Abstract

A late diagenetic dolomitization pervasively affected Carboniferous carbonates of the Variscan Cantabrian Zone (NW Spain). The process generated replacive and void-filling dolomite phases, spatially related with various calcite cements. The nature of the diagenetic fluids has been investigated by cryo-Raman spectroscopy, i.e., a combination of Raman spectroscopy and low-temperature microthermometry, which reveals in great accuracy the salinity and the major types of dissolved cations and anions in single fluid inclusions. Fluid properties obtained only from microthermometry are distinctively different. The study demonstrates that this improved analytical method of fluid inclusions is a valuable contribution for the interpretation of fluids in dolomite research. In primary fluid inclusions of the Cantabrian dolomites, hydrohalite and two unknown salt hydrates, one of which resembles MgCl₂·12H₂O, were detected by cryo-Raman spectroscopy, whereas only hydrohalite appeared in the calcite primary inclusions. The presence of CaCl₂ hydrates is suspected only from low eutectic temperatures. Dolomite formed from an evolving fluid, as reflected by the highly variable equivalent Na/Ca ratios calculated, at high and nearly constant total salinities. The approximately constant Na/Ca ratio in the first calcite cement reflects a homogeneous source for the salts. Cryo-Raman spectroscopy reveals that different cooling procedures may induce the formation of different phase assemblages within the same fluid inclusion in both dolomite and calcite. Consequently, fluid inclusions display different melting behaviors, corresponding to different values of calculated salinities. Salinity calculation only from microthermometry may lead to an underestimation of true salinities. According to an assumed hydrostatic geothermal gradient, maximum formation conditions for the dolomites are 150 ± 30°C and 40 ± 10 MPa, corresponding to a depth of 3.9 ± 1.0 km. The first calcite formed at 130 ± 20°C and 35 ± 5 MPa, corresponding to a depth of 3.4 ± 0.6 km. The minimum T–P of formation is defined by the homogenization conditions.