Serpentine minerals and brucite in ultramafic rocks from the South Chamorro Seamount were characterized chemically to investigate the serpentinization of the Mariana forearc mantle. Relict primary minerals of the serpentinites are olivine, enstatite and minor Cr-spinel and diopside. The secondary minerals are mostly serpentine and brucite with minor magnetite. The serpentine minerals, mostly lizardite and chrysotile, display large compositional variations. Al2O3 and Cr2O3 contents depend generally upon the nature of the primary mineral from which the serpentine was derived. Both serpentine minerals and brucite exhibit wide Mg, Fe and Mn substitution: the Mg# ranges are 95.1–77.2 and 88.9–60.8, respectively. These mineralogical and chemical features allowed us to estimate an upper temperature limit for serpentinization of ~200–300°C, in agreement with recent thermal models which suggest that the serpentinized mantle wedge of the Izu-Bonin-Mariana subduction zone is cold. The high degree of serpentinization (40–100%, average >75%), and the serpentine + brucite paragenesis of these ultramafics imply that the Mariana forearc mantle has a significantly reduced density and strength down to ~30 km, which provides a driving mechanism for serpentinite diapirism. Pervasive serpentinization of the forearc by fluids released from the décollement zone also explains the low seismicity of the Izu-Bonin-Mariana subduction zone.