LOW TEMPERATURE DOLOMITE SYNTHESIS: REACTION KINETICS, UREA CATALYSIS AND ISOTOPIC ANALYSES

Abstract

Dolomite is a naturally abundant carbonate mineral possessing important links to economic mineral deposits, petroleum reservoirs and carbonate geochemistry, yet the geochemical conditions by which it forms remain a mystery. Abundant attempts to synthesis dolomite at temperatures below 100 °C have proved unsuccessful, forming the paradox known as the “dolomite problem”. This study demonstrates a newly developed method capable of synthesizing dolomite at temperatures as low as 60 °C. This method, involving the addition of solid phase Na2CO3 to Ca-Mg2+ cation solutions, seemingly overcomes the kinetic barriers known to inhibit dolomite formation by rapid replacement of Na2CO3. Furthermore, although previously proposed to encourage dolomite formation, the role of urea facilitating dolomite synthesis is confirmed. The addition of varying urea concentrations is found to improve the stoichiometry and cation ordering of dolomite between 50 - 80 °C, and notably, dolomite synthesis at 50 °C requires a 252 mmolal urea concentration in solution. These findings provide a foundational tool which will greatly benefit future research into answering the “dolomite problem”. Due to the inability to synthesize dolomite at low temperatures, understanding of isotope fractionation between dolomite and water has been poorly constrained. Therefore, calibrations of the dolomite-water oxygen isotope paleothermometer are scarce, relying on data from either the extrapolation of high temperature (> 100 °C) dolomite studies or protodolomite synthesized at more ambient temperatures. Provided here is a new dolomite-water oxygen isotope fractionation curve, constructed from dolomite synthesized between 50 - 80 °C. Although certain dolomite synthesized by the method developed in this study display apparent isotopic heterogeneity, a correction is applied to more closely resemble isotopic equilibrium fractionation between dolomite and water. Initial investigations into isotope effects associated with dolomite synthesis by the method developed here are promising and should strengthen the role of dolomite in carbonate geochemistry research.