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Magmatic CO2 in natural gases in the Permian Basin, West Texas: identifying the regional source and filling history
Journal of Geochemical Exploration 69–70 (2000) 59–63 www.elsevier.nl/locate/jgeoexp
Magmatic CO2 in natural gases in the Permian Basin, West Texas: identifying the regional source and filling history C.J. Ballentine a,*, M. Schoell b, D. Coleman c, B.A. Cain d b
a IGMR, Dept. Erdwissenschaften, ETH Zentrum NO CO 61.7, 8092 Zu¨rich, Switzerland Chevron Research and Technology Company, 6001 Bollinger Canyon, San Ramon, CA 94583, USA c Isotech Laboratories Inc., 1308 Parkland Court, Champaign, IL 61821-1826, USA d Altura Energy LLP, Houston, TX, USA
Abstract The origin of regional CO2 occurrences within basinal fluids cannot be resolved with stable isotopes alone because magmatic and crustal d C 13(CO2) ranges are non-unique. 3He/CO2 from magmatic systems such as mid-ocean ridges fall within a tight range (10 9 –10 10) compared with values found in crustal fluids (10 5 –10 13). Although crustal fluids with 3He/CO2 within the magmatic range clearly have a significant magmatic component, the possibility of crustal CO2 addition or reactive loss of CO2 makes a quantitative assessment of the magmatic contribution non-trivial. In this paper we present a combination of highprecision d C 13(CO2) and 3He/CO2 from the CO2-rich (20–55%) JM-Brown Basset gas field, West Texas. In this field 3He/CO2 is in the magmatic range and varies between 4.1 and 6:2 × 109 : This variation is directly correlated with %CO2. We use the very small range in d C 13(CO2) (⫺2.70 to ⫺3.06‰) to rule out either precipitation or crustal addition of CO2 being the cause of the 3 He/CO2 variation. 3He has no significant crustal sources or sinks. A model of variable magmatic outgassing (up to 20%) is consistent with the range in observed 3He/CO2 and d C 13(CO2). Within the context of this model, all of the CO2 in this gas field is magmatic. Furthermore samples preserving the highest 3He/CO2 represent the earliest phase of magmatic degassing and provide a temporal and spatial filling history for the gas field. In the regional context we show that our results are consistent with a regional magmatic CO2 source to the North of the Marathon Thrust Fault system. This CO2 must have been in place before significant generation of CH4 and other hydrocarbons filled these reservoirs from the same direction, overprinting the magmatic CO2. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Helium; Carbon; Dioxide; Isotopes; Magma; Orogenic
1. 3He/CO2 in magmatic and crustal systems Approximately 90% of mantle volatile degassing occurs in mid-ocean spreading ridge settings, ⬃10% from ocean island volcanism, and ⬍1% from continental regions (Javoy, 1997). Because He is lost from the atmosphere, with a residence time of approximately 10 6 years (Torgersen, 1989), the concentration * Corresponding author. E-mail address: [email protected] (C.J. Ballentine).
of 3He in air is two orders of magnitude smaller than in typical magmatic gases. The clearest indication of a mantle-derived fluid is therefore shown by the presence of 3He, and has been used to quantify the flux of mantle-derived volatiles through extensional basin settings (Sano et al., 1986, Martel et al., 1989; Stute et al., 1992; Ballentine, 1997). The relationship between CO2 and 3He at midocean ridge settings is reasonably well defined with a CO2/ 3He range of between 1 and 7 × 109 (Marty and Jambon, 1987; Javoy and Pineau, 1991; Burnard et al.,
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00045-5
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Fig. 1. CO2/ 3He vs. CO2 content. The JM-B field has CO2/ 3He indistinguishable from magmatic values, but displays a clear trend with CO2 content. Simple mixing of CO2 with CH4 containing radiogenic He ( 3He-defficient) would preserve a constant CO2/ 3He. Vector B shows the effect of CH4 addition. The vector labeled A in the trajectory samples would take on simple CO2 loss. Neither process accounts for this data set and magmatic source CO2/ 3He variation is the preferred explanation.
1997). The range from ocean island systems is somewhat larger, between 7 × 108 and 3 × 1010 (Trull et al., 1993), with variations from the estimated source value of ⬃ 1–2 × 109 explained by solubility degassing fractionation and varied magma chemistry. Back-arc volcanism produces a much wider range of CO2/ 3He, between 10 10 –10 13. The CO2/ 3He higher than midocean ridge values is attributed either to the addition of subducted marine carbonates to the mantle wedge supplying the magmatic volatiles (Van Soest et al., 1998), or to shallow CO2 addition, again derived from the decarbonation of marine carbonates (Pedroni et al., 1999). The carbon isotopic composition of the MORB and OIB source is indistinguishable with both magmatic systems showing a wide range in carbon isotopic value, between ⫺1.6 and ⫺9.8‰, most reasonably attributed to phase fractionation and magma chemistry (Pineau and Javoy, 1983; Marty and Jambon, 1987; Javoy and Pineau, 1991; Mattey, 1991; Trull et al., 1993). Samples from Arc settings show a far larger range, depending on the isotopic composition
of the carbonate and the extent of organic carbon contribution to the excess CO2 (e.g. Van Soest et al., 1998; Pedroni et al., 1999). Because the bulk crustal carbon isotopic composition of between ⫺5 and ⫺7‰ (Deines, 1980) overlaps with the range of magmatic values, carbon isotopes alone do not enable magmatic vs. crustal CO2 sources to be unambiguously resolved. However, the CO2/ 3He range for oceanic and mid-ocean ridge magmatism is very small compared with typical crustal gases with values ranging between CO2 =3 He 105 –1013 (Sherwood Lollar et al., 1997 and references therein). Notwithstanding exceptional coincidence, CO2 in natural gases that preserve CO2 =3 He ⬃ 1–10 × 109 can be assumed to be dominantly magmatic in origin. Indeed, several gas reservoirs with CO2 concentrations approaching 100% have 3He/ 4He, Xe isotope ratios and CO2/ 3He approaching magmatic endmember values. The CO2 in these fields can reasonably be considered to be dominantly magmatic in origin, and appears to be derived from partial degassing of deepseated magmas (Ballentine, 1997).
C.J. Ballentine, M. Schoell / Journal of Geochemical Exploration 69–70 (2000) 59–63
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Fig. 2. Location of JM-BB field showing regional increase in %CO2 towards the Marathon Thrust.
In the case of natural gases with CO2 =3 He q 1010 ; the CO2 can only reasonably be produced from crustal (carbonate) sources (Fig. 1). The origin of(CO2 with CO2 =3 He ⱕ 1–10 × 109 is more difficult to assess, because magmatic CO2 can be lost in the subsurface by carbonate precipitation or reduction to form CH4 (Fig. 1). The possibility of a subsequent crustal CO2 overprint in these gases must also be considered. Systematic behavior of CO2/ 3He, 3He/ 4He and stable isotopes in such a deposit provides the basis for evaluating more ambiguous systems (e.g. Sherwood Lollar et al., 1997). The CO2 variation in the JMBrown Bassett (JM-BB) field reflects regional trends
in natural gas CO2 content (Fig. 2) and provides an ideal location to combine stable isotope work with the information available from noble gases.
2. The JM-BB field and results The JM-BB field is located in the Val-Verde basin in West Texas, on the foredeep margin of the late Paleozoic Marathon Allochthon Thrust Belt (Shumaker, 1992) (Fig. 2). The main producing formation in this section of the Permian Basin is brecciated Ellenberger Dolomite. The gas composition
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C.J. Ballentine, M. Schoell / Journal of Geochemical Exploration 69–70 (2000) 59–63
Fig. 3. Evolution of d C 13(CO2) and CO2/ 3He calculated for the gas phase of a degassing magma body. Two models are shown. The first is the composition of the gas evolving from the magma by a Rayleigh fractionation process. The second is the composition of this gas in an accumulating reservoir. The model undegassed magma is taken to have d C 13/CO2 ⫺4.7‰ and CO2 =3 He 2 × 109 : The tick marks are the percent loss of CO2 from the magma body. d C 13(CO2) fractionation between a CO2 gas phase and magma is taken to be 2‰ (Mattey, 1991), and the relative solubility of He/CO2 5 (Bottinga and Javoy, 1991). Both CO2/ 3He and d C 13(CO2) of the JM-BB field are consistent with this simple magma degassing model.
varies between 20 and 55% by volume CO2, the remaining gas being dominated by CH4. In addition to high-precision stable isotope analysis of these gases we have determined the He, Ne, Ar isotope and abundance composition in twelve of these gases. 3He/ 4He and CO2/ 3He correlates directly with CO2 content, varying between 3 He=4 He 0:2–0:55Ra; CO2 =3 He 4:2–6:2 × 109 for CO2 20–55%: The 3He/ 4He ratio is a result of resolvable magmatic–crustal twocomponent mixing. Between 3.2 and 6.8% of the 4 He is estimated to be mantle-derived. The remainder is crustal-radiogenic in origin. Measured 20Ne/ 22Ne, 21 Ne/ 22Ne and 40Ar/ 36Ar vary between 9.28 and 10.02, 0.037 and 0.073 and 1030 and 5660, respectively, and enable air-derived Ne and Ar to be resolved from crustal radiogenic components. Samples free of modern air contamination have low groundwater (dissolved air)-derived Ne and Ar concentrations that are unfractionated from typical water values. This result shows that the groundwater system played no resolvable role in either the gas transport to the trapping structure or composition variation (e.g. Ballentine et al., 1991). Resolved crustal-radiogenic gases are found at their predicted
4
He/ 21Ne/ 40Ar average crustal production ratios. The crustal radiogenic gases are associated with both CO2 and CH4 endmembers. The crustal noble gas ratios are typical of a gas phase that is derived from regions of the crust at temperatures ⬎250⬚C, and has migrated to the trapping structure without any transport- or phaserelated fractionation (Ballentine et al., 1994). CO2/ 3He are within the range found in typical mantle environments 1–10 × 109 ; compared to the range in crustal fluids of 10 5 –10 13, and are reasonably dominated by magmatic CO2. The variation in CO2/ 3He can be explained by three mechanisms: (i) crustal CO2 addition; (ii) precipitation of CO2; or (iii) magmatic source variation. However, high-precision d C 13(CO2) (^0.015‰) shows ⬍0.3‰ variation across the entire field. Exceptional coincidence notwithstanding, this is not consistent with crustal CO2 addition or fractionation caused by CO2 precipitation. A simple model of Rayleigh-style magmatic degassing has been constructed. The 10–20% partial degassing of a deep-seated magma body is consistent with both CO2/ 3He and d C 13(CO2) variation (Fig. 3). Within the context of the magmatic degassing model, JM-BB samples with the highest 3He/CO2 preserve
C.J. Ballentine, M. Schoell / Journal of Geochemical Exploration 69–70 (2000) 59–63
the earliest stages of magmatic outgassing and, assuming a simple filling history, are likely to be the furthest from the CO2 source. On the JM-BB field scale the highest CO2/ 3He values are found in the samples with the highest %CO2. On a regional scale, the highest %CO2 natural gases are found on the Northern and Eastern margins of the Marathon Thrust Belt. Assuming these gases follow the JM-BB trend, it would suggest that the Marathon Thrust Belt is not spatially related to the CO2 source, and that the source itself must be to the North of this feature. This in itself is not surprising. Magmatic fluids, traced by the presence of 3He, are typically associated with regional extension rather than compression or loading. The identification of extensional features, probably associated with the timing of the Marathon Thrust Fault, will provide the most likely source/s for this regional CO2 occurrence. Identification of the general direction of CO2 migration also constrains the timing of the CO2 input relative to the CH4. Assuming simple filling, samples nearest the source of the CO2 might be expected to have the highest %CO2 concentrations, if this was implaced after CH4 production. These in fact have the highest %CH4 content. The distribution of CO2/ 3He and CH4/CO2 is consistent with an initial input of CO2, from the North or East of the Marathon Thrust Belt, with subsequent CH4 generation in the Val-Verde and Southern Delaware basin foredeeps. This results in the observed overprinting and dilution of the CO2 containing the most evolved magmadegassing signature.
References Ballentine, C.J., 1997. Resolving the mantle He/Ne and crustal 21Ne/22Ne in well gases. Earth Planet. Sci. Lett. 152, 233–250. Ballentine, C.J., O’Nions, R.K., Oxburgh, E.R., Horvath, F., Deak, J., 1991. Rare gas constraints on hydrocarbon accumulation, crustal degassing and groundwater flow in the Pannonian Basin. Earth Planet. Sci. Lett. 105, 229–246. Ballentine, C.J., Mazurek, M., Gautschi, A., 1994. Thermal constraints on rare gas release and migration: Evidence from Alpine fluid inclusions. Geochim. Cosmochim. Acta 58, 4333–4348.
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Bottinga, Y., Javoy, M., 1991. MORB degassing: bubble growth and ascent. Chem. Geol. 81, 255–270. Burnard, P., Graham, D., Turner, G., 1997. Vesicle-specific noble gas analysis of “popping rock”: implications for primordial noble gases in earth. Science 276, 568–571. Deines, P., 1980. The isotopic composition of reduced organic carbon. In: Fritz, P., Fontes, J. (Eds.). Handbook of Environmental Isotope Geochemistry, Elsevier, Amsterdam, pp. 329– 406. Javoy, M., 1997. The major volatile elements of the Earth: their origin, behavior, and fate. Geophys. Res. Lett. 24, 177–180. Javoy, M., Pineau, F., 1991. The volatile record of a ‘popping’ rock from the Mid-Atlantic Ridge at 14⬚N: chemical and isotopic composition of gas trapped in vesicles. Earth Planet. Sci. Lett. 107, 598–611. Martel, D.J., et al., 1989. Leakage of helium from the Pannonian Basin. Nature 432, 908–912. Marty, B., Jambon, A., 1987. C/3He in volatile fluxes from the solid Earth: implications for carbon geodynamics. Earth Planet. Sci. Lett. 83, 16–26. Mattey, D.P., 1991. Carbon-dioxide solubility and carbon isotope fractionation in basaltic melt. GCA 55, 3467–3473. Pedroni, A., Hammerschmidt, K., Friedrichsen, H., 1999. He, Ne, Ar, and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc. Geochim. Cosmochim. Acta 63, 515–532. Pineau, F., Javoy, M., 1983. Carbon isotopes and concentrations in mid-oceanic ridge basalts. Earth Planet. Sci. Lett. 62, 239–257. Sano, Y., Wakita, H., Huang, C.W., 1986. Helium flux in a continental land area estimated from 3He/ 4He in northern Taiwan. Nature 323, 55–57. Sherwood Lollar, B., Ballentine, C.J., O’Nions, R.K., 1997. The fate of mantle-derived carbon in a continental sedimentary basin: integration of C/He relationships and stable isotope signatures. Geochim. Cosmochim. Acta 61, 2295–2307. Shumaker, R.C., 1992. Paleozoic Structure of the Central Basin Uplift and Adjacent Delaware Basin, West Texas. AAPG 76 (11), 1804–1824. Stute, M., Sonntag, C., Deak, J., Schlosser, P., 1992. Helium in deep circulating groundwater in the Great Hungarian Plain: flow dynamics and crustal and mantle helium fluxes. Geochim. Cosmochim. Acta 56, 2051–2067. Torgersen, T., 1989. Terrestrial helium degassing fluxes and the atmospheric helium budget—implications with respect to the degassing processes of continental-crust. Chem. Geol. 79, 1–14. Trull, T., Nadeau, S., Pineau, F., Polve, M., Javoy, M., 1993. C–He systematics in hotspot xenoliths: implications for mantle carbon contents and carbon recycling. Earth Planet. Sci. Lett. 118, 43– 64. Van Soest, M.C., Hilton, D.R., Kreulen, R., 1998. Tracing crustal and slab contributions to are magmatism in the Lesser Antilles island are using helium and carbon relationships in geothermal fluids. Geochim. Cosmochim. Acta 62, 3323–3335.
Magmatic CO2 in natural gases in the Permian Basin, West Texas: identifying the regional source and filling history C.J. Ballentine a,*, M. Schoell b, D. Coleman c, B.A. Cain d b
a IGMR, Dept. Erdwissenschaften, ETH Zentrum NO CO 61.7, 8092 Zu¨rich, Switzerland Chevron Research and Technology Company, 6001 Bollinger Canyon, San Ramon, CA 94583, USA c Isotech Laboratories Inc., 1308 Parkland Court, Champaign, IL 61821-1826, USA d Altura Energy LLP, Houston, TX, USA
Abstract The origin of regional CO2 occurrences within basinal fluids cannot be resolved with stable isotopes alone because magmatic and crustal d C 13(CO2) ranges are non-unique. 3He/CO2 from magmatic systems such as mid-ocean ridges fall within a tight range (10 9 –10 10) compared with values found in crustal fluids (10 5 –10 13). Although crustal fluids with 3He/CO2 within the magmatic range clearly have a significant magmatic component, the possibility of crustal CO2 addition or reactive loss of CO2 makes a quantitative assessment of the magmatic contribution non-trivial. In this paper we present a combination of highprecision d C 13(CO2) and 3He/CO2 from the CO2-rich (20–55%) JM-Brown Basset gas field, West Texas. In this field 3He/CO2 is in the magmatic range and varies between 4.1 and 6:2 × 109 : This variation is directly correlated with %CO2. We use the very small range in d C 13(CO2) (⫺2.70 to ⫺3.06‰) to rule out either precipitation or crustal addition of CO2 being the cause of the 3 He/CO2 variation. 3He has no significant crustal sources or sinks. A model of variable magmatic outgassing (up to 20%) is consistent with the range in observed 3He/CO2 and d C 13(CO2). Within the context of this model, all of the CO2 in this gas field is magmatic. Furthermore samples preserving the highest 3He/CO2 represent the earliest phase of magmatic degassing and provide a temporal and spatial filling history for the gas field. In the regional context we show that our results are consistent with a regional magmatic CO2 source to the North of the Marathon Thrust Fault system. This CO2 must have been in place before significant generation of CH4 and other hydrocarbons filled these reservoirs from the same direction, overprinting the magmatic CO2. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Helium; Carbon; Dioxide; Isotopes; Magma; Orogenic
1. 3He/CO2 in magmatic and crustal systems Approximately 90% of mantle volatile degassing occurs in mid-ocean spreading ridge settings, ⬃10% from ocean island volcanism, and ⬍1% from continental regions (Javoy, 1997). Because He is lost from the atmosphere, with a residence time of approximately 10 6 years (Torgersen, 1989), the concentration * Corresponding author. E-mail address: [email protected] (C.J. Ballentine).
of 3He in air is two orders of magnitude smaller than in typical magmatic gases. The clearest indication of a mantle-derived fluid is therefore shown by the presence of 3He, and has been used to quantify the flux of mantle-derived volatiles through extensional basin settings (Sano et al., 1986, Martel et al., 1989; Stute et al., 1992; Ballentine, 1997). The relationship between CO2 and 3He at midocean ridge settings is reasonably well defined with a CO2/ 3He range of between 1 and 7 × 109 (Marty and Jambon, 1987; Javoy and Pineau, 1991; Burnard et al.,
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00045-5
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Fig. 1. CO2/ 3He vs. CO2 content. The JM-B field has CO2/ 3He indistinguishable from magmatic values, but displays a clear trend with CO2 content. Simple mixing of CO2 with CH4 containing radiogenic He ( 3He-defficient) would preserve a constant CO2/ 3He. Vector B shows the effect of CH4 addition. The vector labeled A in the trajectory samples would take on simple CO2 loss. Neither process accounts for this data set and magmatic source CO2/ 3He variation is the preferred explanation.
1997). The range from ocean island systems is somewhat larger, between 7 × 108 and 3 × 1010 (Trull et al., 1993), with variations from the estimated source value of ⬃ 1–2 × 109 explained by solubility degassing fractionation and varied magma chemistry. Back-arc volcanism produces a much wider range of CO2/ 3He, between 10 10 –10 13. The CO2/ 3He higher than midocean ridge values is attributed either to the addition of subducted marine carbonates to the mantle wedge supplying the magmatic volatiles (Van Soest et al., 1998), or to shallow CO2 addition, again derived from the decarbonation of marine carbonates (Pedroni et al., 1999). The carbon isotopic composition of the MORB and OIB source is indistinguishable with both magmatic systems showing a wide range in carbon isotopic value, between ⫺1.6 and ⫺9.8‰, most reasonably attributed to phase fractionation and magma chemistry (Pineau and Javoy, 1983; Marty and Jambon, 1987; Javoy and Pineau, 1991; Mattey, 1991; Trull et al., 1993). Samples from Arc settings show a far larger range, depending on the isotopic composition
of the carbonate and the extent of organic carbon contribution to the excess CO2 (e.g. Van Soest et al., 1998; Pedroni et al., 1999). Because the bulk crustal carbon isotopic composition of between ⫺5 and ⫺7‰ (Deines, 1980) overlaps with the range of magmatic values, carbon isotopes alone do not enable magmatic vs. crustal CO2 sources to be unambiguously resolved. However, the CO2/ 3He range for oceanic and mid-ocean ridge magmatism is very small compared with typical crustal gases with values ranging between CO2 =3 He 105 –1013 (Sherwood Lollar et al., 1997 and references therein). Notwithstanding exceptional coincidence, CO2 in natural gases that preserve CO2 =3 He ⬃ 1–10 × 109 can be assumed to be dominantly magmatic in origin. Indeed, several gas reservoirs with CO2 concentrations approaching 100% have 3He/ 4He, Xe isotope ratios and CO2/ 3He approaching magmatic endmember values. The CO2 in these fields can reasonably be considered to be dominantly magmatic in origin, and appears to be derived from partial degassing of deepseated magmas (Ballentine, 1997).
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Fig. 2. Location of JM-BB field showing regional increase in %CO2 towards the Marathon Thrust.
In the case of natural gases with CO2 =3 He q 1010 ; the CO2 can only reasonably be produced from crustal (carbonate) sources (Fig. 1). The origin of(CO2 with CO2 =3 He ⱕ 1–10 × 109 is more difficult to assess, because magmatic CO2 can be lost in the subsurface by carbonate precipitation or reduction to form CH4 (Fig. 1). The possibility of a subsequent crustal CO2 overprint in these gases must also be considered. Systematic behavior of CO2/ 3He, 3He/ 4He and stable isotopes in such a deposit provides the basis for evaluating more ambiguous systems (e.g. Sherwood Lollar et al., 1997). The CO2 variation in the JMBrown Bassett (JM-BB) field reflects regional trends
in natural gas CO2 content (Fig. 2) and provides an ideal location to combine stable isotope work with the information available from noble gases.
2. The JM-BB field and results The JM-BB field is located in the Val-Verde basin in West Texas, on the foredeep margin of the late Paleozoic Marathon Allochthon Thrust Belt (Shumaker, 1992) (Fig. 2). The main producing formation in this section of the Permian Basin is brecciated Ellenberger Dolomite. The gas composition
62
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Fig. 3. Evolution of d C 13(CO2) and CO2/ 3He calculated for the gas phase of a degassing magma body. Two models are shown. The first is the composition of the gas evolving from the magma by a Rayleigh fractionation process. The second is the composition of this gas in an accumulating reservoir. The model undegassed magma is taken to have d C 13/CO2 ⫺4.7‰ and CO2 =3 He 2 × 109 : The tick marks are the percent loss of CO2 from the magma body. d C 13(CO2) fractionation between a CO2 gas phase and magma is taken to be 2‰ (Mattey, 1991), and the relative solubility of He/CO2 5 (Bottinga and Javoy, 1991). Both CO2/ 3He and d C 13(CO2) of the JM-BB field are consistent with this simple magma degassing model.
varies between 20 and 55% by volume CO2, the remaining gas being dominated by CH4. In addition to high-precision stable isotope analysis of these gases we have determined the He, Ne, Ar isotope and abundance composition in twelve of these gases. 3He/ 4He and CO2/ 3He correlates directly with CO2 content, varying between 3 He=4 He 0:2–0:55Ra; CO2 =3 He 4:2–6:2 × 109 for CO2 20–55%: The 3He/ 4He ratio is a result of resolvable magmatic–crustal twocomponent mixing. Between 3.2 and 6.8% of the 4 He is estimated to be mantle-derived. The remainder is crustal-radiogenic in origin. Measured 20Ne/ 22Ne, 21 Ne/ 22Ne and 40Ar/ 36Ar vary between 9.28 and 10.02, 0.037 and 0.073 and 1030 and 5660, respectively, and enable air-derived Ne and Ar to be resolved from crustal radiogenic components. Samples free of modern air contamination have low groundwater (dissolved air)-derived Ne and Ar concentrations that are unfractionated from typical water values. This result shows that the groundwater system played no resolvable role in either the gas transport to the trapping structure or composition variation (e.g. Ballentine et al., 1991). Resolved crustal-radiogenic gases are found at their predicted
4
He/ 21Ne/ 40Ar average crustal production ratios. The crustal radiogenic gases are associated with both CO2 and CH4 endmembers. The crustal noble gas ratios are typical of a gas phase that is derived from regions of the crust at temperatures ⬎250⬚C, and has migrated to the trapping structure without any transport- or phaserelated fractionation (Ballentine et al., 1994). CO2/ 3He are within the range found in typical mantle environments 1–10 × 109 ; compared to the range in crustal fluids of 10 5 –10 13, and are reasonably dominated by magmatic CO2. The variation in CO2/ 3He can be explained by three mechanisms: (i) crustal CO2 addition; (ii) precipitation of CO2; or (iii) magmatic source variation. However, high-precision d C 13(CO2) (^0.015‰) shows ⬍0.3‰ variation across the entire field. Exceptional coincidence notwithstanding, this is not consistent with crustal CO2 addition or fractionation caused by CO2 precipitation. A simple model of Rayleigh-style magmatic degassing has been constructed. The 10–20% partial degassing of a deep-seated magma body is consistent with both CO2/ 3He and d C 13(CO2) variation (Fig. 3). Within the context of the magmatic degassing model, JM-BB samples with the highest 3He/CO2 preserve
C.J. Ballentine, M. Schoell / Journal of Geochemical Exploration 69–70 (2000) 59–63
the earliest stages of magmatic outgassing and, assuming a simple filling history, are likely to be the furthest from the CO2 source. On the JM-BB field scale the highest CO2/ 3He values are found in the samples with the highest %CO2. On a regional scale, the highest %CO2 natural gases are found on the Northern and Eastern margins of the Marathon Thrust Belt. Assuming these gases follow the JM-BB trend, it would suggest that the Marathon Thrust Belt is not spatially related to the CO2 source, and that the source itself must be to the North of this feature. This in itself is not surprising. Magmatic fluids, traced by the presence of 3He, are typically associated with regional extension rather than compression or loading. The identification of extensional features, probably associated with the timing of the Marathon Thrust Fault, will provide the most likely source/s for this regional CO2 occurrence. Identification of the general direction of CO2 migration also constrains the timing of the CO2 input relative to the CH4. Assuming simple filling, samples nearest the source of the CO2 might be expected to have the highest %CO2 concentrations, if this was implaced after CH4 production. These in fact have the highest %CH4 content. The distribution of CO2/ 3He and CH4/CO2 is consistent with an initial input of CO2, from the North or East of the Marathon Thrust Belt, with subsequent CH4 generation in the Val-Verde and Southern Delaware basin foredeeps. This results in the observed overprinting and dilution of the CO2 containing the most evolved magmadegassing signature.
References Ballentine, C.J., 1997. Resolving the mantle He/Ne and crustal 21Ne/22Ne in well gases. Earth Planet. Sci. Lett. 152, 233–250. Ballentine, C.J., O’Nions, R.K., Oxburgh, E.R., Horvath, F., Deak, J., 1991. Rare gas constraints on hydrocarbon accumulation, crustal degassing and groundwater flow in the Pannonian Basin. Earth Planet. Sci. Lett. 105, 229–246. Ballentine, C.J., Mazurek, M., Gautschi, A., 1994. Thermal constraints on rare gas release and migration: Evidence from Alpine fluid inclusions. Geochim. Cosmochim. Acta 58, 4333–4348.
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