Light Hydrocarbon Gases in Guaymas Basin Hydrothermal Fluids: Thermogenic Versus Abiogenic Origin

J. A. Welhan (2), J. E. Lupton (3)

Abstract:

Light hydrocarbon gases (methane through pentane) in high-temperature hydrothermal fluids of the Guaymas basin, Gulf of California, are predominantly derived from thermocatalysis of organic carbon in sediments intruded by mid-ocean ridge volcanic rocks. The spectrum of C₁-C₅ gases in Guaymas basin hydrothermal fluids is characterized by a preponderance of alkanes, with essentially no alkenes present. Comparison with high-temperature fluids from 21°N on the East Pacific Rise, where geochemical evidence indicates a distinct lack of thermogenic gas and where methane is derived abiogenically from the basalts, shows that the 21°N hydrocarbon gases are characterized by a prominent ethylene signature. Stable isotope compositions of Guaymas basin m thane differ from 21°N gas; Guaymas basin has ^dgr¹³C values of -43 to -51 ^pmil vs. PDB, compared to 21°N where values as high as -15^pmil have been measured. These differences reflect the different origins of light hydrocarbon gases in the two systems, with Guaymas methane being overwhelmingly of an organic, thermogenic derivation and showing no evidence of the abiogenic, basalt-derived gas that characterizes the 21°N environment. Carbon isotope analyses of dissolved inorganic carbon in Guaymas basin hydrothermal fluids indicate that high-temperature hydrocarbon oxidation may be an important process acting in conjunction with high-temperature thermocatalytic hydrocarbon generation.

Text:

INTRODUCTION

Continental geothermal gases appear to be characterized by an abundance of thermocatalytically derived (thermogenic) hydrocarbons, mainly methane through butane (DesMarais et al, 1981), indicating the ubiquity of organic matter in the rocks in which these hydrothermal systems have developed. Unlike these systems, the 21°N East Pacific Rise (EPR) hydrothermal system has evolved in virgin oceanic crust that is devoid of sediment, so sedimentary carbon sources are virtually nonexistent (although the prolific biology associated with these submarine hot springs can be an important local source of organic carbon in bottom waters, but not in high-temperature hydrothermal waters).

In contrast to the 21°N system, hydrothermal convection on the northern extension of the EPR, in the Guaymas basin, Gulf of California, developed in thick sedimentary strata overlying the basaltic basement. Basaltic volcanism is characterized by sills and dikes intruding into the sediments (Einsele et al, 1980; Lonsdale and Becker, 1985) and high-temperature hydrothermal fluids circulating through these organic-rich sediments. Although the Guaymas basin system seems to have slightly higher water/rock ratios than at 21°N (Lupton, 1983), maximum temperatures (315°C) are comparable to those at 21°N (350°C), and as at 21°N, methane and hydrogen concentrations are very high (Welhan and Craig, 1982), as are concentrations of mantle-derived ³He (Lupto , 1983).

Evidence for an abiogenic methane source in the high-temperature hydrothermal fluids at 21°N (Welhan and Craig, 1983) has raised the question of its relative contribution in other hydrothermal systems and its importance in global mantle carbon outgassing and the carbon cycle. Simoneit (1983, 1985) reported on hydrocarbons in Guaymas basin hydrothermal fluids, demonstrating the complex hydrocarbon spectrum generated by high-temperature thermocatalysis in this system. Light hydrocarbons, including methane, have also been reported in Guaymas basin sediments (Einsele et al, 1980; Galimov and Simoneit, 1982), and isotopic evidence indicates that both microbiologic (biogenic) and thermogenic sources of gas are present in the sediments. Because of the similarities between Guaymas basin nd 21°N in terms of hydrothermal temperatures, tectonic environment, and basaltic volcanism, and because of the predominantly abiogenic character of 21°N methane and its derivation from the mid-ocean ridge basalts (Welhan and Craig, 1983), we compared the relative contributions of abiogenic vs. thermogenic and biogenic methane in Guaymas basin hydrothermal fluids.

SAMPLES

Sample locations for sediment gases and hydrothermal fluids are shown in Figure 1. Sediment gases from Deep Sea

Drilling Project (DSDP) Leg 64, hole 477, were obtained from J. Gieskes, Scripps Institution of Oceanography. Free gases within core barrel liners were sampled with a syringe on shipboard and transferred to septum-sealed Vacutainer glass vials. Hydrothermal fluid samples from the Guaymas basin were collected by J. Lupton and K. Von Damm during the Pluto expedition, January 1982, with the deep submersible RV Alvin. In addition, four samples of 21°N hydrothermal fluid discussed here were collected by H. Craig, J. Edmond, K. Kim, and J. Welhan on Pluto expedition, November 1981, also with DSRV Alvin.

Hydrothermal fluids debouching from constructional "chimneys" (Von Damm et al, 1985) were sampled in 750-ml all-titanium syringes, along an 8-km segment of the southern trough of the Guaymas basin. Alvin dive numbers (sample locations) are shown in Figure 1. Different vent samples collected on each dive are designated by a numerical suffix in tables and figures. Sample locations and fluid chemistry are described in Von Damm et al (1985). Fluid aliquots were drawn into evacuated flasks (sealed with high-vacuum glass stopcocks) or copper tube samplers (sealed by crimping with steel clamps) for gas analysis. Fluid was also collected in 100-ml glass bottles and poisoned with HgCl₂ for total dissolved inorganic carbon (DIC) isotopic analysis. Unfortunately, due to the high gas p rtial pressures in the Guaymas basin fluids, extensive degassing of the samples occurred within the titanium syringes prior to their opening, so the original gas concentrations in the fluids cannot be reconstructed accurately.

EXPERIMENTAL METHODS

Core liner gases stored in Vacutainer vials were extracted at the Isotope Laboratory, Scripps Institution of Oceanography, on a high-vacuum system using a valve and syringe needle interface to access the Vacutainer. Hydrothermal fluid samples were extracted on the same system, using ultrasonic agitation and a capillary orifice to "pump" dissolved gases out of the sample container. All gases were dried and separated into residual gas and condensable (CO₂, H₂S, C₂⁺ hydrocarbon) gas fractions in a train of dry ice-acetone and liquid nitrogen cold traps. Splits were saved for gas chromatography and stable isotope analysis of methane. DIC analyses were performed by acidifying an aliquot of solution under vacuum, degassing the solution, and collec ing the dry condensable gases at -195°C. Carbon isotope analyses could be performed directly on this gas because H₂S was removed by precipitation with excess dissolved mercury.

Gas chromatographic analyses on residual gas splits were made with a Tracor ultrasonic detector; reported precision is ± 3% of stated amounts. Methane isotope analyses were performed on pure CO₂ derived from combustion of methane, which was separated chromatographically from other gases (Welhan, 1981). Carbon and hydrogen isotope ratios were reported as deviations in per mil from the PDB and SMOW standards, respectively. Splits of total gas (residual plus condensable, dried over acetone-dry ice traps) and condensable gas were analyzed for light hydrocarbons (up to C₅) by Global Geochemistry, Canoga Park, California, using a temperature-programmed Varian flame ionization gas chromatograph. Precision of duplicate gas analyses was ±4% of reported concentra ion. Splits of some residual gases were also analyzed by Global Geochemistry for methane isotope ratios using methods described above.

RESULTS

Analyses of DSDP hole 477 core liner gases are summarized in Table 1; all of these samples showed extensive air contamination. Guaymas basin hydrothermal gas compositions (uncorrected for degassing fractionation) are shown in Table 2. In sample 1175-5/8, the observed gas mixture was corrected for fractionation experienced during degassing within the titanium syringe. This correction was made by normalizing to dissolved atmospheric argon in seawater and assuming that degassing of all species occurred in an equilibrium manner at 15°C. Henry's Law coefficients were taken from the compilation of Wilhelm et al (1977). As is evident from a comparison of the uncorrected and corrected total gas concentrations and residual/condensable gas ratios in this example, the correction is large. T us, the gas analyses can only qualitatively indicate the relative proportions of species present.

Although reliable total gas compositions could not be obtained from these samples due to the degassing problem, the relative hydrocarbon proportions should not be fractionated significantly with respect to one another because their Henry's Law solubility coefficients are similar. For example, at 15°C the coefficients for CH₄, C₂H₆, and C₃H₈ are, respectively, 3.24, 2.22, and 2.63 (all × 10⁴ atm). Compared to Henry's Law coefficients for N₂, H₂, and CO₂, (7.25, 6.62, and 0.123, respectively), the relative differences in solubilities for the light hydrocarbons are small, indicating that the relative fractionation of the hydrocarbon species with respect to each other during equili rium degassing should be minor.

Relative hydrocarbon concentrations in four Guaymas basin hydrothermal samples and four 21°N EPR samples are shown in Table 3. These two sets of samples represent total gas (residual plus condensable) dried over dry-ice-acetone only, in identical fashion. Thus, except for minor fractionation of relative hydrocarbon abundances due to degassing in the Guaymas samples, the two sets of hydrocarbon data are directly comparable. Additional hydrocarbon data on Guaymas basin condensable gas splits (i.e., C₂-C₅ only) are shown in Table 4. These hydrocarbon concentrations have not been corrected for degassing.

Stable isotope data on methane in Guaymas basin hydrothermal and sedimentary gases are summarized in Table 5, with published data on 21°N EPR hydrothermal fluids and Salton trough geothermal wells. Carbon isotope compositions and concentrations of total DIC for Guaymas basin hydrothermal fluids are shown in Table 6.

DISCUSSION

Tables 1 and 2 show that Guaymas basin sediments and hydrothermal fluids are all rich in methane. Despite extensive air contamination of the core liner gases, CH₄ contents approach 50% of the residual gas content. Similarly, apparent CH₄ concentrations in Guaymas basin hydrothermal fluids exceed 40 cm³ (STP)/kg, and are > 150 cm³ (STP)/kg when corrected for degassing loss. As shown in Figure 1, the gas samples of Tables 1 and 2 span most of the hydrothermally active segment of the Guaymas basin southern trough. That methane is ubiquitous in this area

Fig. 1. Location of DSDP hole 477 core gas samples and hydrothermal fluid samples in active Guaymas basin rift zone. Hachures indicate downthrown side of faults. Thermal anomalies are shown as dots, hydrothermal sample locations as circled dots. After similar maps by F. Grassle and P. Lonsdale (1983, personal communication). C.I. = 10 m.

supports previous hypotheses regarding its hydrothermal-thermogenic origin in the sediments (Einsele et al, 1980; Galimov and Simoneit, 1982). The similarity of C₂H₆/CH₄ and ¹³C/¹²C ratios in hydrothermal fluids and sediments (Tables 1, 3, 5), further corroborates this hypothesis.

The spectrum of light hydrocarbon gases observed in Guaymas basin hydrothermal fluids is also consistent with a thermogenic origin of these gases. Simoneit (1983) showed that the hydrocarbon spectrum continues well beyond the gasoline range and reflects high-temperature thermocatalysis of buried organic matter in the rift-valley sediments. Concerning the hydrocarbon data of Tables 3 and 4 and Figure 2, several important points should be emphasized:

1. Based on the similarity of Henry's Law solubility coefficients for light hydrocarbon gases (Wilhelm et al, 1977), the relative proportions of these gases should approximate actual ratios, despite the extent of degassing that occurred in the sample bottles.

2. Light hydrocarbon concentrations in Guaymas basin fluids are high, with corrected CH₄ concentrations of about 150 cm³ (STP)/kg, compared with about 1-2 cm³ (STP)/kg in 21°N fluids (Welhan and Craig, 1983).

3. Alkanes in the C₁-C₅ range in Guaymas basin fluids appear to predominate, almost totally excluding alkenes. Only two of nine samples of Guaymas gases showed any C₂H₄ or C₃H₆, and then only traces were detected. In contrast, 21°N fluids contained almost as much unsaturated C₂ gas as C₂ and C₃ alkanes, but with little of the C₄-C₅ gases.

The carbon and hydrogen isotope ratios in Guaymas basin hydrothermal methane (Table 5) are similar to thermocatalytically derived natural gases. In the Schoell (1980) classification, these gases plot in or near the thermogenic field and are distinct from biogenic gases and from abiogenic methane in 21°N fluids (Figure 3). Geothermal gases of thermogenic origin tend to fall outside the thermogenic field of low-temperature natural gases (e.g., Welhan, 1981; Lyon and Hulston, 1984) (Figure 3), probably due to hydrogen-isotope equilibration with deuterium-depleted meteoric water at elevated temperatures (Welhan, 1981), rather than due to genetic or source controls. Comparison of Guaymas basin hydrothermal methane with 21°N EPR and Salton trough methanes (Table 5) further undersc res the difference between Guaymas basin and 21°N hydrothermal methane, and the similarity of Guaymas basin and high-temperature geothermal methanes from the Salton trough. Light hydrocarbon gases in high-temperature continental geothermal systems, including Salton trough wells, were derived principally via thermocatalysis (DesMarais et al, 1981). Thus, methane in the Cerro Prieto geothermal wells is overwhelmingly of thermogenic origin and, as in the Guaymas basin, reflects a large input of organic carbon from local sedimentary sources. The magnitude of the organic carbon input in such hydrothermal systems completely masks any abiogenic methane component that may be present, and emphasizes the special geologic conditions at 21°N that permitted identification of this component.

DIC concentrations and carbon isotope compositions of Guaymas basin hydrothermal fluids represent a mixture of

Table 1. Analyses of Core Liner Gases, DSDP Leg 64, Hole 477, Guaymas Basin

Table 2. Analyses of Dissolved Gases in Guaymas Basin Hydrothermal Fluids

DIC indigenous to the high-temperature vent fluid (itself a mixture of original seawater DIC and added hydrothermal CO₂) and ambient cold seawater DIC entrained to varying degrees during sample collection. Unfortunately, rigorous analysis of the DIC data is precluded due to isotopic fractionation accompanying degassing within the syringes. However, the magnitude of this effect can be estimated. Sample 1172-2B, a duplicate sample from titanium syringe 1172-2, was drawn after the syringe had been opened and initially sampled, and displays an isotopic fractionation effect accompanying additional loss of CO₂ from solution. The calculated apparent fractionation factor based on these data is about 15 ^pmil, similar to equilibrium fractionation between CO₂ an HCO₃^- at 25° (Robinson, 1975). Thus, for a 10% loss of CO₂ from solution--based on corrected vs. uncorrected gas analyses in Table 2--the ^dgr¹³C of remaining DIC would be about 1-2 ^pmil heavier.

As an approximation, carbon isotope values of hydrothermal CO₂ added to circulating hot seawater have been calculated for these vents by neglecting such fractionation effects. These values are shown in the last column of Table 6, and represent upper limits of the actual values. The lowest values, -10.5 ^pmil, are much lower than values for seawater DIC or for basaltic carbon added to EPR and Galapagos hydrothermal seawater (-5 to -7 ^pmil) (Craig et al, 1980); however, they are similar to the lowest values observed in DSDP Leg 64 sediments (Table 5) and indicate that hydrocarbon oxidation is occurring in this system. Thornton and Seyfried (1985) showed experimentally that elevated temperatures promote oxidation of sedimentary organic matter (and production of CO₂ via MnO₂ and Fe₂O₃ reduction, with the extent of reaction dependent on the relative organic carbon and MnO₂ contents according to such reactions as:

MnO₂ + C_org + 2H⁺ = Mn⁺⁺ + CO₂ + H₂O.

The CO₂ produced in such processes would be expected to be isotopically light.

If we assume that only basaltic carbon (^dgr¹³C ^approx -5 ^pmil) and oxidized organic carbon (^dgr¹³C ^approx -25 ^pmil) are being added to circulating seawater, the carbon isotope values for excess DIC in the hydrothermal fluids indicate that oxidized hydrocarbons contribute up to 25% of the added carbon. This view is overly simplistic, however, since the highest ^dgr¹³C values calculated in Table 6 indicate that a third CO₂ source--sedimentary carbonate--is probably involved.

Although CH₄ concentrations are unavailable for many samples, available C₂⁺ alkane concentrations (Table 4) appear to correlate with the calculated ^dgr¹³C values of the added DIC component (Figure 4). This trend could reflect a

Table 3. Comparison of Light Hydrocarbon Gases in Guaymas Basin Hydrothermal Fluids and Fluids at 21°N on East Pacific Rise

Table 4. Hydrocarbon Analyses of Condensable Gas Fraction of Guaymas Basin Hydrothermal Fluids degassing fractionation effect acting on seawater DIC, although a 10 ^pmil isotope effect due to CO₂ loss would imply that more than half the initial DIC in solution would have been lost; thus, essentially all the hydrocarbons and air gases would have exsolved. However, sample 1173-14, which contains low C₂⁺ concentrations (Table 4), still retains high DIC levels (Table 6) and substantial air gases as well as H₂ and CH₄ (Table 2), so degassing alone seems an unlikely explanation for the trend in Figure 4. An alternative is that high-temperature hydrocarbon oxidation is most pronounced in those vents that also have the greatest production of thermogenic hydrocarbons. That is, high temperatures may promote hydrocarbon generation and destruction in the same system. Thus, hydrocarbon oxidation may be responsible not only for the low ^dgr¹³C values of added inorganic carbon, but considering the correlation in

Table 5. Stable Isotope Composition of Guaymas Basin Methane, Compared to 21°N East Pacific Rise Hydrothermal Fluids and High-Temperature Geothermal Wells in Salton Trough

Table 6. Total Dissolved Inorganic Carbon (DIC) Concentrations and Carbon Isotope Ratios in Guaymas Basin Hydrothermal Fluids

Figure 4, also may correlate with thermogenic hydrocarbon production, since both processes are favored by higher temperatures.

Although the wide range in ^dgrD values of hydrothermal methane in Guaymas basin vents (-123 to -265 ^pmil SMOW) (Table 4; Figure 3) may reflect differences in thermal maturity of different gases, the effects of high-temperature hydrocarbon oxidation should also be considered. Although experimental data are lacking, high-temperature hydrocarbon oxidation may promote a kinetic isotope effect analogous to that produced by low-temperature bacterial oxidation (e.g., Coleman et al, 1981). Then variations in ^dgrD (and ^dgr¹³C) of hydrothermal methane in Guaymas basin vents (Figure 3) may be due to differences in the extent of hydrocarbon oxidation in different vents.

The overwhelming dominance of thermogenic hydrocarbons in the Guaymas basin environment is indicated by: (1) the geologic evidence for production in the sediments; (2) the high CH₄ and C₂⁺ concentrations, both in the hydrothermal fluids and in the sediments; (3) the spectrum of C₁-C₅ hydrocarbon gases, and the predominance of alkanes over alkenes; (4) the carbon and hydrogen isotopic compositions of methane; and (5) the presence of ¹³C-depleted DIC, apparently derived from hydrocarbon oxidation and which correlates with dissolved hydrocarbon gas concentrations.

The relative importance of these thermogenic hydrocarbon gases in Guaymas basin fluids, or rather their minimal contribution to 21°N fluids, is shown in Tables 3 and 5 and Figure 2. These comparisons show that 21°N hydrocarbons are distinctly richer in ethylene but are much lower in total C₁-C₅ gases, and are much more ¹³C- and D-enriched. Indeed, these criteria (high relative C₂ alkene content, low C₄-C₅ concentrations, and heavy isotope enrichment) distinguish 21°N EPR hydrocarbon gases from those of conventional thermogenic derivation in general.

The lack of unsaturated light hydrocarbons in Guaymas basin gases is an indication of their thermogenic origin, because alkenes are relatively unstable during thermocatalysis. The presence of ethylene in 21°N fluids, in amounts comparable to ethane and propane, suggests that nonthermogenic processes are responsible for the hydrocarbon spectrum in these fluids. Ethylene found in the 350°C vents on the East Pacific Rise is indigenous to the hot waters and is not an artifact of sampling or biologic contamination. However, ethylene may either reflect an abiotic synthesis within the basalt, similar to methane, or it may be an intermediate species reflecting thermal decomposition of other (abiogenic?) hydrocarbons at elevated temperatures.

In light of the basaltic origin postulated for methane at 21°N (Welhan and Craig, 1983), the C₂ hydrocarbon concentrations observed in Mid-Atlantic Ridge (MAR) basalts (Zolotarev et al, 1981) suggest that ethylene in 21°N hydrothermal fluids also may be primarily of basaltic origin. Ethylene/ethane ratios in 21°N fluids (Table 3) average about 0.6, comparable to the mean of 0.1 (with values to 0.3) observed in MAR basalts (Zolotarev et al, 1981). In contrast, these values are at least two orders of magnitude greater than those observed in Guaymas basin thermogenic gases.

In the sediment-free, basalt-dominated, high-temperature hydrothermal systems of the East Pacific Rise, methane is the major hydrocarbon gas. Its carbon isotopic composition and concentration relative to primordial (mantle) helium indicate a predominantly basaltic, abiogenic origin (Welhan and Craig, 1982, 1983), possibly due to high-temperature chemical equilibrium within the rock (Welhan et al, 1984). Despite the geochemical evidence for abiogenic methane outgassing from the mantle, the relative

Fig. 2. Comparison of C₂⁺ hydrocarbon concentrations relative to methane in Guaymas basin (shaded) and 21°N East Pacific Rise (unshaded) high-temperature hydrothermal fluids. Bar height less than 0.2 indicates a value below detection level. Data from Table 3. Guaymas basin hydrocarbons are characterized by much higher gas concentrations in general, much higher ethane/methane ratios, and essentially no ethylene, compared to gases at 21°N on East Pacific Rise.

Fig. 3. Methane isotopic compositions of Guaymas basin hydrothermal fluids compared with ranges of biogenic and thermogenic natural gases (Schoell, 1980) and DSDP Leg 64 sediment gases (Schoell, 1982), high-temperature methane from Salton trough geothermal wells (Welhan, 1981), and high-temperature abiogenic methane from hydrothermal fluids at 21°N on the East Pacific Rise (Welhan and Craig, 1983). Guaymas basin methane is characterized by isotopic compositions indicating thermogenetically derived gas, and is distinctive from the isotopic compositions observed in 21°N East Pacific Rise methanes, which are believed to be of abiogenic (basaltic) origin.

importance of this flux in the global carbon cycle is uncertain. For example, CH₄/CO₂ ratios in the 21°N EPR fluids are about 0.01 (Welhan and Craig, 1983). Assuming that 21°N is representative of the high-temperature submarine hydrothermal carbon flux, and the present global submarine CO₂ flux is sufficient to supply the total crustal carbon inventory since the earth originated (DesMarais, 1984), then the equivalent total abiogenic (mantle) CH₄ supply over this time would be about 10²¹ g, equivalent to about 10% of the earth's sedimentary organic carbon reservoir (Hunt, 1972). However, hydrothermal methane injected into the oceans is rapidly oxidized (Kim, 1983), so its potential role in augmenting the crustal hydrocarb n inventory is severely diminished.

Nevertheless, the sharp geochemical contrasts between hydrocarbon gases of thermogenic origin and those of abiogenic (basaltic) origin can be applied to source characterization of natural gases, and they strengthen claims for the existence of deep earth hydrocarbons (e.g., Gold, 1985). In the Guaymas basin, however, the importance of crustal thermogenic gas production seems unquestionable, although this source is undoubtedly augmented by mantle outgassing, given its high ³He/⁴He ratio (Lupton, 1983).

Although the geochemical differences between abiogenic and thermogenic hydrocarbons presented here are substantial, such criteria alone are insufficient to estimate relative proportions reliably, even in a system where both components coexist. The relative importance of abiogenic vs. thermogenic gas in the Guaymas system can be estimated by comparing Guaymas CH₄ abundances with those of the 21°N EPR system. Argon is convenient for normalizing CH₄ abundances because it has similar solubility--thus minimizing degassing fractionation effects--and because it is essentially all of atmospheric origin. Neglecting the air-contaminated analysis in Table 2, CH₄/Ar ratios in the Guaymas fluids are about 400-2,000, or about 100-500 times higher than CH_{4< SUB>/Ar ratios in 21°N EPR vents (Welhan and Craig, 1983). If seawater circulates through both systems at similar rates and if similar quantities of abiogenic methane are extracted from the basalt in both systems, then abiogenic methane must comprise less than 1% of the hydrothermal methane in the Guaymas fluids. Therefore, discerning such small mixing ratios on the basis of carbon isotopes or other hydrocarbon characteristics would be exceedingly difficult, and unambiguous identification of a mantle hydrocarbon component in crustal environments will remain an elusive goal.}

SUMMARY

Data on gases and methane isotopes in Guaymas basin sediments and hydrothermal fluids demonstrate several important points concerning the genesis of light hydrocarbon gases in mid-ocean ridge hydrothermal systems. First, high CH₄ concentrations occur in sedimentary gases and hydrothermal fluid samples, spanning 8 km of the hydrothermally active rift valley of the Guaymas basin, and support the hypothesis of a thermocatalytic CH₄ origin within the organic-rich sediments in response to the high temperatures and thermal gradients. Similar carbon isotope ratios and C₂H₆/CH₄ ratios in sediments and hydrothermal fluids corroborate this conclusion.

Second, the C₁-C₅ gases in Guaymas basin hydrothermal fluids are predominantly alkanes, with little or no alkenes, and also reflect a thermogenic origin. This thermogenic spectrum is distinct from that in 21°N EPR fluids, where methane is predominantly of abiogenic origin, derived from basalt, and where ethylene is a significant hydrocarbon component, possibly also of abiogenic origin.

Third, comparison of stable isotope compositions of Guaymas basin and 21°N methane indicates a different origin of methane in these two systems. Methane isotope ratios of pore gases and hydrothermal gases in the Guaymas basin are typical of thermocatalytically synthesized gas. Besides ruling out any significant microbiological source in these particular hydrothermal gases, these isotopic ratios help demonstrate that the ¹³C-enriched methane found in 21°N EPR fluids of similar temperature is not of thermogenic origin.

Finally, isotopic compositions of dissolved inorganic carbon in Guaymas basin hydrothermal fluids indicate that high-temperature hydrocarbon oxidation is a significant source of DIC in these fluids, and that both hydrocarbon production and oxidation may be occurring at high temperature within this system.

The contrasts between Guaymas basin and 21°N CH₄ isotope ratios, hydrocarbon gas concentrations, and C₂-C₃ alkane vs. alkene distributions may eventually lead to diagnostic criteria for distinguishing abiogenic hydrocarbons in such hydrothermal environments. At present, these parameters

Fig. 4. C_2⁺ hydrocarbon gas concentrations (Table 4) vs. calculated carbon isotopic composition of added CO₂ (Table 6) in Guaymas basin hydrothermal vents. Correlation implies either a large degassing fractionation effect, or more likely, a coupling between high-temperature thermocatalytic hydrocarbon production and the amount of isotopically light CO₂ produced from high-temperature oxidation of these hydrocarbons.

emphasize the differences in hydrocarbon genesis and carbon sources in these high-temperature hydrothermal systems, and support the concept of a predominantly thermogenic, organic origin for Guaymas basin methane and a predominantly abiogenic, basaltic origin at 21°N.

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Acknowledgments:

(2) Isotope Laboratory, Scripps Institution of Oceanography, La Jolla, California 92093. Present address: Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada.

(3) Isotope Laboratory, Scripps Institution of Oceanography, La Jolla, California 92093. Present address: Marine Science Institute and Department of Geological Sciences, University of California at Santa Barbara, Santa Barbara, California 93106.

The authors are grateful to the crew and pilots of DSRV Alvin and RV Lulu for their skill in collecting the hydrothermal samples, and to J. Gieskes for collecting the DSDP core liner gases. We also thank K. Von Damm and J. Edmond for assistance in sampling the Guaymas basin hydrothermal fluids, and H. Craig for obtaining the 21°N hydrothermal samples. We gratefully acknowledge the cooperation and assistance of I. Kaplan and P. Mankiewicz of Global Geochemistry Corporation, who analyzed the hydrocarbon gases. The manuscript benefited substantially from comments and suggestions by M. Schoell, W. Orr, and two anonymous reviewers. This work was supported by grants from the National Science Foundation.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.