Application of Radiogenic Isotope Systems to the Timing and Origin of Hydrothermal Processes
Jeremy P. Richards, Stephen R. Noble, 1998. "Application of Radiogenic Isotope Systems to the Timing and Origin of Hydrothermal Processes", Techniques in Hydrothermal Ore Deposits Geology, Jeremy P. Richards, Peter B. Larson
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The potential use of radiogenic isotopes in the study of geological problems was recognized at an early stage in the investigation of nuclear science. At the turn of the century, F. Soddy and E. Rutherford first proposed the law of radioactive decay, and in 1905, Rutherford obtained the first age estimates of uraniferous minerals by measuring their helium content. The first U-Pb chemical dates for uraninites were published two years later byB.B. Boltwood (1907). F.W. Aston's development of the mass spectrometer shortly after the end of World War I led to the confirmation that many elements consist of isotopes having different atomic mass (Aston, But it was A.O. Nier's refinements of mass spectrometer design during and after World War II that provided the technological breakthrough required for routine geochronological measurements (Nier, 1940). Subsequent instrumental developments have principally involved improvements in precision and sensitivity, with the current generation of thermal ionization multi-collector mass spectrometers (TIMS) offering rapid simultaneous measurement of several isotopes from nanogram-sized samples.
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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.