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Water-Rock Interaction – Birkle & Torres-Alvarado (eds) © 2010 Taylor & Francis Group, London, ISBN 978-0-415-60426-0 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. 199 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 200 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. 201 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. 6.A comparison of fresh rock equivalent with tuffs shows that most elements are immobile within a core sample, with the exception of Na2O which is highly mobile, and CaO, P2O5, K2O, Rb and V to a lesser extent. Crovisier, J.-L., Honnorez, J., Fritz, B. & Petit, J.-C. 1992. Dissolution of Subglasial Volcanic Glasses from Iceland: Laboratory Study and Modelling, Applied Geochemistry 7(1): 55–81. Franzson, H., Gudlaugsson, S.Th & Fridleifsson, G.O. 2001. Petrophysical properties of Icelandic rocks. Proc. 6th Nordic Symp. on Petrophysics. Throndheim, Norway: 1–14. Franzson, H., Zierenberg, R. & Schiffman, P. 2008. Chemical transport in geothermal systems in Iceland. Evidence from hydrothermal alteration. Journal of Volcanology and Geothermal Research 173: 217–229. Frolova, J., Ladygin, V., Franzson, H., Sigurdsson, O., Stefansson, V. & Sustrov, V. 2005. Petrophysical properties of fresh to mildly altered hyaloclastite tuffs. Proc. World Geothermal Congress, Antalya, Turkey: 15p. Helgadottir, H.M. 2005. Formation of Palagonite. Petrographical analysis of hyaloclastite tuffs from the Western Volcanic Zone in Iceland. BSc. Dissertation, 40 p. Schiffman, P., Spero, H.J., Southard, R.J. & Swanson, D.A. 2000. Control on palagonitization versus pedogenic weathering of basaltic tephra: Evidence from the consolidation and geochemistry of the Keanakako’i Ash Member, Kilauea Volcano. Geochemistry Geophysics Geosystems, vol. 1, paper no. 2000GC000068. Sigurdsson, O., Gudmundsson, A., Fridleifsson, G.O., Franzson, H., Gudlaugsson, S.Th. & Stefansson, V. 2000. Database on igneous rock properties in Icelandic geothermal systems, status and unexpected results. Proc. World Geothermal Congress, Kyushu – Tohuku, Japan: 2881–2886. Stefansson, V., Sigurdsson, O., Gudmundsson, A., Franzson, H., Fridleifsson, G.O. & Tulinius, H. 1997. Core measurements and geothermal Modelling. Proc. Second Nordic Symp. on Petrophysics. Fractured reservoir. Nordic Petroleum Series: One: 198–220. Stroncik, N.A. & Schmincke, H.U. 2001. Evolution of Palagonite: Crystallization, chemical changes, and element budget. Geochemistry, Geophysics, Geosystems 2(7), 1017, doi: 10.1029/2000GC000102. Thorseth, I.H., Furnes, H., & Tumyr, O. 1992. A textural and chemical study of Iceland palagonite of varied composition and its bearing on the mechanism of the glass-palagonite transformation. Geochimica et Cosmochimica Acta 56(2): 845–850. 202