A DECADE OF LABORATORY AND FIELD STUDIES: LESSONS LEARNED FROM INVESTIGATING DOLOMITE MINERALOGY

Dolomite [CaMg(CO₃)₂] is often perceived as the trouble-making younger sibling of the better behaved and more level headed Mg-calcite [(Ca,Mg)CO₃]. This mistake causes confusion and severely limits our scientific appreciation and application of this complicated mineral. Despite the potential to have similar compositions, Mg-calcite and dolomite are structurally quite distinct. Whereas Mg-calcite has randomly arranged Ca and Mg within the crystal lattice, dolomite is characterized by an ordered, alternating arrangement of Mg and Ca. Moreover, dolomites in the rock record and those produced in the lab exhibit a wide range of compositions (38-52 mole% MgCO₃) and structural characteristics (very poorly to well-ordered), mineralogical attributes that provide important geological clues. Empirical data show that dolomite mineralogy is directly linked to thermodynamic stability, crystal growth mechanisms, internal microstructure, trace element incorporation, and other fundamental phenomena. Such mineralogical data are not, however, routinely collected or integrated into diagenetic studies. Armed with knowledge about the various controls on stoichiometry and cation ordering gathered from lab experiments, such as formation temperature, fluid Mg/Ca, precursor mineralogy, fluid salinity, etc., dolomite mineralogy is gaining recognition as a potentially powerful proxy in our quest to develop and test diagenetic models. In this talk, a series of examples will be presented to show how dolomite mineralogy, in conjunction with other petrological data, is being applied to evaluate: (i) large-scale, top-down reflux in thick evaporite-capped marine carbonates, (ii) cycle-scale, evaporative reflux in peritidal carbonates, (iii) dolomitization in lacustrine settings, (iv) late-stage, fault-controlled hydrothermal dolomitization in deep basin settings, (v) terrigenous sediment sources and distribution patterns in glacially influenced depositional settings, and (vi) models of mass dependent stable isotope fractionation in Mg-Ca-carbonates.