Equilibrium between dolomitization and dedolomitization of a global set of surface water samples: A new theoretical insight on the dolomite inorganic formation mechanism

Abstract

The dolomite problem has confused scientists for nearly two centuries because the classic saturation index cannot explain the failure of dolomite precipitation in carbonate solutions including 1000-fold super saturation for 32 years and oversaturation of modern seawater for millions of years. Although the dolomite problem is still controversial, the chemistry of a global set of surface water samples reported in dolomitization (N = 54) and dedolomitization (N = 433) is indispensable, and firstly summarized to explore the mechanism of dolomite inorganic precipitation. This study indicates that: (1) saturation index of dolomite in the surface water of dolomitization is closing to dedolomitization, suggesting the failure of SI to reflect the real precipitation driving force of dolomite; (2) the primary difference is that most of water in dedolomitization has negative SI of gypsum, while most of water in dolomitization has positive SI of gypsum; (3) the inherent stoichiometry competition relationship between calcite and dolomite is proposed to explain the incompleteness of classic saturation index model in predicting inorganic dolomitization; (4) A multi-component calcite - dolomite - gypsum phase equilibrium (CDGPE) model is recommended to replace SI in predicting the dolomitization and dedolomitization of a global set of surface waters; (5) CDGPE model is proved reliable and useful in predicting the surface water reported in dolomitization and dedolomitization (including the modern seawater); (6) the evaporation modelling of the modern seawater through the stepwise detraction of water shows that the surface waters (Persian Gulf, Mediterranean, Abu Dhabi, Coorong Region at Southern Australia, and LagoaVermelha at Brazil) observed with abundant dolomitization are all located at the different evaporation paths of modern seawater, suggesting that the dolomitization at these seawater are resulted by the evaporation, which is consistent with the recent finding that the dolomitization event at the oceanic chemocline occurred during the Permian-Triassic transition period when the temperature was hot on the earth surface.