Episodic mantle wedge carbonation induced by infiltration of oxidizing fluids

Abstract:

Slab-mantle interface is most active site of fluid-rock interactions, which affects the mass transfer and mechanical properties along subduction zone megathrust. Mantle wedge is one of the potential sinks of CO₂, but the nature of mantle wedge carbonation essentially remains unconstrained due to a lack of natural geological evidence. Here we present observations of carbonation of serpentinite body from the Sanbagawa belt, Kanto Mountain, Japan. The depleted composition of Cr spinel and temperature of ~400 ˚C indicates that this body was originated from the leading edge of the mantle wedge. This body was completely serpentinized with 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 was formed after ultramafic body and chlorite rocks were formed from pelitic schists. The isotope compositions of carbonates and thermodynamic calculation reveal that carbonic fluid was derived from carboniferous materials in sediments under oxidizing condition (higher than QFM). We show that serpentinized mantle wedge is carbonated by reaction-induced fracturing assisted by solid volume contraction and high mobility of Mg. Moreover, our findings suggest that repeated brittle failure to form carbonate veins and viscos flow of carbonate and by-product talc occurs within the subduction zone. As ultramafic bodies in the Sanbagawa belt is not always carbonated, mantle wedge carbonation would be heterogeneous. 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.