Low temperature behaviour of natural saline fluid inclusions in saddle dolomite (Paleozoic, NW Spain)

The origin of many dolomites is still a matter of debate because of many possible chemical and hydrological conditions of formation. Fluid inclusion studies have been applied in order to improve knowledge about paleofluids responsible for the precipitation of dolomite, and used to define temperatures and salinities. The combination of Raman Spectroscopy and microthermometry is tested here to improve the analytical method to identify the main ion species present in individual inclusions. Natural samples of saddle dolomite from the Cambrian Lancara Fm., Cantabrian Mountains (NW Spain), contain zoned crystals with two-phase aqueous fluid inclusions (liquid-rich). The most stable phase assemblage in these inclusions at) -150 degrees C consists of ice, hydrohalite and an unknown salt hydrate. The latter melts between) -47 and -41 degrees C, probably representing a eutectic temperature. Subsequently, ice melts in the range of -32.5 to -29 degrees C and, finally, hydrohalite melts between -9 and -3.5 degrees C. Salinities can be calculated in the fluid system H2O-NaCl with addition of another salt, either CaCl2 or MgCl2, and result in 7.5-10.6 eq. mass% NaCl and 17.0-21.0 eq. mass% CaCl2. Dependent on the rate of cooling runs, three different types of metastability may occur, i.e. the absence of hydrohalite, the unknown salt-hydrate is not formed, and the nucleation of only ice. Salinity calculations from those melting temperatures differ substantially from equilibrium behaviour values. The unknown salt-hydrate needs to be further specified by comparison to standard solutions. The method gives an opportunity to characterize the major compounds in complex fluid systems active during dolomitization, thus contributing to a better understanding of the 'dolomite problem'.