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Geol. Mag. 144 (4), 2007, pp. 643–659.
c
2007 Cambridge University Press 643
doi:10.1017/S0016756807003470 First published online 29 May 2007 Printed in the United Kingdom
Hybrid magma generation preceding Plinian silicic eruptions at
Hekla, Iceland: evidence from mineralogy and chemistry of two
zoned deposits
GUDRUN SVERRISDOTTIR
∗
Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland
(Received 21 November 2005; accepted 9 November 2006)
Abstract – Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between
repose periods and the composition of the first-erupted magma. The two largest explosive silicic
eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions
were sampled in detail and analysed for major and trace elements, along with microprobe analyses of
minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO
2
,
with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but
disequilibrium in the later erupted basaltic andesite component. The eruptions started with production
of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the
intermediate magma and predominant reverse zoning of minerals, trending towards a common surf ace
composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The
suggested model involves partial melting of older tholeiitic cr ust to produce silicic magma, which
segregated and accumulated in deep crustal reservoir. Silicic magma eruption is triggered by basaltic
andesite dyke injection, with a proportion of the dyke magma contributing to the production and
eruption of a mixed hybrid magma. Both the volume of the silicic partial melt, and the proportion of
the hybrid magma depend on the pre-eruptive repose time.
Keywords: hybrid magma, magma mixing, magma chamber, zoned deposit.
1. Introduction
The large propor tion of silicic magma produced by
Icelandic central volcanoes is rare for oceanic ridge
environments. Detailed studies of several volcanoes
in Iceland (Kerlingarfjoll, Askja, Torfaj
¨
okull, Krafla)
have shed light on the processes producing the evolved
magma, most of them involving some reworking of
the crust (K. Gronvold, unpub. Ph.D. thesis, Univ.
Oxford, 1972; Sigurdsson & Sparks, 1981; McGarvie
et al. 1990; Gunnarsson, Marsh & Taylor, 1998;
Jonasson, 1994). Rock suites of these volcanoes are
predominantly bimodal, but Hekla produces a higher
proportion of intermediate magma, from basaltic
andesite to rhyolite, according to the terminology of
Le Bas et al. (1986).
The volcanism of Hekla has been studied us-
ing tephrochronology and detailed historical records
(Thorarinsson, 1967). The known dated eruptions
are shown in Table 1, along with roughly estimated
volumes. The SiO
2
content of the large eruptions
generally exceeds 70 %, and only a minor proportion
of erupted magma contains as little as 55 % SiO
2
.
The proportions are almost reverse for the smaller
eruptions, where compositions are predominantly
intermediate (ranging from 54 % to 63 % SiO
2
).
Compositional variation is seen in all erupted material
∗
Author for correspondence: gsv@hi.is
except that from the last four eruptions since 1970.
The most pronounced variation is found in the largest
eruptions, with a range of 16 % SiO
2.
Estimates based
on calculations and approximations shown in Table 1
indicate that the basaltic andesite productivity of Hekla
has increased in the past 1000 years.
Research on the chemistry and petrology of Hekla
has been extensive, although key aspects remain
poorly understood. Bunsen (1851) suggested that silicic
and basaltic magmas were interacting beneath the
volcano. Einarsson (1950) suggested differentiation of
the Hekla magma by crystal fractionation. Tryggvason
(1965) described the mineralogy and chemistry of
the products, and Tomasson (1967) proposed two
magma chambers beneath Hekla, and favoured mixing
of silicic and basic magma to explain the chemical
characteristics. Thorarinsson (1967) pointed out that
the silica content of the initial phase of each eruption
was proportional to the length of the repose period.
Baldridge et al. (1973), in their study of the 1970
Hekla eruption, favoured fractional crystallization.
Sigvaldason (1974) proposed mixing of two different
magmas produced by crustal melting. Sigmarsson,
Condomines & Fourcade (1992) made a detailed
isotope study of Hekla products and showed that two
unrelated magmas, assumed to have formed by partial
melting of the deep crust, produced the rock suite by
mixing and later modification by crystallization.
644 G. SVERRISDOTTIR
Table 1. Known Hekla eruptions
Eruption Date Dating method km
3
DRE References for volume estimate
H5 >7000 BP Estimated 0.7 Larsen & Thorarinsson, 1977
H4 ∼4200
BP Corr.
14
C 1.8 Larsen & Thorarinsson, 1977
H-Sv ∼3900
BP Corr.
14
C 0.4–0.5 Sverrisdottir (unpub. data)
H3 ∼3000
BP Corr.
14
C 2.2 Larsen & Thorarinsson, 1977
HX ∼15 eruptions ?? <0.5 × 15 B. Robertsdottir, pers. comm.
Tephrochronology
+ historical records
1104
AD " ∼0.5 Modified after Thorarinsson, 1967
1158
AD " ∼0.15 "
1206
AD " ∼0.15 "
1222
AD " ∼0.15 "
1300
AD " ∼0.57 "
1341
AD " ∼0.15 "
1389
AD " ∼0.25 "
1510
AD " ∼0.79 "
1597
AD " ∼0.56 "
1636
AD " ∼0.2 "
1693
AD " ∼0.74 "
1766
AD " ∼1.25 "
1845
AD " ∼0.63 "
1947
AD " ∼0.76 "
1970
AD " ∼0.2 Thorarinsson & Sigvaldason, 1972
1980
AD " ∼0.15 Mod. after Gronvold et al. 1983
1991
AD " ∼0.15 Gudmundsson et al. 1992
2000
AD "0.22A.H
¨
oskuldsson et al. in press
Estimated dense rock equivalent volumes of known Hekla eruptions. HX are about 15 tephra layers of intermediate compositions, and
volumes an order of magnitude less than H4 and H3. According to ongoing mapping, some older layers of this size are also found (B.
Robertsdottir, pers. comm). Most of the volumes older than
AD 1845 are very rough estimates. DRE – dense-rock equivalents.
The present work aims to define and account for
the compositional variation within the two largest
explosive eruptions of Hekla, H4 and H3. These two
eruptions were selected because almost the whole range
of magma compositions from Hekla is represented. The
aim is to use these eruptions to construct a model of
the Hekla magma system and outline the processes that
take place during the evolution of silicic magma.
2. Geological setting
Hekla is located at the western margin of a propagating
rift (Fig. 1) that extends southward from the east-
ern rift zone in Iceland (Oskarsson, Sigvaldason &
Steinthorsson, 1982; Gudmundsson et al. 1992). The
volcano forms a ridge, rising approximately 1000 m
above the surrounding lava plateau, to about 1500 m
above sea level. The Hekla ridge is superimposed on a
cluster of basaltic Pleistocene hyaloclastite ridges that
make up the western flank of the volcano and form a
low mountain range to the west, parallel with the Hekla
ridge. Eruptions of evolved magma occur on a 5 km
long fissure along the spine of the Hekla ridge, and on
short fissures on its flanks (Fig. 2). Basaltic, effusive
eruptions occur on a fissure system parallel to the
ridge. All the late Pleistocene hyaloclastite formations
analysed in the Hekla region are basaltic (Fig. 2).
Lava flows, most widespread south of the mountain,
are of similar composition. Because they are covered
by younger lava flows in the vicinity of Hekla, their
Figures 1. Volcanic zones of Iceland. The figure outlines the
neovolcanic Quaternary formations of Iceland. WRZ denotes
the Western Rift Zone of Iceland, and ERZ denotes the Eastern
Rift Zone. SISZ is the South Icelandic Seismic Zone. Hekla is
shown on the western margin of a propagating rift that extends
south to the offshore Vestmannaeyjar volcanic centre. Volcanism
on the propagating rift can be traced back to early Quaternary
times. Rock suites of the propagating rift, which are alkalic
to the south, and tholeiitic with alkalic affinities at the north
termination, rest discordantly on older tholeiitic crust.
origin cannot be traced, but topographic relations and
chemical similarities of the ridges and the lavas indicate
that the productive basaltic volcanism of the basement
surrounding Hekla continued a few thousand years into
the Holocene.
Hybrid magma generation 645
Figure 2. Geological outlines of the Hekla volcanic ridge.
Evolved activity of Hekla is mostly confined to a fissure that
extends about 5 km along the volcanic ridge. Most of the basaltic
andesites originate in the fissure, and H4 and H3 deposits were
erupted within the central segment of this fissure. Craters and
fissures to the north and south of Hekla are mostly basaltic. The
location of the H3 tephra profile is shown by X, and Y with
arrow indicates the profile location of H4 20 km NE of the edge
of the map.
The earliest activity of Hekla as an evolved volcanic
centre (erupting basaltic andesite to rhyolite) probably
occurred during early Holocene time. The first recorded
silicic eruption from Hekla, H5, occurred roughly
7000 years ago. During the early Holocene, infrequent
rhyolitic to dacitic Plinian eruptions dominated. No
intermediate lavas are known from this early Holocene
phase of volcanism, and rock fragments found in the
H5 deposit are exclusively basaltic. However, xenoliths
of basaltic andesite composition are abundant in the H4
deposit, which indicates that the first significant basaltic
andesite production in the Hekla system followed the
H5 eruption.
H4 was erupted 3826
14
C years ago (Dugmore et al.
1995), or 4178–4202 years
BP (Stuiver et al. 1998).
The dense-rock equivalent (DRE) volume of the airfall
deposit is estimated at 1.8 km
3
(Larsen & Thorarinsson,
1977). The deposit is compositionally graded from
56 to 72 % SiO
2
. H3 is the most voluminous Hekla
fallout, erupted 2879
14
C years ago (Dugmore et al.
1995), or 2950–3041 years
BP (Stuiver et al. 1998). The
dense-rock volume is estimated at 2.2 km
3
(Larsen &
Thorarinsson, 1977). The deposit is compositionally
graded from 56 to 70 % SiO
2
.
Data from other Hekla eruptions are included in
this study for comparative purposes only; H5, the
first known major explosive eruption of Hekla, is at
least 0.7 km
3
dense-rock equivalent volume (Larsen &
Thorarinsson, 1977), with composition ranging from
65 to 75 % SiO
2
; new analyses from four other
eruptions were done, from
AD 1766, 1845, 1947 and
1970. The first three range from 63 to 54 % SiO
2
,
whereas the last one has a uniform composition of 54–
55 % SiO
2
.
3. Results
3.a. Sampling and analytical methods
Samples were selected to represent the compositional
variation within each eruption. Samples were collected
from tephra layers, and if available, from lava flows as
well. The samples from H4 and H3 were collected by
Gudrun Larsen. Samples from H5 were collected by
Gudrun Larsen and Karl Ingolfsson, and sampled at
equal intervals through the unstratified layer. Samples
from other Hekla products analysed for comparison
were collected by Gudrun Larsen and colleagues,
from tephra layers and their respective lava flows. All
samples were fresh and unaltered.
Samples of H4 tephra were collected from a 0.7 m
thick profile at Sigalda, 30 km NE of Hekla. The
sampling was done on the basis of colour gradation,
and bulk samples were collected systematically through
the Plinian deposit, each representing a 3 to 6 cm thick
horizon. All samples consist of the largest pumice
fragments from each bulk sample. The top of the
deposit was somewhat eroded, so that the topmost
sample probably does not represent the end of the
explosive eruption. Sample H4-1 represents the first
erupted material, but H4-12 represents the last erupted
material available. No abrupt interface is seen between
colour classes, but as the gradation is a distinctive
feature of these deposits, they are regarded as ‘zoned’
deposits. H3 was sampled at
´
Ofærugil, 8.5 km NW
of Hekla, in a 5 m thick complete section close to
the axis of maximum thickness of the Plinian deposit.
Division into zones was made on the basis of colours,
which changed more moderately than in H4. Samples
consist either of pieces of pumice collected from 10 cm
horizons, or of fragments of larger pumice clasts. H3-1
and H3-13 represent the assumed first and last erupted
material, respectively.
Whole-rock major element composition was determ-
ined by the author, by combined wet chemical and XRF
techniques at the geochemical laboratory of the Nordic
Volcanological Institute. X-ray analyses of major
oxides were performed using the method of Norris &
Chappell (1977), and the alkalis and Mg by AAS after
acid solution of the rocks. Trace element analyses were
carried out on the same samples at the U.S. Geological
Survey, Reston, VA. This part of the analytical work
was done by Susan L. Russell-Robinson. Most of the
trace elements were analysed by the INAA technique.
The procedure is described in Roelands (1977). Cu,
Ni, Cd and Pb were determined by ICP-AES, Mo
and Nb by spectrophotometry, and B by emission
spectroscopy. Samples from H5 were analysed by
ICP-AES at the Nordic Volcanological Institute, by
646 G. SVERRISDOTTIR
Figure 3. Crystallinity profile and composition of the most common phenocrysts in the H4 tephra layer. The layer is shown schematically
as reversed stratigraphy in order to represent the conditions in the magma chamber, and colour changes are indicated by hatching as
follows: zones 1–4 – white; zone 5 – yellowish white; zones 6–8 – greyish yellow or greyish pink; zones 9–12 – greyish brown to dark
brown. Sample number 10 was not used in the research.
Niels Oskarsson. ICP analyses were performed on
solutions of rock-powder fluxed with Li-metaborate
and dissolved in 5 % nitric acid with 1.3 % hydrochloric
and oxalic acids as described in Govindaraju & Mevelle
(1987). Microprobe analyses of minerals were made on
every other zone and microprobe glass analyses were
made on a few zones for comparison with the whole
rock compositions. Microprobe analyses were made
on an electron microprobe (ARL-Q30) at the Nordic
Volcanological Institute, using analytical conditions
of 15 kV accelerating voltage, and 15–20 nA sample
current for basaltic glass and minerals respectively, and
a beam diameter of 2–3 µ. For silicic glasses, sample
current was lowered to 10 nA in order to minimize Na-
loss, and the samples moved under the beam for the
same purpose.
3.b. H4 and H3 deposits
3.b.1. Zones and petrography
The existence of a magma chamber beneath Hekla has
been assumed for a long time, based on geochemical
characteristics, eruption mechanism, and ground de-
formation (Kjartansson & Gronvold, 1983; Soosalu &
Einarsson, 2004). The Hekla rock suite is different
from the tholeiitic basalt erupted from the adjoining
fissure system as confirmed by isotopic evidence
(Sigmarsson, Condomines & Fourcade, 1992), and
compositions change systematically between erup-
tions. Petrological constraints suggest the pre-eruptive
equilibrium depth of the Hekla 2000 magma was
about 14 km (Hoskuldsson et al. in press). Several
attempts have also been made to locate the chamber by
geophysical methods. Kjartansson & Gronvold (1983)
estimated the centre of deflation after the 1980 eruption
at 8 km depth. Recent seismic evidence favours a depth
of 14 km or more (Soosalu & Einarsson, 2004).
As a starting point of the present study, the different
fallout zones were assumed to represent the relative
positions of material in a magma chamber before erup-
tion, practically undisturbed by eruption withdrawal
(Blake & Fink, 1987). Accordingly, compositions are
plotted against schematic reversed stratigraphic height
in Figures 3, 4, 5 and 6. In the discussion of these
deposits in the following chapters, ‘upper’ and ‘lower’
always refer to location in a hypothetical magma
chamber, not to the respective tephra layers. Zonation
of H4 is shown in Figure 3. Zones 1 to 5 consist of
white to yellowish-white pumice. In these first five
zones, some crystals, mainly feldspars, are visible in
hand specimen. In the sixth zone, an abrupt change
to greyish pumice is encountered, and no crystals are
seen. Colour gradation is seen in most zones; the last
one is almost black. No clear interface separates the
zones. Zonation of H3 is shown in Figure 4. The first
erupted pumice is not as white as in H4, but even more
crystals are visible. Zones 1 to 4 display faint colour
gradation from almost white to beige. Colour change
in the H3 layer is continuous apart from pale streaks
or bands in the dark-grey zone number 11. Grading
through the deposit is more moderate, turning to black
in the last zone. No distinct interfaces are seen. Pumice
in zones 5 to 13 seems aphyric.
Whole-rock analyses of major and trace elements
in the H4 fallout are listed in Table 2, and glass
Hybrid magma generation 647
Figure 4. Crystallinity profile and composition of the most common phenocrysts in the H3 tephra layer. The layer is shown schematically
as reversed stratigraphy and colour changes are indicated by hatching as follows: zones 1–4 – white to yellowish white; zones 5–9 –
greyish yellow to greyish pink; zones 10–12 – dark brown to greyish brown; zone 13 – black.
compositions are given in Table 5. The respective
analyses of the H-3 fallout are in Tables 3 and 5.
Table 4 gives compositions of four young eruptions
analysed for comparison, and Table 6 shows major and
some trace element analyses of H5, also analysed for
comparison.
3.b.2. Petrology and mineral composition
The crystal content of H4 is up to 7 % in zones 3 and
4, b ut is less than 1 % in zones 6 to 12 (Fig. 3). In
H3 the most silicic zones 1 to 5 contain from 11 to 2 %
crystals, but zones 5 to 13 less than 1 % (Fig. 4). In both
deposits plagioclase, olivine, clinopyroxene, magnetite
and occasional ilmenite are present as phenocrysts,
plagioclase being the most abundant. Tiny grains
of zircon are found as an accessory mineral within
plagioclase crystals in most zones of both layers. All
phenocrysts are small; the plagioclase ranges up to
1 mm, but the others are smaller than 0.5 mm. The
zircon crystals are of micrometre scale.
The composition of plagioclase and olivine in the
two deposits H4 and H3 is outlined in Figures 3 and
4. The compositions of plagioclase in the most silicic
parts of the layers are fairly uniform. In H4 it ranges
from An24 to An34 and in H3 from An38 to An45.
The olivine composition is also uniform, Fa97 in H4
and Fa87 in H3. These compositions are consistent with
the respective whole rock and glass compositions and
indicate equilibrium conditions. In the less evolved, less
crystallized zones of the deposits, the compositional
range increases for both minerals, with reversed zoning
dominating throughout the sequence. Cores of the most
sodic plagioclases, of about An25 in H4 and An40 in
H3, occur through the sequence. In this more basaltic
part of H4, olivine was only found in zone 6, showing
a range of composition from Fa87 to Fa97, and in H3
in zones 7 and 9, showing compositional range of Fa77
to Fa89. The zoning of the olivines is mostly reverse.
3.b.3. Geochemistry
Several major element sections through both deposits,
H4 and H3, are shown in Figure 5. Glass compositions
are indicated by arrows for some elements if different
from whole rock compositions. Trace element concen-
trations are plotted in Figure 6. For comparison, Zr
content in samples from the H5 eruption is also plotted
in the same figure.
The most outstanding feature of H4 is a composi-
tional break at the boundary between zones H4-5 and
H4-6, where the silica content drops from 71 % to 65 %
(Fig. 5). This break is also reflected in most elements,
and follows a decrease in crystal content between
zones 5 and 6. The change in composition of the
plagioclase from homogeneous An25 to reverse zoning
occurs at a similar level (Fig. 3). As chemical analyses
of minerals were only done in every other zone, the
location of a compositional break was confirmed by
microscopic features. In zones 1 to 5, the concentrations
of major elements remain virtually constant, whereas
in zones 6 to 12 they show considerable gradation. A
slight difference in whole-rock and glass composition
reflects crystallization in the most silicic part of both
layers. No significant difference is seen in the darker,
less crystallized part. Light bands, only seen in zone
6 in H4 and zone 11 in H3, have the composition of
the most silicic material in the respective layer. An
648 G. SVERRISDOTTIR
Table 2. The H4 tephra
Zones1234567891112
SiO
2
71.74 72.36 72.14 71.74 71.34 65.25 64.09 62.87 61.61 57.82 56.99
TiO
2
0.20 0.16 0.17 0.17 0.21 0.47 0.55 0.70 0.79 1.27 1.50
Al
2
O
3
12.87 13.12 13.00 13.06 13.04 14.16 14.31 14.30 14.16 14.69 14.58
Fe
2
O
3
– 0.38 0.34 0.48 0.45 1.05 1.46 1.93 2.14 2.32 3.55
FeO – 1.81 1.79 1.80 2.42 5.67 6.20 7.22 7.47 8.17 7.50
MnO – 0.08 0.08 0.09 0.11 0.22 0.23 0.25 0.25 0.23 0.24
MgO – 0.06 0.07 0.08 0.08 0.14 0.33 0.57 0.98 1.88 2.05
CaO 1.49 1.48 1.48 1.50 1.78 3.42 3.67 4.18 4.37 5.18 5.68
Na
2
O – 4.73 4.71 4.78 4.85 4.80 4.78 4.58 4.71 4.07 3.84
K
2
O 2.81 2.81 2.77 2.80 2.69 2.01 1.91 1.74 1.66 1.39 1.31
P
2
O
5
0.00 0.00 0.00 0.00 0.02 0.07 0.12 0.19 0.25 0.52 0.62
H
2
O 2.28 2.16 2.97 2.69 2.53 1.24 1.16 0.90 0.65 0.74 0.92
Cl 680 660 680 580 520 400 360 320 320 290 260
F 1600 1480 1360 1360 1320 1120 960 920 880 1040 880
B 6.1 4.6 5.3 16 5.2 4.2 3.6 3.5 3.3 2.4 3.6
Rb 63 63 58 61 64 49 52 44 51 39 36
Cs 0.9 0.8 0.8 0.8 0.7 1.2 1.1 0.7 <2.0 0.5 0.6
Ba 699 701 635 622 588 591 484 458 474 390 360
Zr 394 394 425 270 460 1000 1150 1390 1410 645 620
Hf 11.5 11.4 11 11 13.2 23.7 24.9 28.4 27 13.6 12.4
Nb 80 78 79 78 79 74 73 63 67 56 53
Ta 6.27 5.89 5.67 5.61 5.64 4.8 4.79 4.38 4.06 3.74 3.88
Pb 7 4.3 5.7 6.8 9.4 5.4 4.4 6 2.6 4 3.4
Th 11.2 11.5 11 12.1 11 8.7 8.1 7.4 6.8 5.8 5.3
U 3.4 2.7 2.7 2.5 2.5 2.3 2.2 1.3 2.2 1.7 1.3
Sc 4.54 4.29 3.97 4.32 5.08 16.2 19.8 24.3 23.6 18 19
Cr 3.4 2.2 <8.0 <7.0 3.2 5.4 <20 <20 5.7 <10 5.4
Co 1.3 0.7 0.7 0.9 1.1 1.9 3 3.8 4 13 15
Ni 42234473136
Cu 14 14 15 13 14 19 20 27 21 22 23
Zn 130 130 130 130 140 170 180 180 170 180 170
Mo – 4.8 4.8 4.6 4.6 4.2 4.2 2 3.6 3 2.4
Cd 0.34 0.36 0.32 0.3 0.32 0.31 0.34 0.45 0.28 0.28 0.26
La 79 94 79 123 94 75 69 63 61 55 54
Ce 159 193 165 239 189 157 142 128 126 117 113
Nd 91 90 87 112 94 89 79 72 75 72 70
Sm 16.6 16.9 14.2 19 17.1 15.8 15.3 14.1 14.2 12.5 14.1
Eu 2.87 3.07 2.88 3.47 3.32 4.28 4.17 4.17 4.28 4.23 4.15
Gd 18.6 20.3 18.2 19.8 18.2 17.6 15.8 15.1 15.8 15.4 14.4
Tb 2.48 2.79 2.69 2.22 2.28 2.2 2.04 2.28 1.95 1.97 1.89
Tm 1.46 1.39 1.11 1.31 1.26 1.2 1.48 1.11 1.16 1.06 0.76
Yb 9 9.1 8.8 8.9 9.1 9.5 9.5 9 9 7.1 6.5
Lu 1.24 1.23 1.18 1.18 1.26 1.41 1.38 1.33 1.29 1 0.95
Chemical composition of the H4 tephra layer (whole rock analyses). Major element concentrations are given as weight % of the oxides; trace
element concentrations are given in ppm.
outstanding change in H4 at the compositional break is
a decrease in the content of residual volatiles and some
trace elements, such as F, Cl and Th in zones 6 to 12,
and significant enrichment in Zr, Sc (Fig. 6), Hf and
Yb in zones 6 to 9. Exceptional enrichment of the trace
elements Th and La is observed in zone 4 as discussed
below (Fig. 6).
In H3 a break in chemical trends between zones 4
and 5 is much less pronounced than the compositional
break in H4, but is visible for CaO, K
2
O and FeO
between zones 4 and 5 (Fig. 5). Changes in the crystal
content and mineral composition occur at a similar
level (Fig. 4). Zones H3-1 to 5 are lower in silica than
the respective zones in H4, and the glass composition
differs slightly from the whole-rock composition. In
contrast to H4, the major element concentrations of H3
vary only moderately through zones 1 to 12. A gap
associated with a colour change from greyish-brown to
nearly black where the SiO
2
content drops from 63 %
to 56 % separates zones 12 and 13, and is reflected in
several elements (Figs 5, 6). H3-11 is the only zone in
which the light bands could be analysed. Their glass
composition is identical to the earliest erupted pumice
(Table 5). Trace element concentrations vary only
slightly through most of the H3 profile (Fig. 6). Most
are similar to those of H4, except for lower halogens,
Th and La concentrations, and only slight enrichment
in Zr (Fig. 6). The Zr content of H5 increases abruptly
below the level of 69 % SiO
2
, showing a similar trend
to H4, but with less enrichment (Fig. 6).
Chondrite-normalized REE patterns are shown for
all zones in H4 and H3 on Figure 7a and Figure 7b,
respectively. In H4, LREE enrichment increases from
zone 1 to zone 4, and moderate Eu depletion occurs.
Hybrid magma generation 649
Table 3. The H3 tephra
Zones12345678910111213
SiO
2
67.82 68.46 69.31 68.69 66.96 67.68 66.48 66.26 65.10 63.55 62.49 62.71 56.15
TiO
2
0.35 0.33 0.34 0.33 0.39 0.41 0.43 0.46 0.51 0.70 0.76 0.69 1.70
Al
2
O
3
14.42 14.12 14.55 14.28 14.57 14.87 14.65 14.78 14.88 14.92 15.24 15.01 14.73
Fe
2
O
3
0.70 0.81 0.68 0.65 0.81 0.60 1.38 1.07 1.31 3.12 2.02 1.46 4.47
FeO 3.62 3.52 3.73 3.55 4.41 4.97 4.56 4.87 5.22 5.00 6.48 6.50 7.34
MnO 0.13 0.13 0.14 0.13 0.16 0.17 0.18 0.18 0.18 0.21 0.21 0.21 0.24
MgO 0.27 0.19 0.24 0.21 0.33 0.30 0.36 0.37 0.47 0.91 0.98 0.85 2.30
CaO 2.59 2.48 2.59 2.46 3.08 3.20 3.27 3.29 3.59 4.13 4.36 4.07 6.09
Na
2
O 4.66 4.83 4.93 4.89 4.90 4.88 4.82 4.83 4.83 4.88 4.69 4.66 3.96
K
2
O 2.28 2.31 2.35 2.30 2.09 2.13 2.07 2.05 1.99 1.75 1.74 1.75 1.27
P
2
O
5
0.05 0.04 0.06 0.03 0.08 0.07 0.10 0.09 0.11 0.22 0.14 0.19 0.81
H
2
O 1.94 1.63 1.15 1.68 0.99 0.94 1.08 0.96 1.16 0.73 0.42 0.72 0.75
Cl 400 400 450 400 380 350 370 390 370 340 280 310 260
F 1040 1080 1120 1080 1000 1000 1360 1040 1040 960 960 960 1000
B 4.4 3.3 4.2 4.5 4.4 4.8 4.8 5 4.1 3.4 4 3.9 2.6
Rb 60 51 55 58 48 48 51 51 49 44 47 48 36
Cs 0.8 0.7 0.8 0.8 0.7 0.9 0.5 0.5 0.7 0.8 0.6 0.7 0.8
Ba 602 609 612 603 563 651 569 588 523 545 491 533 427
Zr 591 690 649 748 822 758 864 803 747 726 630 524 420
Hf 14.4 15.8 14.5 15 17.5 17.6 17.7 17.3 16 14.6 14.2 14.4 10.7
Nb 64 65 67 65 66 67 66 66 63 55 53 57 53
Ta 4.98 4.93 4.99 5.22 5.19 5.06 5.1 4.99 4.63 4.71 4.59 4.58 4.3
Pb <1.0 <1.0 <1.0 4 <1.0 4.6 <1.0 5 <1.0 4 7 2 5.4
Th 9.4 9.6 9.5 9.6 9 8.9 8.8 8.5 8.1 7.7 7.4 7.5 5.4
U 2.9 2.9 2.9 2.9 2.8 3.1 2.7 2.6 2.5 2.7 2.6 2.4 1.8
Sc 9.24 9.17 9.27 8.8 11.7 12.1 12.2 12.3 12.9 14.6 15.2 14.3 18.8
Cr <9.0 <10.0 3.6 <10.0 <9.0 <20.0 <20.0 <20.0 <10.0 <20.0 5.4 <10.0 <20.0
Co 2.6 2.4 2 1.9 1.9 2.2 2.6 2.9 3.4 5 4.9 5 17
Ni <1 <12<13<1 <12<15<1 <19
Cu 20 16 17 14 18 17 17 19 17 19 21 19 26
Zn 120 120 120 120 140 140 150 160 160 160 150 150 180
Mo 4 – 4.4 4.4 4 4.1 4.2 4.4 4 3.8 3.2 – 3
Cd 0.19 0.19 0.23 0.18 0.21 0.17 0.2 0.16 0.2 0.13 0.15 0.08 0.06
La 72 73 73 73 69 68 68 66 64 60 59 60 53
Ce 148 151 143 149 141 142 140 136 130 123 121 122 111
Nd 85 83 79 84 80 81 80 74 69 71 77 70 74
Sm 15.8 14.2 15.4 16 15.3 16 15.3 13.1 14.6 14.3 13.2 14.4 14.1
Eu 3.54 3.5 3.32 3.55 3.69 3.82 3.55 3.6 3.62 3.75 3.8 3.74 4.21
Gd 15.6 18.5 19.4 16.6 16.6 17.2 15.9 15.8 14.3 15.2 16.2 14 15.6
Tb 2 2.05 2.26 2.26 2.23 2.13 2.17 2.18 2.1 2.2 2.25 2.18 2.21
Tm 1.33 1.36 1.33 1.42 1.36 1.44 1.54 1.36 1.25 1.24 1.23 1.1 1.1
Yb 8.3 8.6 8.2 8.2 8.3 8.4 8.4 8.1 7.5 7.3 7.2 7.3 6.6
Lu 1.16 1.19 1.17 1.18 1.18 1.19 1.17 1.15 1.11 1.04 1.04 1.06 0.93
Chemical composition of the H3 tephra layer (whole rock analyses). Major element concentrations are given as weight % of the oxides; trace
element concentrations are given in ppm.
Zones 6 to 12 display less LREE enrichment and
no Eu anomaly, and zones 11 and 12 are HREE-
depleted. The REE patterns for H3 are more moderate
and gradual through the zones. Zone 3 shows the
most evolved composition; LREE enrichment is not as
prominent and diminishes continously from zone 3 to
zone 13. The same patterns are shown for the faint Eu-
anomaly and HREE depletion. All chemical gradations
through the layers are more moderate in H3 than
H4.
4. Discussion
4.a. Evidence from mineralogy
The mineralogical characteristics described in the
preceding section provide the strongest clues by far to
the processes responsible for the chemical gradation of
the large Hekla layers. The most silicic part of the layers
contain 2 to 11 % crystals, mostly plagioclase and some
fayalitic olivine. In H4, crystallinity is highest in zones
3 and 4, about 7 %, but in H3, zone 1 has the highest
crystallinity, 11 % (Figs 3, 4). The mineral composition
is fairly uniform, and apparently crystal nucleation has
occurred in conditions close to equilibrium.
It was shown on the basis of Th isotopes (Sigmarsson
et al. 1991; Sigmarsson, Condomines & Fourcade,
1992), that the silicic Hekla magma is neither related
to the basaltic andesites nor the basalts of the volcanic
system. Therefore, production of the silicic magma by
crystal fractionation of the basaltic andesite can be
ruled out. As the feldspars plot close to the eutectic
in the Ab–Or–Qz diagram, and equilibrium conditions
are indicated by homogeneous crystal composition, it
is strongly suggested that the silicic magma is produced
by partial melting of the local crust. Segregation and
650 G. SVERRISDOTTIR
Table 4. Four Hekla eruptions from the last 250 year period
Samples H1766-T H1766-L H1845-T H1845-L H1947-T H1947-L H1970-T H1970-L
SiO
2
58.99 54.35 59.70 54.80 61.55 56.13 54.90 54.36
TiO
2
1.30 1.96 1.17 1.88 0.98 1.75 1.89 2.05
Al
2
O
3
15.20 14.50 15.20 14.66 15.36 14.54 14.61 14.64
Fe
2
O
3
1.88 1.74 1.72 1.23 0.74 2.08 4.15 2.64
FeO 8.13 10.42 7.39 10.54 7.67 9.25 7.32 9.70
MnO 0.23 0.25 0.22 0.25 0.21 0.24 0.25 0.25
MgO 1.50 2.73 1.77 2.97 1.42 2.75 2.76 3.03
CaO 5.42 6.68 5.06 6.53 4.58 6.29 6.56 6.89
Na
2
O 4.41 4.20 4.52 4.20 4.74 4.27 4.17 4.15
K
2
O 1.54 1.26 1.60 1.31 1.72 1.31 1.28 1.23
P
2
O
5
0.54 0.97 0.41 0.94 0.32 0.83 0.79 1.03
H
2
O 0.00 0.35 0.00 0.32 0.13 0.17 0.32 –
Cl 340 330 370 420 380 320 320 460
F 1160 1600 1000 1120 1040 920 1120 1360
B 2.1 2.1 1.9 1.7 3.9 2.7 2.6 2.1
Rb 35 35 38 32 40 36 36 34
Cs <1.0 0.4 0.5 0.4 0.7 0.8 <2.0 <1.0
Ba 429 417 426 377 469 418 384 388
Zr 320 490 500 320 604 470 410 500
Hf 13.1 11.5 12.3 11.2 13.4 11.2 10.9 10.6
Nb 68 63 64 64 70 66 67 68
Ta 4.64 4.5 4.29 4.26 4.85 4.6 6.56 6.35
Pb 2.4 3.2 3.2 3.4 2.8 3 1.3 3.2
Th 5.7 4.9 5.9 5 6.7 5.2 4.9 4.7
U 1.5 1.8 1.2 1.4 2.1 2 2 1.6
Sc 18.5 21.6 16.6 20.6 16.1 19.65 21.1 21.3
Cr <20.0 <20.0 <20.0 <20.0 <20.0 <20.0 7.4 <20.0
Co 12 20 11 18 8 14 36 23
Ni 224147269
Cu 11 11 18 10 15 12 17 8
Zn 190 210 180 200 180 200 200 200
Mo – 2.6 – 2.9 3.4 2.9 – –
Cd 0.21 0.17 0.34 0.15 0.33 0.23 0.25 0.39
La 56 54 56 50 59 53 52 52
Ce 128 124 118 121 125 121 114 113
Nd 76 80 66 80 75 80 71 73
Sm 12.4 13.8 11.7 12.2 12.8 13 12.6 13.1
Eu 4.71 4.91 4.21 4.71 4.3 4.74 4.59 4.54
Gd 18.3 17 14.8 15.5 14.3 15 17 14.9
Tb 2.3 2.44 2.03 1.92 2.16 2.13 2.05 2.33
Tm 1.08 1.06 0.97 0.87 1.02 1.09 0.83 1
Yb 7.1 7 6.8 6.7 7.3 6.6 6.6 6.5
Lu 1.01 1 0.95 0.94 1.03 0.95 0.91 0.91
Chemical composition of products from four young Hekla eruptions (whole rock analyses). T denotes tephra sample. L denotes lava sample.
Major element concentrations are given as weight % of the oxides; trace elements are given in ppm.
displacement of magma near its source into a space
created by cr ustal spreading is favoured (Gunnarsson,
Marsh & Taylor, 1998). Since a silicic melt produced
by anatexis is inevitably close to equilibrium with
several mineral phases, crystal nucleation is favoured
through the entire magma body. As silicic magma is the
first erupted part of each eruption and this magma is
followed by less evolved, sparsely crystallized magma,
crystallization was probably enhanced near the (cooler)
roof of the magma chamber. Crystallization has little
effect on the major element chemistry of near-eutectic
melts. Accordingly, the silica range in the uppermost
zones of the chamber is only 1.04 % and 2.35 % in H4
and H3, respectively.
In the dacitic to basaltic andesitic part of the deposits,
where the crystal content is less than 1 % in most
zones, the disequilibrium mineral composition reveals
the hybrid character of the magma in the deeper par t
of the reservoir. Reverse zoning of most plagioclase
crystals provides suppor ting evidence for mixing of
two magmas. Crystals are of similar size as in the
silicic part, but in some zones no crystals were found.
The main mineralogical indicator for magma mixing is
the ubiquitous occurrence of cores of sodic plagioclase
through the sections, reversely zoned to equilibrium
with the glass composition of each layer. Resorption
of minerals is more common as the hybrid magma
becomes more mafic. These outstanding disequilibrium
characteristics all indicate mixing of the evolving silicic
magma with a basaltic magma intruded from below.
Similar reversed zoning has indeed been observed in
the intermediate rocks of the Krafla volcanic centre,
where mixing between rhyolite and tholeiitic basalt was
suggested (Jonasson, 1994).
Accessory phases have been suggested as indicators
of magma chamber processes (Robinson, Miller &
Hybrid magma generation 651
Table 5. H4 and H3, glass compositions
Zone / no.
of shards SiO
2
TiO
2
Al
2
O
3
FeO MnO MgO CaO Na
2
OK
2
OP
2
O
5
Total
H4-1 / 6 73.60 0.09 13.10 1.90 0.10 0.00 1.41 4.94 2.76 0.00 97.90
Std dev. 2.03 0.07 0.18 0.08 0.06 0.00 0.08 0.24 0.10 0.00
H4-3 / 8 73.61 0.07 12.71 1.93 0.11 0.02 1.60 4.64 2.74 0.00 97.43
Std dev. 0.77 0.06 0.39 0.17 0.06 0.02 0.23 0.29 0.08 0.00
H4-6 Lb / 3 72.43 0.09 12.78 1.72 0.06 0.08 1.14 4.46 2.77 0.33 95.86
Std dev. 0.64 0.03 0.09 0.09 0.05 0.05 0.23 0.39 0.03 0.15
H4-6 / 5 66.50 0.45 14.15 6.05 0.32 0.20 3.55 4.47 1.94 0.17 97.80
Std dev. 0.97 0.17 0.20 0.52 0.12 0.05 0.25 0.55 0.12 0.11
H4-9 / 10 61.22 0.79 14.72 9.60 0.30 0.84 4.70 4.59 1.54 0.30 98.60
Std dev. 0.61 0.06 0.26 0.42 0.07 0.11 0.33 0.26 0.16 0.11
H4-12 / 4 58.32 1.44 14.63 9.70 0.30 2.02 5.73 3.94 1.28 0.71 98.07
Std dev. 1.05 0.07 0.67 0.29 0.07 0.22 0.22 0.15 0.12 0.12
H3-1 / 6 71.86 0.16 13.58 2.88 0.17 0.12 1.95 4.43 2.49 0.02 97.66
Std dev. 0.54 0.06 0.52 0.22 0.06 0.01 0.14 0.25 0.15 0.03
H3-3 / 5 71.80 0.23 13.70 2.93 0.10 0.14 2.15 5.18 2.50 0.00 98.73
Std dev. 0.58 0.00 0.32 0.21 0.08 0.08 0.14 0.29 0.05 0.00
H3-7 / 8 68.24 0.49 14.74 5.46 0.19 0.33 3.65 4.03 1.95 0.07 99.15
Std dev. 0.78 0.09 0.24 0.27 0.07 0.10 0.28 0.48 0.04 0.08
H3-9 / 8 65.92 0.47 14.89 6.36 0.21 0.59 3.61 4.50 1.89 0.15 98.59
Std dev. 0.74 0.11 0.31 0.25 0.06 0.10 0.31 0.33 0.04 0.05
H3-11 Lb / 4 71.23 0.15 13.37 3.67 0.13 0.11 1.56 4.22 2.56 0.06 97.06
Std dev. 0.27 0.06 0.73 0.56 0.06 0.01 0.10 0.27 0.13 0.06
H3-11 / 4 61.36 0.76 14.90 7.71 0.26 0.75 4.59 5.03 1.44 0.24 97.04
Std dev. 1.20 0.15 0.17 0.54 0.07 0.19 0.13 0.23 0.20 0.10
H3-13 / 7 57.79 1.78 14.41 10.97 0.28 2.01 6.22 3.79 1.32 0.87 99.44
Std dev. 0.37 0.17 0.23 0.30 0.06 0.28 0.23 0.29 0.09 0.09
Representative microprobe glass analyses (wt %). Lb = light bands in the pumice. Each analysis represents the average of several shards.
Table 6. The H5 tephra
Sample 2 3 4 5 7 8 9 10 11 12 13 14 15
SiO
2
72.52 73.03 74.30 75.01 74.17 69.58 69.53 73.43 73.10 68.73 67.84 65.40 66.73
TiO
2
0.41 0.40 0.21 0.18 0.24 0.89 0.85 0.42 0.38 0.46 0.49 0.59 0.54
Al
2
O
3
13.98 13.90 13.72 13.33 14.02 13.81 13.72 13.24 13.34 14.35 14.37 15.44 14.37
FeO 3.30 3.16 2.49 2.24 2.41 4.95 4.63 2.99 2.99 5.55 5.51 6.88 6.74
MnO 0.09 0.10 0.08 0.08 0.08 0.10 0.11 0.09 0.09 0.16 0.17 0.20 0.20
CaO 2.21 2.15 1.83 1.78 1. 78 2.72 3.04 2.08 2.12 3.
16 3.41 3.75 3.38
MgO 0.37 0.42 0.16 0.12 0.15 0.98 0.90 0.33 0.30 0.35 0.36 0.51 0.50
Na
2
O4.33 4.02 4.39 4.28 4.56 4.04 4.27 4.32 4.46 4.36 4.98 4.35 4.63
K
2
O2.58 2.63 2.65 2.83 2.43 2.65 2.69 2.88 3.01 2.54 2.53 2.44 2.52
P
2
O
5
0.08 0.07 0.04 0.03 0.04 0.15 0.14 0.09 0.09 0.16 0.17 0.25 0.22
Ba 643 601 635 646 724 617 596 622 628 536 502 538 501
Co 6.37.616.41.32.718.117.94.85.63.315.24.98.5
Cr 9.66.26.37.24.214.714.36.77.66.87.05.15.7
Cu 30.722.314.912.914.439.233.015.515.716
.618.722.518.0
Ni 14.212.79.76.79.516.412.87.68.61.93.84.94.3
Sc 7.27.15.95.65.811.610.87.27.313.112.814.713.2
Sr 145 136 125 127 136 163 167 149 147 216 213 222 205
V14.521.23.32.11.768.566.115.612.25.76.514.710.0
Y 868188889378757980908687 81
Zn 111 105 101 100 91 115 110 111 110 146 145 152 138
Zr 288 268 262 260 282 230 249 299 314 833 842 887 837
Chemical composition of the H5 tephra layer (whole rock analyses). Major element concentrations are given as weight % of the oxides; trace
element concentrations in ppm.
Ayers, 1997; Gunnarsson, Marsh & Taylor, 1998). A
chemical fingerprint of an accessory phase most pro-
nounced in the fourth uppermost zone in H4, although
not confirmed by mineralogy, is high enrichment of the
light REE and Th. This is distinctive in the chondrite
normalized plot (Fig. 7) and in Figure 6. Therefore,
small-scale accumulation of the accessory mineral
allanite is suggested, since distribution coefficients for
La, Ce, Nd, Sm, Gd and Th in allanite are very high,
and hence can account for the enrichment (Henderson,
1984; Hildreth, 1979, 1981). This is more decisive as
the Th enrichment is not compatible with a Zr anomaly
(Fig. 6). Taking such phases into account without
mineralogical confirmation was also found necessar y
by Furman, Frey & Meyer (1992), in their study on the
evolution of Icelandic central volcanoes.
652 G. SVERRISDOTTIR
Figure 5. Major element variation in H4 and H3 deposits shown as reversed stratigraphy. Glass compositions are only shown if different
from whole-rock composition of the corresponding zone. In some cases, two glass compositions were found in the same zone, as
discussed in the text.
Figure 6. Trace element variation in H4 and H3 ash layers shown as reversed stratigraphy (ppm). Zr content in H5 is s hown for
comparison at similar stratification levels as in H4 and H3. Although no visible zonation, the layer was sampled in several steps from
bottom to top. No basaltic andesite compositions were found.
In H4, anomalous enrichment of Zr, Hf, Sc and Yb
in the four zones 6 to 9 below the crystallizing zones
can only be explained by settling of plagioclase crystals
with zircon embedded within them, as very small zircon
grains were found as inclusions in plagioclase in zones
H4-2 to H4-9, and in zones 1 to 11 in H3. The Zr
enrichment is plotted against SiO
2
on Figure 8 for all
zones of H4 and H3. For H5 only Zr is plotted on
the figure. Figure 8 also shows the elements Hf, Sc
and Yb in H4 and H3. The crystal chemistry of zircon
allows Hf to incorporate by simple substitution, but Sc
and Yb substitute by a coupled mechanism (Hoskin &
Schaltegger, 2003). Liquidus temperatures calculated
by the MELTS program (Ghiorso & Sack, 1995), and
M-value as presented by Hanchar & Watson (2003),
were used to evaluate the zircon saturation of the
samples. These zircons could only have been captured
by mixing from the silicic magma which is saturated in
zircon. The hybrid magma is strongly undersaturated in
zircon, hence the zircon crystals could not have formed
in that magma.
The pronounced negative Eu anomaly seen in the
silicic part almost disappears in zone 6 (Fig. 7a). This
change in Eu is attributed to plagioclase settling, and
possibly dissolution of the smaller crystal population
upon mixing. The zircon-enriched feldspar began to
settle since the plagioclase phenocrysts are heavier
than the silicic melt (s.g. 2.6 v. 2.2), according to
calculations done with the MELTS program (Ghiorso &
Sack, 1995). It has been suggested that static conditions
allow mineral settling, based on density difference
alone (Sparks, Huppert & Turner, 1984). According to
Hybrid magma generation 653
Figure 7. (a, b) Chondrite-normalized REE-concentrations for
the whole rock analyses of (a) H4. Zones 1, 2, 4 and 5 are
shownbywholelines,zone3bygreysolidline,andzones
6–12 by dashed lines. Arrows indicate which zone is the most
Eu-depleted (zone 3), and the most LREE-enriched (zone 4).
(b)H3.Zones1,2,4,5,6,7and8areshownbysolidlines,zone
3 by grey solid line, and zones 9–13 by dashed lines. Arrows
indicate the zones which are the most Eu-depleted (zone 3), and
the most LREE-enriched (zone 3).
ion probe analyses of melt inclusions in plagioclase in
H3 (Niels Oskarsson, pers. comm.), the Hekla magmas
contain about 5 % H
2
O. Viscosity of the H4 silicic melt
was calculated by MELTS about 10
4.5
Pa·sat5%H
2
O.
4.b. Chemistry of mixing
In accordance with the chemical and mineralogical
characteristics of H4 and H3 described above, a model
of magma mixing may be constructed and some time
constraints on the process may be set.
Several major elements in all H4 and H3 samples
were plotted against SiO
2
in order to construct a
simple mixing line. The samples with the highest silica
content were selected as the silicic end-members. Since
no unmixed representatives of the pure basaltic end-
members are known, a best fit line through the samples
pointing to a hypothetical basaltic composition was
composed. The line was forced through the silicic
composition and the resulting best fit lines are shown
on Figure 9. The lines show binary mixing for CaO and
K
2
O, but for Na
2
O and Al
2
O
3
some deviations occur.
The plagioclase settling discussed above is suggested
as an explanation for the enrichment of the Na and
Figure 8. Four trace elements plotted against SiO
2
in the H4
and H3 samples. The elements were selected to demonstrate
exceptional enrichment in the hybrid zones in H4. The same
elements show more moderate enrichment at higher silica
content in H3, and in H5, Zr follows the same trend.
Al in the hybrids, whereas Ca and K would not be
affected by crystallization of the oligoclase–andesine
compositions. The more evolved characteristics of the
H4 silicic magma when compared to the respective H3
magma are also explicitly reflected in Na and Al. The
MgO and TiO
2
abundances are mirroring the deviations
attributed to plagioclase settling. This is attributed to
settling of magnetite, which is the first crystallizing
654 G. SVERRISDOTTIR
Figure 9. Selected major elements plotted on a Harker diagram for all zones of H3 and H4. Lines are best fit lines forced through the
most silicic sample of the respective deposit. Whole line for H4, dashed line for H3.
phase in the rhyolite, according to MELTS calculations
(Ghiorso & Sack, 1995). The magnetite has obviously
settled further down in the chamber as the deepest
zones are enriched, whereas the middle zones are
depleted. The faint enrichment in FeO in the middle
zones may seem controversial as magnetite is a Fe-
rich mineral, but by evidence from the corresponding
but amplified enrichment in MnO, it is concluded that
the fayalitic olivines found in the rhyolite have settled
down to the same depth as the plagioclase. These
fingerprints are so decisive that the fact that almost
no olivines were found in the hybrids indicates that the
crystals were small and had dissolved while the hot
basaltic andesite magma mixed with the rhyolites. The
absence of small plagioclases probably indicates their
dissolution. All these features are explicit in H4 but are
also visible to a lesser extent in H3. This confirms that
some crystal settling had also started in H3, although
not reflected in the crystallinity. It is concluded from
mineralogical and chemical evidence that even small-
scale cr ystallization and settling within a magma body
puts its clear signature on the compositions. These
Hybrid magma generation 655
observations also imply that the variation in the deposits
reflects the almost undisturbed stratification in the
magma chamber prior to the eruptions.
The overall chemical gradation of the H4 and H3
deposits indicates a magma mixing process that formed
a binary mixture of the silicic magma and a hypothetical
basaltic andesite composition without destroying a fine-
scale chemical stratification established by pre-mixing
crystal settling in the silicic reservoir.
4.c. Mechanism of mixing
4.c.1. Dyke injection
A process that confor ms with such a delicate pattern
is constrained by several f actors. Whether the term
‘zonation’ is used for these compositional gradients
is a matter of definition, as no abrupt changes or
clear interfaces are seen between most of the ‘zones’
in the Hekla products. It is concluded from the
uniform composition and equilibrium mineralogy that
the rhyolitic magma was almost homogeneous during
and after emplacement in the magma chamber, and that
nucleation of phases started soon after emplacement.
Important supporting evidence for crystal settling in
the silicic end-member before mixing is that, in the H5
deposit, which appears to be unmixed rhyolite–dacite,
settling of mineral phases had already started. This is
confirmed by the Zr anomaly in the less-evolved part of
that fallout (Figs 6, 8). This puts some time constraints
on the evolution within the silicic reservoir, implying
that the crystallization occurred before the basaltic
andesite injection. The homogeneous composition of
glass within zones, along with the disequilibrium
mineralogy, indicates that the mixing was very effective
within each level, and that the time between injection
and eruption was too short to allow significant
growth of equilibrium minerals in the hybrid magma.
Nevertheless, some resorption and growth of reversed
zoning on the plagioclases entrained with the silicic
end-member, and the complete dissolution of smaller
crystals had enough time to occur. This indicates that
residence time after mixing was short.
Liquidus temperatures were calculated using the
MELTS program (Ghiorso & Sack, 1995) for all H4
and H3 samples, and plotted against SiO
2
(Fig. 10).
A mixing line was constructed in the same way as for
the elements. Most of the calculated temperatures for
the hybrids plot slightly above the mixing line, which
indicates that mixing will drive crystallization in the
hybrid magma. This conforms with the reversed zoning
of the phenocrysts. The lowermost hybrid zones plot
slightly below the mixing line, which could indicate
super-heating of a few degrees. This is consistent with
the increasing resorption of crystals at that depth.
Since the basaltic andesite end-member is of unknown
composition, the exact location of the mixing lines for
the basaltic andesite field is less significant.
Figure 10. Calculated liquidus temperatures for the H4 and H3
samples, plotted against SiO
2
. Calculations were done by the
MELTS program (Ghiorso & Sack, 1995). Lines are best fit
lines forced through the most silicic sample of the respective
deposit. Whole line for H4, dashed line for H3.
Injection of hotter basaltic andesite magma into
the residing silicic magma and their intimate mixing
conforms well with characteristics of the H4 and
H3 fallout. Both magmas are clearly divided into
an unmixed part situated in the topmost part of
the reservoir, and a lower part forming a range of
hybrid compositions. This suggests that the injection
of basaltic andesite gradually mixed with the resident
silicic magma up to a defined level. Chemical and
mineralogical observations can be explained by a
fountain mechanism in a magma chamber as described
by Campbell & Turner (1989). The critical height
reached by an intruding fountain in a resident magma
depends on several factors which are not easily
measured, such as width of the feeder, and the injective
force of the incoming magma. However, as long as
the inflow is turbulent, resident magma is entrained
into the rising fountain and a stratified hybrid layer is
formed.
The shape and size of a magma chamber are also
critical factors affecting the mixing process. Clemens &
Petford (1999) conclude that tectonics and local
structures are more determinative factors for magma
emplacement styles than physical properties of the
magma, and Eichelberger et al. (2000) emphasize that
even highly silicic magma is likely to intrude upper
crustal reservoirs as dykes. The volcano-tectonics of
the Hekla volcano (Gudmundsson et al. 1992), as well
as the prolonged volcanic activity along this tectonic
lineament (Johannesson & Saemundsson, 1989), lend
strong support to a dyke emplacement model. There are
no signs of local tectonic subsidence, and the absence
of hydrothermal activity excludes a shallow magma
chamber and shallow plutons. Crustal spreading at
the margin of the propagating rift is probably the
process that creates space for the magma reservoir
of Hekla, along with the effect of the South Iceland
seismic zone, which intersects the propagating rift at
656 G. SVERRISDOTTIR
this location. Evidence from other regions suggests that
crustal spreading creates elongate magma reservoirs
(Bjoernsson et al. 1977).
A propagating rift model for the volcanic zone in
South Iceland is consistent with geodetic GPS studies,
which suggest ‘tapering’ of spreading southwards
along the eastern volcanic zone. The spreading rate
was estimated to be 8 mm a
−1
on a profile south of
Hekla, compared to a rate of 19 mm a
−1
on a profile
crossing the eastern rift zone 50 km to the north
(LaFemina et al. 2005).
It is suggested that the magma displacement events,
both the segregation of the silicic magma, and the
injection of the basaltic andesitic magmas, are driven by
periodic crustal spreading. This implies that the mixing
events were short-lived injections as described in the
fluid dynamics model. Another implication is that
the rhyolitic chamber was actually expanding during the
injection. The most likely crustal deformation process
is upward migration of the whole magma body, causing
inflation similar to that which preceded recent eruptions
(Kjartansson & Gronvold, 1983; Sturkell et al. 2006)
but on a larger scale. The basaltic andesite injection
may thus be modelled as a dyke which penetrated
through the rhyolitic magma, produced hybrid magma
by mixing, and finally triggered eruptions.
4.c.2. The magma chamber
The assumption that the magma chamber was com-
pletely drained in each eruption only holds for a
hypothetical chamber containing the silicic magma
and the hybrid magma. Whether the basaltic andesite
magma which intrudes the silicic reservoir resides in
the ‘bottom’ of the same reservoir, or if there exist some
kind of a ‘double chambers’ (Gudmundsson, 1988),
is uncertain. A dyke-formed magma chamber could
also contribute to an explanation of the fact that the
stratification was not disturbed by eruption withdrawal
(Blake & Fink, 1987).
Frequent dyke injections continued after H3, gradu-
ally building the Hekla ridge, but with a successively
decreasing contribution from the silicic source. After
the last silicic eruption which occurred in
AD 1158,
approximately two eruptions every 100 years produced
less than 10 % of dacite of up to 63 % SiO
2
each, and
90 % basaltic andesite. Compositional range of these
eruptions is 54 to 63 % SiO
2
(Table 4). The increased
frequency and smaller size of eruptions dominated
by basaltic andesite composition could mark a new
epoch in the influence of rift propagation on Hekla. A
hypothetical model of the Hekla magma chamber from
its early formation before the H5 eruption until recent
conditions is suggested (Fig. 11a–c).
The time-dependent evolution of the Hekla mag-
mas in the last 1000 year period, as described by
Thorarinsson (1967), involves a positive correlation
between the SiO
2
content of the initial phase of erup-
Figure 11. (a–c) Model of the Hekla magma chamber.
(a) Suggested formation of silicic reservoir, until the H5
eruption. (b) The large silicic eruptions H4 and H3. (c) Possible
evolution in the last 1000 year period.
tions and the repose time of the volcano. Evaluation
of this observation according to the present model
leads to the conclusion that the real time-dependent
process is the segregation and accumulation of the
Hybrid magma generation 657
silicic end-member. Assuming that the injection of the
basaltic andesite is driven by crustal spreading as it is
for most rift-zone volcanoes, the available quantity of
the shallower silicic end-member will be intruded by
basaltic andesite during rifting episodes. The mixing
process forming the eruptions in the period from
AD
1947 back to about 1000 years ago is similar to that
described here, except that the end-member proportions
are different; the estimated component of the silicic
magma is less than 5 %. The last four Hekla eruptions
(since 1970) occur after short repose periods (10–
20 years) which are apparently too short for significant
segregation of silicic melts.
Crustal deformation measurements across the east-
ern rift zone (ERZ) and western rift zone (WRZ)
(Fig. 1) were performed using geodimeters in the
years 1967 to 1986 (Decker, Einarsson & Mohr,
1971), and by GPS technique from 1986 to 1994
(Jonsson, Einarsson & Sigmundsson, 1997). These
measurements, combined with the history of rifting
episodes in south Iceland, indicate more crustal
extension across the eastern rift zone than across the
subparallel western rift zone during the last 1000 years
than during the rest of Holocene time (Sigmundsson
et al. 1995; Jonsson, Einarsson & Sigmundsson, 1997).
This supports the theory that spreading in south Iceland
is presently focused on the eastern rift zone, but is not
conclusive on the long term partitioning of the activity
of the two overlapping rift zones.
The apparent increased production of basaltic
andesite in Hekla during the last 1000 year period is
consistent with increased activity of the propagating
rift.
5. Concluding remarks
The main conclusions of the study of chemical
gradation of the two Hekla tephra layers H4 and H3 can
be summarized in three stages of magmatic evolution.
Stage 1. Old tholeiitic crust was reactivated by heat
flow within a propagating rift; silicic magma formed by
partial melting of the crust, segregated and collected in
a dyke-formed chamber at considerable depth.
Stage 2. The silicic magma started to crystallize and
settle out crystals, but before the phenocryst content
reached the average level of 5 %, basaltic andesite
magma intruded the chamber from below and mixed
with the resident magma, forming a sequence of hybrid
magmas.
Stage 3. Eruption was triggered and the pristine rhy-
olite erupted first, followed by the hybrid compositions.
This course of events was probably repeated prior to all
Hekla eruptions producing variable compositions.
It is also concluded that the basaltic andesite, which
is defined as the mafic end-member of mixing, was
not erupted in these two eruptions, but became more
common in later eruptions as the proportion of silicic
magma diminished.
Availability of silicic magma within the Hekla sys-
tem is time-dependent, while the injection frequency of
basaltic andesite occurs in response to crustal spread-
ing. The result is displayed in a sequence of hybrid
eruptions composed of different proportions of similar
end-members. Exceptions are the first silicic eruption
(H5) where probably unmixed silicic magma was erup-
ted, and the four last eruptions where almost unmixed
basaltic andesite has er upted with intervals too short
for significant segregation of the silicic end-member.
Intermediate hybrid rocks occur frequently within
the Torfaj
¨
okull centre east of Hekla, where magma
mixing between tholeiitic basalts and rhyolite is con-
firmed (McGarvie et al. 1990; Gunnarsson, Marsh &
Taylor, 1998). In the Krafla volcanic centre the
intermediate rocks are also hybrids of tholeiite and
rhyolite (Jonasson, 1994). Hekla is by no means
unusual in having magma mixing. However, production
of basaltic andesite is not common within the Icelandic
rift system. Basaltic andesite, with higher Fe and lower
Al than in intermediate rocks, in general, was given the
name ‘Icelandite’ by Carmichael (1964). It is indeed
a rare rock type on a global scale, but occurs, for
example, in some volcanic centres in Iceland, and
in a propagating rift environment in the Galapagos
islands (McBirney & Williams, 1969). In Icelandic
volcanoes it is produced during a short-lived stage in
their evolution. It is inferred that Hekla is undergoing
that stage of evolution. The high proportion of basaltic
andesite magma in Hekla, compared to many other
Icelandic volcanoes, may reflect its tectonic setting on
a propagating rift margin. The rare occurrence of this
rock type worldwide may be explained by the fact that
the tectonic conditions which are favourable to its pro-
duction are relatively restricted in time and space during
the evolution of a propagating rift into older crust.
Acknowledgements. I am grateful to many people who
have provided assistance during this work. Gudmundur E.
Sigvaldason gave valuable advice in planning the project.
Gudr
´
un Larsen did the detailed sampling work, and Susan
Russell-Robinson made the WR trace-element analyses. Karl
Gr
¨
onvold helped with the microprobe analytical work. N
´
ıels
´
Oskarsson is thanked for valuable help through the course of
this work, both as regards practical advice on the chemical
and petrological work, and constructive discussions. Amy E.
Clifton kindly read the manuscript and improved the English,
and gave several useful comments. R
´
osa
´
Olafsd
´
ottir provided
help with the graphic work. J
´
on
¨
Orn Bjarnason is thanked
for continuing encouragement.
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