Abstract
High δ13C values, up to +5.7‰ (PDB), were measured in marbles associated with ultrahigh‐pressure eclogites in the Dabie Mountains, East China. Such a carbon isotope anomaly is interpreted to represent the carbon isotope composition of premetamorphic limestone (i.e., the protolith of the marbles), because there is no known process to enrich carbonate in 13C during regional metamorphism. The high δ13C also corresponds to high δ18O of up to +22.9‰ (SMOW), suggesting that premetamorphic δ13C values would be about +5 to +6‰ for their protolith (limestones). The carbon isotope anomaly in the protolith of the marbles implies a local change in the redox state of water during deposition of the limestones. Preservation of the unusually high positive δ13C values in the marbles supports the previous conclusion that there was no significant isotopic exchange with the mantle during subduction of supracrustal rocks to mantle depths of greater than 100 km.
Published Version
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