THE CARBONATE CLUMPED ISOTOPE PALEOTHERMOMETER AS A POSSIBLE RECORDER OF LOW-TEMPERATURE BASIN DIAGENESIS

The clumped isotope paleothermometer has proven to be an invaluable tool for paleo-climate and paleo-environmental reconstructions, tectonics, and sedimentology. The recording and retention of primary ¹³C-¹⁸O bond ordering in the carbonate lattice at surface temperatures is well understood. However, what controls the degree of resetting of this preferential bonding of the rare isotopes of carbon and oxygen during burial is incompletely understood. Typically low Δ₄₇ (high temperature) results are considered unusable for paleo-environmental studies. Here we explore the use of clumped isotope paleothermometry as a tool to reconstruct sedimentary basin low temperature diagenetic histories when the starting conditions of carbonate deposition are reasonably well-constrained.

The Eocene Jialazi and Quxia Formations (Tso Jianding Group, southern Tibet) record the marine to non-marine transition of the leading edge of Eurasia just prior to the onset of India-Asia collision (Orme et al., 2014). Low latitude, Eocene marine carbonate should yield a δ¹⁸O_c of 0 to -2‰. However, the intertidal to near shore calcareous sandstones to siltstones and marine fossils of the Tso Jianding yield highly depleted δ¹⁸O_c values of -13 to -21‰. Clumped isotope measurements yield both cool and hot paleo-temperatures, ranging from ~7 to 86°C. This wide range of paleo-temperatures suggests that some secondary carbonate cement precipitated during burial while the low T(Δ₄₇)-low δ¹⁸O_csamples must have recrystallized at surface temperatures after the Indus Yarlung Suture Zone reached high elevations.

The samples with known marine fossils are good candidates for water-rock exchange reaction modeling, such as that described in Banner & Hanson (1990) and implemented in Huntington et al. (2011), because we know the isotopic starting point of marine carbonate within 1-2‰. We model the possible alteration trajectories/relationships of diagenetic waters and marine carbonate in oxygen isotope space using the upper bound of T(Δ₄₇) as the diagenetic/minimum burial temperature. We compare these model results to measured δ¹⁸O_c and T(Δ₄₇) of carbonate shell, matrix, and vein material of individual samples to place bounds on the isotopic composition of altering waters during Cenozoic basin deformation and uplift.