No direct effect of F, Cl and P on REE partitioning between carbonate and alkaline silicate melts

This study presents new insights into the effects of halogens (F and Cl) and phosphorous (P) on rare earth element (REE) partitioning between carbonatite and alkaline silicate melts. F, Cl and P are elements that are abundant in carbonatites and alkaline magmatic systems and they are considered to play an important role on the REE behaviour. Nonetheless, their effect on REE partitioning between carbonate and alkaline silicate melts has not yet been constrained.Here we present new experimental data on REE partitioning between carbonate and alkaline silicate melts doped in F, Cl and P, in order to (1) test the Nabyl et al. [2020] REE partitioning model in F-, Cl- and P-rich systems, and (2) identify the possible role of F, Cl and P in carbonate melt REE enrichments during alkaline–carbonatite magma differentiation. The experiments were performed at 850–1050 °C and 0.8 GPa using piston-cylinder devices. Starting materials consisted of carbonatite and phonolite compositions ${\pm }$ doped in F, Cl and P. The experimental results show that REE partitioning is similar in F-Cl-P-rich and -poor systems. The silicate melt composition and its molecular structure (i.e. SiO$_{2}$ contents, the alumina saturation index and the alkali/alkaline-earth element ratio), which have already been identified as controlling REE partitioning in F-, Cl- and P-poor systems, still operate in doped systems. No direct effect of the F, Cl or P melt concentrations on REE partitioning has been identified. We also propose an application to natural systems

Experimental characterization of rare metals behavior during the differentiation of carbonatites and alkaline magma

Les carbonatites et les roches magmatiques alcalines constituent une faible fraction du magmatisme terrestre et sont essentiellement produits en contexte intraplaque. Ces magmas sont particulièrement riches en éléments volatils (dioxyde de carbone, halogènes et eau) et présentent aussi des fortes concentrations en métaux rares (REE, Hf, Zr, Nb, Ta). Les gisements associés à ces roches magmatiques sont souvent affectés par des stades hydrothermaux tardifs brouillant les relations entre les processus magmatiques à l’origine de la formation de ces magmas (immiscibilité, différenciation, cristallisation fractionnée) et cet enrichissement métaux et en constituants volatils. Des expériences haute pression et haute température ont été réalisées afin d’évaluer le comportement des métaux rares pendant la différenciation de ces magmas. Ces expériences adressent le comportement des métaux rares pendant les processus d’immiscibilité entre liquides silicatés et liquides carbonatés et pendant la cristallisation de ces magmas. Les conditions optimales d’enrichissement en REE des liquides carbonatés au cours de la différenciation des magmas alcalins sont identifiées : les liquides carbonatés les plus enrichis sont formés par immiscibilité avec des liquides silicatés très différenciés et polymérisés de type phonolite/phono-trachyte. Un modèle d’enrichissement en REE des liquides carbonatés basé sur la composition des liquides silicatés est proposé et permet d’identifier i) le potentiel en REE des carbonatites pouvant être formées par immiscibilité avec des magmas alcalins, et ii) le stade de genèse par immiscibilité des carbonatites tout au long de la différenciation alcaline. De plus, le degré de différenciation et de polymérisation des liquides silicatés joue également un rôle sur l’enrichissement en métaux rares des cristaux (clinopyroxène, grenat, titanite, calcite et apatite) : les métaux rares sont plus concentrés dans les minéraux coexistant avec des liquides silicatés plus différenciés et polymérisés. Ceci implique que les liquides silicatés alcalins tendent à s’appauvrir en REE au cours de la différenciation, en comparaison aux liquides carbonatés et aux cristaux.Carbonatites and alkaline magmatic rocks occur in intraplate context and constitute a small fraction of the earth magmatism. Those magmas are particularly enriched in volatiles (carbon dioxide, halogens, water) and also in rare metals (REE, Hf, Zr, Ta, Nb). The associated deposits are often affected by hydrothermal and supergen processes which erase any relation to the magmatic processes at the origin of these magmas (immiscibility, differentiation, fractional crystallization) and responsible of the rare metal and volatile enrichments.High pressure and high temperature experiments have been performed to characterize the behavior of rare metals during both magma differentiation. These experiments simulate the immiscibility between carbonate and alkaline silica-undersaturated melts, during the crystallization of the magma.The optimum of carbonate melt REE enrichments across alkaline magma differentiation course is identified : carbonate melts immiscible with highly differentiated and polymerised silicate melts of phonolitic/phono-trachytic compositions are the REE richest. A modelling of carbonate melts REE enrichment based on the silicate melt composition is suggested, to identify the REE potential of carbonatites which may be immiscible with an alkaline magmatic rock, or to identify at which differentiation stage the immiscibility has occurred. Moreover, the silicate melt degree of differentiation and polymerisation has also an impact on crystal rare metal enrichments : crystals which coexist with highly differentiated and polymerized silicate melts are highly enriched in rare metals. This implies that silicate melts become depleted in REE across the differentiation compare to crystals and carbonate melts

Caractérisation expérimentale du comportement des métaux rares au cours de la différenciation des carbonatites et des magmas alcalins

Carbonatites and alkaline magmatic rocks occur in intraplate context and constitute a small fraction of the earth magmatism. Those magmas are particularly enriched in volatiles (carbon dioxide, halogens, water) and also in rare metals (REE, Hf, Zr, Ta, Nb). The associated deposits are often affected by hydrothermal and supergen processes which erase any relation to the magmatic processes at the origin of these magmas (immiscibility, differentiation, fractional crystallization) and responsible of the rare metal and volatile enrichments.High pressure and high temperature experiments have been performed to characterize the behavior of rare metals during both magma differentiation. These experiments simulate the immiscibility between carbonate and alkaline silica-undersaturated melts, during the crystallization of the magma.The optimum of carbonate melt REE enrichments across alkaline magma differentiation course is identified : carbonate melts immiscible with highly differentiated and polymerised silicate melts of phonolitic/phono-trachytic compositions are the REE richest. A modelling of carbonate melts REE enrichment based on the silicate melt composition is suggested, to identify the REE potential of carbonatites which may be immiscible with an alkaline magmatic rock, or to identify at which differentiation stage the immiscibility has occurred. Moreover, the silicate melt degree of differentiation and polymerisation has also an impact on crystal rare metal enrichments : crystals which coexist with highly differentiated and polymerized silicate melts are highly enriched in rare metals. This implies that silicate melts become depleted in REE across the differentiation compare to crystals and carbonate melts.Les carbonatites et les roches magmatiques alcalines constituent une faible fraction du magmatisme terrestre et sont essentiellement produits en contexte intraplaque. Ces magmas sont particulièrement riches en éléments volatils (dioxyde de carbone, halogènes et eau) et présentent aussi des fortes concentrations en métaux rares (REE, Hf, Zr, Nb, Ta). Les gisements associés à ces roches magmatiques sont souvent affectés par des stades hydrothermaux tardifs brouillant les relations entre les processus magmatiques à l’origine de la formation de ces magmas (immiscibilité, différenciation, cristallisation fractionnée) et cet enrichissement métaux et en constituants volatils. Des expériences haute pression et haute température ont été réalisées afin d’évaluer le comportement des métaux rares pendant la différenciation de ces magmas. Ces expériences adressent le comportement des métaux rares pendant les processus d’immiscibilité entre liquides silicatés et liquides carbonatés et pendant la cristallisation de ces magmas. Les conditions optimales d’enrichissement en REE des liquides carbonatés au cours de la différenciation des magmas alcalins sont identifiées : les liquides carbonatés les plus enrichis sont formés par immiscibilité avec des liquides silicatés très différenciés et polymérisés de type phonolite/phono-trachyte. Un modèle d’enrichissement en REE des liquides carbonatés basé sur la composition des liquides silicatés est proposé et permet d’identifier i) le potentiel en REE des carbonatites pouvant être formées par immiscibilité avec des magmas alcalins, et ii) le stade de genèse par immiscibilité des carbonatites tout au long de la différenciation alcaline. De plus, le degré de différenciation et de polymérisation des liquides silicatés joue également un rôle sur l’enrichissement en métaux rares des cristaux (clinopyroxène, grenat, titanite, calcite et apatite) : les métaux rares sont plus concentrés dans les minéraux coexistant avec des liquides silicatés plus différenciés et polymérisés. Ceci implique que les liquides silicatés alcalins tendent à s’appauvrir en REE au cours de la différenciation, en comparaison aux liquides carbonatés et aux cristaux

Des terres si rares ?

International audienc

Des terres si rares ?

International audienc

Des terres si rares ?

International audienc

No direct effect of F, Cl and P on REE partitioning between carbonate and alkaline silicate melts

International audienceThis study presents new insights into the effects of halogens (F and Cl) and phosphorous (P) on rare earth element (REE) partitioning between carbonatite and alkaline silicate melts. F, Cl and P are elements that are abundant in carbonatites and alkaline magmatic systems and they are considered to play an important role on the REE behaviour. Nonetheless, their effect on REE partitioning between carbonate and alkaline silicate melts has not yet been constrained.Here we present new experimental data on REE partitioning between carbonate and alkaline silicate melts doped in F, Cl and P, in order to (1) test the Nabyl et al. [2020] REE partitioning model in F-, Cl- and P-rich systems, and (2) identify the possible role of F, Cl and P in carbonate melt REE enrichments during alkaline–carbonatite magma differentiation. The experiments were performed at 850–1050 °C and 0.8 GPa using piston-cylinder devices. Starting materials consisted of carbonatite and phonolite compositions doped in F, Cl and P. The experimental results show that REE partitioning is similar in F-Cl-P-rich and -poor systems. The silicate melt composition and its molecular structure (i.e. SiO contents, the alumina saturation index and the alkali/alkaline-earth element ratio), which have already been identified as controlling REE partitioning in F-, Cl- and P-poor systems, still operate in doped systems. No direct effect of the F, Cl or P melt concentrations on REE partitioning has been identified. We also propose an application to natural systems

Crystallisation sequence of a REE-rich carbonate melt: an experimental approach

International audienceCarbonatites host Earth's main REE deposits, with bastnaesite (LREE)CO 3 F being the main economic REE-bearing mineral. However, bastnaesite mineralisation processes are debated between hydrothermal or magmatic origin. This study aims to assess if bastnaesite can be magmatic, and to characterise the REE behaviour during carbonatite crystallisation. Crystallisation experiments have been performed from 900 to 600 °C at 1 kbar, on a REE-rich calciocarbonatitic composition. REEbearing calcite is the dominant crystallising mineral, driving the residual melt towards natrocarbonatitic compositions. Both halogens (i.e., Cl and F) and water decrease the temperature of calcite saturation. REE are slightly incompatible with calcite: for all REE, partition coefficients between carbonate melt and calcite are comprised between 1 and 11, and increase with temperature decrease. Britholite (REE, Ca) 5 ((Si,P)O 4) 3 (F,OH) crystallises at high temperatures (700-900 °C), while pyrochlore (Ca,Na,REE) 2 Nb 2 O 6 (OH,F) crystallises at low temperatures (600-700 °C), as well as REE-rich apatite (600-650 °C). No bastnaesite is found in crystallisation experiments. We thus performed a bastnaesite saturation experiment at 600 °C. The bastnaesite-saturated melt contains 20 wt% of REE: such magmatic saturation is unlikely to happen in nature. Textural evidences imply a Na, Cl, REE-rich fluid at high temperatures and hydrous conditions. We propose that fluids are the main mineralising agent for bastnaesite at hydrothermal stage (<600 °C)