The δ13C–δ18O variations in marble in the Hida Belt, Japan

Abstract

AbstractMarble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid‐crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite‐facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ13C [VPDB] and δ18O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ13C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO2 releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ13C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ18O values of calcites are remarkably lower than the marine‐carbonate values. The large δ18O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ18O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (87Sr/86Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin‐section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H2O‐rich fluid infiltration at a relatively high‐temperature condition. Our isotope study implies that the upper amphibolite‐facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.