HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS

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HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS ( handbook-onphysics-and-chemistry-rare-earths )

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150 Handbook on the Physics and Chemistry of Rare Earths 15 20 25 30 50 75 100 900 Temperature ( C) 1000 1100 1200 Amp lherzolite + CO2 Dol-amp-gnt lherzolite Amp lherzolite + carbonatite melt Amp Sp lherzolite + Ol-nephellinitic melt Sp Grt Grt lherzolite + Ol-melilititic melt FIG. 13 enrichment in carbonatites (Table 4 and Fig. 14), because the partition coeffi- cients between carbonatite melt and garnet are low (<0.3) for LREEs but high (0.3–10) for HREEs based on the melting experiment of garnet lherzolite (Dasgupta et al., 2009). Carbonatites are known to contain the highest amounts of REEs ($500 to >10,000 ppm; Table 4) of any igneous rocks, although only a few carbonatites attain minable level of REE concentration (>4 wt.%). Carbonatites are classified into calcite-carbonatite, dolomite-carbonatite, ferro-carbonatite, and natro-carbonatite, based on the most abundant carbonate minerals (Woolley and Kempe, 1989). In these carbonatites, REEs reside mainly in the Ca-bearing phases, ie, carbonates, apatite, Ca-Nb-oxides, and Ca-silicates, where they substitute with Sr2+ for Ca2+. Specific REE minerals such as ancylite, bastnäsite, britholite, monazite, parisite, etc., are either rare or volumetrically insignificant accessory phases (Hornig-Kjarsgaard, 1998). Carbonatites are gener- ally show systematic enrichment form Lu to La with some exceptions that show Ce-negative anomalies. It is evident that the concentration and relative enrich- ment of the REEs in most carbonatites is controlled by carbonate phases and apa- tite because of their high modal amounts and moderate REE abundances. Rare earth rich accessories such as perovskite and pyrochlore influence the bulk-rock REE pattern only if they occur in sufficient amounts (Hornig-Kjarsgaard, 1998). Kimberlite is a complex group of hybrid volatile-rich (dominantly CO2), potassic, or ultrapotassic rocks with a fine-grained matrix and macrocrystals Phase relationships in metasomatized lherzolitic mantle modified from Wallace and Green (1988) and Chakhmouradian (2006). Gray area shows where carbonatite magmas are in equilibrium with amphibole lherzolite. Broken line separates the stability fields of spinel and garnet. Solid thin lines show the stability fields of other minerals. Gray line shows amphibole stability field. Abbreviations: Amp: amphibole, Cpx: clinopyroxene, Dol: dolomite, Grt: garnet, Mgn: magnetite, Ol: olivine, Opx: orthopyroxene, Sp: spinel. Solidus Ol + Cpx + CO 2 Dol + Opx P (kbar) Depth (km) Dol + Opx Mgn + Cpx

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