Fluid inclusions and microstructures

Paleofluids (fluid inclusions) are an accompanying research topic in geoscience and in studies related with deep geothermal energy. Studied are fluids of different origin, preserved in various fluid inclusion assemblages, as well as the possible relations with free (sub) recent fluids. In addition, we may reveal information about the chemical evolution of geological fluids, the changes of geothermal gradients and uplift paths during the geological history.

Fluid compositions can be obtained by microthermometry (MT), Laser-Raman-micro-spectrometry (LRM) and REM/EDX. Microthermometry is a basic method for studying fluid inclusions. Based on the microthermometry results, samples are selected for further analysis. Raman analysis allows the detection of the most important components in fluid inclusions like gases (e.g. CO2, CH4, N2, H2), as well as anions in solution (e.g. SO42-), solids, e.g., graphite, and daughter crystals. By means of REM/EDX the crystallized content (decrepitates) of opened fluid inclusions can be analyzed. Fluid inclusion studies allow the interpretation of pressure and temperature as well as the geochemical conditions during fluid entrapment.



Fig. 1 : High-salinity fluid inclusions (FI) in carbonate cement (cc). Rotliegend quartz sandstone.

Microthermometry implies the observation and recording of phase transitions in fluid inclusions at varying temperature. Fluid compositions and densities of the inclusions can be determined from (Tm) and homogenization (Th) temperatures. Fluid pressure can be calculated from an equation of state for the appropriate fluid system. The isochore is the graphical presentation of the pressure changes with temperature (Fig. 2). The intersection of the isochore and geothermal gradient defines the forming conditions (Tt, Pt) of the fluid inclusions.


Fig. 2 (schematic): The stability fields of solid (S), liquid (L) and gas phases for a simple aqueous system. After freezing, the fluid inclusions are heated, and show successively the melting of ice (Tm) and the homogenization (Th) of the fluid and liquid phases. In the one-phase stability field, pressure and temperature are defined by the isochore and crosscut the geothermal gradient (formation conditions). The gradient may represent lithostatic or hydrostatic conditions

Application of cathodoluminescence microscopy
By means of optical cathodoluminescence microscopy (O-CL) the genetic relations between fluid inclusions and host crystal can be visualized. On the one hand quartz and carbonate generations with different fluid inclusion inventory can be differentiated. On the other hand, microstructures may indicate secondary modifications of fluid inclusions and paleo-fluid migration paths. These structures are the result of the forming of authigenic quartz or carbonate during uplift. Electron microprobe (SEM-CL) imaging allows the visualization of finest structures, as well as the combined analysis of trace elements. In sediments, cement generations (evtl. with fluid inclusions) can be distinguished and the sequence of diagenetic processes explained.

Fig. 3 : Example of fluid-induced structures in quartz (drilling Bad Urach, basement rocks). The left image shows the quartz in crossed-polarized light, the right image shows the same area in CL (image height = ca. 2 mm). Paleo-fluid-migration paths can be recognized by the forming of secondary quartz, which developed here from open vugs into microfractures.

Literature:
Van den Kerkhof A.M. and Hein U.F. (2001) Fluid inclusion petrography. Lithos 55, 27-47.
Van den Kerkhof A.M. (2003) Advancements of cathodoluminescence microscopy and related techniques with application to the study of fluid-rock interaction. Göttinger Arb. Geol. Paläont., Sb5, 111-120.