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Water-Rock Interaction – Birkle & Torres-Alvarado (eds)
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Porosity, density and chemical composition relationships
in altered Icelandic hyaloclastites
H. Franzson, G.H. Guðfinnsson & H.M. Helgadóttir
Iceland GeoSurvey, Reykjavík, Iceland
J. Frolova
Geological Department, Moscow State University, Russia
ABSTRACT: Basaltic hyaloclastite tuffs play an important role in hosting groundwater and geothermal
systems in Iceland. Their porosity is high and may exceed 60%. Alteration starts by palagonitization of
the glass during eruption and is usually complete at relatively low temperatures. Petrophysical measurements show that grain density decreases with increasing alteration but increases again when alteration
has reached the chlorite-epidote facies. Porosity changes during alteration, with macro-pores filling at the
same time as micro-porosity increases. When glass alters, most chemical components are released from
the rock. This study shows, however, that chemical mobility during this process is very limited, even at the
highest alteration state. Only Na2O shows high mobility and CaO, K2O, P2O5, Rb and V to a lesser extent.
Other elements show no apparent mobility.
1 InTroduction
2 summary of data
Icelandic volcanic rocks host vast water resources
ranging from cold groundwater to high-temperature
systems, and are utilized through wells drilled into
these reservoirs. The type and longevity of these
reservoirs are highly dependent on the petrophysical character of the rocks. Representative rock
samples are needed for the study of these characteristics in the laboratory. However, because of
the lack of cores taken during drilling, a research
project, financed by National Energy Authority
and Reykjavik Energy, was undertaken in 1993
to study the petrophysical character of Icelandic
rocks of all types and degrees of alteration from
surface samples to deeply eroded crustal sections.
About 500 samples were collected and studied for
these purposes (Stefansson et al. 1997, Sigurdsson
et al. 2000). An additional 120 samples were taken
in 1999 to study variations in petrophysical characteristics within a single fresh lava flow of olivine
tholeiitic composition (Franzson 2001).
The present project, which is ongoing, focuses
on basaltic hyaloclastite tuffs, ranging from relatively fresh to totally altered samples. An additional
100 samples of tuffs were collected in 2002, bringing the total up to some 140 samples. Duplicate
samples were analyzed at Moscow State University
for petrophysical properties (Frolova 2005). This
paper summarizes the analytical methods and the
relationship between ­permeability, porosity, density, alteration and chemistry of hyaloclastite tuffs.
Hyaloclastite tuffs are fragmental rocks formed
during sub-glacial volcanic eruptions where magma
contacts glacial meltwater.
Palagonitization is the first stage of glass alteration
and is postulated to start during the eruption. Three
time-dependent types of palagonite have been recognized as shown in Figures 2 and 3; rind palagonite,
isolated spherical palagonite and layers of spherical
palagonites, all of which we recognize as hydrated
glass. When palagonite has altered into smectite, the
glass becomes vulnerable to further alteration, and
deposition of alteration minerals start to fill voids in
the rock and enhance consolidation as the alteration
proceeds (Helgadottir 2005, Thorseth et al. 1992,
Stroncik & Schminke 2001, Schiffman et al. 2000).
Each thin section was point counted (2000
points) to establish quantitatively the changes
occurring in the rock during the gradual alteration, and for comparison to geochemical and
petrophysical data. This includes the proportion of
glass, primary crystals, rock alteration, open voids
and mineral deposition. SEM images were also collected to study the porosity structures of the tuff.
All samples were measured for total and effective porosity, gas and klinkenberg permeability,
and grain density. Several other parameters were
measured at Moscow State University, some of
which have been published (Frolova 2005).
All samples were analyzed for major and basic trace
elements, along with LOI, CO2 and FeO ­analysis.
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Figure 1. Geological map of Iceland with the sample
areas outlined.
Figure 2. Time relation of the progressive alteration of
basaltic glass.
Ten samples were analyzed for H2O+ to establish the
relation between LOI and CO2 analyses.
The porosity of tuff was determined by direct
measurement in air and petrographically by point
counting. A comparison of the results of the two
methods is shown in Figure 4. It shows that the
measured porosity is significantly higher than the
thin- section porosity. Even in samples where thinsection inspection shows all pores to be filled with
secondary minerals, measured porosity is up to a
third of the bulk rock. The minimum size of pores
observed in thin sections is related to the thickness of the section, indicating that the method
only includes pores larger than 30 µm while the
measured porosity includes all pores. This allows
a distinction of pores into macro and micro-pores
(<30 µm). Several lines of evidence suggest that, as
the alteration of the glass proceeds and macro-pores
fill with minerals, micro-pores are created within
the alteration minerals, such as low-temperature
clays. The creation of micro-pores is clearly seen at
the intersection of glass and spherical palagonite
in Figure 3.
Figure 3. A backscatter-electron image of tuff showing
rind palagonite, irregular spherical palagonite fresh glass,
primary pores and secondary micro porosity at the intersection of the glass and palagonite.
Figure 4. A comparison between measured porosity and
porosity assessed from petrography (counted porosity).
Hydrous minerals form as glass is altered, thus
water in the rock increases with increased alteration as shown in Figure 5, where water increases
from less than 2% at 30% alteration to over 10%
at intense alteration. An interesting feature is the
marked loss of water in samples belonging to the
chlorite-epidote alteration zone, which is explained
by the transformation from smectite to less hydrous
chlorite, and the disappearance of zeolites. It is
interesting to note that the lower boundary of the
alteration indicates the minimum rock consolidation
needed to acquire core samples from the tuffaceous
rocks (around 30% alteration as seen in Fig. 4).
Rock grain density (g/cm3) is the average density of the minerals in the rock, excluding pores.
Grain density of basaltic glass ranges from 2.7 to
2.8. When alteration starts, lower density palagonite and clays replace the glass along with zeolite
precipitation in the voids, resulting in an overall
decrease of density. Figure 6 shows this relation
where the density gets progressively lower with
increasing content of water. However, a distinct
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Figure 5. Graph of water content (LOI–CO2) versus
alteration volume% (determined form point counts).
alteration minerals formed. A comparison of palagonite composition to fresh glass (Fig. 7) shows a
very small difference between them with the exception of Na2O, which is clearly highly mobile during
palagonitization. A small loss of CaO also occurs.
Other elements, such as FeO and TiO2, are highly
resistant to mobilization as shown in other studies of palagonitization (e.g. Thorseth et al. 1992,
Crovisier et al. 1992), which support the finding of
limited mobility of base cations on a microscale.
Another method of evaluating element mobility
in the tuffs is to compare them with chemical trends
produced by magmatic processes. If elements are
mobile, alteration will lead to deterioration of such
correlation, especially if the elements that produce
the trends behave differentially during the hydrothermal alteration. Figures 8 and 9 show examples
of such a comparison.
Figure 6. Graph of grain density versus water content.
density increase occurs in samples containing
chlorite-epidote alteration, coinciding with a drastic decrease in water content. The decrease in the
water content is caused by the disappearance of
the more hydrous alteration minerals (clays, zeolites) along with the formation of higher density
alteration minerals such as epidote.
Hydrothermal alteration is a change in the mineral content as a result of reaction of the rock with
hydrothermal fluids. The rate of change depends
to some extent on the stability of the minerals in
the fresh rock. Basaltic volcanic glass is very sensitive to alteration, and made more vulnerable to
­alteration by the fur extreme porosity and permeability of the tuff it is contained in. Chemical
transport during hydrothermal alteration in basaltic lavas and intrusions shows different mobility
of the elements in the rocks partly related to the
primary mineralogy of the rocks (Franzson et al.
2008). In basalt glass, the element mobility is not
constrained by the different stabilities of crystal lattices, and the elements should all become mobile as
the glass alters. The distance of element transport
from glass thus depends more on the particular
Figure 7. Palagonite composition (corrected for LOI)
in comparison with fresh basaltic glass showing substantial leaching of Na from glass as palagonite forms.
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Figure 8. Variation diagram showing bulk concentration of Zr and TiO2 in fresh rock samples from the
Reykjanes-Langjökull volcanic zone (x) and in fresh to
altered tuff from this study (filled circles). The compositional overlap shows that Zr and Ti are not significantly
changed by alteration.
Acknowledgement
This project has mainly been financed by the
National Energy Authority and Reykjavik Energy.
A NATO grant from the Icelandic Research Council and Russian Foundation for basic research
(grant 05-03-64842) is also acknowledged. The
manuscript benefitted greatly by the review of
Halldór Armannsson and Mark Reed.
References
Figure 9. Variation diagram showing bulk rock concentrations of Zr and Na2O in fresh rock samples from
the Reykjanes-Langjökull volcanic zone (x) and fresh to
altered tuff from this study (o).
The relation between TiO2 and Zr shows that
they conform closely with the magmatic evolutionary trend of the Reykjanes-Langjökull volcanic
zone as seen in Figure 8, whereas Na2O shows
increasing loss with progressive alteration (Fig. 9).
Other elements that show signs of mobility are
CaO, P2O5, K2O and the trace elements of Rb and V.
3 conclusions
Volcaniclastic tuffs are unusual in that the rocks
have large porosity and permeability, but in nature
they behave more like aquicludes where they form
cap rocks for many high-temperature systems in
Iceland. This study of about 140 fresh to totally
altered tuff samples shows the following:
1. Porosity values are up to 60%.
2. Palagonitization starts during the eruption and
progresses to total alteration at relatively low
temperatures.
3. While macropores are filled during alteration,
secondary microporosity is formed.
4. Grain density diminishes with increasing alteration, but increases again at the chlorite-epidote
alteration state.
5.Only Na2O shows strong mobility during palagonitization, while other elements remain relatively stable.
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core sample, with the exception of Na2O which
is highly mobile, and CaO, P2O5, K2O, Rb and
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