© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. Metaharzburgite and metadunite in the ultramafic body of the Naran Massif in the Khantaishir Ophiolite, western Mongolia, record multi-stage processes of serpentinization (antigorite, lizardite + brucite, then chrysotile). Bulk-rock chemistry and the compositions of primary olivine (P-olivine) and Cr-spinel suggest that the alteration occurred in the forearc mantle. In the metaharzburgite, a novel occurrence of fine-grained (10–50 μm) secondary olivine (S-olivine) takes the form of aggregates (a few millimeters across) with bands of antigorite. The S-olivine has higher Mg# values (0.96–0.98) than the P-olivine (Mg# = 0.92–0.94) and contains inclusions of clinopyroxene and magnetite. The P-olivine has been replaced by antigorite and magnetite. Mesh textures of lizardite + brucite are developed in both P- and S-olivine. The microtextures and chemical compositions of minerals suggest that S-olivine aggregates were formed by pseudomorphic replacement of orthopyroxene related to multi-stage hydration processes. Assuming the mantle wedge conditions beneath a thin crust, orthopyroxene was first replaced by S-olivine + talc at high temperatures (500–650 °C at ~ 0.5 GPa). With cooling to ca. 400–500 °C and fluid supply, talc transformed to antigorite with the release of silica. During this stage, P-olivine was also transformed to antigorite by consumption of silica released from orthopyroxene decomposition. At temperatures below 300 °C, lizardite + brucite ± magnetite formed from the remaining P- and S-olivine grains. The formation of S-olivine presented in this study contrasts with the commonly ascribed process of deserpentinization. Taking into account the geochemical data for the studied ultramafic rocks and those previously reported for mafic rocks, our results suggest that mantle wedge beneath thin crust was hydrated in response to continuous cooling and fluid supply from a subducting slab after subduction initiation.