No abstract

No abstract

A combination of Raman spectrometry and microthermometry has been applied to synthetic fluid inclusions filled with pure H 2 O, a NaCl brine and a MgCl 2 brine, in order to analyze spectra between -190 and +100 C. The combined technique allows:(1) the determination of the types of dissolved salts from the presence of salt hydrates at low temperatures, and (2) an accurate estimate of true temperatures of melting, even of phases that are difficult to observe within fluid inclusions. Raman spectra of water, brines, ice, glass and salt hydrates were analyzed by combined Gaussian-Lorentzian fitting of components. These fits illustrate the presence of singularities in the water spectra, around -35 C in a NaCl brine and around -30 C in a MgCl 2 brine. During freezing experiments, inclusions may contain different configurations of phases at the same temperature. Rapid freezing of a MgCl 2 brine inhibits the formation of a MgCl 2 hydrate, and in such inclusions, ice and supersaturated brine are present down to -190 C. The phase MgCl 2 •12H 2 O forms only during slow cooling. Temperatures of phase changes, including eutectic point and final melting, were accurately determined by changes in measured Raman spectra of fluid inclusions. The variable freezing behavior of the same fluid inclusion, depending on cooling rates and cycling procedures, indicates the care with which natural fluid inclusions should be treated to obtain true salinities.

No abstract

No 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 MgCl 2 ?12H 2 O, were detected by cryo-Raman spectroscopy, whereas only hydrohalite appeared in the calcite primary inclusions. The presence of CaCl 2 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 6 30uC and 40 6 10 MPa, corresponding to a depth of 3.9 6 1.0 km. The first calcite formed at 130 6 20uC and 35 6 5 MPa, corresponding to a depth of 3.4 6 0.6 km. The minimum T-P of formation is defined by the homogenization conditions. CHARACTERIZATION OF DOLOMITIZING FLUIDS 1307 J S R

No abstract

No abstract