Origin of trace gases in submarine hydrothermal vents of the Kolbeinsey Ridge, north Iceland

EPSL ELSEVIER

Earth and Planetary Science Letters 171 (1999) 83-93 www.elsevier.com/locate/epsl

Origin of trace gases in submarine hydrothermal vents of the Kolbeinsey Ridge, north Iceland R. B o t z a,., G . W i n c l d e r b, R. B a y e r b, M . S c h m i t t c, M . S c h m i d t a, D . G a r b e - S c h f n b e r g

a,

E S t o f f e r s a, J . K . K r i s t j a n s s o n d a lnstitutfiir Geowissenschafien, Universitgit Kiel, Olshausenstrafle 40, D-24118 Kiel, Germany b Institutfiir Umweltphysik, Universitiit Heidelberg, lm Neuenheimer Feld 229, D-69120 Heidelberg, Germany c Geochemische AnaIysen, Gliickaufstrafle 50, D-31319 Sehnde-Ilten, Germany d Technological Institute oflceland, Keldnaholt, IS-112 Reykjavik, Iceland

Received 25 September 1998; revised version received 22 March 1999; accepted 31 May 1999

Abstract Two hydrothermal fields of the Kolbeinsey Ridge area, north of Iceland, show vent gas characteristics which can be related to the subsurface conditions. Helium isotopes (R/Ra~r = 9.8, 10.9) indicate a mantle-derived origin and can be considered as a mixture of MORB helium and a deep-mantle plume helium component. The carbon isotope composition of CO2 ranges between -2.4 and -7.8%0. The less negative ~13C-CO2 values were found at Grimsey. The data from Grimsey are very similar to those previously published and regarded as being characteristic for the Icelandic magmatic source. However, small amounts of biogenic CO2 and/or subsurface calcite precipitation are responsible for the lighter isotope values of CO2 from Kolbeinsey. CH4/3He ratios which are higher than in MORB indicate an additional (sedimentary) methane source for Kolbeinsey and Grimsey hydrothermal gases. The presence of higher hydrocarbons up to butane, together with the carbon isotope values of methane (~13C = -26.1 to -39.8%~) suggest a probably high-mature organic source within thick sediments of the Tjtimes Fracture Zone and smaller depressions on the west side of the Kolbeinsey Ridge crest. Geochemical characteristics of hydrocarbons present in KR hydrothermal fluids are, however, typical for a mixed (thermogenic and high-temperature hydrothermal, e.g. EPR-type) origin. Moreover, it is likely that secondary processes such as bacterial oxidation and thermal cracking determined the geochemical characteristics of the gases. © 1999 Elsevier Science B.V. All rights reserved. Keywords: hydrothermal vents; hydrocarbons; carbon dioxide; helium; isotope ratios; organic carbon; chemical ratios

1. Introduction Kolbeinsey Ridge (KR) is part of the Mid-Atlantic Ridge north of Iceland reaching from approximately 66°N (Tjfrnes Fracture Zone, TFZ) to the Jan Mayen * Corresponding author. Fax: +49 431 880 4376; E-mail: [email protected]

Fracture Zone near 71°N (Fig. 1). The ridge is named after the Island of Kolbeinsey which formed during an volcanic eruption about 10,000 years ago. The southern part of KR is volcanically active [1,2] with the latest documented eruption in the year 1372. Most of the seismic activity of Iceland is related to the Mid-Atlantic plate boundary. In Iceland the plate boundary is superimposed on a large hot spot

0012-821X/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S001 2-82 1X(99)00 128-4

84

R. Botz et al,/Earth and Planetary Science Letters 171 (1999) 83-93

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Fig. 1. Map of working area showing locations of Grimsey- (GHF) and Kolbeinsey (KHF) hydrothermal fields. Strike and extent of the Tj6rnes fracture zone is indicated by dashed lines. The North Irminger Current (NIC) is responsible for sediment transport into the Kolbeinsey Ridge area.

responsible for high heat flow and excessive volcanism leading to complex and unstable tectonic situations [3]. At least one ridge jump has occurred in north Iceland where the TFZ developed. The TFZ forms an approximately 80 km wide and 150 km long seismic zone on the north Icelandic shelf between Iceland and Kolbeinsey Island [4,5]. The zone is dominated by three N-S striking lineaments: the KR-Eyjafjardarall trough, the KR-Skjalfandadjup trough complex (east of Grimsey Island), and the Axarfjardardjup trough [5]. Sediments have accumulated further out on the shelf off Skjalfandafloi during the last 1 m.y. They consist of shallow-marine or near-shore sediments alternating with terrestrial sediments and volcanics [6-8]. Although sediment thickness within the Skjalfandi-deep is not known, seismic surveys in related deep areas suggest that sediments may reach more than 2 km (up to 4 km) in thickness [9,10]. Grimsey Island lies in the center

of the TFZ and it consists of SW-dipping plateau basalts probably younger than Mid-Gauss with intercalated conglomeratic sediment and hyaloclastite [4,5]. High-temperature hydrothermal venting (up to 250°C; this study) was observed within the Skjalfandadjup trough northeast of Grimsey Island where most intense faulting and volcanic activity occurs [5]. The second hydrothermal field investigated has already been described by [2,11]. The vents are located south of Kolbeinsey Island on the KR. Similar to Grimsey area, the Kolbeinsey hydrothermal field is related to tectonic lineaments and fissure swarms in the subsurface. Chimneys, fissures and crater-like dips were found. South of Kolbeinsey Island the ridge is characterized by a rough morphology [12]. The sediment distribution in the southern KR area has been investigated by seismic reflection [13]. Sediment thickness in the vicinity of the Kolbeinsey

85

R. Botz et al. /Earth and Planetary Science Letters 171 (1999) 83-93

hydrothermal field is from <100 m up to several hundreds of meters. The thickness is increasing to the northwest and even reaches >1300 m within a depression centered near 67°30'N and 19°0TW (on the western side of the ridge about 20 to 30 n.mile north of the hydrothermal field investigated). Sedimentation in this area is controlled by the W E flowing North Irminger Current (Fig. 1) whereas further south sedimentation is largely influenced by Iceland [14]. Submarine hydrothermally active regions of the Earth's crust show enrichments of trace gases (plumes) in the water column above vent systems [15,16]. Although methane is the predominating hydrocarbon, higher homologues (ethane, propane, etc.) were also detected when a high heat flux caused thermal degradation of sedimentary organic matter [17]. Not only the gas composition reflects (gas) genetic processes but also the stable isotope concentrations of the gaseous compounds give reliable information on their formation processes [18,19]. Moreover, helium is a sensitive tracer for fluid movements. Its isotope ratios vary by more than four orders of magnitude, from 3He/4He ~ 10 -9 to 3He/4He ~ 10 -5 [20]. Three major components are usually mixed to various degrees: atmospheric helium with 3He/4He = 1.384 x 10 -6 [21], radiogenic helium which is significantly enriched in 4He relative to atmospheric helium (with an 3He/4He ratio of 10 .7 to 10 -8 [22] and isotopically light primordial mantle helium [23]. In Iceland 3He/4He ratios up to 29Rair were found [24-26]. These ratios are significantly in excess of ratios found in normal mid-ocean ridge basalt (MORB). The range of 3He/4He variations (6 to 29 × Rair) reflects interaction of a 3He-enriched plume source (R/Rair > 29) and normal MORB-type mantle (R/Rair -----8 -4- 1), and local addition of atmospheric or radiogenic crustal helium with R/Rair < 7. The effect of the 3He mantle plume signature is observed not only on Iceland [25] but also 1000 km to the south on the Reykjanes Ridge and 500 km north along the Kolbeinsey Ridge [27]. 3He/4He ratios and related vent gas characteristics (for instance the ratio of CH4 to mantle-derived 3He) provide information about the origin of hydrothermal gas components [28]. Thus, mantle-derived gases can be differentiated from trace gases generated by thermal

degradation of sedimentary organic carbon within the geothermal system.

2. Sampling and methods During the R / V Poseidon cruise in 1997, research submersible JAGO operated in the KR region north of Iceland at water depths between approximately 100 and 400 m (Table 1) in order to directly sample emerging hydrothermal fluids. In the present paper two hydrothermally active areas were investigated (Fig. 1). Here they are called 'Kolbeinsey hydrothermal field' (KHF) which is located south of Kolbeinsey Island and 'Grimsey hydrothermal field' (GHF) situated northeast of Grimsey Island. A detailed description of hydrothermalism in the KR area will be presented elsewhere. Water samples were taken using the common Niskin bottles carried by JAGO. In general, the bottles were directly placed into the outflows (except for helium sampling; see below). Gas bubbles have been collected using a water-filled (1.5 1, inverted) glass bottle positioned in the gas stream. The water in the bottles was partly replaced by gas which further expanded during uplift. When JAGO reached the surface a diver closed the sample bottle under water and then disconnected it from the submersible. Water samples were degassed on board applying a combined ultrasonic/vacuum degassing technique [29]. Compositional analyses of light hydrocarbons Table 1 Position, water depths and maximum fluid temperatures measured in Grimseyand Kolbeinseyhydrothermalfields Station Area 1 2 3 4 5 6 ASW a 8 9 10 11

Lat. (°N)

Kolbeinsey 67°05.54J Kolbeinsey 67005.48/ Kolbeinsey 67°05.461 Kolbeinsey 67005.46, Kolbeinsey 67°05.46' Kolbeinsey 67°05.46' Grimsey 66°37.00' G r i m s e y 66036.56, G r i m s e y 66037:59' G r i m s e y 66°36.56' Grimsey 66°36.49~

a Ambient sea water.

Long. (°W)

Tmax (°C)

18°42.81' 18%2.701 18042.64, 18042.64' 130 18042.64/ 131 18042.64, > 100 17040.34' 17°39.12, 200 17°39.151 249 17°39.13' 250 17°39.21' 238

Depth (m) 113 104 102 104 105 104 50 404 407 406 395

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R. Botz et aL /Earth and Planetary Science Letters 171 (1999) 83-93

(C1-C6) and the stable carbon isotopic compositions of methane and carbon dioxide (note that ~13C-CO2 was determined in samples of free gas bubbles only) were performed applying standard techniques [30]. Quantitative chromatographic analyses of gaseous hydrocarbons were made using a Packard ® model 438 GC (50 m long A1203-KC1 PLOT ® quartz capillary column; flame ionization detector). Carbon isotope analyses of hydrocarbons were performed by 'continuous flow isotope ratio analysis' (CF-IRMS). One ml of the gas sample was transferred into a gas loop and then injected into a gas chromatograph (SHIMADZU GC 14A equipped with a 30 m long 53 tzm ID GS-Q coated quartz capillary; carrier gas helium 6.0; temperature program 40°C-200°C). Separated individual hydrocarbon compounds were quantitatively oxidized using an oxidation furnace filled with a copper wire and platinum as a catalyst (T = 900°C). The time span between the elution of each hydrocarbon was >2 rain. Liberated water was trapped using a stainless-steel loop cooled with dry ice (-78°C) while the carbon dioxide was transferred by carrier gas via a splitting system directly into the ion source of a high-precision mass spectrometer FINNIGAN MAT 251. Reproducibility (ls, n = 2) of carbon isotope analysis is -4-0.5%o for methane, ±0.6%0 for ethane and propane and :1:0.3%o for carbon dioxide. Isotope ratios 13C/12C are reported in the common g-notation relative to the PDB standard. Water samples for helium isotope analyses were taken separately from the free gas samples. Due to helium fractionation effects in the course of bubble formation we took the water samples in centimetre to metre distances from the vents (dilution effect with ambient sea water). Water samples for helium isotope analyses were stored in pinch-off copper tubes. We avoided contamination of the helium water samples as we did not use helium stripping techniques on board (compare above). Gases were extracted by vacuum degassing techniques and measured on a VG MM 3000 mass spectrometer using analytical standard procedures [31]. Concentrations of dissolved magnesium were determined in on-board filtrated (0.4 ~tm, Nuclepore) aliquots of the Niskin water samples by ICP-AES. Accuracy was controlled by measurement of IAPSO sea water. Precision was <2% rel. determined from replicate analyses of sea water.

3. Results and discussion

3.1. General observations During the R/V Poseidon cruise boiling sea water was observed and sampled from Grimsey and Kolbeinsey hydrothermal fields. At the given water depths of 100 to 400 m the boiling point is between 180°C and 250°C [32]. This is the approximate range of (maximum) temperatures in the vents measured by JAGO (Table 1). However, lower temperatures measured during water sampling reflect various mixing situations with ambient sea water. Table 2 shows all results of geochemical analyses such as hydrothermal gas composition, isotope data of carbon dioxide and associated light hydrocarbons, the helium isotope values and dissolved magnesium (Mg). Carbon dioxide concentrations in free gas bubbles are between 0.98 vol.% and (the exceptionally high value) of 41 vol.%. Methane concentrations of hydrothermal gases from Grimsey field are distinctly higher (up to 24.3 vol.%) compared with Kolbeinsey hydrothermal field where only concentrations up to 1.5 vol.% were measured. Appreciable amounts of gaseous higher hydrocarbons to n-butane were detected in Grimsey hydrothermal fluids. Much lower amounts of higher hydrocarbons were found in gases from the Kolbeinsey vents. There stations 4, 5 and 6 (located within distances of a few meters) showed detectable amounts of ethane (1-41 ppm V - - parts per million by volume) and propane (1-7 ppm V). The hydrocarbon gas composition is reflected by the C1/(C2 + C3) ratios which are generally low but more variable (between 62 and 450) for Kolbeinsey samples than for Grimsey gases (51 to 85). The carbon isotope values of CO2 are between - 2 . 4 and -7.8%o (free gas bubbles only). The isotopic compositions of methane range from -26.1 to -52.3%0. Four (Grimsey) ethane ~13C-values fall between -15.5 and -16.2%o. Two samples which contained sufficient amounts for determination of the isotope composition of propane gave 313C (C3) values of - 13.6 and - 17.0%o, respectively. Ten fluid samples from the two hydrothermal fields were analysed for He isotopes. Fig. 2 shows the isotopic ratio as a function of the He concentration of the fluids. Helium is enriched in the fluid in respect to air-dissolved He in ambient sea water

87

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4 He [10 -8 ccSTP / g ] Fig. 2. 3He/4He isotope ratios vs. the 4He concentration of fluids from Grimsey- and Kolbeinsey hydrotherrnal vent fields (uncertainty of single data points is smaller than the symbol used). The cmwes represent mixing lines between ambient sea water (ASW) and fluid endmembers with 3He/4He ratios of 9.8Rai r (Grimsey) and 10.9Rair (Kolbeinsey). The shaded area gives the maximum range of 3He/4He ratios MORB (8 4- 1).

sea water. In Fig. 3 4He concentrations are plotted against the magnesium content of hydrothermal fluids. This diagram allows us to infer the dilution of the hydrothermal endmember with ambient sea wa-

(ASW). Helium supersaturations are caused by the injection of a hydrothermal fluid component. Variable 4He concentrations reflect the (variable) dilution of the hydrothermal fluid endmember with ambient 600

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magnesium[mM] Fig. 3.4He concentrations vs. the Mg content of hydrothermal fluids from KR hydrothermal systems•

R. Botz et al./ Earth and Planetary Science Letters 171 (1999) 83-93

ter. For each vent field a well-defined regression line is obtained. Assuming that the hydrothermal fluid is free of Mg, extrapolation of the linear correlation defines a 4He endmember concentration in the range between 1 x 10.5 cm 3 STP/g (Kolbeinsey) and 1.8 x 10 -5 cm 3 STP/g (Grimsey), close to 4He concentrations of 1.2 to 2.6 x 10.5 known from hydrothermal fluids at the MAR [33]. 3.2. Mantle-derived helium

The lines shown in Fig. 2 are mixing lines between ambient sea water (ASW) and two slightly different endmembers with 3He/4He ratios of 9.8Rair (Grimsey) and 10.9Rair (Kolbeinsey). The 3He/4He ratio obtained for the hydrothermal fluid from the Kolbeinsey field is slightly lower than the value (12Rair) that has been published previously [11]. The ratios are significantly higher than the normal MORB ratios (R/Rair = 8 ~ 1). This coincides with former helium isotope studies where variations with latitude were detected for the area north of Iceland [24]. Most of the existing helium isotope analyses were made from basaltic glasses [27,34,35] or from subaerial geothermal fluids on iceland [25,26,36]. The present study deals with submarine hydrothermal fluids. The 3He/4He ratios of the Grimsey and Kolbeinsey fluids are remarkably similar to the 3He/4He ratios in basalt glasses dredged at Kolbeinsey Ridge between 67 ° and 70°N ( R / R a i r = 10.3 to 12) [27]. The similarity between helium isotope ratios of recent hydrothermal fluids and basalt glasses suggests time-constancy in the 3He/4He characteristics and, moreover, it indicates that, if at all, helium isotopes suffer only minor fractionations within this tectonic system. Characteristic 'plume-type' 3He/4He ratios ( R > 8Rair) are related to the Icelandic mantle plume which is observable along the entire length of the Reykjanes Ridge and to 70°N on the Kolbeinsey Ridge. Hence, the mantle helium component of Kolbeinsey and Grimsey hydrothermal fluids can be interpreted as a mixture of MORB helium and a deep-mantle plume helium component. 3.3. Carbon dioxide and hydrocarbons

The measured 313C-CO2 values from Grimsey and Kolbeinsey hydrothermal fields are typical for

89

geothermal CO2 sampled from numerous locations worldwide [19,28,37-39]. A range of ~13C-CO2 values between - 4 and -7%o was measured in hydrothermal fluids from the EPR [40]. Icelandic high-temperature geothermal fields (reservoir temperatures up to 320°C) revealed 313C-CO2 values between -2.5 and -22.6%0 [25]. More specifically, a ~13C-CO2 endmember value of -3.8 4- 0.7%o was found for Icelandic geothermal fluids with the highest CO2 concentrations. This value is very similar to MORB (-4.3 + 0.1%o) and is thought to be representative for magmatic CO2 of the Iceland plume [25]. CO2 from the Grimsey hydrothermal field has a mean ~13C value of -2.8%0 which is somewhat less negative compared with the CO2 (endmember) source given above. A reason for a positive isotope shift may be crustal carbonate dissociation (see below). On the other hand, Kolbeinsey hydrotherreal fluids appear to contain less amounts of CO2 than Grimsey samples. Moreover, the most negative ~13C-CO2 value of -7.8%o was measured for the sample with the lowest CO2 concentration. Thus, the value of -7.8%o is not believed to be representative for the magmatic source. The reason for low CO2 concentrations associated with relatively minor isotopic shifts to about -7%0 within Iceland geothermal systems may be calcite precipitation in the geothermal flow system as suggested by Poreda et al. [25] who observed this effect for fluids with low CO2 concentrations and low (subsurface) fluid temperatures. Although the latter seems to be the case for the Kolbeinsey fluid (Table 1), we want to stress that the (sampling) temperatures of these submarine vents are controlled by the boiling point of sea water (and, therefore, water/sampling depth) and do not reflect the actual subsurface reservoir temperature. Similarly, the theoretical CO2/CH4 carbon isotope equilibrium temperatures of approximately 200 ° to 350°C [41] do not correspond to subsurface reservoir temperatures, as coexisting carbon dioxide and methane from the KR area are not cogenetic in origin (as inferred from the following discussion on CO2 and CH4 formation). Nevertheless, such a range of subsurface temperatures cannot be excluded by the present data, so calcite precipitation [25,42] could indeed cause the low gl3C-CO2 values. Alternatively, small contributions of 12C-rich CO2 from bacterial oxidation of organic matter may be respon-

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R. Botz et al. /Earth and Planetary Science Letters 171 (1999) 83-93

sible for the low 8~3C-CO2 values of Kolbeinsey vents. Special (diluted hydrothermal fluid) conditions were required for He sampling. Thus, only two water samples (station 5 Kolbeinsey and station 10 Grimsey field; Table 2) could be selected for parallel analyses of helium isotopes and dissolved hydrocarbon gas. The CH4/3He ratios of these samples are however considered to be representative for the two hydrothermal vent fields. As 3He certainly is mantle-derived and, in contrast to the hydrocarbons, not affected by biogeochemical processes, CH4/3He ratios are indicative for potential sources of methane. The ratio reflects the relative importance of abiogenic (mantle-derived) CH4, and CH4 from other sources such as thermal breakdown of organic matter and microbial activity. The CH4/3He ratio of 1.3 x 107 obtained for Grimsey fluids is lower than the value of 7 x 107 determined for Kolbeinsey vent fluids. Both values fall within the upper range of values (3 x 104 to 108) published so far for geothermal systems on Iceland [25]. The observed CH4/3He values are higher than the mean ratio in mid-ocean ridge hydrothermal systems (1 to 5 x 106) [43] and much higher than in oceanic mantle plume environments as, for instance, Loihi seamount (1 to 4 x 105) [44]. The excess (relative to a single mantle reservoir) in CH4/3He ratios point to (an) additional methane source(s). This is also indicated by the carbon isotope composition of CH4. Relating the 313C values of methane to the CH4/3He ratio (Table 2; stations 5 and 10) the data follow the general trend known for geothermal systems on Iceland of decreasing ~13C-CH4 values with increasing CH4/3He ratios [25]. Again it is indicated that fluids from both hydrothermal systems (e.g. Kolbeinsey and Grimsey fields) are not only derived from a single mantle source (with typical characteristics of 3t3C = - 15 to -18%o; CH4/3He = 1 to 5 x 106). Hence, CH4 formation by thermogenic degradation of organic matter and/or microbial methanogenesis are responsible for the elevated (relative to MOR) CH4/3He ratios of both hydrothermal fluid systems. The genesis of methane can be deduced from the stable isotope composition [18,19]. Relatively 13C-rich (~13C >-20%0) methane was reported from various hydrothermal environments such as the East Pacific Rise [15], Iceland [45], New Zealand

[46] and the Aegean Sea [47]. In contrast, biogenic methane sources in hydrothermal environments should exhibit low ~13C values < -60%0 and gD < -150%o (rel. SMOW), respectively. Typical g-values for methane in high-temperature hydrothermal systems range from -25 to -30%e [28]. The offset to less negative 8-values for most hydrothermal CH4 samples (except for instance CH 4 generated from the relatively immature Guaymas Basin sediments) may reflect differences in type and maturity of the parent organic material. Hydrothermal methane and higher gaseous hydrocarbons in the Guaymas Basin (GB) derived from thennocatalysis of organic carbon in sediments which were intruded by mid-ocean ridge volcanic rocks [48]. This conclusion was drawn from the 813C values of CH4 between -43 and -51%o and the presence of higher hydrocarbons up to pentane [17,49]. However, secondary oxidation and mixing processes may further influence the geochemical signature of trace gases

[50]. A comparison of light gaseous hydrocarbons and isotope compositions of GB, Gulf of California and 21°N East Pacific Rise (EPR) hydrothermal systems was made [28]. Beside the much higher methane quantities found in GB hydrothermal fluids (compared to EPR) significant differences were also found in the ratios of CH4/C2H6 (CI/C2), CH4/3He (C1/3He) and the 313C-CH4 (813C-C1) values of gases from both regions. In a similar approach KR data can be compared with the geochemical characteristics of the 'endmembers gases' GB and EPR with the aim to possibly quantify (mixed) sources of Kolbeinsey and Grimsey hydrothermal hydrocarbons (M. Schoell, pers. commun.). Hence, based on fluid concentrations, a three-dimensional diagram was constructed by combining the molecular hydrocarbon gas compositions C1/C2, C1/3He ratios and the ~13C-C1 values; mean values [28] of both 'endmembers' (GB, EPR) are included in Fig. 4. A theoretical line was calculated which describes variable mixing situations of GB with EPR gas types. Selected geochemical data (stations 5 and 10) from Kolbeinsey and Grimsey vents are also included in the diagram. KR hydrothermal gases have relatively low C1/C2 values which are quite similar to the GB gases of known thermocatalytic origin. The CH4/3He ratios, however, are distinctly lower than

R. Botz et aL /Earth and Planetary Science Letters 171 (1999) 83-93 C1/C2

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\c~/ m

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those derived from the GB. Furthermore, the 813C values of hydrothermal CH4 from the KR area are distinctly less negative than those measured in the GB. Fig. 4 shows that KHF values fall close to the theoretical mixing line whereas data from GHF deviate from this GB-EPR mixing line. Obviously not only mixing of high-temperature hydrothermal methane with sediment-derived hydrocarbons is responsible for hydrothermal gas composition from the KR area. From Fig. 4 it can be concluded that KR fluids are predominated (>90%) by EPR-type. However, secondary processes such as thermal cracking of hydrocarbons [51 ] and bacterial oxidation [52] are known to cause changes of primary geochemical gas characteristics (see also below). The relative amounts of ethane, propane and butane in fluids from the Grimsey and Kolbeinsey vent

91

fields (Table 2) indicate thermal breakdown of sedimentary organic matter to contribute to hydrocarbon formation. Variations in type and maturity of parent organic matter could (at least partly) be responsible for the isotope geochemical data of hydrothermal CH4 from Kolbeinsey and Grimsey vent fields. Sediments in the TFZ (Grimsey field) are mainly derived from the south (Iceland). Hence, terrigenous sediment transport into the basins [12] probably containing significant amounts (up to 1.8% C-org., K. Lackschewitz, pets. commun.) of humic organic matter (possibly redeposited sediments containing plant debris and lignite [54]) led to the formation of relatively thick sediments in the TFZ. Kolbeinsey hydrothermal field, however, is located west of the ridge and influenced by the North Irminger Current. The ridge morphology and hydrodynamics play a major role in sediment distribution [55]. Hence sediments deposited in smaller local depressions of the Kolbeinsey Ridge are thinner compared to the Grimsey area and they are probably derived from a marine liptinitic source. A high subbottom temperature regime is indicated for the KR hydrothermal fields by venting hot fluids which have (measured) temperatures up to 250°C (Table 1). As a consequence, prevailing high subsurface temperatures cause a high maturation of the organic source. Common maturation estimations based on the 813C values of higher gaseous hydrocarbons [53] are probably not reliable, however, as the very 'heavy' isotope values of ethane and propane (Table 2) are most likely caused by secondary processes such as bacterial oxidation [52] and thermal cracking reactions [51]. Similarly, 813C-C1 and C1/3He (C~/C2) ratios may have been influenced by those secondary processes. Hence, the exact primary mixing situation responsible for KR hydrothermal fluids still remains unsolved.

4. Conclusions

Phase separation (boiling sea water) leads to the formation of free gas bubbles emerging from the sea floor at two hydrothermal sites of the Kolbeinsey Ridge area. The gas contains COs probably of magmatic origin which is slightly influenced by subsurface carbonate dissociation (Grimsey) or subsurface

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R. Botz et aI./ Earth and Planetal~y Science Letters 171 (1999) 83-93

calcite precipitation and/or additions of biogenic CO2 at Kolbeinsey hydrothermal vents. Hydrothermal He components can readily be explained by mantle helium, which represents a mixture between MORB helium and a deep-mantle plume helium component, diluted with variable amounts of ambient sea water. Higher CH4/3He ratios (compared with the EPR situation) suggest an additional sedimentary source for methane. Accordingly, the C1/C2 ÷ C3 values together with the isotopic composition of CH4 indicate that (beside the high-temperature hydrothermal methane) thermal degradation processes of sedimentary organic matter deposited in basins of the Tj6rnes Fracture Zone and within local depressions on the west side of the Kolbeinsey Ridge crest are most likely responsible for hydrocarbon genesis. However, beside gas mixing, secondary processes such as bacterial oxidation and thermal cracking may have influenced both hydrocarbon molecular composition and isotope values.

Acknowledgements We are grateful to the crew of R/V Poseidon and the JAGO team (J. Schauer and K. Hissmann) for their large efforts made during sampling. J. Sass did the 3D-modeling. We thank E. Faber, U. Berner and K. Lackschewitz for helpful discussions and a review of the manuscript. Valuable suggestions and helpful comments were made by the journal referees M. Schoell and M. Trieloff. The project was funded by the Deutsche Forschungsgemeinschaft (DFG). [AC]

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