Halogen concentrations and stable Cl isotopes in apatite as a fluid probe: mapping regional-scale fluid pulses by Cl-isotopes

流体-岩石相互作用导致元素的移动和分馏。它与经济矿床的形成有关，并对受影响岩石的岩石物理性质和相关大系统的地球动力学产生影响。与流体-岩石相互作用相关的一个基本问题是流体如何在低渗透结晶岩石中流动——流体是如何通道的，以及在区域尺度上流体流动的地球化学特征是什么？为了更好地理解流型的形成和反应前沿的传播，我们需要流体的化学示踪剂（例如卤素比和稳定的Cl同位素）和可作为流体探针的流体成分指示矿物。磷灰石存在于各种岩石类型中，是一种典型的副脉状矿物，存在于温度范围广的热液系统中。磷灰石通过溶解-再沉淀对流体成分的变化反应相当敏感，使其成为流体探针的合理候选者。磷灰石的另一个优点是，与许多其他矿物不同，它通常含有可测量浓度的卤素。本项目旨在利用现场数据，结合实验得出的分配系数和同位素分馏数据，开发一种基于磷灰石的流体探针。最后，对磷灰石的卤素浓度和稳定的Cl同位素组成进行结构控制的原位测量，将有助于定量地破译反应流体的化学演化，探测不同的流体脉冲、主要的流体路径以及流体流动的方向和程度，并结合交代传质数据。这将是了解结晶岩流体流动系统以及研究经济矿床形成的有力工具。

Fluid-rock interaction causes element mobilisation and fractionation. It is related to the formation of economic mineral deposits and has consequences for the petrophysical properties of the affected rocks and the geodynamics of the related large-scale systems. One fundamental question associated with fluid-rock interaction is how fluids flow in lowpermeability crystalline rocks – how are fluids channelled and what is the geochemical signature of fluid flow on a regional scale? To better understand flow pattern formation and reaction front propagation, we need chemical tracers for the fluid (e.g., halogens – ratios and stable Cl isotopes) and fluid-composition indicative minerals that can be used as fluid probes. Apatite occurs within all kinds of rock types and is a typical accessory vein-mineral found in hydrothermal systems over a wide range of temperatures. Apatite reacts rather sensitively to changes of fluid compositions by dissolution-reprecipitation, making it a reasonable candidate as a fluid probe. Another advantage of apatite is that, unlike many other minerals, it routinely incorporates measurable concentrations of halogens. This project aims to develop an apatite-based fluid probe by applying field data combined with experimentally derived partition coefficients and isotopic fractionation data. Finally, textural controlled in situ measurements of halogen concentrations and the stable Cl isotopic compositions of apatites will allow to decipher quantitatively the chemical evolution of a reacting fluid, to detect distinct fluid pulses, the main fluid pathways, and the direction and the extent of fluid flow in a combination with data about the metasomatic mass transfer. This could be a powerful tool for understanding fluid flow systematics in crystalline rocks as well as for the research on the formation of economic mineral deposits.