86 SEDIMENTARY ROCKS/Dolomites Figure Outcrop photograph of Upper Carboniferous carbon ates from the southwestern Cantabrian Zone, Spain: high temperature dolomitization of limestones The dolostone appears dark where covered with lichen (upper right corner), yet light beige where cleaned of lichen (centre) The lime stone (left) has a medium grey colour Note the sharp contacts between limestones and dolostones, and that sedimentary and diagenetic textures visible in the limestones are obliterated in the dolostones Hammer for scale or whether thermochemical sulphate reduction contributed carbon to the carbonates Also, there is a secular carbon isotope trend that may be used in the dating of marine dolostones, but only under very favourable circumstances Radiogenic isotopes are less commonly used in studies of carbonate diagenesis, mainly because they are analytically much more expensive Yet, strontium isotopic compositions (usually quoted as 87Sr/86Sr ratios) are an excellent parameter to deduce compositional changes and, especially, flow directions of the fluids from which the diagenetic carbonates have formed This is because strontium isotopes, as opposed to the more commonly used stable isotopes of oxygen and carbon, are not fractionated by pressure, temperature, and (as in the case of carbon) microbial processes Figure 10 Outcrop photograph of Upper Carboniferous car bonates from the southwestern Cantabrian Zone, Spain: high temperature dolomitization of limestones The dolostone is light coloured and at the bottom Note the sharp contacts between limestones and dolostones, and that sedimentary and diage netic textures visible in the limestones are obliterated in the dolostones Hammer for scale The direction of fluid flow can also be determined using trace elements, which is especially attractive because trace element analysis is the cheapest of all the common geochemical methods Trace element trends have been documented in several Phanerozoic dolostone sequences For all practical applications, i.e., the determination of fluid composition and/or fluid flow direction, the absence, presence, and/or degree of recrystallization is important If changes via recrystallization in texture, structure, composition, and/or palaeomagnetic properties are so small that the total data range after recrystallization is the same as when the dolomite first formed, a dolomite/dolostone is said to be ‘insignificantly recrystallized’ (Figure 14, top), and its properties are still representative of the fluid and environment of dolomitization On the other hand, if these changes result in data ranges that are larger than