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

白云石实验变形及碳酸钙镁体系流动机制

• 批准号: 0107078
• 负责人: Julie Newman
• 金额: $ 14.54万
• 依托单位: Texas A&M Research Foundation
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-06-01 至 2004-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0107078&HistoricalAwards=false
• 关键词: Experimental; Deformation; Dolomite; Mechanisms; Flow

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.

0107078Newman碳酸盐岩是山地带常见的岩性单元，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的断裂强度相当；然而，在深度变形的碳酸盐的流动强度相对较低，并且穿透应变局限于其中。 主要由方解石组成的变形石灰石和大理石通过机械孪生、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应变证据。 高温下恢复和动态重结晶的证据随处可见。 这些过程也发生在实验变形的方解石中，在已知的温度、压力和应变率下进行。 这些实验测量的机械性能为变形所需的环境条件以及模拟大陆碰撞的构造和结构发展所需的流变性提供了约束。主要由白云石 (CaMg(CO3)2) 组成的碳酸盐单元具有显着不同的机械性能方解石 (CaCO3) 的特性。 对白云石和方解石的现场和实验观察表明，白云石的强度显着超过方解石，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证据。 大多数白云石变形的实验研究都与断裂特性有关，只有少数特殊的研究提供了流动强度和穿透变形的机制。白云石的早期高温实验提供了有关晶体塑性变形机制的信息，但缺乏可用于确定这种碳酸盐高温流变学的流动强度的系统测量。 本研究通过受控变形实验解决白云石的高温变形问题；目标包括确定白云石的流动规律、确定与流变学相关的变形过程以及评估其在造山带变形所需的条件。正在对天然细粒白云石和热压合成白云石进行三轴压缩实验，围压为400 MPa，温度为400 至900 度，应变速率为10-4 至10-7 s-1。光学显微镜、SEM 和 TEM 用于表征微观结构并记录变形机制。除了本研究解决的构造和结构问题外，不同成分的菱面体碳酸盐的机械性能比较还解决了涉及矿物物理和变形化学。 白云石相对于方解石的高强度引发了许多有趣的问题，包括：（1）一种阳离子 Mg 替代另一种阳离子 Ca 是如何影响变形过程的？ (2) 粘合强度如何影响变形过程的强度和热激活势垒？ (3) 阳离子排序如何影响变形？； (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 (MgCO3) 以及一些端元样品组成的样本的结果比较方解石在与白云石实验相当的条件下变形，可以阐明变形如何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常见条件下的强度、每种碳酸盐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。