Geochemical fingerprints of dolomitization in Upper Jurassic Arabian carbonate platform: Evidence from carbon, oxygen, strontium and magnesium isotopes

The formation of extensive dolomite across ancient carbonate platform is widely attributed to the replacement of the calcium-carbonate so-called dolomitization. However, it is still unclear whether the extensive dolomite formation is controlled by one single or multiple mechanisms, as whether the mineralogical, petrological, and geochemical records of dolomite preserve the pristine signature or diagenetic effect is still under debate. In this study, we integrate carbon, oxygen, strontium and magnesium isotopes to provide new insights into ancient dolomite origin interpretation. 
Dolomite in the Upper Jurassic Arab Formation on the Arabian Plate has widely been studied due to the complete carbonate-evaporite cycle and relevance to oil reservoirs. Sabkha and seepage reflux dolomitization models have been proposed despite the high temperatures inferred from clumped isotopes and fluid inclusions, as well as some geochemical data such as oxygen and strontium isotopes, are inconsistent with near-surface dolomitization conditions. Here we performed integrated isotope study of the Upper Jurassic Arab Formation, and found that (1) Mg isotope data range from -2.25 ‰ to -1.78 ‰, suggesting the Mg cations for Arab dolomite formation were delivered from Late Jurassic seawater, which is consistent with the idea that dolomite is a valid archive of seawater Mg isotope composition through geological times; (2) in each cycle, more depleted Mg isotope signatures in dolomite formed at shallower depth than that formed at deeper depth, indicating top-down dolomitization process which is controlled by percolation of dense and saline fluids, suggesting the advective flow model and related seepage reflux dolomitization; (3) parts of Sr isotopic ratios are out of Late Jurassic seawater range, revealing hydrothermal or burial dolomitization probably altered the dolomite formed from near-surface conditions, which can also be verified by depleted oxygen isotopic values. This study indicates that ancient carbonate platform with massive dolomite distribution are probably generated by multi-mechanism dolomitization processes and it is necessary to integrate Mg isotope with traditional isotope methods to unravel the dolomite problem.