Noble and reactive gases of Palinpinon geothermal field (Philippines): Origin, reservoir processes and geodynamic implications

Chemical Geology 339 (2013) 4–15

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Noble and reactive gases of Palinpinon geothermal field (Philippines): Origin, reservoir processes and geodynamic implications Gabriella Magro a, Fabrizio Gherardi a,⁎, Francis Edward B. Bayon b a b

Istituto di Geoscienze e Georisorse (IGG), Consiglio Nazionale delle Ricerche (CNR), Via Moruzzi 1, 56124, Pisa, Italy Energy Development Corporation (EDC), Ortigas Business Center, Philippines

a r t i c l e

i n f o

Article history: Accepted 23 September 2012 Available online 1 October 2012 Keywords: Gas geochemistry Noble gases He isotopes Palinpinon geothermal field Philippines

a b s t r a c t Palinpinon is a high-temperature, liquid-dominated volcano-geothermal system located on southern Negros Island, Philippines, associated with subduction of Negros-Sulu arc (Early Pliocene to Recent). In 2001, eleven (11) producing wells of the Palinpinon geothermal field were analyzed for major gas components and for noble gases isotopic composition. Geothermal gases are dominated by H2O, with CO2 and H2S being the most abundant species of the dry fraction. Chemical and isotope data indicate that two main components feed the geothermal system: (i) a deep component, enriched in CO2, H2S, H2 and He, related to volcanohydrothermal interactions occurring in the roots of the geothermal system, and (ii) a surficial component, enriched in N2, Ar, Ne, related to natural meteoric recharge of the reservoir. The noble gas fraction is dominated by argon of atmospheric origin, as denoted by 40Ar/36Ar ratios between 295 and 310. Helium, in excess above the reference concentrations in air and air-saturated water (ASW), has an isotopic signature (3He/4He ratios between 6.96 to 7.94 RA) in the range of values normally observed for subduction-related volcanism. 3 He/ 4He and CO2/ 3He (between 12.1 × 10 9 to 28.7 × 109) ratios support the hypothesis that most of the deep gases are directly derived from a magmatic source and/or from the scavenging of an organic-depleted, basalt-rich crust. Water–rock interactions cause some geothermal overprinting of the deep magmatic component, allowing redox conditions in the reservoir to be controlled by the Fe(II)–Fe(III) buffer. Based on CO2/CH4 and H2/Ar ratios, maximum equilibrium temperatures between 300 and 350 °C have been estimated in the geothermal reservoir. Chemical data indicate that the geothermal reservoir is largely flushed by steam derived from the boiling of waters of meteoric recharge and reinjected brines. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Located in southern part of the Negros Island, Philippines, the Palinpinon high-enthalpy geothermal resource consists of two main producing sectors, Palinpinon I and Palinpinon II. The Negros Island is dominated by two andesitic complexes, the Mt. Silay–Mt. Mandalagan volcanic complex to the north, and the Cuernos de Negros volcanic complex to the south. The Palinpinon geothermal field is located on the northern slopes of the Cuernos de Negros volcanic complex (Fig. 1). The geothermal area has been exploited for production of electricity since 1983 (Palinpinon I; current installed capacity 112.5 MWe), and successive improvements during the early 1990s led to the development of new modular plants in the Palinpinon II sector (total installed capacity 80 MWe). To date, a total of 73 wells, 48 producers and 25 reinjectors, have been drilled in the geothermal area (Pamatian et al., 2003).

⁎ Corresponding author. E-mail address: [email protected] (F. Gherardi). 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2012.09.036

The field is divided in three geographical areas: the Puhagan sector on the east, hosting the Palinpinon I producing area, the Nasuji and Sogongon sectors on the west, hosting the Palinpinon II (Fig. 2). Reservoir temperatures are generally between about 200 and 300 °C, but locally exceed 325 °C. Geothermal brines are neutral-pH, and moderately saline (reservoir chloride between 4000 to 5000 ppm), with low gas content (generally below 0.1 mol%). The main upflow area of the deep parent geothermal fluid is located close to Lagunao, southwest and east of Puhagan and Nasuji sectors, respectively. The main outflow is along the fault-controlled Okoy Valley, northeastward from the upflow. Some secondary outflow occurs towards Nasuji and Sogongon sectors, on the northwest of Lagunao (Fig. 2). Along both outflow sectors, fluid temperatures and Cl concentrations decline due to dilution with groundwater (D'Amore et al., 1993). Surface exploration and early development of the geothermal field have been summarized by Maunder et al. (1982) and Bromley and Española (1982). Fluid reinjection at Palinpinon started in parallel with production, and Harper and Jordan (1985) assessed geochemical changes induced by early exploitation of the field. Fluid circulation paths in the reservoir were reconstructed on the basis of tracer tests

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Fig. 1. Map of Philippines showing the tectonic setting, the Philippine fault zone (Ph.f), and the locations of Palinpinon (yellow circle) and Bacon–Manito (red circle) geothermal fields. Also shown are the locations of other geothermal and volcano-hydrothermal systems (white circles) investigated by Giggenbach and Poreda (1993), and of major magmatic arcs. Modified after Yumul et al. (2008). CA = Cagua; MP = Mount Pinatubo; TA = Taal; BI = Biliran; M = Mambucal; AP = AltoPeak; H = Mahagnao; A = Mount Apo; TL = Matinloc; NI = Mount Nido; LO = Lourdes; LC = Santa Lucia.

(Urbino et al., 1986) and preliminary numerical modeling studies (Sta. Ana and O'Sullivan, 1988). Based on stable isotope evidence, the geothermal reservoir is fed by a deep parent water originating from a mixture of local meteoric (80%) and magmatic (20%) water, with meteoric recharge derived from ~ 1000 m (Gerardo et al., 1993). Stable isotope studies also provide evidence of injection return in the reservoir. Due to exploitation and depressurization, boiling has developed within the reservoir, and currently most of the producing wells are discharging high-enthalpy fluids, i.e. fluids with measured enthalpy in excess compared to that of steam saturated water at aquifer temperature. D'Amore et al. (1993) used gas geochemistry to investigate physical processes due to exploitation from 1985 to 1991: (i) pressure drawdown in the southern part of the field, with local increase in reservoir vapor fraction; (ii) vapor loss from an original liquid phase during the fluid ascent through fractures; (iii) mixing and cooling due to injection returns located in the northeastern part of the field. The pressure differential between production and injection sectors caused injected brine to enter the production zones. The Lagunao, Ticala, Puhagan and Nasuwa faults (Fig. 2) have been identified as preferential paths for brine returns (e.g. Rae et al., 2004). The return of reinjected brines has been documented since the beginning of the exploitation both in Palinpinon I (Seastres et al., 1995), and in Palinpinon II (Ramos-Candelaria et al., 1997). In order to attain and maintain stable reservoir pressures (approximately steadily decreased from 7 to 5 MPa, depending on the sectors of the field, during the 1992–2010 period), the

reinjection strategy has been changed over time in terms of both location of the reinjection wells and rates of reinjection (Malate and Aqui, 2010; and references therein). Production and brine injection rates ranged between ~ 450 and ~ 700 kg s − 1 and between 0 and 150 kg s − 1, respectively, during the 1992–2010 period, with a minimum of less than 200 and of 0 kg s − 1, respectively, between June 1997 and June 1998 (Malate and Aqui, 2010). In the framework of a technical co-operation project funded by the International Atomic Energy Agency (IAEA), in 2001 eleven (11) production wells of the Palinpinon geothermal field have been analyzed for major gas components and for chemical and isotopic composition of noble gases. At the time of this sampling campaign, most of the injection load (about 80 kg s −1) was concentrated in the peripheral sectors of Ticala and Malaunay, away from the central production sector of Puhagan (Fig. 2). Steam production (about 600 kg s −1) was occurring from an expanded two-phase region extending over most of the geothermal field, although wells discharging single-phase gaseous fluid were also present. This study is aimed at combining reactive and inert gas data for the evaluation of fluid origin and the assessment of main physical and chemical processes governing the geochemical evolution of the Palinpinon geothermal field. By comparing noble gas data from Palinpinon with data from other geothermal and volcano-hydrothermal features along the Philippines arc (Giggenbach and Poreda, 1993; Bayon et al., 2008), the geodynamical setting of the geothermal systems is also discussed.

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Fig. 2. a: map of the Palinpinon Geothermal field, with location of production (red) and injection (blue) wells sampled during the 2000–2010 period. The main active faults are represented by the thick dashed red lines. Outflow directions are indicated by orange arrows. b: sketch SW-NE geological section of the geothermal field (unpublished PNOC/ EDC internal reports). Main features are: recent domes and intrusive bodies (brown areas, 1); recent acid alteration (green areas, 2); relict acid alterations (purple areas, 3); Nasuji pluton (gray to orange area, 4). SG-2 and OK-5 are production wells; OK-3R is an injection well.

2. Geological framework The Philippines Archipelago is a complex assemblage of island arcs located between two major tectonic plates, the Pacific and the Eurasian plates. The opposing subduction zones generated a discontinuous belt of Pliocene to Quaternary volcanoes, which extends from Luzon Island in the north to Mindanao Island in the south. The Philippine fault zone cuts the archipelago from Luzon through eastern Mindanao (Fig. 1). Seismic activity along the fault indicates left-lateral strike–slip motion; it has been estimated that the central segment of the fault has been active since Pliocene, resulting in a total displacement between 40 and 100 km (Barrier et al., 1991). Recent estimates indicate that crustal thickness beneath the Philippines ranges between 17 and 30 km, with maximum values corresponding to the

central portion of the Luzon Island and the Bicol–Masbate–Panay– Central Mindanao area (Dimalanta and Yumul, 2006). The thermal anomaly of the Palinpinon geothermal field is related to the last episodes of magmatism coincident with the commencement of subduction at Negros-Sulu arc, in Early Pliocene time. Main features of magmatism are represented by the Puhagan dikes and the Nasuji Pluton that intruded most of the overlying volcaniclastic formations. The area of highest thermal anomaly is located near the Lagunao–Puhagan sector (Fig. 2; Rae et al., 2004, 2011; and references therein). The geology of Palinpinon, along with whole rock geochemistry, radiometric and paleontological ages, has been documented by Philippine National Oil Company—Energy Development Corporation (PNOC-EDC) scientists and consultants in unpublished company reports and files,

G. Magro et al. / Chemical Geology 339 (2013) 4–15

and summarized by Seastres (1982), Rae et al. (2003, 2004, 2011). The local stratigraphic column comprises: – Cuernos Volcanics (CV) — extrusive rocks of Quaternary age represent the youngest stratigraphic unit in Palinpinon (thicknessb 620 m). Basaltic andesites and andesites of the CV are related to subduction along the Negros-Sulu arc (Early Pliocene to Recent); – Southern Negros Formation (SNF) —– moderately to intensely altered andesite lavas, hydrothermally-altered tuffs and lahars of Pliocene to Pleistocene age (thickness between 1500 to 2000 m); – Okoy Sedimentary Formation (OSF) — thick sequence of interbedded calcareous sedimentary rocks and volcanic breccias of Late Miocene to Early Pliocene age, present in the north-eastern portion of the geothermal field (thickness>1000 m); – Puhagan Volcanoclastic Formation (PVF) — altered andesitic breccias of Middle Miocene age (thickness > 800 m); this formation is absent in northwestern part of the field, where the Nasuji Pluton (NP) is located, – Nasuji Pluton (NP) — quartz monzodiorite body (maximum thickness 1410 m) of Middle Pleistocene age (40Ar/39Ar plateau age of 0.54± 0.09 Ma; Rae et al., 2004), enveloped by a contact metamorphic aureole (Contact Zone; maximum thickness 505 m; secondary mineralogy dominated by plagioclase, biotite, actinolite, magnetite, epidote, albite); – Puhagan dikes — dioritic dikes of Early Pliocene age (39Ar/40Ar plateau age of 4.17± 0.05 Ma; Rae et al., 2004) intruding OSF and PVF in the Puhagan area, in proximity to the intersection of the NW trending Lagunao Fault and NE-trending Ticala Fault.

3. Sampling and results Gas samples of reservoir fluids were obtained from production wells in different sectors of the field (Fig. 2). Wells OK-5, OK-9D, BL-3D and LG-3D represent the main upflow zone, located southwest of Puhagan area, close to Lagunao sector. Wells OK-2, PN-15D and PN-22D are located in the central Puhagan sector, a two-phase zone extending at relatively shallow depths between −400 and +100 m s.l. (Vidal and Hermoso, 2000). Wells NJ-5D, NJ-8D, SG-2 and SG-3D are located in the Nasuji and Sogongon sectors, with well SG-3D located in the westernmost periphery of the field. Four wells, BL3D, OK2, SG2 and PN15D, were discharging a single gaseous phase at the time of sampling. Based on water chemistry evidence (Cl concentrations), production data, and tracer analysis, well NJ5D fluids are influenced by the rapid return of hot injected brines (e.g. Bayon and Ogena, 2005; Malate and Aqui, 2010; Maturgo et al., 2010; and references therein). Steam samples were collected from the pipelines by means of a small, portable (Webre) separator. Major gases were collected in evacuated glass flasks containing alkaline solution (4N NaOH; Giggenbach

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and Goguel, 1989). Noble gases were collected in a copper tube crimped using a cold-welder sealing device. Gas analyses were performed at the chemistry laboratories of PNOC-EDC, now Energy Development Corporation (EDC) using wet chemistry (H2O, CO2 and H2S), and gas chromatography (CH4, H2, N2, O2, Ar and He). These analyses are reported in Table 1. Typical uncertainties are ~ ±1% for CO2, ~ ±2 to 4% for other gases. Noble gas isotope analyses were performed at the Rare Gases Laboratory of the Istituto di Geoscienze e Georisorse (CNR-Pisa, Italy). These gas samples were processed on a stainless steel vacuum line equipped with cold (active charcoal at liquid N2 temperature) and hot traps (Ti getter) to separate noble gases from the other gases (Magro and Pennisi, 1991; Magro et al., 2003). The extraction line was connected to both a magnetic mass spectrometer (MAP 215-50) equipped with an ion counting device, and a quadrupole mass spectrometer (Spectralab 200, VG-Micromass). The magnetic mass spectrometer resolution is close to 600 AMU for HD-3He at 5% of the peak. Typical blanks for a magnetic mass spectrometer are on the order of 0.6 to 1 × 10 −9 cc STP for 4He, with a ratio close to that of air. No blank corrections are applied to the 3He/ 4He ratios of geothermal gases, as the He concentration is relatively high (a few hundred parts per million), generally several orders of magnitude higher than the He blank (at parts per billion levels). The no blank corrections are applied to He, Ne and Ar concentration, which were at least one order of magnitude higher than blank. The noble gas concentrations were obtained by comparison of the unknown peak height with peak height of known air standard volume. The standard volume of air at different known pressures was introduced into the extraction line and treated in the same way as the samples. The results together with the reproducibility of air isotopic ratios are reported in Table 2. 3.1. Chemical composition of major components The Palinpinon geothermal gases are dominated by H2O, with carbon dioxide, CO2, and hydrogen sulfide, H2S, being the most abundant components of the dry fraction. The gas/steam ratio time series plot (Fig. 3) shows that all the sampled wells are characterized by relatively low gas contents, and that during the period 2000–2010 the gas/steam ratios underwent minor temporal variations across the field. Wells PN15D, PN22D, SG3D, OK2 and LG3D (group 1) can be classified as relatively gas-rich, compared to wells SG2, NJ8D, NJ5D, OK5, BL3D and OK9D (group 2). The occurrence of minima of the gas/steam ratio around or lower than 1 mmol/mol indicates that gaseous discharges at Palinpinon are made up by almost pure steam, with trace amounts of non-condensable gases. Based on relative proportions of the three major components of the Palinpinon gases, H2O, CO2 and H2S, most of the variability in the composition of the 2001 dataset is due to variations in H2O contents,

Table 1 Major gas concentrations of Palinpinon geothermal discharges. Concentrations are given in μmol/mol on a dry basis. Gas/steam ratio is calculated on a molar basis (mol/mol). Measurements performed by gas-chromatography and wet-chemical analysis on two-phase samples collected in evacuated glass bulbs partially filled with 4 M NaOH aqueous solutions. Well

Date

Lata

Longa

Ha

gas/steam

CO2

H2S

H2

N2

CH4

Ar

N2/Ar

OK2 PN15D PN22D NJ5D NJ8D SG2 SG3D OK5 BL3D OK9D LG3D

5/21/01 5/22/01 5/23/01 5/24/01 5/24/01 5/22/01 5/22/01 5/23/01 5/23/01 5/23/01 5/21/01

518.743 518.835 518.197 518.074 515.616 515.625 514.131 517.887 517.349 517.768 517.742

1027.788 1027.604 1025.393 1026.958 1025.306 1025.823 1025.413 1026.480 1026.064 1025.157 1025.712

2700 2685 1982 1358 1507 1952 1952 2440 2697 1462 2706

0.0079 0.0081 0.0084 0.0024 0.0028 0.0032 0.0049 0.0042 0.0026 0.0033 0.0057

681729 895367 910264 914898 925451 917696 935095 927264 881677 872253 931860

56316 59151 68893 69892 59585 69469 46611 53637 95004 71573 52338

3166 10334 10163 4604 1602 3665 2557 7861 9015 6586 7404

247301 24845 2600 5765 8678 6994 13432 5800 8766 30137 3153

3117 4413 4004 1362 666 498 945 1404 536 8498 1026

2448 246 24 92 96 106 123 176 66 401 44

101 101 109 63 90 66 109 33 133 75 72

a

Lat is latitude north (Universal Transverse Mercator, UTM, km); Long is longitude east (UTM, km); H is total fluid enthalpy (kJ kg−1).

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Table 2 Noble gas chemical and isotopic composition of Palinpinon geothermal discharges. Concentrations are given in μmol/mol on a dry basis. R/Ra is the 3He/4He ratio normalized to the air value (1.39 × 10−6). Measurements performed by magnetic and quadrupole mass spectrometry on gas samples collected in copper tubes. Well

Date

He

Ne

Ar

R/Ra

4

OK2 PN15D PN22D NJ5D NJ8D SG2 SG3D OK5 BL3D OK9D LG3D

5/21/01 5/22/01 5/23/01 5/24/01 5/24/01 5/22/01 5/22/01 5/23/01 5/23/01 5/23/01 5/21/01

3.83 4.05 4.01 3.94 6.44 6.81 8.01 6.24 2.97 6.23 3.76

2.97 0.90 0.15 0.35 0.27 0.16 0.21 0.15 0.17 1.18 0.06

4149 488 84 264 203 187 237 121 137 704 49

7.58 7.55 7.31 7.68 7.55 7.06 6.96 7.74 7.45 7.84 7.56

1.43 5.00 28.77 12.51 26.45 45.88 42.55 45.66 18.97 5.84 71.63

310 295 304 303 295 295 295 308 302 300 310

9490 ± 650 9340 498 284

1 ± 0.03 1 1 1

0.31 ± 0.03 0.32 0.24 0.27

295 ± 25 295 295 295

air standard air ASW 0 °C ASW 25 °C

5.3 ± 0.36 5.24 0.049 0.0441

at relatively constant CO2/H2S molar ratios of 15 ± 5 (Fig. 4). Data from separate surveys during the period 2000–2010 confirm this trend, suggesting that the observed variations in the gas/vapor ratio, and water contents, are large compared to variations in relative concentrations of non-condensable gases. The variations in relative contents of water vapor point to existence of large sinks/sources for this component, the first candidate for this being the admixture of variable amounts of vapor deriving from the evaporation of exogenous liquid water entering the reservoir possibly in the form of reinjected brine or natural meteoric recharge. In agreement with this hypothesis, samples from the sector most heavily affected by reinjection returns (e.g. samples from NJ5D well; Fig. 4d) show the highest H2O relative contents. More generally, higher H2O relative concentrations are observed in areas and/or during periods of enhanced reinjection return (Fig. 4b and c). After CO2 and H2S, nitrogen, N2, is the most abundant component of the dry gas fraction (H2S /N2 molar ratios between 0.15 and 91), followed by hydrogen, H2, (H2S/H2 molar ratios between 3.7 and 58), and methane, CH4, (H2S/CH4 molar ratios between 5.9 and 240). 3.2. Noble gases isotopic composition The noble gas fraction of Palinpinon geothermal discharges is dominated by argon, with concentrations up to 4150 μmol/mol dry

He/20Ne

40

Ar/36Ar

4

He/36Ar

20

0.29 2.44 14.41 4.52 9.34 10.71 9.96 15.89 6.54 2.66 23.75

0.20 0.49 0.50 0.36 0.35 0.23 0.23 0.35 0.34 0.45 0.33

0.166 0.029 0.046

Ne/36Ar

0.52 0.12 0.17

gas. Helium and neon also occur in detectable amounts, with He (up to 8 μmol/ mol dry gas) well above the reference concentrations in air and air-saturated water (ASW). 40Ar/36Ar ratios between 295 and 310 primarily reflect an atmospheric origin. Neon isotopes represent an atmospheric source and are not reported and discussed in this paper. In contrast, 3He/4He ratios normalized to air value (1.39× 10-6) are between 6.96 and 7.84, in the range of values normally observed for subduction-related volcanism (Hilton and Craig, 1989; Marty et al., 1989; Poreda and Craig, 1989). He and N2 contents of the fluids are up to about 500 times, and up to about 3.5 times that expected for ASW, respectively (Fig. 5).

4. Discussion Reservoir T- redox conditions, together with reactions and physical processes affecting the chemical composition of gases, are inspected by means of suitable groups of gas components combined into reliable geoindicators. Following a widely accepted approach pioneered by Giggenbach (1980, 1987), analytical concentrations of Palinpinon gases are compared with theoretical patterns indicative of equilibrium conditions attained by geothermal fluids under different temperature, redox and phase saturation conditions.

Fig. 3. Time series plot of gas/steam ratio (mol/mol) for 11 production wells at Palinpinon over the period 2000–2010.

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Fig. 4. Relative H2O, CO2 and H2S contents for Palinpinon gases. a: samples from 2001 survey compared with data for 11 production wells over the period 2000–2010; b–d: time series for 3 selected wells. Wells OK2 (b), OK5 (c), and NJ5D (d) represent sectors of the field with low, intermediate and high reinjection impact, respectively. Numbers indicate the last two digits of the year of sampling.

4.1. CO2–CH4 and H2–Ar ratios The CO2–CH4 vs. H2–Ar2 geoindicator (Giggenbach, 1993) allows a preliminary assessment of thermal and redox conditions in the geothermal reservoir. It is based on the coupling between the H2–Ar

(Giggenbach and Goguel, 1989) and the CH4–CO2 geothermometers (Giggenbach, 1987). The H2–Ar geoindicator is based on the assumption that Ar content of the hydrothermal liquid phase is equal to that of air-saturated water (ASW), and that H2/Ar ratios respond rapidly to variations in temperature and redox conditions. The equilibrium

Fig. 5. Plot of (4He/36Ar) versus (20Ne/36Ar) (box a) and of (N2/36Ar) versus (20Ne/36Ar) (box b) for Palinpinon geothermal discharges (yellow circles; 2001 survey). Also plotted are the air and air-saturated water (ASW, 25 °C) end-members.

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concentrations are calculated by means of the following revised equations (modified after Caliro et al., 2007): Log ðX H2 =X Ar ÞVAP ¼ 3:809–12707=T–

1

/

log f O2 –log BAr

ð1aÞ

Log ðX H2 =X Ar ÞLIQ ¼ 3:809–12707=T–

1

/

log f O2 –log BH2

ð1bÞ

2

2

where Bi is the unitless vapor–liquid distribution coefficient of the gas i. The CH4/CO2 geothermometer has a slow kinetics, and it was found to attain equilibrium concentrations in mature systems (Giggenbach, 1980). The equilibrium concentrations are calculated by means of the following equations: Log ðX CH4 =X CO2 ÞVAP ¼ −11:145 þ 5181=T–2 log f O2 þ log BCH4 – log BCO2

ð2aÞ

Log ðX CH4 =X CO2 ÞLIQ ¼ −11:145 þ 5181=T–2 log f O2

ð2bÞ

CO2/CH4 and H2/Ar ratios are plotted in Fig. 6. The analytical values of Palinpinon plot at temperatures between 250 and 350 °C, in close agreement with temperatures measured in the field. Wells PN22D, LG3D, OK5 and BL3D, located in close proximity of the main upflow area, are characterized by the highest apparent equilibrium temperatures, between 300 and 350 °C, and plot near the equilibrium lines predicted for the Fe(II)–Fe(III) redox buffer. Most of the remaining wells plot at temperatures between 250 and 280 °C, close to equilibrium lines of the D'Amore and Panichi (1980) empirical redox buffer. Finally, the representative point of well OK2 shows a marked shift towards lower values of the XH2/XAr ratio, with an apparent equilibrium temperature of about 200 °C. A possible scenario which explains all the measured data is that the deep parent water feeding the geothermal system effectively attains a condition of full equilibrium with rock matrix at temperatures between 300 and 350 °C under redox conditions buffered by di- and trivalent ironbearing minerals. The shift towards lower temperatures at almost constant XCH4/XCO2 values of all the remaining representative points likely reflects a larger contribution of vapors relatively depleted in H2, or enriched in Ar, possibly related to reinjected brine returns and/or meteoric recharge, respectively. The presence of slightly oxidizing fluid conditions in the reservoir is further supported by the finding of abundant anhydrite in veins and alteration assemblages.

Fig. 6. Plot of Log (XH2/XAr) versus Log (XCH4/XCO2) for Palinpinon gases (yellow circles; 2001 survey). The liquid and vapor equilibrium lines are calculated for different temperatures for the following redox buffers: (i) fayalite–magnetite; (ii) Fe(II) –Fe(III); (iii) D'Amore and Panichi (1980); (iv) calcite-anhydrite, calculated for CO2/H2S = 100 molar ratio.

4.2. N2/Ar and Ne/Ar ratios In order to gain further insights on reservoir physical processes, in Fig. 7 the N2/Ar and Ne/Ar molecular ratios of Palinpinon gas discharges have been compared with compositional trajectories for residual liquids and associated vapors produced by boiling of air-saturated waters at temperatures between 10 and 80 °C. The analytical uncertainty and the correction of Ne/Ar ratios for a hypothetical atmospheric contamination of up to 30% by vol. have been also plotted to exclude that analytical and/or sampling artifacts might affect the interpretation of results. 1 Meteoric recharge is expected to occur between 15 and 30 °C at Palinpinon, and values up to 60 °C may be expected for reinjected waters from surface lagoons (cold reinjection, average temperature 40 °C). The reference curves forming the compositional grid of Fig. 7 are representative of an isothermal single-step boiling process at 280 °C, which is an average temperature for the reservoir. Although vapor separation is likely to take place over the temperature range delineated by the equilibrium temperature and the measured downhole temperature, the choice of different temperature values in the range of 260 to 300 °C does not change the calculated patterns. Again, over the 260–300 °C T-ran ge, the assumption of different mechanisms of vapor separation, e.g. open system vapor separation, may have a maximum effect of extending the theoretical equilibrium grid towards the air end-member, on the vapor side, and towards the left corner of near zero values, on the residual liquid side, without any appreciable effect on the trends already outlined in Fig. 7. All field data plot outside the equilibrium compositional grid expected for air-saturated waters, but some wells, namely NJ5D, SG2, LG3D and OK9D, plot in a roughly intermediate position between the grid for evaporated air-saturated waters and the air end-member, suggesting a mixing relationship between vapors derived from these two atmospheric sources. Apart from the anomalous OK5 sample, all the remaining samples plot at near-atmospheric Ne/Ar ratios, but higher than atmospheric N2/Ar values, supporting the hypothesis of a nitrogen excess likely related to the degassing of subducted sediments, as widely recognized in arc-related volcanic and geothermal systems (e.g. Matsuo et al., 1978; Giggenbach, 1992, 1995; Hilton et al., 2002 and references therein). At Palinpinon, most of reinjection is carried out at 160 °C, after separation of steam at the separators (Malate and Aqui, 2010). Due to solubility effects, flashed brines used for reinjection are expected to be strongly depleted in noble gases, and should plot in the left, lowermost corner of the diagram. The lack of representative points in this part of the diagram suggest that, at least at the time of 2001 survey, inert gases concentrations were not significantly affected by the return of such high-temperature reinjected brines. This is in apparent contrast with evidences from water chemistry. Stable isotopes, Cl and tracers concentrations indicate in fact that reinjected brine return is a widely recognized process at Palinpinon (e.g. Urbino et al., 1986; Gerardo et al., 1993; Sanchez, 2010). A likely explanation for this phenomenon stems from the higher mobility of the gas phase compared to liquid phase. In zones affected by exploitation, the pressure decrease may cause liquid water to boil even in the producing layers. Depending on production and recharge rates, and 1 The comparison between uncorrected and corrected data confirms that the use of analytical data coming from two separate gas samples and determined by different analytical methods (⁎) does not affect the interpretation of results even in the worst, unrealistic case that gas samples collected in copper tubes have suffered a large (up to +30% by vol.) atmospheric (air and/or air-saturated water) contamination. Corrections are made with the following formula: CCORR = CMS − [(XATM × CATM) / (1 − XATM)], where: CCORR, CATM and CMS are the Ar (or Ne) concentrations in the samples after correction, in the atmospheric component, and in the basic field sample analyzed by mass spectrometry (i.e. without corrections; Table 2), respectively; X is the fraction of atmospheric contaminant in CMS , arbitrarily comprised between 0 and 0.3 in our calculation; the subscript ATM stands for atmospheric, i.e. air or air-saturated water. (⁎) N2/Ar ratios were measured by gas-chromatography on gas samples collected with glass bulbs partially filled with 4M NaOH aqueous solution (“soda samples”); Ne/Ar ratios were measured by mass spectrometry on gas samples collected with copper tubes.

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Fig. 7. Plot of N2/Ar versus Ne/Ar for Palinpinon gases. The compositional grid is calculated by assuming a single-step isothermal vapor separation at 280 °C from a set of aqueous solutions having the same composition of air-saturated water at temperatures variables between 10 and 80 °C. Liquid–vapor equilibria have been calculated by means of solubility coefficients given by Harvey (1996). 1) Samples with ±10% maximum analytical uncertainty; 2) Ne/Ar analytical values corrected for hypothetical differential entrainement of air in copper tubes (0 to +30%) compared to glass bulbs samples used for the determination of N2/Ar ratios; 3) same correction of point 2) made under the assumption of 0 to +30% hypothetical differential entrainement of vaporized air-saturated water in copper tubes.

5. Origin of gases and geodynamic implications

Points for Palinpinon geothermal wells plot roughly in the same area of most of the samples analyzed by Giggenbach and Poreda (1993), being different in their 4He/ 20Ne values, i.e. higher percentages of the atmospheric component. The marked separation between representative points for the Palinpinon and Bacon Manito geothermal fields are notable. Gases from Bacon Manito have lower and quite variable 3He/ 4He ratios reflecting different contributions of the radiogenic helium (Bayon et al., 2008). The occurrence of R/RA values close to or slightly lower than MORB value at Palinpinon indicates instead a predominant He contribution from mantle wedge (around or greater than 90%), and negligible contributions from both subducted sediments and/or crust slices, and arc lithosphere. However, whereas it is widely accepted that sediments associated with subducting slabs cannot contribute helium to arc magmatism (e.g.. Staudacher and Allègre, 1988; Hilton et al., 1992; Hiyagon, 1994), the lack of radiogenic He contribution from arc lithosphere deserves further discussion. Based on geophysical evidences (Yumul et al., 2008; and references cited therein), it appears in fact that the Palinpinon geothermal field lies on the same zone of crust thickening (up to 30 km) as the Bacon Manito geothermal field, which extends beneath Bicol, Masbate, Panay, Negros, Central Mindanao islands. This evidence seems to be in contrast with the generally accepted axiom that crustal thickness is the major factor controlling the escape of mantle helium in subduction areas. A possible reconciling hypothesis for this apparent inconsistency could be that the segment arcs to which the two geothermal fields belong, are in fact characterized by different ages and stratigraphic sequences. The Bacon–Manito geothermal system is hosted by a thick sequence (greater than 2800 m) of volcanic rocks, mainly andesitic but ranging from basaltic to dacitic, overlying a thick sedimentary and volcanic succession of Miocene to Eocene age (Fragata et al., 2001; Ramos, 2002). In contrast, the occurrence of intrusive and volcanic rocks of Middle Miocene to Pleistocene age with adakitic 2 signature is thought to be a convincing evidence of

The chemical and isotopic composition of noble gases is primarily used to identify the deep source of volatile components in the reservoir. The correlation between 3He/ 4He and 4He/ 20Ne ratios indicates that Palinpinon noble gases derive from a binary mixing between atmospheric and deep, “arc-type” magmatic components (Fig. 9).

2 Adakitic magmas are generally produced by the partial melting of hot and young subducted oceanic crust. They are characterized by: SiO2 >56%, Al2O3 > 15%, Na2O > 3.5%, Sr >400 ppm, Y b 18 ppm, and important amounts of plagioclase and amphibole (e.g., among many others, Defant and Drummond, 1990; Martin, 1999; Defant and Kapezhinskas, 2001; Martin et al., 2005).

hydraulic properties of host rocks, boiling may extend further from the well causing a two-phase zone to develop within the reservoir. Wells continue to produce water and steam mixtures until mobile liquid water exists in the rocks, or pressure drop is insufficient to cause a complete vaporization of the liquid water. Pressure gradients are expected to force the free gas phase to move preferentially to the well, and, under conditions of favorable lateral permeability, as locally claimed for Palinpinon (Malate and Aqui, 2010), to drain lateral sectors of the field, possibly until the margins of the reservoir are reached. Accepting this hypothesis, the chemical composition of the gas phase sampled at wellhead should be considered as the result of a combination of different sources and evolutionary mechanisms. In particular, three major sources are expected to contribute to the final gas inventory (Fig. 8): (i) boiling of natural meteoric recharge or reinjected cold brines from lagoons open to atmosphere (Fig. 8, trend 1); (ii) boiling of hot (160 °C) reinjected brines (Fig. 8, trend 2); (iii) direct upwelling of deep magmatic/hydrothermal gases (Fig. 8, DHC). The first mechanism is expected to produce relatively huge amounts of steam and atmospheric gases (N2, Ar, Ne), and to dilute the deep magmatic and/or hydrothermal component (CO2, H2S, H2, CH4, He). Conversely, the second mechanism is expected to produce steam alone, and then to dilute both the magmatic/hydrothermal and the atmospheric components. Based on all these considerations, the meteoric signature of Palinpinon gases, can be then interpreted as evidence for the predominance, at least at the time of 2001 survey, of the first mechanism (natural meteoric recharge and/or cold reinjection). Major gradients in noble gas composition of steam are expected to occur under different injectate and natural meteoric relative recharge rates. Noble gases analyses could be then used as tracers for early detection of injectate breakthrough and continuous monitoring of varying proportions of injectate return in the production stream with time.

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Fig. 8. H2O, CO2 and N2 relative contents for Palinpinon gases. Three main components/differing processes are also shown: 1) DHC = deep hydrothermal component; 2) arrow 1 = dilution with vapor from boiling of natural meteoric recharge or reinjected cold brines from lagoons opent to atmosphere; 3) arrow 2 = dilution with vapor from boiling of hot reinjected brines. a: data for 11 production wells over the period 2000–2010; b: data from 2001 survey.

partial melting of subducted oceanic basalts at southern Negros (Rae et al., 2004). The geodynamic model consequently devised considers that in the Negros area the subduction of young (b 20 Ma) oceanic crust (Defant and Drummond, 1990; Sajona et al., 1993; Peacock et al., 1994), with slab temperatures in excess of 700 °C at relatively shallow depths (75–85 km; Sajona et al., 1993), is still ongoing. The existence of zones of recent and surficial magma upwelling, and the availability of huge amounts of young basalts, should be then considered as a plausible condition for the production of 3He-rich fluids at Negros, and their upwelling into the Palinpinon geothermal reservoir without any significant addition of radiogenic helium. The aforementioned differences between the two fields are further supported by the occurrence of larger and younger intrusive bodies at Palinpinon, compared to Bacon Manito (PNOC-EDC unpublished internal reports).

5.1. 3He/ 4He and CO2/ 3He ratios The range of CO2/ 3He ratios for Palinpinon gases, between 12.1 and 28.7 × 10 9, overlaps the lower end of that observed for other subduction-related systems worldwide (Giggenbach et al., 1993; Sano and Marty, 1995; Snyder et al., 2001; Hilton et al., 2002, and references cited therein), and it is about one order of magnitude greater than the MORB ratio of 1.2×109 (Marty and Jambon, 1987). In Fig. 10, Palinpinon gases cluster near the deep magmatic component, distinct from Bacon Manito samples. CO2 and He contents seem to be correlated at Palinpinon, suggesting that a minor crustal contribution dilutes the magmatic signature. We estimate that the crustal end-member has CO2/3He ratios greater than 2×1013, similar to the CO2/3He crustal end-member hypothesized for gases from volcaniclastic reservoirs of Green Tuff Basin, Japan (1×1013; Wakita et al., 1990), and to the New Zealand convergent plate boundary (1×1014; Giggenbach et al.,1993). Based on simple mass balance considerations, we estimate that the crustal end-member contributes less than 8% to the total He and CO2 budget of all the Palinpinon wells, except for wells SG3 and SG2, where maximum values of 12% are possible. Carbonate precipitation may preferentially remove CO2 and affect the CO2/3He ratios, however, no clear evidence of massive calcite deposition in the reservoir exists (e.g. Sanchez, 2010). 5.2. CH4– 3He– 4He and N2– 3He– 4He Diagrams

Fig. 9. 3He/4He versus. 4He/20Ne ratios (normalized to air) for Palinpinon gases. Samples from Bacon Manito geothermal field (Bayon et al., 2008) and other volcanic-hydrothermal areas in the Philippines (Giggenbach and Poreda, 1993) are plotted for comparison. Dashed curves represent binary mixing trends between air-saturated water at 25 °C and mixtures of arc (R/Ra=8) and crust (R/Ra=0.02) sourced gas.

Fig. 11 shows relative CH4– 3He– 4He contents of Philippines samples. Gases from Palinpinon groups close to the 3He corner, and partially overlap with most of the volcano-hydrothermal features sampled along the Philippines archipelago by Giggenbach and Poreda (1993). In contrast, Bacon Manito geothermal gases are characterized by a marked CH4-excess and reflect the contribution of three possible sources: 3Herich magmatic gases (arc-type mantle end-member), 4He-rich radiogenic gases from crust (4He-rich pole of the crustal component), and CH4-rich gases from subducted materials or arc crust (thermogenic 3 end-member). The CH4-excess of Bacon Manito can be likely related to processes of thermal degradation of sedimentary organics exposed to magma intrusions or other heat sources. This scenario is supported 3 Thermogenic gases are produced by thermal decomposition of organic matter at temperatures generally >100 °C (e.g. Schoell, 1980, 1988; DesMarais et al., 1981).

G. Magro et al. / Chemical Geology 339 (2013) 4–15

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Fig. 10. 3He/4He versus CO2/3He ratios in Palinpinon gases. Samples from Bacon Manito geothermal field (Bayon et al., 2008) and other volcanic-hydrothermal areas in the Philippines (Giggenbach and Poreda, 1993) are plotted for comparison. Curves represent mixing between the deep magmatic component ( “MANTLE”) and an array of crustal endmembers with fixed 3He/4He ratios (0.02 Ra), but variable CO2/3He ratios. Data from others arc-related volcano-geothermal systems are from Giggenbach et al. (1993), Sano and Marty (1995), Snyder et al. (2001), and references therein.

by the presence of organic-bearing materials in the local stratigraphic column (Fragata et al., 2001; Ramos, 2002). The occurrence of low methane contents at Palinpinon is a typical feature of high-temperature hydrothermal systems. Methane is unlikely of primary mantle or magmatic origin, and has to be generated within the crust. A plausible scenario for Palinpinon is that CH4 is formed inorganically from CO2, under redox conditions likely controlled by the Fe(II)–Fe(III) buffer, in absence of significant thermogenic contribution. This scenario is consistent with the geological setting of the Negros region, dominated by volcanic rocks and apparently devoid of organic-bearing materials (Rae et al., 2004).

The relative N2– 3He– 4He contents (Fig. 12) of Philippine gases further support this general picture. Palinpinon gases align in fact on a binary mixing trend between the magmatic and the atmospheric component, whereas Bacon–Manito denote variable contributions of crustal and thermogenic sources. Again, the differences between the two geothermal systems can be easily explained in terms of different geological settings. Due to the minor contribution from subducted sedimentary sources, the low N2/ 3He ratios of Palinpinon reflect an intermediate signature between typical arc-related, high-N2 gases, and low-N2 ,high- 3He, mantle-type volatiles, mirrored by extremely variable CO2/N2 ratios, generally comprised between 3 to about 350.

Fig. 11. Relative CH4, 3He and 4He contents for Palinpinon gases. Samples from Bacon Manito geothermal field (Bayon et al., 2008) and other volcanic-hydrothermal areas in the Philippines (Giggenbach and Poreda, 1993) are plotted for comparison.

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Fig. 12. Relative N2, 3He and 4He contents for Palinpinon gases. Samples from Bacon Manito geothermal field (Bayon et al., 2008) and other volcanic-hydrothermal areas in the Philippines (Giggenbach and Poreda, 1993) are plotted for comparison.

6. Conclusions The principal conclusions of this work can be summarized as follows: i. the high-enthalpy Palinpinon geothermal system is a low-gas, waterdominated geothermal field, fed by magmatic fluids, and largely flushed by steam derived from the boiling of (a) waters of meteoric recharge and (b) reinjected brines; ii. the non-condensable gases mainly derive from two sources: (i) deep supply of gases of likely magmatic origin, contributing for most of CO2 and He; (ii) natural meteoric recharge, contributing for most of N2, Ar and Ne; iii. water–rock interactions cause some geothermal overprinting of the deep magmatic component, allowing (a) redox conditions to be controlled by the Fe(II)-Fe(III) buffer, and (b) minor CH4 to be inorganically formed from CO2; iv. He isotopes, He/Ne, CO2/3He, N2/3He and CH4/3He ratios provide internally consistent information, supporting the hypothesis that most of the deep gases are directly derived from a magmatic source and/or from the scavenging of an organic-depleted, basalt-rich crust. The still ongoing process of subduction of young (b20 Ma) oceanic crust, together with the recent and surficial upwelling of andesitic magmas in the Negros area, strongly supports this scenario.

Acknowledgements The authors gratefully acknowledge EDC for assistance in sampling and laboratory analyses, as well as access to chemistry data. This research was supported by the International Atomic Energy Agency of Vienna (contract no. TC PHI/8/023). The manuscript has been reviewed by S. Simmons and an anonymous reviewer under the supervision of the Associate Editor T. Fischer.

References Barrier, E., Huchon, P., Aurelio, M., 1991. Philippine fault: a key for Philippine kinematics. Geology 19, 32–35. Bayon, F.E.B., Ogena, M.S., 2005. Handling the problem of rapid reinjection returns in Palinpinon-I and Tongonan, Philippines. Proceedings World Geothermal Congress 2005, Antalya, Turkey, 24–29 April 2005, Paper 1217. Bayon, F.E.B., See, F.S., Magro, G., Pennisi, M., 2008. Noble gas and boron isotopic signatures of the Bacon–Manito geothermal fluid, Philippines. Geofluids 8, 230–238. Bromley, C.J., Española, O.S., 1982. Resistivity methods applied to geothermal exploration in the Philippines. Proceedings Pacific Geothermal Conference, pp. 447–452. Caliro, S., Chiodini, G., Moretti, R., Avino, R., Granieri, D., Russo, M., Fiebig, J., 2007. The origin of the fumaroles of La Solfatara (Campi Flegrei, South Italy). Geochimica et Cosmochimica Acta 71, 3040–3055. D'Amore, F., Panichi, C., 1980. Evaluation of deep temperature of hydrothermal systems by a new gas-geothermometer. Geochimica et Cosmochimica Acta 44, 549–556. D'Amore, F., Ramos-Candelaria, M.N., Seastres Jr., J.S., Ruaya, J.R., Nuti, S., 1993. Applications of gas chemistry in evaluating physical processes in the Southern Negros (Palinpinon) Geothermal Field, Philippines. Geothermics 22, 535–553. Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665. Defant, M.J., Kapezhinskas, P., 2001. Evidence suggests slab melting in arc magmas. EOS Transactions 82, 65–69. DesMarais, D.J., Donchin, J.H., Nehring, M.L., Truesdell, A.H., 1981. Molecular carbon isotopic evidence for the origin of geothermal hydrocarbons. Nature 292, 826–828. Dimalanta, C.B., Yumul Jr., G.P., 2006. Magmatic and amagmatic contributions to crustal growth of the Philippines island-arc systems: comparison of the Cretaceous and post-Cretaceous periods. Geosciences Journal 10, 321–329. Fragata, J.J., Gonzalez, R.C., See, F.S., Delfin, F.G., 2001. The Manito low enthalpy geothermal field: geologic setting and development history. Journal of the Geological Society of Philippines 56, 144–162. Gerardo, J.Y., Nuti, S., D'Amore, F., Seastres Jr., J.S., Gonfiantini, R., 1993. Isotopic evidence for magmatic and meteoric water recharge and the processes affecting reservoir fluids in the Palinpinon geothermal system, Philippines. Geothermics 22, 521–533. Giggenbach, W.F., 1980. Geothermal gas equilibria. Geochimica et Cosmochimica Acta 44, 2021–2032. Giggenbach, W.F., 1987. Redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zealand. Applied Geochemistry 2, 143–161. Giggenbach, W.F., 1992. The composition of gases in geothermal and volcanic systems as a function of tectonic setting. Proceedings of the VII International Symposium on Water–Rock Interaction, Park City, USA, pp. 873–878. Giggenbach, W.F., 1993. Redox control of gas compositions in Philippine volcanichydrothermal systems. Geothermics 22, 575–587. Giggenbach, W.F., 1995. Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research 68, 89–116.

G. Magro et al. / Chemical Geology 339 (2013) 4–15 Giggenbach, W.F., Goguel, R.L., 1989. Collection and Analysis of Geothermal and Volcanic Water and Gas Discharges. Report no.CD2401. Chemistry Division DSIR, New Zealand, p. 81. Giggenbach, W.F., Poreda, R.J., 1993. Helium isotopic and chemical composition of gases from volcanic-hydrothermal systems in the Philippines. Geothermics 22, 369–380. Giggenbach, W.F., Sano, Y., Wakita, H., 1993. Isotopic composition of helium, and CO2 and CH4 contents in gases produced along the New Zealand part of a convergent plate boundary. Geochimica et Cosmochimica Acta 57, 3427–3455. Harper, R.T., Jordan, O.T., 1985. Geochemical changes in response to production and reinjection for Palinpinon I geothermal field, Negros Oriental, Philippines. Proceedings 7th New Zealand Geothermal Workshop, pp. 39–44. Harvey, A.H., 1996. Semiempirical correlation for Henry's constants over large temperature ranges. American Institute of Chemical Engineers Journal 42, 1491–1494. Hilton, D.R., Craig, H., 1989. A helium isotope transect along the Indonesian archipelago. Nature 342, 906–908. Hilton, D.R., Hoogewerff, J.A., van Bergen, M.J., Hammerschmidt, K., 1992. Mapping magma sources in the east Sunda–Banda arcs, Indonesia: constraints from helium isotopes. Geochimica et Cosmochimica Acta 56, 851–859. Hilton, D.R., Fischer, T.P., Marty, B., 2002. In: Porcelli, D., Ballentine, C.J., Wieler, R. (Eds.), Noble gases and volatile recycling at subduction zones, pp. 319–370. Hiyagon, H., 1994. Retentivity of solar He and Ne in IDPs in deep sea sediments. Science 263, 1257–1259. Magro, G., Pennisi, M., 1991. Noble gases and nitrogen: mixing and temporal evolution in the fumarolic fluids of Vulcano, Italy. Journal of Volcanology and Geothermal Research 47, 237–247. Magro, G., Ruggieri, G., Gianelli, G., Bellani, S., Scandiffio, G., 2003. Helium isotopes in paleofluids and present-day fluids of the Larderello geothermal field: constraints on the heat source. Journal of Geophysical Research 108 (B1) http://dx.doi.org/ 10.1029/2001JB00159. Malate, R.C.M., Aqui, A.A., 2010. Steam production from the expanded two-phase region in the Southern Negros Geothermal Production Field, Philippines. Proceedings of World Geothermal Congress 2010, Bali, Indonesia, 25–29 April 2010, Paper 2411. Martin, H., 1999. Adakitic magmas: modern analogues of Archaean granitoids. Lithos 46, 411–429. Martin, H., Smithies, R.H., Rapp, R., Moyen, J.F., Champion, D., 2005. An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24. Marty, B., Jambon, A., 1987. C/3He in volatile fluxes from the solid Earth: implications for carbon geodynamics. Earth and Planetary Science Letters 83, 16–26. Marty, B., Jambon, A., Sano, Y., 1989. Helium isotopes and CO2 in volcanic gases of Japan. Chemical Geology 76, 25–40. Matsuo, S., Suzuki, M., Mizutani, Y., 1978. Nitrogen to argon ratio in volcanic gases. In: Alexander Jr., E.C., Ozima, M. (Eds.), Terrestrial Rare Gases. Japan Scientific Societies Press, Tokyo, pp. 17–25. Maturgo, O.O., Sanchez, D.R., Barroca, G.B., 2010. Tracer test using naphthalene disulfonates in Southern Negros geothermal production field, Philippines. Proceedings World Geothermal Congress 2010, Bali, Indonesia, 25–29 April 2010, Paper 2406. Maunder, B.R., Brodie, A.J., Tolentino, B.S., Maunder, B.R., Brodie, A.J., Tolentino, B.S., 1982. The Palinpinon geothermal resources, Negros, Republic of the Philippinesan exploration case history. Proceedings Pacific Geothermal Conference, pp. 87–92. Pamatian, P.I., Vidal, B.C., Hermoso, D.Z., 2003. Continuing reinjection reteurn in the Palinpinon geothermal production field, Negros Oriental, Philippines. Proceedings 2003 PNOC-EDC Geothermal Conference, pp. 49–57. Peacock, S.M., Rushmer, T., Thompson, A.B., 1994. Partial melting of subducting oceanic crust. Earth and Planetary Science Letters 121, 227–244. Poreda, R., Craig, H., 1989. Helium isotopes in circum-Pacific volcanic arcs. Nature 338, 473–478.

15

Rae, A.J., Cooke, D.R., Phillips, D., Yeats, C., Ryan, C., Hermoso, D., 2003. Spatial and temporal relationships between hydrothermal alteration assemblages at the Palinpinon Geothermal Field, Philippines: implications for porphyry and epithermal ore deposits. In: Simmons, S.F., Graham, I.J. (Eds.), Volcanic, geothermal, and ore-forming fluids: rulers and witnesses of processes within the earth. Littleton, Colorado, USA: Society of Economic Geologists, Special publication, 10, pp. 233–246. Rae, A.J., Cooke, D.R., Phillips, D., Zaide-Delfin, M., 2004. The nature of magmatism at Palinpinon geothermal field, Negros Island, Philippines: implications for geothermal activity and regional tectonics. Journal of Volcanology and Geothermal Research 129, 321–342. Rae, A.J., Cooke, D.R., Brown, K.L., 2011. The trace metal chemistry of deep geothermal water, Palinipinon geothermal field, Negros Island, Philippines: implications for precious metal deposition in epithermal gold deposits. Economic Geology 106, 1425–1446. Ramos, S.G., 2002. Potential constraints to the development of the Rangas sector based on petrologic evaluation of the Bacman geothermal field, Philippines. Proceedings 27th Workshop of Geothermal Reservoir Engineering, Stanford University, California, p. 4. Ramos-Candelaria, M.N., Garcia, S.E., Hermozo, D.Z., Bayrante, L.F., Mejorada, A.V., 1997. Application of geochemical techniques to deduce the reservoir performance of the Palinpinon Geothermal field, Philippines — an update. Geothermal Resources Council Transactions 21, 231–239. Sajona, F.G., Maury, R.C., Bellon, H., Cotton, J., Denfant, M.J., Pubellier, M., 1993. Initiation of subduction and the generation of slab melts in western and eastern Mindanao, Philippines. Geology 21, 1007–1010. Sanchez, D.R., 2010. Calcite deposition of wells affected by re-injected fluids in Palinpinon II, Southern Negros Geothermal Production Field, Philippines. Proceedings World Geothermal Congress 2010, Bali, Indonesia, 25-29-April 2010, paper n. 2718. Sano, Y., Marty, B., 1995. Origin of carbon in fumarolic gas from island arcs. Chemical Geology 119, 265–274. Schoell, M., 1980. The hydrogen and carbon isotopic composition of methane from natural gases of various origins. Geochimica et Cosmochimica Acta 44, 649–661. Schoell, M., 1988. Multiple origins of methne in the Earth. Chemical Geology 71, 1–10. Seastres, J.S., 1982. Subsurface geology of the Nasuji-Sogongon sector, Southern Negros geothermal field, Philippines. Proceedings of the 4th New Zealand Geothermal Workshop, Auckland, pp. 173–178. Seastres, J., Hermoso, D., Candelaria, M., Gerardo, J., 1995. Application of geochemical techniques in evaluating the reservoir response to exploitation at Palinpinon geothermal field, Philippines. Proceedings World Geothermal Congress 1995, pp. 1025–1030. Snyder, G., Poreda, R., Hunt, A., Fehn, U., 2001. Regional variations in volatile composition: isotopic evidence for carbonate cycling in the Central American volcanic arc. Geochemistry, Geophysics, Geosystems 2 2001GC000163. Sta. Ana, F.X.M., O'Sullivan, M.J., 1988. Computer modeling of the Southern Negros geothermal field, Philippines. Proceedings 10th New Zealand Geothermal Workshop, pp. 85–93. Staudacher, T., Allègre, C.J., 1988. Recycling of oceanic crust and sediments: the noble gas subduction barrier. Earth and Planetary Science Letters 89, 173–183. Urbino, M.E.G., Zaide, M.C., Malate, R.C.M., Bueza, E.L., 1986. Structural flowpaths of reinjected fluids based on tracer tests — Palinpinon I, Philippines. Proceedings 8th New Zealand Geothermal Workshop, pp. 53–68. Vidal Jr., B., Hermoso, D., 2000. Estimating reservoir steam fraction from CO2, H2S, and H2 concentrations in the Southern Negros Geothermal field, Philippines. Proceedings World Geothermal Congress 2000, Kyushu-Tohoku, Japan, pp. 2969–2974. Wakita, H., Sano, Y., Urabe, A., Nakamura, Y., 1990. Origin of methane-rich natural gas in Japan: formation of gas fields due to large scale submarine volcanism. Applied Geochemistry 5, 263–278. Yumul Jr., G.P., Dimalanta, C.B., Maglambayan, V.B., Marquez, E.J., 2008. Tectonic setting of a composite terrane: a review of the Philippine island arc system. Geosciences Journal 12, 7–17.