92 SEDIMENTARY ROCKS/Dolomites crystals suggests temperatures of formation or recrystallization in excess of the critical roughening temperature of about 50 C; and the presence of saddle dolomite suggests temperatures of formation in excess of about 80 C All burial (subsurface) models for dolomitization essentially are hydrological models They differ mainly in the hydrological drives and direction(s) of fluid flow Four main types of fluid flow take place in subsurface diagenetic settings: (1) compaction flow; (2) thermal convection; (3) topography driven flow; and (4) tectonically driven flow Combinations of these flow regimes and associated fluid compositions are possible under certain circumstances The oldest burial model of dolomitization is the compaction model, according to which seawater and/or its subsurface derivative(s), that were buried along with the sediments, are pumped through the rocks at several tens to several hundreds of metres as a result of compaction dewatering The compaction model in its original form was never especially popular because burial compaction can only generate fairly limited amounts of dolostone due to the limited amounts of expelled water However, despite this mass balance constraint, the compaction model remains a viable alternative for burial/ subsurface dolomitization where funnelling of the compaction waters is/was possible Thermal convection is driven by the temperature gradient prevailing across sedimentary strata, which is vertical in most geological situations, except in cases of vigorous advection, igneous intrusions, or in the proximity to plate boundaries and/or orogenic fronts, all of which can ‘distort’ the normal subvertical temperature gradient Where the temperature gradient and average rock permeability are high enough, convection cells may become established In principle, there are two types of convection, i.e., open and closed, although mixed cases are possible Open convection cells (also called half-cells) may form in carbonate platforms that are open to seawater recharge and discharge laterally and at the top, respectively Numerical modelling has shown that the magnitude and distribution of permeability are the most important parameters governing flow and dolomitization, and that this type of convection can be active to a depth of about 2–3 km, provided that the sequence does not contain effective aquitards, such as (overpressured) shales or evaporites The amounts of dolomite that can be formed are theoretically very large, i.e., dolomite can be formed as long as convection is sustained, because Mg is constantly (re-)supplied from the surrounding seawater However, even at a moderate width of only 40 km, complete dolomitization in a km thick sequence takes about 30–60 million years, which is much longer than the time during which most carbonate platforms remain laterally open to seawater recharge Hence, most carbonate platforms, even if subjected to thermal convection by seawater, would at best become only partially dolomitized during the time that they were open to seawater recharge Closed convection can occur, in principle, in any sedimentary basin over tens to hundreds of metres of thickness, provided that the temperature gradient is high enough relative to the permeability of the strata As a rule of thumb, however, such convection cells will only be established, and capable of dolomitizing a carbonate sequence of interest, if a sequence is highly permeable and not interbedded with aquitards Such conditions are rarely fulfilled in typical sedimentary basins, most of which contain effective aquitards Furthermore, even if closed thermal convection cells are established, the amounts of dolomite that can be formed are limited by the pre-convection Mg content of the fluids, even more so than in the case of compaction flow, as no new Mg is supplied to the system and ‘compaction funnelling’ is not possible Therefore, extensive, pervasive dolomitization by closed cell thermal convection is highly unlikely Convection cells invariably have rising limbs that penetrate the overlying and cooler strata, linking thermal convection to hydrothermal dolomitization There are well-documented examples of hydrothermal dolomite on a local and regional scale Most cases of hydrothermal dolomitization are rather small and restricted to the vicinity of faults and fractures and/or localized heat sources One striking case of this type is the Pb–Zn mineralized Navan dolomite plume in Ireland, and another is a dolomitized plume in the Latemar in the Italian Alps There are also some cases of larger scale, even regionally extensive hydrothermal dolomitization, such as the Middle Devonian Presqu’ile barrier, which forms an aquifer in northwestern Canada that contains abundant saddle dolomite as a replacement and as a cement, including MVT-type mineralization near the discharge area at Pine Point Texturally, most true hydrothermal dolomite is saddle dolomite Topography driven flow takes place in all uplifted sedimentary basins that are exposed to meteoric recharge With time, topography can drive enormous quantities of meteoric water through a basin, often concentrated by water–rock interaction (especially salt dissolution), and preferentially funnelled through aquifers Volumetrically significant dolomitization can only take place, however, where the meteoric water dissolves enough Mg en route before encountering limestones This does not appear to be common