Carbon dioxide transport through Taal volcano’s hydrothermal system and Main Crater Lake (Philippines)

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Université Libre de Bruxelles

Département Géosciences, Environnement et Société

Laboratoire de Volcanologie

Carbon dioxide transport through Taal volcano’s

hydrothermal system and Main Crater Lake (Philippines)

by

Katharine Maussen

A dissertation submitted in partial fulfilment of the requirements for the degree of ‘Docteur en Sciences’

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Abstract

The presence of a hydrothermal system at Taal volcano is evident from the presence of a crater lake (Main Crater Lake, MCL), a caldera lake (Lake Taal) and several hot springs on the flanks of Taal volcano island and in the crater. Taal MCL, covering an area of 1.2 km², is acidic (pH = 3), warm (T = 30-33 °C) and its composition is dominated by Cl, Na and SO4. This thesis

aims at understanding the geochemistry of Taal volcano’s hydrothermal system and the way CO2 is transported through the hydrothermal system and MCL towards the atmosphere.

The long-term geochemical evolution of MCL indicates that the hydrothermal system is made of two reservoirs, one being volcanic and one geothermal in origin. The geothermal component in Taal MCL has stayed rather constant since 1991, while the volcanic component has decreased.

The low pH makes Taal volcano the perfect natural laboratory to study the behaviour of CO2,

because there is no dissociation of CO2. A combined approach of total CO2 flux measurements

via accumulation chamber and gaseous CO2 flux measurements via echo sounder shows that

more than 90% of the total CO2 output of Taal volcano is due to the influx of dissolved CO2,

migrating from the hydrothermal system to MCL via thermal springs under the lake surface.

After verification of both horizontal and vertical homogeneity of dissolved CO2 concentrations,

a continuous monitoring station was installed in 2013, measuring dissolved CO2 using an

infrared gas analyser protected by an ePTFE membrane, as well as several meteorological and environmental parameters. Several environmental and lacustrine processes influence CO2

transport in MCL, including stratification, solar heating and rainfall.

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were recorded before the start of seismic or deformation activity, which makes continuous CO2

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Résumé

La présence d’un système hydrothermal au volcan Taal se manifeste par la présence d’un lac de cratère (Main Crater Lake, MLC) ainsi qu’un lac de caldera (Lake Taal) et de multiples sources d’eau chaudes sur les flancs et dans le cratère. Le MCL, avec une surface de 1.2 km², est acide (pH = 3), chaud (T = 30-33 °C) et composé principalement de Cl, Na et SO4. Le but

de cette thèse est de comprendre la géochimie du système hydrothermal du Taal et la manière dont le CO2 est transporté à travers de celui-ci ainsi qu’à travers le MCL vers l’atmosphère.

L’évolution géochimique à long terme indique que le système hydrothermal est composé de deux réservoirs, un d’origine volcanique et un autre d’origine géothermale. Le composant géothermal est resté plutôt constant depuis 1991, tandis que le composant volcanique a diminué.

Le pH plutôt bas fait que le volcan Taal est le laboratoire naturel parfait pour étudier le comportement du CO2, parce qu’il n’y a pas de dissociation de CO2. Une approche combinée

du flux de CO2 total via chambre d’accumulation, et flux de CO2 gazeux via echo sondeur

montre que plus que 90% du flux de CO2 total est dû au CO2 dissout, qui migre depuis le

système hydrothermal au MCL via des sources thermales sous la surface du lac.

Après vérification de l’homogénéité horizontale et verticale du CO2 dissout, une station de

monitoring en continu a été installée en 2013. Cette station mesure le CO2 dissout à l’aide d’un

analyseur de gaz infrarouge protégé par une membrane en ePTFE, ainsi que de multiples paramètres météorologiques et environnementaux. Le transport de CO2 dans le MCL est

influencé par plusieurs processus environnementaux et lacustre, comprenant la stratification, l’échauffement solaire et la pluie.

Le volcan Taal connait régulièrement des périodes de crises caractérisées par une activité sismique, par une déformation du sol et par un flux élevé du CO2. En 1991-1994, ceux-ci ont

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monitoring en continu a enregistré des données toutes les heures pendant la crise en 2015 et a montré que des concentrations particulièrement élevées en CO2 dissout ont été enregistrées

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Acknowledgements

My sincerest gratitude goes out to a number of people who have challenged me, supported me and in one way or another contributed in making this thesis possible.

First and foremost, I would like to thank my supervisor, Alain Bernard, for suggesting this project and believing in me from the start. His door has always been wide open for me and I have always been able to walk in anytime with any type of question, comment or frustration I had. Alain has spent a lot of his time applying for funding that gave me the opportunity to go on numerous fieldwork trips to the Philippines, an experience which I will never forget.

This work has benefitted from a collaboration between ULB and the Philippine Institute of Volcanology and Seismology (PHIVOLCS). First, I would like to thank Renato Solidum and Mariton Bornas for enabling us to work in the Philippines and providing us with logistical support during fieldwork. I am immensely grateful to PHIVOLCS engineers Edgardo Villacorte and Ronald Pigtain, core members of the Taal fieldwork team, for their valuable help in setting up the continuous monitoring station, providing us with power, telemetry, concrete platforms, staircases, casings for the equipment and much more. I would like to thank Raymond, Ericson, Elisa, Alan, Djenan, Julien and the staff of Taal Volcano Observatory: Paolo, Ric, Lawrence and Jojo for their help during fieldwork and for the animated conversations afterwards. The entire PHIVOLCS staff, and the Volcano Monitoring and Eruption Prediction Division in particular, have been extremely kind during our stays in the Philippines.

I have worked with many types of analytical instruments over the course of these six years, and I am grateful to Patrick, Morgane, Saida, Marie, Wendy and Sabrina who have taught me how to use them properly.

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1 Introduction

Humans have always had a rather dual relationship with volcanoes, expressing their admiration for volcanoes through art and stories and growing crops on the fertile grounds near volcanoes. On the other hand, volcanic eruptions are responsible for loss of life and property, that can extend up to a global scale for large eruptions. Even though volcanic eruptions cannot be prevented at this moment, it is possible to estimate the probability of an impending eruption and, in some cases, the type and location of the eruption, through careful monitoring of the precursor signals. Knowing when, where and how an eruption is most likely to occur is crucial in the prevention of socioeconomic loss.

A large variety of techniques can be used to assess the activity of a volcano, of which most belong to either seismic, deformation or geochemical monitoring. Several changes are expected to occur when magma and/or volcanic gas and fluids migrate towards the surface. The ascending magma causes pressure changes in the earth’s crust, which causes faults and fractures in the surrounding rock that generate earthquakes measured by seismic monitoring. The increased pressure of the intruding magma disturbs the ground surface (deformation monitoring), and volcanic gas gets released due to depressurisation of the magma (geochemical monitoring).

Geochemical monitoring involves measurement of fluxes, concentrations and isotopic ratios of gas and water at the surface of a volcano in order to study degassing processes of the magma itself. These interpretations are relatively straightforward for volcanoes with open vents that degas directly to the atmosphere (for example for lava lakes, which are in direct contact with the atmosphere), but become more complicated when other processes occur. When the volcano in question possesses a hydrothermal system (a hot groundwater reservoir), volcanic gas reacts with water which might alter fluxes, concentrations and isotopic ratios of the gas species involved.

Taal volcano (Philippines) has a long history of unrests associated with high CO2 fluxes from

its Main Crater Lake (MCL) to the atmosphere, although they do not usually lead to an eruption. The objective of this work is to study the migration of CO2 through the hydrothermal system

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1.1 What is a hydrothermal system

The two components of a hydrothermal system are water (Greek: hydros) and heat (Greek: thermos). A hydrothermal system is a hot groundwater reservoir which consists of different components. In the case of volcano-hydrothermal systems, the heat is provided by a magmatic intrusion, and the water is made up of deep fluids which originate from the degassing magma, and surface fluids, usually meteoric water.

1.1.1 Magmatic degassing

The major constituents of magmatic gas are H2O, CO2 and SO2, followed by the minor

components H2, H2S, HCl and HF (Symonds et al. 1994; Symonds et al. 2001). These gases

are released from the magma due to decompression. These gases all have different solubilities and will therefore separate from the magma in a certain order, depending on magma composition, temperature and pressure (Burnham 1979). Carbon dioxide is relatively insoluble in most magmas, and will therefore usually be the first gas to be released from the magma at depth, followed by water (Holloway 1976; Giggenbach 1996; Wallace 2005). Sulphur and halogen gases are more soluble and get released at shallower levels, generally in the order S gases , HCl and HF, for which HF is being released at the shallowest level (Carroll and Webster 1994; Giggenbach 1996). This order of degassing is used in volcanic gas monitoring, where increased fluxes of gas species (usually SO2) and changes in gas species ratios (CO2/SO2 or

SO2/Cl) are seen before eruptions, at least for open vent systems (Daag et al. 1996; Edmonds

et al. 2001; Aiuppa et al. 2009).

1.1.2 Interactions of magmatic gases with surface water

Surface water, usually of meteoric (rainwater) origin and in some cases seawater for specific volcanoes close to the sea (Chiodini et al. 1995; Delmelle et al. 1998; Christenson et al. 2017), infiltrates the volcanic soil and forms a groundwater reservoir that is heated by volcanic fluids. It will react with magmatic gases to form a hydrothermal reservoir. Different gases will react differently with water, according to the following equations:

𝐻𝐶𝑙_{( )} ⇌ 𝐻 ₍ ₎+ 𝐶𝑙 ₍ ₎ ( 1.1 )

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3 4 𝑆𝑂 ( )+ 4 𝐻 𝑂( , ) ⇌ 3 𝑆𝑂 ( )+ 6 𝐻 ( )+ 𝐻 𝑆( ) ( 1.3 )

3 𝑆𝑂 _{( )}+ 2 𝐻 𝑂_{( , )} ⇌ 2 𝑆𝑂 ₍ ₎+ 4 𝐻 ₍ ₎+ 𝑆_{( , )} ( 1.4 ) 𝐶𝑂 ( ) + 𝐻 𝑂( , ) ⇌ 𝐻 𝐶𝑂 ( ) ⇌ 𝐻 ( ) + 𝐻𝐶𝑂 ₍ ₎ ( 1.5 )

Furthermore, in oxidising conditions, H2S reacts to sulphate according to the following

reaction:

𝐻 𝑆_{( )}+ 2 𝑂 _{( )} ⇌ 2 𝐻 ₍ ₎+ 𝑆𝑂 ₍ ₎ ( 1.6 )

During these reactions, numerous H+_{ions are formed, acidifying the hydrothermal fluid. The}

acidic fluid, in turn, reacts with the silicate host rock to form an alteration mineral assemblage. This process progressively neutralises the hydrothermal fluid, for example via the following reaction where potassium feldspar reacts to pyrophyllite and silica:

𝐾𝐴𝑙𝑆𝑖 𝑂 _{( )}+ 𝐻 ₍ ₎⇌ 0.5 𝐴𝑙 𝑆𝑖 𝑂 (𝑂𝐻) _{( )}+ 𝑆𝑖𝑂 _{( )}+ 𝐾 ₍ ₎ ( 1.7 )

K-feldspar Pyrophyllite

Hydrochloric (HCl), fluoric (HF) and sulphuric acid (H2SO4) are strong acids, meaning they

will dissociate completely in contact with water. Carbonic acid (H2CO3), however, is a weak

acid and only partially dissociates, depending on pH. At sufficiently low pH (<4), it does not dissociate and the last step of reaction 1.5 can therefore be omitted (see chapter 4 for more details).

Symonds et al. (2001) define the term ‘magmatic gas scrubbing’ as any process that reduces gas emissions during reactions between magmatic gas, water, and sometimes rock. This includes dissolution into the aqueous phase and formation of mineral precipitates from gas-water or gas-gas-water-rock interactions. When this process is effective, most volcanic gases (H2O,

SO2, HCl, HF) get trapped in the hydrothermal system and do not reach the surface. Carbon

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1.1.3 Zonation of hydrothermal systems

As explained in the previous section, the pH of a hydrothermal system is determined by two processes: dissociation of volcanic gas and water-rock interactions. The relative importance of these processes depends on the location of the fluids in the volcanic edifice and will influence the composition of that part of the hydrothermal system (White 1957; Ellis and Mahon 1977; Giggenbach 1988; Nicholson 1993). A schematic cross-section of a hydrothermal system is given in Figure 1.1. Right above the magma chamber, volcanic gas dissociation is strongest, and this will likely dominate over water-rock interactions. In these areas, reactions 1.1 to 1.4 will be dominant, and the resulting waters will be acidic and rich in sulphate and chloride. When these fluids migrate laterally, the influence of volcanic gas input decreases and the water progressively neutralises. During boiling of the hydrothermal system, H2S and CO2 transfer

into the gas phase and can get trapped in a superficial reservoir creating respectively acid sulphate (reaction 1.6) or neutral bicarbonate (reaction 1.5) steam-heated reservoirs. The remaining liquid phase often resurfaces as neutral chloride springs.

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1.2 Taal volcano: an overview

Taal volcano is situated in central Luzon about 60 km south of Manila, capital of the Philippines. Volcanism in Taal is related to the subduction of the South China Sea plate under the Philippine Archipelago along the Manila Trench (Hamburger et al. 1983), which extends from Mindoro Island, Philippines, all the way to Taiwan. Convergence rates for the Manila Trench are estimated at between 91 mm/yr at the northern tip of Luzon to 55 mm/yr north of Mindoro, based on GPS, gravity anomaly and bathymetric data (Hsu et al. 2012). A generalised map of tectonic structures is shown in Figure 1.2.

Figure 1.2: Generalised tectonic framework of Luzon island (A) and the Philippines (B). ELT = East Luzon Trough; EMB = East Mindoro Block; LI = Lubang Island; MC = Macolod Corridor; MB = Masbate Island; MF = Marikina Fault; MN = Mindoro Island; NT = Negros Trench; PF = Philippine Fault; PML = Macolod Lineament; PMP = Palawan-Mindoro platform: SSBoPF = Sibuyan Sea branch of the Philippine Fault: VIPF = Verde Island Passage Fault, TFZ = Taal fracture zone. After Förster et al. (1990)

Taal volcano is one of several volcanoes in the Macolod corridor, a ENE-WSW oriented pull-apart basin (Förster et al. 1990). Lavas are typically basaltic to andesitic and calc-alkaline, typical for subduction type volcanism (Miklius et al. 1991; Listanco 1993; Mukasa et al. 1994).

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Williams 1999). It is now filled with a freshwater lake named Lake Taal. Historic eruptive activity is concentrated on Taal Volcano Island (TVI, max. elevation 311 m), an island of about 6 km diameter in the middle of Lake Taal. Eruptive centres include the main crater and a series of fissures and cones along the flanks of the volcano (Figure 1.3). The main crater is filled with the Main Crater Lake (MCL), shown in Figure 1.4, with a surface area of about 1.2 km². The Main Crater Lake hosts a small islet, called Vulkan Point, a remnant from what previously was the crater floor.

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7 Figure 1.4: Photo of Taal Main Crater Lake, with the Vulkan Point islet on the right. The other end of MCL, where no trees are growing, is the Solfatara area

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No eruptions have occurred since 1977, although Taal volcano has exhibited several episodes of increased unrest. These are characterised by increased seismicity, ground deformation, carbon dioxide degassing and electrical and magnetic anomalies, in some cases accompanied by opening of fissures and a temperature increase of MCL (Global Volcanism Program 1994; Torres et al. 1995; Poussielgue 1998; Lowry et al. 2001; Bartel et al. 2003; Zlotnicki et al. 2009a; Maeda et al. 2013; Arpa et al. 2013; Galgana et al. 2014; Sayco et al. 2017; Zlotnicki et al. 2017). These will be discussed in detail in chapter 2. The nature of all these unrests is poorly understood. Several authors consider magmatic intrusions to be the cause of increased activity at Taal volcano (Bartel et al. 2003; Maeda et al. 2013; Arpa et al. 2013; Galgana et al. 2014; Kumagai et al. 2014; Hernández et al. 2017). On the contrary, others have considered changes in the hydrothermal system (Lowry et al. 2001; Zlotnicki et al. 2009a).

Self-potential (SP), total magnetic field (TMF), temperature and soil CO2 anomalies have been

observed at two sites, one on the north shore of MCL (Solfatara area) and another on the northern flank of TVI along the Daang Kastila trail where cracks have opened after the 1994 inflation period (Harada et al. 2005; Zlotnicki et al. 2009b). Both these areas are characterised by strong alteration of rocks and fumarolic activity, with geysers and hot springs in the main crater, and are interpreted as areas where the hydrothermal system is active near the surface. An electrical resistivity tomography (ERT) study by Fikos et al. (2012) confirmed the earlier studies and revealed a pathway for upward migration of hydrothermal fluids near the northern side of MCL. Other parts of the volcanic edifice, including the area right under MCL, are characterised by high resistivity and were interpreted as dense rock impermeable to hydrothermal fluids.

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9 MCL and thermal waters have been analysed by Delmelle et al. (1998) and Hernández et al. (2017). MCL waters are acidic (pH=2-3) and extraordinarily rich in Na and Cl compared to other acidic volcanic systems. Delmelle et al. (1998) analysed MCL and hot spring samples between 1991 and 1995 and attributed the high NaCl content to incorporation of marine fluids in the hydrothermal system, based on Br/Cl ratio and sulphur isotopic signatures. The temperature of the hydrothermal reservoir was estimated at 300°C based on sulphur isotopic fractionation. A cross-section representing the hydrothermal system by Delmelle et al. (1998) is given in Figure 1.5. Hernández et al. (2017) sampled MCL waters in March and June 2011 at different depths. They found an abnormally high Mg/Cl ratio and concluded this was due to increased magmatic activity during the 2010-2011 unrest.

Figure 1.5: NE-NW cross-section through Taal volcano indicating the possible hydrological structure and water flow (not to scale), after Delmelle et al. (1998)

1.3 Objectives and outline

This thesis aims at understanding the geochemistry of Taal volcano’s hydrothermal system and the way carbon dioxide is transported through the hydrothermal system and the Main Crater Lake (MCL) towards the atmosphere. An understanding of the behaviour of carbon dioxide in the hydrothermal system is necessary to develop a correct method for continuous monitoring of carbon dioxide at Taal volcano.

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hydrothermal system and its response to volcanic unrest. This chapter includes an assessment of the hydrothermal structure of Taal volcano based on long-term lake water monitoring data to verify if the model by Delmelle et al. (1998) is still valid now that more data is available. The causes of both long-term and short-term changes in volcanic activity on the geochemistry of lake and hot spring waters are investigated in order to determine the nature of non-eruptive unrests at Taal volcano.

In the third chapter, detailed surveys of total carbon dioxide degassing between the lake surface and the atmosphere measured using an accumulation chamber, and the presence of gas bubbles on the lake bottom measured using echo sounder, are presented. Spatial analysis of CO2 fluxes calculated based on accumulation chamber and echo sounder surveys are performed

to check whether degassing is equally distributed over the entire lake or if degassing originates in one or more areas. The relative proportion of gaseous vs dissolved CO2 relative to total flux

is estimated based on echo sounder vs accumulation chamber outputs, and reproducibility of CO2 flux measurements via accumulation chamber is tested using sequential analysis on the

same location.

Chapter Four focuses on measuring dissolved carbon dioxide (dCO2) in lake water. Several

methods for measuring dCO2 concentration are being presented and compared. The aim of this

chapter is to discuss the spatial variability of dissolved carbon dioxide over the surface of MCL, and to get an idea of the variability of dCO2 over time. Homogenisation of lake waters in depth

is discussed using vertical conductivity-temperature-pH vs depth profiles. Carbon isotopic data is used as a tracer for the origin of CO2 in MCL, and the residence time of carbon dioxide and

its potential for volcano monitoring are discussed.

Finally, in chapter five, a station for continuous monitoring of dCO2 and different

environmental variables is presented. The behaviour of CO2 in response to environmental

parameters is studied in order to analyse which factors affect transport of CO2 from the

hydrothermal system to the atmosphere. The relationship between dCO2 and total CO2 flux is

investigated, and an equation to calculate CO2 flux based on dCO2 data is proposed. The value

of continuous dCO2 concentration and CO2 flux monitoring can then be assessed comparing

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15 Symonds RB, Rose WI, Bluth GJS, Gerlach TM (1994) Volcanic gas studies: methods, results and applications, in Volatiles in Magmas. In: Carroll MR, Hollaway JR (eds) Mineralogical Society of America Reviews in Mineralogy 30. pp 1–66

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2 Geochemical

characterisation

of

Taal

volcano-hydrothermal system and temporal

evolution during continued phases of unrest

(1991-2017)

Katharine Maussen a_{, Edgardo Villacorte}b_{, Ryan R. Rebadulla}b_{, Raymond Patrick Maximo} a,b_{, Vinciane Debaille}a_{, Ma. Antonia Bornas}a,b_{, Alain Bernard}a

a_{Laboratoire G-Time, DGES, Université Libre de Bruxelles, CP 160/02, Avenue Franklin}

Roosevelt 50, 1050 Brussels, Belgium

b_{Philippine Institute of Volcanology and Seismology, C.P. Garcia Avenue, University of the}

Philippines, Diliman Campus, Quezon City 1101 Philippines

This chapter is published as a research paper in Journal of Volcanology and Geothermal Research 352 (2018) pp 38-54. https://doi.org/10.1016/j.jvolgeores.2018.01.007

2.1 Abstract

Taal volcano (Luzon Island, Philippines) has last erupted in 1977 but has known some periods of increased activity, characterised by seismic swarms, ground deformation, increased carbon dioxide flux and in some cases temperature anomalies and the opening of fissures. We studied major, trace element and sulphur and strontium isotopic composition of Taal lake waters and hot springs over a period of 25 years to investigate the geochemical evolution of Taal volcano’s hydrothermal system and its response to volcanic unrest.

Long-term evolution of Main Crater Lake (MCL) composition shows a slow but consistent decrease of acidity, SO4, Mg, Fe and Al concentrations and a trend from light to heavy sulphate,

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Sulphate and strontium isotopic data suggest this neutral chloride-rich component represents input of geothermal water into Taal hydrothermal system. A significant deviation from the long-term baseline can be seen in two samples from 1995. That year, pH dropped from 2.6 to 2.2, F, Si and Fe concentrations increased and Na, K and Cl concentrations decreased. Sulphate was depleted in 34_{S and temperature was 4 °C above baseline level at the time of sampling. We}

attribute these changes to the shallow intrusion of a degassing magma body during the unrest in 1991-1994.

More recent unrest periods have not caused significant changes in the geochemistry of Taal hydrothermal waters and are therefore unlikely to have been triggered by shallow magma intrusion. A more likely cause for these events is thus pressurisation of the hydrothermal reservoir by increasing degassing from a stagnant magma reservoir. Our study indicates that new magmatic intrusions that might lead to the next eruption of Taal volcano are expected to change the geochemistry of MCL in the same way as in 1994-1995, with the most notable effects being changes in temperature, pH, F and Si concentrations.

2.2 Introduction

Taal volcano is located on the southwest side of Luzon Island, Philippines, and consists of a large water-filled caldera which hosts an active post-caldera complex (Taal Volcano Island, TVI, Figure 2.1). The presence of a large hydrothermal system in the subsurface of TVI is evident from the existence of a crater lake, geysers and hot springs.

Geochemical changes of crater lake waters and hot springs are often related to changes in volcanic activity, and can therefore effectively be used as a monitoring parameter (Christenson 2000; Taran et al. 2000; Martı́nez et al. 2000; Mazot et al. 2008; Ohba et al. 2008; Christenson et al. 2010; Agusto et al. 2016; Christenson et al. 2017). Taal volcano has not erupted since 1977, but has gone through a series of unrests, characterised by seismic swarms, deformation and increased degassing (Lowry et al. 2001; Bartel et al. 2003; Zlotnicki et al. 2009a; Arpa et al. 2013; Galgana et al. 2014). The origin of this activity is poorly known.

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19 difficult to properly distinguish the geochemical signatures during quiescent periods from those during volcanic unrest.

The objective of this paper is to study the evolution of the hydrothermal system from a geochemical point of view over a period of 25 years, to assess the cause of both long-term and short-term changes in volcanic activity on the geochemistry of the Main Crater Lake and hot springs, and to determine the nature of non-eruptive unrests at Taal volcano.

2.3 Volcano-tectonic setting and eruptive history

Volcanism in Taal (Figure 2.1) is related to the subduction of the South China Sea plate under the Philippine Archipelago along the Manila Trench (Hamburger et al. 1983). Taal volcano is one of several volcanoes in the Macolod corridor, a NE-SW oriented pull-apart basin located in the southwest of Luzon Island, Philippines (Förster et al. 1990). Lavas are typically basaltic to andesitic and calc-alkaline, typical for subduction type volcanism (Miklius et al. 1991; Listanco 1993; Mukasa et al. 1994). Taal caldera is about 25 x 30 km wide and formed through several large ignimbrite-forming eruptions between 5.6 and 140 ka (thousands of years; Oles 1991; Listanco 1993; Martinez and Williams 1999). It is now filled with a freshwater lake named Lake Taal. Historic eruptive activity is concentrated on Taal Volcano Island (TVI, max. elevation 311 m), an island of about 6 km diameter in the middle of Lake Taal. Eruptive centres include the main crater and a series of fissures and cones along the flanks of the volcano. The main crater is filled with the Main Crater Lake (MCL). Several hot springs can be found inside the crater around the Solfatara areas and one is present on the TVI flank near the Lake Taal shoreline (Solfataras and Balantok Hot Spring, Figure 2.1).

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Figure 2.1: Location of Taal Volcano. (A) Location of Taal volcano on Luzon Island in the Philippines. Mac. Cor. = Macolod Corridor. (B) Digital Elevation Model of Taal volcano, with Lake Taal and Main Crater Lake (MCL) in blue, TVI = Taal Volcano Island, BHS = Balantok Hot Spring. Digital elevation model was provided by National Mapping and Resource Information Authority (Philippines) through PHIVOLCS based on 2013 Interferometric Synthetic Aperture Radar (IFSAR) data.

Increased seismicity, ground deformation and temperature anomalies have first been described for a period of non-eruptive unrest in 1952 (Torres et al. 1995). From 1991 to 1994, Taal volcano went through a series of seismic crises characterised by a dramatic increase in volcano-tectonic (VT) events, of which several were felt by local residents, combined with episodes of strong uplift in 1994 (Global Volcanism Program 1994; Solidum et al. 2011). Fissures of up to 1 km long opened at the north and southeast sides of TVI in 1992 and 1994 (Global Volcanism Program 1994; Gabinete 1999; Sabit 2010). The Main Crater Lake (MCL) temperature rose gradually from 27 °C in early 1991 to 39 °C in the middle of 1994 (Poussielgue 1998). Evacuation of TVI was ordered twice during this period, once in 1992 and again in 1994.

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21 earthquakes per day and geysering activity. The source of the inflation was modelled at 4 to 5 km depth below the main crater for both episodes. Another period of self-potential, total magnetic field, ground temperature and soil CO2 concentration anomalies coincides with a

seismic swarm between September 2004 and February 2005, with most of the activity concentrated in the solfatara area of the Main Crater and near the summit of the northern flank (PHIVOLCS 2004; Zlotnicki et al. 2009b; Sabit 2010). Inflation was also observed from June 2004 to February 2005 and point-source model inversion by Galgana et al. (2014) indicated a depth of 5 km below the main crater.

The last time PHIVOLCS raised the alert level for Taal was during a seismic crisis from December 2010 to March 2011, with a strong increase in long-period (LP) seismic events from December 2010 to March 2011, followed by an increase in shallow VT events from April 2011 to August 2011. The LP events during this crisis are thought to be caused by the contact of hot vapours traveling upwards through fissures at the northern side of MCL with cold aquifers (Maeda et al. 2013). Carbon dioxide emitted by the MCL increased by four to ten times the baseline level during 5 measurements between August 2010 and June 2011 and SO2/H2S ratios

in fumaroles increased from 0.45 in 2009 to 4.5 and 5.14 in 2010 and 2011 (Arpa et al. 2013). Geothermal activity on the northern flank of TVI increased in 2010, according to self potential and ground temperature surveys (Zlotnicki et al. 2017). Most recently, a period of uplift between December 2014 and May 2015 was followed by VT swarms (Sayco et al. 2017). During this time, carbon dioxide partial pressure in MCL waters rose to above 0.25 atm, the highest since the beginning of continuous monitoring in 2013 (Bernard et al. 2017).

The nature of all these unrests is poorly understood. Several authors consider magmatic intrusions to be the cause of increased activity at Taal volcano (Bartel et al. 2003; Maeda et al. 2013; Arpa et al. 2013; Galgana et al. 2014; Kumagai et al. 2014; Hernández et al. 2017). On the contrary, others have considered changes in the hydrothermal system (Lowry et al. 2001; Zlotnicki et al. 2009b).

Self-potential (SP), total magnetic field (TMF), temperature and soil CO2 anomalies have been

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crater, and are interpreted as areas where the hydrothermal system is active near the surface. An electrical resistivity tomography (ERT) study by Fikos et al. (2012) confirmed the earlier studies and revealed a pathway for upward migration of hydrothermal fluids near the northern side of MCL. Other parts of the volcanic edifice, including the area right under MCL, are characterised by high resistivity and were interpreted as dense rock impermeable to hydrothermal fluids.

Alanis et al. (2013) and Yamaya et al. (2013) have confirmed the existence of a hydrothermal reservoir thanks to magnetotelluric observations and placed the seal rock up to 1 km depth and the hydrothermal reservoir between 1 and 4 km deep. A zone of low seismic velocity (low-Q) was found in 1993 and 2011-2013 up to 2 km depth under the MCL (Nishigami et al. 1994; Kumagai et al. 2014). Whereas Nishigami et al. (1994) interpret this area as being the hydrothermal reservoir, Kumagai et al. (2014) interpret this zone to be an actively degassing magma body directly underneath the MCL. A seismic reflector is present at 6 km depth and is interpreted as the top of the magma reservoir by Nishigami et al. (1994). The partially melted zone and deep magma origin for Taal volcano is estimated at 18 km depth based on a shear-wave velocity model (Besana et al. 1995).

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23 incorporate data from Delmelle et al. (1998), to complement the data between 1996 and 2017 published here and to study the long-term evolution of the hydrothermal system of Taal volcano.

2.4 Sampling and analytical methods

Routine water sampling consists of two samples for anion and cation analysis, in 50 to 100 ml low-density polyethylene (LDPE) bottles. For samples with pH exceeding 4.5, a third sample is taken for in situ alkalinity titration. Samples for sulphur and strontium isotopic analysis are stored in larger 250 or 500 ml LDPE bottles. MCL samples are taken in the middle area of the lake, and spring samples as close to the outlet as possible. Vertical conductivity-temperature-depth profiles of MCL, using a SeaCAT SBE 19plus profiler, were regularly obtained since 2013 and do not show significant variations in conductivity, temperature or pH with depth, except for a thin layer at the top that is affected by solar heating. Sampling rate is 4 Hz and measurements are performed while the instrument goes down as well as up, in order to evaluate accuracy of the measurements. An example is shown in Figure 2.2. Variations are very minor, indicating MCL is entirely mixed and water composition at the surface is representative for the entire water body. pH, temperature and conductivity are measured in situ with a WTW© portable multimeter, in combination with a thermocouple for higher temperatures. The pH electrode is calibrated with standard solutions of pH 1.00, 4.00 and 7.00. Alkalinity is measured by Gran titration with HCl and expressed as HCO3-.

Major element analysis is performed by ion chromatography (for Cl, SO4, F, Br, Na, K, Mg,

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Figure 2.2: Temperature and specific conductance of MCL vs depth on January 28th_{, 2014.}

Both downcast and upcast are represented, giving two separate profiles. No significant variations can be seen in the water column excluding stratification of MCL.

For strontium isotope measurements, 100 ml of each water sample was evaporated and the residue was dissolved in successive steps with intermediary evaporation using (1) concentrated HNO3/HF (ratio 4:1), (2) 6 M HCl and (3) 25 % HNO3. For rock samples, around 100 mg of

sample was crushed in an agate mortar and dissolved in concentrated HNO3/HF (ratio 3:1) and,

after evaporation, in 6 M HCl. Small aliquots of the initial dissolutions were diluted in 5 % HNO3 for trace element analysis (Sr but also Rb, Ba, Pb, U, REE, Hf, Zr, Nb, Ta,Th) using an

Agilent 7700 Quadrupole ICP-MS. For isotope measurements, Sr was then purified using a Sr SpecTM_{chromatographic resin. The sample was loaded onto the column with 2 x 1 ml 2 M}

HNO3. The column was then rinsed with 2 x 1 ml 2 M HNO3, 3 x 1 ml 7 M HNO3 and 1 ml 3

M HNO3. Finally, the sample was collected using 2 x 1 ml 0.05 M HNO3. The separation

procedure was repeated twice to ensure maximum effectiveness. Strontium isotope ratios were determined on a Nu Plasma multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Results are expressed as 87_Sr/86_{Sr and for each sample a 2SE confidence}

interval was calculated.

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25 Sulphate in samples for sulphur isotopic analysis was converted to BaSO4 according to the

procedure described in Révész et al. (2012). Isotopic ratios were then measured using an EA-IRMS, where the BaSO4 is combusted and resulting SO2 is separated by gas chromatography

in an elemental analyser (EA) before measurement of isotopic ratios by isotope ratio mass spectrometry (IRMS). Sulphur isotope ratios are expressed in the delta notation relative to Vienna-Cañon Diablo Troilite (V-CDT). Standard deviation of repeated analysis on 2 standards is less than 0.2 ‰.

Twenty-two fresh- and altered-looking rock samples from various places within the Solfatara area and the islet were analysed for bulk mineralogy, as well as a layer rich in altered lithics from recent pyroclastic deposits along the Daang Kastila trail on the north side of the crater and sediments from the MCL lake floor that got trapped in our instruments. Analysis was performed on powdered samples via X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray spectrometry (EDS). XRD spectra were analysed using DIFFRAC.EVA software.

2.5 Results

2.5.1 Major element geochemistry

Results of chemical analysis of MCL, hot spring and Lake Taal waters are listed in Table 2.1. The data acquired in the present study is consistent with earlier data published by Delmelle et al. (1998). Major element analyses are available from 1991 until 2017. Main Crater Lake waters are acidic, with pH between 2.2 and 3.1. Dominant anions are Cl and SO4 in a ratio of about 5

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Main Crater Lake compositions have changed a bit over the years, and some elements show a distinctive long-term trend (Figure 2.3). Whereas the MCL concentrations of Cl, Na and K have not significantly evolved in 25 years, others have decreased considerably, including SO4,

Mg, Fe and Al, while pH has increased. The difference in behaviour between these two groups of elements is consistent with mixing of two endmembers with different geochemical signatures for which the proportions change over time, one rich in chloride, sodium and potassium and another associated with sulphate-rich acidic fluids. Leaching and precipitation of Mg-, Fe- and Al-rich minerals depend on pH so the concentration differences in these elements might be due to pH changes. This supports the findings of Delmelle et al. (1998) that the hydrothermal waters feeding MCL result from a mixture of at least two components, one made of sodium-potassium-chloride-rich fluids and one of acidic sulphate-dominated fluids.

A significant deviation from this long-term trend can be seen in October 1995, after the 1991-1994 unrest (Figure 2.3). Temperature at the time was at its highest at 37.0 °C, pH at its lowest at 2.2. Concentrations of Cl, Na and K decreased significantly and SiO2, Al and Fe increased.

Fluoride was well above the detection limit in 1995 whereas it has not been detected in 1992 or before, or after 1995. All these parameters indicate a dramatically increased influx of warm acidic fluids into the MCL, diluting the chloride-rich component. Main Crater Lake geochemistry was back to background values in April 1996. No significant short-term changes in geochemistry have been observed since 1995.

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Hot springs can be found in two different areas. One spring is situated on Balantok Beach near Alas-As village on the northwestern shore of TVI adjacent to Lake Taal (BHS in Figure 2.1). Several hot springs can be found inside the crater near the MCL shores, on the northeast and the southwest sides in zones strongly affected by hydrothermal alteration (Solfataras in Figure 2.1). Flow rates are usually low and springs often dry up. The presence of sublacustrine springs in the Main Crater Lake is evident from visible zones of upwelling near the north-eastern shores, from an area with a distinct colour change from dark green to milky blue near the south-western Solfatara and from the presence of positive thermal anomalies (Bayani Cardenas et al. 2012). Furthermore, the exceptionally high Cl content of MCL indicates a significant input of hydrothermal waters since Cl is not effectively transported in hydrothermal vapour (Symonds et al. 2001).

The chemical composition of the different hot springs varies among them. All hot springs have either similar concentrations than MCL samples or are more diluted. Delmelle et al. (1998) identified two solfatara hot springs in the 1990s with a high SO4/Cl ratio as steam-heated, these

were no longer found in recent sampling missions. All other springs in the Solfatara area are Na-Cl-SO4 dominated, like MCL. Balantok is the only hot spring with significant bicarbonate

content. Lake Taal waters are slightly alkaline (pH between 8 and 9) and of the Na-Cl-HCO3

type with considerable sulphate.

2.5.2 Trace element geochemistry

Trace element concentrations in one lava sample, one Lake Taal sample and 12 MCL samples between 2003 and 2016 are reported in Table 2.2.

Rare Earth Elements (REE) are visualised in a spider diagram in Figure 2.4. Samples are normalised to lava from the 1965 lava flow to study the effects of water/rock interaction in the hydrothermal reservoir. In this type of diagram, horizontal lines represent simple congruent dissolution of host rocks, while more complicated patterns appear in case of preferential leaching of certain elements. Water/rock interaction typically enriches the fluids in light rare earth elements (LREE) which are more mobile than heavy rare earth elements (Kogiso et al. 1997). Main Crater Lake samples are however depleted in LREE (La to Sm) compared to lava, and follow a rather horizontal trend for the heavier REE.

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35 Figure 2.4: Spider diagram for rare earth elements for MCL and Lake Taal samples. REE concentrations are normalised to 1965 lava flow. Seawater data from Elderfield and Greaves (1982)

2.5.3 Sulphur isotopes

The δ34_{S isotopic composition of sulphate in MCL waters has increased slightly from around}

9.8-10.8 ‰ in 1992 to +12.4 ‰ in 2016, with 1995 samples being slightly lower at 9.4 and 9.6 (Figure 2.5). Lake Taal composition has remained constant at around +16.9 to +18.4 ‰, very similar to Balantok hot spring (16.9 ‰) at a couple of meters from the Lake Taal shoreline. However, in the hot springs in the Solfatara area, the δ34_{S varies strongly from -3.6 to +15.5}

‰. This wide range of δ34_{S is due to samples from 1991 and 1995 when many hot springs were}

present in the Solfatara area. More recent field surveys showed that only a few hot springs remain and the sulphur isotopic composition only ranges from +12.6 to +14.5 ‰.

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Figure 2.5: Sulphur isotopic evolution of sulphate in Taal samples over time. Red bands represent unrests. HSB = Hot Spring Balantok, HSS = Hot Spring Solfatara, LT = Lake Taal, MCL = Main Crater Lake.

2.5.4 Strontium isotopes

Strontium isotopes can be used as tracers for Sr-containing fluids, because different sources have different affinities for 86_{Sr and}87_{Sr. Strontium isotopic data for MCL and Lake Taal are}

plotted in Figure 2.6, alongside literature data for Taal rocks and seawater. Local rocks analysed by Mukasa et al. (1994) include samples from TVI as well as from the caldera wall and range in age from 1969 to pre-caldera. The 1969 lava flow that was analysed in this study falls within the range of Sr isotopic ratios found by Mukasa et al. (1994). Seawater isotopic ratios were measured by Hess et al. (1986) on modern samples and on well-preserved fossil foraminifera from the Deep Sea Drilling Project cores for prehistoric samples. Samples up to 600 ka were considered for comparison, which is well before caldera formation. All MCL samples plot near the range of local rocks. Lake Taal however is more enriched in heavy Sr than MCL and has a Sr isotopic ratio intermediary between local rocks and seawater.

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37 Figure 2.6: 87_Sr/86_{Sr ratio of different Taal samples, including MCL and Lake Taal samples,}

rock samples (Mukasa et al. 1994, this study) and seawater data for the last 600 ka (Hess et al. 1986). Error bars are smaller than symbol size for all data except Lake Taal.

2.5.5 Alteration mineralogy

The degree of alteration of the rocks sampled for this study varies strongly between samples. All alteration minerals are summarised in Table 2.3. Fresh lavas and pyroclastics contain plagioclase and augite, as well as volcanic glass. More altered samples are typically very fine-grained and predominant colours are white, dark red and greenish. Amorphous silica and sulphate minerals (alunite, gypsum, anhydrite, jarosite and alunogen) are the most dominant alteration phases. Native sulphur, pyrite and kaolinite were found in one or two samples each. Altered lithics are especially interesting for this study because they represent the rocks that have been directly sampled from the hydrothermal system during eruptions. They contain pyrite, alunite and anhydrite. Lake bottom sediments are made up of amorphous silica, pyrite and amorphous iron hydroxides.

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Primary Minerals (Solfatara)

Plagioclase (Na,Ca)(Si,Al)4O8

Augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6

Volcanic glass

Secondary Minerals (Solfatara)

Gypsum CaSO4•2(H2O)

Anhydrite CaSO4

Jarosite KFe3(SO4)2(OH)6

Alunite KAl3(SO4)2(OH)6

Sulfur S0

Amorphous Silica SiO2 (a)

Amorphous iron hydroxides

Kaolinite Al2Si2O5(OH)4 Alunogen Al2(SO4)3·17H2O Pyrite FeS2 Altered lithics Pyrite FeS2 Anhydrite CaSO4

Alunite KAl3(SO4)2(OH)6

MCL Sediments

Amorphous silica SiO2 (a)

Pyrite FeS2

Amorphous iron hydroxides

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39

2.6 Discussion

2.6.1 Origin of hydrothermal fluids

Based on different evolution of Na, K and Cl concentration in MCL in comparison to other elements like H+_{, SO}₄_{, Mg, Fe, Al we have concluded that the MCL composition is the result}

of varying mixing proportions between two types of fluid, one being neutral waters rich in chloride, sodium and potassium, and one being acidic and sulphate-rich. In order to identify the origin of these elements, every component is studied in more detail.

A Na + K versus Cl diagram (Figure 2.7a) for all Taal samples shows a linear trend between chloride concentration and the sum of sodium and potassium concentrations passing through the origin, which is typical for a dilution trend between waters rich in Na, K and Cl with meteoric waters.

Furthermore, all Taal samples plot near the electrical balance line, which represents a theoretical electrically neutral fluid containing only Na, K and Cl, meaning concentrations of other elements in the parent fluid are likely to be low. This applies to H+_{ions as well, indicating}

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Figure 2.7: Na, K, Cl plots for Taal samples and seawater. (a) Na+K vs Cl plot. All samples lie near a trend whereby chloride is balanced by sodium and potassium along the electrical balance line. (b) K vs Na plot. All Taal samples follow a linear trend, which indicates a fixed Na/K ratio. Dashed line is a regression line for all Taal samples, excluding seawater. Seawater is much more enriched in Na than Taal samples. HSB = Hot Spring Balantok, HSS = Hot Spring Solfatara, LT = Lake Taal, MCL = Main Crater Lake, SW = Seawater.

This hypothesis is supported by strontium isotopes and REE. All MCL samples exhibit

87_Sr/86_{Sr ratios similar to local rocks and plot far from seawater (Figure 2.6), demonstrating Sr}

in Taal hydrothermal system is derived from interaction with local rocks and not from seawater. Lake Taal however shows higher Sr isotope ratios. This could imply the involvement of seawater, likely when a channel between Lake Taal and Balayan Bay existed in the eighteenth century (Murillo Velarde 1734; Bellin 1752; Herre 1927). The REE pattern of MCL samples is distinctly different from the REE pattern in seawater. REE are usually trivalent, although Ce and Eu can also occur as +II or +IV, respectively. Seawater typically has negative Ce and Eu anomalies compared to lavas, caused by solubility differences between the different redox

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41 states of Ce and Eu (Elderfield and Greaves 1982). No such anomalies can be seen when MCL waters are normalised to local lavas, implying the REE are likely originating from local rocks. Solubility of REE depends strongly on pH with high solubilities in acidic conditions and low solubilities in near neutral conditions (Michard 1989; Lewis et al. 1997; Takano et al. 2004; Inguaggiato et al. 2015). Therefore, the REE are likely associated with the acidic sulphate-rich component.

In Figure 2.8, δ34_{S is plotted in function of the weight fraction of SO}₄_{to major anions. This}

type of plot is used to study isotope fractionation between the chloride-rich and the sulphate-rich endmembers. Two Solfatara hot spring samples from the early 1990s are very sulphate-rich in sulphate and have very low δ34_{S and low pH, typical for depleted H}₂_{S that has either condensed}

or oxidised in shallow meteoric waters (Ellis and Mahon 1977; Kusakabe et al. 2000; Mazot et al. 2008; Marini et al. 2011; Delmelle and Bernard 2015). MCL and other Solfatara hot springs are aligned, with high Cl content associated with high δ34_{S and SO}₄_{-rich samples that are more}

depleted (Figure 2.8). This behaviour is consistent with mixing between a neutral chloride component with heavy sulphate and an acidic sulphate component with lighter sulphate. Delmelle et al. (1998) interpreted the high isotopic signature of chloride waters as evidence for incorporation of seawater, which typically has a sulphur isotopic composition of around 20 ‰ and has often been proposed as enriched sulphur source in hydrothermal systems in coastal areas in Japan and Iceland (Sakai and Matsubaya 1977; Sakai et al. 1980; Gunnarsson-Robin et al. 2017). However, the sulphur isotopic composition of the chloride fluids is also consistent with mature geothermal waters. These fluids typically occur in volcanic systems when acidic sulphate-rich volcanic fluids undergo extensive fluid/rock interaction, progressively neutralise and equilibrate with the alteration assemblage of the host rock. They are usually rich in Na, K and Cl with very low concentrations of Ca and Mg and can be sampled from distal springs on the flanks of volcanoes or from deep geothermal wells (White 1957; Ellis and Mahon 1977; Giggenbach 1988; Giggenbach 1997; Peiffer et al. 2015). Sulphur isotopic composition of geothermal waters represents a temperature-dependent equilibrium between H2S and SO4, with

H2S usually around 0 ‰ and SO4 being more enriched. The fractionation factor increases with

decreasing reservoir temperature (Ohmoto and Lasaga 1982; Marini et al. 2011). Bayon and Ferrer (2005) analysed well fluids and minerals from four different geothermal fields in the Philippines (Mt. Apo, Mindanao; Palinpinon, Negros; Mahanagdong, Leyte and Bacon-Manito, Luzon), and found that δ34_{S of deep well waters was typically between +15 and +25}

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in chloride have been obtained from Yellowstone, USA and drillholes at Wairakei and Tokaanu-Waihi geothermal areas, New Zealand (Steiner and Rafter 1966; Truesdell et al. 1978; Robinson and Sheppard 1986) although none of these systems are particularly close to the coast.

Figure 2.8: Sulphur isotopic ratio in function of SO4 concentration as a percentage of major

anions (SO4+Cl in mg/kg). Grey rectangle represents data range for mature geothermal waters

(pH>6 and Cl>>SO4) from the Philippines (Bayon and Ferrer 2005), grey circle represents volcanic water as suggested by Delmelle et al. (1998). HSB = Hot Spring Balantok, HSS = Hot Spring Solfatara, LT = Lake Taal, MCL = Main Crater Lake, SW = Seawater.

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43 on rock dissolution alone, suggesting that Na and K are added to the system after mixing with mature geothermal waters. Other elements, including Al, Fe, Si and Ca, lie below the 20 g/l line, implying these are either less easily leached from the host rock than Mg and Mn, or have precipitated as secondary mineral phases. Alteration minerals such as alunite, jarosite, amorphous silica, gypsum, anhydrite and pyrite are common within the Taal main crater altered rocks and could explain the low concentration of these elements in MCL waters.

Figure 2.9: Log-log compositional diagram of rock forming elements in host rock and MCL fluids.

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Figure 2.10: Silica concentration vs temperature diagram. Dashed line is the equilibrium solubility curve for amorphous silica (Fournier 1977). HSB=Hot Spring Balantok, HSS=Hot Spring Solfatara, LT=Lake Taal, MCL=Main Crater Lake.

In volcanoes with a large hydrothermal system like Taal, magmatic SO2 is typically

transformed to other sulphur species when it condenses into the hydrothermal reservoir according to the following disproportionation reactions:

4 SO2(g) + 4 H2O(g,l) → 3 SO42−(aq) + 6 H+(aq) + H2S(g)

3 SO2(g) + 2 H2O(g,l) → 2 SO42−(aq) + 4 H+(aq) + S(l,s)

During this process, sulphate is typically enriched in 34_{S and hydrogen sulphide and elemental}

sulphur are depleted (Kusakabe et al. 2000; Marini et al. 2011; Delmelle and Bernard 2015). The more enriched sulphate is typically trapped in volcanic waters. In Taal however, the volcanic water component is significantly more depleted than geothermal waters, having a maximum δ34_{S of about +7 ‰, according to Figure 2.8. The isotopic composition of magmatic}

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45 A possible explanation for the apparent link between pH and sulphur isotopic fractionation could be the presence of sulphur-oxidising bacteria like Thiobacillus thiooxidans. These have been identified in several moderately acidic crater lakes (Ivanov and Karavaiko 1966; Takano et al. 1997) and optimal conditions for bacteria growth are pH between 2 and 3.5 and temperatures between 28 and 30 °C (Takano et al. 1997), which makes MCL an ideal environment. They do not survive in conditions where pH is lower than 1 (Takano et al. 1997). When 34_{S-depleted H}₂_{S, formed by disproportionation of SO}₂_{, is oxidised near the surface, the}

net effect on sulphur isotopic fractionation of these volcanic waters would be negligible. The lack of an equilibrium between heavy aqueous sulphate and light sulphur in the hydrothermal system makes it impossible to use sulphur isotopes as a geothermometer, as proposed by Kusakabe et al. (2000).

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Concentration of REE is relatively low in MCL fluids. Whereas the dissolution rate based on major elements is around 20 g of host rock per litre fluid (Figure 2.9), this decreases to on average around 50 mg/l for LREE and 150 mg/l for HREE, meaning REE concentration is over 100 times lower than expected based on isochemical rock dissolution. It is therefore suspected that REE are preferentially incorporated in the alteration mineral assemblage, much like Fe, Al and Si for major elements.

2.6.2 Temperature of the Taal hydrothermal system

The temperature of hydrothermal reservoirs is commonly determined via geothermometers based on thermodynamic equilibrium between dissolved species and minerals or gases. However, in the case of Taal where the hydrothermal reservoir consists of a mixture between two components, equilibrium is not necessarily obtained. Fluids are acidic and have therefore not attained equilibrium with the host rock mineral assemblage, and sulphur species originate from different reservoirs. Therefore, fractionation between light and heavy isotopes is not necessarily due to an equilibrium in the fluid. Nevertheless, Na and K are thought to originate from the same geothermal reservoir and the Na/K ratio is similar for all MCL and Solfatara samples, excluding the two steam-heated hot springs. Even though water/rock interaction once the hydrothermal waters are mixed might change the Na/K ratio due to alunite and jarosite precipitation, a rough estimation of the geothermal reservoir temperature based on the Na/K ratio, using the relationship defined by Giggenbach (1988), is around 260 °C.

Europium has two different oxidation states and its behaviour can therefore vary from the other REEs in situations where divalent Eu is stable rather than the trivalent kind. For MCL fluids, no Eu anomaly is present which indicates that the Eu in the hydrothermal system of Taal volcano is in the trivalent state like other REEs. A study by Sverjensky (1984) describes the strong temperature dependence of Eu oxidation state, using 250 °C as an upper limit for stability of Eu3+_{in hydrothermal systems. The absence of an Eu anomaly in Taal MCL fluids}