Mantle hydration initiated by Ca metasomatism in a subduction zone: An example from the Chandman meta-peridotite, western Mongolia

Abstract

Fluids in subduction zones contain various aqueous species and cause metasomatic reactions as well as hydration (i.e., serpentinization) at the slab–mantle interface. However, the nature of elemental transfer during fluid infiltration into the dry mantle in the subduction zone is poorly understood. In this study, we describe novel textures related to orthopyroxene (Opx) pseudomorphs and antigorite veins in the Chandman meta-peridotite body in the Alag Khadny accretionary wedge, western Mongolia. This body consists mainly of meta-harzburgite and meta-dunite, and hydration proceeded pervasively within olivine domains associated with the development of antigorite vein networks. Pseudomorphs after orthopyroxene consist of secondary clinopyroxene (S-Cpx) + tremolite (Tr) ± [secondary olivine (S-Ol) + antigorite (Atg)] whereas the primary clinopyroxene (P-Cpx) is almost unaltered. The pseudomorphs after Opx are rarely cut by Atg veins. The mineral occurrences indicate two main stages of mantle alteration: (1) Opx replacement (Ca-metasomatism) and (2) Atg formation induced by Si-bearing fluid infiltration (i.e., the main hydration event), followed by low-temperature (T) alteration that formed lizardite, chrysotile, and brucite. The mineralogical sequence (S-Cpx + Tr ± S-Ol ± Atg) related to Opx replacement can be explained by replacement at 550–650 °C (assuming a pressure of 1.0–2.0 GPa), which is broadly consistent with temperatures experienced by eclogites in the Chandman area during exhumation. Atg in P-Ol domains could have formed at lower temperatures (<500 °C; assuming a pressure of 1.0–2.0 GPa). Mass balance calculations show that Opx replacement was characterized by gains of Ca and a small amount of H2O, whereas Ol hydration was characterized by gains of Si and H2O. A modeled slab-derived fluid, which was calculated to be in equilibrium with basaltic crustal rocks, indicates the time-integrated fluid flux required for Ca metasomatism of the Chandman meta-peridotite body, with an assumption of the alteration thickness of L = 1000 m, was 0.18–1.5 × 106 m3 fluid m−2 rock, whereas the time-integrated fluid flux required for Si metasomatism was 0.3–1.7 × 103 m3 fluid m−2 rock, which is 102–103 lower than that of the Ca metasomatism. The characteristics of the Chandman meta-peridotite body suggest that: (1) the alteration type and trapped elements in the mantle derived from slab-derived fluids vary with temperature and local Si activity; and (2) intense fracture networks facilitated extensive serpentinization related to the Si metasomatism.