Abstract
More than one teramole of carbon per year is subducted as carbonate or carbonaceous material. However, the influence of carbonation/decarbonation reactions on seismic activity within subduction zones is poorly understood. Here we present field and microstructural observations, including stable isotope analyses, of carbonate veins within the Higuchi serpentinite body, Japan. We find that the carbon and oxygen isotope compositions of carbonate veins indicate that carbonic fluids originated from organic materials in metasediments. Thermodynamic calculations reveal that carbonation of serpentinite was accompanied by a solid volume decrease, dehydration, and high magnesium mobility. We propose that carbonation of the mantle wedge occurs episodically in a self-promoting way and is controlled by a solid volume contraction and fluid overpressure. In our conceptual model, brittle fracturing and carbonate precipitation were followed by ductile flow of carbonates and hydrous minerals; this might explain the occurrence of episodic tremor and slip in the serpentinized mantle wedge.
Highlights
More than one teramole of carbon per year is subducted as carbonate or carbonaceous material
Slow slip events include long-term slip events that occur in relatively shallow parts of the subduction interface, and episodic tremor and slip (ETS) in relatively deep parts
In warm subduction zones such as Nankai and northern Cascadia, ETS is abundant in the corner of the mantle wedge[20,21,22,23], a region that could be dominated by serpentinite formed by fluids released from the subducting slab
Summary
Carbonation of serpentinite associated with brittle fracturing. The Sanbagawa belt is a HP metamorphic belt that extends ~800 km along the Median Tectonic Line from the Kanto Mountains in the east to Kyushu in the west along Japan (Supplementary Fig. 1)[41]. The quantitative mass balance analysis of the carbonation of the serpentinite body is difficult due to the heterogeneous distribution of carbonate veins, the carbonation reactions were characterized by the formation of carbonates (magnesite, dolomite, or calcite) + talc at the expense of antigorite, and accompanied by gains in CO2, SiO2, and CaO, and losses of H2O and MgO (Fig. 3c). With an increase in the proportion of antigorite, the mineral assemblage evolves as follows: Chl ± Qtz ± Cpx/Chl + Tr/Cal + Talc/Dol + Talc/Mgs + Talc/Mgs + Talc + Atg (Fig. 4b, c) Such a mineralogical sequence reflects the fluiddominated system at the boundary of the serpentinite body and close to the large veins, and the rock-dominated system in the interior of the serpentinite body with a fine vein network (Fig. 4c) observed within the Higuchi serpentinite body The reaction proceeds along the margins and fractures within the serpentinite body in response to diffusive flux of metasomatic agents (i.e., CO2 species or silica), which is saturated in the pelitic
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