Quantification of vesicle characteristics in some diatreme-filling deposits, and the explosivity levels of magma-water interactions within diatremes

摘要

Vesicles within juvenile fragments in mafic pyroclastic deposits contain important information about the state of the magma at the time of fragmentation. There have been few vesicle studies of juvenile pyrodasts from mafic phreatomagmatic deposits, however, and none we can find from maar-diatreme volcanoes. In this paper we document the vesicularity and vesicle-population characteristics of juvenile fragments sampled from non-bedded lithified deposits of the Coombs Hills diatreme complex, part of the Ferrar large igneous province, Antarctica. The diatreme-filling pyroclastic deposits, dominated by lapilli tuffs and tuff breccias, contain typically abundant lithic clasts derived mostly from the enclosing sedimentary sequence, and several types of juvenile clasts ranging from blocky to fluidal or "raggy". In the samples measured, 77-80% of the juvenile pyroclasts ranging in size from 0.5 mm to fine lapilli is in the 'non-vesicular' to 'incipiently vesicular' range (<20% vesicles). Such low vesicularities are expected for pyroclasts from maar-diatreme volcanoes where fragmentation takes place at depth in the diatreme or root zone due to magma-water interaction. A few juvenile clasts, however, are more vesicular, and seven of these were chosen and sectioned for 2D analysis of vesicle shapes and orientation, vesicle number densities (Nv), and vesicle volume distributions. The shapes of the vesicles in the studied sections are mostly elliptical (sometimes polylobate), with mean aspect ratios ranging between 0.67 and 0.72. Circular statistics are used to test for trends in the vesicle long-axis orientation data; non-uniformity of orientations is found in most cases, but the trends are weak. Vesicle volume distributions are often bimodal due to variable coalescence. Total N_v values range from 1.0×10~2 to 5.7 × 10~3 mm~(-3); taking the effects of bubble coalescence into account, these values are similar to those found in pyroclasts from other phreatomagmatic volcanoes, although they also overlap partly with those seen in fire fountain deposits and some basaltic Plinian eruptions. Fluidal- or rag-shaped juvenile clasts, some circular vesicles, and the lack of microlites all suggest that the Coombs Hills magma had a relatively low viscosity prior to fragmentation, despite the basaltic andesite composition. This low viscosity allowed parts of the magma to be fragmented in a non-brittle fashion during phreatomagmatic explosions and to form fluidal clasts. Phreatomagmatic explosions in diatremes can therefore produce diverse types of juvenile clasts simultaneously, and the proportions of each will depend on the explosivity of the magma-water (slurry) interaction and other factors. Recycling of fragments is also thought to be an important factor in generating mixtures of different types of juvenile fragments in diatremes.