Deformation-controlled cation diffusion in tourmaline: A microanalytical study on trace elements and boron isotopes
Deformation-controlled cation diffusion in tourmaline: A microanalytical study on trace elements and boron isotopes
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电气石中变形控制的阳离子扩散：微量元素和硼同位素的微分析研究
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发表时间：
2007
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通讯作者：
S. Kasemann
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作者：
S. Büttner;S. Kasemann
摘要不同强度的塑性变形改变了来自阿根廷西北部Sierras Pampeas的糜棱伟晶岩中电气石的原生结构、成分和元素分布。在角闪岩相P-T条件下，同心电气石原生分带被叠加，部分被同动阳离子扩散破坏。结构变化包括晶体弯曲、亚晶形成和亚晶旋转，导致电气石局部重结晶。部分变形晶体受塑性变形和二次阳离子扩散的影响较小或不受影响。晶内扩散和基质交换是造成变形电气石微区成分变化的两个主要过程。晶内扩散部分或完全破坏了从高浓度区到低浓度区的阳离子扩散，破坏了原生微量元素的分带。占据Y位的小阳离子即使在弱变形的情况下也很容易被活化，而X位上的大阳离子需要较高的变形强度才能被活化，并且表现出不那么明显的原生分带或没有均质。电气石与共存的固体或流体之间的基质交换通常会导致变形的电气石微区中的微量元素流失。在糜棱岩化过程中，基质交换的程度主要受活化阳离子物种与基质相的相容性控制。相反，变形和未变形晶区的B同位素组成是一致的，表明在晶体生长和塑性变形过程中没有明显的B扩散和同位素分馏；即电气石中移动B的激活能必须太高，即使强烈的塑性变形也无法克服。
Abstract Plastic deformation of variable intensity has modified primary textures, composition, and element distribution in tourmaline from a mylonitic pegmatite from the NW Argentinean Sierras Pampeanas. The primary concentric tourmaline zonation has been overprinted and partly destroyed by synkinematic cation diffusion that occurred under amphibolite-facies P-T conditions. Textural changes include bending of crystals, formation of subgrains, and subgrain rotation, leading locally to recrystallization of tourmaline. Some parts of deformed crystals were less affected or unaffected by plastic deformation and secondary cation diffusion. Two principal processes, intracrystal trace-element diffusion and matrix exchange of trace-elements, caused the compositional variations in deformed tourmaline domains. Intracrystal diffusion partly or completely destroyed the primary trace-element zonation by cation diffusion from zones of high to low concentration. Small cations occupying the Y-position are readily mobilized even by weak deformation, whereas large cations on the X-site require higher deformation intensity to be mobilized and show less pronounced or no homogenization of the primary zonation. Matrix exchange between tourmaline and co-existing solid or fluid phases generally leads to trace-element loss from deformed tourmaline domains. The extent of the matrix exchange is primarily controlled by the compatibility of mobilized cation species with matrix phases during mylonitization. In contrast, the B-isotopic compositions in deformed and undeformed crystal domains are uniform, indicating absence of significant B diffusion and isotope fractionation during crystal growth and plastic deformation; i.e., the activation energy to move B in tourmaline must be too high even for intense plastic deformation to overcome.