Rock-buffered recrystallization of Marion Plateau dolomites at low temperature evidenced by clumped isotope thermometry and X-ray diffraction analysis

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(Delta(47dol)) ranges between 12 and 35 degrees C. The apparent fluid composition (delta O-18(w(app))) falls in the range of sea water composition, but a correlation between T(Delta(47dol)), delta O-18(dol), and delta O-18(w(app)) exists: the higher T (Delta(47dol)), the higher delta O-18(w(app)). T(Delta(47dol)) and delta O-18(w(app)) increase with depth, whereas delta O-18(dol) and delta C-13(dol) tend to both decrease with depth. We interpret the correlation between T(Delta(47dol)) and delta O-18(w(app)) as evidence of shallow burial recrystallization via dissolution/re-precipitation. Modelling of the T(Delta(47dol)), delta O-18(dol), and delta O-18(w(app)) indicates that the recrystallization happened at very low water to rock ratio. Carbon isotopes delta C-13 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(Delta(47dol)) 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(Delta(47dol)) before entering the solid-state reordering realm, and that in deeper buried basins, the range of measured T(Delta(47dol)) could still to a large extent result from recrystallization via dissolution/re-precipitation processes. (C) 2019 Elsevier Ltd. All rights reserved.