A process by which serpentinite fault-gouge protruded onto the seafloor in suprasubduction-zone environments was first suggested by Lockwood (1971, 1972) as mechanism for formation of "sedimentary serpentinite" in a marine setting. The discovery of active serpentinite mud volcanism on the Mariana forearc and recent mud volcanism in the Izu-Bonin forearc during Leg 125 revived the study of this phenomenon. This type of mud volcanism only occurs in nonaccretionary convergent margins, and forearc extensional deformation is key to emplacement of the mud flows at the seafloor. The drilling results from serpentine seamounts drilled during Legs 125 and 195 show us that composition of the serpentinite muds and the metamorphosed peridotite clasts included in them can vary regionally. The slab-derived fluids in these muds are unique among pore fluids from oceanic sediments. The fluids show systematic variation in composition with distance from the trench and are thus related to depth to the slab. The complexity in distribution of various slab-derived volatile-mobile trace elements suggests that the suprasubduction-zone mantle that is altered by them may be being recycled downdip to greater depth beneath the arc. The serpentinite clasts in the muds erupted at sites in the forearc of the Izu-Bonin system shows that the mantle of that forearc has experienced a different history of both magmatic and melt-fluid interaction than that of the Mariana system. Mafic rocks brought up in the Mariana serpentinite mud flows derive from both the subducted slab and the overlying forearc crust (none were recovered from the Izu-Bonin seamount drilled). The slab-derived metabasites have mineralogic and compositional characteristics that have been used to constrain the temperature and depth conditions of their metamorphism and place them in the incipient blueschist facies. The mud flows with their included rock clasts provide a window into suprasubduction-zone mantle processes, as well as those at work within the décollement itself.
Serpentinite mélange deposits occur world-wide, and those that formed by mud volcanism in forearc environments can be recognized by careful analysis of the sheared and unconsolidated matrix serpentines, the variously metamorphosed ultramafic and mafic clasts entrained within the mud flows, and the composition of the of the serpentinites. The chief characteristics that distinguish the serpentinite massifs of convergent margins from those of other tectonic settings in the oceans are the overprint of subduction-derived constituents in the peridotitic protoliths and their derivative melts, the altered and metamorphosed metabasic rocks associated with the serpentinites, and possibly the subtle geochemical and mineralogical consequences of the unique biological activity that characterizes these remarkable sites. Serpentinite mud volcanism may be the origin of some of the world's most complex and voluminous serpentinite terranes on former convergent margins.