Abstract:

In recent years, quantitative textural studies of igneous rock have shown that by using conventional rock and mineral identification equipment, one can obtain two-dimensional petrographic photos of different scales of igneous rock, so that mineral particles can be identified with naked eyes. Furthermore, by using image processing and textural analysis software the textural characteristics of igneous rocks can be accurately quantify. In this paper, we summarized the advantages and disadvantages of various observation methods commonly used in the two-dimensional quantitative textural analysis in igneous petrology. The quantitative textural parameters of igneous rocks with grain size below millimeter level can be analyzed by two or more observation methods using polarized microscope with transmission, reflective light or cathodoluminescence and back scatter-electron imaging techniques to obtain digital images of high precision and accuracy. All kinds of mafic minerals with crystal size less than 0.03 mm can be analyzed by reflective light and back scatter-electron image. One can assign different colors to look-alike gray-scale mafic mineral images by image processing software to improve mineral identification. Polarized microscope cathodoluminescence can be used for the analyses of felsic and most accessory minerals which are difficult to distinguish under conventional polarized microscope. The rock-forming minerals with mm or above grain size can be quantitatively analyzed by polished large sections or field measurement. In order to facilitate researchers in related fields to use this method, we listed the specific analysis steps in detail. Some detailed analyses are made by combining the quantitative textural data of olivine phenocryst in a basalt sample, focusing on how to accurately identify the boundaries of mineral particles, to determine mineral content and shape, analysis area and crystal numbers, and to distinguish different crystal population. The results show that the intercept and slope of crystal size distribution (CSD), mineral content, alignment factor and crystal shape of granular minerals are not significantly different for particle numbers between 100-500, but the maximum crystal length and degree of particle aggregation are underestimated. For particle numbers less than 300, the R value of crystal spatial distribution pattern will be overestimated by 0.05-0.2, which were largely ignored in previous studies. All textural parameters tend to be stable and precision and accuracy are significantly improved for particle numbers above 500. At the present, most of the textural parameters provided by researchers are usually statistical results and often related to the observation and statistical methods, and the intercept and slope of CSD lack the corresponding original data, which is not convenient for the comparative study among peers. We suggest that detailed analysis steps be provided with the original data when publishing results related to textural parameters in the future. The recommended analysis steps are as follows: (1) Identify boundaries of aggregated minerals or deal with it concretely; (2) Determine crystal three-dimensional shape and whether CSD parameter change among samples is caused by shape parameter change; (3) Obtain minimum particle size of minerals that can be accurately measured; and (4) Estimate sample homogeneity and precision of analysis on samples with large particle numbers so 3-5 repeated measurements can be performed. The original data should (1) contain analysis of at least two different areas of the sample. For multiple authors, at least two independent analyses by different authors is recommended for precision and accuracy evaluation. And (2) the number of crystals in different crystal size intervals of each sample, the original high-resolution mineral outline of the sample or the relevant original parameters of image analysis should be provided in the text or appendix of the article. When multiple crystal populations appear in igneous rocks, the quantitative textural parameters often reflect the mixed characteristics of multiple crystal populations and are related to the proportion of different crystal populations. Future research needs to focus on identifying textural parameters of different crystal populations by combining multiple observation methods and micro-area composition analysis. More effective methods should be developed to identify crystal populations with similar grain size and composition, which is of great significance for the understanding of the genesis of the textural diversity of igneous rocks and magmatic processes.

Key words: igneous rock, quantitative textural analysis, petrography, crystal size distribution, polarized microscope