Bubble-Crystal Interactions in a Young Basalt Lava Flow – Preliminary Data from 3D X-Ray Microtomography

English Abstract:

The Tseax monogenetic volcano lies at the southern margin of the Northern Cordillera volcanic province, BC, Canada. Its last eruption (1691±23 CE) produced a 32 km long basaltic lava flow which may have been triggered by the Cascadia megathrust earthquake in 1700 CE (Higgins, 2009).

The lava contains a framework of tabular plagioclase crystals with smaller interstitial equant olivine crystals, all set in a glassy matrix. Samples a few mm in diameter were imaged by X-ray µ-CT at APS Chicago, with the aim of studying the relations between the crystals and the gas bubbles (vesicles).

Olivine crystals are accompanied by small bubbles, located on their surface asperities, or within them, impeding crystal growth in that direction. Large bubbles, perhaps a first generation that had more time to grow and coalesce, are defined and restricted in size and shape by a framework of plagioclase crystals. They adhere perfectly to the longer plagioclase faces (010) but there is a melt film between them and the shorter faces as well as in the corners of two touching plagioclase crystals. This suggests that we should perhaps distinguish the process of nucleation of bubbles on crystal surfaces and their enlargement by surface wetting, for different crystals and different faces.

The texture of the large bubbles suggests that the pressure wave of the Cascadia megathrust earthquake may have enabled the growth of bubbles by wetting of plagioclase surfaces even if no new bubbles nucleated. The reduction in density of the magma could then have triggered the eruption.

Our next stage is to further quantify the morphological relations and to measure the contact angles between bubbles and different mineral phases. Combining these with mineral composition and structure differences will hopefully lead to a better understanding of bubbles and non-oxide crystal dynamics.