An Experimental Study of the Breakdown of Dolomite in Mixed-Volatile Metamorphic Environments

变质石灰岩和白云岩中矿物的组成和丰度通常被用来确定变质流体的性质。重要的属性是流体的组成、流体流量和流体流动的地形尺度模式。在变质体系中，矿物组合的发展依赖于几个速率常数的相对值，如导水率、导热率、扩散和分散性以及反应进程。在这项研究中，我们计划确定在混合挥发环境中影响白云石稳定性的反应动力学。在富水条件下变质的白云岩中，一个常见的反应顺序是：白云石=方解石+方镁石+二氧化碳，其次是方镁石+水镁石。在混合挥发流体中超镁铁质岩石的变质过程中也会发生类似的反应。白云岩体系是研究反应动力学的理想之选，因为这些组合赋存于接触和区域变质地体中的各种贫硅岩石中。我们计划的一系列实验包括三个部分。首先，我们将在热液容器中对粉状原料进行时间序列实验，以提供矿物和同位素反应速率的上限。其次，我们将借助激光拉曼光谱，在水热金刚石-压腔中原位观察快速和不可熄灭的反应。在实验研究的第三部分，我们计划使用圆柱状的白云岩作为起始材料，以更接近地模拟自然反应。在所有实验中，我们计划使用同位素标记的流体来确定同一实验中的矿物学反应和同位素交换反应的速率。我们将把实验研究的结果，特别是岩心实验的结果，与来自德克萨斯州大理石峡谷的接触变质白云岩的观察结果进行比较，该白云岩经历了方镁石形成反应和水镁石形成退变反应。

LabotkaEAR-0087553The compositions and abundances of minerals in metamorphosed limestone and dolomite rock are commonly used to determine properties of the metamorphic fluid. The important properties are the composition of the fluid, the fluid flux, and terrain-scale patterns of fluid flow. In metamorphic systems, the development of mineral assemblages depends on the relative values of several rate constants, such as hydraulic conductivity, thermal conductivity, diffusion and dispersivity, and reaction progress. In this study, we plan to determine the kinetics of reactions that affect the stability of dolomite in mixed-volatile environments. A common sequence of reactions in dolomitic rocks metamorphosed under H2O-rich conditions is dolomite = calcite + periclase + CO2, followed by periclase + H2O = brucite. Similar reactions occur during metamorphism of ultramafic rocks in mixed-volatile fluids. The dolomite system is ideal for the study of reaction kinetics because the assemblages occur in a variety of silica-poor rocks in contact and regional metamorphic terrains. Our planned set of experiments consists of three parts. First, we will conduct time-series experiments on powdered starting materials in hydrothermal vessels to provide upper limits on mineral and isotope reaction rates. Second, we will observe fast and unquenchable reactions in situ in the hydrothermal diamond-anvil cell with the aid of laser Raman spectroscopy. In the third part of experimental investigation we plan to use cylinders of dolomite rock as starting material to mimic more closely the natural reactions. In all experiments, we plan to use isotopically labeled fluid to determine the rates of not only the mineralogic reactions, but also the isotope-exchange reactions in the same experiments. We will compare the results of the experimental study, particularly of the rock core experiments, with observations of the contact-metamorphosed dolomite from Marble Canyon, Texas, which experienced both the periclase-forming reaction and the brucite-forming retrograde reaction.