Journal article

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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 Miocene 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 the crystallization temperature, the more negative the fluid composition is. T(Δ47dol) and δ18Ow (app) increase with depth, whereas δ18Odol and δ13Cdol tend to both decrease with depth. We interpret the negative 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 are inherited from the dolomitization process, and not reset during recrystallization. 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(ΔΔ47dol) could still to a large extent result from recrystallization via dissolution/re-precipitation processes.

Publication date: 
Wednesday, February 20, 2019