Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey

VOLGEO-05994; No of Pages 12 Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

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Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey Daniele L. Pinti a,⁎, M. Clara Castro b, Aida Lopez-Hernandez c, Guolei Han b,d, Orfan Shouakar- Stash e, Chris M. Hall b, Miguel Ramírez-Montes f a

GEOTOP, Département des Sciences de la Terre et de l'Atmosphère, Université du Québec à Montréal, QC, Canada Dept. of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA Facultad de Ingeniería Civil, UMSNH, Morelia, Mich., Mexico d School of Earth Sciences and Resources, China University of Geosciences, Beijing, China e Isotope Tracer Technologies Inc., Waterloo, ON, Canada f Gerencia de Proyectos Geotermoeléctricos, CFE, Mexico b c

a r t i c l e

i n f o

Article history: Received 12 October 2016 Received in revised form 16 January 2017 Accepted 18 January 2017 Available online xxxx Keywords: Noble gases Helium Argon Reinjection fluids Los Humeros Geothermal Field Mexico

a b s t r a c t Los Humeros Geothermal Field (LHGF) is one of four geothermal fields currently operating in Mexico, in exploitation since 1990. Located in a caldera complex filled with very low-permeability rhyolitic ignimbrites that are the reservoir cap-rock, recharge of the geothermal field is both limited and localized. Because of this, planning of any future geothermal exploitation must be based on a clear understanding of the fluid circulation. To this end, a first noble gas survey was carried out in which twenty-two production wells were sampled for He, Ne, Ar, Kr, and Xe isotope analysis. Air-corrected 3He/4He ratios (Rc) measured in the fluid, normalized to the helium atmospheric ratio (Ra; 1.384 × 10−6), are consistently high across the field, with an average value of 7.03 ± 0.40 Ra. This value is close to that of the sub-continental upper mantle, indicating that LHGF mines heat from an active magmatic system. Freshwater recharge does not significantly affect He isotopic ratios, contributing 1–10% of the total fluid amount. The presence of radiogenic 40Ar* in the fluid suggests a fossil fluid component that might have circulated within the metacarbonate basement with radiogenic argon produced from detrital dispersed illite. Solubility-driven elemental fractionation of Ne/Ar, Kr/Ar, and Xe/Ar confirm extreme boiling in the reservoir. However, a combined analysis of these ratios with 40Ar/36Ar reveals mixing with an air component, possibly introduced by re-injected geothermal fluids. © 2017 Published by Elsevier B.V.

1. Introduction The Los Humeros Geothermal Field (LHGF), located ca. 200 km SE of Mexico City, is one of four geothermal fields currently operating in Mexico. The field, administrated by the Comisión Federal de Electricidad (CFE) of Mexico, has a total capacity of 68.4 MWe, with one 25 MWe unit under construction. The field is located within a complex caldera system, formed by ignimbrite eruptions initiated 460 ka ago and interrupted by the construction of rhyolitic domes and andesitic volcanoes. Activity ceased 20 ka ago, with the magma chamber reaching the final hydrothermal stage (Ferriz and Mahood, 1984). With geothermal fluids reaching 400 °C in the northern production area and steam enthalpy higher than 2400 kJ/kg (Gutiérrez-Negrín and IzquíerdoMontalvo, 2010), LHGF has great potential for further development. However, the field is characterized by a low-permeability reservoir and is isolated from regional recharge (Cedillo Rodríguez, 2000). ⁎ Corresponding author. E-mail address: [email protected] (D.L. Pinti).

Twenty-six years of exploitation have already caused boiling, phase separation, and condensation, while the effect of used geothermal fluid reinjection is still unclear (Arellano et al., 2015). Planning of future geothermal exploitation must be based on a clear understanding of fluid circulation within the system. Although numerous studies on the geology, as well as on the fluid geochemistry have been carried out, several questions remain unanswered. Some of the most relevant, concerning the dynamics of this geothermal system include: a) whether LHGF consists of two reservoirs or of one fed by different production zones (e.g., Arellano et al., 2003; Gutiérrez-Negrín and Izquíerdo-Montalvo, 2010); b) what the origin of the very acidic fluids deeper in the main productive northern zone is (Izquíerdo et al., 2009); and c) the extent of recharge and the location of the contributing areas. To further constrain the origin and mixing of LHGF fluids as well as the physical processes (e.g., boiling, condensation, re-injection) affecting the reservoir, a noble gas (He, Ne, Ar, Kr, and Xe) survey was conducted at LHGF in January 2015. This work is part of a larger project aiming to study fluid circulation in the four operating geothermal fields in Mexico (i.e., Cerro Prieto, Los Azufres, Los Humeros, and Las Tres

http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015 0377-0273/© 2017 Published by Elsevier B.V.

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

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D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

Virgenes). Major ions, trace elements, and 87Sr/86Sr, H, O, C, S, Cl, Br, and noble gas isotopes are analyzed to trace the sources of heat and fluids, to estimate fluid residence times and to identify physical processes induced by (over)-exploitation of the fields. Noble gases are particularly useful in the study of geothermal systems (e.g., Mazor and Truesdell, 1984; Smith and Kennedy, 1985; Kennedy et al., 1991; Magro et al., 2013; Pinti et al., 2013; Birkle et al., 2016). Helium has two isotopes. One is primordial 3He, and the other is radiogenic 4He. 3He is enriched in the mantle and is the most important volatile tracer of mantle-derived fluids in volcanic and hydrothermal areas (e.g., Sano and Wakita, 1985; Kennedy et al., 2000; Saar et al., 2005; Kennedy and Van Soest, 2007; Sano et al., 2015). Radiogenic 4 He produced by the decay of U and Th, coupled with other chronometers, can indicate the residence time of fluid in the reservoir (e.g., Birkle et al., 2016). Radiogenic 40Ar* produced by the decay of K in rocks and released in fluids at high temperatures can also indicate the extent of water-rock interaction and long fluid residence times (e.g., Pinti et al., 2013). Finally, atmosphere-derived noble gases (ANG) dissolved in water in recharge zones following their solubility (Air-Saturated Water (ASW) composition) can indicate recent recharge in the field. Departure of ANG concentrations from initial ASW values could be caused by either boiling and phase separation at depth (e.g. Mazor and Truesdell, 1984; Pinti et al., 2013) or by injection of used geothermal fluids (Kennedy et al., 1999; Kennedy and Shuster, 2000). These

phenomena can be distinguished and quantified using elemental and isotopic ratios of noble gases. 2. Geological context of the Los Humeros Geothermal Field The LHGF is located on the eastern portion of the Plio-Pleistocene Trans-Mexican Volcanic Belt, near the border of this province with the Sierra Madre Oriental province. The field is located inside a caldera complex system (Fig. 1; Ferriz and Mahood, 1984). The basement rocks of the LGHF are granites and schists of Paleozoic age, covered by a thick series of Jurassic and Cretaceous limestones, metamorphosed during the Laramide orogeny and by Oligocene magmatic intrusions (De la Cruz, 1983). Fissural volcanic activity in the area started in the Miocene (~10 Ma), producing the Alseseca Andesites that outcrop in the northeastern part of the Los Humeros caldera. Further volcanic activity did not take place until the Pliocene, when the volcanism associated with the Mexican Volcanic Belt started, producing the Teziutlán Andesites in the area (3.5–1.9 Ma ago) (Yáñez García and Casique Vázquez, 1980). Los Humeros caldera formation started 460 ka ago, when a highly differentiated magmatic chamber was emplaced beneath the Mesozoic calcareous sequence. This process continued until 20 ka ago, ultimately leading to two nested calderas (Los Humeros and Los Potreros), and several rhyolitic domes and basaltic-andesite volcanoes within them. The largest caldera, which formed 460 ka ago following an eruption of

Fig. 1. Simplified tectonic map of the central part of the Los Humeros caldera with the major faults and the positions of the sampled production (black circles: North Zone; white diamonds: Median Zone; grey squares: South Zone) and re-injection wells (triangles) indicated. Numbers on the X- and Y-axis indicate respectively the easting and northing geographical coordinates on the World Geodetic System 1984 (WGS84).

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

115 km3 of rhyolitic ignimbrites (Xaltipan Ignimbrite), has a diameter of 21 km (Ferriz and Mahood, 1984). Since this last major explosive episode, between 360 and 240 ka ago, several rhyolitic domes and rhyodacitic-andesitic plinian deposits were emplaced. At 100 ka, andesitic to rhyodacitic ignimbrites with an equivalent magma volume of 20 km3 erupted, creating a new collapsed area inside the Los Humeros caldera. This collapse, called Los Potreros (Fig. 1), has a diameter of ~7–10 km. Finally, volcanic activity ended 20 ka ago with basaltic and andesitic lava flows, several scoria cones, and some phreato-magmatic explosions (Ferriz and Mahood, 1984). Since then, a geothermal system has been active, the heat source of which is the magmatic chamber at the terminal hydrothermal stage (Gutiérrez-Negrín and IzquíerdoMontalvo, 2010). A third depressed area, inside the Los Potreros caldera, called the Colapso Central and currently corresponding to the main productive geothermal area (Fig. 1), is likely a morphological arrangement of lava flows rather than a volcanic caldera (Garduño Monroy et al., 1985). The geothermal reservoir of the Los Humeros system consists of a sequence of blocks surrounded by fractures and faults arranged as graben and horst and associated with the collapse process of the Los Humeros Caldera formation. Two main structural systems, consisting of normal faults, are visible in the field (Fig. 1): the oldest one has a NE-SW to EW direction, such as the Las Papas and Las Cruces faults (Fig. 1); the younger one is a set of NW-SE to N-S normal faults, such as the Maztaloya, Los Humeros, and La Antigua faults (Fig. 1), some of which cross-cut faults from the older system (Garduño Monroy et al., 1985). The La Antigua fault is an old basement structure, which was later reactivated and defines the western limit of the reservoir. 3. The Los Humeros Geothermal Field The ring-fracture border of Los Potreros caldera delimitates the LHFG (Fig. 1). Currently, the main production wells are concentrated in the North Zone (Fig. 1). The reservoir consists of medium- to low-permeability pre-caldera (10–1.9 Ma) andesites. Numerous studies have suggested the presence of two reservoirs: a deeper low liquid saturation reservoir in basalts and hornblende andesites, with low pH fluids at temperatures of between 300 and 400 °C, and a liquid-dominated shallower one in augite andesites, with neutral pH fluids at 300– 330 °C (e.g., Arellano et al., 2003). Based on field observations, Gutiérrez-Negrín and Izquíerdo-Montalvo (2010) have suggested that, rather than two separate reservoirs, one single reservoir with distinct production zones might be present. The distribution of low permeability granites and clayey limestone around the Los Humeros caldera, combined with the presence of annular faults, isolated the geothermal reservoir from regional recharge (Cedillo Rodríguez, 2000). Recharge might occur locally, from rainfall infiltrating the reservoir through its fault and fracture systems (Cedillo Rodríguez, 2000). LHGF wells mainly produce steam with enthalpy over 2400 kJ/kg, except for well H-1, which has always produced water with a lower enthalpy, of 1500–1700 kJ/kg. Water is chemically homogeneous, from Na-Cl type to H2CO3-SO4 type, with high boron, ammonia, and arsenic contents (Izquíerdo et al., 2009; Bernard et al., 2011), and is saturated in silica and calcite (Barragán et al., 1991). Wells producing water show a large ionic unbalance, perhaps due to the presence of condensate, which contains relatively high concentrations of bicarbonates and sulfates that are subtracted from the ionic balance (Arellano et al., 2003). One special feature of LHGF is the presence of very low pH fluids in wells drilled in the North Zone, at N 1800 m depth (Izquíerdo et al., 2009). It has been proposed that these acidic fluids come from a deep acid geothermal reservoir located in the hornblende andesites, but this theory has since been discarded (Izquíerdo et al., 2000). Instead, the formation of low pH fluids is explained as a post-exploitation process. It could be related to the migration of deep magmatic volatile species, CO2, H2S, Cl, and F, induced by the extraction of fluids from the

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reservoir, which react with aqueous fluids, producing aqueous corrosive species (Izquíerdo et al., 2009). First exploration of the LHGF took place in 1968. The first deep well was drilled in 1981, and the commercial exploitation of the resource began with the installation of the first 5 MWe unit in 1990. From 1990 to 2015, N 40 wells were drilled, four of them (H-13D, H-29D, H-38 and H-40) re-injection wells. During this period, the capacity of the field was increased to a net value of 68.4 MW (Arellano et al., 2015). The most profitable areas of the field are the North Zone and the corridor formed between the La Antigua and Mastaloya faults (Fig. 1) (Gutiérrez-Negrín and Izquíerdo-Montalvo, 2010). After 26 years of commercial exploitation, the extraction and reinjection of fluids in a low-permeability reservoir with very limited recharge have induced five physical processes: (a) moderate boiling with increasing steam; (b) substantial boiling with steam condensation; (c) production of returns from injection, either as liquid or as steam, and the production of steam and sometimes condensed steam from injection fluid boiling; (d) interaction with deep fluids; and (e) decrease in liquid saturation due to insufficient recharge (Arellano et al., 2015). 4. Sampling and analytical methods Sampling was carried out in January 2015. Twenty-two productive wells were initially sampled (Table 1). However, analyses from well H-37 were discarded because the casing is cracked, allowing modern freshwater to enter the well, modifying the pristine isotopic composition of the fluid (H-37 results are therefore not reported in Table 1). Water samples for stable isotope analyses were collected directly at the exit of the steam/water separator using a metallic container. The water was allowed to cool to ambient temperature prior to being transferred into Nalgene® bottles. Samples for noble gas analyses were collected in standard refrigeration-grade 3/8″ copper tubes (~14 cm3), sealed by stainless steel pinchoff clamps after gas was flowed through for several minutes. In wells not equipped with a steam/water separator, gas samples were collected directly at the wellhead, using a mini-separator and a cooling coil. In wells equipped with a steam/water separator, the copper tube was fixed to a small stool aligned with one of the output valves of the steam separator conduit. A single copper tube was extended from the sampler to the NPT-type male connector screwed on the steam conduit valve. The clamps were closed using electric drills. Since this fluid sampling method has been implemented at the GEOTOP Noble Gas Laboratory, the risk of accidental noble gas air contamination has been reduced to nearly zero (e.g., Saby et al., 2016). The noble gas analyses were carried out at the Noble Gas Laboratory of the University of Michigan (Wen et al., 2016). Gas samples in the copper tubes were attached to a vacuum extraction and purification system. The copper tube was connected to a vacuum system at a pressure of ~2 × 10−5 Torr. Once this pressure was achieved and the system was isolated from its turbo-molecular vacuum pump, the lower clamp was opened to release gas into a low He diffusion glass flask. Subsequently, the gas sample was dried by the water adsorption properties of the molecular sieve and most active gas components were removed by letting samples into the first getter with a Ti sponge at 600 °C for ~60 min. Remaining gases were subsequently admitted into a cleanup section of the line equipped with the second getter pump, which removed the remainder of the active gases. Noble gases were then quantitatively extracted and sequentially allowed to enter the Thermo Scientific® Helix SFT (for He and Ne isotopes) and ARGUS VI (for Ar, Kr, and Xe isotopes) mass spectrometers using a computer-controlled cryo-separator. Ar, Kr, and Xe were pumped into the high temperature (high-T) chamber of the cryo-separator at a temperature of 104 K, while He and Ne were pumped into the low temperature (low-T) cryo-separator chamber at a temperature of ~10 K. The cryo-temperature was then increased sequentially to release He, Ne, Ar, Kr, and Xe, at temperatures of 49 K, 84 K, 210 K, 245 K, and

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

4

Easting

Northing

Depth

4 He ±1σ ccSTP/cctot

20

Well

WGS84 zone 14

WGS84 zone 14

m

×10−6

×10−8

H-03 H-06 H-06 H-07 H-09 H-12 H-15 H-17 H-19 H-19 H-20 H-30 H-31 H-33 H-34 H-35 H-41 H-42 H-44 H-45 H-48D H-49 Aira

660633.4 663504.7 663504.7 661843.6 660625.3 663803.4 661639.2 662296.3 662885.0 662885.0 663325.8 661490.1 661826.3 661859.3 662964.8 661057.8 663568.3 663321.5 662534.5 661602.9 662058.5 661891.7

2177886.5 2173548.7 2173548.7 2175858.1 2178211.7 2173053.3 2178808.2 2178602.0 2176645.6 2176645.6 2177492.1 2178545.5 2179043.6 2178200.8 2177224.4 2178366.3 2173281.7 2173507.0 2178355.1 2176391.0 2175611.0 2175049.9

1860 2541 2541 2340 2500 3104 1502 1700 1850 1850 2402 1917 1926 1600 1800 1690 2200 2200 1770 2280 2010 2030

3.011 2.623 27.099 12.261 0.145 12.046 0.163 0.091 13.652 20.216 3.386 15.659 5.066 11.570 4.722 0.262 0.040 3.556 11.964 22.745 6.072 24.684 5.24

0.045 0.039 0.406 0.184 0.002 0.181 0.002 0.001 0.205 0.303 0.051 0.235 0.076 0.174 0.071 0.004 0.001 0.053 0.179 0.341 0.091 0.370

Ne

0.164 b.l. 8.148 3.642 0.014 0.055 0.415 1.029 0.159 0.132 0.174 227.674 1.116 422.680 1.034 0.783 b.l. 0.078 377.724 0.070 0.388 25.388 1645

±1σ

36

Ar

±1σ

×10−8 0.002 – 0.106 0.047 0.001 0.003 0.005 0.013 0.002 0.002 0.002 2.960 0.015 5.495 0.013 0.010 – 0.001 4.910 0.001 0.005 0.330

0.508 0.484 17.817 8.226 0.191 0.553 3.339 6.289 0.423 0.590 1.083 481.658 4.171 805.430 3.897 5.180 b.l. 0.312 864.870 0.415 0.999 43.196 3161

84

Kr

±1σ

×10−10 0.013 0.017 0.232 0.107 0.007 0.007 0.043 0.082 0.010 0.008 0.014 6.262 0.054 10.471 0.051 0.067 – 0.006 11.243 0.007 0.013 0.562

1.70 1.44 41.99 20.16 1.17 2.09 13.39 25.80 1.61 2.31 4.52 953.41 15.62 1655.72 14.54 20.50 0.36 1.24 1837.87 1.38 2.68 85.50 20000

132

±1σ

F(4He)

±1σ

F(20Ne)

±1σ

F(84Kr)

±1σ

F(132Xe)

±1σ

0.10

0.00 0.04 0.02 0.00 0.00 0.02 0.04 0.00 0.00 0.01 0.81 0.02 1.33 0.02 0.03 0.00 0.00 1.51 0.00 0.00 0.06

1.036 1.267 0.181 0.177 0.177 2.593 0.006 0.002 5.480 4.082 0.372 0.004 0.145 0.002 0.144 0.006 – 1.710 0.002 7.707 0.723 0.068

0.617 n.d. 0.874 0.846 0.136 0.188 0.237 0.312 0.720 0.427 0.307 0.903 0.511 1.002 0.507 0.289 n.d. 0.475 0.834 0.322 0.741 1.123 0.5235

0.017 – 0.016 0.016 0.007 0.009 0.004 0.006 0.020 0.008 0.006 0.017 0.009 0.018 0.009 0.005 – 0.012 0.015 0.009 0.014 0.021

1.621 1.436 1.140 1.186 2.972 1.828 1.939 1.985 1.838 1.897 2.016 0.957 1.811 0.994 1.805 1.915 n.d. 1.924 1.028 1.614 1.300 0.957 0.0207

0.047 n.d. 0.023 0.024 0.116 0.036 0.038 0.039 0.052 0.038 0.040 0.019 0.036 0.020 0.036 0.038 – 0.048 0.020 0.038 0.026 0.019

2.690 – 1.214 1.290 9.065 2.665 3.457 3.580 3.377 3.408 4.038 1.024 3.077 1.012 3.069 3.264 n.d. 3.748 1.063 2.842 1.624 0.865 0.00074

0.090

1.61 0.79 0.13 0.11 0.86 1.68 0.11 0.15 0.33 36.72 0.96 60.68 0.89 1.26 0.06 0.09 68.43 0.09 0.12 2.78 233.9

35.52 32.48 9.12 8.94 4.55 130.65 0.29 0.09 193.63 205.66 18.74 0.19 7.28 0.09 7.27 0.30 n.d. 68.41 0.08 328.92 36.45 3.43 0.1667

Xe

×10−10 0.03 0.02 0.63 0.30 0.02 0.03 0.20 0.39 0.02 0.03 0.07 14.30 0.23 24.84 0.22 0.31 0.01 0.02 27.57 0.02 0.04 1.28

Last four numerals are the atmospheric elemental i/36Ar ratios (with i being 4He, 20Ne, 84Kr and 132Xe). The F(i) values for air are obviously equal to 1 by definition. a Isotopic abundances of noble gases are reported as volume fraction. Data from Ozima and Podosek (1983).

0.031 0.033 0.382 0.068 0.088 0.091 0.110 0.087 0.103 0.026 0.079 0.026 0.078 0.083 – 0.111 0.027 0.081 0.041 0.022

D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

Table 1 Noble gas isotopic abundances and F(i) values for the Los Humeros fluids.

D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

5

reported, as the measured errors are N50%. Samples H-06 and H-19 were analyzed twice (H-06a and H-06b, and H-19a and H-19b respectively). Both the He and the Ar isotopic compositions measured in samples H-06a and H-06b are distinct. In particular the 40Ar/36Ar ratios were 398.2 in H-06a and 317.6 in H-06b. The higher 40Ar/36Ar value in the first measurement likely indicates a more pristine sample, as opposed to the second one where a small amount of air contamination might have occurred. In contrast, He, Ne, and Ar isotopic compositions of H19a and H-19b were identical, within the stated uncertainties (Table 2). Air-normalized helium isotopic ratios, R/Ra, range from 6.09 ± 0.08 for well H-17 to 7.62 ± 0.07 for well H-6 (but repeated analysis of well H-06 shows a lower R/Ra of 7.34 ± 0.06) (Table 2). These values clearly indicate a dominant mantle origin for helium. 20Ne/22Ne ratios are close to the atmospheric value of 9.80, within uncertainties, except for H-03 and H-42, which show values lower than the atmospheric ratio. 21 Ne/22Ne ratios range from atmospheric values (0.0290; Ozima and Podosek, 1983) to 0.0517 ± 0.0159, indicating the addition of terrigenic 21 Ne* of crustal and/or mantle origin. Finally, 40Ar/36Ar ratios range from the atmospheric value of 295.5, within uncertainty, to 865.3 ± 15.6 for well H-45, indicating the addition of radiogenic and/or mantle 40 Ar*.

290 K respectively. Specifically, at the He release temperature, He was introduced into the SFT mass spectrometer and the signal intensity of 4 He was determined for the He concentration estimate. This estimate was then used by the automated system to optimize the amount of He that should be introduced for the 3He/4He ratio measurement. Complete measurement procedures involved estimating the concentration of each noble gas component, as well measuring He, Ne, Ar, Kr, and Xe isotopic ratios. Standard errors for concentrations are 1.5, 1.3, 1.3, 1.5, and 2.2%, respectively. First, a known volume of air was introduced into the molecular sieve section of the extraction system, and all noble gases were measured in turn with the Helix SFT and ARGUS VI mass spectrometers. This calibrates the mass spectrometer signal size for each noble gas. Following the air calibration run, the same measurement procedure was performed on a portion of the unknown sample. All noble gas isotopes were measured using a Faraday detector, except for 3He, which was measured using an electron multiplier in ion counting mode. Additional sampling, extraction, and purification procedures can be found in the literature (Castro et al., 2009, Hall et al., 2012). 5. Results Sample location, well's depth, and noble gas isotopic concentrations (4He, 20Ne, 36Ar, 84Kr and 132Xe) are reported in Table 1. Noble gas relative abundances are given using the F-value notation (Table 1), in which measured 4He, 20Ne, 84Kr, and 132Xe abundances, “i”, are normalized to the air abundances, with 36Ar as the reference isotope (e.g., F(i) = [(i/36Ar)sample/(i/36Ar)air]). F-values are fractionation factors that provide a measure of enrichment or depletion relative to the atmospheric composition, and can be useful fingerprints of boiling and gas depletion in the reservoir. Table 2 reports He, Ne, and Ar isotopic compositions, as well as the stable isotopic composition of the water (δD and δ18O) and 3He/4He ratios (R) normalized to the air value (Ra = 1.384 × 10−6). For wells H-09, H-35, and H-41, stable isotopes were not measured because no water phase was available at the wellhead (Table 2). Kr and Xe isotopic compositions were atmospheric with minimal overall fractionation and are reported in Appendix Table A1. A few samples (H-09, 12, 20, and 41) display concentrations close to blank levels, leading to poor measurement precision of the Ne isotopic composition. These values are not reported in Table 2. The 36Ar concentration of sample H-41 was very low, and thus both the Ar isotopic composition and F(i) values are not

6. Discussion 6.1. Stable isotopes: Freshwater and pristine fluids Fig. 2 reports the stable isotopic composition of LHGF fluids discharged from productive wells in a classic δD vs. δ18O diagram, together with fluids from two cold springs sampled in the north (Huitchotita) and east (Calzacateno) of the field, as well as one water well (PZ-107) in the southeast, outside of the Los Humeros caldera, close to the town of Perote. Stable isotopic compositions of other springs and domestic wells around LHGF are also available in Portugal et al. (1994). The theoretical local water meteoric line (LWML) calculated using the Online Isotope Precipitation Calculator (OIPC; Bowen and Wilkinson, 2002) is also reported for reference. The geothermal fluids discharged from the LHGF result mainly from mixing between a meteoric and an “andesitic” water component (Fig. 2), the latter having been interpreted by Giggenbach (1992) as recycled seawater carried to the source region of arc magmas on the top of the subducting slab. Previous work by Arellano et al. (2003) and Barragán et al. (2010) has

Table 2 He, Ne and Ar isotopic composition, together with stable isotopes of water for the Los Humeros fluids. Well

δD

±

δ18O

±

R/Ra

± 1σ

Rc/Ra

± 1σ

20

Ne/22Ne

H-03 H-06a H-06b H-07 H-09 H-12 H-15 H-17 H-19a H-19b H-20 H-30 H-31 H-33 H-34 H-35 H-41 H-42 H-44 H-45 H-48D H-49 Air

−62.5 −66.7

0.3 0.1

−1.86 −3.3

0.03 0.06

−57.1 n.d. −64.1 −66.3 −47.0 −64.8

0.2 – 0.4 0.2 0.2 0.4

−0.08 n.d. −2.97 −5.02 0.26 −3.19

0.05 – 0.06 0.02 0.1 0.08

−68.2 −65.0 −63.6 −63.6 −61.6 n.d. n.d. −59.8 −51.6 −67.4 −69.6 −52.1

0.2 0.3 0.4 0.2 0.4 – – 0.2 0.2 0.3 0.2 0.3

−3.51 −4.05 −3.55 −4.63 −2.65 n.d. n.d. −0.66 −0.66 −5.1 −4.38 −0.8

0.09 0.04 0.18 0.02 0.1 – – 0.08 0.1 0.12 0.09 0.06

6.35 7.62 7.34 7.32 6.86 6.95 6.96 6.09 7.27 7.37 7.29 7.00 7.18 6.69 7.18 7.09 6.13 6.42 6.64 7.22 6.80 7.28 1.00

0.05 0.07 0.06 0.08 0.05 0.07 0.06 0.08 0.07 0.06 0.05 0.05 0.06 0.06 0.06 0.05 0.07 0.06 0.08 0.06 0.07 0.06

6.35 7.62 7.34 7.33 6.86 6.95 7.00 6.25 7.27 7.37 7.29 7.25 7.18 7.32 7.18 7.14 6.13⁎ 6.42 7.17 7.22 6.80 7.30 1.00

0.05 0.07 0.06 0.08 0.05 0.07 0.06 0.10 0.07 0.06 0.05 0.13 0.06 0.34 0.06 0.05 0.07 0.06 0.29 0.06 0.07 0.06

9.39 n.d. 9.81 9.82 n.d. n.d. 9.69 9.71 9.93 10.11 n.d. 9.81 9.76 9.85 9.84 9.79 n.d. 9.32 9.80 10.66 9.84 9.86 9.80

± 1σ

21

Ne/22Ne

0.23 – 0.02 0.03 – – 0.11 0.08 0.20 0.23 – 0.02 0.05 0.02 0.05 0.06 – 0.25 0.01 0.72 0.11 0.02

0.0252 n.d. 0.0291 0.0286 n.d. n.d. 0.0276 0.0247 0.0327 0.0323 n.d. 0.0290 0.0270 0.0291 0.0257 0.0296 n.d. 0.0404 0.0289 0.0517 0.0252 0.0292 0.0290

± 1σ

38

Ar/36Ar

± 1σ

40

Ar/36Ar

0.0077 – 0.0001 0.0003 – – 0.0034 0.0018 0.0082 0.0045 – 0.0001 0.0013 0.0001 0.0011 0.0015 – 0.0093 0.0000 0.0159 0.0044 0.0001

0.1949 n.d. 0.1904 0.1886 0.1874 n.d. 0.1910 0.1880 0.1770 0.1691 0.1894 0.1882 0.1868 0.1878 0.1872 0.1891 n.d. 0.1717 0.1879 0.1842 0.2022 0.1889 0.1880

0.0244 – 0.0006 0.0012 0.0234 – 0.0032 0.0041 0.0215 0.0119 0.0080 0.0001 0.0023 0.0001 0.0024 0.0019 – 0.0213 0.0001 0.0234 0.0110 0.0001

507.2 398.2 317.6 317.4 343.7 774.7 302.4 302.6 726.3 726.9 340.6 296.0 313.0 295.0 312.2 301.1 n.d. 518.7 295.4 865.3 378.2 304.3 295.5

± 1σ 12.6 14.7 0.2 0.3 12.4 9.3 1.3 1.6 17.3 8.0 2.8 0.1 1.1 0.1 1.1 0.6 – 10.2 0.1 15.6 4.9 0.1

⁎ Because Ne and Ar are close to blank levels, 3He/4He ratio of H-41 fluid has been not corrected for the air component.

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

6

D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

Fig. 2. The δD versus δ18O of the LHGF production fluids, and cold springs and water wells (Calzacateno, Huitchotita spring, and PZ-107 well at Perote; half-tone circles; this study), and of cold springs and private water wells measured by Portugal et al. (1994) (triangles). The local meteoric water line (LMWL; dashed line) was calculated using the model of Bowen and Wilkinson (2002). Primary magmatic water and andesitic water isotopic compositions are from Sheppard and Epstein (1970) and Giggenbach (1992) respectively. The straight trend line is calculated using the geothermal wells data points.

shown that the isotopic composition of the water molecule is dominated by two processes. The first is mixing of recharge water with a deep fluid-type andesistic water, leading to a positive correlation between δD and δ18O, as shown in Fig. 2. The second process is boiling and phase separation. At fluid temperatures higher than 220 °C, 18O is preferentially partitioned into the fluid phase, while deuterium is slightly partitioned into the vapor phase (Arellano et al., 2003). The resulting fractionation scatters the points a few per mil perpendicular to the main mixing trend (Fig. 2; Arellano et al., 2003; Barragán et al., 2010). 6.2. Mantle helium: Homogeneity of the source Helium has three main sources: the mantle, the crust, and the atmosphere (e.g., Ozima and Podosek, 1983; Ballentine et al., 2002). Air-normalized 3He/4He ratios, R/Ra, were plotted against the 4He/20Ne ratios (Fig. 3). Most of sampled fluids are compatible with a two-component mixing between freshwater introducing atmospheric helium dissolved in equilibrium with water (Air Saturated Water or ASW) and a deeper fluid carrying terrigenic helium, mainly of mantle origin (Fig. 3). The mixing line between the terrigenic source and the present recharge (ASW composition) indicates that N90% of the fluid is of deep origin, with a maximum contribution of 10% derived from freshwater (Fig. 3). Interestingly, the wells located in the North Zone appear to have a higher recharge component than those in the South Zone (Fig. 3). This could indicate that limited local recharge occurs mainly along the circular fractures bordering the Los Potreros Caldera in the North Zone. The South Zone could essentially be isolated from this recharge, and thus its further exploitation might lead to a rapid decline in production. Air-normalized 3He/4He ratios (R/Ra) were corrected for the atmospheric component, (Rc/Ra) (Table 2), using the 4He/20Ne ratio, as follows (e.g., Sano et al., 2006; Pinti et al., 2013): 

 R −r Rc Ra ¼ ð1−rÞ Ra 4

He



20 Ne

r ¼ 4

ð1Þ

 ASW

He 20 Ne meas

ð2Þ

Fig. 3. The 3He/4He ratio (R), normalized to that of the atmosphere (Ra), versus 4He/20Ne ratios. All data can be interpreted as a mixture between water at equilibrium with the atmosphere (ASW, i.e., local modern recharge water) and a mantle-derived helium fluid with an average R/Ra of 6.96 ± 0.42 (average of measured values in the LHGF). Percentages are the volume percentage amounts of freshwater in the mixing. A few wells lie below the mixing curves, indicating the addition of b10% crustal helium.

A (4 He/ 20Ne) ASW value of 0.273 was calculated with solubility equations of Smith and Kennedy (1983) and used for the average annual local recharge temperature of 25 °C. The (4He/20Ne)meas is the measured ratio calculated from data reported in Table 1. Because neon was below the detection level in sample H-6a, the helium ratio was corrected for the air component by replacing the (4 He/ 36 Ar) ratio in Eqs. (1) and (2), as suggested by Marty et al. (1993). In sample H-41, which is heavily degassed and for which both Ne and Ar abundances could not be accurately measured, the R/Ra ratio of 6.13 ± 0.07 was not corrected (Table 2). Errors on the calculated Rc/Ra values account for the analytical errors on the measured 3He/4He ratio and on the 4He and 20Ne concentrations (ranging from 2 to 4%), and were calculated using equations developed in Sano et al. (2006). Air-corrected Rc/Ra ratios range from 6.25 ± 0.10 to 7.67 ± 0.07 (Table 2), with an average of 7.03 ± 0.40. This average value is within the range measured in MORBs (8 ± 1; Farley and Neroda, 1998; Graham, 2002) or of the sub-continental mantle reservoir (6.1–7; Ballentine and Burnard, 2002; Gautheron and Moreira, 2002). Bernard et al. (2011) reported measured Rc /R a values between 7 and 7.5 Ra for the LHGF, similar in range to those measured in this work. That the helium isotopic composition of LHGF fluids is close to that of pristine mantle values indicates that they derive heat from an active, near-surface magmatic system (Kennedy and Van Soest, 2006). It is important to note that four samples (H-03, H-17 from the North zone and H41 and H-42 from the South zone) are outliers, with Rc/Ra ratios below the 7.03 ± 0.40 range. To explain these low Rc /Ra values, a small amount of crustal radiogenic 4 He must be added to the mixture. Assuming the average R c/Ra value of 7.03 to represent the sub-continental mantle end-member in the region and using a simple binary mixing equation, the crustal radiogenic helium component is calculated to account for 9–13% of the mixture. It is also worth noting that samples H-03 and H-42 have 40Ar/36Ar ratios well above the atmospheric value of 295.5, suggesting the presence of radiogenic crustal argon. This is further discussed in Section 6.3. These samples might represent areas of low fluid circulation, where stagnant fossil waters had time to accumulate the crustallyproduced noble gas isotopes, 4He and 40Ar*.

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

6.3. Radiogenic argon: Lithological control and fluid residence time Several sampled fluids show 40Ar/36Ar ratios higher than the atmospheric ratio of 295.5, which could be accounted for by either mantle or crustal 40Ar* addition. Mantle argon is rarely detected in geothermal fluids. Indeed, the atmosphere contains a significant argon component (1%) and very little 3 He, while in the mantle, argon is present at low levels together with high 3He contents. This leads to the rapid dilution of mantle argon, with 40Ar/36Ar ratios indistinguishable from the atmospheric value. This rapid dilution renders the identification of mantle argon, based on the 40Ar/36Ar ratios alone, nearly impossible. In contrast, 3He/4He is essentially unaffected by the presence of freshwater, until the proportion of freshwater in the mixture reaches at least 80% or more of the total amount of fluid present. This dilution effect is shown in Fig. 4, where the measured R/Ra ratios have been plotted against the 40 Ar/ 36 Ar ratios. In this plot, the mixing between a mantle source and an ASW source is described by a hyperbole, where the curvature parameter r is equal to the ratio between the 4 He/ 36 Ar values measured for the two end-members (Langmuir et al., 1978; Fig. 3). Here, a sub-continental mantle 40 Ar/ 36 Ar ratio of 30,000 is assumed (Dunai and Baur, 1995) and a 4 He/ 36 Ar ratio of 39,000 is calculated using a mantle 3 He/ 36 Ar ratio of 0.4 (Moreira et al., 1998) and a maximum 3 He/ 4 He of 7.43R a (this study). This would result in a mantle 4 He/40Ar ratio of 1.3, in line with that expected in the upper mantle (Graham, 2002). For the ASW end-member, the 4He/36Ar ratio calculated at 25 °C is 0.0463 using solubility data from Smith and Kennedy (1983). Most of the helium and argon ratios measured in LHGF fluids can be accounted for by mixing between a deep mantle and meteoric sources (ASW). The amount of mantle argon in the mixture is b2%. Several samples do not plot on the mixing hyperbola (Fig. 4). The isotopic composition of these fluids might be explained by either 1) mixing between an ASW and a mantle source having different 4He/36Ar ratios; or 2) mixing with a radiogenic component, the

Fig. 4. The R/Ra vs 40Ar/36Ar ratios mixing plot. The mixing hyperbola with different curvatures, r, have been plotted. Note that the Y-axis is split for clarity. Numbers on the hyperbolae correspond to the calculated 4He/40Ar expected values in each mixture. The calculated values range from 1.3 to b0.001, and are inconsistent with those calculated in Fig. 6, which vary from 2.5 to 8.7. The arrow indicates the possible mixing between a fluid containing a mixture of mantle-ASW helium and argon and another fluid containing ASW helium and radiogenic argon, the composition of which is somewhere in the left upper region of the plot.

7

isotopic composition of which is located somewhere in the left upper hand side of the plot (Fig. 4). If the first hypothesis is correct, mantle end-member 4He/36Ar ratios 100–1000 times smaller than the theoretical value of 39,000 should be assumed in order to fit the data (dashed curves in Fig. 4). In Fig. 4, numbers in the plot are the expected 4He/40Ar ratios of the mantle source for the different mixing hyperbolas. These values are inconsistent with those calculated for the LHGF fluids, as reported in Fig. 6 (see below for a detailed discussion). 4 He/ 40 Ar ratios for the LHGF fluids range from 2.5 to 8.7 (Fig. 6), which would lead to an even steeper slope of the mixing hyperbola than that calculated for a mantle ratio of 1.3 (Fig. 4). The second hypothesis suggests that a third fluid, carrying radiogenic 40Ar*, is added to the mixture. This radiogenic component is clearly identified when the stable isotopic composition of the water molecule (δ18O, δD) is plotted against 40Ar/36Ar ratios (Fig. 5a, b). By plotting δ18O and δD versus 40Ar/36Ar ratios, it can be noticed that more pristine “andesitic” water, with δ18 O of higher than 0‰ (Fig. 5a) and δD higher than − 50‰ (Fig. 5b), shows atmospheric 40 Ar/ 36 Ar ratios or slightly higher, while samples with lighter 18 δ O and δD values, down to − 5‰ and − 70‰, respectively (i.e.,

Fig. 5. a) The δ18O versus 40Ar/36Ar ratios and b) δD versus 40Ar/36Ar ratios. A three component mixing can be observed: 1) andesitic water and 2) modern recharge, with 40 Ar/36Ar ratios mainly atmospheric or slightly enriched in mantle 40Ar but discriminated by different δ18O and δD values: andesitic heavier, modern recharge lighter. The third component is fossil water, enriched in 40Ar/36Ar of crustal origin with lighter δ18O and δD values.

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

8

D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

more affected by freshwater recharge), show both atmospheric 40 Ar/36Ar ratios and 40Ar/36Ar ratios of N 800 (Fig. 4). This suggests a three-component mixing: recent freshwater with lighter δ 18 O and δD and atmospheric 40 Ar/ 36Ar ratios; fossil water carrying radiogenic 40Ar* but δ18O and δD compatible with a meteoric origin; and aqueous fluids related to the magma source with heavier “andesitic” δ18O and δD values, and 40Ar/36Ar ratios similar to the atmospheric value of 295.5 or slightly higher (because of the dilution described above; Fig. 4). Except for well H-03 in the North Zone, all other samples that show clear radiogenic 40 Ar* anomalies (H-12, H-19, H-42 and H-45) are from the Median and South Zones of the LHGF (Fig. 5a,b). Assuming an initial δ 18 O and δD of − 13.5‰ and − 96.3‰, respectively, extrapolated for modern recharge from the fitting of data shown in Fig. 2, the regression line through data points H-03D, H-19, H-42, and H-45 allows a crustal 40 Ar/36 Ar ratio to be determined in pre-production fluids, ranging from 1640 (from the 40Ar/36Ar- δ18O trend; Fig. 5a) to 1703 (from the 40Ar/36Ar- δD trend; Fig. 5b). In order for large amounts of radiogenic 40Ar* to accumulate in groundwater, it is expected that the reservoir temperature is higher than the closure temperatures of the main K-bearing minerals, such as muscovite, biotite, or sanidine (250–300 °C), so that radiogenic 40 Ar* produced by 40 K radioactive decay in the rock can be transferred efficiently to the fluid through diffusion (e.g., O'Nions and Ballentine, 1993; Ballentine et al., 1994). However, in the LHGF, temperature does not seem to be the key parameter to explain the regional distribution of radiogenic 40Ar* in the geothermal fluids. Indeed, the hottest reservoir temperatures (400 °C), higher than most of the closure temperatures of K-bearing minerals, have been recorded in the North Zone, where the amount of radiogenic 40 Ar* is either minimal or absent (Fig. 5a,b). To explain the observed spatial distribution of radiogenic Ar in the field, it can be postulated that fluid residence time in the Median and South Zones is significantly longer than in the North Zone, allowing for radiogenic Ar accumulation in the fluids due to prolonged water-rock interactions. However, it is noteworthy that the initial regional distribution of the argon isotopic signature in the pre-production fluids might have been significantly altered by recent dilution from atmospheric argon-rich re-injected fluids, as discussed below (cf., Section 6.3). These two hypotheses can't therefore be entirely rejected. An alternative hypothesis is that wells showing anomalous 40Ar* are located near the hydrothermal alteration halos, where K-bearing minerals might occur. In the LHGF, three alteration zones have been identified containing K-bearing illite at low (b 400 m) and median (b 1800 m) depths, and biotite at the deepest depth (N1800 m) (Martínez-Serrano, 2002). Illite can be affected by diffusive Ar loss into pore water at temperatures N 150 °C (e.g., Hamilton et al., 1989). However, the shallower alteration zones can be excluded as possible sources of radiogenic argon. Indeed, preliminary strontium isotopic data obtained during this survey show 87Sr/86 Sr ratios ranging from 0.70407 ± 0.00001, characteristic of the andesite-rhyolite rocks of the area (Verma, 2000), to 0.70885 ± 0.00001, typical of carbonates (e.g., Veizer et al., 1997). Interestingly, fluids from wells H-12, H-19, and H-45, which show the highest 40Ar/36Ar ratios, ranging from 726.3 to 865.3, have 87 Sr/86 Sr ratios ranging from 0.70853 to 0.70885, typical of carbonates. This suggests that radiogenic argon-rich fluids circulated into the deeper basement, made of metacarbonates, primarily acquiring strontium from the local carbonates, and possibly argon from detrital illite, which is commonly found in carbonates. A plot of 40 Ar/ 36 Ar ratios versus 4 He/ 36 Ar ratios (Fig. 6) shows that fluids from the three zones of the field (i.e., North, Median and South) lie on three straight lines representing mixing between freshwater carrying 4 He and 40 Ar of atmospheric composition, and a fluid enriched in both terrigenic (crustal and/or mantle) 4 He and

Fig. 6. 40Ar/36Ar versus 4He/36Ar (normalized to that of the atmosphere or F-notation). Fluids from the three production zones (North, Median, and South) are aligned on three different slopes, suggesting a different mantle/crustal source responsible for the Ar and He origin in the reservoir (see text for details).

40 Ar*. In Fig. 6, the fluid 4 He/36Ar ratio has been normalized to the atmospheric ratio for simplicity. In this plot, atmospheric F( 4 He) ratio is 1. It is worth noting that noble gas elemental fractionation driven by solubility differences during phase separation (boiling; see Section 6.3 for details) can displace points along horizontal trajectories. However, solubility-driven boiling will not affect the argon isotopic composition (e.g., Kennedy et al., 1991). Although diffusive kinetic effects might occur, this has been shown to be minimal (e.g., Kaneoka, 1994). Elemental fractionation will lead to lower F( 4 He) in the residual fluid, due to the lower solubility of helium relative to argon. However, at the observed high temperatures of the LHGF reservoir, He and Ar solubility values are similar (e.g., Potter and Clynne, 1978; Smith, 1985). Birkle et al. (2016) have shown that the effect of Rayleigh distillation should be minimal. It is expected that the relationship between 40 Ar/ 36 Ar and 4He/ 36 Ar will not be significantly affected by solubility-driven noble gas elemental fractionation. The lack of a clear relationship between F( 4 He) and F( 84 Kr) or F( 132 Xe) (not shown) supports the hypothesis that F( 4 He) is indeed not significantly fractionated. The slopes of the three straight lines provide the average 4He/40Ar ratios for each group of samples. Using the U/4He and K/40Ar* production rate equations (see e.g., Ballentine and Burnard, 2002), the K/U ratio of each fluid source can be extrapolated from the resulting 4 He/40Ar ratios (corrected for the atmospheric ratio because of the normalization used). Resulting 4 He/40Ar and K/U ratios from the fluid source in the North Zone are 2.8 and 20,000, respectively. These values are within the ranges of those reported for the upper mantle, of 2.5 and 19,000 ± 2600 respectively (Graham, 2002; Arevalo Jr et al., 2009), suggesting that local mantle is the source of these two isotopes, and that little interaction with external fluids occurs. 4He/40Ar ratios calculated for the two other zones are higher than the mantle value of 2.5, up to 8.7 for the South Zone, and with K/U of the hypothetical source lower than the mantle, down to 6767 (Fig. 6). This could indicate different local crustal sources (different lithologies) adding radiogenic 40Ar* into a He-Ar mantle-dominated fluid. K/U ratios lower than those measured in silicate rocks could indicate the influence of the metacarbonate basement, as suggested by the measured 87Sr/ 86 Sr ratios, of up to 0.708850 in the fluids. Indeed, carbonate rocks show K/U ranging from 8000 down

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

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9

relationships between ln(4He/40Ar*) and ln(40Ar*) (Burnard et al., 2004) and this is not the case here. Further, this mechanism presumes that most Ar is of mantle origin, which seems not to be the case in LGHF fluids (Fig. 4).

6.4. Atmospheric noble gas ratios: Boiling and re-injection effects in the field Phase separation at depth (boiling and steam condensation) resulting from continuous exploitation can be identified through elemental fractionation of noble gases caused by solubility-driven Rayleigh distillation processes (e.g., Mazor and Truesdell, 1984; Pinti et al., 2013). Mixing with re-injected used geothermal fluids (injectate) can be also identified with noble gases, as the amount and isotopic signatures of noble gases in the injectate will be distinct from that of the pre-flashed production fluid. Generally, the total amount of noble gases in re-injected fluids is 100 to 1000 times less than that expected in waters in equilibrium with air or ASW. The isotopic signature is close to that of the atmosphere, due to degassing of the production fluid at the surface and its partial

Fig. 7. (a) The F( 20 Ne) versus F( 84 Kr) values and (b) the F( 132 Xe) versus F( 84 Kr) values measured in LHGF production fluids. Initial pre-production fluid noble gas composition is assumed here to be the ASW value at 25 °C. The F( 20 Ne) versus F( 84 Kr) variations can be explained by boiling and phase separation, and then mixing with an injectate enriched in atmospheric noble gases. The F( 132 Xe) versus F( 84 Kr) variations can be explained by both boiling and mixing with an injectate.

to 1, typically on the order of 1–100 (Pinti and Marty, 1998; Ephraim, 2012). It is worth noting that the different 4He/40Ar* ratios could be also related to the degassing conditions of the magma source (e.g., Burnard et al., 2004; Paonita et al., 2012). Hydrothermal fluids often show 4He/40Ar ratios higher than that expected for the upper mantle (1.2 to 4.2; Graham, 2002). This is caused by the fractional degassing of the magma during melting. Fractionation between gas species is consistent with their solubilities in silicate melts (Burnard et al., 2004). Argon, which is less soluble than He in silicate melts, will be preferentially lost. Resulting 4He/40Ar ratios in fluids exsolved from silicate melts degassed at different depths will thus be higher than those in the original magma source (e.g., Paonita et al., 2012). However, if this is the main mechanism controlling the He/Ar fractionation in the system, we should observe clear

Fig. 8. (a) F( 20 Ne) versus 40 Ar/ 36 Ar ratios and (b) F( 84 Kr) versus 40 Ar/ 36 Ar ratios measured in LHGF production fluids. Dashed areas delimitate fluids only affected by processes of phase separation (boiling). Boiling affects elemental ratios but not isotopic ratios. All other samples can be explained by mixing between a fluid with ASW composition and an injectate enriched in air bubbles.

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

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D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

re-equilibration with the atmosphere prior to re-injection (Kennedy et al., 1999; Kennedy and Shuster, 2000; Pinti et al., 2013). Fig. 7a and b show the light 20Ne versus the heavy 84 Kr, as well 132 as Xe versus 84 Kr using the F-value notation (Table 1). The advantage of normalizing to the air ratio is that any contamination from air, as expected in injectate (Pinti et al., 2013), will translate into an F(i) value approaching 1 (Fig. 7a,b). Because noble gas solubility in water increases with mass (solubility Ne b Ar b Kr b Xe; e.g., Smith and Kennedy, 1983), phase separation during boiling is translated into a decrease of 20Ne/ 36Ar ratios in the residual liquid (Fig. 7a). Indeed, Ne is less soluble than Ar, and tends to be partitioned into the steam condensate. On the other hand, the 84 Kr/36 Ar and 132Xe/36 Ar ratios will increase in the residual liquid after boiling, since Kr and Xe are more soluble than Ar (Fig. 7b). The initial F-values in the pre-production reservoir fluid were not measured, but they are usually assumed to be those of ASW (e.g., Kennedy et al., 1999). For the LHGF pre-production fluids, the ASW F-values were calculated as those corresponding to an ambient average temperature of ~ 25 °C (i.e., the expected temperature of local meteoric recharge during the rainy season in the area). Noble gas solubility data used for computations are those of Smith and Kennedy (1983) for the initial ASW composition and Smith (1985) for reservoir temperatures. In Fig. 7a and b, the expected evolution curve of the F-values in the residual fluid after boiling (plain curves) and in the steam condensate (dashed curves) are reported. It is interesting to note in Fig. 7b that there are several populations of production fluids. Production fluids from wells H-09, H-17, H-19, H-20, H-31, H-34, and H-35 have F( 84Kr) and F(132 Xe) values compatible with those expected in residual fluid having been affected by boiling at reservoir temperatures between 310 and 100 °C. Fluid from well H-12 departs from the general trend. This could be due an error in measuring Xe, as the Kr/Ar ratio is compatible with an ASW value (Fig. 7b). At least some of the fluid from wells H-03D, H-06, H-07, H-30, H-33, H44, H-45, and H-49 might represent either a steam condensate (dashed line in Fig. 7b), or simply mixing with air (see Air end-member in Fig. 7b). Well H-09 has undergone a dramatic boiling effect. Indeed, the high F( 84 Kr) and F( 132Xe) values measured in this well could indicate that the residual fluid in the reservoir is only 0.2% that of the initial amount prior to boiling. Fig. 7a shows similar results, although the data points are more scattered. Fluid from wells H-09, H-12, H15, H-17, H35, and H-45 could be compatible with ASW values or with residual fluids after boiling. Fluids from wells H-03D, H-06, H-07, H-30, H-33, H-34, H38, H-44, and H-48 could result from a residual fluid after boiling (5% residual of the total volume), mixed with air (F-values of 1; Fig. 7a). Fig. 8a and b show that more than half of the productive wells of the LHGF displays a relationship between measured 40Ar/36Ar ratios and calculated F(20Ne) and F(84Kr), as well as between 40Ar/36Ar and F(132Xe), although the latter is not shown here. As noted in Section 6.2, solubility-driven boiling and condensate processes have the potential to affect the noble gas elemental ratios, but not the isotopic ratios (e.g., Kennedy et al., 1991). This is the case for wells H-09, H15, H-17, H-20, H-34, and H-35 (Fig. 8a, b), for which a correlation between F-values and 40Ar/36Ar ratios is absent (dashed area in Fig. 8a, b). All other samples represent mixing between a fluid with an ASW composition and 40Ar/36Ar ratios higher than 800 and a fluid containing atmospheric noble gases (F-values close to 1) and an atmospheric 40Ar/36Ar ratio of 295.5 (Fig. 8a, b). The first fluid could represent the pre-production pristine fluid (i.e., the original fluid source). The second one could be injectate. Indeed, it is unlikely that used geothermal fluids in LHGF would have enough time to reequilibrate with the atmosphere prior to re-injection. Re-injection is done by gravity-controlled flow in unused boreholes, and it is

expected that injectate traps air bubbles, leading to an air-dominated (rather than ASW-dominated) noble gas composition in the injected fluid as a whole. It is worth noting that the wells where fluid is largely mixed with air (F-values between 0.75 and 1.1) are those located close to the re-injection wells H-29D/H-38 in the north (H-30, H-33 and H-44) and between the re-injection wells H-29D/H-38 and H-13D/H-40 in the Median and South zone (H-7, H-48 and H-49) of the field (Fig. 1). These results suggest that re-injection is starting to severely impact the LHGF. If injectate is responsible for the dilution of the argon isotopic ratios (Fig. 8a,b), a larger volume of ancient water rich in radiogenic 40Ar* might have been present in the field in preproduction time. Determining the precise volume of this water and in which area of the geothermal field it was concentrated is complicated by 26 years of uninterrupted production, as the present fluid isotopic signature seems to be strongly affected by the re-injection of used brines. 7. Conclusions A first comprehensive noble gas survey at the LHGF reveals new insight into the circulation of production fluids in this geothermal field. Helium and argon isotopes, as well as noble gas elemental abundance ratios, together with water stable isotope compositions suggest that the LHGF production fluids are a complex mixture of at least four components. The first represents local modern water recharge, with an ASW noble gas elemental and isotopic composition. Helium isotopic systematics suggest that modern recharge is very limited, likely b1–10% in volume of the total production fluid, and mainly concentrated in the North Zone, where the annular faults of the Los Potreros Scarp might act as a conduit. The second component is representative of deep-seated fluid mining heat and helium from an active magma source. The 3 He/ 4 He ratios are consistently high, with an average R c /R a value of 7.03 ± 0.40, and heavier δD-δ 18 O indicating the occurrence of “andesitic” water (i.e., water having interacted with an active magma source at depth and accumulating mantle helium). The third component is representative of fossil water characterized by high 40 Ar/ 36 Ar ratios resulting from the accumulation of locally-produced radiogenic 40 Ar*. High 87 Sr/ 86 Sr ratios of up to 0.70885 in fluids with high 40 Ar/ 36 Ar ratios suggest that fossil fluids circulated within the metacarbonate basement. This fossil component is clearly preserved in a few wells (H-12, H-19, and H-45). Finally, the fourth component is of atmospheric origin, and it likely represents re-injected fluids (injectate) containing large amounts of air-derived noble gases. The wells showing the highest injectate content (62 to 100% of the production fluid, based on noble gas elemental ratios) are located within 1 km of the re-injection wells in the North (H-30, H-33, and H-44) and Median (H-7, H-48, and H-49) Zones, except for well H-06, which is located along the northern border of the Los Potreros Scarp. Further water chemistry data and H, O, Br, Cl, and C stable isotopic compositions are currently being analyzed and might help to determine the exact proportions of each of these fluid components in the reservoir by mass and isotopic balance computation. Acknowledgements Detailed comments and suggestions from two anonymous reviewers greatly improved the manuscript. The authors thank the personnel at the Gerencia de Proyectos Geotermoeléctricos for facilitating sample collection and at the LHGF for their assistance during sampling. This work was supported by CeMIEGeo (Centro Mexicano de Innovacion en Energia Geotermica), Grant No. 207032 SENERCONACYT, Project 20.

Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015

0.101 0.007 0.015 0.055 0.091 0.013 0.015 0.075 0.054 0.032 0.003 0.011 0.003 0.014 0.010 0.123 0.086 0.003 0.085 0.114 0.005 2.013 2.185 2.215 2.188 2.299 2.173 2.158 2.083 2.189 2.224 2.183 2.197 2.193 2.203 2.183 2.240 2.336 2.188 2.186 2.291 2.173 2.176 0.120 0.009 0.019 0.063 0.107 0.016 0.018 0.088 0.064 0.038 0.005 0.014 0.004 0.017 0.012 0.146 0.100 0.005 0.103 0.134 0.007 2.518 2.568 2.606 2.622 2.725 2.562 2.546 2.494 2.573 2.630 2.567 2.587 2.584 2.592 2.564 2.663 2.702 2.581 2.490 2.644 2.557 2.563 0.277 0.022 0.047 0.152 0.253 0.038 0.043 0.218 0.160 0.095 0.010 0.032 0.010 0.042 0.029 0.338 0.250 0.010 0.239 0.323 0.016 6.160 6.620 6.700 6.703 6.976 6.590 6.568 6.389 6.670 6.760 6.622 6.650 6.660 6.674 6.610 6.842 6.984 6.657 6.403 6.895 6.585 6.607 0.225 0.016 0.037 0.121 0.201 0.030 0.034 0.177 0.126 0.075 0.006 0.025 0.007 0.033 0.022 0.275 0.199 0.008 0.186 0.262 0.010 4.924 5.220 5.285 5.306 5.478 5.223 5.201 5.118 5.228 5.379 5.254 5.229 5.270 5.248 5.230 5.503 5.482 5.271 5.050 5.549 5.195 5.213 0.273 0.019 0.044 0.150 0.245 0.036 0.041 0.215 0.158 0.092 0.008 0.030 0.010 0.040 0.027 0.324 0.243 0.011 0.231 0.322 0.012 6.085 6.495 6.586 6.586 6.836 6.479 6.444 6.315 6.553 6.645 6.528 6.520 6.549 6.543 6.487 6.719 6.810 6.553 6.274 6.843 6.468 6.496 0.0398 0.0032 0.0061 0.0205 0.0364 0.0067 0.0063 0.0327 0.0170 0.0088 0.0009 0.0057 0.0008 0.0056 0.0029 0.0720 0.0344 0.0007 0.0462 0.0553 0.0019 0.4926 0.4701 0.4776 0.4986 0.4644 0.4773 0.4604 0.4455 0.4618 0.4671 0.4695 0.4674 0.4661 0.4698 0.4692 0.3078 0.4290 0.4664 0.4609 0.5514 0.4722 0.4715 0.00190 0.00196 0.00191 0.00227 0.00183 0.00204 0.00198 0.00201 0.00135 0.00214 0.00096 0.00193 0.00099 0.00194 0.00205 0.00445 0.00189 0.00103 0.00202 0.00182 0.00187 0.30058 0.30324 0.30135 0.30046 0.30016 0.30103 0.30080 0.29727 0.30088 0.30079 0.30464 0.30065 0.30519 0.30074 0.30120 0.28974 0.30083 0.30531 0.30267 0.30487 0.30617 0.30524 0.00173 0.00118 0.00118 0.00154 0.00105 0.00129 0.00126 0.00148 0.00095 0.00129 0.00056 0.00122 0.00059 0.00122 0.00126 0.00592 0.00179 0.00061 0.00151 0.00156 0.00114 0.19966 0.20080 0.20168 0.20169 0.20192 0.20202 0.20172 0.20359 0.20197 0.20175 0.20172 0.20165 0.20231 0.20157 0.20172 0.19118 0.20168 0.20243 0.20532 0.20374 0.20074 0.20136 0.00133 0.00124 0.00125 0.00162 0.00133 0.00126 0.00135 0.00188 0.00089 0.00130 0.00062 0.00130 0.00063 0.00130 0.00129 0.00620 0.00153 0.00066 0.00150 0.00130 0.00119 0.20371 0.20384 0.20500 0.20394 0.20559 0.20451 0.20438 0.20588 0.20488 0.20420 0.20239 0.20516 0.20262 0.20483 0.20464 0.20773 0.20385 0.20285 0.20418 0.20529 0.20176 0.20217 0.03984 0.04040 0.04086 0.04024 0.04080 0.04090 0.04050 0.04042 0.04072 0.04035 0.04015 0.04064 0.04020 0.04063 0.04084 0.03086 0.04148 0.04030 0.04108 0.04133 0.03960 0.03960 H-03 H-06 H-07 H-09 H-12 H-15 H-17 H-19 H-19 H-20 H-30 H-31 H-33 H-34 H-35 H-41 H-42 H-44 H-45 H-48D H-49 Air

0.00139 0.00025 0.00028 0.00087 0.00084 0.00031 0.00032 0.00105 0.00056 0.00055 0.00012 0.00028 0.00013 0.00028 0.00029 0.00517 0.00099 0.00013 0.00120 0.00077 0.00024

± Kr/84Kr 82

± Kr/84Kr 80

Well

Table A1 Kr and Xe isotopic composition for Los Humeros fluids.

Appendix A

83

Kr/84Kr

±

86

Kr/84Kr

±

128

Xe/130Xe

±

129

Xe/130Xe

±

131

Xe/130Xe

±

132

Xe/130Xe

±

134

Xe/130Xe

±

136

Xe/130Xe

±

D.L. Pinti et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxx–xxx

11

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Please cite this article as: Pinti, D.L., et al., Fluid circulation and reservoir conditions of the Los Humeros Geothermal Field (LHGF), Mexico, as revealed by a noble gas survey, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.jvolgeores.2017.01.015