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

0107078纽曼碳酸盐岩是山地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岩的断裂强度相当;然而，碳酸盐岩在深部变形的流动强度相对较低，渗透应变局限于其中。 主要由方解石构成的变形石灰岩和大理岩通过机械孪生、微裂纹、溶液转移、位错滑移、位错蠕变和扩散蠕变表现出丰富的内部应变证据。 在高温下回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的方解石，在已知的温度，压力和应变速率进行。 从这些实验中测得的力学性能提供的变形所需的环境条件的约束，以及所需的流变学建模构造和大陆碰撞的结构发展。碳酸盐单位主要由白云石（CaMg（CO 3）2）有显着不同的力学性能从那些由方解石（CaCO 3）。 白云石和方解石的野外和实验观察表明，白云石的强度超过方解石的显着，但在高温下变形的白云石也表现出孪生，位错蠕变，和动态重结晶的证据。 大多数白云岩变形的实验研究都与断裂性质有关，只有少数例外的研究提供了渗透变形的流动强度和机制。早期的高温实验提供的晶体塑性变形机制的信息，但缺乏系统的测量流动强度，可用于确定这种碳酸盐的高温流变性。 本研究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，确定与这种流变学相关的变形过程，并评估其在造山带中变形所需的条件。在围压400 MPa、温度400 ℃ ~ 900 ℃、应变速率10-4 ~ 10-7 s-1条件下，对天然细粒白云石和热压合成白云石进行了三轴压缩试验。光学显微镜，扫描电镜和透射电镜被用来表征微观结构和文件变形mechanism.In除了构造和结构问题，这项研究地址，比较不同组成的菱面体碳酸盐岩的机械性能解决涉及矿物物理和化学变形的基本问题。 白云石相对于方解石的高强度引起了许多有趣的问题，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)粘接强度如何影响变形过程的强度和热激活势垒？ (3)阳离子有序化如何影响变形？(4)与有序/无序和层状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（MgCO 3），以及一些样品的端元方解石，在可比的条件下变形的白云石实验的标本的结果比较，可以阐明如何变形取决于晶体化学和结构。 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激活能。

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.