Carbon isotope systematics and CO2 sources in The Geysers-Clear Lake region, northern California, USA

Geothermics 30 (2001) 303±331 www.elsevier.com/locate/geothermics

Carbon isotope systematics and CO2 sources in The Geysers-Clear Lake region, northern California, USA Deb Bergfeld a,*, Fraser Go€ a, Cathy J. Janik b a

Los Alamos National Laboratory, EES-1, MS D462, Los Alamos NM 87545, USA US Geological Survey, MS 910, 345 Middle®eld Road, Menlo Park, CA 94025, USA

b

Received 15 November 1998; accepted 29 November 1999

Abstract Carbon isotope analyses of calcite veins, organic carbon, CO2 and CH4 from 96 rock and 46 gas samples show that metamorphic calcite veins and disseminated, organically-derived carbon from Franciscan Complex and Great Valley Sequence rocks have provided a primary carbon source for geothermal ¯uids during past and present hydrothermal activity across The Geysers-Clear Lake region. The stable isotope compositions of calcite veins vary widely on a regional scale, but overall they document the presence of 13C-poor ¯uids in early subductionrelated vein-precipitating events. d13C values of calcite veins from the SB-15-D corehole within The Geysers steam ®eld indicate that carbon-bearing ¯uids in the recent geothermal system have caused the original diverse d13C values of the veins to be reset. Across The Geysers-Clear Lake region the carbon isotope composition of CO2 gas associated with individual geothermal reservoirs shows a general increasing trend in d13C values from west to east. In contrast, d13C values of CH4 do not exhibit any spatial trends. The results from this study indicate that regional variations in d13C±CO2 values result from di€erences in the underlying lithologies. Regional CO2 contains signi®cant amounts of carbon related to degradation of organic carbon and dissolution of calcite veins and is not related to equilibrium reactions involving CH4. CO2 from degassing of underlying magma chambers is not recognizable in this region. Published by Elsevier Science Ltd on behalf of CNR. Keywords: Geothermal; The Geysers; Clear Lake; Carbon isotopes; CO2; CH4; USA

* Corresponding author. Tel.: +1-505-667-1812; fax: +1-505-665-3285. E-mail address: [email protected] (D. Bergfeld). 0375-6505/01/$20.00 Published by Elsevier Science Ltd on behalf of CNR. PII: S0375-6505(00)00051-1

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1. Introduction Quaternary volcanism in northern California resulted in the formation of a premier vapor-dominated geothermal system at The Geysers and several smaller, liquid-dominated reservoirs in the Clear Lake region. The combined Geysers-Clear Lake region hosts a large number of springs, fumaroles, and gas vents that release copious amounts of CO2 to the atmosphere (Bergfeld et al., 1997). To date, no regional study has determined the provenance for the CO2, although topical studies have been conducted by Lambert and Epstein (1992) and by Go€ and Janik (1993). The considerable quantity of CO2 discharged in this area makes it an excellent site in which to study the relations between, and the production of, greenhouse gases in a large geothermal region. Evaluation of the possible CO2 sources requires examination of carbon species within the geothermal system, those originally present in the host rock, and those introduced via circulating ¯uids. Stable isotope analyses provide a tool for resolving interrelations among the carbon species because of the extreme variation that is inherent in the d13C values of carbon derived from di€erent source materials (Fig. 1). This research establishes a regional carbon isotope inventory of rock and vein samples collected from outcrop, and gas samples collected from vents, fumaroles, springs and wells (Fig. 2). The outcrop samples provide background information on the early isotopic composition of Franciscan rocks before the inception of the recent geothermal systems. Core samples of graywacke and argillite from the SB-15-D well in The Geysers geothermal ®eld yield information regarding the isotopic composition of whole rocks and veins from the largest of several active geothermal systems. The objectives of this study are to use the background data provided by the outcrop samples to evaluate the role of geothermal ¯uids on the modern carbon isotope systematics. This estimate will provide us with a better understanding of the processes that have in¯uenced this region since the inception of geothermal activity and may give us a window into processes governing the widespread production of CO2 today. 2. Geology The geology of The Geysers-Clear Lake region (Fig. 2) consists of a complex stratigraphy of subduction-related, late Jurassic to Eocene marine metasedimentary rocks and ophiolites, and late Pliocene to Holocene rocks of the Clear Lake volcanic ®eld. Two coeval formations, the Franciscan Complex and the overthrust Great Valley sequence, are juxtaposed along the NNW trending Coast Range thrust. In The Geysers-Clear Lake region the dominant lithology of both sequences consists of turbidite facies metagraywackes and argillites, which were deposited into trench and fore-arc basin settings, and later deformed and variably metamorphosed along an obliquely convergent subduction margin (McLaughlin, 1981; McLaughlin and Ohlin, 1984; Thompson, 1989). In much of the study area the basement rocks are part of the central belt of the Franciscan Complex which range in degree of meta-

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Fig. 1. The range in d13C values for a variety of carbon-bearing substances as they compare to the 13C values of mantle carbon (vertical bar). Samples from this study are shown in grey. The ®gure is modi®ed from Rollinson (1993).

morphism from prehnite-pumpellyite to lawsonite grade (McLaughlin, 1981). Rocks of the Great Valley sequence are weakly metamorphosed to laumontite grade (Blake et al., 1988) and are generally constrained to the southern and eastern portions of the ®eld area (Hearn et al., 1995). The Clear Lake volcanic ®eld located north of the Miocene to Pliocene Sonoma volcanic ®eld is the youngest of two regional volcanic episodes (Hearn et al., 1981; McLaughlin, 1981). The volcanism is thought to be related to northward migration and passage of the Mendocino triple junction with subsequent upwelling of asthenosphere in a slabless window (Dickinson and Snyder, 1979; McLaughlin, 1981; Johnson and O'Neil, 1984; Fox et al., 1985; Benz et al., 1992). In The Geysers-Clear Lake region volcanism occurred from about 2.1 to 0.01 Ma (Donnelly-Nolan et al., 1981). The center of volcanic activity migrated in a generally northward direction with an early widespread distribution of minor volumes of basaltic andesite lavas followed by emplacement of larger volumes of localized silicic eruptions (Hearn et al., 1981). The youngest volcanic rocks in the region, dated at 0.09±0.01 Ma, are

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Fig. 2. Generalised geologic map of the Clear Lake region, California, showing gas and vein sample locations. SB-15-D=corehole; BSFZ=Bartlett Springs fault zone; BSCFZ=Big Sulphur Creek fault zone; CFZ=Collayomi fault zone; CSFZ=Cross Springs fault zone; KBFZ=Konocti Bay fault zone; RFZ=Resort fault zone; IVR=Indian Valley reservoir; LP=Lakeport; CL=Clearlake. Gas features: AS=Anderson hot spring; BGS=Borax gas seep; BG=Big Geysers; BM=Big Mother Spring; BSS=Baker soda spring; CT= Crabtree Hot Spring; EM=Elgin Mine Spring; ES=Elbow spring; Gas=Gas spring; GS=Grizzly spring; HB=Horseshoe Bay Spring; HH=Hog Hollow spring; JS=Jones Hot spring; KEL= Kelseyville methane well; LG=Little Geysers; SBM=Sulphur Bank Mine; SCS=Sulphur Creek spring; SM=Sulphur Mound mine; WS=Wilbur hot springs. Figure modi®ed from

located along the eastern margin of Clear Lake in the Sulphur Bank Mine/Borax Lake area (Donnelly-Nolan et al., 1981). These rocks overlie and intrude graywacke and shale of the Franciscan Complex, which hosts a liquid-dominated geothermal system with ¯uid temperatures as hot as 218 C at 503 m depth (Go€ et al., 1995). The Geysers geothermal ®eld resides predominantly in Late Mesozoic Franciscan rocks that were intruded by a Cenozoic (>1.3±0.9 Ma; Donnelly-Nolan et al., 1981; Pulka, 1991; Dalrymple, 1992) composite felsic pluton. Porosity within The Geysers steam ®eld is in¯uenced by shear and hydraulic fractures related to emplacement of the intrusion (Thompson and Gunderson, 1989; Sternfeld, 1989) as well as by fractures related to the San Andreas fault system, and numerous older low-angle faults (Thompson and Gunderson, 1989). The evolution of The Geysers geothermal ®eld from a 400 C liquid-dominated system (Moore, 1992; Gunderson and Moore, 1994)

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to the present-day steam ®eld involved formation of secondary porosity produced by dissolution of Franciscan metamorphic quartz and calcite veins (Hulen et al., 1991, 1992; Gunderson, 1992; Thompson and Gunderson, 1992). Subsequent precipitation of hydrothermal bladed calcite in upper levels of the reservoir is thought to play a role in producing the caprock within the steam ®eld (Hulen et al., 1991, 1992; Moore, 1992). The Geysers Coring Project, SB-15-D, produced 236.8 m of continuous core consisting predominately of metagraywacke with lesser amounts of interbedded argillite (Hulen et al., 1995). During drilling, the ®rst of two ¯uid-loss zones was encountered at  417 m (Hulen et al., 1995). In the core, this depth marks the transition between less permeable caprock above, and upper horizons of the steam reservoir below (Hulen et al., 1995). Two generations of calcite veins, older Franciscan regional metamorphic veins (CF) and more recent hydrothermal veins (CH) (described below), are present throughout the length of the core (Hulen and Nielson, 1995b). The metamorphic veins are rare to absent in deeper sections of The Geysers reservoir due to dissolution in the early water-dominated system (Walters et al., 1988; Hulen et al., 1992; Moore, 1992). Argillaceous sections of the core increase in competency with depth (Hulen and Nielson, 1995a), host thin hydrothermal veins (Hulen and Nielson, 1995b) and contain up to 2.6 wt.% of organically-derived carbon (Corg) (analysis by DGSI Labs, TX). The Geysers and Clear Lake thermal regimes are separated by the northwesttrending Collayomi fault zone (Go€ et al., 1977). Although drilling in the Clear Lake region revealed high temperatures at depth, the chemistry of thermal waters indicated that the liquid-dominated geothermal reservoirs are restricted in their areal extent and are, in general, noncommercial in nature (Go€ et al., 1993). Gases discharged at most springs and vents across the Clear Lake area are dissimilar to hightemperature (5 200 C) geothermal gases because they usually contain higher concentrations of CO2 and may contain signi®cantly higher concentrations of CH4 (Table 1) while usually containing much lower concentrations of H2S and H2 (Go€ and Janik, 1993; Go€ et al., 1993). In the subsequent tables and ®gures, sample locations are generally grouped into one of ®ve areas. From west to east these areas are: 1. Hopland, located approximately 20 km northwest of the Big Sulphur Creek fault zone, away from any known geothermal in¯uence. The three locations include mineralized springs and a domestic water well. 2. The Geysers geothermal ®eld and its famous vapor-dominated system. Sample locations include boiling springs, other thermal waters, fumaroles, and geothermal production wells (mostly from the northwest Geysers). Carbon isotope data for four geothermal well samples from earlier work by Shigeno et al. (1987) are also discussed. 3. The general Clear Lake region. Sample locations include gas vents, and thermal and mineralized springs. 4. Sulphur Bank Mine, a localized, liquid-dominated geothermal system (4 218 C) within the Clear Lake region. Sample locations include gas vents and an abandoned geothermal well. 5. Wilbur Springs District, an area that hosts a localized, liquid-dominated geothermal system (4 140 C) in the northeastern extent of the Clear Lake volcanic ®eld. Sample locations include thermal springs located in Great Valley sequence rocks.

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Table 1 Partial gas geochemistry for samples from The Geysers and Clear Lake geothermal regions and the Hopland area. Analyses in mol% dry gas Sample no.

Location

Date

Temp. C CO2 CH4

The Geysers G90-12 G90-4 G91-10 G91-1 G91-5 G90-10 CL95-2 G96-01 GYS95-1 GYS95-2 GYS95-3 GYS95-4

CA State 92-6 Well DX 4596- 45 Well Prati 25 Well Prati 37 Well Prati State 12 Well Sulfur Bank- 15 Well Anderson Hot Spring Big S. Creek Old Geysers Hot Springs Creek Little Geysers Little Geysers in Stream

12/12/90 10/12/90 25/06/91 24/06/91 24/06/91 12/12/90 30/03/95 18/01/96 08/08/95 08/08/95 08/08/95 08/08/95

na na na 181 na 132 76.6 98 40.5 95.2 97.6 95.8

72.2 6.46 74.1 3.16 74.6 9.51 64.8 11.3 59.8 4.82 54.5 5.78 90.5 3.85 76.8 5.94 83.6 6.57 56.5 9.39 51.2 1.03 56.6 1.37

Clear Lake region, General CL96-6 Baker Soda Spring SBM95-8 Big Mother Spring CL95-4 Borax Lake Gas Seep CL95-13 Crabtree Gas Seep CL97-1 Gas Spring CL95-8 Grizzly Spring CL95-16 Hog Hollow Spring CL95-14 Horseshoe Spring CL95-1 Kelseyville Gas Well CL95-12 Sulphur Creek Spring CL95-11 Sulphur Mound Mine

17/06/96 13/09/95 31/03/95 21/07/95 13/06/97 17/07/95 24/07/95 23/07/95 30/03/95 19/07/95 19/07/95

22.2 26 11.3 29 17.1 21.5 31 40 13.7 29 20

98.5 0.924 2.9 53.0 75.5 22.3 93.1 1.36 95.4 1.21 98.0 1.20 98.3 1.35 93.5 2.28 64.7 31.1 98.7 0.053 91.2 6.17

Sulphur Bank Area SBM95-6 SBM95-7 CL93-56 CL93-60 SBM95-1 SBM95-3 CL95-15

Basalt Pit, S end Green Bubbling Pool Herman Pit Herman Pit Herman Pit Abandoned Well Near Green Bubbling Pool

12/09/95 12/09/95 04/12/93 04/12/93 11/09/95 11/09/95 24/07/95

25 25.6 14 14 23.4 27 26

76.3 18.2 82.6 9.8 88.8 8.89 84.0 8.80 89.9 7.98 92.8 5.42 86.7 10.4

18/07/95 15/06/96 06/12/93 15/06/96 16/06/96 16/06/96 18/07/95

70.5 73 69 67 57 57 56.5

95.6 1.61 95.8 1.82 97.1 0.752 97.0 0.639 46.8 49.9 48.5 48.1 91.1 4.88

CL96-4 (1) CL96-4 (2)

Elbow Hot Spring Elbow Hot Spring Elgin Mine, Main Spg Elgin Mine, Main Spg Jones Hot Spring Jones Hot Spring Wilbur Springs Near Source Wilbur Hot Spring Wilbur Hot Spring

16/06/96 56 16/06/96 56

83.8 85.4

2.87 2.89

Hopland CLH96-6 CLH96-3

Lucchetti well Soda Spring

22/08/96 19.5 21/08/96 16

97.3 98.2

0.882 0.233

Wilbur Springs District CL95-10 CL96-3 CL93-67 CL96-1 CL96-5 (1) CL96-5 (2) CL95-9

R/Raa

7.96 (K) 8.32 (K) 7.17 (K)

4(P); 5.2 (G) 7.9 (G) 3 (G)

0.8 (G)

7.5 (G)

1.6(P); 1.7(G) 1.3 (P)

a R/Ra values for samples from this region are from Go€ et al., 1993 (G), Kennedy and Truesdell, 1996 (K), Peters, 1991 (P). R=Ra is a measure of the 3 He=4 He ratio in the sample relative to the ratio in the air. na, data not available. Gas data from The Geysers (except CL95-2) are from Lowenstern et al. (1999); the remaining gas data are from Janik and Go€, in prep.

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3. Methods 3.1. Stable isotope analyses and ¯ux measurements on gases Gas samples were collected in evacuated doubleport glass bottles containing a 4 N NaOH solution. The procedures for collection and analysis of gas samples is described in full in Fahlquist and Janik (1992). All gas geochemistry was performed at the US Geological Survey, Menlo Park, CA. Carbon isotope analyses of methane were performed by combusting CH4 to CO2 with O2 in a CuO furnace at 800 C. An internal methane standard yielded 17.7% compared to an expected value of 17.30.5%. Reproducibility of replicate methane analyses for samples from different bottles collected on the same day was 0.3%. CO2 gas was liberated from the caustic solutions by reaction with phosphoric acid. Internal CO2 standards, Na2CO3 and Na2CO3 dissolved in NaOH, yielded d13C values of 7.40.1% and 8.00.2% compared to the expected values of 7.40.15% and 7.90.2%, respectively. Because of the sample collection method, isotopic analysis of the oxygen in CO2 is not performed. Isotope analyses are reported in d notation relative to the PDB standard for carbon and the SMOW standard for oxygen. Flux measurements were made by placing an inverted funnel of known volume over a gas source and timing the rate of water replacement by the gas. Between ®ve and ten measurements were made at each site. The average gas ¯ux values are reported in units of liters per hour. Standard error for the rate of ®ll is 1.2 s. 3.2. Stable isotope analyses of carbonate veins and organic carbon Carbon isotope analyses of calcite veins and Corg from the SB-15-D core and on outcrop samples of metamorphic calcite veins from across the Clear Lake region were analyzed using the Finnigan Mat Delta-E isotope ratio mass spectrometers at New Mexico Institute of Mining and Technology and the University of Missouri, Columbia. Prior to crushing with a mortar and pestle, calcite veins from core samples were visually identi®ed as to type. Hydrothermal calcite veins are commonly found in vugs and high-angle fractures and can be distinguished from the highly deformed, massive Franciscan calcite veins by their linear orientation, the ease of vein separation from the host rock and their bladed morphology (Hulen and Nielson, 1995b). Calcite powders from core samples were hand picked under a lowpower microscope to remove small amounts of cubic pyrite crystals. CO2 was liberated from powdered calcite by reaction with phosphoric acid following the methods of McCrea (1950). Four analyses of the NBS-19 standard yielded an average value of 1.950.02% for carbon and 28.650.11% for oxygen compared to expected values of 1.95 and 28.65%. Analytical precision for calcite veins based on replicate analyses of NBS and laboratory standards was 0.02% for carbon and 0.15% for oxygen. Rock samples were trimmed of weathered portions, washed with distilled water, dried, and crushed to a ®ne powder. Organic carbon for isotope analysis was separated from the rock powders by ¯otation on distilled water. The extracts were treated with 50% HCl to remove any carbonate minerals and then rinsed with dis-

310

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tilled water and dried. Samples of approximately 2 mg of ``puri®ed'' organic material were placed into annealed quartz tubing with excess ( 0.5 g) of a 2:1 ratio of CuO and Cu metal (Rumble and Hoering, 1986). The tubes were evacuated, torchsealed, and heated in a furnace at 900 C for 2 h. Following combustion the samples cooled slowly to room temperature for a period of at least 8 h. The produced CO2 was puri®ed of water and non-condensible gases on a vacuum line and then analyzed by mass-spectrometry. Analyses of laboratory standard spectrographic graphite averaged 24.8% compared to an expected value of 24.9%. Reproducibility of replicate analyses of Corg was 0.4%. 3.3. Transmission electron microscopy of organic carbon Organic carbon in selected powdered rock samples was concentrated by HCl and HF digestion by the DGSI laboratory in Texas. Two samples containing between 0.65 and 0.75 wt.% total organic carbon were selected for TEM analysis to provide a qualitative estimate of the degree of structural ordering, or crystallinity of Corg. The analyses were performed on a JEOL 2010 high resolution transmission electron microscope operating at 200 kV by Dr. Adrian Brearley at the University of New Mexico, Albuquerque. 4. Results 4.1. Carbon isotope composition of CO2 and CH4 in gases Thirty-three locations (Fig. 2) consisting of springs, wells, fumaroles and gas vents across The Geysers-Clear Lake region were sampled to obtain the d13C values of CO2 and CH4 in the geothermal gases. As isotopic analyses of CO2 have been undertaken in earlier investigations of the area, the decision to resample sites was based on data indicating locations with gases that contain at least 0.5 mol% methane (Go€ et al., 1993; Go€ and Janik, 1993). Three additional ``background'' locations in the Hopland area were also sampled to examine the isotopic composition of CO2 and CH4 outside of the geothermal region. Partial chemical analyses of the gases and the d13C values for CH4 and CO2 from this study and four analyses of Shigeno et al. (1987) are listed in Tables 1 and 2. Analyses of CO2 in the geothermal gas samples show some regional variation in isotopic composition (Fig. 3a). Although there is overlap in the data, d13C±CO2 values are generally lowest in The Geysers region and are comparatively high in the Wilbur Springs District. Average d13C-CO2 values for The Geysers (including the data of Shigeno et al. 1987), Clear Lake, Sulphur Bank Mine and Wilbur Springs geothermal areas are equal to 12.50.7%, 12.32.2%, 10.50.6%, and 10.10.5%, respectively. The average d13C±CO2 value for the Clear Lake region excluding the CH4-rich sample from a vent on the lake bottom (Big Mother Spring) is 11.70.9%. d13C values for the three samples of CO2 from the Hopland area average 12.90.7%, similar to values from The Geysers. Samples collected from submerged gas vents at di€erent loca-

Table 2 Carbon isotope data for CO2 and CH4 in The Geysers and Clear Lake geothermal regions and in the Hopland area with calculated temperatures using the carbon isotope exchange thermometer of Ohmoto and Rye (1979) and gas geochemistry thermometer of D'Amore and Panichi (1980) Locationa

Source

d13Cb CO2

The Geysers G90-12 G90-4 G91-10 G91-1 G91-5 G90-10 CL95-2 G96-01 GYS95-1 GYS95-2 GYS95-3 GYS95-4 A B C D

CA State 92-6 DX 4596-45 Prati 25 PS-37 (The Geysers) PS-12 (The Geysers) Sulfur Bank-15 Anderson Hot Spring Big S. Creek Old Geysers (2 of 2) Hot Springs Creek (2 of 2) Little Geysers (1 of 2) Little Geysers Stream Geysers * Geysers * Geysers * Geysers *

Well, NW Geysers Well, NW Geysers Well, NW Geysers Well, NW Geysers Well, NW Geysers Well, Central Geysers Spring Fumarole Gas Vent Fumarole, Drowned Fumarole, Drowned Gas Vent Well Well Well Well

11.8 11.8 12.3 12.5 12.5 13.1 12.3 12.7 12.5 12.6 14.3 13.6 12.0 12.6 11.7 12.0

Clear Lake Region, General CL96-6 SBM95-8 CL95-4 CL95-13 CL97-1 CL95-8 CL95-16 CL95-14 CL95-1

Baker Soda Spring Big Mother Spring Borax Gas Seep Crabtree Hot Spring Gas Spring Grizzly Spring Hog Hollow Horseshoe Bay Kelseyville Methane Well

Spring Gas Vent, Drowned Gas Vent Spring Spring Spring Spring Spring Gas Vent

11.0 18.6 11.1 11.1 11.6 11.7 13.2 11.7 10.8

d13C CH4


CO2±CH4

Ohmoto and Rye T C

D'Amore and Panichi T C

35.9 33.9 31.8 37.0 35.3 32.2 32.6 30.0 35.8 40.3 37.7 32.1 31.7 31.4 35.8 31.2

24.2 22.1 19.5 24.5 22.8 19.1 20.3 17.2 23.3 27.7 23.4 18.5 19.7 18.8 24.1 19.2

309 342 390 305 330 398 375 440 323 263 322 410 386 404 311 397

227 236 210 229 256 263 205 297 268 232 256 263 nac na na na

33.2 41.5 30.2 37.4 37.3 31.2 30.9 35.5 28.5

22.2 22.9 19.1 26.3 25.7 19.5 17.7 23.8 17.7

342 328 399 281 288 391 430 315 429

<25 150 116 117 <33 79 <17 69 59

311

(continued on next page)

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Sample

312

Table 2 (continued) Locationa

Source

CL95-12 CL95-11

Sulphur Creek Spring Sulphur Mound Mine

Spring Gas Vent

Sulphur Bank Area SBM95-6 SBM95-7 CL93-56 CL93-60 SBM95-1 SBM95-3 CL95-15

Basalt Pit Green Bubbling Pool Herman Pit, Dec. 1993 Herman Pit, Dec. 1993 Herman Pit, Sept. 1995 Abandoned Well Near Green Bubbling Pool

Gas Vent, Gas Vent, Gas Vent, Gas Vent, Gas Vent, Well Gas Vent,

Wilbur Springs District CL95-10 CL96-3 CL93-67 CL96-1 CL96-5 CL96-5 CL95-9 CL96-4 CL96-4

Elbow Hot Spring, July 1995 Elbow Hot Spring, June 1996 Elgin Mine, Dec. 1993 Elgin Mine, June 1996 Jones Hot Spring (1 of 2) Jones Hot Spring (2 of 2) Wilbur Springs Near Source Wilbur Springs Source (1 of 2) Wilbur Springs Source (2 of 2)

Hopland area CLH96-6 CLH96-4 CLH96-3

Lucchetti well Coppage Spring Soda Spring

a b c

Samples marked with * are from Shigeno et al., 1987. Carbon isotope values in % PDB. na, data not available.

d13Cb CO2

d13C CH4


CO2±CH4

Ohmoto and Rye T C

D'Amore and Panichi T C

13.4 11.6

68.6 32.1

55.1 20.5

72 370

135 133

10.2 9.6 10.3 10.2 11.1 11.1 10.9

41.4 37.4 32.0 31.6 42.7 45.2 29.9

31.2 27.8 21.7 21.4 31.6 34.1 19.0

225 262 349 353 221 198 401

<60 <48 118 101 119 197 119

Spring Spring Spring Spring Spring Spring Spring Spring Spring

9.9 9.7 9.6 9.7 10.0 10.0 10.8 ( 11.1?) 10.7

53.8 65.4 31.8 30.1 38.7 38.3 25.0 31.3 31.5

43.9 55.8 22.2 20.4 28.6 28.3 14.2 20.2 20.8

128 69 342 372 252 257 525 377 365

270 264 183 181 115 119 137 144 159

Well Spring Spring

12.1 13.3 13.3

53.5 72.7 43.2

41.4 59.4 29.9

143 55 238

29 na <22

Drowned Drowned Drowned Drowned Drowned Drowned

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Sample

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Fig. 3. a. d13C values for CO2 in gas samples and the mol% CO2 concentration. Samples from The Geysers geothermal region show a positive correlation between d13C±CO2 and mol% CO2. Most samples from the Clear Lake region (excluding Big Mother spring) show a negative correlation between d13C±CO2 and mol% CO2. b. d13C values for CH4 in gas samples and the mol% CH4 concentration. There is no correlation between d13C values of CH4 and mol% CH4 for any region.

314

Table 3 Carbon and oxygen isotope data for calcite veins and d13C values for organic carbon in rock samples from SB-15-D core from The Geysersa Hydrothermal calcite veins

Caprock 258.9 282.4 293.8 312.3 347.0 382.3 391.3 402.2 406.4 Steam reservoir 417.7 418.7 446.9 454.2 459.3 476.1 a

d C calcite

Franciscan calcite veins 18

d O calcite

12.6 12.0 12.5 11.1 12.8 13.0 11.7 11.8 12.6

8.3 8.9 7.5 8.0 9.0 8.2 9.8 7.1 7.8

13.6 14.3 13.2

8.1 6.9 8.2

14.9 14.6

8.5 9.4

13

d C Corg

23.4 22.0 23.7

Core depth (m)

13

d C calcite

Mixed calcite veins 18

d O calcite

256.9 282.7 282.8 282.9 302.0 364.7 383.4

11.4 12.4 12.1 9.6 11.6 13.0

12.0 13.7 13.2 12.2 13.2 11.4

419.2 473.7

13.3 12.2

8.1 12.9

23.8

Isotope values in % PDB for carbon and % SMOW for oxygen.

13

d C Corg 23.7 23.6

Core depth (m)

d13C calcite

d18O calcite

258.6 322.6

12.6 12.6

10.8 8.7

433.1

11.9

7.3

23.7

23.7 24.0

d13C Corg

21.3

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Core depth (m)

13

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tions inside Herman Pit at Sulphur Bank Mine show essentially no variation; however, a comparison of samples collected in December 1993 shows a 0.9% enrichment in d13C-CO2 compared with a sample collected in September 1995. Repeat sampling at two sites in Wilbur Springs District shows essentially no temporal carbon isotope variation for CO2 at these locations. The carbon isotope signatures of CO2 from wells, fumaroles and springs in The Geysers di€er from samples collected in the Clear Lake region in that there is a positive correlation (r=0.64) between d13C values and the percentage of CO2 in the gas (Fig. 3a). In contrast, gas samples collected from the Sulphur Bank Mine area and most locations across the Clear Lake region are negatively correlated for these parameters. Gas samples from springs in the Wilbur Springs District show no relation between CO2 contents and d13C values. Results from this study signi®cantly expand the knowledge of the carbon isotope composition of CH4 in The Geysers-Clear Lake region. d13C±CH4 values across the study area show considerable variability, ranging between 72.7 and 25.0% (Fig. 3b, Table 2). There is no correlation between mol% CH4 and d13C values of CH4 for any of the areas. Microbially produced CH4 has d13C values < 55% (Schoell, 1980; Poulson et al., 1995). Elbow Spring in the Wilbur Springs District, Sulphur Creek Spring, just east of the Collayomi fault zone, and Coppage Spring in the Hopland area have d13C±CH4 values 4-54%. Gas ¯ux from these springs is relatively subdued (4 24 L/h) and the low d13C values suggest there is some microbial production of CH4. A comparison of two Elbow Spring samples collected in June and July of successive years ( 65.4 and 53.8%) also indicates an input of microbial CH4, because the variation in d13C±CH4 (total) is most readily explained by di€erences in the relative proportion of microbially-generated CH4. A seasonal (or yearly) variation in d13C values of CH4 was also noted in samples collected inside Herman Pit at Sulphur Bank Mine. Samples collected in cold water (14 C) in December 1993 show a 11% enrichment in d13C values compared with a sample collected in warmer water (23 C) in Sept. 1995. Gas ¯ux at many locations inside Herman Pit is high (4 950 L/h in September 1995), which creates many areas of sustained vigorous bubble trains in the water column. The dynamic ¯ux would seem to be evidence of predominantly inorganic gas generation but the change in the d13C±CH4 values may indicate that some component of the gas is microbially derived from the pit sediments. 4.2. Analyses of carbon and oxygen in rock and vein samples Calcite veins and organic carbon from the SB-15-D core and from graywacke and argillite outcrop samples were analyzed to determine the relation between carbon sources in the host rocks and CO2 outgassed from the geothermal systems. d13C values for the samples are listed in Tables 3 and 4. d13C values for calcite from core samples above depths of 417 m show little variation between the two generations of veins (Franciscan metamorphic and recent hydrothermal) and no systematic variation in d13C with depth (Fig. 4). d13C values of CF and CH from the caprock average 11.7 ( 1.2) and 12.2 ( 0.6) %, respectively. The average d13C value for CF is skewed by one sample, without which the average value is 12.1 ( 0.6) %. Below

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Table 4 Carbon and oxygen isotope data for calcite veins and one limestone and d13C values for organic carbon in rock samples from The Geysers-Clear Lake geothermal regions and the Hopland areaa Franciscan graywacke veins Sample

d13C

d18O

Sample

Veins in mixed Franciscan metamorphic rocks d13C

d18O

Sample

CL95-1 CL95-3 CL95-6 CL95-10 CL95-12 CL95-13 CL95-14 CL95-16 CL95-22 CL95-23 CL95-24 CL95-26 CL96-4

11.1 6.4 8.5 7.4 2.7 4.8 4.0 7.6 7.2 6.4 3.8 6.7 8.4

18.9 14.7 15.9 13.8 14.5 22.9 18.0 15.7 13.1 13.0 12.1 12.4 10.9

CL96-22 CL96-23 CL96-24 CL96-26 CL96-28 CL96-31 CL96-33 CL96-34 CL96-36 CL96-37 CL96-40 CL96-43 CL97-1

10.2 10.0 12.0 10.5 9.5 3.4 3.3 5.6 6.8 4.0 7.1 12.2 5.7

15.1 15.2 15.0 16.1 14.3 13.3 21.3 19.1 16.5 17.7 15.2 12.2 17.5

CL96-11 CL96-12 CL96-13 CL96-18 CL96-19 CL96-20

9.3 7.6 7.9 10.0 10.6 4.7

14.0 16.4 21.7 14.0 14.0 12.3

CL97-3 CL97-4 CLH96-1 CLH96-2 GYS95-1 GYS95-2

10.7 3.6 8.8 8.9 8.5 12.1

13.8 12.7 CL95-16 15.5 CL95-7 14.4 CL95-9 13.1 15.6

Franciscan argillite veins Sample CL95-4 CL95-5 CL95-9 CL95-21 CL96-1 a b

13

d C 8.8 11.2 9.9 4.5 8.5

18

d O 15.5 14.1 14.2 14.2 16.6

Sample CL96-14 CL96-21 CL96-32 CL96-39

d13C

GYS95-3 CL96-15 CL96-16 CL96-25 CL96-27 CL96-29 CL96-30 CL96-35 CL96-41 CL96-42 CL97-5

6.0 9.6 8.0 12.3 1.2 9.0 2.8 7.8 0.2 2.6 4.6

d18O 13.9 14.9 14.7 15.9 16.5 16.1 15.2 16.6 14.8 16.8 15.0

Rock type Glaucophane schist Metabasalt Metabasalt Metabasalt Metabasalt Sheared calcite nodule Metagraywacke Sandstone Metabasalt Chert breccia Metabasalt

Corg in whole rock outcrop samples 25.5 25.6 24.8

Metagraywacke Argillite Argillite

Veins and limestone in mixed Great Valley rocks 13

d C 9.3 9.6 7.9 8.1

18

d O 15.0 13.8 13.4 14.6

Sample

d13C

CL96-5 CL96-6b CL96-7 CL96-9 CL96-10 CL97-7

1.2 19.9 9.3 4.0 1.8 8.9

d18O 22.9 32.7 16.2 17.4 19.6 18.8

Rock type Metabasalt Limestone Graywacke Graywacke Mudstone Serpentine

Isotope values in % PDB for carbon and % SMOW for oxygen. Average value from three laboratories.

the ®rst ¯uid-loss zone d13C values of CH are shifted approximately 1.9% to lower values. There are only two analyses of CF from the reservoir and there does not appear to be any change in the carbon isotope composition of these veins. Similarly, oxygen isotope analyses of the calcite veins show no shift across the caprock/reservoir boundary. d18O values for CF and CH throughout the core average 12.1 ( 1.8) and 8.3 ( 0.8) %, respectively (Table 3). Organic carbon in the argillaceous sections of the core consists of roughly equal portions of unstructured lipid organic matter and terrigenous humic material (Hulen and Moore, 1996). Total organic carbon concentrations for eleven selected samples ranged between 0.4 and 2.6 wt.%. TEM analyses of Corg from two core samples at 283 m (caprock) and 474 m (reservoir) identify the presence of kerogen (unstructured, insoluble

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317

Fig. 4. d13C calcite values for CF and CH from SB-15-D and the corresponding d13C values for organic carbon plotted vs. depth in the core. Fluid-loss zones are indicated by arrows. The ®rst ¯uid-loss zone (highlighted in grey) demarcates the boundary between the caprock and the reservoir.

carbonaceous material) in both the caprock and steam reservoir. Moderately ordered graphite-like material, indicative of post-depositional alteration of the organic matter (Buseck and Huang, 1985), was also observed in the deeper reservoir sample (Brearley, personal communication). The carbon isotope values of Corg from the argillite core samples exhibit some variation, ranging between 24.0 and 21.3% (Fig. 4). These values are slightly higher than the average value of 25% for sedimentary organic carbon (Hoefs, 1980; Rollinson, 1993) and are higher than the values of three Franciscan argillite samples collected from outcrops ( 25.6 to 24.8%). Metamorphic calcite veins analogous to CF in the core are found in outcrops throughout the region (McNitt, 1968; Thompson and Gunderson, 1992; Lambert and Epstein, 1992). Fifty-eight samples of calcite veins from outcrops of Franciscan rocks are considerably diverse in their carbon and oxygen isotope compositions (Table 4; Fig. 5). Calcite veins from Great Valley Sequence rocks have a smaller range in d13C and d18O values, but only ®ve samples were collected and analyzed. Calcite veins from Franciscan argillites generally have lower d13C values than Franciscan graywackes; however there are no distinct associations between rock type and carbon isotope composition. d13C values of veins from some outcrop samples overlap d13C values of veins from caprock portions of SB-15-D. No outcrop veins have the low d13C values exhibited by hydrothermal calcite veins from reservoir sections of the SB-15-D core.

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Fig. 5. Comparison of the d13C and d18O values for calcite from outcrop and core samples. Data for the box representing the range in isotopic composition of Jurassic and Cretaceous marine carbonates are from Veizer and Hoefs (1976).

Across The Geysers-Clear Lake region there are only a few localities containing outcrops of marine limestone. As such, only one sample of a Cretaceous limestone was collected. d13C values for the limestone from three laboratories were identical for carbon, with d13C equal to 19.9%. d18O values were between +32.6 and +32.8%. 5. Discussion 5.1. Carbon isotopes in gases The presence of elevated 3 He=4 He ratios in gas samples from The Geysers and Clear Lake regions has generated considerable discussion regarding the relative importance of magmatic components in the geothermal systems (Lowenstern et al., 1999; Kennedy and Truesdell, 1996; Go€ et al., 1995). The carbon isotope value of mantle-derived carbon ranges between 8 and 3% (Fig. 1) (Hoefs, 1980; Rollinson, 1993). As such, the positive correlation between d13C±CO2 and mol% CO2 for The Geysers samples could be explained by an increasing contribution of magmatic CO2 in the geothermal gases. However, the stable isotope results do not support the idea of magmatic gases as a primary source for the CO2 for any of the

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locations in the ®eld area. Only samples from the Wilbur Springs District have d13C± CO2 values approaching the carbon isotope composition of mantle-derived CO2 and 3 He/4He data (R=Ra  1.5) do not indicate that large percentages of these gases are from a magmatic source (Table 1). Relatively 13C-rich CO2 was also collected in the Sulphur Bank Mine area. The gas vents and wells around the mine are partially surrounded by 40 ka andesite ¯ow that overlies the Franciscan Complex. R=Ra values of  7.5 from earlier investigations by Go€ and co-workers (Go€ and Janik, 1993; Go€ et al., 1993, 1995) show this to be an area where a convincing argument can be made for a magmatic component in the gas (Table 1). Simple mass balance calculations for a two component system where X is the mol fraction of CO2 from a speci®c source yield an estimate of the maximum amount of magmatic CO2 at Sulphur Bank Mine. The following assumptions and equations were used: XCO2 13

A

‡ XCO2

‰ C CO2

…A†

B

ˆ1

 XCO2

…A†

Š ‡ ‰13 C CO2

…B†

 XCO2

…B†

Š ˆ 13 C CO2 …total†

Magmatic CO2 (A) was assumed to range between 8 to 3%, d13C±CO2 (B)= 12.9% (Hopland non-geothermal CO2), and d13C±CO2 total= 10.5%, (the average value for Sulphur Bank Mine). For a simple two-component system, carbon isotope values indicate that the fraction of magmatic CO2 in Sulphur Bank Mine gases could be as high as 24±49%. Similar calculations for The Geysers where d13C±CO2 total= 12.5% yield between 4 and 9% magmatic CO2. More conservative estimates for both areas are obtained if a third component of CO2 derived from dissolution of calcite veins is considered. Studies of the chemical composition of Sulphur Bank Mine and northwest Geysers gases have revealed that, in comparison to magmatic gases, these locations have relatively high CH4 and low H2S (Go€ et al. 1993, 1995; Lowenstern et al., 1999). These studies found that although helium and heat in the geothermal systems are magmatically derived, the major components in the ¯uids and gases originate from metamorphism of the Franciscan rocks. As such, our maximum estimates of magmatic CO2 assuming a two-component system are probably too high. Although magmatic gases may contribute some CO2 at a few locations in The Geysers and Clear Lake geothermal regions, the relatively low d13C±CO2 values from all sites in this study point to a di€erent source for the bulk of the emitted CO2. CH4 provides a potential source of isotopically light CO2 in geothermal systems, as CO2 can be produced at elevated temperatures via the reaction CH4+2H2O!CO2+ 4H2 (Panichi et al., 1977; Lyon and Hulston, 1984). If CO2 is a byproduct of the thermal breakdown of CH4 and carbon isotopes are not subsequently reset, then one would expect some relation between d13C±CO2 and d13C±CH4. Alternatively, CO2 could be derived from a source other than CH4. Given sucient time and elevated temperatures, carbon isotope exchange between the two gases could obscure the original isotope values, resulting in isotopically light CO2 that appears to be related to CH4. Much research has been devoted to resolving this issue with the intention of developing a carbon isotope-exchange thermometer. Application of a carbon isotope

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geothermometer requires that temperatures were sucient for isotopic exchange to occur (> 300 C; Giggenbach, 1982), that the duration of exchange was long enough to achieve equilibrium between the two carbon species, and that there was no subsequent input of either gas from another source. The validity of the CH4±CO2 carbon isotope thermometer in geothermal environments has been widely disputed as, in some systems, temperatures derived from theoretical isotope models exceed temperatures determined from ¯uid chemistry or down-hole well measurements. The temperature discrepancy could be a function of isotope disequilibrium, implying that the thermometer is ill-suited for some geothermal systems, or that the isotope temperatures may be re¯ective of earlier conditions no longer present in the reservoir (Lyon and Hulston, 1984). The isotherms calculated for equilibrium temperatures using the carbon isotope exchange thermometer of Ohmoto and Rye (1979) are shown in Fig. 6. The carbon isotope temperatures for most locations in this study are greater than those calculated from the empirical gas equilibria thermometer of D'Amore and Panichi (1980) (Table 2). When the temperatures for all locations are examined, only the well samples from The Geysers have a restricted range in carbon isotope temperatures (305±398 C).

Fig. 6. d13C±CO2 and d13C±CH4 values for gas samples in The Geysers-Clear Lake region. Isotherms were generated using the ®tting curve of Ohmoto and Rye (1979) to the data of Bottinga (1969). Samples with d13C±CH4 values less than 56% were omitted for clarity. Data for four of the well samples in The Geysers are from the work of Shigeno et al. (1987).

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The calculated isotope temperatures for the well samples approach measured downhole temperatures in the NW Geysers (up to 342 C, Walters et al., 1988; Moore, 1992; Moore and Gunderson, 1995) but are somewhat higher than what is considered to be reasonable for the reservoir. For all other locations there is much greater scatter in the data, complicated by sites with possible microbial CH4. The scatter in the temperature data mostly re¯ects large di€erences in d13C±CH4 values, suggesting that, in part, CH4 sources are non-unique. Overall there is little evidence that CO2 in the gases has attained isotopic equilibrium with CH4. The majority of CH4 in high-temperature geothermal systems is thought to be derived from the thermal degradation of complex hydrocarbons in organic-bearing sedimentary rocks (Craig et al., 1966; Gunter and Musgrave, 1971; Gunter, 1978; Des Marais et al., 1981, 1988; Welhan, 1988). In general, d13C values of thermogenic CH4 are low relative to the d13C values of the original Corg in the host rocks due to preferential incorporation of 12C into CH4 (Hoefs, 1980). Schoell (1980, 1983) noted that the carbon isotope composition of CH4 produced from humic source rocks has higher d13C values ( 30 to 25%) relative to CH4 produced from marine source rocks ( 40 to 30%). Laboratory studies and investigations of ¯uids from hightemperature geothermal wells and oil ®eld wells have shown that the d13C values of CH4 increase with increasing thermal maturity of the source rocks (Des Marais et al., 1981, 1988; Lyon and Hulston, 1984; Sackett, 1984; Poulson et al., 1995). More recent experiments have shown that the relative di€erence between the carbon isotope composition of the precursor Corg and the resulting CH4 may be a€ected by the presence of liquid water in addition to temperatures and pressures (Seewald et al., 1994). As previously discussed, some of the variation in the d13C±CH4 values in this study may be related to microbial processes. Other factors a€ecting d13C±CH4 values are likely the di€erences in metamorphic grade and the diversity of rock types that underlie the region. The absence of any correlation between d13C±CO2 and d13C±CH4 values indicates that either isotopic equilibrium between the gases never occurred, that carbon isotope values have been reset, or that there are multiple (unequilibrated) sources of CH4. The lack of isotopic equilibrium eliminates carbon isotope thermometry even as a guide to apparent subsurface temperatures at any of the vent, fumarole or spring sites in this study. It also suggests that CH4 is not the source for the bulk of the CO2 in this region and that CO2 is more likely derived from a source such as calcite veins or Corg. 5.2. Vein and whole rock analyses; implications for SB-15-D core d13C values of hydrothermal calcite veins are in¯uenced by ¯uid pH, temperature, oxygen fugacity, and the d13C values of the carbon sources in the system (Deines et al., 1974). The isotope values of metamorphic calcite veins sampled from outcrops of Franciscan rocks are less than the d13C and d18O values of marine carbonates of similar age (Veizer and Hoefs, 1976) (Fig. 5). The outcrop veins document the presence of 13C-poor sources in vein-depositing ¯uids during regional metamorphism. The large variations in d13C values in the metamorphic veins are thought to represent variable mixing of ¯uids from two carbon sources, marine carbonate and Corg

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(Lambert and Epstein, 1992). Well cuttings of massive calcite veins from depths <1000 m in the central Geysers, east of SB-15-D, have d13C values between +2 to 15% (Lambert and Epstein, 1992), similar to values for outcrop samples from this study. The authors concluded that isotopic heterogeneity in shallow well samples was retained due to the CO2-poor nature of ¯uids in liquid-dominated portions of The Geysers geothermal system. A comparison of CF from the SB-15-D core with massive calcite veins from other wells in The Geysers and metamorphic outcrop veins from this study shows a much more restricted range in the d13C values of the SB-15-D core samples ( 13.3 to 9.6%). While it is possible that this narrow range in d13C values represents homogeneity of the original vein-depositing ¯uids, the diversity of d13C values for massive calcite in other wells and the similarity of d13C-calcite values of both CF and CH in SB-15-D caprock samples, suggests that the isotopic composition of CF re¯ects changes imposed by the recent hydrothermal system. If this is so, then the recent hydrothermal ¯uids must have contained 13C depleted carbon in sucient concentrations to produce the observed isotopic homogenization. Dissolution of CF during the early liquid-dominated geothermal system at The Geysers is thought to be responsible for secondary porosity development in the host rock (Gunderson, 1992; Hulen et al., 1991, 1992; Thompson and Gunderson, 1992; Hulen and Nielson, 1993). This process would yield dissolved carbon to the hotwater system and could promote isotopic homogenization of the relic calcite veins. More recently, development of the vapor-dominated system at about 300 ka in The Geysers and boiling of geothermal ¯uids resulted in precipitation of CH in the caprock and upper portions of the steam reservoir (Hulen and Nielson, 1995a; Moore and Gunderson, 1995). Calculations for the d13C±CO2 values of recent geothermal ¯uids using a value for d13C±C®nal of 12.1% (average for all calcite veins in the caprock), the CO2±calcite fractionation factors of Bottinga (1968b), and assuming paleotemperatures determined from ¯uid inclusions in the core between 195 and 311 C (Hulen and Moore, 1996) indicate that the d13C value of CO2 in equilibrium with calcite in the caprock had fairly low values between 11.8 and 10.2%. Dissolved carbonate and bicarbonate in this aqueous ¯uid would have d13C values 2 to 4% lower than the CO2 (Ohmoto and Rye, 1979). Given that metamorphic calcite veins from outcrop samples generally have higher d13C values than this, either there was another carbon source besides metamorphic calcite veins in the recent geothermal ¯uids, or some mechanism is necessary to explain a loss of 13C from the system. Coupled 13C and 18O loss in carbonates is often attributed to metamorphic decarbonation reactions due to preferential incorporation of these isotopes into CO2. The magnitude of the isotopic depletion is controlled by the temperature of the reaction, the ``style'' of degassing and the amount of volatilization as the reaction progresses (Valley, 1986). Batch volatilization involves single-stage loss of isotopically exchanged gas (CO2) in contrast to Rayleigh volatilization, a process of continuous isotope exchange and gas loss (Rayleigh, 1896; Epstein, 1959). At low amounts of volatilization the degree of isotopic change in the remaining rocks is similar for both methods. With increasing reaction progress the isotopic shift resulting from Rayleigh degassing becomes large. In modeling decarbonation reactions the reaction progress is de®ned as

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F (from 1 to 0), the mol fraction of the element of interest remaining. Valley (1986) de®ned a ``calc-silicate limit'' of 0.6 for F of oxygen based on reaction stoichiometries and oxygen isotope exchange with H2O and silicate minerals usually present in the system. In contrast, F values for carbon are thought to range between 1 and 0. The calculated limits of carbon and oxygen isotope shifts for batch and Rayleigh decarbonation are shown in Fig. 7 for an ``average'' outcrop metamorphic calcite vein (d13C= 7%, d18O=+15.5%) with FCarbon from 1 to 0 and FOxygen from 1 to 0.6. The calculations were performed at 240 and 340 C to encompass the present and past temperatures of maximum hydrothermal activity (Hulen and Moore, 1996). The equations for mass balance as written by Valley (1986) are: f

…Rayleigh†

ˆ i ‡ 1000…F…

1†

-1† and f

…batch†

ˆ i

…1

F†1000 ln :

df and di are the ®nal and initial isotope values of the rock, and a is the temperaturedependent fractionation factor for oxygen and carbon isotope exchange (¯uid-rock) (Bottinga 1968a,b). Rayleigh decarbonation reactions at high temperature could result in large shifts in d13C values of the remaining calcite but only after signi®cant amounts (>80%) of the original veins were lost. Calculations at higher temperatures allow for lower d13C values of the remaining veins with smaller amounts of calcite loss. Decarbonation reactions are insucient to explain the low d18O values of the hydrothermal veins unless a low d18Oi value (4 12%) is assumed. Although a detailed investigation of the d18O values of the veins is beyond the scope of this work, a more probable explanation for the low d18O values in CH is that they result from interaction with meteoric waters in the geothermal ¯uids (Criss and Taylor, 1986). Another source for 13C-poor ¯uids in The Geysers geothermal system is thermal degradation of disseminated organic carbon that is primarily concentrated in the argillites. During metamorphism, isotopic exchange between Corg and calcite is promoted when poorly organized carbon is restructured into more crystalline graphitic material (Buseck and Huang, 1985). Fully ordered graphite was not observed in the SB-15-D core, but the TEM analysis of the sample from the reservoir showed an increase in the degree of structural ordering of some of the Corg as compared to the sample from the caprock (Brearley, personal communication). These data are consistent with the apparent increase of argillite competency in the steam reservoir (Hulen and Nielson, 1995a; Hulen et al., 1995) and can explain the higher d13C values for Corg in the core as compared to outcrop samples. Graphitization of Corg and carbon isotope exchange with existing calcite would result in an increase in the d13C values of Corg and a decrease in the d13C values of calcite (Bottinga, 1969). The low d13C values for CH in reservoir sections of the core may have occurred when the percentage of Corg in the boiling geothermal ¯uids increased as a result of carbon restructuring. 5.3. Gas±mineral carbon isotope equilibria Organically derived carbon may be incorporated into geothermal ¯uids when it converts to CO2 via oxidation (C+O2 ! CO2) and/or hydrolysis (2C+2H2O !

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CO2+CH4) reactions (Ohmoto and Rye, 1979). Both reactions would produce d13C-poor CO2, probably with values <-10% (Ohmoto and Rye, 1979). Experimental studies by Seewald et al. (1994, 1998) document the production of signi®cant quantities of CO2 during thermal alteration of organic matter in the presence of liquid water. Comparisons of closed-system dry and wet experiments show that more CO2 is produced in the wet experiments and at lower temperatures than in the dry experiments (Seewald et al., 1998). The ®nal amount of free CO2 gas in natural systems is ultimately controlled by the balance between CO2 sources and CO2 sinks. CO2 sinks include dissolution of the gas into ¯uids with subsequent migration from the system, and precipitation of carbonate minerals (Seewald et al., 1998). Evaluation of CO2±Corg isotope equilibria is dicult as the fractionation factors for carbon isotope exchange between the two compounds are not known. Theoretical fractionation factors for exchange between CO2 and graphite have been determined by Bottinga (1968a). If we use graphite as a proxy for Corg we can model the equilibrium temperatures necessary to produce the observed CO2. For initial Corg= 23.5% (average for Corg in the core) and ®nal d13C±CO2 at The Geysers and

Fig. 7. d13C and d18O values for calcite from outcrop and core samples from Fig. 5 with trajectories calculated for isotopic change resulting from decarbonation reactions. Lines at F=0.2 and 0.1 de®ne the fraction of carbon remaining during the reactions. Batch (straight lines) and Raleigh (curved lines) decarbonation are calculated for 240 C (dashed lines) and 330 C (solid lines).

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the Wilbur Springs district equal to 12.7 and 10.1%, the graphite model would indicate temperatures of 460 and 280 C, respectively. Calculations using the average values for Corg from outcrop samples ( 25.3%) yield more reasonable temperatures for The Geysers (330 C) but would suggest temperatures that are too low for Wilbur Springs ( 100 C). It is our opinion that these estimates are not reasonable. Fractionation factors for carbon isotope exchange between CO2 and calcite are better de®ned than those for the CO2±Corg system as more experimental data are available to evaluate uncertainties in the theoretical equilibrium values (Ohmoto and Rye, 1979). Calculations using the CO2-calcite fractionation factors of Bottinga (1968b) and the average d13C values for all calcite veins in the SB-15-D caprock (d13C= 12.1%) yield carbon isotope equilibria temperatures of 170 C for The Geysers and 280 C for Wilbur Springs. The smaller temperature range seems more reasonable than temperatures from calculations for CO2±Corg equilibria, but calculated temperatures are lower than measured temperatures in The Geysers steam reservoir and are much higher than those at Wilbur Springs. Given the current understanding of The Geysers reservoir temperatures, the results from the carbon isotope analyses of the SB-15-D core do not identify a single source for The Geysers CO2. Dissolution of the calcite veins in upper parts of The Geysers reservoir can produce 13C-poor CO2 but the very low carbon isotope values for The Geysers CO2 requires that Corg must also contribute to the total gas. The di€erence between carbon isotope values of CO2 from the Hopland samples and those from most samples from The Geysers suggests that Corg is not the sole source for The Geysers CO2. As previously discussed, the second source of CO2 may be derived from magmatic degassing or from the dissolution of the calcite veins. Since there is evidence for the presence of carbon derived from Corg and calcite in earlier geothermal ¯uids, and calcite veins are still found in upper sections of The Geysers reservoir, it is conceivable that both calcite veins and Corg contribute to present-day generation of CO2. 5.4. Regional CO2 variation The regional gas samples provide more evidence that CO2 is partially derived from a relatively 13C-rich source such as the calcite veins. Although overall d13C±CO2 values are generally low, the increase in the carbon isotope values of CO2 in Clear Lake, Sulphur Bank Mine, and Wilbur Springs gases demonstrates that a larger percentage of the total CO2 is derived from a source other than Corg. The regional increases in d13C±CO2 values could result from addition of CO2 from dissolution of marine carbonate rocks, if such rocks were abundant. Geologic mapping throughout the region (McLaughlin, 1975; McLaughlin et al., 1989) reveals that marine limestone occurs rarely in the Franciscan complex and Great Valley sequence, and usually as exotic blocks in melange and detrital serpentinite (Lockwood, 1971; McLaughlin et al., 1989). The largest marine limestone block known in the region is about 0.19 km2 in dimension and occurs in sheared detrital serpentinite about 0.5 km southeast of Wilbur Springs. This block is extremely fossiliferous and has a surprising carbon isotope value of 19.9% (Table 4). Similar but much smaller

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limestone bodies in this region were studied by Campbell and Bottjer (1995a,b) who determined that they were deposited from relatively cold, nutrient-rich, submarine seeps. These authors report d13C values of 24 to 19% for ®ve carbonate samples and argue that these values result from seawater-methane ®xation. There are no typical marine carbonates with the usual d13C values near 03% known in the region. Thus, dissolution of minor amounts of marine limestone from this region would apparently add isotopically light rather than 13C-rich carbon to any resulting CO2. From the results of our work the simplest explanation for the regional di€erences in d13C±CO2 values is that they result from variations in the percentage of CO2 derived from degradation of Corg and dissolution of metamorphic calcite veins. The di€erences in the local lithologies must also be considered as a factor in the isotopic variability of the regional CO2. Although additional study of cuttings or core samples from these areas is necessary, it is arguable that the carbon isotope signature of Corg in the relatively unmetamorphosed Great Valley Sequence rocks may be di€erent from that in the highly deformed metasedimentary rocks of the Franciscan Complex. 6. Summary Carbon isotope analyses of the SB-15-D core samples indicate that two carbon reservoirs, Corg and calcite veins, have provided a source and a sink for carbonbearing ¯uids in the geothermal system at The Geysers. Dissolution of CF provided carbon to early geothermal ¯uids and likely contributed to homogenizing the originally diverse isotopic composition of metamorphic calcite veins. The present-day low d13C values for calcite veins in the SB-15-D core could result from extensive Rayleigh-type decarbonation reactions. However, shifted d13C values of disseminated Corg, the observed increased structural ordering in Corg at depth, and very low d13C values for CH in the reservoir sections of the core, point to Corg as an important constituent in The Geysers geothermal ¯uids. The carbon isotope data from this study do not delineate an exclusive source for regional CO2 emissions because gas±gas and gas±mineral isotope equilibria yield temperatures that are in disagreement with present-day reservoir temperatures. The isotope data do however provide constraints to permit evaluation of individual carbon reservoirs as potential CO2 sources. d13C values of CH4 show no correlation with d13C±CO2; as such, there is no isotopic evidence that CO2 is derived from thermal degradation of CH4. A comparison of d13C±CO2 values and 3 He=4 He ratios indicates that some sites in The Geysers and Sulphur Bank Mine may have a minor magmatic CO2 component that results in a rise in the total d13C±CO2 values. However, ¯uid and gas geochemistry and overall low d13C values rule out magmatic gases as a primary CO2 source. In general, our data show that most of the CO2 produced in The Geysers-Clear Lake region re¯ects thermal e€ects on the various metasedimentary rocks. Organic carbon within the Franciscan metagraywackes and argillites has played a continuous role in in¯uencing the isotopic composition of carbon species in the past and present-day geothermal system at The Geysers and provides a source for CO2 across the ®eld area. Dissolution of metamorphic calcite veins is also an important process

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contributing to the total CO2 emissions. The variability of d13C±CO2 values across the region likely re¯ects changes in the local lithologies and di€erences in the relative proportions of CO2 derived from these two carbon sources.

Acknowledgements D. Bergfeld was partially supported on this project by grants and scholarships from the following: Associated Western Universities, Geological Society of America, the Oce of Graduate Studies at the University of New Mexico, the Department of Earth and Planetary Sciences (UNM) and the Student Resources Allocations Committee (UNM). Additional support came from the US Department of Energy, Oce of Geothermal Technologies, and from the US Geological Survey Volcano Hazards Program. Electron microscopy was performed in the Electron Microbeam Analysis Facility, Department of Earth and Planetary Sciences and Institute of Meteoritics, University of New Mexico. The authors would like to thank Je€ Hulen and the Energy and Geoscience Institute for providing samples from the SB-15-D corehole; Tom Powell and Brian Koenig, both formerly of UNOCAL, for help with sampling at The Geysers; C. Kendall and L.D. White for mass spectrometry and use of the Water Resources Division isotope lab, and M. Huebner for CO2 extraction from gas samples for carbon isotope analyses, all at the US Geological Survey, Menlo Park, CA; A. Brearley for TEM analyses, and P. I. Nabelek and A. Campbell at the University of Missouri, Columbia, and New Mexico Institute of Mining and Technology for the use of their stable isotope facilities. Thoughtful reviews by S. Silva and T. Lorenson of the US Geological Survey, by D. Cole and other Geothermics reviewers helped in the preparation of the ®nal manuscript.

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