Chlorine in amphibole: intracrystalline diffusion and partitioning between amphibole and silicate melt

氯（Cl）是地球和其他行星（如火星）内外层中含量丰富的挥发性元素，在温度和压力范围内的各种地质和环境过程中起着重要作用。在热液系统中，Cl浓度可以使用流体包裹体来追踪。然而，作为一种挥发性元素，Cl通常从岩浆系统中部分释放（随流体），只有很少的相可用作天然岩浆系统中Cl浓度的示踪剂。角闪石是一种常见的容卤矿物，广泛存在于岩石体系和地质条件中，是氯演化的重要示踪剂。然而，由于复杂的结构和化学的角闪石和大的组成变化的熔体，可以与角闪石共存，氯之间的分配角闪石和熔体仍然知之甚少。本文通过一系列实验研究了氯在角闪石和硅酸盐熔体（KCl）之间的分配和氯在角闪石（DCl）中的扩散速率。对于玄武质到流纹英安岩系统，将在角闪石稳定的大压力和温度范围内（100-700 MPa和750-1000 °C）研究Cl在角闪石和硅酸盐熔体之间的分配。初步研究表明，自然系统的整体组成可能会强烈影响Cl的分配。使用12种不同的起始材料，我们计划限制控制Cl的分配（角闪石的晶体化学和熔体组成）的主要组成参数。Cl分配的实验将通过具有挑战性的实验来补充，旨在了解这种元素在角闪石中的扩散性。以大的角闪石晶体为起始原料，实验设计为在没有其它阳离子交换的情况下实现角闪石与熔体之间的Cl/OH交换。这项研究的结果对火山系统特别感兴趣，但有可能应用于地球和其他行星上的岩浆过程。 Cl在角闪石和熔体之间的分配对于根据角闪石成分准确估计岩浆中Cl含量和定量解释岩浆系统中发生的过程至关重要。氯在角闪石中的扩散率是利用角闪石中氯浓度剖面提取岩浆体系地质作用时间尺度的关键。

Chlorine (Cl) is an abundant volatile element in the interior and outer layers of the Earth and other planets (e.g. Mars), which plays important roles in a variety of geological and environmental processes within a wide range of temperature and pressure. In hydrothermal systems, Cl concentrations can be traced using fluid inclusions. However, as a volatile element, Cl is usually partly released (with fluids) from magmatic systems and there are only little phases which can be used as tracer for Cl concentrations in natural magmatic systems. Amphibole is a common halogen-hosting mineral which exists in a large range of rock systems and of geological conditions, and which can serve as an important tracer of Cl evolution. However, because of the complex structure and chemistry of amphiboles and of the large compositional variation of melt which can coexist with amphibole, the partitioning of Cl between amphibole and melt is still poorly understood. Two key issues will be addressed in this proposal with series of experiments aimed at constraining (1) the partitioning of Cl between amphibole and silicate melt (KCl), and (2) the diffusion rate of Cl in amphibole (DCl). The partitioning of Cl between amphibole and silicate melt will be investigated in a large pressure and temperature range within the stability of amphibole (100-700 MPa and 750-1000 °C) for basaltic to rhyodacitic systems. Preliminary studies show that the bulk composition of natural systems may influence strongly the partitioning of Cl. Using 12 different starting materials, we plan to constrain the main compositional parameters controlling the partitioning of Cl (crystal chemistry of amphibole and melt composition). The experiments on Cl partitioning will be complemented by challenging experiments aimed at understanding the diffusivity of this element in amphibole. Large amphibole crystals will be used as starting materials and the experiments will be designed to achieve Cl/OH exchange between amphibole and melt without other cation exchange. The results of this study are of particular interest for volcanic systems, but have the potential to be applied to magmatic processes on Earth and other planets. The partitioning of Cl between amphibole and melt is crucial to estimate accurately Cl contents in magmas based on amphibole composition and to interpret quantitatively processes occurring in magmatic systems. Chlorine diffusivity in amphibole is crucially needed to extract time scales of geological processes in magmatic systems using Cl concentration profiles in amphiboles.