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REE Mineralogy and Resources Chapter 279 155 A 4 km B m 8 7 6 5 4 3 2 1 Alkali feldspar Phase III eudialyte-bearing nepheline syenite Phase II layered nepheline syenite Phase I nepheline syenite 00 20 40 60 80 100 Modal abundance (vol %) FIG. 15 Simplified geological map of the Lovozero peralkaline complex (A) and constituent mineral abundance of a unit in the Phase II layered nepheline syenite (B). After Femenias, O., Coussaert, N., Brassinnes, S., Demaiffe, D., 2005. Emplacement processes and cooling history of layered cyclic unit II-7 from the Lovozero alkaline massif (Kola Peninsula, Russia). Lithos 83, 371–393. 2.4.2 Immiscibility There is a long-standing debate about whether carbonatite magmas are pro- ducts of liquid immiscibility or are derived directly from melting of the mantle. However, many recent experimental studies have demonstrated that carbonatite melts can form from carbonated lherzolite at moderate or high pressures. Although several studies suggest that carbonatite melts form by immiscibility from silicate melts (eg, Demeny et al., 2004; Halama et al., 2005; Krasnova et al., 2004), most of the carbonatites are considered to be a product of direct melting of lithospheric mantle composed of garnet lherzo- lite that is metasomatized by volatile-rich melts/fluids of a sublithospheric source (Bell and Simonetti, 2010). 2.4.3 Fractional Crystallization Because of incompatible characteristics of REEs, fractional crystallization may increase concentration of REEs in the melts which form late-stage REE-enriched rocks (eg, granites). However, the process of REE enrichment is not simple, because REE distribution is controlled by a variety of REE-bearing rock- forming minerals. Carbonatites, the major source of REE resources, are believed to be enriched in REEs during the fractional crystallization. When carbonatites crystallize, REE-bearing phosphate minerals, including apatite and monazite, precipitate earlier than REE-bearing carbonate (calcite) and REE carbonate Nepheline Apatite and loparite Distance from the contact with the lower lujavrite Aegirine ArfvedsonitePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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