© Institute of Geological and Nuclear Sciences Limited, 2013

ISSN 1177-2425 ISBN 978-1-972192-49-8

I. Chambefort, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352 G. Bignall, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352

BIBLIOGRAPHIC REFERENCE

Chambefort, I; Bignall, G. 2013. Preliminary stable isotope study on the Lahendong geothermal system, Indonesia, GNS Science Report 2013/14. 9p.

GNS Science Report 2013/14 i

CONTENTS

ABSTRACT ........................................................................................................................... II

KEYWORDS .......................................................................................................................... II

1.0 INTRODUCTION ........................................................................................................ 1

2.0 STABLE ISOTOPE ANALYSES ................................................................................ 1

2.1 Sampling ........................................................................................................................ 1 2.2 Analytical methods ........................................................................................................ 1 2.3 Results ........................................................................................................................... 2

3.0 POSSIBLE FLUID SOURCES .................................................................................... 3

4.0 CONCLUSIONS ......................................................................................................... 5

5.0 REFERENCES ........................................................................................................... 5

FIGURES

Figure 1: δD and δ18O isotopic values of the calculated fluids in equilibrium with Lahendong clay and δ18O values of water in equilibrium with quartz and calcite .................................................... 4

TABLES

Table 1 Lahendong samples selected and hand-picked for isotopic analyses. ......................................... 1 Table 2 Carbon, oxygen and hydrogen stable isotope values for selected Lahendong samples. ............. 2 Table 3 Carbon isotope values for Lahendong calcite with respect to VSMOW. ....................................... 2 Table 4 δ13CCO2 in equilibrium with Lahendong calcite sample at 250°C................................................... 4

GNS Science Report 2013/14 ii

ABSTRACT

Five core samples from two wells (LHD1 and LHD5) from Lahendong geothermal system were selected for stable isotope analyses. Oxygen, hydrogen and carbon isotopes were analysed on quartz, clay and calcite.

δ18OVSMOW of quartz is 10.32, 2.78 and 11.51‰. Calcite δ18OVPDB ranges from -20.30 to -15.76‰, with δ13CVPDB from -3.33 to -0.8‰. Clay (mainly chlorite) have a δ18OVSMOW = 6.1‰ and δDVSMOW = -74.9‰.

Assuming temperature of precipitation between 250°C and 270°C, fluid in equilibrium with hydrothermal minerals can be recalculated.

Oxygen value of the fluid that precipitated quartz, calcite and clay is generally enriched in oxygen, with isotopic value around 3‰ for pure quartz, 4.8‰ for clay and between 2 and 7‰ for calcite. The range of δ18Ofluid points to a mixed origin between depleted meteoric, neutral chloride thermal water, and magmatic oxygen. Based on these preliminary results, it is challenging to differentiate between a direct magmatic input with a high δ18Ofluid, or a low water-rock ratio interaction where δ18Ofluid is derived from isotopic exchange with 18O-enriched magmatic rocks.

The value of δ13CCO2 in isotopic equilibrium with calcite is relatively light with δ13CCO2 varying between -2.03 and 0.47‰. Such CO2 values are above the assumed isotopic value of magmatic-derived CO2 and indicate that the carbon in the system is also influenced by from near-by marine sediments.

KEYWORDS

Lahendong geothermal system, Indonesia, oxygen, carbon and hydrogen isotopes

GNS Science Report 2013/14 1

1.0 INTRODUCTION

This scientific report presents preliminary stable isotope analyses (oxygen, hydrogen, carbon) of five core samples selected from the Lahendong geothermal system, North Sulawesi, Indonesia. The aim of this study is to identify potential fluid source(s) of the geothermal fluids and to investigate whether there could be a mixture of fluid components from magmatic and meteoric sources.

2.0 STABLE ISOTOPE ANALYSES

2.1 SAMPLING

Four samples were selected from well LHD-1 from cores at 750, 801, 900, 1452 m and one sample from well LHD-5 at 900 m. Table 1 presents the detailed mineralogy hand-picked for stable isotope analyses.

Table 1 Lahendong samples selected and hand-picked for isotopic analyses.

Sample Mineral Isotope analysed

LHD-1_750 quartz δ18O

LHD-1_801-802 calcite δ18O + δ13C

LHD-1_801-802 clay# δ18O + δD

LHD-1_900-901 calcite δ18O + δ13C

LHD-1_900-901 quartz δ18O

LHD-1_1452-1453 calcite δ18O + δ13C

LHD-5_900-901 quartz δ18O

# Clay mineralogy mainly consists of chlorite with rare illite.

2.2 ANALYTICAL METHODS

Samples were hand-picked for quartz and calcite, and the <2 micron fraction was separated using centrifugation for the clay analyses. Stable isotope ratios were measured at the National Isotope Centre (NIC), GNS Science, New Zealand.

Oxygen in quartz samples was extracted from sample powders for isotope analyses using a CO2-laser and BrF5 (Sharp, 1990). Oxygen isotope values are reported in the familiar δ18O notation, relative to VSMOW (Vienna Standard Mean Ocean Water). Samples were normalised to the international quartz standard NBS-28 using a value of +9.6 per mil (‰). Values for four NBS-28 analysed with the samples had values that varied by less than 0.15‰. Samples and standards were heated overnight to 150°C prior to loading into the vacuum extraction line. These were then evacuated for approximately 6 hours. Blank BrF5 runs were done until yield was less than 0.2 μmoles of oxygen. Oxygen yields were recorded and CO2 gas analysed on a Geo20-20 mass spectrometer.

Clay samples were analysed on a HEKAtech high temperature elemental analyser coupled with a GV Instruments IsoPrime mass spectrometer. Samples were pyrolysed at 1450°C, in silver capsules. All samples were analysed in triplicate. All results are reported with respect to VSMOW, normalised to international standards IAEA-CH-7, NBS30 and NBS22 with

GNS Science Report 2013/14 2

reported δD values of -100‰, -66‰ and -118‰. The external precision for these measurements is better than 2‰.

Carbonates were analysed on the GVI IsoPrime Carbonate Preparation System at a reaction temperature of 25°C for 24 hours and run via dual inlet on the IsoPrime mass spectrometer. Results are reported with respect to VPDB (Vienna Pee Dee Belemnite), normalised to our internal standards GNS Marble with reported values of 2.04‰ for δ13C and -6.40‰ for δ18O. The external precision for these measurements are better than 0.1‰ for δ13C and 0.2‰ for δ18O.

2.3 RESULTS

Table 2 presents the raw stable isotope analyses for five samples selected from wells LHD-1 and LHD-5. Quartz oxygen values ranges between 10.32 to 11.51% with the exception of sample LHD-1_900-901 which has a value of 2.78‰. This sample is considered as not pure quartz and has experienced some contamination. The clay separate from LHD-1_801-802 has a δ18O of 6.1‰ and a δD of -74.9‰. This sample is mainly composed of chlorite with rare illite (Utami, 2011).

Table 2 Carbon, oxygen and hydrogen stable isotope values for the selected Lahendong samples.

Sample Mineral δ13C Value δ18O Value δD Value

LHD-1_750 quartz

10.32

LHD-1_801-802 calcite -1.49 -15.76

LHD-1_801-802 clay#

6.1 -74.9

LHD-1_900-901 calcite -3.33 -18.87

LHD-1_900-901* quartz

2.78

LHD-1_1452-1453 calcite -0.83 -20.30

LHD-5_900-901 quartz 11.51 # Clay mineralogy mainly consists of chlorite with rare illite. * This sample is probably contaminated and is likely not pure quartz.

The calcite oxygen value varies between -20.30 to -15.76‰ with respect to VPDB, with δ13C composition between -3.33 to -0.83‰.

For comparison, δ18OVPDB Calc is recalculated with respect to VSMOW standard (Table 3), using this equation: δ18OVSMOW = 1.03091 * δ18OVPDB + 30.91 ‰ (Coplen et al., 1983).

Table 3 Carbon isotope values for Lahendong calcite with respect to VSMOW.

Sample Mineral δ18O VPDB δ18O SMOW

LHD-1_801-802 calcite -15.76 14.66

LHD-1_900-901 calcite -18.87 11.46

LHD-1_1452-1453 calcite -20.30 9.98

GNS Science Report 2013/14 3

3.0 POSSIBLE FLUID SOURCES

Mineral stable isotope values can be used to estimate the different fluid sources that may have contributed to their compositions. The equilibrium fractionation between mineral species and fluids is dependent on temperature following the generic equation:

103lnαxy = Axy (106/T2) + Bxy,

where x and y represent two phases in equilibrium, A and B are constants unique to the pair of phases x and y, and αxy is the equilibrium fractionation factor.

If two phases are in equilibrium then

∆xy ≈ δx – δy = 103lnαxy

We can thus estimate the isotopic value of the fluid, assuming equilibrium fractionation between the fluid (e.g. fluid CO2) and the mineral if we know the temperature of precipitation, for example,

δwater = δquartz – 103lnαquartz-water

Fluid isotopic values have been recalculated for Lahendong samples using estimates of the temperature of the two wells based on fluid inclusion data and measured well temperature (Utami, 2011).

The oxygen compositions of the fluid in equilibrium with quartz, calcite and clay were calculated using the fractionation factors from Zheng (1993), O’Neil et al. (1969), Friedman and O’Neil (1977) and Cole and Ripley (1998), respectively. The hydrogen value of the fluid in equilibrium with chlorite was calculated for pure chlorite using Graham et al. (1984). The carbon isotope value of the fluid in equilibrium with calcite was calculated using Ohmoto and Rye (1979).

The oxygen and hydrogen ranges of isotopic values of the fluid in equilibrium with quartz, calcite and clay samples are reported in Figure 1. Assuming a range of temperature between 250 and 270°C for the precipitation of the quartz, the fluid in equilibrium has δ18O varying from 1.36 to 2.21‰ for LHD1_750, 2.55 to 3.4‰ for LHD5_900-901, and -6.18 to -5.33‰ for LHD1_900-901, however, the last sample is assumed to be impure quartz (most probably calcite contamination). Oxygen isotopes of water in equilibrium with the calcite are calculated using the δ18OSMOW values of the calcite and range between 2.7 and 7‰ at 250°C (Figure 1).

The calculated fluids are isotopically heavier than the meteoric water or the oxygen value of local spring water and are interpreted to reflect mixing with magmatic oxygen. This is consistent with the oxygen and hydrogen isotopic values of the fluid in equilibrium with the chlorite (±illite) in sample LHD1_801-802 (Figure 1). The isotopic value of the fluid was recalculated at 250°C for pure chlorite and also reflects a mixture of magmatic and meteoric oxygen sources. These sources can be represented by either: 1) a magmatic source where a large component of the fluid is directly exsolved from a degassing magma chamber or; 2) a large “O-shift” due to a low water-rock ratio between 18O-depleted meteoric and/or chloride hot spring water and Lahendong volcanic rocks with an anticipated oxygen value >6‰. The composition of the fluid in equilibrium with chlorite in sample LHD1-801, assuming that a proportion of illite is present in the sample, is slightly enriched in deuterium compared to the primary magmatic rocks, which indicates some direct magmatic oxygen in the system.

GNS Science Report 2013/14 4

Figure 1 δD and δ18O isotopic values of the calculated fluids in equilibrium with Lahendong clay (diamonds),

and δ18O values of water in equilibrium with quartz and calcite (blue and red bars, respectively, calculated between 250 and 270°C). The temperatures are estimated from fluid inclusions and well temperatures from Utami (2011). White diamonds are Indonesian spring waters from Utami (2011). Volcanic vapours field from Giggenbach (1992), PMW (primary magmatic waters) from Taylor (1979).

The carbon isotope value of the fluid was recalculated at 250°C assuming that the calcite was in equilibrium with CO2 (Table 4). δ13CCO2 ranges between -2.03 and 0.47‰. Mantle (e.g. magmatic) carbon has an isotopic value varying between -8 and -4‰, oceanic carbon has a value of ~0‰. The authors suggest that a portion of the carbon is most likely to have been assimilated by calcite originated from nearby marine sediments.

Table 4 δ13CCO2 in equilibrium with Lahendong calcite samples at 250°C.

Sample names Mineral δ13C VPDB δ13C CO2, 250°C

LHD-1_801-802 calcite -1.49 -0.19

LHD-1_900-901 calcite -3.33 -2.03

LHD-1_1452-1453 calcite -0.83 0.47

GNS Science Report 2013/14 5

4.0 CONCLUSIONS

This preliminary isotopic study of Lahendong minerals shows that a large portion of the Lahendong geothermal system incorporated oxygen originating from magmatic and meteoric sources. At the preliminary stage of this study, the distinction between a direct magmatic fluid source and/or oxygen exchange between the rocks at a low-18O fluid cannot be determined. The propylitic assemblage and propylitically altered rocks are generally characterised by 18O-depletion associated with 18O-enrichment in sodium chloride hot spring fluids. This can explain the heavy isotopic values of some hydrothermal minerals at Lahendong.

One interesting observation of this preliminary study is the low δ13C of the calcite, which can be attributed to low δ13C in CO2, and shows that some of the carbon is likely derived from marine sediments. Sulfur isotopic analysis of anhydrite at Lahendong will constrain further the possible sedimentary influence. Without more detailed analysis of the O, C, H and S isotopic systems these preliminary results represent only an indication of the different possible fluid sources.

5.0 REFERENCES Cole D.R. and Ripley E.M. 1998. Oxygen isotope fractionation between chlorite and water

from 170-350C : A preliminary assessment based on partial exchange and fluid/rock experiments, Geochimica et Cosmochimica Acta, 63. P. 449-457.

Coplen, T.B., Kendall, C., and Hopple, J. 1983. Comparison of stable isotope reference samples. Nature, v.302, p.236-238.

Friedman I. and O'Neil J.R. 1977 Compilation of stable isotope fractionation factors of geochemical interest. U.S. Geol. Surv. Prof. Pap. 440-KK, 49 p.

Giggenbach, W. F. 1992. Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Earth and Planetary Science Letters, v. 113 (4), p. 495-510.

Graham, C.M., Atkinson, J. and Harmon, R.S. 1984. Hydrogen isotope fractionation in the system chlorite-water. NERC 6th Progress Report of Research 1981-1984, NERC Publication Series D, No. 25, p. 139.

Ohmoto, H. and Rye, R. O. 1979. Isotope of sulfur and carbon, in Barnes, H. L. Ed., Geochemistry of Hydrothermal deposits, John Wiley & Sons, p. 509-567.

O'Neil, J.R., Clayton, R.N. and Mayeda, T.K. 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys. 51, pp. 5547-5558.

Sharp, Z. D. 1990. Laser-based microanalytical method for the in situ determination of oxygen isotope ratios of silicates and oxides: Geochimica et Cosmochimica Acta, v. 54, p. 1353-1357.

Taylor, H.P.Jr. 1979. Oxygen and hydrogen isotope relationships in hydrothermal minerals deposit: In geochemistry of hydrothermal ore deposits, 2nd ed., Barnes, H.J. (ed), New York, John Wiley, p.236-277.

Utami, P. 2011. Hydrothermal alteration and the evolution of the Lahendong geothermal system, North Sulawesi, Indonesia. Unpublished PhD thesis, University of Auckland.

Zheng, Y.-F. 1993. Calculation of oxygen isotope fractionation in anhydrous silicate minerals. Geochim. Cosmochim. Acta 57, pp. 1079-1091.