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

几十年来，两种硅酸盐熔体(铁玄武岩-流纹岩)之间的不混溶对分异和成矿过程的作用一直存在争议。自然系统中两种硅酸盐熔体分离的明确证据在岩石中很少被记录下来，因为在冷却和凝固过程中不混溶的瞬变性质。因此，实验方法对于检验熔体不混溶在岩浆和相关成矿过程中的重要性至关重要。在这项研究中，我们建议使用两种方法来理解硅酸盐液体不混溶在Fe(±Ti±P±F±REE)矿床形成中的作用：(1)我们将研究捕获在一些主要铁矿的磷灰石中的熔体包裹体，这将给我们提供对平衡液体成分的直接限制；(2)我们将进行详细的实验方法来了解不混溶开始的条件(岩浆成分、温度、压力、氧逸度和流体含量)，以及主要元素在成对熔体之间的分配。熔体不混溶的作用将通过三个著名的自然例子来检验，这些例子都含有富Fe和P的组合，并且覆盖了广泛的成分范围：拉斑玄武岩系统(Bushveld杂岩)，苦味铁质/高钛玄武岩系统(峨眉山大火成岩省)和基鲁纳型系统(Vergenig矿床)。在选定的实验中，我们还将研究不混溶对富铁和贫铁硅酸盐熔体之间微量元素分馏的影响。这些结果将为我们定量地讨论熔体不混溶对玄武岩体系中的分异作用、一些重要的经济铁矿的形成以及微量元素(特别是稀土)的富集性的作用提供重要的数据。

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).