Low temperature behaviour of natural saline fluid inclusions in saddle dolomite (Paleozoic, NW Spain) Fabio Lapponi, 1 Ronald J. Bakker 2 and Thilo Bechstaedt 1 1 Geological-Paleontological Institute, University of Heidelberg, INF 234, D-69120 Heidelberg, Germany; 2 Department of Applied Geosciences and Geophysics, Mineralogy and Petrology, University of Leoben, Peter-Tunner Str. 5, A-8700 Leoben, Austria Introduction One of the essential problems in understanding dolomite origin is the chemical composition of the fluids from which the dolomite precipitated. Two types of dolomite formation processes are common in diagenetic environments: cementation and replacement of carbonates. Dolomite cementation in pore space and vugs can occur directly from aqueous solu- tions of variable salinities in different diagenetic environments. Fluid inclu- sions provide the only direct evidence of the fluid responsible for dolomiti- zation. To characterize the properties of this fluid, microthermometry has been applied as a routine method in some studies concerning dolomite for- mation (see also Morrow et al., 1986; Coniglio et al., 1994; Qing and Mountjoy, 1994; Zeeh et al., 1995; Boni et al., 2000). Major element composition and salinity can be esti- mated from the eutectic temperature and the final melting temperature of ice respectively. Limitations of the optical microscope make the identifi- cation of these phase changes some- times impossible. Furthermore, it is generally difficult to be confident that all the phase assemblages, observed during the heating of a frozen inclu- sion represent an equilibrium phase configuration (see also Bakker, 2004). Anomalous behaviour of fluid inclu- sions during freezing has been observed in calcium-rich inclusion solutions in pegmatite hydrothermal systems (Kozlowsky, 1984). The crush-leach method (e.g. Roed- der, 1984, Banks and Yardley, 1992) provides additional information about the major and minor elemental solute composition of the aqueous phase in fluid inclusions. Disadvantages of this bulk analytical method, however, are the inevitable analysis of all fluid inclusion generations present in the samples, and quantitative salinity calculations still depend on micro- thermometric data. The combination of Raman spec- troscopy with microthermometry rep- resents a powerful method to estimate better the major-ion composition of individual fluid inclusions (Dubessy et al., 1982; Bakker, 2004). Most salt- hydrates that nucleate at low temper- ature in fluid inclusions (i.e. NaClÆ 2H 2 O, CaCl 2 Æ6H 2 O) can be identified with Raman spectroscopy. In this study, this method has been tested on natural inclusions in dolo- mite from the La´ncara Fm., Canta- brian Zone, NW Spain (Fig. 1). Eutectic and other phase change tem- peratures are established, in addition to the identification of metastable phase assemblages that can lead to erroneous interpretation of the salin- ity of the fluids. Geological setting and dolomite occurrences The Cantabrian Zone (Lotze, 1945) is the foreland fold-and-thrust belt of the Variscan orogen in NW Iberia (Julivert, 1971). The succession was deformed during Late Carboniferous by thin-skinned tectonics. A dolomi- tization event post-dates the emplacement of the main Variscan structures. Two main carbonate intervals of the Paleozoic sedimen- tary succession, i.e. La´ ncara Fm. (Cambrian) and Barcaliente and Valdeteja Fms. (Carboniferous, see Gasparrini, 2003) were affected by dolomitization. The dolostones in the ABSTRACT 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 La ´ ncara 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 °C consists of ice, hydrohalite and an unknown salt hydrate. The latter melts between )47 and )41 °C, probably representing a eutectic temperature. Subse- quently, ice melts in the range of )32.5 to )29 °C and, finally, hydrohalite melts between )9 and )3.5 °C. Salinities can be calculated in the fluid system H 2 O–NaCl with addition of another salt, either CaCl 2 or MgCl 2 , and result in 7.5–10.6 eq. mass% NaCl and 17.0–21.0 eq. mass% CaCl 2 . 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 substan- tially 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 Ôdolomite problemÕ. Terra Nova, 19, 440–444, 2007 Correspondence: Dr Fabio Lapponi, Geo- logical-Paleontological Institute, Univer- sity of Heidelberg, INF 234, Heidelberg 69120, Germany. Tel.: +49 6221 544891; fax: +49 6221 545503; e-mail: flapponi@ uni-hd.de 440 Ó 2007 Blackwell Publishing Ltd doi: 10.1111/j.1365-3121.2007.00769.x