High-Resolution X-ray Computed Tomography as a Tool for Visualization and Quantitative Analysis of Igneous Textures in Three Dimensions
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High-resolution X-ray computed tomography of igneous rocks provides a rapid, non-destructive means to acquire textural data and imagery in three dimensions. Tomographic images are maps of the spatial variations within a specimen of the average linear attenuation coefficient for the input X-ray spectrum, a quantity that depends strongly on mass density and also on atomic number. Visualization of these data benefits from the ability to view sections through the specimen in any location and orientation, and from the ability to extract features of particular interest and display perspective views of them in arbitrary orientation. These images can then be animated to facilitate comprehension of three-dimensional textural relationships. The numerical character of the method also makes these data sets inherently rich in the metrical information required for quantitative characterization and statistical analysis of the sizes, shapes, locations, and interrelations of textural components in igneous rocks.
Examples of the scientific value of tomographic textural analysis are numerous. Images of the distribution of metal veins and particles in a lodranite meteorite define scales and mechanisms of transport during silicate-metal segregation resulting from partial melting of primitive compositions. Diamonds in eclogites are seen in tomographic imagery to have a close spatial association with a network of subplanar alteration zones, implying that the diamond does not have an igneous origin, but is instead a product of later metasomatism. In a slowly cooled basalt flow, tomographic images reveal plagioclase crystals linked in monomineralic chains, forming continuous networks that affect the flow's rheology and its chemical differentiation, and record its compaction history. Comingling of compositionally dissimilar magmas occurs across an interface that exhibits delicate interfingering of the contrasting melts at all observed length scales. Measurement of the size and shape distributions of vesicles in basalts yields information on the principal driving forces for volcanic eruptions by revealing details of bubble growth, nucleation, and coalescence, and permits estimates of paleoelevation at the time of emplacement of the flows. Images of leucosome/melanosome/mesosome relationships in migmatites provide quantitative constraints on the topology of melt flow paths during anatexis.
Key wordsX-ray computed tomography textural analysis visualization
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