Carbon and Oxygen Stable Isotopes in Isochemical Contact Metamorphism (Case Study of the Kochumdek Aureole, East Siberia)

––Carbon and oxygen isotope compositions have been determined in (CO3)-groups of Ca carbonate–silicate minerals (spurrite, tilleyite, and scapolite) and calcite from samples of the Kochumdek contact aureole (East Siberia). The observed small δ13C and δ18O difference between the Kochumdek marbles and their protoliths (Δδ13C ≤ 1.5‰ and Δδ18O ≤ 2.0‰) is consistent with moderate high-temperature metamorphic decarbonation of the protolith (0.1–0.2) and with almost absent metasomatic alteration in the aureole. Significant 18O depletion was inferred only for vein scapolite (δ18O from +17.0 to +17.3‰ V-SMOW) and calcite from recrystallized limestone layers (δ18O from +18.9 to +20.6‰), which is evidence for limited infiltration of magmatic aqueous fluids from the cooling sill into the host sediments. The C and O isotope compositions of (CO3)-bearing spurrite and tilleyite show lower 18O depletion and insignificant 13C depletion relative to calcite, especially in highest-temperature merwinite marble (Δδ18OSpu–Cal = +3.3‰; Δδ13CSpu–Cal = –0.8‰). Gas chromatography-mass spectrometry (GC-MS) provided constraints on the relative percentages of H2O and CO2 in the fluid at highest-temperature spurrite-merwinite metamorphism. The fluid phase entrapped in calcite from merwinite marble is rich in CO2(XCO2=0.4−0.6).