MINERALOGICAL CONTROL ON CARBONATE CLUMPED ISOTOPE TEMPERATURES FROM A NEOPROTEROZOIC CAP CARBONATE IN BRAZIL

Carbonate clumped isotope measurements of ex-aragonite crystal fans, micrite drapes, and dolomitic microbialaminite facies from a post-glacial cap carbonate (Sête Lagoas Formation) in south-central Brazil reveal a wide range of temperatures that appear to be primarily controlled by carbonate mineralogy. The calculated Δ₄₇ temperatures range from 116 to 250^oC while derived oxygen isotope compositions of water in equilibrium with these carbonates range from 4.8 to 15.2‰ (SMOW). The dolomicrite samples occupy the high end of these ranges, and are warmer than the dolomicrites associated with ¹³C depleted samples of the Doushantuo Formation cap carbonate where the involvement of hydrothermal fluids is suspected. Fine-grained micrite and ex-aragonite crystal fan samples with low Mn/Sr, high Sr abundances, and non-radiogenic ⁸⁷Sr/⁸⁶Sr compositions define the lower end of these ranges. While it is clear that thermal effects during deep burial resulted in the redistribution of clumped isotope compositions in all of these carbonates, we suggest that other geochemical measures of depositional conditions are retained in both limestone and dolomite facies. The mineralogical dichotomy in Δ₄₇ values may result from differences in blocking temperatures or susceptibility to recrystallization between dolomite and calcite where the latter continued to rearrange C-O bonds as the succession cooled from peak burial temperatures more closely approximated by the dolomites.