Fluids venting from the submarine portion of the Marmara Main Fault (part of the North Anatolian Fault system, Turkey) were sampled in Ti bottles deployed by submersible. The fluids consist of mixtures of fault derived gases, fault related cold seep fluids, and ambient seawater; these components can readily be distinguished using the isotopes of He and the He/Ne ratios. He-3/He-4 ratios range between 0.03 +/- 0.1 and 4.9 +/- 0.4 Ra, indicating that both crustal and mantle derived sources of helium are sampled by the fault. The dominant gas in all the samples analyzed is methane with the abundance of CO2 below detection (<= 2%) in the mantle rich (high He-3/He-4) fluids. This is in contrast to nearly all mantle derived fluids where the C species are dominated by CO2. While high CH4/CO2 ratios may reflect organic or inorganic reactions within the crust which reduce mantle derived CO2 to methane, this is not a priori necessary: we show that simple dilution of mantle fluids with methane produced within local sediments could result in the high He-3/He-4, methane rich gases currently emanating from the fault. This observation is supported by an anticorrelation between He-3/He-4 and C/He-3, which is consistent with addition of C and He-4 simultaneously to the fluids.