Do we actually understand geologic processes? New technology brings new information and perceptions, which sometimes overturn imaginations based on simple observation and estimation, in conjunction with common sense inference. In 1902– 1904, Pierre Curie and Ernest Rutherford first formulated the idea of using radioactive transformation of nuclides as a geologic chronometer. After a century of working with such tools, geology has advanced from a descriptive science to an analytic science that formulates conclusions based on exact values. The technology of radiogenic isotope geology has created a branch of science that considers the Earth as a planet generated within a Solar system and studies the subsequent evolution of geologic processes that has resulted in the present formation of our planet’s continents and oceans.<...>

The rare earth elements, lanthanum to lutetium (atomic numbers 57—71), are members of Group IIIA in the periodic table (Fig. 1.1) and all have very similar chemical and physical properties. This uniformity arises from the nature of their electronic configurations, leading to a particularly stable 3+ oxidation state and a small but steady decrease in ionic radius with increase in atomic number for a given co-ordination number. Despite the similarity in their chemical behaviour, these elements can be partially fractionated, one from the other, by several petrological and mineralogical processes. The wide variety of types and sizes of the cation co-ordination polyhedra in rock-forming minerals provides the means for this chemical fractionation: it is this phenomenon which has important consequences in geochemistry <...>

Geochemists recognize the utility of the rare earth elements (REE) as especially powerful tools for tracing geochemical processes within the earth. Interest in the REEs reflects, in large part, their unique and chemically coherent properties that arise as a result of their trivalent charge and the similar ionic radii of all 14 naturally occurring rare earths. For example, owing to their trivalent charge, the REEs are chemically fractionated from divalent Ba and tetravalent Hf, their nearest neighbors in the Periodic Table of the Elements.

Garnets occur in more than 30 natural mineral parageneses forming the rocks of magmatic, metamorphoric and metasomatic origin, which suggests their crystallization in a wide range of physicochemical parameters. By the ratio of main components (Ca, Mg, Al, Fe, Cr, Mn), garnets are divided into two major groups: almandine (pyrope, almandine, and spessartite) and andradite (grossular, andradite, and uvarovite). Pyropes are contained mainly in high-temperature peridotites and eclogites from deep xenoliths carried by kimberlite and alkaline-basaltic melts. In high-temperature and mesobaric metamorphic complexes (eclogites, granulites, gneisses, and schist), as well as in metasomatic rocks (skarns) garnets are represented by the varieties of almandine-grossular-pyrope series. When systematizing garnets by chemical compositions and parageneses in which they occur, normally different binary diagrams are used, including the diagrams in CaO–Cr2O3 coordinates [Sobolev, 1964; Sobolev et al., 1973]. <...>