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BACKGROUND Vesicular 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. OBJECTIVES To quantitatively characterize the content and size distribution of vesicles in volcanic rocks. METHODS On 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. RESULTS The 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. CONCLUSIONS The 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.

Olivine compositions are widely used to constrain magmatic thermodynamic conditions such as magmatic temperature, oxygen fugacity, and H2O content. However, elemental diffusion may change the initial compositions and lead to large uncertainty on the estimation of these thermodynamic conditions. In this study, we conducted LA-ICP-MS elemental mapping and EPMA analysis of olivine phenocrysts and olivine-hosted spinel from the Jiagedaqi (JGD) alkaline basalts in northeast China to evaluate the influence of elemental difusion on olivine-composition-based geothermometry, oxybarometry, and hygrometry. The JGD olivines show normal Fo [Mg/(Mg + Fe) × 100 in moles] zoning, with cores having Fo of 77–87 and rims having Fo of 67–73. The constant P contents from core to rim indicate that these compositional zonings were caused mainly by difusion. Because Al is a slow-diffusing element and its content is relatively constant from core to rim, the temperature calculated by the Al-in-olivine thermometer is not influenced by elemental difusion and preserves the JGD olivine crystallization temperature up to 1150 °C. The temperatures calculated using the Sc/Y-in-olivine thermometer, the oxygen fugacity calculated using the olivine–spinel oxybarometer, and the H2O content calculated on the basis of Ca partitioning between olivine and melt are strongly influenced by the diffusion of Fo, Sc/Y, and Ca. However, the compositional plateaus in olivine cores, which were not influenced by elemental diffusion, preserve the magmatic temperature (1150 °C), oxygen fugacity (QFM+1.4), and H2O content (4 wt%) that applied during the formation of the JGD olivines. Together, these findings suggest that the mantle source of the JGD basalts was metasomatized by fluids released from the subducted slab. This study highlights that elemental difusion in olivine phenocrysts can strongly afect the application of olivine-composition-based geothermometers, oxybarometers, and hygrometers. However, primitive olivine cores that have not been influenced by difusion preserve the initial magmatic thermodynamic conditions. [ABSTRACT FROM AUTHOR]

Determination of Pb isotopes in melt inclusions by Laser Ablation -Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry (LA-MC-ICP-MS) is limited to samples formed later than Mesozoic because U and Th signals are not monitored. In this study, Pb isotopic compositions of two reference glasses and ancient melt inclusions which have undergone significant U-Th decay are determined by LA-MC-ICP-MS. The effect of the U/Pb and Th/Pb analytical errors on the calculation of initial Pb isotopic ratios was also evaluated. In the experiment USGS NKT-1G is used as the external calibrant and a standard-sample-standard bracketing method is used to correct instrument drift and mass bias. Analytical precisions of 208Pb/206Pb and 207Pb/206Pb of USGS standards, BHVO-2G and TB-1G, are better than 0.30% (2RSD) and both agree within 0.30% of the preferred values. However, 232Th/206Pb and 238U/206Pb ratios are highly variable and yield an external precision of 5%. Error propagation calculations indicate that the ages of the samples have significant effects on the errors of the initial lead isotopic ratios. For Phanerozoic samples that are earlier than 540 Ma, even the differences between the measured 232Th/206Pb and 238U/206Pb ratios and the real values reach 10%, yet the age-corrected initial lead isotopic ratios are still within 0.80% deviations relative to the real values. Therefore, the proposed method in this study can determine the lead isotopic compositions of melt inclusions in samples from Cenozoic to Paleozoic, expanding the geological application.