Several recently discovered active mud volcanoes on the nonaccretionary Mariana convergent plate margin are erupting slab-derived fluids, serpentine mud, and metamorphosed rocks from depths of as great as 25 km. Blueschist materials from the metamorphosed subducted plate are contained in the muds. Pore fluids indicate a depth dependence for decarbonation. The mud volcanoes record in situ conditions along the decollement, including pressure and temperature conditions and physical properties within the subduction zone. Similar mud-flow material occurs worldwide as “sedimentary serpentinite” deposits in accreted fragments of former convergent margins, making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized.