Non-destructive and Fast Analysis of Content and Size Distribution of Vesicles in Volcanic Rock by X-ray Computed Tomography
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摘要: 火山岩的气孔构造记录了岩浆中挥发性气体出溶、膨胀和逃逸的过程。通过对火山岩气孔特征的详细研究，有助于了解岩浆源区的挥发份含量和岩浆的上升喷发过程。目前用来研究火山岩中气孔的方法普遍存在耗时费力、采集气孔数据较少、易破坏样品等问题。本文在通过计算机断层扫描（工业CT）技术获取玄武岩投影数据的基础上，使用商用软件VG Studio MAX对样品进行三维重构和气孔体积测量，再由开源软件ImageJ对CT切片作图像处理和二维形态学运算，同时开发程序代码批量处理CT切片，快速获取气孔的含量及大小分布情况。结果表明：南海玄武岩样品在三维空间中的气孔体积分数为12.32%，大小分布呈现出对数正态分布的特点，等效球直径和最大外接圆直径分别集中分布在180~200μm、340~360μm的区间内。剖面上二维切片中的气孔含量有较大变化，但各个数值围绕体积分数波动的幅度不大，并且与气孔数密度呈显著的正相关关系。同时，通过改进海底环境下火山岩中挥发份质量分数的计算方法，得到该样品气孔体积全部转换为CO2或H2O的质量分数分别为0.233%、0.099%。研究认为，工业CT扫描结合图像处理软件可以实现火山岩气孔的无损快速统计和分析，该方法有望提高火山岩中气孔数量、体积以及挥发份含量的计算精度，为研究火山岩成因及其岩浆过程提供帮助。Abstract:
BACKGROUNDVesicular structure of volcanic rocks records the processes of the dissolution, expansion and escape of volatile gases in the ascending magma. The detailed study of characteristics of vesicles in volcanic rocks will be helpful to understand volatile content of magma and the ascent and erupting process of magma. Although a number of methods have been developed in the last decades to study the vesicle characteristics of volcanic rocks, they generally have the problems of low efficiency, less data collection, and sample destruction. OBJECTIVESTo quantitatively characterize the content and size distribution of vesicles in volcanic rocks. METHODSOn the basis of three-dimensional reconstruction of vesicular basalt by X-ray computed tomography, the content and size distribution of vesicles were calculated with three software programs (VG Studio MAX, ImageJ, Matlab), and an improved method for conversion of vesicle volume to volatile mass fraction in volcanic rock was also proposed. RESULTSThe vesicle content in three-dimensional space for a basalt sample from the South China Sea in water depth of 1488 meter was 12.32%, and the vesicle size showed a lognormal distribution. The majority of vesicles were 180-200μm in equivalent sphere diameter and 340-360μm in maximum diameter. The content of vesicles in the two-dimensional slices on the profile varied greatly, but the amplitude of each value around the volume fraction fluctuated little, and there was a significant positive correlation with the number density of vesicles. Based on the known vesicle content, the calculated mass fractions of CO2 and H2O in the sample were 0.233% and 0.099%, respectively. CONCLUSIONSThe study demonstrates that industrial CT scanning combined with image processing software can produce non-destructive rapid statistics and analysis of volcanic vesicles. The proposed method will be an efficient tool to study the genesis of volcanic rocks and their magmatic processes. -
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表 1 本文与其他学者获得的气孔体积-质量分数转换结果对照
Table 1. Comparison of conversion results of vesicle volume to mass fraction between this paper and another scholar
水深(m) 气体全部为CO2(%) 气体全部为H2O(%) 本文 Head等(2003)[37] RSD(%) 本文 Head等(2003)[37] RSD(%) 500 1.913 1.935 0.57 0.801 0.802 0.06 1000 3.678 3.761 1.12 1.575 1.575 0 1500 5.343 5.520 1.63 2.335 2.337 0.04 2000 6.914 7.216 2.14 3.084 3.087 0.05 2500 8.401 8.852 2.61 3.821 3.825 0.05 3000 9.809 10.432 3.08 4.545 4.553 0.09 3500 11.145 12.147 4.30 5.258 5.359 0.95 注：气体体积分数ηCO2/H2O(vol/%)= ηvesicle(vol/%)=75%，深水压强P(MPa)=1026(kg/m3)×9.8(m/s2)×Depth(采样水深, m)/106+0.101325(MPa, 标准大气压)，岩浆温度T=1255℃，熔体密度ρlava=2700(kg/m3)；气体密度ρCO2/H2O(kg/m3)采用的数值不同，Head等[37]由理想气体状态方程计算得到，本文是先在系统中安装好Nist Refprop9.1，并将其加载到Excel宏中，然后在单元格中输入气体的密度函数[38]调用NIST数据库特定温压下的气体密度得到；岩石密度ρrock(kg/m3)= ρlava(kg/m3)×[1- ηCO2/H2O(vol/%)]+ ρCO2/H2O(kg/m3)× ηCO2/H2O(vol/%)，气体质量分数ηCO2/H2O(wB/%)= ρCO2/H2O(kg/m3)× ηCO2/H2O(vol/%)/ ρrock(kg/m3)×100%。RSD为两种方法CO2质量分数计算值的相对标准偏差。 
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