Rare-metal enrichment in carbonatite-bearing magmatic systems:Part B. Understanding the role of fractional crystallization and liquid immiscibility by experimental simulations of silicate-carbonatite systems

碳酸岩是稀土和铌的主要来源。大多数含矿碳酸岩杂岩与硅质不饱和碱性火成岩密切相关，并被认为是通过晶体分馏从地幔成因的碳酸盐玄武质-霞石质岩浆中分化出来的。这种分馏过程，特别是硅酸盐和碳酸盐熔体之间的不可渗透性在分化的某个阶段的作用，以及共存熔体之间的微量元素的分配是不理解的。现有的少量实验数据表明，微量元素应该分配到硅酸盐熔体，这是从自然案例研究的调查所获得的数据相比。 拟议的工作计划旨在填补我们知识中的这些空白。实验将在高温和高压下进行，以了解在何种条件下碳酸化霞岩岩浆的结晶导致不混溶熔体的形成，以及硅酸盐-碳酸盐不混溶性是否可以绕过不混溶性，最终导致方解石和其他碳酸盐矿物的直接结晶。压力条件将从1.5到0.1 GPa不等，使我们能够模拟岩浆从地幔上升到近地表条件的过程。结晶实验的产物将用于限制微量元素在硅酸盐熔体、碳酸盐熔体和矿物相（例如，单斜辉石、钙钛矿、磷灰石）。也将进行试点实验，以了解额外的配体（F-，Cl-，PO 43-和SO 42-）的影响，硅酸盐和碳酸盐液体之间的不可渗透性差距，并确定和表征可能的交代反应之间的残留natrocarbonatitic熔体和矿物结晶的霞石岩浆在早期分化阶段。 将选择实验的起始材料，以便将结果应用于自然案例研究，特别是科拉碱性省，这是碳酸岩和相关矿床的典型和暴露良好的案例。

Carbonatites constitute the main source of REE and Nb. Most ore-bearing carbonatitic complexes are closely related to silica-undersaturated alkaline igneous rocks and are considered to differentiate from a parental carbonated basanitic-nephelinitic magma of mantle origin by crystal fractionation. This fractionation process, and in particular the role of immiscibility between a silicate and a carbonate melt at some stage of the differentiation as well as the partitioning of trace elements between co-existing melts is not understood. The few available experimental data indicate that trace element should partition to the silicate melt, which is contrasting with the data obtained from the investigation of natural case studies. The proposed work program is designed to fill these gaps in our knowledge. Experiments will be conducted at high temperatures and pressures to understand at which conditions crystallization of carbonated nephelinitic magma leads to the formation of immiscible melts and if silicate-carbonate immiscibility can bypass immiscibility, ending up with the direct crystallization of calcite and other carbonate minerals. Pressure conditions will vary from 1,5 to 0.1 GPa, allowing us to simulate processes in magmas ascending from mantle to near-surface condition. The products from crystallization experiments will be used to constrain the trace element partitioning between silicate melt, carbonate melt and mineral phases (e.g., clinopyroxene, perovskite, apatite). Pilot experiments will also be conducted to understand the effects of additional ligands (F-, Cl-, PO43- and SO42-) on the immiscibility gap between silicate and carbonate liquids and to identify and characterize possible metasomatic reactions between residual natrocarbonatitic melt and minerals crystallized from nephelinitic magmas in early differentiation stages . The starting material for the experiments will be selected so that the results can be applied to natural case studies, especially the Kola Alkaline Province, representing a classical and well-exposed case of carbonatites and related deposits.