Gieré and Sorensen reported in 2004 that epidote-group minerals rich in rare earth elements (REE), in particular allanite, are common accessory phases in igneous, metamorphic, metasomatic, and sedimentary rocks. Small amounts of REE are present in most epidote-group minerals, but in allanite—and the related minerals dissakisite, ferriallanite, dollaseite, khristovite and androsite—the REE are essential structural constituents. An important characteristic of REE-rich epidote-group minerals is that their octahedrally coordinated M sites contain major amounts of divalent cations. The authors summarize literature data for these minerals and discusses their chemistry, occurrence, phase relations, and petrologic and geologic significance. The chapter emphasizes allanite, because it is the most common and best-studied of the REE-rich epidote-group minerals.
Many directions for future research are suggested by the current state of knowledge of allanite and related minerals, and by recent technological improvements of microanalysis of trace elements, in particular the REE. Existing studies have documented a large number of the most critical variables for understanding the substitutions of various elements into the allanite structure. However, the residence sites and compositional limits of three petrogenetically significant elements, namely P, Be, and V, are at present poorly understood. The V-richer allanite minerals could add to the conclusion that is suggested by Mn and Fe behavior, namely: ƒO2 and T, rather than variations in P and element concentrations appear to be the most critical variables for determining transition metal and REE ratios in the epidote-group minerals. In particular, the characterization of allanite and REE-rich epidote minerals for their full complement of trace elements will likely be greatly enhanced as SIMS and LA-ICP-MS technology becomes more widely available. Many compositional parameters discussed in this chapter are poorly understood simply because they cannot readily be analyzed by the electron microprobe. High-contrast BSE imaging is also needed to conduct a first-rate study of these minerals, because it is so common for allanite and REE-rich epidote minerals to be zoned and altered.
The ratio Fe3+/Fetot in allanite can be changed via the oxidation-dehydrogenation reaction. Experimental studies, however, were carried out under metastable conditions. The Fe3+/Fe2+ and H contents, therefore, need to be accurately measured for epidote-group minerals equilibrated within their stability fields, so that a thermodynamic model applicable to natural magmatic samples can be formulated. Such experimental data would be invaluable for a better understanding of igneous rocks, in particular rocks that contain coexisting amphibole, mica, and ferri-allananite-allanite-epidote-clinozoisite solid solutions. In examples such as the Bergell tonalite, where detailed chronological data for allanite are available, one can hope to additionally gain insight into how some petrologically important parameters of the tonalitic melt (e.g., T, P, fH2O) changed with time. Allanite, therefore, could become a powerful geothermobarometer as well as a hygrometer and chronometer for calc-alkaline systems.
In addition to such experiments, we suggest that the following topics be studied in the future:
1. Radiation damage: We anticipate that both the initial and the critical amorphization doses of allanite could be easily determined, because allanite is commonly zoned with respect to Th, and different zones thus would have received different α doses. Moreover, such studies would help in understanding the relationship between metamictization, hydration and release of REE, Th and Pb to fluids during alteration. Examples of such studies are given for other minerals.
2. Incorporation of Eu: the presence or absence of distinct Eu anomalies, both positive and negative, for different REE-rich epidote-group minerals in different environments has so far not been much discussed. This is in part because low values of Eu have been difficult to measure in situ. The use of either SIMS or LA-ICP-MS for microanalytical studies of Eu-incorporation into REE-rich epidote and allanite could reveal the behavior of this element in different petrogenetic environments.
3. Experiments are needed to determine the partition coefficients between allanite and various fluids at different pressures and temperatures. We anticipate that the composition of allanite could be used as a tool for monitoring REE compositions of fluids, once such data are available.
4. Thorium zoning in igneous allanite: it is not clear why some allanites display core-to-rim decreases in Th content, whereas others show the opposite zoning. Since allanite exerts such an important control on the behavior of Th in some igneous rocks, it would be desirable to study this feature in more detail and in various types of igneous rocks (e.g., with/without monazite).
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