Rare earth element (REE) enrichments in carbonatites are often described as resulting from late magmatic-hydrothermal or supergene processes. However, magmatic pre-enrichment linked to the igneous processes at the origin of carbonatites are likely to contribute to the REE fertilisation. Experimental constraints reveals that immiscibility processes between carbonate and silicate melts can lead to both REE enrichments and depletions in carbonatites making the magmatic processes controlling REE enrichments unclear. We link REE contents of carbonatites to the magmatic stage at which carbonatites are separated from silicate magma in their course of differentiation. We present results of experiments made at pressure and temperature conditions of alkaline magmas and associated carbonatites differentiation (0.2 to 1.5 GPa; 725 to 975 °C; FMQ to FMQ +2.5), simultaneously addressing crystal fractionation of alkaline magmas and immiscibility between carbonate (calcio-carbonate type) and silicate melts (nephelinite to phonolite type). The experimental data shows that the degree of differentiation, controlling the chemical composition of alkaline melts, is a key factor ruling the REE concentration of the coexisting immiscible carbonate melts. In order to predict carbonate melt REE enrichments during alkaline magma differentiation, we performed a parameterisation of experimental data on immiscible silicate and carbonate melts, based exclusively on the silica content, the alumina saturation index and the alkali/alkaline-earth elements ratio of silicate melts. This parameterisation is applied to more than 1600 geochemical data of silicate magmas from various alkaline provinces (East African Rift, Canary and Cape Verde Islands) and show that REE concentrations of their potential coeval carbonatite melts can reach concentration ranges similar to those of highly REE enriched carbonatites (∑REE > 30 000 ppm) by immiscibility with phonolitic/phono-trachytic melt compositions, while more primitive alkaline magmas can only be immiscible with carbonatites that are not significantly enriched in REE.

中文翻译：

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碱性岩浆分异过程中的一个窗口，有利于不混溶的富稀土碳酸岩

碳酸岩中的稀土元素 (REE) 富集通常被描述为晚期岩浆热液或表生过程的结果。然而，与碳酸岩起源处的火成岩过程相关的岩浆预富集可能有助于稀土元素的肥化。实验限制表明，碳酸盐和硅酸盐熔体之间的不混溶过程会导致碳酸岩中稀土元素的富集和枯竭，使得控制稀土元素富集的岩浆过程变得不清楚。我们将碳酸岩的稀土元素含量与碳酸岩在分化过程中与硅酸盐岩浆分离的岩浆阶段联系起来。我们展示了在碱性岩浆和相关碳酸盐岩分化（0.2 至 1.5 GPa；725 至 975 °C；FMQ 至 FMQ +2.5）的压力和温度条件下进行的实验结果，同时解决碱性岩浆的晶体分馏以及碳酸盐（碳酸钙型）和硅酸盐熔体（霞石到响辉石型）之间的不混溶性。实验数据表明，控制碱性熔体化学成分的分化程度是决定共存不混溶碳酸盐熔体稀土元素浓度的关键因素。为了预测碱性岩浆分异过程中碳酸盐熔体 REE 的富集，我们对不混溶的硅酸盐和碳酸盐熔体的实验数据进行了参数化，完全基于硅酸盐熔体的二氧化硅含量、氧化铝饱和指数和碱/碱土元素比. 这种参数化应用于来自不同碱性省份（东非裂谷、