As a part of the 1988 NOAA VENTS Program, CH4 and Mn tracers were used to identify and compare hydrothermal plumes found above the TAG Field (26-degrees-N) and in the rift valley at 15-degrees-N close to the eastern intersection of the ridge axis with the 15-degrees 20'N Fracture Zone at the Mid-Atlantic Ridge (MAR). Active hydrothermal venting was confirmed at TAG, based on elevated concentrations of total dissolved Mn (TDM up to 30 nmol/kg), high CH4 concentrations (up to 200 nL/L), and elevated nephelometry signals. Plumes of a different composition were identified at 15-degrees-N with high CH4 concentrations (up to 400 nL/L), low total dissolved Mn concentrations (TDM < 1 nmol/kg) and no significant nephelometry signal. The different properties of these tracers and the different tracer ratios can be used to deduce vent fluid characteristics and compare one hydrothermal area to another. TDM/CH4 and Nephel/CH4 ratios at TAG are of the same order of magnitude as those observed at other spreading axis hydrothermal fields. At 15-degrees-N, the low TDM/CH4 ratio provides evidence of fluid circulation into ultrabasic rocks and offers a potentially useful and single method of exploring for hydrothermal activity associated with serpentinization. Mantle degassing through hydrothermal activity associated with serpentinization is an important process with respect to chemical and thermal exchanges between the upper mantle and the ocean. Different ratios of hydrothermal tracers (i.e., TDM/CH4) provide a useful framework for identifying subseafloor processes along mid-oceanic ridges.
