our CH 4 (nM) versus Si ( yM) regression (Fig. 6a) was 5.6 x 10~ 3 , 

 a value which is in the range of the previously published 

 Galapagos Rift data. 



The CH4/excess EC02 molar ratio estimated for the water 

 samples collected from Pele's Vent is 2.3 x 10"^ (Fig. 7), 

 compared to a value of 3 x 10"^ that was predicted from the 

 CH4/PH relationships of Loihi Seamount hydrothermal plumes 

 (Sakai, Tsubota, Nakai, Ishibashi, Akagi, Gamo, Tilbrook, 

 Igarashi, Kodera, Shitashima, Nakamura, Fujioka, McMurtry, 

 Malahoff, and Ozima 1987). It is highly unlikely that there is 

 either CH4 production or net CO2 uptake in the hydrothermal plume 

 that might account for the different ratios. One possible 

 explanation is that the CH^/excess £C02 ratio of Pele's Vent may 

 not be characteristic of Loihi Seamount as a whole. In support 

 of this interpretation, Gamo, Ishibashi, Sakai and Tilbrook 

 (1987) have suggested that there exist at least two identifiable 

 and chemically-distinct hydrothermal plumes at Loihi. Relative 

 to the content of 3 He, the deep plume had approximately 3-4 times 

 more CH4 than the plume detected at shallower depths (1000-1050 

 m). This chemical distinction is also evident in the CH4-PH 

 relationships (Gamo, Ishibashi, Sakai, and Tilbrook 1987). Two 

 mixing lines, representing two end-member fluid compositions, are 

 evident; the shallow plume again has less CH4 (relative to pH 

 [i.e., CO2] ) than the deep plume. 



At Pele's Vent, there are at least two potential sources for 

 hydrothermal CH4: (1) abiogenic sources, including volatile 

 release from mantle sources, thermocatalysis of deposited organic 

 matter, and high-temperature exchange reactions with the basalt- 



seawater system (i.e., 4H2 + CO2 > CH 4 + 2H 2 0) and (2) 



biogenic reactions involving the activities of thermophilic, 

 methanogenic bacteria. Welhan and Craig (1983) suggest that the 

 observed enrichments of CH4 in hydrothermal fluids are the result 

 of direct extraction of CH4 from basalt by circulating seawaters. 

 Their conclusion is based upon several lines of evidence, 

 including: the association of CH 4 with 3 He and excess l CO2 

 which are known to be derived from ridge crest basalts, the lack 

 of known sources of organic matter for thermocatalysis, the 

 general absence of C2 + hydrocarbon homologues in vent solutions, 

 the S^C enrichment of vent-derived CH4, and the similarity in 

 the CH4/ 3 He ratios of the EPR hydrothermal fluids and mid-ocean 

 ridge basalts. 



On the other hand, the CH4 measured at Pele's Vent and 

 enriched in hydrothermal solutions in general could, at least in 

 part, be formed as a result of the reduction of CO2 by the 

 activities of methanogenic bacteria. Baross, Lilley and Gordon 

 (1982) have demonstrated the production of CH4 (and other 

 "biogenic" gases) at 100+2°C by 21°N EPR hot water and sulfide- 

 chimney associated bacterial communities. Because their 



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