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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