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）内层和外层丰富的挥发性元素，在广泛的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，可以利用流体包裹体追踪Cl浓度。然而，作为一种挥发性元素，Cl通常是部分地（随流体）从岩浆体系中释放出来的，在天然岩浆体系中，只有很少的相可以作为Cl浓度的示踪剂。角闪孔是一种常见的含卤素矿物，广泛存在于各种岩石体系和地质条件中，是氯演化的重要示踪物。然而，由于角闪石的复杂结构和化学性质，以及与角闪石共存的熔体成分变化较大，对角闪石与熔体之间Cl的分配仍知之甚少。本文将通过一系列实验来解决两个关键问题，旨在限制(1)氯在角闪孔和硅酸盐熔体之间的分配（KCl），以及(2)氯在角闪孔中的扩散速率（DCl）。在较大的压力和温度范围内（100-700 MPa, 750-1000℃），研究玄武岩-流纹酸体系中角闪孔和硅酸盐熔体之间Cl的分配。初步研究表明，自然系统的体积组成可能对Cl的分配有强烈的影响。使用12种不同的起始材料，我们计划约束控制Cl分配的主要成分参数（角闪孔晶体化学和熔体成分）。氯分配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洞中的扩散。大的角闪孔晶体将作为起始材料，实验将设计为在没有其他阳离子交换的情况下，实现角闪孔与熔体之间的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的岩浆过程。角闪孔和熔体之间Cl的划分对于根据角闪孔组成准确估计岩浆中Cl含量和定量解释岩浆系统中发生的过程至关重要。利用角闪洞氯离子浓度剖面提取岩浆系统地质过程的时间尺度，需要角闪洞氯离子扩散系数。

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.