures (Albarède et al.
1997; Bureau et al. 1999).
Since historical times, volcanic activity has been
dominated by small- to moderate-volume (small-scale)
eruptions, which occur intermittently between major,
large-volume eruptions. Six major eruptions have occurred
in the twentieth and twenty-first centuries: 1931, 1961,
1977, 1986, 1998, and 2007 and 21 small-scale eruptions
occurred between the major eruptions of 1998 and 2007.
The volumes of material expelled during these small-scale
eruptions are one to two orders of magnitude smaller than
those expelled during the major eruptions. Major and smallscale events can also be distinguished on the basis of
seismic data. The 1998 and 2007 major eruptions were
accompanied by earthquakes at depths >5 km b.s.l.,
whereas small-scale events are associated with earthquake
depths of ≤2 km b.s.l. (Observatoire Volcanologique du
Piton De La Fournaise (OVPDLF), unpublished data).
There is no correlation between eruptive volume and
effusion rate. Some major eruptions had low effusion rates
(3.5 m3/s; 1998) and erupted nearly aphyric basalt, whereas
numerous small-scale eruptions since 1998 have had high
eruption rates (20.8 m3/s; February 2005) and ejected
olivine-rich basalts (Vlastélic et al. 2005, 2007). Vlastélic
et al. (2007) and Peltier et al. (2009) have identified the
1998–2007 period as an eruptive cycle during which the
eruption of steady-state basalts with little entrained olivine
antecrysts (crystals foreign to the erupting melt but
originating from the magmatic system) was progressively
replaced, starting in June 2001, by the eruption of olivinerich lavas containing olivine antecrysts of cumulate origin.
The November 2002 eruption
After 3 months of intermittent seismic activity, a large
seismic crisis involving several hundred earthquakes
located above sea level and strong summit deformation
started at Piton de la Fournaise on November 15, 2002
and lasted 5 h (Longpré et al. 2007). The eruption tremor
began on November 16, with fissures opening on the east
flank of the volcano at elevations of 1,900 to 1,600 m,
followed by the flow of lava down the same flank. A small
cone, called Piton Guanyin, formed on one of the most active
fissures at an elevation of ~1,600 m (Fig. 1). Hawaiian-type
activity commenced immediately and was characterized by
lava fountains reaching up to 80 m in height. On the
following day (November 17), the fountains were smaller,
only up to 30 m high, due partly to drag imposed by a small
lava lake that had developed within the cone’s interior. By
then, the eruption tremor had decreased by a factor of four,
1080
Bull Volcanol (2009) 71:1077–1089
and the fissures located at ~1,850 and ~1,750 m elevation
had ceased to be active, leaving Piton Guanyin as the only
active center.
Activity remained generally constant until November
29th, when the eruptive tremor, number of seismic events,
and the height of the lava fountains increased suddenly
while the summit began to deflate. Shallow seismic events
beneath the summit were frequent and particularly intense
with up to 6,000 events per day until December 23, when a
pit crater appeared within the Dolomieu crater followed by
a rapid decline of seismicity (Longpré et al. 2007). Lava
emissions ended on December 3, after 18 days of eruption
that had produced 8×106 m3 of material at an average
output rate of 5.1 m3/s (Vlastélic et al. 2007).
Rock and melt inclusion descriptions
Samples were collected at various times during the 2 weeks
spanned by the eruption and comprised spatter cone tephra,
lava quenched in a bucket of water and the quickly cooled
tops of ‘a’a lava flows collected along levees. Information
on sample type, location, and time of emplacement is
provided in Table 1. Sample REU0211-161, which is
spatter cone tephra released on the first day of the eruption,
is referred to hereafter as the “early-stage” sample, and all
other samples, which are from lavas erupted between the
25th of November and the 2nd of December, are referred to
as the “late-stage” samples. This division corresponds to a
change in the composition and proportion of the olivine
crystals in the samples.
The groundmass of lava samples contains visible microlites
(<300 μm in diameter) of Cr-spinel, plagioclase, and
clinopyroxene, whereas the tephra groundmass is glassier
and contains significantly less Cr-spinel (early-stage sample
REU0211-161). All samples contain olivine crystals that
increased in size as the eruption progressed, from <0.5 mm in
length in the early tephra to 4 mm in length in the latest lavas.
Olivine crystals in the late-stage lavas contain Cr-spinel
inclusions with a Cr # of 54–63 (Cr #=100Cr/(Cr+Al)), have
visible healed fractures, and are cut by numerous secondary
melt and fluid inclusion planes (Fig. 2). Some display kink
banding, a feature found in other olivine crystals from Piton
de la Fournaise identified as cumulates (Albarède et al. 1997;
Bureau et al. 1999). The lack of these features in the earlystage olivine suggests that this olivine is not of cumulate
origin.
Primary melt inclusions are variable in size, ranging
from ~25 to ~150 μm in diameter. They are isolated or
occur in clusters, whereas secondary melt inclusions are
generally smaller and occur along healed cracks. The
dominant shape is ellipsoidal but round, highly elongate,
and negative crystal shapes also occur. The primary melt
inclusions display varying degrees of crystallization due
either to cracking and volatile loss upon ascent or slow
cooling during flow of the host lava at the surface. Some
primary inclusions are not visibly crystallized and were
targeted for analysis. However, there is evidence that they
have undergone post-entrapment crystallization of olivine
on their walls (see below). All primary melt inclusions
contain a shrinkage bubble.
Methodology
X-ray fluorescence
Whole-rock major element analyses were conducted on
fused beads prepared from ignited samples using a Phillips
PW2400 3-kW automated X-ray fluorescence spectrometer
system at the Geochemical Laboratories of McGill University. The accuracy for SiO2 was within 0.5 wt.% of the
amount reported and for all other major elements was 1%
of the amount present; the overall precision was 0.5%.
Table 1 Sample descriptions
Sample
Early phase
REU0211-161
REU0211-171
Late phase
REU0211-252
REU0211-254
REU0212-014
REU0212-034
REU0212-021
Date emplaced
Date sampled
Location
Type
Nov. 16, 2002
Nov. 17, 2002
Nov. 16, 2002
Nov. 17, 2002
S fissure, 1,600 m elevation
N fissure, 1,550 m elevation
Spatter cone tephra
Water-quenched lava
Nov. 25, 2002
Nov. 25, 2002
Nov. 29, 2002
Nov. 29, 2002
Dec. 2, 2002
Nov. 25, 2002
Nov. 25, 2002
Dec. 1, 2002
Dec. 3, 2002
Dec. 2, 2002
S channel levee, 700 m elevation
S channel, 700 m elevation
S channel levee, 110 m elevation
S channel levee, 120 m elevation
N channel levee, 120 m elevation
Surficially cooled ‘a’a
Water-quenched lava
Surficially cooled ‘a’a
Surficially cooled ‘a’a
Surficially cooled ‘a’a
Only the last three digits of each sample name are used to identify the samples in subsequent tables
lava
lava
lava
lava
Bull Volcanol (2009) 71:1077–1089
1081
Fourier-transform infrared spectroscopy
Fig. 2 Olivine crystal from sample REU0212-014 containing glassy
primary melt inclusions, Cr-spinel inclusions, and trails of secondary
melt inclusions
Electron microprobe
Analyses of major elements in olivine and major and
volatile elements (S and Cl) in the primary melt inclusions
were carried out at McGill University using a Jeol JXA8900L electron microprobe and the University of Oregon
using a Cameca SX-100, both with an atomic number,
absorption, and fluorescence correction procedure. Operating conditions for melt inclusion analysis at McGill
University were as follows: 15 kV, 15 nA, 20 s peak
counting times for all elements except Fe and Mn (30 s), Ti,
Ni, and P (40 s), and Cl (50 s). The beam diameter was
defocused to 10 μm. A basaltic glass standard (BMAK
Smithsonian 113498) was used for major elements except
Na and K, for which an obsidian glass standard (KN9) was
used. Operating conditions for olivine analysis were as
follows: 15 kV, 40 nA, 20 s peak counting times for all
elements, and a beam diameter of 2 μm. An olivine
standard was used for all elements. The average precision
(2σ) is 6% or better for all elements except for MnO (30%),
P2O5 and S (20%), and Cl (35%). At the University of
Oregon, 60 s counting times were used for all elements
except Ti, Al, Mg, and P (40 s) and Ca, K (20 s). For melt
inclusion analysis, the beam was set to 20 kV with a current
of 30 nA and a diameter of 5 μm. Linear regression of
count rate to time zero was performed on Na and K to
correct for the effects of alkali migration. The basaltic glass
standard VG-2 (USGS) was used for all elements. For
olivine analysis, the beam conditions were as follows:
15 kV, 30 nA, and a focused beam. An olivine standard was
used for all elements. The average precision (2σ) is 5% or
better for all the elements analyzed except MnO for which
the precision is ≤25%.
The water and carbon contents of primary melt inclusions
were measured using a Nicolet Magna 560 Fouriertransform infrared spectrometer (FTIR) with a continuum
IR microscope and OMNIC software at the University of
Oregon. Doubly polished olivine wafers were prepared
such that the melt inclusions were intersected on both sides.
The average thickness of these wafers was 30 μm and the
average aperture size used in the FTIR microscope was
50×30 μm. Transmission spectra were collected in the
wavelength range of 6,000–650 cm−1 and then converted to
absorption values. For H2O absorption measurements, the
peak height at 3,535 cm−1 was measured above the
background. For carbonate measurements, an unpublished
peak fitting program supplied by S. Newman was used to
obtain absorbance values at wavelengths of 1,430 and
1,515 cm−1. Owing to the thinness of the wafers, most
spectra exhibited interference fringes, which in many cases
masked any possible signal of the CO32− doublet. For this
reason, only two inclusions yielded spectra in which the
background was smooth enough to confidently analyze
CO2.
Absorbance was converted to concentration using the Beer–
Lambert law, a melt density of 2.8 g/cm3 and the following
absorption coefficients: 394 L mol−1 cm−1 for CO32−
(calculated from the melt inclusion compositions using the
method of Dixon and Pan 1995) and 63 L mol−1 cm−1 for OH
and H2O at 3,535 cm−1 (Dixon et al. 1995). The values of
these parameters are lower than those employed by Bureau et
al. (1998b, 1999) but are more appropriate for the Piton de la
Fournaise basalts. Melt inclusion thickness was determined by
mounting the wafers on a thin needle and immersing them in
oil of refractive index 1.657. The wafers were then viewed
under the microscope at 100× magnification and tilted such
that the field of view was perpendicular to the width of the
wafer. This allowed for a cross-sectional view of the wafer
and inclusion and measurement of the inclusion thickness
using the objective reticule.
Results
Whole-rock chemistry
Two water-quenched lava samples were collected by a team
of scientists from the Observatoire Volcanologique du Piton
de la Fournaise, one on November 17th (REU0211-171)
and the other on November 25th (REU0211-254). Their
chemical compositions are reported in Table 2. Sample
REU0211-254 is more magnesian than sample REU0211171, containing 8.7 wt.% MgO compared to 6.9 wt.%, and
has slightly lower concentrations of most other oxides. The
1082
Bull Volcanol (2009) 71:1077–1089
a
20
Fo80
All whole rock
Nov 2002 whole rock
Nov 2002 olivine crystals
18
FeOtotal (wt.%)
data agree well with other published whole-rock compositions for the November 2002 eruption (Vlastélic et al. 2007)
and point to an increase in olivine content as the eruption
progressed (increase in MgO content and dilution of most
other oxides due to accumulation of olivine Fo84; Fig. 3a).
The whole-rock data for the November 2002 eruption fall
along the compositional trend defined by other lavas
erupted from Piton de la Fournaise (Fig. 3; cf. Albarède et
al. 1997; Bureau et al. 1999; Vlastélic et al. 2005, 2007).
16
82
84
14
86
Olivine control line
12
Dec. 2
10
Nov. 16-17
Nov. 25
8
6
Olivine and melt inclusion major element chemistry
0
10
20
30
40
50
MgO (wt.%)
Table 2 Whole-rock compositions in weight percent
Sample
Eruption date
REU0211-171
Nov. 17, 2002
REU0211-254
Nov. 25, 2002
SiO2
TiO2
Al2O3
FeOa
MnO
MgO
CaO
Na2O
K2O
P2O5
48.70
2.69
14.32
11.09
0.17
6.85
11.31
2.72
0.77
0.33
48.32
2.56
13.60
11.32
0.17
8.72
10.77
2.54
0.72
0.32
a
Total iron reported as FeO
b
Clinopyroxene
fractionation
0.90
Olivine control line
CaO/Al2O3
All olivine crystals have cores varying in composition from
Fo83 to Fo84, except in the early-stage sample (REU0211161), which is from tephra released on the first day of the
eruption; cores of olivine crystals from this sample have a
composition of Fo81. Most of the crystals have a compositionally distinct rim varying in thickness from 10–15 μm
and in composition from Fo76 to Fo82.
Only primary melt inclusions in olivine were analyzed,
and their compositions are reported in Table 3. The most
conspicuous feature of the analyses is the highly variable
MgO (2.2–5 wt.%) and FeOtotal (5.5–10 wt.%, all iron
expressed as FeO) content of the inclusions. If these
compositions were representative of the original magma,
they would require that the host olivine have a composition
of Fo68–78, which is more Fe rich than the observed values
(Fo81–84). This indicates that the melt inclusions are out of
equilibrium with their olivine hosts and implies that the
inclusions have undergone post-entrapment modification
(Table 3; uncorrected compositions).
After entrapment of a melt inclusion, olivine crystallizes
along the walls of the inclusion as temperature decreases
and there is diffusion of iron from the melt into the host
olivine (Danyushevsky et al. 2000). To account for these
changes, the compositions of the melt inclusions were
0.80
0.70
Early phase 2002
Late phase 2002
Past major eruptions
0.60
0
10
20
30
MgO (wt.%)
Fig. 3 a MgO vs. FeOtotal contents of whole-rock samples from Piton
de la Fournaise compiled from Albarède et al. (1997), Bureau et al.
(1999), and Vlastélic et al. (2005, 2007) and including the November
2002 eruption (data from Vlastélic et al. 2007 and this study; eruption
dates are indicated). The compositions of olivine from the 2002
eruption are also plotted (black squares) and their corresponding
forsterite contents are indicated. The whole-rock data fall along an
olivine control line representing accumulation of olivine (dominantly
Fo83–84) in the lavas. Lavas extruded after November 17, 2002 show
evidence of olivine accumulation, the proportion of which increased
as the eruption progressed. b MgO vs. CaO/Al2O3 contents of wholerock samples and compositionally restored melt inclusions (see text
for a description of the correction procedure) from the November 2002
eruption and past major eruptions (Bureau et al. 1998b). Both melt
inclusions and whole-rock data show evidence of clinopyroxene
fractionation. The lack of clinopyroxene phenocrysts in the November
2002 lavas (only minor groundmass/microphenocrystic clinopyroxene
is present) does not preclude clinopyroxene fractionation from the
liquid at depth. Albarède et al. (1997) argued that fractionation of
clinopyroxene in the deeper parts of the edifice accompanied early
differentiation of the Piton de la Fournaise lavas, and this process is
likely responsible for the variability in the CaO/Al2O3 ratios of melt
inclusions (Bureau et al. 1999)
corrected following the methods described in Danyushevsky
et al. (2000). The extent of this post-entrapment crystallization and iron loss for the melt inclusions of the
November 2002 eruption is readily seen by comparing their
compositions with those of the lavas (Fig. 4). If a melt
inclusion is in equilibrium with its host olivine, its
composition should plot at the intersection of the liquid
line of descent for olivine fractional crystallization (curves
labeled L.l.d.) and the line representing liquid compositions
c
b
a
48.80
2.81
14.44
1.43
10.26
0.15
7.46
11.75
2.44
0.66
0.33
0.075
0.019
50.03
3.15
16.18
9.81
0.16
3.60
13.16
2.73
0.74
0.38
0.085
0.021
81
12
1,099
1,181
161-04
48.76
2.75
14.36
1.42
10.28
0.15
7.47
11.69
2.47
0.68
0.29
0.080
0.022
50.02
3.09
16.14
9.96
0.17
3.50
13.14
2.78
0.76
0.32
0.090
0.025
81
12
1,097
1,182
161-05
48.29
2.81
14.29
1.38
10.35
0.15
7.53
11.46
2.41
0.66
0.30
0.079
0.026
49.63
3.19
16.25
9.80
0.17
3.20
13.03
2.74
0.75
0.34
0.090
0.029
81
14
1,092
1,184
161-06
48.70
2.70
14.30
1.37
10.38
0.13
7.54
11.51
2.41
0.68
0.30
0.080
0.028
50.15
3.08
16.33
9.75
0.15
3.08
13.15
2.75
0.78
0.34
0.091
0.032
81
14
1,088
1,184
161-07
Corrected for both iron loss by diffusion and post-entrapment crystallization
Percentage of post-entrapment crystallization
Total iron as FeO
48.91
2.70
14.27
1.38
10.35
0.15
7.53
11.79
2.43
0.65
0.28
0.073
0.025
Corrected compositions in wt.%c
SiO2
48.97
49.01
2.75
2.71
TiO2
14.38
14.33
Al2O3
1.42
1.39
Fe2O3
FeO
10.29
10.34
MnO
0.14
0.15
MgO
7.47
7.51
CaO
11.82
11.60
2.40
2.45
Na2O
K2O
0.69
0.68
0.31
0.34
P2O5
S
0.077
0.080
Cl
0.025
0.028
161-03
50.34
3.07
16.23
10.07
0.17
3.18
13.42
2.77
0.75
0.32
0.083
0.029
81
14
1,089
1,182
50.28
3.10
16.19
9.99
0.16
3.44
13.31
2.71
0.78
0.35
0.087
0.028
81
13
1,095
1,181
SiO2
TiO2
Al2O3
FeOa
MnO
MgO
CaO
Na2O
K2O
P2O5
S
Cl
Host Fo
% p.e.cb
Tquench °C
Ttrapping °C
161-02
50.43
3.08
16.26
9.73
0.18
3.24
13.17
2.78
0.77
0.39
0.091
0.032
81
14
1,092
1,182
161-01
Sample
Table 3 Analyzed melt inclusion compositions in weight percent
47.47
2.60
13.42
1.38
10.15
0.16
8.90
11.50
2.27
0.52
0.62
0.081
0.021
48.72
2.99
15.42
10.09
0.19
4.04
13.21
2.61
0.60
0.71
0.094
0.024
84
15
1,109
1,213
252-01
48.32
2.68
13.61
1.30
10.35
0.10
9.32
10.64
2.45
0.80
0.44
0.101
0.021
50.08
3.18
16.17
8.39
0.11
3.36
12.63
2.91
0.95
0.53
0.120
0.025
84
19
1,093
1,220
252-04
48.76
2.63
13.84
1.32
10.28
0.11
8.97
10.52
2.70
0.69
0.36
0.081
0.023
50.47
3.08
16.24
9.05
0.13
3.45
12.34
3.17
0.81
0.46
0.095
0.026
84
17
1,093
1,211
252-07
47.43
3.12
13.44
1.30
10.31
0.12
8.95
10.88
2.57
0.80
0.70
0.054
0.020
48.96
3.68
15.84
9.18
0.14
3.27
12.83
3.02
0.94
0.83
0.064
0.024
83
18
1,092
1,214
252-08
48.70
2.76
13.94
1.48
9.96
0.11
8.53
10.20
2.77
1.07
0.51
0.116
0.038
49.69
3.05
15.40
9.44
0.12
4.99
11.27
3.06
1.18
0.56
0.128
0.042
83
11
1,133
1,207
014-01
49.07
2.38
13.39
1.20
10.60
0.10
9.63
11.47
2.33
0.54
0.23
0.064
0.023
51.47
2.93
16.50
8.15
0.12
2.56
14.13
2.87
0.67
0.29
0.079
0.028
84
23
1,073
1,222
034-04
49.17
2.46
13.58
1.25
10.44
0.11
9.25
11.38
2.36
0.75
0.36
0.134
0.031
51.28
2.96
16.34
8.07
0.14
2.93
13.69
2.84
0.90
0.44
0.161
0.037
84
20
1,082
1,215
034-11
48.47
2.46
13.92
1.27
10.41
0.09
9.17
11.59
2.43
0.58
0.30
0.086
0.025
50.38
2.94
16.66
8.39
0.11
3.00
13.88
2.91
0.70
0.36
0.103
0.030
84
20
1,084
1,214
034-12
48.83
2.84
14.34
1.21
10.55
0.06
9.33
12.15
2.50
0.87
0.40
0.050
0.027
51.06
3.46
17.49
5.52
0.08
2.59
14.82
3.05
1.06
0.49
0.061
0.033
84
22
1,072
1,213
021-03
48.48
2.73
14.02
1.26
10.44
0.11
9.41
11.54
2.24
0.75
0.32
0.095
0.022
50.45
3.29
16.89
7.56
0.13
3.01
13.90
2.69
0.90
0.39
0.115
0.027
84
21
1,084
1,218
021-05
50.30
2.23
14.28
1.17
10.67
0.07
9.42
10.51
2.37
1.34
0.07
0.033
0.011
53.08
2.76
17.67
5.90
0.08
2.29
13.00
2.93
1.66
0.08
0.040
0.014
84
24
1,070
1,220
021-12
49.59
2.69
14.63
1.16
10.69
0.05
9.43
10.13
2.38
1.45
0.07
0.032
–
52.24
3.34
18.15
5.50
0.07
2.23
12.56
2.95
1.79
0.08
0.040
–
84
24
1,072
1,223
021-13
Bull Volcanol (2009) 71:1077–1089
1083
1084
Bull Volcanol (2009) 71:1077–1089
in equilibrium with the host olivine (lines labeled Foxx).
Any deviation from this point of intersection defines a
vector, the magnitude and direction of which depend on the
extent and relative proportions of post-entrapment crystallization (vector parallel to the liquid line of descent) and
diffusive iron loss (vector parallel to the ordinate). From
Fig. 4, it is evident that there were substantial postentrapment changes in the compositions of the November
2002 melt inclusions, particularly in the concentration of
iron for some inclusions. As mentioned above, these melt
inclusions have analyzed compositions that are in equilibrium with olivine of composition Fo68–78 (average Fo71) but
are hosted in olivine of composition Fo81 or Fo83–84. By
comparison, the compositions of melt inclusions from
major eruptions appear to have undergone relatively minor
change. They are in equilibrium with olivine of composition Fo80–83 (average Fo81) but are hosted in olivine of
composition Fo83–85.
To correct for the post-entrapment changes, we added
Fe2+ back into the inclusion until its composition reached
the liquid line of descent for olivine fractionation and then
incrementally added olivine to the melt until the Mg/Fe2+
ratio of the latter reflected equilibrium with its host
olivine. For this purpose, the partition coefficient, KD =
Early phase 2002
Late phase 2002
Past major eruptions
14
FeOtotal (wt.%)
12
Corrected compositions
L.l.d.
L.l.d.
10
8
Fo85
.E.C.)
6
Fo71
Fo81
L.l.d . (P
Fo83
4
Iron loss
Foxx (host)
2
0
2
4
6
8
10
12
MgO (wt.%)
Fig. 4 Uncorrected FeOtotal vs. MgO contents of melt inclusions from
the November 2002 eruption and from past major eruptions (Bureau et
al. 1998b). Also shown are the corresponding compositions after
correction for post-entrapment crystallization and Fe2+ loss. The
shaded field represents the whole-rock compositions shown in Fig. 3a.
A liquid in equilibrium with a given olivine composition will lie along
the line labeled with the olivine composition (Foxx) assuming an FeO/
FeOtotal ratio of 0.86 (Bureau et al. 1998b). These lines were calculated
using a KD of 0.306 (Roeder and Emslie 1970; Fisk et al. 1988). Liquid
lines of descent (L.l.d.) for olivine fractionation are shown starting with
liquids in equilibrium with olivine Fo84 and Fo81. A melt inclusion in
equilibrium with its host olivine (Foxx) and with the appropriate liquid
line of descent (implying no Fe2+ loss) will plot at the intersection of
these two lines. The effects of post-entrapment crystallization and Fe2+
loss by diffusion are illustrated in the inset. P.E.C.=post-entrapment
crystallization along the inclusion–host interface
(Mg/Fe2+)melt/(Mg/Fe2+)olivine, was assumed to be equal to
0.306 based on experimental studies of Reunion magmas
and other basalts (Roeder and Emslie 1970; Fisk et al.
1988). We used a melt FeO/FeOtotal ratio of 0.86 based on
olivine–spinel-liquid equilibrium (Maurel and Maurel
1982) and Cr-spinel Fe2+/Fe3+ ratios reported by Bureau
et al. (1998b) for past summit eruptions at Piton de la
Fournaise (1.81±0.03 for Dolomieu crater samples). The
latter data imply fO2 conditions ~1 log unit above FMQ at
1,200°C. However, this estimate, which is on the high end
of estimates for other oceanic hot spot magmas (e.g.,
Óskarsson 1994; Rhodes and Vollinger 2005) should be
treated with some caution as Cr-spinel re-equilibrates
rapidly with the melt and an incomplete analysis of minor
elements like Ti and V can significantly affect the
calculated Fe2+/Fe3+ value (Clynne and Borg 1997). An
error in fO2 of ±1 log unit could change the corrected
MgO content of a melt inclusion by up to 10% (relative).
We modeled the melt inclusions as closed systems (Fe3+
behaves incompatibly), and consequently, the FeO/FeOtotal
ratio of the melt increased slightly during incremental
addition of equilibrium olivine.
For all of the November 2002 melt inclusions, the
proportion of Fe2+ lost to diffusion and olivine crystallization after melt entrapment is estimated to have been 14% to
56% and 11% to 24% respectively. The compositions of
melt inclusions hosted in olivine (Fo83–85) from past major
eruptions (Bureau et al. 1998b) were corrected in the same
manner; the iron loss and olivine crystallization were
estimated to be 0% to 16% and 3% to 8%, respectively.
Corrected major element compositions for the November
2002 primary melt inclusions are presented in Table 3 and
compared to those of melt inclusions from major eruptions
(hosted by Fo83–85 olivine) in Fig. 4.
The melt inclusions hosted in Fo83–84 olivine from the
2002 eruption have corrected MgO contents from 8.5 to
9.6 wt.%, SiO2 contents from 47 to 50 wt.%, and total
alkali (Na2O+K2O) contents from 3 to 4 wt.%. These
compositions fall within the range of corrected compositions of melt inclusions hosted in Fo83–84 olivine from
major eruptions (Figs. 3b and 4; Bureau et al. 1998b). The
melt inclusions hosted in Fo81 olivine have a restricted
compositional range with a MgO content of ~7.5 wt.%,
SiO2 contents from 48 to 49 wt.%, and total alkali contents
of 3 wt.% and are more evolved than the average melt
inclusion hosted in Fo83–84 olivine (Figs. 3b and 4).
Volatile content
Owing to differences in the H2O and CO2 absorption
coefficients used in this study and those used by Bureau et
al. (1998b, 1999), the H2O and CO2 contents of the melt
inclusions in previous studies were underestimated; we
Bull Volcanol (2009) 71:1077–1089
1085
Table 4 H2O and CO2 contents
a
Corrected for post-entrapment crystallization. Melt inclusions with no associated major element analysis were corrected using the average
amount of post-entrapment crystallization for November 2002 melt inclusions. Most CO2 values in the November 2002 melt inclusions are below
detection limit (–) due to analytical difficulties discussed in the text
b
The difference between the absorption coefficients used by Jendrzejewski et al. (1996a, b) and Bureau et al. (1998b, 1999) and the coefficients
from Dixon and Pan (1995) results in an error (underestimate) of 19% for H2O and 1% for CO2
have recalculated the published values using the more
commonly accepted coefficients (Table 4). FTIR analysis of
primary melt inclusions from samples spanning the November 2002 eruption yielded H2O contents in the range 0.1 to
1.8 wt.%; most values were between 0.2 and 0.8 wt.%
(Table 4). Only two melt inclusions yielded reliable CO2
values due to the analytical difficulties discussed above.
These values are 165 and 1,010 ppm and are for inclusions
from samples REU0212-014 and REU0211-252, respectively
(sample REU0211-252 also has the highest H2O content).
The corresponding vapor saturation pressures are 425 and
2,630 bars, respectively (calculated with VolatileCalc; Newman and Lowenstern 2002). Vapor saturation is inferred from
the presence of fluid inclusions in these samples, as well as in
olivine (Fo83–87) from earlier eruptions studied by Bureau et
al. (1998b, 1999). It is also consistent with the wide range of
H2O and CO2 contents reported by them for melt inclusions
in these samples. Our calculated saturation pressures are
considered minimum values because we did not analyze the
volatile content of the shrinkage bubbles. Contents of both
H2O and CO2 overlap with those obtained by Bureau et al.
(1998b) for melt inclusions in Fo83–85 olivine from major
eruptions.
The concentrations of sulfur in melt inclusions analyzed
in our study span a wide range, from 330 to 1,380 ppm.
One of the inclusions (sample REU0211-252-3) contains a
visible sulfide globule with appreciable proportions of Cu
and Ni (10 and 8 wt.%, respectively; data from electron
microprobe energy dispersive spectrometry). However, no
sulfide globules are present in the host olivine crystal.
Previous studies of Piton de la Fournaise have reported the
presence of sulfide globules in both melt inclusions and the
host olivine. The sulfur content of the melt inclusions
investigated in these studies averages 1,140±260 ppm
(Bureau et al. 1998a, b). The chlorine contents of our
melt inclusions vary from below detection to 390 ppm and
are comparable to those of melt inclusions from major
eruptions, which are in the range 270±50 ppm (Bureau et
al. 1998a, b).
Discussion
Trapping and eruption temperatures of melt inclusions
The initial composition of a primary melt inclusion hosted
by olivine may be assumed to be the same as that of the
corresponding magma and reflect equilibrium with the
olivine at the time of entrapment. When the host olivine
begins to cool, the melt inclusion will evolve by crystalliz-
1086
ing olivine along the rim of the inclusion, diffusing Fe2+
into the host and growing a shrinkage bubble due to
thermal contraction of the melt and the volume decrease
caused by crystallization. This evolution will terminate
once the inclusion cools fast enough (i.e., upon eruption) to
inhibit further diffusion of elements, not only from the
inclusion but also within it (thereby preventing further
crystallization of olivine). The temperature at which this
occurs is referred to as the quenching temperature. If the
inclusion were to cool slowly during an eruption, such as
might be the case in the interior of a lava flow, the
quenching temperature would be significantly lower than
the eruption temperature. However, if it cooled quickly, for
example because of its location in a small pyroclast, the two
temperatures could be quite similar.
To estimate temperatures of melt inclusion trapping and
quenching, we used the empirical MgO-in-liquid geothermometer of Sugawara (2000; Eq. 6a), which has been
calibrated for a range of pressures, temperatures, and
compositions appropriate for the Reunion magmas. This
geothermometer incorporates the effects of pressure and
H2O content on the equilibrium temperature of olivinesaturated liquids. For Hawaiian lavas, the model yields
results similar to those obtained with the experimentally
derived geothermometers of Montierth et al. (1995) and
Helz and Thornber (1987); these lavas include compositions (low MgO) similar to those analyzed in the November
2002 inclusions.
The Sugawara geothermometer was applied to the
corrected melt inclusion compositions at a pressure of
1,200 bars (average trapping pressure calculated from the
H2O and CO2 contents of the 2002 melt inclusions). If the
water content of a melt inclusion had not been determined,
the average analyzed water content of the 2002 melt
inclusions was assumed. This value (0.55 wt.%) is similar
to the average value obtained for melt inclusions previously
analyzed in Fo83–85 olivine by Bureau et al. (1998b;
0.86 wt.%, recalculated in Table 4). The uncertainty in the
calculated pressure and water content of the melt inclusions
is less than the standard error of the geothermometer, which
is ±40ºC. The resulting calculated trapping temperatures are
given in Table 3. Melt inclusions from the early stage of the
eruption (Fo81 olivine hosts) yield an average trapping
temperature of 1,182±1°C, whereas the trapping temperatures of those from the late-stage (Fo83–84 olivine hosts)
range from 1,207°C to 1,223°C. These temperatures are
equivalent within the error of the geothermometer.
However, given the systematic differences in calculated
temperatures, olivine host compositions, and melt MgO
contents between the early and late-stage melt inclusions,
we conclude that the difference in trapping temperatures of
melt inclusions sampled in the two stages is real. We also
calculated trapping temperatures for melt inclusions from
Bull Volcanol (2009) 71:1077–1089
major eruptions using the raw data of Bureau et al. (1998b),
corrected using the same procedure employed for the 2002
inclusions and a pressure of 1,200 bars. The calculated
trapping temperatures vary with the olivine composition
and range from 1,199°C to 1,227°C.
Quenching temperatures were calculated by applying the
Sugawara geothermometer to the uncorrected melt inclusion compositions using the same pressure and water
content estimates as above. This assumes no water loss
from the melt inclusions due to pre-eruptive diffusion and
no drop in pressure within the inclusions. The resulting
temperatures are 1,088°C to 1,099°C for the early-stage
melt inclusions and from 1,070°C to 1,133°C for the latestage inclusions (Fig. 5). For the past major events, the
quenching temperatures are in a narrow range from 1,163°C
to 1,181°C (Fig. 5).
Although the quenching temperatures for the November
2002 melt inclusions are all equivalent within the error of
the geothermometer, there is a significant amount of scatter
in the temperatures recorded by the late-stage melt
inclusions. If the melt inclusions cooled from the same
magma at a similar rate, they should all record the same
quenching temperature, and given rapid cooling (explosively erupted), this temperature should be similar to the
eruption temperature. In order to evaluate this cooling
history, we plotted the amount of iron loss by diffusion
experienced by each inclusion (FeO in weight percent)
versus its calculated quenching temperature (Fig. 5). The
amount of iron loss is a function of the time-averaged
cooling rate and the size of the inclusion (Danyushevsky et
al. 2000, 2002). Based on a comparison of the data for
Reunion melt inclusions to cooling rate-dependent curves
determined for an average inclusion radius of 25 μm, we
conclude that the inclusions from past major eruptions and
those from the early phase of the 2002 eruption, all from
explosively erupted tephra samples, cooled at timeaveraged rates of ≥100°C/day. Some of the late-stage
inclusions from the 2002 eruption, which were hosted by
lava, also experienced similarly fast cooling. However,
other late-stage inclusions cooled more slowly, i.e., at a
time-averaged rate of <100°C/day. Much of this slow
cooling likely occurred in the lava flow, but we cannot
rule out the possibility that some of it may have occurred
during prolonged storage in the plumbing system.
For the purpose of this study, we based our estimate of
the eruption temperature for the November 2002 event on
the quenching temperatures of those inclusions that cooled
relatively quickly (≥100°C/day). Temperatures for the
early-stage melt inclusions (sample REU0211-161) and
those from sample REU0211-252, which was erupted on
the 25th of November, cluster around a value of 1,100°C. A
single inclusion from sample REU0212-014 also falls along
the rapid cooling curve of 100°C/day and appears to record
Bull Volcanol (2009) 71:1077–1089
1087
a higher eruption temperature than the other inclusions
(1,133°C). However, in the absence of supporting data, it is
uncertain whether or not this lava, which was extruded on
November 29th (the onset of renewed explosive activity
during the 2002 eruption), was indeed hotter. Therefore,
assuming that the magma erupted in the November 2002
event was at a temperature of 1,100°C, it follows that this
magma was cooler (possibly by up to 70°C) than magma
erupted during past major eruptions, an interpretation that is
consistent with the presence of more evolved olivine
phenocrysts in the products of the early phase of the
November 2002 eruption.
Nov. 2002
7
Major eruptions
1 ˚C/day (150 days)
10 (15)
4
3
100 (1)
2
1000 (0.17)
10000 (0.02)
Trapping Fo84
5
Trapping Fo81
FeO loss (wt.%)
6
1
100 (1)
0
1000
1050
1100
1150
1200
1250
Quenching Temperature (°C)
Fig. 5 Calculated quenching temperature vs. FeO loss by diffusion
(calculated initial FeO minus analyzed FeO assuming FeO/FeOtotal =
0.86) for each inclusion of the November 2002 eruption and the
inclusions from past major eruptions (Bureau et al. 1998b). Symbols as
in Fig. 3. The solid time-averaged cooling curves are from
Danyushevsky et al. (2002) for a melt inclusion radius of 25 μm
and a cooling interval of 150°C, starting at 1,220°C, which is the
trapping temperature of the melt inclusions hosted by Fo83–84 olivine
crystals. The number next to each curve represents the time-averaged
cooling rate (°C/day) and the number in brackets is the time in days
that it would take the inclusions to cool to 1,070°C. The dashed curve
represents a time-averaged cooling rate of 100°C/day for an inclusion
radius of 25 μm, a starting temperature of 1,180°C and a cooling
interval of 90°C, appropriate for melt inclusions hosted by olivine of
composition Fo81 from the early stage of the 2002 eruption. Trapping
temperatures for melt inclusions hosted in olivine of compositions
Fo81 and Fo83–84 are indicated by the vertical lines (the lower end of
the range is shown for Fo83–84 olivine-hosted melt inclusions). High
time-averaged cooling rates and limited pre-eruptive cooling characterize inclusions from past major eruptions which quenched rapidly
upon eruption (tephra samples). The gray shaded field encompassing
these inclusions represents the range of eruption temperatures for past
major events. A portion of the November 2002 inclusions, including
all of the early phase inclusions, appears to have quenched at similarly
high rates, suggesting limited post-eruptive modification. The gray
shaded field encompassing these inclusions provides the best
representation of the range of eruption temperatures for the November
2002 event. The remaining inclusions appear to have experienced a
lower time-averaged cooling rate. These inclusions were probably
affected by slow cooling in the lava flow. There is also one late-stage
melt inclusion that records an anomalously high eruption temperature
(see text for discussion)
Comparison between small-scale and major eruptions
The depth of crystallization and accumulation of olivine
(Fo83–84) sampled by major eruptions is estimated to have
ranged from 5 km b.s.l. to <1 km below the summit of the
volcano (Bureau et al. 1998b). The November 2002 melt
inclusions were trapped at depths varying from 1.5 km a.s.l.
to 6 km b.s.l. (values obtained from partial pressures of
H2O and CO2 recorded by melt inclusions). As has been the
case for major eruptions, the November 2002 event also
sampled Fo83–84 olivine from a variety of levels within and
below the volcanic edifice.
Although the November 2002 eruption may have
sampled a similar population of olivine crystals to those
sampled by major eruptions, there is seismic evidence for
brittle crustal deformation and magma movement possibly
having been initiated at shallower depths. The November
2002 eruption, as well as the three small eruptions
preceding it (March 2001, June 2001, and January 2002),
had seismic event hypocenters (volcano-tectonic earthquakes) located at depths varying between 1.5 and 0.5 km
b.s.l. prior to eruption, suggesting a buildup of magma
pressure at those depths (Staudacher and Cheminée 2001;
Staudacher and OVPDLF 2002; OVPDLF unpublished
report 2002). By comparison, the last major eruption of
1998 was accompanied by pre-eruptive volcano-tectonic
earthquakes originating from 6 km b.s.l., suggesting magma
replenishment from deep levels (Staudacher et al. 1998;
Battaglia et al. 2005).
The late-stage olivine sampled by the November 2002
eruption appears to be of cumulate origin based on the
presence of numerous healed fractures filled with secondary
fluid and melt inclusions, kink banding, and disequilibrium
between olivine (Fo83–84) and whole-rock compositions
(calculated to be in equilibrium with olivine of composition
Fo80–81). Olivine of similar composition expelled during
past major eruptions has also been interpreted to be of
cumulate origin (Bureau et al. 1998a, b, 1999). It thus
appears that olivine crystals not expelled during a major
eruption remain in the shallow plumbing system as
cumulate piles until they are expelled by a later magma,
and consequently, each eruption carries with it phenocrysts
and antecrysts of varying proportions (cf. Bureau et al.
1998a, b, 1999; Vlastélic et al. 2007). This process has also
been documented at Etna, Sicily, by Armienti et al. (1994),
who showed that each erupted batch of magma contained
new phenocrysts as well as crystals left behind during the
preceding eruption. Vlastélic et al. (2007) used the Pb
isotopic signature of the recent lavas emitted from Piton de
la Fournaise to suggest that the November 2002 eruption,
along with other small- to modest-scale eruptions since
1998, were the result of extended magma storage at depth,
whereas the major eruption of 1998 was the result of deep
1088
magma injection into the shallow plumbing system. The
results of our study suggest that magma ejected during
these shallow modest eruptions is also of lower temperature
compared to magma ejected during major eruptions.
Cumulate olivine is entrained in this lower temperature,
more evolved magma and carried to the surface together
with other olivine phenocrysts. Interestingly, the 1998
magma, which erupted from the Kapor vent and contains
minor amounts of Fo84 olivine (Bureau et al. 1999), is
chemically similar to melt trapped as inclusions by Fo81
olivine crystals from the early stage of the November 2002
eruption. This supports the idea that the November 2002
magma comprised a mix of residual 1998 melt, olivine
phenocrysts possibly crystallized from the initial batch of
1998 magma and olivine antecrysts incorporated from a
zone of cumulate mush.
Summary
The November 2002 eruption was typical of the summit
eruptions that characterized volcanic activity at Piton de la
Fournaise between 1999 and 2006. The composition of the
olivine crystals varied from Fo81 in the early phase of the
eruption to Fo83–84 thereafter. Major element analysis has
revealed that melt inclusions from the November 2002
eruption are in chemical disequilibrium with their host
olivine. Some melt inclusions from lava flows experienced
substantially greater degrees of post-entrapment modification than melt inclusions from tephra samples that cooled
very quickly. This is consistent with the observation that
these melt inclusions record the lowest quenching temperatures. By comparing the amounts of diffusive iron loss
experienced by melt inclusions to those predicted for
different time-averaged cooling rates, we classified our
melt inclusions into slower and faster cooled populations
(i.e., with time-averaged cooling rates of <100°C/day and
>100°C/day). Based on the compositions of the latter
population of inclusions and the MgO-in-liquid geothermometer of Sugawara (2000), we estimate that the magma
erupted during the November 2002 event was at a
temperature of ~1,100°C.
Trapping temperatures of melt inclusions were calculated
by applying the Sugawara geothermometer to compositions
that were corrected for post-entrapment crystallization of
olivine and diffusive iron loss. Inclusions from the early
stage of the November 2002 eruption, which are hosted by
Fo81 olivine, were trapped at an average temperature of
1,182°C, whereas those from the later stage of the eruption,
which occur in Fo83–84 olivine, were trapped at temperatures of 1,207°C to 1,223°C. Significantly, the latter
temperature range is very similar to that for melt inclusions
from major eruptions (1,199°C to 1,227°C), as is the
Bull Volcanol (2009) 71:1077–1089
composition of the host olivine (Fo83–84), although the
eruption temperature calculated for major eruptions is
~1,170°C, i.e., 70°C higher than that of the November
2002 eruption. The eruption of cumulate olivine with a
composition similar to most of the 1998 olivine (Fo83–84)
and the eruption of olivine with a composition in
equilibrium with the whole rock of the 1998 eruption at
the Kapor vent (Fo81) are consistent with the model for
magma evolution at Piton de la Fournaise proposed by
Bureau et al. (1998a, b, 1999) and Vlastélic et al. (2007).
Evidence from this and previous studies indicates a system
in which primitive magma is supplied from depth to the
shallow parts of the system and evolves through interaction with olivine cumulate piles and further crystallization
of olivine. Small-scale eruptions, such as the November
2002 event, expel magma which cools and evolves
following the last major replenishment and entrains
variable amounts of cumulate olivine depending on the
eruptive flux.
Acknowledgments We would like to thank Eric Delcher (Université
de la Réunion), Jean-Louis Cheminée (Observatoire Volcanologique
du Piton de la Fournaise), and Marc-Antoine Longpré (McGill
University) for their help in the field and in sample collection. Patrick
Bachèlery (Université de la Réunion) provided important guidance
and support. We are very grateful to Jim Clark (McGill University)
and John Donovan (University of Oregon) for performing the
microprobe analyses. Discussions with Don Baker and David Dolejs
(McGill University) helped clarify issues of olivine crystallization.
Comments by Hélene Bureau and formal reviews by Keith Putirka,
Ivan Vlastélic, and Peter Michael greatly improved the final
manuscript.
The project was supported by a Natural Sciences and Engineering
Research Council of Canada Undergraduate Student Research Award
to NV and Discovery grant to AEW-J as well as travel funds provided
by the Office Franco-Québécois de la Jeunesse awarded to NV.
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