Fluid Inclusion Techniques of Analysis
Few areas of geochemistry have challenged the ingenuity and patience of researchers as much as the analysis of fluid inclusions (see reviews by Roedder, 1972, 1984, 1990; Hollister, 1981; Shepherd et al., 1985; Boiron and Dubessy, 1994). From simple optical techniques to the use of particle accelerators, no stone has been left unturned in the search for analytical perfection. Progress has been painfully slow and, by comparison with methods for the analysis of rocks and minerals, we are still in the rudimentary stages of development. The last five years, however, have seen a quantum leap in progress, largely as a result of rapid advances in microbeam technology that have established new standards in sensitivity, precision, and accuracy. Though rooted historically in the study of ore deposits, many of the recent breakthroughs have been pioneered by analysts in the petroleum and materials science industries. in the wider field of chemistry, techniques tend to fall into two categories: those for organic and those for inorganic constituents. The situation is very similar with regard to elemental and isotope analysis. This has tended to polarize studies of the composition of fluid inclusions much to the detriment of all concerned, in particular those geologists concerned with the genesis of low-temperature, sediment-hosted hydrothermal deposits where organic material plays an important role in the distribution of ore minerals. Fortunately, technology transfer is now resulting in hybrid instruments that offer multiple capabilities and should lead to a substantial broadening of opportunities and applications.
Figures & Tables
Anyone studying an ore deposit winds up with a lot of data: field observations in the form of maps, sections and drill logs, chemical analyses, isotope analyses, fluid inclusion data, paragenetic relations, and so on. In addition, there is a vast amount experimental data in the literature on systems relevant to the deposit being studied, in the form of data on the chemical and physical properties of solids and fluids. The investigator then tries to come up with a model of ore formation that is consistent with all these data. Naturally, the model must also be consistent with accepted principles of chemistry and physics, and one of the subjects most useful, in fact essential, in assembling all these data into consistent models is thermodynamics.
The purpose of this chapter is to introduce the concepts and terms of chemical thermodynamics that are useful in constructing models of hydrothermal systems. These will be used extensively in the chapters to follow. The concepts covered in this chapter normally occupy a complete book; the coverage is therefore necessarily brief. We can save considerable space, for example, by assuming that we are all familiar with the concepts of energy, work, heat, and temperature. These are in fact quite difficult subjects, but an intuitive understanding is usually sufficient for us.
Some Basic Definitions
In this chapter we will not describe any natural system, and only one model of a simple natural system (H2-N2). Most of the discussion will be about a model of energy relationships, called thermodynamics. Although natural systems and thermodynamic models of natural systems are described using many of the same terms, there are some subtle differences. To begin with, a natural system is any part of the universe we choose to consider, such as the contents of a beaker, a crystal of quartz, the solar system, or a bacterium. Thermodynamic systems, on the other hand, are not real but conceptual and mathematical, and are of three types. The three types are used to distinguish between the ways that changes in composition and energy content can be effected, and therefore they are defined basically by the nature of their boundaries.
Isolated systems can exchange neither matter nor energy with their surroundings. They are therefore described as having walls that are rigid (preventing any change in volume and hence any energy change due to work), and impermeable to matter and energy.