Experimental Deformation of Dolomite and Mechanisms of Flow in the Calcium-Magnesium Carbonate System

碳酸盐岩是山带中常见的岩性单元，其变形对大型构造的发育起着重要作用。现场和实验研究表明，碳酸盐岩在近地表条件下的断裂强度与硅质碎屑岩相当；而深度变形碳酸盐的流动强度相对较低，穿透应变局部化。主要由方解石组成的变形灰岩和大理岩表现出大量的内部应变迹象，包括机械孪晶、微裂、溶液转移、位错滑动、位错蠕变和扩散蠕变。在高温下恢复和动态再结晶的证据是普遍存在的。这些过程也发生在实验变形方解石中，在已知的温度、压力和应变速率下进行。从这些实验中测量的力学特性提供了变形所需的环境条件的约束，以及建模大陆碰撞的构造和结构发展所需的流变学。主要由白云石（CaMg(CO3)2）组成的碳酸盐单元与由方解石（CaCO3）组成的碳酸盐单元具有明显不同的力学性能。对白云石和方解石的现场和实验观察表明，白云石的强度明显超过方解石，但高温变形的白云石也表现出孪晶、位错蠕变和动态再结晶的迹象。大多数关于白云岩变形的实验研究都与裂缝特性有关，只有少数例外的研究提供了渗透变形的流动强度和机制。白云岩的早期高温实验提供了晶体塑性变形机制的信息，但缺乏系统的流动强度测量，可用于确定这种碳酸盐的高温流变性。通过控制变形实验研究白云岩的高温变形；目的包括确定白云岩的流动规律，确定与这种流变性相关的变形过程，并评估其在造山带变形所需的条件。对天然细粒白云岩和热压合成白云岩进行了三轴压缩实验，围压为400 MPa，温度为400℃~ 900℃，应变速率为10-4 ~ 10-7 s-1。光学显微镜，扫描电镜和透射电镜被用来表征微观结构和记录变形机制。除了本研究解决的构造和构造问题外，不同组成的菱形碳酸盐的力学特性的比较还解决了涉及矿物物理和变形化学的基本问题。白云石相对于方解石的高强度提出了许多有趣的问题，包括：(1)一种阳离子Mg取代另一种阳离子Ca如何影响变形过程？(2)粘结强度如何影响变形过程的强度和热活化障碍？(3)阳离子有序如何影响变形？(4)与有序/无序和片层成分变化相关的微观结构如何影响变形和强度？由镁端元碳酸盐、菱镁矿（MgCO3）以及少量端元方解石组成的样品在与白云石实验相当的条件下变形的结果的比较，可能会阐明变形是如何依赖于晶体化学和结构的。这项研究比较了这三种碳酸盐在一组共同条件下的强度，在每种条件下激活的变形机制，以及可能告诉我们速率控制过程的活化能。

0107078NewmanCarbonates are common lithologic units in mountain belts, and their deformation plays an important role in the development of large-scale structures. Field and experimental studies show that fracture strengths of carbonates at near-surface conditions are comparable to those of siliciclastic rocks; however, flow strengths of carbonates deformed at depth are relatively low and penetrative strains are localized within them. Deformed limestones and marbles made up primarily of calcite exhibit abundant evidence of internal strain by mechanical twinning, microcracking, solution transfer, dislocation glide, dislocation creep, and diffusional creep. Evidence of recovery and dynamic recrystallization at elevated temperatures is ubiquitous. These processes also occur in experimentally deformed calcite, carried out at known temperatures, pressures and strain rates. The measured mechanical properties from these experiments provide constraints on the environmental conditions required for deformation, and the rheologies needed to model the tectonics and structural development of continental collisions.Carbonate units made up primarily of dolomite (CaMg(CO3)2) have significantly different mechanical properties from those made up of calcite (CaCO3). Field and experimental observations of dolomite and calcite indicate that the strength of dolomite exceeds that of calcite significantly, yet dolomites deformed at high temperatures also exhibit evidence of twinning, dislocation creep, and dynamic recrystallization. Most experimental studies of dolomite deformation bear on fracture properties and only a few, exceptional studies, have been done that provide flow strengths and mechanisms of penetrative deformation. Early high temperature experiments on dolomites provide information on crystal plastic deformation mechanisms, but systematic measurements of flow strengths that can be used to determine the high temperature rheology of this carbonate are lacking. This research addresses the high temperature deformation of dolomite through controlled deformation experiments; the objectives include determining the flow law for dolomite, determining the deformation processes associated with this rheology, and evaluating the conditions required for its deformation in orogenic belts. Experiments are being conducted on natural fine-grained dolomite and hot pressed synthetic dolomite in triaxial compression at confining pressure of 400 MPa, temperatures of 400 degree to 900 degree, strain rates of 10-4 to 10-7 s-1. Optical microscopy, SEM, and TEM are being used to characterize microstructures and document deformation mechanisms.In addition to the tectonic and structural problems this research addresses, the comparison of the mechanical properties of rhombohedral carbonates of differing composition addresses fundamental questions involving the mineral physics and chemistry of deformation. The high strength of dolomite relative to that of calcite raises a number of interesting questions, including: (1) How does substitution of one cation, Mg, for another, Ca, affect the deformation process?; (2) How do bond strengths affect the strengths and thermal activation barriers to deformation processes?; (3) How does cation ordering affect deformation?; (4) How do microstructures associated with order/disorder and lamellar compositional variations affect deformation and strength?Comparison of results for specimens made up of the magnesium end-member carbonate, magnesite (MgCO3), as well as a few samples of end-member calcite, deformed at comparable conditions to those of the dolomite experiments, may elucidate how deformation depends on crystal chemistry and structure. This research compares the strengths of these three carbonates at a common set of conditions, the deformation mechanisms that are activated in each, and the activation energies that presumably tell us about the rate-controlling processes.