Extremophilic archaeal communities living in serpentinized muds influenced by pH 12.5 deep-slab derived fluids were detected and their richness and relatedness assessed from across seven serpentinite mud volcanoes located along the Mariana forearc. In addition, samples from two near surface core sections (Holes D and E) at ODP Site 1200 from South Chamorro were subjected to SSU rDNA clone library and phylogenetic analysis resulting in the discovery of several novel operational taxonomic units (OTUs). Five dominant OTUs of Archaea from Hole 1200D and six dominant OTUs of Archaea from Hole 1200E were determined by groups having three or more clones. Terminal-restriction fragment length polymorphism (T-RFLP) analysis revealed all of the dominant OTUs were detected within both clone libraries. Cluster analysis of the T-RFLP data revealed archaeal community structures from sites on Big Blue and Blue Moon to be analogous to the South Chamorro Hole 1200E site. These unique archaeal community fingerprints resulted from an abundance of potential methane-oxidizing and sulfate-reducing phylotypes. This study used deep-sea sediment coring techniques across seven different mud volcanoes along the entire Mariana forearc system. The discovery and detection of both novel Euryarchaeota and Marine Benthic Group B Crenarcheaota phylotypes could be efficacious archaeal indicator populations involved with anaerobic methane oxidation (AMO) and sulfate reduction fueled by deep subsurface serpentinization reactions.