Gases and helium isotopes in high temperature solutions sampled before and after ODP Leg 158 drilling at TAG hydrothermal field (26 degrees N, MAR)

Hydrothermal fluids were sampled for dissolved gases at TAG (26 degrees N - MAR) during two Alvin dive series, in April-May 1993 and March 1995, respectively 17 months before and 4 months after Ocean Drilling Program Leg 158. Total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude, even if some increase in H2S, CO2 and CH4 was noticeable in the 1995 samples. No significant difference was observed in helium concentration and helium isotopic ratio (He-3/He-4=8.2 +/-0.1 Ra). The CH4/He-3 ratio found in black smokers sampled in 1993 is around 9 x 10(6), close to ratio found in MAR basalts. In 1995, this ratio is around 4 times higher, due to the CH4 increase. The delta(13)C in CO2, measured on two black smokers sampled in 1993, is uniform at -8.4 to - 8.8 parts per thousand (versus PDB), while a delta(13)C value of -13 parts per thousand is measured at a new site sampled in 1995, located 40 meters east of the main Black Smokers Complex (BSC). CH4/He-3 and C-13 values both point to a mainly magmatic (abiogenic) origin of the carbon species in the system. Overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium, CO2 and CH4 of magmatic origin, even if some thermogenic CH4 contribution seems likely in the 1995 samples collected in the new active area. We can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the TAG mound and temporarily enhanced the hydrothermal circulation, by opening new pathways favouring transient input of CH4 rich fluids, before coming back to a new steady state.