Extreme alteration by hyperacidic brines at Kawah Ijen volcano, East

Journal of Volcanology and Geothermal Research 198 (2010) 253–263
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Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
Extreme alteration by hyperacidic brines at Kawah Ijen volcano, East Java, Indonesia:
I. Textural and mineralogical imprint
Vincent van Hinsberg a,⁎, Kim Berlo b, Manfred van Bergen c, Anthony Williams-Jones a
a
b
c
Hydrothermal Geochemistry Laboratory, Department of Earth & Planetary Sciences, McGill University, 3450 University Street, Montréal, Québec, H3A 2A7, Canada
Volcanology Group, Department of Earth & Planetary Sciences, McGill University, 3450 University Street, Montréal, Québec, H3A 2A7, Canada
Petrology Group, Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584CD, Utrecht, the Netherlands
a r t i c l e
i n f o
Article history:
Received 18 August 2009
Accepted 1 September 2010
Available online 16 September 2010
Keywords:
water–rock interaction
alteration
Kawah Ijen
crater lake
hyperacidic waters
a b s t r a c t
Kawah Ijen volcano, located on the eastern tip of Java and renowned for its large hyperacidic crater lake, poses
significant volcanic and environmental hazards to its immediate surroundings. Crater lake brines seep
through the flanks of the volcano to form the Banyu Pahit river, which is used in irrigation downstream,
resulting in extensive pollution, sharply reduced crop yields and health problems. The impact on the
environment comes mainly from the high element load, which is derived from leaching of rocks by the acid
fluids and transported downstream. Our detailed study of water–rock interaction in different parts of the
Kawah Ijen system indicates that there are three settings for this alteration; the crater lake and Banyu Pahit
riverbed, the hydrothermal system below the lake, and the solfatara of the active rhyolite dome. In all three
settings, the silicates are leached and altered to amorphous silica in the order olivine + glass N An-rich
plagioclase N ortho-pyroxene N clino-pyroxene N Ab-rich plagioclase. In contrast, the alteration of titanomagnetite is characterised by dissolution in the surficial setting, replacement by pyrite and Ti-oxide in the
hydrothermal system and pyritisation + Ti-mobility in the fumarole conduits. Alteration progresses along
crystallographically controlled planes in all phases, and shows strong compositional control in plagioclase and
titanomagnetite. No secondary minerals develop, except for minor barite, cristobalite, pyrite and jarosite. This
indicates that, despite its high element load, the waters are undersaturated with respect to most secondary
minerals typically produced during alteration of these magmatic rocks by acid chloride-sulphate brines, and
that water–rock interaction at Kawah Ijen is not a sink of elements, but rather contributes to the element load
transported downstream.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Summit crater lakes on active volcanoes are rare, but they pose
substantial natural hazards for their local environment (Delmelle and
Bernard 2000a; Mastin and Witter 2000). Phreato-magmatic eruptions involving crater lake fluids can be devastating to life and land
(e.g. the 1919 eruption of Kelut volcano — Kemmerling, 1921a),
especially when augmented by the commonly acidic and toxic nature
of these fluids. Crater lakes can also affect their surroundings during
periods of volcanic quiescence, for example as a result of seepage of
acid fluids through the flanks of the volcano. Where these fluids
interact with the environment, as in the case of Kawah Ijen volcano,
soil productivity plummets and harmful effects on human health
ensue (Löhr et al., 2005). On the other hand, crater lake systems are
prime geochemical field laboratories where the interaction between
fluids and rocks in a volcanic-hydrothermal setting can be studied
⁎ Corresponding author.
E-mail address: [email protected] (V. van Hinsberg).
0377-0273/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2010.09.002
directly and in-situ. Such information not only improves our
understanding of water–rock interaction processes such as those
operating during ore deposition (e.g. Hedenquist et al., 1993), but also
provides a window into the deep magmatic-hydrothermal system,
which can be used to monitor changes in volcanic activity (e.g.
Christenson, 2000; Martínez et al., 2000; Ohba et al., 2008). To
evaluate the response of volcanic-hydrothermal fluids to changes in
volcanic activity and their potential as ore-forming solutions, a
detailed understanding of the mechanisms operating during the
interaction between volcanic fluids and rocks is essential.
In this contribution we report the results of a study of water–rock
interaction involving the hyperacidic fluids of the Kawah Ijen crater
lake and Banyu Pahit river in East Java, Indonesia. The crater lake of
Kawah Ijen volcano represents the largest body of natural hyperacidic
brine in the world (Delmelle and Bernard, 1994) and has been present
since at least 1789 (cf. Bosch, 1858). It is characterised by a singularly
high dissolved element load (N100 g/l) and very low pH. Fluids from
the crater lake seep through the western flank to form the acid Banyu
Pahit river, the water of which is eventually used for irrigation 40 km
downstream of the lake in the coastal plain of Asambagus. This has led
254
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
to a strong, and recently worsening, loss of agricultural productivity,
as well as health problems including fluorosis (Heikens et al., 2005;
Löhr et al., 2005; van Rotterdam-Los et al., 2008). Although the
chemistry, genesis and evolution of fluids in the Kawah Ijen system
have been studied extensively (Woudstra, 1921; Delmelle and
Bernard, 1994, 2000b; Delmelle et al., 2000; Takano et al., 2004),
the complementary changes in the rocks have received less attention,
and inferences on rock alteration are based mainly on model
estimates or bulk-rock measurements. This is unfortunate as it
prohibits an accurate assessment of the mineral alteration mechanisms and hence the mineral-scale element sources, sinks and
reservoirs in the system. Here we focus specifically on the textural
and mineralogical characteristics of Kawah Ijen's eruption products
and their altered derivatives to provide this key complementary story.
2. Geomorphological setting
2.1. The Ijen volcanic complex
The Ijen highland, on the eastern tip of Java, occupies a 20 km wide
caldera formed more than 50,000 years ago as a result of the collapse
of the Ijen stratovolcano (Kemmerling, 1921b; Sitorus, 1990). It is
bounded in the north by the semicircular Kendeng caldera wall and in
the south by the post-caldera rim volcanoes Merapi, Ranteh, and
Jampit (Fig. 1). A number of smaller vents are present within the
caldera, aligned along an east–west trend extending from Kawah Ijen
to Suket. Blau volcano, the oldest intra-caldera vent, lies markedly
north of this trend and may represent a separate stage of activity. The
northern half of the caldera is devoid of post-caldera collapse volcanic
activity. Lacustrine deposits attest to the past presence of a lake in this
area. A fault-bounded gap in the Kendeng caldera wall now drains this
area as well as the rest of the caldera, including the acid fluids released
from the Kawah Ijen crater lake. Volcanic activity in the Ijen caldera is
limited at present to Kawah Ijen. The last major eruption took place in
1817, which resulted in complete expulsion of the lake, thick deposits
of ash, and mud flows down the outer slopes towards the southeast, as
well as along the Banyu Pahit river valley (Bosch, 1858; Kemmerling,
1921b). There are also accounts of a significant deepening and
enlargement of the crater during this eruption (“Oudgast”, 1820;
Bosch, 1858). More recent activity has been confined to the crater lake
and includes strong events in 1916–1917 (Hengeveld, 1920; Kemmerling, 1921b), 1936, 1952 and 1993–1994 (Delmelle et al., 2000).
2.2. Kawah Ijen volcano
Kawah Ijen volcano occupies the western flank of Gunung Merapi,
just inside the caldera rim (Fig. 1). Its summit consists of two
interlocking craters, forming an elongated depression that is filled by
the Kawah Ijen crater lake (Fig. 2). The lake is bordered by steep cliffs
rising circa 250 m above the lake, except on the western side, where a
break in the crater rim extends almost to lake level. The latter is
topped by a dam that was built in the early 20th century to control
discharge from the lake (Hengeveld, 1920). Current lake levels are,
however, well below the level of the dam. The rocks exposed in the
crater walls consist of layered pyroclastics and lavaflows, covered by a
veneer of consolidated, sulphur-bearing mud derived from recent
activity. Altered and fresh ballistics are abundant on the outer slopes
and flat summit areas. The path that leads up on the outer flank from
the Pondok (shelter) to the crater rim crosscuts scoria layers and
small lavaflows (up to 1 m in thickness), as well as altered phreatic
and phreato-magmatic material close to the rim that lies discordantly
on top of the magmatic deposits.
The southeastern inner flank of the crater has a gentler slope that
exposes thick, horizontal deposits of altered pyroclastics and lake
sediments (cf. Kemmerling, 1921b; Delmelle and Bernard, 1994;
Delmelle et al., 2000; Takano et al., 2004). Immediately to the west of
these deposits, adjacent to the lake, there is a small rhyolitic dome
measuring approximately 100 m in diameter and 20 m in height,
which is the locus of strong fumarolic activity (Fig. 2). The white to
yellow fumes emitted by this activity reach temperatures up to 600 °C
(van Hinsberg et al., 2009). Pipes, driven into the fumaroles by
sulphur miners, direct these fumes down to the base of the dome
where they condense to yield liquid sulphur. The dome consists of
dense, grey to white rock, crosscut by native sulphur veins in the
cooler sections. Brittle sulphur needles coat all surfaces around the
fumaroles and black pyrite and alunite–jarosite precipitates are
Fig. 1. Overview map of the Ijen caldera showing the location of the main volcanic centres and outline of the Ijen caldera.
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
255
Fig. 2. Detailed map showing sample localities around Kawah Ijen volcano and in the upstream part of the Banyu Pahit river valley. Based on the1:20000 scale map by the
topographical survey (1918), updated with our GPS measurements. Contour interval is 10 m, with thick lines for 100 m contours.
present around the vents. Minor flows of sulphur descend from the
pipes towards the lake. The eroded remnants of an older alteration
centre are exposed in the valley next to the active dome (Fig. 2) and
consist of soft, white material (alunite and cristobalite) crosscut by
black veins of pyrite in a box-work texture. This material appears to
represent the feeder system for a fumarole field located about 150 m
above it and destroyed in the 1817 eruption (“Oudgast”, 1820; Bosch,
1858).
2.3. Banyu Pahit river
The Banyu Pahit river forms from the confluence of springs
emerging in the steep valley that descends Kawah Ijen on its western
flank (Fig. 2). The uppermost spring emerges from the foundations of
the dam. Gypsum needles and laths precipitate from the spring water
upon evaporation, forming a smooth, cascading cover over the rocks
that is up to 35 cm thick and ends in a 100 m long and 20 m wide
slope completely covered in gypsum terraces. Approximately 650 m
downstream of the dam, the water seeps into the loose riverbed
sediment (Fig. 2). The water re-emerges a further 100 m downstream
from a porous scoria deposit, only to seep back gradually into the
sediment another 150 m downstream. The Banyu Pahit is subsequently dry for 600 m, before re-appearing as a result of being fed by
four springs from below a lavaflow in the southern flank of the valley.
From here on, the river flows continuously, cascading down two
major waterfalls before reaching the Plalangan–Paltuding road
(Figs. 1 and 2).
Rocks exposed in the valley wall consist of layered pyroclastics,
lavaflows, lahar deposits, and ash and mudflows, whereas the valley
floor is made up of their altered equivalents. The orientation and
lateral extent of these deposits suggest that they descended through
this same valley. In fact, the river does not truncate the lavaflows of
the two lowermost waterfalls, as it does the lavaflows upstream. The
continuity in valley deposits is truncated for the uppermost 700 m
where the deposits follow a different valley up to the crater wall
(Fig. 2). This valley is barred at the lake by a thick lava flow, which
forced the river into its present course. The deposits in this new
section are markedly different and composed mainly of altered scoria
and lava similar to that found in the layered deposits on the path up to
the volcano from the Pondok. Consolidated lahar deposits, which are
common downstream, are conspicuously absent. The bedrock is only
visible in the Banyu Pahit riverbed where rapids are present.
Elsewhere, the river flows on a bed of mm- to m-sized, rounded
rock fragments, both altered and fresh. Plant and wood debris are
common, and, interestingly, wood and bamboo appear to be relatively
resistant to the acid water. Where the river cuts through unconsolidated pyroclastic material, steep-sided canyons have developed, with
cliffs rising tens of metres above the level of the river. Rock falls are
common in such sections. From the intersection with the road
downstream to Watucapil, the river has a more gradual slope and is
confined to a Kawah Ijen lavaflow. At Watucapil the river plunges
from this lavaflow into mostly unconsolidated pyroclastics, merges
with neutral streams at the village of Blawan and then flows out of the
caldera through the gap in the Kendeng caldera wall (Fig. 1).
3. Methods
Samples were collected at various localities both within and on the
flanks of Kawah Ijen volcano during fieldwork in 1999, 2007 and 2008
(Figs. 1 and 2). Unaltered material, representative of all major Kawah
Ijen lavas as well as a subset of lava and scoria layers of the smaller
cone-building eruptions were sampled. Ballistics were collected from
a flat, triangular area on the southern rim of the crater, as well as
around the old observatory, both of which are strewn with rounded
boulders up to half a metre in diameter. These boulders range from
breadcrust and pumice bombs of juvenile material to rounded and
angular fragments of altered material. Additional altered ballistics
were collected on the western flank of the crater, where they are
present as rounded boulders in a lahars matrix (sample KV99-854).
Samples with varying degrees of alteration were taken in the Banyu
Pahit riverbed below the dam, and where the river arrives at the
Plalangan–Paltuding road in a series of rapids (sites 2 and 3 in Fig. 1
respectively). At the rapids, a detailed transect was sampled from the
unaltered lava to its altered equivalent below the waterline. This
transect is characteristic of the alteration observed along the entire
length of the lava flow. Rocks from the surface of the rhyolitic dome
and up to 30 cm depth in an active vent (gas temperature circa
600 °C) were also sampled.
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V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
Back-scattered electron images and quantitative mineral compositions were collected on polished thin sections using a JEOL JXA 8900
electron microprobe at the Geochemisches Institut of Georg-August
University Göttingen and at the Department of Earth and Planetary
Sciences of McGill University. Operating conditions were 15 kV
acceleration voltage, 15 nA beam current and 5 μm spotsize, calibrating on wollastonite (Si, Ca), rutile (Ti), corundum or albite (Al),
eskolaite (Cr), hematite or olivine (Fe), periclase or diopside (Mg),
rhodonite (Mn), barite (Ba), celestite or Sr-feldspar (Sr), albite (Na),
orthoclase (K), vanadinite or galena (Pb), topaz (F), halite (Cl), barite
or pyrite (S), with peak count times up to 60 s for minor and 20 s for
major elements. A 20 kV, 50 nA beam was used for sulfides. Analyses
were conducted in WDS mode and the results corrected using the phirho-z correction routine for silicates and oxides, and ZAF for sulphates
and sulfides. The median detection limits of the various microprobe
sessions are (in ppm) Si — 290, Ti — 500, Al — 585, Cr — 270, Fe — 67,
Mg — 360, Mn — 190, Ca — 235, Ba — 510, Sr — 170, Na — 285, K — 112,
Pb — 30, F — 320, Cl — 90 and S — 100.
Trace element analyses were performed on a Geolas 193 nm
excimer laser ablation system coupled to a Micromass Platform
quadrupole ICP-MS at Utrecht University, using a 60 μm diameter
crater and fluence of 2 to 5 Jcm− 2. Data were calibrated against NIST
SRM 612 glass, with Ca, Mn or Fe (determined by microprobe) as
internal standards. Accuracy was assessed by analysing BCR2G and
was better than 10% for the trace elements reported here (using the
reference values of Gao et al., 2002). Precision was determined from
repeated analyses of NIST SRM 612 glass bracketing every three
mineral analyses and was less than 5% RSD for all elements. Detection
limits for the samples were variable due to fluctuations in the
respective background. The median detection limit for the analyses
reported here at the 0.99 confidence level is V 0.3 ppm, Cr 1.1 ppm,
Mn 0.9 ppm, Ni 3.9 ppm, Cu 7.7 ppm, Zn 9.0 ppm, Sr 0.1 ppm, Ba
0.6 ppm, Pb 0.2 ppm, Th 0.01 ppm, U 0.01 ppm.
4. Results
All rocks that were in contact with the acid Kawah Ijen fluids, both
within and outside the crater, show signs of alteration. They appear to
be bleached, with colour converging on beige with progressive
alteration. Competent lavas are transformed into a friable material,
and break into sheets and blocks (Fig. 3). Alteration progresses inward
from the exposed rock surface, cracks and vesicles, and scoria is
therefore replaced more rapidly than lava. Still, the final alteration
products are similar in appearance and texture, suggesting a common
alteration mechanism.
Unaltered magmatic rocks from Kawah Ijen contain plagioclase,
ortho- and clino-pyroxene and titanomagnetite phenocrysts in a matrix
of glass and microlites. The microlites, dominated by plagioclase,
commonly define a flow texture around the phenocrysts. Olivine
phenocrysts are present in the most mafic samples and as rare relicts
rimmed by ortho-pyroxene in andesite. Plagioclase and clino-pyroxene
crystals are concentrically zoned and titanomagnetite grains show a
dense network of ülvospinel-dominated exsolution lamellae in a
magnetite-rich matrix. Ortho-pyroxene is confined mostly to symplectitic intergrowths with magnetite, formed by oxidative replacement of
olivine. Rare ortho-pyroxene rims on clino-pyroxene are observed in
some samples. Apatite is the main accessory phase in these rocks.
Mineral compositions vary between An90Ab10 and An40Ab55 for
plagioclase, En56Fs41 and En67Fs30 for ortho-pyroxene, and Usp20Mt73Sp7 and Usp50Mt45Sp5 for titanomagnetite (Table 1 and supplementary
material). Clino-pyroxene compositions are relatively constant, clustering around Ca0.8Mg0.8Fe0.3Al0.1Si1.97O6. Olivine has a composition of
Fo65. Plagioclase crystals show an overall trend towards a more albite
rich composition from core to rim, although both normal and reverse
zoning is present. The alteration product of plagioclase, pyroxene and
matrix glass is indistinguishable, consisting of a silica-dominated
Fig. 3. Photographs of in-situ alteration sequences in the Banyu Pahit riverbed below
the dam around a bomb in a scoria deposit (top) and lavaflow (bottom). The original
black basalt is progressively bleached and fractures into sheets and blocks. These
textures are typical for lava alteration along the entire length of the Banyu Pahit from
the dam to Watucapil.
amorphous material. No significant concentrations of elements other
than silica were determined, but low totals indicate the presence of up to
20 wt.% water.
The alteration is accompanied by the formation of new phases.
Intergrown jarosite crystals and idiomorphic grains of barite are
observed in the altered riverbed, and gypsum and Al-sulphates on the
banks of the river and in cracks. The ballistic samples contain pyrite,
cristobalite and Al-sulphates in cracks and vesicles, as well as (partial)
replacement of titanomagnetite by pyrite. Ti-oxide, barite and pyrite
are present in the altered magmatic material of the active dome.
5. Mineral alteration textures
5.1. Banyu Pahit riverbed section
The transition from unaltered material to its altered equivalent can
be observed in-situ in the riverbed of the Banyu Pahit river at the
rapids, where fluctuations in water level combined with spray result
in a progression of alteration stages towards the water level.
Alteration starts with the matrix glass, which is replaced by a nonisotropic beige to brown material. The microlites are not affected by
this alteration, although they are more difficult to identify due to the
changes in the groundmass. Even the least altered samples show a
complete replacement of the glass, indicating that it is a rapid process.
Olivine, which was only observed as a single grain rimmed by orthopyroxene, is altered concurrently. The grain is crosscut by fractures
filled by the same beige material as the altered groundmass, but it is
unclear whether this represents an alteration product of the olivine or
an infill. The dominant alteration mechanism is dissolution extending
from the fractures and progressing along crystallographic planes
(Fig. 4a). Dissolution is fastest along the β-axis, consistent with the
dissolution experiments of Awad et al. (2000).
Sample
mineral
KV07-708
KV99-805
KV99-802b
KV99-853a
KV99-805
KV99-802b
KV99-805
KV99-802b
KV99-853a
KV99-853a
KV07-602
KV07-602
KV99-854
KV07-802
KV99-879
KV99-879
ol
mag-1
mag-2
mag-3
opx
opx
cpx
cpx
cpx
plag
barite
silica
Al-sulf
alunite
py-1
py-2
SiO2 - wt.%
TiO2
Cr2O3
Al2O3
MgO
MnO
FeO
CaO
PbO
BaO
SrO
Na2O
K2O
Cl
F
SO3
Total
V - ppm
Cr
Mn
Ni
Cu
Zn
Sr
Ba
Pb
Th
U
n
Corr
35.60
0.00
n.a.
0.01
31.91
0.77
31.19
0.09
b d.l.
0.02
b d.l.
0.02
0.02
n.a.
n.a.
0.03
99.65
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.24
12.27
0.02
1.94
1.62
0.62
77.04
0.07
n.a.
bd.l.
n.a.
bd.l.
0.03
bd.l.
bd.l.
n.a.
93.89
787.8
17.79
4097.0
389.41
265.4
351.9
1.23
0.99
bd.l.
0.24
0.06
2
Mn
0.10
16.11
0.06
2.96
1.66
0.45
73.56
bd.l.
n.a.
bd.l.
n.a.
bd.l.
bd.l.
bd.l.
bd.l.
n.a.
94.90
4524.1
105.61
3469.7
619.04
2127.7
989.1
5.30
bd.l.
bd.l.
bd.l.
0.10
2
Mn
0.12
18.36
0.03
1.34
0.50
0.78
74.62
0.08
n.a.
bd.l.
n.a.
bd.l.
0.03
bd.l.
bd.l.
n.a.
95.88
472.3
bd.l.
6056.5
16.58
4534.0
313.8
5.43
1.49
bd.l.
1.36
1.66
1
Mn
53.25
0.32
b d.l.
0.64
22.71
0.83
21.52
1.34
n.a.
b d.l.
n.a.
0.02
b d.l.
b d.l.
b d.l.
n.a.
100.62
60.2
1.96
3800.5
36.88
34.0
198.4
1.20
2.63
1.67
0.11
0.02
1
Ca
53.16
0.22
bd.l.
0.68
21.79
0.93
22.52
1.48
n.a.
bd.l.
n.a.
0.03
0.02
bd.l.
bd.l.
n.a.
100.83
65.7
bd.l.
7187.3
33.03
33.2
345.3
5.74
1.37
0.53
0.50
0.17
2
Mn
50.99
0.73
0.02
2.99
14.27
0.34
8.96
20.62
n.a.
b d.l.
n.a.
0.34
b d.l.
0.02
b d.l.
n.a.
99.26
169.3
1.88
1293.9
11.01
38.0
36.9
20.61
0.46
0.12
0.06
0.03
2
Ca
52.39
0.38
bd.l.
1.70
14.90
0.43
9.49
20.04
n.a.
bd.l.
n.a.
0.29
bd.l.
0.01
bd.l.
n.a.
99.65
147.0
2.14
2382.0
16.42
32.8
54.6
18.23
19.37
bd.l.
bd.l.
bd.l.
1
Ca
52.63
0.28
b d.l.
1.13
12.92
0.64
12.47
20.07
n.a.
b d.l.
n.a.
0.29
b d.l.
0.01
b d.l.
n.a.
100.45
67.7
b d.l.
3199.4
8.85
22.8
78.1
15.16
0.00
b d.l.
0.05
b d.l.
1
Ca
57.62
0.02
b d.l.
26.24
0.02
b d.l.
0.29
8.52
n.a.
0.13
n.a.
5.92
0.75
b d.l.
b d.l.
n.a.
99.52
b d.l.
b d.l.
20.45
b d.l.
b d.l.
b d.l.
356.53
305.84
5.67
0.58
0.18
2
Ca
2.76
2.16
n.a.
0.10
0.00
0.07
0.20
0.14
2.69
55.08
2.33
0.16
0.18
n.a.
n.a.
32.02
97.86
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
82.32
0.02
n.a.
0.01
b d.l.
b d.l.
0.02
0.03
b d.l.
b d.l.
b d.l.
0.01
0.02
0.05
0.06
0.09
82.62
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.42
b d.l.
b d.l.
57.97
b d.l.
b d.l.
b d.l.
0.05
n.a.
0.01
b d.l.
2.10
2.06
0.04
n.a.
31.53
94.19
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
0.07
0.95
n.a.
17.27
0.31
0.00
9.07
0.05
0.00
0.04
0.00
0.15
0.48
n.a.
n.a.
48.67
77.06
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
Si
Ti
0.01
0.39
0.05
0.01
Fe
43.43
43.13
S
Total
51.73
95.18
248.5
3.07
1409.8
80.76
337.2
29.5
3.59
4.56
9.18
0.11
0.08
2
Fe
51.63
94.80
63.2
2.80
289.4
970.34
201.3
16.7
6.22
8.18
16.07
0.51
b d.l.
2
Fe
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
Table 1
Characteristic major and trace element compositions of minerals as determined by electron microprobe for major elements and laser ablation ICP-MS for trace elements (Ti in sulfides by laser ablation ICP-MS). Trace element results represent
averages on multiple grains as shown by the number of analyses (n). Details of minerals analysed; ol — olivine relict rimmed by ortho-pyroxene, mag-1 — symplectitic magnetite, mag-2,3, opx, cpx and plag — phenocrysts in the riverbed series
samples, barite, silica and Al-sulf — most altered material in the Banyu Pahit riverbed at the dam, alunite — active dome, py-1 — pyrite replacing magnetite in a ballistic, py-2 — pyrite in cavity in a ballistic. Corr lists the element used as the
internal standard in the laser ablation data correction.
257
258
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
Fig. 4. Optical (PPL and XPL) and back-scattered electron (BSE) images of typical textures developing during progressive alteration of Kawah Ijen deposits, with a–h from riverbed samples,
i–j from ballistics and k–m from dome samples; a — dissolution of olivine, preferentially along its β-axis, b — selective alteration of plagioclase progressing along An-rich zones and twinning
planes, c — an advanced stage of alteration showing complete replacement of the matrix glass and An-rich plagioclase domains, but persistence of its Ab-rich rim, d — jarosite (light
coloured) lining a fracture in an altered plagioclase phenocryst, e — selective dissolution of the magnetite-rich component of titanomagnetite, leaving a framework of ülvospinel lamellae,
f — relict framework of ülvospinel lamellae in altered titanomagnetite, partially leached to form Ti-oxide, g — clino-pyroxene alteration along cracks with a very sharp front crosscutting the
compositional zoning and forming a layered alteration product, h — synchronous alteration of clino- and ortho-pyroxene along shared fractures, with a wider alteration zone in orthopyroxene indicating faster replacement compared to associated clino-pyroxene, i — cristobalite and pyrite in a cavity in a ballistic, j — pseudomorph of pyrite after magnetite with original
ülvospinel lamellae preserved as Ti-oxide, k — hopper crystal of titanomagnetite in the magmatic material of the fumarole mound, l — pyrite growth surrounding an altered titanomagnetite
grain, accompanied by recrystallisation of Ti-oxide, m — preferential alteration of An-rich zones in plagioclase, accompanied by pyrite precipitation (black grains), n — yellow graphite
occupying fractures in a titanomagnetite phenocryst.
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
259
Fig. 4 (continued).
Plagioclase is the first phenocryst phase to be affected by
alteration. It is replaced by a material similar to that of the altered
matrix. Alteration starts along twin-boundaries and has a strong
preference for the An-rich zones of the phenocrysts (Fig. 4b),
consistent with observations by Africano and Bernard (2000) for
Usu volcano and experimental results of Casey et al. (1991). This
preferred alteration produces remnants with completely altered
cores, but unaffected rims (e.g. Fig. 4c), similar to observations of
altered plagioclase in the Valley of Ten Thousand Smokes (Spilde et al.,
1993). Jarosite is locally present in strongly altered plagioclase grains
along fractures (Fig. 4d). The plagioclase microlites are Ab-rich and
therefore persist during the initial stages of alteration. Titanomagnetite grains are subsequently affected by alteration, which is characterised by dissolution of the magnetite dominated parts, leaving the
ülvospinel lamellae unaffected (Fig. 4e). The symplectitic oxides are
poor in Ti due to their origin by replacement of olivine and are
dissolved. Interestingly, this dissolution takes place without any effect
on the symplectitic ortho-pyroxene, similar to the non-participation
of ortho-pyroxene during olivine dissolution (Fig. 4a).
Unlike alteration of titanomagnetite and plagioclase, alteration of
pyroxene phenocrysts is not guided by compositional heterogeneity.
Instead, a sharp alteration front develops along outer surfaces and
cracks, crosscutting compositional zoning. This front moves inward
from these surfaces, enclosing progressively rounded relicts. The
alteration front is very sharp (bb1 μm thick, Fig. 4g) and replacement
is abrupt (i.e., no intermediate stages of alteration have been observed
within the resolution of the electron microprobe). Ortho- and clinopyroxene are altered simultaneously, as evidenced by uninterrupted
alteration zones crosscutting ortho-pyroxene-rimmed clino-pyroxene
(Fig. 4h). However, the rate of ortho-pyroxene alteration is faster,
resulting in a thicker alteration zone in the ortho-pyroxene compared
to the equivalent zone in clino-pyroxene. The alteration product has a
layered appearance (Fig. 4g, h), which suggests that the transformation was episodic, due possibly to intermittent submersion of the rock
in the Banyu Pahit waters. There is also evidence of preferential attack
along the long crystallographic axis, resulting in a ragged outer
surface and elongated pits (e.g. Fig. 4g), but this texture is not as welldeveloped as observed by Spilde et al. (1993).
260
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
Distinct alteration stages cannot be defined, because replacement is
gradual due to compositional heterogeneity in the phenocrysts.
However, overall, alteration starts with the glass matrix and olivine
grains, progresses to the An-rich part of plagioclase, is followed by
dissolution of the magnetite-rich parts of titanomagnetite, and finally
attacks the pyroxene grains, with ortho-pyroxene alteration proceeding
more rapidly than that of clino-pyroxene. The most albitic parts of the
plagioclase decompose synchronously with the last pyroxene remnants.
In the most altered samples, only ülvospinel lamellae remain.
Interestingly, this sequence is completely opposite to the pyroxeneN plagioclase N glass sequence reported by Rowe and Brantley (1993)
for Poas and by Africano and Bernard (2000) for Usu volcanoes. The fate
of apatite is unclear. Where present as inclusions, no preferential
alteration of the inclusions is observed, but neither is apatite present
after its host has been altered. A concurrent alteration is therefore
suggested.
5.2. Banyu Pahit riverbed at the dam
The material collected in the riverbed of the Banyu Pahit below the
dam represents altered equivalents of more diverse precursors. In-situ
alteration sequences are present (e.g. Fig. 3), but these are thin
compared to the thickness of the alteration zone that is observed
between the rapids and Watucapil (mm- to cm-scale rather than mscale). Magmatic precursors range from basaltic, through andesitic to
dacitic lava, scoria and bombs. Despite this compositional variety, the
original mineralogy and textures are similar, with the main
differences being in the mineral proportions and the appearance of
olivine phenocrysts in the more mafic rocks. Phenocrysts cluster in
the andesitic to dacitic material, giving the rock a glomero-porphyritic
appearance.
Alteration textures are identical to those observed for the rapids
section, but alteration progresses beyond what is observed there.
Titanomagnetite alteration proceeds beyond magnetite dissolution to
leaching of Fe in the ülvospinel lamellae, replacing them by Ti-oxide
(Fig. 4f). Identification of this phase was not possible due to its small
size. This leaching results in disintegration of the lamellae leaving
voids. In several samples, we observed small (10–50 μm) precipitates
of barite, consisting of a coalesced mass of smaller grains, some of
which have idiomorphic rims. Gypsum is present in several samples
as a surface coating and infill of cracks, but this is unrelated to
alteration. Rather, it is the product of evaporative saturation of the
fluids with gypsum (Delmelle and Bernard, 2000b).
5.3. Altered Kawah Ijen ballistics
The ballistics show a range of textures and mineralogy indicating
that they were subjected to several different styles of alteration. The
overall appearance and texture of several samples from a phreatic
deposit on the southwestern flank of Kawah Ijen, closely resemble the
alteration style discussed above. Alteration in the phreatic deposit is,
however, distinguishable from that in the Banyu Pahit valley in that
the altered material is cut by cracks filled with Al-sulphate laths,
which show oscillatory zoning along their length. Their stoichiometry
is close to that of alunite, with the zoning changing the composition
among the H3O+–Na–K endmembers.
The alteration of ballistics collected on the crater rim is markedly
different. Matrix glass has been replaced by an amorphous, but
competent, grey mass, enclosing abundant voids and fractures that are
partially filled with framboidal silica and irregularly shaped pyrite grains
(Fig. 4i). However, the most distinctive feature is the progressive
replacement of titanomagnetite by pyrite, culminating in perfect pyrite
pseudomorphs after titanomagnetite preserving the original ülvospinel
lamellae as a Ti-oxide phase (Fig. 4j). The alteration style of the
plagioclase and pyroxene phenocrysts is identical to that described for
the Banyu Pahit river. Other samples from the crater rim display
alteration intermediate between these two types, i.e. containing both
pyritisation of the titanomagnetite and Al-sulphate needles and laths.
The variations in alteration textures and mineralogy cannot be related to
differences in the precursor as the remnant textures and mineralogy of
all ballistics are similar, and indistinguishable from those observed in
unaltered Kawah Ijen magmatic deposits.
5.4. Alteration of rocks in the rhyolitic dome
The dome contains two main rock types representing the inactive
and actively degassing areas respectively. Rocks from inactive areas
consist of (sub)angular altered rock fragments cemented and crosscut
by multi-stage sulphur veins. Discontinuity between adjacent fragments attests to this being a non-juvenile deposit. Alteration of the
rock fragments is complete in that all minerals and matrix glass have
been replaced by a homogeneous mass of amorphous silica.
Precipitates of pyrite and idiomorphic grains of barite are present
within this matrix as well as small grains of pure Ti-oxide. Several of
these Ti-oxide grains contain central cavities bridged by Ti-oxide laths
intersecting at 60° angles that reminisce the ülvospinel framework
observed in altered primary oxide grains and suggest that they
represent replacements after titanomagnetite. However, the mass
proportion of Ti-oxide indicates that substantial addition of Ti from
the fumarole fluids must have taken place, and that the remnants of
the original oxide mainly acted as a seed surface. Similar Ti-oxide
grains have also been observed in the native sulphur veins. Significant
mobility of Ti is uncommon in nature and it may be related here to the
elevated F-contents of Kawah Ijen fluids, as F is known to mobilise
normally refractory elements (e.g. Rhyzhenko et al., 2006).
The rocks of the active vents consist of the altered equivalent of a
distinct magmatic precursor characterised by hopper crystals of
titanomagnetite (Fig. 4k). This titanomagnetite habit is markedly
absent in the deposits of the surrounding cliffs and has elsewhere only
been identified in the juvenile component of relatively recent
phreato-magmatic layers in the Banyu Pahit river valley. Together
with the presence of pristine, unaltered areas in the dome rocks and
strong similarity in samples from different parts of the dome, we
interpret this deposit as in-situ juvenile material. Alteration is
localized and confined to the actual gas pathways in the sample,
resulting in sharp alteration gradients and less adherence to the
alteration sequence observed in the Banyu Pahit riverbed samples.
Additionally, deposition of fine-grained pyrite accompanies replacement of the matrix glass and plagioclase (Fig. 4m). Titanomagnetite
grains in dome surface samples are commonly fractured and filled
with graphite (Fig. 4n). In the deeper samples, pyrite has partially
replaced titanomagnetite, but most titanomagnetite grains appear
simply to have acted as a seeds for pyrite nucleation and growth. The
cores of these pyrite grains contain abundant inclusions of altered
material, whereas the rims show well-defined crystal faces and are
inclusion-free (Fig. 4l). Where replacement of titanomagnetite was
extensive, the pyrite encloses recrystallised, and in some cases
idiomorphic grains of Ti-oxide, suggesting mobilisation of Ti from
the titanomagnetite (Fig. 4l). The well-developed crystal shape allows
this Ti-oxide to be identified as either brookite or rutile.
6. Discussion
6.1. Three styles of alteration at Kawah Ijen
Petrological observations indicate that there are at least three
distinct alteration styles in the Kawah Ijen system. All are characterised
by replacement of the matrix glass, plagioclase and pyroxene by
homogeneous, amorphous silica, but differ in their alteration of
titanomagnetite. Alteration of titanomagnetite in the Banyu Pahit
riverbed samples is dominated by dissolution. In the ballistics,
titanomagnetite grains are replaced by pyrite, forming pseudomorphs
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
in which the original ülvospinel lamellae are preserved as Ti-oxide. This
indicates that Ti was not mobile during replacement and the chemistry
confirms this interpretation (see below). In contrast, Ti is mobile in the
dome as evident in precipitation of Ti-oxide in altered titanomagnetite
grains and as separate grains. These differences in titanomagnetite
alteration are accompanied by differences in the secondary mineralogy.
Taken in combination, this puts constraints on the physicochemical
environment and can thus be used to position the alteration styles
within the Kawah Ijen system.
The first style of alteration has been observed in-situ in the
riverbed of the Banyu Pahit and can be related directly to interaction
with surface fluids. Alteration textures and mineralogy are the same in
samples from the river valley at the dam to Watucapil, indicating a
single over-riding control on alteration despite local variations in
primary bulk chemistry and rock texture. Identical textures are found
in material from the lake shore at the dam, suggesting that this style of
alteration extends to the (shallow) levels of the lake. This is not
surprising given the similarity in fluid chemistry between the lake and
upstream part of the Banyu Pahit river (e.g. Delmelle and Bernard,
2000b). The third alteration style is linked directly to interaction of
the rocks with fumarole emissions. Titanium mobility is characteristic
of this setting, and it further displays fine-grained pyrite precipitates
in altered plagioclase and matrix glass. Dome surface samples also
contain graphite as infill of cracks in titanomagnetite (Fig. 4n), most
likely related to a redox controlled deposition involving Fe2+ in
magnetite and CO2 in the fumes.
We propose a hydrothermal setting for the second style of
alteration. The presence of an extensive hydrothermal system at
Kawah Ijen is well-established (Delmelle and Bernard, 1994; Delmelle
et al., 2000), and it is likely that material is sampled from this system
during phreatic and phreato-magmatic eruptions. The observed
restriction of this style of alteration to ballistic samples is consistent
with this interpretation. Moreover, the presence of Al-sulphates and
pyrite in ballistic samples is incompatible with a lake or river water
origin, as both are unstable in these fluids (Delmelle and Bernard,
2000b). Alteration in the ballistics is also not consistent with that of
the active dome as it is pervasive and homogenous, in contrast to the
localized alteration that is characteristic for the active vents.
Furthermore, titanium is specifically retained in the ballistic samples.
A deeper level in the dome can also be excluded as an origin for the
ballistic samples, because replacement by alunite dominates alteration in this setting as evident in the exposed feeder system of the
pre-1817 fumarole field. We therefore conclude an origin for these
samples in the hydrothermal system underlying the dome and lake,
and interpret the variability of alteration in the ballistics to reflect
sampling of different parts of the hydrothermal system.
6.2. Alteration sequence and controls
The Banyu Pahit riverbed samples represent an exceptionally wellconstrained, in-situ alteration sequence in this system and will be
used in the following paragraphs to probe the controls and effects of
the alteration in more detail.
The end product of alteration of the silicate fraction in the Banyu
Pahit riverbed is hydrated silica, virtually devoid of other elements.
Therefore, this alteration style can be thermodynamically modeled as
dissolution of the silicate minerals in the precursor lava, accompanied
by precipitation of amorphous silica. The fact that silica is saturated in
the surface waters (e.g. Delmelle and Bernard, 2000b) strengthens
this approximation. In this model, the mineral alteration sequence
depends on the extent of instability of each mineral relative to the
aqueous solution, which, in turn, is governed by a competition for the
elements between the original solid phases and the solution. This
competition is controlled by the activity of a given element in the solid
and the water, as well as the element's thermodynamic stability in the
solid phase and respective aqueous species.
261
Using the above interpretation of the system, the relative instability
of each phase was determined from its activity in the water after
dissolution of a rock aliquot and allowing for precipitation of amorphous
silica. These calculations were performed at 25 °C for the precursor
andesite lava at the Banyu Pahit rapids site and associated river water
composition (calculated using FactSage — Bale et al., 2002, confined to a
HCl–H2SO4–HF–H2O starting water and the major elements for the
solid, and using the approximate proportions of the minerals in the
unaltered rock). The predicted alteration sequence is olivine N anorthite N magnetite N clino-pyroxene N ortho-pyroxene N albite.
Glass was not included in these calculations as its thermodynamic
properties at these conditions are poorly known. With the exception of
the reversal in the alteration order for the pyroxenes, which may be
kinetically controlled, this sequence is in good agreement with the
observations reported earlier.
The above calculations and the lack of remnants of the original
minerals in the final alteration product indicate that none of these are
in equilibrium with the aqueous solution. The observed progressive
alteration is therefore not the result of a sequence of alteration
equilibria (cf. Marini et al., 2003), but rather a record of the passage of
the system though a set of alteration barriers. Plagioclase is the first
observable barrier (glass is already completely altered in these
samples) and as a result of its compositional zoning it slides with
progress of alteration to more Ab-rich compositions. In contrast,
alteration of the pyroxenes appears to be governed by a threshold
mechanism. Once crossed, alteration occurs abruptly and is not
guided by compositional zoning in the grains. Both pyroxenes are
altered simultaneously, but ortho-pyroxene at a higher rate (e.g.
Fig 4h). Alteration of progressively more Ab-rich plagioclase accompanies this, and may extend beyond the exhaustion of the pyroxenes.
When all silicates have been altered, the rock starts to disintegrate
and voids develop, but it is unclear whether this is a chemical or
physical process. Silicate alteration is accompanied by dissolution of
the magnetite component of the oxides, with the persistence of
ülvospinel lamellae. Upon exhaustion of this alteration barrier, Fe is
leached from the ülvospinel framework, which is replaced by Ti-oxide
and eventually also dissolves.
Silicate alteration in the dome and hydrothermal system appears
to be governed by the same controls as observed for the Banyu Pahit
riverbed. However, a high H2S activity results in the replacement of
magnetite by pyrite followed by ülvospinel to pyrite + Ti-oxide. This
latter reaction can only take place when the replacement of magnetite
by pyrite has been completed, as is indeed observed.
The alteration sequence established here is different from that
inferred by Rowe and Brantley (1993) for Poas fluids and observed by
African and Bernard (2000) for Usu volcano. In the case of Usu, the
discrepancy could be the result of heterogeneous alteration with local
persistence of glass. This is also observed for equivalent dome samples
from Kawah Ijen where it is related to localised passage of altering fluids
through these rocks. In the case of Poas, the substantially higher pH of
these fluids at spring outflow compared to Kawah Ijen may impact
results. Furthermore, this sequence was obtained from modeling of fluid
compositions rather than the study of progressively altered samples.
6.3. Mineral chemistry
Leaching dominates the changes in mineral chemistry that
accompany the textural alteration described above. The starting
chemistry of minerals in the magmatic deposits of Kawah Ijen is
typical for basaltic to dacitic arc rocks, both in terms of major and trace
elements, whereas the alteration product is near-pure silica (Fig. 5
and Table 1). This change in chemistry is abrupt and no intermediate
compositions were observed within electron microprobe resolution.
Unfortunately, instability of the alteration product prohibited accurate determination of its trace element content, but qualitative results
suggest low concentrations of all elements quantified here.
262
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
Fig. 5. Compilation of the compositions of fresh plagioclase and clino-pyroxene and their alteration products from all analysed Kawah Ijen magmatic deposits, as determined by
electron microprobe. Plagioclase composition shows a wide range from An90 to An30, whereas clino-pyroxene composition is reasonably constant. Both minerals are altered to a
silica-dominated material that shows no memory of its precursor mineral and no intermediate compositional stages are present.
Dissolution of titanomagnetite also releases its full content of
elements into the surface water during alteration, and only the
secondary phases may retain some of the original rock chemistry.
High concentrations of Pb and Sr are indeed observed for barite, and
pyrite replacing magnetite retains the trace element signature of its
precursor, including its high Ti content (confirming the immobility of
Ti during pyritisation). Cristobalite, on the other hand, does not
contain any trace elements above the detection limit. The complete,
and leaching dominated alteration and lack of secondary minerals
indicate that the Kawah Ijen fluids are undersaturated with respect to
most solid product phases. This is in agreement with thermodynamic
calculations of secondary mineral saturation by Delmelle and Bernard
(1994) and Delmelle et al. (2000).
Combining the alteration sequence established above with the
mineral chemistry, allows the sequential release of elements to be
reconstructed, as well as an alteration signature for each phase to be
identified. Plagioclase alteration is characterized by release of Al, Ca,
Na, Ba, Sr and Pb, pyroxene by Mg, Fe, Ca and Mn, and magnetite by Fe,
Ti, Cr, Ni, Cu and Zn. However, these sequential signatures will only be
recorded in the fluid when there is interaction with fresh material,
because the actual riverbed has already been completely altered. The
common rock falls and slides in the steep sections of the Banyu Pahit
valley provide the opportunity for this interaction and a chemical
imprint of such events on the river water chemistry is therefore
expected. Similar chemical signatures should also develop in the lake,
where rock falls from the steep crater sides are common, but these
will be difficult to recognize due to the large lake volume. An ideal
time to establish whether chemical signatures of alteration can be
observed would be the first rains following the dry season, as these
wash large amount of material into the lake and river.
7. Conclusions and implications
The Kawah Ijen–Banyu Pahit system provides a unique opportunity
for in-situ investigation of alteration by acid sulphate-chloride brines.
Direct observations firmly establish the mineral alteration sequence,
mechanisms and controls, and this is confirmed by thermodynamic
modelling. These results indicate that progressive alteration at Kawah
Ijen is principally controlled by thermodynamic mineral stability under
evolving element activities in the aqueous solution as subsequent
alteration barriers are exhausted. In contrast, alteration textures are
governed by compositional zoning and preferential leaching along
crystallographic planes. Three distinct alteration settings are present;
surficial, in the Banyu Pahit riverbed and lake shore, hydrothermal in the
system feeding the lake and dome, and fumarolic in the active vents of
the dome. Results from this study indicate that silicate alteration is
identical in all environments and that the in-situ surface manifestations
provide a direct proxy for the mechanisms and chemical impact of
alteration in the underlying magmatic-hydrothermal system. Alteration
in the Kawah Ijen–Banyu Pahit system is extremely aggressive with
convergence of a diversity of basaltic to dacitic magmatic deposits to a
mass of silica that is virtually devoid of other elements. Therefore,
water–rock interaction at Kawah Ijen is not a sink of elements. This is the
case both for the major elements, and for the majority of trace elements.
This uniform leaching shows that the Kawah Ijen waters are
undersaturated in these primary minerals and, combined with the
absence of most secondary minerals, that any additional flux of rocks
into the river will therefore result in alteration and transport of their
element load downstream. Alteration is therefore not only ineffective in
neutralizing the toxicity of these fluids, but actively increases the
element load of the system.
V. van Hinsberg et al. / Journal of Volcanology and Geothermal Research 198 (2010) 253–263
Acknowledgements
We thank L. Marini and P. Delmelle for their thoughtful reviews, Sri
Sumarti, Rudi Hadisantono, Guillaume Mauri, Stephanie Palmer,
Nathalie Vigouroux, and Glyn Williams–Jones for their help in the
field and discussions on the Ijen system, Paul Mason for insightful
comments and Jeroen Kraan for help with the laser ablation
measurements. We further acknowledge logistical support by the
Volcanological Survey of Indonesia and financial support from
WOTRO, NSERC, and the Trajectum and Amoco funds.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.1016/j.
jvolgeores.2010.09.002.
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