Silicate liquid immiscibility and the formation of Fe (±Ti ±P ±F ±REE) ore deposits

两种硅酸盐熔体（铁玄武岩-流纹岩）的不混溶在分异和成矿过程中的作用几十年来一直争论不休。在岩石中很少记录到两种硅酸盐熔体在自然体系中分离的明确证据，因为在冷却和凝固过程中不混溶的瞬态性质。因此，实验方法对于测试岩浆和相关成矿过程中熔体不混溶的重要性至关重要。在本研究中，我们建议使用两种方法来理解硅酸盐液体不混溶性在Fe（±Ti±P±F±REE）矿床形成中的作用：(1)我们将研究一些主要铁矿的磷灰石中的熔体包裹体，这将为我们提供平衡液体成分的直接约束；(2)我们将采用详细的实验方法来了解不混相发生的条件（岩浆成分、温度、压力、氧逸度和流体含量），以及成对熔体之间主要元素的分配。熔体不混溶的作用将在三个著名的自然例子中进行测试，它们都含有富铁和富磷组合，涵盖了广泛的组成范围：拉斑玄武岩系统（Bushveld杂岩），铁/高钛玄武岩系统（峨眉山大火成岩省）和kirhna型系统（vergeneg矿床）。在选定的实验中，我们还将研究不混相对富铁和贫铁硅酸盐熔体中微量元素分馏的影响。这些结果将提供重要的数据，使我们能够定量地讨论熔体不混相在玄武岩体系分异过程中的作用，在一些经济上重要的铁矿床的形成以及微量元素（特别是REE）的富集机制。

The role of immiscibility between two silicate melts (ferrobasalt-rhyolite) on differentiation and ore-forming processes is under debate since decades. Clear evidence for the separation of two silicate melts in natural systems is rarely recorded in rocks because of the transient nature of immiscibility during cooling and solidification. Thus, experimental approaches are crucial to test the importance of melt immiscibility in magmatic and associated ore-forming processes. In this study, we propose to use a twofold approach to understand the role of silicate liquid immiscibility in the formation of Fe (±Ti ±P ±F ±REE) ore deposits: (1) we will study melt inclusions trapped in apatite of some major iron deposits which will give us direct constraints on the equilibrium liquid compositions and (2) we will perform a detailed experimental approach to understand the conditions for the onset of immiscibility (magma compositions, temperature, pressure, oxygen fugacity and fluid content), and the partitioning of major elements between paired melts. The role of melt immiscibility will be tested for three well known natural examples, all containing Fe- and P-rich associations and covering a broad compositional range: tholeiitic basaltic system (Bushveld complex), ferropicritic/high-Ti basaltic system (Emeishan large igneous province), and Kiruna-type systems (Vergenoeg deposit). In selected experiments, we will also study the effect of immiscibility on trace elements fractionation between Fe-rich and Fe-poor silicate melts. The results will provide crucial data allowing us to discuss quantitatively the role of melt immiscibility on differentiation processes in basaltic systems, on the formation of some economically important Fe ore deposits and on enrichment mechanisms of trace elements (especially REE).