Title: Rock-buffered recrystallization of Marion Plateau dolomites at low temperature evidenced by clumped isotope thermometry and X-ray diffraction analysis
Abstract: Much debate exists on the extent to which early dolomites recrystallize and preserve the signature of their primary diagenetic setting. Here, we combine clumped isotopes thermometry with X-ray diffraction and thin section petrography to study dolomite recrystallization under shallow burial (<1 km) conditions. We analysed 26 dolomite samples from two Miocene carbonate platforms on the Marion Plateau, NE Australia. Marion Plateau dolomites provide an ideal case study to examine the effects of recrystallization because of the relative simplicity of the geological setting, with simple subsidence, and several episodes of early dolomitization by normal sea water. Results show that Marion Plateau dolomites are very rich in calcium and their formation temperature inferred from clumped isotopes T(Δ47dol) ranges between 12 and 35 °C. The apparent fluid composition (δ18Ow(app)) falls in the range of sea water composition, but a correlation between T(Δ47dol), δ18Odol, and δ18Ow(app) exists: the higher T(Δ47dol), the higher δ18Ow(app). T(Δ47dol) and δ18Ow(app) increase with depth, whereas δ18Odol and δ13Cdol tend to both decrease with depth. We interpret the correlation between T(Δ47dol) and δ18Ow(app) as evidence of shallow burial recrystallization via dissolution/re-precipitation. Modelling of the T(Δ47dol), δ18Odol, and δ18Ow(app) indicates that the recrystallization happened at very low water to rock ratio. Carbon isotopes δ13C are not reset during recrystallization and are probably inherited from the dolomitization process. This study shows that dolomite recrystallization has the potential to affect T(Δ47dol) at depths shallower than previously demonstrated. It emphasizes the fact that high calcium dolomites (and possibly aragonite and high Mg-calcite) can have a range of T(Δ47dol) before entering the solid-state reordering realm, and that in deeper buried basins, the range of measured T(Δ47dol) could still to a large extent result from recrystallization via dissolution/re-precipitation processes.