Experimental Investigation of Chlorine Substitution into Calcic Amphiboles

这项研究的结果应有助于深入了解海水与大洋地壳岩石的相互作用、富氯流体在地壳深部岩石中产生矿物学变化的作用以及与成矿有关的流体的组成等基本过程。在广泛的地质环境中观察到了富含氯的角闪石，但对它们是如何形成的知之甚少。这项建议的重点是实验研究氯在常见成岩矿物角闪石和含氯卤水中的分配。通过校准角闪石中氯的浓度随宿主角闪石的组成(如其铁含量)和周围卤水的氯含量的变化，地质学家可以确定曾经与角闪石接触但不再存在的流体的氯含量。该项目的重要成果将是确定角闪石中的哪些成分变量对控制其结合氯的能力最重要，并量化氯在矿物和流体之间的分布。这项研究项目将通过检验文献中提出的一些晶体-化学关联，提供一些关于氯取代角闪石的组成控制的初步实验数据。将研究四个特定的关联：(I)八面体位置(即Fe#)上的Fe_2对Mg的替代以及确定氯含量是否与Fe#成线性变化；(Ii)K在晶体A位的占有率；(Iii)四面体Al替代Si；以及(Iv)八面体Fe_3替代八面体Al。这些组成变量将主要使用闪锌矿和闪石来研究，这两种闪石在自然界中都有很高的氯含量，可以合成出足够大的尺寸来进行常规的电子探针分析。将进行一套补充实验，在不同类型和氯浓度的盐水中合成角闪石，以确定在给定的压力、温度和氧逸度条件下在角闪石中获得的最大氯含量或饱和水平。这项研究的结果将有助于我们更好地理解哪些组成变量对控制氯在角闪石中的取代作用最重要，并量化氯在钙角闪石和环境卤水之间的分配。这些信息应用于地质科学的不同领域，包括海水-岩石相互作用、与氯离子交代有关的流体中的氯离子浓度、盐水溶液在形成高级变质岩中的作用，以及确定产生重要经济矿床的流体的特征。

The results from this study should provide insights into such basic processes as the interaction of seawater with oceanic crustal rocks, the role of chlorine-rich fluids in producing mineralogical changes in deep-crustal rocks, and the compositions of fluids associated with ore-mineral formation. Chlorine-rich amphiboles have been observed in a wide range of geological settings yet little is known about how they form. The focus of this proposal is to investigate experimentally the partitioning of chlorine between the common rock-forming mineral amphibole and chlorine-bearing brines. Calibrating how the concentration of chlorine in the amphibole varies with both the composition of the host amphibole (such as its iron content) and the chlorine content of the ambient brine permits geologists to determine the chlorine content of fluids that were once in contact with the amphibole but are no longer present. Important outcomes from this project will be to identify what compositional variables in the amphibole are most important in controlling its ability to incorporate chlorine, and quantifying the distribution of chlorine between mineral and fluid. This research project will provide some of the first experimental data on the compositional controls on the substitution of chlorine into amphibole by examining some of the crystal-chemical correlations that have been proposed in the literature. Four specific correlations will be investigated: (i) substitution of Fe2+ for Mg at octahedral sites (i.e., the Fe#) and determining whether or not the Cl content varies linearly with the Fe#, (ii) occupancy of K at the crystallographic A site, (iii) substitution of tetrahedral Al for Si, and (iv) substitution of octahedral Fe3+ for octahedral Al. These compositional variables will be studied primarily using the amphiboles ferro-pargasite and hastingsite, both of which have been observed with high Cl contents in nature and which can be synthesized with sufficient size for routine electron microprobe analysis. A complementary set of experiments will be done where amphiboles are synthesized in brines of varying types and Cl concentration in order to determine the maximum Cl content, or saturation level, obtained in the amphibole at a given set of pressure?temperature?oxygen-fugacity conditions. The results of this study should improve our understanding of what compositional variables are most important in controlling the substitution of Cl into amphibole and quantify the partitioning of Cl between calcic amphiboles and an ambient brine. This information has applications to diverse areas in the geological sciences, including seawater-rock interaction, chorine concentration in fluids associated with Cl-metasomatism, the role of saline solutions in forming high-grade metamorphic rocks, and characterizing fluids that yield economically important deposits.