Abstract
The mechanism and efficiency of CO2 release from subducting carbonates lie at the heart of quantifying carbon cycling in subduction zones. Most previous studies focused on CO2 release from calcite/aragonite, while decarbonation behavior of another critical carbonate—dolomite—is poorly constrained. Here, we investigated reaction veins and zones in dolomitic marbles from the Dabie ultrahigh-pressure (UHP) terrane, China. Petrology, geochemistry, O-C isotopes, and U-Pb dating reveal that the reaction veins and zones record the metasomatic interaction of dolomitic marbles with episodic Si-Al-Ti−bearing aqueous fluids from UHP eclogites and substantial release of CO2. Mass-balance calculations indicate that 67%−84% of initial CO2 was released from the altered marbles, whereas the losses of Ca (34%−67%) and Mg (almost immobile) are much lower, suggesting that CO2 removal was not caused by simple (bulk) dissolution of dolomite. Instead, it reflects that ∼50% of initial CO2 was removed via replacement reactions of dolomite by Mg-silicates + CaCO3, and another ∼17%−34% CO2 extraction was by dissolution of reaction-formed CaCO3. Enrichment of solutes (e.g., Si and Al) in slab fluids facilitates the decarbonation by solute load in fluid-dolomite interactions, and pulsed fluid infiltration helps enhance CO2 liberation. Considering the wide presence of dolomite in subducted slabs and its high decarbonation efficiency during fluid infiltration, the findings highlight that slab dolomite plays a significant role in carbon release from subduction zones.
Published Version
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