Rupture of serpentinized mantle wedge by self-promoting carbonation: insights from Sanbagawa metamorphic belt

The slab-mantle interface is one of the most active sites of fluid-rock interaction, which affects the mass transfer and mechanical properties along subduction zone megathrust. However, the effects of CO₂ fluids and carbonation/decarbonation reactions on seismic activity are still poorly understood. In addition, although mantle peridotite is known as a large sink of CO₂, the nature of carbonation at mantle wedge condition remains unconstrained. In this study, we show the characteristics of carbonation of serpentinite body from the Sanbagawa metamorphic belt, Kanto Mountain, Japan [1]. The Higuchi serpentinite body (8 x15 m) is mainly composed of antigorite, and has not relics of olivine and pyroxenes. Massive antigorite parts are segmented by talc + carbonates (magnesite, dolomite and calcite) veins. At the boundary between serpentinite body and pelitic schists, actinolite-chlorite schist and chlorite rocks were formed. The location, depleted composition of Cr-rich spinel in the Higuchi body and temperature of ~400 ˚C of carbonation suggest that this body was originated from the leading edge of the mantle wedge. The carbon and oxygen stable isotope compositions of carbonates within the Higuchi body reveal that carbonic fluid was derived from carboniferous materials in sediments. Carbonation of serpentinite body is characterized by gains of CO₂, silica and Ca, and losses of H₂O and Mg. The thermodynamic calculations on mineral-fluid equilibria reveal that (1) the carbonic fluid produced under the oxidizing conditions (QFM +0.3) explains the systematic mineralogical variations within the Higuchi body, and that (2) carbonation of serpentinite proceeded with solid volume contraction, high fluid pressure and high mobility of Mg, which is largely consistent with the experimental carbonation at the mantle wedge condition [2]. This is consistent with the tree-like patterns of carbonate veins within the Higuchi body. Brittle failure to form carbonate veins was followed by a viscos flow of carbonate and talc. We infer that episodic infiltration of oxidizing fluids causes self-promoting carbonation of mantle wedge with solid volume change, which could affect the mechanical properties of slab-mantle interface.

[1] Okamoto, A, et al., 2021. Com Env Earth, 58, 4831-4839. doi.org/10.1038/s43247-021-00224-5

[2] Sieber et al. 2020. J. Petrol., 1-24. doi: 10.1093/petrology/egaa035